Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
Department of Geology and Geophysics, Yale University, New Haven, CT 06511.
Proc Natl Acad Sci U S A. 2022 Feb 8;119(6). doi: 10.1073/pnas.2116101119.
Earth's surface has undergone a protracted oxygenation, which is commonly assumed to have profoundly affected the biosphere. However, basic aspects of this history are still debated-foremost oxygen (O) levels in the oceans and atmosphere during the billion years leading up to the rise of algae and animals. Here we use isotope ratios of iron (Fe) in ironstones-Fe-rich sedimentary rocks deposited in nearshore marine settings-as a proxy for O levels in shallow seawater. We show that partial oxidation of dissolved Fe(II) was characteristic of Proterozoic shallow marine environments, whereas younger ironstones formed via complete oxidation of Fe(II). Regardless of the Fe(II) source, partial Fe(II) oxidation requires low O in the shallow oceans, settings crucial to eukaryotic evolution. Low O in surface waters can be linked to markedly low atmospheric O-likely requiring less than 1% of modern levels. Based on our records, these conditions persisted (at least periodically) until a shift toward higher surface O levels between 900 and 750 Ma, coincident with an apparent rise in eukaryotic ecosystem complexity. This supports the case that a first-order shift in surface O levels during this interval may have selected for life modes adapted to more oxygenated habitats.
地球表面经历了漫长的氧化过程，这通常被认为对生物圈产生了深远的影响。然而，这一历史的基本方面仍存在争议，尤其是在藻类和动物出现前的数十亿年里，海洋和大气中的氧气水平。在这里，我们使用铁（Fe）在铁岩中的同位素比值（富铁的沉积岩，在近岸海洋环境中沉积）作为浅海海水 O 水平的代用指标。我们表明，溶解的 Fe(II)的部分氧化是前寒武纪浅海环境的特征，而较年轻的铁岩则通过 Fe(II)的完全氧化形成。无论 Fe(II)的来源如何，部分 Fe(II)氧化都需要浅海海洋中的低 O 水平，这对真核生物的进化至关重要。浅海水中的低 O 水平与大气中 O 的含量明显降低有关，可能需要不到现代水平的 1%。根据我们的记录，这些条件至少在周期性地持续存在，直到 900 至 750 百万年前，浅海表面 O 水平的这种变化与真核生物生态系统复杂性的明显上升相吻合。这支持了这样一种观点，即在这一时期，表面 O 水平的一级变化可能选择了适应含氧环境的生命模式。
