ABRUPT AND SHORT-LIVED GLOBAL MARINE ANOXIA ASSOCIATED WITH THE LATE ORDOVICIAN (HIRNANTIAN) MASS EXTINCTION DETECTED USING U ISOTOPES OF MARINE CARBONATES

The Ordovician witnessed an explosion in marine biodiversity followed by the first of the ‘big-5’ Phanerozoic mass extinctions, the Late Ordovician mass extinction (LOME). The LOME consists of two discrete pulses occurring at the beginning and end of the Hirnantian. Lithologic and geochemical evidence suggests widespread marine anoxia triggered the second LOME pulse; however, most of these redox proxies record local bottom water or porewater conditions rather than global seawater conditions. To evaluate global redox trends, we utilize uranium (U) isotopes of marine carbonates as a global marine redox proxy. This is possible because the residence time of U in the ocean is significantly longer than ocean mixing times and U isotopes fractionate under reducing conditions.

Bulk carbonate samples were collected from the upper Katian, Hirnantian, and through the lowermost Rhuddanian on Anticosti Island, Quebec. The sampled time interval records multiple orders (~20 ky to 1.2 My durations) of glacio-eustatic sea-level change and an unusually thick globally recognized positive δ¹³C excursion (HICE). Analyzed U isotope values are relatively high (average = -0.22‰) in the late Katian-early Hirnantian representing more oxic seawater conditions; they abruptly decrease (average = -0.47‰) in the late Hirnantian-earliest Rhuddanian indicating more anoxic conditions, then abruptly return to higher values (average = +0.28‰) in the Rhuddanian. The onset of the <500 ky-long anoxic interval is coincident with the second LOME pulse but continues after the biotic recovery; the anoxic event persisted throughout the peak Hirnantian glaciation and continues for ~200 ky after the HICE end

These results support earlier interpretations that global anoxia triggered the second LOME pulse and suggest that the mechanisms controlling the burial of organic matter (δ¹³C record) and the areas of anoxic sediment deposition (U isotope record) were decoupled. These results also indicate that global anoxia persisted (but fluctuated) despite ~20 ky to 1.2 My-scale glacial-interglacial ocean circulation and climatic changes and through varying glacio-eustatic sea-level positions.