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Clues
to African Archaeology Found in Lead Isotopes
By Jeff Harrison
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Copper
artifacts from West Africa. Photo by Thomas Fenn
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Microscopic specs of lead
are offering clues about the enormous cultural changes that swept
across northern Africa a thousand years ago.
At The
University of Arizona in Tucson, a young archaeologist is
analyzing lead traces in artifacts to shed light on the
relatively little-understood archaeology of Africa, especially
the period marked by the spread of the new religion of Islam.
Thomas R. Fenn, a doctoral student in the UA anthropology
department, is unraveling evidence of centuries-old trade
patterns across the Sahara Desert by identifying smelted metal
artifacts, mainly copper, found in the continent's sub-Saharan
regions.
Fenn will report the results of his work
("Getting to the source of the problem: Lead isotope
analysis and provenance determination of ancient African copper
artifacts") on Sunday, March 26, at 2 p.m., U.S. Eastern
Time at the annual meeting of the American Chemical Society in
Atlanta. Fenn's presentation is in the Georgia World Congress
Center, Room C-108.
As Islamic forces moved across
northern Africa, they set in motion trading opportunities between
the arid lands bordering the Mediterranean and the dense jungles
and savannahs south of the Sahara.
One of the questions
Fenn wants to answer concerns the sources of copper and other raw
materials that became manufactured goods that were traded
throughout the region. Specifically, why were metal workers in a
sophisticated metallurgical industry in the sub-Sahara importing
copper ingots when there were perfectly good copper ore deposits
nearby?
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Credit: Ken
Feisel, Archaeology Magazine
Map shows sites in northern
and western Africa and the trade routes that connected them.
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Knowing where these and
other materials originated, said Fenn, may offer larger insights
about not only trade, but also about technologies, economics and
social organization. Who controlled bankable natural resources
and transportation routes? How was labor distributed in these
societies?
David J. Killick, a UA associate professor of
anthropology and expert on the archaeology of metallurgy in
Africa, said tracing metals is a crucial part of understanding
the development of trade in Africa.
"Most of the
money circulating in the western half of the Islamic world
between the 11th and 16th centuries was minted with gold from
sub-Saharan west Africa, and competition for the wealth generated
by the trade fueled the growth of major West African states like
Ghana, Mali and Songhai," Killick said.
Using a
process called lead isotope ratio analysis, or LIA, Fenn has
examined more than 100 Iron-Age artifacts, most of them copper,
from sub-Saharan Africa. The experiments were done in the W.M.
Keck Isotope and Trace Element Laboratory at the UA. The lab is
partially funded by the National Science Foundation and run by
Joaquin Ruiz, a professor of geosciences and dean of the UA
College of Science.
"LIA is extremely accurate as a
forensic tool in identifying lead traces found in metal ores,"
said John Chesley, a research scientist in the UA department of
geosciences who developed the laboratory and analytical
techniques for Fenn's project.
Lead has four different
isotopes, three of which occur as the natural decay of uranium
and thorium. The isotopic ratios change as a function of time.
Smelting doesn't change the ratios, making them a virtual
fingerprint for a metal's source of origin. Scientists need only
about 100 billionth of a gram for analysis.
The trick,
said Chesley, is making sure the sample remains completely free
of contamination. The process takes about two weeks, but offers a
high degree of certainty of linking objects to their source. LIA
has been used successfully to determine the sources of
non-ferrous metals from sites in other parts of the world for
years, but its use in African archaeology is fairly recent.
"In
reality, I am dating the deposition of the ores on a geological
timescale - millions of years - but I am not dating them within
an archaeological time scale," Fenn said. "I am, in
fact, using the geological age, derived from the lead isotope
ratios, as a means of provenancing raw and refined copper metals,
and metallurgical debris, to a potential ore source based on the
similarity of their geological age, i.e., their lead isotope
ratios, as well as by examining and comparing their chemical
compositions."
From his analysis, Fenn theorizes
that the ore used to make the copper ornaments and other items
found in the sites in West Africa likely came from North Africa.
He said merchants there traded gold from regions like present-day
Niger for copper from the north via camel caravans across the
desert.
Refined copper, Fenn said, likely was prized as a
commodity that fit in with the value system of the region, where
it was easily worked into ornamental objects and other items that
could be bartered for other goods and services.
Fenn will
be a fellow in the IGERT Program in Archaeological Sciences at UA
during 2006-07. IGERT (Integrative Graduate Education and
Research Traineeship) is sponsored by the National Science
Foundation. The UA's grant supports interdisciplinary graduate
education across a broad range of disciplines related to absolute
dating, past climates and subsistence practices, and materials
and technologies.
Source
/ Credit: University of Arizona
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