Co-evolution of atmosphere-hydrosphere redox states and life at the end of the Lomagundi-Jatuli Event: A failed natural experiment for a prelude of the Cambrian Explosion of life?

西非加蓬共和国的古元古代Francevillian沉积序列被描述为记录了约2.46-2.32Ga前的大氧化事件(GOE)的最显著的余辉。这些事件包括(1)Lomagody-Jatuli事件(LE)─，这是地球历史上最正的碳同位素漂移，以及(2)与氧气充足的表层海洋有关的假定的大型真核生物群体的证据。然而，一些基于地球化学的研究表明，这个约2.1Ga的老Francevillian群的上部，覆盖了下部化石，沉积在缺氧水柱下，也与LE的末端重合。这种上法兰西维利亚海洋的缺氧现象与一种新兴的范式特别一致，该范式认为在GOE开始约200-300 Ma后，全球脱氧事件进入我们的星球。此外，最近的研究调查了深海富含有机页岩的Se同位素系统，并推断表层海洋只有在GOE开始时才处于亚氧状态，因此氧气水平不够高，不足以发展真核生物。在这里，我们打算研究(I)有机碳和碳酸盐碳的C和O同位素组成，以进一步了解全球C-org和C-Carb之间的平均分馏可能有助于提高我们对GOE和LE后期大气-海洋系统释放的氧量的理解，(Ii)法国沉积物的主要和微量元素变化以及Fe-Mo同位素组成，以更好地了解水柱化学和相关的氧化还原状态，特别强调是什么控制了GOE和LE后期海洋中过渡金属的循环，(3)详细的稳定Se同位素系统学，以进一步测试浅海是否有足够的氧气，使复杂生命得以发展和扩散。然而，这项研究的最终目标是测试海洋环境条件在GOE和LE开始期间创造更高生命形式的尝试，以及为什么它可能不起作用，是什么结束了LE。

The Paleoproterozoic Francevillian sedimentary successions of the Republic of Gabon, West Africa were described to record the most prominent aftermaths of the Great Oxidation Event (GOE) constrained at ca. 2.46-2.32 Ga ago. These include (1) the Lomagundi-Jatuli Event (LE) ─ the most positive carbon isotope excursion of the Earth’s history, and (2) the evidence of putative colonies of large eukaryotes associated with a well-oxygenated surface ocean. However, a number of geochemical-based studies shown that the upper part of this ca. 2.1 Ga old Francevillian Group, overlaying the lower fossil-bearing part, was deposited under anoxic water column which also coincided with the end of the LE. Such an upper Francevillian ocean anoxia is particularly consistent with an emerging paradigm arguing for a global deoxygenation event into our planet some 200-300 Ma after the beginning of the GOE. Moreover, recent studies investigated Se isotope systematics for deep marine organic-rich shales and inferred that surface oceans were only suboxic during the onset of the GOE and thus O2 levels were not high enough to allow the development of eukaryotic life. Here we intend to investigate (i) the C and O isotopic compositions of organic- and carbonate carbon for bringing further insights on the global average fractionation between C-org and C-carb likely contributing to improve our understanding on the amount of oxygen released in the atmosphere-ocean system during the late stages of the GOE and the LE, (ii) the major and trace elements variations and Fe-Mo isotopic compositions of Francevillian sediments for a better understanding of the water column chemistry and associated redox states with particular emphasis on what controlled the cycling of transitional metals in the oceans during the late stages of the GOE and the LE, (iii) detailed stable Se isotope systematics to further test whether shallow oceans were oxygenated enough to allow the development and proliferation of complex life. The ultimate goal of this study however is to test the attempt of marine environmental conditions at creating higher life forms during the onset of the GOE and LE as well as why it probably did not work and what ended the LE.