华南地区南沱冰期海洋氧化还原环境研究

新元古代晚期地球经历了两次“雪球地球”事件，对冰期后埃迪卡拉纪的海洋氧化和多细胞生物的多样化产生了重大影响。然而由于冰期化学沉积岩的缺乏，迄今为止对南沱冰期海洋氧化还原条件了解不多。碳酸盐岩稀土元素地球化学特征能有效地反映其形成时的环境条件，已广泛用于恢复地质历史时期古环境条件。在贵州省松桃地区发现了一套南沱组自生沉积的碳酸盐岩，为探讨华南地区南沱冰期海洋古环境提供了理想的地质材料。该白云岩产于南沱组下部，厚约1.6 m，夹于两套冰碛砾岩中间。采用微钻技术钻取白云岩粉末，稀醋酸溶样，利用等离子体质谱仪（ICP?MS）进行稀土和其他微量元素分析。分析结果显示稀土元素含量较高，ΣREE为（23.0~46.6）×10-6，PAAS标准化后稀土元素显示Ce的弱负异常或无异常（平均值0.90），可变的Eu和La异常，中稀土富集，Y/Ho比值平均为34.6。白云岩稀土配分特征与现代氧化的海水明显不同，结合样品中高的Fe、Mn含量特征，反映了南沱冰期华南地区表层海水整体处于缺氧富铁的环境，可能制约了冰期后的海洋氧化和多细胞真核生物的演化。

Ocean Redox Environment in the Nantuo Ice Age of South China: An indication of the rare earth element geochemistry in the dolomites from the Nantuo Formation in Guizhou province

In the late Neoproterozoic， the Earth experienced two "Snowball Earth" events， which had a significant impact on the marine oxidation and the diversification of multicellular organisms in the subsequent Ediacaran period. However， due to the lack of chemical sedimentary rocks during the glacial period， little is known about the marine redox conditions of the Nantuo Ice Age. The authors discovered a bed of carbonate deposits from the Nantuo Formation in the Songtao area of Guizhou Province， which provides an ideal window for exploring the marine paleoenvironment of the Nantuo Ice Age in South China. The geochemical characteristics of rare earth elements （REEs） in carbonate rocks can effectively reflect the environmental conditions at the time of their formation and have been widely used to reconstruct the marine redox conditions in geological history. The dolomite occurs in the lower part of the Nantuo Formation， about 1.6 m thick， and is sandwiched between two sets of diamictites. Dolomite powder was drilled by micro?drilling and dissolved in dilute acetic acid. REEs and other trace elements were analyzed using a ionization coupled plasma mass spectrometer （ICP?MS）. The analytical results show that the contents of REEs are high ranged from 23.0×10-6 to 46.6×10-6. After PAAS（Post?Archaean Australian Shale） normalization， REEs show negative or no anomalies of Ce （average 0.90）， variable Eu and La anomalies， MREE（Middle REEs） enrichment and Y /Ho ratio averages 34.6. The characteristics of REEs in dolomite are significantly different from those of modern oxic seawater. Combined with the high Fe and Mn contents in the sample， it reflects the fact that the surface seawater in South China was an iron?rich and anoxic environment in the Nantuo Ice Age. It may restrict the ocean oxidation and the evolution of multicellular eukaryotes after the glacial period.