Terrestrial record of carbon-isotope shifts across the Norian/Rhaetian boundary: A high-resolution study from northwestern Sichuan Basin, South China

Abstract

A series of biotic crises and turnovers have been recorded at the Norian/Rhaetian boundary (NRB), together with a putative perturbation to the global carbon cycle in the form of one or more negative carbon-isotope shifts. Despite this evidence, the full validation of Norian/Rhaetian climate changes has not been possible. In particular, no clear record of Norian/Rhaetian environmental change has yet been recovered from a terrestrial setting. The eruption of the Angayucham Large Igneous Province has been considered as a possible cause for the putative NRB negative carbon-isotope shifts. Hg as a volcanism proxy has been well-studied in many ancient marine and terrestrial sediments, but to date no Hg record has been obtained across the NRB. In this study, we focus on a terrestrial Norian-Rhaetian section (Xujiahe section) located north of Guangyuan in the northwestern Sichuan Basin, which has been correlated to the NRB using cyclostratigraphy and magnetostratigraphy. A long-lived negative trend in δ13Corg, superimposed with four negative shifts, is recognized across the NRB interval. This can be correlated to the uppermost part of the third negative δ13Corg shift (named S3) recognized in profiles from marine sites in Europe. Contemporaneous Hg concentration, Hg/Al and Hg/TOC anomalies in the Xujiahe section were possibly caused by large-scale volcanic activity. The large amplitude changes in δ13Corg (ca. −5.2‰) that we observe are suggested to be associated with local wildfire and enhanced coarse sediment input, indicating a significant environmental change during the NRB interval. Similar paleoclimatic perturbations have been noted across other extinction events (e.g., end-Permian mass extinction event) as a result of the emplacement of large igneous provinces, or because of felsic volcanism.