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Wallace, Douglas

Permanent URI for this collectionhttps://hdl.handle.net/10222/22184

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  • ItemOpen Access
    Nitrous oxide in the North Atlantic Ocean
    (2006) Walter, S.; Bange, H. W.; Breitenbach, U.; Wallace, D. W. R.
    In order to get a comprehensive picture of the distribution of nitrous oxide (N(2)O) in the North Atlantic Ocean, measurements of dissolved nitrous oxide were made during three cruises in the tropical, subtropical and cold-temperate North Atlantic Ocean in October/November 2002, March/April 2004, and May 2002, respectively. To account for the history of atmospheric N(2)O, we suggest a new depth-dependent calculation of excess N(2)O (Delta N(2)O). N(2)O depth profiles showed supersaturation throughout the water column with a distinct increasing trend from the cold-temperate to the tropical region. Lowest nitrous oxide concentrations, near equilibrium and with an average of 11.0 +/- 1.7 nmol L(-1), were found in the cold-temperate North Atlantic where the profiles showed no clear maxima. Highest values up to 37.3 nmol L(-1) occurred in the tropical North Atlantic with clear maxima at approximately 400 m. A positive correlation of nitrous oxide with nitrate, as well as excess nitrous oxide with the apparent oxygen utilization (AOU), was only observed in the subtropical and tropical regions. Therefore, we conclude that the formation of nitrous oxide via nitrification occurs in the tropical region rather than in the cold-temperate region of the North Atlantic Ocean.
  • ItemOpen Access
    Distribution of N2O in the Baltic Sea during transition from anoxic to oxic conditions
    (2006) Walter, S.; Breitenbach, U.; Bange, H. W.; Nausch, G.; Wallace, D. W. R.
    In January 2003, a major inflow of cold and oxygen-rich North Sea Water terminated an ongoing stagnation period in parts of the central Baltic Sea. In order to investigate the role of North Sea Water inflow in the production of nitrous oxide (N2O), we measured dissolved and atmospheric N2O at 26 stations in the southern and central Baltic Sea in October 2003. At the time of our cruise, water renewal had proceeded to the eastern Gotland Basin, whereas the western Gotland Basin was still unaffected by the inflow. The deep water renewal was detectable in the distributions of temperature, salinity, and oxygen concentrations as well as in the distribution of the N2O concentrations: Shallow stations in the Kiel Bight and Pomeranian Bight were well-ventilated with uniform N2O concentrations near equilibrium throughout the water column. In contrast, stations in the deep basins, such as the Bornholm and the Gotland Deep, showed a clear stratification with deep water affected by North Sea Water. Inflowing North Sea Water led to changed environmental conditions. especially enhanced oxygen (O-2) or declining hydrogen sulphide (H2O) concentrations, thus, affecting the conditions for the production of N2O. Pattern of N2O profiles and correlations with parameters like oxygen and nitrate differed between the basins. Because of the positive correlation between Delta N2O and AOU in oxic waters the dominant production pathway seems to be nitrification rather than denitrification. Advection of N2O by North Sea Water was found to be of minor importance. A rough budget revealed a significant surplus of in situ produced N2O after the inflow. However, due to the permanent halocline, it can be assumed that the N2O produced does not reach the atmosphere. Hydrographic aspects therefore are decisive factors determining the final release of N2O produced to the atmosphere.
  • ItemOpen Access
    Estimating the monthly pCO(2) distribution in the North Atlantic using a self-organizing neural network
    (2009) Telszewski, M.; Chazottes, A.; Schuster, U.; Watson, A. J.; Moulin, C.; Bakker, D. C. E.; Gonzalez-Davila, M.; Johannessen, T.; Koertzinger, A.; Lueger, H.; Olsen, A.; Omar, A.; Padin, X. A.; Rios, A. F.; Steinhoff, T.; Santana-Casiano, M.; Wallace, D. W. R.; Wanninkhof, R.
    Here we present monthly, basin-wide maps of the partial pressure of carbon dioxide (pCO(2)) for the North Atlantic on a 1 degrees latitude by 1 degrees longitude grid for years 2004 through 2006 inclusive. The maps have been computed using a neural network technique which reconstructs the non-linear relationships between three biogeochemical parameters and marine pCO(2). A self organizing map (SOM) neural network has been trained using 389 000 triplets of the SeaWiFS-MODIS chlorophyll-a concentration, the NCEP/NCAR reanalysis sea surface temperature, and the FOAM mixed layer depth. The trained SOM was labelled with 137 000 under-way pCO(2) measurements collected in situ during 2004, 2005 and 2006 in the North Atlantic, spanning the range of 208 to 437 mu atm. The root mean square error (RMSE) of the neural network fit to the data is 11.6 mu atm, which equals to just above 3 percent of an average pCO(2) value in the in situ dataset. The seasonal pCO(2) cycle as well as estimates of the interannual variability in the major biogeochemical provinces are presented and discussed. High resolution combined with basin-wide coverage makes the maps a useful tool for several applications such as the monitoring of basin-wide air-sea CO2 fluxes or improvement of seasonal and interannual marine CO2 cycles in future model predictions. The method itself is a valuable alternative to traditional statistical modelling techniques used in geosciences.
  • ItemOpen Access
    Carbon cycle research after Kyoto
    (1999-04) Tans, PP; Wallace, DWR
    Recent progress in research of the global carbon cycle is reviewed and research needs for the immediate future are discussed, in light of the challenge posed to society to come to grips with the problem of man-made climate change. The carbon cycle in the oceans and on the land is reviewed, and how the atmosphere functions to couple them together. Major uncertainties still exist for any projection of the future atmospheric burden of carbon dioxide resulting from postulated emission scenarios of CO2. We present some ideas on how future policies designed to limit emissions or to sequester carbon can possibly be supported by scientific evidence of their effectiveness.
  • ItemOpen Access
    Contrasting biogeochemistry of nitrogen in the Atlantic and Pacific Oxygen Minimum Zones
    (2012) Ryabenko, E.; Kock, A.; Bange, H. W.; Altabet, M. A.; Wallace, D. W. R.
    We present new data for the stable isotope ratio of inorganic nitrogen species from the contrasting oxygen minimum zones (OMZs) of the Eastern Tropical North Atlantic, south of Cape Verde, and the Eastern Tropical South Pacific off Peru. Differences in minimum oxygen concentration and corresponding N-cycle processes for the two OMZs are reflected in strongly contrasting delta N-15 distributions. Pacific surface waters are marked by strongly positive values for delta N-15-NO3-) reflecting fractionation associated with subsurface N-loss and partial NO3- utilization. This contrasts with negative values in NO3- depleted surface waters of the Atlantic which are lower than can be explained by N supply via N-2 fixation. We suggest the negative values reflect inputs of nitrate, possibly transient, associated with deposition of Saharan dust. Strong signals of N-loss processes in the subsurface Pacific OMZ are evident in the isotope and N2O data, both of which are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively low (epsilon(d)=11.4 parts per thousand) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N2O vs. AOU for waters with oxygen concentrations ([O-2] < 5 mu mol l(-1)) in both regions reflect a nitrification source. Sharp decrease in N2O concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O-2 < 5 mu mol l(-1).
  • ItemOpen Access
    Estimating mixed layer nitrate in the North Atlantic Ocean
    (2010) Steinhoff, T.; Friedrich, T.; Hartman, S. E.; Oschlies, A.; Wallace, D. W. R.; Koertzinger, A.
    Here we present an equation for the estimation of nitrate in surface waters of the North Atlantic Ocean (40 degrees N to 52 degrees N, 10 degrees W to 60 degrees W). The equation was derived by multiple linear regression (MLR) from nitrate, sea surface temperature (SST) observational data and model mixed layer depth (MLD) data. The observational data were taken from merchant vessels that have crossed the North Atlantic on a regular basis in 2002/2003 and from 2005 to the present. It is important to find a robust and realistic estimate of MLD because the deepening of the mixed layer is crucial for nitrate supply to the surface. We compared model data from two models (FOAM and Mercator) with MLD derived from float data (using various criteria). The Mercator model gives a MLD estimate that is close to the MLD derived from floats. MLR was established using SST, MLD from Mercator, time and latitude as predictors. Additionally a neural network was trained with the same dataset and the results were validated against both model data as a 'ground truth' and an independent observational dataset. This validation produced RMS errors of the same order for MLR and the neural network approach. We conclude that it is possible to estimate nitrate concentrations with an uncertainty of +/- 1.4 mu mol L(-1) in the North Atlantic.
  • ItemOpen Access
    Methodological Underestimation of Oceanic Nitrogen Fixation Rates
    (2010-09) Mohr, Wiebke; Grosskopf, Tobias; Wallace, Douglas W. R.; LaRoche, Julie
    The two commonly applied methods to assess dinitrogen (N(2)) fixation rates are the (15)N(2)-tracer addition and the acetylene reduction assay (ARA). Discrepancies between the two methods as well as inconsistencies between N(2) fixation rates and biomass/ growth rates in culture experiments have been attributed to variable excretion of recently fixed N(2). Here we demonstrate that the (15)N(2)-tracer addition method underestimates N(2) fixation rates significantly when the (15)N(2) tracer is introduced as a gas bubble. The injected (15)N(2) gas bubble does not attain equilibrium with the surrounding water leading to a (15)N(2) concentration lower than assumed by the method used to calculate (15)N(2)-fixation rates. The resulting magnitude of underestimation varies with the incubation time, to a lesser extent on the amount of injected gas and is sensitive to the timing of the bubble injection relative to diel N(2) fixation patterns. Here, we propose and test a modified (15)N(2) tracer method based on the addition of (15)N(2)-enriched seawater that provides an instantaneous, constant enrichment and allows more accurate calculation of N(2) fixation rates for both field and laboratory studies. We hypothesise that application of N(2) fixation measurements using this modified method will significantly reduce the apparent imbalances in the oceanic fixed-nitrogen budget.