Biological markers and stable carbon isotope studies of bitumen impregnating Quaternary sediments at Starunia palaeontological site and vicinity (Carpathian region, Ukraine)

Abstract

Biological markers and stable carbon isotopic compositions of bitumen impregnating the Pleistocene and Holocene sediments around the Starunia palaeontological site (Carpathian region, Ukraine) near discovered remnants of a mammoth and three woolly rhinoceroses, and one almost completely preserved rhinoceros carcass found in 1907 and 1929, were investigated. The research was carried out to assess genetic connections between the bitumen and oils from the nearby deep accumulations of the Boryslav-Pokuttya and Skyba units of the Carpathians. Another aim of these geochemical studies was to reveal the secondary geochemical processes (oxidation and biodegradation) influencing the bitumen, and to determine which environments have been favourable or unfavourable for the preservation of large Pleistocene vertebrates. Bitumen included within the near-surface rocks are not genetically connected with residual organic matter present in Quaternary sediments. Geochemical features, such as distributions of pentacyclic triterpanes and steranes, biomarker parameters and isotope composition indicate that all bitumens originate from oil-prone, Type II kerogen with insignificant admixture of terrestrial Type III kerogen in the middle stage of catagenesis. The oils occurring in deep accumulations in the Starunia area were the sole source of bitumen found in the near-surface sediments. Input of immature organic matter to bitumen from Pleistocene and Holocene sediments has not been found. The main factors differentiating the bitumen were: biodegradation, water washing and/or weathering. Additional influence of transport of bitumen by brine was found. The most favourable conditions for preservation of large, extinct mammals within the Pleistocene muds exist in the vicinity of Nos 22 and 23 boreholes, where bitumen is best preserved. The worst conditions were found in the vicinity of Nos 1, 4, 4' and 15 boreholes. Intensification of biodegradation and weathering effects were followed by intensive, chaotic changes of the remnants of large Pleistocene mammals.