The extensive continental shelf seas encircling much of the Arctic Ocean play a critical role in modifying and conditioning the waters of the interior basin. However, the precise mechanisms and rates by which water is transported off shelf remain largely unknown. One possible mechanism is via eddy formation at or near the continental shelf-break in the Chukchi/Beaufort Seas, in the relatively energetic peripheral currents, with subsequent migration into the central basin. This paper discusses the role that such eddies appear to play in the transport of carbon and nutrients into the Canada Basin, and some of the implications that this transport can have on the carbon budget, oxygen utilization, and maintenance of the upper halocline layer in the western Arctic Ocean.
1.1. Regional Hydrography, Biogeochemistry, and Eddy Properties
1.1.1. Regional Hydrography
 In the western Arctic Ocean, nutrient-rich Pacific and fluvial waters flow through Bering Strait into the Chukchi Sea where they undergo modifications through seasonal primary production, physical forcing (i.e., cooling in winter, densification through brine rejection) and interactions with the sediments [Chen et al., 2004]. This area is subject to strong seasonal and interannual variability in sea ice cover, freshwater input from rivers, light availability and meteorological forcing.
 The volume transport through Bering Strait is on average 0.8 Sv (1 Sv = 106 m3 s−1), although this varies on a variety of timescales [Roach et al., 1995; Weingartner et al., 1998; Woodgate et al., 2005]. On the eastern side of the strait, the inflow is dominated in late-spring to early-fall by the warm, relatively fresh Alaskan Coastal Current [Paquette and Bourke, 1974; Mountain, 1974]. The western side of the inflow contains water from the Anadyr Current, with the lowest temperatures, highest salinities and highest nutrient concentrations. The central Bering Strait contains Bering Sea shelf water that is intermediate in temperature and salinity. Observations suggest that water flows across the Chukchi Shelf as two or three distinct branches dictated by the bathymetry of the shoals and canyons [Paquette and Bourke, 1981; Weingartner et al., 1998; Woodgate et al., 2005], which is consistent with recent model results (M. A. Spall, Circulation and water mass transformation in the Chukchi Sea, submitted to Journal of Geophysical Research, 2006, hereinafter referred to as Spall, submitted manuscript, 2006) (Figure 1). North of Bering Strait some seasonal flow also enters the region from the East Siberian Current [Muenchow et al., 1999], but it is not clear what impact this water has on the system (Figure 1).
 When the inflow through Bering Strait is reduced during winter due to intensified northerly winds, sea ice advances southward, covering the Chukchi Sea and parts of the Bering Sea. During this time, waters on the Chukchi Shelf are cooled and densified [Weingartner et al., 1998], and the water entering the strait is also dense due to similar modification in the Bering Sea [Muench et al., 1988]. Different classes of winter water are formed, and the largest volumetric mode on the Chukchi shelf is referred to here as “winter-transformed” Pacific water [see Weingartner et al., 1998; Pickart, 2004]. At the end of winter, the newly formed dense water drains off the shelf [Mountain et al., 1976; Muenchow and Carmack, 1997; Pickart et al., 2005], and models predict that it should turn eastward as a shelf-break current along the edge of the Chukchi and Beaufort Seas [Winsor and Chapman, 2004; Spall, submitted manuscript, 2006].
 Recent observational studies indicate that such an eastward-flowing boundary current does indeed exist. In the Beaufort Sea a shelfedge current is present year-round, with distinct seasonal configurations [Pickart, 2004; A. Nikolopoulos and R. S. Pickart, The western Arctic Boundary Current at 152°W: Transport, structure, and variability, submitted to Deep Sea Research, Part II, 2006]. In late-winter to early-summer the current is bottom-intensified and advects winter-transformed Pacific water eastward. Some of this water likely emanated from Barrow Canyon [Muenchow and Carmack, 1997; Pickart et al., 2005]. In the Chukchi Sea the situation is less certain due to limited observations. However, synoptic vertical sections across the continental slope suggest that a similar shelfedge current exists during the spring and summer months. For example, an absolute geostrophic velocity section occupied in early August 2002 on the Polar Star near 158°W shows winter-transformed Pacific water flowing eastward as a bottom-intensified current along the outer-shelf and shelf-break (Figure 2) [see also Pickart et al., 2005]. The flow at 100 m depth is >15 cm s−1 and corresponds to a core of cold water <−1.7°C and elevated concentrations of silicate (not shown). Another absolute geostrophic velocity section occupied two months earlier at a nearby location showed a similarly configured boundary current (R. Pickart, personal communication, 2006). This is consistent as well with the temperature signal and density shear in historical hydrographic sections (e.g., see Figure 10 of Weingartner et al. ). At other times of the year the flow along the shelfedge can be very different, and during upwelling winds the current can even flow to the west [e.g., Muenchow et al., 2000]. However, in spring and summer, under light winds, the shelfedge current appears to flow eastward along both the Chukchi and Beaufort Seas.
 As the winter-transformed Pacific water moves across the Chukchi shelf it is modified through chemical and biological processes from both the surface and from the sediments [Cooper et al., 1997]. In late spring and early summer, when sea ice retreat exposes the nutrient-rich surface waters of the Chukchi Sea to sunlight, a brief but intense phytoplankton bloom occurs with rates of water column primary production >300 g C m2 yr−1 [Sambrotto et al., 1984; Hansell et al., 1993; Hill and Cota, 2005], exhausting most of the nitrate in the surface layer. Some of the sinking particles are entrained or remineralized in the winter-transformed Pacific water and transported off the shelf [Bates et al., 2005a, 2005b] while the rest are deposited in the highly productive sediments of the Chukchi Sea [Moran et al., 1997; Grebmeier and Dunton, 2000]. Along with modifications from the surface, waters draining off the shelf are also modified through interaction with the benthos [Moran et al., 2005; Lomstein et al., 1989; Grebmeier and Barry, 1991; Henriksen et al., 1993] and through stirring of the organic-rich sediments. The sediments of the Chukchi Sea are highly productive, and remineralized nutrients, dissolved organic carbon, and resuspended sedimentary particles are entrained by the overlying dense water as it moves northward [Cooper et al., 1997] (Figure 3). When this modified water reaches the edge of the shelf, it gives the shelf-break current a strong biogeochemical “shelf signature” [e.g., Pickart et al., 2005]. Jones and Anderson  and Moore et al.  suggested that nutrients released from Bering and Chukchi shelf sediments could play a role in the development of the Arctic Ocean nutrient maximum [Aagaard et al., 1981; Aagaard and Carmack, 1989; Jones and Anderson, 1986; Macdonald et al., 1989; Salmon and McRoy, 1994; Melling and Moore, 1995] and organic matter transported from the shelf to the basin could drive down halocline O2 concentrations.
1.1.3. Eddy Properties
 Seaward of the Chukchi and Beaufort Seas, the Canada Basin is populated with a large number of submesoscale eddies [Manley and Hunkins, 1985; Muenchow et al., 2000]. Most of theses eddies are anticyclones filled with Pacific-origin water, ranging in age from weeks [Kadko et al., 2006] to a more than a year [Muench et al., 2000]. It has been hypothesized that the eddies are formed at the mouth of Barrow Canyon [D'Asaro, 1988; Chao and Shaw, 2003], but recent observations suggest that they are formed along the entire length of the shelf-break, both to the east and west of Barrow Canyon [Pickart et al., 2005; Pickart, 2005].
 A likely mechanism of formation is hydrodynamic instability of the eastward-flowing shelf-break jet, which was first hypothesized by Manley and Hunkins . Pickart  showed that the springtime potential vorticity configuration of the current satisfies the necessary conditions for baroclinic instability, and Figure 2 shows an eddy of winter-transformed Pacific water being spawned from the current which supports this hypothesis. The lens of cold water (delineated by the white box in Figure 2) is circulating anticyclonically as it separates from the boundary. A similar pinching of boundary current water was observed during the same cruise on a section roughly 100 km to the east [Pickart et al., 2005].
 Three types of eddies have been characterized in the southern Canada Basin from hydrographic and drifting-buoy observations. Warm-core surface-intensified eddies, containing Alaska Coastal Current water, have been observed in the vicinity of Barrow Canyon. These eddies seem to be seasonal (late-summer/early-fall), and are dissimilar to the eddy discussed in this study. The other two types of eddies are subsurface features that can have either a cold or a warm core As shown above, the former is spawned from the boundary current during spring and early-summer when the current is advecting dense winter-transformed water. The latter are formed later in the year (into autumn) when the boundary current has been replaced with warmer summertime Chukchi shelf water (R. Pickart, personal communication, 2006). The cold-core eddies are centered near a depth of 100–150 m, and are dense enough to provide a means for transport of Pacific-origin winter water into the upper halocline (S = 33.1; T = −1.7°C) of the Canada Basin. Hence, these eddies may play an important role in the maintenance of the halocline lying between the warm Atlantic layer (S = 34.8; T = 0.4°C) and the polar surface layer (S < 30; T = −1.6°C) [Muench et al., 2000].
 In this paper we examine a cold-core, anticyclonic eddy surveyed just seaward of the Chukchi Sea in late-summer 2004, and compare its features to water in the boundary current from which the eddy was likely formed as well to those of a similar eddy that was observed in 1997 [Muench et al., 2000]. Finally, we discuss the role that such eddies could play in the biogeochemistry of the Chukchi Sea and adjacent Canada Basin by estimating the off-shelf transport of nutrients and carbon, and the impact that this has on the basin interior.