Carbon isotope study of conodont elements: Applications and limitations

Abstract

Conodont elements composed of protein-apatite nanocomposites are considered a promising geochemical archive due to their high resistivity to weathering and diagenetic processes. This article introduces some possible applications and limitation of the carbon isotope investigations in conodont research. It is based on the author's collections of conodonts comprising stratigraphic interval from the Upper Devonian to the Mississippian. Total number of the studied conodont elements is 282. The data obtained suggest that isotopic compositions of carbon of conodont elements probably reflect isotopic composition of the conodont diet with some insignificant taphonomical and histological biases. The variations in δ13Ccon were probably controlled by changes in the C-isotope composition of the phytoplankton as the main part of conodont food, which, in turn, was affected by global and local changes in the pelagic ecosystem. Decrease in δ13Ccon may be provoked by shortening of the food chains, enhancing of C-isotope fractioning by phytoplankton, and a decrease in δ13C of atmospheric CO2. Significant changes in δ13Ccon mark probable perturbations in the pelagic ecosystems. Decoupling carbon isotope composition of carbonates and conodont elements is a proxy of the water temperature, CO2 concentration in atmosphere and hydrosphere, and phytoplankton growth rate. It is likely that the CO2 concentration was a leading factor affecting decoupling carbon isotope composition of carbonates and conodont elements in the late Famennian – Tournaisian interval. However, the CO2 signal can be obscured by variations in primary bioproductivity. Fluctuations in decoupling carbon isotope composition of carbonates and conodont elements reflect global and local changes in pelagic marine ecosystems, and prominent excursions can be used as stratigraphic markers.