鄂西新元古代陡山沱组六水碳钙石假晶矿物的成因及其古气候意义研究

Neoproterozoic climate changes are characterized by episodic glaciations. However, due to the absence of directly glaciogenic sedimentary records (e.g. diamictite) and controversies in interpreting different geochemical proxies for paleoclimatic reconstruction. It remains uncertain to illustrate stratigraphic record in the Doushantuo Formation (South China) potentially correlated with any Ediacaran cooling event like the Gaskiers glaciation. We unprecedentedly discovered and reported the occurrence of glendonite in the Doushantuo Formation, western Hubei. It is widely accepted that ikaite, as precursor mineral of glendonite, originally precipitates under a condition with near-freezing temperature. With further constrains of stratigraphic correlations and isotopic geochronological data, the occurrence of glendonite is interpreted to mineralogically indicate climate cooling possibly related to the Ediacaran Gaskiers glaciation. In this proposal, we will carry out comprehensive investigations into glendonite records in the Doushantuo Formation at multiple sections, western Hubei, in comparison with other reference sections across South China. Relevant research is aimed at uncovering occurrence characteristics and spatial-temporal distribution of glendonite. In addition, in terms of the origin and change for ikaite in modern marine sediments, we try to explain primary formation of precursor ikaite and its diagenetic transformation to pseudomorph for current glendonite records in the Doushantuo Formation. Finally, potential paleoclimatic implications of Doushantuo glendonite are discussed in this study.

新元古代气候变化以多幕次冰川作用为特征。其中，由于直接冰成沉积记录（如冰碛砾岩）的缺乏和地化指标古气候重建的多解性，华南陡山沱组地层对晚新元古代埃迪卡拉纪Gaskiers冰期等气候变冷事件的记录尚不明确。申请人首次报道了鄂西陡山沱组六水碳钙石假晶的存在，基于六水碳钙石原生形成条件具有接近冰点温度的环境特征，通过综合地层对比和同位素年代学约束，初步推测六水碳钙石假晶可作为Gaskiers冰期相关的华南气候变冷的直观矿物学证据。本项目拟针对鄂西多个含六水碳钙石假晶剖面，采用地层学、沉积学、矿物学和地球化学等多学科综合的研究方法，并与华南其他经典剖面进行区域对比；从而总结鄂西陡山沱组六水碳钙石假晶的产出特征和时空分布规律；此外，调研和对比现代海洋环境六水碳钙石的形成和变化，根据多种地化指标来综合评价陡山沱组六水碳钙石原始形成环境条件，重建其假晶化转变过程，探讨陡山沱组六水碳钙石假晶的古气候意义。

埃迪卡拉纪是后生动物起源和演化的关键地质历史时期，其古气候变化的探讨通常依赖地球化学指标，鄂西陡山沱组六水碳钙石假晶存在的首次报道为利用自生矿物地质记录指示该时期古气候变冷事件提供了新的研究视角和思路。本项目在前期研究基础上，利用鄂西地区露头和岩芯样品，通过陡山沱组地层学和沉积学特征的野外调查以及六水碳钙石假晶新材料矿物学和地球化学组成的室内分析，围绕陡山沱组六水碳钙石假晶时空分布规律、原生矿物形成环境因素、假晶转变过程和自生矿物古气候变化指示意义等科学问题开展了系统和深入研究。结果表明1）六水碳钙石假晶在鄂西地区的空间分布具有广泛性，但并不是无处不在；假晶在陡山沱组地层中的分布层位具有一致性，可对应地层无机碳同位素变化曲线EP1区间上部，晚于WANCE负偏事件，显著早于Shuram负偏事件。2）六水碳钙石假晶原生矿物的形成受到该时期古海洋沉积物高磷酸盐浓度、缺氧硫化状态和底层水低温条件等环境因素的共同影响；假晶方解石相碳-锶同位素组成记录了早期成岩阶段甲烷-有机质厌氧氧化作用和海底硅酸盐风化作用相关信息，方解石相团簇同位素古温度重建结果表明埋藏成岩阶段地层埋藏热效应对假晶方解石化部分固相重排过程可能产生了显著影响；此外，假晶硅化作用发生于方解石化作用之后。3）含六水碳钙石假晶剖面和不含假晶剖面的陡山沱组化学蚀变指数（CIA）变化曲线与自生矿物记录一致指示华南埃迪卡拉纪早期存在一次气候变冷事件，证明陡山沱组六水碳钙石假晶相关研究具有重要的古气候意义。下一步工作重点可以围绕陡山沱组蓝铁矿-黄铁矿-碳酸盐（如六水碳钙石假晶）自生矿物组合如何揭示埃迪卡拉纪古海洋富甲烷和有机质沉积物早期成岩过程碳-硫-磷生物地球化学循环展开。