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Soils are the largest terrestrial pool of organic carbon and can potentially be used to sequester atmospheric carbon. In the western US, SOC stocks in mountainous terrain are strongly modulated by the thickness and properties of the critical zone. Although most studies focus on SOC storage in the top 1m, deep SOC stocks are substantial where soils are highly weathered. For example, slow or non-eroding, low-gradient landforms, such as deep-seated landslides, develop thick soils, where chemical and physical weathering break down and alter mineral grains and create a large accommodation space for SOC. Critical zone thickness increases with soil age, resulting in a greater degree of weathering and production of pedogenic minerals, which can increase mineral reactivity and carbon stabilization. In western Oregon, deep-seated landslide deposits exhibit thick, highly weathered critical zones and make up at …