The Arctic Ocean is by volume the dominating component of the Arctic Mediterranean Sea, which additionally includes the Nordic Seas. The Arctic Ocean is landlocked, bordered by the Pacific Ocean at the Bering Strait and by the Nordic Seas in the Fram Strait and by the Barents Sea. The Arctic Mediterranean Seas are linked to the North Atlantic via exchange over the Greenland-Scotland Ridge system, where dense overflow to the south, primarily through the Denmark Strait and the Faroe Bank Channel, is an important component of the North Atlantic Deep Water and the global thermohaline circulation. The water that feeds the deep overflow contains components from both the Nordic Seas as well as deep waters from the Arctic Ocean, thus linking the Arctic to the global conveyor belt. Recent evidence suggests that water from the Arctic Ocean is becoming an increasingly important component of the deep water of the Nordic Sea as well as in the overflow water [e.g., Karstensen et al., 2005; Tanhua et al., 2005]. This provides a direct link between changing conditions in the Arctic Ocean and the World Ocean. In this paper we present measurements of the transient tracers chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF6) made in the Arctic Ocean during more that two decades. We use these data to calculate the ventilation of the Arctic Ocean, and we present estimates of the anthropogenic CO2 (Cant) content. The ventilation time of a water parcel is the time since it was last in contact with the atmosphere, where exchange of gases can take place. Depending on what assumptions are being made with regard to mixing, surface saturation, influence of bomb-derived 14C, a range of ventilation times can be obtained from the same data. In this study we use the mean age of the transit time distribution, see below, as a measure of ventilation time.
 The volume of the central Arctic Ocean is close to 12.34 × 106 km3. Together with several important shelf seas having a combined volume of about 0.65 × 106 km3, the total volume is roughly 1% of the volume of the World Ocean [Jakobsson, 2002]. The Central Arctic Ocean is divided by ocean ridges into several sub-basins. The prominent Lomonosov Ridge divides the Arctic Ocean into the Eurasian and Canadian basins (the latter sometimes referred to as the Amerasian Basin, [Björk et al., 2007]). The Eurasian Basin is divided into the Amundsen and the Nansen basins by the Gakkel Ridge, and the Canadian Basin is divided into the Canada and Makarov basins by the Alpha/Mendeleev Ridge. The shelves cover a proportionally significant area of the Arctic Ocean, comprising about 53% of the Arctic Ocean area, but contain only about 5% of its volume [Jakobsson, 2002].
1.1. Ventilation and Circulation of the Arctic Ocean
 The circulation of the intermediate water and the Atlantic Water is primarily along the topographic boundaries and the ocean ridge systems in the Arctic Ocean [e.g., Rudels et al., 1994; Jones et al., 1995]. Atlantic Water enters the Arctic through the Fram Strait and the Barents Sea. Important mixing and entrainment regions are located along the shelf, particularly east of the St. Anna Trough in the Kara Sea. The deep and intermediate layers of the Arctic Ocean are mainly ventilated via boundary convection processes that transport brine-enriched water formed over the shelf to the deep central basins as sinking plumes that descend the continental slope, entraining surrounding waters during the process [e.g., Aagaard and Carmack, 1989; Wallace et al., 1992; Rudels et al., 1994; Jones et al., 1995; Bönisch and Schlosser, 1995; Anderson et al., 1999; Smethie et al., 2000]. The deep inflow through the Fram Strait of water originating in the Greenland Sea is an additional significant process of Arctic Ocean deep water ventilation [e.g., Smethie et al., 1988; Rudels et al., 2005]. Deep water is also exported from the Arctic Ocean through Fram Strait to the Greenland Sea and to the deeper part of the East Greenland Current [e.g., Aagaard et al., 1991; Rudels et al., 2005].
 There are only a handful of estimates of the ventilation time of the deep Arctic Ocean available in the literature. One reason is that the atmospheric time history for the CFCs is too short to capture the length residence times of the deep water [e.g., Jones et al., 1995], with the exception of the most recent CFC data, as we shall see in this work. The CFC concentrations in the deep Eurasian Basin are typically close to, or below, the detection limit; whereas the CFC concentrations in the Canadian Basin are most often below the detection limit for the deep water [e.g., Jones et al., 1995]. However, the related tracer carbon tetrachloride, CCl4 has typically been found throughout the Arctic Ocean deep profiles [e.g., Krysell and Wallace, 1988; Jones et al., 1995], indicating that CFCs should also be detected in the deep waters in the near future (CCl4 has longer atmospheric history compared to the CFCs, making it a good tracer in slightly older water). Estimates of tracer ages from CFC and/or CCl4 data have been incompatible with mean ages from isotope measurements, mainly because the different input functions and the unresolved issue of mixing in the “tracer age” calculations (generally simple comparison of the seawater concentrations to the corresponding atmospheric concentration for a specific time, see below).
 Even though the CFCs have been of somewhat limited value for old waters, estimates of mean ages, or ventilation times, of the Arctic Ocean deep waters have been calculated from isotope measurements. On the basis of a few 14C profiles taken during the Oden cruise in 1991 in the Arctic Ocean, Jones et al.  calculated the ventilation time of the Makarov Basin to be 360 years. Similarly, Macdonald et al.  used 14C from the Makarov Basin sampled during 1989 to calculate an effective deep water age of about 500 years. Further more, 39Ar and 14C data suggest that the Eurasian Basin deep water is 250–300 years, whereas the deep water in the Canadian Basin is approximately 450 years [Schlosser et al., 1994, 1997]. Although this estimate is slightly lower than the 500–800 years ventilation time for the deep Canada Basin obtained by Östlund et al.  based on 14C profiles, it is reasonable to assume that the discrepancy can be explained by the somewhat higher uncertainties in the data set of Östlund et al. . Even though there is some uncertainty in these estimates due to scarcity of data and uncertainty in, for instance, the influence of bomb-14C, there is certainly general agreement on the magnitude of the deep Arctic Ocean ventilation.
1.2. Arctic Ocean Anthropogenic CO2 Inventory
 The storage of Cant in the World Ocean is relatively high due to the buffering capacity of seawater; the ocean contains roughly 50 times more inorganic carbon than the atmosphere and is a significant sink of Cant [Canadell et al., 2007]. Good understanding and monitoring of the oceanic sink and uptake of Cant is essential for our ability to understand and correctly assess the rapidly changing atmospheric, oceanic and terrestrial carbon reservoirs. However, because of scarcity of hydrochemical data in the Arctic Ocean, there are only a few estimates of the Arctic Ocean anthropogenic CO2 inventory.
 Current observational based estimates of the global oceanic Cant inventory for the year 1994 (excluding the Arctic Ocean and adjacent seas) range from 106 ± 17 petagrams of carbon (Pg = 1015 g) [Sabine et al., 2004] to 94–121 Pg-C [Waugh et al., 2006] using two very different techniques on the same data set (GLODAP [Key et al., 2004]).
 Even though the Arctic Ocean volume contains only ∼1% of the World Ocean volume [Jakobsson, 2002], it possibly contains as much as 5% of the global CFC-11 inventory [Willey et al., 2004]. The Arctic Ocean has thus the potential to contribute significantly to the overall global Cant inventory. A study based on data collected in 1991 calculated an Arctic Ocean inventory of 1.35 (1.29 to 1.47) Pg-C in 1991 [Anderson et al., 1998b], corresponding to ∼1.68 Pg-C in 2005, accounting only for the Cant inventory below 500-m depth in the central basin. Since the Global Ocean stores half of its Cant inventory above a depth of 400- to 500 m-depth [Sabine et al., 2004; Waugh et al., 2006], this approach underestimates the Arctic Ocean total Cant inventory. In the ground breaking paper on the global Cant inventory by Sabine et al. , the Arctic Mediterranean Seas Cant inventory was scaled to 5% of the global Cant inventory. This estimate corresponds to ∼4.9 Pg-C in the Arctic Ocean in 2005 (∼6.4 Pg-C including the Nordic Seas). The scaling was based on the calculation of the global CFC-11 inventory from WOCE data [Willey et al., 2004]. However, even if the presence of CFCs in the water column is an indicator for the presence of Cant, the CFC inventory is an imperfect scaling factor for the Cant inventory due to the differences in temperature and salinity dependency between Cant and CFC. Whereas the CFC equilibrium concentration in seawater increases with decreasing temperature and salinity, the anthropogenic carbon equilibrium concentration behaves the opposite way due to the chemistry of the carbonate system in seawater [Thomas and England, 2002], which might be critical for the fresh and cold surface waters of the Arctic. Since the inventory estimate by Sabine et al.  for the Arctic Ocean did not account for this effect, their calculation is likely an overestimate.