华南新元古代-早寒武世海洋生命与环境共演化的分子及碳氢同位素地球化学记录

In recennt years, many advances have been achieved in understanding the fundamental revolutions of the marine environmental and biotic structures during the period from the middle Neoproterozoic to early Cambrian, accompanying with many studies focused on the chemistry and biogeochemistry of the oceans. However, the results from molecular organic geochemical study are very limited. Following our previous study, this project aims to understand the processes and genesis mechanisms of the coevolution of life and environment in the Neoproterozoic - early Cambrian south China ocean, by using molecular and C- and H-isotopic geochemical methods, and based on the spatially and temporally expanded sedimentary sections. The drill-core samples of the Doushantuo Formation from the east Chongqing region will be studied with comparison with the Jiulongwan section in Yichang region, with respect of spatial variation of sedimentary facies. We will supply the organic geochemical study on the siliceous Liuchapo Formation which belongs to the Neoproterozoic-Cambrian transition in the dee-water facies. We will carry out a systematic study on the early Cambrian Aijiahe section and Xiaotan section. This study will focus on understanding the algal primary production and the secondary production and the water column stratification of the oceans during interglacial and deglacial periods, by revealing the molecular and compound-specific C- and H-isotopic signals from the Datangpo Fm shales and the Nantuo Fm diamictites. The characteristis and genesis of the C-isotopic coupling and decoupling in key carbon cycle fluctuations will be studied by adding more carbonate-kerogen C-isotope data set, and by combining the data of biomarker and compound-specific C-isotope analysis, we will discuss the geochemical mechanisms of the carbon cycling pertubation, which may be related to primary production, chemical and microbial geochemical stratification of the oceans as well as the possibility of the oxidation of the DOC in the oceans. In addition, will pay more attention to the detection of the 24-isoprenylsteranes diagnostic of sponge animals across the whole section to reassess the molecular evidence for the early animal evolution.

近几年来，人们对新元古代-早寒武世地球生命与环境的剧烈变革过程有了深入认识，特别是对这一时期海洋化学及生物地球化学的多样性有了更多了解。然而，来自分子有机地球化学的成果还极为有限。本申请拟在前期探索性研究基础上，扩展和深化对华南新元古代-早寒武世经典地层剖面的有机地球化学研究：对比渝东地区钻井岩芯与九龙湾剖面陡山沱组因相变引起的分子地球化学差异；着力分析大塘坡间冰期页岩和南沱冰期冰积岩的分子及其碳氢同位素信息，研究间冰期及溶冰期海洋中细菌和藻类发育及水体盐度分层特征；补充对留茶坡组硅质岩段、肖滩剖面和艾家河剖面早寒武世地层的研究；系统分析关键层段碳酸盐岩-干酪根碳同位素耦合与非耦合特征，结合分子及其碳同位素组成研究，剖析碳循环剧烈波动与初级生产、海洋化学-微生物地球化学分层及DOC氧化等过程的关系；系统检测新元古代-早寒武世地层中24-异丙基胆甾烷的分布，为海绵动物起源演化提供依据。

新元古代-早寒武世是地球生命与环境剧烈演变时期，了解海洋化学及生物地球化学过程对动物演化的作用具有重要科学意义。本项目以分子有机地球化学为主，结合同位素及微量元素地球化学，研究并揭示了新元古代-早寒武世华南洋有关生物与环境演化的重要信息。.大塘坡间冰期页岩和南沱冰期冰积岩的碳酸盐及有机质碳同位素组成研究显示碳同位素存在非耦合变化，但海洋中DOC应弱于DIC库；碳氧同位素组成明显偏负的特征可能指示了冰川融水及海洋甲烷氧化的不同程度影响。.对三峡地区陡山沱组碳同位素负漂移事件做了重新评价，揭示了这一过程中无机-有机碳同位素为耦合变化关系，而非非耦合。进而揭示这一耦合性的负漂移起源于DOC库的氧化，从而确证这一碳同位素负漂移明确是全球性海洋-大气的氧化事件。分子地球化学证据也支持了这一变化及成因机制。.灯影组石板滩段暗色灰岩以有机质含量相对高、含有宏观藻化石为特征。碳酸盐岩δ13Ccarb平均在3‰，显示海洋初级生产率较高。分子地球化学揭示该环境为厌氧条件，进一步证明为一局限环境，由此造成海洋硫酸盐的缺乏，致使黄铁矿硫同位素特别偏正。.下寒武统岩家河组的δ13Corg和δ13Ccarb 演化都显示了阶段性，特别是主要呈现了两个δ13Ccarb 波动旋回。δ13Corg与δ13Ccarb变化呈现长期的非耦合特征，其中，在III段底部出现短期的δ13Corg异常偏负。与陡山陀组相比，岩家河期大部分层段的真核生物相对贡献明显偏高，这与浮游藻类繁盛有直接关系。黄铁矿硫同位素及分子有机地化参数等都显示岩家河期海洋环境变化的复杂性。.水井沱组黑色页岩良好地保存了生物标志物，特别是芳烃地化参数结合硫同位素及微量元素的研究取得良好结果。总体上，水井沱组与陡山陀组IV黑色页岩具有类似性，反映了该时期海洋硫酸盐水平的增加以及底水缺氧到部分硫化的环境。并进一步证明了陡山陀IV期海洋含氧水平有了重要提高。