CARBON ISOTOPE STRATIGRAPHY OF TOURNAISIAN (LOWER MISSISSIPPIAN) OF NE LAURUSSIA REVEALED FROM ORGANIC MATTER OF CONODONT ELEMENTS

The ability to correlate marine deposits using carbon isotope ratios in the organic matter of conodont elements (δ¹³C_{org_con}) relies on the assumption that these ratios had varied regionally (or, probably globally) over time due to climate-driven changes in pCO₂, and probably due to perturbations in trophic structure of the marine ecosystems. Conodont elements preserve the organic matter better than a host rock, and provide the more definite isotope signal.

The study region comprises eastern part of the Pechora Plate and northern Cis-Uralian, locating in the north-east of Europe. Paleogeographically this region occupies NE shelves of Laurussia. Three measured sections (Kozhva-Kamenka, Izyayu, and Kuz-Yama) represent various facies belts in a range from the carbonate platform to deep intra-shelf depression. The Tournaisian (Lower Mississippian) is represented by clay and limestone alternation in the platform facies and by calciturbidites in the deep shelf facies. Conodont elements obtained are of good preservation and have CAI of 1-3, that promises preservation of the primary isotope signal.

Cleaned conodont elements, belonging to genera Neopolygnathus, Polygnathus, Hindeodus, Ligonodina, and Siphonodella, were used for analyses of carbon isotope composition of organic matter. Average Middle-Late Paleozoic value of δ¹³C_{org_con} is about -26‰. In the stratigraphic interval under investigation, four prominent negative shifts in δ¹³C_{org_con} were distinguished. All studied taxa demonstrate conform δ¹³C_{org_con} variations but of different amplitude. The first shift corresponds to the terminal Famennian (down to -27.5 ... -32‰), the second shift coincides with the upper part of sulcata Zone (-27 ... -30‰), the third minor shift is correlated with the middle part of the Lower crenulata Zone (-27 ... -28‰), and the fourth shift corresponds to the middle part of the Upper crenulata-isosticha Zone (-29.5‰). Similar δ¹³C_{org_con} pattern in all the study sections promises that these variations may be used as an isotope-stratigraphy tool, at least in regional scale.