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Abstract
Reconstructing past climate change is critical for understanding future variability. Unfortunately, instrumental records are relatively short, with the longest known record in Australia existing from the late 19th Century. Records on such a narrow timescale limit our understanding of natural variability, and inhibit the detection of rare or extreme events on different timescales. Climate proxies provide an opportunity to extend instrumental records over millennia. The use of proxies such as ice-cores, tree-rings, lake and ocean sediments, have revealed important climate variability during the past 3000 years, particularly in the Northern Hemisphere; these studies have identified multi-centennial events such as the Medieval Climate Anomaly (approximately AD 800-1250) and the Little Ice Age (approximately AD 1350-1850). In the Southern Hemisphere, however, our understanding of climate over this period is considerably less well known; with a particular dearth of records in the tropics.
This study provides new, high-resolution climate proxy data on the climate of the Atherton Tableland region of tropical northern Queensland during the last 3000 years. A dearth of climate proxy data exists from the global tropics, limiting our understanding of climate dynamics. Dominated by ocean cover, the tropics serve as a significant global heat reservoir and play a major role in global atmospheric circulation. The West-Pacific Warm Pool (WPWP) is the largest of these heat reservoirs stretching from the eastern Indian Ocean through to the western portion of the equatorial Pacific and including the tropical waters off the north-east coast of Queensland.
The Atherton Tableland is ideally placed to reconstruct changes of the past 3000 years, located at the junction of several major climatic and oceanic boundaries including the WPWP the region is influenced by the Inter-Tropical Convergence Zone (ITCZ), the El Niño Southern Oscillation (ENSO) and the Australian-Indonesian Summer Monsoon (AISM). In the remnant tropical forests of the Atherton Tablelands, volcanic crater lakes contain sediments deposited over thousands of years; offering the potential of high-resolution palaeoclimate analysis of the last 3000 years.
In this study, proxy data was obtained from cores extracted from two extinct volcanic craters; Quincan Crater and Bromfield Swamp. Humification, geochemical analysis (ITRAX), carbon:nitrogen ratio, charcoal and pollen analysis were undertaken at a fine resolution, with the ITRAX (XRF) core scanner providing sub-annually resolved data. The multi-proxy analysis was undertaken within a robust geochronological framework using comprehensive AMS radiocarbon (14C) dating. At Quincan Crater sediment accumulated at a rate of 1 cm per 6.8 years on average, and was found to be three times more rapid than that of Bromfield Swamp.
This research revealed two distinct climatic periods operating over the last 3000 years. From 3000 – 1700 Y BP, ENSO appears to be the dominant climate mode, with strong ENSO periodicities expressed across multiple proxy datasets. After 1700 years ago, however, southward migration of the ITCZ appears to have been established, delivering highly seasonal rainfall via the monsoon and minimising the influence of ENSO. The timing and significance of these changes to global climate are discussed.