Radionuclides in Arctic sea ice: Tracers of sources, fates and ice transit time scales

Arctic sea ice can incorporate sediment and associated chemical species during its formation in shallow shelf environments and can also intercept atmospherically transported material during transit. Release of this material in ice ablation areas (e.g. the Fram Strait) enhances fluxes of both sediments and associated species in such areas. We have used a suite of natural (Be, Pb) and anthropogenic (Cs, Pu, Pu) radionuclides in sea ice, sea-ice sediments (SIS), sediment trap material and bottom sediments from the Fram Strait to estimate transit times of sea ice from source to ablation areas, calculate radionuclide fluxes to the Fram Strait and investigate the role of sea-ice entrained sediments in sedimentation processes. Sea ice intercepts and transports the atmospherically supplied radionuclides Be and Pb, which are carried in the ice and are scavenged by any entrained SIS. All of the Be and most of the excess Pb measured in SIS collected in the Fram Strait are added to the ice during transit through the Arctic Ocean, and we use these radionuclides as chronometers to calculate ice transit times for individual ice floes. Transit times estimated from the Pb inventories in two ice cores are 1–3 years. Values estimated from the Be/Pbexcess activity ratio of SIS are about 3–5 years. Finally, equilibrium values of the activity ratio of Pb to its granddaughter Po in the ice cores indicate transit times of at least 2 years. These transit times are consistent with back-trajectory analyses of the ice floes. The latter, as well as the clay-mineral assemblage of the SIS (low smectite and high illite content), suggest that the sampled sea-ice floes originated from the eastern Siberian Arctic shelf seas such as the eastern Laptev Sea and the East Siberian Sea. This result is in agreement with the relatively low activities of Pu and Cs and the Pu/Pu atom ratios ( 0.18, equivalent to that in global fallout) in SIS, indicating that prior global atmospheric fallout, rather than nuclear fuel reprocessing facilities, forms the main source of these anthropogenic radionuclides reaching the western Fram Strait at the time of sampling (1999). Transport of radionuclides by sea ice through the Arctic Ocean, either associated with entrained SIS or dissolved in the ice, accounts for a significant flux in ablation areas such as the Fram Strait, up to several times larger than the current atmospheric flux in the area. Calculated fluxes derived from sea-ice melting compare well to fluxes obtained from sediment traps deployed in the Fram Strait and are consistent with inventories in bottom sediments. Pu/Pu atomic ratios lower than 0.18 in bottom sediments from the Fram Strait provide evidence that plutonium from a source other than front matter r 2007 Elsevier Ltd. All rights reserved. r.2007.04.016 ng author. Fax: +3493 581 3331. ss: [email protected] (P. Masqué).