一个国际研究小组称，来自澳大利亚皮尔巴拉带状含铁岩层的铁矿石核心样本将大气中氧气首次出现的时间追溯到了24.8亿年前。
带领这个研究小组的马克·巴利教授说，他们的发现基于岩石样本的可靠性。这项研究发表在《自然》杂志上。
地理学家称，大氧化事件发生在24.8亿年前至23.2亿年前，地球大气中的氧气就是在那个时期形成的。
巴利教授说：“这证明了有氧呼吸生命的最初形态有氧呼吸细菌氧化了黄铁矿，产生出溶解岩石和土壤的酸，然后被水流带到海洋里。”
“有氧呼吸细菌需要与蓝细菌共生产生氧气才能实现这一过程，”巴利教授说，地理生物学家正在建立更完善的数据库，为这种与氧气含量提高有关的细菌到底何时发生作用寻找更多证据。
他说：“我们从带状含铁岩层中找到了大量样本，分析了铬同位素和其他元素，因为那可以提供氧化最有力的证据。”（生物谷Bioon.com）
doi:10.1038/nature10511
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Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event
Kurt O. Konhauser, Stefan V. Lalonde, Noah J. Planavsky, Ernesto Pecoits, Timothy W. Lyons, Stephen J. Mojzsis, Olivier J. Rouxel, Mark E. Barley, Carlos Rosìere, Phillip W. Fralick, Lee R. Kump & Andrey Bekker
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 surface1, 2. 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 formations3. 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 soils7. 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 sulphate8 and sulphide-hosted trace elements to the oceans around that time1, 9. Our model adds to amassing evidence that the Archaean-Palaeoproterozoic boundary was marked by a substantial shift in terrestrial geochemistry and biology.
