CALCITE RAFT GEOCHEMISTRY AS A GROUNDWATER LEVEL PROXY FOR PALEOINDIAN ACCESS IN LA MINA (SAGITARIO) AND HOYO NEGRO (SAC AKTUN) CAVES, QUINTANA ROO, MEXICO

Calcite raft accumulations provide important paleoenvironmental information on Holocene groundwater level in the Yucatan Peninsula which can be used to assess age dating of Paleoindian archaeological sites. Rising Holocene water level and cave passage morphology restricts access providing a date before which humans could enter the cave. Calcite rafts precipitate at the air/water interface in caves and caverns from CaCO₃ saturated water through CO₂ degassing and/or evaporation, and sink through agitation of the water surface either by movements of hanging roots in the cavern, dropping bat guano, or drip water from the cave ceiling. This point source disturbance and deposition of calcite rafts then forms piles or cones on the cave bottom or larger sheet-like accumulations. These accumulations have been used as a record of paleohydrology using elemental (Sr/Ca, Cl/Ca) and isotopic proxies (δ¹⁸O, δ¹³C, ⁸⁷Sr/⁸⁶Sr) recording changing groundwater chemistry, and can be radiocarbon dated.

The calcite raft sheets found in La Mina cover the bottom of the cave in various spots and are found in narrow overhangs of limestone and within ochre mine pits. Individual fragments of calcite rafts (~ 10 cm in diam.) were once interconnected in a relatively continuous sheet on the water surface prior to deposition on the cave bottom, which likely resulted from a drop in groundwater of ~ 10-20 cm. Because the calcite rafts could only form within a narrow height range below the overhangs, they provide a very accurate water level estimate.

The Paleoindian age of La Mina (~11-12 ka) is corroborated by the timing of the flowstone formation and of the submergence of the cave by sea-level rise. At multiple locations, flowstone can be seen encrusting mined surfaces; sheets of flowstone cap many of the charcoal concentrations that were sampled and dated. This indicates that these mined areas had been abandoned for many years prior to the submergence of La Mina. Calcite rafts that formed when the cave bottom was flooded with Holocene sea-level rise produced dates between 8,020 and 7,760 cal BP, consistent with evidence from Hoyo Negro that sea level surmounted 9-11 m below modern sea level between 8.0 and 7.0 ka. Groundwater levels in the region track sea level closely, so water levels in the La Mina and Hoyo Negro, 8 and 6 km inland, respectively, should be equivalent. Therefore, the calcite raft evidence indicates that mining activities in La Mina were abandoned well before 8.0 ka.