Geology Club Seminar

The oxidation state of fixed nitrogen (N), a major limiting nutrient for marine primary production, is dictated by the ambient environmental redox conditions: in an anoxic ocean, inorganic N would be stable as ammonium, while in the presence of dissolved O2 nitrate is stable. We have developed a method of determining nitrate content in carbonates, Carbonate Associated Nitrate (CAN), as a proxy for the oceanic nitrate content. To investigate changes in the global O2 and marine nitrogen cycles through time, concentrations of CAN have been evaluated in both limestones and dolostones from multiple localities around the world, spanning the ages from ~3 Ga through modern. The highest CAN values were found as several distinct peaks in the late Neoproterozoic carbonates from two locations: Caborca in Sonora, Mexico, within a stratigraphic sequence deposited during the Ediacaran, and within the Rainstorm Member of the Johnnie Formation in the Death Valley, California, likely deposited at the onset of the Ediacaran Shuram δ13C excursion. The δ15N of nitrate in these rocks is consistent with oxidation of the pre-existing oceanic ammonium pool, perhaps driven by increasing pO2 during this time. Transformation of fixed N from the reduced to the oxidized form (from ammonium to nitrate) may have caused a major restructuring of the global ocean N cycle, potentially contributing to the diversification of the eukaryotic phytoplankton, which were forced to adapt to nitrate instead of ammonium as the major nitrogen source.