Crossing the Borders: New Methods and Techniques in the Study of Archaeological Materials from the Caribbean

220 / Booden et al.
Table 15.1. Mean Value for Strontium Ratios of the Four Sample Categories.
Number of samples mean 87 Sr/ 86 Sr SD
Rice rats 4 0.709173 0.000038
Soil 4 0.709156 0.000030
All individuals 50 0.708989 0.000359
Local individuals 36 0.709137 0.000051
the variation in strontium isotope ratios in a single enamel crown. The spread in
strontium ratios within the multiple sampled teeth is considerably smaller (nearly
two orders of magnitude in the case of these extreme samples) than the spread in
isotopic ratios between the local and nonlocal samples, and approximately half
that of the standard deviation of the local group. We conclude that isotopic composition
does not vary significantly within well- preserved single teeth.
Statistical Analysis
The subset of possible local individuals was distinguished from the nonlocal individuals
by iteratively excluding outliers from the data set, until no further out liers
are identified. Outliers were defined as values outside the 95 percent confidence interval.
For the whole series the enamel strontium isotope ratios show a distribution
(Figure 15.2) that is skewed to higher ratios (-2.63) and is leptokurtic (7.77).
Two successive iterations and the exclusion of 14 outliers were sufficient to reduce
the data set to an approximately normal distribution (n = 36, skewness -0.51 with
a standard error of skewness of 0.39, kurtosis -0.18, mean and median coincide)
(Figure 15.3). The local population is defined as having a mean 87 Sr: 86 Sr ratio of
0.70914 and standard deviation of 0.00005 (five times higher than the analytical
reproducibility). Samples with a strontium ratio between 0.70903 and 0.70924 are
defined as local.
Discussion
87
Sr/ 86 Sr Provenance
In the enamel samples a relatively homogenous group, with normally distributed
strontium isotope ratios, was recognized. The mean strontium ratio of this homogenous
group was in agreement with the local strontium signature determined
from soil and rice rat enamel samples. Thus the homogenous group was considered
to represent the local population. The samples of 14 individuals have been categorized
as nonlocal because of strontium ratios outside the range of a normal distribution.
These 14 individuals represent the minimum number of nonlocal individuals
in the sample. As is visible in the distribution of strontium ratios in
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