Kipp Michael A, Stüeken Eva E, Bekker Andrey, Buick Roger
Department of Earth & Space Sciences and Astrobiology Program, University of Washington, Seattle, WA 98195-1310;
Virtual Planetary Laboratory, NASA Astrobiology Institute, Seattle, WA 98195-1310.
Proc Natl Acad Sci U S A. 2017 Jan 31;114(5):875-880. doi: 10.1073/pnas.1615867114. Epub 2017 Jan 17.
It has been proposed that an "oxygen overshoot" occurred during the early Paleoproterozoic Great Oxidation Event (GOE) in association with the extreme positive carbon isotopic excursion known as the Lomagundi Event. Moreover, it has also been suggested that environmental oxygen levels then crashed to very low levels during the subsequent extremely negative Shunga-Francevillian carbon isotopic anomaly. These redox fluctuations could have profoundly influenced the course of eukaryotic evolution, as eukaryotes have several metabolic processes that are obligately aerobic. Here we investigate the magnitude of these proposed oxygen perturbations using selenium (Se) geochemistry, which is sensitive to redox transitions across suboxic conditions. We find that δSe values in offshore shales show a positive excursion from 2.32 Ga until 2.1 Ga (mean +1.03 ± 0.67‰). Selenium abundances and Se/TOC (total organic carbon) ratios similarly show a peak during this interval. Together these data suggest that during the GOE there was pervasive suboxia in near-shore environments, allowing nonquantitative Se reduction to drive the residual Se oxyanions isotopically heavy. This implies O levels of >0.4 μM in these settings. Unlike in the late Neoproterozoic and Phanerozoic, when negative δSe values are observed in offshore environments, only a single formation, evidently the shallowest, shows evidence of negative δSe. This suggests that there was no upwelling of Se oxyanions from an oxic deep-ocean reservoir, which is consistent with previous estimates that the deep ocean remained anoxic throughout the GOE. The abrupt decline in δSe and Se/TOC values during the subsequent Shunga-Francevillian anomaly indicates a widespread decrease in surface oxygenation.
有人提出，在古元古代早期的大氧化事件（GOE）期间，伴随着被称为洛马冈迪事件的极端正碳同位素偏移，出现了“氧过冲”现象。此外，还有人认为，在随后的极为负向的顺加-弗朗斯维尔碳同位素异常期间，环境氧含量随后骤降至极低水平。这些氧化还原波动可能对真核生物的进化过程产生了深远影响，因为真核生物有几个代谢过程绝对依赖氧气。在这里，我们利用对亚oxic条件下的氧化还原转变敏感的硒（Se）地球化学来研究这些假定的氧扰动的幅度。我们发现，近海页岩中的δSe值从23.2亿年到21亿年呈正向偏移（平均+1.03±0.67‰）。硒丰度和Se/TOC（总有机碳）比率在此期间同样出现峰值。这些数据共同表明，在大氧化事件期间，近岸环境普遍存在亚oxic状态，使得非定量的硒还原作用驱动残留的硒氧阴离子在同位素上变重。这意味着这些环境中的氧含量>0.4μM。与新元古代晚期和显生宙不同，那时在近海环境中观察到负的δSe值，只有一个地层（显然是最浅的地层）显示出负δSe的证据。这表明没有来自含氧深海储层的硒氧阴离子上涌，这与之前关于整个大氧化事件期间深海一直处于缺氧状态的估计一致。在随后的顺加-弗朗斯维尔异常期间，δSe和Se/TOC值的突然下降表明地表氧化作用普遍减弱。
