A process-based model for production and evolution of sediment particles by physical and chemical weathering in mountain catchments

Landscapes evolve through interactions between subsurface processes that move and deform bedrock, and surface processes that redistribute mass through erosion, transport, and deposition of sediment. Sediment is composed of discrete particles that are produced from bedrock and modified during transport by physical and chemical weathering. Sediment particle attributes, including size, angularity, and durability, therefore depend on the climatic, tectonic, and lithologic factors that regulate weathering processes. These attributes, in turn, influence rates and modes of sediment transport, and the tools and cover effects that control rates of river incision into bedrock. Thus the production of sediment helps set the slopes of river channels and the relief structure of landscapes, making it central to the feedbacks between tectonics, climate, and erosion that create topography. Despite their importance, sediment particles are rarely included explicitly in landscape evolution modeling due to gaps in understanding of sediment production on hillslopes, the particle evolution that occurs on hillslopes and in channels, and the implications of sediment attributes for river incision into bedrock. Although these processes have been studied in isolation, they have not been combined together in a comprehensive model of the role of sediment in climate-tectonic-erosion feedbacks. 

Here we present results from a new, spatially-explicit model that predicts the evolution of individual particle attributes, including size, angularity, and durability. The model also predicts the resulting distributions of particle attributes as sediment from different sources is mixed, and as particles evolve during transport through catchments. The model has two components. The first predicts the initial particle attributes as sediments are produced from bedrock on hillslopes. The initial particle size distribution depends on the spacing of fractures and sizes of mineral grains in crystalline rocks, and on the spacing of bedding planes and the size of cemented particles in clastic sedimentary rocks. Initial size, as well as particle angularity and durability, are also influenced by chemical weathering, which depends on the fraction of soluble minerals, the local climate (parameterized as mean temperature and precipitation), and the residence time of bedrock as it is exhumed through the hillslope weathering engine.

The second model component quantifies how particles change as they are transported across hillslopes and through channel networks. Particle sizes are reduced by abrasion as a function of three factors: the potential energy lost in transport; particle angularity; and particle durability, which depends on initial rock tensile strength and subsequent loss of strength due to chemical weathering. Mass lost from abrasion of coarse particles is converted to sand and silt. Particles become less angular as a function of cumulative mass loss. However, high rates of energy loss on steep slopes cause fragmentation, which creates new coarse particles and resets particle angularity. Model relationships are parameterized using published data as well as newly acquired data from laboratory experiments and field studies in the Sierra Nevada, California. We couple the model with the saltation abrasion/bedrock river incision model to simulate evolution of river longitudinal profiles, and explore potential feedbacks between rock uplift, climate, and sediment production.