Transcript Bone chemistry

Isotope analysis
in archaeology
Carbon 14 Dating
• Carbon-14 is radioactive.
• It has a half life of 5,730 years.
• This means that after 5,730 years have
passed only half of the original amount
of C14 will remain.
• After 2 x 5730 = 11,460 years there will
be ½ x ½ = ¼.
• After 3 x 5730 = 17190 years there will
be ½ x ½ x ½ = 1/8…..
• C14 is made by the
action of cosmic rays.
• Life is based on
carbon.
• Whilst organisms are
alive C14 will be
absorbed at the same
rate as C12.
• As C14 decays it is
continually replaced.
• But after death no
more C14 is absorbed.
• It is as if a stop clock
is started.
• Archaeologists have
only to measure the
C14 in bones, wood,
hair…. to date them.
• The technique can be
used for objects up to
48,000 years old.
• But when the ages of
historic artefacts
several millennia old
were compared to the
radiocarbon dates they
were found to be too
young.
• It seems that C14 is not
always produced at the
same rate.
• So the dates have been
calibrated using the
wood of the Bristlecone
Pine, which lives for
over 7,000 years!
Stable Isotopes of elements
Many elements have
isotopes that do not
radioactively decay.
Isotope proportions
often vary from
place to place.
If bone composition matches the contemporary isotope
ratii of a site’s soil and water the people are likely to
have been local.
Bone is being continually broken down and
built up…
so its isotopic ratii reflect the last 5 – 10 years of life.
Tooth enamel forms during childhood and
adolescence, when the milk teeth are replaced.
Premolars typically
form at 3–6 years,
but the third molars
are not in place until
c9-13 years.
A comparison of tooth enamel
with bone isotopes allows
individual mobility to be traced.
Oxygen
Oxygen has two stable isotopes.
8 protons
8 neutrons
16O
18O
8 protons
10 neutrons
Usually 18O accounts for only c0.2%.
18O
forms the same compounds as 16O.
H2
16O
H2
18O
So drinking water in hotter climates has relatively more 18O .
H2
16O
H216O is lighter and
easily evaporates.
H2
18O
H218O is heavier, so
needs more energy
to evaporate
Oxygen in bone comes from drinking water, which is
ultimately derived from precipitation.
The 16O : 18O ratio
depends on such
factors as;
1) Temperature
-8.5% in
the north
east.
2) Altitude
3) Amount of precipitation
4) Distance from ocean.
In the British Isles it
varies from…
-4.5% in
the west
Be
Strontium (Sr)
Strontium is in the same group of
the periodic table as calcium.
Mg
So it can replace
Ca in bone
minerals.
Ca
Ca
Sr
Sr
Ba
Sr
Ca
Strontium has four stable isotopes
84Sr
0.56%
86Sr
9.86%
87Sr
7%
88Sr
82.58%
Living organisms have no preferences between
the Sr isotopes in their food and drink.
87Sr
forms from
decay of 87Rb, which
has a half life of 4.75
x1010 years.
The 86Sr : 87Sr ratio
depends on the
underlying rocks.
These data can be used to
track down people’s origins
from their bones.
It is greatest, c0.720,
in ancient rocks of
highland Britain >100
million years old, eg;
granite and gneiss.
In the younger
rocks of lowland
Britain it is
c.704.
Limitations
• Post mortem
changes may effect
the stable isotope
ratii.
• Although bone can
be treated, eg by
washing with acid,
to minimise these
effects.
• Modern water may
not have exactly
the same ratii due
to climate change.
Case Study; Who were the “Beaker People who had
helped build the megalithic stone circles…?
And introduced metal into
Britain?
…buried their dead under round
barrows with their typical pots?
The Archer’s enamel isotopes
indicated he came from a
colder region than Britain…
… probably from
central Europe ( dark
In 2002 Wessex Archaeology
excavated the Amesbury
Archer near Stonehenge.
blue-green).
The Archer had a companion…
His isotopic signature was
generally consistent with southern
England and Ireland (yellow).
But the enamel of his wisdom teeth
was different, indicative of the
Midlands or north east Scotland (mid
green)
In 2003 Wessex
Archaeology excavated
another Beaker Burial at
Boscombe, Wiltshire.
A mass grave which
contained the remains of
two adult men, an
adolescent male and a
child.
But are typical
of Cornwall,
Man, Lake
District and
Highland
Scotland.
The Boscombe burials had
very high ratios of 87Sr:86Sr
that are not matched in
Wessex.
Oxygen isotopes narrow down their origins.
Scotland was too
cold, with not
enough 18O.
So their childhood
home is likely to
have been in the
Lake District, North
or South-West
Wales.
Cornwall was too
warm, with too
much 18O.