Famennian negative carbon isotope excursion in the Izyayu River section (Tchernyshev Uplift, Cis-Uralian Foredeep)

   Research subject. The article is focused on manifestations of the Early-Middle Famennian negative excursion in carbon isotope composition in the Izyayu River section (south of the Tchernyshev Uplift).   Aim. The study is aimed to reconstructing of the probable causes of this isotope excursion.   Materials and methods. The article is based on the results of study of the deep-shelf lower-middle Famennian sequence of the Izyayu River section. The carbon isotopic composition was studied for the carbonate samples and conodont elements (32 and 9 samples respectively).   Results. The lightening of the carbon isotopic composition of carbonates by 2.5 %, and of the carbon isotopic composition of conodont elements by 4 % were found in the Pal. gracilis gracilis–Pal. marginifera marginifera zonal interval. The difference between the carbon isotopic compositions of carbonates and conodont elements increases in this interval by more than 2.5 %. Possible reasons for the observed variations could be the intensification of terrigenous runoff due to regression, a relatively high content of carbon dioxide in the atmosphere, and, possibly, a local decline in primary bioproductivity.   Conclusions. The negative carbon isotope shift spans Pal. gracilis gracilis–Pal. marginifera marginifera zonal interval in the Izyayu River section. The shift corresponds to the global decreasing in the δ13Ccarb values in the early/middle Famennian boundary interval. The increase in supply of the isotopically light carbon with terrigenous runoff and decrease in the primary bioproductivity are considered as probable causes of the carbon isotope excursion.