Preliminary results of studying the carbon isotope composition of conodont elements at the border of Devonian and Carboniferous periods (Kamenka river sections, Pechora carbonate platform)

Abstract

Research subject. Changes in the trophic structure of shallow-water pelagic ecosystems at the Devonian/Carboniferous border were investigated by studying the carbon isotope composition of conodont organic matter.Materials and methods. Two Devonian-Carboniferous shallow-water clayey-carbonate sections located in the southern part of the Pechora-Kozhva Uplift (Pechora Plate) were analysed. The Devonian-Carboniferous boundary was detected by the first occurrences of Siphonodella sulcata, S. semichatovae and Patrognathus crassus, as well as by the last occurrence of Pseudopolygnathus graulichi. The carbon isotope composition was investigated both in whole-rock carbonate samples and the conodont organic matter of two dominant species (Polygnathus parapetus and P. communis communis).Results. The distribution of stable carbon isotopes in the organic matter of conodont elements accompanied by the data on carbonate isotope composition allowed us to suggest changes in the food composition of the dominant taxa during the Late Famennian-Early Tournaisian transition. It was assumed that the latest Famennian representatives of Polygnathus parapetus and P. communis communis consumed largely phyto- and zooplankton, which is characterized by a light isotopic composition of organic carbon. The nutrition based on phyto- and zooplankton with a heavier isotopic composition of organic carbon was suggested for the early Tournaisian representatives of these species.Conclusions. The discovered variations in the carbon isotope composition of conodont organic matter in shallow-water facies may correspond to the change from the eutrophic pelagic ecosystem to the oligotrophic ecosystem, and/or global perturbation of the carbon cycle due to climatic changes. Since the available data is limited to two geological sections, it is impossible to unambiguously interpret the scale (local, regional, global) of these variations and their correlation potential.