Aerobic nitrogen cycling in a molybdenum-limited, redox-stratified Mesoproterozoic epeiric sea

Nitrogen is a critically important element for life on Earth, and its cycling through the ocean-atmosphere system is a biologically mediated process that requires a suite of transition metals as enzyme cofactors. For example, the most common form of nitrogenase (the enzyme that mediates nitrogen fixation by prokaryotes) requires molybdenum and iron as structural components. It has been proposed, however, that ‘alternative nitrogenase’ enzymes may have been important under conditions of expanded euxinia and Mo drawdown that may have characterized parts of the Proterozoic oceans. Furthermore, it has been proposed that bioavailable nitrate was scarce in the Mesoproterozoic oceans, inhibiting the evolution of eukaryotes that lack the ability to fix their own nitrogen. To test these hypotheses, we examined nitrogen isotopes (δ¹⁵N) in black and grey shale of the ∼1.1 Ga Atar and El Mreiti groups, Taoudeni Basin, Mauritania as a proxy for nitrogen cycling in the Mesoproterozoic oceans. These strata were previously investigated for iron speciation and trace metal content, and they record a period of substantial Mo drawdown in a redox-stratified epeiric sea. In the Atar and El Mreiti groups, both shallow epicratonic and deeper pericratonic environments record strongly positive δ¹⁵N values (median = +4.6‰), indicative of an aerobic nitrogen cycle characterized by nitrification and partial denitrification, and thus surface waters that were replete in bioavailable nitrate. Even in the most Mo-limited euxinic environments, positive δ¹⁵N values suggest nitrate-replete surface waters. These results are consistent with a large compilation of δ¹⁵N data from other Mesoproterozoic basins, which collectively do not support the presence of a metal-nitrogen co-limited biosphere in the Mesoproterozoic oceans. Instead, we suggest that nitrogen fixation likely proceeded via Mo-Fe-nitrogenase and nitrate was readily available in the oceans during much of the Mesoproterozoic, with phosphorus acting as the limiting macronutrient on the marine biosphere.