Palaeo-Environmental Approaches to Reconstructing Sea Level Changes in Estuaries

Modern estuaries owe their origin to post-glacial, eustatic sea level rise of the order of 120–130 m. Superimposed on this global effect are local or regional factors which need to be established for each study location, such as many Southern Hemisphere locations have shown a highstand and then a regressive stage. In the last century, sea level has been rising on many coastlines around the world, and where local or regional tectonic and/or glacio-isostatic influences prevail, relative sea level varies from global trends. The sedimentary evidence of these changes varies according to the processes dominant within an individual estuary, whether wave, tide or river dominant, or a mixture. There are two parts to sea-level reconstruction, first, determining index points to define a local sea level curve, and second, interpreting the causes of these. Sea level index points use indicators of some feature, erosional or depositional, abiotic or biotic, that formed during a period when it was under the influence of intertidal processes, and which can be dated. Quantitative transfer function approaches to achieving sea level reconstructions from estuarine deposits are shown using a case study from eastern Tasmania. Here, foraminiferal occurrences across the present intertidal marsh surface can be used to define former sea level positions in sediment cores at depth. These can be dated to provide a sea level envelope that can verify isostatic modelling results. In turn these can improve planning for future sea level change by incorporating local relative sea level trends into global projections.