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Iron solubility driven by speciation in dust sources to the ocean

An Erratum to this article was published on 21 May 2009

Abstract

Although abundant in the Earth’s crust, iron is present at trace concentrations in sea water and is a limiting nutrient for phytoplankton in approximately 40% of the ocean1,2. Current literature suggests that aerosols are the primary external source of iron to offshore waters, yet controls on iron aerosol solubility remain unclear3,4. Here we demonstrate that iron speciation (oxidation state and bonding environment) drives iron solubility in arid region soils, glacial weathering products (flour) and oil combustion products (oil fly ash). Iron speciation varies by aerosol source, with soils in arid regions dominated by ferric (oxy)hydroxides, glacial flour by primary and secondary ferrous silicates and oil fly ash by ferric sulphate salts. Variation in iron speciation produces systematic differences in iron solubility: less than 1% of the iron in arid soils was soluble, compared with 2–3% in glacial products and 77–81% in oil combustion products, which is directly linked to fractions of more soluble phases. We conclude that spatial and temporal variations in aerosol iron speciation, driven by the distribution of deserts, glaciers and fossil-fuel combustion, could have a pronounced effect on aerosol iron solubility and therefore on biological productivity and the carbon cycle in the ocean.

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Figure 1: Cumulative Fe solubility of successive leaches of sediment samples with 250 ml of MilliQ water.
Figure 2: Iron speciation of aerosol source samples used in this study.
Figure 3: Representative EXAFS spectra and theoretical fits used to determine Fe species partitioning in aerosol source materials.

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Acknowledgements

This project was financially supported as part of a Mendenhall Postdoctoral Fellowship awarded to A.W.S. by the US Geological Survey (USGS) and financial support to A.W.S. and J.C. by the Coastal and Marine Geology and Earth Surface Dynamics Programs of the USGS. E.R.S. was supported by NSF. Synchrotron work was done on beamline 11-2 at Stanford Synchrotron Radiation Laboratory (SSRL): a national user facility operated by Stanford University on behalf of the Department of Energy, Office of Basic Sciences. We thank SSRL staff for their help, particularly J. Bargar and J. Rogers, as well as S. Birdwhistell of the WHOI Plasma Facility. Discussions with A. Aguilar-Islas, K. Bruland, J. Bratton, A. Kolker, P. Lam and O. Rouxel improved this manuscript. We thank B. Puecker-Ehrenbrink and M. Seggiani for providing us with samples of Chinese loess and oil fly ash, respectively.

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Contributions

A.W.S., J.C. and E.R.S. designed the study. A.W.S. and J.C. conducted the leaching experiments. A.W.S. interpreted the leach data with input from J.C. and E.R.S. A.W.S. and B.C.B. collected and interpreted XAS spectra. A.W.S. wrote the manuscript with valuable input from all authors.

Corresponding author

Correspondence to Andrew W. Schroth.

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Schroth, A., Crusius, J., Sholkovitz, E. et al. Iron solubility driven by speciation in dust sources to the ocean. Nature Geosci 2, 337–340 (2009). https://doi.org/10.1038/ngeo501

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