The 2013-15 temporal variation in the 129I concentration in seawater in the southern Canada Basin

Abstract

129I concentrations were measured in surface waters and for two depth profiles in the Chukchi Borderland (74.5°N 162.0°W) and the southern Canada Basin (72.5°N 155.4°W) during the R/V Mirai cruises in 2014 and 2015. The 129I concentrations in the surface waters of the North Pacific Ocean and the Bering and Chukchi seas in 2014 and 2015 were 1.35 ± 0.19 and 1.18 ± 0.17 × 107 atoms L−1, respectively. The depth profiles of 129I were almost identical in 2014 and 2015. 129I concentrations were 10–20 × 107 atoms L−1 in the surface layer, less than 10 × 107 atoms L−1 (i.e., the minimum value) around 100–150 m, and 65–120 × 107 atoms L−1 below 300 m (below 200 m in 2015). Although the peak 129I concentrations exhibited slightly fluctuating values of 90–120 × 107 atoms L−1, the average 129I concentrations at water depths in ranging from 300 to 800 m were an almost constant (83–86 × 107 atoms L−1) from 2014 to 2015. Both the low 129I concentration level relative to other areas of the Arctic Ocean and the constant 129I concentrations over time may correspond to the ages of low discharge rate, until the mid-1990s, from the nuclear fuel-reprocessing facilities at Sellafield (U.K.) and La Hague (France) to the Atlantic Ocean.