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Outlet-glacier, ice-stream, and ice-shelf velocities
Glaciological delineation of the dynamic coastline
Richard S. Williams, Jr., U.S. Geological Survey, Woods
Hole, Massachusetts 02543
Jane G. Ferrigno, U.S. Geological Survey, Reston,
Charles Swithinbank, Scott Polar Research Institute,
Cambridge, United Kingdom
Baerbel K. Lucchitta, U.S. Geological Survey, Flagstaff,
Barbara A. Seekins, U.S. Geological Survey, Woods
Hole, Massachusetts 02543
Christina E. Rosanova, U.S. Geological Survey, Flagstaff,
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:
Changes in the area and volume of polar ice sheets are intricately linked
to changes in global climate. It is not known whether the ice sheet is
growing or shrinking (NRC 1985). As a result, measurement of changes in
the antarctic ice sheet was given a very high priority in recommendations
by the Polar Research Board of the National Research Council (1986) and
the Scientific Committee on Antarctic Research (SCAR) (1989).
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);
Index map to the planned 24 1:1,000,000-scale Coastal-Change
and Glaciological Maps of Antarctica. Bakutis Coast map shown in gray.
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.
The primary steps in the compilation
of the coastal-change and glaciological maps of Antarctica are as follows:
The following discussion of the recently completed Bakutis Coast map (currently
being readied for printing) is used as an example of the types of coastal-change
and glaciological information that can be derived from analysis of Landsat
MSS and TM images.
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.
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.
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
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,
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.
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