• journal_title：Geology
• Contributor：Linda V. Godfrey ; Simon W. Poulton ; Gray E. Bebout ; Philip W. Fralick
• Publisher：Geological Society of America
• Date：2013-06-01
• Format：text/html
• Language：en
• Identifier：10.1130/G33930.1
• journal_abbrev：Geology
• issn：0091-7613
• volume：41
• issue：6
• firstpage：655
• section：Articles

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 δ¹⁵N compositions up to ∼3‰ 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 N₂ by denitrification or anammox (anaerobic ammonium oxidation) led to the observed increase in δ¹⁵N close to shore. Any deficit in biologically available N was overcome by N₂-fixing organisms, but the input of N with low δ¹⁵N from this process did not overwhelm the increase in δ¹⁵N 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 O₂ 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 N₂ fixation on δ¹⁵N increased to more distal parts of the shelf, consistent with models invoked for modern upwelling zones over broad continental margins.