Low altitude permafrost from temperate regions: response to climate variability and environmental drivers – FrozenCORE

Permafrost is ground which remains at or below 0°C for at least two consecutive years (van Everdingen, 1998), while mountain permafrost refers to its position in mountain areas in opposition to latitudinal permafrost. The study of mountain permafrost is a relatively young research field (Haeberli et al., 2010). Discontinuous mountain permafrost is found on a broader spatial scale in the interior of the mean annual air temperature (MAAT) isotherm of -3°C. However, significant areas of permafrost can also be found down to the -1°C MAAT isotherm (Schwindt, 2013). Outside this climatic zone, but still close to the upper treeline, only sporadic or isolated patches of permafrost can be encountered (Kneisel et al., 2000). Recently, it was discovered that perennially frozen ground can be found well below the regional altitudinal limit of discontinuous mountain permafrost in particular local settings that are still insufficiently understood in the present. At a few sites with low altitudes, down to 500 m asl (Bohemian Mountains), 770 m asl (Swiss Alps) or 1050 m asl (Romanian Carpathians) corresponding to MAAT above +7°C, porous talus slopes with persistent underground ice deposits can host extrazonal permafrost (Gude et al., 2003; Morard et al., 2010; Popescu et al., 2017).

Previous research focused on documenting the permafrost occurrence at these “frozen screes” and on explaining the mechanisms causing such atypical phenomenon. Continuous thermal monitoring revealed large negative annual thermal anomalies (ATA, understood as the difference between mean annual ground surface temperature (MAGST) and MAAT) in the lower parts of the talus slopes and positive ATA in the upper sectors (Delaloye et al., 2003; Raska et al., 2011). Morard et al (2008) performed a detailed thermal monitoring across the units of a talus slope – relict rock glacier system and Morard et al (2010) presented multiannual thermal data and ATA variability at numerous cold screes from Switzerland. Geophysical investigations (electrical resistivity tomography – ERT) were performed in order to document the permafrost presence in talus slopes close to the timberline at medium altitude (Kneisel et al., 2000) and at low altitude (Stiegler et al., 2014; Popescu et al., 2017). The favoring effect of thick and humid peat moss layer development on talus thermal regime and permafrost development was also investigated (Schwindt, 2013). Short term and shallow permafrost was proved to exist at a cold talus slope from the Swiss Alps based on multiannual continuous thermal monitoring in boreholes at different depths (Morard et al., 2010). Most of the studies indicated that the so called “chimney effect” is the main mechanism of such large negative ATA (to -7°C and even more) and extrazonal permafrost preservation. Chimney circulation consists in cold air suction during winter through the lower parts of the talus induced by a pressure low that is caused by “warm” air exist in the upper parts of the talus slope. This can occur even through a thick and dry snow cover (Morard et al., 2010). The “cold reservoir” is thus formed in the cold season resulting in the ground ice buildup especially in the early springtime. In the warm season, the cold air from the debris descends and exit through the lower parts of the talus while warm air is absorbed in the upper parts of the scree. Consequently, the cold conditions persist in the lower parts for the entire year allowing for a frozen core to persist in some years for the entire summer.

Despite the main mechanisms and causes of chimney circulation being known, some aspects and thresholds values for the favoring parameters are still unknown. So far there were rare direct air currents measurements on chimney circulation with inconclusive results (Lambiel, 2006) and, as a consequence, some issues are still insufficiently known. For example, the cold air suction and the cold air exhalation areas from the lower part of a talus overlap completely or only partially? The warm and cold areas are completely static, migrate intra-seasonal or expand and shrink around an “epicenter” where the air circulation is the most intense? Also, besides the general assumption that chimney circulation weakens during summer, the variation in intensity of chimney circulation and its drivers are unknown. Another question is related to the typology of circulation at the beginning of the springtime and during fall when the air flow reverses. Is there a transition period when the circulation flow is interchangeable? What are the threshold values for atmospheric and ground temperature and pressure for these phases to switch? What is the influence of topography in the distribution of thermal units on a complex debris system? So far, only simple morphologies consisting in talus slopes and sometimes small rock glaciers below were investigated. In a complex morphology site like Detunata we found that only a small point could work as a warm exhalation zone (natural chimney) located on the rock glacier ridge. Is there an air speed contrast between that point and the other (probably more diffuse) evacuation area located in the upper half of the talus slope? In Popescu et al., 2017a it was show that debris porosity triggers the ground air circulation but could there be established some relationships between surface debris texture and air speed? Similarly, we found that the slope threshold of 25° is not necessary for chimney effect to occur as previously assumed and slope down to 5° can allow it. But is there any correlation between slope and ground air speed (efficiency) or the length of cold air adsorption and exhalation periods or not? Regarding slope morphometry, even though it was suggested that concave foot slopes are more favorable to cold conditions (Raska et al., 2011), this hypothesis was poorly supported with data, so, I intend to investigate the relation between slope profile (concave, linear, convex) and thermal efficiency. The investigations proposed in this project are much more detailed and accurate in comparison to those used in the study published in 2017: the high accuracy digital elevation model (DEM) will replace the simple topographic profiles and the complex aims described above will be tested using the dense network of thermal sensors, more extensive ERT profiles, more smoke air tracing experiments and adding new instruments like ultrasonic anemometers and pressure sensors. Drilling will allow the verification of permafrost presence, will allow the investigations of the frozen ground thickness fluctuation from year to year and thus the impact of climate change which is a completely new field of study in Romania. Dendrologic investigations performed in Popescu et al., 2017 indicated the long term inverse relation between tree growth rates and mean annual air temperature. This project could allow testing this finding in other sites by analyzing the samples already taken from a few other sites like Toteisboden (Austria) (Stiegler et al., 2014), Klic (Bohemian Mountains) (Gude et al., 2003), Stredehori (Czech Republic) Raska et al., 2011 and also to take new ones and to compare the growth rates with meterological data available online. Thus, this project has an intensive measurements component (in Detunata site) and an inter-sites investigations and comparisons component both for a better understanding of the process and its variability in manifestations.

Conceptual model of the seasonal chimney circulation in a TS–RG (talus slope and– rock glacier) system and the corresponding cold and warm units based on thermal anomalies during the cold season (A) and the warm season (B) (Popescu et al., 2017).

The importance of the project from a scientific point of view results from the nature of permafrost as one of the 13 terrestrial essential climate variables (ECVs) whose monitoring helps understanding the cumulated effects of climate impact on the environment. Also, it could provide precious information regarding the past environmental changes either by ice and organic matter dating (the permafrost functions as a palaeoclimatic archive) or by dendrological investigations (tree-ring widths evolution) which may highlight the large amplitudes in ground air circulation induced by climate variability (Popescu et al., 2017). From socio-economic perspective, low altitude permafrost sites could and should be organized as geomorphosites and to be both protected and promoted for visiting. The spectacular nature of these sites comes from the unusual summer ice occurrence, cold air blowing and the dwarf trees which are in fact centuries old. From this perspective, this project intends to make an inventory of other potential sites with cold screes and low altitude permafrost by remote sensing analysis and field visits. From a cultural point of view, these sites usually are well fixed in the collective memory of the people because they are most of the times spectacular sites and starting points from local legends.

The difficulty elements of studying low altitude permafrost and associated processes concern i) the instruments protection in the field from potential malicious persons; ii) the placing of the instruments at the right position in the field in order to capture the manifestation of some processes occurring sometimes on short periods of time; iii) the laborious and complex field investigations like ERT that is difficult to perform on such rough terrain that inhibits the connection between electrodes and the ground and especially the borehole drillings that necessitate bringing heavy equipment in the site, proper facilities for storing frozen samples before thawing. The current approach is field oriented and the ambitious objectives necessitate long periods of continuous and careful investigations at least at certain phases of the projects. The limitation of the current approach in relation to the state of the art is related to the lack of a 3D modeling of subsurface processes (air flow and temperature, ice buildup) in relation to debris porosity and structure, topography and vegetation.

O1: Advancing the knowledge on the ground air circulation (and subsurface permafrost changes) in relation to environmental drivers

We previously proved with the use of thermal measurements and smoke air experiments the occurrence of an intense ground air circulation; the specific thermal regimes of cold air absorption zones with large negative ATA and of warm air evacuation zones with positive ATA were also presented together with other relevant site observations and measurements during both the cold and warm seasons (Popescu et al., 2017a). Till present, the ground air currents were directly monitored using anemometers only in one site (Dreveneuse, Swiss Alps) but for a too short period of time (including instruments failure in the field) which did not allow for significant conclusions to be drawn (Lambiel, 2006). In these conditions, we plan to install several ultrasonic 3D-anemometers with high sensitivity and accuracy in order to monitor both aspiration and exhalation during the cold and warm seasons and to identify ground air circulation patterns in relation to temperature offset (between atmosphere and ground temperatures), talus slope morphometry and debris texture. Detunata is a perfect site as a natural laboratory to study the ground air circulation in relation to ATA variability and already mentioned controlling factors due to its complex and rare morphology (talus slope – rock glacier system) expanded on a large surface and to regular basaltic clasts imposing extraordinary high porosities (Popescu et al, 2017a). Within the current project we expect to bring a substantial progress for the global understanding of the mechanisms governing the chimney circulation at low altitude permafrost sites by using accurate and detailed DEM, smoke experiments, ultrasonic anemometers and thermal sensors. In addition, several BTS campaigns will be performed during winter to capture the hypothesized thermal zones expansion and contraction. Intensive ERT survey will be performed in order to detect the perfect site for borehole.

O2: Starting a long-term temperature monitoring program of the permafrost and cold reservoir at Detunata site as a key site for monitoring the climate variability impact on cold screes;

Mountain permafrost is usually investigated by indirect measurements, with thermal and geophysical methods but the ideal case is represented by drilling. The latter provide direct evidence of permafrost and offer a unique opportunity to get insight of permafrost characteristics that cannot be otherwise known: temperature profile, active layer depth, zero annual amplitude depth, ice content and mean clast-size. Moreover, by installing thermistors arrays in a sealed and stabilized borehole, one can monitor the permafrost changes (seasonal, annual, and multiannual) and the impact of climate variability. Even though we know how temperature, precipitation, snow cover and other meteorological variables evolve in time by accurate measurements, we cannot precisely estimate the impact on ground thermal regime which is the result of the complex interplay between the meteorological factors, land cover types, topography and vegetation. That is why permafrost is one of the 13 terrestrial essential climate variables (ECVs) and why the Global Terrestrial Network for Permafrost (GTN-P) was developed in 1990s by the International Permafrost Association with the “long-term goal of obtaining a comprehensive view of the spatial structure, trends and variability of changes in the active layer thickness and permafrost temperature”. Thus, taking the opportunity of relatively accessible Detunata scree, we aim to perform the first borehole in permafrost in the Carpathians (and in the Eastern Europe) as a contribution of Romania to the GTN-P data base and in a relatively rare geologic feature (rectangular basaltic columns deposit) and complex geomorphologic landform (talus slope-rock glacier system). One or two drillings will be performed and the stratigraphy will be described in situ in order to infer the formation and palaeoevolution of the deposit and to draw palaeoclimatic information later on. Further, terrestrial cosmogenic nuclides (TCN) datings will be performed in order to determine the absolute age of the landform. The borehole(s) will be equipped with thermistors array connected to a main logger at the ground surface.

O3: Inter-site analysis and comparison of multiple LAP sites

Several studies have concentrated on documenting the permafrost occurrence at low altitude cold screes but only a few attempted to look comparatively at several locations (e.g. Morard, 2011). Thus, the causes that determine different thermal anomalies in cold screes remain insufficiently understood. In this context, we plan to test our hypothesis that lithology via debris texture and porosity is the key factor in establishing the magnitude of thermal anomalies. Our preliminary inventory of cold screes already indicated that large ATAs can be found on basalts, granites and marbles (-6…-9.1°C), medium ATA on limestone (-4.06… -5.25°C) and reduced ATA on crystalline schist (> -3.05°C). Thus, we plan to assess the influence of different rock types and topography on ATA by several investigations at several cold screes from Central Europe by: (i) detailed DEMs (basic morphometry), (ii) clast-size and the surface debris porosity measurements and (iii) determination of the mechanical (hardness, joint network, cleavage) and thermal (conductivity) properties of rocks. Identification and analyzing of other cold screes susceptible for permafrost preservation in different lithological contexts from Romanian Carpathians is also intended as an original component of the project.

The second inter-site analysis will be focused on dendrologic investigations. We previously proposed a method to evaluate the climate fluctuations impact on low altitude permafrost sites thermal regime by dendrogeomorphology (tree ring width analysis; Popescu et al., 2017a) showing that during colder time intervals the cold scree environments are slightly warmer and the dwarf trees have a better growth and vice versa. We now intend to test the validity of this finding in other cold screes sites by analyzing the collected samples and by collecting and analyzing new ones. The dendrologic series could be also used to document natural events impact on climate change like major volcanic eruptions around the world from the last centuries.