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

1 Introduction

The current rise in atmospheric CO2, and its link with the global climate change, provides a strong motivation to understand the natural processes that control the nature and magnitude of geological CO2 cycling (Evans, 2011). The release of carbon dioxide into the atmosphere via Earth degassing has played a crucial role in controlling global planetary temperature over geological time via the greenhouse effect (Foster et al., 2017). The modes and rates of geological CO2 release are thus crucial to understand the compositional evolution of the atmosphere through geological time, life on Earth, and climate changes (Berner and Lasaga, 1989; Kerrick, 2001; Dasgupta, 2013; Aiuppa et al., 2019; Fischer and Aiuppa, 2020; Guo et al., 2021). Despite continuous improvements via direct measurements, models, and global extrapolations, the CO2 Earth degassing output remains poorly constrained, hampering full understanding of the geological carbon cycle (Berner and Lagasa, 1989; Burton et al., 2013; Fischer, 2013; Fischer et al., 2019; Fischer and Aiuppa, 2020). The release of CO2 from the Earth’s interior into the atmosphere occurs in different tectonic settings (Lee et al., 2019), through volcanic and non-volcanic sources, and on a global scale, it is known that CO2 discharges are associated with tectonically/seismically active zones (Barnes et al., 1978; Chiodini et al., 2004; Tamburello et al., 2018). Quantitative estimates of CO2 outgassing fluxes in different tectonic settings are thus critical for decoding the link between the global carbon budget and climate evolution from a whole-Earth carbon cycling perspective (Zhang et al., 2021). In the last decades, the number of studies on CO2 degassing in non-volcanic areas has risen exponentially, emphasizing the important contribution of these areas to the Earth carbon budget (e.g., Chiodini et al., 2020, 2004; Minissale, 2004; Becker et al., 2008; Italiano et al., 2008; Groppo et al., 2022, 2017, 2013; Rolfo et al., 2015; Lee et al., 2016; Tamburello et al., 2018; Caracausi and Sulli, 2019; Frondini et al., 2019). The first regional-scale CO2 Earth degassing studies led to the catalogue of Italian CO2-rich gas emissions (googas.ov.ingv.it and www.magadb.net) and to the regional map of deeply derived CO2 degassing in central Italy that uses the quantification of carbon dissolved in regional groundwater systems (Chiodini et al.,. 2000; 2004; 2011). Some studies (Chiodini et al., 2004, 2020; Miller et al., 2004) also demonstrated a relation between CO2 degassing and seismogenesis in the Italian Apennines, pointing to the presence of gas triggering earthquakes. The Mt. Pollino region, at the southern end of the Apennines (southern Italy), has been historically recognized as one of the most hazardous seismic gaps in the intra-Appenine seismogenic belt (Napolitano et al., 2021), but it has recently been affected by seismic sequence occurred between 2010 and 2014 and characterized by about 10,000 earthquakes with highly variable magnitude (strongest events ML 4.3 and ML 5.0; De Matteis et al., 2021; Pastori et al., 2021). Moreover, recent studies identified fluid-related dynamics responsible for historical and recent seismicity of the area (Sketsiou et al., 2021). The Calabrian arc, further to the south, is one of the most active seismogenetic areas in Italy (Italiano et al., 2010; Neri et al., 2020), which has been repeatedly affected by catastrophic seismic events with 5.9 < M < 7.2 during the last centuries (18 times from 1626 to 1908; Gruppo di Lavoro CPTI, 2004; Boschi et al., 2000). The two areas are characterized by the presence of several springs, some representing low-enthalpy geothermal resources (Zarlenga, 2011; Vespasiano et al., 2014, 2015a, 2015b, 2015c, 2016, 2021; Apollaro et al., 2015, 2016, 2020). The geochemical and isotopic compositions of Calabrian and Pollino waters have previously been investigated to define their geochemical features and geothermal potential (Bencini and Ciracò, 1982; Duchi et al., 1991), to investigate a link with seismicity and implications for a fluid-fault relationship (Gurrieri et al., 1984; Calcara and Quattrocchi, 1993; Italiano et al., 2010; Apollaro et al., 2020) and to evaluate potential natural metal contamination of spring waters (Margiotta et al., 2012, 2014; Paternoster et al., 2021). However, no attempt has been made so far to model the water-gas interaction processes and to quantify the regional-scale budget of CO2 sequestrated/transported by aquifers at depths and released into the atmosphere upon spring discharge.

In this study, we present the results of a geochemical study of cold and thermal springs from both the Calabrian arc and the Pollino region. Our goals are to 1) investigate the relationships between Earth degassing and geological features in the two areas; 2) assess the presence and eventual origin of deep volatiles released in the hydrothermal basins and the surrounding areas; 3) model the processes at depths that can modify the pristine chemistry of deeply rising volatiles, potentially affecting the deep carbon budget; and 4) estimate the total deeply derived CO2 output. For this aim, we combine helium isotopes (3He/4He), total dissolved inorganic carbon (TDIC), and dissolved carbon isotopes (δ13CDIC) of groundwaters to explore the origin of carbon and to develop a model of water-gas-rock interaction. The results are then compared with the CO2 output from some active tectonic regions and volcanic areas worldwide.