TCThe CryosphereTCThe Cryosphere1994-0424Copernicus PublicationsGöttingen, Germany10.5194/tc-12-2741-2018West Antarctic sites for subglacial drilling to test for past ice-sheet collapseWest Antarctic sites for subglacial drillingSpectorPerryspectorp@gmail.comhttps://orcid.org/0000-0003-2820-4526StoneJohnPollardDavidHillebrandTrevorLewisCameronGombinerJoelDepartment of Earth and Space Sciences, University of Washington,
Seattle, WA, USABerkeley Geochronology Center, 2455 Ridge Road,
Berkeley, CA, USAEarth and Environmental Systems Institute,
Pennsylvania State University, University Park, PA, USACenter for Remote Sensing of Ice Sheets (CReSIS),
University of Kansas, Lawrence, KS, USAPerry Spector (spectorp@gmail.com)24August20181282741275730April20184June201831July20188August2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://tc.copernicus.org/articles/12/2741/2018/tc-12-2741-2018.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/12/2741/2018/tc-12-2741-2018.pdf
Mass loss from the West Antarctic Ice Sheet (WAIS) is
increasing, and there is concern that an incipient large-scale deglaciation
of the marine basins may already be underway. Measurements of cosmogenic
nuclides in subglacial bedrock surfaces have the potential to establish
whether and when the marine-based portions of the WAIS deglaciated in the
past. However, because most of the bedrock revealed by ice-sheet collapse
would remain below sea level, shielded from the cosmic-ray flux, drill sites
for subglacial sampling must be located in areas where thinning of the
residual ice sheet would expose presently subglacial bedrock surfaces. In
this paper we discuss the criteria and considerations for choosing drill
sites where subglacial samples will provide maximum information about WAIS
extent during past interglacial periods. We evaluate candidate sites in West
Antarctica and find that sites located adjacent to the large marine basins of
West Antarctica will be most diagnostic of past ice-sheet collapse. There are
important considerations for drill site selection on the kilometer scale that
can only be assessed by field reconnaissance. As a case study of these
considerations, we describe reconnaissance at sites in West Antarctica,
focusing on the Pirrit Hills, where in the summer of 2016–2017 an 8 m
bedrock core was retrieved from below 150 m of ice.
Introduction
There is strong but indirect evidence for a diminished West Antarctic Ice
Sheet (WAIS) during some past warm interglacial periods of the Pleistocene.
Coastal records of former sea-level highstands see reviews
in, along with geological evidence from Antarctica
, ice-sheet modeling experiments (e.g.,
), and other lines of evidence (e.g.,
; see review in
) suggest large-scale deglaciation of the WAIS
within the past ∼1 million years (Myr), which may have occurred as
recently as the last interglacial period, ∼125 thousand years before
present (kyr BP). It has also been suggested that the WAIS, along with the
Greenland Ice Sheet and parts of the East Antarctic Ice Sheet, disappeared
during the mid-Pliocene (∼3 Myr BP), the last time atmospheric
CO2 concentrations reached modern levels. However, the evidence for
this, which largely consists of relative sea-level data and marine oxygen
isotope records, has large uncertainties, which preclude robust estimates of
sea-level and ice-sheet volume during this period (see reviews in
and ).
Large-scale deglaciation of the WAIS (so-called “collapse”) is theorized to
occur because much of the ice sheet overlies deep marine basins in a
configuration that makes it susceptible to a feedback involving marginal
retreat, flow acceleration, thinning, and flotation
. Increasing ice loss is
presently occurring through these mechanisms in the Amundsen Sea sector of
the WAIS (Fig. ) see review in,
and numerical modeling suggests that an incipient collapse of the ice sheet
is underway in this sector e.g.,. How much and
how quickly future sea level will rise due to WAIS deglaciation remains
unknown , but continued ice loss could eventually
increase global mean sea level by 3–4 m .
Knowledge of ice-sheet extent during interglacials warmer and more prolonged
than the Holocene would be invaluable for understanding, and potentially
predicting, the future stability or instability of the WAIS. Geological
observations from West Antarctica which constrain the configuration of the
WAIS during former interglacial periods remain scarce because evidence of its
limits during these times is concealed beneath the present-day ice sheet.
Maps of West Antarctica. The left map is colored to identify areas
where the bed is presently below sea level and areas where it is above
. The right map is colored to show the velocity of
grounded ice . White circles show the locations of
possible drill sites discussed in the text. Black circles show the locations
of other sites discussed in the text.
One potential source of evidence is the presence or absence of long-lived
cosmogenic nuclides in subglacial bedrock. Because most cosmic radiation is
absorbed by as little as 5–10 m of ice cover, discovering significant
concentrations of these nuclides would provide unambiguous evidence for
former ice-free conditions and could establish whether the WAIS collapsed
during the past few million years. At sites affected by a single collapse,
cosmogenic-nuclide measurements can directly date that event. In the case of
more complex deglaciation histories, the same data record the cumulative
exposure time of the bedrock surface. Although cosmogenic nuclides have the
potential to unambiguously indicate past ice-sheet collapse on timescales
ranging from the Holocene to the Pliocene, the power of the method depends on
careful site selection. In this paper we describe the criteria and
considerations for choosing subglacial sampling sites where
cosmogenic-nuclide data will provide the maximum amount of information about
WAIS extent during past interglacials.
In central Greenland, a bedrock core was opportunistically retrieved from
below the full thickness of the ice sheet at the GISP2 drilling site.
Concentrations of cosmogenic 10Be and 26Al in the core
require periods of prolonged exposure during the Pleistocene when the ice
sheet was largely absent . In
contrast to this work, bedrock recovered from below the thick portions of the
WAIS would not be capable of providing equivalent information because the bed
in these areas is located far below sea level (Fig. ) and
would remain submerged, shielded from the cosmic-ray flux, if the ice sheet
collapsed. Establishing whether the thick, marine-based portions of the WAIS
disappeared in the past will therefore require drilling through adjacent,
thinner portions of the ice sheet into subglacial highlands that would be
exposed by ice thinning during collapse events.
In the 2016–2017 austral summer, the first cores of subglacial bedrock from
West Antarctica were recovered from the Pirrit Hills and the Ohio Range
(Fig. ), which was made possible by recent advances in
sub-ice drilling technology e.g.,. For these as
well as future drilling efforts to provide meaningful information about past
WAIS configurations, the measurements made on the recovered bedrock must be
representative of the past ice thickness at the drill site, which in turn
must be linked to the extent of the broader ice sheet. In Sect. 2 of this
paper, we describe cosmogenic-nuclide considerations that guide drill site
selection. In Sect. 3, we use an ice-sheet model to predict the areas of the
WAIS where significant thinning (and thus exposure of presently subglacial
bedrock) would occur during collapse events. In Sect. 4, we evaluate a group
of candidate drill sites throughout West Antarctica. Finally, in Sect. 5, we
describe reconnaissance work at three sites in West Antarctica, with emphasis
on the Pirrit Hills, which we present as a case study of drill site selection
on the scale of an individual nunatak.
Cosmogenic nuclide considerations for drill site selectionStrategies for subglacial bedrock sampling and analysis
Because subglacial drilling is expensive and time consuming, drill site
selection, drilling operations, and analysis of recovered samples should be
designed to maximize the information provided by the inherently limited
amount of subglacial bedrock. There are several strategies to accomplish
this. At a given drill site, collecting multiple bedrock cores in an
elevation transect below the modern ice surface can establish the magnitude
of past deglaciations. By locating drill sites near outcropping mountains,
elevation transects can be extended up to and above the limits of the
thicker, ice-age WAIS, thereby constraining ice-thickness variations over the
full glacial–interglacial cycle. Drilling near outcrops also allows the
subglacial rock type to be inferred with confidence, which is important as
not all lithologies are suitable for cosmogenic-nuclide measurements.
Although measuring a single cosmogenic nuclide (e.g., 10Be or
36Cl) in subglacial bedrock samples is enough to detect past
exposure, measuring several nuclides that have different half lives yields
considerably more information about the glacial history, in both the recent
and the distant past (e.g., ). Independent
constraints on the most recent period of exposure can potentially be added by
(i) collecting and dating the basal ice and (ii) luminescence dating of the
subglacial bedrock surface. Because cosmogenic nuclides are primarily
produced within the topmost few meters of the bedrock surface, preservation
of the record of past exposure requires drill sites to be located in areas
where erosion below or above the ice has been minimal. Knowledge of the
erosion history, which is required for accurate interpretation of the glacial
history, can be gained by analyzing not only surface samples of the
subglacial bedrock but by measuring depth profiles in rock cores that extend
several meters below the surface .
Subglacial bedrock lithology
The central portion of West Antarctica is composed of three tectonic and
topographic blocks, the Ellsworth–Whitmore Mountains, Marie Byrd Land, and
what is known as the Thurston Island region, which are separated by low-lying
areas of the West Antarctic Rift System (Fig. ). The
portions of the bed that would be above sea level if the WAIS collapsed
(accounting for isostatic effects) are primarily located in these three
tectonic regions; however there are isolated peaks and plateaus, such as
subglacial Mt. Resnik , which rise above sea level
from the deep marine basins (Fig. ).
Drilling into these subglacial highlands must target rock types in which
useful cosmogenic nuclides can be measured. Commonly measured nuclides (and
their half-lives) include 10Be (1.4 Myr), 26Al
(0.7 Myr), 21Ne (stable), 36Cl (0.3 Myr), 3He
(stable), and 14C (5.7 kyr). If more of these nuclides can be measured,
more restrictive time constraints can be placed on exposure episodes that may
have occurred in the distant and/or recent past. Because (i) all of these
nuclides (except 36Cl) can be measured in quartz, and (ii) their
production rates are best known for quartz, quartz-rich rocks would allow the
glacial history to be constrained over a large range of timescales. Igneous
or metamorphic rocks of granitic composition would permit the greatest
variety of analyses because, in addition to containing quartz, they also
contain potassium feldspar and (commonly) Cl-rich mica in which
36Cl can be measured. Other analytical strategies could be applied
to the volcanic rocks that underlie large areas of the WAIS. For example,
cosmogenic 3He, 21Ne, and 36Cl could be measured
in basaltic pyroxene, or in a combination of quartz and sanidine from more
felsic volcanic rocks. However, the relative abundance of useful minerals
such as these is also important because of the small amount of rock that can
be retrieved by subglacial drilling .
The necessity of recovering a suitable rock type implies that it will likely
be advantageous to target the subglacial extension of outcropping mountains,
where the rock type at depth can be inferred with confidence. Although
geophysical surveys can narrow the range of possible lithologies, the precise
identity and mineralogy of underlying bedrock generally remains unknown
e.g.,. In this paper, we
largely restrict our consideration of potential drill sites to areas near
mountains of granitic composition. Figure shows the
location of some of the granitic nunataks in West Antarctica. Although this
map is not comprehensive, it is generally representative of their geographic
distribution. Scattered granitic nunataks outcrop between the Ellsworth
Mountains and the southern Transantarctic Mountains, predominately in the
Weddell Sea sector of the WAIS. Granitic nunataks are also located (i) at the
base of the Antarctic Peninsula, (ii) in the Thurston Island region, and
(iii) along sections of the Marie Byrd Land coast. In addition to granitic
sites, in Fig. 1 we also include quartzite peaks of the northern Ellsworth
Mountains, the isolated quartzite nunatak of Mt. Johns, and subglacial Mt.
Resnik, which, although likely volcanic , is a
tempting drill target because (i) it lies upstream of where
found evidence for a large-scale Pleistocene
deglaciation of the WAIS, (ii) its conical form and high relief suggest that
it erupted subaerially when the ice sheet was absent ,
and (iii) its summit is only ∼330 m below the ice surface
.
Preservation of the cosmogenic-nuclide record
Because the cosmogenic-nuclide record is primarily produced in the topmost
few meters of an exposed bedrock surface, its survival requires that the
bedrock remains continuously protected from erosion. This is most likely to
be the case in areas that are surrounded by slow-flowing ice and where
thickening of the ice sheet during past glacial periods such as the Last Glacial Maximum (LGM) was
minimal. Other factors which promote cold-based ice are high accumulation
rates, low surface temperatures, and a low geothermal heat flux. As discussed
in Sect. , sites in the WAIS
interior are likely to have preserved subglacial bedrock surfaces hundreds of
meters below the modern ice level. Preserved subglacial surfaces also likely
exist near the ice-sheet margin; however, it may be more difficult to
identify these sites with confidence. Although subglacial bedrock samples
that have remained continuously uneroded will provide the greatest
constraints on the glacial history, samples that have experienced low rates
of erosion may also be of use, provided that the erosion history can be
estimated. As mentioned above, this can be accomplished by measuring cosmogenic
nuclides not only in the subglacial bedrock surface but also in depth
profiles along the length of short (∼3–6 m) bedrock cores
.
A related concern to bedrock erosion is the possibility that presently
subglacial surfaces remained concealed by till, soil, or snow when the ice
sheet disappeared in the past. Failure to account for past surface cover
would cause the true exposure history to be underestimated. Analysis of the
subglacial bedrock core from the GISP2 site in central Greenland suggests
that the present-day bedrock surface was covered by a thin layer of material
when the ice sheet disappeared during the Pleistocene
. Soil accumulation there is plausible because
debris-rich basal ice in the GISP2 and other Greenland ice cores contains
evidence for a vegetated landscape during one or more interglacial periods in
the past million years . In Antarctica, however, fossil organisms and pollen
from sites spanning the continent show that a tundra landscape went extinct
around the mid-Miocene, and that the climate has been continuously polar
since that time . Although soil is unlikely
to have covered drill targets in Antarctica during former interglacial
periods, accumulated till is possible. In the vicinity of outcropping
mountains where drill sites will probably be located, englacial debris
commonly accumulates in blue-ice areas, and subsequent thinning could drape
the underlying bedrock with a layer of till. As discussed below in
Sect. , this concern can be mitigated by
locating drill sites above subglacial ridges where the likelihood of till or
snow cover is minimal.
Where will WAIS collapse cause the largest changes?
WAIS collapse is theorized to occur via a feedback in which initial retreat
of the grounding line into deeper water causes more ice to flow across the
grounding line . This accelerates
the flow of the ice sheet upstream, causing it to thin, which in turn causes
previously grounded ice to float as the grounding line recedes farther
inland. The thinning of upstream grounded ice is important because it is the
link between withdrawal of ice from the marine basins and the exposure of
presently subglacial bedrock surfaces in the WAIS interior. Therefore, an
overarching criterion is that drill sites be located in areas that experience
the largest change in ice thickness during collapse events, bearing in mind
the need to avoid sites where ice flow may have been erosive in the past.
Recent observations in the Amundsen Sea sector show that thinning induced by
grounding-line retreat is greatest near the ice-sheet margin, but remains
detectable hundreds of kilometers upstream .
This suggests that although large portions of the WAIS are prone to thinning
during deglaciations, some sites will be more or less diagnostic of past
ice-sheet collapse.
The geological and glaciological constraints on WAIS configuration during
times of reduced ice volume are scant and therefore of limited use for
assessing the response of candidate drill sites during collapse events. A
much more complete record exists for the deglaciation of marine-based ice in
the Ross Sea following the LGM, which may provide a useful analog. Withdrawal
of ice from the outer Ross Sea, where the ice sheet was relatively thin, led
to moderate thinning of ∼300 m in adjacent areas of northern Victoria
Land (Brent Goehring, personal
communication, 2018). Moderate thinning also occurred at Siple Dome
, which overlies a broad
area of high topography, as well as at the Ohio Range ,
which remains well upstream of the modern grounding line. The greatest
thinning (>1 km), however, is recorded at sites in the southern
Transantarctic Mountains that directly abut a deep marine basin in the
western Ross Sea which lost ∼1.5–2 km of ice . Collectively, these observations suggest that if the WAIS
collapsed during past interglacial periods, the greatest thinning likely
occurred at sites directly adjacent to the deep marine basins in West
Antarctica that became ice free, such as Bentley Subglacial Trench
(Fig. ).
In order to examine the transient response of different parts of the WAIS to
collapse events, and thereby identify which areas will be most diagnostic of
past deglaciations, we use the Penn State University ice-sheet model (PSU-3D)
to simulate the Antarctic Ice Sheet continuously over the past 5 Myr. This
time period, from the early Pliocene to the present, covers a large range of
glacial–interglacial climates and is comparable to the time period that can
be investigated with cosmogenic-nuclide measurements. In comparison, most
existing collapse simulations depict only a single deglaciation (e.g.,
) or are equilibrium models (e.g.,
), which may not represent ice-sheet behavior during
short-lived events such as Pleistocene interglacial periods.
The model we use solves a hybrid combination of the scaled dynamical
equations for the flow of grounded and floating ice
. It
is similar to that presented in ; however it uses
basal sliding coefficients derived from inverse calculations, as well as
improved parameterizations of ice-shelf calving and sub-ice oceanic melting
. A parameterization by allows
for reasonably accurate simulations of grounding-line migration on the coarse
grids that are required for multimillion year model runs. We use a 40 km
horizontal grid, which, although coarse, is comparable to the resolution with
which the bed is known in the vicinity of many of the candidate drill sites
. Bedrock deformation in the model is treated as
an elastic lithosphere above a viscous asthenosphere that relaxes toward
isostatic equilibrium. The model includes two processes that exacerbate
retreat during warm climates: (i) hydrofracture of ice shelves due to surface
water draining into crevasses and (ii) structural failure of ice cliffs at
the grounding line .
The model is forced with parameterizations of surface temperature,
precipitation, sub-ice-shelf melting, and sea level, which have been
described in previous publications . These parameterizations are largely functions of a
stacked benthic δ18O record and
orbital insolation variations . In this run, we add the
influence of long-term atmospheric CO2 decline by prescribing a
linear ramp from 400 to 280 ppmv CO2 between 3 and 2 Ma, with
corresponding small uniform shifts to atmospheric and oceanic temperatures.
This results in generally smaller model ice volumes prior to 3 Myr compared
to . The parameters for the simulation shown in
Fig. have been calibrated in previous model experiments
(e.g., ).
Because many of the parameters related to the climate forcing and aspects of
the model physics are uncertain, and alternative values can affect the size
of the ice sheet and its rate of change, we use this simulation as a guide to
how ice thickness at candidate drill sites responds to deglaciation of the
marine basins rather than an accurate depiction of the ice sheet through
time.
Results of a 5 Myr ice-sheet simulation. (a) Variations in
the mass of the WAIS, where the WAIS is taken to be the portion of the ice
sheet between 180∘ and 50∘ W. The maps show the WAIS at its smallest (b) and largest
(c) extents of the past 1 Myr. These occur at 205 and 625 kyr BP,
respectively. The modern grounding line is shown for comparison. The maps are
colored to show how much thinner or thicker the ice sheet was at these times
relative to present. (e–s) The relationship between WAIS mass and
local ice thickness at candidate drill sites. Each point represents a single
5000-year model time step and is colored to distinguish ice-sheet behavior
during the Late Pleistocene (blue: 0.8 Myr BP–present) from the Early
Pleistocene (yellow: 2.58–0.8 Myr BP) and Pliocene (red:
5.0–2.58 Myr BP). White circles represent the conditions at 205 and
625 kyr BP, which correspond to the maps in panels (b) and
(c), respectively. Horizontal dashed lines on some plots represent
complete deglaciation of the site. Note that although the vertical axes are
limited to values between -1000 and 1000 m, during large deglaciations Mt.
Resnik and Pagano Nunatak become completely ice free, and the local ice thins
by
up to ∼2600 and ∼1700 m, respectively. As shown in
Fig. and as discussed in the text, the modeled
present-day ice sheet places the grounding line upstream of Robin Subglacial
Basin (compare to Fig. ). A result of this misfit is that,
for sites upstream of this area, ice-thickness changes shown in panels
(e)–(s) underestimate the thinning during interglacial
periods and overestimate the thickening during glacial periods.
The model is sampled every 5000 years, which sufficiently captures the
ice sheet's variations and results in minimal aliasing. The modeled ice sheet
transitions rapidly between expanded and contracted configurations on orbital
frequencies that reflect the climate forcing (Fig. ).
The simulated ice sheet was smaller than present prior to ∼2.7 Myr BP,
at which time its average size began to increase, and collapsed
configurations with little or no marine-based ice in West Antarctica became
less frequent. Some warm periods of the Pleistocene resulted in full
deglaciation of the marine basins, leaving small ice sheets on areas of high
topography, as shown in Fig. b. During other
interglacial periods, thinning and grounding-line retreat were more limited
and seaways were unable to link the Amundsen, Ross, and Weddell seas.
Although accumulation rates increase over the residual ice sheet during
deglaciations, in most areas this is insufficient to offset the dynamic
thinning. Thinning is typically greatest directly upstream of the retreating
grounding line, especially in areas adjoining deep marine basins that become
ice free, as expected from the considerations described above.
In most areas of central West Antarctica, the modeled present-day ice
thickness closely matches observations (Fig. ). The
one important exception is in the Weddell Sea sector, where the grounding
line in the model is located inland of Robin Subglacial Basin (compare to
Fig. ). Because ice thickness in
Fig. e–s is shown relative to the modeled present-day
ice sheet, this misfit causes thinning during interglacial periods to be
underestimated and thickening during glacial periods to be overestimated at
sites upstream of this area. The magnitude of this effect is difficult to
quantify due to the transient nature of the model, but at sites directly
upstream (e.g., Pirrit Hills, Nash Hills, and Pagano Nunatak) it will likely
exceed 100 m. For individual sites in Fig. e–s, this
would have the effect of shifting points uniformly along the Y axis, but
not changing their position relative to each other.
The difference between the modeled modern thickness of grounded ice
and the thickness of grounded ice in Bedmap2 . The
grounding line for the modeled ice sheet is inland of Robin Subglacial
Basin.
Evaluation of candidate drill sites
In this section we use the ice-sheet model described above and available
geologic information to evaluate candidate drill sites in terms of (i) how
changes in local ice levels are related to the extent and configuration of
the broader ice sheet and (ii) whether the cosmogenic-nuclide record of past
exposure is likely to have remained preserved by cold-based ice cover.
Sensitivity of sites to deglaciation of different parts of the WAIS
During collapse episodes simulated by the ice-sheet model, the grounding line
retreats rapidly over deep marine basins and is ultimately halted once it
reaches shallow water. Because further retreat can only occur slowly and in
the presence of a warm atmosphere, the ice-sheet margin is commonly pinned to
a narrow band that closely follows the perimeter of the highland regions
(Figs. and ). This is not unique to
our simulation; rather it is a robust feature of ice-sheet models forced with
interglacial climates (e.g., ). The model demonstrates that in areas that remain
glaciated, thinning occurs as the grounding line approaches, but then stops
once the grounding line stabilizes, even if deglaciation continues in other
sectors of the WAIS. The implication of this is that the magnitude of
thinning at candidate drill sites is most directly controlled by the
proximity of the grounding line and the thickness of ice lost from the marine
basins immediately downstream and less by ice-sheet changes elsewhere in
West Antarctica.
At sites not adjoined to large marine basins, such as the Ford Ranges and the
Jones Mountains, it may be difficult to know whether subglacial evidence of
past exposure resulted from large-scale WAIS deglaciation or from modest
retreat of the ice-sheet margin locally. In a related way, ice thinning at
the Pirrit Hills, Nash Hills, and Pagano Nunatak is likely to be controlled
most strongly by deglaciation of Robin Subglacial Basin, which underlies
the
Institute and Möller ice streams (Fig. ). Therefore,
these sites may not be as sensitive to the presence or absence of grounded
ice in the much larger basins of the Amundsen Sea and Ross Sea sectors of the
WAIS. A potential caveat to this is that our simulation, as well as others
using the PSU-3D ice-sheet model (e.g., ),
shows that the large marine basins deglaciate in unison, implying that
evidence for deglaciation from one sector could be extrapolated to other
areas in West Antarctica. At sites on the periphery of the large basins in
the Ross and Amundsen sectors (i.e., Mt. Murphy, Mt. Resnik, the Ohio Range,
the Whitmore Mountains, Mt. Woollard, Mt. Johns, and the northern Ellsworth
Mountains), ice levels will likely be diagnostic of whether full collapse of
the WAIS has occurred in the past.
Magnitude, timing, and frequency of thinning
The areas where the model predicts large drawdowns, not only during the most
severe interglacial periods (which occur during the Pliocene in the
simulation) but also during briefer warm periods of the Pleistocene, are the
most likely areas to find evidence of past deglaciation in the form of
previously exposed subglacial bedrock. The two sites that exhibit the
greatest and most consistent ice loss are Mt. Woollard and Mt. Johns, which
thin by up to ∼800 m during the Pliocene, the Early Pleistocene, and
the Late Pleistocene (Fig. e, i). The frequency of
deglaciation at these sites is perhaps not surprising as they are located at
the head of the Thwaites Glacier catchment, where (i) there is concern at
present about the stability of the grounding line (e.g.,
), and (ii) there are no major topographic obstacles
to impede the grounding line until it reaches the Ellsworth–Whitmore
Mountains. The large magnitude of deglaciation at these sites results from
their proximity to Bentley Subglacial Trench, where more than 3 km of ice
would be lost during large deglaciations. Most of the other sites in the
Ellsworth–Whitmore Mountains are predicted to thin consistently and
significantly during collapse episodes; however, the greatest thinning is
commonly restricted to the prolonged warm climates of the Pliocene, and does
not occur, or does so only rarely, during the briefer Pleistocene
interglacial periods (Fig. ).
Almost no thinning is predicted at Mt. Petras or the Thiel Mountains, which
are not located upstream of deep marine basins that are vulnerable to
deglaciation, and thus are not expected to experience dynamic thinning as a
result of WAIS collapses. At Mt. Moulton, a peak ∼175 km from Mt.
Petras, ice has been found in an ablation area which dates to the last
interglacial period, and as far back as ∼500 kyr BP
. Although the
presence of this ice does not require continuous glaciation of other portions
of Marie Byrd Land, it is consistent with the simulation of stable ice levels
shown in Fig. q. Minimal thinning is also predicted at
Haag Nunataks, which is surprising because, in contrast, this site is
surrounded by marine basins that deglaciate during collapse episodes. The
site is unique, however, in that during simulated collapse events it is
located within a small ice cap that is represented by as few as six grid cells
(Fig. b). Unlike the majority of the WAIS, these cells
exhibit high spatial variability, with adjacent cells thinning and
thickening, respectively, at times (which cancel out to produce the modest
response shown in Fig. h). This example serves as a
cautionary reminder that model results should not be overinterpreted,
especially in areas where inadequate grid resolution results in spatial
patterns of thinning and thickening that do not vary smoothly.
As shown in Fig. c, one possible complication predicted
by the model is modest thinning in the WAIS interior during glacial periods
. This is potentially important at the Whitmore
Mountains, Mt. Woollard, and Mt. Johns because bedrock surfaces, covered by
less than ∼100 m of ice, may have been exposed not only during collapse
events but also when the ice sheet was larger. Although Mt. Resnik is also
located in the region expected to thin, its summit is ∼330 m below the
surface and would likely remain fully ice covered. The
thinning is caused by reduced accumulation over the ice-sheet interior due to
the decreased ability of the cold glacial atmosphere to carry moisture.
Although similar thinning has been simulated previously (e.g.,
), most models of the LGM, including ones using
the PSU-3D ice-sheet model, depict thicker-than-present ice
e.g.,. The only existing geologic constraints come
from exposure dating at Mt. Waesche and the Ohio Range, sites on the margin
of the region predicted to thin. These data indicate that ice was modestly
thicker at ∼10 kyr BP .
However, because this is several thousand years after accumulation rates
began to rise in West Antarctica , the data do not
preclude thinner ice prior to ∼10 kyr BP. Although lower ice levels
during glacial periods could complicate the search for evidence of past
ice-sheet collapse, determining whether ice levels in the WAIS interior were,
in fact, lower during the LGM would be significant in its own right. Because
thinning in the interior is expected to be less than ∼200 m
(Fig. c), it would likely be possible to drill to deeper
depths to encounter bedrock that may have only been exposed during collapse
episodes.
Preservation of subglacial bedrock surfaces
The ice-sheet model could, in theory, be used to predict erosion at candidate
drill sites; however, the 40 km grid resolution is too coarse for this
purpose, given that drill sites will likely be located near nunataks where
topographic relief is high. Even if the model was run at higher resolution,
insufficient knowledge of the geothermal heat flux and other factors that
influence basal conditions would preclude accurate erosion predictions.
Instead we use the model as a guide to the relationship among three
factors: ice thickness, surface velocity, and basal velocity
(Fig. ). Thickness and surface velocity are
known or easily measured, while basal velocity, a strong indicator of erosion
or preservation, is generally unknown or difficult to infer from
measurements. Figure shows that at the
depths of interest for subglacial drilling in West Antarctica (up to
∼1 km), significant glacial erosion is unlikely in areas where the
surface velocity is less than ∼10 m yr-1. Such areas are common
in the ice-sheet interior, as well as in some areas near the margin
(Fig. ). This result is consistent with other ice-sheet
model experiments investigating the distribution of subglacial erosion in
Antarctica .
(a) The relationship between surface velocity, ice
thickness, and basal velocity, as predicted by the 5 Myr ice-sheet model.
Results for the full ice sheet are binned by surface velocity and ice
thickness, and each bin is colored by its average basal velocity. This
averaging hides considerable variability in the actual range of basal
velocities within each bin. Therefore, in panels (b)–(d), we
provide histograms of basal velocity for points in the model where the ice is
less than 1000 m thick, for surface velocities of ∼1, ∼10, and
∼100 m yr-1. At depths relevant to subglacial drilling,
significant subglacial erosion is unlikely in areas where the surface
velocity is less than ∼10 m yr-1 (see Fig. ).
Although slow-flowing ice is present near many of the candidate drill sites
shown in Fig. , the lowest velocities occur at the ice
divides in the interior, near sites such as the Whitmore Mountains and Mt.
Woollard. At these interior sites, where (i) surface temperatures are very
low and (ii) ice either thickened modestly during glacial periods
(;
Sect. ) or potentially thinned in some
areas (Fig. ), subglacial drill targets, to depths of at
least a few hundred meters, have likely remained continuously frozen. This is
supported by field observations and exposure dating at the Pirrit Hills, Ohio
Range, Nash Hills, and Whitmore Mountains (; Sect. ), which
show that bedrock surfaces near the modern ice level are commonly weathered
and have exposure ages of hundreds of thousands of years, implying that
preserved bedrock surfaces likely extend below modern ice levels.
Sites near the modern ice-sheet margin, such as Haag Nunataks, Mt. Murphy,
and the Jones Mountains, are predicted to thicken by up to ∼800–1000 m
during glacial periods (Fig. ), which suggests that
bedrock surfaces there may be vulnerable to erosion beneath thick ice that is
sliding at its base. Geologic constraints on former highstands are not
available at these sites; however field observations from a site near Mt.
Murphy indicate that ice was at least ∼300 m thicker than present
during the LGM . Of all the candidate sites that are
located near the coast, the most extensive investigation of former ice cover
has been conducted at the Ford Ranges , a group of peaks which extend ∼100 km inland from
the modern grounding line (Fig. ) and span multiple grid
cells in the ice-sheet model. The model predicts moderate thickening during
glacial periods at the upstream edge of the Ford Ranges
(Fig. s), but considerably more near the modern
grounding line (up to ∼900 m; not shown in Fig. ),
which is consistent with geologic constraints on LGM ice levels
. reported evidence of
wet-based glacial erosion on the lower flanks of many of the peaks in the
Ford Ranges, especially those closest to the modern grounding line.
Therefore, if uneroded subglacial bedrock surfaces exist here, they will most
likely be found at shallow depths near the inland peaks, where past
thickening was more limited and ice velocities are lower.
In contrast to the evidence for glacial erosion, it should be noted that
other sites near the ice-sheet margin, such as the Pensacola Mountains
(Fig. ), show evidence for surface preservation by
cold-based ice cover .
Additionally, radar data over ice rises around the perimeter of Antarctica
indicate that the majority of them are currently frozen to their beds
. Although this was not necessarily the case
during past glacial periods, the radar data indicate that cold-based ice may,
in fact, be common near slow-flowing areas of the ice-sheet margin. Taken
together, these observations indicate that although preserved subglacial
bedrock surfaces likely exist at some coastal sites, it may be challenging to
predict their locations with confidence.
Bedrock lithology
With the possible exception of Mt. Resnik, which is fully ice covered, the
sites shown in Figs. and all have
quartz-bearing bedrock that would allow for measurements of a wide range of
cosmogenic nuclides. However, at some sites, the exposed rock also includes
lithologies in which the ability to make cosmogenic-nuclide measurements
would be limited, or, in some cases, impossible. These sites are the Nash
Hills (see Sect. ), Mt. Petras
, the Jones Mountains
, and Mt. Johns . Drilling
at these sites may therefore remain risky unless the subglacial distribution
of rock types can be determined.
Drill site reconnaissance
(a) Bedrock ridge located southeast of Mt. Axtell at the
Pirrit Hills (Fig. ). The granite is oxidized and
displays large cavernous weathering pits (ice axe in foreground for scale).
Such weathering features are common at the Pirrit Hills, both on higher
mountain flanks and intersecting the modern ice surface as shown here.
(b) Photo looking up the NE ridge of Mt. Axtell. Glacially deposited
boulders rest on the more oxidized bedrock of the ridge. The depositional
limit is ∼15 m above the boulders in the foreground. (c) Photo
of Harter Nunatak, showing the location of bedrock samples collected for
cosmogenic-nuclide measurements. (d) Backside of the bedrock ridge
shown in panel (a). The ridge is orthogonal to the surface winds.
Ice levels on the upwind side of the ridge are ∼90–130 m higher than
at the blue-ice ablation zone in the lee. The ridge is directly flanked by a
wind scoop on its upwind side (not visible in panel a) and a snow
apron on its downwind side.
(a) WorldView satellite imagery (copyright DigitalGlobe,
Inc.) of the Pirrit Hills. Bright areas on the ice sheet generally correspond
to areas of steeper surface slopes and overlie or are in the lee of
subglacial ridges. The black box shows the location of panel (b).
(b) Map of Harter Nunatak (center) and the surrounding area.
Elevation contours are derived from WorldView satellite imagery (DEM created
by the Polar Geospatial Center). Colors represent ice thickness as measured
with the GSSI radar system and arrows represent ice velocity as measured by
repeat stake surveys (refer to the Appendix for measurement details). The
location of the radar profile in Fig. is not shown on
this map, but it intersects the RB-2 borehole and is oriented approximately
perpendicular to the ice-flow direction. The ice velocity measured closest to
the RB-2 drill site is ∼0.25 m yr-1. Changes in ice-surface
elevation were also measured at these stakes. The winds flow from the
southwest, but are forced to diverge around the Pirrit Hills. The range of
wind directions shown in panel (a) reflects the fact that the wind
orientation varies with location around the mountains. The wind direction is
relatively constant within the domain of panel (b) and so a single
vector is used to represent the wind direction.
10Be and 26Al data for the bedrock samples from
Harter Nunatak shown in Fig. c. Nuclide concentrations
are normalized to the surface production rate for each sample (N*=N/P,
where N is the 10Be or 26Al concentration and P is
the local production rate of that nuclide). Ellipses represent 1σ
uncertainty regions. Continuously exposed and uneroded surfaces will plot
along the black line near the top. Continuously exposed and eroding surfaces
will plot between the black line and the solid gray line. Samples plotting
below the solid gray line, such as those from Harter Nunatak, require at
least one episode of ice cover following prior exposure. Dashed contours show
lower limits on the cumulative exposure and cumulative ice cover experienced
by the samples.
As discussed in Sect. there are
advantages to drilling in the neighborhood of exposed nunataks. However, the
effects of nunataks on the local ice flow and meteorology cannot be captured
at the scale of the ice-sheet model used in
Sects. 3 and
4, introducing considerations
that need to be addressed by field reconnaissance. Fieldwork prior to
drilling also allows for (i) sampling of exposed bedrock to test its
antiquity with cosmogenic-nuclide measurements, (ii) determining the limits
of glacial-to-present ice-sheet fluctuation, (iii) examining weathering
features for evidence of rock surface preservation, and (iv) locating
potential drill sites using ice-penetrating radar.
We visited three sites in West Antarctica – the Pirrit Hills, Nash Hills,
and Mt. Seelig in the Whitmore Mountains – in the 2012–2013 austral summer
to evaluate their potential for subglacial drilling. These three sites were
selected as lying as closely as possible to a single flow line, with the
(perhaps optimistic) idea of subglacial drilling along such a transect. In
the end, most reconnaissance work was carried out at the Pirrit Hills; time
constraints and problems with radar equipment limited exploration at the
other two sites. A brief reconnaissance at the Nash Hills revealed rock types
not suitable for cosmogenic-nuclide measurements and complicated bedrock
structure that will require geophysical surveying to locate drill sites above
subglacial granite. At the Whitmore Mountains, long-lived cosmogenic-nuclide
measurements (which will be described in a forthcoming publication) show
evidence of prolonged exposure and limited ice-thickness changes. This,
combined with the implication from the ice-sheet model that modest thinning
may occur at the Whitmore Mountains during glacial periods (see
Fig. ,
Sect. ), discouraged us from
selecting this site for initial subglacial drilling. Like the Nash Hills, the
Pirrit Hills rise from the ice sheet approximately half way from divide to
grounding line, where regional ice flow velocities are less than
5 m yr-1. Here we were able to obtain evidence
of low bedrock erosion, sizable glacial–interglacial fluctuations in ice
cover, and radar profiles that revealed potential drill sites. Based on these
and other factors discussed below, we chose to drill at two sites near Harter
Nunatak, a minor outcrop ∼5 km north of the Pirrit Hills massif.
Evidence for exposure and preservation of subglacial bedrock surfaces
At the Pirrit Hills, minimally weathered glacial deposits were found up to
∼330 m above the modern ice surface, marking the LGM highstand. This is
similar to the thickening predicted by the ice-sheet model
(Fig. f). Despite this evidence for thicker ice, there
is almost no indication of recent glacial erosion. Bedrock surfaces are
oxidized, and, in places, exhibit case hardening, wind polish, and cavernous
weathering pits (Fig. ). Weathered surfaces occur both
high on mountain flanks as well as near the modern ice surface, where they
commonly intersect and appear to descend below the ice. Similar evidence for
lower ice levels in the past has been found in other parts of East and West
Antarctica . Although these observations demonstrate that ice
levels were lower in the past, they establish neither the magnitude nor the
timing of thinning.
We collected two samples of weathered bedrock from Harter Nunatak
(Figs. , ) for analysis of
cosmogenic 10Be and 26Al to determine the exposure,
ice cover, and erosional history of the nunatak. Analytical methods are
described in the Appendix. Cosmogenic nuclide data
for other samples collected from the Pirrit Hills will be described in a
subsequent publication. Data from these samples require minimum cumulative
exposure of ∼580–600 kyr and minimum cumulative ice cover of
∼390–500 kyr (Fig. ). Together with the
geomorphic observations, this indicates that (i) the ice at the Pirrit Hills
has been both thinner and thicker than present for prolonged periods in the
past ∼1 Myr, and (ii) during this time bedrock surfaces have remained
preserved by the polar climate and cold-based ice cover.
Firn temperatures in the vicinity of the Pirrit Hills are approximately
-26∘C and the ice is undoubtedly frozen to bedrock within a few
hundred meters of the surface. Increasing ice thickness by ∼330 m
during the LGM is unlikely to have raised basal temperatures to near melting.
As discussed below, ice surface velocities at the site where we ultimately
decided to drill are <1 m yr-1. Given the expected relation between
surface velocity, ice thickness, and basal velocity shown in
Fig. , this suggests that uneroded bedrock
surfaces likely extend hundreds of meters below the modern ice level.
Local meteorology, accumulation, and ablation
As noted above, mean annual temperature in the region of the Pirrit Hills is
approximately -26∘C. Firn depths vary from zero over blue-ice
areas to at least ∼36 m, as measured in access holes for subglacial
drilling described below. Ice-motion stakes placed around Harter
Nunatak in 2015 and resurveyed 1 year later in 2016 (Fig. ) showed
changes in the ice surface of ±0.4 m, comparable to the height of
sastrugi in the area. Given these values, accumulation in the vicinity of the
nunatak appears to be low, suggesting that the firn and ice column overlying
the targeted drill sites accumulated upstream, but nearby.
Regionally, snow-bearing winds descend the ice sheet and cross the Pirrit
Hills from southwest to northeast. Snow has accumulated into an embankment
upwind of the Pirrit Hills, which rises over a distance of ∼5–10 km to
the level of the col between Mt. Tidd and Mt. Goodwin
(Fig. a). The ice surface drops ∼600 m across this
obstruction to the northeast, where the massif is bordered by a 1–2 km wide
blue-ice ablation zone. This geometry results from descending warm,
turbulent, foehn-like winds that ablate the ice surface in the lee of the
mountains see. Any changes in wind
direction during collapse events could modify this pattern of accumulation
and ablation and potentially induce ice-thickness changes comparable to
amounts expected from dynamical thinning (Fig. ).
Atmospheric modeling suggests that surface winds near the Pirrit Hills may
vary slightly in direction and magnitude during collapse events
; however a fundamental reconfiguration of
accumulation and ablation areas appears unlikely.
At smaller scales around the Pirrit Hills, obstructions such as minor peaks
and low bedrock ridges can reverse the spatial pattern of snow erosion and
deposition, with wind scoops on the upwind side and aprons of snow and ice in
the lee. The ridge shown in Fig. d exhibits a
combination of such features. Their distribution around bedrock uncovered by
small-scale deglaciation is difficult to predict and could confuse
cosmogenic nuclide records by shielding rock above or exposing rock below
the regional ice sheet surface. When drilling to shallow bedrock these
potential complications are probably best avoided by targeting the crests of
subglacial ridges.
Selected drill sites near Harter Nunatak
The subglacial topography northeast of the Pirrit Hills appears to be that of
a large cirque, its central basin flanked by subglacial ridges descending
from Mt. Tidd and Mt. Turcotte and re-emerging at Harter and John
nunataks, respectively (Fig. ). Regional ice flow crosses these
ridges obliquely from west to east, producing steep, locally crevassed slopes
along much of their length, precluding drilling into the underlying bedrock.
However, in the course of radar reconnaissance in 2013 we circled both
outlying nunataks and identified a subglacial ridge extending northwest of
Harter Nunatak. Unlike the major ridges radiating from the Pirrit Hills massif,
this ridge lies roughly parallel to ice flow and is overlain by a featureless
firn surface dipping gently towards the nunatak. As shown in
Fig. , the ridge is asymmetric with steeply dipping
southwest and gently dipping (∼20∘) northeast flanks. This
ridge became the chosen target for two drill holes in 2016–2017. Radar
methods and additional survey data are given in the Appendix.
The trend in the ridge crest (Fig. ) is almost
perpendicular to the prevailing wind direction. Combined with the steepness
of the upwind face, this might be expected to lead to a lee-side ablation
zone in any deglaciation that removed hundreds of meters of regional ice,
mimicking the gross morphology of ice surfaces around the Pirrit Hills
discussed above. However, smaller deglaciations that only exposed tens of
meters of the ridge crest may have left a lee-side snowbank, similar to that
downwind of Harter Nunatak at the present day. In siting a shallow subglacial
drill hole here, we therefore aimed to drill into the crest of the ridge
itself.
Unmigrated radar profile collected with the CReSIS radar showing the
location of the 150 m RB-2 borehole to the bed. Profile is oriented
perpendicular to the ridge.
During the 2016–2017 summer, we planned to drill at two sites above this
ridge with ice thicknesses of 100 and 200 m. The target for the
100 m borehole was the ridge crest at site RB-1 shown in
Fig. b. Because we were unable to image deeper portions
of the ridge crest, we sited the second borehole northeast of the crest, above
the gently dipping ridge flank. We used the Agile Sub-Ice Geological (ASIG)
Drill, a modified wireline mineral exploration drill, which is similar in
design to the much larger Rapid Access Ice Drill (RAID)
. The ASIG Drill is designed to be able to drill 15 m
of rock core beneath 700 m of ice. An unexplained hydrofracture of the basal
ice of the RB-1 borehole occurred when the bottom of the hole was within 10 m of the bed,
forcing the borehole to be abandoned. Because the ice flow at this site is
oblique to the ridge, it is possible that the basal ice is subject to
extensional stresses, which could facilitate brittle failure. To maximize the
likelihood of reaching the bed, we moved the drill to site RB-2
(Fig. b), which is upstream of the ridge crest with
respect to ice flow. The firn surface here dips toward the ridge, suggesting
that the basal ice may be in a state of compression and more resistant to
fracturing. An 8 m subglacial bedrock core was successfully recovered at
this site from a depth of 150 m. The core surface shows no smoothing or
striations indicative of subglacial abrasion, consistent with cold-based ice
cover, nor is there evidence of oxidation or other subaerial weathering
features. Details of the core and analyses on it will be described in
forthcoming publications. If past ice levels at this site are representative
of regional ice levels, the measurements on the bedrock core should provide
constraints on whether Robin Subglacial Basin, below Institute Ice Stream and Möller
Ice Stream, deglaciated in the past. Because we were unable to recover
bedrock from a shallower site, the measurements will not necessarily be able
to detect deglaciations in which less than 150 m of thinning occurred.
Conclusions
Measurements of cosmogenic nuclides in bedrock retrieved from below the WAIS
have the potential to establish whether and when marine-based portions of the
ice sheet deglaciated in the past. The potential of this method, however,
requires that drill sites meet three basic criteria: (i) local ice levels
must be drawn down significantly during collapse events, (ii) the subglacial
bedrock must contain minerals in which useful cosmogenic nuclides can be
measured, and (iii) the cosmogenic-nuclide record, which is primarily
produced in the top few meters of exposed bedrock, must remain continuously
protected from erosion. These criteria are also applicable to subglacial
drilling projects testing whether marine basins in East Antarctica
deglaciated in the past. Sites that are expected to be most indicative of
past ice-sheet extent are located adjacent to deep marine basins, such as
Bentley Subglacial Trench, where maximum ice loss would occur during a
collapse event. Because ice levels at each of the potential drill sites
discussed above are sensitive to the deglaciation of different sectors of the
WAIS, subglacial samples from multiple sites will ultimately be required to
fully determine the configuration of the ice sheet during past interglacial
periods.
The Pirrit Hills are located in the Weddell Sea sector,
midway between the grounding line and the divide. Ice-sheet modeling suggests
that deglaciation of Robin Subglacial Basin induces thinning of a few hundred
meters at the Pirrit Hills. Field observations and cosmogenic-nuclide
measurements indicate that ice levels at this site have, indeed, been lower
in the past, and multiple lines of evidence suggest that uneroded bedrock
extends hundreds of meters below the modern ice surface. Ice-penetrating
radar surveys revealed a gently plunging subglacial ridge extending from
nearby Harter Nunatak, and, in the 2016–2017 summer, an 8 m bedrock core
was extracted from the ridge from below 150 m of ice. If ice levels at the
drill site are representative of the region, measurements on the core should
constrain past drawdowns of grounded ice in the Weddell Sector of the ice
sheet.
The code for the PSU-3D ice-sheet model is available on
request from David Pollard. Ice-thickness, strain, and accumulation data are
available from the corresponding author.
Geochemical and geophysical measurements10Be and 26Al measurements
Samples were prepared for 10Be/9Be and 26Al/Al
measurements at the University of Washington. We crushed the rock samples,
sieved them at 250–500 µm, and purified quartz using surfactants
and dilute hydrofluoric acid etching . Samples were then dissolved
in hydrofluoric acid, after which total Al concentrations were measured on aliquots of the
solution via inductively coupled plasma optical emission spectrometry. Be and
Al were isolated using ion-exchange chromatography
, and Be and Al isotope ratios were measured
at the Lawrence Livermore National Laboratory Center for Accelerator Mass
Spectrometry (LLNL-CAMS). Be isotope ratios were measured relative to the ICN
01-5-4 Be standard, assigned a 10Be/9Be ratio of 2.851×10-12 by . We compute production rates for
10Be and 26Al using the method of as implemented
in the version 3 exposure age calculator .
Production rates are calibrated from the CRONUS-Earth primary calibration
data sets . Atmospheric pressure at sample
sites is calculated using the relation between elevation and Antarctic
atmospheric pressure of . Production by muons is
calculated using the method of .
Ice-penetrating radar surveys
We used a radar built by the Center for Remote Sensing of Ice Sheets (CReSIS)
with a center frequency of 750 MHz. This radar has a cross-track antenna
array consisting of two widely spaced transmitters and eight receivers
designed to identify and locate off-nadir reflections. After processing, a
full tomographic reconstruction of the subglacial topography was constructed.
However, prior to the 2016–2017 subglacial drilling season at the Pirrit
Hills, ice-thickness errors were discovered in this reconstruction, and so
the bed at the drill site was re-surveyed with a Geophysical Survey Systems
Inc. (GSSI) SIR-4000 control unit and a 100 MHz monostatic transceiver.
These data are shown in Fig. b. The survey
Sample information and cosmogenic-nuclide
concentrations. Errors (±1σ) include laboratory procedural
uncertainties and individual AMS measurement errors.
consisted of parallel lines spaced 50 m
apart and oriented obliquely to the subglacial ridge, as well as other
exploratory lines around Harter Nunatak. The survey was conducted by towing
the antenna on foot at a pace of less than 0.5 m s-1. The data were
processed using GSSI RADAN software. Distance and elevation corrections were
applied to the data. To reduce noise, the data were stacked and a bandpass
filter was applied. To calibrate radar wave propagation velocities and
thereby improve ice-thickness estimates, we measured firn density to a depth
of 38 m and used the relationship of to estimate how
the real dielectric constant varied with depth. We extrapolated the
depth–density relationship to deeper depths by fitting the data using a
firn-densification model . Drilling at the RB-2 site
(Fig. b) encountered the bed at a depth of 150 m,
confirming ice-thickness estimates from radar surveys of 152±10 m.
Strain and accumulation measurements
A total of 16 stakes were arranged in a grid around Harter Nunatak, and two
additional stakes were placed over the subglacial ridge
(Fig. b). Stakes were installed and surveyed in
December 2015, and measurements were repeated in December 2016. Stake
positions were surveyed with a Trimble 5700 GPS receiver with a Zephyr
Geodetic antenna. No base station was used for the first survey; the second
survey was able to use a POLENET GPS station located on Harter Nunatak as a
base station. Mean velocity uncertainty is 0.23 m yr-1.
Change in snow surface height was measured at each stake. Over the 1-year
period, height change varied from -39 to +43 cm yr-1, with a mean
of -5 cm yr-1. There is no apparent spatial pattern to the results,
which are comparable to the amplitude of sastrugi in the area.
The authors declare that they have no conflict of interest.
Acknowledgements
Support for this work was provided through US National Science Foundation
(NSF) grants 1142162 and 1341728 and the United States Antarctic Program.
Perry Spector received funding from the NSF Graduate Research Fellowship Program. We
thank Maurice Conway and Paul Koubek for assistance in the field, Seth
Campbell for guidance in collection and processing of radar data, and Taryn Black
and Mika Usher for field and laboratory assistance. Geospatial support for
this work provided by the Polar Geospatial Center under NSF OPP awards
1043681 and 1559691.Edited by: Arjen Stroeven
Reviewed by: Nathaniel A. Lifton and one anonymous referee
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