Savoye Nicolas

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  • Article
    Th-234 deficit and excess in the Southern Ocean during spring 2001 : particle export and remineralization
    (American Geophysical Union, 2004-06-16) Savoye, Nicolas ; Buesseler, Ken O. ; Cardinal, Damien ; Dehairs, Frank
    234Th deficit and excess were examined in the upper 500 m of the Southern Ocean from Sub-Antarctic to Seasonal Ice Zones (Australian sector) during austral spring 2001. 234Th fluxes at 100 m indicate that particle export was low in the North (46.9–51.0°S), minimal in the Polar and Inter-Polar Frontal Zones and high in the South (≥61°S). These results are in tight agreement with new production estimates from the same cruise. Our data indicate that Polar and Inter-Polar Frontal Zones were not zones of intense export in the Australian sector at this time of year, in contrast with other sectors of the Southern Ocean. Also, we highlight the usefulness of 234Th excess below the mixed layer as a tool to study mesopelagic remineralization.
  • Preprint
    A review of present techniques and methodological advances in analyzing Th-234 in aquatic systems
    ( 2005-10-10) Rutgers van der Loeff, Michiel M. ; Sarin, Manmohan M. ; Baskaran, Mark ; Benitez-Nelson, Claudia R. ; Buesseler, Ken O. ; Charette, Matthew A. ; Dai, Minhan ; Gustafsson, Orjan ; Masqué, Pere ; Morris, Paul J. ; Orlandini, Kent ; Rodriguez y Baena, Alessia ; Savoye, Nicolas ; Schmidt, Sabine ; Turnewitsch, Robert ; Voge, Ingrid ; Waples, James T.
    The short-lived thorium isotope 234Th (half-life 24.1 days) has been used as a tracer for a variety of transport processes in aquatic systems. Its use as a tracer of oceanic export via sinking particles has stimulated a rapidly increasing number of studies that require analyses of 234Th in both marine and freshwater systems. The original 234Th method is labour intensive. Thus, there has been a quest for simpler techniques that require smaller sample volumes. Here, we review current methodologies in the collection and analysis of 234Th from the water column, discuss their individual strengths and weaknesses, and provide an outlook on possible further improvements and future challenges. Also included in this review are recommendations on calibration procedures and the production of standard reference materials as well as a flow chart designed to help researchers find the most appropriate 234Th analytical technique for a specific aquatic regime and known sampling constraints.
  • Preprint
    Primary, new and export production in the NW Pacific subarctic gyre during the vertigo K2 experiments
    ( 2008-01-28) Elskens, Marc ; Brion, N. ; Buesseler, Ken O. ; Van Mooy, Benjamin A. S. ; Boyd, Philip W. ; Dehairs, Frank ; Savoye, Nicolas ; Baeyens, W.
    This paper presents results on tracer experiments using 13C and 15N to estimate uptake rates of dissolved inorganic carbon (DIC) and nitrogen (DIN). Experiments were carried out at station K2 (47°N, 161°E) in the NW Pacific subarctic gyre during July-August 2005. Our goal was to investigate relationships between new and export production. New production was inferred from the tracer experiments using the f ratio concept (0-50m); while export production was assessed with neutrally buoyant sediment traps (NBSTs) and the e ratio concept (at 150m). During trap deployments, K2 was characterized both by changes in primary production (523 to 404 mg C m-2 d-1), new production (119 to 67 mg C m-2 d-1), export production (68 to 24 mg C m-2 d-1) and phytoplankton composition (high to low proportion of diatoms). The data indicate that 17 to 23% of primary production is exportable to deeper layers (f ratio) but only 6 to 13% collected as a sinking particle flux at 150m (e ratio). Accordingly, > 80% of the carbon fixed by phytoplankton would be mineralized in the upper 50m (1 – f), while < 11% would be within 50-150m (f – e). DIN uptake flux amounted to 0.5 mM m-2 h-1, which was equivalent to about 95% particulate nitrogen (PN) remineralized and/or grazed within the upper 150 m. Most of the shallow PN remineralization occurred just above the depth of the deep chlorophyll maximum (DCM), where a net ammonium production was measured. Below the DCM, while nitrate uptake rates became negligible because of light limitation, ammonium uptake did continue to be significant. The uptake of ammonium by heterotrophic bacteria was estimated to be 14-17% of the DIN assimilation. Less clear are the consequences of this uptake on the phytoplankton community and biogeochemical processes, e.g. new production. It was suggested that competition for ammonium could select for small cells and may force large diatoms to use nitrate. This implies that under Fe stress as observed here, ammonium uptake is preferred and new production progressively suppressed despite the surplus of nitrate.
  • Preprint
    An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of 234Th as a POC flux proxy
    ( 2005-06-18) Buesseler, Ken O. ; Benitez-Nelson, Claudia R. ; Burd, Adrian B. ; Charette, Matthew A. ; Cochran, J. Kirk ; Coppola, L. ; Fisher, Nicholas S. ; Fowler, Scott W. ; Gardner, Wilford D. ; Guo, L. D. ; Gustafsson, Orjan ; Lamborg, Carl H. ; Masqué, Pere ; Miquel, Juan Carlos ; Passow, Uta ; Santschi, Peter H. ; Savoye, Nicolas ; Stewart, G. ; Trull, Thomas W.
    Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the 234Th activity distribution (in order to calculate 234Th fluxes) and an estimate of the C/234Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C/234Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C/234Th either increases or is relatively invariant with increasing particle size (size classes >1 to 100’s μm). Observations also suggest that C/234Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from 234Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2-3). We recommend sampling for C/234Th from a standard depth of 100 m, or at least one depth below the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple 234Th profiles and C/234Th samples during the course of longer observation periods to better sample temporal variations in both 234Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column.
  • Preprint
    Th-234 sorption and export models in the water column : a review
    ( 2005-10-10) Savoye, Nicolas ; Benitez-Nelson, Claudia R. ; Burd, Adrian B. ; Cochran, J. Kirk ; Charette, Matthew A. ; Buesseler, Ken O. ; Jackson, George A. ; Roy-Barman, Matthieu ; Schmidt, Sabine ; Elskens, Marc
    Over the past few decades, the radioisotope pair of 238U/234Th has been widely and increasingly used to describe particle dynamics and particle export fluxes in a variety of aquatic systems. The present paper is one of five review articles dedicated to 234Th. It is focused on the models associated with 234Th whereas the companion papers (same issue) are focused on present and future methodologies and techniques (Rutgers van der Loeff et al.), C/234Th ratios (Buesseler et al.), 234Th speciation (Santschi et al.) and present and future applications of 234Th (Waples et al.). In this paper, we review current 234Th scavenging models and discuss the relative importance of the non steady state and physical terms associated with the most commonly used model to estimate 234Th flux. Based on this discussion we recommend that for future work the use of models should be accompanied by a discussion of the effect that model and data uncertainty have on the model results. We also suggest that future field work incorporate repeat occupations of sample sites on time scales of 1-4 weeks in order to evaluate steady state versus non steady state estimates of 234Th export, especially during high flux events (> ca. 800 dpm m-2 d-1). Finally, knowledge of the physical oceanography of the study area is essential, particularly in ocean margins and in areas of established upwelling (e.g. Equatorial Pacific). These suggestions will greatly enhance the application of 234Th as a tracer of particle dynamics and flux in more complicated regimes.
  • Preprint
    Barium in twilight zone suspended matter as a potential proxy for particulate organic carbon remineralization : results for the North Pacific
    ( 2008-01-22) Dehairs, Frank ; Jacquet, Stéphanie H. M. ; Savoye, Nicolas ; Van Mooy, Benjamin A. S. ; Buesseler, Ken O. ; Bishop, James K. B. ; Lamborg, Carl H. ; Elskens, Marc ; Baeyens, W. ; Boyd, Philip W. ; Casciotti, Karen L. ; Monnin, Christophe
    This study focuses on the fate of exported organic carbon in the twilight zone at two contrasting environments in the North Pacific: the oligotrophic ALOHA site (22°45' N 158°W; Hawaii; studied during June–July 2004) and the mesotrophic Subarctic Pacific K2 site (47°N, 161°W; studied during July-August 2005). Earlier work has shown that non-lithogenic, excess particulate Ba (Baxs) in the mesopelagic water column is a potential proxy of organic carbon remineralization. In general Baxs contents were significantly larger at K2 than at ALOHA. At ALOHA the Baxs profiles from repeated sampling (5 casts) showed remarkable consistency over a period of three weeks, suggesting that the system was close to being at steady state. In contrast, more variability was observed at K2 (6 casts sampled) reflecting the more dynamic physical and biological conditions prevailing in this environment. While for both sites Baxs concentrations increased with depth, at K2 a clear maximum was present between the base of the mixed layer at around 50m and 500m, reflecting production and release of Baxs. Larger mesopelagic Baxs contents and larger bacterial production in the twilight zone at the K2 site indicate that more material was exported from the upper mixed layer for bacterial degradation deeper, compared to the ALOHA site. Furthermore, application of a published transfer function (Dehairs et al., 1997) relating oxygen consumption to the observed Baxs data indicated that the latter were in good agreement with bacterial respiration, calculated from bacterial production. These results corroborate earlier findings highlighting the potential of Baxs as a proxy for organic carbon remineralization. The range of POC remineralization rates calculated from twilight zone excess particulate Ba contents did also compare well with the depth dependent POC flux decrease as recorded by neutrally buoyant sediment traps, except in 1 case (out of 4). This discrepancy could indicate that differences in sinking velocities cause an 3 uncoupling of the processes occurring in the fine suspended particle pool from those affecting the larger particle pool which sustains the vertical flux, thus rendering comparison between both approaches risky.