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

Abstract

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%o increase in the smectite δ 18O values synchronous with a ∼ 3.5%o increase in δ 18O values of pedogenic calcite at 8.5–6.5 Ma (previously observed by Quade et al., 1989). The covariation of the δ 18O values of smectite with calcite gd 18O values through time strongly suggests a pedogenic origin of smectite and that its δ 18O values reflect the oxygen isotope ratio of the soil water present during pedogenesis. These data show that the δ 18O values of clay minerals in paleosols in aggrading sedimentary sequences are viable paleoclimate indicators. Kaolinite δ 18O values are similar to smectite δ 18O 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, ∼3%o 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%o increase in oxygen isotope ratios of smectite and calcite at 8.5–6.5 Ma indicates that this 18O/ 16O increase is not due to diagenesis, but rather results from a climate change. The most likely causes of the 18O/ 16O increase in smectite and calcite are either increased aridity resulting in increased soil water evaporation (thus 18O 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., Harrison et al., 1992), may have acted as an effective orographic barrier to low δ 18O precipitation originating from central Asia, causing higher δ 18O values of the precipitation falling on the Potwar Plateau. Currently it is impossible to distinguish between these two potential causes of the 18O/ 16O increase at 8.5–6.5 Ma, but these clay mineral δ 18O values support that there was a significant climate change in this region.