PALEOHYDROLOGY OF MIDDLE HOLOCENE LAGOONAL & LACUSTRINE DEPOSITS IN THE ENRIQUILLO VALLEY, DOMINICAN REPUBLIC: PORE MORPHOMETRICS AND ISOTOPE GEOCHEMISTRY OF OSTRACODA

The southwestern region of the Dominican Republic (Enriquillo Valley) contains an exceptionally well-preserved, relict marine, saline lake deposit. Abundant euryhaline ostracodes include Cyprideis salebrosa, C. mexicana, C. similis, and C. edentata. Morphometric and geochemical analyses performed on Cyprideis spp. indicate positive d¹⁸O  and d¹³C coincident with relative abundances of  irregular shape pores that permeate the ostracode carapace. We recognize 3 stratigraphic intervals with distinct ostracode normal pore circularity and stable isotope  trends:  (I) 4.5-5.0 m interval that contains ostracodes with highly irregular shaped pores (multiradiate) with high amplitude variability d¹⁸O and d¹³C values; (II) 5.0-5.6 m interval that contains ostracodes with circular pores with an overall trend towards negative isotope values; and (III) 5.6-6.5 m interval that yields ostracodes with upward increasing abundance in circular pore shapes coincident with a positive-to negative shift in the d¹⁸O and d¹³C values. When the lagoon was first separated from the sea, arid conditions contributed to evaporative, hypersaline waters in the restricted lagoon. By middle-to late Holocene time, increased precipitation in the valley resulted in a coastal lake system that became progressively oligohaline, yet, moderate to small scale amplitude variability in the salinity proxy data suggest short term oscillations in the precipitation-evaporation budgets. At least 2 marine inundations of marine waters contributed to the observed variability in the d¹⁸O and d¹³C values. Evidence for abrupt changes in baselevel indicates that tectonic activity may have contributed to the observed paleoenvironmental trends recorded in the deposit.