Dating Methods

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Dating Methods Anthropology 101 Scott A. Lukas,
Ph.D.
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Introduction
The first problem encountered when trying to
determine the date of an ancient object is that
in prehistoric times there were no written
records to document the cultures of the past, so
archaeologists relied on a system of relative
dating to put things into context. In relative
dating, a series of techniques are used that
compares artifacts to determine which is older.
Paleolimnology involves some of the relative
dating techniques used. Unfortunately, relative
dating techniques give us no idea of the actual
age of an artifact or site.
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Text References
Here are some of the textbooks references to
dating methods Stratigraphy (p. 30-32) Seriation
(p. 340-342)/ Tempering (p. 289) Faunal
Assemblage (p. 51) Radiometric Methods (p.
48-52) Potassium Argon (p. 415) Fission Track (p.
415) Radiocarbon (p. 151, 355)
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The Issues in Dating
When archaeologists study dating techniques
(relative and absolute), the following factors
relate to all such techniques Material which
dating method should be used given the
composition of the artifact (bone, stone,
pottery, organic, etc.)? Age given the
approximate age of the object, which method can
be used to ascertain a more certain date?
Damage will the dating method process destroy or
damage the artifact? Cost how much will the
dating set me back?
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Relative v.s. Absolute Dating
When an archaeologist first begins to collect
artifacts on a given site, they must use both
Relative and Absolute Dating techniques. Both
techniques are used to find out how old a
specific site is, or how old an artifact may be.
Evaluating a site using dating techniques aids
the archaeologist in finding a specific context
or use for artifacts. Both relative and absolute
dating techniques require the analysis of
artifacts, whether they be stone, bone, pollen,
or tools, etc. The difference between relative
and absolute dating is that, absolute dating can
find an exact date of how old a specific object
is (say 1,000 years old approx.), where as
relative dating is an estimation based on other
factors of a given site.
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Relative Dating
Relative dating is an archeological dating
technique which assigns a speculative date to an
artifact based upon many factors such as
location, type, similarity, geology and
association. Types of relative dating techniques
include, dendrochrology, pollen analysis, ice
core sampling, stratigraphy, seriation,
linguistic dating, and climate chronology, in
addition to many other types of dating. All of
the dating techniques above are directly
concerned with discovering the correct order of
events at a site, and rely heavily on
associations. Relative dating usually relies upon
several standard factors within a site.
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Factors in Relative Dating
The reliance upon stratigraphy in relative
dating, is concerned with how far apart
geographically, one tool type or artifact is from
another, or which tool type occupies which type
of strata. This is best summarized in the law of
supposition, which states that lower layers of
earth or artifacts are older than those which lay
on top. For example, a tool type found within a
strata or level of sandy soil, is below a
geographic strata of clay on a site, and can be
considered to be older or in the sense that it
was created before the tool type above it.
Seriation works upon the same principles
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Factors in Relative Dating
Another relative dating method is the
geologic-climatic method. Since geology can give
clues to climate, analysis of artifacts which are
associated with geology can be dated if the age
of a climatic event (such as an ice age) is
known. Plant and animal fossils can also give a
clue to the climatic period. For example, a
modern animal of the jungle found in the middle
of a desert site can signify that the animal
supersedes the climatic change from jungle
environment to desert. If the rate of
desertification is known, than the fossil can be
dated.
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Factors in Relative Dating
Pollen grains can also be used to date artifacts.
Pollen can be analyzed to identify the plant life
and the climate of the era which can contribute
to creating a relative date for the items found
with the grains of pollen. Again, association is
used to connect two items found together.
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The Value of Relative Dating
Using known processes of change, relative dating
can be used to predict the age of artifacts found
under a variety of circumstances when absolute
techniques are either not available or not
conclusive. It is a very valuable tool used by
archaeologists in the quest to place our history
in order, and although it has its limits, it will
be employed by archaeologists the world over as
long as the science exists.
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Examples of Relative Dating
Let us review some of the most popular forms of
relative dating.
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Stratigraphy
Stratigraphy is the oldest of the methods that
archaeologists use to date things. Stratigraphy
is based on the rule of superposition--like a
layer cake, the lowest layers must have been
created first. That is, artifacts found in the
upper layers of a site will have been deposited
more recently than those found in the lower
layers. Cross-dating of sites, when one compares
geologic strata at one site with another
location, and extrapolates relative ages in that
manner is still used today, primarily when sites
are far too old for absolute dates to have much
meaning. .
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Stenos Laws
It was recognized in the 1600's that in a
sedimentary sequence, the older beds are on the
bottom, and the younger beds are on the top. This
has come to be called the Principle of
Superposition. You can visualize how this occurs
if you imagine a stack of newspapers in the
corner of a room. Every day you put another
newspaper on the pile. After several weeks have
passed, you have a considerable stack of
newspapers, and the oldest ones will be on the
bottom of the pile and the most recent ones will
be on the top. This fairly obvious, but very
important fact about layering was first noted by
Nicholaus Steno, and is the first of three
principles which have come to be known as Steno's
Laws. .
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Stenos Laws
Steno's second law is the Principle of Original
Horizontality, which states that sediments are
deposited in flat, horizontal layers. We can
recognize this easily if we consider a
sedimentary environment such as the sea floor or
the bottom of a lake. Any storm or flood bringing
sediment to these environments will deposit it in
a flat layer on the bottom because of the
sedimentary particles settling under the
influence of gravity. As a result, a flat,
horizontal layer of sediment will be
deposited. .
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Stenos Laws
Steno's third law is the Principle of Original
Lateral Continuity. If we consider again the
sediment being deposited on the seafloor, the
sediment will not only be deposited in a flat
layer, it will be a layer that extends for a
considerable distance in all directions. In other
words, the layer is laterally continuous. .
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Rate of Accumulation
Rate of Accumulation is a relative dating
technique which uses stratigraphy to determine
the relative ages of artifacts. The basic
principle of Rate of Accumulation is based on the
geologic theory that the rock layers in the earth
accumulated over time and that the deeper the
layer, the older. In archeology, the artifacts
are the basis on which the rate of accumulation
is determined over time. So it would follow that
the deeper the artifact, the older it is. .
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Cross-dating
Cross-dating is a technique used to take
advantage of consistencies in stratigraphy
between parts of a site or different sites, and
objects or strata with a known relative
chronology. A specialized form of cross-dating,
using animal and plant fossils, is known as
biostratigraphy. The following animation will
provide you with an example of how cross-dating
is used. Click on the button below to start the
animation. .
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Faunal Assemblage
Note the reference on page 51 of the text. As
presented, archaeologists often use faunal
remains (animals) to construct relative
chronologies of early human ancestors. If we
know the relative time periods during which
certain species lives, we can then connect these
faunal finds to relative periods in the lives of
early humans and archaeological sites.
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Seriation
Seriation, on the other hand, was a stroke of
genius. First used, and probably invented by
Flinders-Petrie in 1899, seriation (or sequence
dating) is based on the idea that artifacts
change over time. Like fins on the back end of a
Cadillac, artifact styles and characteristics
change over time, coming into fashion, then
fading in popularity. Generally, seriation is
manipulated graphically. The standard graphical
result of seriation is a series of "battleship
curves," which are horizontal bars representing
percentages plotted on a vertical axis. Plotting
several curves can allow the archaeologist to
develop a relative chronology for an entire site
or group of sites.
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Cation Ratio Dating
Cation ratio dating is used to date rocks that
have a modified surface such as prehistoric rock
carvings (petroglyphs). This is a relative dating
technique and is not considered to be an accurate
method of dating in some professional views.
Rocks are covered by a kind of varnish, a
chemically-changed layer caused by weathering
that builds up over time. The change in the rock
varnish is due to calcium and potassium seeping
out of the rock. The cation ratio is determined
by scraping the varnish from the carved or
petroglyph surface back to the original rock
surface and making a comparison of the two using
a positively charged ion. Like dendrochronology,
this ratio is affected by soil and moisture.
Thus, a leaching curve is created by geographical
area.
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Cultural Affiliation Dating
Cultural Affiliation dating is complex. There is
no one way to go about using this technique. If a
community of people uses Cultural Affiliation to
gauge its history, they might be differently
attached to a place, body, or artifact. In
addition to this, sometimes there are different
levels of Cultural Affiliation. Debates are also
made as to which kind of Cultural Affiliation is
the best
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Cultural Affiliation Dating
The different levels of Cultural Affiliation are
known as temporal levels. Native peoples of the
Southwest have a few different types of these
levels. The first type is the Mythic Period,
which was the time before the creation of fully
formed humans, and even has some continuation
into the present. The next type is the
Traditional Period. This Affiliation has fully
formed humans living where they are now. Historic
Period Affiliation follows, and began when the
first European person, animal, tool, grain, or
disease arrived. Next is the Aboriginal Period.
This is defined by the U.S. Indian Claims
Commission as when Indian ethnic groups lost
their land to the U.S. federal government. And
the last is the Contemporary Period, which is
where Indian peoples are living today.
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Flourine Dating
Fluorine is an element that is found in most
ground water around the world. It can be used as
a relative dating technique. Skeletal remains
buried in the earth are subject to a wide range
of chemical changes. One of these changes can
occur when percolating ground water comes into
contact with the remains. The ground water
inundates the bone remains with a solution of
minerals drawn from local soils. This can cause a
change in the mineral composition of the bone.
Hydroxyl ions are displaced with a form of
soluble fluorides. These ions form fluorapatite
which is markedly less soluble. Over time, more
and more fluorides are accumulated. The rate
varies depending on the specific condition in the
soil of the area and increases with age.
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Flourine Dating (and Piltdown)
Fluorine dating is chiefly of value in
determining whether bone implements or human
skeletal remains found in association with other
bones were buried at the same time. It was
fluorine dating that was instrumental in the
debunking of Piltdown Man. The Piltdown
controversy lasted until 1949 when the Piltdown
skull and jawbone were subjected to fluorine
testing. The levels of fluorine in the skull and
jawbone were significantly lower then in other
bone specimens collected from the same area.
After further testing the jawbone and a canine
tooth proved to contain no more fluorine than
fresh bones and teeth. The skull contained enough
fluorine to indicate that it was not modern.
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Obsidian Hydration Analysis
Developed in 1960, Obsidian Hydration Analysis
(OHA) is an inexpensive technique archaeologists
and geoarchaeologists use to find the age of a
site they have excavated. This method is most
often used as a means of relative dating , but an
absolute date may also be estimated in some
circumstances. When obsidian is newly exposed to
the atmosphere, its surface begins to absorb
water from the air, which gradually seeps into
the interior of the stone. Several factors can
affect the obsidian's water absorption, including
soil type, climate, time and geochemistry. When
viewed under a microscope, the layer permeated by
moisture (known as a "rind") becomes visible as a
rim when the rind reaches a width of 0.5 microns
(a micron being one millionth of a meter). The
greater the rind thickness, the greater the age
of the exposed obsidian.
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Patination
Patination is a technique involving the measuring
of the patina on an artifact. The patina is the
outermost surface of the artifact that differs in
color, texture, luster or composition from the
rest of the artifact. This difference is the
result of chemical, physical or biological change
in response to the surrounding soil and
environmental condition. Although it is not an
actual dating technique, patination is used when
multiple artifacts of the same type are found in
the same area and under the same conditions. The
use of this technique is to determine the age of
the artifacts, relative to the others, by
comparing the thickness of the patina on them.
There are many variables that have to be
calculated, and this makes dating lithics from
patina formations a relative dating technique.
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Pollen Analysis
Pollen analysis, study of vegetation history
using the microfossils (pollen grain and spores
of size 15-50 um), can give us useful information
about the target area's condition in the present
and past. Since the outside of the pollen grain
wall is made of highly resistant material, the
pollen spores from 400 million years ago can be
found today. Each pollen grain and spore is
different in structure and shape, thus, the
morphology is the key to understanding the kinds
of vegetation that existed and their evolutionary
development.
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Varve Analysis
Varve analysis is the process of counting varves
or annually laminated sediments to determine the
rates of change in climate and various
ecosystems. Varves form when glacial advances
come in contact with bodies of water such as
lakes. When this process occurs, layers of
sediment form on the floor of the body of water.
This technique provides an opportunity to acquire
detailed chronological information about the
composition, displacement, and climate of that
region, at that time. It was first developed by
the Swedish scientist Baron de Geer in 1878.
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Varve Analysis
Significant advances have been made to improve
accuracy in determining varves. Using
audio/visual equipped computers and digital video
cameras, differences in hues can be determined
and analyzed. Through such a procedure, accurate
counts are made to determine different layers,
which in turn gives varve analysis more
credibility. Also, with the aid of computers and
digitally enhanced photography, content and
classification are accurate and efficient.
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Absolute Dating Methods
Absolute dating, the ability to attach a specific
chronological date to an object or collection of
objects, was a breakthrough for archaeologists.
Until the 20th century, with its multiple
developments, only relative dates could be
determined with any confidence. Since the turn of
the century, several methods to measure elapsed
time have been discovered.
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Chronological Markers
The first and simplest method of absolute dating
is using objects with dates inscribed on them,
such as coins, or objects associated with
historical events or documents. For example,
since each Roman emperor had his own face stamped
on coins during his realm, and dates for
emporer's realms are known from historical
records, the date a coin was minted may be
discerned by identifying the emperor depicted.
Many of the first efforts of archaeology grew out
of historical documents--for example, Schliemann
looked for Homer's Troy, and Layard went after
the Biblical Ninevah--and within the context of a
particular site, an object clearly associated
with the site and stamped with a date or other
identifying clue was perfectly useful. n
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Astronomical Dating
Sir Norman Lockyer (1836-1920), a professional
astronomer, tried to date Stonehenge by
astronomical means. The technique he attempted to
use was astronomical dating. Over a period of
several thousand years, the midsummer sunrise
position had changed. Today, it rises slightly
east of the position that it rose in prehistoric
times. The sighting line was marked by the axis
of the Avenue, and pegs were set at intervals
along the Avenue. Since the sun no longer rises
in the same position, we have to measure the
alignment of the stones with optical instruments
to calculate the position of the sunrise as it
used to be.
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Dendrochronology
The use of tree ring data to determine
chronological dates, dendrochronology, was first
developed in the American southwest by astronomer
Andrew Ellicott Douglass. In 1901, Douglass began
investigating tree ring growth as an indicator of
solar cycles. Douglass believed that solar flares
affected climate, and hence the amount of growth
a tree might gain in a given year. His research
culminated in proving that tree ring width varies
with annual rainfall. Not only that, it varies
regionally, such that all trees within a specific
species and region will show the same relative
growth during wet years and dry years. Each tree
then, contains a record of rainfall for the
length of its life, expressed in density, trace
element content, stable isotope composition, and
intra-annual growth ring width.
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Dendrochronology
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Dendrochronology
In order for this to be a reliable method for
dating, four factors must be present 1. The
species studied must only produce one ring per
growing season or year. 2. Only one dominant
environmental factor can be the cause of hindered
or increased growth. 3. The dominant
environmental factor should vary each year so we
can see the changes clearly in every ring. 4. And
lastly, the environmental factor must affect a
large geographic area so testing can be compared
easily (Stokes and Smiley 6).
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Dendrochronology
The main drawback to dendrochronology is its
reliance on the existence of relatively
long-lived vegetation with annual growth rings.
Secondly, annual rainfall is a regional climatic
event, and so tree ring dates for the southwest
are of no use in other regions of the world.
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Archaeomagnetism
The field of study concerned with ancient
geo-magnetic phenomena and the use of
archaeological material in determining past
variation in the earth's magnetic field is called
archaeomagnetism. These variations have been
recorded in London, Paris and Rome over the past
four centuries. The fact of these variations in
the earth's magnetic field from time to time in
direction and intensity are the basis of
archaeomagnetic dating. This magnetism occurs
naturally and is called fossil magnetism.
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Electron Spin Resonance Dating (Radiometric
Dating)
Electron Spin Resonance Dating falls into the
group of dating methods that uses radiation
exposure to date many materials found at
archaeological sites. It is also known as a
Radiometric Dating Method. This technique is
mostly used to date minerals.It has been used to
date such things as sedimentary quartz,
fossilized teeth, flint, and calcium carbonate in
limestone, coral and egg shells.
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Fission Track Dating
Fission track dating was developed in the mid
1960s by three American physicists, who noticed
that micrometer-sized damage tracks are created
in minerals and glasses that have minimal amounts
of uranium. These tracks accumulate at a fixed
rate, and are good for dates between 20,000 and a
couple of billion years ago. This description is
from the Geochronology unit at Rice University.
Fission-track dating was used at Zhoukoudian. A
more sensitive type of fission track dating is
called alpha-recoil.
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Optically Stimulated Luminescence
The OSL (Optically Stimulated Luminescence)
technique is similar to thermal dating. Like
thermal, optical dating can be thought of as a
"clock setting" event. The minerals in the
sediment grains are sensitive to light, and when
exposed to light, electrons vacate the sediment
grains. This process is known as recombination,
which is the "clock setting" event. To detect the
age, a comparison must be made between sediment
grains with a known amount of added radiation,
and sediment grains that were acted upon
naturally.
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Optically Stimulated Luminescence
Optical dating is suitable for a variety of
unheated sediments that are no older than 500,000
years. This includes silty and sandy sediments
that were deposited by water. The technique does
not apply to sediments that were deposited with
little or no exposure to light. Glacial deposits
fall into this category.
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Potassium-Argon
Potassium-Argon This method, like radiocarbon
dating, relies on measuring radioactive
emissions. The Potassium-Argon method dates
volcanic materials and is useful for sites dated
between 50,000 and 2 billion years ago. It was
first used at Olduvai Gorge. A recent
modification is Argon-Argon dating, used recently
at Pompeii. .
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Racemization
A controversial dating method is Racemization.
This method utilizes the presence of amino acids
in organic materials. It is applied to organic
materials (or fossil materials) such as human and
animal bones, teeth, plants, ostrich egg shells,
mollusks, marine sediments, freshwater and marine
shells, oyster shells, carbonate shells
(foraminifera, snails, clams, ostracods,)
calcareous sediments and peats. Racemization
can date samples to approximately 5,000 - 100,000
years old. However, there are dates as old as
200,000 years produced by this method. It is said
that Racemization can date older materials than
what Carbon 14 can date. As a dating technique,
racemization has practical application in the
fields of Archaeology, Geochemistry, Marine
Geology, Geochronology or Geoscience .
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Thermoluminescence
Thermoluminescence (called TL) dating was
invented around 1960 by physicists, and is based
on the fact that electrons in all minerals emit
light (luminesce) after being heated. It is good
for between about 300 to about 100,000 years ago,
and is a natural for dating ceramic vessels. The
Laboratory for Thermoluminescence and Dating at
the University of Tolun in Poland provides a
short description of the technique. TL dates have
recently been the center of the controversy over
dating the first human colonization of Australia.
.
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Uranium-Thorium Dating
Uranium-Thorium dating is an absolute dating
technique which uses the properties of the
radio-active half-life of Uranium-238 and
Thorium-230. The half-life of uranium--238 is
4,470,000,000 years, that is, in that many years
half of the original amount is still uranium- the
other half has lost protons to form a different
element which is more stable. The half-life of
thorium-230 is only 75,380 years. When the
amounts of uranium and thorium are compared an
accurate estimation of the age of an object can
be obtained. .
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Uranium-Thorium Dating
Uranium-Thorium dating was first used on fossil
bones in 1956, however, it had been used for
dating wood before this. This dating technique
has been used effectively on marine sediment,
bone, wood, coral, stone and soil. One of the
benefits of uranium-thorium dating is that the
sample sizes can be less than 20 grams, in fact
bone samples can be 3-5 grams for an accurate
date. .
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Cautionary Dating
Radiocarbon samples are easily contaminated by
rodent burrowing or during collection.
Thermoluminescence dates may be thrown off by
incidental heating long after the occupation has
ended. Site stratigraphies may be disturbed by
earthquakes, or when human or animal excavation
unrelated to the occupation disturbs the
sediment. Seriation, too, may be skewed for one
reason or another. For example, in our sample we
used the preponderance of 78 rpm records as an
indicator of relative age. Say a Californian lost
her entire 1930s jazz collection in the 1993
earthquake, and the broken pieces ended up in a
landfill which opened in 1985. Heartbreak, yes
accurate dating of the landfill, no. .
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Cautionary Dating
Dates derived from dendrochronology may be
misleading if the occupants used relict wood to
burn in their fires or construct their
houses. Obsidian hydration counts begin after a
fresh break the obtained dates may be incorrect
if the artifact was broken after the occupation.
Even chronological markers may be deceptive.
Collecting is a human trait and finding a Roman
coin a ranch style house which burned to the
ground in Peoria, Illinois probably doesn't
indicate the house was built during the rule of
Caesar Augustus. .
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So How Do They Do It?
So how do archaeologists resolve these issues?
Context and cross-dating. Since Michael
Schiffer's work in the early 1970s,
archaeologists have come to realize the critical
significance of understanding site context. The
study of site formation processes, understanding
the processes that created the site as you see it
today, has taught us some amazing things. If at
all possible, the archaeologist will have several
dates taken, and cross check them by using
another form of dating. This may be simply
comparing a suite of radiocarbon dates to the
dates derived from collected artifacts, or using
TL dates to confirm Potassium Argon readings.
.
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