RESOLVING PLEISTOCENE STRATIGRAPHY AND SEA-LEVEL FLUCTUATION IN SOUTHERN FLORIDA: AN AMINOSTRATIGRAPHIC APPROACH

Amino acid racemization data provide chronostratigraphic information for sedimentary facies deposited since the early Pleistocene. Racemization is the biochemical process by which amino acids in fossil organisms convert from the L-form, occurring in living specimens, to the D-form stereoisomer, which increases through time in geologic samples until equilibrium is reached. Racemization data are presented here for four sample localities across southern Florida, representing exposures of the Caloosahatchee (Plio-Pleistocene), Bermont (lower to mid-Pleistocene) and the Fort Thompson (upper Pleistocene) ”Formations”. Fossil mollusks, including Chione elevata and Prunum apicinum, were extracted from bulk samples and analyzed using HPLC reverse-phase liquid chromatography. Kinetic data are currently being established for modern Chione and Mercenaria to better constrain numerical ages and local paleotemperature gradients. Amino acid concentration data are also evaluated. Averaged data for each stratigraphic unit demonstrates that samples collected from the Caloosahatchee display the highest ratios (0.98 ± 0.04 for valine and 0.90 ± 0.05 for glutamic acid), whereas fossils collected from the Fort Thompson correspond to the lowest reported D/L ratios (0.58 ± 0.04 for valine and 0.61 ± 0.04 for glutamic acid). Bermont samples yield D/L ratios between those values, in agreement with stratigraphy. Furthermore, a relative aminostratigraphy is presented for the Bermont-correlative Longan Lakes locality to test the ability of the technique to resolve the timing of relatively small-scale sea-level fluctuations recorded in the strata. Aspartic acid D/L values range from 0.79 ± 0.02 to 0.87 ± 0.03, mostly conforming to observed stratigraphy, although ratios from the middle of the sequence are higher than those at the bottom. Sedimentary reworking may explain this inversion of D/L ratios, as older fossils were reworked through successive sea-level cycles. Analysis of additional samples combined with modern kinetic data will allow further testing of the temporal resolution in this stratigraphic system.