re-translated from PDF June 28
LAKE CONNECTIVITY
The extent to which Antarctic subglacial lakes are interconnected has important implications for their scientific interest as well as for their environmental stewardship.
If the lakes are hydraulically isolated, then in principle a variety of distinct subglacial environments could develop and the consequences of chemical or biological contamina-tion would be localized. If the lakes are highly interconnected, as they would be if they were part of a pervasive subglacial hydrologic system, then environmental diversity would be decreased, the effects of dilution would be increased, and in terms of impacted area, the consequences of contamination would be increased. These extremes of con-nectivity might be termed the “repository model” (as in Figure 2.9) and the “watershed model” (as in Figure 2.9).
Much of the speculation concerning the scientific interest of Lake Vostok has been guided by the repository model. Yet there is little compelling evidence to support this assumption. The behavior of water beneath warm-based valley glaciers and ice caps is well described by the watershed model, and examples of long-term subglacial water
GEOLOGICAL AND GEOPHySICAL SETTING received and delivered from the subglacial environment and from melting and freezing processes operating at the roof. (b) Input discharge from upstream subglacial melt sources is balanced by output discharge to the downstream subglacial environment. (c) Melting and freezing processes in the lake exactly balance with no external sources or sinks. (d) Input discharge from the subglacial environment is balanced by freeze-on at the lake roof. (e) Roof melting is balanced by discharge from the lake to the downstream subglacial environment. SOURCE: G. Clarke, Committee Member.
repositories are rare or nonexistent. Evidence suggests that Lake Vostok is long-lived and, on the millennial scale, stable, but this cannot be generally true of subglacial lakes.
Wingham et al. (2006) present strong evidence for the episodic discharge from a deeply buried Antarctic subglacial lake into two receiver lakes situated at least 290 km down-stream from the source lake. This observation is consistent with previous (Gray et al.
2005) and very recent (Fricker et al. 2007) observations of subglacial water discharge events occurring beneath West Antarctic ice streams. As a default position, one must accept that lake-to-lake connectivity could be the norm rather than the exception. The implication of this is that a basin-scale analysis of hydrologic catchments is necessary before lake-to-lake interactions can be critically examined.
Water System at Ice-Bed Interface
The fate of flowing subglacial water is strongly constrained by the well-established physics of fluid mechanics. It can be shown that the flow of water at the ice-bed interface is driven by the bed slope gradient gbed and the ice surface slope gradient gsurf and that the surface slope rice /(rwater – rice) is approximately 8 times more effective in directing subglacial water flow than the bed slope. This result has important ramifications for subglacial water routing because it implies that, to a first approximation, subglacial
EXPLORATION OF ANTARCTIC SUBGLACIAL AQUATIC ENVIRONMENTS
2.10
FIGURE 2.10 Locations of known Antarctic subglacial lakes and predicted major drainage routings. SOURCE: Siegert et al. 2007.
water flow follows the ice surface slope. Ice flow is known to follow the surface slope so the implication of the above is that subglacial water routing is closely similar to the routing of the ice flow, both being determined by the ice surface topography.
With knowledge of both the ice surface topography (e.g., Liu et al. 1999) and the bed topography (e.g., Lythe et al. 2000) and the use of a standard water flow routing algorithm, it is possible to delineate large-scale drainage basins for subglacial water flow (Figure 2.10). The same approach can be applied at a finer spatial resolution in regions where high-resolution surface and bed elevation data are available (Figure 2.11).
It has yet to be demonstrated that flow routing algorithms that have been developed for subaerial flows can reliably predict subglacial routings because the situations are not completely analogous. Subglacial flows are not exclusively guided by geometry but are subject to permeability barriers (e.g., frozen beds) that have no subaerial counter-part. Nevertheless such analyses can provide useful information on the expected water pathways and the possibility of interconnection between lakes. For example, according to Figure 2.10, water flowing from Lake Vostok would be routed toward the outlet
GEOLOGICAL AND GEOPHySICAL SETTING
2.11
FIGURE 2.11 Calculated routings for subglacial meltwater based on surface and bed topography in the Dome C and Ridge B region of East Antarctica and application of a flow routing algo-rithm. SOURCE: Siegert et al. 2007.
EXPLORATION OF ANTARCTIC SUBGLACIAL AQUATIC ENVIRONMENTS
of Byrd Glacier in West Antarctica, and according to Figure 2.11, lakes in the Ridge B-Dome C region of East Antarctica would be expected to manifest a large degree of interconnectivity.
Finally, using satellite radar altimetry, very accurate digital elevation models (DEMs) of the ice sheet surface can be generated, and from these, the faint traces of subglacial hydrological networks can be extracted using a surface curvature detection algorithm (Rémy and Legrésy 2004). Whether interconnectivity is continuous or sporadic is a separate question. The observation of Wingham et al. (2006), is best interpreted as a subglacial outburst flood that, over the course of 16 months, transferred 1.8 km3 of water from one basin to others downslope from it. The situation is somewhat analo-gous to jökulhlaups that catastrophically release meltwater stored beneath Icelandic ice caps (Björnsson 2002) and for which there is a sizable observational and theoretical literature. Inspired in part by the Wingham et al. result, efforts are under way to adapt existing theory to deal with such floods (Evatt et al. 2006).
Groundwater Routing
Groundwater flow is driven by gradients in the hydraulic potential that, in turn, are governed by the surface and bottom geometry of the ice sheet. Water transport fol-lowing a groundwater routing is driven by the same hydraulic potential gradient that drives water flow along the ice-bed interface, so the relative importance of groundwater flow depends on which mechanism is more effective and under what circumstances.
In regions where the bed is at the melting temperature, the sheet and groundwater flow systems can coexist and both systems experience roughly the same hydraulic potential gradient. Which system dominates will depend on the hydraulic conduc-tivity of the subglacial materials. As is apparent from Table 2.1, the natural range for hydraulic conductivity is extremely large. For back-of-the-envelope estimates, a value of 10–6 m s–1 is taken. There are doubtless regions in which significantly higher values apply, but regions where the hydraulic conductivity is low are likely to control the overall flow rate. By a simple calculation it can be shown that a 1-mm-thick water sheet is as effective as a 460-m-thick aquifer, having a hydraulic conductivity of 10–6 m s–1. Thus, when sheet flow is possible it is far more effective at transporting water than groundwater flow unless the aquifer is highly conductive or very thick.
TABLE 2.1 Representative Values of Hydraulic Conductivity of Various Rock Types
Rock Type Hydraulic conductivity (m s–1)
Limestone, dolomite 1×10–9 – 6×10–6
Sandstone 3×10–10 – 6×10–6
Siltstone 1×10–11 – 1.4×10–8
Shale 1×10–13 – 2×10–9
Permeable basalt 4×10–7 – 2×10–2
Fractured igneous and metamorphic rock 8×10–9 – 3×10–4
Weathered granite 3.3×10–6 – 5.2×10–5
Weathered gabbro 5.5×10–7 – 3.8×10–6
Basalt 2×10–11 – 4.2×10–7
Unfractured igneous and metamorphic rocks 3×10–14 – 2×10–10 SOURCE: Excerpted from Domenico and Schwartz (1990).
GEOLOGICAL AND GEOPHySICAL SETTING
Where the ice-bed interface is below the freezing point, a sheet flow system cannot develop, but groundwater flow can proceed through unfrozen material beneath the ice sheet (Figure 2.4). A representative value of the geothermal gradient in frozen subglacial rock or sediment is 30ºC km–1.
Thus, if the base of the ice sheet is 3ºC below the freezing point, the subglacial material is expected to be frozen to a depth of approximately 100 m below the base of the ice sheet. For this case, the upper boundary of the groundwater flow system would therefore lie 100 m below the bed of the glacier.