THE IMPLICATIONS AND LIMITATIONS OF PHREATIC OVERGROWTHS OF SPELEOTHEMS AS SEA LEVEL INDICATORS: QUINTANA ROO, MEXICO

Phreatic Overgrowths of Speleothems (POS) are calcite crusts deposited on cave formations, typically found at similar elevations in disparate areas of a cave. They are thought to form where CO₂ outgasses at the water table and calcite precipitates in a band representing seasonal and tidal water table fluctuations. U-Th dating provides precise ages of POS and, in low-gradient systems, their elevation is expected to be nearly equivalent to sea level during the time of their deposition. Where sea level records are accurate, the difference between the former and modern elevation of a sample provides a measure of uplift or subsidence.

We have obtained U-Th ages of 15 POS and 15 stalactites from three caves in the northeastern Yucatan Peninsula, a region assumed to have remained tectonically stable at least since the late Pleistocene (Szabo, 1978). Based on this assumption, the caves in which they were collected were thought to have formed at their present elevations above modern sea level during the high stands of the late Pleistocene. Samples were collected in cave passages from 1-7 km inland, at elevations 1-7 m above modern sea level. POS dates ranged from 49 +/- 2.0 ka to 287 +/- 3.6 ka. According to North Atlantic composite curves (Spratt and Leisieki, 2016), all correspond to times of lower sea level than today, suggesting that uplift has occurred. Outliers could reflect rapid sea level changes not detected in ice or ocean sediment cores. The dates of interior stalactites range from 110 +/- 0.7 ka to 646 +/- 125 ka, which demonstrates that cave passages were fully developed and contained formations much earlier than any Pleistocene high stand.

Our data suggest that these caves formed at multiple sea levels with uplift of the Yucatan Platform at 250 mm/ka – 390 mm/ka. Uplift could be the result of tectonic movement, isostatic rebound due to mass loss from denudation, and/or changes in buoyant support. Modern water table gradients are very low, between 0.5 m/km (Jenson, unpublished) and 0.05 m/km (Beddows, 2007); past gradients may have been steeper, and changing positions of the coastline could introduce variability in the observed elevations of POS. Additional POS will be collected from above, at, and below the water table to refine our data and to test hypotheses about the geochemical conditions required for POS deposition.