Iron analysis in bottle profiles off Mauritania

Iron solubility measurements in the Mauritanian upwelling and the adjacent Open Ocean of the Tropical Atlantic show for all stations lower values in the surface mixed layer than at depth below the pycnocline. We attribute this distribution to a combination of loss terms, chiefly photo-oxidation of organic ligands in the surface, and supply terms, predominantly from the release of ligands from the decomposition of organic matter. Significant correlations with pH, oxygen and phosphate for all samples below the surface mixed layer indicate that biogenic remineralisation of organic matter results in the release of iron binding ligands into the dissolved phase. The comparison of the cFeS/PO4**3- ratio with other published data from intermediate and deep waters in the Pacific suggests an enhanced release of iron chelators in the more productive Mauritanian upwelling zone.

Iron in the Sargasso Sea

We report iron measurements for water column and aerosol samples collected in the Sargasso Sea during July-August 2003 (summer 2003) and April-May 2004 (spring 2004). Our data reveal a large seasonal change in the dissolved iron (dFe) concentration of surface waters in the Bermuda Atlantic Time-series Study region, from ~1-2 nM in summer 2003, when aerosol iron concentrations were high (mean 10 nmol/m**3), to ~0.1-0.2 nM in spring 2004, when aerosol iron concentrations were low (mean 0.64 nmol/m**3). During summer 2003, we observed an increase of ~0.6 nM in surface water dFe concentrations over 13 days, presumably due to eolian iron input; an estimate of total iron deposition over this same period suggests an effective solubility of 3-30% for aerosol iron. Our summer 2003 water column profiles show potentially growth-limiting dFe concentrations (0.02-0.19 nM) coinciding with a deep chlorophyll maximum at 100-150 m depth, where phytoplankton biomass is typically dominated by Prochlorococcus during late summer.

Iron and aluminium and hydrogen peroxide measured on water bottle samples during Marion Dufresne cruise ANTARES-II

Total dissolvable iron (TDFe), particulate iron (PFe) and hydrogen peroxide (H2O2 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 H2O2 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 x 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, H2O2 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.