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Quantifying landslide activity and contribution to sediment fluxes with cosmogenic radionuclides and grain-size distributions

Periodic Reporting for period 1 - LandFlux (Quantifying landslide activity and contribution to sediment fluxes with cosmogenic radionuclides and grain-size distributions)

Berichtszeitraum: 2018-06-01 bis 2020-05-31

Landslides are a primary erosion process in steep landscapes, one of the most sensitive surface process to tectonic and climatic perturbations, and are among our most deadly and damaging geohazards. However, it is extremely difficult to constrain long-term or past rates of landslide activity, and hence landslide-derived sediment generation, because the physical records of landsliding are often removed in <10^2 yrs. This limited record prevents accurate predictions of their activity in the face of climate change, and hinders landscape evolution studies. The LandFlux project aim was to develop and apply a new methodology to quantify long term landslide activity and its contribution to sediment fluxes. The main scientific objectives of the project were to test if Cosmogenic Radionuclide (CRN) concentrations from landslide deposits and fluvial sediment could be used to reconstruct landslide frequencies on >10^2 yr timescales, and whether these concentrations were representative of the erosional depths (i.e. landslide depths) and the intensity of landslide activity. We focused on measuring the concentrations of two in-situ produced CRNs in quartz: 10Be, a very well-established CRN; and 14C, a CRN whose applications are still under development due to the challenges of its extraction from non-organic materials, and the scarce availability of production-rate estimates. The combination of these two nuclides was ideal for addressing our project objectives because these nuclides have different production mechanisms, which means that 14C/10Be ratios are expected to be sensitive to depth.
We collected samples from recent (<30 yr) landslide deposits and from fluvial sediment of the Fiordland and Southern Alps of New Zealand, exploiting their known order-of-magnitude differences in exhumation rates and landslide frequencies. We have analysed 10Be concentrations from 31 catchments and 17 landslides, and 14C concentrations from 9 landslides. For each of the sampled landslides, we collected drone imagery to build digital elevation models of the landslide scars, and measured the grain-size distributions of the landslide deposits. The main findings of the project are: (a) we show, for the first time, that 10Be concentrations in landslide deposits can be used to estimate landslide recurrence intervals and frequency on >10^2 yr timescales, when combined with detailed DEMs of the landslide scars built from photogrammetry; (b) we have produced the first empirical data set proving that 14C/10Be ratios increase with landslide depth, and hence can be used to trace erosional depth; and (c) we demonstrate that catchment-averaged erosion rates on the Southern Alps over 10^2 yr timescales span an order of magnitude, from 0.8 to 9 mm/yr, and that the variability among these rates is best explained by the proportion of the catchment area at elevations of 1500 to 2000 m. Hence, the project has successfully achieved its main scientific objectives.
This project contributes to opening new possibilities to quantify landslide frequency and sediment generation from landslides on >10^2 yr timescales, which complements the more common aerial imagery-based landslide studies that capture landslide frequency only over 10^1 yr timescales. This new application of CRNs will facilitate future projects on landscape evolution and landslide hazard assessments. Additionally, the LandFlux project contributes to the development and promotion of in-situ 14C as a useful CRN for geomorphological studies, given that it represents one of the first and largest data sets of in-situ 14C applications. This project also represents the first comprehensive data set on catchment-averaged erosion rate estimates over 10^2-10^3 yr timescales for the Southern Island of New Zealand, which will enable future landscape evolution studies in an area that is of particular interest given its very active tectonic setting and glaciation history.
Recent landslides (rockfalls) on the walls of Fox glacier valley, New Zealand