Hypogenic caves of Syracuse area, Sicily (Italy): geomorphological evidence of CO₂ degassing, freshsalt water mixing, and late condensaAon corrosion

Description

Many caves in Sicily have been shown to have a sulfuric acid or other hypogenic origin. We studied three caves (Palombara, Scrivilleri, Monello) near Syracuse (eastern Sicily), in an area that was strongly uplioed and faulted, creaKng mulKple Pleistocene marine terraces. Mineralogy, stable isotopes and daKng methods (paleomagneKsm, U/Th) were used to characterize cave sediments, some of which were related to the iniKal hypogenic phase (Fe and Mn oxides, calcite spar), others were introduced by surface runoff later. Many other sediments are the result of in situ weathering, such as lime sands produced by condensaKon-corrosion processes on the calcarenite walls. Phosphates, kaolinite and montmorillonite are related to bat guano decay. Stable isotopes show that the speleothems derive from surface seepage with temperatures similar to the present, with no evidence of a hydrothermal origin. Other deep sources of aggressivity are also excluded. We obtained an age of 603 ka (+285/-91) for a marine notch deposit near Palombara, as well as a possible paleomagneKc inversion (>780 ka) for clasKc allogenic sediments. These ages are discussed, raising the quesKon of the reliability of calculaKons extrapolated from marine terrace daKng and the possibility that the caves may be older than expected.

Cave morphologies clearly indicate a hypogenic phase, with aggressive ascending flows creaKng the typical Morphologic Suites of Rising Flow (MSRF). The bubble trails and acid notches are formed by carbonic degassing and subsequent acidificaKon in more or less closed aerated environments at the water table. CO₂ probably derived from both the bedrock and the oxidaKon of surface-derived organic carbon at the density boundaries of the freshwater lens. We propose a mixed Flank Margin Cave and hypogenic speleogenesis model, where dissoluKon was concentrated in areas of greater CO₂ concentraKon, producing phreaKc maze paRerns recording past sea-level posiKons. We suggest that aggressiveness of the rising fluids could have partly originated at a shallow depth, in the mixing zone between fresh and salt water.

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