Author's personal copy Active erosion-deposition cycles in the hyperarid Atacama Desert of Northern Chile

Be 21 Ne a b s t r a c t There is significant debate over the rates and types of fluvial activity at the Plio-Pleistocene boundary in the hyperarid Atacama Desert of Chile. To quantify fluvial processes and help resolve this debate, we measure terrestrial cosmogenic nuclide (TCN) ( 10 Be and 21 Ne) concentration depth profiles in three settings representing a chronosequence: (1) a late Pliocene alluvial fan representative of major regional deposits, (2) a modern, active channel and (3) an adjacent low terrace inset into the Pliocene alluvium. Late Pliocene deposits that are widely preserved in the region contain TCN profiles consistent with relatively rapid stripping of upland sediment at the Plio-Pleistocene boundary. Deposits inset into these Late Pliocene features record cut and fill cycles that rework sediment throughout the Quaternary. The TCN profile in the modern channel is best explained by sediment aggradation at 2.1 m Myr −1 during the last 250,000 yr. Similarly, the adjacent low terrace sediments contain TCN concentration profiles consistent with aggradation of 2.0 m Myr −1 over a period of 250,000-750,000 yr prior to the last 250,000 yr of stability. In summary, depth profiles of two TCNs provide constraints on the rates of sediment deposition, sources of sediment and transport history, as well as the subsequent exposure conditions of the sediment following deposition. Our results are consistent with early Quaternary initiation of hyperaridity for the region. During the Quaternary, winter precipitation events experienced at our sites' latitude (241S) drive active erosion-deposition cycles. The northward migration of the subtropical front during Quaternary glacial cycles may have enhanced precipitation at 241S, leading to more active fluvial processes during cooler periods