A 15 Ky High Resolution 10be-Record of Denudation Rate Change from an Alpine Catchment in the Eastern European Alps

Friday, 19 December 2014
Reto Grischott1, Florian Kober2, J├╝rgen Reitner3, Kristina Hippe4, Susan Ivy-Ochs4, Irka Hajdas4 and Sean Willett1, (1)ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, (2)NAGRA National Cooperative for the Disposal of Radioactive Waste, Wettingen, Switzerland, (3)Geological Survey of Austria, Vienna, Austria, (4)ETH Zurich, Laboratory of Ion Beam Physics, Zurich, Switzerland
The influence of climate on erosion of alpine catchments has been sparsely understood due to the missing temporal or spatial resolution of archives and the quantification of processes. Sediment budget studies in the Alps show severalfold increased lateglacial denudation compared to the present being in line with the concept of paraglacial cycle.

Here, we present results coupling a 15-ky record of cosmogenic 10Be-derived paleo-denudation rates (n=42) from a 160 m Stappitz lake archive in the Austrian Alps (Seebach-Valley, 34 km2) and a two-year timeseries of the modern stream. The age-depth chronology was established using 14C dates and relative pollen-stratigraphy. Postdepositional correction for the core samples was not necessary due to sufficient shielding while deposited in a lake. Catchment mapping combined with glacial extents reconstructions of revealed prominent lateglacial moraines which likely decoupled the sediment flux from upper valley flanks to the trunk stream since Younger Dryas. Thus, we interpret the denudational pattern as being dominated by the lower hillslopes for the Holocene with only minimal admixing of glacial material. The latter was probably the dominant sediment source in the lateglacial. The beginning of the Holocene shows decreasing denudation rates down to a minimum at 5 ky BP from 1.0 to 0.3 mm/yr. The transition from 5 to 3 ky BP is marked with increasing rates up to the level of the last 3 ky with 0.6 mm/yr, similar to the modern rate. Given the stratigraphy of the core, we attribute the rather low alpine denudation rates to the stabilising effect of vegetation on the hillslopes. Based on an assumed lag time, the denudation rates increase due to the cooler and wetter climate in the late Holocene resulting in less vegetation (on the slopes), more shallow debris-flows and frost cracking. Our results suggest that in the study area climate modulates denudation by dictating vegetation on the hillslopes and efficient frost cracking processes. Importantly, the state of coupling of sediment storage compartments e.g. glacial deposits to the trunk stream has to be considered otherwise nuclide concentrations might be solely a result of mixing. Furthermore, our data seem to contradict the concept of paraglacial cycle. Denudation rate and sediment flux might be not as tightly coupled as commonly assumed.