High-resolution δ13C data of organic carbon from a continuous section of the Late Ordovician–Early Silurian reveal two positive δ13C excursions that are associated with the mass extinction in South China. The first stratigraphic δ34S measurements on pyrite tied to well-established biostratigraphy indicate a large perturbation of the sulfur cycle, consistent with major sea-level changes related to the glaciation. The elevated δ34S values of pyrites and a large, short-lived negative δ34S excursion of ~20‰ associated with the decay of the glaciation suggest deep-water anoxia during the Hirnantian Stage, in contrast to the conventional view that the global oceans were oxygenated. We suggest that deep-water anoxia may have contributed to the Late Ordovician mass extinction in South China and possibly elsewhere.

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