Publication Date:
2020-06-15
Description:
Meteoric Beryllium-10 (10Bem) accumulates in soil and floodplain sediment over time, and therefore can theoretically be used to calculate river sediment transit time, provided that 1) the flux at which 10Bem is deposited onto the floodplain (Fmet) is known, and 2) grain size and hydrodynamic sorting effects can be quantified. In this study, we measured 10Bem and 9Be concentrations in the chemically extracted phase of river suspended sediment (SS) samples from the Rio Bermejo, which traverses the Andean foreland basin in NW Argentina. SS was collected at five locations from the mountain front to nearly 700 km downstream, and from different channel depths. A depth-integrated [10Bem] value was calculated for each location by Rouse modeling of SS concentrations with water depth, accounting for sorting effects. Grain size effects were detected by a strong correlation between [10Bem] and specific surface area. Normalizing to stable 9Be concentrations eliminates this surface area dependency, resulting in a 230% increase in [10Bem] from upstream to downstream, over the 665 km linear transit distance (1221 km along channel distance). This downstream increase is used to estimate the mean transit time of sediment transported downstream through the active lowland alluvial river. Measured increases in 10Bem can be converted to mean transit times if the mean depth of channel remobilization and Fmet are known. 10Bem inventories were measured in OSL-dated soil profiles from the Bermejo floodplain to estimate Fmet, and mean channel remobilization depths between 1 and 8 m were measured by DEM analysis. Resulting estimates of sediment transit time through the Rio Bermejo yield 1.4 kyr, 5.6 kyr, and 11.2 kyr for channel remobilization depths of 1 m, 4 m, and 8 m, respectively. Additionally, we find that the rate of [10Bem] increase with distance downstream changes as the river crosses three morphotectonic domains, namely the foredeep, forebulge, and backbulge of the Andean foreland basin. These changes suggest that the timescales of sediment transport and storage are influenced by foreland basin dynamics. This application of 10Bem as a sediment transit time tracer shows the utility of this method for constraining the timescales of alluvial sediment dynamics in response to different geomorphic processes.
Language:
English
Type:
info:eu-repo/semantics/conferenceObject
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