EXTRACTING A PALEOENVIRONMENTAL RECORD OF MID-HOLOCENE CLIMATE FROM DOMINICAN REPUBLIC CARBONATE SERPULID TUBE GEOCHEMISTRY

The Enriquillo Valley, Dominican Republic is home to a largely undisturbed fossil Holocene coral reef capped by large serpulid-tufa bioherms dating ~5 to 4 ka. Bioherms consist of massive aggregates of CaCO₃ tubes formed by filter-feeding annelids and associate tufa. A northward migration of the Intertropical Convergence Zone during the Holocene Thermal Maximum shifted Caribbean climate to warm and wet conditions. This resulted in termination of open circulation with the Caribbean Sea due to increased sedimentation at the mouth of the Enriquillo embayment, and an end to normal marine water chemistry. Change in climate and shifting landscape established an environment in which serpulid aggregations thrived. Trace and minor element-to-calcium ratios of modern and early- to mid-Holocene open marine Dominican serpulid tubes and the Holocene bioherms were analyzed to quantify evolution from normal marine conditions to a hyposaline and then hypersaline environment. Li/Ca, B/Ca, Mg/Ca, Mn/Ca, Zn/Ca, Sr/Ca, Cd/Ca, U/Ca, Al/Ca, and Fe/Ca of radiocarbon and U/Th dated samples were measured. Elemental ratios of serpulids collected from the modern and Holocene “normal marine” setting are markedly different from those of bioherms. Preliminary trace element data show that average Mg/Ca of bioherm serpulids (~100 mmol/mol) is 25 times greater than normal marine serpulid Mg/Ca. Bioherm serpulids also have higher Li/Ca (~8.97 umol/mol) than normal marine serpulids (~5.72 umol/mol). In contrast, normal marine serpulids exhibit higher Sr/Ca (~9.15 umol/mol) and U/Ca (~1.25 umol/mol) than bioherm serpulids (~3.77 umol/mol and 0.34 umol/mol, respectively). It is unknown if normal marine and mound-building species have similar partition coefficients with respect to ambient water. If so, the data reflect chemical evolution of seawater after normal marine circulation was terminated and a lake formed in the Valley. The partition coefficients may reveal a more definitive picture of the geochemical conditions in which the Lago Enriquillo serpulids thrived. Regardless, the relatively high and wide range of elemental ratio values (compared to other biogenic carbonates) of mound-forming, eurytopic serpulids indicate these organisms were able to adapt and thrive in a high-stress ephemeral environment.