Nitrogen Availability in Proterozoic Seawater

Abstract:

Recent bulk δ¹⁵N measurements transecting the Mesoproterozoic Belt Supergroup basin by Stüeken (2013) showed a trend of heavier δ¹⁵N values in shallow and peritidal environments to lighter δ¹⁵N values in deep water environments. This indicates a gradient in nitrogen metabolisms from quantitative nitrification, partial denitrification, and assimilation of the residual NO₃^- onshore (with most δ¹⁵N values greater than 2‰), to nitrogen fixation offshore (δ¹⁵N values less than 2‰). Here we present nitrogen isotope data from the roughly contemporaneous Bangemall Group in northwestern Australia and the Roper Group in northern Australia, both showing an average ~1‰ enrichment from deep to shallow facies. A maximum range from 0.9‰ (deep) to 5.8‰ (shallow) and 1.4‰ (deep) to 4.8‰ (shallow) is observed in the Bangemall and Roper Groups respectively. Viewed together with the maximum 7‰ basinal trend of the Belt Supergroup, these results suggest (a) that bioavailable nitrate limitation offshore may have been widespread in the Mesoproterozoic, and (b) that there was significant variability between basins in the degree of onshore-offshore nitrogen speciation. Prolonged nitrate limitation in offshore environments may have globally prevented the radiation of eukaryotes into deeper waters. Differences between the isotope profiles of the Belt Supergroup and the Bangemall Group may be attributed to varying degrees of restriction of each basin to the ocean.