The use of oxygen and carbon isotope composition of pedogenic carbonates from pleistocene palaeosols in NW Bangladesh, as palaeoclimatic indicators

Abstract

δ 13C and δ 18O values of pedogenic carbonate concretions from Pleistocene palaeosol sequences of the northwestern part of Bangladesh are presented. The major goal of this study is to test (1) the reproducibility of the δ 13C and δ 18O signals within palaeosols and (2) their variation from one palaeosol to the other. The palaeosols that are expected to have formed under different palaeoenvironmental, palaeoclimatic or diagenetic conditions provide isotopic signatures ( δ 13C and z δ 18O)) that can be clearly distinguished. In other words the differences between analytical results from the same palaeosol are much smaller than between samples from different palaeosols. This would mean that climatic or environmental changes are recorded as significant differences in isotopic signature. Carbon isotopic compositions vary between −11.5 and +2.0‰. The relatively larger δ 13C variations may be explained by climatically induced changes in floral composition. Changes in humidity are translated into variations of C 3 and C 4 plant proportions, resulting in changes in soil CO 2 δ 13C values. The extreme δ 13C values found would correspond to nearly single ‘phase’ C 3 ( δ 13C: −11.5%.) or C 4 ( δ 13C: +2.0‰). Variations in the oxygen isotopic composition are much smaller, and range between 6̄.5‰ and −3.5‰. These smaller differences can be induced by temperature changes.