Þorsteinn Sæmundsson

The Höfðahólar rock avalanche (sturzström): Chronological constraint of paraglacial landsliding on an Icelandic hillslope

The Höfðahólar rock avalanche, in the Skagafjörður area of northern Iceland, was investigated on the basis of a geomorphological analysis of its landforms and close surrounding environment. Thanks to sound chronological constraints (14C dating from birch remnants in peat areas that developed within depressions over the chaotic rock-avalanche deposit, tephrochronological sequences resulting from subsequent ash fallouts over the deposit, calibration of an age–depth model of peats and previously dated raised beaches), we define the rock-avalanche implementation with a wider timeframe between 10,200 and 7975 cal. yr BP and with a narrower frame between 9000 and 8195 ± 45 cal. yr BP. Such a well constrained timing proposes one of the most precise datings of an early-Holocene major slope failure in Iceland. This result fits well in the known chronology of the deglaciation in this area and in the prevailing Icelandic theory of a generalized phase of landsliding that occurred shortly after the deglaciation of the area. The main driver for the rock-avalanche occurrence is associated to a paraglacial origin; glacio-isostatic rebound, associated to rockwall debuttressing, is thought to be the main factor in the genesis of this Boreal major disequilibrium.

Debris Flow Triggered by Rapid Snowmelt: A Case Study in The Glei .arhjalli Area, Northwestern Iceland

Abstract Debris flows in the Gleivarhjalli area in northwestern Iceland occurred after a sudden and intensive snowmelt period during 10–12 June, 1999. The area, in the northwestern part of the town of Ísafjörvur, was chosen for a detailed study. Meteorological data and bedrock conditions, triggering mechanisms and geomorphological and human impacts were examined. This paper describes and emphasises the role of rapid snowmelt as a mechanism for the release of debris flows in a subpolar basaltic fjord setting. Post‐event mapping of erosional and depositional landforms showed strong geomorphic impacts of debris flows and their role in mass transfer in a mountainous environment. The estimated denudation rate for the singleevent is 0.29 mm/km2. The use of a new lichen growth curve provides relative dating of previous unreported events. Finally, the paper estimates the mean return period for debris‐flow events in the Gleivarhjalli area as 4–5 years, thus constituting a serious threat to the community.

Focusing on the spatial non-stationarity of landslide predisposing factors in northern Iceland Do paraglacial factors vary over space?

Most studies focusing on landslide spatial analysis have considered the relationships between predictors and landslide occurrence as fixed effects. Yet spatially varying relationships, i.e. non-stationarity, often occur in any spatial data set and should be theoretically considered in statistical models for a better fit. In Skagafjörður, a landslide-rich north–south oriented area located in northern Iceland, we investigated whether spatial non-stationarity in the relationships between paraglacial variables (glacio-isostatic rebound and post-glacial debuttressing, both captured in this area by latitude) and landslide locations is detectable. To explore the non-stationarity of factors that predispose landslide occurrence, we performed two logistic regression models, one global (GLR) and the other enabling the regression parameters to vary locally (geographically weighted logistic regression, GWLR). Each model was computed with two types of outcome, one based on the entire masses of landslides and the other only on the scarps of landslides. GLR results reveal that increasing latitude is associated with increasing probability of landslide occurrence, confirming that post-glacial rebound is of prime importance at the regional scale. Nevertheless, GWLR indicates that this relationship is absent or reversed at some locations, meaning that the influence of paraglacial and other predisposing factors of landsliding (slope, valley depth and curvature) vary at the local scale. This result sheds light on the spatial clustering of three subzones where landsliding drivers are homogeneous. We conclude that a GWR-based approach provides some significant inputs for spatial analysis of mass movement processes, by identifying multi-scale process control zones and by highlighting local drivers, indecipherable in global models.

DISTRIBUTION AND FREQUENCY OF SNOW-AVALANCHE DEBRIS TRANSFER IN THE DISTAL PART OF COLLUVIAL CONES IN CENTRAL NORTH ICELAND

Abstract. This paper emphasizes the importance of studying diffuse rock debris accumulation in the far distal part of colluvial cones with the aim of defining the spatial distribution and frequency of extreme snow avalanches. These deposits are located at some distance from the slope and have rarely been described in the literature. The field based methods used in this study confirm the snow‐avalanche origin of these deposits by (i) characterizing the distribution of the deposits along the colluvial cones and to their furthest extent, some way from the foot of the slope, and (ii) clearly defining the orientation of the long axis of the furthest and largest boulders as parallel with the main cone axis. A relative age of the deposits is obtained by assessing the vegetation cover of the boulder surfaces and by measuring the rock hardness using a Schmidt hammer. The study concludes that there were frequent extreme snow avalanches which have occurred since the Little Ice Age, and that they have decreased in magnitude during the past decade.

Dendrogeomorphology as a tool to unravel snow-avalanche activity: Preliminary results from the Fnjóskadalur test site, Northern Iceland

Snow avalanches are a major hazard for many settlements and transportation corridors in northern Iceland. At many sites the occurrence of snow avalanches during the past century has not been recorded. Visible damage, such as tilting, scars and decapitation of trees and shrubs (Betula pubescens) growing on colluvial cones in a remote area in Central North Iceland clearly identifies snow-avalanche paths of a given magnitude and frequency. An analysis of tree-ring data was made using the chronology of ring sizes and wood reaction in snow-avalanche tracks subject to frequent avalanches. Abnormal growth, correlated with abrupt increases or decreases in growth rates, is related to snow-avalanche impact. The preliminary results provide reliable dendrogeomorphological data that show the spatial extent and frequency of snow avalanches in the study area. Further investigation that includes a broader sampling strategy and dendrochronological laboratory analysis is required.

METEOROLOGICAL CONDITIONS DURING SLUSH-FLOW RELEASE AND THEIR GEOMORPHOLOGICAL IMPACT IN NORTHWESTERN ICELAND: A CASE STUDY FROM THE BÍLDUDALUR VALLEY

ABSTRACT. This paper examines triggering factors and geomorphic significance of slush flows in the Bíldudalur valley, northwestern Iceland. The area is prone to release slush flows from two confined gullies, and at least ten flows have been reported since the beginning of the twentieth century. Despite their short path (600 m) and their moderate magnitude (from 6000 to 8000 m3), slush flows in the Bíldudalur valley represent a serious threat for the local community that is situated within the runout and deposition zones. With the help of meteorological data, the release of known slush flows is examined, highlighting the role of heavy rainfall and rapid snow‐melt during winter cyclonic activity. The geomorphological impact of slush flows is assessed through the characteristics of the landforms produced during the 1997 and 1998 slush‐flow events. It appears that the most obvious characteristics of slush flows in the Bíldudalur valley are the entrainment and deposition of debris, spatially differentiated. Chaotic sedimentation occurs chiefly in the middle part of a clearly concave cone, even if the flows continue beyond the cone.

Reconnaissance surveys of contemporary permafrost environments in central iceland using geoelectrical methods: implications for permafrost degradation and sediment fluxes

Abstract. Four different sites in the highlands of central Iceland have been investigated for permafrost occurrence using two‐dimensional resistivity imaging. The results of the surveys indicate the presence of shallow permafrost of low to medium resistivity. The distribution pattern is spatially heterogeneous which is consistent with permafrost at the fringe of seasonal frost. These sites are likely to react rapidly to changes of the environmental boundary conditions, therefore future research should include monitoring for detecting the early impact of climate change on permafrost degradation. The extent to which periglacial morphodynamics and sediment fluxes are influenced by permafrost and/or seasonal frost and potential permafrost degradation is hard to determine. Hence, long‐term monitoring approaches for both permafrost and sediment dynamics are essential.

CHANGES IN DEPOSITION ON A COLLUVIAL FAN DURING THE UPPER HOLOCENE IN THE TINDASTÓLL MOUNTAIN, SKAGAFJÖRÐUR DISTRICT, NORTH ICELAND: PRELIMINARY RESULTS

Abstract. This study investigates variability in depositional processes and landforms on the Innstaland colluvial fan, northern Iceland, over the late Holocene. This is completed using geomorphological mapping, historical records and relative‐age dating tools (vegetation survey, rock hardness and tephrochronology). Debris flows are the main contributors to fan development. Six main phases of deposition are distinguished, varying in deposit magnitude along a general trend of decreasing magnitude over time. With the help of dated tephra layers, the aggradation rates of the fan are calculated for the upper Holocene and in particular over the historical period (post‐ AD 1104). An episodic sediment transfer system is reconstructed, with several phases of low activity between more rapid aggradational episodes. The interpretation of the colluvial deposits suggests that the main phase of aggradation on the fan ended during the historical period after which incision started (between AD 1300 and 1766). Incision tracks act as the main pathways for debris distribution at the present time. These variations are related to the changing response of the catchment system over time.

An overview of postglacial sediment records from colluvial accumulations in Northwestern and North Iceland

Abstract Active denudation processes occurring on slopes in north and northwestern Iceland have contributed to the buildup of large colluvial cones. These processes have been active since around 10,000 14C yr BP when the ice sheet retreated during the last deglaciation. Stratigraphic records provide information of the kind of sedimentary transfer processes that have been active on slopes through time. Vertical sections in colluvial cones in north and northwestern Iceland exhibit a characteristic stratigraphy with successions of material from mass-movements interbedded with soil horizons occurring throughout the Holocene, under periglacial conditions of varying intensity. The alternating organic and minerogenic units are indicators of phases of slope activity and stability. The dating of the deposits is possible with tephrochronology and 14C dating. The quantitative analysis of sediment on colluvial cones shows the relative importance of aggradation due to slope processes vs. soil formation during the Holocene. Increasing accumulation rates have been observed over historical time since at least a.d. 1104. The clastic deposits observed in north and northwestern Iceland are thought to provide information on extreme events during the Holocene, as the occurrence of mass-wasting release cannot be clearly related to Holocene climatic trends.

Sedimentary fluxes and budgets in changing cold environments: the global IAG/AIG sediment budgets in cold environments (SEDIBUD) programme

1Geological Survey of Norway (NGU), Quaternary Geology and Climate group, Trondheim, Norway 2Norwegian University of Science and Technology (NTNU), Department of Geography, Trondheim, Norway 3Queen’s University, Department of Geography, Kingston, Canada 4University of Clermont-Ferrand, Laboratory of Physical and Environmental Geography GEOLAB, CNRS, Clermont-Ferrand, France 5Natural Research Centre of North-western Iceland, Saudarkrokur, Iceland 6University of Arkansas, Department of Geosciences, Fayetteville AR, USA 7 University of Otago, Department of Geography, Dunedin, New Zealand 8University of Salzburg, Department of Geography and Geology, Salzburg, Austria 9University of Colorado, Institute of Arctic and Alpine Research, Boulder, CO, USA 10Durham University, Department of Geography, Durham, UK 11Adam Mickiewicz University, Institute of Paleogeography and Geoecology, Poznan, Poland