  The periodical degassing from CO2 over-pressured reservoirs may have serious
consequences for the environment making urgent understanding the processes and
forecasting the frequency. Prediction though needs methods that depends from
temporal and spatial properties of hydro-chemical and physical reservoir
characteristics that unfortunately are often lacking. We have analyzed surface
emissions of CO2 attributed to over-pressured CO2-rich reservoirs in the
Central Italian Apennines a zone characterized by significant periodical CO2
degassing. Here aquifers are hosted in Mesozoic limestone with high pCO2
groundwater and travertine deposits. We analyzed a 10-year temporal series and
found that in the Apennines, CO2 flux and aquifer fluid composition are
correlated with the lunar tides specific to the geographic zone. In particular,
our study reveals that low CO2 flux corresponds with low lunar tidal potential
values. We found a similar trend for dissolved calcium and water alkalinity,
while pH values display a linear correlation with tidal cycles. The forces
associated with tidal potentials are not capable of fracturing rock. However,
they can, under certain conditions, drive the flow of fluids in over-pressured
reservoirs, triggering sub-surface fluid movements that in turn modify the
water-rock reactivity. In the central Apennines, these movements result in
increased dolomite dissolution and an eventual return to calcite equilibrium.
In this case, dolomite dissolution breaks the rock releasing calcium into
ground water, which leads to calcite equilibrium and in turn to the formation
of significant quantities of travertine and the concomitant release of CO2 in
the atmosphere.
