National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)


Suggested Citation:"PLANT MEGAFOSSIL EVIDENCE FOR CLIMATIC CHANGE." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 179
Suggested Citation:"PLANT MEGAFOSSIL EVIDENCE FOR CLIMATIC CHANGE." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 180
Suggested Citation:"PLANT MEGAFOSSIL EVIDENCE FOR CLIMATIC CHANGE." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
Page 181

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THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 179 megafossil record are therefore (1) the Middle Miocene fluctuation and (2) the terminal Eocene event. Figure 10.4 Oxygen isotope curves calibrated tentatively against a temperature scale. Shaded envelope covers low latitude Pacific values from surface (to the right) planktonic foraminifera and from bottom (to the left) bethonic foraminifera. The overall trend toward cooling, the reversals, and the progressive differentiation between surface and bottom waters are all more important than the actual temperature values. Black envelope: high southern latitude oceanic profiles; again, surface to the right and bottom to the left. Shading and arrows: events and intervals of global significance. Right: paleobiological record of events of relevance to Australia. (Figure modified from Frakes et al., 1987.) PLANT MEGAFOSSIL EVIDENCE FOR CLIMATIC CHANGE The first area for which to consider plant megafossil evidence is the cooling cycle within the Middle Eocene. As is seen in Figure 10.5, there are a large number of known Eocene localities in southern Australia. Four of them—Maslin's Bay (Christophel and Blackburn, 1978), Golden Grove (Christophel and Greenwood, 1987), Anglesea (Christophel et al., 1987), and Nelly Creek (Christophel et al., 1991)—are late Middle Eocene and represent the return to warmth shown on Figure 10.4. They include classical tropical rain forest taxa such as Elaeocarpus/Sloanea, Lauraceae, and Gymnostoma. These four floras show a physiognomic signature consistent with that of the litter from Webb's Complex Notophyll Vine Forest or his Complex Mesophyll Vine Forest (Christophel and Greenwood, 1988). A little-documented site at Dean's Marsh (Douglas and Ferguson, 1988) (Figure 10.5), approximately 80 km east of the Anglesea locality, displays a physiognomic signature almost identical to that of the Anglesea locality. Whereas the Anglesea fossils occur high in the Eastern View Formation and are considered late Middle Eocene, the Dean's Marsh locality is basal Eastern View Formation and is late Early Eocene (Douglas and Ferguson, 1988). An examination of the taxa collected from the two localities has shown no taxa held in common. Although the deposits of the late Middle Eocene scattered across southern Australia show similar mixes of families and in many cases genera, the Dean's Marsh locality shows no such correlation. One explanation that fits nicely with the known data is that although they occur in very similar environmental situations, and hence have similar physiognomic signatures, the two deposits occur on either side of the 8-m.y. cooling period discussed by McGowran (1989); hence, the evolution and natural selection that occurred during that time selected totally different taxa for the same niches. The major cooling event at the end of the Eocene is much easier to document. Palynological studies have highlighted it for many years (e.g., Kemp, 1978)—the most obvious signal being the shift from the tropical Nothofagus brassii pollen type to the Fusca and Menzesii types that

THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 180 represent the small toothed-leaf Antarctic beeches known from the modern cool temperate floras in New Zealand, southern South America, and Tasmania. The only megafossil record of leaves and cupules of the N. brassii type comes from the Eocene of Tasmania (Hill, 1987), whereas the small-leafed types are prevalent in sediments from then onward. Hill and Carpenter (1991) have also provided data suggesting that some of the smaller-leafed conifers may also show a leaf size reduction across this crucial boundary. Figure 10.5 Map of eastern Australia showing Eocene megafossil localities cited in the text.

THE IMPACT OF CLIMATIC CHANGES ON THE DEVELOPMENT OF THE AUSTRALIAN FLORA 181 In general the late Middle and Upper Eocene floras mentioned above all show physiognomic signatures indicating warm, wet climates, whereas those known from the Oligocene-Miocene show much reduced, sclerophyllous signatures. Localities displaying these features include Kiandra in New South Wales as well as Berwick and Bacchus Marsh in Victoria (Figure 10.6). Not only does the physiognomic signature change, but so does the taxonomic composition of the flora. As can be seen in Table 10.1, the domination by the Gondwanic Proteaceae, Lauraceae, and Gymnostoma evident in the Eocene floras has been lost by Oligocene time and is replaced by elements of the flora now found in different modern communities—namely, Eucalyptus, Acacia, and Epacridaceae. The Proteaceae and Casuarinaceae are still prevalent, but the Proteaceae is now dominated by Banksia and sclerophyllous forms, and Allocasuarina and Casuarina (sensu Johnson, 1982) have now replaced the more mesic Gymnostoma. All of the above changes are well documented in the pollen record, but frustratingly, very few occurrences of Eucalyptus and Acacia are known from the megafossil record. Thus, two distinct significant global climatic events (Figure 10.4) can be seen to be reflected in the Australian megafossil record and, when considered floristically, appear to have had a major effect on the vegetation development of the continent. Figure 10.6 Map of eastern Australia showing Oligocene-Miocene plant megafossil localities cited in the text: (1) Warrumbungle Mountains, New South Wales; (2) Kinadra, New South Wales; (3)Bacchus Marsh, Victoria; (4) Berwick, Victoria; (5) Morewell, Victoria; (6) Yallourn, Victoria; (7) Pioneer, Tasmania; and (8) New Norfolk, Tasmania.

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What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

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