PALEOENVIRONMENTAL IMPLICATIONS OF MID-HOLOCENE SERPULID TUBE/TUFA MOUNDS AND UNDERLYING CORAL COLONIES, ENRIQUILLO VALLEY, DOMINICAN REPUBLIC

Rising sea level in early Holocene time (~9.8 ka) flooded the Enriquillo Valley of southwestern Dominican Republic, creating an embayment with a thriving fringing coral reef. About 5 ka sediment accumulation at the embayment mouth cut off circulation to the Caribbean Sea, resulting in drastic environmental changes and demise of the corals. Today much of the valley is occupied by Lago Enriquillo, a large hypersaline lake lying ~40 m below present sea level. Arroyos in the valley walls reveal a superbly preserved fossil coral reef.

Resting directly on coral substrates at many sites around the valley are well developed, meter-scale mounds composed of serpulid tubes and tufa, with internal pockets of molluscan shells and sediment. Radiocarbon dates of the youngest corals and oldest serpulid tubes at well-exposed contacts reveal a 700- to 1300-year hiatus between coral death and mound development. Likely during and certainly after this hiatus, ancient Lago Enriquillo experienced water-level fluctuations owing to changing precipitation patterns that may have been associated with a more northerly presence of the Intertropical Convergence Zone. Within the ancient lake, salinity levels decreased with increased precipitation, permitting opportunistic serpulid worms favoring hyposaline conditions to form large aggregates.

In most areas, serpulid aggregation was coupled with tufa precipitation. Serpulid tube/tufa mounds occur around the periphery of the valley and can be classified into four distinct morphologies: individual, “island”, terraced, and tiered. Controlling factors on mound shape are substrate type, elevation, and slope angle. Tufa precipitation is frequently associated with groundwater springs, and locations and morphologies of these mounds may be controlled by freshwater influxes into the ancient lake.

Results of δ¹³C and δ¹⁸O stable isotope analyses of the youngest Montastraea annularis corals suggest an inverse relationship between salinity and elevation. This relationship may be due to development of a hyposaline lens within the ancient bay. Later on, such a lens also would facilitate tufa precipitation in the lacustrine setting. The widespread association of tufa precipitation with serpulid aggregation indicates that both are favored by specific and localized paleoenvironmental conditions.