Description: Measurements of Fe(II) and H2O2 were carried out in the Atlantic sector of the Southern Ocean during EisenEx, an iron enrichment experiment. Iron was added on three separate occasions, approximately every 8 days, as a ferrous sulfate (FeSO4) solution. Vertical profiles of Fe(II) showed maxima consistent with the plume of the iron infusion. While H2O2 profiles revealed a corresponding minima showing the effect of oxidation of Fe(II) by H2O2, observations showed detectable Fe(II) concentrations existed for up to 8 days after an iron infusion. H2O2 concentrations increased at the depth of the chlorophyll maximum when iron concentrations returned to pre-infusion concentrations (〈80 pM) possibly due to biological production related to iron reductase activity. In this work, Fe(II) and dissolved iron were used as tracers themselves for subsequent iron infusions when no further SF6 was added. EisenEx was subject to periods of weak and strong mixing. Slow mixing after the second infusion allowed significant concentrations of Fe(II) and Fe to exist for several days. During this time, dissolved and total iron in the infusion plume behaved almost conservatively as it was trapped between a relict mixed layer and a new rain-induced mixed layer. Using dissolved iron, a value for the vertical diffusion coefficient Kz=6.7±0.7 cm**2/s was obtained for this 2-day period. During a subsequent surface survey of the iron-enriched patch, elevated levels of Fe(II) were found in surface waters presumably from Fe(II) dissolved in the rainwater that was falling at this time. Model results suggest that the reaction between uncomplexed Fe(III) and O2? was a significant source of Fe(II) during EisenEx and helped to maintain high levels of Fe(II) in the water column. This phenomenon may occur in iron enrichment experiments when two conditions are met: (i) When Fe is added to a system already saturated with regard to organic complexation and (ii) when mixing processes are slow, thereby reducing the dispersion of iron into under-saturated waters.

Description: Here we present the concentrations of inorganic nutrients, dissolved organic carbon, nitrogen and phophorus and dissolved inorganic carbon (DIC) from discrete water samples collected with a CTD-rosette during the European Iron Fertilization Experiment (EIFEX). The experiment was carried out from February 11 to March 20, 2004 in the 60-km diameter, rotating core of an eddy, formed by a meander of the Antarctic Polar Front (centred at around 49°10' S and 2°10' E). Samples were taken within the eddy inside and outside the fertilized patch, and in a few cases outside the eddy. Inorganic nutrients (silicate, phosphate, nitrate, nitrite and ammonium) were measured with a Technicon Autoanalyser II system using standard methods. Dissolved organic carbon (DOC) was determined by high temperature combustion using a TOC-VCPH/CPN (Shimadzu) according to Skoog et al. (1997). Dissolved organic nitrogen (DON) was measured on an Evolution continuous flow analyser (Alliance Instruments) after Valderrama (1981). Dissolved inorganic carbon was measured by coulometric titration (Johnson et al., 1987) using a SOMMA system with gas loop calibration with a reproducibility of 2 mmol/kg. DIC was calibrated against certified reference materials from Andrew Dickson at Scripps Institution of Oceanography (SIO).