An Integrated ChemoStratigraphic Approach to Understanding the Siluro-Devonian Positive Carbon Isotope Excursion

Abstract

Through comparative analysis of the stable isotope and trace element stratigraphy across three sections in the NY Helderberg Group, this study aims to better constrain the processes responsible for the carbon isotope excursion spanning the Silurian-Devonian boundary. Stable isotope and trace element profiles of stratigraphic sections from Cherry Valley and the Hudson Valley fold and thrust belt are correlated with the U/Pb dated Cobleskill section to provide absolute time constraints for interpretation of the positive carbon isotope excursion. Chemostratigraphic correlations between Cherry Valley and Hudson Valley sections support the hypothesis of a diachronous transgressive depositional history and provide evidence of locally anoxic conditions occurring at distinct times in the two sections studied. Additionally, comparison of the stable isotope signal recorded by various carbonate phases within the sections gives novel results suggesting that crinoids, corals and brachiopods are comparable as carbon isotope recorders. Trace element analysis of fossils at the single-sample scale provide a rich picture of diagenetic alteration of fossils away from the micrite end-member composition and towards that of a reducing, DIC-charged fluid. The anti-correlated organic vs. inorganic carbon isotope record from Cherry Valley provides additional evidence supporting a period of locally high organic weathering input and organic burial flux. Taken all together, the time-constrained chemostratigraphic data support a model of locally occurring anoxic conditions and high organic weathering and burial flux in the 4 Myr time period from 424-420 Ma spanning the 4.88hS-D carbon isotope excursion.