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 (delta C-13(org)) and sulfur isotopic compositions of iron sulfide (delta S-34(sulfide)) 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 delta C-13(org) values vary from about -30.8 parts per thousand. VPDB in the mid-Ashgill to -27.6 parts per thousand in the upper Hirnantian interval, which abruptly return to the pre-Hirnantian values. At Nanbazi section in northern Guizhou, delta C-13(org) values vary from -30.5 parts per thousand to -26.6 parts per thousand from the mid-Ashgill to the upper Himantian horizons, which shift negatively to the low spike at -29.2 parts per thousand in the lowermost Rhuddanian, then increase slightly to relatively persistent values around -28 parts per thousand. Similar variation patterns of delta S-34(sulfide) 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 delta S-34(sulfide) 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. (C) 2009 Elsevier B.V. All rights reserved.