Description: Author Posting. © Association for the Sciences of Limnology and Oceanography, 2013. This article is posted here by permission of Association for the Sciences of Limnology and Oceanography for personal use, not for redistribution. The definitive version was published in Limnology and Oceanography: Methods 11 (2013): 62-78, doi:10.4319/lom.2013.11.62.

Description: Atmospheric deposition of trace elements and isotopes (TEI) is an important source of trace metals to the open ocean, impacting TEI budgets and distributions, stimulating oceanic primary productivity, and influencing biological community structure and function. Thus, accurate sampling of aerosol TEIs is a vital component of ongoing GEOTRACES cruises, and standardized aerosol TEI sampling and analysis procedures allow the comparison of data from different sites and investigators. Here, we report the results of an aerosol analysis intercalibration study by seventeen laboratories for select GEOTRACES-relevant aerosol species (Al, Fe, Ti, V, Zn, Pb, Hg, NO3 , and SO42 ) for samples collected in September 2008. The collection equipment and filter substrates are appropriate for the GEOTRACES program, as evidenced by low blanks and detection limits relative to analyte concentrations. Analysis of bulk aerosol sample replicates were in better agreement when the processing protocol was constrained (± 9% RSD or better on replicate analyses by a single lab, n = 7) than when it was not (generally 20% RSD or worse among laboratories using different methodologies), suggesting that the observed variability was mainly due to methodological differences rather than sample heterogeneity. Much greater variability was observed for fractional solubility of aerosol trace elements and major anions, due to differing extraction methods. Accuracy is difficult to establish without an SRM representative of aerosols, and we are developing an SRM for this purpose. Based on these findings, we provide recommendations for the GEOTRACES program to establish consistent and reliable procedures for the collection and analysis of aerosol samples.

Description: This work was partially funded by the following sources: US National Science Foundation (NSF) grant OCE- 0752832 (PLM, WML, and AM), National Science Council Taiwan grant 100-2628-M-001-008-MY4 (SCH), US NSF grant OCE-1137836 (AMA-I), United Kingdom Natural Environmental Research Council (NERC) grant NE/H00548X/1 (AR Baker), Australian Government Cooperative Research Centres Programme (AR Bowie), US NSF grant OCE-0824304 (CSB and Adina Paytan), US NSF grants OCE-0825068 and OCE- 0728750 (SG and Robert Mason), US NSF grant OCE-0961038 (MGH), US NSF grant OCE-0752609 (MH and Christopher Measures), US NSF grant ATM-0839851 (AMJ), US NSF grant OCE-1031371 (CM), UK NERC grant NE/C001931/1 (MDP and Eric Achterberg), US NSF grant OCE-1132515 (GS and Carl Lamborg), US NSF grant OCE-0851462 (AV and Thomas Church), and US NSF grant OCE-0623189 (LMZ). This paper is part of the Intercalibration in Chemical Oceanography special issue of L&O Methods that was supported by funding from the US National Science Foundation, Chemical Oceanography Program (grant OCE-0927285 to Gregory Cutter).

Description: The island of South Georgia is situated in the iron (Fe) depleted Antarctic Circumpolar Current of the Southern Ocean. Iron emanating from its shelf system fuels large phytoplankton blooms downstream of the island, but the actual supply mechanisms are unclear. To address this we present the first inventory of Fe, manganese (Mn) and aluminium (Al) in shelf sediments, pore waters and the water column in the vicinity of South Georgia, alongside data on zooplankton-mediated Fe cycling processes. The seafloor sediments were the main particulate Fe source to shelf bottom waters as indicated by Fe / Mn and Fe / Al ratios for shelf sediments and suspended particles in the water column. Less than 1 % of the total particulate Fe pool was leachable surface adsorbed (labile) Fe, and therefore potentially available to organisms. Pore waters formed the primary dissolved Fe (DFe) source to shelf bottom waters supplying 0.1–4 μmol DFe m−2 d−1. However, only 0.41 ± 0.26 μmol DFe m−2 d−1 was transferred to the surface mixed layer by vertical diffusive and advective mixing. Other trace metal sources to surface waters included glacial flour released by melting glaciers and zooplankton excretion processes. On average 6.5 ± 8.2 μmol m−2 d−1 of labile particulate Fe was supplied to the surface mixed layer via krill faecal pellets, with further DFe released by krill at around 1.1 ± 2.2 μmol m−2 d−1. The faecal pellets released by krill constituted of seafloor derived lithogenic material and settled algae debris, in addition to freshly ingested suspended phytoplankton specimen. The phytoplankton Fe requirement in the blooms ca. 1250 km downstream the island of South Georgia was 0.33 ± 0.11 μmol m−2 d−1, with the DFe supply by horizontal/vertical mixing, deep winter mixing and via aeolian dust estimated as ~ 0.12 μmol m−2 d−1. We suggest that additionally required DFe was provided through recycling of biogenically stored Fe following luxury Fe uptake by phytoplankton on the Fe rich shelf. This process would allow Fe to be retained in the surface mixed layer of waters downstream of South Georgia through continuous recycling and biological uptake, and facilitate the large scale blooms.

Description: During the winter of 2006 we measured nifH gene abundances, dinitrogen (N2) fixation rates and carbon fixation rates in the eastern tropical and sub-tropical North Atlantic Ocean. The dominant diazotrophic phylotypes were filamentous cyanobacteria, which may include Trichodesmium and Katagnymene, with up to 106 L−1 nifH gene copies, unicellular group A cyanobacteria with up to 105 L−1 nifH gene copies and gamma A proteobacteria with up to 104 L−1 nifH gene copies. N2 fixation rates were low and ranged between 0.032–1.28 nmol N L−1 d−1 with a mean of 0.30±0.29 nmol N L−1 d−1 (1σ, n = 65). CO2-fixation rates, representing primary production, appeared to be nitrogen limited as suggested by low dissolved inorganic nitrogen to phosphate ratios (DIN:DIP) of about 2±3.2 in surface waters. Nevertheless, N2 fixation rates contributed only 0.55±0.87% (range 0.03–5.24%) of the N required for primary production. Boosted regression trees analysis (BRT) showed that the distribution of the gamma A proteobacteria and filamentous cyanobacteria nifH genes was mainly predicted by the distribution of Prochlorococcus, Synechococcus, picoeukaryotes and heterotrophic bacteria. In addition, BRT indicated that multiple a-biotic environmental variables including nutrients DIN, dissolved organic nitrogen (DON) and DIP, trace metals like dissolved aluminum (DAl), as a proxy of dust inputs, dissolved iron (DFe) and Fe-binding ligands as well as oxygen and temperature influenced N2 fixation rates and the distribution of the dominant diazotrophic phylotypes. Our results suggest that lower predicted oxygen concentrations and higher temperatures due to climate warming may increase N2 fixation rates. However, the balance between a decreased supply of DIP and DFe from deep waters as a result of more pronounced stratification and an enhanced supply of these nutrients with a predicted increase in deposition of Saharan dust may ultimately determine the consequences of climate warming for N2 fixation in the North Atlantic.

Description: Highlights • Aerosol time-series measurements on the Cape Verde reveal seasonal variations • High aerosol loadings in winter at Cape Verde related to dust transport in trade winds • Dust supply during summer originating mainly from the Sahel region • Dust supply during winter originating mainly from the North and West Saharan region • Clay dominant mineral in aerosols over Cape Verde, influencing iron solubility Abstract The North Atlantic receives the largest dust loading of any of the world’s oceans due to its proximity to North African deserts and prevailing wind patterns. The supply of biologically important trace elements and nutrients via aerosols has an important influence on biogeochemical processes and ecosystems in this ocean region. In this study we continuously sampled aerosols between July 2007 and July 2008 at the Cape Verde Atmospheric Observatory (CVAO), which is situated on an island group close to the North African continent and under the Saharan/Sahelian dust outflow path. The aim of our work was to investigate temporal variations in aerosol concentration, composition and sources in the Cape Verde region over a complete seasonal cycle, and for this purpose we undertook mineralogical and chemical (42 elements) analyses of the aerosol samples and air mass back-trajectory calculations. Aerosol samples were also collected during a research cruise in the (sub-) tropical Northeast Atlantic Ocean in January 2008. The concentration of atmospheric Al, a proxy for mineral aerosol concentration, at CVAO was in the range 0.01 – 66.9 μg.m- 3 (maximum on 28-30 January 2008) with a geometric mean of 0.76 μg.m- 3. It showed distinct seasonal variations, with enhanced Al concentrations in winter (geometric mean 1.3 μg.m- 3), and lower concentrations in summer (geometric mean 0.48 μg.m- 3). These observations have been attributed to dust transport occurring in higher altitude air layers and mainly north of the Cape Verde during summer, whilst in winter the dust transport shifts south and occurs in the lower altitude trade winds with consequent greater influence on the Cape Verde region. The elemental composition of the aerosols closely agreed with mean upper crustal abundances, with the exception of elements with pronounced anthropogenic sources (e.g. Zn and Pb) and major constituents of sea water (Na and Mg). Mineral analysis showed that clays were the most abundant mineral fraction throughout the whole sampling period, with an increase in quartz and clays during strong dust events and an associated decrease in calcite. This could have important implications for the estimation of release of for example Fe from mineral dust with clays having a higher Fe solubility than quartz. The elemental composition and mineralogy of aerosol samples collected during the cruise were indistinguishable from those collected at the CVAO during the same period, although mean atmospheric Al was 65% higher at the CVAO than those measured on the ship due to the irregular and uneven nature of dust transport. Air mass back-trajectories showed an important role for southern source regions of the North African deserts during summer, with 92.5% of the samples indicating a contribution from the Sahel. Significantly elevated ratios of V, Ni, Cu, Zn, Cd and Pb with Al were present in samples originating from the Sahel compared with samples with a more northerly origin. This was likely due to enhanced anthropogenic emissions related to the greater population densities in the Sahel compared with the less developed Saharan regions further north. Ratios of other elements and trends in rare earth elements could however not be used to distinguish differences in source regions. Similar source material compositions, the mixing of dust from different regions during transport, and the pooling of samples over a 1-3 day collection period appears to have diluted specific signals from source regions.