The mean residence time of river water in the Canada Basin

Seawater was collected from the western Arctic Ocean for measurements of ¹⁸O, ²²⁶Ra and ²²⁸Ra. The fractions of river runoff and sea ice melt-water in water samples were estimated by using δ ¹⁸O-S-PO* tracer system. The mean residence time of river water in the Canada Basin was calculated based on the relationship between ²²⁸Ra/²²⁶Ra)_A.R. and the fractions of river runoff in the shelf and deep ocean. Our results showed that the river runoff fractions in the Canada Basin were significantly higher than those in the shelf regions, suggesting that the Canada Basin is a major storage region for Arctic river water. ²²⁸Ra activity concentrations in the Chukchi shelf and the Beaufort shelf ranged from 0.16 to 1.22 Bq/m³, lower than those reported for shelves in the low and middle latitudes, indicating the effect of sea ice melt-water. A good positive linear relationship was observed between ²²⁸Ra/²²⁶Ra)_A.R. and the fraction of river runoff for shelf waters, while the ²²⁸Ra/²²⁶Ra)_A.R. in the Canada Basin was located below this regressive line. The low ²²⁸Ra/²²⁶Ra)_A.R. in the Canada Basin was ascribed to ²²⁸Ra decay during shelf waters transporting to the deep ocean. The residence time of 5.0–11.0 a was estimated for the river water in the Canada Basin, which determined the time response of surface freshening in the North Atlantic to the river runoff into the Arctic Ocean. Supported by the Program for New Century Excellent Talents in University of China (Grant No. NCET-04-0593) and the Chinese First Arctic Expedition Foundation