HIGH RESOLUTION STABLE ISOTOPE PROFILES OF HOLOCENE BIVALVE SHELLS FROM THE DOMINICAN REPUBLIC

Various species of bivalves from a pristinely preserved, subaerially exposed Holocene coral reef in the Enriquillo valley, Dominican Republic were sampled for stable carbon and oxygen isotope and trace element analysis. The reef was deposited in a shallow, low energy embayment and the bivalves were taken from a location with largely in situ Acropora cervicornis coral in a carbonate mud matrix. Coarse resolution d¹⁸O and d¹³C profiles, and Mg/Ca and Sr/Ca ratios of bivalves were used to detect long term changes in the depositional environment throughout the section while high resolution d¹⁸O and d¹³C profiles of Tellina sp. shells were constructed to function as “snapshots” of the climate during key periods. The reef was deposited between ~9.3 and ~6 k.a., when insolation was greater in the northern hemisphere than today and there was increased seasonality in the Caribbean region. Oxygen isotope values from Barbatia tenera, Barbatia candida, Arcopsis adamsi, Barbatia domingensis. and Tellina sp. indicate a significant evaporative component to d¹⁸O variability throughout the seasonal cycle. This indicated by relatively high d¹⁸O values (-.28-1.56 per mil) which yield unreasonably low paleotemperature estimates of Caribbean surface water unless ambient d¹⁸O values are assumed to be relatively high. Moreover, high resolution d¹⁸O and d¹³C profiles have relatively short and pronounced negative excursions which are too large to indicate changes in temperature alone. The semi-arid climate with a weak rainy season of the modern Dominican Republic is not likely to be able to account for such shifts either, but increased seasonality at the time of deposition may. The data could also reflect the further northward migration of the Inter-Tropical Convergence Zone (ITCZ), which is predicted by some climate models for this period, or it could be indicative of severe storms. Lack of information about the degree of restriction of the embayment during the time of deposition makes quantitative analysis of this system difficult, but information about the timing of the negative excursions will aid in differentiating between these hypotheses.