Australia: The Land Where Time Began

A biography of the Australian continent 

The Australian Pollen Record - Moist Tropics

The sites at Lynchs Crater on the Atherton Tablelands and the marine core off Cairns, ODP820, are the most intensely studied cores in Australia (at least up to 2006). They indicate that rainforest existed at Lynchs Crater until at least 78,000 BP. Araucaria and Podocarpus, and other rainforest conifers, increased during cooler, drier periods, and Angiosperms increasing in the interglacials when conditions were warmer and wetter. (Kershaw, 1986; Moss & Kershaw, 2000).

46,000 BP was the beginning of the replacement of the rainforest by sclerophyll forests, mixed eucalypts and acacias, the trend increasing between 40,000 and 35,000 years ago (Turney, 2001). Just before the last glacial maximum (LGM) Poaceae increased, indicating a trend towards swamp grasses as the lake became shallower. They declined at the LGM, suggesting a complete drying of the lake. The first major appearance of charcoal in the record at 46 thousand years ago Turney et al., 2001b).

It is believed the pollen in the ODP820 core was moved to the site by runoff from the ranges and coastal areas, that included the Lynchs Crater. Changes in the pollen record over the last 1.5 million years are described by Kershaw et al. (1993), Moss & Kershaw (2000), describe changes over the last 200,000 years. The pollen in these cores indicate that rainforest was dominant, with a large rainforest conifer component, for most of the last 1.5 million years. There were also sclerophyll forests dominated by casuarinas, that included banksias and acacia, but not many eucalypts, until some time in the last 200,000 years.

Freshwater aquatic plants, grasses and sedges and ferns were also present. Indications of high sea levels are the times when mangroves are present. The mangroves were present occasionally, not continuously. It was in the last 200,000 years that there is evidence of changing vegetation, such as declining rainforest conifers and increasing eucalypt-dominated sclerophyll vegetation. At this site, but not at Lynchs Crater, the pollen record records 2 times of change, one of which occurred in each of the last 2 glacial phases. Throughout the LGM Poaceae underwent a continued increase, but sclerophyll vegetation continued to decline. This suggests the regional spread of grasslands or savannas. Charcoal increased close to the end of both glacial phases.

At least 4 glacial phases are believed to be covered by a core of Lake Coomboo on Fraser Island. Periods when the lake was dry correspond to increases in dry forest and increases of rainforest at times when the lake was full. (Longmoore & Heijnis, 1999). Casuarina formed a large component of the dry forest for most of the record, together with some eucalypts and Angophora. At 22,000 BP, during a dry period, the lake was surrounded by a dry sclerophyll forest dominated by eucalypts, the casuarinas having disappeared. From 12,000 BP casuarinas returned, becoming very common. At the start of the sequence rainforest was present, but declined steadily throughout the sequence, being gradually replaced by wet sclerophyll forests dominated by eucalypts and melaleuca. About 350,000 BP saw a major change from rainforest to wet sclerophyll forests. At about 20,000 BP charcoal reached a maximum, having increased mostly steadily throughout the sequence.

Pollen from Indonesia and Australia is found in the marine cores from the Lombok Ridge and the Banda Sea. It was suggested by Wang et al. (1999) that the pollen, though mixed from the 2 regions, were mostly of Australian origin because of the high component of grasses, eucalypts and sedges. According to the cores, the interglacials were associated with chenopods, sedges and grasses and the interglacials are associated with rainforest vegetation, mangroves and ferns.

185,000 BP was the beginning of the most severe phase of the 2nd last glacial cycle, when it was very dry and cold. This time saw a major sustained change from eucalypts to grasses. It is believed the change indicates a major expansion of savanna in northwestern Australia. There is an associated  increase of charcoal that was sustained. A major change from eucalypt to grassland is also indicated by the Banda Sea marine core that occurs at about the time of the LGM (van der Kaars et al., 2000) 

Sources & Further reading

• Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
• Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006