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 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 δ³⁴S_CAS and ~ 25‰ for δ³⁴S_pyr). This pattern of isotopic variability is also found between sites from the same carbonate platform, where the magnitude and isotopic variability in δ³⁴S_CAS and δ³⁴S_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 δ³⁴S_CAS and δ³⁴S_pyr. Increased reworking and proximity to the shoreline results in local sulfide oxidation, seen as a decrease in δ³⁴S_CAS in the most proximal settings. Platform-scale evolution of isotopically distilled pore-fluids associated with dolomitization results in increased δ³⁴S_CAS in deep water settings. Correlations in paired δ³⁴S_CAS-δ¹⁸O_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 δ³⁴S_CAS 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 δ³⁴S_CAS records that cannot otherwise be reconciled with behavior of the marine sulfate reservoir.