Estimation of Atlantic Water transit times in East Greenland fjords using a 233U-236U tracer approach

Abstract

Water mass composition and transit times of outflowing waters from the Arctic Ocean can reflect changes of polar climate and ocean circulation upstream. In this study we apply a novel approach using anthropogenic uranium tracers (233U and 236U), combined with salinity, and nutrients (nitrate and phosphate) to estimate transit times of waters from the Atlantic passing through the Arctic and into East Greenland fjords. In Polar Surface Water (PSW, typically found in surface ~150 m of the fjords) the dominant source of 236U is European reprocessing plants (63%) while in Arctic Atlantic Water (AAW, typically directly below PSW in these fjords) it is much less (26%) and the 236U signal is dominated by the global fallout contribution. Here we isolate the 236U signal from reprocessing plants using 236U/233U ratios and use the temporal development in 236U discharges to estimate transit times for Atlantic Water entering the Arctic Ocean and exiting as either PSW or AAW on the Greenland Shelf. PSW, which flows into the fjords from the shelf, has a transit time of between 6 and 14 years from the Arctic entrance (Barents Sea Opening, 74°N, 19°E). The transit time of AAW, which is entrained into upwelling subglacial discharge in the inner parts of the fjords, is in the order of 24–25 years since entrance in the Barents Sea. The findings indicate the potential of this novel 233U-236U approach to trace Atlantic Water circulation in the Arctic Ocean. The method offers independent transit time estimates to compare with circulation models and indicates the potential time lag between documented recent change in properties of water leaving the Arctic Ocean and the upstream processes contributing to these changes.