i SS
GEOCHEMICAL AND PETROLOGIC CHARACTERIZATION OF PUMICE ARTIFACTS FROM
THE MIAMI CIRCLE-BRICKELL POINT ARCHAEOLOGICAL SITE PLUS OTHER SITES IN
FLORIDA - POTENTIAL PROVENANCE LOCATIONS

STEPHEN A. KISH

Department of Geosciences, Florida State University, Tallahassee, FL 32306-4100

E-Mail: kish@gly fsu.edu

 

Introduction

Pumice is a light-colored, highly vesicular igneous rock that
normally has sufficient buoyancy to float on water. The
formation of pumice is associated with the violent eruption of
volcanoes, features that are not present in Florida. Wheeler
(2006, this issue) reported that at least 121 pumice artifacts
have been recovered from excavations at the Miami Circle-
Brickell Point archaeological site (8DA12) and there are at least
30 other sites in southern Florida that contain a limited number
of pumice artifacts (Figure 1). Several pumice artifacts (~3-4)
also were discovered on the northwestern Gulf Coast of Florida
at the Wynnhaven Beach site (80K239) during recent site
studies (Ryan J. Wheeler, personal communication). A single
location, the Governor Martin site (8LE853B), in the inland
portion of the Florida Panhandle, near Tallahassee, Florida also
produced a single pumice fragment. A majority of the pumice
artifacts appear to be associated with the Glades I period
(Wheeler 2006), however, material from younger sites has
been discovered, including early European contact period
locations suchas Governor Martin site (DeSoto winter encamp-
ment); the early eighteenth century Higgs site and the early
nineteenth century Indian Key site (Wheeler 2006). Radiocar-
bon (‘C) dating of charcoal from the Miami Circle and
adjacent Brickell Point — Sheraton site, yield calibrated ages in
the range of BC 500 — AD 400 (Carr and Ricisak 2000). A
similar range of '“C ages has been obtained from Lucina shells
taken from different stratigraphic intervals at the site (Widmer
2004). At the Miami Circle site no other types of volcanic
material, such as obsidian, dense rhyolite, or basalt, have been
discovered. Ground stone celts found at the Brickell Point
excavations are composed of dark, aphanitic igneous rock that
has a basaltic composition, however, the material most likely
came from Mesozoic intrusive (diabase) dikes exposed in the
Piedmont region of Georgia (Dixon et al. 2000).

As a working hypothesis, it is assumed that the pumaceous
material found at archeological sites in Florida is derived from
oceanic drift material that washed ashore on beaches and may
have been locally redistributed to inland communities. Inrecent
times (1950s — present) a limited number of pumice clasts have
washed ashore along the Atlantic coastline of Florida (Bryan
1972; Dennis 2000), however, the presence of such a large
number of pieces of this exotic geologic material at the Miami
Circle site may indicate that a relatively massive, short-term
influx of pumice occurred along this portion of the shoreline of
southeast Florida approximately 2000 years ago, or perhaps

there was a long-term interest by native inhabitants in collecting
sporadic occurrences of this material for use as tools or for
ornamental purposes.

The purpose of this study is to provide information on the
physical and chemical characteristics of selected pumice
samples for the Miami Circle site and other selected sites in
Florida. Specific objectives of the study are to: (1) characterize
the petrology (rock type) of the pumice artifacts; (2) use the
mineralogy and chemistry of the samples to test for the homo-
geneity of samples at one site and petrologic diversity between
sites, and (3) test for the ultimate source of the pumice by
comparing the observed petrologic and chemical characteristics
of pumice artifacts with pumice samples collected from areas of
recent volcanic activity in the circum-Atlantic, Caribbean and
material from areas adjoining the western Gulf of Mexico.

Sample Selection

Twenty-two samples were selected for petrographic and
geochemical studies. Five samples were selected from the
Miami Circle site (8DA12). Pumice artifacts from sites in close
proximity to the Miami Circle also were analyzed. These
included two samples from the Brickell Point site (8DA98) and
one from the Granada site (8DA11). An additional eleven
pumice samples were analyzed for other sites in southern
Florida and the Keys. Three pumice samples from the
Wynnhaven Beach (80K2339) site, located on the Gulf Coast of
northwestern Florida, also were analyzed. The locations of
these sites, plus the locations of all other archeological sites
known to contain pumice artifacts are shown in Figure 1. The
specific site names and individual sample repository numbers
are listed in Table 1.

Sample Preparation

Individual pumice samples were cut into slabs with a
diamond saw to prepare the material for petrographic and
geochemical analyses. The fresh surfaces of the pumice were
observed using a low power, binocular microscope and physical
characteristics (color, vesiculation and presence of pheoncrysts)
were noted. Three samples were selected for scanning electron
microscopy (SEM) studies. A small chip from each of these
samples. was ultrasonically cleaned in acetone and then pro-
cessed using standard SEM procedures. Petrographic thin
sections for a selected subset of samples were prepared by
vacuum impregnation of the porous rock with a blue dye epoxy,

ere eeeneenr

VOL, 59(3-4)

THE FLORIDA ANTHROPOLOGIST

SEPTEMBER- DECEMBER 2006