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Sedimentational Geochemical Characteristics of the Sinian Cap Carbonate from the Upper Yangtze Region

YANG Rui-dong1 2, WANG Shi-jie1, DONG Li-rain1, JIANG Li-jun2, ZHANG Wei-hua2, GAO Hui2   

  1. 1. The State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002; 2. Guizhou University of Technology, Guiyang 550003, China
  • Received:2003-03-20 Revised:2003-03-20 Online:2003-03-20 Published:2003-03-20

Abstract: A global glaciation happened at about 600 Ma.The earth became a “snowball earth”,which ended abruptly when subaerial volcanic outgassing raised atmospheric carbon dioxide to about 350 times the modern leve1.The rapid termination would have resulted in warming of the snowball earth to extreme greenhouse conditions.The transfer of atmospheric carbon dioxide to the ocean would result in the rapid precipitation of calcium carbonate in warm surface waters and consequently formation of the cap carbonate rocks observed globally.Regionally persistent,thin intervals of carbonate rocks directly and ubiquitously overlie Proterozoic glacial deposits almost on every continent,and are commonly referred to as cap carbonates.Their unusual facies,stratigraphically abrupt basal and upper contacts, and strongly negative carbon isotopic signature(δ13C values between –7.0‰ and 0‰ )suggest a chemical oceanographic origin,the details of which remain unsolved. Here we propose that these enigmatic deposits are related to the destabilization of gas hydrate in terestrial permafros t following rapid post.glacial warming and flooding of widely exposed continental shelves and interior basins.The authors studied geochemistry ,sedimentology and palaeontology of the Sinian cap carbonates in Guizhou and Hunan Province,including common occurrence within the cap carbonates of unusual fabrics,strongly negative carbon isotopic signature,and large amount of asphalt nodule.According to these results,we suggest that the cap carbonates be produced by solid methane seeps.This is in agreement with Kennedy’s view (2001).In contrast,we disagree with Hoffman et al’s view (1998).They suggested that the cap carbonate be produced by the transfer of atmospheric carbon dioxide to ocean which would result in the rapid precipitation of calcium carbonate in WaF1TI surface waters.