Ninth International Conference on Permafrost ... - IARC Research

Surface Ice and Snow Disappearance in Alpine Cirques and Its PossibleSignificance for Rock Glacier Formation: Some Observations from Central AustriaAndreas Kellerer-PirklbauerInstitute of Geography and Regional Science, University of Graz, AustriaIntroductionActive rock glaciers consist of two components: ice(congelation and/or sedimentary ice including “glacier”ice) and lithological material (periglacially and/or glaciallyderivedrock fragments of different grain size). Consideringtheir long formation period (centuries to millennia) andcommon climate variability (e.g., temperature, precipitation)in such long time scales, rock glaciers experience strongvariations in the rate of nourishment as well as the ratiobetween ice and debris input to the rock glacier system.Studies on the climate of rock glaciers reveal that themean annual air temperature at the rooting zone of activerock glaciers is usually only slightly higher (if at all) than atnearby equilibrium line altitudes (ELA) of normal glaciersand/or the annual precipitation is only slightly lower thanat nearby ELA (e.g., Haeberli 1983). This indicates the highsensitivity of rock glacier development to cooler and/ormore humid conditions and their close relationship to normalglaciers. However, knowledge regarding incipient formation,entire development period, variations in nourishment rate,and the ratio between ice and rock input seen over a longtime span is still far from being complete.This paper presents data on surface ice and snowdisappearance since the Little Ice Age (LIA) at twoneighboring cirques in the Central Alps of Austria. Today,one houses a rock glacier and one, a debris-covered glacierremnant. Observations and measurements on thicknessvariations and thermal conditions of the supraglacial debriscover, on landforms formed from this supraglacial material,and on buried sedimentary ice add to the understanding ofrock glacier formation and development.Study Area and Applied MethodsKögele Cirque and Hinteres Langtal CirqueThe study area consists of the two neighboring cirquesKögele Cirque (KC) and Hinteres Langtal Cirque (HLC) locatedin the central part of the Schober Mountains (46°59′N,12°47′E), Central Alps (Fig. 1). The study area is characterizedby crystalline rocks and a continental climate (1500mmat 2000m asl, 0°C mean annual air temperature at 2300 ma.s.l.) causing minor glaciation but a high abundance ofrock glaciers. Both cirques are oriented towards the westnorthwest,each with comparable high crests and mountainsummits (>3000 m a.s.l.) to the south and east (Fig. 1). TheHLC is dominated by the Hinteres Langtalkar Rock Glacier(HLRG) which at its front indicates the local lower limit ofdiscontinuous permafrost at 2450 m a.s.l. In contrast, theKC is located some 50 m higher, lacks a rock glacier, buthouses a glacier remnant (for details see Kellerer-Pirklbauer& Kaufmann 2007).MethodsThe spatial extent of surface areas covered by glaciers andperennial snow at both cirques was reconstructed for the fourstages—c. 1850 (LIA-maximum), 1969, 1997, and 2006—by using morphological evidences (LIA-moraine ridges), airphotographs (1969, 1997), and field mapping (Sept. 2006).Massive sedimentary ice outcrops in the HLC weremapped between summer 2003 and 2007 during fieldwork.In 2006, the thickness of the supraglacial debris mantle inthe KC was quantified by digging through and measuringthe debris layer to solid ice at 34 sites. These data were usedto interpolate the debris cover thickness in the KC, applyingthe Inverse Distance Weighted interpolator in ArcGIS.To monitor near-surface temperatures in the supraglacialdebris cover, two 3-channel miniature temperature dataloggers(MTLs) (Ge oPr e c i s i o n) were installed in 2006 at 2690m a.s.l. (KC1) and 2710 m a.s.l. (KC2) in the KC (Fig. 2).Three temperature sensors (PT1000; accuracy +/-0.05°C)were connected to each MTL. The two MTLs operated correctlyduring the entire period 12.09.2006–25.07.2007, loggingvalues every 0.5 h at depths 0–50 cm (KC1: 0, 10, and50 cm; KC2: 0, 10, and 20 cm). The lowest sensor at siteKC2 was placed at the debris/ice boundary.Results and Some Related CommentsSurface ice disappearance since 1850In 1850 A.D., both cirques were covered by glaciers and perennialsnow fields covering 0.21 km² of the KC and 0.18 km²of the HLC. In 1969, ice/snow still covered 0.12 km² of theKC and 0.06 km² of the HLC, but by 1997 these values werereduced to 0.06 km² (KC) and 0.01 km² (HLC). In 2006, theextent of surface ice/snow was reduced to a few small patchesin the HLC and covered less than 0.01 km² of the KC (Fig. 1).Supraglacial debris cover and buried sedimentary iceThe supraglacial debris cover in the KC was formedduring the last decades by periglacially- and paraglaciallyderivedsediments from the steadily enlarging supraglacialslopes above the shrinking ice mass. The debris cover iscurrently relatively thin and increases in thickness downvalley, exceeding 70 cm at the lower part of the mapped area(Fig. 2). In contrast to the KC, surface and near-surface (