Former Subglacial Thermal Regimes

Understanding former subglacial thermal regimes (see Section 3.4.1) is vital for an understanding of former ice sheets and the landforms they created (Figure 12.5). This is because the former subglacial thermal regime (warm-based or thawed-bed conditions versus cold-based or frozen-bed conditions) determines whether land-forms are created (thawed bed) or preserved (frozen bed) beneath an ice sheet. One productive approach to this question is to compare the patterns of ice flow and subglacial thermal organisation of contemporary ice sheets with the patterns of ice flow indicated by landforms on the beds of former ice sheets such as the Laurentide or Fennoscandian Ice Sheets. By adopting this approach it is possible to develop conceptual models of the subglacial thermal organisation beneath ice sheets (Figure 12.6). Using the Antarctic Ice Sheet as an example, it appears that there are four major dynamic components of ice sheets that are important to understanding ice dynamics, landform creation and preservation beneath ice sheets. These are related directly to the spatial organisation of frozen- and thawed bed zones under the ice sheet.

Ice shelf Shear margins

Ice shelf Shear margins

Figure 12.5 The range of possible subglacial thermal regimes in a former ice sheet. The figure shows a hypothetical ice sheet with a terrestrial dome (T), a marine dome (M), and an ice shelf. White areas indicate frozen-bed conditions and black areas indicate thawed-bed conditions. [Modified from: Kleman and Glasser (2007), Quaternary Science Reviews, 26,

Figure 12.5 The range of possible subglacial thermal regimes in a former ice sheet. The figure shows a hypothetical ice sheet with a terrestrial dome (T), a marine dome (M), and an ice shelf. White areas indicate frozen-bed conditions and black areas indicate thawed-bed conditions. [Modified from: Kleman and Glasser (2007), Quaternary Science Reviews, 26,

Ice streams, Mühlig-Hofmann Mountains, Palaeoflow traces, Yukon, Cordilleran Ice sheet East Antarctica

Ice streams, Mühlig-Hofmann Mountains, Palaeoflow traces, Yukon, Cordilleran Ice sheet East Antarctica

Pa la eo flow traces, Williams Lake, Cordilleran Ice sheet Whillans Ice Stream tributaries. West Antarctica

Figure 12.6 Similarities between the frozen- and thawed-bed patterns in formerly glaciated areas (A and C) and the surface of the contemporary Antarctic Ice Sheet (B and D). (A) The organisation of flow traces (red lines) in the north-central Cordilleran Ice Sheet. (B) Ice streams passing through the MUhlig-Hofmann Mountains in East Antarctica. Black lines indicate the boundaries between ice-streams and low-velocity areas. Number 1 indicates an ice-stream tributary draining ice from a large frozen-bed patch (labelled '2'). (C) Flow traces south of Prince George, British Columbia, Canada. Symbols as in (A). Numbers 1-5 indicate inferred frozen-bed patches without flow traces comparable to the main flow pattern (6). (D) Major tributaries to the Whillans Ice Stream in West Antarctica. Number 1 indicates lenticular frozen-bed patches. Number 2 indicates the upper end of an ice-stream tributary. [Modified from: Kleman and Glasser (2007), Quaternary Science Reviews, 26, figure 8, p. 590]

Figure 12.6 Similarities between the frozen- and thawed-bed patterns in formerly glaciated areas (A and C) and the surface of the contemporary Antarctic Ice Sheet (B and D). (A) The organisation of flow traces (red lines) in the north-central Cordilleran Ice Sheet. (B) Ice streams passing through the MUhlig-Hofmann Mountains in East Antarctica. Black lines indicate the boundaries between ice-streams and low-velocity areas. Number 1 indicates an ice-stream tributary draining ice from a large frozen-bed patch (labelled '2'). (C) Flow traces south of Prince George, British Columbia, Canada. Symbols as in (A). Numbers 1-5 indicate inferred frozen-bed patches without flow traces comparable to the main flow pattern (6). (D) Major tributaries to the Whillans Ice Stream in West Antarctica. Number 1 indicates lenticular frozen-bed patches. Number 2 indicates the upper end of an ice-stream tributary. [Modified from: Kleman and Glasser (2007), Quaternary Science Reviews, 26, figure 8, p. 590]

1. Frozen-bed patches are present under both contemporary and palaeo-ice sheets. In the contemporary Antarctic Ice Sheet, for example, frozen-bed patches occur on the ridges of higher land that lie between fast-flowing outlet glaciers and ice streams. Under former mid-latitude ice sheets the evidence for frozen-bed patches includes areas of the ice sheet bed that display a complete absence of glacial erosion or preserved landforms. Cosmogenic isotope dating has been used to confirm quantitatively the absence of erosion in many areas where preserved landforms are thought to represent relict surfaces. Often these preserved patches are adjacent to zones showing intense reshaping of the bed (e.g., the formation of glacial lineations) or glacial erosion (Box 12.3).

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