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International audience; The silicon isotopic composition of silicic acid (δ 30 Si(OH) 4) and biogenic silica (δ 30 Si-bSiO 2) were measured for the first time in marine Arctic waters from the Mackenzie River delta to the deep Canada Basin in the late summer of 2009. In the upper 100 m of the water column, δ 30 Si(OH) 4 signals (+1.82‰ to +3.08‰) were negatively correlated with the relative contribution of Mackenzie River water. The biogenic Si isotope fractionation factor estimated using an open system model, 30 ε = À0.97 ± 0.17‰, agrees well with laboratory and global-ocean estimates. Nevertheless, the δ 30 Si dynamics of this region may be better represented by closed system isotope models that yield lower values of 30 ε, between À0.33‰ and À0.41‰, depending on how the contribution of sea-ice diatoms is incorporated. In the upper 400 m, δ 30 Si-bSiO 2 values were among the heaviest ever measured in marine suspended bSiO 2 (+2.03‰ to +3.51‰). A positive correlation between δ 30 Si-bSiO 2 and sea-ice cover implies that heavy signals can result from isotopically heavy sea-ice diatoms introduced to pelagic assemblages. Below the surface bSiO 2 production zone, the δ 30 Si(OH) 4 distribution followed that of major water masses. Vertical δ 30 Si(OH) 4 profiles showed a minimum (average of +1.84 ± 0.10‰) in the upper halocline (125-200 m) composed of modified Pacific water and heavier average values (+2.04 ± 0.11‰) in Atlantic water (300-500 m deep). In the Canada Basin Deep Water (below 2000 m), δ 30 Si(OH) 4 averaged +1.88 ± 0.12‰, which represents the most positive value ever measured anywhere in the deep ocean. Since most Si(OH) 4 enters the Arctic from shallow depths in the Atlantic Ocean, heavy deep Arctic δ 30 Si(OH) 4 signals likely reflect the influx of relatively heavy intermediate Atlantic waters. A box model simulation of the global marine δ 30 Si(OH) 4 distribution successfully reproduced the observed patterns, with the δ 30 Si(OH) 4 of the simulated deep Arctic Ocean being the heaviest of all deep-ocean basins.

International audience; The accelerated decline in Arctic sea ice and an ongoing trend toward more energetic atmospheric and oceanic forcings are modifying carbon cycling in the Arctic Ocean. A critical issue is to understand how net community production (NCP; the balance between gross primary production and community respiration) responds to changes and modulates air-sea CO2 fluxes. Using data collected as part of the ArcticNet-Malina 2009 expedition in the southeastern Beaufort Sea (Arctic Ocean), we synthesize information on sea ice, wind, river, water column properties, metabolism of the planktonic food web, organic carbon fluxes and pools, as well as air-sea CO2 exchange, with the aim of documenting the ecosystem response to environmental changes. Data were analyzed to develop a non-steady-state carbon budget and an assessment of NCP against air-sea CO2 fluxes. During the field campaign, the mean wind field was a mild upwelling-favorable wind (~ 5 km h-1) from the NE. A decaying ice cover (< 80% concentration) was observed beyond the shelf, the latter being fully exposed to the atmosphere. We detected some areas where the surface mixed layer was net autotrophic owing to high rates of primary production (PP), but the ecosystem was overall net heterotrophic. The region acted nonetheless as a sink for atmospheric CO2, with an uptake rate of -2.0 ± 3.3 mmol C m-2 d-1 (mean ± standard deviation associated with spatial variability). We attribute this discrepancy to (1) elevated PP rates (> 600 mg C m-2 d-1) over the shelf prior to our survey, (2) freshwater dilution by river runoff and ice melt, and (3) the presence of cold surface waters offshore. Only the Mackenzie River delta and localized shelf areas directly affected by upwelling were identified as substantial sources of CO2 to the atmosphere (> 10 mmol C m-2 d-1). Daily PP rates were generally < 100 mg C m-2 d-1 and cumulated to a total PP of ~ 437.6 × 103 t C for the region over a 35-day period. This amount was about twice the organic carbon delivery by river inputs (~ 241.2 × 103 t C). Subsurface PP represented 37.4% of total PP for the whole area and as much as ~ 72.0% seaward of the shelf break. In the upper 100 m, bacteria dominated (54%) total community respiration (~ 250 mg C m-2 d-1), whereas protozoans, metazoans, and benthos, contributed to 24, 10, and 12%, respectively. The range of production-to-biomass ratios of bacteria was wide (1-27% d-1), while we estimated a narrower range for protozoans (6-11% d-1) and metazoans (1-3% d-1). Over the shelf, benthic biomass was twofold (~ 5.9 g C m-2) the biomass of pelagic heterotrophs (~ 2.4 g C m-2), in accord with high vertical carbon fluxes on the shelf (956 ± 129 mg C m-2 d-1). Threshold PP (PP at which NCP becomes positive) in the surface layer oscillated from 20 to 152 mg C m-2 d-1, with a pattern from low-to-high values as the distance from the Mackenzie River decreased. We conclude that (1) climate change is exacerbating the already extreme biological gradient across the Beaufort shelf-basin system; (2) the Mackenzie Shelf acts as a weak sink for atmospheric CO2, suggesting that PP might exceed the respiration of terrigenous and marine organic matter in the surface layer; and (3) shelf break upwelling can transfer CO2 to the atmosphere, but CO2 outgassing can be attenuated if nutrients brought also by upwelling support diatom production. Our study underscores that cross-shelf exchange of waters, nutrients and particles is a key mechanism that needs to be properly monitored as the Arctic transits to a new state.

International audience; The accelerated decline in Arctic sea ice combined with an ongoing trend toward a more dynamic atmosphere is modifying carbon cycling in the Arctic Ocean. A critical issue is to understand how net community production (NCP; the balance between gross primary production and community respiration) responds to changes and modulates air-sea CO2 fluxes. Using data collected as part of the ArcticNet-Malina 2009 expedition in southeastern Beaufort Sea (Arctic Ocean), we synthesize information on sea ice, wind, river, water column properties, metabolism of the planktonic food web, organic carbon fluxes and pools, as well as air-sea CO2 exchange, with the aim of identifying indices of ecosystem response to environmental changes. Data were analyzed to develop a non-steady-state carbon budget and an assessment of NCP against air-sea CO2 fluxes. The mean atmospheric forcing was a mild upwelling-favorable wind (~5 km h-1) blowing from the N-E and a decaying ice cover (600 mg C m-2d-1) over the shelf prior to our survey, (2) freshwater dilution by river runoff and ice melt, and (3) the presence of cold surface waters offshore. Only the Mackenzie River delta and localized shelf areas directly affected by upwelling were identified as substantial sources of CO2 to the atmosphere (>10mmol C m-2d-1). Although generally

ISI Document Delivery No.: 148AY Times Cited: 1 Cited Reference Count: 34 Cited References: AMAP, 2011, SNOW WAT IC PERM ARC Anderson LG, 2010, DEEP-SEA RES PT I, V57, P869, DOI 10.1016/j.dsr.2010.03.012 Anderson LG, 2004, J GEOPHYS RES-OCEANS, V109, DOI [10.1029/2003JC002120, 10.1029/2003JC001773] Arrigo KR, 2008, GEOPHYS RES LETT, V35, DOI 10.1029/2008GL035028 Barber DG, 2009, GEOPHYS RES LETT, V36, DOI 10.1029/2009GL041434 Bates NR, 2006, GEOPHYS RES LETT, V33, DOI 10.1029/2006GL027028 Bates NR, 2009, BIOGEOSCIENCES, V6, P2433 Cai WJ, 2010, SCIENCE, V329, P556, DOI 10.1126/science.1189338 DICKSON AG, 1987, DEEP-SEA RES, V34, P1733, DOI 10.1016/0198-0149(87)90021-5 Else B. G. T., 2013, GLOBAL BIOG IN PRESS Else BGT, 2011, J GEOPHYS RES-OCEANS, V116, DOI 10.1029/2010JC006760 Else BGT, 2012, J GEOPHYS RES-OCEANS, V117, DOI 10.1029/2012JC007901 Else BGT, 2012, J GEOPHYS RES-OCEANS, V117, DOI 10.1029/2011JC007346 Galley R. J., 2013, ARCTIC, V66 Hutchings JK, 2012, J GEOPHYS RES-OCEANS, V117, DOI 10.1029/2011JC007182 Jutterstrom S, 2010, MAR CHEM, V122, P96, DOI 10.1016/j.marchem.2010.07.002 Lansard B, 2012, J GEOPHYS RES-OCEANS, V117, DOI 10.1029/2011JC007299 Lewis E., 1998, ORNLCDIAC105 US DEP Macdonald RW, 2002, DEEP-SEA RES PT I, V49, P1769, DOI 10.1016/S0967-0637(02)00097-3 Mathis JT, 2012, GEOPHYS RES LETT, V39, DOI 10.1029/2012GL051574 McPhee MG, 2009, GEOPHYS RES LETT, V36, DOI 10.1029/2009GL037525 MEHRBACH C, 1973, LIMNOL OCEANOGR, V18, P897 Miller LA, 2011, J GEOPHYS RES-OCEANS, V116, DOI 10.1029/2009JC006058 Morison J, 2012, NATURE, V481, P66, DOI 10.1038/nature10705 Mucci A, 2010, J GEOPHYS RES-OCEANS, V115, DOI 10.1029/2009JC005330 Pierrot D, 2009, DEEP-SEA RES PT II, V56, P512, DOI 10.1016/j.dsr2.2008.12.005 Rysgaard S, 2009, J GEOPHYS RES-OCEANS, V114, DOI 10.1029/2008JC005088 Shadwick EH, 2011, LIMNOL OCEANOGR, V56, P303, DOI 10.4319/lo.2011.56.1.0303 Toole JM, 2010, J GEOPHYS RES-OCEANS, V115, DOI 10.1029/2009JC005660 Tremblay JE, 2011, GEOPHYS RES LETT, V38, DOI 10.1029/2011GL048825 Tremblay JE, 2008, J GEOPHYS RES-OCEANS, V113, DOI 10.1029/2007JC004547 Wanninkhof R, 2002, GEOPH MONOG SERIES, V127, P351 Yamamoto-Kawai M, 2009, SCIENCE, V326, P1098, DOI 10.1126/science.1174190 Yamamoto-Kawai M, 2009, J GEOPHYS RES-OCEANS, V114, DOI 10.1029/2008JC005000 Else, Brent G. T. Galley, R. J. Lansard, B. Barber, D. G. Brown, K. Miller, L. A. Mucci, A. Papakyriakou, T. N. Tremblay, J. -E. Rysgaard, S. NCE; NSERC; CIHR; SSHRC; Canada Excellence Research Chair in Arctic Geomicrobiology and Climate Change; Centre for Earth Observation Science at the University of Manitoba Authors of this paper are members of ArcticNet (funded in part by NCE, NSERC, CIHR, and SSHRC) and the Arctic Science Partnership. Additional support was provided through the Canada Excellence Research Chair in Arctic Geomicrobiology and Climate Change, and from the Centre for Earth Observation Science at the University of Manitoba. Many thanks are owed to the captains, crew, and research technicians onboard the CCGS Amundsen. 1 AMER GEOPHYSICAL UNION WASHINGTON GEOPHYS RES LETT; Using data collected in 2009, we evaluated the potential for the southeastern Canada Basin (Arctic Ocean) to act as an atmospheric CO2 sink under the summertime ice-free conditions expected in the near future. Beneath a heavily decayed ice cover, we found surprisingly high pCO(2sw) (similar to 290-320 mu atm), considering that surface water temperatures were low and the influence of ice melt was strong. A simple model simulating melt of the remaining ice and exposure of the surface water for 100 days revealed a weak capacity for atmospheric CO2 uptake (mean flux: -2.4mmol m(-2) d(-1)), due largely to warming of the shallow mixed layer. Our results confirm a previous finding that the Canada Basin is not a significant sink of atmospheric CO2 under summertime ice-free conditions and that increased ventilation of the surface mixed layer due to sea ice loss is weakening the sink even further.

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Macdonald, Robie W. Gratton, Yves Macdonald, Robie/A-7896-2012 Macdonald, Robie/0000-0002-1141-8520 CCGS Amundsen; CASES (Canadian Arctic Shelf Exchange Study) NSERC network (Natural Sciences and Engineering Research Council of Canada); Canadian Fund for Innovation; Canadian Coast Guard; Department of Fisheries and Oceans Canada; NSERC Discovery We thank the officers and crew of the CCGS Amundsen for their support and dedication to the CASES expedition. We are indebted to Constance Guignard, Nes Sutherland, Pascale Collin, Simon Belanger, Jens Ehn, Mike Arychuk and Owen Owens for their care and perseverance in collecting and analyzing the TA, TIC and pH samples at sea. Thanks must go to the CTD data acquisition group for these basic but critical measurements and the calibration of the various probes. Most of the plots and maps in this study were created with the ODV Software [Schlitzer, 2009]. We also thank A. Proshutinsky and two anonymous reviewers who provided constructive comments that helped to improve our manuscript. This study was funded through the CASES (Canadian Arctic Shelf Exchange Study) NSERC network (Natural Sciences and Engineering Research Council of Canada) and a Canadian Fund for Innovation grant to support the upgrade and operation of the CCGS Amundsen. Additional financial contributions were provided by the Canadian Coast Guard, the Strategic Science Fund of the Department of Fisheries and Oceans Canada, and NSERC Discovery grants to A. Mucci and Y. Gratton. 10 AMER GEOPHYSICAL UNION WASHINGTON J GEOPHYS RES-OCEANS; We examined the seasonal variability of water mass distributions in the southeastern Beaufort Sea from data collected between September 2003 and August 2004. Salinity, total alkalinity (TA) and isotopic composition (delta O-18) of seawater were used together as tracers of freshwater input, i.e., meteoric water and sea ice meltwater. We used an optimum multiparameter analysis to identify the different water masses, including the Mackenzie River, sea ice melt (SIM), winter polar mixed layer (PML), upper halocline water (UHW) with core salinity of 33.1 psu (Pacific origin) and Atlantic Water. Computed values of CO2 fugacity in seawater (fCO(2)-sw) show that the surface mixed layer (SML) remains mostly undersaturated (328 +/- 55 mu atm, n = 552) with respect to the average atmospheric CO2 concentration (380 +/- 5 mu atm) over the study period. The influence of the Mackenzie River (fCO(2-SW) > 500 mu atm) was relatively small in the southeastern Beaufort Sea, and significant fractions were only observed on the inner Mackenzie Shelf. The contribution of sea ice melt (fCO(2-SW) < 300 mu atm) to the SML could reach 30% beyond the shelf break and close to the ice pack in autumn. The density of the PML increased through the winter due to cooling and brine rejection. The winter PML reached a maximum depth of 70 m in late April. The UHW (fCO(2-SW) > 600 mu atm) was usually located between 120 and 180 m depth, but could contribute to the SML during wind-driven upwelling events, in summer and autumn, and during brine-driven eddies, in winter.

International audience

The dispersion and fate of the Rhone River inputs to the Gulf of Lions (Northwestern Mediterranean Sea) have been studied through the spatial and temporal distributions of plutonium isotopes in continental shelf sediments. Plutonium isotopes (Pu-238 and Pu-239,Pu-240) are appropriate tracers to follow the dispersion of particulate matter due both to their high affinity for particles and their long half-lives. In the Rhone River valley, plutonium isotopes originate from both the weathering of the catchment basin contaminated by global atmospheric fallout, and the liquid effluents released from the Marcoule reprocessing plant since 1961.;This work presents a first detailed study on Pu-238 and Pu-239,Pu-240 distributions in sediments from the Rhone prodelta to the adjacent continental shelf, since the decommissioning of Marcoule in 1997. The vertical distribution of Pu isotopes has been analysed in a 4.75 in long core sampled in 2001 at the Rhone mouth. Despite this length, plutonium is found at the last 10 cm, manifesting the high sedimentation rate of the prodeltaic area and its ability for trapping fine-grained sediments and associated contaminants. The highest Pu-238 and Pu-239,Pu-240 concentrations reached 1.26 and 5.97 Bq kg(-1) respectively and were found within the layer 280-290 cm. The Pu-238/Pu-239,Pu-240 activity ratios (AR) demonstrated an efficient and huge trapping of the Pu isotopes derived from Marcoule. The fresh sediments, located on the top of the core, show lower plutonium activity concentrations and lower Pu-238/Pu-239,Pu-240 ratios. This decrease is in close relation with the shut down of the Marcoule reprocessing plant in 1997.;In 2001, plutonium isotopes were also analysed in 21 surface sediments located offshore and concentrations ranged from 0.03 to 0.17 Bq kg(-1) for Pu-238 and from 0.33 to 1.72 Bq kg(-1) for Pu-239,24(0). The Pu-238/Pu-239,Pu-240 AR ranged from 0.24 close to the river mouth to 0.06 southwards, indicating the decreasing influence of the Marcoule releases (global fallout AR 0.03-0.05 and Marcoule AR 0.30). This is in good agreement with the main direction spread of the Rhone River plume and the bottom current. This dataset has been compared to those obtained in the same area in 1984 and 1990 in order to follow the time trend in Pit concentrations. This comparison highlights the decrease with time in plutonium concentrations close to the Rhone River mouth, but further away this reduction is not so evident. (c) 2007 Elsevier B.V. All rights reserved.