Seismic architecture and sedimentology of a major grounding zone system deposited by the Bjørnøyrenna Ice Stream during Late Weichselian deglaciation

Quaternary Science Reviews 30 (2011) 2776e2792

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Seismic architecture and sedimentology of a major grounding zone system deposited by the Bjørnøyrenna Ice Stream during Late Weichselian deglaciation Denise Christina Rüther*, Rune Mattingsdal, Karin Andreassen, Matthias Forwick, Katrine Husum Department of Geology, University of Tromsø, Dramsveien 201, N-9037 Tromsø, Norway

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 December 2010 Received in revised form 7 June 2011 Accepted 14 June 2011 Available online 5 July 2011

A 280 km wide sediment wedge in outer Bjørnøyrenna (Bear Island Trough), south-western Barents Sea, has been investigated using 2D and 3D seismic data, sediment gravity cores, as well as regional swath and large scale bathymetry data. The bathymetry data indicate a division into an up to 35 m high frontal wedge with large depressions, and an upstream part characterized by mega scale glacial lineations (MSGL). From seismic sections increasing erosion is demonstrated for the upstream part, coinciding with the location of MSGL. Whether the latter are depositional features postdating an extensive erosional event or formed by erosion remains inconclusive. Based on the distinct morphology and internal structures, we infer that the system was deposited during a rapid readvance whereby the ice front pushed and bulldozed predominantly soft, diluted proglacial sediments. Analyses in the eastern part of the sediment system reveal the existence of imbricated thrust sheets in the frontal part of the wedge. This is suggested to imply upstream erosion of sedimentary rock and incorporation of thrusted blocks into the moraine, forming a composite ridge locally. We argue that observed large scale depressions are dead-ice features in the marine environment. It is envisioned that intense englacial thrusting may have developed into a decollement as the cold glacier snout got overrun by ice masses from the interior, thereby enabling the inclusion of slabs of ice in the push moraine mass. Radiocarbon dates indicate that the sediment wedge was deposited around 17,090 cal yrs BP (14,530 14C yrs BP) and that the ice front probably remained stable until 16,580 cal yrs BP (13,835 14C yrs BP). Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Grounding zone system Ice stream readvance Barents Ice Sheet Late Weichselian deglaciation

1. Introduction As the most dynamic parts of ice sheets (Bamber et al., 2007), the margins of the Greenland and Antarctic Ice Sheets are of key importance in the light of anticipated climatic warming and sea level rise (Oppenheimer, 1998; Overpeck et al., 2006). The majority of ice, meltwater, and sediment are discharged through ice streams e corridors of fast-flowing ice (Bentley, 1987). Subglacial elongate bedforms are indicative of fast flow, and have received much attention in the literature (e.g. Solheim et al., 1990; Clark, 1993; Canals et al., 2000; Stokes and Clark, 2001). However, they are only one of several geomorphological features which can be employed in the reconstruction of past ice sheet extent and dynamics (Stokes and Clark, 1999). Sediment deposits resulting from focused sediment delivery at the mouth or grounding line of ice streams can also be useful indicators of periods of stand-still * Corresponding author. Tel.: þ47 776 45065; fax: þ47 776 45600. E-mail addresses: [email protected] (D.C. Rüther), [email protected] (R. Mattingsdal), [email protected] (K. Andreassen), matthias.forwick@uit. no (M. Forwick), [email protected] (K. Husum). 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.06.011

and/or readvance (e.g. O’Brien et al., 1999; Anderson et al., 2001; Mosola and Anderson, 2006). We refer to such deposits as grounding zone systems. This is a non-generic term for sediment deposited at or near the grounding line of a glacier or ice stream terminating subaqueously, which, unlike terms such as morainal bank or grounding zone wedge, does not make specific inferences about depositional mechanisms (Powell and Alley, 1997). Grounding zone systems are thought to have a stabilizing effect against retreat of the grounding line in response to relative sea level rise of at least several metres (Alley et al., 2007; Anandakrishnan et al., 2007). Furthermore, the position of palaeo grounding lines may be used to validate the predictability of grounding line positions with numerical ice sheet models (e.g. Vieli and Payne, 2005; Schoof, 2007; Nick et al., 2010). Geomorphological mapping of grounding zone systems plays an important role in the reconstructions of palaeo-ice sheet configuration and pattern of deglaciation (e.g. in the Barents Sea area: Ottesen et al., 2005, 2008a; Andreassen et al., 2008; Winsborrow et al., 2010). The Barents Sea experienced repeated glaciations during the late Cenozoic (Elverhøi and Solheim, 1983; Vorren et al., 1988).

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During the most recent Late Weichselian glaciation, grounded ice reached the shelf break twice on the western margin (Laberg and Vorren, 1996): prior to 26.5 cal ka BP (22 14C ka BP) and between 21.5 and 18.1 cal ka BP (18e15 14C ka BP; Vorren and Laberg, 1996). It is well established that Bjørnøyrenna was the main drainage area of the Barents Sea Ice Sheet during subsequent glacial maxima (Vorren and Laberg, 1997; Andreassen et al., 2008) and that the last deglaciation of the Barents Sea shelf took place stepwise (Polyak et al., 1995; Landvik et al., 1998; Winsborrow et al., 2010). The outer Bjørnøyrenna sediment wedge (OBSW), explored in detail in this study, was identified by Andreassen et al. (2008) and coincides with the ice marginal position of stage 2 in the Late Weichselian deglaciation reconstruction by Winsborrow et al. (2010). In this paper, high resolution bathymetric data complements previously published large scale bathymetry (Andreassen et al., 2008; Winsborrow et al., 2010), enabling us, for the first time, to distinguish two geomorphological facies in the frontal and upstream part of the studied sediment wedge. Its internal characteristics are analysed based on hitherto unpublished single channel seismic lines as well as 2D and 3D industry seismic data. Seismic analyses includes mapping of the eastern, buried part of OBSW and observations have important implications for the genesis of the system. Furthermore, studied sediment gravity cores yielded unprecedented deglaciation ages for outermost Bjørnøyrenna. We argue that inferred upstream erosion and the deposition of a frontal arcuate rim encompassing large depressions can be best explained

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with a rapid readvance scenario. We suggest this ice stream readvance is dominated by ductile failure as upstream ice masses push diluted proglacial sediments and overrun slabs of the frozen glacier snout which are incorporated in the moraine mass. Brittle failure does, however, occur in the eastern area where outcropping Cretaceous bedrock is found upstream. Finally, we give a sediment flux estimate for the grounding line. 2. Study area The epicontinental Barents Sea covers one of the widest continental shelves in the world (Fig. 1B). It is bounded to the north and west by continental slopes, to the east by Novaja Zemlja, and to the south by the Fennoscandian coast. The most prominent geomorphological feature is the 500 km long, 150e200 km wide and 300e500 m deep Bjørnøyrenna cross-shelf trough. Bjørnøyrenna is flanked by shallow bank areas (<300 m): Spitsbergenbanken and Sentralbanken to the north and east, as well as Tromsøflaket and Nordkappbanken to the south (Fig. 1B). Two north-east to northwest trending troughs just off the coast of Norway (Ingøydjupet and Djuprenna) reach water depths of 450 m. Quaternary sediment cover in the Barents Sea is generally thin (<100 m), but local areas of high accumulation with thicknesses of up to 200e300 m occur, particularly off the Finnmark coast (Elverhøi and Solheim, 1983; Vorren et al., 1989; Sættem et al., 1992). Over most of the Barents continental shelf an erosional boundary, referred to as Upper

Fig. 1. (A) Bathymetric map showing the studied sediment wedge system together with the locations of sediment gravity cores, seismic data, as well as acquired swath bathymetry data. Large scale bathymetry of the south-western Barents Sea based on the interpretation of seismic seafloor reflections on a relatively dense grid of industry 2D multi-channel seismic data (Andreassen et al., 2008; Winsborrow et al., 2010). (B) Larger overview map and place names for the Barents Sea shelf area, western slope and northern Fennoscandia (bathymetry with 100 m contour interval). Maximum extend of the Late Weichselian Fennoscandian and Barents Sea Ice Sheets is based on Svendsen et al. (2004) and encompasses the maximum ice sheet extent in all areas, which did not necessarily occur time synchronously. Trough mouth fans along the western margin are digitized from Dahlgren et al. (2005) and based on the analyses of Vorren and Laberg (1997). Sr: Storfjordrenna, Nb: Nordkappbanken, Id: Ingøydjupet, Tf: Tromsøflaket, Hd: Håkjerringdjupet. (C) Seismic transect across the studied sediment deposit.

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Regional Unconformity (URU), can be identified. It separates seaward-dipping stratified sedimentary rocks from overlying subparallel Quaternary sediments (Solheim and Kristoffersen, 1984; Vorren and Kristoffersen, 1986). Trough mouth fans (TMF; Vorren and Laberg, 1997) have formed off the western and northern Barents Sea margins. TMFs are inferred to consist of stacked debris flow lobes deposited at the mouth of transverse troughs on glaciated continental shelves during subsequent glacial maxima. With an area of about 215,000 km2 the Bjørnøya TMF is the largest on the north-west European glaciated continental margin (Fig. 1B). The southern flank of Bjørnøyrenna is around one hundred metres shallower than the northern flank. This inclination is partly tectonically controlled since the Bjørnøyrenna Fault Complex e one of the main JurassiceCretaceous faults bounding the deep Cretaceous basins e runs along the southern axis of Bjørnøyrenna (Faleide et al., 1993). On large scale maps the tilt obscures the approximately 35 m high OBSW. 3. Material and methods This study is based on a variety of marine geological data including large scale bathymetry, regional scale swath bathymetry, single and multi-channel 2D seismic lines, 3D seismic data, as well as sediment gravity cores (Fig. 1A). 3.1. Acoustic data A 2700 km2 box of swath bathymetry data (Fig. 1A) has been acquired during a scientific cruise with R/V Jan Mayen in 2008 using a hull-mounted Kongsberg Simrad EM 300 multibeam echo sounder with a nominal sonar frequency of 30 kHz (KongsbergSimrad, 2002). The swath width is in the range of five times water depth in shallow waters (<500 m). Water masses in the area showed little variation so that the depth conversion could be based on a single sound velocity profile. The grid was processed in Neptune, gridded to 25 m in IVS 3D Fledermaus, and exported for the use in ArcGIS which is the platform used for mapping of landforms in this study. Along with the swath bathymetry data, chirp data were acquired using a hull-mounted Edgetech 3300 HM 4  4 transducer array which was operated at an energy level of 4 kW, a shot rate of 1 s, and a 40 ms long signal with a frequency which is gradually increased from 1.5 kHz to 9 kHz. This setup did, however, not permit any penetration which is indicative of the hard nature of the sediment at or near the sea surface in the studied area. Single channel seismic lines (Fig. 1A) were acquired on two cruises with R/V Jan Mayen in 2008 and 2010. On both occasions a controlled bubble air gun was used (Mini GI 60 and GI 210) e a pneumatic seismic source consisting of two independent air guns, generator and injector, fired shortly after each other in order to control and reduce bubble oscillations (Sercel, 2006). Processing was performed in Promax and includes bandpass filtering and Kirchhoff time migration. Amongst the industry seismic data used in this study (Table 1) are three 3D seismic datasets (Fig. 1A) and

Table 1 Overview of conventional industry multi-channel seismic surveys used in this study. Survey name

Operator

Year of acquisition

Survey type

Size

NH0608 SG9717 SG9804 SG9810 SG9817 ST8611 ST8813

Norsk Hydro Saga Saga Saga Saga Statoil Statoil

2006 1997 1998 1998 1998 1986 1988

3D 2D 3D 3D 2D 2D 2D

570 km2 488 km 1020 km2 1170 km2 63 km 3874 km 567 km

a selection of 2D multi-channel seismic lines (Fig. 1A). The 3D data attain a vertical resolution of around 10 m (a quarter of the dominant wavelet assuming a velocity of 1600 m/s and a frequency of 60 Hz). Due to a spatial sampling rate of 12.5 m and 3D migration techniques a horizontal resolution of approximately 12 m can be obtained. To convert depth from two-way-travel time to metres we assume a velocity in glacigenic sediments of 1800 m/s (Sættem et al., 1992). 3.2. Sediment cores and radiocarbon dating A total of eleven sediment cores from outer Bjørnøyrenna were collected with a 6 m long gravity corer onboard R/V Jan Mayen in 2007, 2008, and 2009 (Table 2), where plastic liners have an inner diameter of 10.2 cm. Eight of these are located downstream of the studied sediment deposit while three were collected from the top of the wedge and further upstream (Fig. 1A). Prior to opening, physical properties of the sediments (including wet bulk density) were measured with a Geotek Multi Sensor Core Logger (Weber et al., 1997). After opening, the cores were described, x-rayed (half cores), and measured for undrained shear strength with the fall cone test as described by Hansbo (1957). Clasts larger than one millimetre were counted for two centimetre intervals on x-radiographs (compare with Grobe, 1987). It is important to be aware of distortions that can result from bird-eye projection of x-radiographs, masking of underlying density anomalies, and inherent subjectivity in counting clasts as density anomalies. Subsamples, 1 cm-thick slices, were analysed for water content and grain size distribution which included wet sieving, analyses of the fine fraction (<63 mm) using a Micrometics SediGraph 5100, and dry sieving of the coarse fraction (>63 mm). Existing macrofossils were collected for Accelerator Mass Spectrometry (AMS) radiocarbon dating. In order to pinpoint the deglaciation of outermost Bjørnøyrenna, foraminifera were picked from the bottom of glacimarine units overlying subglacial tills. All samples were prepared at the Radiological Dating Laboratory in Trondheim, Norway, and measured at the Ångström Laboratory in Uppsala, Sweden (Table 4). The radiocarbon ages were calibrated with the software Calib 6.0.1 (Stuiver and Reimer, 1993) using the Marine09 calibration curve (Reimer et al., 2009). To express the regional difference with respect to the global mean we use a DR value of 67  41 as derived from a sample near Bjørnøya (Mangerud and Gulliksen, 1975). Furthermore, we recalibrated all cited radiocarbon dates using the Marine09 calibration curve in order to make them comparable with the calibrated ages of this study. Where not specified by cited authors we use a standard deviation of 100 yrs to recalibrate previously published dates. Pooled DR values as presented by Mangerud et al. (2006) are deemed most representative for

Table 2 Overview of sediment cores; for simplicity the core ID extension “-GC” (gravity core) is left out in text and figures. Core ID

Latitude (N)

Longitude (E)

Water depth (m)

Recovery (m)

JM08-0306-GC JM08-0307-GC JM08-0308-GC JM08-0309-GC JM07-09-GC JM07-10-GC JM09-KA01-GC JM09-KA01(2)-GC JM09-KA02-GC JM09-KA03-GC JM09-KA04-GC

72 59.342 72 41.345 72 41.385 72 29.358 72 19.673 73 32.010 72 18.246 72 18.265 72 24.173 72 44.144 73 09.798

19 30.958 18 00.070 17 59.976 17 00.743 17 30.715 16 58.087 17 01.051 17 01.190 16 42.594 16 11.906 16 14.569

416 366 366 385 378 385 383 383 389 427 480

2.12 1.0 1.95 2.11 2.69 2.27 1.17 1.80 1.57 3.07 1.66

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recalibrated ages from Svalbard (105  24) and the southern Barents Sea (71  21). For the purpose of recalibration of radiocarbon ages from the eastern Barents Sea DR values off Novaja Zemlja and the Kola Peninsula were averaged to 44  56. Uncorrected radiocarbon ages (14C yrs BP), radiocarbon ages corrected for reservoir effect (corr yrs BP), as well as calibrated calendar ages (cal yrs BP) are reported in years before present (yrs BP) or kilo years before present (ka BP); where the present is defined as A.D. 1950 by convention. 4. Geomorphology of the wedge system Despite its relatively poor horizontal resolution, the large scale bathymetry data (Figs. 1A and 2A) allows for the unique possibility to study the entire OBSW. During the mapping of the seafloor morphology the swath bathymetry data and seafloor interpretation from 3D block SG9810 were used to “ground truth” the mapping performed on the basis of the large scale bathymetry. Profiles in Figs. 2B-D illustrate the effect the poorer resolution has on the representation of the seafloor morphology. Geomorphological features smaller than 4 km are at the limit of detection and often smoothed to a considerable degree or not resolved at all. On the basis of the comparison with the swath bathymetry we are, however, confident that the large scale bathymetry reveals the principle and major trends on the seafloor.

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4.1. Geomorphology of the wedge system e description The glacial landforms of the OBSW include a main arcuate sediment deposit, depressions and elongate ridge/groove features within the system (Fig. 2A). Superimposed on observed larger scale structures, the seafloor in outermost Bjørnøyrenna is heavily affected by iceberg ploughing (Fig. 2E), possibly obscuring any preexisting smaller scale structures. The frontal outline of the OBSW is irregular and lobate, while its overall shape is arcuate. In Bjørnøyrenna the deposit is up to 35 m high, whereas it can rise up to 70 m above downstream seafloor, where it meets the overdeepened trough Ingøydjupet to the south. To the south-east, the wedge continues underneath Nordkappbanken western distal lobe as observed in earlier studies (Andreassen et al., 2008; Winsborrow et al., 2010). In our study, we map the extent of the buried part of the wedge (Fig. 2A) based on a relatively dense grid of 2D industry seismic data (Fig. 1A). Low vertical seismic resolution, however, makes interpretation of the wedge margin difficult, and hence the interpreted buried margin represents a minimum extent. This establishes a total width of 280 km measured from margin to margin as compared to 200 km width for the wedge visible on the seafloor today. In the area covered by swath bathymetry data, the frontal slope of the OBSW does not exceed 0.36 to 0.66 (Figs. 2B-D), while the slope of the

Fig. 2. (A) Overview map showing the results of geomorphologic mapping exposed and buried features in the wedge area. Outline of the wedge is shown as a continuous line where exposed on the seafloor and a dashed line for the part buried beneath Norkappbanken where the mapping represents a minimum estimate. Locations of (B), (C), and (D) are indicated in their respective colour, while location of (E) as well as Figs. 4 and 5 are shown by black boxes. Depth profiles (BeD) reflect swath bathymetry data plotted as continuous thin lines compared to the large scale bathymetry data as represented by the dashed lines. (E) is showing the acquired swath bathymetry data in more detail and gives reference to the location of sediment gravity cores as well as the seismic transect and cross section (Fig. 3A and B). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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stoss side is even gentler. The resolution of the large scale bathymetry is insufficient for detailed slope analyses. However, based on qualitative observations it can be differentiated between areas with a clear slope where the wedge is unconfined and a subdued front where the wedge meets more elevated areas towards Tromsøflaket and Nordkappbanken (Fig. 1B). The bathymetry data indicate a division into a frontal wedge with a complex morphology characterized by large scale depressions, and an upstream part of the wedge where elongate ridge/ groove features occur (Figs. 2A, 2E, 4A and 5A). The frontal rim of the sediment deposit, stretching over a length of approximately 30 km, reveals pronounced depressions often flanked by particularly elevated areas. The maximum lateral extent of single depressions is approximately 8 km and their vertical relief varies from several metres to a maximum of 30 m. Some of these depressions may be overdeepened and incised into underlying strata as illustrated by a seismic section through a depression from the part of the OBSW buried under Nordkappbanken (Fig. 5B). The elongate ridge/groove features have a vertical relief of 5e10 m, elongate ridges are 20e40 km long, 1.5e4 km wide and their crest-to-crest spacing is approximately 5 km. They are most pronounced in the central part of the wedge deposit. 4.2. Geomorphology of the wedge system e interpretation The genesis of the OBSW will be treated in the discussion under Sections 8.2 and 8.3 while we aim at clarifying the significance of the observed elongate ridge/groove features here. We interpret these to be streamlined bedforms; the term encompassing positive

and negative geomorphic features indicative for the action of grounded, relatively fast-flowing ice (Clark, 1993). It has been established empirically that progressive elongation of subglacial landforms suggests increasing flow velocities (e.g. Rose, 1987; Ó Cofaigh et al., 2002). An elongation ratio (length:width) of >10:1 has been introduced by Stokes and Clark (1999) to characterize streamlined bedforms of palaeo-ice streams, indicative of fast ice flow. The correlation of palaeo-ice streams and MSGL has been established through the early work of Clark (1993), and is further supported by numerous studies (e.g. Shipp et al., 1999; King et al., 2009). The described ridge/groove features fulfil the elongation ratio criterion for fast flow (Stokes and Clark, 1999) and lie within the suggested length range (8e70 km) characteristic for MSGL, while being wider than the suggested width of 200e1300 m (Clark, 1993). The latter may be a reflection of a larger morphological variety of MSGL. 5. Internal characteristics of the wedge system 5.1. Internal characteristics of the wedge system e description The seismic profile (Fig. 3A) is situated immediately north of a larger depression (for location see Fig. 1A). Due to the dense occurrence of iceberg ploughmarks (Fig 2E) the seafloor reflection is very rugged. Internal seismic reflections of the sediment wedge are diffuse to chaotic with a weak and discontinuous, undulating reflection at its base (reflection a in Fig. 3A). Reflection b (Fig. 3A) is subparallel to the seafloor reflection downstream, while it has a convex shape beneath the sediment wedge, suggesting the

Fig. 3. (A) Interpreted seismic transect JM08KA001, vertical black line indicating where (B) intersects. Seismic reflections c and d are correlated to Intra GIIIb and Intra GIIIa seismic horizon, respectively (Andreassen et al., 2007; Andreassen and Winsborrow, 2009) and slope stratigraphy consists of the latest glacial package GIII of Mid-Late Pleistocene age (Faleide et al., 1996). (B) Interpreted seismic cross section 10JM-KA001 with palaeo ice flow towards the reader. Red arrows designate the position of notches and vertical black line marks the intersection with (A). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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existence of an older overridden transverse ridge at this location. The division into zones IeIII (Fig. 3A) is based on thickness variations between seafloor and reflection b. Downstream of the wedge (zone I, Fig. 3A) sediment thickness is relatively constant at 50 ms (45 m). In the depositional frontal 30 km of the wedge (zone II, Fig. 3A) sediment thickness varies from 60 to 80 ms (54e72 m), while it is gradually reduced to 10 ms (9 m) further upstream until the two reflections become indistinguishable at 70 km upstream from the front (zone III in Fig. 3A). Seismic reflection e delineates a distinct knob located upstream from the sediment wedge. The seismic signal hardly penetrates beneath reflection e, is discontinuous and delineates an irregular surface, particularly at the flanks of the knob. The cross sectional seismic line (Fig. 3B) begins downstream from the wedge in the north-west, crosses two large depressions

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within the frontal part of the wedge, and extends over a notch of the lobate wedge front in the south-east (Figs. 1A, 2E). Basal wedge reflection a can be identified in one instance, illustrating that there is virtually no sediment in the depressions (Fig. 3B). Internal wedge reflections are diffuse to chaotic with low amplitudes. Reflection b can be identified where the seismic lines intersect, but is difficult to track all the way due to distortions caused by a ghost at 43 ms below seafloor reflection (Fig. 3B; dashed blue line where interpretation is more tentative). Reflections c and e are correlated from the intersection and the existence of tilted strata beneath reflection e illustrates better penetration of the cross sectional line. Root-mean-square (RMS) amplitude maps (Figs. 4B and 5D) generated over specified seismic windows (Figs. 4C, 5E and 5F) are a conclusive way of mapping the distribution and strength of

Fig. 4. (A) Seafloor morphology from 3D seismic data in the southern part of the wedge; location of survey SG9810 indicated in Figs. 1A and 2A. (B) Root-mean-square (RMS) seismic amplitude map created over the volume indicated by the grey shaded area in (C). (C) Seismic profile illustrating the seismic signature of the wedge in this area; its location is indicated in (A) and (B).

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Fig. 5. (A) Interpreted horizon with a large depression corresponding to the upper surface of the wedge buried underneath Nordkappbanken; location within 3D seismic survey NH0608 indicated in lower left corner, for larger scale location see Figs. 1A and 2A. (B) Seismic line crossing the depression visible on the upper surface of the wedge; its location is indicated in (A). (C) Seismic sketch illustrating the seismic stratigraphy in the eastern part of sediment wedge. The extent of survey NH0608 along this line is indicated by a black box, while the location of this seismic sketch transect is shown in Fig. 1A. (D) Root-mean-square (RMS) seismic amplitude map created over the volume indicated in (E) and (F) by the grey shaded area; location within 3D seismic survey NH0608 indicated in lower left corner. (E) Seismic inline 3443 through the area where upper and lower surface of the wedge merge; its location is indicated in (D). (F) Zoom into inline 3443 as displayed in (D) and (E). Interpreted thrusted blocks are colour-coded and their location is indicated in (D).

amplitudes in a seismic volume and of visualizing potential internal features. The RMS map shown in Fig. 4B was calculated for a window of 15e35 ms below the smoothed seafloor in 3D block SG9810 (indicated by a shaded grey zone in Fig. 4C). RMS amplitude values are generally low and uniform for the wedge area, while areas of depressions have slightly higher RMS amplitudes. In the north-westernmost corner RMS seismic amplitude values are significantly higher and it can be concluded that stronger seismic reflections underlying the wedge are included in the calculation. The RMS map shown in Fig. 5D is generated over a volume 70e95 ms below the smoothed seafloor (Figs. 5E and 5F) in 3D block NH0608, corresponding to the frontal part of the buried OBSW (Fig. 2A). The seismic sketch in Fig. 5C illustrates that the wedge was pushed strongly uphill in the area of Nordkappbanken. In this area, the outer margin of the wedge is characterized by the occurrence of blocks of a different material (higher product of density and velocity) than the surrounding till (Fig. 5D). These blocks have irregular shapes with sizes ranging from 50 to 1000 m.

Seismic profiles indicate that some of them are stacked on top of each other (Figs. 5E and 5F), attaining thicknesses of 15e20 m. The amplitude anomalies associated with these blocks give rise to a chaotic seismic reflection pattern that interferes with reflections from the upper and lower wedge surface (dotted yellow and green lines on Fig. 5C). 5.2. Internal characteristics of the wedge system e interpretation The overridden transverse ridge confined by reflection b (Fig. 3A), possibly an older grounding line system, was a regional bathymetric high prior to OBSW deposition. We suggest that it might have functioned as a pinning point for the ice resulting in the deposition of the OBSW. The existence of an up to 36 m thicker sediment package above reflection b in zone I as compared to zone III may indicate that significant amounts of glacimarine sediments were deposited beyond the glacier front during the formation of the wedge. Yet, all sediment cores that were retrieved downstream

Glaciproximal

Subglacial

8. D(SC)ms: Stratified diamict, matrix supported (sandy mud) 9. D(SC)mm: massive diamict, matrix supported (sandy mud) 7. D(SC)ms(bio): stratified bioturbated diamict, matrix supported (sandy mud)

Glacimarine Lag deposit Palimpsest sediment Depositional environment

3. SCtc: trough cross laminated sandy clay

Palimpsest sediment

7. D(SC)ms(bio): stratified bioturbated diamict, matrix supported (sandy mud)

Sediment plume deposit

1. Cm(bio): massive bioturbated clay 6. SCs: stratified bioturbated sandy mud

10. D(S)mm: crudely stratified diamict, matrix supported (sand) 11. D(G)cm: massive gravelly diamict, clast-supported 4. CSil/CSim: faintly laminated to massive (clayey/sandy) silt 5. CSl/SiSl: faintly laminated to massive (clayey/silty) sand Facies codes

2. SCl/Cl: laminated (sandy) clay

None Occasional, max. 2 mm Burrows Frequent, max. 5 mm None None Bioturbation Dark mud clasts

Intense None Intense None

Clasts (# >1 mm/2 cm interval) Shear strength (kPa) Water content (weight %) Wet bulk dens. (g/cm3) Upper unit boundary Lower unit boundary

0e17 (6) 1e5 (3) 22e43 (30) 1.2e2.0 Sharp Sharp

Burrows Frequent, max. 10 mm

0e17 (3) 3e7 (5) 35e39 (37) w1.6 Sharp Gradational from unit E Burrows Frequent, max. 5 mm

7e55 (25) 6e40 (21) 24e34 (30) w1.8 Gradational into unit D Sharp

75e185 D(SC) Dark grey to very dark grey (2.5Y 4/1e2.5Y 3/1) 2e53 (21) 4e63 (26) 21e34 (25) 1.8e2.0 Sharp Not recovered 60 D(SC) Very dark grey (2.5Y 3/1)

Unit E Unit D

75 C Dark grey (2.5Y 4/1)

20e160 SCeD(SC) Dark grey to very dark grey (2.5Y 4/1e2.5Y 3/1) 0e44 (16) 1e10 (4) 28e42 (35) 1.5e1.6 Sharp Sharp, erosional

Unit C Unit B

5e15 D(SC), D(G) Darker greyish brown to greyish brown (2.5Y 4/2e2.5Y 5/2) 4e52 (22) 2e35 (10) 13e29 (22) w1.4/w1.9 Sharp Erosional 30e40 SC, SiC, C Darker greyish brown (2.5Y 4/2)

Unit A2 Unit A1

6.1.2. Interpretation We interpret unit F as a subglacial traction till (cf. Evans et al., 2006), formed by a combination of deformation and lodgement processes under relatively high pore water pressures. Shear strength values observed in this study are in the range of values reported by Ó Cofaigh et al. (2007) for a soft till from an Antarctic palaeo-ice stream, and display very similar structures including attenuated soft sediment clasts, banding, aligned clasts, and shear planes. The observation of shear planes points to brittle deformation, while sediment banding may indicate early stages of ductile deformation (van der Meer et al., 2003). From the existence of attenuated soft sediment pellets it can be inferred that deformation has not been pervasive. Intervals of laminated sediment observed within the subglacial

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5e15 SiS, CS, SSi, Si Olive brown to light olive brown (2.5Y 4/4e2.5Y 5/4) 0e11 (2) 2e7 (4) 22e47 (31) 1.2e1.8 Top of core Sharp

6.1.1. Description Unit F is the lowermost recovered unit, being at least 75e185 cm thick. It contains a dark grey to very dark grey, massive, matrixsupported diamict with a sandy mud matrix. It is characterized by low water content and high, but variable, shear strength. In seven cores the uppermost 30e100 cm of unit F have lower shear strength with a mean of 13 kPa as compared to a mean of 32 kPa for the remaining unit. The sediment is generally massive, apart from occasional intervals with laminated sediment, inclined shear planes, sediment banding, aligned clasts, as well as attenuated black soft sediment pellets. Five to ten cm thick intervals with laminated sediment are observed in three cores and seem to be associated with low shear strength. Shear planes are predominantly found in the upper part with generally lower shear strength, and often occur in connection to larger clasts. Bioturbation is absent.

Thickness (cm) Lithology Colour (Munsell)

6.1. Unit F (subglacial sediment)

Lithological units

Eleven different sediment facies have been observed in a total of eleven sediment gravity cores (Table 2). The division into lithological units AeF and corresponding depositional environments is based on a variety of additional parameters including water content, undrained shear strength and clast content (Table 3).

Parameters

6. Lithostratigraphy

Table 3 Sediment properties, division into lithological units AeF as well as corresponding facies and depositional environments. Lithological abbreviations are modified from Eyles et al. (1983).

from the OBSW penetrate into subglacial facies at 0.6e2.9 m depth, suggesting that the largest part of the downstream sequence predates wedge formation. We argue, therefore, that contrasting thicknesses in zones I and III rather point to extensive upstream sediment erosion. Whether MSGL in zone III are depositional features postdating extensive erosion in this area or erosional features remains inconclusive. Reflection e (Fig. 3A) is thought to circumscribe an outcrop of tilted sedimentary rock typically underlying the glacial stratigraphy in the area. The blocks observed within the buried part of OBSW characterized by higher RMS amplitude values (Fig. 5D) are interpreted as glacitectonic megablocks and rafts (Aber and Ber, 2007) of a different sediment type compared to the surrounding sediments (much higher acoustic impedance). The stacked nature of some of these sediment blocks (Figs. 5E-F) is interpreted as glacitectonic imbricated thrust sheets (Rafaelsen et al., 2007). The observed amplitude anomalies bear similarities with megablocks and rafts as described from several stratigraphic levels in 3D block NH9803 from the outer parts of Bjørnøyrenna outside OBSW (Fig. 1A; Andreassen et al., 2004, 2007). On 2D multi-channel seismic profiles, much of the outer margin of the buried eastern part of OBSW is characterized by diffuse, discontinuous seismic reflections, indicating that irregular glacitectonic features might be common at the margin in this area. Vertical resolution of the 2D seismic data is, however, too low to establish this conclusively.

Unit F

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Fig. 6. Summary of results for sediment gravity cores JM08-0309 and JM09-KA03 together with the legend. Lithofacies codes are modified from Eyles et al. (1983) and explained in Table 3.

traction till in three cores (JM07-10, JM09-KA01, JM09-KA01(2)) are interpreted as short periods of ice-bed decoupling resulting in the deposition of subglacially sorted sediment (Piotrowski and Tulaczyk, 1999; Larsen et al., 2004). On a macroscopic scale unit F is not significantly different in the cores located downstream compared to those situated upstream and on the wedge.

6.2. Units E and D (glaciproximal sediment and sediment plume deposit) 6.2.1. Description Unit E is a bioturbated stratified, matrix-supported diamict with sandy mud matrix occurring 60 cm thick in core JM07-09 (Fig. 7).

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Up to 5 mm large, black, organic-rich mud pellets occur in variable amounts. Unit E has higher water content and lower undrained shear strength than unit F (Table 3). The upper contact of unit E into unit D is gradual and stretches over an interval of 20 cm in which clast content is progressively decreasing and colour changes from very dark grey to dark grey. Unit D is a 75 cm thick bioturbated, massive clay found in core JM07-09 (Fig. 7). Thread-like burrows and up to 5 mm large, black, organic-rich mud pellets occur frequently. Water content is high, undrained shear strength low, and clasts larger than 1 mm are absent. Semi-quantitative analyses of the foraminiferal assemblages in samples from the top of unit E as well as bottom and top of unit D were undertaken in order to guide interpretation. Both planktic and benthic foraminferal faunas were observed. The planktic fauna was dominated by the subpolar to polar species Neogloboquadrina pachyderma (Bé and Tolderlund, 1971). The benthic foraminiferal fauna was mainly dominated by Elphidium excavatum f. clavata and Cassidulina reniforme together with Cassidulina neoteretis and Melonis barleanus reflecting cold, glacimarine, well oxygenated environmental conditions (e.g. Hald and Steinsund, 1996; Wollenburg and Mackensen, 1998; Korsun and Hald, 2000). 6.2.2. Interpretation Grain size variations in front of tidewater glaciers are complex, but display two major trends. In the inner part of the ice-proximal zone, suspension settling often dominates over rain-out from icebergs resulting in deposition of relatively fine grained material and vice versa in the outer part of the ice-proximal zone where diamicts are more common (Bennett and Glasser, 2009). Moreover, Elverhøi et al. (1980) specify that sediments at 200e300 m from the grounding line have relatively high sand and gravel content, while the next 10e15 km are dominated by mud, corresponding to the inner part of the ice-proximal zone as defined by Bennett and Glasser (2009). These muds tend to lack clastic lamination which can be attributed to flocculation of clays in marine waters and consequently particle behaviour similar to that of silt (Sauramo, 1923). Core JM07-09, in which units D and E occur, is situated 1.5 km from the frontal slope. We therefore suggest that the observed units mark a gradual transition from glaciproximal conditions under the influence of an advancing ice stream, where rain-out from icebergs was intense, into more stable conditions of sediment plume deposits in the inner part of the ice-proximal zone. Observed bioturbation indicates that benthic life was possible throughout the deposition of unit D. Observed organic-rich mud pellets could be fecal pellets which form as zooplankton grazes on flocs and agglomerates, typically occurring in settings rich in suspended sediment (Syvitski, 1989, 1991). Alternatively, these pellets may be erosional products of the Late Jurassic “Hot Shale” occurring on Svalbard (Agardhfjellet Formation), Spitsbergenbanken and the central Barents Sea (Elverhøi et al., 1995). Under the premise of fast sedimentation it can be envisioned that the contained organic matter is not degraded.

6.3. Unit C (glacimarine sediment) 6.3.1. Description Unit C encompasses dark grey to very dark grey, laminated to stratified sandy mud with scattered clasts, and a stratified matrixsupported diamict with a sandy mud matrix with higher clast content. Bioturbation is common in form of thread-like burrows. Up to 10 mm large mud pellets, predominantly black in colour and organic-rich, typically occur in varying amounts. High water content and low undrained shear strength characterize this unit (Table 3).

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As part of preliminary stratigraphic investigations semiquantitative analyses of the foraminiferal assemblages were also carried out on selected levels in unit C. The same planktic and benthic fauna that was found in units E and D was also observed in unit C reflecting a comparably cold, glacimarine environment. 6.3.2. Interpretation The existence of stratification with varying amounts of sand as well as gravel in cores close to the shelf edge is suggested to reflect ice rafting. Based on the general scarcity of sedimentation on the outer shelf, we deem proximity to a glacier front the most likely source as is supported by the observed foraminiferal assemblages. Unit C encompasses both diamicts and sandy mud with occasional gravel clasts. Whereas the deposition of muds presumably reflects conditions in the inner part of the ice-proximal zone, where suspension settling dominates over rain-out from icebergs (e.g. JM08-0309 in Fig. 6), we assume that the opposite conditions occurred in the outer part of the zone, resulting in the deposition of diamicts (e.g. JM09-KA03 in Fig. 6; cf. Bennett and Glasser, 2009). The existence of abundant burrows indicates favourable conditions for benthic organisms. We assume that sedimentation rates have been relatively high, since stratification is not totally obscured by bioturbation. The frequent occurrence of organic-rich mud pellets may further support the idea of fast sedimentation in a proglacial environment.

6.4. Unit B (lag sediment) 6.4.1. Description Unit B is composed of a 5e15 cm thick, greyish brown to darker greyish brown diamict. Unit B can be a crudely stratified, upwards coarsening diamict with sandy matrix where it overlays unit C (e.g. core JM09-KA03 in Fig. 6), while a clast-supported, asymmetrically bedded diamict with erosive base is observed where it overlays unit F. 6.4.2. Interpretation The sandy diamict and gravelly diamict facies are suggested to be derived from winnowing of underlying sediments comprising units C and F, respectively. According to Vorren et al. (1983) the ploughing of icebergs causes deformation, reworking, and some sorting or winnowing of the bottom sediments, which may lead to a complete depletion in fines. We suggest that, depending on the lithology of the sediment that is being ploughed, the resulting lag deposit can consist of gravel over subglacial tills (as described for unit dB in Vorren et al., 1984) or of a sandy diamict over glacimarine sediments.

6.5. Units A1 and A2 (palimpsest sediment) 6.5.1. Description Unit A1 is present in eight sediment cores with thicknesses varying from 5 to 15 cm. It consists of light olive brown to greyish brown, massive to faintly laminated sandy silt to clayey sand. In four of the cores A1 is underlain by 30e40 cm thick unit A2 (e.g. Fig. 7). Unit A2 consists of darker greyish brown trough cross bedded clays and sands followed by laminated sandy or silty clays. Unit A is characterized by low undrained shear strength, relatively high water content and relatively large variations in wet bulk density (Table 3). Clast content is comparatively low in unit A, while highest amounts occur along bedding planes in the lower cross bedded sandy clays of unit A2.

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Fig. 7. Summary of results for sediment gravity core JM07-09. The legend is shown in Fig. 6. Lithofacies codes are modified from Eyles et al. (1983) and explained in Table 3.

6.5.2. Interpretation Based on comparison with a study of over 200 sediment gravity cores from the southern Barents Sea (Vorren et al., 1984), we interpret unit A as palimpsest sediments. According to Vorren et al. (1984) erosional activity and winnowing on the banks and other exposed areas has ceased by 7.8 cal ka BP. A presumable less pronounced erosional regime during late Holocene resulted in the deposition of a mixture of reworked sediments and recent predominantly biogenic sands, referred to as palimpsest sediment (Unit dA in Vorren et al., 1978, 1984). 7. Chronology The transition from grounded ice (subglacial traction till) in outer Bjørnøyrenna into glacimarine conditions was dated in three cores (JM07-09, JM08-0309, JM09-KA03) located downstream from the sediment wedge. Two AMS dates on bulk benthic foraminifera from the base of glacimarine sediment (unit C) in cores JM08-0309 and JM09-KA03 (Fig. 6) yielded ages of 17,090 cal yrs BP (14,530  65 14C

yrs BP) and 16,580 cal yrs BP (13,835  60 14C yrs BP), respectively (Table 4). A sample from the transition from glaciproximal sediments (unit E) to sediment plume deposit (unit D) in core JM07-09 (Fig. 7) provided an age of 16,920 cal yrs BP (14,320  80 14C yrs BP; Table 4). Bioturbation throughout the sediment plume deposit (unit D) indicates that the settling from suspension took place slowly enough for benthic organisms to thrive. This makes us confident that the dated bulk benthic foraminifera at the base of the sediment plume deposit were mostly living fauna, rather than reworked, and reflect the age of the wedge deposition. In the upstream core (JM08-0306), Holocene palimpsest sediment (unit A2) directly overlies subglacial traction till (unit F) and an AMS date on the planktic foraminifera species N. pachyderma (sin) resulted in an age of 6900 cal yrs BP (6485  45 14C yrs BP; Table 4). Two radiocarbon dates on macrofossils with ages of 3065 cal yrs BP (3310  40 14C yrs BP; Table 4) and 2020 cal yrs BP (2455  40 14C yrs BP; Table 4) render further age control for unit A2 and create the impression of a complete absence of Early Holocene sediments.

TRa-263 TRa-262 TRa-261 TUa-7773 TUa-7774 TUa-8193 15,869e16,853 16,857e17,467 16,743e17,174 2890e3238 1862e2171 6733e7084 16,316e16,753 16,923e17,218 16,822e17,030 2975e3155 1944e2096 6814e6982 16,580 17,090 16,920 3065 2020 6900 1.0 1.0 1.0 2.7 2.1 1.0 60 65 80 40 40 45 13,835 14,530 14,320 3310 2455 6485 Base of unit C Base of unit C Transition from unit E to D Base of unit A2 Unit A2 Base of unit A2 JM09-KA03, 178e180 cm JM08-0309, 68e70 cm JM07-09, 126e128 cm JM07-09, 45 cm JM08-0306, 29 cm JM08-0306, 42e44 cm

Bulk benthic foraminifera Bulk benthic foraminifera Bulk benthic foraminifera Macandrevia cranium Cyclopecten imbrefer Neogloboquadrina pachyderma (sin)

2s range 1s range Cal yrs BP (mode)

d13C & 1s (uncorrected) C yrs BP 14

Material Lithological unit Core ID and level

Table 4 Uncorrected and calibrated radiocarbon ages presented in this study; calibration based on the Marine09 calibration curve (Reimer et al., 2009) and a DR value of 67  41 (Mangerud and Gulliksen, 1975).

Lab ID

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8. Discussion 8.1. Timing and pattern of retreat of the western Barents Ice Sheet margin Deglaciation ages from Tromsøflaket and Ingøydjupet reflect the decline of the Fennoscandian Ice Sheet (Rokoengen et al., 1977; Vorren et al., 1978; Vorren and Kristoffersen, 1986; Junttila et al., 2010), whereas the retreat of the Barents Sea Ice Sheet is documented in Storfjordrenna in the north and in the south-eastern Barents Sea (Polyak et al., 1995; Rasmussen et al., 2007; Fig. 8). Direct deglaciation ages from the Barents Sea shelf (indicated by blue dots in Fig. 8) are rare. This implies that the timing of the Barents Sea Ice Sheet retreat in the south-western part is so far merely based on evidence from continental slope records (indicated by blue triangles in Fig. 8). The dates from the bottom of glacimarine or glaciproximal units in three cores downstream from OBSW (17,090 cal yrs BP, 16,920 cal yrs BP, and 16,580 cal yrs BP) are suggested to represent an age for OBSW deposition as well as a reliable minimum deglaciation age for outer Bjørnøyrenna. For the western Svalbard margin there is wide agreement for a distinct meltwater peak at 17.6 cal ka BP (14.5 corr ka BP; Elverhøi et al., 1995; Knies et al., 1999; Vogt et al., 2001). On the other hand, some authors point out the significance of a smaller, short-termed ice rafted debris (IRD) pulse as distinct deglaciation signal around or shortly before 20 cal BP (Knies et al., 1999 for the northern margin; Jessen et al., 2010 for the western margin). Outermost Storfjordrenna was deglaciated by approximately the same time period as revealed by an IRD peak in the slope record from about 21.2 to 19.8 cal ka BP (Jessen et al., 2010) and by a series of radiocarbon dates in hemipelagic sediments indicating open water since at least 19.4 cal ka BP (16.8 14C ka BP; Rasmussen et al., 2007). Based on IRD analyses from several continental slope records, Bischof (1994) identified a strong meltwater pulse derived from the southern part of the Barents Sea Ice Sheet between about 17.6 and 17.0 cal ka BP (14.5e14 corr ka BP). Our results indicate glacimarine conditions and deposition of a wide sediment wedge in outermost Bjørnøyrenna at about 17.1e16.6 cal ka BP, thus immediately postdating the pronounced meltwater spike from the continental slope. The regional IRD event described by Bischof (1994) may therefore reflect the first oscillation of the Bjørnøyrenna Ice Stream rather than its final retreat. Minimum estimates of glacial retreat from Tromsøflaket range from 16.7 to 16.5 cal ka BP (13.9 14C ka BP in Vorren et al., 1978; 13.8 14C ka BP in Rokoengen et al., 1977, Fig. 8). Winsborrow et al. (2010) suggest the Coast-parallel Trough Ice Stream off the Finnmark coast to be active contemporaneous with OBSW deposition during stage 2 in their deglaciation reconstruction (Fig. 8). It was fed by the Fennoscandian Ice Sheet and terminated in a lobate sedimentary deposit in southern Ingøydjupet. Deglaciation ages presented by Junttila et al. (2010) are conformable with this interpretation suggesting open water downstream from (north of) the Ingøydjupet lobate sedimentary deposit from 18.6 cal ka BP (15.8 14C ka BP) and open water in the lobe area from 15 cal ka BP (13.1 14C ka BP). An earlier study by Vorren and Kristoffersen (1986) established a deglaciation age of 16.5 cal ka BP (13.7 14C ka BP) for the lobe area, indicating that the lobate sedimentary deposit in Ingøydjupet may predate OBSW deposition (Fig. 8). Moreover, Winsborrow et al. (2010) point out two sets of MSGL and a sedimentary deposit in Håkjerringdjupet 15 km from the shelf break (Fig. 8), which are thought to represent a contemporaneous development of ice stream retreat and readvance, albeit at smaller scale than in outer Bjørnøyrenna (Fig. 8). Between the glacial margins in Håkjerringdjupet and southern Ingøydjupet, grounded ice is thought to have extended from the Norwegian mainland about halfways onto Tromsøflaket leaving a corridor of

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Fig. 8. Compilation of published direct and indirect deglaciation ages as well as suggested stages of ice stream retreat for the western Barents Sea region. Blue triangles show reference and calibrated indirect deglaciation ages from continental slope records. Blue dots represent references and calibrated direct minimum deglaciation ages on the Barents shelf. The dotted orange and yellow lines refer to deglaciation stages 2 and 3, respectively, as suggested by Winsborrow et al. (2010). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

open water towards the readvancing Bjørnøyrenna Ice Stream to the north (Winsborrow et al., 2010; Fig. 8). It can therefore be concluded that Barents Sea and Fennoscandian ice sheets were not confluent any more at this stage of the deglaciation. 8.2. Analogues and existing depositional models The OBSW has been termed grounding zone wedge previously (Winsborrow et al., 2010) which is a generic term introduced by Powell and Domack (1995) encompassing and replacing the terms till tongue (King et al., 1991) and till delta (Alley et al., 1987). Grounding zone wedges are typically found in polar settings with confined meltwater, and build up as a combination of topsets of stacked or amalgamated deforming beds and foresets to bottomsets of density flows, possibly slumps and rain-out (Powell and Alley, 1997). While dimension and geometry of the OBSW are comparable to observed grounding zone wedges in Antarctica (e.g. Alley et al., 1989; O’Brien et al., 1999; Shipp et al., 2002), its distinct morphology of large depressions in the frontal part has, to our knowledge, not been described in the literature; possibly because analyses are seldom based on extensive sets of bathymetry data. In

the systematization of sediments deposited at or near the grounding line of marine-terminating glaciers, morainal banks are understood as the opposite end-member to grounding zone wedges (Powell and Alley, 1997). Morainal banks are most commonly associated with temperate and subpolar glaciers where meltwater is more readily available resulting in positive topographic features. The OBSW has a wedge-geometry and the term morainal bank is therefore inappropriate. Andreassen et al. (2008) suggested overlapping and timetransgressive deposition of lobes in their account of the OBSW. However, the fan-shaped outline, similar length of all lobes, as well as absence of onlapping reflections on seismic cross profile (Fig. 3B) may indicate that the OBSW was deposited as one body of sediment. With a gentler slope on the stoss than the lee side, push, squeeze, and lodgement seem the most likely involved depositional mechanisms (Powell and Alley, 1997). Analysis of the seismic section has revealed erosion of sediment in the upstream part of the wedge with most extensive removal of 36 m of sediment coinciding with the onset area of MSGL, while the frontal part is depositional (Fig. 3A). The inferred extensive redistribution of sediment may further support pushing as depositional mechanism

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and suggests a rather rapid readvance of the ice stream (e.g. Boulton et al., 1996; Bennett et al., 1999). The fan-shaped geometry is another indication that a rapid readvance of the ice stream may have taken place, since typical calving margins in the relatively deep shelf waters are straight (Boulton et al., 2001). The idea of a rapid ice stream readvance raises the question whether the Bjørnøyrenna Ice Stream behaviour bears resemblance to a surge. Surges are internally triggered, cyclic glacier flow instabilities and, although most commonly associated with smaller glaciers, this definition also applies to ice stream behaviour (Meier and Post, 1969). At this point, our palaeo data does not support the existence of a surge cyclicity including long quiescent and short active surge phase. Several studies of glacier surges in fjords on Svalbard, reveal that surge moraines are typically accompanied by large muddy debris flow lobes on their distal sides (Boulton et al., 1996; Plassen et al., 2004; Ottesen and Dowdeswell, 2006; Ottesen et al., 2008b; Kristensen et al., 2009). A debris flow deposit distal to the OBSW has not been observed. Crevasse-squeeze ridges are another landform typically associated with surging tidewater glaciers (Boulton et al.,1996; Ottesen and Dowdeswell, 2006; Ottesen et al., 2008b) and are thought to be indicative of surges (Sharp, 1985). A heavily ploughed seafloor in the study area does, however, not allow any conclusions as to the existence of crevasse-squeeze ridges. While acknowledging that there is no conclusive evidence for an ice stream surge in outer Bjørnøyrenna, we argue that certain aspects of surge landform models might advance the understanding of the observed wedge system. The morphology and internal structures of moraines deposited through glacial surges vary with foreland properties as illustrated by the continuum of proglacial tectonics of surging glaciers suggested by Kristensen et al. (2009). Where proglacial materials have high shear strengths, e.g. permafrozen ground, sediment coherence may be maintained resulting in the deposition of thrust block moraines (e.g. Boulton et al., 1999). For low shear strength material, such as marine muds, proglacial pushing will lead to glacitectonic deformation, oversteepening, and subsequent failure, such that the glacier front is pushing a continuously deforming slurry. Low, homogenous seismic RMS amplitudes are observed for the central part of the wedge in 3D block SG9810 (Figs.1A and 4B). The seismic profile and cross profile (Fig. 3A, B) also reveal a diffuse seismic signature without abrupt changes in seismic amplitude. Therefore, large parts of the wedge seem to be dominated by ductile deformation as expected when relatively soft marine muds are pushed by the ice front (Kristensen et al., 2009). In the part of the wedge buried under Nordkappbanken megablocks and rafts occur at the margin and are associated with thrust sheets as revealed by analysis of 3D block NH0608 (Figs. 1A and 5DeF). As indicated in Fig. 5C, Cretaceous sedimentary rocks are subcropping upstream of the area where amplitude anomalies are observed. We, therefore, assume that blocks of sedimentary rock were detached and incorporated in the moraine at this location, forming a composite ridge (Aber and Ber, 2007). The margin in this area is fuzzy and characterized by thrusted, imbricated megablocks and rafts, which is indicative of the importance of brittle deformation in this part of the wedge. To our knowledge, neither surge landform models nor concepts about continuous grounding line deposition explain and discuss the occurrence of up to 8 km wide depressions. It could be envisioned that these depressions are giant pockmarks (Solheim and Elverhøi, 1993; Kelley et al., 1994). While pockmarks are known to be circular to semicircular, more irregularly shaped craters up to 4 km in diameter are also reported (Cole et al., 2000). However, in outer Bjørnøyrenna no indications of fluid migration are found in association with large scale depressions (Figs. 4C and 5B). We note that the strong reflection underneath OBSW depression in Fig. 5B has the same positive polarity as the seafloor reflection, and is

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therefore not indicative of the presence of gas. Alternatively, depressions may form following ice sheet break-up as large icebergs ground and create wallows. While observed depressions are beyond the scale of any reported iceberg ploughmark (e.g. Kuijpers et al., 2007), the disintegration of an ice sheet is likely to produce an exceptional amount of large icebergs. Wallows created by grounded icebergs are known to create pronounced hummocky rims (Lien et al., 1989). In outer Bjørnøyrenna the sediment supposedly removed from potential wallows is not matched by sediment accumulated on the rims (Figs. 2E, 4A and 5A). It seems that the frontal part of the wedge bears most resemblance to deadice moraine landscapes as known from terrestrial settings (e.g. Maizels, 1992; Evans et al., 1999). However, kame and kettle topographies (Paul, 1983) or hummocky moraines (Benn, 1992) in terrestrial settings are 1e2 orders of magnitude smaller than observed depressions and elevated areas in our study area. More important, the buoyancy of ice implies that processes involved in the marine environment during burial and melt-out of ice must be fundamentally different from the stepwise and lengthy processes on land (Kjær and Krüger, 2001). In the following section we propose a new conceptual model of rapid ice stream readvance. Crucially, and perhaps counter-intuitively, this model incorporates the idea of ice-burial in a marine environment. 8.3. Conceptual model It is likely that the Barents Ice Sheet had thinned considerably in response to a warming event at around 22e21.4 cal ka BP (18.3e17.9 14 C ka BP; Alm, 1993) as identified in the palynostratigraphy from Andøya, northern Norway (Fig. 1B). This has consequences for the anticipated ice dynamics as basal freeze-on tends to be rapid beneath thin glaciers either upon climatic cooling (Alley et al., 1997) or through horizontal advection of cold ice (Christoffersen and Tulaczyk, 2003). Based on geotechnical studies by Sættem et al. (1992, 1996), basal freeze-on has been identified to be a common process on the glaciated south-western Barents shelf. As basal freeze-on is thought to be the primary process of ice stream slow down and termination (Tulaczyk et al., 2000), we argue that the Bjørnøyrenna Ice Stream may have slowed down or stopped prior to its readvance. The grounding line position is suggested to coincide with the bathymetric high above a sedimentary rock outcrop (Fig. 3A), i.e. slightly up-glacier from the onset of well-defined MSGL. In the suggested conceptual model, the early stage of the rapid advance is characterized by intense proglacial and englacial thrusting (Fig. 9A). This may have been facilitated by the existence of a cold ice front which is being pushed by warm ice masses from the interior (e.g. Larsen et al., 2010). Large slabs of ice at the cold glacier snout are suggested to get overrun by the advancing ice stream, whereby englacial thrust planes develop into a decollement. During a later stage of the readvance (Fig. 9B) blocks of ice have therefore been incorporated into the moraine body despite the buoyancy of ice. Upon the final retreat of the Bjørnøyrenna Ice Stream, the buried blocks of ice are unlikely to melt in situ, since overlying sediment is not sufficient to compensate for buoyancy. Rather these blocks are uplifted and create large scale, irregularly shaped depressions in the frontal depositional part of the OBSW (Fig. 9C). The upstream part of the resulting landsystem is characterized by extensive erosion and occurrence of MSGL which may be erosional and/or depositional features (Fig. 9C). The conceptual model shown here differentiates between two glacitectonic end-members that we find evidence for in our analyses. The cartoon to the left in Fig. 9 is characteristic for the central part of the wedge where ductile failure of soft sediment prevails. In the east, buried under Nordkappbanken, incorporation of more rigid material through englacial or proglacial thrusting suggests itself,

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Fig. 9. Conceptual model involving early (A), late (B), and post- (C) stage of an ice stream readvance. Two end-members are distinguished: one typical for the central part of the wedge as shown on the left and one typical for the eastern, buried part of the wedge as shown on the right. Ductile failure of soft sediments prevails in the central part, while elements of brittle failure occur in the east, where rigid material is incorporated in the moraine through proglacial and/or englacial thrusting.

resulting in interplay of brittle and ductile deformation (right side in Fig. 9). Once advanced and drawn down to a thin state the ice sheet possibly returned to a cold state relatively quickly (Alley et al., 1997). It was probably stable for a while since the moraine may have had a stabilizing effect with respect to variations in sea level (Alley et al., 2007; Anandakrishnan et al., 2007). Our analyses indicate that the ice stream front probably occupied outer Bjørnøyrenna from about 17.1 to 16.6 cal ka BP, and final break-up of the Barents Ice Sheet presumably occurred shortly after. A rough estimate indicates that the volume of the 30 m high, 280 km wide and 30 km long sediment wedge is approximately 450,000 m3 per unit width. Sediment flux at the grounding line can be calculated to be on the order of 105 m3 m1 a1 in case of the push moraine being deposited almost instantaneously in the course of a couple of years or 102 m3 m1 a1 for a more conservative time estimate of half a millennium. The latter is within the range of reported grounding line sediment fluxes (Anandakrishnan et al., 2007; Nygård et al., 2007), however we consider it conceivable that the same sediment volume was moved during one rapid readvance event. 9. Conclusions Based on seafloor geomorphology, acoustic characteristics, lithology, and radiocarbon ages for a 280 km wide sediment wedge in outer Bjørnøyrenna, a conceptual model for rapid marine ice stream readvances during deglaciation is proposed. Observed upstream erosion and formation of MSGL as well as deposition of a frontal rim with a complex morphology of depressions and elevated areas challenges existing depositional models.

Depressions, up to 8 km across and 30 m deep, are explained as the result of submarine burial of ice. During a suggested rapid readvance scenario warm ice masses from the interior advance towards a presumably cold glacier snout, such that intense englacial thrusting develops into a decollement. Parts of the ice front thereby become isolated and overrun by the advancing glacier which subsequently pushes and bulldozes a combination of soft diluted proglacial sediment and slabs of ice. While dominated by ductile deformation the ice stream advance also includes elements of brittle failure most likely in locations where the ice stream erodes and incorporates blocks of sedimentary rock in the moraine. AMS dates on bulk benthic foraminifera from the bottom of glacimarine units in three cores situated downstream from the sediment wedge yielded calibrated radiocarbon ages ranging from 17,090 to 16,580 cal yrs BP. This indicates that the ice stream front has been stable over at least 500 yrs before it retreated further. Acknowledgements We are grateful to the PhD Trainee School in Arctic Marine Geology and Geophysics for funding the position of D. Rüther (project: The palaeoenvironment of the Euro-Arctic Margin during the last deglaciation and Holocene). We further acknowledge the Norwegian Research Council, as well as oil companies Det Norske Oljeselskap, BG Norge and Statoil for funding the position of R. Mattingsdal (PetroMaks Project number 200672: Glaciations in the Barents Sea area: GlaciBar). We thank Erik Mauring at the Norwegian Geological Survey for compiling the large scale bathymetry data and Statoil for making it available to us as an ASCII file. Thanks to the captains and crews

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onboard R/V Jan Mayen and to engineer Steinar Iversen who provided much appreciated technical help during and after cruises. Steffen Aagaard Sørensen carried out the semi-quantitative analyses of foraminiferal assemblages. Katarzyna Zamelczyk and Chiara Consolaro assisted in identification of foraminifera for AMS dating. We are grateful to the editor Neil Glasser, referee Colm Ó Cofaigh, and one anonymous referee for their constructive critique which we feel has improved this paper significantly. References Aber, J.S., Ber, A., 2007. Glaciotectonism. Elsevier, Amsterdam. Alley, R.B., Blankenship, D.D., Bentley, C.R., Rooney, S.T., 1987. Till beneath ice stream B. 3. Till deformation: evidence and implications. Journal of Geophysical Research 92 (B9), 8921e8929. Alley, R.B., Blankenship, D.D., Rooney, S.T., Bentley, C.R., 1989. Sedimentation beneath ice shelves e the view from ice stream B. Marine Geology 85 (2e4), 101e120. Alley, R.B., Cuffey, K.M., Evenson, E.B., Strasser, J.C., Lawson, D.E., Larson, G.J., 1997. How glaciers entrain and transport basal sediment: physical constraints. Quaternary Science Reviews 16 (9), 1017e1038. Alley, R.B., Anandakrishnan, S., Dupont, T.K., Parizek, B.R., Pollard, D., 2007. Effect of sedimentation on ice-sheet grounding-line stability. Science 315 (5820), 1838e1841. Alm, T., 1993. Øvre Æråsvatn e palynostratigraphy of a 22,000 to 10,000 BP lacustrine record on Andøya, northern Norway. Boreas 22 (3), 171e188. Anandakrishnan, S., Catania, G.A., Alley, R.B., Horgan, H.J., 2007. Discovery of till deposition at the grounding line of Whillans Ice Stream. Science 315 (5820), 1835e1838. Anderson, J.B., Wellner, J.S., Lowe, A.L., Mosola, A.B., Shipp, S.S., 2001. Footprint of the expanded West Antarctic ice sheet: ice stream history and behavior. GSA Today 11 (10), 4e9. Andreassen, K., Winsborrow, M.C., 2009. Signature of ice streaming in Bjørnøyrenna, Polar North Atlantic, through the Pleistocene and implications for ice-stream dynamics. Annals of Glaciology 50 (52), 17e26. Andreassen, K., Nilssen, L.C., Rafaelsen, B., Kuilman, L., 2004. Three-dimensional seismic data from the Barents Sea margin reveal evidence of past ice streams and their dynamics. Geology 32 (8), 729e732. Andreassen, K., Ødegaard, C.M., Rafaelsen, B., 2007. Imprints of former ice streams, imaged and interpreted using industry three-dimensional seismic data from the south-western Barents Sea. In: Davies, R.J., Posamentier, H.W., Wood, L.J., Cartwright, J.A. (Eds.), Seismic Geomorphology: Applications to Hydrocarbon Exploration and Production. Geological Society, Special Publications, London, pp. 151e169. Andreassen, K., Laberg, J.S., Vorren, T.O., 2008. Seafloor geomorphology of the SW Barents Sea and its glaci-dynamic implications. Geomorphology 97 (1e2),157e177. Bamber, J.L., Alley, R.B., Joughin, I., 2007. Rapid response of modern day ice sheets to external forcing. Earth and Planetary Science Letters 257, 1e13. Bé, A.W.H., Tolderlund, O.L., 1971. Distribution and ecology of living planktonic foraminifera in surface waters of the Atlantic and Indian Oceans. In: Funnell, B.M., Riedel, W.R. (Eds.), Micropaleontology of the Oceans. Cambridge University Press, London, pp. 105e149. Benn, D.I., 1992. The genesis and significance of ‘hummocky moraine’: evidence from the Isle of Skye, Scotland. Quaternary Science Reviews 11 (7e8), 781e799. Bennett, M.R., Glasser, N.F., 2009. Sedimentation in marine environments. In: Bennett, M.R., Glasser, N.F. (Eds.), Glacial Geology: Ice Sheets and Landforms, second ed. John Wiley & Sons Ltd, Chichester. Bennett, M.R., Hambrey, M.J., Huddart, D., Glasser, N.F., Crawford, K., 1999. The landform and sediment assemblage produced by a tidewater glacier surge in Kongsfjorden, Svalbard. Quaternary Science Reviews 18, 1213e1246. Bentley, C.R., 1987. Antarctic ice streams: a review. Journal of Geophysical Research 92 (B9), 8843e8858. Bischof, J.F., 1994. The decay of the Barents ice sheet as documented in Nordic seas ice-rafted debris. Marine Geology 117 (1e4), 35e55. Boulton, G.S., van der Meer, J.J.M., Hart, J., Beets, D., Ruegg, G.H.J., Van Der Wateren, F.M., Jarvis, J., 1996. Till and moraine emplacement in a deforming bed surge e an example from a marine environment. Quaternary Science Reviews 15 (10), 961e987. Boulton, G.S., van der Meer, J.J.M., Beets, D.J., Hart, J.K., Ruegg, G.H.J., 1999. The sedimentary and structural evolution of a recent push moraine complex: Holmstrømbreen, Spitsbergen. Quaternary Science Reviews 18 (3), 339e371. Boulton, G.S., Dongelmans, P., Punkari, M., Broadgate, M., 2001. Palaeoglaciology of an ice sheet through a glacial cycle: the European ice sheet through the Weichselian. Quaternary Science Reviews 20 (4), 591e625. Canals, M., Urgeles, R., Calafat, A.M., 2000. Deep sea-floor evidence of past ice streams off the Antarctic Peninsula. Geology 28 (1), 31e34. Christoffersen, P., Tulaczyk, S., 2003. Signature of palaeo-ice-stream stagnation: till consolidation induced by basal freeze-on. Boreas 32 (1), 114e129. Clark, C.D., 1993. Mega-scale glacial lineations and cross-cutting ice-flow landforms. Earth Surface Processes and Landforms 18 (1), 1e29. Cole, D., Stewart, S.A., Cartwright, J.A., 2000. Giant irregular pockmark craters in the Palaeogene of the outer Moray Firth Basin, UK North Sea. Marine and Petroleum Geology 17, 563e577.

2791

Dahlgren, K.I.T., Vorren, T.O., Stoker, M.S., Nielsen, T., Nygård, A., Sejrup, H.P., 2005. Late Cenozoic prograding wedges on the NW European continental margin: their formation and relationship to tectonics and climate. Marine and Petroleum Geology 22 (9e10), 1089e1110. Elverhøi, A., Solheim, A., 1983. The Barents Sea ice sheet e a sedimentological discussion. Polar Research 1 (1), 23e42. Elverhøi, A., Liestøl, O., Nagy, J., 1980. Glacial erosion, sedimentation and microfauna in the inner part of Kongsfjorden, Spitsbergen. Norsk Polarinstitutt Tidskrift 172, 33e61. Elverhøi, A., Andersen, E.S., Dokken, T., Hebbeln, D., Spielhagen, R., Svendsen, J.I., Sørflaten, M., Rørnes, A., Hald, M., Forsberg, C.F., 1995. The growth and decay of the Late Weichselian ice sheet in Western Svalbard and adjacent areas based on provenance studies of marine sediments. Quaternary Research 44 (3), 303e316. Evans, D.J.A., Lemmen, D.S., Rea, B.R., 1999. Glacial landsystems of the southwest Laurentide Ice Sheet: modern Icelandic analogues. Journal of Quaternary Science 14 (7), 673e691. Evans, D.J.A., Phillips, E.R., Hiemstra, J.F., Auton, C.A., 2006. Subglacial till: formation, sedimentary characteristics and classification. Earth-Science Reviews 78 (1e2), 115e176. Eyles, N., Eyles, C.H., Miall, A.D., 1983. Lithofacies types and vertical profile models; an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences. Sedimentology 30 (3), 393e410. Faleide, J.I., Vågnes, E., Gudlaugsson, S.T., 1993. Late MesozoiceCenozoic evolution of the south-western Barents Sea in a regional rift-shear tectonic setting. Marine and Petroleum Geology 10 (3), 186e214. Faleide, J.I., Solheim, A., Fiedler, A., Hjelstuen, B.O., Andersen, E.S., Vanneste, K., 1996. Late Cenozoic evolution of the western Barents Sea-Svalbard continental margin. Global and Planetary Change 12 (1e4), 53e74. Grobe, H., 1987. A simple method for the determination of ice-rafted debris in sediment cores. Polarforschung 57 (3), 123e126. Hald, M., Steinsund, P.I., 1996. Benthic Foraminifera and carbonate dissolution in the surface sediments of the Barents and Kara Seas. In: Stein, R., Ivanov, G.I., Levitan, M.A., Fahl, K. (Eds.), SurfaceeSediment Composition and Sedimentary Processes in the Central Arctic Ocean and along the Eurasian Continental Margin. Polarforschung, Berlin, pp. 285e307. Hansbo, S., 1957. A New Approach to the Determination of the Shear Strength of Clay by the Fall-cone Test. Royal Swedish Geotechnical Institute, Stockholm. Jessen, S.P., Rasmussen, T.L., Nielsen, T., Solheim, A., 2010. A new Late Weichselian and Holocene marine chronology for the western Svalbard slope 30,000e0 cal years BP. Quaternary Science Reviews 29 (9e10), 1301e1312. Junttila, J., Aagaard-Sørensen, S., Husum, K., Hald, M., 2010. Late GlacialeHolocene clay minerals elucidating glacial history in the SW Barents Sea. Marine Geology 276 (1e4), 71e85. Kelley, J.T., Dickson, S.M., Belknap, D.F., Barnhardt, W.A., Henderson, M., 1994. Giant sea-bed pockmarks: evidence for gas escape from Belfast Bay, Maine. Geology 22, 59e62. King, E.C., Hindmarsh, R.C.A., Stokes, C.R., 2009. Formation of mega-scale glacial lineations observed beneath a West Antarctic ice stream. Nature Geoscience 2, 585e588. King, L.H., Rokoengen, K.R., Fader, G.B.J., Gunleiksrud, T., 1991. Till-tongue stratigraphy. Geological Society of America Bulletin 103 (5), 637e659. Kjær, K.H., Krüger, J., 2001. The final phase of dead-ice moraine development: processes and sediment architecture, Kötlujökull, Iceland. Sedimentology 48 (5), 935e952. Knies, J., Vogt, C., Stein, R., 1999. Late Quaternary growth and decay of the Svalbard/ Barents Sea ice sheet and paleoceanographic evolution in the adjacent Arctic Ocean. Geo-Marine Letters 18, 195e202. Kongsberg-Simrad, 2002. EM300 Multibeam echo sounder e operator manual. Korsun, S., Hald, M., 2000. Seasonal dynamics of benthic foraminifera in a glacially fed fjord of Svalbard, European Arctic. Journal of Foraminiferal Research 30 (4), 251e271. Kristensen, L., Benn, D.I., Hormes, A., Ottesen, D., 2009. Mud aprons in front of Svalbard surge moraines: evidence of subglacial deforming layers or proglacial glaciotectonics? Geomorphology 111 (3e4), 206e221. Kuijpers, A., Dalhoff, F., Brandt, M.P., Hümbs, P., Schott, T., Zotova, A., 2007. Giant iceberg plow marks at more than 1 km water depth offshore West Greenland. Marine Geology 246, 60e64. Laberg, J.S., Vorren, T.O., 1996. The Middle and Late Pleistocene evolution and the Bear Island Trough Mouth Fan. Global and Planetary Change 12 (1e4), 309e330. Landvik, J.Y., Bondevik, S., Elverhøi, A., Fjeldskaar, W., Mangerud, J., Salvigsen, O., Siegert, M.J., Svendsen, J.-I., Vorren, T.O., 1998. The last glacial maximum of Svalbard and the Barents sea area: ice sheet extent and configuration. Quaternary Science Reviews 17 (1e3), 43e75. Larsen, N.K., Piotrowski, J.A., Kronborg, C., 2004. A multiproxy study of a basal till: a time-transgressive accretion and deformation hypothesis. Journal of Quaternary Science 19 (1), 9e21. Larsen, N.K., Kronborg, C., Yde, J.C., Knudsen, N.T., 2010. Debris entrainment by basal freeze-on and thrusting during the 1995e1998 surge of Kuannersuit Glacier on Disko Island, west Greenland. Earth Surface Processes and Landforms 35 (5), 561e574. Lien, R., Solheim, A., Elverhøi, A., Rokoengen, K., 1989. Iceberg scouring and sea bed morphology on the eastern Weddell Sea shelf, Antarctica. Polar Research 7, 43e57. Maizels, J., 1992. Boulder ring structures produced during Jökulhlaup flows. Origin and hydraulic significance. Geografiska Annaler 74A (1), 21e33.

2792

D.C. Rüther et al. / Quaternary Science Reviews 30 (2011) 2776e2792

Mangerud, J., Gulliksen, S., 1975. Apparent radiocarbon ages of recent marine shells from Norway, Spitsbergen, and Arctic Canada. Quaternary Research 5, 263e273. Mangerud, J., Bondevik, S., Gulliksen, S., Hufthammer, A.K., Høisæter, T., 2006. Marine 14C reservoir ages for 19th century whales and molluscs from the North Atlantic. Quaternary Science Reviews 25 (23e24), 3228e3245. Meier, M.F., Post, A., 1969. What are glacier surges? Canadian Journal of Earth Sciences 6 (4), 807e817. Mosola, A.B., Anderson, J.B., 2006. Expansion and rapid retreat of the West Antarctic Ice Sheet in eastern Ross Sea: possible consequence of over-extended ice streams? Quaternary Science Reviews 25 (17e18), 2177e2196. Nick, F.M., Van der Veen, C.J., Vieli, A., Benn, D.I., 2010. A physically based calving model applied to marine outlet glaciers and implications for the glacier dynamics. Journal of Glaciology 56 (199), 781e794. Nygård, A., Sejrup, H.P., Haflidason, H., Lekens, W.A.H., Clark, C.D., Bigg, G.R., 2007. Extreme sediment and ice discharge from marine-based ice streams: new evidence from the North Sea. Geology 35 (5), 395e398. O’Brien, P.E., Santis, L.D., Harris, P.T., Domack, E., Quilty, P.G., 1999. Ice shelf grounding zone features of western Prydz Bay, Antarctica: sedimentary processes from seismic and sidescan images. Antarctic Science 11 (01), 78e91. Ó Cofaigh, C., Pudsey, C.J., Dowdeswell, J.A., Morris, P., 2002. Evolution of subglacial bedforms along a paleo-ice stream, Antarctic Peninsula continental shelf. Geophysical Research Letters 29 (8), 1199. Ó Cofaigh, C., Evans, J., Dowdeswell, J.A., Larter, R.D., 2007. Till characteristics, genesis and transport beneath Antarctic paleo-ice streams. Journal of Geophysical Research 112, F03006. Oppenheimer, M., 1998. Global warming and the stability of the West Antarctic Ice Sheet. Nature 393 (6683), 325e332. Ottesen, D., Dowdeswell, J.A., 2006. Assemblages of submarine landforms produced by tidewater glaciers in Svalbard. Journal of Geophysical Research 111 (F1), F01016. Ottesen, D., Dowdeswell, J.A., Rise, L., 2005. Submarine landforms and the reconstruction of fast-flowing ice streams within a large Quaternary ice sheet: the 2,500 km-long Norwegian-Svalbard margin (57 to 80 N). Geological Society of America Bulletin 117, 1033e1050. Ottesen, D., Stokes, C.R., Rise, L., Olsen, L., 2008a. Ice-sheet dynamics and ice streaming along the coastal parts of northern Norway. Quaternary Science Reviews 27 (9e10), 922e940. Ottesen, D., Dowdeswell, J.A., Benn, D.I., Kristensen, L., Christiansen, H.H., Christensen, O., Hansen, L., Lebesbye, E., Forwick, M., Vorren, T.O., 2008b. Submarine landforms characteristic of glacier surges in two Spitsbergen fjords. Quaternary Science Reviews 27 (15e16), 1583e1599. Overpeck, J.T., Otto-Bliesner, B.L., Miller, G.H., Muhs, D.R., Alley, R.B., Kiehl, J.T., 2006. Palaeoclimatic evidence for future ice-sheet instability and rapid sea-level rise. Science 311 (5768), 1747e1750. Paul, M.A., 1983. The supraglacial landsystem. In: Eyles, N. (Ed.), Glacial Geology. Pergamon, Oxford, pp. 71e90. Piotrowski, J.A., Tulaczyk, S., 1999. Subglacial conditions under the last ice sheet in northwest Germany: ice-bed separation and enhanced basal sliding? Quaternary Science Reviews 18 (6), 737e751. Plassen, L., Vorren, T.O., Forwick, M., 2004. Integrated acoustic and coring investigation of glacigenic deposits in Spitsbergen fjords. Polar Research 23 (1), 89e110. Polyak, B., Lehman, S.J., Gataullin, V., Timothy Jull, A.J., 1995. Two-step deglaciation of the southeastern Barents Sea. Geology 23 (6), 567e571. Powell, R.D., Alley, R.B., 1997. Grounding-line systems: processes, glaciological inferences and the stratigraphic record. In: Geology and Seismic Stratigraphy of the Antarctic Margin, Part 2, Antarctic Research Series, vol. 71 169e187. Powell, R.D., Domack, E.W., 1995. Modern glacimarine environments. In: Menzies, J. (Ed.), Modern Glacial Environments: Processes, Dynamics, and Sediments. Butterworth-Heinemann, Oxford. Rafaelsen, B., Andreassen, K., Hogstad, K., Kuilman, L.W., 2007. Large-scale glaciotectonic-imbricated thrust sheets on three-dimensional seismic data: facts or artefacts? Basin Research 19 (1), 87e103. Rasmussen, T.L., Thomsen, E., Slubowska, M.A., Jessen, S., Solheim, A., Koç, N., 2007. Paleoceanographic evolution of the SW Svalbard margin (76 N) since 20,000 14C yr BP. Quaternary Research 67 (1), 100e114. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Ramsey, C.B., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0e50,000 years cal BP. Radiocarbon 51 (4), 1111e1150. Rokoengen, K., Bell, G., Bugge, T., Dekko, T., Gunleiksrud, T., Lien, R.L., Løfaldli, M., Vigran, J.O., 1977. Prøvetaking av fjellgrunn og løsmasser utenfor deler av NordNorge i 1976. Institutt for kontinentalsokkelundersøkelser. Rose, J., 1987. Drumlins as part of a glacial bedform continuum. In: Menzies, J., Rose, J. (Eds.), Drumlin Symposium. Balkema, Rotterdam. Sauramo, M., 1923. Studies on the Quaternary varve sediments in southern Finland. Fennia 44, 1e164. Sercel, 2006. Marine Sources. Sound Science. Reliable Results. Product Guide. Sercel, Toulon. Schoof, C., 2007. Ice sheet grounding line dynamics: steady states, stability, and hysteresis. Journal of Geophysical Research 112, F03S28. Sharp, M.J., 1985. ‘Crevasse-fill’ ridges: a landform type characteristic of surging glaciers? Geografiska Annaler 67A, 213e220.

Shipp, S., Anderson, J., Domack, E., 1999. Late PleistoceneeHolocene retreat of the West Antarctic Ice-Sheet system in the Ross Sea: Part 1 e geophysical results. Geological Society of America Bulletin 111 (10), 1486e1516. Shipp, S.S., Wellner, J.S., Andersen, J.B., 2002. Retreat signature of a polar ice stream: sub-glacial geomorphic features and sediments from the Ross Sea, Antarctica. In: Dowdeswell, J.A., Ó Cofaigh, C. (Eds.), Glacier-influenced Sedimentation on High-latitude Continental Margins: Introduction and Overview. Geological Society, London, pp. 277e304. Solheim, A., Kristoffersen, Y., 1984. The physical environment in the Western Barents Sea, 1:5,000,000. Sediments above the upper regional unconformity: thickness, seismic stratigraphy and outline of the glacial history. Norsk Polarinstitutt Skrifter 179B, 1e26. Solheim, A., Elverhøi, A., 1993. Gas-related sea floor craters in the Barents Sea. GeoMarine Letters 13, 235e243. Solheim, A., Russwurm, L., Elverhøi, A., Berg, M.N., 1990. Glacial geomorphic features in the northern Barents Sea: direct evidence for grounded ice and implications for the pattern of deglaciation and late glacial sedimentation. In: Dowdeswell, J.A., Scourse, J.D. (Eds.), Glacimarine Environments: Processes and Sediments. The Geological Society, London, pp. 253e268. Stokes, C.R., Clark, C.D., 1999. Geomorphological criteria for identifying Pleistocene ice streams. Annals of Glaciology 28, 67e74. Stokes, C.R., Clark, C.D., 2001. Palaeo-ice streams. Quaternary Science Reviews 20 (13), 1437e1457. Stuiver, M., Reimer, P.J., 1993. Extended 14C database and revised CALIB radiocarbon calibration program. Radiocarbon 35, 215e230. Svendsen, J.I., Alexanderson, H., Astakhov, V.I., Demidov, I., Dowdeswell, J.A., Funder, S., Gataullin, V., Henriksen, M., Hjort, C., Houmark-Nielsen, M., Hubberten, H.W., Ingólfsson, Ó., Jakobsson, M., Kjær, K.H., Larsen, E., Lokrantz, H., Lunkka, J.P., Lyså, A., Mangerud, J., Matiouchkov, A., Murray, A., Möller, P., Niessen, F., Nikolskaya, O., Polyak, L., Saarnisto, M., Siegert, C., Siegert, M.J., Spielhagen, R.F., Stein, R., 2004. Late Quaternary ice sheet history of northern Eurasia. Quaternary Science Reviews 23 (11e13), 1229e1271. Syvitski, J.P.M., 1989. On the deposition of sediment within glacier-influenced fjords: oceanographic controls. Marine Geology 85 (2e4), 301e329. Syvitski, J.P.M., 1991. Towards an understanding of sediment deposition on glaciated continental shelves. Continental Shelf Research 11 (8e10), 897e937. Sættem, J., Rise, L., Westgaard, D.A., 1992. Composition and properties of glacigenic sediments in the southwestern Barents Sea. Marine Geotechnology 10, 229e255. Sættem, J., Rise, L., Rokoengen, K., By, T., 1996. Soil investigations, offshore mid Norway: a case study of glacial influence on geotechnical properties. Global and Planetary Change 12 (1e4), 271e285. Tulaczyk, S., Kamb, W.B., Engelhardt, H.F., 2000. Basal mechanics of Ice Stream B, West Antarctica 1. Till mechanics. Journal of Geophysical Research 105 (B1), 463e481. van der Meer, J.J.M., Menzies, J., Rose, J., 2003. Subglacial till: the deforming glacier bed. Quaternary Science Reviews 22 (15e17), 1659e1685. Vieli, A., Payne, A.J., 2005. Assessing the ability of numerical ice sheet models to simulate grounding line migration. Journal of Geophysical Research 110, F01003. Vogt, C., Knies, J., Spielhagen, R.F., Stein, R., 2001. Detailed mineralogical evidence for two nearly identical glacial/deglacial cycles and Atlantic water advection to the Arctic Ocean during the last 90,000 years. Global and Planetary Change 31 (1e4), 23e44. Vorren, T.O., Kristoffersen, Y., 1986. Late Quaternary glaciation in the south-western Barents Sea. Boreas 15, 51e59. Vorren, T.O., Laberg, J.S., 1996. Late glacial air temperature, oceanographic and ice sheet interactions in the southern Barents Sea region. In: Andrews, J.T., Austin, W.E.N., Bergsten, H., Jennings, A.E. (Eds.), Late Quaternary Palaeoceanography of the North Atlantic Margins. Geological Society Special Publication, London, pp. 303e321. Vorren, T.O., Laberg, J.S., 1997. Trough mouth fans e palaeoclimate and ice-sheet monitors. Quaternary Science Reviews 16 (8), 865e881. Vorren, T.O., Strass, I.F., Lind-Hansen, O.W., 1978. Late Quaternary sediments and stratigraphy on the continental shelf off Troms and West Finnmark, Northern Norway. Quaternary Research 10, 340e365. Vorren, T.O., Hald, M., Edvardsen, M., Lind-Hansen, O.W., 1983. Glacigenic sediments and sedimentary environments on continental shelves. In: Ehlers, J. (Ed.), Glacial Deposits in North-West Europe. A.A. Balkema, Rotterdam. Vorren, T.O., Hald, M., Thomsen, E., 1984. Quaternary sediments and environments on the continental shelf off northern Norway. Marine Geology 57 (1e4), 229e257. Vorren, T.O., Hald, M., Lebesbye, E., 1988. Late Cenozoic environments in the Barents Sea. Paleoceanography 3 (5), 601e612. Vorren, T.O., Lebesbye, E., Andreassen, K., Larsen, K.B., 1989. Glacigenic sediments on a passive continental margin as exemplified by the Barents Sea. Marine Geology 85, 251e272. Weber, M.E., Niessen, F., Kuhn, G., Wiedicke, M., 1997. Calibration and application of marine sedimentary physical properties using a multi-sensor core logger. Marine Geology 136, 151e172. Winsborrow, M.C.M., Andreassen, K., Corner, G.D., Laberg, J.S., 2010. Deglaciation of a marine-based ice sheet: Late Weichselian palaeo-ice dynamics and retreat in the southern Barents Sea reconstructed from onshore and offshore glacial geomorphology. Quaternary Science Reviews 29 (3e4), 424e442. Wollenburg, J.E., Mackensen, A., 1998. Living benthic foraminifers from the central Arctic Ocean: faunal composition, standing stock and diversity. Marine Micropaleontology 34, 153e185.