Permafrost, Phillips, Springman & Arenson (eds)
© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7
Methods for absolute and relative age dating of rock-glacier surfaces
in alpine permafrost
W. Haeberli, D. Brandova, C. Burga, M. Egli, R. Frauenfelder, A. Kääb & M. Maisch
Geography Department, University of Zurich, Switzerland
B. Mauz and R. Dikau
Geography Department, University of Bonn, Germany
ABSTRACT: Rock glacier surfaces reflect debris accumulations produced, deposited and deformed during historical and Holocene time periods. Dating of such surfaces is difficult but can best be achieved by using a combination of absolute and relative age-determination methods. Photogrammetry of present-day flow fields yields
trajectories along which inverse velocities can be integrated and compared with radiocarbon dates from organic
samples providing absolute ages. Weathering rinds and Schmidt-hammer rebound values can be calibrated with the
time scales calibrated in this manner. This helps to interpret lichen-cover distribution under marginal growth conditions. Luminescence dating of fine material becoming exposed in upper parts of rock-glacier fronts has the potential of providing true travel times. Relict “rockglacierized” features can be dated by cosmogenic (exposure) effects.
1 BACKGROUND AND CONCEPT
perennially frozen scree
1
2
debris accumulation
frost heave
The formation and preservation of ground ice over the
typical time scales (millennia) involved with the steady
flow over long time intervals of large and welldeveloped rock glaciers require the existence of perennially negative ground temperatures, i.e., permafrost
by definition (Haeberli, 2001). This climatically determined ground thermal condition makes rock glaciers
interesting in view of quantitative paleoclimatic reconstructions (Frauenfelder et al., 2001). Moreover, the
debris accumulated in rock glaciers reflects centuries
and millennia of past frost weathering and rock-fall
activity (Barsch, 1977; Olyphant, 1987). In order to
decipher the corresponding information, rock glaciers
must be dated.
The following briefly outlines a strategy which combines a variety of applicable methods. It uses a basic
concept of permafrost creep, which relates to results
from extensive drilling, borehole observation, geophysical soundings, photogrammetric analyses, permafrost mapping etc., at Murtèl rock glacier (Figure 1;
Haeberli et al., 1998). The initial condition is a talus
cone with characteristic vertical sorting of grain sizes.
Frost heave and creep of the ice-supersaturated finer
material originally deposited on the upper part of the
talus cone then forms a bulge (protalus rampart) which
steadily develops into a larger rock glacier. Coarse
blocks continue to accumulate at the base of the talus
cone and are carried along on the back of the further
advancing rock glacier until reaching its front. There,
they fall down the oversteepened scree of re-exposed
and thawed fine material. These coarse blocks from the
rock-glacier surface are subsequently overriden by the
creeping ice-super-saturated fine material. In contrast
gra
in
es
ort
3
supersaturated permafrost
steady-state creep
structured permafrost
damped creep
permafrost
creep
siz
protalus rampart
ing
rock glacier - compression
in overdeepening
4
rock glacier - extension
on steep slope
surface blocks from foot of scree slope
stiff basal layer
blocks falling from surface
n
tio
ua
ac
ev
Figure 1. Model of rock glacier development as a basis
for dating purposes (after Haeberli et al., 1998).
to the latter with ages increasing along flow paths,
they form a stiff basal layer in which age decreases
towards the rock glacier-front. The age distribution at
depth in rock-glacier permafrost, therefore, is likely
to contain sharp time inversions.
2 METHODS
2.1
Photogrammetry
A first indication of age distributions is provided by
surface-flow fields using modern photogrammetric
methods (Kääb et al., 1997, 1998; Kääb & Vollmer,
2001). Such measurements enable time (age) to be
integrated for particle paths along flow trajectories
constructed for present-day conditions assuming a
343
rock wall
00
28
direct and absolute dating by the radiocarbon method.
The primary problem consists in the necessity to drill
or dig through the active layer which is often composed of coarse blocks. Moreover, only very small
amounts of organic remains may indeed exist in the
ice. In the core recovered from the drilling through the
permafrost of the active rock glacier Murtèl (Figure 2),
only one layer within massive ice contained moss
remains at a depth of about 6 metres below surface,
providing a mean conventional 14C age of 2250 100
years. The relatively well-preserved habitus of the
moss as well as of the pollen scavenged by them suggest an insignificant residence time of the material on
the rock glacier surface before isolation from its ambient environment by incorporation into the ice matrix.
Hence, the 14C age of the moss may not be too strongly
influenced by other carbon pools and is expected to
represent the age of the hosting ice layer well. These
results confirm that the photogrammetrically estimated ages on Murtèl are reasonable, that rock glacier
surfaces are indeed millennia old and that the flow of
the creeping permafrost was quite constant over this
time period (Haeberli et al., 1999). Similar results
have been obtained by Konrad et al. (2001) for Galena
Creek rock glacier (US), a feature of relatively warm
(1°C?) permafrost probably affected in its upper
part by a former (possibly Little Ice Age, Holocene)
glacier or glacieret. It is interesting to note that perennially frozen, “rock-glacierized” ice-cored (push)
moraines in northern Sweden provided radiocarbon
dates of quite similar age (Østrem, 1965).
A. Kääb
0
270
1000 a 100 a
N
20m
100m
Figure 2. Flow trajectories and age calculation from photogrammetric flow analysis on rock glacier Murtèl (after
Kääb et al., 1998). The square in the center of the rock glacier tongue indicates the location of core drilling in 1987.
steady-flow condition as a first-order approximation.
On the Murtèl rock glacier (Figure 2), this approximation seems to be justified by the fact that the curvature
of the isochrones is similar to the curvature of the
ogive-like transverse ridges, the formation and cumulative deformation of which obviously takes up several millennia; their wavelength of about 20 m
corresponds to an age difference between ridges of
about 300 to 400 years (Kääb et al., 1998).
The surface age at the rock glacier front as calculated from the present-day velocity field amounts to
approximately 5–6 ka. Similar velocities and flow fields
measured on other rock glaciers indicate that this result
is probably of general significance for comparable
forms of actively creeping permafrost (Frauenfelder
and Kääb, 2000; Kääb et al., 1997, 2002). The example shown in Figure 2 demonstrates that photogrammetrically derived estimates yield highly detailed spatial
patterns of age distribution and thus represent a unique
tool for interpolating results from other dating methods. The main concerns are temporal variations in rates
of permafrost creep and the behaviour during initial
stages of rock glacier development with flow velocities growing from zero to present-day values. The latter
point makes photogrammetrically determined ages minimum rather than maximum values.
2.3
Weathering rinds
Rock particles are increasingly subjected to weathering processes along the flowpaths of rock-glacier surfaces. The thickness of the weathering rind, a reddish
outer crust-layer around individual rock components,
should therefore increase along flow trajectories. The
use of weathering rates from rinds on sandstones,
basalts, andesite boulders, etc., for relative to absolute
age dating of quaternary deposits have been discussed,
for instance, by Chinn (1981), Gellatly (1984), Ricker
et al. (1993), Ivy Ochs et al. (1996) or Oguchi (2001).
First measurements on rock glaciers of the Upper
Engadine (Swiss Alps) were carried out for surfaceexposed clasts selected at a series of cross-sectional
transects along flowlines from the talus to the front.
Around 50 to 100 rind samples per transect were
chipped with a hammer from gneiss or granite rock
debris and rind thicknesses were measured normal
to the surface to the innermost discernible edge of
weathering using a 0.1 mm scale-graded magnifying
glass. The median as well as the modal values are
equally suitable to delineate the increasing tendency of
weathering rind thickness on gneiss or granite with the
2.2 Radiocarbon
The ice within the perennially frozen material may
contain well-preserved organic remains which allow for
344
r-value (Murtèl)
weathering rind (Gianda Grischa)
50
45
2
40
1
35
0
0
2000
4000
6000
time (years)
8000
r-value (Schmidt-hammer)
weathering rind thickness [mm]
millennia old (Haeberli et al., 1999; Frauenfelder &
Kääb, 2000), r – values measured by Castelli (2001)
closely correlate with the chronology estimated by
photogrammetry and radiocarbon dating. Furthermore,
the results are in good agreement with weathering rind
measurements effectuated on the same transects
(Figure 3; cf. Laustela et al., 2003). These promising
results provide an opportunity to calibrate age dating
by Schmidt-hammer measurements for assessing at
least relative chronologies for Holocene debris surfaces
at other sites.
weathering rind (Murtèl)
3
30
10000
2.5
Figure 3. Weathering rind thickness (modal values) and
schmidt-hammer rebound values as a function of photogrammetrically estimated time since exposure to weathering
on rock glacier Murtèl.
The radial growth of crustose lichens as an indicator
of substrate age (Beschel, 1950; Locke et al., 1979)
has been most widely used in dating glacial deposits
in tundra environments where lichens often form the
dominant vegetation cover (cf., for instance, Beschel,
1961; Andersen & Sollid, 1971; Erikstad & Sollid,
1986; Matthews, 1992). Growth rates vary from one
region to another and may decline after initial colonization to an almost constant value. Lichenometry
has a useful range of ca. 500 years (Innes, 1985) with
exceptional lifetimes up to 4500 years or more under
cold and dry continental conditions (Beschel, 1958).
The crustose lichen Rhizocarpon geographicum has
been most commonly used (see growth curves of
Rhizocarpon geographicum and R. alpicola e.g. in
King & Lehmann, 1973; Bradley, 1985). Climate type
(mainly moisture supply, solar radiation/rock temperatures and altitude a.s.l.) is a major factor affecting
lichen growth rates (see e.g. Calkin & Ellis, 1980).
Permanent snow cover and unstable blocks within
zones of extending flow on rock glaciers may lead to
reduced or lichen-free zones (Haeberli et al., 1979).
Lichens on the surface of Alpine rock glaciers have
been studied by Haeberli et al. (1979) and Burga
(1987). Detailed mapping on the surface of the active
rock glacier Murtèl by Ruffet (2001) clearly demonstrated that lichen diameters correlate with relative age
differences as estimated from photogrammetric flow
determinations. However, lichen growth is limited to
the frontal parts of the rock glacier, whereas no lichens
of major size can be found in its upper part. This striking feature is most probably due to the adverse effects
of rock-fall activity, extending flow and long snowcover duration within the root zone of the rock glacier.
Much larger lichen diameters and denser surface cover
can be observed on inactive and relict features.
duration of exposure to weathering. The chronofunctions developed by Laustela et al. (2003) show that the
weathering rinds tend to follow a logistic trend with
an asymptotic value after t ! and are closely correlated with Schmidt-hammer rebound values (cf.
below and Fig. 3). Chemical analyses of the weathering rinds show a continuous formation of iron oxihydroxides and a steady increase of dithionite-extractable
Fe with age (Laustela et al., 2003). Rock weathering
rinds found in the Swiss Alps however, were usually
thinner than those observed on sandstones (New
Zealand) and reached characteristic growth rates of
0.2 mm/ky (Murtèl site), 0.3 mm/ky (Gianda Grischa
site) or 1 mm/ky (Suvretta site).
2.4
Lichenometry
Schmidt-hammer rebound
The Schmidt-hammer is a portable instrument originally designed to measure the surface strength of concrete by recording the rebound of a spring-loaded bolt
impacting a surface (Schmidt, 1951). The rebound
value (r) gives a relative measure of the surface hardness and as such provides information on the time of
surface exposure and the degree of weathering. The
method has been successfully applied for relative age
dating of moraines (e.g. Matthews & Shakesby, 1984;
Winkler & Shakesby, 1995; Rune & Sjåstad, 2000),
rockfall deposits (Nesje et al., 1994) and debris flow
deposits (Lippert, 2001). A random sample of fifty
measurements is usually recorded on as many different boulders as possible, selecting surfaces which
have comparable lithology and which are dry, flat, clean
and free of lichens, visual fissures and cracks (cf.
Williams & Robinson, 1983). The mean or the median
of the values can then be considered representative for
the effective hardness of the analysed surface. On the
Murtèl rock glacier, mainly composed of gneisses,
basica and ultrabasica and with surfaces up to several
2.6
Luminescence
Luminescence is used for dating purposes, because natural minerals such as quartz and feldspars are dosimeters. These minerals are capable of accumulating some
345
to ionising radiation, the extremely inhomogeneous
radiation field surrounding the dosimeter and the
absorption of ionising energy alternately by water and
ice in a not-constant pore volume.
2.7
Cosmogenic (exposure) dating
The concentration of “cosmogenic isotopes” such as
10
Be (t1/2 1,500,000 years), 26Al (t1/2 716,000
years) and 36Cl (t1/2 301,000 years; cf. Lal, 1991;
Cerling & Craig, 1994; Kurz & Brook, 1994; Zreda &
Phillips, 1994) depends on the period of time the surface has been exposed, the local production rate, the
decay constant of the radionuclide, the rock density, the
erosion rate and the cosmic-ray attenuation length.
Quartz is well suited to 10Be- and 26Al-exposure dating
studies because of its inherently low 27Al content which
allows the measurement of the 26Al/27Al ratio (Kohl &
Nishiizumi, 1992). Since the method was introduced
and tested more profoundly (e.g. Klein et al., 1986;
Nishiizumi et al., 1986) it has been applied to more and
more specific questions in different geomorphic contexts (Cerling & Craig, 1994; Kurz & Brook, 1994).
Applications in Switzerland are quite recent (Ivy Ochs
et al., 1996; Tschudi, 2000) and enable significant
improvement in determining the chronology of lateWürmian and Holocene glacier fluctuations. The
Egesen moraines of the Lagrev glacier at the Julier Pass
were dated and yielded exposure ages of 11,100 years
(Ivy-Ochs et al., 1996). The main potential for exposure
dating of rock glaciers and other creep phenomena in
Alpine permafrost concerns blocks on the surface and
blocks from the talus apron at the foot of the front. In
the case of relict rock glaciers, dated blocks from the
surface provide a minimum time between the deposition of the block at the surface until today, whereas
blocks which had fallen – and thereby most probably
been turned over – to the talus apron at the foot of the
now inactive front indicate the time since rock glacier
movement came to a stop. The difference between the
two ages could give a minimum travel time of the
blocks from deposition at the surface to their presentday position. First samples were analysed from a relict
rock-glacierized late-glacial moraine at La Veduta near
Julier pass. The exposure age of the top surface of a
prominent large boulder on the frontal ridge was determined using the cosmogenic radionuclide 10Be. It
yielded a preliminary mean exposure age of 6800 years,
which may possibly indicate continued creep of perennially frozen late-glacial morainic material well into the
Holocene. Two other boulders contained various kinds
of minerals which could not be eliminated from the
quartz fraction, making it impossible to prepare pure
quartz so that 10Be could not be measured. The interpretation of cosmogenic dating at the investigated rockglacierized feature, therefore, remains uncertain.
Figure 4. Sampling for luminescence dating at the front
of rock glacier Muragl.
of the environmental and cosmic ionising radiation to
which they are exposed. By stimulating the dosimeter
using light (optically stimulated luminescence, OSL)
a luminescence signal is monitored which is proportional to the absorbed dose. The event that can be
dated (where t 0) is the last exposure to daylight.
Essential preconditions for successful dating are (i)
sufficient reduction of the latent luminescence signal
at time of deposition, (ii) thermal stability of the luminescence signal during burial, (iii) signal growth
according to the energy absorbed and (iv) radioactive
equilibrium in the burial environment. Precondition
(i) is crucial for dating rock glaciers. It should be satisfied if flows start from a talus cone, which is fed by
sand falling from the rock wall above the cone. The
sand is subsequently buried by new material and
migrates to the upper part of the front where it is again
exposed to daylight (cf. Figure 1). Thus, OSL-ages of
samples from the frontal debris (Figure 4) should provide true travel times of buried near-surface particles
which have participated in the long-term creep process
of the perennially frozen talus.
In future, spatially high-resolution positioning of
OSL samples and successful dating could give detailed
information about the flow behaviour in time.
Samples from the rock glaciers Murtèl, Muragl and
La Veduta were collected in 2001 using tubes under a
black cover shielding the sampling site from daylight.
In the laboratory sample preparation focused on
extracting quartz grains of 90–200 m size. Routine
procedures were used which are described in detail
elsewhere (Mauz et al., in press).
The OSL-study of the samples are in progress. First
results indicate that some samples are datable, others
are not. All datable samples give Holocene ages
between ⬃8 ka and ⬃4 ka. This preliminary result
clearly confirms the concept of rock glaciers developing at time scales of millennia as deduced from flow
measurements and radiocarbon dating. To estimate
accurate and precise OSL-ages, problems have still to
be solved concerning the low sensitivity of the quartz
346
3 RECOMMENDED STRATEGY AND
CONCLUSIONS
Beschel, R.E. 1961. Dating rock surfaces by lichen growth
and its application in glaciology and physiography
(lichenometry). In Raasch, G.O. (ed), Geology of the
Arctic 2: 1044–1062.
Bradley, R.S. 1985. Quaternary Paleoclimatology. Methods
of Paleoclimatic Reconstruction. Boston: Allen &
Unwin, 472 pp.
Burga, C.A. 1987. Gletscher- und Vegetationsgeschichte
der Südrätischen Alpen seit der Späteiszeit.
Denkschriften der Schweizerischen Naturforschenden
Gesellschaft 101: 162 pp.
Calkin, P.E. & Ellis, J.M. 1980. A lichenometric dating
curve and its application to Holocene glacier studies
in the Central Brooks Range, Alaska. Arctic Alpine
Research 12: 245–264.
Castelli, S. 2000. Geomorphologische Kartierung im Gebiet
Julierpass, Val Suvretta und Corvatsch (Oberengadin,
GR), sowie Versuche zur Relativdatierung der morphologischen Formen mit der Schmidt-Hammer Methode.
Zurich: unpublished master thesis, Dept. of Geography,
University of Zurich.
Cerling, T.E. & Craig, H. 1994. Geomorphology and in-situ
cosmogenic isotopes. Annual Review of Earth and
Planetary Sciences 22: 273–317.
Chinn, T.J.H. 1981. Use of rock weathering-rind thickness
for Holocene absolute age-dating in New Zealand.
Arctic and Alpine Research 13(1): 33–45.
Erikstad, L. & Sollid, J.L. 1986. Neoglaciation in South
Norway using lichenometric methods. Norsk
geografisk Tidsskrift 40: 85–105.
Frauenfelder, R. & Kääb, A. 2000. Towards a paleoclimatic
model of rock-glacier formation in the Swiss Alps.
Annals of Glaciology 31: 281–286.
Frauenfelder, R., Haeberli, W., Hölzle, M. & Maisch, M.
2001. Using relict rockglaciers in GIS-based modelling to reconstruct Younger Dryas permafrost distribution patterns in the Err-Julier area, Swiss Alps.
Norwegian Journal of Geography 55: 195–202.
Gellatly, A.F. 1984. The use of rock weathering-rind thickness to redate moraines in Mount Cook National Park,
New Zealand. Arctic and Alpine Research 16(2):
225–232.
Haeberli, W. 2000. Modern research perspectives relating
to permafrost creep and rock glaciers. Permafrost and
Periglacial Processes 11(4): 290–293.
Haeberli, W., Hoelzle, M., Kääb, A., Keller, F., Vonder
Mühll, D. & Wagner, S. 1998. Ten years after drilling
through the permafrost of the active rock glacier
Murtèl, Eastern Swiss Alps: answered questions and
new perspectives. Proceedings of the 7th International
Conference on Permafrost., Yellowknife, 23–27 June
1998, Collection Nordicana 57: 403–410.
Haeberli, W., Kääb, A., Wagner, S., Vonder Mühll, D.,
Geissler, P., Haas, J.N., Glatzel-Mattheier, H. &
Wagenbach, D. 1999. Pollen analysis and 14C age of
moss remains in a permafrost core recovered from the
active rock glacier Murtèl-Corvatsch, Swiss Alps:
geomorphological and glaciological implications.
Journal of Glaciology 45 (149): 1–8.
Haeberli, W., King, L. & Flotron, A. 1979. Surface movement and lichen cover studies at the active rock glacier
The key and starting point for the proposed dating strategy is the fact that the thermal inertia of ice-rich permafrost causes active rock glaciers to flow and develop
steadily over centuries and millennia of relatively constant (Holocene) climatic conditions. Time since clast
release and deposition through rock fall onto the rockglacier surface systematically increases along flow trajectories, enabling highly differentiated age patterns to
be derived from photogrammetric flow analysis. Such
patterns can be calibrated by luminescence dating of
fine material reappearing at the oversteepened front,
and by radiocarbon dating of (sparse) organic matter
found in drillings or excavations in the permafrost itself.
This helps limiting the uncertainty about effects from
past changes in flow rates. Weathering-rind thicknesses
and Schmidt-hammer rebound values can now be tied
to the thus defined time scales. These easily applied
field methods in turn facilitate the selection of blocks
which have been overturned when falling from the surface over the steep front to the frontal talus of relict rock
glaciers immediately before movement came to its final
stop. Cosmogenic exposure dating of such selected
blocks defines the point in time when this important climate- or topography-induced event took place, and
luminescence dating of fine material from the front may
add information about total travel times and possibly
even involved processes (especially the often discussed
participation of small glaciers and large rockfalls or
landslides in initial debris transport). A thus calibrated
methodology involving weathering-rind thicknesses,
Schmidt-hammer rebound values, exposure and luminescence dates may also be applied on other characteristic cold-mountain deposits such as moraines, debris
flows or rock-fall deposits. It is evident that the longterm creep of mountain permafrost together with a
whole set of newly developed dating methodologies
opens most interesting perspectives for chronological
work about late-glacial and Holocene landscape evolution in climate-sensitive high-mountain areas.
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