ELSEVIER
Marine Micropaleontology 27 ( 1996) 253-27 1
Miocene and Pliocene paleoclimate of the Dry Valleys region,
Southern Victoria land: a geomorphological approach
David R. Marchant aT*,George H. Denton a,b
aInstitutefor Quaternary Studies, University of Maine, Orono, ME 04469, USA
’ Department of Geological Sciences, University of Maine, Orono, ME 04469, USA
Received 1 October 1994; accepted 1 December 1994
Abstract
The Dry Valleys region is a hyper-arid, cold polar desert. Modem climate varies systematically with increasing elevation and
distance from the coast. We distinguish three microclimate zones on the basis of varying precipitation, wind direction, relative
humidity, temperature, and soil-moisture content. Zone 1 represents coastal, Zone 2 intermediate, and Zone 3 far-western areas
of the Dry Valleys region.
Soil-moisture content and relative humidity are the key parameters that control the area1 distribution of solifluction terraces,
gelifluction lobes, polygonal ground, scree slopes, and soil development in Zones 1,2 and 3. The coastal Zone I shows active
solifluction terraces, gelifluction lobes, levees, streams, debris flows, mudflows, and subxerous soils. The intermediate Zone 2
contains little evidence for modem downslope movement; here active gelifluction lobes, debris flows, and streams are largely
restricted to north-facing slopes with high moisture content. The inland Zone 3 lacks evidence for significant modem downslope
movement. There are no active solifluction terraces, stream channels, debris flows, or levees in Zone 3. Instead, Zone 3 shows
Miocene- and Pliocene-age sand wedges, avalanche cones, and desert pavements. The mid-Miocene landsurface of Zone 3 is
preserved to a remarkable degree. The antiquity and longevity of paleoforms in Zone 3 can be readily demonstrated by the
topographic position of dated ashfall deposits. Our chronology comes from laser-fusion 40Ar/39Ar analyses of in-situ ashfall
deposits that rest at, or just below, the ground surface in Zones 2 and 3 (Marchant et al., 1993a,b,c, 1995).
The lack of gelifluction lobes, solifluction terraces, rills, levees, and stream channels on in-situ Miocene- and Pliocene-age
deposits in Zone 3 indicates that here mean-annual air temperature, soil moisture content, and relative humidity did not reach
levels that now occur in Zones 1 and 2. The present mean-annual air temperature and relative humidity of Zones 1 and 2 are
about - 17”C/ 75% and - 27”C/ 45%. respectively. The implication is that climatic warming of the magnitude necessary for
East Antarctic Ice Sheet deglaciation predicted by some glaciological models (e.g., about 20°C above present values according
to Huybrechts, 1993) and growth of vascular vegetation in the Transantarctic Mountains (e.g., Webb and Harwood, 1993)
could not have occurred during Pliocene time. In addition, the preservation of Miocene-age ashfall deposits, avalanche cones,
and delicate desert pavements strongly suggest that no wet-based, erosive glaciers advanced into the far western Dry Valleys
region above 1200 m elevation during late Pliocene time. Overall, our paleoclimate record from the Dry Valleys region implies
an enduring East Antarctic Ice Sheet since Middle Miocene time. This makes it difficult to ascribe large-scale Pliocene sea-level
fluctuations to ice-volume variations on the East Antarctic craton.
* Present address: Department of Barth Sciences, Boston University, 675 Commonwealth Avenue, Boston, MA 022 IS, USA
0377-8398/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved
.SSDlO377-8398(95)00065-8
254
D.R. Marchant, G.H. Denton /Marine Micropaleontology 27 (1996) 253-271
1. Introduction
A detailed late Cenozoic paleoclimate record for the
Transantarctic Mountains is necessary background to
assess the problem of East Antarctic Ice Sheet stability
during Pliocene time. Our purpose here is to present
geomorphic data from the Dry Valleys region of southern Victoria Land that indicate persistent cold-desert
conditions and imply an enduring East Antarctic Ice
Sheet since middle Miocene time. We acknowledge
that some warming probably occurred in the Dry Valleys region during Pliocene time, but that this warming
was likely less than 3°C to 8°C (Marchant et al., 1993a;
Denton et al., 1993) and, as described below, was insufficient to cause significant geomorphic changellandscape evolution of the Dry Valleys region. Our
paleoclimate record does not incorporate the effects of
potential surface uplift in the Dry Valleys region. However, recent work by Wilch et al. (1993) and Denton
et al. (1993) show minimal surface uplift of the Dry
Valleys region during the last 3 Ma.
Two hypotheses have been developed with regard to
Pliocene paleoclimate and East Antarctic Ice Sheet
dynamics. Based on the ecology of marine diatoms and
Nothofagus (Southern Beech) wood within Sirius
Group glacial deposits in the Transantarctic Mountains,
one hypothesis holds that deglaciation of the East Antarctic craton occurred around 3.0 Ma as well as earlier
in Pliocene time (Webb et al., 1984; Webb and Harwood, 1987, 1991, 1993; Barrett et al., 1992; Hambrey
and Barrett, 1993; McKelvey et al., 1991). This
hypothesis relies on one fundamental assumption;
namely, that reworked marine diatoms now within Sirius Group deposits originated in ocean basins in the
interior of East Antarcticaand were subsequently transported into the Transantarctic Mountains (which then
supported Nothofagus) by an expanded East Antarctic
ice sheet. According to Huybrechts (1993), atmospheric temperatures during the interval(s) of limited
ice cover in East Antarctica was likely to have been at
least 20°C above present values to accommodate icesheet deglaciation. In sharp contrast, the other hypothesis holds that the East Antarctic Ice Sheet has been
robust and relatively stable since Middle Miocene time
(Shackleton and Kennett, 1975; Savin et al., 1975;
Miller et al., 1987; Kennett, 1982). This latter hypothesis is based on interpretations of the marine-oxygen
isotope record, which show no unambiguous evidence
for significant Pliocene deglaciation of East Antarctica
(Kennett and Hodell, 1993).
Resolution of the intertwined problems of ice-sheet
stability and paleoclimate has important implications
for records of eustatic sea-level change, ocean temperature, and atmospheric circulation (Cronin and Dowsett, 199 1) . If the hypothesis of East Antarctic Ice Sheet
deglaciation is correct, then the implication is that
future ice-sheet collapse might also occur in response
to postulated atmospheric warming in the next century
(Barrett et al., 1992). On the other hand, if the hypothesis of ice-sheet stability is correct, then the implication
is that East Antarctic Ice Sheet deglaciation from the
magnitude of atmospheric warming predicted to occur
in the next century is highly unlikely (Marchant et al.,
1993a).
2. Geologic setting
The Transantarctic Mountains form a major morphological and geological boundary between the East
and West Antarctic ice sheets (Dalziel et al., 1987).
The mountain front is adjacent to the Ross Embayment,
which represents a downfaulted region of thin continental crust (20-25 km thick, Kadmina et al., 1983)
that has experienced Cretaceous ( ?) through Cenozoic
extension (Cooper and Davey, 1987a, b; Fitzgerald et
al., 1986).
Glacier ice covers much of the Transantarctic Mountains, but there are a few predominantly ice-free areas
(Fig. 1) . The largest is the Dry Valleys region of southern Victoria Land, with 4000 km2 of high-relief, mountain desert topography between McMurdo Sound and
the East Antarctic polar plateau. The bedrock here consists of nearly flat-lying Devonian-to-Triassic sandstones, siltstones, and conglomerates of ‘the Beacon
Supergroup and a basement complex of lower Paleozoic igneous and metamorphic rocks, all of which are
intruded by Jurassic-age Ferrar Dolerite and Cenozoic
volcanics. Taylor, Wright, and Victoria are the main
valleys that transect the region. These valleys are separated by the Asgard/Olympus Ranges and the Quartermain Mountains. The mountains are highest at the
western rim of the Dry Valleys (where they exceed
2000 m elevation) and become progressively lower and
D.R. Marchant, G.H. Denton/Marine Micropaleontology 27 (1996) 253-271
7T30'
255
7T45'
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256
D.R. Marchant, G.H. Denton / Marine Micropaleontology 27 (I9%) 253-271
more dissected towards the coast. Mountain slopes
show a thin, patchy veneer of glacial till, talus, and
colluvium.
Large, offshore stratovolcanoes occur within 100 km
of the Dry Valleys region. Volcanic detritus recovered
in drill cores in the western Ross Embayment indicates
that volcanism may extend back to late Eocene time
(Kyle, 1990). The oldest dated ashfall in the Dry Valleys region is isotopically dated (40Ar/39Ar) at
15.15 40.02 Ma (Mat-chant et al., 1993~).
The Dry Valleys now feature a hyper-arid, colddesert climate. Details of the present Dry Valleys climate are scarce, but mean annual temperature and
precipitation, both recorded at 100 m elevation on the
floor of central Wright Valley, are about - 20°C and
80 mm water equivalent, respectively (Schwerdtfeger,
1984). Katabatic winds flow across intervalley mountain blocks towards the Ross Sea. Alpine glaciers occur
where wind-blown snow is concentrated in the lee of
topographic highs. Because they are small and occur in
a cold-desert climate, these glaciers are frozen to underlying beds and are nearly free of debris (Meserve Glacier in Wright Valley has a basal temperature of
- 18°C [Bull and Carnein, 19681). Glaciers and snow
patches now lose mass predominantly by sublimation,
although some melting occurs below about 1000 m
elevation ( < 10% of the total ablation, Chinn, 1980).
Although the entire Dry Valleys region is a hyper-arid
cold polar desert, there are substantial intravalley differences in climate (Keys, 1980). The modern climate
varies systematically with increasing elevation and distance from the coast. Mean annual temperature and
relative humidity are highest near the coast; katabatic
winds, which descend from the polar plateau, are
strongest inland; and precipitation is greatest near the
coast. Topography also plays a role in the Dry Valleys
climate. Winds of varying moisture content and temperature are directed and funnelled around topographic
highs.
3. Division of morphologic and climatic zones
Climate is a major factor in controlling landform
development. In the hyper-arid Dry Valleys region,
soil-moisture content and relative humidity are key parameters in controlling the area1 distribution of
solifluction terraces, gelifluction lobes, polygonal
ground, and scree slopes. Changes in relative humidity
and soil-moisture content, both strongly influenced by
atmospheric temperature and wind direction (Keys,
1980), regulate salt weathering, frost action, and soil
development (Campbell and Claridge, 1982). In turn,
these processes control rates of downslope movement,
fan development, and gullying. Westerly winds from
the polar plateau are very dry, with an average relative
humidity during summer months of 45% (Bull, 1966)
and a minimum of < 10% (Campbell and Claridge,
1987, p. 57; Chinn, 1980). In contrast, easterly winds
from the Ross Sea show an average relative humidity
during summer months of about 75% (Bull, 1966). We
distinguish three microclimate zones on the basis of
varying precipitation, wind direction, relative humidity, temperature, and soil-moisture content. Zone 1 represents coastal, Zone 2 intermediate, and Zone 3
far-western areas of the Dry Valleys region (Fig. 1) .
At issue is the relative geographic displacement of these
climate zones through time.
3.1. Zone 1
Zone 1 is the coastal zone. It includes the area
between sea level and about 1000 m elevation near Mt.
Barnes and descends iniand along valley bottoms to
about 100 m elevation at Lakes Bonney and Vanda
(Fig. 1). Relative humidity during summer months
averages about 75%, reflecting the predominance of
southeasterly winds, Precipitation probably exceeds 80
mm of water equivalent per year. Mean annual air temperature is similar to that of McMurdo Station on Ross
Island and is about - 17’C (Schwerdtfeger, 1984).
Given a lapse rate of l”C/ 100 m elevation rise (Prentice et al., 1993; Robin, 1988)) mean annual temperatures in Zone 1 are from about - 17°C at sea level to
about - 27°C at 1000 m elevation. Average Si8O values for snow patches and small glaciers in Zone 1 are
between - 25%0and - 30%0.
The relatively mild climate of Zone 1 permits development of gelifluction lobes, solifluction terraces, ice
wedges, rills, channels, debris flows, levees, and
ephemeral ponds, lakes, and rivers (Fig. 2). Soils
developed in Zone 1 are subxerous and contain salts
enriched in sodium chloride, reflecting the presence of
liquid water and easterly winds from the Ross Sea
D.R. Marchant, G.H. Denton / Marine Micropaleontology 27 (19%) 253-271
251
Fig. 2. Surface morphologic features common in Zone 1. Photograph taken near south stream at Marble Point. Small solifluction terraces occur
on distant slopes.
Fig. 3. Surface morphologic forms in Zone 2. Two fans on the south wall of central Wright Valley. Snowmelt on dolerite cliffs feed small
streams that now dissect these fans. Shorelines associated with high levels of Lake Vanda are superimposed on these fans.
Fig. 4. Example of the ground surface in Zone 3. Photograph shows west-central Arena Valley (about 1500 m elevation). Note absence of rills
and streams on dolerite-rich talus slopes.
(Campbell and Claridge, 1987). The depth to frozen
sediment is generally ~50 cm. Solifluction terraces
occur near the snout of Wright Lower Glacier (Nichols,
1968) and along the Onyx River flood plain (Selby,
I97 1) ; soil movement occurs within a saturated layer
that overlies ice-cemented ground (e.g. Dylik, 195 1).
The land surface of Zone 1 shows an array of highrelief polygons separated by deep V-shaped troughs.
Ice commonly fills such polygon troughs (Berg and
Black, 1966; Black, 1976; McSaveney
and McSaveney, 1972). This ice reflects the relatively high
moisture content of soils in Zone 1 and contrasts
sharply with polygon troughs in Zones 2 and 3, which
generally are filled with sand rather than with ice (see
below). Glaciers in Zone 1 show some surface melting
during summer months (about lo%, the rest by sublimation, Chinn, 1980).
3.2. Zone 2
Zone 2 is the intermediate zone. It includes moderate-to-low-elevation
areas in the central Dry Valleys
region and high-elevation areas near the coast. At the
coast, the lower limit of Zone 2 is about 1000 m; Zone
2 includes all summits of the eastern Asgard/ Olympus
258
D.R. Marchant, G.H. Denton /Marine Micropaleontology
Ranges. Inland, Zone 2 extends from the floor of central
Wright Valley to about 800 m elevation at the Dais/
Labyrinth (Fig. 1) . In Taylor Valley, Zone 2 extends
westward to the Cavendish Rocks (Fig. 1). Relative
humidity is quite variable, probably ranging from about
10% to 70%. This variation reflects the influence of
alternating westward-flowing katabatic winds and eastward-flowing winds from the Ross Sea. Mean annual
air temperatures probably range from about - 21” to
- 27°C in the west and from about - 23” to - 35°C in
the east (based on a lapse rate of l”C/ 100 m elevation
rise and a recorded mean-annual temperature of about
-20°C at 100 m elevation in central Wright Valley,
Schwerdtfeger, 1984). The predominance of katabatic
winds in Zone 2 ensure relatively cold and very dry
climatic conditions. Consequently,
meltwater is rare,
although some occurs on perennial snowbanks and glaciers (see below). Snowfall is less than in Zone 1 (Bull,
1966; Keys, 1980).
The climate of Zone 2 precludes significant soil
moisture. Gelifluction lobes, debris flows, levees, and
small streams are rare and are probably active only
during extreme climatic events (Chinn, 1980). On the
basis of field observations and air photograph interpretation, we calculate that less than 5% of the area in
Zone 2 has isolated streams and small ponds for two to
three weeks each year. These streams do not erode the
land surface significantly; they are essentially restricted
to north-facing slopes and where perennial snowbanks
and glaciers terminate on dark-colored rocks (predominantly dolerite and McMurdo
Group volcanics,
Fig. 3). Meltwater in Zone 2 contains a high concentration of water-soluble salts, principally solutions of
calcium and magnesium chlorides, with very low freezing points (Campbell and Claridge, 1987). In some
areas, saline meltwater migrates through soils at temperatures well below 0°C (Campbell and Claridge,
1982). These hyper-saline solutions may become concentrated on an impermeable layer, for example on the
interface between ice-cemented and unfrozen ground,
and facilitate slumping or accelerated down-slope
movement on otherwise stable slopes. This phenomenon is not widespread, and consequently the landsurface of Zone 2 is not dynamic. Instead, dry climatic
conditions favor the development and preservation of
extensive desert pavements and sand-wedge polygons.
The polygons, which form by periodic contraction of
perennially frozen ground, are similar to the ice-wedge
27 (1996) 253-271
polygons of Zone 1, except that inter-polygonal troughs
are generally filled with vertically stratified sand-andgravel deposits rather than with ice (Pew& 1959;
Svensson, 1988; Watson, 1981; Black, 1976; Berg and
Black, 1966).
3.3. Zone 3
Zone 3 is the inland zone. It includes all ice-free
areas above 800 m elevation along the western rim of
the Dry Valleys region (Fig. 1). Relative humidity
probably averages less than 45% and reflects the predominance of dry, westerly katabatic winds. Mean
annual temperatures are below -27°C; at the top of
high-elevation peaks, such as Mt. Feather (2965 m) in
the Quartermain Mountains, mean annual temperatures
are below - 35” to - 40°C. Precipitation is rare, but
snow blown off the polar plateau accumulates on small
glaciers and perennial snow banks in the lee of topographic obstacles (Keys, 1980). Average 6’*0 values
for snow patches at about 1700 m elevation in the
Quartermain Mountains are from - 30%0 to - 40%0
(Sugden et al., 1995). Meltwater is absent in Zone 3.
Glaciers and drifted snowbanks lose mass entirely by
sublimation. Relative soil moisture content is < 1.5%
(weight percent, Table 1; Cameron and Conrow,
1969). Importantly, the extreme hyper-arid climate of
the far-western Dry Valleys region stems from persistent katabatic winds, which are contingent on the presence of a large, inland ice sheet. The cold and dry winds
that descend from the polar plateau flow out across
Zone 3 and prevent moist easterly winds from penetrating far into the Dry Valleys region.
The hyper-arid and very cold environmental conditions of the western Dry Valleys region favor the development of sand-wedge polygons with tightly knit
ventifact pavements along with the preservation of thin
talus relicts unmarked by rills, channels, mudflows, and
levees; surface clasts show extensive secondary quartz
overgrowths
(quartzification
of Weed and Norton,
1991) and desert varnish. There are no solifluction
terraces (Fig. 4). Active polygons in Zone 3 are areally
restricted to the margins of alpine glaciers and perennial
snowbanks, or to unsorted rock debris on stagnant ice.
Upstanding cobbles and boulders commonly line the
margins of these active troughs. Relict polygons are
only recognizable in stratigraphic sections as V-shaped,
forms
‘Data from tables 34 and 35 in Keys ( 1979).
‘Data from Cameron and Conrow ( 1969).
both in-situ and reworked ashfall
< 7.1 Ma and may be active today; in
Taylor Valley, 7.1 Ma ashfall is incorpo
rated into gelifluction lobes at 600 to
800 m elevation.
None
< 12,000 yrs; morphologic forms occur
superimposed on Ross Sea drift.
in-situ ashfall
Relict landsurface > 10 to 15 Ma.
Complete preservation of in-situ
ashfall in sand wedges and on desert
pavements.
> 100 cm
I .04%
45cmto
3.51%
<45 cm
12.73%
LOOcm
absent
absent
absent
absent
absent
absent
absent
absent
absent
absent
absent
rare
common
rare
common
Zone 3 Inland
absent
rare
uncommon
uncommon
uncommon
uncommon
uncommon
uncommon
uncommon
uncommon
uncommon
common
rare
uncommon
common
Zone 2 Intermediate
common
common
common
common
common
common
common
common
common
common
common
uncommon
absent
common
rare
I Coastal
I, 2, and 3
Zone
forms in Zones
Solifluction terraces
Gelifluction lobes
Debris flows
Rills
Levees
Gullies
Channels
Lakes
Ephemeral ponds
Streams
Subxerous soils
Xerous soils
Ultraxerous soils
Surface-salt encrustations’ Halite (NaCI)
Surface-salt encrustations’ Epsomite
( MgSO, 7Hz0)
Depth to ice-cemented soil
Average in-situ soil moisture (weight
percent)’
Ashfall deposits
AGE
Surface morphologic
Table 1
Summary of surface morphologic
260
D.R. Marchant, G.H. Denton /Marine Micropaleontology 27 (I9%) 2.53-271
vertically stratified sand-and-gravel
deposits (Marchant et al., 1993b,c; McSaveney and McSaveney,
1972). Soils developed in Zone 3 are ultraxerous and
contain salts enriched in nitrates, reflecting the influence of westerly katabatic winds from the polar plateau
(Campbell and Claridge, 1987; Bockheim, 1990). The
depth to frozen ground generally exceeds 100 cm (The
top meter of sediment in Zone 3 lacks ice; there is no
active layer in Zone 3).
4. Age of unconsolidated
landforms
4.1. Zone I
Zone 1 contains the youngest suite of landforms in
the Dry Valleys. Here, most solifluction terraces, gelifluction lobes, levees, rills, and fans are unequivocally
younger than 12,000 years old, because they occur
superimposed on late Wisconsin Ross Sea drift (Denton et al., 1989). It is likely that all such unconsolidated
landforms throughout Zone 1 are evolving today; but
it is not immediately clear if the landforms on older
deposits within Zones 2 and 3 are active or relict. To
determine the age of surface landforms in Zones 2 and
3, we employed laser-fusion 40Ar/39Ar analyses of insitu ashfall deposits that rest on, or just below, the
surface of unconsolidated
deposits in the central and
western
Dry Valleys
region
(Marchant
et al.,
1993a,b,c, 1995).
Table 2 lists the isotopic age and depositional setting of several ashfall deposits in the Dry Valleys
region. The ashes are all concentrated in Zones 2 and
3 and are greater than 3.9 Ma old. The absence of
surface ash in Zone 1 is consistent with the relatively
high rate of solifluction/gelifluction
and with the fact
that most unconsolidated
sediment along the coast is
of late Wisconsin age (Denton et al., 1989).
In our opinion, most ash deposits in Zones 2 and 3
are in-situ to near in-situ and represent direct ashfall
during volcanic eruptions. They contain very little
detrital contamination
(basal sections show less than
5% non-volcanic contamination,
e.g. Marchant et al.,
1993a). Glass shards exhibit delicate spires and intact
bubble vesicles, which indicate limited reworking.
Ashes of different isotopic ages and geochemical com-
positions are not mixed together, as one would expect
if the ashes were reworked and retransported by wind.
Finally, as reported in detail elsewhere (Marchant et
al., 1996) the bi-modal grain size distribution and poor
sorting of the ash deposits indicates limited winnowing
by wind. Such limited winnowing is consistent with
ashfall deposition and inconsistent with aeolian erosion
and redeposition. We recognize that some ash deposits,
particularly those that fell into active contraction cracks
(see below), may have been displaced 5 cm to 10 cm
by continued ground contraction following ashfall deposition. Even so, the use of the term in-situ is valid
because the ashes have not been retransported since
initial deposition; they still rest within contraction
cracks (see below). In this regard, ashfall deposits of
the Dry Valleys region are similar to many late Quaternary ashfall deposits in New Zealand, Iceland, and
the Pacific Northwest USA. Such deposits are considered in-situ in many places and are important stratigraphic markers even though they have been disturbed
slightly by bioturbation (roots, worms, burrows, etc) .
4.2. Zone 2
Zone 2 contains ashes of late Miocene to mid-Pliocene age. In Wright Valley, ashfall deposits are concentrated on the north and south valley walls between
about 300 and 500 m elevation (near the Hart, Bartley,
and Goodspeed Glaciers), The ashes crop out just
above the upper limit of climate Zone I. They rest on
steep (25”) slopes and are overlain by 10 to 25 cm of
glacial drift with erratics (Hall et al., 1993) or a thin
ventifact pavement 1 to 2 cm thick. Glass shards
removed from in-situ ash near the Hart Glacier are
dated by the potassium-argon
method to 3.9 + 0.3 Ma
(Hall et al., 1993). The preservation of in-situ ashfall
is consistent with limited mass wasting (gelifluction,
solifluction, or slumping) since mid-Pliocene time. In
Taylor Valley, ash of late Miocene age (7.1 + 0.35 Ma)
crops out on the north valley wall between 800 and
1050 m elevation. At its upper-elevation limit, the ash
is in place and rests on ancient colluvium and till. At
lower elevations, particularly on slopes exceeding 20”,
the ash is disseminated in large gelifluction lobes that
extend to within 300 m of the northern margin of Taylor
Glacier. The lobes have been active during the last 7.1
Ma, and may be active today; but the presence of 7.1
D. R. Marchunt, G. H. Denton / Marine Micropaleontology
27 (1996) 253-271
261
Table 2
‘“‘Ar/S”Ar analyses Dry Valley ashes (adapted
Sample
‘7&,29Ar
% j"Ar
‘6Ar/39Ar
40Ar / “Ar
%40Ar
0.00023
0.00020
0.00001
0.00032
0.00052
0.00019
0.00005
0.00015
0.00027
0.0003 1
0.00013
0.00023
0.00018
0.00021
0.00003
0.00009
0.00008
0.00025
0.8177
0.8257
0.8672
0.8201
0.8277
0.8824
0.8697
0.8524
0.8278
0.8320
0.8648
0.8406
0.8540
0.8272
0.8844
0.8578
92.4
4.191
4.232
93.3
99.7
4.444
4.203
98.4
84.3
4.242
4.522
94.1
4.457
98.2
94.9
4.368
4.242
91.1
4.264
90.0
4.432
96.0
92.5
4.308
4.377
94.1
4.239
93.1
4.532
99.3
4.396
97.0
4.375
97.2
4.347
91.9
Weighted mean = 4.33 f 0.07
15.8278
15.9122
15.8924
15.9245
15.9054
97.4
99.3
97.7
97.4
13.589
13.661
13.644
13.672
0.0000
0.00145
0.00040
0.00126
0.00146
0.00052
0.1593
0.6002
0.4866
0.1247
0.1654
0.1388
0.1292
0.1485
0.0829
0.0841
0.1364
0.00498
0.00247
0.00540
0.00141
0.00166
0.00185
0.00155
0.02264
0.04104
0.00216
0.00365
17.4518
16.9513
16.5883
16.9101
17.3040
17.2699
17.1905
17.1369
17.1700
17.1869
17.1088
92.3
96.1
91.4
97.6
97.3
97.0
97.5
72.0
58.6
96.5
94.1
Plateau age =
14.978
14.550
14.240
14.5 15
14.851
14.822
14.754
14.709
14.737
14.75 1
14.685
14.75 * 0.03 SE
0.0984
0.1368
0.1135
0.1176
0.1050
0.0998
0.1086
0.0798
0.0734
0.1071
0.00201
0.00151
0.00039
0.00199
0.00224
o.M)197
0.00108
0.00475
0.00269
0.00148
18.0002
17.2810
17.4501
17.6940
17.7621
17.5596
17.2757
17.4196
17.2758
17.3039
15.446
96.8
14.832
97.5
14.976
99.4
15.185
96.8
15.243
96.5
15.070
96.8
14.827
98.2
14.950
92.6
14.827
95.6
14.851
97.6
Weighted mean= 15.15f0.02
Ash on buried desert pavement
DMS-8686B (scmidine/6lA2)
5075-O I
0.0302
5075-02
0.0381
5072-03
0.0485
5075-04
0.0189
5075-05
0.0266
5075-06
0.0305
5075-07
0.0259
5075-08
0.0200
5075-09
0.0335
5075-10
0.0370
5075. I I
0.0388
5075- I2
0.0349
5075-13
0.0407
5075-14
0.02 14
5075-15
0.0444
5075-16
0.0314
5075. I7
0.0307
1075-18
0.01 I8
Ash-filled sand wedges
DME9 I -4 I (sanidine/98A
7 174-02
7 174-07
7 174-08
7 174-09
7174-10
DME91-41
7185-OIA
7185.OIB
7185-OIC
7185.OlD
7185-OIE
7185.OIF
7185-016
7185.OIH
7185-011
7185-015
7185.OlK
(glass/98A)
14.5
I.0
0.5
1.3
9.6
15.9
9.5
6.4
18.2
8.3
15.1
DM.S9 l-22 (scmidine/98A)
7183-01
7 183-02
7183-04
7 183-05
7183-06
7183-07
7183-09
7183-10
7183-1 I
7183-12
from Marchant et al., 1993h,c, 1996)
0.8538
0.8482
Age (Ma)
SD
0.203
0.237
0.250
0.239
0.278
0.554
0.422
0.283
0.418
0.528
0.27 I
0.293
0.334
0.495
0.366
0.169
0.359
0.409
SE
)
0.0213
0.0582
0.0000
0.0000
0.197
0.147
0.095
0.187
13.655
0.119
99.0
Weighted mean = 13.65 + 0.06 SE
0.302
0.243
0.417
0.175
0.061
0.072
0.065
0.090
0.093
0.057
0.055
0.045
0.145
0.187
0.063
0.073
0.064
0.059
0.148
0.144
0.110
SE
262
Sample
D.R. Marchant,
%‘“Ar
=Ar/ 29Ar
Analvses not used in mean calculation
13.0090
7 IS,03
0.1086
7183-08
0.1126
7183-13
DMS91-22 (gluss/98A)
7179-OIA
9.3
0.8831
7179.OlB
21.8
0.1235
0.1314
7179.OIC
33.2
7179-OlD
22.4
0.1285
7179-OlE
4.7
0.1347
7179.OlF
1.9
0.1069
7179-016
I .4
0.1751
7179.OlH
I.1
7179-011
2.4
0.2166
0.1167
7179-013
1.5
7179-OIK
0.4
4.4100
G.H. Denton /Marine
Micropaleontology
27 (1996) 253-271
“Ar13’Ar
@A#“Ar
%40Ar
Age (Ma)
SD
0.00516
0.00112
0.00354
13.2935
20.3787
23.1014
96.6
98.4
95.7
11.420
0.07 I
0.077
0.102
0.10562
0.05 125
0.02563
0.01316
0.01435
0.02336
0.03174
0.04576
0.0435 1
0.02872
0.08557
16.0059
16.5810
16.5815
16.6009
15.9215
18.3097
15.0362
13.5650
15.1982
17.0089
12.3835
33.9
52.3
68.7
81.1
79.0
72.6
61.6
50.1
54.2
66.7
33.1
Plateau age =
13.741
0.290
14.233
0.122
14.234
0.090
14.250
0.079
13.669
0.193
15.71 I
0.494
12.912
0.686
11.653
0.823
13.050
0.442
14.599
0.538
10.641
2.378
14.24 f 0.05 SE
17.478
19.800
DMS9 I- 107 (scmidine)
7 182-02
12.5394
7 182-04
10.7856
7182-05
IO.4845
7 182-06
9.5960
7 182-08
IO.4079
7 182-09
9.6967
1782-10
10.7869
I .55483
0.02569
0.01234
0.04598
0.00000
0.07168
0.13244
0.01119
0.00474
0.00372
0.00133
0.00368
0.00089
0.00477
9.368
9.387
9.387
9.206
9.319
9.439
9.389
74.6
8.055
87.0
8.071
89.5
8.072
95.9
7.916
89.5
8.013
97.3
8.116
87.0
8.073
Weighted Mean 8.07 f 0.06
Arithmetic Mean 8.05 + 0.07
0.293
0.222
0.184
0.158
0.477
0.080
0.208
NfS88-2 (sunidine)
2497-O 1
4.4866
2497-02
3.1188
2497-03
7.9987
2497-04
3.1251
2497-08
3.1431
2497-09
3.3614
2497. IO
3.1806
0.04201
0.23502
0.04416
0.02010
0.15182
0.03697
0.00638
0.0005 1
0.01614
0.00117
0.00097
0.00188
0.00120
2.985
2.968
3.247
2.779
2.855
2.817
2.826
61.3
8.026
95.2
7.981
40.6
8.729
88.9
7.475
90.8
7.676
7.577
83.8
88.9
7.601
Weighted Mean 7.87 f 0.43
Arithmetic Mean 7.78 + 0.05
0.402
0.083
0.346
0.123
0.093
0.170
0.196
DMS89-132B
3180-01
3 180-02
3 180-04
3 180-05
3 180-06
0.0100
0.0240
0.0136
0.0106
0.0128
0.00034
0.9170
0.9418
0.9453
0.9721
0.9817
89.9
9.767
0.063
88.7
10.031
0.069
90.2
10.068
0.069
94.4
10.353
0.066
97.0
IO.454
0.072
Weighted mean = 10.12 f 0.03 SE
0.0780
0.0767
0.0759
0.0809
0.0760
0.0720
0.06607
0.07810
0.00852
0.02101
0.00116
0.00083
2.0449
2.0312
1.9960
I .9841
2.0296
2.0563
10.237
I .595
9.5
0.739
8.1
10.169
44.3
9.993
0.224
24.2
9.934
0.979
85.8
10.161
0.278
0.5 16
89.5
10.294
Plateau age = IO.08 i 0.17 SE
DMS90-36B
511 I-OIA
511 I-OIB
51 I I-01c
Sill-OID
511 I-01E
511 I-OIF
(xmidine/41B)
8.4
26.6
26.8
13.6
16.0
8.7
D.R. Marchant, G.H. Denton / Marine Micropaleontology 27 (1996) 253-271
263
Table 2 (continued)
Sample
%3”Ar
=‘Ar/=Ar
3=Ar/39Ar
“OAr/ “Ar
%aAr
0.0218
0.0438
0.0370
0.0304
0.0182
0.0003 1
0.00086
0.00072
0.00021
0.00249
2.4285
2.3712
2.4049
2.4401
2.4570
96.5
12.151
0.193
90.4
11.865
0.277
92.0
12.033
0.290
97.6
12.209
0.380
76.8
12.137
0.927
Weighted mean = 12.07 rt 0.03 SE
0.00175
0.00078
0.00102
4.2405
4.4076
4.4810
0.0643
0.0340
0.0174
0.0176
0.0365
0.0313
0.00416
0.00262
0.00738
0.00358
0.00238
0.01340
57.5321
57.7051
75.8994
87.4462
98.9741
102.5835
89.2
10.894
0.104
95.1
11.322
0.088
93.8
11.510
0.085
Weighted mean = 11.28 f 0.05 SE
97.9
142.476
0.576
98.7
142.888
0.440
97.2
185.690
0.334
98.8
212.337
0.365
99.3
238.553
0.799
0.846
96.3
246.683
(‘&ss/illB)
0.5
22.1
54.4
20.8
2.0
0.2980
0.1209
0.1142
0.1366
0.4047
0.00081
0.00079
0.00031
0.00005
0.00079
2.8567
2.4965
2.5066
2.4822
2.3834
92.9
91.7
96.8
99.7
92.0
Plateau age =
14.718
12.369
12.920
12.795
12.287
12.90 f 0.06 SE
3.439
0.146
0.060
0.141
2.688
Sample
40139
37/39
36139
40139
%Rad.
Age
lrrad.
J
J”Ar
7W.S87-142C
1795-09
1795-04
1795-06
1795-07
1795-05
1795-10
1795-08
1795-O I
3.0914
3.1682
3.1026
3.0367
0.1430
3.1762
3.0782
3.1755
0.16183
0.09562
0.13082
0.12588
0.10361
0.08127
0.11593
0.11376
0.00129
0.00141
0.00119
0.00092
0.00126
0.00136
0.00101
0.00130
2.7219
2.7579
2.7606
2.7737
2.7769
2.7779
2.7864
2.7994
88.0
87.0
89.0
91.4
88.3
87.5
90.5
88.1
7.000
7.093
7.100
7.133
7.142
7.144
7.165
7.131
20
20
20
20
20
20
20
20
0.00142836
0.00142838
0.00142836
0.00142836
0.00142835
0.00142836
0.00142835
0.00142836
0.529
0.818
I.052
I.469
0.826
0.678
0.969
1.615
DMSIO-38B
5095-01
5095-02
5095-03
5095-04
5095-05
Arena Valley ash avalance deposit
DMS87-113 (sanidine/20)
1794-01
0.1144
1794-08
0.0432
1794-02
0.1265
1794-04
1794-06
1794-03
1794-09
1794-05
1794-07
DMS86-113
5089-01 A
5089-OIB
5089-O IC
5089-01 D
5089-OlE
Age (Ma)
SD
All sanidine 4”Ar/3”Ar ages are total fusion analyses.
lrradiation data: 20, J =0.001428 _LO.OOOOOl;41B, J= 0.005920 +0.000009; 61B, J = 0.002783 f0.000002;
61A2. J =0.002X44 f 0.000004:
98A, J = 0.000478 f 0.000001.
Note: ‘“Ar/“Ar
analyses of Dry Valleys ashes (from Marchant et al., 1993b,c, 1995). 40Ar/39Ar laser total-fusion of volcanic crystals and
incremental heating of volcanic glasses follow previously outlined methods and procedures in Marchant et al. ( 1993a) and Swisheret al. ( 1992).
All ages are based on a date of 27.84 Ma for the Fish Canyon Sanidine monitor mineral as intercalibrated at the Berkeley Geochronology Center
with MMhb- I, which has a published age of 520.4 + 1.7 Ma (Samson and Alexander, 1987). To obtain an accurate age we analyzed one to
three sanidine crystals for each date (weighted means are based on several dates). To verify that crystals were related to the vitric component
of each ash, we dated volcanic glass in several of the samples. Where both sanidine and glass were analyzed, none of the glasses yielded
significantly different ages from the crystal dates. We consider these findings to verify that the crystals and glass were most likely co-magmatic,
and pertain to the same volcanic event. The slight variation in age between analyses of crystals and glass shards is in some cases likely due to
the mobility and subsequent uniform loss of potassium during low-temperature hydration (Cerling et al., 1985).
264
D.R. Marchant, G.H. Denton / Marine Micropaleontology 27 (1996) 253-271
million year old ash on steep walls indicates that the
rate of mass wasting has been very low and that such
downslope movement has had minimal effect on slope
evolution in central Taylor Valley during the last 7.1
Ma (see also Sugden et al., 1995).
4.3. Zone 3
Ashfalls of middle Miocene and mid-Pliocene age
occur in Zone 3. These ashes are most common within
sand-wedge troughs, which are excellent geomorphic
traps (Marchant et al., 1995). The premise is that volcanic ashfall is trapped in deep, V-shaped thermal contraction cracks, just as modern snowfall is trapped in
active contraction cracks today (Fig. 5). As the loci of
thermal contraction sites vary over time, inactive/relict
troughs are infilled with slumped sand-and-gravel
deposits that cover and protect previous ashfall (Marchant et al., 1993b,c, 1995). Individual sand wedges in
the western Asgard Range contain ash isotopically
dated
at
15.15+0.02
Ma,
13.65kO.06
Ma,
12.07f0.13
Ma, 10.12+0.03,
and 10.08_+0.17 Ma.
Likewise, ash within three different sand wedges in
Beacon Valley is isotopically dated at 10.66 + 0.29 Ma,
8.07_+0.06Ma,and7.87&-0.43Ma(Table
2;Fig. 1).
As shown in Fig. 5, ancient sand wedges in the western Asgard Range and the Quartermain Mountains preserve detailed sedimentary structures that date back to
Miocene time. These include the V-shaped wedge
form, the near-vertical layering of sands and gravels,
and the sharp stratigraphic contacts with enclosed volcanic ashfall. There is no evidence for extensive solifluction or development of an active (liquid) surface
layer following ash deposition in any of the dated sandwedge deposits. Likewise, there is no evidence for i.e.
rills, channels, mudflows, solifluction terraces, or levees superimposed on unconsolidated
deposits that are
coeval and older than in-situ Miocene ash deposits. One
implication is that the relatively warm and wet climate
conditions necessary for development of such unconsolidated landforms have not occurred in the western
Asgard Range during the last 15.0 Ma.
Volcanic ash in Zone 3 is also concentrated in lobate
avalanche deposits on valley walls inclined up to 30
(Marchant et al., 1993b). One example is on the west
wall of lower Arena Valley in the Quartermain Mountains (Fig. 6). Here, an avalanche deposit extends
downslope from a narrow bedrock couloir at about
1650 to 1625 m elevation and terminates at 1100 m
elevation. The avalanche cone stratigraphically overlies older colluvium devoid of volcanic ash. In handdug sections, the ash-avalanchedeposit
shows a chaotic
mixture of unweathered sandstone gravel ( 10%)) dolerite ventifacts ( 15%), quartz sand and dolerite grus
(30%)) granite erratics ( < I %) , and coarse-grained
(1.0-1.5 mm) volcanic ash (45%); sandstone, dolerite, and granite clasts show no preferred orientation.
Volcanic ash within the avalanche deposit is isotopitally dated to 11.28 f 0.05 Ma (Table 2). Soils within
the ash-avalanchedeposit
contain thick salt pans (Stage
5 of Bockheim, 1990) that are consistent with the
inferred age of the deposit. Several in-situ moraines
associated with Quatemary fluctuations of Taylor Glacier unconformably
overlie the ash-avalanche deposit
(Marchant et al., 1994; Fig. 7).
The geomorphic setting, morphology, and poor sorting together suggest rapid emplacement of the Arena
Valley ash-avalanche deposit over pre-existing colluvium. The avalanche most likely originated from collapse of unstable accumulations of volcanic ash (either
trapped in bedrock couloirs or resting on oversteepened
valley slopes). It probably formed near the time of
volcanic eruption because katabatic winds would have
dispersed unprotected and exposed volcanic ash, and
because ash probably would have become unstable and
avalanched shortly after initial buildup in steep bedrock
couloirs. Hence the lobate avalanche deposit in lower
Arena Valley probably formed about 11.3 Ma ago (isotopic age of enclosed ash, Table 2). The slopes of
Arena Valley are unmarked by traces of liquid water.
There are no mudflows, channels, or solifluction terraces superimposed either on the avalanche deposit or
on adjacent colluvium, which by stratigraphic considerations must antedate 11.28 f 0.05 Ma.
In-situ ashfall in Zone 3 also occurs directly on ventifact pavements at two localities. One example is in
Arena Valley, where a 4.33 f0.07 Ma ashfall rests on
a tightly knit desert pavement at about 1400 m elevation. A modern pavement overlies the ash and prevents
aeolian deflation (Marchant et al., 1993a). The preservation of the underlying pavement and detailed sedimentary structures within the ash deposit (Marchant
et al., 1993a), along with the absence of geomorphic
evidence for liquid water on the surface of unconsolidated deposits adjacent to the ashfall deposit, indicate
D.R. Marchant, G.H. Denton /Marine Micropaleontology 27 (19%) 253-271
265
Fig. 5. (a) Active sand wedge polygons in Zone 3; modem snowfall fills troughs between polygon centers. (b) Oblique photograph of the land
surface in Beacon Valley showing ash trapped in a relict sand wedge (the dole&e-rich desert pavement has been scraped off to reveal the outline
of the underlying ash wedge. White plaque is about 35 cm wide. (c) Cross section across sand wedge shown in (b). The ash wedge is about 35
cm wide at the surface. (d) Cross section across a relict sand wedge in Koenig Valley (Koenig Valley Ash, 13.65 f 0.06 Ma, Marchant et al.,
1993b and Table 2). Ash (center stripe) is about 10 cm wide. Note vertically oriented ventifacts to the left of the ash.
that the surface morphology of central Arena Valley
antedates 4.33 + 0.07 Ma (Marchant et al., 1993a,b).
In summary, gelifluction lobes, solifluction terraces,
channels, rills, and levees are active along the coast in
Zone 1. Such landforms are rare in Zone 2, but are
concentrated locally in favorable areas. Under the present climate of Zone 2 these landforms are either inactive
or are forming at extremely slow rates. The landsurface
in Zone 3 is essentially paralyzed under persistent
hyper-arid, cold-desert conditions. Apart from minor
headword erosion of some glacial valleys, very limited
wind deflation and polygon development, the land surface in Zone 3 had taken on its present day form by
Middle Miocene time. Campbell and Claridge ( 1987)
reached a similar conclusion on the basis of ancient
soils preserved on steep valley slopes that occur at
266
D.R. Marchant, G.H. Denton /Marine Micropaleontology 27 (1996) 253-271
inland sites adjacent to the East Antarctic Ice Sheet.
We note that there is some geomorphic evidence for
warmer-than-present climates in Zone 3. This evidence
consists of widespread glacial tills that we infer (on the
basis of underlying striated bedrock, till texture, and
morphologic form of moraines) were deposited from
wet-based ice. However, these tills are isotopically
dated (by 40Ar/39Ar analyses of overlying ashfall
deposits) at > 15.0 Ma (i.e. Sessrumnir till, Mat-chant
et al., 1993b,c).
5. Discussion
Fig. 6. Oblique area1 view of ash-avalanche deposit in lower Arena
Valley. The avalanche deposit is overlain by moraines and drift
associated with Plio-Pleistocene
advances of Taylor Glacier into
Arena Valley ( Marchant et al., 1994). Ash within the avalanche lobe
is isotopically dated at 11.28 f 0.05 Ma (DMS-86- 113,Table 2).
5. I. Miocene and Pliocene paleoclimate
The preservation of in-situ Miocene and Pliocene
ashfall deposits in the far-western Dry Valleys region
Fig. 7. Stratified, 7.1 Ma ashfall deposit in Central Taylor Valley (location no. 2 in Fig. 1; TWE-87- 142C). The ash is ca. 45 cm thick (Wilch,
1991).
D.R. Marchant, G.H. Denton /Marine Micropaleontology 27 (1996) 253-271
indicates that the less severe climate of the coastal zone,
which permits extensive downslope movement and
active surface layers, did not occur in the Asgard/
Olympus Ranges, or in the Quartermain Mountains,
during Pliocene time. If atmospheric temperature and
relative humidity had risen to levels now experienced
at the coast, then ashfall deposits and unconsolidated
diamictons in Zone 3 would show evidence for solifluction, gelifluction,
and erosion by liquid water,
which we show here is not the case. One cannot argue
that such features formed in the far-western Dry Valleys region during Pliocene time and have since been
eroded; this is because the preservation of in-situ Miocene and Pliocene ashfall deposits, sand wedges, avalanche cones, and desert pavements indicates that the
surface morphology of Zone 3 is ancient and that it was
not modified extensively during late Miocene, Pliocene, or Quaternary time. Hence, the volcanic data of
Zone 3 strongly suggests persistent hyper-arid, cold
desert conditions since at least late Miocene time (see
also Table 1)
Ashfall deposits can also be used to resolve local
climate conditions (temperature and soil moisture) at
the time of volcanic eruption (Gerrard, 1985). For
example, sand wedges form only in cold, dry continental climates with mean annual temperature well below
- 6“ to - 8°C (Romanovskij,
1973; PCwt, 1959,1966,
1973; Black, 1976) and low relative humidity. Hence,
in-situ ashfall deposits that occur in sand-wedge
troughs in the western Asgard Range indicate cold and
dry
climatic
conditions
at
15.15 &-0.02 Ma,
13.65 TO.06 Ma, 12.07+0.03
Ma, 10.12f0.03,
and
10.08 f 0.17 Ma. Likewise, in-situ ashfall in wedges in
the Quartermain Mountains indicate cold and dry climates there at 10.66 + 0.29 Ma, 8.07 f 0.06 Ma, and
7.87 kO.43 Ma (Table 2; Fig. 1). The presence of
undeformed stratigraphic contacts between volcanic
ashfall and adjacent sand and gravel in all of these dated
sand-wedge troughs strongly suggests that the wedges
lacked associated ice lenses at the time of ashfall deposition; otherwise differential melting/sublimation
of
the ice lenses would have produced characteristic
deformation patterns. The implication is that very cold
and dry conditions (with average relative humidity
< 45%) prevailed in the western Asgard Range and in
the Quartermain Mountains at times of the dated volcanic eruptions.
267
Volcanic ashfall that rests directly on ventifact pavements also yields paleoclimate data. The Arena Valley
Ash indicates a very dry climate in central Arena Valley
at 4.33 $- 0.07 Ma. This is because the ash rests directly
on a tightly knit pavement of sandstone and doleritc
ventifacts, identical to modern pavements now forming
in this hyper-arid valley (Marchant et al., 1993a).
The chemical stability of in-situ ashfall deposits also
bears on the question of climate change and Norhofa~us
growth. Volcanic glass is chemically unstable at the
ground surface and alters to clay at a rate dependent on
atmospheric temperature and the abundance of pore
water (rates increase at high atmospheric temperatures
and high pore-water pressures; Lowe and Nelson, 1983;
Lowe, 1986). Under humid temperate conditions in
New Zealand, which are compatible with growth of
Nothofugus, volcanic ashes older than about 50,000
years have weathered to > 60% clay (Birrell and Pullar, 1973; Lowe and Nelson, 1983; Lowe, 1986). The
Dry Valleys ashes contain less than 5% clay, and the
volcanic crystals lack evidence for chemical etching
(Marchant et al., 1995). The absence of clay-sized
grains in Miocene and Pliocene surficial ash deposits
suggests that warm, wet climate conditions suitable for
Nothofugus growth did not occur in the Dry Valleys
region during the last 15 Ma.
5.2. Implications for ice sheet dynanzics
Two mutually exclusive hypothesis have been developed with regard to late Cenozoic East Antarctic ICC
Sheet dynamics. The first postulates limited ice cover
in East Antarctica during much of Pliocene time, followed by late Pliocene/early
Pleistocene glacial ovcrriding of the Transantarctic Mountains by a wet-based.
erosive ice sheet (Webb et al., 1984, 1986; Webb and
Hat-wood, 1987, 1991, 1993; Barrettet al.. 1992; Hambrey and Barrett, 1993). East Antarctic atmospheric
temperatures during intervals of limited ice cover were
likely at least 20°C above present values to accommodate interior seaways (Huybrechts,
1993) with warm
sea-surface temperatures (2” to 6°C. Harwood, 1986)
and growth of Nothofagus to 86”s latitude (Webb and
Harwood, 1993; and below). The second hypothesis
postulates an enduring East Antarctic ICC Sheet since
Middle Miocene time.
268
D.R. Marchant, G.H. Denton /Marine
Here we argue that preservation of Miocene-andPliocene-age
surlicial ashfall deposits, desert pavements, sand wedges, and avalanche lobes in the
far-western Dry Valleys region strongly suggests that
wet-based, erosive glaciers could not have covered
Zone 3 during late Pliocene/ early Pleistocene time.
The implication is that the wet-based glaciers required
for deposition of the Sirius Group, which crops out in
the Dry Valleys region at 2650 m elevation on Mt.
Feather in the Quartermain Mountains and at 2300 m
elevation on Mt. Fleming in the western Asgard Range,
must pre-date the isotopic age of the oldest in-situ surficial ash deposit. If this is correct, then Sirius Group
deposits at Mt. Feather and Mt. Fleming are probably
> 15.0 Ma old. Furthermore, because we see no evidence for significant climatic warming in the far-western Dry Valleys region during the last 15.0 Ma, we
argue that Pliocene collapse of the East Antarctic Ice
Sheet is unlikely. This is because the East Antarctic Ice
Sheet is terrestrial (grounded above sea level and not
influenced directly by changes in ocean temperature
and circulation), and therefore may only collapse from
extensive surface melting, which requires atmospheric
temperatures about 20°C above present values (Huybrechts, 1993; Marchant et al., 1993a). Finally, the
hyper-arid, cold desert climate of the extreme western
Dry Valleys region stems largely from dry katabatic
winds that descend from the polar plateau; hence,
hyper-aridity and ultraxerous soils of Zone 3 are most
likely contingent on the presence of an inland ice sheet.
Therefore, our data for continuous hyper-arid, colddesert conditions in the western Dry Valleys region call
for an enduring polar East Antarctic Ice Sheet since
Middle Miocene time.
6. Conclusions
The Dry Valleys region is a hyper-arid, cold polar
desert. Substantial variations of temperature, precipitation, wind, and relative humidity occur throughout
the region. Modern precipitation, temperature, and relative humidity are highest along the coast and diminish
inland. Easterly winds from the Ross Sea are moist,
and westerly katabatic winds, which descend from the
polar plateau, are extremely arid. Intravalley microcli-
Micropaleontology
27 (19%) 253-271
mate Zones 1,2, and 3 reflect variations in these climate
parameters.
Soil-moisture content and relative humidity are key
parameters that control the areal distribution of solifluction terraces, gelifluction lobes, polygonal ground,
scree slopes, and soil development. The coastal zone
(Zone 1) shows numerous active solifluction terraces,
gelifluction lobes, levees, streams, and debris flows.
The intermediate zone (Zone 2) features late Mioceneand Pliocene-age landforms, as well as some modem
gelifluction lobes and debris flows that occur sporadically on north-facing slopes and/or in protected areas
with high moisture content. The inland zone (Zone 3)
exhibits an ancient micro-topography.
There are no
solifluction terraces, streams, mudflows, or levees in
Zone 3. Instead the region shows Miocene-age sand
wedges, avalanche cones, and desert pavements that
are unmodified by active surface layers or erosion by
liquid meltwater.
The absence of solifluction terraces, rills, levees, and
stream channels on in-situ Miocene- and Pliocene-age
sediment in the far-western Dry Valleys region indicates that the relatively mild climate of Zones 1 and 2
(with mean annual temperature and relative humidity
of about - 17”C/ 75% and -27”Cl
45%, respectively) has not existed in the Quartermain Mountains
nor in the western Asgard/ Olympus Ranges during at
least the last 10.0 Ma. We recognize that some warming
may have occurred in the Dry Valleys region during
Pliocene time, but in Zone 3 this probably amounted to
a rise in mean annual air temperature of less than 3°C
(e.g., Marchant et al., 1993a). The implication is that
the warm climate conditions necessary for development of extensive surface-melting
ablation zones
(required for terrestrial ice-sheet collapse by most
models, e.g., Huybrechts,
1993; Marchant et al.,
1993a) and growth of vascular vegetation (e.g., Webb
and Harwood, 1993) in the Dry Valleys region could
not have occurred during Pliocene time. In addition,
the preservation of in-situ ashfall deposits, avalanche
cones, and delicate desert pavements, all of Miocene
and/or Pliocene age, strongly suggest that no wetbased, erosive glaciers advanced into the far western
Dry Valleys region (above 1200 m elevation) during
late Pliocene time (a requirement of the hypothesis of
ice sheet instability, see also Marchant et al., 1993b,c;
Denton et al., 1993). If this is correct, then deposition
of the Sirius Group in the Quartermain Mountains and
D. R. Marchant. G. H. Denton /Marine Micropaleontology 27 (19%) 253-271
the western Asgard Range during late Pliocene time is
unlikely. Overall, our paleoclimate record for the Dry
Valleys implies an enduring polar East Antarctic Ice
Sheet since Middle-Miocene time, and makes it difficult to ascribe large-scale Pliocene sea-level fluctuations (30 to 40 m sea level rise, Brigham-Grette and
Hopkins, 1994) to ice-volume variations on the East
Antarctic craton (e.g., Dowsett and Cronin, 1990;
Krantz, 1991; Cronin et al., 1995).
Acknowledgements
This work was funded and supported by the Division
of Polar Programs of the United States National
Science Foundation. Richard Kelly drafted Fig. 1.
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