中国湖相碳酸盐岩碳氧同位素时空特征及其古湖泊学意义

湖相碳酸盐岩是一种分布极其广泛的陆相碳酸盐岩。中国湖相碳酸盐岩沉积始于二叠纪—三叠纪,经侏罗纪—白垩纪发展,鼎盛于古近纪,具有沉积时间跨度长、分布面积广的特征。碳酸盐岩发育方式多样,常以多个单层、薄夹层状、互层状、韵律沉积为主,也见以结核状或钙质微体化石等薄层赋存于泥岩、页岩等细粒碎屑岩中。湖相碳酸盐岩δ¹³C值介于-10.0×10^-3~+10.0×10^-3之间,多数为-5.0×10^-3~+5.0×10^-3,正负值均有。二叠系—三叠系δ¹³C发生负偏,侏罗系—白垩系先正向漂移随后负偏,古近系—新近系再次正偏,达最大值约+5.0×10^-3。氧同位素δ¹⁸O值介于-20.0×10^-3~+2.0×10^-3之间,多数为-15.0×10^-3~-0.0×10^-3,几乎全部为负值。二叠系—三叠系、侏罗系—白垩系碳酸盐岩δ¹⁸O值在-15.0×10^-3~-3.0×10^-3之间变化,古近系—新近系δ¹⁸O值明显正偏移,多数位于-10.0×10^-3~0.0×10^-3。中国湖相碳酸盐岩碳氧同位素相关性分析表明,δ¹³C与δ¹⁸O密切相关,且δ¹³C值多正值或轻微负值,指示湖泊为封闭型咸水—半咸水环境;δ¹³C和δ¹⁸O不相关,δ¹³C值为负值,指示湖泊为开放型淡水环境。

Temporal and spatial characteristics of C-O isotopes of Lacustrine carbonate in China and the Paleolimnological significance

Lacustrine carbonate rock is a widely distributed continental facies. In China it is also widely distributed and precipitated in long terms beginning in Permian-Triassic period, developing in Jurassict-Cretaceous period and flourishing in Paleogene. It occurs in different forms, such as the dominant rhythmic multi-single layers, thin intercalith and interbeded layers, thin nodular and calcareous micro-fossil layers in mudstone or shale. The value is in range -10.0×10^-3~+10.0×10^-3 and most of the value in range -5.0×10^-3~+5.0×10^-3 in Permian-Triassic period tends to be negative, in Jurassic-Cretaceous period first to be positive and then negative, in Paleogene to be positive with top value +5.0×10^-3. The value is in range -20.0×10^-3~+2.0×10^-3 and most of the value in range -15.0×10^-3~0.0×10^-3 of which almost are negative. The value of Permian-Triassic period and Jurassic-Cretaceous period is in range -15.0×10^-3~-3.0×10^-3, the value of Paleogene -10.0×10^-3~0.0×10^-3 tend to be positive. Analysis of correlation of C and O isotopes shows that and are closely correlated and most positive, slightly negative indicating that the lakes is under closed environment with saline-semisaline water, are not correlated and value negative indicating that the lakes under open environment with fresh water.