Carbon and sulphur isotopes in marine evaporites from the late Permian to Late Triassic, Yorkshire, UK

Abstract

Seawater sulphate has been used widely as a tool for palaeoceanographic reconstructions, but more recently it has been used for stratigraphic correlation. The sulphur isotope ratio (δ34S) of seawater is controlled predominantly through evaporite and/or pyrite formation and burial. δ34S can be measured from evaporites, pyrite, and carbonate-associate- sulphate (CAS). Organic carbon isotope ratios (δ13C) have been widely used for stratigraphic purposes in geologic time. In this study, a marine sulphur and organic carbon isotope curve are produced from 389 evaporite/sediment samples from the Staithes S-20 borehole, Yorkshire, UK, which spans the late Permian to Late Triassic time interval. The δ34S and δ13C curves produced in this study are used to ascertain their use for stratigraphic correlation of Triassic sedimentary successions in the UK which are poorly dated due to the type of depositional environment. The Permian–Triassic boundary that records a major negative δ13C excursion and positive δ34S excursion are not recorded in the Staithes S-20 core due to a major hiatus in the upper Sherwood Sandstone Group. The δ34S data indicate that the Sherwood Sandstone Group is dominantly latest Permian in age in this region and only the uppermost part of the group is Early Triassic in age. The Mercia Mudstone Group ranges in age from the Middle to Late Triassic and provide additional data to fill in major gaps in the global δ34S curve — which will aid in global correlation as the general undated Staithes-20 core do not allow a detailed sedimentary correlation with other sections of the UK. Although the results from this study are promising, more Triassic cores from around the UK need to be analysed to better understand and constrain the lithostratigraphic relationships that are currently used to correlate Triassic sequences. Additional isotopic analyses (e.g, strontium isotopes) can be performed on the evaporite samples for a more accurate correlation. The combined application of biostratigraphy, sedimentology and chemostratigraphy will help to track the overall changes of marine sulphate, carbon and pO2, thus develop a greater understanding of the climate and environment of the Triassic.