Millennial erosion rates across the Pamir based on 10Be concentrations in fluvial sediments: Dominance of topographic over climatic factors


Millennial erosion rates across the Pamir based on 10Be concentrations in fluvial sediments: Dominance of topographic over climatic factors

Fuchs, M. C.; Gloaguen, R.; Merchel, S.; Pohl, E.; Sulaiman, V.; Andermann, C.; Rugel, G.

The understanding of erosion processes is fundamental to study the evolution of actively deforming mountain ranges, whereas the relative contributions tectonic and climatic factors and their feedbacks are debated. The Pamir is peculiar in both, high deformation rates induced by the India-Eurasia collision and its position at the transition between Westerlies and Monsoon. In order to contribute to this debate we quantify basin-wide erosion rates from cosmogenic 10Be concentrations in modern river sediments measured by accelerator mass spectrometry. Sample locations represent the Panj basin at six sites along its trunk stream, and the major, east-west elongated tributary basins at five sites. An average erosion of 0.64 mm/yr for the entire Pamir reveals a rapid landscape evolution.
Erosion rates of tributary sub-basins highlight the strong contrast between the plateau (0.05 mm/yr to 0.16 mm/yr) and the Pamir margins (0.54 mm/yr to 1.45 mm/yr). The intensity of erosion is primarily (R2 of 0.81) correlated to slope steepness (0.75 quartiles) suggesting either tectonic uplift or base level lowering. Multiple linear regression reveals that precipitation may contribute also to the efficiency of erosion (R2 of 0.93) to a lesser extent. Dry conditions and low slopes hinders sediment transport and consequently, erosion on the plateau. The highest erosion coincides with the predominant winter precipitation from the Westerlies. The concentrated discharge during spring and early summer favors pronounced erosion along the north-western Pamir margin by driving the sediment flux out of the basins. The magnitude of erosion in Pamir is similar to rates determined in the south Himalayan escarpment, whereas climatic and tectonic conditions are very different. Millennial erosion does not balance the roughly ten times higher fluvial incision implying a transient landscape. We propose that river captures are responsible for the strong base level drop driving the incision along the Panj and consequently, initiate steep hillslopes that will contribute to high erosion at the Pamir margins. Precipitation may act as limiting factor to hillslope adjustment and consequently to erosion processes.

Keywords: accelerator mass spectrometry; erosion rate; Pamir; cosmogenic nuclides; fluvial sediments

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Publ.-Id: 19190