Glacial geology and geophysics
Glacial geology and geophysics Evidence from Wright Valley for the response of the antarctic ice sheet to climate warming M.L. PRENTICE and G.H. DENTON Institute for Quaternary Studies
and
Department of Geological Sciences University of Maine Orono, Maine 04469
L.H. BURCKLE Lamont-Doherty Geological Observatory Columbia University Palisades, New York 10964
D.A. HODELL Department of Geology University of Florida Gainesville, Florida 32611
Peleus till, a distinctive basal ice deposit, outcrops throughout the central and eastern portion of one of the dry valleys, Wright Valley, up to an elevation of 1,150 meters (Prentice 1982). The glacier that deposited this till flowed eastward down Wright Valley toward the Ross Sea with much of its base at the pressure melting point (Prentice et al. 1985). Peleus till along with an array of other glacial features was originally assigned to a tremendous antarctic ice sheet that submerged this portion of the Transantarctic Mountains and so occupied both East and West Antarctica (Denton et al. 1984; Prentice 1985). However, new data collected during the past year suggest an alternative hypothesis for Peleus till which is the subject of this paper. This alternative has important implications for past and future antarctic ice dynamics. Our alternate hypothesis is that Peleus till was deposited by a thin temperate glacier that was wet-based because of a warm climate. This hypothesis differs from the original one principally concerning the maximum thickness and thermal characteristics attained by Peleus ice. The temperate valley glacier alternative is important, because it suggests that, in a warmerthan-present climate, the west antarctic ice sheet was absent and that the east antarctic ice sheet could have been larger than at any time during the last several hundred thousand years. The 56
potential configuration resembles the output of an ice sheet sensitivity experiment run at a sea-level air temperature 100 warmer than modern (Oerlemans 1982). The temperate glacier alternative has implications for how the antarctic ice sheet may respond to future carbon-dioxide-induced warming. Temperate glacier evidence. The evidence that favors the temperate valley glacier alternative is as follows: • Peleus till has not been found in reconnaissance of the mountains adjacent to central and eastern Wright Valley. Had the Peleus ice mass overridden these mountains, it probably should have deposited some till there. • Sandstone gravel is virtually absent in Peleus till. Sandstone gravel should be abundant if Peleus ice overrode the mountains adjacent to the valley from the southwest (Denton et al. 1984). This is because sandstone outcrop in these mountains is extensive and transport distance from the mountains into the valley is too short for complete gravel comminution. • Tongues of Peleus-equivalent till project out of western Wright Valley southward into the mouths of cirques in the Asgard Range (Ackert in preparation). The morphology of these tongues suggests deposition at the margin of an ice lobe restricted to Wright Valley. • The valley glacier alternative is replicated in nearby Beacon Valley (Potter personal communication). Here, till similar to Peleus till reaches an upper limit that outlines an ice lobe projecting from eastward-flowing ice in Taylor Valley up into Beacon Valley. • Subantarctic marine diatoms in a fjord deposit directly beneath Peleus till suggest that this fjord was significantly warmer (approximately 5° C) than modern fjords in the region (Denton, Prentice, and Burckle in press). We infer from this that the local climate prior to the Peleus glaciation was significantly warmer than it is today. These data lead us to prefer the valley glacier alternative explanation of Peleus till to the giant ice sheet hypothesis. However, the latter as well as other more complicated hypotheses cannot as of yet be ruled out. Temperate glacier reconstruction. Figure 1 depicts a sketch of the ice mass that deposited Peleus till consistent with the temperate glacier alternative. Because the upper ice surface intersects the highest Peleus outcrops in the valley, the displayed thickness is a minimum for peak Peleus glaciation. An important question is whether this ice lobe was part of an alpine glacier system restricted to the Transantarctic Mountains or was supplied by the east antarctic ice sheet. To answer this question, we assessed the mass balance of the Peleus ice system for the case of its restriction to the Transantarctic Mountains. A highly negative mass balance would suggest that such an ice system was prohibitively ephemeral. Figure 2 shows Peleus ice surface area and suggested mass balance versus elevation for the alpine system case. The upper limit of Peleus till in the central valley approximates the altitude of the Peleus equilibrium line. The field of Peleus balance gradients ANTARCTIC JOURNAL
Figure 1. Schematic profile of the temperate valley glacier that deposited Peleus till in Wright Valley. The modern profile of the Upper Wright Glacier is dashed. Ice temperatures, based on the presence of basal till, are at or very near the pressure melting point. Arrows at the ice base reflect ice flow downward to a wet-melting bed. This reconstruction is one extreme alternative to explain Pefeus till. ("m" denotes "meter?' "km" denotes "kilometer?')
(activity index equals approximately 3 millimeters per meter) incorporates gradients for many glaciers in southwestern Greenland (Weidick 1984; Braithwaite 1986) which we consider good modern analogs for this Peleus alternative (Kuhn 1984). Mass balances calculated from these considerations are all high ly negative (figure 2). Mass balances calculated by a different scheme based simply on equilibrium line altitudes and an average effective balance gradient (Braithwaite 1984) are even more negative. We infer that the Peleus ice lobe could not have been maintained by Transantarctic Mountain accumulation alone but required considerable influx from the east antarctic ice sheet. Ice sheet contours inland of the Peleus ice lobe were recon structed so as to increase the accumulation area of the Peleus ice lobe sufficiently for this ice system to achieve a plausible mass balance. Figure 3 presents one possible mesoscale reconstruction of the Peleus ice lobe and adjacent ice sheet consistent with available data. Peleus ice surface contours probably paralleled the Transantarctic Mountain crest. Note that, by this reconstruction, the paleo-ice sheet was sufficiently thick that no local ice dome, such as the McMurdo Ice Dome, existed. Age control. Marine diatoms in the fjord deposit directly beneath Peleus till at Prospect Mesa suggest that Peleus till is no older than late Pliocene, about 3 million years (Burckle et al. 1986). However, preliminary strontium-87/strontium-86 data from pecten shells within this same deposit suggest a maximum age of 4 to 5 million years for the Peleus till. The latter estimates are preferred because of the probability for mid-tohigh latitude diachroneity of biostratigraphic datums. Peleus till is constrained to be older than 2.0 million years on the basis of radiometric dates on volcanic rocks stratigraphically above Peleus till. Conclusion. A temperate valley glacier hypothesis is preferred to explain Peleus till. From this we suggest that, under warmerthan-present climate, the proximal sector of the east antarctic ice sheet was thicker than it is today and that sea-level temperatures in the Ross Sea were too warm for a marine ice sheet to have existed. If these inferences can be extended to the rest of East and West Antarctica by further investigation, a new warm1987 REVIEW
climate configuration for the antarctic ice sheet emerges. Such ice sheet configurations represent the proving grounds for numerical ice sheet models employed to predict antarctic ice sheet response to future carbon-dioxide-induced warming. This work was supported by National Science Foundation grant DPP 83-18808. We thank Mauri Pelto for mass-balance discussions.
3600
Area (km2) 0 100 200
3300 3000 2700 E C 0 0 >
2400 200 1800
W 1500 1200 900 600 300 0
-12 -10 -8 -6 -4 -2 0 +2 +4 Balance (m)
Figure 2. Mass balance of the Peleus temperate ice system if it were restricted to the Transantarctic Mountains. Ice surface area versus elevation is to the left. Mass balance versus elevation is to the right. A range of balance gradients midway between that of the polar Taylor Glacier (Robinson 1984) and the maritime temperate Taku Glacier (Mayo 1984) is suggested for the Peleus glacier. The mass balance for this hypothetical alpine glacier system (upper right) is prohibitively negative. ("m" denotes "meter." "km 2" denotes "square kilometers?') 57
77°S
"
Modern Ice Surface
w 0
Peleus
0 0 (D
I
I
\
V
(\
Ice Surface
(0
0 0 To
2300 - 0 0
g
CV
0
C4
N
C)C)
0
—4
\%JpP W 0 (0 to
o
10 20 30 40 50
I_I
I I
KM 78°s
Figure 3. Sketch of the preferred configuration for the east antarctic ice sheet margin that deposited Peleus till (uncorrected for isostatic depression). Contours with solid lines reflect Peleus ice surface elevation in meters. Dashed contours are the modern ice topography from Drewry (1982). In this particular configuration, no ice dome existed just inland of the mountains. ("km" denotes "kilometer:')
References Ackert, F. In preparation. Glacial history of the western Asgard Range. (Masters Thesis, University of Maine, Orono, Maine.) Braithwaite, R.J. 1984. Can the mass balance of a glacier be estimated from its equilibrium-line altitude? Journal of Glaciology, 30, 364-368. Braithwaite, R.J. 1986. Assessment of mass-balance variations within a sparse stake network, Qamanarssup sermia, West Greenland. Journal of Glaciology, 32, 50-53. Burckle, L.H., M.L. Prentice, and G.H. Denton. 1986. Neogene Antarctic glacial history: New evidence from marine diatoms in continental deposits. EQS Transactions of the American Geophysical Union,
67(16), 295. Denton, G.H., M.L. Prentice, and L.H. Burckle. In press. Cenozoic history of the Antarctic Ice Sheet. In R. Tingey (Ed.), The geology of Antarctica. London: Oxford University Press. Denton, GH., M.L. Prentice, D.E. Kellogg, andT.B. Kellogg. 1984. Late Tertiary history of the Antarctic ice sheet: Evidence from the Dry Valleys. Geology, 12, 263-267. Drewry, D.J. 1982. Ice flow, bedrock, and geothermal studies from radio-echo sounding inland of McMurdo Sound, Antarctica. In C.
58
Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Kuhn, M. 1984. Mass budget imbalances as criterion for a climatic classification of glaciers. Geografiska annaler, 66A, 229-238. Mayo, L.R. 1984. Glacier mass balance and runoff research in the U.S.A. Geograjiska annaler, 66A, 215-227. Oerlemans, J . 1982. A model of the Antarctic Ice Sheet. Nature, 297, 550-553. Potter, N., Jr. 1987. Personal communication. Prentice, M.L. 1982. Surficial geology and stratigraphy in central Wright ValleL/, Antarctica: Implications for Antarctic Tertiary glacial history. (Mas-
ters Thesis, University of Maine, Orono, Maine.) Prentice, M.L. 1985. Peleus glaciation of Wright Valley. South African Journal of Science, 81(5), 241-243. Prentice, M.L., S.C. Wilson, J.G. Bockheim, and G.H. Denton. 1985. Geologic evidence for pre-late Quaternary east antarctic glaciation of central and eastern Wright Valley. Antarctic Journal of the U.S., 20(5), 61-62. Robinson, P. H. 1984. Ice dynamics and thermal regime of Taylor Glacier, south Victoria Land, Antarctica. Journal of Giaciology, 30, 153-160. Weidick, A. 1984. Studies of glacier behaviour and glacier mass balance in Greenland—A review. Geografiska annaler, 66A, 183-195.
ANTARCTIC JOURNAL
and
Department of Geological Sciences University of Maine Orono, Maine 04469
L.H. BURCKLE Lamont-Doherty Geological Observatory Columbia University Palisades, New York 10964
D.A. HODELL Department of Geology University of Florida Gainesville, Florida 32611
Peleus till, a distinctive basal ice deposit, outcrops throughout the central and eastern portion of one of the dry valleys, Wright Valley, up to an elevation of 1,150 meters (Prentice 1982). The glacier that deposited this till flowed eastward down Wright Valley toward the Ross Sea with much of its base at the pressure melting point (Prentice et al. 1985). Peleus till along with an array of other glacial features was originally assigned to a tremendous antarctic ice sheet that submerged this portion of the Transantarctic Mountains and so occupied both East and West Antarctica (Denton et al. 1984; Prentice 1985). However, new data collected during the past year suggest an alternative hypothesis for Peleus till which is the subject of this paper. This alternative has important implications for past and future antarctic ice dynamics. Our alternate hypothesis is that Peleus till was deposited by a thin temperate glacier that was wet-based because of a warm climate. This hypothesis differs from the original one principally concerning the maximum thickness and thermal characteristics attained by Peleus ice. The temperate valley glacier alternative is important, because it suggests that, in a warmerthan-present climate, the west antarctic ice sheet was absent and that the east antarctic ice sheet could have been larger than at any time during the last several hundred thousand years. The 56
potential configuration resembles the output of an ice sheet sensitivity experiment run at a sea-level air temperature 100 warmer than modern (Oerlemans 1982). The temperate glacier alternative has implications for how the antarctic ice sheet may respond to future carbon-dioxide-induced warming. Temperate glacier evidence. The evidence that favors the temperate valley glacier alternative is as follows: • Peleus till has not been found in reconnaissance of the mountains adjacent to central and eastern Wright Valley. Had the Peleus ice mass overridden these mountains, it probably should have deposited some till there. • Sandstone gravel is virtually absent in Peleus till. Sandstone gravel should be abundant if Peleus ice overrode the mountains adjacent to the valley from the southwest (Denton et al. 1984). This is because sandstone outcrop in these mountains is extensive and transport distance from the mountains into the valley is too short for complete gravel comminution. • Tongues of Peleus-equivalent till project out of western Wright Valley southward into the mouths of cirques in the Asgard Range (Ackert in preparation). The morphology of these tongues suggests deposition at the margin of an ice lobe restricted to Wright Valley. • The valley glacier alternative is replicated in nearby Beacon Valley (Potter personal communication). Here, till similar to Peleus till reaches an upper limit that outlines an ice lobe projecting from eastward-flowing ice in Taylor Valley up into Beacon Valley. • Subantarctic marine diatoms in a fjord deposit directly beneath Peleus till suggest that this fjord was significantly warmer (approximately 5° C) than modern fjords in the region (Denton, Prentice, and Burckle in press). We infer from this that the local climate prior to the Peleus glaciation was significantly warmer than it is today. These data lead us to prefer the valley glacier alternative explanation of Peleus till to the giant ice sheet hypothesis. However, the latter as well as other more complicated hypotheses cannot as of yet be ruled out. Temperate glacier reconstruction. Figure 1 depicts a sketch of the ice mass that deposited Peleus till consistent with the temperate glacier alternative. Because the upper ice surface intersects the highest Peleus outcrops in the valley, the displayed thickness is a minimum for peak Peleus glaciation. An important question is whether this ice lobe was part of an alpine glacier system restricted to the Transantarctic Mountains or was supplied by the east antarctic ice sheet. To answer this question, we assessed the mass balance of the Peleus ice system for the case of its restriction to the Transantarctic Mountains. A highly negative mass balance would suggest that such an ice system was prohibitively ephemeral. Figure 2 shows Peleus ice surface area and suggested mass balance versus elevation for the alpine system case. The upper limit of Peleus till in the central valley approximates the altitude of the Peleus equilibrium line. The field of Peleus balance gradients ANTARCTIC JOURNAL
Figure 1. Schematic profile of the temperate valley glacier that deposited Peleus till in Wright Valley. The modern profile of the Upper Wright Glacier is dashed. Ice temperatures, based on the presence of basal till, are at or very near the pressure melting point. Arrows at the ice base reflect ice flow downward to a wet-melting bed. This reconstruction is one extreme alternative to explain Pefeus till. ("m" denotes "meter?' "km" denotes "kilometer?')
(activity index equals approximately 3 millimeters per meter) incorporates gradients for many glaciers in southwestern Greenland (Weidick 1984; Braithwaite 1986) which we consider good modern analogs for this Peleus alternative (Kuhn 1984). Mass balances calculated from these considerations are all high ly negative (figure 2). Mass balances calculated by a different scheme based simply on equilibrium line altitudes and an average effective balance gradient (Braithwaite 1984) are even more negative. We infer that the Peleus ice lobe could not have been maintained by Transantarctic Mountain accumulation alone but required considerable influx from the east antarctic ice sheet. Ice sheet contours inland of the Peleus ice lobe were recon structed so as to increase the accumulation area of the Peleus ice lobe sufficiently for this ice system to achieve a plausible mass balance. Figure 3 presents one possible mesoscale reconstruction of the Peleus ice lobe and adjacent ice sheet consistent with available data. Peleus ice surface contours probably paralleled the Transantarctic Mountain crest. Note that, by this reconstruction, the paleo-ice sheet was sufficiently thick that no local ice dome, such as the McMurdo Ice Dome, existed. Age control. Marine diatoms in the fjord deposit directly beneath Peleus till at Prospect Mesa suggest that Peleus till is no older than late Pliocene, about 3 million years (Burckle et al. 1986). However, preliminary strontium-87/strontium-86 data from pecten shells within this same deposit suggest a maximum age of 4 to 5 million years for the Peleus till. The latter estimates are preferred because of the probability for mid-tohigh latitude diachroneity of biostratigraphic datums. Peleus till is constrained to be older than 2.0 million years on the basis of radiometric dates on volcanic rocks stratigraphically above Peleus till. Conclusion. A temperate valley glacier hypothesis is preferred to explain Peleus till. From this we suggest that, under warmerthan-present climate, the proximal sector of the east antarctic ice sheet was thicker than it is today and that sea-level temperatures in the Ross Sea were too warm for a marine ice sheet to have existed. If these inferences can be extended to the rest of East and West Antarctica by further investigation, a new warm1987 REVIEW
climate configuration for the antarctic ice sheet emerges. Such ice sheet configurations represent the proving grounds for numerical ice sheet models employed to predict antarctic ice sheet response to future carbon-dioxide-induced warming. This work was supported by National Science Foundation grant DPP 83-18808. We thank Mauri Pelto for mass-balance discussions.
3600
Area (km2) 0 100 200
3300 3000 2700 E C 0 0 >
2400 200 1800
W 1500 1200 900 600 300 0
-12 -10 -8 -6 -4 -2 0 +2 +4 Balance (m)
Figure 2. Mass balance of the Peleus temperate ice system if it were restricted to the Transantarctic Mountains. Ice surface area versus elevation is to the left. Mass balance versus elevation is to the right. A range of balance gradients midway between that of the polar Taylor Glacier (Robinson 1984) and the maritime temperate Taku Glacier (Mayo 1984) is suggested for the Peleus glacier. The mass balance for this hypothetical alpine glacier system (upper right) is prohibitively negative. ("m" denotes "meter." "km 2" denotes "square kilometers?') 57
77°S
"
Modern Ice Surface
w 0
Peleus
0 0 (D
I
I
\
V
(\
Ice Surface
(0
0 0 To
2300 - 0 0
g
CV
0
C4
N
C)C)
0
—4
\%JpP W 0 (0 to
o
10 20 30 40 50
I_I
I I
KM 78°s
Figure 3. Sketch of the preferred configuration for the east antarctic ice sheet margin that deposited Peleus till (uncorrected for isostatic depression). Contours with solid lines reflect Peleus ice surface elevation in meters. Dashed contours are the modern ice topography from Drewry (1982). In this particular configuration, no ice dome existed just inland of the mountains. ("km" denotes "kilometer:')
References Ackert, F. In preparation. Glacial history of the western Asgard Range. (Masters Thesis, University of Maine, Orono, Maine.) Braithwaite, R.J. 1984. Can the mass balance of a glacier be estimated from its equilibrium-line altitude? Journal of Glaciology, 30, 364-368. Braithwaite, R.J. 1986. Assessment of mass-balance variations within a sparse stake network, Qamanarssup sermia, West Greenland. Journal of Glaciology, 32, 50-53. Burckle, L.H., M.L. Prentice, and G.H. Denton. 1986. Neogene Antarctic glacial history: New evidence from marine diatoms in continental deposits. EQS Transactions of the American Geophysical Union,
67(16), 295. Denton, G.H., M.L. Prentice, and L.H. Burckle. In press. Cenozoic history of the Antarctic Ice Sheet. In R. Tingey (Ed.), The geology of Antarctica. London: Oxford University Press. Denton, GH., M.L. Prentice, D.E. Kellogg, andT.B. Kellogg. 1984. Late Tertiary history of the Antarctic ice sheet: Evidence from the Dry Valleys. Geology, 12, 263-267. Drewry, D.J. 1982. Ice flow, bedrock, and geothermal studies from radio-echo sounding inland of McMurdo Sound, Antarctica. In C.
58
Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Kuhn, M. 1984. Mass budget imbalances as criterion for a climatic classification of glaciers. Geografiska annaler, 66A, 229-238. Mayo, L.R. 1984. Glacier mass balance and runoff research in the U.S.A. Geograjiska annaler, 66A, 215-227. Oerlemans, J . 1982. A model of the Antarctic Ice Sheet. Nature, 297, 550-553. Potter, N., Jr. 1987. Personal communication. Prentice, M.L. 1982. Surficial geology and stratigraphy in central Wright ValleL/, Antarctica: Implications for Antarctic Tertiary glacial history. (Mas-
ters Thesis, University of Maine, Orono, Maine.) Prentice, M.L. 1985. Peleus glaciation of Wright Valley. South African Journal of Science, 81(5), 241-243. Prentice, M.L., S.C. Wilson, J.G. Bockheim, and G.H. Denton. 1985. Geologic evidence for pre-late Quaternary east antarctic glaciation of central and eastern Wright Valley. Antarctic Journal of the U.S., 20(5), 61-62. Robinson, P. H. 1984. Ice dynamics and thermal regime of Taylor Glacier, south Victoria Land, Antarctica. Journal of Giaciology, 30, 153-160. Weidick, A. 1984. Studies of glacier behaviour and glacier mass balance in Greenland—A review. Geografiska annaler, 66A, 183-195.
ANTARCTIC JOURNAL
