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

Abstract

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 14C‐10Be‐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 14C‐10Be steady‐state assumption, we dated sediment storage units within one of the previously investigated catchments using radiocarbon dating, cosmogenic 10Be‐26Al isochron burial dating, and 10Be‐26Al 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 14C‐10Be disequilibrium measured in the channel. Instead we propose that the 14C‐10Be 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 14C‐10Be 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.

Cosmogenic in situ 14C-10Be reveals abrupt Late Holocene soil loss in the Andean Altiplano

“…5 ). Similarly, temporary burial of the fluvial sediment during transport can be excluded as a cause of the 14 C offset, because the floodplains are too young and too thin to provide for significant 14 C decay 51 . Notwithstanding that gullying potentially contributes deeply buried particles with low 14 C, the consistency of the 14 C- 10 Be offset (45–65 %) in both our hilltop soils and fluvial sediments suggests that a shared landscape-wide signal of transience has been transmitted from the hillslopes to the river system.…”

“…Soils stripped from hillslopes were transported via rainsplash and sheetwash into temporary storages at the base of slopes and in floodplains, and these accumulations exist along the Belén valley today. Six conventional radiocarbon dates 51 spanning 3440–2540 cal. yr BP (2 σ ) or 2110–1180 cal.…”

“…5 ). The floodplains are currently incised presumably due to the decline in sediment supply rate from hillslopes 51 . This fall in sediment supply is a scenario described by our spike model (Fig.…”

The Transport History of Alluvial Fan Sediment Inferred From Multiple Geochronometers

“…The mode at 10 Kyr may reflect a basin‐wide depositional event around 14 ka (Figure 9a) that was followed by quiescence for about 10 Kyr. Measurement of in situ 14 C and 10 Be from sand to test grain size dependence (e.g., Fülöp et al., 2020; Hippe et al., 2018; Lukens et al., 2016) or p‐IR IRSL dating of the undersides of boulders (e.g., Brill et al., 2020; Freiesleben et al., 2015) would be informative. Regardless, our results suggest that sand grains within a catchment experience multiple storage histories, while cobbles and boulders given their quasi‐normally distributed storage durations all behave similarly within the landscape prior to final deposition on the fan surface.…”

Assessing non‐steady‐state erosion processes using paired 10Be–26Al in southeastern Tibet

“…Paired analyses of 10 Be and 26 Al could help to detect the complex exposure/burial history of surfaces governed by non‐erosive glaciers (e.g., Andersen et al, 2020; Beel et al, 2016; Corbett, Bierman, & Rood, 2016), estimate the timing of sediment burials (e.g., Çiner et al, 2015; Hippe et al, 2019; McPhillips et al, 2016), and constrain the exposure/erosion history of long‐lived surfaces (e.g., Chen et al, 2018; Glasser et al, 2012; Zhou et al, 2020). A common practice is to plot 26 Al/ 10 Be ratios versus 10 Be concentrations on a two‐nuclide diagram.…”