Stability of the nitrogen cycle during development of sulfidic water in the redox-stratified late Paleoproterozoic Ocean

Nitrogen cycling has been evaluated across a depth transect in the late Paleoproterozoic Animikie Basin (North America), spanning the end of Earth's final period of global iron precipitation, and a major transition to euxinic conditions in areas of high productivity. Sediments from near shore, where productivity was highest, have d(15)N compositions up to similar to 3 parts per thousand higher than at more distal sites. This suggests that as NH4+ mixed vertically upward into the oxic photic zone from the anoxic ocean interior, it was either assimilated by organisms or oxidized. Subsequent enhanced production of N-2 by denitrification or anammox (anaerobic ammonium oxidation) led to the observed increase in d(15)N close to shore. Any deficit in biologically available N was overcome by N-2-fixing organisms, but the input of N with low d(15)N from this process did not overwhelm the increase in d(15)N from denitrification. Because there is no evidence for conditions of severe N stress arising from trace metal limitation (particularly Mo) of N fixation during the transition to euxinic conditions, losses of N were either very small (potentially because low O-2 levels limited NH4+ oxidation), or alternative pathways that retained N were important. The fact that Mo appears to have remained bioavailable for N fixation, either suggests that the extent or severity of sulfidic water column conditions was not sufficient to quantitatively sequester Mo on a global scale, or that rivers directly delivered Mo to surface waters on the inner shelf. The effects of N-2 fixation on d(15)N increased to more distal parts of the shelf, consistent with models invoked for modern upwelling zones over broad continental margins.