Iron in north-east Pacific waters

Abstract

Although Fe is an element of great biological¹ and geochemical² importance, little is known about its distribution in the sea. The reasons for this are: (1) contamination is extremely difficult to avoid during sampling and laboratory procedures, not only because of man's wide use of this element, but also because it is fourth most abundant element in the Earth's crust (5.63%)³; (2) the chemistry of Fe is very complex, and its form (or forms) in seawater is poorly known, hence whether one preconcentration technique will work for existing species is questionable. Iron also appears to be very insoluble⁴ in oxygenated ocean water, and most (90%)⁵ precipitates out in association with dissolved organics during estuarine mixing processes^5–8. Indeed, some argue that truly dissolved Fe does not exist in seawater and that the fraction found in filtrates is totally colloidal⁹. We have been attempting oceanic dissolved Fe measurements for the past four years and report here three vertical Fe profiles (Fig. 1) that have the following features in common: Fe is severely depleted (0.15–0.30 nmol kg⁻¹) in surface waters; Fe maxima (up to 2.6 nmol kg⁻¹) occur in association with oxygen minima; and, Fe levels appear to vary little in mid-depth waters (0.5–1.0 nmol kg⁻¹).