T240 Excursions, Extinctions, and Environmental Change: New Advances in Geochemical Tools Linked to Critical Intervals of Environmental and Biotic Change in Earth History (Posters)

Tuesday, 27 September 2016
9:00 AM - 6:30 PM
Exhibit Hall E/F

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

Paper number: T240-15
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-285161

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