Molybdenum isotopic evidence for anoxic marine conditions during the end-Permian mass extinction

Abstract

The end-Permian mass extinction was the most severe biotic crisis of the Phanerozoic. Oceanic anoxia has long been considered one of the main drivers of the biotic crisis. We examined the molybdenum isotope compositions of bulk carbonate samples from two Permian–Triassic boundary sections in southern China: the Meishan Global Stratotype Section and Point and the Dajiang section (an isolated carbonate platform). We also measured the Mo contents of the carbonate phases, oxides, and sulfides of the carbonate samples via sequential extraction to investigate the samples' Mo-bearing minerals. In the Meishan section, from Bed 23 to the middle of Bed 24, the δ98/95Mocorr.Al values decrease slowly from +0.89‰ to +0.35‰, while from the middle of Bed 24 to Bed 25, the δ98/95Mocorr.Al values abruptly increase from +0.35‰ to +1.78‰. In the Dajiang section, samples below the mass extinction boundary have relatively low δ98/95Mocorr.Al values with a narrow variation range (−0.05‰ to +0.46‰; average + 0.19‰); the δ98/95Mocorr.Al values of the five samples directly above the extinction boundary increase abruptly (vary from +1.38‰ to +1.63‰; average of +1.53‰), and the δ98/95Mocorr.Al values of other samples above the mass extinction boundary fluctuate slightly before returning to pre-extinction values. The results of the sequential extraction procedure indicate that carbonate-phase Mo accounts for a small portion of the total Mo (average of 13.6 and 18.4 wt% for the Meishan and Dajiang sections, respectively), oxidized and sulfided Mo are the dominant forms in the carbonate samples. Moreover, the carbonate samples with high δ98/95Mo values tend to have high contents of sulfided Mo. We suggest that marine carbonate sediments record paleo-ocean Mo isotopic signals by a mechanism similar to that of other open-ocean sediments (e.g., shales): higher δ98/95Mo values in the sediments indicate a more reducing sedimentary environment. Based on this assumption, the δ98/95Mo values of the studied sections indicate that during the Permian–Triassic transition, oceanic anoxia coincided with or slightly predated the mass extinction, and the ocean anoxic event lasted for a relatively short time, likely less than 0.061 Myr.