Isotopic evidence for an anomalously low oceanic sulfate concentration following end-Permian mass extinction

摘要

The cataclysmic end-Permian mass extinction was immediately followed by a global expansion of microbial ecosystems, as demonstrated by widespread microbialite sequences (disaster facies) in shallow water settings. Here we present high-resolution carbonate carbon (δ~(13)C_(carb)) and carbonate-associated sulfate-sulfur isotope (δ~(34)S_(CAS)) records from the microbialite in the Cili Permian-Triassic (P-Tr) section in South China. A stepwise decline in δ~(13)C_(carb) begins in the underlying skeletal limestone, predating the main oceanic mass extinction and the first appearance of microbialite, and reaches its nadir in the upper part of the microbialite layer. The corresponding δ~(34)S_(CAS), in the range of 17.4‰ to 27.4‰, is relatively stable in the underlying skeletal limestone, and increases gradually from 2m below the microbialite rising to a peak at the base of the microbialite. Two episodes of positive and negative shifts occurred within the microbialite layer, and exhibit a remarkable co-variance of sulfur and carbon isotope composition. The large amplitude of the variation in δ~(34)S_(CAS), as high as 7‰ per 100kiloyears, suggests a small oceanic sulfate reservoir size at this time. Furthermore, the δ~(13)C_(carb) and δ~(34)S_(CAS) records co-vary without phase lag throughout the microbialite interval, implying a marine-driven C cycle in an anoxic ocean with anomalously low oceanic sulfate concentrations. On the basis of a non-steady-state box model, we argue that the oceanic sulfate concentration may have fallen to less than 15%, perhaps as low as 3%, of that in the modern oceans. Low oceanic sulfate concentration likely was the consequence of evaporite deposition and widespread anoxic/sulfidic conditions prior to the main mass extinction. By promoting methanogenesis and a build-up of atmospheric CH_4 and CO_2, low oceanic sulfate may have intensified global warming, exacerbating the inimical environmental conditions of the latest Permian.