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

摘要

Carbon isotope data of terrestrial organic matter (?′13CTOM) 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 ?′13CTOM events in the Middlea€“Upper Turonian of the Yezo Group. ?′13CTOM 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 ?′13CTOM events are correlated with previously documented ?′13Ccarbonate events in Europe (the Lulwortha€“Round Down, Glyndea€“Pewsey, and Late Turonian Events) based on global biostratigraphy. Our chemostratigraphic correlations strengthen the use of these ?′13C events for global correlation of the Turonian marine successions. In addition, global correlation of Turonian marine and terrestrial ?′13C events identifies changes in isotopic difference between ?′13CTOM and ?′13Ccarbonate (?”TOMa€“carbonate), which are interpreted to reflect changes in atmospheric pCO2 levels, and climate-driven stresses of humidity and soil processes. In earlier stages of Turonian, ?”TOMa€“carbonate values are increased. Elevated atmospheric pCO2, and increased humidity and soil processes in enhanced greenhouse conditions during mid-Turonian, are interpreted to enlarge ?”TOMa€“carbonate values. In later stages of Turonian, ?”TOMa€“carbonate values are at a constant level, and the lowering of atmospheric pCO2 or decrease of climate stress related to the diverse paleoclimatic cooling is interpreted to have restored the ocean-atmosphere ?′13C trends.