archaeology > Methods and maps
The application of isotope analysis
in tooth enamel to the study of population migration and movement
Strontium has four naturally occurring isotopes, of which three are
stable, and one (87Sr) forms
by the decay of 87Rb. Strontium
is chemically similar to calcium, and enters the biosphere primarily
through the uptake of Sr by plants. The isotope composition of Sr in
the biosphere is controlled by the isotope composition of the leachable
component of the soil on which the plants grow. This, in turn, varies
depending upon a combination of factors which include the age of the
underlying rocks, the rubidium content, weathering conditions and the
susceptibility of different minerals in a heterogeneous rock to weathering
Strontium isotopes provide a powerful tool for studying the movement
of humans and animals across isotopically different terrains because
strontium is absorbed by the body and deposited, with calcium, in
teeth and bones. Tooth enamel forms during the early years of life
and its composition is not altered subsequently; hence, it constitutes
a recoverable archive of childhood diet.
The 87Sr/86Sr biosphere map for Britain to be used in conjunction rainfall map in coastal areas.
See Evans, J.A. Montgomery, J. Wildman, G. & Boulton, N (2010) Spatial variations in biosphere 87Sr/86Sr in Britain. Journal of Geological Society vol 167, pp1-4
Average rainfall, with permission from the Met office.
In archaeological skeletal
remains, enamel is also the tissue most resistant to post-mortem
contamination, thus retaining its isotopic integrity over thousands
of years. Once incorporated into plants, Sr passes up the food chain
Sr-isotope analysis of tooth enamel provides a tool to differentiate
between people or animals based on where they obtained their food and
water at the time of tooth mineralisation. Combined with oxygen analysis,
which is related to climate zones (see below), these isotopes can provide
powerful constraints on the childhood origin of individuals, and can
be used to assess population heterogeneity and track migration.
Million International Geological Map of Europe and Adjacent Areas (Asch, K., 2005. IGME 5000)
geological map of the UK.
Voerkelius, S., Lorenz G.D., Rummel, S., Quetel, C.R., Heiss, G., Baxter, M., C. , B.-P., Deters-Itzelsberger, P., Hoelzl, S., Hoogewereff, J., Ponzervera, E., Bocxstaele, M.V. & Ueckermass, H., 2010, Strontium isotopic signatures of natural mineral waters, the reference to a simple geological map and its potential for authentication of food: Food Chemistry, 118, 933-940.
| Oxygen isotopes
The dominant source of oxygen isotopes within the body is from drinking
water and this, in turn, reflects the isotope composition of rainwater
which is modified, through fractionation, within the body.
There is a general global trend in the oxygen isotope composition of
rainwater of increased depleting from the equator to the poles, however
this pattern is modified regionally by weather systems. In Europe the
main variation in rainwater is from west to east as seen below. The
analytical method used at NIGL is to extract the oxygen from the phosphate
radical of the bio-apatite that makes up the majority of the tooth.
The measured ratio is presented in a standard delta notion given relative
to Standard Mean Ocean Water (SMOW). Eg δ18O
Oxygen isotope map of drinking water variations across Europe compiled
Lecolle P, (1985) The oxygen isotope composition of land snail shells
as a climatic indicator - applications to hydrogeology and Palaeoclimatology.
Chemical Geology, 58 (1-2): 157-181.
Forstel H and Henkel C, (1982), Simulation function of the local climate
(Julich, Federal-Republic-of-Germany) from a 10-year observation period
(1972-1981). Landwirtschaftliche Forschung 35 (3-4): 275-279
Oxygen isotope map of drinking water variations across the UK compiled
from data from
Darling, W.G. and Talbot, J.C., 2003. The O & H stable isotopic composition
of fresh waters in the British Isles: 1, Rainfall. Hydrology and earth
System Sciences, 7(2): 163-181.
Because of the fractionation of the oxygen within the body, this
value has to be recalculated, using experimentally generated calibrations,
to determine the composition of the water an individual imbibed,
thus making it possible to compare the individual with drinking water
datasets. Several calibrations for this conversion exist and it can
be a source of error and uncertainty and so must be used with care.
At NIGL, in addition to using the drinking water maps, we are developing
datasets of δ18SMOW values
for human tooth enamel both across the country and through time,
in order to make direct comparisons of measured population values
with the aim of understanding inter-population differences which
may reflect such factors, as cultural practices, and water sources,
as well as the main effects of climate region.
Lead is a toxin, but it can become incorporated into the body and the
isotope composition can be used to trace the source of lead 'pollution'
to which an individual was exposed. The diagram (right) shows a graph
of lead isotope composition measured in human tooth enamel vs. the
concentration of lead within the tooth enamel of five British populations.
The Neolithic and Iron Age tooth enamel tends to show a wide spread
of lead isotope compositions associated with low concentration of
lead (<1ppm) within the tooth. Later populations show a reduction
in the range of lead isotope values but an increase in lead concentrations
recorded in the enamel. Populations which pre-date metallurgy, or
have little exposure to metalware (i.e. the more ancient populations)
have lead concentrations that reflect the natural background levels
picked up from the environment and which reflect the variable isotope
composition of the geology on which the individuals lived.
The advent of trade and the wider use of metalware is reflected in
the populations increased Pb concentrations, however, because the exposure
is now related to man-made items the natural diversity of the lead
isotope compositions is reduced to an average value that reflects the
overall composition of the metal sources and reworked material within
the particular culture.
Lead focussing diagram - courtesy Janet Montgomery
This feature of increased lead concentration with reduced isotope range
has been coined 'cultural focussing' (Montgomery et al., 2005).
Montgomery, J, Evans, J A, Powlesland, D, and Roberts, C A, 2005. Continuity
or colonisation in Anglo-Saxon England? Isotope evidence for mobility,
subsistence practice, and status at West Heslerton. American Journal
of Physical Anthropology, 126(2), 123-138.