Decadal, sediment-driven sulfur isotope evolution in Lake Cadagno
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Keywords: sulfur isotope composition, pyrite, euxinia
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Abstract
Figures
 Figure 1 SO42− and H2S concentrations and δ34S values in the water column (upper panels) and the porewaters (lower panels) of Lake Cadagno, from four different sampling campaigns. (a) SO42− and H2S concentrations in the water column, (b) δ34S of SO42−* and H2S in the water column, (c) SO42− and H2S concentrations in the sediment porewaters, (d) δ34S of H2S in the sediment porewaters. Error bars are smaller than symbols. The colour shading indicates the sampling year. Data published in *Canfield et al. (2010) and **Berg et al. (2025). |  Figure 2 Sulfur isotope compositions of water column SO42− and H2S, porewater H2S, CRS and pyrite grains in the sediment uppermost 5 cm. (a) δ34S values of the aforementioned species. Deep water-column data are represented by diamonds on the upper edge of the plot. Porewater H2S (downward pointing triangles) and CRS (upward pointing triangle) data are represented on the lower boundary of the plot. SIMS data are represented by the histogram with n the number of analyses. (b) Δpyr values of CRS and pyrite grains (Δpyr = δ34Ssulfate − δ34SCRS or pyr). The mean δ34S value of deep water-column SO42− (+24.7 ‰) is reported at 0 ‰ for comparison. The maximum microbial isotope fractionation εmic = 71.8 ‰ between SO42− and H2S in surface sediments measured by Canfield et al. (2010) is represented by the grey band. The microbial isotope fractionation is defined as εmic = 1000 × ((34S/32S)SO4/(34S/32S)H2S − 1). (c, d) SEM micrographs of framboidal pyrite analysed by SIMS and their δ34S values (the scalebar is 5 μm). |  Figure 3 Schematic representation of (a) the traditional model of euxinia showing the reservoir effect during MSR (adapted from Gomes and Hurtgen, 2015), and (b) H2S accumulation in Lake Cadagno. The concentration and δ34S trends of SO42− and H2S are shown as a function of water and sediment depth. The dashed lines represent the initial δ34S value of SO42−. The isotopic offset between SO42− and H2S in (a) corresponds to the microbial isotope fractionation εmic, while the isotopic offset Δ is smaller in (b). |
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Introduction
Earth’s biogeochemical sulfur (S) cycle is predominantly driven by microbial activity (Jorgensen, 1982
Jørgensen, B.B. (1982) Mineralization of organic matter in the sea bed—the role of sulphate reduction. Nature 296, 643–645. https://doi.org/10.1038/296643a0
). This cycle regulates organic matter remineralisation and Earth’s surface redox balance through sedimentary burial of oxidised and reduced S species, with microbial sulfate reduction (MSR) as the dominant S-based metabolic pathway (Jørgensen et al., 2019Jørgensen, B.B., Findlay, A.J., Pellerin, A. (2019) The Biogeochemical Sulfur Cycle of Marine Sediments. Frontiers in Microbiology 10, 849. https://doi.org/10.3389/fmicb.2019.00849
). The sulfide (H2S) produced during MSR is depleted in 34S compared to initial SO42−, leading to the precipitation of 34S-depleted sulfide minerals, such as pyrite (Berner, 1984Berner, R.A. (1984) Sedimentary pyrite formation: An update. Geochimica et Cosmochimica Acta 48, 605–615. https://doi.org/10.1016/0016-7037(84)90089-9
).Sulfate concentrations were proposed as a dominant factor controlling the extent of microbial isotope fractionation during MSR (Habicht et al., 2002
Habicht, K.S., Gade, M., Thamdrup, B., Berg, P., Canfield, D.E. (2002) Calibration of Sulfate Levels in the Archean Ocean. Science 298, 2372–2374. https://doi.org/10.1126/science.1078265
; Gomes and Hurtgen, 2015Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
). For example, large apparent S isotope fractionation is observed in modern stratified systems harbouring SO42− concentrations larger than 15 mM (between 50 and 60 ‰; e.g., in the Black Sea, Cariaco Basin and Green Lake; Sweeney and Kaplan, 1980Sweeney, R.E., Kaplan, I.R. (1980) Stable isotope composition of dissolved sulfate and hydrogen sulfide in the Black Sea. Marine Chemistry 9, 145–152. https://doi.org/10.1016/0304-4203(80)90064-X
; Li et al., 2010Li, X., Gilhooly III, W.P., Zerkle, A.L., Lyons, T.W., Farquhar, J., Werne, J.P., Varela, R., Scranton, M.I. (2010) Stable sulfur isotopes in the water column of the Cariaco Basin. Geochimica et Cosmochimica Acta 74, 6764–6778. https://doi.org/10.1016/j.gca.2010.08.020
; Zerkle et al., 2010Zerkle, A.L., Kamyshny Jr., A., Kump, L.R., Farquhar, J., Oduro, H., Arthur, M.A. (2010) Sulfur cycling in a stratified euxinic lake with moderately high sulfate: Constraints from quadruple S isotopes. Geochimica et Cosmochimica Acta 74, 4953–4970. https://doi.org/10.1016/j.gca.2010.06.015
; Gomes and Hurtgen, 2015Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
). Below this threshold, the apparent S isotope fractionation decreases to 5 ‰ at micromolar SO42− levels (Gomes and Hurgten, 2015Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
). Microbiological factors, such as the cell specific sulfate reduction rate, which is itself a function of the microbial community, the temperature, the electron donor and the amount of sulfate, have also been recognised as key drivers of S isotope fractionation (Brunner and Bernasconi, 2005Brunner, B., Bernasconi, S.M. (2005) A revised isotope fractionation model for dissimilatory sulfate reduction in sulfate reducing bacteria. Geochimica et Cosmochimica Acta 69, 4759–4771. https://doi.org/10.1016/j.gca.2005.04.015
; Sim et al., 2011Sim, M.S., Ono, S., Donovan, K., Templer, S.P., Bosak, T. (2011) Effect of electron donors on the fractionation of sulfur isotopes by a marine Desulfovibrio sp. Geochimica et Cosmochimica Acta 75, 4244–4259. https://doi.org/10.1016/j.gca.2011.05.021
; Bradley et al., 2016Bradley, A.S., Leavitt, W.D., Schmidt, M., Knoll, A.H., Girguis, P.R., Johnston, D.T. (2016) Patterns of sulfur isotope fractionation during microbial sulfate reduction. Geobiology 14, 91–101. https://doi.org/10.1111/gbi.12149
). Recently, empirical and modelling studies have pointed to the role of local sedimentary conditions, such as sedimentation rate or reactive iron availability, in modulating the S isotope composition recorded by pyrite (e.g., Pasquier et al., 2017Pasquier, V., Sansjofre, P., Rabineau, M., Revillon, S., Houghton, J., Fike, D.A. (2017) Pyrite sulfur isotopes reveal glacial−interglacial environmental changes. Proceedings of the National Academy of Sciences 114, 5941–5945. https://doi.org/10.1073/pnas.1618245114
; Marin Carbonne et al., 2022Marin-Carbonne, J., Decraene, M.-N., Havas, R., Remusat, L., Pasquier, V., Alléon, J., Zeyen, N., Bouton, A., Bernard, S., Escrig, S., Olivier, N., Vennin, E., Meibom, A., Benzerara, K., Thomazo, C. (2022) Early precipitated micropyrite in microbialites: A time capsule of microbial sulfur cycling. Geochemical Perspectives Letters 21, 7–12. https://doi.org/10.7185/geochemlet.2209
; Bryant et al., 2023Bryant, R.N., Houghton, J.L., Jones, C., Pasquier, V., Halevy, I., Fike, D.A. (2023) Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios. Science 382, 912–915. https://doi.org/10.1126/science.adg6103
; Halevy et al., 2023Halevy, I., Fike, D.A., Pasquier, V., Bryant, R.N., Wenk, C.B., Turchyn, A.V., Johnston, D.T., Claypool, G.E. (2023) Sedimentary parameters control the sulfur isotope composition of marine pyrite. Science 382, 946–951. https://doi.org/10.1126/science.adh1215
). Taken together, these studies highlight the complexity in interpreting S isotope signatures in sedimentary pyrite.Pyrite texture, size, trace element composition and S isotope composition (δ34S) have been widely used to reconstruct the extent and duration of euxinic conditions in past oceans (Fakhraee et al., 2025
Fakhraee, M., Crockford, P.W., Bauer, K.W., Pasquier, V., Sugiyama, I., Katsev, S., Raven, M.R., Gomes, M., Philippot, P., Crowe, S.A., Tarhan, L.G., Lyons, T.W., Planavsky, N. (2025) The history of Earth’s sulfur cycle. Nature Reviews Earth & Environment 6, 106–125. https://doi.org/10.1038/s43017-024-00615-0
) — environments characterised by water column oxygen depletion and dissolved H2S enrichment. Widespread euxinia is thought to repeatedly occur during the Phanerozoic and was likely associated with several mass extinctions (Lyons et al., 2009Lyons, T.W., Anbar, A.D., Severmann, S., Scott, C., Gill, B.C. (2009) Tracking Euxinia in the Ancient Ocean: A Multiproxy Perspective and Proterozoic Case Study. Annual Review of Earth and Planetary Sciences 37, 507–534. https://doi.org/10.1146/annurev.earth.36.031207.124233
). Permanent euxinia is currently scarce and only occurs in stratified lakes, fjords, coastal upwelling zones and restricted marine basins (Meyer and Kemp, 2008Meyer, K.M., Kump, L.R. (2008) Oceanic Euxinia in Earth History: Causes and Consequences. Annual Review of Earth and Planetary Sciences 36, 251–288. https://doi.org/10.1146/annurev.earth.36.031207.124256
).The alpine meromictic Lake Cadagno has been extensively used as a model for Proterozoic low sulfate oceans and euxinic conditions (e.g., Canfield et al., 2010
Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
; Ellwood et al., 2019Ellwood, M.J., Hassler, C., Moisset, S., Pascal, L., Danza, F., Peduzzi, S., Tonolla, M., Vance, D. (2019) Iron isotope transformations in the meromictic Lake Cadagno. Geochimica et Cosmochimica Acta 255, 205–221. https://doi.org/10.1016/j.gca.2019.04.007
; Janssen et al., 2022Janssen, D.J., Rickli, J., Wille, M., Sepulveda Steiner, O., Vogel, H., Dellwig, O., Berg, J.S., Bouffard, D., Lever, M.A., Hassler, C.S., Jaccard, S.L. (2022) Chromium Cycling in Redox-Stratified Basins Challenges δ53Cr Paleoredox Proxy Applications. Geophysical Research Letters 49, e2022GL099154. https://doi.org/10.1029/2022GL099154
). A continuous supply of dense, solute-rich waters from subaquatic springs over the past 9000 years has maintained strong water column stratification (Del Don et al., 2001Del Don, C., Hanselmann, K.W., Peduzzi, R., Bachofen, R. (2001) The meromictic alpine Lake Cadagno: Orographical and biogeochemical description. Aquatic Sciences 63, 70–90. https://doi.org/10.1007/PL00001345
; Wirth et al., 2013Wirth, S.B., Gilli, A., Niemann, H., Dahl, T.W., Ravasi, D., Sax, N., Hamann, Y., Peduzzi, R., Peduzzi, S., Tonolla, M., Lehmann, M.F., Anselmetti, F.S. (2013) Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps). Geochimica et Cosmochimica Acta 120, 220–238. https://doi.org/10.1016/j.gca.2013.06.017
). Moderate SO42− levels (up to 2 mM) fuel MSR in the anoxic deep waters. A previous analysis of S multiple-isotope composition of dissolved SO42− and H2S in the water column highlighted the predominant role of MSR in driving the S cycle in Lake Cadagno and revealed a large S isotope fractionation during MSR in surface sediment (71.8 ‰ during incubation experiments; Canfield et al., 2010Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
). Our study explores whether comparable isotope fractionation occurs under natural conditions and whether such fractionation is preserved in pyrite. To answer those questions, we present microscale S isotope compositions of pyrite from surface sediments along with the corresponding bulk S isotope composition of chemically extracted pyrite. In addition, new (i.e. sampled in 2022) multiple S isotope composition of water column and porewater H2S are compared to previously published isotope compositions of porewater H2S and water column H2S (Berg et al., 2025Berg, J.S., Rodriguez, P.C., Magnabosco, C., Deng, L., Bernasconi, S.M., Vogel, H., Morlock, M., Lever, M.A. (2025) Microbial sulfur cycling across a 13 500-year-old lake sediment record. Biogeosciences 22, 5483–5496. https://doi.org/10.5194/bg-22-5483-2025
; Canfield et al., 2010Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
, respectively). This comparison enables, for the first time, a reconstruction of the multi-decadal evolution of S isotopes in the water column and their subsequent preservation in sediments. Our results reveal that the H2S pool in Lake Cadagno is highly dynamic and that near-equilibrium microbial isotope fractionation is poorly recorded in surface sediment pyrite. These results have strong implications for ancient environmental reconstructions of euxinia and SO42− concentrations.top
Results
New S isotope data were obtained on water column and porewater H2S samples collected in 2022, as well as on pyrite in surface sediments (0–5 cm depth). These data are compared with earlier isotopic measurements from samples collected in 1991 (Berg et al., 2025
Berg, J.S., Rodriguez, P.C., Magnabosco, C., Deng, L., Bernasconi, S.M., Vogel, H., Morlock, M., Lever, M.A. (2025) Microbial sulfur cycling across a 13 500-year-old lake sediment record. Biogeosciences 22, 5483–5496. https://doi.org/10.5194/bg-22-5483-2025
), and 2006, 2007 and 2008 (Canfield et al., 2010Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
). Considering the local sedimentation rate of 2.4 mm yr−1 for surface sediments (Wirth et al., 2013Wirth, S.B., Gilli, A., Niemann, H., Dahl, T.W., Ravasi, D., Sax, N., Hamann, Y., Peduzzi, R., Peduzzi, S., Tonolla, M., Lehmann, M.F., Anselmetti, F.S. (2013) Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps). Geochimica et Cosmochimica Acta 120, 220–238. https://doi.org/10.1016/j.gca.2013.06.017
), the uppermost 5 cm of the sediments correspond to the past 20 years.Sulfate concentration in the water column in 2006 increased with depth, exceeding 2 mM below 16 m (Fig. 1a). Between 2006 and 2008, sulfate δ34S values were positive and roughly constant within the oxic surface waters (+16.7 ± 2.8 ‰, n = 26). They increased in the euxinic interval to reach +24.7 ± 4.9 ‰ above the sediment-water interface (SWI; Fig. 1b). In 2022 sediments, porewater SO42− concentrations decreased with depth, from 0.9 mM at the SWI to less than 0.2 mM at 25 cm (Fig. 1b).
Figure 1 SO42− and H2S concentrations and δ34S values in the water column (upper panels) and the porewaters (lower panels) of Lake Cadagno, from four different sampling campaigns. (a) SO42− and H2S concentrations in the water column, (b) δ34S of SO42−* and H2S in the water column, (c) SO42− and H2S concentrations in the sediment porewaters, (d) δ34S of H2S in the sediment porewaters. Error bars are smaller than symbols. The colour shading indicates the sampling year. Data published in *Canfield et al. (2010
Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
) and **Berg et al. (2025Berg, J.S., Rodriguez, P.C., Magnabosco, C., Deng, L., Bernasconi, S.M., Vogel, H., Morlock, M., Lever, M.A. (2025) Microbial sulfur cycling across a 13 500-year-old lake sediment record. Biogeosciences 22, 5483–5496. https://doi.org/10.5194/bg-22-5483-2025
).In 2006, H2S concentration in the water column also increased with depth to values higher than 0.25 mM (Fig. 1a). In 2022, water column H2S concentrations were lower (<100 μM). During all sampling campaigns, δ34S values of water column H2S were negative. Values slightly increased in the chemocline and, when each campaign is considered individually, remained nearly constant below (variability <2 ‰, 2 s.d.). Nevertheless, comparison of the deepest water column samples shows an increase in δ34S values of H2S with time of over 15 ‰, from −19.62 ‰ in 2006 to −4.40 ‰ in 2022 (Fig. 1b). A comparable trend is observed in surficial sediment porewater H2S (Fig. 1d), with δ34S values increasing by ∼20 ‰ from 1991 to 2022. In 1991, δ34S values of porewater H2S were negative near the SWI and progressively becoming positive with depth (Fig. 1d). In 2022, they were consistently positive and comparable to those measured below 15 cm in 1991 (Fig. 1d).
Sedimentary pyrite is present in the uppermost 5 cm surface sediment mostly as framboidal pyrite (8 to 20 μm in diameter; Fig. 2c,d), and euhedral grains (20 to 30 μm in size). Pyrite was analysed by secondary ion mass spectrometry (SIMS, see Supplementary Information) and displays a wide range of δ34S values, from −54.9 ± 0.2 ‰ to +4.6 ± 0.2 ‰ (2 s.d.). The SIMS median value (δ34S = −10.5 ‰) aligns well with bulk chemically extracted pyrite (δ34SCRS = −4.0 ± 1.2 ‰; weighted average of 3 bulk measurements between 1 and 5 cm deep; Fig. 2a). Over 37 pyrite grains analysed by SIMS, one framboidal grain displays a δ34S value of −54.9 ± 0.2 ‰, while the remaining grains range from −28.7 ± 0.2 ‰ to +4.6 ± 0.2 ‰. Furthermore, total organic carbon (TOC) content decreases strongly with depth, from 15 wt. % in the uppermost 10 cm of the sediment, where sediments derive from settling water column particles and include chemocline-produced organic matter, to 2 wt. % below 20 cm, where sediments consist of remobilised shallow deposits rich in detrital material (Fig. S-1; Wirth et al., 2013
Wirth, S.B., Gilli, A., Niemann, H., Dahl, T.W., Ravasi, D., Sax, N., Hamann, Y., Peduzzi, R., Peduzzi, S., Tonolla, M., Lehmann, M.F., Anselmetti, F.S. (2013) Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps). Geochimica et Cosmochimica Acta 120, 220–238. https://doi.org/10.1016/j.gca.2013.06.017
).Figure 2 Sulfur isotope compositions of water column SO42− and H2S, porewater H2S, CRS and pyrite grains in the sediment uppermost 5 cm. (a) δ34S values of the aforementioned species. Deep water-column data are represented by diamonds on the upper edge of the plot. Porewater H2S (downward pointing triangles) and CRS (upward pointing triangle) data are represented on the lower boundary of the plot. SIMS data are represented by the histogram with n the number of analyses. (b) Δpyr values of CRS and pyrite grains (Δpyr = δ34Ssulfate − δ34SCRS or pyr). The mean δ34S value of deep water-column SO42− (+24.7 ‰) is reported at 0 ‰ for comparison. The maximum microbial isotope fractionation εmic = 71.8 ‰ between SO42− and H2S in surface sediments measured by Canfield et al. (2010)
Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
is represented by the grey band. The microbial isotope fractionation is defined as εmic = 1000 × ((34S/32S)SO4/(34S/32S)H2S − 1). (c, d) SEM micrographs of framboidal pyrite analysed by SIMS and their δ34S values (the scalebar is 5 μm).top
Discussion
Sedimentary origin of the large isotopic variability of sulfide. Over the past three decades, we observe a 34S enrichment of 15 to 20 ‰ of water column H2S, shifting its δ34S values towards water column SO42− (Fig. 1d). The sharp decline in SO42− concentrations between the bottom waters and the uppermost sediment, together with the peak in H2S concentration within the sediment (Fig. 1a,c) indicate intense MSR activity near and below the SWI. Furthermore, the steep gradient in H2S concentration in surface sediment and the non-zero H2S concentration at the SWI (Fig. 1c) implies that microbially generated H2S within the sediment porewaters is diffusing into the water column (see also Hanselmann and Hutter, 1998
Hanselmann, K., Hutter, R. (1998) Geomicrobiological coupling of sulfur and iron cycling in anoxic sediments of a meromictic lake: sulfate reduction and sulfide sources and sinks in Lake Cadagno. Documenta dell’Istituto Italiano di Idrobiologia 63, 85–98.
). This benthic H2S flux exceeds the integrated rate of H2S production via MSR in the water column by three orders of magnitude, highlighting the significant contribution of H2S produced in the sediment to the water column pool (see Supplementary Information). Considering the 34S enrichment of porewater H2S between 1991 and 2022 (Fig. 1d), our mass balance calculations show that the benthic H2S flux drives an increase in δ34S of water column H2S (see Supplementary Information). Consequently, we suggest that the decadal δ34S evolution of water column H2S is likely driven by the δ34S increase of the benthic flux of H2S.The observed δ34S increase of porewater H2S over the past decades is possibly related to the dynamic sedimentation regime of Lake Cadagno. As SO42− is nearly consumed within the uppermost 20 cm of sediment (Fig. 1c) through organoclastic SO42− reduction and SO42− reduction coupled to anaerobic oxidation of methane (SR-AOM; Canfield et al., 2010
Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
; Su et al., 2020Su, G., Zopfi, J., Yao, H., Steinle, L., Niemann, H., Lehmann, M.F. (2020) Manganese/iron-supported sulfate-dependent anaerobic oxidation of methane by archaea in lake sediments. Limnology and Oceanography 65, 863–875. https://doi.org/10.1002/lno.11354
), residual porewater SO42− likely becomes 34S enriched over the same depth interval (Jorgensen et al., 2004Jørgensen, B.B., Böttcher, M.E., Lüschen, H., Neretin, L.N., Volkov, I.I. (2004) Anaerobic methane oxidation and a deep H2S sink generate isotopically heavy sulfides in Black Sea sediments. Geochimica et Cosmochimica Acta 68, 2095–2118. https://doi.org/10.1016/j.gca.2003.07.017
; Berg et al., 2025Berg, J.S., Rodriguez, P.C., Magnabosco, C., Deng, L., Bernasconi, S.M., Vogel, H., Morlock, M., Lever, M.A. (2025) Microbial sulfur cycling across a 13 500-year-old lake sediment record. Biogeosciences 22, 5483–5496. https://doi.org/10.5194/bg-22-5483-2025
). Sulfide produced by MSR in the same layer is expected to follow a comparable trend, as observed in 1991 (Fig. 1d). In 2022, however, the constant δ34S values of porewater H2S are similar to the 1991 δ34S values at depth (Fig. 1d) and could reflect upward diffusion of H2S produced from residual 34S enriched SO42− at depth, as observed elsewhere (Jorgensen et al., 2004Jørgensen, B.B., Böttcher, M.E., Lüschen, H., Neretin, L.N., Volkov, I.I. (2004) Anaerobic methane oxidation and a deep H2S sink generate isotopically heavy sulfides in Black Sea sediments. Geochimica et Cosmochimica Acta 68, 2095–2118. https://doi.org/10.1016/j.gca.2003.07.017
; Liu et al., 2021Liu, J., Pellerin, A., Antler, G., Izon, G., Findlay, A.J., Røy, H., Ono, S., Kasten, S., Turchyn, A.V., Jørgensen, B.B. (2021) Early diagenesis of sulfur in Bornholm Basin sediments: The role of upward diffusion of isotopically “heavy” sulfide. Geochimica et Cosmochimica Acta 313, 359–377. https://doi.org/10.1016/j.gca.2021.08.018
; Pasquier et al., 2025Pasquier, V., Marin-Carbonne, J., Giunta, T., Ruffine, L., Halevy, I. (2025) Microscale iron and sulphur isotopic compositions reveal pyritization pathways during early diagenesis. Communications Earth & Environment 6, 248. https://doi.org/10.1038/s43247-025-02213-4
). In Lake Cadagno, the ongoing deposition of an organic-rich sedimentary layer above comparatively organic-poor turbidites may cause an upward migration of the sulfate-methane transition zone with time. The absence of dilution by turbiditic deposition in the uppermost 10 cm enhances the organic flux to the sediment, allowing a greater proportion of organic carbon to escape heterotrophic respiration and fuel methanogenesis in deeper layers (Kasten et al., 2003Kasten, S., Zabel, M., Heuer, V., Hensen, C. (2003) Processes and Signals of Nonsteady-State Diagenesis in Deep-Sea Sediments and their Pore Waters. In: Wefer, G., Mulitza, S., Ratmeyer, V. (Eds.) The South Atlantic in the Late Quaternary. Springer, Berlin, Heidelberg, 431–459. https://doi.org/10.1007/978-3-642-18917-3_20
). The resulting larger ascending flux of methane would bring SR-AOM closer to SWI, thus increasing the production of SR-AOM derived H2S and consequently modifying the S isotope composition of the H2S reservoir due to mixing and pooling. Such non-steady state diagenetic processes could explain fluctuations in the H2S pool, although further investigation is needed to confirm this.H2S pool dynamics control the S isotope composition of pyrite. Sulfur isotope fractionation during MSR can be recorded at the microscale by individual pyrite grains and approached through the calculation of Δpyr, which corresponds to the δ34S difference between water column SO42− and individual pyrite grains (Bryant et al., 2023
Bryant, R.N., Houghton, J.L., Jones, C., Pasquier, V., Halevy, I., Fike, D.A. (2023) Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios. Science 382, 912–915. https://doi.org/10.1126/science.adg6103
). In the uppermost 5 cm of sediment, one framboidal pyrite grain Δpyr value (79.6 ‰) is roughly consistent with the near-equilibrium microbial isotope fractionation (71.8 ‰) previously measured in Lake Cadagno sediment incubation (Fig. 2b; Canfield et al., 2010Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
). This observation has important implications for our understanding of MSR activity in natural environments, as despite elevated TOC content (15 wt. %) and intense SO42− reduction rates (SRR) at the community level in Lake Cadagno surface sediment, such large microbial isotope fractionation implies slow cell-specific SRR (Wing and Halevy, 2014Wing, B.A., Halevy, I. (2014) Intracellular metabolite levels shape sulfur isotope fractionation during microbial sulfate respiration. Proceedings of the National Academy of Sciences 111, 18116–18125. https://doi.org/10.1073/pnas.1407502111
; Halevy et al., 2023Halevy, I., Fike, D.A., Pasquier, V., Bryant, R.N., Wenk, C.B., Turchyn, A.V., Johnston, D.T., Claypool, G.E. (2023) Sedimentary parameters control the sulfur isotope composition of marine pyrite. Science 382, 946–951. https://doi.org/10.1126/science.adh1215
).Nevertheless, the microbial S isotope fractionation was overall rarely recorded in the surface sediment, as all other microscale Δpyr values are <55 ‰. Most microscale δ34Spyr values and the δ34SCRS value fall within the δ34S range of dissolved H2S measured from 1991 to 2022, encompassing H2S both in deep water-column and porewater (Fig 2a). Therefore, pyrite in the uppermost 5 cm of sediment recorded the δ34S evolution of H2S over the past decades. Consistently, framboid size exceeds 8 μm, which indicate a sedimentary origin of pyrite (Rickard, 2019
Rickard, D. (2019) How long does it take a pyrite framboid to form? Earth and Planetary Science Letters 513, 64–68. https://doi.org/10.1016/j.epsl.2019.02.019
), in line with the lack of water column pyrite observation (Dahl et al., 2010Dahl, T.W., Anbar, A.D., Gordon, G.W., Rosing, M.T., Frei, R., Canfield, D.E. (2010) The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland. Geochimica et Cosmochimica Acta 74, 144–163. https://doi.org/10.1016/j.gca.2009.09.018
). This interpretation is further supported by microscale δ34Spyr values, some of which are more positive than water column H2S but still lie within the δ34S range of porewater H2S (Fig. 2a). We therefore infer that framboids formed in the sediment porewaters, thereby recording the temporal δ34S evolution of the pooled porewater H2S reservoir (Pasquier et al., 2025Pasquier, V., Marin-Carbonne, J., Giunta, T., Ruffine, L., Halevy, I. (2025) Microscale iron and sulphur isotopic compositions reveal pyritization pathways during early diagenesis. Communications Earth & Environment 6, 248. https://doi.org/10.1038/s43247-025-02213-4
).The poor preservation of the microbial S isotope fractionation together with the constant porewater sulfide δ34S values suggest that sulfide was isotopically homogenised before it precipitated as pyrite (Fig. 2b). Sulfide accumulates in the surface porewaters up to 0.9 mM, likely driven by rapid bulk sulfate reduction rates relative to sulfide sequestration as pyrite. Under these conditions, the mixing of H2S produced at different depths in the sediment — originating from a residual SO42− pool with varying degrees of 34S enrichment — would dilute the local, microbially produced 34S-depleted H2S in surface sediment (Fike et al., 2015
Fike, D.A., Bradley, A.S., Rose, C.V. (2015) Rethinking the Ancient Sulfur Cycle. Annual Review of Earth and Planetary Sciences 43, 593–622. https://doi.org/10.1146/annurev-earth-060313-054802
; Pasquier et al., 2021Pasquier, V., Fike, D.A., Halevy, I. (2021) Sedimentary pyrite sulfur isotopes track the local dynamics of the Peruvian oxygen minimum zone. Nature Communications 12, 4403. https://doi.org/10.1038/s41467-021-24753-x
). Consequently, sedimentary pyrite δ34S values are expected to correspond to those of pooled H2S, masking near-equilibrium isotope fractionation signatures (Pasquier et al., 2025Pasquier, V., Marin-Carbonne, J., Giunta, T., Ruffine, L., Halevy, I. (2025) Microscale iron and sulphur isotopic compositions reveal pyritization pathways during early diagenesis. Communications Earth & Environment 6, 248. https://doi.org/10.1038/s43247-025-02213-4
).An alternative model for pyrite isotope record of euxinic systems. Our study offers an alternative perspective to the traditional view that attributes H2S reservoir dynamics in euxinic environments to in situ water column MSR (Habicht et al., 2002
Habicht, K.S., Gade, M., Thamdrup, B., Berg, P., Canfield, D.E. (2002) Calibration of Sulfate Levels in the Archean Ocean. Science 298, 2372–2374. https://doi.org/10.1126/science.1078265
; Gomes and Hurtgen, 2015Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
). The traditional model relies on preferential consumption of 32S during MSR in the water column, which yields a progressive 34S enrichment of the residual SO42− and produced H2S (Fig. 3a). When the SO42− reservoir is small, the 34S enrichment becomes significant and the δ34S offset between surface SO42− and water column, microbially produced H2S can be reduced (Gomes and Hurtgen, 2015Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
). δ34S values of H2S are therefore controlled by the fraction of SO42− consumed during MSR. Under such conditions, if pyrite forms in the water column and records the δ34S values of local H2S, then, ignoring potential diagenetic overprinting, the Δpyr values preserved in sediment can be used to reconstruct water column MSR and/or sulfur cycle. In this model, diagenetic pyrite formation is considered minimal, as Fe is largely pyritised in the water column, leaving the sediment relatively Fe-limited (Gomes and Hurtgen, 2015Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
; Zerkle et al., 2010Zerkle, A.L., Kamyshny Jr., A., Kump, L.R., Farquhar, J., Oduro, H., Arthur, M.A. (2010) Sulfur cycling in a stratified euxinic lake with moderately high sulfate: Constraints from quadruple S isotopes. Geochimica et Cosmochimica Acta 74, 4953–4970. https://doi.org/10.1016/j.gca.2010.06.015
).Figure 3 Schematic representation of (a) the traditional model of euxinia showing the reservoir effect during MSR (adapted from Gomes and Hurtgen, 2015
Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
), and (b) H2S accumulation in Lake Cadagno. The concentration and δ34S trends of SO42− and H2S are shown as a function of water and sediment depth. The dashed lines represent the initial δ34S value of SO42−. The isotopic offset between SO42− and H2S in (a) corresponds to the microbial isotope fractionation εmic, while the isotopic offset Δ is smaller in (b).In Lake Cadagno, euxinia is maintained by the influx of SO42−-rich waters to the lake and sedimentary production of H2S (Fig. 3b). Elevated community-level SRR in the sediment drive the build up and mixing of H2S, from which pyrite rapidly forms and thereby records the δ34S of the pooled H2S reservoir. Excess sulfide can diffuse into the water column and up to the chemocline, where it undergoes oxidation. This model may be relevant for aqueous systems where surface sediments feature steep H2S gradients, with higher H2S concentrations in the sediment than in the overlying water column.
Sulfide accumulation is inherent to all euxinic systems, including euxinic sediment overlain by non-euxinic waters. In such settings, the pooling of H2S and isotopic homogenisation also lead to the formation of sedimentary pyrite that is relatively enriched in 34S (Liu et al., 2021
Liu, J., Pellerin, A., Antler, G., Izon, G., Findlay, A.J., Røy, H., Ono, S., Kasten, S., Turchyn, A.V., Jørgensen, B.B. (2021) Early diagenesis of sulfur in Bornholm Basin sediments: The role of upward diffusion of isotopically “heavy” sulfide. Geochimica et Cosmochimica Acta 313, 359–377. https://doi.org/10.1016/j.gca.2021.08.018
; Pasquier et al., 2025Pasquier, V., Marin-Carbonne, J., Giunta, T., Ruffine, L., Halevy, I. (2025) Microscale iron and sulphur isotopic compositions reveal pyritization pathways during early diagenesis. Communications Earth & Environment 6, 248. https://doi.org/10.1038/s43247-025-02213-4
). Consequently, the S isotope composition of sedimentary pyrite alone may not be able to distinguish between euxinic conditions in the water column and those restricted to the sediment.top
Conclusions
The comparison of H2S and SO42− isotope data from Lake Cadagno over three decades revealed a strikingly dynamic H2S pool. The δ34S increase in both porewater and water column H2S is best explained by sedimentary processes, and this increase is faithfully recorded in pyrite that formed over this period. This study emphasises the role of local sedimentary dynamics in shaping pyrite δ34S signatures. By combining microscale analyses with conventional bulk S measurements and long term monitoring, we provide new insights into the complex pathways of pyrite formation and the biogeochemical S cycle in dynamic sedimentary systems, highlighting the need for a more integrated approach when studying such modern systems.
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Acknowledgements
We thank Jasmine Berg, Alice Bosco Santos, Giulia Ceriotti, Dave Janssen and Nina Zeyen for their assistance during the field campaign. We would also like to acknowledge the contribution of Kurt Hanselmann for providing the sediment peepers and for field assistance that made possible the sampling of the porewaters in 1991. We acknowledge the use of the research facility provided by the Alpine Biology Center during the field campaign. We thank Anne-Sophie Bouvier, Florent Plane and Thomas Bovay for their support during SIMS analytical sessions. We also thank Caroline Gorge for her assistance with the ionic chromatography measurements. We are grateful to the two anonymous reviewers for their constructive comments on the manuscript and to Claudine Stirling for editorial handling. JD and JMC acknowledge the financial support of the European Research Council (ERC) under the European Union’s Horizon program (STROMATA; Grant 759289). JD was also supported by the Swiss National Science Foundation (grant number 200020_212159). SEM and SIMS analyses were performed at the Center for Advanced Surface Analysis, a collaborative research centre between the University of Lausanne and EPFL.
Editor: Claudine Stirling
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References
Berg, J.S., Rodriguez, P.C., Magnabosco, C., Deng, L., Bernasconi, S.M., Vogel, H., Morlock, M., Lever, M.A. (2025) Microbial sulfur cycling across a 13 500-year-old lake sediment record. Biogeosciences 22, 5483–5496. https://doi.org/10.5194/bg-22-5483-2025
Show in context In addition, new (i.e. sampled in 2022) multiple S isotope composition of water column and porewater H2S are compared to previously published isotope compositions of porewater H2S and water column H2S (Berg et al., 2025; Canfield et al., 2010, respectively).
View in article
New S isotope data were obtained on water column and porewater H2S samples collected in 2022, as well as on pyrite in surface sediments (0–5 cm depth). These data are compared with earlier isotopic measurements from samples collected in 1991 (Berg et al., 2025), and 2006, 2007 and 2008 (Canfield et al., 2010).
View in article
Data published in *Canfield et al. (2010) and **Berg et al. (2025).
View in article
As SO42− is nearly consumed within the uppermost 20 cm of sediment (Fig. 1c) through organoclastic SO42− reduction and SO42− reduction coupled to anaerobic oxidation of methane (SR-AOM; Canfield et al., 2010; Su et al., 2020), residual porewater SO42− likely becomes 34S enriched over the same depth interval (Jorgensen et al., 2004; Berg et al., 2025).
View in article
Berner, R.A. (1984) Sedimentary pyrite formation: An update. Geochimica et Cosmochimica Acta 48, 605–615. https://doi.org/10.1016/0016-7037(84)90089-9
Show in context The sulfide (H2S) produced during MSR is depleted in 34S compared to initial SO42−, leading to the precipitation of 34S-depleted sulfide minerals, such as pyrite (Berner, 1984).
View in article
Bradley, A.S., Leavitt, W.D., Schmidt, M., Knoll, A.H., Girguis, P.R., Johnston, D.T. (2016) Patterns of sulfur isotope fractionation during microbial sulfate reduction. Geobiology 14, 91–101. https://doi.org/10.1111/gbi.12149
Show in context Microbiological factors, such as the cell specific sulfate reduction rate, which is itself a function of the microbial community, the temperature, the electron donor and the amount of sulfate, have also been recognised as key drivers of S isotope fractionation (Brunner and Bernasconi, 2005; Sim et al., 2011; Bradley et al., 2016).
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Brunner, B., Bernasconi, S.M. (2005) A revised isotope fractionation model for dissimilatory sulfate reduction in sulfate reducing bacteria. Geochimica et Cosmochimica Acta 69, 4759–4771. https://doi.org/10.1016/j.gca.2005.04.015
Show in context Microbiological factors, such as the cell specific sulfate reduction rate, which is itself a function of the microbial community, the temperature, the electron donor and the amount of sulfate, have also been recognised as key drivers of S isotope fractionation (Brunner and Bernasconi, 2005; Sim et al., 2011; Bradley et al., 2016).
View in article
Bryant, R.N., Houghton, J.L., Jones, C., Pasquier, V., Halevy, I., Fike, D.A. (2023) Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios. Science 382, 912–915. https://doi.org/10.1126/science.adg6103
Show in context Recently, empirical and modelling studies have pointed to the role of local sedimentary conditions, such as sedimentation rate or reactive iron availability, in modulating the S isotope composition recorded by pyrite (e.g., Pasquier et al., 2017; Marin Carbonne et al., 2022; Bryant et al., 2023; Halevy et al., 2023).
View in article
Sulfur isotope fractionation during MSR can be recorded at the microscale by individual pyrite grains and approached through the calculation of Δpyr, which corresponds to the δ34S difference between water column SO42− and individual pyrite grains (Bryant et al., 2023).
View in article
Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
Show in context The alpine meromictic Lake Cadagno has been extensively used as a model for Proterozoic low sulfate oceans and euxinic conditions (e.g., Canfield et al., 2010; Ellwood et al., 2019; Janssen et al., 2022).
View in article
A previous analysis of S multiple-isotope composition of dissolved SO42− and H2S in the water column highlighted the predominant role of MSR in driving the S cycle in Lake Cadagno and revealed a large S isotope fractionation during MSR in surface sediment (71.8 ‰ during incubation experiments; Canfield et al., 2010).
View in article
In addition, new (i.e. sampled in 2022) multiple S isotope composition of water column and porewater H2S are compared to previously published isotope compositions of porewater H2S and water column H2S (Berg et al., 2025; Canfield et al., 2010, respectively).
View in article
New S isotope data were obtained on water column and porewater H2S samples collected in 2022, as well as on pyrite in surface sediments (0–5 cm depth). These data are compared with earlier isotopic measurements from samples collected in 1991 (Berg et al., 2025), and 2006, 2007 and 2008 (Canfield et al., 2010).
View in article
Data published in *Canfield et al. (2010) and **Berg et al. (2025).
View in article
As SO42− is nearly consumed within the uppermost 20 cm of sediment (Fig. 1c) through organoclastic SO42− reduction and SO42− reduction coupled to anaerobic oxidation of methane (SR-AOM; Canfield et al., 2010; Su et al., 2020), residual porewater SO42− likely becomes 34S enriched over the same depth interval (Jorgensen et al., 2004; Berg et al., 2025).
View in article
In the uppermost 5 cm of sediment, one framboidal pyrite grain Δpyr value (79.6 ‰) is roughly consistent with the near-equilibrium microbial isotope fractionation (71.8 ‰) previously measured in Lake Cadagno sediment incubation (Fig. 2b; Canfield et al., 2010).
View in article
Dahl, T.W., Anbar, A.D., Gordon, G.W., Rosing, M.T., Frei, R., Canfield, D.E. (2010) The behavior of molybdenum and its isotopes across the chemocline and in the sediments of sulfidic Lake Cadagno, Switzerland. Geochimica et Cosmochimica Acta 74, 144–163. https://doi.org/10.1016/j.gca.2009.09.018
Show in context Consistently, framboid size exceeds 8 μm, which indicate a sedimentary origin of pyrite (Rickard, 2019), in line with the lack of water column pyrite observation (Dahl et al., 2010).
View in article
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Show in context A continuous supply of dense, solute-rich waters from subaquatic springs over the past 9000 years has maintained strong water column stratification (Del Don et al., 2001; Wirth et al., 2013).
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Ellwood, M.J., Hassler, C., Moisset, S., Pascal, L., Danza, F., Peduzzi, S., Tonolla, M., Vance, D. (2019) Iron isotope transformations in the meromictic Lake Cadagno. Geochimica et Cosmochimica Acta 255, 205–221. https://doi.org/10.1016/j.gca.2019.04.007
Show in context The alpine meromictic Lake Cadagno has been extensively used as a model for Proterozoic low sulfate oceans and euxinic conditions (e.g., Canfield et al., 2010; Ellwood et al., 2019; Janssen et al., 2022).
View in article
Fakhraee, M., Crockford, P.W., Bauer, K.W., Pasquier, V., Sugiyama, I., Katsev, S., Raven, M.R., Gomes, M., Philippot, P., Crowe, S.A., Tarhan, L.G., Lyons, T.W., Planavsky, N. (2025) The history of Earth’s sulfur cycle. Nature Reviews Earth & Environment 6, 106–125. https://doi.org/10.1038/s43017-024-00615-0
Show in context Pyrite texture, size, trace element composition and S isotope composition (δ34S) have been widely used to reconstruct the extent and duration of euxinic conditions in past oceans (Fakhraee et al., 2025) — environments characterised by water column oxygen depletion and dissolved H2S enrichment. Widespread euxinia is thought to repeatedly occur during the Phanerozoic and was likely associated with several mass extinctions (Lyons et al., 2009).
View in article
Fike, D.A., Bradley, A.S., Rose, C.V. (2015) Rethinking the Ancient Sulfur Cycle. Annual Review of Earth and Planetary Sciences 43, 593–622. https://doi.org/10.1146/annurev-earth-060313-054802
Show in context Under these conditions, the mixing of H2S produced at different depths in the sediment — originating from a residual SO42− pool with varying degrees of 34S enrichment — would dilute the local, microbially produced 34S-depleted H2S in surface sediment (Fike et al., 2015; Pasquier et al., 2021).
View in article
Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
Show in context Sulfate concentrations were proposed as a dominant factor controlling the extent of microbial isotope fractionation during MSR (Habicht et al., 2002; Gomes and Hurtgen, 2015).
View in article
For example, large apparent S isotope fractionation is observed in modern stratified systems harbouring SO42− concentrations larger than 15 mM (between 50 and 60 ‰; e.g., in the Black Sea, Cariaco Basin and Green Lake; Sweeney and Kaplan, 1980; Li et al., 2010; Zerkle et al., 2010; Gomes and Hurtgen, 2015).
View in article
Below this threshold, the apparent S isotope fractionation decreases to 5 ‰ at micromolar SO42− levels (Gomes and Hurgten, 2015).
View in article
Our study offers an alternative perspective to the traditional view that attributes H2S reservoir dynamics in euxinic environments to in situ water column MSR (Habicht et al., 2002; Gomes and Hurtgen, 2015).
View in article
When the SO42− reservoir is small, the 34S enrichment becomes significant and the δ34S offset between surface SO42− and water column, microbially produced H2S can be reduced (Gomes and Hurtgen, 2015).
View in article
In this model, diagenetic pyrite formation is considered minimal, as Fe is largely pyritised in the water column, leaving the sediment relatively Fe-limited (Gomes and Hurtgen, 2015; Zerkle et al., 2010).
View in article
Schematic representation of (a) the traditional model of euxinia showing the reservoir effect during MSR (adapted from Gomes and Hurtgen, 2015), and (b) H2S accumulation in Lake Cadagno.
View in article
Habicht, K.S., Gade, M., Thamdrup, B., Berg, P., Canfield, D.E. (2002) Calibration of Sulfate Levels in the Archean Ocean. Science 298, 2372–2374. https://doi.org/10.1126/science.1078265
Show in context Sulfate concentrations were proposed as a dominant factor controlling the extent of microbial isotope fractionation during MSR (Habicht et al., 2002; Gomes and Hurtgen, 2015).
View in article
Our study offers an alternative perspective to the traditional view that attributes H2S reservoir dynamics in euxinic environments to in situ water column MSR (Habicht et al., 2002; Gomes and Hurtgen, 2015).
View in article
Halevy, I., Fike, D.A., Pasquier, V., Bryant, R.N., Wenk, C.B., Turchyn, A.V., Johnston, D.T., Claypool, G.E. (2023) Sedimentary parameters control the sulfur isotope composition of marine pyrite. Science 382, 946–951. https://doi.org/10.1126/science.adh1215
Show in context Recently, empirical and modelling studies have pointed to the role of local sedimentary conditions, such as sedimentation rate or reactive iron availability, in modulating the S isotope composition recorded by pyrite (e.g., Pasquier et al., 2017; Marin Carbonne et al., 2022; Bryant et al., 2023; Halevy et al., 2023).
View in article
This observation has important implications for our understanding of MSR activity in natural environments, as despite elevated TOC content (15 wt. %) and intense SO42− reduction rates (SRR) at the community level in Lake Cadagno surface sediment, such large microbial isotope fractionation implies slow cell-specific SRR (Wing and Halevy, 2014; Halevy et al., 2023).
View in article
Hanselmann, K., Hutter, R. (1998) Geomicrobiological coupling of sulfur and iron cycling in anoxic sediments of a meromictic lake: sulfate reduction and sulfide sources and sinks in Lake Cadagno. Documenta dell’Istituto Italiano di Idrobiologia 63, 85–98.
Show in context Furthermore, the steep gradient in H2S concentration in surface sediment and the non-zero H2S concentration at the SWI (Fig. 1c) implies that microbially generated H2S within the sediment porewaters is diffusing into the water column (see also Hanselmann and Hutter, 1998).
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Janssen, D.J., Rickli, J., Wille, M., Sepulveda Steiner, O., Vogel, H., Dellwig, O., Berg, J.S., Bouffard, D., Lever, M.A., Hassler, C.S., Jaccard, S.L. (2022) Chromium Cycling in Redox-Stratified Basins Challenges δ53Cr Paleoredox Proxy Applications. Geophysical Research Letters 49, e2022GL099154. https://doi.org/10.1029/2022GL099154
Show in context The alpine meromictic Lake Cadagno has been extensively used as a model for Proterozoic low sulfate oceans and euxinic conditions (e.g., Canfield et al., 2010; Ellwood et al., 2019; Janssen et al., 2022).
View in article
Jørgensen, B.B. (1982) Mineralization of organic matter in the sea bed—the role of sulphate reduction. Nature 296, 643–645. https://doi.org/10.1038/296643a0
Show in context Earth’s biogeochemical sulfur (S) cycle is predominantly driven by microbial activity (Jorgensen, 1982).
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Jørgensen, B.B., Böttcher, M.E., Lüschen, H., Neretin, L.N., Volkov, I.I. (2004) Anaerobic methane oxidation and a deep H2S sink generate isotopically heavy sulfides in Black Sea sediments. Geochimica et Cosmochimica Acta 68, 2095–2118. https://doi.org/10.1016/j.gca.2003.07.017
Show in context As SO42− is nearly consumed within the uppermost 20 cm of sediment (Fig. 1c) through organoclastic SO42− reduction and SO42− reduction coupled to anaerobic oxidation of methane (SR-AOM; Canfield et al., 2010; Su et al., 2020), residual porewater SO42− likely becomes 34S enriched over the same depth interval (Jorgensen et al., 2004; Berg et al., 2025).
View in article
In 2022, however, the constant δ34S values of porewater H2S are similar to the 1991 δ34S values at depth (Fig. 1d) and could reflect upward diffusion of H2S produced from residual 34S enriched SO42− at depth, as observed elsewhere (Jorgensen et al., 2004; Liu et al., 2021; Pasquier et al., 2025).
View in article
Jørgensen, B.B., Findlay, A.J., Pellerin, A. (2019) The Biogeochemical Sulfur Cycle of Marine Sediments. Frontiers in Microbiology 10, 849. https://doi.org/10.3389/fmicb.2019.00849
Show in context This cycle regulates organic matter remineralisation and Earth’s surface redox balance through sedimentary burial of oxidised and reduced S species, with microbial sulfate reduction (MSR) as the dominant S-based metabolic pathway (Jørgensen et al., 2019).
View in article
Kasten, S., Zabel, M., Heuer, V., Hensen, C. (2003) Processes and Signals of Nonsteady-State Diagenesis in Deep-Sea Sediments and their Pore Waters. In: Wefer, G., Mulitza, S., Ratmeyer, V. (Eds.) The South Atlantic in the Late Quaternary. Springer, Berlin, Heidelberg, 431–459. https://doi.org/10.1007/978-3-642-18917-3_20
Show in context The absence of dilution by turbiditic deposition in the uppermost 10 cm enhances the organic flux to the sediment, allowing a greater proportion of organic carbon to escape heterotrophic respiration and fuel methanogenesis in deeper layers (Kasten et al., 2003).
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Li, X., Gilhooly III, W.P., Zerkle, A.L., Lyons, T.W., Farquhar, J., Werne, J.P., Varela, R., Scranton, M.I. (2010) Stable sulfur isotopes in the water column of the Cariaco Basin. Geochimica et Cosmochimica Acta 74, 6764–6778. https://doi.org/10.1016/j.gca.2010.08.020
Show in context For example, large apparent S isotope fractionation is observed in modern stratified systems harbouring SO42− concentrations larger than 15 mM (between 50 and 60 ‰; e.g., in the Black Sea, Cariaco Basin and Green Lake; Sweeney and Kaplan, 1980; Li et al., 2010; Zerkle et al., 2010; Gomes and Hurtgen, 2015).
View in article
Liu, J., Pellerin, A., Antler, G., Izon, G., Findlay, A.J., Røy, H., Ono, S., Kasten, S., Turchyn, A.V., Jørgensen, B.B. (2021) Early diagenesis of sulfur in Bornholm Basin sediments: The role of upward diffusion of isotopically “heavy” sulfide. Geochimica et Cosmochimica Acta 313, 359–377. https://doi.org/10.1016/j.gca.2021.08.018
Show in context In 2022, however, the constant δ34S values of porewater H2S are similar to the 1991 δ34S values at depth (Fig. 1d) and could reflect upward diffusion of H2S produced from residual 34S enriched SO42− at depth, as observed elsewhere (Jorgensen et al., 2004; Liu et al., 2021; Pasquier et al., 2025).
View in article
In such settings, the pooling of H2S and isotopic homogenisation also lead to the formation of sedimentary pyrite that is relatively enriched in 34S (Liu et al., 2021; Pasquier et al., 2025).
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Lyons, T.W., Anbar, A.D., Severmann, S., Scott, C., Gill, B.C. (2009) Tracking Euxinia in the Ancient Ocean: A Multiproxy Perspective and Proterozoic Case Study. Annual Review of Earth and Planetary Sciences 37, 507–534. https://doi.org/10.1146/annurev.earth.36.031207.124233
Show in context Pyrite texture, size, trace element composition and S isotope composition (δ34S) have been widely used to reconstruct the extent and duration of euxinic conditions in past oceans (Fakhraee et al., 2025) — environments characterised by water column oxygen depletion and dissolved H2S enrichment. Widespread euxinia is thought to repeatedly occur during the Phanerozoic and was likely associated with several mass extinctions (Lyons et al., 2009).
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Marin-Carbonne, J., Decraene, M.-N., Havas, R., Remusat, L., Pasquier, V., Alléon, J., Zeyen, N., Bouton, A., Bernard, S., Escrig, S., Olivier, N., Vennin, E., Meibom, A., Benzerara, K., Thomazo, C. (2022) Early precipitated micropyrite in microbialites: A time capsule of microbial sulfur cycling. Geochemical Perspectives Letters 21, 7–12. https://doi.org/10.7185/geochemlet.2209
Show in context Recently, empirical and modelling studies have pointed to the role of local sedimentary conditions, such as sedimentation rate or reactive iron availability, in modulating the S isotope composition recorded by pyrite (e.g., Pasquier et al., 2017; Marin Carbonne et al., 2022; Bryant et al., 2023; Halevy et al., 2023).
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Meyer, K.M., Kump, L.R. (2008) Oceanic Euxinia in Earth History: Causes and Consequences. Annual Review of Earth and Planetary Sciences 36, 251–288. https://doi.org/10.1146/annurev.earth.36.031207.124256
Show in context Permanent euxinia is currently scarce and only occurs in stratified lakes, fjords, coastal upwelling zones and restricted marine basins (Meyer and Kemp, 2008).
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Pasquier, V., Sansjofre, P., Rabineau, M., Revillon, S., Houghton, J., Fike, D.A. (2017) Pyrite sulfur isotopes reveal glacial−interglacial environmental changes. Proceedings of the National Academy of Sciences 114, 5941–5945. https://doi.org/10.1073/pnas.1618245114
Show in context Recently, empirical and modelling studies have pointed to the role of local sedimentary conditions, such as sedimentation rate or reactive iron availability, in modulating the S isotope composition recorded by pyrite (e.g., Pasquier et al., 2017; Marin Carbonne et al., 2022; Bryant et al., 2023; Halevy et al., 2023).
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Pasquier, V., Fike, D.A., Halevy, I. (2021) Sedimentary pyrite sulfur isotopes track the local dynamics of the Peruvian oxygen minimum zone. Nature Communications 12, 4403. https://doi.org/10.1038/s41467-021-24753-x
Show in context Under these conditions, the mixing of H2S produced at different depths in the sediment — originating from a residual SO42− pool with varying degrees of 34S enrichment — would dilute the local, microbially produced 34S-depleted H2S in surface sediment (Fike et al., 2015; Pasquier et al., 2021).
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Pasquier, V., Marin-Carbonne, J., Giunta, T., Ruffine, L., Halevy, I. (2025) Microscale iron and sulphur isotopic compositions reveal pyritization pathways during early diagenesis. Communications Earth & Environment 6, 248. https://doi.org/10.1038/s43247-025-02213-4
Show in context In 2022, however, the constant δ34S values of porewater H2S are similar to the 1991 δ34S values at depth (Fig. 1d) and could reflect upward diffusion of H2S produced from residual 34S enriched SO42− at depth, as observed elsewhere (Jorgensen et al., 2004; Liu et al., 2021; Pasquier et al., 2025).
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We therefore infer that framboids formed in the sediment porewaters, thereby recording the temporal δ34S evolution of the pooled porewater H2S reservoir (Pasquier et al., 2025).
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Consequently, sedimentary pyrite δ34S values are expected to correspond to those of pooled H2S, masking near-equilibrium isotope fractionation signatures (Pasquier et al., 2025).
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In such settings, the pooling of H2S and isotopic homogenisation also lead to the formation of sedimentary pyrite that is relatively enriched in 34S (Liu et al., 2021; Pasquier et al., 2025).
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Rickard, D. (2019) How long does it take a pyrite framboid to form? Earth and Planetary Science Letters 513, 64–68. https://doi.org/10.1016/j.epsl.2019.02.019
Show in context Consistently, framboid size exceeds 8 μm, which indicate a sedimentary origin of pyrite (Rickard, 2019), in line with the lack of water column pyrite observation (Dahl et al., 2010).
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Sim, M.S., Ono, S., Donovan, K., Templer, S.P., Bosak, T. (2011) Effect of electron donors on the fractionation of sulfur isotopes by a marine Desulfovibrio sp. Geochimica et Cosmochimica Acta 75, 4244–4259. https://doi.org/10.1016/j.gca.2011.05.021
Show in context Microbiological factors, such as the cell specific sulfate reduction rate, which is itself a function of the microbial community, the temperature, the electron donor and the amount of sulfate, have also been recognised as key drivers of S isotope fractionation (Brunner and Bernasconi, 2005; Sim et al., 2011; Bradley et al., 2016).
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Su, G., Zopfi, J., Yao, H., Steinle, L., Niemann, H., Lehmann, M.F. (2020) Manganese/iron-supported sulfate-dependent anaerobic oxidation of methane by archaea in lake sediments. Limnology and Oceanography 65, 863–875. https://doi.org/10.1002/lno.11354
Show in context As SO42− is nearly consumed within the uppermost 20 cm of sediment (Fig. 1c) through organoclastic SO42− reduction and SO42− reduction coupled to anaerobic oxidation of methane (SR-AOM; Canfield et al., 2010; Su et al., 2020), residual porewater SO42− likely becomes 34S enriched over the same depth interval (Jorgensen et al., 2004; Berg et al., 2025).
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Sweeney, R.E., Kaplan, I.R. (1980) Stable isotope composition of dissolved sulfate and hydrogen sulfide in the Black Sea. Marine Chemistry 9, 145–152. https://doi.org/10.1016/0304-4203(80)90064-X
Show in context For example, large apparent S isotope fractionation is observed in modern stratified systems harbouring SO42− concentrations larger than 15 mM (between 50 and 60 ‰; e.g., in the Black Sea, Cariaco Basin and Green Lake; Sweeney and Kaplan, 1980; Li et al., 2010; Zerkle et al., 2010; Gomes and Hurtgen, 2015).
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Wing, B.A., Halevy, I. (2014) Intracellular metabolite levels shape sulfur isotope fractionation during microbial sulfate respiration. Proceedings of the National Academy of Sciences 111, 18116–18125. https://doi.org/10.1073/pnas.1407502111
Show in context This observation has important implications for our understanding of MSR activity in natural environments, as despite elevated TOC content (15 wt. %) and intense SO42− reduction rates (SRR) at the community level in Lake Cadagno surface sediment, such large microbial isotope fractionation implies slow cell-specific SRR (Wing and Halevy, 2014; Halevy et al., 2023).
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Wirth, S.B., Gilli, A., Niemann, H., Dahl, T.W., Ravasi, D., Sax, N., Hamann, Y., Peduzzi, R., Peduzzi, S., Tonolla, M., Lehmann, M.F., Anselmetti, F.S. (2013) Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps). Geochimica et Cosmochimica Acta 120, 220–238. https://doi.org/10.1016/j.gca.2013.06.017
Show in context A continuous supply of dense, solute-rich waters from subaquatic springs over the past 9000 years has maintained strong water column stratification (Del Don et al., 2001; Wirth et al., 2013).
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Considering the local sedimentation rate of 2.4 mm yr−1 for surface sediments (Wirth et al., 2013), the uppermost 5 cm of the sediments correspond to the past 20 years.
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Furthermore, total organic carbon (TOC) content decreases strongly with depth, from 15 wt. % in the uppermost 10 cm of the sediment, where sediments derive from settling water column particles and include chemocline-produced organic matter, to 2 wt. % below 20 cm, where sediments consist of remobilised shallow deposits rich in detrital material (Fig. S-1; Wirth et al., 2013).
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Zerkle, A.L., Kamyshny Jr., A., Kump, L.R., Farquhar, J., Oduro, H., Arthur, M.A. (2010) Sulfur cycling in a stratified euxinic lake with moderately high sulfate: Constraints from quadruple S isotopes. Geochimica et Cosmochimica Acta 74, 4953–4970. https://doi.org/10.1016/j.gca.2010.06.015
Show in context For example, large apparent S isotope fractionation is observed in modern stratified systems harbouring SO42− concentrations larger than 15 mM (between 50 and 60 ‰; e.g., in the Black Sea, Cariaco Basin and Green Lake; Sweeney and Kaplan, 1980; Li et al., 2010; Zerkle et al., 2010; Gomes and Hurtgen, 2015).
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In this model, diagenetic pyrite formation is considered minimal, as Fe is largely pyritised in the water column, leaving the sediment relatively Fe-limited (Gomes and Hurtgen, 2015; Zerkle et al., 2010).
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Supplementary Information
The Supplementary Information includes:
- 1. Lake Cadagno
- 2. Materials and Methods
- 3. Sulfide Multiple Isotopes
- 4. Data Compilation
- 5. Sulfide Mass Balance Calculation
- Tables S-1 to S-4
- Figures S-1 to S-5
- Supplementary Information References
Download the Supplementary Information (PDF)
Download Table S-4 (.xlsx)
Figures
Figure 1 SO42− and H2S concentrations and δ34S values in the water column (upper panels) and the porewaters (lower panels) of Lake Cadagno, from four different sampling campaigns. (a) SO42− and H2S concentrations in the water column, (b) δ34S of SO42−* and H2S in the water column, (c) SO42− and H2S concentrations in the sediment porewaters, (d) δ34S of H2S in the sediment porewaters. Error bars are smaller than symbols. The colour shading indicates the sampling year. Data published in *Canfield et al. (2010
Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
) and **Berg et al. (2025Berg, J.S., Rodriguez, P.C., Magnabosco, C., Deng, L., Bernasconi, S.M., Vogel, H., Morlock, M., Lever, M.A. (2025) Microbial sulfur cycling across a 13 500-year-old lake sediment record. Biogeosciences 22, 5483–5496. https://doi.org/10.5194/bg-22-5483-2025
).
Figure 2 Sulfur isotope compositions of water column SO42− and H2S, porewater H2S, CRS and pyrite grains in the sediment uppermost 5 cm. (a) δ34S values of the aforementioned species. Deep water-column data are represented by diamonds on the upper edge of the plot. Porewater H2S (downward pointing triangles) and CRS (upward pointing triangle) data are represented on the lower boundary of the plot. SIMS data are represented by the histogram with n the number of analyses. (b) Δpyr values of CRS and pyrite grains (Δpyr = δ34Ssulfate − δ34SCRS or pyr). The mean δ34S value of deep water-column SO42− (+24.7 ‰) is reported at 0 ‰ for comparison. The maximum microbial isotope fractionation εmic = 71.8 ‰ between SO42− and H2S in surface sediments measured by Canfield et al. (2010)
Canfield, D.E., Farquhar, J., Zerkle, A.L. (2010) High isotope fractionations during sulfate reduction in a low-sulfate euxinic ocean analog. Geology 38, 415–418. https://doi.org/10.1130/G30723.1
is represented by the grey band. The microbial isotope fractionation is defined as εmic = 1000 × ((34S/32S)SO4/(34S/32S)H2S − 1). (c, d) SEM micrographs of framboidal pyrite analysed by SIMS and their δ34S values (the scalebar is 5 μm).
Figure 3 Schematic representation of (a) the traditional model of euxinia showing the reservoir effect during MSR (adapted from Gomes and Hurtgen, 2015
Gomes, M.L., Hurtgen, M.T. (2015) Sulfur isotope fractionation in modern euxinic systems: Implications for paleoenvironmental reconstructions of paired sulfate–sulfide isotope records. Geochimica et Cosmochimica Acta 157, 39–55. https://doi.org/10.1016/j.gca.2015.02.031
), and (b) H2S accumulation in Lake Cadagno. The concentration and δ34S trends of SO42− and H2S are shown as a function of water and sediment depth. The dashed lines represent the initial δ34S value of SO42−. The isotopic offset between SO42− and H2S in (a) corresponds to the microbial isotope fractionation εmic, while the isotopic offset Δ is smaller in (b).