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

Geochemical Techniques in Discriminating Chert Materials / 57
Comparing chert from the Central Plain Group tuff and Antigua Formation
limestone of the Antigua Island, allows distinction as based on Al/ K ratios, reflecting
mixed land derived/ marine and mainly marine clay mineral assemblages,
respectively (Table 4.2 and Figure 4.6). Chert that formed in tuff displays higher
intrasource variability, reflecting formation in beds of low terrigeneous input (low
Al/ K ratio), as well as in beds with high terrigeneous input (high Al/ K ratio). Comparing
the Al/ K ratio of the St. Kitts chert (flint) to the Antigua Formation chert
(flint), suggests that also the former has a marine origin and was probably formed
in limestone, although possibly of different type than the Antigua Formation, or
else of the same type but affected by differences in later weathering, as follows
from the differences between the trace element suites.
Weathering also has a significant impact on the trace- element composition. Prolonged
exposure, by this we mean in geological rather than archaeological terms,
to chemical processes in soils proved to be most severe, more than the effects of
seawater. Comparing trace element composition of primary and secondary chert
(flint) from the Antigua formation, suggests that Na, trapped as interstitial water,
is lost in chert from secondary context (Table 4.3). The lower Ca and Mg concentration
in chert from secondary context suggests dissolution of carbonates, increase
of porosity and specific surface area, further promoting quartz dissolution
and change to lighter color, during weathering. Porosity of the rock also induces
iron and manganese oxidation. Depending on the water transport in the flint, Fe
(and Mn) may be precipitated in bands in the outer parts of the rock. Slowly clay
minerals and other terrestrial minerals, trapped in the quartz and calcite, are dissolved
resulting in lower Al, K, and Ti concentrations. K and Ti decrease relatively
more in concentration than Al, as they are more reactive. Decrease in Li and Cr
indicate actual quartz dissolution. These elements are least affected by weathering
suggesting relatively little quartz is dissolved. Depending on the Fe contents
in the surrounding soil new Fe (and Mn) may be precipitated in the voids that are
formed after solution. If Fe content is higher in the surrounding soil than in the
flint this will result in an increase of Fe in the flint, as has been the case at the secondary
Antigua Formation Blackman’s Point source and some of the secondary
Puerto Rican sources.
For some sources, intrasource variability may be low enough to assign samples
to a single source, for others intrasource variability may be so high that trace element
values of different sources overlap, and samples cannot be clearly assigned to
a single source. This is particularly true for the secondary chert sources on Puerto
Rico and St. Kitts.
Source Discrimination
The acquisition of quantifiable variables, the trace elements in chert, offers the
possibility to employ multivariable statistical analysis, of which Discriminant
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