LOW NITRATE LEVELS IN THE MESOPROTEROZOIC OCEAN

Eukaryogenesis may have occurred during the Paleoproterozoic era, but eukaryote proliferation and rise to ecological dominance is not evident until the Neoproterozoic. Nitrogen limitation in the open ocean may have contributed to this delay because fixed nitrogen (primarily nitrate) is essential for eukaryotic growth, as eukaryotes cannot fix atmospheric N₂. This hypothesis has been tested once before in the Mesoproterozoic Belt Supergroup (~1.4 Ga) using nitrogen isotopes as a proxy for nitrogen limitation. Results from that study suggest nitrate was scarce offshore relative to nearer-shore facies. Here, we expand the Mesoproterozoic marine nitrogen isotope database to include two more basins, the Roper (~1.4 Ga) and Bangemall (~1.5 Ga), in order to get a broader picture of nitrogen cycling during the “boring billion”. Both basins show an average ∼1–2‰ enrichment in δ¹⁵N_bulk from deep to shallow facies. Unlike the Belt basin, the Bangemall and Roper basins show some offshore δ¹⁵N_bulk values that are enriched beyond the isotopic range associated with biological N₂ fixation alone. This suggests the presence of some free fixed nitrogen, with a mixture of aerobic and anaerobic metabolisms offshore. In shallow waters, where δ¹⁵N_bulk enrichment peaks, an aerobic nitrogen cycle was evidently dominant. Even though isotopic signatures of aerobic nitrogen cycling are seen in all parts of the Bangemall and Roper basins, our data are consistent with a lateral gradient in nitrate availability within the photic zone, with higher concentrations in near-shore environments. This trend is now seen in all three Mesoproterozoic basins so far examined, and contrasts with the Paleoproterozoic and Neoproterozoic where nearly all δ¹⁵N_bulk data are above the N₂ fixation window. Thus, we propose that the Mesoproterozoic ocean was characterized by a nitrate deficit, with the lowest concentrations in offshore environments. This inference is consistent with a Mesoproterozoic O₂ decline following a temporary Paleoproterozoic O₂ high, and it further supports the idea that nitrate limitation offshore may have contributed to the restriction of photosynthetic eukaryotes to near-shore environments, delaying their rise to ecological dominance until the Neoproterozoic Era.