The recent history of the Antarctic ice sheet : constraints from sea-level change
Date
1995
Authors
Zwartz, Daniel Peter
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Abstract
The rheology of the earth and the history of ice sheets, which have a major
contributing role in climate change, are both subjects of considerable interest
in earth sciences, and the study of sea-level change provides insight into
both. Sea-level change since the last glacial maximum (LGM), about 18,000
years ago, can be explained as the sum of three contributions: the sea-level
rise due to melting of the Pleistocene ice sheets; the isostatic and
gravitational response to the melting of these ice sheets; and the isostatic
and gravitational response to the water added to the oceans. Thus, sea-level
change varies with location, and is dependent on the volume and
distribution of the ice removed, the shape of the oceans, and the rheological
structure of the earth. Using sea-level records from appropriate locations,
and numerical models of the response of the earth to surface loads,
constraints can be placed on some of these parameters. In this thesis, I use
new sea-level observations from Antarctica and Queensland to estimate the
former distribution of ice at several Antarctic sites, the total amount of extra
ice which was stored in Antarctica at the LGM compared to the present, and
the eustatic sea-level change which has occurred in the last 6,000 years.
I present a new high-resolution sea-level record from the Vestfold Hills,
Antarctica, obtained by dating the lacustrine-marine and marine-lacustrine
transitions in sediment cores from lakes which were formerly connected to
the sea. A sea-level maximum ~9 m above present sea-level 6,000 years ago
is documented. Sea-level observations from other Antarctic sites have been
compiled and compared with predictions derived from simplified models of
melting at the ice sheet margin. The results indicate that at the LGM the
East Antarctic ice sheet margin was 25 - 100 km beyond its present position,
resulting in ice thicknesses of 500 - 1000 m at sites now on the coast.
Eustatic sea-level change in the last 6,000 years can be estimated from the
difference between sea-level predictions and observations at sites unaffected
by details of the ice sheet reconstructions. Using new sea-level observations
and a compilation of published data from north Queensland, a eustatic sealevel
rise of 3 - 6 m in the last 6,000 years is inferred, with the rate of rise
decreasing towards the present.
The reconstructions described here, if applied to the entire Antarctic Ice
Sheet margin, would amount to 8.6 - 12.1 m equivalent sea-level. When
combined with the ~90 m contribution from the better-constrained northern
hemisphere ice sheets, this is insufficient to make up the observed -120 m
of postglacial eustatic sea-level rise. The additional water may have been
stored (i) in the Antarctic Ice Sheet at locations far from the sites studied
here, such as the West Antarctic Ice Sheet, or (ii) in another reservoir
elsewhere on the planet. When corrections are made for all other known
contributions, the observed sea-levels in eastern Australia show a gradient
in latitude consistent with the removal of a large volume of ice from
Antarctica, supporting the former scenario.
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