Oxygenated Mesoproterozoic lake revealed through magnetic mineralogy
Oxygenated Mesoproterozoic lake revealed through magnetic mineralogy
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DOI：
10.1073/pnas.1813493115
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发表时间：
2018-12
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影响因子：
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通讯作者：
S. Slotznick;N. Swanson‐Hysell;E. Sperling
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文献类型：
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作者：
S. Slotznick;N. Swanson‐Hysell;E. Sperling
限制元古界海洋和湖泊的氧水平和氧化还原化学的意义对于对早期好氧真核生物进化施加环境限制至关重要。最近的工作使用了以铁为基础的地球化学替代物--然而，由于与湖泊沉积物基线有关的不确定性以及与经验校准的替代物有关的模棱两可的区域，对这种测量结果的解释可能很困难。我们集成了磁学、地球化学和微尺度成像技术来分析有11亿年历史的古湖诺努苏赫的铁矿物学，这是这个时代为数不多的湖泊记录之一。用这些方法，我们解决了模糊的地球化学信号，并记录了一个氧跃层，其浅水含氧，氧随深度减少。这些结果表明，11亿年前陆地上存在稳定的充氧环境。在元古界海洋以缺氧为主的时期，陆地环境被认为是真核生物和生物多样性的有毒避难所。然而，铁的形态和铁/铝的数据来自大约11亿年前的诺内什组，沉积在一个大湖中，含有不同的早期真核生物组合，被解释为持续的缺氧条件。为了阐明这些不同的假设，我们分析了两个钻芯，这些岩芯跨越了海侵进入湖中和随后的浅水。虽然在沉积物中高活性的铁与全铁的比例(Fehr/Fet)是一致的，并且通常在缺氧和缺氧条件之间不明确的范围内，但磁性实验和岩石学数据表明，铁存在于三种不同的矿物组合中，这三种矿物组合都是由一个氧跃层产生的。在最深的水域，铁氧化物的还原溶解记录了缺氧环境。然而，沉积序列的其余部分具有指示氧化环境的铁氧化物组合。在中等水深，赤铁矿和磁铁矿的混相指示低氧条件。在湖水最浅的地方，几乎所有的氧化铁都被氧化成了氧化程度最高的赤铁矿。结合磁学和结构分析，可以更细致入微地理解模糊的地球化学信号，并表明在其大部分时间内，以及在其大部分水柱中，古湖的水中都有氧气。
Significance Constraining oxygen levels and redox chemistry of Proterozoic oceans and lakes is vital for placing environmental constraints on early aerobic eukaryotic evolution. Recent work has used iron-based geochemical proxies—however, interpretation of such measurements can be difficult due to uncertainties related to baselines for lake sediments and equivocal zones associated with empirically calibrated proxies. We integrate magnetic, geochemical, and microscale imaging techniques to analyze the iron mineralogy of the 1.1-billion-year-old Paleolake Nonesuch, one of the few lacustrine records of this era. With these methods, we resolve ambiguous geochemical signals and document an oxycline with oxygenated shallow waters and decreasing oxygen with depth. These results indicate a stable oxygenated environment in the terrestrial realm 1.1 billion years ago. 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 (FeHR/FeT) 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.