Glacier geophysics at Taylor Dome: Year 4
Glacier geophysics at Taylor Dome: Year 4 E.D. WADDINGTON and D.L. MORSE,
Geophysics Program, University of Washington, Seattle, Washington 98195
G.D. GLOW, U.S. Geological Survey, Menlo Park, California 94025
aylor Dome, centered west of the Transantarctic Mounal iceflow line, however, provide a better opportunity to T tains at 77°50'S 159°00'E, is a local ice dome that supplies investigate the accumulation record. Using the ground-based ice to glaciers in the McMurdo Dry Valleys. Although the site radio echo sounding system reported earlier (Weertman 1993; has also been known as "McMurdo Dome," the name "Taylor Waddington et al. 1991; Morse and Waddington 1992, 1993), Dome" has recently been given official status by the U.S. we gathered echo sounding data along a 30-km profile Board on Geographic Names. approximating the iceflow line through the drill site. These Following 3 years of planning and reconnaissance to data will be analyzed for accumulation patterns using the select a site (Waddington et al. 1991, 1993; Grootes, Steig, and methods introduced byWeertman (1993). Massey 1991; Morse and Waddington 1992, 1993; Grootes and Earlier work showed that the depositional environment Steig 1992), an ice core reaching bedrock at 554 meters (m) and degree of preservation of the environmental record vary depth was recovered in January 1994 (Grootes, Steig, and across Taylor Dome (Grootes et al. 1991; Grootes and Steig Stuiver, Antarctic Journal, in this issue). Geophysical work 1992) and that this may be associated with microclimate continued at the site in 1993-1994, gathering information zones related to winds (Waddington and Morse in press). needed to interpret the ice-core paleoclimate record accuData from automatic weather stations (AWS5) provide insight rately and to derive additional climatic data to complement into the processes that deposit snow and the associated envithe ice-core records. The field party of Gary Glow, Kurt Cuffey, ronmental record found in the ice core. We collected weather David Morse, Brian Peterka, Andrew Stirling, and Edwin data from our three existing AWS units (figure 1) and installed Waddington spent late November in Taylor Valley and a fourth AWS 20 km farther west, at the site marked by a December and January at Taylor Dome. square in figure 1 (77°45'S 157°00'E). During the first three field seasons, we had established a The strong wind on the antarctic plateau probably causes surface strain network of approximately 200 markers (figure some deposition, removal, or redistribution of climatically 1). During the 1993-1994 season, we resurveyed the positions important chemical species in the porous snow and firn after of all the markers east of 158°20'E. This includes all the markthe snow is deposited. Surface bumps may create airflow ers within 10 kilometers (km) of the drill site. Preliminary through the firn. We measured snow permeability, detailed analysis indicates that the ice at the drill site moves a few temperature histories, and pressure gradients in the upper 1 decimeters per year. Using these measured ice velocities, we m of snow, to help us understand the impact of the wind on will be able to test the accuracy of iceflow models needed to the ice-core records. calculate the age of the ice recovered in the core and to calculate the expected pat159 00 E '59 E 16050E+ 161 DO E 162 00 tern of isochrons (buried for% mer ice-sheet surfaces) , throughout the dome. The shape of these layers, as -. detected by radio echo sounding, is an indication of spatial patterns of snow accumulation rate, which can affect the climate record preserved in the ice core. Gathering radio echo sounding data on a regular grid is the most efficient way to map the bedrock and internal layers for ice-core site
I ILL
+ 159 00 0
158 00 0
Ca + 160 00 0
161000
78 00 S l6200
se1 p i'tinn liirinci thp thrioa
previous field seasons we completed such a grid (Morse and Waddington, 1992, 1993). Data collected along the actu-
Figure 1. Map showing Taylor Dome and its relation to Taylor Glacier. Dots show ice-motion markers surveyed between 1990 and 1994. Star shows AWS installed in December 1990. The triangle marks the 554-m borehole, the AWS installed in December 1991, and a 130-rn dry hole logged for temperature. The circle at the entrance to Taylor Valley marks the AWS installed in December 1992 and a 100-rn dry hole logged for temperature. The square marks the AWS installed in January 1994.
ANTARCTIC JOURNAL - REVIEW 1994 82
One objective of the ice-core project is to relate 162 00 160 00 164 00 the ice-core paleoclimate record to the radiometrical77 15+ ly dated climate history of the McMurdo Dry Valleys % ,/ 0 / (Denton et al. 1989). Taylor Glacier provides the path,,, 7, - / • "//",////,,Loke ";z" /.// O• ///////////////'•'' /74(/7/,'/ way between the climate forcing at its source (Taylor Dome) and the terminus response, which can create a / Ron *?//// geomorphological record through moraines and ice7•;' 77: 30 blocked drainage channels. The ice dynamics of Tay- 11111111 A br Glacier introduce a lag; for example, the terminus 7/1/ advance culminates thousands of years after the sgor '//,,,,i /,Loke' //////'./ 7/7////// //' / Y,'..f/ ///// •/7//////7 ' 7/7/7 /7/ snowfall rate increases at Taylor Dome. In addition, the factor relating snowfall changes to glacier-margin • :( advance must be derived from an iceflow model. We determined ice velocities in November 1993 by resurveying markers emplaced in December 1991 on two profiles (G and H) across Taylor Glacier and a third (F) across the Taylor-Ferrar Confluence (figure 1). We Figure 2. Map showing Taylor Dome and its relation to Taylor Glacier and the dry also measured the ice depths along each profile with valleys. Dashed lines show topographic divides. Hatched areas are largely icethe ground-based radio echo sounding system. From free. DVDP boreholes are marked by solid triangles. Highest priority hole, DVDP11, was successfully logged to 282 m depth. Rising lake levels in Lake Vanda and ice velocity and ice thickness, we now know the ice Lake Vida have encroached on boreholes DVDP-14 and 6 at those sites. Holes 10 flux through each profile; this information is needed (at New Harbor) and 12 (at Lake Hoare) were found to be plugged by ice. to calibrate the iceflow models. This research was supported by National Science FounAn additional objective of the Taylor Dome geophysics dation grants OPP 89-15924 and OPP 92-20261. We thank program is the reconstruction of surface paleotemperatures at Glen Rowe and the New Zealand Department of Survey and Taylor Dome and in the nearby McMurdo Dry Valleys from Land Information (NZDOSLI) for providing information precision subsurface temperature measurements made in about the dry valleys survey control network and prior Taylor boreholes. At Taylor Dome, comparison of the derived surface Glacier surveys and for making long-range survey instrutemperature history with the isotopic oxygen-18 (180) record ments available to us for the 1993 survey. The New Zealand will provide a means for detecting nonthermal influences on 6 18 0 at this site. Because borehole temperatures respond to Antarctic Program (NZAP) and NZDOSLI also provided field assistance in 1991, through the services of Garth Falloon, the year-round changes in surface temperature, the surface-temNZAP/DOSLI surveyor. We are indebted to Garth who joined perature history derived for the dry valleys will complement our field team for the 1991 Taylor Glacier survey and docuother paleoclimate proxies for the valleys (e.g., lake levels), mented the results of that initial survey. We also thank all proxies that primarily reflect paleotemperatures during the other members of our field teams who helped to acquire the summer season. During December 1993, temperature logs data we report here, and all those in the U.S. Antarctic Prowere acquired in a 130-rn corehole 1.5 km from the site of the gram who provided the logistical and scientific support that main Taylor Dome core (triangle in figure 1) and in a 100-rn made our research possible. corehole at the entrance to Taylor Valley (circle in figure 1). Five of the Dry Valley Drilling Project (DVDP) permafrost References boreholes were targeted for temperature logging during the 1993-1994 field season (figure 2). Of these, DVDP-10 (New Denton, G.H., J.G. Bockheim, S.C. Wilson, and M. Stuiver. 1989. Late Harbor) and DVDP-12 (Lake Hoare) were found to be blocked Wisconsin and early Holocene glacial history, inner Ross Embayat shallow depths, presumably by ice and, thus, could not be ment, Antarctica. Quaternary Research, 31(2), 151-182. logged. Borehole D\TDP-14 could not be located; it appears to Grootes, P.M., and E.J. Stieg. 1992. Taylor Dome ice-core study. have been completely inundated when Lake Vanda overAntarctic Journal of the U.S., 27(5), 57-58. Grootes, P.M., E.J. Steig, and C. Massey. 1991. "Taylor Ice-Dome" flowed into the North Fork Basin during January 1983. DVDPstudy: Reconnaissance 1990-1991. Antarctic Journal of the U.S., 6 (Lake Vida) has also been inundated by rising lake levels; the 26(5), 69-71. damaged wellhead was still visible within the lake ice in Grootes, P.M., E.J. Steig, and M. Stuiver. 1994. Taylor Ice Dome study November 1993. Our highest-priority borehole in the Dry Val1993-1994: An ice core to bedrock. AntarcticJournal of the U.S., 29(5). leys (DVDP-11, near the Commonwealth Glacier) was sucMorse, D.L., and E.D. Waddington. 1992. Glacier geophysical studies for an ice core site at Taylor Dome: Year two. Antarctic Journal of cessfully logged to 282 m depth during November. An AWS the U.S., 27(5), 59-61. was also installed within a few meters of the DVDP- 11 boreMorse, D.L., and E.D. Waddington. 1993. Glacier geophysical studies hole to investigate the coupling between air and ground temat Taylor Dome: Year three. Antarctic Journal of the U.S., 28(5), peratures at this site. 67-69. The ongoing geophysical studies and ice-core research Waddington, E.D., and D.L. Morse. In press. Spatial variations of local climate at Taylor Dome, Antarctica: Implications for paleoclimate promise to provide a useful paleoclimate history for this from ice cores. Annals of Glaciology, 20. region of southern Victoria Land.
ANTARCTIC JOURNAL - REVIEW 1994 83
in the climate system (NATO ASI Series I, Global Environmental Change, Vol. 12). Berlin: Springer-Verlag. Weertman, B. 1993. Interpretation of ice sheet stratigraphy: a radio echo sounding study of the Dyer Plateau, Antarctica. (Ph.D. dissertation, Geophysics Program, University of Washington, Seattle.)
Waddington, E.D., D.L. Morse, M.J. Balise, and J. Firestone. 1991. Glacier geophysical studies for an ice core site at "Taylor Dome." Antarctic Journal of the U.S., 26(5), 71-73. Waddington, E.D., D.L. Morse, P.M. Grootes, and E.J. Steig. 1993. The connection between ice dynamics and paleoclimate from ice cores: A study of Taylor Dome, Antarctica. In W.R. Peltier (Ed.), Ice
Preliminary report on the physical and stratigraphic properties of the Taylor Dome ice core J.J. FITZPATRICK,
U.S. Geological Survey, Denver, Colorado 80225
uring the 1993-1994 austral field season, a 554-meter (m) D core to bedrock was recovered from Taylor Dome drill site (77041.7'S 158°43.1'E). Site information and details of the core recovery are presented elsewhere (see Grootes and Steig, Antarctic Journal, in this issue) and in earlier reports (Grootes, Steig, and Massey 1991; Waddington et al. 1991, 1993, pp. 499-516; Grootes and Steig 1992; Morse and Waddington 1992, 1993). Physical properties, including density, grain- and bubble-sizes, temperature, and differential ultrasonic p-wave velocity, were measured in the field within a few hours of core recovery in a subsurface laboratory near the drill site. The preliminary results of these analyses are presented here. The visual appearance of the core was noted as drilling progressed both from whole core and from thick sections taken shortly after recovery. The stratigraphy follows a predictable diagenetic sequence from highly porous firn to ice with large, irregularly shaped bubbles and then to ice with spherical bubbles that decrease in size with increasing depth. No notable bubble-free lenses, layers, or glands were seen, although the presence of thin (less than 1 millimeter), clear-ice crusts is ubiquitous in the upper part of the core. These crusts were observed forming on the surfaces of wind-packed sastrugi as a result of solar-induced sintering. They are easily discernible in the core to a depth of 200 m. Beyond this depth, they become increasingly difficult to see, and by 250 m, they cease to be visible at all. Subdued density contrast due to varying depositional conditions and possible postdepositional diagenesis is also present in the upper part of the core, but like the clear-ice crusts, becomes indistinct below 250 meters. No clear annual signal is present in the stratigraphy. Visual clues to the annual stratigraphy will require the eventual comparison of the visual and isotopic records. A zone of anomalously elongated bubbles between a depth of 360 to 390 m was noted in the field. This zone is judged to be anomalous because ice both above and below this interval contains only spherical bubbles. The bubble population in this anomalous interval consists of a mixture of both spherical and aligned, elongated bubbles. The elongated bubbles vary in aspect ratio from 3:1 to 5:1 and, upon reconstruction of their absolute direction from the core azimuth, point in the direction of current surface motion at the site (or 180 0 away from it), that is, toward the Skelton Névé (Waddington personal communication). The
cross-sectional diameter of the elongated bubbles appears to be about the same size as the diameter of the average spherical bubble measured in the same section. Elongated bubbles in this depth interval display plunge values near 0 0. This is in contrast to the zone of elongated bubbles that is close to the bed in which bubbles display chaotic plunges as much as 350. The characteristics and significance of the anomalous zone of elongated bubbles are the subject of ongoing study. The ice began to exhibit brittle behavior by a depth of about 300 m. By a depth of 335 m, ice was consistently broken and fractured as it was unloaded from the core barrel. The onset of this behavior corresponds to a load pressure of about 27 atmospheres. The deepest ice at this site experiences a hydrostatic load of 47 atmospheres and is too warm (approximately -26°C) to allow the formation of air hydrate clathrates. As a result, all core recovered below a depth of 335 m displayed brittle behavior. The density/load curve for the core is shown in figure 1. Densities were measured in the field within a few hours of core recovery. Densities above 85 m were determined volumetrically; those below 85 m were determined by immersion in saturated iso-octane. The depth of the firn/ice transition was interpolated from a linear fit of the volumetric data above 85 m. The depth of this transition at approximately 72 m was confirmed 1000
50
900.0
40
800.0
30
700.0
20
600.0
10
3
500.0 0 100 200 300 400 500 600 Depth (m)
Figure 1. Density and load profile, Taylor Dome main core. (Kg-m-3 denotes kilograms per cubic meter. atm denotes atmospheres.)
ANTARCTIC JOURNAL 84
REVIEW 1994
Geophysics Program, University of Washington, Seattle, Washington 98195
G.D. GLOW, U.S. Geological Survey, Menlo Park, California 94025
aylor Dome, centered west of the Transantarctic Mounal iceflow line, however, provide a better opportunity to T tains at 77°50'S 159°00'E, is a local ice dome that supplies investigate the accumulation record. Using the ground-based ice to glaciers in the McMurdo Dry Valleys. Although the site radio echo sounding system reported earlier (Weertman 1993; has also been known as "McMurdo Dome," the name "Taylor Waddington et al. 1991; Morse and Waddington 1992, 1993), Dome" has recently been given official status by the U.S. we gathered echo sounding data along a 30-km profile Board on Geographic Names. approximating the iceflow line through the drill site. These Following 3 years of planning and reconnaissance to data will be analyzed for accumulation patterns using the select a site (Waddington et al. 1991, 1993; Grootes, Steig, and methods introduced byWeertman (1993). Massey 1991; Morse and Waddington 1992, 1993; Grootes and Earlier work showed that the depositional environment Steig 1992), an ice core reaching bedrock at 554 meters (m) and degree of preservation of the environmental record vary depth was recovered in January 1994 (Grootes, Steig, and across Taylor Dome (Grootes et al. 1991; Grootes and Steig Stuiver, Antarctic Journal, in this issue). Geophysical work 1992) and that this may be associated with microclimate continued at the site in 1993-1994, gathering information zones related to winds (Waddington and Morse in press). needed to interpret the ice-core paleoclimate record accuData from automatic weather stations (AWS5) provide insight rately and to derive additional climatic data to complement into the processes that deposit snow and the associated envithe ice-core records. The field party of Gary Glow, Kurt Cuffey, ronmental record found in the ice core. We collected weather David Morse, Brian Peterka, Andrew Stirling, and Edwin data from our three existing AWS units (figure 1) and installed Waddington spent late November in Taylor Valley and a fourth AWS 20 km farther west, at the site marked by a December and January at Taylor Dome. square in figure 1 (77°45'S 157°00'E). During the first three field seasons, we had established a The strong wind on the antarctic plateau probably causes surface strain network of approximately 200 markers (figure some deposition, removal, or redistribution of climatically 1). During the 1993-1994 season, we resurveyed the positions important chemical species in the porous snow and firn after of all the markers east of 158°20'E. This includes all the markthe snow is deposited. Surface bumps may create airflow ers within 10 kilometers (km) of the drill site. Preliminary through the firn. We measured snow permeability, detailed analysis indicates that the ice at the drill site moves a few temperature histories, and pressure gradients in the upper 1 decimeters per year. Using these measured ice velocities, we m of snow, to help us understand the impact of the wind on will be able to test the accuracy of iceflow models needed to the ice-core records. calculate the age of the ice recovered in the core and to calculate the expected pat159 00 E '59 E 16050E+ 161 DO E 162 00 tern of isochrons (buried for% mer ice-sheet surfaces) , throughout the dome. The shape of these layers, as -. detected by radio echo sounding, is an indication of spatial patterns of snow accumulation rate, which can affect the climate record preserved in the ice core. Gathering radio echo sounding data on a regular grid is the most efficient way to map the bedrock and internal layers for ice-core site
I ILL
+ 159 00 0
158 00 0
Ca + 160 00 0
161000
78 00 S l6200
se1 p i'tinn liirinci thp thrioa
previous field seasons we completed such a grid (Morse and Waddington, 1992, 1993). Data collected along the actu-
Figure 1. Map showing Taylor Dome and its relation to Taylor Glacier. Dots show ice-motion markers surveyed between 1990 and 1994. Star shows AWS installed in December 1990. The triangle marks the 554-m borehole, the AWS installed in December 1991, and a 130-rn dry hole logged for temperature. The circle at the entrance to Taylor Valley marks the AWS installed in December 1992 and a 100-rn dry hole logged for temperature. The square marks the AWS installed in January 1994.
ANTARCTIC JOURNAL - REVIEW 1994 82
One objective of the ice-core project is to relate 162 00 160 00 164 00 the ice-core paleoclimate record to the radiometrical77 15+ ly dated climate history of the McMurdo Dry Valleys % ,/ 0 / (Denton et al. 1989). Taylor Glacier provides the path,,, 7, - / • "//",////,,Loke ";z" /.// O• ///////////////'•'' /74(/7/,'/ way between the climate forcing at its source (Taylor Dome) and the terminus response, which can create a / Ron *?//// geomorphological record through moraines and ice7•;' 77: 30 blocked drainage channels. The ice dynamics of Tay- 11111111 A br Glacier introduce a lag; for example, the terminus 7/1/ advance culminates thousands of years after the sgor '//,,,,i /,Loke' //////'./ 7/7////// //' / Y,'..f/ ///// •/7//////7 ' 7/7/7 /7/ snowfall rate increases at Taylor Dome. In addition, the factor relating snowfall changes to glacier-margin • :( advance must be derived from an iceflow model. We determined ice velocities in November 1993 by resurveying markers emplaced in December 1991 on two profiles (G and H) across Taylor Glacier and a third (F) across the Taylor-Ferrar Confluence (figure 1). We Figure 2. Map showing Taylor Dome and its relation to Taylor Glacier and the dry also measured the ice depths along each profile with valleys. Dashed lines show topographic divides. Hatched areas are largely icethe ground-based radio echo sounding system. From free. DVDP boreholes are marked by solid triangles. Highest priority hole, DVDP11, was successfully logged to 282 m depth. Rising lake levels in Lake Vanda and ice velocity and ice thickness, we now know the ice Lake Vida have encroached on boreholes DVDP-14 and 6 at those sites. Holes 10 flux through each profile; this information is needed (at New Harbor) and 12 (at Lake Hoare) were found to be plugged by ice. to calibrate the iceflow models. This research was supported by National Science FounAn additional objective of the Taylor Dome geophysics dation grants OPP 89-15924 and OPP 92-20261. We thank program is the reconstruction of surface paleotemperatures at Glen Rowe and the New Zealand Department of Survey and Taylor Dome and in the nearby McMurdo Dry Valleys from Land Information (NZDOSLI) for providing information precision subsurface temperature measurements made in about the dry valleys survey control network and prior Taylor boreholes. At Taylor Dome, comparison of the derived surface Glacier surveys and for making long-range survey instrutemperature history with the isotopic oxygen-18 (180) record ments available to us for the 1993 survey. The New Zealand will provide a means for detecting nonthermal influences on 6 18 0 at this site. Because borehole temperatures respond to Antarctic Program (NZAP) and NZDOSLI also provided field assistance in 1991, through the services of Garth Falloon, the year-round changes in surface temperature, the surface-temNZAP/DOSLI surveyor. We are indebted to Garth who joined perature history derived for the dry valleys will complement our field team for the 1991 Taylor Glacier survey and docuother paleoclimate proxies for the valleys (e.g., lake levels), mented the results of that initial survey. We also thank all proxies that primarily reflect paleotemperatures during the other members of our field teams who helped to acquire the summer season. During December 1993, temperature logs data we report here, and all those in the U.S. Antarctic Prowere acquired in a 130-rn corehole 1.5 km from the site of the gram who provided the logistical and scientific support that main Taylor Dome core (triangle in figure 1) and in a 100-rn made our research possible. corehole at the entrance to Taylor Valley (circle in figure 1). Five of the Dry Valley Drilling Project (DVDP) permafrost References boreholes were targeted for temperature logging during the 1993-1994 field season (figure 2). Of these, DVDP-10 (New Denton, G.H., J.G. Bockheim, S.C. Wilson, and M. Stuiver. 1989. Late Harbor) and DVDP-12 (Lake Hoare) were found to be blocked Wisconsin and early Holocene glacial history, inner Ross Embayat shallow depths, presumably by ice and, thus, could not be ment, Antarctica. Quaternary Research, 31(2), 151-182. logged. Borehole D\TDP-14 could not be located; it appears to Grootes, P.M., and E.J. Stieg. 1992. Taylor Dome ice-core study. have been completely inundated when Lake Vanda overAntarctic Journal of the U.S., 27(5), 57-58. Grootes, P.M., E.J. Steig, and C. Massey. 1991. "Taylor Ice-Dome" flowed into the North Fork Basin during January 1983. DVDPstudy: Reconnaissance 1990-1991. Antarctic Journal of the U.S., 6 (Lake Vida) has also been inundated by rising lake levels; the 26(5), 69-71. damaged wellhead was still visible within the lake ice in Grootes, P.M., E.J. Steig, and M. Stuiver. 1994. Taylor Ice Dome study November 1993. Our highest-priority borehole in the Dry Val1993-1994: An ice core to bedrock. AntarcticJournal of the U.S., 29(5). leys (DVDP-11, near the Commonwealth Glacier) was sucMorse, D.L., and E.D. Waddington. 1992. Glacier geophysical studies for an ice core site at Taylor Dome: Year two. Antarctic Journal of cessfully logged to 282 m depth during November. An AWS the U.S., 27(5), 59-61. was also installed within a few meters of the DVDP- 11 boreMorse, D.L., and E.D. Waddington. 1993. Glacier geophysical studies hole to investigate the coupling between air and ground temat Taylor Dome: Year three. Antarctic Journal of the U.S., 28(5), peratures at this site. 67-69. The ongoing geophysical studies and ice-core research Waddington, E.D., and D.L. Morse. In press. Spatial variations of local climate at Taylor Dome, Antarctica: Implications for paleoclimate promise to provide a useful paleoclimate history for this from ice cores. Annals of Glaciology, 20. region of southern Victoria Land.
ANTARCTIC JOURNAL - REVIEW 1994 83
in the climate system (NATO ASI Series I, Global Environmental Change, Vol. 12). Berlin: Springer-Verlag. Weertman, B. 1993. Interpretation of ice sheet stratigraphy: a radio echo sounding study of the Dyer Plateau, Antarctica. (Ph.D. dissertation, Geophysics Program, University of Washington, Seattle.)
Waddington, E.D., D.L. Morse, M.J. Balise, and J. Firestone. 1991. Glacier geophysical studies for an ice core site at "Taylor Dome." Antarctic Journal of the U.S., 26(5), 71-73. Waddington, E.D., D.L. Morse, P.M. Grootes, and E.J. Steig. 1993. The connection between ice dynamics and paleoclimate from ice cores: A study of Taylor Dome, Antarctica. In W.R. Peltier (Ed.), Ice
Preliminary report on the physical and stratigraphic properties of the Taylor Dome ice core J.J. FITZPATRICK,
U.S. Geological Survey, Denver, Colorado 80225
uring the 1993-1994 austral field season, a 554-meter (m) D core to bedrock was recovered from Taylor Dome drill site (77041.7'S 158°43.1'E). Site information and details of the core recovery are presented elsewhere (see Grootes and Steig, Antarctic Journal, in this issue) and in earlier reports (Grootes, Steig, and Massey 1991; Waddington et al. 1991, 1993, pp. 499-516; Grootes and Steig 1992; Morse and Waddington 1992, 1993). Physical properties, including density, grain- and bubble-sizes, temperature, and differential ultrasonic p-wave velocity, were measured in the field within a few hours of core recovery in a subsurface laboratory near the drill site. The preliminary results of these analyses are presented here. The visual appearance of the core was noted as drilling progressed both from whole core and from thick sections taken shortly after recovery. The stratigraphy follows a predictable diagenetic sequence from highly porous firn to ice with large, irregularly shaped bubbles and then to ice with spherical bubbles that decrease in size with increasing depth. No notable bubble-free lenses, layers, or glands were seen, although the presence of thin (less than 1 millimeter), clear-ice crusts is ubiquitous in the upper part of the core. These crusts were observed forming on the surfaces of wind-packed sastrugi as a result of solar-induced sintering. They are easily discernible in the core to a depth of 200 m. Beyond this depth, they become increasingly difficult to see, and by 250 m, they cease to be visible at all. Subdued density contrast due to varying depositional conditions and possible postdepositional diagenesis is also present in the upper part of the core, but like the clear-ice crusts, becomes indistinct below 250 meters. No clear annual signal is present in the stratigraphy. Visual clues to the annual stratigraphy will require the eventual comparison of the visual and isotopic records. A zone of anomalously elongated bubbles between a depth of 360 to 390 m was noted in the field. This zone is judged to be anomalous because ice both above and below this interval contains only spherical bubbles. The bubble population in this anomalous interval consists of a mixture of both spherical and aligned, elongated bubbles. The elongated bubbles vary in aspect ratio from 3:1 to 5:1 and, upon reconstruction of their absolute direction from the core azimuth, point in the direction of current surface motion at the site (or 180 0 away from it), that is, toward the Skelton Névé (Waddington personal communication). The
cross-sectional diameter of the elongated bubbles appears to be about the same size as the diameter of the average spherical bubble measured in the same section. Elongated bubbles in this depth interval display plunge values near 0 0. This is in contrast to the zone of elongated bubbles that is close to the bed in which bubbles display chaotic plunges as much as 350. The characteristics and significance of the anomalous zone of elongated bubbles are the subject of ongoing study. The ice began to exhibit brittle behavior by a depth of about 300 m. By a depth of 335 m, ice was consistently broken and fractured as it was unloaded from the core barrel. The onset of this behavior corresponds to a load pressure of about 27 atmospheres. The deepest ice at this site experiences a hydrostatic load of 47 atmospheres and is too warm (approximately -26°C) to allow the formation of air hydrate clathrates. As a result, all core recovered below a depth of 335 m displayed brittle behavior. The density/load curve for the core is shown in figure 1. Densities were measured in the field within a few hours of core recovery. Densities above 85 m were determined volumetrically; those below 85 m were determined by immersion in saturated iso-octane. The depth of the firn/ice transition was interpolated from a linear fit of the volumetric data above 85 m. The depth of this transition at approximately 72 m was confirmed 1000
50
900.0
40
800.0
30
700.0
20
600.0
10
3
500.0 0 100 200 300 400 500 600 Depth (m)
Figure 1. Density and load profile, Taylor Dome main core. (Kg-m-3 denotes kilograms per cubic meter. atm denotes atmospheres.)
ANTARCTIC JOURNAL 84
REVIEW 1994
