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

• Published in: Palaeogeography, palaeoclimatology, palaeoecology palaeoclimatology, palaeoecology, 2009-04, Vol.274 (1), p.32-39
• Main Authors: Yan, Detian, Chen, Daizhao, Wang, Qingchen, Wang, Jianguo, Wang, Zhuozhuo
• Format: Article
• Language: English
• Subjects: Anoxia; Carbon–sulfur isotope; Climate change; Marine; Mass extinctions; Ordovician–Silurian boundary; Stable isotopes; Yangtze Platform
• ISSN: 0031-0182; 1872-616X
• DOI: 10.1016/j.palaeo.2008.12.016

Abundance of organic matter, carbon isotopic compositions of organic matter ( δ 13C org) and sulfur isotopic compositions of iron sulfide ( δ 34S 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 δ 13C org 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, δ 13C org 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 δ 34S 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 δ 34S 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.