Search

Abstract: Aerosol samples were collected in the North Atlantic Ocean during June–August, 2003. Aerosols were divided into nine size fractions ranging from >18 μm to <0.056 μm. Total element concentrations were measured by energy dispersive X-ray fluorescence to determine the size distribution of biogeochemically important elements. The solubility of Fe and other elements was measured in ultrapure deionized water to investigate the relationship between particle size and aerosol solubility. We found that the majority of soluble aerosol Fe was on particles of ≥1 μm aerodynamic diameter. Aerosol Fe solubility was somewhat variable but in general, aerosol Fe solubility did not increase with decreasing particle size. [Copyright &y& Elsevier]

Abstract: Samples of total suspended matter were collected from the upper 1000m 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 (>3.3nM) and Al (>10nM) in surface waters between 10 and 20°N. A broad mid-depth enrichment of particulate Fe (>5.4nM) and Al (>19nM) 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 (8days and <1year, 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 50–200m 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. [Copyright &y& Elsevier]

Atmospheric deposition is a source of potentially bioavailable iron (Fe) and thus can partially control biological productivity in large parts of the ocean. However, the explanation of observed high aerosol Fe solubility compared to that in soil particles is still controversial, as several hypotheses have been proposed to explain this observation. Here, a statistical analysis of aerosol Fe solubility estimated from four models and observations compiled from multiple field campaigns suggests that pyrogenic aerosols are the main sources of aerosols with high Fe solubility at low concentration. Additionally, we find that field data over the Southern Ocean display a much wider range in aerosol Fe solubility compared to the models, which indicate an underestimation of labile Fe concentrations by a factor of 15. These findings suggest that pyrogenic Fe-containing aerosols are important sources of atmospheric bioavailable Fe to the open ocean and crucial for predicting anthropogenic perturbations to marine productivity.