Global and Planetary Change 79 (2011) 61–72
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Global and Planetary Change
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g l o p l a c h a
Elevated East Antarctic outlet glaciers during warmer-than-present climates in
southern Victoria Land
Kate M. Swanger a,⁎, David R. Marchant a, Joerg M. Schaefer b, Gisela Winckler b, James W. Head III
a
b
c
c
Department of Earth Sciences, Boston University, 675 Commonwealth Avenue, Boston, MA 02215, USA
Lamont-Doherty Earth Observatory, Route 9W, Palisades, NY 10964, USA
Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912, USA
a r t i c l e
i n f o
Article history:
Received 10 December 2010
Accepted 26 July 2011
Available online 3 August 2011
Keywords:
McMurdo Dry Valleys
Taylor Dome
Taylor Glacier
cosmogenic
Pliocene
Pleistocene
a b s t r a c t
We document Plio-Pleistocene changes in the level of Taylor Glacier, an outlet glacier in southern Victoria
Land that drains Taylor Dome on the periphery of the East Antarctic Ice Sheet (EAIS). Chronologic control
comes from 3He cosmogenic-nuclide analyses of 27 boulders sampled from drifts and moraines in Kennar
Valley, a small hanging valley that opens onto a peripheral lobe of Taylor Glacier in the Quartermain
Mountains. Assuming a constant boulder-erosion rate of 10 cm Myr−1, our preferred age model spans the last
3.1 Myr and calls for stepped ice recession from a local highstand ~ 200 m above the present base of Taylor
Glacier at the mouth of Kennar Valley. The texture and sedimentology of all mapped moraines and drifts
indicate deposition from cold-based ice, analogous with the modern Taylor Glacier at the mouth of Kennar
Valley.
The Kennar Valley glacial record shows an uncharacteristic relationship with average global temperatures,
exhibiting higher-than-present ice levels during globally warm periods, including the Pliocene climatic
optimum (~ 3.1 Ma) and Marine Oxygen Isotope Stage (MIS) 31 (~1.07 Ma). The Kennar Valley record also
suggests that the rate of ice-surface lowering accelerated after the mid-Pleistocene transition at ~0.9 Ma.
Correlation of our moraine record with published reports for fluctuations of Taylor Glacier elsewhere in the
Quartermain Mountains, and with a dated moraine record from Ferrar Glacier (a second outlet for Taylor
Dome), reveals similar ice-surface changes, highlighting minor, but widespread ice recession in southern
Victoria Land since the mid- to late-Pliocene. Our record for minimal variability in the East Antarctic Ice Sheet
contrasts with recent data from nearby marine cores that call for dynamic fluctuations in the volume of
grounded ice in the Ross Embayment, and significant reduction of the West Antarctic Ice Sheet (WAIS) during
warmer-than-present intervals. Taken together, these records from the Ross Embayment call for considerable
variation in the response of marine-based West Antarctic ice and terrestrial East Antarctic outlet glaciers
during Plio-Pleistocene time.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
One of the most important issues facing climate scientists today
concerns the response of Antarctica's ice sheets to global climate
change. Recent interpretations of sediments recovered from the AND1B marine core in the western Ross Embayment (78° S) (collected
under the auspices of the multinational ANDRILL program, ANtarctic
DRILLing) call for dynamic, obliquity-paced fluctuations in the volume
and areal extent of the West Antarctic Ice Sheet (WAIS) during PlioPleistocene time (Naish et al., 2009). The findings imply full to partial
collapse of the WAIS during warmer-than-present climate intervals,
including marine oxygen isotope stage 31 (~1.07 Ma), and perhaps
even during MIS 11 (~400 ka) and MIS 5.5 (~125 ka; Scherer et al.,
⁎ Corresponding author at: Department of Environmental, Earth and Atmospheric
Science, University of Massachusetts, Lowell, MA 01854, USA. Tel.: +1 978 934 2664.
E-mail address: Kate_Swanger@uml.edu (K.M. Swanger).
0921-8181/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.gloplacha.2011.07.012
2008; Naish et al., 2009; Pollard and DeConto, 2009). Unknown,
however, is whether nearby outlet glaciers draining the East Antarctic
Ice Sheet (EAIS) experienced similar fluctuations, or whether in the
Ross Embayment only marine-based portions of Antarctica's ice
sheets underwent dynamic behavior during Plio-Pleistocene time
(Denton and Hughes, 1981; see also Bentley et al., 2010). To begin to
address this question, we mapped and dated glacial moraines and
drifts deposited from Taylor Glacier, an outlet glacier that drains
Taylor Dome on the periphery of the East Antarctic Ice Sheet. Taylor
Glacier terminates on land, ~ 40 km from the coast, and about
~150 km from the AND 1B core (Fig. 1). Consequently, the record
from Taylor Glacier affords an ideal opportunity to observe the
phasing and dynamics between the marine-based West Antarctic Ice
Sheet and the larger, terrestrial East Antarctic Ice Sheet. Our
chronologic control comes from 3He cosmogenic-nuclide analyses of
27 boulders sampled from nine moraines and drifts in Kennar Valley, a
small hanging valley that opens onto a peripheral lobe of Taylor
62
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
Taylor
Depot
Nunatak
Turnabout
Valley
Finger
Mountain
ium
Lin
yM
Be
a
co
n
e
aV
alle
y
ibr
Kukri Hills
Are
n
uil
Tabular
Mountain
cie
r
Lashly
Glacier
la
Vernier Valley
Mount
Feather
Ferra
G
oun
tain
s
r
ie r
ac
l
G
y
Eq
Las
hl
Friis Hills
Ta
ylo
Fig.
2
Va
lle
Taylor Dome
r
Kennar Valley
Ca
ssi
dy
Gla
cie
r
15 km
Glacie
r
Ross Sea
New
Harbour
DC TD
TA
M
N
E
Taylor Glacier
McMurdo
Dry Valleys
W
Taylor
Dome
Ferrar Glacier
0
5
Ross Island
2A
1B
Ross Ice Shelf
10 km
0
1000
2000 km
0
50
100 km
Fig. 1. Landsat 7 satellite image of upper Taylor Glacier and upper Ferrar Glacier, both sourced from Taylor Dome. The dotted black line shows the equilibrium line (here the general boundary
between dry snow and wind-swept blue ice) (Chinn, 1980). The black rectangle corresponds to the region of Kennar Valley depicted in Fig. 2a. Lower left inset: Antarctica with the location of the
McMurdo Dry Valleys indicated with the black rectangle. DC = Dome Circe (Dome C), TAM = Transantarctic Mountains, and TD = Taylor Dome. Lower right inset: Eastern Ross Sea region
showing relative locations of Taylor Dome, Ross Ice Shelf, McMurdo Dry Valleys, outlet glaciers, and ANDRILL offshore marine cores (1B and 2A) (see Naish et al., 2009). Black rectangle shows
location of the satellite image.
Glacier in the Quartermain Mountains, McMurdo Dry Valleys (MDV)
(Figs. 1 and 2).
2. Background and setting
2.1. Taylor Dome and Taylor Glacier
Taylor Dome is one of several peripheral domes along the margins
of the East Antarctic Ice Sheet. Taylor Dome (77°47′47″ S, 158°43′26″
E) merges with a broad ice divide that extends inland to Dome Circe, a
major dome in interior East Antarctica (Fig. 1) (Drewry, 1982). Given
this configuration, changes in the level of Taylor Dome reflect local
changes in precipitation (Steig et al., 2000; Grootes et al., 2001) as
well as major fluctuations in the level of interior East Antarctic ice
(Chinn, 1980; Marchant et al., 1994). The 75-km long Taylor Glacier
extends eastward from Taylor Dome and passes across a series of
high-level bedrock steps before terminating in central Taylor Valley
~ 40 km from the coast (Fig. 1). In its upper reaches near Kennar
Valley, Taylor Glacier is ~1000-m thick; on the basis of repeat GPS
surveys and synthetic aperture radar interferometry (InSAR), icesurface velocities in this region are ~ 5–10 m yr −1 (Kavanaugh et al.,
2009). East of Kennar Valley, Taylor Glacier is funneled through
narrow bedrock constrictions and accelerates to a maximum velocity
of 15–20 m yr −1. Apart from strain-induced melting in these regions
of accelerated ice flow, Taylor Glacier is cold based, largely nonerosive, and frozen to its bed (Robinson, 1984; Staiger et al., 2006;
Kavanaugh et al., 2009). Although we cannot preclude some level of
basal entrainment beneath cold-based ice (e.g., Cuffey et al., 2000;
Atkins et al., 2002) the absence of dirty basal ice at the margin of
Taylor Glacier alongside Kennar Valley suggests that basal plucking is
likely insignificant in the upper reaches of the modern Taylor Glacier.
The noted debris carried at the surface of Taylor Glacier today most
likely arises from direct rock fall onto the ice surface and from
windblown sands (Marchant et al., 1994; Staiger et al., 2006; see also
Swanger et al., 2010).
Evidence for past changes in the elevation and areal extent of
Taylor Glacier comes from mapped moraines and drifts that crop out
alongside Taylor Glacier in lower Kennar Valley, as well as in lower
Arena and Beacon valleys (Fig. 1) (Brook et al. 1993; Marchant et al.,
1994).
2.2. Kennar Valley
2.2.1. Physical setting
Kennar Valley is located along the western margin of the Quartermain
Mountains, where Taylor Glacier first bends eastward toward the coast
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
K2
0
170
63
K1
K3
K3
K2
b
F
Fig. 2b
200
0
a)
190
0
18
1700
00
1900
1
Ice-cored moraine
1700
Moraine
without ice core
Inferred moraine/ridge
Contour (50-m interval)
Glacier
Ice-cored drift
N
01
0.5
km
36
07
03
43
K2
01
g.
06
08
09
42
35
K4
K5
50
04
62
05
UD 61
0
60
41
02
K7
0
0
16
20
39
38
3
ig.
0
150
K6
K8
47
K2 37
1400
K5K4
K6
K7
UD
K1
49
46
45
Fi
3a
63
K8 55
N
b)
0
250 500 m
Fig. 2. (a) Sketch map of Kennar Valley on a topographic map base, showing the location of the Taylor Glacier lobe (light gray), moraines (black lines), and ice-cored drift (dark gray);
contour interval = 50 m. Enclosed box indicates area of coverage in panel b. (b) Aerial photograph (USGS TMA 3072 series) of lower Kennar Valley, showing the moraine sequence
(white lines) and the locations of cosmogenic exposure samples (red/black circles with sample numbers). Locations for Fig. 3a and b are shown as numbered brackets that open in
the direction of view.
(~77°45′ S and 161°24′ E). Kennar Valley is predominantly free of surface
ice and bounded to the west and south by a continuous bedrock cliff
reaching ~1700-m elevation (Fig. 2). The peripheral lobe of Taylor Glacier
that occupies the valley mouth displays a clean ice ramp and terminates
at ~1400 m elevation (Fig. 2). Adiabatically warmed katabatic winds are
funneled through Kennar Valley and produce a local blue-ice ablation
zone on Taylor Glacier that drives continuous southward ice flow into
Kennar Valley (Figs. 1 and 2).
As noted earlier, given that Taylor Glacier is cold-based in this
region, the most obvious sources for debris entrainment are icemarginal bedrock cliffs. Extensive bedrock cliffs of Ferrar Dolerite,
150 m high, overlook Taylor Glacier just southwest of Kennar Valley
(McElroy and Rose, 1987; Elliot and Fleming, 2004). At more distant
locations alongside Taylor Glacier, e.g., at Tabular Mountain, Depot
Nunatak, and the Lashly Mountains (all b25 km from Kennar Valley,
Fig. 1), similar cliffs are incised in Feather Conglomerate, Weller Coal
Measures, and the Lashly Formation (fine-grained sandstones and
gray shales with carbonaceous bands) (Barrett and Fitzgerald, 1986;
McElroy and Rose, 1987); all such lithologies are present within the
drifts in lower Kennar Valley (see below). Beyond these regions of
cliffed bedrock, the East Antarctic Ice Sheet is sufficiently thick to
overtop all bedrock topography and prevent debris entrainment from
direct rockfall onto the ice surface. Assuming that all possible rockfall
sources are within ~25 km, the maximum potential time for transport of
supraglacial-debris to Kennar Valley is ~5 kyr (assuming a minimum
ice-flow velocity of ~5 m yr−1 for Taylor Glacier and continuous
supraglacial exposure during transport).
2.2.2. Climate
Environmental conditions in Kennar Valley are among the coldest and
driest in the MDV. Mean annual atmospheric temperatures are b−20 °C
and summertime temperatures rarely, if ever, exceed 0 °C (Kowalewski
et al., 2006; Marchant and Head, 2007). Precipitation is b50 to 100 mm of
water equivalent per year (Clow et al., 1988; Beyer et al., 1999; Doran
et al., 2002; Fountain et al., 2009). Such dry conditions limit soil moisture
and active-layer processes; cryoturbation is largely restricted to
gravitational sliding along the margins of sublimation-type contractioncrack polygons (e.g., Marchant et al., 2002). In general, rocks at the ground
surface tend to stay at the surface, since they are not subjected to repeated
episodes of burial and exposure as occurs in cold regions with saturated
active layers (Hallet and Waddington, 1992; Marchant and Head, 2007;
Morgan et al., 2010). Erosion is essentially restricted to salt weathering,
wind abrasion, and thermal fracture (Marchant and Head, 2007; see
Results section). Estimates for erosion rates, via cosmogenic-nuclide
analyses, range from 5 to 10 cm Ma−1 in high-elevation (2000–2500 m)
nunataks (Summerfield et al., 1999) to ~20 cm Ma−1 in nearby Arena
Valley (Morgan et al., 2010) (see Fig. 1).
3. Methods
3.1. Mapping and sedimentological analyses
We employed geomorphic analyses of orthorectified aerial photographs and detailed fieldwork in 2004 and 2006 to map drifts deposited
from Taylor Glacier in lower Kennar Valley (Figs. 2 and 3). Elevation
control was established using hand-held GPS units, a Garmin 3000 with
a reported accuracy of ±5 m horizontal and ±10 m vertical and a
Trimble GeoExplorer 3000 with ±1 m horizontal/vertical accuracy.
During the course of fieldwork we collected multiple sediment samples
at 20- to 30-cm depth intervals (each 2 kg) for standard grain-size
analyses. We examined the 16- to 64-mm fraction (gravel) for lithologic
constituents and evidence for surface modification (such as glacial scour
and/or weathering). At Boston University, we employed standard wet
and dry sieving procedures to calculate weight percentages for fine
gravel, sand, and mud-sized components (Table 1).
64
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
K3
K2d
K7
K2m
K5
K6
K4
a)
200 m
b)
K4
200 m
K8
D
is
ta
l
Distal
c)
2m
d)
2m
Fig. 3. (a) Oblique aerial photograph looking north, showing K2 drift and bounding moraine. Note the high-centered sublimation polygons in the ice-cored drift (K2d) (location
shown in Fig. 2b). (b) Oblique aerial photograph of upper moraines K4–K7; view to the south (location shown in Fig. 2b). (c) Outer limit of K4 drift; view is to the south. (d) Outer
limit of K8 drift; view is to the northwest.
3.2. Measuring weathering parameters
To measure progressive changes in the magnitude of surface
weathering across Kennar drifts, we examined a minimum of 40 clasts
on each drift for morphologic evidence of salt weathering (solution
pits), wind abrasion (ventifacts and/or wind-abraded facets), and
thermal fracture (fresh rock cores surrounded by thermally cleaved
and spalled rock fragments (e.g., “puzzle rocks” of Marchant and
Head, 2007)). We restricted analyses to clasts N10 4 cm 3 (to ensure
sufficient surface area for study), but otherwise samples were selected
randomly. We also measured the a-axes of the largest clasts at the
surface of each drift (40 clasts per drift). Qualitative measures for
wind abrasion and thermal fracture were noted in the field by the
presence of wind-polished facets and fractured clasts, whereas the
effects of salt weathering (solution pits) were quantified through
direct measurement of pit dimensions using digital calipers. For each
measured clast, we noted the width and depth of the largest solution
pits with a measurement precision of ±1 mm.
3.3. Cosmogenic 3He sample collection
We collected 27 surface clasts (Ferrar Dolerite) from nine mapped
units for cosmogenic-nuclide analyses (Figs. 2 and 3). Each cosmogenic
sample was at least 103 cm3. To minimize the effects of potential rock
displacement associated with the development of contraction-crack
polygons, we restricted sample collection to areas without polygons, or
if necessary, to the center of the largest polygons (e.g., Marchant et al.,
2002). We also collected samples along ridge crests, in order to reduce
Table 1
Physical characteristics of Kennar Valley drifts.
Drift
Elev.a
(m)
Height above Taylor Glacierb
(m)
Reliefc
(m)
dol:sst:
cond
Grain
sizee
Zingg E:O:
P:Bf
Average maximum clast size (cm)g
(cm)
Qualitative
measures of
weatheringh
K2
K3
K4
K5
K6
K7
K8
UDi
1460
1475
1500
1490
1500
1495
1610
N/A
55
70
95
85
95
90
205
N/A
8
1.5
2.5
1
1.5
1
1
N/A
80:20:0
82:18:0
78:22:0
90:0:10
94:4:2
68:25:7
97:0:3
88:4:8
19:76:5
32:65:3
16:74:10
58:38:4
59:34:7
11:83:6
18:80:2
30:60:10
16:49:15:20
15:45:17:23
13:62:5:20
13:44:20:23
22:51:11:16
20:40:25:15
15:44:12:29
18:40:25:17
200–300
200
100
100
100
50
50
40
t
t, h,
t, h,
t, h,
t, h,
t, h,
t, h,
t, h,
a
w
w, v,
w, v,
w, v,
w, v,
w, v,
w, v,
p
p
p
p
p
p
Meters above sea level.
Maximum elevation of moraine compared to present-day average elevation of base of Taylor Glacier at Kennar Valley mouth (1405 masl).
Maximum relief of moraine crests.
d
Ratio of lithologies in the N 16-mm fraction of dolerite:sandstone:rounded quartz pebbles from conglomerates.
e
Ratio of gravel:sand:mud in the b16-mm fraction.
f
Ratio of equant:oblate:prolate:blade in the N 16-mm fraction. dl = diameter of long axis, di = diameter of intermediate axis, ds = diameter of short axis. Equant = di/dl N 0.67,
ds/di N 0.67, Oblate = di/dl N 0.67, ds/di b 0.67. Prolate = di/dl b 0.67, ds/di N 0.67, Blade = di/dl b 0.67, ds/di b 0.67
g
Average maximum clast size determined by visual analyses in the field and of ground pictures.
h
Letter indicates presence of: t: thermal fracture (puzzle rocks), h: weathered (oxidized) soil horizon, w: wind faceting on surface clasts, v: varnish N1-mm thick on surface
dolerite clasts, and p: development of solution pits N 1 mm in diameter.
i
Undifferentiated drift that lies stratigraphically below moraines K4–K7.
b
c
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
the chances of burial beneath wind-blown snow (e.g., Margerison et al.,
2005).
Table 2a
Sample location, shielding and cosmogenic 3He data from the Kennar Valley drifts.
Drift
Sample
Longitudea
Latitudea
Altitudea
(masl)
Shielding
factorb
3
Hec
(108 at/g)
K1
DXP-04-03
DXP-06-20
KSX-06-39
DXP-04-01
KSX-06-35
KSX-06-41
KSX-06-42
KSX-04-43
KSX-06-47
KSX-06-49
DXP-04-02
KSX-06-36
KSX-06-37
KSX-06-38
KSX-06-45
KSX-06-46
DXP-04-06
DXP-04-07
KSX-06-50
KSX-06-63
DXP-04-04
DXP-04-05
DXP-04-08
KSX-06-62
KSX 06-55
DXP-04-09
KSX-06-61
160.142
160.141
160.434
160.425
160.416
160.432
160.433
160.438
160.422
160.423
160.419
160.421
160.423
160.428
160.418
160.417
160.402
160.403
160.395
160.395
160.387
160.387
160.384
160.384
160.341
160.383
160.383
− 77.751
− 77.751
−77.742
− 77.752
−77.752
− 77.751
− 77.751
−77.752
−77.747
−77.746
− 77.752
−77.751
− 77.750
− 77.750
− 77.747
− 77.746
− 77.751
− 77.752
− 77.751
−77.751
− 77.751
− 77.751
− 77.750
−77.749
−77.751
− 77.750
−77.749
1405
1405
1395
1415
1415
1410
1410
1410
1450
1460
1395
1400
1400
1400
1470
1475
1500
1490
1487
1482
1500
1503
1492
1494
1610
1490
1488
0.912
0.912
0.883
0.992
0.993
0.996
0.996
0.996
0.995
0.995
0.992
0.993
0.987
0.991
0.994
0.995
0.995
0.995
0.994
0.995
0.995
0.995
0.995
0.995
0.985
0.995
0.995
0.088 ± 0.003
0.089 ± 0.003
0.094 ± 0.003
0.692 ± 0.031
0.522 ± 0.021
0.624 ± 0.025
0.917 ± 0.037
0.487 ± 0.019
1.220 ± 0.037
0.669 ± 0.020
0.786 ± 0.014
1.388 ± 0.056
1.533 ± 0.061
0.429 ± 0.017
1.070 ± 0.032
1.770 ± 0.053
2.810 ± 0.042
3.290 ± 0.058
5.390 ± 0.162
2.410 ± 0.072
3.135 ± 0.065
2.370 ± 0.066
9.160 ± 0.140
8.890 ± 0.267
15.12 ± 0.60
15.39 ± 0.25
16.40 ± 0.49
3.4. Mineral separation and gas extraction
Whole-rock, cosmogenic samples were cut at the Lamont-Doherty
Earth Observatory (LDEO) and then crushed at Boston University (BU)
using a Spex Certiprep 8515 Shatterbox; fragments were then sieved
to isolate the N150 μm and b300 μm fraction. Typical sample weights
were about 30 mg. At LDEO, pyroxene grains were separated using
magnetic and heavy liquid techniques, followed by handpicking.
Separated pyroxenes were then analyzed for helium concentrations
and isotopic composition at LDEO on a MAP 215-50 noble gas mass
spectrometer calibrated with a known volume of a Yellowstone
helium standard (MM) with a 3He/ 4He ratio of 16.45Ra, where Ra =
( 3He/ 4He)air = 1.384 × 10 −6 (following protocols outlined in Winckler
et al., 2005; Schaefer et al., 2006). Hot procedural blanks contained
less than 5000 atoms of 3He with approximately atmospheric helium
isotopic composition. Blank corrections for 3He were smaller than 2%
(for samples with low 3He concentrations, K1), and in most cases
smaller than 1%.
K2md
K2dd
K3
K4
K5
K6
K7
3.5. Exposure age calculations and erosion rates
We use the sea level, high latitude production rate from spallation
reactions of 120 at g−1 yr−1 for 3He in pyroxene (Goehring et al., 2010).
At each sample site we recorded local and regional shielding by
measuring horizon geometry. Shielding factors were calculated for each
sample after Balco et al. (2008) (Table 2a). Atmospheric pressures over
Antarctica are anomalously low compared to typical pressure–elevation
relationships. Therefore, we employed the Antarctic-specific equations
in Stone (2000) to scale production rates to sample elevation. Choice of
these production rates and scaling schemes relative to other reported
production rates (i.e. Lal, 1991; Licciardi et al., 1999) does not impact our
chronology or our main conclusions. Nuclide-measurement depths in all
surface clasts were b5 cm. Minimum, no-erosion, exposure ages were
calculated using the following equation:
N = Pt
ð1Þ
where N is number of cosmogenic nuclides (at g −1), P is the
production rate (at g −1 yr −1), and t is exposure time (yr).
By convention, exposure ages are typically reported assuming zero
erosion. This assumption may be valid for some Holocene and youngeraged samples, but for older samples, surface erosion typically removes
the outer crusts of rocks and reduces the total cosmogenic-nuclide
inventory. As a consequence of this reduction, exposure ages not
corrected for erosion are typically viewed as minimum constraints. If
one assumes a constant erosion rate (see below), the relationship
between exposure age and erosion rate is governed by the following
equation:
N=
PL −Et
1− e L
E
ð2Þ
where N is number of cosmogenic nuclides (at g −1), P is the
production rate (at g −1 yr −1), L is the attenuation length (g cm −2),
E is the erosion rate (g cm −2 yr −1), and t is time (yr). Solving Eq. (2)
assuming an infinite exposure age (t) yields the maximum possible
erosion rate for a given sample (Table 2b). We used an attenuation
length of 155 g cm −2 (after Sarda et al., 1993) and an average rock
density of 2.7 g cm −3.
65
K8
UDe
a
Longitude, latitude and altitude (masl = meters above sea level) were measured at
each sample location using a Garmin 3000 (measurement error of ± 5 m horizontal and
±10 m vertical). Due to this relatively high vertical error, we compared these data to
topographic maps and measured moraine ridge elevations using a Trimble GeoExplorer
3000 (vertical error ± 1 m).
b
Shielding factors were calculated after Balco et al. (2008) from horizon geometry
measurements recorded for each sample in the field.
c
1σ errors of 3He concentrations reflect propagated analytical uncertainties, based
on statistical errors and variability in the sensitivity of the mass spectrometer.
d
K2 ice-cored moraines (K2m) and K2 ice-cored drift (K2d) (Figs. 2 and 3).
e
Undifferentiated drift distal to moraine K7 and stratigraphically below moraines
K4–K7.
4. Results
4.1. The spatial distribution and weathering characteristics of Kennar
Valley drifts
The mapped pattern of drifts in lower Kennar Valley indicates
deposition from southward advance(s) of Taylor Glacier. In most
cases, drifts are composed of scattered and isolated erratics whose
concentration increases toward a single, major bounding moraine
ridge (Figs. 2 and 3); a similar pattern was also noted for drifts
deposited from the Ferrar outlet glacier in nearby Vernier Valley
(Fig. 1) (Staiger et al., 2006). The Kennar Valley drifts are numbered
sequentially from K1 (proximal to Taylor Glacier) to K8 (distal)
(Fig. 2). As noted below, K2 is atypical in that it includes widespread,
matrix-supported debris over stagnant, glacier ice. A second, matrixsupported drift of unknown origin (UD) underlies K4–K7 drifts and
crops out extensively on an upper-level bench beyond K7 (Fig. 2).
Kennar Valley drifts show an overall reduction in maximum clast
size with increasing distance from Taylor Glacier (Table 1). The a-axes
of surface clasts decline from an average maximum of ~200 cm for K2,
to ~100 cm for K5, and to ~50 cm for both K7 and K8 (Table 1). Some
of the reduction in clast size likely reflects intermittent fracture
(Fig. 4d–f) and pitting (Fig. 4a–c; Fig. 5). None of the clasts show
evidence for transport beneath wet-based ice (e.g., striations, polish,
molding, and/or faceting). For clarity we group Kennar drifts into two
categories, ice cored (K1 and K2) and non-ice cored (K3–K8).
66
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
Table 2b
Cosmogenic exposure ages and maximum erosion rates from the Kennar Valley drifts.
Drift
Sample
3
Hea
(108 at/g)
Elevation
scaling
factorb
Minimum 3He agec
(ka)
3
He-age 5 cm/Myrd
(ka)
3
He-age 10 cm/Myrd
(ka)
Max erosion ratee
(cm/Myr)
K1
DXP-04-03
DXP-06-20
KSX-06-39
DXP-04-01
KSX-06-35
KSX-06-41
KSX-06-42
KSX-04-43
KSX-06-47
KSX-06-49
DXP-04-02
KSX-06-36
KSX-06-37
KSX-06-38
KSX-06-45
KSX-06-46
DXP-04-06
DXP-04-07
KSX-06-50
KSX-06-63
DXP-04-04
DXP-04-05
DXP-04-08
KSX-06-62
KSX 06-55
DXP-04-09
KSX-06-61
0.088 ± 0.003
0.089 ± 0.003
0.094 ± 0.003
0.692 ± 0.031
0.522 ± 0.021
0.624 ± 0.025
0.917 ± 0.037
0.487 ± 0.019
1.220 ± 0.037
0.669 ± 0.020
0.786 ± 0.014
1.388 ± 0.056
1.533 ± 0.061
0.429 ± 0.017
1.070 ± 0.032
1.770 ± 0.053
2.810 ± 0.042
3.290 ± 0.058
5.390 ± 0.162
2.410 ± 0.072
3.135 ± 0.065
2.370 ± 0.066
9.160 ± 0.140
8.890 ± 0.267
15.12 ± 0.60
15.39 ± 0.25
16.40 ± 0.49
4.58
4.58
4.54
4.62
4.62
4.60
4.60
4.60
4.75
4.78
4.54
4.56
4.56
4.56
4.82
4.84
4.94
4.90
4.89
4.87
4.94
4.94
4.91
4.92
5.38
4.90
4.90
17.5 ± 0.5
17.7 ± 0.5
19.5 ± 0.5
126 ± 6
95 ± 4
114 ± 5
167 ± 7
89 ± 4
215 ± 6
117 ± 4
145 ± 3
255 ± 10
284 ± 11
79 ± 3
186 ± 6
306 ± 9
477 ± 7
562 ± 10
923 ± 28
414 ± 12
532 ± 11
402 ± 11
1562 ± 24
1513 ± 45
2378 ± 94
2630 ± 43
2803 ± 84
18
18
20
127
95
114
168
89
217
118
146
258
287
79
188
310
490
580
960
420
550
410
1700
1600
2700
3000
3200
18
18
20
127
96
115
170
89
220
120
150
260
290
80
190
320
500
590
1000
430
560
420
1800
1800
3100
3500
3900
3600
3600
3300
460
600
500
350
650
270
500
400
220
200
730
310
190
120
100
60
140
110
140
37
38
25
22
21
K2mf
K2df
K3
K4
K5
K6
K7
K8
UDg
a
1σ errors of 3He concentrations reflect propagated analytical uncertainties, based on statistical errors and variability in the sensitivity of the mass spectrometer.
Cosmogenic production rates were scaled for elevation using equations from Stone (2000) for Antarctica.
c
We used a sea level, high-latitude cosmogenic 3He production rate of 120 at g−1 yr−1 (pyroxene) after Goehring et al. (2010). Minimum ages assume no erosion, accounting only
for production rates and shielding factors at each sample location.
d
Ages calculated with constant erosion rates of 5 and 10 cm Myr−1, with an attenuation length of 155 g cm−2 and an average rock density of 2.7 g cm−3.
e
Maximum erosion rates are calculated from the measured cosmogenic 3He assuming an infinite exposure time.
f
K2 ice-cored moraines (K2m) and K2 ice-cored drift (K2d) (Fig. 2 and 3).
g
Undifferentiated drift distal to moraine K7 and stratigraphically below moraines K4–K7.
b
4.1.1. Ice cored drift: K1 and K2
K1 drift includes the modern ice-cored moraine alongside Taylor
Glacier and all visible clasts embedded in the margin of Taylor Glacier
at the mouth of Kennar Valley (Fig. 2). The modern ice-cored moraine
is sharp crested, ~ 2 m wide, and ~ 3 m high. The only evidence for
surface alterations are thin iron-oxide stains that coat some rock
surfaces (Fig. 4); otherwise the rocks at the surface, and those partly
embedded in Taylor Glacier ice, are fresh, angular, and resemble those
found in rockfall deposits elsewhere in the MDV (e.g., Swanger &
Marchant, 2007).
K2 drift reaches a maximum elevation of ~ 1460 m, ~55 m above
the base of nearby Taylor Glacier (Figs. 2 and 3). It includes an
extensive sheet of matrix-supported, rocky debris that rests directly
on stagnant glacier ice (designated K2d); the drift is bound by
multiple, largely ice-cored, and partially cross-cutting moraines (K2m)
(Figs. 2 and 3). K2 drift displays well-developed sublimation-type
polygons (Marchant et al., 2002; Marchant and Head, 2007), with a
relief of ~ 3 m from elevated polygon centers to deep marginal
troughs. The contacts between drift (typically 25–50 cm thick) and
underlying glacier ice are sharp, dry, and planar. The largest ice-cored
moraine in K2 drift is 8 m high (with almost all of the relief arising
from the ice core itself); this moraine circumscribes a ~ 0.4 km 2 region
of ice-cored drift (K2d) (Figs. 2 and 3a).
Clasts at the surface of K2 drift are composed of Ferrar Dolerite
(80–90%), Feather Conglomerate, and undifferentiated sandstones,
siltstones, and shales. The clasts of dolerite are typically ~ 1 to 2 m in
diameter and exhibit weakly developed rock varnish (b1-mm thick)
(Table 1).
4.1.2. Non-ice cored drift: K3–K8
K3 drift reaches a maximum elevation of 1475 m, ~70 m above the
base of nearby Taylor Glacier. Its bounding moraine ridge is sharpcrested, 1–2 m high, and composed of dolerite (90%) and sandstone
cobbles (Figs. 2 and 3). The surface dolerites exhibit minor windabraded facets and thin (b1-mm thick) rock varnish (Table 1).
K4–K7 drifts include a suite of closely spaced moraines and erratic
cobbles that crop out between 1490 and 1500 m elevation, 85–95 m
above the base Taylor Glacier (Fig. 2). The moraines and scattered
erratics rest unconformably on an older, undifferentiated and matrixsupported drift sheet (UD). The bounding moraine that marks the outer
edge of K4 drift is 2- to 3-m high, whereas moraines that mark the outer
limits of drifts K5, K6 and K7 are relatively low and diffuse, reaching a
maximum height of ~1 m. Lithologies within K4–K7 drifts are uniformly
composed of ~80–90% dolerite and ~10–20% sandstone (with elevated
numbers of isolated quartz pebbles from the Feather Conglomerate).
Clast size decreases from an average maximum of ~100 cm (a axis) on
the surface of K4 drift, to ~50 cm on K7 drift. Solution pits from salt
weathering on the surface of clasts of Ferrar Dolerite increase from a
maximum depth of ~15 mm on K4 drift to ~29 mm on K7 drift (Figs. 4
and 5; Table 1). Ventifacts and puzzle rocks are also relatively common
(Fig. 4). Salt-cemented horizons bind iron-oxide stained quartz grains in
the upper 20 cm of K4–K7 drifts (e.g., Bockheim, 2010).
K8 drift reaches a maximum elevation of 1610 m, ~ 205 m above
the base Taylor Glacier at the mouth of Kennar Valley (~2.5 km away).
It terminates in a narrow moraine ridge ~ 400-m long and 1-m high
(Fig. 3). As for the other Kennar drifts, the concentration of surface
erratics increases up to the bounding moraine (Fig. 3). The clasts
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
a)
20 cm
67
d)
b)
e)
c)
f)
Fig. 4. Relative weathering observed for clasts of Ferrar Dolerite on Kennar Valley drifts. Left column (a–c), shows the progressive increase in the development of solution pits: (a) no
pits observed (fresh-appearing dolerite from K1 drift); (b) slightly pitted clast from K4 drift; (c) well developed solution pits on a clast from the surface of undifferentiated drift (UD)
beyond K7 drift. Right column (d–f) shows the effects of thermal fracture: (d) fractured, but unbroken, boulder from K4 drift; (e and f) fractured and spalled clasts from K5 and K7
drifts; arrows indicate direction of inferred movement away from central core cobble/boulder.
within K8 are N95% dolerite and display an average maximum size of
~ 50 cm (Table 1); most clasts of dolerite exhibit thick rock varnish
(≫1 mm thick) and large weathering pits (~20 mm deep). Sand
grains stained with iron oxides occur in the upper ~ 20 cm of the
moraine and are cemented with visible salt encrustations.
4.1.3. Undifferentiated drift
As noted earlier, an undifferentiated drift (UD) crops out on the
upper-level bench between K7 and K8 drifts (Fig. 2). In hand-dug
sections, this drift stratigraphically underlies erratics and moraines
associated with K4, K5, K6, and K7 drifts. Clasts at the surface are
typically b40 cm long (a axis) and exhibit solution pits as much as
~ 45 mm deep, by far the largest for any mapped deposit in our study
area (Figs. 4 and 5, Table 1).
clasts lack evidence for weathering beyond slight iron-oxide stains.
Clasts at the surface of K2 drift show slightly greater weathering, with
rock varnish replacing iron-oxide stains (e.g., Staiger et al., 2006;
Kowalewski et al., 2011). Thereafter, the development and progressive
growth of solution pits, wind-polished facets, and puzzle rocks (thermal
fracture) suggest that drift ages increase sequentially from K3 to K8
(Fig. 4). Consistent with this assertion is the overall reduction in
maximum clast size from ~200 cm on the surface of K2 drift to ~50 cm
on K8 drift. The reduction in clast size likely reflects the cumulative
effects of surface weathering, especially episodic thermal fatigue and
rock fracture (Fig. 4). The undifferentiated drift (UD) is assigned the
oldest relative age, with maximum surface clast sizes of b 40 cm and
solution pits as much as 45 mm deep (Fig. 5, Table 1).
4.3. Numerical chronology
4.2. Relative chronology
A relative chronology for Kennar moraines comes from noted
changes in surface and subsurface weathering parameters, as well as the
presence/absence of an underlying glacier ice. At the surface of K1 drift,
Our cosmogenic 3He analyses of 27 surface cobbles from Kennar
drifts corroborate our relative chronology. In our preferred age model
(see Section 5.1.3), we use the age of the oldest dated boulder from
each drift to approximate drift age. As noted below, we adopt this
68
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
a)
50
My
r -1
yr -1
mm
5. Discussion and wider implications
40
Weathering Pit Diameter (mm)
60
mM
K4
K5
K6
K7
UD
(K7), and 2378±94 ka (K8). The oldest clast on the undifferentiated drift
just beyond the K7 yielded an uncorrected age of 2803±84 ka (Table 2b).
60
m
70
-1
5.1. Sources of scatter in cosmogenic data and calculating a preferred
age model
yr
m
20
40
M
m
30
20
10
0
0
500
1000
1500
2000
2500
3000
3500
Uncorrected Drift Age (ka)
My
yr -1
M
m
20
m
40
mm
K4
K5
K6
K7
UD
30
Weathering Pit Depth (mm)
50
r -1
b)
30
r-1
10
20
mm
My
The chronology presented here ranges from 20,000 years to ~3 million
years. With one exception (K6), cosmogenic samples show a general trend
of greater exposure age with increasing distance from the modern Taylor
Glacier, consistent with our relative chronology. However, there is scatter
in the dataset. As noted in several prior cosmogenic studies (e.g., Brook
et al., 1993; Schaefer et al., 1999; Gosse and Phillips, 2001; Ackert and
Kurz, 2004; Margerison et al., 2005; Staiger et al., 2006; Balco et al.,
2008), the scatter in measured nuclide inventories is likely associated
with some combination of (1) inheritance during prior exposure on cliff
walls and/or during transport to ice margins, (2) inclusion of noncosmogenic 3He in analytical measurements, (3) diffusive loss of 3He
from minerals, (4) shielding by snow and ice, and (5) loss of the
cosmogenic-nuclide inventory due to physical erosion. Of these, the first
two typically result in an overestimation of deposit ages, whereas the
latter three can cause great underestimation of ages. Potential impacts of
each of the above sources of error are discussed below.
Regarding the potential for diffusive loss of 3He from mineral grains,
recent analytical studies have demonstrated that pyroxenes in Ferrar
Dolerite quantitatively retain 3He (Schaefer et al., 2000); therefore, we do
not consider diffusive loss to be a significant source of error. In addition,
because precipitation in the MDV is extremely low (Fountain et al., 2009)
episodic burial beneath snowfall is unlikely, however it cannot be
definitively ruled out and could cause underestimation of exposure ages
due to occasional shielding of the samples (Margerison et al., 2005). This
leaves the cumulative effects of cosmogenic-nuclide inheritance, noncosmogenic sources, and erosional processes as the most likely factors
that could cause significant scatter in our exposure ages.
10
0
0
500
1000
1500
2000
2500
3000
3500
Uncorrected Drift Age (ka)
Fig. 5. Plots showing pit evolution over time. Panel (a) shows the change in maximum
pit diameter and panel (b) shows the change in maximum pit depth; all pits measured
on clasts of Ferrar Dolerite. Data are plotted as a function of minimum (no-erosion)
exposure ages. Dotted lines indicate maximum pit dimensions over time given specific
rates of pit deepening and widening. In general, maximum weathering pit diameters
increase by ~ 30–40 mm Myr −1 and maximum pit depths increase by ~ 15–
20 mm Myr−1.
procedure because erosion, especially over the multi-million-year
timescales considered here, leads to a reduction in the overall
inventory of cosmogenic nuclides and significant underestimation of
deposit age; hence, we assume all cosmogenic ages are minimum age
estimates (see Section 5.1.2 for more information on the impact of
erosion on exposure ages). In our discussion below, we highlight the
oldest dated sample on each drift; see Tables 2a and 2b for all results.
With the exception of K5 drift, all ages on clasts from individual drifts
are internally consistent (Tables 2a and 2b).
4.3.1. Uncorrected ages
The oldest sample from K2 drift yielded an uncorrected age of 284±
11 ka. Likewise, the oldest sample from K3 drift yielded an uncorrected
age of 306±9 ka. The upper-elevation drifts, K4–K8, contain samples that
are considerably older, with the oldest samples yielding uncorrected ages
of 562±10 ka (K4), 923±28 ka (K5), 532±11 ka (K6), 1562 ±24 ka
5.1.1. Nuclide inheritance and non-cosmogenic 3He
Earlier cosmogenic noble-gas studies from Ferrar Dolerite in the Dry
Valleys have demonstrated consistent 3He and 21Ne exposure ages (e.g.
Bruno et al., 1997, Schaefer et al., 1999, Staiger et al., 2006), indicating only
minor contributions by non-cosmogenic 3He. Here we expand this
argument by measuring the 3He concentrations in three samples
entrained in the terminus of Taylor Glacier in Kennar Valley (K1 drift)
(Figs. 2 and 4). Because these clasts were entrained in the glacier, any 3He
in the pyroxenes must originate from prior exposure at the rock-fall
source area (inherited nuclides) and/or from a non-cosmogenic source.
All three samples yielded very low concentrations of 3He, ~9×106 at g−1,
which translates to an exposure age (if assumed to be entirely
cosmogenic) of ~18 ka. Based on these data, we conclude that the noncosmogenic and pre-exposure signal for the Kennar Valley samples is
b20 kyr, which is minor relative to the timescales discussed here and
therefore, not a significant source of uncertainty.
5.1.2. Effects of erosion on cosmogenic exposure ages
By convention, exposure ages are typically reported assuming zero
erosion. This assumption may be valid for some late-Pleistocene and
Holocene samples, but for older samples, surface erosion typically
removes the outer crusts of rocks (and/or causes exposure of entirely
new, fresh rock faces) and reduces the total cosmogenic-nuclide inventory
(Fig. 4). As a consequence of this reduction, exposure ages that are not
corrected for erosion are typically viewed as minimum-age constraints.
Published erosion rates for the MDV are typically ≤1 m Myr−1, and may
be as low as ~5 cm Myr−1 for high-elevation regions like Mt. Feather at
2000–2500 m (Fig. 1) (Ivy-Ochs et al., 1995; Schaefer et al., 1999;
Summerfield et al., 1999; Margerison et al., 2005).
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
As noted previously, clasts at the ground surface in Kennar Valley
show evidence for surface erosion. Our quantitative analyses indicate
that the depth and diameter of solution pits increases with distance
from Taylor Glacier, itself a proxy for increasing exposure age (Fig. 5).
Similar changes were observed in the size of weathering pits in dolerites
along a moraine sequence in Vernier Valley that spans ~4 Myr (Fig. 1)
(Staiger et al., 2006). Although rates of weathering-pit deepening do fall
below the slowest reported erosion rates in the MDV of 5 cm Myr −1
(Summerfield et al., 1999), pit formation represents only one type of
weathering process, with others including wind abrasion and thermal
fracture, for which our qualitative measures suggest increasing values
from K2 to K8 (Table 1, Fig. 4). As shown in Fig. 6, thermal fracture
produces rock fragments with limited nuclide inventories and causes an
increase in age scatter with deposit age. For the oldest samples dated in
this study, on K8 drift and the undifferentiated drift (UD) the maximum
possible erosion rates are 21–25 cm Myr−1 (erosion rates calculated
following equations in Gosse and Phillips, 2001). As noted below, when
calculating our preferred age model we assume erosion rates of
10 cm Myr−1, a value that is consistent with other cosmogenic-nuclide
studies in this region (Ivy-Ochs et al., 1995; Schaefer et al., 1999).
5.1.3. The preferred age model
Except for K5 drift, exposure ages within a single drift are internally
consistent. For K5, we received an age of ~430 ka for one clast and
~1000 ka for another clast ~100 m distant (Table 2b). Our working
hypothesis is that the relatively young age of ~430 ka reflects nuclide loss
due to episodic erosion via thermal fracture/spallation (Fig. 6) (though
there were no obvious signs of recent erosion/spalling exceeding that of
nearby clasts). Although the old age could reflect some level of prior
exposure, our findings of very low nuclide inheritance for clasts within
the modern Taylor Glacier (K1 drift) suggest that the effects of prior
b)
Initial Rock
Fracture
After First Spalling Event
Fracture
Measured Exposure Age (ka)
a)
Measured Exposure Age (ka)
After Third Spalling Event
exposure may be minimal. In support of this assertion we note that the
internally consistent ages derived from dated cobbles on drifts K4, K6, K7
and the undifferentiated drift (UD) all indicate similar levels of exposure,
an unexpected result if prior inheritance played a major role in altering
cosmogenic-nuclide inventories.
Therefore, our preferred age model for the cosmogenic data is
generated by (1) applying a correction for constant erosion equivalent to
10 cm Myr−1 (Table 2b), which is typical for the region (Ivy-Ochs et al.,
1995; Schaefer et al., 1999) and (2) selecting the oldest cosmogenic age
for each mapped unit (Table 3). Assuming our preferred age model is
correct, the ages are as follows: ~290 ka for the K2 drift, ~320 ka for K3
drift, ~590 ka for K4 drift, ~1000 ka for K5 drift, ~560 ka for K6 drift,
~1800 ka for K7 drift, and ~3100 ka for K8 drift (see Fig. 7 for all samples
ages). The preferred age for undifferentiated drift (UD) that lies beyond
(and stratigraphically below) the K7 moraine is ~3900 ka (Table 3). K1 is
the modern ice-cored moraine.
5.2. Did Taylor Glacier fluctuate between periods of moraine formation?
Due to the episodic deposition of moraines alongside cold-based
outlet glaciers, a key question is whether Taylor Glacier, and by inference
Taylor Dome, could have experienced large-scale fluctuations during
intervals of non-deposition in Kennar Valley. To address this question, we
compare our dataset with other climate records in the MDV region. The
Kennar Valley record, which indicates overall recession of Taylor Glacier
since a highstand ~3.1 Ma, is fully consistent with previously published
chronologies for outlet glaciers in the MDV (see Fig. 1) (Brook et al., 1993;
Marchant et al., 1994; Staiger et al., 2006). Indeed, cosmogenic analyses of
an extensive sequence of 39 moraines in nearby Arena Valley call for
overall ice recession of Taylor Glacier since the late-Pliocene (Brown et al.,
1991; Brook et al., 1993; Marchant et al., 1994). Likewise, cosmogenic
nt
Event
g Eve
vent
palling ond Spallin d Spalling E
ir
h
Sec
T
First S
500
Age
range:
300 to
500 ka
400
300
200
100
0
0
200
100
300
400
500
Deposit Age (ka)
c)
After Second Spalling Event
Fracture
69
nt
Event
ng Eve
Event
palling
d Spalli
palling
First S
Secon
Third S
2000
Age
range:
1000 to
2000 ka
1500
1000
500
0
0
500
1000
1500
2000
Deposit Age (ka)
Fig. 6. Cartoon showing the effect of episodic thermal fracture on exposure histories and cosmogenic-nuclide inventories. Panel (a) shows an initial coherent rock that undergoes
three spalling events over time. During each spalling event, a “buried-rock surface” is exposed. In the case of the blue/light gray fragment, its upper surface is continually exposed;
without other forms of erosion (e.g., no pitting or wind abrasion) its cosmogenic analyses would yield the most accurate age. However, with each spalling/thermal-fracture event, a
new surface is exposed to cosmic rays; each newly exposed surface contains a reduced cosmogenic inventory relative to that of the blue/light gray surface. In this case, the green/
black slab, most recently exposed by thermal fracture, would contain the lowest nuclide inventory. Panels (b) and (c) show plots of “measured” exposure age vs. drift age as a
function of spalling/thermal fracture; lines are color coded to spalling events/thermal fracture as noted in panel (a). Only the blue/light gray line shows equivalent exposure age and
deposit age through time. The lines for the other three fragments show exposure ages that increasingly underestimate deposit ages. Panel (b) assumes spallation/thermal fracture at
100-kyr intervals for a 500 ka deposit, whereas panel (c) assumes spallation/thermal fracture every 500 kyr for a 2 Ma deposit. Compare results with thermal fracture as observed in
Fig. 4; our assumption is that some of the scatter in exposure ages likely reflects the effects of intermittent thermal fracture (see text and exposure ages, Table 2b).
70
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
Table 3
Kennar Valley drift ages.
Deposit
Heighta
(m)
Mean exposure ageb
(ka)
Preferred age modelc
(ka)
K1
K2
K3
K4
K5
K6
K7
K8
UDd
0
55
70
95
85
95
90
205
n/a
19
160
260
550
720
490
1800
3100
3700
20
290
320
590
1000
560
1800
3100
3900
and persistent rockfall from exposed dolerite cliffs at the base of Finger
Mountain and/or potentially persistent higher-than-average ablation
rates along the surface of Taylor Glacier ice at the mouth of Arena Valley
(which may drive increased ice flow into the valley and result in more
frequent moraine formation).
5.3. Response of Taylor Dome to warmer-than-present conditions and
the mid-Pleistocene transition
a
Maximum elevation of deposit/moraine above current elevation of the base of
Taylor Glacier.
b
Average of all clast exposure ages for each moraine/drift, in which a constant
erosion rate of 10 cm Myr−1 is assumed for each sample.
c
Preferred age model using oldest dated clast from each moraine/drift, assuming a
constant erosion rate of 10 cm Myr−1.
d
Undifferentiated drift dated ~ 20 m distal to moraine K7. Drift is stratigraphically
below K7 drift.
dates on a series of moraines in Vernier Valley (~25 km southeast of
Kennar Valley) indicate gradual lowering of Ferrar Glacier (a second
outlet glacier draining Taylor Dome) since the mid-Pliocene (Staiger et al.,
2006; Johnson and Staiger, 2007) (Fig. 1). Although these records share
similar first-order trends, they differ in the precise number and age of
individual moraines and drifts; individual drifts cannot be correlated with
certainty from valley to valley. The variation likely arises from stochastic
factors associated with spatially variable and intermittent rockfall. In
addition, temporal and spatial changes in rates of ice ablation, and
resulting ice flow, might also influence moraine deposition. The unusually
large number of moraines in Arena Valley most likely reflects extensive
The 3.1 million-year glacial record from Kennar Valley (as well as from
Arena and Vernier valleys) implies overall ice recession for outlet glaciers
draining Taylor Dome since mid- to late-Pliocene time. In detail, the
Kennar Valley record indicates that the ice-surface elevation of Taylor
Glacier, and hence of Taylor Dome, stood at higher-than-present levels
during significant, globally warm intervals: the mid-Pliocene climatic
optimum (~3.0–3.1 Ma) and MIS 31 (~1.07 Ma). However, these findings
contrast with recent reports for significant reductions in the volume of
grounded, marine-based ice from the WAIS in the Ross Embayment
(Scherer et al., 2008; Naish et al., 2009; Pollard and DeConto, 2009; see
also Miller et al., 2005). The findings call for considerable variability in the
response of Antarctic ice to global climate change.
In addition, the combined glacier records from Kennar, Arena, and
Vernier valleys suggest that the rate of change accelerated at the midPleistocene transition (or just shortly after), with both Taylor and Ferrar
glaciers experiencing most (50–80%) of their total vertical recession
after ~0.9 Ma (Table 3). One possible explanation calls on reduction in
snowfall at Taylor Dome, which could reflect overall cooling of
atmospheric temperatures throughout the late Pleistocene (Lisiecki
and Raymo, 2005) and/or northward displacement of open water in the
Ross Embayment associated with expanding sea ice and/or increasing
frequency of WAIS expansion (Denton and Marchant, 2000; Steig et al.,
2000; Grootes et al., 2001; Naish et al., 2009).
5.4. Implications for paleoclimate
K3
320
190
18
K1
20
120
220
80
K2
115
150
K7
89
290
260
K6
170
18
K2
127
96
K4
K5
500
590
1000
430
560
1800
1800
420
3500
3900
UD
The mapped drifts in Kennar Valley (as well as in nearby Arena and
Vernier valleys) were deposited from cold-based ice (Brook et al., 1993;
Marchant et al., 1994; Staiger et al., 2006). Had wet-based conditions
occurred, clasts within the Kennar Valley drifts would show evidence for
glacial abrasion, including striations, polish, and faceting, which is not the
case (see also Marchant et al., 1994; Staiger et al., 2006). Also, had the ice
surface experienced significant melting, outwash and/or stratified
sediments would be commonplace (e.g., Denton et al., 1993). Instead,
all moraines are texturally and morphologically identical to those found
today alongside cold-based margins of outlet and alpine glaciers that pass
across the central and southern Transantarctic Mountains (Denton et al.,
1989; Marchant et al., 1994; Staiger et al., 2006; Kowalewski et al., 2011).
The implication is that climate conditions during moraine deposition
were similar to present-day conditions, and not as warm as those inferred
for the central Ross Embayment (Scherer et al., 2008; Naish et al., 2009).
In addition, the presence of in-situ moraine ridges at the base of steep
valley walls in all mapped valleys (K8 drift in Kennar Valley, for example)
imply limited slope development for the last ~3.1 Myr. To be sure, a
record of morphologic change does exist, and is most notably expressed
in the overall reduction in clast size on drifts K2 to K8 and a gradual
lowering of moraine heights with increasing exposure age (e.g., Morgan
et al., 2011).
6. Conclusions
K8 3100
N
0
250 500 m
Fig. 7. Results from our preferred-age model (assuming 10 cm Myr−1 of erosion) for all
27 exposure samples in Kennar Valley. Ages are listed in thousands of years (ka).
The areal distribution of drifts in lower Kennar Valley, along with a
relative and numerical chronology afforded by surface-weathering
characteristics and 3He exposure ages, call for overall thinning of upper
Taylor Glacier over the last 3.1 Myr, although subtle readvances cannot
be precluded. At ~3.1 Ma, the margin of upper Taylor Glacier in Kennar
Valley stood at ~1610 m elevation (~205 m higher than at present).
K.M. Swanger et al. / Global and Planetary Change 79 (2011) 61–72
Between ~1.8 and ~0.56 Ma, a series of four closely spaced moraines
were deposited between ~1500- and 1490-m elevations (~85–95 m
above present Taylor Glacier). During the last ~0.32 Myr, the level of
upper Taylor Glacier in Kennar Valley retreated from ~1475-m elevation
(~70 m above present values) to its present value of ~1405 m elevation.
Throughout all intervals of moraine deposition, the glacier margin
remained cold-based (frozen to its bed) and lacked significant surface
melting. There are no textural features within drifts to suggest clast
transport beneath wet-based ice (i.e., no striated, molded, or polished
clasts) and moraines lack associated outwash sediments. The
implication is that climate conditions during drift deposition at this
site were essentially similar to modern conditions.
Comparison of our moraine record with published reports for
fluctuations of Taylor Glacier elsewhere in the Quartermain Mountains,
and with a dated moraine record from Ferrar Glacier (also sourced from
Taylor Dome), reveals consistent ice-surface changes, highlighting minor,
but widespread ice recession in southern Victoria Land since the mid- to
late-Pliocene. The combined records show an atypical relationship with
average global temperatures, with higher-than-present ice levels during
globally warm periods, including the Pliocene climatic optimum (~3.0–
3.1 Ma), MIS 31 (~1.0 Ma), and MIS 5.5 (~125 ka) (Brook et al., 1993;
Marchant et al., 1994; Higgins et al., 2000; Staiger et al., 2006). The Kennar
Valley glacial record highlights the potentially complex and non-uniform
response of Antarctic ice to climate change. The record suggests lowamplitude fluctuations of Taylor Dome during the last 3.1 Myr, while
offshore sediment cores indicate considerable variability in the extent of
grounded marine-based ice from the WAIS in the Ross Embayment
(Fig. 1) (Naish et al., 2009).
Acknowledgements
We thank Douglas Kowalewski and David Shean for excellent
assistance in the field and for insight regarding GPS data. We also
thank two anonymous reviewers whose comments helped improve
this manuscript greatly. Funding for this research was provided by
NSF Polar Programs Grants ANT-1043706 and ANT-0944702 to DRM
and ANT-1043724 to KMS.
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