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[1] We analyzed d 29 Si of dissolved silicate for eight water column profiles across the Southern Ocean (south of Australia in spring 2001) from the Seasonal Ice Zone (SIZ) north to the Subantarctic Zone (SAZ), including the first isotopic compositions measured for Si-depleted seawaters. All profiles display mixed layer enrichments in heavy Si isotopes relative to deep water in accordance with preferential uptake of the light isotope by diatoms. As silicate levels decrease from the SIZ northward across the Polar Front Zone (PFZ) to the SAZ, surface and mesopelagic d 29 Si signatures generally become progressively heavier, but the most Si-depleted SAZ waters do not exhibit d 29 Si values heavier than in the PFZ. This intricacy appears to derive from variations in the vertical and horizontal supply of silicate to surface waters, and by applying a steady state open system model, we estimate a fractionation factor, 29 e, between diatoms and seawater of À0.45 ± 0.17%, independently of zones and phytoplankton community. Though encouraging, these results are related to latitudinal changes in mesopelagic d 29 Si values, complexity in surface silicateÀd 29 Si correlations, and differences from previous studies, which underline the need for caution in the use of silicon isotopes in paleoceanographic studies until systematic efforts have been undertaken to better understand modern variations.

The cation-exchange purification technique used here does not remove anions (in our case, mostly Cl-, SO 4 2-and to a lesser extent NO 3-) from solutions. In this case, the addition of a known artificial matrix in excess in both the sample and standard solution can be used to dilute the natural concentration of the contaminant and to homogenize sample and standard matrices (doping method, Georg et al., 2006; Hughes et al., 2011). Indeed, dissimilar matrices will affect differently the plasma and ionization efficiency and will induce artificial bias in the delta measurements, invalidating the use of the standard-sample bracketing technique. In our samples, Cl-originating from seawater can be neglected compared to Cladded as HCl (Merck Suprapur) to dissolve the brucite; and as solutions were analyzed in a HCl matrix largely in excess (up to 0.5 mol L-1) compared to natural Cl-concentration. Similarly, the occurrence of NO 3-in seawater was resolved by the use of HNO 3 (Merck Suprapur, 0.5 mol L-1) as a solvent in both the samples and standards. For Depth Depth (m) (m)

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The quantification of silicon isotopic fractionation by biotic and abiotic processes contributes to the understanding of the Si continental cycle. In soils, light Si isotopes are selectively taken up by plants, and concentrate in secondary clay-sized minerals. Si can readily be retrieved from soil solution through the specific adsorption of monosilicic acid (H 4 SiO 4 0 ) by iron oxides. Here, we report on the Si-isotopic fractionation during H 4 SiO 4 0 adsorption on synthesized ferrihydrite and goethite in batch experiment series designed as function of time (0-504 h) and initial concentration (ic) of Si in solution (0.21-1.80 mM), at 20°C, constant pH (5.5) and ionic strength (1 mM). At various contact times, the d 29 Si vs. NBS28 compositions were determined in selected solutions (ic = 0.64 and 1.06 mM Si) by MC-ICP-MS in dry plasma mode with external Mg doping with an average precision of ±0.08& (±2r SEM ). Per oxide mass, ferrihydrite (74-86% of initial Si loading) adsorbed more Si than goethite (37-69%) after 504 h of contact over the range of initial Si concentration 0.42-1.80 mM. Measured against its initial composition (d 29 Si = +0.01 ± 0.04& (±2r SD )), the remaining solution was systematically enriched in 29 Si, reaching maximum d 29 Si values of +0.70 ± 0.07& for ferrihydrite and +0.50 ± 0.08& for goethite for ic 1.06 mM. The progressive 29 Si enrichment of the solution fitted better a Rayleigh distillation path than a steady state model. The fractionation factor 29 e (±1r SD ) was estimated at À0.54 ± 0.03& for ferrihydrite and À0.81 ± 0.12& for goethite. Our data imply that the sorption of H 4 SiO 4 0 onto synthetic iron oxides produced a distinct Si-isotopic fractionation for the two types of oxide but in the same order than that generated by Si uptake by plants and diatoms. They further suggest that the concentration of light Si isotopes in the clay fraction of soils is partly due to H 4 SiO 4 0 sorption onto secondary clay-sized iron oxides.

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

The determination of the plant-induced Si-isotopic fractionation is a promising tool to better quantify their role in the continental Si cycle. Si-isotopic signatures of the different banana plant parts and Si source were measured, providing the isotopic fractionation factor between plant and source. Banana plantlets (Musa acuminata Colla, cv Grande Naine) were grown in hydroponics at variable Si supplies (0.08, 0.42, 0.83 and 1.66 mM Si). Si-isotopic compositions were determined on a multicollector plasma source mass spectrometer (MC-ICP-MS) operating in dry plasma mode. Results are expressed as d 29 Si relative to the NBS28 standard, with an average precision of ± 0.08& (±2r D ). The fractionation factor 29 e between bulk banana plantlets and source solution is -0.40 ± 0.11&. This confirms that plants fractionate Si isotopes by depleting the source solution in 28 Si. The intra-plant fractionation D 29 Si between roots and shoots amounts to -0.21 ± 0.08&. Si-isotopic compositions of the various plant parts indicate that heavy isotopes discrimination occurs at three levels in the plant (at the root epidermis, for xylem loading and for xylem unloading). At each step, preferential crossing of light isotopes leaves a heavier solution, and produces a lighter solution. Si-isotopic fractionation processes are further discussed in relation with Si uptake and transport in plants. These findings have important implications on the study of continental Si cycle.