Award: OCE-1029299

The global marine nitrogen balance consists of input of fixed nitrogen (mostly NO3-) by nitrogen fixation--the process by which bacteria change N2, the most abundant gas in the atmosphere, into NH4+ and ultimately to NO3- in ocean surface waters, and the output is denitrification--the process in which NO3- is reduced to N2 gas by bacteria in regions of low or no oxygen concentration. It is important to know the values of the inputs and outputs because they control the NO3- concentration in the ocean, and this molecule is the main limiting nutrient for ocean photosynthesis. This NSF project was designed to determine the role of deintrification in sediments of the deep ocean (> 1000 m). We observed before this grant was awarded that the N2/Ar ratio of ocean deep waters increases by about 0.5 % as the deep waters transit the ocean from the North Atlantic to the North Pacific. Part of this increase may be due to denitrification and part is possibly caused by interactions of deep waters with the atmosphere and ice in the Antarctic Ocean when the deep waters surface on their way from the Atlantic to the Pacific. In order to sort out the role of physical (air-sea interaction) and biological (denitrification) processes that cause the observed N2/Ar increase, we measured the concentration of noble gases: Ne, Ar, Kr, Xe. These gases are tracers of air-sea interaction so if they also increase in the deep waters between the Atlantic and Pacific then we will know that that is the reason for the N2/Ar increase—not denitrificaiton. On the other hand if the noble gas concentrations do not increase, then the N2/Ar ratio change is due to denitrification in the sediments, which is the only location where oxygen concentrations are low enough to permit this reaction. To answer this question we measured the N2/Ar ratio and concentrations of noble gases in depth profiles from the surface to 5000 meters depth at six locations in the ocean: The Labrador Sea, the Bermuda Atlantic Time Series, the Drake Passage between the southern tip of South America and Antarctica, the Hawaii Ocean Time-series, the South Pacific off Samoa, and the subarctic Pacific at Ocean Station Papa. The sampling and measurements were completed in June of 2013, and we are now in the process of interpreting the results. Interpreting these measurements to answer to the question we set out to resolve will be part of our research for the coming year. Last Modified: 08/20/2013 Submitted by: Steven R Emerson