MULTIPLE, SHORT-LIVED OCEAN ANOXIC EVENTS ACROSS THE LATE DEVONIAN MASS EXTINCTION DETECTED USING URANIUM ISOTOPES OF MARINE CARBONATES

The Late Devonian mass extinction, one of the ‘big five’ Phanerozoic extinction events, decimated the most extensive reef ecosystems in Earth history. The extinction occurs as five distinct diversity loss pulses, with the earliest two associated with two globally recognized positive δ¹³C shifts. Geochemical and lithologic evidence suggest that marine anoxia played a role in the extinction; however, these proxies only track local bottom water or porewater redox conditions and cannot evaluate global seawater conditions.

This study utilizes uranium (U) isotopes of marine carbonates to evaluate global marine redox trends across the Late Devonian extinction interval. Under reducing conditions, ²³⁸U is preferentially sequestered into anoxic sediments, leaving seawater and calcite precipitating from that seawater depleted. Because the marine residence time of uranium is significantly longer than ocean mixing times, U isotopes (δ²³⁸U) represent a global-ocean redox signal. Preliminary results from isotopic analysis of bulk marine carbonates from the Upper Devonian (late Frasnian-early Famennian) Devil’s Gate Formation, central Nevada document three negative δ²³⁸U excursions (reducing conditions). The first negative δ²³⁸U excursion (~-0.3‰ shift) coincides with the first extinction pulse and positive δ¹³C shift near the lower-upper rhenana conodont Zone boundary. The second negative δ²³⁸U excursion (~-0.4‰ shift) occurs in the mid linguiformis Zone (latest Frasnian) and overlaps with the second extinction pulse and the start of a positive δ¹³C shift. The youngest negative δ²³⁸U excursion (~-0.3‰ shift) spans the middle triangularis Zone (earliest Famennian) and is not associated with a known positive δ¹³C shift. These data support prior interpretations that marine anoxia played a major role in the Late Devonian extinction and suggests that a short-lived anoxic event continued into the earliest Famennian but was decoupled from processes controlling carbon burial because it is not associated with a known positive δ¹³C shift.