LARGE TUFA-COATED SERPULID MOUNDS SIGNAL AN ABRUPT MID-HOLOCENE TRANSITION FROM MARINE TO RESTRICTED HYPOSALINE CONDITIONS, LAGO ENRIQUILLO, DOMINICAN REPUBLIC

Unique aggregates of serpulid worm tubes from the southwestern Dominican Republic record paleoclimate conditions following the abrupt cessation of normal marine circulation in the Enriquillo embayment during mid to late Holocene time. Stable carbon and oxygen isotopic data in conjunction with ²³⁴U/²³⁰Th and ¹⁴C dating suggest a transition to hyposaline conditions ~4900 ybp. High-resolution stable isotope data from individual tubes indicate considerable variability in temperature and salinity conditions throughout the period of tufa-coated serpulid mound formation.

These mounds (up to 2.9 m tall) occur in multiple tiers along the paleoshoreline of Lago Enriquillo. Initially, serpulids colonized hard coral substrates such as large Montastraea annularis colonies and Acropora cervicornis rubble, but some may have even colonized live coral. The mounds are coated with a thick tufa rind; however, the relative proportion of primary precipitate versus post-depositional weathering product is still under investigation.

Stable isotope values of the mound serpulids range from -3.0 to -4.0 permil δ¹³C, and -0.8 to -1.4 permil δ¹⁸O, in contrast with higher carbon and oxygen values for similar serpulid species from older coral reef deposits (1.0 permil δ¹³C and 0.0 to 1.2 permil δ¹⁸O). High resolution geochemical analyses indicate that sub-annual paleoenvironmental data may be extracted from individual serpulid tubes, and seasonal paleoclimate variability is greater in mound tubes than in tubes formed under normal marine conditions. ²³⁴U/²³⁰Th dating of tubes from an older, unconsolidated 5 cm-thick layer of serpulids indicates a trial colonization period ~6000 ybp, coinciding with the Holocene Thermal Maximum. This suggests the changing regional climate patterns may have contributed to final restriction of the Enriquillo embayment and termination of coral reef growth.