Pulsed carbon export from mountains by earthquake-triggered landslides explored in a reduced-complexity model

Abstract

Abstract. In mountain ranges, earthquakes can trigger widespread landsliding and mobilise large amounts of organic carbon by eroding soil and vegetation from hillslopes. Following a major earthquake, the landslide-mobilised organic carbon can be exported from river catchments by physical sediment transport processes, or stored within the landscape where it may be degraded by heterotrophic respiration. The competition between these physical and biogeochemical processes governs a net transfer of carbon between the atmosphere and sedimentary organic matter, yet their relative importance following a large landslide-triggering earthquake remains poorly constrained. Here, we propose a model framework to quantify the post-seismic redistribution of soil-derived organic carbon. The approach combines predictions based on empirical observations of co-seismic sediment mobilisation, with a description of the physical and biogeochemical processes involved after the earthquake. Earthquake-triggered landslide populations are generated by randomly sampling a landslide area distribution, a proportion of which is initially connected to the fluvial network. Initially disconnected landslide deposits are transported downslope and connected to rivers at a constant velocity in the post-seismic period. Disconnected landslide deposits lose organic carbon by heterotrophic oxidation, while connected deposits lose organic carbon synchronously by both oxidation and river export. The modelling approach is numerically efficient and allows us to explore a large range of parameter values that exert a control on the fate of organic carbon in the upland erosional system. We explore the role of the climatic context (in terms of mean annual runoff and runoff variability) and rates of organic matter degradation using single and multi-pool models. Our results highlight that the redistribution of organic carbon is strongly controlled by the annual runoff and the extent of landslide connection, but less so by the choice of organic matter degradation model. In the context of mountain ranges typical of the southwest Pacific region, we find that model configurations allow for more than 90 % of the landslide-mobilized carbon to be exported from mountain catchments. A simulation of earthquake cycles suggests efficient transfer of organic carbon out of a mountain range during the first decade of the post-seismic period. Pulsed erosion of organic matter by earthquake-triggered landslides therefore offers an effective process to promote carbon sequestration in sedimentary deposits over thousands of years.

Estimated Amounts and Rates of Carbon Mobilized by Landsliding in Old‐Growth Temperate Forests of SE Alaska

“…So, although C mobilization amount and rates in SE Alaska are generally low compared to areas with more extreme disturbances, the high connectivity of landslide deposits to the ocean and fjords via small mountain streams likely facilitates a high rate of carbon sequestration over geologic timescales, with valley configuration playing a role in how the material is exported. This is consistent with observations of earthquake-triggered landslides in southeast Asia, where high connectivity to streams make landslides net sink in the years following an earthquake event, as a result of efficient export of sediment to the ocean (Croissant et al, 2020). However, some C in the form of dissolved organic carbon (DOC) can be lost to the atmosphere as it transports.…”

Measuring the grain‐size distributions of mass movement deposits

“…The methodological uncertainties associated with comparing GSDs and percentiles obtained using different methods can have consequences for accurate process interpretation. For example, the factor of two difference in grain‐size percentile estimates from survey tape counts relative to sieving for a fine deposit could shift the D 50 value from suspended load to bedload, which would have implications for estimates of sediment export rates and onward transport (Croissant et al, 2021; Marc et al, 2021). Similarly, by excluding up to 20% by weight of the finest grains, all non‐sieving methods are unable to find evidence for processes where the proportion of sand and silt is influential (de Haas et al, 2015; Kaitna et al, 2016; Makris et al, 2020).…”

“…Similarly, by excluding up to 20% by weight of the finest grains, all non‐sieving methods are unable to find evidence for processes where the proportion of sand and silt is influential (de Haas et al, 2015; Kaitna et al, 2016; Makris et al, 2020). The rates and calibre of hillslope sediment supply to channels have also been used increasingly to drive landscape evolution and fluvial modelling (Attal et al, 2015; Croissant et al, 2021; Egholm et al, 2013; Roda‐Boluda et al, 2018). Given that mass movement‐derived sediment is an essential component in these problems (Sklar & Dietrich, 2006), improvements are needed in our ability to characterize this material to provide robust conclusions about the timescales and rates of bedrock incision and sediment transport.…”

Residence Time of Over‐Steepened Rock Masses in an Active Mountain Range

“…Rock masses in excess topography are mechanically unstable and prone to failure (Blöthe et al., 2015; Montgomery, 2001). Landslides from these unstable rock masses regulate landscape evolution and sediment and carbon fluxes (Croissant et al., 2021; Emberson et al., 2016; Korup, 2007; Marc et al., 2019), making excess topography a nexus between tectonics, climate, and surface processes.…”