Fe and H 2O 2 distributions in the upper water column in the Indian sector of the Southern Ocean

Abstract

Total dissolvable iron (TDFe), particulate iron (PFe) and hydrogen peroxide (H 2O 2 measurements were performed along a N-S transect in the upper 250 m in the Southern Ocean (62°00E/66°42S–49°00S, ANTARES II cruise, February 1994). TDFe was organically extracted (APDC/DDDC-chloroform) and analysed by Graphite Furnace Atomic Absorption Spectrometry (GFAAS), PFe was analysed by GFAAS following a strong mixed-acid leach, and H 2O 2 was analysed on board by fluorometry. The respective detection limits are equal to 0.13 nmol/kg, 0.02 nmol/kg, and 3.0 nmol/kg. TDFe concentrations vary from 0.4 to 6.2 nmol/kg and profiles are not completely depleted in the surface. PFe concentrations vary from 0.02 to 0.2 nmol/kg. Iron/carbon (Fe/C) uptake ratios for phytoplankton were calculated either from seawater or particle measurements. They are variable along the transect but are consistent when they could be compared. All the observed ratios are within the range of values proposed for the Fe/C uptake ratios by phytoplankton. Using our uptake ratio calculated in the Permanent Open Ocean Zone (4 · 10 −6 mol/mol), we estimate that the primary production which can be supported by the iron input flux into the surface waters is two times higher than the measured primary production in the same area. In the surface waters, H 2O 2 concentrations vary from 5.0 to 19.7 nmol/kg. Such low concentrations are due to strong vertical mixing, low dissolved organic matter concentrations and the latitude of the site.