# Cenozoic record of δ³⁴S in foraminiferal calcite implies an early Eocene shift to deep-ocean sulfide burial

Victoria C. F. Rennie<fnref n="1" />*, Guillaume Paris²,³, Alex L. Sessions², Sigal Abramovich⁴, Alexandra V. Turchyn©<fnref n="1" /> and Jess F. Adkins²

Understanding the changes in, and drivers of, isotopic variability of sulfur in seawater sulfate (δ³⁴S₅₀₄-sw) over geological time remains a long-standing goal, particularly because of the coupling between the biogeochemical sulfur and carbon cycles. The early Cenozoic has remained enigmatic in this regard, as the existing seawater sulfate isotopic records appear to be decoupled from the well-defined carbon isotope composition of the ocean. Here, we present a new Cenozoic record of sulfur isotopes, using carbonate-associated sulfate hosted in the calcite lattice of single-species foraminifera. The vastly improved stratigraphy afforded by this record demonstrates that carbon and sulfur cycles, as recorded by their isotopes, are not fully decoupled in the early Cenozoic. With a model driven by partial coupling of the carbon and sulfur cycles, we demonstrate that a change in sulfur isotopic fractionation of the pyrite burial flux best explains the large increase in δ³⁴S₅₀₄-sw ~53 million years ago (Ma) and the subsequent long steady state. We suggest that the locus of pyrite burial changed from shallow epicontinental seas and shelf environments to more open-ocean sediments around 53 Ma. Loss of extensive shelf environments corresponds to Cretaceous-Palaeogene sea-level changes and tectonic reorganization, occurring as the Himalayan arc first collided with Asia.

The balance between fluxes of reduced and oxidized sulfur and carbon to and from Earth's surface control the oxygen content of the ocean-atmosphere system on geological timescales<fnref n="1" />. At Earth's surface, both oxidative weathering and microbially mediated reactions readily change the valence state of sulfur between sulfate and sulfide. Microbial sulfate reduction produces sulfide, enriched in ³²S, which, when removed as pyrite, helps maintain a high ratio of ³⁴S/³²S (reported as δ³⁴SₗCDP where VCDT is Vienna Canyon Diablo Troilite) in seawater sulfate (δ³⁴S₅₀₄-sw). Records of δ³⁴S₅₀₄-sw are therefore the primary tool for reconstructing the balance between reduced and oxidized fluxes in the sulfur cycle². Similarly, photosynthesis reduces atmospheric CO₂ and dissolved inorganic carbon (DIC) in seawater to organic carbon (the reduced sink in the carbon cycle), which is enriched in ¹²C. The removal of isotopically light organic carbon by burial in sediments leaves the residual DIC enriched in ¹³C (herein reported as δ¹³CₗVPDB, where VPDB is Vienna PeeDee Belemnite), thus providing a proxy for the burial flux of organic carbon. It has long been hypothesized that the carbon and sulfur cycles are linked via these reduced burial fluxes, with increases in burial of organic carbon driving increases in pyrite burial³. This suggests that the δ¹³CₗDIC and δ³⁴S₅₀₄-sw should largely co-vary over geological time. When records of δ¹³CₗDIC and δ³⁴S₅₀₄-sw do not co-vary, explanations invoking the decoupled burial of carbon and sulfur are required, with related implications for the redox state of Earth's surface³,⁴.

Published records of seawater δ¹³CₗDIC and δ³⁴S₅₀₄-sw show large systematic changes in the early Cenozoic (Fig. 1). The latter, measured in marine barium sulfate minerals (barite), exhibits a 5‰ (±1‰) increase in the mid-Eocene⁵, whereas the former, recorded in carbonate minerals, oscillates by 2‰ (±0.5‰) over the early

Cenozoic⁶. Both records suggest that seawater δ¹³CₗDIC and δ³⁴S₅₀₄-sw have remained more or less constant from 47 to 10 Ma. Despite the anticipated coupling between the two cycles, their relationship in the Palaeogene has been hard to determine, due to the lack of a marine archive from which both could be analysed simultaneously. Establishing the precise relationship between δ³⁴S and δ¹³C would strongly constrain the co-evolution of both cycles.

## The foraminiferal CAS δ³⁴S record

Foraminiferal calcite is ideal for measuring both carbon and sulfur isotope ratios. We extracted carbonate-associated sulfate (CAS) from single-species planktonic foraminifera samples from six globally distributed cores spanning the past 67 million years (Supplementary Fig. 3). We measured δ³⁴S by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), a technique that is significantly more sensitive than conventional isotope-ratio mass spectrometry (IRMS)-based methods⁷,⁸. Cultures⁹ and core-top calibrations (Supplementary Section 2.2) indicate that different species record δ³⁴S with small, species-specific isotopic offsets from seawater sulfate, highlighting the need to analyse δ³⁴S in single-species foraminiferal samples and to cross-correct between species. Our species-adjusted record reproduces the large δ³⁴S increase shown in Cenozoic barite (Fig. 1, Supplementary Sections 1–4). The δ³⁴S record is offset by +1‰ from δ³⁴Sₗbarite for most of the Cenozoic; this is within the variability of both core-top barites and among species of foraminifera⁵. The systematic offset between barite and single-species foraminiferal δ³⁴S probably results from δ³⁴S fractionation on sulfate incorporation into one or both mineral phases.

We use our record to directly compare foraminiferal δ³⁴S to boxcar-averaged global benthic foraminiferal δ¹³C⁶ (Supplementary

<footnote n="1">¹Department of Earth Sciences, University of Cambridge, Cambridge, UK. ²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA. ³Centre de Recherches Pétrographiques et Géochimiques, CRPG, UMR 7358, CNRS, Université de Lorraine, Vandoeuvre-lès-Nancy, France. ⁴Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva, Israel. *e-mail: vcr22@cam.ac.uk</footnote>
<figure><img src="image_1.png" /></figure>

**Fig. 1** | Cenozoic evolution of δ³⁴Sₕ₀₄⁻sw from foraminiferal CAS, compared with barite δ³⁴S, benthic foraminiferal δ¹³C and δ¹⁸O, sea level and India-Tibet convergence. **a**, δ³⁴S from foraminiferal carbonate-associated sulfate and smoothed barite record⁵ (dark grey line). Error bars (2σ) for foraminiferal data are smaller than the symbols (±0.2‰) except for unspliced data (±1‰); barite quoted uncertainty is ±0.3‰ (lighter grey shading). ODP, Ocean Drilling Program. **b**, δ¹³C (top) and δ¹⁸O (bottom) in foraminiferal calcite (grey dots) and global averages (black lines), compared with this study (diamonds), the benthic stack⁶ and bulk carbonate δ¹³C (ref. ⁴⁷) adjusted by the 1‰ offset from foraminifera⁴⁸. Yellow bars highlight the change in the long-term δ¹³C average. VSMOW, Vienna Standard Mean Ocean Water. **c**, Convergence rate data compiled by ref. ⁴⁶ (symbols and dotted line), Indian Ocean spreading rates⁴⁹ (thin black line), and computed India-Asia convergence rates (thick black line)⁴⁶ (top), with vertical grey bars highlighting the timing of the arc and continental collisions and (bottom) global sea-level estimates (grey line)⁵⁰ and 5-pt running mean (black line). Q, Quaternary.

Section 5) and to stratigraphically tied benthic foraminiferal δ¹³C measured from the same suite of cores as δ³⁴S (Figs. 1b and 2c). The relationship between the records of the biogeochemical carbon and sulfur cycles changes fundamentally around 53–52 Ma. From 67 to 52.6 Ma, δ¹³C varies rapidly relative to δ³⁴S, as demonstrated by a steep δ¹³C/δ³⁴S gradient and an increase in δ¹³C (of +1‰ at 61–58 Ma) with no corresponding increase in δ³⁴S. Minima in δ³⁴S and δ¹³C occur synchronously at 52.6 Ma (Figs. 1 and 2c), after which both δ³⁴S and δ¹³C increase, with a much shallower δ¹³C/δ³⁴S slope than previously. A further change of slope in the δ¹³C/δ³⁴S crossplot is observed at 50 Ma, when δ¹³C stops increasing whereas δ³⁴S continues to increase to a new steady-state value. Thus, the relationship between the isotope records of carbon and sulfur changes after ~53 Ma, from independent isotope records beforehand, to records that co-vary afterwards.

## Drivers of isotopic variability within the sulfur cycle

The 5‰ increase in δ³⁴Sₕ₀₄⁻sw after 53 Ma has been suggested to result from a three- to fourfold increase in pyrite burial sustained for up to

six million years⁴, or through a large pulse of evaporite weathering¹⁰. However, such marked changes in the biogeochemical sulfur cycle would impact ocean alkalinity and the carbon cycle; this seems to be inconsistent with the geological record. Pyrite burial increases alkalinity, driving carbonate mineral formation and favouring enhanced preservation of calcium carbonate¹¹; gypsum/anhydrite weathering also increases marine calcium concentrations, which would increase the carbonate saturation state and lead to the increased preservation of calcium carbonate minerals. Yet improved calcium carbonate preservation is evidenced in the geological record only 15 Myr later¹². More generally, increasing δ³⁴Sₕ₀₄⁻sw through changes in the input or output of marine sulfate requires a non-transient change that must be sustained for the remainder of the Cenozoic, given the long δ³⁴Sₕ₀₄⁻sw steady state after the 53 Ma event. There are other possible mechanisms for an irreversible increase in δ³⁴Sₕ₀₄⁻sw such as decreasing the evaporite burial flux. However, no decline in environments conducive to evaporite burial is observed over the remainder of the Cenozoic¹³. Alternatively, an increase in the δ³⁴S of the riverine sulfur influx could generate a +5‰ shift in δ³⁴Sₕ₀₄⁻sw. This increase could be due either to a rise in the δ³⁴S of weathered evaporite (for the same weathering influx) or a sustained increase in the proportion of evaporites relative to pyrite in the weathering flux. However, the former would be hard to sustain through the Cenozoic, and the latter is unlikely; as shelf areas become exposed during Cenozoic sea-level decline, additional pyrite weathering would be as likely as additional evaporite weathering. Furthermore, there is no evidence for a step-change in the δ³⁴S of riverine sulfate at this time¹⁴. A final option to explain the 5‰ increase in δ³⁴Sₕ₀₄⁻sw is a shift to more negative average δ³⁴S values of pyrite (δ³⁴Sₕ₀₄⁻sw).

The sulfur isotope fractionation between sulfate and sulfide, and thus pyrite (³⁴εₕ₀₄⁻pyr), depends on a variety of physical and biological variables (see, for example, refs ¹⁵,¹⁶). At present, δ³⁴Sₕ₀₄⁻ values are, on average, higher (³⁴εₕ₀₄⁻pyr is smaller) in shallow sediments than in deeper open-ocean sediments¹⁷. This difference in the sulfur isotope composition of pyrite in shallow versus deep environments is probably due to many factors, including faster rates of microbial sulfate reduction (for example, at high sulfate reduction rates, sulfur isotope fractionation is lower ³⁴εₕ₀₄⁻pyr = -10 to -20‰; ref. ¹⁸), frequent sedimentary reworking promoting reoxidation (for example, in deltaic settings, δ³⁴Sₕ₀₄⁻pyr = 5–20; refs ¹⁹,²⁰), an abundance of sulfate-driven anaerobic methane oxidation (for example, ³⁴εₕ₀₄⁻pyr = -10 to -25‰; ref. ²¹) and/or the near-quantitative closed-system conversion of sulfate to pyrite that allows δ³⁴Sₕ₀₄⁻ to approach δ³⁴Sₕ₀₄⁻sw (for example, the South China Sea, where δ³⁴Sₕ₀₄⁻ = 20‰ at depth in the sediments²²). Most recently, the δ³⁴Sₕ₀₄⁻ in one sedimentary site was shown to have changed systematically with sea level over glacial-interglacial cycles (δ³⁴Sₕ₀₄⁻ varies from -14‰, when sea level was lower, to -42‰, when sea level was higher²³). Furthermore, in organic carbon-rich sediments, sulfide may be buried as organosulfide, which is generally less depleted in ³⁴S than pyrite (for example, the Cariaco Basin, where δ³⁴Sₕ₀₄⁻organic fractions = -20‰ to -15‰, whereas δ³⁴Sₕ₀₄⁻ = -35‰; refs ²⁴⁻²⁶). In theory, if the dominant global burial environment for pyrite changed from shallower waters to deeper waters, this could result in a global shift towards more negative δ³⁴Sₕ₀₄⁻ values, which, if sustained, would lead to an increase in δ³⁴Sₕ₀₄⁻sw.

## Environmental transitions at ~53 Ma and changes in ³⁴εₕ₀₄⁻pyr

The Eocene marks the transition between the sea-level highstand of the greenhouse Cretaceous and the sea-level lowstand of the icehouse remainder of the Cenozoic, with accompanying biotic, palaeogeographic and ocean circulation changes. These reorganizations would influence the biogeochemistry of sedimentary depocentres, and thus affect pyrite burial. One possible sedimentary change at this time is enhanced export production due to the proliferation of pelagic calcifiers and silicifiers²⁷,²⁸, which would increase organic carbon export to the deep ocean (where ³⁴εₕ₀₄⁻pyr is likely to
<figure><img src="image_2.png" /></figure>

**Fig. 2** | Model parameters and outputs for fully and partially coupled carbon-sulfur models, compared with $\delta^{34}\text{S}_{\text{SO4-sw}}$ data and carbon-sulfur cross-plots. **a**, Organic carbon (green line) and pyrite burial fluxes used as model input parameters for fully (black line) and partially (red line) coupled models (for split pyrite burial fluxes see Supplementary Fig. 20). $\delta^{34}\text{E}_{\text{globalAverage}}$ is the sum of $\delta^{34}\text{E}_{\text{SO4_pyr_nearshore}}$ and $\delta^{34}\text{E}_{\text{SO4_pyr_deep}}$, weighted by deep/total pyrite burial (initially 70%, partially coupled, and 55%, fully coupled). C:S = 1, evaporite burial flux = $1.4 \times 10^{12} \text{ mol yr}^{-1}$ (coupled) and $1.7 \times 10^{12} \text{ mol yr}^{-1}$ (partially coupled). **b**, Concentration and $\delta^{34}\text{S}$ model outputs, compared with $\delta^{34}\text{S}_{\text{SO4-sw}}$ from this study (filled circles) and the barite record$^5$ (open circles: 1‰ offset). **c**, Foraminiferal $\delta^{34}\text{S}_{\text{SO4-sw}}$ against boxcar-averaged $\delta^{13}\text{C}$ record$^6$ (coloured circles, see colour scale) and our stratigraphically tied data (black diamonds). Error bars reflect 2σ analytical uncertainty in $\delta^{34}\text{S}_{\text{SO4-sw}}$. **d**, Model outputs for fully and partially coupled models (black and red lines, respectively). Arrows represent direction of time, compared with sulfur-carbon cross-plot data from **c** (grey symbols).

be larger). Another possibility is enhanced deep-ocean ventilation$^{29}$ driving pyrite burial deeper into the sediment column in deeper waters. Alternatively, the loss of shallow sedimentary environments due to the closure of epicratonic seas, resulting from sea-level and/or tectonic reorganization, would drive the locus of pyrite burial to deeper settings. A shift in the locus of pyrite formation from mixed shallow and deep to dominantly deep environments via any of these mechanisms would increase the average $\delta^{34}\text{E}_{\text{SO4-pyr}}$ (hereafter $\delta^{34}\text{E}_{\text{globalAverage}}$), thereby elevating $\delta^{34}\text{S}_{\text{SO4-sw}}$.

We favour the tectonically driven loss of shallow sedimentary environments driving pyrite burial to deeper ocean settings for several reasons. Increases in organic carbon export to the deep ocean suggested near this time are probably slightly offset by the increase in deep ocean ventilation that is also suggested. Furthermore, the timing and onset of pelagic calcifiers are thought to occur gradually$^{27}$ and, in the case of silicifiers, later than 50 Ma (ref. $^{28}$). Increases in deep ocean ventilation alone would also probably lead to a decrease in the total amount of deep pyrite burial (even if the $\delta^{34}\text{S}$ of that pyrite was lower), whereas the increase in $\delta^{13}\text{C}$ and $\delta^{34}\text{S}$ from 53 Ma instead suggests a slight global increase in pyrite burial.

The proportion of shallow shelf environments has been shown to correlate with the difference between the $\delta^{34}\text{S}$ of sulfate and pyrite over much of geological time$^{18}$. Estimates of global shelf area extrapolated from North American stratigraphy show a decline across the Cenozoic, with an accelerated loss of shelf environments at this time$^{13}$. In addition, the loss of epicratonic and shallow shelf environments

has been documented in the cessation of marine sediments in epicratonic seas in Africa–Arabia in the late Palaeogene–early Eocene$^{30}$ and Eurasia at ~50 Ma (ref. $^{31}$). There is also contemporaneous evidence for the narrowing and closure of the Neotethys$^{32}$ and rapid relative sea-level declines on the Peri-Tethyan margin$^{33–35}$. A global decrease in sea level across the early Cenozoic (Fig. 1) has also been inferred from sediment cores$^{36}$, geological reconstructions of marginal environments$^{37}$ and geochemical records from marine and marginal marine settings$^{38}$. A decline in sea level can rapidly change the distribution of burial environments, as changes in the position of sea level on the hypsometric distribution of land would have a nonlinear effect on the proportion of shallow environments (for example, ref. $^{39}$), forcing deposition to rapidly move from shallow to deeper environments.

Furthermore, the India–intra-oceanic Kohistan–Ladakh arc collision has been estimated$^{40}$ to have occurred at $50.2 \pm 1.5$ Ma. This would have resulted in the loss of a particularly tectonically active sedimentary basin (with high sedimentation rates and probable high pyrite burial). It has furthermore been suggested that continental collisions, by adding mass to the continental crust, lead to observed decreases in global sea level$^{41}$. The collision and uplift of the Himalayas, coupled with subaerial exposure of previously submerged shallow environments, would remobilize carbon and iron buried in these sediments to the ocean. Enhanced iron delivery to the ocean would promote more pyrite burial in wider and deeper environments, as sedimentary pyrite formation is typically limited by the supply of iron to sedimentary environments$^{42}$.
Modelling the Cenozoic sulfur cycle

The improved temporal connection between δ³S and δ¹³C records
allows us to quantitatively explore the Palaeogene 5‰ increase in
δ³Sₐ₅₄₃ₙ. We developed a one-box model of the marine sulfur and
carbon cycles, with constant influxes, and two burial fluxes for each
element (Supplementary Section 6). The burial flux of oxidized spe-
cies (calcium sulfate and calcium carbonate) have the same isoto-
pic composition as the ocean, and the reduced burial fluxes (pyrite
and organic carbon) have lower δ³S and δ¹³C values, respectively.
The carbon cycle portion of the model uses carbonate δ¹³C to cal-
culate steady-state organic carbon burial fluxes⁶ at each time step.
To do so, we assume a constant ¹³C/¹²C fractionation of -27‰ and
that the burial of carbonate minerals varies as a function of DIC
concentration. The initial calculated organic carbon burial flux
is 5.5×10¹² molyr⁻¹. Sulfur influx is constant at 3.5×10¹² molyr⁻¹
and +4‰ (Supplementary Section 6). The reduced burial fluxes are
coupled by assuming a sedimentary reduced C:S burial ratio of 1:1 (at
the sulfur-rich end of marine shales⁴³, although a value of the ratio
between 0.8 and 1.5 has only a minor impact on our main conclusion;
see Supplementary Section 6). Pyrite burial is split into two fluxes,
one representing burial in the deep ocean and another in nearshore
environments. Deep-ocean pyrite (³⁴εₐ₅₄₃ₚ-rd_p_p) has a sulfur isotope
composition close to, or more negative than, that observed in mod-
ern open-ocean sediments, and represents more than half of the total
pyrite burial flux (~70%). Nearshore environments are collectively
ascribed a much smaller sulfur isotope fractionation (³⁴εₐ₅₄₃ₚ-nr_p_p)
and more positive δ³Sₚ₃r values. Values of ³⁴εₐ₅₄₃ₚ-deep and
³⁴εₐ₅₄₃ₚ-nr_p_p for the model runs shown in Fig. 2 are -41.2‰ and
-10‰, respectively, although a range of values from -33‰ to -60‰
for ³⁴εₐ₅₄₃ₚ-deep and -20‰ to -3‰ for ³⁴εₐ₅₄₃ₚ-nr_p_p was tested, and
also reproduces the data (Supplementary Section 6).

We set evaporite burial, and the ratio of the pyrite burial fluxes,
such that the marine sulfur cycle starts in an isotopic steady state
at 67 Ma, resulting in an initial $^{34}\text{ε}_{\text{global\_average}}$ of -27‰ to -47‰
(Supplementary Section 6). The evaporite flux is kept constant.
After 53 Ma, shallower pyrite burial is suppressed and $^{34}\text{ε}_{\text{global\_average}}$
becomes equal to $^{34}\text{ε}_{\text{SO4-pyr-deep}}$ (Fig. 2a, Supplementary Fig. 20).
In our model, seawater sulfate concentrations gradually increase
across the Cenozoic, consistent with constraints from fluid inclu-
sions²⁷. The modelled burial fluxes for organic carbon and pyrite are
shown in Fig. 2a. The change in nearshore and deep pyrite burial for
this model run are shown in Supplementary Fig. 20.

Fully coupled runs (C:S of the reduced output remains constant throughout), do not reproduce the increasing δ¹³C with decreasing δ³S from 61 to 57 Ma, but do reproduce the 5‰ increase in δ³Sₐ₅₄₃ₙ after 53 Ma (black line in Fig. 2a,b,d), suggesting the carbon and sulfur cycles were partially decoupled until 53 Ma, and additional processes contributed to the δ¹³C record⁴,¹⁴. Although the oscillations in δ¹³C before 53 Ma are not directly coupled to changes in δ³S, there is a longer-wavelength decrease in δ¹³C, mirrored by a smaller (~1‰) decrease in δ³S (Supplementary Section 6). We therefore assume a pyrite burial flux that tracks the long-term decrease in δ¹³C from 66 to 53 Ma (Fig. 2a, 'partially coupled' scenario, red line), before pyrite burial becomes 'fully coupled' to organic carbon burial at 53 Ma. This change in carbon-sulfur coupling more readily reproduces the data for a range of sedimentary C:S ratios and accompanying changes in $^{34}\text{ε}_{\text{global\_average}}$ (for example, Fig. 2a,d; Supplementary Table 2).

The model results suggest that carbon and sulfur isotope co-
variations are partially governed by their coupled burial as organic
carbon and pyrite, but additional factors (which have a larger
impact on δ¹³C than δ³S¹⁷) sway the carbon cycle before 53 Ma
(for example, volcanism⁴⁴ or terrestrial carbon burial¹). Critically,
at most only half of the 5‰ increase in δ³Sₐ₅₄₃ₙ can be explained
by a transient increase in pyrite burial through sedimentary
C:S coupling (Supplementary Section 6). The remainder of the
change in δ³Sₐ₅₄₃ₙ must be related to changing the average isotope

composition of the burial flux, which we calculate must change
by ~10‰, resulting in a final $^{34}\text{ε}_{\text{global\_average}}$ of approximately -41‰
(Fig. 2). Our sensitivity analysis gives a full range of final $^{34}\text{ε}_{\text{global\_average}}$
values between -33‰ and -60‰ (with a decrease in $^{34}\text{ε}_{\text{global\_average}}$
of 8.5–15‰ at 53 Ma; see Supplementary Section 6). This range of
$^{34}\text{ε}_{\text{global\_average}}$ lies within reported sulfur isotope fractionations for
sulfate reduction to sulfide and subsequent preservation as pyrite⁴⁵.

Thus, a fundamental change in the location of carbon and sulfur sinks is indicated at 53 Ma, diminishing shallow, organic-rich burial environments in which carbon and sulfur can be more decoupled (for example, terrestrial carbon sinks, epicratonic seas) and increasing the dominance of deeper marine burial environments more reminiscent of modern conditions. The relocation of pyrite burial to deeper settings will increase $^{34}\text{ε}_{\text{global\_average}}$ and bury reduced sulfur with more negative δ³S values, resulting in higher $^{34}\text{S}_{\text{SO4-sw}}$ values. The shallow burial environments of the early Cenozoic never reoccur post sea-level fall, preventing a return to the previous steady state. The long-term shift between the Cretaceous mean δ¹³C value of 1.5‰ and the Cenozoic mean of 0.5‰ (Fig. 1) also coincides with the increase in $^{34}\text{S}_{\text{SO4-sw}}$.

The timing of the India–Asia convergence corresponds closely to variability in both the δ¹³C and δ³S records. The acceleration of the convergence rate is synchronous with the large-amplitude variability in δ¹³C before 53 Ma, and the subsequent increase in δ³Sₐ₅₄₃ₙ occurs at the same time as the initial arc collision and closing of the foreland basin between India and Asia⁴⁶ (Fig. 1). The accelerated convergence and collision would rapidly create and destroy burial environments for carbon (and sulfur), and could have a global impact on sea level— and therefore burial environments—that may contribute to changes in carbon–sulfur coupling at 53 Ma. The deeper insights afforded by the foraminiferal δ³Sₐ₅₄₃ₙ record into variations between the sulfur and carbon cycles over the Cenozoic highlight the strengths of the foraminiferal δ³Sₐ₅₄₃ₙ proxy, which can in future be extended to examine the relationship between δ³Sₐ₅₄₃ₙ and other foraminiferal-hosted proxies, as well as across other time intervals of interest.

**Data availability.** The authors declare that all the data and MATLAB
codes supporting the finding of this study are available within the
article and its supplementary information files.

Received: 4 February 2018; Accepted: 5 July 2018;
Published online: 13 August 2018

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## Acknowledgements

We thank M. Vautravers for her expertise with Palaeogene foraminifera and S. Misra, G. Antler and W. Fisher for helpful discussions and advice. This work was supported by a 'Small Sulfur' NERC grant (NERC NE/H011595/1), the ERC (ERC StG 307582, CARBONSINK to A.V.T.) and a NERC studentship to V.C.F.R.

## Author contributions

V.C.F.R. designed the cleaning tests, did the lab work and analysis for trace element analysis and sulfur isotopes, and wrote the model and the paper. G.P. advised on the cleaning tests, did sulfate and sulfur isotope preparation and analysis, was heavily involved in the modelling and wrote the paper. A.L.S. provided equipment and fruitful discussions. S.A. picked the foraminifera for the stable isotope analyses. A.V.T. provided extensive advice and funding at all stages, had the idea for the shift in $^{34}$S$^{SO_4^{2-}}$ and wrote the paper. J.F.A. provided guidance and lab equipment for lab work and analysis, advised extensively on the model and wrote the paper.

## Competing interests

The authors declare no competing interests.

## Additional information

Supplementary information is available for this paper at https://doi.org/10.1038/s41561-018-0200-y.

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