Late Pleistocene and Holocene sea-level change along the Australian coast

Abstract

Tectonically, the Australian continent is relatively stable and, as such, it provides a good platform for studying Late Pleistocene and Holocene sea-level change. The observations indicate that present sea-level was reached at about 6000 years ago and that since then level has remained constant to within a few metres. Considerable spatial variability in the amplitude of the emergence and in the time at which sea-levels first reached their present values have been observed. Spatial variability has also been observed in the sea-level during the last glacial maximum and during the late phase of the Holocene transgression. Much of this variaiblity is the result of the Earth's adjustment to the glacial melting in the Late Pleistocene-Early Holocene time and the concomitant distribution of this meltwater into the oceans. Conclusions that are drawn from matching observations with this glacio-hydro-isostatic model include the following. (1) The mantle viscosity increases with depth: the average upper mantle viscosity is about (2–3)10 20 Pa s and the average lower mantle (defined as below 670 km depth) viscosity is about 10 22 Pa s. The average lithospheric thickness for the coastal and offshore region is about 70–80 km. Lateral variations in these mantle parameters are indicated. (2) The addition of melwater into the oceans did not cease 6000 years ago but continued at a reduced rate up to more recent times, adding an amount of water equivalent to a 3 m sea-level rise in this interval. (3) The amplitude of the Holocene emergence is a function, inter alia, of coastal geometry and significant variations occur over distances of a few hundred kilometres and more. Holocene highstands of 1–2 m are generally well developed at the continental margins but are less well developed or do not developed at all at offshore islands and reefs. (4) Variations in the time of maximum emergence also occur, being earlier at upstream tidal river sites than at coastal sites. At offshore reefs or islands the sea-level curve lags the coastal curves by 1000 years or more. (5) Considerable spatial variability occurs in the sea-levels at the time of the last glacial maximum with variations of up to 30 m occurring across the continental shelves. (6) There is generally no need to invoke tectonic movements to explain the sea-level data along the Australian coast but possible exceptions occur for the Perth region and for Tasmania.