South Australian case study
The rapid rise in sea level following the last glacial cycle can be illustrated for Gulf St Vincent (figure 2.19) in South Australia where, by 9700 years BP, the rising sea had broken through what is now known as Backstairs Passage. By 9500 years BP the sea had cut off what is now known as Kangaroo Island, and by around 6500 years BP had reached its present level in Gulf St Vincent. Since then, coastal and shelf processes and coastal sedimentation have progressively stabilised, producing a range of depositional styles that relate
Figure 2.19 Rapid post-glacial sea level rise illustrated for Gulf St Vincent
Source: Harvey et al. 2001b, after Lampert 1981
Figure 2.20 Holocene sedimentation rate, LeFevre Peninsula
Source: Harvey and Bowman 1987
increasingly to the modem hydrodynamic regime. For example, a detailed geochronological study of a Holocene beach ridge system for the Lefevre Peninsula near Port Adelaide (Bowman & Harvey 1986, Harvey & Bowman 1987), demonstrated that the large volumes of sediment being moved onshore in the early Holocene have progressively declined, to the extent that beaches have to be artificially supplied by modem management (figure 2.20).
At the same time that coastal sediments were adjusting to the new sea level, hydro-isostatic loading of the continental shelf by the rising sea also resulted in different degrees of coastal warping and a geographically variable apparent sea level regression. This loading factor (see Barnett and Harvey 2001) results from the flexure of the continental margins because of the added weight of the post-glacial sea water once it has risen and moved across the continental shelf. Major factors affecting the variable response to this hydro-isostatic loading are the different water depths and the spatial extent of the continental shelf.
The variation of this relative mid-Holocene highstand has been described in detail for the South Australian coast by Belperio et al. (2002). This highstand was at a maximum of 4.5 m in the northern Spencer Gulf (the greatest distance from the continental shelf), 3.0 m in northern Gulf St Vincent, and 1.0 m along the south-eastern and western Eyre Peninsula coastlines. The variation in levels is directly attributable to different amounts of coastal warping and land adjustment following the post-glacial sea level rise.
Modern sea level changes
More recent, or historic, sea level change is measured using modem tide gauges. A number of these have been maintained around the South Australian coast, and a few of them are considered reliable enough for establishing long-term sea level trends. Australian tide gauge records from 1897 to the present (figure 2.21) have been monitored and analysed by the National Tidal Facility (NTF) at Flinders University in South Australia. Although there is a variation in the quality of tidal records available, the NTF analysis of tidal data from gauges with an acceptable datum stability indicate an Australian average rate of rise of 0.68 [±] 0.08 mm/year at the 95% confidence level. South Australian sea level trends based on tide gauge data have been presented by Mitchell (1991) and discussed in relation to geological effects (Belperio 1993, Harvey et al. 1999b, 2001b, 2002).
Figure 2.21 Sea level trends for the Australian coastline (least squares analysis)
Source: National Tidal Facility
It is useful to examine just two tide gauge locations, Port Pirie and Port Adelaide, to illustrate how sea level trends measured from tide gauge data alone can be misleading without correcting for other factors. The tidal records for these sites are among the longest (Port Adelaide, over 55 years; Port Pirie, over 63 years) and most reliable in South Australia. These records have also been used in calculations of global average sea level rise. Figure 2.22 shows that the sea level trends at these two sites are quite different, even though they are only 200 km apart. At Port Pirie it appears that sea level is falling fairly slowly at -0.19 mm per year, whereas at Port Adelaide the sea-level appears to be rising very rapidly at 2.08 mm per year. Obviously, there must be some local or regional reason for this difference.
Figure 2.22 Sea level trends at Port Pirie and Port Adelaide
Source: National Tidal Facility