BIOGEOCHEMISTRY IN THE LATE PALEOZOIC - EVIDENCE FROM SULFUR ISOTOPES

Profound geological and biological changes characterize the late Paleozoic. These include the assembly of Pangea, the change from greenhouse to icehouse conditions, and the colonization of continents by land plants. Expected changes in sedimentary biogeochemistry are evident from sulfur isotope measurements of sedimentary sulfur.

The sulfur isotopic composition of seawater sulfate displays substantial temporal variations during the Paleozoic. This is clearly documented by a new sulfur isotope record based on the analysis of structurally substituted sulfate in biogenic calcites and whole rock carbonates. Strongly positive d³⁴S values around +30 ‰ in the early Paleozoic are followed by a continuous decline to values around +11 ‰ at the end of the Permian. Additional variations exist on much shorter time scales. In the Devonian, an average sulfur isotope value of +24 ‰ for the Lochkovian is followed by a decrease towards an average value of +17 ‰ for the Eifelian, followed by an increase to an average value of +24 ‰ for the Frasnian. The Carboniferous displays a decline in d³⁴S from +21 ‰ in the Hastarian to +12 ‰ in the Gzelian.

Biogenic pyrite in sedimentary successions of late Paleozoic age displays highly variable sulfur isotope values. These reveal variable geochemical conditions during sedimentation and diagenesis. A maximum sulfur isotope fractionation of 50 ‰ towards more negative d³⁴S values clearly points to the process of bacterial sulfate reduction. Early Paleozoic sediments, however, also show strongly positive sulfur isotope values for biogenic pyrites, almost always exceeding the value for contemporaneous seawater sulfate. This suggests a sulfate turnover that exceeds sulfate replenishment. No indication for this is present in late Paleozoic sediments. It is suggested that this change in sulfate turnover is linked to the increasing input of less metabolizable terrestrial organic matter following the colonization with land plants.