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Ice Fronts and Icebergs in the Ross and Weddell Seas ROSS

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Ice Fronts and Icebergs in the Ross and Weddell Seas ROSS This research was supported by National Science Foundation steady-state thickness and basal ice configurations of the central grant DPP 84-15207, the C.A. Lindbergh Fund, and the NATO Ronne Ice Shelf. Annals of Glaciology. geosciences program. Lewis, E.L., and R.G. Perkin. In press. Ice pumps and their rates. Journal of Geophysical Research. References McIntyre, N. 1986. Discharge of ice into the Filchner-Ronne ice shelves. In H. Kohnen (Ed.), Filchner-Ronne Ice Shelf Programme, (Report No. 3). Crabtree, R.D., and C.S.M. Doake. 1986. Radio-echo investigations of Bremerhaven: Alfred-Wegener-Institute for Polar and Marine Ronne Ice Shelf. Annals of Glaciology, 8, 37-41. Research. Dieckmann, C., C. Rohardt, H. Helimer, and J . Kipfstuhl. 1986. The Robin, C. de Q., C.S.M. Doake, H. Kohnen, R.D. Crabtree, S.R. Jor- occurrence of ice platelets at 250 m depth near the Filchner Ice Shelf dan, and D. Mo;diller. 1983. Regime of the Filchner-Ronne ice and its significance for sea ice biology. Deep-Sea Research, 33(2), shelves, Antarctica. Nature, 302(5909), 582-586. 141-148. Thyssen, F. 1986. The central part of the Filchner-Ronne Ice Shelf. In H. Engelhardt, H., and J. Determann. 1987. Borehole evidence for a thick Kohnen (Ed.), Filchner-Ronne Ice Shelf Programme, (Report No. 3). layer of basal ice in the central Ronne Ice Shelf. Nature, 373, 318-319. Bremerhaven: Alfred-Wegener-Institute for Polar and Marine Lange, M.A., and D.R. MacAyeal. In press. Numerical models of Research. Ice fronts and icebergs in the Ross Approximately 11 percent of the 14 million square kilometer antarctic ice sheet floats on and interacts with the continental and Weddell seas shelf seas of the southern ocean. The seaward margins of the ice shelves and glacier tongues evolve at various rates in response to inflowing ice streams, thermal and mechanical oceanic forc- STANLEY S. JACOBS ing, and the calving of icebergs. Where growth is not balanced by small-scale attrition, decadal periods of slow advance can be Lamon t- Doherty Geological Observatory punctuated by major ice front retreats of many kilometers in a Columbia University few days. For the past 25-75 years, most of the Ross Ice Shelf, Palisades, New York 10964 the Drygaiski Ice Tongue, and Erebus Glacier Tongue have 161° 163° 165° 167°E 800 o0 O0 75°S TERRA NOVA BAY ROSS 600 DAVWJ....................... 1t GLACIER DRYGALSKI ICE TONGUE : SEA 0 0 600 I- rodo -TOO 0 20 40 60 km I I I 76° Figure 1. Profiles of the Drygalski Ice Tongue in October 1960 (dotted) February 1973 (dot-dash), early 1980 (dashed), and February 1987 (solid). Sea floor topography in meters. Modified from Holdsworth (1985). 1987 REVIEW 91 advanced into the Ross Sea without significant retreat. Major continued to advance, with its eastern tip located at 165°27E calving events occurred in the Weddell Sea and the Ross Sea 75°27S in February 1987. Profiles of the Drygaiski Ice Tongue at during 1986 and 1987, with the Larsen, Filchner and Ross Ice 7-year intervals between 1960 and 1987 (figure 1) suggest a fairly Shelves producing some of the largest icebergs on record (Jac- steady eastward motion averaging around 750 meters per year obs 1987). since 1960. Several expendable bathythermograph casts re- Working from the U.S. Coast Guard icebreaker Polar Sea in vealed that water very near the glacial ice was warmer than at a February 1987, J. Ardai and P. McNutt (Columbia University) short distance north of it in Terra Nova Bay. Numerous ice caves recovered a current-meter mooring set by Oregon State Univer- were apparent at the Drygalski Ice Tongue water line, perhaps sity 2 years earlier near the Ross Ice Shelf at 173°W. In addition, indicative of erosion enhanced by wave action and penetration they made 200 expendable bathythermograph casts, most to full of seawater into firn layers. continental shelf depth, and assisted C. Stearns (University of The Ross Ice Shelf front in February, 1987 was everywhere Wisconsin) in the servicing of automatic weather stations on the north of its 1983 position. The advance was greatest near 1750E ice shelf and on Inexpressible Island. Taking advantage of the and east of the Bay of Whales. A large iceberg calved from the ships track and of the interest of Polar Sea Quartermaster H. latter section in late September or early October 1987 (figure 2) Garcia, they were also able to update positions of major ice and began to drift northwest at about 3 centimeters per second. fronts in the Ross Sea. Relatively small changes occurred from the Bay of Whales west In February 1987 the seaward end of the Erebus Glacier to approximately 172°W, where a north-south aligned sub- Tongue was at 166°21.6E 77425S, about 2 kilometers due marine ridge is the focus of relatively intense exchange between south of Tent Island and approximately 1 kilometer west of its the open Ross Sea and the sub-ice-shelf cavity (Visser and February 1979 location (Jacobs et al. 1981). This new position is Jacobs 1987). A persistent 33-kilometer peninsula around 173°E consistent with its calculated flow rate of about 150 meters per appeared to have thinned to an average width of approximately year and indicates that Erebus Glacier Tongue was probably 6 kilometers and developed a domed north-south profile. Pre- longer in 1987 than at any previous time in its 85-year recorded liminary calculations show an apparent 1983-1987 advance rate history (Holdsworth 1982). The Drygalski Ice Tongue has also for the entire ice front of 0.9 kilometers per year, close to the 0.84 fr: - I Figure 2. Satellite image of the large iceberg (named "B-9") which calved in the Bay of Whales in late September or early October 1987. 92 ANTARCTIC JOURNAL kilometers per year average reported by Jacobs, MacAyeal, and equivalent to more than twice the annual accummulation on the Ardai (1986) for the 1962-1985 period. antarctic ice sheet (e.g., Ferrigno and Gould 1987). Following The major 1986 and 1987 calving events redressed many the Larsen breakout, the large iceberg split into several parts, years of ice shelf advance, and resulted in icebergs ranging up two of which were closely tracked northward across the South to 95 kilometers on a side and containing a total ice volume Scotia Ridge and into the Scotia Sea (figure 3). However, the KOM 50° W 550 55°S 600 60° 65° 65° 60° 50° Figure 3. Solid squares show positions of the largest Larsen iceberg along its track from 6 March 1986, shortly after it calved from the hatched area, until 20 August 1986. By 5 September 1986 it had broken into two parts, the separate tracks of which are shown by triangles and circles up to 18 March 1987. Iceberg locations from Navy/National Oceanic and Atmospheric Administration Joint Ice Center weekly charts and other Defense Meteorological Satellite Program data. Sea floor bathymetry is in meters. Modified from Jacobs and Barnett (1987). 1987 REVIEW 93 A perspective on relative volumes above, S. Brower and B. Batchelder helped in the acquisition of equipment and data and in the preparation of this report. Volume (in Location/parameter cubic kilometers)a References Filchner and Larsen icebergs, 1986 5,100 Ferrigno, J., and W. Gould. 1987. Substantial changes in coastline of Lake Michigan 4,900 Antarctica revealed by satellite imagery. Polar Record, 23 (146), Annual runoff, Soviet Union 4,400 577-583. Annual world water use by man, 1980 3,600 Holdsworth, G. 1982. Dynamics of Erebus Glacier Tongue. Annals of World reservoir storage capacity by the year 2000 3,100 Glaciology, 3, 131-137. Annual accumulation on the antarctic ice sheet 2,200 Holdsworth, G. 1985. Some effects of ocean currents and wave motion Trolltunga iceberg, 1973 1,000 on the dynamics of floating glacier tongues. In Oceanology of the Sea level rise per year during the past century 500 Antarctic Continental Shelf, (Antarctic Research Series, Vol. 43). Wash- ington, D.C.: American Geophysical Union. Jacobs, S. 1986. The polar ice sheets: A wild card in the deck? Oceanus, a Values rounded to nearest 100 cubic kilometers water equivalent, as- 29(4), 50-54. suming an ice density of 0.84 and a sea level rise of 1.25 millimeters per Jacobs, S. 1987. Antarctic ice: How much smaller? EOS, 68(51), 1793. year. Source material in Jacobs and Barnett (1987). Jacobs, S. In press. Marine controls on modern sedimentation on the Antarctic Continental Shelf. Marine Geology. Filchner icebergs did not move far from their source area during Jacobs, S., and D. Barnett. 1987. On the draughts of some large Ant- the June 1986 to June 1987 period. They may have been trapped arctic icebergs. Iceberg Research. 14, 3-13. for a time by the westward coastal current in the cul-de-sac Jacobs, S., H. Huppert, G. Holdsworth and D. Drewry. 1981. Ther- formed by the new Filchner ice front, and by their greater mohaline steps induced by melting of' the Erebus Glacier Tongue. draughts relative to a shoal area near Berkner Island (Jacobs and Journal of Geophysical Research, 86, C7, 6547-6555. Jacobs, S., D. MacAyeal, and Ardai. 1986. The recent advance of the Barnett 1987). Icebergs generated from already floating ice J. Ross Ice Shelf, Antarctica. Journal of Glaciology, 32(112), 464-474. shelves have no immediate impact upon sea level (Jacobs 1986; Robin, G. 1986. Changing the sea level. In The greenhouse effect, climatic Robin 1986), but the larger ones contain impressive volumes of change, and ecosystems.
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