Other Dating Techniques

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Other Dating Techniques


Luminensence Dating - Thermoluminescence (TL), OSL and Optically Stimulated Luminescence (OSL)
Electron-spin resonance
Fission track dating
Amino acid racmisation and epirmisation
Obisdian hydration dating - volcanic glass
Cosmogenic Nuclide Exposure Dating

comparison of timescales of other dating methodss
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.

Luminensnce Dating - Thermoluminescence (TL), OSL and Optically Stimulated Luminescence (OSL)

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 daylight.

sample releases ligh upon heating

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 studies.

Like 14C dating, thermoluminescence is related to radioactive decay.

exposure to light resets clock

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 dates.

Dating Range: 1,000 - 300,000,000 yrs

Materials Dated: pottery, hearths, tephra, radiolaria, speliothems, sediments

  1. measure native TL (OSL)
  2. measure sensitivity TL/rad (OSL/rad) (radiation absorbed dose)
  3. determine dose rate at site (rad/yr)

native TL / sensitivity (TL/rad) rad
age (yr) = ----------------------------- = ---------

site dose rate (rad/yr) (rad/yr)

mineral sample giving off light on heating

Electron-spin resonance

Australian megafauna teeth 4.2myo
Australian megafauna teeth in jawbone 4.2myo (see Earth Science Australia  - Expeditions...)

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.
  1. measure TD Total Dose with spectrometer
  2. measure ED External Dose rate in field
  3. age (Te) = TD/ED

Fission track dating

Fission (radioactive decay) of 238Uranium atom produces high energy particles which leave straight "tracks" (10 - 20 µm) in glassy material

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 cooling.
Zircon is common in volcanic ash, and its crystals contain very small amounts of the uranium-238 isotope6. As the uranium decays,subatomic particles7
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.

U-series decay

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)

  1. etch with acid, count the "native" tracks (microscope 200 - 500 X)
  2. heat sample to remove tracks
  3. calculate amount of 238U by irradiating sample with neutron beam to produce artificial fission tracks from 235U
  4. calculate age from native tracks and decay rate of 238U
      age= #tracks / tracks/yr

1 ps g ld σ I φ
Age = --- ln[ 1 + ------------------------]

LD pi lf

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 yr-1)

Dating Range: (30,000) 100,000 - 20,000,000 yr
Materials Dated: apatite, mica, sphene, zircon, volcanic glass

Amino acid racmisation and epirmisation

Based on the rate of a chemical process effected both by time and temperature

Dating Range: 2000 - 2,000,000 ± 15%

Materials Dated: organic, exchangeability very important



Obisdian hydration dating -  volcanic glass

Example: Ken Pierce et al. (1976) date Rocky Mountain glacial chronology


Obsidian Hydration Readings

Surface dating
Cosmogenic Nuclide Exposure Dating

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 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:
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 dating.
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.

cosmogenic nuclide exposure ages overview

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 timescales.
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.