Tracing the stepwise oxygenation of the Proterozoic ocean
Tracing the stepwise oxygenation of the Proterozoic ocean
复制标题
追踪元古代海洋的逐步氧化过程
DOI：
10.1038/nature06811
复制
发表时间：
2008-03-27
期刊：
影响因子：
64.8
通讯作者：
Anbar, A. D.
中科院分区：
文献类型：
--
作者：
Scott, C.;Lyons, T. W.;Anbar, A. D.
保存在古代沉积岩中的生物地球化学特征为地球大气氧化的性质和时间提供了线索。地球化学数据（1-6）表明，氧化作用在元古代（2，500至5.42亿年前）开始和结束时分两个大步骤进行。元古代海洋在这两个步骤之间的氧化状态和深海氧化的时间对地球上生命的进化过程具有重要意义，但仍然知之甚少。在这里，我们提出了一个新的观点，海洋氧化的基础上自生积累的氧化还原敏感的过渡元素钼在硫化物黑色页岩。在26.5亿年前的页岩中已经发现了来自海水的自生钼的积累;然而，这些富集的小幅度反映了大约2,200万年前溶解钼的微弱或短暂来源，与大陆的最小氧化风化一致。指示持续和强烈氧化风化的富集出现在大约21.5亿年前沉积的页岩中，在大气氧最初上升之后超过2亿年（1，2）。约18亿年前（参考文献8、9）之后硫化物条件的后续扩张使中元古代钼储层保持在现代储量的20%以下，这反过来可能起到营养反馈的作用，限制了富氧（硫化物）海底沃茨的时空分布，也可能限制了真核生物的进化和生态扩张（10）。到5.51亿年前，由于深海的氧化作用以及沉积物和水柱中硫化物条件的相应降低，钼含量反映了海洋储层的极大扩展。
Biogeochemical signatures preserved in ancient sedimentary rocks provide clues to the nature and timing of the oxygenation of the Earth's atmosphere. Geochemical data(1-6) suggest that oxygenation proceeded in two broad steps near the beginning and end of the Proterozoic eon ( 2,500 to 542 million years ago). The oxidation state of the Proterozoic ocean between these two steps and the timing of deep- ocean oxygenation have important implications for the evolutionary course of life on Earth but remain poorly known. Here we present a new perspective on ocean oxygenation based on the authigenic accumulation of the redox- sensitive transition element molybdenum in sulphidic black shales. Accumulation of authigenic molybdenum from sea water is already seen in shales by 2,650 Myr ago; however, the small magnitudes of these enrichments reflect weak or transient(7) sources of dissolved molybdenum before about 2,200 Myr ago, consistent with minimal oxidative weathering of the continents. Enrichments indicative of persistent and vigorous oxidative weathering appear in shales deposited at roughly 2,150 Myr ago, more than 200 million years after the initial rise in atmospheric oxygen(1,2). Subsequent expansion of sulphidic conditions after about 1,800 Myr ago ( refs 8, 9) maintained a mid- Proterozoic molybdenum reservoir below 20 per cent of the modern inventory, which in turn may have acted as a nutrient feedback limiting the spatiotemporal distribution of euxinic ( sulphidic) bottom waters and perhaps the evolutionary and ecological expansion of eukaryotic organisms(10). By 551 Myr ago, molybdenum contents reflect a greatly expanded oceanic reservoir due to oxygenation of the deep ocean and corresponding decrease in sulphidic conditions in the sediments and water column.