ISOTOPE VARIABILITY IN EARLY-HOLOCENE ACROPORA CERVICORNIS FROM THE ENRIQUILLO VALLEY, DOMINICAN REPUBLIC

Previous studies of d¹⁸O and d¹³C values of scleractinian coral skeletons have emphasized the importance of recognizing the variety of factors that influence skeletal isotope values (salinity, temperature, calcification rate, isotope values of ambient seawater, etc.). Sampling strategy is critical to the scale of any study since low-resolution sampling may attenuate any primary high-resolution isotope variability. In addition, previous work has demonstrated that different skeletal fabrics within a specimen exhibit marked variation in isotope values. Early-Holocene Acropora cervicornis were sampled at a resolution of ~12 samples/year (assuming a growth rate of ~7mm/yr) along discrete linear transects up the major growth axis and time-equivalent corallite to evaluate sampling methodologies in branching corals for isotope studies. A statistically significant offset of 0.2 ‰ in the d¹³C was observed between the two skeletal elements. Such heterogeneity in the d¹³C values suggests the two fabrics are influenced by different vital effects. Coral skeletons precipitate with isotope values lower than predicted by thermodynamic equilibrium. Since the major growth axis d¹³C values of Enriquillo corals are generally lighter than those of the corallites, we propose calcification may have been slower along the corallites allowing for skeletal material to approach isotopic equilibrium with ambient seawater. d¹⁸O data from this study are equivocal as a record of seasonal variability. This may be due to a general lack of appreciable seasonal variation in a tropical setting. ¹⁴C dating of the corals indicates that growth-axis d¹⁸O and d¹³C values decreased by ~0.5 ‰ from ~9.3 to 7.5 ka, suggesting: 1) decrease in water oxygen and carbon isotope values due to increased freshwater influx from the regional watershed of the Rio Yaque del Sur; 2) warming during the Holocene thermal maximum; or 3) subaerial diagenesis affecting younger corals more strongly than older ones. Continued investigation of Acropora biogeochemistry may provide valuable insight into the environmental evolution of the Enriquillo Valley paleo-embayment on a millennial scale.