Late Quaternary surface fluctuations of Beardmore Glacier, Antarctica
Late Quaternary surface fluctuations of Beardmore Glacier, Antarctica
G. H. DENTON Institute for Quaternary Studies
and
Department of Geological Sciences University of Maine Orono, Maine 04469
B.C. ANDERSEN Department of Geology University of Oslo Oslo, Noru'ay
H.W. CONWAY Department of Chemical and Process Engineering University of Canterbury, Private Bag Christchurch, New Zealand
The Beardmore Glacier drains ice from the east antarctic polar plateau through the Transantarctic Mountains to the Ross Ice Shelf (figures 1 and 2). Because of its interior location and adjacent ice-free areas, the Beardmore Glacier is particularly well suited to document antarctic ice-sheet behavior during late Quaternary time. This behavior, in turn, can illustrate the role of the antarctic ice sheet in global ice ages. During the austral field season of 1985-1986, we mapped glacial drift sheets to resolve conflicting interpretations of late Quaternary surface-level changes of Beardmore Glacier. From Beardmore lateral moraines, Mercer (1972) inferred extensive grounding of the Ross Ice Shelf accompanied by little or no change in interior East Antarctica during the late Quaternary ice ages. In sharp contrast, Mayewski (1975) concluded that the lateral moraines reflect former thickening of east antarctic ice accompanied by only minor grounding-line advance along the inland periphery of the Ross Ice Shelf. These two interpretations of past changes in Beardmore Glacier imply fundamentally different controls of the antarctic ice sheet during late Quaternary ice ages. Our research strategy combines geologic mapping with soil studies that document postdepositional weathering of drift sheets. This strategy allowed drift sheets to be differentiated in each ice-free area and then to be correlated on a local and regional scale. Differentiation of drift sheets was by morphology, cross-cutting geometric relationships, depth of oxidation, solum thickness, morphogenetic salt stage, and weathering stage (Bockheim, Wilson, and Leide, Antarctic Journal, this issue). Four drift sheets were found to mantle the present valley walls and, in places, rest on Sirius drift (Prentice et al., Antarctic 90
Journal, this
issue). These four drifts are from 10 centimeters to several meters thick. They are composed largely of unconsolidated gravel. Numerous included striated clasts were probably reworked from Sirius drift. Thin boulder-belt moraines commonly mark drift surfaces and define outer edges of drift sheets. The thin drift sheets overlie well-preserved morphological features, particularly in Sirius deposits. Figure 3 shows former surfaces of Beardmore Glacier represented by the four drift sheets. The upper limit of Plunket drift parallels the present surface of Beardmore Glacier along its entire length. It also fringes the snout of Rutkowski Glacier, which drains the local ice cap on the Dominion Range. This drift configuration shows similar behavior of these two glaciers during deposition of Plunket drift. The upper limit of Beardmore and Meyer drifts are close to the present surface of Beardmore Glacier near the polar plateau but systematically rise above the present surface in the downglacier direction. Further, the areal patterns of Beardmore and Meyer drifts show recession of Rutkowski Glacier concurrent with expansion of Beardmore Glacier. Dominion drift occurs on the northern flank of the Dominion Range, where it reaches high above Beardmore Glacier (Prentice et al., Antarctic Journal, this issue). We draw several inferences from the configuration, physical characteristics, and weathering of these four drift sheets. The first involves their age. From advanced soil development (Bockheim et al, Antarctic Journal, this issue), we infer that Dominion drift is pre-late Quaternary in age. Beardmore and Meyer drifts are judged to be late Quaternary in age on the basis of soil development (Bockheim et al., Antarctic Journal, this issue). Most likely, Beardmore drift correlates with Britannia I and II (Hatherton and Darwin Glaciers) and Ross Sea (McMurdo Sound region) drifts, which are radiocarbon dated to late Wisconsin age (Stuvier et al. 1981; Denton, Prentice, and Burckle in press). Meyer drift most likely corresponds to stage 6 drift in the McMurdo Sound area (Denton et al. in press). Plunket drift shows less soil development than Beardmore drift (Bockheim et al., Antarctic Journal, this issue) and is probably Holocene in age. Plunket drift is similar in position, morphology, and soil development to ice-cored lateral moraines in the ice-free valleys of southern Victoria Land that have maximum radiocarbon dates of 3,100 years (Denton et al. in press). From their physical characteristics, we infer deposition of these four drift sheets by polar ice with a frozen bed. They thus stand in marked contrast to underlying Sirius drift, which was deposited under temperate conditions with woody vegetation and extensive summer ice melting (Prentice et al., Antarctic Journal, this issue). These two contrasting styles of glaciation mark a profound climatic change (see also Mercer 1972 and Prentice et al., Antarctic Journal, this issue) that was pre-late Quaternary in age. We conclude that Dominion drift represents extension thickening of polar ice subsequent to this climatic change. The longitudinal ice-surface profiles derived from Beardmore and Meyer drift show thick blocking ice near the Ross Ice Shelf and little elevation change in the polar plateau during late Quaternary glaciations (figure 3). In fact, our data permit a decrease in the level of the polar plateau inland of Beardmore Glacier. This is consistent with a dual control of the antarctic ice sheet by eustatic sea-level lowering (causing widespread grounding of the Ross Ice Shelf) and by a decrease in precipitation due to colder atmospheric temperatures (resulting in little change or even slight decline of the polar plateau). This dual control is in accord with the out-of-phase behavior of Beardmore Glacier and the Rutkowski outlet of the ice cap on ANTARCTIC JOURNAL
Figure 1. Index map of Antarctica.
the Dominion Range. When Beardmore Glacier thickened (grounding of the Ross Ice Shelf), Rutkowski Glacier contracted (decreased precipitation). Comparison of profile C-C" in figure 3 with profiles A-A" and B-B' in figures 2 and 3 of Prentice et al. (Antarctic Journal, this issue) shows that late Quaternary fluctuations of interior antarctic ice were far less severe than those of pre-late Quaternary time. Our inferences concerning late quaternary drift sheets are in substantial agreement with those of Mercer (1972). Mercer's Beardmore III drift is largely equivalent to our Beardmore drift, and his Beardmore II drift corresponds with our Meyer drift. 1986 REVIEW
Mercer (1972) likewise concluded that sea-level lowering and grounding of the Ross Ice Shelf caused thickening of the lower reaches of Beardmore Glacier shown by these two drifts. Hence, our Beardmore data and those of Mercer (1972) both support our conclusion based on field work further north that widespread grounding occurred in the area of the present Ross Ice Shelf and Ross Sea during late Quaternary global ice ages (Stuiver et al. 1981). This research was supported by National Science Foundation grant DPP 83-18808. We are very grateful to VXE-6 for helicopter support in the Beardmore Glacier area. 91
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Figure 2. Sketch map of the Beardmore Glacier region. C-C" shows position of longitudinal profile in figure 3. A-A" and B-B' show profiles in Prentice et at. (Antarctic Journal, this issue).
C C' C"
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Figure 3. Present and former longitudinal profiles of the surface of Beardmore Glacier. The position of C-C" shown in figure 2. Moraine evaluations are projected onto the profile from ice-free areas designated above the profiles.
92
References Bockheim, J. G., S.W. Wilson, and J. E. Leide. 1986. Soil development in the Beardmore Glacier region, Antarctica. Antarctic Journal of the U. S., 21(5). Denton, G.H., M. L. Prentice, and L. H. Burckle. In press. Late Cenozoic history of the Antarctic Ice Sheet. In R.J. Tingey (Ed.), The geology of Antarctica. Cambridge: Oxford University Press. Mayewski, P.A. 1975. Glacial geology and late Cenozoic history of the Transantarctic Mountains, Antarctica. (Institute of Polar Studies, Report No. 56.) Columbus: Ohio State University. Mercer, J.H. 1972. Some observations on the glacial geology of the Beardmore Glacier area. In R.J. Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. Prentice, ML., G.H. Denton, TV. Lowell, H.C. Hughes and L.E. Heusser. 1986, Pre-late Quaternary glaciation of the Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S. 21(5). Stuiver, M., G. H. Denton, T.J. Hughes, and J. L. Fastook, 1981. History of the marine ice sheet in West Antarctica during the last glaciation: A working hypothesis. InG.H. Denton, andT.J. Hughes, (Eds.), The last great ice sheets. New York: Wiley-Interscience.
ANTARCTIC JOURNAL
G. H. DENTON Institute for Quaternary Studies
and
Department of Geological Sciences University of Maine Orono, Maine 04469
B.C. ANDERSEN Department of Geology University of Oslo Oslo, Noru'ay
H.W. CONWAY Department of Chemical and Process Engineering University of Canterbury, Private Bag Christchurch, New Zealand
The Beardmore Glacier drains ice from the east antarctic polar plateau through the Transantarctic Mountains to the Ross Ice Shelf (figures 1 and 2). Because of its interior location and adjacent ice-free areas, the Beardmore Glacier is particularly well suited to document antarctic ice-sheet behavior during late Quaternary time. This behavior, in turn, can illustrate the role of the antarctic ice sheet in global ice ages. During the austral field season of 1985-1986, we mapped glacial drift sheets to resolve conflicting interpretations of late Quaternary surface-level changes of Beardmore Glacier. From Beardmore lateral moraines, Mercer (1972) inferred extensive grounding of the Ross Ice Shelf accompanied by little or no change in interior East Antarctica during the late Quaternary ice ages. In sharp contrast, Mayewski (1975) concluded that the lateral moraines reflect former thickening of east antarctic ice accompanied by only minor grounding-line advance along the inland periphery of the Ross Ice Shelf. These two interpretations of past changes in Beardmore Glacier imply fundamentally different controls of the antarctic ice sheet during late Quaternary ice ages. Our research strategy combines geologic mapping with soil studies that document postdepositional weathering of drift sheets. This strategy allowed drift sheets to be differentiated in each ice-free area and then to be correlated on a local and regional scale. Differentiation of drift sheets was by morphology, cross-cutting geometric relationships, depth of oxidation, solum thickness, morphogenetic salt stage, and weathering stage (Bockheim, Wilson, and Leide, Antarctic Journal, this issue). Four drift sheets were found to mantle the present valley walls and, in places, rest on Sirius drift (Prentice et al., Antarctic 90
Journal, this
issue). These four drifts are from 10 centimeters to several meters thick. They are composed largely of unconsolidated gravel. Numerous included striated clasts were probably reworked from Sirius drift. Thin boulder-belt moraines commonly mark drift surfaces and define outer edges of drift sheets. The thin drift sheets overlie well-preserved morphological features, particularly in Sirius deposits. Figure 3 shows former surfaces of Beardmore Glacier represented by the four drift sheets. The upper limit of Plunket drift parallels the present surface of Beardmore Glacier along its entire length. It also fringes the snout of Rutkowski Glacier, which drains the local ice cap on the Dominion Range. This drift configuration shows similar behavior of these two glaciers during deposition of Plunket drift. The upper limit of Beardmore and Meyer drifts are close to the present surface of Beardmore Glacier near the polar plateau but systematically rise above the present surface in the downglacier direction. Further, the areal patterns of Beardmore and Meyer drifts show recession of Rutkowski Glacier concurrent with expansion of Beardmore Glacier. Dominion drift occurs on the northern flank of the Dominion Range, where it reaches high above Beardmore Glacier (Prentice et al., Antarctic Journal, this issue). We draw several inferences from the configuration, physical characteristics, and weathering of these four drift sheets. The first involves their age. From advanced soil development (Bockheim et al, Antarctic Journal, this issue), we infer that Dominion drift is pre-late Quaternary in age. Beardmore and Meyer drifts are judged to be late Quaternary in age on the basis of soil development (Bockheim et al., Antarctic Journal, this issue). Most likely, Beardmore drift correlates with Britannia I and II (Hatherton and Darwin Glaciers) and Ross Sea (McMurdo Sound region) drifts, which are radiocarbon dated to late Wisconsin age (Stuvier et al. 1981; Denton, Prentice, and Burckle in press). Meyer drift most likely corresponds to stage 6 drift in the McMurdo Sound area (Denton et al. in press). Plunket drift shows less soil development than Beardmore drift (Bockheim et al., Antarctic Journal, this issue) and is probably Holocene in age. Plunket drift is similar in position, morphology, and soil development to ice-cored lateral moraines in the ice-free valleys of southern Victoria Land that have maximum radiocarbon dates of 3,100 years (Denton et al. in press). From their physical characteristics, we infer deposition of these four drift sheets by polar ice with a frozen bed. They thus stand in marked contrast to underlying Sirius drift, which was deposited under temperate conditions with woody vegetation and extensive summer ice melting (Prentice et al., Antarctic Journal, this issue). These two contrasting styles of glaciation mark a profound climatic change (see also Mercer 1972 and Prentice et al., Antarctic Journal, this issue) that was pre-late Quaternary in age. We conclude that Dominion drift represents extension thickening of polar ice subsequent to this climatic change. The longitudinal ice-surface profiles derived from Beardmore and Meyer drift show thick blocking ice near the Ross Ice Shelf and little elevation change in the polar plateau during late Quaternary glaciations (figure 3). In fact, our data permit a decrease in the level of the polar plateau inland of Beardmore Glacier. This is consistent with a dual control of the antarctic ice sheet by eustatic sea-level lowering (causing widespread grounding of the Ross Ice Shelf) and by a decrease in precipitation due to colder atmospheric temperatures (resulting in little change or even slight decline of the polar plateau). This dual control is in accord with the out-of-phase behavior of Beardmore Glacier and the Rutkowski outlet of the ice cap on ANTARCTIC JOURNAL
Figure 1. Index map of Antarctica.
the Dominion Range. When Beardmore Glacier thickened (grounding of the Ross Ice Shelf), Rutkowski Glacier contracted (decreased precipitation). Comparison of profile C-C" in figure 3 with profiles A-A" and B-B' in figures 2 and 3 of Prentice et al. (Antarctic Journal, this issue) shows that late Quaternary fluctuations of interior antarctic ice were far less severe than those of pre-late Quaternary time. Our inferences concerning late quaternary drift sheets are in substantial agreement with those of Mercer (1972). Mercer's Beardmore III drift is largely equivalent to our Beardmore drift, and his Beardmore II drift corresponds with our Meyer drift. 1986 REVIEW
Mercer (1972) likewise concluded that sea-level lowering and grounding of the Ross Ice Shelf caused thickening of the lower reaches of Beardmore Glacier shown by these two drifts. Hence, our Beardmore data and those of Mercer (1972) both support our conclusion based on field work further north that widespread grounding occurred in the area of the present Ross Ice Shelf and Ross Sea during late Quaternary global ice ages (Stuiver et al. 1981). This research was supported by National Science Foundation grant DPP 83-18808. We are very grateful to VXE-6 for helicopter support in the Beardmore Glacier area. 91
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$0 100 Km 0 20 40 SO I • •
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Source: Antarctic Map Folio
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Folio 12
Figure 2. Sketch map of the Beardmore Glacier region. C-C" shows position of longitudinal profile in figure 3. A-A" and B-B' show profiles in Prentice et at. (Antarctic Journal, this issue).
C C' C"
C" C"
3800 0 Ml,,imo,,, Ice eo,tac. of Dominion drill 3000
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IC. .o,faC. of PlonImI drift 17T771'7lll Present surface of Beardmore Gliol., 2800
-
113
—
0 -
•D
2000-:
Lu
0 20 40 80 80 100 120 140 180 180 200 220 240 Distance From Ross Ice Shell (Km)
Figure 3. Present and former longitudinal profiles of the surface of Beardmore Glacier. The position of C-C" shown in figure 2. Moraine evaluations are projected onto the profile from ice-free areas designated above the profiles.
92
References Bockheim, J. G., S.W. Wilson, and J. E. Leide. 1986. Soil development in the Beardmore Glacier region, Antarctica. Antarctic Journal of the U. S., 21(5). Denton, G.H., M. L. Prentice, and L. H. Burckle. In press. Late Cenozoic history of the Antarctic Ice Sheet. In R.J. Tingey (Ed.), The geology of Antarctica. Cambridge: Oxford University Press. Mayewski, P.A. 1975. Glacial geology and late Cenozoic history of the Transantarctic Mountains, Antarctica. (Institute of Polar Studies, Report No. 56.) Columbus: Ohio State University. Mercer, J.H. 1972. Some observations on the glacial geology of the Beardmore Glacier area. In R.J. Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. Prentice, ML., G.H. Denton, TV. Lowell, H.C. Hughes and L.E. Heusser. 1986, Pre-late Quaternary glaciation of the Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S. 21(5). Stuiver, M., G. H. Denton, T.J. Hughes, and J. L. Fastook, 1981. History of the marine ice sheet in West Antarctica during the last glaciation: A working hypothesis. InG.H. Denton, andT.J. Hughes, (Eds.), The last great ice sheets. New York: Wiley-Interscience.
ANTARCTIC JOURNAL
