Transcript dating methods

DATING METHODS
One of the most important questions asked about any ancient object is just how old it really is.
There are a number of scientific techniques which can be used to date antiquities
DENDROCHRONOLOGY
RADIOCARBON DATING
POTASSIUM ARGON DATING
LUMINESCENCE DATING
ESR DATING
FISSION TRACK DATING
PROTEIN AND AMINO ACID DIAGENESIS DATING
OBSIDIAN HYDRATIAN DATING
ARCHEOMAGNETIC DATING
SURFACE DATING USING ROCK VANISH
TARİHLENDİRME YÖNTEMLERİ
Organik veya inorganik materiyalleri
Radyokarbon (C-14),
Dendrokronoloji-Tree ring dating,
Elektron Spin Rezonans (ESR),
Termolüminesans (TL) ve OSL gibi
arkeometrinin en önemli uygulamaları arasında sayılabilecek yöntemlerle tarihlendirmek mümkündür.
Potasyum Argon Metodu (KA):
Radyoaktif olan (potasyumun) radyoaktif olmayan Argon40 gazına dönüşmesine dayanır.
Özellikle jeolojik tabakalar içinde bulunan fosil kalıntılarına uygulanır.
100.000 yılı aşkın volkanik kayalara da uygulanmaktadır.
Radyokarbon Metodu (C-14): 1955' te Amerika'da Chicago Üniversitesi'nde W. Libby
ve arkadaşları bu metodu uygulamışlardır. Bu tarihten itibaren en geçerli, en yaygın tarihlendirme
metodudur. Özellikle tarih öncesi arkeolojide kullanılır. Tüm organik maddelerde bulunan
radyoaktif karbonun, bunların canlılıklarını kaybetmelerinden sonra belirli bir tempoda azaldığı
gözlenmiştir. Bu oran bilindiğinden, bulunan organic maddenin yaşı, bu gözönünde tutularak bulunur.
Ölçülere göre yaklaşık olarak organik maddelerin ömürlerinin yarısı boyunca yılda 5568 karbon kaybettikleri
anlaşılmıştır. Sakıncalı yanı tam doğru netice vermemesidir.Nedeni de atmosferin her zaman aynı miktarda
karbon ihtiva etmemesidir.
Dendrokronoloji: Amerikalı A.E. Douglass tarafından bulunan, ağaç gövdelerinin enine kesitinde görülen yıllık ha
Douglass eski evlerde kullanılan ağaçlardan özel bir teknikle kesit alarak,
üzerlerindeki halkaları sayıp yapıların tarihini saptamayı başarmıştır.
Termolüminesans Metodu: Taş, keramik, cam gibi kristal yapıya sahip maddelerin yaş tayininde
kullanılır.
DENDROCHRONOLOGY
Dendrochronology or tree-ring dating
is the method of scientific dating based
on the analysis of tree-ring growth
patterns. This technique was invented
and developed during the 20th century
originally by A. E. Douglass,the founder
of the Laboratory of Tree-Ring
Research at the University of Arizona.
The technique can date wood to exact
calendar years.
Visible rings result from the change in
growth speed through the seasons of
the year, thus one ring usually marks
the passage of one year in the life of
the tree.
Radiocarbon dating is a radiometric dating method
that uses the naturally occurring isotope isotope carbon-14 (14C)
to determine the age of carbonaceous materials up to about 60,000 years
The technique of radiocarbon dating was discovered by
Willard Libby and his colleagues in 1949 during his tenure as a professor at the University of
Chicago.
Probably the best known and most frequently used is radiocarbon or 14C dating.
Radiocarbon dating can only be applied to organisms that were once alive and is a means of determining how
long ago they died.
Radiocarbon dating is possible because of the existence in nature of a tiny amount of 14C, or radiocarbon,
a radioactive isotope of carbon.
By measuring how much 14C remains in ancient organic materials, it is possible to calculate how long ago they
died.
To do this requires extensive chemical processing, carried out in laboratories
To convert the carbon in the ancient objects to a form in which the very low level radioactivity can be measured.
Most radiocarbon dating is carried out
on bone or charcoal,
as these are the organics that most frequently survive from the past,
but many other materials can also be dated using this technique.
RADIOCARBON DATING
Carbon has two stable, nonradioactive isotopes : carbon-12 (12C), and
carbon-13 (13C).
In addition, there are trace amounts of the unstable isotope
carbon-14(14C) on Earth.
Carbon-14 has a half-life of 5730 years and would have long ago
vanished from Earth were it not for the unremitting cosmic ray impacts on
nitrogen in the Earth's atmosphere, which create more of the isotope.
This 14C isotope is produced in
the upper atmosphere by the
action of cosmic rays on 14N.
This 14C combines with oxygen
to produce carbon dioxide
(CO2) and is taken in by plants
during photosynthesis.
From plants this 14C is
absorbed into the tissues of
every living thing via the food
chain.
Since it is radioactive, it is
unstable and decays away at a
known rate. While any plant or
animal is alive
the 14C lost by radioactive
decay is constantly replaced
through the food chain,
but when that organism dies,
no more 14C is taken in, and
the amount present in the
tissues goes down.
(from Aitken 1990)
POTASSIUM ARGON DATING
Potassium-Argon Dating
is the only technique for dating very old
archaeological materials.
Geologists have used this method to date rocks as
much as 4 billion years old. It is based on the fact
that some of the radioactive isotope of Potassium,
Potassium-40 (K-40) ,decays to the gas Argon as
Argon-40 (Ar-40).
By comparing the proportion of K-40 to Ar-40 in a
sample of volcanic rock, and knowing the decay rate
of K-40, the date that the rock formed can be
determined.
In principle all the materials containing quartz or feldspars and submitted to
heating to several hundreds of degrees centigrate can be dated by this way
LUMINESCENCE DATING
Materials with suitable luminescence
properties can be dated because at
some point in the past traps are emptied
of their charge by sufficient exposure to
heat or light.
Subsequently, traps become refilled
because of continued ionization by
radioactivity and a latent luminescence
signal steadily accumulates
Optical dating is method of determining
how long ago minerals were last exposed
to daylight, It is useful to geologists and
archaeologists who want to know when
Thermo luminescence dating is
such an event occurred. Time clock
method of determining how long ago
become zero for a sediment of
minerals heated to 800-1000°C degrees; earthquake line,or sediment of loess.
then when heated time clock become
zero (for making pottery or baked brick)
In phosphorescence the energy difference between the excited state and the
meta stable state is generally so small that detrapping occurs by lattice vibrations at ambient temperature, i.e. no external supply of energy is required.
However, in TL and OSL the energy difference between the excited state
and the meta stable state is so large that external energy must be applied
to detrap the electrons.
In TL the luminescence emission is accelerated by
thermal stimulation whereas
in OSL the luminescence emission is accelerated by optical stimulation.
LUMINESCENCE DATING
As far as dating is concerned, the
phenomenon of luminescence can be
subdivided according to the kind of
energy supply during stimulation into
ELSEC 9010 osl device
•
thermoluminescence
stimulated by heat)
•
optical stimulated luminescence (OSL,
stimulated by visible light)
•
infrared stimulated luminescence
(IRSL, stimulated by infrared light)
(TL,
Basic Concepts in Luminescence
Luminescence is a generic term for the electromagnetic radiation
(usually in the form of visible light) emitted as a consequence of
an atomic or molecular non-thermal excitation. Thus,
luminescence is often described as cold light to distinguish it from
incandescent light emission, which occurs when a material is
excited thermally. Luminescent materials are able to absorb
energy, store part of it and convert it into light; these materials
usually have a crystalline structure.
Luminescence can broadly be categorised as either Fluorescence or phosphorescence. Fluorescence: the light emission resulting from the relaxation of an
electron from an excited state to the ground state (possibly through a metastable
state from which transition to the ground state is allowed). The delay between the
absorption of energy resulting in the excited state and the emission is determined by
the life time of the excited state.The life time can be as short as picoseconds and as
long as milliseconds in special cases. Photoluminescence, cathodo luminescence,
chemi-luminescence, bioluminescence and triboluminescence are all examples of
different fluorescence processes with different means of excitation(i.e. photons,
electrons, chemical energy, biochemical and mechanical energy, respectively)
Phosphorescence: the relaxation back to the ground state is delayed by a relaxation
to the ground state is not permitted. These meta stable states function as electron
traps, and energy must be supplied to detrap (release) the electrons back to the
excited state from where they can relax to the ground state (McKinlay, 1981). The
return to the ground state is thus delayed for period of time; the length of which
depends on the life time of the electron in the meta stable state.
The two types of luminescence
are distinguished by the atomic
mechanisms
whereby the light is emitted.
How stimulation occurs
With optical dating,the signal is
obtained by exposure to a beam of
blue /green light or infrared
radiation.
Optically-stimulated
luminescence(OSL) is commonly
used , also it is calling as Photon
stimulated luminescence (PSL)
and Photoluminescence(PL)
Optically stimulated luminescence relies on the same basic concepts as
TL,
but in OSL the stimulation energy is supplied by photons instead of heat.
Thus, the physical principles of OSL are closely related to those of TL.
However, it is not clear that the same defect centres are involved in both
processes
in any one material (McKeever, 2001). OSL has several advantages over
TL.
When dealing with unheated materials (i.e. materials zeroed by light exposure) the most important of these advantages are that in OSL only the
trapping levels most sensitive to light are sampled; that is the charge
population most e®ectively zeroed. In many samples, it is believed that 99% of
the
initial OSL signal originates from the 325 ±C TL peak in quartz (Murray and
Wintle, 1999). Another advantage of OSL over TL is that stimulation can be
performed at room temperature (although some advantages may be
obtained
Trap mechanism
Trap schematic
Trap mechanism, namely the timedependent accumulation of electrons and
holes in the crystal lattice of certain common
minerals (trap). The minerals are acting as
natural radiation dosimeters. When a
mineral is formed or reset, all electrons are
in the ground state. Naturally occurring
radioactive isotopes (U, Th, and K) emit a
variety of rays which ionize atoms.
Negatively charged electrons are knocked
off atoms in the ground state (valence band)
and transferred to a higher energy state
(conduction band); positively charged holes
remain near the valence band.
The trapped electrons and holes forms luminescence centres which can be
stimulated. For the measurement of a luminescence signal, the trapped electrons
have to be either thermally (by heating) or optically (by light exposure) activated.
The electrons return to the conduction band and most of them will recombine with
the holes. If such holes are luminescence centres, light emission (luminescence)
is observed.
Zeroing a trap
•
Figure show the basic principles
of the dating process. A zeroing
event resets any previously
stored trapped electrons - this
resetting may be heating,
exposure to sunlight or mineral
formation. After zeroing, new
electrons and holes are trapped
as a result of natural radiation
References
• Chronometric Dating in Archaelogy (Edited
by R.E.Taylor and Martin j.Aitken)
• Archaeological dating using physical
phenomena(M.J. Aitken)
Electron spin resonance
ESR DATING
Electron spin resonance (ESR)
was proposed as a dating method
by Zeller as early as 1967, but its
practical application began with
the work of Ikeya in 1975.Study
based on carbonate materials,
bones, and quartz
Figure 1: Created by ionising
radiation, point defects
accumulate predictably in
enamel. Imperfections in the
crystal lattice exist between
the valence and conduction
bands, trapping free radicals
ESR-dating is a powerful tool to
date the time of mineralization,
sedimentation or last heating of
minerals. The dating method has
the same dosimetric basis as
TL/OSL, but in contrast to these
methods only paramagnetic
centers are detected with ESR. As
the measurement does not
destroy the centers however, it
can be repeated several times.
FISSION TRACK DATING
Fission tracks in an apatite
crystal (top) and in a muscovite
mica (bottom).
•
Fission track dating is a
radiometric dating technique based
on analyses of the damage trails, or
tracks, left by fission fragments in
certain uranium bearing minerals
and glasses.
•
Uranium-238 undergoes
spontaneous fission decay at a
known rate. The fragments emitted
by this fission process leave trails
of damage in the
crystal structure of the minerals
enclosing the uranium
FISSION TRACK DATING
•
Etching of polished surfaces of these minerals reveals the spontaneous fission
tracks for counting by optical microscopic means. The number of tracks
correlates directly with the age of the sample and the uranium content
Fission Tracks (FT) are micrometer-sized, linear damage tracks that occur in
insulating minerals and that are caused by the spontaneous fission of heavy,
unstable nuclides (mostly 238U in natural minerals). The spontaneous
fission of 238U occurs at a specific rate, described by the decay constant (l f
= 8.46 ´ 10-17a-1). This implies that when the uranium concentration (CU) of
a sample is known, the spontaneous FT density (r s = number of
tracks/cm2) in that sample is an indication for the sample’s age. r s is
determined by counting the tracks under an optical microscope (at 1250 ´
magnification). Since FTs are features at an atomic scale, they require
chemical techniques (etching) to make them observable under the optical
microscope. CU is measured by irradiating the sample in a nuclear reactor
with thermal neutrons
PROTEIN AND AMINO ACID
DIAGENESIS DATING
•
At a widely publicized news conference in August of 1972, Dr. Jeffrey Bada
of Scripps Institute of Oceanography announced the "discovery" of a new
dating method based on the rate of racemization of amino acids in fossil
material. He was quoted as saying that he had discovered the basis of the
method in 1968, and that it was so obvious and simple he was amazed it hadn't
been discovered earlier.
Amino acids are the "building
blocks," or sub-units, of proteins.
About 20 different kinds of amino
acids are found in proteins. All
amino acids in proteins (except
glycine) are L-amino acids. These
amino acids spontaneously tend
to slowly change to the D-form.
The D-form tends to revert to the
L-form, and eventually an
equilibrium is obtained, as
illustrated here for alanine
PROTEIN AND AMINO ACID
DIAGENESIS DATING
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The process by which an L-amino acid changes into a mixture of the L- and Dforms (or the D-form changes into a mixture of the L- and D-forms) is called
racemization. Racemization is complete when equal amounts of the L- and Dforms are obtained.
Hare and Mitterer3 measured the rate of racemization of L-isoleucine to
D-alloisoleucine in modern shell fragments heated in water at high
temperatures and extrapolated these data to lower temperatures in order
to estimate the rate of racemization of L-isoleucine in fossil shells to obtain
what they believed to be an approximate age for these fossil shells.
These data are believed to yield the rates at which L-isoleucine was
converted to Dalloisoleucine in the sediment through geological time. The
extent of conversion of L-isoleucine to D-alloisoleucine in core sediment
samples from various depths was then determined and conclusions
based on the above rates were used to estimate the ages of the
sediments from various core depths
OBSIDIAN HYDRATIAN DATING
The obsidian hydration dating method
was introduced to the archaeological
community in 1960 by Irving Friedman
and Robert Smith of the U. S. Geological
Survey (Friedman and Smith 1960). The
potential of the method in archaeological
chronologic studies was quickly
recognized and research concerning the
effect of different variables on the rate of
hydration has continued to the present
day by Friedman and others
Obsidian hydration rim
(between arrows)
.
•
OBSIDIAN HYDRATIAN DATING
When a new surface of obsidian is exposed to the atmosphere, such as during the
manufacture of glass tools, water begins to slowly diffuse from the surface into the
interior of the specimen. When this hydrated layer or rind reaches a thickness of
about 0.5 microns, it becomes recognizable as a birefringent rim when observed as
a thin section under a microscope. Hydration rims formed on artifacts can vary in
width from less than one micron for items from the early historic period to nearly
30 microns for early sites in Africa (Michels et al. 1983a; Origer 1989).
Once a hydration layer has been measured, it can be used to determine the
relative ages of items or, in some circumstances, can be converted into an
estimated absolute age. In order to transform the hydration rim value to a
calendar age, the rate of the diffusion of water into the glass must be
determined or estimated. The hydration rate is typically established empirically
through the calibration of measured samples recovered in association with
materials whose cultural age is known or whose age can be radiometrically
determined, usually through radiocarbon dating methods (Meighan 1976).
ARCHEOMAGNETIC DATING
Paleomagnetism is concerned with the history of the Earth's magnetic
field during geologic time,and applications to geological and
geophysical problems.Archeomagnetism involves the study of the
Earth's magnetic field during archaelogical time,and the application of
paleomagnetic techniques and principles to archaeological features
and artifacts (Tarling 1983)
An absolute dating method based on the earth's shifting magnetic poles.
When clays and other rock and soil materials are fired to approximately
1300°F (700°C) and allowed to cool in the earth's magnetic field, they
retain a weak magnetism which is aligned with the position of the poles
at the time of firing. This allows for dating, for example, of when a fire
pit was used, based on the reconstruction of pole position for earlier
times.
SURFACE DATING USING RACK
VANISH
•
Rock vanish ,a dark colored,magnesium-,iron-,and silica-rich coating that forms on
exposed rock surfaces over time, especially in arid and semi-arid regions,has been
used as a chronometric dating tool in both archeology and geology.
•
The methods most commonly employed are cation-ratio dating ,using differential
leaching of cations in the varnish coating ,and accelerator mass spectrometry-based
radiocarbon dating of organic material contained within or trapped beneath the
varnish coating.
Some terminology
Chronology
is the science of locating events in time.
An arrangement of events from either earliest to latest or
the reverse
Archaeological stratigraphy
•
In the field of archaeology, soil
stratigraphy is used to better understand
the processes that form and protect
archaeological sites. The law of
superposition holds true, and this can
help date finds or features from each
context, as they can be placed in
sequence and the dates interpolated.
Phases of activity can also often be seen
through stratigraphy, especially when a
trench or feature is viewed in section
(profile). As pits and other features can
be dug down into earlier levels, not all
material at the same absolute depth is
necessarily of the same age, but close
attention has to be paid to the
archeological layers.