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

Abundant pedogenic carbonate nodules are present in the Shimonoseki Subgrou p in the Kanmon Basin of Kyushu and Honshu, Japan. The oxygen isotope compo sitions of these pedogenic carbonates range from -20.1 to -22.8 parts per t housand (PDB), which is 7 parts per thousand lower than those of the septar ian crystallaria in the carbonate nodules. The meteoric water composition e stimated from the oxygen isotope composition of the soil water in equilibri um with the carbonates is much depleted compared with the estimate from oth er sources. This suggests that the oxygen isotope compositions of the Shimo noseki pedogenic carbonates were modified during diagenesis. The carbon iso topic compositions of the Shimonoseki carbonates range from -5.4 to -7.0 pa rts per thousand (PDB), with an average of -6.7 parts per thousand (PDB), s uggesting carbonate formation in soils dominated by C-3 type of vegetation. An analysis of these values using Cerling's model (Cerling, T.E., Wright, V.P., Vanstone, S.D., 1992. Further comments on using carbon isotopes in pa leosols to estimate the CO2 content of the palaeo-atmosphere. J. Geol. Sec. London 148 (1991), 945-947; 149, 673-676) indicates that the partial press ure of CO2 in the Early Cretaceous Shimonoseki atmosphere was about 1700-32 00 ppmV. (C) 1999 Elsevier Science B.V. All rights reserved.