Active degassing of crustal CO2 in areas of tectonic collision: A case study from the Pollino and Calabria sectors (Southern Italy)

Carbon dioxide (CO2) is released from the Earth’s interior into the atmosphere
through both volcanic and non-volcanic sources in a variety of tectonic
settings. A quantitative understanding of CO2 outgassing fluxes in different
geological settings is thus critical for decoding the link between the global
carbon budget and different natural processes (e.g., volcanic eruption and
earthquake nucleation) and the effects on the climate evolution over
geological time. It has recently been proposed that CO2 degassing from
non-volcanic areas is a major component of the natural CO2 emission
budget, but available data are still sparse and incomplete. Here, we report
the results of a geochemical survey aimed at quantifying CO2 emissions
through cold and thermal springs of the tectonically active Pollino Massif
and Calabrian arc (Southern Italy). The chemical ad isotopic (He and C)
composition of fifty-five dissolved gas samples allows to identify two
different domains: 1) a shallow system dominated by gas components of
atmospheric signature (helium, hereafter He) and biogenic origin (C), and 2) a
deeper system in which crustal/deep fluids (CO2 and He) are dominant. The
measured He isotope ratios range from 0.03 to 1.1 Ra (where Ra is the He
isotopic ratio in the atmosphere) revealing a variable atmospheric
contamination. Furthermore, the He isotopic data indicate the presence of
traces of mantle He contributions (2%–3%) in the thermal groundwater. The
prevailing low R/Ra values reflect the addition of crustal radiogenic 4He during
groundwater circulation. Using helium and carbon isotope data, we explore
the possible sources of fluids and the secondary processes (dissolution/
precipitation) that act to modify the chemistry of pristine volatiles. For the
thermal springs, we estimate a deep C output of 2.3 x 107 to 6.1 x
108 mol year−1. These values correspond to deep CO2 fluxes per square km
comparable with those estimated in several active and inactive volcanic areas
and in continental regions affected by metamorphic CO2 degassing (e.g., the
southern margin of the Tibetan Plateau).