Anth 140
Summer 2007
DATING TECHNIQUES
DATING
TECHNIQUE
METHOD
AGE RANGE
(B.P.)
DATABLE
MATERIAL
DATED EVENT
(What is event
dated?)
Death of tree
STRENGTHS
LIMITATIONS
Dendrochronology
Counting of annual growth rings
0 - 8,000 years
Tree rings in
preserved logs and
lumber.
Precise to a year
Wood, charcoal,
bone, carbonate,
shell, organic
sediments
Death of organism
Well-established
method, relatively
cheap
100 - 50,000 years
Wood, charcoal,
bone, carbonate,
shell, organic
sediments, seeds
Death of organism
35,000 years
Obsidian objects
The time the surface
of the obsidian object
was exposed to air
(e.g., the time a flake
was knocked off).
Uses very small
samples so more
things can be dated;
background
radiation not a
problem
Very cheap.
Measurements can
be made with a
microscope. Dates
manufacturing
event.
Region specific. Can
only be used in areas
where master
chronology is
developed; areas
where annual rings of
trees vary enough.
Wood, charcoal, bone
may not preserve well;
needs large amounts
(>25 g). Needs to be
calibrated.
Contamination a
problem.
Wood, charcoal, bone
may not preserve well;
needs to be calibrated;
costs 2-3 times more
than conventional 14C
dating
The composition of
obsidian can vary
greatly. Hydration rate
responds to
temperature and
environmental factors.
It is often difficult to
compare hydration
measurements between
location. Requires
local calibration.
Radiocarbon
(Conventional)
Radioactive decay of Carbon 14 (14C)
(Counts beta emissions)
100 - 40,000 years
Accelerator Mass
Spectrometry
Radioactive decay of Carbon 14 (14C)
(Counts actual 14C atoms)
Obsidian
Hydration
Accumulation of weathering rind on
artifact. Over time obsidian absorbs
water. Water form a hydration layer. The
thickness of the hydration later is a
measure of how old an object.
1
Anth 140
Summer 2007
DATING TECHNIQUES
DATING
TECHNIQUE
Potassium-Argon
Dating
METHOD
AGE RANGE (B.P.)
Radioactive decay of Potassium-40
(40K) to Argon-40 (40Ar). Half life of
40K is 1.3 billion ± 40 million years
Unlimited (usually over
100,000 years).
Archaeomagnetism
Alignment with changes in location of
the Earth’s magnetic field [Relies
upon the fact that the earth’s magnetic
field varies through time. Changes in
the direction of the earth’s magnetic
field is recorded in heated materials
(such as baked clay) that contain iron
particles. By comparing the magnetic
orientation of clays that are dated
using radiocarbon with the magnetic
orientation of a new sample, a date can
be assigned.]
Accumulation of TL in crystals.
[Ionizing radiation dislodges electrons
from atoms. Some of these atoms
become trapped in the crystal structure
of the material. The longer a sample
has been exposed to radiation the more
trapped electrons will accumulate.
When material is heated, these
electrons are released and give off
energy seen as light. The amount of
light given off is a measure of how old
the object is.]
Thermoluminescence
(TL)
DATABLE
MATERIAL
Geological materials
that contain potassium.
(e.g., volcanic rocks and
minerals)
DATED EVENT
STRENGTHS
LIMITATIONS
Time of geological
deposition
2,000 years
Intact hearths, kilns, or
burned areas. Fired
clay such as pottery or
bricks but only if found
in original firing
position.
Heating of the clay.
Can be used to
date geological
formations
associated with the
remains of fossil
hominids and
Lower Paleolithic
tools.
Relatively
inexpensive, can
be used to date
deposits with no
charcoal.
Appropriate
samples are rare.
Less precision.
Dates the age of
the geological
formation not the
artifacts/bones
themselves.
Few labs. Must
be carefully
calibrated . Still
being developed.
Hard to determine
original firing
position.
1,000,000 years
Fired clay, pots, bricks,
heated stones, calcite.
Heating of the material
which causes the
“zeroing” out of the
trapped electrons.
Exposure to light (in
the case of sediments)
can also zero out the
material.
Can date
manufacturing
event of an object
(such as pottery).
Extends past the
range of
radiocarbon
dating.
Still needs a
developed
protocol. Fairly
expensive.
2