The trace element composition of suspended particulate matter in the upper 1000 m of the eastern North Atlantic Ocean: A16N

Marine Chemistry 142–144 (2012) 41–53

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The trace element composition of suspended particulate matter in the upper 1000 m of the eastern North Atlantic Ocean: A16N Pamela M. Barrett a, b,⁎, Joseph A. Resing b, Nathaniel J. Buck b, Clifton S. Buck c, William M. Landing c, Christopher I. Measures d a

School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195, USA Joint Institute for the Study of the Atmosphere and Ocean, University of Washington PMEL/NOAA, 7600 Sand Point Way NE, Seattle, WA 98115, USA Department of Earth, Ocean, and Atmospheric Science, Florida State University, 117 N. Woodward Avenue, Tallahassee, FL 32306, USA d Department of Oceanography, University of Hawai'i at Manoa, 1000 Pope Road, Marine Sciences Building, Honolulu, HI 96822, USA b c

a r t i c l e

i n f o

Article history: Received 12 April 2012 Received in revised form 27 July 2012 Accepted 30 July 2012 Available online 4 August 2012 Keywords: North Atlantic Ocean Trace elements Iron Aluminum Suspended particulate matter Aerosols Pollution effects

a b s t r a c t Samples of total suspended matter were collected from the upper 1000 m of the eastern North Atlantic between 62°N and 5°S during the CLIVAR/CO2 Repeat Hydrography section A16N from June to August 2003. Particulate matter samples were analyzed by energy-dispersive X-ray fluorescence for Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, and Pb. Intense seasonal deposition of Saharan dust produces maxima in particulate Fe (>3.3 nM) and Al (>10 nM) in surface waters between 10 and 20°N. A broad mid-depth enrichment of particulate Fe (>5.4 nM) and Al (>19 nM) between the equator and 20°N is sustained by vertical transport of lithogenic particles and scavenging of dissolved Fe released by remineralization. Surface distributions of particulate Fe and Al show maxima over a narrower, northerly shifted latitude range and are consistent with the seasonal location of atmospheric deposition associated with the Intertropical Convergence Zone, while the location of the mid-depth maximum reflects the full annual latitude range of surface inputs and suggests similar winter and summer atmospheric fluxes. Spatial offsets between surface maxima in particulate and dissolved Al distributions indicate relatively short residence times (8 days and b 1 year, respectively) for both phases of Al in the equatorial Atlantic, and suggest that temporal sampling biases could have significant effects in models of dust deposition and surface-ocean chemistry. Efficient scavenging of dissolved Al by biogenic particles following the spring bloom in subpolar latitudes results in elevated mixed-layer particulate Al concentrations despite low aeolian inputs. A subsurface minimum in particulate Fe and Al concentrations at 50–200 m throughout the transect likely results from efficient transport of lithogenic particles out of the surface layer by aggregation into large organic aggregates. Relative depletion of Fe in suspended particulate matter is observed in vertical profiles coincident with maxima in fluorescence and biogenic particle concentrations. At these depths, dissolved Fe increases from ~10–30% to 50–70% of the total Fe pool, suggesting a biological influence on the partitioning of Fe between particulate and dissolved forms. Metal-to-Al ratios indicate major anthropogenic sources for Cr, Ni, Cu, Zn, and Pb inputs to the surface ocean at latitudes outside of the low-latitude Saharan dust plume. Increased aerosol-Fe solubility in these regions likely contributes to relatively depleted Fe:Al ratios in surface-ocean particulates. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Iron has been shown to limit primary production in high-nutrient, low-chlorophyll regions of the world ocean, playing an important role in the carbon cycle and climate feedbacks (Boyd et al., 2007; Martin, 1990). Improved understanding of the sources and distribution of iron delivered to the open ocean, which are expected to shift with changing global climate and increasing emissions of anthropogenic pollutants

⁎ Corresponding author at: Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, WA 98115, USA. Tel.: +1 206 526 6452; fax: +1 206 526 6054. E-mail addresses: [email protected] (P.M. Barrett), [email protected] (J.A. Resing), [email protected] (N.J. Buck), [email protected] (C.S. Buck), [email protected] (W.M. Landing), [email protected] (C.I. Measures). 0304-4203/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.marchem.2012.07.006

(Mahowald et al., 2009; Sholkovitz et al., 2009) will better constrain biogeochemical models. In areas of open ocean, away from coastal environments and regions of intense upwelling, the deposition of atmospheric aerosols typically represents a significant, if not dominant, source of iron to the surface ocean (Duce and Tindale, 1991). Although the magnitude is poorly constrained due to the logistical difficulty of direct measurement of long-term aerosol flux (Duce et al., 1991), the chemistry of dissolved and particulate matter in the surface ocean reflects an integrated imprint of atmospheric deposition and can be used to infer aerosol delivery. Relative to the dissolved phase, the residence time of particulate metals in the surface ocean is typically short. Thus, particulate distributions can provide a sense of mesoscale spatial or temporal variation in surface-ocean dynamics and are less influenced by

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P.M. Barrett et al. / Marine Chemistry 142–144 (2012) 41–53

advective processes when assessing local trace metal budgets. However, partitioning of trace metals between dissolved and particulate phases is specific to individual elements and dependent on both adsorptive properties and biological demand in surface waters. For some readily scavenged or biologically important trace metals, most notably Fe, the dissolved phase may have extremely short residence times and uniformly low concentrations in surface waters relative to the particulate phase. As a result, the availability of Fe and other biologically important trace metals to phytoplankton from atmospheric deposition is thought to be determined by the solubility of particles deposited on the surface ocean and the adsorptive/desorptive interactions of these metals with particles throughout the water column. Particle distributions in surface waters reflect both inputs and patterns of biological uptake, passive scavenging, and vertical transport, which largely control the removal of dissolved Fe and other trace metals. Thus, observations of the chemical composition and distribution of ocean particulates are important to our understandings of the sources and cycling of Fe and other trace metals in the oceans. A high-resolution dataset is presented for the trace metal composition of water column particulates in the eastern North Atlantic. Sampling was conducted along the CLIVAR/CO2 repeat hydrography A16N transect from June to August 2003 on the NOAA R/V Ronald H. Brown. The top 1000 m of the water column from 62°N to 5°S was sampled with a resolution of approximately 1–2°. Total suspended particulate samples were collected on 0.4 μm polycarbonate filters under trace metal-clean conditions and analyzed for concentrations of Al, Si, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, and Pb by energy-dispersive X-ray fluorescence. Distributions of dissolved Fe and Al and the chemical composition of bulk and size-fractionated aerosol samples were also analyzed and have been reported previously (Buck et al., 2010a,b; Measures et al., 2008a). The magnitude and source of particulate iron and other trace metal fluxes to the surface ocean in the North Atlantic are highly variable. In subpolar and mid-latitudes, aerosol fluxes are low and particles deposited on surface waters are heavily impacted by emissions from industrialized Europe and North America (Buck et al., 2010a). At low latitudes, prevailing winds over the African deserts carry significant loads of lithogenic material, representing up to 50% of global dust flux to the oceans (Mahowald et al., 1999). The distributions of particulate Fe and other trace metals reflect the diversity of environments across the North Atlantic and can be used to investigate various controls on the oceanic Fe budget. Particulate Al is considered as a proxy for the magnitude of lithogenic contributions to oceanic particulate loads while V, Pb, and other metals prevalent in anthropogenic pollutants can be used to assess anthropogenic impacts on surface-ocean chemistry. Aerosols and surface-ocean suspended matter are compared to assess the importance of aerosol deposition relative to other processes that contribute to the surface-ocean particulate Fe pool, including biological production and authigenic particle formation. Where aerosol-derived particulate Fe dominates, comparisons between aerosols and suspended matter are used to examine post-deposition compositional changes to particles and infer trends in the relative solubility of Fe. Finally, the availability of particulate Fe to biota is considered. 2. Methods 2.1. Sample collection Particulate samples were collected during the CLIVAR/CO2 Repeat Hydrography section A16N from Reykjavik, Iceland, to Natal, Brazil, from 20 June to 7 August, 2003 (Fig. 1). Seawater samples were collected from the surface ocean to depths of 750–1000 m at 60 stations, then sub-sampled for dissolved-phase trace elements and suspended particulate matter under trace-element-clean conditions (see Measures et al., 2008a,b for details). Briefly, suspended particulate matter samples were collected by pressurizing GO-FLO bottles with ≤10 psi filtered,

Fig. 1. Map of the trace metal particulate sampling stations along the CLIVAR/CO2 repeat hydrography section A16N cruise track 20 June–7 August 2003.

compressed air. Samples were filtered through acid-cleaned 0.4 μm polycarbonate filters (Nuclepore) in polypropylene holders. Acidcleaned backing filters of mixed cellulose esters were used to ensure even loading on sample filters. Samples were rinsed while on the filter holders with 15–20 mL deionized (DI) water adjusted to pH 8 with ammonium hydroxide from the edges to the center, with a low vacuum applied to avoid loss or re-distribution of particles. Filtration was started approximately 30 to 60 min after water samples were collected and was generally completed within 60 min. The relatively shallow cast depth and short time between sample collection and filtration should minimize particle settling, although this potential loss term is not quantified. The average filtration volume per sample was 8 L. 2.2. XRF analysis Particulate metal (PMe) concentrations in total suspended matter samples were determined by energy-dispersive X-ray fluorescence (EDXRF) using a thin film technique described by Feely et al. (1991) and employed in previous studies of the composition of marine particulate material (Feely et al., 1994, 1996, 1999; Resing et al., 2007). Analysis by EDXRF is non-destructive and requires little sample preparation, allowing for efficient quantification of a wide range of elements in large sample sets. An optical subsample of each filter was analyzed on a Thermo Fisher Quant'X equipped with a Rhodium Target X-ray tube and an electronically cooled, lithium-drifted solid state detector. X-rays for primary sample excitation were filtered for optimum control of peak-to-background ratios. Standards for calibration

P.M. Barrett et al. / Marine Chemistry 142–144 (2012) 41–53

consisted of commercial thin film standards (MicroMatter), geochemical reference sample material MAG-1 (Gladney and Roelandts, 1988) finely ground and loaded onto polycarbonate filters (Feely et al., 1991), and a series of standards prepared using a modification of the method reported by Holynska and Bisiniek (1976) using sodium diethyldithiocarbamate (NaDDTC) to quantitatively precipitate trace metals from a solution of known concentration. These standards were prepared using a 1% NaDDTC solution that was added to trace metal solutions at pH 4 with a ligand-to-metal ratio of 4:1. The resulting precipitate was filtered over an acid-cleaned, 0.4 μm polycarbonate Nuclepore filter. Analysis of the filtrate by graphite furnace atomic absorption spectrophotometry confirmed quantitative precipitation of trace metals. Procedural blanks were made by addition of a NaDDTC solution to a sample of acidified DI water followed by filtration. MicroMatter and MAG-1 standards were used to create individual standard curves for all elements; standard curves for Fe and Mn also included NaDDTC standards. Four different excitation conditions, all conducted under a vacuum atmosphere, were used for sample analysis and are detailed along with minimum determination limits (MDL) and method blanks for individual elements in Table 1. MDLs are defined as 3 times the square root of the background intensity measured from a standard of known concentration: 
 MDL ¼ 3  √Ib = Ip =conc

ð1Þ

where Ib is the background intensity, Ip is the peak intensity, and conc is the concentration of the standard. MicroMatter standards were used to calculate MDLs. Method-blank values for individual elements were distinguishable from MDLs for Al, Ca, Fe, and Ni only. Reference material NIST SRM 2783, air particulate on filter media, is analyzed on a monthly basis; recoveries for individual elements are shown in Table 2. Additionally, a subset of total suspended matter samples was analyzed by graphite furnace atomic absorption spectrophotometry after a microwave digestion procedure to confirm accuracy of the EDXRF analysis (Barrett, P.M., Buck, N.J., Resing, J.A., unpublished results). 3. Results and discussion The following discussion is organized into three main themes. First, the particulate Fe and Al (pFe and pAl) distributions are presented in detail and compared to the distributions of their dissolved phase counterparts (dFe and dAl) in an attempt to identify particle sources and Table 1 XRF excitation conditions, minimum determination limits (MDL), and method blank values (n=11) for elements of interest given as filter concentrations (ng cm−2) and equivalent seawater concentrations (nmol L−1) for the average sample filtration volume of 8 L. For a majority of elements, the method blank signal peak was indistinguishable from the background. Analyte

Al Si K Ca Ti V Cr Mn Fe Ni Cu Zn Pb

Limits of detection

Method blanks

(ng/cm2)

(nM)

(ng/cm2)

(nM)

9.4 4.61 3.15 3.99 2.52 2.41 1.39 1.27 0.95 0.77 1.25 1.28 1.58

0.54 0.25 0.12 0.15 0.08 0.07 0.04 0.04 0.03 0.02 0.03 0.03 0.01

11.04 – – 7.22 – – – – 2.11 – 1.41 – –

0.63 – – 0.28 – – – – 0.06 – 0.03 – –

Excitation conditions, 1° X-ray filter, voltage, and current Low Za, thin graphite, 10 kV, 1.98 mA Low Zb, thick graphite, 12 kV, 1.98 mA Mid Za, thin Pd, 30 kV, 1.66 mA

Mid Zb, thick Pd, 50 kV, 1 mA

43

Table 2 Results from monthly EDXRF analysis of reference material NIST SRM 2783, air particulate on filter media, comparing measured values and standard deviations to certified reference values for individual elements. Analyte

Certified value (ng cm

Al Si K Ca Ti V Cr Mn Fe Ni Cu Zn Pb

−2

2330.32 5883.53 530 1325.3 149 4.86 13.55 32.13 2660 6.82 40.5 179.72 31.8

)

Measured +/−1 SD

(ng cm−2)

+/−1 SD

% recovery

53 160 52 170 24 0.6 0.25 0.12 160 1.2 0.42 13 5.4

2149.65 5682.73 541.23 1338.65 163.8 BDL 14.75 33.51 2864.02 6.41 43.67 153.36 32.9

82.27 185.88 11.21 63.98 5.26 – 0.06 0.81 69.19 0.45 2 4.19 3.35

92 97 102 101 110 – 109 104 108 94 108 85 103

Notes: BDL = below detection limit.

calculate surface–ocean residence times in different oceanic regimes. Second, the distributions of trace elements Cr, Ni, Cu, Zn, and Pb are considered, for which aerosol inputs from anthropogenic sources prove to dominate over much of the transect. Finally, the distributions of biogenic particles are presented, which suggest increased mobilization of particulate Fe coincident with the subsurface chlorophyll maximum and rapid, biologically mediated transport of lithogenic particles out of the euphotic zone. 3.1. Distribution of particulate Fe The distribution of pFe in the upper 1000 m of the water column is shown in Fig. 2a. Three regions can be broadly defined by variation in the source and magnitude of Fe inputs as inferred from the particulate distribution. In the first, pFe is enriched at depth at high-latitude stations over the Icelandic continental shelf. In the second, low inputs of pFe are evident through the mid-latitudes, and in the third, robust seasonal dust deposition at low latitudes sustains high surface and subsurface pFe concentrations. The highest measured values for pFe along the entire transect are found in the deepest samples (>600 m) at 59°N and 62°N where concentrations reach 11.4 nM. The high pFe observed here likely results from sediment re-suspension and scavenging of dFe remobilized from sediments as bottom depth shoals close to the Icelandic shelf. Vertical profiles of suspended matter composition at these high-latitude stations show deep subsurface maxima in particulate Ti and Al (Fig. 3), which is consistent with re-suspension of sediments (Blomqvist and Larsson, 1994). These samples also have elevated pFe:pAl molar ratios (up to 0.6) compared to average crustal values (0.26) (Wedepohl, 1995) that can be attributed to scavenging of dFe (Fig. 3). The local maximum in dFe evident below 800 m at stations north of 60°N (Measures et al., 2008a) indicates an epibenthic flux of dFe produced by diagenetic processes in the sediments. The dFe pool remobilized from sediments includes both truly soluble (b0.02 μm) Fe that may be scavenged by particles in the overlying water column and colloidal (>0.02 to b0.4 μm) Fe subject to aggregation (Moore and Braucher, 2008), producing the pFe enrichment observed in suspended matter samples. The timing of sampling suggests that the release of sedimentary dFe may result from delivery of organic matter and onset of anoxic conditions following the spring bloom (Dehairs et al., 1989; Sundby et al., 1986). Throughout the mid-latitudes in the North Atlantic (25 to 60°N), pFe concentrations in surface waters are relatively low with an average concentration of 0.64 ± 0.39 nM, ranging from 0.29 to 1.71 nM. Concentrations of pFe in open ocean surface waters primarily reflect the magnitude of deposition of atmospheric aerosols on the surface ocean, and the low pFe observed here is consistent with the observed

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Fig. 2. Distribution of particulate Fe (a), dissolved Fe (b), particulate Al (c), and dissolved Al (d) in nmol L−1 for the upper 1000 m from 5°S to 62°N. Dissolved Fe and dissolved Al data are from Measures et al. (2008a). Sampling was restricted to the upper 750 m at stations between 27 and 62°N due to problems with the weight handling ability of the winch. Plotted using Ocean Data View (Schlitzer, 2010).

P.M. Barrett et al. / Marine Chemistry 142–144 (2012) 41–53

0

0

Al Fe Ti Mn

200

Depth (m)

200

Depth (m)

45

400

400

600 600

800 Al, Fe 0 Ti, Mn 0

5

10 1

15 2

20

25

3

800

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Fe:Al (mol/mol)

4

Fig. 3. Vertical profiles of particulate Ti, Fe, Mn, and Al concentrations in nmol L−1 (left) and particulate Fe:Al ratios (right) at 62°N. The dotted line in the right panel represents the average crustal Fe:Al ratio (Wedepohl, 1995). Error bars shown represent +/−3 standard deviations; where no error bar is shown, the error is contained within the symbol.

in this region, attributable to the release of Fe during the remineralization of organic matter below the photic zone (Measures et al., 2008a). The particle-reactive nature of Fe predicts that scavenging of this dFe pool by settling dust particles should contribute to pFe concentrations at depth. Measures et al. (2008a) found low Fe:N ratios in subsurface waters, indicating the preferential loss of dFe relative to N during remineralization, presumably the result of scavenging of dFe by settling particulate matter. The average Fe:Al ratio of suspended matter within this subsurface plume shows a slight enrichment (0.29 ± 0.02) over average crustal values, indicating that the subsurface pFe maximum is sustained principally from transport of dust particles out of the surface layer, but scavenging likely also contributes to elevated pFe values. Subsurface minima in pFe are observed at depths of 100 to 200 m throughout the transect. Concentrations typically decrease 0.1–0.5 nM between the surface and the mid-depth minimum; at low latitudes where surface concentrations are high due to dust deposition, the change in pFe concentration between the surface and pFe minimum was observed to be as great as 1–2 nM. This feature likely represents rapid transport of dust particles out of the surface layer by the

10 this study (Jun-Aug)

8

pFe (nM)

relatively weak aeolian Fe source (Buck et al., 2010a). In the northernmost latitudes (> 50°N), surface concentrations of pFe increase slightly, averaging 0.84 ± 0.55 nM, likely in part due to mixing with river discharge near the southern coast of Iceland (discussed below in Section 3.2). Surface water pFe increases significantly toward the equatorial Atlantic, where waters are heavily impacted by atmospheric deposition of dust from the African continent (Mahowald et al., 1999). A region of high surface concentrations (2.4–3.4 nM) extends from 9 to 18°N. South of this feature, surface pFe decreases sharply to 0.2–0.3 nM. Aerosol samples collected concurrently with water column sampling show peak flux of aerosol Fe from Saharan dust at latitudes between 9 and 23°N (Buck et al., 2010a), coincident with the location of the surface–ocean pFe maximum. The transport of Saharan dust across the Atlantic follows the seasonal migration of the Intertropical Convergence Zone (ITCZ), typically alternating between 0 and 10°N during the Northern Hemisphere winter and 10–20°N throughout the summer (Husar and Prospero, 1997). Hence, the location of the low-latitude, surface particulate maximum is consistent with the expected location of seasonal transport of Saharan dust during the July and August sampling dates. In Fig. 4, surface pFe concentrations from this study are plotted together with Fe data reported from the eastern North Atlantic in October and November of 2005 by Pohl et al. (2011). Surface concentrations of pFe from Pohl et al. (2011) were determined by taking the difference between unfiltered (total) and filtered (dissolved) Fe concentrations to infer pFe and likely represent a minimum estimate of the particulate fraction. Surface concentrations of pFe in mid-latitudes from Pohl et al. (2011) range from 0.1 to 1.2 nM (excluding negative values), comparable to the concentrations of 0.4 to 1.1 nM observed in this study. Maximum pFe concentrations at low latitudes are also of similar magnitude, but the seasonal shift in Saharan aerosol transport is evident from the more southerly location of the surface maximum in the Pohl et al. (2011) data compared to this study. A large subsurface maximum in pFe is observed between ~ 2 and 18°N from depths of ~ 150 to 1000 m, where pFe concentrations reach 5.5 nM. The subsurface pFe maximum would be expected to develop as dust particles deposited on the surface ocean are transported out of the surface layer through the water column. Disaggregation of large particles and release of small lithogenic particles from aggregates during degradation processes at depth are likely a source for suspended pFe between 200 and 1000 m. Elevated subsurface dFe is also observed

Pohl et al. (Oct-Nov)

6 4 2 0 -2 -5

0

5

10

15

20

25

30

35

40

45

50

Latitude ( N) Fig. 4. Comparison of surface pFe concentrations (nmol L−1) in the eastern North Atlantic during summer (this study) and fall (Pohl et al., 2011). Data from Pohl et al. (2011) represent the difference between filtered (dissolved Fe) and unfiltered (total Fe) to determine the particulate Fe fraction. The calculated pFe for some stations is negative and could be due to contamination during processing of the filtered samples (Pohl et al., 2011).

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incorporation of small lithogenic particles into larger aggregates. This process and the role of the biological processes in facilitating this transport are addressed further in the discussion of biogenic particulates in Section 3.5. 3.2. Distribution of particulate Al The distribution of pAl in the upper 1000 m of the water column (Fig. 2c) provides further insight into the sources and cycling of oceanic particulate matter. The three regions identified in the discussion of pFe distributions can also be used to understand the distribution of pAl. In high latitudes, local inputs from Icelandic rivers and shelf environments are the most prominent features in the pAl distribution. At mid-latitudes, weak aeolian inputs contribute little pAl to surface waters, but enhanced scavenging of dAl by biological particles results in elevated pAl in the mixed layer. As with Fe, substantial aerosol deposition dominates pAl distributions in low latitudes. North of 58°N, high pAl (up to 19.7 nM) in surface waters can be attributed to mixing with inputs from the glacial rivers of southern Iceland, which carry high sediment loads and are expected to be near peak sediment transport at the time of sampling (Lawler et al., 1992; Louvat et al., 2008). Intense surface maxima are also evident in the profiles of particulate Si, K, and Ca at stations closest to the Icelandic coast (Supplementary data). Aerosol samples collected at these latitudes reflect low dust conditions (Buck et al., 2010a), indicating that deposition of aerosol Al is minimal and metal concentrations likely reflect the influence of river drainage. Dissolved Fe and Al in these high-latitude surface waters are low (Measures et al., 2008a), confirming that while fluvial inputs contribute to the particulate metal budget, they are not an important source of dissolved metals to surface waters. The low dAl concentrations (2–3 nM) are likely maintained by intense scavenging of dAl by both particles delivered by coastal runoff and the high concentrations of biogenic particles in the productive coastal zone, a process which further elevates pAl concentrations. Low surface water dAl concentrations (0.2 to 2 nM) have previously been reported in offshore waters in the Gulf of Alaska (Brown et al., 2010) and attributed to high rates of scavenging in the high-latitude glacial river system. The re-suspension of bottom sediments from the Icelandic shelf described above can also be clearly seen in the elevated pAl at depth (Fig. 3). The relatively elevated pAl (4.4–12.2 nM) observed in the surface layer between 40 and 50°N is unexpected both because of the low delivery of dust and the low content of Al in aerosols at these latitudes (Buck et al., 2010a). MODIS imagery from the Giovanni online data system, for both the Terra and Aqua satellites, confirms low dustiness in the region for the period in question and the two preceding months. A local maximum in the Al:Ti ratio of surface-ocean particulates (data not shown) in this region suggests that, in addition to aerosol dust, another source for pAl must be present. We believe that this source is scavenging of dAl from the surface layer, primarily by biogenic particles. Dissolved Al often has a longer residence time in the surface ocean than its particulate phase, particularly when scavenging rates are comparatively low (Orians and Bruland, 1986). As a result, dAl and pAl may become decoupled as lithogenic particles are removed from the surface ocean by settling while dAl persists and can be transported with water masses and/or later scavenged onto particles. Given the low rates of dust deposition in this region, particle concentrations, and, hence, scavenging rates in the mixed layer are expected to be largely dominated by seasonal cycles of primary productivity. Following the accumulation of dAl during winter months, increased scavenging by biological particles at the onset of the spring bloom in the North Atlantic transfers Al from dissolved to particulate pools. Fluorescence and biogenic particle concentrations (discussed below; Section 3.5) in surface waters were high at the time of sampling, which took place shortly after the 2003 spring bloom at these latitudes (Henson et al., 2009). Rapid removal of dAl from surface waters and coincident increases in pAl concentrations

have previously been observed during seasonal bloom conditions in the North Atlantic (Kremling and Hydes, 1988; Moran and Moore, 1988), and preferential scavenging of Al onto biogenic particles in the subpolar Atlantic has been credited with producing elevated Al:Ti ratios in surface–ocean particles (Kuss and Kremling, 1999). Model results assign a short residence time (b 1 year) for mixed layer dAl in this region due to efficient scavenging associated with high biological productivity (Han et al., 2008). Additionally, depth profiles for dAl north of ~40ºN display minimum dAl values in the surface and increasing dAl with depth (Fig. 2d), indicating strong removal of dAl in the surface mixed layer. The low concentrations of dAl (Measures et al., 2008a) and elevated pAl at the surface from ~40 to 50°N most likely represent scavenging of dAl by biogenic particles following intense seasonal productivity. In the equatorial North Atlantic, the distribution of pAl in the surface waters closely follows that of pFe, with pFe and pAl well-correlated (r2 = 0.88) between 5°S and 26°N. Peak surface pAl is found between ~9 and 19°N where concentrations range from 6.1 to 10.9 nM, while south of this maximum, pAl concentrations at the surface decline sharply, averaging 2.6 ± 1.1 nM between 8°N and 5°S. The location of the pAl surface maximum described here is consistent with expected summer deposition of continental dust from the Sahara. Concentrations of Al in aerosol samples collected concurrently with water sampling confirm maximum dust transport occurring at relatively northern latitudes (centered at ~15°N) (Buck et al., 2010a). Residence times for particles in the surface ocean can be calculated from the surface inventory of pAl and measured concentrations of aerosol Al. Buck et al. (2010a) found the average aerosol Al concentration over 24 h sampling intervals at latitudes within the Saharan dust plume (~ 9–24°N) to be 41 nmol m −3. Following their arguments, this is equivalent to a daily aerosol Al flux of 42,000 nmol m −2 d −1 which, assuming an aerosol Al content of 8.1%, yields an annual dust flux of approximately 5 g dust m −2 yr −1, consistent with other estimates of 2 to 20 g dust m −2 yr −1 in this region (Mahowald et al., 1999; Zender et al., 2003). The average Al solubility for these aerosols is 10% (Buck et al., 2010a) and thus 90%, or 38,000 nmol Al m −2 d −1, remains in the particulate phase. Using this dust input, the observed average surface (upper 50 m) concentration of pAl (6 nM) returns an estimate of eight days for surface–ocean particle residence time at these latitudes. This estimate of residence time may be lower than the true value due to the short duration of the dust event and/or from an overestimate of the dust flux calculated by Buck et al. (2010a). Nonetheless, it is close to previous estimates of 1–4 weeks for the North Pacific (Orians and Bruland, 1986), 30 days for the Sargasso Sea (Wallace et al., 1981), and 10–40 days in the subtropical North Atlantic (Bory and Newton, 2000). The implication of the residence time estimate is that particulates are removed from the surface waters on shorter time scales than the seasonal shifts in the latitudes of maximum dust deposition. Thus, the location of the observed surface maxima in pFe and pAl distributions at low latitudes of the North Atlantic is controlled largely by seasonal patterns of atmospheric transport and associated deposition of aerosols. Active biological uptake of Al is not significant and passive scavenging onto the surface of biogenic particles in this region is likely a minor contribution to surface pAl compared to the aerosol flux. At depths below 200 m, elevated pAl concentrations (≤19.5 nM) are observed between ~ 0 and 20°N. This deep particulate maximum spans the latitudinal range of the seasonal migration of the Saharan dust plume and must represent a temporally integrated signal of annual deposition. A rough estimate of the residence time for pAl between 200 and 1000 m from 0 to 20°N is ~ 1 year, based on the average pAl concentration over this depth (11 nM) and the atmospheric inputs described above. Although there is an apparent shoaling of the subsurface pAl maximum between 9 and 11°N, the distribution of pAl (and pFe) observed in Fig. 2c is relatively constant along isobars between 0 and 20°N despite the shift in the location of aeolian inputs throughout the year, suggesting that the residence time may be longer than one year.

P.M. Barrett et al. / Marine Chemistry 142–144 (2012) 41–53

47 100000

3.5

a

p Fe d Fe

pFe, dFe (nM)

2.5

10000

a Fe

1000 1.5 100 0.5 10

-0.5

-5

0

5

10

15

20

25

30

35

40

45

50

55

60

1 65

Latitude (°N) 1000000

40 p Al

b

35

a Al

10000

25 1000 20 100 15

a Al (pmol/m3)

pAl, dAl (nM)

100000

d Al

30

10

10

1

5 0

0.1 -5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

Latitude ( N)

c

35

d Al

12

30

10

25

8

20 6 15

pAl (nM)

p Al

dAl (nM)

Dissolved Fe and Al are introduced to surface waters primarily by the partial dissolution of aerosols deposited on the surface ocean and removed by passive particle scavenging, and, in the case of Fe, active biological uptake (Boyd and Ellwood, 2010; Orians and Bruland, 1986). While dFe is expected to be relatively short-lived in the mixed layer, longer residence times for dAl have made surface concentrations useful for tracking dust deposition to the open ocean (Measures and Vink, 2000). Comparing the surface distributions of Fe and Al in the particulate and dissolved phases can indicate the extent to which temporal variability in aeolian inputs is reflected in surface-ocean trace metal concentrations, and can aid interpretation of models coupling dust flux and surface-ocean chemistry. Peak surface concentrations of dFe were found coincident with the location of maxima in pAl and pFe surface concentrations due to deposition of Saharan dust at low latitudes (~10–20°N) with much lower concentrations outside this region. Enhanced scavenging from the lithogenic particle load combined with biological uptake leads to estimates of a relatively short residence time (2–4 weeks) for dFe in surface waters of the eastern tropical North Atlantic (Sarthou et al., 2003), which is comparable to the residence time calculated above for particles in the surface ocean. Thus, the rapid removal of dFe from surface waters leads to distributions for dFe that are similar to those for pFe and pAl and deposition patterns for aerosols both locally (Buck et al., 2010a) and seasonally (Husar and Prospero, 1997). Fig. 5a shows the close association between the distributions of pFe, dFe, and concentrations of aerosol Fe, reflecting recent, high rates of aerosol deposition to the surface ocean in the region between approximately 10 and 20°N, and relatively low deposition rates elsewhere. There is a local peak in aerosol Fe concentration at 21°N that is not reflected in surface-ocean Fe chemistry. This aerosol signal, found at the outer limit of the climatological mean extent of the summer dust plume (Husar and Prospero, 1997), is likely an effect of the highly variable nature of the aerosol signal and thus reflects a short-term, transient signal with a small net input of dust. Unlike the close association between distributions of surface particulates and dFe, there is a distinct spatial offset between the particulate surface maxima located at ~10–20°N and the dAl surface maxima found at ~0–10°N (Measures et al., 2008a) (Fig. 5b, c). We offer two possible scenarios in an attempt to explain this observation. First, if a residence time of several years for dAl is assumed (Jickells et al., 1994), surface concentrations would integrate seasonal and spatial variations in dust delivery. This interpretation of dAl distributions suggests that deposition of Saharan dust occurring at more southerly latitudes during the winter is significantly greater than deposition occurring during the summer. Previous studies of aerosol delivery to the North Atlantic disagree about seasonal variations in dust-flux magnitude at low latitudes (Chen and Siefert, 2004; Gao et al., 2001). A strong latitudinal gradient over ~0–20°N is not apparent in global models of annual dust delivery to the surface ocean (Duce et al., 1991; Ginoux et al., 2001; Jickells et al., 2005; Mahowald et al., 1999), which instead indicates that the total annual flux is relatively constant over this latitude range. Nor is a trend of greater dust flux at more southerly latitudes evident from particle distributions deeper in the water column in this data set. Mid-depth (200 to 1000 m) distributions of pFe and pAl are fairly uniform between the equator and 20ºN (Fig. 2a, c), suggesting that the seasonal dust fluxes associated with the summer and winter positions of the ITCZ are of similar magnitude. Hence, the observed gradient surface water dAl concentrations between 0 and 20ºN is unlikely to result if the residence time for dAl is several years. Alternatively, dAl surface concentrations observed between ~ 0 and 10°N can be interpreted as a memory effect of the preceding season's aerosol Al deposition that develops due to relatively short residence times for dAl in this region. Previous studies have revealed a seasonal shift in the position of maximum dAl concentrations in the

aFe (pmol/m3)

3.3. Comparison of particulate Fe and Al with distributions of dissolved phases

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Latitude ( N) Fig. 5. (a) Distribution of particulate (pFe) and dissolved (dFe) iron (Measures et al., 2008a) in nmol L−1 in surface waters and aerosol iron (aFe) concentrations (Buck et al., 2010a) in pmol m−3 plotted on a logarithmic scale. (b) Distribution of particulate (pAl) and dissolved (dAl) aluminum (Measures et al., 2008a) in nmol L−1 in surface waters and aerosol aluminum (aAl) concentrations (Buck et al., 2010a) in pmol m−3 plotted on a logarithmic scale. Aerosol concentrations plotted on the x-axis represent samples at or below the instrumental detection limit. (c) Distribution of particulate (pAl) and dissolved (dAl) (Measures et al., 2008a) aluminum in nmol L−1 in surface waters in the equatorial region from 4°S to 26°N.

eastern tropical North Atlantic (Bowie et al., 2002; Helmers and van der Loeff, 1993) and have suggested that the dAl residence time must be on the order of several months. A recent study by Dammshäuser et al. (2011), in explaining discrepancies between dissolved Al and Ti, concluded that dAl concentrations reflect seasonal patterns of Saharan dust deposition, and calculated surface-ocean residence times for dAl to be 0.2–1.8 yr in this region. Modeled dynamics of dAl also produce comparatively short residence times of several months to one year for dAl in the eastern tropical North Atlantic (Gehlen et al., 2003; Han et al., 2008). Such short residence times in this region are likely the result of fairly intense absorptive scavenging due to high dust inputs and biological productivity stimulated by equatorial upwelling (Charette and Moran, 1999). In this scenario, dAl accumulates in surface waters at ~0–10°N over the season of high deposition (winter) then slowly decays over several months after inputs cease (summer). As a result, dAl

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surface concentrations lag the shift in dust deposition to higher latitudes and create a “memory effect” in surface waters at low latitudes (Helmers and van der Loeff, 1993). Estimated residence times for pAl are sufficiently short to neglect potential effects of lateral advection on particle distributions. In the “memory effect” scenario, residence times for dAl in surface waters are assumed to be shorter than one year, but could be long enough to be affected by advection. However, surface circulation in the tropical northeast Atlantic is dominated by a system of zonal currents, and the comparatively weak meridional transport in this region is unlikely to produce such a large southerly shift in the surface–ocean dAl concentrations (Stramma et al., 2005). Although it is clear that the distributions of dAl in surface waters reflect trends in dust delivery to the oceans, it is essential to consider the residence time and temporal variability of surface dAl to best use dAl distributions to identify patterns and magnitude of aerosol deposition on the surface ocean. Seasonal variability in dAl concentrations may lead to temporal sampling biases that contribute to discrepancies between observations and model results in the equatorial Atlantic (Gehlen et al., 2003; Han et al., 2008). For example, Han et al. (2008) note that when their Biogeochemical Elemental Cycling ocean model is used to generate surface–ocean dAl distributions from dust input fields, “the predicted maximum at the North Atlantic is too far north” (their Fig. 6) compared to the existing dAl datasets. While existing datasets provide good spatial coverage of surface–ocean dAl in the eastern equatorial Atlantic, most of these were collected in the spring and summer months. Under the scenario outlined above, the seasonal memory effect would result in the location of the observed dAl maximum to be found at relatively southern latitudes during these sampling times. This temporal effect would contribute to the apparent bias in model output, which is based on integrated annual dust inputs. The equatorial Atlantic, or other ocean regions with high aerosol deposition or biological productivity, may have sufficiently short dAl residence times to be able to assess interannual variability in the magnitude and location of dust deposition, as also suggested in recent work by Dammshäuser et al. (2011). 3.4. Anthropogenic impacts on particulate composition Increasing attention has been focused on anthropogenic impacts on the global iron budget. Recent work reveals high aerosol-Fe solubilities in regimes heavily impacted by anthropogenic activity, and that anthropogenic emissions may be an important factor controlling the input of labile iron to the surface ocean (Chuang et al., 2005; Luo et al., 2008; Sedwick et al., 2007; Sholkovitz et al., 2009). To assess anthropogenic impacts on surface-ocean particulate chemistry, particulate samples were analyzed for V, Cr, Ni, Cu, Zn, As, and Pb, trace elements that are characteristic of emission products from coal-fired power plants, oil combustion,

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and biomass burning (Smith, 1980; Hildemann et al., 1991; Duce et al., 1991; Wang et al., 2003; Desboeufs et al., 2005). Trends in the chemical composition of particulate samples show significant anthropogenic impacts on the surface chemistry of the eastern North Atlantic (Fig. 6). The highest concentrations for all metals are found in surface waters at ~45–60°N where air-mass back-trajectories indicate aerosol sources impacted by North American and European industrial emissions (Buck et al., 2010a). Because these metals are also present in lithogenic material and there is a large variation in the magnitude of total aerosol flux along the A16N transect, it is useful to examine metal concentrations relative to expected contributions from lithogenic sources. Particulate Al concentrations are dominated by inputs from lithogenic sources and are not expected to be significantly affected by anthropogenic contributions. Therefore, the metal:Al ratios in particulate samples can be compared with the ratio of average crustal material to isolate contributions from anthropogenic sources. In areas such as the region between 40 and 50°N where scavenging of dAl likely contributes to measured pAl concentrations, these ratios represent minimum estimates of anthropogenic contributions. When the metal:Al ratios in surface-ocean particle samples are calculated for Cr, Ni, Cu, Zn, and Pb, similar latitudinal trends emerge, e.g., for Ni and Cu (Fig. 7). The highest ratios are found in the northern latitudes, generally between 45 and 55°N. At latitudes between 10 and 20°N where Saharan dust dominates aerosol inputs, metal:Al ratios decrease to a minimum, approaching the crustal ratio, before increasing again to the south of the dust plume. The distributions of anthropogenically sourced metals in surfaceocean particles generally follow similar latitudinal patterns as anthropogenic metals found in aerosols over the North Atlantic. Buck et al. (2010a) reported vanadium enrichment in aerosol samples as an indicator of anthropogenic impacts on atmospheric aerosol loads. Aerosol V:Al ratios are well above the crustal average of 0.0002 (Wedepohl, 1995) at all latitudes outside the region of high Saharan dust input between 10 and 20°N (Fig. 7). These trends seen in both aerosol and surface–ocean particulate chemical composition strongly suggest that deposition of aerosols on the surface ocean is a major pathway for input of anthropogenic trace metals to the open ocean. Water column particulate samples were also analyzed for vanadium; however, concentrations were below the instrumental detection limit due to a combination of low average crustal abundance and high aerosol V solubility. Using the average aerosol V concentration (0.33 pmol/m 3) and average aerosol V solubility (29%) reported by Buck et al. (2010a), a depositional velocity of 1000 m/day, a mixed layer depth of 50 m, and a particle residence time of 10 days, the expected surface particulate V concentration would be 0.005 nM, an order of magnitude below the instrumental detection limit of 0.07 nM for the average sample filtration volume (Table 1). Surface concentrations of Cu, Ni, Zn, and Pb measured in this study are consistently higher than those reported by Kuss and Kremling (1999) for the eastern North Atlantic (Table 3). Between 40 and 60°N, trace metal concentrations are 2–3 times the magnitude of previous measurements; between 30 and 40°N, values are up to an order of magnitude higher. Short residence times for surface-ocean particulates and high variability in dust events complicate comparisons of particulate Cr, Ni, Cu, Zn, and Pb signals to previous studies. Additionally, differences in sample collection approaches between studies may lead to systematic discrepancies in measured concentrations. The study of Kuss and Kremling (1999) sampled surface particulates using a flow-through centrifuge apparatus, a technique that typically excludes particles smaller than 1 μm (Schüβler and Kremling, 1993). Analysis of size-fractionated aerosol samples collected along the A16N transect (Buck et al., 2010b) showed that the fraction of aerosols with a diameter of b 1 μm is significant; on average, 55% of measured aerosol Fe was found on fractions with particle sizes of less than 1 μm. In regions outside of the Saharan dust plume, where anthropogenic contributions to trace metal budgets are highest, aerosol particles b1 μm in diameter comprised an average of 70% of the aerosol

P.M. Barrett et al. / Marine Chemistry 142–144 (2012) 41–53 0.08 V:Al aerosol

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Latitude ( N) Fig. 7. Molar ratio of V:Al in aerosol samples (Buck et al., 2010a) (top panel) and Cu:Al and Ni:Al in surface ocean particulate samples (bottom panel) along the A16N transect from 5°S to 62°N. Error bars represent +/− three standard deviations. Average crustal ratios (Wedepohl, 1995) are indicated by dashed lines.

Fe pool. Aerosols emitted from fossil fuel combustion are typically smaller than lithogenic aerosols (Duce et al., 1991); thus, previous reports may underestimate concentrations of anthropogenically sourced particulate trace metals in the surface ocean due to exclusion of small size classes. Differences between the two datasets are higher and more variable between 30 and 40°N than those observed between 40 and 60°N. Although speculative, this may be due to higher biological productivity between 40 and 60°N leading to more efficient packaging of small particles into fecal pellets and biological aggregates large enough to be captured by the centrifuge technique. The molar ratio of Fe:Al in both aerosols and surface-ocean particles is shown in Fig. 8. Within the Saharan dust plume (9–23°N) aerosol samples have an average Fe:Al ratio of 0.30± 0.05 which is similar to the average crustal ratio (0.26) and to the average Fe:Al ratio of surface-ocean particulates at these latitudes (0.28± 0.06). By comparison, aerosol samples immediately north and south of the Saharan dust plume have elevated Fe:Al ratios (0.42± 0.17) that are attributed to air masses containing Fe-rich particulates emitted from anthropogenic

49

combustion processes. However, surface-ocean particles at these latitudes have appreciably lower Fe:Al ratios, averaging 0.17 ± 0.06. Several processes likely contribute to the observed relative depletion of Fe in surface-ocean suspended matter. First, depleted Fe:Al of suspended particulate matter samples may result from higher relative solubility of Fe in anthropogenically sourced aerosols. Buck et al. (2010a) observed an average aerosol-Fe solubility of 8% for samples within Saharan air masses, while much higher aerosol-Fe solubilities (typically 10–47%) were observed north and south of the Saharan dust in regions significantly impacted by anthropogenic emissions. This is consistent with previous work that has found high aerosol-Fe solubility (>80%) in combustion products (Desboeufs et al., 2005; Schroth et al., 2009). Surface concentrations of dFe are low in these regions (typically b2.5 nM), suggesting that leachable aerosol Fe is quickly assimilated by phytoplankton and/or transported out of the surface layer by large sinking organic particles or aggregates. The Fe:Al ratio of suspended matter increases with depth in the water column, indicating that Fe is preferentially removed from the surface layer. Secondly, as discussed previously (Section 3.2), a comparison of aerosol-Al flux and surfaceocean pAl concentrations indicates that scavenging of dAl contributes to pAl concentrations in regions that experience high seasonal productivity. This process should also lead to depleted Fe:Al ratios in surface particulates, most notably between 40 and 50°N. For this process to account for depleted surface-ocean Fe:Al ratios would require an average 200% increase in pAl concentrations from scavenging of the dAl pool across the transect; this is likely far beyond the average contribution of scavenging processes at most latitudes, particularly in the oligotrophic gyre where productivity is dominated by small, non-siliceous phytoplankton. However, the maximum observed aerosol-Fe solubility (47%) is insufficient to explain the full degree of Fe depletion in surface-ocean particulates; measured aerosol-Fe solubilities can account for an average of 20–40% of the observed depletion in surfaceocean particulate Fe:Al ratios. Thus, a combination of these two processes likely leads to the changes in chemical composition relative to aerosol Fe and Al inputs. 3.5. Biogenic particulates and Fe distributions In addition to atmospheric controls, the distribution of pFe and dFe species in seawater is also affected by surface-ocean processes, including biological influences such as Fe uptake and production of organic Fe-binding ligands (Baker and Croot, 2010). This dataset reveals that Fe:Al ratios in particulate samples from the top 200 m are dynamic and correspond to changes in the biologic character of samples and measured chlorophyll concentrations. Concentrations of particulate Si and Ca reveal trends in the biologic character of the suspended matter. These effects are highlighted by

1.0

Table 3 Comparison of surface–ocean suspended matter concentrations of Cu, Ni, Pb, and Zn in pmol L−1 from this study and the work of Kuss and Kremling (1999). 30–40°N

Cu (pmol L−1) Ni (pmol L−1) Pb (pmol L−1) Zn (pmol L−1) 1

44–60°N

Kuss and Kremling (1999)1

This study

Kuss and Kremling (1999)1

This study

17.3 + 7.4 11.0 + 2.3 1.09 + 0.27 18.6 + 5.6

250 + 78 43.6 + 19.8 15.2 + 14.3 56.6 + 26.1

254 + 372 40.5 + 19.1 10.7 + 16.4 74.3 + 33.7

540 + 411 111 + 95 28.0 + 35.9 169 + 158

Data from ANT VII/7 (May 1990).

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isolating the biogenic component of Si (SiB) and Ca (CaB) from the lithogenic fraction by using average crustal molar ratios of Si:Al and Ca:Al (Wedepohl, 1995) as follows:   SiB ¼ Simeasured –ðSi : AlÞcrust  Almeasured

ð2Þ

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ð3Þ

The biogenic fraction of measured particulate Ca ranges from 60 to 100% throughout the transect; in the upper 200 m, CaB accounts for an average of 96% of the particulate Ca pool. The estimated biogenic component of pSi is more variable and ranges from b0 to 96%; Eq. (2) does result in negative values for some samples, particularly in the oligotrophic subtropical gyre where diatoms represent only a small percentage of the phytoplankton population. Calculated values for SiB likely underestimate actual biogenic opal concentrations due to scavenging of dAl onto particles, an effect that is expected to increase with depth. Estimates of SiB in the surface ocean may also be affected by solubility differences between aerosol Al and Si. However, the distribution of SiB (Fig. 9) follows expected geographical trends for diatom productivity and calculated values generally agree with other available measurements of biogenic silica in the North Atlantic. LeBlanc et al. (2009) measured surface biogenic silica concentrations of b0.2 to 0.8 μmol L−1 between the Icelandic Basin and 45°N during June 2005, comparable to the SiB concentrations up to 0.5 μmol L−1 observed in this dataset for the same latitudes. In subpolar waters, the SiB signal indicates the presence of a relatively intense bloom at the surface with a resultant flux of biogenic opal out of the mixed layer, representing a vehicle for significant carbon export following the spring bloom (Martin et al., 2011). In contrast, the waters of the oligotrophic subtropical gyre have only a weak subsurface SiB signal at depths of 100–150 m. Mixed layer concentrations of SiB increase near the equator where upwelling stimulates production with some indication of export

out of the surface ocean. The SiB signal in the surface ocean correlates strongly with fluorescence measured along the transect (Fig. 9). Biological processes appear to affect both the vertical transport of particles and the partitioning of Fe between the particulate and dissolved phases in the surface layer. Between ~ 10 and 20°N, anthropogenic effects on Fe inputs and speciation should be minimal, allowing isolation of a biological influence. At these latitudes, vertical profiles of pFe typically exhibit a subsurface minimum in the upper 200 m, a common feature in pFe distributions throughout the transect (Fig. 2a). Shallow subsurface minima are also observed in individual pAl profiles (Fig. 2c). This signal could arise in part from the pulsed, episodic nature of atmospheric aerosol deposition. However, the subsurface minima in pFe and pAl distributions may also result from increased vertical transport of lithogenic material associated with biological production. Shallow subsurface minima in pAl and pFe are typically found between 50 and 100 m, coincident with peak fluorescence and SiB concentrations (Fig. 10a, b). Increased export of aerosols and other lithogenic particles has been documented during high biological production as a result of packaging into fecal pellets and aggregate formation with biogenic particles and marine snow, processes which incorporate individual dust particles into larger particles that sink rapidly out of the surface layer (Deuser et al., 1983; Honjo, 1982; Mignon et al., 2002). Biological processes also appear to affect the phase partitioning of Fe in the surface layer, facilitating the solubilization of Fe for subsequent uptake by phytoplankton and resulting in preferential vertical export of Fe. pFe:pAl ratios decrease by an average of 35% within the subsurface chlorophyll maximum, dropping from 0.32 ± 0.05 at the surface to 0.20 ± 0.05 at 50–70 m (Fig. 10c). Minimum values for pFe:pAl coincide with peak SiB and chlorophyll concentrations (Fig. 10a, b). When considering solely the effect of an increased proportion of biogenic particles on pFe:pAl, the molar ratio may be predicted to increase within the chlorophyll maximum due to biological

Fig. 9. Concentrations of biogenic particulate Si (SiB) in nmol L−1 over a limited concentration range (≤ 200 nM) (top panel). Calculated values may be less than zero from the overestimation of lithogenic pAl concentrations due to scavenging of dAl in the surface ocean. Chlorophyll concentrations were recorded by an uncalibrated fluorometer (in volts) on the trace metal rosette (bottom panel).

P.M. Barrett et al. / Marine Chemistry 142–144 (2012) 41–53

uptake of the relatively high concentrations of dFe at the surface at these latitudes, whereas significant biological utilization of dAl is not expected. Instead, the relative depletion of pFe appears to result in part from increased solubilization of pFe. Although profiles of dFe show a minimum associated with the chlorophyll maximum, dFe increases as a relative portion of the entire Fe pool (particulate + dissolved). Between ~ 10 and 20°N, dFe comprises approximately 10–30% of the total Fe at the surface, but increases rapidly to 50–70% of total Fe between 50 and 100 m before decreasing at depths below the chlorophyll maximum (Fig. 10d). The relative increase in the dFe pool appears to be mediated by biology and may result from the production and release of organic ligands in seawater, which has been shown to be a strong control on the solubility of both aerosol Fe and ferric hydroxides (Wagener et al., 2008; Ye et al., 2011; Yoshida et al., 2002). Measures et al. (2008a) noted that the subsurface minimum in dFe corresponding to the chlorophyll maximum represented a partitioning of dFe into a biological particulate phase. The increased solubilization likely does lead to increased uptake of dFe by phytoplankton, but may be quickly transported out of the surface layer, leading to the overall deficit of total Fe (particulate + dissolved) observed at the subsurface chlorophyll maximum. Similar dynamics of solubilization of Fe, biological uptake, and rapid vertical transport have been inferred from low Fe:Al molar ratios observed in the euphotic zone in the North Pacific (Johnson et al., 1997). 4. Conclusions The concentrations of pFe and pAl in surface waters of the North Atlantic generally reflect patterns of atmospheric aerosol transport while concentrations at depth are controlled by particle settling, sediment re-suspension, and the scavenging of dFe released during

remineralization of organic matter. The most prominent feature in the particulate distributions is the surface maxima in particulate Fe and Al between 10 and 20ºN produced by intense seasonal deposition of Saharan dust. Below this surface signal is a broader subsurface maximum of elevated particulate Fe and Al between 0 and 20ºN that is an integrated signal of annual dust flux. The fairly even particulate distribution across the subsurface feature suggests that seasonal variation in total dust flux is minimal, which contrasts with the gradient in surface dAl concentrations observed across these latitudes. This discrepancy is interpreted to be a result of short residence times (b 1 year) for both phases of Al and Fe in the surface layer of the equatorial Atlantic. As a result of short residence times and seasonal shifts in the location of atmospheric deposition, temporal biases in sampling may be an important consideration when modeling dust delivery based on the chemistry of the surface ocean. Although surface–ocean Al chemistry serves as an imprint of atmospheric deposition of aerosols, fractionation between the particulate and dissolved phases of Al is also linked to productivity, which is temporally variable and independent of aerosol delivery. Unlike Fe, whose dissolved phase is typically present at extremely low concentrations in surface waters due to high biological demand, the variable intensity of scavenging of dAl can have significant effects on surface–ocean Al distributions. An example seen clearly in this dataset is the region of elevated pAl in the surface mixed layer between 40 and 50°N where the supply of dAl from delivery of aerosols is low, thus implicating seasonal scavenging of dAl by biogenic particles as a major pAl source. This dataset also reinforces the significant biological role in the fractionation of Fe between particulate and dissolved phases. Vertical profiles of the chemical composition of settling particulate matter show relative depletion of Fe in the surface mixed layer and an increase in the proportion of dFe relative to the total Fe pool. This shift between

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Fe phases is coincident with elevated concentrations of biogenic particles and the subsurface fluorescence maximum (Fig. 10). This biological influence on the solubilization of particulate Fe could conceivably be facilitated by increased concentrations of organic Fe-binding ligands. Complexation with ligands would prevent scavenging by the heavy particulate loads and increase active biological uptake of Fe. Biological uptake and rapid transport out of the surface layer would maintain the low Fe:Al of particulates and the overall Fe deficit in the subsurface chlorophyll maximum. Anthropogenic impacts on surface–ocean particulate chemistry are evidenced by significant enrichment of Cr, Ni, Cu, Zn, and Pb relative to average crustal composition in suspended matter throughout the North Atlantic. As expected, impacts are largest in the mid-latitudes due to significant emissions from industrialized Europe and North America; however, they are also detectable at low latitudes, where source regions have relatively low levels of industrialization and lithogenic material likely dominates aerosols delivered to the surface ocean. Evidence of anthropogenic influence on aerosols and surface–ocean chemistry correlates well with increased solubility of aerosol Fe and depleted Fe:Al ratios in surface–ocean particulates. With continued global industrialization and increased combustion of fossil fuels, the high fractions of labile Fe associated with anthropogenic emissions are likely to have significant impacts on the surface–ocean Fe budget. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.marchem.2012.07.006. Acknowledgements Analyses and visualizations used in this paper were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. Contour plots presented were created using Ocean Data View (http://odv.awi-bremerhaven.de). We also acknowledge the MODIS mission scientists and associated NASA personnel for the production of the data used in this research effort and thank two anonymous reviewers for their helpful comments on the manuscript. This work was supported by NSF grant OCE-0649505 (JAR), OCE-0223378 (WML), and OCE 0223397 (CIM). This work was also partially funded by the Joint Institute for the Study of the Atmosphere and Ocean (JISAO). This is JISAO publication number 1893 and PMEL publication number 3787. References Baker, A.R., Croot, P.L., 2010. Atmospheric and marine controls on aerosol iron solubility in seawater. Mar. Chem. 120, 4–13. Blomqvist, S., Larsson, U., 1994. Detrital bedrock elements as tracers of settling resuspended particulate matter in a coastal area of the Baltic Sea. Limnol. Oceanogr. 39, 880–896. Bory, A.J.-M., Newton, P.P., 2000. Transport of airborne lithogenic material down through the water column in two contrasting regions of the eastern subtropical North Atlantic Ocean. Global Biogeochem. Cycles 14, 297–315. Bowie, A.R., Whitworth, D.J., Achterberg, E.P., Mantoura, R.F.C., Worsfold, P.J., 2002. Biogeochemistry of Fe and other trace elements (Al, Co, Ni) in the upper Atlantic Ocean. Deep-Sea Res.I 49, 605–636. Boyd, P.W., Ellwood, M.J., 2010. The biogeochemical cycle of iron in the ocean. Nat. Geosci. 3, 675–682. Boyd, P.W., Jickells, T., Law, C.S., Blain, S., Boyle, E.A., Buesseler, K.O., et al., 2007. Mesoscale iron enrichment experiments 1993–2005: synthesis and future directions. Science 315, 612–617. Brown, M.T., Lippiatt, S.M., Bruland, K.W., 2010. Dissolved aluminum, particulate aluminum, and silicic acid in northern Gulf of Alaska coastal waters: glacial/riverine inputs and extreme reactivity. Mar. Chem. 122, 160–175. Buck, C.S., Landing, W.M., Resing, J.A., Measures, C.I., 2010a. The solubility and deposition of aerosol Fe and other trace elements in the North Atlantic Ocean: observations from the A16N CLIVAR/CO2 repeat hydrography section. Mar. Chem. 120, 57–70. Buck, C.S., Landing, W.M., Resing, J.A., 2010b. Particle size and aerosol iron solubility: a high-resolution analysis of Atlantic aerosols. Mar. Chem. 120, 14–24. Charette, M.A., Moran, S.B., 1999. Rates of particle scavenging and particulate organic carbon export estimated using 234Th as a tracer in the subtropical and equatorial Atlantic Ocean. Deep-Sea Res. II: Top. Stud. Oceanogr. 46, 885–906. Chen, Y., Siefert, R.L., 2004. Seasonal and spatial distributions and dry deposition fluxes of atmospheric total and labile iron over the tropical and subtropical North Atlantic Ocean. J. Geophys. Res. 109, D09305.

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