DERIVED CHARACTERISTICS OF THE ROSS ICE
SHELF, ANTARCTICA
(Abstract only)
by
R. H.Thomas
(OAO Corporation, 7 5 0 0 Greenway Center Drive, Greenbelt, Maryland 2 0 7 7 0 , U.S.A.)
and D. R. MacAyeal
(Geophysical Fluid Dynamics Program, Box 3 0 8 , Princeton University, Princeton,
New Jersey 0 8 5 4 0 , U.S.A.)
ABSTRACT
Results of the Ross Ice Shelf Geophysical and
Glaciologlcal Survey (RIGGS) provide the most
complete data set a v a i l a b l e f o r any large p o r t i o n of
the polar ice sheets. I n t h i s paper, we use RIGGS
data to c a l c u l a t e some i c e - s h e l f c h a r a c t e r i s t i c s .
These include steady-state p a r t i c l e t r a j e c t o r i e s
through the ice shelf and the depth of isochronous
surfaces, which are of p a r t i c u l a r importance i n
choosing a d r i l l i n g s i t e where ice from the grounded
West A n t a r c t i c ice sheet i s l i k e l y to be near the
surface. Our estimates f o r depth to ice o r i g i n a t i n g
from the 500 m elevation contour show good agreement
with depths to a glaciochemical t r a n s i t i o n i n four
ice cores t h a t i s believed to be associated w i t h t h i s
e l e v a t i o n . This suggests t h a t , f o r much of the ice
s h e l f , there have been no dramatic and sustained
departures from steady state during the past 1.5 to
2.5 ka. With the RIGGS data and an assumed bottommelting rate d i s t r i b u t i o n we c a l c u l a t e steady-state
temperature p r o f i l e s a t each of the measurement
s t a t i o n s . Then, adopting an i c e - f l o w law deduced from
laboratory experiments and i c e - s h e l f measurements, we
obtain an e f f e c t i v e flow-law parameter f o r each of
these s i t e s . Using these values, the measured s t r a i n rate f i e l d i s transformed to an equivalent stress
f i e l d over the ice s h e l f . The stresses are determined
by the i c e - s h e l f freeboard and by the force f i e l d
exerted on the ice shelf by i t s sides and by ice r i s e s ,
and our analysis y i e l d s estimates of these r e s t r a i n ing forces F f o r the Ross Ice Shelf. An apparent
increase i n F very close to the ice f r o n t suggests
t h a t the ice shelf possesses a narrow seaward f r i n g e
of anomalously s t i f f i c e . We suspect t h a t t h i s represents the e f f e c t s of increased bottom-melting rates
(and therefore colder and s t i f f e r ice) very close to
the ice f r o n t .
In order to i l l u s t r a t e the r o l e of the r e s t r a i n ing forces i n c o n t r o l l i n g i c e - s h e l f behavior, we
c a l c u l a t e the s t r a i n - r a t e f i e l d f o r an u n r e s t r i c t e d
Ross Ice Shelf, i . e . one t h a t i s detached from i t s
sides and contains no ice r i s e s . Currently the creept h i n n i n g rate f o r most of the ice shelf i s 0.5 to
1 m a " 1 f o r an u n r e s t r i c t e d ice s h e l f , but i t would
increase t o 1 to 10 m a - 1 , w i t h values up to 60 m a - 1
up-stream of the ice r i s e s .
This paper has been accepted f o r p u b l i c a t i o n in
t h e Journal
of
Glaoiology.
THE IMPORTANCE OF PRESSURIZED SUBGLACIAL WATER
IN SEPARATION AND SLIDING AT THE GLACIER BED
(Abstract only)
by
Robert Bindschadler
(Goddard Laboratory for Atmospheric Sciences, NASA/Goddard Space Flight Center, Greenbelt,
The e f f e c t of pressurized sub-glacial water on
the s l i d i n g process i s q u a n t i f i e d by c a l c u l a t i n g a
"bed separation index". The water pressure d i s t r i bution i s calculated assuming the existence of a
Rothlisberger channel at the bed. Kamb's formulation
i s used to describe the v a r i a t i o n of normal stress
over periodic bed undulations. The hypothesis i s t h a t
as e i t h e r basal shear stress or water pressure i s
increased the extent of ice-bedrock separation (on
the down-glacier side of undulations) increases and
enhanced s l i d i n g occurs.
Data from three g l a c i e r s of widely varying size
are used to t e s t t h i s hypothesis. For Columbia
Glacier and "Ice Stream B" the importance of i n c l u d ing the e f f e c t s of water pressure i n any " s l i d i n g
law" are pronounced. More complete data from the
t h i r d t e s t case, Variegated G l a c i e r , are used to
compare a number of possible formulations of s l i d i n g
law which encompass the above hypothesis. A modified
Weertmantype law appears to be most preferable while
while some p o s s i b i l i t i e s , i n c l u d i n g Budd's l u b r i cation f a c t o r hypothesis, are t e n t a t i v e l y r e j e c t e d .
Consideration of the temporal v a r i a t i o n s of the
"bed separation index" reemphasize t h a t , especially
i n the short time scale, v a r i a t i o n s of water pressure
can dominate the s l i d i n g process. An order of magnitude increase i n water discharge causes a hundredf o l d t r a n s i e n t increase i n the water pressure.
This paper has been accepted f o r p u b l i c a t i o n i n
the Journal of
Glaaiology.
349
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