Browsing by Author "Srivastava, P"

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    Late quaternary fluvial incision and aggradation in the Lesser Himalaya, India
    (Elsevier, 2018-10-01) Dosseto, A; May, JH; Choi, JH; Swander, ZJ; Fink, D; Korup, O; Hesse, PP; Singh, T; Mifsud, C; Srivastava, P
    Reconstructing how rivers respond to changes in runoff or sediment supply by incising or aggrading has been pivotal in gauging the role of the Indian Summer Monsoon (ISM) as a geomorphic driver in the Himalayas. Here we present new chronological data for fluvial aggradation and incision from the Donga alluvial fan and the upper Alaknanda River, as well as a compilation of previous work. In addition to conventional OSL-SAR (Single-Aliquot Regenerative-Dose) dating method, we have tested and applied pulsed OSL (POSL) dating for quartz samples that include K-rich feldspar inclusions, which is expected to improve the applicability and validity of OSL ages in the Lesser Himalaya. For previously dated deposits, our OSL ages are shown to be systematically older than previously reported ages. These results suggest periods of aggradation in the Alaknanda and Dehradun Valleys mainly between ∼25 and 35 ka. This most likely reflects decreased stream power during periods of weakened monsoon. In addition, in-situ cosmogenic beryllium-10 was used to infer bedrock surface exposure ages, which are interpreted as episodes of active fluvial erosion. Resulting exposure ages span from 3 to 6 ka, suggesting that strath terraces were exposed relatively recently, and incision was dominant through most of the Holocene. In combination, our results support precipitation-driven fluvial dynamics, which regulates the balance between stream power and sediment supply. On a larger spatial scale, however, fluvial dynamics are probably not spatially homogeneous as aggradation could have been taking place in adjacent catchments while incision dominated in the study area. © 2018 Elsevier Ltd.
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    Past changes in sediment dynamics in the Himalayas inferred from uranium-series isotopes
    (Australian Geosciences Council, 2012-08-05) Dosseto, A; Hesse, PP; Fink, D; Srivastava, P
    Climate variability has been setting the rhythm of the Earth’s history, but how does the landscape, in particular rivers, adapt to these changes? One possible approach to tackle this question is to investigate how sedimentary deposits have recorded changes in fluvial dynamics. As a proxy for this dynamics, we use the sediment residence time, i.e. the time spent by sediments in a catchment before export (storage in soils + fluvial transport). Recently, it has been proposed that uranium-series isotopes can be used to quantify past variations in sediment residence time. In this study, this approach is applied to alluvial deposits from the upper Ganges River, the Yamuna River and the Dun valley in the foothills of the Himalayas. The age of these deposits has been previously constrained by optically-stimulated luminescence dating and range from 0 to 40 ka. Preliminary work using lithium isotopes has shown a strong link between monsoon activity and weathering intensity. Results from this study will investigate how hillslope erosion and fluvial transport may have responded to past climatic change.