@article{fdi:010046024,
  title = {{T}heoretical cosmogenic nuclide concentration in river bed load clasts : does it depend on clast size ?},
  author = {{C}arretier, {S}{\'e}bastien and {R}egard, {V}. and {S}oual, {C}.},
  editor = {},
  language = {{ENG}},
  abstract = {{T}errestrial {C}osmogenic {N}uclides ({TCN}s) have been widely used to date the exposure of alluvial surfaces and to estimate catchment-scale erosion rates. {H}owever, {TCN} concentration differences in samples of different grain sizes remain to be fully understood. {I}n order to explore the possibility that river processes generate such differences, we develop a numerical model to calculate along-stream clast-scale {TCN} concentrations. {U}sing the hillslope model, there is a progressive detachment of successive clasts of specific sizes followed by their instantaneous fall into the river. {I}n the river, transport velocity and {TCN} concentration evolution in a clast depend 1) on the probability of being trapped in the sediment mixing layer of the river or within an adjacent terrace; 2) on its size which decreases downstream by attrition. {T}he size-dependent transport law corresponds to the partial transport state in a river. {W}e model the distribution of {TCN} concentrations in different clast size fractions in the 0-5 cm radius range for catchments in steady-state erosion, and for catchments experiencing sedimentation. {W}e propose that clast attrition tends to increase the variance of {TCN} concentrations of the small clast size fractions because these fractions incorporate initially big clasts that travelled a long distance in addition to small clasts contributed near the outlet. {W}e obtained numerous clast size-{TCN} concentration correlations, positive or negative, the significance of which depends on the initial clast size distribution, hillslope erosion rate, river length and lithology. {F}or an equilibrium catchment, even large, we found that the addition of {TCN} concentration acquired during river transport is negligible compared to {TCN} concentration acquired on a hillslope, although a clast size-{TCN} concentration relationship can result from or be modified by clast attrition. {O}n the contrary, aggrading catchments may show a significant clast size-dependent {TCN} concentration increase during river transport. {T}his may introduce a small bias in the {TCN}-derived catchment erosion rate, but it could be used positively to quantify the mean transport velocity of clasts of different sizes over thousands of years. {I}n addition, the lack of correlation between {TCN} concentration and clast size does not imply that the mean transport velocity is the same for all clast size fractions. {O}verall, our study provides an alternative explanation for observed clast size-dependent {TCN} concentrations and brings to the fore the need for measuring {TCN} concentration in larger clast size fractions than is usually done. {T}o see if the byproducts of abrasion dilute or increase the {TCN} concentration of sand, all products should be included in a future study.},
  keywords = {{C}osmogenic nuclide ; {R}iver ; {S}ediment transport ; {C}atchment-scale erosion ; rate},
  booktitle = {},
  journal = {{Q}uaternary {G}eochronology},
  volume = {4},
  numero = {2},
  pages = {108--123},
  ISSN = {1871-1014},
  year = {2009},
  DOI = {10.1016/j.quageo.2008.11.004},
  URL = {https://www.documentation.ird.fr/hor/fdi:010046024},
}