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