The evolving front of the Ross Ice Shelf
The evolving front of the Ross Ice Shelf Department of Conservation, Turangi, New Zealand Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964 LAWSON W. BRIGHAM, U.S. Coast Guard, Seattle, Washington, FPO AP 96698-3919 HARRY (J. R.) KEYS,
STANLEY S. JACOBS,
east of Roosevelt Island where the giant B-9 iceberg calved in 1987 (Keys et al. 1990). At its western extremity, the ice front is now north of Cape Bird on Ross Island, substantially beyond any position recorded in the last 150 years. The maximum advance rate calculated, 1.6 kilometers per year northward relative to a 1987 position, is larger than that estimated for the 1962-1985 interval (Jacobs et al. 1986), and occurred at the northeast headland of a new feature we refer to as "Polar Sea Bay." This bay has widened and lengthened since 1987 at the site of a rift visible on EOSAT thematic mapper image 045/116, dated 25 December 1989. In places, this rifting has offset flow traces, showing that the velocity field is not uniform in space or time and that rifting is one reason for such variations. The height and, therefore, thickness of the ice front show a general decrease from east to west (figure 2). Heights are greatest along the B-9 calving front and least in Coast Guard Bay and in a newly forming Bay of Whales. Heights are relatively low from approximately 171°W to 178 0W, where a "warm" inflow enters and leaves the sub-ice cavity at depths of 200-300 meters. We infer that this causes preferential melting and thinning of the ice shelf in the central region. The heights we measured are similar to those recorded in 1901 and 1911 on the Discovery and Terra Nova expeditions; both data sets are characterized by considerable small-scale spatial variability. This has implications for calving, basal melting near the ice front, and monitoring by satellite sensors. Rifts near ice fronts probably exert the primary control on ice-shelf calving. The northeast 1 60E 170E180E 170W 160•W 75S 75S Ross Ice Shelf could be similar to the "throttle" area between Crary Ice Rise and the Transantarctic Mountains (Whillans and van der Veen 1993). Dynamic changes in ice stream E and/or compression up-glacier and tension down-glacier between Roosevelt Island and Shirase Coast could have led to the rifting that created B-9. (See figure 3.) Away from such effects and from lateral and basal drag, ice-shelf creep thinning and 8OS basal melting may lead to rifting 8OS 160E 170E 180E 170W 160W below some critical ice thickFigure 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 presence of two large rifting feature, a more southerly 1994 position results from the "B-9" calving event.
hanges recently observed in parts of the west antarctic C ice sheet (Bindschadler 1993) re-emphasize questions about its possible instability and role in global climate change. The floating ice shelves are of special interest to investigators in several fields. For example, the mass balance at the bottom of ice shelves has important influences on the ocean circulation, which in turn helps to control the ice thickness. The ice shelves may retard the flow of grounded ice streams. The ice fronts generate icebergs that can track the subsurface currents (Keys, Jacobs, and Barnett 1990) and may impede the ocean circulation in areas of large-scale grounding. But how stable are ice front positions over periods of several decades? In February and March of 1994, we measured the position and height of several west antarctic ice sheet fronts, using radar ranging and a sextant from the USCGC Polar Sea and the R/V Nathaniel B. Palmer. The fronts of the Getz Ice Shelf and other features displayed a wide variety of changes from previously mapped locations but no consistent overall trend. The observed fluctuations result mainly from cycles of slow advance and rapid retreat, driven by the ice-stream forcing, thickness changes, and aperiodic calving events. During the last few decades some of the records have become accurate and frequent enough for estimates of short-term temporal changes in the ice fronts. Here we focus upon the Ross Ice Shelf edge (figure 1), which has continued its northward advance, except to the
ANTARCTIC JOURNAL - REVIEW 1994 125
Ross Ice Shelf - height of ice front
Height (metres) 60 -r
bays
161058.8W
50 ?
40 30 20
. : B-9 calving region S
0 155.00
•.'•
178e59.E
D
+ •
4
10
t. ••
C
.
if
Bay of Whales
0
•••
• opposite Ross Is
I
176°09'E -. in CG Bay -4
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 assistance 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 perspectives. 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), 243-257. Whillans, I., and C.J. van der Veen. 1993. New and improved determination of velocity of ice streams B and C, West Antarctica. Journal of Glaciology, 39(133), 483-490.
Figure 3. The eastern Ross Ice Shelf front, from NOAA-12 AVHRR vlsible image acquired 5 February 1994 on the USCGC Polar Sea. Moving westward from its 0830L position that day, the ship reached 78029.9'S 1630 45.2'W in the notch northwest of Roosevelt Island, which is imbedded in the ice shelf beneath the oval outline.
ANTARCTIC JOURNAL 126
- REVIEW 1994
STANLEY S. JACOBS,
east of Roosevelt Island where the giant B-9 iceberg calved in 1987 (Keys et al. 1990). At its western extremity, the ice front is now north of Cape Bird on Ross Island, substantially beyond any position recorded in the last 150 years. The maximum advance rate calculated, 1.6 kilometers per year northward relative to a 1987 position, is larger than that estimated for the 1962-1985 interval (Jacobs et al. 1986), and occurred at the northeast headland of a new feature we refer to as "Polar Sea Bay." This bay has widened and lengthened since 1987 at the site of a rift visible on EOSAT thematic mapper image 045/116, dated 25 December 1989. In places, this rifting has offset flow traces, showing that the velocity field is not uniform in space or time and that rifting is one reason for such variations. The height and, therefore, thickness of the ice front show a general decrease from east to west (figure 2). Heights are greatest along the B-9 calving front and least in Coast Guard Bay and in a newly forming Bay of Whales. Heights are relatively low from approximately 171°W to 178 0W, where a "warm" inflow enters and leaves the sub-ice cavity at depths of 200-300 meters. We infer that this causes preferential melting and thinning of the ice shelf in the central region. The heights we measured are similar to those recorded in 1901 and 1911 on the Discovery and Terra Nova expeditions; both data sets are characterized by considerable small-scale spatial variability. This has implications for calving, basal melting near the ice front, and monitoring by satellite sensors. Rifts near ice fronts probably exert the primary control on ice-shelf calving. The northeast 1 60E 170E180E 170W 160•W 75S 75S Ross Ice Shelf could be similar to the "throttle" area between Crary Ice Rise and the Transantarctic Mountains (Whillans and van der Veen 1993). Dynamic changes in ice stream E and/or compression up-glacier and tension down-glacier between Roosevelt Island and Shirase Coast could have led to the rifting that created B-9. (See figure 3.) Away from such effects and from lateral and basal drag, ice-shelf creep thinning and 8OS basal melting may lead to rifting 8OS 160E 170E 180E 170W 160W below some critical ice thickFigure 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 presence of two large rifting feature, a more southerly 1994 position results from the "B-9" calving event.
hanges recently observed in parts of the west antarctic C ice sheet (Bindschadler 1993) re-emphasize questions about its possible instability and role in global climate change. The floating ice shelves are of special interest to investigators in several fields. For example, the mass balance at the bottom of ice shelves has important influences on the ocean circulation, which in turn helps to control the ice thickness. The ice shelves may retard the flow of grounded ice streams. The ice fronts generate icebergs that can track the subsurface currents (Keys, Jacobs, and Barnett 1990) and may impede the ocean circulation in areas of large-scale grounding. But how stable are ice front positions over periods of several decades? In February and March of 1994, we measured the position and height of several west antarctic ice sheet fronts, using radar ranging and a sextant from the USCGC Polar Sea and the R/V Nathaniel B. Palmer. The fronts of the Getz Ice Shelf and other features displayed a wide variety of changes from previously mapped locations but no consistent overall trend. The observed fluctuations result mainly from cycles of slow advance and rapid retreat, driven by the ice-stream forcing, thickness changes, and aperiodic calving events. During the last few decades some of the records have become accurate and frequent enough for estimates of short-term temporal changes in the ice fronts. Here we focus upon the Ross Ice Shelf edge (figure 1), which has continued its northward advance, except to the
ANTARCTIC JOURNAL - REVIEW 1994 125
Ross Ice Shelf - height of ice front
Height (metres) 60 -r
bays
161058.8W
50 ?
40 30 20
. : B-9 calving region S
0 155.00
•.'•
178e59.E
D
+ •
4
10
t. ••
C
.
if
Bay of Whales
0
•••
• opposite Ross Is
I
176°09'E -. in CG Bay -4
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 assistance 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 perspectives. 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), 243-257. Whillans, I., and C.J. van der Veen. 1993. New and improved determination of velocity of ice streams B and C, West Antarctica. Journal of Glaciology, 39(133), 483-490.
Figure 3. The eastern Ross Ice Shelf front, from NOAA-12 AVHRR vlsible image acquired 5 February 1994 on the USCGC Polar Sea. Moving westward from its 0830L position that day, the ship reached 78029.9'S 1630 45.2'W in the notch northwest of Roosevelt Island, which is imbedded in the ice shelf beneath the oval outline.
ANTARCTIC JOURNAL 126
- REVIEW 1994
