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Pine Island Glacier and Its Drainage Basin: Results from Radio Echo-Sounding

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Pine Island Glacier and Its Drainage Basin: Results from Radio Echo-Sounding Annals of Glaciology 3 1982 © International Glaciological Society PINE ISLAND GLACIER AND ITS DRAINAGE BASIN: RESULTS FROM RADIO ECHO-SOUNDING by R. D. Crabtree and C. S. M. Doake (British Antarctic Survey, Natural Environment Research Council, Madingley Road, Cambridge CB3 OET, England) ABSTRACT tica, Pine Island Glacier, and Thwaites Glacier, Retreat of the grounding lines of West Antarctic calve directly into the bay without the restraining ice streams may lead to the collapse of the West Ant­ effect of an ice shelf. Stuiver and others (1981) arctic ice sheet. Pine Island Glacier has been pin­ suggest that, provided that no high bedrock sill pointed as an ice stream in which rapid retreat is exists, the grounding line of Pine Island Glacier in likely, especially as it is not buttressed by an ice particular could retreat along the Bentley Subglacial shelf. Radio echo-sounding flights have produced a Trench and across the base of the Antarctic Peninsula. longitudinal thickness profile for the glacier. The A similar retreat could take place up Rutford Ice ice presently rests on a bedrock sill which may play Stream (Stuiver and others 1981) and ultimately the a crucial role in controlling the position of the collapse of the ice sheet would lead to the expansion grounding line. The profile can be fitted to a steady- of Pine Island Bay and/or Ronne Ice Shelf into the state model but this alone is not adequate to deter­ Byrd Subglacial Basin. mine steady- or non-steady-state behaviour. Landsat Clearly, field data on the ice thickness and images show that the ice front undergoes periodic bedrock topography of this region are crucial to calving. Mass-balance calculations suggest that the argument. In February 1981 the British Antarctic accumulation in the catchment may exceed ablation by Survey carried out radio echo-sounding in Ellsworth a factor of 2. However, accumulation data are poor Land as part of a joint project with I W D Dalziel and there is no firm evidence of a build-up of ice of Lamont-Doherty Geological Observatory to invest­ within the Pine Island Glacier drainage basin. igate the geological relationship between Greater and Lesser Antarctica. Based at the US National 1. INTRODUCTION Science Foundation's (NSF) unoccupied Ellsworth Most of the West Antarctic ice sheet rests on Mountains field camp a total of 16 000 km was flown rock that is below sea-level. As such it may be using fuel left by NSF the previous year. A flight unstable (Weertman 1974) and may become decoupled on 6 February crossed Pine Island Glacier from north­ from its bed by a variety of processes (Hughes 1973). east to south-west, returning to the middle of the Grounding-line retreat of the large fast-flowing ice glacier and turning upstream from there. On 9 February streams which drain the interior may be the most the entire length of the glacier was sounded and a important mechanism whereby collapse of the ice sheet transverse line flown close to the ice front. These could occur (Hughes 1977). Since the position of the flights are shown superimposed on a Landsat image of grounding line is determined by the condition of Pine Island Glacier (Fig.l). hydrostatic equilibrium, it will advance outwards if the ice thickness increases or if sea-level falls, ultimately to the edge of the continental shelf if conditions allow. Conversely it is argued (Hughes 1977) that if the ice thins or sea-level rises, ice- stream grounding lines may retreat over low sills into the heart of the West Antarctic ice sheet. Grounding-line retreat will be halted if a high bed­ rock sill is reached or if an ice shelf forms and restrains the flow of ice across the grounding line. Thus the Ronne, Filchner, and Ross ice shelves are probably impeding flow of most of the ice streams which drain the present West Antarctic ice sheet, thereby tending to render the ice sheet stable (Thomas and Bentley 1978). The CLIMAP ice-sheet disintegration model (Stuiver and others 1981) indicates that Pine Island Bay in the Amundsen Sea may be the area most likely Fig.l. Landsat image of Pine Island Glacier showing to control any collapse of the West Antarctic ice flight lines of 6 February (dashed line) and sheet both in the Holocene and also today. Two large 9 February 1981 (solid line). The dotted line ice streams, which drain the interior of West Antarc­ shows the position of the inferred grounding line. 65 Downloaded from https://www.cambridge.org/core. 30 Sep 2021 at 11:12:57, subject to the Cambridge Core terms of use. Crabtree and Doake: Pine Island Glacier and its drainage basin 2. PROFILES (a) Figure 2(a) shows a longitudinal profile of the surface and bottom elevations and Figure 3 shows two 100 ] transverse profiles. Surface elevations were obtained o from pressure altimetry, allowing for terrain clear­ ance. The difference in radio altimeter readings -100 crossing the ice front gave the glacier elevation a.s.l. and provided control for the pressure readings. -200 Surface elevations should be within ±10 m. Ice ice shelf thicknesses were measured from the radio echo film -300 and should be within ±30 m; the velocity of radio -400 waves in ice was taken as 169 m ys"1. By using measured values of surface elevation and -500 ice thickness, the average ice density was calculated on the assumption that the glacier was floating in -600 hydrostatic equilibrium on sea-water of density 1.028 Mg m"3. Where the glacier was floating, the -700 average density was approximately constant at around 8 12 16 20 24 28 0.89 Mg m-3. Going up-stream, there was a sharp DISTANCE (km) reduction in the apparent average density coinciding with a prominent break in slope of the surface, suggesting that the glacier was aground. To confirm the position of the grounding line, the surface (b) elevation data were used to calculate the thickness of ice of average density 0.89 Mg m~3 that would 200 float in hydrostatic equilibrium with sea-water. Where the glacier is aground the "equilibrium" thick­ ness was of course greater than the measured thick- -200 LLI S-400 < CO 600 -800 -1000 10 20 30 40 50 60 70 80 90 Distance km (a) 0 8 12 16 20 24 28 32 36 40 DISTANCE ( km ) D Fig.3. Surface and bottom elevations across Pine j»500 -P J^~-~"~~ Island Glacier. (Letters refer to Fig.l.) (a) Near the ice front (there is a break in the radio echo between the glacier and ice shelf to V << the west). 1000 (b) About 20 km down-stream from the grounding n line. 1500 A A-A/ B ness (Fig.2). In this way the position of the ground­ ing line was found to within ±1 km. 70 80 90 By comparing the position of the ice front on (to) Distance km three Landsat images taken over a two-year period between 1973 and 1975 the average velocity at the ice Fig.2. Longitudinal profiles of Pine Island Glacier. front was found to be 2.1±0.2 km a"1. This compares (Letters refer to Fig.l.) with an independent estimate, using the same data, of (a) Surface and bottom elevations of Pine Island 2.2 km a"1 (R S Williams personal communication to Glacier. The dashed lines are the profile of C W M Swithinbank). The shape of the ice front is 6 February, the solid lines from the flight of basically unchanged, indicating that there were no 9 February. The dotted lines indicate the thickness major calving events during this period. of ice needed to float in hydrostatic equilibrium. The position of the ice front in February 1981 They follow the bottom profiles up to a point of was close to where a large rift was seen to run sudden divergence, indicating the position of the across the glacier in a 1973 Landsat image. With an grounding line (see text). average velocity of more than 2 km a"1 the ice front (b) Thickness profiles of Pine Island Glacier. The should have been at least 16 km further forward by solid line shows the profile as measured on the 1981. The ice front must calve periodically, maintain­ flight of 9 February 1981, the dashed line as ing the same approximate position. derived from the model. The longitudinal profile can be modelled by using 66 Downloaded from https://www.cambridge.org/core. 30 Sep 2021 at 11:12:57, subject to the Cambridge Core terms of use. Cvabtvee and Doake: Pine Island Glaaiev and its drainage basin a slightly modified version of the procedure describ­ although we do not know the form of the thickness ed by Sanderson (1979) for calculating equilibrium over x (which we are trying to calculate), we have profiles of ice shelves. The general equation for the still constrained the total length X, but it is thickness profile of an ice shelf confined in a bay unknown until the full profile has been calculated. with non-parallel sides (Sanderson 1979: equation 27) We have made no more assumptions than before is and have no more degrees of freedom (i.e. adjustable 6H u tanf parameters) than in the backward integration method. = {P.(a-m)/p - H(e +— )}/(u(x)+/ e dx}, (1) All that has been done is to replace an initial bound­ fix n xx X X xx ary condition for the ice-shelf length by a similar condition for the total ice-shelf side-wall area.
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