The impact of changing surface ocean conditions on the dissolution of aerosol iron

Abstract

The proportion of aerosol iron (Fe) that dissolves in seawater varies greatly and is dependent on aerosol composition and the physicochemical conditions of seawater, which may change depending on location or be altered by global environmental change. Aerosol and surface seawater samples were collected in the Sargasso Sea and used to investigate the impact of these changing conditions on aerosol Fe dissolution in seawater. Our data show that seawater temperature, pH, and oxygen concentration, within the range of current and projected future values, had no significant effect on the dissolution of aerosol Fe. However, the source and composition of aerosols had the most significant effect on the aerosol Fe solubility, with the most anthropogenically influenced samples having the highest fractional solubility (up to 3.2%). The impact of ocean warming and acidification on aerosol Fe dissolution is therefore unlikely to be as important as changes in land usage and fossil fuel combustion. Our experimental results also reveal important changes in the size distribution of soluble aerosol Fe in solution, depending on the chemical conditions of seawater. Under typical conditions, the majority (77-100%) of Fe released from aerosols into ambient seawater existed in the colloidal (0.02-0.4 μm) size fraction. However, in the presence of a sufficient concentration of strong Fe-binding organic ligands (10 nM) most of the aerosol-derived colloidal Fe was converted to soluble Fe (<0.02 μm). This finding highlights the potential importance of organic ligands in retaining aerosol Fe in a biologically available form in the surface ocean.

A review of colloidal iron partitioning and distribution in the open ocean

“…North versus South Atlantic Ocean basins; Ussher et al, 2013). This has been validated by studies that show a preferential release of aerosol Fe into the cFe pool Fishwick et al, 2014).…”

“…size, stability and functional group chemistry) limits our understanding of their interactions with Fe (Gledhill and Buck, 2012). However, voltammetric methods have been used to distinguish between strong (L1) and weak (L2) Febinding ligand classes (Rue and Bruland, 1995), and high concentrations of soluble sized L1 ligands have been shown to stabilise aerosol-derived Fe in the soluble size fraction (Fishwick et al, 2014).…”

Toward a Regional Classification to Provide a More Inclusive Examination of the Ocean Biogeochemistry of Iron-Binding Ligands

“…This exchangeable pool is often referred to as the dissolved iron pool (Gledhill and Buck, 2012), a possibly misleading assumption as not all the dissolved iron might be exchangeable and as particulate iron can also react with organic ligands such as siderophores and HS (Kraemer et al, 2005;Paris and Desboeufs, 2013;Fishwick et al, 2014). It is also to be noted that the co-existence of multiple metals has implications for iron biogeochemistry as organic ligands may bind metals with different binding affinities, potentially leading to competition, representing a research direction mostly unexplored for in-situ oceanic ligands.…”

An Intercomparison of Dissolved Iron Speciation at the Bermuda Atlantic Time-series Study (BATS) Site: Results from GEOTRACES Crossover Station A

“…Thus, the higher DFe in November 2011 compared to June 2010 in the upper water column may reflect the deposition of North American aerosols with higher fractional solubility of Fe at BATS in the late fall/early winter compared to the predominant deposition of relatively insoluble Saharan dust in the summer months (Sedwick et al, 2007). Previous studies indicate that particles, whether atmospheric, authigenic, biological or resuspended, can greatly influence DFe concentrations and size fractionation in the surface ocean (Sedwick et al, 2005;Buck et al, 2010a,b;Fitzsimmons et al, 2015), and that these influences are largely mediated by the concentrations and conditional stability constants of Fe-binding organic ligands in the water column (Rijkenberg et al, 2008;Wagener et al, 2008;Aguilar-Islas et al, 2010;Boyd et al, 2010;Bundy et al, 2014;Fishwick et al, 2014). The trend of higher DFe in the November 2011 water column was not restricted to the surface ocean though (Figure 2A) and we cannot explain these differences in deeper water masses from seasonality.…”

“…These ligand classes are operationally defined by their associated conditional stability constants (K cond show considerable overlap, and it has been proposed that these ligand classes be defined by specific ranges in log K cond FeL i ,Fe ′ values (Gledhill and Buck, 2012). The complexation capacity or competition strength of these ligands is described by α FeL , which is a function of the free (not bound to Fe, "excess") ligand concentration ([L ′ > 12) and siderophores in particular have been shown to play a distinct role in Fe solubility (Cheah et al, 2003;Buck et al, 2007;Rijkenberg et al, 2008;Wagener et al, 2008;Aguilar-Islas et al, 2010;Mendez et al, 2010;Bundy et al, 2014;Fishwick et al, 2014), though characterizing more than one ligand class during the interpretation of CLE-AdCSV titration datasets is challenging (Hudson et al, 2003;Wu and Jin, 2009;Laglera et al, 2013). In addition to requiring sufficient titration points to quantify more than one ligand class with suitable error estimates, resolving multiple ligand classes also requires that the alpha coefficients of the proposed ligand classes are suitably different (Hudson et al, 2003;Wu and Jin, 2009) and larger than the center of the analytical window (α FeL of the added competing ligand) employed (Laglera et al, 2013).…”