New data on isotope composition of carbon and oxygen of Jurassic ammonite shells (Kostroma Volga region)

We have studied conditions influencing the results of the isotopic composition analysis of oxygen and carbon of carbonate skeletons of fossil ammonites, taking into account that they had been open for post-sedimentation processes. The object of study was shells of Ringsteadia, Eurasenia, Perisphinctes, Dichotomoceras, Cardioceras ammonites from outcrops on the right bank of the Unzhi River, near the city of Makaryev of Kostroma Region. The most probable reason for the difference in d13Сcarb and d18Ocarb values of ammonite shells from coeval belemnites, except for mineral affinity (calcite, aragonite), was the presence of «pure» authigenic calcite shells. The indicator of the isotopic composition of carbonates was determined not only by the nature and origin of source substances, but also by the fractionation of the isotopes in various processes during their formation. We supposed that the multistage sulfate reduction could have changed isotopic characteristics of the carbonate of the shells.