Subglacial Lakes

Location The earliest evidence of subglacial lakes was from Russian aircraft pilots flying missions over the Antarctic continent, claims subsequently verified by airborne radio-echo sounding during the 1960s and 1970s. We now know that more than 150 lakes exist beneath the Antarctic ice sheet (Figure 1), many of which may be connected by large subglacial rivers. Approximately 81% of the detected lakes lie at elevations less than ^200 m above mean sea level, while the majority of the remaining lakes are 'perched' at higher elevations. Sixty-six percent of the lakes lie within 50 km of a local ice divide and 88% lie within 100 km of a local divide. The high density of lakes in the Dome-C region implies that they may be hydro-logically connected within the same watershed and would be an important system to study from the standpoint of subglacial hydrology and biological and geochemical diversity.

Formation and diversity The association of subglacial lakes with local ice divides leads to a fundamental question concerning the evolution of subglacial lake environments:

Does the evolving ice sheet control the location of subglacial lakes or does the fixed lithospheric character necessary for lake formation (e.g., basal morphology,

Figure 1 Locations of the lakes discussed in the text. Red stars show the locations of specific lakes or regions; triangles denote the location of known subglacial lakes; yellow dashed lines represent the approximate location of several of the major ice divides on the continent; blue circle denotes the Dome-C subglacial lake cluster.

Figure 1 Locations of the lakes discussed in the text. Red stars show the locations of specific lakes or regions; triangles denote the location of known subglacial lakes; yellow dashed lines represent the approximate location of several of the major ice divides on the continent; blue circle denotes the Dome-C subglacial lake cluster.

Table 1 Location, air temperature, and ice cover characteristics of selected lake regions discussed in this article

District

Latitude

Mean temperature Annual Summer

Winter

Ice cover

Notes

Subantarctic Islands

Signy Island

60° 43' S

-4.2

1.4

-9.6

Variable (8-12 mo, 1-2 m)

Maritime climate zone

Continental ice-free zones

Shirmacher

70° 45' S

-10.8

-2.0

-16.1

Permanent (~3 m)

Freshwater and epishelf lakes

Hills

Larsemann

69° 03' S

-10.5

4.0

-15.0

Variable (8-10 mo), ~2 m

Freshwater lakes

Hills

Vestfold Hills

68° 33' S

-10.0

-0.9

-16.9

Variable (8-10 mo), 0.5-2 m

Saline lakes formed by isostatic

rebound

Bunger Hills

66° 17' S

-9.0

0.4

-16.3

Variable (8-12 mo, 2-4 m)

Many tidally influenced epishelf lakes

McMurdo

77°30' S

-17.7

-3.1

-25

Permanent (4-19 m)

Many chemically stratified with

Dry Valleys

ancient bottom water brines

Sources

Jacka TH, Budd WF, and Holder A (2004) A further assessment of surface temperature changes at stations In the Antarctic and Southern Ocean, 1949-2002. Annals of Glaciology 39: 331-338.

Simmons GM, Vestal JR, and Wharton RA (1993) Environmental regulators of microbial activity In continental Antarctic lakes. In: Friedmann I (ed.) Antarctic Microbiology, pp. 491-451. New York: Wiley-Liss.

Heywood RB (1984) Inland waters. In: Laws RM (ed.) Antarctic Ecology, vol. 1, pp. 279-334.

Gibson JAE and Anderson DT (2002) Physical structure of epishelf lakes of the southern Bunger Hills, East Antarctica. Antarctic Science 14(3): 253-261.

Sources

Jacka TH, Budd WF, and Holder A (2004) A further assessment of surface temperature changes at stations In the Antarctic and Southern Ocean, 1949-2002. Annals of Glaciology 39: 331-338.

Simmons GM, Vestal JR, and Wharton RA (1993) Environmental regulators of microbial activity In continental Antarctic lakes. In: Friedmann I (ed.) Antarctic Microbiology, pp. 491-451. New York: Wiley-Liss.

Heywood RB (1984) Inland waters. In: Laws RM (ed.) Antarctic Ecology, vol. 1, pp. 279-334.

Gibson JAE and Anderson DT (2002) Physical structure of epishelf lakes of the southern Bunger Hills, East Antarctica. Antarctic Science 14(3): 253-261.

geothermal flux or the nature of sub-ice aquifers) constrain the evolution of ice sheet catchments? With the exception of central West Antarctica (where lakes are few), we know little about either the lithospheric character along these catchment boundaries or the history of their migration, given by layering within the ice sheet. Subglacial lake environments rest at the intersection of continental ice sheets and the underlying lithosphere. This unique location sets the stage for generating a spectrum of subglacial environments reflective of the complex interplay of ice sheets and the lithosphere.

Antarctic subglacial lakes have been categorized into three main types: (1) lakes in subglacial basins in the ice-sheet interior; (2) lakes perched on the flanks of subglacial mountains; and (3) lakes close to the onset of enhanced ice flow. The bedrock topography of the ice-sheet interior involves large subglacial basins separated by mountain ranges. The lakes in the first category are found mostly in and on the margins of subglacial basins. These lakes can be divided into two subgroups. The first subgroup is located where subglacial topography is relatively subdued, often toward the center of subglacial basins; the second subgroup of lakes occurs in significant topographic depressions, often closer to subglacial basin margins, but still near the slow-flowing center of the Antarctic Ice Sheet. Where bed topography is very subdued, deep subglacial lakes are unlikely to develop. Lake Vostok (surface area of ~14 000 km2, maximum depth ^800 m; volume ^5400 km3) is the largest known subglacial lake and the only one that occupies an entire section of a large subglacial trough. Theoretical models reveal that the subglacial environment may hold ~10% of all surface lake water on Earth, enough to cover the whole continent with a uniform water layer with a thickness of ~1m. These models further reveal that the average water residence time in the subglacial zone is ^1000 years.

Much attention is currently focused on the exciting possibility that the subglacial environments of Antarctica may harbor microbial ecosystems isolated from the atmosphere for as long as the continent has been glaciated (20-25 My). The recent study of ice cores comprised of water from Lake Vostok frozen to the overlying ice sheet has shown the presence, diversity, and metabolic potential of bacteria within the accreted ice overlying the lake water. Estimates of bacterial abundance in the surface waters range from 150 to 460 cells ml-1 and small subunit rDNA gene sequences show low diversity. The sequence data indicate that bacteria in the surface waters of Lake Vostok are similar to present day organisms. This similarity implies that the seed populations for the lake were incorporated into the glacial ice from the atmosphere and were released into the lake water following the downward transport and subsequent melting from the bottom of the ice sheet. Subglacial lakes present a new paradigm for limnology, and once sampled, will produce exciting information on lakes that have been isolated from the atmosphere for more than 10 My.

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