Landscape evolution in Namibia and Antartica

Abstract

Abstract provided by author:

Quantifying denudation rates is fundamental to understanding the way landscapes evolve. Such data are required to constrain numerical models of long-term landscape evolution, the development of which is outrunning our ability to constrain them empirically. In-situ cosmogenic isotope analysis is a valuable addition to the range of techniques currently available to measure denudation rates and is especially useful in providing information over periods of 10 -3 to 10 -6 a. In-situ cosmogenic 10Be, 26A1 and 21Ne concentrations from locations in central Namibia and the Transantarctic Mountains, Antarctica have been used to investigate rates of landscape change in passive margin settings and contrasting hot and cold arid conditions

Concentrations of cosmogenic l0Be and 26A1 in quartz separated from 14 samples from the Gamsberg in central Namibia have been measured using accelerator mass spectrometry. The Gamsberg is a flat-topped residual forming part of the Great Escarpment of central Namibia. Rates of summit denudation range from 0. 2 to 0. 8. m Ma-1 and rates of slope retreat have been~10 m Ma-1 over the past 0. 04 to 1. 5 Ma. The data indicate that backwearing is much more significant than downwearing but that summit denudation is occurring. Low rates of escarpment retreat of ~10 m Ma-1 are incompatible with the idea that retreat from the coast has been at a uniform rate since rifting occurred ~130 Ma BP. The data are consistent with geological evidence and data from apatite fission track thermochronology that suggest that the escarpment retreated rapidly soon after rifting but has not retreated significantly during the Tertiary

Measurements of cosmogenic I0Be and 26 Al from three granite bornhardts on the coastal plain of Namibia seaward of the Great Escarpment in the arid/hyper-arid central Namib Desert indicate mean rates of summit lowering ranging from 2. 2 to 6. 3 m Ma-1 over the past 1-3 x lO5 a. Low variability in estimated denudation rates between the sampling sites and the long-term persistence of an arid climate implies that a rate of summit lowering of ~5 m Ma -1 has characterised bedrock exposures in the central Namib for at least the past 10 Ma, and possibly throughout much of the Cenozoic. The complex exposure history of some samples appears to be due to the mode of inselberg weathering and mass wasting

Cosmogenic 10Be, 26A1 and 2lNe denudation rate estimates from high-elevation surfaces and rectilinear slopes in the Dry Valleys region of Antarctica suggest very slow rates of denudation of 1 m Ma -1 for at least the past 2-5 Ma. The rates from all three isotopes for each sample are generally consistent and where paired isotopic data are available exposure histories appear to be simple. The data are fully compatible with existing cosmogenic isotope data and independent evidence from the region that imply minimal landscape modification and a polar, hyper-arid climate since the late Miocene. The data support a hypothesis that the East Antarctic Ice Sheet has been essentially stable over this period rather than experiencing significant fluctuations as recently as the Pliocene

It has been demonstrated that in-situ cosmogenic isotope analysis can provide previously unobtainable data on denudation rates over timescales intermediate between short-term process studies and long-term estimates from techniques such as thermochronology. Used in conjunction with other techniques, cosmogenic isotopes analysis has significant potential for evaluating controls on denudation rates in a range of geomorphic settings and constraining models of long-term landscape evolution