Stable isotopic composition of pedogenic carbonates of the Early Cretaceous Shimonoseki Subgroup, western Honshu, Japan

Abundant pedogenic carbonate nodules are present in the Shimonoseki Subgroup in the Kanmon Basin of Kyushu and Honshu, Japan. The oxygen isotope compositions of these pedogenic carbonates range from −20.1 to −22.8‰ (PDB), which is 7‰ lower than those of the septarian crystallaria in the carbonate nodules. The meteoric water composition estimated from the oxygen isotope composition of the soil water in equilibrium with the carbonates is much depleted compared with the estimate from other sources. This suggests that the oxygen isotope compositions of the Shimonoseki pedogenic carbonates were modified during diagenesis. The carbon isotopic compositions of the Shimonoseki carbonates range from −5.4 to −7.0‰ (PDB), with an average of −6.7‰ (PDB), suggesting carbonate formation in soils dominated by C3 type of vegetation. An analysis of these values using Cerlingʹs model (Cerling, T.E., Wright, V.P., Vanstone, S.D., 1992. Further comments on using carbon isotopes in paleosols to estimate the CO2 content of the palaeo-atmosphere. J. Geol. Soc. London 148 (1991), 945–947; 149, 673–676) indicates that the partial pressure of CO2 in the Early Cretaceous Shimonoseki atmosphere was about 1700–3200 ppmV.