Browsing by Subject "Sulfur isotopes"

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    Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate: Insights from paired S and O isotopic analyses
    (Elsevier, 2021) Richardson, Jocelyn A.; Lepland, Aivo; Hints, Olle; Prave, Anthony R.; Gilhooly, William P., III; Bradley, Alexander S.; Fike, David A.; Earth and Environmental Sciences, School of Science
    Carbon, sulfur and oxygen isotope profiles in Silurian strata of the Baltoscandian Basin (Estonia), coincident with the Ireviken Bioevent, provide insights into basin-scale and platform-specific depositional processes. Paired carbon isotope records preserve a positive isotope excursion during the early Wenlock, coincident with faunal turnover, yet δ13C variability of this excursion compared to other locations within the paleobasin reflects local depositional influences superimposed on a global signal. In comparison, sulfur isotope records do not preserve a systematic isotopic excursion over the same interval. Instead, sulfur isotope records have high sample-to-sample stratigraphic variability, particularly in shallow-water carbonate rocks (scatter up to ~10‰ for δ34SCAS and ~ 25‰ for δ34Spyr). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in δ34SCAS and δ34Spyr differ depending on relative local sea level (and therefore facies). Such facies-dependent variability reflects more closed- versus more open-system diagenetic conditions where pulses of increased sedimentation rate in the shallow water environments generates greater isotopic variability in both δ34SCAS and δ34Spyr. Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in δ34SCAS in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased δ34SCAS in deep water settings. Correlations in paired δ34SCAS-δ18OCAS data support these conclusions, demonstrating the local alteration of CAS during deposition and early marine diagenesis. We present a framework to assess the sequence of diagenetic and depositional environmental processes that have altered δ34SCAS and find that δ34S of ~27–28‰ approximates Silurian seawater sulfate. Our findings provide a mechanism to understand the elevated variability in many deep-time δ34SCAS records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.
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    On the Biogeochemistry of Modern Euxinia: From the Origin and Controls of Sulfurization Pathways to Trace Elements as Indicators of Environmental Changes
    (2025-05) Fouskas, Fotios; Gilhooly, William P., III; Druschel, Gregory K.; Macris, Catherine A.; Filippelli, Gabriel
    Euxinic lakes are analogues of the chemical and microbial sulfur (S) cycling that was prevalent in the anoxic conditions of ancient Earth. Mahoney Lake (Canada) and Green Lake (USA) were studied for their sulfate reducing and sulfide oxidizing bacteria that respectively produce high concentrations of sulfide and organic matter (OM) that are preserved as pyrite and organo-sulfur compounds (OSCs) in the sediments. Isotope and elemental proxies were used to evaluate the origin and controls of the sulfurization reactions that drive pyrite and OSCs burial, and the elements that influence localized environmental changes. The wide range in sulfate and sulfide concentrations make the lakes ideal settings for comparing S cycling between a hyper-euxinic (Mahoney) and a moderate (Green) end member of euxinia. The S isotope offset between dissolved sulfate and sulfide is identical (~50‰) showing that sulfate availability did not influence the isotope fractionation. The S isotopes of pyrite and OSCs in Mahoney sediments did not exhibit diagenetic effects observed in other studies. The S isotopes of these two phases are nearly identical, suggesting that pyrite and OSCs are formed within the water column. In contrast, diagenetic reactions preferentially formed pyrite in Green Lake sediments with an average 10‰ S isotope offset from OSCs. Reactive Fe and trace element patterns are consistent with euxinic conditions in both lakes. Redox sensitive trace metals (i.e., Mo) can track temporally and spatially localized changes in redox and broader climatic changes during the Holocene. These climate changes, including tephra from an eruption, might have influenced the variability of OM and ecology in Mahoney Lake. Molecular analysis of Mahoney Lake water showed a diverse stoichiometry of OSCs that suggests the sulfurization rates of iron and OM are competitive. These OM compounds can contribute to rapid rates of OSCs formation. Kinetic modelling supports our hypothesis that high concentrations of reactive OM play a significant role to competitive sulfurization reactions, which subsequently influenced the observed unconventional isotope patterns within the sedimentary record of Mahoney Lake.