Sulphuric acid speleogenesis and landscape evolution: Montecchio cave, Albegna river valley (Southern Tuscany, Italy)

Montecchio cave (Grosseto province, Tuscany, Italy) opens at 320 m asl, in a small outcrop of Jurassic limestone (Calcare Massiccio Fm.), close to the Albegna river. This area is characterised by the presence of several thermal springs and the outcropping of travertine deposits at different altitudes. The Montecchio cave, with passage length development of over 1700 m, is characterised by the presence of several sub-horizontal passages and many medium- and small-scale morphologies indicative of sulphuric acid speleogenesis (SAS). The thermal aquifer is intercepted at a depth of about 100 m below the entrance: the water temperature exceeds 30 °C and sulphate content is over 1300 mg l− 1. The cave hosts large gypsum deposits from 40 to 100 m below the entrance that are by-products of the reaction between sulphuric acid and the carbonate host rock. The lower part of the cave hosts over 1 m thick calcite cave raft deposits, which are evidence of long-standing, probably thermal, water in an evaporative environment related to significant air currents. Sulphur isotopes of gypsum have negative δ34S values (from − 28.3 to − 24.2‰), typical of SAS. Calcite cave rafts and speleogenetic gypsum both yield young U/Th ages varying from 68.5 ka to 2 ka BP, indicating a rapid phase of dewatering followed by gypsum precipitation in aerate environment. This fast water table lowering is related to a rapid incision of the nearby Albegna river, and was followed by a 20–30 m fluctuation of the thermal water table, as recorded in the calcite raft deposits and gypsum crusts