Manganese and iron in Indian Ocean waters

Abstract

The first vertical profiles of dissolved Mn and Fe for the (NW) Indian Ocean are reported. The area is characterized by seasonal upwelling and a broad oxygen minimum zone in intermediate waters. The dissolved Fe-profile exhibits a maximum (5.1 nM) in the oxygen minimum zone, with low values both in surface waters (0.3 nM) and deep waters (around 1 nM). Mn concentrations in the surface waters are elevated (2.0-4.3 nM), and decrease rapidly in an offshore direction. Below the first 25 m, concentrations decrease dramatically (0.5-1.3 nM), indicating removal by oxidation and particle scavenging. Further down, various Mn maxima are observed which can be related to hydrographic features (sigma-θ): 1. 1. Intermediate water originating from the Red Sea lost its dissolved O 2 while flowing northward along the Omani coast and exhibits a strong Mn maximum (4.6-6.5 nM) coincident with the deep O 2 minimum. 2. 2. At the two inshore stations in the Gulf of Oman this is overlain by relatively modest Mn maxima (±2.7 nM) related to Arabian Gulf overflow water. 3. 3. Finally the strong Mn maxima (4.4-5.6 nM) in the oxygen minimum zone at the two offshore stations are related to yet another watermass. Below these various maxima, concentrations decrease gradually to values as low as 90 pM at 2000 meters depth. Towards the seafloor concentrations increase again, leading to a modest bottom water maximum (0.7-1.5 nM). The overall vertical distributions of Mn and Fe are strikingly similar, also in actual concentrations, to those previously reported for the eastern equatorial Pacific, an area also characterized by an extensive O 2-minimum zone.