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 (δ13Ccarb), organic carbon (δ13Corg), carbonate-associated sulfate (δ34SCAS) and pyrite (δ34Spyr) isotopes, along with trace element concentrations. Following the End-Ordovician glaciation and the Hirnantian carbon isotope excursion, δ13Ccarb values rise to an early Llandovery carbon isotope maximum (+2‰). Subsequently, δ13Ccarb and coeval δ13Corg record a large ∼+4.5‰ excursion in the early Wenlock, associated with a regression and biotic turnover known as the Ireviken bioevent (IBE). Overall, δ34SCAS 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 δ34SCAS values are associated with deep-water facies, while δ34SCAS becomes higher and more variable in the shallow-water facies. Similarly, δ34Spyrvalues 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 δ34SCAS are observed where CAS abundance is low. A large, well-resolved positive excursion in δ34SCAS 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 δ34SCAS 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.