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 Barents Sea. Iodine 129 results from halocline waters (water depths of 59 and 134 m) collected virtually synoptically throughout the central Arctic Ocean during the USS Cavalla cruise clearly show the front between Atlantic origin water having high I-129 levels (>100 x 10(7) atoms L-1) and Pacific origin water labeled mainly by fallout (<5 x 10(7) atoms L-1). This front is aligned with the Mendeleyev Ridge and is displaced toward the Canada Basin with increasing water depth. Iodine 129 levels decrease on isohaline surfaces from stations over the continental slope and Mendeleyev Ridge to stations in the interior of the Makarov Basin. These results are consistent with circulation mechanisms in which the inferior of the Makarov Basin is ventilated by lateral mixing from topographically steered boundary currents. Low I-129 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. The much higher I-129 levels (>30 x 10(7) atoms L-1) measured in central regions of the Canada Basin indicate that the interior is more efficiently ventilated than previously thought, possibly by separation from boundary currents 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 provided estimates of 6-7 years (+/-0.5 years) for the passage of halocline and Atlantic Water from the Norwegian Coastal Current (60 degrees N) to the continental slope of the Makarov Basin and a lower limit of 8 years for transport to interior regions of the Makarov and Amundsen Basins.