Ocean tides beneath the Ross Ice Shelf
Extrapolation of Dorrer's ice velocities (Dorrer et at., 1969) to the base 'camp area gives an approximate ice velocity of between 100 ma' and 200 ma- 1 . With these values in the first equation, we have
Ocean tides beneath the Ross Ice Shelf
B —10 (± 15) cm of ice per year.
HUGO A. C. NEUBURG, and
Until further data is available, we thus are unable to predict, even for a steady state ice shelf, whether there is melting or freezing at the bottom surface. However, bottom freezing appears unlikely unless the ice shelf is not in steady state.
Department of Geological Sciences Virginia Polytechnic Institute and State Univrsity Blacksburg, Virginia 24061
We hope that the work described here will be continued over the next 2 or 3 austral summers. This should result in a complete coverage of the Ross Ice Shelf by a network of measurements of strain rates, of ice velocity, of snow accumulation rates, of ice thickness, and of oxygen isotope ratios. From these it will be possible to calculate steady state 'bottom melting and freezing rates for the entire ice shelf; comparison with independent measurements at the RISP drill site will help to indicate the actual state of the ice shelf. This research was supported by National Science Foundation grant GV-4073X.
EDWIN S. ROBINSON, RICHARD WILLIAMS
A study of the ocean 'tide beneath the R s Ice Shelf began during the 1973-1974 austral suinmer. This research is in conjunction with the interdisciplinary Ross Ice Shelf Project (RIsP). Prelinimary results indicate that the range of the sprin tide exceeds 1 meter in the southern extremities f the Ross Sea em'bayment. The range is similar to alues obtained earlier at Little America V (Thiel et al., 1960) and in the McMurdo Sound area (Fleath, 1971). The tidal variation of gravity was measured continuously from December 19, 1973, to Februry 2, 1974, at the RISP base camp (near 82°30'S. 166 W.). A recording gravimeter (Geodynamics model o-1) was used to obtain the data. The instrumen was located in a 5- by 5-meter Jamesway and op rated from 110-volt, 60 cycles per second line pow r. A base platform for the instrument was mount, on four 10- by 10-centimeter timbers each extending 3 meters into firn. In addition to base station observa-
References Bentley, C. R., J . W. Clough, and J . D. Robertson. 1974. RISP geophysical work. Antarctic Journal of the U.S., IX(4): 157. Clough, John W. 1974. RISP radio-echo soundings. Antarctic Journal of the U.S., IX (4): 159. Crary, A. P., E. S. Robinson, H. F. Bennett, and W. W. Boyd. 1962. Glaciological studies of the Ross Ice Shelf, Antarctica: 1957-1960. New York, American Geographical Society. ICY Glaciological Report Series, 6.
Dorrer, E., W. Hofmann, and W. Seufert. 1969. Geodetic results of the Ross Ice Shelf survey expeditions, 19621963 and 1965-1966. Journal of Glaciology, 8(52): 6790. Gaylord, D. R., and J . D. Robertson. In press. Sediments exposed on the surface of the Ross Ice Shelf, Antarctica. Journal of Glaciology. Hughes, T. 1972. Is the antarctic ice sheet disintegrating? Columbus, The Ohio State University, Institute of Polar Studies. Scientific justification, ISCAP Bulletin, 1. Weertman, H. 1974. Stabiilty of the junction of an ice sheet and an ice shelf. Journal of Glaciology, 13(67): 3-11. Zumberge, J . H., M. Giovinetto, R. Kehle, and J . Reid. 1960. Deformation of the Ross Ice Shelf near the Bay of Whales, Antarctica. New York, American Geographical Society. IGY Glaciological Report Series, 3. 162
Figure 1. Tidal observation sites on the Ross Ic 1973-1974 base camp; Bi: survey site occupied c and 24, 1974; LAS: Little America V; M I : survey from November 2 to 5, 1959.
ANTARCTIC JOUR?AL
tions, hree or more gravity measurements were made with a gravimeter (LaCoste and Romberg) at approxi ate hourly intervals for periods of between 3 and 6 hours at 46 sites on the southern Ross Ice Shelf. These sites were occupied as part of a geophysic 1 survey. The field party remained 45 hours at one o these sites; 33 gravity readings were obtained. The 1 cations of site B 1 (84 0 S. 180 0 W.) and of the base amp are shown in fig. 1. Tid 1 variation of gravity over the Ross Ice Shelf an be attributed primarily to the ocean tide, with mall secondary effects related directly to lunar and solar masses, and to tidal deformation of the solid earth. The ocean tide introduces a periodic change in the elevation of a gravimeter located on the floating shelf, and a periodic change in the subjacent water mass. These effects were combined by Thiel et al. (1960) to obtain an equation relating tidal variation of water level Ah, expressed in meters, and gravity changes Ag, expressed in milligals: (1) Ah = 3.765 Ag The combined effects of lunar and solar masses, and the corresponding solid earth deformation, cause spring tidal gravity fluctuations of less than 0.02 milligal at latitudes greater than 80°. As the observed tidal gravity fluctuation on the Ross Ice Shelf exceeds 0.50 milligal during the spring tide the ocean tide clearly is the primary cause of gravimetric fluctuations, accounting for more than 95 percent of the measured gravity tide. The 46-day tidal record obtained at the base camp was read at hourly intervals for subsequent harmonic analysis, the results of which are not yet available. A segment of this record, converted to equivalent water level fluctuation using equation (1), is presented in
Cl) Er Lu I-
z U
-J
41
0
(I) -J 4 0
cc iu > 2
Jan. 23, 1974 Jan. 24, 1974 Figure 2. Tidal water level fluctuations determined at two sites from tidal variations of gravity. The points indicate individual observations at site Bi.
fig. 2. It is compared with a simultaneous record of water level fluctuation at site B 1 prepared from 33 individual gravimeter readings. These measurements indicate a spring tide during a new moon on January 23. Both records reveal the dominance of diurnal constituents, a fact recognized from earlier studies of the Ross Sea tide. The ranges of the spring tide at these two sites on the southern part of the shelf are approximately the same as the range at Little America V that was reported by Thiel et al. (1960). It is smaller than the range of 2.2 meters obtained by Thiel (1960) at 80 0 25'S. 169°35'E. from gravimeter observations during a 3-day period near the time of spring tide in November 1959. This record and a segment of the record obtained at Little America V are presented in fig. 3 for comparison with data from the southern part of the shelf. The different recording sites are seen in fig. 1.
LITTLE AMERICA
9
JUNE /0 2
04
Figure 3. Tidal water level fluctuations determined at two sites on the northern part of the Ross Ice Shelf from tidal gravity measurements. Little America V data is from Thiel et al. (1960). The data from 80 0 25'S. 169 0 35'E. is from Thiel (1960).
4100 U) I2 D 3900
LOCAL TIME
9 JUNE II 9 JUNE 12 9 JUNE 13 9 JUNE 14 9 Q FULL MOON
80025'S - 169°35'E
ILl
I— U
—J 4
July—August 1974
3700
NOV3
NOV.4
NOV5
MCMURDO LOCAL TIME (1959) 163
It is clear from these preliminary results that the tide persists under the southern portion of the Ross Ice Shelf with a range comparable to that thsezved for the northern portion. Following harmonic analysis of the 46-day record it will be possible to compare phases of the harmonic constituents with corresponding data from Little America V and from McMurdo Sound. In subsequent field seasons, tidal gravity will be measured for periods of a month or more at several sites to better define the spatial variation of the ocean tide in this region. This research was sponsored by National Science Foundation grant ov-40434. We appreciate the help of Mr. Thomas M. Kolich in thtaining gravity data used in this report. Other Ross Ice Shelf Project participants aided in making the field program a success.
Buckling of the Meserve Glacier sirface M. J . MCSAVENEY
Institute of Polar Studies
The Ohio State University Columbus, Ohio 43210
In January 1974, Dr. Stephen J . Derksen land I concluded field study of formation of surfac wave ogives on Meserve Glacier, Wright Valley, y remeasuring an ablation and strain network that first was measured in January 1972. Analysis of surface strain is continuing. Ana ysis of tilt for a line of nine shallow, cased holes (1. to 5 meters in depth) across a wave within the n twork shows that buckling is taking place (fig. 1). The buckling is consistent in magnitude and direction wi h that predicted from the measured longitudinal comp ession of the undulations on the glacier surface. That is, the measured buckling is a consequence of compression of an existing irregular surface, and not the other way around. The smaller wavelength of buckling (about 33 meters) apparently is the result of interference between two interdigitating sets of parallel wave ogive trains of about 66 meters in wavelength (fig. 2). The 66-meter wavelength is not apparent in the buckling data, but a larger wavelength is present that is greater than the length of the line of cased holes. A 400-meter long profile of the glacier surface through this region
References Heath, R. A. 1971. Tidal constants for McMurdo Sound,
Antarctica. New Zealand Journal of Marine and Fresh Water Resources, 3(2): 376-380.
Thiel, E. C., A. P. Crary, R. A. Haubrick, and J. C. Behrendt. 1960. Gravimeter determination of the ocean tide, Weddell and Ross seas. Journal of Geophysical Research, 65(2): 629-636. Thiel, E. C. 1960. Tides on the Ross Ice Shelf. Journal of Geophysical Research, 65(8): 2561-2562.
5'2
62
olo'^O^ 197
0 0 1 1.9
2-3 O'3
O'7
Figure 1. Rotational velocity of shallow holes on Meserve Glacier. Horizontal and vertical scales are divided into tens of meters, rotational velocity is X10 4 rad. a in the direction indicated. The lower line of rotational velocities are residuals to the larger fiexure indicated by the upper line.
164
ANTARCTIC JOURNAL
Ocean tides beneath the Ross Ice Shelf
B —10 (± 15) cm of ice per year.
HUGO A. C. NEUBURG, and
Until further data is available, we thus are unable to predict, even for a steady state ice shelf, whether there is melting or freezing at the bottom surface. However, bottom freezing appears unlikely unless the ice shelf is not in steady state.
Department of Geological Sciences Virginia Polytechnic Institute and State Univrsity Blacksburg, Virginia 24061
We hope that the work described here will be continued over the next 2 or 3 austral summers. This should result in a complete coverage of the Ross Ice Shelf by a network of measurements of strain rates, of ice velocity, of snow accumulation rates, of ice thickness, and of oxygen isotope ratios. From these it will be possible to calculate steady state 'bottom melting and freezing rates for the entire ice shelf; comparison with independent measurements at the RISP drill site will help to indicate the actual state of the ice shelf. This research was supported by National Science Foundation grant GV-4073X.
EDWIN S. ROBINSON, RICHARD WILLIAMS
A study of the ocean 'tide beneath the R s Ice Shelf began during the 1973-1974 austral suinmer. This research is in conjunction with the interdisciplinary Ross Ice Shelf Project (RIsP). Prelinimary results indicate that the range of the sprin tide exceeds 1 meter in the southern extremities f the Ross Sea em'bayment. The range is similar to alues obtained earlier at Little America V (Thiel et al., 1960) and in the McMurdo Sound area (Fleath, 1971). The tidal variation of gravity was measured continuously from December 19, 1973, to Februry 2, 1974, at the RISP base camp (near 82°30'S. 166 W.). A recording gravimeter (Geodynamics model o-1) was used to obtain the data. The instrumen was located in a 5- by 5-meter Jamesway and op rated from 110-volt, 60 cycles per second line pow r. A base platform for the instrument was mount, on four 10- by 10-centimeter timbers each extending 3 meters into firn. In addition to base station observa-
References Bentley, C. R., J . W. Clough, and J . D. Robertson. 1974. RISP geophysical work. Antarctic Journal of the U.S., IX(4): 157. Clough, John W. 1974. RISP radio-echo soundings. Antarctic Journal of the U.S., IX (4): 159. Crary, A. P., E. S. Robinson, H. F. Bennett, and W. W. Boyd. 1962. Glaciological studies of the Ross Ice Shelf, Antarctica: 1957-1960. New York, American Geographical Society. ICY Glaciological Report Series, 6.
Dorrer, E., W. Hofmann, and W. Seufert. 1969. Geodetic results of the Ross Ice Shelf survey expeditions, 19621963 and 1965-1966. Journal of Glaciology, 8(52): 6790. Gaylord, D. R., and J . D. Robertson. In press. Sediments exposed on the surface of the Ross Ice Shelf, Antarctica. Journal of Glaciology. Hughes, T. 1972. Is the antarctic ice sheet disintegrating? Columbus, The Ohio State University, Institute of Polar Studies. Scientific justification, ISCAP Bulletin, 1. Weertman, H. 1974. Stabiilty of the junction of an ice sheet and an ice shelf. Journal of Glaciology, 13(67): 3-11. Zumberge, J . H., M. Giovinetto, R. Kehle, and J . Reid. 1960. Deformation of the Ross Ice Shelf near the Bay of Whales, Antarctica. New York, American Geographical Society. IGY Glaciological Report Series, 3. 162
Figure 1. Tidal observation sites on the Ross Ic 1973-1974 base camp; Bi: survey site occupied c and 24, 1974; LAS: Little America V; M I : survey from November 2 to 5, 1959.
ANTARCTIC JOUR?AL
tions, hree or more gravity measurements were made with a gravimeter (LaCoste and Romberg) at approxi ate hourly intervals for periods of between 3 and 6 hours at 46 sites on the southern Ross Ice Shelf. These sites were occupied as part of a geophysic 1 survey. The field party remained 45 hours at one o these sites; 33 gravity readings were obtained. The 1 cations of site B 1 (84 0 S. 180 0 W.) and of the base amp are shown in fig. 1. Tid 1 variation of gravity over the Ross Ice Shelf an be attributed primarily to the ocean tide, with mall secondary effects related directly to lunar and solar masses, and to tidal deformation of the solid earth. The ocean tide introduces a periodic change in the elevation of a gravimeter located on the floating shelf, and a periodic change in the subjacent water mass. These effects were combined by Thiel et al. (1960) to obtain an equation relating tidal variation of water level Ah, expressed in meters, and gravity changes Ag, expressed in milligals: (1) Ah = 3.765 Ag The combined effects of lunar and solar masses, and the corresponding solid earth deformation, cause spring tidal gravity fluctuations of less than 0.02 milligal at latitudes greater than 80°. As the observed tidal gravity fluctuation on the Ross Ice Shelf exceeds 0.50 milligal during the spring tide the ocean tide clearly is the primary cause of gravimetric fluctuations, accounting for more than 95 percent of the measured gravity tide. The 46-day tidal record obtained at the base camp was read at hourly intervals for subsequent harmonic analysis, the results of which are not yet available. A segment of this record, converted to equivalent water level fluctuation using equation (1), is presented in
Cl) Er Lu I-
z U
-J
41
0
(I) -J 4 0
cc iu > 2
Jan. 23, 1974 Jan. 24, 1974 Figure 2. Tidal water level fluctuations determined at two sites from tidal variations of gravity. The points indicate individual observations at site Bi.
fig. 2. It is compared with a simultaneous record of water level fluctuation at site B 1 prepared from 33 individual gravimeter readings. These measurements indicate a spring tide during a new moon on January 23. Both records reveal the dominance of diurnal constituents, a fact recognized from earlier studies of the Ross Sea tide. The ranges of the spring tide at these two sites on the southern part of the shelf are approximately the same as the range at Little America V that was reported by Thiel et al. (1960). It is smaller than the range of 2.2 meters obtained by Thiel (1960) at 80 0 25'S. 169°35'E. from gravimeter observations during a 3-day period near the time of spring tide in November 1959. This record and a segment of the record obtained at Little America V are presented in fig. 3 for comparison with data from the southern part of the shelf. The different recording sites are seen in fig. 1.
LITTLE AMERICA
9
JUNE /0 2
04
Figure 3. Tidal water level fluctuations determined at two sites on the northern part of the Ross Ice Shelf from tidal gravity measurements. Little America V data is from Thiel et al. (1960). The data from 80 0 25'S. 169 0 35'E. is from Thiel (1960).
4100 U) I2 D 3900
LOCAL TIME
9 JUNE II 9 JUNE 12 9 JUNE 13 9 JUNE 14 9 Q FULL MOON
80025'S - 169°35'E
ILl
I— U
—J 4
July—August 1974
3700
NOV3
NOV.4
NOV5
MCMURDO LOCAL TIME (1959) 163
It is clear from these preliminary results that the tide persists under the southern portion of the Ross Ice Shelf with a range comparable to that thsezved for the northern portion. Following harmonic analysis of the 46-day record it will be possible to compare phases of the harmonic constituents with corresponding data from Little America V and from McMurdo Sound. In subsequent field seasons, tidal gravity will be measured for periods of a month or more at several sites to better define the spatial variation of the ocean tide in this region. This research was sponsored by National Science Foundation grant ov-40434. We appreciate the help of Mr. Thomas M. Kolich in thtaining gravity data used in this report. Other Ross Ice Shelf Project participants aided in making the field program a success.
Buckling of the Meserve Glacier sirface M. J . MCSAVENEY
Institute of Polar Studies
The Ohio State University Columbus, Ohio 43210
In January 1974, Dr. Stephen J . Derksen land I concluded field study of formation of surfac wave ogives on Meserve Glacier, Wright Valley, y remeasuring an ablation and strain network that first was measured in January 1972. Analysis of surface strain is continuing. Ana ysis of tilt for a line of nine shallow, cased holes (1. to 5 meters in depth) across a wave within the n twork shows that buckling is taking place (fig. 1). The buckling is consistent in magnitude and direction wi h that predicted from the measured longitudinal comp ession of the undulations on the glacier surface. That is, the measured buckling is a consequence of compression of an existing irregular surface, and not the other way around. The smaller wavelength of buckling (about 33 meters) apparently is the result of interference between two interdigitating sets of parallel wave ogive trains of about 66 meters in wavelength (fig. 2). The 66-meter wavelength is not apparent in the buckling data, but a larger wavelength is present that is greater than the length of the line of cased holes. A 400-meter long profile of the glacier surface through this region
References Heath, R. A. 1971. Tidal constants for McMurdo Sound,
Antarctica. New Zealand Journal of Marine and Fresh Water Resources, 3(2): 376-380.
Thiel, E. C., A. P. Crary, R. A. Haubrick, and J. C. Behrendt. 1960. Gravimeter determination of the ocean tide, Weddell and Ross seas. Journal of Geophysical Research, 65(2): 629-636. Thiel, E. C. 1960. Tides on the Ross Ice Shelf. Journal of Geophysical Research, 65(8): 2561-2562.
5'2
62
olo'^O^ 197
0 0 1 1.9
2-3 O'3
O'7
Figure 1. Rotational velocity of shallow holes on Meserve Glacier. Horizontal and vertical scales are divided into tens of meters, rotational velocity is X10 4 rad. a in the direction indicated. The lower line of rotational velocities are residuals to the larger fiexure indicated by the upper line.
164
ANTARCTIC JOURNAL
