Des problèmes de blanchiment chez les
all black où Impact et enregistrement de
la dynamique de l’incision fluviatile (Rangitikei,
Nouvelle Zélande) sur les géochronomètres
OSL, IRSL

Stéphane Bonnet; Tony Reimann; Dimitri Lague; Jakob Wallinga; Philippe Davy; Rieser Uwe

Abstract : Landscape response to climate induces periods of enhanced sediment supply to rivers promoting the formation of climatic-aggradation fluvial terraces whose age and height are used for estimating mean fluvial incision rates. These rates must however be taken with caution. Numerical simulations actually suggest that incision rates are not linear between the formation of two successive terraces. The Rangitikei River (New Zealand) is downcutting in an uplifting basement. Entrenchment is punctuated by terraces, the more recent one (T1) being classically considered to formed from 30 to 15 ka. Since abandonment of T1, the river incised 75-meter deep gorges within the bedrock. We investigated this entrenchment using optical dating (IRSL on feldspars) of 17 alluvial deposits from strath terraces cut into the bedrock. We performed measurements using Multiple Aliquots or Single Aliquot Regenerative protocols on large aliquots of fine grains of feldspars (4-11 um) as well as measurements on single-grains on a coarser fraction (125-200 um) of the same samples but using Post-Infrared IRSL (p-IRIR) protocol. We show that : (1) abandonment of terrace T1 occurred 10 kyr ago, more recently than previous estimates (2) a period of very fast incision follows the formation of T1. Incision rate then slows down ; this decrease coincides with the development of extensive post-T1 terraces. Our data support numerical findings showing that the ratio of vertical incision to lateral planation is driven largely by variations in vertical incision rate (3) The distribution of single-grains ages depend on the river incision rate, periods of fast incision being characterized by a larger proportion of unbleached grains in the samples. Accordingly, IRSL ages obtained using classical methods that consider measures on aliquots instead of on singles grains, all show an overestimation of ages whose magnitude depends on incision rate. We show that ion IRLL geochronology, the proportion of mixing between bleached and unbleached grains is intimately linked to variations in the rate of bedrock erosion and related supply of unbleached grains to the river. Our study consequently suggests that the shape and width of IRSL age distributions obtained through single-grain measurements could illuminate the past dynamics of rivers.

Des problèmes de blanchiment chez les all black où Impact et enregistrement de la dynamique de l’incision fluviatile (Rangitikei, Nouvelle Zélande) sur les géochronomètres OSL, IRSL

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