新元古代不同沉积环境的白云岩帽碳同位素特征及成因

Carbon isotopes and origin of the Neoproterozoic cap dolostone in different sedimentary environments

贵州印江永义剖面白云岩帽的碳同位素值,与全球广布的新元古代碳酸盐岩帽的一样,发生了一次强的负偏移,可能为“雪球地球”冰释后大量的大陆风化产物流入海洋中形成。其δ18OPDB均值和δ13CPDB均值分别比处于碎屑潮坪环境的清镇阿坝寨剖面低6‰和6.5‰,比处于台地斜坡环境的大庸田坪剖面高2.3‰和2.5‰,而与同处于浅海陆棚环境的铜仁龙井剖面一致,这反映了沿碎屑潮坪相-台地斜坡相-浅海陆棚相古环境剖面的一个碳同位素梯度。前人认为这种碳同位素分层可能由于冰后期海洋生物泵吸作用的结果,作者则认为这些不同环境沉积的碳酸盐岩可能不是同时形成的,其碳同位素梯度可能为大陆风化产物不断输入、CCD界面变化及生物繁盛等共同作用的结果。

A remarkably negative excursion is recognized for the carbon isotopic values of the cap dolostone from the Yongyi secton in Yinjiang, Guizhou just like those of the Neoproterozoic cap carbonates on a global scale. The cap dolostone may be originated from abundant continental weathering products entering into oceans after the deglaciation of the ＂snowball earth＂. The mean isotopic values of δ^18OPDB and δ^13CPDB for the Yongyi section are lower at 6‰ and 6.5‰, respectively than those for the Abazhai section in Qingzhen, Guizhou in the elastic tidal-fiat environment; higher at 2.3‰ and 2.5‰, respectively than those for the Tianping section in Dayong, Hunan in the platform foreslope environment, and consistent with those for the Longjing section in Tongren, Guizhou in the same shallow-sea and shelf environments. All these values indicate carbon isotopic gradients along the palaeoenvironmental transection through the elastic tidal-fiat facies to the platform foreslope facies and to the shallow-sea and shelf facies. The previous authors contended that the C isotopic stratification may be attributed to the effect of biological pumping in a postglaeial ocean. The author in this paper, however, argues that the precipitation of the carbonate rocks cited above may not be synchronous in different environments. Instead the carbon isotopic gradients may result from the combination of the constant input of the continental weathering products, variations in CCD interface and bloom of biotas.