Storage and weathering of landslide debris in the eastern San Gabriel Mountains, California, USA: Implications for mountain solute flux

Abstract

AbstractThe weathering of silicate minerals in mountain landscapes provides a critical source of chemical solutes in the global biogeochemical cycles that sustain life on Earth. Observations from across Earth's surface indicate that the greatest flux of chemical solute is derived from rapidly eroding landscapes, where landsliding often limits the development of a continuous soil cover. In this study, we evaluate how weathering of landslide debris deposits may supplement the chemical solute flux from rapidly eroding, bedrock‐dominated landscapes. We present new measurements of depositional surface and soil morphology, soil geochemistry, and luminescence‐based depositional ages from debris stored in Cow Canyon, a tributary to the East Fork of the San Gabriel River in the eastern San Gabriel Mountains of California. Cow Canyon deposits include locally derived debris emplaced by dry colluvial and debris flow processes. Deposits have planar, low‐angle, sloping surfaces with soils exhibiting a greater degree of weathering than nearby soils formed on bedrock. A ~33–40 ka depositional age of Cow Canyon deposits exceeds the estimated recurrence time for the largest landslides in the San Gabriel Mountains, suggesting the stored landslide debris may be a persistent source of chemical solute in this landscape. To quantitatively explore the significance of landslide debris on the landscape solute flux, we predict the flux of chemical solute from bedrock and debris soils using a generic, time‐dependent model of soil mineral weathering. Our modeling illustrates that debris soils may be a primary source of chemical solute for a narrow range of conditions delimited by the initial landslide debris porosity and the comparative soil age. Broadly, we conclude that while landslide debris may be an important local reservoir of chemical solute, it is unlikely to dominate the long‐term solute flux from rapidly eroding, bedrock‐dominated landscapes. © 2018 John Wiley & Sons, Ltd.