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

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.

The Sensitivity of Terrestrial δ18O Gradients to Hydroclimate Evolution

“…Following the Cascades case, peak elevation corresponds with the median height of the ridgeline (other elevations are tested in supporting information section S7). The δ 18 O observational data come from previous publications (Grujic et al, ) and compilations (Caves et al, ; L. Li & Garzione, ; which include Bershaw et al, ; Ding et al, ; Hren et al, ; International Atomic Energy Agency/World Meteorological Organization, ; Q. Liu, ; Z. Liu et al, ; Quade et al, ; Tian et al, ; Yao et al, ; X. Zhang et al, ) and published and unpublished data from the Waterisotopes database (http://www.waterisotopes.org; Waterisotopes Database, ; Florea et al, ). These data include both surface water and precipitation samples.…”

“…Following the Cascades case, peak elevation corresponds with the median height of the ridgeline (other elevations are tested in supporting information section S7). The δ 18 O observational data come from previous publications(Grujic et al, 2018) and compilationsL. Li & Garzione, 2017; which include Bershaw et al, 2012;Ding et al, 2009;Hren et al, 2009; International Atomic Energy Agency/World Meteorological Organization, 2015;Q.…”

Deformational Temperatures Across the Lesser Himalayan Sequence in Eastern Bhutan and Their Implications for the Deformation History of the Main Central Thrust

“…(a) Temperatures obtained by RSCM (in blue) plotted with their structural distance from the MBT according to the cross section (Figure 2). Temperatures in green from the Siwaliks group sediments are vitrinite reflectance data (Grujic et al, 2018). Yellow: metamorphic peak temperatures in the Jaishidanda formation and at the base of the GHS in eastern Bhutan (Daniel et al, 2003).…”

Variation of deuterium excess in surface waters across a 5000-m elevation gradient in eastern Nepal

“…In the Himalaya, precipitation from the ISM is marked by low deuterium excess values while WWD-sourced precipitation has relatively high deuterium excess values (Balestrini et al, 2016;Bershaw et al, 2012;Hren et al, 2009;Jeelani et al, 2013;Pande et al, 2000). Previous studies of surface water isotope signatures in the Himalaya largely sampled in the post-monsoon season and reflect a dominant ISM moisture source, and little is known about the temporal evolution of surface water oxygen and hydrogen stable isotope values driven by seasonal moisture source variation, particularly the influence of seasonal snowmelt (Grujic et al, 2018;Hren et al, 2009;Meese et al, 2018;van der Veen et al, 2018;Varay et al, 2017).…”