Nitrogen cycling during the Mesoproterozoic as informed by the 1400 million year old Xiamaling Formation

Nitrogen is an essential nutrient for all life on Earth. It is, and likely has been, a critical limiting nutrient controlling primary production rates in the oceans. The marine N budget is regulated by the balance between nitrogen loss as N₂ (and N₂O) in low-oxygen settings in the oceans including marine sediments and coastal oxygen-minimum zones and nitrogen inputs, mainly from nitrogen fixation. How this balance has changed through Earth history, particularly considering changing degrees of ocean oxygenation, is a frontier research area. Here, we explore ancient ocean nitrogen dynamics as revealed through stable nitrogen isotopes in the ca. 1400 million year old (Ma) Xiamaling Formation of North China. We compare these dynamics to other Mesoproterozoic-aged settings and to modern environments. Through these comparisons, we see that Mesoproterozoic (1600 to 1000 Ma) oceans experienced both nitrogen replete- and nitrogen-deplete conditions with isotope systematics very similar to modern analogues. We provide models explaining the ancient ocean nitrogen dynamics, and we conclude that Mesoproterozoic oceans generally maintained extensive regions of nitrate-replete conditions. These nitrate-deplete conditions could have corresponded with extensive areas of deoxygenated seawater or vast nitrate-replete marine environments similar to modern oxygen-minimum zone environments. We also evaluate, generally, what nitrogen isotope systematics can and cannot tell us about nitrogen limitation in ancient oceans.