Santander, February 19th-22nd 2008 - Aranzadi

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KATERINA DOUKA, THOMAS F. G. HIGHAM AND ROBERT E. M. HEDGES
rred substrates and food selection are generally wellstudied,
hence, retrieval of this type of material from an
archaeological site offers important local habitat and
environmental information as well as insights into several
behavioural features of the linked human community
(e.g. exploitation of shellfish for food, for use in the
production of tools or weapons, or utilization for
symbolic reasons).
As dating material, molluscs offer both relative and
absolute chronometric information.
Seasonality studies fall into the former category.
Relative growth-ring and oxygen isotope measurements
provide information regarding the age-at-death
of shellfish within archaeological sites, and enable a
seasonal signal to be diagnosed (e.g. Koike 1973,
1979, Deith 1983, Deith 1985, 1986, Milner 2001,
Dupont 2006). This is critical information for the construction
of seasonal subsistence patterns and the
movement of people within specific regions in the past.
Absolute methods such as Radiocarbon ( 14 C)
dating, Amino Acid Racemisation (AAR), Uraniumseries
and Electron Spin Resonance (ESR), can be all
used for the direct dating of molluscan remains, allowing
secure inter and intra-site correlations when
humans are responsible for their accumulation (e.g.
Kaufman 1971, Wehmiller 1984, Skinner 1988;
Bezzera et al 2000, Penkman et al. 2008).
This paper specifically deals with 14 C dating and
reviews the basis of the method in so far as it is used to
date marine and estuarine shell carbonates. It will discuss
the fundamental assumptions, related uncertainties
and problems, along with new developments and
their application to a significant problem of the European
prehistory, the Middle to Upper Palaeolithic transition.
2. FUNDAMENTALS ON MARINE SHELLS
Molluscan skeletons are polycrystalline biominerals
mainly composed of calcium carbonate
(CaCO3) precipitated as distinct layers within an
organic proteinaceous matrix. In marine shells
CaCO3 comprises high-Mg calcite and aragonite, in
several formations.
The two polymorphs, aragonite and calcite,
share identical chemical compositions but quite different
crystal structures and thermodynamic equilibria,
with calcite being the stable form and aragonite
the metastable form at present earth-surface temperatures
and pressures.
Carbon (C) is one of the main elements
molluscs make use of to form their exoskeletons.
The origin of this carbon derives from various sources
and the mixing of different carbon pools leads
to their isotopic deviation from the ambient environment.
Shells incorporate carbon deriving from
two pools: oceanic dissolved inorganic carbon
(DIC) and carbon from respiratory CO2 mainly
stemming from food metabolism (Tanaka et al.
1986, McConnaughey et al. 1997, Gillikin et al.
2007). For a full summary on the current status of
research regarding the C isotopes of biological
carbonates the reader is referred to recent publication
by McConnaughey and Gillikin (2008) and
references therein.
3. RADIOCARBON DATING OF SHELLS
Radiocarbon dating is the most commonly used
chronometric technique in archaeology. Carbon (C)
is found in the atmosphere in the form of three isotopes
1 12 C, 13 C and 14 C, which account for the element’s
natural abundance. The first two, 12 C and 13 C,
are stable isotopes and can be found in atmospheric
concentrations of approximately 98.89% and
1.11% respectively, whereas 14 C is weakly radioactive
and has an extremely low atmospheric activity, of
about one per trillion 12 C atoms.
Radiocarbon is produced in the stratosphere
and shortly after is oxidized to 14 CO2. It rapidly
enters circulation and exchanges with the oceans
and biosphere (Fairbanks et al. 2005, Bronk
Ramsey 2008). Each living organism will incorporate
14 C atoms through photosynthetic, respiratory,
metabolic, and other biological pathways and will
reach isotopic equilibrium with the ambient environment.
This ceases to exist when the organism
dies. 14 C is no longer incorporated into its tissues,
and the radioactive 14 C isotope undergoes nuclear
decay. The decay progresses exponentially at
a regular rate expressed as the constant known
as the radiocarbon half-life, often quoted to be
5730 2 years.
1
Isotopes are varieties of the same element, which contain the same number of electrons and protons (same atomic number) but different
number of neutrons (hence different mass number) in their nuclei. Most elements are found in mixtures of two or more, stable and unstable
(radioactive) isotopes. Variations in isotope abundance of an element are caused either by radioactive decay of the unstable isotopes or by
isotope fractionation (Hoefs 2003: 1–5).
2
Two values are often quoted for the rate of 14 C decay. The first to be published was the Libby half-life 5568 ± 30 years (Anderson and Libby
1951) and the second is the Cambridge half-life of 5730 ± 40 years (Godwin 1962), often thought to be more accurate. However, very recently
(Fairbanks et al. 2005) supported the idea that the half-life may be significantly different, as much as 6000 years, although this idea is not
widely accepted (see for example, Roberts and Southon 2007).
MUNIBE Suplemento - Gehigarria 31, 2010
S.C. Aranzadi. Z.E. Donostia/San Sebastián