Quantifying the Cosmogenic Radionuclide Concentration in Landslide Dominated Catchments
Authors:   , Fieman Dina 
Published: October 2024

<p><strong>The measurement of cosmogenic radionuclides (CRNs) in alluvial and fluvial sediments has become one of the most popular methods for quantifying erosional processes across a wide variety of landscapes. Despite the prevalent use of CRNs for determining catchment-averaged erosion rates, deep-seated co-seismic landslides can bias the CRN concentration in detrital sediment through the input of low concentration CRN sediment in the channel. The influence of co-seismic landslide magnitude and frequency on the CRN concentration in tectonically active catchments however, is still relatively unknown. Through a combination of field measurements and numerical modelling, this thesis aims to understand the impact of landsliding on the in-situ CRN concentration in detrital sediment. In particular, this study aims to investigate how 10Be dilution subsequent a major earthquake event can provide insights into sediment transport dynamics and determine the influence of earthquake events on the CRN concentration over millennial timescales. This thesis begins by presenting the 10Be concentrations in two catchments, the Hapuku and Kowhai, that were greatly affected by the 2016 7.8 Mw Kaikoura earthquake event, New Zealand. The 10Be concentration was measured in multiple grain sizes and compared to the detailed landslide inventories and sediment fluxes calculated from the geomorphic change detection. The landslides triggered from the earthquake delivered low enough 10Be concentrations to the channel to dilute the concentration in both the sand and pebble grain sizes. The results indicated that the 10Be concentration in the sand and pebble fraction accurately reflect the suspended and bedload flux, respectively. To understand the factors controlling the 10Be dilution in each catchment, a numerical model is built to accurately calculate the CRN concentration through time with various erosional processes and landslide events. By varying the residence time and hillslope sediment contribution using the landslide distribution from the Kaikoura earthquake, the model is able to predict the measured dilution factors. The model results suggested that magnitude of dilution is controlled by the ratio of landslide to background sediment volume. As one earthquake event is able to influence the CRN concentration, the model is then utilised to explore the influence of recurrent earthquake events on the mean 10Be concentration in the channel over a millennial-scale. This result is particularly prevalent in catchments where frequent large earthquakes occur that potentially generate significantly large landslides. The results from this study indicated that regardless of the existing background erosion rate, the CRN concentration at any point in time is dictated by the production rate of CRNs, and the time from the last earthquake event. The model results highlight the importance that the catchment-averaged erosion rate derived from the CRN concentration can either underestimate or overestimate the true catchment-averaged erosion rate if landslides are the dominate driver of erosion. Hence, the CRN concentration may never accurately reflect the background erosion rate in tectonically active catchments that are frequently perturbed by mass-wasting events. Previous studies that have published catchment-averaged erosion rates derived from the CRN concentration in catchments where perturbed by landslides and other mass-wasting events may therefore need reassessment. This thesis demonstrates that deciphering catchment average erosion rates using the CRN concentration in tectonically active landscapes is complex and provides important guidance on how CRN signals may be interpreted.</strong></p>