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

Abstract

Seawater dissolved iron isotope ratios (δ56Fe) have been measured in the North Atlantic near Bermuda. In a full‐depth profile, seawater dissolvedδ56Fe is isotopically heavy compared to crustal values throughout the water column (δ56FeIRMM‐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 δ56Fe increases below this to a maximum of +0.71‰ at 2500 m, decreasing again to +0.35‰ at 4200 m. The δ56Fe profile is very different from the iron concentration profile; in the upper water column [Fe] is variable while δ56Fe is relatively constant, and in the deeper water column δ56Fe varies while [Fe] remains relatively constant. The δ56Fe 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 betweenδ56Fe 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.