Periglacial preconditioning of debris flows in the Southern Alps, New Zealand

<p>The lower boundary of alpine permafrost extent is considered to be especially sensitive to climate change. Ice loss within permanently frozen debris and bedrock as a consequence of rising temperature is expected to increase the magnitude and frequency of potentially hazardous mass wasting processes such as debris flows. Previous research in this field has been generally limited by an insufficient understanding of the controls on debris flow formation. A particular area of uncertainty is the role of environmental preconditioning factors in the spatial and temporal distribution of debris flow initiation in high-alpine areas. This thesis aims to contribute by investigating the influence of permafrost and intensive frost weathering on debris flow activity in the New Zealand Southern Alps. By analysing a range of potential factors, this study explores whether debris flow systems subjected to periglacial influence are more active than systems outside of the periglacial domain. A comprehensive debris flow inventory was established for thirteen study areas in the Southern Alps. The inventory comprises 1534 debris flow systems and 404 regolith-supplying contribution areas. Analysis of historical aerial photographs, spanning six decades, identified 240 debris flow events. Frequency ratios and logistic regression models were used to explore the influence of preconditioning factors on the distribution of debris flows as well as their effect on sediment reaccumulation in supply-limited systems. The preconditioning factors considered included slope, aspect, altitude, lithology, Quaternary sediment presence, neo-tectonic uplift rates (as a proxy for bedrock fracturing), permafrost occurrence, and frost-weathering intensity. Topographic and geologic information was available in the form of published datasets or was derived from digital elevation models. The potential extent of contemporary permafrost in the Southern Alps was estimated based on the statistical evaluation of 280 rock glaciers in the Canterbury region. Statistical relationships between permafrost presence, mean annual air temperature, and potential incoming solar radiation were used to calculate the spatially distributed probability of permafrost occurrence. Spatially distributed frost-weathering intensities were estimated by calculating the number of annual freeze-thaw cycles as well as frost-cracking intensities, considering the competing frost-weathering hypotheses of volumetric ice expansion and segregation ice growth. Results suggest that the periglacial influence on debris flow activity is present at high altitudes where intense frost weathering enhances regolith production. Frost-induced debris production appears to be more efficient in sun-avert than sun-facing locations, supporting segregation ice growth as the dominant bedrock-weathering mechanism in alpine environments. No indication was found that permafrost within sediment reservoirs increases slope instability. Similarly, the presence of permanently frozen bedrock within the debris flow contribution areas does not appear to increase regolith production rates and hence debris flow activity. Catchment topography and the availability of unconsolidated Quaternary deposits appeared to be the cardinal non-periglacial controls on debris flow distribution. This thesis contributes towards a better understanding of the controls on debris flow formation by providing empirical evidence in support of the promoting effect of intense frost weathering on debris flow development. It further demonstrates the potential and limitations of debris flow inventories for identifying preconditioning debris flow controls. The informative value of regional-scale datasets was identified as a limitation in this research. Improvement in the spatial parameterisation of potential controls is needed in order to advance understanding of debris flow preconditioning factors.</p>

Periglacial preconditioning of debris flows in the Southern Alps, New Zealand

<p>The lower boundary of alpine permafrost extent is considered to be especially sensitive to climate change. Ice loss within permanently frozen debris and bedrock as a consequence of rising temperature is expected to increase the magnitude and frequency of potentially hazardous mass wasting processes such as debris flows. Previous research in this field has been generally limited by an insufficient understanding of the controls on debris flow formation. A particular area of uncertainty is the role of environmental preconditioning factors in the spatial and temporal distribution of debris flow initiation in high-alpine areas. This thesis aims to contribute by investigating the influence of permafrost and intensive frost weathering on debris flow activity in the New Zealand Southern Alps. By analysing a range of potential factors, this study explores whether debris flow systems subjected to periglacial influence are more active than systems outside of the periglacial domain. A comprehensive debris flow inventory was established for thirteen study areas in the Southern Alps. The inventory comprises 1534 debris flow systems and 404 regolith-supplying contribution areas. Analysis of historical aerial photographs, spanning six decades, identified 240 debris flow events. Frequency ratios and logistic regression models were used to explore the influence of preconditioning factors on the distribution of debris flows as well as their effect on sediment reaccumulation in supply-limited systems. The preconditioning factors considered included slope, aspect, altitude, lithology, Quaternary sediment presence, neo-tectonic uplift rates (as a proxy for bedrock fracturing), permafrost occurrence, and frost-weathering intensity. Topographic and geologic information was available in the form of published datasets or was derived from digital elevation models. The potential extent of contemporary permafrost in the Southern Alps was estimated based on the statistical evaluation of 280 rock glaciers in the Canterbury region. Statistical relationships between permafrost presence, mean annual air temperature, and potential incoming solar radiation were used to calculate the spatially distributed probability of permafrost occurrence. Spatially distributed frost-weathering intensities were estimated by calculating the number of annual freeze-thaw cycles as well as frost-cracking intensities, considering the competing frost-weathering hypotheses of volumetric ice expansion and segregation ice growth. Results suggest that the periglacial influence on debris flow activity is present at high altitudes where intense frost weathering enhances regolith production. Frost-induced debris production appears to be more efficient in sun-avert than sun-facing locations, supporting segregation ice growth as the dominant bedrock-weathering mechanism in alpine environments. No indication was found that permafrost within sediment reservoirs increases slope instability. Similarly, the presence of permanently frozen bedrock within the debris flow contribution areas does not appear to increase regolith production rates and hence debris flow activity. Catchment topography and the availability of unconsolidated Quaternary deposits appeared to be the cardinal non-periglacial controls on debris flow distribution. This thesis contributes towards a better understanding of the controls on debris flow formation by providing empirical evidence in support of the promoting effect of intense frost weathering on debris flow development. It further demonstrates the potential and limitations of debris flow inventories for identifying preconditioning debris flow controls. The informative value of regional-scale datasets was identified as a limitation in this research. Improvement in the spatial parameterisation of potential controls is needed in order to advance understanding of debris flow preconditioning factors.</p>

Sediment export in marly badland catchments modulated by frost-cracking intensity, Draix-Bléone Critical Zone Observatory, SE France

At the interface between the lithosphere and the atmosphere, the critical zone records the complex interactions between erosion, climate, geologic substrate and life, and can be directly monitored. Long data records collected in the sparsely vegetated, steep marly badland catchments of the Draix-Bléone Critical Zone Observatory (CZO), SE France, allow analysing potential climatic controls on long-term regolith dynamics and sediment export. Although widely accepted as a first-order control, rainfall variability does not fully explain the observed inter-annual variability in sediment export, suggesting that regolith production and its controls may modulate the observed pattern of sediment export. Here, we define sediment-export anomalies as the residuals from a predictive model with annual rainfall intensity above a threshold as the control. We then use continuous soil-temperature data, recorded at different locations over multiple years, to highlight the role of frost weathering in regolith production. Several proxies for different frost-weathering processes have been calculated from these data and compared to the sediment-export anomalies, with careful consideration of field data quality. Our results suggest that frost-cracking intensity (linked to ice segregation) can explain about half (47–64 %) of the sediment-export anomalies. In contrast, the number of freeze-thaw cycles (linked to volumetric expansion) has only a minor impact on catchment sediment response. The time spent below 0 °C also correlates well with the sediment-export anomalies and requires fewer field data to be calculated than the frost-cracking intensity. Thus, frost-weathering processes modulate sediment export by controlling regolith production in these catchments and should be taken into account when building a predictive model of sediment export from these badlands under a changing climate.

Ground thermal contrasts and variability at an alpine pyramidal peak in central Austria (Innerer Knorrkogel, Venediger Mountains)

&lt;p&gt;Ground temperatures in alpine environments are severely influenced by slope orientation (aspect), slope inclination, local topoclimatic conditions, and thermal properties of the rock material. Small differences in one of these factors may substantially impact the ground thermal regime, weathering by freeze-thaw action or the occurrence of permafrost. To improve the understanding of differences, variations, and ranges of ground temperatures at single mountain summits, we studied the ground thermal conditions at a triangle-shaped (plan view), moderately steep pyramidal peak over a two-year period (2018-2020).&lt;/p&gt;&lt;p&gt;We installed 18 monitoring sites with 23 sensors near the summit of Innerer Knorrkogel (2882m asl), in summer 2018 with one- and multi-channel datalogger (Geoprecision). All three mountain ridges (east-, northwest-, and southwest-facing) and flanks (northeast-, west-, and south-facing) were instrumented with one-channel dataloggers at two different elevations (2840 and 2860m asl) at each ridge/flank to monitor ground surface temperatures. Three bedrock temperature monitoring sites with shallow boreholes (40cm) equipped with three sensors per site at each of the three mountain flanks (2870m asl) were established. Additionally, two ground surface temperature monitoring sites were installed at the summit.&lt;/p&gt;&lt;p&gt;Results show remarkable differences in mean annual ground temperatures (MAGT) between the 23 different sensors and the two years despite the small spatial extent (0.023 km&amp;#178;) and elevation differences (46m). Intersite variability at the entire mountain pyramid was 3.74&amp;#176;C in 2018/19 (mean MAGT: -0.40&amp;#176;C; minimum: -1.78&amp;#176;C; maximum: 1.96&amp;#176;C;) and 3.27&amp;#176;C in 2019/20 (mean MAGT: 0.08&amp;#176;C; minimum: -1.54&amp;#176;C; maximum: 1,73&amp;#176;C;). Minimum was in both years at the northeast-facing flank, maximum at the south-facing flank. In all but three sites, the second monitoring year was warmer than the first one (mean +0.48&amp;#176;C) related to atmospheric differences and site-specific snow conditions. The comparison of the MAGT-values of the two years (MAGT-2018/19 minus MAGT-2019/20) revealed large thermal inhomogeneities in the mountain summit ranging from +0.65&amp;#176; (2018/19 warmer than 2019/20) to -1.76&amp;#176;C (2018/19 colder than 2019/20) at identical sensors. Temperature ranges at the three different aspects but at equal elevations were 1.7-2.2&amp;#176;C at ridges and 1.8-3.7&amp;#176;C at flanks for single years. The higher temperature range for flank-sites is related to seasonal snow cover effects combined with higher radiation at sun-exposed sites. Although the ground temperature was substantially higher in the second year, the snow cover difference between the two years was variable. Some sites experienced longer snow cover periods in the second year 2019/20 (up to +85 days) whereas at other sites the opposite was observed (up to -85 days). Other frost weathering-related indicators (diurnal freeze-thaw cycles, frost-cracking window) show also large intersite and interannual differences.&lt;/p&gt;&lt;p&gt;Our study shows that the thermal regime at a triangle-shaped moderately steep pyramidal peak is very heterogeneous between different aspects and landforms (ridge/flank/summit) and between two monitoring years confirming earlier monitoring and modelling results. Due to high intersite and interannual variabilities, temperature-related processes such as frost-weathering can vary largely between neighbouring sites. Our study highlights the need for systematic and long-term ground temperature monitoring in alpine terrain to improve the understanding of small- to medium-scale temperature variabilities.&lt;/p&gt;

Geophysical monitoring of temperature variation and water movement in a frost weathering cave

&lt;p&gt;One third of the caves in the north-eastern part of the Eastern Alps are assumed to be created by frost weathering. The geomorphological process of frost weathering is linked to temperature variations around the freezing point and a sufficient amount of water in the inner of a rock. Fractured areas are highly sensitive to frost weathering and are characterised by high variations of temperature and water content. Geophysical electrical methods are widely used to monitor variations of temperature and water content with time, considering the sensitivity of the electrical resistivity to both properties. In this study, we present imaging results for electrical monitoring conducted in February 2020 in the ceiling of the Untere Traisenbacherh&amp;#246;hle, a frost weathering cave, which is located in the foothills of the Eastern Calcareous Alps. In total, 77 imaging measurements were conducted during the monitoring period of approximately 60 hours with an electrode separation of about 10 cm to gain data with high temporal and high spatial resolution during and after a raining event. Simultaneously, temperature was measured at one point in different rock depths. Geophysical data was pre-processed by a four-step filtering procedure to identify and remove spatial and temporal outliers. Then the data was inverted with the open-source library Pybert. Inversion results reveal that during the entire monitoring the resistivity varies up to &amp;#177;30% compared with the values at the start of the monitoring. To investigate in more detail the temporal changes, we extracted pixel values in 16 areas. These pixels show a strong negative linear correlation with the temperature (correlation coefficients up to 99%), which ranges between 2 &amp;#176;C and 8 &amp;#176;C. However, in some areas a simple linear model seems to not represent the relationship of both parameters in the low temperature range adequately. Based on such correlation, the resistivity data was temperature-corrected to investigate water content changes affecting the resistivity of the ceiling of the cave. Such analysis permitted to delineate clear anomalies related to water seeping into the rock as well as drying processes at the inner parts of the rock wall. Further, geophysical measurements were conducted by means of the low-induction number electromagnetic method in June 2020 to evaluate the applicability of this to map the entire rock wall of the cave. Electrical resistivity (ERT) data differs strongly from the low-induction number electromagnetic (EMI) mapping, likely because of a contamination of the EMI data due to the presence of the metallic electrodes used for the ERT monitoring and due to different weather conditions. Our study reveals the possibility to quantify water content changes in caves in an imaging framework. Further, this information can be used to delineate fractured zones in carbonate rocks, which are supposed to be more sensitive to frost weathering.&lt;/p&gt;

Topographic controls on frost and thermal weathering processes and implications for rockwall erosion

&lt;p&gt;Mechanical weathering by freezing and thermal processes are influenced by climate. Topography modulates this climatic influence due to altitudinal decrease of temperature, modifying insolation due to rockwall aspects and insulation by snow cover. In this study, we (i) quantify rock fracture damage in the field, (ii) monitor rock surface temperature and snow cover, (iii) model frost weathering processes, (iv) quantify fracture kinematics and (v) assess how these processes contribute to rockwall erosion. For this purpose, we conducted measurements on rockwalls with different aspects along an altitudinal gradient ranging from 2,500 to 3,200 m in the Hungerli Valley, Swiss Alps, between 2016 and 2019.&lt;/p&gt;&lt;p&gt;(i) The geology of the Hungerli Valley comprises schisty quartz slate with inclusions of aplite and amphibolite. We conducted Rock Mass Strength (RMS) measurements and used fracture spacing and uniaxial compressive strength (UCS) measurements as proxies for mechanical weathering. RMS ranges from 62 to 77 for schisty quartz slate rockwalls, up to 73 for aplite and 74 for amphibolite. Fracture spacing and UCS reflect lithological differences of the catchment area suggesting a geological control on weathering efficacy.&amp;#160;&lt;/p&gt;&lt;p&gt;(ii) Rock surface temperatures (RST) were monitored using temperature loggers. RST decreases with elevation from 2,500 to 2,900 m, however, increases again at 3,150 m potentially due to higher insolation on ridges. Snow cover duration shows a similar altitudinal trend. Due to aspect, RSTs are 2 to 4 &amp;#176;C warmer on south facing rockwalls with significant shorter snow cover period.&lt;/p&gt;&lt;p&gt;(iii) We used measured RST to drive frost cracking models by Walder and Hallet (1985) and Rempel et al. (2016). Both models show near surface frost weathering at lower altitudes, which should results in lower UCS. The models show significantly higher frost cracking at higher altitudes with peaks at rock depths between 0.5 and 2 m suggesting a higher fracture spacing.&lt;/p&gt;&lt;p&gt;(iv) Rockwalls between 2,500 and 2,900 m were equipped with crackmeters and show higher daily temperature changes and crack deformation at lower altitudes or south facing aspects due to higher insolation compared to higher located rockwalls. Seasonal crack displacement depends on dipping of monitored blocks and is controlled by both thermal and cryogenic processes (Draebing, 2020).&lt;/p&gt;&lt;p&gt;(v) In summary, low-altitudinal rockwalls show a higher weathering at the surface due to a combination of thermal processes and near surface frost weathering resulting in release of small blocks and lower erosion rates. In contrast, rockwalls at higher altitudes reveal higher seasonal thermal changes propagating deeper into the rock in combination with frost cracking in higher depths, which results in larger blocks and higher erosion rates.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Draebing, D.: Identification of rock and fracture kinematics in high Alpine rockwalls under the influence of altitude, Earth Surf. Dynam. Discuss., 1-31, 2020.&lt;/p&gt;&lt;p&gt;Rempel, A. W., Marshall, J. A., &amp; Roering, J. J.: Modeling relative frost weathering rates at geomorphic scales. Earth and Planetary Science Letters, 453, 87-95, 2016.&lt;/p&gt;&lt;p&gt;Walder, J., and Hallet, B.: A Theoretical-model of the fracture of rock during freezing, Geological Society of America Bulletin, 96, 336-346, 1985.&lt;/p&gt;

Block fields and block streams deposits in the Pirin Mountains

The block fields and block streams are widespread in the Pirin Mountains. They are significant in size and are distributed on both relatively flat and sloping surfaces. Their origin is associated with frost weathering, which can subsequently be supplemented by other processes related to the destruction of the material – chemical weathering, fluvial, cryogenic processes, etc. The main aim of the study is to analyze the block field and block stream deposits which will give us a better understanding of their origin and evolution.