Coupled Cr and Mo isotope investigations of post-GOE Paleoproterozoic sediments: Testing the hypothesis of an oxygen overshoot in atmospheric O2 levels

通过对俄罗斯佩琴加绿岩带(Pechenga Greenstone Belt，PGB)和奥涅加盆地(Oenera盆地，OB)2.5-2.0Gyr年代沉积物中氧化还原敏感元素铬和钼的联合同位素研究，将重建大氧化事件(GOE)以来大气和海洋的氧化还原演化。GOE标志着大气氧气水平的第一次显著增加，并定义了从缺氧环境条件到氧气环境条件的转变。GOE的后果的特点是在2.22-2.05-Gyr-2.05-Gyr古老的Lomagody事件和随后的Shunga事件期间，氧、碳和硫循环-可能是全球的-发生重大扰动。虽然GOE后的沉积物记录了大量有机碳的埋藏、海水硫酸盐水库的扩张和收缩、提出的氧超调和有效的大陆风化的开始，但大气和海洋的氧合作用之间的时间和反应受到的限制很小。此外，更令人费解的是，尽管GOE标志着富氧大气条件的开始，随后可能会出现氧超调，但被认为是游离氧示踪剂的铬同位素未能确定古元古代的富氧条件。该项目旨在对Kuetsjärvi、Kolosjoki(PGB)和Zaonezhskaya(OB)地层的各种近岸沉积环境进行详细的铬和钼同位素耦合调查。该项目的第一部分将通过常量元素、微量元素和碳同位素来表征古环境条件，而第二部分将重点放在铬和钼同位素的联合调查上。将对(博茨瓦纳)2.2-2.1岁的比勒陀利亚群的近岸沉积物进行补充地球化学调查，因为这些沉积物也捕捉到了洛马芒迪事件，并将有助于提供对古元古代大气-海洋系统的更详细的、可能是全球的了解。

Coupled isotope investigations of the redox-sensitive elements Cr and Mo in 2.5 – 2.0-Gyr-old sediments of the Pechenga Greenstone Belt (PGB) and the Onega Basin (OB) in Russia will be used to reconstruct the redox evolution of the atmosphere and oceans postdating the Great Oxidation Event (GOE). The GOE marks the first significant increase in atmospheric oxygen levels and defines the transitions from anoxic to oxic environmental conditions. The aftermath of the GOE is characterized by major perturbations of the oxygen, carbon, and sulfur cycle - probably global - during the 2.22 – 2.05-Gyr-old Lomagundi and the following Shunga events. While the post-GOE sediments archive evidence for the burial of vast amounts of organic carbon, the expansion and contraction of the seawater sulfate reservoir, a proposed oxygen overshoot and the onset of effective continental weathering, the timing and response between the oxygenation of the atmosphere and oceans is poorly constrained. Moreover and more puzzling, even though the GOE marks the onset of oxygen-rich atmospheric conditions that was probably followed by an oxygen overshoot, Cr isotopes that are suggested as a tracer for free oxygen fail to identify oxygen-rich conditions in the Paleoproterozoic. This project seeks to perform detailed coupled Cr and Mo isotope investigations on various near-shore sedimentary environments from the Kuetsjärvi, Kolosjoki (PGB) and Zaonezhskaya (OB) formations. The first part of the project will characterize the paleoenvironmental conditions by major and trace elements and carbon isotopes, whereas the second part focuses on the combined Cr and Mo isotope investigations. Complementary geochemical investigations will be carried out on near-shore sediments from the 2.2 – 2.1-Gyr-old Pretoria Group (Botswana), because these sediments also capture the Lomagundi Event and will help to provide a more detailed, and probably global, understanding of the Paleoproterozoic atmosphere-ocean system.