Effects of early marine diagenesis and site-specific depositional controls on carbonate-associated sulfate: Insights from paired S and O isotopic analyses

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.

Automated summary

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.