Shelf–basin interactions in the Arctic Ocean based on 210Pb and Ra isotope tracer distributions

Abstract

Measurements of the naturally occurring radionuclides, 210Po, 210Pb, 228Ra and 226Ra were conducted on water and sediment samples collected during the Arctic Ocean Section (1994) and Beaufort Sea (1995) expeditions and are used to assess circulation and shelf/slope scavenging in the Western Arctic Ocean. 228Ra concentrations are higher over the continental shelves of the Chukchi and Beaufort Seas compared to interior waters, consistent with a shelf source for 228Ra. However, 228Ra/ 226Ra ratios for Pacific-origin water have a lower end member value (ca. 1.0) compared to literature values of 3.6 for end member 228Ra/ 226Ra ratios for Atlantic-origin water passing across the shelf of the Laptev Sea. The shift during the 1990s in the position of the axis of the Trans-Polar Drift and the consequent change in the pathways for the flow of freshwater from the Russian continental shelves is reflected in a shift of elevated 228Ra concentrations in surface water from the Lomonosov Ridge to the Mendeleyev Ridge. 210Pb water column distributions in the Eurasian Basin are similar to those measured in the North Atlantic and indicate scavenging residence times of 15–20 yr. 210Pb concentrations are lower in intermediate waters of the Makarov and Canada Basins, because of enhanced scavenging at the continental margin. The lowest 210Pb concentrations (<0.2 dpm/100 l) were in the halocline and are a result of scavenging over the continental shelves. Low 210Pb levels in upper halocline waters of Pacific origin extend hundreds of kilometers into the interior of the Makarov and Canada Basins and reflect scavenging over the Chukchi and Beaufort Sea shelves. Low 210Pb levels in lower halocline waters of Atlantic origin reflect both scavenging over the Barents and Kara Sea shelves and removal during boundary current transport along the continental margin in the Makarov and Canada Basins. Focusing factors calculated from comparisons of 210Pb sediment inventories with 210Pb water column deficits confirmed the net scavenging of 210Pb at the continental margins.