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

摘要

The Llandovery-Wenlock carbonate-marl succession of the Baltoscandian Basin has been analyzed in the Viki drill core, western Estonia, for carbonate carbon (delta C-13(carb)). organic carbon (delta C-13(org)), carbonate associated sulfate (delta S-34(CAS)) and pyrite (delta S-34(pyr)) isotopes, along with trace element concentrations. Following the End-Ordovician glaciation and the Hirnantian carbon isotope excursion, delta C-13(carb) values rise to an early Llandovery carbon isotope maximum (+2 parts per thousand). Subsequently, -org delta C-13(carb) and coeval delta C-13(org) record a large similar to+4.5 parts per thousand excursion in the early Wenlock, associated with a regression and biotic turnover known as the Ireviken bioevent (IBE). Overall, delta S-34(CAS) values generally fall within a range (25 parts per thousand to 35 parts per thousand) similar to that reported in other sections of comparable age. Two distinct trends are observed: low and stable delta S-34(CAS) values are associated with deep-water facies, while delta S-34(CAS) becomes higher and more variable in the shallow-water facies. Similarly, delta S-34(pyr) values are low and invariant (similar to-15 parts per thousand) in deep-water facies but become higher and more variable (ranging between -35 parts per thousand and 16 parts per thousand) 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 delta S-34(CAS) are observed where CAS abundance is low. A large, well-resolved positive excursion in delta S-34(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 delta S-34(CAS) profiles in coeval sections. The S-i