Fluvial dynamics and 14 C‐ 10 Be disequilibrium on the Bolivian Altiplano

Determining sediment transfer times is key to understanding source‐to‐sink dynamics and the transmission of environmental signals through the fluvial system. Previous work on the Bolivian Altiplano applied the in situ cosmogenic 14 C‐ 10 Be‐chronometer to river sands and proposed sediment storage times of ~10–20 kyr in four catchments southeast of Lake Titicaca. However, the fidelity of those results hinges upon isotopic steady‐state within sediment supplied from the source area. With the aim of independently quantifying sediment storage times and testing the 14 C‐ 10 Be steady‐state assumption, we dated sediment storage units within one of the previously investigated catchments using radiocarbon dating, cosmogenic 10 Be‐ 26 Al isochron burial dating, and 10 Be‐ 26 Al depth‐profile dating. Palaeosurfaces appear to preserve remnants of a former fluvial system, which has undergone drainage reversal, reduction in catchment area, and local isostatic uplift since ~2.8 Ma. From alluvium mantling the palaeosurfaces we gained a deposition age of ~580 ka, while lower down fluvial terraces yielded ≤34 ka, and floodplains ~3–1 ka. Owing to restricted channel connectivity with the terraces and palaeosurfaces, the main source of channel sediment is via reworking of the late Holocene floodplain. Yet modelling a set of feasible scenarios reveals that floodplain storage and burial depth are incompatible with the 14 C‐ 10 Be disequilibrium measured in the channel. Instead we propose that the 14 C‐ 10 Be offset results from: (i) non‐uniform erosion whereby deep gullies supply hillslope‐derived debris; and/or (ii) holocene landscape transience associated with climate or human impact. The reliability of the 14 C‐ 10 Be chronometer vitally depends upon careful evaluation of sources of isotopic disequilibrium in a wide range of depositional and erosional landforms in the landscape. © 2018 John Wiley & Sons, Ltd.