Oskar Knudsen

Ice fracturing during jökulhlaups: implications for englacial floodwater routing and outlet development.

Theoretical studies of glacial outburst floods (jokulhlaups) assume that: (i) intraglacial floodwater is transported efficiently in isolated conduits; (ii) intraglacial conduit enlargement operates proportionally to increasing discharge; (iii) floodwater exits glaciers through pre-existing ice-marginal outlets; and (iv) the morphology and positioning of outlets remains fixed during flooding. Direct field observations, together with historical jokulhlaup accounts, confirm that these theoretical assumptions are not always correct. This paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jokulhlaups; secondly, it identifies and explains the mechanisms controlling the position and morphology of supraglacial jokulhlaup outlets; and finally, it presents a conceptual model of the controls on supraglacial outbursts. Field observations are presented from two Icelandic glaciers, Skeiðararjokull and Solheimajokull. Video footage and aerial photographs, taken before, during and after the Skeiðararjokull jokulhlaup and immediately after the Solheimajokull jokulhlaup, reveal changes in floodwater routing and the positioning and morphology of outlets. Field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed. Rapid increases in jokulhlaup discharge generate basal hydraulic pressures in excess of ice overburden. Under these circumstances, floodwater can be forced through the surface of glaciers, leading to the development of a range of supraglacial outlets. The rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop. Steady increases in basal hydraulic pressure can retro-feed pre-existing englacial drainage, whereas transient increases in pressure can generate hydraulic fracturing. The position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding. The development of supraglacial jokulhlaup outlets provides a new mechanism for rapid englacial debris entrainment.

Glaciohydraulic supercooling in Iceland

We present evidence of glaciohydraulic supercooling under jokulhlaup and ablation- dominated conditions from two temperate Icelandic glaciers. Observations show that freezing of sediment-laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes. Intraglacial frazil ice propagation under normal ablation-dominated conditions can trap copious volumes of sediment, which forms anomalously thick sections of debris-rich ice. Glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development. Extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling, which may explain the concentration of englacial sediments deposited in Heinrich layers in the North Atlantic during the last glaciation.

Morphology and sedimentology of a giant supraglacial, ice-walled, jökulhlaup channel, Skeiðarárjökull, Iceland: implications for esker genesis

Abstract This paper examines the sedimentary infill of a spectacular, 500-m-long, 100-m-wide ice-walled supraglacial channel, excavated into the snout of Skeiðararjokull, Iceland during the November 1996 jokulhlaup. The ice-walled channel developed in an area of the glacier, which was extensively fractured during the jokulhlaup. Sculpting of the ice-walled channel into the active snout of Skeiðararjokull suggests that the presence of stagnating glacier ice is not a prerequisite for the development of ice-walled channels. The ice-walled channel occupied an inter-lobate location, which acted as a focus for meltwater during the November 1996 jokulhlaup. The geometry of the supraglacial ice-walled channel system acted as a major control on the morphology and sedimentology of jokulhlaup deposits, through the tremendous spatial variability of resultant flow conditions. Maximum calculated jokulhlaup powers and shear stresses for the supraglacial ice-walled channel reached 40,000 W m −2 and 5000 N m −2 , respectively, with associated mean flow velocities between 7 and 11 m s −1 . Within the main ice-walled channel, Ground Penetrating Radar and outcrop exposure provide evidence of an ∼8-m-thick progradational and aggradational gravel macroform succession. The supraglacial ice-walled channel system is therefore analogous to a bedrock-confined fluvial system. This study provides a new analogue for the interpretation of ice-contact glaciofluvial deposits associated with former ice margins in Iceland and other areas subject to high magnitude discharges. Former supraglacial ice-walled channels resulting from tunnel collapse and ice margin break-up during high magnitude jokulhlaups will be associated with extensive coarse-grained, heavily kettled proglacial outwash surfaces. It is clear that the relationship between the characteristics of former ice-walled channels labeled as eskers and the prevailing glaciological and hydrological conditions needs to be modified in light of our knowledge of a modern flood-related large-scale supraglacial channel and its sedimentary infill. Such re-evaluation may provide a valuable new insight into former ice margin positions, modes of glacier retreat, and the role of high magnitude floods within the sedimentary record of former proglacial areas. This study therefore improves our understanding of the meltwater magnitude and frequency regime of former glaciers.

Concertina eskers, Brúarjökull, Iceland: An indicator of surge-type glacier behaviour

Abstract Bruarjokull is a surging outlet lobe of Vatnajokull in SE Iceland. Geological evidence indicates that during quiescent phases, water drainage takes place in ice-walled and ice-roofed channels. As the glacier retreats, the courses of these channels are represented by eskers. Eskers which were englacial prior to surges become deformed during a surge. The deformation of eskers reflects strong longitudinal compression of the ice in the terminal zone. The wavelength of one of the compressed eskers indicates that ice in a 4 km wide margin was compressed by 50% of its original length. Compressed, or concertina eskers, date from the 1963–1964 and the 1890 surges of Bruarjokull. They have not been found from the 1810 surge, indicating that they may not survive if overrun by subsequent surges.

GPR derived architecture of November 1996 jökulhlaup deposits, Skeiðarársandur, Iceland

Abstract Skeiðarársandur in southeastern Iceland, with an area of > 1000 km2, is the world’s largest active proglacial outwash plain. In July–August 2000, a total of over 10 km of ground penetrating radar (GPR) profile data (at 50 MHz and 100 MHz) was collected from a variety of proglacial outwash sediments across the Gígjukvísl channel region of the Skeiðarársandur plain. GPR-profile results and their corresponding facies interpretations are presented for the flood deposits of a single supraglacial outwash fan and its associated source-proximal ice-walled canyon created entirely by the November 1996 jökulhlaup event. By combining the GPR data with ground surveying, photogrammetry and detailed sedimentary outcrop evidence, this study adds a new perspective to the large-scale analysis of single, high-magnitude flood events and the sedimentary record of former, ice-proximal outwash plains. The GPR derived architectures point to a higher degree of sediment reworking than predicted by previous sedimentary models and may provide a useful analogue for the study of sedimentation within similar bedrock fluvial and alluvial fan feeder systems.

A Polygenetic Landform At Stígá, Öræfajökull, Southern Iceland

Abstract Recent research has identified problems inherent in the identification and description of landforms. Morphologically similar small‐scale glacial and periglacial landforms can be misinterpreted, thus hindering environmental reconstruction. This study reveals that a landform resembling a moraine at Stígárjökull, southern Iceland, is the product of both glacial deposition and mass movement. The landform has two distinct morphological and sedimentological components: a basal, lithologically diverse component, and an upper, lithologically homogenous component. Clast lithological analysis, particle shape and particle size measurements demonstrate that the basal component of the landform consists of sediment whose characteristics match nearby moraines. In contrast, the source of the upper component is a narrow outcrop of rock above the valley floor. Evidence suggests that frost‐shattered material was transported across a perennial snow patch to a small moraine, leading to growth of the ‘moraine’. This combination of processes is unlikely to be unique, but the geological peculiarities of the field site permitted their identification. It is possible that many similar ‘moraines’ could be enlarged by subaerial feeding, leading to false reconstruction of glacier form and/or associated rates of erosion and sedimentation. Such polygenetic landform genesis therefore has implications for environmental reconstruction.

Jökulhlaup‐related Ice Fracture and Supraglacial Water Release During the November 1996 Jökulhlaup, Skeiðarárjökull, Iceland

During the initial stages of the November 1996 jökulhlaup at Skeiðarárjökull, Iceland, floodwaters burst onto the glacier surface via a series of fractures. This supraglacial drainage led to the formation of a number of distinct ice surface depressions, one of which is investigated in detail. The morphology and structural characteristics of this feature are described, as well as the sedimentology of an associated assemblage of debris‐filled fractures. This work suggests that debris‐charged subglacial floodwaters travelled up to the glacier surface, where supraglacial flow occurred initially via an extensive network of fractures, orientated parallel to the glacier margin. Supraglacial discharge became progressively more focused into a series of discrete outlets, leading to the mechanical erosion of a number of depressions on the glacier surface. The associated transfer of subglacially derived floodwaters to high levels within the glacier resulted in the rapid entrainment of large volumes of sediment which may influence the patterns, processes and products of ice‐marginal sedimentation in the future.

7. Icelandic jökulhlaup impacts

Iceland contains the worlds largest and best-documented active glacial outwash plains or sandar that have been studied since the 19th century. Vigorous subglacial volcanic activity and the presence of numerous ice-dammed lakes, make Iceland the prime location for the study of glacier outburst floods or jokulhlaups and their geomorphological and sedimentological impact. Increasing attention is being focused on large jokulhlaup channels related to both modem and ancient processes. Jokulhlaup impact within Icelandic bedrock channels has so far received little attention despite the fact that such channels are abundant as sandar in Iceland. Despite clear descriptions within Icelandic literature of jokulhlaup impact on glacier margins, there have been attempts to link jokulhlaup feeder system dynamics with processes and products in the proglacial outwash system. This chapter presents the latest research on the impacts of jokulhlaups generated by three main mechanisms: volcanic, ice-dammed lake drainage, and glacier surge. This chapter identifies and discusses the main advances in the understanding of jokulhlaup impact in Iceland.