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Coupled models of glacial isostasy and ice sheet dynamics


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Zweck, Christopher 1997 , 'Coupled models of glacial isostasy and ice sheet dynamics', PhD thesis, University of Tasmania.

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This thesis deals with the incorporation of isostatic processes into realistic models
of ice sheet dynamics. A viscoelastic half-space model of isostatic adjustment
is developed, and as an initial exercise is coupled to a model of the Antarctic
ice sheet simulating the last glacial cycle. The ice sheet model is a three dimensional,
time-dependent model originally formulated by Jenssen (1977)
where the driving input data are net accumulation of snow and eustatic sea
level change. This allows examination of the sensitivity of the ice sheet simulation
to changes in the parameters of the isostatic model. In general, the
maximum ice volume generated over a glacial cycle decreases with increasing
mantle viscosity and increasing lithospheric rigidity.
To obtain realistic values for the isostatic parameters of mantle viscosity and
lithospheric rigidity the retreat of the Northern Hemisphere ice sheets and the
subsequent isostatic adjustment since the last ice age is simulated. The isostatic
parameters are adjusted until the overall model provides the best match to
relative sea level data, with the eustatic component of the relative sea level
change prescribed. (The maximum value of the amplitude of the prescribed sea
level change is 130 m as determined from the Huon Peninsula in Papua New
Guinea). Initially the simulation and matching procedure is performed using a
simple ice sheet model whose time dependent extent is set by the ICE4G dataset
(Peltier, 1994) and whose thickness and volume is set on the assumption of a
parabolic profile of thickness. From these trials the model parameters that most
realistically reproduce the observed isostatic adjustment associated with the
retreat of the Laurentide ice sheet are 3 x 1021 Pa s for lower mantle viscosity,
2 x 1021 Pa s for upper mantle viscosity and 1 x 1025 N m for lithospheric
rigidity. For the Fennoscandian ice sheet the corresponding parameter values
are 6 x 1021 Pa s, 4 x 1021 Pa s and 6 x 1024 N m. The trials are then repeated
with the parabolic profile ice sheet assumption replaced by generation of ice
sheet thickness using the Jenssen ice sheet model. For the Laurentide ice sheet
the same earth model parameters are recovered. For the Fennoscandian ice sheet
the use of the Jenssen model to simulate ice thickness produces earth model
parameters of 1.3 x 1021 Pas for both the lower and upper mantle viscosity and
2 x 1025 N m for the lithospheric rigidity. A problem with the analysis is that
the maximum volume of the combined ice sheets corresponds only to 50 m of
eustatic sea level change in the case of the parabolic profile simulation and to
40 m when using the Jenssen model.
The sensitivity of the Antarctic ice sheet to regional variations in lithospheric
rigidity is examined. Using a range of simple relations between crustal
thickness (for which there exists data on geographic distribution) and lithospheric
thickness, it is determined that the main effect of non-uniform lithospheric
thickness is on the extent of the Ronne and Amery ice shelves.
The constraint of prescribed eustatic sea level change since the last ice age
is removed by linking the Laurentide, Fennoscandian and Antarctic ice sheet
models via the common sea level change determined by the deglaciation of the
combined ice sheets. The constraint on Northern Hemisphere ice sheet extent is
also removed by allowing the ice sheet model (the Jenssen model) t<;> determine
its own extent when driven by climatology and the Milankovitch cycles of solar
input. This overall model produces a realistic eustatic sea level change since
the last ice age (130 m), but unrealistic changes in relative sea level. In some
locations the calculated relative sea level changes are too large by 200 m.
The problem of obtaining a consistent simulation of both eustatic and relative
sea level change is not resolved. There are three possible explanations.
First there may have been an extensive ice sheet over Siberia, which has not
been accounted for in this or any other analysis. Second the calculations here
assume linearity between isostatic ·disequilibrium and rate of adjustment. This
may not be the case. Third, significant changes in ice volume may have occurred
before the relative sea level record was laid down in the geological record.

Item Type: Thesis - PhD
Authors/Creators:Zweck, Christopher
Copyright Holders: The Author
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Copyright 1997 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Additional Information:

Examines the incorporation of isostatic processes into realistic models of ice sheet dynamics, using a three-dimensional, time-dependent ice sheet model. Thesis (Phh.D.)--University of Tasmania, 1998. Includes bibliographical references. Examines the incorporation of isostatic processes into realistic models of ice sheet dynamics, using a three-dimensional, time-dependent ice sheet model

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