Biology Reference
In-Depth Information
individuals have elevated
d
15
N levels compared to nonstressed individuals. These data
caution researchers against making interpretations related to diet in populations from arid
regions. In these instances,
d
15
N levels may be enriched for reasons other than dietary choice.
Strontium
In 1985, Ericson published the first example of the anthropological use of strontium
isotopes (
87
Sr/
86
Sr). The application of strontium isotope analysis to problems in skeletal
biology is straightforward. In essence, strontium is a trace element that exists in geological
bedrock. The concentration or abundance of strontium in geological substrate is variable
and depends on the type of bedrock present. Numerous authors (see
Bentley (2006)
for
a review) have discussed the way in which strontium is transferred throughout an
ecosystem. Essentially, the geological composition of bedrock subsequently influences the
concentrations of strontium in groundwater and soil, which are taken up or absorbed by local
flora and fauna. In other words, the concentration in local flora and fauna mimics the under-
lying strontium concentration contained in local bedrock.
Strontium becomes incorporated into the body's skeletal and dental tissues through the
ingestion of water, plants, and animals. It then replaces some calcium in the hydroxyapatite
in bones and teeth. Unlike other isotopes, such as
d
15
N, no change or fractionation occurs as
the isotopes move from water, plants, and animals to humans. As a result, an individual's
skeletal and dental tissues mirror the strontium concentration in soil and groundwater of
their local area. It is important to note here, however, that numerous scholars (
Knudson
et al. (2005)
for example) emphasize that strontium isotopes in tooth enamel reflect the
geological bedrock of the local area that the individual lived in while their teeth were devel-
oping (because unlike bone, once the teeth are formed, they do not remodel), predominantly
during the first 12 years of life. As a result,
87
Sr/
86
Sr is often construed to indicate an individ-
ual's place of origin. See Hammerl (Chapter 10), this volume, for an example of how stron-
tium analysis has been used with teeth.
Strontium isotope analysis is therefore used to detect residential mobility or migration in
the archaeological record, as those individuals who are migrants oftentimes present
87
Sr/
86
Sr
ratios that are outside of the expected local level where their remains were recovered. Local
“baseline” levels have traditionally been calculated by first surveying geological literature for
previously reported
87
Sr/
86
Sr ratios. Those levels are then compared to “baseline” data that
are created by analyzing archaeological local fauna, or by taking modern examples from
wherever the local area might be. Regarding calculation of the local
87
Sr/
86
Sr level, it is
important to note that researchers are encouraged to choose locally raised fauna. In the
corpus of work from the Andean region of South America, scholars have utilized guinea
pigs or
cuy
(in Spanish) to serve as baseline data for their respective areas of study.
Cuy
are ubiquitous throughout the Andes and are an excellent example of a locally raised species,
both from prehistory and the present day. As a result, samples from
cuy
are preferential to
highly migratory camelids (i.e., llamas, alpacas, vicu ˜ a). Once local samples are obtained,
calculating a local signature requires the researcher to calculate the mean
two standard
deviations of the entire local faunal sample. Any human that falls outside of this local range
is considered a migrant or nonlocal individual.
One example that illustrates the role of strontium isotopes in identifying nonlocal individ-
uals comes from the work of Knudson and colleagues (2005). In this paper, Knudson et al.
