Geochemistry of the Narayani river sediments tracing the evolution of erosion of Central Nepal over the LGM-Holocene and last decades
Abstract
Himalaya is one region of the planet where the interplay between tectonic and climate exerts primary controls on topography and erosion processes over millions of years. This includes (1) the relatively steady Himalayan uplift that maintains high elevation of the range and steep slopes, the more variable monsoon precipitation that controls part of landslide activity and the sediment export, and (3) earthquake which trigger sporadic intensification of erosion. More recently, human activity triggered changes initially in the soil surface through terracing and deforestation and, since the beginning of this century, by development of road and river hydropower infrastructure. Such changes impact the erosion of hillslopes and the export of sediments.
In this study we compare geochemical compositions of river sediments exported by the Narayani river to explore the change of the distribution of erosion in Central Nepal Himalaya. We compare sediment characteristics from 50-kyr-long sedimentary archives drilled in the Gandak foreland basin with river sediments sampled at the front of the Himalaya in Narayanghat since 25 years. Samplings from 2010, 2015-18 are daily high resolution of the surface suspended sediment during the monsoon season. Major elements geochemistry has characteristics very similar to those of the Himalayan bedrocks implying that physical erosion of bedrock is by far the dominant process of erosion over surficial soil erosion. This situation appears steady over the long term and not impacted by anthropic activities. Sr and Nd isotopic compositions and carbonate concentrations document the geological provenances of sediments from the three main Himalayan formations. These compositions remained stable from the last Glacial period to the Holocene and until the end of the 20th century. About 80% of the sediments were derived from erosion of the High Himalayan regions (Tethys H. and HHC) and 20% from erosion of the Lesser Himalaya (LH). Since the turn of the century, a shift in the proportion of LH progressively occured to reach 30-45%. While the erosion distribution of the basin appeared to be very stable in spite of the LGM-Holocene climate change, the recent rise of the erosion of the Lesser Himalaya appears to coincide with the rapid increase of population density and road cut in the Middle Hills of Nepal.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGC53G1246F
- Keywords:
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- 1630 Impacts of global change;
- GLOBAL CHANGE;
- 1655 Water cycles;
- GLOBAL CHANGE;
- 4902 Anthropogenic effects;
- PALEOCEANOGRAPHY;
- 4932 Ice cores;
- PALEOCEANOGRAPHY