The vertical distribution of iron stable isotopes in the North Atlantic near Bermuda

Seawater dissolved iron isotope ratios (delta Fe-56) have been measured in the North Atlantic near Bermuda. In a full-depth profile, seawater dissolved delta Fe-56 is isotopically heavy compared to crustal values throughout the water column (delta Fe-56(IRMM-014) = +0.30 parts per thousand to +0.71 parts per thousand). Iron isotope ratios are relatively homogenous in the upper water column (between +0.30 parts per thousand to +0.45 parts per thousand above 1500 m), and delta Fe-56 increases below this to a maximum of +0.71 parts per thousand at 2500 m, decreasing again to +0.35 parts per thousand at 4200 m. The delta Fe-56 profile is very different from the iron concentration profile; in the upper water column [Fe] is variable while delta Fe-56 is relatively constant, and in the deeper water column delta Fe-56 varies while [Fe] remains relatively constant. The delta Fe-56 profile is also not well correlated with other hydrographic tracers in the North Atlantic such as temperature, salinity, or the concentrations of oxygen, phosphate, silica, and CFC-11. The dissimilarity between delta Fe-56 profiles and profiles of [Fe] and other hydrographic tracers shows that Fe isotope ratios provide a unique sort of information about ocean chemistry, and they suggest that Fe isotopes may therefore be a valuable new tool for tracing the global sources, sinks, and biogeochemical cycling of Fe.