Award: OCE-1232901

Nitrogen (N) is an essential nutrient whose availability in the ocean often limits biological production and its capacity to pump carbon from the surface for sequestration at depth. This nitrogen control on the oceanÆs biological pump thus also influences atmospheric CO2 concentration. The oceanÆs inventory of bioavailable N in the ocean (mostly in the form of nitrate; NO3-) is uniquely driven by biological processes such as nitrogen fixation (which creates bioavailable N from unavailable N2 gas) and denitrification and anammox, which convert bioavailable N back into unavailable gaseous forms (N2O and N2). Oxygen deficient zones (ODZs) are important sites in the ocean for bioavailable N loss (up to half of global total), as well as N2O (a greenhouse gas) emission to the atmosphere, and they are projected to expand and intensify in the coming years as global warming increases ocean stratification and decreases O2 renewal rates. It is important to understand the current distribution of relevant chemical forms of nitrogen and their isotopic composition and how these signals are spread from the ODZ out to the greater South Pacific Basin in order better constrain the operation of these N transformation processes at present and into the future. The stable isotope signals of nitrate in ODZs are some of the highest values observed anywhere in the global ocean, as a result of isotopic fractionation during nitrate consumption. These regions have far-reaching impacts on the mean isotopic composition of deep ocean nitrate. How these signals are transmitted and modified after leaving the ODZ is poorly understood. The GEOTRACES Peru-Tahiti (EPZT) section (Fig. 1) provided a rare opportunity to track the fate of the isotopic signals of N loss in one of the largest water column ODZs. Our 3-PI project measured isotopic composition of nitrate, nitrite, and nitrous oxide in seawater samples collected along this transect as well as in nitrate from aerosol and rain samples. In addition, we have measured biogenic N2 and its isotope composition to close the N mass and isotope budgets (see Figs. 2 &3). Nitrate isotopic composition is a GEOTRACES "core parameter" that complements other measurements, such as bioactive trace element concentrations and speciation, and by itself provides important constraints on the oceanographic processes that drive its variation: N fixation, denitrification, and lateral nitrate transport. Combined, these measurements yielded insight into modern biogeochemical processes and provide first order background information for both modern physical oceanographic and paleoceanographic applications. For example, studies of sediments in the eastern tropical south Pacific suggest past changes in the rate of denitrification, based on variations over time in N isotopic composition. Progress in paleoceanographic interpretations relies on a more complete understanding of the determinants of isotope signals in oxygen deficient zone nitrate, and the transmission or attenuation of these signals though transit into the South Pacific. For UMD's portion of the project, the major field activity was first collection of samples for biogenic N2 and N2 isotope analysis along the Pacific GeoTraces transect. The major shore-based activity was analysis of these samples in the laboratory. So far we have been able to infer from these data that 1) The slope of N deficit versus biogenic N2 (x2) is slightly less than 1 (Fig. 3), suggesting that nitrogen to phosphorous (N:P) remineralization ratios may be less than the canonical 16:1 in ODZ waters. Previous studies have suggested organic matter remineralized in the ETSP ODZ may be similar to but slightly less than the canonical Redfield ratio. 2) Estimates of the N:P of remineralized organic matter in the middle and lower ODZ (sigma theta density surfaces 26.4 to 26.8) may increase slightly away from the ODZ, As the N:P of phytoplankton can vary, longitudinal gradie...