Surface ocean variability at Galapagos from 1936–1982: Calibration of geochemical tracers in corals

A variety of geochemical tracers has been developed in reef corals as a means of reconstructing the natural variability of the tropical surface ocean over wide ranging time frames. The purpose of this paper is to calibrate the performance of five of these tracers in a modern coral colony, over an extended period. A 47‐year growth interval (1936–1982) in a colony of Pavona clavus taken from San Cristobal Island, Galapagos Islands, was sectioned into 187 quarterly intervals and analyzed for δ¹⁸O, δ¹³O, Ba/Ca, Cd/Ca, and Mn/Ca. The resulting time series are compared with eastern Pacific sea surface temperatures—one of few instrumental data bases available for calibration over this time period. Linear least squares regressions of four of the tracers (δ¹⁸O, δ¹³O, Ba/Ca, Cd/Ca) against Peruvian SST show highly significant correlations (p<0.0001). Over the annual, biennial, and ENSO (3.8 years) frequency bands, cross‐spectral comparisons show even stronger coherency, with SST frequently accounting for 70–80% of the variance in these tracers. These results as well as the phase relationships determined from the spectral analyses suggest that the coupling between SST and δ¹⁸O and the nutrientlike tracers Ba and Cd is controlled principally by seasonal upwelling cycles and interannual interruptions of these cycles brought about by El Niño and anti‐El Niño phenomena. An inverse relationship between SST and δ¹³C has its primary origins in the co‐occurrence of the warm SST phase with high light levels during the first half of the calendar year. The association with sunlight derives from the photosynthetic response of algal symbionts which results in fractionation of light and heavy carbon isotopes between soft tissues and CaCO₃ skeleton. The fifth tracer, the transition metal Mn, exhibits poorer correlations against both SST and the other tracers. Slight shifts in phasing of the historical Mn/Ca annual cycle suggest that the general water column distribution of this tracer, which is the reverse of a nutrient, may not be stable. The erratic behavior of Mn illustrates how complex hydrography near Galapagos may influence tracers with diverse source signatures.