Legacy sediment: Definitions and processes of episodically produced anthropogenic sediment

Extensive anthropogenic terrestrial sedimentary deposits are well recognized in the geologic literature and are increasingly being referred to as legacy sediment (LS). Definitions of LS are reviewed and a broad but explicit definition is recommended based on episodically produced anthropogenic sediment. The phrase is being used in a variety of ways, but primarily in North America to describe post-settlement alluvium overlying older surfaces. The role of humans may be implied by current usage, but this is not always clear. The definition of LS should include alluvium and colluvium resulting to a substantial degree from a range of human-induced disturbances; e.g., vegetation clearance, logging, agriculture, mining, grazing, or urbanization. Moreover, LS should apply to sediment resulting from anthropogenic episodes on other continents and to sediment deposited by earlier episodes of human activities.Given a broad definition of LS, various types of LS deposits are described followed by a qualitative description of processes governing deposition, preservation, and recruitment. LS is deposited and preserved where sediment delivery (DS) exceeds sediment transport capacity (TC). This can be expressed as a storage potential ratio that varies within and between basins and through time. When DS/TC < 1, recruitment and transport of LS dominate, but if DS/TC > 1, deposition and preservation are likely. When DS/TC ≫ 1, abundant deposition and graded deposits are likely even without barriers or sinks. Thus, spatial patterns of LS deposits may reveal information about past land-use history and hydrodynamics in a catchment.