Tufa and travertines of southern Italy: deep-seated, fault-related CO2 as the key control in precipitation.

Continental carbonates of Quaternary age in southern Italy commonly exhibit the facies of calcareous tufa, often reported as related to shallow aquifers fed by meteoric waters and to organic processes. A close spatial relationship exists between the mappable tufa deposits and major Quaternary extensional faults. With respect to the Ca-Mg- HCO₃ composition of limestone aquifers' springs, tufa-depositing springs exhibit higher salinity and alkalinity, are slightly warmer, have lower pH and are enriched in SO₄ and CO₂. Their δ¹³C values are systematically positive and compatible with a deep-seated carbon source. A clear input of soil-derived organic carbon is indicated only for small, non-mappable tufas deposited by perched springs. The dataset indicates that the large tufa deposits owe their origin to a supplementary source of CO₂ advected by degassing through active faults, as a necessary prerequisite for inducing a rise of total dissolved salts and alkalinity. Meteoric waters that have come from a shallow aquifer are able to precipitate only limited amount of carbonates. [ABSTRACT FROM AUTHOR]