Carbon and Oxygen Isotope Compositions of Lower-Middle Ordovician Carbonate Rocks in the Northwestern Russian Platform

摘要

Carbon and oxygen isotope compositions of Lower-Middle Ordovician carbonate rocks in the northwestern Russian Platform (eastern Ladoga Klint, Lynna River, and Babino quarry sections) are considered. In the studied section interval, average δ~(13)C and δ~(18)O values are 0 ± 0.5 and -5 + 0.5‰ (V-PDB), respectively. Two closely-spaced negative carbon isotope excursions with the amplitude of 2‰ are established near the Lower-Middle Ordovician boundary (between the Floian and Dapingian stages). The lower part of the Darriwilian Stage is marked by the gradual decrease in δ~(13)C values to 1 ‰. Excursions of δ~(13)C do not correlate with δ~(18)O variations and can be considered as primary. The carbon isotope event defined at the Lower-Middle Ordovician boundary is traceable at the interregional level and represents a promising stratigraphic reference level. It may likely be explained by decrease in the relative rate of organic matter burial due to sea level fall and expansion of well-aerated shallow-water basins with a low primary production of phytoplankton.