Geology Reference
In-Depth Information
If for any reason a deposit containing an inven-
tory of cosmogenic radionuclides is deeply bur-
ied, and taken out of the cosmogenic radionuclide
production zone (say 4 m), another twist on the
method may be employed. Here we take advan-
tage of two radionuclides with different half-
lives: 26 Al, with a half-life of 0.705 Myr, and 10 Be,
with its half-life of 1.387 Myr. We know their pro-
duction rates at the surface in quartz differ by a
factor of 6.75. Because the decay rates differ, this
ratio will decline with time and becomes a clock
once the sample is taken out of the production
zone. The method based on this ratio clock is
called the burial method (e.g., Granger and
Smith, 2000; Granger and Muzikar, 2001). Burial
dating allows us to date caves in which sedi-
ments exposed to nuclide production during
hillslope and river transport in the headwaters
are sequestered in the cave tens to thousands of
meters below the ground surface (e.g., Granger
et al. , 1997, 2001; Stock et al. , 2004, 2005)
(Fig. 3.21). The burial method has also been used
to date very old deposits that have been buried
deeply since deposition, such as early tills in the
North American glacial sequence (Balco et al. ,
2005). Especially where recent excavations, such
as deep road cuts or landslides, provide access to
previously hard-to-date strata, burial ages pro-
vide a practical method to  determine deposi-
tional ages (e.g., Harkins et  al. , 2007). Because
the age depends on a ratio of two different iso-
topic concentrations, and each concentration
measurement has its own error, burial ages are
not highly precise, but they allow a distinction to
be drawn between strata that are, for example,
500, 700, and 900 ka (Fig. 3.21).
Landscape features may also be dated using
garden-variety CRNs produced in the atmo-
sphere and subsequently rained out in
precipitation (see review in Morris, 1991). In
particular, 10 Be is found to be useful in that its
chemistry is such that it is held tightly by clays
in soils (Pavich et al. , 1984; see review in
Willenbring and von Blankenburg, 2010). The
soil, therefore, acts as a reservoir within which
the 10 Be slowly builds up with the age of the
surface. If surfaces are well chosen to limit
the role of soil erosion, i.e., are nearly flat, then
the total 10 Be inventory on grain surfaces
A
Simpleveld-2
Margraten
Dated
Terrace
Succession
150 m
Sibbe-1&2
Valkenburg-1
Valkenburg-2
Geertruid-1
Geertruid-2
Geertruid-3
West Meuse Valley
Pietersberg-2
Pietersberg-3
Gravenvoeren
Pietersberg-1
100 m
Rothem-2
Rothem-1
Caberg-1
Caberg-2
Caberg-3
50 m
Eijsden-Lanklaar
Mechelen a/d Maas
NW
SE
Holocene floodplain
100
B
Past Erosion Rates
80
60
40
20
0
0
0.5
1.0
1.5
Age (Ma)
Fig. 3.20 Past erosion rates from 10 Be inheritance.
A. Elevation cross-profile of the West Meuse valley,
Netherlands, showing numerous fluvial terraces dated
using 10 Be produced in situ . B. Erosion rates deduced
from the inherited component of the 10 Be profile for
each terrace, revealing a history of erosion rates in the
catchment. Modified after Schaller et al . (2004).
One may also use the shift in the profile on
a depositional surface - the inheritance - to
deduce the erosion rate of the catchment at
the time of deposition, using Eqn 3.5. The
inheritance has been exploited (e.g., Schaller
et al. , 2001, 2002, 2004) to derive paleoero-
sion rates through time from river terrace
sequences on European rivers draining the
Alps) (Fig. 3.20).
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