Australia: The Land Where Time Began
Australian Palaeoclimate - the Tertiary - Evidence of climate from the palaeobotanical record.
Leaf fossils that had accumulated in streams form a detailed record of past vegetation, but this source of fossils is biased for a number of reasons (Fergus, 1985; Spicer, 1989; Greenwood, 1991a). Problems associated with fossil leaves include:
The proportions of different leaves in a deposit are influenced by different rates of leaf fall, as between deciduous and evergreen species, and the degree of correlation with periods of greater sedimentation. Leaves that fall at times of high sediment load, as in floods or heavy rain, are more likely to be buried before they can be degraded by detritovores.
Whether the leaves of a plant are dehiscent or non-dehiscent, as found in many palms, forbs and herbs. Dehiscent leaves are more likely to be fossilised.
The distance a leaf, or other plant structure, travels from the plant before landing is determined by the mass per unit area, those with low weight per unit area tend to travel further from the tree before landing, denser leaves landing closer to the tree.
Different types of leaves, as from shade plants or open sun species, decay at different rates. Those that take longer to decay have a greater chance of being buried than those that decay more easily.
Differences in numbers of leaves dropped by different species skew the ratios of the species recorded in fossil deposits. An example is the high number of leaves dropped by canopy trees compared with those of the shade beneath them.
Deposits containing fossil leaves therefore represent only a subset of the species in the surrounding area, and often only the immediately adjacent area, especially with large, comparatively heavy plant parts (Greenwood, 1991a). Geologically instantaneous records may be represented by accumulations of smaller plant macrofossils. Because of the way leaves fall from different species at different times, as in seasonal changes or dry spells, etc., individual layers in a deposit can contain fossils from different components of the vegetation around a single deposit, suggesting that the different species were present sequentially rather than the actual situation, that they are present simultaneously in a mixed population of species that shed their leaves at different times (Christophel et al., 1987). Macrofloras that are stratigraphically correlated may be separated in time by thousands of years, providing a record of the original vegetation, which indicates the probable climate at the time of fossilisation.
Local climates can be determined by studying assemblages of terrestrial plant fossils, a single datum being represented by a particular plant assemblage. The local variation can be determined from the separate assemblages from a single flora. In places such as the Latrobe Valley (D.R. Greenwood in Hill (ed.), 1994), where there is continuous deposition in a single basin over millions of years, the deposits can preserve the record of vegetation changes that can indicate climatic changes taking place over extended lengths of time.
Analysis of the macroflora record indicates that during the Middle Miocene the climate was not uniform across Australia, and in the Oligocene and Early Miocene, the climate, and hence the plant communities, were influenced by local topography in southeastern Australia. According to Nix (1982), the main driving force of biotic change in Australia in the early to mid Tertiary was increasing seasonality of temperature extremes and rainfall, and to a lesser extent, marked changes of the thermal regime. This is supported by the palaeobotanical evidence for the climate and vegetational patterns (Christophel & Greenwood, 1989; Read et al., 1990).
Deposits at Maslin Bay, Golden Grove, Anglesea and Nerriga (Figs.4.1 & 4.3, Hill (ed.), 1994) contain macrofloras from the Middle Miocene suggesting a predominantly mesothermal-megathermal climate that ranged from MAT (mean annual temperature) values of about 16o C at Anglesea (mesothermal) to possibly up to 25o C at Maslin Bay (megathermal) on the coastal lowlands to moderate elevations. In Middle Miocene South Australia the climates were noticeably warmer than the present. Under these climatic conditions there were various types of rainforest such as SNVF (simple notophyll vine forest), CNVF (complex notophyll vine forest) and CMVF (complex mesophyll vine forest) (Christophel & Greenwood, 1989).
Evidence has been found of a transitional zone, an irregular belt of mesothermal climates across southeast Australia, with climates that are microthermal to the south and megathermal to the north (Nix, 1982; Christophel & Greenwood, 1989). The actual location of this band is uncertain, requiring more evidence such as more fossils of macrofloras have been found over a wider area and subjected to detailed study. The existence in the Middle Eocene of a possible megathermal climate in the interior of the continent, with a seasonal climate of wet and dry periods, is suggested by macrofloras from the Lake Eyre Basin. There is evidence that by that time a flora that was physiognomically pre-adapted to aridity was present in the interior of the continent (Greenwood et al., 1990; Christophel et al., 1992).
It appears southeast Australia had a climate with lower temperatures in the Oligocene than than in the Eocene (Hill & Gibson, 1986, Hill, 1990a). In the Oligocene, microthermal lineages, especially Nothofagus, were increasingly important compared with those of the Middle Eocene (Hill, 1990a; Read et al., 1990. Humid microthermal climates are suggested by the foliar physiognomic and floristic character of these macrofloras. Modern Australasian tropical montane communities are better analogues of the Cethana vegetation of the Oligocene than are modern humid microthermal forests of southern Australia, such as the MFF (moist foothill forest) dominated by Nothofagus (Hill, 1990a; Read et al., 1990). The Pioneer and Miocene Bacchus Marsh macrofloras suggest a microthermal to modern MFF dominated by Nothofagus, especially the montane forests of New South Wales dominated by N. moorei. Increasing seasonality, possibly with greater temperature extremes in the Early Miocene than in the Oligocene, is suggested by the absence of some key Oligocene taxa from the Early Miocene macrofloras (Hill & Read, 1987; Hill, 1990a). In Tasmania at this time subalpine woodlands at moderate altitudes (Hill & Gibson, 1986; Macphail et al., 1991).
Some speculation on the nature of the vegetation and climate of Australia in the Early Miocene has been made by Wolfe (1985), but the small number of well-documented macrofloras from southeast Australia in the Oligocene and Miocene provide insufficient evidence on which to base a firm conclusion.
Mostly microthermal climates are indicated for southeast Australia in the Middle Miocene, as indicated by the macrofloras of the Yallourn Clay. It is believed there was probably mesothermal humid vegetation, CNVF, in places in the Latrobe Valley in the Middle Miocene, and remains there at the present. Based on anecdotal evidence, it is believed the climates in the Latrobe Valley during the Middle Miocene may have been more equable than at present, seasonal dryness becoming established by the Late Miocene, and possibly much earlier in the central parts of the continent (Lange, 1982; Quilty, 1984; Greenwood et al., 1990).
microthermal - Typical of modern continental climates as presently found in North America and Eurasia, in the Northern Hemisphere, the only parts of the world where there are broad land masses in the higher latitudes , with cold winters that can maintain snow cover and hot summers.
mesothermal - Typical of modern temperate climates - moderate heat, winters not cold enough to sustain snow cover, summers are warm in oceanic climate regimes and hot in continental climate regimes.
megathermal - Tropical, equatorial (outer tropical) and tropical
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