Carbon isotope stratigraphy of terrestrial organic matter for the Turonian (Upper Cretaceous) in northern Japan: Implications for ocean-atmosphere δ13C trends during the mid-Cretaceous climatic optimum

Carbon isotope data of terrestrial organic matter (delta C-13(TOM)) obtained in Hokkaido, northern Japan, from the marine Cretaceous Yezo Group along the northwestern Pacific margin elucidated a detailed chemostratigraphy for the Turonian Stage in this region of East Asia. Chemostratigraphic intra-basin correlation reveals three positive delta C-13(TOM) events in the Middle-Upper Turonian of the Yezo Group. delta C-13(TOM) fluctuations in these events show similar patterns in the Yezo Group, indicating that terrestrial organic matter is mixed sufficiently before deposition in the Yezo Basin. These delta C-13(TOM) events are correlated with previously documented delta C-13(carbonate) events in Europe (the Lulworth-Round Down, Glynde-Pewsey, and Late Turonian Events) based on global biostratigraphy. Our chemostratigraphic correlations strengthen the use of these delta C-13 events for global correlation of the Turonian marine successions. In addition, global correlation of Turonian marine and terrestrial delta C-13 events identifies changes in isotopic difference between delta C-13(TOM) and delta C-13(carbonate) (Delta(TOM-carbonate)), which are interpreted to reflect changes in atmospheric pCO(2) levels, and climate-driven stresses of humidity and soil processes. In earlier stages of Turonian, Delta(TOM-carbonate) values are increased. Elevated atmospheric pCO(2), and increased humidity and soil processes in enhanced greenhouse conditions during mid-Turonian, are interpreted to enlarge Delta(TOM-carbonate) values. In later stages of Turonian, Delta(TOM-carbonate) values are at a constant level, and the lowering of atmospheric pCO(2) or decrease of climate stress related to the diverse paleoclimatic cooling is interpreted to have restored the ocean-atmosphere delta C-13 trends.