GEOCHEMICAL ANALYSIS OF TWO DIFFERENT MOLLUSCS REVEAL FEEDING PATTERNS AFFECT PRESERVED TEMPERATURE RECORDS

Typical paleoclimatic reconstructions using oxygen isotopes of molluscs have been based on single species, which may provide only a truncated temperature record. Here we examine oxygen isotope records from shells of modern marine gastropods whose seasonal activity levels are known and assess their potential value as paleoclimate indicators in the recent geologic past.

Live specimens of Busycon contrarium and Fasciolaria tulipa were collected from Tampa Bay, Florida, near a monitoring station that records water quality parameters. Previous observations show that B. contrarium feeds during the winter but is inactive during the summer months. Conversely, F. tulipa exhibits increased activity in the summer. Combined isotope records of these species should provide a record encompassing the full range of annual temperatures.

Oxygen isotope profiles revealed 3 years of growth for B. contrarium and 2 for F. tulipa, with sinusoidal cycles in the profiles having a δ¹⁸O amplitude of 1.97‰ for B. contrarium and 3.50‰ for F. tulipa. Comparisons of observed to predicted isotopic values revealed deviations from the expected patterns of seasonal growth. Although B. contrarium is active during cooler months, its isotopic values matched predicted values only for warm months. F. tulipa recorded the full range of annual predicted values for the site, despite its apparent inactivity during the Florida winters.

Our hypothesis that seasonal patterns of calcification can be inferred from seasonal patterns of activity is falsified. This may be explained by feeding strategies. B. contrarium uses its shell to chip open the bivalve prey. Precipitating new, thin shell, ineffective at chipping, during the active feeding season would be disadvantageous. Therefore, new shell growth is limited to the warmer months when there is sufficient time to deposit and thicken shell material. Conversely, F. tulipa requires only a thin shell lip to wedge open bivalves. Since the success of this feeding style is not dependent on a thick shell, the F. tulipa is able to secrete shell material throughout the year.

Knowing which species to sample for a given time interval is still problematic, even when seasonal patterns of activity of the potential species pool are well understood. Our results indicate, though, that F. tulipa alone is an adequate paleoclimate archive source for local Plio-Pleistocene shell beds.