Abstract
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 (δ18O, δ13C and Δ47) of a variety of subaqueous carbonate speleothems (e.g. calcite spars, dogtooth calcite crystals and calcite coatings). Clumped isotopes (Δ47) provide quantitative estimates of carbonate formation temperatures (and thus water temperatures) that are independent of the oxygen isotope composition of water (δ18Ow). Then, the δ18Ow of the paleo-water has been reconstructed from the clumped isotope temperature (TΔ47) and the δ18O of the carbonate (δ18Oc). We find that some high-temperature calcite spars formed prior to 600 ka at temperatures above ~120 °C. Lower-temperature spars (~70 °C) precipitated at ~400 ka, and cold-water subaqueous speleothems (~10–20 °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 ~120 m from ~400 to ~250 ka, with a relatively rapid rate of ~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 δ18O 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 δ18O signal of meteoric water, with no significant contributions from metasomatic and metamorphic waters. The observed δ18Ow variability can be explained by glacial/interglacial paleoclimate changes affecting rainfall δ18O (and thus groundwater δ18O). We conclude that clumped isotope thermometry on subaqueous carbonate speleothems is a useful tool for tracking paleo-aquifer temperatures and δ18Ow reconstructions.
Highlights
Hypogene speleogenesis caused by rising of CO2/H2S-rich fluids is, contrary to what was thought only a decade years ago, a common karstification process that generated dozens of caves worldwide
U/Th results indicate that speleothems in the Mount San Giovanni karst system formed between > 600 ka and 82.3 ± 0.3 ka BP (Table 1)
The δ18Ow of the speleothem-forming water at ~264 ka, during the colder MIS 8 (−7.7 ± 0.4‰,) was ~2‰ lower than the modern δ18Ow in the cave. These results suggest that the δ18Ow of the cave water may have varied at least ~2‰ during the past 400 ka, including warmer (MIS 11, MIS 7 and MIS 5c) and colder (MIS 8) periods
Summary
Hypogene speleogenesis caused by rising of CO2/H2S-rich fluids is, contrary to what was thought only a decade years ago, a common karstification process that generated dozens of caves worldwide (see Klimchouk et al, 2017, for a recent review on hypogenic caves). Ca2+–HCO3−-rich waters can precipitate carbonate minerals (i.e. calcite or aragonite) when the equilibrium reaction is displaced to the left. This is generally accompanied by CO2-degassing, evaporation and/ or water temperature lowering (Audra et al, 2009). Such conditions are prone to occur when the fluids rise to the upper level of hydrothermal aquifers and the hydrostatic pressure decreases, as water approaches the top of the aquifer, moving away from the deeper thermal source. The shallower sectors of many hypogene karst systems generally show greater abundance of secondary carbonate formations, compared to deeper areas that often display mainly dissolution features (Audra et al, 2009; Gázquez and Calaforra, 2013)
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