Work Package 4
Seasonal fluxes of trace gases and their governing physical and microbial controls
Upwelling fosters the release of the greenhouse gases (GHG) methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) to the atmosphere. Although the underlying biogeochemical processes are quite distinct, the key variable to best describe these effects is oxygen content or AOU (Naqvi et al., 2010). High subsurface pCO2 is related to the integrated mineralization in the upwelling-feeding waters, and N2O is produced mainly by nitrification and inversely correlated to oxygen concentration. At very low oxygen levels, N2O strongly increases and denitrification becomes the dominant production pathway. Recent studies show an intensified N2O production when sulfide-based chemoautotrophic denitrification dominates (Dalsgaard et al. 2013). Under anoxic conditions, N2O is used as an electron acceptor and mostly absent. While N2O investigations in the BUS are sparse, various studies have investigated the methane cycle, mostly on the shelf, where the highest methane concentrations of open coastal waters have been reported (Brüchert et a., 2009).
General Questions and Research topics:
- How can the seasonal air-sea fluxes of greenhouse gases be related to variability in both physical forcing and microbiological processes?
We will survey the seasonal cycle of trace gas sea-air fluxes by operating our continuous fast surface trace gas measurement system (N2O, CH4, CO2, δ13C-CO2) along the Namibian monitoring line (with SP1) for an entire year. These fluxes will be analyzed with the detailed information on upwelling strength and underlying biogeochemical patterns, and interpreted in correlation to measurements at the Gobabeb Namib Desert Atmospheric Observatory. During the major expeditions, we will investigate the role of the differing source waters, the hypoxic Angola Basin Central Water off Cape Frio and less oxygen depleted Cape Basin South Atlantic Central Water off Lüderitz (Monteiro et al., 2008), on trace gas formation, and determine the fluxes of CH4 and N2O from the sediments. High resolution profiles will be obtained by coupling of equilibration/CEAS instruments to IOW’s pump CTD (demonstrated during a SCOR WG 143 intercalibration cruise). Sites with increased N2O concentrations will be sampled for microbiological analysis (DNA, mRNA) to assess microbial diversity and activity. Here, incubation experiments with varying concentrations of oxygen and sulfide (with SP5) will be conducted and combined with an analysis of the active microbial populations (with SP3). At sites where anoxic, methane-rich water intersects with oxygen- or nitrate-containing water masses, the efficiency of methane removal by microbial oxidation will be measured, and involved microbial populations determined.