Geomorphology 109 (2009) 17–26
Contents lists available at ScienceDirect
Geomorphology
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h
Luminescence dating of hillslope deposits—A review
M. Fuchs a,⁎, A. Lang b
a
b
Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA
Department of Geography, University of Liverpool, Liverpool L69 7ZT, UK
a r t i c l e
i n f o
Article history:
Received 18 December 2006
Received in revised form 15 May 2007
Accepted 9 August 2008
Available online 10 March 2009
Keywords:
Optical dating
Colluvium
Geomorphology
Geoarchaeology
Climate change
a b s t r a c t
Luminescence dating determines the last exposure to light of quartz and feldspar mineral grains and thus the
time a sediment was laid down. Prerequisite is that the sediment grains were sufficiently exposed to light
prior to deposition. For eolian deposits this seems a fair assumption, although exceptions may well exist. For
other depositional environments, light exposure prior to burial is often restricted in duration, intensity and
spectral composition and may result in an age overestimation.
Recent developments now allow dating of sediments that were exposed to only little light prior to
deposition. The main developments are single-aliquot techniques and statistical tests that allow extraction of
valid age estimates even from deposits in which bleached and unbleached grains are mixed. These
developments, in combination with the fact that the great majority of sediments near the Earth's surface are
not of eolian origin, have resulted in the large interest in the luminescence dating of non-eolian deposits.
Here, we review possibilities and challenges of luminescence dating of colluvial deposits. We summarize
techniques for the detection and handling of insufficiently bleached sediments—a common condition in
colluvial deposits. We give an overview how luminescence dating has been applied on hillslope sediments
for reconstructing landscape response to climate change, to analyze past fault activity and unravel human
induced soil erosion. We discuss sampling strategies and give guidelines as to which techniques are best
suited to establish luminescence chronologies for colluvium.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Hillslope deposits are an important sedimentary archive for geomorphological and paleoenvironmental research. They have extensively been
used for reconstructing landscape response to climate change (e.g.,
Hanson et al., 2004), anthropogenic induced soil erosion (e.g., Fuchs et al.,
2004) and tectonic activities (e.g., Fattahi et al., 2006). Hillslope deposits
often are called colluvium. Unfortunately, various definitions for
colluvium are in use, and different scientific communities apply different
criteria. In general, the term describes sediments that are eroded from
and transported along hillslopes by running water or gravity, and that
usually form wedge-shaped deposits on the foot-slope. Typically,
colluvial sediments are considered to be derived from spatially diffusive
processes and are distinguished from alluvial sediments, which are
transported in spatially confined channels. The limited transport distance
within colluvial systems reduces the complexity of such systems, allows
easy delineation of the sediments' source area and often enables
straightforward identification of causes for hillslope erosion.
A prerequisite for the use of colluvial sediments as archives of paleoprocesses is the knowledge about their temporal evolution. A review of
⁎ Corresponding author. Chair of Geomorphology, University of Bayreuth, D-95440
Bayreuth, Germany.
E-mail address: [email protected] (M. Fuchs).
0169-555X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.geomorph.2008.08.025
techniques available for the dating slope deposits can be found in Lang
et al. (1999a). To establish chronologies of late Quaternary colluviation,
luminescence dating is one of the preferred dating tools as it may allow
dating of the formation of the slope deposits and thus direct sediment
dating. In contrast, indirect dating of slope sediments like 14C dating of
organic matter, may result in age overestimation, due to frequent
reworking of old carbon and long time lags before incorporation of the
organic material in the sediments (Lang and Hönscheidt, 1999). The
prerequisite for successful age determination in luminescence dating
requires sufficient resetting of the previous luminescence signal by
exposure of mineral grains to daylight. If the last process of erosion,
transport and deposition does not provide sufficient bleaching of the
luminescence signal (e.g., Stokes, 1999; Lian and Roberts, 2006), a
remnant signal remains, that if undetected will cause an age overestimation (Fuchs and Wagner, 2005). The limited transport distance
within colluvial systems that on the one hand is advantageous for
geomorphology and paleoecology, on the other hand renders colluvial
sediments prone to insufficient bleaching. The limited transport
distance can usually be associated with only short pre-depositional
light exposure. Nevertheless, in several cases colluvial sediments were
successfully dated using relatively slowly bleaching thermoluminescence (TL) signals (e.g., Wintle and Catt, 1985; Forman et al., 1988). With
the introduction of optical stimulated luminescence (OSL) dating by
Huntley et al. (1985) and infrared stimulated OSL (IR OSL) dating of
feldspars by Hütt et al. (1988), which access order of magnitude more
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M. Fuchs, A. Lang / Geomorphology 109 (2009) 17–26
Fig. 1. Hillslope erosion model after Lang and Hönscheidt (1999). The hillslope system represents a cascade system with temporary sinks. Thus, sediment may not be transported
downslope in a single event, but may temporarily be stored on the slope and only later remobilized and transported to the foot-slope where it is accumulated as colluvium. The
repeated cycles of erosion and sedimentation increase the probability of bleaching.
M. Fuchs, A. Lang / Geomorphology 109 (2009) 17–26
light sensitive luminescence signals, luminescence dating of colluvial
sediments became more popular. Under favorable conditions OSL
signals can be fully bleached within seconds of light exposure
(Godfrey-Smith et al., 1988). The process leading to colluviation will
not always provide such conditions and processes like attenuation of
daylight by clouds and suspended particles in turbid water, or
coagulation of mineral grains during transport hamper bleaching of
grains. Thus, complete resetting of the OSL signal is the exception rather
than the rule. In consequence, detecting and eliminating insufficient
bleaching is a necessary requirement in luminescence dating of
sediments (Fuchs and Wagner, 2005).
There are several possibilities for the identification of insufficient
bleaching (e.g., Li, 1994; Clarke, 1996; Olley et al., 1999; Wallinga,
2002; Fuchs and Wagner, 2003). Different bleaching characteristics of
different luminescence signals can also be compared: OSL and TL
(Wintle et al., 1993), OSL from feldspar and quartz (e.g., Sørensen et
al., 2001), OSL from coarse and fine grains (e.g., Kadereit et al., 2006a),
fast and slow OSL components (e.g., Bailey et al., 1997) or different
luminescence emissions (e.g., Krause et al., 1997). Most widely used
are statistical analyses of equivalent dose (De) distributions obtained
on single aliquots (Duller, 1991) or single grains (Lamothe et al., 1994).
These approaches also allow extraction of a best estimate of the
equivalent dose from an insufficiently bleached sample (e.g., Galbraith
et al., 1999; Fuchs and Lang, 2001; Lepper and McKeever, 2002).
Here we review luminescence dating of colluvial sediments. We
discuss techniques designed to detect and handle insufficient
bleaching, present results from case studies where luminescence
dating has been used on colluvial sediments, and give a perspective of
their application in geomorphology and geoarchaeology.
19
3) The catchment morphometry and the erosion process define the
sediment path, sediment travel distance and sediment sink. Due to
the usually rather small catchments of colluvial systems, these
parameters are rather well known and the travel distance tends to
be relatively small. This decreases the probability of efficient
bleaching, because the duration of light exposure is short and
events may occur during unfavorable daylight conditions (e.g.,
transport during the night).
Despite the above mentioned shortcomings, there are many
examples of successful luminescence dating of colluvium. Two factors
influence this: (a) hillslope sediments are often eroded, transported
and accumulated not in a single event, but temporary storage on the
hillslope occurs, before a further mobilization takes place (Fig. 1; Lang
and Hönscheidt, 1999). This increases the probability of exposure to
daylight and thus bleaching. (b) bioturbation and mechanical
processes in the soil as well as cultivation ensure that mineral grains
are frequently exposed to daylight before the sediment grains are
eroded and in many cases also after deposition before the sediment
grains are covered through further sedimentation. According to Berger
and Mahaney (1990) this process is more important for sediment
bleaching than the colluvial transport itself.
Nevertheless, as in other sedimentary environments, excluding
insufficient bleaching is a necessary requirement for successful
luminescence dating of colluvial sediments. This is demonstrated by
investigations of modern colluvial sediments, where residual luminescence signals were identified, which would result in age overestimations of hundreds (Lu et al., 2002) or even thousands of years
(Porat et al., 2001).
2.1. Detection of insufficiently bleached sediments
2. Luminescence dating of colluvial sediments
The advantage of luminescence dating over other dating methods
is its ability to directly date the deposition of clastic sediments (e.g.,
Lian and Roberts, 2006). The process resetting the luminescence
‘clock’ is the last exposure of quartz or feldspar mineral grains to
daylight, a process which is likely to occur while sediments are
eroded, transported and deposited. For eolian sediments, sufficient
exposure to daylight to reset the luminescence signal is usually given
(Hilgers et al., 2001). For colluvial sediments, complete signal
resetting is often hampered by various factors.
1) Several processes are involved in sediment reworking, leading to
colluviation. The important processes are soil erosion (e.g.,
plowing, sheet and rill erosion), mass movements (e.g., sliding,
falling) and soil creep (e.g., solifluction). While these processes act
on hillslope material to move it downslope, the sediment itself
may move as a rigid block as in the case of sliding. Thus, in respect
of daylight bleaching, only a small fraction of sediment involved in
the colluviation process (e.g. only the outside layer of sediment
grains on a rigid block) is exposed to daylight. If material is
transported by water (e.g., sheet erosion), the intensity and
spectral composition of the daylight is subdued by the usually
high suspended load (Ditlefsen, 1992).
2) A large variety of hillslope materials—the source of colluvial
sediments—exist, leading to a large variety in sediment characteristics, e.g. grain-size distribution, mineral composition or organic
content. Thus, certain sediments tend to be transported as single
mineral grains, like pure sand, others are prone to coagulation, like
clay-dominated sediments. The latter often hampers sufficient
zeroing of the luminescence signal, because the inner grains of an
aggregate are shielded from daylight exposure (Lang and Wagner,
1996). The same may result from mineral coatings, where e.g. iron,
manganese or carbonate coatings hamper daylight penetration
into the mineral grains (Singhvi et al., 1986; Quickert et al., 2003).
Recent years have seen big improvements for detecting and handling
insufficient bleaching. The techniques available to detect insufficient
bleaching are based on comparing different luminescence signals or the
degree of bleaching of different mineral grains. Earlier studies explored
the lower light sensitivity of TL compared to OSL (Fig. 2; Godfrey-Smith
et al., 1988). For example Wintle et al. (1993) compare TL and IR-OSL
(infrared stimulated luminescence) of the same colluvial samples from
Natal/South Africa, and find that TL ages are up to 10 ka older than IROSL ages. However, IR-OSL ages are also overestimating the expected
age, and Wintle et al. (1993) conclude that in this setting IR-OSL is only
suitable for dating older colluvial deposits where the residual age is
insignificant.
Various grain-size fractions may also show different degrees of
bleaching. In luminescence dating, generally two different fractions are
used: fine grains (4–11 µm) and coarse grains (90–200 µm). For alluvial
sediments, several case studies have shown that the coarser grains seem
to be better bleached than finer fractions (e.g., Olley et al., 1998;
Wallinga, 2002). For colluvial sediments, there are no studies comparing
coarse and fine grains of the same mineral type. Kadereit et al. (2006a)
compare results of coarse grain quartz and fine grain feldspar extracted
from colluvial sediments from a tell in Romania. The fine grains return
significantly higher OSL ages than the coarse grains. This might also be
caused by the different bleaching properties of the minerals, with quartz
bleaching faster than feldspar (Fig. 2; Godfrey-Smith et al., 1988). A
comparison of different minerals within a grain-size fraction of alluvial
sediments confirms faster bleaching of quartz for modern fluvial
sediments (Fuchs et al., 2005). If this behavior also holds for colluvial
sediments has yet to be established.
Differences in the bleaching properties of different OSL components can also be utilized to detect insufficient bleaching. Initially
differentially bleached signal components within an OSL shine-down
curve were compared (Aitken and Xie, 1992). If De is constant with
stimulation time sufficient bleaching should have occurred. This
technique was used for multiple aliquot protocols. However, it has also
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M. Fuchs, A. Lang / Geomorphology 109 (2009) 17–26
Fig. 2. TL and OSL bleaching characteristics of quartz and feldspar (after Godfrey-Smith
et al., 1988). The graph shows that the OSL signal is faster and more readily bleachable
then the TL signal. It is also shown that under full daylight conditions quartz OSL
bleaches faster than feldspar OSL.
threshold based on the experimental error obtained for this sample
simulating well-bleached conditions (Fuchs and Lang, 2001), (3) the
minimum age model (Galbraith et al., 1999) and a simplified version
thereof (Juyal et al., 2006), and (4) the determination of a threshold via
the leading edge technique (Lepper and McKeever, 2002).
Bailey and Arnold (2006) evaluate the different statistical
approaches and show that significantly different estimates of the burial
dose are obtained using the different models. Based on their results, the
authors provide decision support criteria for which model it is best to
choose for different types of dose distribution.
The underlying problem with these techniques is the uncertainty
about the causes of a broad distribution. Besides the bleaching history,
there are also other causes for De scatter, such as microdosimetry or
luminescence characteristics (e.g., Murray and Roberts, 1997; Kalchgruber et al., 2003). Thus, when insufficient bleaching is detected, a
conservative approach is to determine a maximum age estimate for a
deposit based on the median of all measured aliquots. Application of
the statistical approaches requires knowledge of the variability in the
case of well bleached samples. Using the lowermost Des may be
erroneous as shown by Rodnight et al. (2006). They use the finite
mixture model (Galbraith and Green, 1990) to identify and exclude
low dose outliers that are measurement artifacts due to the
luminescence characteristics of these aliquots.
been shown that the presence of such a plateau is necessary but not
sufficient to decide if a sediment can successfully be OSL dated (Lang
et al., 1999b). The OSL signal of quartz consists of fast-, medium-, and
slow-components (Bulur, 1996; Bailey et al., 1997; Bailey, 2000), with
the fast component bleaching most rapidly. Using single aliquot
protocols, this is shown by Singarayer et al. (2005) for colluvial coarse
grain quartz extracts from Oxon, UK, and Kadereit et al. (2006a) show
a similar behavior for polymineral fine grain extracts from colluvial
deposits in southwest Germany.
The introduction of single aliquot techniques (e.g., Duller, 1991;
Roberts et al., 1998; Murray and Wintle, 2000) made it possible to
develop alternative approaches to detect insufficient bleaching, especially in cases where sediments contain a mixture of grains bleached to
various degrees (‘differential bleaching’ after Duller, 1994). Here, the
differences between aliquots or single grains are used. The earlier
attempts were based on comparing an aliquots' (or single grains')
natural luminescence intensity and its De (e.g., Li, 1994; Colls et al.,
2001). The analyses of De scatter can also be used to unravel insufficient
bleaching (e.g. Clarke, 1996), if aliquots consist of a small number of
grains only (Olley et al.,1999; Wallinga, 2002; Fuchs and Wagner, 2003).
When plotting the Des as histograms (Fig. 3), well bleached samples
show narrow and almost symmetrical distributions, whereas broad
distributions are characteristic for insufficiently bleached samples. Often
strongly skewed distributions are obtained when the majority of the
grains were well bleached but also a number of poorly bleached grains is
present (e.g., Olley et al., 1999; Thomas et al., 2005; Fuchs et al., 2007).
Unfortunately, all scatter based techniques can only be used for
coarse grain separates. For fine grains the number of grains is too large
(N106) and any scatter will be averaged out.
2.2. De determination of insufficiently bleached sediments
Several techniques were developed to derive a true equivalent
dose from a distribution of De values in an insufficiently bleached
sample. All these techniques are based on statistical analyses of De
variations and try to differentiate between insufficiently and sufficiently bleached aliquots. These approaches rely on a certain (usually
large) number of Des determined on coarse grains using small aliquots
or single grains. The common idea of all techniques is to identify the
well bleached population towards the lower end of a De distribution.
Techniques in use include (1) the use of the lowest 5% of a dose
distribution as best De estimate (Olley et al., 1998), (2) a sample specific
Fig. 3. Frequency histograms of equivalent doses (De) obtained on two samples of
colluvial quartz extracts from Oman. Graph a.) shows a narrow distribution of a well
bleached sample, Graph b.) shows a broad and skewed distribution, typical of an
insufficiently bleached sample.
M. Fuchs, A. Lang / Geomorphology 109 (2009) 17–26
3. Applications in geomorphology, geoarchaeology and paleoclimate
research
Even though luminescence dating of colluvium is rather challenging, results from a variety of case studies are available from all over
the world. These include all climatic regions from polar to tropical, and
represent a variety of applications in geomorphology, geoarchaeology
and paleoenvironmental research (Table 1). Among the first who
successfully applied luminescence dating to colluvial sediments were
Forman et al. (1988) using TL dating techniques. Recently TL is very
rarely applied in the dating of sediments, and optical methods (OSL,
IR-OSL) are generally preferred.
3.1. Landscape response to past climatic change
The first report of successful optical dating of colluvial sediments is
presented by Wintle et al. (1993). In this and successive papers (Botha
et al., 1994; Wintle et al., 1995a,b) the authors were able to correlate
sediments and soils with dryer or wetter climatic periods of the Late
Quaternary in South Africa and describe in detail the complexity of
slope evolution processes. First, TL and IR-OSL multiple-aliquot
protocols on polymineral fine-grain extracts were used, and later
also single-aliquot protocols on coarse-grain feldspar extracts. These
studies provided chronologies for the deposits, but also showed that
the light exposure history of individual colluvial sediments can be
dramatically different. Using a single-aliquot IR-OSL protocol on
coarse-grain feldspar separates, a wide range of De values was
obtained for the upper part of a colluvial sequence, whereas a much
21
smaller range in Des was found in the lower part (Wintle et al., 1995b).
Comparison with radiocarbon age constraints showed that IR-OSL
ages for the upper colluvium overestimated the burial age, whereas in
the underlying colluvium—where grains were bleached more uniformly—the obtained IR-OSL ages were correct. TL results for the same
samples showed that the TL-signals were not fully bleached at the
time of deposition, but that the grains from the lower sediment had
experienced considerably more light exposure prior to burial than the
grains from the overlying unit.
Similar sediments have been studied by Clarke et al. (2003). They
report IR-OSL ages as old as 100 ka obtained on coarse-grained
feldspars and using a single-aliquot additive-dose protocol. IR-OSL
ages in the younger units are consistent with 14C ages from soil organic
matter extracted from paleosols.
In a study on colluvial deposits from Tanzania, Eriksson et al.
(2000) identified two major colluvial periods. The first period of
deposition took place during the late Pleistocene, probably related to
climate change from dry to wet conditions. The second period of
colluvial formation took place after a long phase of stability and soil
formation and started ca. 900 years ago. This period of colluviation
seems to be related to human activity, such as agricultural intensification, livestock keeping and iron smelting, and the associated land
clearance. For age determination, an OSL single aliquot protocol was
applied to the quartz coarse grain fraction. Measuring many aliquots
per sample, the Des showed a large scatter which indicated
insufficient bleaching. To extract sufficiently bleached aliquots,
Eriksson et al. (2000) applied the minimum age model of Galbraith
et al. (1999).
Table 1
Overview on luminescence dating studies of colluvial sediments.
Task
Region
Luminescence technique
Reference
Methodology
California, USA
Mount Kenya, Kenya
Natal, South Africa
Natal, South Africa
Germany
Greece
Oman
Mount Kenya, Kenya
Himalaya/Pakistan
Tanzania
Mongolia
Natal/South Africa
Wyoming, USA
Vietnam
Tasmania/Australia
South Africa
Tibet
Tanzania
Zambia
Australia
Utah/USA
Utah/USA
Utah/USA
Israel
Israel
Israel
Mongolia
China
Iran
Germany
Germany
Germany
Germany
Germany
Greece
Greece
Romania
Belgium
TL
TL, MA, P, f
TL, IR-OSL, MA, SA, Fsp, c
IR-OSL, SA, Fsp, c
IR-OSL, MA, P, f
OSL, SA, Q, c
OSL, SA, Q, c
TL, MA, P, f
TL, MA, P, f
OSL, SA, Q, c
TL, MA, P, Q, c, f
IR-OSL, SA, Fsp, c
OSL, SA, Q, c
OSL, SA, Q, c
OSL, SA, Q, c
OSL, Q, SA, c
IR-OSL, MA, Fsp, f
OSL, IR-OSL, SA, Q, Fsp, c
OSL, SA, Q, c
TL, MA, Q, c
TL, MA, P, f
TL, MA, P, f
TL, MA, P, f
IR-OSL, SA, Fsp, c
IR-OSL, SA, Fsp, c
OSL, MA, Q, c
TL, IR-OSL, OSL, MA, SA, Q, Fsp, c, f
OSL, IR-OSL, MA Q, P, c, f
OSL, SA, Q, c
IR-OSL, MA, P, f
IR-OSL, MA, P, f
IR-OSL, MA, P, f
IR-OSL, MA, SA, P, f
IR-OSL, MA, P, f
OSL, SA, Q, c
OSL, SA, Q, c
OSL, IR-OSL, MA, SA, Q, P, c, f
OSL, SA, Q, f
Berger and Huntley (1987)
Berger and Mahaney (1990)
Wintle et al. (1993, 1995a,b)
Li (1994)
Lang and Wagner (1996)
Fuchs and Wagner (2003)
Fuchs et al. (2007)
Berger and Mahaney (1990)
Owen et al. (1992)
Eriksson et al. (1999, 2000)
Lehmkuhl and Lang (2001)
Clarke et al. (2003)
Hanson et al. (2004)
Zuchiewicz et al. (2004)
Duller (2004)
Boardman et al. (2005)
Kaiser et al. (2006)
Sørensen et al. (2001)
Thomas and Murray (2001)
Hutton et al. (1994)
Forman et al. (1988, 1989, 1991)
McCalpin and Forman (1991)
McCalpin et al. (1994)
Amit et al. (1996)
Porat et al. (1996, 1997, 2001)
Zilberman et al. (2000)
Prentice et al. (2002)
Lu et al. (2002)
Fattahi et al. (2006)
Lang and Wagner (1996, 1997)
Lang and Hönscheidt (1999)
Lang et al. (1999b, 2003)
Kadereit et al. (2002, 2006a)
Lang (1994, 2003)
Fuchs et al. (2004)
Fuchs and Wagner (2005)
Kadereit et al. (2006b)
Rommens et al. (2007)
Paleoclimate
Fault activity
Geoarchaeology/human induced soil erosion
SA: Single Aliquot; MA: Multiple Aliquot; Q: Quartz; Fsp: Feldspar; P: Polymineral; c: coarse grain; f: fine grain.
22
M. Fuchs, A. Lang / Geomorphology 109 (2009) 17–26
On Tasmania, Duller (2004) was investigating colluvium associated
with a raised marine terrace in a relatively tectonically stable region.
The raised terrace, associated with a global high sea-level stand, was
OSL dated using single aliquot and single grain protocols and coarse
grain quartz. Measurements on large aliquots (N1000 grains per
aliquot) resulted in an age estimate of ca. 20 ka, a time when eustatic
sea-level was below instead of above present sea level. Additionally,
the broad De distribution indicated insufficient bleaching. When small
aliquots (ca. 200 grains per aliquot) and single grains were investigated, the variability in De values increased and the weighted mean
De decreased. The De histograms obtained show a tendency to more
positively skewed De distributions with a more pronounced ‘tail’
towards higher Des when the number of grains per aliquots decreases
(Fig. 4). This can be explained by the presence of poorly or unbleached
grains within many grains that were bleached at deposition. For age
calculation, Duller (2004) used only the well bleached grains from the
lower part of the De distribution based on single grain measurements.
The estimated OSL age for the raised beach was between 5 ka and 6 ka,
which fits with a global high sea-level stand in the mid-Holocene. The
insufficient bleaching of the material could be explained by the reworking of the raised terrace sediments, resulting in colluviation.
3.2. Past fault activity and paleoearthquake reconstruction
The vertical displacement along a fault scarp leads to its erosional
degradation, thus colluviation. The dating of these colluvial sediments enables the reconstruction of fault activity and earthquake
recurrence intervals, which are important information for natural
hazard assessment.
Porat et al. (1996) were the first who applied optical dating to
colluvial sediments associated with fault scarp activities for estimating
seismic hazards. In their studies at the Arava fault, Israel, they were able
to establish an earthquake chronology with four large earthquakes
(M N 7) in the period of 37 ka–14 ka and five smaller earthquakes
(M = 6.2–6.6) in the following time until recently. Based on these data,
Porat et al. (1996) derived a recurrence interval of earthquakes with a
magnitude ≥6.2 of ca. 4 ka. For De determination of the colluvial
sediments, coarse grain feldspar extracts were measured with a single
aliquot protocol. Due to no correlation between De and natural signal
intensity (Li, 1994), the colluvial samples seemed to be sufficiently
bleached. This was supported by only minor residual ages of modern
colluvial sediments, showing an age of ca. 550 a. In general, the
suitability of colluvial sediments for investigating fault activities at the
Arava fault in southern Israel was confirmed by Porat et al. (1997).
However, Porat et al. (2001) recognized a slope-face dependency for the
bleaching degree of colluvial sediments, with colluvial sediments from
south-facing scarps being better bleached than colluvium derived from
north-facing scarps.
In a study from Xiyangfang, China, Lu et al. (2002) applied OSL and
IR-OSL multiple aliquot protocols on polymineral fine grain (4–11 µm)
extracts from colluvial sediments, to reconstruct the earthquake
recurrence interval of the last 30 ka. Based on De estimated from
modern colluvial sediments, a residual luminescence signal with a
mean De of ca. of 1.37 ± 0.28 Gy could be obtained for the IR-OSL
measurements and 1.76 ± 0.13 Gy for the OSL measurements. To
correct for insufficient bleaching, these values were subtracted from
De estimates of the dated samples. From the resulting OSL age
estimates for paleoearthquake activities of the last 30 ka, a rough
recurrence interval of ca. 7 ka could be given. However, luminescence
dated paleoearthquakes could only partially be correlated to historically documented earthquakes.
Fattahi et al. (2006) report OSL dating of colluvial sediments from
the Sabzevar thrust fault in northeastern Iran. They used a single
aliquot regenerative-dose (SAR) protocol on quartz extracts in the
grain size range of 38–63 µm and obtained narrow and symmetrical
De distributions, indicating sufficient bleaching of the colluvial
Fig. 4. Equivalent doses of a colluvial sample from Tasmania, using different aliquot
sizes. (a) Large aliquots of ca. 1000 grains per aliquot, (b) small aliquots of ca. 200 grains
per aliquot, (c) single grain measurements. With decreasing aliquot sizes, the De
variability increases, while the weighted mean of De decreases. The positively skewed
De distribution with a ‘tail’ towards larger Des is explained by insufficient bleaching of
some grains (after Duller, 2004. Copyright John Wiley & Sons Limited. Reproduced with
permission).
sediments. The OSL ages link the fault activity to the 1052 AD Baihaq
earthquake.
3.3. Past human-induced soil erosion
Next to naturally induced soil erosion due to e.g. climate change or
fault activity, human impact is one of the main factors responsible for
Holocene soil erosion and colluviation. The reason for this anthropogenic
dominance as a factor for Holocene colluviation is related to the socioeconomic change from the Paleo- and Mesolithic to the Neolithic, when
humans started to build permanent settlements, to domesticate animals
and to cultivate land. This change resulted in widespread woodland
M. Fuchs, A. Lang / Geomorphology 109 (2009) 17–26
clearance and associated soil erosion and colluviation. Therefore, colluvial
sediments are an important archive in geoarchaeological research and
have successfully been used to unravel past human activities.
Among the first who successfully dated human induced colluvial
sediments derived from a loess-covered landscape in southwest
Germany was Lang (1994). In this study, colluvial sediments in the
vicinity of an archaeological site were investigated, applying a multiple
aliquot protocol for polymineral fine grain extracts (4–11 µm), using the
subtraction technique by Aitken and Xie (1992). The results showed that
even for the trench fillings with a maximum sediment transport
distance of 100 m, the IR-OSL signal must have been bleached during the
last sediment reworking. This was proven by archaeological evidence,
stratigraphic consistency and an agreement between a TL and IR-OSL age
of a heated sample from a fireplace. The successful application of the IROSL multiple aliquot polymineral fine grain technique to loess-derived
colluvium could be confirmed at several sites from the loess hills of
south Germany (e.g., Kadereit et al., 2002). Lang (2003) constructed a
frequency distribution of 60 IR-OSL ages from southwest Germany.
Phases of increased colluviation coincide with phases of higher
population density and show that the intensity of farming activities is
the main trigger for soil erosion.
For timing past soil erosion, OSL performs in a superior way when
compared to indirect dating techniques that rely on material
incorporated in the sediments like artifacts or organic remains. In
these environments, the time from when a sediment particle first
entered the erosion–transport–deposition pathway until it is finally
laid down at the foot-slope can be very long and reworking occurs
frequently. This was shown for example by Lang and Hönscheidt
(1999), where time lags of several thousand years exist between the
age of an object and the age of deposition (Fig. 5).
Kadereit et al. (2006b) adopted a single aliquot regeneration
protocol for IR-OSL on polymineral fine grains. In this protocol IRstimulation time is varied to differentiate between harder-to-bleach
and easier-to-bleach components. The comparison of the two enables
the detection of insufficiently bleached samples. Applied to colluvial
sediments from an archaeological site in southwest Germany, the
single aliquot IR-OSL protocol identified insufficient bleaching, which
was not possible using the multiple aliquot additive dose approach.
The single aliquot dating results indicate that colluviation coincides
Fig. 5. Age versus depth plot for colluvial sediments. OSL ages, calibrated AMS 14C ages
of charcoal fragments, and the age of a ceramic fragment are plotted according to the
depth of sampling. OSL ages are in stratigraphic order, whereas age information based
on objects incorporated in the sediments shows a rather chaotic pattern due to
reworking, interim storages of the objects on the slope and their much later
incorporation into the sediments (after Lang and Hönscheidt, 1999).
23
Fig. 6. Holocene sedimentation rates derived from OSL-dated colluvia from Greece. Bars
marked with a star-symbol represent minimum sedimentation rates. Additionally, the
Holocene chronology of the cultural periods in the study area is given (after Fuchs et al.,
2004).
with woodland clearance in the Bronze Age/Early Iron Age and
probably was related to the construction of a Celtic rampart.
In a study in Greece, Fuchs et al. (2004) applied an OSL single
aliquot protocol to the quartz coarse grain fraction for reconstructing
Holocene soil erosion and to elucidate the interaction of man and
environment since Neolithic times. In their study they used the
statistical parameter of the coefficient of variation ν in combination
with small aliquots to differentiate between sufficiently and insufficiently bleached samples (Fuchs and Wagner, 2003). Based on the
calculated OSL ages from several foot-slope localities, a highresolution chronology was established and sedimentation rates
calculated (Fig. 6; Fuchs et al., 2004). Colluviation strongly fluctuated
in the course of the Holocene, with a sharp increase during the Early
Neolithic, the onset of agricultural activities. Further periods of
increased colluviation are the Middle and Late Bronze Age, the
Roman period and the period since the sixteenth century AD. The
good correlation between sedimentation rate and cultural activity
shows that colluviation is dominated by anthropogenic drivers.
However, the first phase of colluviation occurred during a period
when Neolithic human impact coincided with enhanced precipitation.
Probably the combination of these factors caused the sudden increase
in soil erosion with the onset of the Neolithic (Fuchs and Wagner,
2005; Fuchs, 2007).
In a comparative study from Romania, Kadereit et al. (2006a)
report age estimates of colluvial sediments developed at the footslope of a tell mound situated in an alluvial plain. Both, polymineral
fine grain and quartz coarse grain separates were extracted. De
determination for the quartz extracts were based on the OSL
regenerative single aliquot protocol (SAR) using small aliquots.
Insufficient bleaching was detected and Des calculated following
Fuchs and Lang (2001). For the polymineral fine grain extracts, an IROSL multiple-aliquot protocol was employed. The results show that
OSL ages based on the best bleached quartz Des are significantly
younger than the ages derived from the IR-OSL polymineral fine
24
M. Fuchs, A. Lang / Geomorphology 109 (2009) 17–26
grains. Thus, techniques available for coarse grain quartz seem to be
better suited than techniques available for the fine grain extracts. Even
so, compared to independent age control, both age estimates seem to
overestimate the true sedimentation age.
Fine grained quartz extracts and a SAR protocol have been used by
Rommens et al. (2007) to establish the history of soil erosion and
Holocene slope evolution in central Belgium. Here, the first sediment
deposition occurred in the early Iron Age. The sedimentation rate
increased from 2.9 ± 0.9 t ha− 1 a− 1 to 5.2 ± 1.5 t ha− 1 a− 1 during the
Roman Period and further to 18.0 ± 2.0 t ha− 1 a− 1 in the Middle-Ages.
The study also shows the limits of OSL dating: OSL dating of soil
erosion seems to be successful only if the depositional style is of a lowmagnitude high-frequency character. Then sediment grains have long
residence times on the slope where they are overturned by cultivation
and bioturbation and effectively bleached. In such cases, only a small
(if any) residual signal has to be bleached during erosion, transport,
and deposition to allow for successful OSL dating. If, on the contrary,
the intensity of soil erosion increases and during high magnitude
events sub-soil particles are mobilized that have not been bleached on
the slope, then age overestimates occur and not even SAR based OSL
techniques allow extracting the ‘true’ depositional age.
4. Conclusion and perspective
Recent developments in luminescence dating enable dating of noneolian sediments with often limited pre-depositional daylight exposure.
Here we review OSL-dating of colluvial sediments and its application in
geomorphology, paleoclimate reconstruction and geoarchaeology.
Colluvial sediments represent an important paleoenvironmental geoarchive and being able to establish robust chronologies for such deposits
is essential. Luminescence dating studies have shown that in many
environments OSL dating of colluvium is possible despite the often short
transport distances. OSL dating of colluvial sediments still represents a
challenge and as for other non-eolian environments sufficient bleaching
must be checked for. This can be achieved by (1) the study of the De
distribution of small aliquots and single grains, (2) comparison of
luminescence signals with different light sensitivity, (3) stratigraphic
consistency of OSL age estimates, and (4) comparison of OSL-ages and
independent age control. Because of (1), luminescence dating of sandy
sediments and using coarse grain techniques are preferable. Fine grain
aliquots consist of too many grains and grain to grain variations as a
result of insufficient bleaching cannot be detected. Caution has to be
taken as skewed dose distributions can also be produced by inhomogeneous β-radiation fields (which will be a severe problem in the
usually poorly sorted colluvial deposits) and incompletely understood
luminescence properties of some sediments. This can be seen in high De
scatter even for well bleached samples (Thomas et al., 2005).
The differences in luminescence and bleaching behavior (2)
demonstrate that certain signal components, mineral types and grain
size fractions are favorable for colluvium dating due to their higher light
sensitivity. This can also be exploited to detect insufficient bleaching by
comparing luminescence ages obtained from different luminescence
signals like fast and slow components of quartz (e.g., Larsen et al., 2000;
Singarayer et al., 2005; Li and Li, 2006), coarse and fine grain fractions, or
quartz and feldspar minerals (Kadereit et al., 2006a). If the comparison
returns the same luminescence age for each of the components, the
sediment grains should have experienced sufficient bleaching to
completely reset both luminescence signals. A difference in the age
estimates would point towards insufficient bleaching of the component
returning the higher age estimate. Unfortunately in such a case, the age
estimate based on a faster bleaching component may still be an age
overestimate as this signal may still contain a residual signal. Also, in the
case of no-bleaching during transport, two similar age estimates should
be derived, both overestimates.
Therefore, establishing a chronology for colluvial sediments should
always be based on several samples and—where possible—different
techniques. This will allow checking the stratigraphic consistency of
the obtained results (3), and (4) comparing OSL-ages and independent age control.
To reduce possible difficulties with poor bleaching, besides a
thorough sedimentological analysis, a careful geomorphic evaluation of
a sampling site and its catchment is also necessary before samples are
taken for luminescence dating. Taking into account the following parameters significantly reduces the occurrence of insufficient bleaching.
The transport distance of the sediments should be as large as
possible, because this enhances the possibility of intermittent sediment storage on the slope and thus increases the exposure time of the
sediment grains to daylight.
Locations where sediments can be deposited by high magnitude
events should be avoided, because high magnitude processes provide
limited daylight exposure of the sediments. Additionally, high
magnitude events may incorporate sub-surface sediments with high
residual luminescence.
The slope aspect and topographic shading of the sediment pathway should be taken into account, as for example south facing slopes
in the northern hemisphere receive more sunlight and thus provide
better bleaching conditions.
Obviously not all of these parameters can always be considered
and especially the conditions at the time of deposition remain
unknown. Thus, the suitability of luminescence dating should be
tested for every sampling site and multi-sample, multi-technique
approaches should be employed. Nevertheless, recent results are very
promising and shown that in many cases colluvial sediments can
successfully be dated using luminescence dating, and thus opening up
colluvia as valuable geoarchives for paleoenvironmental research
within the last glacial–interglacial cycle.
Acknowledgments
We thank the two anonymous referees for their helpful comments.
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