The cycling and redox state of nitrogen in the Archaean ocean
The cycling and redox state of nitrogen in the Archaean ocean
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DOI：
10.1038/ngeo633
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发表时间：
2009-10-01
影响因子：
18.3
通讯作者：
Falkowski, Paul G.
中科院分区：
文献类型：
--
作者：
Godfrey, Linda V.;Falkowski, Paul G.
通过光合作用产生氧气的生物体存在于太古宙晚期，大约距今 25 亿年前，但有争议的证据表明它们可能已经进化了几亿年。氧气预计会与海洋氮发生反应，改变其氧化还原状态。该反应在有机物中结合的氮的同位素组成中留下了特征。在这里，我们展示了从南非 Campbellrand-Malmani 平台的微小蚀变页岩中提取的干酪根的氮同位素组成记录。从古太古宙到大约 26.7 亿年前，干酪根的 Delta N-15 值上升了约千分之二。我们将这种增加解释为海洋表面耦合硝化和反硝化或厌氧氨氧化反应开始的迹象，这需要游离氧的存在。大约25.2亿年前氮同位素组成的第二次增加意味着氮循环的不稳定和固定氮的损失。这一海洋中可用氧的证据出现在地球化学表明存在大量大气氧之前至少2亿年。我们认为耦合的硝化和反硝化作用导致了固定无机氮的损失，导致氮限制，并得出结论，生物可利用氮的低水平限制了产氧浮游生物的生长，延缓了大气中氧气的积累。
Organisms that produce oxygen through photosynthesis existed during the late Archaean eon, about 2,500 million years ago, but controversial evidence suggests that they may have evolved several hundred million years earlier. Oxygen is expected to react with oceanic nitrogen, altering its redox state. The reaction leaves a signature in the isotopic composition of the nitrogen bound in organic matter. Here we present a record of the nitrogen isotopic composition of kerogen extracted from minimally altered shales from the Campbellrand-Malmani platform in South Africa. Between the Palaeo-Archaean and about 2,670 million years ago, the delta N-15 values of the kerogen rose by about 2 parts per thousand. We interpret this increase as an indication of the onset of coupled nitrification and denitrification or anammox reactions in the surface oceans, which require the presence of free oxygen. A second increase in nitrogen isotopic composition around 2,520 million years ago implies instability of the N cycle with loss of fixed N. This evidence for available oxygen in the oceans occurs at least 200 million years before geochemical indications of the presence of significant levels of atmospheric oxygen. We suggest that coupled nitrification and denitrification drove the loss of fixed inorganic nitrogen, leading to nitrogen limitation, and conclude that the low levels of biologically available nitrogen limited the growth of oxygen-producing plankton, delaying the accumulation of oxygen in the atmosphere.