Iron speciation in coastal rainwater: concentration and deposition to seawater

More than half of the dissolved iron in rain collected in Wilmington, NC, U SA, occurred as Fe(II)(aq). More than 80% of the dissolved iron in marine r ain from several marine storms in both North Carolina and New Zealand was F e(II)(aq). In almost all rain events Fe(II)(aq) was in excess of Fe(III)(aq ). Rainwater is a significant source of iron to surface seawater and contri butes approximately 10(10) mol year(-1) of dissolved plus particulate iron to surface seawater on a global scale, which is more than 30 times the amou nt of iron resident in the surface 10 m of seawater. The length of time atm ospherically deposited dissolved iron remains in surface seawater is critic al to its role as a phytoplankton nutrient because it is predominately the soluble form of Fe that is bioavailable. Earlier studies have demonstrated that Fe(II)(aq) oxidizes rapidly in seawater. Our experiments utilizing aut hentic rainwater with ambient concentrations and speciation of iron clearly demonstrate, however, that rainwater Fe(II)(aq) is stabilized against oxid ation for more than 4 h in seawater and rainwater Fe(III)(aq) is protected against rapid precipitation when added to coastal or oligotrophic seawater. These results are significant because they show rainwater deposited Fe doe s not behave as previously thought based on earlier kinetic work on non-rai nwater Fe(II) oxidation in seawater. Rainwater, therefore, is an important source of soluble, stable Fe(II)(aq) to surface seawater. (C) 2001 Elsevier Science B.V. All rights reserved.