Discordance between cosmogenic nuclide concentrations in amalgamated sands and individual fluvial pebbles in an arid zone catchment

Codilean, Alexandru T.; Fenton, Cassandra R.; Fabel, Derek; Bishop, Paul; Xu, Sheng

doi:10.1016/j.quageo.2012.04.007

Cited by 40 publications

(33 citation statements)

References 43 publications

(20 reference statements)

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“…Our numerical simulations using empirically derived abrasion rates agree with recent research that highlights grain size biasing as one of the factors controlling the mineralogy and, therefore, the grain information of sands transported by rivers (e.g., Aguilar et al, ; Carretier et al, ; Codilean et al, ). The importance of abrasion in distorting grain age distributions is, however, more debatable.…”

Section: Discussionsupporting

confidence: 87%

Does Pebble Abrasion Influence Detrital Age Population Statistics? A Numerical Investigation of Natural Data Sets

et al. 2018

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Pebble abrasion is a key factor controlling the release of minerals into sand, but few attempts have been made to model how it could influence the liberation of minerals into the size fraction used in detrital geochronology. We perform a series of experiments with an abrasion model to test this influence using natural and synthetic data sets. Our results demonstrate that pebble abrasion can change the zircon mixing proportions of upstream source units as well as the age distribution of mixed fluvial sands. This change is particularly significant when there is strong contrast in rock resistance within the watershed. Pebble abrasion is one of many factors that can change the mixing proportion of sands, including hillslope gravel supply, erosion rates, and mineral fertility. In our study case (Marsyandi watershed, Himalaya), the abrasion model predicts age distributions that are statistically indistinguishable from those predicted by a no‐abrasion model. However, the relative erosion rates estimated by our model largely differ from the results of a no‐abrasion model and are closer to those from other studies that suggest a strong correlation between modern erosion rates, tectonics, and precipitation intensity in the Marsyandi watershed. These findings highlight that, even in cases where there is no statistical evidence of change between the modeled age distributions, abrasion can affect the erosion rates estimated from them. Therefore, quantifying the influence of abrasion on sand production is an essential step not only to predict mixing proportions but also to accurately retrieve erosion rates from the measured grain age distributions.

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Section: Discussionsupporting

confidence: 87%

Does Pebble Abrasion Influence Detrital Age Population Statistics? A Numerical Investigation of Natural Data Sets

et al. 2018

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“…Negative NSGI values were only measured in catchments with median slopes below 25° and are predominantly found in the northern, slowly denuding areas. We suggest that these negative values are a result of slower transport of gravel compared to sand on the gentle slopes, as has been noted in other low‐slope regions (Codilean et al, ). If transient sand and gravel are equally distributed with depth in the temporary sedimentary deposits, such that they are exposed to similar 10 Be production rates during downstream transport, then negative NSGI values could potentially be used to infer relative differences in sand and gravel residence times.…”

Section: Discussionmentioning

confidence: 99%

“…In areas characterized only by diffusive hillslope processes like soil creep and sheetwash, [ 10 Be] sand should be equal to or lower than [ 10 Be] gravel ( NSGI = ‐1 to 0). Higher [ 10 Be] in gravel relative to sand, which would result in negative NSGI values, has rarely been reported, but has been attributed to the accumulation of 10 Be during slower transport of gravel compared to sand on gentle slopes (Codilean et al, ).…”

Section: Introductionmentioning

confidence: 99%

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

Tofelde

Duesing

Schildgen

et al. 2018

Earth Surf Processes Landf

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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.

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“…For example, many catchments lack datable minerals or systematic age‐elevation relationships for thermochronometric fingerprinting of sediment source elevations. Meanwhile, cosmogenic nuclides can vary with sediment size due to other complications besides a grain size bias [ Matmon et al ., ; Belmont et al ., ; Codilean et al ., , ; Puchol et al ., ; Aguilar et al ., ; Attal et al ., ], making interpretations of data from multiple sediment sizes inherently equivocal. For instance, variations in 10 Be concentrations with sediment sizes could reflect the preferential breakdown of coarse particles originating from higher elevations, where nuclide production rates are higher [ Matmon et al ., ]: This may help explain why coarse clasts have relatively low cosmogenic nuclide concentrations in some streams [ Matmon et al ., ; Belmont et al ., ].…”

Section: Discussionmentioning

confidence: 99%

Grain size bias in cosmogenic nuclide studies of stream sediment in steep terrain

et al. 2016

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Cosmogenic nuclides in stream sediment are widely used to quantify catchment‐average erosion rates. A key assumption is that sampled sediment is representative of erosion from the entire catchment. Here we show that the common practice of collecting a narrow range of sizes—typically sand—may not yield a representative sample when the grain size distribution of sediment produced on slopes is spatially variable. A grain size bias arises when some parts of the catchment produce sand more readily than others. To identify catchments that are prone to this bias, we used a forward model of sediment mixing and erosion to explore the effects of catchment relief and area across a range of altitudinal gradients in sediment size and erosion rate. We found that the bias increases with increasing relief, because higher‐relief catchments have a larger fraction of high elevations that are underrepresented in the sampled sand when grain size increases with altitude. The bias also increases with catchment area, because sediment size reduction during transport causes an underrepresentation of more distal, higher elevations within the catchment. Our analysis indicates that grain size bias may be significant at many sites where cosmogenic nuclides have been used to quantify catchment‐average erosion rates. We discuss how to quantify and account for the bias using cosmogenic nuclides and detrital thermochronometry in multiple sediment sizes.

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Discordance between cosmogenic nuclide concentrations in amalgamated sands and individual fluvial pebbles in an arid zone catchment

Cited by 40 publications

References 43 publications

Does Pebble Abrasion Influence Detrital Age Population Statistics? A Numerical Investigation of Natural Data Sets

Does Pebble Abrasion Influence Detrital Age Population Statistics? A Numerical Investigation of Natural Data Sets

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

Grain size bias in cosmogenic nuclide studies of stream sediment in steep terrain

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Discordance between cosmogenic nuclide concentrations in amalgamated sands and individual fluvial pebbles in an arid zone catchment

Cited by 40 publications

References 43 publications

Does Pebble Abrasion Influence Detrital Age Population Statistics? A Numerical Investigation of Natural Data Sets

Does Pebble Abrasion Influence Detrital Age Population Statistics? A Numerical Investigation of Natural Data Sets

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

Grain size bias in cosmogenic nuclide studies of stream sediment in steep terrain

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