Seasonality and variations in moisture-supply on different timescales (1-104 yrs) and their relation to surface processes in the NW Himalaya
Zusammenfassung der Projektergebnisse
Present-day conditions and calibration. This project utilized 10Be measurements in fluvial sands from modern river beds to determine basinwide erosion rates in two major catchments in the Himalaya (Arun and Sutlej) with the aim to characterize millennial-scale erosion processes in a N-S transect and across the pronounced topographic and climatic gradients of the Himalaya. These efforts were accompanied by DEM analysis, an assessment of vegetation covers and regional rainfall patterns using satellite data. In addition to the N-S transects we also carried out an along-strike analysis of erosion rates in the Himalaya by merging our newly obtained data with re-calculated erosion rates compiled from published studies. Palaeoclimate reconstruction. This project received funds from HIMPAC for analytical work and field sampling of sediments with the goal to determine basin-wide erosion rates. Our study of millennial-scale erosion rates using cosmogenic nuclide dating suggests that our newly determined (34 new 10Be-derived catchment-mean denudation rates obtained in the Arun Valley of Nepal, and 12 new denudation rates from the Sutlej Valley of India) and compiled erosion rates along the Himalaya are most highly correlated with climatically modulated indices, such as seasonality or vegetation indices, rather than with topographic metrics, as been commonly argued in the past. A total of 195 cosmogenic erosion-rate measurements along strike of the Himalaya were recalculated with the newest available production rates and combined with remotely sensed data on precipitation, extreme rainfall events, vegetation density, seasonal changes in vegetation, and diverse topographic metrics (slope angles, channel steepness, etc.). In the Arun region catchment-mean denudation rates in tributaries to the Arun increase northward, from <0.2 mm yr^-1 to ~1.5 mm yr^-1. It was also found that 10Be concentrations and denudation rates obtained in mainstem sectors of the Arun do not uniformly represent the processes of the upstream area. Importantly, a W-E along-strike decrease in denudation-rate variability positively correlates with the seasonality of vegetation density (R=0.95), and negatively correlates with mean vegetation density (R=-0.84). Vegetation density modulates the topographic response to changing denudation rates, such that the functional relationship between denudation rate and topographic steepness becomes increasingly linear as vegetation density increases. Thus, while tectonic processes locally control the pattern of denudation rates across strike of the Himalaya (i.e., S-N), along strike of the orogen (i.e., E-W) climate exerts a measurable influence on how denudation rates scatter around long-term, tectonically-controlled erosion, and on the functional relationship between topography and denudation.
