A cadmium-based quantification of marine organic carbon burial during the PETM

A cadmium-based quantification of marine organic carbon burial during the PETM | 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 cadmium-based quantification of marine organic carbon burial during the PETM Hannah Elms, Ross Whiteford, James Brakeley, Sev Kender, Ying Cui, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6680768/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 The Paleocene-Eocene Thermal Maximum (PETM, ~ 56 Ma) was driven by rapid, large-scale carbon release into the oceans and atmosphere. Environmental recovery post-PETM must have been associated with climate feedback mechanisms to remove carbon from Earth’s surface reservoirs, including silicate weathering and organic carbon burial. However, the amount of carbon removed by organic matter burial has been difficult to quantify due to ambiguity in many carbon-cycle proxies, thus limiting accurate paleoclimate modelling. We have measured the isotopic composition of cadmium (δ 114 Cd), a novel proxy for organic carbon burial, in sedimentary rocks deposited across the PETM in four separate marine basins. A ~ 0.2‰ positive excursion in δ 114 Cd sw is interpreted to reflect a global-scale increase in organic cadmium burial in marine sediments. We simulate the global cadmium cycle using a box model to quantify the marine organic carbon burial flux required to drive this isotopic shift. Our median estimate of ~ 40,000 ± 15,500 Pg C total excess organic carbon burial across the PETM suggests that organic carbon burial was highly important in balancing the PETM carbon cycle budget. Earth and environmental sciences/Climate sciences/Ocean sciences/Marine chemistry Physical sciences/Chemistry/Environmental chemistry/Geochemistry Earth and environmental sciences/Environmental sciences/Environmental chemistry/Marine chemistry Earth and environmental sciences/Ocean sciences/Marine chemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction The Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) was a period of rapid global warming associated with the release of thousands of petagrams of isotopically light carbon into the Earth’s atmosphere and oceans. However, the magnitude of marine organic carbon (C org ) burial following carbon release is not well constrained, despite its importance as a CO 2 feedback mechanism 1–3 . Evidence of short-term imbalances in the geological C cycle is preserved in the C isotope (δ 13 C) signatures of organic matter and carbonates 4 . Previous studies have modelled the rate of change of δ 13 C records 2 , extrapolated C org content in marine rocks 5 , or modelled coupled C org -marine nutrient cycles 6 to estimate anywhere between 2,000 Pg and 10,300 Pg of C org burial over the PETM. Better constraint on this range is limited, however, because δ 13 C records are a synthesis of multiple co-occurring feedbacks, making it difficult to attribute specific processes 7,8 . More direct proxies that may circumvent this issue are lacking, which is problematic for fully understanding climate dynamics because marine C org burial is an important feedback mechanism for removing carbon from the ocean-atmosphere system. The stable cadmium (Cd) isotope system, recently utilised in paleo-productivity studies 9–12 , has strong potential as an independent C org burial proxy. Seawater Cd systematics are controlled by input of Cd to the ocean (almost wholly riverine), and a series of sedimentary burial fluxes: oxyhydroxides, clays, carbonates, sulfides, and organic matter (Figure 1), the balance of which ultimately determines the isotopic composition of Cd dissolved in the ocean. Previously published mass balance models for oceanic Cd are defined by Cd burial by oceanic regions - such as continental margins 13–16 , by specific oceanic basins or transects therein 17 , or by the type of bulk sedimentary deposit, which may include several isotopically-distinct sedimentary phases. It is thus difficult to utilise existing calculations to achieve a geochemically plausible mass balance when considering the entire global marine system. We have instead calculated cadmium burial fluxes using an isotopic mass balance approach, assuming oceanic steady-state with a balance between riverine input and sedimentary output fluxes, and utilising known isotopic compositions of river waters 18 and sedimentary phases 13,14,19–24 . Our best estimate of the modern-day steady state is depicted in Figure 2 (calculations for which can be found in the Methods section). Together, incorporation into organic matter and precipitation of authigenic CdS likely account for approximately 95% of seawater cadmium burial, and both are capable of changing the cadmium isotope ratio of seawater (δ 114 Cd sw ). In contrast, the oxyhydroxide and clay burial fluxes are both small and, over geological timescales, assumed to remove cadmium in isotopic equilibrium with seawater, so do not affect δ 114 Cd sw . Cadmium adsorbed or substituted onto/into carbonates or FeMn oxyhydroxides is fractionated 20,21,23,25 , but again the Cd flux is small (~2% of the total Cd burial flux 21 ), so this has only a small impact on δ 114 Cd sw . Of the five burial pathways, only two currently have the potential to drive significant changes in δ 114 Cd sw : burial of Cd associated with C org , and precipitation of authigenic CdS (Figures 1, 2). The Cd isotope composition of authigenic CdS depends on whether Cd is completely precipitated from precursor fluid during its formation. If authigenic CdS precipitation does not completely deplete seawater Cd, it will be isotopically light (driving δ 114 Cd sw to become heavier), but if precipitation does remove all the available dissolved Cd, then the authigenic CdS will have the same isotopic signature as seawater. Cd has been shown to be highly reactive in the presence of trace levels of HS - 26,29 , and thus under sulfidic depositional conditions CdS precipitation will completely fix dissolved Cd into sediments with minimal isotopic fractionation 30 . While we assume no isotopic fractionation between sediments and seawater in the euxinic depositional settings used in this study, the extent to which incomplete Cd burial into authigenic CdS contributes to the global isotope mass balance is not well known, nor is its potential variability through time (Figure 1). Our isotope-constrained mass balance indicates that Cd buried in association with organic matter accounts for roughly 20% of modern steady state Cd burial. Organic matter has a lighter Cd isotope composition than modern deep-ocean seawater (-0.15‰ to -0.8‰ offset 10,15,31 ), so increases in C org burial may be postulated to drive seawater cadmium towards heavier values. All else being equal, driving the same perturbation with less isotopically offset cadmium would require greater carbon burial – so we use the larger value of -0.8‰, to ensure our estimates are conservative. We leverage the two main sinks of Cd together: changes in C org burial as the driver of changes in δ 114 Cd sw , and authigenic sulfides as an archive of δ 114 Cd sw under euxinic conditions 30 . We measure bulk Cd isotopes in four sedimentary successions that span the PETM interval, then combine these data with numerical modelling to quantify the magnitude of C org burial. The four shale sections used for this study have all been previously studied and are well-characterised for their geochemical and paleogeographical context 32–37 . The sections also span variable distances from the North Atlantic Igneous Province, proposed as the main trigger for the PETM 36,38,39 , and originate in basins with differing levels of hydrographic restriction from the open ocean (Figures 3, 4). Three sections (Svalbard, North Sea, and Arctic) were deposited in euxinic depositional conditions with abundant sulfide, while one section (Kheu River) was deposited at a few hundred metres depth in an open-ocean setting with episodic euxinia during the early part of the PETM 33,35,42 . The different depositional settings provide a useful framework to identify common trends in the cadmium isotope data that carry supra-regional significance. Carbon-isotope records were used to align each of the four sections, with the event itself divided into pre-PETM, onset, core, and recovery segments based on inflections in the δ 13 C curves and changes in TOC content (Figure 5a-b). A chronology was transferred to the aligned records using the age model of Charles et al., 2011 43 . Results Sedimentary cadmium isotope records Cadmium concentrations (Fig. 5c) vary between the four sections, with overall ranges of 0.3–0.6 µg g − 1 (Svalbard), 0.3–3 µg g − 1 (Arctic), 0.3–13 µg g − 1 (North Sea) and 0.01–475 µg g − 1 (Kheu River). Total Organic Carbon content is high in three sections (1–3% Arctic, 0–7% North Sea, 0–9% Kheu River), but lower in Svalbard (1–2%). The low TOC content in Svalbard is likely due to post-depositional maturation and/or high level of sediment dilution; however, previous studies show that thermal maturity has no effect on the isotopic compositions of Cd and other trace metals 46 , 47 . Therefore, even if the concentrations are affected by burial maturation, the isotopic data on which our estimates of organic carbon burial flux are based (Fig. 5d) remain robust. All four sites exhibit a positive Cd isotope excursion of similar magnitude despite the order of magnitude differences in Cd concentrations (Fig. 5d). Burial phase differences at the local scale superimpose noise onto the global seawater signal. The heaviest δ 114 Cd shale values coincide with the onset or ‘peak’ of the PETM. Pre-onset values hover around − 0.1‰ and then increase by 0.05–0.2‰ by the core of the event. Peak values are even higher, at 0.6‰ (Kheu River), 0.5‰ (Svalbard), 0.4‰ (North Sea), and 0.2‰ (Arctic). During the recovery phase, the Cd isotope compositions decrease back to -0.2 to -0.1‰ (Fig. 5d). Quantifying C burial using cadmium isotope modelling Our δ 114 Cd sw record (Fig. 5d) indicates a pulse of additional marine C org burial across the PETM (Fig. 6 ). To quantify that flux, we first extract the global signal from our four sites using a Gaussian Process (Fig. 8). We then combine this extracted global trend in δ 114 Cd sw with an isotope mass balance model of the seawater cadmium system (CdBURY). We use an inverse strategy, which allows us to calculate the amount of marine organic carbon burial required to drive the observed change in δ 114 Cd sw . We calculate a scaling to convert the predicted fluxes of Cd to C org , using our modern day estimate of organic bound Cd burial flux (5,287,500 mol yr − 1 ) and marine C org burial flux (0.1 Pg C yr − ). This allows us to estimate the background organic carbon burial pre-PETM to be between 0 and 0.5 Pg C yr − 1 (median 0.2 Pg C yr − 1 ), and peak organic carbon burial to be between 0.3 and 1.5 Pg C yr − 1 (median 0.8 Pg C yr − 1 ). We estimate the cumulative magnitude of additional C org burial over the PETM to be between 15,225 Pg C and 73,600 Pg C at 95% confidence, with a median and most likely estimate of ~ 40,000 Pg C (see Methods). Discussion Our median estimate of ~ 40,000 Pg C is notably higher than previous estimates of organic carbon burial, which range from 2,000–10,300 Pg C 2 , 5 , 6 . However, it is reconcilable with estimates of PETM carbon emissions. Estimates of potential carbon emissions directly from the NAIP are generally in the 10,000’s Pg C range (18,000–60,000 Pg C 38 , ~ 27,000 Pg C 48 , 40,000 Pg C 49 ), with additional indirect carbon sources contributing several thousand Pg C more (3000–15,000 Pg C from magmatic interaction with sediments 38 and 100–10,000 Pg C from oxidative weathering 50 ). In contrast, the emissions required to drive measured trends in δ 13 C, pH, and CO 2 , are only between 3000 and 12,000 Pg C 38 , 51 . However, these estimates typically assume constant (or near constant) organic burial. Our estimate of excess marine organic carbon burial across the PETM (15,525 Pg C − 73,600 Pg C) could reconcile the large carbon source that NAIP could provide with the smaller carbon excess required to drive the surficial carbon cycle changes. Our median estimate of ~ 40,000 Pg of additional C burial suggests that organic carbon burial was a significant carbon sink across the PETM. This estimate is given in terms of excess carbon burial, which is inherently dependent on our quantification of background organic carbon burial. If background organic carbon burial was higher than our calculated ~ 0.2 Pg C yr − 1 , the excess carbon burial estimate would decrease without affecting the total carbon burial we reconstruct. A feature of our simulated C org burial curve is that maximum burial fluxes occur early during the PETM. The timing of C org burial probably records a fast response of the oceanic biosphere to changing nutrient conditions delivered from rapid continental weathering or oceanic recycling 32 , 33 , 40 . It is consistent with pulses of continental weathering recorded by both lithium and osmium records early in the PETM 32 , 52 , 53 , and with barite flux estimates of the timing of peak export productivity during the PETM 54 , 55 . Ma et al. 54 estimated that about 2000 Pg C could have entered the refractory oceanic carbon pool as a result of biological C export, from a total export of ~ 500,000 Pg C. There is ample evidence for expanded low oxygen conditions during the PETM 35 , 40 , 45 , 56 , 57 , particularly in marginal ocean basins and oxygen minimum zones that could have increased the burial efficiency of exported organic C, making the estimate of Ma et al. a minimum value and consistent with the Cd-based estimates presented here. The modern day marine organic C burial rate is ~ 0.1 Pg C yr − 1 , and we estimate background organic C burial pre-PETM was between 0.1 and 0.5 Pg C yr − 1 , and peak marine organic C burial during the PETM was between 0.3 and 1.5 Pg C yr − 1 (Fig. 6 ). The peak emission rate across the PETM is between 0.3 and 1.1 Pg C yr − 58 – the highest known rate since the Mesozoic (and, as this is an average rate, quite possible higher on shorter timescales), but nonetheless dwarfed by anthropogenic carbon release which is up to 10 Pg C yr − 58 . Our results suggest that burial of additional marine organic carbon during climate perturbations could act as a stabilising climate feedback, with burial rate potentially scaling by an order of magnitude relative to its modern-day level. However, even if modern organic carbon burial were to increase by an order or magnitude, it would bury only a tenth of current emissions each year. Methods Geochemical methods Organic-rich, sulfidic marine shale samples (~ 200 mg powder) spanning each studied record were oxidised and then fully digested following standard inverse aqua regia-HF:HNO 3 digestion techniques 19 in the metal-free, ultra-clean geochemical laboratory facilities at Royal Holloway, University of London, using clean PFA vials. Concentration analyses were obtained via ICP-MS or (post-July 2024) ICP-OES, calibrated using in-house multi-element standards. Digest aliquots were then precisely weighed to capture 50 ng of natural cadmium, and homogenised with a 111 Cd- 113 Cd double-spike to obtain a double-spike sample ratio of ~ 1.5, before drying and taking up in 1ml 1M HF:0.5M HCl. Cadmium was isolated via a two-column procedure using AG1-X8 anion exchange resin followed by TRU-spec extraction chromatography resin following Bryan, 2018 59 . Samples were centrifuged prior to loading onto the columns to remove any fluoride precipitates. Purified cadmium isolates were then dried down and taken up in sufficient 3% (~ 0.5M) HNO 3 to dilute to 10 ppb and analysed on a Thermo Neptune MC-ICP-MS in dry plasma mode, using an Aridus III desolvating nebuliser, at Royal Holloway University of London. Data were deconvolved offline and final isotopic ratios reported relative to NIST 3108, run at the same concentration and sample-spike ratio as the samples. OXCAD was used as a secondary isotope standard, and all sample batches included an aliquot of USGS SGR-1b (δ 114 Cd = -0.85 and 0.062 relative to NIST3108, respectively 60 , 61 ), digested and analysed as a total procedural rock standard. Our measured values for OXCAD were 0.86 ± 0.06 (2sd, n = 20) and SGR-1b were 0.08 ± 0.03 (2sd, n = 11). Blank contributions were low-pg level, comprising < 1% of the cadmium content. Detrital Cd contributions to each sample, assessed using Cd/Al ratios obtained from the ICP-OES measurements, were similarly negligibly low enough to make no difference to the final δ 114 Cd compositions. Cadmium isotope system modelling As described in the introduction, the cadmium system in seawater is a balance of the input flux (predominantly riverine) and five output fluxes (Fig. 2). However, only two of the output fluxes are substantial enough to drive large scale shifts in the cadmium cycle. The clay flux is small and assumed to be isotopically in equilibrium with seawater (so unable to have significant influence). The carbonate and oxyhydroxide fluxes are potentially isotopically offset from seawater, but small in magnitude (so unable to have significant influence). The sulfide flux is large, but does not isotopically fractionate Cd where precipitation of CdS from a precursor fluid is ‘complete,’ as would typically occur in sulfidic depositional settings. Organic carbon is a relatively large flux and has a large isotopic offset from seawater. Cd has a nutrient-like profile in the modern ocean, so the shallow seawater exchanged into marginal basins may carry a Cd isotope signature that is offset from the global deep-ocean average. However, if the vertical profile in seawater Cd remained constant through the PETM, δ 114 Cd shale records will still faithfully record the relative change in δ 114 Cd sw, and it is the relative change in δ 114 Cd sw which drives our record of organic carbon burial. Also of note is that global mass balance requires seawater δ 114 Cd (recorded by ~ δ 114 Cd shale ) to be equal to or heavier than the input fluxes to the ocean. The background value of our estimated global trend in δ 114 Cd shale (Fig. 5d) is ~ -0.2‰, implying an even lighter input flux. However, the bulk silicate Earth has a Cd isotope ratio of ~ 0‰ and measured riverine values are in the range of 0.1–0.3‰ 18 . We suggest that there is likely to be a systematic offset between the sediments and basin seawater. This offset does not, however, alter the relative shape or magnitude of temporal change in δ 114 Cd shale, and therefore doesn’t affect the reconstructed trend in marine organic carbon burial that is based on these relative changes. We extract the global trend in δ 114 Cd sw from our δ 114 Cd shale data using a Gaussian Process (detailed below), which smooths the data on a 50kyr timescale – chosen as a balance between smoothing and retaining detail in our record. From the Gaussian Process we draw 10,000 possible δ 114 Cd sw curves which are consistent with our measured δ 114 Cd and smoothing timescale. To turn our smoothed δ 114 Cd sw curves into estimates of organic burial, we simulate the fluxes of Cd into and out of the ocean (Fig. 2) using a newly configured box model (CdBURY). We invert this model, such that it varies the organic bound Cd flux to drive the estimated change in δ 114 Cd sw . All other output fluxes are assumed to be linearly related to seawater Cd concentrations, and the input flux is assumed to remain constant (which is conservative in terms of reconstructed organic carbon burial). We propagate uncertainty in the initial conditions ([Cd] sw : 0.3x to 3x modern, [Cd] rivers : 0.3x to 3x modern, and δ 114 Cd rivers : -0.4‰ to -0.1‰). We rule out simulations in which δ 114 Cd sw changes too quickly to be compatible with the dynamics of the oceanic Cd system, leaving us with a posterior dataset of many potential curves of organic bound Cd burial which could have driven our estimated δ 114 Cd sw . CdBURY CdBURY is a single box model which simulates the fluxes of cadmium into and out of the ocean (as depicted in Figs. 1 , 2). It predicts the mass of cadmium in seawater and its isotopic composition. The mass balance equation is: $$\:\frac{dCd}{dt}={Cd}_{input}-{Cd}_{oxyhydroxide}-{Cd}_{clay}-{Cd}_{carbonate}-{Cd}_{organic}-{Cd}_{sulfide}$$ Similarly for each isotope: \(\:\frac{d{}^{114}Cd}{dt}={}^{114}{Cd}_{input}-{}^{114}{Cd}_{oxyhydroxide}-{}^{114}{Cd}_{clay}-{}^{114}{Cd}_{carbonate}-{}^{114}{Cd}_{organic}-{}^{114}{Cd}_{sulfide}\) \(\:\frac{d{}^{110}Cd}{dt}={}^{110}{Cd}_{input}-{}^{110}{Cd}_{oxyhydroxide}-{}^{110}{Cd}_{clay}-{}^{110}{Cd}_{carbonate}-{}^{110}{Cd}_{organic}-{}^{110}{Cd}_{sulfide}\) Where Cd represents the mass of cadmium, and 114 and 110 are the two pertinent isotopes. CdBURY performs full isotope mass balance, accounting for each flux entering and leaving the ocean. Figure 7 shows a demonstration of asking CdBURY to either double marine carbon burial, or turn it off completely, and the resulting changes in the seawater cadmium system. Gaussian Process To combine the data from our four sites and extract a global trend in δ 114 Cd sw , we use a Gaussian Process to smooth our data (Figs. 6 , 8). We use a Radial Basis Function kernel, which encodes the belief that the δ 114 Cd sw is smooth on a prescribed timescale using two hyperparameters. We choose 50 kyr as the length scale parameter, which allows us to extract a global signal from multisite data by removing of the noise from local effects in our datasets. The noise scale parameter is set to 2.0‰, but this choice has minimal impact on our reconstruction because of the high data density. From this Gaussian Process we draw 10,000 curves for δ 114 Cd sw which are compatible with our dataset. CdBURY Inversion We ask the CdBURY model to recreate each of these 10,000 δ 114 Cd sw curves by changing organic carbon burial and having all other fluxes change only as a linear function of [Cd] sw . Calculated organic bound cadmium burial flux is driven by: the target δ 114 Cd sw evolution, the initial amount of cadmium in seawater, and the input flux of cadmium to seawater (including its isotopic composition). Uncertainties associated with each of these parameters are propagated through our model using a Monte Carlo approach. Samples for the initial amount of cadmium in seawater and the input flux of cadmium to seawater are randomly drawn, covering a range of 0.333x to 3x modern values, and the riverine cadmium isotope composition ranges from − 0.4 – -0.1‰. Having picked these random initial conditions, the model is rebalanced to steady state by adjusting the organic carbon and sulfide burial fluxes such that the input and output fluxes are balanced both in terms of mass and isotopes. From this starting steady state, CdBURY changes the organic carbon burial flux to drive the estimated trend in δ 114 Cd sw . The posterior set of initial conditions indicates that it is easier to drive the measured trend in δ 114 Cd sw when the input flux into the ocean is high relative to modern, the input isotopic composition is low (< 0.2‰), and the mass of cadmium in the ocean is low relative to modern. Carbon Burial Each of the 3737 posterior curves which pass all checks are converted into estimates of marine organic carbon burial. We may look at this estimate through several different lenses, as a time variable flux (Fig. 6 ), or as total excess carbon burial across the interval. Here we show a histogram (Fig. 10 ) of total excess carbon burial from all posterior samples. We may calculate this in two ways: a) assuming that [Cd] sw controls the magnitude of each flux (constant stoichiometry), or b) assuming that [Cd] sw controls the cadmium content of each flux (variable stoichiometry). The former is the more conservative assumption but is not realistic because [Cd] sw is not a major control on the fluxes sensu stricto . Instead, it is more realistic to imagine that changing [Cd] sw controls how much Cd is incorporated into each output flux. Variable stoichiometry calculations suggest greater carbon burial than constant stoichiometry calculations, but on the scale of the estimates the increase is minor. Both constant and variable stoichiometry estimates have peak likelihood of organic carbon burial of around 40,000 Pg C. Declarations Author Contributions AJD designed the study and assisted with laboratory analyses. HCE performed geochemical analyses and drafted the manuscript. RW developed the CdBURY model. YC and SK provided samples and contributed to interpretations. JB provided laboratory analyses and technical assistance. All authors contributed to data interpretation and manuscript editing. HCE and RW contributed equally to this study. 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Nat Lett 44 Charles AJ et al (2011) Constraints on the numerical age of the Paleocene-Eocene boundary. Geochem. Geophys. Geosystems 12, n/a-n/a Stein R, Boucsein B, Meyer H (2006) Anoxia and high primary production in the Paleogene central Arctic Ocean: First detailed records from Lomonosov Ridge. Geophys Res Lett 33 Dickson AJ, Cohen AS, Coe A (2012) L. Seawater oxygenation during the Paleocene-Eocene Thermal Maximum. Geology 40:639–642 Dickson AJ, Idiz E, Porcelli D, Van Den Boorn SH, J. (2020) M. The influence of thermal maturity on the stable isotope compositions and concentrations of molybdenum, zinc and cadmium in organic-rich marine mudrocks. Geochim Cosmochim Acta 287:205–220 Dickson AJ et al (2022) No effect of thermal maturity on the Mo, U, Cd, and Zn isotope compositions of Lower Jurassic organic-rich sediments. Geology 50:598–602 Eldholm O, Thomas E (1993) Environmental impact of volcanic margin formation. Earth Planet Sci Lett 117:319–329 Saunders AD (2016) Two LIPs and two Earth-system crises: the impact of the North Atlantic Igneous Province and the Siberian Traps on the Earth-surface carbon cycle. Geol Mag 153:201–222 Lyons SL et al (2019) Palaeocene–Eocene Thermal Maximum prolonged by fossil carbon oxidation. Nat Geosci 12:54–60 Zeebe RE, Zachos JC, Dickens GR (2009) Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming. Nat Geosci 2:576–580 Ravizza G, Norris RN, Blusztajn J, Aubry M-P (2001) An osmium isotope excursion associated with the Late Paleocene thermal maximum: Evidence of intensified chemical weathering. Paleoceanography 16:155–163 Von Pogge PAE et al (2021) Lithium isotope evidence for enhanced weathering and erosion during the Paleocene-Eocene Thermal Maximum. Sci Adv 7:eabh4224 Ma Z et al (2014) Carbon sequestration during the Palaeocene–Eocene Thermal Maximum by an efficient biological pump. Nat Geosci 7:382–388 Paytan A, Averyt K, Faul K, Gray E, Thomas E (2007) Barite accumulation, ocean productivity, and Sr/Ba in barite across the Paleocene–Eocene Thermal Maximum. Geology 35:1139 Clarkson MO et al (2021) Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes. Nat Commun 12:399 Zhou X, Thomas E, Rickaby REM, Winguth AME (2014) Lu, Z. I/Ca evidence for upper ocean deoxygenation during the PETM. Paleoceanography 29:964–975 Zeebe RE, Ridgwell A, Zachos JC (2016) Anthropogenic carbon release rate unprecedented during the past 66 million years. Nat Geosci 9:325–329 Bryan A (2018) Investigation of the controls on the cadmium isotope composition of modern marine sediments. Thesis Abouchami W et al (2013) A Common Reference Material for Cadmium Isotope Studies – NIST SRM 3108. Geostand Geoanalytical Res 37:5–17 Jochum KP et al (2005) GeoReM: A New Geochemical Database for Reference Materials and Isotopic Standards. Geostand Geoanalytical Res 29:333–338 Additional Declarations There is NO Competing Interest. Supplementary Files ElmsetalPETMCdcarbonburialdata.xlsx Dataset 1 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6680768","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":459942618,"identity":"315d1f8d-793b-4c54-ac76-f126b2e9332c","order_by":0,"name":"Hannah Elms","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA8klEQVRIiWNgGAWjYBACAxDB2MDAwAekHgAZMhDxA0RoYWNgYDYAMnhI0sImQZQWc/azxx783MEgxyZ2/Fl15Y7DPAzshx8w85zBrcWyJy/dsPcMgzGbdI7ZzbNngFp40gyYeW7gcdiBHDMJ3jaGxDbpHLabjW1ALQw5DMw8H/BoOf/GTPJvG0N9m3T6s0KwFv43BLTcyDGTBtqSwCadYMYI1iIBsgWfw268MZOWbZMwBDrMWLKxLZ2HTeKZwcE5eLxvcD7HTPJtm408v3T6w4+NbdZy/PzJDx+8OYZbCxRIIJjACMIXK6NgFIyCUTAKiAEASs9J6dk19ucAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0003-0103-044X","institution":"Royal Holloway, University of London","correspondingAuthor":true,"prefix":"","firstName":"Hannah","middleName":"","lastName":"Elms","suffix":""},{"id":459942619,"identity":"9f9803c2-b556-46aa-8717-9c26f2d796ec","order_by":1,"name":"Ross Whiteford","email":"","orcid":"https://orcid.org/0000-0002-2178-3476","institution":"University of St Andrews","correspondingAuthor":false,"prefix":"","firstName":"Ross","middleName":"","lastName":"Whiteford","suffix":""},{"id":459942620,"identity":"4ec77228-eaa3-490b-8021-a6a535ee5653","order_by":2,"name":"James Brakeley","email":"","orcid":"","institution":"Royal Holloway, University of London","correspondingAuthor":false,"prefix":"","firstName":"James","middleName":"","lastName":"Brakeley","suffix":""},{"id":459942621,"identity":"c01e4a2c-227a-4c25-9f95-44689c631698","order_by":3,"name":"Sev Kender","email":"","orcid":"https://orcid.org/0000-0003-4216-3214","institution":"University of Exeter","correspondingAuthor":false,"prefix":"","firstName":"Sev","middleName":"","lastName":"Kender","suffix":""},{"id":459942622,"identity":"632d04c0-fc64-45bf-9aed-32009a837519","order_by":4,"name":"Ying Cui","email":"","orcid":"https://orcid.org/0000-0003-1057-4369","institution":"Department of Earth and Environmental Studies, Montclair State University","correspondingAuthor":false,"prefix":"","firstName":"Ying","middleName":"","lastName":"Cui","suffix":""},{"id":459942623,"identity":"857d9439-b203-416e-ab4c-6f7a1bdd179b","order_by":5,"name":"Alexander Dickson","email":"","orcid":"https://orcid.org/0000-0001-8928-4799","institution":"Royal Holloway University of London","correspondingAuthor":false,"prefix":"","firstName":"Alexander","middleName":"","lastName":"Dickson","suffix":""}],"badges":[],"createdAt":"2025-05-16 12:30:21","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6680768/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6680768/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":83332372,"identity":"3850cca8-99e2-4f48-be32-927541d686db","added_by":"auto","created_at":"2025-05-23 08:08:35","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":52684,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eCadmium fluxes are plotted for modern day steady state conditions in terms of both mass removed as a proportion of input and isotope offset from seawater. The magnitude of the impact of each flux on the cadmium system is shown by the contours, with darker colours indicating greater impact. Vertical bars are used to show uncertainty in isotopic fractionation factors.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/5be21d889eba305678b8fa71.jpg"},{"id":83331814,"identity":"806b3a2a-bca6-41fe-881b-5c0f265a3393","added_by":"auto","created_at":"2025-05-23 08:00:37","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":41882,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eThe modern-day steady state isotope mass balance of the seawater cadmium system. The mass and isotope composition of cadmium in seawater is shown, along with estimates of the fluxes (in 10\u003c/em\u003e\u003csup\u003e\u003cem\u003e6\u003c/em\u003e\u003c/sup\u003e\u003cem\u003emol yr\u003c/em\u003e\u003csup\u003e\u003cem\u003e-1\u003c/em\u003e\u003c/sup\u003e\u003cem\u003e) and their isotopic signatures. Sources for these estimates are: oxyhydroxide\u003c/em\u003e\u003csup\u003e20,25\u003c/sup\u003e\u003cem\u003e, clay, sulfide\u003c/em\u003e\u003csup\u003e19,24,26\u003c/sup\u003e\u003cem\u003e, carbonate\u003c/em\u003e\u003csup\u003e21,23\u003c/sup\u003e\u003cem\u003e, organic\u003c/em\u003e\u003csup\u003e27\u003c/sup\u003e\u003cem\u003e, riverine\u003c/em\u003e\u003csup\u003e15,18\u003c/sup\u003e\u003cem\u003e, and seawater\u003c/em\u003e\u003csup\u003e28\u003c/sup\u003e\u003cem\u003e. Fluxes shown in black are assumed to occur in isotopic equilibrium with seawater over geological timescales, those in grey are sometimes offset from seawater, and those in red are always offset from seawater. Greater removal of isotopically light cadmium (for instance by increasing organic carbon burial) drives seawater to heavier values.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/d429c0a0818b48bb3c62a415.jpg"},{"id":83331794,"identity":"63b9156e-3ce8-4a16-9a7e-641e35353358","added_by":"auto","created_at":"2025-05-23 08:00:35","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":33321,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eLocation map of the four sections analysed in this study, as they were during the PETM\u003c/em\u003e\u003csup\u003e40\u003c/sup\u003e\u003cem\u003e. Locations vary from the high Arctic (IODP Site M0004A, BH9-05) to the mid-latitude Peri-Tethys region (Kheu River), and include both restricted (BH9-05, E-8X) and open (Kheu River) oceanographic settings.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/8285cd1c51d82aa7c735518c.jpg"},{"id":83332374,"identity":"a2a6e846-94e5-4fcc-a59b-17b8756568be","added_by":"auto","created_at":"2025-05-23 08:08:35","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":50598,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eGeochemical proxy evidence for variable restriction (closed symbols) vs. open-ocean (open symbols) paleoceanographic settings for each basin in this study, after Sweere et al., 2016\u003c/em\u003e\u003csup\u003e41\u003c/sup\u003e\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/8e8cd47e2f7f64a38aa4cef0.jpg"},{"id":83331815,"identity":"121fcd70-b277-4ea0-85a2-8989f1387507","added_by":"auto","created_at":"2025-05-23 08:00:37","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":67190,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eCompiled (a) δ\u003c/em\u003e\u003csup\u003e\u003cem\u003e13\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003eorg\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e \u003c/em\u003e\u003csup\u003e31–36,40,42,43\u003c/sup\u003e\u003cem\u003e and (b) TOC records\u003c/em\u003e\u003csup\u003e32–37,42,44,45\u003c/sup\u003e\u003cem\u003e for the four PETM sections used in this study, aligned using inflection points and existing age models\u003c/em\u003e\u003csup\u003e43\u003c/sup\u003e\u003cem\u003e, with (c) Cd concentrations, (d) cadmium isotope data for the four sections (this study). 2σ uncertainty shown in panel (d) is the analytical reproducibility of δ\u003c/em\u003e\u003csup\u003e\u003cem\u003e114\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eCd (determined via repeat analysis of the USGS SGR-1b international rock standard, n= 9).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/d30982f1cf4f123aede21e0b.jpg"},{"id":83332375,"identity":"f6853154-2f2b-4f9b-9841-113f47a5e818","added_by":"auto","created_at":"2025-05-23 08:08:35","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":52859,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eThe δ\u003c/em\u003e\u003csup\u003e\u003cem\u003e13\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003eorg\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e (upper panel), δ\u003c/em\u003e\u003csup\u003e\u003cem\u003e114\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eCd\u003c/em\u003e\u003csub\u003e\u003cem\u003esw\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e (central panel), and resulting organic carbon burial rate predicted by the CdBURY model (lower panel). Our measured data are shown by the points in the central panel, coloured by site. The δ\u003c/em\u003e\u003csup\u003e\u003cem\u003e114\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eCd\u003c/em\u003e\u003csub\u003e\u003cem\u003esw\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e predicted by our model is shown by the black line, with uncertainty in the grey window. The organic carbon burial rate required to drive this change is shown in the lower panel, with the median shown by the blue line, and the 95% confidence interval shown by the blue shaded region.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/8bc746778506df247675c7d0.jpg"},{"id":83331806,"identity":"126dc1a8-df5c-4f2e-9828-e90ceea10451","added_by":"auto","created_at":"2025-05-23 08:00:35","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":38876,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eThree simulations from the CdBURY model are shown in each panel. In black is a steady state run where nothing is changed. In orange and blue the organic carbon burial flux is perturbed at 55.5Ma for 0.2Myr, before being reset to the initial condition. In orange the flux is doubled, and in blue it is set to zero. The predicted perturbation to δ\u003c/em\u003e\u003csup\u003e\u003cem\u003e114\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eCd\u003c/em\u003e\u003csub\u003e\u003cem\u003esw\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e (middle panel) and the mass of cadmium in seawater (lower panel) is shown.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/4abb2825296471ee11c80241.jpg"},{"id":83331801,"identity":"5ac82eb5-560d-42f3-8c9b-0da40c11b9f1","added_by":"auto","created_at":"2025-05-23 08:00:35","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":50094,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eδ\u003c/em\u003e\u003csup\u003e\u003cem\u003e114\u003c/em\u003e\u003c/sup\u003e\u003cem\u003eCd\u003c/em\u003e\u003csub\u003e\u003cem\u003esw\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e record across the PETM, with datapoint colour coded by site, and the prediction of the Gaussian Process shown in black and grey. The black line is the mean prediction and the grey window is the 95% confidence interval.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/e3578e1120d0bbab4e7a8ac5.jpg"},{"id":83331809,"identity":"b3293a30-c7a8-403a-8417-655b6fd9e521","added_by":"auto","created_at":"2025-05-23 08:00:35","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":42200,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003ePrior vs posterior estimates of the initial conditions.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/e036fe9d0b5a2cca4965f451.jpg"},{"id":83332377,"identity":"50a562be-e175-4fe6-84e6-5aa3303bca50","added_by":"auto","created_at":"2025-05-23 08:08:35","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":42900,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eEstimates of total excess carbon burial across the PETM, with either constant or variable stoichiometry\u003c/em\u003e.\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/4eca357214a2898851cd6aab.jpg"},{"id":86938024,"identity":"326da35f-f67e-44ad-a286-26ecb7d4f401","added_by":"auto","created_at":"2025-07-17 11:14:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1083062,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/ca2332c5-6949-402d-b038-a2fb44a9ccea.pdf"},{"id":83331796,"identity":"50892b10-c0de-4fba-845f-fc979c3b4608","added_by":"auto","created_at":"2025-05-23 08:00:35","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":34612,"visible":true,"origin":"","legend":"Dataset 1","description":"","filename":"ElmsetalPETMCdcarbonburialdata.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-6680768/v1/7358990d6cb2630741882d54.xlsx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"A cadmium-based quantification of marine organic carbon burial during the PETM","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma) was a period of rapid global warming associated with the release of thousands of petagrams of isotopically light carbon into the Earth\u0026rsquo;s atmosphere and oceans. However, the magnitude of marine organic carbon (C\u003csub\u003eorg\u003c/sub\u003e) burial following carbon release is not well constrained, despite its importance as a CO\u003csub\u003e2\u003c/sub\u003e feedback mechanism\u003csup\u003e1\u0026ndash;3\u003c/sup\u003e. Evidence of short-term imbalances in the geological C cycle is preserved in the C isotope (\u0026delta;\u003csup\u003e13\u003c/sup\u003eC) signatures of organic matter and carbonates\u003csup\u003e4\u003c/sup\u003e. Previous studies have modelled the rate of change of \u0026delta;\u003csup\u003e13\u003c/sup\u003eC records\u003csup\u003e2\u003c/sup\u003e, extrapolated \u0026nbsp;C\u003csub\u003eorg\u003c/sub\u003e content in marine rocks\u003csup\u003e5\u003c/sup\u003e, or modelled coupled C\u003csub\u003eorg\u003c/sub\u003e-marine nutrient cycles\u003csup\u003e6\u003c/sup\u003e to estimate anywhere between 2,000 Pg and 10,300 Pg of C\u003csub\u003eorg\u003c/sub\u003e burial over the PETM. Better constraint on this range is limited, however, because \u0026delta;\u003csup\u003e13\u003c/sup\u003eC records are a synthesis of multiple co-occurring feedbacks, making it difficult to attribute specific processes\u003csup\u003e7,8\u003c/sup\u003e. More direct proxies that may circumvent this issue are lacking, which is problematic for fully understanding climate dynamics because marine C\u003csub\u003eorg\u003c/sub\u003e burial is an important feedback mechanism for removing carbon from the ocean-atmosphere system.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe stable cadmium (Cd) isotope system, recently utilised in paleo-productivity studies\u003csup\u003e9\u0026ndash;12\u003c/sup\u003e, has strong potential as an independent C\u003csub\u003eorg\u003c/sub\u003e burial proxy. Seawater Cd systematics are controlled by input of Cd to the ocean (almost wholly riverine), and a series of sedimentary burial fluxes: oxyhydroxides, clays, carbonates, sulfides, and organic matter (Figure 1), the balance of which ultimately determines the isotopic composition of Cd dissolved in the ocean.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePreviously published mass balance models for oceanic Cd are defined by Cd burial by oceanic regions - such as continental margins\u003csup\u003e13\u0026ndash;16\u003c/sup\u003e, by specific oceanic basins or transects therein\u003csup\u003e17\u003c/sup\u003e, or by the type of bulk sedimentary deposit, which may include several isotopically-distinct sedimentary phases. It is thus difficult to utilise existing calculations to achieve a geochemically plausible mass balance when considering the entire global marine system. We have instead calculated cadmium burial fluxes using an isotopic mass balance approach, assuming oceanic steady-state with a balance between riverine input and sedimentary output fluxes, and utilising known isotopic compositions of river waters\u003csup\u003e18\u003c/sup\u003e and sedimentary phases\u003csup\u003e13,14,19\u0026ndash;24\u003c/sup\u003e. Our best estimate of the modern-day steady state is depicted in Figure 2 (calculations for which can be found in the Methods section).\u003c/p\u003e\n\u003cp\u003eTogether, incorporation into organic matter and precipitation of authigenic CdS likely account for approximately 95% of seawater cadmium burial, and both are capable of changing the cadmium isotope ratio of seawater (\u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e). In contrast, the oxyhydroxide and clay burial fluxes are both small and, over geological timescales, assumed to remove cadmium in isotopic equilibrium with seawater, so do not affect \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e. Cadmium adsorbed or substituted onto/into carbonates or FeMn oxyhydroxides is fractionated\u003csup\u003e20,21,23,25\u003c/sup\u003e, but again the Cd flux is small (~2% of the total Cd burial flux\u003csup\u003e21\u003c/sup\u003e), so this has only a small impact on \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e. Of the five burial pathways, only two currently have the potential to drive significant changes in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e: burial of Cd associated with C\u003csub\u003eorg\u003c/sub\u003e, and precipitation of authigenic CdS (Figures 1, 2).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe Cd isotope composition of authigenic CdS depends on whether Cd is completely precipitated from precursor fluid during its formation. If authigenic CdS precipitation does not completely deplete seawater Cd, it will be isotopically light (driving \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e to become heavier), but if precipitation does remove all the available dissolved Cd, then the authigenic CdS will have the same isotopic signature as seawater. Cd has been shown to be highly reactive in the presence of trace levels of HS\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e26,29\u003c/sup\u003e, and thus under sulfidic depositional conditions CdS precipitation will completely fix dissolved Cd into sediments with minimal isotopic fractionation\u003csup\u003e30\u003c/sup\u003e. While we assume no isotopic fractionation between sediments and seawater in the euxinic depositional settings used in this study, the extent to which incomplete Cd burial into authigenic CdS contributes to the global isotope mass balance is not well known, nor is its potential variability through time (Figure 1).\u003c/p\u003e\n\u003cp\u003eOur isotope-constrained mass balance indicates that Cd buried in association with organic matter accounts for roughly 20% of modern steady state Cd burial. Organic matter has a lighter Cd isotope composition than modern deep-ocean seawater (-0.15\u0026permil; to -0.8\u0026permil; offset\u003csup\u003e10,15,31\u003c/sup\u003e), so increases in C\u003csub\u003eorg\u003c/sub\u003e burial may be postulated to drive seawater cadmium towards heavier values. All else being equal, driving the same perturbation with less isotopically offset cadmium would require greater carbon burial \u0026ndash; so we use the larger value of -0.8\u0026permil;, to ensure our estimates are conservative. We leverage the two main sinks of Cd together: changes in C\u003csub\u003eorg\u003c/sub\u003e burial as the driver of changes in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e, and authigenic sulfides as an archive of \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e under euxinic conditions\u003csup\u003e30\u003c/sup\u003e. We measure bulk Cd isotopes in four sedimentary successions that span the PETM interval, then combine these data with numerical modelling to quantify the magnitude of C\u003csub\u003eorg\u003c/sub\u003e burial.\u003c/p\u003e\n\u003cp\u003eThe four shale sections used for this study have all been previously studied and are well-characterised for their geochemical and paleogeographical context\u003csup\u003e32\u0026ndash;37\u003c/sup\u003e. The sections also span variable distances from the North Atlantic Igneous Province, proposed as the main trigger for the PETM\u003csup\u003e36,38,39\u003c/sup\u003e, and originate in basins with differing levels of hydrographic restriction from the open ocean (Figures 3, 4).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThree sections (Svalbard, North Sea, and Arctic) were deposited in euxinic depositional conditions with abundant sulfide, while one section (Kheu River) was deposited at a few hundred metres depth in an open-ocean setting with episodic euxinia during the early part of the PETM\u003csup\u003e33,35,42\u003c/sup\u003e. The different depositional settings provide a useful framework to identify common trends in the cadmium isotope data that carry supra-regional significance. Carbon-isotope records were used to align each of the four sections, with the event itself divided into pre-PETM, onset, core, and recovery segments based on inflections in the \u0026delta;\u003csup\u003e13\u003c/sup\u003eC curves and changes in TOC content (Figure 5a-b). A chronology was transferred to the aligned records using the age model of Charles et al., 2011\u003csup\u003e43\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSedimentary cadmium isotope records\u003c/h2\u003e \u003cp\u003eCadmium concentrations (Fig.\u0026nbsp;5c) vary between the four sections, with overall ranges of 0.3\u0026ndash;0.6 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Svalbard), 0.3\u0026ndash;3 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Arctic), 0.3\u0026ndash;13 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (North Sea) and 0.01\u0026ndash;475 \u0026micro;g g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Kheu River). Total Organic Carbon content is high in three sections (1\u0026ndash;3% Arctic, 0\u0026ndash;7% North Sea, 0\u0026ndash;9% Kheu River), but lower in Svalbard (1\u0026ndash;2%). The low TOC content in Svalbard is likely due to post-depositional maturation and/or high level of sediment dilution; however, previous studies show that thermal maturity has no effect on the isotopic compositions of Cd and other trace metals\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. Therefore, even if the concentrations are affected by burial maturation, the isotopic data on which our estimates of organic carbon burial flux are based (Fig.\u0026nbsp;5d) remain robust.\u003c/p\u003e \u003cp\u003eAll four sites exhibit a positive Cd isotope excursion of similar magnitude despite the order of magnitude differences in Cd concentrations (Fig.\u0026nbsp;5d). Burial phase differences at the local scale superimpose noise onto the global seawater signal. The heaviest δ\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003eshale\u003c/sub\u003e values coincide with the onset or \u0026lsquo;peak\u0026rsquo; of the PETM. Pre-onset values hover around \u0026minus;\u0026thinsp;0.1\u0026permil; and then increase by 0.05\u0026ndash;0.2\u0026permil; by the core of the event. Peak values are even higher, at 0.6\u0026permil; (Kheu River), 0.5\u0026permil; (Svalbard), 0.4\u0026permil; (North Sea), and 0.2\u0026permil; (Arctic). During the recovery phase, the Cd isotope compositions decrease back to -0.2 to -0.1\u0026permil;\u003c/p\u003e \u003cp\u003e(Fig.\u0026nbsp;5d).\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eQuantifying C burial using cadmium isotope modelling\u003c/h3\u003e\n\u003cp\u003eOur δ\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e record (Fig.\u0026nbsp;5d) indicates a pulse of additional marine C\u003csub\u003eorg\u003c/sub\u003e burial across the PETM (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e6\u003c/span\u003e). To quantify that flux, we first extract the global signal from our four sites using a Gaussian Process (Fig.\u0026nbsp;8). We then combine this extracted global trend in δ\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e with an isotope mass balance model of the seawater cadmium system (CdBURY). We use an inverse strategy, which allows us to calculate the amount of marine organic carbon burial required to drive the observed change in δ\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e. We calculate a scaling to convert the predicted fluxes of Cd to C\u003csub\u003eorg\u003c/sub\u003e, using our modern day estimate of organic bound Cd burial flux (5,287,500 mol yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and marine C\u003csub\u003eorg\u003c/sub\u003e burial flux (0.1 Pg C yr\u003csup\u003e\u0026minus;\u003c/sup\u003e). This allows us to estimate the background organic carbon burial pre-PETM to be between 0 and 0.5 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (median 0.2 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), and peak organic carbon burial to be between 0.3 and 1.5 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (median 0.8 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). We estimate the cumulative magnitude of additional C\u003csub\u003eorg\u003c/sub\u003e burial over the PETM to be between 15,225 Pg C and 73,600 Pg C at 95% confidence, with a median and most likely estimate of ~\u0026thinsp;40,000 Pg C (see Methods).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eOur median estimate of ~\u0026thinsp;40,000 Pg C is notably higher than previous estimates of organic carbon burial, which range from 2,000\u0026ndash;10,300 Pg C\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e,\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. However, it is reconcilable with estimates of PETM carbon emissions. Estimates of potential carbon emissions directly from the NAIP are generally in the 10,000\u0026rsquo;s Pg C range (18,000\u0026ndash;60,000 Pg C\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e, ~\u0026thinsp;27,000 Pg C\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e, 40,000 Pg C\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e), with additional indirect carbon sources contributing several thousand Pg C more (3000\u0026ndash;15,000 Pg C from magmatic interaction with sediments\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e and 100\u0026ndash;10,000 Pg C from oxidative weathering\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e). In contrast, the emissions required to drive measured trends in δ\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003eC, pH, and CO\u003csub\u003e2\u003c/sub\u003e, are only between 3000 and 12,000 Pg C\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e,\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. However, these estimates typically assume constant (or near constant) organic burial. Our estimate of excess marine organic carbon burial across the PETM (15,525 Pg C \u0026minus;\u0026thinsp;73,600 Pg C) could reconcile the large carbon source that NAIP could provide with the smaller carbon excess required to drive the surficial carbon cycle changes. Our median estimate of ~\u0026thinsp;40,000 Pg of additional C burial suggests that organic carbon burial was a significant carbon sink across the PETM. This estimate is given in terms of excess carbon burial, which is inherently dependent on our quantification of background organic carbon burial. If background organic carbon burial was higher than our calculated\u0026thinsp;~\u0026thinsp;0.2 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, the excess carbon burial estimate would decrease without affecting the total carbon burial we reconstruct.\u003c/p\u003e \u003cp\u003eA feature of our simulated C\u003csub\u003eorg\u003c/sub\u003e burial curve is that maximum burial fluxes occur early during the PETM. The timing of C\u003csub\u003eorg\u003c/sub\u003e burial probably records a fast response of the oceanic biosphere to changing nutrient conditions delivered from rapid continental weathering or oceanic recycling\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. It is consistent with pulses of continental weathering recorded by both lithium and osmium records early in the PETM\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e,\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e,\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e, and with barite flux estimates of the timing of peak export productivity during the PETM\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e,\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e. Ma et al.\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e estimated that about 2000 Pg C could have entered the refractory oceanic carbon pool as a result of biological C export, from a total export of ~\u0026thinsp;500,000 Pg C. There is ample evidence for expanded low oxygen conditions during the PETM\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e,\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e,\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e,\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e, particularly in marginal ocean basins and oxygen minimum zones that could have increased the burial efficiency of exported organic C, making the estimate of Ma et al. a minimum value and consistent with the Cd-based estimates presented here.\u003c/p\u003e \u003cp\u003eThe modern day marine organic C burial rate is ~\u0026thinsp;0.1 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and we estimate background organic C burial pre-PETM was between 0.1 and 0.5 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and peak marine organic C burial during the PETM was between 0.3 and 1.5 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The peak emission rate across the PETM is between 0.3 and 1.1 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;58\u003c/sup\u003e \u0026ndash; the highest known rate since the Mesozoic (and, as this is an average rate, quite possible higher on shorter timescales), but nonetheless dwarfed by anthropogenic carbon release which is up to 10 Pg C yr\u003csup\u003e\u0026minus;\u0026thinsp;58\u003c/sup\u003e. Our results suggest that burial of additional marine organic carbon during climate perturbations could act as a stabilising climate feedback, with burial rate potentially scaling by an order of magnitude relative to its modern-day level. However, even if modern organic carbon burial were to increase by an order or magnitude, it would bury only a tenth of current emissions each year.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\n \u003ch2\u003eGeochemical methods\u003c/h2\u003e\n \u003cp\u003eOrganic-rich, sulfidic marine shale samples (~\u0026thinsp;200 mg powder) spanning each studied record were oxidised and then fully digested following standard inverse aqua regia-HF:HNO\u003csub\u003e3\u003c/sub\u003e digestion techniques\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e in the metal-free, ultra-clean geochemical laboratory facilities at Royal Holloway, University of London, using clean PFA vials. Concentration analyses were obtained via ICP-MS or (post-July 2024) ICP-OES, calibrated using in-house multi-element standards. Digest aliquots were then precisely weighed to capture 50 ng of natural cadmium, and homogenised with a \u003csup\u003e111\u003c/sup\u003eCd-\u003csup\u003e113\u003c/sup\u003eCd double-spike to obtain a double-spike sample ratio of ~\u0026thinsp;1.5, before drying and taking up in 1ml 1M HF:0.5M HCl. Cadmium was isolated via a two-column procedure using AG1-X8 anion exchange resin followed by TRU-spec extraction chromatography resin following Bryan, 2018\u003csup\u003e59\u003c/sup\u003e. Samples were centrifuged prior to loading onto the columns to remove any fluoride precipitates.\u003c/p\u003e\n \u003cp\u003ePurified cadmium isolates were then dried down and taken up in sufficient 3% (~\u0026thinsp;0.5M) HNO\u003csub\u003e3\u003c/sub\u003e to dilute to 10 ppb and analysed on a Thermo Neptune MC-ICP-MS in dry plasma mode, using an Aridus III desolvating nebuliser, at Royal Holloway University of London. Data were deconvolved offline and final isotopic ratios reported relative to NIST 3108, run at the same concentration and sample-spike ratio as the samples. OXCAD was used as a secondary isotope standard, and all sample batches included an aliquot of USGS SGR-1b (\u0026delta;\u003csup\u003e114\u003c/sup\u003eCd = -0.85 and 0.062 relative to NIST3108, respectively\u003csup\u003e\u003cspan class=\"CitationRef\"\u003e60\u003c/span\u003e,\u003cspan class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e), digested and analysed as a total procedural rock standard. Our measured values for OXCAD were 0.86\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06 (2sd, n\u0026thinsp;=\u0026thinsp;20) and SGR-1b were 0.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 (2sd, n\u0026thinsp;=\u0026thinsp;11). Blank contributions were low-pg level, comprising\u0026thinsp;\u0026lt;\u0026thinsp;1% of the cadmium content. Detrital Cd contributions to each sample, assessed using Cd/Al ratios obtained from the ICP-OES measurements, were similarly negligibly low enough to make no difference to the final \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd compositions.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003eCadmium isotope system modelling\u003c/h2\u003e\n \u003cp\u003eAs described in the introduction, the cadmium system in seawater is a balance of the input flux (predominantly riverine) and five output fluxes (Fig.\u0026nbsp;2). However, only two of the output fluxes are substantial enough to drive large scale shifts in the cadmium cycle. The clay flux is small and assumed to be isotopically in equilibrium with seawater (so unable to have significant influence). The carbonate and oxyhydroxide fluxes are potentially isotopically offset from seawater, but small in magnitude (so unable to have significant influence). The sulfide flux is large, but does not isotopically fractionate Cd where precipitation of CdS from a precursor fluid is \u0026lsquo;complete,\u0026rsquo; as would typically occur in sulfidic depositional settings. Organic carbon is a relatively large flux and has a large isotopic offset from seawater.\u003c/p\u003e\n \u003cp\u003eCd has a nutrient-like profile in the modern ocean, so the shallow seawater exchanged into marginal basins may carry a Cd isotope signature that is offset from the global deep-ocean average. However, if the vertical profile in seawater Cd remained constant through the PETM, \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003eshale\u003c/sub\u003e records will still faithfully record the relative change in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw,\u003c/sub\u003e and it is the relative change in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e which drives our record of organic carbon burial. Also of note is that global mass balance requires seawater \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd (recorded by\u0026thinsp;~\u0026thinsp;\u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003eshale\u003c/sub\u003e) to be equal to or heavier than the input fluxes to the ocean. The background value of our estimated global trend in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003eshale\u003c/sub\u003e (Fig. 5d) is ~ -0.2\u0026permil;, implying an even lighter input flux. However, the bulk silicate Earth has a Cd isotope ratio of ~\u0026thinsp;0\u0026permil; and measured riverine values are in the range of 0.1\u0026ndash;0.3\u0026permil;\u003csup\u003e18\u003c/sup\u003e. We suggest that there is likely to be a systematic offset between the sediments and basin seawater. This offset does not, however, alter the relative shape or magnitude of temporal change in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003eshale,\u003c/sub\u003e and therefore doesn\u0026rsquo;t affect the reconstructed trend in marine organic carbon burial that is based on these relative changes.\u003c/p\u003e\n \u003cp\u003eWe extract the global trend in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e from our \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003eshale\u003c/sub\u003e data using a Gaussian Process (detailed below), which smooths the data on a 50kyr timescale \u0026ndash; chosen as a balance between smoothing and retaining detail in our record. From the Gaussian Process we draw 10,000 possible \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e curves which are consistent with our measured \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd and smoothing timescale.\u003c/p\u003e\n \u003cp\u003eTo turn our smoothed \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e curves into estimates of organic burial, we simulate the fluxes of Cd into and out of the ocean (Fig. 2) using a newly configured box model (CdBURY). We invert this model, such that it varies the organic bound Cd flux to drive the estimated change in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e. All other output fluxes are assumed to be linearly related to seawater Cd concentrations, and the input flux is assumed to remain constant (which is conservative in terms of reconstructed organic carbon burial). We propagate uncertainty in the initial conditions ([Cd]\u003csub\u003esw\u003c/sub\u003e: 0.3x to 3x modern, [Cd]\u003csub\u003erivers\u003c/sub\u003e: 0.3x to 3x modern, and \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003erivers\u003c/sub\u003e: -0.4\u0026permil; to -0.1\u0026permil;). We rule out simulations in which \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e changes too quickly to be compatible with the dynamics of the oceanic Cd system, leaving us with a posterior dataset of many potential curves of organic bound Cd burial which could have driven our estimated \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e.\u003c/p\u003e\n\u003c/div\u003e\n\u003ch3\u003eCdBURY\u003c/h3\u003e\n\u003cp\u003eCdBURY is a single box model which simulates the fluxes of cadmium into and out of the ocean (as depicted in Figs. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e, 2). It predicts the mass of cadmium in seawater and its isotopic composition. The mass balance equation is:\u003c/p\u003e\n\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e$$\\:\\frac{dCd}{dt}={Cd}_{input}-{Cd}_{oxyhydroxide}-{Cd}_{clay}-{Cd}_{carbonate}-{Cd}_{organic}-{Cd}_{sulfide}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eSimilarly for each isotope:\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{d{}^{114}Cd}{dt}={}^{114}{Cd}_{input}-{}^{114}{Cd}_{oxyhydroxide}-{}^{114}{Cd}_{clay}-{}^{114}{Cd}_{carbonate}-{}^{114}{Cd}_{organic}-{}^{114}{Cd}_{sulfide}\\)\u003c/span\u003e\u003c/span\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\frac{d{}^{110}Cd}{dt}={}^{110}{Cd}_{input}-{}^{110}{Cd}_{oxyhydroxide}-{}^{110}{Cd}_{clay}-{}^{110}{Cd}_{carbonate}-{}^{110}{Cd}_{organic}-{}^{110}{Cd}_{sulfide}\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\n\u003cp\u003eWhere Cd represents the mass of cadmium, and 114 and 110 are the two pertinent isotopes. CdBURY performs full isotope mass balance, accounting for each flux entering and leaving the ocean. Figure 7 shows a demonstration of asking CdBURY to either double marine carbon burial, or turn it off completely, and the resulting changes in the seawater cadmium system.\u003c/p\u003e\n\u003ch3\u003eGaussian Process\u003c/h3\u003e\n\u003cp\u003eTo combine the data from our four sites and extract a global trend in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e, we use a Gaussian Process to smooth our data (Figs. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e, 8). We use a Radial Basis Function kernel, which encodes the belief that the \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e is smooth on a prescribed timescale using two hyperparameters. We choose 50 kyr as the length scale parameter, which allows us to extract a global signal from multisite data by removing of the noise from local effects in our datasets. The noise scale parameter is set to 2.0\u0026permil;, but this choice has minimal impact on our reconstruction because of the high data density. From this Gaussian Process we draw 10,000 curves for \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e which are compatible with our dataset.\u003c/p\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003eCdBURY Inversion\u003c/h2\u003e\n \u003cp\u003eWe ask the CdBURY model to recreate each of these 10,000 \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e curves by changing organic carbon burial and having all other fluxes change only as a linear function of [Cd]\u003csub\u003esw\u003c/sub\u003e. Calculated organic bound cadmium burial flux is driven by: the target \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e evolution, the initial amount of cadmium in seawater, and the input flux of cadmium to seawater (including its isotopic composition). Uncertainties associated with each of these parameters are propagated through our model using a Monte Carlo approach. Samples for the initial amount of cadmium in seawater and the input flux of cadmium to seawater are randomly drawn, covering a range of 0.333x to 3x modern values, and the riverine cadmium isotope composition ranges from \u0026minus;\u0026thinsp;0.4 \u0026ndash; -0.1\u0026permil;. Having picked these random initial conditions, the model is rebalanced to steady state by adjusting the organic carbon and sulfide burial fluxes such that the input and output fluxes are balanced both in terms of mass and isotopes. From this starting steady state, CdBURY changes the organic carbon burial flux to drive the estimated trend in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e.\u003c/p\u003e\n \u003cp\u003eThe posterior set of initial conditions indicates that it is easier to drive the measured trend in \u0026delta;\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e when the input flux into the ocean is high relative to modern, the input isotopic composition is low (\u0026lt;\u0026thinsp;0.2\u0026permil;), and the mass of cadmium in the ocean is low relative to modern.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003eCarbon Burial\u003c/h2\u003e\n \u003cp\u003eEach of the 3737 posterior curves which pass all checks are converted into estimates of marine organic carbon burial. We may look at this estimate through several different lenses, as a time variable flux (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e), or as total excess carbon burial across the interval. Here we show a histogram (Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e) of total excess carbon burial from all posterior samples. We may calculate this in two ways: a) assuming that [Cd]\u003csub\u003esw\u003c/sub\u003e controls the magnitude of each flux (constant stoichiometry), or b) assuming that [Cd]\u003csub\u003esw\u003c/sub\u003e controls the cadmium content of each flux (variable stoichiometry). The former is the more conservative assumption but is not realistic because [Cd]\u003csub\u003esw\u003c/sub\u003e is not a major control on the fluxes \u003cem\u003esensu stricto\u003c/em\u003e. Instead, it is more realistic to imagine that changing [Cd]\u003csub\u003esw\u003c/sub\u003e controls how much Cd is incorporated into each output flux.\u003c/p\u003e\n \u003cp\u003eVariable stoichiometry calculations suggest greater carbon burial than constant stoichiometry calculations, but on the scale of the estimates the increase is minor. Both constant and variable stoichiometry estimates have peak likelihood of organic carbon burial of around 40,000 Pg C.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contributions\u003c/h2\u003e \u003cp\u003eAJD designed the study and assisted with laboratory analyses. HCE performed geochemical analyses and drafted the manuscript. RW developed the CdBURY model. YC and SK provided samples and contributed to interpretations. JB provided laboratory analyses and technical assistance. All authors contributed to data interpretation and manuscript editing. HCE and RW contributed equally to this study.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis study was supported by a UKRI Frontier research grant (EP/X022080/1). We are grateful to Tali Babila, Sarah Green and Pam Vervoort for insightful discussions during model development and manuscript preparation. We have no competing interests to declare.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eBurdige DJ (2007) Preservation of Organic Matter in Marine Sediments: Controls, Mechanisms, and an Imbalance in Sediment Organic Carbon Budgets? Chem Rev 107:467\u0026ndash;485\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBowen GJ, Zachos JC (2010) Rapid carbon sequestration at the termination of the Palaeocene\u0026ndash;Eocene Thermal Maximum. 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Geostand Geoanalytical Res 29:333\u0026ndash;338\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-6680768/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6680768/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe Paleocene-Eocene Thermal Maximum (PETM, ~\u0026thinsp;56 Ma) was driven by rapid, large-scale carbon release into the oceans and atmosphere. Environmental recovery post-PETM must have been associated with climate feedback mechanisms to remove carbon from Earth\u0026rsquo;s surface reservoirs, including silicate weathering and organic carbon burial. However, the amount of carbon removed by organic matter burial has been difficult to quantify due to ambiguity in many carbon-cycle proxies, thus limiting accurate paleoclimate modelling. We have measured the isotopic composition of cadmium (δ\u003csup\u003e114\u003c/sup\u003eCd), a novel proxy for organic carbon burial, in sedimentary rocks deposited across the PETM in four separate marine basins. A\u0026thinsp;~\u0026thinsp;0.2\u0026permil; positive excursion in δ\u003csup\u003e114\u003c/sup\u003eCd\u003csub\u003esw\u003c/sub\u003e is interpreted to reflect a global-scale increase in organic cadmium burial in marine sediments. We simulate the global cadmium cycle using a box model to quantify the marine organic carbon burial flux required to drive this isotopic shift. Our median estimate of ~\u0026thinsp;40,000\u0026thinsp;\u0026plusmn;\u0026thinsp;15,500 Pg C total excess organic carbon burial across the PETM suggests that organic carbon burial was highly important in balancing the PETM carbon cycle budget.\u003c/p\u003e","manuscriptTitle":"A cadmium-based quantification of marine organic carbon burial during the PETM","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-23 08:00:30","doi":"10.21203/rs.3.rs-6680768/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":"d6d113ea-9ef3-477c-9f3e-8ada28da10e9","owner":[],"postedDate":"May 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":48859215,"name":"Earth and environmental sciences/Climate sciences/Ocean sciences/Marine chemistry"},{"id":48859216,"name":"Physical sciences/Chemistry/Environmental chemistry/Geochemistry"},{"id":48859217,"name":"Earth and environmental sciences/Environmental sciences/Environmental chemistry/Marine chemistry"},{"id":48859218,"name":"Earth and environmental sciences/Ocean sciences/Marine chemistry"}],"tags":[],"updatedAt":"2025-07-17T11:06:13+00:00","versionOfRecord":[],"versionCreatedAt":"2025-05-23 08:00:30","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6680768","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6680768","identity":"rs-6680768","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}