A validation of OSL and 14C dating of initial soils in LateHolocene polycyclic drift-sand deposits (S.E. Netherlands)
Madeleine L. M. Schilder
Extended abstract
Optically stimulated luminescence (OSL) dating and radiocarbon (14C) dating were applied to
Late-Holocene polycyclic drift-sand deposits from the southern Netherlands. In preliminary
studies (Castel et al., 1989; Van Mourik, 1988; Van Mourik et al., 1995), dating of polycyclic drift
sand sequences was based on palynology and radiocarbon ages of humic horizons of buried soils.
With these techniques, it was impossible to separate between periods of active sand drifting and
periods of soil development. Combining OSL dating of mineral grains and 14C dating of soil
organic matter (SOM) was expected to improve the accuracy of the chronology of such
polycyclic sequences. This study was also aimed at finding the most essential technique(s) for
reading and reconstructing drift-sand areas in general, which will be useful for future research.
Castel et al. (1989) studied Holocene drift-sand deposits in different parts of Western Europe
(Drenthe, Veluwe, Brabant, Germany and Poland). All areas were found to have polycyclic soil
profiles representing an alternation of periods of active sand drifting and soil formation.
Eventually, all knowledge of the chronology and landscape development of the drift-sand
deposits in the southern Netherlands is expected to be useful for research in west European driftsand areas with the same history of plaggen agriculture.
In drift-sand areas, alternation of periods of sand drifting and periods of landscape stability
resulted in an intercalation of aeolian sediments and humic soil horizons. In the Dutch drift-sand
area Weerterbergen, these humic soil horizons are visible as buried initial soils (micropodzols) in
Kootwijk-2 sediment. Based on former research in the area, the expectation was that these initial
soils were developed in post-Medieval times, between 1423 and 1633 AD, and that they were
related to a recovering of heath land.
To refine the existing knowledge about the chronology, OSL and 14C dating were performed on
the humic soil horizons in the polycyclic drift-sand deposits. The combination of the two
techniques was supposed to provide more information about the chronology of the humic soil
horizons that were expected to be initial soils. Validation of both OSL and 14C dating results was
done to find out what dating technique is the most essential one for reading and reconstructing
drift-sand landscapes and to learn more about the soil development processes. To validate,
pyrolysis gas chromatography/mass spectrometry, soil micromorphology, pollen analysis, and
phytolith analysis were performed. The time available for landscape stability and thereby for soil
development (pedogenesis) was determined. Also, it was investigated whether the humic
horizons were in-situ developed micropodzols, as was expected from field observations and
former research. Thereby, it was studied whether the development of the humic horizons was
due to regional or local processes, respectively resulting in one single period or in different
periods of landscape stability. To learn more about the processes of landscape development, a
characterisation was made of the vegetation that was present during soil development, since
vegetation acted as trigger for landscape stability and soil formation. In addition to their
importance for validating OSL and 14C results, pollen analysis and phytolith analysis were
expected to provide a better insight in the paleovegetation and thereby in landscape development.
Furthermore, it was investigated whether phytolith analysis could complete pollen analysis in
reconstructing the paleovegetation.
14
C ages provided an estimation of the age of the humic acids of the humic soil horizons. The
radiocarbon ages are correlated with the SOM of the humic soil horizons. OSL ages represented
the burial ages of the mineral samples above and below the micropodzol. A large discrepancy
between the OSL and 14C ages was observed (table A). Surprisingly, in all profiles 14C ages turned
out to be older than burial (OSL) ages.
Table A
OSL and 14C results for all sampled profiles in Weerterbergen. OSL ages in y AD
with 1σ (NCL, Delft) and radiocarbon ages in cal y AD.
Profile
Sample
Depth (m)
Technique
Ages (cal y AD)
Def-1
unknown
unknown
OSL
1910 +/- 10
Def-1
GrN-14458
unknown
radiocarbon
1423 - 1633
Def-1
unknown
unknown
OSL
1650 +/- 30
Bos-1
5106010
0.63
OSL
1961 +/- 2
Bos-1
GrN-29960
0.67
radiocarbon
1528 - 1952
Bos-1
5106011
0.72
OSL
1819 +/- 10
Mad-1
5106012
0.38
OSL
1922 +/- 5
Mad-1
GrN-29959
0.42
radiocarbon
971 - 1225
Mad-1
5106013
0.47
OSL
1896 +/- 5
Mad-2
5106014
0.34
OSL
1981 +/- 1
Mad-2
GrN-29961
0.39
radiocarbon
1222 - 1395
Mad-2
5106015
0.43
OSL
1890 +/- 6
Mad-3
5106016
0.35
OSL
1993 +/- 1
Mad-3
GrN-29962
0.38
radiocarbon
1955-1956
Mad-3
5106017
0.42
OSL
1989 +/- 1
Mad-4
5106018
0.39
OSL
1943 +/- 3
Mad-4
GrN-29964
0.42
radiocarbon
1297 - 1440
Mad-4
5106019
0.47
OSL
1878 +/- 6
Mad-5
5106020
0.40
OSL
1795 +/- 11
Mad-5
GrN-29963
0.43
radiocarbon
340 - 766
Mad-5
5106021
0.48
OSL
1801 +/- 11
OSL ages were interpreted to be the most reliable ages for determining the chronology of the
polycyclic drift-sand deposits. First, because all samples were very suitable for OSL dating.
Second, because the origin of the OSL samples was not important, but that of the 14C samples
was. Moreover, the 14C ages turned out to represent ages of a mix of relatively young organic
material, charcoal, and relatively old organic material, of which the latter two were expected to be
blown-in from older sources (burial mounds and Twente podzols respectively) in the
Weerterbergen area. The presence of charcoal in the humic soil horizons was confirmed by the
soil micromorphological results. Third, because results of the soil micromorphological study
concerning in-situ soil development were in line with the OSL ages. Last, pollen results, especially
the presence of Pinus pollen, provided another proof of the reliability of the OSL ages.
According to the results of the soil micromorphological study, profiles Def-1, Bos-1, Mad-1,
Mad-2, Mad-3, and Mad-4 turned out to be in-situ developed initial soils (micropodzol), since
they all contained in-situ formed organic material of excremental origin. Profile Mad-5 turned out
to be one big sediment in which parts contained blown-in organic material and roots. All
sediments turned out to be deposited very recently: between 1650 and 1993 AD. This was not in
range with results of former studies performed in the drift-sand area Weerterbergen (Van
Mourik, 1988; Van Mourik et al., 1995). In these studies the humic soil horizons were stated to be
developed in post-Medieval times (between 1423 and 1633 AD); these conclusions were based on
14
C ages and palynology. Because of the reliability of OSL ages, results of this present study were
expected to be more accurate and reliable than results of these former studies. Both the length
and the start of the time available for landscape stability and soil development did vary among the
different profiles, from 292 years (Def-1) to 3 years (Mad-3). The differences in ratio of tree –
shrubs/herbs pollen of the different profiles could be linked successfully to these differences in
time available for soil development. There turned out to be not one single period of landscape
stability. Consequently, development of the humic soil horizon was expected to be due to local
conditions, and not to regional fluctuations in landscape stability.
Pollen results showed that the vegetation did differ among the different profiles. The
paleovegetation was a mix of regional and local species. Pinus pollen were present in all samples,
except for Mad-5, which turned out to be no micropodzol. Since Pinus was planted by men only
from 1800 AD on (Van Mourik and Dijkstra, 1995), presence of Pinus pollen in the humic soil
horizon confirmed the development of the horizon after 1800 AD. The soil development of the
initial soils (micropodzols) studied in this present research, was not only expected to be related to
a recovering of heath land, as was stated by former research (Van Mourik, 1988; Van Mourik et
al., 1995), but for all but one profile (Mad-5), it was expected to be correlated with presence of
both Pinus and Ericaceae during soil development.
Phytolith spectra could not be prepared for three reasons: reference phytoliths could not be
clearly distinguished in reference slides; during the micromorphological study, phytoliths could
not be found in thin sections; the prepared phytolith slides were extremely disturbed by crystals
or clay particles. Unfortunately, it had to be concluded that phytolith analysis could not complete
pollen analysis and the reconstruction of paleovegetation in this research.
This research did indeed refine the existing knowledge of the chronology and landscape
development of the drift-sand deposits in the southern Netherlands by applying OSL dating, 14C
dating, pyrolysis gas chromatography/mass spectrometry, soil micromorphology, and pollen
analysis. The research contributed to utilization of OSL dating in paleo-ecological research. OSL
dating turned out to be an essential tool for getting insight in Late-Holocene polycyclic drift-sand
deposits. 14C dating of these humic soil horizons in the aeolian environment alone would lead to
erroneous interpretation, due to the complex composition of the SOM. However, 14C ages,
combined with pyrolysis gas chromatography/ mass spectrometry, are expected to help providing
a better insight in the complexity of the SOM. The combination of pollen analysis and
micromorphology showed to be essential for learning more about the vegetation that acted as a
trigger for and was present during local periods of landscape stability and soil development.
Studying phytoliths in soil sediments was, like OSL dating, quite pioneering. Results of phytoliths
analysis were expected to complete pollen results in the reconstruction of paleovegetation, but
this turned out not to be the case. This innovative multidisciplinary research showed that the
combination of OSL dating, 14C dating, pyrolysis gas chromatography/mass spectrometry, soil
micromorphology and pollen analysis is essential for reading and reconstructing drift-sand
landscapes.
Keywords:
polycyclic drift-sand deposits, initial podzol, OSL dating,14C dating, paleo-ecological
reconstruction, soil micromorphology, soil organic material