Velazquez Diana, Sheldon Nathan D, Hren Michael T, Kharbush Jenan J
Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan, USA.
Department of Earth Sciences, University of Connecticut, Storrs, Connecticut, USA.
Geobiology. 2025 Sep-Oct;23(5):e70033. doi: 10.1111/gbi.70033.
The widespread, stepwise oxygenation of Earth's atmosphere in the Precambrian led to a transformation of the global carbon (C) and nitrogen (N) cycles. While the temporal evolution of these nutrient cycles has been studied extensively in marine environments, lacustrine environments are understudied. This study first examines how water column oxygen conditions impact sedimentary carbon (δC) and nitrogen (δN) isotope signals in modern lakes. Subsequently, we use these patterns to interpret past changes in the geological record of lacustrine δN during atmospheric oxygenation. The compiled modern lake sediment dataset reveals average (± standard deviation) δN values of +2.9‰ ± 3.2‰ and δC values of -25.99‰ ± 3.77‰, as well as thresholds in δC for oxic versus anoxic conditions, and in δN for circumneutral versus alkaline pH conditions. In contrast to the stepwise oxygenation of the atmosphere, the lacustrine δN record does not directly reflect major oxygenation events, but instead increases gradually in response to the evolution of new aerobic N metabolic pathways, with a notable shift in the Phanerozoic. While we found that intrasite variability at a single modern anoxic lake is expected to remain within ~5‰ for δN, alkaline lakes in both the ancient and modern deviate from this range. We observe δN > +10‰ for approximately half of total ancient alkaline lake sediments and some modern lake sediments. This is consistent with previous applications of enriched δN as a basicity proxy. The lacustrine δN record aligns well with the evolution of microbial metabolic pathways in addition to providing information pertaining to environmental conditions of the depositional setting.
前寒武纪地球大气中广泛的、逐步的氧化作用导致了全球碳（C）和氮（N）循环的转变。虽然这些养分循环的时间演变在海洋环境中已得到广泛研究，但湖泊环境的研究却较少。本研究首先考察水柱氧气条件如何影响现代湖泊中的沉积碳（δC）和氮（δN）同位素信号。随后，我们利用这些模式来解释湖泊δN地质记录在大气氧化过程中的过去变化。汇编的现代湖泊沉积物数据集显示，平均（±标准差）δN值为+2.9‰±3.2‰，δC值为-25.99‰±3.77‰，以及有氧与缺氧条件下δC的阈值，以及中性与碱性pH条件下δN的阈值。与大气的逐步氧化不同，湖泊δN记录并不直接反映主要的氧化事件，而是随着新的有氧氮代谢途径的演变而逐渐增加，在显生宙有明显的转变。虽然我们发现单个现代缺氧湖泊内的站点内变异性预计δN将保持在~5‰以内，但古代和现代的碱性湖泊都偏离了这个范围。我们观察到，大约一半的古代碱性湖泊沉积物和一些现代湖泊沉积物的δN> +10‰。这与之前将富集δN用作碱度指标的应用一致。湖泊δN记录除了提供有关沉积环境的环境条件的信息外，还与微生物代谢途径的演变非常吻合。
