Carbon and sulfur isotopic anomalies across the Ordovician–Silurian boundary on the Yangtze Platform, South China

Abundance of organic matter, carbon isotopic compositions of organic matter (δ13Corg) and sulfur isotopic compositions of iron sulfide (δ34Ssulfide) across the Ordovician–Silurian (O–S) boundary was analysed from two sections (Wangjiawan, western Hubei; Nanbazi, northern Guizhou) on the Yangtze Platform, South China. The organic abundance across the O–S boundary at these two sections is generally high, except for the Hirnantian interval. At Wangjiawan section in western Hubei, the δ13Corg values vary from about − 30.8‰ VPDB in the mid-Ashgill to − 27.6‰ in the upper Hirnantian interval, which abruptly return to the pre-Hirnantian values. At Nanbazi section in northern Guizhou, δ13Corg values vary from − 30.5‰ to − 26.6‰ from the mid-Ashgill to the upper Hirnantian horizons, which shift negatively to the low spike at − 29.2‰ in the lowermost Rhuddanian, then increase slightly to relatively persistent values around − 28‰. Similar variation patterns of δ34Ssulfide values are unravelled at the two sections, showing a positive excursion from the mid-Ashgill to the upper Hirnantian, then a sharp negative shift in the lowermost Rhuddanian, although with different background values of δ34Ssulfide from these two sections. These data, together with those from other areas, demonstrate large climatic fluctuations from warming to cooling, then to warming, oceanic changes from and anoxic to oxygenated, to anoxic water columns, during the O–S transition. Climatic fluctuations, together with multiple oceanic anoxia and sea-level fluctuations, were likely responsible for the stepwise massive demise of the O–S biotic crisis.