PHREATIC OVERGROWTHS ON SPELEOTHEMS IN THE LITTORAL CAVES OF MALLORCA, WESTERN MEDITERRANEAN: HYDRO- AND GEOCHEMICAL MONITORING

Mallorca’s phreatic overgrowths on speleothems (POS) are useful paleo-sea level and paleoclimate proxies because they precipitate from meteoric-marine mixed water in littoral caves, where the water table is coincident with sea level. POS alternation of calcite and aragonite precipitates mirrors changes from meteoric-vadose to meteoric-marine mixing zone environments. POS are documented above, below, and at the current Mediterranean sea level in Mallorca and can be accurately dated using U/Th methods, thus constraining the elevation of each sea-level stand.

Gaps exist in our understanding of the geochemical processes responsible for the precipitation of POS in Mallorca, specifically: differences between pools that currently precipitate aragonite instead of calcite. This study monitors aragonite-precipitating pools in Vallgornera and calcite-precipitating pools in Drac. Dissolved oxygen, temperature, conductivity, redox potential, and pH are recorded in-situ in the upper 3 cm of each pool. Water samples are collected monthly for total alkalinity, total hardness, total sulfate δ¹³C, δ¹⁸O and trace element analyses. Cave atmospheric loggers are used to correct the water level recorded in each cave pool for barometric pressure fluctuations. Relative humidity, CO₂, and temperature are also recorded in the cave atmosphere. Artificial supports installed in each pool collect precipitates formed during to the monitoring period.

Preliminary data (December 2011-June 2012)

Site	Conductivity (mS/cm)	pH	Average Pool Temperature (°C)	Average Air Temperature (°C)	CO2 (ppm)	Alkalinity (mg/L)
Vallgornera	9-12.5	7.2-7.6	19.4	19.3	873-2044	168-232
Drac	10.8-14.3	7.1-7.9	18.5	18.7	252-579	204-292

The carbonate system chemical data and the continuous CO₂ record document the role of CO₂ degassing at the air-water interface in POS deposition. Comparing the geochemical variation of pool surface waters with that of their POS will constrain the conditions necessary for precipitation, and potential equilibrium or kinetic isotope effects. A better understanding of calcite and aragonite precipitation in the freshwater/seawater mixing zone (i.e., controlling factors such are CO₂ degassing, salinity, CO₃^2- supply, saturation index, etc.) is crucial for POS used in either paleoclimate or sea-level reconstructions.