COUPLED URANIUM AND MOLYBDENUM ISOTOPE INSIGHTS ON LOCAL AND GLOBAL OCEAN REDOX CONDITIONS DURING DEPOSITION OF THE FRASNIAN-FAMENNIAN KETTLE POINT FORMATION, ONTARIO, CANADA

We report Mo isotope, U isotope, and elemental data from organic-rich mudrocks of the Kettle Point Formation (southwestern Ontario, Canada) to evaluate ocean redox conditions around the time of the Frasnian-Famennian mass extinction event. High total organic carbon (TOC) (3.1-15.6 wt%), total sulfur (1.2-6.0 wt%), Mo (40-459 ppm), and U (10-62 ppm) concentrations, high Mo/U ratios (about 3 times the molar Mo/U ratio of modern seawater), and well-correlated Mo and U enrichments are observed in black shales from the Gore of Chatham core, suggesting a locally euxinic depositional environment. The δ⁹⁸Mo values in the Kettle Point Formation range from 0.5‰ to 1.7‰ (relative to standard NIST SRM 3134 = 0.25‰) and δ²³⁸U values vary from –0.1‰ to 0.5‰ (relative to standard CRM145 = 0‰). A distinctive decrease in δ⁹⁸Mo and increase in δ²³⁸U occurs in the Famennian upper part of the formation, resulting in a general inverse correlation between Mo and U isotope values. This stratigraphic change in isotope systematics probably does not arise from changes in global ocean redox conditions, which should shift both isotope systems in the same direction. Instead, changes in the local depositional were important. Notably, samples with low δ⁹⁸Mo typically have strong Mo and V enrichments that along with high Mo/U ratios suggest enhanced operation of an Fe-Mn particulate shuttle that delivered isotopically light Mo to sediments. Changes in U enrichment are modest yet unusually high δ²³⁸U (up to 0.5‰) occurs upsection and may reflect an increase in local seawater-sediment U isotope fractionation associated with a greater proportion of U reduction and removal in the sulfidic water column compared to the underlying sediments. The highest δ⁹⁸Mo values, indicative of extensive global ocean oxygenation, occur around the Frasnian-Famennian boundary in the lower Kettle Point Formation, consistent with recent studies that suggest ocean anoxia was not the key driver of the mass extinction.