Constant denudation rates in a high alpine catchment for the last 6 kyrs. Issue 7 (22nd November 2016) - Grischott, R.
Abstract: Terrestrial cosmogenic nuclides (TCN) have widely been used as proxies in determining denudation rates in catchments. Most studies were limited to samples from modern active streams, thus little is known about the magnitude and causes of TCN variability on millennial time scales. In this work we present a 6 kyrs long, high resolution record of 10 Be concentrations ( n = 18), which were measured in sediment cores from an alluvial fan delta at the outlet of the Fedoz Valley in the Swiss Alps. This record is paired with a 3‐year time series ( n = 4) of 10 Be measured in sediment from the active stream currently feeding this fan delta. The temporal trend in the 10 Be concentrations after correction for postdepositional production of 10 Be was found to be overall constant and in good agreement with the modern river 10 Be concentration. The calculated mean catchment‐wide denudation rate amounts to 0.73 ± 0.18 mm yr −1 . This fairly constant level of 10 Be concentrations can be caused by a constant denudation rate over time within the catchment or alternatively by a buffered signal. In this contribution we suggest that the large alluvial floodplain in the Fedoz Valley may act as an efficient buffer on Holocene time scales in which sediments with different 10 Be signatures are mixed. Therefore, presumable variations in the 10 Be signals derived from changes in denudation under a fluctuating Holocene climate are only poorly transferred to the catchment outlet and not recorded in the 10 Be record. However, despite the absence of high frequency signals, we propose that the buffered and averaged 10 Be signal could be meaningfully and faithfully interpreted in terms of long‐term catchment‐averaged denudation rate. Our study suggests that alluvial buffers play an important role in regulating the 10 Be signal exported by some alpine settings that needs to be taken into account and further investigated. Copyright © 2016 John Wiley & Sons, Ltd.