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Abstract(s)
Only a few studies have examined the variation of oxygen and hydrogen isotopes of seawater in
NE Atlantic water masses, and data are especially sparse for intermediate and deep-water masses.
The current study greatly expands this record with 527 δ18O values from 47 stations located
throughout the mid- to low-latitude NE Atlantic. In addition, δD was analyzed in the 192 samples
collected along the GEOTRACES North Atlantic Transect GA03 (GA03_e = KN199-4) and the
115 Iberia-Forams cruise samples from the western and southern Iberian margin. An
intercomparison study between the two stable isotope measurement techniques (cavity ring-down
laser spectroscopy and magnetic-sector isotope ratio mass spectrometry) used to analyze GA03_e
samples reveals relatively good agreement for both hydrogen and oxygen isotope ratios. The
surface (0-100 m) and central (100-500 m) water isotope data show the typical, evaporation
related trend of increasing values equatorward with the exception for the zonal transect off Cape
Blanc, NW Africa. Off Cape Blanc, surface water isotope signatures are modified by the
upwelling of fresher Antarctic Intermediate Water (AAIW) that generally has isotopic values of
0.0 to 0.5‰ for δ18O and 0 to 2‰ for δD. Along the Iberian margin the Mediterranean Outflow
Water (MOW) is clearly distinguished by its high δ18O (0.5-1.1‰) and δD (3-6‰) values that
can be traced into the open Atlantic. Isotopic values in the NE Atlantic Deep Water (NEADW)
are relatively low (δ18O: –0.1 to 0.5‰; δD: –1 to 4‰) and show a broader range than observed
previously in the northern and southern convection areas. The NEADW is best observed at
GA03_e Stations 5 and 7 in the central NE Atlantic basin. Antarctic Bottom Water isotope values
are relatively high indicating modification of the original Antarctic source water along the flow
path. The reconstructed δ18O-salinity relationship for the complete data set has a slope of 0.51, i.e.
slightly steeper than the 0.46 described previously by Pierre et al. (1994) for the tropical to
subtropical Northeast Atlantic. This slope decreases to 0.46 for the subtropical North Atlantic
Central Water (NACW) and the MOW and to 0.32 for the surface waters of the upper 50 m. The
δD-salinity mixing lines have estimated slopes of 3.01 for the complete data, 1.26 for the MOW,
3.47 for the NACW, and 2.63 for the surface waters. The slopes of the δ18O-δD relationship are
significantly lower than the one for the Global Meteoric Water Line with 5.6 for the complete
data set, 2.30 for the MOW, 4.79 for the NACW, and 3.99 for the surface waters. The lower slopes in all the relationships clearly reflect the impact of the evaporation surplus in the
subtropics.
Description
Keywords
Stable isotopes North Atlantic Seawater samples Geotraces Oxygen isotopes Deuterium Water mass signals