TRACKING GLOBAL SEAWATER REDOX TRENDS DURING THE LATE DEVONIAN EXTINCTION USING U ISOTOPES OF UPPER DEVONIAN MARINE CARBONATES

The Late Devonian extinction ranks as one of the ‘big five’ Phanerozoic extinctions affecting up to 80% of marine species and occurred during 5 distinct pulses. The leading hypotheses explaining the pulsed extinctions are global cooling and/or widespread marine anoxia. We test the marine anoxia hypothesis by analyzing uranium isotopes (δ²³⁸U) across a ~7 My interval of well-dated Upper Devonian marine carbonates in Nevada (Devil’s Gate section).

The measured δ²³⁸U curve shows no co-variation with water-depth dependent facies changes, redox-sensitive trace metals, Mg/Ca and Mn/Sr ratios, or detrital elements (K, Al) indicating the δ²³⁸U curve was not controlled by local depositional or diagenetic processes and represents global seawater redox conditions. Two negative δ²³⁸U shifts indicating more reducing seawaters are observed with -0.2‰ to -0.3‰ magnitudes and with durations of ~2.7 My and ~1 My, respectively. The late Frasnian negative shift is broadly coincident with the first extinction pulse (late rhenana Zone), whereas the early Famennian negative shift (lower-middle triangularis zones) does not coincide with the most intense Frasnian-Famennian (F-F) extinction pulses. Sediment-type compilations of all available Upper Devonian data with conodont zonation-level age control indicates that the extinction pulses were coincident with widespread anoxic deposits accumulating in subtropical epeiric seaways. The temporal relationships among global ocean redox conditions represented by the new δ²³⁸U curve, subtropical epeiric sea redox conditions (represented by anoxic versus oxic sediments), and biodiversity trends supports previous interpretations that more regional epeiric sea anoxia was an important driver of Late Devonian extinctions, however that these seaways were only partially coupled with global ocean redox trends.