Current glacier recession causes significant rockfall increase: The immediate paraglacial response of deglaciating cirque walls

Abstract

In the European Alps almost half the glacier volume disappeared over the past 150 years. The loss is reflected in 10 glacier retreat and ice surface lowering even at high altitude. In steep glacial cirques surface lowering exposes rock to atmospheric conditions for the very first time in many millennia. Instability of rockwalls has long been identified as one of the direct consequences of deglaciation, but so far cirque-wide quantification of rockfall at high-resolution is missing. Based on terrestrial LiDAR a rockfall inventory for the permafrost-affected rockwalls of two rapidly deglaciating cirques in the Central Alps of Austria (Kitzsteinhorn) is established. Over six-years (2011-2017) 78 rockwall scans were acquired to generate data 15 of high spatial and temporal resolution. 632 rockfalls were registered ranging from 0.003 to 879.4 m³, mainly originating from pre-existing structural rock weaknesses. 60 % of the rockfall volume detached from less than ten vertical meters above the glacier surface, indicating enhanced rockfall activity over tens of years following deglaciation. Debuttressing seems to play a minor effect only. Rather, preconditioning is assumed to start inside the Randkluft (gap between cirque wall and glacier) where sustained freezing and ample supply of liquid water likely cause enhanced physical weathering and high plucking stresses. 20Following deglaciation, pronounced thermomechanical strain is induced and an active layer penetrates into the formerly perennially frozen bedrock. These factors likely cause the observed paraglacial rockfall increase close to the glacier surface. This paper presents the most extensive dataset of high-alpine rockfall to date and the first systematic documentation of a cirquewide erosion response of glaciated rockwalls to recent climate warming.

Production and Transport of Supraglacial Debris: Insights From Cosmogenic 10Be and Numerical Modeling, Chhota Shigri Glacier, Indian Himalaya

“…Nevertheless, the total amount of such low‐concentrated material is likely small, due to the small extent of these areas. Although, repeat Lidar observations from the Kitzsteinhorn in Austria suggest that formerly glaciated bedrock, adjacent to the Bergschrund, may undergo a period of accelerated erosion upon recent exposure (Hartmeyer et al, 2020), thus accounting for a relatively large supply of material, despite the small area. It should be noted that a relative increase in the supply of material from close to the glacier surface would reduce the aggregated 10 Be concentration of debris derived from that hillslope, even without considering former ice cover shielding, simply because of the lower production rates compared to higher up on the hillslope.…”

“…Better understanding of the coupling between ice‐free bedrock hillslopes and glaciers in steep mountains requires means to assess headwall erosion rates. Some studies applied repeat laser scanning of steep rock walls to determine the amount of erosion (e.g., Abellàn et al, 2014; Hartmeyer et al, 2020). Although important, such studies typically cover small areas and short time spans, and may be biased by interannual variations and the effects of recent climate change.…”

Point-by-point response

“…5.1 and 5.2. Our measurements/observations suggest sustained freezing and ample water supply inside the Randkluft at the same time (during summer) -two key requirements for ice segregation (and thus for frost cracking) (see also analyses of Sanders et al 2012, Alley et al 2019, Evans et al 2020 who all assume enhanced frost cracking inside the Randkluft). We feel that some confusion was arising from our use of the term "subcritical fracture propagation" (which results from ice segregation).…”

High mountain rockfall dynamics: rockfall activity and runout assessment under the aspect of a changing cryosphere

“…Brenva (2 × 10 6 m³, 1997), Punta Thurwiese (2 × 10 6 m³, 2007), Pizzo Cengalo in the European Alps (3 × 10 6 m³, 2017) (Deline 2001;Sosio et al 2008;Walter et al 2020). Also, an enhanced activity of rockfalls with magnitudes ranging from debris falls (<10 m³) to cliff falls (10 4 -10 6 m 3 ) have been observed and studied in peri-and proglacial areas of high mountain regions (Rabatel et al 2008;Fischer et al 2012;Ravanel et al 2017;Hartmeyer et al 2020).…”

“…Some recent studies have focused on the quantification of low-magnitude failures through field studies (e.g. Heckmann et al 2016;Vehling et al 2016), photo comparisons (Ravanel and Deline 2011) or terrestrial laser scanning (Rabatel et al 2008;Kenner et al 2011;Hartmeyer et al 2020). These systematic rockfall investigations allow a highly accurate assessment of rockfall activity but are limited to local areas.…”