THE RESPONSE OF VEGETATION TO ICE VOLUME, MONSOON AND ENSO CLIMATE FORCING AND HUMAN IMPACT IN THE SOUTHERN INDO-PACIFIC WARM POOL REGION

Marine palynological records from the seas to the north of the Australian continent provide evidence of vegetation dynamics along a gradient from arid to humid at Milankovitch timescales covering the past 100,000 to 500,000 years. Each record shows many distinctive features reflecting spatial variation in the vegetation, the complexity of the climate of the region and different and changing pollen catchment areas. Most pollen components reflect northern hemisphere forcing primarily through changes in land/sea distributions as a result of sea level fluctuations but also through changes in solar radiation and/or atmospheric carbon dioxide levels. Consequently interglacials are generally wetter and warmer than glacial periods. Superimposed on this pattern are cyclical variations that appear to reflect ENSO forcing within the Pacific region and Australian (southern hemisphere) monsoon forcing in the Indian Ocean region. The influence of regional forcing also increases away from the extensive Sunda continental shelf. A trend towards continental drying or increased climatic variability is demonstrated by changes in both vegetation and biomass burning within all records. Specific events can be identified, centred on 40,000-45,000 years ago, that strongly suggest initial impact of people, and this proposed timing for the migration of people into the Australian continent is consistent with both archaeological evidence and the dates for the demise of megafauna. However, people did not initiate this trend that is likely to have resulted from altered atmospheric and oceanic circulation patterns due to tectonic activity within the Indonesian gateway about 350,000 years ago. There is evidence that this trend resulted in a severe reduction in the distribution of what may have been extensive drier rainforests with feedbacks accelerating the drying of the Australian continent. A number of regional and total plant extinctions are documented. In terms of carbon storage, this would have been reduced over the period of the trend but, because of the extension of vegetation over continental shelves during glacial periods, there may have been little fluctuation in the overall carbon budget.