Geology Reference
In-Depth Information
yield an underestimate of the Late Pleistocene
sample age. This tool has been recently exploited
to tell the tale of the demise of giant birds in
Australia upon the arrival of humans on that
continent (Miller
et al.
, 1999, 2005).
0.6
forward and backward
reactions roughly equal
Amino Acid
Racemization
Clock
1.2
0.4
thermal effect
on 125-ka
deposits
0.6
0.2
Luminescence dating
0.0
-10
The applicability of many of the dating techniques
discussed thus far is often limited by the
availability of suitable material for dating, such as
carbonaceous debris, shells, or corals. In most
terrestrial settings, shells or corals are absent, and
even where carbonaceous material is preserved,
the age of the deposit of interest may exceed the
practical range of
14
C. Luminescence dating has
the potential both to be broadly applicable to
terrestrial deposits and to extend well beyond the
age limitations of radiocarbon dating.
The basis of luminescence is the trapping and
release of energy by electrons within crystals
(Aitken, 1985, 1998; Duller 1996; Wintle, 1993).
When energy is added to electrons through
radiation, they tend to move from a lower energy
level (the valence band) to a higher energy level
(the conduction band). Some of these energized
electrons can be trapped metastably by crystal
defects between these two energy levels.
Trapped electrons can be made to drop back to
their valence band upon the addition of a small
amount of energy, at which time they release
photons of light equivalent to the change in
their energy state. This released light is termed
luminescence
. The radiation that initially causes
the energy state of electrons to increase within
a crystal comes largely from the decay of nearby
radioisotopes within a sedimentary deposit.
That this rate of decay should be approximately
constant is the basis of the luminescence clock.
The electrons can be reset (zeroed) to their low-
energy state by exposure to either intense heat
or sunlight. When luminescence is caused by
the addition of heat in the laboratory, it is
termed
thermoluminescence
(TL), whereas
when it results from the addition of light, it is
termed
optically stimulated luminescence
(OSL).
In TL dating, luminescence is measured as the
sample is incrementally heated (Fig. 3.16A). In
OSL, the sample is exposed to light of a certain
10
30
MAT (°C)
0.0
0
200
400
600
800
1000
Time (ka)
Fig. 3.15
Theoretical curve of amino acid racemization
through time.
For early times, L-D transitions dominate, whereas
back-reactions D-L become roughly equal at later times.
Inset shows effects of higher mean annual temperatures
(MAT) on reaction rates, and hence of D/L ratio
expected at a given age (125 ka). Modified after Kaufman
and Miller (1992) and Hearty and Miller (1987).
history to which a sample has been subjected
subsequent to death. For instance, isoleucine
attains its equilibrium value of about 1.3 in
100-300kyr in equatorial sites, whereas in the
Arctic it may take 10Myr (Miller and Brigham-
Grette, 1989) (Fig. 3.15). The thermal dependence
is both a problem to be dealt with in the absolute
dating of a sample, and a paleoclimatic tool in itself
if the date is already known through other dating
or correlation methods (inset in Fig. 3.15). Because
the rate constant is apparently influenced by the
type of animal (the taxonomic effect), research-
ers typically focus on a few common genera.
Typical application of racemization to absolute
dating involves sampling a site of known age
(say, a Holocene site that has been independently
dated using
14
C) and a nearby site that is
presumed to be Late Pleistocene. The same
fossil genera are collected from each site. The
D/L ratio of the Holocene sample yields an
empirical rate constant that is both geographically
and taxonomically specific. The interpretation of
the Late Pleistocene D/L ratio takes into account
the fact that the average temperatures since
the Late Pleistocene are likely lower than the
average since the Holocene sample was
deposited. This temperature contrast indicates
that the use of the Holocene rate constant should
