Ice shelf moraines

This is a page about an article written by Michael Hambrey, Bethan Davies and colleagues on ice shelf moraines on Alexander Island in Antarctica[1]. You can download the article here (open access).

Full reference:

Hambrey MJ, Davies BJ, Glasser NF, Holt TO, Smellie JL, Carrivick JL. Structure and sedimentology of George VI Ice Shelf, Antarctic Peninsula: implications for ice-sheet dynamics and landform development. Journal of the Geological Society 172, 599-613 (2015).

Ice shelves

Ice shelves are common in Antarctica; in fact, they fringe around 75% of its coastline. In most places, ice shelves are unconstrained, and flow outwards away from the landmass.

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

An ice shelf is the floating extension of on-land glacier ice. Ice shelves are firmly attached to the land and receive ice from inflow from glaciers, snow falling directly onto the ice surface, and marine water freezing on from below.

Ablation Point Massif, Alexander Island

In rare cases, ice shelves may impinge on the land around them, forming ice shelf moraines. For example, an ice shelf sourced from glaciers on the mainland (Palmer Land) flows into in George VI Sound, Antarctic Peninsula. It flows north and south through the sound, and butts up against Alexander Island.

In 2012, Mike Hambrey, myself and Ian Hey, our wonderful field assistant, spent a field season looking at the ice shelf moraines at Ablation Point Massif, on Alexander Island, Antarctic Peninsula. You can read my blog about the fieldwork here. Our work is now published[1, 2].

Ablation Point, Alexander Island, Antarctic Peninsula

Alexander Island is an island off the coast of the Antarctic Peninsula. The island is separated from the Antarctic Peninsula by a trough (George VI Sound) which is around 900 m deep. This trough was likely scoured out by Marguerite Trough Ice Stream over repeated glacial periods.

Constrasting geology

The geology of the two areas is strikingly different, with the Antarctic Peninsula being dominated by plutonic igneous intrusions, and Alexander Island being dominated by Cretaceous/Jurassic mudstones and sandstones, with some volcanic basalts.

George VI Ice Shelf, Alexander Island

Ice shelf moraines

Ice shelf moraines are curious, and rare, geomorphological features. On Alexander Island, they are prominent, but they have a low preservation potential as they are ice-cored. The thin depth of the debris cover means that once the ice-core is melted, only a very subtle drift will be left, which may hamper identification in the palaeo-landscape record.

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The ice-shelf moraines contain granite boulders from Palmer Land

You can explore the study area (Ablation Point Massif) in Google Earth/Google Maps yourself.

What do ice-shelf moraines look like?

Ice-shelf moraines are very different to ordinary moraines at the edge of glaciers. The processes that lead to their formation are very different. At Ablation Point Massif, seven distinct zones can be identified between the source ice stream from the Antarctic Peninsula (Bertram Glacier), across the inner ice shelf, the ‘pressure ridges’ adjacent to Alexander Island and the actively forming moraines[1].

Ablation Point Massif, Alexander Island. Image from Google Earth Pro.

Palmer Land Ice Streams (Bertram Glacier)

In this sector, the most important tributary glacier to George VI Ice Shelf is Bertram Glacier. From Landsat imagery, there are a number of transverse crevasses and intersecting crevasses. As Bertram Glacier approaches George VI Ice Shelf, the crevasses become water-filled. They then close up and are draped by snow.

Ice-shelf surface (central)

George VI Ice Shelf is characterised by broad, smooth, snow-covered ridges and depressions containing elongated ponds, connected by supraglacial streams. These supraglacial lakes travel along the surface of the ice shelf in the manner of a wave, with a velocity that is different to the local ice-flow velocity[3]. Relief is only of the order of several metres, and the distance between the ridges is >100 m.

Ice-shelf surface (western margin)

Close to the margin of Alexander Island, the ice shelf is characterised by a zone of rough ice, 0.5-2 km wide, with a series of shore-parallel steep-sided ridges and troughs. After we were deployed to the ice shelf, we had to cross this zone to make our way to basecamp. This was challenging as we were man-hauling sledges, which had a tendency to slide away from us down the steep slopes and try and pull us off our feet!

These ridges are hundreds of metres long, and have a relief of 10-15 m, and an amplitude of 50-200 m.

Superimposed on this relief are the degraded ‘ice ships’ or pinnacles of melted ice. There are also supraglacial streams, cryoconite holes with well sorted sand and mud, and ponds.

In Moutonnée Lake, a rock bar prevents the ice shelf from entering the lake, meaning that a vertical ice cliff rises some 20 m above the frozen lake surface. In Ablation Lake, the ice tongue extends into the middle of the lake, creating 5 m high pressure ridges.

Ice-cored moraine

There are two main zones characterising the edge of George VI Ice Shelf: active and ice-cored, and inactive with no apparent ice core (old, degraded moraines).

The ice-cored moraine has a veneer of clast-rich sandy diamicton and sandy gravel, from a few centimetres to a few metres in thickness, overlying ice-shelf ice. The ice-shelf ice has layers in the ice (foliation) that is near-vertical or dips steeply shelf-wards.

Ice shelf moraines with full and drained frozen lakes within the moraines, Ablation Point Massif

The ice-shelf moraine has a relief of around 20 m, with some sharp-crested ridges. Sediment sublimating from the ridge crests is blocky, but collapses to form debris flows during periods of flow.

However, the amount of debris within the ice is low (circa <10% by volume). The debris layers are parallel to the foliation. There are scattered boulders, commonly more than 1 m in diameter, including many of granite and gneiss from Palmer Land. Many of the cobbles are faceted and some are striated.

Provenance of clasts

Stones collected from the actively-forming moraines near the margin of Alexander Island between Ablation and Moutonnée lakes clearly have either a Palmer Land or Alexander Island origin. Arkose and rarer volcanilithic sandstone, derived from the Fossil Bluff Group, indicate an Alexander Island provenance[1]. The metamorphic lithologies were carried through the ice shelf and originated from Bertram Glacier on the Antarctic Peninsula.

To protect basal debris from melting out in the marine waters at the grounding line, basal freeze-on occurred in the past, though this may be limited today[5].  Folding within the glacier ice could also have moved debris to a high englacial position within the ice.

Granite on the ice-shelf moraine – the author’s favourite rock. Photo credit: Mike Hambrey

Old, degraded moraine

These are inactive moraines that are composed of sandy gravel and clast-rich sandy diamicton. There is no apparent buried ice. There is a ‘lag’ of dispersed boulders, including Palmer Land erratics, on these surfaces.

Looking downslope over scree and colluvium with angular blocks, then old, degraded moraine, then fresh, ice-cored, ice-shelf moraine, then the ice shelf ridges. Ice shelf in the background, mountains from Antarctic Peninsula in the far distance.

Cliffs and scree

Actively forming scree and colluvium forms on Alexander Island below cliffs of volcanic and sedimentary strata of the Fossil Bluff Formation (Jurassic-Cretaceous age). These occur inland of the degraded ice-shelf moraines, and include angular local boulders that have fallen from the cliffs above.

Structural Glaciology

The ice here has several different glaciological characteristics. It is foliated (S1); layered with coarse-bubbly and fine ice, found as remnants of layers that have been folded. This structure is probably derived from folded flow stripes inherited from Bertram Glacier.

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Glacier ice in the moraine. Note the foliation with coarse clear ice and white bubble-rich ice.

The dominant glaciological structure is S2 foliation, with coarse bubbly and coarse-clear ice, aligned parallel to the ridges. There is a strong near-vertical preferred orientation parallel to the coast. In the moraine zone, the coarse-clear layers contain disseminated mud, sand and gravel. The coarse bubbly ice is interpreted as glacier ice, and the coarse-clear ice is interpreted as frozen water-ice from transposed water-filled crevasses from the heavily crevassed Bertram Glacier.

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Glacier ice with foliation

We observed no evidence of thrusting, although Clapperton & Sugden[4] observed thrusting 3 km to the south of Moutonnée Lake, so this may be an important process elsewhere.

Formation of the ice-shelf ridges

The ice-shelf moraines at Ablation Point Massif are formed directly from the ridges in George VI Ice Shelf[1]. Most ice shelves have flow-parallel features, such as flow-lines or flow-stripes. These features are inherited from the inland ice that supplies them. These flow features are the surface manifestation of longitudinal foliation[6, 7] (S1). This is the product of folding of primary stratification; as flow converges, the ice becomes folded about flow-parallel axes. This folding explains how basal debris makes its way into higher englacial levels within a glacier.

Formation of longitudinal foliation

George VI Ice Shelf is unusual because it is constrained. Extending flow with calving only occurs at the northern and southern extremities of the ice shelf[8, 9]. In the central zone, where this study is focused, the ice shelf is fed by Bertram Glacier, which is characterised by flow stripes and transverse crevassing.

This first foliation (S1) is largely overprinted by the crevasse-related features that form in the lower reaches of Bertram Glacier. Compressive flow heals the crevasses, and their traces are transposed as they cross the ice shelf. This gives rise to a new foliation (S2) perpendicular to the first one. On the ice shelf, these features control the formation of the supraglacial lakes, because of the resulting uneven topography[1].

By the time this S2 foliation reaches the Alexander Island margin at Ablation Lake, the darker water-ice is exposed, with varying amounts of coarse-bubbly (glacier) ice and coarse-clear (water) ice, all dipping steeply. The lower albedo of the coarse-clear water ice preferentially melts faster, resulting in the high-relief topography observed at Ablation Point Massif.

In other places along the margin of Alexander Island, the ridges may be ‘pressure ridges’ related to thrusting rather than differential melting[4].

Formation of ice-shelf moraines

The ice-shelf moraine comprises a series of irregular ridges of debris, forming just a thin veneer over ice. The source of the debris is the disseminated mud, sand and gravel in the coarse clear layers in foliation (S2). The ice is very debris-poor, suggesting that tens of metres have melted out to produce the amount of debris on the surface. The debris melts out from the near-vertical foliation, and thus forms unstable ridges of sediment which flow during the ablation season.

The ice-shelf moraines thus form in a very different manner to those observed on McMurdo Ice Shelf[10] (Ross Sea, Antarctica). Here, the McMurdo moraines form by the accretion of older subglacial sediment on the sea floor as a result of basal freeze-on of marine waters.

Both these moraines are different to moraines formed by glaciers, which are the product of a variety of processes, such as thrusting, delivery of debris to the margin, or ice stagnation.

The low debris concentrations in the S2 foliation suggest that accretion of debris is slow. The moraine debris is thin and likely to flatten out over time due to debris flows. Any resultant landforms are likely to be subdued.

Conceptual model of ice-shelf-moraine formation for George VI Ice Shelf, showing transition from grounded ice on Palmer Land to the
floating ice-shelf reach, and its impingement on Alexander Island. Circles show details of inferred structural relationships. From: Hambrey et al. 2015

Links

  • You can view Mike Hambrey’s excellent photographs from this season here.
  • I have blogged about the fieldwork expedition here.
  • Read more about structural glaciology here.

References

  1. Hambrey, M.J., et al., Structure and sedimentology of George VI Ice Shelf, Antarctic Peninsula: implications for ice-sheet dynamics and landform development. Journal of the Geological Society, 2015. 172: p. 599-613.
  2. Davies, B.J., et al., Ice-dammed lateral lake and epishelf lake insights into Holocene dynamics of Marguerite Trough Ice Stream and George VI Ice Shelf, Alexander Island, Antarctic Peninsula. Quaternary Science Reviews, 2017. 177: p. 189-219.
  3. LaBarbera, C. and D. MacAyeal, Traveling supraglacial lakes on George VI Ice Shelf, Antarctica. Geophysical Research Letters, 2011. 38(24).
  4. Sugden, D.E. and C.M. Clapperton, An ice-shelf moraine, George VI Sound, Antarctica. Annals of Glaciology, 1981. 2(1): p. 135-141.
  5. Graham, A.G.C. and J.A. Smith, Palaeoglaciology of the Alexander Island ice cap, western Antarctic Peninsula, reconstructed from marine geophysical and core data. Quaternary Science Reviews, 2012. 35(0): p. 63-81.
  6. Reynolds, J.M. and M.J. Hambrey, The structural glaciology of George VI Ice Shelf, Antarctic Peninsula. British Antarctic Survey Bulletin, 1988. 79: p. 79-95.
  7. Hambrey, M.J. and J.A. Dowdeswell, Flow regime of the Lambert Glacier-Amery Ice Shelf system, Antarctica: structural evidence from Landsat imagery. Journal of Glaciology, 1994. 20(401-406): p. 401-406.
  8. Holt, T., N. Glasser, and D. Quincey, The structural glaciology of southwest Antarctic Peninsula Ice Shelves (ca. 2010). Journal of Maps, 2013. 9(4): p. 523-531.
  9. Holt, T.O., et al., Speedup and fracturing of George VI Ice Shelf, Antarctic Peninsula. The Cryosphere, 2013. 7: p. 797-816.
  10. Glasser, N., et al., Debris characteristics and ice-shelf dynamics in the ablation region of the McMurdo Ice Shelf, Antarctica. Journal of Glaciology, 2006. 52(177): p. 223-234.

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