Spatiotemporal Variability in the Salinity-Oxygen Isotope Relationship of Seawater Across the Tropical Pacific Ocean

Abstract:

The relationship between sea surface salinity and the stable oxygen isotope composition of seawater (δ¹⁸O_sw) is of utmost importance to the quantitative reconstruction of past changes in sea surface salinity and sea surface temperature from δ¹⁸O values of marine carbonates. This relationship is often considered to be a uniform across ocean basins, but the constancy of the salinity-oxygen isotope relationship across space and time is highly uncertain due to a dearth of paired salinity-δ¹⁸O_sw measurements. Here we present new linear salinity-δ¹⁸O_sw relationships from weekly seawater sampling programs at sites spanning the tropical Pacific Ocean. New data from Palau, Papua New Guinea, and Galápagos show a strong and statistically significant relationship between salinity and δ¹⁸O_sw. We were unable to detect a δ¹⁸O_sw-salinity relationship at Kiritimati due to the low seasonal variability in salinity and δ¹⁸O_sw, although lagoonal samples did show a significant salinity-δ¹⁸O_sw relationship. The slope of the δ¹⁸O-salinity relationship was largest in Palau and Papua New Guinea and smallest in the Galápagos, consistent with increasing salinity-δ¹⁸O_sw slopes from east to west in previous isotope-enabled model simulations. These results suggest that modeling δ¹⁸O carbonate using a constant salinity-δ¹⁸O_sw slope may underestimate the contribution of δ¹⁸O_sw to δ¹⁸O carbonate variance in the western tropical Pacific, and overestimate its contribution in the eastern tropical Pacific. A comparison of salinity-δ¹⁸O_sw relationships derived from spatial surveys and time series at Papua New Guinea and Galápagos suggests space-for-time substitution is reasonable at these sites, at least within the time period of the investigation. However, the salinity-δ¹⁸O_sw relationship varied temporally at Palau, with a larger slope occurring during a period of enhanced ENSO variability from 1998-2000 versus a period of more subdued ENSO variability from 2013-2014.