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The response of surface processes to variations in climate: cosmogenically-derived erosion rates in the Andes, and numerical models

English title The response of surface processes to variations in climate: cosmogenically-derived erosion rates in the Andes, and numerical models
Applicant Schlunegger Fritz
Number 107448
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geologie Universität Bern
Institution of higher education University of Berne - BE
Main discipline Geology
Start/End 01.01.2006 - 31.12.2008
Approved amount 471'715.00
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Keywords (5)

Surface Processes; Climate and Surface Erosion; Cosmogenic Nuclides; Numerical Modeling ; Peru

Lay Summary (English)

Lead
Lay summary
This research explores at what scale highly episodic precipitation events control the evolution of drainage basins, and how the effects can be seen and measured in natural landscapes. These questions are addressed by a regional (PhD project) and a conceptual study (Postdoc program). The regional study concentrates on the Piura drainage basin in northern Peru where warm ENSO events have repeatedly perturbed the pattern of precipitation. For this drainage basin, long-term process rates are measured using terrestrial cosmogenic nuclides (TCNs), and the effects on the landscape are quantified based on morphometric analyses. The conceptual part of the project explores how variabilities in sediment flux relate to the hydrological regime, and how this can be seen in the long-term record (e.g., the cosmogenic nuclides).
According to the scope, cosmogenic samples were collected from bedloads of tributary systems (PhD project). The sites were selected to cover all relevant scales and climate zones. The samples are currently prepared for the dissolution of quartz in the clean lab and will be measured in summer 2007. The conversion of nuclide concentrations into erosion rates requires that nuclide production rates at the weathering sites are known.
Accordingly, nuclide production rates of the sample sites were modelled in GIS thereby using the morphometric properties of the upstream drainage basin as criteria. This was done using a digital elevation model with a 20-m resolution. In addition, the results of the morphometric modelling of the Piura basin yielded a consistent pattern of topographic parameters.
Specifically, higher channel concavities are calculated for the lower reaches that have been strongly affected by highly episodic precipiation events in relation to warm ENSOs. Lower channel concavities are determined for the upper portions of the transverse drainages where climate has been seasonal with low contrasts between ENSO and ‚normal’ years.
The development of a one-dimensional numerical model (Postdoc program) allowed to quantify the expected statistical dispersion (scatter) and bias in TCN-based erosion rates caused by complex (non steady) erosion histories. The model is capable to account for both continuous erosion with time but variant erosion rates, and discontinuous erosion events with variable thickness of the detached layers (i.e. particle spalling and / or shallow landslides). The results show that effects caused by discontinuous erosion events do not compensate, but bias the erosion rate estimates obtained from cosmogenic nuclide concentrations. For the general case, the magnitude of this bias strongly depends on the incision depth associated with the discontinuous events, as well as on the routing pathway that the detached material undergoes before sampling. The effects caused by continuous but time variant erosion histories may (but not necessarily do) compensate, when the net deviation from a constant erosion rate amounts to zero for the relevant integration interval. However, this question strongly depends on the time window under investigation and the magnitude of erosion rate fluctuations.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
121680 Chronology of sediment accumulation and surface erosion in the western Peruvian Andes in response to climate variations during the Quaternary: cosmogenic nuclide analysis, OSL dating and modelling 01.01.2009 Project funding (Div. I-III)

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