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Abstract
Several methods to date a soil are developed in the field of soil sciences. The most commonly
used method is radiocarbon (14C) dating. 14C-dating is based on the fact that 14C, contrary to
12
C and 13C, is radioactive. 14C is taken up by living organisms and therefor the concentration
in organisms is constant. As soon as an organism dies, radioactive decay results in a lowering
of the 14C concentration. The specific mean- and half-life rates for 14C make it possible to
determine the time of death of an organism.
Many materials found in soils can be used for radiocarbon dating, as long as they were once
part of the biosphere. Charcoal is an often used material. Charcoal is the residue of incomplete
burning of wood and consists out of stacked polycyclic aromatic sheets. Although the
biological inertness and physical stability of the charcoal make it a suitable material for
radiocarbon dating, some difficulties with the radiocarbon dating of charcoal appeared.
Charcoal is known to adsorb organic material. The adsorption of even one percent of the mass
of young organic material to charcoal with ‘an infinite age’ causes a measured age of 37k
year. Several methods have been developed to remove this organic contamination, with
diverse results. Some methods seem to work better for older samples than others.
One event in recent history that copes with dating problems is the transition from the Allerød
to the Younger Dryas about 12.900 year ago. The cause of this transition is heavily debated
and studies have given several hypothesis as reorganization of the oceans or an extraterrestrial
impact. A point of discussion in this scientific debate is a soil layer found in the United States
as well as in Europe. This so-called Usselo layer consists or organic material in between sand
deposits and is characterized by the presence of charcoal. This charcoal is the result of
wildfires that characterize the transition from Allerød to Younger Dryas
Radiocarbon dating of this charcoal coming from a driftsand area in the Netherlands with the
standard ABA pretreatment method for samples younger than 20k years have shown
remarkable results. The 14C-dating resulted in ages of 8500-9500 year, which is significantly
younger than the expected age. In this area, the Usselo horizon was located close under a
podzol. Podzols are known for the leaching of organic material. We hypothesized that this
leached organic material has adsorbed to the charcoal, and cannot be removed by the
‘standard’ ABA removing technique. The goal of this study was to characterize the organic
material that adsorbed to this charcoal and determine whether different pretreatment methods
would result in a varying charcoal composition and eventually determine the suitability of the
standard ABA-pretreatment. Therefore, the charcoal was pretreated with the conventional
ABA- and ABOx-pretreatment and the newly introduced TO375- and ASE- methods. The
treated charcoal samples were characterized with TMAH-Py-GC/MS and compared to
samples from the B-horizon of the podzol and fiber material in order to study the possible
adsorbed fraction of soil material. CHN-analysis was performed on the charcoal- and soil
samples to determine the elemental composition. After pretreatments, the charcoal samples
were 14C –dated to determine the impact of the pretreatments on the radiocarbon age.
This research has shown that the composition of the charcoal varies significantly between the
differently pretreated charcoal samples. The use of more aggressive pretreatment methods
result in charcoal samples with a higher fraction of ‘original charcoal’ material due to the
desorption of organic material. This is supported by the CHN-analysis, that shows elevated
levels of carbon for the pretreated charcoal samples.
Dating of these charcoal samples has shown that there is a significant difference between the
treated- and untreated charcoal samples. The ABA, ABOx and TO375 provide the oldest
dating results with an estimated age for the charcoal of ± 10.500 cal. years BP. Although the
composition of these charcoal shows significant differences, there are no significant
differences found in the radiocarbon ages of these three pretreatment methods. This is caused
by the low sample size and low formation temperature of the charcoal.
This research has not shown that the ABA-pretreatment is not suitable for charcoal samples
with an expected age of <20k years found in soils with leaching properties, but does show that
in the worst case scenario, the ABOx and TO375 provide results at least as good as the ABA.
Therefore, it can be concluded that the ABOx is more optimal pretreatment methods for future
charcoal radiocarbon dating
Regarding the Allerød - Younger Dryas transition, this research cannot provide extra
information due to the still unexpected low radiocarbon age of the charcoal. Nevertheless, the
ABOx- and TO375- pretreatments show a tendency towards older provided radiocarbon ages.
In a debate that focusses on a age difference of 200 years, the findings in this research could
be of crucial importance.