Calibration of stable oxygen isotope signatures in Galápagos corals

Abstract

A 2‐year (1993‐1994) study was conducted in the Galápagos Islands (Ecuador) to determine the relationship between δ18O in skeletal carbonate and sea surface temperature (SST) in three species of reef‐building corals: Pavona clavus, Pavona gigantea, and Porites lobata. Coral samples were grown at 3, 10, and 3 m depth at Bartolomé Island, Champion iIsland, and Urvina Bay (Isabela Island), respectively. Hourly measurements of SST and sea surface salinity (SSS) were taken at each site immediately adjacent to colonies which were stained biannually to establish the chronology of growth. In addition, surface waters were sampled periodically (bimonthly to monthly) at each site to determine variation in δ18O seawater. Results indicate the mean annual SSTs were similar between sites, varying from 22.9°C at Champion to 23.8°C at Urvina Bay. Comparisons of monthly SST averages between instrumental and remote sensing (satellite, 1° × 1° grid) data show a high correspondence (r2 ranging from 0.84 to 0.94), indicating that remote sensing data are useful for interpreting the δ18O record in corals when instrumental data are lacking. Here δ18Ocoral analyses of eight specimens show that coralline aragonite is a reliable indicator of SST in Galápagos. In general, higher‐resolution coral sampling/year resolved more of the monthly variation in SST, up to 97% at a sampling resolution of 1.4 samples per millimeter of linear skeletal growth. Comparisions of the δ18Ocoral signal among and between species at the same site showed consistent seasonal patterns of variation closely tracking SST. In addition, comparisons between sites were highly concordant, with some differences reflecting local variation in SST. Seasonal patterns, however, were essentially the same over the entire region. Thus we conclude that the δ18Ocoral signal from coral skeletons in Galápagos can be used to interpret regional changes in SST variation.