South Africa - Inkaba.org

Cosmogenic nuclide-based perspectives on long-term
landscape evolution models and short-term soil erosion rates
in southern Africa
J.E. Decker 1* , S. Niedermann 2 , M.J. de Wit 3
1. AEON and Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa,
geojedenator@gmail.com
2. Deutsches GeoForschungsZentrum, 14473 Potsdam, Germany, nied@gfz-potsdam.de
3. AEON and Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa,
Maarten.DeWit@uct.ac.za
*Present address: Petroleum Agency SA, 7 Mispel Road, Tygerpoort Building, Bellville 7789, South Africa,
deckerj@petroleumagencysa.com
ABSTRACT
Southern Africa’s landscape is characterised by a high plateau (elevation > 1,000 m) bound by escarpments and
extensional continental margins, and the region has been influential in the development of geomorphological
theory. The abundances of cosmogenic noble gas nuclides ( 3 He, 21 Ne and 38 Ar) in pyroxene separates from 22
Early Jurassic (~183 Ma) Karoo dolerite surface samples from southern Africa indicate minimum exposure ages
that range from ~20 ka to ~500-700 ka. Corresponding maximum erosion rates range from ~0.6 m/Myr to ~20
m/Myr, with most values < ~4 m/Myr. This data implies a decline in denudation rates over time, as these recent
surface process rates are too low to account for the form of topographic features. Low maximum erosion rates are
interpreted as weathering rates, but do not correlate with lithological and present-day climatic variables. However,
dolerite weathering rates are similar to theoretical basalt weathering fluxes modelled as a function of present-day
temperature and run-off. Regional discrepancies between measured and model values may be interpreted as an
indication of a variable late Quaternary palaeoclimate. Dolerite weathering rates are also similar to stratigraphically
determined Holocene denudation rates, suggesting that denudation has been weathering-limited over this time
period. Finally, if Karoo dolerite maximum erosion rates are interpreted as soil production rates, it is estimated that
modern rates of soil erosion in southern Africa are up to two orders of magnitude higher than rates of soil
production, implying that modern land-use practices are unsustainable.
Key words: cosmogenic nuclides, Karoo dolerite, landscape evolution, soil erosion, noble gases
22