Evaluating the geogenic CO2 flux from geothermal areas by analysing quaternary travertine masses. New data from western central Italy and review of previous CO2 flux data

Quantification of carbon fluxes between solid Earth and its atmosphere is necessary to understand the global geological carbon cycle. Some of the main CO2 contributors are metamorphism and magmatic mantle degassing. CO2 is discharged from active and quiescent volcanoes, fault zones, geothermal systems and CO2 rich groundwater. Here a new method for the estimation of the geogenic flux of CO2 from tectonically active regions, based on the volume, composition and age of travertine deposits, is proposed. The method is applied to the travertine deposits of western Central Italy where travertine deposition is driven by degassing of CO2 charged groundwater. Results show that the study areas are characterized, since Middle Pleistocene, by diffuse CO2 degassing processes with time averaged CO2 fluxes ranging between 1.24+/-0.12 10(6) mol y(-1) km(-2) and 1.38 +/- 0.42 106 mol y(-1) km(-2). These values are of the same order of magnitude of carbon dioxide fluxes measured by different methods in western central Italy and are higher than the global baseline CO2 flux from high heat flow regions. The review of the available U-234/Th-230 and C-14 data shows that the CO2 degassing processes that affects western Central Italy nowadays were already active at least 350 Ka ago, proving that this region is a globally relevant case for the study of Earth degassing. Considering the widespread occurrence of travertine deposits in tectonically active areas worldwide, the proposed approach can be used as a reliable tool to estimate the CO2 flux in different geodynamic settings within system Earth.