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Chimaera, a Large Abiotic Gas Seep in Turkey

Abstract

Abiotic gaseous hydrocarbons are generated by chemical reactions, without organic matter being involved, under a wide range of temperatures (Etiope and Sherwood Lollar, 2013): trace amounts occur in volcanic and geothermal systems, but larger amounts were recently discovered in an increasing number of sites, in low temperature continental ultramafic rocks (e.g., Etiope et al., 2013). An outstanding occurrence of dominantly abiotic methane is that of the Chimaera seep, in Turkey, which is probably the biggest onshore abiotic gas seep on Earth. Chimaera (from the mythological fire-breathing creature told by Homer in The Iliad) is an archeological site near Çirali, in the Antalya Gulf, famous for the natural flames issuing from the ground. The ground is formed by serpentinized peridotite rocks (belonging to the Tekirova ophiolite) cut by a large fault; the gas, composed by about 87% of methane and 10% of hydrogen, escapes from this fault producing at least 20 large flames, up to a half meter in height (Hosgormez et al., 2008). Methane is dominantly abiotic (δ13C: −12 ‰; δ2H: −129 ‰) and it escapes from both focused (visible) vents in rock fractures and pervasively as an invisible seepage throughout the area; at least 190 tonnes of CH4 are released every year into the atmosphere (Etiope et al., 2011). In a survey conducted in 2012, we found a second site, about 300 m far from Chimaera, with two actively burning gas vents and numerous burned trees over an area of at least 2000 m2. The trees were likely killed by episodic burning of gas issuing from the ground, as suggested by clear signs in the surrounding burned soil. The total emission of methane into the atmosphere could be of several hundred tonnes per year. Radiocarbon (14C) analyses of CH4 have demonstrated that gas is fossil, i.e., older than 50,000 years. This fact and the enormous continuous flux of gas imply that abiotic methane must come from a pressurized reservoir at depth. Considering that the “eternal flames” are active since at least 2 millennia (they were documented by Pliny the Elder in Naturalis Historia, <79 AD) simple calculations suggest that the total amount of methane emitted so far would be in the order of 400 millions of cubic meters. The initial amount of methane stored in the reservoir (the ultimate reserve) could have been in the order of thousands of million of cubic meters, similar to a conventional biotic gas field. No studies have been made to assess the present-day reserve.