Global change and Australian coastal processes
It was noted earlier in this chapter that the 1PCC has been instrumental in providing climate change projections related to the predicted effects of enhanced global greenhouse warming and associated sea-level rise (Houghton et al. 1991, 1992, 1996, 2001). These predictions and their implications for Australia's future coastal management are discussed in chapter 5 of this book. However, it is also important in the context of coastal evolution to look at past global changes, specifically the rapid environmental changes that took place during the Quaternary period. These changes have had the most dramatic effects on the appearance of the modem Australian coastline, such as the creation of extensive coastal barrier and dune systems, back-barrier lagoons, tidal plains, and subtidal sediment accretion associated with reef and seagrass growth.
Impact of Quaternary global changes on the Australian coast
Being the most recent geological period (the last 1.8 million years), the Quaternary has had a significant effect on the Australian coast because of the impacts of oscillating sea levels and alternation between wet and dry phases in the landscape. Unlike many northern hemisphere coastlines, the Australian coast has not been affected by ice during the Quaternary. However, global climate changes have caused repetitive build-up and decay of continental-scale ice sheets, resulting in alternate flooding and exposure of continental margins, associated with periods of erosion and sedimentation.
These global climatic changes demonstrate a pattern that is linked to variations in the Earth's orbit. The resulting differences in solar radiation were predicted by Milankovitch (1930) and subsequently termed Milankovitch cycles. The Earth's orbit has three key elements that influence climate, each with a different period. The orbital eccentricity cycle (a stretching of the Earth's orbit) occurs about every 100 000 years; the axial tilt cycle (a rolling of the axis of rotation) occurs about every 41 000 years and the precessional cycle (wobble of the axis of rotation) occurs about every 23 000 years. The influence of these cycles has varied. In the early part of the Quaternary (up to 850 000 years ago) the axial tilt cycle was dominant, but since then the orbital eccentricity cycle has been dominant. The result has been a series of glacial cycles with a period of around 100 000 years, punctuated by warmer interglacial events such as our modern-day climate.
The major features of the glacial-interglacial fluctuations have been outlined by Emiliani (1955), who measured oxygen isotope ratios (180 : 160) of planktonic foraminifera extracted from deep ocean sediments. Using these ratios he was able to interpret global temperature changes over time. He established a quasiperiodic cycling of climate that had recurred many more times than the four major glaciations recognised from previous northern hemisphere stratigraphic studies.
These global climatic changes affected the Australian coastal environment by repeated sea level transgressions (rising sea level) and regressions (falling sea level) across the continental shelf, leaving an associated sedimentary record. Today, the Quaternary sedimentary legacy can be seen around the coast in sedimentary deposits such as relict and active coastal dunes, beach ridges, and estuarine and back-barrier sediments. In addition to the exposed coastal deposits, there is significant sedimentary evidence of former sea levels below
Figure 2.10 Blue hole on Molar Reef in the Pompey Complex. This hole is more than 30 m deep and more than 250 m wide. It is also related to karst processes during the Quaternary, when fluctuating sea levels alternately exposed the reef structure a number of times subjecting it to periods of erosion.
(photography Nick Harvey)
present sea level because the high-stands associated with each transgression were rarely higher than the high-stand of today.
Elsewhere on the Great Barrier Reef there is good anecdotal evidence of the effects of these repeated sea level transgressions and regressions through successive periods of coral accretion and erosion. This is most marked in the Pompey Complex (about 250 km east of Mackay) where blue holes (see figure 2.10) occur within reefs of 100 km- or more (Backshall et al. 1979). These blue holes are dolines or collapsed caves caused from multiple periods of limestone weathering during each regressive phase.
In South Australia there is a spectacular series of uplifted coastal barriers, which dates back to around 800 000 years BP on the evidence of the Bruhnes-Matuyama magnetic reversal data (see figure 2.11). These major bar-
Figure 2.11 Sedimentary evidence of higher sea level events from the south-east
Source: modified from Gilbertson & Foale 1977
riers were formed by the accumulation of calcareous sand dunes during interglacial periods of a relative high-stand of sea level. In the intervening glacial periods, the sea retreated and a slow tectonic uplift of the region resulted in the stranding of the former barrier. During the following interglacial cycle, sea level rose again to form a new barrier some distance seaward of the earlier stranded barrier. Thermoluminescent dating of the whole series of barriers has demonstrated a correlation between the barriers and the deep sea oxygen-isotope records, with evidence of the 100 000 year cycle of climate and sea-level change (see figure 2.12). Of approximately 27 major glacial-interglacial cycles recognised in the Quaternary, at least 13 sea level highstands are preserved as elevated shorelines (high-energy coastal barriers) on the south-east coastal plain of South Australia, one of the best on-land preserved sequences in the world.
2 and 8 m higher than today (Chappell 1987). Since then, sea level fluctuations have always been lower than at present, with evidence from the Australian region of a lowest sea level of between 130 and 165 m below the present level, 18 000 yrs BP (Chappell 1987).
Evidence of former shorelines is often very patchy with only occasional sedimentary exposures, but the last interglacial shoreline, which stood higher than the present sea level, is sufficiently preserved around the Australian coast to enable a good reconstruction (Murray-Wallace & Belperio 1991). This not only indicates the position of this previous coastline, but also provides evidence of geological movements that have taken place since that time. Whereas Murray-Wallace and Belperio have mapped this at a broad scale for Australia, it is also possible to see more detailed evidence at the regional level, such as in South Australia (Bourman et al. 1999, Harvey et al. 2001b). The coherence in palaeo-shoreline elevation from site to site in South Australia allows geological warping since the last interglacial to be confidently determined. The lack of upwarping on the west coast and Eyre Peninsula indicates relative tectonic stability in the region. Localised upwarping in parts of the Mount Lofty Ranges demonstrates large relative land or sea-level changes over short distances. Tectonic effects are most noticeable along the south-east coastal plain, between Lake Alexandrina and Mount Gambier, where Quaternary volcanism has resulted in ongoing uplift and tilting of the coastal plain (Murray-Wallace et al. 1998). The scale and variability of this upwarp can be illustrated by the chang-
Figure 2.13 Geological warping of the last interglacial shoreline
Source: Harvey et al. 2002, after Murray-Wallace & Belperio 1991 and Bourman et al. 1999
ing elevation of the last interglacial shoreline, which rises progressively southwards from 5 m above present sea level at Salt Creek to over 18 m near Port MacDonnell. Uplift rates in the Port MacDonnell region are 0.07 mm/year if averaged out over this entire time period.
Following the higher sea level and warm climatic conditions of the last interglacial, sea level began to fall as the Earth entered the last glacial period. Although sea level rose and fell a number of times during this time, it was always lower than it is today and remained so for around 100 000 years, it is difficult to find detailed evidence for sea level changes during the glacial period because much of the evidence is either below modem sea level or is buried under sediments. Some of the best evidence of detailed sea level changes during the most recent glacial cycle (from around 125 000 years BP to the present day) can be found to the north of Australia, on the Huon Peninsula in New Guinea. Here a flight of coral terraces is preserved on a tectonically uplifted coast, where the rapidity of the uplift has stranded each coral fringing reef produced during significant relative sea level still-stands (periods long enough for a new fringing reef to grow). From this series of terraces it has been possible, using modem dating methods, to plot the pattern of sea level change during the last glacial cycle (Bloom et al. 1974, Chappell 1974).
As noted above, sea level was at its lowest during the maximum of the last glacial cycle (around 20 000 years BP), exposing the continental shelf and creating land bridges across areas of former inundation (figure 2.15). After this event, the rapid global warming and retreating polar ice caps produced a sea level rise with an average rate of between 6 and 12 mm/year before reach-
Figure 2.14 Global sea level change during the last glacial cycle
Source: modified from Church et al. 2001
ing its present level between 6000 and 7000 years BP. This rapidly rising sea cut off Tasmania around 12 000-13 500 BP, and New Guinea around 6500-8000 years BP (Jennings 1971). Coast-dwelling Australian Aborigines would have witnessed this rapidly rising sea, and would have had to retreat inland. In the South Australian gulfs region the rapidly rising sea created Kangaroo Island and cut off the indigenous population from the mainland. There is also good sedimentary evidence from the South Australian gulfs to demonstrate the rapidity of the sea level rise in this region (see figure 2.16).
Figure 2.15 Approximate position of the Australian coastline 20 000 years BP (left) and the present day (right)
Figure 2.16 Sedimentary evidence from the South Australian gulfs demonstrating the rapidity of sea level rise
Source: Harvey et al. 2001b, after Belperio 1995
It is this significant postglacial sea level transgression that has had a major impact on the Australian coast, and much of the coast is still responding to this event in various ways. In addition to the sea-level rise, there has also been a climatic change that has produced different river flows and altered the amounts of sediment being transported to the coast. In some parts of the coast, historical supplies of sediment are still being redistributed and moved along the coast following the postglacial marine transgression. Elsewhere, the effect of water loading on the continental shelf has caused localised coastal warping (known as hydro-isostatic adjustment) causing relative land-sea level changes, ft is important to note that the effect of this rising sea has not been uniform around the globe because of different crustal responses to the water loading. This has been modelled in some detail on a global basis by Peltier (1999), and for Australia by Nakada and Lambeck (1989). This is discussed in more detail in the next section.