TCThe CryosphereTCThe Cryosphere1994-0424Copernicus PublicationsGöttingen, Germany10.5194/tc-12-2167-2018Brief communication: Candidate sites of 1.5 Myr old ice 37 km southwest of the Dome C summit, East AntarcticaCandidate sites of 1.5 Myr old ice SW of Dome CPassalacquaOlivierolivier.passalacqua@univ-grenoble-alpes.frCavitteMariehttps://orcid.org/0000-0002-5777-0933GagliardiniOlivierhttps://orcid.org/0000-0001-9162-3518Gillet-ChauletFabienhttps://orcid.org/0000-0001-6592-3840ParreninFrédérichttps://orcid.org/0000-0002-9489-3991RitzCatherineYoungDuncanhttps://orcid.org/0000-0002-6866-8176Université Grenoble Alpes, CNRS, IRD, IGE, 38000 Grenoble, FranceUniversity of Texas Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USADepartment of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USAOlivier Passalacqua (olivier.passalacqua@univ-grenoble-alpes.fr)28June20181262167217422January201826February201818May20184June2018This 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/12/2167/2018/tc-12-2167-2018.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/12/2167/2018/tc-12-2167-2018.pdf
The search for ice as old as 1.5 Myr requires the identification
of places that maximize our chances to retrieve old, well-resolved,
undisturbed and datable ice. One of these locations is very likely southwest
of the Dome C summit, where elevated bedrock makes the ice thin enough to
limit basal melting. A 3-D ice flow simulation is used to calculate five selection criteria,
which together delineate the areas with the most appropriate glaciological properties. These selected areas
(a few square kilometers) lie on the flanks of a bedrock high, where a balance is found
between risks of basal melting, stratigraphic disturbances and sufficient age
resolution. Within these areas, several sites of potential 1.5 Myr old
ice are proposed, situated on local bedrock summits or ridges. The
trajectories of the ice particles towards these locations are short, and the
ice flows over a smoothly undulating bedrock. These sites will help to choose
where new high-resolution ground radar surveys should be conducted in
upcoming field seasons.
Introduction
Antarctic ice is an exceptional archive of the Earth's paleoclimates across
all the glacial–interglacial periods, and the only one that contains direct
samples of ancient atmospheres. The oldest available ice archive goes back
0.8 Myr in time EPICA Dome C ice core;
but is not old enough to study a main climatic transition that occurred
between 1.2 and 0.9 Ma, known from the temporal variations
of the isotopic composition of benthic sediments (mid-Pleistocene
transition, MPT; ). The main climatic periodicity and
the amplitude of climate cycles seem to radically change during the MPT.
There is presently no general agreement on the processes responsible for this
transition and its origin; the influence of the trend of atmospheric
CO2, the dynamics of ice sheets in the Northern Hemisphere, the sea ice
extent and the dust content of the atmosphere have been proposed. Considering
these scientific issues, locating a future 1.5 Myr old ice drill site
was identified as one of the main goals of the ice-core community
seefor an overview.
Thermal and mechanical ice flow simulations have to be carried out to assess
the potential of possible drill sites . The present
study is part of a set of three modeling exercises led by the IGE (Institut
des Géosciences de l'Environnement, Grenoble) group and collaborators
around Dome C, and more specifically on the bedrock high lying ∼40km southwest of the Dome C summit, suspected to be a good
old-ice candidate , under the ice ridge
linking Dome C to the Vostok region seefor a precise
description. First, a 1-D thermal model was run over the
last 0.8Myr to determine the present state (temperate or cold) of
basal ice, which was compared to the reflectivity map in the region of
Dome C . The value of the local geothermal flux was
inferred, and the spatial distribution of interpreted subglacial water was
explained. The results of these transient simulations show that the top of
this bedrock high is likely to be melt-free on long timescales, or to undergo
very limited basal melting. Hence, this subregion is thermally compatible
with the archiving process on glacial–interglacial timescales. Second, a
kinematic 1-D ice flow model was used to evaluate the age and age resolution
of the deepest portion of the ice sheet . The distance
between the dated isochrones and the modeled ones was minimized to infer a
thinning parameter that characterizes the vertical deformation through the
ice column. We showed that the observed isochrones are compatible with high
basal age and provide sufficient resolution. However, these two previous
studies neglect the horizontal motion of ice, but the trajectories of the ice
particles are not only vertical. The travel time of these particles, and
consequently their age, is strongly influenced by the shape of the bedrock
and the ice surface. These prior studies lacked the description of the local
3-D state of stress of the ice, where the geometry of the terrain and the
strain history of the ice particles can be properly taken into account.
To evaluate the quality and position of deep old ice, we here proceed to a
steady-state 3-D ice flow simulation, at a regional scale. Whereas these two
previous 1-D modeling results alone could not help us determine the best
oldest-ice targets, we here intend to provide objective criteria that
together delimit kilometer-scale areas of old, well-resolved, undisturbed
basal ice. Note that the goal of this study is not to assess the best age
estimate but to evaluate which sites show more suitable glaciological
characteristics than others. The bottommost ice recovered should be older
than the MPT, ideally as old as 1.5 Myr such that several climate
cycles pre-MPT can be recorded. The vertical age resolution has to be better
than 10 kyrm-1 to detect high-frequency climatic variability in
the ice core (Jérôme Chappellaz, personal communication, 2017). Moreover, basal ice
will probably be disturbed, similarly to the deepest 60 m of the
EPICA Dome C ice core . We
have no better evaluation of the height of disturbed ice in the region, and
we will similarly take 60 m as a safety distance for the drilling
minimum distance from the bedrock, but we should keep in mind this cutoff
height could be an underestimation. One should look for places where the
mechanisms responsible for stratigraphy disturbances (cumulated basal shear,
bedrock roughness) should be minimal. Convergent flow should be avoided as
well, because it tends to thicken basal layers. This is unfavorable to
recovering the oldest ice as it will shift older layers downwards and makes the
dating process more complex . Finally, the
location of the future drill site should be above the highest subglacial lake
detected by the radar survey ; otherwise the risk
of basal melting could be drastically increased. We will call this threshold
the “water limit”, above which there is no evidence of the presence of water in
the radargrams.
Another modeling study has been recently led at a continental scale to
locate a future >1Myr old drill site . These authors
used a transient thermodynamical model to compute the highest geothermal flux
value that would keep basal ice frozen. By comparison with available
continental geothermal flux datasets, they locate areas where basal ice has
likely remained frozen over 1.5 Myr. The authors also included a
further mechanical constraint representing limited impact of bedrock
roughness on the preservation of the bottom ice stratigraphy. Our study and
the one of differ in the way they account for the heat
budget of ice and for the mechanical constraints on the basal ice. We should
consider these differences as an interesting source of information for the
decision-making process. This will be discussed in Sect. .
The decision-making process for a future drill site needs both field survey
and modeling, the former feeding the latter with geophysical constraints,
and the latter reducing the areas of interest for new field surveys, focusing
more and more on promising sites. The information brought by the present
study should be sufficient for a last high-resolution (few-hundred-meter
scale) radar survey to be led on promising sites during the next field
seasons. Then the community should be able to make a final decision for a
1.5 Myr old drill site in the Dome C region.
Ice flow modelModel description
The Stokes equations are solved on a 83×114km domain,
approximately centered on Dome C (Fig. ), using the
finite-element model Elmer/Ice. The surface and bed geometries are provided by the
Bedmap2 datasets , except on the bedrock high
southwest of Dome C, where a dense airborne radar survey has been recently
collected . The model works in ice equivalent, and
we adjust the surface height by assuming that the density profile of the firn
is that of Dome C on the whole domain .
Horizontal resolution of the corresponding mesh is 1 km. The
resolution of the 20 vertical elements of the mesh evolves linearly, so that
the deepest one is 25 times finer than the upper one, and to ensure that the
velocity field is better resolved near the bottom.
Our domain sits in the center of the Antarctic Plateau, and lateral boundary
conditions do not correspond to physical boundaries, but to virtual vertical
surfaces. As the domain contains the dome summit, the ice flow is divergent
and no input flow should be considered. We impose velocity boundary
conditions corresponding to the shallow-ice approximation
SIA;. The ice surface as observed today may
not correspond to the chosen rheology and is relaxed for 50 years such that
the present surface slope does not induce an unrealistic velocity field. The
ice surface is very flat, and 50 years is enough to accommodate the surface
altitude to the ice rheology up to ∼ 1 m, without radically
changing the orientation of the ice ridge, on which we have little control.
The part of ice motion attributed to basal sliding is not known precisely in
the Dome C region and depends on water circulation. We here focus on a
region where basal melting is probably not present or limited, and horizontal
velocities are very small, so that, for the sake of simplicity, a no-sliding
condition is imposed at the bottom of the ice column. Vertical velocities at
the base are equal to the basal melt rate output from previous modeling work
Fig. 7 in.
Mesh, bedrock dataset
and basal melt rate
used for the simulation on a 83×114km domain. The red patch on the context map (bottom left) shows
the domain used for the calculation, and the blue rectangle at the top of the
image is the location of Fig. .
As we know that the basal ice in the Dome C region is at or near the melting
point , the temperature
profile measured in the EPICA Dome C borehole is a good representation of
the thermal structure of the ice in the Dome C vicinity. Hence, we account
for ice temperature by using the same normalized temperature profile on all
the domain. Solving the coupled thermomechanical equations would require
excessive computing resources, without radically changing the ice fluidity –
which is mainly controlled by temperature. Similarly, we do not account here
for long-term evolutions of the ice sheet surface but are aware that this
assumption might considerably affect the trajectories of the ice particles.
As the main interest of this work focuses on the deepest ice, mechanical
anisotropy of the ice has to be accounted for. The relation between strain
rates and stresses is described by the generalized orthotropic linear flow
law GOLF;, given a certain vertical profile of ice
fabric. By introducing a dependence on the second invariant of the deviatoric
stress, this law can be extended to the nonlinear case
. The fabric profile is only known at the dome
summit and shows that the ice mainly undergoes
vertical compression but also undergoes longitudinal extension in the deep layers
. However, there is no reason to use the very same
fabric profile everywhere else, where shearing might have more influence
and/or the bed shape is different. In this short study we will not discuss
the influence of the chosen rheology, but we first made sure that the
computed surface velocities correctly simulated the horizontal velocity field
measured at the surface by for different n values
and fabric profiles. Hence, we decided to use the widely accepted value of
3 for n and a synthetic vertical fabric profile, for which the
eigenvalues of the second-order orientation tensor evolve linearly with
normalized depth, from isotropy at the surface to a single maximum fabric at
the bottom.
Model outputs
Back trajectories are computed from the 3-D velocity field using a Lagrangian
scheme such that the age is known along the forward trajectory of the ice
particles, and an age field is generated. The age resolution could be
calculated from the vertical derivative of this age field, but we found it
more accurate to track the annual layer thickness λ [ma-1] from the ice surface using
dλdt=λϵ˙zz,
where ϵ˙zz is the vertical strain rate. This formulation
neglects vertical rotation effects that tend to overturn internal layers.
This assumption is reasonable if internal layers are mainly horizontal, which
is the case over the studied bedrock high. The age resolution is then stated
as 1/λ.
The way ice strains by flowing over a rough bed differs depending on the
shape of the bedrock underneath. Similarly, a given bedrock shape can be a
convergence or a divergence area, depending on the orientation of horizontal
ice flow. Once the velocity field is known, a local coordinate system (X,Y,Z) can be defined at each point, for which the x axis is oriented along
flow. The curvature of the bed perpendicular to the flow is then computed
everywhere, and convergence areas are identified where the bed curvature is
positive.
Beyond the computation of ages and age resolutions, the 3-D simulation is
also useful to detect where deep ice is more likely to be folded. Shearing
will tend to amplify small wrinkles in the ice layers and so disturb the
ice's basal stratigraphy, whereas longitudinal extension will tend to flatten
them. Competition between shear and longitudinal stresses can be represented
by a dimensionless shear number :
S=2ϵ˙XZϵ˙XX-ϵ˙ZZ,
where ϵ˙XZ is the shear strain rates along ice flow, and
ϵ˙XX and ϵ˙ZZ are the local longitudinal and
vertical strain rates, respectively. used this shear number
as a criterion to detect if a given wrinkle can be amplified by shear. More
simply here, we use it to predict the presence of undisturbed ice, whereby
the smallest shear number is best.
Selection of favorable locations
If the absolute value of the age, age resolution or strain rates can be
discussed regarding the choices of the model parameters, we reckon their
spatial variabilities are robust since they mainly depend on the shape of the
bedrock and of the ice surface. As a consequence, this study focuses on
comparing between promising locations rather than on discussing the
sensitivity to model parameters.
The five selection criteria (age, age resolution, bed curvature, shear number
and bed height) are used to compute five masks, thresholded as follows. For
bed curvature, 0m-1 would have been the natural threshold, but
it was too restrictive, and we decided for a slightly higher value (2×10-5m-1). The shear number threshold appeared naturally by
studying its spatial evolution (see Sect. ). The bed height
threshold corresponds to the water limit, at 480 m. Furthermore, our
results show that most of the subglacial elevated bed high southwest of
Dome C is favorable to the existence of 1.5 Myr old ice, so we
adopted more conservative age and age resolution thresholds for the selection
of smaller suitable locations (1.8 Myr for the age and
8.5 kyrm-1 for the age resolution, 60 m above the bed). A
Boolean combination of the five masks delineates the areas fulfilling all our
five selection criteria.
Results and interpretationAge at 60m above the bed
The area identified as possibly hosting the oldest ice is elongated along the
bedrock high and stands at an intermediate bed elevation (mainly between
400 and 550m above seal level, Fig. a).
Neither the very top of the bedrock high nor its lowest foothills appear to
be suitable for the archiving process of very old ice. The imposed basal melt
rates are zero on the upper part of the bedrock high; therefore infinite age
is calculated for the very basal ice. In that case, the age of the ice
standing 60 m above the bed is strongly dependent on the ice
thickness. On the top of the bedrock high, the ice is at its thinnest, and the
old ice is at less than 60 m above the bed. On the foothills of the
bedrock high, the ice is thicker, basal melt rates are above zero and the
basal ice is therefore continuously being melted from the bottom.
Selection criteria thresholded as follows (yellow areas): (a) age >1.8Myr, (b) age resolution <8.5kyrm-1, (c) bed
curvature <2×10-5m-1, (d) shear number S<40,
(e) Boolean combination of the selection criteria. The water limit
corresponds to a bed height of 480m and is shown here by a thick yellow
contour. In panel (d) the elliptical wavy area corresponds to a region of
higher shear. Magenta boxes A, B and C correspond to areas that could be
considered as our best oldest-ice targets. Dashed lines show trajectories of
ice particles; the red one corresponds to the profile presented in
Fig. . Colored points locate possible drill sites, discussed in
the text. Hatched areas show the best areas of . All the
panels cover the same area. This figure focuses on the bedrock high designed
as such in Fig. , located ∼40km southwest of the
Dome C summit. Projection: WGS84/Antarctic Polar Stereographic – EPSG:3031
(kilometers).
Some places may host ice even older than 2Myr, but they all stand
below the water limit of Young et al. (2017; Fig. , yellow line).
The presence of very old ice in those areas is not impossible, but it may also
be the consequence of insufficiently imposed basal melting. The transition
between melting and frozen ice should stand somewhere on the flanks of the
bedrock high, but it is difficult to pinpoint the precise location of this
threshold. Despite the promising thick ice in this region that could allow
old ages and sufficient age resolution, the risk of basal melting is real.
Age resolution at 1.5 Myr
Age resolution in the deepest part of the ice column is influenced by two
factors. First and obviously, the thicker the ice, the better the age
resolution. As a consequence, the tops of the bedrock high are rarely
compatible with a sufficient age resolution of the oldest ice. Bedrock flanks
should be preferred, but some of the thickest ice areas on the flanks will be
discarded as well because of an increased risk of basal melting. Second, for
ice positioned close to the ice divide, age resolution may benefit from a
thickening effect of the deeper layers so-called Raymond
effect;. This results in a band of well-resolved
ice, oriented along the ice ridge and perpendicular to the bedrock high.
However, no Raymond arch is visible in the radargrams that would indicate a
strong Raymond effect here. One explanation is that the shape of the ice
surface at these sites is much more rounded than at the Dome C summit, and
the produced along-ridge flow tends to dampen the amplitude of the Raymond
arches . Moreover, the characteristic time for a
Raymond arch to form here would be several 100 kyr, during which the surface ridge may have moved,
smoothing out the developing arches. Unfortunately, the past position and
shape of the ridge are unknown, and drilling far from its present position
would not guarantee a better resolution.
Stratigraphic disturbances
At a divide, the shear stress perpendicular to the divide is zero, so that in
general the shear number S of the deep ice close to the ridge is
low (∼ 10). However, a series of subglacial peaks lie immediately west
of the ice divide. As a consequence the shear number increases very sharply
and reaches much higher values (∼ 100), and this zone of higher shear
should be discarded for a future drill site (Fig. d, wavy
area). All the areas for which the basal ice crossed this zone of higher
shear should be discarded as well, so the trajectories of the ice particles
need to be evaluated.
Trajectories for oldest-ice spots
The best combination of age, age resolution, folding, convergence and melting
criteria is shown in Fig. e, revealing several spots of ice
meeting the five criteria. The areas for which the trajectories are
shorter should be preferred, highlighted here by three magenta boxes. For
boxes A, B and C the ice originates from the divide, guided by the strong
lateral divergence resulting from the shape of the ice surface. Locations
within boxes C and A are closer to the ice ridge and should be considered
first for future oldest-ice drilling because of shorter trajectories and
less risk of stratigraphy disturbances. Box B in particular lies on a
relatively flat bedrock platform (black point in Fig. e),
which may ensure stability of ice flow. Figure shows the paths taken
by ice from the surface to that site, demonstrating that the horizontal
distance traveled does not exceed 10 km from the surface. However, as there
is probably no basal melting here, deep ice would closely follow the bed for
several kilometers, in a depth range dominated by strong vertical shear,
enhanced by an undulating bed underneath. To minimize the bed influence, we
could also consider a drill site located 3 km upstream, where the ice
age would still exceed 1.5 Myr (red point in Fig. e,
red dashed line in Fig. ).
Of course, locating a unique “best” drill site within one of these three
boxes is not possible with our 3-D-modeling approach only. However, it
allows a restricted area to be defined where a new set of observations will
be the most valuable. We should focus on local bedrock summits or crest
lines, because local troughs make not only the ice flow but also heat flow converge
, increasing the possibility of insufficient age and
positive basal melt. Considering that, only a few set of favorable drill
sites remain in boxes A, B and C (blue, orange, pink and yellow points in
Fig. e). Pink and blue points have less risk of basal
melting, while yellow and orange have less risk of stratigraphic
disturbances. The best choice between these sites should now be guided by
local radar surveys characterizing the internal layering of the ice and the
vertical strain rate profile .
Trajectories of the ice particles from the ice surface towards the
black point in Fig. (dashed red trajectory). Red numbers
indicate the age of the ice, in millions of years. The bed profile is shown in grey.
The thin red line corresponds to one possible drill site (red point in
Fig. ).
Comparison with results based on thermodynamical modeling
identified a 8 km long area covering the SE upper
part of the bedrock high, which crosses our box A, but they do not overlap
perfectly (Fig. e, hatched blue and yellow areas). They also
identified a site within our magenta box B, but no site in box C. These
comparative results highlight the complementarity of the two approaches. The
1-D thermodynamical model of has a better control over the
thermal aspect of the problem than over its mechanical aspect, and it selects
sites that are more conservative from a heat budget point of view, i.e.,
preferentially local bedrock heights. In contrast, our 3-D approach accounts
for the horizontal strain of the ice and selects sites that are more
conservative from a mechanical point of view. The upper part of box A or
the left part of box B validates the constraints of both approaches. In
our approach, the bedrock summit in box C is the safest mechanically;
however, it was not selected by because of a local bedrock
roughness exceeding their threshold of 20 m, despite the fact that their
thermal criterion was fulfilled.
Conclusions
The three-dimensional ice flow simulation presented here aims at defining and
calculating several objective criteria which represent ideal conditions for
the retrieval of old, well-resolved, undisturbed ice. The influence of the
bedrock high and of the position of the ice ridge allows us to define only
few sites compatible with all our selection criteria. The ages calculated at
the base by our simulation predict ice older than 1.5 Myr high enough
above the bedrock, which gives us confidence that the community's target of
1.5 Myr should be attainable, with the required age resolution.
However, the modeling approach implicitly assumes that the ice flow is
regular down to the bedrock, but there is no guarantee that it is actually
the case. A ground radar survey focusing on several few-hundred-meter-scale areas
presented here is essential to explore the structure of the deep layers.
Finally, a rapid-access drill is currently planned
to be deployed during the 2018/19 season to assess ice quality and age for a
chosen target site, before the final site location is decided upon.
Observation data used in this paper come from publicly available datasets. Simulation results can be accessed on demand from the authors.
The authors declare that they have no conflict of
interest.
Acknowledgements
We would like to thank Jean-Louis Tison for the editing process, and we
appreciate the helpful comments of Michelle Koutnik, Tas Van Ommen and
Bryn Hubbard as referees. This publication was generated in the frame of
Beyond EPICA-Oldest Ice (BE-OI). The project has received funding from the
European Union's Horizon 2020 research and innovation program under grant
agreement no. 730258 (BE-OI CSA). It has received funding from the Swiss
State Secretariat for Education, Research and Innovation (SERI) under
contract no. 16.0144. It is furthermore supported by national partners and
funding agencies in Belgium, Denmark, France, Germany, Italy, Norway, Sweden,
Switzerland, the Netherlands and the United Kingdom. Logistic support is
mainly provided by AWI, BAS, ENEA and IPEV. The opinions expressed and
arguments employed herein do not necessarily reflect the official views of
the European Union funding agency, the Swiss Government or other national
funding bodies. This is BE-OI publication no. 4. The Australian Antarctic
Division provided funding and logistical support (AAS 3103, 4077, 4346). This
work was supported by the Australian Government Cooperative Research Centres
Program through the Antarctic Climate and Ecosystems Cooperative Research
Centre (ACE CRC); support for UTIG came from the G. Unger Vetlesen Foundation
and NSF grant PLR-1443690. This paper is UTIG contribution number 3269. This
work was granted access to the HPC resources of CINES under the allocation
A0020106066 made by GENCI. This publication also benefitted from support by
the Université Grenoble Alpes in the framework of the proposal called
Grenoble Innovation Recherche AGIR. We also thank Brice Van Liefferinge for
making his datasets available. Edited by:
Jean-Louis Tison Reviewed by: Michelle Koutnik, Tas van
Ommen,
and Bryn Hubbard
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