Rock glacier development in the Northern Calcareous Alps at the Pleistocene-Holocene boundary

    Rock glacier development in the Northern Calcareous Alps at the PleistoceneHolocene boundary Andrew P. Moran, Susan Ivy Ochs, Christof Vockenhuber, Hanns Kerschner PII: DOI: Reference:

S0169-555X(16)30264-1 doi: 10.1016/j.geomorph.2016.08.017 GEOMOR 5731

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

2 May 2016 18 July 2016 9 August 2016

Please cite this article as: Moran, Andrew P., Ochs, Susan Ivy, Vockenhuber, Christof, Kerschner, Hanns, Rock glacier development in the Northern Calcareous Alps at the Pleistocene-Holocene boundary, Geomorphology (2016), doi: 10.1016/j.geomorph.2016.08.017

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ACCEPTED MANUSCRIPT Rock glacier development in the Northern Calcareous Alps at the

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Pleistocene-Holocene boundary

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Andrew P. Moran1*, Susan Ivy Ochs2, Christof Vockenhuber2, Hanns Kerschner1 1

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Institute of Geography, University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria

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Laboratory of Ion Beam Physics, ETH Zurich, Otto-Stern-Weg 5, 8093 Zurich, Switzerland

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*Corresponding author: Email: [email protected]

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Abstract

Relict rock glaciers provide information on past discontinuous permafrost and former mean annual air temperatures. A lack of records showing former permafrost distribution along the

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northern Alpine fringe prompted the investigation and numerical dating of a belt of relict rock

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glaciers in the Karwendel Mountains of the Northern Calcareous Austrian Alps. In two 36

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neighbouring cirques that were still glaciated during the early Younger Dryas, eleven

exposure ages from boulder surfaces were obtained. The ages imply the onset of rock glacier activity around ~12.3 ka with subsequent stabilization and permafrost melt out no

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later than ~10.1 ka. Hence, rock glacier formation coincided with glacier retreat in the cirques around the mid-Younger Dryas and continued into the early Holocene. As permafrost induced features, the rock glacier termini indicate the local past lower limit of discontinuous permafrost in open cirque floors at ~2000 m asl, which is around 400 m lower than during the mid-twentieth century at comparable locations in the Karwendel Mountains. Thus, a mean annual air temperature reduction of ~-2.6 to -3.8°C relative to the mid-twentieth century is inferred. Based on a minimum glacier equilibrium line altitude in the cirques, a summer temperature reduction of less than -2.6 to -1.8°C is shown, suggesting an increased seasonality at the time of rock glacier activity. Keywords: rock glaciers; palaeoclimate; 36Cl; Alps

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1. Introduction

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In high mountain environments, cryogenic processes account for the development of a

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variety of geomorphological landforms. Among those features, rock glaciers, as defined by

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Wahrhaftig and Cox (1959), Haeberli (1985), and Barsch (1996), constitute one of the most obvious and widespread indicators of periglacial activity in the Alps (Haeberli, 2000). Rock glaciers usually exhibit a tongue or lobe-shaped geometry with surfaces marked by

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transverse and longitudinal ridges, furrows, and undulations and consist of clasts, unconsolidated regolith, and ice of varying contents and multiple origins (Haeberli, 1996).

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Their development is dependent on a sufficient provision of debris (Barsch, 1988), often provided by talus at the foot of steep rock faces (Humlum, 1988), moraines (Haeberli, 1979),

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and protalus lobes (Johnson et al., 2007), combined with the occurrence of discontinuous

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permafrost conditions (Haeberli, 1985). Rock glaciers have been mapped by many authors throughout the Alpine region (e.g., Lieb, 1991, 1998; Schoeneich, 1992; Baroni et al., 2004;

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Kellerer-Pirklbauer, 2005, Kellerer-Pirklbauer et al., 2012; Krainer and Ribis, 2012; Scotti et al., 2013) and have been monitored over continuous periods of time at several locations

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worldwide (e.g., Hoelzle et al., 1998; Ikeda and Matsuoka, 2003; Roer et al., 2005, 2008; Hausmann et al., 2007; Bodin et al., 2009; PERMOS, 2013). Rock glaciers, as permafrost-induced geomorphic features, are climate indicators because the occurrence of permafrost is a function of climatic parameters. On a large scale their occurrence depends in particular on mean annual air temperature (MAAT; Kerschner, 1985; Humlum, 1988; Haeberli et al., 1993; Steig et al., 1998; Haeberli, 2000; Refsnider and Brugger, 2007), while on a more local scale short-wave radiation is of importance. The latter can be parameterized by aspect and topographic shielding (Geiger, 2013). The rock glaciers occur primarily in dry mountain regions where the equilibrium line altitude of glaciers (ELA) is at high elevations relative to the lower limit of permafrost (Haeberli, 1985). Consequently, a shift in climate toward warmer conditions can induce the depletion of permafrost leading 2

ACCEPTED MANUSCRIPT progressively to the melt out of ice and a ceasing of rock glacier motion. According to Haeberli (1985) and Barsch (1996), warming brings about a transition from an active rock glacier (occurrence of permafrost with movement), via an inactive state (some permafrost, no

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movement), to a relict rock glacier (neither permafrost, nor movement). Therefore the investigation of relict rock glaciers in various mountain areas contributes to the interpretation

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and reconstruction of past climate (Kerschner, 1978; Konrad et al., 1999; Sailer and Kerschner, 1999; Frauenfelder et al., 2001; Hughes et al., 2003; Aoyama, 2005). In recent 10

Be was

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years first surface exposure dating of rock glaciers with the cosmogenic nuclide

carried out in the Alps (Ivy-Ochs et al., 2009; Böhlert et al., 2011b). However, hitherto the

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dating of rock glaciers (Ivy-Ochs et al., 2009) as a climate proxy has focused on locations in the central Alps. Conversely, no absolute ages have been determined for moraines and rock glaciers in the Northern Alps. This region is of particular interest as the northern Alpine fringe

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receives considerably more precipitation (Schwarb et al., 2001; Tirol Atlas, 2013) than the central Alps and can be expected to show deviations from the central Alps reflected by

the first

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permafrost distribution and ELAs of palaeoglaciers (Kerschner et al., 2000). Here we present Cl ages of rock glacier stabilization in the Northern Alps. The purpose of this paper

is to derive novel chronological and climate information from past rock glaciers in this region.

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The obtained ages also pose age constraints for below- and above-lying moraine systems, where currently still no absolutely dated moraine stratigraphy exists.

2. Research area In this study we investigate two high-elevation sites in the Northern Calcareous Alps of western Austria. They are located in the Karwendel Mountains (Fig. 1), which consist of four chiefly parallel mountain ridges each extending in an east-west direction. The highest peaks rise to elevations around 2300 to 2750 m asl. The orographic effect they have on moist, incoming air masses from the Atlantic northwest induces high precipitation sums ranging between 1800 and 2300 mm/a at our research sites, which is ~300% higher than in some dry 3

ACCEPTED MANUSCRIPT inner Alpine locations (Tirol Atlas, 2013). Contemporary treelines are found at elevations of up to ~1800 m asl but based on Lotter et al. (2006) can be expected to have reached above 2250 m asl in the Northern Alps during warm periods of the Holocene. Lieb (1998) estimated

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the modern lower limit of discontinuous permafrost in the Northern Alps above ~2400 m asl Currently, most of the Karwendel Mountains are free of permafrost with exceptions restricted

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to small, high elevational, and well-shaded north-facing cirques and exposed peaks. Recent assessments on the occurrence of permafrost in a comparable region have been performed

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in the neighbouring Wetterstein Mountains by Gude and Barsch (2005), who show that discontinuous permafrost reaches there generally 200 to 300 m lower than in the central

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Alps.

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Fig. 1. Location of the research area in the Karwendel Mountains of the Northern Calcareous

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Austrian Alps.

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Our research sites show evidence of former glacial and periglacial activity in this mountain group (Fig. 2). The corresponding landforms are located in the southern part of the Karwendel Mountains in the neighbouring Mandl (11°24’00’’ E; 47°19’20’’ N) and Pfeis

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cirques (11°25’20’’ E; 47°19’36’’ N) ~6 km north of the center of Innsbruck. They were first mapped by Kerschner (1993). The cirque floors are found at elevations of between ~2000 and 2100 m asl. All surrounding peaks and ridges consist of bedded to massive Middle Triassic Wetterstein limestone (Mutschlechner, 1949; Heissel, 1994). The lithology supports the formation of steep rock walls rising above extensive fine scree slopes and talus cones. The peaks surrounding the Mandl cirque reach elevations around 2300 to 2400 m asl (e.g., Gleirschtaler Brandjoch, 2372 m asl), whereas the Pfeis cirque lies at the foot of the somewhat higher summit of Rumer Spitze Mountain (2454 m asl). The Nordkette is characterized by an east-west strike dipping 30° to 60° to the north, whereas the area north of the Mandlkar is marked by a much more gentle dip of 5° to 30° to the southwest forming a large syncline (Ampferer and Hammer, 1898). 4

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2.1. Mandl cirque rock glaciers

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The Mandl cirque contains four tongue- or lobe-shaped relict rock glaciers delimited by

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sharp ridges at their outer margins rising up to ~5 m on their proximal sides and to ~10 m on

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their distal sides. Their frontal slopes often have inclinations slightly lower than the angle of repose. The surfaces show rudimentary pedogenesis and a patchy high-montane vegetation cover typical for the elevation. As depicted in Fig. 2, the largest rock glacier (R1) lies beneath

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Gleirschtaler Brandjoch peak (2372 m asl) and covers the northern part of the cirque. It

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emerges from the foot of extensive scree slopes in the north and east and extends from its upper onset at ~2090 m asl to its terminus at ~1970 m asl. It is ~550 m long, exhibits a surface marked by frequent undulations and furrows in its lower parts, and consists of

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unsorted, unconsolidated material and only some scattered boulders. A second tongue-

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shaped rock glacier (R2) lies beneath Mandlspitze (2366 m asl) and is located ~200 m south

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of R1. It extends ~270 m northwestward and is ~170 m wide. The margin of a third rock glacier (R3) trends generally parallel to the southern scree slopes forming a small tongue at its mid-section. Altogether it is ~600 m wide and extends a distance of up to 160 m from the

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foot of the slope to its furthest terminal position. The fourth rock glacier (R4) forms an arc of ~600 m in length, fully surrounding R3, and lying tangent to R2. From their position in the field, the rock glaciers described above correspond to typical talus-derived rock glaciers (Barsch, 1996). Some 800 to 900 m downvalley from these landforms at Grubach (Fig. 2) a further tongue-shaped rock glacier (R6) consisting of several consecutive transversal ridges and furrows is present. It reaches its terminal position at 1770 m asl and is currently completely covered by dwarf mountain pines (Pinus mugo subsp. mugo). This feature can be best identified by visualized LIDAR (Laser Illuminated Detection and Ranging) data. It likely formed out of a series of closely deposited moraine ridges deformed by permafrost-induced creep. Higher up, the landform gradually resembles a former glacier bed with glacial till. It 5

ACCEPTED MANUSCRIPT can likely be interpreted in the sense of a rock glacier-glacier landscape continuum (Giardino and Vitek, 1988). A small former glacier with its not so well-defined end above the closely spaced transversal ridges was mapped by Kerschner (1993). To the south and west of R6, a

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series of four relict rock glacier deposits (R7 to R10) extends down to ~1750 m asl (Fig. 2). The individual features exhibit steep outer embankments at their fronts reaching heights of

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up to ~15 to 25 m above the lower surrounding settings. A well-preserved lateral moraine segment (M1) of ~600 m length is found along the lower slopes of the orographic right side of

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the Mandl Valley constraining all these landforms.

2.2. Pfeis cirque rock glaciers

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Neighbouring to the east of the Mandl cirque, the Pfeis cirque also contains several

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talus-derived rock glaciers (Fig. 2). The most impressive one is a large rock glacier system (R5) at the northern foot of the Rumer Spitze Mountain (2454 m asl). Its margin is formed by

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a ridge characterized by a distinct, steep outer embankment, which rises above the surrounding surface by ~5 m in its lateral positions and between 10 and 15 m at its terminus. Forming two lobes extending northward, the entire margin of R5 adds up to a length of

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~1800 m. With its highest onset at an elevation of ~2080 m asl, the greater western lobe reaches 1950 m asl, whereas the shorter eastern one extends downvalley to 1980 m asl. Similar to the landforms in Mandl cirque, the Pfeis rock glacier exhibits a gently undulating surface with some boulders and unconsolidated material consisting mainly of poorly sorted diamicton. Large transversal ridges and furrows are found in the lower area of both tongues. In its active process phase, the rock glacier was fed by talus deposited at the foot of the Rumer Spitze headwalls. Other relict rock glaciers (R11 and R12) developed from the extensive slopes of talus below the eastern and northeastern slopes of Gleirschtaler Brandjoch (Fig. 2). 6

ACCEPTED MANUSCRIPT Fig. 2. Orthorectified aerial image map of the relict rock glaciers in the Mandl and Pfeis cirques in the southern Karwendel Mountains. Source of aerial images: TIRIS, Map Service

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of the Federal State of Tyrol. Image: 2013.

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3. Methods 3.1. Field investigations

Geomorphological investigations were carried out at both research sites based on

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remote sensing technologies and field reconnaissance. Mapping was performed at a scale of 1:10,000 and was conducted on the basis of orthorectified aerial images (0.25x0.25 m

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resolution) and hillshades derived from a high-resolution LIDAR digital elevation model (1x1 m resolution). These geodata sources were provided by the Map Service of the Federal

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State of Tyrol (TIRIS) and are the foundation for a Geographical Information System (GIS)-

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driven analysis of surface topography.

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3.2. Surface exposure dating with the cosmogenic nuclide 36Cl All ages were determined with the surface exposure dating method using the

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cosmogenic radionuclide 36Cl, which is suitable for dating in limestone environments (Alfimov and Ivy-Ochs, 2009). This method assumes a continuous production of

36

Cl in rock surfaces

exposed to cosmogenic radiation, predominantly by means of spallation processes and to a lesser extent by muon reactions and low-energy neutron capture (Alfimov and Ivy-Ochs, 2009). Common target elements for the production of

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Cl in carbonates are Ca, K, and Cl

(Stone et al., 1996; Dunai, 2010).

3.2.1. Sample extraction A total of 11 samples were extracted from the upper surfaces of large (at least 1 m³), stable limestone boulders located on relict rock glaciers. Sample thickness was 2 cm or less for all samples. For each sample ~0.5 kg of rock material was removed in the field with a hammer, chisel, and battery-powered saw. Following the guidelines of Ivy-Ochs and Kober 7

ACCEPTED MANUSCRIPT (2008) and Akçar et al. (2011), care was taken to sample only boulders clearly related to the rock glaciers. The boulders were also examined to reduce the chance of post-depositional exhumation, tilting, or displacement; whereas the sampled rock surfaces were inspected to

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minimize the possibility of spalling. Rock surfaces showing karst-weathering were preferred, as this is an indicator that a boulder surface has been exposed for a significant amount of

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time. Topographic shielding as well as strike and dip of the rock surfaces were determined on-site with a clinometer. Care was taken to choose positions with minimum topographic

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shielding.

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3.2.2. Sample positions

All relevant sample properties are listed in Table 1. Primarily boulders at the front or along the margin were selected for sampling. However, contrary to rock glaciers in the

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crystalline central Alps, large boulders suitable for dating are much less frequent. This is mainly owing to the weathering characteristics of Wetterstein limestone, which usually

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shatters into small pieces. Therefore, more central positions on the rock glaciers also were considered, if necessary. Seven rock samples were extracted from within Mandl cirque (Fig. 3). There three samples were collected from R1, the northern most relict rock glacier. MAND-

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1 represents a large block on the crest of a transverse ridge ~40 m within the lower margin of the rock glacier. MAND-2 was taken from the proximal side of the same ridge at a location ~35 m south therefrom. MAND-3 stems from a boulder in a central position on the rock glacier surface ~300 m from the frontal area. MAND-4 and MAND-5 are located at the lower margin of R2, with the former boulder situated on the proximal side of the marginal crest and the latter on the distal part of the outer embankment. MAND-6 lies ~25 m within the limits of R3 and is sufficiently distanced from the rock walls in the south. MAND-7 is a medium-sized boulder located within a central position on the surface of R4, ~60 m beyond R3. In the neighboring Pfeis cirque, four samples were collected from R5 (Fig. 4). PFS-1 is situated 40 m within the lower margin of the eastern lobe. PFS-2 is an extraordinary large clast-supported block (~30 m³) on the surface of the western rock glacier lobe near the rock 8

ACCEPTED MANUSCRIPT glacier margin. Situated in a depression near the crest of the western lobe, PFS-3 represents a small boulder exceeding the surrounding setting by only 0.5 m. PFS-4 is a medium-sized boulder on the western margin of R5 located ~380 m from and ~80 m higher than the

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terminus of the relict rock glacier.

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Fig. 3. View of the relict rock glaciers R1 to R4 in the Mandl cirque to the northwest with

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sample locations. Photo: A. Moran.

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Fig. 4. View of the relict rock glacier R5 in the Pfeis cirque to the southwest with sample

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locations. Photo: A. Moran.

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Fig. 5. Geomorphological map of the Mandl and Pfeis cirques. Twenty-meter contour lines derived from the digital elevation model of Tyrol provided by the Map Service of the Federal

Table 1

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State of Tyrol (TIRIS).

Sample properties; shielding correction encompasses topographic shielding by the surrounding cirque rim as well as dip and strike of the boulder surfaces Sample

Lat.

Long.

Elevation

Sample

Height

Size of

Topographic

no.

(DD)

(DD)

(m asl)

thickness

above

boulder (m³)

shielding

(cm)

ground (m)

Mandl cirque

MAND-1

47.326

11.399

1992

1

1.5

15

0.956

MAND-2

47.325

11.399

1989

1

0.8

1.5

0.952

9

ACCEPTED MANUSCRIPT 47.324

11.402

2041

1

1.2

2.5

0.967

MAND-4

47.321

11.399

2068

1.5

0.6

1

0.987

MAND-5

47.322

11.399

2063

1

1.2

1.2

0.973

MAND-6

47.320

11.398

2065

1

0.5

1.2

MAND-7

47.321

11.398

2053

1.5

1.5

PFS-1

47.327

11.425

1989

2

PFS-2

47.327

11.423

1991

2

PFS-3

47.327

11.423

1979

1.5

PFS-4

47.325

11.420

2023

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4

0.902

5

30

0.959

0.5

1.2

0.971

1

2.5

0.961

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1.3

0.973

0.988

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Pfeis cirque

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2

T

MAND-3

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3.2.3. Sample preparation and determination of 36Cl ages

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Sample preparation was carried out at the Laboratory of Ion Beam Physics, ETH Zurich. There the crushed and sieved rock samples, with a grain size of <0.6 mm, were processed following standard laboratory procedures based on Zreda et al. (1994), Stone et al. (1996),

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and Ivy-Ochs et al. (2004) and as recently demonstrated in the Alpine region by Claude et al. (2014) and Martin et al. (2014). Accordingly, the process of

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Cl extraction comprised (i)

sample leaching with diluted HNO3 and washing in ultrapure water to eliminate meteoric Cl, (ii) spiking the sample with 2.4 mg of

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Cl carrier for improving the

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Cl/Cl measurements, (iii)

dissolving ~60 g of rock in concentrated HNO3, (iv) precipitating AgCl by adding AgNO3, and (v) removing sulfur with Ba(NO3)2. Lastly (vi), the AMS measurements (Ivy-Ochs et al., 2004) were conducted at the 6 MV HVEC EN-Tandem facility (Synal et al., 1997; Christl et al., 2013) of the Laboratory of Ion Beam Physics, ETH Zurich. The applied (K381/4N) assumes a ratio of 17.36 x 10-12. As the production rate of

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Cl/Cl standard

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Cl is dependent on

the composition of the rock sample (Alfimov and Ivy-Ochs, 2009), measurements of major and trace elements for each rock sample (Table 3) were conducted by ICP-MS (Inductively 10

ACCEPTED MANUSCRIPT Coupled Plasma Mass Spectrometry) at SGS S.A. (Ontario, Canada). The topographic shielding parameters were determined in CRONUS® Earth calculator, version 2.2 (Balco et al., 2008; http://hess.ess.washington.edu/math). The ages were computed on the basis of

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the Alfimov and Ivy-Ochs (2009) technique and carried out with a MATLAB-based Cl calculator developed at ETH Zurich. The ages were calculated with the

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Cl production rates

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of 48.8 ± 3.4 atoms/g/a for spallation in Ca and 5.3 ± 1.0 atoms/g/a for muon capture in Ca (Stone et al., 1996). A neutron capture rate of 760 ± 150 neutrons/gr_air (Alfimov and Ivy36

Cl half-life of

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Ochs, 2009) was assumed. All ages were calculated under consideration of a

301 ka (Gosse and Phillips, 2001). The scaling of the production rates to the geographical

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location and elevation followed Stone (2000) and Balco et al. (2008).

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3.3. Relict rock glaciers as palaeoclimatic proxies

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Relict rock glaciers serve as valuable proxies for reconstructing past climates, especially mean annual temperatures. As shown in Haeberli (1985) and Barsch (1996), rock glaciers

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provide evidence of the existence of permafrost during the time of their activity. Hence the lowest elevation of a band of rock glaciers gives a maximum altitude of the lower limit of

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discontinuous permafrost (LLP) in a given region allowing the estimation of quantitative temperature information (Humlum, 1988; Frauenfelder and Kääb, 2000). Under the assumption of constant boundary conditions (e.g. aspect and solar radiation, morphology, supply and size of debris; Frauenfelder et al., 2001), the elevation difference between relict rock glaciers and currently active rock glaciers of modern times (e.g. Little Ice Age or present conditions) can be converted to changes in MAAT with a standard temperature lapse rate (e.g. 0.65°C/100 m). Alternatively, following Barsch (1978), Haeberli (1985), and Kerschner (1985), the LLP corresponds to a MAAT of -1 to -2°C. Therefore, ΔMAAT between the time of past rock glacier activity and modern times can also be determined as shown in Eq. (1), using the MAAT of surrounding climate stations and a standard temperature lapse rate to adjust

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ACCEPTED MANUSCRIPT differences in elevations. Both approaches require rock glaciers in comparable topographic positions to ensure constant boundary conditions.

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Eq. (1) Calculation of ΔMAAT at the past lower limit of discontinuous permafrost (LLP) based

Tm st

a tst a t 100

Tp

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MAAT

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on a nearby climate station (after Kerschner, 1985).

MAAT Change in mean annual air temperature at the past LLP

Tp

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Tm st : Modern mean annual air temperature at a nearby climate station [°C] : Past mean annual air temperature at past LLP [°C]

: Altitude of past LLP [m asl]

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at

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a tst: Altitude of nearby climate station [m asl]

4. Results

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: Temperature lapse rate [°C/100 m].

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4.1. Field mapping

Geomorphological field mapping (Fig. 5) of the investigated rock glaciers yielded geographic properties as listed in Table 2. The lowest rock glacier elevations, correlating with the past LLP, range from 2050 m asl (R2) to 1950 m asl (R5) and feature a mean elevation of ~2000 m asl whereby the largest rock glaciers (R1 and R5) also reach the lowest elevations. The upper part of each rock glacier is located within 60 elevational meters from one another, averaging altogether an elevation of 2085 m asl. All rock glaciers exhibit nearly identical surface gradients between 12° and 13° along the respective central flow lines (Table 2).

Table 2 12

ACCEPTED MANUSCRIPT Rock glaciers and their geographic properties

Aspect

glacier

Lowest

Highest

Δ

Length

Area

Average

elevation

elevation

Elevation

(m)

(m² x 10³)

surface

(m asl)

(m asl)

(m)

R1

W

1970

2090

120

MAND

R2

NW

2050

2110

60

MAND

R3

NE

2050

2090

MAND

R4

NE

2050

PFS

R5

N

1950

550

gradient (°)

165

12

270

47

13

40

180

73

13

2090

40

270

49

13

2080

130

550

230

12

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MAND

T

Rock

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Cirque

a

a

MA

Average surface gradients were derived along the central flow lines of the rock glaciers.

4.2. Exposure ages

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In Table 4, all sample exposure age calculations are presented (Alfimov and Ivy-Ochs,

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2009), with and without an average limestone weathering correction of 5 mm/ka (Reber et al., 2014). As the depth profile of in situ

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Cl production is rock dependent (Table 3) and

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nonlinear (Ivy-Ochs and Schaller, 2009) with the highest production rates a few decimeters beneath the rock surface (Liu et al. 1994), the samples show younger ages (mostly <5%)

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when considering the effects of surface erosion. A higher weathering rate within the range provided by André (2002) would not lead to significantly different ages than those with and without erosion shown here (Table 4). In the following discussion, all considerations are based on ages with an erosion correction of 5 mm/ka. In the Mandl Cirque, ages range from 12,930 ± 660 (MAND-5) to 9610 ± 560 (MAND-6) years and in the Pfeis cirque from 12,050 ± 810 (PFS-1) to 9160 ± 600 (PFS-3) years. The mean ages of the various rock glaciers are listed in Table 5. Where only a single age is available, it is given instead of a mean. The mean of all samples in Mandl Cirque is 11,070 ± 1130 and in Pfeis cirque 10,900 ± 790 years. Table 3 Major and trace elements of the sampled rocks (SGS S.A., Ontario, Canada) 13

ACCEPTED MANUSCRIPT Al2O3

CaO

Cr2O3

Fe2O3

K2O

MgO

MnO

Na2O

P2O5

SiO2

TiO2

B

Gd

Sm

U

Th

Mand-1 No. Mand-2

0.05

55.8

<0.01

0.05

<0.01

0.44

<0.01

<0.10

<0.01

0.19

<0.01

<0.01

0.10

0.10

2,09

0.20

0.02

55.5

<0.01

0.06

<0.01

0.35

<0.01

<0.10

<0.01

0.05

<0.01

<0.01

0.10

0.10

2,09

0.20

Mand-3

0.07

51.8

<0.01

0.06

<0.01

3.14

<0.01

<0.10

<0.01

0.10

<0.01

<0.01

0.10

0.10

2,09

0.20

Mand-4

<0.01

54.6

<0.01

0.04

<0.01

0.89

<0.01

<0.10

<0.01

0.01

<0.01

<0.01

0.10

0.10

2,09

0.20

Mand-5

<0.01

54.4

<0.01

0.02

<0.01

1.43

<0.01

<0.10

<0.01

<0.01

<0.01

<0.01

0.10

0.10

2,09

0.20

Mand-6

<0.01

56.2

<0.01

0.02

<0.01

0.38

<0.01

<0.10

<0.01

<0.01

<0.01

<0.01

0.10

0.10

2,09

0.20

Mand-7

0.18

53.9

<0.01

0.16

0,01

0.87

<0.01

<0.10

<0.01

0.89

<0.01

<0.01

0.10

0.10

2,09

0.20

PFS-1

0.12

51.7

<0.01

0.17

<0.01

3.63

<0.01

<0.10

<0.01

0.54

<0.01

<0.01

<0.05

<0.01

1.29

<0.10

PFS-2

0.09

54.9

<0.01

0.10

<0.01

0.53

<0.01

<0.10

<0.01

0.50

<0.01

<0.01

<0.05

<0.01

1.29

<0.10

PFS-3

0.06

56.1

<0.01

0.09

<0.01

0.40

<0.01

<0.10

<0.01

0.31

<0.01

<0.01

<0.05

<0.01

1.29

<0.10

PFS-4

0.22

56.3

<0.01

0.15

0.02

0.45

<0.01

<0.10

<0.01

1.00

0,01

<0.01

<0.05

<0.01

1.29

<0.10

NU

SC R

IP

T

Sample

MA

Table 4

Exposure ages in years *with topographic shielding but without erosion correction, **including topographic shielding and surface erosion correction (5 mm/ka); uncertainties

D

represent 1σ confidence range that comprises AMS counting errors and errors based on the

Sample

36

No. R1

R1

Cl in rock

Exposure age without

Exposure age with

(atoms 106 g-1)

(ppm)

erosion correction (a)

erosion correction** (a)

MAND-1

1.476±0.038

66.5±0.3

12,780±570

12,670±620

MAND-2

1.346±0.055

112.8±0.7

10,610±630

10,300±670

MAND-3

1.187±0.036

77.9±0.3

10,110±500

9960±540

MAND-4

1.479±0.065

67.8±0.3

11,900±680

11,830±720

R2

MAND-5

1.604±0.043

71.69±0.4

13,080±600

12,930±660

R3

MAND-6

1.350±0.043

108.5±0.3

9880±520

9610±560

R4

MAND-7

1.417±0.044

112.5±0.4

10,530±570

10,190±610

R5

PFS-1

1.509±0.047

128. 8±0.5

12,670±740

12,050±810

R5

PFS-2

1.287±0.036

50.2±0.4

11,670±520

11,750±550

R5

PFS-3

1.306±0.046

141.0±0.76

9540±560

9160±600

R2

AC

R1

Cl

CE P

Rock glacier

TE

normalization to blanks and standards

14

ACCEPTED MANUSCRIPT R5

PFS-4

1.438±0.045

107.91±0.49

10,910±580

10,640±630

T

Table 5

IP

Mean ages of the sampled relict rock glaciers in Mandl cirque (MC) and Pfeis cirque (PC)

Cirque

Rock glacier

Mean

SC R

*5 mm/ka age

with

Amount

erosion* (a)

samples

R1

10980±710

3

MC

R2

12380±380

2

MC

R3

9610±560

1

MC

R4

10190±610

1

MC

R1-R4

PF

R5

MC+PF

R1-R5

11070±1130

7

10900±790

4

11010±1380

11

TE

D

MA

NU

MC

of

CE P

4.3. Palaeotemperature estimations

The rock glaciers in the Mandlkar and Pfeis cirques extend downward to elevations between 1950 and 2050 m asl (Table 5). On average a former LLP of ~2000 m asl can be

AC

deduced for the research sites. The ca cu ation of ΔMAAT was performed by two methods.

4.3.1. Elevational shift of rock glacier bands/LLP Presently active rock glaciers in the Karwendel Mountains are confined to a single, wellshaded, north-facing cirque in the central chain, where they reach down to 2300 m asl. From a larger sample, Lieb (1998) estimated the LLP in the northern Austrian Alps at elevations around 2400 m asl in the northern sector, which appears to be a more representative reference value for a comparison with our relict rock glaciers. Thus, a rise in LLP of around 400 m is ensued. With a standard lapse rate of 0.65°C/100 m, a minimum ΔMAAT of -2.6°C is estimated.

15

ACCEPTED MANUSCRIPT 4.3.2. Assumption of a MAAT at the LLP A ternative y, the ΔMAAT at the former

was estimated in accordance with Eq. 1 and

with a lapse rate of 0.65°C/100 m (Table 6). Temperature data were applied from the nearby

IP

T

high-elevation climate station, Hafelekar (2260 m asl; 47°18’43.4’’ N, 11°23’01.0’’ E; Fliri, 1975), distanced 1.5 and 3 km, respectively, to the southwest of the research localities.

SC R

Based on the assumption of a MAAT of -1 to -2°C at the LLP (Barsch, 1978), a temperature depression of approximately -2.8 to -3.8°C is calculated for the past LLP in relation to the

NU

1931-1960 standard normal period, which is representative of the twentieth century (Auer et al., 2007). To account for recent warming in the Alpine region, an additional ~1°C (Auer et

MA

al., 2014) can be added to the ΔMAAT in re ation to present day conditions.

Table 6

TE

D

Calculation of the mean annual air temperature (MAAT) increase between past rock glacier activity and the mid-twentieth century (1931-1960; Fliri, 1975) at the lower limit of

CE P

discontinuous permafrost (LLP) based on Eq. 1; scenario 1 assumes an MAAT of -1°C at the LLP and scenario 2 an MAAT of -2°C at the LLP (Barsch, 1978); vertical lapse rate of

AC

0.65°C/100 m is applied throughout (*calculated values)

Scenario 1

Scenario 2

Hafelekar weather station

Research site (LLP)

Research site (LLP)

2260 m asl

Altitude [m asl]

2000

2000

2260

1931-1960 MAAT [°C]

1.8*

1.8*

0.08

Past MAAT [°C]

-1

-2

Δ

2.8°C*

3.8°C*

260 m*

5. Discussion 5.1. Boulder ages Despite a rigorous sampling strategy (Ivy-Ochs and Kober, 2008), given the spectrum of possible, no longer visible disturbances that could have affected the continual buildup of

36

Cl

16

ACCEPTED MANUSCRIPT nuclides in the sampled rock surfaces, cosmogenic exposure ages are treated as minimum ages (Hallet and Putkonen, 1994; Böhlert et al., 2011a; Heyman et al., 2011). Considering the individual sampled boulders, we assume that PFS-3 (9160 ± 600 years) possibly may be

IP

T

too young because the sampled rock surface was only 0.5 m above the surrounding setting making shielding by an excessive winter snow cover or exhumation a possibility.

SC R

For other sampled boulders, cosmogenic nuclide buildup likely occurred already during transportation at the rock glacier surface (Haeberli et al., 1998, 2003). In such cases,

NU

boulders with the longest residence times at the surface most likely coincide with the farthest transport distances and show the oldest ages. These boulders reflect the sum of the time

MA

since the end of rock glacier activity plus the transport time on the rock glacier surface. This can be seen best by the three oldest ages obtained (12,930 ± 660, MAND-5; 12,670 ± 620, MAND-1; and 12,050 ± 810, PFS-1) that stem from boulders situated directly on or near the

TE

D

crest of the lower rock glacier margins.

CE P

5.2. Age of rock glacier formation and stabilization The relict rock glaciers in Mandlkar cirque developed in the former accumulation area of a glacier (M1; Fig. 2, 5), which reached down to 1600 m asl. Its ELA was at 1990 m asl,

AC

which is slightly below the elevation of the cirque floors. While the LIA (Little Ice Age) ELA in the northern Karwendel Mountains and the neighbouring Wetterstein Mountains was around 2300 m asl (Hirtlreiter, 1992), we may estimate the LIA ELA in our research area, which is already somewhat sheltered from the northwest, between 2300 and 2400 m asl. The ELA lowering of the glacier was hence in the range of 300 to 400 m, which is typical for the Egesen maximum advance (early Younger Dryas) in the Northern Alps (Kerschner et al., 2000; Kerschner and Ivy-Ochs, 2008). A similar-sized glacier had to exist in the Pfeis cirque but, based on topography, field evidence is only rudimentary. Located at about the same elevation, R1 to R5 form a common band of rock glaciers. Based on their positions, they developed synchronously in response to the same climatic signal. Likewise, a common warming phase appears responsible for the final melt out of 17

ACCEPTED MANUSCRIPT permafrost and cessation of rock glacier activity. In regard to the morphology and the relative small size of the rock glaciers, we assumed that these features formed during a single quasicontinuous period of activity. Eleven exposure ages obtained from the neighbouring relict

IP

T

rock glaciers span a time period of ~3 ka with ages ranging from 12,930 ± 660 to 9160 ± 600 years. The average age of all samples in both cirques is 11,010 ± 1380 years. If PFS-3 were

SC R

omitted based on the reliability of the sampled boulder, the average age would increase slightly to 11,190 ± 1240.

NU

The probability density functions of the sample ages (Fig. 6) show that the various ages relate to two distinct groups. The older group (MAND-1, -4, -5 and PFS-1, -2) is centered

MA

around 12,250 ± 830 and spans a 1σ range from 11,400 to 13,080 years. The younger group (MAND-2, -3, -6, -7 and PFS-4) is centered around 10,140 ± 690, showing a 1σ range from 9420 to 10,820 years. The younger age group seems to represent the youngest age

TE

D

constraint for landform stabilization and permafrost melt out in our research area. This is in good accordance with the present-day knowledge of climate during the late Preboreal and

CE P

Boreal period in the Alps (Nicolussi and Patzelt, 2000; Heiri et al., 2014). As boulders are commonly transported on the surface of the rock glacier, we suggest that the older age group centered around 12.2 ka (mid-Younger Dryas) represents the onset of rock glacier formation

AC

in the Mandlkar and Pfeis cirques. Hence, the beginning of rock glacier development likely coincides with the commencement of the second phase of the Younger Dryas. A similar development was reported from the central Alps, when a phase of glacier advances during the early Younger Dryas was followed by a period of rock glacier development within the former glacier beds (Sailer and Kerschner, 1999, Frauenfelder et al., 2001, Ivy-Ochs et al., 2009). Both age groups point to an active phase of the rock glaciers of ~2000 years. As the longest rock glaciers in our study area are ~500 m long, an average advance rate of ~25 cm/a is inferred (Barsch, 1996; Kääb et al., 1997; Berger et al., 2004). The ages imply that rock glacier development started during the last phase of the Pleistocene and came to an end in the early Holocene. 18

ACCEPTED MANUSCRIPT Farther downvalley but still within the former glacier-covered area, rock glaciers developed from glacier-transported debris (e.g. R6, R8, and R9; Giardino and Vitek, 1988) and talus slopes (e.g. R10). The final recession of the glacier tongue of the Egesen

IP

T

maximum glacier enabled the transition to rock glacier activity in this area. From their comparatively sheltered and well-shaded position, we assume that their period of activity

NU

5.3. Rock glaciers and their climatic inferences

SC R

coincides with the active period of the dated rock glaciers higher up.

Temperatures during the final Alpine Lateglacial cold period of the Younger Dryas

MA

(~12.9-11.7 ka; Rasmussen et al., 2006; Fiedel, 2011) were lowered significantly for ~1.3 ka in the Alpine region with the MAAT ~4 to 5°C lower than present (Wurth et al., 2004). This event led to marked multiphased glacier advances dated throughout the Alpine region at their

TE

D

maximum positions to the early Younger Dryas (e.g., Federici et al., 2008; Ivy-Ochs et al., 2009; Schindelwig et al., 2011) and named Egesen stadial (Heuberger, 1966; Ivy-Ochs et al.,

CE P

2008). These advances are associated with glacier equilibrium line altitude depressions (ΔE A) of -200 to -250 m in relation to the LIA ELA in the central Alps and a ΔE A of -300 to -450 m along the northern Alpine fringe (Ivy Ochs et al., 2008).

AC

Rock glacier activity in the Mandl and Pfeis cirques had to commence subsequently to the early Younger Dryas glacier advances corresponding with an ELA rise above the cirque floors to an elevation of at least 2150 m asl. Depending on an LIA ELA of 2300 to 2400 m asl, this conforms to a ΔE A of ~-150 to -250 m or less in relation to the LIA and less than ~-230 to -330 m referenced to the mid-twentieth century (Maisch, 2000). Kuhn (1981) showed that an ELA shift of +100 m is induced by a summer temperature increase of +0.8°C, providing precipitation did not decrease. Precipitation during the mid-twentieth century was presumably comparable to that of the early Holocene (Magny et al., 2007; Ivy-Ochs et al., 2009) and higher than precipitation in the late Younger Dryas (Kerschner et al., 2000; Frauenfelder et al., 2001). Therefore a mean summer temperature depression relative to the mid-twentieth century mean of -2.6 to -1.8°C or less can be assumed. Concomitantly, the 19

ACCEPTED MANUSCRIPT estimated ΔMAAT between -2.6 and -3.8°C in relation to mid-twentieth century values shows climatic conditions considerably colder than today during the active process in the rock glacier. The less pronounced mean summer temperature depression points to an increased

IP

T

seasonality during the late Younger Dryas and early Holocene, possibly in connection with elevated northern hemispheric summer insolation (Berger and Loutre, 1991; Denton et al.,

SC R

2005; Renssen et al., 2009).

NU

5.4. Alpine rock glacier formation

The investigated rock glaciers had to form as glaciers melted back during the latter part

MA

of the Younger Dryas and the early Holocene (Ilyashuk et al., 2009). Such development conforms well to research on other relict rock glaciers carried out in the central Alps (Frauenfelder et al., 2001; Ivy-Ochs et al., 2009). In the course of the Younger Dryas,

TE

D

increasingly dry conditions set in as also seen in lake level fluctuations on the northwestern fringe of the Alps (Magny and Ruffaldi, 1995; Magny and Richoz, 2000; Fig. 6). A decrease in

CE P

precipitation coinciding with the sustainment of low temperatures caused glaciers to melt back from their maximum Egesen positions enabling permafrost development and rock glacier formation in areas covered by glaciers during the early stages of the Younger Dryas.

AC

Such rock glaciers in the Tyrolean Larstig Valley (western Austria; Fig. 1) were 10Be dated to the Younger Dryas-early Holocene transition period (Ivy-Ochs et al., 2009). It is representative of a band of relict rock glaciers formed directly in subsequence to the melting of glaciers from their maximum Younger Dryas positions (Haeberli, 1985; Sailer and Kerschner, 1999). This band is smaller in the humid Northern Alps as lower glacier equilibrium line altitudes limited the area of suitable locations free of ice and yet still conducive for the existence of permafrost (Kellerer-Pirklbauer et al., 2012). As a result of their location, apparently beneath the lowest limits of any subsequent glacier advances, the rock glaciers in the Mandl and Pfeis cirques could be well-preserved throughout the entire Holocene.

20

ACCEPTED MANUSCRIPT Fig. 6. Comparison of palaeoclimatic records in the Alpine region. (A) Greenland NGRIP

T

stable oxygen isotope record (Rasmussen et al., 2006; Svensson et al., 2008), (B) Lake

IP

Ammersee ostracod stable oxygen isotope record (von Grafenstein, 2003), (C) Lake

SC R

Mondsee ostracod stable oxygen isotope record (Lauterbach et al., 2011), (D) Lake Morat lake level record (Magny and Richoz, 2000; Magny, 2001), (E) probability density functions of

NU

the exposure ages of relict rock glaciers in the Karwendel Mountains.

MA

6. Conclusion

The activity of relict rock glaciers at an elevation of ~2000 m asl in the Northern Calcareous Alps was numerically dated to a period from ~12.3 to 10.1 ka. The lower age

D

represents the latest age constraint for the stabilization of the landforms. Concomitantly, the

TE

research sites were still completely glaciated during the cold and humid early Younger Dryas.

CE P

It is likely that the melt back of glaciers followed by the generation of permafrost and subsequent rock glacier development occurred during the dry, but still cold late Younger Dryas. Indicating the lower level of discontinuous permafrost, the locations of these rock

AC

glaciers can be used to infer a ΔMAAT between -2.6 and -3.8°C, whereas a minimum glacier equilibrium line altitude shows a mean summer temperature depression from -2.6 to -1.8°C or less. In accordance with our exposure ages, rock glacier activity in the Northern Alps around elevations of ~2000 m asl ceased in the early Holocene with the onset of increasingly warm temperatures leading to permafrost depletion in these positions.

Acknowledgements The project P23601-N21 was generously funded by the Austrian Science Foundation FWF. Additionally, a short-term scientific mission to ETH Zurich for sample preparation was supported by the INTIMATE EU COST Action. We thank the staff members of the Laboratory of Ion Beam Physics, ETH Zurich for their kind lab support and the carrying out of AMS 21

ACCEPTED MANUSCRIPT measurements. The reviews by Rick Giardino and Wilfried Haeberli and the comments by Richard Marston are greatly appreciated. They contributed to a deeper understanding and

IP

T

considerable improvement of the manuscript.

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