Lilja R. Bjarnadóttir

Geophysical constraints on the dynamics and retreat of the Barents Sea ice sheet as a paleobenchmark for models of marine ice sheet deglaciation

Our understanding of processes relating to the retreat of marine-based ice sheets, such as the West Antarctic Ice Sheet and tidewater-terminating glaciers in Greenland today, is still limited. In particular, the role of ice stream instabilities and oceanographic dynamics in driving their collapse are poorly constrained beyond observational timescales. Over numerous glaciations during the Quaternary, a marine-based ice sheet has waxed and waned over the Barents Sea continental shelf, characterized by a number of ice streams that extended to the shelf edge and subsequently collapsed during periods of climate and ocean warming. Increasing availability of offshore and onshore geophysical data over the last decade has significantly enhanced our knowledge of the pattern and timing of retreat of this Barents Sea ice sheet (BSIS), particularly so from its Late Weichselian maximum extent. We present a review of existing geophysical constraints that detail the dynamic evolution of the BSIS through the last glacial cycle, providing numerical modelers and geophysical workers with a benchmark data set with which to tune ice sheet reconstructions and explore ice sheet sensitivities and drivers of dynamic behavior. Although constraining data are generally spatially sporadic across the Barents and Kara Seas, behaviors such as ice sheet thinning, major ice divide migration, asynchronous and rapid flow switching, and ice stream collapses are all evident. Further investigation into the drivers and mechanisms of such dynamics within this unique paleo-analogue is seen as a key priority for advancing our understanding of marine-based ice sheet deglaciations, both in the deep past and in the short-term future.

Large subglacial meltwater features in the central Barents Sea

During the last glacial period large parts of the Arctic, including the Barents Sea, north of Norway and Russia, were covered by ice sheets. Despite several studies indicating that melting occurred beneath much of the Barents Sea ice sheet, very few meltwater-related landforms have been identified. We document ∼200 seafloor valleys in the central Barents Sea and interpret them to be tunnel valleys formed by meltwater erosion beneath an ice sheet. This is the first account of widespread networks of tunnel valleys in the Barents Sea, and confirms previous predictions that large parts of the ice sheet were warm based. The tunnel valleys are interpreted to be formed through a combination of steady-state drainage and outburst floods close to the ice margin, as a result of increased melting within a period of rapid climate warming during late deglaciation. This is the first study documenting widespread tunnel valley formation at the northern reaches of a Northern Hemisphere paleo–ice sheet, during advanced deglaciation and beneath a much reduced ice sheet. This indicates that suitable conditions for tunnel valley formation may have occurred more widely than previously reported, and emphasizes the need to properly incorporate hydrological processes in current efforts to model ice sheet response to climate warming. This study provides valuable empirical data, to which modeling results can be compared.

Unusual iceberg ploughmarks on the Norwegian continental shelf

Iceberg ploughmarks are produced when the keels of drifting icebergs impinge upon and cut into seafloor sediments. They are common landforms of high-latitude shelves and fjords, especially in water shallower than about 500 m where they are easily detected using modern multibeam echo-sounding and earlier side-scan sonar systems (e.g. Lien 1983; Dowdeswell et al. 1993). In addition, similar buried morphological features have been imaged within Quaternary sediments on palaeo-shelves using 3D seismic methods (e.g. Dowdeswell & Ottesen 2013). Five examples of unusual linear to curvilinear features from the seabed of the Norwegian continental shelf are presented (Fig. 1). The first and rather small feature is from outside Brasvellbreen (Fig. 1a), an outlet glacier of Austfonna in E Svalbard. The looped feature is about 15 m wide and 1 m deep and has small berms on either side. The diameter of the loop is about 1.2 km. The feature is found on top of a dense criss-crossing pattern of similar features on the seafloor in water depths of around 40 m. Fig. 1. Iceberg ploughmarks on the Norwegian continental shelf and upper slope (located in (g)). ( a ) An almost-circular iceberg ploughmark outside Brasvellbreen, Austfonna, Svalbard. ( b ) Peculiarly shaped iceberg ploughmark in the central Barents Sea. ( c ) Iceberg ploughmark demonstrating rotation, central Barents Sea. The many small round …

Enigmatic needle-like seafloor features in the Bear Island Trough, central Barents Sea

Mapping of well-preserved submarine glacial landforms has been used to decipher the pattern and dynamics of the retreat of the Bear Island Trough ice stream that drained the Barents Sea Ice Sheet during the Late Weichselian deglaciation (e.g. Andreassen et al. 2008, 2014; Winsborrow et al. 2010; Ruther et al. 2011; Bjarnadottir et al. 2014). However, the formation processes of some of the landforms identified are not fully understood; one example is that of an enigmatic linear sedimentary seafloor feature termed ‘needle’ by Bjarnadottir et al. (2014). A variety of subglacial landforms co-exist in the Bear Island Trough (BIT), including landforms typically associated with both active ice streams, such as mega-scale glacial lineations (MSGLs) and grounding-zone wedges (GZWs), and ice stagnation (e.g. Ruther et al. 2011; Andreassen et al. 2014; Bjarnadottir et al. 2014). Included among these are remarkably straight and elongate features, termed ‘needles’ by Bjarnadottir et al. (2014), which are described here. Needle-like features occur in the upper BIT at water depths of 230–280 m (Fig. 1a). They are <40 km long, <700 m wide and protrude <40 m from …

Ice-stream landform assemblage in Kveithola, western Barents Sea margin

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; Ruther et al. 2012; Bjarnadottir et al. 2013). 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 Bjarnadottir et al. (2013). Here, the acoustic-stratigraphic units defined by Bjarnadottir et al. (2013) are used, with a focus on seafloor morphology relating to the ice-stream landform assemblage. Fig. 1. 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 …

National Programmes: Geomorphological Mapping at Multiple Scales for Multiple Purposes

A better understanding of marine geomorphology is a common goal for seabed mapping programmes, with various mapping approaches, methodologies and challenges associated with systematically describing geomorphological features. To address these issues, and highlight the overall value of geomorphological mapping, a group of representatives from the seabed mapping programmes of the geological surveys of Norway, Ireland and the United Kingdom have formed a partnership to share their knowledge, expertise and technologies. Here we describe the first year of collaboration by outlining the background to and motivation for the groups’ national seabed mapping programmes, and presenting several case studies as well as tests to potentially adopt a harmonised classification scheme.

Retreat patterns and dynamics of the former Bear Island Trough Ice Stream

Covering one of the widest continental shelves in the world, the epicontinental Barents Sea is characterized by several shallow banks separated by troughs that open towards the Norwegian Sea in the west and the Arctic Ocean in the north (Fig. 1a). The bank areas have typical water depths of 100–200 m and the troughs 300–500 m. The most prominent cross-shelf trough, the Bear Island Trough (Bjornoyrenna), extends over 750 km from Storbanken (‘the large bank’ in Norwegian) in the NE to the shelf break in the SW (Fig. 1a). It is 150–200 km wide and spans water depths of 300–500 m. Ice sheet reconstructions over the last decades have recognized that a major ice sheet covered the whole Barents Sea during the Last Glacial Maximum (LGM; Mangerud et al. 1992; Svendsen et al. 2004). Large trough-mouth fans (TMFs; Vorren et al. 1989) appear as seaward-convex bulges in the bathymetry at the mouth of troughs that extend to the shelf break. The largest of these, the Bear Island Trough-Mouth Fan (Fig. 1a), contains up to 3–4 km of Plio-Pleistocene glacial sediments. The location of a major ice stream in the Bear Island Trough, draining the former Barents Sea and Fennoscandian ice sheets and delivering large amounts of sediments to the Bear Island TMF during the LGM (Fig. 1a), has been inferred from seafloor geomorphology and palaeo-ice-sheet geometry (Denton & Hughes 1981; Solheim et al. 1990; Ottesen et al. 2005; Andreassen et al. 2007), from studies of the fan itself (e.g. Vorren & Laberg 1997) and from borehole data (Saettem 1994). Two late glacial maxima have been inferred in the SW Barents Sea, one before 22 cal ka BP and one after 19 cal ka (Vorren & Laberg 1996). Fig. 1. Bathymetry, geomorphology, Last Glacial …