Carbon isotope stratigraphy of terrestrial organic matter for the Turonian (Upper Cretaceous) in northern Japan: Implications for ocean-atmosphere 13C trends during the mid-Cretaceous climatic optimum

Abstract

Carbon isotope data of terrestrial organic matter (δ 13 C 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 δ 13 C TOM events in the Middle–Upper Turonian of the Yezo Group. δ 13 C 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 δ 13 C TOM events are correlated with previously documented δ 13 C 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 δ 13 C events for global correlation of the Turonian marine successions. In addition, global correlation of Turonian marine and terrestrial δ 13 C events identifies changes in isotopic difference between δ 13 C TOM and δ 13 C carbonate (Δ TOM-carbonate ), which are interpreted to reflect changes in atmospheric p CO 2 levels, and climate-driven stresses of humidity and soil processes. In earlier stages of Turonian, Δ TOM-carbonate values are increased. Elevated atmospheric p CO 2 , and increased humidity and soil processes in enhanced greenhouse conditions during mid-Turonian, are interpreted to enlarge Δ TOM-carbonate values. In later stages of Turonian, Δ TOM-carbonate values are at a constant level, and the lowering of atmospheric p CO 2 or decrease of climate stress related to the diverse paleoclimatic cooling is interpreted to have restored the ocean-atmosphere δ 13 C trends.