Iron and manganese in the wake of the Kerguelen Islands (Southern Ocean)

Abstract

As part of the ANTARES 3/F-JGOFS cruise, the distributions of dissolved iron and manganese were measured in October 1995 in the north–east wake of the Kerguelen archipelago (48°40′–49°40′S, 68°70′–70°50′E), an area that shows high phytoplankton biomass (CZCS and SeaWiFS data) in the middle of the High Nutrient Low Chlorophyll (HNLC) Southern Ocean . The study area (about 25,000 km 2 ) comprised a branch of the Polar Front with Antarctic surface water (AASW) intruding northward, shouldering the shelf break of the Kerguelen Plateau. The coastal zone was clearly affected by material of lithogenic origin (riverine discharges, soil leaching by rain waters, aeolian inputs), as well as by inputs from the sediments (effluxes from the sediment–water interface, resuspension from the sediments), its near surface waters showing considerable enrichment in dissolved iron (5.3–12.6 nM) and in dissolved manganese (2.9–8.6 nM). The offshore waters, although less enriched in trace-metals, were also affected by trace-metal inputs from coastal and continental shelf origin. Dissolved iron and manganese concentrations in these waters were 0.46–0.71 and 0.68–1.3 nM, i.e. far over typical antarctic open ocean surface water concentrations of 0.16 nM for iron [Martin, J.H., Gordon, R.M., Fitzwater, S.E., 1990. Iron in Antarctic waters. Nature, 345: 156–158.] and around 0.1 nM for manganese [Martin, J.H., Gordon, R.M., Fitzwater, S.E., 1990. Iron in Antarctic waters. Nature, 345: 156–158; Sedwick, P.N., Edwards, P.R., Mackey, D.J., Griffiths, F.B., Parslow, J.S., 1997. Iron and manganese in surface waters of the Australian subantarctic region . Deep-Sea Res., 44: 1239–1253.]. The dissolved iron enrichment in coastal waters of the Kerguelen Islands is much more important (about 10 times for dissolved iron) than for the Galapagos Islands, another oasis in the HNLC Equatorial oceanic system, where the concentration increase in dissolved iron in the surface waters around the islands is mostly driven by upwelling of the Equatorial Under Current (EUC) as it reaches the Galapagos Platform.