Recent sea level change
Following the low sea level some 20 000 years ago, the sea rose rapidly until approximately 6500 years ago, when it reached its current position. Over the same time, there were changes in coastal sedimentation and land movement which complicate the interpretation of the position of former sea levels. For example, an area of rapid tectonic uplift such as New Guinea might show evidence of a 'relative' sea level fall for a specific location which is larger than any global sea level rise. Thus it is necessary to incorporate both land and sea level movements in order to get an accurate picture of 'relative' sea level change.
Australia is often referred to as a stable continent from which absolute sea levels may be measured, but in reality there are subtle land movements caused by structural, geoidal and isostatic processes, together with factors such as sediment compaction. These affect the Australian coast at different spatial and temporal scales and complicate the interpretation of past sea level changes. A number of reviews of Australian Holocene sea level changes (see Barnett & Harvey 2001, Hopley 1983) have attempted to compile some of the extensive geomorphological and geological evidence for Holocene sea-level variations around Australia's margin. Barnett and Harvey (2001) noted the difficulties of interpreting past sea levels because of (a) the need to establish the relationship
Figure 2.18 Variability of relative highstand of the 6000 year BP shoreline around the Australian coast
of each indicator to sea level at the time of its formation, (b) the relationship of the submerged or elevated shoreline indicator to present sea level datum, and (c) dating errors.
Barnett and Harvey (2001) also demonstrated the variability of the relative highstand of the 6000 years BP shoreline around the Australian coast (figure 2.18). The majority of this evidence is based on geomorphic, sedimentological or biological data such as chenier ridges, beach ridges, microatolls, coral reefs, oysters, intertidal calcareous worms, in situ tree stumps, mangroves, and shell bed evidence. As can be seen from figure 2.18, the evidence of the 6000 year BP sea level gives values that vary from 0 to 6 m above the present level.
Most of these studies provide evidence of variations in Holocene sea levels around Australia, but il is not easy to link these to present sea-level variations estimated from tidal data. The reason for this is that very few of the studies have been conducted close enough to modem tide gauges to produce any meaningful results. Three exceptions to this are studies from Townsville (Harvey et al. 2002), Port Adelaide (Belperio 1993) and Port Pirie (Harvey et al. 1999a). The latter two are both from South Australia in an area where several of Australia's longest tidal records exist. In the following case study, isostatic and anthropogenic contributions to vertical land movements were deduced from evidence within the tidal sediments deposited near the tide gauge stations.
