Sedimentary and Geochemical Characteristics of Sinian Cap Carbonates in the Upper Yangtze Region

A global-scale glaciation occurred at about 600 Ma ago. As a result, the Earth became the “Snowball Earth“. The glaciation came to the end abruptly when atmospheric carbon dioxide increased to such an extent as to be about 350 times the modem level because of subaerial volcanic degassing. The rapid termination of glaciation would have led to warming of the Snowball Earth and extreme greenhouse conditions would have been created. The transfer of atmospheric carbon dioxide to oceans would give rise to the rapid precipitation of calcium carbonate in warm surface seawaters, thus forming the cap carbonate rocks as observed worldwide today. Regionally persisting, thin layers of carbonate rocks directly and ubiquitously overlie Proterozoic glacial deposits almost on every continent, and are commonly referred to as cap carbonates. Their unusual litho- fabrics, stratigraphically abrupt basal and upper-level contacts and strongly negative carbonate isotopic signatures ( δ ^13Cearb. values range from -7.0‰ —0‰) suggest a chemical oceanographic origin, the details of which remain unknown. It is proposed that these enigmatic deposits are related to the destabilization of gaseous hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and internal basins. The authors carried out studies on the geochemistry, sedimentology and palacontology of the Sinian cap carbonates in Guizhou and Hunan provinces, including the occurrence of cap carbonates of unusual fabrics, strongly negative carbon isotopic signatures, and a lot of bitumen nodules.From the results it is suggested that the cap carbonates were formed from solid methane seepage, and it is in agreement with Kennedy‘‘ s viewpoint (2001) that the cap carbonates resulted from the rapid precipitation of calcium carbonate in response to solid methane seepage.

Sedimentary and geochemical characteristics of Sinian cap carbonates in the Upper Yangtze region

A global-scale glaciation occurred at about 600 Ma ago. As a result, the Earth became the “Snowball Earth”. The glaciation came to the end abruptly when atmospheric carbon dioxide increased to such an extent as to be about 350 times the modern level because of subaerial volcanic degassing. The rapid termination of glaciation would have led to warming of the Snowball Earth and extreme greenhouse conditions would have been created. The transfer of atmospheric carbon dioxide to oceans would give rise to the rapid precipitation of calcium carbonate in warm surface seawaters, thus forming the cap carbonate rocks as observed worldwide today. Regionally persisting, thin layers of carbonate rocks directly and ubiquitously overlie Proterozoic glacial deposits almost on every continent, and are commonly referred to as cap carbonates. Their unusual lithofabrics, stratigraphically abrupt basal and upper-level contacts and strongly negative carbonate isotopic signatures (δ¹³C_carb values range from −7.0‰ −0‰) suggest a chemical oceanographic origin, the details of which remain unknown. It is proposed that these enigmatic deposits are related to the destabilization of gaseous hydrate in terrestrial permafrost following rapid postglacial warming and flooding of widely exposed continental shelves and internal basins. The authors carried out studies on the geochemistry, sedimentology and palaeontology of the Sinian cap carbonates in Guizhou and Hunan provinces, including the occurrence of cap carbonates of unusual fabrics, strongly negative carbon isotopic signatures, and a lot of bitumen nodules. From the results it is suggested that the cap carbonates were formed from solid methane seepage, and it is in agreement with Kennedy’s viewpoint (2001) that the cap carbonates resulted from the rapid precipitation of calcium carbonate in response to solid methane seepage. Funded jointly by the National Natural Science Foundation of China (Grant No. 40062001), the Funds for Qualified Scientists and Technicians in Guizhou, the State Key Basic Research Program (No. 2002 CCC 02600) and the Provincial Governor’s Special Funds.