Effects of deep‐seated versus shallow hillslope processes on cosmogenic ¹⁰Be concentrations in fluvial sand and gravel

Abstract

Terrestrial cosmogenic nuclide (TCN) concentrations in fluvial sediment, from which denudation rates are commonly inferred, can be affected by hillslope processes. TCN concentrations in gravel and sand may differ if localized, deep‐excavation processes (e.g. landslides, debris flows) affect the contributing catchment, whereas the TCN concentrations of sand and gravel tend to be more similar when diffusional processes like soil creep and sheetwash are dominant. To date, however, no study has systematically compared TCN concentrations in different detrital grain‐size fractions with a detailed inventory of hillslope processes from the entire catchment. Here we compare concentrations of the TCN 10Be in 20 detrital sand samples from the Quebrada del Toro (southern Central Andes, Argentina) to a hillslope‐process inventory from each contributing catchment. Our comparison reveals a shift from low‐slope gullying and scree production in slowly denuding, low‐slope areas to steep‐slope gullying and landsliding in fast‐denuding, steep areas. To investigate whether the nature of hillslope processes (locally excavating or more uniformly denuding) may be reflected in a comparison of the 10Be concentrations of sand and gravel, we define the normalized sand‐gravel index (NSGI) as the 10Be‐concentration difference between sand and gravel divided by their summed concentrations. We find a positive, linear relationship between the NSGI and median slope, such that our NSGI values broadly reflect the shift in hillslope processes from low‐slope gullying and scree production to steep‐slope gullying and landsliding. Higher NSGI values characterize regions affected by steep‐slope gullying or landsliding. We relate the large scatter in the relationship, which is exhibited particularly in low‐slope areas, to reduced hillslope‐channel connectivity and associated transient sediment storage within those catchments. While high NSGI values in well‐connected catchments are a reliable signal of deep‐excavation processes, hillslope excavation processes may not be reliably recorded by NSGI values where sediment experiences transient storage. © 2018 John Wiley & Sons, Ltd.

Applications of SAR Interferometric Coherence Time Series: Spatiotemporal Dynamics of Geomorphic Transitions in the South‐Central Andes

“…We interpret this as a proxy for the pattern of long‐term rates of tectonic and erosional activity along the profile of the Quebrada del Toro. Longer‐term geomorphic activity, therefore, is highest in the lower Quebrada del Toro where precipitation and millennial erosion rates are also high (Bookhagen & Strecker, ; Tofelde et al, ), moderate in the middle reaches of the Quebrada del Toro (40–110 km), where storms still penetrate into the basin, but overall precipitation has decreased, and lowest in the upper Quebrada del Toro, above the knickpoint at 110 km, where the topography has not adjusted to the conditions below the knickpoint.…”

“…Positive relationship between millennial denudation rates measured from terrestrial cosmogenic 10‐Beryllium (Bookhagen & Strecker, ; Tofelde et al, ) and seasonal landscape stability measured from coherence range within each basin. Basins are colored by mean hillslope gradient within each basin (m/m), demonstrating a relationship between short‐term landscape stability and long‐term erosion that does not depend on basin morphology.…”

“…The Quebrada del Toro in particular serves as a representative example of the climatic and topographic gradients found throughout the south‐central Andes (e.g., Bookhagen & Strecker, , ; Castino et al, , ; Purinton & Bookhagen, ; Tofelde et al, , ). The lower Quebrada del Toro is typified by a steep gorge with relatively high precipitation, moderate vegetation, and a steep hillslope gradient (Figure ).…”

“…Recent developments in geochemical methods, for example, meteoric and in situ cosmogenic nuclides (Carretier et al, 2015;Reusser & Bierman, 2010;Tofelde et al, 2018), have made advances in quantifying the relative contributions of different transport domains, but the methodology involved (e.g., measurements on multiple grain sizes) is costly, labor-intensive, and cannot easily be applied to a wide area. Another method to quantify the contributions of each transport domain to the total erosional budget of a system is to map the transition between transport domains either through field observations (e.g., Montgomery & Dietrich, 1989) or through the analysis of digital topography (e.g., Band, 1986;Clubb et al, 2014;Dietrich et al, 1993;Grieve et al, 2016;Tarolli & Dalla Fontana, 2009).…”

Lithology and orographic precipitation control river incision in the tropical Andes

“…Thus, CRN-derived erosion rates might both overestimate and underestimate long term incision rates in these catchments. Overestimation results from the occurrence of recent, deep-seated landslide events, that deliver sediments with low CRN concentration to rivers (Tofelde et al, 2018). Underestimation might occur if long-term hillslope lowering is accomplished by landslides characterized by the occurrence of rare, large events with a return period exceeding the integration time of CRN-derived erosion rates, (Niemi et al, 2005;Yanites and Tucker, 2010).…”