The recent history of the Antarctic ice sheet : constraints from sea-level change

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.