THE END-TRIASSIC INITIAL CARBON ISOTOPIC EXCURSION: NOT YOUR GRANDPA’S CIE (Invited Presentation)

The end-Triassic mass extinction event (ETE) 201.6 million years ago displays one of the hallmarks of mass extinctions - a global negative carbon isotopic excursion (CIE). The canonical δ¹³C_org CIE at St Audrie’s Bay was preserved in an otherwise marine to restricted marine environmental setting in the Bristol Channel rift basin in Somerset, United Kingdom. It is stratigraphically situated at the marine invertebrate and palynological turnover event marking the regional ETE. However, the CIE itself occurs in a thin interval exhibiting abundant desiccation cracks completely lacking any marine or marginal marine invertebrate fossils. Based on biomarker data, an oxic microbial mat developed in freshwater to brackish shallow water. Relative to underlying marine strata, this mat was dominated by cyanobacteria, with an increase in contributions from methanotrophs and a decrease in sulfate-reducing bacteria, with minor inputs from purple sulfur bacteria and green sulfur bacteria (Chlorobiaceae). Green sulfur bacteria, purple sulfur bacteria, and sulfate-reducing bacteria, coupled with a decrease in cyanobacteria and methanotrophs, support the rapid development of an anoxic mat in the later stage of the CIE. The CIE is succeeded by an increase in marine input as recorded by biomarker evidence of a short-lived episodic interval of shallow-water photic zone euxinia. This was followed by a return to more open marine conditions prior to the fully marine to restricted marine conditions of the overlying Blue Lias Formation. The St. Audrie’s CIE was directly related to a regional and abrupt elimination of marine waters by either a drop in sea level or closure of a gateway, and development of a shallow, ephemeral, and short-lived lake. Furthermore, based on paleomagnetic, astrochronological, and U-Pb correlations to the lacustrine eastern North American rifts and marine Pucara Group of Peru, the St. Audrie’s CIE is part of a 405-ky-eccentricity-paced cyclical δ¹³C sequence (also seen in Peru) during which the initial phase of the massive Central Atlantic Magmatic Province was emplaced. Despite its anomalous environmental fluctuations, the St. Audrie’s CIE evidently correlates to the marine extinction level in the Pucara Group and the continental extinction in eastern North America within some small fraction of a 405 kyr cycle.