Icebergs rework sediments on antarctic shelf
Icebergs rework sediments on antarctic shelf PETER W. BARNES
U.S. Geological Survey Menlo Park, California 94025 REIDAR LIEN
A/S
øvre Flataøsv. 10 7079 Flataoos Norway
Ice-related geologic processes are important in forming seabed morphology on high-latitude continental shelves. Previous studies of the antarctic shelf have characterized the rugged relief, great depths, and marginal troughs (Vanney and Johnson 1985). Glacial processes dominate sedimentary processes and are responsible for large-scale topographic features (Anderson et al. 1983). The shelf area, less than 1,000 meters deep, excluding submerged portions of the continents covered by glaciers and ice shelves, is about 3 million square kilometers (Barnes 1986a; table). Depth data are not available for 25 percent of the shelf mainly along the coast in areas with persistent ice. Of the remaining three quarters of the shelf, 54 percent or 1.2 million square kilometers is less than 500 meters deep. Weddell Sea studies by Orheim and Elverhøi (1981) describe well-preserved "plough marks" in water depths down to 600 meters. The fresh appearance of the various features suggests their recency; however, slow rates of erosion or sedimentation are also implicated. Lien (1981) delineates seabed morphologies in water depths of less than 400 meters. Ice gouging or plough marks are prevalent on the bank tops and down to 320 meters with maximum widths reaching 250 meters and maximum depths 25 meters, below the surrounding seafloor. A washboard pattern of parallel ridges attributed to wobbling icebergs is associated with the gouging that extends down to at least 400 meters, the maximum
depth of the survey. The Norwegian Antarctic Research Expedition (NARE) 1984-1985 seabed surveys indicate that ice gouging expanded to areas previously dominated by washboard patterns while areas of undisturbed seabed have diminished. Studies off the Wilkes Land coast from the S.P. Lee in 1984 indicate a moderately ice-gouged surface with micro-relief of less than 3 meters (Barnes 1986b). The eastern slope of Mertz Bank is an irregular surface where >5 meter micro-relief to depths of 500 meters is common (figure). The most common bedform on both the bank top and slope are "crisp" 50 to 100 meters circular and semicircular, flat-floored depressions less than 2 meters deep. Commonly, several depressions of equal size occur en-echelon, to form a single linear feature superimposed to the northwest. Many superimposed depressions were associated with multiple parallel northwest-southeast linear ice gouge furrows (Barnes 1986b). The potential draft of antarctic tabular icebergs based on limited observations is 330 meters (Orheim 1980). Because nontabular and tilted tabular icebergs make up the major portion of the antarctic iceberg population (Keys 1983) and can increase their draft up to 50 percent by rolling (Lewis and Bennett 1984), modern iceberg drafts may approach 500 meters. The grooves on the seafloor of the Antarctic are similar to ice gouges generated by sea ice and glacial ice in the Arctic (Reim-
Shelf areaa Square kilometers Percent
No data 0-200 meters 200-500 meters 500 meters-1 kilometer
730,600 138,600 1,070,500 1,042,100
24.5 4.6 35.9 34.9
Total
2,981,900
99.0
a From Barnes 1986a.
- -.......................................--.-
300
-.
-
.
mk
MISE
Sonograph of ice gouges and overlapping iceberg depressions on the eastern slope of Mertz Bank off the Wilkes Land coast at 530-meter water depth.
130
ANTARCTIC JOURNAL
nitz and Barnes 1974; Josenhans, Zevenhuizen, and Klassen 1986). The similarity of form and the availability of deep draft ice indicate to us that the antarctic seabed features were also formed by iceberg keels. The origin of the sub-circular, flat-floored depressions are also postulated to be related to modern iceberg keels (Lien 1981; Barnes 1986). Subglacial processes, permafrost collapse, slope failure, and mass movement are not favored as mechanisms to form these depressions. Their close association with ice gouging and wobbling also suggests iceberg keels are responsible for these features. Iceberg with drafts between 400 and 500 meters are probably uncommon; however, recurrence intervals of tens to a few hundred years may be sufficient to maintain the observed morphologies. Vorren et al. (1983) introduced the term iceberg turbate for seabed strata developed from the tilling action of iceberg keels on the seafloor. We suggest that ice keel turbates are widespread on the antarctic shelf. Ice reworking the surficial sediment may be forming a surficial unstructured marine diamicite which may correspond to the glacial marine deposits reported by Anderson et al. (1983). Side-scan sonar data from the Weddell Sea and the Wilkes Land shelf indicate ice gouge incisions a few meters deep and tens of meters in width, down to depths of over 500 meters. In the Weddell Sea, a "washboard" pattern and hummocky bed features have been formed by the interaction between ice keels and the seabed to depths exceeding 300 meters. The freshness of seabed morphology, and Holocene sediment ponding at depths greater than 500 meters, indicate that the seafloor is presently being reworked by iceberg keels. We conclude that a modern ice keel turbate deposit may be widespread on the portion (54 percent) of the Antarctic Shelf less than 500 meters deep, probably in the form of an unstratified to poorly stratified marine diamicton. The field work aboard the S.P. Lee was in part supported by the National Science Foundation's program at McMurdo Station and Christchurch, New Zealand.
References Anderson, J. B., C. Brake, E. W. Domack, N. Myers, and R. Wright. 1983. Development of a polar glacial-marine sedimentation model from Antarctic Quaternary deposits and glaciological information. In B.F. Molnia (Ed.), Glacial-marine sedimentation. New York: Plenum Press. Barnes, P.W. 1986a. Distribution of water depths on the Antarctic continental shelf seaward of the continental land and ice mass. (U. S. Geological Survey Open-File Report, 86-599.) Washington, D.C.: U.S. Government Printing Office. Barnes, P.W. 1986b. Morphologic studies of the Wilkes Land continental shelf glacial and iceberg effects. In S.L. Eittreim and M.A. Hampton (Eds.), The Antarctic continental margin: Geology and geophysics of offshore Wilkes Land. (Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, Vol. 5A.) Houston, Texas. Josenhans, H. W., J . Zevenhuizen, and R. A. Klassen. 1986. The Quaternary geology of the Labrador Shelf. Canadian Journal of Earth Sciences,
23, 1190-1213. Keys, J.R. 1983. Iceberg quantities, shapes, and sizes in western Ross and D'Urville Seas. Antarctic Journal of the U.S., 18(5), 125-127. Lewis, J.C., and G. Bennett. 1984. Monte Carlo calculations of iceberg draft changes caused by roll. Cold Regions Science and Technology, 10, 1-10. Lien, R. 1981. Seabed features in the Baaenga area, Weddell Sea, Antarctica. Port and ocean engineering under Arctic conditions 81, Proceedings.
Quebec, Canada, University of Laval. Orheim, 0. 1980. Physical characteristics and life expectancy of tabular Antarctic icebergs. Annals of Glaciology, 1, 11-18. Orheim, 0., and A. Elverhoi. 1981. Model for submarine glacial deposition. Annals of Glaciology, 2, 123-128. Reimnitz, E., and P.W. Barnes. 1974. Sea ice as a geologic agent. In J. Reed and J . Sater (Eds.), The coast and shelf. Arlington, Va. Vanney, JR., and G.L. Johnson, 1985. GEBCO bathymetric sheet 5-18. In S.S. Jacobs (Ed.), Oceanology of the Antarctic continental shelf. (Antarctic Research Series, Vol. 43.) Washington, D.C.: American Geophysical Union. Vorren, TO., M. Hald, M. Edvardsen, and 0. Lind-Hansen. 1983. Glaciogenic sediments and sedimentary environments on continental shelves: General principles with a case study from the Norwegian Shelf. In J. Ehlers (Ed.), Glacial deposits of northwest Europe. Rotterdam: Balkema.
Preliminary results of marine geological and geophysical investigations in the Bransfield Strait, Antarctic Peninsula JOHN D. JEFFERS
Department of Geology and Geophysics Rice University Houston, Texas 77251
The 1986-1987 austral summer marked the first season iIv (figure 1) operated in antarctic waters as a fully equipped marine geophysical research vessel. Newly installed equipment included a single-channel seismic reflection data acquisition system, nuclear precession magnetometer, and 3.5and 12-kilohertz echo sounders. Seismic reflection, magnetic, and bathymetric profiles were collected in the Bransfield Strait Polar Duke
1987 REVIEW
Figure 1. R/V Polar Duke at Palmer Station, Anvers Island.
131
U.S. Geological Survey Menlo Park, California 94025 REIDAR LIEN
A/S
øvre Flataøsv. 10 7079 Flataoos Norway
Ice-related geologic processes are important in forming seabed morphology on high-latitude continental shelves. Previous studies of the antarctic shelf have characterized the rugged relief, great depths, and marginal troughs (Vanney and Johnson 1985). Glacial processes dominate sedimentary processes and are responsible for large-scale topographic features (Anderson et al. 1983). The shelf area, less than 1,000 meters deep, excluding submerged portions of the continents covered by glaciers and ice shelves, is about 3 million square kilometers (Barnes 1986a; table). Depth data are not available for 25 percent of the shelf mainly along the coast in areas with persistent ice. Of the remaining three quarters of the shelf, 54 percent or 1.2 million square kilometers is less than 500 meters deep. Weddell Sea studies by Orheim and Elverhøi (1981) describe well-preserved "plough marks" in water depths down to 600 meters. The fresh appearance of the various features suggests their recency; however, slow rates of erosion or sedimentation are also implicated. Lien (1981) delineates seabed morphologies in water depths of less than 400 meters. Ice gouging or plough marks are prevalent on the bank tops and down to 320 meters with maximum widths reaching 250 meters and maximum depths 25 meters, below the surrounding seafloor. A washboard pattern of parallel ridges attributed to wobbling icebergs is associated with the gouging that extends down to at least 400 meters, the maximum
depth of the survey. The Norwegian Antarctic Research Expedition (NARE) 1984-1985 seabed surveys indicate that ice gouging expanded to areas previously dominated by washboard patterns while areas of undisturbed seabed have diminished. Studies off the Wilkes Land coast from the S.P. Lee in 1984 indicate a moderately ice-gouged surface with micro-relief of less than 3 meters (Barnes 1986b). The eastern slope of Mertz Bank is an irregular surface where >5 meter micro-relief to depths of 500 meters is common (figure). The most common bedform on both the bank top and slope are "crisp" 50 to 100 meters circular and semicircular, flat-floored depressions less than 2 meters deep. Commonly, several depressions of equal size occur en-echelon, to form a single linear feature superimposed to the northwest. Many superimposed depressions were associated with multiple parallel northwest-southeast linear ice gouge furrows (Barnes 1986b). The potential draft of antarctic tabular icebergs based on limited observations is 330 meters (Orheim 1980). Because nontabular and tilted tabular icebergs make up the major portion of the antarctic iceberg population (Keys 1983) and can increase their draft up to 50 percent by rolling (Lewis and Bennett 1984), modern iceberg drafts may approach 500 meters. The grooves on the seafloor of the Antarctic are similar to ice gouges generated by sea ice and glacial ice in the Arctic (Reim-
Shelf areaa Square kilometers Percent
No data 0-200 meters 200-500 meters 500 meters-1 kilometer
730,600 138,600 1,070,500 1,042,100
24.5 4.6 35.9 34.9
Total
2,981,900
99.0
a From Barnes 1986a.
- -.......................................--.-
300
-.
-
.
mk
MISE
Sonograph of ice gouges and overlapping iceberg depressions on the eastern slope of Mertz Bank off the Wilkes Land coast at 530-meter water depth.
130
ANTARCTIC JOURNAL
nitz and Barnes 1974; Josenhans, Zevenhuizen, and Klassen 1986). The similarity of form and the availability of deep draft ice indicate to us that the antarctic seabed features were also formed by iceberg keels. The origin of the sub-circular, flat-floored depressions are also postulated to be related to modern iceberg keels (Lien 1981; Barnes 1986). Subglacial processes, permafrost collapse, slope failure, and mass movement are not favored as mechanisms to form these depressions. Their close association with ice gouging and wobbling also suggests iceberg keels are responsible for these features. Iceberg with drafts between 400 and 500 meters are probably uncommon; however, recurrence intervals of tens to a few hundred years may be sufficient to maintain the observed morphologies. Vorren et al. (1983) introduced the term iceberg turbate for seabed strata developed from the tilling action of iceberg keels on the seafloor. We suggest that ice keel turbates are widespread on the antarctic shelf. Ice reworking the surficial sediment may be forming a surficial unstructured marine diamicite which may correspond to the glacial marine deposits reported by Anderson et al. (1983). Side-scan sonar data from the Weddell Sea and the Wilkes Land shelf indicate ice gouge incisions a few meters deep and tens of meters in width, down to depths of over 500 meters. In the Weddell Sea, a "washboard" pattern and hummocky bed features have been formed by the interaction between ice keels and the seabed to depths exceeding 300 meters. The freshness of seabed morphology, and Holocene sediment ponding at depths greater than 500 meters, indicate that the seafloor is presently being reworked by iceberg keels. We conclude that a modern ice keel turbate deposit may be widespread on the portion (54 percent) of the Antarctic Shelf less than 500 meters deep, probably in the form of an unstratified to poorly stratified marine diamicton. The field work aboard the S.P. Lee was in part supported by the National Science Foundation's program at McMurdo Station and Christchurch, New Zealand.
References Anderson, J. B., C. Brake, E. W. Domack, N. Myers, and R. Wright. 1983. Development of a polar glacial-marine sedimentation model from Antarctic Quaternary deposits and glaciological information. In B.F. Molnia (Ed.), Glacial-marine sedimentation. New York: Plenum Press. Barnes, P.W. 1986a. Distribution of water depths on the Antarctic continental shelf seaward of the continental land and ice mass. (U. S. Geological Survey Open-File Report, 86-599.) Washington, D.C.: U.S. Government Printing Office. Barnes, P.W. 1986b. Morphologic studies of the Wilkes Land continental shelf glacial and iceberg effects. In S.L. Eittreim and M.A. Hampton (Eds.), The Antarctic continental margin: Geology and geophysics of offshore Wilkes Land. (Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, Vol. 5A.) Houston, Texas. Josenhans, H. W., J . Zevenhuizen, and R. A. Klassen. 1986. The Quaternary geology of the Labrador Shelf. Canadian Journal of Earth Sciences,
23, 1190-1213. Keys, J.R. 1983. Iceberg quantities, shapes, and sizes in western Ross and D'Urville Seas. Antarctic Journal of the U.S., 18(5), 125-127. Lewis, J.C., and G. Bennett. 1984. Monte Carlo calculations of iceberg draft changes caused by roll. Cold Regions Science and Technology, 10, 1-10. Lien, R. 1981. Seabed features in the Baaenga area, Weddell Sea, Antarctica. Port and ocean engineering under Arctic conditions 81, Proceedings.
Quebec, Canada, University of Laval. Orheim, 0. 1980. Physical characteristics and life expectancy of tabular Antarctic icebergs. Annals of Glaciology, 1, 11-18. Orheim, 0., and A. Elverhoi. 1981. Model for submarine glacial deposition. Annals of Glaciology, 2, 123-128. Reimnitz, E., and P.W. Barnes. 1974. Sea ice as a geologic agent. In J. Reed and J . Sater (Eds.), The coast and shelf. Arlington, Va. Vanney, JR., and G.L. Johnson, 1985. GEBCO bathymetric sheet 5-18. In S.S. Jacobs (Ed.), Oceanology of the Antarctic continental shelf. (Antarctic Research Series, Vol. 43.) Washington, D.C.: American Geophysical Union. Vorren, TO., M. Hald, M. Edvardsen, and 0. Lind-Hansen. 1983. Glaciogenic sediments and sedimentary environments on continental shelves: General principles with a case study from the Norwegian Shelf. In J. Ehlers (Ed.), Glacial deposits of northwest Europe. Rotterdam: Balkema.
Preliminary results of marine geological and geophysical investigations in the Bransfield Strait, Antarctic Peninsula JOHN D. JEFFERS
Department of Geology and Geophysics Rice University Houston, Texas 77251
The 1986-1987 austral summer marked the first season iIv (figure 1) operated in antarctic waters as a fully equipped marine geophysical research vessel. Newly installed equipment included a single-channel seismic reflection data acquisition system, nuclear precession magnetometer, and 3.5and 12-kilohertz echo sounders. Seismic reflection, magnetic, and bathymetric profiles were collected in the Bransfield Strait Polar Duke
1987 REVIEW
Figure 1. R/V Polar Duke at Palmer Station, Anvers Island.
131
