Silurian records of carbon and sulfur cycling from Estonia: The importance of depositional environment on isotopic trends

The Llandovery–Wenlock carbonate-marl succession of the Baltoscandian Basin has been analyzed in the Viki drill core, western Estonia, for carbonate carbon (δ ¹³ C _carb ), organic carbon (δ ¹³ C _org ), carbonate-associated sulfate (δ ³⁴ S _CAS ) and pyrite (δ ³⁴ S _pyr ) isotopes, along with trace element concentrations. Following the End-Ordovician glaciation and the Hirnantian carbon isotope excursion, δ ¹³ C _carb values rise to an early Llandovery carbon isotope maximum (+2‰). Subsequently, δ ¹³ C _carb and coeval δ ¹³ C _org record a large ∼+4.5‰ excursion in the early Wenlock, associated with a regression and biotic turnover known as the Ireviken bioevent (IBE). Overall, δ ³⁴ S _CAS values generally fall within a range (25‰ to 35‰) similar to that reported in other sections of comparable age. Two distinct trends are observed: low and stable δ ³⁴ S _CAS values are associated with deep-water facies, while δ ³⁴ S _CAS becomes higher and more variable in the shallow-water facies. Similarly, δ ³⁴ S _pyr values are low and invariant (∼−15‰) in deep-water facies but become higher and more variable (ranging between −35‰ and 16‰) in shallow-water deposits. Despite abundant recrystallization, no obvious evidence for large-scale alteration of proxies is apparent in concentrations of Fe, Sr, Mn and Mg, or in isotopic correlations, although increased variability and lower values of δ ³⁴ S _CAS are observed where CAS abundance is low. A large, well-resolved positive excursion in δ ³⁴ S _CAS observed during the IBE from a time-correlative section on Gotland (Sweden) is absent in the data presented here. This difference demonstrates that, counter to general understanding, local environmental variations can give rise to divergent δ ³⁴ S _CAS profiles in coeval sections. The S-isotope fluctuations in CAS and pyrite in the Viki drill core can be explained by local processes involving facies changes, early diagenetic processes and associated open vs. closed system behavior that can overprint and potentially mask basin or global signals.