Long-term dispersion of river gravel in a canyon in the Atacama Desert, Central Andes, deduced from their 10Be concentrations

Abstract

Intense storms or earthquakes in mountains can supply large amounts of gravel to rivers. Gravel clasts then travel at different rates, with periods of storage and periods of displacement leading to their downstream dispersion over millennia. The rate of this dispersion controls the long-term downcutting rate in mountainous rivers as well as the grain-size signature of climate and tectonic variations in sedimentary basins. Yet, the millennial dispersion rates of gravel are poorly known. Here, we use 10Be concentrations measured in individual pebbles from a localized source along a 56 km-long canyon in the Central Andes to document the distribution of long-term gravel transit rates. We show that an inverse grain-size velocity relationship previously established from short-term tracer gravel in different rivers worldwide can be extrapolated to the long-term transit rates in the Aroma River, suggesting some universality of this relationship. Gravel are also dispersed by large differences in the mean transport rates independent of gravel size, highlighting that some gravel rest at the river surface over tens of thousands of years. These different transport rates imply a strong spreading of the gravel plumes, providing direct proof for the long-term river buffering of sediment signals between mountainous sources and sedimentary basins. The inferred distribution of residence times suggests the first evidence of anomalous diffusion in gravel transport over long timespans.

Times Associated With Source-to-Sink Propagation of Environmental Signals During Landscape Transience

“…This conceptual figure represents a summary of experimental and numerical works (see text for references). measured in sand-and silt-sized quartz grains (Schaller et al, 2004;Val et al, 2016;Lenard et al, 2020), although comparisons of the same cosmogenic nuclide in different grain size fractions at the same location have revealed great variability between grain size fractions (Puchol et al, 2014;Carretier et al, 2015Carretier et al, , 2019Schildgen et al, 2016;Tofelde et al, 2018;van Dongen et al, 2019). Finally, fluvially transported organic compounds, for example leaf waxes from terrestrial plants, are regularly analyzed for their hydrogen (δD) and carbon (δ 13 C) isotope composition (Galy and Eglinton, 2011;Garcin et al, 2012;Sachse et al, 2012;Schefuß et al, 2016;Diefendorf and Freimuth, 2017).…”

“…The dominant transport mode of a distinct hydraulic grain size fraction exerts a major control on sediment transport times along SRSs. Sediment transport times, in turn, have direct implications for signal propagation and modification within SRSs (Chabaux et al, 2012;Carretier et al, 2019Carretier et al, , 2020Watkins et al, 2020). Therefore, we propose to investigate signal propagation not for bulk sediments, but for hydraulic grain size fractions individually.…”

“…Rather than measuring the time during which the sediment has been stored, Carretier and Regard (2011) demonstrated that the concentration in terrestrial cosmogenic nuclides of boulders can be used to determine the rate of transport of a grain, when abrasion is limited. Building on this approach, Carretier et al (2019) provide field evidence that sediment can be stored for a substantial amount of time (tens of thousands of years) on their way to the sedimentary basins simply by the way rivers transport sediments. A new tool for quantifying sediment transport times is being developed using subaqueous bleaching rates of optically stimulated luminescence (OSL) in quartz and feldspar.…”

Times associated with source-to-sink propagation of environmental signals during landscape transience

“…In contrast, high and low temperature detrital geo-/thermochronology is commonly analyzed on sand-sized heavy minerals, such as zircon or apatite (Weislogel et al, 2006;Heberer et al, 2011;O'Sullivan et al, 2018;Sharman et al, 2018). Paleo-denudation rates inferred from in-situ cosmogenic nuclides in detrital sediments are mostly measured in sand-and silt-sized quartz grains (Schaller et al, 2004;Val et al, 2016;Lenard et al, 2020), although comparisons of the same cosmogenic nuclide in different grain size fractions at the same location have revealed great variability between grain size fractions (Puchol et al, 2014;Carretier et al, 2015Carretier et al, , 2019Schildgen et al, 2016;Tofelde et al, 2018;van Dongen et al, 2019). Finally, fluvially transported organic compounds, for example leaf waxes from terrestrial plants, are regularly analyzed for their hydrogen (δD) and carbon (δ 13 C) isotope composition (Galy and Eglinton, 2011;Garcin et al, 2012;Sachse et al, 2012;Schefuß et al, 2016;Diefendorf and Freimuth, 2017).…”

“…The dominant transport mode of a distinct hydraulic grain size fraction exerts a major control on sediment transport times along SRSs. And sediment transport times, in turn, have direct implications for signal propagation and modification within SRSs (Chabaux et al, 2012;Carretier et al, 2019Carretier et al, , 2020Watkins et al, 2020). Therefore, we propose to investigate signal propagation not for bulk sediments, but for hydraulic grain size fractions individually.…”