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

30 / Hofman, Isendoorn, Booden, and Jacobs
Figure 2.5 reports Y/ Zr ( Yttrium/ Zirconium) and Ba (Barium) of the studied
clays and potsherds. Y/ Zr is a ratio of two trace elements that are insoluble in
water under earth surface conditions, meaning their abundances are relatively unaffected
by chemical weathering processes.
An element ratio is used to compensate for any sorting effects that might alter
these elements’ concentrations. Ba, together with Pb (lead), is the element that
most strongly shows a multimodal distribution in concentrations. Both Ba and
Pb are relatively soluble elements, so they are potentially mobile during chemical
weathering. However, any weathering effects are apparently swamped by the naturally
occurring variation between islands. Pb and Ba are also potentially sensitive
to changes caused by firing of clay pots, so different production techniques may
have played a role in at least part of the observed differences in abundance.
In a Y/ Zr vs. Ba diagram, the clays from Saba and St. Eustatius define two distinct
clusters. The four Saban clay samples plot in the lower right of the diagram.
The two Statian clays plot in the upper left. Clays from Anguilla and St. Martin
are scattered over much of the diagram. Sherds appear in two general clusters: one
surrounding the Saban clays in the lower right, and one in the upper left half. In all
likelihood, the sherds associated with the clay cluster in the lower right have been
locally produced, while the sherds outside this cluster have different compositions
and are probably of nonlocal origin.
The local population has been defined by determining the largest subset of
samples in each site’s data set that is normally distributed in both Ba and Y/ Zr.
This was done by iteratively excluding outlying samples from the set, based on
95 percent confidence, until the remaining population was symmetric and mesoor
platykurtic (meaning its distribution has small tails). Sherds in this remaining
group were termed “local.” Excluded sherds were arbitrarily termed “associated
nonlocal” (ANL) if they appeared to belong to the general cluster around
the Saban clays, or “nonassociated nonlocal” (NANL) if they did not. The results
are shown in Table 2.1. In general, nearly half of the total population has been excluded.
However, in the case of Spring Bay and The Bottom, only a quarter or less
of these nonlocals is actually nonassociated. The associated nonlocal sherds may
or may not be local in origin. However, it is impossible to determine whether their
composition is really different or if their exclusion from the “local” set is an artifact
of the small numbers of samples in these analyses. The nonassociated nonlocals
are very different from the local sherds and clays. It is suggested that these
cannot have been produced from Saban clays and infer that they must have been
imported from another island.
The very large spread, in both Ba and Y/ Zr, of both the nonlocal sherds and
clays from the neighboring islands of St. Martin and Anguilla, preclude the assignment
of a definite source for these sherds at this moment. Clays from St. Eustatius,
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