Santander, February 19th-22nd 2008 - Aranzadi

Radiocarbon dating of shell carbonates: old problems and new solutions
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6. RADIOCARBON DATING OF EUROPEAN
PALAEOLITHIC SHELLS
The Middle to Upper Palaeolithic transition still
remains one of the most vividly debated issues in
European prehistory. It broadly revolves around the
nature and timing of the replacement of
Neanderthals by Anatomically Modern Humans
(AMHs) around 35-45,000 years ago.
As early as 1960, Harvard prehistorian Hallam
Movius states “Time alone is the lens that can
throw it [the Palaeolithic] into focus”. (Movius 1960:
355). Dating the relevant evidence, however, has
proven to be more challenging than originally
thought, mainly due to the need for well-preserved,
contaminant-free samples at the limit of the radiocarbon
method.
Sample pretreatment chemistry is a continuously
revised field with new protocols being
developed by laboratories around the world. When
the preservation state of a sample is particularly
poor, however, pretreatment chemistry cannot provide
effective solutions. Bad sample preservation
often occurs in hot and humid environments, and in
particular around the Mediterranean zone, where
the organic biomolecules used to date bone or
wood (collagen, cellulose respectively) quickly oxidize,
disintegrate and/or bind with contaminants.
Shell carbonates, on the other hand, are abundant
in most archaeological sites around the
Mediterranean. Since the biomineral (CaCO3) and
not the organic matter is used for 14 C dating, marine
shells are valuable chronological indicators with
great potential.
At the onset of the European Upper Palaeolithic,
marine shells become visible in the archaeological
record. Their first appearance in Europe -in the form
of shell artifacts- coincides with the spread of the
Aurignacian, often associated with the physical
expansion of modern human populations into
Europe (Mellars 2005).
A new project aimed at the dating of shell artifacts
from the early Upper Palaeolithic began at the
ORAU in 2006. The main objective of the project is to
establish a chronology for sites along the “southern
dispersal route” of AMHs, all across the
Mediterranean Rim, which contain evidence for the
earliest, or else, “Proto” Aurignacian culture of the
region (Mellars, 2004). This involves dating sites with
a great geographic distribution, from the Levant to
the Iberian Peninsula, and includes material from key
sites in Lebanon, Turkey, Greece, Italy, France and
Spain. The vast majority of these sites have never
been reliably dated before due to problems in the
preservation of bone collagen and the difficulties in
effectively dating small residue remains of charcoal.
We have selected over 200 specimens of shell
ornaments, analyzed their production methods,
determined whether or not they were obtained from
fossil sources and assayed their mineralogical phases
using powder X-Ray Diffraction and SEM analysis.
The shells that yielded diagenetic calcite in their
XRD scans (approximately 5% of the total number of
individual specimens) underwent an extra pretreatment
step (CarDS protocol, in Douka et al.) to ensure
the removal of the contaminating carbonate and
ensure reliability in derived AMS determinations.
The first radiocarbon results show great consistency
and the majority of them will be reported soon after
the completion of the project at the end of 2009.
7. CONCLUSIONS
If precautions are taken and a strict set of criteria
and scientific protocols are used, shell carbonates
can be excellent environmental, cultural and
chronometric indicators for both historic and prehistoric
sites. Archaeologists, archaeometrists and
marine biologists all need to be aware of the
potentials and pitfalls during the physical and chemical
analyses of this material thus close collaboration
of all workers is vital in the extraction of precise
and accurate information from archaeological
molluscan remains.
8. ACKNOWLEDGEMENTS
This work is financially supported by IKY
(Greece), Keble College and RLAHA (University of
Oxford), the Leventis Foundation and the
Malacological Society of London.
We would like to thank the organizers of the 2nd
ICAZ- Archaeomalacology Working Group Meeting,
all of our colleagues at the ORAU and RLAHA, the
Materials Science Department (University of Oxford)
which granted us permission to use the XRD facilities
and the Centre for Microscopy at the University
of Reading for the ESEM. Finally we would like to
acknowledge the significant input of the two referees
who provided us with helpful suggestions that
improved the initial manuscript.
9. BIBLIOGRAPHY
ANDERSON, E. C. & LIBBY, W.F.
1951 “World-wide distribution of natural radiocarbon”. Physical
Review, 81(1): 64–69.
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