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

[1] Seawater dissolved iron isotope ratios (dFe) have been measured in the North Atlantic near Bermuda. In a full-depth profile, seawater dissolved dFe is isotopically heavy compared to crustal values throughout the water column (dFeIRMM-014 = +0.30‰ to +0.71‰). Iron isotope ratios are relatively homogenous in the upper water column (between +0.30‰ to +0.45‰ above 1500 m), and dFe increases below this to a maximum of +0.71‰ at 2500 m, decreasing again to +0.35‰ at 4200 m. The dFe profile is very different from the iron concentration profile; in the upper water column [Fe] is variable while dFe is relatively constant, and in the deeper water column dFe varies while [Fe] remains relatively constant. The dFe 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 dFe 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.