Repeated occurrences of marine anoxia under high atmospheric O₂ and icehouse conditions.

Abstract

The Late Paleozoic Ice Age (~340 to 260 Ma) occurred under peak atmospheric O₂ (1.2 to 1.7 PIAL, pre-industrial atmospheric levels) for Earth history and CO₂ concentrations comparable to those of the preindustrial to that anticipated for our near future. The evolution of the marine redox landscape under these conditions remains largely unexplored, reflecting that oceanic anoxia has long been considered characteristic of carbon cycle perturbation during greenhouse times. Despite elevated O₂, a 10⁵-y period of CO₂-forced oceanic anoxia was recently identified, but whether this short-term interval of widespread oceanic anoxia was anomalous during this paleo-ice age is unexplored. Here, we investigate these issues by building a high-resolution record of carbonate uranium isotopes (δ²³⁸U_carb) from an open-marine succession in South China that permits us to reconstruct the global marine redox evolution through the deep glacial interval (310 to 290 Ma) of near peak O₂. Our data reveal repeated, short-term decreases in δ²³⁸U_carb coincident with negative C isotopic excursions and rises in paleo-CO₂, all superimposed on a longer-term rise in δ²³⁸U_carb. A carbon-phosphorus-uranium biogeochemical model coupled with Bayesian inversion is employed to quantitatively explore the interplay between marine anoxia, carbon cycling, and climate evolution during this paleo-glacial period. Although our results indicate that protracted, enhanced organic carbon burial can account for the long-term O₂ increase, seafloor oxygenation, and overall low CO₂, episodic pulses of C emissions had the potential to drive recurring short-term periods of marine anoxia (with 4 to 12% of seafloor anoxia) despite up to 1.7 times higher atmospheric O₂ than present day.