Department of Geosciences and Earth and Environmental Systems Institute The Pennsylvania State University University Park PA

71.1 Introduction

Local changes in the West Antarctic ice sheet (WAIS) have regional effects on ice-sheet mass balance with the potential for global impacts through cryospheric influence on eustatic sea level and ocean circulation patterns. Although a majority of the ice in the WAIS is tied up in the slow-moving (1-10 myr-1) interior, attention is focused on the fast-moving (100 myr-1) ice streams that rapidly discharge ice to the ocean through three main outlet systems.

Despite major ice-age advance of grounded ice into the Ross Embayment to near the continental-shelf edge followed by an approximately 1300-km retreat (Conway et al., 1999), the Siple Coast ice streams maintained roughly their same locations and their anomalously low-elevation, low-slope surface profiles (Alley & White, 2000; Anderson & Shipp, 2001). The apparent long-term persistence (Anderson & Shipp, 2001) and short-term variability (e.g. Rose, 1979; Retzlaff & Bentley, 1993; Bindschadler & Vornberger, 1998) of these ice streams are probably tied to regional thermal conditions and local basal lubrication (Parizek et al., 2002, 2003).

Previous research has indicated that available basal water is almost certainly essential to till lubrication and therefore to the past, present and future of these fast-flowing ice streams (e.g. Engelhardt & Kamb, 1998; Tulaczyk et al., 2000b; Kamb, 2001). Furthermore, recent observations and theoretical studies suggest that local basal freeze-on is occurring along stretches of several Siple Coast ice streams (e.g. Tulaczyk et al., 2000b; Raymond, 2000; Kamb, 2001; Joughin et al., 2003a). Significant slow-down of the Kamb Ice Stream occurred ca. 150yr ago (Rose, 1979; Retzlaff & Bentley, 1993), and more recent deceleration is under

Figure 71.1 Flowline-model domains, from divides down Mercer Ice Stream (A), Whillans Ice Stream (B), Kamb Ice Stream (C) and Bindschadler Ice Stream (D) and across the Ross embayment to the continental-shelf edge. Topographic data from BEDMAP data set (sponsored by the Scientific Committee on Antarctic Research and coordinated by the British Antarctic Survey, Cambridge). (Based on fig. 1 in Parizek et al., 2002, 2003.)

contour interval = 500 m

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Figure 71.1 Flowline-model domains, from divides down Mercer Ice Stream (A), Whillans Ice Stream (B), Kamb Ice Stream (C) and Bindschadler Ice Stream (D) and across the Ross embayment to the continental-shelf edge. Topographic data from BEDMAP data set (sponsored by the Scientific Committee on Antarctic Research and coordinated by the British Antarctic Survey, Cambridge). (Based on fig. 1 in Parizek et al., 2002, 2003.)

way on Whillans Ice Stream (Bindschadler & Vornberger, 1998; Joughin et al., 2002) and possible Mercer Ice Stream (Joughin & Tulaczyk, 2002; Joughin et al., 2002). It has been suggested that dewatering subglacial tills quickly causes deceleration, indicating impending shutdown of Whillans Ice Stream (e.g. Joughin et al., 2003a).

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