Quantifying sediment discharge from the Bolivian Andes into the Beni foreland basin from cosmogenic 10Be-derived denudation rates

Hella Wittmann,Jean-Loup Guyot,Peter Kubik,Laurence Maurice,Friedhelm Von Blanckenburg

doi:10.25249/0375-7536.2011414629641

Abstract

Enormous volumes of sediment are produced in the Central Andes and are then delivered into the foreland basins of large tributaries to the Amazon basin. While cosmogenic nuclides in sediment are a suitable tool to measure the denudation rates of sediment-producing areas, the requirement of steady state between nuclide production and nuclide removal by denudation appears to make this method less obvious in depositional foreland basins, where sediment storage may alter 10 Be-based erosion signals from the sediment-providing areas. A published cosmogenic nuclide-based modeling approach however predicts that source-area cosmogenic nuclide concentrations are not modified by temporary sediment storage. We tested this approach in the large Beni foreland basin by measuring cosmogenic 10 Be nuclide concentrations in detrital sediment along a 600 km long floodplain reach. The outcome of our study is that t he 10 Be-based denudation rate signal of the Bolivian Andes is preserved in the Beni floodplain even though this basin stores the sediment for thousands of years. For the floodplain part of the Beni basin, the cosmogenic nuclide-derived denudation rate is 0.45 ± 0.07 mm/yr, and the respective Andean source area erodes at a very similar rate of 0.37 ± 0.06 mm/yr. We thus suggest that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area. This finding opens the unique possibility of constraining paleo-sediment budgets for these large basins using cosmogenic nuclides as the denudation rate signal of the sediment-producing area is preserved in sedimentary archives.

Highlights

Cosmogenic nuclides in detrital material, especially 10Beryllium (Be), have been used over the past decade to determine catchment-wide mountain erosion and weathering rates (Bierman& Nichols 2004, Granger & Riebe 2007, von Blanckenburg 2005)
This study shows that denudation rates measured from cosmogenic nuclide concentrations in the floodplain of the Beni basin (Bolivia) are, within a variability expected of high-relief areas that tend to erode stochastically, identical to those obtained from the sediment-providing Bolivian Andes
Cosmogenic nuclide concentrations in the floodplain are not altered by storage of sediment that is longer than the integration times of the cosmogenic nuclide method

Summary

Introduction

Cosmogenic nuclides in detrital material, especially 10Beryllium (Be), have been used over the past decade to determine catchment-wide mountain erosion and weathering rates (Bierman& Nichols 2004, Granger & Riebe 2007, von Blanckenburg 2005). We will test whether this method can be used to infer basin-wide denudation rates from sediment routed through large depositional settings (Bierman & Steig 1996, Nichols et al 2005, Nichols et al 2002). In such basins, sediment is continuously deposited in the floodplain and reincorporated in the active fluvial system by bank erosion. The cosmogenic nuclide signal of the high Bolivian Andes is well established by a large dataset from Safran et al (2005) For this area, fission track-derived long-term rates of erosion are available (Safran et al 2006). We will show here that it is possible to calculate meaningful catchmentwide denudation rates for the hinterland in large depositional basins, and we do so by providing new cosmogenic nuclide concentrations from Beni River floodplain sediment

Methods

Discussion

Conclusion