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

The relationship between salinity and the stable oxygen isotope ratio of seawater (δO) is of utmost importance to the quantitative reconstruction of past changes in salinity from δO values of marine carbonates. This relationship is often considered to be uniform across water masses, but the constancy of the δO-salinity relationship across space and time remains uncertain, as δO responds to varying atmospheric vapor sources and pathways, while salinity does not. Here we present new δO-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 δO-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 δO-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 δO-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 δO-salinity relationship. Applying local δO-salinity relationships in a coral δO forward model shows that using a constant, basin-wide δO-salinity slope can both overestimate and underestimate the contribution of δO to carbonate δO variance at individual sites in the western tropical Pacific.