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The Evolving Front of the Ross Ice Shelf

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The Evolving Front of the Ross Ice Shelf The evolving front of the Ross Ice Shelf HARRY (J. R.) KEYS, Department of Conservation, Turangi, New Zealand STANLEY S. JACOBS, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964 LAWSON W. BRIGHAM, U.S. Coast Guard, Seattle, Washington, FPO AP 96698-3919 hanges recently observed in parts of the west antarctic east of Roosevelt Island where the giant B-9 iceberg calved in C ice sheet (Bindschadler 1993) re-emphasize questions 1987 (Keys et al. 1990). At its western extremity, the ice front is about its possible instability and role in global climate now north of Cape Bird on Ross Island, substantially beyond change. The floating ice shelves are of special interest to any position recorded in the last 150 years. The maximum investigators in several fields. For example, the mass balance advance rate calculated, 1.6 kilometers per year northward at the bottom of ice shelves has important influences on the relative to a 1987 position, is larger than that estimated for the ocean circulation, which in turn helps to control the ice 1962-1985 interval (Jacobs et al. 1986), and occurred at the thickness. The ice shelves may retard the flow of grounded northeast headland of a new feature we refer to as "Polar Sea ice streams. The ice fronts generate icebergs that can track Bay." This bay has widened and lengthened since 1987 at the the subsurface currents (Keys, Jacobs, and Barnett 1990) and site of a rift visible on EOSAT thematic mapper image may impede the ocean circulation in areas of large-scale 045/116, dated 25 December 1989. In places, this rifting has grounding. But how stable are ice front positions over peri- offset flow traces, showing that the velocity field is not uni- ods of several decades? form in space or time and that rifting is one reason for such In February and March of 1994, we measured the posi- variations. tion and height of several west antarctic ice sheet fronts, using The height and, therefore, thickness of the ice front show radar ranging and a sextant from the USCGC Polar Sea and a general decrease from east to west (figure 2). Heights are the R/V Nathaniel B. Palmer. The fronts of the Getz Ice Shelf greatest along the B-9 calving front and least in Coast Guard and other features displayed a wide variety of changes from Bay and in a newly forming Bay of Whales. Heights are rela- previously mapped locations but no consistent overall trend. tively low from approximately 171°W to 178 0W, where a The observed fluctuations result mainly from cycles of slow "warm" inflow enters and leaves the sub-ice cavity at depths advance and rapid retreat, driven by the ice-stream forcing, of 200-300 meters. We infer that this causes preferential melt- thickness changes, and aperiodic calving events. During the ing and thinning of the ice shelf in the central region. The last few decades some of the records have become accurate heights we measured are similar to those recorded in 1901 and frequent enough for estimates of short-term temporal and 1911 on the Discovery and Terra Nova expeditions; both changes in the ice fronts. data sets are characterized by considerable small-scale spatial Here we focus upon the Ross Ice Shelf edge (figure 1), variability. This has implications for calving, basal melting which has continued its northward advance, except to the near the ice front, and monitoring by satellite sensors. Rifts near ice fronts proba- bly exert the primary control on 1 60E 170E180E 170W 160•W ice-shelf calving. The northeast 75S 75S Ross Ice Shelf could be similar to the "throttle" area between Crary Ice Rise and the Trans- antarctic Mountains (Whillans and van der Veen 1993). Dy- namic changes in ice stream E and/or compression up-glacier and tension down-glacier be- tween Roosevelt Island and Shi- rase Coast could have led to the rifting that created B-9. (See fig- ure 3.) Away from such effects and from lateral and basal drag, ice-shelf creep thinning and 8OS 8OS basal melting may lead to rifting 160E 170E 180E 170W 160W below some critical ice thick- Figure 1. Sketch map of changes to the Ross Ice Shelf front between 1962 and 1994. West of the Bay of ness. In this regard, it will be of Whales, the ice front has contined the northward advance reported in Jacobs et al. (1986). East of that interest to observe whether the feature, a more southerly 1994 position results from the "B-9" calving event. presence of two large rifting ANTARCTIC JOURNAL - REVIEW 1994 125 Ross Ice Shelf - height of ice front Height (metres) 60 -r 161058.8W bays 50 178e59.E 40 ? if . : 30 B-9 calving region • opposite Ross Is •• C t. D • • • 20 S •.• 0 I 4. + • 176°09E -. in CG Bay 10 Bay of Whales - 4 - 0 155.00 160.00 165.00 170.00 175.00 180.00 185.00 190.00 Longitude West 1901 ° 1911 • 1994 - 1994 west from inside Coast Guard Bay Figure 2. Heights of the front of Ross Ice Shelf measured by sextant and radar range from Polar Sea (±2 meters) and historic data (Jones personal communication). Small-scale variability is apparent along the ice front, particularly in the Bay of Whales and other bays, most of which are controlled by rifting. bays on the thin northwest portion of the ice shelf leads to major calving in this sector in the near future and, if it does, whether the resulting icebergs will be carried westward into McMurdo Sound by the coastal current. We thank the officers and crews of the USCGC Polar Sea and the R/V Nathaniel B. Palmer for their considerable assis- tance and W. Gallagher of the Antarctic Library, Christchurch, New Zealand, for invaluable maps of the antarctic coastline. The fieldwork was supported by National Science Foundation grant OPP 92-20009. References Bindschadler, R. 1993. Siple Coast Project research of Crary Ice Rise and the mouths of ice streams B and C: Review and new perspec- tives. Journal of Glaciology, 39(133), 538-552. Jacobs, S.S., D.R. MacAyeal, and J.L. Ardai, Jr. 1986. The recent advance of the Ross Ice Shelf, Antarctica. Journal of Glaciology, 32(112),464-474. Jones, A.G.E. 1988. Personal communication. Keys, H.J.R., S.S. Jacobs, and D. Barnett. 1990. The calving and drift of Iceberg B-9 in the Ross Sea, Antarctica. Antarctic Science, 2(3), Figure 3. The eastern Ross Ice Shelf front, from NOAA-12 AVHRR vlsi- 243-257. ble image acquired 5 February 1994 on the USCGC Polar Sea. Moving Whillans, I., and C.J. van der Veen. 1993. New and improved determi- westward from its 0830L position that day, the ship reached 78029.9S nation of velocity of ice streams B and C, West Antarctica. Journal 163045.2W in the notch northwest of Roosevelt Island, which is of Glaciology, 39(133), 483-490. imbedded in the ice shelf beneath the oval outline. ANTARCTIC JOURNAL - REVIEW 1994 126.
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