The CO2 output from the Sperchios Basin area (central Greece): the role of hidden degassing from streams

The Sperchios Basin is an actively spreading rift area with deeply rooted extensional faults and also a site of quaternary volcanic activity. Such geologic conditions favoured the formation of many hydrothermal systems, whose surface expressions are among the biggest thermal springs of the whole Greece. Degassing of deeply derived geogenic CO2 is highlighted by strong bubbling within the main pools of the springs. Flux measurements were made in the Thermopyles spring with the floating chamber method and results showed that bubbling gases in the spring release about 1 ton/day of CO2. The outgoing stream has a flow of more than 250 l/s of water rich in CO2 (about 16 mmol/l). Although no bubbl…

Origin of methane and light hydrocarbons in the gas manifestations of Greece.

The geologic emissions of greenhouse gases (CO2 and CH4) give an important natural contribution to the global carbon budget. However, the contribution of these emissions to the global carbon cycle and their possible role on the climate change remain still poorly quantified (Guliyev and Feizullayev, 1997; Milkov, 2000; Etiope et al., 2015 and references therein). Methane, the most abundant organic compound in Earth's atmosphere, may be created either from existing organic matter or synthesized from inorganic molecules. Accordingly, it can be differentiated in two main classes: a) biotic (either microbial or thermogenic) and b) abiotic. For this study, 115 gas samples of fumarolic, thermal an…

Potentially harmfull elements accumulation in fumarolic alteration products at three hydrothermal systems of Greece

Samples of efflorescences and encrustations of hydrothermal origin have been collected at three fumarolic areas in Greece. The three sites are Sousaki, Thiafi (Methana) and Kokkino Nero (Kos) and all belong to the South Aegean Active Volcanic Arc. Samples were analysed for their mineralogical (XRD and SEM-EDS) and chemical composition. Solutions obtained from both mineralization with HNO3 and leaching with distilled water, were analysed for major (ICP-OES), minor and trace metals (ICP-MS) and for sulfate contents (IC). Results show that their composition is mainly controlled by the petrological composition of the substrate (ultramafic rocks at Sousaki, felsic volcanic rocks at Methana and l…

Gas geochemistry and preliminary CO2 output estimation from the island of Kos (Greece)

Several gas samples have been collected from natural gas manifestations at the island of Kos. Most of them are found underwater along the southern coast of the island. On land two anomalous degassing areas have been recognized. These are characterised by lack of vegetation and after long dry periods by the presence of sulfate salts efflorescences. Almost all the gases are CO2-dominated (CO2 ranging from 88 to 99%) with minor amounts of N2 (up to 7%) and CH4 (up to 2.6%). Only the on-land manifestations have also significant contents of H2 (up to 0.2%) and H2S (up to 0.3%). Only one underwater manifestation is N2-dominated (61-99%) with CH4 (0.6-11%) and low CO2 (0.1-26%). The isotopic compo…

Investigating diffuse CO2 degassing in tectonically active areas by groundwater

Gas geochemistry and CO2 output estimation of Milos Island (Greece)

Several gas samples have been collected from natural gas manifestations in the island of Milos. Most of them are located underwater along its coasts, whereas three anomalous degassing fumarolic areas (Kalamos, Paleochori and Adamas) were identified on land. Almost all the gases have CO2 as the prevailing gas species, with concentrations ranging from 88 to 99% for the samples taken underwater, while the on-land manifestations present a wider range (15-98%), being sometimes heavily contaminated by air. Methane reaches up to 1.0%, H2 up to 3.2% and H2S up to 3.5% indicating a hydrothermal origin of the gases. The isotope composition of He shows values ranging from 2.55 to 3.39 R/RA, highlighti…

Origin of the geogenic gases and preliminary estimation of the carbon release of Greece

Volatiles are transported from the deep crust or mantle to the surface in geodynamically active areas where seismic, volcanic and geothermal activity is present; the circulation of hydrothermal fluids in the crust is enhanced. In such areas, faults may act as preferential pathways for advective gas-carrying fluid transport. Towards the surface, pressure decrease allows the gases to escape from the fluids into soil gas and eventually into the atmosphere (King, 1986). The migration of carbon-bearing crustal and mantle fluids contributes to Earth’s carbon cycle (Berner & Kothavala 2001). However, till now, the mechanisms, magnitudes and time variations of carbon transfer from depth to the …

Geochemical characterization of groundwater quality in Hellenic karst systems

Karst aquifers are considered to be one of the most important aquifer types, as they constitute the main drinking water resource for the majority of the global population (Ford et al., 2007). They are generated from the dissolution of carbonate rocks (e.g. limestone, dolomite, marble etc.), a phenomenon commonly known as “karstification”. This process is mainly caused by the acidity of water enriched in dissolved CO2, with the concentration of the latter being dependent on both the temperature and the CO2 partial pressure of the atmosphere in contact with the water (Bakalowicz, 2005). Carbonate rocks cover about 35% of the land surface of Greece and are mainly located in the western, centra…

Geochemical characterization of water quality in karst systems of Greece

Karst systems represent the main drinking water source for 20-25% of world’s population, although covering only 7-10% of the global land surface (Ford & Williams, 2007). Karst aquifers are highly vulnerable to external pollution, so their protection and management is of critical importance to sustain water resources. In Greece, starting from the 1970’s, water demands for agricultural, domestic and industrial use increased significantly, mainly in coastal areas (Daskalaki & Voudouris, 2008). The main Hellenic aquifers are hosted in alluvial deposits, in Neogene deposits and in carbonate rocks. The latter cover about 35% of the country and are located in Western, Central and Southern …

Gas hazard related to CO2 degassing at Loutra Ypatis, Greece

Earthquakes and volcanic eruptions represent a hazard. However, the impact of gases released in geodynamically active areas should not be underestimated. It is commonly known that geogenic sources release great amounts of gases, which, apart from having an important influence on the global climate, can also have a strong impact on human health causing both acute and chronic effects. In particular, CO2 and sulphur gases (mainly H2S and SO2) are the main compounds responsible for acute mortality due to their asphyxiating and/or toxic properties. One of the most known and also worst episodes occurred, took place on the 21th of August 1986 at Lake Nyos, Cameroon, when about 1700 people were kil…

Gas geochemistry of shallow submarine vents in the Aegean sea (Greece)

The Aegean area, which is geodynamically very active, is characterised by intense seismic activity, presence of active volcanic systems and anomalous geothermal gradients. Like other regions of intense geodynamic activity it is also characterized by extensive geogenic degassing. Gas manifestations are not only widespread on land but are also very frequent underwater. Many of these, as for example those of Milos and Santorini, are known since long time and have been previously studied although the targets were generally the hot waters or the sediments affected by the emissions (Smith and Cronan 1978, Dando et al. 1995, Price et al. 2013, Megalovassilis 2014).The present study aims at produci…

Microbial methane oxidation leading to extreme isotopic fractionation in thermal springs of central Greece

The Greek territory belongs to the geodynamically active Alpine-Himalayan orogenic belt. As such, it shows intense seismic activity, active volcanic systems and areas of enhanced geothermal fluxes. One of these areas is the Sperchios basin and the northern part of Evia island in central Greece, which present widespread thermal manifestations (D’Alessandro et al., 2014). Five of them with temperatures from 33 to 80°C present bubbling gases whose dominating species are either CO2 or N2. All gases contain from 27 to 4000 ppm of CH4. The isotopic composition of CH4 in these gases covers a wide range with δ13C values ranging from -21.7 to +16.9 ‰ and δ2H values ranging from -124 to +301 ‰. The h…

Mineralogy of the soils altered by fumarolic activity at Nisyros volcano, Greece

Nisyros is the easternmost island of the South Aegean Active Volcanic arc in Greece. It is an active stratovolcano known for its intense hydrothermal activity. In this study we present the results of the mineralogical analyses of 20 soil samples collected in the caldera area. Samples were analysed through X-ray diffraction and the results allow us to divide them in two groups: Lakki Plain and Stefanos Crater. In their majority the soils of Lakki Plain have a main mineralogical assemblage that consists of quartz, feldspar and gismondine (Ca2Al4Si4O16·9H2O). Gismondine as well as stellerite (CaAl2Si7O18·7H2O), which is appearing in some samples, derive from hydrothermal alteration, whilst wol…

Origin of He and CO2 in the gas manifestations of Greece.

In the period from 2004 to 2017, more than 350 samples of free and dissolved gases were collected along the whole Hellenic area. Some literature data have also been taken into consideration (Rizzo et al., 2016). Samples have been analysed for their chemical and isotope composition. The concentrations range from 0.10 to 3370 μmol/mol for He, 600 to 995,000 μmol/mol for N2, 0.60 to 915,000 μmol/mol for CH4 and 17 to 1,002,000 μmol/mol for CO2, whereas the isotope values range from 0.01 to 7.10 for R/RA and -29.91 to +6.00 for δ13C-CO2. Considering the R/RA and 4He/20Ne ratios the atmospheric, mantle and crustal contributions for He have been calculated (Sano and Wakita, 1985). The highest man…

Light hydrocarbons as a proxy to identify the origin of the gas manifestations in Greece

The geologic emissions of greenhouse gases (CO2 and CH4) have an important natural contribution in the global carbon budget. Tectonics, through faults in geothermal and oil producing areas, play a significant role in the release of C-gases in many non-volcanic regions of the Earth. Methane, the most abundant organic compound in Earth’s atmosphere, has a potential global warming that is 28 times higher than that of CO2 on a 100-year time horizon. In this study, δ13C-CH4, δ2H-CH4 and light hydrocarbon (alkane: CH4, C2H6, C3H8, C3H6, i-C4H10, n-C4H10; alkene C3H6, iC4H8; and aromatic C6H6) gas concentration data of 119 gas samples (103 unpublished data and 16 literature data) from volcanic-hyd…

Etna International Training School of Geochemistry, 2018. Science meets practice.

Mount Etna, located in eastern Sicily, is the largest stratovolcano in Europe and one of the most intensely degassing volcanoes of the world (Allard et al., 1991; Gerlach, 1991). In particular, previous estimates highlighted that Mt Etna emits about 1.6 % of global H2O fluxes from arc volcanism (Aiuppa et al., 2008) and 10 % of global average volcanic emission of CO2 and SO2 (D’Alessandro et al., 1997; Caltabiano et al., 2004). Furthermore, Gauthier and Le Cloarec (1998) underscored that Mt. Etna is an important source of volcanic particles, having a mass flux of particle passively released from the volcano during non-eruptive period estimated between 7 to 23 tons/day (Martin et al., 2008; …