We have generated a 49-year-long, high-resolution, stable isotope record from a fossil coral, Solenastrea bournoni, to evaluate regional sea surface conditions during the middle Pliocene (3.0¿0.5 Ma) in southwest Florida. Continuous routing of the coral slab at an interval of 0.76 mm produced ~5 to 6 samples per year. The annual cycle is well defined in Δ18O and Δ13C records confirming the average yearly growth rate estimated from density banding of 4 to 5 mm/yr. High-density bands are synchronous with the highest Δ18O and Δ13C values for 79% and 45% of the annual cycles recorded, respectively. The annual range in Δ18O is 0.70 to 2.27% and in Δ13C it is 0.31 to 2.21%. Partitioning of the coral Δ18O signal into sea surface temperature (SST) changes and Δ18Owater changes is difficult to estimate in the ancient. However, isotopic mass-balance calculations, using modern climate data, suggest a partitioning of the Δ18O signal into 8 to 12% salinity and 92 to 88% temperature at the sea surface. If 100% of the Δ18O variation is attributed to temperature, the observed mean annual range in Δ18O (mean=1.54¿0.37%) corresponds to a seasonal SST range of ~7.0¿1.7 ¿C (0.22%/1 ¿C), which is a mean of 3.5 ¿C less than that of the present day.

Isotope data are consistent with the hypothesis that Δ13C values covary with the number of sunshine hours. Annually, peak Δ18O values are often observed to precede peak Δ13C values by one sample (i.e., about 2 months). We infer from the isotope pattern that periods of reduced SST were followed about 2 months later by periods of increased number of sunshine hours. This seasonal pattern is similar to that of southwest Florida today. Cross-spectral analysis of fossil coral Δ18O and Δ13C documents coherency peaks, above the 95% confidence interval, at annual and subdecadal frequencies (1.0, 1.6, 2.9 and 5 year). The spectral phase angle between Δ18O and Δ13C at the annual frequency is -60¿10¿, at the 95% confidence interval. The temporal equivalent of this angle is approximately 2 months, which confirms visual observations of seasonality from the isotope pattern. ¿ American Geophysical Union 1995