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We present here the first mercury speciation study in the water column of the Southern Ocean, using a high-resolution south-to-north section (27 stations from 65.50°S to 44.00°S) with up to 15 depths (0-4440 m) between Antarctica and Tasmania (Australia) along the 140°E meridian. In addition, in order to explore the role of sea ice in Hg cycling, a study of mercury speciation in the "snow-sea ice-seawater" continuum was conducted at a coastal site, near the Australian Casey station (66.40°S; 101.14°E). In the open ocean waters, total Hg (Hg T ) concentrations varied from 0.63 to 2.76 pmol L À1 with "transient-type" vertical profiles and a latitudinal distribution suggesting an atmospheric mercury source south of the Southern Polar Front (SPF) and a surface removal north of the Subantartic Front (SAF). Slightly higher mean Hg T concentrations (1.35 ± 0.39 pmol L À1 ) were measured in Antarctic Bottom Water (AABW) compared to Antarctic Intermediate water (AAIW) (1.15 ± 0.22 pmol L À1 ). Labile Hg (Hg R ) concentrations varied from 0.01 to 2.28 pmol L À1 , with a distribution showing that the Hg T enrichment south of the SPF consisted mainly of Hg R (67 ± 23%), whereas, in contrast, the percentage was half that in surface waters north of PFZ (33 ± 23%). Methylated mercury species (MeHg T ) concentrations ranged from 0.02 to 0.86 pmol L À1 . All vertical MeHg T profiles exhibited roughly the same pattern, with low concentrations observed in the surface layer and increasing concentrations with depth up to an intermediate depth maximum. As for Hg T , low mean MeHg T concentrations were associated with AAIW, and higher ones with AABW. The maximum of MeHg T concentration at each station was systematically observed within the oxygen minimum zone, with a statistically significant MeHg T vs Apparent Oxygen Utilization (AOU) relationship (p < 0.001). The proportion of Hg T as methylated species was lower than 5% in the surface waters, around 50% in deep waters below 1000 m, reaching a maximum of 78% south of the SPF. At Casey coastal station Hg T and Hg R concentrations found in the "snow-sea ice-seawater" continuum were one order of magnitude higher than those measured in open ocean waters. The distribution of Hg T there suggests an atmospheric Hg deposition with snow and a fractionation process during sea ice formation, which excludes Hg from the ice with a parallel Hg enrichment of brine, probably concurring with the Hg enrichment of AABW observed in the open ocean waters. Contrastingly, MeHg T concentrations in the sea ice environment were in the same range as in the open ocean waters, remaining below 0.45 pmol L À1 . The MeHg T vertical profile through the continuum suggests different sources, including atmosphere, seawater and methylation in basal ice. Whereas Hg T concentrations in the water samples collected between the Antarctic continent and Tasmania are 0016-7037/$ -see front matter Ó Geochimica et Cosmochimica Acta 75 (2011) 4037-4052 comparable to recent measurements made in the other parts of the World Ocean...

Iron limits primary productivity in vast regions of the ocean. Given that marine phytoplankton contribute up to 40% of global biological carbon fixation, it is important to understand what parameters control the availability of iron (iron bioavailability) to these organisms. Most studies on iron bioavailability have focused on the role of siderophores; however, eukaryotic phytoplankton do not produce or release siderophores. Here, we report on the pivotal role of saccharides-which may act like an organic ligand-in enhancing iron bioavailability to a Southern Ocean cultured diatom, a prymnesiophyte, as well as to natural populations of eukaryotic phytoplankton. Addition of a monosaccharide (>2 nM of glucuronic acid, GLU) to natural planktonic assemblages from both the polar front and subantarctic zones resulted in an increase in iron bioavailability for eukaryotic phytoplankton, relative to bacterioplankton. The enhanced iron bioavailability observed for several groups of eukaryotic phytoplankton (i.e., cultured and natural populations) using three saccharides, suggests it is a common phenomenon. Increased iron bioavailability resulted from the combination of saccharides forming highly bioavailable organic associations with iron and increasing iron solubility, mainly as colloidal iron. As saccharides are ubiquitous, present at nanomolar to micromolar concentrations, and produced by biota in surface waters, they also satisfy the prerequisites to be important constituents of the poorly defined "ligand soup," known to weakly bind iron. Our findings point to an additional type of organic ligand, controlling iron bioavailability to eukaryotic phytoplankton-a key unknown in iron biogeochemistry.trace metals | carbohydrates | organic matter | exopolymeric substances | plankton

The GEOTRACES Intermediate Data Product 2014 (IDP2014) is the first publicly available data product of the international GEOTRACES programme, and contains data measured and quality controlled before the end of 2013. It consists of two parts: (1) a compilation of digital data for more than 200 trace elements and isotopes (TEls) as well as classical hydrographic parameters, and (2) the eGEOTRACES Electronic Atlas providing a strongly inter-linked on-line atlas including more than 300 section plots and 90 animated 3D scenes. The IDP2014 covers the Atlantic, Arctic, and Indian oceans, exhibiting highest data density in the Atlantic. The TEI data in the IDP2014 are quality controlled by careful assessment of intercalibration results and multi-laboratory data comparisons at cross-over stations. The digital data are provided in several formats, including ASCII spreadsheet, Excel spreadsheet, netCDF, and Ocean Data View collection. In addition to the actual data values the IDP2014 also contains data quality flags and 1-sigma data error values where available. Quality flags and error values are useful for data filtering. Metadata about data originators, analytical methods and original publications related to the data are linked to the data in an easily accessible way. The eGEOTRACES Electronic Atlas is the visual representation of the IDP2014 data providing section plots and a new kind of animated 3D scenes. The basin-wide 3D scenes allow for viewing of data from many cruises at the same time, thereby providing quick overviews of large-scale tracer distributions. In addition, the 3D scenes provide geographical and bathymetric context that is crucial for the interpretation and assessment of observed tracer plumes, as well as for making inferences about controlling processes. (C) 2015 The Authors. Published by Elsevier B.V