Radionuclide tracer profiles at the CESAR Ice Station and Canadian Ice Island in the western Arctic Ocean

Abstract

Water column profiles of naturally-occurring ( 210Pb, 210Po, 226Ra) and fallout ( 137Cs, 239,240Pu radionuclides measured at the Canadian Ice Island in 1985, 1986 and 1989 over the Arctic Ocean continental shelf were compared to profiles measured at the CESAR ice station in the central Arctic Ocean in 1983. Cesium-137 concentrations at CESAR decreased from a mean value of 4.2 Bq/m 3 in the surface mixed layer to less than 1.0 Bq/m 3 in the Atlantic layer and deep waters. Cesium-137 concentrations were similar to CESAR values in the surface mixed layer at the Ice Island, but in halocline water they were lower than values measured at comparable water depths at CESAR, indicating that the arrival of fallout 137Cs in the Ice Island halocline had been delayed compared to its arrival in the CESAR halocline. These data are consistent with a combination of mixing box and lateral advection models with mixing occurring over the Russian continental shelves followed by lateral advection through the interior halocline waters of the Canada Basin. Dissolved 210Pb and 210Po concentrations were higher in nutrient maximum water at the Ice Island compared to CESAR. Low values at CESAR are caused by scavenging of particle-reactive radionuclides during modification of Bering Sea water in continental shelf environments. The elevated 210Pb and 210Po concentrations measured in nutrient maximum water at the Ice Island are assumed to result from ingrowth of 210Pb from 226Ra during lateral advection of upper halocline water between the CESAR and Ice Island locations, a process that is estimated to take approximately 11 years. Plutonium-239,240 is also depleted, relative to 137Cs in the surface mixed layer and upper halocline at the Ice Island owing to its scavenging during modification of Pacific water in shelf environments. The 239,240Pu 137Cs ratio increases to values approaching fallout levels in lower halocline and Atlantic water owing to both reduced 239,240Pu removal in source waters and vertical transport of 239,240Pu with sinking particles.