A zero-exposure time test on an erratic boulder : evaluating the problem of pre-exposure in surface exposure dating

The method of surface exposure dating using in-situ produced cosmogenic nuclides has become an important and widely applied tool in Quaternary science. One application is the dating of erratic boulders on moraines. An important problem however remains: the evaluation of potential pre-exposure time for samples from boulder surfaces. We have tested pre-exposure by sampling all sides of a recently exposed boulder in order to measure inherited nuclides from prior exposure periods. The sampled erratic boulder rests on the right lateral moraine of the most recent advance of the Glacier de Tsijiore Nouve in the Arolla Valley, Switzerland. Mapping of the area was done to reconstruct the Holocene fl uctuations of the glacier. This glacier is especially useful for such a test as it is characterized by an ideal geometric relationship between accumulation and ablation area and, therefore, responds rapidly to mass-balance changes. The sampled boulder was deposited in 1991. Assuming no prior exposure the expected concentration of a given cosmogenic nuclide should be near zero. The 10Be/9Be ratios of the fi ve measured samples were indistinguishable from blank values within the given errors, demonstrating that the samples did not experience pre-exposure. Three samples measured for 21Ne reveal 21Ne/20Ne and 22Ne/20Ne ratios similar to those of air, with no detectable prior cosmogenic Ne accumulation. [Ein Experiment an einem 1991 abgelagerten erratischen Block: Annäherung an das Problem der Vorbestrahlung in der Oberfl ächendatierung] Kurzfassung: Die Oberfl ächendatierung mittels in-situ produzierten kosmogenen Nukliden hat sich in den letzten Jahren in der Quartärgeologie zu einer wichtigen und häufi g angewandten Methode entwickelt. Eine Anwendung ist die Altersbestimmung von erratischen Blöcken auf Moränen. Ein wesentliches Problem ist jedoch die Ermittlung einer eventuellen vorherigen Bestrahlungsperiode eines Blockes. Wir haben dies getestet, indem wir alle Seiten eines kürzlich exponierten Blockes beprobten und die Proben auf schon vorhandene Nuklide hin untersuchten. Der untersuchte Block liegt auf der rechten lateralen Moräne des jüngsten Gletschervorstosses des Glacier de Tsijiore Nouve im Val d’Arolla in der Schweiz. Die holozänen Gletscherschwankungen wurden durch eine Kartierung rekonstruiert. Durch die ideale geometrische Verteilung von Akkumulationsund Ablationsgebiet reagiert dieser Gletscher schnell auf Änderungen der Massenbilanz. Daher ist er besonders gut für einen solchen Test geeignet. Der beprobte Block wurde 1991 durch den Gletscher abgelagert. Durch die Annahme, dass er vorher keiner kosmogenen Strahlung ausgesetzt war, sollte seine Nuklidkonzentration annähernd null sein. Die gemessenen 10Be/9Be Verhältnisse der fünf Proben waren innerhalb der Fehler nicht zu unterscheiden von Blindprobenwerten. Dies zeigt, dass die Proben keiner Vorbestrahlung ausgesetzt waren. Gemessene 21Ne/20Ne und 22Ne/20Ne Verhältnisse bei drei Proben sind ähnlich derer in der Luft, mit keiner nachweisbaren vorhergehenden kosmogenen Ne Anreicherung.


Introduction
The method of surface exposure dating has become a useful and widespread tool in Quaternary science (for a review of the method see GOSSE & PHILLIPS 2001). Surface exposure dating has been most extensively applied to studies directly dating the advance of a glacier from boulders on moraine ridges. By measuring the nuclide concentration resulting from reactions of cosmic ray particles with target elements in the rock surfaces, a surface's exposure and, therefore, resting time can be determined. Estimating the correct exposure age, however, also requires knowledge of previous surface exposure histories. There are many possible scenarios for a rock to have been exposed prior to reaching its present position, e.g., in a cliff before falling onto the glacier or in an older moraine ridge which became reworked by a more recent advance (BROWN et al. 1991, BROOK et al. 1995. The eventual case of inherited nuclides from prior exposure is usually neglected in the application of surface exposure dating and therefore the estimated ages may be incorrect. The goal of this study is to examine the exposure time of each side of a specifi c boulder of known, very young exposure age. If the samples have no prior exposure history, then concentrations of cosmogenic nuclides should be very low or even undetectable as the exposure time is too short to build up a signifi cant amount of nuclides. To cross-check the possibility of a long-term prior exposure followed by a signifi cant shielding period, we estimated the concentrations of the cosmogenic nuclides 10 Be and 21 Ne. While the unstable 10 Be decays during times of burial, the stable cosmogenic 21 Ne records the total accumulated exposure time. Thus, the comparison of 21 Ne to 10 Be is suitable for detecting pre-exposure of the surface by revealing a higher 21 Ne concentration than expected compared to the 10 Be concentration (GRAF et al. 1991). We sampled a boulder resting on the right lateral moraine crest of the maximum stand of the most recent advance (1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991) of the debrisrich Glacier de Tsijiore Nouve near Arolla, in the uppermost part of the Val d'Hérens in the south-western Swiss Alps (Fig. 1). All sides of the boulder were sampled in order to test for possible pre-exposure for all possible past orientations of the boulder (Fig. 2). The boulder was identifi ed during careful geological mapping of the Holocene fl uctuations of the glacier (ABBÜHL et al. 2002). It is very suitable due to its enormous size, its stable position since deposition and its known deposition age (1991). The Glacier de Tsijiore Nouve has ideal characteristics for this test as it reacts rapidly to mass balance changes due to its relatively small extent (and therefore short transport distance of the debris) and to its geometry of accumulation to ablation area. This case study is the fi rst to test pre-exposure in a high Alpine setting by estimating the concentrations of cosmogenic nuclides ( 10 Be and 21 Ne) in different faces of an individual boulder. It is a contribution to the question as to whether it is possible to rule out pre-exposure of boulder surfaces whenever sampling boulders of unknown age.

Overall setting
The uppermost part of the Val d'Hérens, the Val d'Arolla, is characterized by an open basin and steep valley walls. The village of Arolla, situated on the valley fl oor, has an altitude of 2000 m a.s.l. and the highest of the surrounding mountains, the Pigne d'Arolla, reaches 3796 m a.s.l. A relatively steep east and a more open west valley wall terminate the valley. Three glaciers terminate in close distance to Arolla, namely the Glacier de Tsijiore Nouve, the Glacier de Pièce and the Glacier d'Arolla. The Glacier d'Arolla is the main valley glacier while the others meet the valley of Arolla at approximately 90°. Well-defi ned moraines of glacial advances since the last glaciation cover the area (Fig. 1). In particular the Glacier de Tsijiore Nouve has accumulated moraine ridges of up to 80 m height because of its reduced sediment transfer. Its most extensive, still identifi able, Holocene advance is dated to 8400 ± 200 14 C yr BP (e.g. moraines at Hôtel Kurhaus in

Sampling in the fi eld
Five sides of the boulder were sampled, including bottom and top surfaces (Fig. 2). The aim was to sample in the middle part of each surface to avoid edge effects due to neutron leakage (MASARIK & WIELER 2003). The rock slabs were taken in the fi eld using dynamite because a large rock sample of 2-3 kg was needed. Small charges were set around a given area and an intact rock plate was blasted free. A large separate of pure quartz is desirable for determining the expected low concentration of 10 Be in these samples. Before blasting, strike and dip (Tab. 1), the top position of the sample and the characteristics (e.g. possible erosion, snow and/or sediment cover, quartz veins) were carefully noted. Shielding by surrounding mountains was measured with an inclinometer (Fig. 2c) and the exact position and altitude of the boulder was recorded by GPS. After sampling, photographs for documentation were taken ( Fig. 2a and b).

Origin of the boulder
The erratic boulder sampled rests on the crest of the right lateral moraine of the Glacier de Tsijiore Nouve's advance of 1975 to 1991.
From thin section analysis of sample A1, the lithology is a quartz-rich metadiorite, which crops out in much of the glacier catchment area ( Fig. 1). This lithology belongs to the Série d'Arolla in the crystalline basement of the Dent Blanche nappe, which is part of the Austroalpine units of the Alps (LABHART 1998;BURRI et al. 1999). The incorporation of the boulder into the glacier system was most likely by falling on the glacier's surface from a collapsing rock cliff, a process, which can be observed today on the rock walls surrounding the steep icefall of the glacier (Fig. 1). Below the icefall, the glacier surface is covered by blocky surface debris, indicating that the production of sediment here mainly occurs by rock fall from the surrounding rock walls. The shape of the boulder itself is not typical of glacial transport as it lacks smooth surfaces and glacial polish. However on a smaller scale, partial smoothing and polishing are visible. This implies a supra-or englacial transport of the boulder. The possibility of being reworked out of older lateral moraine depositions can not be excluded. Its fi nal deposition on the moraine crest occurred during the formation of the moraine. A fi ne-to coarse-grained gravelly till covers depressions on the boulder top as small and irregular patches (5-6 cm thick). No movement of the boulder is observed since deposition.

Sample data
The elevation of the sampled boulder is 2220 m a.s.l. and the geographical coordi- The height of the sample location is above the moraine surface. *Snow height (own observations) is given above the moraine surface, not on the sampled surface of the boulder itself. nates are E 7° 28' 21.16" and N 46° 1' 3.08" (= 602626/096220 in the Swiss grid). Its height is 6.80 m, the length 9.50 m and the width 4.10 m. Based on thin section analysis, the rock consists of 50% quartz, occurring as dynamically recrystallized fi ne-grained crystals alongside quartz porphyroclasts. Quartz-rich layers alternate with mica layers. Accessory minerals including apatite, zircon and opaque minerals are also identifi able (BURRI et al. 1999). In addition, a reaction with HCl during the fi rst quartz purifi cation step points to the presence of calcite in the rock.

Production rate systematics and age calculation
Cosmogenic nuclides are produced in the upper surface of a rock by nuclear reactions induced by cosmic rays (LAL & PETERS 1967). P 0 (atoms/yr!g SiO 2 ) is the isotope production rate valid for the exposure time and normalized to sea-level, high geomagnetic latitude ("60°) and open sky conditions (GOSSE & PHILLIPS 2001). A sea level, high latitude 10 Be production rate of 5.1 atoms/g SiO 2 !yr (STONE 2000) and a sea level, high latitude 21 Ne production rate of 20.3 ± 3.7 atoms/g SiO 2 !yr (NIEDERMANN 2000) were used in this study.
Latitude (geographic) and altitude scaling of P 0 follows STONE (2000). As sample thickness did not exceed 5 cm, it was not necessary to correct P 0 for thickness (MASARIK & REEDY 1995). Corrections were also not made for magnetic fi eld changes, as the exposure time of the boulder is negligible compared to the time scales of magnetic fi eld fl uctuations (MASARIK et al. 2001). The surrounding mountain ranges with an average elevation of ca. 3000 m a.s.l. partially shield the boulder from the incoming cosmic ray fl ux. Therefore, only a fraction of the total incoming fl ux is available for the production of the cosmogenic nuclides. The circle in Fig. 2c illustrates the measured shielding values for sample AO/AO2 in degrees. The dip of the sampled surfaces of the other samples reduces their horizon and thus production rates of the cosmogenic nuclides to even lower values (Tab. 2). To the northeast, the "open valley" allows a higher amount of cosmic rays to pass while, to the south, the Pigne d'Arolla blocks the cosmic ray fl ux. The bottom-boulder sample AU has the lowest production rate as a result of the corrections for the overall shielding by the boulder itself. Shielding corrections were calculated following DUNNE, ELMORE & MUZIKAR (1999). For all samples, no correction due to vegetation cover was necessary. We observed snow on the moraine in winter of about 1.5 m thickness. Winter snow cover on the surfaces of the Note: The AMS measurement uncertainty for sample and blank are at the 1"-level. The error of the used standard (±2.5%) is not included. A weighted mean blank of 10 Be/ 9 Be = 0.012*10 -12 with an error of 18.3% was taken for the subtraction step. A sea level, high latitude production rate of 5.1 atoms/g SiO 2 !yr (STONE 2000) was used. A zero-exposure time test on an erratic boulder boulder is 30 cm or less and on the steep sides of the boulder no snow remains. All samples experience less than 30 cm of snow cover (in the case of sample AU location height is 1.20 m above moraine surface while snow cover on the moraine is 1.50 m) (Tab. 1). After the model of MASARIK & REEDY (1995), the cosmogenic production rate on the boulder surface is not affected until snow cover exceeds 40 cm (assuming a snow density of 0.3 g/cm 3 ). Therefore, no correction for snow cover was performed. The ages were calculated using the equation in GOSSE & PHILLIPS (2001), assuming no erosion.

Laboratory analysis
For extracting 10 Be from dissolved quartz, the methods described in OCHS & IVY-OCHS (1997) were followed. The pure quartz mineral separate was produced by selective chemical dissolution using hydrofl uoric acid, following the procedure of KOHL & NISHIIZUMI (1992). Laboratory work was carried out at the Institute of Geological Sciences at the University of Bern. Measurements of 10 Be/ 9 Be ratios of the samples and the necessary blanks were done by accele-rator mass spectrometry at the ETH/PSI tandem facility in Zurich. Pure quartz was handpicked for Ne analysis. Noble gas concentrations were measured with a 90° sector fi eld static noble gas mass spectrometer at the noble gas laboratory at ETH Zurich (BEYERLE et al. 2000). This spectrometer features a modifi ed Baur/Signer ion source equipped with a compressor that enhances the sensitivity for helium and neon by two orders of magnitude (BAUR 1999). We applied stepwise heating to enrich the cosmogenic neon fraction in the low-temperature steps.

Samples
The 10 Be ages are shown in Fig. 3a and Tab.2. All ages are zero within uncertainties. Their relatively high error is due to the low 10 Be concentrations in the samples, which are similar to the measured blank values. The mean age of sample AU is negative and only the upper limit of the age is realistic. This is due to the subtraction of the mean blank value, which has Sample Step (  a higher 10 Be/ 9 Be ratio of 0.012*10 -12 than the sample ratio. The high shielding effect by the surrounding mountains and the boulder itself and the consequential small production rate also contribute to the particularly low 10 Be concentration of this bottom sample. The 21 Ne/ 20 Ne and the 22 Ne/ 20 Ne ratios of the samples A3, AO and AU are similar to air ratios (Fig. 3b, Tab. 3). Accordingly, no excess of cosmogenic 21 Ne and 22 Ne, which would have been produced in a previous period of exposure, were detected. No differences between the Ne isotope ratios of the three samples are observed. Be measurements were within 2 sigma of their blank value. DAVIS et al. (1999) concluded that glacial erosion must have been suffi cient to remove the nuclide signal gained in a previous period of exposure, e.g. during an interglacial or interstadial. In an Alpine environment with a higher relief, data from the Nägelisgrätli in the Grimsel region in Central Switzerland show that nuclide inheritance is also negligible (KELLY et al. 2006). Based on these data and on the fact that at 3 m depth in the rock production of nuclides is only a few percent of the value at the surface (LAL 1991), the authors concluded that at least 3 m of rock was removed during the Last Glacial Maximum (LGM) and that therefore nuclide inheritance originating from exposure prior to the LGM in the Alps is low (IVY-OCHS, KERSCHNER & SCHLÜCHTER 2007). Sample AO2 of our study would require at least 300 years of pre-exposure to yield a 10 Be/ 9 Be ratio clearly outside the uncertainties of the blank value. For the Ne data, at least 60,000 years of exposure to cosmic rays would be required to build up a minimal detectable amount for sample AO (Fig. 3c). Assuming that the clock has been "zeroed" by LGM glacial erosion, nuclide inheritance could have only accumulated during low glacial stands following the LGM. In case of the Ne data, time since the LGM was too short to build up a detectable amount of cosmogenic Ne. For 10 Be, considering the Holocene glacial history of this region and of the Alps in general (SCHNEEBELI & RÖTH-LISBERGER 1976;JÖRIN, STOCKER & SCHLÜCHTER 2006), the required time span of at least 300 years of phases with a shorter glacial extent than today would easily be reached. Therefore, it can be concluded that the 10 Be concentration of the boulder was zero when it was incorporated into the glacier system.

Conclusion
The aim of this work was to test a boulder for inherited nuclides from previous periods of exposure. We are aware that we are dealing with one single boulder only; however, in a fi rst approach a multi-isotope and multi-surface study is an absolutely necessary step in evaluating the case of pre-exposure of erratic boulder surfaces. As the known exposure time of the block is about 12 years, the nuclide concentration built up since exposure should be undetectable if no previous exposure of the surfaces has occurred. The 10 Be and the 21 Ne results demonstrate that none of the sides of the boulder that we sampled experienced pre-exposure. The build-up of the cosmogenic nuclides started with its deposition on the moraine crest, as all of the samples show zero exposure ages within error. The uncertainties are relatively high due to the high error in the measuring step which again results from the low 10 Be concentration in the samples. The several hundred years of pre-exposure required for building up a minimal detectable amount of 10 Be would be easily reached in geological time scales, as well as the 60,000 years required for 21 Ne. This implies that the concentration of cosmogenic nuclides in the boulder was zero by the time of its incorporation in the glacier sys-A zero-exposure time test on an erratic boulder tem; possible subsequent gain of cosmogenic nuclides is in the undetectable range or was immediately lost during transport.