All Supporting information may be found in the online version of this article.

grl50221-sup-0001-2012GL054615text01.docWord document86KDetailed description of methods used for the research reported in the manuscript.
grl50221-sup-0002-2012054615ts01.docWord document73KTable presenting major and trace element composition of ship-board collected ash samples. EYJ 04 ash sample was collected from atmospheric deposits onboard the RRS Discovery on May 8, 2010. EYJ07 was collected from Iceland at an earlier stage of the summit eruption (17/04/2010). Comparison of the results for the EYJ 04 and EYJ 07 ash indicates that the iron concentration had not changed much over the course of the eruption. JA-1 and JA-2 are Japanese Andesite standards used to determine the accuracy. RSD is the relative standard deviation of the measured standards.
grl50221-sup-0003-2012GL054615fs01.jpgJPEG image938KFigure S1 presents real colour satellite images of movement of Eyjafjallajökull ash plume for the period just before and after our sampling immediately under the plume on May 8, 2010. The track of the vessel (RRS Discovery) on the images for May 6 and 8 is indicated by a red line.
grl50221-sup-0004-2012GL054615fs02.jpgJPEG image407KFigure S2 presents the Iron deposition at 63.1°N, 18.5°W (location sampled by RRS Discovery on 8 May, 2010) estimated from modelled volcanic ash deposition. Based on salt layer of 20 nm thickness. Black dot indicates occupation at location under ash plume.
grl50221-sup-0005-2012GL054615fs03a.JPGJPEG image140KFigure S3: a) Nitrate, b) phosphate, c) silicate present drawdown response in bioassay experiments to iron and ash additions, showing results for controls (in blue triangle), additions of 2 nM FeCL3 (in red circle), additions of 9 mg l-1 volcanic ash (in grey square), and additions of 9 mg l-1 volcanic ash + 2 nM FeCL3 (in black diamond). Shown are mean values (±1 standard error, n = 3).
grl50221-sup-0006-2012GL054615fs03b.JPGJPEG image152KSupporting information
grl50221-sup-0007-2012GL054615fs03c.JPGJPEG image143KSupporting information
grl50221-sup-0008-2012GL054615fs04.jpgJPEG image391KFigure S4 presents size distribution of particles (as % mass) in ash sample collected on the RRS Discovery on May 8, 2010.
grl50221-sup-0009-2012GL054615fs05.jpgJPEG image337KFigure S5 presents the relationship of settling velocities to ash grain size for particles collected at sea under the Eyjafjallajökull plume on May 8, 2010. Particle settling experiments were conducted in laboratory in Southampton at 10°C with seawater of salinity 35. The settling experiments with our collected ash suggested a size dependent particle residence time of 1.5 h – 2 day in the observed 30 m mixed layers, which is thus likely to be sufficient for the majority of metals to be leached out of the surface salt layer [Duggen et al., 2010], but insufficient for significant dissolution of the volcanic glass.
grl50221-sup-0010-2012GL054615fs06.jpgJPEG image387KFigure S6: Upper and lower bound estimates of iron deposition (nM) over course of eruption. a) Based on iron solubility of 0.042 %, b) based on a salt layer of thickness 90 nm estimated from modelled volcanic ash deposition.
grl50221-sup-0011-2012GL054615fs07.jpgJPEG image471KFigure S7 presents a) Fluorescence line height and b) aerosol optical thickness derived from MODIS satellite data, averaged over the region where iron deposition >0.2 nM. Thick black line is 2010; thin grey line is 2003-2009 mean; grey shading is standard deviation of 2003-2009. Black horizontal line marks period of volcanic eruption.

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