The rise of oxygen To understand the evolution of the biosphere, we need to know how much oxygen was present in Earth's atmosphere during most of the past 2.5 billion years. However, there are few proxies sensitive enough to quantify O2 at the low levels present until slightly less than 1 billion years ago. Lu et al. measured iodine/calcium ratios in marine carbonates, which are a proxy for dissolved oxygen concentrations in the upper ocean. They found that a major, but temporary, rise in atmospheric O2 occurred at around 400 million years ago and that O2 levels underwent a step change to near-modern values around 200 million years ago. Science, this issue p. 174 The I/Ca ratio in marine carbonates tracks atmospheric oxygen levels for the past 2.5 billion years. Rising oceanic and atmospheric oxygen levels through time have been crucial to enhanced habitability of surface Earth environments. Few redox proxies can track secular variations in dissolved oxygen concentrations around threshold levels for metazoan survival in the upper ocean. We present an extensive compilation of iodine-to-calcium ratios (I/Ca) in marine carbonates. Our record supports a major rise in the partial pressure of oxygen in the atmosphere at ~400 million years (Ma) ago and reveals a step change in the oxygenation of the upper ocean to relatively sustainable near-modern conditions at ~200 Ma ago. An Earth system model demonstrates that a shift in organic matter remineralization to greater depths, which may have been due to increasing size and biomineralization of eukaryotic plankton, likely drove the I/Ca signals at ~200 Ma ago.

中文翻译：

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弹性含氧上层海洋的晚期开始

氧气的增加 要了解生物圈的演变，我们需要知道在过去 25 亿年的大部分时间里地球大气中存在多少氧气。然而，在不到 10 亿年前，几乎没有足够敏感的指标来量化目前存在的低水平 O2。卢等人。测量了海洋碳酸盐中的碘/钙比率，这是上层海洋溶解氧浓度的代表。他们发现，大约 4 亿年前，大气中 O2 的主要但暂时性升高，并且 O2 水平在大约 2 亿年前发生了阶跃变化，接近现代值。科学，这个问题 p。174 海洋碳酸盐中的 I/Ca 比率跟踪过去 25 亿年的大气氧含量。随着时间的推移，海洋和大气中氧气含量的上升对于提高地球表面环境的宜居性至关重要。很少有氧化还原代理可以跟踪上层海洋后生动物生存阈值水平附近溶解氧浓度的长期变化。我们提供了海相碳酸盐中碘钙比 (I/Ca) 的广泛汇编。我们的记录支持大约 4 亿年前 (Ma) 大气中氧分压的显着上升，并揭示了大约 200 Ma 前，上层海洋的氧合作用发生了阶跃变化到相对可持续的近现代条件。地球系统模型表明，有机质再矿化向更深层次的转变，这可能是由于真核浮游生物的大小和生物矿化的增加，可能在大约 200 Ma 之前驱动了 I/Ca 信号。