HIGH RESOLUTION DATING AND CHARACTERIZATION OF ACROPORA CERVICORNIS GROWTH IN THE ENRIQUILLO VALLEY, 9.45 – 6.06 KA (U/TH)

Located in the Bahia de Neiba in southwestern Dominican Republic, the Enriquillo Valley is distinguished for the pristine preservation of Holocene coral reef outcrops. Towards the end of the last glacial period, as global sea level rose, the Caribbean Sea flooded the valley, producing a seaway in which fringing reefs developed (~9-5 ka). For 3,500 years of that time, extensive (~11m in height), monospecific reefs of Acropora cervicornis accumulated. By 5 ka, serpulid worms had replaced Acropora cervicornis as the dominant reef species.

Because the cervicornis outcrop in the Enriquillo Valley is so well exposed, collection of samples at high resolution (2 cm intervals) is possible. X-Ray diffraction analyses have shown that 68 of 122 samples from the outcrop are pure aragonite, with the lower part of the outcrop more poorly preserved. ²³⁴U/²³⁰Th dating indicates that Acropora cervicornis flourished from ~9.45 to ~6.06 ka. The 9.45 ka date, however, may not suggest a seawater environment. Based upon these dates, Acropora cervicornis accumulated twice as rapidly from 9.45 to 8.66 ka as it did during later periods. ¹⁴C dates are also being determined for 50 samples at 20 cm intervals throughout the outcrop. The first ten of these conventional radiocarbon dates (8.72 to 7.96 ka) demonstrate that Acropora cervicornis accumulated consistently in correct stratigraphic sequence with few interruptions. The only evidence of mixing may suggest that a large storm event occurred ~8.40 ka.

The stable isotope values of Acropora cervicornis samples from this outcrop reveal significant environmental changes during reef formation. Data show that mean δ¹⁸O decreased from -0.9 to -2.2‰ while mean δ¹³C increased from -4.5 to -1.4‰ from ~9.45 to ~6.06 ka. These data suggest a decrease in salinity of the Lago Enriquillo embayment due to an increase in precipitation associated with the thermal maximum at 6 ka. High resolution stable isotope data may also reveal changes in seasonality throughout the Holocene.