GROWTH RATES AND TEMPERATURES OF FLORIDA RECENT AND PLEISTOCENE MOLLUSKS USING d¹⁸O PROFILES

Oxygen stable isotope profiles were used as a proxy for measuring growth span and rates of the Recent and Aftonian Pleistocene bivalves from South Florida from similar environments. Pleistocene specimens of Anadara floridana and Dosinia elegans, from the Loxahatchee member of the Bermont Formation and Pleistocene and Recent Codakia orbicularis were sampled across their growth increments with an average number of about 50 samples per shell. The isotope profile of the recent C. orbicularis from Florida Keys shows a shell growth period of 1.3 to 1.5 years of essentially uniform growth. The Pleistocene C. orbicularis growth curve matches very well to its recent counterpart, with total of 1.5 years of growth. Growth profiles of Recent and Pleistocene C. orbicularis are very similar in shape and indicate that shell growth in both cases started in the beginning of winter and ended at the end of the following winter. Isotopic values in Recent C. orbicularis vary from 0.85 to 2.5 and from 0.41 to 2.35 in the Pleistocene specimen. Temperature estimates from the Recent C. orbicularis are significantly below the recorded seasonal temperatures in the locality which implies significant salinity variations. The A. floridana growth curve displays a fast initial summer growth spurt which slows down after its 1st year, followed by continual growth during both summer and winter totaling 2.5 years of growth. D. elegans has the longest growth span which occurs at a constant rate for 4.5 years. The A. floridana isotopic values vary from 0.42 to 1.75. The isotopic values for D. elegans range from -0.35 to 2.48. The values from both shells were affected by salinity variations. Calibration of isotopic data with known temperature values for recent specimen have a potential to provide a calibration tool for evaluating the influence of salinity on the composition of calcareous shells in the subtropical coastal waters.