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-preserve d bivalves and gastropods from the Pliocene Pinecrest Beds near Saraso ta, Florida, provide detailed records of the paleoenvironmental and pa leodimatic conditions under which these high-density shell beds accumu lated, as well as offer insight into shell-growth rates and longevity. Eight turriteilid gastropods were analyzed from within and surroundin g two Turritella-rich beds, an upper bed within unit 2 (2.5-2.0 Ma) an d 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 lo wer horizon (middle of unit 6/7), considered to represent off shore, n ormal-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 re flect annual variations in water temperature which on average ranged f rom 15 to 24 degrees C; minimum paleoseasonality contrasts varied from 7 to 9 degrees C. Summer paleotemperatures compare favorably with tho se determined from ostracodes, whereas winter temperatures appear slig htly cooler. Except for one pectinid specimen, temperature differences between horizons and specimens are small. Annual delta(18)O cycles in dicate: Turritella apicalis lived longer and grew slightly faster than Turritella gladeensis; and, shell growth rates and increment formatio n in Mercenaria campechiensis are comparable to modem Florida populati ons. Coincident episodes of heavy delta(18)O and light delta(13)O in b oth 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.