TY  - JOUR
A1  - Grujic, Djordje
A1  - Govin, Gwladys
A1  - Barrier, Laurie
A1  - Bookhagen, Bodo
A1  - Coutand, Isabelle
A1  - Cowan, Beth
A1  - Hren, Michael T.
A1  - Najman, Yani
T1  - Formation of a Rain Shadow
BT  - O and H Stable Isotope Records in Authigenic Clays From the Siwalik Group in Eastern Bhutan
JF  - Geochemistry, geophysics, geosystems
N2  - 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.
KW  - authigenic clay
KW  - stable isotope
KW  - orographic precipitation
KW  - Siwaliks
KW  - Himalaya
KW  - foreland basin
Y1  - 2018
U6  - https://doi.org/10.1029/2017GC007254
SN  - 1525-2027
VL  - 19
IS  - 9
SP  - 3430
EP  - 3447
PB  - American Geophysical Union
CY  - Washington
ER  -