GEOCHEMISTRY OF CARBON-DIOXIDE IN 6 TRAVERTINE-DEPOSITING WATERS OF ITALY

The chemical compositions of six travertine-depositing hot spring wate rs in Italy are described with emphasis on the carbon dioxide system. All springs contained high concentrations of CO2 (>20 mM l(-1)) with e quilibrium partial pressures well above those which could have been fo rmed in contact with a soil atmosphere. After surfacing, the CO2 is ra pidly lost to the atmosphere, with evasion rates close to the springs ranging from 0.45-4.41 mM m(-2) s(-1). Partial pressures of CO2 showed an exponential decline with distance, which is consistent with the st atic film model where temperature and turbulence are constant. Downstr eam CO2 transfer coefficients, which ranged from 66 to 360 cm h(-1) we re consistent with moderately turbulent flow, however, there was no co rrelation between turbulence, measured as the mean shear stress, and C O2 evasion rate. The channels investigated had all been modified by ma n and most possessed even widths and gradients. All waters became incr easingly supersaturated with aragonite and calcite downstream and both of these minerals were present in fresh travertine deposits. The supe rsaturation was driven almost exclusively by gas evasion. Comparison o f daytime and nighttime evasion rates demonstrated that photosynthetic activity was an insignificant source of CO2 flux in the reaches inves tigated. Carbon dioxide evasion is therefore primarily responsible for the supersaturation and probably also the deposition of travertine at these sites. The CaCO3 content of the travertines ranged from 91.3 to 96.0 wt% with 1.7-4.1% CaSO4, traces of organic matter and acid-insol uble minerals.