THE USE OF OXYGEN AND CARBON-ISOTOPE COMPOSITION OF PEDOGENIC CARBONATES FROM PLEISTOCENE PALEOSOLS IN NW BANGLADESH, AS PALEOCLIMATIC INDICATORS

delta(13)C and delta(18)O values of pedogenic carbonate concretions fr om Pleistocene palaeosol sequences of the northwestern part of Banglad esh are presented. The major goal of this study is to test (1) the rep roducibility of the delta(13)C and delta(18)O signals within palaeosol s and (2) their variation from one palaeosol to the other. The palaeos ols that are expected to have formed under different palaeoenvironment al palaeoclimatic or diagenetic conditions provide isotopic signatures (delta(13)C and z delta(18)O) that can be clearly distinguished. In o ther words the differences between analytical results from the same pa laeosol are much smaller than between samples from different palaeosol s. This would mean that climatic or environmental changes are recorded as significant differences in isotopic signature. Carbon isotopic com positions vary between -11.5 and +2.0 parts per thousand. The relative ly larger delta(13)C variations may be explained by climatically induc ed changes in floral composition. Changes in humidity are translated i nto variations of C-3 and C-4 plant proportions, resulting in changes in soil CO2 delta(13)C values. The extreme delta(13)C values found wou ld correspond to nearly single 'phase' C-3 (delta(13)C: -11.5 parts pe r thousand) or C-4 (delta(13)C: +2.0 parts per thousand). Variations i n the oxygen isotopic composition are much smaller, and range between -6.5 parts per thousand and -3.5 parts per thousand. These smaller dif ferences can be induced by temperature changes. (C) 1997 Elsevier Scie nce Ltd.