Extreme sulphur isotope fractionation in the deep Cretaceous biosphere

Isotopically light sulphur in sedimentary pyrite was generally predominant during the Cretaceous as a consequence of volcanic and hydrothermal activity and bacterial sulphate reduction (BSR). However, we report super-high sulphur isotopic compositions of sedimentary pyrite (δ34S + 89.3‰) from late mid- Cretaceous sediments from the Central Apennines, Italy. These exceptionally high δ34S values are not consistent with current models for interpreting the geological record for sulphur isotopes because these cannot explain the extreme isotopic fractionation observed in our study. Precise details of the mechanism for producing pyrite so highly enriched in 34S remain elusive, but the large variation in δ34S values (>70‰) within the analysed samples and the considerable micro-scale variations in δ34S (up to 50‰ even within a single nodule) indicate a complex mechanism of sulphate reduction and pyrite precipitation. This includes early removal of isotopically light H2S by pyrite precipitation in the water column, diffusion and advection of H2S and SO4 2-, and finally BSR. Similar but less severe conditions occur in the Black Sea and in deep ocean sediments. These new findings provide new insights for the interpretation of the palaeoceanographic conditions that prevailed during the Mesozoic. © 2010 Geological Society of London.