Abstract
A positive carbon isotopic excursion across the Cenomanian/Turonian boundary in the Kotanbetsu area, Hokkaido, Japan provides accurate positioning of the boundary. A microscopic study based on organic petrology reveals that the organic matter included in mudstones of the Kotanbetsu River section is exclusively terrestrial. The results of stratigraphic time-series analysis of stable carbon isotopes from these mudstone samples can be translated as representing an average of a terrestrial plant community signal. The isotopic fluctuation through this time interval records information on the global ocean-atmosphere system. Two internationally recognized events characterize the uppermost Cenomanian through middle Turonian. On the basis of this study the Cenomanian/Turonian boundary can be recognized within a stratigraphic range of ∼ 14 meters. This horizon of the boundary is concordant with that from biostratigraphy (ammonoids, inoceramids and planktonic foraminifers). Above the middle Turonian strata, the isotopic pattern supports the biochronology of planktonic foraminifers rather than that of inoceramids.
