Sea Level Changes - the last 6000 years
The Fixed Biological Indicator Method of detecting the height and fluctuations of former higher sea levels. Certain common shell species that are attached to rocky cliffs especially tube worm and barnacles are tightly constrained to fixed zones in the inter-tidal assemblage of shell fish on rocky ocean and bay shorelines. They form pronounced and easily identifiable bands of shell. When sea levels fall from previous high points these bands of shells are left behind, but only if the fall is rapid, so their very presence is an indication of how the sea fell, fast or slowly. The remnant exposed shell on the rock faces are mostly eroded away by weather, except in caves and crevices and where rock collapses protect them. Most spectacular are the finds under the immense collapsed sandstone visors in the indented coastline of the Sydney region but there are numerous sites around Australia, Southeast Australia and Brazil recording former higher sea levels
|
Tubeworms (Source : WA Museum) |
(Source : Baker and Haworth 2000)
(Source; Baker et al. 2005)
This diagram is a polynomial regression curve of carbon-14 dated shell samples, showing one possible interpretation of the cluster of dates as a fluctuating sea level curve over 5000 years, with a net sea level fall of c. 2 metres over this period. The Y axis is years BP (Before Present). The X axis is height above present equivalent sea level. All dates are calibrated to ‘real’ calendar years from tree ring data. For a long time it was thought that the elevated Rottnest shellcrust and marine platforms were a result of tectonic uplift: our work has shown a similar pattern along the entire southern WA coast, a pattern not dissimilar to the SE Australian coast. Hence, the well known elevated shell crust deposits of Rottnest Island’s lakes are typical of the whole Australian coastline. The debate is about whether this fall—and possibly fluctuating fall---is a result of hydro-isostatic influence or eustatic (world-wide) fall in sea level from presumably climatic influences.
For a step by step introduction of how the work is done, have a look at King Island.