Iron (Fe)

Atomic number:	26
Atomic weight:	55.847
Average concentration in ocean:	540 pmol/kg
Residence time:	200-500 yrs

Distribution in ocean
Johnson et al. (1997a, b) have reviewed the distribution of dissolved iron in seawater using the MLML data set. Iron has a nutrient-like vertical profile with concentrations depleted at the surface (mean surface concentration 70 pmol/kg) (data). Concentrations rise with depth, generally forming a mid-depth maximum in many of the stations. Despite the typical nutrient-like distribution, iron distributions are anomalous for two reasons. There is little or no inter-ocean fractionation (a difference between deep Atlantic and deep Pacific concentrations) as seen for the distributions of other nutrient-like elements (see Zn profile). Elements with very short residence times, such as iron has (Bruland et al., 1994), typically do not form nutrient-like profiles. They are enriched at the surface and depleted at depth (see Al profile). External inputs may have a large impact on the distribution of elements with short residence times. Concentrations of iron may be greatly increased in coastal systems above any values found in open ocean systems when upwelling brings the iron-enriched benthic boundary layer to the surface.

Speciation
The chemical speciation of iron may explain much of its anomalous behaviour. Iron is stable as the +III oxidation state in oxic seawater as Fe(OH)₂^-. This form of iron is very insoluble and concenrations should be less than 200 pmol/kg (Millero, 1998). However, several studies (e.g. Rue and Bruland, 1995) have shown that an organic ligand, present in open ocean seawater at concentrations near 0.6 nmol/L, binds iron very strongly. This ligand may prevent iron from being lost to the sediments and it must act to set the mean concentration of iron at a level much higher than the solubility (Johnson et al., 1997a).

Residence time
The riverine input cannot easily be used to estimate residence time because much of the dissolved iron in rivers is lost in estuaries. Estimates of the residence time are derived from estimates of the rate at which dissolved iron is scavenged onto sinking particles (see Johnson et al., 1997a for summary).

Analysis
Dissolved Fe has typically been measured by an organic extraction to preconcentrate metals and separate them from the seasalt matrix, followed by graphite furnace atomic absorption spectrophotometry to quantify the Fe in the preconcentrated solution. This work is extremely demanding because of the potential for iron contamination at many of the steps. Recent work has shown that iron can be measured by flow injection analysis with chemiluminescence or kinetic spectrophotometric detection with much less chance of contamination (Elrod et al., 1992; Obata et al., 1993; Measures et al., 1995)

References