Thursday, March 14, 2013

Isotopic Evidence for a Guadalupian Mass Extinction From China

Carbon and sulfur isotopic fluctuations associated with the end-Guadalupian mass extinction in South China


1. Yan Detain (a, b)
2. Zhang Liqin (c)
3. Qiu Zhen (b)


a. Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China

b. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

c. Key Laboratory of Geospace Environment and Geodesy of Ministry of Education, Wuhan University, Wuhan 430079, China


Concentrations of total organic matter (TOC), carbon isotopic compositions of carbonate and organic matter (δ13Ccarb, δ13Corg), and sulfur isotopic compositions of carbonate associated sulfate (δ34Ssulfate) across the Guadalupian-Lopingian (G–L) boundary were analysed from identical samples of Tieqiao section, Laibin, Guangxi province, South China. The δ13Ccarb values show a positive excursion from -0.45‰ to the peak of 3.80‰ in the Laibin limestone member, followed by a drastic drop to -2.60‰ in the lowest Heshan Formation, then returned to about 1.58‰. Similar to the trends of the δ13Ccarb values, △13Ccarb-org values also show a positive excursion followed by a sharp negative shift. The onset of a major negative carbon isotope excursion postdates the end Guadalupian extinction that indicates subsequent severe disturbance of the ocean-atmosphere carbon cycle. The first biostratigraphic δ34Ssulfate values during the G-L transition exhibit a remarkable fluctuation: a dramatic negative shift followed by a rapid positive shift, ranging from 36.88‰ to -37.41‰. These sulfate isotopic records suggest the ocean during the G-L transition was strongly stratified, forming an unstable chemocline separating oxic shallow water from anoxic/euxinic deep water. Chemocline excursions, together with subsequent rapid transgression and oceanic anoxia, were likely responsible for the massive demise of the G–L biotic crisis.

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