Spatiotemporal variability in the δ 18 O‐salinity relationship of seawater across the tropical Pacific Ocean

The relationship between salinity and the stable oxygen isotope ratio of seawater (δ 18 O sw ) is of utmost importance to the quantitative reconstruction of past changes in salinity from δ 18 O values of marine carbonates. This relationship is often considered to be uniform across water masses, but the constancy of the δ 18 O sw ‐salinity relationship across space and time remains uncertain, as δ 18 O sw responds to varying atmospheric vapor sources and pathways, while salinity does not. Here we present new δ 18 O 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 δ 18 O sw ‐salinity relationship is higher in the western tropical Pacific versus the eastern tropical Pacific in observations and in two isotope‐enabled climate model simulations. A comparison of δ 18 O sw ‐salinity relationships derived from short‐term spatial surveys and multiyear 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 δ 18 O 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 δ 18 O sw ‐salinity relationship. Applying local δ 18 O sw ‐salinity relationships in a coral δ 18 O forward model shows that using a constant, basinwide δ 18 O sw ‐salinity slope can both overestimate and underestimate the contribution of δ 18 O sw to carbonate δ 18 O variance at individual sites in the western tropical Pacific.