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