Constraints on the salinity-oxygen isotope relationship in the central tropical Pacific Ocean

Uncertainties surround the relationship between salinity and the stable isotopic composition of seawater, largely due to a dearth of modern seawater isotope data. Here we report 191 new, paired measurements of salinity and seawater oxygen isotopes (δ18Osw) taken from the central tropical Pacific in May 2012, from the surface to 4600 m depth. We observe significant correlations between δ18Osw and salinity across the study region, with slopes ranging from 0.23 to 0.31‰/psu for the mixed layer, and 0.35-0.42‰/psu for waters between the mixed layer and 500 m depth. When considering δ18Osw-salinity across averages of individual water masses in the region, slopes range from 0.21 to 0.40‰/psu, albeit with appreciable scatter. Surface salinity and δ18Osw data corresponding to the North Equatorial Countercurrent are significantly higher than previously observed, which we attribute to a weak westerly current and dry conditions in the region during the May 2012 cruise. Subsurface (80-500 m) salinity values from 2012 are significantly lower than corresponding values from pre-existing regional data, highlighting a different latitudinal sampling distribution, while subsurface δ18Osw is not significantly different. Thus, in May 2012, δ18Osw in this region could not be used to distinguish between subsurface water masses of different salinities. Unlike other regions where the surface 'freshwater endmember' is close to the δ18O value of regional precipitation, the freshwater endmember implied by our dataset (− 10.38‰) is consistent with a strong evaporative influence. Paired δ18O-δD values of precipitation and surface seawaters have similar slopes (5.0, 5.1), and relatively low intercepts (1.4, 0.8) indicating isotopic variability in both reservoirs is also partly controlled by evaporation.