Ross Ice Shelf

The Ross Ice Shelf is the largest ice shelf on earth having an area of 536,000 km2 (twice the area of New Zealand) and a volume of 23,000 km3 (Stuiver et al., 1981;

Drewry, 1983). The ice shelf drains 2.86 million km2 (including the ice shelf itself) or 21% of Antarctica (Giovinetto and Bentley, 1985). Extensive field work in the 1970s, mainly under the Ross Ice Shelf Project (Thomas, 1979b), has made this the best known ice shelf.

The thickness of Ross Ice Shelf is anything but regular (Bentley et al., 1979; Bentley, 1985) (Fig. 4.2). Broad intrusions of thick ice enter the shelf from West Antarctic ice streams (e.g., B and C) and East Antarctic outlet glaciers (e.g., Byrd Glacier). Thicknesses are greater upstream of ice rises because these present major obstacles to flow. Roosevelt Island forces ice to flow around it, causing a thinner convergence zone downstream in which (and mainly because of which) the quasipermanent indentation(s) known as the Bay of Whales is formed (Fig. 4.2). In this zone, compression, rifting and differential thinning cause probably the most complex topography on the shelf. Complex topography is also created where large glaciers float free of their bed as they flow into the ice shelf, forming poorly understood transverse crevasse patterns (Lucchita and Ferguson, 1986).

The shelf is thinnest along the ice front but here also thickness is quite variable. At present the ice front is consistently less than 100 m thick west of 173°E (Fig. 4.3) (Jacobs et al., 1986) and thicknesses of 70 to 133 m were quoted for Bay of Whales by Markov et al. (1970). The ice front is thickest in the central portion between 178°E and 167°W (Bentley et al., 1979) — but even here it may not be much thicker than 100 m after periods of prolonged basal melting in places (Fig. 4.3). The ice

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Fig. 4.3. Two cross-sections through the seaward margin of the Ross Ice Shelf showing the pronounced thinning which occurs towards the ice front (drawn from radio echo sounding data obtained in 1974 by the Scott Polar Research Institute, courtesy of P. Cooper). The positions of sections are shown on Fig. 4.2.

shelf generally slopes down towards the ice front (Fig. 4.3) because of loss of ice at depth.

The height of the ice front above sea level varies from a few metres to about 50 metres (Wright and Priestley, 1922) reflecting ice front thickness and vertical profile, ice shelf density and hydrostatic forces. Shabtaie and Bentley (1982) derived buoyancy relationships between height and thickness for the ice shelf and icebergs from it. Such relationships may be least accurate for thin ice fronts (and icebergs) where basal melting has removed the densest basal ice accentuating the effect of density variations in the snow and firn above.

The dynamics and mass balance of Ross Ice Shelf have been major themes of study. Glacier flow lines (Fig. 4.2) and velocity vectors have been mapped (Bentley et al., 1979; Neal, 1979) and show that velocity increases to a maximum of 1.1 km yr1 in the centre of the shelf at the ice front. Surface accumulation has been determined by Clausen et al. (1979) and varies from 0.1 m yr1 (water equivalent) in the interior to a maximum of 0.2 to 0.3 m yr1 along the ice front. Total ice input has been estimated to be between 224 and 327 km3 yr1 (Bentley, 1985; Doake, 1985). Bottom mass balance is complex with some evidence for saline bottom ice due to brine infiltration and bottom freezing (Neal, 1979; Zotikov et al., 1980). There is also good evidence for bottom melting especially near the ice front, for instance near Ross and Roosevelt Islands (Bentley et al., 1979; Neal, 1979; MacAyeal and Thomas, 1986). Modelling and mass balance considerations suggest that these melt rates are as large as 2.5 m yr1 at the ice front decreasing to + 0.1 m yr1100 km inland (Thomas and MacAyeal, 1982; Jacobs et al., 1986). Basal melting may total 60 to 155 km3 yr1 similar to the estimated 150 km3 yr1 lost on average by calving and minor attrition (Doake, 1985; Jacobs et al., 1986; Bentley et al., 1987). However, these estimates are not yet accurate enough to give a reliable estimate of mass balance.

Ice thickness patterns and basal mass balance vary so much across the shelf that it is not likely to be in equilibrium everywhere (Bentley, 1985). Dynamic changes may occur on a time scale of a century or so. However, the shelf is believed to be in, or close to, steady state (i.e., accumulation rate, thickness, velocity, temperature do not change with time) and has been so for perhaps 200 years in the east and 1,500 to 2,500 years elsewhere (Thomas and MacAyeal, 1982; Jezek and Bentley, 1984; Bentley, 1985).

Nevertheless, the position of the ice front has moved north over recent years, although the general trend of its coastline has been maintained (Jacobs et al., 1986) (Fig. 4.2). The ice front has reached its most northern point (77°10'S) in recorded history (i.e., since 1841) at longitude 170°30'E. The Bay of Whales has persisted for most, if not all, of this period.

Between 1962 and 1985, the whole ice front moved an average of 19.3 km north while between 170°E and 178°E the advance was 13.9 km (Jacobs et al., 1986). The average annual rate of advance in this period agrees quite well with measured ice velocities indicating that no major calving has occurred in those 23 years.

Major calving is propably episodic with intervals of several decades, when hundreds of cubic kilometres of ice shelf break off. This happened in late September-early October 1987 when a giant iceberg (B-9) calved from the eastern

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