南华盆地南沱冰期海水氧化还原特征

新元古代冰期冰盖可能覆盖整个地球表面，冰期海洋环境条件以及真核生物如何度过“雪球地球”的极端环境条件，是学术界普遍关心的科学问题。同冰期化学沉积岩的缺乏，严重制约了对冰期海洋环境的了解。贵州东部南沱组中发育的同冰期白云岩，为探讨这一时期海洋氧化还原环境提供了理想的材料。利用电感耦合等离子体质谱（ICP-MS）对采集于贵州松桃一钻孔岩心南沱组中同冰期白云岩进行了稀土和微量元素分析，结果表明:研究区南沱组白云岩稀土元素含量在（10.33~67.91）×10-6之间；Ce/Ce*值范围为0.72~1.12，平均值为0.97；Fe含量（1 470~37 200）×10-6，平均值为9 902×10-6；Mn含量（4 040~17 500）×10-6，平均值为10 476×10-6。白云岩中高的铁、锰含量以及Ce无异常，反映南华盆地南沱冰期海洋基本处于富铁缺氧的状态。与贵州其他两个钻孔岩心中南沱组白云岩的沉积特征和Ce异常特征的对比分析表明，南华盆地南沱冰期近岸海水氧化程度较高，可能反映了冰川融水是同冰期氧气进入海洋的重要渠道，有氧的滨海区域可能为冰期真核生物提供了栖息场所。

Redox Characteristics of Marine Environment of Nantuo Glaciation,Nanhua Basin

During the Neoproterozoic “Snowball Earth” glaciations， ice sheets probably covered the entire surface of the Earth. Knowledge of marine environmental conditions during the glaciation and the survival of eukaryotic life forms in such conditions has been severely limited， is a scientific question of general interest to the academic community. However， the apparent lack of primary marine precipitates limited the understanding of the syn-glacial ocean chemistry during the Snowball Earth. Synglacial dolomites in the Nantuo Formation in eastern Guizhou province are ideal materials for exploring the ocean redox environment during this period. A bed of synglacial dolomite was collected from a drill core in the Nantuo Formation， Songtao， Guizhou province， and samples were analyzed by inductively coupled plasma mass spectrometry （ICP-MS） for rare earth and other trace elements. It was found that the rare earth content ranged from 10.33×10–6 to 67.91×10–6. After post-Archean Australian shale （PAAS） normalization， the content of Ce anomalies ranged between 0.68 and 0.91 （average 0.82）； iron concentrations 1 470×10-6?37 200×10-6 （average 9 902×10-6）； and manganese concentrations 4 040×10–6?17 500×10–6 （average 10 476×10–6）. The high Fe and Mn contents together with very low Ce anomalies in the dolomite indicate that iron-rich， anoxic （ferruginous） conditions were predominant in the Nanhua Basin during the Nantuo glaciation. Comparison with the sedimentary characteristics and Ce anomalies in two other drill cores from Guizhou province suggest that near-shore seawater was stratified during this period， with oxic conditions at the ocean surface overlying ferruginous water at depth. This implies that subglacial melt water supplied oxygen for the survival of eukaryotes during extreme Marinoan glaciation.