ICE SHEET DESCRIPTION

The Antarctic ice sheet accounts for roughly 90 percent of Earth’s present ice vol-ume (Church et al. 2001); it is bisected by the Transantarctic Mountains, which divide it into the high-standing and voluminous East Antarctic ice sheet (EAIS) and the low-standing but more dynamic West Antarctic ice sheet (WAIS). The platform upon which these two ice sheets lie is predominantly above sea level for the EAIS and below sea level for the WAIS, leading to the generalization that the EAIS is a terrestrial ice sheet and the WAIS is a marine ice sheet (Figure 2.3). Maximum ice thicknesses, exceeding 4 km, are encountered in the interior regions of the EAIS where the surface elevation is high and the resulting surface temperature is low.

FIGURE 2.3 Cross-sectional profile of the Antarctic ice sheet based on BEDMAP bed topogra-phy (Lythe et al. 2001) and surface topogratopogra-phy from Liu et al. (1999). Inset: Location of profile end points. SOURCE: Lythe, M. B., D. G. Vaughan, and C. BEDMAP: A new ice thickness and subglacial topographic model of Antarctica, Journal of Geophysical Research B: Solid Earth, 106 (B6): pp. 11335-11351 (2001). Reproduced with permission of American Geophysical Union. SOURCE: G. Clarke, committee member.

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fixed image R01011 Antarctic

EXPLORATION OF ANTARCTIC SUBGLACIAL AQUATIC ENVIRONMENTS

The ice sheet flows by a combination of processes, some occurring within the ice mass and others at the boundary between the ice and its base. Plastic creep contributes significantly to the flow of the interior regions and tends to be a slow process, yielding flow rates in the range 1 cm to 10 m per year. Closer to the margins, a dendritic network of fast-flow arteries, termed “ice streams,” can develop. These have typical flow rates between 500 and 2000 m per year, and their fast flow is attributed to a combination of sliding over the bed and deformation of subglacial sediment. Where the Antarctic ice sheet flows into the surrounding ocean, ice can come afloat but remain intact to form an ice shelf that flows by thinning and spreading under the influence of gravity.

Prominent examples are the Ross and Filchner-Ronne ice shelves that fringe the WAIS.

The modes of flow outlined above are referred to as sheet flow, stream flow, and shelf flow, where different flow mechanisms dominate for each case. For the most part, shelf flow is relevant only to the floating margins of the Antarctic ice sheet, but it also oper-ates over large subglacial lakes such as Lake Vostok where, as for ice shelves, the ice column is afloat and the lower boundary is free of frictional resistance.

The temperature distribution within an ice sheet is determined by surface tempera-ture, ice thickness, ice flow, and geothermal heat flux. The mean annual surface air temperature for Antarctica is −36°C (Giovinetto et al. 1990), with mean temperatures of the high interior region being as low as −55°C. For the most part, the sediment and rock that form the continental platform of Antarctica are much warmer because they are shielded from cold surface temperatures by a kilometers-thick insulating blanket of glacier ice. The general tendency is for ice temperature to increase with depth until the bed is reached or the ice melting temperature is attained (Figure 2.4). This melting temperature depends on the ambient pressure (which is determined by the height of the ice column) and on the impurity content of the ice, both of which lower the melt-ing temperature. For ice sheets, the pressure influence dominates and accounts for a 0.0742ºC decrease in melting temperature per megapascal of pressure increase, roughly equivalent to a decrease of 0.654ºC per kilometer of ice thickness.

Much of the base of the ice sheet is thought to be at or near the ice melting tempera-ture. This conjecture is consistent with the bottom temperatures at the Byrd Station, Vostok, and Dome Concordia deep ice core sites (Gow et al. 1968; Petit et al. 1999;

EPICA community members 2004) and is likely to be the case at the Dome Fuji site as well (Saito and Abe-Ouchi 2004). Melting temperatures also prevail at the bottom of all holes thus far drilled to the bed of active ice streams. As described earlier, extensive airborne radar sounding surveys, which allow the base of the ice sheet to be mapped in detail (e.g., Lythe et al. 2001), also indicate the extent of melting as revealed by the existence of subglacial lakes (Siegert et al. 2005a).

To extend these observations, other sources of information must be used. Compu-tational models of the dynamics of the Antarctic ice sheet (Figures 2.5 and 2.6) indicate that large areas of the ice sheet base are at the ice melting temperature and that most areas are within 5ºC of the melting point (which depends on ice pressure). The distri-bution of Antarctic subglacial lakes (Figure 2.6, black triangles) is broadly consistent with the predicted bed temperatures in Figure 2.6 whereas Figure 2.5 seems to be cold biased potentially resulting from modeling parameters. It is reasonable to conclude that over large areas the underlying sediment and rock are unfrozen and water saturated, and that storage and transport of subglacial water are to be expected.

The rate of basal melting corresponds to the rate at which a subglacial meltwater layer would thicken (e.g., in millimeters per year) if water flow was prevented. This

GEOLOGICAL AND GEOPHySICAL SETTING

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FIGURE 2.4 Diagrammatic profiles of ice temperature and ice melting temperature through an idealized ice sheet and its bed. (a) Frozen glacier bed with subglacial permafrost layer. (b) Melt-ing glacier bed. SOURCE: G. Clarke, Committee Member.

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FIGURE 2.5 Map of bed temperature of Antarctic ice sheet (in degrees Celsius relative to the pressure melting temperature) as predicted by the numerical ice dynamics model. SOURCE:

Huybrechts (1990).

0 EXPLORATION OF ANTARCTIC SUBGLACIAL AQUATIC ENVIRONMENTS

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FIGURE 2.6 Map of basal temperature of Antarctic ice sheet (in degrees Celsius) as predicted by the numerical ice dynamics model of Siegert et al. (2005). Black triangles indicate the loca-tions of known subglacial lakes, with the largest triangle corresponding to Lake Vostok. Because of the dependence of ice melting temperature on pressure, the melting temperature over large areas of the ice sheet interior is at or below –2ºC. SOURCE: Global and Planetary Change, 45, Martin J. Siegert, Justin Taylor, Antony J. Payne, Spectral roughness of subglacial topography and implications for former ice-sheet dynamics in East Antarctica Bristol Glaciology Centre, School of Geographical Sciences, 249-263, 2005, with permission from Elsevier.

rate is not directly observable but can be predicted using a thermomechanical ice dynamics model. Figure 2.7 presents results from one such model (Siegert et al. 2005).

The figure shows the simulated average depth of the subglacial water layer but should be interpreted qualitatively rather than quantitatively because the calculation assumes a highly simplified subglacial hydrologic system. Nevertheless, to a first approxima-tion, regions where the simulated water depth is large correspond to regions where the estimated melt rate is high.

GEOGRAPHICAL LOCATION OF ANTARCTIC SUBGLACIAL LAKES