Tracing water masses with 129I and 236U in the subpolar North Atlantic along the GEOTRACES GA01 section
Authors: Castrillejo Maxi , Casacuberta Núria , Christl Marcus , Vockenhuber Christof , Synal Hans-Arno , García-Ibáñez Maribel I. , Lherminier Pascale , Sarthou Géraldine , Garcia-Orellana Jordi , Masqué Pere 
Published: September 2018

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) ×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, 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.