th 3E&G / Vol. 65 / No. 2 / 2016 / th 3–5 / DOI 10.3285/eg.65.2.th2 / © Authors / Creative Commons Attribution License E&G Quaternary Science Journal Volume 65 / Number 2 / 2016 / th 3–5 / DOI 10.3285/eg.65.2.th2www.quaternary-science.net GEOZON SCIENCE MEDIAISSN 0424-7116 THESIS ABSTRACT Spatially and temporally variable catchment-wide denudation rates – clues from the Alps Reto Grischott E-Mail: reto.grischo@phys.ethz.ch University: Laboratory for Ion Beam Physics, ETH Zürich, Switzerland Supervisors: Florian Kober, Sean D. Wille, Susan Ivy-Ochs, Rainer Wieler, Mahias Hinderer Dissertation online: hp://e-collection.library.ethz.ch/view/eth:49001 (abstract only). Fulltext will be available in May 2017. Catchment-wide denudation rates (CWDR) can be ob- tained by measuring in-situ produced terrestrial cosmo- genic nuclides in alluvial sediments (G˜™š›œ˜ et al. 1996). For alpine catchments, such “modern” samples commonly provide a mean estimate of denudation over the last two millennia, but their sensitivity to recording climatic ¢uc- tuations on the Holocene time-scale is limited. In order to overcome these temporal limitations of modern CWDRs, paleo-CWDRs were derived from sediment cores collected in valley £lls. ¤e main goal of the thesis was therefore to establish 10Be records of paleo-CWDRs in alpine seings, in order to determine controls on denudation over the Ho- locene and the latest Pleistocene. Additionally, for one set- ting it was aimed to independently validate the cosmogenic paleo-CWDRs by a sediment budget approach. A major part of the thesis was dedicated to the £rst goal of establishing 10Be records of paleo-CWDRs in the European Alps. ¤e approach entailed selecting two al- pine catchments with di§erent altitude ranges, in order to compare and test them for climatic impacts on denudation. ¤e lower altitudinal range (1200–2400 m) is represented by the Seebach catchment in the eastern Austrian Alps (G˜©ª«¬®¯¯ et al., submied). ¤e geomorphological anal- ysis indicates that that sediment released from the upper hillslopes above ~2400 m are geomorphologically discon- nected to the trunk stream for the Holocene. Repeated sed- iment samples from the active stream during three years were analysed to obtain estimates about modern CWDRs. Paleo-CWDRs were obtained from 10Be measurements de- rived from a previously drilled 160 m-long lake sediment core, which contains the Late Glacial to present signal. ¤e combined record of paleo and modern CWDRs show three di§erent stages of denudation. ¤e Late Glacial to the Ear- ly Holocene period is characterized by a decreasing trend of anomalously high CWDRs of ~7 mm/yr towards typical alpine rates of ~0.6 mm/yr, which can explained by the in- corporation of low-dosed, glacial sediments found in the sediment core (Fig. 1A). ¤e temporal trend of our 10Be data indicate paraglacial sediment ¢uxes for the Seebach Valley from at least ~15 to ~7 kyr BP which show a similar paern like the paraglacial cycle postulated by C¬´˜«¬ µ R¶·œ˜ (1972). Holocene CWDRs for the last 7 kyrs correlate with the frequency of extreme precipitation events and anti-cor- relate with mean annual temperature. Minimal CWDRs of ~0.4 mm/yr were found during the Middle Holocene, and are best explained by weaker hillslope erosion induced by rare extreme precipitation events, dense vegetation cover and thus stabilized soils on the hillslope (Fig. 1B). In the Late Holocene, CWDRs quickly raised again and then re- mained at a constant rate of 0.65–0.85 mm/yr, due to in- creased hillslope erosion likely caused by frequent extreme precipitation events and lower timberlines and vegetation cover (Fig. 1C). ¤e second study area is the Fedoz catchment in the eastern Swiss Alps, which represents the higher altitudinal range (2000–3000 m). Samples from three alluvial sediment cores integrating over the last 6 kyr, and a three-year time series of samples from the active stream were analysed for 10Be (G˜©ª«¬®¯¯ et al. 2016). Derived paleo-CWDRs show a surprisingly constant temporal trend of 0.7 mm/yr for the last 6 kyr. We propose that the elongated alluvial ¢oodplain in the Fedoz Valley may act as an e»cient sediment bu§er on the Holocene time-scales (Fig. 2). Sediments from glacial and hillslope erosion containing low and high 10Be concen- trations, respectively, are mixed and their variability in 10Be is dampened during transport. ¤erefore, variations in the 10Be signal that may be produced by Holocene climate ¢uc- tuations are only poorly transferred to the outlet and are, thus, not recorded in the 10Be record of the studied sediment archive. Despite this loss of signal variability, it is proposed that the bu§ered 10Be signal still could be meaningfully in- terpreted as a robust, longer-term denudation rate. ¤e data from the Fedoz catchment have additionally been used to independently validate the cosmogenic denu- dation rates by a sediment budget approach (E©šªœ¼œ µ H©š½ ·œ˜œ˜ 1998). Geophysical measures of the subaquatic and th 4 E&G / Vol. 65 / No. 2 / 2016 / th 3–5 / DOI 10.3285/eg.65.2.th2 / © Authors / Creative Commons Attribution License subaerial delta slope were used to de£ne the sediment vol- umes (Fig. 2). ¤e sediment cores from both parts were taken to validate the data as ground truth and integrate them in a chronostratigraphical framework. However, due to several technical limitations of the geophysical techniques and a lack of sediment cores with reliable age depth models, the uncertainty of the derived sediment budget is signi£cant. Mean denudation rates based on the sediment budget model are a magnitude lower during the Middle Holocene com- pared with the cosmogenic CWDRs. ¤is o§set might result from sediment storage caused by a strongly reduced trans- port capacity of the Fedoz River, likely due to a decreased occurrence of ¢ood events. During the Late Holocene, CW- DRs calculated from the sediment budget are half of the cos- mogenic nuclide derived, which may imply that less stor- age in the catchment occurred. Frequent ¢ood events might have increased the ¢uvial transport capacity and provided a stronger connection between the river and hillslopes. Altogether, this work shows that great progress was made in isolating climate control on denudation over a Fig. 1: Sematic, temporal evolution of the 10Be hillslope prole, hillslope cover and interpreted sediment uxes for the Holocene. During the Late Pleis- tocene and Early Holocene (A), reworking of the glacial sediments and zeroed hillslopes due to glacial erosion leads to low 10Be concentrations of exported sediments. †e Mid-Holocene (B) is aracterized by rare ood events and warm temperatures leading to the development of soil and vegetation cover and related stabilization of hillslopes. Later, increase of the ood frequency and colder temperatures during the Late Holocene (C) leads to higher hillslope ero- sion and sediment ux. Fig. 2: Sematic sket of con- tributing sediment sources and their assumed 10Be signature in the Fedoz catment shown along with the annel length prole of the Fedoz River, the locations of the 10Be samples and sediment cores. †e al- luvial ood plain is suggested to act as an important sedi- ment buŽer and mixing system (indicated by the retrograde arrows). †e sediment budget is estimated based on sediment volumes of the delta plain and slope. (Figure modied from G’“”•–—˜˜ et al. 2016) th 5E&G / Vol. 65 / No. 2 / 2016 / th 3–5 / DOI 10.3285/eg.65.2.th2 / © Authors / Creative Commons Attribution License 103–104 year timescale by the extended analysis of 10Be in sediment archives of various catchments. Furthermore, it has been proven once more that 10Be can serve as an ideal sediment tracer even for seings or time periods where this proxy is not representative for hillslope erosion. References C¬´˜«¬, M., R¶·œ˜, J.M. (1972): Paraglacial sedimentation – A considera- tion of ¢uvial processes conditioned by glaciation. – Geological Soci- ety of America Bulletin 83, 3059-&. E©šªœ¼œ, G., H©š·œ˜œ˜, M. (1998): ¿antifying denudation and sediment- accumulation systems (open and closed lakes): basic concepts and £rst results. – Palaeogeography Palaeoclimatology Palaeoecology 140, 7–21. G˜™š›œ˜, D.E., K©˜«¬šœ˜, J.W. µ F©šÀœ¼, R. (1996): Spatially averaged long-term erosion rates measured from in situ-produced cosmogenic nuclides in alluvial sediment. – Journal of Geology 104, 249–257. G˜©ª«¬®¯ ,¯ R., K®Áœ˜, F., L´ÂÀœ˜, M., H©Âœ, K., Iö½O«¬ª, S., H™Ä·™ª, I., S™¼«¬œ˜, B. µ C¬˜©ª¯¼, M. (2016): Constant denudation rates in a high alpine catchment for the last 6 kyrs. – Earth Surf Processes, 10.1002/esp.4070 G˜©ª«¬®¯ ,¯ R., K®Áœ˜, F., L´ÂÀœ˜, M., Rœ©¯šœ˜, J.M., D˜œª«¬œ˜½S«¬šœ©½ ·œ˜, R., H™Ä·™ª, I., C¬˜©ª¯¼, M. µ W©¼¼œ¯ ,¯ S.D. (submied): Millen- nial scale variability of denudation rates for the last 15 kyrs inferred from the detrital 10Be record of lake Stappitz in the Hohe Tauern mas- sif, Austrian Alps. – ¤e Holocene.