A heavy molybdenum reservoir in Neoarchean seawater tracks extensive iron oxidation

A heavy molybdenum reservoir in Neoarchean seawater tracks extensive iron oxidation | 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 A heavy molybdenum reservoir in Neoarchean seawater tracks extensive iron oxidation Kurt Konhauser, Changle Wang, Feiyu Dong, Leslie Robbins, Lianchang Zhang, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7181523/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Progressively heavier molybdenum isotope compositions (δ 98 Mo) in Neoarchean marine sediments have been interpreted as evidence for widespread surface oxygenation prior to the Great Oxidation Event. Here, we assess whether the deposition of banded iron formations (BIFs) — iron-rich sedimentary rocks formed predominantly in the Neoarchean — can account for this isotopic signal through processes operating under pervasively anoxic conditions. BIF samples analyzed here possess a wide range of δ 98 Mo values, which are attributed to the combined effects of Mo adsorption onto primary ferric iron (Fe) oxyhydroxides and subsequent diagenetic incorporation into Fe-Mo-sulfides. Given the isotopic fractionation associated with Mo adsorption, we estimate that Neoarchean seawater δ 98 Mo ranged from 1.5‰ and 1.6‰, and was more stable than previously suggested. If Mo in Neoarchean rivers had an average isotopic composition like today, a mass balance model predicts that only a modest manganese oxide sink is required to generate these heavy δ 98 Mo values. Instead, the dominant control may have been the removal of isotopically light Mo via adsorption to abundant ferric oxyhydroxide particles setting through a ferruginous water column. These findings imply that the Neoarchean Mo isotope record may track extensive photosynthetic iron oxidation rather than pervasive oxygen accumulation in the surface ocean. Earth and environmental sciences/Biogeochemistry/Element cycles Earth and environmental sciences/Ocean sciences/Marine chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Understanding the evolution of atmospheric and marine redox conditions on early Earth is essential for reconstructing the biogeochemical evolution that made the planet habitable. The transition from a pervasively anoxic to an oxic atmosphere occurred across the Archean to Paleoproterozoic boundary and culminated in the Great Oxidation Event (GOE; 2.45-2.22 Ga) 1-3 . Notably, geochemical evidence from heavy stable isotopes and redox-sensitive trace elements indicates that marine oxygenation may have preceded the GOE by up to several hundred million years 4-9 . These signals, interpreted as local to regional oxygen (O 2 ) oases, suggest a decoupling of atmospheric and marine oxygenation. However, the spatial extent and persistence of oxygenated surface waters in the Neoarchean remain poorly constrained, largely due to the challenges of extrapolating from geochemical records that are often tied to productive continental margins 4,10-13 . To address this problem, multiple attempts have been made to track the oxygenation of Earth’s shallow ocean during the runup to the GOE. One widely used proxy for reconstructing paleomarine oxygenation is the molybdenum (Mo) isotope composition (δ 98 Mo) of seawater 14-16 . This approach is grounded in modern marine Mo systematics, where Mo predominantly exists as the highly soluble and stable molybdate oxyanion (MoO 4 2- ). Due to its low chemical reactivity, Mo has a long residence time on the order of 450,000 to 800,000 yrs, which greatly exceeds the ocean’s mixing time (~1,500 yrs) 17 , resulting in a globally homogeneous concentration of approximately 110 nM 18 . The δ 98 Mo of modern seawater is similarly uniform (~2.3‰ 19 ), and significantly heavier than the average continental crust (0.0-0.4‰ 20 ). This isotopic offset is mainly due to the preferential removal of lighter Mo isotopes (as MoO 4 2- ) during co-precipitation with Fe(III)- and Mn(IV)-oxyhydroxides (from herein referred to as Fe and Mn oxides) in oxic environments 19,21-22 . Consequently, the initial rise in O 2 in Earth’s ocean would have driven the formation and burial of these oxides, enriching seawater in heavy Mo isotopes. If Mo was then quantitatively scavenged from this seawater without further fractionation, sedimentary archives could retain a record of this isotopically heavy signal. Indeed, Neoarchean shales and carbonates commonly yield δ 98 Mo values above 1‰ 11-12, 23-24 , compared to generally crustal δ 98 Mo values of older sediments 25-26 (Fig. 1). These elevated values have been interpreted as evidence for at least regional accumulation of O 2 in Neoarchean marine environments. However, such interpretations remain debated, as alternative mechanisms — such as the formation of Fe oxides via photoferrotrophy in the absence of free O 2 27 — could potentially have generated a significant sink for isotopically light Mo isotope in the Neoarchean 22 , similar to modern Fe oxides 28 . These alternative pathways complicate the use of δ 98 Mo signatures in shales and carbonates as unambiguous indicators of early oxygenation. Banded iron formations (BIFs) are ancient chemical sediments characterized by high Fe contents (15-40 wt.% Fe) dominated by Fe oxide mineral phases such as magnetite and hematite, interbedded with microcrystalline SiO 2 29-30 . These sediments become volumetrically abundant in the Neoarchean and early Paleoproterozoic, coinciding with a generally increasing trend in Archean marine δ 98 Mo values (Fig. 1). Geochemical evidence strongly suggests that BIFs formed under ferruginous (anoxic, Fe(II)-rich) water column conditions 29-32 , and it is widely inferred that anoxygenic photosynthetic Fe(II) oxidation — i.e., photoferrotrophy — likely played a dominant role in Fe oxide precipitation, at least on the outer shelf where O 2 was largely absent 31-32 . In this context, we examine the Mo isotope compositions of Neoarchean BIFs in the North China Craton (NCC) to assess if Fe deposition mediated by photoferrotrophy could have acted as a sink for isotopically light Mo, thereby contributing to the buildup of an isotopically heavy Mo reservoir in coeval seawater. The NCC hosts numerous Neoarchean BIFs, most of which are confined to late Archean (2.6-2.5 Ga) lithotectonic assemblages 33 (Fig. 2). These Archean BIFs, similar to those in other cratons worldwide, typically occur within successions of supracrustal rocks (e.g., greenstone belts), and are commonly associated with metamorphosed mafic volcanic rocks 34 (Fig. 3). Most BIFs in the NCC have undergone amphibolite-facies metamorphism, with some reaching granulite facies and exhibiting prominent structural deformation 35 . These BIFs are generally interpreted to have formed within arc or back-arc basin settings 33 , an interpretation supported by the geochemical characteristics of surrounding metamorphosed volcanic and sedimentary rocks 34 . Mineralogically, the BIFs consist of magnetite, quartz, various types of Fe silicate minerals (i.e., greenalite, stilpnomelane, cummingtonite-grunerite, and pyroxene), with minor carbonates (i.e., siderite, ankerite, and calcite) and sulfides (i.e., pyrite), reflecting extensive alteration of the primary mineral assemblages during diagenesis and metamorphism. In this work, we investigate seven large BIFs distributed across four different lithotectonic assemblages of the NCC (Fig. S1). These BIFs have been long exploited as China’s principal source of industrial Fe, with samples principally collected in active open pit mines, enabling access to fresh material. These targeted formations include: (1) ~2.57 Ga Xiaolaihe BIF (Qingyuan); (2) ~2.56 Nanfen BIF (Anshan-Benxi); (3) ~2.55 Ga Gongchangling BIF (Anshan-Benxi); (4) ~2.54 Ga Dong’anshan BIF (Anshan-Benxi); (5) ~2.54 Ga Sijiaying BIF (Eastern Hebei); (6) ~2.53 Ga Ekou BIF (Wutai); and (7) ~2.53–2.51 Ga Qidashan BIF (Anshan-Benxi) (Fig. 3). Approximately 180 fresh BIF samples were collected and analyzed petrographically using reflected light and scanning electron microscopy 35-37 , with representative samples selected for further elemental and Fe isotopic analysis 32 . Geological, petrographic, and sedimentological details for each BIF deposit are provided in the Supplementary Information. Following screening to avoid obvious secondary alteration (e.g., veining, chert recrystallization, and martite formation) of BIFs, 70 samples were selected for Mo isotope analyses. Given the sample mineralogy, it is possible that Mo was preferentially scavenged by either sulfides or organic matter rather than major Fe-bearing phases 11,38 . To assess this possibility, total organic carbon (TOC) and total sulfur (TS) concentrations were also determined for each sample. We further compared Mo isotopic compositions of the Neoarchean BIF analyzed here with published data from coeval carbonate and shale records across various sedimentary basins to shed new light on marine Mo cycling and the relative magnitude and spatial extent of surface oxygenation at that time. Notably, our study represents the first application of the Mo isotope proxy to a comprehensive suite of BIFs from the NCC, offering a coherent view of the ocean redox structure coinciding with the terminal stage of Archean BIF deposition. Results Total organic carbon and total sulfur All BIF samples are organic-lean, with TOC ranging from 0.01 to 0.22 wt% (mean 0.02 wt%) (Table S2), which is comparable with most Precambrian BIFs (<0.5 wt%) 39 . In contrast, their TS contents show significant variability, ranging from levels below the detection limit (<0.01 wt%) to 2.12 wt% (mean 0.23 wt%). Samples from the Xiaolaihe BIF are strongly enriched in TS, ranging between 0.06 and 2.12 wt% (mean 0.78 wt%). This is in stark contrast to those of other BIFs (mean 0.15 wt%). These observations are consistent with the sample mineralogy, which is dominated by magnetite and quartz, with minor amounts of organic matter and pyrite present in only a few samples 35-37 . Molybdenum concentration and isotopic composition Molybdenum concentrations in the BIF samples are consistently low, peaking at 2.4 ppm, with the most values falling below average upper continental crust (~1.1 ppm) 40 (Table S2). Despite these low concentrations, a clear relationship emerges between Mo concentrations and δ 98 Mo values: samples with higher Mo concentrations tend to exhibit relatively heavy Mo isotope compositions (Fig. 4a-b). Moreover, the δ 98 Mo values span a broad range, from near-crustal signatures (0.0-0.4‰) 20 to as high as 1.52‰ (mean 0.64‰). Discussion Verifying authigenic signatures Before discussing the paleoenvironmental implication of our data, it is important to consider the potential effects of post-depositional metamorphism and weathering 41-42 . Although our BIF samples are collected from outcrops, they were freshly exposed due to ongoing mining activity, minimizing surface alteration. Importantly, no correlation is observed between δ 98 Mo values and shale-normalized Ce anomalies 32 (Fig. S2), suggesting that modern weathering had negligible impact on primary δ 98 Mo values. In addition, δ 98 Mo values do not systematically vary with metamorphic grade (Fig. S3), indicating that high-temperature metamorphic processes did not significantly fractionate Mo isotopes in these samples. It is also necessary to consider the potential influence of syn-depositional clastic contamination on the Mo isotope composition of the BIFs. To evaluate terrestrial input, we examined typical indicators, such as Al 2 O 3 concentrations. Although all BIF samples have low Al 2 O 3 (<1 wt%) 32 , there still exists a positive correlation between Al 2 O 3 and both Mo contents and δ 98 Mo values in the Gongchangling and Sijiaying BIFs (Fig. 4c-d), indicating some degree of terrigenous Mo contribution. Close examination of this correlation reveals that the δ 98 Mo values of these samples become especially variable when Al 2 O 3 contents exceed 0.2 wt%, with the high-Al endmember corresponding to isotopically heavy signatures greater than typical continental crustal values between 0 and 0.4‰ 20 . This implies that any detrital influence on the Mo isotopic composition is likely minor. Following established approaches 12 , we calculated authigenic δ 98 Mo values by correcting samples for the detrital component using Al normalization (Table S2). However, the corrections were generally small (<0.1‰) and did not significantly affect the overall dataset. Furthermore, such corrections introduce additional uncertainty due to assumptions about the composition of crustal Mo. Therefore, given this uncertainty, all subsequent discussion is based on the original, uncorrected dataset. Molybdenum isotope fractionation during BIF deposition The δ 98 Mo values for the NCC BIF samples span a relatively wide range, yet most samples fall well below marine δ 98 Mo estimates (>1‰) derived from contemporaneous shales and carbonates 12,24 . This suggests that BIFs preferentially accumulate isotopically light Mo from seawater. This interpretation is consistent with the weak but positive correlation between Mo concentrations and δ 98 Mo values observed in the dataset (Fig. 4a-b). Previous studies of a few early Precambrian (~2.95 Ga and ~2.46 Ga) BIFs have reported significant Mo isotopic variability and identified negative correlations between δ 98 Mo and Mn/Fe ratios 5,43 . Such correlations have been attributed to the local formation of Mn oxides – in the presence of water column O 2 – and preferential adsorption of isotopically light Mo from dissolved MoO 4 2- , resulting in Mn enriched samples having the lowest δ 98 Mo values. In contrast, the NCC BIF samples are characterized by extremely low Mn/Fe ratios (<0.01) 32 and show no correlation (R 2 < 0.25) between δ 98 Mo and Mn/Fe ratios for any of the studied BIFs (Fig. 4e-f), pointing to the absence of Mn redox cycling in the basin’s water column. Instead, we propose three alternate mechanisms that may have contributed to the relatively lighter δ 98 Mo values observed in our samples: (i) Mo adsorption onto Fe oxides . Experimental studies show that Mo in the form of MoO 4 2- can adsorb onto ferrihydrite – the likely precursor Fe mineral of BIFs – with a fractionation factor of Δ 98 Mo ferrihydrite-solution = -1.11 ± 0.15‰ 22 . This suggests that marine Fe redox cycling might have played an important role in regulating the δ 98 Mo values of sediments. Mo drawdown by adsorption onto newly forming Fe oxides might have significantly affected the local dissolved MoO 4 2- reservoir and driven its isotopic composition toward heavier values. As Fe(II) oxidation and Mo adsorption proceed, the remaining seawater would become increasingly enriched in isotopically heavy Mo, and this composition could have been archived in contemporaneous shales and carbonates (Fig. 1) 12,24 . (ii) Mo adsorption into biomass . The precipitation of BIF requires oxidation of Fe(II), either indirectly via oxygenic photosynthesis or directly via anoxygenic photoferrotrophy. Both mechanisms would have produced some biomass (i.e., organic carbon) that settled to the seafloor together with primary Fe oxides such as ferrihydrite 30 . Organic matter serves as an important marine sink for Mo, scavenging MoO 4 2- and triggering a coordination change from tetrahedral to octahedral geometry depending on the specific incorporation mechanism 44 . This change results in the preferential adsorption of isotopically light Mo, such that organically bound Mo is significantly lighter than seawater Mo (△ 98 Mo seawater-organics = 0.2‰ to 1‰) 45 . During diagenesis and subsequent metamorphism, oxidation of this organic matter could have released the light Mo back into sediment porewaters, where it was later sequestered into secondary minerals such as Fe(II)-carbonate and/or magnetite 11 . This process is consistent with the extremely low TOC concentrations in the NCC BIF samples – mostly near the analytical detection limit (0.01 wt%) – and the absence of correlations (R 2 < 0.10) between TOC and either δ 98 Mo values or Mo contents (Fig. 5a-b). Therefore, recycling of light Mo from degraded organic matter may have contributed to the relatively depleted and variable δ 98 Mo values recorded in the BIFs compared to contemporary seawater. (iii) Mo sequestration in sulfides . The settling of organic matter drives early diagenesis within sediments, wherein microorganisms utilize a range of terminal electron acceptors as oxidants. The impact of early diagenesis on Mo cycling is strongly dependent on the depositional environment, particularly the concentrations of porewater sulfide and organic matter 38,44 . In settings where H 2 S is restricted to sediment porewaters, incomplete (non-quantitative) Mo removal can produce sedimentary δ 98 Mo values that are either similar to or lighter than the overlying seawater 38,44 . In the NCC BIF samples, sulfides are present, albeit in minor amounts 32 , and frequently exhibit positive Fe isotope signatures 37 , suggesting that these authigenic pyrite grains likely formed via complete sulfidation of primary Fe oxides during diagenesis 46 . Supporting this hypothesis, we observe a significant correlation between TS concentrations and either δ 98 Mo values or Mo contents, especially in samples with high TS concentrations (>0.2 wt%) (Fig. 5c-d). These trends imply that the formation of authigenic sulfides in porewaters played an important role in controlling Mo isotope compositions, contributing to the variability observed across the BIF dataset. Reconstructing δ 98 Mo of Neoarchean seawater Considering the factors above, the authigenic Mo isotopic composition preserved in our BIF samples likely reflects a combination of Mo initially adsorbed to primary Fe oxides, with potentially minor contributions from biomass-associated Mo and Mo incorporated during diagenetic formation of secondary sulfides. Therefore, we interpret the mean δ 98 Mo values of low-TS (<0.2 wt%) samples from each BIF, coupled with the experimentally determined fractionation factor for Mo adsorption onto ferrihydrite 22 , as a proxy for ambient seawater δ 98 Mo at the time of deposition of Fe oxides that comprise BIF. These calculations suggest a relatively stable seawater δ 98 Mo record of approximately 1.5-1.6‰ between 2.57 and 2.51 Ga (Table S2). This interpretation is further supported by the fact that a few high-TS samples from the NCC BIFs show similarly isotopically heavy signatures exceeding 1.5‰. As discussed above, sequestration of Mo in these samples appears to be partially linked to local sulfide availability, as evident by the overall positive correlation between sedimentary Mo and TS abundances. This suggests non-quantitative scavenging of Mo under variable porewater redox conditions. However, given that dissolved Mo concentrations in Archean seawater were thought to have been much lower than today 47 , the efficiency of Mo removal – especially under high sediment porewater H 2 S – may have occasionally approached quantitative transfer to sediments. Hence, the heaviest δ 98 Mo value of our high-TS BIF samples would serve as a conservative estimate for the late Archean seawater δ 98 Mo signature, and seawater, if anything, may have been heavier in reality. Our seawater δ 98 Mo estimates are likely, at minimum, to be representative of a regionally homogeneous value. This is supported by the spatial distribution of the seven BIFs analyzed here, which are spread across the NCC’s different lithotectonic assemblages and collectively yield a consistent isotopic range. Moreover, previous calculations show that even if the total Mo reservoir in the Archean ocean was only 1% of the modern inventory (<5 nM) 47-48 , its residence time (at least 4500 yr) would still have exceeded the estimated ocean mixing time (~1500 yr today), suggesting the potential for regional to global isotopic homogeneity, although the true mixing time of the late Archean oceans remains unknown. Our inferred seawater δ 98 Mo estimates are broadly comparable to those estimated from contemporary carbonates and shales deposited on the Pilbara and Kaapvaal cratons (Fig. 1) 12,49 , which have yielded maximum values generally between 1.50‰ and 1.58‰, and up to 1.72‰ in a shale sample from the ~2.6 Ga Nauga Formation, South Africa 26 . Nevertheless, we note that the majority of previously published δ 98 Mo data from Archean shales and carbonates fall below these heavier estimates. Similar inconsistencies in δ 98 Mo values between broadly time-equivalent sedimentary archives have also been described in Mesoarchean 24 and late Proterozoic 50 . On first interpretation, these differences might imply fluctuating seawater δ 98 Mo values on short timescales of tens to hundreds of million years during the late Archean. However, most of the shales these estimates are based on were deposited under ferruginous conditions, which may not have enabled quantitative Mo drawdown from seawater during deposition 12 . Moreover, δ 98 Mo values in shales would have been diluted by mixing with detrital lithogenic components given the riverine sourcing of the precursor clay minerals 23 . Therefore, late Archean shales might not capture a quantitative bulk seawater δ 98 Mo with great fidelity. Archean carbonates, likewise, were susceptible to Mo isotopic overprinting through interactions with Fe oxides or during early diagenesis, both of which tend to shift values toward lighter compositions relative to contemporaneous seawater 24,49,51 (see Supplementary Information for details). On the other hand, BIFs were deposited on the continental shelf via upwelling, Fe(II)-enriched hydrothermal waters, and thus they more broadly captured bulk seawater 30 . Taken together, we propose that the late Archean ocean δ 98 Mo composition was more stable than previously inferred, and likely closer to the upper end of the published estimates 12 . Unless future work can definitely resolve whether apparent temporary δ 98 Mo shifts reflect real oceanographic change rather than inter-proxy or inter-archive biases, BIF-based reconstructions may provide the most direct and reliable insight into Archean seawater Mo isotope composition. Implications for ocean oxygenation To decipher the specific mechanism controlling marine δ 98 Mo values, we constructed a Mo isotope mass balance model for the late Archean ocean, modified from previous workers 14,48,52 : F R δ R + F H δ H = F O δ O + F RED δ RED + F E δ E Eq. (1) where F and δ refer to the Mo flux (g m -2 yr -1 ) and Mo isotope composition for each of sources or sinks involved, respectively. The model includes two primary sources of Mo to the oceans: riverine input (R) and upwelling hydrothermal fluxes (H). Three sinks involved are: (i) an oxic sink (O), comprising sediments enriched in Mn oxides deposited under oxygenated waters (dissolved O 2 > 10 µM); (ii) an euxinic sink (E), deposited beneath sulfidic bottom waters (dissolved H 2 S > 11 µM), where quantitative conversion of Mo to MoS 4 2- occurs; and (iii) an intermediate sink (RED), which encompasses suboxic, anoxic, and weakly euxinic depositional environments. The suboxic sink is associated with sediments deposited under water columns with low O 2 (<10 µM). The anoxic sink refers to sediments deposited under anoxic water columns lacking O 2 , with or without dissolved porewater H 2 S. The weakly euxinic sink is associated with sediments deposited beneath low-H 2 S (<11 µM) bottom waters. Presently, rivers – potentially including contributions from groundwater – account for approximately 90% of Mo input to the modern oceans, while the remaining 10% derives from chemical exchange within low-temperature oceanic hydrothermal systems 52 . However, there is evidence suggesting that the Mo in these fluids (~0.8‰) is sourced via reductive dissolution of Mo-bearing sediments overlying oceanic crust 53 . As such, these fluids more closely represent a failed Mo sink rather than a true long-term Mo source. High-temperature hydrothermal fluids were previously estimated to contribute only 1% of the modern riverine Mo flux 17 , but are now understood to act as a net sink owing to the low solubility of Mo sulfides under such conditions 17 . Although this high-temperature input may have been more significant in early Archean oceans 32,54 , its magnitude and isotopic composition are poorly constrained. We therefore excluded this term from our mass-balance equation, and simplify Eq. (1) to: F R δ R = F O δ O + F RED δ RED + F E δ E Eq. (2) Dividing both sides of Eq. (2) by F R we obtain: δ R = f O δ O + f RED δ RED + f E δ E Eq. (3) where f O = F O /F R , f RED = F RED /F R , and f E = F E /F R . If we assume there are no other sinks, i.e., f O + f RED + f E =1 Eq. (4) Note that the Mo isotope composition of each sedimentary environment is additionally constrained by the respective isotope fractionation factors relative to seawater (SW) inferred from field observations and laboratory experiments (i.e., Δ 98 Mo SW-O , Δ 98 Mo SW-RED , and Δ 98 Mo SW-E ). Hence, we can obtain an expression for the marine δ 98 Mo value associated with inputs and outputs: δ SW = δ R + Δ 98 Mo SW-E × f E + Δ 98 Mo SW-O × f O + Δ 98 Mo SW-RED × f RED Eq. (5) This equation indicates that high marine δ 98 Mo values require a higher riverine Mo isotopic composition, and/or relatively larger Mo sinks. Modern rivers are characterized by a wide range of δ 98 Mo value, from -0.1‰ to +2.3‰ 55-56 . With the inclusion of a direct groundwater contribution to the Mo flux to the oceans, the average riverine δ 98 Mo input to the ocean is 0.55‰ 52 . These isotopically heavy values are interpreted to result from Mo isotope fractionation during oxidative weathering, in which light Mo isotopes are preferentially retained in soils by adsorption onto Mn-Fe oxides and clay minerals 55 . However, this mechanism would have limited long-term impact if weathering ultimately releases all soil-bound Mo to rivers on timescales shorter than the oceanic Mo residence time 57 . In contrast, Archean terrestrial weathering conditions were fundamentally different, characterized by very low atmospheric O 2 levels and the absence of significant plant activity 58 . As a result, soil horizons would have been much thinner and surface oxide minerals sparse, rendering Archean oxidative weathering processes – and their associated isotopic fractionations – significantly less than today. Consequently, we interpret the modern average δ R of 0.55‰ as a conservative upper limit for Mo inputs to the late Archean ocean. This precludes a sufficiently heavy source to explain the observed late Archean seawater δ 98 Mo values and instead necessitates a greater sink for isotopically light Mo to satisfy mass-balance constraints. Well-oxygenated settings produce the largest local isotopic offset between seawater and sediments (Δ 98 Mo SW-O of ~3‰), primarily due to Mo adsorption onto Mn oxides 19,21 . This equilibrium fractionation is largely insensitive to variations in temperature and other environmental parameters over geological timescales 59 . In contrast, under strongly euxinic settings, nearly quantitative Mo removal from bottom waters enables preservation of seawater δ 98 Mo in the sediments (Δ 98 Mo SW-E of -0.3‰ to 0‰) 60-61 . Given that weakly and strongly euxinic settings have Mo burial fluxes that are significantly higher than in non-euxinic settings 47,62 , a compromise is to assign a small isotope fractionation Δ 98 Mo SW-E of ~0.5‰ to encompass both settings 60 . The isotopic offset for the intermediate sink (Δ 98 Mo SW-RED ), which spans suboxic to anoxic redox regimes, is less well defined. However, an average Mo isotope fractionation of ~0.7‰ is typically chosen to represent this sink, consistent with the Mo isotope offset between overlying seawater and deposition beneath low-O 2 to anoxic bottom waters on continental margins 63-64 . Accordingly, assuming a global late Archean seawater δ 98 Mo value of 1.6‰, the average δ 98 Mo values of sediments associated with oxic, RED, and euxinic sinks would be approximately -1.4‰, 0.9‰, and 1.1‰, respectively. In addition to considering the burial fluxes (F) for each of the three redox-defined settings, we can also explore their areal extent. These fluxes scale with the size of the global oceanic Mo reservoir and thus can be expressed as: F = F 0 × R/R 0 Eq. (6) where R denotes the size of the global oceanic Mo reservoir and the subscript 0 denotes the modern value. Therefore, each f term in the Mo isotope mass balance equation can be replaced by: f = [(F 0 × R/R 0 ) × (A total × f A )]/F R Eq. (7) where f A denotes the fraction of seafloor represented by the sink and A total denotes the total seafloor area covered by the three sinks. In this way, the global seawater δ 98 Mo values can be modelled as a function of the areal extent of each sink (A O , A RED , and A E ) through incorporating modern Mo accumulation parameters (Table S3). Our modelling results show that high marine δ 98 Mo values, such as those inferred for the late Archean ocean (~1.6‰), can only be achieved if the ocean floor was overwhelmingly dominated by oxic conditions, with the oxic area (A O ) comprising between 92% and 97% of the global seafloor (Fig. 6a). However, such extensive oxygenation seems unrealistic for the late Archean, given the scarcity of Mn-rich sediments from this time 65 . This inference is further supported by the absence of correlation between δ 98 Mo values and Mn/Fe ratios in our BIF samples (Fig. 4e-f) and by invariant, near-crustal thallium isotope compositions observed in contemporaneous shales from western Australia and South Africa 8,66 . Additional constraints from Fe speciation analyses in these shales 4,12 points to a Neoarchean ocean dominated by ferruginous, rather than widespread oxic or euxinic conditions. Unlike the relatively localized sedimentary record from western Australia and South Africa 7 , BIFs were deposited globally during this interval, including China, Canada, and India 29-30,67 . Crucially, the precipitation of BIFs does not require free O 2 30-32,67 , and if O 2 is involved, this would exist only in surface waters where Fe(II) oxidation occurred. Hence, if O 2 was absent or restricted to marine shelf environments, the Neoarchean marine Mo isotope signatures may instead have been significantly shaped by fractionations during Mo adsorption to Fe oxides. Interestingly, this correlates well with a recent study on nitrogen isotopes in Neoarchean BIFs that suggested declining seawater oxygenation and increased denitrification between 2.60 and 2.48 Ga 9 . To test this possibility, the isotopic fractionation assigned to the RED sink in the Neoarchean ocean needs to be adjusted to reflect conditions more consistent with Mo adsorption onto ferrihydrite (Δ 98 Mo SW-Fer of 1.1‰) 22 . Ferrihydrite is widely considered the primary Fe(III) mineral phase of BIFs 30 and has been shown to adsorb Mo approximately 30 times more efficiently than hematite on a per-unit basis 22 . A comparison of model outputs using two different RED fractionation factors (Fig. 6) reveals an important distinction: with Δ 98 Mo SW-RED of 0.7‰, the maximum δ SW value in the absence of a significant oxic sink is approximately 1.25‰, whereas using the adjusted Δ 98 Mo SW-RED of 1.1‰ increases the maximum to about 1.65‰, closely matching the late Archean seawater value. Moreover, when A O is below 97% of the seafloor, δ SW is almost entirely controlled by the extent of the RED sink, as evidenced by the alignment of lines of constant δ SW with those of constant A E /A RED ratios (Fig. 6b). Although the oxic sink results in a considerable fractionation from seawater, the burial rate in this setting is two orders of magnitude lower than in RED settings 47,62 . On the other end of the redox spectrum, euxinic environments also efficiently scavenge Mo from seawater but tend to do so with minimal isotopic offset due to near-quantitative Mo removal 62 . Therefore, the dominant control on δ SW under ferruginous conditions is the extent of RED sinks, unless the deep ocean is fully oxygenated as it is today. Our modelling outcome underscores that elevated δ 98 Mo values are consistent with restricted O 2 accumulations in the Neoarchean and should be interpreted in light of the complex interplay between multiple Mo sinks with differing fractionation behaviors. Accordingly, it is more likely that adsorption of isotopically light Mo to abundant BIFs deposited via photoferrotrophy or possibly low levels of O 2 in surface waters contributed greatly to the late Archean marine heavy Mo reservoir, which, by extension, implies pervasively ferruginous water columns, at least on the outer shelf (Fig. 7). This interpretation is consistent with fluctuating thallium and nitrogen isotope compositions of Neoarchean shales 8 and BIFs 9 , collectively pointing towards oscillating seawater oxygenation. Conclusions Our new geochemical data and model constraints suggest that elevated Neoarchean marine δ 98 Mo value may represent an increased contribution from the intermediate redox sink (i.e., BIF deposition), consistent with the ubiquitous occurrence of BIFs in the Neoarchean sedimentary record 30 , 32 . Although oxic sinks may have existed on the continental shelf and contributed to Mo isotopic fractionation, they are not strictly required to account for the observed seawater signal. An abundant BIF sink can also explain the co-occurrence of positive Mo isotope signatures and negative Fe isotopes at multiple stratigraphic levels in roughly contemporaneous shales and carbonates from South African and western Australia 12 , 68 – 69 . Importantly, while our findings do not preclude the existence of localized oxygenated marine shelves, they highlight that widespread anoxic deep ocean conditions dominated in the immediate lead up to the GOE. These conditions may have significantly impacted the cycling of redox-sensitive elements such as Mo in the Neoarchean ocean. The shale and carbonate record of Mo isotopes may thus be considered as an indirect record of extensive phototrophic Fe(II) oxidation in offshore environments. Methods Total organic carbon and total sulfur contents The TOC and TS contents were measured using a LECO CS844 infrared carbon-sulfur analyzer at ALS Chemex (Guangzhou, China). For each sample, approximately 80–120 mg of dried rock powders were first leached using 2 M HCl to remove possible inorganic carbon. The residual powder was washed using Milli-Q water and then dried at 80℃ before instrumental analysis. Subsequently, these samples were loaded into tin capsules, and the carbonate content was determined by the mass percentage loss during decalcification. The analytical precision of TOC concentrations was better than ± 0.02 wt%. For bulk-rock TS analysis, a sodium carbonate leach was used to remove sulfate, and the insoluble sulfide residue was filtered and analyzed. Multiple analyses of certified reference materials GGC-12, GS310-8, GS913-3, and OREAS 504 present analytical precisions better than ± 0.02 wt%. Mo isotopes Approximately 50 mg of sample powder was dissolved in Teflon beakers using concentrated HCl, HF, and HNO 3 heated at ~ 200℃ for 48 h, dried down, and subsequently dissolved and dried down again using 0.5 mL 10 M H 2 O 2 heated at ~ 60℃ for 1 h to digest residual black materials (e.g., organic matter). Bulk-rock Mo abundances and isotopic compositions of these BIF samples were determined using the double-spike method. The chemical purification of Mo was performed using the ion-exchange chromatography protocol described in Li et al. 70 at the Guizhou Tongwei Analytical Technology Co., ltd. (China). The Mo isotope ratios were analyzed on a multi-collector inductively coupled plasma-mass spectrometer (MC–ICP–MS; Thermo-Fisher Scientific Neptune Plus) at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The Mo isotopic composition was reported relative to the NIST SRM 3134 standard and were then set to + 0.25‰ for ease of comparison with previously published data: δ 98 Mo NIST+0.25 = [( 98 Mo/ 95 Mo) sample /( 98 Mo/ 95 Mo) NIST3134 – 1] × 1000 + 0.25 71 . External reproducibility of the NIST SRM 3134 standard was better than ± 0.05‰ (2SD, n = 54). The geochemical reference materials NOD-A-1 (Mn nodule), IAPSO (seawater), and SGR-1b (shale) were analyzed alongside samples to monitor accuracy, yielding δ 98 Mo NIST+0.25 values of − 0.76 ± 0.05‰ (2SD, n = 2), 2.30 ± 0.06‰ (2SD, n = 6), and 0.36 ± 0.05‰ (2SE, n = 1), respectively. These results are in excellent agreement with previously reported values 70 . Declarations Acknowledgement: This work was supported by grants from the National Key R&D Program of China (2024YFF0810200), the Strategy Priority Research Program (Category B) of Chinese Academy of Sciences (XDB0710000), and the National Natural Science Foundation of China (42325206, 42150104). EES acknowledges funding from a NERC Frontiers grant (NE/V010824/1). References Holland HD (2006) The oxygenation of the atmosphere and oceans. Philosophical Trans Royal Soc B: Biol Sci 361(1470):903–915 Konhauser KO, Lalonde SV, Planavsky NJ, Pecoits E, Lyons TW, Mojzsis SJ, Rouxel OJ, Barley ME, Rosìere C, Fralick PW, Kump LR, Bekker A (2011) Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event. Nature 478(7369):369–373 Poulton SW, Bekker A, Cumming VM, Zerkle AL, Canfield DE, Johnston DT (2021) A 200-million-year delay in permanent atmospheric oxygenation. 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Geostand Geoanal Res 38(3):345–354 Nägler TF, Anbar AD, Archer C, Goldberg T, Gordon GW, Greber ND, Siebert C, Sohrin Y, Vance D (2014) Proposal for an international molybdenum isotope measurement standard and data representation. Geostand Geoanal Res 38(2):149–151 Additional Declarations There is NO Competing Interest. Supplementary Files TableS1final.xlsx TableS1 TableS2final.xlsx TableS2 TableS3final.xlsx TableS3 SupplementaryInformationfinal.docx Supplementary Information Cite Share Download PDF Status: Posted 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. 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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-7181523","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":489836285,"identity":"80224d70-39d7-4f01-82c7-deec02bba22c","order_by":0,"name":"Kurt Konhauser","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzUlEQVRIiWNgGAWjYBACCSBmbKhgYOCTIEELY2PDGQYGNpCWA0RraWwjRYvkjNznD2fOOyzPJt187POHGgZ5/gYCWqQl0g0bN247bNgmcyx5xoFjDIYzCFklJ5HG2Phw22HGNokcY4YDbAwJBF0H0TLnsH2bRP5nhgP/GBLkCWmRBmnZ2HA4EWgLM8PBNoYEA0JaJHueMc6ccSw9GegXY4azfRKGGwlpkTiexvCxp8batl+6+TFDxTcbeTlCWjCMIFH9KBgFo2AUjAKsAABQcUDWeGBIMwAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-7722-7068","institution":"University of Alberta","correspondingAuthor":true,"prefix":"","firstName":"Kurt","middleName":"","lastName":"Konhauser","suffix":""},{"id":489836286,"identity":"315a4275-f833-4d8a-a45b-160b7730624b","order_by":1,"name":"Changle Wang","email":"","orcid":"","institution":"Institute of Geology and Geophysics","correspondingAuthor":false,"prefix":"","firstName":"Changle","middleName":"","lastName":"Wang","suffix":""},{"id":489836287,"identity":"677e5eff-9f26-4307-98d7-80128fd0c9f1","order_by":2,"name":"Feiyu Dong","email":"","orcid":"","institution":"Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Feiyu","middleName":"","lastName":"Dong","suffix":""},{"id":489836288,"identity":"2caba772-e742-407b-a4c2-ad24d54fdae9","order_by":3,"name":"Leslie Robbins","email":"","orcid":"","institution":"University of Regina","correspondingAuthor":false,"prefix":"","firstName":"Leslie","middleName":"","lastName":"Robbins","suffix":""},{"id":489836289,"identity":"086f17f0-6f2d-4b99-88f6-2a90cae7505e","order_by":4,"name":"Lianchang Zhang","email":"","orcid":"","institution":"Institute of Geology and Geophysics, Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Lianchang","middleName":"","lastName":"Zhang","suffix":""},{"id":489836290,"identity":"e8c04fcb-fe71-4d62-b8bb-62f83a7d33c7","order_by":5,"name":"Jie Li","email":"","orcid":"","institution":"State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry","correspondingAuthor":false,"prefix":"","firstName":"Jie","middleName":"","lastName":"Li","suffix":""},{"id":489836291,"identity":"8c4e6c4c-cd53-4e37-a3f0-c31a7c0bbea0","order_by":6,"name":"Eva Stüeken","email":"","orcid":"https://orcid.org/0000-0001-6861-2490","institution":"University of St Andrews","correspondingAuthor":false,"prefix":"","firstName":"Eva","middleName":"","lastName":"Stüeken","suffix":""},{"id":489836292,"identity":"8c60af36-a0f3-4081-90e3-43b46ca6f5e7","order_by":7,"name":"Bo Wan","email":"","orcid":"https://orcid.org/0000-0002-5896-9485","institution":"Institute of Geology and Geophysics，Chinese Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Bo","middleName":"","lastName":"Wan","suffix":""}],"badges":[],"createdAt":"2025-07-22 01:45:28","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7181523/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7181523/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":87480982,"identity":"6a9124d5-1b6e-4703-8fe5-c8a123488b66","added_by":"auto","created_at":"2025-07-24 09:58:39","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":66066,"visible":true,"origin":"","legend":"\u003cp\u003eThe Mo isotope compositions of shales, carbonates, and BIFs deposited in the Paleoarchean to Paleoproterozoic (see the Supplementary Table S1 for detailed data and sources). Additionally, BIF deposition is shown in billion metric tonnes (Gt) with bar widths corresponding to 50 Ma intervals (modified from Bekker et al.\u003csup\u003e29\u003c/sup\u003e).\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/015bc181f6404ece8854fcdb.jpg"},{"id":87481186,"identity":"c7fee15b-7bca-4591-b084-c8d883ee3575","added_by":"auto","created_at":"2025-07-24 10:06:39","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":151169,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of banded iron formations (BIFs) in the North China Craton (NCC). The insert shows the location of the NCC (yellow) relative to the rest of China.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/fef4c65cf947ba8a03f8d7c6.jpg"},{"id":87480983,"identity":"e821e133-7d4f-4a58-bfe1-7c05e43ff373","added_by":"auto","created_at":"2025-07-24 09:58:39","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":57121,"visible":true,"origin":"","legend":"\u003cp\u003eStratigraphic columns from the studied locations across the NCC showing stratigraphic variations and the relationship of the BIFs to other stratigraphic units. Note the strong association of BIFs with metamorphosed volcano-sedimentary strata.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/c604eb34eec628afa815bc1a.jpg"},{"id":87480986,"identity":"143c16a9-d791-4ed3-9c9c-b63cb48fca2e","added_by":"auto","created_at":"2025-07-24 09:58:39","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":113238,"visible":true,"origin":"","legend":"\u003cp\u003eBulk δ\u003csup\u003e98\u003c/sup\u003eMo values plotted against Mo concentrations (a-b), Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e contents (c-d), and Mn/Fe ratios (e-f) for the NCC BIFs. Note that Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and Mn/Fe data are from Wang et al.\u003csup\u003e32\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/c9229c936c9d6d911e76fc1b.jpg"},{"id":87480991,"identity":"43c19a38-fdca-49c8-9c4b-768bbb45e474","added_by":"auto","created_at":"2025-07-24 09:58:40","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":70691,"visible":true,"origin":"","legend":"\u003cp\u003eRelationships between bulk δ\u003csup\u003e98\u003c/sup\u003eMo values and total organic carbon (a-b) and total sulfur (c-d) contents for the NCC BIFs.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/eae4720ea148c69cc14d883c.jpg"},{"id":87482381,"identity":"f788fbc5-bee6-4985-8590-0d4c320b98c2","added_by":"auto","created_at":"2025-07-24 10:22:40","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":50820,"visible":true,"origin":"","legend":"\u003cp\u003eThree-component diagrams of the marine Mo isotope system showing relative seafloor area of the different redox sinks, where A\u003csub\u003eO\u003c/sub\u003e, A\u003csub\u003eE\u003c/sub\u003e, and A\u003csub\u003eRED\u003c/sub\u003e represent the seafloor areas of oxic, euxinic, and intermediate reducing conditions, respectively. The left panel (a) is based on a fractionation of 0.7‰ for the intermediate sink, while the right panel (b) employs a larger fractionation of 1.1‰ for this sink.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/bbe6f7467b7cf4f804b19d26.jpg"},{"id":87480985,"identity":"97008a8b-9d0e-4c89-949f-713789be1b10","added_by":"auto","created_at":"2025-07-24 09:58:39","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":63883,"visible":true,"origin":"","legend":"\u003cp\u003eMolybdenum isotope fluxes in the late Archean seawater. Molybdenum isotope compositions of waters and continental crusts and associated Mo isotope fractionation between waters and sediments are from refs 20, 22, 52, 60, 61, and this study. Dissolved Fe(II) ([Fe\u003csup\u003e2+\u003c/sup\u003e]\u003csub\u003eaq\u003c/sub\u003e) and O\u003csub\u003e2\u003c/sub\u003e ([O\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003eaq\u003c/sub\u003e) concentrations are schematic.\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/7e9e3d33131317b33270f10f.jpg"},{"id":101880991,"identity":"fbb57ec7-3048-4bbb-a21f-259a9fed1bd0","added_by":"auto","created_at":"2026-02-04 15:08:44","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1310572,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/5c877368-b37b-4f5d-8fe8-b84b1bfe57fe.pdf"},{"id":87480989,"identity":"0d192f9d-d9e6-406d-806b-f0a271b7cd8f","added_by":"auto","created_at":"2025-07-24 09:58:39","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":146939,"visible":true,"origin":"","legend":"TableS1","description":"","filename":"TableS1final.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/d5548c530f39f90337e6063e.xlsx"},{"id":87480999,"identity":"a41c2643-68e1-4539-a28f-dc1dbefc5234","added_by":"auto","created_at":"2025-07-24 09:58:40","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":22441,"visible":true,"origin":"","legend":"TableS2","description":"","filename":"TableS2final.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/c697c50b834ffd43377b5339.xlsx"},{"id":87480988,"identity":"3902e12d-6362-4c56-838d-939fed8ccb09","added_by":"auto","created_at":"2025-07-24 09:58:39","extension":"xlsx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":9827,"visible":true,"origin":"","legend":"TableS3","description":"","filename":"TableS3final.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/d996e8d9267945b00fc18d3b.xlsx"},{"id":87481195,"identity":"37da9485-3146-4cc6-8972-0b21bdbc9379","added_by":"auto","created_at":"2025-07-24 10:06:40","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":412762,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformationfinal.docx","url":"https://assets-eu.researchsquare.com/files/rs-7181523/v1/1cbbb4e72941e84b2cb856df.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"A heavy molybdenum reservoir in Neoarchean seawater tracks extensive iron oxidation","fulltext":[{"header":"Introduction","content":"\u003cp\u003eUnderstanding the evolution of atmospheric and marine redox conditions on early Earth is essential for reconstructing the biogeochemical evolution that made the planet habitable. The transition from a pervasively anoxic to an oxic atmosphere occurred across the Archean to Paleoproterozoic boundary and culminated in the Great Oxidation Event (GOE; 2.45-2.22 Ga)\u003csup\u003e1-3\u003c/sup\u003e. Notably, geochemical evidence from heavy stable isotopes and redox-sensitive trace elements indicates that marine oxygenation may have preceded the GOE by up to several hundred million years\u003csup\u003e4-9\u003c/sup\u003e. These signals, interpreted as local to regional oxygen (O\u003csub\u003e2\u003c/sub\u003e) oases, suggest a decoupling of atmospheric and marine oxygenation. However, the spatial extent and persistence of oxygenated surface waters in the Neoarchean remain poorly constrained, largely due to the challenges of extrapolating from geochemical records that are often tied to productive continental margins\u003csup\u003e4,10-13\u003c/sup\u003e. To address this problem, multiple attempts have been made to track the oxygenation of Earth’s shallow ocean during the runup to the GOE.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOne widely used proxy for reconstructing paleomarine oxygenation is the molybdenum (Mo) isotope composition (δ\u003csup\u003e98\u003c/sup\u003eMo) of seawater\u003csup\u003e14-16\u003c/sup\u003e. This approach is grounded in modern marine Mo systematics, where Mo predominantly exists as the highly soluble and stable molybdate oxyanion (MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e). Due to its low chemical reactivity, Mo has a long residence time on the order of 450,000 to 800,000 yrs, which greatly exceeds the ocean’s mixing time (~1,500 yrs)\u003csup\u003e17\u003c/sup\u003e, resulting in a globally homogeneous concentration of approximately 110 nM\u003csup\u003e18\u003c/sup\u003e. The\u0026nbsp;δ\u003csup\u003e98\u003c/sup\u003eMo of modern seawater is similarly uniform (~2.3‰\u003csup\u003e19\u003c/sup\u003e), and significantly heavier than the average continental crust (0.0-0.4‰\u003csup\u003e20\u003c/sup\u003e). This isotopic offset is mainly due to the preferential removal of lighter Mo isotopes (as MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e) during co-precipitation with Fe(III)- and Mn(IV)-oxyhydroxides (from herein referred to as Fe and Mn oxides) in oxic environments\u003csup\u003e19,21-22\u003c/sup\u003e. Consequently, the initial rise in O\u003csub\u003e2\u003c/sub\u003e in Earth’s ocean would have driven the formation and burial of these oxides, enriching seawater in heavy Mo isotopes. If Mo was then quantitatively scavenged from this seawater without further fractionation, sedimentary archives could retain a record of this isotopically heavy signal. Indeed, Neoarchean shales and carbonates commonly yield\u0026nbsp;δ\u003csup\u003e98\u003c/sup\u003eMo values above 1‰\u003csup\u003e11-12, 23-24\u003c/sup\u003e, compared to generally crustal\u0026nbsp;δ\u003csup\u003e98\u003c/sup\u003eMo values of older sediments\u003csup\u003e25-26\u003c/sup\u003e (Fig. 1). These elevated values have been interpreted as evidence for at least regional accumulation of O\u003csub\u003e2\u003c/sub\u003e in Neoarchean marine environments. However, such interpretations remain debated, as alternative mechanisms — such as the formation of Fe oxides via photoferrotrophy in the absence of free O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e27\u003c/sup\u003e — could potentially have generated a significant sink for isotopically light Mo isotope in the Neoarchean\u003csup\u003e22\u003c/sup\u003e, similar to modern Fe oxides\u003csup\u003e28\u003c/sup\u003e. These alternative pathways complicate the use of\u0026nbsp;δ\u003csup\u003e98\u003c/sup\u003eMo signatures in shales and carbonates as unambiguous indicators of early oxygenation.\u003c/p\u003e\n\u003cp\u003eBanded iron formations (BIFs) are ancient chemical sediments characterized by high Fe contents (15-40 wt.% Fe) dominated by Fe oxide mineral phases such as magnetite and hematite, interbedded with microcrystalline SiO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e29-30\u003c/sup\u003e. These sediments become volumetrically abundant in the Neoarchean and early Paleoproterozoic, coinciding with a generally increasing trend in Archean marine δ\u003csup\u003e98\u003c/sup\u003eMo values (Fig. 1). Geochemical evidence strongly suggests that BIFs formed under ferruginous (anoxic, Fe(II)-rich) water column conditions\u003csup\u003e29-32\u003c/sup\u003e, and it is widely inferred that\u0026nbsp;anoxygenic photosynthetic Fe(II) oxidation\u0026nbsp;— i.e., photoferrotrophy —\u0026nbsp;likely played a dominant role in Fe oxide precipitation, at least on the outer shelf where O\u003csub\u003e2\u003c/sub\u003e was largely absent\u003csup\u003e31-32\u003c/sup\u003e. In this context, we examine the Mo isotope compositions of Neoarchean BIFs in the North China Craton (NCC) to assess if Fe deposition mediated by photoferrotrophy could have acted as a sink for isotopically light Mo, thereby contributing to the buildup of an isotopically heavy Mo reservoir in coeval seawater.\u003c/p\u003e\n\u003cp\u003eThe NCC hosts numerous Neoarchean BIFs, most of which are confined to late Archean (2.6-2.5 Ga) lithotectonic assemblages\u003csup\u003e33\u003c/sup\u003e (Fig. 2). These Archean BIFs, similar to those in other cratons worldwide, typically occur within successions of supracrustal rocks (e.g., greenstone belts), and are commonly associated with metamorphosed mafic volcanic rocks\u003csup\u003e34\u003c/sup\u003e (Fig. 3). Most BIFs in the NCC have undergone amphibolite-facies metamorphism, with some reaching granulite facies and exhibiting prominent structural deformation\u003csup\u003e35\u003c/sup\u003e. These BIFs are generally interpreted to have formed within arc or back-arc basin settings\u003csup\u003e33\u003c/sup\u003e, an interpretation supported by the geochemical characteristics of surrounding metamorphosed volcanic and sedimentary rocks\u003csup\u003e34\u003c/sup\u003e. Mineralogically, the BIFs consist of magnetite, quartz, various types of Fe silicate minerals (i.e., greenalite, stilpnomelane, cummingtonite-grunerite, and pyroxene), with minor carbonates (i.e., siderite, ankerite, and calcite) and sulfides (i.e., pyrite), reflecting extensive alteration of the primary mineral assemblages during diagenesis and metamorphism.\u003c/p\u003e\n\u003cp\u003eIn this work, we investigate seven large BIFs distributed across four different lithotectonic assemblages of the NCC (Fig. S1). These BIFs have been long exploited as China’s principal source of industrial Fe, with samples principally collected in active open pit mines, enabling access to fresh material. These targeted formations include: (1) ~2.57 Ga Xiaolaihe BIF (Qingyuan); (2) ~2.56 Nanfen BIF (Anshan-Benxi); (3) ~2.55 Ga Gongchangling BIF (Anshan-Benxi); (4) ~2.54 Ga Dong’anshan BIF (Anshan-Benxi); (5) ~2.54 Ga Sijiaying BIF (Eastern Hebei); (6) ~2.53 Ga Ekou BIF (Wutai); and (7) ~2.53–2.51 Ga Qidashan BIF (Anshan-Benxi) (Fig. 3). Approximately 180 fresh BIF samples were collected and analyzed petrographically using reflected light and scanning electron microscopy\u003csup\u003e35-37\u003c/sup\u003e, with representative samples selected for further elemental and Fe isotopic analysis\u003csup\u003e32\u003c/sup\u003e. Geological, petrographic, and sedimentological details for each BIF deposit are provided in the Supplementary Information.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFollowing screening to avoid obvious secondary alteration (e.g., veining, chert recrystallization, and martite formation) of BIFs, 70 samples were selected for Mo isotope analyses. Given the sample mineralogy, it is possible that Mo was preferentially scavenged by either sulfides or organic matter rather than major Fe-bearing phases\u003csup\u003e11,38\u003c/sup\u003e. To assess this possibility, total organic carbon (TOC) and total sulfur (TS) concentrations were also determined for each sample. We further compared Mo isotopic compositions of the Neoarchean BIF analyzed here with published data from coeval carbonate and shale records across various sedimentary basins to shed new light on marine Mo cycling and the relative magnitude and spatial extent of surface oxygenation at that time. Notably, our study represents the first application of the Mo isotope proxy to a comprehensive suite of BIFs from the NCC, offering a coherent view of the ocean redox structure coinciding with the terminal stage of Archean BIF deposition.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003eTotal organic carbon and total sulfur\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll BIF samples are organic-lean, with TOC ranging from 0.01 to 0.22 wt% (mean 0.02 wt%) (Table S2), which is comparable with most Precambrian BIFs (\u0026lt;0.5 wt%)\u003csup\u003e39\u003c/sup\u003e. In contrast, their TS contents show significant variability, ranging from levels below the detection limit (\u0026lt;0.01 wt%) to 2.12 wt% (mean 0.23 wt%). Samples from the Xiaolaihe BIF are strongly enriched in TS, ranging between 0.06 and 2.12 wt% (mean 0.78 wt%). This is in stark contrast to those of other BIFs (mean 0.15 wt%). These observations are consistent with the sample mineralogy, which is dominated by magnetite and quartz, with minor amounts of organic matter and pyrite present in only a few samples\u003csup\u003e35-37\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMolybdenum\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003econcentration and isotopic composition\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMolybdenum concentrations in the BIF samples are consistently low, peaking at 2.4 ppm, with the most values falling below average upper continental crust (~1.1 ppm)\u003csup\u003e40\u003c/sup\u003e (Table S2). Despite these low concentrations, a clear relationship emerges between Mo concentrations and δ\u003csup\u003e98\u003c/sup\u003eMo values: samples with higher Mo concentrations tend to exhibit relatively heavy Mo isotope compositions (Fig. 4a-b). Moreover, the δ\u003csup\u003e98\u003c/sup\u003eMo values span a broad range, from near-crustal signatures (0.0-0.4‰)\u003csup\u003e20\u003c/sup\u003e to as high as 1.52‰ (mean 0.64‰).\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003e\u003cstrong\u003eVerifying authigenic signatures\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBefore discussing the paleoenvironmental implication of our data, it is important to consider the potential effects of post-depositional metamorphism and weathering\u003csup\u003e41-42\u003c/sup\u003e. Although our BIF samples are collected from outcrops, they were freshly exposed due to ongoing mining activity, minimizing surface alteration. Importantly, no correlation is observed between \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values and shale-normalized Ce anomalies\u003csup\u003e32\u003c/sup\u003e (Fig. S2), suggesting that modern weathering had negligible impact on primary \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values. In addition, \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values do not systematically vary with metamorphic grade (Fig. S3), indicating that high-temperature metamorphic processes did not significantly fractionate Mo isotopes in these samples.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIt is also necessary to consider the potential influence of syn-depositional clastic contamination on the Mo isotope composition of the BIFs. To evaluate terrestrial input, we examined typical indicators, such as Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e concentrations. Although all BIF samples have low Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (\u0026lt;1 wt%)\u003csup\u003e32\u003c/sup\u003e, there still exists a positive correlation between Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e and both Mo contents and \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values in the Gongchangling and Sijiaying BIFs (Fig. 4c-d), indicating some degree of terrigenous Mo contribution. Close examination of this correlation reveals that the \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values of these samples become especially variable when Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e contents exceed 0.2 wt%, with the high-Al endmember corresponding to isotopically heavy signatures greater than typical continental crustal values between 0 and 0.4\u0026permil;\u003csup\u003e20\u003c/sup\u003e. This implies that any detrital influence on the Mo isotopic composition is likely minor. Following established approaches\u003csup\u003e12\u003c/sup\u003e, we calculated authigenic \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values by correcting samples for the detrital component using Al normalization (Table S2). However, the corrections were generally small (\u0026lt;0.1\u0026permil;) and did not significantly affect the overall dataset. Furthermore, such corrections introduce additional uncertainty due to assumptions about the composition of crustal Mo. Therefore, given this uncertainty, all subsequent discussion is based on the original, uncorrected dataset.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMolybdenum\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eisotope fractionation during BIF deposition\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values for the NCC BIF samples span a relatively wide range, yet most samples fall well below marine \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo estimates (\u0026gt;1\u0026permil;) derived from contemporaneous shales and carbonates\u003csup\u003e12,24\u003c/sup\u003e. This suggests that BIFs preferentially accumulate isotopically light Mo from seawater. This interpretation is consistent with the weak but positive correlation between Mo concentrations and \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values observed in the dataset (Fig. 4a-b).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePrevious studies of a few early Precambrian (~2.95 Ga and ~2.46 Ga) BIFs have reported significant Mo isotopic variability and identified negative correlations between \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo and Mn/Fe ratios\u003csup\u003e5,43\u003c/sup\u003e. Such correlations have been attributed to the local formation of Mn oxides \u0026ndash; in the presence of water column O\u003csub\u003e2\u003c/sub\u003e \u0026ndash; and preferential adsorption of isotopically light Mo from dissolved MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e, resulting in Mn enriched samples having the lowest \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values. In contrast, the NCC BIF samples are characterized by extremely low Mn/Fe ratios (\u0026lt;0.01)\u003csup\u003e32\u003c/sup\u003e and show no correlation (R\u003csup\u003e2\u003c/sup\u003e \u0026lt; 0.25) between \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo and Mn/Fe ratios for any of the studied BIFs (Fig. 4e-f), pointing to the absence of Mn redox cycling in the basin\u0026rsquo;s water column. Instead, we propose three alternate mechanisms that may have contributed to the relatively lighter \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values observed in our samples:\u003c/p\u003e\n\u003cp\u003e(i) \u003cem\u003eMo adsorption onto Fe oxides\u003c/em\u003e. Experimental studies show that Mo in the form of MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e can adsorb onto ferrihydrite \u0026ndash; the likely precursor Fe mineral of BIFs \u0026ndash; with a fractionation factor of \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eferrihydrite-solution\u003c/sub\u003e = -1.11 \u0026plusmn; 0.15\u0026permil;\u003csup\u003e22\u003c/sup\u003e. This suggests that marine Fe redox cycling might have played an important role in regulating the \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values of sediments. Mo drawdown by adsorption onto newly forming Fe oxides might have significantly affected the local dissolved MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e reservoir and driven its isotopic composition toward heavier values. As Fe(II) oxidation and Mo adsorption proceed, the remaining seawater would become increasingly enriched in isotopically heavy Mo, and this composition could have been archived in contemporaneous shales and carbonates (Fig. 1)\u003csup\u003e12,24\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(ii) \u003cem\u003eMo adsorption into biomass\u003c/em\u003e. The precipitation of BIF requires oxidation of Fe(II), either indirectly via oxygenic photosynthesis or directly via anoxygenic photoferrotrophy. Both mechanisms would have produced some biomass (i.e., organic carbon) that settled to the seafloor together with primary Fe oxides such as ferrihydrite\u003csup\u003e30\u003c/sup\u003e. Organic matter serves as an important marine sink for Mo, scavenging MoO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e and triggering a coordination change from tetrahedral to octahedral geometry depending on the specific incorporation mechanism\u003csup\u003e44\u003c/sup\u003e. This change results in the preferential adsorption of isotopically light Mo, such that organically bound Mo is significantly lighter than seawater Mo (△\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eseawater-organics\u003c/sub\u003e = 0.2\u0026permil; to 1\u0026permil;)\u003csup\u003e45\u003c/sup\u003e. During diagenesis and subsequent metamorphism, oxidation of this organic matter could have released the light Mo back into sediment porewaters, where it was later sequestered into secondary minerals such as Fe(II)-carbonate and/or magnetite\u003csup\u003e11\u003c/sup\u003e. This process is consistent with the extremely low TOC\u0026nbsp;concentrations in the NCC BIF samples \u0026ndash; mostly near the analytical detection limit (0.01 wt%) \u0026ndash; and the absence of correlations (R\u003csup\u003e2\u003c/sup\u003e \u0026lt; 0.10) between TOC and either \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values or Mo contents (Fig. 5a-b). Therefore, recycling of light Mo from degraded organic matter may\u0026nbsp;have contributed to the relatively depleted and variable\u0026nbsp;\u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values recorded in the BIFs compared to contemporary seawater.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e(iii) \u003cem\u003eMo sequestration in sulfides\u003c/em\u003e. The settling of organic matter drives early diagenesis within sediments, wherein microorganisms utilize a range of terminal electron acceptors as oxidants. The impact of early diagenesis on Mo cycling is strongly dependent on the depositional environment, particularly the concentrations of porewater sulfide and organic matter\u003csup\u003e38,44\u003c/sup\u003e. In settings where H\u003csub\u003e2\u003c/sub\u003eS is restricted to sediment porewaters, incomplete (non-quantitative) Mo removal can produce sedimentary \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values that are either similar to or lighter than the overlying seawater\u003csup\u003e38,44\u003c/sup\u003e. In the NCC BIF samples, sulfides are present, albeit in minor amounts\u003csup\u003e32\u003c/sup\u003e, and frequently exhibit positive Fe isotope signatures\u003csup\u003e37\u003c/sup\u003e, suggesting that these authigenic pyrite grains likely formed via complete sulfidation of primary Fe oxides during diagenesis\u003csup\u003e46\u003c/sup\u003e. Supporting this hypothesis, we observe a significant correlation between TS concentrations and either \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values or Mo contents, especially in samples with high TS concentrations (\u0026gt;0.2 wt%) (Fig. 5c-d). These trends imply that the formation of authigenic sulfides in porewaters played an important role in controlling Mo isotope compositions, contributing to the variability observed across the BIF dataset.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReconstructing \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo of Neoarchean seawater\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eConsidering the factors above, the authigenic Mo isotopic composition preserved in our BIF samples likely reflects a combination of Mo initially adsorbed to primary Fe oxides, with potentially minor contributions from biomass-associated Mo and Mo incorporated during diagenetic formation of secondary sulfides. Therefore, we interpret the mean \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values of low-TS (\u0026lt;0.2 wt%) samples from each BIF, coupled with the experimentally determined fractionation factor for Mo adsorption onto ferrihydrite\u003csup\u003e22\u003c/sup\u003e, as a proxy for ambient seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo at the time of deposition of Fe oxides that comprise BIF. These calculations suggest a relatively stable seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo record of approximately 1.5-1.6\u0026permil; between 2.57 and 2.51 Ga (Table S2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThis interpretation is further supported by the fact that a few high-TS samples from the NCC BIFs show similarly isotopically heavy signatures exceeding 1.5\u0026permil;. As discussed above, sequestration of Mo in these samples appears to be partially linked to local sulfide availability, as evident by the overall positive correlation between sedimentary Mo and TS abundances. This suggests non-quantitative scavenging of Mo under variable porewater redox conditions. However, given that dissolved Mo concentrations in Archean seawater were thought to have been much lower than today\u003csup\u003e47\u003c/sup\u003e, the efficiency of Mo removal \u0026ndash; especially under high sediment porewater H\u003csub\u003e2\u003c/sub\u003eS \u0026ndash; may have occasionally approached quantitative transfer to sediments. Hence, the heaviest \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo value of our high-TS BIF samples would serve as a conservative estimate for the late Archean seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo signature, and seawater, if anything, may have been heavier in reality.\u003c/p\u003e\n\u003cp\u003eOur seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo estimates are likely, at minimum, to be representative of a regionally homogeneous value. This is supported by the spatial distribution of the seven BIFs analyzed here, which are spread across the NCC\u0026rsquo;s different lithotectonic assemblages and collectively yield a consistent isotopic range. Moreover, previous calculations show that even if the total Mo reservoir in the Archean ocean was only 1% of the modern inventory (\u0026lt;5 nM)\u003csup\u003e47-48\u003c/sup\u003e, its residence time (at least 4500 yr) would still have exceeded the estimated ocean mixing time (~1500 yr today), suggesting the potential for regional to global isotopic homogeneity, although the true mixing time of the late Archean oceans remains unknown. Our inferred seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo estimates are broadly comparable to those estimated from contemporary carbonates and shales deposited on the Pilbara and Kaapvaal cratons (Fig. 1)\u003csup\u003e12,49\u003c/sup\u003e, which have yielded maximum values generally between 1.50\u0026permil; and 1.58\u0026permil;, and up to 1.72\u0026permil; in a shale sample from the ~2.6 Ga Nauga Formation, South Africa\u003csup\u003e26\u003c/sup\u003e. Nevertheless, we note that the majority of previously published \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo data from Archean shales and carbonates fall below these heavier estimates. Similar inconsistencies in \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values between broadly time-equivalent sedimentary archives have also been described in Mesoarchean\u003csup\u003e24\u003c/sup\u003e and late Proterozoic\u003csup\u003e50\u003c/sup\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOn first interpretation, these differences might imply fluctuating seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values on short timescales of tens to hundreds of million years during the late Archean. However, most of the shales these estimates are based on were deposited under ferruginous conditions, which may not have enabled quantitative Mo drawdown from seawater during deposition\u003csup\u003e12\u003c/sup\u003e. Moreover, \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values in shales would have been diluted by mixing with detrital lithogenic components given the riverine sourcing of the precursor clay minerals\u003csup\u003e23\u003c/sup\u003e. Therefore, late Archean shales might not capture a quantitative bulk seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo with great fidelity. Archean carbonates, likewise, were susceptible to Mo isotopic overprinting through interactions with Fe oxides or during early diagenesis, both of which tend to shift values toward lighter compositions relative to contemporaneous seawater\u003csup\u003e24,49,51\u003c/sup\u003e (see Supplementary Information for details).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOn the other hand, BIFs were deposited on the continental shelf via upwelling, Fe(II)-enriched hydrothermal waters, and thus they more broadly captured bulk seawater\u003csup\u003e30\u003c/sup\u003e. Taken together, we propose that the late Archean ocean \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo composition was more stable than previously inferred, and likely closer to the upper end of the published estimates\u003csup\u003e12\u003c/sup\u003e. Unless future work can definitely resolve whether apparent temporary \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo shifts reflect real oceanographic change rather than inter-proxy or inter-archive biases, BIF-based reconstructions may provide the most direct and reliable insight into Archean seawater Mo isotope composition.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eImplications for ocean oxygenation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo decipher the specific mechanism controlling marine \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values, we constructed a Mo isotope mass balance model for the late Archean ocean, modified from previous workers\u003csup\u003e14,48,52\u003c/sup\u003e:\u003c/p\u003e\n\u003cp\u003eF\u003csub\u003eR\u003c/sub\u003e\u0026delta;\u003csub\u003eR\u003c/sub\u003e + F\u003csub\u003eH\u003c/sub\u003e\u0026delta;\u003csub\u003eH\u003c/sub\u003e = F\u003csub\u003eO\u003c/sub\u003e\u0026delta;\u003csub\u003eO\u003c/sub\u003e + F\u003csub\u003eRED\u003c/sub\u003e\u0026delta;\u003csub\u003eRED\u003c/sub\u003e + F\u003csub\u003eE\u003c/sub\u003e\u0026delta;\u003csub\u003eE\u003c/sub\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Eq. (1)\u003c/p\u003e\n\u003cp\u003ewhere F and \u0026delta; refer to the Mo flux (g m\u003csup\u003e-2\u003c/sup\u003e yr\u003csup\u003e-1\u003c/sup\u003e) and Mo isotope composition for each of sources or sinks involved, respectively. The model includes two primary sources of Mo to the oceans: riverine input (R) and upwelling hydrothermal fluxes (H). Three sinks involved are: (i) an oxic sink (O), comprising sediments enriched in Mn oxides deposited under oxygenated waters (dissolved O\u003csub\u003e2\u003c/sub\u003e \u0026gt; 10 \u0026micro;M); (ii) an euxinic sink (E), deposited beneath sulfidic bottom waters (dissolved H\u003csub\u003e2\u003c/sub\u003eS \u0026gt; 11 \u0026micro;M), where quantitative conversion of Mo to MoS\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e occurs; and (iii) an intermediate sink (RED), which encompasses suboxic, anoxic, and weakly euxinic depositional environments. The suboxic sink is associated with sediments deposited under water columns with low O\u003csub\u003e2\u003c/sub\u003e (\u0026lt;10 \u0026micro;M). The anoxic sink refers to sediments deposited under anoxic water columns lacking O\u003csub\u003e2\u003c/sub\u003e, with or without dissolved porewater H\u003csub\u003e2\u003c/sub\u003eS. The weakly euxinic sink is associated with sediments deposited beneath low-H\u003csub\u003e2\u003c/sub\u003eS (\u0026lt;11 \u0026micro;M) bottom waters.\u003c/p\u003e\n\u003cp\u003ePresently, rivers \u0026ndash; potentially including contributions from groundwater \u0026ndash; account for approximately 90% of Mo input to the modern oceans, while the remaining 10% derives from chemical exchange within low-temperature oceanic hydrothermal systems\u003csup\u003e52\u003c/sup\u003e. However, there is evidence suggesting that the Mo in these fluids (~0.8\u0026permil;) is sourced via reductive dissolution of Mo-bearing sediments overlying oceanic crust\u003csup\u003e53\u003c/sup\u003e. As such, these fluids more closely represent a failed Mo sink rather than a true long-term Mo source. High-temperature hydrothermal fluids were previously estimated to contribute only 1% of the modern riverine Mo flux\u003csup\u003e17\u003c/sup\u003e, but are now understood to act as a net sink owing to the low solubility of Mo sulfides under such conditions\u003csup\u003e17\u003c/sup\u003e. Although this high-temperature input may have been more significant in early Archean oceans\u003csup\u003e32,54\u003c/sup\u003e, its magnitude and isotopic composition are poorly constrained. We therefore excluded this term from our mass-balance equation, and simplify Eq. (1) to:\u003c/p\u003e\n\u003cp\u003eF\u003csub\u003eR\u003c/sub\u003e\u0026delta;\u003csub\u003eR\u003c/sub\u003e = F\u003csub\u003eO\u003c/sub\u003e\u0026delta;\u003csub\u003eO\u003c/sub\u003e + F\u003csub\u003eRED\u003c/sub\u003e\u0026delta;\u003csub\u003eRED\u003c/sub\u003e + F\u003csub\u003eE\u003c/sub\u003e\u0026delta;\u003csub\u003eE\u003c/sub\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Eq. (2)\u003c/p\u003e\n\u003cp\u003eDividing both sides of Eq. (2) by F\u003csub\u003eR\u003c/sub\u003e we obtain:\u003c/p\u003e\n\u003cp\u003e\u0026delta;\u003csub\u003eR\u003c/sub\u003e = f\u003csub\u003eO\u003c/sub\u003e\u0026delta;\u003csub\u003eO\u003c/sub\u003e + f\u003csub\u003eRED\u003c/sub\u003e\u0026delta;\u003csub\u003eRED\u003c/sub\u003e + f\u003csub\u003eE\u003c/sub\u003e\u0026delta;\u003csub\u003eE\u003c/sub\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Eq. (3)\u003c/p\u003e\n\u003cp\u003ewhere f\u003csub\u003eO\u003c/sub\u003e = F\u003csub\u003eO\u003c/sub\u003e/F\u003csub\u003eR\u003c/sub\u003e, f\u003csub\u003eRED\u003c/sub\u003e = F\u003csub\u003eRED\u003c/sub\u003e/F\u003csub\u003eR\u003c/sub\u003e, and f\u003csub\u003eE\u003c/sub\u003e = F\u003csub\u003eE\u003c/sub\u003e/F\u003csub\u003eR\u003c/sub\u003e. If we assume there are no other sinks, i.e.,\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ef\u003csub\u003eO\u003c/sub\u003e + f\u003csub\u003eRED\u003c/sub\u003e + f\u003csub\u003eE\u003c/sub\u003e =1 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Eq. (4)\u003c/p\u003e\n\u003cp\u003eNote that the Mo isotope composition of each sedimentary environment is additionally constrained by the respective isotope fractionation factors relative to seawater (SW) inferred from field observations and laboratory experiments (i.e., \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-O\u003c/sub\u003e, \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-RED\u003c/sub\u003e, and \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-E\u003c/sub\u003e). Hence, we can obtain an expression for the marine\u0026nbsp;\u0026delta;\u003csup\u003e98\u003c/sup\u003eMo value associated with inputs and outputs:\u003c/p\u003e\n\u003cp\u003e\u0026delta;\u003csub\u003eSW\u003c/sub\u003e = \u0026delta;\u003csub\u003eR\u003c/sub\u003e + \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-E\u003c/sub\u003e \u0026times; f\u003csub\u003eE\u003c/sub\u003e + \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-O\u003c/sub\u003e \u0026times; f\u003csub\u003eO\u003c/sub\u003e + \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-RED\u003c/sub\u003e \u0026times; f\u003csub\u003eRED\u003c/sub\u003e\u0026nbsp; \u0026nbsp; Eq. (5)\u003c/p\u003e\n\u003cp\u003eThis equation indicates that high marine \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values require a higher riverine Mo isotopic composition, and/or relatively larger Mo sinks.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eModern rivers are characterized by a wide range of \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo value, from -0.1\u0026permil; to +2.3\u0026permil;\u003csup\u003e55-56\u003c/sup\u003e. With the inclusion of a direct groundwater contribution to the Mo flux to the oceans, the average riverine \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo input to the ocean is 0.55\u0026permil;\u003csup\u003e52\u003c/sup\u003e. These isotopically heavy values are interpreted to result from Mo isotope fractionation during oxidative weathering, in which light Mo isotopes are preferentially retained in soils by adsorption onto Mn-Fe oxides and clay minerals\u003csup\u003e55\u003c/sup\u003e. However, this mechanism would have limited long-term impact if weathering ultimately releases all soil-bound Mo to rivers on timescales shorter than the oceanic Mo residence time\u003csup\u003e57\u003c/sup\u003e. In contrast, Archean terrestrial weathering conditions were fundamentally different, characterized by very low atmospheric O\u003csub\u003e2\u003c/sub\u003e levels and the absence of significant plant activity\u003csup\u003e58\u003c/sup\u003e. As a result, soil horizons would have been much thinner and surface oxide minerals sparse, rendering Archean oxidative weathering processes \u0026ndash; and their associated isotopic fractionations \u0026ndash; significantly less than today. Consequently, we interpret the modern average \u0026delta;\u003csub\u003eR\u003c/sub\u003e of 0.55\u0026permil; as a conservative upper limit for Mo inputs to the late Archean ocean. This precludes a sufficiently heavy source to explain the observed late Archean seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values and instead necessitates a greater sink for isotopically light Mo to satisfy mass-balance constraints.\u003c/p\u003e\n\u003cp\u003eWell-oxygenated settings produce the largest local isotopic offset between seawater and sediments (\u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-O\u003c/sub\u003e of ~3\u0026permil;), primarily due to Mo adsorption onto Mn oxides\u003csup\u003e19,21\u003c/sup\u003e. This equilibrium fractionation is largely insensitive to variations in temperature and other environmental parameters over geological timescales\u003csup\u003e59\u003c/sup\u003e. In contrast, under strongly euxinic settings, nearly quantitative Mo removal from bottom waters enables preservation of seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo in the sediments (\u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-E\u003c/sub\u003e of -0.3\u0026permil; to 0\u0026permil;)\u003csup\u003e60-61\u003c/sup\u003e. Given that weakly and strongly euxinic settings have Mo burial fluxes that are significantly higher than in non-euxinic settings\u003csup\u003e47,62\u003c/sup\u003e, a compromise is to assign a small isotope fractionation \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-E\u003c/sub\u003e of ~0.5\u0026permil; to encompass both settings\u003csup\u003e60\u003c/sup\u003e. The isotopic offset for the intermediate sink (\u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-RED\u003c/sub\u003e), which spans suboxic to anoxic redox regimes, is less well defined. However, an average Mo isotope fractionation of ~0.7\u0026permil; is typically chosen to represent this sink, consistent with the Mo isotope offset between overlying seawater and deposition beneath low-O\u003csub\u003e2\u003c/sub\u003e to anoxic bottom waters on continental margins\u003csup\u003e63-64\u003c/sup\u003e. Accordingly, assuming a global late Archean seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo value of 1.6\u0026permil;, the average \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values of sediments associated with oxic, RED, and euxinic sinks would be approximately -1.4\u0026permil;, 0.9\u0026permil;, and 1.1\u0026permil;, respectively.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn addition to considering the burial fluxes (F) for each of the three redox-defined settings, we can also explore their areal extent. These fluxes scale with the size of the global oceanic Mo reservoir and thus can be expressed as:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eF = F\u003csub\u003e0\u003c/sub\u003e \u0026times; R/R\u003csub\u003e0\u003c/sub\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Eq. (6)\u003c/p\u003e\n\u003cp\u003ewhere R denotes the size of the global oceanic Mo reservoir and the subscript 0 denotes the modern value. Therefore, each f term in the Mo isotope mass balance equation can be replaced by:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ef = [(F\u003csub\u003e0\u003c/sub\u003e \u0026times; R/R\u003csub\u003e0\u003c/sub\u003e) \u0026times; (A\u003csub\u003etotal\u003c/sub\u003e \u0026times; f\u003csub\u003eA\u003c/sub\u003e)]/F\u003csub\u003eR\u003c/sub\u003e\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Eq. (7)\u003c/p\u003e\n\u003cp\u003ewhere f\u003csub\u003eA\u003c/sub\u003e denotes the fraction of seafloor represented by the sink and A\u003csub\u003etotal\u003c/sub\u003e denotes the total seafloor area covered by the three sinks. In this way, the global seawater \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values can be modelled as a function of the areal extent of each sink (A\u003csub\u003eO\u003c/sub\u003e, A\u003csub\u003eRED\u003c/sub\u003e, and A\u003csub\u003eE\u003c/sub\u003e) through incorporating modern Mo accumulation parameters (Table S3).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOur modelling results show that high marine \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values, such as those inferred for the late Archean ocean (~1.6\u0026permil;), can only be achieved if the ocean floor was overwhelmingly dominated by oxic conditions, with the oxic area (A\u003csub\u003eO\u003c/sub\u003e) comprising between 92% and 97% of the global seafloor (Fig. 6a). However, such extensive oxygenation seems unrealistic for the late Archean, given the scarcity of Mn-rich sediments from this time\u003csup\u003e65\u003c/sup\u003e. This inference is further supported by the absence of correlation between \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values and Mn/Fe ratios in our BIF samples (Fig. 4e-f) and by invariant, near-crustal thallium isotope compositions observed in contemporaneous shales from western Australia and South Africa\u003csup\u003e8,66\u003c/sup\u003e. Additional constraints from Fe speciation analyses in these shales\u003csup\u003e4,12\u003c/sup\u003e points to a Neoarchean ocean dominated by ferruginous, rather than widespread oxic or euxinic conditions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eUnlike the relatively localized sedimentary record from western Australia and South Africa\u003csup\u003e7\u003c/sup\u003e, BIFs were deposited globally during this interval, including China, Canada, and India\u003csup\u003e29-30,67\u003c/sup\u003e. Crucially, the precipitation of BIFs does not require free O\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e30-32,67\u003c/sup\u003e, and if O\u003csub\u003e2\u003c/sub\u003e is involved, this would exist only in surface waters where Fe(II) oxidation occurred. Hence, if O\u003csub\u003e2\u003c/sub\u003e was absent or restricted to marine shelf environments, the Neoarchean marine Mo isotope signatures may instead have been significantly shaped by fractionations during Mo adsorption to Fe oxides. Interestingly, this correlates well with a recent study on nitrogen isotopes in Neoarchean BIFs that suggested declining seawater oxygenation and increased denitrification between 2.60 and 2.48 Ga\u003csup\u003e9\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eTo test this possibility, the isotopic fractionation assigned to the RED sink in the Neoarchean ocean needs to be adjusted to reflect conditions more consistent with Mo adsorption onto ferrihydrite (\u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-Fer\u003c/sub\u003e of 1.1\u0026permil;)\u003csup\u003e22\u003c/sup\u003e. Ferrihydrite is widely considered the primary Fe(III) mineral phase of BIFs\u003csup\u003e30\u003c/sup\u003e and has been shown to adsorb Mo approximately 30 times more efficiently than hematite on a per-unit basis\u003csup\u003e22\u003c/sup\u003e. A comparison of model outputs using two different RED fractionation factors (Fig. 6) reveals an important distinction: with \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-RED\u003c/sub\u003e of 0.7\u0026permil;, the maximum \u0026delta;\u003csub\u003eSW\u003c/sub\u003e value in the absence of a significant oxic sink is approximately 1.25\u0026permil;, whereas using the adjusted \u0026Delta;\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eSW-RED\u003c/sub\u003e of 1.1\u0026permil; increases the maximum to about 1.65\u0026permil;, closely matching the late Archean seawater value. Moreover, when A\u003csub\u003eO\u003c/sub\u003e is below 97% of the seafloor, \u0026delta;\u003csub\u003eSW\u003c/sub\u003e is almost entirely controlled by the extent of the RED sink, as evidenced by the alignment of lines of constant \u0026delta;\u003csub\u003eSW\u003c/sub\u003e with those of constant A\u003csub\u003eE\u003c/sub\u003e/A\u003csub\u003eRED\u003c/sub\u003e ratios (Fig. 6b). Although the oxic sink results in a considerable fractionation from seawater, the burial rate in this setting is two orders of magnitude lower than in RED settings\u003csup\u003e47,62\u003c/sup\u003e. On the other end of the redox spectrum, euxinic environments also efficiently scavenge Mo from seawater but tend to do so with minimal isotopic offset due to near-quantitative Mo removal\u003csup\u003e62\u003c/sup\u003e. Therefore, the dominant control on \u0026delta;\u003csub\u003eSW\u003c/sub\u003e under ferruginous conditions is the extent of RED sinks, unless the deep ocean is fully oxygenated as it is today.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOur modelling outcome underscores that elevated \u0026delta;\u003csup\u003e98\u003c/sup\u003eMo values are consistent with restricted O\u003csub\u003e2\u003c/sub\u003e accumulations in the Neoarchean and should be interpreted in light of the complex interplay between multiple Mo sinks with differing fractionation behaviors. Accordingly, it is more likely that adsorption of isotopically light Mo to abundant BIFs deposited via photoferrotrophy or possibly low levels of O\u003csub\u003e2\u003c/sub\u003e in surface waters contributed greatly to the late Archean marine heavy Mo reservoir, which, by extension, implies pervasively ferruginous water columns, at least on the outer shelf (Fig. 7). This interpretation is consistent with fluctuating thallium and nitrogen isotope compositions of Neoarchean shales\u003csup\u003e8\u003c/sup\u003e and BIFs\u003csup\u003e9\u003c/sup\u003e, collectively pointing towards oscillating seawater oxygenation.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eOur new geochemical data and model constraints suggest that elevated Neoarchean marine δ\u003csup\u003e98\u003c/sup\u003eMo value may represent an increased contribution from the intermediate redox sink (i.e., BIF deposition), consistent with the ubiquitous occurrence of BIFs in the Neoarchean sedimentary record\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e,\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e. Although oxic sinks may have existed on the continental shelf and contributed to Mo isotopic fractionation, they are not strictly required to account for the observed seawater signal. An abundant BIF sink can also explain the co-occurrence of positive Mo isotope signatures and negative Fe isotopes at multiple stratigraphic levels in roughly contemporaneous shales and carbonates from South African and western Australia\u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e,\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e–\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e\u003c/sup\u003e. Importantly, while our findings do not preclude the existence of localized oxygenated marine shelves, they highlight that widespread anoxic deep ocean conditions dominated in the immediate lead up to the GOE. These conditions may have significantly impacted the cycling of redox-sensitive elements such as Mo in the Neoarchean ocean. The shale and carbonate record of Mo isotopes may thus be considered as an indirect record of extensive phototrophic Fe(II) oxidation in offshore environments.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cb\u003eTotal organic carbon and total sulfur contents\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe TOC and TS contents were measured using a LECO CS844 infrared carbon-sulfur analyzer at ALS Chemex (Guangzhou, China). For each sample, approximately 80–120 mg of dried rock powders were first leached using 2 M HCl to remove possible inorganic carbon. The residual powder was washed using Milli-Q water and then dried at 80℃ before instrumental analysis. Subsequently, these samples were loaded into tin capsules, and the carbonate content was determined by the mass percentage loss during decalcification. The analytical precision of TOC concentrations was better than ± 0.02 wt%.\u003c/p\u003e\u003cp\u003eFor bulk-rock TS analysis, a sodium carbonate leach was used to remove sulfate, and the insoluble sulfide residue was filtered and analyzed. Multiple analyses of certified reference materials GGC-12, GS310-8, GS913-3, and OREAS 504 present analytical precisions better than ± 0.02 wt%.\u003c/p\u003e\u003cp\u003e\u003cb\u003eMo isotopes\u003c/b\u003e\u003c/p\u003e\u003cp\u003eApproximately 50 mg of sample powder was dissolved in Teflon beakers using concentrated HCl, HF, and HNO\u003csub\u003e3\u003c/sub\u003e heated at ~ 200℃ for 48 h, dried down, and subsequently dissolved and dried down again using 0.5 mL 10 M H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e heated at ~ 60℃ for 1 h to digest residual black materials (e.g., organic matter). Bulk-rock Mo abundances and isotopic compositions of these BIF samples were determined using the double-spike method. The chemical purification of Mo was performed using the ion-exchange chromatography protocol described in Li et al.\u003csup\u003e\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e at the Guizhou Tongwei Analytical Technology Co., ltd. (China). The Mo isotope ratios were analyzed on a multi-collector inductively coupled plasma-mass spectrometer (MC–ICP–MS; Thermo-Fisher Scientific Neptune Plus) at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences. The Mo isotopic composition was reported relative to the NIST SRM 3134 standard and were then set to + 0.25‰ for ease of comparison with previously published data: δ\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eNIST+0.25\u003c/sub\u003e = [(\u003csup\u003e98\u003c/sup\u003eMo/\u003csup\u003e95\u003c/sup\u003eMo)\u003csub\u003esample\u003c/sub\u003e/(\u003csup\u003e98\u003c/sup\u003eMo/\u003csup\u003e95\u003c/sup\u003eMo)\u003csub\u003eNIST3134\u003c/sub\u003e – 1] × 1000 + 0.25\u003csup\u003e71\u003c/sup\u003e. External reproducibility of the NIST SRM 3134 standard was better than ± 0.05‰ (2SD, n = 54). The geochemical reference materials NOD-A-1 (Mn nodule), IAPSO (seawater), and SGR-1b (shale) were analyzed alongside samples to monitor accuracy, yielding δ\u003csup\u003e98\u003c/sup\u003eMo\u003csub\u003eNIST+0.25\u003c/sub\u003e values of − 0.76 ± 0.05‰ (2SD, n = 2), 2.30 ± 0.06‰ (2SD, n = 6), and 0.36 ± 0.05‰ (2SE, n = 1), respectively. These results are in excellent agreement with previously reported values\u003csup\u003e\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgement:\u003c/h2\u003e\u003cp\u003eThis work was supported by grants from the National Key R\u0026amp;D Program of China (2024YFF0810200), the Strategy Priority Research Program (Category B) of Chinese Academy of Sciences (XDB0710000), and the National Natural Science Foundation of China (42325206, 42150104). EES acknowledges funding from a NERC Frontiers grant (NE/V010824/1).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHolland HD (2006) The oxygenation of the atmosphere and oceans. 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Geostand Geoanal Res 38(2):149\u0026ndash;151\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7181523/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7181523/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eProgressively heavier molybdenum isotope compositions (δ\u003csup\u003e98\u003c/sup\u003eMo) in Neoarchean marine sediments have been interpreted as evidence for widespread surface oxygenation prior to the Great Oxidation Event. Here, we assess whether the deposition of banded iron formations (BIFs) — iron-rich sedimentary rocks formed predominantly in the Neoarchean — can account for this isotopic signal through processes operating under pervasively anoxic conditions. BIF samples analyzed here possess a wide range of δ\u003csup\u003e98\u003c/sup\u003eMo values, which are attributed to the combined effects of Mo adsorption onto primary ferric iron (Fe) oxyhydroxides and subsequent diagenetic incorporation into Fe-Mo-sulfides. Given the isotopic fractionation associated with Mo adsorption, we estimate that Neoarchean seawater δ\u003csup\u003e98\u003c/sup\u003eMo ranged from 1.5‰ and 1.6‰, and was more stable than previously suggested. If Mo in Neoarchean rivers had an average isotopic composition like today, a mass balance model predicts that only a modest manganese oxide sink is required to generate these heavy δ\u003csup\u003e98\u003c/sup\u003eMo values. Instead, the dominant control may have been the removal of isotopically light Mo via adsorption to abundant ferric oxyhydroxide particles setting through a ferruginous water column. These findings imply that the Neoarchean Mo isotope record may track extensive photosynthetic iron oxidation rather than pervasive oxygen accumulation in the surface ocean.\u003c/p\u003e","manuscriptTitle":"A heavy molybdenum reservoir in Neoarchean seawater tracks extensive iron oxidation","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-24 09:58:35","doi":"10.21203/rs.3.rs-7181523/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2d0c08a5-cd7b-4dff-97b8-f69532055728","owner":[],"postedDate":"July 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":52005823,"name":"Earth and environmental sciences/Biogeochemistry/Element cycles"},{"id":52005824,"name":"Earth and environmental sciences/Ocean sciences/Marine chemistry"}],"tags":[],"updatedAt":"2026-01-28T10:31:27+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-24 09:58:35","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7181523","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7181523","identity":"rs-7181523","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}