Recent evolution of 129I levels in the Nordic Seas and the North Atlantic Ocean

Highlights: • Update of ¹²⁹I distribution in Central North Atlantic (NA) and the Nordic Seas. • Evolution of ¹²⁹I from 2002 to 2012 is associated to changes in releases from NFRPs. • Results suggest deep water formation in the eastern area of the Nordic Seas. • ¹²⁹I from NFRPs partly reaches Central NA and are not carried northwards by the NCC. Most of the anthropogenic radionuclide ¹²⁹I released to the marine environment from the nuclear fuel reprocessing plants (NFRP) at Sellafield (England) and La Hague (France) is transported to the Arctic Ocean via the North Atlantic Current and the Norwegian Coastal Current. ¹²⁹I concentrations in seawater provides a powerful and well-established radiotracer technique to provide information about the mechanisms which govern water mass transport in the Nordic Seas and the Arctic Ocean and is gaining importance when coupled with other tracers (e.g. CFC, ²³⁶U). In this work, ¹²⁹I concentrations in surface and depth profiles from the Nordic Seas and the North Atlantic (NA) Ocean collected from four different cruises between 2011 and 2012 are presented. This work allowed us to i) update information on ¹²⁹I concentrations in these areas, required for the accurate use of ¹²⁹I as a tracer of water masses; and ii) investigate the formation of deep water currents in the eastern part of the Nordic Seas, by the analysis of ¹²⁹I concentrations and temperature-salinity (T-S) diagrams from locations within the Greenland Sea Gyre. In the Nordic Seas, ¹²⁹I concentrations in seawater are of the order of 10⁹ at·kg^{− 1}, one or two orders of magnitude higher than those measured at the NA Ocean, not so importantly affected by the releases from the NFRP. ¹²⁹I concentrations of the order of 10⁸ atoms·kg^{− 1} at the Ellet Line and the PAP suggest a direct contribution from the NFRP in the NA Ocean. An increase in the concentrations in the Nordic Seas between 2002 and 2012 has been detected, which agrees with the temporal evolution of the ¹²⁹I liquid discharges from the NFRPs in years prior to this. Finally, ¹²⁹I profile concentrations, ¹²⁹I inventories and T-S diagrams suggest that deep water formation occurred in the easternmost area of the Nordic Seas during 2012.