Stratigraphic data, microfossil assemblages, and stable carbon and oxygen isotopic compositions of microfossils and corals record abrupt paleoenvironmental changes in the mid-Holocene coral reef sequence of Lago Enriquillo, southwestern Dominican Republic. These proxy indicators of environmental conditions reveal rapid transition from a normal marine environment to a highly variable salinity environment. Reefal beds are exposed in gullies located around the margins of Lago Enriquillo, at elevations of 30 to 3 meters below present sea level. Hemispherical serpulid worm mounds cap the reef up to a height of 2.3 meters above sea level. Serpulid layers of 1-3 centimeters in thickness are also found within the reef sequence, first occurring at 18.5 meters below sea level. Data suggests that serpulid layers and mounds form under brackish water conditions. The stratigraphy of the layers and mounds suggest multiple transitions in the salinity history of the area. Stable carbon and oxygen isotope data taken from the serpulid layers and mounds indicate a decrease in salinity throughout the evolution of the reef sequence from ~6 Ka to ~4 Ka. The salinity change is mirrored in the microfossil assemblages found in reefal deposits. Marine deposits found early in the sequence are dominated by normal marine microfossils (ostracodes: Trachyleberididae sp., Bairdia sp.; forams: Pyrgo sp., Triloculina sp.). Microfossil assemblages from more recent deposits indicate brackish conditions (ostracodes: Cypria sp., Cyprideis sp.; forams: Ammonia beccarii; hydrobiid gastropods: Littoridnops monroensis; charophytes: Chara sp.).

The source of salinity fluctuations in the paleo-embayment is attributed to the progressive restriction of the Lago Enriquillo valley from the Caribbean Sea. As the rate of sea-level rise slowed in the mid to late Holocene, continued alluvial shedding from the surrounding mountains closed the eastern end of the embayment ~4 Ka. During the mid-Holocene humid period, the Enriquillo valley served as the drainage basin for the Rio Yaque del Sur. Therefore, high levels of precipitation created influxes of isotopically light freshwater that entered the valley via the river. After the closure of the bay, more arid conditions prevailed, in which net evaporation exceeded net precipitation.