华北元古代中期（17—13亿年）大气氧气和海洋氧化还原演变研究

The earth environment and biosphere experienced boring billion years in the mid-Proterozoic. The time period from 1.7 Ga to 1.3 Ga is the most boring part due to the stasis of eukaryote evolution as well as the atmosphere and ocean chemical conditions. However, a number of fossil records recently discovered globally including from North China indicate that the eukaryotes have experienced an early evolutionary diversification during this time. We hypothesize that the simplest default reason for this mid-Proterozoic renovation is the oxygenation in earth surface system. However, the knowledge about the oxygen level in the atmosphere and the redox conditions in the ocean during the mid-Proterozoic is still very limited and thus will be our scientific research issue. In order to achieve that, we are going to focus on the well-preserved and low-grade metamorphosed strata in North China spanning a period from 1.7 Ga to 1.3 Ga. On the basis of carbon and strontium isotope stratigraphy, we will emphasize analyzing the iron speciations of black rocks and the chromium isotope of carbonates, as well as the redox-sensitive trace metals (such as molybdenum, uranium and vanadium) and the TOC of the black shales and the cerium anomaly, iodine/calcium ratios of carbonates throughout the sediments from Chuanlinggou Formation to Xiamaling Formation. By synthesis of different proxies, we are trying to reveal the atmosphere oxygen level and the ocean redox conditions and their evolution through time, and discuss the relationship between the atmosphere and ocean redox conditions and the eukaryote evolution.

以前认为地球环境和生物在元古代中期经历了沉寂枯燥的十亿年，其中距今17—13亿年又是最“枯燥”时段，真核生物演化停滞，大气和海洋化学无所变化。但近期包括华北在内的全球一些化石证据显示，该时段出现了真核生物早期分异演化。本项目的工作假设为：促使该演化的环境因素可能是地球表层系统中氧气的增长。但目前学界对该时段大气含氧程度和海水氧化还原状态认识还非常有限，本项目拟研究这一重要科学问题。为此，本项目拟以华北出露良好、变质程度低的元古界17—13亿年的地层为研究对象，在C、Sr同位素地层学基础上，重点分析从串岭沟组到下马岭组整套地层中黑色岩系的Fe组分，碳酸盐岩的Cr同位素组成，黑色页岩的氧化还原敏感元素Mo、U、V等和有机碳含量及碳酸盐岩的氧化还原指标Ce异常、I/Ca。通过综合分析，揭示该时间段大气含氧量变化、海洋氧化还原状态演变，探讨大气和海洋氧化还原环境与真核生物演化之间的关系。

地球环境和生物在元古代中期经历了沉寂枯燥的十亿年，其中距今17—13亿年又是最“枯燥”的时段，真核生物演化停滞，大气和海洋化学无所变化。但近期包括华北、澳大利亚、美国、东欧、印度在内的全球一些化石证据显示，该时段出现了真核生物早期分异演化。促使该演化的环境因素包括营养盐、温度等，但更可能是地球表层系统中氧气的增长。对该时段大气含氧程度和海水氧化还原状态认识还需要完善，这也是本项目要着重研究的内容。本项目以华北出露良好、变质程度低的元古界17—13亿年的地层为研究对象，在开展C、Sr同位素地层学基础上，结合大型真核生物化石产出层位，完善年代地层框架，再重点分析铁岭组碳酸盐岩的Cr同位素和Ce异常、高于庄组碳酸盐岩的U同位素、下马岭组黑色岩系的Fe组分和氧化还原敏感元素Mo、U、V等和有机碳含量。这些氧化还原地球化学指标有针对性地指示全球或局域海，定量或定性地指示大气、海水不同深度的氧化还原程度。数据结果表明15.6亿年前的地层记录显示海水已经出现氧气含量升高迹象，与被认为是真核生物的大型藻类化石相对应，大约14.4亿年前大气和海洋的氧气含量出现了短暂的提升，大气含氧量可能短暂高于现代水平的1%，其后海水氧化程度回落，真核生物的演化也出现停滞。其后新元古代两次大冰期之内及之后大气含氧量可能出现大于现代值1%，直到寒武纪早期深海氧气动荡之后才有了更加适合后生动物演化的环境基础。