Dissolved manganese in ocean waters: analytical and biogeochemical studies

Abstract

The aim of this study was to improve our understanding of the natural sources and processes of the dissolved manganese associated with the Southern Ocean and the Tropical North-Eastern (NE) Atlantic Ocean. A flow injection analyser (FIA) with chemiluminescence (CL) detection was first set up for the determination of dissolved manganese (DMn, ? 0.2 ?m). Extension work was undertaken to solve problems relating to our limited level of understanding of the CL reaction, and the behaviour of the resins used to either preconcentrate the manganese (i.e. Toyopearl AF-Chelate 650M) or to remove the interfering elements (cleaning resin, i.e. NTA Superflow and 8-Hydroxyquinoline). Using the system built in our laboratory, the determination of the interference of other dissolved trace metals (e.g. Fe, Ni, Cu, Co, Zn and Cd) were possible with dissolved manganese measurement in order to obtain a good measurable CL peak for dissolved manganese. The distribution of manganese around the Crozet Islands was examined and was used to provide a conceptual framework for future studies. Dissolved manganese concentrations were measured in samples collected from nine vertical profiles taken across the Crozet Plateau (80 - ~4500 m water depth) that show evidence of a range of processes influencing the manganese distributions. Dissolved manganese varied between 0.1 and 2.44 nM, and the resulting detailed section showed evidence of an island souce is identified which suggests that the plateau and the associated sediments are a source of manganese. Waters further north also appear to be affected by this input of both coastal and shelf origin, although dissolved manganese decrease as a function of distance to the north of the plateau with a gradient of 0.096 nM/km as a result of dispersion and mixing. This gradient was then combined with short-lived Radium isotopes profiles, allowing the determination of a lateral advective flux of manganese (up to 538 nmol/m2/d). Estimates of atmosphere and vertical fluxes of manganese to surface waters were also calculated. It was then possible to estimate a pre-bloom concentration of ~0.4 nM. A set of surface samples were collected from the Tropical NE Atlantic Ocean, and were analysed for dissolved manganese. Results suggest the land-sources of manganese near to the Canary Islands, the Cape Verde Islands and the African Continent, where high dissolved manganese concentrations were determined, with the highest is ~3.90 nM. The lateral advective flux of manganese was higher (47 ?mol/m2/d) than the atmospheric flux of manganese (0.17 ?mol/m2/d), thus making the shelf+sediment as the most prominent sources of dissolved manganese in the seawater close to the islands. From this atmospheric flux of manganese, it was then possible to estimate the manganese enrichment around the further offshore dust event regions of 0.73 nM/yr and consistent with the dissolved manganese background concentrations. This support the low residence time calculated in dust event regions of around 1 year.

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