IDENTIFYING THE CAUSES OF EARLY PALEOZOIC MASS EXTINCTIONS USING NEW CARBONATE GEOCHEMICAL PROXIES: LOCAL EVIDENCE OF MARINE ANOXIA USING I/CA RATIOS

The early Paleozoic experienced several mass extinctions and biotic crises. These includes the Late Ordovician and Late Devonian mass extinctions as well as the recurrent Cambrian trilobite extinctions (i.e., biomeres) and Silurian conodont/graptolite extinctions. Environmental changes like global warming/cooling and/or marine anoxia, inferred by geochemical proxies, are attributed as causal mechanisms of many these extinctions. Traditional proxies like stable carbon isotopic trends (δ¹³C) measured from carbonate rocks and fossils are used to identify periods of elevated organic carbon burial rates, potentially linked to anoxia but lack mechanistic certainty. Several redox proxies focus on sampling organic-rich shales to confirm whether anoxia once existed, but these records are limited in the geologic record. Recent advances in the marine iodine proxy (I/Ca) recorded within carbonate rocks and fossils fingerprints local water column anoxia when low I/Ca values are recorded. When δ¹³C and I/Ca values are combined from multiple localities, they can provide strong evidence for water column anoxia at local and potentially global scales, which suggests a mechanism for significant biological evolution including extinction events.

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.