Rainwater as a source of Fe(II)-stabilizing ligands to seawater

Rainwater hydrophobic extractable dissolved organic matter (EDOM) contains ligand(s) that completely prevent Fe(II) oxidation for at least 4 h after rain is mixed with seawater. The EDOM Fe(II) complex is at least of comparable strength to the ferrozine complex, indicating that this rainwater Fe(II) ligand is among the strongest Fe(II) ligands ever observed in natural waters. In addition to the strong class of ligands that prevent oxidation, rainwater EDOM also contains weaker Fe(II) ligands that slow oxidation of Fe(II) in seawater. Rainwater EDOM is not a single molecule but rather a complex mixture of relatively hydrophobic compounds, even in marine rain with minimal continental influence. When EDOM from the nearby Cape Fear River was extracted using the same method as for rainwater, the river EDOM could not prevent or even slow the oxidation of Fe(II) on mixing with seawater. Therefore, rainwater EDOM is fundamentally different than surface-water EDOM with respect to the strength of Fe(II) ligands. The stability of EDOM complexed Fe(II) most likely affects the bioavailability of rainwater-derived Fe in the surface ocean because the length of time atmospherically deposited Fe remains dissolved in seawater is critical to its role as a phytoplankton nutrient. There is tremendous interest in the distribution and cycling of iron in seawater because of its role as a limiting nutrient in certain regions of the world’s oceans. The speciation of iron in seawater is of particular interest because speciation determines stability and residence time and hence affects bioavailability. Unusually high and unexplained concentrations of Fe(II) are occasionally observed in seawater even though many experiments and calculations indicate that Fe(II) should be quickly oxidized by hydrogen peroxide to Fe(III) at the pH of seawater (Moffett and Zika 1987; Millero and Sotolongo 1989). Nannomolar concentrations of Fe(II) were observed during spring plankton blooms several years in a row in oxic suface waters near Japan, perhaps produced by photochemical reduction of Fe(III) transported from sediments (Kuma et al. 1996). Fe(II) was also observed in bottom waters near Peru (Hong and Kester 1986), near the chlorophyll maximum in the Equatorial Pacific (O’Sullivan et al. 1991), in the surface waters of the North Sea (Gledhill et al. 1995), in surface water in the Boston Harbor (Zhuang et al. 1995), and in coastal seawater around Okinawa Island (Okada et al. 2005). Fe(II) can be photochemically produced from Fe(III); however, not all these studies were in surface seawater. Unexpectedly high concentrations of Fe(II) have also been observed during iron enrichment experiments. Persistent (5 d) nmol L21 concentrations of Fe(II) after a fourth iron addition in the Southern Ocean Iron Release Experiment in February 1999 were reported, perhaps resulting from photochemical production from Fe(III), slow oxidation (cold temperatures 3uC), low concentrations of H2O2 (10 nmol L21), and possible organic complexation of Fe(II) (Croot et al. 2001). During another Southern Ocean iron addition experiment in the summer of 2000 (EisenEx), high levels of Fe(II) (tenths of nmol L21) were observed up to 8 d after iron additions (Croot et al. 2005). This was attributed to cold temperatures, limited vertical mixing, and reduction of Fe(III) by superoxide. The authors also suggested that the high Fe(II) concentrations might result from intense rainfall during the latter part of this experiment. They were able to collect one rain sample using trace metal clean conditions in which they determined that total dissolved iron (TFe) was 260 6 20 nmol L21. This high concentration was explained by back-trajectory calculations that indicated that the air mass traveled from southern Patagonia and hence probably contained ironrich dust. They reported a minimum rainwater concentration of 40 6 20 nmol L21 Fe(II). The rain event collected by Croot et al. (2005) during the EisenEx Experiment was obtained in early summer (27 November 2000) between 15:30 h and 18:00 h. The iron concentration in this precipitation is consistent with data generated in the summer of 1999 for rainwater collected on the South Island of New Zealand (Kieber et al. 2001a). For rains collected between noon and 18:00 h, Kieber et al. (2001a) found a volume-weighted average concentration of 240 nmol L21 for total iron and 62 nmol L21 for Fe(II), indicating that the sample collected by Croot et al. (2005) 1 yr later during EisenEx was representative of rain over the Southern Ocean during this season and time of day. Fe(II) concentrations that are higher than expected have therefore been observed in several marine environments by investigators working in different laboratories using various analytical techniques. Although analysis for Fe(II) 1 Corresponding author ([email protected]).