A MULTI-DECADAL RECORD OF RAINFALL IN THE PLIOCENE NEARSHORE ENVIRONMENT IN SOUTH FLORIDA

Isotope and trace element analysis of a Pliocene Siderastrea radians coral reveals a hydrodynamic regime for the nearshore marine environment in the southwest Florida Gulf Coast that was markedly different from today. Sampling was conducted via continuous routing at an interval of 0.25 mm to produce a multi-decadal record with ~5-8 samples per year, as defined by growth band counts and the cyclicity of d¹⁸O and Sr/Ca records.

To calculate the d¹⁸O_water values, and thus determine seasonality of precipitation, we used the Sr/Ca temperature equation derived for a modern Siderastrea spp. coral in conjunction with the measured Sr/Ca and d¹⁸O_aragonite values from the fossil coral. Variations in the d¹⁸O_water estimates reveals a consistent record of depletion during the winter months (to -2.75 ‰), and enrichment during the summer months (to -0.90 ‰). Modern models of Rayleigh distillation in coastal environments show that, when compared to seawater, precipitation is more depleted in d¹⁸O. Therefore, the depleted winter d¹⁸O_water values derived from Pliocene Siderastrea radians represent a meteoric influence into the nearshore marine environment. Coupled with the Sr/Ca-derived temperature record, we find that the periods of higher precipitation occurred primarily during the winter months.

Excessive rainfall in Florida during winter and spring today is characteristic of conditions during an El Niño event, which influences the region through global climate teleconnections. A stable El Niño-like hydrodynamic regime for the Florida Pliocene, as indicated by our coral records, is consistent with recent reports that the Pliocene equatorial Pacific Ocean lacked an east-west temperature gradient and was in a “permanent” El Niño-like state.