Temporal variability of rainwater iron speciation at the Bermuda Atlantic Time Series Station

Abstract

Diurnal and seasonal variations in rainwater iron speciation were investigated during a summer and winter cruise aboard the RV Endeavor at the Bermuda Atlantic Time Series Station. Concentrations of total Fe, Fe(II), and Fe(III) were all higher in rainwater falling at BATS during the summer relative to winter cruise. Iron speciation displayed marked diel variability with a 200-fold higher Fe(II)(aq)/Fe(III)(aq) ratio in rain received between 1200 and 1800 local time (LT) relative to late evening or early morning summer rain. The higher Fe(II)/Fe(III) ratio in the afternoon indicates Fe(II) is produced at the expense of Fe(III) during the course of the day, suggesting photochemical processes are important in controlling iron speciation in marine rains falling over the open ocean. Experiments utilizing authentic rainwater collected at BATS during both summer and winter cruises demonstrate that rainwater Fe(II)(aq) is stabilized against oxidation and precipitation for more than 4 hours after mixing with BATS seawater. Preliminary results suggest this stabilization is the result of organic complexation of Fe in rainwater. Owing to the isolation of BATS from riverine sources of Fe, rainwater is the predominant source of soluble and stable Fe(II) in this region of the North Atlantic.