Department of Earth and Planetary Science, University of California, Berkeley, CA 94720;
Department of Earth and Planetary Science, University of California, Berkeley, CA 94720.
Proc Natl Acad Sci U S A. 2018 Dec 18;115(51):12938-12943. doi: 10.1073/pnas.1813493115. Epub 2018 Dec 3.
Terrestrial environments have been suggested as an oxic haven for eukaryotic life and diversification during portions of the Proterozoic Eon when the ocean was dominantly anoxic. However, iron speciation and Fe/Al data from the ca. 1.1-billion-year-old Nonesuch Formation, deposited in a large lake and bearing a diverse assemblage of early eukaryotes, are interpreted to indicate persistently anoxic conditions. To shed light on these distinct hypotheses, we analyzed two drill cores spanning the transgression into the lake and its subsequent shallowing. While the proportion of highly reactive to total iron (Fe/Fe) is consistent through the sediments and typically in the range taken to be equivocal between anoxic and oxic conditions, magnetic experiments and petrographic data reveal that iron exists in three distinct mineral assemblages resulting from an oxycline. In the deepest waters, reductive dissolution of iron oxides records an anoxic environment. However, the remainder of the sedimentary succession has iron oxide assemblages indicative of an oxygenated environment. At intermediate water depths, a mixed-phase facies with hematite and magnetite indicates low oxygen conditions. In the shallowest waters of the lake, nearly every iron oxide has been oxidized to its most oxidized form, hematite. Combining magnetics and textural analyses results in a more nuanced understanding of ambiguous geochemical signals and indicates that for much of its temporal duration, and throughout much of its water column, there was oxygen in the waters of Paleolake Nonesuch.
陆地环境被认为是真核生物生命和多样化的含氧避难所，特别是在元古代时期的一部分，当时海洋主要是缺氧的。然而，来自约 11 亿年前的诺森克组的铁形态和 Fe/Al 数据表明，该地区持续处于缺氧状态，该组沉积在一个大型湖泊中，含有早期真核生物的多样组合。为了阐明这些截然不同的假设，我们分析了两个跨越湖泊进积和随后变浅的钻芯。虽然高反应性铁与总铁（Fe/Fe）的比例在整个沉积物中一致，通常在被认为是缺氧和有氧条件之间存在歧义的范围内，但磁实验和岩相数据表明，铁存在于三个不同的矿物组合中，这是由于氧化层的存在。在最深的水域，铁氧化物的还原溶解记录了缺氧环境。然而，其余的沉积序列具有指示富氧环境的氧化铁组合。在中等水深，赤铁矿和磁铁矿的混合相表明低氧条件。在湖泊最浅的水域，几乎每一种氧化铁都被氧化成最氧化的形式——赤铁矿。将磁性和纹理分析相结合，对模棱两可的地球化学信号有了更细致的理解，并表明在 Paleolake Nonesuch 的大部分时间和整个水柱中，水中都有氧气。
