The role of nitrate in the ocean carbon cycle and its relatively short residence time make it crucial to understand the marine nitrogen cycle; however, there is currently insufficient experimental evidence to accurately determine present day fluxes. Denitrification and nitrogen fixation are the main sink and source for dissolved inorganic nitrogen in the sea.
In this study a research team at the University of Washington will collaborate with colleagues at the University of Victoria to study changes in the N2/Ar ratio in seawater caused by denitrification. Previous research has demonstrated the utility of this tracer in the oxygen minimum zones of the Pacific and Indian Ocean, but they will investigate observed changes in the "background" distribution of the ratio. The investigators already have unpublished data that indicate the N2/Ar ratio increases by about 0.5 % from the Atlantic to Pacific Oceans in waters below 1000 meters. If this increase is assumed to be caused by denitrification in deep ocean sediments it amounts to roughly 80 Tg/yr of denitrification. This is a significant portion of estimated global denitrification (between 200 and 400 Tg/yr) and within the range of the largely untested predictions of deep-ocean sediment denitrification using global sediment diagenesis models. Presently it is not possible to unequivocally attribute the observed deep water column N2/Ar increase to denitrification because it could also be caused by deep-water formation processes in the Antarctic.
The investigators will separate the fraction of the N2/Ar ratio increase due to the physical processes of atmosphere or ice-water interaction from that due to denitrification by measuring other noble gas ratios (primarily Ne/Ar and Kr/Ar) that change only in response to ocean surface cooling and bubble processes. They will measure deep water-column profiles of N2/Ar, Ne/Ar and Kr/Ar in strategically-located sites where there are ships of opportunity: the Labrador Sea, the North Atlantic at the Bermuda time-series site, the Drake Passage, the Indian Ocean south of Madagascar, the subtropical North Pacific at the Hawaii Ocean time-series site, and the subarctic North Pacific at Station P. Preliminary measurements of all of the gas ratios have been made, and extensive testing has been done to identify sources of contamination in the sampling methods. This proposal involves a two-laboratory collaboration to make it possible to sample a large number of ocean sites, minimize atmospheric contamination by rapid sample analysis, and create maximum accuracy through laboratory intercalibration.
Broader Impacts: This project will promote international ocean science collaboration between the U.S.and Canada. It will support the research of an assistant professor to apply analytical methods that she has helped develop to an important problem in oceanography. A PhD candidate at the University of Washington will be trained in the area of chemical oceanography using analytical methods of gas ratio and isotope ratio mass spectrometry.
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.
