Vanishing Coastlines

 An easy way to understand 'natural' climate change is to imagine an Ice Age coming every 100,000 years or so. Climate change normally occurs in reasonably regular cycles. This has been the pattern over the last 7-800,000 years.

The scientific for Ice Age is Glacial. This name refers to major cold episodes in the Earth's history, when continental ice sheets form, glaciers advance, and sea levels fall. The periods between Ice Ages are called Interglacials. These are times when polar ice caps shrink and glaciers retreat. Their released melt-water then raises sea levels again. We currently reside in one of these warmer phases.

The last complete glacial cycle took place between 130,000 and 20,000 years ago. The particular details of this approximately 110,000 year long event have been 'mapped' according to Marine Isotopic Stages (MIS). This involves comparing oxygen isotope ratios preserved in the carbonate shells of long dead marine organisms.

Geologists argue that different amounts of H₂¹⁶0 and H₂¹⁸0 in ocean waters reflect the variations between Glacial and Interglacial conditions. During glacial phases, large quantities of lighter H₂¹⁶0 are trapped in accumulating masses of ice, leaving the world's oceans enriched in H₂¹⁸0. In contrast, during interglacials large volumes of H₂¹⁶0 are released back into the sea as ice sheets melt, resulting in much lower H₂¹⁸0 ratios.

The diagram below represents the marine isotope record for the past 350,000 years. The high points, where isotopic values are positive, indicate warmer interglacial positions. The low isotopically negative points indicate colder glacial times.

The MIS graph shows the Last Glacial Maximum (LGM) and the Penultimate Glacial Maximum (PGM) at roughly 20,000 years and 130,000 years before present. The corresponding interglacials follow on in what appears to be a comparatively short time. The striking inference here is that once the 'big melt' is on, nothing much can stop it. The MIS record shows a steep straight line from one climatic extreme to the other, virtually without pause or correction.

Another method of 'mapping' climate change is to sum other dating information, like pollen counts, speleoform growth, and sediment stratification. A combination of global and local data has been used to relate the past 130,000 years with the rise and fall of sea levels. This produces a very similar curve line to the most recent cycle in the MIS information.

This diagram shows the period from the Penultimate Interglacial to the Last Interglacial. The composite dating data has been presented as a relative sea level curve (blue line) and also grouped into 10 metre sea level depth bins (red line). The additional approach was introduced to account for dating uncertainties from results published all over the world, but it also helps to give a sense of the more frequent or repeated elevations over time. 

It can be seen that there are a number of intermediate low points, called Stadials or 'low stands', and high points, called Interstadials or 'high stands'. Stadials are stages of cooler and drier conditions whereas Interstadials mark warmer and moister times.

The overall picture presented by both diagrams for the last 130,000 years is of a steadily deteriorating climate, with many 'pauses' and 'pulses' along the way, before a final 'rapid' recovery to modern conditions. The significance of these interruptions to a 'smooth curve' is that they allow new coastlines to form.

There is sound bathymetric evidence for a paleo-shoreline about 6 kilometres off the present-day coast of western Victoria, at 60 metres below sea level. Features identified include substantial cliffs, shore platforms, and sea stacks. They were probably formed by wave erosion during a stable sea level 'high stand' around 60,000 YBP.

The Twelve Apostles are a familiar sight on the Great Ocean Road near Port Campbell. Standing offshore from 50 metre high cliffs and exposed to the full force of southern gales, these 45 metre tall limestone pillars now number only eight (the ninth having tumbled into the sea in 2009).

The Twelve Apostles were formed as weaker parts of the Port Campbell Limestone were eroded by wind and wave, leading to their isolation as headlands. As erosion continued,  these headlands were gradually dissected by caves and then arches. Further undercutting of the rock led to collapse of the connecting arches, leaving the free-standing arches now known a sea stacks.

This  contemporary landscape of marine sculpture is actually replicated along part of the earlier shoreline. A section of submerged cliff is fronted seaward by five 'drowned apostles'.

The old cliff line is reduced to an average 13 or 14 metres in height, the limestone pillars to 4 or 5 metres, but there is little doubt that they are an earlier version of the famous tourist attraction.

The paleo-coast makes constant reference to its visible counterpart. There are three discernible cliff-lines. Two of these, each 5 kms long, form the opposite sides of a huge embayment 7 kms across, which narrows towards Port Campbell Bay. The western cliff has a river channel running through it that can be traced back to the mouth of the Curdies River near Peterborough. There are horizontal rock platforms extending from the base lines at the westernmost and easternmost sections of these cliffs, echoing again what occurs on the 'new' surface.

The group of five separate sea stacks are admittedly about ten times shorter and four times thicker than the surviving eight on the current coast. However, they still maintain a clear columnar profile and have a proximate relationship with their 'parent' cliffs. 

The difference is that all these drowned cliffs, platforms and stacks are deep beneath the present ocean surface. They were 'carved' (calved?) six millennia ago, during 'a warm stand with a fairly wet climate'. Sea level rise was relatively fast during this Interstadial, at 6mm per year compared to the modern rate of 3mm. Because of their quicker submergence, the five drowned apostles were possibly saved from a similar fate to the younger 12 Apostles. That paleo-environment spared these massive 'stumps' from inevitable collapse under exposed conditions.

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KEY RESOURCE

Rhiannon Bezore, 2019, 'The Morphology and Evolution of Rock Coasts Over Eustatic Cycles in Temperate, Wave Dominated Environments', PhD thesis, Melbourne University, <rest.neptune-prod.its.unimelb.edu.au/>

(including Rhiannon Bezore, 2016, 'The Drowned Apostles: The Longevity of Sea Stacks over Eustatic Cycles', Journal of Coastal Research, 75, 592-596)

Supplementary Sources

C Sloss & P Moss, 2008, Australian Quaternary Association's submission to the "Inquiry into climate change and environmental impacts on coastal communities".

Brooke, Nichol, Huang, Beaman, 2017, 'Paleoshorelines on the Australian continental shelf: Morphology, sea-level relationship and applications to environmental management and archaeology', Continental Shelf Research, 134, 26-38.