CH4, N2O, and CO2 measured on discrete water samples during METEOR cruise M157

The high surface productivity triggered by nutrient-rich Benguela upwelled waters results in significant enrichment of organic carbon in the sub-surface waters due to enhanced mineralization in the water column and benthic fluxes. Hence, microbial O2-consuming processes are promoted, driving oxygen depletion that favors trace gases i.e. methane (CH4) and nitrous oxide (N2O) production at relatively shallow depths. During upwelling, gas-rich subsurface waters are also transported towards the surface waters, enhancing trace gas sea-air fluxes. We investigate the variability of these fluxes on seasonal and shorter timescales to understand the intensity of the Benguela upwelling system in gas emissions. The data might serve as a base for projections under a changing climate. The fieldwork took place during the cruise M157 (August 4th – September 16th, 2019) onboard the R/V METEOR, which encompassed close-coastal and open ocean regions between Mindelo (Cape Verde) and Walvis Bay. The main transect lines around 18, 23 and 25°S represents the Angola-Benguela frontal zone, Walvis Bay and Lüderitz upwelling cells respectively, which are suggested to represent some regional hotspots of trace gas emissions to the atmosphere, in particular in the vicinity of the upwelling cells. To explore further, nearly 300 discrete water samples were collected from the Niskin bottles at different stations for determination of the concentrations of CH4, N2O, and total inorganic carbon (CT). Analysis for CH4 and N2O was performed using an in-house designed purge and trap system with a dynamic headspace. In brief, a subsample of the water is purged with an inert ultrapure carrier gas of Helium, and the gases are focused on a cryo-trap operated at about -120°C. The volatile compounds are desorbed by rapid heating and analyzed by a gas chromatograph (Agilent 7890 B), equipped with a Flame Ionization Detector for CH4 and an Electron Capture Detector for N2O measurements, respectively. Samples for CT were taken to investigate the carbonate system. CT was measured using an automated Infra-Red Inorganic Carbon Analyzer (AIRICA) system (Marianda e.K., 24145 Kiel) from discrete 250 ml samples. In brief, a subsample is drawn into a volume-calibrated syringe and injected into a purge vessel, where the discrete sample is acidified. All species of the inorganic carbon system are converted to CO2, which is purged from the water using a carrier gas that streams through the acidified probe. Then the gas flows through a Peltier cooler and a NAFION dryer to be dried. The concentration of CO2 is then measured by an infrared detector (LICOR 7000), which integrates the peak of the purged sample. The integrated signal is directly proportional to the carbon released, allowing the CT concentration to be calculated with high precision. Certified reference material (CRM) of known CT-concentration is used for standardization and to account for drift of the sensor response