Temporal variation of ¹²⁹I concentration in seawater of the Arctic Ocean

¹²⁹I concentrations were measured in surface waters and for three 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 2013, 2014 and 2015. The ¹²⁹I concentrations in the surface waters of the North Pacific Ocean and the Bering and Chukchi seas in 2013, 2014 and 2015 were 1.44 ± 0.28, 1.35 ± 0.19 and 1.18 ± 0.17 × 10⁷ atoms L⁻¹, respectively. The depth profiles of ¹²⁹I were almost identical in 2013 to 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 2013 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. (author)