IDENTIFYING THE CAUSES OF EARLY PALEOZOIC MASS EXTINCTIONS USING NEW CARBONATE GEOCHEMICAL PROXIES: LOCAL EVIDENCE OF MARINE ANOXIA USING I/CA RATIOS
New and published I/Ca datasets from several Paleozoic sections suggest that marine anoxia, in shallow carbonate platforms settings, was an important driver of several mass extinctions. An Early Ordovician mass extinction, a continuation of the Cambrian biomere events, appears to have been caused by the expansion of anoxic waters overlying nearshore environments based on low I/Ca values coincident with a positive δ13C excursion. This pattern extends into the latest Ordovician where I/Ca values reach a minimum during the Hirnantian δ13C excursion and mass extinction. The late Silurian Lau-Kozlowskii extinction event also exhibits this pattern of local water column anoxia coinciding with marine mass extinction. Finally, I/Ca data across the Late Devonian Frasnian-Famennian biotic crisis interval also show that local water column conditions were anoxic during this time. The I/Ca proxy shows promise for future studies where δ13C trends suggest enhanced organic carbon burial may have coincided with faunal turnover in marine settings but has yet to be investigated.