Post-seismic evolution of organic carbon mobilised by landsliding

In mountain ranges, widespread landsliding triggered by earthquakes can mobilise large amounts of organic carbon by stripping soil vegetation and bedrock from hillslopes. During the post-seismic period, the organic carbon is either transported out of the river catchment by physical sediment transport, stored transiently in landslide deposits, or degraded by heterotrophic respiration when it is not directly available for transport. However, the relative importance of these processes on post-seismic organic carbon evolution, and their implications for the time scales of carbon transfer, remains to be better understood. Here, we present a statistical analysis to quantify the post-seismic redistribution of organic carbon in atmospheric and offshore reservoirs over decadal to centennial timescales. This approach combines empirical observations informed predictions of co-seismic sediment mobilisation, with descriptions of the physical processes involved during the post-seismic phase. Earthquakes-triggered landslide populations are generated by randomly sampling characteristic landslide area distributions, and a proportion of the landslides are initially connected to the fluvial network. The remaining part of the population is transferred to the river network at a velocity controlled by the distance of the landslide to the network. The landslide connectivity status controls the organic carbon export process: disconnected landslides lose organic carbon by oxidation in the landscape, while connected ones see their mass of organic carbon synchronously decreasing by both oxidation and physical export. This statistical analysis is numerically efficient and allows us to explore a wide range of parameters that exert a control on organic carbon redistribution. Amongst them, we focus in particular on the influence of climatic context (mean annual runoff and runoff variability), tectonic context (earthquake magnitude), and river transport capacity on the residence time of organic carbon. Our results show that contexts that promote the efficient physical export of sediments, 40% of the organic carbon is oxidised whereas in less advantageous configurations, this value can reach up to 90%.