Tracing anthropogenic aerosol Fe sources in the North Atlantic Ocean using dissolved Fe isotope ratios

Tim M Conway, University of South Florida, Rachel Shelley, LEMAR/UBO, Plouzané, France; Florida State University, Department of Earth, Ocean, and Atmospheric Science, Tallahassee, FL, United States, Ana M Aguilar-Islas, University of Arkansas, Fayetteville, AR, United States, William M Landing, Florida State University, Earth, Ocean, and Atmospheric Science, Tallahassee, FL, United States, Natalie M Mahowald, Cornell University, Department of Earth and Atmospheric Sciences, Ithaca, NY, United States and Seth John, University of South Carolina, Columbia, SC, United States; University of Southern California, Earth Sciences, Los Angeles, CA, United States
Abstract:
Supply of iron (Fe) to the surface ocean from atmospheric deposition plays a vital role in marine biogeochemical cycles, especially in Fe-limited areas or regions close to dust sources. However, large uncertainties remain over the fluxes, solubility and bioavailability of Fe supplied by aerosol dust. Additionally, aerosol Fe is likely to consist of a mixture of natural and anthropogenic (urban, biomass burning and combustion) components, which may have very different solubilities in seawater [e.g. 1]. To constrain soluble Fe supply to the oceans, it is thus vitally important to understand the relative contributions of different types of aerosol Fe, their solubilities and spatial distributions. Stable Fe isotopes (δ56Fe) may offer a way to discriminate between different dust sources [2], because of differential fractionation during a range of chemical processes. In this study, we measured δ56Fe in North Atlantic marine aerosols collected during two US GEOTRACES GA03 cruises (Lisbon to Woods Hole via Cape Verde, 2010-11) and we present δ56Fe measurements (relative to IRMM-014) from both the bulk aerosol (HF-HNO3 digested) and the water-soluble (10s ultrapure water leach) fractions. Aerosols collected from different air-masses (Saharan, European and N. American) allowed us to investigate the variability in δ56Fe due to different regional dust sources. The bulk phase was characterized by near-crustal δ56Fe values of +0.1±0.2‰, indicating the dominance of mineral dust. In contrast, the water-soluble fraction showed great variability; aerosols from European and North American air-masses were very isotopically light (-1.2±0.2‰ and -1.1±0.7‰) while those from Saharan air-masses were crustal (+0.1‰). Comparison of this data with isotope-informed model predictions of soluble Fe from mineral and anthropogenic sources (combustion, biofuels and biomass burning) [1], suggests that the data is consistent with mixing of either 1) Fe from mineral dust (+0.1‰) and a distinctly light anthropogenic Fe (-1.6‰), or 2) Fe from crustal mineral dust, isotopically heavy combustion Fe (+0.1 to +0.3‰), and biomass Fe that is both isotopically light (-1.6‰) and very soluble (>50%).

[1] Luo, C. et al. (2008), Glob. Biogeochem. Cyc., 22, GB1012.

[2] Mead, C. et al. (2013), Geophys. Res. Lett., 40, 5722-5727.

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