Circulation features in the central Arctic Ocean revealed by nuclear fuel reprocessing tracers from Scientific Ice Expeditions 1995 and 1996

Measurements of the tracer radionuclides I-129 and Cs-137 were conducted on seawater samples collected during the Scientific Ice Expedition cruises to the Arctic Ocean of the U.S. Navy nuclear submarines, USS Cavalla and USS Pogy in 1995 and 1996, respectively. These radionuclides are derived mainly from discharges from the Sellafield (England, United Kingdom) and La Hague (France) nuclear fuel reprocessing plants and are subsequently transported with Atlantic water into the Arctic Ocean through Fram Strait and the Bare nts Sea. Iodine 129 results from halocline waters (water depths of 59 and 1 34 m) collected virtually synoptically throughout the central Arctic Ocean during the USS Cavalla cruise clearly show the front between Atlantic origi n water having high I-129 levels (>100 x 10(7) atoms L-1) and Pacific origi n water labeled mainly by fallout (<5 x 10(7) atoms L-1). This front is ali gned with the Mendeleyev Ridge and is displaced toward the Canada Basin wit h increasing water depth. Iodine 129 levels decrease on isohaline surfaces from stations over the continental slope and Mendeleyev Ridge to stations i n the interior of the Makarov Basin. These results are consistent with circ ulation mechanisms in which the inferior of the Makarov Basin is ventilated by lateral mixing from topographically steered boundary currents. Low I-12 9 levels (<2 x 10(7) atoms L-1) measured in Atlantic and intermediate water in the northern Canada Basin and over the Alpha Ridge during the USS Pogy cruise indicate that ventilation rates are extremely low in this region. Th e much higher I-129 levels (>30 x 10(7) atoms L-1) measured in central regi ons of the Canada Basin indicate that the interior is more efficiently vent ilated than previously thought, possibly by separation from boundary curren ts flowing over the continental margin north of the Chukchi Plateau. The I- 129 and Cs-137 data were interpreted using a transit time model that provid ed estimates of 6-7 years (+/-0.5 years) for the passage of halocline and A tlantic Water from the Norwegian Coastal Current (60 degrees N) to the cont inental slope of the Makarov Basin and a lower limit of 8 years for transpo rt to interior regions of the Makarov and Amundsen Basins.