Temperature measurements in the margin of ice stream B, 1992 ...
(figure 3). This unusual reversed temperature dependence with altitude clearly leads to the absence of colder ice below the surface. This absence and the basal heat generation create the peculiar situation that ice stream B is relatively warm compared to other ice masses flowing down from higher altitudes at lower temperatures, for example Jakobshavns Isbr, where ? = 0.0070 CIm (Echelmeyer et al. 1992). For comparison, in figure 3 we included the temperature profile (dashed line) calculated by Lingle and Brown (1987). This profile is calculated for a location about 20 kilometers downstream from our boreholes. It is not immediately clear what it is about the Lingle and Brown temperature modeling calculation that causes the considerable discrepancy with observations because the basal and surface temperature boundary conditions are correct. The relative warmth of west antarctic ice streams may be a factor in the very existence of the ice streams and could contribute to the possible instability of the west antarctic ice sheet.
(Ed.), Antarctic map folio series. New York: American Geographical Society. Carslaw, H.S., and J.C. Jaeger. 1959. Conduction of heat in solids. Oxford: Clarendon Press. Echelmeyer, K., W.D. Harrison, T.S. Clarke, and C. Benson. 1992. Surficial glaciology of Jakobshavns Isbr, West Greenland: Part II. Ablation, accumulation and temperature. Journal of Glaciology, 38(128),169-181. Engelhardt, H., M. Fahnestock, N. Humphrey, and B. Kamb. 1989. Borehole drilling to the bed of ice stream B, Antarctica. Antarctic Journal of the U.S., 24(5), 83-84. Engelhardt, H., N. Humphrey, B. Kamb, and M. Fahnestock. 1990. Physical conditions at the base of a fast moving antarctic ice stream. Science, 248, 57-59. Humphrey, N. 1991. Estimating ice temperatures from short records in thermally disturbed boreholes. Journal of Glaciology, 37(127), 414-419. Lachenbruch, A.H., and M.C. Brewer. 1959. Dissipation of temperature effect of drilling a well in Arctic Alaska. In Experimental and theoretical geophysics (U.S. Geological Survey bulletin 1083-C). Washington, D.C.: U.S. Government Printing Office. Lingle, C.S., and T.J. Brown. 1987. A subglacial aquifer bed model and water pressure dependent basal sliding relationship for a west antarctic ice stream. In C.J. van der Veen and J. Oerlemans (Eds.), Dynamics of the west antarctic ice sheet. Dordrecht: D. Reidel. Radok, U., D. Jenssen, and W. Budd. 1970. Steady-state temperature profiles in ice sheets. IAHS Publications, 86, 151-165. Robin, G. de Q. 1955. Ice movement and temperature distribution in glaciers and ice sheets. Journal of Glaciology, 2(18), 523-532. Robin, G. de Q. 1983. The climatic record in polar ice sheets. Cambridge: Cambridge University Press.
References Alley, R.B., and C.R. Bentley. 1988. Ice-core analysis on the Siple Coast of West Antarctica. Annals of Glaciology, 11, 1-7. Bentley, C.R., R.L. Cameron, C. Bull, K. Kojima, and A.J. Gow. 1964. Physical characteristics of the antarctic ice sheet. In V.C. Bushnell
Temperature measurements in the margin of ice stream B, 1992-1993 KEITH ECHELMEYER
and WILLIAM HARRISON, Geophysical Institute, University ofAlaska, Fairbanks, Alaska 99775-0800
ice stream. The work began near Upstream B Camp in the 1992-1993 austral summer. Severe crevassing in the margins posed a major challenge to drilling operations, which were performed with the California Institute of Technology's hot-water rig. We used a careful program of probing and subsurface exploration of buried crevasses to find a safe route for the drill rig, then operated as close to the south margin of the ice stream as possible. From this point about 1,700 meters (m) of hot-water hose was dragged into the chaotic crevasses of the margin, where the drilling was performed with a light hose-handling winch and a single heater, which was used to boost the temperature of the water arriving from the distant drill rig. Three holes were drilled, one each at the remote, intermediate, and pad sites. The first was in the chaotically crevassed portion of the margin, the third at the site of the main drill rig about 1,800 m closer to the center of the ice stream, and the second at a site roughly halfway between the other two. In the preliminary results (shown in the figure), the temperatures have been corrected to an accuracy of about 0.5 Kelvin or bet-
he low shear stress at the bottom of ice stream B suggestT ed both by soft subglacier sediment samples acquired by the California Institute of Technology near Upstream B Camp (Engelhardt et al. 1990) and by recent theoretical analyses of transverse profiles of velocity across the ice stream (Echelmeyer et al. in preparation; Van der Veen and Whillans in preparation) indicates that the margins of the ice stream probably play a significant role in the dynamics of flow, perhaps exerting more drag on the ice stream than does the bed itself. This situation would require a relatively large shear stress at the margins of the ice stream—large enough that the effects of shear heating should be detectable by temperature measurements at several hundred meters depth. The most important unknowns are the rate of convergence of ice into the ice stream (which controls the residence time of the ice in the active part of the margins), the stability of the positions of the margins, and the shear stress itself. We are examining these unknowns using a program of temperature measurements in the margins and a surveying program to improve our knowledge of the rate of convergence of the ice into the
ANTARCTIC JOURNAL - REVIEW 1993 66
There are two striking preliminary results. First, the nearsurface drops as the margin is approached (see figure). This very large effect, one apparently not observed before, is probably due to the ponding of cold air in crevasses. Second, at greater depths, the intermediate site is the warmest of the three, indicating that there is not a simple relationship between deep temperature and proximity to the margin. This is a surprising result for which at least two hypotheses may be entertained: a complex history of the ice (perhaps related to different paths through upstream crevasse patterns) or even erratic migration of the position of the boundary of the ice stream. In 1993-1994, we hope to extend this transverse profile of holes all the way through the margin. We are grateful for the support of the personnel from many different institutions who contributed to the field operations. Financial support was from National Science Foundation grant OPP 91-22783.
TEMPERATURE (deg C) -30-29 -28 -27 -26 -25 -24 -23 -22 -21 -20 0 1
1
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150 200 250 300 350 400
References
Temperature at three sites near the south margin of ice stream B. The remote site is near the inner edge of the chaotically crevassed portion of the margin; the pad site is about 1,800 m nearer the center of the ice stream, and the intermediate site is approximately halfway between the other two.
Engelhardt, H., N. Humphrey, B. Kamb, and M. Fahnestock. 1990. Physical conditions at the base of a fast moving antarctic ice stream. Science, 248, 57-59. Echelmeyer, K.A., W.D. Harrison, J.E. Mitchell, C. Larsen. In preparation. Velocity across ice stream B and the role of the margins in ice stream dynamics. Van der Veen, C.J., and I.M. Whillans. In preparation. Controls on the west antarctic ice sheet.
ter for the disturbance caused by drilling. The successful recovery of dataloggers left at the sites would improve the accuracy.
Glacier geophysical studies at Taylor Dome: Year three D.L. MORSE and E.D. WADDINGTON, Geophysics Program, University of Washington, Seattle, Washington 98195
equipment were transported to Taylor Dome by LC-130; a remote camp near Mount DeWitt (77 0 15'S 159°50'E) was supported by helicopter. We arrived at the field site on 10 December 1992 and returned to McMurdo Station on 20 January 1993. The key element of the ice dynamics part of this program is the emplacement and survey of a network of marker poles by which to determine ice surface topography, motion, and strain. We have successively increased the density of this network to zero in on the final drilling location (figure 1). In this third season, we placed a grid of markers over a 2.5 kilometers (km) by 2.5 km area around the site determined to be the optimal drilling location (figures 1 and 2). These poles are spaced at 500-600 meters (m), a distance approximately equal to the local minimum ice thickness. Beyond this region, the grid was extended 10 km downstream of the core site using 1,200-m spacing. Additionally, poles were emplaced to cover the region upstream of the star at the entrance to Taylor Glacier in figure 1, where a 100-m ice core was drilled during the
aylor Dome (77050'S 159°00'E) is the site of an ongoing ice T core/paleoclimate project. The 1992-1993 austral summer was this project's third consecutive field season. Previous work has been reported earlier (Grootes et al. 1991; Waddington et al. 1991, 1993; Grootes and Steig 1992; Morse and Waddington 1992). The primary goal of the previous seasons was the identification of an optimal site from which to extract an ice core. Site determination is based both on ice dynamics and depositional environment considerations (Waddington et al. 1993). Logistical constraints required the drilling operation, once scheduled for the 1992-1993 season, to be postponed until 1993-1994. The main activities of the 1992-1993 season included surveys by ground-based optical methods, surveys using satellite receivers, radio-echo sounding of bedrock topography, and depositional environment characterization. The six field team members for the 1992-1993 season were Edwin Waddington, David Morse, Mike Balise, Peter Balise, Phil Trowbridge, and Matt Duvall. Team members and
ANTARCTIC JOURNAL - REVIEW 1993 67
(Ed.), Antarctic map folio series. New York: American Geographical Society. Carslaw, H.S., and J.C. Jaeger. 1959. Conduction of heat in solids. Oxford: Clarendon Press. Echelmeyer, K., W.D. Harrison, T.S. Clarke, and C. Benson. 1992. Surficial glaciology of Jakobshavns Isbr, West Greenland: Part II. Ablation, accumulation and temperature. Journal of Glaciology, 38(128),169-181. Engelhardt, H., M. Fahnestock, N. Humphrey, and B. Kamb. 1989. Borehole drilling to the bed of ice stream B, Antarctica. Antarctic Journal of the U.S., 24(5), 83-84. Engelhardt, H., N. Humphrey, B. Kamb, and M. Fahnestock. 1990. Physical conditions at the base of a fast moving antarctic ice stream. Science, 248, 57-59. Humphrey, N. 1991. Estimating ice temperatures from short records in thermally disturbed boreholes. Journal of Glaciology, 37(127), 414-419. Lachenbruch, A.H., and M.C. Brewer. 1959. Dissipation of temperature effect of drilling a well in Arctic Alaska. In Experimental and theoretical geophysics (U.S. Geological Survey bulletin 1083-C). Washington, D.C.: U.S. Government Printing Office. Lingle, C.S., and T.J. Brown. 1987. A subglacial aquifer bed model and water pressure dependent basal sliding relationship for a west antarctic ice stream. In C.J. van der Veen and J. Oerlemans (Eds.), Dynamics of the west antarctic ice sheet. Dordrecht: D. Reidel. Radok, U., D. Jenssen, and W. Budd. 1970. Steady-state temperature profiles in ice sheets. IAHS Publications, 86, 151-165. Robin, G. de Q. 1955. Ice movement and temperature distribution in glaciers and ice sheets. Journal of Glaciology, 2(18), 523-532. Robin, G. de Q. 1983. The climatic record in polar ice sheets. Cambridge: Cambridge University Press.
References Alley, R.B., and C.R. Bentley. 1988. Ice-core analysis on the Siple Coast of West Antarctica. Annals of Glaciology, 11, 1-7. Bentley, C.R., R.L. Cameron, C. Bull, K. Kojima, and A.J. Gow. 1964. Physical characteristics of the antarctic ice sheet. In V.C. Bushnell
Temperature measurements in the margin of ice stream B, 1992-1993 KEITH ECHELMEYER
and WILLIAM HARRISON, Geophysical Institute, University ofAlaska, Fairbanks, Alaska 99775-0800
ice stream. The work began near Upstream B Camp in the 1992-1993 austral summer. Severe crevassing in the margins posed a major challenge to drilling operations, which were performed with the California Institute of Technology's hot-water rig. We used a careful program of probing and subsurface exploration of buried crevasses to find a safe route for the drill rig, then operated as close to the south margin of the ice stream as possible. From this point about 1,700 meters (m) of hot-water hose was dragged into the chaotic crevasses of the margin, where the drilling was performed with a light hose-handling winch and a single heater, which was used to boost the temperature of the water arriving from the distant drill rig. Three holes were drilled, one each at the remote, intermediate, and pad sites. The first was in the chaotically crevassed portion of the margin, the third at the site of the main drill rig about 1,800 m closer to the center of the ice stream, and the second at a site roughly halfway between the other two. In the preliminary results (shown in the figure), the temperatures have been corrected to an accuracy of about 0.5 Kelvin or bet-
he low shear stress at the bottom of ice stream B suggestT ed both by soft subglacier sediment samples acquired by the California Institute of Technology near Upstream B Camp (Engelhardt et al. 1990) and by recent theoretical analyses of transverse profiles of velocity across the ice stream (Echelmeyer et al. in preparation; Van der Veen and Whillans in preparation) indicates that the margins of the ice stream probably play a significant role in the dynamics of flow, perhaps exerting more drag on the ice stream than does the bed itself. This situation would require a relatively large shear stress at the margins of the ice stream—large enough that the effects of shear heating should be detectable by temperature measurements at several hundred meters depth. The most important unknowns are the rate of convergence of ice into the ice stream (which controls the residence time of the ice in the active part of the margins), the stability of the positions of the margins, and the shear stress itself. We are examining these unknowns using a program of temperature measurements in the margins and a surveying program to improve our knowledge of the rate of convergence of the ice into the
ANTARCTIC JOURNAL - REVIEW 1993 66
There are two striking preliminary results. First, the nearsurface drops as the margin is approached (see figure). This very large effect, one apparently not observed before, is probably due to the ponding of cold air in crevasses. Second, at greater depths, the intermediate site is the warmest of the three, indicating that there is not a simple relationship between deep temperature and proximity to the margin. This is a surprising result for which at least two hypotheses may be entertained: a complex history of the ice (perhaps related to different paths through upstream crevasse patterns) or even erratic migration of the position of the boundary of the ice stream. In 1993-1994, we hope to extend this transverse profile of holes all the way through the margin. We are grateful for the support of the personnel from many different institutions who contributed to the field operations. Financial support was from National Science Foundation grant OPP 91-22783.
TEMPERATURE (deg C) -30-29 -28 -27 -26 -25 -24 -23 -22 -21 -20 0 1
1
1
1
I1
1I
50 100
E I 0
150 200 250 300 350 400
References
Temperature at three sites near the south margin of ice stream B. The remote site is near the inner edge of the chaotically crevassed portion of the margin; the pad site is about 1,800 m nearer the center of the ice stream, and the intermediate site is approximately halfway between the other two.
Engelhardt, H., N. Humphrey, B. Kamb, and M. Fahnestock. 1990. Physical conditions at the base of a fast moving antarctic ice stream. Science, 248, 57-59. Echelmeyer, K.A., W.D. Harrison, J.E. Mitchell, C. Larsen. In preparation. Velocity across ice stream B and the role of the margins in ice stream dynamics. Van der Veen, C.J., and I.M. Whillans. In preparation. Controls on the west antarctic ice sheet.
ter for the disturbance caused by drilling. The successful recovery of dataloggers left at the sites would improve the accuracy.
Glacier geophysical studies at Taylor Dome: Year three D.L. MORSE and E.D. WADDINGTON, Geophysics Program, University of Washington, Seattle, Washington 98195
equipment were transported to Taylor Dome by LC-130; a remote camp near Mount DeWitt (77 0 15'S 159°50'E) was supported by helicopter. We arrived at the field site on 10 December 1992 and returned to McMurdo Station on 20 January 1993. The key element of the ice dynamics part of this program is the emplacement and survey of a network of marker poles by which to determine ice surface topography, motion, and strain. We have successively increased the density of this network to zero in on the final drilling location (figure 1). In this third season, we placed a grid of markers over a 2.5 kilometers (km) by 2.5 km area around the site determined to be the optimal drilling location (figures 1 and 2). These poles are spaced at 500-600 meters (m), a distance approximately equal to the local minimum ice thickness. Beyond this region, the grid was extended 10 km downstream of the core site using 1,200-m spacing. Additionally, poles were emplaced to cover the region upstream of the star at the entrance to Taylor Glacier in figure 1, where a 100-m ice core was drilled during the
aylor Dome (77050'S 159°00'E) is the site of an ongoing ice T core/paleoclimate project. The 1992-1993 austral summer was this project's third consecutive field season. Previous work has been reported earlier (Grootes et al. 1991; Waddington et al. 1991, 1993; Grootes and Steig 1992; Morse and Waddington 1992). The primary goal of the previous seasons was the identification of an optimal site from which to extract an ice core. Site determination is based both on ice dynamics and depositional environment considerations (Waddington et al. 1993). Logistical constraints required the drilling operation, once scheduled for the 1992-1993 season, to be postponed until 1993-1994. The main activities of the 1992-1993 season included surveys by ground-based optical methods, surveys using satellite receivers, radio-echo sounding of bedrock topography, and depositional environment characterization. The six field team members for the 1992-1993 season were Edwin Waddington, David Morse, Mike Balise, Peter Balise, Phil Trowbridge, and Matt Duvall. Team members and
ANTARCTIC JOURNAL - REVIEW 1993 67
