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

• Published in: Biogeosciences
• Main Authors: Castrillejo, Maxi, Casacuberta, Nuria, Christl, Marcus, Vockenhuber, Christof, Synal, Hans-arno, Garcia-ibanez, Maribel, Lherminier, Pascale, Sarthou, Geraldine, Garcia-orellana, Jordi, Masque, Pere
• Format: Text
• Language: English
• Published: Copernicus Gesellschaft Mbh
• Subjects: envir; geo; Greenland; Denmark Strait; East Greenland; east greenland current; Iceland; North Atlantic
• Online Access: https://doi.org/10.5194/bg-15-5545-2018; https://archimer.ifremer.fr/doc/00457/56873/58725.pdf; https://archimer.ifremer.fr/doc/00457/56873/58726.pdf; https://archimer.ifremer.fr/doc/00457/56873/58728.pdf; https://archimer.ifremer.fr/doc/00457/56873/58729.pdf; https://archimer.ifremer.fr/doc/00457/56873/58730.pdf; https://archimer.ifremer.fr/doc/00457/56873/58727.xlsx

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) × 107atkg−1 (40–2350) × 10−12 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, ...