Australia: The Land Where Time Began
23.7-1.6 Ma a period of transition
The climate was warm to hot for the first third of the Neogene, reversing the trend towards the ice age. The permanent southern ice cap formed about 15 Ma in the Middle Miocene. After this the trend toward the ice age resumed. The world was in an ice age by the end of the Neogene. As usual in cooling phases, Australia underwent a period of aridification, leading to a change in the vegetation. As the arid phase intensified the flora and fauna had to adapt to survive as the once ubiquitous rainforests contracted and central Australia progressively became a desert.
Comprising the Neogene:
Overall, the environmental changes that took place at this time changed the Australian landscape from rainforest dominated to a landscape that was mostly more open vegetation communities, with the remanent rainforests being restricted mostly to the wetter areas of the present. The changes also led to the development of a strong latitudinal gradient, the rainforest elements being restricted mostly to the eastern coastal areas and savannas in northern Australia. By the end of the Pliocene most extant communities had developed or begun to develop, their distribution and relative importance being greatly affected by changes that took place in the Pleistocene. One of the most significant changes has been the expansion of the eucalypts, replacing dry rainforests and forests dominated by Casuarinaceae. It has been suggested that, more recently, they may have replaced or augmented more rainforest-related trees in the savannas, and in wet sclerophyll forests, replaced other Myrtaceae in the canopy. There was also an expansion of a grassy understorey in eucalypt-dominated forests and woodlands, largely replacing the Asteraceae, and temperate grasslands developed.
Scrub of a sclerophyllous myrtaceous type is often found on or near sites were sediment is accumulating, while swamp rainforests are rare at the present, even in rainforest areas. Most of these changes can be attributed to burning. In addition to higher levels of disturbance that are believed to be associated with extreme glacial/interglacial climatic oscillations. Aboriginal firestick-farming methods are also believed to have played a part in the changes that took place from the Late Pleistocene.
During the transitional time that characterises the Neogene, when the rainforest regarded as 'Tertiary' was progressively reduced in extent, it also became progressively more difficult to document these changes with the aid of the pollen record, because one result of aridification was the reduction of the number of places were pollen could accumulate in deposits where it could be preserved. There is also the problem of a relative dearth of palynological research on those deposits that did form in the period. For much of the Neogene there is a lack of dated biostratigraphic evidence, especially from the latter part of the Quaternary, that is often of less than optimal quality. The result of these problems is that knowledge of this critical time in the biological history of Australia is fragmentary and the quality of the evidence on which to base a reconstruction the vegetation of the time is often not good.
The definition of the Neogene is also problematic, especially gaining agreement on when the period ended. The Miocene is accepted to have begun at at about 25 Ma, though this may not be of great significance with regard to palnostratigraphy and biogeography. The end of the period is disputed, the actual point where the Pliocene/Pleistocene boundary should be placed. It has usually been placed at the top of the Olduvai palaeomagnetic event at about 1.6 Ma (Berggren et al., 1985), though there is has been a move to shift the transition to the Gauss/Matuyama palaeomagnetic reversal boundary about 2.4 Ma, because this correlates more closely with the start of the climatic fluctuations and cooling associated with the Pleistocene (Zagwin, 1985; Kukla, 1989.
The northward drift of the Australian continent and the changes in the global climate at this time affected the entire continent during the Neogene. A steepening of the temperature gradients between the poles and the equator, and the formation of the Antarctic Circum Polar Current, leading to the development of the current atmospheric circulation pattern (Kemp, 1978), at least partially led to the cooling event and climatic fluctuations characteristic of the Pleistocene. The changes were believed to have begun in the Tertiary, the major changes occurring in the Miocene. In the Middle Miocene major changes in ice accumulation took place, as indicated by the ocean core oxygen isotope (18O) record and the eustatic sea level curve.
In Australia the northward drift of the continent, at an estimated 50-70 mm/year during the last 30 Million years, partially offset the cooling that was taking place, reduced precipitation resulting from the lower sea levels as well as lower sea temperatures, together with the movement of the continent into the belt of atmospheric high pressure in the mid-latitudes. The marine regression from the continental basins, especially the Murray Basin, were were instrumental in lowering the moisture levels in the interior of the continent.
In the Pliocene, expansion of the Northern Hemisphere ice sheets was accompanied by further global cooling and increased desiccation (Shackleton & Opdyke, 1977). A period of major, long lasting aridity in Australia was noted by Bowler (1982) between 6-2.5 Ma. Based on the weathering profiles in the sediments of lakes, he associated the aridity with the initiation and intensification of the high pressure system. Prior to initiation of the high pressure system rainfall was not seasonal, being distributed approximately evenly throughout the year, following the initiation the climate changed to one in which the rain was seasonal, falling mostly in the summer, as occurs in present-day Australia. The movement of the high pressure system to the north at a higher rate than the drift north of the continent, as well as intensification of the high pressure system, meant that the southern parts of the continent was moved into the latitudes where they were influenced by the westerly winds by about 2.5 Ma. This correlates broadly with the climatic fluctuations that occurred in the Quaternary.
The Australian continent underwent little tectonic activity during the later Tertiary, unlike the activity that occurred on most other continents at the time. The eastern highlands were highlands long before this time, but they underwent periods of volcanic activity at this time, but it is believed there was probably no significant uplift associated with this activity (Williams, 1993). In the Middle-Late Miocene was a time when the New Guinea Highlands were uplifted as the Australian Plate collided with the Asian Plate, effectively bringing Australia into contact with the region of Southeast Asia.
Palynological fossil deposits
Almost all known deposits containing pollen from this periods are found in eastern Australia, areas that are, or were prior to clearing, covered by rainforest, very few being known in drier areas outside the humid/subhumid fringe of areas of present-day rainforest, eucalypt forest and woodlands. Few pollen-containing sites in drier areas are known from periods other than lake and marginal marine sites from the Miocene, that are associated with inland basins of the Tertiary. Most research has been carried out in the Murray Basin and its catchment, associated with the search for groundwater. Most analysis has used bore cuttings, though some continuous coring has been undertaken. Close to the Murray Basin embayment, of Miocene age, to the fluvial and related swamp deposits of tributary valleys on the western slopes of the Eastern Highlands in the east, a marginal marine and lacustrine sequence has been studied.
The Latrobe Valley in Victoria is another area that has been well studied, detailed reconstructions of vegetation and environments have been possible because of the large coal deposits of the area (Blackburn & Sluiter, Chapter 14, Hill, 1994). Very limited temporal records have been obtained in sediments of lake, swamp or fluvial origin, a capping of basalt often being responsible for their preservation. The record from Lake George is the only pollen record that can be traced back to the Late Tertiary, though there are often sediments in this deposit that lack pollen.
Generalised vegetation histories of some areas have been found in a number of offshore deposits, and for north Australia this is almost the only palynological evidence known. An advantage of these sites is that they provide time scales to allow the pollen record to be correlated with the well-dated foraminiferal record.
Zonaztions of pollen have been established for a number of areas, eastern Queensland (Hekel, 1972) and the Otway and Gippsland Basins in Victoria (Harris, 1971; Stover & Partridge, 1973); A.D.Partridge. Within the southeast Australian region the Gippsland stratigraphy has been extensively applied.
Marginal marine sediments from the western Murray Basin, representing the Murray Basin in the earlier part of the Neogene, has been related to the global microfaunal stratigraphy (Truswell et al., 1985). Comparisons with the record of the Gippsland Basin, and the climatic changes inferred from the marine oxygen isotope record, have allowed a less firm dating of the record from the Murray Basin for the later Neogene. K-Ar dating of associated basalts has allowed some time control of pollen assemblages that are more isolated, and the sequences at Lake George have been dated by palaeomagnetic stratigraphy (McEwan Mason, 1989).
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