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    Rights statement: An edited version of this paper was published by AGU. Copyright 2018 American Geophysical Union. Grujic, D., Govin, G., Barrier, L., Bookhagen, B., Coutand, I., Cowan, B., et al. (2018). Formation of a rain shadow: O and H stable isotope records in authigenic clays from the Siwalik Group in eastern Bhutan. Geochemistry, Geophysics, Geosystems, 19, 3430–3447. https://doi.org/10.1029/2017GC007254

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Formation of a Rain Shadow: O and H Stable Isotope Records in Authigenic Clays From the Siwalik Group in Eastern Bhutan

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<mark>Journal publication date</mark>09/2018
<mark>Journal</mark>Geochemistry, Geophysics, Geosystems
Issue number9
Volume19
Number of pages18
Pages (from-to)3430-3447
Publication StatusPublished
Early online date26/09/18
<mark>Original language</mark>English

Abstract

We measure the oxygen and hydrogen stable isotope composition of authigenic clays from Himalayan foreland sediments (Siwalik Group), and from present day small stream waters in eastern Bhutan to explore the impact of uplift of the Shillong Plateau on rain shadow formation over the Himalayan foothills. Stable isotope data from authigenic clay minerals (<2 μm) suggest the presence of three paleoclimatic periods during deposition of the Siwalik Group, between ∼7 and ∼1 Ma. The mean δ18O value in paleometeoric waters, which were in equilibrium with clay minerals, is ∼2.5‰ lower than in modern meteoric and stream waters at the elevation of the foreland basin. We discuss the factors that could have changed the isotopic composition of water over time and we conclude that (a) the most likely and significant cause for the increase in meteoric water δ18O values over time is the “amount effect,” specifically, a decrease in mean annual precipitation. (b) The change in mean annual precipitation over the foreland basin and foothills of the Himalaya is the result of orographic effect caused by the Shillong Plateau's uplift. The critical elevation of the Shillong Plateau required to induce significant orographic precipitation was attained after ∼1.2 Ma. (c) By applying scale analysis, we estimate that the mean annual precipitation over the foreland basin of the eastern Bhutan Himalayas has decreased by a factor of 1.7–2.5 over the last 1–3 million years. ©2018. American Geophysical Union. All Rights Reserved.

Bibliographic note

An edited version of this paper was published by AGU. Copyright 2018 American Geophysical Union. Grujic, D., Govin, G., Barrier, L., Bookhagen, B., Coutand, I., Cowan, B., et al. (2018). Formation of a rain shadow: O and H stable isotope records in authigenic clays from the Siwalik Group in eastern Bhutan. Geochemistry, Geophysics, Geosystems, 19, 3430–3447. https://doi.org/10.1029/2017GC007254