Legacy Sediments As Novel Soil Profiles.

Legacy sediments are a widespread consequence of post-colonial upland erosion in the United States, and are ubiquitous in the lowlands of the southeastern Piedmont. While these deposits have received attention for their impact on fluvial dynamics and water quality, they have not been studied in the context of pedogenesis and soil formation. In this study, we characterized legacy sediment in order to understand how pedogenic processes are overprinting these deposits within a particular 40-hectare floodplain in South Carolina. Radioisotope analysis of Pb-210 and C-14 constrained the timing of deposition on mineral soil and buried charcoal samples. Results show that a regenerating hardwood forest has driven the development of strong depth-dependent gradients of soil carbon and nitrogen. In one hundred years of forest regeneration, vegetation and fluctuating redox conditions have allowed these sediment deposits to evolve as soil profiles both texturally and compositionally. Given the estimated millennia it will take to erode legacy sediment from Piedmont floodplains, it is important to think of these deposits as new stable environments on their own trajectory of soil evolution.