Oxygen isotope evidence of climate change from pedogenic clay minerals in the Himalayan molasse

In this study, we examine the oxygen isotope ratios of smectite and kaolinite in a stratigraphic sequence of paleosols in the Siwalik Himalayan molasse on the Potwar Plateau, northern Pakistan. There is a 3-4 parts per thousand increase in the smectite delta(18)O values synchronous with a similar to 3.5 parts per thousand increase in delta(18)O values of pedogenic calcite at 8.5-6.5 Ma (previously observed by Quade et al., 1989). The covariation of the delta(18)O values of smectite with calcite delta(18)O values through time strongly suggests a pedogenic origin of smectite and that its delta(18)O values reflect the oxygen isotope ratio of the soil water present during pedogenesis. These data show that the delta(18)O values of clay minerals in paleosols in aggrading sedimentary sequences are viable paleoclimate indicators. Kaolinite delta(18)O values are similar to smectite delta(18)O values, as would be expected for pedogenic kaolinite, but there is greater variation in these data which we attribute to incomplete mineral separation. The measured permil oxygen isotope fractionation between calcite and smectite within individual paleosols is, on average, similar to 3 parts per thousand higher than the equilibrium fractionation. This isotopic disequilibrium can be explained by the calcite forming predominantly during the dry winter season and the smectite forming predominantly during the wet summer season. The synchronous 3.5 parts per thousand increase in oxygen isotope ratios of smectite and calcite at 8.5-6.5 Ma indicates that this O-18/O-16 increase is not due to diagenesis, but rather results from a climate change. The most likely causes of the O-18/O-16 increase in smectite and calcite are either increased aridity resulting in increased soil water evaporation (thus O-18 enrichment) and/or a change from more continental to more marine-sourced precipitation. The Tibetan Plateau, which is suggested to have undergone rapid uplift around 8 Ma (e.g., Hanison et al., 1992), may have acted as an effective orographic barrier to low delta(18)O precipitation originating from central Asia, causing higher delta(18)O values of the precipitation falling on the Potwar Plateau. Currently it is impossible to distinguish between these two potential causes of the O-18/O-16 increase at 8.5-6.5 Ma, but these clay mineral delta(18)O values support that there was a significant climate change in this region. Copyright (C) 1997 Elsevier Science Ltd.