Oxygen isotope evidence of climate change from pedogenic clay minerals in the Himalayan molasse
- Dartmouth College, Hanover, NH (United States)
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{per_thousand} increase in the smectite {delta}{sup 18}O values synchronous with a {approximately}3.5{per_thousand} increase in {delta}{sup 18}O values of pedogenic calcite at 8.5-6.5 Ma. The covariation of the {delta}{sup 18}O values of smectite with calcite {delta}{sup 18}O values through time strongly suggests a pedogenic origin of smectite and that its {delta}{sup 18}O values reflect the oxygen isotope ratio of the soil water present during pedogenesis. These data show that the {delta}{sup 18}O values of clay minerals in paleosols in aggrading sedimentary sequences are viable paleoclimate indicators. Kaolinite {delta}{sup 18}O values are similar to smectite {delta}{sup 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, {approximately}3{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{per_thousand} increase in oxygen isotope ratios of smectite and calcite at 8.5-6.5 Ma indicates that this {sup 18}O/{sup 16}O increase is not due to diagenesis, but rather results from a climate change. The most likely causes of the {sup 18}O/ {sup 16}O increase in smectite and calcite are either increased aridity resulting in increased soil water evaporation (thus {sup 18}O enrichment) and/or a change from more continental to more marine-sourced precipitation. 83 refs., 8 figs., 2 tabs.
- OSTI ID:
- 567953
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 4 Vol. 61; ISSN GCACAK; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
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