CO2 release variations during the last 2,000 years at the Colli Albani volcano (Roma, Italy) from speleothems studies

The Colli Albani is the quiescent volcano that dominates the southwestern skyline of Roma (Italy). The last eruption occurred during the Holocene, from the eccentric Albano maar, along its western slope. The volcano is presently affected by cyclic seismic swarms, ground uplift and diffuse CO2 degassing. The degassing has caused several deadly incidents during the last years and constitutes a major civil protection concern, as the volcano slopes are densely inhabited. Nevertheless, the volcano does not have a permanent monitoring network, and the background level and anomalous CO2 levels, the relationship between the gas release and the seismic and ground deformation activity at the Colli Albani are still to be defined. The aim of this work is to define the historical record of CO2 release. Evidences of deep CO2 periodic release during the last 2000 years in the area of Colli Albani volcano (Roma, Italy) are offered from speleothems studies. A Roman-age stone mine, now used for mushroom cultivation, is decorated with actively growing speleothems, characterised by depositional hiatuses. Different levels of four stalactites, separated by depositional unconformities, and several samples from a single depositional cycle belonging to a stalagmite have been dated by U/Th method and analysed for their O and C isotopic composition. Eight cycles of deposition have been identified from 90–110 A. D. to 1350–1370 A.D., some of which are recognised across different speleothems. The age gap dividing different growth layers is in the order of one to few hundred years giving a temporal span for periodic interruption of speleothems deposition. O and C isotopic analyses performed on the samples collected from a single cycle (the oldest) have shown that the composition of the mother solutions was initially mainly meteoric and that a progressive increase in the input of a deep component rich in CO2 (up to a proportion of 20–30%) occurred just before the interruption of the speleothem deposition. This could be due to a progressive increase of the acidity of the water solutions that caused the undersaturation of fluids. If we extrapolate this mechanism to the other cycles of deposition, being characterised by analogue isotopic compositions, we can hypothesise periods of deposition interrupted by episodes of CO2 release which in the Colli Albani volcano are often recorded in coincidence with earthquakes. Therefore we have correlated the hiatuses with some of the largest historical earthquakes interesting to the city of Rome.