Oxygenation and Alkalinity Drive the Lacustrine Nitrogen Isotope Record Throughout the Past 3.2 Billion Years

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 (δ13Corg) and nitrogen (δ15N) isotope signals in modern lakes. Subsequently, we use these patterns to interpret past changes in the geological record of lacustrine δ15N during atmospheric oxygenation. The compiled modern lake sediment dataset reveals average (± standard deviation) δ15N values of +2.9‰ ± 3.2‰ and δ13Corg values of -25.99‰ ± 3.77‰, as well as thresholds in δ13Corg for oxic versus anoxic conditions, and in δ15N for circumneutral versus alkaline pH conditions. In contrast to the stepwise oxygenation of the atmosphere, the lacustrine δ15N 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 δ15N, alkaline lakes in both the ancient and modern deviate from this range. We observe δ15N > +10‰ for approximately half of total ancient alkaline lake sediments and some modern lake sediments. This is consistent with previous applications of enriched δ15N as a basicity proxy. The lacustrine δ15N record aligns well with the evolution of microbial metabolic pathways in addition to providing information pertaining to environmental conditions of the depositional setting.