The nitrogen isotopic composition of sedimentary rocks (δ¹⁵N) can trace redox-dependent biological pathways and early Earth oxygenation^1,2. However, there is no substantial change in the sedimentary δ¹⁵N record across the Great Oxidation Event about 2.45 billion years ago (Ga)³, a prominent redox change. This argues for a temporal decoupling between the emergence of the first oxygen-based oxidative pathways of the nitrogen cycle and the accumulation of atmospheric oxygen after 2.45 Ga (ref. ³). The transition between both states shows strongly positive δ¹⁵N values (10–50‰) in rocks deposited between 2.8 Ga and 2.6 Ga, but their origin and spatial extent remain uncertain^4,5. Here we report strongly positive δ¹⁵N values (>30‰) in the 2.68-Gyr-old shallow to deep marine sedimentary deposit of the Serra Sul Formation⁶, Amazonian Craton, Brazil. Our findings are best explained by regionally variable extents of ammonium oxidation to N₂ or N₂O tied to a cryptic oxygen cycle, implying that oxygenic photosynthesis was operating at 2.7 Ga. Molecular oxygen production probably shifted the redox potential so that an intermediate N cycle based on ammonium oxidation developed before nitrate accumulation in surface waters. We propose to name this period, when strongly positive nitrogen isotopic compositions are superimposed on the usual range of Precambrian δ¹⁵N values, the Nitrogen Isotope Event. We suggest that it marks the earliest steps of the biogeochemical reorganizations that led to the Great Oxidation Event.

沉积岩的氮同位素组成 (δ ¹⁵ N) 可以追踪依赖于氧化还原的生物途径和早期地球氧化^1,2 。然而，在大约24.5亿年前的大氧化事件(Ga) ³期间，沉积物δ ¹⁵ N记录没有发生实质性变化，这是一个显着的氧化还原变化。这表明氮循环的第一个基于氧的氧化途径的出现与2.45 Ga之后大气氧气的积累之间存在时间脱钩（参考文献³ ）。两种状态之间的转变在 2.8 Ga 和 2.6 Ga 之间沉积的岩石中显示出强烈的正 δ ¹⁵ N 值（10-50‰），但它们的起源和空间范围仍然不确定^4,5 。在此，我们报告了巴西亚马逊克拉通 Serra Sul 地层⁶的 2.68 戈古浅至深海洋沉积物中的 δ ¹⁵ N 强正值（>30‰）。我们的发现最好地解释为与神秘的氧循环相关的铵氧化成 N ₂或 N ₂ O 的区域变化程度，这意味着含氧光合作用在 2.7 Ga 下进行。分子氧的产生可能改变了氧化还原电位，从而产生了中间的 N 循环基于在地表水中硝酸盐积累之前形成的氨氧化。当强正氮同位素组成叠加在前寒武纪 δ ¹⁵ N 值的通常范围上时，我们建议将这一时期命名为氮同位素事件。我们认为它标志着导致大氧化事件的生物地球化学重组的最早步骤。