Glaciological delineation of the dynamic coastline of Antarctica
Jane G. Ferrigno, U.S. Geological Survey, Reston, Virginia 22092
Charles Swithinbank, Scott Polar Research Institute, Cambridge, United Kingdom
Baerbel K. Lucchitta, U.S. Geological Survey, Flagstaff, Arizona 86001
Barbara A. Seekins, U.S. Geological Survey, Woods Hole, Massachusetts 02543
Christina E. Rosanova, U.S. Geological Survey, Flagstaff, Arizona 86001
In spite of their importance to global climate and sea level, the mass balance of the antarctic ice sheet and the dynamics of the coast of Antarctica are largely unknown. In 1990, the U.S. Geological Survey, in cooperation with the Scott Polar Research Institute (SPRI), began a long-term coastal-mapping project in Antarctica that is based on analysis of Landsat images and ancillary sources (Williams et al. 1995). The project has five objectives:
- to determine coastline changes that have occurred between the mid-1970s and the late 1980s/early 1990s;
- to establish an accurate baseline series of 24 1:1,000,000-scale maps that defines, from analysis (at a scale of 1:500,000) of Landsat images, the glaciological characteristics (e.g., floating ice, grounded ice, and so forth) of the coastline of Antarctica during the two time periods (figure);
- to determine velocities of outlet glaciers, ice streams, and ice shelves from comparison of Landsat images of the same areas taken over time;
- to compile a comprehensive inventory of named (from published maps and Landsat images) and unnamed (from analysis of Landsat images) outlet glaciers and ice streams in Antarctica that are mappable from Landsat images or from ancillary sources (e.g., maps, gazetteers, CD-ROMs, and so forth) (Swithinbank 1980, 1985, 1988; Alberts 1981, 1995; NSF 1989; BAS, SPRI, and WCMC 1993); and
- to compile a 1:5,000,000-scale map of Antarctica derived from the 24 maps.
Index map to the planned 24 1:1,000,000-scale Coastal-Change and Glaciological Maps of Antarctica. Bakutis Coast map shown in gray. |
Methodology
The primary steps in the compilation of the coastal-change and glaciological maps of Antarctica are as follows:- Identification of optimum Landsat multispectral scanner (MSS) or thematic mapper (TM) images for the two time periods (mid-1970s and late 1980s/early 1990s) and enlargement to a nominal scale of 1:500,000.
- Identification and plotting of ground control points and pass points on Landsat images from geodetic field-survey information (e.g., field notebooks, tables, and vertical and trimetrogon aerial photographs and maps) archived in the U.S. Geological Survey's SCAR Library (Reston, Virginia 22092). Plotting of pass points on overlapping Landsat images and transfer of control points and pass points to transparent overlays to provide ties between images in areas where geodetic ground control does not yet exist.
- Manual annotation of glaciological features by SCAR Code (SCAR 1980) or Antarctic Digital Database (ADD) Geocode (BAS et al. 1993) on 1:500,000-scale transparent overlays of Landsat images for both time periods. [The ADD project provides a digitized coastline and other cartographic information of Antarctica generalized to a scale of 1:1,000,000 that is available on a CD-ROM (BAS et al. 1993); the ADD CD-ROM provides the best existing coastline information for Antarctica.]
- Manual transfer of the combined (MSS with TM) annotated overlays to 1:500,000-scale oblique Mercator maps of each map sheet. TM images provide the most geometrically accurate base for combining the annotations derived from analysis of the MSS and TM images.
- Digitization, at 1:500,000-scale, using the U.S. Geological Survey's MAPGEN software (Evenden and Botbol 1985) and a digitizing program called "digin" written by G.I. Evenden (unpublished), of glaciological annotations and other related information on the oblique Mercator projections by SCAR Code or ADD Geocode.
- Transformation of digitized annotations to a 1:1,000,000-scale polar stereographic map base (standard parallel at 71°S) using the U.S. Geological Survey's MAPGEN software (Evenden 1990).
- Addition of glacier velocities, geographic place names, including codes for unnamed outlet glaciers and ice streams identified on Landsat images and modification of selected topographic form lines (BAS et al. 1993) and bathymetric contours using Adobe Illustrator software.
- Analysis of coastal changes, glaciological features, and outlet-glacier, ice-stream, and ice-shelf velocities.
Glaciological features
The Bakutis Coast (Swithinbank et al. in preparation) shows two dominant glaciological features: relatively narrow fringing ice shelves (Getz, Dotson, and Crosson Ice Shelves) and the Thwaites Glacier system (Thwaites Glacier, Thwaites Glacier Tongue, and Thwaites Iceberg Tongue). The Bakutis Coast map is divided into five ice-front segments by four islands (Dean, Siple, Carney, and Wright) located between DeVicq Glacier and Martin Peninsula. Siple Island, Carney Island, Martin Peninsula, and Bear Peninsula also contain small ice shelves separated by ice walls. Twenty-seven named and 14 unnamed outlet glaciers and ice streams flow into the ice shelves or directly into the Amundsen Sea; three other named glaciers are located in interior mountain ranges.Coastal change
As would be expected, the ice fronts, iceberg tongues, and glacier tongues are the most dynamic and changeable features in the coastal regions of Antarctica. Seaward of the grounding line of outlet glaciers, ice streams, and ice shelves, the floating ice margin is subject to frequent and large calving events or rapid flow. Both of these situations lead to annual and decadal changes in the position of ice fronts on the order of several kilometers, even tens of kilometers in extreme cases of major calving events. Although calving does occur along ice walls, the magnitude of change on an annual to decadal basis is generally not discernible on Landsat images; therefore, ice walls can be used as relatively stable reference features against which to measure other changes along the coast; only a single observation date is given for the position of ice walls.An analysis of changes from Wrigley Gulf on the western part of the Bakutis Coast map to the western part of Pine Island Bay on the east (130-104°W) indicates the following. West and north of Dean Island, the Getz Ice Shelf advanced from 3 to 12 kilometers (km) between 11 January 1973 and 25 February 1988 across a 51-km-wide ice front. The eastern part of the tongue of DeVicq Glacier (mostly on the Saunders Coast map) receded 6 km. West and east of Carney Island small parts of the Getz Ice Shelf receded from 1 to 5 km between 22 December 1972 and 25 February 1988 and between 23 November 1973 and 25 December 1986, respectively. The 46-km-wide ice front of Dotson Ice Shelf also receded 1 to 5 km between 16 January 1973 and 23 January 1990. The largest changes, however, occurred in the Thwaites Glacier Tongue and in the adjacent Crosson Ice Shelf. From the southeastern end of the ice wall of Hamilton Ice Piedmont (about 110°W) to the ice wall west of Pine Island Glacier (about 104°W) is a distance of 186 km. Along a 62-km-wide front of Crosson Ice Shelf that includes the confluence of Smith, Pope, and Vane Glaciers, the ice front receded from 5 to 13 km between 27 December 1972 and 22 January 1988. The irregular 83-km-wide terminus of Thwaites Glacier Tongue advanced about 10 km between 27 December 1972 and 22 January 1988; between 22 January 1988 and 9 February 1989, it advanced another 2 km.
Outlet-glacier, ice-stream, and ice-shelf velocities
Velocities of floating glaciers (e.g., glacier tongues, ice streams, and ice shelves) were determined by two methods: an interactive one in which crevassed patterns are traced visually on images (Lucchitta et al. 1993) and an auto-correlation program developed by Bindschadler and Scambos (1991) and Scambos et al. (1992). Under optimum conditions, errors can be as small as ±0.02 km per year, but for most Landsat image pairs, where registration of features is accurate to only two or three pixels, the accuracy of velocity vectors is ±0.1 km per year. The larger glacier tongues and ice shelves have well-developed rift patterns that can be used for velocity measurements. From 10 to 50 measurement points were made for each glacier tongue or ice shelf. Thwaites Glacier Tongue has an average velocity of 2.8 km per year, on the basis of Landsat images acquired on 2 December 1984 (50276-14524) and 9 January 1990 (42734-14552) (Ferrigno et al. 1993). On the basis of Landsat images acquired on 13 January 1973 (1174-14325) and 22 January 1988 (42016-14343), the floating tongue of Smith Glacier moved at an average rate of 0.6 km per year, although the velocity decreased to 0.5 km per year near the grounding line. The Smith Glacier tongue increased in velocity to an average of 0.7 km per year between 19 January 1988 and 23 January 1990. Dotson Ice Shelf, into which several named (Singer, McClinton, Dorchuk, Keys, Kohler, Boschert, True, Zuniga, Brush, and Sorenson Glaciers) and other unnamed glaciers flow, has an average velocity of 0.4 km per year (Lucchitta et al. 1993, 1994).Glacier inventory
Producing a sophisticated glacier inventory of Antarctica according to the requirements of the World Glacier Monitoring Service, as part of their ongoing "World Glacier Inventory" program, is impossible with the present state of glaciological knowledge about Antarctica (Swithinbank 1980). It is, however, possible to use Landsat images, supplemented by other satellite images and photographs south of 81.5°S (e.g., recently declassified Corona photographs, Systeme Probatoire d'Observation de la Terre images, Soyuzkarta images and photographs, National Oceanic and Atmospheric Administration advance very-high-resolution radar images, and so forth), and available maps to produce a reasonably complete preliminary inventory of named and unnamed outlet glaciers and ice streams and also to define more accurately related glaciological features, such as ice domes, ice piedmonts, ice shelves, ice rises, ice rumples, glacier tongues, iceberg tongues, and so forth. Satellite images and photographs also permit a better distinction to be made of islands and peninsulas, physical features that were often incorrectly identified and defined on earlier maps because of the lack of appropriate data.References
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