Amount and Depositional Fate of Carbon Mobilized by Landsliding in SE Alaska

Forest disturbances in the form of landslides mobilize carbon (C) sequestered in vegetation and soils. The mobilized C has two basic depositional fates, deposition onto hillslopes or into water, which sequester C from and release C to the atmosphere at different time scales. The C-dense old-growth temperate forests of SE Alaska are a unique location to quantify the C mobilization rate by frequent landslide events. In this study, we estimate the amount of C mobilized by debris flows over historic time scales by combining a landslide inventory with maps of modeled biomass and soil carbon. We then infer depositional fate over geologic time scales via simulated debris flow deposition modelling with DFLOWZ, calibrated to the study area. In August 2015, a single storm initiated 66 debris flows near Sitka, AK and mobilized 57,651 ± 3,266 tC, while historic storms over a 55-year period in SE Alaska mobilized a total of 4.69 ± 0.21 MtC. Approximately 21% of historic debris flows intersected the stream network, which was consistent with long-term modeled connectivity and suggests that debris flows likely contribute to measured high dissolved organic C in streams and globally significant amounts of C buried in local fjord sediments. Moderate landslide frequency in areas with high C density mobilized C at the highest rates. Our results are consistent with an emerging consensus that disturbances which mobilize organic carbon may play an important role in the global carbon cycle over geologic time, in addition to geochemical processes such as silicate weathering