Santander, February 19th-22nd 2008 - Aranzadi

22
KATERINA DOUKA, THOMAS F. G. HIGHAM AND ROBERT E. M. HEDGES
effect may add hundreds of years to the apparent
age of a sample (Pilcher 1991, Bezzera et al. 2000)
although more severe effects have been reported
(e.g. Gischler et al. 2008). Similarly a hard-water
effect may be generated by magmatic CO2 brought
into lakes and rivers from volcanic sources.
In open waters, the hard-water effect on shells
is usually unnoticeable because of the overwhelming
preponderance of marine dissolved inorganic
carbon (DIC), mainly in the form of HCO3−,
which obscures the products of the carbonate
substrate solubility reaction 3 .
The effect is larger in molluscan shells growing
in localities (a) with restricted water circulation (b)
where there is considerable mixing of fresh and
oceanic water, (c) where the geological substrate
is highly carbonaceous and (d) in areas with high
abundance of terrestrial organic matter (Forman
and Polyak 1997). In a paper published by Hogg
et al. (1998), the dating of modern estuarine and
subtidal shellfish showed that those living in rock
pools give different radiocarbon ages from
molluscs living in the open sea.
The ways in which species obtain their C and
especially the way they feed (filter/ suspension-feeders,
deposit/ detritus-feeders) is probably the most
defining parameter on the manifestation of a hardwater
effect although not many studies have addressed
the issue in its full extent. It is generally assumed
the deposit-feeders should be avoided, but since
most gastropods fall in this category such a generalization
seems rather limiting and certainly arbitrary,
as several reliable dates have been produced by
such species. Nonetheless, when possible, identification
of species, their feeding patterns and growth
localities are very important factors in the reliable
dating of shells from enclosed seas.
In cases where the local reservoirs and the
hard-water effect is thought to seriously affect the
dates, a test can be made by the dating of few paired
samples, i.e. pairs of contemporaneous marine
and terrestrial material such as shell and charcoal,
from the exact same depositional context
(Kennett et al. 2002), or when this is not possible,
pre-bomb modern shells of the same species as
the archaeological ones and from localities where
the latter were most likely collected, can be used.
4. Time-averaging or the “old shell” problem.
It is often assumed that shellfish used as food was
harvested alive and was brought to the archaeological
site shortly after death, therefore the radiocarbon
activity of the exoskeleton should reflect
the time passed since the animal’s death, and
would therefore date the human activity.
However, in the case of dating ornamental
shells or shells used for the production of tools,
cutlery or other, a considerable amount of time
may have elapsed between the animal’s death
and the time of use. This shell material may well
have been picked up from fossil outcrops or longdead
beach assemblages and thanatocenoses.
Dates based on such samples will always overestimate
the age of the deposit thus can be only
used as termini post quem.
Several studies have shown that molluscan
shells have the potential of a long post mortem life
and the fossil record is clearly biased in favour of
organisms with such hard parts. The notion of
“time-averaging”, as the process by which biogenic
remains from different time intervals come to
be preserved together, has been long used in
paleoecology, taxonomy, biostratigraphy and evolutionary
studies; recently in AMS 14 C shell dating
as well (for full summaries see: Flessa &
Kowalewski 1994, Kidwell and Bosence 1991,
Kidwell 1998).
Based on observations over the geological
context of fossil shell assemblages, paleontologists
have reported that the phenomenon of timeaveraging
operates over a broad range of timescales.
In actively forming death-assemblages on
beaches, tidal flats, and nearshore sub-tidal habitats
(10m depth) to the continental slope
(approximately 600 m) appear to have the same
minimum and maximum ages, however the
median age there appears to be approximately
10000 years.
As a result, according to this estimate, the
modal age for a typical shell collected from a
nearshore locality will be less than 1000 years old,
which is often not manifestly larger than the typical
standard deviations for a Palaeolithic radiocarbon
date, although still a significant value.
At this point a note needs to be made. The studies
mentioned above concern palaeontological
material, which lack human agency in their deposi-
3
CaCO3 ⇔ Ca 2+ + CO3 2 − ==> Ksp = [Ca 2+ ][CO3 2 −].
MUNIBE Suplemento - Gehigarria 31, 2010
S.C. Aranzadi. Z.E. Donostia/San Sebastián