Kristina Hippe

Debris-flow–dependent variation of cosmogenically derived catchment-wide denudation rates

Catchment-wide denudation rates (CWDRs) obtained from cosmogenic nuclides are an efficient way to determine geomorphic processes quantitatively in alpine mountain ranges over Holocene time scales. These rate estimations assume steady geomorphic processes. Here we use a time series (3 yr) in the Aare catchment (central Swiss Alps) to test the impact of spatially heterogeneous stochastic sediment supply on CWDRs. Our results show that low-frequency, high-magnitude debris-flow events significantly perturb cosmogenic nuclide ( 10 Be, 14 C) concentrations and thus CWDRs. The 10 Be concentrations decrease by a factor of two following debris-flow events, resulting in a doubling of inferred CWDRs. The variability indicates a clear time and source dependency on sediment supply, with restricted area-weighted mixing of sediment. Accordingly, in transient environments, it is critical to have an understanding of the history of geomorphic processes to derive meaningful CWDRs. We hypothesize that the size of debris flows, their connectivity with the trunk stream, and the ability of the system to sufficiently mix sediment from low- and high-order catchments control the magnitude of CWDR perturbations. We also determined in situ 14 C in a few samples. In conjunction with 10 Be, these data suggest partial storage for colluvium of a few thousand years within the catchment prior to debris-flow initiation.

Constant denudation rates in a high alpine catchment for the last 6 kyrs

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 10Be 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 three-year time series (n = 4) of 10Be measured in sediment from the active stream currently feeding this fan delta. The temporal trend in the 10Be concentrations after correction for postdepositional production of 10Be was found to be overall constant and in good agreement with the modern river 10Be concentration. The calculated mean catchment-wide denudation rate amounts to 0.73 ± 0.18 mm/yr. This fairly constant level of 10Be 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 10Be signatures are mixed. Therefore, presumable variations in the 10Be signals derived from changes in denudation under a fluctuating Holocene climate are only poorly transferred to the catchment outlet and not recorded in the 10Be record. However, despite the absence of high frequency signals, we propose that the buffered and averaged 10Be 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 10Be signal exported by some alpine settings that needs to be taken into account and further investigated. This article is protected by copyright. All rights reserved.

Geomorphology and Age of Large Rock Avalanches in Trentino (Italy): Castelpietra

Within a project aimed at understanding past catastrophic rock slope failure in the Trentino Province of Italy, we studied the Castelpietra landslide. Castelpietra encompasses a main blocky deposit, with an area of 1.2 km2, which is buried on the upper side by more recent rockfall debris. The release area is the Cengio Rosso rock wall, which is comprised of Dolomia Principale and overlying Calcari Grigi Group dolomitized limestones. 36Cl exposure dates from two boulders in the main blocky deposit indicate that the landslide occurred at 1060 ± 270 AD (950 ± 270 yr ago). The close coincidence in time of the Castelpietra event with several events that lie within a maximum distance of 20 km, including Kas at Marroche di Dro, Pra da Lago and Varini (at Lavini di Marco) landslides, strongly suggests a seismic trigger. Based on historical seismicity compilations, we have identified the “Middle Adige Earthquake” at 1046 AD as the most likely candidate. Its epicenter lies right in the middle of the spatial distribution of the discussed landslides.

Fluvial dynamics and 14C-10Be disequilibrium on the Bolivian Altiplano: Isotopic disequilibrium in fluvial landscapes

Determining sediment transfer times is key to understanding source‐to‐sink dynamics and the transmission of environmental signals through the fluvial system. Previous work on the Bolivian Altiplano applied the in situ cosmogenic 14C‐10Be‐chronometer to river sands and proposed sediment storage times of ~10–20 kyr in four catchments southeast of Lake Titicaca. However, the fidelity of those results hinges upon isotopic steady‐state within sediment supplied from the source area. With the aim of independently quantifying sediment storage times and testing the 14C‐10Be steady‐state assumption, we dated sediment storage units within one of the previously investigated catchments using radiocarbon dating, cosmogenic 10Be‐26Al isochron burial dating, and 10Be‐26Al depth‐profile dating. Palaeosurfaces appear to preserve remnants of a former fluvial system, which has undergone drainage reversal, reduction in catchment area, and local isostatic uplift since ~2.8 Ma. From alluvium mantling the palaeosurfaces we gained a deposition age of ~580 ka, while lower down fluvial terraces yielded ≤34 ka, and floodplains ~3–1 ka. Owing to restricted channel connectivity with the terraces and palaeosurfaces, the main source of channel sediment is via reworking of the late Holocene floodplain. Yet modelling a set of feasible scenarios reveals that floodplain storage and burial depth are incompatible with the 14C‐10Be disequilibrium measured in the channel. Instead we propose that the 14C‐10Be offset results from: (i) non‐uniform erosion whereby deep gullies supply hillslope‐derived debris; and/or (ii) holocene landscape transience associated with climate or human impact. The reliability of the 14C‐10Be chronometer vitally depends upon careful evaluation of sources of isotopic disequilibrium in a wide range of depositional and erosional landforms in the landscape.

Postglacial to Holocene landscape evolution and process rates in steep alpine catchments: Postglacial to Holocene alpine landscape evolution and process rates

Climate change and high magnitude mass wasting events pose adverse societal effects and hazards, especially in alpine regions. Quantification of such geomorphic processes and their rates is therefore critical but is often hampered by the lack of appropriate techniques and the various spatiotemporal scales involved in these studies. Here we exploit both in situ cosmogenic beryllium-10 (Be) and carbon-14 (C) nuclide concentrations for deducing exposure ages and tracing of sediment through small alpine debris flow catchments in central Switzerland. The sediment cascade and modern processes we track from the source areas, through debris flow torrents to their final export out into sink regions with cosmogenic nuclides over an unprecedented five-year time series with seasonal resolution. Data from a seismic survey and a 90m core revealed a glacially overdeepened basin, filled with glacial and paraglacial sediments. Surface exposure dating of fan boulders and radiocarbon ages constrain the valley fill from the last deglaciation until the Holocene and show that most of the fan existed in early Holocene times already. Current fan processes are controlled by episodic debris flow activity, snow (firn) and rock avalanches. Field investigations, digital elevation models (DEMs) of difference and geomorphic analysis agree with sediment fingerprinting with cosmogenic nuclides, highlighting that the bulk of material exported today at the outlet of the subcatchments derives from the lower fans. Cosmogenic nuclide concentrations steadily decrease from headwater sources to distal fan channels due to the incorporation of material with lower nuclide concentrations. Further downstream the admixture of sediment from catchments with less frequent debris flow activity can dilute the cosmogenic nuclide signals from debris flow dominated catchments but may also reach thresholds where buffering is limited. Consequently, careful assessment of boundary conditions and driving forces is required when apparent denudation rates derived from cosmogenic nuclide analysis are upscaled to larger regions.