三峡地区早寒武世海水氧化还原环境的变化——来自罗家村剖面碳同位素解耦的证据

Evolution of environmental oxidation and reduction of sea water in Three Gorges area in Early Cambrian: evidence from decoupled carbon isotopes in Luojiacun section

汪瑾 吝祎勃 杨涛

WANG Jin LIN Yibo YANG Tao

南京大学 地球科学与工程学院 内生金属矿床成矿机制研究国家重点实验室, 南京 210023

State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China

新元古代晚期到寒武纪早期是地球历史上生命演化和环境变化的重要转折期，此期间深海被认为广泛缺氧，而表层海水存在重大氧化还原环境的波动。选取宜昌三峡地区罗家村剖面下寒武统岩家河组、水井沱组为研究对象，基于生物地层学和无机碳同位素地层学对比，岩家河组、水井沱组分别对应纽芬兰统幸运阶至第二阶和第二统第三阶。根据此时间框架，系统研究了无机碳同位素和有机碳同位素解耦对水体氧化还原环境转变的响应。研究显示，海侵过程中底部缺氧水体的上升为初级生产力提供了充足的营养元素和生物必需微量元素，净生产力的提高导致了水体氧化还原环境的变化。光合作用产生的氧气使得表层海水处于暂时充氧的环境，这部分溶解氧随着溶解有机碳(DOC)储库的氧化而逐步消耗，海洋缺氧面积逐渐增加，有利于有机质的保存。底水上涌将深部缺氧水体中的DOC输入到表层水体，此氧化环境下DOC的持续氧化，导致形成的自生碳酸盐岩贫13C，而早寒武世海水中容量可观的DOC库缓冲了δ13Corg的变化，最终表现为早寒武世第二期—第三期过渡时期的碳同位素解耦现象。

The Late Neoproterozoic to Early Cambrian is a remarkable time-interval that witnessed significant biolo-gical and environmental evolutions, during which, the deep ocean was extensively anoxic, whereas the surface ocean was characterized by considerable fluctuations in redox conditions. The Lower Cambrian Yanjiahe and Shuijingtuo formations in the Luojiacun section from the Three Gorges area situated in Yichang city of central China were studied in this paper. According to biostratigraphic and carbonate carbon isotope stratigraphic correlation, the Yanjiahe Formation and Shuijingtuo Formation were assigned to the Cambrian Terreneuvian Stage 1-2 and Series 2 Stage 3, respectively. In view of this time-frame, the response of inorganic and organic carbon isotopes decoupling to water redox environment transition was systematically studied. The rise of anoxic bottom water during transgression provided sufficient nutrients and essential trace elements for primary productivity in the surface ocean. The increase of net productivity led to oxygen consumption increase and the changes of redox environment in the water column. Photosynthetic oxygen temporarily oxygenated the surface seawater, and dissolved oxygen was gradually consumed by the oxidation of DOC reservoirs, resulting in gradual expansion of oceanic anoxia, which was conducive for the preservation of organic matters. The oxidation of DOC in the surface water accounted for the formation of authigenic carbonates that were 13C-depleted, while the large oceanic DOC reservoir in the Early Cambrian buffered the change in δ13Corg, which eventually manifested as decoupled carbon isotope during the transition time of Cambrian Stage 2-3.

国家自然科学基金重点项目 41830425;中国科学院战略性先导科技专项B类子课题 XDB26000000

https://doi.org/10.11781/sysydz202301157