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

• Published in: Earth Surface Processes and Landforms
• Main Authors: Hippe, Kristina, Gordijn, Tiemen, Hajdas, Irka, Jansen, John, Christl, Marcus, Vockenhuber, Christof, Maden, Colin, Akcar, Naki, Ivy-Ochs, Susan
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2019
• Subjects: C-14-Be-10 chronometer; CENTRAL ANDES; COSMOGENIC RADIONUCLIDES; DEBRIS-FLOW; DENUDATION RATES; GLACIAL HISTORY; HYDROLOGIC VARIATION; ICE-CORE; LAKE TITICACA; LATE PLEISTOCENE; SEDIMENT PRODUCTION; cosmogenic nuclide dating; radiocarbon dating; sediment; steady-state; storage; ice core
• Online Access: https://pure.au.dk/portal/da/publications/fluvial-dynamics-and-14c10be-disequilibrium-on-the-bolivian-altiplano(3de15aad-e4a8-48a5-8f93-c6562e993d16).html; https://doi.org/10.1002/esp.4529; http://www.scopus.com/inward/record.url?scp=85056777629&partnerID=8YFLogxK

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.