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The size partitioning of dissolved iron and organic iron-binding ligands into soluble and colloidal phases was investigated in the upper 150 m of two stations along the GA03 U.S. GEOTRACES North Atlantic transect. The size fractionation was completed using cross-flow filtration methods, followed by analysis by isotope dilution inductively-coupled plasma mass spectrometry (ID-ICP-MS) for iron and competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) for iron-binding ligands. On average, 80% of the 0.1–0.65 nM dissolved iron (< 0.2 μm) was partitioned into the colloidal iron (cFe) size fraction (10 kDa < cFe < 0.2 μm), as expected for areas of the ocean underlying a dust plume. The 1.3–2.0 nM strong organic iron-binding ligands, however, overwhelmingly (75–77%) fell into the soluble size fraction (< 10 kDa). As a result, modeling the dissolved iron size fractionation at equilibrium using the observed ligand partitioning did not accurately predict the iron partitioning into colloidal and soluble pools. This suggests that either a portion of colloidal ligands is missed by current electrochemical methods because they react with iron more slowly than the equilibration time of our CLE-ACSV method, or part of the observed colloidal iron is actually inorganic in composition and thus cannot be predicted by our model of unbound iron-binding ligands. This potentially contradicts the prevailing view that greater than > 99% of dissolved iron in the ocean is organically complexed. Disentangling the chemical form of iron in the upper ocean has important implications for surface ocean biogeochemistry and may affect iron uptake by phytoplankton. [ABSTRACT FROM AUTHOR]

The long-lived uranium decay products 230 Th and 231 Pa are widely used as quantitative tracers of adsorption to sinking particles (scavenging) in the ocean by exploiting the principles of radioactive disequilibria. Because of their preservation in the Pleistocene sediment record and through largely untested assumptions about their chemical behavior in the water column, the two radionuclides have also been used as proxies for a variety of chemical fluxes in the past ocean. This includes the vertical flux of particulate matter to the seafloor, the lateral flux of insoluble elements to continental margins (boundary scavenging), and the southward flux of water out of the deep North Atlantic. In a section of unprecedented vertical and zonal resolution, the distributions of 230 Th and 231 Pa across the North Atlantic shed light on the marine cycling of these radionuclides and further inform their use as tracers of chemical flux. Enhanced scavenging intensities are observed in benthic layers of resuspended sediments on the eastern and western margins and in a hydrothermal plume emanating from the Mid-Atlantic Ridge. Boundary scavenging is clearly expressed in the water column along a transect between Mauritania and Cape Verde which is used to quantify a bias in sediment fluxes calculated using 230 Th-normalization and to demonstrate enhanced 231 Pa removal from the deep North Atlantic by this mechanism. The influence of deep ocean ventilation that leads to the southward export of 231 Pa is apparent. The 231 Pa/ 230 Th ratio, however, predominantly reflects spatial variability in scavenging intensity, complicating its applicability as a proxy for the Atlantic meridional overturning circulation. [ABSTRACT FROM AUTHOR]

The size partitioning of dissolved Fe (dFe<0.2 µm) into soluble (sFe<0.02 µm) and colloidal (0.02 µm

Size-fractionated aerosol impactor samples (n = 3) representing North American, North African, and marine air mass influences were collected over the North Atlantic Ocean as part of the 2011 US GEOTRACES cruise (GEOTRACES GA03), and a molecular-level chemical characterization of the water soluble organic matter (WSOM) was obtained using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Remarkable differences were found between the molecular characteristics of WSOM from the fine (< 0.49 μm) and coarse size fractions demonstrating the importance of collecting size-fractionated aerosols in order to obtain a full molecular characterization of aerosol WSOM. The greatest molecular complexity, as defined by the number of formulas in each fraction, was found in the < 0.49 μm fraction of North American and North African-influenced aerosols, and in the > 7.2 μm fraction of marine-influenced aerosols. Principal component analysis (PCA) showed the < 0.49 μm fraction of the North American-influenced WSOM to be differentiated from all other size fractions by the prevalence of highly oxidized compounds, an indicator of atmospheric aging. The coarse fractions of the North American-influenced sample showed evidence of important contributions from low O/C (< 0.2) and condensed aromatic compounds indicative of primary combustion-influenced organic matter (OM). The fine fractions of the marine and North African-influenced samples showed characteristics of a primary marine biological source including peptide-like, phospholipid-like, and carbohydrate-like molecular formulas. The coarse fractions of the marine sample and the 0.49–3.0 μm fraction of the North African-influenced sample were defined by molecular formulas at intermediate O/C (0.2 to 0.6) and high H/C (> 1.5) ratios including many heteroatom (N, S, P) containing formulas. Condensed aromatic compounds, which originate from anthropogenic sources and can affect the global climate by absorbing solar radiation, as well as compounds that can be attributed to secondary organic aerosols (SOA), were found in all 17 WSOM fractions analyzed. This indicates the ubiquitous nature of these types of compounds across aerosol size fractions and air mass influences, even in aerosols collected over the middle of the ocean. These results provide an indication of size-fractionated aerosol composition, and future molecular characterization work should be paired with physical characteristics to inform size-fractionated and source-resolved models of aerosol OM impacts in the atmosphere. [ABSTRACT FROM AUTHOR]