Radionuclides as tools to study the role of the Arctic Sea Ice in the interception, transport and redistribution of particulate matter and chemical species

The Arctic Ocean is characterized by being covered by sea ice with a large degree of seasonal variability between summer and winter. Along the whole life cycle of sea ice, diverse physical and chemical processes determine the concentration of the sea-ice sediments (SIS) and the chemical species entrapped in it and their final fate. Initially, sea ice incorporates particles (SIS) and associated chemical species (metals, nutrients, contaminants, etc.) during its formation mainly in the continental shelves, while dissolved solutes are excluded. As sea ice drifts offshore to the central Arctic Basin, it intercepts chemical species from the atmosphere and, the sediments in the ice may also incorporate some chemical solute compounds from the surface waters by scavenging (although this is likely small). Eventually, transported components, chemical species and SIS, are released to the underlying water column during melting. Thus, sea ice becomes an important transpo rt and distribution agent. However, the efficiency of interception of atmospheric fluxes by sea ice, the origin of the entrapped SIS and transit times of sea ice in the Arctic, as well as the importance of the transport of chemical species and particulate matter (SIS) and its release in the ablation area are all poorly understood. In an attempt to address these questions, a suite of natural (7Be and 210Po-210Pb) and artificial (137Cs, 239,240Pu) radionuclides were analysed in samples from precipitation, sea ice, surface water, water beneath ice and sea-ice sediments collected during the ARK XXII/2 expedition in 2007. The distributions of 7Be and 210Pb showed enrichment in sea ice (129 ± 90 and 5.1 ± 2.9 Bq·m-3, respectively) with respect to surface water (7.1 ± 1.3 and 1.1 ± 0.36 Bq·m-3, respectively). Given that only 4% of the total amount of 210Pb in sea ice comes from seawater and that any 7Be (T1/2 = 53 days) trapped in sea ice during its formation has decayed during drift, the direct atmospheric flux appears as the most important source of both radionuclides in sea ice. From mass balance calculations we estimate that sea ice intercepts about 30% of the 7Be atmospheric flux. This figure may be extrapolated to other chemical species with atmospheric sources, such as metals, nutrients, and contaminants. Given that 7Be and 210Pb are intercepted and accumulated during sea ice transit and also scavenged by SIS, we can use both radionuclides to assess sea ice transit time. Using the 210Pb inventory in ice floes respect to the 210Pb atmospheric flux intercepted by sea ice and the 7Be/210Pbex activity ratio in SIS, we estimated transit times from less than 0.5 to 3 years along the Eurasian Basin. Results are consistent with information reported by satellite maps and back-trajectories analysis of the sampled sea ice floes. Indeed, the SIS presence indicates that the ice floes come from continental shelves, and their origin can be constrained using artificial radionuclides (137Cs and the 239,240Pu) in SIS. Data shows that most of the SIS in the Eurasian Basin originated from the Siberian shelves, in agreement with back-trajectory analyses and main drift patterns. The relevance of sea ice as a significant transport and source of radionuclides in melting areas, such as the Fram Strait, is reflected in the annual fluxes of dissolved 7Be and 210Pb carried by sea ice (67 ± 55 and 13 ± 7 Bq·m-2·y-1, respectively), which are comparable to atmospheric inputs in this region (113-131 and 10-18.3 Bq·m-2·y-1, for 7Be and 210Pb). In addition, the annual mass flux of SIS to the Fram Strait, assessed using a 7Be mass balance and the mean annual ice area efflux through the Fram Strait, is on average 240 (4.5 - 1700) ·106 tons. As a reference, the discharge of sediment load from Arctic rivers is of about 115·106 tons per year.