ALBERT

Description: The GEOTRACES section (GN04, TransArc-II) on board the German R/V Polarstern covered a full transect in the Arctic Ocean from the Barents Sea to the Makarov Basin, crossing the Nansen and Amundsen basins. The distributions of 129I concentrations and 236U/238U atomic ratios obtained from more than 300 seawater samples are consistent with different water masses. Atlantic Waters flowing into the Arctic Ocean bring the signal of the two European nuclear reprocessing plants of Sellafield and La Hague, with the highest 129I concentrations (〉 1500 x107 at·kg-1) and 236U/238U ratios (〉 3000 x10-12) in the Norwegian shelf, where the Norwegian Coastal Current penetrates into the Barents Sea. Lowest 129I concentrations (〈 5 x107 at·kg-1) and 236U/238U ratios (〈 10 x10-12) were observed in the deep and bottom waters of the Makarov Basin, proving the long-term isolation of these waters. The combination of 129I/236U and 236U/238U atomic ratios can be used as a dual tracer to understand sources of artificial radionuclides to the Arctic Ocean [1]. This dataset confirms that global fallout and reprocessing plants are the main suppliers of 129I and 236U to the Arctic Ocean, while Siberian rivers would be minor contributors [2]. Different from previous assumptions, results show that the Barents Sea Branch Water (BSBW) has a higher 129I/236U atom ratio than expected from the mixed signal of Sellafield and La Hague. This high 129I/236U ratio suggests that the contribution of La Hague relative to Sellafield is larger than expected in this water mass. Together with the data from other GEOTRACES cruises, a synoptic distribution of 129I and 236U in the Arctic Ocean will be available soon, which will help understanding major water circulation patterns in the Arctic Ocean. [1] M. Christl, N. Casacuberta, C. Vockenhuber, E. C., P. Bailly du Bois, J. Herrmann, H.A. Synal, Reconstruction of the 236U input function for the Northeast Atlantic Ocean: Implications for 129I/236U and 236U/238U-based tracer ages, Journal of Geophysical Research: Oceans, (2015). [2] N. Casacuberta, P. Masque, G. Henderson, M.R. van der Loeff, D. Bauch, C. Vockenhuber, A. Daraoui, C. Walther, H.A. Synal, M. Christl, First 236U data from the Arctic Ocean and use of 236U/238U amd 129I/236U as a new dual tracer, Earth and Planetary Science Letters, 440 (2016) 127-134.

Description: Artificial radionuclides have been introduced to the oceans from nuclear weapons tests (1950’s-1960’s), nuclear accidents (e.g. Chernobyl, 1986) and nuclear reprocessing plants (Sellafield, and La Hague, from 1960s until today) and since then they are used as transient tracers for ocean circulation. Among these radionuclides, 129I and 236U have become of special interest in the last decade and the combination of 129I/236U to 236U/238U atom ratios has been suggested as a tool to determine transit times in the Arctic Ocean [1] and to constrain the source of the anthropogenic contamination [2]. The performance of this new dual tracer has now been tested as data from samples collected during several cruises performed in the North Atlantic and Arctic oceans. Here we present the results of a number of expeditions to the Arctic Ocean (2011, 2012 and 2015) and the North Atlantic Ocean (2010, 2014). Atlantic waters entering the Arctic Ocean are tagged with high 129I/236U ratios (〉100) and 236U/238U ratios (〉1000 x10-12), originating from more recent (〈15 years) releases from reprocessing plants. Deep and bottom waters in the Arctic Ocean have much lower 129I/236U and 236U/238U ratios (supported by low 14C concentrations), representing the isolation of these waters from anthropogenic signals. New data from Barents Sea Branch Waters indicate the presence of a stronger signal from waters tagged by La Hague while Fram Strait Branch Waters, show a mixture of both Sellafield and La Hague endmembers. In order to better disentangle the sources and fate of 129I and 236U to the North Atlantic and Arctic oceans, results from a global ocean model will be presented and discussed in the context of the experimental data. [1] Christl et al. (2015) J. Geophys. Res. Oceans, 120. [2] Casacuberta et al. (2016) Earth Planet. Sc. Lett. 440.