The effect of Indian Summer Monsoon rainfall on surface water δD values in the central Himalaya

Abstract

Stable isotope proxy records, such as speleothems, plant-wax biomarker records, and ice cores, are suitable archives for the reconstruction of regional palaeohydrologic conditions. But the interpretation of these records in the tropics, especially in the Indian Summer Monsoon (ISM) domain, is difficult due to differing moisture and water sources: precipitation from the ISM and Winter Westerlies, as well as snow-and glacial meltwater.In this study, we use interannual differences in ISM strength (2011)(2012) to understand the stable isotopic composition of surface water in the Arun River catchment in eastern Nepal. We sampled main stem and tributary water (n = 204) for stable hydrogen and oxygen isotope analysis in the postmonsoon phase of two subsequent years with significantly distinct ISM intensities. In addition to the 2011/2012 sampling campaigns, we collected a 12-month time series of main stem waters (2012/2013, n = 105) in order to better quantify seasonal effects on the variability of surface water δ 18 O/δD. Furthermore, remotely sensed satellite data of rainfall, snow cover, glacial coverage, and evapotranspiration was evaluated. The comparison of datasets from both years revealed that surface waters of the main stem Arun and its tributaries were D-enriched by~15‰ when ISM rainfall decreased by 20%. This strong response emphasizes the importance of the ISM for surface water run-off in the central Himalaya. However, further spatio-temporal analysis of remote sensing data in combination with stream water d-excess revealed that most high-altitude tributaries and the Tibetan part of the Arun receive high portions of glacial melt water and likely Winter Westerly Disturbances precipitation. We make the following two implications: First, palaeohydrologic archives found in high-altitude tributaries and on the southern Tibetan Plateau record a mixture of past precipitation δD values and variable amounts of additional water sources. Second, surface water isotope ratios of lower elevated tributaries strongly reflect the isotopic composition of ISM rainfall implying a suitable region for the analysis of potential δD value proxy records.

Validation and calibration of soil δ2H and brGDGTs along (E-W) and strike (N-S) of the Himalayan climatic gradient

“…The Alaknanda surface water δ H values ranged from -51‰ to -102‰. The Arun water data (Meese et al, 2018) was sampled together with the soils and additional river sediments (Hoffmann et al, 2016). A negative correlation was observed between δ 2 H and mean catchment elevation in the Sutlej, Alaknanda and Arun river catchments (R 2 = 0.79, p < 0.001 and R 2 = 0.82, p < 0.001, R 2 =0.73, p<0.01, respectively).…”

Late Holocene Landscape Collapse of a Trans‐Himalayan Dryland: Human Impact and Aridification

“…First, the isotopic composition of ISM precipitation, the principal moisture source in the upper KG valley, is inversely correlated with the amount of precipitation in the tropics (Dansgaard, 1964) and imprinted on plant source waters in the Himalayas (Meese et al, 2018). First, the isotopic composition of ISM precipitation, the principal moisture source in the upper KG valley, is inversely correlated with the amount of precipitation in the tropics (Dansgaard, 1964) and imprinted on plant source waters in the Himalayas (Meese et al, 2018).…”

“…Instead, we identify two mechanisms related to late Holocene aridification. First, the isotopic composition of ISM precipitation, the principal moisture source in the upper KG valley, is inversely correlated with the amount of precipitation in the tropics (Dansgaard, 1964) and imprinted on plant source waters in the Himalayas (Meese et al, 2018). Modeling suggests that a 50% decrease in ISM intensity between the middle and late Holocene has resulted in a 1.5‰ increase in precipitation δ 18 O values or 12‰ in δD (LeGrande & Schmidt, 2009).…”

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

“…In the Himalaya, the combination of decreasing temperatures and increasing rainout with increasing elevation accentuates fractionation processes and results in water that is isotopically lighter (depleted in 18 O and 2 H relative to VMSOW). This relationship, reported as δ 18 O and δD lapse rates, ranges in surface water in Himalayan rivers from -0.45 to -4.4 ‰ km -1 for δ 18 O and -8.3 to -33.0 ‰ km -1 for δD, but these estimates largely draw on surface water samples from streams in low and mid elevations less than 5000 m (Florea et al, 2017;Hren et al, 2009;Meese et al, 2018;Poage & Chamberlain, 2001;Racoviteanu et al, 2013;van der Veen et al, 2018;Varay et al, 2017;Wilson et al, 2015). High-elevation samples are sparse but essential to accurately characterize mountain-sourced surface waters, especially in regions with significant contributions from isotopically heavy snow and glacial melt.…”

“…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).…”