EURHOMALEA EXALBIDA (BIVALVIA): A RELIABLE CLIMATE ARCHIVE OF SOUTHERN SOUTH AMERICA? (Invited Presentation)

Due to the lack of suitable high-resolution archives, regional and continental-scale climate dynamics of southern South America are not well understood. Shells of the long-lived, shallow-marine bivalve mollusk, Eurhomalea exalbida (Dillwyn) are likely to contain information on the past water temperatures. As yet, however, no rigorous calibration study has been presented so that growth history traits and the reliability of shell oxygen isotope-based temperature estimates remain unknown. Shell growth patterns and oxygen isotope ratios of four young specimens of E. exalbida from the Falkland Islands (Southwest Atlantic) were analyzed and cross-calibrated with environmental parameters. Results indicate that E. exalbida likely captured the full seasonal temperature amplitude in its shell. Annual growth line formation occurred between fall and early winter. The most remarkable finding, however, was that E. exalbida formed its shell with an offset of -0.48‰ to -1.91‰ from expected oxygen isotopic equilibrium with the ambient water. If this remained unnoticed, paleotemperature estimates would overestimate actual water temperatures by 2.1°-8.3°C. With increasing ontogenetic age, the discrepancy between measured and reconstructed temperatures increases exponentially, irrespective of the seasonally varying shell growth rates. We attribute this finding to a pH increase in the extrapallial fluid during ontogeny favoring a dominance of the (isotopically lighter) carbonate ions over (isotopically heavier) bicarbonate ions. When this disequilibrium fractionation effect is taken into account, E. exalbida can serve as a high-resolution paleoclimate archive for mid to high latitudes of southern South America providing quantifiable temperature estimates, even from single fossil specimens.