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 ka olinite in a stratigraphic sequence of paleosols in the Siwalik Himala yan molasse on the Potwar Plateau, northern Pakistan. There is a 3-4 p arts per thousand increase in the smectite delta(18)O values synchrono us with a similar to 3.5 parts per thousand increase in delta(18)O val ues of pedogenic calcite at 8.5-6.5 Ma (previously observed by Quade e t al., 1989). The covariation of the delta(18)O values of smectite wit h 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 aggr ading sedimentary sequences are viable paleoclimate indicators. Kaolin ite delta(18)O values are similar to smectite delta(18)O values, as wo uld be expected for pedogenic kaolinite, but there is greater variatio n in these data which we attribute to incomplete mineral separation. T he measured permil oxygen isotope fractionation between calcite and sm ectite 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 d uring the dry winter season and the smectite forming predominantly dur ing the wet summer season. The synchronous 3.5 parts per thousand incr ease in oxygen isotope ratios of smectite and calcite at 8.5-6.5 Ma in dicates that this O-18/O-16 increase is not due to diagenesis, but rat her results from a climate change. The most likely causes of the O-18/ O-16 increase in smectite and calcite are either increased aridity res ulting 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 upli ft around 8 Ma (e.g., Hanison et al., 1992), may have acted as an effe ctive orographic barrier to low delta(18)O precipitation originating f rom central Asia, causing higher delta(18)O values of the precipitatio n falling on the Potwar Plateau. Currently it is impossible to disting uish 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 E lsevier Science Ltd.