Slow Rates of Rock Surface Erosion and Sediment Production across the Namib Desert and Escarpment, Southern Africa

Abstract

Slow erosion has characterized the Namib Desert, the Namibian escarpment, and the adjacent Namibian highlands over the Pleistocene. Paired analyses (n‫)66؍‬ of in-situ-produced 10 Be and 26 Al in quartz-bearing samples of bedrock primarily from inselbergs, of sediment from dry river and stream channels, and of clasts from desert surfaces reveal large inventories of these cosmogenic nuclides indicating significant landscape stability over at least the past million years.Bedrock samples (n ‫؍‬ 47) collected in three transects from the coast, across the escarpment, and into the highlands, show no spatial pattern in elevation-normalized nuclide abundance despite a difference in mean annual precipitation (MAP) between sample sites at the coast (MAP <25 mm yr ؊1 ) and those in the highlands (MAP >400 mm yr ؊1 ). Average model erosion rates inland of the escarpment (3.2 ؎ 1.5, n ‫؍‬ 9) are indistinguishable from average rates seaward of the escarpment (3.6 ؎ 1.9, n ‫؍‬ 38) indicating that rock on the pedimented coastal plain is eroding at the same rate as rock in the highlands. Sediment samples (n ‫؍‬ 3) from small streams suggest that the landscape as a whole is eroding more rapidly than the bedrock outcrops and that a basin in the steep escarpment zone is eroding several times faster (16 m my ؊1 ) than either a basin in the highlands (5 m my ؊1 ) or a basin in the coastal plain (8 m my ؊1 ). Data from large rivers (n ‫؍‬ 4) constrain erosion rates, averaged over 10 5 yrs and 10 4 to 10 5 km 2 , between 3 and 9 m my ؊1 . Small quartz clasts (n ‫؍‬ 12) collected from four desert surfaces record extraordinarily long, variable, and in some cases complex exposure histories. Simple 10 Be model ages are as high as 1.8 my; some minimum total histories, considering both 10 Be and 26 Al and including both burial and exposure, exceed 2.7 my. As a group, the Namibian cosmogenic data do not support the model of significant and on-going escarpment retreat.The similarity of erosion rates calculated from 10 Be analysis of fluvial sediments and longer-term (10 7 yr), average mass removal rates estimated by others using fission track analysis of rock suggests that Namibian erosion rates have reached a steady state and are changing little over time. At outcrop scales, the concordance of 10 Be and 26 Al in most bedrock samples suggests that the model of steady, uniform bedrock erosion is valid; there is no indication of intermittent burial, shedding of thick rock slabs, or stripping of previous cover. At an intermediate scale, a transect of bedrock samples north of Gobabeb demonstrates that the northern boundary of the massive Namib Sand Sea has been steady and unshifting. Similarly low cosmogenically estimated erosion rates across west and central Namibia suggest that the landscape is in geomorphic steady state, its overall appearance changing only slowly through time.

Erosion in northwest Tibet from in‐situ‐produced cosmogenic 10Be and 26Al in bedrock

“…Thus the cosmogenic nuclide dating technique confines erosion rates with a shorter and more recent time interval and the fission-track technique confines erosion rates with a longer time interval. Although the basis of the two methods is different, results published in the literature show that erosion rates confined by the two methods are consistent for tectonically stable landforms (Kirchner et al, 2001;Bierman and Caffee, 2001).…”

Applications of morphochronology to the active tectonics of Tibet

“…However, this is only the case for very old samples with negligible erosion rates (Bierman and Caffee, 2001;Nishiizumi et al, 1991). Erosion has an effect similar to increasing the decay constant and samples experiencing rapid erosion reach steady state before the 26 Al/ 10 Be ratio can deviate significantly from the production ratio.…”

The Kalahari Epeirogeny and climate change: differentiating cause and effect from core to space

“…The land surface of southern Africa was uplifted in the Mesozoic and is presently still in a complex stage of active, but slow peneplanation (King, 1967;Partridge and Maud, 2000;Bierman and Caffee, 2001), but with low vertical denudation rates (<2m/Ma), and horizontal escarpment retreat (<10m/Ma), as determined from cosmogenic dating of present day exposed surfaces (Flemming et al, 1999;Cockburn et al, 2000;Bierman and Caffee;2001;Bierman and Nichols, 2004;Kounov et al, 2007).…”