Romanian mud volcanoes – main features and methane flux to the atmosphere

Studies performed in the last decade have shown the importance of geological sources in releasing methane, an important greenhouse gas, following only to carbon dioxide in the ranking of global warming producing gases. The IPCC Fourth Assessment Report, released in 2007, for the first time considers the geologic source of methane beside the other natural sources taken into account in the previous reports.
   Mud volcanoes are important methane releasing geological features, occurring onshore and offshore in many parts of the world. Most of them are located in compressional settings, although in some cases they may be found in other tectonic environments. Most commonly, the onshore mud volcanoes are cone-shaped, with variable dimensions, from a few meters in diameter and less than one meter in height, to several kilometres in diameter and hundreds of meters in height. The shape of the mud volcanoes depends on the nature of the expelled fluids. Convex shapes are formed when the mud is viscous, but very frequently, circular pools with muddy water occur when the mineral fraction/water ratio is very low. In Europe, mud volcanoes are distributed in some specific areas. Such features were identified in Italy, Romania, and their occurrence continues eastward on the northern shore of the Black Sea (Ukraine, Russia), and in the Caucasian – Caspian area, where the world’s most impressive mud volcanoes were described.
   The most important Romanian mud volcanoes are located in Berca area (Carpathian Foredeep), close to the bending zone of the Carpathian chain. These mud volcanoes are distributed in four distinct areas: Paclele Mari, Paclele Mici, Fierbatori, and Beciu, and seem to be the biggest in Europe, excepting the giant structures in Azerbaijan. In Transylvania, quite numerous methane releasing structures were identified. The mud volcanoes here are generally small, not exceeding a few meters in height and tens of meters in diameter. In some spots, dry gas emissions occur.
   In the past years, the methane flux was measured by using the classical closed chamber method. After the chamber deployment, gas samples were collected by syringes and analysed in the laboratory by gas chromatography. Recently, an innovative measuring method was introduced by using a portable methane and carbon dioxide fluxmeter. Specific sensors for the two gases are connected to the accumulation chamber, and after deploying the device in the field, the gas concentrations are measured and fluxes derived. This new method has been used until now in Transylvania and a total flux of about 680 t CH₄^y-1 was estimated for theinvestigated areas in the Miocene basin of Transylvania. The total methane flux in Berca area is exceeding 1000 t CH₄^y-1.
   Acknowledgments. The work described in this report was financially supported by the Romanian National Authority for Research (ANCS) within the Project 31-094 FLUX.