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Nitrous Oxide In the Atlantic Ocean

Nitrous Oxide In the Atlantic Ocean. Grant Forster, G ű nther Uher and Robert C. Upstill-Goddard. Department of Marine Science and Technology, University of Newcastle upon Tyne, UK. Introduction:. Water masses sampled during AMT 12 and 13.

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Nitrous Oxide In the Atlantic Ocean

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  1. Nitrous Oxide In the Atlantic Ocean Grant Forster, Gűnther Uher and Robert C. Upstill-Goddard Department of Marine Science and Technology, University of Newcastle upon Tyne, UK. Introduction: Water masses sampled during AMT 12 and 13 The N2O flux estimates for the tropical and equatorial regions and for the African upwelling were in the range 3.7 to 6.2 μmol m-2 d-1 and 3.5 to 6.2 μmol m-2 d-1 respectively. In comparison the gyre regions had much lower N2O fluxes, in the range 0.2 to 0.3 μmol m-2 d-1 (NAG) and 0.8 to 1.1 μmol m-2 d-1 (SAG). However because of their greater surface area these regions are likely greater contributors of atmospheric N2O than the tropical and equatorial upwellings, at least on the basis of data presented here. Data interpretation is however still ongoing, including estimates of the areal extent of these biogeochemical provinces based on other data collected during the AMT programme. By way of comparison, fluxes calculated by Popp et al (2002) for the subtropical North Pacific gyre, 1.1 ± 0.7 μmol m-2 d-1, are similar to our estimates for both the NAG and SAG. Oudot et al (2002) report air-sea fluxes in the range 1.1 to 1.8 μmol m-2 d-1 for the Atlantic ocean between 7°30’N and 4°30’S. Our estimates for that region are around three times higher than those reported by Oudot et al (2002). A recent synthesis of available data and modelling results reveals an increase in the average global surface temperature of ~0.6ºC since the late 19th century (IPPC, 2001), which is linked not only to the growth in atmospheric CO2 but also to the growth in other important greenhouse gases including methane (CH4) and nitrous oxide (N2O). The globally averaged tropospheric mixing ratio of N2O in 1998 was 314 ppbv (IPPC, 2001), whereas the pre-industrial mixing ratio was ~ 285 ppbv (Dickinson and Cicerone, 1986). The current rate of tropospheric N2O increase is ~ 0.3 % yr-1. The oceans are thought to currently account for ~ 30% of atmospheric N2O, with upwelling regions considered to be among the most important contributors (e.g. Bange et al, 1996). Marine N2O is produced by two main mechanisms that are both microbially mediated: nitrification (the oxidation of NH4+ to NO3-) and denitrification (the reduction of NO3-to N2 gas). During research cruises of the NERC AMT programme onboard RRS James Clark Ross vertical profiles of N2O to 300m were obtained. The cruise tracks are shown in Figure 1 below. Figure 2 is a Sigma-T (σt) plot used to distinguish between water masses sampled during AMT-12 and AMT-13, based on their characteristic salinities and temperatures. Waters from the North and South Atlantic are clearly distinguishable from each other; most waters sampled corresponded either to South Atlantic Central Water (SACW) or Eastern North Atlantic Central Water (ENACW), whose boundaries are shown in the figure. µmol L-1 µmol L-1 µmol L-1 µmol L-1 Figures 4a (top left), 4b (top right), 4c (bottom left) & 4d (bottom right) (above) show oxygen concentration (µmol L-1) for AMT-12 and AMT-13, Nitrate + Nitrite (µmol L-1) for AMT-12 and AMT-13, respectively. Nutrient data collected and analysed by Malcolm Woodward (PML) and Katie Chamberlain (PML). O2 Data from Carol Robinson (PML) and Niki Gist (PML). b. a. References: Bange, H.W., Rapsomanikis, M.O. and M.O. Andreae. (1996) “ The Aegean Sea as a source of atmospheric nitrous oxide and methane.” Marine Chemistry 53: pp41-49. Dickinson, R.D. and R.J. Cicerone. (1986). “ Future global warming from atmospheric trace gases.” Nature 319: pp109-115. IPPC, Climate Change 2001. “The scientific basis. Contribution of working group I to the third assessment report of the Intergovernmental Panel on Climate Change (IPCC).” Cambridge University Press, Cambridge (UK) and New York NY, USA). Liss, P.S. and L. Merlivat. (1986) “Air sea gas exchange rates: Introduction and synthesis” , in The Role of Air-Sea Exchange in Geochemical Cycling. Edited by P.B. Menard, pp113-127, D. reidel, Norwell, Mass. Naqvi, S.W.A. (1991) “N2O production in the ocean.” Nature 349: pp373-374. Oudot, C., Jean-Baptiste, P., Fourré, E., Mormiche, C., Guevel, M., Ternon, J-F., Le Corre, P. (2002) “Transatlantic equatorial distribution of nitrous oxide and methane.” Dees-Sea Research I 49: pp1175-1193. Popp, B.N., Westley, M.B., Toyoda, S., Miwa, T., Dore, J.E., Yoshida, N., Rust, T.M., Sansone, F.J., Russ, M.E., Ostrom, N.E., Ostrom, P.H. (2002) “Nitrogen and oxygen isotopomeric constraints on the origins and sea-to-air flux of N2O in the oligotrophic subtropical North Pacific gyre.” Global Biogeochemical Cycles 16: pp 1064-1073. Upstill-Goddard, R.C., Rees, A.P. and Owens, N.J.P. (1996) “Simultaneous high-precision measurements of methane and nitrous oxide in water and seawater by single phase equilibration gas chromatography.” Deep-Sea Research I, 43: pp1669-1682. Wanninkhof, R. (1992) “Relationship between wind speed and gas exchange over the ocean.”Journal of Geophysical Research 97:pp 7373-7382. Figure 9 Figure 2 - σt plot showing water masses samples during AMT-12 and AMT-13. AMT-12 southern stations (triangles filled) AMT-12 Northern stations (black squares filled), AMT-13 Southern stations (empty triangles), AMT-13 Northern stations (empty squares). c. d. Results and Conclusions: AMT-12 AMT-13 Figures 3 & 4 (below) show dissolved N2O saturations to be highest in the tropical equatorial region and off the West coast of Africa. The overall close positive correspondence between N2O and the sum of NO3- plus NO2-, and the strong negative correspondence between N2O and O2 indicate N2O production via nitrification. Figure 1- AMT-12 and AMT-13 cruise tracks. Figure 5a, b, c & d (above) shows N2O concentration vs. NO3- + NO2- for AMT-12 cruise track (top left) and for AMT-12 Tropical Equatorial Region only (top right). Bottom left and bottom right show delta N2O vs. AOU for the whole AMT-12 cruise track and the TER, respectively. The AMT cruises traverse a range of “Biogeochemical Ocean Provinces”, including the North and South Atlantic Gyres (NAG & SAG), the Equatorial upwelling and the West African coastal upwelling. In this poster we present N2O concentration data from vertical profiles to 300m depth. We show relationships between N2O and NO3- + NO2- and between N2O and AOU that provide evidence for mechanisms of N2O production in the Atlantic Ocean. Finally we estimate sea-to-air N2O fluxes for the biogeochemical provinces listed above. The plots in Figure 5 are further evidence of the nitrification source. The area of enhanced N2O production in figures 3a and 3b is known as the Tropical and Equatorial Region (TER). % saturation When the data from the TER are plotted separately (Figure 5b & 5d) from the rest of the cruise data the correlations become stronger suggesting that the N2O in this region may be from a more intense nitrification source. Methodology: Air-Sea N2O fluxes % saturation • Seawater samples were collected using a 24×20 Litre CTD rosette system (SEABIRD) and dissolved N2O was determined using single-phase equilibration gas chromatography (Upstill-Goddard et al, 1996). Acknowledgements This study was supported by the UK Natural Environment Research Council through the Atlantic Meridional Transect consortium (NER/O/S/2001/00680). We would like to thank all of the crew from the RRS James Clark Ross on AMT-12 and AMT-13 and all of the scientists without whom this data analysis would not have been possible. Air-sea N2O fluxes were estimated using mean mixed layer N2O concentrations (nmol L-1) and underway wind speeds (m-2s-1), based on the formulations of Wanninkhof (1992) and Liss and Merlivat (1986). Figure 3 shows N2O % Saturation for AMT-12 (top) and AMT-13 (bottom).

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