Epishelf lakes

This page is mostly sourced from Davies et al. 2017, and focuses on the epishelf lakes at Ablation Point Massif, Alexander Island, Antarctic Peninsula. Read the full paper here:

Davies, B.J., Hambrey, M.J., Glasser, N.F., Holt, T., Rodés, A., Smellie, J.L., Carrivick, J.L., Blockley, S.P.E., 2017. 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 177, 189-219.

Epishelf lakes: dammed by ice shelves

Sometimes, a floating ice shelf (the seaward extension of glaciers that terminate in the ocean) may block the mouth of a fjord, creating a unique type of lake called an “Epishelf Lake”. Freshwater from the ice shelf and any water draining from the land are dammed behind the ice shelf. Since the water is fresh and cold, it floats on top of the sea water.

Schematic illustration of an epishelf lake, a freshwater lake dammed by an ice shelf. The ice shelf is calving icebergs into the freshwater lake, which lies over a layer of denser, saltier marine water.

Epishelf lakes maintain a direct hydraulic connection to the sea under the base of the ice shelf; the surface elevation of the lake therefore changes tidally[1]. The depth of the freshwater layer is limited to the draft of the ice shelf. Any excess freshwater will be evacuated away underneath the ice shelf.

Schematic of Epishelf Lake from Wikimedia Commons. 1. Freshwater layer. 2. Marine layer. 3. Ice shelf. 4. Ice cover on lake. 5. Land mass.

A proxy for ice shelf presence/absence?

If the ice shelf disappears, then the fresh water will no longer be dammed and the freshwater epishelf lake will disappear. The sedimentary record from an epishelf lake can therefore be used as an indicator of ice-shelf absence[2]. Epishelf lakes, which maintain a connection to the ocean, are also important indicators of modern and palaeo sea level [3, 4].

However, awareness of the importance of these lakes in the Arctic and the Antarctic is growing [2, 3, 5, 6]. Detailed descriptions of palaeo epishelf lake shorelines are therefore required in order to facilitate their recognition elsewhere in the landscape.

Alexander Island and George VI Ice Shelf

Alexander Island is a large island on the western Antarctic Peninsula. George VI Sound, between the island and the main land, is covered by George VI Ice Shelf.

George VI Ice Shelf is sourced from large glaciers on the Antarctic Peninsula and flows out of the sound in its northern and southern ends. It is an usual ice shelf as it is laterally confined in the sound, and is grounded on the margin of Alexander Island.

The map below shows the geology of Alexander Island and the Antarctic Peninsula, with George VI Ice Shelf in George VI Sound, between the island and main land. Arrows indicate direction of flow on the ice shelf.

At Ablation Point Massif, the ice shelf blocks the mouths of two valleys (Ablation Valley and Moutonnee Valley), resulting in the formation of two epishelf lakes: Ablation Lake and Moutonnee Lake.

Ablation and Moutonnée lakes, Alexander Island

At Ablation Point Massif, George VI Ice Shelf impounds two epishelf lakes in Ablation and Moutonnée valleys. These epishelf lakes have surfaces close to sea level with a direct hydraulic connection to the sea [7]. Both lakes are subject to tidal displacement, resulting in a tidal crack around the edge of the perennial frozen lakes.

At the mouths of the lakes, George VI Ice Shelf is partially in contact with (i.e. grounded on) a submerged bedrock ridge. The grounding zone is expressed by a raised crevassed area on the surface of the ice shelf[7].

Part of the ice shelf flows westwards over the ridge and into Ablation Lake as a prominent, heavily fractured ice tongue extending 2.8 km into Ablation Lake, resulting in 5 m high ridges of ice [7, 8].

The interactive Google Map below allows you to explore Ablation Point Massif, with the two epishelf lakes: Ablation Lake and Moutonnee Lake. You can clearly observe the fractured ice tongue extending into Ablation Lake.

Stratified water column

Within the epishelf lakes, meltwater from onshore Alexander Island and George VI Ice Shelf forms a layer of fresh water across the epishelf lakes. Meltwater and snow from the catchment will typically accumulate in epishelf lakes until the thickness of the freshwater layer is equal to the minimum draft of the ice shelf.

Excess freshwater inflow is exported below the base of the ice shelf to the sea[3]. Marine waters are advected from beneath the ice shelf. Perennial thick ice cover on the lake surface and strong density stratification prevents mixing[9].

These two epishelf lakes are characterised by a stratified water column, with a less dense, cold freshwater layer overlying marine water, and are nutrient limited and deficient in phytoplankton [7].

Epishelf Lake glaciology

The grounding zone of George VI Ice Shelf is clearly visible in Ablation and Moutonnée epishelf lakes as a series of ice ridges, which extend well into the lakes. The ice shelf gradually fragments into Ablation Lake as icebergs, which sublimate to form low ridges, as has been observed in other ice-dammed lakes in Antarctica [12].  These low ridges of calved icebergs, originally formed perpendicular to the calving front, become increasingly arcuate towards the centre of the lake.

Glaciology of an epishelf lake, Ablation Lake, Alexander Island, Antarctic Peninsula

Mapped features on Ablation Lake, Alexander Island. From Davies et al., 2017

Surrounding the margin of Ablation Valley epishelf lake are two small ice-ridges where the lake ice has been thrust up to 2 m high around the lake margin. These ridges are associated with a tidal crack, where displacement occurs daily in accordance with the tides. The inner thrusted lake ice has a vertical crystal structure and mostly is very clean ice, with limited debris load.

Epishelf lake geomorphology

The outer ridge of lake ice is composed of thrusted and reworked local sediments, most prominently along the southern margin of the lake, closer to the ice shelf. Here, north-facing slopes of the thrusted ice are covered with a thin layer of coarse diamicton or coarse, poorly sorted material overlying finer sediments.

When de-iced, these form a low, rounded, hummocky ridge of poorly sorted sediment overlain by coarse cobbles and gravels, including both locally derived and Palmer Land erratic cobbles and small boulders. In most places, these ridges comprise simply reworked local material. There is no evidence of pebble edge-rounding or sorting by wave action on the lake.

Lake-ice conveyors

We argue that a lake-ice conveyor operates on Ablation and Moutonnée lakes [10, 12]. In this mechanism, icebergs calved into the lake ice from George VI Ice Shelf gradually sublimate and are subsumed by the lake ice.

Icebergs and glacially transported material are moved to the lake edge due to pressure exerted on the lake ice by the glacier/ice shelf. Seasonal warming at the lake margins, where the ice is thinner, can cause melting at the margins. A convection current occurs within the ice; the lake is coldest close to the ice shelf, where fresh meltwater easily freezes. A moat occurs at the lake shoreline during the height of summer; the ice melts and debris within the lake ice is released to the shoreline.

Epishelf lake and lake-ice conveyor

Schematic of a lake-ice conveyor operating on Ablation Lake, Alexander Island. From Davies et al., 2017

The ridges of calved icebergs become increasingly arcuate because the lake-ice conveyor moves faster in the centre of the lake than at the edges [cf. 12]. Debris content of the ice shelf and lake ice is low but erratics and local clasts entrained by the ice shelf [cf. 8] are encased by the lake ice.

Pressure from the ice shelf and limited thermally driven convection currents within the lake therefore drive the lake ice conveyor, delivering Palmer Land erratics and local clasts to the shoreline. However, the strongly stratified nature of the lake suggests that there are likely to be limited convection currents and mixing [cf. 13].

Further reading

More fieldwork photographs from Mike Hambrey: Glaciers Online

Madzu: Ellesmere

Epishelf Lake research projects

George VI Ice Shelf

Further reading on Antarctic Ice Shelves

References

  1. 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.
  2. Antoniades, D., et al., Holocene dynamics of the Arctic’s largest ice shelf. Proceedings of the National Academy of Sciences of the United States of America, 2011. 108(47): p. 18899-18904.
  3. Hamilton, A.K., et al., Dynamic response of an Arctic epishelf lake to seasonal and long-term forcing: implications for ice shelf thickness. The Cryosphere Discuss., 2017. 2017: p. 1-34.
  4. Galton-Fenzi, B.K., et al., A decade of change in the hydraulic connection between an Antarctic epishelf lake and the ocean. Journal of Glaciology, 2012. 58(208): p. 223-228.
  5. England, J.H., M.F. Furze, and J.P. Doupé, Revision of the NW Laurentide Ice Sheet: implications for paleoclimate, the northeast extremity of Beringia, and Arctic Ocean sedimentation. Quaternary Science Reviews, 2009. 28(17): p. 1573-1596.
  6. Van Hove, P., et al., Farthest north lake and fjord populations of calanoid copepods Limnocalanus macrurus and Drepanopus bungei in the Canadian high Arctic. Polar Biology, 2001. 24(5): p. 303-307.
  7. Smith, J.A., et al., Limnology of Two Antarctic Epishelf Lakes and their Potential to Record Periods of Ice Shelf Loss. Journal of Paleolimnology, 2006. 35(2): p. 373-394.
  8. 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.
  9. Veillette, J., et al., Arctic epishelf lakes as sentinel ecosystems: Past, present and future. Journal of Geophysical Research: Biogeosciences, 2008. 113(G4).
  10. Hendy, C., et al., Proglacial lake‐ice conveyors: a new mechanism for deposition of drift in polar environments. Geografiska Annaler: Series A, Physical Geography, 2000. 82(2-3): p. 249-270.
  11. Smith, J.A., et al., Oceanic and atmospheric forcing of early Holocene ice shelf retreat, George VI Ice Shelf, Antarctic Peninsula. Quaternary Science Reviews, 2007. 26: p. 500-516.
  12. Hall, B.L., C.H. Hendy, and G.H. Denton, Lake-ice conveyor deposits: Geomorphology, sedimentology, and importance in reconstructing the glacial history of the Dry Valleys. Geomorphology, 2006. 75(1): p. 143-156.
  13. Laybourn‐Parry, J., et al., Life on the edge: the plankton and chemistry of Beaver Lake, an ultra‐oligotrophic epishelf lake, Antarctica. Freshwater Biology, 2001. 46(9): p. 1205-1217.

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