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Distributions of dissolved (DOM) and suspended (POM) organic matter, and their chromophoric (CDOM) and fluorescent (FDOM) fractions, are investigated at high resolution (< 10 km) in the Cape Verde Frontal Zone (CVFZ) during fall 2017. In the epipelagic layer (< 200 m), meso- and submesoscale structures (meanders, eddies) captured by the high resolution sampling dictate the tight coupling between physical and biogeochemical parameters at the front. Remarkably, fluorescent humic-like substances show relatively high fluorescence intensities between 50 and 150 m, apparently not related to local mineralization processes. We hypothesize that it is due to the input of Sahara dust, which transports highly re-worked DOM with distinctive optical properties. In the mesopelagic layer (200-1500 m), our results suggest that DOM and POM mineralization occurs mainly during the transit of the water masses from the formation sites to the CVFZ. Therefore, most of the local mineralization seems to be due to fast-sinking POM produced in situ or imported from the Mauritanian upwelling. These local mineralization processes lead to the production of refractory CDOM, an empirical evidence of the microbial carbon pump mechanism. DOM released from these fast-sinking POM is the likely reason behind the observed columns of relatively high DOC surrounded by areas of lower concentration. DOM and POM dynamics in the CVFZ has turned out to be very complex, in parallel to the complexity of meso- and submesoscale structures present in the area. On top of this high resolution variability, the input of Sahara dust or the release of DOM from sinking particles have been hypothesized to explain the observed distributions.

A fast adsorptive stripping voltammetric procedure for the determination of ultratrace amounts of iron in unpurged oceanic waters is reported. The method is based on that developed by van den Berg et al. [l]. It was modified to increase the sensitivity, to measure in the presence of oxygen and to reduce drastically the analysis time at the ultratrace level (pM). The scan rate, pH and concentrations of reagents were optimized to avoid interferences using background correction. The catalytic effect of the hydrogen peroxide was not found using high scan rate. The instability of the peak was related to colloidal iron. Acidified or heated samples showed stable peaks. The method is insensitive to the turbulence of the solution during the scan. The reproducibility (140 pM) and detection limits (120 pM) of the method were calculated using 60s deposition time and oceanic waters (600pM). The detection limit can be lowered to 70 pM using 3 min accumulation.

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Abstract. The circulation patterns in the confluence of the North Atlantic Subtropical and Tropical gyres delimited by the Cape Verde Frontal Zone (CVFZ) in summer 2017 were examined. Hydrology, dissolved oxygen (O2) and inorganic nutrients data collected in a closed box embracing the CVFZ allowed estimating transports of water masses, O2 and inorganic nutrients for the first time. Higher transports occurred mainly at the surface and central waters, and were moderately affected by the Cape Verde Front located in the southeastern part of the domain. Thus, the front conditioned the meridional transports, acting as a barrier between North and South Atlantic Central waters. Specifically, −3.2±1.7 Sv entered through north and east and 6.7±1.7 Sv left through west and south transects. At intermediate levels, the most important source came from the south with −2.2±1.5 Sv of modified Antarctic Intermediate water, moderately affected by the circulation pattern above. The transports of O2 and inorganic nutrients conditioned by their distributions behaved quite similar to mass transports. The most intense and important transports of O2 and inorganic nutrients occurred in the deepest layer of central waters and in the shallowest two layers of intermediate waters where inorganic nutrients accumulated and large differences in concentrations of O2 were found, especially in the deepest layer of central waters between the northeast and southeast zones. In these three layers, transports of O2 and inorganic nutrients came from the east and south and they left northward and westward. This circulation pattern delivers inorganic nutrient from east and south to oligotrophic waters of north and west of CVFZ. Nevertheless, it can also hinders the ventilation of the deepest layer of central waters and the shallowest two layers of intermediate waters.