Landslides generally occur on steep hillslopes, when gravity overcomes the ability of rocks and soil to hold together. Periods of intense rain, vegetation loss and earthquakes are just some of the factors that can make landslides more likely. “Landslides are among the most hazardous geological phenomena,” explains LandFlux project coordinator Duna Roda-Boluda, a postdoctoral researcher at the German Research Centre for Geosciences. “Up to 20 000 km of Europe’s roads and railways are threatened. Climate change, and human impact on landscapes, will make landslides more likely in the future.” Despite this danger, our knowledge of how often landslides occur, and how their rates of recurrence have fluctuated in the past, remains limited. Physical records are often removed through erosion and revegetation, and aerial imagery only allows us to track landslide activity over the past few years.
A long-term perspective
The LandFlux project, which was undertaken with the support of the Marie Skłodowska-Curie Actions programme, was launched with the aim of addressing this knowledge gap. “I wanted to estimate landslide activity over timescales of centuries and millennia,” says Roda-Boluda. “This will provide much-needed context to assess shorter-term records of landslide activity.” To achieve this, Roda-Boluda sampled recent landslide deposits under 20 years old, at the bottom of hillslopes in New Zealand. Concentrations of two cosmogenic nuclides found in quartz – 10Be and 14C – were then measured. Measuring 10Be is a well-established technique for estimating how long the rock or sediment has been exposed at the surface. Finding the concentration of 10Be gave the project team an idea of the time span between the sampled landslide, and the previous one. Measuring 14C content was carried out to see if 14C/10Be ratios increase with the erosional depth of landslides. “Sediment derived from landslides is sourced from deeper in the bedrock, while sediment derived from other processes is primarily sourced from the rock surface,” adds Roda-Boluda. “So high values of the 14C/10Be ratio could potentially constitute a new tracer for landslide-derived sediment.”
Measuring landslide recurrence
The application of these techniques enabled Roda-Boluda to obtain, for the first time, direct estimates of landslide recurrence intervals on a timescale of thousands of years. “Our recurrence estimates range between 1 000 and 6 000 years,” she says. “As expected, intervals are longer for larger landslides and shorter for smaller ones.” Also as predicted, the project found that 14C/10Be ratios correlate with landslide depth. “This means that these nuclides could potentially be used to identify deep-sourced material, and hence fingerprint landslide-derived sediment,” explains Roda-Boluda. “However, the extraction of inorganic 14C from sediment is challenging and can only be done in a few laboratories around the world. While this technique is still in development, our results highlight the potential of this new technique.” The project team is currently measuring 14C/10Be ratios in river sediment, on the hypothesis that regions which have more landslide activity should have higher 14C/10Be ratios. If true, this could provide scientists with a tool to examine how landslide activity has fluctuated since the Ice Age, or in periods of known earthquakes. “I would certainly hope to see our approach for estimating long-term landslide recurrence intervals, and for identifying landslide-derived sediment, used routinely in future studies,” adds Roda-Boluda. “This could allow us to track if, and how much, landslide activity is changing in regions that are experiencing intensive land-use changes, or where severe droughts are followed by storms and flooding.”
LandFlux, landslide, sediment, climate, flooding, droughts, quartz, cosmogenic nuclides