Reconstruction of the ~(236)U input function for the Northeast Atlantic Ocean: Implications for ~(129)I/~(236)U and ~(236)U/~(238)U-based tracer ages

摘要

A reconstruction of historical discharges of ~(236)U into the Northeast Atlantic Ocean by nuclear installations is presented. The nuclear reprocessing facilities Sellafield (SF), Great Britain (GB) and La Hague (LH), France and potentially also the nuclear fuel processing installation Springfields (SP), GB represent the main contributors of ~(236)U in the Northeast Atlantic Ocean. Because data on ~(236)U releases is lacking, ~(236)U discharges from SP and SF are estimated based on the U-isotopic systematics found in the discharges from LH. The resulting reconstruction of ~(236)U releases indicates that, until 2013, a total of (95±32) kg of ~(236)U was discharged from SF, SP, and LH. In a second step, the reconstructed ~(236)U releases are combined with ~(129)I data from literature and oceanic and atmospheric box models are used to derive the ~(129)I/~(236)U and ~(236)U/~(238)U input functions that, for example, can be used to calculate tracer ages of Atlantic Waters in the Arctic Ocean. Our conceptual results show that the combination of ~(129)I/~(236)U and ~(236)U/~(238)U generally allows the estimation of tracer ages over the past approximately 25 years if contributions of ~(236)U from global fallout are considered. Finally, as a proof of concept, the new method is applied to calculate tracer ages of Arctic Ocean surface samples (collected in 2011/2012) and the results are in good agreement with literature data. We conclude that the combination of ~(129)I/~(236)U with ~(236)U/~(238)U in a dual tracer approach provides a sensitive tool for the calculation of tracer ages and ventilation rates in the North Atlantic region.