Santander, February 19th-22nd 2008 - Aranzadi

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KATERINA DOUKA, THOMAS F. G. HIGHAM AND ROBERT E. M. HEDGES
unsuitable for dating. With the help of Low-
Vacuum SEM (LV-SEM) or Environmental SEM
(ESEM) that do not require surface coating, we
are able to use the same fragments for imaging
and AMS 14 C dating.
Figure 2. Close-up photograph and ESEM micrograph of a diagenetic
Glycymeris sp. specimen from a Palaeolithic context. The mineralogical changes
are observed macroscopically (left, red rectangle) and are confirmed
microscopically (right), in the form of distinct calcite crystals that cover the entire
surface of the sample and replace the original aragonite microstructure.
Figure 1. XRD spectra of (a) mixture of 0.1% calcite in 99.9% aragonite; (b) pure
geological calcite; and (c) pure biogenic aragonite. Despite the broad peak overlap,
the (114) peak at 29.4 O 2Θ angle (range indicated by shadowing) is characteristic
of calcite and is evident in binary mixtures of as low as 0.1% calcite.
SEM can be used to obtain high-resolution
images of the shell surface and to directly document
the preservation state of the aragonite
phase (Fig. 2). The disadvantages of the method
are that (i) minor contamination cannot be completely
ruled out since an SEM micrograph is restricted
to a small portion of the structure and cannot
be used to check all crystalline phases; (ii)
some experience is required with different shell
microstructures (crossed- lamellar, nacreous,
prismatic, homogenous) and (iii) CaCO3 being
non-conductive material, requires coating with
carbon or gold which then renders the sample
It is, therefore, advisable that every calcareous
sample designed to undergo 14 C dating should have
an XRD scan and/or an SEM micrograph associated
with it so that an evaluation of the obtained result –in
terms of sample quality- is possible. This should be
provided either by the submitter or the dating facility,
given that such a service is available.
In the ORAU, we have recently developed and
proposed an additional, novel pretreatment step in
case that secondary recrystallization is identified:
that of the physical separation of the new protocol
is termed CarDS (Carbonate Density Separation)
and makes use of lithium polytungstate, a harmless,
new-generation heavy liquid, to separate aragonite
from secondary calcite (Douka et al. 2010, in
press). Since both phases are found in the form of
CaCO3 with very similar composition, the application
of chemical methods to eliminate one of the two
may result in non-discriminatory, uncontrolled
effects. Certain physical properties of the polymorphs,
on the other hand, such as crystal structure
and especially specific gravity are quite distinct;
therefore they can be used as a basis to distinguish
between calcite and aragonite. Aragonite is slightly
heavier than calcite thus the former will sink and the
latter will float when both are present in a solution of
an intermediate density. After decanting the lighter
mineral, we are able to isolate and date only the original,
autochthonous phase.
The initial results of this new separation
method are very promising and technical details
are described in a forthcoming publication (Douka
et al. 2010, in press).
MUNIBE Suplemento - Gehigarria 31, 2010
S.C. Aranzadi. Z.E. Donostia/San Sebastián