Sedimentary pyrite sulfur isotopes track the local dynamics of the Peruvian oxygen minimum zone

Abstract

Sulfur cycling is ubiquitous in sedimentary environments, where it mediates organic carbon remineralization, impacting both local and global redox budgets, and leaving an imprint in pyrite sulfur isotope ratios (δ34Spyr). It is unclear to what extent stratigraphic δ34Spyr variations reflect local aspects of the depositional environment or microbial activity versus global sulfur-cycle variations. Here, we couple carbon-nitrogen-sulfur concentrations and stable isotopes to identify clear influences on δ34Spyr of local environmental changes along the Peru margin. Stratigraphically coherent glacial-interglacial δ34Spyr fluctuations (>30‰) were mediated by Oxygen Minimum Zone intensification/expansion and local enhancement of organic matter deposition. The higher resulting microbial sulfate reduction rates led to more effective drawdown and 34S-enrichment of residual porewater sulfate and sulfide produced from it, some of which is preserved in pyrite. We identify organic carbon loading as a major influence on δ34Spyr, adding to the growing body of evidence highlighting the local controls on these records.

Multiple sulphur isotope record of Paleoarchean sedimentary rocks across the Onverwacht Group, Barberton Greenstone Belt, South Africa

“…Petrography suggests that Fe‐bearing phases (haematite, jasperlite, siderite, and ankerite) are very rare to absent in our samples, and therefore differences in Fe‐availability between our samples cannot explain the variability in MIF‐S. The concentration, availability, and source of organic carbon is another important control, with studies of δ 34 S in sedimentary pyrite from the modern day Peruvian margin finding that organic carbon burial rates controls MSR rates, effective drawdown and 34 S‐enrichment of residual porewater sulphate and sulphide, some of which is preserved in sedimentary pyrite (Pasquier et al, 2021). TOC data show some variability between and within the black cherts sampled in this study (McLoughlin & Grosch, 2014; Walsh & Lowe, 1999), but to test this hypothesis further would require cm‐scale sampling of individual chert horizons bearing sulphides and more detailed characterization of the contained organics.…”

“…For example, S 8 particles comprise more than 90% These modelling results would predict that our Noisy Formation sediments might therefore contain more MIF-S variability, compared with some of our more distal cherts, for example, Hc2, if all other variables remain constant. Considering also variation in depositional rates, which are higher closer to the continental source of sediments and lower further offshore (Fike et al, 2015;Pasquier et al, 2021), this has implications for the efficiency of diffusive exchange between the sediment porewater and the overlying seawater and for the availability of particulate sources of iron to scavenge porewater sulphide and precipitate pyrite. Variability in depositional rates is also therefore a likely source of MIF-S heterogeneity in our samples.…”

Diagenetic Geochemistry of Iron, Sulfur, and Molybdenum in Sediments of the Middle Okinawa Trough Impacted by Hydrothermal Plumes and/or Cold Seeps

“…On the other hand, high csSRRs stimulated by labile OC tend to suppress the S isotope fractionations (Bradley et al., 2016; Leavitt et al., 2013; Sim, Ono, et al., 2011). In addition, very sulfate‐limited systems also dampen the fractionations due to the Rayleigh fractionation (i.e., reservoir effect) (X. Liu et al., 2020; Pasquier et al., 2017; Pasquier, Bryant, et al., 2021; Pasquier, Fike, & Halevy, 2021), as long as csSRR is not exceptionally low (Wing & Halevy, 2014). The Δδ 34 S sulfate‐pyrite values (59‰–70‰) in G06 and G08 (Figure S4 in Supporting Information ) are close to the equilibrium fractionation value of SR (∼70‰), which strongly suggests that organoclastic sulfate reduction (OSR) proceeds at very low rates in sulfate‐ample porewater environments, and that anaerobic respiration is not an effective pathway of carbon cycle in CH 4 ‐free sediments of the basin.…”

“…It is noted that sedimentary Mo enrichment is sensitive to sulfidic condition of porewater or bottom water and specific depositional/diagenetic processes involving the mechanisms of particulate Mo delivery to the sediments, and Mo recycling and burial in the sediments (Eroglu et al., 2020; Scholz et al., 2017, 2018; Scott & Lyons, 2012). Similarly, pyrite‐S isotope (δ 34 S py ) is also sensitive to either global S biogeochemical evolution or local depositional/diagenetic environments (Fike et al., 2015; J. Liu et al., 2021; Pasquier, Bryant, et al., 2021; Pasquier, Fike, & Halevy, 2021). Thus, enrichment of Mo and 34 S‐enriched pyrite at specific zones of modern/ancient sediments can be used as important proxies to identify modern SR‐AOM and to trace the intensity of ancient SR‐AOM and the upward/downward migrations of the SMTZ responding to the variations of the past climatic or depositional environments (F. Chen et al., 2016; Lin et al., 2021; X. Liu et al., 2020).…”

“…In this study, we used four sediment cores located between the Minami‐Ensei Knoll (MEK) hydrothermal field and a cold‐seep site to: (a) evaluate the potential impacts of hydrothermal Fe input on Fe geochemistry in the sediments, (b) unravel the diagenetic features of Fe, S, and Mo in the sediments between the two extreme environments. The results could provide an insightful look at how and to what extent the local extreme depositional and/or diagenetic environment influences sedimentary sulfide (particularly pyrite) isotopes, with implications whether the ancient stratigraphic variations in δ 34 S py are better interpreted to reflect the local depositional environment or global S‐cycle evolution (Fike et al., 2015; Pasquier et al., 2017, Pasquier, Bryant, et al., 2021; Pasquier, Fike, & Halevy, 2021).…”