Early Aptian carbon and sulphur isotope signatures at ODP Site 765

Abstract

Current carbon and sulphur isotope ratios (δ13C and δ34S) suggest there were major shifts in partitioning between reduced and oxidised reservoirs of carbon and sulphur during the Early Cretaceous. However, the δ13C and δ34S records are composed from different Ocean Drilling Program sites and are hard to correlate at high resolution. We present high-resolution Aptian δ13Corg and δ34Sbarite values derived from the same set of samples, enabling a higher certainty correlation than previously possible. Two major hypotheses aim to explain the Early Aptian S-isotope excursion: increased volcanic degassing and/or fluctuations in the marine sulphate concentration. Our S-isotope data provide tight constraints on the timing and magnitude of volcanic flux required. We show that the observed S-isotope signature can be explained by a 2 Ma pulse of increased volcanic flux, injecting ∼4.5×1018 mol C into the atmosphere. Further work is needed to evaluate whether these fluxes are compatible with the existing C-isotope record.