Chemical characterisation of the gases released at Gyali Island, Dodecanese, Greece and preliminary estimation of the CO2 output

Abstract

Greece belongs to the most geodynamically active regions of the world and as such, it has to be considered an area of intense geogenic degassing. In terms of carbon, the territory is characterized by the high hydrothermal and volcanic activity of the South Aegean Active Volcanic Arc (SAAVA), and by widespread geological seeps of buried carbon dioxide and methane. In the present work, we investigate the island of Gyali located in the volcanic system Kos-Gyali-Nisyros. Nine gas samples have been collected on the island of Gyali in areas found both on land, in a small lake (~2000 m2) along its beach, and in the sea close to the shore at shallow depths (<20 m). Results show that CO2 is the prevailing gas species with concentrations above 833,000 μmol/mol, whilst minor enrichments in N2 are noticed in some of the samples. Helium isotope ratio suggests an up to 70% mantle origin (considering a MOR type source) with the contribution of the atmospheric component being negligible, whereas C presents a mixed mantle-limestone origin with organic sediment being unimportant. Such isotope values fall in the range that characterizes the Kos-Gyali-Nisyros Volcanic Field. First estimations on the CO2 release suggest an output in the range from 18 to 28 t/d for Gyali Island, which is much lower than the total output of Kos ~75 t/d and Nisyros ~100 t/d. However, such results highlight that Gyali is an active system and, despite its small size, its degassing is not trivial.