Perturbations of the global carbon cycle across the Cretaceous–Palaeogene boundary

Perturbations of the global carbon cycle across the Cretaceous–Palaeogene boundary | 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 Perturbations of the global carbon cycle across the Cretaceous–Palaeogene boundary Mingming Ma, Mengdi Wang, Huixin Huang, Xiuming Liu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3893195/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 10 May, 2024 Read the published version in Communications Earth & Environment → Version 1 posted You are reading this latest preprint version Abstract Two hyperthermal events with different carbon cycle perturbations occurred across the Cretaceous-Palaeogene (K-Pg) boundary, i.e., the late Maastrichtian Warming Event (LMWE) and the early Danian Dan-C2 event. However, the roles played by Deccan volcanism and orbital forcing in these two hyperthermals are still debated. Here, we obtain a new terrestrial δ 13 C carb record in the Nanxiong Basin (southeastern China) and compare it with marine records. The results show that both the LMWE and Dan-C2 event can be well distinguished in the terrestrial record and that the Dan-C2 event is characterized by a typical hyperthermal event; however, the specificity of the context under which this event occurred has resulted in inconsistencies in the marine records. In addition, the δ 13 C excursion during the LMWE was more muted and prolonged than that during the Dan-C2 event, and the short-eccentricity cycle disappeared in the marine record during the LMWE, indicating that Deccan volcanism perturbed the carbon cycle during the LMWE, while the Dan-C2 event was less influenced by volcanic perturbation. Earth and environmental sciences/Climate sciences/Palaeoclimate Earth and environmental sciences/Solid Earth sciences/Geology Terrestrial record Cretaceous-Palaeogene boundary Global carbon cycle Deccan Traps Orbital forcing Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction A series of hyperthermal events characterized by transient warming and significant negative carbon isotope excursion (CIE) occurred during the late Cretaceous–early Palaeogene 1 , 2 . Among them, the Late Maastrichtian Warming Event (LMWE) and the early Danian Dan-C2 event occurred across the Cretaceous-Palaeogene (K-Pg) boundary. Both were related to significant climatic and biotic changes; for instance, the LMWE was probably related to mass extinction at the end of the Cretaceous 3 – 5 , while the Dan-C2 event promoted biotic recovery after the extinction 4 , 6 . The Dan-C2 event has been distinguished in the Atlantic Ocean 1 , 6 – 9 , the Tethys Ocean 10 – 12 , and the middle and lower latitudes of Eurasia 4 , 13 – 16 . Compared to other hyperthermal events, the δ 13 C negative excursions during the Dan-C2 event were restricted to planktonic foraminifera and bulk records in parts of the Atlantic and Tethys Oceans, while benthic foraminifera rarely recorded this event 1 , 9 . Furthermore, the warming indicated by oxygen isotopes (δ 18 O) during this event was also limited to surface waters in parts of the North Atlantic, with evidence of warming in bottom waters generally lacking 1 , 9 , 17 (supporting information Text S1, Fig. S1 ). Therefore, the global significance of the Dan-C2 event and even whether it should be regarded as a true hyperthermal are still controversial 1 , 7 , 17 . Two main sources of the massive 12 C-rich CO 2 triggering of the carbon cycle during the LMWE and Dan-C2 event were proposed: Deccan volcanism and carbon pools controlled by orbital forcing. The extremely negative values of the CIE for the LMWE and Dan-C2 event are located at the maxima of the 405-kyr long-eccentricity cycle 18 , 4 , 12 . Moreover, the two CIEs of the Dan-C2 event correspond to two 100-kyr short-eccentricity maxima 1 , indicating that orbital forcing contributed to the CIE during both the LMWE and the Dan-C2 event. The overlapping occurrences of Deccan volcanism and two hyperthermals suggest that the large amount of volcanic carbon emitted could have also played a role in driving hyperthermals, especially the LMWE 3 , 5 , 6 , 18 . However, this speculation is plausible and contemporaneous and lacks causal connection 19 . Recent modelling efforts to simulate CO 2 emission scenarios from Deccan volcanism have yielded conflicting results, with either more CO 2 20 , half of the CO 2 6 , or one-third of the CO 2 21 released before the K-Pg boundary. Therefore, the roles that Deccan volcanism played in perturbing the carbon cycle in the LMWE and the Dan-C2 event are still unclear. To date, most of the carbon isotope records are from marine sediments, whereas terrestrial records are lacking. Our previous work showed that the LMWE and Dan-C2 event were recorded by red strata in the Nanxiong Basin (Southeastern China, Fig. 1 ). Hg and its isotopes indicate that they formed during Deccan volcanism; moreover, both hyperthermals were related to the 405-kyr long eccentricity cycle 4 , 15 , 16 . However, due to the low resolution of the δ 13 C carb data, detailed comparisons with marine records and in-depth analyses of carbon cycle processes are limited. Here, a total of 274 fresh samples from the upper part of the Zhenshui Formation to the lower Xiahui part of the Shanghu Formation (Fig. 1 b) were collected, then we obtain a new δ 13 C carb record across the K-Pg boundary in the basin and compare it with other published records to reveal 1) the terrestrial records of the LMWE and Dan-C2 event, as well as the global significance of the Dan-C2 event; and 2) the relative contributions of Deccan volcanism and orbital forcing to the carbon cycle during the two hyperthermal events. Results The δ 13 C value of the bulk carbonate from the Nanxiong Basin (Fig. 2 c) is consistent with the previous δ 13 C values of pedogenic carbonate nodules that were not affected by diagenesis 22 , 23 (Fig. S2a); moreover, there is no significant correlation between δ 13 C carb and δ 18 O carb (Fig. S2b), suggesting that the influence of diagenesis on our bulk carbonate δ 13 C can be ruled out. δ 13 C carb shows a steady decrease from 66.4 to 66.2 Ma, reaches a minimum at approximately 66.2 Ma, with a negative CIE of ~ 1.5‰, and then slowly increases to the K-Pg boundary, corresponding to the LMWE. δ 13 C carb decreases sharply immediately after the K-Pg boundary and then shows an overall increase, with several negative excursions, such as the double CIEs at ~ 65.8 and ~ 65.7 Ma, which indicate that the Dan-C2 event characterized by the first CIE (~ 3‰) was larger than the second CIE (~ 2‰). In addition, another excursion of δ 13 C carb (~ 2‰) occurred at ~ 65.3 Ma, which should correspond to the Lower C29n event. Overall, the δ 13 C carb values in the Nanxiong Basin can be well compared with the δ 13 C bulk values at ODP site 1262 (Fig. 2 e); both show more muted CIEs during the LMWE than during the Dan-C2 event. In addition, the total duration of the onset, peak, and recovery of the LMWE (~ 200–300 kyr) is significantly longer than that of each of the CIEs during the Dan-C2 event (~ 100 kyr) (Figs. 2 c and 2 e). However, the magnitudes of the CIEs in the Nanxiong Basin (1.5-3‰) are greater than those in ODP Site 1262 (0.5-1‰), which is also consistent with the findings of previous studies showing that the CIE magnitude is greater in the terrestrial record than in the marine record for the same hyperthermal event 24 , 25 . Evolutionary power spectral analysis revealed that both the δ 13 C carb of the Nanxiong Basin and the δ 13 C bulk of ODP site 1262 exhibited significant 405-kyr long eccentricity cycles from 66.6 to 65 Ma (Fig. 3 a). The dramatic negative excursion of global δ 13 C immediately after the K-Pg boundary was probably related to the large amount of CO 2 injected into the atmosphere by the Chicxulub impact 30 , as well as the vital effects caused by mass extinction 31 . This dramatic excursion could lead to the insignificance of the 100-kyr short eccentricity cycle in both terrestrial and marine records under a 500 kyr sliding window during 66.2 to 65.8 Ma (Fig. 3 a). To eliminate this bias, two discrete time windows without dramatic excursions were divided and then analysed separately with a sliding window of 150 kyr. The results reveal significant 100-kyr short eccentricity cycles both below and above the K-Pg boundary in the Nanxiong Basin (Fig. 3 b); however, this cycle is complicated at ODP site 1262: the short eccentricity cycle was significant from 66.6 to 66.3 Ma, after which the significance level decreased and even disappeared from 66.2–66.0 Ma (Fig. 3 c, right), after which the significance returned again after the K-Pg event (Fig. 3 c, left). Discussion Was the Dan-C2 event a global hyperthermal event? Our study provides a new terrestrial δ 13 C record of the LMWE and Dan-C2 event, expanding their global distribution, especially the Dan-C2 event. However, there are still inconsistencies in marine records of the Dan-C2 event (supporting information, Text S1, Fig. S1 ), which are summarized in the following aspects: 1) the negative excursion of δ 13 C is mainly recorded by bulk samples and planktonic foraminifera, while benthic foraminifera are rarely recorded; 2) the distribution is restricted to parts of the Atlantic Ocean and the Tethys Ocean; and 3) evidence of bottom-seawater warming is generally lacking. To explore the possible reasons for the inconsistencies in the Dan-C2 event, we compared it to the Palaeocene-Eocene Thermal Maximum (PETM), which is the most studied hyperthermal event. The CIE of the PETM can reach 7‰ in terrestrial records and only 3‰ in marine records 24 , 25 , 32 . Two mechanisms have been proposed: 1) marine CIE records are commonly truncated by carbonate dissolution, so the most extreme values are not represented, leading to an incomplete CIE 32 , 33 ; and 2) the terrestrial CIE is amplified by environmental changes and fractionation effects caused by photosynthesis in plants 34 . A similar phenomenon is also shown with the Dan-C2 event. The CIEs of δ 13 C organic and δ 13 C carb in Boltysh crater (Fig. S1 l) and the Nanxiong Basin (Fig. 1 c) can reach ~ 3.0‰, whereas the marine CIEs are less than 1.5‰ (Fig. S1 ). Differences in the carbon isotope records during the PETM are not only manifested in marine and terrestrial records but also among different types of carbonate records (bulk samples, planktonic foraminifera, or benthic foraminifera) and even within the same type of carbonate but in different marine regions 33 (supporting information Fig. S3 and Fig. 4 ). Ocean acidification during the PETM led to carbonate dissolution and shoaling of the carbonate compensation depth (CCD), causing the sediments to be clay-rich. The thickness of the clay layer increases with increasing palaeodepth 32 , 33 , indicating enhanced carbonate dissolution. The CIEs of δ 13 C bulk and δ 13 C benthic have significant negative correlations with palaeodepth (Figs. 4 a and 4 b), suggesting that the dissolution of carbonate suppressed the amplitude of the CIE. In addition to carbonate dissolution, the sedimentation rate also affects the CIE. The CIEs of δ 13 C bulk and δ 13 C benthic have positive correlations with sediment thickness (Figs. 4 c and 4 d). The greater the sediment thickness is, the greater the sedimentation rate, the more complete the carbonate isotope record, and the greater the CIE. Similarly, for the Dan-C2 event, the CIEs of δ 13 C bulk show a significant negative correlation with palaeodepth (Fig. 4 e) but a positive correlation with sediment thickness (Fig. 4 f) and carbonate concentration (Fig. 4 g), indicating that the CIEs of the Dan-C2 event are also influenced by the water depth, sedimentation rate, and carbonate dissolution, leading to some differences in the CIE records from different regions. However, compared with the PETM, the Dan-C2 event was a short-lived and muted warming event that occurred under a specific environmental background, for instance, biotic turnover and drastic ecosystem changes caused by mass extinction. Under normal oceanic conditions, phytoplankton preferentially convert 12 CO 2 into organic matter through photosynthesis (primary productivity), and then, 12 C-rich organic carbon is transported to the bottom water through a biological pump (export productivity), thus promoting carbon exchange between the surface and the deep ocean 35 . After the K-Pg boundary, a collapse occurred in the surface-bottom δ 13 C gradient 36 , which was initially thought to reflect either a collapse in primary productivity (“Strangelove Ocean” 36 ) or export productivity (“Living Ocean” 37 ) after the mass extinction. However, benthic faunal records show a lack of significant extinction of phytoplankton-dependent benthic foraminifera and an increased food flux to the seafloor in the southeastern Atlantic Ocean and the Pacific Ocean 31 . In addition, biogenic barium records indicate geographic heterogeneity in export productivity, an increase in the central Pacific Ocean, no changes in upwelling or shelf Atlantic sites, and decreases in the northeast and southwest Atlantic Ocean, Southern Ocean, and Indian Ocean 38 . Thus, the “Resilient Ocean” and “Heterogeneous Ocean” have been proposed to explain these phenomena 39 . The spatial heterogeneity of primary/export productivity could cause heterogeneous carbon cycling processes between the surface and deep ocean, leading to inconsistent marine δ 13 C records from the Dan-C2 event. However, this heterogeneity was proposed to be due to the limited number of sites, which is insufficient to reveal a robust pattern, and the mechanism responsible for this heterogeneity is still unclear 38 . The geographic location 40 , circulation, nutrient runoff from land, and stratification 38 are potential drivers of the spatially heterogeneous ocean and inconsistent records of the Dan-C2 event. For example, ODP site 1049C is located at a productive coastal upwelling site 41 in the western North Atlantic, where a great deal of terrestrial materials are transported by surface ocean currents 42 ; in addition, the export productivity is stable 43 , leading to strong negative excursions from all δ 13 C bulk , δ 13 C planktic and δ 13 C benthic (Fig. S1 a), especially negative excursions from δ 13 C benthic . Although increased export productivity was recorded at ODP sites 1209 and 1210 in the Central Pacific Ocean 24 , 38 , there are no records of Dan-C2 event, probably due to pelagic and oligotrophic environments, as well as the lack of nutrients from terrestrial sources. Evidently, the mass extinction and the dramatic changes it brought are the indispensable causes of the spatial heterogeneity of the Dan-C2 records, but proposing a reasonable model to explain this mechanism requires additional records in the future. The roles of Deccan volcanism and orbital forcing in the carbon cycle The large amount of CO 2 released by large igneous provinces (LIPs) can cause perturbations to the global carbon cycle. In addition to LIPs, other carbon pools, such as peat and methane hydrates, which are modulated by eccentricity forcing, could also contribute to the carbon cycle during hyperthermals. For instance, during eccentricity minima, seasonally uniform annual precipitation is more suitable for carbon burial, whereas during eccentricity maxima, short wet seasons and prolonged dry seasons caused by “monsoon-like” precipitation could promote carbon release 2 . In addition, methane hydrates buried in the marine shelf become unstable and decompose in response to orbital-driven warming, leading to large quantities of light carbon being emitted to the atmosphere-ocean system, further perturbing the global carbon cycle 44 . Previous work has shown that the total mercury (Hg) concentration in the Nanxiong Basin has been anomalous from 66.4 to 65.6 Ma (Fig. 2 b); combined with Hg isotope data, Ma et al. 4 attributed these anomalies to volcanism in the central Deccan Traps. Both the LMWE and the Dan-C2 event temporally overlapped with the central Deccan volcanism (Fig. 2 ). Moreover, the LMWE and Dan-C2 event occurred during the last 405-kyr long eccentricity of the Maastrichtian and the first 405-kyr long eccentricity of the Danian, respectively (Figs. 2 c, 2 d, and 2 e), and their CIE maxima were all within the maxima of the 405-kyr eccentricity cycle 12 . Moreover, 100-kyr short eccentricity cycles were significant in both terrestrial and marine records (except for the LMWE of ODP 1262; Fig. 3 ). These findings imply that both Deccan volcanism and orbital forcing contributed to the LMWE and Dan-C2 event. However, there are noticeable differences between the LMWE and Dan-C2 event: 1) the magnitude of the CIE during the LMWE (~ 1.5‰ in the Nanxiong Basin,<0.5‰ in ODP 1262) is more muted than in the Dan-C2 event (2–3‰ in the Nanxiong Basin, 0.6‰ in ODP 1262); 2) the LMWE was characterized by both surface and deep sea warming 1 , 18 , while the Dan-C2 event was characterized by surface ocean warming, accompanied by little appreciable deep sea warming; 3) each of the double CIEs of the Dan-C2 event corresponds to a maxima of the 100-kyr eccentricity cycle, while the total duration of the onset, peak and recovery of the LMWE (200 ~ 300 kyr) was significantly longer than each CIE of the Dan-C2 event (Fig. 2 ); 4) the short eccentricity cycles are significant during the Dan-C2 event, whereas they are insignificant and even disappear during the LMWE in the marine record (Fig. 3 ); and 5) although the CIE maxima of both the Dan-C2 and the LMWE were all within the maxima of the 405-kyr eccentricity cycle, the onset of the LMWE occurred at the minima of the 405-kyr eccentricity cycle 12 , 18 (Figs. 2 c and 2 e). These apparent differences suggest that Deccan volcanism and orbital forcing played different roles in driving the LMWE and Dan-C2 event, as well as in the global carbon cycle. High-precision chronologies indicate that both the eruption rate and volume were low during the early stage of the central Deccan Traps 26 , 27 (Fig. 2 a). However, CO 2 release has the potential to decouple from rates of surface volcanism because large amounts of CO 2 can be released through passive degassing 6 , 27 , especially from intrusive magmas 20 . The reconstructed atmospheric CO 2 concentration based on the pedogenic carbonate nodules showed higher p CO 2 values during the LMWE than during the Dan-C2 event 5 , 45 (Fig. S4), which is consistent with direct measurements of melt-inclusion CO 2 concentrations, suggesting that early Deccan magmas were enriched with more CO 2 20 . Although the onset of the LMWE occurred at the minima of the 405-kyr eccentricity 18 (Figs. 2 c and 2 e), several thousand Gt of carbon that degassed from the early Deccan magmas was sufficient to trigger the LMWE 6 , 20 . The δ 13 C composition of volcanic CO 2 (~-5‰ 46 ) is much more positive than that of other sources, such as peat (δ 13 C≈-25‰ 47 ) and methane hydrates (δ 13 C ≈-60‰ 48 ); therefore, the massive amount of volcanic CO 2 emitted through passive degassing of early Deccan magmas could have led to muted and prolonged δ 13 C negative excursions during the LMWE (Fig. 2 ), as well as disruption of the short-eccentricity cycle in the oceanic record (Fig. 3 c, right). Although passive degassing triggered the LMWE, whether the amount of released CO 2 was sufficient to cause ~ 4°C of warming is still debated 19 , 20 . Notably, 405-kyr long eccentricity cycles were significant during the LMWE according to both terrestrial and marine records (Fig. 3 a), and even 100-kyr short eccentricity cycles were significant according to the terrestrial record (Fig. 3 b, right), suggesting that Deccan CO 2 outgassing likely enhanced the climate sensitivity to orbital forcing, leading to a global warming of ~ 4°C 12 , 20 . After the K-Pg boundary, with a decrease in CO 2 released from the Deccan magma 20 , 27 , the carbon cycle was mainly controlled by orbitally driven carbon pools with more negative δ 13 C values, leading to larger CIEs (Fig. 2 ) and significant short-eccentricity cycles in both terrestrial and marine records (Figs. 3 b left and 3c left). Due to the decrease in CO 2 emissions, p CO 2 and its warming effects were muted during the Dan-C2 event. The above speculated release scenario of Deccan volcanic CO 2 was further substantiated by long-term ocean-atmosphere-sediment carbon cycle reservoir (LOSCAR) 49 model simulations, which showed that prior to the K-Pg boundary, more CO 2 was released through passive degassing 20 (intrusive:extrusive = 5:1) or that half of the CO 2 was released (50:50 outgassing scenario) but with a higher emission rate 6 . The greater volume and higher rate of CO 2 release before compared to after the K-Pg boundary confirm that Deccan volcanism likely contributed to both the LMWE and Dan-C2 event but contributed more to the LMWE. In conclusion, we provide a new terrestrial δ 13 C record of the LMWE and Dan-C2 event in low-latitude East Asia, which can be compared with marine records, further expanding the global distribution of these events. The inconsistency of marine records for the Dan-C2 event is related to the drastic ecosystem changes caused by mass extinction, especially in the heterogeneous ocean, while the specific mechanism remains to be revealed by additional studies. We hypothesize that Deccan volcanism and orbital forcing played different roles in the carbon cycles during the LMWE and Dan-C2 event. Deccan volcanic CO 2 triggered the LMWE through passive degassing, disturbed the carbon cycle and amplified the sensitivity of the climate to orbital forcing, whereas the Dan-C2 event was mainly controlled by orbital forcing, with weakening of the volcanic perturbation. Materials and Methods The Nanxiong Basin is in southeastern China (Fig. 1 a); it is elongated, with its axis oriented from northeast to southwest. Continuous red fluvial–lacustrine clastics spanning from the Upper Cretaceous to the lower Palaeocene are preserved in the basin. Extensive chronostratigraphic, stratigraphic, palaeontological, and palaeoclimatic works have been carried out in the CGD-CGY section (also called the Datang section) 15 , 16 , 22 , 23 , which is dominated by muddy siltstone and silty mudstone with interbedded sandstone and conglomerate (Fig. 1 b). Many fossils have been preserved, for instance, dinosaurs, dinosaur footprints, dinosaur eggs, and mammals. Various palaeosol layers with pedogenic carbonate formed on the red clastics 22 . Previous studies have shown that the palaeoclimate in this basin across the K-Pg boundary was mainly hot and (semi)arid; moreover, the climatic evolution was consistent with marine records 4 , 15 . A total of 274 fresh samples from the upper part of the Zhenshui Formation to the lower Xiahui part of the Shanghu Formation (CGD section; Fig. 1 b) were collected at approximately 1 m intervals. The chronology of the CGD section follows that of Ma et al. 4 and is based on the palaeomagnetic framework of Clyde et al. 22 combined with a chronological control point on the K-Pg boundary. The carbon isotopic composition of the bulk carbonate was determined using a Thermo Fisher Isotope ratio mass spectrometer (Mat 253) coupled with a GasBench II at the Laboratory for Stable Isotope Geochemistry, Institute of Geology and Geophysics, IGGCAS, through the production of CO 2 after reaction with phosphoric acid. Acid digestion was performed in a GasBench II in continuous flow mode at a temperature of 72 ± 0.1°C and a reaction time of 60 min, through which the generated CO 2 was transferred by high-purity (99.999%) He carrier gas into the mass spectrometer. The standard deviation of δ 13 C values was calculated from replicate analyses of an internal laboratory calcite standard, which is better than 0.15‰. The measured δ 13 C values are reported relative to those of the Vienna Pee Dee Belemnite (V-PDB). To determine the contribution of orbital forcing to the carbon cycle, evolutionary power spectra were generated through the δ 13 C records of both the Nanxiong Basin and ODP 1262 using Acycle software 50 . In preparation for this analysis, the δ 13 C records were interpolated linearly at 5-kyr and 2-kyr intervals for the Nanxiong Basin and ODP 1262 records, respectively, and detrended using local regression smoothing (LOWESS). The fast Fourier transform (FFT) method was selected, and the sliding windows were 500 kyr and 150 kyr for the complete and discrete time windows, respectively. Declarations Competing interests: The authors declare no competing interests. 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A humid climate state during the Palaeocene/Eocene thermal maximum. Nature 432, 495–499 (2004). Hilting, A. K., Kump, L. R. & Bralower, T. J. Variations in the oceanic vertical carbon isotope gradient and their implications for the Paleocene-Eocene biological pump. Paleoceanogr. Paleoclimatology 23, PA3222 (2008). Hsü, K. J. et al. Environmental and evolutionary consequences of mass-mortality at the end of the Cretaceous. Science 216, 249–256 (1982). D’Hondt, S. Consequences of the Cretaceous/Paleogene mass extinction for marine ecosystems. Annu. Rev. Ecol. Evol. Syst. 36, 295–317 (2005). Hull, P. M. & Norris, R. D. Diverse patterns of ocean export productivity change across the Cretaceous-Paleogene boundary: New insights from biogenic barium. Paleoceanography 26, PA3205 (2011). Esmeray-Senlet, S. et al. Evidence for reduced export productivity following the Cretaceous/Paleogene mass extinction. Paleoceanography 30, 718–738 (2015). Jiang, S., Bralower, T. J., Patzkowsky, M. E., Kump, L. R. & Schueth, J. D. Geographic controls on nannoplankton extinction across the Cretaceous/Palaeogene boundary. Nat. Geosci. 3, 280–285 (2010). Alegret, L. & Thomas, E. Cretaceous/Paleogene boundary bathyal paleo-environments in the central North Pacific (DSDP Site 465), the Northwestern Atlantic (ODP Site 1049), the Gulf of Mexico and the Tethys: The benthic foraminiferal record. Palaeogeogr. Palaeoclimatol. Palaeoecol. 224, 53–82 (2005). Nauter-Alves, A. et al. Biotic turnover and carbon cycle dynamics in the early Danian event (Dan-C2): New insights from Blake Nose, North Atlantic. Glob. Planet. Change 221, 104046 (2023). Faul, K. L., Anderson, L. D. & Delaney, M. L. Late Cretaceous and early Paleogene nutrient and paleoproductivity records from Blake Nose, western North Atlantic Ocean. Paleoceanography 18, PA000722 (2003). Lunt, D. J. et al. A model for orbital pacing of methane hydrate destabilization during the Palaeogene. Nat. Geosci. 4, 775–778 (2011). Nordt, L. C., Atchley, S. & Dworkin, S. Terrestrial evidence for two greenhouse events in the latest Cretaceous. Geol. Soc. Am. 13, 4–9 (2003). Grard, A., Francois, L. M., Dessert, C., B. Dupré & Y. Goddéris. Basaltic volcanism and mass extinction at the Permo-Triassic boundary: Environmental impact and modeling of the global carbon cycle. Earth Planet. Sci. Lett. 234, 207–221 (2005). DeConto, R. M. et al. Past extreme warming events linked to massive carbon release from thawing permafrost. Nature 484, 87–91 (2012). Dickens, G. R., Castillo, M. M. & Walker, J. C. A blast of gas in the latest Paleocene: Simulating first-order effects of massive dissociation of oceanic methane hydrate. Geology 25, 259–262 (1997). Zeebe, R. E. LOSCAR: Long-term Ocean-atmosphere-Sediment CArbon cycle Reservoir Model v2.0.4. Geosci. Model Dev. 5, 149–166 (2012). Li, M., Hinnov, L. & Kump, L. Acycle: Time-series analysis software for paleoclimate research and education. Comput. Geosci. 127, 12–22 (2019). Additional Declarations There is NO Competing Interest. Supplementary Files SI.docx Cite Share Download PDF Status: Published Journal Publication published 10 May, 2024 Read the published version in Communications Earth & Environment → 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-3893195","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":269454465,"identity":"f0f74921-500a-4dba-ad55-c493c5d71898","order_by":0,"name":"Mingming Ma","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6klEQVRIiWNgGAWjYJCCA0DMw8DA2P7hQ4WEnDwJWpjbGGecsTA2bCDeMvY2Zt62ikSwCfiAbvvpxMMFv7bJmPMvbHvMO08igbGB+eGjG3i0mJ3J3XB4Zt9tHssZD9sN526TyGNnYDM2zsGn5QBQC2/PbR6DGwcbJN5ukyhmbOBhk8ar5fxbJC28cyQSGw4Q0nIDaAvPD6CW841tkrwNRGkB2dIAsoWx2XDGMQljw2ZCfjmfu/kzz5/b9gbnjz988KGmTk6evfnhY3xawICxDUhIJEB5zISUg8EfIOY/QJTSUTAKRsEoGIEAAApaWHFU6NrVAAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0003-4896-6886","institution":"Fujian Normal University","correspondingAuthor":true,"prefix":"","firstName":"Mingming","middleName":"","lastName":"Ma","suffix":""},{"id":269454466,"identity":"3dc148b1-22d2-42ae-a229-3c941c42e8b6","order_by":1,"name":"Mengdi Wang","email":"","orcid":"","institution":"Fujian Normal University","correspondingAuthor":false,"prefix":"","firstName":"Mengdi","middleName":"","lastName":"Wang","suffix":""},{"id":269454467,"identity":"6d2367b2-6a79-4d53-9b0f-87742869c4ea","order_by":2,"name":"Huixin Huang","email":"","orcid":"","institution":"Fujian Normal University","correspondingAuthor":false,"prefix":"","firstName":"Huixin","middleName":"","lastName":"Huang","suffix":""},{"id":269454468,"identity":"332762d8-0796-497d-90a6-3b65206bc074","order_by":3,"name":"Xiuming Liu","email":"","orcid":"","institution":"Fujian Normal University","correspondingAuthor":false,"prefix":"","firstName":"Xiuming","middleName":"","lastName":"Liu","suffix":""}],"badges":[],"createdAt":"2024-01-24 06:45:48","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3893195/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3893195/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s43247-024-01425-4","type":"published","date":"2024-05-10T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":50841416,"identity":"b03054dd-5fd9-4cdf-b282-20a9fef76892","added_by":"auto","created_at":"2024-02-08 07:58:42","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":255643,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Palaeogeographic reconstruction at 66 Ma (from the Ocean Drilling Stratigraphic Network Paleomap Project, https://www.odsn.de/odsn/services/paleomap/paleomap.html); the black dots indicatethe studied sites of the Dan-C2 event; (b) stratigraphy of the CGD-CGY section in the Nanxiong Basin. CGD, Chinese–German Datang section; CGY, Chinese–German Yuanpu section.\u003c/p\u003e","description":"","filename":"F1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3893195/v1/f64606864b4a4ccced2af565.jpg"},{"id":50841413,"identity":"92175cac-7908-4cc6-a230-5f1da824f056","added_by":"auto","created_at":"2024-02-08 07:58:42","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":304913,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Central Deccan eruption models based on the chronology of Schoene et al. (2019) \u003csup\u003e26\u003c/sup\u003eand Sprain et al. (2019)\u003csup\u003e27\u003c/sup\u003e; (b) Hg/Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e ratio of bulk samples from the Nanxiong Basin\u003csup\u003e4\u003c/sup\u003e; (c) d\u003csup\u003e13\u003c/sup\u003eC record of carbonates in the Nanxiong Basin; (d) La2010b orbital solution (blue) filtered by long eccentricity (405-kyr), illustrated in red\u003csup\u003e28\u003c/sup\u003e; (e) d\u003csup\u003e13\u003c/sup\u003eC record of bulk samples from ODP 1262\u003csup\u003e1,29\u003c/sup\u003e. LMWE: Late Maastrichtian Warming Event; Dan-C2: Dan-C2 event; L.C29n: Lower C29n event.\u003c/p\u003e","description":"","filename":"F2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3893195/v1/deaa3eb6aefdf36b832a88c9.jpg"},{"id":50841975,"identity":"a67d9b9b-8a80-479c-be83-ae18ce26192d","added_by":"auto","created_at":"2024-02-08 08:06:42","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":427739,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Evolutionary power spectral analysis of the d\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e of Nanxiong Basin (left) and d\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003ebulk\u003c/sub\u003e of ODP 1262 (right) for the complete records; (b) evolutionary power spectral analysis of the d\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e from the Nanxiong Basin for two discrete time windows: 65.85-65.0 Ma (left) and 66.6-66.0 Ma (right); (c) evolutionary power spectral analysis of the d\u003csup\u003e13\u003c/sup\u003eC\u003csub\u003ebulk\u003c/sub\u003e of ODP 1262 for two discrete time windows: 65.9-65 Ma (left) and 66.6-66.0 Ma (right). Blue represents low spectral power, and yellow represents high spectral power.\u003c/p\u003e","description":"","filename":"F3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3893195/v1/4ef05265f6877af568780835.jpg"},{"id":50841414,"identity":"ce68d9dc-1a66-45e5-ad5f-b6165f8ba6bf","added_by":"auto","created_at":"2024-02-08 07:58:42","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":171935,"visible":true,"origin":"","legend":"\u003cp\u003eScatter plots between the CIE and palaeodepth, sediment thickness, and carbonate concentration for the PETM (a-d) and the Dan-C2 event (e-g). The PETM data are from ODP 1262, ODP 1263, ODP 1265, ODP 1266, ODP 1267, DSDP 525, and DSDP 527. The Dan-C2 event data are from ODP 1049C, ODP 1262, ODP 1267, DSDP 516F, DSDP 528, and IODP U1403. More details are given in supporting information Tables S2 and S3.\u003c/p\u003e","description":"","filename":"F4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-3893195/v1/91a3a4520ef289ee3fd38484.jpg"},{"id":56296107,"identity":"01c3fed9-b3be-4f7e-8331-f7f20fc13652","added_by":"auto","created_at":"2024-05-11 07:06:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1049940,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3893195/v1/417ac438-93d3-465b-ba16-69b7bbd21f59.pdf"},{"id":50841417,"identity":"49464b17-a918-4bef-83be-b9904793c9fe","added_by":"auto","created_at":"2024-02-08 07:58:42","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":1439201,"visible":true,"origin":"","legend":"","description":"","filename":"SI.docx","url":"https://assets-eu.researchsquare.com/files/rs-3893195/v1/dd4a2e8e66c9d0943aa7e3ed.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Perturbations of the global carbon cycle across the Cretaceous–Palaeogene boundary","fulltext":[{"header":"Introduction","content":"\u003cp\u003eA series of hyperthermal events characterized by transient warming and significant negative carbon isotope excursion (CIE) occurred during the late Cretaceous\u0026ndash;early Palaeogene\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. Among them, the Late Maastrichtian Warming Event (LMWE) and the early Danian Dan-C2 event occurred across the Cretaceous-Palaeogene (K-Pg) boundary. Both were related to significant climatic and biotic changes; for instance, the LMWE was probably related to mass extinction at the end of the Cretaceous\u003csup\u003e\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e, while the Dan-C2 event promoted biotic recovery after the extinction\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe Dan-C2 event has been distinguished in the Atlantic Ocean\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan additionalcitationids=\"CR7 CR8\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e, the Tethys Ocean\u003csup\u003e\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e, and the middle and lower latitudes of Eurasia\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan additionalcitationids=\"CR14 CR15\" citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Compared to other hyperthermal events, the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC negative excursions during the Dan-C2 event were restricted to planktonic foraminifera and bulk records in parts of the Atlantic and Tethys Oceans, while benthic foraminifera rarely recorded this event\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. Furthermore, the warming indicated by oxygen isotopes (δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO) during this event was also limited to surface waters in parts of the North Atlantic, with evidence of warming in bottom waters generally lacking \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e(supporting information Text S1, Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Therefore, the global significance of the Dan-C2 event and even whether it should be regarded as a true hyperthermal are still controversial\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e,\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTwo main sources of the massive \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003eC-rich CO\u003csub\u003e2\u003c/sub\u003e triggering of the carbon cycle during the LMWE and Dan-C2 event were proposed: Deccan volcanism and carbon pools controlled by orbital forcing. The extremely negative values of the CIE for the LMWE and Dan-C2 event are located at the maxima of the 405-kyr long-eccentricity cycle\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Moreover, the two CIEs of the Dan-C2 event correspond to two 100-kyr short-eccentricity maxima\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, indicating that orbital forcing contributed to the CIE during both the LMWE and the Dan-C2 event. The overlapping occurrences of Deccan volcanism and two hyperthermals suggest that the large amount of volcanic carbon emitted could have also played a role in driving hyperthermals, especially the LMWE\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. However, this speculation is plausible and contemporaneous and lacks causal connection\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Recent modelling efforts to simulate CO\u003csub\u003e2\u003c/sub\u003e emission scenarios from Deccan volcanism have yielded conflicting results, with either more CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e20\u003c/sup\u003e, half of the CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e6\u003c/sup\u003e, or one-third of the CO\u003csub\u003e2\u003c/sub\u003e \u003csup\u003e21\u003c/sup\u003ereleased before the K-Pg boundary. Therefore, the roles that Deccan volcanism played in perturbing the carbon cycle in the LMWE and the Dan-C2 event are still unclear.\u003c/p\u003e \u003cp\u003eTo date, most of the carbon isotope records are from marine sediments, whereas terrestrial records are lacking. Our previous work showed that the LMWE and Dan-C2 event were recorded by red strata in the Nanxiong Basin (Southeastern China, Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Hg and its isotopes indicate that they formed during Deccan volcanism; moreover, both hyperthermals were related to the 405-kyr long eccentricity cycle\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. However, due to the low resolution of the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e data, detailed comparisons with marine records and in-depth analyses of carbon cycle processes are limited. Here, a total of 274 fresh samples from the upper part of the Zhenshui Formation to the lower Xiahui part of the Shanghu Formation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) were collected, then we obtain a new δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e record across the K-Pg boundary in the basin and compare it with other published records to reveal 1) the terrestrial records of the LMWE and Dan-C2 event, as well as the global significance of the Dan-C2 event; and 2) the relative contributions of Deccan volcanism and orbital forcing to the carbon cycle during the two hyperthermal events.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eThe δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC value of the bulk carbonate from the Nanxiong Basin (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec) is consistent with the previous δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC values of pedogenic carbonate nodules that were not affected by diagenesis \u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e(Fig. S2a); moreover, there is no significant correlation between δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e and δ\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003eO\u003csub\u003ecarb\u003c/sub\u003e (Fig. S2b), suggesting that the influence of diagenesis on our bulk carbonate δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC can be ruled out. δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e shows a steady decrease from 66.4 to 66.2 Ma, reaches a minimum at approximately 66.2 Ma, with a negative CIE of ~\u0026thinsp;1.5\u0026permil;, and then slowly increases to the K-Pg boundary, corresponding to the LMWE. δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e decreases sharply immediately after the K-Pg boundary and then shows an overall increase, with several negative excursions, such as the double CIEs at ~\u0026thinsp;65.8 and ~\u0026thinsp;65.7 Ma, which indicate that the Dan-C2 event characterized by the first CIE (~\u0026thinsp;3\u0026permil;) was larger than the second CIE (~\u0026thinsp;2\u0026permil;). In addition, another excursion of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e (~\u0026thinsp;2\u0026permil;) occurred at ~\u0026thinsp;65.3 Ma, which should correspond to the Lower C29n event. Overall, the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e values in the Nanxiong Basin can be well compared with the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebulk\u003c/sub\u003e values at ODP site 1262 (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee); both show more muted CIEs during the LMWE than during the Dan-C2 event. In addition, the total duration of the onset, peak, and recovery of the LMWE (~\u0026thinsp;200\u0026ndash;300 kyr) is significantly longer than that of each of the CIEs during the Dan-C2 event (~\u0026thinsp;100 kyr) (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). However, the magnitudes of the CIEs in the Nanxiong Basin (1.5-3\u0026permil;) are greater than those in ODP Site 1262 (0.5-1\u0026permil;), which is also consistent with the findings of previous studies showing that the CIE magnitude is greater in the terrestrial record than in the marine record for the same hyperthermal event\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eEvolutionary power spectral analysis revealed that both the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e of the Nanxiong Basin and the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebulk\u003c/sub\u003e of ODP site 1262 exhibited significant 405-kyr long eccentricity cycles from 66.6 to 65 Ma (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The dramatic negative excursion of global δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC immediately after the K-Pg boundary was probably related to the large amount of CO\u003csub\u003e2\u003c/sub\u003e injected into the atmosphere by the Chicxulub impact\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, as well as the vital effects caused by mass extinction\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. This dramatic excursion could lead to the insignificance of the 100-kyr short eccentricity cycle in both terrestrial and marine records under a 500 kyr sliding window during 66.2 to 65.8 Ma (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). To eliminate this bias, two discrete time windows without dramatic excursions were divided and then analysed separately with a sliding window of 150 kyr. The results reveal significant 100-kyr short eccentricity cycles both below and above the K-Pg boundary in the Nanxiong Basin (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb); however, this cycle is complicated at ODP site 1262: the short eccentricity cycle was significant from 66.6 to 66.3 Ma, after which the significance level decreased and even disappeared from 66.2\u0026ndash;66.0 Ma (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, right), after which the significance returned again after the K-Pg event (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, left).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eWas the Dan-C2 event a global hyperthermal event?\u003c/h2\u003e \u003cp\u003eOur study provides a new terrestrial δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC record of the LMWE and Dan-C2 event, expanding their global distribution, especially the Dan-C2 event. However, there are still inconsistencies in marine records of the Dan-C2 event (supporting information, Text S1, Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), which are summarized in the following aspects: 1) the negative excursion of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC is mainly recorded by bulk samples and planktonic foraminifera, while benthic foraminifera are rarely recorded; 2) the distribution is restricted to parts of the Atlantic Ocean and the Tethys Ocean; and 3) evidence of bottom-seawater warming is generally lacking.\u003c/p\u003e \u003cp\u003eTo explore the possible reasons for the inconsistencies in the Dan-C2 event, we compared it to the Palaeocene-Eocene Thermal Maximum (PETM), which is the most studied hyperthermal event. The CIE of the PETM can reach 7\u0026permil; in terrestrial records and only 3\u0026permil; in marine records\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Two mechanisms have been proposed: 1) marine CIE records are commonly truncated by carbonate dissolution, so the most extreme values are not represented, leading to an incomplete CIE\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e; and 2) the terrestrial CIE is amplified by environmental changes and fractionation effects caused by photosynthesis in plants\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. A similar phenomenon is also shown with the Dan-C2 event. The CIEs of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003eorganic\u003c/sub\u003e and δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e in Boltysh crater (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003el) and the Nanxiong Basin (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec) can reach\u0026thinsp;~\u0026thinsp;3.0\u0026permil;, whereas the marine CIEs are less than 1.5\u0026permil; (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Differences in the carbon isotope records during the PETM are not only manifested in marine and terrestrial records but also among different types of carbonate records (bulk samples, planktonic foraminifera, or benthic foraminifera) and even within the same type of carbonate but in different marine regions\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e (supporting information Fig. S3 and Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Ocean acidification during the PETM led to carbonate dissolution and shoaling of the carbonate compensation depth (CCD), causing the sediments to be clay-rich. The thickness of the clay layer increases with increasing palaeodepth\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e, indicating enhanced carbonate dissolution. The CIEs of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebulk\u003c/sub\u003e and δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebenthic\u003c/sub\u003e have significant negative correlations with palaeodepth (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), suggesting that the dissolution of carbonate suppressed the amplitude of the CIE. In addition to carbonate dissolution, the sedimentation rate also affects the CIE. The CIEs of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebulk\u003c/sub\u003e and δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebenthic\u003c/sub\u003e have positive correlations with sediment thickness (Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec and \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). The greater the sediment thickness is, the greater the sedimentation rate, the more complete the carbonate isotope record, and the greater the CIE. Similarly, for the Dan-C2 event, the CIEs of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebulk\u003c/sub\u003e show a significant negative correlation with palaeodepth (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee) but a positive correlation with sediment thickness (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ef) and carbonate concentration (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eg), indicating that the CIEs of the Dan-C2 event are also influenced by the water depth, sedimentation rate, and carbonate dissolution, leading to some differences in the CIE records from different regions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eHowever, compared with the PETM, the Dan-C2 event was a short-lived and muted warming event that occurred under a specific environmental background, for instance, biotic turnover and drastic ecosystem changes caused by mass extinction. Under normal oceanic conditions, phytoplankton preferentially convert \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003eCO\u003csub\u003e2\u003c/sub\u003e into organic matter through photosynthesis (primary productivity), and then, \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003eC-rich organic carbon is transported to the bottom water through a biological pump (export productivity), thus promoting carbon exchange between the surface and the deep ocean\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. After the K-Pg boundary, a collapse occurred in the surface-bottom δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC gradient\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e, which was initially thought to reflect either a collapse in primary productivity (\u0026ldquo;Strangelove Ocean\u0026rdquo;\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e) or export productivity (\u0026ldquo;Living Ocean\u0026rdquo;\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e) after the mass extinction. However, benthic faunal records show a lack of significant extinction of phytoplankton-dependent benthic foraminifera and an increased food flux to the seafloor in the southeastern Atlantic Ocean and the Pacific Ocean\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e. In addition, biogenic barium records indicate geographic heterogeneity in export productivity, an increase in the central Pacific Ocean, no changes in upwelling or shelf Atlantic sites, and decreases in the northeast and southwest Atlantic Ocean, Southern Ocean, and Indian Ocean\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. Thus, the \u0026ldquo;Resilient Ocean\u0026rdquo; and \u0026ldquo;Heterogeneous Ocean\u0026rdquo; have been proposed to explain these phenomena\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe spatial heterogeneity of primary/export productivity could cause heterogeneous carbon cycling processes between the surface and deep ocean, leading to inconsistent marine δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC records from the Dan-C2 event. However, this heterogeneity was proposed to be due to the limited number of sites, which is insufficient to reveal a robust pattern, and the mechanism responsible for this heterogeneity is still unclear\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e. The geographic location\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e, circulation, nutrient runoff from land, and stratification\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e are potential drivers of the spatially heterogeneous ocean and inconsistent records of the Dan-C2 event. For example, ODP site 1049C is located at a productive coastal upwelling site\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e in the western North Atlantic, where a great deal of terrestrial materials are transported by surface ocean currents\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e; in addition, the export productivity is stable\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e, leading to strong negative excursions from all δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebulk\u003c/sub\u003e, δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003eplanktic\u003c/sub\u003e and δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebenthic\u003c/sub\u003e (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea), especially negative excursions from δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ebenthic\u003c/sub\u003e. Although increased export productivity was recorded at ODP sites 1209 and 1210 in the Central Pacific Ocean\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e,\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e, there are no records of Dan-C2 event, probably due to pelagic and oligotrophic environments, as well as the lack of nutrients from terrestrial sources. Evidently, the mass extinction and the dramatic changes it brought are the indispensable causes of the spatial heterogeneity of the Dan-C2 records, but proposing a reasonable model to explain this mechanism requires additional records in the future.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eThe roles of Deccan volcanism and orbital forcing in the carbon cycle\u003c/h3\u003e\n\u003cp\u003eThe large amount of CO\u003csub\u003e2\u003c/sub\u003e released by large igneous provinces (LIPs) can cause perturbations to the global carbon cycle. In addition to LIPs, other carbon pools, such as peat and methane hydrates, which are modulated by eccentricity forcing, could also contribute to the carbon cycle during hyperthermals. For instance, during eccentricity minima, seasonally uniform annual precipitation is more suitable for carbon burial, whereas during eccentricity maxima, short wet seasons and prolonged dry seasons caused by \u0026ldquo;monsoon-like\u0026rdquo; precipitation could promote carbon release\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. In addition, methane hydrates buried in the marine shelf become unstable and decompose in response to orbital-driven warming, leading to large quantities of light carbon being emitted to the atmosphere-ocean system, further perturbing the global carbon cycle\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e. Previous work has shown that the total mercury (Hg) concentration in the Nanxiong Basin has been anomalous from 66.4 to 65.6 Ma (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb); combined with Hg isotope data, Ma et al.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e attributed these anomalies to volcanism in the central Deccan Traps. Both the LMWE and the Dan-C2 event temporally overlapped with the central Deccan volcanism (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Moreover, the LMWE and Dan-C2 event occurred during the last 405-kyr long eccentricity of the Maastrichtian and the first 405-kyr long eccentricity of the Danian, respectively (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee), and their CIE maxima were all within the maxima of the 405-kyr eccentricity cycle\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. Moreover, 100-kyr short eccentricity cycles were significant in both terrestrial and marine records (except for the LMWE of ODP 1262; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). These findings imply that both Deccan volcanism and orbital forcing contributed to the LMWE and Dan-C2 event.\u003c/p\u003e \u003cp\u003eHowever, there are noticeable differences between the LMWE and Dan-C2 event: 1) the magnitude of the CIE during the LMWE (~\u0026thinsp;1.5\u0026permil; in the Nanxiong Basin,\u0026lt;0.5\u0026permil; in ODP 1262) is more muted than in the Dan-C2 event (2\u0026ndash;3\u0026permil; in the Nanxiong Basin, 0.6\u0026permil; in ODP 1262); 2) the LMWE was characterized by both surface and deep sea warming\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e, while the Dan-C2 event was characterized by surface ocean warming, accompanied by little appreciable deep sea warming; 3) each of the double CIEs of the Dan-C2 event corresponds to a maxima of the 100-kyr eccentricity cycle, while the total duration of the onset, peak and recovery of the LMWE (200\u0026thinsp;~\u0026thinsp;300 kyr) was significantly longer than each CIE of the Dan-C2 event (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e); 4) the short eccentricity cycles are significant during the Dan-C2 event, whereas they are insignificant and even disappear during the LMWE in the marine record (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e); and 5) although the CIE maxima of both the Dan-C2 and the LMWE were all within the maxima of the 405-kyr eccentricity cycle, the onset of the LMWE occurred at the minima of the 405-kyr eccentricity cycle\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). These apparent differences suggest that Deccan volcanism and orbital forcing played different roles in driving the LMWE and Dan-C2 event, as well as in the global carbon cycle.\u003c/p\u003e \u003cp\u003eHigh-precision chronologies indicate that both the eruption rate and volume were low during the early stage of the central Deccan Traps\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). However, CO\u003csub\u003e2\u003c/sub\u003e release has the potential to decouple from rates of surface volcanism because large amounts of CO\u003csub\u003e2\u003c/sub\u003e can be released through passive degassing\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, especially from intrusive magmas\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. The reconstructed atmospheric CO\u003csub\u003e2\u003c/sub\u003e concentration based on the pedogenic carbonate nodules showed higher \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e values during the LMWE than during the Dan-C2 event\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e (Fig. S4), which is consistent with direct measurements of melt-inclusion CO\u003csub\u003e2\u003c/sub\u003e concentrations, suggesting that early Deccan magmas were enriched with more CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e20\u003c/sup\u003e. Although the onset of the LMWE occurred at the minima of the 405-kyr eccentricity\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e (Figs.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec and \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee), several thousand Gt of carbon that degassed from the early Deccan magmas was sufficient to trigger the LMWE\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. The δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC composition of volcanic CO\u003csub\u003e2\u003c/sub\u003e (~-5\u0026permil;\u003csup\u003e46\u003c/sup\u003e) is much more positive than that of other sources, such as peat (δ\u003csup\u003e13\u003c/sup\u003eC\u0026asymp;-25\u0026permil;\u003csup\u003e47\u003c/sup\u003e) and methane hydrates (δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC \u0026asymp;-60\u0026permil;\u003csup\u003e48\u003c/sup\u003e); therefore, the massive amount of volcanic CO\u003csub\u003e2\u003c/sub\u003e emitted through passive degassing of early Deccan magmas could have led to muted and prolonged δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC negative excursions during the LMWE (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), as well as disruption of the short-eccentricity cycle in the oceanic record (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, right). Although passive degassing triggered the LMWE, whether the amount of released CO\u003csub\u003e2\u003c/sub\u003e was sufficient to cause\u0026thinsp;~\u0026thinsp;4\u0026deg;C of warming is still debated\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Notably, 405-kyr long eccentricity cycles were significant during the LMWE according to both terrestrial and marine records (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), and even 100-kyr short eccentricity cycles were significant according to the terrestrial record (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, right), suggesting that Deccan CO\u003csub\u003e2\u003c/sub\u003e outgassing likely enhanced the climate sensitivity to orbital forcing, leading to a global warming of ~\u0026thinsp;4\u0026deg;C\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. After the K-Pg boundary, with a decrease in CO\u003csub\u003e2\u003c/sub\u003e released from the Deccan magma\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, the carbon cycle was mainly controlled by orbitally driven carbon pools with more negative δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC values, leading to larger CIEs (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and significant short-eccentricity cycles in both terrestrial and marine records (Figs.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb left and 3c left). Due to the decrease in CO\u003csub\u003e2\u003c/sub\u003e emissions, \u003cem\u003ep\u003c/em\u003eCO\u003csub\u003e2\u003c/sub\u003e and its warming effects were muted during the Dan-C2 event. The above speculated release scenario of Deccan volcanic CO\u003csub\u003e2\u003c/sub\u003e was further substantiated by long-term ocean-atmosphere-sediment carbon cycle reservoir (LOSCAR)\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e model simulations, which showed that prior to the K-Pg boundary, more CO\u003csub\u003e2\u003c/sub\u003e was released through passive degassing\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e (intrusive:extrusive\u0026thinsp;=\u0026thinsp;5:1) or that half of the CO\u003csub\u003e2\u003c/sub\u003e was released (50:50 outgassing scenario) but with a higher emission rate\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. The greater volume and higher rate of CO\u003csub\u003e2\u003c/sub\u003e release before compared to after the K-Pg boundary confirm that Deccan volcanism likely contributed to both the LMWE and Dan-C2 event but contributed more to the LMWE.\u003c/p\u003e \u003cp\u003eIn conclusion, we provide a new terrestrial δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC record of the LMWE and Dan-C2 event in low-latitude East Asia, which can be compared with marine records, further expanding the global distribution of these events. The inconsistency of marine records for the Dan-C2 event is related to the drastic ecosystem changes caused by mass extinction, especially in the heterogeneous ocean, while the specific mechanism remains to be revealed by additional studies. We hypothesize that Deccan volcanism and orbital forcing played different roles in the carbon cycles during the LMWE and Dan-C2 event. Deccan volcanic CO\u003csub\u003e2\u003c/sub\u003e triggered the LMWE through passive degassing, disturbed the carbon cycle and amplified the sensitivity of the climate to orbital forcing, whereas the Dan-C2 event was mainly controlled by orbital forcing, with weakening of the volcanic perturbation.\u003c/p\u003e"},{"header":"Materials and Methods","content":"\u003cp\u003eThe Nanxiong Basin is in southeastern China (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea); it is elongated, with its axis oriented from northeast to southwest. Continuous red fluvial\u0026ndash;lacustrine clastics spanning from the Upper Cretaceous to the lower Palaeocene are preserved in the basin. Extensive chronostratigraphic, stratigraphic, palaeontological, and palaeoclimatic works have been carried out in the CGD-CGY section (also called the Datang section)\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, which is dominated by muddy siltstone and silty mudstone with interbedded sandstone and conglomerate (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). Many fossils have been preserved, for instance, dinosaurs, dinosaur footprints, dinosaur eggs, and mammals. Various palaeosol layers with pedogenic carbonate formed on the red clastics\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Previous studies have shown that the palaeoclimate in this basin across the K-Pg boundary was mainly hot and (semi)arid; moreover, the climatic evolution was consistent with marine records\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. A total of 274 fresh samples from the upper part of the Zhenshui Formation to the lower Xiahui part of the Shanghu Formation (CGD section; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb) were collected at approximately 1 m intervals. The chronology of the CGD section follows that of Ma et al.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e and is based on the palaeomagnetic framework of Clyde et al.\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e combined with a chronological control point on the K-Pg boundary.\u003c/p\u003e \u003cp\u003eThe carbon isotopic composition of the bulk carbonate was determined using a Thermo Fisher Isotope ratio mass spectrometer (Mat 253) coupled with a GasBench II at the Laboratory for Stable Isotope Geochemistry, Institute of Geology and Geophysics, IGGCAS, through the production of CO\u003csub\u003e2\u003c/sub\u003e after reaction with phosphoric acid. Acid digestion was performed in a GasBench II in continuous flow mode at a temperature of 72\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u0026deg;C and a reaction time of 60 min, through which the generated CO\u003csub\u003e2\u003c/sub\u003e was transferred by high-purity (99.999%) He carrier gas into the mass spectrometer. The standard deviation of δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC values was calculated from replicate analyses of an internal laboratory calcite standard, which is better than 0.15\u0026permil;. The measured δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC values are reported relative to those of the Vienna Pee Dee Belemnite (V-PDB).\u003c/p\u003e \u003cp\u003eTo determine the contribution of orbital forcing to the carbon cycle, evolutionary power spectra were generated through the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC records of both the Nanxiong Basin and ODP 1262 using Acycle software\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. In preparation for this analysis, the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC records were interpolated linearly at 5-kyr and 2-kyr intervals for the Nanxiong Basin and ODP 1262 records, respectively, and detrended using local regression smoothing (LOWESS). The fast Fourier transform (FFT) method was selected, and the sliding windows were 500 kyr and 150 kyr for the complete and discrete time windows, respectively.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests:\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor contribution:\u003c/h2\u003e \u003cp\u003eMingming Ma and Xiuming Liu designed this study; Mingming Ma and Mengdi Wang carried them out; Huixin Huang conducted the time-series analyses; Mingming Ma prepared the manuscript with contributions from all co-authors.\u003c/p\u003e\u003ch2\u003eAcknowledgement:\u003c/h2\u003e \u003cp\u003eThis research was supported by the National Natural Science Foundation of China (awards 42277440 and 42130507), and the IGCP Project 679.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBarnet, J. S. 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Geosci. 127, 12\u0026ndash;22 (2019).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"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":"Terrestrial record, Cretaceous-Palaeogene boundary, Global carbon cycle, Deccan Traps, Orbital forcing","lastPublishedDoi":"10.21203/rs.3.rs-3893195/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3893195/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTwo hyperthermal events with different carbon cycle perturbations occurred across the Cretaceous-Palaeogene (K-Pg) boundary, i.e., the late Maastrichtian Warming Event (LMWE) and the early Danian Dan-C2 event. However, the roles played by Deccan volcanism and orbital forcing in these two hyperthermals are still debated. Here, we obtain a new terrestrial δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC\u003csub\u003ecarb\u003c/sub\u003e record in the Nanxiong Basin (southeastern China) and compare it with marine records. The results show that both the LMWE and Dan-C2 event can be well distinguished in the terrestrial record and that the Dan-C2 event is characterized by a typical hyperthermal event; however, the specificity of the context under which this event occurred has resulted in inconsistencies in the marine records. In addition, the δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC excursion during the LMWE was more muted and prolonged than that during the Dan-C2 event, and the short-eccentricity cycle disappeared in the marine record during the LMWE, indicating that Deccan volcanism perturbed the carbon cycle during the LMWE, while the Dan-C2 event was less influenced by volcanic perturbation.\u003c/p\u003e","manuscriptTitle":"Perturbations of the global carbon cycle across the Cretaceous–Palaeogene boundary","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-02-08 07:58:37","doi":"10.21203/rs.3.rs-3893195/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-earth-and-environment","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commsenv","sideBox":"Learn more about [Communications Earth and Environment](https://www.nature.com/commsenv/)","snPcode":"","submissionUrl":"","title":"Communications Earth \u0026 Environment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"c5d5ba4c-b380-41d5-8275-7e05571292bb","owner":[],"postedDate":"February 8th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":28395907,"name":"Earth and environmental sciences/Climate sciences/Palaeoclimate"},{"id":28395908,"name":"Earth and environmental sciences/Solid Earth sciences/Geology"}],"tags":[],"updatedAt":"2024-05-11T07:06:29+00:00","versionOfRecord":{"articleIdentity":"rs-3893195","link":"https://doi.org/10.1038/s43247-024-01425-4","journal":{"identity":"communications-earth-and-environment","isVorOnly":false,"title":"Communications Earth \u0026 Environment"},"publishedOn":"2024-05-10 04:00:00","publishedOnDateReadable":"May 10th, 2024"},"versionCreatedAt":"2024-02-08 07:58:37","video":"","vorDoi":"10.1038/s43247-024-01425-4","vorDoiUrl":"https://doi.org/10.1038/s43247-024-01425-4","workflowStages":[]},"version":"v1","identity":"rs-3893195","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3893195","identity":"rs-3893195","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}