Quantifying rates of erosion in Namibia using cosmogenic nuclides

Quantifying rates of erosion on millennial timescales is fundamental to understanding the processes that shape the Earth's surface. Cosmogenic nuclide analysis is probably the most suitable technique for this purpose as it is capable of quantifying rates of erosion over a variety of spatial scales and over timescales of the order of thousands to millions of years.

Cosmogenic nuclide analysis is a relatively new technique with the first studies being published in the early 1990's. Cosmogenic nuclides are minute amounts of nuclides such as He-3, Be-10, C-14, Ne-21, Al-26, and Cl-36, and are produced by the interaction of high-energy cosmic particles (mainly neutrons) with minerals in the Earth's upper crust. The technique is based on the principle that the concentration of cosmogenic nuclides is proportional to the amount of time the mineral (or host rock) is exposed to cosmic radiation. The production of cosmogenic nuclides is confined to the upper few metres of the crust and the production rate is strongly dependent on elevation. The production rate also depends on other factors, such as geomagnetic latitude and the configuration of surrounding topography, but these are somewhat less important. The strong dependence of the production rate on altitude means that the total cosmogenic nuclide concentration acquired by a grain, before being detached from bedrock, is sensitive to variations in bedrock erosion rate (i.e., how long the grain spends within the upper few metres of the Earth's surface) and also to changes in surface elevation (or where the grain is exposed).

Cosmogenic nuclides have been used extensively to estimate rates of erosion at both the outcrop- and the landscape-scales. The Namibian sector of the Great Escarpment, especially the area in the vicinity of the Gamsberg Mountain, has been extensively studied, and given a comprehensive published cosmogenic nuclide dataset, this area is ideal for illustrating the power of the technique.

Namibia's landscape is dominated by three elements: the Namib Desert, ~2000 km long and ~200 km wide; the Great Escarpment, a major escarpment zone parallel to the coast; and an upland plateau inland of the escarpment, characterised by low relief and gentle slopes. Namibia has an arid climate as subtropical easterly winds loose moisture while crossing the African continent before descending the escarpment and drying further. The south Atlantic anticyclone, the cold Benguela current, and coastal upwelling offshore of Namibia mean that little precipitation reaches the coast. Rivers and streams originating in Namibia are ephemeral, being dry most of the year except during floods that may last up to two weeks.

Cosmogenic nuclide analyses in samples collected from bedrock outcrops from the coastal plain and upland plateau by Bierman and Caffee (2001) and van der Wateren and Dunai (2001) have yielded very low bedrock erosion rates, averaging around 2.5 meters per million years. Bedrock samples collected by Cockburn et al. (2000) along a profile perpendicular to the Great Escarpment in the Gamsberg area have yielded slightly higher (although still relatively very low) bedrock erosion rates averaging around 7.9 meters per million years. Basin-wide erosion rates obtained by Bierman and Caffee (2001) and Codilean et al. (2008) based on cosmogenic nuclide analyses of sediment are higher than their bedrock counterparts but exhibit a similar regional pattern: 6.4 and 5.8 meters per million years on the coastal plain and upland plateau respectively, and 12.9 meters per million years on the escarpment.

The results of the cosmogenic nuclide analyses in both bedrock and river sediment samples suggest that the landscape of central-western Namibia has virtually remained unchanged in the last couple of million years. Comparing the bedrock- and sediment-based erosion rates indicates that bedrock outcrops are more resistant to erosion than the landscape as a whole, suggesting that relief may fluctuate over time as inselbergs grow and then decay due to lateral erosion (Twidale and Bourne, 1975). Despite this local variability, illustrated by the difference between the rates recorded by bedrock and those recorded by sediment, both bedrock and sediment tell a similar regional story, that is, the steeper escarpment area is eroding more rapidly than either the more gently sloping coastal plain or the upland plateau.

Further, Bierman and Caffee's (2001) and Codilean et al.'s (2008) sediment-based cosmogenic nuclide data show a strong linear correlation with the average slope of the sediment's source drainage basins. This finding is important, confirming other studies that have identified mean basin slope as a dominant control on rates of erosion and the associated development of topography.

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