ON THE NANOGEOCHEMISTRY AND SIZE FRACTIONATION OF IRON AND COUPLED TRACE ELEMENTS IN THE LAND-OCEAN INTERACTION ZONE

Iron is one of the most abundant elements in the earth crust and large fluvial fluxes of iron reach the worlds river estuaries as both dissolved, colloidal and particulate fractions. Dissolved inorganic iron (III) has a very low solubility in both freshwater but especially in seawater with its higher pH. River waters with high concentrations of natural organic matter can sustain substantial levels of colloidal iron, both as stabilized iron oxide colloids and also as iron complexed to colloidal organic matter. It is also known that colloidal iron is largely transferred to settling particles in esuarine mixing and that concenquently the iron level is as low as sub nanomolar levels in the open ocean waters.

It has been shown that iron is actually the limiting micronutrient for primary production in large parts of the global ocean. The speciation and size fractionation of iron in these waters and possible feedbacks from biology is not yet understood, and further studies are hampered by the picomolar low concentrations of species and fractions and logistical problems of sampling and analysis.

Therefore we have carried out seasonal studies with high temporal and depth resolution in a coastal fjord with slightly higher iron concentrations and easy logistical access. The methods used were metal analysis of unfiltered and filtered fractions as well as size fractionated analysis by field flow fractionation coupled to ICP-MS. The physicochemical characterization of the nanomaterials were complemented by atomic force microscopy determine sizes and shapes of the nanomaterials.

The colloidal size spectra of the elements, and the atomic force microscopy images, revealed that three to four classes of colloids were present in the samples, differing in size, shape, and element-binding properties. Small (0.5–3 nm) spherical macromolecules of chromophoric dissolved organic matter, binding most elements, occurred at all depths and on all sampling occasions. Populations of 3–7-nm globular or slightly elongated colloids, binding Cu, Ag, and Pb, and 7–40-nm fibrillar colloids, binding Fe and Pb, appeared in the upper mixed-water layer during late spring and summer, thereby changing the colloidal size spectra of associated elements.