Submarine landforms produced by drifting icebergs are common on the sedimentary beds of the polar seas. High-resolution multibeam-bathymetric images have revealed a variety of iceberg-related landforms, ranging from 1 to 2 m high corrugation ridges (e.g. Jakobsson et al. 2011; Graham et al. 2013) to large linear to curvilinear ploughmarks several tens of kilometres long and tens of metres deep, that are often distributed chaotically on polar continental shelves (e.g. Dowdeswell et al. 2010). Extensive lateral erosion of submarine ridge-top sediments in the central Arctic Ocean has also been attributed to the impact of deep-drafted icebergs that were once much more abundant in the Arctic than they are today (Jakobsson et al. 2008).
Description
Submarine terraces with remarkably flat surfaces were observed on the crest and along the distal slope of a large terminal-moraine ridge at the mouth of the Hambergbukta (Fig. 1a, c), a fjord in southeastern Spitsbergen (Fig. 1b). The terraces were recorded at several bathymetric levels from 29–66 m in water depth with their surface areas varying from c. 0.04 km2 to >2 km2 (Fig. 1a, d).
(a) Swath-bathymetric imagery of a terminal-moraine ridge in Hambergbukta, Svalbard, modified by iceberg processes that may include both ploughing and broader-scale planing-off to form smooth terraces. Acquisition system Kongsberg EM2040-04. Frequency 200 kHz. Grid-cell size 5 m. (b) Location of study area (red box, from IBCAO v. 3.0). (c) Overview image of seafloor morphology of Hambergbukta. (d) Cross-section along terminal-moraine ridge showing possible iceberg-produced terraces at different levels. VE×16. (e) Central part of the terminal-moraine crest flattened by possible iceberg erosion. Note small terraces on distal side of the ridge. Steep slopes at the junction between terraces and moraine (arrowed in (e) and (f)) suggest iceberg pushing. (f) Oblique view of the southern part of the terminal moraine. (g) Oblique aerial photograph of the 30 m high terminal ice cliffs of Austfonna (located in (b)). (h) Oblique aerial photograph of the tidewater margin of Negribreen, showing partly overturned icebergs (about 200 m across) embedded in winter sea-ice.
The terrace with largest surface area is located in the central part of the terminal-moraine crest at a water depth of about 49 m (Fig. 1a). Smaller terraces are found along the crest and to either side of it at somewhat higher elevations, and on the distal slope of the terminal-moraine ridge (Fig. 1c, d). Transitions between the terraces on the distal slope and the moraine ridge form wall-like structures with steep to near-vertical slopes up to 4–5 m high (Fig. 1d–f).
Although generally remarkably flat, the surfaces of the submarine terraces also show small undulations and locally pitted or hummocky morphologies (Fig. 1e, f). Narrow and linear to curvilinear iceberg-keel ploughmarks are also numerous on the moraine crest where the terraces are absent (Fig. 1a) and on the upper part of both sides of the terminal moraine, although a few ploughmarks also dissect the terraces (Fig. 1a, e, f).
Interpretation
The terraces on the crest and distal slope of the terminal-moraine ridge in Hambergbukta may have been produced through erosion of the sedimentary seafloor by flat-based tabular icebergs. If this conjecture is correct, then the flattened crest of the central part of the moraine ridge implies icebergs of considerable size, possessing sufficient mass in order to erode large amounts of glacial sediment from the ridge top.
The occurrence of small terraces on the distal slope of the moraine ridge suggests that the icebergs drifted into the ridge from the open-marine waters of Storfjorden to the east (Fig. 1b), eroding the central part of the crest and resulting in small terraces indented into its distal slope (Fig. 1a, d–f).
No lineations were observed on the surface of the terraces in Hambergbukta. However, elsewhere in the Arctic where broad-scale ice erosion has been suggested (e.g. on Yermak Plateau and Lomonosov Ridge), the erosional surfaces often feature glacial lineations (Jakobsson et al. 2010). The absence of lineations could be explained by a combination of short-distance linear and rotational movement of the icebergs, driven by currents and winds. The drift of icebergs was probably facilitated by tidally controlled vertical motion. The small undulations and pitted appearance of the terrace surfaces support this idea (Fig. 1e, f).
The large iceberg-keel ploughmarks along the glacier-distal edge of the ridge-crest terrace have locally been filled in and flattened over a distance of c. 450 m (Fig. 1a, e, f). The locally flattened iceberg-keel ploughmarks suggest that icebergs with flat bases, as well as those with relatively pointed keels, have impacted the moraine ridge intermittently. The numerous smaller iceberg-keel ploughmarks that cover the crest and slopes of the moraine ridge may originate both from the glaciers draining into Hambergbukta as well as those produced by the calving glaciers in the other fjords in Storfjorden and elsewhere in the northern Barents Sea area (Fig. 1b).
Rather few large tabular icebergs have been reported calving from Svalbard tidewater glaciers (Dowdeswell 1989), although large and sometimes tabular bergs have been observed offshore of the calving margins of both Austfonna, an 8000 km2 ice cap on Nordaustlandet in eastern Svalbard (Dowdeswell et al. 2008) (Fig. 1b), and Negribreen at 79° N, which is the largest outlet glacier on the west side of Storfjorden (Fig. 1g, h). It may be that the short active-phase of the surge cycle in tidewater glaciers such as Negribreen is the most likely time to produce such relatively large icebergs (Dowdeswell 1989; Dowdeswell et al. 1991). This may help explain why submarine terraces similar to those in Hambergbukta appear to be relatively uncommon in Svalbard waters.
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
References
Possible iceberg-produced submarine terraces in Hambergbukta, Spitsbergen
