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: The distribution of dissolved aluminium (dAl) in the water column of the North Atlantic and Labrador Sea was studied along GEOTRACES section GA01 to unravel the sources and sinks of this element. Surface water dAl concentrations were low (median of 2.5 nM) due to low aerosol deposition and removal by phytoplankton. However, surface water dAl concentrations were enhanced on the Iberian and Greenland shelves (up to 30.9 nM) due to continental inputs (rivers, glacial flour and ice melt). A negative correlation was observed between dAl in surface waters and primary production, phytoplankton community structure and biogenic opal production. The abundance of diatoms exerted a significant (p 〈 0.01) control on the surface particulate Al (pAl) to dAl ratios by decreasing dAl levels and increasing pAl levels. Dissolved Al concentrations generally increased with depth and correlated strongly with silicate (R2 〉 0.76) west of the Iberian Basin, suggesting net release of dAl at depth during remineralization of sinking biogenic opal containing particles. Enrichment of dAl at near-bottom depths was observed due to resuspension of sediments near the sediment-water interface. The highest dAl (up to 38.7 nM) concentrations were observed in Mediterranean Overflow Waters which act as a major source of dAl to mid depth waters of the eastern North Atlantic. This study clearly shows that the vertical and lateral distribution of dAl in the North Atlantic differs when compared to other regions of the North Atlantic and global ocean due to the large spatial differences both in the main source of Al, atmospheric deposition, and the main sink for Al, particle scavenging, between different oceanic regions.

Description: The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this Special Issue, results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among seventeen articles. Here, the scientific context, project objectives and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue.

Description: Trace element deposition from desert dust has important impacts on ocean primary productivity, the quantification of which could be useful in determining the magnitude and sign of the biogeochemical feedback on radiative forcing. However, the impact of elemental deposition to remote ocean regions is not well understood and is not currently included in global climate models. In this study, emission inventories for eight elements primarily of soil origin, Mg, P, Ca, Mn, Fe, K, Al, and Si are determined based on a global mineral data set and a soil data set. The resulting elemental fractions are used to drive the desert dust model in the Community Earth System Model (CESM) in order to simulate the elemental concentrations of atmospheric dust. Spatial variability of mineral dust elemental fractions is evident on a global scale, particularly for Ca. Simulations of global variations in the Ca / Al ratio, which typically range from around 0.1 to 5.0 in soils, are consistent with observations, suggesting that this ratio is a good signature for dust source regions. The simulated variable fractions of chemical elements are sufficiently different; estimates of deposition should include elemental variations, especially for Ca, Al and Fe. The model results have been evaluated with observations of elemental aerosol concentrations from desert regions and dust events in non-dust regions, providing insights into uncertainties in the modeling approach. The ratios between modeled and observed elemental fractions range from 0.7 to 1.6, except for Mg and Mn (3.4 and 3.5, respectively). Using the soil database improves the correspondence of the spatial heterogeneity in the modeling of several elements (Ca, Al and Fe) compared to observations. Total and soluble dust element fluxes to different ocean basins and ice sheet regions have been estimated, based on the model results. The annual inputs of soluble Mg, P, Ca, Mn, Fe and K associated with dust using the mineral data set are 0.30 Tg, 16.89 Gg, 1.32 Tg, 22.84 Gg, 0.068 Tg, and 0.15 Tg to global oceans and ice sheets.

Description: Dissolved iron (DFe; 〈0.2 µm) and dissolved manganese (DMn; 〈0.2 µm) concentrations were determined in the water column of the Bay of Biscay (eastern North Atlantic Ocean) in March 2002. The samples were collected along a transect traversing from the European continental shelf over the continental slope. The highest DFe and DMn concentrations (2.39 nM and 6.10 nM, respectively) were observed in the bottom waters on the shelf at stations closest to the coast. The release of trace metal from resuspended particles and the diffusion from pore waters were probably at the origin of elevated DFe and DMn concentrations in the Bottom Boundary Layer (BBL). In the slope region, the highest total dissolvable iron (TDFe), DFe and DMn values (24.6 nM, 1.58 nM and 2.12 nM, respectively) were observed close to the bottom at depth of ca.~600–700 m. Internal wave activity and slope circulation are thought to be at the origin of this phenomenon. These processes were also very likely the cause of elevated concentrations (DFe: 1.27 nM, DMn: 2.34 nM) measured in surface waters of stations located in the same area. At stations off the continental slope, the vertical distribution of both metals were typical of open ocean conditions, indicating that inputs from the continental margin did not impact the metal distributions in the offshore waters.

Description: Coccolithophores are a key functional group in terms of the pelagic production of calcium carbonate (calcite), although their contribution to shelf sea biogeochemistry, and how this relates to environmental conditions, is poorly constrained. Measurements of calcite production (CP) and coccolithophore abundance were made on the north-west European shelf to examine trends in coccolithophore calcification along natural gradients of carbonate chemistry, macronutrient availability and plankton composition. Similar measurements were also made in three bioassay experiments where nutrient (nitrate, phosphate) and pCO2 levels were manipulated. Nanoflagellates (〈 10 μm) dominated chlorophyll biomass and primary production (PP) at all but one sampling site, with CP ranging from 0.6 to 9.6 mmol C m−2 d−1. High CP and coccolithophore abundance occurred in a diatom bloom in fully mixed waters off Heligoland, but not in two distinct coccolithophore blooms in the central North Sea and Western English Channel. Coccolithophore abundance and CP showed no correlation with nutrient concentrations or ratios, while significant (p 〈 0.01) correlations between CP, cell-specific calcification (cell-CF) and irradiance in the water column highlighted how light availability exerts a strong control on pelagic CP. In the experimental bioassays, Emiliania-huxleyi-dominated coccolithophore communities in shelf waters (northern North Sea, Norwegian Trench) showed a strong response in terms of CP to combined nitrate and phosphate addition, mediated by changes in cell-CF and growth rates. In contrast, an offshore diverse coccolithophore community (Bay of Biscay) showed no response to nutrient addition, while light availability or mortality may have been more important in controlling this community. Sharp decreases in pH and a rough halving of calcite saturation states in the bioassay experiments led to decreased CP in the Bay of Biscay and northern North Sea, but not the Norwegian Trench. These decreases in CP were related to slowed growth rates in the bioassays at elevated pCO2 (750 μatm) relative to those in the ambient treatments. The combined results from our study highlight the variable coccolithophore responses to irradiance, nutrients and carbonate chemistry in north-west European shelf waters, which are mediated by changes in growth rates, cell-CF and species composition.

Description: In this study we report diapycnal diffusive fluxes of dissolved iron (dFe), dissolved aluminium (dAl) and the major macronutrients to the surface waters of the North Atlantic subpolar gyre. Turbulent diffusivities at the base of the summer mixed layer ranged from 0.01 to 0.5 (median 0.07) cm2 s−1 and daily macronutrient fluxes into the surface mixed layer typically represented 〈 0.5% of integrated mixed layer inventories, although fluxes were highly variable. Elevated nutrient fluxes of up to 4% of mixed layer inventories were identified on the Greenland Shelf, where integrated nutrient pools were lowest due to localised shoaling of the mixed layer. Diffusive fluxes of dFe and dAl were typically 〈0.1% of mixed layer inventories but were also highly variable between stations. Approximations of daily phytoplankton nutrient and Fe uptake indicate that the diffusive flux may at best represent 〈10% of phytoplankton macronutrient uptake, and only 1% of daily phytoplankton Fe uptake. The daily turbulent diffusive flux of dFe was comparable in magnitude to coincident estimates of aeolian Fe supply but despite shallower than normal convective mixing in winter 2010 the diffusive supply was 22 and 59 times smaller than the annual convective supply of Fe to the Irminger and Iceland basins respectively. The general picture obtained from this study is one of small magnitude diffusive nutrient and Fe fluxes to the subpolar North Atlantic during the period of annual nutrient minima and indicates that the diffusive supply mechanism is unlikely to alleviate the recently identified presence of seasonal iron limitation within the North Atlantic subpolar gyre; a condition exacerbated by low dFe:NO3− ratios in subsurface source waters.

Description: Mineral dust supplied to remote ocean regions stimulates phytoplankton growth through delivery of micronutrients, notably iron (Fe). Although attention is usually paid to Fe (hydr)oxides as major sources of available Fe, Fe-bearing clay minerals are typically the dominant phase in mineral dust. The mineralogy and chemistry of clay minerals in dust particles, however, are largely unknown. We conducted microscopic identification and chemical analysis of the clay minerals in Asian and Saharan dust particles. Cross-sectional slices of dust particles were prepared by focused ion beam (FIB) techniques and analyzed by transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDXS). TEM images of FIB slices revealed that clay minerals occurred as either nano-thin platelets or relatively thick plates. Chemical compositions and lattice fringes of the nano-thin platelets suggested that they included illite, smectite, illite–smectite mixed layers, and their nanoscale mixtures (illite–smectite series clay minerals, ISCMs) which could not be resolved with an electron microbeam. EDXS chemical analysis of the clay mineral grains revealed that the average Fe content was 5.8% in nano-thin ISCM platelets assuming 14% H2O, while the Fe content of illite and chlorite was 2.8 and 14.8%, respectively. In addition, TEM and EDXS analyses were performed on clay mineral grains dispersed and loaded on micro-grids. The average Fe content of clay mineral grains was 6.7 and 5.4% in Asian and Saharan dusts, respectively. A comparative X-ray diffraction analysis of bulk dusts showed that Saharan dust was more enriched in clay minerals than Asian dust, while Asian dust was more enriched in chlorite. Clay minerals, in particular nanocrystalline ISCMs and Fe-rich chlorite, are probably important sources of Fe to remote marine ecosystems. Further detailed analyses of the mineralogy and chemistry of clay minerals in global mineral dusts are required to evaluate the inputs of Fe to surface ocean microbial communities.

Description: Comprehensive synoptic datasets (surface water down to 4000 m) of dissolved cadmium (Cd), copper (Cu), manganese (Mn), lead (Pb) and silver (Ag) are presented along a section between 34° S and 57° S in the southeastern Atlantic Ocean and the Southern Ocean to the south off South Africa. The vertical distributions of Cu and Ag display nutrient-like profiles similar to silicic acid, and of Cd similar to phosphate. The distribution of Mn shows a subsurface maximum in the oxygen minimum zone, whereas Pb concentrations are rather invariable with depth. Dry deposition of aerosols is thought to be an important source of Pb to surface waters close to South Africa, and dry deposition and snowfall may have been significant sources of Cu and Mn at the higher latitudes. Furthermore, the advection of water masses enriched in trace metals following contact with continental margins appeared to be an important source of trace elements to the surface, intermediate and deep waters in the southeastern Atlantic Ocean and the Antarctic Circumpolar Current. Hydrothermal inputs may have formed a source of trace metals to the deep waters over the Bouvet Triple Junction ridge crest, as suggested by relatively enhanced dissolved Mn concentrations. The biological utilization of Cu and Ag was proportional to that of silicic acid across the section, suggesting that diatoms formed an important control over the removal of Cu and Ag from surface waters. However, uptake by dino- and nano-flagellates may have influenced the distribution of Cu and Ag in the surface waters of the subtropical Atlantic domain. Cadmium correlated strongly with phosphate (P), yielding lower Cd / P ratios in the subtropical surface waters where phosphate concentrations were below 0.95 μM. The greater depletion of Cd relative to P observed in the Weddell Gyre compared to the Antarctic Circumpolar Current could be due to increase Cd uptake induced by iron-limiting conditions in these high-nutrient–low-chlorophyll waters. Similarly, an increase of Mn uptake under Fe-depleted conditions may have caused the highest depletion of Mn relative to P in the surface waters of the Weddell Gyre. In addition, a cellular Mn-transport channel of Cd was possibly activated in the Weddell Gyre, which in turn may have yielded depletion of both Mn and Cd in these surface waters.