Uranium isotope evidence for Mesoarchean biological oxygen production in shallow marine and continental settings

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 δ²³⁸U 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 δ²³⁸U_auth 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 δ²³⁸U_auth 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.