Assessing the biodiversity crisis within the Triassic-Jurassic boundary interval using redox sensitive trace metals and stable carbon isotope geochemistry

Abstract: Around the Triassic-Jurassic-Boundary time interval, a severe mass extinction event occurred, which has been associated to volcanism of the Central Atlantic Magmatic Province (CAMP). Significant amounts of greenhouse gases as carbon dioxide (CO2) or methane (CH4) and other volatiles are suggested to have been released into the atmosphere, leading to global warming with subsequent impact on the biosphere. The marine realm was affected to a greater extent than observed from the terrestrial record and there, the biotic recovery was slow and prolonged. The causes of this severe extinction and protracted recovery in the oceans are still not well understood. One recurring hypotheses is the presence of extended anoxic conditions in neritic shallow seas. The aim of this study was to address redox conditions of the European epicontinental sea during this time interval. Two cores from the Triassic-Jurassic boundary sedimentary successions were sampled with the target to gain further knowledge on the oxygen conditions that prevailed during time of deposition. One core comes from Stenlille, well nr. 4 from the Danish Basin (Denmark) and the other one, from Schandelah. Schandelah is located within the north German Lower Saxony Basin. Successions from both localities constitute shallow marine to coastal sandstones, mudstones and shales. Variations in oxygen grades of these successions was addressed with elemental analysis, evaluating enrichments of V, Mo, U, Zn, Cr, Co, Cu, Mn and their covariation with Al and TOC. The results indicate that anoxic conditions where generally not met in the seas at those localities during this time interval. Hg and Ni were used as markers of volcanic activities. In Stenlille, one level of anomalous concentrations of Hg and Ni is present, and in Schandelah two anomalous levels are present. At both localities, abnormal accumulation of both Hg and Ni coincide with a negative δ13Corg excursion associated to CAMP, which may indicate that the enrichment of these elements was due to volcanic activity. δ13Corg values are presented here for Stenlille 4, and serve as a tool for stratigraphic correlation between TJB successions.