Spatiotemporal variability in the δ¹⁸O-salinity relationship of seawater across the tropical Pacific Ocean

The relationship between salinity and the stable oxygen isotope ratio of seawater (δ¹⁸O_sw) is of utmost importance to the quantitative reconstruction of past changes in salinity from δ¹⁸O values of marine carbonates. This relationship is often considered to be uniform across water masses, but the constancy of the δ¹⁸O_sw-salinity relationship across space and time remains uncertain, as δ¹⁸O_sw responds to varying atmospheric vapor sources and pathways, while salinity does not. Here we present new δ¹⁸O_sw-salinity data from sites spanning the tropical Pacific Ocean. New data from Palau, Papua New Guinea, Kiritimati, and Galápagos show slopes ranging from 0.09 ‰/psu in the Galápagos to 0.32‰/psu in Palau. The slope of the δ¹⁸O_sw-salinity relationship is higher in the western tropical Pacific versus the eastern tropical Pacific in observations and in two isotope-enabled climate models. A comparison of δ¹⁸O_sw-salinity relationships derived from short-term spatial surveys and multi-year time series at Papua New Guinea and Galápagos suggests spatial relationships can be substituted for temporal relationships at these sites, at least within the time period of the investigation. However, the δ¹⁸O_sw-salinity relationship varied temporally at Palau, likely in response to water mass changes associated with interannual El Niño-Southern Oscillation (ENSO) variability, suggesting nonstationarity in this local δ¹⁸O_sw-salinity relationship. Applying local δ¹⁸O_sw-salinity relationships in a coral δ¹⁸O forward model shows that using a constant, basin-wide δ¹⁸O_sw-salinity slope can both overestimate and underestimate the contribution of δ¹⁸O_sw to carbonate δ¹⁸O variance at individual sites in the western tropical Pacific.