Home Research Vita Publications Teaching Links Grad Students Home Geosciences Home		Peter J. DiFiore Graduate Student Department of Geosciences  321a Guyot Hall Princeton University Princeton , NJ 08544 Phone: (609) 258-7544 E-Mail:pdifiore@princeton.edu Advisor: Daniel M. Sigman
	***Always Being Updated*** Dual isotopic composition of Nitrate - N uptake and Nitrification in the AZ mixed layer? The Antarctic zone appears to have a lower isotope effect of nitrate assimilation than the SAZ, 5‰ vs. > 7‰. Is there a physiological explanation or are there additional processes which may affect the isotopic composition of nitrate? This work attempts to place constraints on the nitrification (NH4+ -> NO3-) pathway and estimate the relative importance of nitrification for "new" production in the Polar and Subpolar mixed layer. New constraints on Atlantic Nitrogen Fixation - Bringing ocean general circulation models and isotopic observations together with C. Deutsch, D. Sigman, and A. Knapp. I have been running a modified version of the GFDL MOM 3.0 general circutlation model to generate fields of newly fixed nitrate (see sample output below). These models are forced with distributions based on previous estimates of Nitrogen fixation. These distributions are then compared to profiles and transects of the isotopic composition of nitrate to learn about the importance of heterogeniety in the flux distributions, upward limits on total Nitrogen fixation flux, and the importance of physical circulation on development of ocean biogeochemical distributions. Oxygen - Argon ratios in Seawater Research in the distribution of dissolved gases in seawater as tracers of biological activity using a new a equilibrator inlet mass spectrometer (EIMS) designed by N. Cassar and other members of the Bender Lab. Oxygen and Argon have very similar saturation coeffficents in seawater, therefore variation in the O2/Ar ratio of water can be used to determine net biological production (photosynthesis) or consumption (respiration) of O2. Below are some uncorrected observations made during the SAZ SENSE 2007 cruise South of Tasmania with the EIMS. Marine Nitrogen Isotopes Biologically available nitrogen is an extremely important nutrient in the ocean, limiting phytoplankton growth in most open ocean environments, yet its cycle is poorly understood by directly measured elemental fluxes. Recent work suggests that the global marine nitrogen cycle is very dynamic and has a grossly underestimated input/output budget.  Our current view of the marine nitrogen budget is that it has a relatively short residence time, ~1,000yr; therefore, small changes in source or sink may yield large changes in global primary production or atmospheric CO2 levels. N Isotope Budget in the Southern Ocean The Southern Ocean represents a critical junction in the communication of the deep ocean and the atmosphere.  The effect of geostrophy and lack of water column stratification combined with strong winds results in the surfacing of deep nutrient-rich (high C, N) water in the Antarctic. Summer westerly winds advect water northward across the Polar frontal zone and into the Subantarctic zone (SAZ) where biological uptake draws nutrient concentrations to lower values northward. This area represents a possible window for reintroduction of carbon into the ocean. Several inhibitory mechanisms may limit the consumption and complete drawdown of nutrients by phytoplankton including iron and light limitation. This leads to variability in the efficiency of the biological pump.  The seasonal formation of Subantarctic Mode Water in this region is critical in that it feeds the low latitude thermocline--a large extent of consumption in this region would lower the supply of nutrients to the low latitudes.   For these reasons a quantitative understanding of biological productivity and export in the SAZ is a necessary constraint for understanding the modern global nutrient cycles.  The Utility of Nitrogen isotopes  The ^15N/^14N of nitrate (NO3^- ) provides a tracer of source, mixing, transport, and uptake in the Southern ocean.  N isotope data is presented in differential notation referenced to the isotopic composition of N2 in air. The delta definition is used because the natural variations in N are very small: delta ^15N = ((^15N/^14N) sample/(^15N/^14N) ref.) – 1) * 1000 (‰ vs. air) Due to kinetic effects, phytoplankton preferentially consume light isotopes for incorporation into biomass leaving the source pool enriched in ^15N.  The preferential uptake can be quantified by an isotope effect, epsilon.  Defined in terms of the reaction rate coefficients, k: epsilon = (^15k/^14k) *1000 Epsilon values from high latitude high nutrient low chlorophyll areas have been shown to be ~ 4 - 5‰ (Sigman 1999).  Recent studies of sediment trap material suggest that the Subantarctic may be 2-3‰ higher (Lourey et al 2003, Karsh et al 2003).  Using a simple model of isotope dynamics we can place constraints on the source of nutrients, circulation, and uptake in the SAZ and also investigate realistic epsilon values.  Our results suggest that a significant portion of the summertime SAZ nitrate is supplied from south of the Subantarctic Front. Our approach also identifies the necessity of an isotope effect for nitrate assimilation in the SAZ of ≥ 7‰ and probably ~ 8 - 9 ‰. Comparison to laboratory results suggests this relatively high isotope effect may result from light limitation in the SAZ (DiFiore et. al, in Review).