The end-Triassic negative δ¹³C excursion: A lithologic test

The end-Triassic mass extinction is associated with a large negative carbon isotope excursion, which has been interpreted as reflecting the rapid injection of ¹³C depleted CO₂ or methane associated with the emplacement of the Central Atlantic Magmatic Province. However, in a number of sections in central Europe, the negative excursion is associated with a carbonate-poor lithology, and the most isotopically depleted values are associated with the lowest percent carbonate, raising the possibility of a lithologic control on δ¹³C_carb.Here we test the uniqueness of the relationship between the carbonate-poor lithology and the δ¹³C signal by comparing the geochemistry of the extinction marl with two Upper Triassic carbonate-poor beds from lower positions within the same stratigraphic sections. We find that the extinction and non-extinction marls overlap nearly completely in terms of their carbonate content, but differ substantially in their isotopic trends. The extinction marl sections show strong depletions in the δ¹³C and δ¹⁸O of carbonate, and enrichment in δ¹³C of bulk organic carbon, while the non-extinction marls show almost no change in these metrics. Accordingly, the difference in isotopic content must lie in differences inherent to the beds themselves and the circumstances of their deposition and early diagenesis. Although a range of primary drivers for the isotopic trends is possible, an acidification origin for the marl, and oceanic origin for the carbon isotope excursion in carbonate are compatible with our data and supported by the broader context of the extinction.