Quantification of paleo-aquifer changes using clumped isotopes in subaqueous carbonate speleothems

Here we track the water-table position and temperature of the Mount San Giovanni aquifer (Iglesiente-Sulcis mining district, SW Sardinia, Italy) during the past 600 ka by determining the ages (U/Th dating) and stable isotope compositions (delta O-18, delta C-13 and Delta(47)) of a variety of subaqueous carbonate speleothems (e.g. calcite spars, dogtooth calcite crystals and calcite coatings). Clumped isotopes (Delta(47)) provide quantitative estimates of carbonate formation temperatures (and thus water temperatures) that are independent of the oxygen isotope composition of water (delta O-18(w)). Then, the delta O-18(w) of the paleo-water has been reconstructed from the clumped isotope temperature (T Delta(47)) and the delta O-18 of the carbonate (delta O-18(c)). We find that some high-temperature calcite spars formed prior to 600 ka at temperatures above similar to 120 degrees C. Lower-temperature spars (similar to 70 degrees C) precipitated at similar to 400 ka, and cold-water subaqueous speleothems (similar to 10-20 degrees C) formed in perched ponds at different levels of the karst systems between 410 ka and 110 ka, while coeval precipitation of subaerial flowstones occurred in the upper level of the shallower caves until 82 ka. We infer that the groundwater level dropped by similar to 120m from similar to 400 to similar to 250 ka, with a relatively rapid rate of similar to 0.8 mm/yr. Considering the tectonic stability of Sardinia during the Quaternary, this high rate derives from climate driven geomorphological processes at the surface rather than tectonic uplift. The d18O values of the paleo-aquifer water range from -6.0 +/- 0.7% during MIS 5c, and similar to modern cave water values (-5.1%), to -7.7 +/- 0.4% during the colder MIS 8. These values indicate that the groundwater reflected the delta O-18 signal of meteoric water, with no significant contributions from metasomatic and metamorphic waters. The observed delta O-18(w) variability can be explained by glacial/interglacial paleoclimate changes affecting rainfall delta O-18 (and thus groundwater delta O-18). We conclude that clumped isotope thermometry on subaqueous carbonate speleothems is a useful tool for tracking paleo-aquifer temperatures and delta O-18(w) reconstructions.