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

Grujic, D. and Govin, Gwladys and Barrier, L. and Bookhagen, B. and Coutand, I. and Cowan, B. and Hren, M.T. and Najman, Yani (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 (9). pp. 3430-3447. ISSN 1525-2027

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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.

Item Type:
Journal Article
Journal or Publication Title:
Geochemistry, Geophysics, Geosystems
Additional Information:
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
Uncontrolled Keywords:
/dk/atira/pure/subjectarea/asjc/1900/1908
Subjects:
ID Code:
129477
Deposited By:
Deposited On:
08 Feb 2019 10:35
Refereed?:
Yes
Published?:
Published
Last Modified:
29 Mar 2020 06:18