REVISITING A TREMADOCIAN (EARLY ORDOVICIAN) MASS EXTINCTION EVENT AND OCEAN ANOXIA: NEW INSIGHTS FROM TRACE METAL ABUNDANCES AND URANIUM ISOTOPE COMPOSITIONS OF CARBONATES FROM THE GREAT BASIN REGION, WESTERN USA

An Early Ordovician (Tremadocian) mass extinction event is well documented in the Ibex area (Utah), accompanied by a globally correlated positive carbon isotope excursion (CIE) near the base of the North American Stairsian Series. A previous study presented geochemical proxies (I/Ca ratios and C and S isotopes) of carbonates from the Ibex (IB), Shingle Pass (SP), and Meiklejohn Peak (MP) sections of the Great Basin and proposed expanded ocean anoxia triggered the mass extinction. However, spatiotemporal variations in I/Ca ratios among the three sections and the extent of global ocean anoxia remain poorly understood. In this study, trace metal abundances and U isotope compositions (δ²³⁸U) were analyzed from the same carbonate samples to further constrain the coeval local and global ocean redox conditions.

Published near-zero I/Ca ratios from the distal SP and MP sections during the CIE suggest an episode of local ocean anoxia, whereas fluctuating I/Ca signals from the proximal IB section suggest dynamic oxic/anoxic conditions. Although no correlation exists between I/Ca and U values, a decrease in U concentrations is observed in all three sections, suggesting a lower global seawater U reservoir during an episode of expanded ocean anoxia. Low Mo concentrations are observed in these sections, implying locally non-euxinic conditions during deposition.

Different δ²³⁸U trends are observed in the three sections and no strong correlations exist between δ²³⁸U and redox proxies. The IB section shows a negative δ²³⁸U excursion (down to −0.90‰), whereas relatively invariant δ²³⁸U trends (mean = −0.49‰) are observed for the SP (except for a sample with −0.75‰) and MP sections. Low δ²³⁸U values from non-euxinic carbonates in the IB section could be driven by expanded ocean euxinia in a local restricted basin (i.e., preferential removal of ²³⁸U to euxinic sediments). Local basin restrictions could be related to regional rifting event. On the other hand, the δ²³⁸U trends from the distal SP and MP sections likely record global seawater δ²³⁸U signals. With an offset of 0.24‰, the coeval seawater δ²³⁸U could be −0.73‰. A three-sink δ²³⁸U mass balance model estimates a maximum of 12% euxinic seafloor area. It suggests expanded anoxic and ferruginous seawater during the ocean anoxic event evidenced by the positive C and S isotope excursions.