Origin, transport and discharge of CO 2 in central Italy

Abstract

This paper reviews published geochemical and isotopic data (together with new data) on: (1) thermal and cold springs emerging after long circulation in a regional Mesozoic carbonate reservoir; (2) gas vents (mostly CO 2) associated with thermal and cold springs and those emerging as dry gas emissions; and (3) active and fossil travertine deposits in a large sector of central–southern Italy, from Florence to south of Naples. The origin and transport of CO 2 and its effects on water–rock interactions during transfer through the shallow crust and discharge to the atmosphere are described. Diagrams and cross-comparison of isodistribution maps of: (1) heat-flow; (2) δ 18O of thermal springs; (3) elevation of thermal springs; (4) calculated pCO 2 in thermal springs; (5) 3He/ 4He ratio in gas vents; (6) Sr concentration in thermal springs; (7) Sr concentration in travertines; (8) δ 13C in CO 2 gas emissions; and (9) δ 13C of fossil and active travertines, show systematic geographic zonation in values of δ 13C in CO 2 gas and travertines, Sr concentration in thermal springs and travertines, pCO 2 in thermal springs and the 3He/ 4He ratio in gas phases sampled at surface in central–southern Italy from the western back-arc volcanic Tyrrhenian sector towards the eastern foredeep Adriatic sector. In their lateral westward flow towards the Tyrrhenian coast, meteoric waters precipitating in the high eastern Apennine range mix with ascending magmatic, metamorphic and geothermal fluids in the highly permeable Mesozoic limestones. This fluid mixing causes transfer of poorly condensable components (among which CO 2 and H 2S) into the regional aquifer and enhances dissolution of limestone to occur faster than in typical, non-thermal karst systems. The carbon isotopic compositions of travertines precipitated along the peri-Tyrrhenian sector are correlated with the carbon isotopic compositions of parent CO 2 (metamorphic or organic), whereas the 87Sr/ 86Sr ratios of travertines inherit the isotopic signature of the dissolved Mesozoic carbonate rock. Although subduction is presently inactive in the northern–central Apennines, the distribution of fluids is typical of active volcanic arcs. Because travertine precipitation is clearly related to climate conditions and all dated travertines in Italy are younger than 500 ka, the analysis of data, in particular the parallel areal distribution of Sr concentration in present thermal springs and fossil travertines, suggests that the westward regional fluid motion has been ongoing since at least 500 ka.