Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada.
Nature. 2011 Oct 19;478(7369):369-73. doi: 10.1038/nature10511.
The enrichment of redox-sensitive trace metals in ancient marine sedimentary rocks has been used to determine the timing of the oxidation of the Earth's land surface. Chromium (Cr) is among the emerging proxies for tracking the effects of atmospheric oxygenation on continental weathering; this is because its supply to the oceans is dominated by terrestrial processes that can be recorded in the Cr isotope composition of Precambrian iron formations. However, the factors controlling past and present seawater Cr isotope composition are poorly understood. Here we provide an independent and complementary record of marine Cr supply, in the form of Cr concentrations and authigenic enrichment in iron-rich sedimentary rocks. Our data suggest that Cr was largely immobile on land until around 2.48 Gyr ago, but within the 160 Myr that followed--and synchronous with independent evidence for oxygenation associated with the Great Oxidation Event (see, for example, refs 4-6)--marked excursions in Cr content and Cr/Ti ratios indicate that Cr was solubilized at a scale unrivalled in history. As Cr isotope fractionations at that time were muted, Cr must have been mobilized predominantly in reduced, Cr(III), form. We demonstrate that only the oxidation of an abundant and previously stable crustal pyrite reservoir by aerobic-respiring, chemolithoautotrophic bacteria could have generated the degree of acidity required to solubilize Cr(III) from ultramafic source rocks and residual soils. This profound shift in weathering regimes beginning at 2.48 Gyr ago constitutes the earliest known geochemical evidence for acidophilic aerobes and the resulting acid rock drainage, and accounts for independent evidence of an increased supply of dissolved sulphate and sulphide-hosted trace elements to the oceans around that time. Our model adds to amassing evidence that the Archaean-Palaeoproterozoic boundary was marked by a substantial shift in terrestrial geochemistry and biology.
在古代海洋沉积岩中，氧化还原敏感微量元素的富集被用于确定地球陆地表面氧化的时间。铬 (Cr) 是追踪大气氧合作用对大陆风化影响的新兴示踪剂之一；这是因为其向海洋的供应主要由陆地过程控制，这些过程可以在前寒武纪铁建造的 Cr 同位素组成中记录下来。然而，控制过去和现在海水 Cr 同位素组成的因素知之甚少。在这里，我们提供了一种独立且互补的海洋 Cr 供应记录，形式为富铁沉积物中 Cr 的浓度和自生富集。我们的数据表明，Cr 在大约 24.8 亿年前在陆地上基本上是不活动的，但在随后的 1.6 亿年中——与与大氧化事件相关的氧化作用的独立证据同步（例如，参见参考文献 4-6）——Cr 含量和 Cr/Ti 比值的明显波动表明 Cr 在历史上前所未有的规模下被溶解。由于当时 Cr 同位素分馏作用较弱，Cr 必须主要以还原态、Cr(III) 的形式被溶解。我们证明，只有通过需氧呼吸、化能自养细菌对大量且以前稳定的地壳黄铁矿储层的氧化，才能产生溶解来自超镁铁质源岩和残余土壤中 Cr(III) 的酸度。这种始于 24.8 亿年前的风化机制的深刻转变构成了最早已知的嗜酸需氧生物和由此产生的酸性岩石排水的地球化学证据，并解释了当时海洋中溶解硫酸盐和硫化物为主的微量元素供应增加的独立证据。我们的模型增加了越来越多的证据，证明太古宙-古元古代边界的标志是陆地地球化学和生物学的重大转变。
