THE ROLE OF CLIMATE CHANGE IN MASS EXTINCTIONS: USING STROMATOLITES TO CONSTRAIN TEMPERATURES DURING TIMES OF BIOTIC CRISIS

Stromatolites are laminated accretionary structures commonly thought to be microbial in origin, formed via the trapping and binding of sediment or metabolically-induced precipitation of minerals. However, stromatolite formation is poorly understood. In general, stromatolite form abundance was higher in the Proterozoic than the Phanerozoic, but notable increases in stromatolite abundance occur in association with Phanerozoic mass extinction events. Here, we focus on stromatolites from the latest Triassic Cotham Marble of the Lilstock Formation (United Kingdom) that occur at or very near the extinction horizon. The end-Triassic mass extinction is coincident with large-scale volcanism in the Central Atlantic Magmatic Province (CAMP) and the associated breakup of Pangea. Some hypothesize that CAMP-associated increases in atmospheric CO₂ led to a rise in global temperatures and ocean acidification that caused or enhanced the extinction. In order to quantify the role of climate change with respect to the end-Triassic mass extinction, we applied the carbonate “clumped” isotope paleothermometer as well as conventional carbon and oxygen stable isotope analyses to the well-preserved Cotham Marble stromatolites. The stromatolites alternate between a laminated and dendrolitic microstructure on the cm scale. In order to ascertain temperatures of formation during extinction and recovery interval, the stromatolites were sequentially drilled along growth axes. The dendritic and laminated microstructures record different temperatures of formation, where the dendrolitic portions in general grew under cooler conditions than laminated layers, and younger layers grew in cooler conditions than older layers. Our results suggest that there were quite warm temperatures coincident or nearly coincident with the extinction horizon, and that climate both fluctuated and potentially trended towards amelioration of the warm temperatures during the deposition of the Cotham Marble.