Early Cretaceous carbon isotopic and marine environmental changes in northern Oyubari area, Hokkaido

Stable carbon isotopic ratio composition (δ¹³C) of terrestrial plants, total organic carbon contents (TOC), and Bisnorhopane/Hopane (BNH/H) ratio were determined for the Cretaceous sequence from the Lower Aptian to the Upper Albian, in the northern Oyubari area, Hokkaido, Japan. Organic matter included in the mudstone is revealed to be mostly terrestrial based on microscopic study, C27-C28-C29 Sterane composition, carbon/nitrogen atomic ratio, and n-alkane distribution in gas chromatograms. The carbon isotopic excursion obtained by terrestrial plants records the global excursion of carbon isotopic ratio of atmosphere. We tried to correlate the measured carbon isotope excursions with those from the Ashibetsu area, Japan, the Piobbico area, Italy, and the Roter Sattel area, Switzerland, within a framework of ammonoid and planktonic foraminiferal biostratigraphy. The carbon isotopic excursions preserved in terrestrial plants in this study area are partly harmoniously correlated with those of carbonates in European Region. The reduced environments were recognized by TOC and BNH/H ratios in the levels between the Lower and Middle Albian. Such reduced environments near the seafloor could be related with the low frequency of occurrence of microfossils and an absence of mega fossils.