GEOCHEMICAL EVIDENCE FOR REDOX VARIATIONS ACROSS THE TRIASSIC-JURASSIC BOUNDARY IN THE TETHYS SEA

Though the mass extinction that occurred at the Triassic-Jurassic boundary (Tr-J; 199.6 Ma) was the second largest such event during the Phanerozoic, it remains one of the least understood.  Here, we present bulk organic d¹⁵N and d¹³C records obtained from a drill core near Mingolsheim, southern Germany.  Based on biostratigraphic reconstruction, the samples were located on a shallow restriction on the Tethyan seaway, and cross the Tr-J boundary. We interpret the wide range of isotopic values for these samples (d¹⁵N from +0.5‰ to +2.2‰; d¹³C from -30‰ to -25‰), as indicative of highly variable water column redox variations at this location. The d¹⁵N and d¹³C profiles are inversely correlated throughout the core. The late Triassic is characterized by relatively low d¹⁵N and high d¹³C values, characteristic of an oxic environment, with the exception of a short excursion of heavy d¹⁵N/light d¹³C just preceding the biostratigraphically defined boundary. The early Jurassic features a gradual enrichment of d¹⁵N coincident with depleted d¹³C values, indicative of an increase in water column denitrification and decrease in oxygen concentration. These variations in redox state most likely resulted from changes in water column stratification. Palynology provides supporting evidence for our interpretation of the isotopic values, while preliminary evaluation of trace metals also provides additional information regarding the redox environment across the Tr-J boundary.  We will examine the possible environmental drivers of the changes in redox state and their potential influence and feedbacks on the mass extinction associated with this boundary.