Aluminum has nine isotopes whose mass numbers range from 23 to 30. Only
27Al (stable isotope) and 26Al (radioactive isotope;
t1/2 = 0.72x106 yr) occur naturally. 26Al
is produced from argon in the atmosphere by spallation caused by cosmic-ray
protons. Aluminum isotopes have found practical applications in dating
marine sediments, Mn-nodules, glacial ice, quartz in rock exposures, and
the terrestrial age of meteorites. The ratio of 26Al to 10Be
has been used to study the role of transport, deposition, sediment storage
and burial times, and erosion on 105 to 106 yr timescales.
Cosmogenic 26Al were first applied in studies of the Moon
and meteorites. Meteorite fragments, after departure from their parent
bodies, are exposed to intense cosmic-ray bombardment during their travel
through space, causing substantial 26Al production. After falling
to Earth, atmospheric shielding protects meteorite fragments from further
26Al production, and its decay can then be used to determine
a terrestrial age (Dicken, 1995).
Erosion rates calculated using 26Al of sediments are consistent
with long-term rates of denudation estimated by using the volume of basin
fills, the depth of basalt incision, and fission track analysis. Granger
and Kirchner (1994 a,b) demonstrate that measurements of 10Be
and 26Al in sediments currently being deposited on two small,
northeastern California alluvial fans can be used to calculate basin scale
erosion rates similar to those deduced considering fan volume and age.
In situ produced 10Be and 26Al measured in quartz-rich
sediment samples collected from several tectonically and climatically diverse
environments have been shown to reflect basin-scale rates of erosion (Bierman,
1995). Brown et al. (1992) used a series of 10Be and 26Al
measurements in a core to estimate erosion rates for quartz sandstone in
the Quartermain Mountains. Nishiizumi et al. (1991) used 26Al
and 10Be abundances to calculate maximum steady-state erosion
rates. Measurement of nearly 60 samples of granite from the inselbergs
of south central Australia (Bierman and Turner, 1995) shows that the tops
of these landforms are eroding at rates only slightly higher than some
The preferential decay of 26Al during sediment storage and
transport is well documented. The differences in 10Be/26Al
ratios can also be used to understand qualitatively the transport history
of some sediments. These studies suggest that measurement of 26Al
in sediments may be a new tool for determining long-term rates of landscape
change at the scale of drainage basins and mountain belts.
Source of text: This review was assembled by Eric Caldwell, primarily
from Dicken (1995), Bierman et al. (1998) and Faure (1986).
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