上扬子区震旦纪南沱冰期后碳酸盐岩帽沉积地球化学特征

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

通过对贵州铜仁和湖南大庸两地南沱冰碛岩层之上白云岩层的岩石、结构和构造、地球化学等综合研究 ,认为该白云岩层具有快速沉积所形成的沉积结构和构造 ,碳同位素特征可与全球同期沉积的碳酸盐岩帽 (CapCarbonate)进行对比 ;特别是在白云岩中发现有机碳同位素低的沥青结核体 ,支持了冰期后碳酸盐岩快速沉积是由于海底固态CH4 发生脱气作用 。

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 level. 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 (δ 13C 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 terrestrial permafrost following rapid postglacial 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 warm surface waters.