塔里木北缘震旦纪——寒武纪转折期碳同位素漂移事件及成因机制

Carbon isotope excursion and its genetic mechanism during the Sinian to Cambrian transition in the northern Tarim Basin

震旦纪—寒武纪转折期是地球演化的关键节点,这一时期的碳氧同位素记录在塔里木保留完整却关注较少。选取苏盖特布拉克露头震旦系与寒武系交界地层实测采样,并开展了古生物、镜下鉴定、碳同位素漂移事件的综合分析,探讨了N1(BACE)、P1(ZHUCE)等碳同位素漂移事件的成因机制。样品的δ13Ccarb和δ18Ocarb相关系数(R2=0.05)、δ18Ocarb和Mn/Sr相关系数(R2=0.09)及岩石学特征表明,后期成岩改造并未导致碳氧同位素的显著分馏,原始碳同位素特征得以基本保留。由下至上,在该剖面识别出P-1事件(δ13Ccarb峰值1.9‰~2.4‰)、N1事件(-6.8‰~-10.3‰)、P1事件(1.4‰~4.1‰)、N2a c事件(-0.4‰~-2.8‰)、P2a c事件(0.2‰~0.6‰)和N3事件(-3.4‰)。综合碳同位素演化特征与古生物、年代学数据,确认了玉尔吐斯组底部硅质页岩与奇格布拉克组顶部藻云岩的分界面为塔里木震旦系与寒武系的分界线,并实现了该露头与老林、肖滩、三峡、西伯利亚、阿曼和摩洛哥剖面的地层对比。分析认为,塔里木北缘震旦纪—寒武纪转折期的碳漂移事件,更多受控于古海洋氧气含量变化引起的固碳率forg的波动。玉尔吐斯组早期海侵背景下的大规模缺氧事件导致的初级生产力和固碳率降低,是N1负漂移事件(BACE)的主因;随后海退中氧气含量的增加引起生产力重建和固碳率增加,形成了P1正漂移事件(ZHUCE)。这一成果性认识有助于塔里木盆地寒武纪古环境研究与深层超深层油气远景资源评价。

The Sinian to Cambrian transition is a time node for the Earth evolution,yet carbon and oxygen isotopes,which are well preserved in the Tarim Basin during this period,are barely studied.Here,based on the measurements of the Sugaitebulake outcrop,a comprehensive analysis of paleontology,microscopic identification,and carbon isotope excursions is carried out,and the genetic mechanism of the N1(BACE)and P1(ZHUCE)excursion events is explained.The correlation coefficient betweenδ13Ccarb andδ18Ocarb(R^(2)=0.05)orδ18Ocarb and Mn/Sr(R2=0.09),as well as microscopic characteristics,shows that the later diagenesis did not lead to fractionation of carbon and oxygen isotopes significantly,and the original carbon isotopic characteristics are basically preserved.Six carbon excursions were detected from the bottom to the top of the formation,which comprise,respectively,excursions P-1 withδ13Ccarb between 1.9‰ and 2.4‰,N1 withδ13Ccarb between-6.8‰and-10.3‰,P1 withδ13Ccarb between 1.4‰ and 4.1‰,N2a c withδ13Ccarb between-0.4‰ and-2.8‰,P2a c withδ13Ccarb between 0.2‰ and 0.6‰,and N3 withδ13Ccarb peaked at-3.4‰.Together with paleontological and chronological constraints,the interface between silica-phosphorus shale of the bottom Yuertusi Formation and algal-dolomite of the top Qigebulake Formation is confirmed as the boundary of the Sinian to Cambrian of the northern Tarim Basin.Also completed is a structural correlation framework comparing the northern Tarim with Tabei,Laolin,Xiaotan,Three Gorges,Siberia,Oman,and Morocco plates.It reveals the carbon isotope excursions during the Sinian to Cambrian transition in northern Tarim are more controlled by the fluctuation of carbon fixation rate(forg)due to paleo-marine redox changes.The decreased productivity and carbon fixation,caused by large-scale anoxic events in the transgression of the Yuertusi Formation,is the main reason for the N1 excursion;then the recovered productivity and carbon fixation from oxygen increase during regression forms the P1 excursion.The above results are beneficial for the further study of the Cambrian paleoenvironments and evaluation of the deep to ultra-deep resources in the Tarim Basin.