The discovery of the gap between ice and bed was made with a new
instrument, the JPL/CIT Antarctic Ice Borehole Probe, developed by a
team of engineers at NASAs Jet Propulsion Laboratory (JPL) to study the
basal zone of the ice streams, where the fast-flow mechanism is
believed to operate. The probe carries two video cameras, one looking
straight down and the other looking to the side. The probe is lowered
down boreholes drilled through the ice to the bottom, and can then
observe features and processes in the basal zone. The boreholes,
about 1200 m (3600 ft) deep, are drilled with a hot-water jet drill
developed by Caltech(CIT). The drill development and the Antarctic
field work and interpretation of the probe results were carried out by
a Caltech/JPL team with support of the National Science Foundation
(NSF) Office of Polar Programs (OPP). The team members are listed at
the end of this report.
In the work leading to discovery of the gap, the Caltech/JPL team put
the probe down three boreholes about 2 miles apart, in the vicinity of
camp Ice Stream C at latitude 82o 22'S,
longitude 136o 24'W. In the
first borehole, the ice was frozen to the bed, and there was no gap.
In the second, the ice was not frozen to the bed, and there was a gap
about 1 or 2 inches wide between the bed and the base of the ice. In
the third hole, a gap 62 inches wide was found, far exceeding anything
previously observed or inferred under the ice streams. This wide gap
was recognized from a combination of features seen in the probes
videos. As the probe approached the bottom, the side-looking video
came upon a sharp horizontal boundary, above which was nearly clear ice
containing a scattering of imbedded rock fragments, and below which was
slightly turbid water containing small particles in turbulent motion
(see
image #1
attached to this
report). This boundary could be seen also in down-looking images as a
bright sharp rim below which the borehole wall did not continue (see
image #2). The underside of
the ice was seen in side-looking
images as a planar surface cut by a pattern of grooves that mark
the boundaries between individual ice crystals
(image #3).
(For technical reasons the side-looking video images (#1 and #3) are
upside down, that is, up on the page is down in the borehole.) As the
probe was lowered farther, the bottom of the borehole came into focus
in the down-looking video, and was made particularly clear by the
probe-chainpiled up on the bottom (
image #4).
This chain 0.75 m (30 inches) long, hung from the bottom of the probe
and served to provide to the down-looking video an advanced indication
of the probe's approach to the bottom of the borehole; the chain is
visible in
image #1. For
image #4 the probe was at
depth 1064.55 m (3492 ft), and for
image #1 it was at depth
1063.25 m (3488 ft). Thus the width of the gap, determined from the
combined side-looking and down-looking video observations, was
(1064.55-1063.25+0.2)m = 1.5 m (59 inches). (The 0.2 m is the length
of the chain not resting on the bottom in
image #3 plus the height of
the side-looking camera lens above the bottom of the probe.) The
horizontal dimensions of the gap were beyond the illumination from the
probe's sideward-shining floodlamp, which is thought to be about 0.5 m
under the conditions of the water clarity that prevailed in borehole
no.3.
The discovery of a substantial gap between the base of the ice and the
bed is quite surprising. No such gaps have been previously detected or
inferred under the ice streams. Antarctic subglacial lakes (like Lake
Vostok) involve a substantial ice-bed gap, of course, but it is on a
dimensional scale hundreds to thousands of times greater than that
involved here; also, no such lakes have been found in association with
ice streams. Theoretical calculations that interpreted earlier
(pre-probe) ice-stream observations have assumed the existence of a
gap, but the calculationsbased on the rate at which water exits from
boreholes and enters a basal conduit systemimplied only a narrow gap,
about a tenth of an inch wide. Some lines of reasoning suggest that
such a gap is not present in the natural system at all but is opened up
in the drilling process when borehole water under a high overpressure
is injected into the ice/bed contact and opens up a gap along it. It
seems quite unlikely, however, that a gap 1.5 m (59 inches) wide could
be formed in this way during the short time (approximately two minutes)
during which the high pressure is applied.
Alternatively, such gaps are a known feature of glaciers that move by
sliding of the ice over the bed. The basal sliding can cause cavities
to open up between the ice and the bed, generally in the lee of
protuberances in the bed. The process is called ice-bed separation or
basal cavitation. The size of the cavities is controlled by the basal
sliding speed, the amplitude of the basal roughness, and the basal
water pressure. At high water pressure, slightly below the ice
overburden pressure, the cavities are generally thin in the vertical
dimension compared to their horizontal dimensions, and when penetrated
by the hot-water drill they should appear as a gap between the base of
the ice and the bed. This appears to be the situation encountered by
the probe in boreholes no. 2 and 3.
The significance of the probes observations of a basal gap is thus that
in the neighborhood of boreholes no. 2 and 3 the ice stream moves by
basal sliding. This is contrary to a widely held view that the motion
is by shear deformation of soft sediment underlying the base of the
ice. The mechanical decoupling of the ice from the bed, which is a
necessary consequence of the gap, should contribute to rapid ice-stream
motion, and it will introduce a significant complication into the
ongoing efforts to analyze and model the rapid-flow mechanism.
Another point of significance is the gaps role in the basal water-conduit
system. In down-looking video at the bottom of borehole no. 2 the
probe observed plumes of turbid water exiting from the bottom of the
borehole via the basal gap, proving that the gap was part of the basal
water system. (The water was leaving the borehole in response to clean
water being pumped into the hole near the top.) In borehole no. 3 the
situation was probably similar, producing the observed turbulent
motions of water in the gap. The existence of the wide gap shows that
the basal water system includes large cavities that can store and
release large volumes of water. This must be important in the
functioning of the water system and its influence on the basal water
pressure, which is probably a key parameter in the ice-stream
mechanism.
In addition to observing the basal gaps in boreholes no. 2
and 3, in all three boreholes the probe observed a basal layer of ice
loaded with rock debris, which has not been previously recognized in
studies of the ice streams. The debris-laden layer was 15.8 m (52 ft) thick in
borehole no. 1, 25 m (82 ft) in no. 2, and 11.6 m (38 ft) in no. 3. From their
appearance in the video, various types of debris-laden ice could be
identified, depending on their content of coarse rock fragments and
fine clay particles (see
image #5).
Except for a relatively rock-poor
layer at the base of the ice in borehole no. 3 (see
image #2), and
except for complex interbedding of the different debris-laden ice types
(image #5),
the rock and clay contents generally increase downward
toward the bed, and may reach 60% rock by volume. This would qualify
them as possible products of basal freeze-on, in which water-saturated
subglacial rock debris gets frozen on to the base of the ice.
Detection of basal freeze-on would be an indication that the ice mass
is tending towards becoming frozen to the bed, which could be
responsible for the great slowdown of Ice Stream C that occurred
150 years ago. Basal freeze-on operating for 150 years could add about
1.5 m (5 ft) to the thickness of the basal debris-laden ice
layer, which is much less than the observed 12-25 m (38-82 ft). Other
mechanisms are needed for emplacing rock debris so high above the bed
and for distinctions among the various types of debris-laden ice,
whose role in ice- stream mechanism is not known.
It may be noted that the fine clay particles and coarse rock fragments
seen imbedded in the ice by the borehole probe represent types of
micro-environment that have been suggested as natural microbe
repositories, whose investigation may be an objective of future
research efforts directed toward Lake Vostok and Europa. To study such
micro-environments in detail, the resolution and magnification of the
borehole probes cameras need to be substantially increased, but the
feasibility of such studies appears encouraging from the success of the
ice borehole probe in the present work.
The work was made possible by
support of the JPL effort by NASA and support of the Caltech effort by
NSF. Details of preliminary results including some 70 still images and
30 video clips are posted on web site
http://helios.jpl.nasa.gov/~behar/JPLAntIceProbe.html.
The NSF-NASA research team consisted of the following participants
(CIT=Caltech):
PROBE DESIGN AND CONSTRUCTION
JPL: Alberto Behar, team leader; Frank
Carsey, PI and project manager;
Lonne Lane, Robert Ivlev, Ken Manatt, Kobie Boykins, engineers;
Fabien Nicaise, and Kai Zhu, technician.
CIT: Hermann Engelhardt, science advisor.
PROBE WINCH AND ICE-DRILL DESIGN AND CONSTRUCTION
CIT: Hermann Engelhardt, supervisor; Robin Bolsey,
technician.
ANTARCTIC FIELD WORK AND PROBE DATA INTERPRETATION
JPL: Alberto Behar, probe operator.
CIT: Barclay Kamb,
P.I.; Hermann Engelhardt, Co-PI and field operations supervisor;
Robin Bolsey, technician; Stefan Vogel, Shulamit Gordon, Regina Sterr, Michele
Koppes, Katherine Batten, Matt Bachmann, and Daniel Adams,
field assistants.
NSF/RPS: Sarah Grundlach, camp manager; Alex Papasava, cook; Jim Evans
and Richard Grillo, mechanics;
Sarah Harvey, general assistant. (RPS=Raytheon Polar Services)
IMAGE CAPTIONS
Image #1
(side-looking in borehole no. 3 at depth
1063.25m).Boundary between basal ice and subglacial
water-filled gap. The ice, in the lower 3/4 of the image,
appears black (clear) with numerous bright spots, which are
imbedded rock fragments. The water contains fewer particles.
The boundary is the straight horizontal bright line one-fourth
of the way down the top of the image. Because the image is
upside down (for technical reasons), the top of the image is
toward the bottom of the borehole.
Image #2
(down-looking in
borehole no. 3 at depth 1062.65m).Base of the ice as seen
looking down the borehole from 3 meters above the bed. The ice
in the closer part of the borehole wall appears whitish because
it is heavily laden with rock debris. Rocks partially melted
out of the ice can be seen protruding through the borehole
wall. Below the debris-laden zone is a black-appearing,
clearer ice layer containing some rock debris. Below this
layer is a partially bright, circular ring, whose sharp inner
edge is the line of intersection of the borehole wall with the
base of the ice. The probe-chain is seen hanging down from the
probe in the lower right part of the image.
Image # 3
(side-looking in borehole no. 3 at depth 1063.29m).In the lower
one-quarter of this rather dark image one can see the base of
the ice above the water-filled basal gap. (Remember that
because of image inversion, down is up.) The ice base (or
sole) is a flat surface cut by grooves in a pattern somewhat
resembling mud cracks. The grooves may be formed by a thermal
etching process acting along the grain boundaries between
adjacent ice crystals in the polycrystalline mass.
Image #4
(down-looking in borehole no.3 at depth 1064.55m).Bottom of borehole
no. 3. The lower part of the probe chain, with a plastic marker at the
tip, is curled up lying on the bottom. The bottom consists of small
rock fragments mostly 2-10 mm in size. The two bright spots in left
and right center result from light scattering by faint turbidity of the
water in the gap.
Image #5
(side-looking in borehole no.2 at depth
1224.90m).Interbedding of three debris-laden ice types seen in the
borehole wall just above the bed. The light gray layer in the center
is type-2 ice. Below it in the image is type-1 ice (black with a
scattering of white chunks). Above it is type-3 ice (granular mass of
rock debris with a little interstitial black (clear) ice).
