Continentalshelf morphology and sediments

High-latitude continental shelves range from a few tens to several hundred kilometres in width. The shelves of both the Arctic and Antarctic are typically made up of relatively deep cross-shelf troughs and intervening shallower banks. The Norwegian shelf provides a Northern Hemisphere example (Plate 30.1), as does the Ross Sea in Antarctica (e.g. Anderson, 1999; Shipp et al., 1999; Ottesen et al., 2002). Many troughs are linked to relatively narrow deep-water fjords which dissect the mountainous hinterland of adjacent landmasses (Syvitski et al., 1987). Ice has advanced across most of these shelves to reach the shelf edge during successive full-glacial periods, retreating again in subsequent interglacials (Fig. 30.1).

Some Arctic continental shelves, in particular the Laptev, East Siberian and Beaufort shelves which fringe the Arctic Ocean, are very shallow at tens of metres deep. They are supplied with sediments mainly by major Arctic river systems, rather than from glacial delivery of debris. They were emergent during the lower sea levels of full-glacial intervals. By contrast, in Antarctica the surrounding shelves often deepen landward, from about 500 m at the shelf edge to sometimes in excess of 1000 m close to the margins of the Antarctic Ice Sheet (Anderson, 1999). This is a result of isostatic loading of the crust beneath Antarctica by ice that averages about 2.5 km in thickness over the continent (Drewry et al, 1983), combined with glacial erosion over the late Cenozoic (Anderson, 1999).

Bathymetry Glacier Debris Fanning
Figure 30.2 Morphology of high-latitude continental margins from swath-bathymetric data. (a) Sediment scarp marking a palaeo-grounding line formed during the retreat of ice offshore of the Larsen Ice Shelf, Antarctica (arrowed) (modified from Evans et al., 2005).

The more detailed morphology of polar continental shelves reveals a series of characteristic sea-floor features (Fig. 30.2). These submarine landforms include moraine ridges and grounding-zone deposits marking the former extent of ice, a variety of streamlined glacial landforms indicating past ice-flow directions and dynamics, and ice-keel scours resulting from the grounding of floating icebergs and sea ice (e.g. Ottesen et al., 2005).

Submarine moraines are sedimentary ridges, orientated parallel to the shelf edge, marking either the maximum extent of past ice sheets or still-stands during retreat (e.g. Anderson, 1999; Shipp et al., 2002; Ottesen et al., 2005). Grounding-zone sedimentary wedges, identifiable on acoustic stratigraphical records and sometimes with surface morphological expression, indicate the past locations of the zone where ice begins to float and form an ice shelf (e.g. Evans et al., 2005) (Fig. 30.2a). The development of both moraines and grounding-zone wedges usually requires that ice is stable on the shelf for some time, in order for significant sediment build-up to take place. Past locations of the ice front and grounding zone thus can be reconstructed and, if sediment cores are available, dated to provide a chronology for ice-sheet retreat across high-latitude shelves (e.g. Svendsen et al., 1992; Domack et al, 1999).

Elongate, streamlined submarine landforms, such as megascale lineations (Clark, 1993) and both sedimentary and rock drumlins (Fig. 30.2b), have been used to infer the direction of former ice flow on glaciated continental shelves (e.g. Lowe & Anderson, 2002; Evans et al., 2004, 2005; Ottesen et al., 2005). In addition, megascale glacial lineations have been observed widely in high-latitude cross-shelf troughs (e.g. Shipp et al., 1999; Canals et al., 2000; Wellner et al., 2001; O Cofaigh et al., 2002a), formed in soft, deformable diamictic sediments a few metres thick (Dowdeswell et al., 2004a; O Cofaigh et al., 2005). These lineations, which have wavelengths and amplitudes of hundreds of metres and less than about 5 m, respectively, are often kilometres in length. They are interpreted to indicate the presence of former ice streams, which drained large interior ice-sheet basins to the shelf edge during full-glacial conditions (Stokes & Clark, 1999).

Figure 30.2 Continued (b) Megascale glacial lineations in Marguerite Trough, Antarctic Peninsula (modified from Dowdeswell et al., 2004a). (c) Irregular pattern of scours produced by the keels of drifting icebergs impinging on sea-floor sediments. (i) Iceberg scours on the East Greenland continental shelf offshore of the Scoresby Sund fjord system. (ii) Iceberg scours on the continental shelf west of the Antarctic Peninsula. Note the irregular pattern of the scours in plan view and the morphological contrast with the subglacially produced, megascale glacial lineations (arrowed) to either side.

Figure 30.2 Continued (b) Megascale glacial lineations in Marguerite Trough, Antarctic Peninsula (modified from Dowdeswell et al., 2004a). (c) Irregular pattern of scours produced by the keels of drifting icebergs impinging on sea-floor sediments. (i) Iceberg scours on the East Greenland continental shelf offshore of the Scoresby Sund fjord system. (ii) Iceberg scours on the continental shelf west of the Antarctic Peninsula. Note the irregular pattern of the scours in plan view and the morphological contrast with the subglacially produced, megascale glacial lineations (arrowed) to either side.

Adaptive Cruise Control
Figure 30.2 Continued (d) Submarine channels in the Greenland Basin, east of Greenland at 73-76°N. (Modified from O Cofaigh et al., 2004.)

Scouring of high-latitude shelves by the intermittent grounding of iceberg and sea-ice keels produces very large numbers of irregular grooves on the sea floor ranging from metres to tens of metres in width, up to several metres deep and from tens to thousands of metres in length (e.g. Belderson et al., 1973; Barnes, 1987; Dowdeswell et al., 1993) (Fig. 30.3c). The keels of modern icebergs can reach up to 500-600 m and, as a result, most polar shelves are heavily scoured, leading to extensive reworking of sea-floor sediments and the destruction or severe disturbance of the stratigraphical record (Vorren et al., 1983; Dowdeswell et al., 1992, 1993, 1994; O Cofaigh et al., 2002b). Sea-ice keels are usually <10 m in depth, although pressure-ridging can produce occasional keels up to about 20-30m in thickness. Even so, the shallow shelves fringing much of the Arctic Ocean, the Beaufort, Laptev and East Siberian shelves are severely scoured by sea ice (Barnes et al., 1982; Reimnitz et al., 1994).

Typical sedimentary facies can be found on high-latitude continental shelves over which ice advanced at the last glacial maximum and retreated during deglaciation (e.g. Svendsen et al., 1992). Erosion during ice advance produces a sharp erosional contact overlain by diamictic sediments with a relatively high shear strength deposited at the ice-sheet base. If a fast-flowing ice stream develops, a soft deforming diamict layer of high porosity and low shear strength forms (Dowdeswell et al., 2004a; Evans et al., 2005; O Cofaigh et al., 2005). As ice retreats across the shelf during deglaciation, grounding-zone deposits comprising inter-laminated gravel, sand and finer grained layers are produced during still-stands in retreat, followed by increasingly finegrained silts and clays with progressively fewer iceberg-rafted pebbles as the shelf becomes more distal to the retreating ice front. The whole sequence may be capped by relatively organic-rich interglacial muds (Domack et al., 1999; Evans & Pudsey, 2002). Where the shelf is shallow enough for iceberg keels to rework sediments, massive diamict is common (Dowdeswell et al., 1994).

Current reworking may lead to the removal of fine material and armouring with a lag made up of shells and iceberg-derived pebbles (e.g. Andruleit et al., 1996). In the coldest Antarctic waters, close to the freezing point of saline ocean water at -1.8°C, no meltwater-transported sediment is delivered. Icebergs traverse the area without melting to release their debris load and slow biogenic sedimentation dominates (Domack, 1988).

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