Degassing at the Volcanic/Geothermal System of Kos (Greece): Geochemical Characterization of the Released Gases and CO2 Output Estimation

Abstract

Forty five gas samples have been collected from natural gas manifestations at the island of Kos, the majority of which is found underwater along the southern coast of the island. On land, two anomalous degassing areas have been recognized. These areas are mainly characterised by lack of vegetation and after long dry periods by the presence of sulfate salt efflorescences. Carbon dioxide is the prevailing gas species (ranging from 88 to 99 %), whilst minor amounts of N2 (up to 7.5 %) and CH4 (up to 2.1 %) are also present. Significant contents of H2 (up to 0.2 %) and H2S (up to 0.3 %) are found in the on-land manifestations. Only one of the underwater manifestations is generally rich in N2 (up to 98.9 %) with CH4 concentrations up to 11.7 % and occasionally extremely low CO2 amounts (down to 0.09 %). Isotope composition of He ranges from 0.85 to 6.71 R/RA, indicating a sometimes strong mantle contribution; the highest values measured are found in the two highly degassing areas of Paradise Beach and Volcania. C-isotope composition of CO2 ranges from -20.1 to 0.64 ‰ vs V-PDB, with the majority of the values being concentrated around -1 ‰ and therefore proposing a mixed mantle – limestones origin. Isotope composition of CH4 ranges from -21.5 to +2.8 ‰ vs V-PDB for C and from -143 to +36 ‰ vs V-SMOW for H, pointing to a geothermal origin with sometimes evident secondary oxidation processes. The dataset presented in this work consists of sites that were repeatedly sampled in the last few years, with some of which being also sampled just before and immediately after the magnitude 6.6 earthquake that occurred on the 20th of July 2017 about 15 km ENE of the island of Kos. Changes in the degassing areas along with significant variations in the geochemical parameters of the released gases were observed both before and after the seismic event, however no coherent model explaining those changes was obtained. CO2-flux measurements showed values up to about 104 g×m-2×d-1 in the areas of Volcania and Kokkinonero, 5×104 g×m-2×d-1 at Paradise beach and 8×105 g×m-2×d-1 at Therma spring. CO2 output estimations gave values of 24.6, 16.8, 12.7 and 20.6 t×d-1 respectively for the above four areas. The total output of the island is 74.7 t×d-1 and is comparable to the other active volcanic/geothermal systems of Greece (Nisyros, Nea Kameni, Milos, Methana and Sousaki).