MODERN CALIBRATIONS OF TEMPERATURE AND NUTRIENT PROXIES FOR PALEOENVIRONMENTAL RECONSTRUCTIONS IN TROPICAL MOLLUSKS

Mollusk shell O-, C-, and N-isotope values are useful indicators of nutrient conditions in nearshore environments. Using measurements of seawater temperature, δ¹⁸O (δ_w), dissolved inorganic carbon δ¹³C (δ¹³C_DIC), and salinity values collected biweekly over a year, we tested the fidelity of shell δ¹⁸O and δ¹³C profiles to record δ_w, δ¹³C_DIC, and nutrient variations in tropical waters. Preliminary results from five Pacific cone snail (Conus) specimens suggest measured shell δ¹⁸O correlates with predicted shell δ¹⁸O calculated using δ_w and water temperature. Seasonal upwelling in the Pacific drove higher amplitudes in shell δ¹⁸O (~1.5‰) compared with the Caribbean (<0.5‰). However, both Pacific and Caribbean shell δ¹³C profiles are relatively invarient, likely reflecting reduced seasonality in the Caribbean and overall lack of variability in DIC source in shallow environments on both sides of the Isthmus of Panama. Predicted shell δ¹³C curves approximated using measured δ¹³C_DIC are ~1-2‰ higher than actual shell δ¹³C profiles, suggesting the incorporation of light metabolic C in Conus shells. In addition, we tested N-isotope ratios (¹⁵N/¹⁴N) in tissues and shell organic matrix of modern bivalves (Isognomon alatus, Pinctada imbricata, and Brachidontes exustus) as a proxy for natural and anthropogenic nutrient fluxes in coastal environments in the Bocas del Toro Archipelago (Caribbean), Panama. For all species, there is no consistent Δ¹⁵N_tissue-shell offset between stomach, gill, mantle, and muscle tissues and shell δ¹⁵N values. Two species, P. imbricata and I. alatus show significant δ¹⁵N tissue-shell correlation. Heavily populated and river influenced sites show higher δ¹⁵N values on average (4.4‰) compared with open-ocean sites (3.2‰) in the Bocas del Toro Archipelago. Shell δ¹⁸O values corroborate δ¹⁵N interpretations of marine versus terrestrial influence with lower values (-1.5‰) in sites influenced by freshwater influx compared with open-ocean sites (-1.0‰). Our results show that O-, C-, and N-isotopes in mollusk shell carbonate and organic matrix are useful indicators of nutrient sources in the southwest Caribbean and eastern tropical Pacific.