Climate of Australia
Notes on the Nature of Australia
Australian Palaeoclimate and Palaeogeography -
Global glaciation as it occurs at the present was apparently rare in most of the Cretaceous, only occurring for short periods of geological time. Conditions in southeast Australia appear to have been cool and humid with some seasonality during the Early Cretaceous. There doesn't appear to have been major glaciation, though there was apparently some sea ice during winter. There was widespread volcanism throughout eastern and southeastern Australia, and a minor amount in southwestern Australia. The temperatures were still cool, but increased towards the northwest. There were large volumes of runoff from the continent, the produced sediment being mostly of terrestrial origin. It was the time of the most widespread marine transgression of the Phanerozoic, extent of sea coverage generally increasing throughout this interval.
The shallow seas retreated from central Australia during the Late Cretaceous, and at this time the drainage divide moved to the north, and drainage of most of Australia flowed to the south central coast. A significant proportion of the sediment being carried by these southward-flowing rivers was of volcanic origin from the eastern Queensland high country, that was carried into the ancestral Darling River system. Cool conditions prevailed in southeast Australia until the Santonian (from about 87.5-83 Ma), at which point the climate became much warmer and more humid towards the close of the Cretaceous.
The evidence from fossils with presence of sediments of mostly biogenic carbonate on the west coast indicate waters that were warm to tropical. In this region the drainage was mainly of a local nature, around the cratonic blocks, in all but the northern parts being relatively minor.
On the northern coast there was a gradient from warm temperatures with high rainfall in the Carnarvon Basin to hotter and drier, possibly even arid conditions, in the northeast. The drainage over much of the continent was mostly internal, the rivers making their way to the newly formed Birdsville Basin.
Long-term temperature and hydrological characteristics are the main elements determining the climate. Several factors are involved in determining long-term temperature, latitude (the distance from the South Pole in the case of Australia), and patterns of circulation in the ocean surrounding the continent. The hydrological characteristics depend on circulation patterns in the oceans, but also on other factors, such as the area of the continent inundated at times of marine transgression, sealevel, relief of the land surface, and the direction and intensity of the wind, variation of the solar constant, cloudiness, etc. (P.G.Quilty in Hill, 1994). Some influences on climate can be detected, directly or indirectly. Wilford and Brown (in Hill, 1994) have reviewed a number of climatic determinants that depend on the tectonic history of the region.
The latitude of the continent, relative to the South Pole, to a large extent determines the general temperature of the ocean and the atmosphere, and major meteorological factors such as the position of the landmass relative to the lows and fronts, as well as their frequency, and the area of the continent in which the climate is influenced by tropical weather systems. Albedo is also relevant to climate, in ways such as variations in intensity from day to day and seasonally, especially when there is winter snow. Long-term albedo changes that result from changes of vegetation cover and aridity cannot be measured directly in the geological record, they can only be guessed. The Middle Miocene is believed to have had a low albedo, because it was a time when the land surface was well-vegetated because of the humid environment, whereas albedo is believed to have been higher in the Late Cretaceous, as a result of the aridity that was prevalent at the time. The changes that resulted from the angle of the sunlight as the continent moved north into different latitudes, and the differing day length from between summer and winter, are factors that cannot be quantified.
During the interval in question, from the Cretaceous to the Recent, there was a large change in the relationship between Australia and the south geographical Pole, the axis or rotation, which is measured by the APWP. The pole was to the southeast or south of Australian throughout the Cretaceous, but at this time movement relative to the Pole was slow, so the palaeolatitude didn't change much. This situation changed in the Palaeocene after the break with Gondwana, when the Australian continent, now independent, began moving north, resulting in the continent moving through a number of climatic zones of decreasing latitude. During this period there were only minor changes in the position of Antarctica. The interpretation of Veevers (1984) was based on a detailed review by Embleton & McElhinny (1982).
The temperature of the oceans strongly influences the temperature of nearshore waters and precipitation. They are also influenced by the direction and strength of the wind, and the relief of the terrain. The interaction is not known well enough to describe it in much detail (Quilty in Hill, 1994). It is believed possible that some aspects of the distribution and evolution of the vegetation is controlled by events occurring in the oceans, resulting from the effects of the oceans on atmospheric composition. The local or regional evolution is impacted most heavily by global events in connection with the oceans.
Many authors, such as Frakes et al., (1987a), believe this interval was of great importance in the evolution of the oceans to their modern mode. At the start of this interval, the temperature gradients between high and low latitudes was not great, and the temperature gradient from deep water to shallow water was very low, and mobility and differentiation was much less than the present. A characteristic of the oceans during the Cretaceous was a series of 'anoxic events'. The number, duration and distribution of the resulting sediments is uncertain. Generally, the ocean temperatures were higher than at present, even in the deeper parts of the ocean, leading to a lower solubility of O2. The modern style appears to have begun in the Late Cretaceous.
The palaeoceanography of the Australian region is not well known. What is known of past water temperatures has been based on foraminifera, whether or not large ones are present and features of planktonic foraminifera such as their diversity, the presence of keels. In the southeast of the continent oxygen isotope evidence is available for part of the Cenozoic. A number of authors have carried out some palaeoceanographic reconstructions, e.g., Burns(1977), Edwards (1975), Kennett (1980), and Savin (1977). These have been generally consistent throughout the Eocene and later. Where they differ is in the Tasman Sea about 21 Ma, at which point Kennett has a clockwise current. The current in the Tasman Sea earlier than 21 Ma is believed to be from south to north, which contrasts with the situation since then, which is believed to be the result of the formation of the Tasman Sea and the Lord Howe Rise, which resulted in a channelling effect. The reconstructions are largely speculative prior to 63 ma, as is Quilty's reconstruction, being based on the assumption that a large counterclockwise current system was a characteristic of all the major Southern hemisphere oceans. There is very little information to base reconstructions on before about 95 Ma as the position of the continents relative the South Pole was very different from what it was later.
Reconstructions of Australian Palaeotemperature estimates are based on data at nearby locations, such as New Zealand (Clayton & Stevens, 1987; Stevens & Clayton,1971) and more scattered oxygen isotope data sets, from sediment features (Frakes & Francis, 1988), and on information from fossils having features that are temperature-dependant. Leg 29 of the Deep Sea Drilling Project (DSDP), in cores from south and southeast of Tasmania, provides the most important oxygen isotope data (Shackleton & Kennett, 1975).
Two factors are involved in inundation, tectonics and sealevel changes. On coastal areas of low relief inundation occurs as marine transgressions when sealevels rise, which not only affects the area covered by the sea, but the biota that is displaced and lives in surrounding terrestrial areas, as well as the climate of the affected area. When inundation occurs, the increased weight on the crust caused it to subside as a result of hydroisostacy. The now depressed areas allow more water to enter from the sea, increasing the depth of the water inundating the land. It is believed hydroisostacy can increase the effects of sealevel rise by as much as 30 %. Inundation can also be caused by the depression of the crust, that then leads to the inundation, even at times of low sea level. The Australian continent underwent major inundations in the Aptian and Albian (Frakes et al.,1987b), during the Potoroo Regime of Veevers (1984), though Australia, that was generally of low relief and tectonically stable during this interval. It is believed tectonic processes were probably responsible, as the sealevels of the time were generally depressed, with the exception of the end of the interval, and because the area affected appears to be too large to result from sealevel rise alone. In the Late Cretaceous, the history of inundation in Australia appears to be almost the antithesis of the global situation in the later Cretaceous (Quilty in Hill, 1994).
Relief, controlled by tectonics, is an important influence on precipitation, as well as the direction that rainwater flows over the surface of the land. Throughout the interval from the Cretaceous to the Recent, the relief of the continent, while being important, has not been as important as it would have been if it hadn't formed as a result of divergent tectonics as it separated from Antarctica. This feature of the formation of the post-breakup continent, that the only parts where compression tectonics played a role are the northern margin through Papua New Guinea and the southeastern margin in New Zealand. The almost complete lack of compressional features, has meant there are no very high mountains. Australia has had very high mountain ranges in the distant past, but they had been worn down long before the Mesozoic (see the Petermann Mountains). This is apparent from the lack of andesitic material in the sediments formed at this time. The only part of the continent where andesitic material is found in significant amounts in the sediments is from southeastern Australia in the Jurassic-Cretaceous transition.
The parts of the continent where there are exceptions to the general low relief are in the eastern highlands, northwestern Australia and Tasmania. This has generally been the case throughout the interval from the Cretaceous to the Recent, though in the early part there were marked exceptions. Evidence has been found in the Perth Basin from the latest Jurassic to the earliest Cretaceous, that the Darling Range was much higher at than time than at present (Johnstone et al., 1973; Veevers, 1984). The evolution of the divergent margin as India separated from southwestern Australia is seen as the cause of this mountain forming event. According to Veevers (1984), the Flinders Ranges, as well as other mountainous regions, had predecessors in the Cretaceous. It is believed that during most of the cretaceous there were possibly earlier ranges where the Eastern Highlands are, especially in Queensland. Not a lot is know of when the highlands, such as the Stirling Ranges, formed on the southern margin of Western Australia.
With the possible exception of southeastern Australia in the earlier Cretaceous, no evidence has been found that could indicate the presence of snow during the interval from the Cretaceous to the recent. There is a great variation of drainage patterns form the interval, from periods during which no evidence has been found of any drainage, to times when drainage was mostly to the central south coast or depocentres in central Australia, earlier equivalents of Lake Eyre, or to the vast inland ocean that separated the high country of the continent into a series of smaller landmasses, the drainage from which formed radial patterns to the surrounding sea by short rivers. Veevers (1984) has summarised the drainage patterns of the period.
Throughout most of the period from the Cretaceous to the Recent, a few hundred kilometre-wide arc along the eastern margin of the continent, that stretched from northern Queensland to Mt Gambier in South Australia, volcanism was a feature of the landscape. In parts of this arc eruptions have occurred as late as 4,600 years ago, suggesting that, though there are no active volcanoes in Australia at the present, leading to the belief that some may actually be dormant, rather than extinct. Through much of the Cenozoic, Tasmania has had a history of major volcanic activity
Much of the geomorphology of eastern Australia has resulted from volcanic activity. The volcanic features in the present landscape include lava plain, see Undara lave field fresh-appearing volcanic cones, volcanic plugs remaining when the softer rock of the surrounding volcano have been eroded away. The volcanic phase is the basis for much of the Australian gemstone industry.
The cause of much of the Australian volcanism is believed to have been the movement of the Australian plate across a series of hotspots, where lava finds ir way to the surface periodically. Evidence for this type of volcanism is seen in linear series of volcanic vents and rocks. There is a lot of evidence for this type of volcanism, but it does not explain all volcanism in Australia. Volcanism is associated with the Eastern Highlands and the Australian plate margin.
|Author: M.H.Monroe Email email@example.com Sources & Further reading|