古元古代"大氧化事件"的生物地球化学后效：华北克拉通下元古界研究及与环苏必利尔盆地对比

The Paleoproterozoic was a period of profound changes in marine environments and biogeochemical cycles due to the Great Oxidation Event (GOE), which was also a time of worldwide expansion of stromatolite reefs and the first deposition of major phosphorites in the rock record. Positive carbon isotope anomalies in carbonates up to 10 ‰ characterize the 2.22-2.06 Ga Lomagundi Event and are not easily reconciled by general models of organic carbon burial, because accumulations of organic matter are found in the aftermath of the GOE. In fact, the following Shunga event featured by >14 ‰ negative excursions and has been related to massive oxidation of organic carbon in increasingly oxygenated environments, but it is still unclear whether it was of global importance. Previous studies have proposed that blooms of primary producing microorganisms might promoted the worldwide formation of stromatolites and phosphorites at that time. Massive burial of biomass produced by these blooms and its subsequent oxidation likely played important roles in the observed perturbations of the carbon cycles. To test this hypothesis, we propose to investigate the sedimentology, geochemistry, C-N-S isotopes and microfossil records of the Paleoproterozoic Hutuo Group in the North China Craton. This work will expand the database of the biogeochemical and fossil record of Paleoproterozoic of the North China Craton. In comparison with the data from contemporaneous successions in the circum-Superior basins, our studies will provide new insights into the processes of environmental changes, sedimentary formation and microbial activities and their genetic links.

古元古代"大氧化事件"后海洋环境和生物地球化学循环发生了强烈扰动，伴随着叠层石和磷块岩的广泛发育。发生于2.22～2.06 Ga间的Lomagundi事件记录了 >10‰的碳酸盐碳同位素正异常，难以用经典的有机碳埋藏模式来解释；随后的Shunga事件显示了高达14‰的碳同位素负偏移，目前认为可能与大规模有机质氧化有关，但其是否为全球性事件尚不清楚。我们的前期研究表明，古元古代可能发生了海洋原始微生物的爆发式增长，形成大规模叠层石建造并促进磷的快速聚集，而大量有机质的埋藏或氧化导致了地球化学循环的强烈扰动。为验证这一模式，我们选择华北克拉通古元古代滹沱群为主要对象，拟通过沉积学、地球化学、C-N-S同位素以及化石记录研究，获取"大氧化事件"后地球化学循环过程和微生物活动的信息。结合与北美环苏必利尔盆地同时代沉积对比研究，探讨古元古代转折期环境变化、沉积建造和微生物过程之间的联系。

对华北克拉通典型古元古代剖面的沉积特征、矿物组成、地球化学、C同位素地层学和碳酸盐晶格S同位素进行了系统研究。发现五台山地区滹沱群碳酸盐岩C同位素显示三阶段特征，下部碳酸盐岩具有一致的轻度C同位素负异常，随后呈现正负波动，最后进入持续的负漂移阶段，显示滹沱群中记录了古元古代Lomagundi碳同位素异常事件的尾声及其向中元古代稳定海洋系统的过渡，与全球性的C同位素异常事件具有较好的对应性。滹沱群白云岩晶格硫（CAS）整体浓度较低，δ34SCAS变化于-1.2‰至+33.1‰间，其较大的波动幅度指示当时古海水具有较低的硫酸盐浓度，通过δ34SCAS变化速率估算该组沉积时期（～2.0Ga）海水硫酸盐浓度处于较低水平（≦1.6mM），相比较Lomagundi事件期间（>2.5mM）出现明显下降，可能是大陆风化硫酸盐输入减少和细菌硫酸盐还原作用（BSR）受到抑制的结果。以上结果暗示大氧化事件之后大气氧含量发生了大幅度下降，导致大陆风化速率和海水硫酸盐输入通量显著降低，这些数据为进一步精细刻画古元古代关键转折时期大气—海洋氧化还原状态变化提供了重要资料。对华南宜昌等地陡山沱组颗粒磷块岩的研究发现，蓝细菌及其分泌的胞外聚合物（EPS）可能对磷酸钙过饱和、磷灰石成核和结晶生长过程起到了促进作用，提出新元古代冰期后的蓝细菌爆发可能导致了大规模成磷事件，并据此建立了一个新的元古宙磷循环模式。研究了古元古代和新元古代转折期硅质岩和磷质岩的成岩作用，发现成岩过程中的微生物活动和有机质氧化等过程对岩石结构构造和化石的保存具有重要作用。