Vignettes > Quantifying rates of erosion in Namibia using cosmogenic nuclides

Quantifying rates of erosion in Namibia using cosmogenic nuclides

Alexandru T Codilean
University of Glasgow
Author Profile

Shortcut URL: http://serc.carleton.edu/31893

Location

Continent: Africa
Country: Namibia

Setting

Climate Setting: Arid
Tectonic setting: Passive Margin
Type: Process

Plot illustrating the different methods available for quantifying erosion rates. It also shows the range of temporal and spatial scales over which these methods are applicable. Any of these methods can in principle be used to quantify erosion over geomorphological timescales but cosmogenic nuclides are the most suitable because: (1) are produced at shallow depths and so unlike low-temperature thermochronometers are highly sensitive to changes in surface morphology, and (2) accumulate over longer timescales and so unlike historical methods (such as using sediment volume data) are not prone to anthropogenic disturbance. Original image courtesy of Roderick Brown (University of Glasgow). Details



Map showing the distribution of hillslope angles in the Gaub River basin located in the vicinity of the Gamsberg Mountain (2351 m). Note the contrast between the low relief upland plateau and highly dissected steep escarpment area. Details


Diagrammatic topographic profile across central-western Namibia (thick line) summarizing the cosmogenic nuclide-based erosion rates of Bierman and Caffee (2001), Cockburn et al. (2000), Codilean et al. (2008) and van der Wateren and Dunai (2001) from bedrock and clast samples (above the topographic profile) and amalgamated sediment samples (below the topographic profile). Original image courtesy of Roderick Brown (University of Glasgow). Details


Plot of Be-10 erosion rates (meters per million years) obtained for Namibian basins by Bierman and Caffee (2001) and Codilean et al. (2008) versus area-weighted mean hillslope angles of these basins. Modified from Codilean et al. (2008). Details


Gamsberg Mountain, a flat-topped granite residual capped by a ~25 m thick quartzite unit. Cosmogenic nuclide analyses indicate that the flat top of the Gamsberg is eroding very slowly, at an astonishing rate of less than one meter per million years. Details


Photograph taken from the summit of the Gamsberg showing the low-relief upland plateau. Details


Photograph taken in the vicinity of the Gamsberg (Farm Weener 193) showing coastal plain and the lip of the escarpment (foreground). Details


Description

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.

Associated References

Bierman, P.R., and Caffee, M., (2001). Slow rates of rock surface erosion and sediment production across the Namib Desert and Escarpment, Southern Africa. American Journal of Science. v. 301, doi: 10.2475/ajs.301.4-5.326. p. 326-358.

Bierman, P.R., and Nichols, K.K., (2004). Rock to sediment - slope to sea with 10Be - rates of landscape change. Annual Review of Earth and Planetary Sciences. v. 32, doi: 10.1146/annurev.earth.32.101802.120539, p. 215-255.

Bishop, P., (2007). Long-term landscape evolution: linking tectonics and surface processes. Earth Surface Processes and Landforms. v. 32, doi: 10.1002/esp.1493, p. 329-365.

Cockburn, H.A.P., Brown, R.W., Summerfield, M.A., and Seidl, M.A., (2000). Quantifying passive margin denudation and landscape development using a combined fission-track thermochronology and cosmogenic isotope analysis approach. Earth and Planetary Science Letters. v. 179, doi: 10.1016/s0012-821x(00)00144-8, p. 429-435.

Codilean, A.T., Bishop, P., Stuart, F.M., Hoey, T.B., Fabel, D., and Freeman, S.P.H.T., (2008). Single-grain cosmogenic 21Ne concentrations in fluvial sediments reveal spatially variable erosion rates. Geology. v. 36, doi: 10.1130/g24360a.1, p. 159-162.

Jacobson, P. J., Jacobson, K. M. and Seely, M. K., (1995) Ephemeral Rivers and Their Catchments: Sustaining People and Development in western Namibia. Desert Research Foundation of Namibia. Windhoek.

Ollier, C.D., and Marker, M.E., (1985). The Great Escarpment of southern Africa: Zeitschrift fur Geomorphologie N. F., Supplement Band. v. 54, p. 37-56.


Twidale, C.R., and Bourne, J.A., (1975). Episodic exposure of inselbergs. Geological Society of America Bulletin. v. 86, p. 1473–1481.

van der Wateren, F.M., and Dunai, T.J., (2001). Late Neogene passive margin denudation history-cosmogenic isotope measurements from the central Namib desert. Global and Planetary Change. v. 30, doi: 10.1016/s0921-8181(01)00104-7, p. 271-307.


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