Subacqueous antistalactites: a new type of speleothem from the old aqueduct of Sassari (Sardinia, Italy).

It is well known that artificial cavities sometimes host speleothems very different from those growing in caves. This is the case of the old aqueduct of Sassari (Sardinia, Italy), 4 km-long tunnel dug in 1880 within Miocene carbonate in order to supply drinking water from the Bunnari dam to the town. At 2 km from the low entrance, it intercepted the groundwater coming from a fracture. Close to this outlet, small subaqueous conical hollow calcite speleothems have been found growing on the floor, in 10 cm-deep water, on a 1-cm calcite crust precipitated on coarse clastic sediments transported by floods. These fragile vertical tubes range from 2 to 5 cm in height and from 0.8 to 1.5 cm in diameter, with wall typically 3-5 mm thick. The development of these underwater speleothems is controlled by the presence of steady gas bubble on top of each of them that seem to be related to the soft sediment present below the hard thin calcite crust lining on it. The air-filled bubble induces diffusion and/or evaporation from the solution into it, allowing for high supersaturation ratio close to the speleothem top. Calcite precipitation occurs by degassing-induced crystallization at the inner margin of the bubble as dictated by surface tension. The ascent gas bubble tends to have a relatively slow degassing rate, maintaining an open canaliculus within the speleothems which acts as the feeding tube of a normal stalactite: for this reason this new type of speleothem has been named "subaqueous antistalactite".