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
Résumé
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.