by Patrick Nunn
Another example of how sea-level change caused widespread and enduring changes to human societies comes from the tropical Pacific islands. Around 700 years ago – ad 1250–1350 is the best estimate – there is evidence that the sea level fell slightly across this vast region. It may have only fallen by a few dozen centimetres, but that was enough, it appears, to cause massive problems for the islanders who had up until this time lived almost entirely along island coasts, eating well for the most part from the rich sources of readily accessible seafoods, supplemented by foods grown along the well-watered coastal plains. Then came disaster. The slight fall in the sea level was enough to expose the surface of the offshore coral reefs – the central link in the seafood-production system – and to lower on-land water tables sufficiently so that crop roots could no longer easily reach them. Reefs died, lagoons went unflushed by seawater, crops failed – and within a few generations people had much less food to eat. In such a situation, when everything else has failed, rather than starve you naturally turn your gaze towards your neighbours. Maybe to eat … but more likely to get your hands on the foods they have to eat, something they would be unlikely to give up without a fight.
Thus the upshot of this sea-level driven food crisis was the outbreak of conflict throughout the tropical Pacific islands. By about ad 1400, people living on high islands across this third of the Earth’s surface had all but abandoned coastal settlements in favour of new ones established in fortifiable locations, usually inland and upslope.17 In island nations like Fiji, where much of my research into hillforts – or koronivalu (literally towns-of-war) as Fijians call them – has been concentrated in recent years, it is clear that conflict following this food crisis endured for hundreds of years.18 When the British began settling in Fiji in earnest around the 1850s, some Fijians were still occupying hillforts. Consider this 1870 report by an Australian newspaperman about what he saw along the coast of Viti Levu Bay, in the north-east of Fiji’s largest island:
In the distance [are] the houses of the mountaineers, perched curiously on the apex of rocky pinnacles, a position singularly secure from invasion... Walking around the base of the bay in a northeasterly direction we came to a thickly populated town on the crest of a hill, east of which could be seen another native village, built on an immense rock. The natives who were all clustered together on a little plateau in front of the town received us very nervously... 19
The plausible connection between climate-induced sea-level fall and societal upheaval for Fiji and other Pacific Island groups about 700 years ago provides an excellent example of how coastal dwellers – in whatever part of the world they may live – pay a price for the advantages of their location, a cost that is occasionally charged when the sea level changes.
The ocean surface has been rising, on and off, for the better part of the last two centuries. The upward trend is clear, although doubters commonly exaggerate the significance of short-lived periods of stasis or even fall. These are to be expected in any natural system being closely monitored over long time periods. The global sea level is currently rising at an average rate of 3.2mm each year, somewhat higher than its average rate of rise of 1.7mm per year over the past 100 years or so.20 Observations explain 87 per cent of the 54mm (about 2in) of sea-level rise between 1993 and 2010 as being caused by the two processes mentioned earlier. Thermal expansion was responsible for 37 per cent, meltwater from glaciers and ice sheets contributed 50 per cent, and the remaining 13 per cent is attributed to the extraction of groundwater by people and its eventual arrival in the ocean.21
During the coldest time of the last ice age (the LGM), the continent of Australia was 30 per cent larger than it is today.22 Where today’s shallow submerged continental shelf is widest, the coastline during the LGM was thousands of kilometres seaward of its present position, all land once occupied by people that has since been drowned. Most of what are now smaller offshore islands, including Tasmania, were connected to the mainland. And significantly, there was a continuous land bridge connecting Australia to what is now the large high island of New Guinea, a land bridge where the Torres Strait and the scatter of islands within it now lie. Across this land bridge, people moved freely for most of the time they occupied Australia. They clustered around large fresh Lake Carpentaria into which the ocean, forced by the rising sea level, finally spilled about 10,500 years ago, leading to the development of the modern Gulf of Carpentaria.
Given that people arrived in Australia around 65,000 years ago, tens of thousands of years before the culmination of the last great ice age about 20,000 years ago, we know for certain that they witnessed the subsequent rise of sea level that trimmed back the extremities of their land so swiftly and by such extraordinary amounts. It has been estimated that people living 10 millennia ago on the low-lying coastal plain south of the Nullarbor Desert would have witnessed the shoreline moving landwards at a rate of a metre (3¼ft) each week. And off the northern shores of modern Australia, it has been calculated that during more rapid periods of postglacial sea-level rise, people would have seen the shoreline retreat landward by 5km (3 miles) every year – a startling thing to contemplate. It would be interesting to know to what degree such dramatic rates of change created a millennialist mindset among contemporary Australians. Did they suppose that the sea level might continue rising independently until it ultimately engulfed all of Australia, condemning its inhabitants to a fate like that of people in the time of Gilgamesh and Noah? Did they accept what was happening as an expression of divine power, an empyrean whim they could not contemplate challenging? Or did they take a more pragmatic view, seeking ways to stop the rise of the waters and halt the contraction of their lands? Much evidence at the moment points to the latter – ancient stories from many parts of Aboriginal Australia recall that people at the time constructed sea defences – and it is likely that such actions were complemented by spiritual activities intended to rationalise and counter what was happening.
We saw in Chapter 3 that there are innumerable extant stories that plausibly recall the nature and effects of postglacial sea-level rise in 21 areas around the fringes of Australia. While extraordinary to ponder, the existence of these stories renders plausible the idea that postglacial sea-level rise had a major impact on the Aboriginal psyche. It is likely that the words of the stories that have come down to us today were merely part of a panoply of beliefs captured at the time in various media – including prose, song, dance and art – which enshrined the memories of sea-level rise and their various explanations in Aboriginal traditions. If the longevity of a particular oral tradition can be taken as a measure of its impact on those who originally conceived it – the eyewitnesses – and a measure of the concomitant need to have it communicated effectively to future generations, then clearly the impact of sea-level rise on the coastal-dwelling Aboriginal peoples who witnessed it was very powerful. But just how powerful? How many years have these stories endured?
To answer this question, we can take as a starting point the fact that the ocean surface – the sea level – reached its present level around Australia about 7,000 years ago. Since that time, steric effects have caused minor oscillations of sea level, but it has not fallen more than 1m (3¼ft) below its present level, nor in most places has it risen much more than 2m (6½ft) above it. So hardly any of the stories recounted in Chapter 3, except maybe those from Moreton and North Stradbroke Islands and that from Bremer Bay, could possibly be based on observations made within the past 7,000 years. Put another way, most of these stories must have endured – as oral traditions – for seven millennia or more.
While we are contemplating the extraordinary proposition that a story – let alone a number of stories – could survive in recognisable form for more than 7,000 years, consider that the comparative stability of the ocean surface within this period would not have allowed any reinforcement of the story using contemporary observations. And yet, it must have appeared increasingly implausible to each successive generation living through these thousa
nds of years that the sea level was ever much lower than it was – as the stories said. Maybe in some communities the stories of the elders were challenged, and the more implausible – like those about the sea level having once risen and drowned lands that are now underwater – were sidelined, and intentionally not transmitted to the next generation. Yet, such speculation aside, it seems indisputable that some groups – perhaps more than 20 of them – were able to effectively transmit drowning stories across a vast slice of time, making their people the greatest of all the oral chroniclers of human history.
As for most parts of the world, the history of postglacial sea-level rise around Australia is known fairly precisely today. A compilation23 is shown in Figure 4.3 and links time (in years ago) to ocean depths. The exact level of the Australian ocean relative to today for any one year in the past is impossible to determine, both because there were apparently minor variations between the levels of the ocean in different parts of the Australian coast at any one time, and because the techniques used to determine past sea levels are unavoidably imprecise. For these reasons, in Figure 4.3 the sea level in any one year can be given only as a range. Alternatively, the time at which the sea level reached a particular depth (or height) relative to today is also expressed as a range. Thus, as you can see, the sea level was 60m (200ft) lower than it is today between about 12,300 and 13,100 years ago – an 800-year range is as good as it gets here. Things are a bit better for the time when the sea level was 30m (100ft) lower, something that occurred around every part of Australia within the 500-year period between 9,950 and 10,450 years ago. Finally, as you can also see in Figure 4.3, the rising ocean surface reached its present level (zero metres) some time between 6,350 and 7,700 years ago. We know that there was a lot of spatial variation in this event, something that reflects the difficulty of determining precise data from the evidence, which (as discussed below) generally becomes more complex about this time.
Figure 4.3 Sea-level envelope for Australia over the last 13,000 years.
You might be wondering how anyone can know how much lower the ocean surface was, say, 10,000 years ago. Along almost every part of the world’s coastline the answer lies underwater, and while finding it is certainly not as difficult as it was a century ago, when scientists first realised it must be there, it is still a challenge to do so.
Every time it rains, soil and other loose particles – sediments – are washed off the land and into the sea. Sometimes if the particles are fine, they float for a while, but most end up sinking to the ocean floor where – over often many years – they accumulate in thick layers. Drilling down through these layers and removing a section gives us a slice through time, a window into the past, and allows us to learn how it incrementally became the present. The different layers of sediment, distinguished perhaps by texture or colour, cohesiveness or structure, tell us about the kinds of terrestrial environments where they originated – whether these were wetter or drier than they are today, for instance, or whether it was windier or stormier, or whether people were around burning the vegetation to form charcoal.
In such sediment drill-cores, it is also possible to identify old coastlines. Perhaps the core will pass through an ancient beach or a diagnostic coastal deposit (like a mangrove peat), which contains the remains of things that live only in such places. What scientists commonly look for in ocean-floor sediments to pinpoint ancient coastlines are concentrations of shells from species that live only between high- and low-tide levels.24 In many parts of the world, other diagnostic organisms are foraminifera, many species of which live only within a metre or two of the ocean surface and contribute their hard parts (tests) to adjoining beaches.
Many uniquely coastal (or intertidal) vegetation groups create sediments that may be prominent in sediment cores. Think of mangroves, for example. They are wonderful communal plants, hugely useful to coastal dwellers, both directly and indirectly, yet are also efficient trappers of sediment. Mangrove ‘muds’ are commonly thick, sticky and unforgettably foul smelling. Similar dense coastal vegetation systems like seagrass beds may also be underlain by distinctive sediments that stand out in ocean-floor sediment cores.
How this works in practice is well illustrated by the Gulf of Carpentaria in northern Australia. If you drill through the sediments on the comparatively shallow ocean floor that separates the modern Gulf from the Arafura Sea (to its west), you invariably find a threefold sequence of sediments. At the deepest level, sitting above much older deposits or bedrock, you find evidence of a terrestrial (dry-land) landscape that was emergent when the ocean surface was much lower during the LGM, and the Gulf was a large freshwater lake. Then, as the sea level rose, so the sediments indicative of terrestrial environments were replaced by those indicative of coastal ones, in which mangroves in particular abounded. As the sea level rose even higher, these diagnostic coastal sediments were replaced by ones more characteristic of the deeper ocean floor. Using radiocarbon dating, the age of the key transition zones in these sediment cores can be calculated.25
In warm ocean waters, coral reefs often provide far more compelling and complete records of past sea-level changes than can be obtained from sediment cores. Coral reefs are extraordinary, both on account of their much-studied and photographed surface expressions, but also because of what lies beneath. On reefs, the living corals we see near the ocean surface today grow on the skeletal remains of their dead ancestors, which grew on the remains of their ancestors, and so on and so on. So drilling through a coral reef takes us on a journey back through time, sometimes for tens of thousands of years. The remains of the corals growing at different periods can be geochemically analysed to tell us something about the temperature and salinity (a proxy for rainfall) of the ocean water in which they were living. Yet more importantly, coral cores can provide us with datable samples that allow us to calibrate the rise of postglacial sea level.26
Our knowledge of the history of postglacial sea-level changes around the Australian continental margin gives us the opportunity to assign ages to each group of Aboriginal stories recounted in Chapter 3. How do we do this? Consider that each story describes a time when the coastline in a particular place was further seaward than it is today. The ancient coastline – its form and even its character – can be reconstructed from information in the story. More importantly, the least amount by which the sea level must have been lower for the story to be true – for this ancient geography to have been truthfully observed by people – can itself be calculated. Thus we can plot the depth of this ancient sea level on the graph in Figure 4.3 and acquire an age range for that observation. Note that the age range is a minimum – the most recent time in Australian history at which observations of the ancient shoreline could have been observed – but that a particular story might actually have originated earlier.
Using this method, minimum age ranges can be assigned to each set of stories discussed in Chapter 3. Let us provide a few representative examples, starting with the Narungga stories of Spencer Gulf (South Australia). Details in several versions of the stories of the drowning of Spencer Gulf talk about it being entirely dry, occupied by ‘lagoons and marshes’ before the day when ‘the sea broke through’. Perhaps the most parsimonious interpretation of these stories is that they describe a time when the ocean was lapping at the lip of the modern Gulf, approximately 50m (165ft) below today’s sea level, in which case the stories must be between 11,200 and 12,460 years old. But of course, with our surveys and our satellites, we are biased when it comes to defining landforms like Spencer Gulf; today we can readily see its limits, but perhaps the people who witnessed its inundation many millennia ago were less spatially prejudiced. Perhaps the Narungga stories describe a time when the ocean had already risen halfway up the modern Gulf, referring instead to the drowning of a coastline 22m (72ft) below the modern one, along the line AB in Figure 3.2, in which case the age range for these stories might be just 9,330–9,700 years ago. This is a formidable achievement of memory however you view it.
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br /> Then we might consider the example of the Great Barrier Reef off the Queensland coast of north-east Australia, about which ‘many tribes … have stories recounting how the shore-line was once some miles further out … where the barrier reef now stands’.27 In the vicinity of Cairns there are numerous stories recalling a time when the coral reef was dry land, even ‘all scrubland’, and readily accessed by the area’s inhabitants. This observation may mean that the contemporary sea level was 40–50m (130–165ft) lower; if the shoreline was where the edge of the barrier reef now stands, then perhaps the sea level was 65m (210ft) lower. Possibly younger stories, like those recalling a time when offshore Fitzroy Island (gabar) was a mainland promontory, could have been true when the sea level was 30m (100ft) or more lower than it is today. As determined from Figure 4.3, the minimum ages for such stories range from 9,960 years ago (the most recent possible time at which the sea level was 30m lower than it is today) to a mind-blowing 13,310 years ago (the oldest time within the postglacial period at which the sea level was 65m lower than it is today).
A final example comes from the islands (Rottnest, Carnac and Garden) that lie off the mouth of the Swan River (Western Australia). George Fletcher Moore’s colourful account recalls an Aboriginal story about a time when the land separating these islands from the mainland was ‘thickly covered with trees’ before ‘the sea rushed in’ and cut them off. As noted in Chapter 3, it would be possible for people taking a meandering path to have accessed these islands from the mainland were the sea level just 5m (16ft) lower than it is today. Yet a sea level 10m (33ft) lower would better satisfy the condition that the area was forested. The lower limit of the age range for the former is 7,450 years ago, the upper limit for the latter 9,140 years ago.
