Calibration of stable oxygen isotope signatures in Galapagos corals

A 2-year (1993-1994) study was conducted in the Galapagos Islands (Ecuador) to determine the relationship between delta(18)O 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 Bartolome 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 delta(18)O seawater. Results indicate the mean annual SSTs were similar between sites, varying from 22.9 degrees C at Champion to 23.8 degrees C at Urvina Bay. Comparisons of monthly SST averages between instrumental and remote sensing(satellite, 1 degrees x 1 degrees grid) data show a high correspondence (r(2) ranging from 0.84 to 0.94), indicating that remote sensing data are useful for interpreting the delta(18)O record in corals when instrumental data are lacking. Here delta(18)O(coral) analyses of eight specimens show that coralline aragonite is a reliable indicator of SST in Galapagos. 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 delta(18)O(coral) 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 delta(18)O(coral) signal from coral skeletons in Galapagos can be used to interpret regional changes in SST variation.