There is ongoing debate as to when oxygenic photosynthesis evolved on Earth^1,2. Geochemical data from ancient sediments indicate localized or ephemeral photosynthetic O₂ production before the Great Oxidation Event (GOE) approximately 2.5–2.3 billion years ago (Ga), and currently suggest Archaean origins, approximately 3 Ga or earlier^{3,4,5,6,7,8,9}. However, sedimentary records of the early Earth often suffer from preservation issues, and poor control on the timing of oxidation leaves geochemical proxy data for the ancient presence of O₂ open to critique^10,11,12,13. Here, we report rare Earth element data from three different Archaean carbonate platforms preserved in greenstone belts of the northwest Superior Craton (Canada), which were deposited by the activity of marine photosynthetic bacteria 2.87 Ga, 2.85 Ga and 2.78 Ga. All three indicate O₂ production before the GOE in the form of significant depletions in cerium (Ce), reflecting oxidative Ce removal from ancient seawater, as occurs today¹⁴. Using ¹³⁸La-¹³⁸Ce geochronology, we show that La/Ce fractionation, and thus Ce oxidation, occurred at the time of deposition, making these the oldest directly dated Ce anomalies. These results place the origin of oxygenic photosynthesis in the Mesoarchaean or earlier and bring an important new perspective on a long-standing debate regarding Earth’s biological and geochemical evolution.

关于含氧光合作用何时在地球上进化的争论^{一直存在 1,2}。来自古代沉积物的地球化学数据表明，在大约 2.5-23 亿年前 （Ga） 的大氧化事件 （GOE） 之前，局部或短暂的光合 O₂ 产生，目前表明起源于太古菌，大约 3 Ga 或更早 ^{3,4,5,6,7,8,9}。然而，早期地球的沉积记录经常存在保存问题，并且对氧化时间的控制不佳使古代存在 O₂ 的地球化学代理数据有待批判 ^10,11,12,13。在这里，我们报告了来自加拿大西北部苏必利尔克拉通绿岩带中保存的三个不同太古宙碳酸盐平台的稀土元素数据，这些数据是由海洋光合细菌 2.87 Ga、2.85 Ga 和 2.78 Ga 的活动沉积的。这三个都表明 O₂ 在 GOE 之前以铈 （Ce） 显着消耗的形式产生，反映了从古代海水中氧化去除 Ce，就像今天发生的那样 ¹⁴。使用 ¹³⁸ 个 ^La-138Ce 地质年代学，我们表明 La/Ce 分馏，以及 Ce 氧化，发生在沉积时，使这些成为最古老的直接测年的 Ce 异常。这些结果将含氧光合作用的起源置于中太古宙或更早，并为关于地球生物和地球化学演化的长期争论带来了重要的新视角。