C51C-01
The rich contributions of A.L. Washburn to permafrost and periglacial studies
Albert Lincoln Washburn, (1911-2007) contributed immensely to the fields of periglacial geomorphology and permafrost research through his studies, publications, leadership, and personal qualities. Link's pioneering discoveries about processes shaping Arctic landscapes are presented in numerous, carefully crafted articles, monographs, and authoritative books that are recognized worldwide. His deep interest in the "intimate workings of nature" is evident in his own meticulous and exhaustive field and laboratory studies of diverse types of periglacial processes. Here, we focus on these productive field studies in Greenland and on Cornwallis Island, and on his research in the Periglacial Laboratory at the University of Washington. The field studies include numerous, detailed surface observations and excavations of diverse types of patterned ground, and extensive long term studies of surface displacements to define quantitatively the kinematics of patterned ground activity, and slope movement. The laboratory work focused on the effects of ice growth/thaw and frost heaving/thaw settling on solifluction, vertical textural segregation, and the development of deformational structures in layered soils. Link's deep interest periglacial processes and his untiring, generous effort to properly present the work and ideas of other researchers are clearly evident in his monographs and renowned books that remain the standard references in the field decades after their publication (Periglacial Processes and Environments, 1973; Geocryology — A survey of periglacial processes and environments, 1979). In addition, Link launched and directed research centers and scientific journals that have nurtured the study of cold landscapes. Significantly, his scientific legacy also includes an exceptional, more personal dimension: the emergence of the unusually cohesive permafrost and periglacial community, as it was fostered in part by Link's and, his wife Tahoe's, warmth, generosity, and friendship that touched many members of this worldwide community.
C51C-02 INVITED
Physical Modelling of Rock Heave and Fracture by ice Segregation
The volumetric expansion of freezing pore water is widely assumed to be a major cause of rock fracture in polar and alpine regions. Physical modelling experiments that simulated natural freezing regimes in large blocks of chalk indicate that fracture results instead from ice segregation. This process was most pronounced in summer, as the rock active layer thawed, releasing meltwater that moved down into the underlying permafrost, where it supplied ice lenses that fractured the rock and heaved up the surface. Progressive heave was interrupted by the European heatwave of summer 2003, which triggered 10 mm settlement of the rock surface as ice lenses melted within the uppermost permafrost. During the following freeze-thaw cycles the ice lenses quickly started to grow again. After simulation of about 20 cycles of active-layer freezing and thawing, fractures in the artificial permafrost matched those in Arctic permafrost and ice-age weathering profiles in mid-latitude Europe. This agreement supports a conceptual model in which ice segregation in near-surface permafrost leads progressively to rock fracture and heave, whereas active-layer deepening leads episodically to melt of segregated ice and rock settlement. Fractured limestones, shales and clays in southern England suggest that ice segregation has been a key driver of landscape evolution during Pleistocene periglacial episodes.
C51C-03
Ice Lenses in Colloidal Clays
A mechanism for the formation of ice lenses in colloidal clays is proposed based on an instability of the ice-clay interface. A mathematical model of freezing saturated montmorillonite is developed which predicts that, given freezing conditions typical of the field, the clay can become thermodynamically supercooled in the same sense as a binary solution such as salt water. A stability analysis predicts that in this case the ice-clay interface is unstable to the formation of a mixed-phase region composed of ice lenses and unfrozen clay. Experimental evidence in support of the theory is discussed and a continuum model of the mixed-phase region is proposed.
C51C-04
Solifluction Processes in Seasonally Frozen Ground, Dovrefjell, Norway
Continuous monitoring of soil temperatures, frost heave, thaw consolidation, pore water pressures and down slope soil movements are reported from a turf-banked solifluction lobe at Seinhoi, Dovrefjell, Norway from August 2002 to August 2006. Mean annual air temperatures over the monitored period were slightly below 0° C, but mean annual ground surface temperatures were around 2° C warmer, due to the insulating effects of snow cover. Winter frost penetration was around 30 cm at the monitoring station, so that the site may be considered close to the limit of active solifluction in this area. Average rates of downs slope surface movement over the four year period 2002-2006 ranged from 0.5 cm yr-1 at the rear of the lobe tread to 1.6 cm yr-1 just behind the lobe front, with corresponding soil transport rates of 5.9 cm3 cm-1 yr-1 and 45.7 cm3 cm-1 yr-1. Pore water pressure measurements indicated seepage of snowmelt beneath the seasonally frozen ground in spring, with artesian pressures beneath the confining layer. Soil thawing was associated with surface settlement and down slope soil displacements. Further surface settlement in early summer was accompanied by retrograde movements. Down slope surface displacements exceeded maximum potential frost creep values and it is concluded that gelifluction was an important component of slow near-surface mass movements at this site. Temporal and spatial variations in solifluction rates across the area are likely to be considerable, and are strongly influenced by snow distribution and thickness.
C51C-05
Sensitivity of Ice-Rich Antarctic Slopes to Climate Change: Are Terrestrial Archives at Risk?
We modeled the sensitivity of ice-rich slope deposits from the western McMurdo Dry Valleys, Antarctica to failure by shallow, thaw-induced planar sliding. The deposits examined have purportedly remained physically stable, without morphologic evidence for downslope movement, for millions of years. Could they fail in the near future from greenhouse-induced warming? To address this question, we first prescribed various increases in mean summertime soil surface temperature (MSSST) and modeled numerically the resultant changes in soil thaw depths using a one-dimensional heat diffusion equation. Second, we calculated the minimum thaw depths required to facilitate failure by shallow planar sliding for each deposit; for all numerical simulations, we maintained present soil-moisture conditions and used a Mohr-Coulomb-based equation of safety factor. Third, we calculated the flow rate of subsurface meltwater assuming Darcy's Law. Our results show that some ice-rich slopes could fail if MSSST, and by inference mean summertime atmospheric temperatures, increase by 5° to 9° C. If we assume that physical evidence for shallow planar slides would be retained in the geomorphic record, and that current soil-moisture conditions can be applied to slope deposits in the distant past (i.e., millions of years ago), then our results suggest that since late Miocene time (the age of the youngest deposits examined) atmospheric temperatures could not have increased by more than 5° to 9° C above present values.
C51C-06
Blockfields of Neogene origin: Challenging the paradigm
The prevailing paradigm for cold-climate in situ blockfields is that they are remnants of Neogene deep weathering profiles. This opinion is frequently based on the presence of large quantities of interstitial silt and clay and/or the presence of clay minerals, such as gibbsite and kaolinite. Using in situ-produced cosmogenic isotopes 10Be and 26Al, XRD, and XRF to study blockfield regolith in the northern Swedish mountains, we challenge this paradigm. Incorporating surface burial by ice sheets, the isostatic response to ice sheet loading and unloading, and subaerial surface erosion, the cosmogenic data indicate that the regolith has been accumulating nuclides for up to 464.5 ka. The ubiquitous presence of chlorite makes it impossible to distinguish kaolinite according to standard XRD techniques. However, gibbsite is present in glacial till in addition to wet- location blockfield regolith. Coupled with the ubiquitous presence of poorly crystallized hydroxides, vermiculization in wet-locations, and an absence of smectite, incipient chemical weathering is indicated. Furthermore, XRF data indicate dominance of the interstitial fine matrix by a foreign component, likely of aeolian origin. All of our observations can be explained by processes operating within the Quaternary timeframe. Because we do not need to appeal to Neogene deep weathering to account for the characteristics of blockfields in the northern Swedish mountains we conclude that these blockfields may have Quaternary origins.
C51C-07
River Response to Deglaciation: a Case-Study of Clyde Fjordhead, Baffin Island, Arctic Canada
River deposits form archives of past dynamics of a number of environmental controls; drainage basin characteristics, climate and sea-level changes. This paper explores long-term river sedimentation based on Late Quaternary deposition in Clyde Inlet on Baffin Island as constrained by 11 cosmogenic and 9 AMS 14C datings. Clyde River drains part of the remaining land ice of Barnes Ice Cap, making it a rare case of a basin in the most advanced stages of deglaciation. A major ice stream of the Laurentide Ice Sheet occupied Clyde Inlet at the Last Glacial Maximum and carved through the U-shaped valley creating a distinct sediment sink. At Early Holocene the system entered deglacial stage; tidewater glaciers retreated rapidly (>100 km in 1000 yrs) through the fjord from 10,4 ka onwards. Grounded ice lobes started retreating from Clyde fjordhead by 9,4 ka coinciding with warm climatic conditions. Then ice contact fans (ICF's) were deposited consisting of flat-topped fan deltas covered with boulder channels and bars. Elevations of their surfaces nowadays vary between 62-77 m above sea level, which is considered to be near contemporaneous sea level. Marine muds have been draped directly onto the ICF complexes and define the minimum marine limit at > 58 m asl. Subsequently, coarse-grained glacio-fluvial valley trains (GVFT's) and deltas were deposited during forced regression. 14C dated bivalves show that relative sea level dropped ~20 m/ka between 9-7 ka. Surfaces of the GVFT's dip downward caused by dominant sea-level fall, despite high sediment supply. At the latest Holocene (3,5 ka) the last remaining lobes of the Laurentide Ice Sheet retreated from the middle parts of Clyde River basin to form the present Barnes Ice Cap. At this phase, strong river incision, despite an order of magnitude slower base-level fall (~1,6 m/ka over the last 3,5 ka), marks reduced sediment supply. Clyde River Basin may not have entered a post-glacial stage by definition, nevertheless the strongly reduced sediment flux is apparent. We speculate that the glacial regime of the retreating proto Barnes Ice Cap switched from a dominantly sediment-producing regime to a non-erosive, cold-based regime. The reconstructed timing of river deposition and incision was compared with climate–driven numerical modeling predictions of water and sediment flux for Clyde River over the Holocene and further supports the dominance of glacial regime and isostatic rebound on river sedimentation.
C51C-08
Influcence of Initial Conditions on Permafrost Localization
Permafrost is a long-term phenomenon and often formed by freezing conditions over hundreds f year or millennia. Modeling its distribution and degradation is challenging because, due to its high thermal inertia, it responds slowly to temperature forcing. Nevertheless a proper assessment of permafrost presence is very relevant to any study which involves climate change and process oriented models have been recently applied to detect the permafrost presence in alpine areas. The objective of this paper is the localization and analysis of permafrost distribution using the model GEOtop (Rigon at al., 2006) in the case of an alpine basin. GEOtop is a distributed hydrological model that jointly solves the water and energy budgets within a catchment and considers the dependencies of temperature and water content on soil thermal and hydraulic dynamics. One of the main difficulties, besides having the appropriate model, lies in the derivation of the initial conditions of temperature and ice content in the ground. Especially during phase change, the response of temperatures to surface signals is null, and, if averaged in the area, slow. Furthermore, surface and ground temperature could be very different from air temperature, and therefore it is difficult to properly initialize the temperature profile of the model domain. In fact, an unrealistic temperature initialization may take several hundred years (of simulated time) to equilibrate with surface conditions. In this case, it becomes difficult (if not impossible) to distinguish between real transient effects represented by a model and the delayed response to an initialization that did not correspond to real conditions. Current methodologies used in climate and hydrological models tend to use simplified initial conditions of both ground temperature and ice content, e.g. constant temperature profile and zero ice content, and then spin the model for several years until a steady condition has been set. This approach, however, is highly computationally expensive and eventually does not assure to gain a realistic profile. In the present work, the initial conditions for the temperature are provided by the analytical solutions of the linearized heat equation, taking into account the mean annual air temperature (MAAT), as derived from accurate kriging of 15 years of hourly temperatures from nine meteo station, and the geothermal gradient . The initial conditions for the ice content take into account the topographical considerations and the temperature profile previously computed. The sensitivity of the model, with respect to various initial conditions, has been tested in the Adamello Presanella Group (Rhaetian Alps, Italy). The results are verified by a DC resistivity survey, GIS technique using updated glacier map with little ice age expansion and empirical topoclimatic approaches in order to infer the correlation between permafrost inferior limit and initial conditions.