Comparison of timescales of other dating techniques
In Australia determining the time of arrival of the first inhabitants at
perhaps 60,000years bp. is a challege. Radio-carbon dating is at it's
extreme upper limit with very large degrees of error due to the tiny
amounts of materials present.
Thermaluminesence (TL) and Optically Stimulated Luminesence (OSL) may
assist in extending age dating timescales though there is a huge challenge
in selecting suitable sampling materials.
TL dating is a matter of comparing the energy stored in a crystal to what
"ought" to be there, thereby coming up with a date-of-last-heated. In the
same way, more or less, OSL (optically stimulated luminescence) dating
measures the last time an object was exposed to sunlight. Luminescence
dating is good for between a few hundred to (at least) several hundred
thousand years, making it much more useful than carbon dating.
Two forms of luminescence dating are used by archaeologists to date events
in the past: thermoluminescence (TL) or thermally stimulated luminescence
(TSL), which measures energy emitted after an object has been exposed to
temperatures between 400 and 500°C; and optically stimulated luminescence
(OSL), which measures energy emitted after an object has been exposed to
Crystalline rock types and soils collect energy from the radioactive
decay of cosmic uranium, thorium, and potassium-40. Electrons from these
substances get trapped in the mineral's crystalline structure, and
continuing exposure of the rocks to these elements over time leads to
predictable increases in the number of electrons caught in the matrices.
But when the rock is exposed to high enough levels of heat or light,
that exposure causes vibrations in the mineral lattices and the trapped
electrons are freed.
Luminescence dating is a collective term for dating methods that
encompass thermoluminescence (TL) and optically stimulated luminescence
(OSL) dating techniques. OSL is also less commonly referred to as
optical dating, photon stimulated luminescence dating or
photoluminescence dating.. Luminescence dating methods are based on the
ability of some mineral grains to absorb and store energy from
environmental ionizing radiation emanating from the immediate
surroundings of the mineral grains as well as from cosmic radiation.
When stimulated these minerals, generally referred to as dosimeters,
will release the stored energy in the form of visible light; hence the
term luminescence. Measuring the energy and determining the rate at
which the energy accumulated allows an age representing the time that
has elapsed since the energy began accumulating to be determined.
Stimulation of energy release using heat is termed TL while stimulation
using light is referred to as OSL. The age range of luminescence methods
generally spans from a few decades to about 100,000 years, though ages
exceeding several hundred thousand years have been reported in some
Like 14C dating, thermoluminescence is related to
Thermoluminescence is produced by radioactive decay particles
(electrons), trapped in mineral grains. Heating the mineral (or exposure
to light) releases electrons, and produces a flash of light, setting the
clock to 0 (maybe only partial). Thereafter, luminescence accumulation
is proportional to age. Used particularly for >50 ka archeological
Electron spin resonance (ESR) has been used for absolute dating of
archaeological materials such as quartz, flints, carbonate crystals, and
fossil remains for nearly 50 years. The technique is based on the fact
that certain crystal behaves as natural dosimeters. This means that
electrons and holes are accumulated over time in the crystal lattice
induced by surrounding radiation. The age is obtained by calculating the
dose received compared to the dose rate generated by the surrounding
environment, mainly radioisotopes K, U, and Th. The dating range is
dependent on the nature and state of conservation of the sample and the
surrounding environment but is between a few thousands and a couple of
million years. Since, ESR dating is best and most commonly applied to
tooth enamel in archaeology
PRINCIPAL: energy trapped in crystal imperfections depends on dose rate
and time. Used particularly for tooth enamel.
Fission (radioactive decay) of 238Uranium atom produces high
energy particles which leave straight "tracks" (10 - 20 µm) in glassy
The tracks are trails of destruction in the crystal lattice formed by
particles emitted during spontaneous fission of 238U. The number of
tracks is proportional to the cooling age as well as to the U content of
the apatite. Track lengths (initially ~ 17 microns in length) are
proportional to the cooling rate as tracks anneal and close during slow
the more tracks the older the crystal
fast cooling leaves longer tracks
Zircon is common in volcanic ash, and its crystals contain very small
amounts of the uranium-238 isotope6. As the uranium
Particles that make up atoms – the building blocks of matter. The three
basic ones are protons, neutrons and electrons. Protons and neutrons are
themselves made of even smaller particles called quarks. split away – this
process is called fission. These particles leave tiny tracks in the
crystal structure of the zircon, which geologists count using a powerful
microscope. The more tracks there are, the longer the uranium has been
decaying for. High temperatures remove the tracks from the crystal, so
when the ash leaves the hot volcano, its fission track ‘clock’ is at zero.
Tracks start building up after the crystals have cooled and settled in a
layer on the ground or at the bottom of the sea.
90 a.m.u & 135 a.m.u particles, 200 MeV
PRINCIPAL: 238U produces nearly all "tracks", 238U
decays at known rate, number of tracks related to age of sample
SET TO 0 when sampled heated (annealing the old tracks)
etch with acid, count the "native" tracks (microscope 200 - 500 X)
heat sample to remove tracks
calculate amount of 238U by irradiating sample with
neutron beam to produce artificial fission tracks from 235U
calculate age from native tracks and decay rate of 238U
age= #tracks / tracks/yr
ps g ld σ I φ
ln[ 1 + ------------------------]
ps = native track density
d = total 238U decay constant
pi = induced track density
f = spontaneous fission 238U
φ = irradiation (neutrons cm-2)
σ = cross-section area
I = isotopic ratio 235U/238U
g = geometry factor
LD = decay rate for 238U (1.551 X 10-10
Dating Range: (30,000) 100,000 - 20,000,000 yr
Materials Dated: apatite, mica, sphene, zircon, volcanic glass
Pearlette Volcanic Ash, Yellowstone Park, WY
2.02 +/-0.08 Ma Huckleberry Ridge
1.27 +/-0.1 Ma Mesa Falls
0.616+/-0.008 Ma Lava Creek
Bishop Ash, Long Valley Caldera, CA (near B/M reversal)
Cosmic Rays: high-energy charged particles from outside solar system
Solar Modulation: (flares produce) solar wind deflects
Geological Modulation: magnetic field, Van Allen Belts, Geography
Collision produces spallation products (e.g. neutrons),
which combine to form both stable and unstable products.
Collisions take place at earth's surface 147N,
105Be, 2613Al, 3617Cl,
eg. 4160 ± 310 3617Cl atoms yr-1
Cosmogenic nuclide dating can be used to determine rates of ice-sheet
thinning and recession, the ages of moraines, and the age of glacially
eroded bedrock surfaces.
Cosmogenic nuclide dating uses the interactions between in glacially
transported boulders or glacially eroded bedrock to provide age estimates
for rock at the Earth’s surface. It is an excellent way of directly dating
glaciated regions. It is particularly useful in Antarctica, because of a
number of factors:
The lack of terrestrial marine organisms makes radiocarbon dating
High winds make burial by snow less likely;
Burial and cover by vegetation is unlikely.
Cosmogenic nuclide dating is effective over short to long timescales
(1,000-10,000,000 years), depending on which isotope you are dating.
Different isotopes are used for different lengths of times. This long
period of applicability is an added advantage of cosmogenic nuclide
Cosmogenic nuclides are rare nuclides that form in surface rocks because
of bombardment by high-energy cosmic rays. These cosmic rays originate
from high-energy supernova explosions in space. Wherever we are on Earth,
when we are outside, we are constantly bombarded by these cosmic rays.
Cartoon illustrating cosmogenic nuclide exposure ages.
A glacier transports an erratic boulder, and then recedes, exposing it
to cosmic rays. Spallation reactions occur in minerals in the rocks upon
bombardment by cosmic rays.
By sampling the rocks and separating certain minerals (such as quartz or
pyroxene) and calculating the amount of these minerals (as a ratio to
other, stable, minerals), we can work out how long the rock has been
exposed on the earth’s surface.
When particular isotopes in rock crystals are bombarded by these energetic
, a spallation reaction results. Spallation reactions are those
where cosmic-ray neutrons collide with particular elements in surface
rocks, resulting in a reaction that is sufficiently energetic to fragment
the target nucleus. These spallation reactions decrease with depth.
Counting the numbers of these isotopes, normally as a ratio to other
isotopes, means that scientists can calculate how long rocks have been
exposed at the Earth’s surface.
These cosmic rays do not penetrate deep into the earth’s surface. This is
important for glacial geologists, as it means that surfaces that have had
repeated glaciations with repeated periods of exposure to cosmic rays can
still be dated, as long as they have had sufficient glacial erosion to
remove any inherited signal.
Geologists must ensure that they choose an appropriate rock. Granite
and sandstone boulders are frequently used in cosmogenic nuclide dating,
as they have large amounts of quartz, which yields Beryllium-10, a
cosmogenic nuclide ideal for dating glacial fluctuations over Quaternary
Beryllium-10 (10Be) does not occur naturally in quartz, and
once it forms following spallation it becomes trapped by quartz’s
regular crystal lattice. For a rock to be suitable for cosmogenic
nuclide dating, quartz must occur in the rock in sufficient quantities
and in the sufficient size fraction. A general rule of thumb is that you
should be able to see the quartz crystals with the naked eye.