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southwest Florida, pliocene, coralline chemistry, oxygen isotope record

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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 similar to 5 to 6 samples per year. The annual cycle is well defined in delta(18)O and delta(13)C 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 delta(18)O and delta(13)C values for 79% and 45% of the annual cycles recorded, respectively. The annual range in delta(18)O is 0.70 to 2.27 parts per thousand and in delta(13)C it is 0.31 to 2.21 parts per thousand. Partitioning of the coral delta(18)O signal into sea surface temperature (SST) changes and delta(18)O(water) changes is difficult to estimate in the ancient. However, isotopic mass-balance calculations, using modern climate data, suggest a partitioning of the delta(18)O signal into and to 12% salinity and 92 to 88% temperature at the sea surface. If 100% of the delta(18)O variation is attributed to temperature, the observed mean annual range in delta(18)O (mean = 1.54 +/- 0.37 parts per thousand) corresponds to a seasonal SST range of similar to 7.0 +/- 1.7 degrees C (0.22 parts per thousand/1 degrees C), which is a mean of 3.5 degrees C less than that of the present day. Isotope data are consistent with the hypothesis that delta(13)C values covary with the number of sunshine hours. Annually, peak delta(18)O values are often observed to precede peak delta(13)C 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 delta(18)O and delta(13)C 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 delta(18)O and delta(13)C at the annual frequency is -60 +/- 10 degrees, 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.

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Paleoceanography, v. 10, no. 3, p. 429-443.