| dc.description.abstract | Ice bodies comes in many forms and sizes in various locations and at different altitudes. The smallest ice bodies (<0.5km2), often referred to as ice patches, glacierets and perennial snow patches are numerous in many alpine areas of the world and are frequently found below regional glaciation thresholds. Despite being numerous, they are rarely included in glacier inventories and seldom subject to individual investigations. Ice patches are interesting components of the cryosphere because they survive at low altitudes despite limited size, and because they can be surprisingly old. Well preserved archeologic artefacts dating back over 5000 years, melting out of ice patches during the last decades have introduced a glaciological conundrum, as preservation of organic artefacts would require near constant ice burial, indicating that the ice itself must be equally old. Understanding ice patch characteristics and their temporal variations from a glaciological stand point is a first step towards answering these questions.
To achieve this goal, field work was conducted on two ice patches and in a pro-ice patch lake on the Dovre-Oppdal mountain plateau in Southern Norway. Detailed geodetic mass balance measurements were conducted on Kringsoll and Storbreen ice patch using a high-resolution terrestrial laser scanner (TLS) over two balance years. In addition, lichenometry measurements (measuring the coverage of lichen) was performed in the ice patches proximal areas and mapping of small scale geomorphological landforms was conducted to evaluate the ice patches previous extent and role as geomorphic agents. Further, internal ice patch debris layers protruding the ice patch surface was sampled and carbon dated to create an age-depth model of Kringsoll ice patch to attempt to establish a minimum ice patch age and analyse temporal ice patch variations. Finally, lake sediment analysis was applied to the pro-ice patch lake Leirtjønna to investigate whether the lacustrine sediment cores contained signals of Holocene ice patch variability, and to interpret that signal.
The TLS data show that the ice patches have large volume variations within the two balance years in study, relative to their mass, as well as their continental setting. The mass balance is largely controlled by topo-climatic factors such as accumulation by wind drift and topographic shade introducing positive and negative feedback mechanisms. Accumulation and ablation patterns were chaotic and no traditional mass balance gradient or equilibrium line altitude (ELA) could be determined. Most of the ice patch surface functioned either as net accumulation or ablation zones during the accumulation and ablation season respectively.
Small fissures in the ice surface, curvature and tilt of debris layers and ice creep calculations suggests small scale internal deformation within the ice. The nature of lichen cover was very limited in the immediate ice patch proximal area, likely strongly affected by the snow kill effect inhabiting lichen from growing. A sharp transition from a sparsely colonized area to a surface where lichens were abruptly very large (>200mm) and mostly merged together was observed and interpreted to represent the ice patch maximum extent during the Little Ice Age. Several small-scale geomorphological landforms were observed in the ice patches marginal zones. These include “scratch-marks” (small scale striation), “mini” flutes, polished bedrock, “push-stones”, pavements, small scale fluvial channels and periglacial landforms such as solifluction.
Radio carbon dating of macro samples from individual debris layers on Kringsoll ice patch revealed an age-depth model with successively older layer age with ice depth. The lowermost debris layer, interpreted to represent an ice patch minimum age, was dated to around 5400 cal BP. Two other main periods were identified interpreted to reflect ice patch growth around 2000 cal BP and 900 cal BP. Results from the sediment core analysis suggests that the presence of an ice patch in a catchment produces a sediment signal in the downstream lake. We propose that the sediment signal originate from aeolian (wind derived) debris accumulated on the patch surface, and weathered material in the ice patch proximal area which is released and transported by seasonal melt water from the ice patch and its proximal area and subject to re-sedimentation in the lake. By interpreting the sediment stratigraphy, deglaciation of the catchment was estimated to around 13000 cal yr BP and an ice patch onset after the Holocene thermal optimum (HTM) to 5700 cal yr BP. Onset of the Little Ice Age (LIA) climate event was estimated to around 500 cal yr BP.
The TLS data revealed the lack of mass surplus and deficit areas on the ice patch surface, which eradicate the need for mass transfer from higher to lower levels needed to maintain quasi-steady topography of the ice patch over time. Without an efficient ice flux, the ice becomes largely stagnant enabling ice to be maintained within the ice patch for long time periods thus partially explaining the ice patches ability to preserve and protect archaeological artefact over long time scales. Although ice creep is most likely present as exemplified by the curved debris layers and ice fissures, deformation rates are considered low ensuring that it is not self-contradictory to find both old well-preserved artefacts and signs of deformation on the ice patches. The existence of old ice is further supported by the ice patch age-depth model confirming the longevity and continuous existence of Kringsoll ice patch over the last 5000 years. Both the ice patch debris layers and lacustrine sediment cores datasets suggests an estimated ice patch onset around 5500 cal yr BP. The general datasets also largely correspond with previous research on Holocene climate- and glacier variation in southern Norway.
Despite evidence suggesting the absence of an ice flux, the geomorphological evidence implies that the ice patches are in fact geomorphologically active, and thus the occurrence of basal slide. Although reported to have a cold temperature regime, supraglacial melt channels without a distinct outlet and the englacial debris layers might allow meltwater to reach the ice patch base, where a thin water film is possibly maintained due to the effect of latent heat release. The ice patches generally shallow and variable thickness may further allow repeated freeze thaw cycles at the base providing initial conditions for basal slide and abrasion.
The evidence presented in this thesis suggest that ice patches are stable features emerging on specific scales in time and space. Their mass balance characteristics alters the properties of the ice mass sufficiently to create features with high climate resilience and attributes that differ from those associated with large glaciers. This ultimately enables long-term ice patch existence and preservation of old ice while simultaneously showing evidence of geomorphic activity, producing small-scale landforms. The sediment record and debris layer age-depth model strengthen the possible applicability of ice patch proxy data for paleo-climate research. More research to evaluate the methods is however recommended given the singularity of the datasets. Nevertheless, the results and theories presented in this thesis strongly underlines the potential of further research on ice patches as it may hold considerable prospects, not only for glaciology and glacial-geomorphology but also other sub disciplines and research areas, such as paleo-climate, climate change and glacial-archaeology. | nb_NO |
