Postglacial to Holocene landscape evolution and process rates in steep alpine catchments

Climate change and high magnitude mass wasting events pose adverse societal effects and hazards, especially in alpine regions. Quantification of such geomorphic processes and their rates is therefore critical but is often hampered by the lack of appropriate techniques and the various spatiotemporal...

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Published in:	Earth surface processes and landforms Vol. 44; no. 1; pp. 242 - 258
Main Authors:	Kober, Florian, Hippe, Kristina, Salcher, Bernhard, Grischott, Reto, Zurfluh, Raphael, Hajdas, Irka, Wacker, Lukas, Christl, Marcus, Ivy‐Ochs, Susan
Format:	Journal Article
Language:	English
Published:	Bognor Regis Wiley Subscription Services, Inc 01-01-2019
Subjects:	Admixtures Alpine climates alpine erosion Alpine regions Avalanches Beryllium Beryllium 10 Boulders Boundary conditions Carbon 14 Catchment area Catchments Climate change cosmogenic nuclides Debris flow Deglaciation Denudation Detritus Digital Elevation Models Driving conditions episodic rates Evolution Field investigations Field tests Fingerprinting Firn Geomorphology Hazards Headwaters Holocene Landslides Mass wasting Meltwater Nuclides Outlets Radiocarbon dating Radiometric dating Sediment Sediment fingerprinting Sediments Seismic surveys Snow avalanches Surveying Switzerland Torrents
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Summary:	Climate change and high magnitude mass wasting events pose adverse societal effects and hazards, especially in alpine regions. Quantification of such geomorphic processes and their rates is therefore critical but is often hampered by the lack of appropriate techniques and the various spatiotemporal scales involved in these studies. Here we exploit both in situ cosmogenic beryllium‐10 (10Be) and carbon‐14 (14C) nuclide concentrations for deducing exposure ages and tracing of sediment through small alpine debris flow catchments in central Switzerland. The sediment cascade and modern processes we track from the source areas, through debris flow torrents to their final export out into sink regions with cosmogenic nuclides over an unprecedented five‐year time series with seasonal resolution. Data from a seismic survey and a 90 m core revealed a glacially overdeepened basin, filled with glacial and paraglacial sediments. Surface exposure dating of fan boulders and radiocarbon ages constrain the valley fill from the last deglaciation until the Holocene and show that most of the fan existed in early Holocene times already. Current fan processes are controlled by episodic debris flow activity, snow (firn) and rock avalanches. Field investigations, digital elevation models (DEMs) of difference and geomorphic analysis agree with sediment fingerprinting with cosmogenic nuclides, highlighting that the bulk of material exported today at the outlet of the subcatchments derives from the lower fans. Cosmogenic nuclide concentrations steadily decrease from headwater sources to distal fan channels due to the incorporation of material with lower nuclide concentrations. Further downstream the admixture of sediment from catchments with less frequent debris flow activity can dilute the cosmogenic nuclide signals from debris flow dominated catchments but may also reach thresholds where buffering is limited. Consequently, careful assessment of boundary conditions and driving forces is required when apparent denudation rates derived from cosmogenic nuclide analysis are upscaled to larger regions. © 2018 John Wiley & Sons, Ltd. Postglacial denudation rates in an alpine landscape are an order of magnitude higher than modern ones testifying the paraglacial cycle of sediment flux. Modern process rates are influenced by episodic processes (rock fall, debris flows) as revealed by the analysis of in situ beryllium‐10 (10Be) and carbon‐14 (14C) cosmogenic nuclide concentration.
ISSN:	0197-9337 1096-9837
DOI:	10.1002/esp.4491