Spatially and temporally variable catchment-wide denudation rates - clues from the Alps
Grischott Reto, Swiss Federal Institute of Technology in Zurich (CH)
The evolution of the Earth's surface is the result of the competing forces of denudation and uplift. Besides tectonics, climate has been considered as a key driver for denudational geomorphic processes, but quantifying its impact has proven to be challenging. Catchmentwide denudation rates (CWDR) can be obtained by measuring in-situ produced terrestrial cosmogenic nuclides in alluvial sediments, and have commonly been used to isolate the factors influencing a landscape. For alpine catchments, such “modern” samples commonly provide a mean estimate of denudation over the last two millennia, but their sensitivity to recording climatic fluctuations 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 fills. The main goal of this thesis was therefore to establish 10Be records of paleo-CWDRs in alpine settings, in order to determine controls on denudation over the Holocene. Additionally, for one setting it was aimed to independently validate the cosmogenic paleo-CWDRs by a sediment budget approach. A further goal was to test the spatial and temporal consistency of modern CWDRs in an alpine setting and determine topographic controls on erosion. The fourth goal was to determine the variability in denudation rates in a landscape with strong glacial morphological inheritance. A major part of this thesis was dedicated to the first goal of establishing 10Be records of paleo-CWDRs in the European Alps. The approach entailed selecting two alpine catchments with different altitude ranges, in order to compare and test them for climatic impacts on denudation. The lower altitudinal range (1200-2400 m) is represented by the Seebach catchment in the eastern Austrian Alps. Paleo-CWDRs were obtained from 10Be measurements derived from a 160 m-long lake sediment core, which contains the Late Glacial to present signal. Combined with a three-year timeseries of modern CWDRs, these CWDRs show three different stages of denudation. The Late Glacial to the Early Holocene period is characterized by a decreasing trend of anomalously high CWDRs towards rates <1 mm/yr, which are explained by the admixing of low-dosed sediments originating from the last glaciation. Holocene CWDRs correlate with the frequency of extreme precipitation events and anti-correlate with temperature. Minimal CWDRs of ~0.4 mm/yr were found during the Middle Holocene, and are explained by weaker hillslope erosion induced by rare flood events and elevated timberlines. In the Late Holocene, CWDRs doubled quickly and then remained at a constant rate of 0.65-0.85 mm/yr, due to increased hillslope erosion caused by frequent extreme precipitation events and decreasing timberlines. The 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 timeseries of samples from the active stream were analysed for 10Be. Derived paleo-CWDRs show a surprisingly constant temporal trend of 0.7 mm/yr for the last 6 kyr. We propose that the large alluvial floodplain in the Fedoz Valley may act as an efficient buffer on Holocene time-scales, during which sediments are mixed and their variability in 10Be is attenuated during transport. Therefore, variations in the 10Be signal that may be produced by Holocene climate fluctuations are only poorly transferred to the outlet and are, thus, not recorded in the 10Be concentration of the studied sediment archive. Despite this loss of signal variability, it is proposed that the buffered 10Be signal could be meaningfully interpreted as a longer-term denudation rate. Data from the Fedoz catchment have additionally been used to independently validate the cosmogenic denudation rates by a sediment budget approach. Geophysical measures of the subaquatic and subaerial delta slope were used to define the sediment volumes. The 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 significant. Mean denudation rates based on the sediment budget model are a magnitude lower during the Middle Holocene compared with the cosmogenic CWDRs. This offset might result from sediment storage caused by a strongly reduced transport capacity of the Fedoz River, likely due to a decreased occurrence of flood events. During the Late Holocene, CWDRs calculated from the sediment budget are half of the cosmogenic nuclide derived ones, which implies that less storage in the catchment might have occurred. Frequent flood events might have increased the fluvial transport capacity and provided a stronger connection between the river and hillslopes. The third study area, the Strimm-Gadria basin in the Italian Alps, was selected because of the diversity of glacial inheritance and geomorphic activity within two of its subcatchments. In order to understand the denudational dynamics, repeated seasonal and high-resolution sampling of modern river sediments was conducted over three years. The CWDR dataset allowed constraining of denudation envelopes for debris flow-dominated (0.38-10.41 mm/yr), semi-alluvial (0.27-0.36 mm/yr) and purely alluvial sub-basins (0.10-0.14 mm /yr). Thanks to the spatially dense sampling, our data could be compared with the index of connectivity, a geomorphometric index that integrates hillslope-to-channel sediment delivery and along-channel sediment conveyance in a basin. The high correlation of denudation rates and connectivity implies that the connectivity is able to distinguish the denudation from landscapes with a different morphology. The observed seasonality of CWDRs measured in the trunk stream of the two sub catchments probably results from human impact by water diversion and retention structures. In a fourth site, we conducted a spatially dense sampling campaign in the densely forested Capilano catchment in the Coast Mountains (British Columbia, Canada), whose landscape has been strongly affected by previous glacial erosion. The aim of this study was to reconstruct the denudational pattern with respect to geomorphic process domains at the sub-basin scale. Preliminary results show unexpected uniform CWDRs within 0.2-0.3 mm/yr, despite a strong gradient of contemporary sediment yield and high rainfall amounts. The highest variability of CWDRs within 0.2-0.6 mm/yr was found in the colluvial domain which represents the headwater domain with small basins. The variability can be explained by the episodic nature of sediment supply in small basins. Further downstream, the semialluvial and fluvial domain have identical values within a range of 0.2-0.3 mm/yr, which might be caused by the dampening effect of dense rain forest on the release of widespread, former glacial sediments. The short-term sediment yields derived from landslide surveys intwo sub-basins strongly differ from the millennial CWDRs, which points to a cyclicity in sediment supply from small sub-basins. Altogether, this work shows that great progress was made in isolating climate control on denudation over the Holocene time scale by the extended analysis of 10Be in sediment archives of various catchments.