Spatial and temporal distribution of Fe(II) and H2O2 during EisenEx, an open ocean mesoscale iron enrichment

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 cm2 s&#8722;1 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&#8722; 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

Determination of superoxide in seawater using 2-methyl-6-(4-methoxyphenyl)-3,7- dihydroimidazo[1,2-a]pyrazin-3(7H)-one chemiluminescence

Superoxide, the one-electron reduced form of dioxygen, is known to be generated in marine environments by photochemical and biological processes. Because of its selective reaction with only a few commonly occurring compounds, superoxide is expected to approach concentrations in the high picomolar or low nanomolar range in seawater. Most currently existing methods do not have both the necessary sensitivity and selectivity to measure naturally occurring concentrations. In contrast, we demonstrate here that the chemiluminescence reagent 2-methyl-6-(4-methoxyphenyl)-3,7-dihydroimidazo[l,2-a]pyrazin-3(7H)-one (MCLA) is selective for superoxide in seawater and can be used with a detection limit of around 50 pM. Although a wide range of potential interferences were shown not to react with MCLA directly, some care must be taken when analyzing samples containing nanomolar concentrations of Fe(II), Cu(I), Mo(V), V(III), or V(IV), since these compounds can react with oxygen to produce superoxide during analysis that is subsequently detected. We describe two methods for calibrating the system, one employing photochemically generated superoxide standards and the other employing the superoxide-generating xanthine/xanthine oxidase system and discuss limitations on the use of each. The method was successfully used in the field to determine steady-state superoxide concentrations in the water column in the eastern equatorial Pacific Ocean