Fe(II) in coastal rainwater: Changing stability and concentrations

Abstract.

The concentration of Fe(II) has decreased more than 50% since 2001 in both winter and summer rain collected in Wilmington, North Carolina, USA. Storage experiments performed on rain collected during the summer of 2008 revealed rapid oxidation of Fe(II) by hydrogen peroxide with second order rate constants between 2 and 48 M⁻¹ s⁻¹. The rapid oxidation of Fe(II) in these samples is in marked contrast to the stability of Fe(II) observed in rain collected 5 to 10 years ago where there was no decrease in the concentration of Fe(II) after 24 hours storage, or 4 hours after mixing with pH 8.1 seawater. We suggest that the lower concentration of Fe(II) observed in summer and winter rainwater in 2008 compared to earlier Fe speciation data is a result of the increased lability of Fe(II) towards oxidation. Evidence suggests that the decreased stability and concentration of Fe(II) compared to a decade ago results from a lowering in the abundance of iron ligands in chromophoric dissolved organic matter (CDOM) in precipitation similar to what has been observed for bulk DOC in precipitation at this location over this time. The decreasing concentration and stability of Fe(II) in rain presented here could have serious implications for marine primary productivity because rain is an important source of this nutrient to open ocean surface seawater. Ultimately the changing speciation of Fe in rainwater may have important consequences for global carbon budgets and climate warming because it implies that there may be less removal of atmospheric CO₂ by marine phytoplankton.