Ice thickness in McMurdo Sound
A perspective on relative volumes
Location/parameter
Volume (in cubic kilometers)a
5,100 Filchner and Larsen icebergs, 1986 4,900 Lake Michigan 4,400 Annual runoff, Soviet Union 3,600 Annual world water use by man, 1980 World reservoir storage capacity by the year 2000 3,100 Annual accumulation on the antarctic ice sheet 2,200 1,000 Trolltunga iceberg, 1973 Sea level rise per year during the past century 500 a Values rounded to nearest 100 cubic kilometers water equivalent, assuming an ice density of 0.84 and a sea level rise of 1.25 millimeters per year. Source material in Jacobs and Barnett (1987).
Filchner icebergs did not move far from their source area during the June 1986 to June 1987 period. They may have been trapped for a time by the westward coastal current in the cul-de-sac formed by the new Filchner ice front, and by their greater draughts relative to a shoal area near Berkner Island (Jacobs and Barnett 1987). Icebergs generated from already floating ice shelves have no immediate impact upon sea level (Jacobs 1986; Robin 1986), but the larger ones contain impressive volumes of water (table), can scar the sea floor (Jacobs in press), and may alter the dynamics of the ice shelves they leave behind. This research was supported by National Science Foundation grants DPP 85-12540 and DPP 85-02386. ITT Antarctic Services, the marine science technicians of Polar Sea, the individuals cited
Ice thickness in McMurdo Sound AMY LEVENTER, ROBERT
B. DUNBAR, and MARIAN R. ALLEN
Earth Systems Institute Department of Geology and Geophysics Rice University Houston, Texas 77251-1892 RICHARD Y. WAYPER
Antarctic Research Centre Victoria University Wellington, New Zealand
In this paper we present data concerning changes in ice thickness throughout McMurdo Sound from October 1986 through February 1987. During a typical year in the Sound, annual sea ice begins to form in March or April. Thin sheets of ice develop and break out during this time, as a consequence of 94
above, S. Brower and B. Batchelder helped in the acquisition of equipment and data and in the preparation of this report. References Ferrigno, J., and W. Gould. 1987. Substantial changes in coastline of Antarctica revealed by satellite imagery. Polar Record, 23 (146), 577-583. Holdsworth, G. 1982. Dynamics of Erebus Glacier Tongue. Annals of Glaciology, 3, 131-137. Holdsworth, G. 1985. Some effects of ocean currents and wave motion on the dynamics of floating glacier tongues. In Oceanology of the Antarctic Continental Shelf, (Antarctic Research Series, Vol. 43). Washington, D.C.: American Geophysical Union. Jacobs, S. 1986. The polar ice sheets: A wild card in the deck? Oceanus, 29(4), 50-54. Jacobs, S. 1987. Antarctic ice: How much smaller? EOS, 68(51), 1793. Jacobs, S. In press. Marine controls on modern sedimentation on the Antarctic Continental Shelf. Marine Geology. Jacobs, S., and D. Barnett. 1987. On the draughts of some large Antarctic icebergs. Iceberg Research. 14, 3-13. Jacobs, S., H. Huppert, G. Holdsworth and D. Drewry. 1981. Thermohaline steps induced by melting of' the Erebus Glacier Tongue. Journal of Geophysical Research, 86, C7, 6547-6555. Jacobs, S., D. MacAyeal, and J. Ardai. 1986. The recent advance of the Ross Ice Shelf, Antarctica. Journal of Glaciology, 32(112), 464-474. Robin, G. 1986. Changing the sea level. In The greenhouse effect, climatic change, and ecosystems. Chichester, W. Sussex: Scientific Committee on Problems of the Environment. Visser, A., and S. Jacobs. 1987. Annual mean estimates of heat transport into the Ross Sea sub-ice cavity. (Paper presented at the High Latitude Oceans Symposia, IAPSO-OPS-2C, International Union of Geodesy and Geophysics, Vancouver, 14 August 1987.)
storms and winds. Once the ice has an opportunity to thicken, it strengthens and becomes more impervious to wind stress. The ice generally continues to thicken until November or December, when it reaches its average thickness of 2 meters. During the summer, the ice edge progressively moves southward until February, when much of the remainder of the sea ice goes out (figure 1); however, many years the ice in the western Sound does not break out at all. Figure 2 illustrates changes in ice thickness at six sites from McMurdo Sound, from October 1986 to February 1987. In the eastern Sound (Tent Island, Barnes Glacier, and Erebus Ice Tongue sites), ice continues to thicken until early to midNovember, and maintains its 2-meter average thickness through mid-December. Ice thins precipitously from late December until ice breakout in January or February. An average melting rate of approximately 2.5 centimeters per day from our data is quite similar to the 2.9 centimeters per day rate measured by Mitchell and Bye (1985) near McMurdo Station. The relatively high melting rates measured in the eastern sound maybe correlated to advection of relatively warm water from the north (Heath 1977; Mitchell and Bye 1985; Tressler and Ommundsen 1962). Although currents were not measured, a steep wire angle (approximately 45°) to the south was observed on our sediment trap mooring at the Erebus Ice Tongue site from 15 January to 27 January. Prior to this time no wire angle was observed. ANTARCTIC JOURNAL
.456
supply. In the eastern Sound, biogenic material previously trapped within the lower portion of the sea ice is continuously being shed into the water column while the sea ice is rapidly thinning. Since primary production within the annual sea ice in McMurdo Sound may exceed 40 grams of carbon per square meter per year (McGrath-Grossi et al. in press), and estimates of annual primary productivity in antarctic waters range between 16 and 100 grams of carbon per square meter per year (HolmHansen et al. 1977). Flux of particulates from the melting ice may provide a significant amount of carbon to sediments in the eastern Sound. In the western Sound, where the amount of ice melting prior to breakout may be inconsequential, particulate material within the ice can be carried out of the Sound more readily. The decreased downward flux of organic material may be a factor influencing the low standing stock of benthic organisms in the western Sound (Dayton and Oliver 1977; Dayton et al. 1986). This work was supported by National Science Foundation grant DPP 85-16911.
0
MCMURDO SOUND
/ ROSS ISLAND ...
t"
1 Tent Island 2 Barnes Glacier 3 Erebus Ice Tongue 4 Granite Harbor inner basin 5 Granite Harbor sill 6 Granite Harbor outer basin
loz
I014
ROSS ICE SHELF
100
78
100
Figure 1. Position of ice edge in McMurdo Sound in September 1986 and on 13 January 1987. Numbers indicate locations for which ice thickness data is presented.
References
Dayton, P.K., D. Watson, A. Palmisano, J.P. Barry, J.S. Oliver, and D. Rivera. 1986. Distribution patterns of benthic microalgal standing stock at McMurdo Sound, Antarctica. Polar Biology, 6, 207-213. Dayton, P.K. and J.S. Oliver. 1977. Antarctic soft bottom benthos in oligotrophic and eutrophic environments. Science, 197, 55-58. Heath, A.R. 1977. Circulation across the ice edge in McMurdo Sound, Antarctica. In M.J. Dunbar (Ed.), Polar oceans, (Proceedings Polar Oceans Conference, McGill University, Montreal.) Holm-Hansesn, 0., S.A. El-Sayed, B.A. Franceschini, and R. Cuhel. 1977. Primary production and the factors controlling phytoplankton growth in the Antarctic seas. In G.A. Llano (Ed.), Adaptations within antarctic ecosystems. Houston, Tex.: Gulf.
Data from the three sites in northwestern McMurdo Sound (Granite Harbor sites) indicate that the sea ice there does not thin appreciably, if at all, prior to its breakout in mid-February. At the time of ice breakout, the average ice thickness in the region was greater than 2 meters. Low melting rates (0.1 centimeter per day) measured in the southwestern Sound (Mitchell and Bye 1985) have been related to northward advection of supercooled water from underneath the Ross Ice Shelf. The difference in melting regime between eastern and western McMurdo Sound has important implications for sediment 3
S.
co co le AQ
.::._.:..1—
.................................................. S.,- £ ... . .. .
: - - - - -
-
v.. —..v Tent Island x----x Barnes Glacier
CD
O0
Erebus Ice Tongue
£--6 Granite Harbor inner basin •......... I
Granite Harbor sill
•--I Granite Harbor outer basin
6 16 26 5
October
15 25
November
5 15 25 4
December
14 24 3
January February
Figure 2. Changes in ice thickness for six sites in McMurdo Sound. The ice broke out at Tent Island on 27 January 1987 and at Barnes Glacier on 17 January 1987. The Granite Harbor area became ice free sometime in mid-February.
1987 REVIEW
95
McGrath-Grossi, S., S.T. Kottmeier, R.L. Moe, G.T. Taylor, and C.W. Oceanology of the Antarctic Continental Shelf, (American Geophysical Sullivan. In press. Sea-ice microbial communities. VI. Growth and Union: Antarctic Research Series, 43, 167-176). primary production in bottom ice under graded snow cover. Marine Ecology. Tressler, W.L., and A.M. Ommundsen. 1962. Seasonal oceanographic Mitchell, W.M., and J.A.T. Bye. 1985. Observations in the boundary studies in McMurdo Sound, Antarctica. (Technical Report TR-125: layer under the sea ice in McMurdo Sound. In S.S. Jacobs (Ed.), 1-141.) Washington, D.C.: U.S. Navy 1-lydrographic Office.
96
ANTARCTIC JOURNAL
Location/parameter
Volume (in cubic kilometers)a
5,100 Filchner and Larsen icebergs, 1986 4,900 Lake Michigan 4,400 Annual runoff, Soviet Union 3,600 Annual world water use by man, 1980 World reservoir storage capacity by the year 2000 3,100 Annual accumulation on the antarctic ice sheet 2,200 1,000 Trolltunga iceberg, 1973 Sea level rise per year during the past century 500 a Values rounded to nearest 100 cubic kilometers water equivalent, assuming an ice density of 0.84 and a sea level rise of 1.25 millimeters per year. Source material in Jacobs and Barnett (1987).
Filchner icebergs did not move far from their source area during the June 1986 to June 1987 period. They may have been trapped for a time by the westward coastal current in the cul-de-sac formed by the new Filchner ice front, and by their greater draughts relative to a shoal area near Berkner Island (Jacobs and Barnett 1987). Icebergs generated from already floating ice shelves have no immediate impact upon sea level (Jacobs 1986; Robin 1986), but the larger ones contain impressive volumes of water (table), can scar the sea floor (Jacobs in press), and may alter the dynamics of the ice shelves they leave behind. This research was supported by National Science Foundation grants DPP 85-12540 and DPP 85-02386. ITT Antarctic Services, the marine science technicians of Polar Sea, the individuals cited
Ice thickness in McMurdo Sound AMY LEVENTER, ROBERT
B. DUNBAR, and MARIAN R. ALLEN
Earth Systems Institute Department of Geology and Geophysics Rice University Houston, Texas 77251-1892 RICHARD Y. WAYPER
Antarctic Research Centre Victoria University Wellington, New Zealand
In this paper we present data concerning changes in ice thickness throughout McMurdo Sound from October 1986 through February 1987. During a typical year in the Sound, annual sea ice begins to form in March or April. Thin sheets of ice develop and break out during this time, as a consequence of 94
above, S. Brower and B. Batchelder helped in the acquisition of equipment and data and in the preparation of this report. References Ferrigno, J., and W. Gould. 1987. Substantial changes in coastline of Antarctica revealed by satellite imagery. Polar Record, 23 (146), 577-583. Holdsworth, G. 1982. Dynamics of Erebus Glacier Tongue. Annals of Glaciology, 3, 131-137. Holdsworth, G. 1985. Some effects of ocean currents and wave motion on the dynamics of floating glacier tongues. In Oceanology of the Antarctic Continental Shelf, (Antarctic Research Series, Vol. 43). Washington, D.C.: American Geophysical Union. Jacobs, S. 1986. The polar ice sheets: A wild card in the deck? Oceanus, 29(4), 50-54. Jacobs, S. 1987. Antarctic ice: How much smaller? EOS, 68(51), 1793. Jacobs, S. In press. Marine controls on modern sedimentation on the Antarctic Continental Shelf. Marine Geology. Jacobs, S., and D. Barnett. 1987. On the draughts of some large Antarctic icebergs. Iceberg Research. 14, 3-13. Jacobs, S., H. Huppert, G. Holdsworth and D. Drewry. 1981. Thermohaline steps induced by melting of' the Erebus Glacier Tongue. Journal of Geophysical Research, 86, C7, 6547-6555. Jacobs, S., D. MacAyeal, and J. Ardai. 1986. The recent advance of the Ross Ice Shelf, Antarctica. Journal of Glaciology, 32(112), 464-474. Robin, G. 1986. Changing the sea level. In The greenhouse effect, climatic change, and ecosystems. Chichester, W. Sussex: Scientific Committee on Problems of the Environment. Visser, A., and S. Jacobs. 1987. Annual mean estimates of heat transport into the Ross Sea sub-ice cavity. (Paper presented at the High Latitude Oceans Symposia, IAPSO-OPS-2C, International Union of Geodesy and Geophysics, Vancouver, 14 August 1987.)
storms and winds. Once the ice has an opportunity to thicken, it strengthens and becomes more impervious to wind stress. The ice generally continues to thicken until November or December, when it reaches its average thickness of 2 meters. During the summer, the ice edge progressively moves southward until February, when much of the remainder of the sea ice goes out (figure 1); however, many years the ice in the western Sound does not break out at all. Figure 2 illustrates changes in ice thickness at six sites from McMurdo Sound, from October 1986 to February 1987. In the eastern Sound (Tent Island, Barnes Glacier, and Erebus Ice Tongue sites), ice continues to thicken until early to midNovember, and maintains its 2-meter average thickness through mid-December. Ice thins precipitously from late December until ice breakout in January or February. An average melting rate of approximately 2.5 centimeters per day from our data is quite similar to the 2.9 centimeters per day rate measured by Mitchell and Bye (1985) near McMurdo Station. The relatively high melting rates measured in the eastern sound maybe correlated to advection of relatively warm water from the north (Heath 1977; Mitchell and Bye 1985; Tressler and Ommundsen 1962). Although currents were not measured, a steep wire angle (approximately 45°) to the south was observed on our sediment trap mooring at the Erebus Ice Tongue site from 15 January to 27 January. Prior to this time no wire angle was observed. ANTARCTIC JOURNAL
.456
supply. In the eastern Sound, biogenic material previously trapped within the lower portion of the sea ice is continuously being shed into the water column while the sea ice is rapidly thinning. Since primary production within the annual sea ice in McMurdo Sound may exceed 40 grams of carbon per square meter per year (McGrath-Grossi et al. in press), and estimates of annual primary productivity in antarctic waters range between 16 and 100 grams of carbon per square meter per year (HolmHansen et al. 1977). Flux of particulates from the melting ice may provide a significant amount of carbon to sediments in the eastern Sound. In the western Sound, where the amount of ice melting prior to breakout may be inconsequential, particulate material within the ice can be carried out of the Sound more readily. The decreased downward flux of organic material may be a factor influencing the low standing stock of benthic organisms in the western Sound (Dayton and Oliver 1977; Dayton et al. 1986). This work was supported by National Science Foundation grant DPP 85-16911.
0
MCMURDO SOUND
/ ROSS ISLAND ...
t"
1 Tent Island 2 Barnes Glacier 3 Erebus Ice Tongue 4 Granite Harbor inner basin 5 Granite Harbor sill 6 Granite Harbor outer basin
loz
I014
ROSS ICE SHELF
100
78
100
Figure 1. Position of ice edge in McMurdo Sound in September 1986 and on 13 January 1987. Numbers indicate locations for which ice thickness data is presented.
References
Dayton, P.K., D. Watson, A. Palmisano, J.P. Barry, J.S. Oliver, and D. Rivera. 1986. Distribution patterns of benthic microalgal standing stock at McMurdo Sound, Antarctica. Polar Biology, 6, 207-213. Dayton, P.K. and J.S. Oliver. 1977. Antarctic soft bottom benthos in oligotrophic and eutrophic environments. Science, 197, 55-58. Heath, A.R. 1977. Circulation across the ice edge in McMurdo Sound, Antarctica. In M.J. Dunbar (Ed.), Polar oceans, (Proceedings Polar Oceans Conference, McGill University, Montreal.) Holm-Hansesn, 0., S.A. El-Sayed, B.A. Franceschini, and R. Cuhel. 1977. Primary production and the factors controlling phytoplankton growth in the Antarctic seas. In G.A. Llano (Ed.), Adaptations within antarctic ecosystems. Houston, Tex.: Gulf.
Data from the three sites in northwestern McMurdo Sound (Granite Harbor sites) indicate that the sea ice there does not thin appreciably, if at all, prior to its breakout in mid-February. At the time of ice breakout, the average ice thickness in the region was greater than 2 meters. Low melting rates (0.1 centimeter per day) measured in the southwestern Sound (Mitchell and Bye 1985) have been related to northward advection of supercooled water from underneath the Ross Ice Shelf. The difference in melting regime between eastern and western McMurdo Sound has important implications for sediment 3
S.
co co le AQ
.::._.:..1—
.................................................. S.,- £ ... . .. .
: - - - - -
-
v.. —..v Tent Island x----x Barnes Glacier
CD
O0
Erebus Ice Tongue
£--6 Granite Harbor inner basin •......... I
Granite Harbor sill
•--I Granite Harbor outer basin
6 16 26 5
October
15 25
November
5 15 25 4
December
14 24 3
January February
Figure 2. Changes in ice thickness for six sites in McMurdo Sound. The ice broke out at Tent Island on 27 January 1987 and at Barnes Glacier on 17 January 1987. The Granite Harbor area became ice free sometime in mid-February.
1987 REVIEW
95
McGrath-Grossi, S., S.T. Kottmeier, R.L. Moe, G.T. Taylor, and C.W. Oceanology of the Antarctic Continental Shelf, (American Geophysical Sullivan. In press. Sea-ice microbial communities. VI. Growth and Union: Antarctic Research Series, 43, 167-176). primary production in bottom ice under graded snow cover. Marine Ecology. Tressler, W.L., and A.M. Ommundsen. 1962. Seasonal oceanographic Mitchell, W.M., and J.A.T. Bye. 1985. Observations in the boundary studies in McMurdo Sound, Antarctica. (Technical Report TR-125: layer under the sea ice in McMurdo Sound. In S.S. Jacobs (Ed.), 1-141.) Washington, D.C.: U.S. Navy 1-lydrographic Office.
96
ANTARCTIC JOURNAL
