Tracing water masses with 129I and 236U in the subpolar North Atlantic along the GEOTRACES GA01 section

Abstract

Pathways and timescales of water mass transport in the subpolar North Atlantic Ocean (SPNA) have been investigated by many studies due to their importance for the meridional overturning circulation and thus for the global ocean. In this sense, observational data on geochemical tracers provide complementary information to improve the current understanding of the circulation in the SPNA. To this end, we present the first simultaneous distribution of artificial ¹²⁹I and ²³⁶U in 14 depth profiles and in surface waters along the GEOVIDE section covering a zonal transect through the SPNA in spring 2014. Our results show that the two tracers are distributed following the water mass structure and that their presence is largely influenced by the global fallout (GF) and liquid effluents discharged to north-western European coastal waters by the Sellafield and La Hague nuclear reprocessing plants (NRPs). As a result, ¹²⁹I concentrations and ²³⁶U∕²³⁸U atom ratios and ¹²⁹I∕²³⁶U atom ratios display a wide range of values: (0.2–256) ×10⁷ at kg⁻¹ (40–2350) $\times {\mathrm{10}}^{-\mathrm{12}}$ and 0.5–200, respectively. The signal from NRPs, which is characterised by higher ¹²⁹I concentrations and ¹²⁹I∕²³⁶U atom ratios compared to GF, is transported by Atlantic Waters (AWs) into the SPNA, notably by the East Greenland Current (EGC)/Labrador Current (LC) at the surface and by waters overflowing the Greenland–Scotland passage at greater depths. Nevertheless, our results show that the effluents from NRPs may also directly enter the surface of the eastern SPNA through the Iceland–Scotland passage or the English Channel/Irish Sea. The use of the ²³⁶U∕²³⁸U and ¹²⁹I∕²³⁶U dual tracer approach further serves to discern Polar Intermediate Water (PIW) of Canadian origin from that of Atlantic origin, which carries comparably higher tracer levels due to NRPs (particularly ¹²⁹I). The cascading of these waters appears to modify the water mass composition in the bottom of the Irminger and Labrador seas, which are dominated by Denmark Strait Overflow Water (DSOW). Indeed, PIW–Atlantic, which has a high level of ¹²⁹I compared to ²³⁶U, appears to contribute to the deep Irminger Sea increasing the ¹²⁹I concentrations in the realm of DSOW. A similar observation can be made for ²³⁶U for PIW entering through the Canadian Archipelago into the Labrador Sea. Several depth profiles also show an increase in ¹²⁹I concentrations in near bottom waters in the Iceland and the West European basins that are very likely associated with the transport of the NRP signal by the Iceland–Scotland Overflow Water (ISOW). This novel result would support current modelling studies indicating the transport of ISOW into the eastern SPNA. Finally, our tracer data from 2014 are combined with published ¹²⁹I data for the deep central Labrador Sea between 1993 and 2013. The results obtained from comparing simulated and measured ¹²⁹I concentrations support the previously suggested two major transport pathways for the AWs in the SPNA, i.e. a short loop through the Nordic seas into the SPNA and a longer loop, which includes recirculation of the AWs in the Arctic Ocean before it enters the western SPNA.