Progress in Oceanography Progress in Oceanography 71 (2006) 145–181 www.elsevier.com/locate/pocean Climate variability and physical forcing of the food webs and the carbon budget on panarctic shelves Eddy Carmack a,*, David Barber b, Jens Christensen c, Robie Macdonald a, Bert Rudels d, Egil Sakshaug e a Department of Fisheries and Oceans, Institute of Ocean Sciences, 9860 West Saanich Road, Sidney, BC, Canada V8L 4B2 b Centre for Earth Observation Science, University of Manitoba, Winnipeg, MB, Canada R3T 2N2 c Danish Meteorological Institute, DK-2100, Lyngbyvej 100, Copenhagen Ø, Denmark d Finnish Institute of Marine Research, PO PL33, FI-00931 Helsinki, Finland e Trondhjem Biological Station, Norwegian University of Science and Technology, Bynesveien 46, N-7018 Trondheim, Norway Available online 17 November 2006 Abstract Brief overviews of the Arctic’s atmosphere, ice cover, circulation, primary production and sediment regime are given to provide a conceptual framework for considering panarctic shelves under scenarios of climate variability. We draw on past ‘regional’ studies to scale-up to the panarctic perspective. Within each discipline a synthesis of salient distributions and processes is given, and then functions are noted that are critically poised and/or near transition and thereby sensitive to climate variability and change. The various shelf regions are described and distinguished among three types: inflow shelves, interior shelves and outflow shelves. Emphasis is on projected climate changes that will likely have the greatest impact on shelf-basin exchange, productivity and sediment processes including (a) changes in wind fields (e.g. currents, ice drift, upwelling and downwelling); (b) changes in sea ice distribution (e.g. radiation and wind regimes, enhanced upwelling and mixing, ice transport and scour resuspension, primary production); and (c) changes in hydrology (e.g. sediment and organic carbon delivery, nutrient supplies). A discussion is given of the key rate-controlling processes, which differ for different properties and shelf types, as do the likely responses; that is, the distributions of nutrients, organic carbon, freshwater, sediments, and trace minerals will all respond differently to climate forcing. A fundamental conclusion is that the changes associated with light, nutrients, productivity and ice cover likely will be greatest at the shelf-break and margins, and that this forms a natural focus for a coordinated international effort. Recog- nizing that the real value of climate research is to prepare society for possible futures, and that such research must be based both on an understanding of the past (e.g. the palaeo-record) as well as an ability to reliably predict future scenarios (e.g. validated models), two recommendations emerge: firstly, a comprehensive survey of circumpolar shelf-break and slope sed- iments would provide long-term synchronous records of shelf-interior ocean exchange and primary production at the shelf edge; secondly, a synoptic panarctic ice and ocean survey using heavy icebreakers, aircraft, moorings and satellites would provide the validation data and knowledge required to properly model key forcing processes at the margins. Ó 2006 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +1 250 363 6585. E-mail addresses: [email protected] (E. Carmack), [email protected] (D. Barber), [email protected] (J. Christensen), [email protected] (R. Macdonald), bert.rudels@fimr.fi (B. Rudels), [email protected] (E. Sakshaug). 0079-6611/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.pocean.2006.10.005 146 E. Carmack et al. / Progress in Oceanography 71 (2006) 145–181 1. Introduction When Nansen set out in 1893 to cross the Arctic Ocean he prepared for and expected to find a shallow sea; so he only took along a few hundred metres of sounding line. In fact, he was at least half right: over 50% of the Arctic Ocean is occupied by shallow continental shelves, by far the largest fraction of all ocean regions (Fig. 1). More to the point, shelves play a critical role in the transformation of water masses that ultimately enter back into the global circulation (cf. Aagaard et al., 1981) and they dominate carbon flux and biogeo- chemical processes within the Arctic system (Stein and Macdonald, 2004a). How will this panarctic shelf sys- tem respond to climate variability and change? Feedback mechanisms within the arctic system demand caution for climate change predictions based on steady change (cf. Overpeck et al., 2005). Attention is thus paid to how arctic shelves may respond abruptly, both physically and biologically, to projected trends in cli- mate change – e.g. to increased extent and duration of open water in summer ice cover - and how such changes on shelves may also impact adjacent deep basins and the thermohaline circulation. It is argued, for example, that three distinct and sudden changes in water mass exchange and biological production would occur (1) if the seasonal ice edge routinely retreated past the shelf break, thus allowing greater upwelling of nutrient-rich waters from offshore basins onto the ice free shelf, (2) if the Arctic Ocean became ice-free in summer, thus eliminating multiyear ice and associated habitat and transport, and (3) if portions of the Arctic Ocean pres- ently frozen in winter were to remain ice free, thus allowing a rearrangement of zoological boundaries. Such changes in climate as these three examples would have both bottom-up (e.g. changes in light climate and nutri- Fig. 1. Map showing the bathymetry of the Arctic Ocean and locations of individual shelves. Isobaths are shown for 400 m and 2000 m; light grey shading denotes depths <400 m. E. Carmack et al. / Progress in Oceanography 71 (2006) 145–181 147 ent availability) and top-down (e.g. changes in ice-based habitat, altered migration cues) ecological conse- quences (cf. Wassmann, 1998); but, what will be the devilish details? The urgency of understanding and developing policy to address climate variability in the Arctic is stressed in recent community overviews (SEARCH SSC, 2001; ACIA, 2004). The growing recognition of the signifi- cance of the panarctic shelves to climate change and carbon flux questions has led to the recent publication of comprehensive reviews of individual shelf regions (for example, see books by Stein and Macdonald, 2004a; Stein et al., 2003; Kassens et al., 1999), and we do not attempt to duplicate these contributions. Instead, emphasis is placed on projected climate changes that will likely have the greatest impact on shelf-basin exchange, productivity and sediment processes at the panarctic scale. The geographical location of the panarctic shelves is shown in Fig. 1; hypsographic data (from Jakobsson et al., 2004) are presented in Table 1. The Arctic Ocean is comprised of two major basins, the Eurasian and the Canadian basins, separated by the Lomonosov Ridge with a sill depth around 1600 m. The Eurasian Basin is further divided by the Nansen–Gakkel Ridge into the 4000 m deep Nansen Basin and the slightly deeper, 4500 m, Amundsen Basin, while the Canadian Basin consists of the 4000 m deep Makarov Basin and the lar- ger, shallower, 3800 m, Canada Basin, separated by the Alpha-Mendeleyev Ridge. The deep basins are sur- rounded by the panarctic shelves. Characteristics of these shelves (starting with the Barents Sea and moving counter clockwise) are presented next. 1.1. A panarctic round-trip The Barents Sea is bounded to the south by the northern coast of Europe, to the north by two archipelagos (Svalbard and Franz Josef Land), to the east by Novaya Zemlya and to the west by the Norwegian Sea. The Barents Sea is the deepest of the arctic shelves with average depth of 200 m and shelf break near 400 m; its area is 1597 · 103 km2. The hydrography of the Barents Sea derives from interactions among warm and saline Atlantic Water, relatively fresh Norwegian Coastal Current waters and river inflows in the southeast (Sever- naya Divna and Pechora rivers). Descriptions of the polar front and its relationship to the ice front are given by Loeng (1990) and Parsons et al. (1996). The southwestern Barents Sea has surface temperatures >0 °C year round and is permanently ice free. It is similar to the Norwegian Sea in terms of primary productivity, in that the onset of thermal stratification in spring allows development of a spring bloom. In the eastern Barents Sea close to Novaya Zemlya dense shelf water is formed by convection during winter, with large interannual vari- ations in salinity (Nansen, 1906; Midttun, 1985; Maus, 2003). The northern Barents Sea is further character- ized by a seasonal ice cover that is highly variable in extent and duration from year to year (cf. Shapiro et al., 2003). Here, the onset of spring stratification is due to melting sea ice, and a spring bloom typically tracks the retreating ice cover (cf. Sakshaug, 2004). The tight coupling of physical forcing and production is shown by Sakshaug and Slagstad (1992) and Slagstad and McClimans (2005). The northern Barents Sea has recurrent polynyas west of Novaya Zemlya and around Franz Josef Land and Storfjorden. The Kara Sea is bounded on the south by the northern coast of Russia, to the west by Novaya Zemlya and to the east by Severnaya Zemlya. The average depth of the Kara Sea is 110 m and its area is 926 · 103 km2; the shelf is indented to the north by two large, deep submarine canyons: the Santa Anna and Voronin canyons. Table 1 Areas, volumes and mean depths of Arctic Shelves (after Jakobsson et al., 2004) Arctic sea Area (103 km2) % Total shelf area Volume (103 km3) % Total shelf volume Mean depth (m) Barents 1597 27 307 37 200 Kara 926 15 121 15 56 Laptev 498 8 24 3 131 East Siberian 987 16 57 7 48 Chukchi 620 10 50 6 58 Beaufort 178 3 22 3 80 Canadian Arctic Archipelago 1032 18 183 22 124 Northern CAA 210 3 65 7 310 Total shelf 6048 100% 829 100% 140 148 E.