Journal
CHEMICAL GEOLOGY
Volume 584, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.chemgeo.2021.120525
Keywords
Carbonate-associated sulfate; CAS; Sulfur isotopes; Sulfate oxygen isotopes; Ireviken bioevent; Silurian
Categories
Funding
- American Chemical Society Petroleum Research Fund [57548-ND2]
- Estonian Research Council [PUT611, PRG836]
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Isotopic profiles of carbon, sulfur, and oxygen in Silurian strata from the Baltoscandian Basin in Estonia during the Ireviken Bioevent reveal local depositional influences superimposed on a global signal. Sulfur isotope records exhibit high stratigraphic variability, affected by local sea level and facies differences. Our findings provide insights into platform-scale evolution and diagenetic processes, offering a mechanism to understand the variability in deep-time sulfur isotope records.
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 delta C-13 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 similar to 10 parts per thousand for delta(34)S(CAS )and similar to 25 parts per thousand for delta S-34(pyr)). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in delta S-34(CAS) and delta S-34(pyr) 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 delta S-34(CAS) and delta S-34(pyr). Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in delta S-34(CAS) in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased delta S-34(CAS) in deep water settings. Correlations in paired delta S-34(CAS)-delta O-18(CAS) 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 delta S-34(CAS) and find that delta S-34 of similar to 27-28 parts per thousand approximates Silurian seawater sulfate. Our findings provide a mechanism to understand the elevated variability in many deep-time delta S-34(CAS) records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.
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