EARLY PLEISTOCENE PALEOTEMPERATURE SEASONAL VARIABILITY: MOLLUSCAN SCLEROCHRONOLOGY FROM SOUTH-CENTRAL FLORIDA

Shells of the bivalve Chione elevata and the gastropod Busycon sp. were collected from the lower Pleistocene Bermont “Formation” exposed within the Longan Lakes Quarry in Collier County, Florida and subjected to stable isotopic analyses. In order to determine seasonal temperature variation within the unit; thirty chionids and five gastropods specimens were selected from bulk samples of similar facies within the six ,either coarsening or fining upward, sequences of the stratigraphic unit. Shells with well-preserved ornamentation and minimal levels of surface erosion and diagenetic alteration were preferentially chosen. In addition, chionids with differing morphology and ornamentation were analyzed separately to determine whether these morphologies reflect varying growth rates or seasons of shell precipitation.

The δ18O data from the chionids and gastropods revealed similar absolute and seasonal temperature values within lithologically equivalent facies as inferred from Grossman and Ku's (1986) aragonite paleotemperature equation. Among the chionids, virtually all specimens show a distinct seasonal fluctuation in temperature. Considered in their totality, they display a seasonal range of temperature that ranges from a maximum of 31.8 °C to a minimum of 17.9 °C. Seasonal temperature variation for individual sequences ranged from 12.5 to 8.8 °C, although this is greater than the seasonal variation displayed by any single specimen. The gastropods show a similar minimum temperature (17.3 °C), but a slightly lower maximum temperature (29.1 °C) suggesting that they may not grow during the warmest temperatures. Based on analyses of modern thermal trends along Florida's west-central coast, these temperatures and their seasonal variation are virtually identical to current temperatures. The modern winter sea-surface temperature (SST) ranges between 16 and 25 °C whereas the summer SST was approximately 30 °C. The analysis of variably ornamented specimens suggests that there is no correlation between the patterns of rib spacing and seasonal variation in chionids.