Australia: The Land Where Time Began
Heading into the Big Dry
Australia was already drying before the world began cooling at the start of the latest glacial phase. As increasing amounts of water were being locked up in the expanding glaciers, the sea around Australia was lowered, exposing large areas of the continental shelf. The flora and fauna adapted as the climate cycled from wet to dry and back again a number of times in the previous 100,000 years. The coming fluctuations were much more extreme than those that occurred during previous cycles.
In the period 128,000 - 124.000 years ago, there is evidence that the sea levels were higher than at present, some coastal lowlands being inundated, as happened the Richmond River Valley in New South Wales. The sediments deposited at this time in the continental seas formed the Gundurimba Clay. The extent of this coastal inundation at this time is indicated by the extent of the Gundurimba Clay, it reached as far are 160 km inland from the present mouth of the Richmond River at Boatharbour, upstream from Lismore, and to as far as a point 4 km east of Casino up another river valley. The clay deposits show that the inundation of the continental margins extended further inland than the present coastline. During this wetter phase of the Pleistocene this epicontinental sea had an irregular shape, extending up tributary valleys, and some highlands being isolated as islands. Ostracods were found in the marine clay deposit. Along the coastal fringe, the Gundurimba Clay is overlain by dunes that extend beneath the sea as sand bars.
The ostracod species found in the Clay indicate that the shallow water the Clay was deposited in was probably a protected bay, with a nearby hinterland with mostly low relief. As the bay appears to have been open to the sea, it implies that the Inner Barrier Beach ridges, that are characteristic of the present-day area, were deposited at a later time of lower sealevel when sand from the exposed continental shelf was redistributed. This barrier of dune sand is composed of the Woodburn Sand. As well as ostracods, a coral fauna from the Late Pleistocene at Evans Head, which is beyond the limits of the Clay, indicate that the environment was more exposed to the open sea than than it is at present, because the corals found require open marine circulation. At the time the corals were living, the Barrier Beach ridges had not been formed. It is believed that water temperature at the time may have been slightly higher than at present.
Parabolic dunes of quartz sand formed during the Pleistocene stretching for 350 km along the coast of southeast Queensland north of the mouth of the Richmond River. Their orientation is northwest-northeast, they run obliquely to the the present coastline, rising in places to 200 m. Off the coast lie the greatest sand deposit in the world. There are 5 barrier islands - North Stradbroke Island, South Stradbroke Island, Moreton Island, Bribie Island and Fraser Island. The dunes at Stradbroke Island are vegetated and fixed, the sand extending to 60 m below sea level. The sand, strongly pigmented in places, contains layers of sand rock and peat.
The Great Sandy Region comprises the Cooloola Sands, Fraser Island, the largest sand island in the world, and the strait separating Fraser Island and the mainland. The sand came originally from erosion along the eastern coast that was carried north along the coast by along-shore currents, then deposited on the continental shelf and rocky ledges. The accumulation of sand that led to the formation of Fraser Island has been occurring throughout the Pleistocene. It is believed the dunes on its eastern side, the oldest, appear to have begun forming at times of low sea level during the Late Pliocene when the continental shelf was exposed. The reason that Fraser Island formed is that the small area of rock of the present island blocked the passage of the sand north when the sea level was high enough for the northward-flowing along shore current to carry sand to that point where it was deposited in the lee of the rocky ridge.
North of Fraser Island, the Capricorn Basin prevents sand being deposited further north on the continental shelf. Dunes along the northern and eastern margins of the continent are composed of quartz, those on the southern, southwestern, and northwestern are composed of calcareous sand. This type of sand forms coastal limestone, only the youngest remaining as mobile dunes.
Beginning about 120,000 years ago, environments began to resemble those of the present. Sea level hasn't changed much since then, and the coastal features and shorelines were similar. The continental interior was dry, with stabilised dunes that had formed in previous dry phases during earlier glacial phases. During the first 40,000 years of the cycle there appear to have been only minor climatic changes. The spread and diversification of eucalypts was being encouraged by the increasing fire incidence. The large amount of variation within species and species groups of Eucalyptus indicates a relatively recent radiation and massive expansion of range.
The chains of playa lakes extending from Lake Hopkins in Western Australia to the Finke River in the Northern Territory, via Lake Neal , Lake Amadeus and many small playas. The remains of a palaeodrainage system, that in the past flowed into the Finke River, is evidenced by these salt lakes, then on to Lake Eyre. Groundwater calcrete deposits, that are often silicified, as well as gypsum and glauberite deposits at places along the palaeodrainage have been dated. They come from a very arid phase.
Evidence of a relatively long arid phase is found at Curtain Springs in the Northern Territory, that lasted from 75,000 to 35,000 years ago. At this time, slow moving groundwater formed a valley floor type silicified calcrete deposit beneath the watertable. This is an aquifer at the present time. The latest glacial maximum initiated a shorter arid phase, from 16,000 to 8,000 years ago.
Between 27,000 and 22,000 years ago, water seeping through soil layers above the watertable, formed vadose calcrete, a characteristic of this phase that is indicative of a wet phase. This time period coincides with a lake-full stage of Lake George. At this time the lakes of the Centre were full. In all the arid regions, much of the continent, the dry periods became seasonal instead of the constant dry times that often characterise the arid regions, though droughts were probably still common. The landscape features that had already been well established, such as high evaporation, wind-blown sands, salinity, swales with a clay substrate and gibber plains, still exercised a control of the vegetation type that was possible.
Though there were some areas of mesic vegetation, even during these wetter phases, there were no corridors that would allow the spread of vegetation to the newly wetter areas, so vegetation patterns tended to remain largely unchanged. The landscape of the central areas had been altered to such an extent by long and recurring periods of aridification that several thousand years of wetter conditions were unable to return the vegetation to the type that had been there prior to the spread of aridification. The main change of vegetation that would have occurred were the fluctuating margins of the various plant communities.
Near Newcastle Waters in the Northern Territory, Lake Woods is a good example of a lake presently set in a semi-arid region, that during the wet phase 22,000 years ago was much larger and 6 m deeper.
The beginning of the arid interval in central Australia that covered the period from 75,000 to 35,000 years ago coincides with the start of the cold interval at Lake George that began 75,000 years ago and continued until 64,000 years ago, as determined by pollen cores from the lake. At this time Lake George was a usually dry lake surrounded by a treeless grassland. The fire frequency decreased because there wasn't sufficient vegetation to feed a fire. Between 64,000 and 22,000 years ago, the area around Lake George was warmer and wetter, dominated by eucalypt woodland. Between 26,000 and 23,000 years ago the lake was overflowing. The oldest aeolian sands at Lake George contain stone tools of the Ngunawal Aborigines, dated to between 23,000 and 17,000 years ago. The beginning of an arid phase is indicted by these beds, when the lake was dry.
The Murray Basin was another area that didn't experience the constant dryness that affected central Australia between between 75,000 and 35,000 years ago. From about 60,000 years ago, the catchments of the Murray Basin on the southern half of the Great Divide that supplies water to the Murrumbidgee and Murray, as well as the southeastern rivers, had plenty of water. All the lakes of the Murray Basin were full by 55,000 years ago. At this time the Willandra Lakes, that are fed by a tributary of the Lachlan River, covered a total area of about 1000 sq km.
One of these lakes, Lake Mungo, is of interest because of its archaeological evidence of Aboriginal occupation. There was a relatively short dry phase about 36,000 years ago when the lakes shrank, becoming increasingly saline, most drying up completely. At this time lunette dunes formed on the eastern side of the lakes, and a lot of salt was carried from the saltpans over the surrounding land, causing soil salination and resulting eventually in a very saline watertable. After this dry phase, wetter conditions returned, and the lakes filled, remaining at moderate levels until about 25,000 years ago. Following the wet phase, drier conditions returned associated with a cooling.
Faunas that are probably older than 30,000 years have been found in lunettes at Lake Tandou and Lake Menindee, among which are Diprotodon, Protemnodon (the giant kangaroo), Sthenurus the browsing kangaroo, Thylacoleo (the marsupial lion), and Procoptodon (the giant browsing kangaroo). There were also modern animals from the arid zone - hare wallabies, nailtail wallabies, bettongs, pig-footed bandicoots and bilbies. At Lake Menura and Lake Victoria, there are similar faunas that also contain koalas, indicating that they lived in a riverine forest in an area subject to seasonally arid conditions. They live in such forests in present-day parts of Queensland.
At sites on the Nepean River near Sydney there is evidence of wet conditions between 60,000 and 36,000 years ago. Between 47,000 and 40,000 years ago a braided river system laid down an alluvial terrace deposit, the Cranebrook Terrace. Aboriginal stone artefacts were found in this deposit. Many rivers around the continent display the same increased activity at this time. Greatly increased river activity is indicated in Tasmania by large alluvial deposits, from the period 44,000 to 22,000 years ago, at Rocky Point and near the mouth of the Welcome River in the northwest of the island.
In the last 40,000 years, lowered sealevel resulting from increased ice accumulation, led in the northernmost pasts of Australia to the exposure of the floor of the Gulf of Carpentaria. This would have required a drop in sealevel of 70 m. The same thing happened at Bass Strait, where it required a drop of 65 m in sealevel. Between about 35,000 and about 12,500 years ago there was a large lake, Lake Carpentaria, in the centre of the Gulf of Carpentaria. It covered an area of about 30,000 km2 with a maximum depth of about 10 m. Cores from the lake sediments indicate that it was surrounded by a savannah-type vegetation, similar to that found on the black soil plains south of the Gulf at present. Sedges and swamp plants and aquatic plants predominated, and grasslands with some Myrtaceae and Callitris and Casuarina were present.
At this time when there was a land bridge to New Guinea there is no evidence from the pollen record that plants from New Guinea invaded the newly exposed land.
Between 50,000 and 30,000 years ago lake levels in southeast Australia were high, the Mungo Lacustral phase. Following this wet phase was a dry phase. Between 25,000 and 14,000 years ago the drying trend on the mainland increased in severity, with maximal aeolian activity between 18,000 and 15,000 years ago, coinciding with the peak of ice accumulation in the northern hemisphere.
The evidence from the rivers of the Riverine Plain in the eastern part for the Murray Basin at this time shows that the lakes changed from full to empty. The river systems crossing the plain during the Mungo Lacustral phase were wide and meandering. There were dunes on their northern and southern margins. This is called the Tallygaroopna Stage of river evolution. Towards the glacial maximum, the rivers narrowed in the drying phase, reducing the amplitude of the meanders, sand sheets spreading from marginal dunes, the Kotupna Stage. After 14,000 years ago, in the interglacial, rivers assumed their modern form of highly sinuous channels, the modern Goulburn-Murray Stage. These patterns also occurred in the ancestral Darling, still characterised by narrow, sinuous channels.
The big meander stage corresponds to wetter phases when the groundwater level was higher and lakes were full. The smaller meanders correspond to the arid phase with lower watertables and empty lakes. The final stage is the return to wetter conditions after the intense aridity of the glacial maximum.
From about 30,000 years ago to the glacial maximum, glacial conditions in the highlands of Tasmania and the southeastern part of the continent were increasing. The snowline was lowered by more than 100 m at the glacial maximum. During the Margaret Glaciation in Tasmania, beginning about 30,000 BP, reaching a maximum about19,000 BP, and finishing about 10,000 BP, the west coast mountains glaciated and there was an ice cap on the central plateau west of the Coast Ranges, that covered an area of up to 6,000 km2 that was at least 700 m thick. The inclusion of the fringe areas would bring the figure to 7,000 km2.
During this time caves in the Florentine River Valley and the Shannon River Valley were occupied by Aborigines. At Bluff Cave, occupation occurred between 31,000 and 11,500 years ago, and at the edge of the Plateau, cave ORS7, in the Shannon River Valley, was occupied between 31,000 and 25,000 years ago. Among the remains in these caves were emu egg shells, wallaby bones, apparently the food eaten by the Aborigines. Artefacts were also found in the caves. The caves show no evidence of megafauna. It is not known if that was because they were already extinct in Tasmania, or because they were not eaten in the caves, possibly being eaten at the site of the kill, or hunted in the summer when the aborigines were most likely to live away from the caves. The megafauna survived on the mainland until about 15,000 years ago. During the glacial maximum the mean temperature was at least 6.5o C lower than at present.
Mt Kosciusko was glaciated, the glaciers covering abut 25 km2. Across the large periglacial area, all the mountainous country in the southeast was affected by frost and winter snow. The sediment load of upland rivers was increased by the material produced by the freeze-thaw erosion of the rocks, often overflowing across the plains as alluvial fans.
Between 25,000 and 20,000 years ago, Antarctic sea ice reached its greatest extent. Open water is thought to have been present for only about 3 months/year south of 55o south latitude.
On a global scale, sealevels were more than 100 m lower than now, exposing vast areas of continental shelves. At this time there was about 20 km of coastal plain between Sydney and the sea, and the Queensland Plateau, site of the Great Barrier Reef, was a coastal lowland. At the glacial maximum, the dry windy conditions resulted in the most severe bout of aridification that Australia had endured in the entire 2.4 million years of the Pleistocene Ice Age.
At this time lakes dried up, often remaining dry ever since. Large gypsum dunes were formed at Lake Eyre and Lake Frome, indicating that conditions were among the worst ever experienced in Australia, as they were not produced before of since the glacial maximum. To produce such dunes it was necessary to have a high watertable prior to a severe drying phase, that mobilised the salts, forming a concentration of sulphate in surface water. When the water evaporated, the salt crystallised and gypsum formed with halite at the water-sediment interface. At present it does this below the surface. In the dry, windy conditions of the glacial maximum these crystals dried and were blown to the downwind side of the lake where they formed very large dunes.
As well as the strong winds usually associated with glaciation in the northern hemisphere, the orientation of clay lunettes indicates the climate of southern Australia may have changed, the seasonal pattern of prevailing westerly winds was shifted, as it was affected by the summer anticyclone that moved 5o further north than at present. There was also the effect of the greater area of land surface causing an increasingly severe continental climate, resulting from lower sealevels exposing more of the continental shelf. Hot dry air from the interior probably reach the coast more frequently in summer during the glacial times, bringing aridity closer to the humid coastal margin. There is evidence from Kangaroo Island and Tasmania that supports this expansion of aridity further south.
In the Seton rock shelter on Kangaroo Island, evidence indicates that between 20,000 and 10,000 years ago, bettongs and wallabies inhabited a open grassland. At present the area is covered by heathland and woodland. The presence of big grey kangaroos and open dry grasslands in northwestern Tasmania is more evidence for the spread of aridity, even to Tasmania.
Between 20,000 and 15,000 years ago the Nullarbor was drier than now, the last vestiges of the mallee vegetation had moved onto its fringes by 20,000 years ago, and woodland had gone for the last time. Evidence from Koonalda Cave and Madura Cave, as well as deposits from a cave near Eucla and the Norina rock shelter, also confirm the increased aridity occurring at the glacial maximum.
Mammoth Cave in the Cape Leeuwin-Naturaliste region of Western Australia has deposits with a faunal record from 37,000 years ago to the present. Devil's Lair covers a timespan from 35,000 to 5,000 years ago. Here there was a very diverse fauna, including such arid land species as the bettong and burrowing rat-kangaroo.
The southeast trade winds are believed to have intensified from about 20,000 years ago onwards towards the glacial maximum in northern Australia, but would have changed direction. The alignment of the dunes in arid central Australia indicate that the direction of the wind was not significantly different from what it is today. The ultimate drying that occurred between 18,000 and 15,000 years ago, coinciding with the peak of glaciation in the Northern Hemisphere, can be seen in the extent of the central dune deserts in present day Australia.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|