Salinity–oxygen isotope relationship during an El Niño (2014–2015) in the southwestern Pacific and comparisons with GEOSECS data (La Niña, 1973–1974)

Abstract

Salinity–oxygen isotope (δ18Osw) relationships in sea surface waters are fundamental to the quantitative interpretation of foraminiferal and coral δ18O records in the analyses of past El Niño–Southern Oscillation (ENSO) variation. However, available δ18Osw data for the western South Pacific are limited to the Geochemical Ocean Sections Study (GEOSECS) data, which were collected during the 1973–1974 La Niña event. Because ocean–atmospheric conditions in the western Pacific change dynamically with La Niña and El Niño events, sample-biased western South Pacific δ18Osw data may not represent the regional mean salinity–δ18Osw relationship. Herein, we present 83 new paired salinity and δ18Osw data from the western Pacific collected during the 2014–2015 El Niño event. These new data indicate that all three regions (40°–20°N, 20°N–30°S, and 30°–66°S) exhibited linear relationships between δ18Osw and salinity. However, each regional linear regression was statistically different from those of the GEOSECS data, indicating that inter-annual ENSO variability impacts the salinity–δ18Osw relationship over a wide area of the western Pacific. The ocean and atmospheric states in the 20°N–30°S region varied dynamically owing to ENSO, which caused distinct differences between the δ18Osw values for 2014–2015 and 1973–1974 datasets. One of the differences was the appearance of distinctly lighter δ18Osw values at 5°–10°N, reflecting a southward shift of the Intertropical Convergence Zone (ITCZ) under typical El Niño conditions. Water masses with heavy δ18Osw values were also observed in the subtropical South Pacific (near ∼30°S), which can be attributed to enhanced evaporation, increased equilibrium fractionation, and increased kinetic effects. This δ18Osw-enriched subtropical water mass advected toward the equator, which produced relatively heavier δ18Osw values in the South Pacific Convergence Zone (SPCZ) (8°–20°S), where high precipitation may also have been a factor causing the decoupling of salinity and δ18Osw. This study shows that salinity and δ18Osw are less correlated in the SPCZ region and that the salinity-based coral δ18Osw interpretations in this region may potentially be uncertain. Therefore, to determine the extent to which ENSO-related salinity variability can be quantitatively reconstructed from δ18O records of corals in the SPCZ, more salinity and δ18Osw data during La Niña and El Niño events in the SPCZ are needed and the temporal variability of the regression slopes and the correlation between salinity and δ18Osw should be evaluated.