INTERPRETATION OF δ13C PROFILES IN MODERN AND PLEISTOCENE SUBTROPICAL PREDATORY GASTROPODS

The relative concentrations of carbon stable isotopes (δ¹³C) in molluscan shells are known to reflect environmental conditions. Most studies of δ¹³C in mollusks have focused on filter feeders and grazers, while little work has been done on predatory gastropods; where energy expense, governed by intermittent feeding, can potentially influence incorporation of ¹³C. Shells of both modern and Aftonian Pleistocene congeneric gastropods: Triplofusus giganteus, Fasciolaria tulipa and F. okeechobeensis, were incrementally sampled over the extent of their growth and analyzed for δ¹³C and δ¹⁸O. Modern living specimens were collected in the Cay Sal Bank (F. tulipa) and Florida Keys (T. giganteus); Pleistocene shell specimens (F. okeechobeensis and ancient T. giganteus) were collected from Bermont Fm. exposures in southern Florida. Both the modern T. giganteus and F. tulipa exhibit seasonality in their δ¹⁸O profiles and empirically calculated temperatures from both modern shells correspond with measured regional seawater temperatures. δ¹³C values from the modern F. tulipa range from 0.41- 2.07‰ and average 1.21‰. The isotopic profiles are antiphase, with positive δ¹³C corresponding to negative (summer) δ¹⁸O excursions. Based on the hydrographic conditions, this presumably signifies seasonal upwelling. δ¹³C values of the modern T. giganteus range from -0.95 – 2.99‰ and average 0.63‰. The profiles are antiphase and δ¹³C values steadily increase as the organism matures. The δ¹⁸O curves of Pleistocene gastropods exhibit seasonal trends and paleotemperatures (δ¹⁸O_w = 1.5‰) within required range of the surrounding biota. δ¹³C values of F. okeechobeensis range from -0.39 – 2.97‰ and average 0.73‰. The δ¹³C curve exhibits a seasonal pattern but does not correlate with the δ¹⁸O curve, which could be attributable to fresh water input and/or metabolic influence. δ¹³C values of the Pleistocene T. giganteus range from 0.34 – 3.82‰ and average 1.94‰. Compared with δ¹⁸O, the δ¹³C curve is inconclusive, however as with its modern cousin, the organism preferentially incorporates ¹³C following maturation. Though care should be taken in their use, predatory gastropod carbon isotope data interpreted in conjunction with oxygen isotopes can yield legitimate paleoenvironmental information.