Records of carbon and sulfur cycling during the Silurian Ireviken Event in Gotland, Sweden

摘要

Early Silurian (similar to 431 Ma) carbonate rocks record a ca. 4.5 parts per thousand positive excursion in the stable isotopic composition of carbonate carbon (delta C-13(carb)). Associated with this isotopic shift is a macroevolutionary turnover pulse known as the 'Ireviken Event'. The onset of this carbon isotope excursion is commonly associated with a shallowing-upward facies transition that may have been accompanied by climatic change, as indicated by a parallel positive shift (similar to 0.6 parts per thousand) in the stable isotopic composition of carbonate oxygen (delta O-18(carb)). However, the relationships among carbon cycle perturbations, faunal turnover, and environmental changes remain enigmatic. Here we present a suite of new isotopic data across the Ireviken Event from multiple sections in Gotland, Sweden. These samples preserve no systematic change in delta O-18(carb) but show positive excursions of equal magnitude in both carbonate (delta C-13(carb)) and organic (delta C-13(org)) carbon. In addition, the data reveal a synchronous perturbation in sulfur isotope ratios, manifest as a ca. 7 parts per thousand positive excursion in carbonate-associated sulfate (delta S-34(CAS)) and a ca. 30 parts per thousand positive excursion in pyrite (delta(34)Spy). The increase in delta S-34(pyr) values is accompanied by a substantial, concomitant increase in stratigraphic variability of delta S-34(pyr).