Carbon and oxygen stable isotopes in the middle-upper miocene and lower pliocene carbonates of the Eastern Paratethys (Kerch-Taman Region): Palaeoenvironments and post-sedimentation changes

Abstract

C and O isotope composition of Middle-Upper Miocene and Lower Pliocene carbonates from Kerch-Taman Region (Eastern Paratethys) have been studied in order to reconstruct palaeoenvironmental variability and post-sedimentation changes. The δ¹³C and δ¹⁸О values of the Upper Sarmatian to Lower Pliocene organogenic carbonates reflect the desalinization of paleobasins, global Late Miocene Cooling, and increase in seasonal temperature fluctuations. Isotopic composition of the Middle Sarmatian organogenic carbonates was strongly influenced by evaporation processes, high bioproductivity, and local submarine methane emissions. Warm climate and low bioproductivity together with unstable hydrological regime during the Late Chokrakian and the Karaganian times influenced the isotope composition of primary carbonates. Calcite shell of Spiratella sp. (δ¹³C =–0.4‰ and δ¹⁸О =–0.4‰) from Tarkhanian sediments was formed in warm marine environment. Dolomitization prevails over other secondary mineralization in the studied carbonate rocks. Two groups of secondary dolomites that are characterized by negative and positive δ¹³C values have been recognized. Lowe δ¹³C values (up to–31.4‰) in dolomites indicate the influence of both dissolved inorganic carbon (DIC) from oxidized organic matter (С_org) and methane. Dolomites with positive δ¹³C values (7.0 and 7.8‰) associat with migration of CO₂- and CH₄-containing saline groundwater.