Iron isotopes suggest significant aerosol dissolution over the Pacific Ocean

This study presents aerosol iron isotopic compositions (δ56Fe) in the western and central equatorial and tropical Pacific Ocean. Aerosols supply iron (Fe), a critical element for marine primary production, to the open ocean. Particulate aerosols, > 1 µm, were sampled during the EUCFe (Equatorial Undercurrent Fe) cruise (RV Kilo Moana, PI: James W. Murray, 2006). One aerosol sample was isotopically lighter than the crust (δ56Fe = −0.16 ± 0.07 ‰, 95 % confidence interval), possibly originating from combustion processes. The nine other aerosol samples were isotopically heavier than the crust, with a rather homogeneous signature of +0.31 ± 0.21 ‰ (2 SD, n= 9). Given (i) this homogeneity compared to the diversity of their modeled geographic origin and (ii) the values of the Fe/Ti ratios used as a lithogenic tracer, we suggest that these heavy δ56Fe signatures reflect isotopic fractionation of crustal aerosols caused by atmospheric processes. Using a fractionation factor of Δsolution-particle= −1.8 ‰, a partial dissolution of ≈ 13 % of the initial aerosol iron content, followed by the removal of this dissolved fraction, would explain the observed slightly heavy Fe isotope signatures. Such fractionation has been observed previously in laboratory experiments but never before in a natural environment. The removal of the dissolved fraction of the aerosols has not been previously documented either. This work illustrates the strong constraints provided by the use of iron isotopes for atmospheric process studies.