三峡地区埃迪卡拉纪的浅海氧化还原环境变化:来自碳酸盐岩Ce异常的证据

EDIACARAN SHALLOW-MARINE REDOX CONDITIONS IN THE YANGTZE GORGES AREA:EVIDENCE FROM CARBONATE CERIUM ANOMALIES

本文系统地统计并评估了扬子板块三峡地区埃迪卡拉纪海相碳酸盐岩的Ce/Ce^(*)比值,并依据能够反映原始海水信息的Ce/Ce^(*)数据探讨了三峡地区埃迪卡拉纪浅海海水的氧化还原性质。数据表明,埃迪卡拉纪早期陡山沱组底部盖帽碳酸盐岩没有明显的Ce异常,结合Fe组分特征,盖帽碳酸盐岩沉积期间水体主要处于铁化环境,并有间歇性的硫化,晚期有向锰化带转变的趋势。埃迪卡拉纪中期,600—590 Ma海水可能具有弱氧化特征,590—580 Ma海水氧化还原条件波动较大,575—570 Ma(Shuram/DOUNCE开始阶段)Ce负异常显著,海水处于氧化环境。埃迪卡拉纪晚期,灯影组Ce负异常明显,表明该时期水体为氧化环境,可能促进了这一时期宏体多细胞动物的繁盛。海相碳酸盐岩的Ce/Ce^(*)比值是判别古海水氧化还原状态的重要指标,目前埃迪卡拉纪可靠的碳酸盐岩Ce/Ce^(*)数据还不够丰富,更多此类研究工作有待开展。

Here we present systematically statistical analysis and evaluation on the published Ce/Ce^(*)values of Ediacaran marine carbonates in the Yangtze Gorges area,Yangtze Platform.Based on the Ce/Ce^(*)values of carbonate which can reflect pristine seawater Ce signals,the shallow-marine redox conditions of the Ediacaran Period in the Yangtze Gorges area are discussed.No Ce anomaly is observed in the cap carbonate at the base of Doushantuo Formation.Combined with iron speciation data,we can infer that cap carbonate was deposited mainly under ferruginous conditions,with intermediate euxinic conditions and tend to manganous conditions in the uppermost part of the cap carbonates.In the middle of Ediacaran Period,redox states of shallow seawater were probably suboxic from 600 Ma to 590 Ma and fluctuated frequently from 590 Ma to 580 Ma.From 575 Ma to 570 Ma(the onset interval of Shuram/DOUNCE anomaly),significant negative Ce anomaly was observed,suggesting that seawater became more oxygenated.In the late Ediacaran,carbonates of Dengying Formation show obvious negative Ce anomalies,indicating that oxygenation of seawater took place during the Dengying deposition interval,which may have facilitated the occurrence of the Ediacaran macroscopic multicellular animals.In sum,the Ce/Ce^(*)values of marine carbonates can be used as an important redox tracer;further investigations are needed to cover the entire Ediacaran sedimentary successions in the area where reliable Ce/Ce^(*)values are absent.