Large Subdued Transverse Ridges: Overrun Moraines

There are a number of large but subdued ridges within Nordvågen, which are orientated approximately perpendicular to the main channel axes and are slightly sinuous in planform (Fig. 2, part b). Three of these large ridges are identified in the deeper western channel: the most distal (R1, Fig. 2, part a) is 3.2 km from the 2009 margin and varies from 10 m to 20 m high and is between 70 m and 120 m wide; the next (R2, Fig. 2, part a) is approximately 3 km from the 2009 margin, is 10 m to 15 m high and varies in width from 40 m to 100 m; the most proximal (R3, Fig. 2, part a) is about 2.2 km from the 2009 margin, is 10 m high, around 130 m wide, and has a length of only approximately 500 m. Some evidence of the most distal and proximal ridges can be found in the narrow channel between the Breøyane islands and Blomstrandhalvøya, but here the ridges are smaller in size. Only one relatively large but subdued ridge (R4) is visible in the eastern channel (Fig. 2, part a), at 3.2 km from the 2009 margin, and is around 15 m high and between 100 to 200 m wide; however, the ridge may extend beyond the data coverage.

The ridges described above are interpreted as terminal moraines, which have been overridden by glacial advances since their formation. They have, therefore, been modified substantially and could mark the extent of previous surges of Blomstrandbreen in the same way that overridden ridges identified in Borebukta were interpreted to represent the former surge margins of Borebreen (Ottesen and Dowdeswell, 2006). These modified ridges probably acted as pinning points for the retreating glacier. The most ice-proximal ridge in the western channel (R3, Fig. 2, part a), with its more discontinuous appearance, has developed a cupola-like hill form and has also been modified by curvilinear features on its surface, at around 45 m water depth.

Large Transverse Ridges: Terminal Moraines

The most distal transverse ridges are large and clearly identifiable on both the western and eastern sides of Blomstrandhalvøya, at 3.6 and 3.4 km from the 2009 margin of Blomstrandbreen, respectively (T1, Fig. 2, part a). The western ridge is approximately 30 m high and 350 m wide; in contrast, the eastern ridge is smaller, with a maximum height of 15 m and is about 250 m wide. The distal ridge in the main western channel adjoins another ridge at the distal end of the narrow channel between Blomstrandhalvøya and the Breøyane islands, with dimensions of approximately 10 m in height and 200–300 m in width. It is located 3.4 km from the 2009 margin. A lobe of sediment is visible on part of the distal slope of this ridge. The lobe has a length up to 250 m and a maximum width of 300 m. There is no evidence of neoglacial subglacial features on the relatively smooth seafloor on the distal side of these ridges.

There is another prominent transverse ridge across Nordvågen further to the east, in addition to the one described above (T2, Fig. 2, part a). This ridge crosses the western channel, at a distance of 2.3 km from the 2009 margin, reaching up to 15 m high and around 100 m wide. Within the topographically constrained passage, which splits off the western channel and passes between Blomstrandhalvøya and the Breøyane islands, this ridge is located 2.2 km from the 2009 margin and is smaller and narrower, with maximum dimensions of 10 m high and 50 m wide. In the eastern channel, the ridge is 1.8 km from the 2009 margin and has the same dimensions as in the topographically constrained channel. The most extensive transverse ridges, located on both sides of Blomstrandhalvøya, are interpreted as terminal moraines (Fig. 2, part b), representing the maximum neoglacial extent of Blomstrandbreen. This is further supported by the survey record from Whittington et al. (1997; Fig. 3), and by historical records (Liestøl, 1988) and NPI aerial photographs, which show lateral moraines on land leading to this maximum extent (Figs. 2, parts a and b). It is interpreted that the glacier extended further offshore in the western channel than the eastern channel. This was probably because the main glacier drainage was into the western channel.

The sediment lobe described above is interpreted as a glacigenic debris flow, representing a slope failure (Ottesen et al., 2008) and may be a result of sediment that has been bulldozed in front of the glacier and subsequently been prone to failure and gravitational spreading (Boulton et al., 1996; Kristensen et al., 2009). The large transverse ridge most proximal to the 2009 margin of Blomstrandbreen is correlated, by the associated occurrence of glacial lineations and through the use of NPI 1966 aerial photography, with the glacier's maximum surge extent culminating in 1967–1968 (Lefauconnier, 1992) and is thus interpreted as a terminal surge moraine (Figs. 4 and 5).

Streamlined Features: Glacial Lineations

In the deeper western channel of Nordvågen, close to the 2009 margin of Blomstrandbreen and extending 2.5 km to the proximal prominent transverse ridge (Figs. 2, parts a and b), there is evidence for another landform type that has had transverse ridges superimposed upon it. This landform appears in the central part of the bay as small curvilinear groove-ridge features, or lineations, running parallel to the main channel flow (orientated southwestwardly) (Fig. 4, part a).

These features have amplitudes around 2 m to 5 m, widths typically between 20 m and 50 m, and can reach lengths of over 500 m. The curvilinear ice-flow parallel features are all found within the deeper western channel on the proximal side of the moraine associated with the 1960s surge (Fig. 2, parts a and b). The superimposition of small transverse ridges over the streamlined landforms (Fig. 4, part a) suggests that the streamlined landforms are older than the ridges.

The description of these landforms leads to their interpretation as glacial lineations, which form as a result of movement of ice on a sedimentary substrate producing low-amplitude streamlined ridges aligned parallel to ice-flow (Boulton, 1976; Gordon et al., 1992; Clark, 1993). In addition, glacial lineations are also indicative of actively surging ice flow in the Svalbard archipelago (Ottesen and Dowdeswell, 2006; Ottesen et al., 2008; Robinson and Dowdeswell, 2011). The lineations were produced subglacially, during a period of high velocity, as Blomstrandbreen surged to its 1960s maximum position. The limited spatial distribution of glacial lineations (Fig. 2) could be due to subsequent burial and destruction as the glacier retreated again (Ottesen et al., 2008). We suggest that these landforms provide further evidence that Blomstrandbreen drained mainly through the deeper western channel at a relatively high velocity.

FIGURE 4.

(a) Detailed bathymetric image showing bifurcations and coalescing of small transverse ridges. Landforms interpreted as lineations and rhombohedral ridges are also marked. (b) Detailed bathymetric image of small transverse ridges with regular morphology and spacing. (c) Seafloor depth profiles across transverse ridges (x–x′, y–y′). (d) Area with extensive depressions, interpreted as iceberg-grounding pits.

Small Transverse Ridges: Retreat Moraines

There are numerous minor subparallel transverse ridges identified throughout Nordvågen (Figs. 2 and 4). A suite of around 65 small transverse ridges is identified within the western channel (Fig. 4, part a). These ridges (Fig. 4, part a, x–x′) have more complex morphologies than those described in the shallower channels (Fig. 4, part b). Away from the fjord sides and topographic highs, the ridges are subdued in comparison with the simple ridges in the eastern channel and frequently bifurcate and coalesce (Fig. 4, part a). These small ridges range from <0.5 and 6 m in height, are typically 20 m wide, and inter-ridge spacing is highly variable between 10 to 70 m (Fig. 4, parts a and c).

From the eastern maximum extent to where the eastern channel converges with the western channel there is a suite of approximately 50 small transverse ridges (Fig. 4, part b). Similarly, there are around 40 small transverse ridges in the narrow channel between the Breøyane islands and Blomstrandhalvøya (Fig. 2). The transverse ridges are uniform in appearance and frequently form one ridge crest across the whole width of the channels with heights from 1 m to 2.5 m and widths typically between 15 and 20 m (Fig. 4, part b, y–y′). However, at certain points they join together forming wider and higher ridges with dimensions of up to 4 m in height and 30 m in width. Ridge spacing varies from <5 m to 50 m, but is typically <20 m (Fig. 4, part b).

The small transverse ridges in Nordvågen, between the terminal moraines and the 2009 margin of Blomstrandbreen, are interpreted as retreat moraines (Fig. 2, parts a and b). The retreat moraines formed ice-marginally, at the grounding line of Blomstrandbreen, during numerous small readvances in the context of the general deglaciation of the bay, similar to features described in Bennett (2001). The ridge spacing of <30 m that occurs in many areas of Nordvågen probably indicates that the features are annual-push retreat moraines formed by small winter readvances (e.g., Boulton, 1986; Bennett, 2001; Ottesen and Dowdeswell, 2006; Dowdeswell et al., 2008), especially in the shallow and narrow channels. In Table 1, we calculate average retreat rates between years when the ice-front position was known; this has been done by dividing the mean distance between two ice-margin positions by the number of years that separate them. The average retreat rate of Blomstrandbreen in the western channel from 1966 to 2009 is 55 m yr^-1 (Table 1). However, the bathymetric data reveal that intermoraine spacing is highly variable, with some sets of moraines being closely spaced (within 5 m), and some ridges being separated by more than 55 m. In addition, the ridges are morphologically more complex than those in the eastern channel. We suggest that this difference is the result of greater water depths in the western channel. Fewer continuous ridges are present in the deepest parts of the western channel (Fig. 2, parts a and c), indicating that the ice margin may have been susceptible to flotation in local depressions during thinning and retreat. The more lobate and bifurcating form of the ridges is somewhat similar to sinuous moraines formed by radially crevassed ice margins (e.g., Sharp, 1984; Bradwell, 2004), and it is also seen in annual retreat moraines as water depths increase in front of other surging tidewater glaciers in Svalbard (Ottesen and Dowdeswell, 2006; their Fig. 9, part E). It is unusual for stagnating or slow-moving ice to have a heavily crevassed surface (Fig. 1, part b) during retreat and, therefore, we interpret this ridge variability to reflect partial lift-off during retreat of the ice margin in local depressions, whereas the ice margin could remain more stable in shallower areas more often forming discontinuous or lobate ridge segments.

FIGURE 5.

Glacier margin fluctuations of Blomstrandbreen. Positions of the former ice-front are from a combination of evidence from Liestøl (1988), Landsat imagery, and aerial photographs. Dashed delineations are from Liestøl (1988). The solid lines represent margins with a higher degree of certainty. The retreat of Blomstrandbreen from its LIA maximum is separated into three retreat phases punctuated by three surges. Tj is Tjukkholmen.

A few of the transverse ridges appear to be linked together by parallel-to-flow ridges (Fig. 4, part a), forming a pattern similar to the better-developed rhombohedral ridges described in several Svalbard fjord and open-marine settings where glaciers have surged across the seafloor (e.g., Solheim and Pfirman, 1985; Solheim, 1991; Ottesen and Dowdeswell, 2006; Ottesen et al., 2008). However, as in the fjord sediments beyond the present terminus of Tunabreen in Isfjorden (Flink et al., 2015), these landforms, interpreted as a result of the squeezing of soft basal sediments into bottom crevasses in the early part of quiescence, are relatively poorly developed at Blomstrandbreen.

TABLE 1

Summary of the retreat or advance between delineations of the former ice-front positions of Blomstrandbreen presented in Figure 5. Measurements have only been made for the eastern channel to where it merges with the western channel. Rate of retreat/advance has been assumed as linear. Negative values represent retreats, while positive values represent advances. Red boxes are glacier surges.

Small Depressions and Minor Scours: Iceberg Pitting

There is evidence of small-scale morphological features on some of the large and small transverse ridges in their shallowest areas and on their distal sides (Fig. 4, part d), particularly on the ridges marking Blomstrandbreen's neoglacial maximum. These features comprise numerous depressions (<0.5 m deep) often surrounded by small berms. Depressions have been identified to a maximum water depth of around 45 m on the distal side of the neoglacial maximum moraine in the eastern channel. By contrast, within Nordvågen, they are concentrated in shallower areas, rarely occurring in water depths greater than 25 m. Occasionally, these depressions form small curvilinear troughs up to around 15 m in length.

The form and bathymetrically constrained distribution of these depressions are consistent with their interpretation as iceberg-grounding pits (Barrie et al., 1992; Dowdeswell et al., 1993; Todd and Shaw, 2012). These pits are produced when the keel of an iceberg or bergy bit comes into contact with the seafloor before lifting off again as a result of tides, currents, or fragmentation. Icebergs can become grounded, melting and rotating to form the pits (Syvitski et al., 1996). The location of the depressions only on topographic highs, and their small surrounding berms, further support this interpretation. Iceberg-grounding pits were also noted on the distal slope of the terminal moraine in side-scan sonar data published by Whittington et al. (1997). The regular presence of icebergs within Kongsfjorden has been noted previously (Dowdeswell, 1989; Dowdeswell and Forsberg, 1992). The maximum depth at which iceberg keel ploughmarks are identified within bathymetric data of the seafloor corresponds with the 40 m iceberg keel depths observed by Dowdeswell and Forsberg (1992). The increase to around 45 m depth on ice-distal slopes is attributed to the higher resolution of bathymetric data used within this study. The presence of grounding pits on the distal side of the terminal moraine in the eastern channel demonstrates that these moraines act as a barrier, preventing larger icebergs from drifting into Nordvågen. The icebergs were calved from glaciers at the head of Kongsfjorden (Kronebreen, Kongsbreen, and Conwaybreen; Fig. 1), which have modern tidewater ice cliffs mostly between 10 and 60 m and are intermittently fast-flowing (Sund et al., 2011).

Imagery

A combination of satellite imagery and aerial photography shows that between 1990 and 2007 there was an increase in crevassing on Blomstrandbreen, then, between 2008 and 2009, there was a small advance indicating the initiation of a change in dynamic regime (Sund and Eiken, 2010). Landsat imagery reveals that the margin of Blomstrandbreen advanced an average of 33 m between 2006 and 2009 (Table 1). Recent imagery further demonstrates this latest surge, with a further advance from 2009 to 2013, resulting in an average advance of 215 m and covering the recently exposed 26 m a.s.l. islet of Tjukkholmen (see Fig. 5 for location of Tjukkholmen).

Landform Assemblage

The full multibeam-bathymetric data set around Blomstrandhalvøya and the assemblage of landforms identified are illustrated in Figures 2, parts a and b, and 4. The superimposition of landforms allows relative ages to be determined, with younger elements cross-cutting older features (Ottesen et al., 2005; Ottesen and Dowdeswell, 2006). A simplified relative age of deposition from oldest to youngest in Nordvågen is as follows: (1) overridden large palaeo-moraines; (2) large terminal moraines; (3) retreat moraines; (4) 1960s surge-related glacial lineations and terminal surge moraine; (5) further retreat moraines (Fig. 2, part b). This is similar to landform assemblages described elsewhere on Svalbard at the margin of surge-type tidewater glaciers (Ottesen and Dowdeswell, 2006; Ottesen et al., 2008); however, because Blomstrandbreen is known to have surged recently and relatively frequently (Liestøl, 1988; Lefauconnier, 1992; Hagen et al., 1993; Sund and Eiken, 2010; Mansell et al., 2012), there is a transition within Nordvågen between pre- and postsurge landforms, with presurge landforms being located west of the surge moraine (cf. Fig. 4) and postsurge landforms within its confines.

The 1960s surge provides an example of a glacier advance, which would have overridden numerous retreat moraines, producing glacial lineations and a terminal surge moraine by bulldozing sediments into a ridge (Figs. 2, part b, and 5). These flow-parallel lineations were subsequently cross-cut by younger transverse retreat moraines, as Blomstrandbreen returned to its quiescent phase and retreated again, forming retreat moraines annually. This largely conforms to the previous models for tidewater surging glaciers (Ottesen and Dowdeswell, 2006; Ottesen et al., 2008), but as the 1960s surge did not extend beyond the LIA maximum there is some divergence from these earlier schematic models. We suggest that, because the surge was only short-lived, it did not produce a large moraine ridge at its outer limit or a steep enough slope to generate debris flows on its distal side. Indeed, there is no morphological evidence to differentiate the 1960s terminal surge moraine (marked as surge moraine on Fig. 2, part b) from a moraine formed during a major stillstand during retreat. The limited distribution of basal crevasse-fill ridges (cf. Fig. 4, part a), which have been identified as an indicative landform linked to surging glaciers elsewhere in Svalbard (Solheim and Pfirman, 1985; Boulton et al., 1996; Ottesen and Dowdeswell, 2006; Ottesen et al., 2008), suggests that the ice margin at Blomstrandbreen may have continued to be active even in the early stages of quiescence, in order for transverse-to-flow push ridges to form close to maximum terminus positions. The lack of crevasse-fill ridges and a terminal surge moraine that has no morphological difference from a moraine formed during a large stillstand have implications for how surge-type glaciers are identified in the bathymetric record across Svalbard and in other high-latitude fjords, highlighting that there are potential variations to surge-related landform assemblages and not all individual landforms of the assemblage may be present in all locations (Ottesen and Dowdeswell, 2006; Ottesen et al., 2008; Flink et al., 2015).

The subsequent glacier retreat resulted in numerous ice-marginal retreat moraines forming on top of the lineations. The variation in morphology of the retreat moraines implies that some areas of the tidewater glacier calving front were more stable than others, supported by Table 1, and demonstrates the nature of the observed unevenness of retreat at the margin (cf. Hanson and Hooke, 2000; Nick et al., 2010; Sundal et al., 2013). Therefore, the submarine landform assemblage of Nordvågen (Fig. 2, part b) provides an insight into more general glacier dynamics as well as landforms produced during a minor post-LIA surge.

Dating the Retreat of Blomstrandbreen

The suite of moraines in Nordvågen is well dated through reference to historical records (Liestøl, 1988), NPI aerial photographs, and Landsat satellite imagery, which show Blomstrandbreen's margin at several times over the past few centuries. This enables a number of moraines to be linked to specific years of formation (Fig. 5). The retreat of Blomstrandbreen, with the three surge-related readvances, is illustrated in Figure 6.

By the end of the 17th century, rudimentary maps from sealing and whaling vessels suggested that Blomstrandhalvøya was an island, clearly separated from the mainland (Liestøl, 1988). However, by the mid-18th century the peninsula was well connected to the north shore of Kongsfjorden (Fig. 5), at a time when many glaciers advanced on Svalbard due to LIA cooling (Werner, 1993). Blomstrandbreen's neoglacial maximum extent was earlier than 1861 (Liestøl, 1988; Fig. 5), which is when detailed scientific observations of the ice margin began, but the actual timing of the neoglacial maximum is unknown. Toward the end of the 19th century the glacier began an initial retreat (Liestøl, 1988; Lefauconnier, 1992), but this was punctuated briefly by a small readvance around the beginning of the 20th century (Lefauconnier, 1992), resulting in the 1907 terminus being less extensive than in 1911 (Fig. 5). There was an average advance across the terminus of approximately 200 m between 1910 and 1928 (Table 1). Gradual retreat occurred until 1956, with an average retreat rate of 55 m yr^-1 in the western channel and 41 m yr^-1 in the eastern channel (Table 1). By 1956, only a narrow corridor of ice still connected Blomstrandbreen to Blomstrandhalvøya (Fig. 5).

Blomstrandbreen's retreat was interrupted between 1960 and 1968 (Liestøl, 1988; Lefauconnier, 1992) by an advance of between 900 m in the west and 550 m in the east, which has been interpreted as the result of a surge observed in the 1960s (Hagen et al., 1993; Figs. 1, part b; 2, part b; and 5). The glacier then reentered a quiescent phase and retreated, with an average rate of 49 m yr^-1 in the western channel and around 35 m yr^-1 for the eastern channel (Lefauconnier, 1992) between 1966 and 1986 (Table 1). Between 1986 and 1989, both the western and eastern ice-fronts of the glacier had retreated again, leaving a thin corridor of ice, only 240 m wide (Liestøl, 1988), connecting the peninsula (Figs. 1, parts b–e, and 5). From 1990 to 2006 the glacier retreated approximately 1 km (Fig. 6), with an average retreat rate of 65 m yr^-1. It has been proposed that this relatively rapid retreat was caused by the grounding-line breaking away from a shallower pinning point on the northern tip of Blomstrandhalvøya and into the deeper water of central Nordvågen (Benn et al., 2007; Mansell et al., 2012), resulting in the peninsula becoming an island between 1990 and 1994 (Figs. 1, parts d and e, and 5).

FIGURE 6.

Mean distance of the margin of Blomstrandbreen from the LIA maximum in the western channel (black) and the eastern channel (green), using recorded extents at dates presented in Table 1 and Figure 5. Rates of retreat and advance have been assumed to be linear between these dates. Colours on graph are the same for the delineations presented in Figure 5. The arrow highlights the 1960s surge event.

Frequency of Surges and Length of Quiescent Phase

The 21st century surge makes Blomstrandbreen one of only five glaciers confirmed to have had two observed surges on Svalbard, and one of two glaciers (Tunabreen; Flink et al., 2015) to most likely have had three surges, resulting in a surge cycle of approximately 51 years (Table 1). This calculated surge cyclicity is very similar to the estimate of 47 years put forward by Mansell et al. (2012) on the basis of glacier surface flow speeds and frontal positions. This is one of the shortest known cycles on the archipelago (Dowdeswell et al., 1991; Flink et al., 2015) and helps explain the suite of landforms, including the overrun moraines, identified within the bay. The relatively high frequency of the surge cycle implies that there have been a number of surge landform assemblages overprinting and modifying earlier seafloor morphologies, including previous surges, within the LIA maximum limit of Blomstrandbreen. In summary, the known surge history of Blomstrandbreen includes a minor surge between 1911 and 1928, a substantial surge in the 1960s, and a surge from 2008 onward.

The relatively short quiescent phase of the surge cycle at Blomstrandbreen (Fig. 6) is likely to be related indirectly to the mass balance of the glacier. Dowdeswell et al. (1995) have shown that, while climate-related changes in mass balance are not triggers of individual surges, such changes may affect the length of time a glacier takes to build up mass in its upper, reservoir area before a new surge can take place. Other factors being equal, and given that most Svalbard glaciers are presently experiencing negative mass balance (e.g., Hagen et al., 2003; Nuth et al., 2010), a relatively less negative balance would yield a more rapid build-up of accumulation-area mass and a new surge would take place relatively sooner than on a glacier with a very negative mass balance. Indeed, at Scott Turnerbreen in central Spitsbergen, which is in the driest part of the archipelago, increased post-LIA surface melting appears to have prevented the glacier from building up the necessary mass for further surges (Dowdeswell et al., 1995).

Influence of Topographic Highs on Glacier Dynamics

Many of the transverse ridges on the seafloor of Nordvågen are linked to topographic highs, which are often associated with recently exposed islands. One clear example of how these highs have influenced moraine formation is the shallow water (5–15 m deep) surrounding a small islet off the northern tip of Blomstrandhalvøya (Fig. 2). Numerous ridges cross the width of the channels and converge at this shallow spur, where there is a decreased spacing between the ridges (around 30 m) from the deep western channel. In contrast, at the deepest point within the western channel (80 m deep), the ridges bifurcate and coalesce more often and have a wider spacing of >40 m. This is further supported by the maximum distances of retreat and relatively high retreat rates (Table 1) that are recorded from the deeper western channel.

The complex morphology of retreat moraines in the deeper parts of Nordvågen is interpreted to represent instability of the ice-front at increased water depths (Benn et al., 2007), and on occasion likely detachment from the bed (Dowdeswell et al., 2008). Detachment seemingly occurred for a large portion of the grounding line from 1995–1999 (Table 1), most likely driven by the glacier retreating from its pinning point on Blomstrandhalvøya (Fig. 1, parts b–e), which had acted as a stabilizing factor.

A Boomer profile from Whittington et al. (1997) spans the data gap in the eastern channel (Fig. 2, part a) and reveals evidence of at least 10 further transverse ridges (Fig. 3). Therefore, the 850-m-wide data gap probably contains a number of ridges not mapped in this study, possibly indicating relatively large stillstands of Blomstrandbreen (Fig. 2, part a).

The 2009 aerial photographs of Nordvågen reveal a number of islands exposed within the bay that were once covered by Blomstrandbreen (Figs. 4 and 5). These islands and associated submarine shallows are most likely to consist of or are underlain by glacially sculpted resistant bedrock. They have had a strong influence on glacial retreat, supporting longer stillstands by acting as pinning points (Benn et al., 2007), and promoting the regular linear morphology of the retreat moraines in the shallow and narrow sections of Nordvågen (Figs. 2, part a, and 4, parts b and c). The associated increase in calving rate at greater depths (Brown et al., 1982; Pelto and Warren, 1991; Van der Veen, 1996; Vieli et al., 2001) is likely to have produced the complex pattern of ridges found in the center of the bay, indicating a more unstable grounding-line and, at times, the possible decoupling of the glacier from the bed. Luckman et al. (2015) reported that some recent tidewater glacier fluctuations in Spitsbergen are linked to subsurface water temperature change (for which we have no data), but Blomstrandbreen appears to respond strongly to water-depth change and dynamic change associated with the surge cycle.