Sea surface trace gases during Maria S. Merian cruise MSM105

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 oxygen demand processes are promoted, driving oxygen depletion that favors trace gas production at relatively shallow depths. During upwelling, gas-rich subsurface waters are 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 MSM105 (January 11th – February 23rd, 2022) onboard the R/V MARIA S. MERIAN, which encompassed close-coastal and open ocean regions between Mindelo (Cape Verde) and Walvis Bay. The working area of the cruise MSM105 was the Namibian shelf between 18°S and 27°S which is suggested to represent some regional hotspots of trace gas emissions to the atmosphere. The partial pressures of CH4, N2O, and CO2 in the sea surface and atmosphere were determined using IOW's self-built Mobile Equilibrator Sensor System (MESS). The system was described in detail elsewhere (Sabbaghzadeh et al., 2021) but in brief, it consists of a custom-built equilibrator (combined shower-head/bubble type) with a water flow rate of about 5 l min-1 and an airflow rate of ~ 4.00 - 5.00 L min-1. The system is linked to two off-axis integrated cavity output laser spectrometers (oa-ICOS, Los Gatos Instruments) for the detection of CH4 / CO2 and N2O / CO. To operate the system, seawater was supplied by a deep-well pump (CAPRARI Desert E4XP30-4 with CAPRARI XPBM1 control unit, ~ 100 L min-1, Italy), installed in the moon-pool at ∼ 6.00 m water depth on board of the R/V MARIA S. MERIAN. All gas analyzers were calibrated against three standard gasses at the beginning and end of each survey daily for data recalibration and drift correction. In addition, one "zero" gas (i.e. Nitrogen 5.00, LINDE) was measured infrequently throughout the survey to check any system deficiency like leakage detection. To quantify sea-air gas fluxes, the atmospheric concentration of studied trace gas was measured at several positions during the cruise using a tube with the inlet positioned at the front of the bow to minimize ship contamination. All other ancillary parameters were synchronized with D-ship data with a simultaneous data reduction to one-minute intervals.