Abstract
Atmospheric deposition is a major source of trace metals in marine surface waters and supplies vital micronutrients to phytoplankton, yet measured aerosol trace metal solubility values are operationally defined, and there are relatively few multi-element studies on aerosol-metal solubility in seawater. Here we measure the solubility of aluminum (Al), cadmium (Cd), cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) from natural aerosol samples in seawater over a 7 days period to (1) evaluate the role of extraction time in trace metal dissolution behavior and (2) explore how the individual dissolution patterns could influence biota. Dissolution behavior occurs over a continuum ranging from rapid dissolution, in which the majority of soluble metal dissolved immediately upon seawater exposure (Cd and Co in our samples), to gradual dissolution, where metals dissolved slowly over time (Zn, Mn, Cu, and Al in our samples). Additionally, dissolution affected by interactions with particles was observed in which a decline in soluble metal concentration over time occurred (Fe and Pb in our samples). Natural variability in aerosol chemistry between samples can cause metals to display different dissolution kinetics in different samples, and this was particularly evident for Ni, for which samples showed a broad range of dissolution rates. The elemental molar ratio of metals in the bulk aerosols was 23,189Fe: 22,651Al: 445Mn: 348Zn: 71Cu: 48Ni: 23Pb: 9Co: 1Cd, whereas the seawater soluble molar ratio after 7 days of leaching was 11Fe: 620Al: 205Mn: 240Zn: 20Cu: 14Ni: 9Pb: 2Co: 1Cd. The different kinetics and ratios of aerosol metal dissolution have implications for phytoplankton nutrition, and highlight the need for unified extraction protocols that simulate aerosol metal dissolution in the surface ocean.
Highlights
IntroductionThe role of atmospheric nutrient deposition in supporting marine phytoplankton growth is well documented (Peierls and Paerl, 1997; Herut et al, 1999; Mills et al, 2004; Duce et al, 2008; Mackey et al, 2012a,c)
A CONTINUUM OF AEROSOL METAL DISSOLUTION The amount of each metal to dissolve from the aerosol samples was measured over time, and Figure 2 shows the dissolution time course with the concentration of each dissolved metal normalized to the mass of aerosol
Metal dissolution behavior occurred along a continuum, where some metals were more strongly influenced by particle reactivity, some dissolved rapidly, and others dissolved gradually over time (Figure 3, Table 3)
Summary
The role of atmospheric nutrient deposition in supporting marine phytoplankton growth is well documented (Peierls and Paerl, 1997; Herut et al, 1999; Mills et al, 2004; Duce et al, 2008; Mackey et al, 2012a,c). Atmospheric deposition of nitrogen (N) and phosphorus (P) provide nutrition in open ocean (Duce et al, 2008; Mackey et al, 2012a,c) and coastal areas (Paerl, 1997; Peierls and Paerl, 1997; Herut et al, 1999), and atmospheric N deposition may supply 40–70% of the total nitrate to phytoplankton in the North Pacific Ocean (Prospero and Savoie, 1989). Deposition of iron (Fe) is another important source of growth-sustaining nutrition for phytoplankton, and atmospheric deposition is a primary source of Fe to many regions of the ocean (Duce and Tindale, 1991; Mills et al, 2004; Mahowald et al, 2005; Moore et al, 2006; Baker and Croot, 2010)
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