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Particulate Organic Carbon Fluxes on the Slope of the Mackenzie Shelf

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Particulate Organic Carbon Fluxes on the Slope of the Mackenzie Shelf Available online at www.sciencedirect.com Journal of Marine Systems 68 (2007) 39–54 www.elsevier.com/locate/jmarsys Particulate organic carbon fluxes on the slope of the Mackenzie Shelf (Beaufort Sea): Physical and biological forcing of shelf-basin exchanges ⁎ Alexandre Forest a, , Makoto Sampei a, Hiroshi Hattori b, Ryosuke Makabe c, Hiroshi Sasaki c, Mitsuo Fukuchi d, Paul Wassmann e, Louis Fortier a a Québec-Océan, Université Laval, Québec, QC, Canada, G1K 7P4 b Hokkaido Tokai University, Minamisawa, Minamiku Sapporo, Hokkaido 005-8601, Japan c Senshu University of Ishinomaki, Ishinomaki, Miyagi 986-8580, Japan d National Institute of Polar Research, 9-10, Kaga 1-chome, Itabashi-ku, Tokyo 173-8515, Japan e Norwegian College of Fishery Science, University of Tromsø, N-9037, Tromsø, Norway Received 27 July 2006; received in revised form 25 October 2006; accepted 27 October 2006 Available online 12 December 2006 Abstract To investigate the mechanisms underlying the transport of particles from the shelf to the deep basin, sediment traps and oceanographic sensors were moored from October 2003 to August 2004 over the 300- and 500-m isobaths on the slope of the Mackenzie Shelf (Beaufort Sea, Arctic Ocean). Seasonal variations in the magnitude and nature of the vertical particulate organic carbon (POC) fluxes were related to sea-ice thermodynamics on the shelf and local circulation. From October to April, distinct increases in the POC flux coincided with the resuspension and advection of shelf bottom particles by thermohaline convection, windstorms, and/or current surges and inversions. Once resuspended and incorporated into the Benthic Nepheloid Layer (BNL), particles of shelf origin were transported over the slope by the isopycnal intrusion of the BNL into the Polar-Mixed Layer off-shelf. Offshore transport of the resuspended particles allowed them to settle over the slope. The resulting vertical POC flux at the shelf-basin boundary amounted to 1.0 g C m−2 y−1 or 58% of the annual POC flux over the 300-m isobath. Consistent with the resuspension of shelf sediments, POC fluxes in fall/winter were characterized by a high terrigenous fraction (25–60%), the dominance of small flagellate cells, and increasingly degraded fecal pellets with time. In late May– early June, a short-duration POC flux maximum characterized by high POC:PON ratio and more positive δ13C resulted from the direct sinking of ice algae and transparent exopolymeric matter flushed from melting sea-ice. In July, a last sedimentation event coincided with the retreat of the sea-ice cover, phytoplankton production from a subsurface bloom, and the sinking of the intact fecal pellets of large herbivorous copepods and appendicularians. Our results confirm the importance of sea-ice thermodynamics and BNL resuspension in promoting the transfer of POC from the shelf to the deep basin in fall/winter. The actual contribution of the summer biological production to the shelf–basin flux of POC remains uncertain. © 2006 Elsevier B.V. All rights reserved. Keywords: Shelf–basin exchange; Particulate flux; Thermohaline convection; Benthic Nepheloid Layer; Biological production; Sediment traps; Arctic Ocean; Beaufort Sea; Mackenzie Shelf; 71°N; 133°W ⁎ Corresponding author. Tel.: +1 418 656 5917; fax: +1 418 656 2339. E-mail address: [email protected] (A. Forest). 0924-7963/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2006.10.008 40 A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 1. Introduction advected offshore (e.g. Backhaus et al., 1997; Ivanov et al., 2004). Based on transmissometer and sequential- Assessing the sequestration of atmospheric CO2 on trap data, O'Brien et al. (2006) suggested that shelf continental shelves is central to our understanding of the sediments are episodically transported to the edge of the role of oceans in regulating climate (e.g. de Haas et al., Mackenzie Shelf in bottom and mid-water nepheloid 2002; Dittmar and Kattner, 2003; Muller-Karger et al., layers. The processes underlying this transport could not 2005). This assessment is particularly important for the be resolved however. extensive shelves of the Arctic Ocean where biogeo- In the present study, sequential sediment traps and chemical cycling and carbon fluxes could be altered oceanographic sensors (temperature, salinity, current dramatically by the on-going reduction of the sea-ice speed, current direction, and turbidity) were moored cover (e.g. Stein and MacDonald, 2004; Macdonald from October 2003 to August 2004 on the slope of the et al., 2005; ACIA, 2005). Documenting the intensity Mackenzie Shelf to document the physical and biological and nature of present fluxes and modeling the response factors that promote the transport of biogenic carbon from of these fluxes to variability and change in sea-ice the continental shelf to the deep Arctic Ocean. The origin regime are central objectives of international programs by source (marine, terrigenous) and nature (protistal, fecal, such as the Canadian Arctic Shelf Exchange Study detrital) of the fluxes recorded on the slope were assessed (CASES) and the American-led Shelf–Basin Interaction through chemical (total POC, POC terrigenous fraction, Study (SBI). C:N ratios and δ13C) and microscopic analyses (protists The immense (ca. 6×106 km2), seasonally ice-covered taxonomy and volume, fecal pellets volume, shape and continental shelves of the Arctic Ocean receive allochtho- degradation state). In particular, we tested the hypothesis nous POC from river runoff, coastal erosion and the that the advection of the BNL on the Mackenzie Shelf melting of landfast ice (Rachold et al., 2004; Wassmann contributes significantly to the transfer of POC from the et al., 2004). In summer, the production of autochthonous shelf to the slope. POC by microalgal photosynthesis is favored along the circum-Arctic coastal polynya system that separates the 1.1. Study area landfast ice and the mobile central ice pack (Stirling, 1980; AMAP, 1998). The allochthonous and autochtho- As part of the Canadian Arctic Shelf Exchange Study nous POC of arctic shelves can be transferred to the food (CASES), the CCGS Amundsen research icebreaker web, to the shelf seabed, or to the deep Arctic Ocean completed a one-year over-wintering expedition to the basins (Belicka et al., 2002; Muller-Karger et al., 2005; Mackenzie Shelf of the Beaufort Sea from September O'Brien et al., 2006). Because of the resulting long-term 2003 to August 2004. The rectangular (120×530 km) sequestration of atmospheric CO2, the transfer of POC Mackenzie Shelf is relatively narrow compared to the from the shelf to the deep basin is of particular relevance broad Siberian shelves (∼500 km width). It is influenced in the present context of Global warming. by the Mackenzie River, the third largest river dischar- In a recent review, McPhee-Shaw (2006) pointed to ging into the Arctic Ocean (330 km3 y− 1; Macdonald the potential importance of the Benthic Nepheloid Layer et al., 1998) and the first in terms of sediment load (BNL) in transferring sediments and POC horizontally (124×106 ty− 1; Holmes et al., 2002). Sea-ice typically from the shelf to the slope and ultimately to the deep starts to form on the shelf in October and reaches its basin. Once bottom particles are resuspended and in- maximum thickness (2–3 m) in March. During winter, corporated into a BNL by some mixing process, the the landfast ice is bounded offshore by the stamukhi, a turbid layer can intrude the ocean interior along its new linear hummock formed along the 20-m isobath by the isopycnal, thereby moving particulate matter horizon- collision of the offshore mobile ice pack onto the landfast tally over the slope (McPhee-Shaw, 2006 and references ice edge. In winter, the stamukhi contains the turbid therein). On ice-free shelves, internal waves, coastal waters of the Mackenzie River, forming the seasonal eddies, hyperpycnal flows caused by coastal storms, Mackenzie freshwater lake under the landfast ice cover boundary currents rushing into topographic disconti- of the coastal zone. Offshore, an intermittent coastal nuities, and other turbulent processes can resuspend polynya (or flaw lead) is formed when winds and/or sediments and mix the BNL with the overlaying water, surface circulation push the central ice pack away from hence modifying its density. At high-latitudes in winter, the stamukhi (e.g. Carmack and MacDonald, 2002; thermohaline convection of dense water resulting from Barber and Hanesiak, 2004). The ice break-up develops ice formation in leads and polynyas is an additional from the coastal polynya around June. The Mackenzie mechanism by which the BNL can be produced and River plume invades the top 5–10 m of the surface layer A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 41 Fig. 1. Bathymetric map of the Mackenzie Shelf in the eastern Beaufort Sea (Arctic Ocean) with the position of moorings CA-04 and CA-07 deployed from October 2003 to August 2004 on the slope. The bold south–north line indicates the oceanographic sections conducted in October 2003 and June 2004. The stippled large rectangle and small square represent the areas over which sea-ice cover percentage was estimated from satellite imagery. The position of the over-wintering station of the CCGS Amundsen in Franklin Bay is shown. of the shelf when the stamukhi breaks. Northwesterly Canadian Ice Service of Environment Canada (http:// winds maintain the plume inshore, whereas easterlies ice-glaces.ec.gc.ca/). Wind speeds and direction at Tuk- push it seaward (Macdonald and Yu, 2006). toyaktuk were retrieved from the Weather Archive of The water masses on the slope of the Mackenzie Environment Canada (http://www.climate.weatheroffice. Shelf are typical of the oligotrophic Canadian Basin ec.gc.ca/). Time-series of weekly-averaged percent ice (Carmack and Kulikov, 1998; McLaughlin et al., 2005).
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