Anne E. Flink

Debris entrainment and landform genesis during tidewater glacier surges

The englacial entrainment of basal debris during surges presents an opportunity to investigate processes acting at the glacier bed. The subsequent melt-out of debris-rich englacial structures during the quiescent phase produces geometrical ridge networks on glacier forelands that are diagnostic of surge activity. We investigate the link between debris entrainment and proglacial geomorphology by analyzing basal ice, englacial structures, and ridge networks exposed at the margins of Tunabreen, a tidewater surge-type glacier in Svalbard. The basal ice facies display clear evidence for brittle and ductile tectonic deformation, resulting in overall thickening of the basal ice sequence. The formation of debris-poor dispersed facies ice is the result of strain-induced metamorphism of meteoric ice near the bed. Debris-rich englacial structures display a variety of characteristics and morphologies and are interpreted to represent the incorporation and elevation of subglacial till via the squeezing of till into basal crevasses and hydrofracture exploitation of thrust faults, reoriented crevasse squeezes, and preexisting fractures. These structures are observed to melt-out and form embryonic geometrical ridge networks at the base of a terrestrially grounded ice cliff. Ridge networks are also located at the terrestrial margins of Tunabreen, neighboring Von Postbreen, and in a submarine position within Tempelfjorden. Analysis of network characteristics allows these ridges to be linked to different formational mechanisms of their parent debris-rich englacial structures. This in turn provides an insight into variations in the dominant tectonic stress regimes acting across the glacier during surges.

Possible iceberg-produced submarine terraces in Hambergbukta, Spitsbergen

Submarine landforms produced by drifting icebergs are common on the sedimentary beds of the polar seas. High-resolution multibeam-bathymetric images have revealed a variety of iceberg-related landforms, ranging from 1 to 2 m high corrugation ridges (e.g. Jakobsson et al. 2011; Graham et al. 2013) to large linear to curvilinear ploughmarks several tens of kilometres long and tens of metres deep, that are often distributed chaotically on polar continental shelves (e.g. Dowdeswell et al. 2010). Extensive lateral erosion of submarine ridge-top sediments in the central Arctic Ocean has also been attributed to the impact of deep-drafted icebergs that were once much more abundant in the Arctic than they are today (Jakobsson et al. 2008). Submarine terraces with remarkably flat surfaces were observed on the crest and along the distal slope of a large terminal-moraine ridge at the mouth of the Hambergbukta (Fig. 1a, c), a fjord in southeastern Spitsbergen (Fig. 1b). The terraces were recorded at several bathymetric levels from 29–66 m in water depth with their surface areas varying from c. 0.04 km2 to >2 km2 (Fig. 1a, d). Fig. 1. ( a ) Swath-bathymetric imagery …

Submarine medial moraines in Hambergbukta, southeastern Spitsbergen

Sediments are incorporated into glaciers mainly by basal erosion and freeze-on, and by debris-fall onto the ice surface along the slopes of adjacent mountainsides. Flow-parallel debris ridges formed at the confluence of glaciers are known as medial moraines (Benn & Evans 2010). Medial moraines have a number of different origins (Eyles & Rogerson 1978; Benn & Evans 2010), including ingestion of basal marginal debris in deep crevasses and redistribution of supra-, en- and subglacial debris in the shear zone between converging glaciers (e.g. Sharp 1988; Vere & Benn 1989; Hambrey & Glasser 2003). Medial moraines up to tens of kilometres long and tens of metres high can be preserved after glacier retreat, marking the former flow path of ice masses. Medial moraines have also been shown to exert significant control over surging glaciers by inhibiting and/or redirecting glacier flow. Locally persistent subglacial meltwater conduits located under the medial moraines can also inhibit glacier surges by draining subglacial meltwater efficiently and decreasing basal water pressure (Benn et al. 2009). Hambergbukta …

Crag-and-tail landforms in outer Rijpfjorden, Nordaustlandet, Svalbard

Streamlined sedimentary and bedrock glacial landforms are widespread on high-latitude continental shelves and in fjords. It has been shown that they are orientated in the direction of past ice flow and are indicative of fast motion linked to basal processes (e.g. Clark 1994). Elongation ratios of these landforms have been suggested to reflect past ice velocity, with higher elongation ratios being characteristic of faster ice flow (e.g. Clark 1993). However, the subglacial substrate has a considerable influence on ice flow and on the resulting glacial landforms (e.g. Heroy & Anderson 2005). Using the character and distribution of streamlined, subglacially produced landforms, the extent and dynamics of former ice sheets and ice streams has been successfully reconstructed (e.g. Ottesen et al. 2005; Dowdeswell et al. 2010; Hogan et al. 2010 a , b ). The seafloor of Rijpfjorden on Nordaustlandet in eastern Svalbard (Fig. 1f) is dominated by crudely streamlined bedrock with south–north to SSW–NNE long-axis …

Annual moraine ridges in Tempelfjorden, Spitsbergen

Surging tidewater glaciers produce a characteristic landform assemblage with glacial lineations, networks of crevasse-fill ridges, large terminal moraine ridges and several recessional moraines, which often override glacial lineations (Solheim 1991; Ottesen et al. 2008). Submarine recessional moraines have also been mapped beyond several Svalbard tidewater glaciers and are interpreted to record an annual to semi-annual signal of glacial retreat during the quiescent phase of the surge cycle (Ottesen & Dowdeswell 2006; Ottesen et al. 2008). The moraine ridges probably form during winter stillstands or minor readvances of the glacier terminus (Boulton et al. 1996; Ottesen et al. 2008). Tunabreen is a tidewater glacier of surge type located in Tempelfjorden, Spitsbergen (Fig. 1e). It has surged three times since the Little Ice Age, in 1930, 1970 and between c. 2002 and 2005 (Forwick et al. 2010) producing large terminal moraines (Fig. 1a). During the latest surge, the glacier margin reached its maximum extent in 2004 and has since retreated about 1 km. Fig. 1. Multibeam-bathymetric images of Tempelfjorden, Svalbard. Image courtesy of the Norwegian Hydrographic Service (permission no. 13/G706). ( a ) Image of inner Tempelfjorden, superimposed on an aerial image from 2009 (Photo …