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Uranium isotope evidence for Mesoarchean biological oxygen production in shallow marine and continental settings
Earth and Planetary Science Letters ( IF 5.3 ) Pub Date : 2020-12-01 , DOI: 10.1016/j.epsl.2020.116583
Xiangli Wang , Frantz Ossa Ossa , Axel Hofmann , Andrea Agangi , Dora Paprika , Noah J. Planavsky

Abstract Oxidative weathering of continental rocks is the major source of redox-sensitive elements to the ocean. The timing for the initiation of oxidative weathering remains strongly debated. Here we report new δ 238 U data for the authigenic component of well-preserved ca. 3.07–2.90 Ga marine and lacustrine shales from the Mozaan and Dominion groups of the Kaapvaal Craton, South Africa. The δ 238 Uauth values in marine shales (−0.71‰ to −0.21‰ relative to CRM 112a) are lower than the bulk silicate Earth (BSE, −0.40‰ to −0.17‰), while those of lacustrine shales (−0.68‰ to 0.05‰) scatter above and below the BSE range. In addition, the δ 238 Uauth values of marine shales deposited in shallow seawater are significantly lower than those of the shales deposited in deep seawater. We propose that U(VI) was adsorbed onto Fe-Mn oxides in oxic shallow water and then shuttled to the seafloor. The U isotope fractionations in lacustrine shales are best explained by reduction of U(VI) to U(IV) below the sediment-water interface. Together, these observations bolster the case that a mildly oxidative weathering regime had been established at the Earth's surface by ∼3.0 Ga, paving the way to the onset of aerobic metabolism in both continental and shallow marine environments.

中文翻译:

浅海和大陆环境中中太古代生物产氧的铀同位素证据

摘要 大陆岩石的氧化风化作用是海洋氧化还原敏感元素的主要来源。氧化风化开始的时间仍然存在激烈争论。在这里,我们报告了保存完好的 ca 的自生成分的新 δ 238 U 数据。来自南非 Kaapvaal Craton 的 Mozaan 和 Dominion 群的 3.07-2.90 Ga 海相和湖相页岩。海相页岩中的 δ 238 Uauth 值(-0.71‰ 至 -0.21‰ 相对于 CRM 112a)低于块状硅酸盐地球(BSE,-0.40‰ 至 -0.17‰),而湖相页岩(-0.68‰ 至0.05‰) 分散在 BSE 范围之上和之下。此外,沉积于浅海的海相页岩的δ 238 Uauth 值明显低于沉积于深海的页岩。我们建议 U(VI) 在含氧浅水中被吸附到 Fe-Mn 氧化物上,然后穿梭到海底。湖相页岩中 U 同位素分馏的最好解释是在沉积物 - 水界面下方将 U(VI) 还原为 U(IV)。总之,这些观察结果支持了地球表面已经建立了大约 3.0 Ga 的温和氧化风化制度的情况,为大陆和浅海海洋环境中的有氧代谢的开始铺平了道路。
更新日期:2020-12-01
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