ИСТИНА

During the TTR-18 cruise authigenic carbonates were collected from different locations of the Central Barents Sea. Two morphological groups of carbonates were found: (1) carbonate cemented shells, corals and tube worms – 10 samples and (2) carbonate rock clasts – 19 samples. The samples were collected from two diapiric structures, the Fedynskii High and from the North-Kildin gas field area. The main aim of this study was to determine the origin of the carbonates and environmental conditions during their formation. To achieve this, stable carbon and oxygen isotope techniques were applied as the main geochemical tool. Based on the obtained data carbonates were subdivided into four groups: (1) carbonate cemented macrofauna and organic remains; (2) carbonate tubes from the North-Kildin gas field; (3) carbonate rock clasts; (4) carbonate tube from top of the Northern diapir. The first group is characterized by δ13C values from 2.5 to 2.19 ‰ VPDB, δ18O values from 3.8 to 6.03 ‰ VPDB. These indicate that carbonate formation was happening in the equilibrium with the ambient seawater. Carbonate tubes of the second group are characterized by lighter stable carbon isotope values, up to -11.19 ‰ VPDB. This suggests that the process of anaerobic oxidation of methane (AOM) was at least in a part the source of 13C-depleted bicarbonate in the system. Carbonate rock clasts of the third group were formed in the regular marine environments, δ13C and δ18O vary from +0.55 to+2.86 ‰ VPDB and from -2.80 to -1.80 ‰ VPDB respectively. The fourth group consists of only a carbonate tube, which is significantly depleted in 13C. δ13C values in the central part of the tube are -37.17 ‰ VPDB getting heavier, to -6.61‰ VPDB in the periphery. Such light stable carbon isotope composition indicates that the carbonate is the product of AOM. The decrease in δ13C values in accompaniment with the increase of dolomite content indicates higher methane oxidation rates in the central part and possibly more intensive sulfate-reduction processes during the accretion of the peripheral part of the tube. According to the variations in the stable carbon and oxygen isotope compositions of morphologically different authigenic carbonates their origin and environmental conditions during their formation were tentatively identified.