Santander, February 19th-22nd 2008 - Aranzadi

Radiocarbon dating of shell carbonates: old problems and new solutions
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tion. Shells used for ornaments were most often
chosen due to their shape, size and mainly vivid
colouration. During deposition, shells will undergo
several taphonomic processes, transporation,
repeated burial/ exhumation cycles, wave or other
predator actions (Kidwell 1998). These influences
will have an effect on the appearance of the shells
and will leave a diagenetic signature on them, in the
form of fragmentation, surface pitting, polishing,
encrustation, discolouration and bioerosion, all of
which are very likely to render these specimens
less appealing in the eyes of the prehistoric man.
Hence, it is sensible to suggest that the effect of
time-averaging on prehistoric shell ornaments is
small and almost minimal compared with the depositional
uncertainties and the errors caused by.
Though the “old shell” problem can perhaps have
the most significant impact, it can usually be identified,
minimized or even eliminated by careful sample
selection and use of only well-preserved specimens
for dating. Specimens with traces of weathering,
abrasion, inclusions, or other marks that may indicate
‘‘old”, “beach-worn” shells should be avoided
(Rick et al. 2005). In addition, dating multiple samples
throughout the stratigraphic sequence of the
archaeological site is an ideal and the most efficient
way to identify anomalously old dates and outliers,
and refine site chronologies.
5. Recrystallization. In a burial environment
shells often behave as open systems, incorporating
exogenous carbon atoms, in the form of
secondary CaCO3. This process is commonly
known as recrystallization or neomorphism, and
alters the original C isotopic ratios and thus the
inferred radiocarbon age.
It is broadly assumed, on thermodynamic
grounds, that diagenesis of aragonite and high-Mg
calcite of shells will result in dissolution and/or precipitation
of low-Mg calcite cement (e.g., Folk and
Assereto 1976, Allan & Matthews 1982, Morse &
Mackenzie 1990, Magnani et al. 2007). Very rare
and notable exceptions to this assumption that
involve recrystallization from aragonite to aragonite
(Enmar et al. 2000, Webb et al. 2007) have been
reported in the literature, but these were attributed to
very specific environmental conditions and
aqueous geochemistry.
When diagenetic alteration occurs, the carbonate
phase being dated will not be autochthonous,
but will include secondary material incorporated in
the system post mortem. This material may have different
carbon isotopic composition from the shell
matrix thus will lead to a erroneous age measurements.
The scale or trend of this effect is unpredictable
so that the age drifts may be of older or younger
direction. Rigorous screening and effective pretreatment
is required to identify evidence of contamination
and reduce the risk of erroneous AMS 14 C
ages (Chappell & Polach 1972).
5. RECENT ADVANCES IN SHELL DATING
The methods routinely used when dating carbonates
include mechanical cleaning of the surface,
occasional acid leaching when this is considered
necessary and selection of aragonite parts, by
using staining methods to differentiate calcite from
aragonite (usually Fiegl’s solution (Friedman 1959,
Dickson 1966). The rest of the chemical pretreatment
(phosphoric acid decomposition of the
CaCO3 and evolution of CO2) has been basically
unchanged since the 1950’s (McCrea 1950) and is
comparable for most radiocarbon laboratories
around the world.
Recent attempts to date Palaeolithic-aged
shells at the ORAU (Camps & Higham, submitted)
have stimulated the development of stricter criteria
to identify and minimize post-depositional alterations.
The identification of diagenesis can be
achieved by determining the mineralogical phases
that are present in the sample, either with X-Ray
diffraction (XRD), Fourier-Transform Infrared
Spectroscopy (FTIR) and/or Scanning Electron
Microscopy (SEM).
FTIR has been used to distinguish between
aragonite and calcite (Subba Rao and Vasudeva
Murthy 1972, Compere 1973), however broad
band overlap hinders most qualitative and quantitative
determinations for the carbonate polymorphs.
For this reason, we do not use it as a
method for high-precision characterization of calcite
and aragonite mixtures.
XRD, on the other hand, can be used to identify,
quantify and characterize phases in complex
mineral assemblages. It is generally believed that
this method is semi-quantitative, however workers
in recent discussions suggest that with careful
specimen preparation, calibration of the equipment
and selection of the optimal conditions (highest
peak/background ratio in optimum time) XRD
scans may offer very accurate quantitative information
(Hakanen and Koskikallio 1982, Chiu et al.
2005, Sepulcre et al. 2009). The detection limits we
have obtained lie routinely in the range of 0.1-0.2%
of calcite (i.e. secondary polymorph) in binary mixtures
with aragonite (Fig. 1) and are comparable to
the ones achieved by Chiu et al. (2005) and
Sepulcre et al. (2009).
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