Abstract
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 (0–100m) and central (100–500m) 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, J. Mar. Syst. 5 (2), 159–170.) 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 50m. 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.
Highlights
High precision stable isotope (δ18O, δD) analyses of fresh and ocean water were first discussed by Epstein and Mayeda (1953) and Friedman (1953). Craig and Gordon (1965) later showed that δ18O can be used as a water mass tracer and that the δ18O-salinity relationship follows a slope of 0.61 in the surface waters of the high-latitude North Atlantic and the North Atlantic Deep Water (NADW) but changes to a slope of 0.22 in the surface waters of the subtropical North Atlantic or even 0.11 in the Atlantic’s equatorial trough
A larger scatter is observed for samples from Stations 5 and 10 compared to results from Station 1, which might be related to the larger range of salinity values encountered at Stations 5 and 10 (Fig. 2,3)
The new large dataset presented here greatly improves our knowledge of oxygen and hydrogen isotope ratios in NE Atlantic water masses
Summary
High precision stable isotope (δ18O, δD) analyses of fresh and ocean water were first discussed by Epstein and Mayeda (1953) and Friedman (1953). Craig and Gordon (1965) later showed that δ18O can be used as a water mass tracer and that the δ18O-salinity relationship follows a slope of 0.61 in the surface waters of the high-latitude North Atlantic and the North Atlantic Deep Water (NADW) but changes to a slope of 0.22 in the surface waters of the subtropical North Atlantic or even 0.11 in the Atlantic’s equatorial trough. Subsequent studies focused on specific regions (e.g., Fairbanks, 1982; Van Donk and Mathieu, 1969; Weiss et al, 1979) and a global study of seawater stable isotope transects and vertical profiles was made by the GEOSECS program (Östlund et al, 1987). In the last two decades, various studies provided new δ18O data that focused on high latitude regions and the influence of meltwater in the Atlantic sector (e.g., Azetsu-Scott and Tan, 1997; Bauch et al, 1995; Cox et al, 2010; Mackensen, 2001; Meredith et al, 1999a; Meredith et al, 1999b), the NW African upwelling region off Cape Blanc (Pierre et al, 1994: Fig. 1b) and the Mediterranean Sea (Pierre, 1999). In the Mediterranean Sea basin with its excess evaporation the slope is lower varying from 0.25 to 0.27 (Pierre, 1999)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callMore From: Deep Sea Research Part II: Topical Studies in Oceanography
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
