Distinct assemblages of landforms are produced by retreating ice streams (e.g. Stokes & Clark 1999; Wellner et al. 2006). The composition of landforms is governed by ice-stream retreat dynamics, implying that ice-stream dynamics can be reconstructed from these geomorphic patterns. This method has been used to decipher the retreat dynamics of an ice stream that occupied the Kveithola Trough on the western Barents Sea margin, presumably during the Late Weichselian deglaciation (Rebesco et al. 2011; Rüther et al. 2012; Bjarnadóttir et al. 2013).
Description
Kveithola Trough contains a diverse assemblage of landforms related to the presence and withdrawal of an ice stream, as well as to postglacial processes (Fig. 1a, b), described in detail by Bjarnadóttir et al. (2013). Here, the acoustic-stratigraphic units defined by Bjarnadóttir et al. (2013) are used, with a focus on seafloor morphology relating to the ice-stream landform assemblage.
Multibeam swath bathymetry from Kveithola Trough, western Barents Sea. (a) Swath-bathymetric image showing trough geomorphology. White polygons indicate locations of detail images (c–f). White lines indicate profiles in Figures 2 and 3. VE×8. (b) Location of study area (red box; map from IBCAO v. 3.0). (c) Oblique view of ice-stream onset zone, showing drumlins and channels. VE×20. (d) Oblique image showing multiple generations of grounding-line fans downstream of a GZW. VE×20. (e) Oblique view of MSGLs on/beneath GZWs. VE×35. (f) Oblique view of diverging MSGLs, shelf edge and gullies on upper slope. VE×16. Images (c–f) are viewed from the black stippled lines. Acquisition system Kongsberg Simrad EM300. Frequency 30 kHz. Grid-cell size 10 m.
Kveithola is on the western margin of the Barents Sea, 40 km NW of Bear Island (Fig. 1b). The trough is c. 90–120 km long, 10–15 km wide (slightly wider at the seaward end) and 200–700 m deep (mostly 200–400 m). It is orientated approximately east–west and cuts through sedimentary rocks, which are oldest in the east (Triassic) and younger (Tertiary) in the west (Sigmond 2002). The seafloor geomorphology of Kveithola appears to be linked in part to the underlying geology, although this alone cannot account for the variety of landforms found (Fig. 1a).
The eastern part of the trough is characterized by rugged seafloor with many fine-scale geomorphic features (Fig. 1a, c). This relatively shallower area (130–240 m) is divided from the deeper part of the trough by a bedrock step. The seafloor in the middle to inner part of the trough has a smoother appearance (Fig. 1a). Meanwhile, the outer half of the trough is characterized by striking linear groove-ridge features and large cross-trough ridges. The upper part of the continental slope immediately west of the trough displays a number of gullies incised into the seafloor (Fig. 1a).
The whole trough is draped by a blanket of acoustically stratified sediments (SU11; Bjarnadóttir et al. 2013) with a relatively uniform thickness of 15–20 m (Fig. 2a). Despite the thickness of this conformable unit, the underlying geomorphology is clearly visible (Fig. 1). Most of the observed morphology arises from the relief beneath SU11, except in the easternmost deeper part of Kveithola (Fig. 1a) where stratigraphically younger sediments (Fig. 2a: SU12–13) are thick enough to mask the underlying relief (Rüther et al. 2012; Bjarnadóttir et al. 2013).
Geoacoustic long- and cross-profiles from Kveithola Trough (modified from Bjarnadóttir et al. 2013). (a) Seismic profile along the long axis of Kveithola Trough. VE×17. (b) Chirp profile across Kveithola Trough. VE×14. (c) Seismic profile across Kveithola Trough. VE×23. Channels are arrowed. Location of profiles shown in Figure 1a. Chirp profiles acquisition system Edgetech 3300 HM. Frequency 1.5–9 kHz. Wavy appearance of the seafloor reflection is due to lack of heave compensation. Seismic profile acquisition system GI-airgun, 20-element single-channel streamer. Frequency c. 90 Hz.
(a) Summary map showing the main landforms in Kveithola Trough (modified from Bjarnadóttir et al. 2013). (b) Schematic landform-assemblage model for the Kveithola Ice Stream system. The legend in (a) also applies here. GZW, grounding-zone wedge.
The relatively rugged eastern part of the trough, east of the bedrock step (Fig. 1a, c), is characterized by numerous fine-scale features. These include slightly elongate ridges shaped like inverted spoons which are 900–1800 m long, 250–500 m wide and 15–25 m high (elongation ratios of 3–7:1). They have tapering distal tails and more blunt proximal heads (Fig. 1c), frequently with crescentic depressions round the head. Other landforms observed in this more rugged part of the trough are linear and/or curvate seafloor depressions or channels (Figs 1c, 3a). They are <12 km long, <1 km wide, up to 25 m deeper than the surrounding seafloor, have overdeepened long profiles and form a connected network.
In the deeper part of innermost Kveithola Trough (west of the bedrock step) the seafloor has a much smoother appearance (Fig. 1a). The topography is characterized by mound-shaped sediment accumulations that consist of acoustically stratified sediments of up to 20 m thickness (SU12, 13, CU2 in Fig. 2a, b), overlying the acoustic drape (SU11, CU3 in Fig. 2a–c). A large number of isolated small ellipsoidal (40–60 m across, <5 m deep) depressions are clustered over much of these smooth sediment mounds. Such features also occur more sparsely in other parts of the trough. Acoustic turbidity and masking occur frequently beneath the depressions on chirp profiles according to Bjarnadóttir et al. (2013), who interpreted them to be pockmarks (Fig. 3a).
Five major transverse ridges stretch across Kveithola Trough (Figs 1a, 2a, 3a). They are about 10–13 km wide and 30–60 m high. In cross-section, the eastern sides of the asymmetrical ridges have less steep slopes (0.23–0.3°). On each ridge, the morphology of the eastern side is dominated by east–west-orientated linear trough-parallel groove-ridge features. The linear groove-ridges occur in two sizes: the first is 350–450 m wide and 2–4 m high and the second 1000–2400 m wide and 5–15 m high (Fig. 1e, f). Their maximum length exceeds 10 km, with elongation ratios of 22–28:1 and 4–10:1, respectively. They appear to diverge at the slightly wider western end of the trough (Fig. 1a, f). Cross-sections over the groove-ridge features vary, ranging from symmetrical or wave-like in form (Fig. 2b) to having flat-bottomed grooves (Fig. 1e), separated by sharp and relatively narrow ridges.
The groove-ridge features occur on the surface of acoustic units characterized by frequent hyperbolic diffractions and internal reflection segments, with bottom reflections representing erosional unconformities (SU1, 2, 4–7, 9 and CU5 in Fig. 2a, b). These units are stacked in a retro-gradational manner, becoming progressively younger towards the top of the sequence and to the east (Fig. 2a); there is one exception where a younger unit completely overlies two older units (SU7 overlies SU5, 6). Similarly, the groove-ridge features associated with the different units become increasingly young towards the east. The units have a hummocky surface and are incised by channels in some cases (Fig. 2a, c).
The relief formed collectively by these grooved acoustic units (Fig. 2a) is overlain by acoustically transparent sediment units (SU3, 8, 10 in Fig. 2a). The thickness of these units varies considerably. Where they are relatively thin (<15 m) on the eastern slopes of the ridges the sediments form a draping unit, mimicking the underlying morphology. Conversely, where they reach their greatest thickness (c. 45 m) on the western, steeper (0.54–0.65°) sides of the ridges, they form fan-shaped accumulations of acoustically transparent sediments that downlap onto older surfaces with groove-ridge features and pond in depressions (Figs 1a, d–f, 2a, c). In general, the fan-like features have sharp, lobate distal boundaries, smooth and slightly convex surfaces and point-source origins (Fig. 1a, d–f). However, their geometry varies considerably from ridge to ridge where they occur either as several small coalescing fans, a few large fans or several overlapping fan-generations (Fig. 1a, d–f).
The surface sediments of the shallowest parts of Kveithola Trough are riddled with curvilinear grooves (a few metres to 10 km long, c. 50 m wide and incised <8 m deep; Fig. 1a, d–f). These features are chaotically orientated and typically consist of several semi-straight segments with abrupt orientation changes occurring frequently between them.
Interpretation
The large transverse ridges are interpreted as composite sediment wedges deposited at an ice-stream grounding zone. Powell & Domack (2002) described a continuum of types of grounding-line deposits ranging from morainal banks (MBs) to grounding-zone wedges (GZWs), where MBs are associated with more free meltwater and consist of coarser sediments while GZWs occur where there is less meltwater and comparatively finer sediments. Here we interpret the transverse ridges as GZWs, that is, of an intermediate character according to Powell & Domack (2002). The groove-ridge features visible on the proximal side of and between the GZWs are interpreted as mega-scale glacial lineations (MSGLs), indicative of fast ice-flow through deformation and/or ploughing of basal sediments (Stokes & Clark 1999; Wellner et al. 2006).
The Kveithola GZWs are interpreted to be formed where an ice stream halted significantly during overall deglacial retreat. They are inferred to consist of a basal and proximal area of subglacial till and an upper and distal area of fine-grained sediment fans deposited from sediment-laden meltwater exiting at the ice-stream grounding line (Bjarnadóttir et al. 2013). Where the fine-grained sediments also occur continually as a thin top layer between the GZWs (Fig. 2a) they are interpreted to have been deposited by the same process during times of steady ice-stream retreat (Bjarnadóttir et al. 2013). The different size and configuration of grounding-line fans indicate that their style of formation varied. Bjarnadóttir et al. (2013) pointed to a relationship between the channels on top of the grooved units and these fan-shaped features, arguing that the observed variations were related to the maturity of the subglacial drainage system at the time of fan deposition, reflecting different combinations of meltwater and sediment input, drainage system efficiency and duration of stillstand.
The influence of meltwater is also seen in other parts of the trough. Numerous gullies on the upper continental slope are interpreted to have been formed by relatively dense plumes of meltwater and sediments when the ice stream was at the shelf edge. Similarly, channel networks in the innermost, rugged part of the trough are thought to result from meltwater erosion (Bjarnadóttir et al. 2013). Meltwater erosion probably also formed the channels or moats along the heads of the small spoon-shaped ridges in the innermost shallowest zone of Kveithola. Bjarnadóttir et al. (2013) interpreted these blunt-nosed ridges as drumlins, based on their shape and elongation ratio (Benn & Evans 2010).
The stratified sediment unit draping the whole trough (Fig. 2a–c: SU11, CU3) is interpreted to consist of glacimarine sediments deposited during and immediately after the ice stream retreated from the trough. Meanwhile, the mound-shaped sediment accumulations in the innermost deep part of Kveithola (Fig. 2a: SU12, 13) are interpreted as drifts deposited by dense bottom currents when the ice-sheet margin had retreated further away from Kveithola (Rüther et al. 2012; Bjarnadóttir et al. 2013). The ellipsoidal depressions on top of these units are interpreted as pockmarks. The curvilinear grooves on the shallowest parts of the trough seafloor are interpreted as iceberg ploughmarks (Bjarnadóttir et al. 2013).
Discussion: an intermediate-type ice-stream landform assemblage?
Relatively similar landform assemblages have been described previously from the Barents Sea (Ottesen et al. 2005; Andreassen et al. 2008; Winsborrow et al. 2010) and from other glaciated regions both onshore and offshore, and attributed to ice streaming (e.g. Stokes & Clark 1999; Wellner et al. 2006). The innermost, shallow part of Kveithola is characterized by drumlins and meltwater channels, which are common in ice-stream onset zones (e.g. Wellner et al. 2006). Likewise, the trunk of the former Kveithola ice stream is characterized by trough-parallel MSGLs, sharp lateral edges and cross-trough GZWs, which are typical geomorphic imprints of ice streams (Stokes & Clark 1999).
Ice-stream landform assemblages characterized by numerous GZWs overlying streamlined trough floors are considered to be indicative of episodic ice-stream retreat between intermittent stillstands (Dowdeswell et al. 2008). This is consistent with Bjarnadóttir et al.’s (2013) reconstruction, which depicted retreat of Kveithola Ice Stream punctuated by several halts and readvances. Bjarnadóttir et al. (2013) attributed some of these oscillations to the interplay of shifting glacial dynamics and reconfiguration of the basal drainage system. Furthermore, they pointed out that the geometry and distribution of grounding-line fans, the occurrence of channels and a geometry of steeper internal moraine ridges is more consistent with morainal banks, whereas the inferred fine-grained, homogeneous nature of the sediments is associated with GZWs. It was, therefore, suggested that the composite grounding-zone deposits in Kveithola should be viewed as an intermediate type of grounding-zone deposit on the continuum described by Powell & Domack (2002), probably governed by the availability of meltwater in the ice-stream system which likely increased as climatic warming occurred.
Several forcing factors may have influenced the style of deglaciation in Kveithola. The Kveithola Ice Stream would have been sensitive to early signs of climatic warming due to its proximity to the deeper ocean and potential influxes of the warm West Spitsbergen Current (Rebesco et al. 2011). Further, rising sea level as deglaciation progressed is likely to have had a destabilizing effect on Kveithola Ice Stream, increasing mass loss by iceberg production. Conversely, the narrow width of the trough may have promoted stillstands during retreat. Finally, ice-stream drawdown driven by internal dynamics may also have been important.
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
