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 organicnmatter (δ13CTOM) obtained in Hokkaido, northernnJapan, from the marine Cretaceous YezonGroup along the northwestern Pacifi c marginnelucidated a detailed chemostratigraphy fornthe Turonian Stage in this region of East Asia.nChemostratigraphic intra-basin correlationnreveals three positive δ13CTOM events in thenMiddle–Upper Turonian of the Yezo Group.nδ13CTOM fl uctuations in these events shownsimilar patterns in the Yezo Group, indicatingnthat terrestrial organic matter is mixed suffinciently before deposition in the Yezo Basin.nThese δ13CTOM events are correlated withnpreviously documented δ13Ccarbonate eventsnin Europe (the Lulworth–Round Down,nGlynde–Pewsey, and Late Turonian Events)nbased on global biostratigraphy. Our chemostratingraphic correlations strengthen the usenof these δ13C events for global correlation ofnthe Turonian marine successions. In addition,nglobal correlation of Turonian marinenand terrestrial δ13C events identifi es changesnin isotopic difference between δ13CTOM andnδ13Ccarbonate (ΔTOM–carbonate ), which are interpretednto refl ect changes in atmospheric pCO2nlevels, and climate-driven stresses of humiditynand soil processes. In earlier stages ofnTuronian, ΔTOM–carbonate values are increased.nElevated atmospheric pCO2, and increasednhumidity and soil processes in enhancedngreenhouse conditions during mid-Turonian,nare interpreted to enlarge ΔTOM–carbonate values.nIn later stages of Turonian, ΔTOM–carbonate valuesnare at a constant level, and the loweringnof atmospheric pCO2 or decrease of climatenstress related to the diverse paleoclimaticncooling is interpreted to have restored thenocean-atmosphere δ13C trends.