129I and 137Cs tracer measurements in the Arctic Ocean

Levels of 137Cs and 129I measured in 1993 in the Barents and Kara Seas and western Arctic Ocean were in excess of fallout levels, owing mainly to inputs of Atlantic-origin water contaminated by discharges from the Sellafield (U.K.) and La Hague (France) nuclear reprocessing plants. A pronounced concentration gradient in 129I extended across the Arctic Ocean, with levels in surface mixed layer water decreasing from 100–200×107 at/l in the Barents and Kara Seas to 80×107 at/l in the Makarov Basin and then to <3×107 at/l in the Canada Basin. Elevated levels of 129I and 137Cs were measured in halocline and Atlantic layer water in the Makarov and Canada Basins, reflecting the labelling of these water masses by discharges from the European reprocessing plants. In contrast, surface mixed layer and upper halocline water layers in the Canada Basin were characterized by relatively low 129I and 137Cs signals associated with Pacific-origin water contaminated mainly by nuclear weapons fallout. A “front” between Atlantic-origin and Pacific-origin water located over the Mendeleyev Ridge was delineated by changes of an order of magnitude in the 129I and 137Cs concentrations. The 129I and 137Cs input functions for Atlantic water entering the Arctic Ocean were estimated from fallout and Chernobyl inputs and reconstructed discharge records for Sellafield and La Hague using transfer factors for a reference location at 60°N in the Norwegian Coastal Current. 129I/137Cs tracer measurements were then used in a simple mixing/advection model to estimate “transit times” to sampling locations in the Arctic Ocean. By 1993, the dynamic range of the model was sufficient to establish transit times of 1–2 yr from the Norwegian Coastal Current to the southern Barents Sea, 2–4 yr to the western Kara Sea and 5–6 yr to the deep waters (>330 m water depth) of the Novaya Zemlya Trough. Although the dynamic range of the model in 1993 was sufficient only to establish a lower limit of 6 yr for transit to the Makarov and Canada Basins, the continuing increase in 129I releases from La Hague will have expanded the transit time range of the 129I/137Cs tracer model to 10 yr by 1997.