RECORDS OF UPWELLING, SEASONALITY AND GROWTH IN STABLE-ISOTOPE PROFILES OF PLIOCENE MOLLUSK SHELLS FROM FLORIDA

Oxygen and carbon isotopic profiles across the shells of well-preserved bivalves and gastropods from the Pliocene Pinecrest Beds near Sarasota, Florida, provide detailed records of the paleoenvironmental and paleodimatic conditions under which these high-density shell beds accumulated, as well as offer insight into shell-growth rates and longevity. Eight turriteilid gastropods were analyzed from within and surrounding two Turritella-rich beds, an upper bed within unit 2 (2.5-2.0 Ma) and a lower bed situated near the top of the main shell bed, unit 6/7 (3.5-2.5 Ma). Three bivalves plus another turritellid specimen from a lower horizon (middle of unit 6/7), considered to represent off shore, normal-marine conditions, were also analyzed. The isotopic profiles of all shells reflect year-round shell growth in a seasonal, subtropical to warm-temperate paleoenvironment. Cyclical patterns in delta(18)O reflect annual variations in water temperature which on average ranged from 15 to 24 degrees C; minimum paleoseasonality contrasts varied from 7 to 9 degrees C. Summer paleotemperatures compare favorably with those determined from ostracodes, whereas winter temperatures appear slightly cooler. Except for one pectinid specimen, temperature differences between horizons and specimens are small. Annual delta(18)O cycles indicate: Turritella apicalis lived longer and grew slightly faster than Turritella gladeensis; and, shell growth rates and increment formation in Mercenaria campechiensis are comparable to modem Florida populations. Coincident episodes of heavy delta(18)O and light delta(13)O in both turritellids and bivalves suggest a pattern of seasonal upwelling. This cool, nutrient-rich, upwelled water may be partially responsible for the great abundance of Pinecrest fossils.