Widespread ancient bedrock landslide deposits facilitate deep weathering for storage and access of organic carbon.

Soils have been proposed as a tool to reduce atmospheric carbon dioxide (CO2). However, sparse field data and soil databases that do not account for geomorphic controls on soil properties hinder accurate quantification of soil organic carbon (SOC) dynamics at the watershed scale. In mountainous terrain, relict landslide deposits are common and feature thick (>5 m), weathered profiles with substantial SOC below the typically measured top 30 cm of soil. We generated a SOC-age relationship using a landslide chronosequence in Western Oregon where these landforms are abundant. We applied our relationships to an inventory of nearly 10,000 previously dated landslides and show that deep SOC stocks constitute ~70% of the total stock for landslides >41.8 thousand years. Our findings also show that SOC stocks associated with deep-seated landslides are >2× larger than estimates from a global model that does not account for geomorphic controls. These results suggest that geomorphology can improve our ability to quantify SOC stocks and could inform watershed management.