埃迪卡拉纪-寒武纪过渡期南华盆地碳硫循环研究

The Ediacaran-Cambrian transition is a most remarkable period for early life evolution on the Earth. Life mainly lived in the ocean during the Ediacaran-Cambrian transition, and thus ocean chemistry was related directly to life emergence, evolution and extinction. Although the redox state of the Nanhua Basin during the Ediacaran-Cambrian transition have been preliminary studied, and consider that the ocean surface was preliminary oxic, slope water was dynamically euxinic, and the deep basin water was anoxic and ferruginous, the water column redox conditions of platform and slope have to be certified further. In addition, the other side of ocean chemistry during the Ediacaran-Cambrian transition, carban and sulfur cycles that was closely related to life in the ocean, were remain unclear. In this project, we will focus on carbonate rocks, black chert and shale deposited in the Nanhua Basin during the Ediacaran-Cambrian transition, and use organic carbon isotope, pyrite sulfur isotope, iron speciation, total organic carbon and sulfur content to clarify platform and slope water column redox conditions, to reconstruct the spatial distribution and the temporal evolution characteristics of carbon and sulfur cycles, and to discuss the relationship of water column redox conditions, carbon and sulfur cycles, and early life in the basin. Our work will deepen understanding on ocean chemistry of the Nanhua Basin during the Ediacaran-Cambrian transition, and create favourable conditions to uncover the relationship between ocean chemistry transition before the Cambrian explosion and early life evolution.

埃迪卡拉纪-寒武纪过渡期是地球早期生命演化的关键时期。那一时期生命赖以生存的环境主要是海洋，海洋化学必然对生命的出现、演化和灭绝产生重要影响。虽然已基本弄清了埃迪卡拉纪-寒武纪过渡期南华盆地的氧化还原环境，认为表层海水初步氧化、斜坡水体动态硫化、深水盆地缺氧富铁，但台地和斜坡水体氧化还原环境须进一步确认。另外，该时期南华盆地海洋化学的另一方面——与早期生命密切相关的古海洋碳硫循环还不是很清楚。本项目拟以埃迪卡拉纪-寒武纪过渡期沉积的碳酸盐岩、硅岩和页岩为研究对象，利用有机碳同位素、黄铁矿硫同位素、铁组分、有机碳和硫含量等指标，确定南华盆地台地和斜坡水体的氧化还原环境，恢复南华盆地碳硫循环过程在空间上的分布和时间上的演化特征，探讨氧化还原环境、碳硫循环和生物之间的关系，加深对埃迪卡拉纪-寒武纪过渡期南华盆地海洋化学的认识，为揭示“寒武纪生命大爆发”前海洋化学转变与生命演化的关系提供依据。

古海洋的化学条件与埃迪卡拉纪-寒武纪（E-C）过渡期地球生命演化关系密切。本项目以华南广泛出露且地质资料丰富的埃迪卡拉纪末期-早寒武世早期（ca. 551-529 Ma）地层为研究对象，系统分析其Fe-C-S-RSTE（氧化还原敏感微量元素）化学等指标并结合前人研究成果来确定南华盆地埃迪卡拉纪末期-早寒武世早期氧化还原环境的时空演变特征和碳硫循环过程，对海洋化学与早期生命演化的关系进行探讨，取得如下重要认识：（1）黄铁矿硫同位素组成大的正漂移（盆地相可达35‰）是E-C界线的良好指示，尤其是深水盆地；（2）埃迪卡拉纪末期-早寒武世早期表层水体氧化，深部水体缺氧富铁，早寒武世早期（ca. 542-529 Ma）在中等深度水体偶尔有短暂硫化环境出现，大体具有似埃迪卡拉纪陡山沱期“硫化楔”型古海洋化学结构；（3）E-C界线处和牛蹄塘组底部记录了两次普遍硫化环境，来自陆源和开阔海的物质供给是其重要控制因素；（4）溶解硫酸盐浓度在盆地内不均一，但总体具有在埃迪卡拉纪末期逐渐增加，然后在E-C界线附近降低并持续低浓度的特征；（5）由于盆底隆起的存在，南华盆地是部分局限的，此背景下的海洋氧化还原分层制约着南华盆地碳硫循环过程；（6）埃迪卡拉纪末期和早寒武世早期硫化缺乏环境分别对应于埃迪卡拉生物群繁盛和小壳动物群及海绵动物繁盛，而E-C界线和牛蹄塘组底部记录的硫化水体普遍发育的时期分别对应于埃迪卡拉生物群灭绝和小壳动物群及海绵动物灭绝，表明古海洋中硫化水体的发展与否对早期动物阶段性灭绝和辐射起到重要控制作用；另外，一些小壳动物和海绵具有较强的耐缺氧能力，能生活在缺氧富铁的环境。我们的研究进一步加深了对E-C过渡期南华盆地海洋化学及其与生命演化之间关系的认识。