Volcanism and basalt weathering drove Ordovician climatic cooling

Volcanism and basalt weathering drove Ordovician climatic cooling | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Volcanism and basalt weathering drove Ordovician climatic cooling Lei Zhang, He Zhao, Thomas Algeo, Zhengyi Lyu, Xiangdong Wang, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6593045/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 12 Dec, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Causal relationships among the major environmental and biological developments of the Ordovician Period (i.e., long-term climatic cooling, Hirnantian Glaciation, Great Ordovician Biodiversification Event, spread of bryophyte-grade land plants, and Late Ordovician Mass Extinction) remain in debate. Here, we present new data for volcanic activity, sea-surface temperatures, and chemical weathering intensity, based respectively on Hg geochemistry, conodont oxygen and strontium isotopes. This dataset documents a ~25-Myr-long interval of climatic cooling (ca. 470-445 Ma), which commenced around the Lower/Middle Ordovician boundary and intensified near the Middle/Upper Ordovician transition, ultimately culminating in the Hirnantian Glaciation. Cooling was associated with long-term intensified weathering of volcanic rocks (basalt) and drawdown of atmospheric pCO 2 , as well as periodic land plant expansion and photic-zone euxinia, during major volcanic intervals and their subsequent phases. These relationships implicate volcanic activity as the primary driver of contemporaneous environmental and climatic changes, with the spread of early land plants as a potential secondary influence, thus revealing complex modulation of life-environment coevolution during the Ordovician Period. Earth and environmental sciences/Planetary science/Geochemistry Earth and environmental sciences/Climate sciences/Palaeoclimate Figures Figure 1 Figure 2 Figure 3 Introduction The Ordovician Period (485.4–443.8 Ma) was bookended by major biotic events, i.e., the early stages of the Great Ordovician Biodiversification Event (GOBE) at its onset and the Late Ordovician Mass Extinction (LOME), the first of the “Big Five” mass extinction, near its termination 1 . During the GOBE, extending from the early Tremadocian (Early Ordovician) to the middle Katian (Late Ordovician) with an early Darriwilian peak 2,3 , the numbers of marine invertebrate species and genera increased by factors of ~6× and 3×, respectively. A key aspect of this diversification event was the proliferation of marine plankton (e.g., primary producers such as photosynthetic cyanobacteria), representing the "Ordovician Plankton Revolution" 4 . The LOME, which had a ~70-80% species-level extinction rate, occurred in two phases corresponding to the onset and termination of the Hirnantian Glaciation: LOME-1 in the early Hirnantian, which eliminated many tropical taxa, and LOME-2 during the mid-Hirnantian, which decimated the cool-water Hirnantia Fauna 5 . A precursor biocrisis (LOME-0) has recently been identified in the mid-Katian 2,6 . On land, the Ordovician witnessed a transition from a microbial cryptopedobiota to sparsely distributed, embryophyte-based terrestrial ecosystems 7 , during which non-vascular (i.e., bryophyte-grade) land plants evolved and began to colonize the continents 8,9 . The fossil record for the earliest bryophyte-grade land plants consists of dispersed spores (i.e., cryptospores) that first appeared in Gondwana during the early Middle Ordovician (Dapingian 10,11 ) and then spread globally 12,13 . The Late Ordovician was a key interval during which the earliest tracheophytes and, possibly, eutracheophytes (i.e., vascular plants) appeared and began to diversify 14 . The driving mechanisms of the Ordovician biological radiations and extinctions have long been speculated upon without achieving a consensus 15-17 . Despite their complex internal structure and chemical composition 18 , the oxygen isotopic composition of conodonts (δ 18 O conodont ) is a valuable tool for study of sea-surface temperatures (SSTs) in the Paleozoic Era 19-21 . Conodont δ 18 O data have established that the GOBE coincided with a long-term global cooling trend with a total magnitude of ~10 °C 20,22,23 , a pattern confirmed by clumped isotope analysis of carbonates 24 . Climatic cooling is postulated to have been a direct driver of evolution among marine invertebrates 20 , as have rising oxygen levels in the atmospheric-oceanic system 15 , although the pattern and trigger of the Ordovician climatic cooling remain contentious. For example, it is debated whether this cooling trend was a continuous long-term event or an episodic, multistage process 20,22,23 . Atmospheric CO 2 levels, a key determinant of the long-term climate evolution of the Earth, are thought to have fallen through enhanced silicate weathering 25 linked to the spread of the earliest land plants 26,27 , or possibly to elevated marine primary productivity 28 , which are not necessarily mutually exclusive mechanisms. Moreover, the patterns of changes in climate and biodiversity in South China (i.e., the most intensively studied craton, and one that may have been the cradle of diversification) versus at a global scale are significantly different 2 , complicating an understanding of the causal relationship between the Ordovician climatic cooling and contemporaneous bioevolutionary developments. Volcanic activity is the principal natural source of mercury (Hg) in the Earth-surface system 29 . Volcanic eruptions can emit substantial quantities of elemental mercury having a residence time of a few years (~1.5 yr) in the atmosphere, which facilitates its long-range transport and eventual deposition in both marine and terrestrial sediments 30,31 . Massive inputs of Hg during major volcanic events may exceed the absorption capacity of organic matter at the sediment-water interface, resulting in positive anomalies of the ratio of total mercury to total organic carbon (TOC) (Hg/TOC) in sedimentary successions that are signals of volcanic Hg inputs 29,32 . The isotopic composition of Hg serves as an additional powerful tool to decipher sources and mechanisms of Hg enrichment (e.g., 31 ). Hg isotopes can reveal both mass-dependent fractionation (MDF; δ 202 Hg) and mass-independent fractionation (MIF; Δ 199 Hg, Δ 200 Hg, Δ 201 Hg). Critically, MIF arises predominantly through aqueous or atmospheric photochemical interactions antecedent to Hg sedimentation, and its isotopic signals are resilient against post-depositional diagenetic alteration. Mercury emissions from subaerial volcanoes typically exhibit near-zero MIF values with minimal variance 29,33 . Volcanic activity may have induced long-term climatic cooling (e.g., Hirnantian Glaciation) during the Late Ordovician 34 , offsetting transient warming associated with greenhouse gas emissions from volcanic eruptions 28 . However, studies integrating paired proxies for both volcanism and temperature are limited, leaving the role of volcanic activity in the long-term cooling trend of the Ordovician unclear 25 . Here, we studied three biostratigraphically well-constrained Ordovician marine successions at Huanghuachang (n.b., the Global Stratotype Section and Point (GSSP) of the Lower/Middle Ordovician boundary), Chenjiahe (n.b., the global auxiliary stratotype section of the same boundary), and Wangjiawan (n.b., the GSSP for the base of the Hirnantian Stage), which are geographically closely spaced (~15 km) near Yichang City (Figs. S1-S2). We generated paired high-resolution profiles of oxygen isotopes for conodont bioapatite (δ 18 O conodont ) as paleotemperature proxies, Hg-system chemistry (Hg/TOC, Δ 199 Hg and δ 202 Hg) as volcanic proxies, and strontium isotopes for conodont bioapatite ( 87 Sr/ 86 Sr conodont ) as a continental weathering proxy. These data allowed us to evaluate the patterns and driving mechanisms of the Ordovician climatic cooling, providing key insights regarding life-environment co-evolution during the Ordovician Period. Results Conodont in-situ oxygen isotopes Our conodont in-situ oxygen isotope record (δ 18 O conodont ) shows a long-term secular increase from ~ + 15–17‰ in the Lower Ordovician Nanjinguan Formation (Fm) to ~ + 16–19‰ in the Middle Ordovician Dawan to Guniutan Fm, and ~ + 18–20‰ in the Upper Ordovician Wufeng Fm (Fig. S3A). We further compiled newly generated data with previously published δ 18 O conodont data using either gas isotope ratio mass spectrometry (GIRMS), sensitive high resolution ion microprobe (SHRIMP) or SIMS. Previous studies suggest a bias between SHRIMP and GIRMS conodont analyses, in which the former is systematically ∼0.6–1.3‰ higher than the later 35 , while none systematically bias between SHRIMP and SIMS analyses. To enhance consistency across different data sources during data compilation, we apply a minimum correction of + 0.6‰ when converting GIRMS results to the SHRIMP standard. In the compiled δ 18 O conodont curve, average values for the latest Cambrian (16.2 ± 0.3‰), and the Lower, Middle and Upper Ordovician (16.2 ± 0.8‰, 17.8 ± 0.7‰ and 19.4 ± 0.9‰, respectively) are used as baseline values for evaluation of cooling intervals (Fig. 1 A). Relative to these baseline values, we recognized two intervals of more rapid increase in δ 18 O conodont superimposed on the long-term secular shift, documenting a stepwise cooling of SSTs during the upper Floian and upper Darriwilian to lower Katian, before the well-known Hirnantian stages. Well preserved δ 18 O conodont profiles (Supplementary Text S1) document three major cooling episodes within the Ordovician (Fig. 2 A), which could be biostratigraphically and lithostratigraphically constrained in study sections: (1) the late Floian cooling event (LFCE), during the Prioniodus honghuayuanensis to Baltoniodus navis zones of the late Folian to middle Dapingian stages (upper Honghuayuan Fm to lower Dawan Fm, ~ 200–215 m), (2) the late Darriwilian cooling event (LDCE), during the Yangtzeplacognathus protoramosus to Amorphognathus ordovicicus zones of the late Darriwilian to early Katian stages (upper Guniutan Fm to lower Baota Fm, ~ 270–280 m), and (3) the Hirnantian Glaciation, during the Normalograptus extraordinarius - N. ojsuensis to N. persculptus zones of the late Katian to Hirnantian stages (upper Lingxiang Fm to middle Guanyinqiao Fm, ~ 305–315 m) (Figs. S2-3). The three major cooling episodes that we integrated are reinforced through comparing with previously published data from individual locations, for example, the LFCE from the Laurentian margins and Argentine Precordillera 21 , 36 , the LDCE from Laurentia and Tarim 21 , 22 and the Hirnantian cooling from the Laurentia and Gondwana 20 , 37 . An offset of ~ 1 to 2‰ between newly generated and previously published data following the LDCE may be attributed to multiple processes, including spatial difference in SSTs, taxon-related effects in conodonts (Supplementary Text S1), and/or biases during data compilation. Although the Hirnantian Glaciation interval was documented in the newly generated data (Fig. S3A), it was not clearly evident in the globally compiled LOESS curve (Fig. 1 A), probably due to the fact that the short-term, pronounced cooling had been obscured by data smoothing techniques. A previous proposed climate warming around the Katian-Hirnantian boundary 37 , 38 is not evident in our new records, possibly because of low data resolution in that interval of the compiled data, smoothing of the data masked the transient fluctuations, or the limited duration or geographic extent of the warming event. Two low δ 18 O conodont values at the top of the study sections indicate temporary warming conditions at the end of the Hirnantian Glaciation 37 . Conodont in-situ strontium isotopes Conodont in-situ strontium isotopes ( 87 Sr/ 86 Sr conodont ) exhibit a slow decrease from ~ 0.7090 to ~ 0.7085 from the lower Tremadocian to the middle Darriwilian, followed by a more rapid decrease to ~ 0.7080 in the upper Katian (Figs. 1 F, S3B). Mercury geochemical data Mercury (Hg) and its ratio to total organic carbon (Hg/TOC) mostly below phanerozoic baseline values (i.e., average values of ~ 60 ppb and ~ 140 ppb/wt%, respectively 29 ), interspersed with peak Hg and Hg/TOC plateaus (as high as ~ 200–400 ppb and ~ 100–600 ppb/wt%, respectively) in the Miaopo Fm (Sandbian) and Wufeng to Longmaxi Fms (Hirnantian) (Figs. 1 D-E, S3E). Besides, the Hg/TOC plateaus or scattered peaks also appeared in the Nanjinguan Fm (lower Tremadocian) and Dawan to Miaopo Fms (upper Floian to Sandbian), corresponding to rise in Hg content even below baseline value. Ratios of mercury to aluminum (Hg/Al), iron (Hg/Fe) and manganese (Hg/Mn) show roughly parallel variations (Fig. S4), marked by general decrease trends (from ~ 50 to < 0.1 ppb/wt%, ~ 50 to 0.1 ppb/wt% and 0.1 to 0.001 ppb/ppm, respectively) in the Nanjinguan to Dawan Fms, followed by increases to maxima (~ 100 ppb/wt%, ~ 300 ppb/wt% and ~ 3 ppb/ppm, respectively) in the upper Guniutan to Miaopo Fms. After decreases to ~ 0.1 ppb/wt%, ~ 0.3 ppb/wt% and ~ 0.001 ppb/ppm, respectively, in the Baota and Lingxiang Fms, these Hg ratios rebound to pre-excursion values in the Wufeng and Longmaxi Fms. Δ 199 Hg exhibits overall positive values in gradually decrease trend from ~ + 0.3‰ to near-zero values (~–0.05 to + 0.05‰) in the Nanjinguan to Dawan Fms (Figs. 1 B, S3C), with a few positive values ( ~ + 0.1 to + 0.3‰) in the Honghuayuan and Dawan Fms. Δ 199 Hg mostly exhibits negative values in the Guniutan and Miaopo Fms (~–0.2 to ~ + 0.05‰), and a range of values from ~–0.05‰ to + 0.2‰ in the Baota and Lingxiang Fms. Accordingly, we subdivided Ordovician Δ 199 Hg profile into four intervals through compilation with previously published data in the overly Wufeng to Longmaxi Fms at Wangjiawan 39 , including (1) a decrease trend of Δ 199 Hg values during the early Tremadocian to middle Folian (Nanjinguan to mid Honghuayuan Fm., ~ 485 − 473 Ma), (2) a dominant of near-zero Δ 199 Hg values during the late to latest Folian, followed by a decrease trend from positive to near-zero values during the earliest to early Dapingian (LFCE interval) (mid Honghuayuan to mid Dawan Fm., ~ 473 − 468 Ma), (3) a gradual decrease trend to slight negative values (~–0.05 to − 0.1‰) through the Darriwilian (~ 468 − 459 Ma), until more negative Δ 199 Hg values (~–0.3 to − 0.2‰) followed by a rebound to near-zero values in the Sandbian (LDCE interval) (upper Dawan to lower Baota Fm., ~ 459 − 453 Ma), and (4) mostly near-zero Δ 199 Hg values followed by both near-zero and slightly positive values ( ~ + 0.05 to + 0.1‰) during the earliest Katian to the Hirnantian glacial interval (upper Baota to lower Longmaxi Fm., ~ 453 − 443 Ma) (age-depth model is given in Supplementary Data). δ 202 Hg mostly shows negative values (~–3 to 0‰) throughout the study sections, except weakly positive values ( ~ + 0.2 to + 0.7‰) in the Guniutan Fm (~ 467 − 462 Ma) and the Miaopo Fm (~ 459 − 454 Ma) (Figs. 1 C, S3D), and is negatively correlated with Δ 199 Hg (Fig. S5). Our new Δ 199 Hg dataset offers much higher temporal resolution than previously published Hg-isotope studies 39 – 41 (Fig. 2 B), revealing a negative shift from the Tremadocian to Sandbian and dominantly near-zero Δ 199 Hg values in the Katian to Hirnantian. Diagenesis evaluations suggest well preservation of both Δ 199 Hg and δ 202 Hg data in study sections (Supplementary Text S2). Discussion Volcanic activity through the Ordovician Volcanic activity, along with other factors, e.g., submarine hydrothermal activity, seawater redox conditions, terrestrial fluxes, and sediment accumulation rates, may collectively influence temporal fluctuations in the Hg content ([Hg]) of sedimentary rock 30 , 31 . In seawater, Hg complexes with organic matter to form methylmercury, and it reacts with sulfide to form mercuric sulfide (HgS), yielding compounds that commonly serving as major repositories of Hg 29 , 42 . However, mercury can also adsorb onto the surfaces of phosphate, clay minerals, and Fe-Mn oxides 29 . The concentration of phosphorus ([P]) can be utilized as an indicator of the relative abundance of phosphate minerals, and the concentration of aluminum ([Al]) can reflect the relative content of clay minerals such as kaolinite and illite, whereas the concentrations of iron ([Fe]) and manganese ([Mn]) indicate the presence of Fe-Mn oxides. In the study units, [Hg] concentrations exhibit pronounced positive covariation with TOC ( r = 0.35, n = 266, p < 0.001 for Huanghuachang section; r = 0.77, n = 50, p < 0.001 for Chenjiahe section; r = 0.49, n = 52, p < 0.001 for Wangjiawan section) but weak or no significant covariation with [Al], [Mn], [Fe], or TS (Fig. S6A-E), implying that organic matter serves as the principal host of mercury. Sources of sedimentary Hg other than volcanic activity are possible. Hydrothermal activity generally results in lower 87 Sr /86 Sr signals for seawater and sedimentary rock (i.e., closer to the mantle endmember, ~ 0.7035) 43 , therefore, conodont 87 Sr /86 Sr ratio may provide critical clues in detection of hydrothermal activity. In the study sections, secular variation in 87 Sr/ 86 Sr conodont (~ 0.7080 to 0.7090) is consistent with primary marine Sr isotope signals (Fig. 1 F) 43 , 44 , providing no evidence of hydrothermal influence. Considering different responses (i.e., capacity and timing in adsorption) of conodont bioapatite and carbonates to hydrothermal sourced Sr or Hg, even hydrothermal Sr is not largely enriched in conodonts, carbonates may still absorb Hg from hydrothermal fluids. Previous studies have suggested that a majority of Sr in the studied conodont specimens (avg. ~13000ppm) source from diagenetic fluids instead of bulk carbonate (avg. ~300ppm) 45 , that means any prominent hydrothermal sourced Sr could be detected. So far, the prioritization of hydrothermal Hg uptake into carbonate and Sr uptake by bioapatite remains unknown, which fundamentally hinders the tracing of Hg provenance in hydrothermal systems. Seawater redox conditions commonly play a pivotal role in sedimentary Hg accumulation, mostly through modulation of microbial activity and the interplay of the C-N-S biogeochemical cycles, thus determining the speciation and concentration of mercury within the aquatic environment 46 , 47 . Therefore, redox oscillations can have a profound effect on Hg diffusion at the sediment-water interface, directly influencing its enrichment in sediment. In the current investigation, molybdenum and uranium-enrichment factors (Mo EF and U EF ) and C org /P ratios 48 document oscillations mainly between suboxic and anoxic conditions in the Lower Ordovician, and oxic conditions in the Middle to Upper Ordovician, except suboxic-anoxic conditions around the OSB (Fig. S4F-H; Supplementary Text S3). No significant correlations exist between [Hg] or Hg/TOC and Mo EF , U EF or C org /P (Fig. S6F-H). Furthermore, the stratigraphic distribution of [Hg] peaks does not coincide with intervals of more reducing watermass conditions (i.e., higher Mo EF , U EF and C org /P values). These observations imply that fluctuations of seawater redox conditions were not important influences on Hg content of sedimentary rocks in the study sections. Sedimentation rate can exert significant influence on anomalous Hg enrichments in sedimentary rock by modulating their physical, chemical, and biogeochemical attributes 49 . A subdued sedimentation rate diminishes physical remixing, thereby fostering a more homogenous distribution and protracted sequestration of Hg within sedimentary matrix, particularly at the sediment-water interface. In the present study, average linear sedimentation rates (LSR) show no correlation with [Hg] at the substage level of resolution (Fig. S7), and positive [Hg] excursions are observed within stratigraphic intervals of uniform lithologic character (i.e., argillaceous limestone at Huanghuachang and Chenjiahe, and shale at Wangjiawan; Fig. S3), suggesting that variations in sedimentation rate were not responsible for the observed [Hg] peaks. However, the lack of high-resolution age models for the study sections precludes detailed examination of the relationship between sedimentation rate and Hg enrichment. In sedimentary rock, the Hg/TOC ratio is usually relatively stable 29 , 32 . Volcanically sourced Hg may be released directly during eruptions or emitted through magmatic heating of organic-rich sediment (e.g., coal beds) 50 , or directly to seawater through ocean-crustal hydrothermal activity. The erupted Hg is predominantly in a gaseous form and is subject to global atmospheric transport over a brief timeframe (~ 0.5 to 2 years), allowing its subsequently integrates into the global ocean's geochemical and biogeochemical cycles, thus abnormal high Hg/TOC ratios (above baseline value) as recorded in sedimentary rock 42 . Sedimentary rock Hg/TOC records have been widely used to evaluate secular variation in volcanic activity at a regional or global scale 29 . In the study sections, positive excursions to high Hg/TOC ratios above baseline values (~ 140 ppm/wt.%) for the Phanerozoic, therefore, indicate major volcanic Hg inputs during the early Tremadocian, late Floian, Sandbian and early Hirnantian stages (Fig. 1 D-E). Apart from Hg anomalies, volcanic tuff layers constitute the most overt manifestation of regional volcanic activity, and such deposits are widely distributed in the Middle to Upper Ordovician of South China 51 and other cratons 52 . The present study sections contain multiple tuff layers in the upper Dawan Fm (a few cm thick), middle Miaopo Fm (nearly 10 cm thick), and Wufeng-Longmaxi Fms (a few mm thick). The identification of these distinct volcanic ash layers supports the inference that concurrent Hg/TOC anomalies are indicative of regional volcanic activity 53 . In addition, hydromica clay rocks, which predominantly form via saturated weathering of volcanic ash and associated pyroclastic materials 54 , are present in the Dawan to Longmaxi Fms (Fig. S3). Notably, these formations also display overall high [Hg] and elevated Hg/TOC ratios. These observations are evidence of significant, sustained regional volcanic activity during the Middle to Late Ordovician (Fig. S3E). So far, no large igneous province (LIP) or continental flood basalt province (CFBP) of Ordovician age has been identified, but a superplume event during the Middle to Late Ordovician has been hypothesized 55 , 56 . Indirect evidence of this superplume event includes carbon cycle and weathering modeling 57 – 59 , a secular decrease of seawater 87 Sr/ 86 Sr 60 , and a lack of geomagnetic reversals (i.e., a polarity superchron) 56 . The hypothetical Ordovician superplume is comparable to the Cretaceous superplume in being part of a long-term supercontinent (Wilson) cycle (Fig. 3 ), also evidenced in the latter case by pulsed oceanic crustal production, formation of large-scale oceanic plateaus (e.g., Ontong Java), intensified magmato-tectonic activity, reduced geomagnetic field reversal frequency (i.e., the Cretaceous Normal Polarity Superchron), and increased hydrothermalism as revealed from various sedimentary geochemical records (e.g., 87 Sr/ 86 Sr) 61,62 . Analogously, the Ordovician Period bears resemblance to the Cretaceous Period, which was marked by the formation of numerous LIPs at an exceptionally high rate (~ 6 LIPs formed within ~ 40 Myr), and several of them were associated with negative excursions of δ 13 C carb in marine sedimentary rock, climate warming and oceanic anoxic events that linked to volcanic activity, and followed by positive excursions of δ 13 C carb , prolonged climate cooling, seawater oxygenation that related to enhanced continental weathering 63 . Processes not directly associated with volcanic activity may also lead to MDF and MIF of mercury isotopes. Hg emanating from remote volcanoes experiences extended atmospheric transport, potentially leading to isotopic fractionation due to atmospheric redox processes. These processes produce positive Hg-MIF compositions (e.g., Δ 199 Hg) and negative δ 202 Hg in atmospheric Hg II and sediment dominated by atmospheric Hg II accumulation. Furthermore, photochemical reduction of aqueous Hg II to Hg 0 in the photic zone of euxinic waters, as observed during the end-Permian mass extinction 46 , can give negative Δ 199 Hg and positive δ 202 Hg in organic-rich sedimentary rock 47 . Negative Δ 199 Hg values in sedimentary rock typically have been ascribed to Hg inputs from photic-zone euxinia or terrestrial soils/plants 47 . Additionally, recent studies have shown that mid-ocean ridge basalt (MORB) and island arc basalt (IAB) exhibit positive Δ 199 Hg values (~ 0.1 to > 0.3‰), while oceanic island basalt (OIB) and continental flood basalt (CFB) have Δ 199 Hg near-zero values 67 (Fig. 2 B). Therefore, basalt weathering likely exerts additional influence on Hg isotopic composition in sedimentary rock. The paired Hg isotopes, [Hg], Hg/TOC and 87 Sr/ 86 Sr conodont ratios collectively revealed episodic intense volcanic activity, enhanced weathering of both volcanic rocks (basalt) (Supplementary Text S4) and soil/plant, and photic-zone euxinia (PZE) throughout the Ordovician (Fig. 1 ). During the ~ 485 − 480 Ma, a rapid decline in Δ¹⁹⁹Hg is likely related to volcanic activity and relevant weathering processes (either continental weathering or/and weathering of different types of basalts), which occurred against a backdrop of volcanic activity (positive Hg/TOC ratios above baseline value ~ 140 ppb/wt.%). During the following ~ 480 − 473 Ma, weathering Hg inputs dominated Hg isotopic fractionation and enrichment in sediment as revealed from negative excursions of Δ¹⁹⁹Hg, and decreases of [Hg] and Hg/TOC below the baseline values. During the LFCE (~ 473 − 468 Ma, Early-Middle Ordovician boundary transition), volcanic Hg signals dominated source of Hg and its enrichment in sediments (relatively constant near-zero Δ¹⁹⁹Hg and negative δ²⁰²Hg values, and positive excursions of Hg/TOC above the baseline) during the first half (~ 473 − 470 Ma), followed by weakened volcanic activity (thus low [Hg] in sediments) and rise in the proportion of atmospheric Hg²⁺ may be related to rise in atmospheric oxygen concentration during the second half (~ 470 − 468 Ma). Over the entire ~ 485 − 468 Ma interval, the prolonged decrease in Δ¹⁹⁹Hg may imply the involvement of early land plants during weathering (see discussion below). During the ~ 468 − 459 Ma, photic zone euxinia dominated marine Hg fractionation (negative Δ¹⁹⁹Hg, positive δ²⁰²Hg) because of enhanced weathering inputs (decrease in 87 Sr/ 86 Sr conodont ) due to expansion of terrestrial plants, and intensified upwelling under cooled climate. This scenario occurred against a backdrop of weak volcanic activity (or higher primary productivity and intensive global oceanic anoxia that resulted in greater Hg drawdown 29 ) which led to sporadic highest peak Hg content and Hg/TOC ratios above the baseline values. During the LDCE (~ 459 − 453 Ma), stronger photic zone euxinia dominated marine Hg fractionation (most negative Δ¹⁹⁹Hg values, rise of δ²⁰²Hg to positive values), while Hg enrichment in the sediments was controlled by volcanic activity (with almost all Hg/TOC values above the baseline value), due to more intensified weathering (sharper decrease in 87 Sr/ 86 Sr conodont ) and cooling driven upwelling partly related to continuous expansion of terrestrial plants. The Middle Ordovician PZE had been reported from Tarim basin and other locality in South China 41 . Photic zone euxinia and volcanic activity weakened while weathering dominated marine Hg fractionation and enrichment in sedimentary rock during the ~ 453 − 446 Ma, as indicated from rise of Δ¹⁹⁹Hg to mostly near-zero values, decline of δ²⁰²Hg to negative values, and drop of Hg/TOC mostly below the baseline value. Volcanic activity briefly driven Hg fractionation and its enrichment in sedimentary rock during the ~ 446 − 443 Ma, resulted in mostly near-zero Δ¹⁹⁹Hg values and peak Hg/TOC ratios above the baseline value 39 . The limitations of the Hg/TOC ratio as a proxy for tracing volcanic signals in sedimentary rock should be considered. Weathering can remove mercury signals in organic-rich sediment, for example, leading to loss of Hg up to ~ 90% in highly weathered shales, while degradation may alter the type and quality of organic matter, especially for samples with low hydrogen and high oxygen index values (e.g., Type II, equivalent to burial temperatures of ~ 60–180°C), thereby affecting the Hg/TOC ratio 68 . Our study successions consist largely of carbonate rocks that underwent low degrees of weathering based on field observations, therefore, weathering is unlikely to have been a dominant influence on Hg/TOC ratios. In addition, the color alteration indices (CAI) of the extracted conodont specimens range from 1 to 3, equivalent to burial temperatures of ~ 60–200°C 18 , suggesting degradation of organic matter may have been an influence on Hg/TOC ratios. However, there is no clear difference in the CAI of conodonts between younger and older strata, indicating a relatively uniform level of thermal alteration throughout the study sections. In addition, TOC is strongly correlated with [Hg], suggesting that degradation had a limited effect on Hg/TOC ratios (Fig. S6A), and positive excursions and peak values in the Hg/TOC profiles were not caused by low TOC content (< 0.2 wt.%) 29 . Note that we regard the Hg/TOC ratio as a proxy only for local/regional volcanism, and Δ 199 Hg as additional proxy for global volcanic activity 29 and weathering of volcanic rocks (e.g., CFB and OIB) 67 . Drivers of the LFCE and LDCE Global volcanism and basalt weathering Long-term Ordovician cooling has been explained by various mechanisms, including orogenic uplift and weathering of silicate rocks 69 and a shift of tectonic plates into more humid regions and subsequent intensified weathering of volcanic rocks 70 . The general problem with such explanations is that there have been many orogenies and shifts in tectonic plates of similar or greater magnitude during the Phanerozoic that have not led to markedly enhanced weathering. Episodic intense volcanic activity followed by prolonged enhanced weathering of basalt represents a more likely explanation for the multistage character of long-term cooling effect, for example, during/after LIP volcanism as shown in mass extinction events 71 and through the Ordovician 25 , although volcanic activity may temporarily cause warming by increasing pCO 2 , acting as immediate effect of volcanisms 72 . This relationship is implied by concurrent positive excursions of δ 18 O carb and peak Hg/TOC above baseline during the LFCE and LDCE, as well as plateau δ 18 O carb value and sporadic Hg/TOC ratios above the baseline value between the two major cooling events (Figs. 1 and 4). These relationships suggest that the Ordovician climatic cooling (e.g., LFCE and LDCE) was primarily a volcanically driven, stepwise cooling event spread out over a ~ 40-Myr-long interval (Fig. 1 ). Moreover, concurrent of climatic cooling to PZE-related weathering of both basalt and terrestrial soil/plant occurred against the backdrop of volcanic activity during the late Dapingian to late Sandbian (~ 468 − 453 Ma) (Figs. 1 – 2 ), supporting our inference of a causal relationship. Chemical weathering of basalt could sequester atmospheric CO 2 , predominantly through secular silicate minerals reacting with CO 2 to produce carbonates. Quantification of ancient basalt weathering reveals an atmospheric CO 2 consumption rate of ~ 1 to 3×10 5 mol C/km²/yr 73 , thus lowering global temperatures. The inferred temporal concurrence of volcanic activity and the Ordovician climatic cooling has previously been ascribed to weathering of volcanic rocks 25 , 58 , 69 . Globally increased production and weathering of volcanic rocks during the Ordovician is likely to have triggered the climatic cooling. Increasing evidence suggests that the subduction of oceanic crust, volcanic activity on Earth’s surface, and magmatic activity in deep earth all have been intensified during the Ordovician (Fig. 3 ). Studies of Phanerozoic geomagnetic polarity suggested a transition from a Middle Cambrian-Middle Ordovician Reversed Polarity Bias Interval to a Late Ordovician-Late Silurian Normal Polarity Bias Interval, forming a Middle Ordovician Polarity Shift that indicates a major transition of Earth’s deep magmatism in the Ordovician 56 . Moreover, extensive basalt lava (e.g., British Isles, northern Iran and West Junggar) 74 – 76 , and global peaks in production of volcanogenic massive sulfide deposits, sulphidic shales and episodic ironstone deposition indicate elevated mantle plume activity during the Ordovician 77 . Recently, a large siliceous LIP has been identified with an areal extent of ∼2.5 Mkm 2 (or an estimated total volume of ∼2.5 Mkm 3 ) (e.g., Pinghe LIP 78 ). However, no large oceanic magmatic event of Ordovician age has been identified or, indeed, is likely to ever be identified because of near-total subduction of oceanic crust of that age. The intense subduction, collision, and orogenic processes in the oceanic lithosphere was evidenced by most frequency occurrence of ophiolite in the Ordovician which higher than other periods in Paleozoic Era 79 . The intense subduction of oceanic lithosphere and/or a potential submarine LIP could lead to intense global hydrothermal activity and influxes, resulting in a prolonged and major negative shift of 87 Sr/ 86 Sr in the Ordovician ocean 43 . In detail, the trajectory of 87 Sr/ 86 Sr in marine sedimentary rock delineates a period of substantial weathering of basalt during the Darriwilian to Sandbian, followed by stabilization in the Katian 60 . Volcanic activity can draw down atmospheric CO 2 and drive secular global cooling by increasing ocean productivity and organic carbon burial rates, through elevated volcanic nutrient supply from weathering of volcanic rocks 28 . In the present study, secular decrease of Δ 199 Hg and rise of Hg/TOC ratio above baseline value during the Middle to Upper Ordovician cooling, correspond to the long-term decline in seawater 87 Sr/ 86 Sr (Fig. 1 ), pointing to a common trigger that may have been related to oceanic plateau volcanism or rapid seafloor spreading 43 , 44 . In addition, negative excursions of Δ 199 Hg signals in the Lower Ordovician can be attributed to changes in the main type of weathered basalt (e.g., from IAB and MORB to OIB) (Fig. 2 B). Simultaneously with the weathering influxes of terrestrial volcanic rocks, increased hydrothermal weathering due to seafloor production prevailed also generated massive basalt as a dominated nutrient P influxes 60 , which elevated marine productivity and enhanced burial rate of organic matter (e.g., peak TOC values in the LDCE interval, Fig. S4B), resulting in a onset of positive excursion and plateau value of δ 13 C carb in the ocean during the Early to early Late Ordovician (Fig. 2 G). These considerations establish connections between the GOBE (Fig. 1 I), the Ordovician climatic cooling, and contemporaneous volcanic activity (e.g., 80 ). Early land plant evolution Some aspects of our Ordovician dataset are not readily explained by volcanic activity and basalt weathering, e.g., the long-term decrease of Δ 199 Hg to negative values (~–0.2‰) and rise of δ 202 Hg to positive values ( ~ + 0.4 to + 0.7‰) in the Middle to Upper Ordovician (Fig. 1 B-C). Photic zone euxinia, which is commonly related to elevated marine productivity in upwelling regions 47 , was prevalent during the early Darriwilian to late Sandbian cooling, as revealed by long-term paired changes in both Δ 199 Hg and δ 202 Hg intervals in Guniutan through Baota Fms. Alternatively, negative D 199 Hg is commonly linked to terrestrially sourced organic matter because land plants predominantly derive Hg 0 in its root from surrounding soil where odd isotopes of mercury (e.g., 199 Hg) are preferentially reduced during photochemical reduction, resulting in negative Δ 199 Hg values in soil carbon 81 . In the present study, negative Δ 199 Hg values (to ~–0.2‰) during the late Dapingian to late Sandbian substages coincided with evolution of the earliest embryophytes (i.e., bryophyte-grade land plants) 10 , 26 (Fig. 1 H). The timing of the first appearance of cryptospores on land in the Dapingian (early Middle Ordovician) 10 , 11 coincided with onset of a shift to negative Δ 199 Hg values (~ 470 − 468 Ma), suggesting a link via terrestrial weathering inputs to marine systems. In addition, we suspect that the long-term decrease of D 199 Hg through the Early Ordovician may also have been related to gradual expansion of land plants as revealed by molecular clock studies 82 . A more significant proliferation of land plants occurred ~ 40 Myr later, during the Early Silurian 83 . The earliest cryptospores of land plants are from the basal Middle Ordovician of Argentina, and similar spore assemblages have been reported globally from the paleo-Equator to high latitudes 66 , 87 , and from the Dapingian to the Llandovery of Early Silurian (Supplementary Text S5), suggesting a uniform vegetation was present for about 30 Myr 11 , before replacement by Silurian plant megafossil assemblages (i.e., vascular plants) (Fig. 1 H). The simple and diminutive earliest land plants would have begun to influence their environment in terms of development of a rudimentary soil, for example, through addition of organic carbon that fostered microbial communities, possibly including free-living fungi. Further evidence of land animals and soil microbes are found throughout the remainder of the Middle and Late Ordovician in many regions of the world 9 , 10 , suggesting a rapid spread of early terrestrial biotic communities. Volcanic activity may have promoted the evolution of land vegetation through delivery of essential nutrients such as P during weathering of volcanic rocks (e.g., basalt) 26 , 88 . The continuous land colonization of vegetation will further enhance the chemical and physical weathering of volcanic rocks, forming positive feedback that is conducive to the continuous input of nutrients. In addition, the most negative Δ 199 Hg value (~–0.2‰) in the late Darriwilian to late Sandbian Substage (~ 459 − 454 Ma) is coincide with both the second climate cooling stage and the most rapid drop of 87 Sr/ 86 Sr in marine sedimentary rock during the Ordovician, implying a causality that expansion of early plants caused intensified terrestrial weathering influxes (thus PZE), and both finally lead to the climate cooling. Our present study shows major volcanic activity and the Ordovician climatic cooling onset during the middle Floian (~ 475 Ma), which is ~ 5 Myr earlier than the land colonization of the earliest embryophytes in the Dapingian (~ 470 Ma) (Fig. 1 ), suggesting land colonization of vegetation does not the trigger but a main driver of the Ordovician climatic cooling. From this perspective, climatic and bio-evolutionary patterns of the Ordovician have an analog in the Devonian, a period also marked by a major decline in atmospheric p CO 2 and a strong cooling trend leading to a terminal glaciation, all of which have been linked to bio-evolutionary developments among terrestrial floras, i.e., the advent of trees and seed plants ( 17 and references therein). Methods Geological background and sample preparation The locations, stratigraphy, and paleogeography of the study sections have been fully documented in prior studies 89 , and here we provide a concise summary focused on key stratigraphic features essential to this analysis. Paleogeographically, the study area was located on the north-central Yangtze Platform, which accumulated shallow-marine carbonate sediment that graded into argillaceous sands in inner-shelf settings to the northwest and slope facies to the southeast 90 . The study area exposure the Wuduhe Fm of the Upper Cambrian, and overlying 12 formations/beds (from base to top, the Xilinxia, Nanjinguan, Fenxiang, Honghuayuan, Dawan, Guniutan, Miaopo, Baota, Lingxiang and Wufeng formations, Guanyinqiao Bed, and Longmaxi Fm) ranging continuously in age from the earliest to latest Ordovician and Ordovician-Silurian boundary transition 90 (Fig. S2 ). Limestone, bioclastic limestone and muddy limestone are dominant in the study successions, except for thin (few meters) black shale intervals with limestone lenses in the Miaopo Fm, and black shales in the Wufeng and Longmaxi Fms. Lithological compositions of the successions were fully described in 89 . Overall, the study successions record a gradual shift from carbonate platform facies in the Lower Ordovician to neritic facies in the Middle Ordovician and deep basinal facies in the Upper Ordovician, reflecting a long-term sea-level rise before the Hirnantian glacio-eustatic fall at the end of the Ordovician 89 . The bulk carbonate rocks, conodont samples and few shales were collected from Huanghuachang, Chenjiahe and Wangjiawan sections in the Yiling District of Yichang area, South China (Fig. S1 ). Continuous bulk-rock sampling (of carbonate rocks and few shale samples) was conducted from the uppermost Wuduhe Fm to lower Longmaxi Fm. Weathered surfaces and diagenetic veins were removed, and the remaining sample was cleaned, air-dried and crushed. Selected samples from each formation/bed were selected for thin-sectioning for petrographic analyses. An aliquot of each sample was powdered using a rock mill to < 200 mesh for bulk-rock geochemical analyses. To extract conodont specimens, lightly crushed samples were dissolved in 10% acetic acid for five days, after which conodont elements were recovered from the insoluble residue using a binocular microscope. Each conodont specimen was identified, including Belodella sp., Drepanodus sp., Pasoistodus sp., Pesiodus sp., Oneotodus sp., Scolopodus sp., Drepanoistodus sp., Baltoniodus sp., Tripodus sp., Sessatognathdus sp., and Trianglodus sp. The color alteration indices (CAI) of the study specimens range from 1 to 3. The samples used in this study represent the same suite that was used in 18 , 89 . Differences in the structure and chemical composition of conodont tissues (i.e., albid crown, hyaline crown, basal body) can result in variable paleotemperature results 18 . In the present study, we systematically analyzed the oxygen isotopic composition of the densest albid crown of each specimen to minimize tissue-related effects. Albid crown is also thought to yield the most faithful record of (near-)primary seawater Sr isotope signals 44 , and it was targeted for the in-situ Sr isotope analyses in this study. In-situ conodont oxygen isotopes and paleotemperature Detailed methods for preparation of conodont resin targets, parameters of instrument analysis by secondary ion mass spectrometry (SIMS), and correction of instrumental mass fractionation factor (IMF) were described in 18 . SIMS measurements were made using a Cameca IMS-1280 SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Oxygen isotopes were measured using the multi-collection mode on two off-axis Faraday cups. Replicate analyses of the Durango apatite standard (which was analyzed after every five sample measurements) yielded an average value of + 9.40 ± 0.16‰ (2σ; n = 78), which is indistinguishable within analytical uncertainty from the reported value of + 9.4 ± 0.3‰ (2σ) 20 . Three δ 18 O measurements were obtained for each specimen, yielding average 2σ variance of 0.8‰. Previous studies have indicated that a 1% increase in δ 18 O conodont corresponds to a decrease of ~ 4°C in seawater surface temperature 18 . In-situ conodont strontium isotopes In-situ 87 Sr/ 86 Sr ratios of conodont albid crown ( 87 Sr/ 86 Sr conodont ) were measured by laser-ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analysis at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) at the China University of Geosciences (Wuhan) 91 . A 193-nm ArF-excimer laser was used during the measurement, with a laser beam diameter of 60 µm. Three 87 Sr/ 86 Sr measurements were obtained for each specimen, yielding average 2σ of ~ 0.0006. Two natural apatite standards, Slyudyanka and MAD (Madagascar apatite), were used to monitor the accuracy of LA-MC-ICP-MS measurements, yielding average values 0.70776 ± 0.00017 (2σ) and 0.71176 ± 0.00016 (2σ), respectively, which are consistent with the reference 87 Sr/ 86 Sr values for Slyudyanka (0.70769 ± 0.00015 (2σ)) and MAD (0.71180 ± 0.00011 (2σ)). Mercury content and isotopes Mercury concentration and its isotopic analyses following methods described in 29 , 30 , 32 . Mercury concentration was measured using a LECO AMA254 mercury analyzer in the GPMR, and its isotopes were determined using a MC-ICP-MS with high sensitivity X skimmer cone at the Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China. For mercury concentration analyses, all the samples were freeze-dried to prevent the decomposition of Hg. About 100 mg for mudstone or shales and 150–200 mg for limestone were analyzed. Data reliability was ensured by analysis of international standard coal samples 502–685 (40 ppb) after every 12 unknowns, yielding reproducibility of sample concentrations being within 10%. Pyrolysis method was used to extract Hg for its isotopic analyses. An international standard NIST SRM 997 Tl was used for simultaneous instrumental mass bias correction of Hg and 4 ng/mL SnCl 2 solution was used to generate elemental Hg 0 before being introduced into the plasma. International standard NIST SRM 3133 was measured after every 3 unknowns to monitor the stability of the instrument. We also analyzed NIST SRM 3177 after every 10 unknowns to examine the instrument accuracy. Hg concentrations of ∼2 ng/mL or 1 ng/mL of NIST SRM 3133 and NIST SRM 3177 solutions were prepared for matching measured sample solutions to reduce the matrix dependent mass bias. Hg isotopic composition is reported in δ 202 Hg notation in units of per mille (‰) relative to the NIST SRM 3133 Hg standard: δ 202 Hg (‰) = [( 202 Hg/ 198 Hg sample )/( 202 Hg/ 198 Hg standard ) − 1] × 1000 Mass independent fractionation (MIF) of Hg isotopes is expressed in Δ notation (Δ xxx Hg), which describes the difference between the measured δ xxx Hg and the theoretically predicted δ xxx Hg value, using the following equations: Δ 199 Hg ≈ δ 199 Hg − (δ 202 Hg × 0.2520) Δ 200 Hg ≈ δ 200 Hg − (δ 202 Hg × 0.5024) Δ 201 Hg ≈ δ 201 Hg − (δ 202 Hg × 0.7520) Replicate analyses of the NIST 3177 Hg isotope reference standard ( n = 4) yielded the following: δ 202 Hg = − 0.42 ± 0.06‰ (2σ); Δ 199 Hg = − 0.04 ± 0.02‰ (2σ); Δ 200 Hg = + 0.01 ± 0.04‰ (2σ); Δ 201 Hg = + 0.01 ± 0.04‰ (2σ). Major elemental and total organic carbon contents Major elements (Al and Fe) were measured using wavelength-dispersive XRF in the GPMR. Average analytical uncertainty is better than 5% (RSD—relative standard deviation) for major elements based on repeated analysis of national standards GBW07132, GBW07133, and GBW07407, and better than 2% (RSD) for trace elements based on international standards AGV-2, BHVO-2, BCR-2, and GSR-1. In the same laboratory, total organic carbon (TOC) and total sulfur content (TS) was measured using an Elementar Vario Micro Cube analyzer. Data quality was assessed through multiple analyses of standard sample DP-1 (65.44 ± 0.33 wt%). A standard sample and a repeat were analyzed after every 12 unknowns, yielding an analytical accuracy of 2.5 wt% of the reported values. Declarations Acknowledgments We thank Zhihong Li, Yang Li, Fei Guo and Pingan Yan for help in the field, and Guangyi Sun, Yunjie Wu, Zhenfeng Luo for their assistance with the laboratory work. This research is supported by the NSFC grants (Nos. 92055212, 42477215), the “CUG Scholar” Scientific Research Funds at China University of Geosciences (No. 2023081), the Postdoctoral Fellowship Program of CPSF under Grant Number (No. GZC20232474, 2024M753028), the “MOST” Special Fund from State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (No. MSFGPMR2024-104). Author contributions H. Z. and L.Z. conceived and designed the study; H.Z., L.Z., T.J.A., Z.Y.L., X.D.W. and F.H. jointly made contributions in the sample collection, data analysis and interpretation, writing, editing and review of the manuscript. Competing interests The authors declare that they have no competing interests. Additional information Supplementary information: The supplementary material is available in this submission. Data availability All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. 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Geology 23:305–308 Yuan W et al (2022) Mercury uptake, accumulation, and translocation in roots of subtropical forest: implications of global mercury budget. Environ Sci Technol 56:14154–14165 Morris JL et al (2018) The timescale of early land plant evolution. Proceedings of the National Academy of Sciences 115, E2274-E2283 Libertin M, Kvacek J, Bek J, Zarsky V, Storch P (2018) Sporophytes of polysporangiate land plants from the early Silurian period may have been photosynthetically autonomous. Nat Plants 4:269–271. https://doi.org:10.1038/s41477-018-0140-y Nechaev VP, Sutherland FL, Nechaeva EV (2022) Phanerozoic evolution of continental large igneous provinces: Implications for galactic seasonality. Minerals 12:1150 Ronov A (1993) Stratisfera ili Osadochnaya Obolochka Zemli: Kolichestvennoye Issledovanie (Stratisphere, or the Sedimentary Sphere of the Earth: Quantitative Research). Moscow (in Russian) Nance RD, Murphy JB, Santosh M (2014) The supercontinent cycle: a retrospective essay. Gondwana Res 25:4–29 Gray J, Theron J, Boucot A (1986) Age of the Cedarberg Formation, South Africa and early land plant evolution. Geol Mag 123:445–454 Parnell J, Foster S (2012) Ordovician ash geochemistry and the establishment of land plants. Geochem Trans 13:1–7 Zhang L et al (2016) Diagenetic uptake of rare earth elements by conodont apatite. Palaeogeogr Palaeoclimatol Palaeoecol 458:176–197 Renbin Z, Jisuo J, Jiayu R (2007) Ordovician-Early Silurian (Llandovery) stratigraphy and palaeontology of the upper Yangtze platform, South China. Science Press Beijing Zhao H et al (2024) Calcium isotope evidence of increased carbonate saturation state during the Frasnian–Famennian boundary event. Earth Planet Sci Lett 642. https://doi.org:10.1016/j.epsl.2024.118876 Additional Declarations There is NO Competing Interest. Supplementary Files DataNC.pdf Data S1, Data S2, Data S3, Data S4, Data S5, Data S6 OrdoviciancoolingandtriggerssupplementarymaterialsforNCv2.docx Supplementary Information for: Volcanism and basalt weathering drove Ordovician climatic cooling Cite Share Download PDF Status: Published Journal Publication published 12 Dec, 2025 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6593045","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":454341375,"identity":"17f2c1cf-d99f-4337-be0d-2149a5d9c030","order_by":0,"name":"Lei Zhang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA7ElEQVRIiWNgGAWjYDACCRBRwMAD5nyAcInRYgDRwjiDFC1gwMxDjLvkZzc/e/jFwEaGgf2M8WubPxZ5/A3MDx/dwKOFcc4xc2MZgzQeBp4cM+vcNoliiQNsxsY5eLQwSySYSUsYHOZhkOAxM85tkEhsOMDDJo1PC5tE+jeglv8QLRZ/JBLnE9LCI5FjJvnB4ABIi/FjoAmJGwhpkZDIKZNmMEgG+iWtjLG3TSJx42ECfpGfkb5N8keFnT0D++HNH378qUucd7z54WN8WkAAHB32B4CugnAJKAcBxh9QrR+IUDwKRsEoGAUjEAAAnfI/SFrQRCUAAAAASUVORK5CYII=","orcid":"","institution":"China University of Geosciences (Wuhan)","correspondingAuthor":true,"prefix":"","firstName":"Lei","middleName":"","lastName":"Zhang","suffix":""},{"id":454341376,"identity":"e0fe4bc5-9c71-4273-914c-c8b4e79ffd7d","order_by":1,"name":"He Zhao","email":"","orcid":"https://orcid.org/0009-0007-3740-928X","institution":"China University of Geosciences (Wuhan)","correspondingAuthor":false,"prefix":"","firstName":"He","middleName":"","lastName":"Zhao","suffix":""},{"id":454341377,"identity":"dafbc0b4-0f7e-4857-9bbd-b87142c3f43b","order_by":2,"name":"Thomas Algeo","email":"","orcid":"https://orcid.org/0000-0002-3333-7035","institution":"University of Cincinnati","correspondingAuthor":false,"prefix":"","firstName":"Thomas","middleName":"","lastName":"Algeo","suffix":""},{"id":454341378,"identity":"570ac388-665f-4a3e-9774-271b95846d2b","order_by":3,"name":"Zhengyi Lyu","email":"","orcid":"","institution":"China University of Geosciences (Wuhan)","correspondingAuthor":false,"prefix":"","firstName":"Zhengyi","middleName":"","lastName":"Lyu","suffix":""},{"id":454341379,"identity":"ed43c8b8-b26f-4725-b036-7779ab70130c","order_by":4,"name":"Xiangdong Wang","email":"","orcid":"","institution":"China University of Geosciences (Wuhan)","correspondingAuthor":false,"prefix":"","firstName":"Xiangdong","middleName":"","lastName":"Wang","suffix":""},{"id":454341380,"identity":"9c4181b7-e1f0-4254-ad14-9a4f3647ce92","order_by":5,"name":"Fang Hao","email":"","orcid":"","institution":"School of Geosciences, China University of Petroleum, East China","correspondingAuthor":false,"prefix":"","firstName":"Fang","middleName":"","lastName":"Hao","suffix":""}],"badges":[],"createdAt":"2025-05-05 09:30:12","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6593045/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6593045/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-025-66316-4","type":"published","date":"2025-12-12T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":82491532,"identity":"f9de8e06-c08f-419c-8e1b-df15b11533b0","added_by":"auto","created_at":"2025-05-12 06:52:15","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":684543,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eComposite geochemical profiles of the study sections (A-E) and major environmental and biological events of the Ordovician (F-G). \u003c/strong\u003e(A) Conodont oxygen isotopes (δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e). (B-C) Mercury isotopes (Δ\u003csup\u003e199\u003c/sup\u003eHg and d\u003csup\u003e199\u003c/sup\u003eHg). (D) Hg content. (E) Ratio of Hg content to total organic carbon (Hg/TOC). (F) Strontium isotopes of conodont (\u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e). (G) Bulk-carbonate carbon isotopes (δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e). (H) Earliest land plant fossil occurrence. (I) Marine biodiversity curves. In panels A-C, dashed and solid fit black curves are based on LOESS analysis using a fixed smoothing factor 0.2. In panel A, the LOESS curve are based on both newly generated and published data, and δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e value analyzed by GIRMS was adjusted by +0.6‰ to integrate these with SHRIMP and SIMS records\u003csup\u003e35\u003c/sup\u003e. Average δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e values of all newly and published data for the latest Cambrian-Early Ordovician (~16.2‰) and Middle Ordovician (~17.8‰) are used as baseline values (dashed blue lines) for identification of the cooling episodes during the Late Floian cooling event (LFCE) and the Late Darriwilian cooling event (LDCE). In panels D-E, Phanerozoic average Hg content (~60 ppb) and Hg/TOC ratio (~140 ppb/%) are adopted as baseline values (dashed blue lines) for evaluation of Hg enrichment episodes\u003csup\u003e29\u003c/sup\u003e. In panel A, published δ\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e data sources from\u003csup\u003e20-22,36,60,64\u003c/sup\u003e. In panels F-I, published \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e profiles source from\u003csup\u003e44\u003c/sup\u003e (orange line) and\u003csup\u003e43\u003c/sup\u003e (black line), δ\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e sources from \u003csup\u003e65\u003c/sup\u003e, paleobotanic data (i.e., cryptospores, glomalean fungus, sporangia and laevigate trilete spores) from review studies in\u003csup\u003e66\u003c/sup\u003e, and biodiversity data from\u003csup\u003e2,3\u003c/sup\u003e. Light pink fields represent cooling episodes during the LFCE, LDCE and the Hirnantian Glaciation.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6593045/v1/c9103c67aebce253543bcf03.jpeg"},{"id":82491534,"identity":"e2c39610-1b0d-4574-ba31-2c5a5705e435","added_by":"auto","created_at":"2025-05-12 06:52:15","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":484338,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eBar-and-whisker plot for Conodont oxygen isotopes (δ\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e18\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003econodont\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e) and mass independent fractionation (MIF) of Hg isotopes (Δ\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e199\u003c/strong\u003e\u003c/sup\u003e\u003cstrong\u003eHg) in Ordovician marine sedimentary rock and some other major reservoirs\u003c/strong\u003e\u003csup\u003e\u003cstrong\u003e33,39-41,67\u003c/strong\u003e\u003c/sup\u003e. In panel A, both newly generated and published data shown in Fig. 1A was used in the plots. OSB = Ordovician-Silurian boundary. Q1 = first quartile. Q3 = third quartile. IQR = Interquartile range.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6593045/v1/610f3735f159473654eae566.jpeg"},{"id":82491535,"identity":"c3079942-b23b-429f-ad95-36409f5c9dca","added_by":"auto","created_at":"2025-05-12 06:52:15","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":739387,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eGeological records for the Phanerozoic, showing secular volcanic activity through the Ordovician.\u003c/strong\u003e (A) Magnetic polarity ratio (%NP = percent normal polarity); (B) large igneous province (LIP) size; (C) volume of volcanic rocks; (D) ophiolite frequency; (E) Seawater \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr; (F) supercontinent cycle. Data sources: A = \u003csup\u003e56\u003c/sup\u003e; B = \u003csup\u003e78,84\u003c/sup\u003e; C = \u003csup\u003e85\u003c/sup\u003e; D = \u003csup\u003e79\u003c/sup\u003e, E = \u003csup\u003e43\u003c/sup\u003e, F = \u003csup\u003e86\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6593045/v1/50b6771e7c3c8377a61e84e0.jpeg"},{"id":99212726,"identity":"4f74f64e-5b4c-4503-9f5a-b2d27050dde7","added_by":"auto","created_at":"2025-12-30 08:27:35","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2962758,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6593045/v1/700e458c-68db-4cc9-b0ae-39842f8b79c4.pdf"},{"id":82491533,"identity":"c1a90049-ec7e-40ad-bb37-b16cc6bcdb2b","added_by":"auto","created_at":"2025-05-12 06:52:15","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1977227,"visible":true,"origin":"","legend":"Data S1, Data S2, Data S3, Data S4, Data S5, Data S6","description":"","filename":"DataNC.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6593045/v1/5490f56deb05b443b6077bf8.pdf"},{"id":82491544,"identity":"5d629591-16a8-4bc8-a964-d96e24a3c7fc","added_by":"auto","created_at":"2025-05-12 06:52:15","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":4976322,"visible":true,"origin":"","legend":"Supplementary Information for: Volcanism and basalt weathering drove Ordovician climatic cooling","description":"","filename":"OrdoviciancoolingandtriggerssupplementarymaterialsforNCv2.docx","url":"https://assets-eu.researchsquare.com/files/rs-6593045/v1/69c547f0c9ff1ddf276454d3.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Volcanism and basalt weathering drove Ordovician climatic cooling","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Ordovician Period (485.4\u0026ndash;443.8 Ma) was bookended by major biotic events, i.e., the early stages of the Great Ordovician Biodiversification Event (GOBE) at its onset and the Late Ordovician Mass Extinction (LOME), the first of the \u0026ldquo;Big Five\u0026rdquo; mass extinction, near its termination\u003csup\u003e1\u003c/sup\u003e. During the GOBE, extending from the early Tremadocian (Early Ordovician) to the middle Katian (Late Ordovician) with an early Darriwilian peak\u003csup\u003e2,3\u003c/sup\u003e, the numbers of marine invertebrate species and genera increased by factors of ~6\u0026times; and 3\u0026times;, respectively. A key aspect of this diversification event was the proliferation of marine plankton (e.g., primary producers such as photosynthetic cyanobacteria), representing the \u0026quot;Ordovician Plankton Revolution\u0026quot;\u003csup\u003e4\u003c/sup\u003e. The LOME, which had a ~70-80% species-level extinction rate, occurred in two phases corresponding to the onset and termination of the Hirnantian Glaciation: LOME-1 in the early Hirnantian, which eliminated many tropical taxa, and LOME-2 during the mid-Hirnantian, which decimated the cool-water \u003cem\u003eHirnantia\u003c/em\u003e Fauna\u003csup\u003e5\u003c/sup\u003e. A precursor biocrisis (LOME-0) has recently been identified in the mid-Katian\u003csup\u003e2,6\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eOn land, the Ordovician witnessed a transition from a microbial cryptopedobiota to sparsely distributed, embryophyte-based terrestrial ecosystems\u003csup\u003e7\u003c/sup\u003e, during which non-vascular (i.e., bryophyte-grade) land plants evolved and began to colonize the continents\u003csup\u003e8,9\u003c/sup\u003e. The fossil record for the earliest bryophyte-grade land plants consists of dispersed spores (i.e., cryptospores) that first appeared in Gondwana during the early Middle Ordovician (Dapingian\u003csup\u003e10,11\u003c/sup\u003e) and then spread globally\u003csup\u003e12,13\u003c/sup\u003e. The Late Ordovician was a key interval during which the earliest tracheophytes and, possibly, eutracheophytes (i.e., vascular plants) appeared and began to diversify\u003csup\u003e14\u003c/sup\u003e. The driving mechanisms of the Ordovician biological radiations and extinctions have long been speculated upon without achieving a consensus\u003csup\u003e15-17\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDespite their complex internal structure and chemical composition\u003csup\u003e18\u003c/sup\u003e, the oxygen isotopic composition of conodonts (\u0026delta;\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e) is a valuable tool for study of sea-surface temperatures (SSTs) in the Paleozoic Era\u003csup\u003e19-21\u003c/sup\u003e. Conodont \u0026delta;\u003csup\u003e18\u003c/sup\u003eO data have established that the GOBE coincided with a long-term global cooling trend with a total magnitude of ~10 \u0026deg;C\u003csup\u003e20,22,23\u003c/sup\u003e, a pattern confirmed by clumped isotope analysis of carbonates\u003csup\u003e24\u003c/sup\u003e. Climatic cooling is postulated to have been a direct driver of evolution among marine invertebrates\u003csup\u003e20\u003c/sup\u003e, as have rising oxygen levels in the atmospheric-oceanic system\u003csup\u003e15\u003c/sup\u003e, although the pattern and trigger of the Ordovician climatic cooling remain contentious. For example, it is debated whether this cooling trend was a continuous long-term event or an episodic, multistage process\u003csup\u003e20,22,23\u003c/sup\u003e. Atmospheric CO\u003csub\u003e2\u003c/sub\u003e levels, a key determinant of the long-term climate evolution of the Earth, are thought to have fallen through enhanced silicate weathering\u003csup\u003e25\u003c/sup\u003e linked to the spread of the earliest land plants\u003csup\u003e26,27\u003c/sup\u003e, or possibly to elevated marine primary productivity\u003csup\u003e28\u003c/sup\u003e, which are not necessarily mutually exclusive mechanisms. Moreover, the patterns of changes in climate and biodiversity in South China (i.e., the most intensively studied craton, and one that may have been the cradle of diversification) versus at a global scale are significantly different\u003csup\u003e2\u003c/sup\u003e, complicating an understanding of the causal relationship between the Ordovician climatic cooling and contemporaneous bioevolutionary developments.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eVolcanic activity is the principal natural source of mercury (Hg) in the Earth-surface system\u003csup\u003e29\u003c/sup\u003e. Volcanic eruptions can emit substantial quantities of elemental mercury having a residence time of a few years (~1.5 yr) in the atmosphere, which facilitates its long-range transport and eventual deposition in both marine and terrestrial sediments\u003csup\u003e30,31\u003c/sup\u003e. Massive inputs of Hg during major volcanic events may exceed the absorption capacity of organic matter at the sediment-water interface, resulting in positive anomalies of the ratio of total mercury to total organic carbon (TOC) (Hg/TOC) in sedimentary successions that are signals of volcanic Hg inputs\u003csup\u003e29,32\u003c/sup\u003e. The isotopic composition of Hg serves as an additional powerful tool to decipher sources and mechanisms of Hg enrichment (e.g.,\u003csup\u003e31\u003c/sup\u003e). Hg isotopes can reveal both mass-dependent fractionation (MDF; \u0026delta;\u003csup\u003e202\u003c/sup\u003eHg) and mass-independent fractionation (MIF; \u0026Delta;\u003csup\u003e199\u003c/sup\u003eHg,\u0026nbsp;\u0026Delta;\u003csup\u003e200\u003c/sup\u003eHg, \u0026Delta;\u003csup\u003e201\u003c/sup\u003eHg). Critically, MIF arises predominantly through aqueous or atmospheric photochemical interactions antecedent to Hg sedimentation, and its isotopic signals are resilient against post-depositional diagenetic alteration. Mercury emissions from subaerial volcanoes typically exhibit near-zero MIF values with minimal variance\u003csup\u003e29,33\u003c/sup\u003e.\u0026nbsp;Volcanic activity may have induced long-term climatic cooling (e.g., Hirnantian Glaciation) during the Late Ordovician\u003csup\u003e34\u003c/sup\u003e, offsetting transient warming associated with greenhouse gas emissions from volcanic eruptions\u003csup\u003e28\u003c/sup\u003e. However, studies integrating paired proxies for both volcanism and temperature are limited, leaving the role of volcanic activity in the long-term cooling trend of the Ordovician unclear\u003csup\u003e25\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eHere, we studied three biostratigraphically well-constrained Ordovician marine successions at Huanghuachang (n.b., the Global Stratotype Section and Point (GSSP) of the Lower/Middle Ordovician boundary), Chenjiahe (n.b., the global auxiliary stratotype section of the same boundary), and Wangjiawan (n.b., the GSSP for the base of the Hirnantian Stage), which are geographically\u0026nbsp;closely spaced (~15 km) near Yichang City (Figs. S1-S2). We generated paired high-resolution profiles of oxygen isotopes for conodont bioapatite (\u0026delta;\u003csup\u003e18\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e)\u0026nbsp;as paleotemperature proxies, Hg-system chemistry (Hg/TOC,\u0026nbsp;\u0026Delta;\u003csup\u003e199\u003c/sup\u003eHg\u0026nbsp;and\u0026nbsp;\u0026delta;\u003csup\u003e202\u003c/sup\u003eHg) as volcanic proxies, and strontium isotopes for conodont bioapatite (\u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e) as a continental weathering proxy. These data allowed us to evaluate the patterns and driving mechanisms of the Ordovician climatic cooling, providing key insights regarding life-environment co-evolution during the Ordovician Period.\u0026nbsp;\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e \u003ch2\u003eConodont in-situ oxygen isotopes\u003c/h2\u003e \u003cp\u003eOur conodont in-situ oxygen isotope record (δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e) shows a long-term secular increase from ~\u0026thinsp;+\u0026thinsp;15\u0026ndash;17\u0026permil; in the Lower Ordovician Nanjinguan Formation (Fm) to ~\u0026thinsp;+\u0026thinsp;16\u0026ndash;19\u0026permil; in the Middle Ordovician Dawan to Guniutan Fm, and ~\u0026thinsp;+\u0026thinsp;18\u0026ndash;20\u0026permil; in the Upper Ordovician Wufeng Fm (Fig. S3A). We further compiled newly generated data with previously published δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e data using either gas isotope ratio mass spectrometry (GIRMS), sensitive high resolution ion microprobe (SHRIMP) or SIMS. Previous studies suggest a bias between SHRIMP and GIRMS conodont analyses, in which the former is systematically \u0026sim;0.6\u0026ndash;1.3\u0026permil; higher than the later\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e, while none systematically bias between SHRIMP and SIMS analyses. To enhance consistency across different data sources during data compilation, we apply a minimum correction of +\u0026thinsp;0.6\u0026permil; when converting GIRMS results to the SHRIMP standard. In the compiled δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e curve, average values for the latest Cambrian (16.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u0026permil;), and the Lower, Middle and Upper Ordovician (16.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u0026permil;, 17.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7\u0026permil; and 19.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u0026permil;, respectively) are used as baseline values for evaluation of cooling intervals (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA). Relative to these baseline values, we recognized two intervals of more rapid increase in δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e superimposed on the long-term secular shift, documenting a stepwise cooling of SSTs during the upper Floian and upper Darriwilian to lower Katian, before the well-known Hirnantian stages.\u003c/p\u003e \u003cp\u003eWell preserved δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e profiles (Supplementary Text S1) document three major cooling episodes within the Ordovician (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA), which could be biostratigraphically and lithostratigraphically constrained in study sections: (1) the late Floian cooling event (LFCE), during the \u003cem\u003ePrioniodus honghuayuanensis\u003c/em\u003e to \u003cem\u003eBaltoniodus navis\u003c/em\u003e zones of the late Folian to middle Dapingian stages (upper Honghuayuan Fm to lower Dawan Fm, ~\u0026thinsp;200\u0026ndash;215 m), (2) the late Darriwilian cooling event (LDCE), during the \u003cem\u003eYangtzeplacognathus protoramosus\u003c/em\u003e to \u003cem\u003eAmorphognathus ordovicicus\u003c/em\u003e zones of the late Darriwilian to early Katian stages (upper Guniutan Fm to lower Baota Fm, ~\u0026thinsp;270\u0026ndash;280 m), and (3) the Hirnantian Glaciation, during the \u003cem\u003eNormalograptus extraordinarius\u003c/em\u003e-\u003cem\u003eN. ojsuensis\u003c/em\u003e to \u003cem\u003eN. persculptus\u003c/em\u003e zones of the late Katian to Hirnantian stages (upper Lingxiang Fm to middle Guanyinqiao Fm, ~\u0026thinsp;305\u0026ndash;315 m) (Figs. S2-3).\u003c/p\u003e \u003cp\u003eThe three major cooling episodes that we integrated are reinforced through comparing with previously published data from individual locations, for example, the LFCE from the Laurentian margins and Argentine Precordillera\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e, the LDCE from Laurentia and Tarim\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e and the Hirnantian cooling from the Laurentia and Gondwana\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. An offset of ~\u0026thinsp;1 to 2\u0026permil; between newly generated and previously published data following the LDCE may be attributed to multiple processes, including spatial difference in SSTs, taxon-related effects in conodonts (Supplementary Text S1), and/or biases during data compilation. Although the Hirnantian Glaciation interval was documented in the newly generated data (Fig. S3A), it was not clearly evident in the globally compiled LOESS curve (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eA), probably due to the fact that the short-term, pronounced cooling had been obscured by data smoothing techniques. A previous proposed climate warming around the Katian-Hirnantian boundary\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e is not evident in our new records, possibly because of low data resolution in that interval of the compiled data, smoothing of the data masked the transient fluctuations, or the limited duration or geographic extent of the warming event. Two low δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e values at the top of the study sections indicate temporary warming conditions at the end of the Hirnantian Glaciation\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eConodont in-situ strontium isotopes\u003c/h2\u003e \u003cp\u003eConodont in-situ strontium isotopes (\u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e) exhibit a slow decrease from ~\u0026thinsp;0.7090 to ~\u0026thinsp;0.7085 from the lower Tremadocian to the middle Darriwilian, followed by a more rapid decrease to ~\u0026thinsp;0.7080 in the upper Katian (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eF, S3B).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eMercury geochemical data\u003c/h3\u003e\n\u003cp\u003eMercury (Hg) and its ratio to total organic carbon (Hg/TOC) mostly below phanerozoic baseline values (i.e., average values of ~\u0026thinsp;60 ppb and ~\u0026thinsp;140 ppb/wt%, respectively\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e), interspersed with peak Hg and Hg/TOC plateaus (as high as ~\u0026thinsp;200\u0026ndash;400 ppb and ~\u0026thinsp;100\u0026ndash;600 ppb/wt%, respectively) in the Miaopo Fm (Sandbian) and Wufeng to Longmaxi Fms (Hirnantian) (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD-E, S3E). Besides, the Hg/TOC plateaus or scattered peaks also appeared in the Nanjinguan Fm (lower Tremadocian) and Dawan to Miaopo Fms (upper Floian to Sandbian), corresponding to rise in Hg content even below baseline value.\u003c/p\u003e \u003cp\u003eRatios of mercury to aluminum (Hg/Al), iron (Hg/Fe) and manganese (Hg/Mn) show roughly parallel variations (Fig. S4), marked by general decrease trends (from ~\u0026thinsp;50 to \u0026lt;\u0026thinsp;0.1 ppb/wt%, ~\u0026thinsp;50 to 0.1 ppb/wt% and 0.1 to 0.001 ppb/ppm, respectively) in the Nanjinguan to Dawan Fms, followed by increases to maxima (~\u0026thinsp;100 ppb/wt%, ~\u0026thinsp;300 ppb/wt% and ~\u0026thinsp;3 ppb/ppm, respectively) in the upper Guniutan to Miaopo Fms. After decreases to ~\u0026thinsp;0.1 ppb/wt%, ~\u0026thinsp;0.3 ppb/wt% and ~\u0026thinsp;0.001 ppb/ppm, respectively, in the Baota and Lingxiang Fms, these Hg ratios rebound to pre-excursion values in the Wufeng and Longmaxi Fms.\u003c/p\u003e \u003cp\u003eΔ\u003csup\u003e199\u003c/sup\u003eHg exhibits overall positive values in gradually decrease trend from ~\u0026thinsp;+\u0026thinsp;0.3\u0026permil; to near-zero values (~\u0026ndash;0.05 to +\u0026thinsp;0.05\u0026permil;) in the Nanjinguan to Dawan Fms (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB, S3C), with a few positive values (\u0026thinsp;~\u0026thinsp;+\u0026thinsp;0.1 to +\u0026thinsp;0.3\u0026permil;) in the Honghuayuan and Dawan Fms. Δ\u003csup\u003e199\u003c/sup\u003eHg mostly exhibits negative values in the Guniutan and Miaopo Fms (~\u0026ndash;0.2 to ~\u0026thinsp;+\u0026thinsp;0.05\u0026permil;), and a range of values from ~\u0026ndash;0.05\u0026permil; to +\u0026thinsp;0.2\u0026permil; in the Baota and Lingxiang Fms. Accordingly, we subdivided Ordovician Δ\u003csup\u003e199\u003c/sup\u003eHg profile into four intervals through compilation with previously published data in the overly Wufeng to Longmaxi Fms at Wangjiawan\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e, including (1) a decrease trend of Δ\u003csup\u003e199\u003c/sup\u003eHg values during the early Tremadocian to middle Folian (Nanjinguan to mid Honghuayuan Fm., ~\u0026thinsp;485\u0026thinsp;\u0026minus;\u0026thinsp;473 Ma), (2) a dominant of near-zero Δ\u003csup\u003e199\u003c/sup\u003eHg values during the late to latest Folian, followed by a decrease trend from positive to near-zero values during the earliest to early Dapingian (LFCE interval) (mid Honghuayuan to mid Dawan Fm., ~\u0026thinsp;473\u0026thinsp;\u0026minus;\u0026thinsp;468 Ma), (3) a gradual decrease trend to slight negative values (~\u0026ndash;0.05 to \u0026minus;\u0026thinsp;0.1\u0026permil;) through the Darriwilian (~\u0026thinsp;468\u0026thinsp;\u0026minus;\u0026thinsp;459 Ma), until more negative Δ\u003csup\u003e199\u003c/sup\u003eHg values (~\u0026ndash;0.3 to \u0026minus;\u0026thinsp;0.2\u0026permil;) followed by a rebound to near-zero values in the Sandbian (LDCE interval) (upper Dawan to lower Baota Fm., ~\u0026thinsp;459\u0026thinsp;\u0026minus;\u0026thinsp;453 Ma), and (4) mostly near-zero Δ\u003csup\u003e199\u003c/sup\u003eHg values followed by both near-zero and slightly positive values (\u0026thinsp;~\u0026thinsp;+\u0026thinsp;0.05 to +\u0026thinsp;0.1\u0026permil;) during the earliest Katian to the Hirnantian glacial interval (upper Baota to lower Longmaxi Fm., ~\u0026thinsp;453\u0026thinsp;\u0026minus;\u0026thinsp;443 Ma) (age-depth model is given in Supplementary Data).\u003c/p\u003e \u003cp\u003eδ\u003csup\u003e202\u003c/sup\u003eHg mostly shows negative values (~\u0026ndash;3 to 0\u0026permil;) throughout the study sections, except weakly positive values (\u0026thinsp;~\u0026thinsp;+\u0026thinsp;0.2 to +\u0026thinsp;0.7\u0026permil;) in the Guniutan Fm (~\u0026thinsp;467\u0026thinsp;\u0026minus;\u0026thinsp;462 Ma) and the Miaopo Fm (~\u0026thinsp;459\u0026thinsp;\u0026minus;\u0026thinsp;454 Ma) (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eC, S3D), and is negatively correlated with Δ\u003csup\u003e199\u003c/sup\u003eHg (Fig. S5). Our new Δ\u003csup\u003e199\u003c/sup\u003eHg dataset offers much higher temporal resolution than previously published Hg-isotope studies\u003csup\u003e\u003cspan additionalcitationids=\"CR40\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB), revealing a negative shift from the Tremadocian to Sandbian and dominantly near-zero Δ\u003csup\u003e199\u003c/sup\u003eHg values in the Katian to Hirnantian. Diagenesis evaluations suggest well preservation of both Δ\u003csup\u003e199\u003c/sup\u003eHg and δ\u003csup\u003e202\u003c/sup\u003eHg data in study sections (Supplementary Text S2).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eVolcanic activity through the Ordovician\u003c/h2\u003e \u003cp\u003eVolcanic activity, along with other factors, e.g., submarine hydrothermal activity, seawater redox conditions, terrestrial fluxes, and sediment accumulation rates, may collectively influence temporal fluctuations in the Hg content ([Hg]) of sedimentary rock\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. In seawater, Hg complexes with organic matter to form methylmercury, and it reacts with sulfide to form mercuric sulfide (HgS), yielding compounds that commonly serving as major repositories of Hg\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. However, mercury can also adsorb onto the surfaces of phosphate, clay minerals, and Fe-Mn oxides\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. The concentration of phosphorus ([P]) can be utilized as an indicator of the relative abundance of phosphate minerals, and the concentration of aluminum ([Al]) can reflect the relative content of clay minerals such as kaolinite and illite, whereas the concentrations of iron ([Fe]) and manganese ([Mn]) indicate the presence of Fe-Mn oxides. In the study units, [Hg] concentrations exhibit pronounced positive covariation with TOC (\u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.35, \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;266, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001 for Huanghuachang section; \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.77, \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;50, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001 for Chenjiahe section; \u003cem\u003er\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.49, \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;52, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001 for Wangjiawan section) but weak or no significant covariation with [Al], [Mn], [Fe], or TS (Fig. S6A-E), implying that organic matter serves as the principal host of mercury.\u003c/p\u003e \u003cp\u003eSources of sedimentary Hg other than volcanic activity are possible. Hydrothermal activity generally results in lower \u003csup\u003e87\u003c/sup\u003eSr\u003csup\u003e/86\u003c/sup\u003eSr signals for seawater and sedimentary rock (i.e., closer to the mantle endmember, ~\u0026thinsp;0.7035)\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e, therefore, conodont \u003csup\u003e87\u003c/sup\u003eSr\u003csup\u003e/86\u003c/sup\u003eSr ratio may provide critical clues in detection of hydrothermal activity. In the study sections, secular variation in \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e (~\u0026thinsp;0.7080 to 0.7090) is consistent with primary marine Sr isotope signals (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eF)\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e, providing no evidence of hydrothermal influence. Considering different responses (i.e., capacity and timing in adsorption) of conodont bioapatite and carbonates to hydrothermal sourced Sr or Hg, even hydrothermal Sr is not largely enriched in conodonts, carbonates may still absorb Hg from hydrothermal fluids. Previous studies have suggested that a majority of Sr in the studied conodont specimens (avg. ~13000ppm) source from diagenetic fluids instead of bulk carbonate (avg. ~300ppm)\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e, that means any prominent hydrothermal sourced Sr could be detected. So far, the prioritization of hydrothermal Hg uptake into carbonate and Sr uptake by bioapatite remains unknown, which fundamentally hinders the tracing of Hg provenance in hydrothermal systems.\u003c/p\u003e \u003cp\u003eSeawater redox conditions commonly play a pivotal role in sedimentary Hg accumulation, mostly through modulation of microbial activity and the interplay of the C-N-S biogeochemical cycles, thus determining the speciation and concentration of mercury within the aquatic environment\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Therefore, redox oscillations can have a profound effect on Hg diffusion at the sediment-water interface, directly influencing its enrichment in sediment. In the current investigation, molybdenum and uranium-enrichment factors (Mo\u003csub\u003eEF\u003c/sub\u003e and U\u003csub\u003eEF\u003c/sub\u003e) and C\u003csub\u003eorg\u003c/sub\u003e/P ratios\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e document oscillations mainly between suboxic and anoxic conditions in the Lower Ordovician, and oxic conditions in the Middle to Upper Ordovician, except suboxic-anoxic conditions around the OSB (Fig. S4F-H; Supplementary Text S3). No significant correlations exist between [Hg] or Hg/TOC and Mo\u003csub\u003eEF\u003c/sub\u003e, U\u003csub\u003eEF\u003c/sub\u003e or C\u003csub\u003eorg\u003c/sub\u003e/P (Fig. S6F-H). Furthermore, the stratigraphic distribution of [Hg] peaks does not coincide with intervals of more reducing watermass conditions (i.e., higher Mo\u003csub\u003eEF\u003c/sub\u003e, U\u003csub\u003eEF\u003c/sub\u003e and C\u003csub\u003eorg\u003c/sub\u003e/P values). These observations imply that fluctuations of seawater redox conditions were not important influences on Hg content of sedimentary rocks in the study sections.\u003c/p\u003e \u003cp\u003eSedimentation rate can exert significant influence on anomalous Hg enrichments in sedimentary rock by modulating their physical, chemical, and biogeochemical attributes\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. A subdued sedimentation rate diminishes physical remixing, thereby fostering a more homogenous distribution and protracted sequestration of Hg within sedimentary matrix, particularly at the sediment-water interface. In the present study, average linear sedimentation rates (LSR) show no correlation with [Hg] at the substage level of resolution (Fig. S7), and positive [Hg] excursions are observed within stratigraphic intervals of uniform lithologic character (i.e., argillaceous limestone at Huanghuachang and Chenjiahe, and shale at Wangjiawan; Fig. S3), suggesting that variations in sedimentation rate were not responsible for the observed [Hg] peaks. However, the lack of high-resolution age models for the study sections precludes detailed examination of the relationship between sedimentation rate and Hg enrichment.\u003c/p\u003e \u003cp\u003eIn sedimentary rock, the Hg/TOC ratio is usually relatively stable\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Volcanically sourced Hg may be released directly during eruptions or emitted through magmatic heating of organic-rich sediment (e.g., coal beds)\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e, or directly to seawater through ocean-crustal hydrothermal activity. The erupted Hg is predominantly in a gaseous form and is subject to global atmospheric transport over a brief timeframe (~\u0026thinsp;0.5 to 2 years), allowing its subsequently integrates into the global ocean's geochemical and biogeochemical cycles, thus abnormal high Hg/TOC ratios (above baseline value) as recorded in sedimentary rock\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. Sedimentary rock Hg/TOC records have been widely used to evaluate secular variation in volcanic activity at a regional or global scale\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. In the study sections, positive excursions to high Hg/TOC ratios above baseline values (~\u0026thinsp;140 ppm/wt.%) for the Phanerozoic, therefore, indicate major volcanic Hg inputs during the early Tremadocian, late Floian, Sandbian and early Hirnantian stages (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eD-E).\u003c/p\u003e \u003cp\u003eApart from Hg anomalies, volcanic tuff layers constitute the most overt manifestation of regional volcanic activity, and such deposits are widely distributed in the Middle to Upper Ordovician of South China\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e and other cratons\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. The present study sections contain multiple tuff layers in the upper Dawan Fm (a few cm thick), middle Miaopo Fm (nearly 10 cm thick), and Wufeng-Longmaxi Fms (a few mm thick). The identification of these distinct volcanic ash layers supports the inference that concurrent Hg/TOC anomalies are indicative of regional volcanic activity\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e. In addition, hydromica clay rocks, which predominantly form via saturated weathering of volcanic ash and associated pyroclastic materials\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e, are present in the Dawan to Longmaxi Fms (Fig. S3). Notably, these formations also display overall high [Hg] and elevated Hg/TOC ratios. These observations are evidence of significant, sustained regional volcanic activity during the Middle to Late Ordovician (Fig. S3E).\u003c/p\u003e \u003cp\u003eSo far, no large igneous province (LIP) or continental flood basalt province (CFBP) of Ordovician age has been identified, but a superplume event during the Middle to Late Ordovician has been hypothesized\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e,\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. Indirect evidence of this superplume event includes carbon cycle and weathering modeling\u003csup\u003e\u003cspan additionalcitationids=\"CR58\" citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e, a secular decrease of seawater \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csup\u003e60\u003c/sup\u003e, and a lack of geomagnetic reversals (i.e., a polarity superchron)\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. The hypothetical Ordovician superplume is comparable to the Cretaceous superplume in being part of a long-term supercontinent (Wilson) cycle (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), also evidenced in the latter case by pulsed oceanic crustal production, formation of large-scale oceanic plateaus (e.g., Ontong Java), intensified magmato-tectonic activity, reduced geomagnetic field reversal frequency (i.e., the Cretaceous Normal Polarity Superchron), and increased hydrothermalism as revealed from various sedimentary geochemical records (e.g., \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr)\u003csup\u003e61,62\u003c/sup\u003e. Analogously, the Ordovician Period bears resemblance to the Cretaceous Period, which was marked by the formation of numerous LIPs at an exceptionally high rate (~\u0026thinsp;6 LIPs formed within ~\u0026thinsp;40 Myr), and several of them were associated with negative excursions of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e in marine sedimentary rock, climate warming and oceanic anoxic events that linked to volcanic activity, and followed by positive excursions of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e, prolonged climate cooling, seawater oxygenation that related to enhanced continental weathering\u003csup\u003e\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eProcesses not directly associated with volcanic activity may also lead to MDF and MIF of mercury isotopes. Hg emanating from remote volcanoes experiences extended atmospheric transport, potentially leading to isotopic fractionation due to atmospheric redox processes. These processes produce positive Hg-MIF compositions (e.g., Δ\u003csup\u003e199\u003c/sup\u003eHg) and negative δ\u003csup\u003e202\u003c/sup\u003eHg in atmospheric Hg\u003csup\u003eII\u003c/sup\u003e and sediment dominated by atmospheric Hg\u003csup\u003eII\u003c/sup\u003e accumulation. Furthermore, photochemical reduction of aqueous Hg\u003csup\u003eII\u003c/sup\u003e to Hg\u003csup\u003e0\u003c/sup\u003e in the photic zone of euxinic waters, as observed during the end-Permian mass extinction\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e, can give negative Δ\u003csup\u003e199\u003c/sup\u003eHg and positive δ\u003csup\u003e202\u003c/sup\u003eHg in organic-rich sedimentary rock\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Negative Δ\u003csup\u003e199\u003c/sup\u003eHg values in sedimentary rock typically have been ascribed to Hg inputs from photic-zone euxinia or terrestrial soils/plants\u003csup\u003e47\u003c/sup\u003e. Additionally, recent studies have shown that mid-ocean ridge basalt (MORB) and island arc basalt (IAB) exhibit positive Δ\u003csup\u003e199\u003c/sup\u003eHg values (~\u0026thinsp;0.1 to \u0026gt;\u0026thinsp;0.3\u0026permil;), while oceanic island basalt (OIB) and continental flood basalt (CFB) have Δ\u003csup\u003e199\u003c/sup\u003eHg near-zero values\u003csup\u003e\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). Therefore, basalt weathering likely exerts additional influence on Hg isotopic composition in sedimentary rock.\u003c/p\u003e \u003cp\u003eThe paired Hg isotopes, [Hg], Hg/TOC and \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e ratios collectively revealed episodic intense volcanic activity, enhanced weathering of both volcanic rocks (basalt) (Supplementary Text S4) and soil/plant, and photic-zone euxinia (PZE) throughout the Ordovician (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). During the ~\u0026thinsp;485\u0026thinsp;\u0026minus;\u0026thinsp;480 Ma, a rapid decline in Δ\u0026sup1;⁹⁹Hg is likely related to volcanic activity and relevant weathering processes (either continental weathering or/and weathering of different types of basalts), which occurred against a backdrop of volcanic activity (positive Hg/TOC ratios above baseline value\u0026thinsp;~\u0026thinsp;140 ppb/wt.%). During the following\u0026thinsp;~\u0026thinsp;480\u0026thinsp;\u0026minus;\u0026thinsp;473 Ma, weathering Hg inputs dominated Hg isotopic fractionation and enrichment in sediment as revealed from negative excursions of Δ\u0026sup1;⁹⁹Hg, and decreases of [Hg] and Hg/TOC below the baseline values.\u003c/p\u003e \u003cp\u003eDuring the LFCE (~\u0026thinsp;473\u0026thinsp;\u0026minus;\u0026thinsp;468 Ma, Early-Middle Ordovician boundary transition), volcanic Hg signals dominated source of Hg and its enrichment in sediments (relatively constant near-zero Δ\u0026sup1;⁹⁹Hg and negative δ\u0026sup2;⁰\u0026sup2;Hg values, and positive excursions of Hg/TOC above the baseline) during the first half (~\u0026thinsp;473\u0026thinsp;\u0026minus;\u0026thinsp;470 Ma), followed by weakened volcanic activity (thus low [Hg] in sediments) and rise in the proportion of atmospheric Hg\u0026sup2;⁺ may be related to rise in atmospheric oxygen concentration during the second half (~\u0026thinsp;470\u0026thinsp;\u0026minus;\u0026thinsp;468 Ma). Over the entire\u0026thinsp;~\u0026thinsp;485\u0026thinsp;\u0026minus;\u0026thinsp;468 Ma interval, the prolonged decrease in Δ\u0026sup1;⁹⁹Hg may imply the involvement of early land plants during weathering (see discussion below).\u003c/p\u003e \u003cp\u003eDuring the ~\u0026thinsp;468\u0026thinsp;\u0026minus;\u0026thinsp;459 Ma, photic zone euxinia dominated marine Hg fractionation (negative Δ\u0026sup1;⁹⁹Hg, positive δ\u0026sup2;⁰\u0026sup2;Hg) because of enhanced weathering inputs (decrease in \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e) due to expansion of terrestrial plants, and intensified upwelling under cooled climate. This scenario occurred against a backdrop of weak volcanic activity (or higher primary productivity and intensive global oceanic anoxia that resulted in greater Hg drawdown\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e) which led to sporadic highest peak Hg content and Hg/TOC ratios above the baseline values. During the LDCE (~\u0026thinsp;459\u0026thinsp;\u0026minus;\u0026thinsp;453 Ma), stronger photic zone euxinia dominated marine Hg fractionation (most negative Δ\u0026sup1;⁹⁹Hg values, rise of δ\u0026sup2;⁰\u0026sup2;Hg to positive values), while Hg enrichment in the sediments was controlled by volcanic activity (with almost all Hg/TOC values above the baseline value), due to more intensified weathering (sharper decrease in \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e) and cooling driven upwelling partly related to continuous expansion of terrestrial plants. The Middle Ordovician PZE had been reported from Tarim basin and other locality in South China\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003ePhotic zone euxinia and volcanic activity weakened while weathering dominated marine Hg fractionation and enrichment in sedimentary rock during the ~\u0026thinsp;453\u0026thinsp;\u0026minus;\u0026thinsp;446 Ma, as indicated from rise of Δ\u0026sup1;⁹⁹Hg to mostly near-zero values, decline of δ\u0026sup2;⁰\u0026sup2;Hg to negative values, and drop of Hg/TOC mostly below the baseline value. Volcanic activity briefly driven Hg fractionation and its enrichment in sedimentary rock during the ~\u0026thinsp;446\u0026thinsp;\u0026minus;\u0026thinsp;443 Ma, resulted in mostly near-zero Δ\u0026sup1;⁹⁹Hg values and peak Hg/TOC ratios above the baseline value\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe limitations of the Hg/TOC ratio as a proxy for tracing volcanic signals in sedimentary rock should be considered. Weathering can remove mercury signals in organic-rich sediment, for example, leading to loss of Hg up to ~\u0026thinsp;90% in highly weathered shales, while degradation may alter the type and quality of organic matter, especially for samples with low hydrogen and high oxygen index values (e.g., Type II, equivalent to burial temperatures of ~\u0026thinsp;60\u0026ndash;180\u0026deg;C), thereby affecting the Hg/TOC ratio\u003csup\u003e\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e\u003c/sup\u003e. Our study successions consist largely of carbonate rocks that underwent low degrees of weathering based on field observations, therefore, weathering is unlikely to have been a dominant influence on Hg/TOC ratios. In addition, the color alteration indices (CAI) of the extracted conodont specimens range from 1 to 3, equivalent to burial temperatures of ~\u0026thinsp;60\u0026ndash;200\u0026deg;C\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, suggesting degradation of organic matter may have been an influence on Hg/TOC ratios. However, there is no clear difference in the CAI of conodonts between younger and older strata, indicating a relatively uniform level of thermal alteration throughout the study sections. In addition, TOC is strongly correlated with [Hg], suggesting that degradation had a limited effect on Hg/TOC ratios (Fig. S6A), and positive excursions and peak values in the Hg/TOC profiles were not caused by low TOC content (\u0026lt;\u0026thinsp;0.2 wt.%)\u003csup\u003e29\u003c/sup\u003e. Note that we regard the Hg/TOC ratio as a proxy only for local/regional volcanism, and Δ\u003csup\u003e199\u003c/sup\u003eHg as additional proxy for global volcanic activity\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e and weathering of volcanic rocks (e.g., CFB and OIB)\u003csup\u003e\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eDrivers of the LFCE and LDCE\u003c/h3\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eGlobal volcanism and basalt weathering\u003c/h2\u003e \u003cp\u003eLong-term Ordovician cooling has been explained by various mechanisms, including orogenic uplift and weathering of silicate rocks\u003csup\u003e\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e and a shift of tectonic plates into more humid regions and subsequent intensified weathering of volcanic rocks\u003csup\u003e\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e. The general problem with such explanations is that there have been many orogenies and shifts in tectonic plates of similar or greater magnitude during the Phanerozoic that have not led to markedly enhanced weathering. Episodic intense volcanic activity followed by prolonged enhanced weathering of basalt represents a more likely explanation for the multistage character of long-term cooling effect, for example, during/after LIP volcanism as shown in mass extinction events\u003csup\u003e\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e\u003c/sup\u003e and through the Ordovician\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e, although volcanic activity may temporarily cause warming by increasing pCO\u003csub\u003e2\u003c/sub\u003e, acting as immediate effect of volcanisms\u003csup\u003e\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e\u003c/sup\u003e. This relationship is implied by concurrent positive excursions of δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003ecarb\u003c/sub\u003e and peak Hg/TOC above baseline during the LFCE and LDCE, as well as plateau δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003ecarb\u003c/sub\u003e value and sporadic Hg/TOC ratios above the baseline value between the two major cooling events (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and 4). These relationships suggest that the Ordovician climatic cooling (e.g., LFCE and LDCE) was primarily a volcanically driven, stepwise cooling event spread out over a\u0026thinsp;~\u0026thinsp;40-Myr-long interval (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Moreover, concurrent of climatic cooling to PZE-related weathering of both basalt and terrestrial soil/plant occurred against the backdrop of volcanic activity during the late Dapingian to late Sandbian (~\u0026thinsp;468\u0026thinsp;\u0026minus;\u0026thinsp;453 Ma) (Figs.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), supporting our inference of a causal relationship. Chemical weathering of basalt could sequester atmospheric CO\u003csub\u003e2\u003c/sub\u003e, predominantly through secular silicate minerals reacting with CO\u003csub\u003e2\u003c/sub\u003e to produce carbonates. Quantification of ancient basalt weathering reveals an atmospheric CO\u003csub\u003e2\u003c/sub\u003e consumption rate of ~\u0026thinsp;1 to 3\u0026times;10\u003csup\u003e5\u003c/sup\u003e mol C/km\u0026sup2;/yr\u003csup\u003e73\u003c/sup\u003e, thus lowering global temperatures. The inferred temporal concurrence of volcanic activity and the Ordovician climatic cooling has previously been ascribed to weathering of volcanic rocks\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e,\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eGlobally increased production and weathering of volcanic rocks during the Ordovician is likely to have triggered the climatic cooling. Increasing evidence suggests that the subduction of oceanic crust, volcanic activity on Earth\u0026rsquo;s surface, and magmatic activity in deep earth all have been intensified during the Ordovician (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Studies of Phanerozoic geomagnetic polarity suggested a transition from a Middle Cambrian-Middle Ordovician Reversed Polarity Bias Interval to a Late Ordovician-Late Silurian Normal Polarity Bias Interval, forming a Middle Ordovician Polarity Shift that indicates a major transition of Earth\u0026rsquo;s deep magmatism in the Ordovician\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. Moreover, extensive basalt lava (e.g., British Isles, northern Iran and West Junggar)\u003csup\u003e\u003cspan additionalcitationids=\"CR75\" citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e76\u003c/span\u003e\u003c/sup\u003e, and global peaks in production of volcanogenic massive sulfide deposits, sulphidic shales and episodic ironstone deposition indicate elevated mantle plume activity during the Ordovician\u003csup\u003e\u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e77\u003c/span\u003e\u003c/sup\u003e. Recently, a large siliceous LIP has been identified with an areal extent of \u0026sim;2.5 Mkm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e (or an estimated total volume of \u0026sim;2.5 Mkm\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e) (e.g., Pinghe LIP\u003csup\u003e\u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e78\u003c/span\u003e\u003c/sup\u003e). However, no large oceanic magmatic event of Ordovician age has been identified or, indeed, is likely to ever be identified because of near-total subduction of oceanic crust of that age. The intense subduction, collision, and orogenic processes in the oceanic lithosphere was evidenced by most frequency occurrence of ophiolite in the Ordovician which higher than other periods in Paleozoic Era\u003csup\u003e\u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e79\u003c/span\u003e\u003c/sup\u003e. The intense subduction of oceanic lithosphere and/or a potential submarine LIP could lead to intense global hydrothermal activity and influxes, resulting in a prolonged and major negative shift of \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr in the Ordovician ocean\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. In detail, the trajectory of \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr in marine sedimentary rock delineates a period of substantial weathering of basalt during the Darriwilian to Sandbian, followed by stabilization in the Katian\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eVolcanic activity can draw down atmospheric CO\u003csub\u003e2\u003c/sub\u003e and drive secular global cooling by increasing ocean productivity and organic carbon burial rates, through elevated volcanic nutrient supply from weathering of volcanic rocks\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. In the present study, secular decrease of Δ\u003csup\u003e199\u003c/sup\u003eHg and rise of Hg/TOC ratio above baseline value during the Middle to Upper Ordovician cooling, correspond to the long-term decline in seawater \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), pointing to a common trigger that may have been related to oceanic plateau volcanism or rapid seafloor spreading\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e,\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. In addition, negative excursions of Δ\u003csup\u003e199\u003c/sup\u003eHg signals in the Lower Ordovician can be attributed to changes in the main type of weathered basalt (e.g., from IAB and MORB to OIB) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). Simultaneously with the weathering influxes of terrestrial volcanic rocks, increased hydrothermal weathering due to seafloor production prevailed also generated massive basalt as a dominated nutrient P influxes\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e, which elevated marine productivity and enhanced burial rate of organic matter (e.g., peak TOC values in the LDCE interval, Fig. S4B), resulting in a onset of positive excursion and plateau value of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e in the ocean during the Early to early Late Ordovician (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eG). These considerations establish connections between the GOBE (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eI), the Ordovician climatic cooling, and contemporaneous volcanic activity (e.g.,\u003csup\u003e\u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e80\u003c/span\u003e\u003c/sup\u003e).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eEarly land plant evolution\u003c/h3\u003e\n\u003cp\u003eSome aspects of our Ordovician dataset are not readily explained by volcanic activity and basalt weathering, e.g., the long-term decrease of Δ\u003csup\u003e199\u003c/sup\u003eHg to negative values (~\u0026ndash;0.2\u0026permil;) and rise of δ\u003csup\u003e202\u003c/sup\u003eHg to positive values (\u0026thinsp;~\u0026thinsp;+\u0026thinsp;0.4 to +\u0026thinsp;0.7\u0026permil;) in the Middle to Upper Ordovician (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eB-C). Photic zone euxinia, which is commonly related to elevated marine productivity in upwelling regions\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e, was prevalent during the early Darriwilian to late Sandbian cooling, as revealed by long-term paired changes in both Δ\u003csup\u003e199\u003c/sup\u003eHg and δ\u003csup\u003e202\u003c/sup\u003eHg intervals in Guniutan through Baota Fms. Alternatively, negative D\u003csup\u003e199\u003c/sup\u003eHg is commonly linked to terrestrially sourced organic matter because land plants predominantly derive Hg\u003csup\u003e0\u003c/sup\u003e in its root from surrounding soil where odd isotopes of mercury (e.g., \u003csup\u003e199\u003c/sup\u003eHg) are preferentially reduced during photochemical reduction, resulting in negative Δ\u003csup\u003e199\u003c/sup\u003eHg values in soil carbon\u003csup\u003e\u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e81\u003c/span\u003e\u003c/sup\u003e. In the present study, negative Δ\u003csup\u003e199\u003c/sup\u003eHg values (to ~\u0026ndash;0.2\u0026permil;) during the late Dapingian to late Sandbian substages coincided with evolution of the earliest embryophytes (i.e., bryophyte-grade land plants)\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eH). The timing of the first appearance of cryptospores on land in the Dapingian (early Middle Ordovician)\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e,\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e coincided with onset of a shift to negative Δ\u003csup\u003e199\u003c/sup\u003eHg values (~\u0026thinsp;470\u0026thinsp;\u0026minus;\u0026thinsp;468 Ma), suggesting a link via terrestrial weathering inputs to marine systems. In addition, we suspect that the long-term decrease of D \u003csup\u003e199\u003c/sup\u003eHg through the Early Ordovician may also have been related to gradual expansion of land plants as revealed by molecular clock studies\u003csup\u003e\u003cspan citationid=\"CR82\" class=\"CitationRef\"\u003e82\u003c/span\u003e\u003c/sup\u003e. A more significant proliferation of land plants occurred\u0026thinsp;~\u0026thinsp;40 Myr later, during the Early Silurian\u003csup\u003e\u003cspan citationid=\"CR83\" class=\"CitationRef\"\u003e83\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe earliest cryptospores of land plants are from the basal Middle Ordovician of Argentina, and similar spore assemblages have been reported globally from the paleo-Equator to high latitudes\u003csup\u003e\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e,\u003cspan citationid=\"CR87\" class=\"CitationRef\"\u003e87\u003c/span\u003e\u003c/sup\u003e, and from the Dapingian to the Llandovery of Early Silurian (Supplementary Text S5), suggesting a uniform vegetation was present for about 30 Myr\u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, before replacement by Silurian plant megafossil assemblages (i.e., vascular plants) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eH). The simple and diminutive earliest land plants would have begun to influence their environment in terms of development of a rudimentary soil, for example, through addition of organic carbon that fostered microbial communities, possibly including free-living fungi. Further evidence of land animals and soil microbes are found throughout the remainder of the Middle and Late Ordovician in many regions of the world\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, suggesting a rapid spread of early terrestrial biotic communities.\u003c/p\u003e \u003cp\u003eVolcanic activity may have promoted the evolution of land vegetation through delivery of essential nutrients such as P during weathering of volcanic rocks (e.g., basalt)\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR88\" class=\"CitationRef\"\u003e88\u003c/span\u003e\u003c/sup\u003e. The continuous land colonization of vegetation will further enhance the chemical and physical weathering of volcanic rocks, forming positive feedback that is conducive to the continuous input of nutrients. In addition, the most negative Δ\u003csup\u003e199\u003c/sup\u003eHg value (~\u0026ndash;0.2\u0026permil;) in the late Darriwilian to late Sandbian Substage (~\u0026thinsp;459\u0026thinsp;\u0026minus;\u0026thinsp;454 Ma) is coincide with both the second climate cooling stage and the most rapid drop of \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr in marine sedimentary rock during the Ordovician, implying a causality that expansion of early plants caused intensified terrestrial weathering influxes (thus PZE), and both finally lead to the climate cooling. Our present study shows major volcanic activity and the Ordovician climatic cooling onset during the middle Floian (~\u0026thinsp;475 Ma), which is ~\u0026thinsp;5 Myr earlier than the land colonization of the earliest embryophytes in the Dapingian (~\u0026thinsp;470 Ma) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), suggesting land colonization of vegetation does not the trigger but a main driver of the Ordovician climatic cooling. From this perspective, climatic and bio-evolutionary patterns of the Ordovician have an analog in the Devonian, a period also marked by a major decline in atmospheric \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e and a strong cooling trend leading to a terminal glaciation, all of which have been linked to bio-evolutionary developments among terrestrial floras, i.e., the advent of trees and seed plants (\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e and references therein).\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eGeological background and sample preparation\u003c/h2\u003e \u003cp\u003eThe locations, stratigraphy, and paleogeography of the study sections have been fully documented in prior studies\u003csup\u003e\u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e89\u003c/span\u003e\u003c/sup\u003e, and here we provide a concise summary focused on key stratigraphic features essential to this analysis. Paleogeographically, the study area was located on the north-central Yangtze Platform, which accumulated shallow-marine carbonate sediment that graded into argillaceous sands in inner-shelf settings to the northwest and slope facies to the southeast\u003csup\u003e\u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e90\u003c/span\u003e\u003c/sup\u003e. The study area exposure the Wuduhe Fm of the Upper Cambrian, and overlying 12 formations/beds (from base to top, the Xilinxia, Nanjinguan, Fenxiang, Honghuayuan, Dawan, Guniutan, Miaopo, Baota, Lingxiang and Wufeng formations, Guanyinqiao Bed, and Longmaxi Fm) ranging continuously in age from the earliest to latest Ordovician and Ordovician-Silurian boundary transition\u003csup\u003e\u003cspan citationid=\"CR90\" class=\"CitationRef\"\u003e90\u003c/span\u003e\u003c/sup\u003e (Fig. \u003cspan refid=\"MOESM2\" class=\"InternalRef\"\u003eS2\u003c/span\u003e). Limestone, bioclastic limestone and muddy limestone are dominant in the study successions, except for thin (few meters) black shale intervals with limestone lenses in the Miaopo Fm, and black shales in the Wufeng and Longmaxi Fms. Lithological compositions of the successions were fully described in\u003csup\u003e\u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e89\u003c/span\u003e\u003c/sup\u003e. Overall, the study successions record a gradual shift from carbonate platform facies in the Lower Ordovician to neritic facies in the Middle Ordovician and deep basinal facies in the Upper Ordovician, reflecting a long-term sea-level rise before the Hirnantian glacio-eustatic fall at the end of the Ordovician\u003csup\u003e\u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e89\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe bulk carbonate rocks, conodont samples and few shales were collected from Huanghuachang, Chenjiahe and Wangjiawan sections in the Yiling District of Yichang area, South China (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Continuous bulk-rock sampling (of carbonate rocks and few shale samples) was conducted from the uppermost Wuduhe Fm to lower Longmaxi Fm. Weathered surfaces and diagenetic veins were removed, and the remaining sample was cleaned, air-dried and crushed. Selected samples from each formation/bed were selected for thin-sectioning for petrographic analyses. An aliquot of each sample was powdered using a rock mill to \u0026lt;\u0026thinsp;200 mesh for bulk-rock geochemical analyses. To extract conodont specimens, lightly crushed samples were dissolved in 10% acetic acid for five days, after which conodont elements were recovered from the insoluble residue using a binocular microscope. Each conodont specimen was identified, including \u003cem\u003eBelodella\u003c/em\u003e sp., \u003cem\u003eDrepanodus\u003c/em\u003e sp., \u003cem\u003ePasoistodus\u003c/em\u003e sp., \u003cem\u003ePesiodus\u003c/em\u003e sp., \u003cem\u003eOneotodus\u003c/em\u003e sp., \u003cem\u003eScolopodus\u003c/em\u003e sp., \u003cem\u003eDrepanoistodus\u003c/em\u003e sp., \u003cem\u003eBaltoniodus\u003c/em\u003e sp., \u003cem\u003eTripodus\u003c/em\u003e sp., \u003cem\u003eSessatognathdus\u003c/em\u003e sp., and \u003cem\u003eTrianglodus\u003c/em\u003e sp.\u003c/p\u003e \u003cp\u003eThe color alteration indices (CAI) of the study specimens range from 1 to 3. The samples used in this study represent the same suite that was used in\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR89\" class=\"CitationRef\"\u003e89\u003c/span\u003e\u003c/sup\u003e. Differences in the structure and chemical composition of conodont tissues (i.e., albid crown, hyaline crown, basal body) can result in variable paleotemperature results\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. In the present study, we systematically analyzed the oxygen isotopic composition of the densest albid crown of each specimen to minimize tissue-related effects. Albid crown is also thought to yield the most faithful record of (near-)primary seawater Sr isotope signals\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e, and it was targeted for the in-situ Sr isotope analyses in this study.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eIn-situ conodont oxygen isotopes and paleotemperature\u003c/h2\u003e \u003cp\u003eDetailed methods for preparation of conodont resin targets, parameters of instrument analysis by secondary ion mass spectrometry (SIMS), and correction of instrumental mass fractionation factor (IMF) were described in\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. SIMS measurements were made using a Cameca IMS-1280 SIMS at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Oxygen isotopes were measured using the multi-collection mode on two off-axis Faraday cups. Replicate analyses of the Durango apatite standard (which was analyzed after every five sample measurements) yielded an average value of +\u0026thinsp;9.40\u0026thinsp;\u0026plusmn;\u0026thinsp;0.16\u0026permil; (2σ; \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;78), which is indistinguishable within analytical uncertainty from the reported value of +\u0026thinsp;9.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u0026permil; (2σ)\u003csup\u003e20\u003c/sup\u003e. Three δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO measurements were obtained for each specimen, yielding average 2σ variance of 0.8\u0026permil;. Previous studies have indicated that a 1% increase in δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003econodont\u003c/sub\u003e corresponds to a decrease of ~\u0026thinsp;4\u0026deg;C in seawater surface temperature\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eIn-situ conodont strontium isotopes\u003c/h2\u003e \u003cp\u003eIn-situ \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr ratios of conodont albid crown (\u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr\u003csub\u003econodont\u003c/sub\u003e) were measured by laser-ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) analysis at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) at the China University of Geosciences (Wuhan)\u003csup\u003e\u003cspan citationid=\"CR91\" class=\"CitationRef\"\u003e91\u003c/span\u003e\u003c/sup\u003e. A 193-nm ArF-excimer laser was used during the measurement, with a laser beam diameter of 60 \u0026micro;m. Three \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr measurements were obtained for each specimen, yielding average 2σ of ~\u0026thinsp;0.0006. Two natural apatite standards, Slyudyanka and MAD (Madagascar apatite), were used to monitor the accuracy of LA-MC-ICP-MS measurements, yielding average values 0.70776\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00017 (2σ) and 0.71176\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00016 (2σ), respectively, which are consistent with the reference \u003csup\u003e87\u003c/sup\u003eSr/\u003csup\u003e86\u003c/sup\u003eSr values for Slyudyanka (0.70769\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00015 (2σ)) and MAD (0.71180\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00011 (2σ)).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eMercury content and isotopes\u003c/h2\u003e \u003cp\u003eMercury concentration and its isotopic analyses following methods described in\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e,\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Mercury concentration was measured using a LECO AMA254 mercury analyzer in the GPMR, and its isotopes were determined using a MC-ICP-MS with high sensitivity X skimmer cone at the Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China.\u003c/p\u003e \u003cp\u003eFor mercury concentration analyses, all the samples were freeze-dried to prevent the decomposition of Hg. About 100 mg for mudstone or shales and 150\u0026ndash;200 mg for limestone were analyzed. Data reliability was ensured by analysis of international standard coal samples 502\u0026ndash;685 (40 ppb) after every 12 unknowns, yielding reproducibility of sample concentrations being within 10%.\u003c/p\u003e \u003cp\u003ePyrolysis method was used to extract Hg for its isotopic analyses. An international standard NIST SRM 997 Tl was used for simultaneous instrumental mass bias correction of Hg and 4 ng/mL SnCl\u003csub\u003e2\u003c/sub\u003e solution was used to generate elemental Hg\u003csup\u003e0\u003c/sup\u003e before being introduced into the plasma. International standard NIST SRM 3133 was measured after every 3 unknowns to monitor the stability of the instrument. We also analyzed NIST SRM 3177 after every 10 unknowns to examine the instrument accuracy. Hg concentrations of \u0026sim;2 ng/mL or 1 ng/mL of NIST SRM 3133 and NIST SRM 3177 solutions were prepared for matching measured sample solutions to reduce the matrix dependent mass bias. Hg isotopic composition is reported in δ\u003csup\u003e202\u003c/sup\u003eHg notation in units of per mille (\u0026permil;) relative to the NIST SRM 3133 Hg standard:\u003c/p\u003e \u003cp\u003eδ\u003csup\u003e202\u003c/sup\u003eHg (\u0026permil;) = [(\u003csup\u003e202\u003c/sup\u003eHg/\u003csup\u003e198\u003c/sup\u003eHg\u003csub\u003esample\u003c/sub\u003e)/(\u003csup\u003e202\u003c/sup\u003eHg/\u003csup\u003e198\u003c/sup\u003eHg\u003csub\u003estandard\u003c/sub\u003e)\u0026thinsp;\u0026minus;\u0026thinsp;1] \u0026times; 1000\u003c/p\u003e \u003cp\u003eMass independent fractionation (MIF) of Hg isotopes is expressed in Δ notation (Δ\u003csup\u003exxx\u003c/sup\u003eHg), which describes the difference between the measured δ\u003csup\u003exxx\u003c/sup\u003eHg and the theoretically predicted δ\u003csup\u003exxx\u003c/sup\u003eHg value, using the following equations:\u003c/p\u003e \u003cp\u003eΔ\u003csup\u003e199\u003c/sup\u003eHg\u0026thinsp;\u0026asymp;\u0026thinsp;δ\u003csup\u003e199\u003c/sup\u003eHg \u0026minus; (δ\u003csup\u003e202\u003c/sup\u003eHg \u0026times; 0.2520)\u003c/p\u003e \u003cp\u003eΔ\u003csup\u003e200\u003c/sup\u003eHg\u0026thinsp;\u0026asymp;\u0026thinsp;δ\u003csup\u003e200\u003c/sup\u003eHg \u0026minus; (δ\u003csup\u003e202\u003c/sup\u003eHg \u0026times; 0.5024)\u003c/p\u003e \u003cp\u003eΔ\u003csup\u003e201\u003c/sup\u003eHg\u0026thinsp;\u0026asymp;\u0026thinsp;δ\u003csup\u003e201\u003c/sup\u003eHg \u0026minus; (δ\u003csup\u003e202\u003c/sup\u003eHg \u0026times; 0.7520)\u003c/p\u003e \u003cp\u003eReplicate analyses of the NIST 3177 Hg isotope reference standard (\u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;4) yielded the following: δ\u003csup\u003e202\u003c/sup\u003eHg\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;0.42\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06\u0026permil; (2σ); Δ\u003csup\u003e199\u003c/sup\u003eHg\u0026thinsp;=\u0026thinsp;\u0026minus;\u0026thinsp;0.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u0026permil; (2σ); Δ\u003csup\u003e200\u003c/sup\u003eHg\u0026thinsp;=\u0026thinsp;+\u0026thinsp;0.01\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u0026permil; (2σ); Δ\u003csup\u003e201\u003c/sup\u003eHg\u0026thinsp;=\u0026thinsp;+\u0026thinsp;0.01\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04\u0026permil; (2σ).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eMajor elemental and total organic carbon contents\u003c/h2\u003e \u003cp\u003eMajor elements (Al and Fe) were measured using wavelength-dispersive XRF in the GPMR. Average analytical uncertainty is better than 5% (RSD\u0026mdash;relative standard deviation) for major elements based on repeated analysis of national standards GBW07132, GBW07133, and GBW07407, and better than 2% (RSD) for trace elements based on international standards AGV-2, BHVO-2, BCR-2, and GSR-1.\u003c/p\u003e \u003cp\u003eIn the same laboratory, total organic carbon (TOC) and total sulfur content (TS) was measured using an Elementar Vario Micro Cube analyzer. Data quality was assessed through multiple analyses of standard sample DP-1 (65.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.33 wt%). A standard sample and a repeat were analyzed after every 12 unknowns, yielding an analytical accuracy of 2.5 wt% of the reported values.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank Zhihong Li, Yang Li, Fei Guo and Pingan Yan for help in the field, and Guangyi Sun, Yunjie Wu, Zhenfeng Luo for their assistance with the laboratory work. This research is supported by the NSFC grants (Nos. 92055212, 42477215), the \u0026ldquo;CUG Scholar\u0026rdquo; Scientific Research Funds at China University of Geosciences (No. 2023081), the Postdoctoral Fellowship Program of CPSF under Grant Number (No. GZC20232474, 2024M753028), the \u0026ldquo;MOST\u0026rdquo; Special Fund from State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (No. MSFGPMR2024-104).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eH. Z. and L.Z. conceived and designed the study; H.Z., L.Z., T.J.A., Z.Y.L., X.D.W. and F.H. jointly made contributions in the sample collection, data analysis and interpretation, writing, editing and review of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary information:\u0026nbsp;\u003c/strong\u003eThe supplementary material is available in this submission.\u003c/p\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eAll data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.\u003c/p\u003e \u003c/div\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eFan JX et al (2020) A high-resolution summary of Cambrian to Early Triassic marine invertebrate biodiversity. 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Science Press Beijing\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhao H et al (2024) Calcium isotope evidence of increased carbonate saturation state during the Frasnian\u0026ndash;Famennian boundary event. Earth Planet Sci Lett 642. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org:10.1016/j.epsl.2024.118876\u003c/span\u003e\u003cspan address=\"https://doi.org:10.1016/j.epsl.2024.118876\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6593045/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6593045/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eCausal relationships among the major environmental and biological developments of the Ordovician Period (i.e., long-term climatic cooling, Hirnantian Glaciation, Great Ordovician Biodiversification Event, spread of bryophyte-grade land plants, and Late Ordovician Mass Extinction) remain in debate. Here, we present new data for volcanic activity, sea-surface temperatures, and chemical weathering intensity, based respectively on Hg geochemistry, conodont oxygen and strontium isotopes. This dataset documents a ~25-Myr-long interval of climatic cooling (ca. 470-445 Ma), which commenced around the Lower/Middle Ordovician boundary and intensified near the Middle/Upper Ordovician transition, ultimately culminating in the Hirnantian Glaciation. Cooling was associated with long-term intensified weathering of volcanic rocks (basalt) and drawdown of atmospheric pCO\u003csub\u003e2\u003c/sub\u003e, as well as periodic land plant expansion and photic-zone euxinia, during major volcanic intervals and their subsequent phases. These relationships implicate volcanic activity as the primary driver of contemporaneous environmental and climatic changes, with the spread of early land plants as a potential secondary influence, thus revealing complex modulation of life-environment coevolution during the Ordovician Period.\u0026nbsp;\u003c/p\u003e","manuscriptTitle":"Volcanism and basalt weathering drove Ordovician climatic cooling","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-12 06:52:10","doi":"10.21203/rs.3.rs-6593045/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"219adf0b-e094-48f8-8b24-57170350bfa4","owner":[],"postedDate":"May 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":48313912,"name":"Earth and environmental sciences/Planetary science/Geochemistry"},{"id":48313913,"name":"Earth and environmental sciences/Climate sciences/Palaeoclimate"}],"tags":[],"updatedAt":"2025-12-30T08:27:26+00:00","versionOfRecord":{"articleIdentity":"rs-6593045","link":"https://doi.org/10.1038/s41467-025-66316-4","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2025-12-12 05:00:00","publishedOnDateReadable":"December 12th, 2025"},"versionCreatedAt":"2025-05-12 06:52:10","video":"","vorDoi":"10.1038/s41467-025-66316-4","vorDoiUrl":"https://doi.org/10.1038/s41467-025-66316-4","workflowStages":[]},"version":"v1","identity":"rs-6593045","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6593045","identity":"rs-6593045","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}