Geogenic CO2 flux calculations from the Late Pleistocene Tivoli travertines (Acque Albule Basin, Tivoli, Central Italy)

The Lapis Tiburtinus travertine of the Acque Albule Basin (Tivoli, Central Italy) is a well-known travertine deposit composed by 10 units separated by unconformity surfaces. This travertine deposition is related to CO2 degassing of thermal waters circulating in deep carbonate-sulphate reservoirs. Since for each mole of calcite precipitated by groundwater, one mole of CO2 is degassing to the atmosphere, the volumes of travertine deposits can be used as an indirect proxy of the amount of CO2 degassed during travertine deposition. In the present work the volumes of the different travertine units and the cumulative amount of CO2 degassed per depositional unit (FCO2, which range between 1.56×108 mol a-1 to 5.93×108 mol a-1) have been computed. Furthermore, the fluxes of carbon dioxide per unit area (φCO2) were computed and compared to the δ18O curve and pollen data. The computed CO2 fluxes, range between 7.11×105 mol a-1 km-2 and 2.70×106 mol a-1 km-2. These values are minimum estimations of the deep CO2 degassing processes because of dissolution and erosion processes and possible CO2 loss from thermal water before their emergence at springs as well as because only the exposed travertine succession was used as proxy for the entire succession. The comparison of the CO2 flux data with the δ18O curve and pollen data shows that smaller variations in carbon dioxide flux are related to changes in climatic conditions, while greater variations are probably caused by the increase of the deep CO2 degassing process of the Acque Albule hydrothermal system, which is, in turn, relate to the activity of Colli Albani volcanic system.