1 BACKGROUND AND CONCEPT

The formation and preservation of ground ice over thetypical time scales (millennia) involved with the steadyflow over long time intervals of large and well-developed rock glaciers require the existence of peren-nially negative ground temperatures, i.e., permafrostby definition (Haeberli, 2001). This climatically deter-mined ground thermal condition makes rock glaciersinteresting in view of quantitative paleoclimatic recon-structions (Frauenfelder et al., 2001). Moreover, thedebris accumulated in rock glaciers reflects centuriesand millennia of past frost weathering and rock-fallactivity (Barsch, 1977; Olyphant, 1987). In order todecipher the corresponding information, rock glaciersmust be dated.

The following briefly outlines a strategy which com-bines a variety of applicable methods. It uses a basicconcept of permafrost creep, which relates to resultsfrom extensive drilling, borehole observation, geo-physical soundings, photogrammetric analyses, perma-frost mapping etc., at Murtèl rock glacier (Figure 1;Haeberli et al., 1998). The initial condition is a taluscone with characteristic vertical sorting of grain sizes.Frost heave and creep of the ice-supersaturated finermaterial originally deposited on the upper part of thetalus cone then forms a bulge (protalus rampart) whichsteadily develops into a larger rock glacier. Coarseblocks continue to accumulate at the base of the taluscone and are carried along on the back of the furtheradvancing rock glacier until reaching its front. There,they fall down the oversteepened scree of re-exposedand thawed fine material. These coarse blocks from therock-glacier surface are subsequently overriden by thecreeping ice-super-saturated fine material. In contrast

to the latter with ages increasing along flow paths,they form a stiff basal layer in which age decreasestowards the rock glacier-front. The age distribution atdepth 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 bysurface-flow fields using modern photogrammetricmethods (Kääb et al., 1997, 1998; Kääb & Vollmer,2001). Such measurements enable time (age) to beintegrated for particle paths along flow trajectoriesconstructed for present-day conditions assuming a

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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. MaischGeography Department, University of Zurich, Switzerland

B. Mauz and R. DikauGeography Department, University of Bonn, Germany

ABSTRACT: Rock glacier surfaces reflect debris accumulations produced, deposited and deformed during his-torical and Holocene time periods. Dating of such surfaces is difficult but can best be achieved by using a combi-nation of absolute and relative age-determination methods. Photogrammetry of present-day flow fields yieldstrajectories along which inverse velocities can be integrated and compared with radiocarbon dates from organicsamples providing absolute ages. Weathering rinds and Schmidt-hammer rebound values can be calibrated with thetime scales calibrated in this manner. This helps to interpret lichen-cover distribution under marginal growth con-ditions. Luminescence dating of fine material becoming exposed in upper parts of rock-glacier fronts has the poten-tial of providing true travel times. Relict “rockglacierized” features can be dated by cosmogenic (exposure) effects.

Permafrost, Phillips, Springman & Arenson (eds)© 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7

perennially frozen scree protalus rampart

supersaturated permafroststeady-state creep

structured permafrostdamped creep

rock glacier - extensionon steep slope

rock glacier - compressionin overdeepening

surface blocks from foot of scree slope

stiff basal layer blocks falling from surface

evacuation

debris accumulationfrost heave

permafrostcreep

grain size sorting

1 2

43

Figure 1. Model of rock glacier development as a basisfor dating purposes (after Haeberli et al., 1998).

steady-flow condition as a first-order approximation.On the Murtèl rock glacier (Figure 2), this approxima-tion seems to be justified by the fact that the curvatureof the isochrones is similar to the curvature of theogive-like transverse ridges, the formation and cumu-lative deformation of which obviously takes up sev-eral millennia; their wavelength of about 20 mcorresponds to an age difference between ridges ofabout 300 to 400 years (Kääb et al., 1998).

The surface age at the rock glacier front as calcu-lated from the present-day velocity field amounts toapproximately 5–6 ka. Similar velocities and flow fieldsmeasured on other rock glaciers indicate that this resultis probably of general significance for comparableforms of actively creeping permafrost (Frauenfelderand Kääb, 2000; Kääb et al., 1997, 2002). The exam-ple shown in Figure 2 demonstrates that photogrammet-rically derived estimates yield highly detailed spatialpatterns of age distribution and thus represent a uniquetool for interpolating results from other dating meth-ods. The main concerns are temporal variations in ratesof permafrost creep and the behaviour during initialstages of rock glacier development with flow veloci-ties growing from zero to present-day values. The latterpoint makes photogrammetrically determined ages min-imum rather than maximum values.

2.2 Radiocarbon

The ice within the perennially frozen material maycontain well-preserved organic remains which allow for

direct and absolute dating by the radiocarbon method.The primary problem consists in the necessity to drillor dig through the active layer which is often com-posed of coarse blocks. Moreover, only very smallamounts of organic remains may indeed exist in theice. In the core recovered from the drilling through thepermafrost of the active rock glacier Murtèl (Figure 2),only one layer within massive ice contained mossremains at a depth of about 6 metres below surface,providing a mean conventional 14C age of 2250 � 100years. The relatively well-preserved habitus of themoss as well as of the pollen scavenged by them sug-gest an insignificant residence time of the material onthe rock glacier surface before isolation from its ambi-ent environment by incorporation into the ice matrix.Hence, the 14C age of the moss may not be too stronglyinfluenced by other carbon pools and is expected torepresent the age of the hosting ice layer well. Theseresults confirm that the photogrammetrically esti-mated ages on Murtèl are reasonable, that rock glaciersurfaces are indeed millennia old and that the flow ofthe creeping permafrost was quite constant over thistime period (Haeberli et al., 1999). Similar resultshave been obtained by Konrad et al. (2001) for GalenaCreek rock glacier (US), a feature of relatively warm(�1°C?) permafrost probably affected in its upperpart by a former (possibly Little Ice Age, Holocene)glacier or glacieret. It is interesting to note that peren-nially frozen, “rock-glacierized” ice-cored (push)moraines in northern Sweden provided radiocarbondates of quite similar age (Østrem, 1965).

2.3 Weathering rinds

Rock particles are increasingly subjected to weather-ing processes along the flowpaths of rock-glacier sur-faces. The thickness of the weathering rind, a reddishouter crust-layer around individual rock components,should therefore increase along flow trajectories. Theuse of weathering rates from rinds on sandstones,basalts, andesite boulders, etc., for relative to absoluteage dating of quaternary deposits have been discussed,for instance, by Chinn (1981), Gellatly (1984), Rickeret al. (1993), Ivy Ochs et al. (1996) or Oguchi (2001).First measurements on rock glaciers of the UpperEngadine (Swiss Alps) were carried out for surface-exposed clasts selected at a series of cross-sectionaltransects along flowlines from the talus to the front.Around 50 to 100 rind samples per transect werechipped with a hammer from gneiss or granite rockdebris and rind thicknesses were measured normal to the surface to the innermost discernible edge ofweathering using a 0.1 mm scale-graded magnifyingglass. The median as well as the modal values areequally suitable to delineate the increasing tendency ofweathering rind thickness on gneiss or granite with the

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rock wall A. Kääb

27002800

1000 a

20m 100m

N

100 a

Figure 2. Flow trajectories and age calculation from pho-togrammetric flow analysis on rock glacier Murtèl (afterKääb et al., 1998). The square in the center of the rock gla-cier tongue indicates the location of core drilling in 1987.

duration of exposure to weathering. The chronofunc-tions developed by Laustela et al. (2003) show that theweathering rinds tend to follow a logistic trend withan asymptotic value after t �! and are closely cor-related with Schmidt-hammer rebound values (cf.below and Fig. 3). Chemical analyses of the weather-ing rinds show a continuous formation of iron oxihy-droxides and a steady increase of dithionite-extractableFe with age (Laustela et al., 2003). Rock weatheringrinds found in the Swiss Alps however, were usuallythinner than those observed on sandstones (NewZealand) and reached characteristic growth rates of0.2 mm/ky (Murtèl site), 0.3 mm/ky (Gianda Grischasite) or 1 mm/ky (Suvretta site).

2.4 Schmidt-hammer rebound

The Schmidt-hammer is a portable instrument origi-nally designed to measure the surface strength of con-crete by recording the rebound of a spring-loaded boltimpacting a surface (Schmidt, 1951). The reboundvalue (r) gives a relative measure of the surface hard-ness and as such provides information on the time ofsurface exposure and the degree of weathering. Themethod has been successfully applied for relative agedating of moraines (e.g. Matthews & Shakesby, 1984;Winkler & Shakesby, 1995; Rune & Sjåstad, 2000),rockfall deposits (Nesje et al., 1994) and debris flowdeposits (Lippert, 2001). A random sample of fiftymeasurements is usually recorded on as many differ-ent boulders as possible, selecting surfaces whichhave comparable lithology and which are dry, flat, cleanand free of lichens, visual fissures and cracks (cf.Williams & Robinson, 1983). The mean or the medianof the values can then be considered representative forthe effective hardness of the analysed surface. On theMurtèl rock glacier, mainly composed of gneisses,basica and ultrabasica and with surfaces up to several

millennia old (Haeberli et al., 1999; Frauenfelder &Kääb, 2000), r – values measured by Castelli (2001)closely correlate with the chronology estimated byphotogrammetry and radiocarbon dating. Furthermore,the results are in good agreement with weathering rindmeasurements effectuated on the same transects(Figure 3; cf. Laustela et al., 2003). These promisingresults provide an opportunity to calibrate age datingby Schmidt-hammer measurements for assessing atleast relative chronologies for Holocene debris surfacesat other sites.

2.5 Lichenometry

The radial growth of crustose lichens as an indicatorof substrate age (Beschel, 1950; Locke et al., 1979)has been most widely used in dating glacial depositsin tundra environments where lichens often form thedominant vegetation cover (cf., for instance, Beschel,1961; Andersen & Sollid, 1971; Erikstad & Sollid,1986; Matthews, 1992). Growth rates vary from oneregion to another and may decline after initial colo-nization to an almost constant value. Lichenometryhas a useful range of ca. 500 years (Innes, 1985) withexceptional lifetimes up to 4500 years or more undercold and dry continental conditions (Beschel, 1958).The crustose lichen Rhizocarpon geographicum hasbeen most commonly used (see growth curves ofRhizocarpon geographicum and R. alpicola e.g. inKing & Lehmann, 1973; Bradley, 1985). Climate type(mainly moisture supply, solar radiation/rock temper-atures and altitude a.s.l.) is a major factor affectinglichen growth rates (see e.g. Calkin & Ellis, 1980).Permanent snow cover and unstable blocks withinzones of extending flow on rock glaciers may lead toreduced or lichen-free zones (Haeberli et al., 1979).Lichens on the surface of Alpine rock glaciers havebeen studied by Haeberli et al. (1979) and Burga(1987). Detailed mapping on the surface of the activerock glacier Murtèl by Ruffet (2001) clearly demon-strated that lichen diameters correlate with relative agedifferences as estimated from photogrammetric flowdeterminations. However, lichen growth is limited tothe frontal parts of the rock glacier, whereas no lichensof major size can be found in its upper part. This strik-ing feature is most probably due to the adverse effectsof rock-fall activity, extending flow and long snow-cover duration within the root zone of the rock glacier.Much larger lichen diameters and denser surface covercan be observed on inactive and relict features.

2.6 Luminescence

Luminescence is used for dating purposes, because nat-ural minerals such as quartz and feldspars are dosime-ters. These minerals are capable of accumulating some

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0

1

2

3

0 2000 4000 6000 8000 10000

35

45

50

40

30

r-value (Murtèl) weathering rind (Gianda Grischa) weathering rind (Murtèl)

time (years)

wea

ther

ing

rind

thic

knes

s [m

m]

r-va

lue

(Sch

mid

t-ha

mm

er)

Figure 3. Weathering rind thickness (modal values) andschmidt-hammer rebound values as a function of photo-grammetrically estimated time since exposure to weatheringon rock glacier Murtèl.

of the environmental and cosmic ionising radiation towhich they are exposed. By stimulating the dosimeterusing light (optically stimulated luminescence, OSL)a luminescence signal is monitored which is propor-tional to the absorbed dose. The event that can bedated (where t 0) is the last exposure to daylight.Essential preconditions for successful dating are (i)sufficient reduction of the latent luminescence signalat time of deposition, (ii) thermal stability of the lumi-nescence signal during burial, (iii) signal growthaccording to the energy absorbed and (iv) radioactiveequilibrium in the burial environment. Precondition(i) is crucial for dating rock glaciers. It should be sat-isfied if flows start from a talus cone, which is fed bysand falling from the rock wall above the cone. Thesand is subsequently buried by new material andmigrates to the upper part of the front where it is againexposed to daylight (cf. Figure 1). Thus, OSL-ages ofsamples from the frontal debris (Figure 4) should pro-vide true travel times of buried near-surface particleswhich have participated in the long-term creep processof the perennially frozen talus.

In future, spatially high-resolution positioning ofOSL samples and successful dating could give detailedinformation about the flow behaviour in time.

Samples from the rock glaciers Murtèl, Muragl andLa Veduta were collected in 2001 using tubes under ablack cover shielding the sampling site from daylight.In the laboratory sample preparation focused onextracting quartz grains of 90–200 �m size. Routineprocedures were used which are described in detailelsewhere (Mauz et al., in press).

The OSL-study of the samples are in progress. Firstresults indicate that some samples are datable, othersare not. All datable samples give Holocene agesbetween �8 ka and �4 ka. This preliminary resultclearly confirms the concept of rock glaciers develop-ing at time scales of millennia as deduced from flowmeasurements and radiocarbon dating. To estimateaccurate and precise OSL-ages, problems have still tobe solved concerning the low sensitivity of the quartz

to ionising radiation, the extremely inhomogeneousradiation field surrounding the dosimeter and theabsorption of ionising energy alternately by water andice in a not-constant pore volume.

2.7 Cosmogenic (exposure) dating

The concentration of “cosmogenic isotopes” such as10Be (t1/2 1,500,000 years), 26Al (t1/2 716,000years) 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 sur-face has been exposed, the local production rate, thedecay constant of the radionuclide, the rock density, theerosion rate and the cosmic-ray attenuation length.Quartz is well suited to 10Be- and 26Al-exposure datingstudies because of its inherently low 27Al content whichallows the measurement of the 26Al/27Al ratio (Kohl &Nishiizumi, 1992). Since the method was introducedand tested more profoundly (e.g. Klein et al., 1986;Nishiizumi et al., 1986) it has been applied to more andmore specific questions in different geomorphic con-texts (Cerling & Craig, 1994; Kurz & Brook, 1994).Applications in Switzerland are quite recent (Ivy Ochset al., 1996; Tschudi, 2000) and enable significantimprovement in determining the chronology of late-Würmian and Holocene glacier fluctuations. TheEgesen moraines of the Lagrev glacier at the Julier Passwere dated and yielded exposure ages of 11,100 years(Ivy-Ochs et al., 1996). The main potential for exposuredating of rock glaciers and other creep phenomena inAlpine permafrost concerns blocks on the surface andblocks from the talus apron at the foot of the front. Inthe case of relict rock glaciers, dated blocks from thesurface provide a minimum time between the deposi-tion of the block at the surface until today, whereasblocks which had fallen – and thereby most probablybeen turned over – to the talus apron at the foot of thenow inactive front indicate the time since rock glaciermovement came to a stop. The difference between thetwo ages could give a minimum travel time of theblocks from deposition at the surface to their present-day position. First samples were analysed from a relictrock-glacierized late-glacial moraine at La Veduta nearJulier pass. The exposure age of the top surface of aprominent large boulder on the frontal ridge was deter-mined using the cosmogenic radionuclide 10Be. Ityielded a preliminary mean exposure age of 6800 years,which may possibly indicate continued creep of peren-nially frozen late-glacial morainic material well into theHolocene. Two other boulders contained various kindsof minerals which could not be eliminated from thequartz fraction, making it impossible to prepare purequartz so that 10Be could not be measured. The inter-pretation of cosmogenic dating at the investigated rock-glacierized feature, therefore, remains uncertain.

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Figure 4. Sampling for luminescence dating at the frontof rock glacier Muragl.

3 RECOMMENDED STRATEGY AND CONCLUSIONS

The key and starting point for the proposed dating strat-egy is the fact that the thermal inertia of ice-rich per-mafrost causes active rock glaciers to flow and developsteadily over centuries and millennia of relatively con-stant (Holocene) climatic conditions. Time since clastrelease and deposition through rock fall onto the rock-glacier surface systematically increases along flow tra-jectories, enabling highly differentiated age patterns tobe derived from photogrammetric flow analysis. Suchpatterns can be calibrated by luminescence dating offine material reappearing at the oversteepened front,and by radiocarbon dating of (sparse) organic matterfound in drillings or excavations in the permafrost itself.This helps limiting the uncertainty about effects frompast changes in flow rates. Weathering-rind thicknessesand Schmidt-hammer rebound values can now be tiedto the thus defined time scales. These easily appliedfield methods in turn facilitate the selection of blockswhich have been overturned when falling from the sur-face over the steep front to the frontal talus of relict rockglaciers immediately before movement came to its finalstop. Cosmogenic exposure dating of such selectedblocks defines the point in time when this important cli-mate- or topography-induced event took place, andluminescence dating of fine material from the front mayadd information about total travel times and possiblyeven involved processes (especially the often discussedparticipation of small glaciers and large rockfalls orlandslides in initial debris transport). A thus calibratedmethodology involving weathering-rind thicknesses,Schmidt-hammer rebound values, exposure and lumi-nescence dates may also be applied on other character-istic cold-mountain deposits such as moraines, debrisflows or rock-fall deposits. It is evident that the long-term creep of mountain permafrost together with awhole set of newly developed dating methodologiesopens most interesting perspectives for chronologicalwork about late-glacial and Holocene landscape evolu-tion in climate-sensitive high-mountain areas.

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