Most information about environmental changes in the Australian continent over the Last Glacial-Interglacial cycle come from traditional continental records such as pollen sequences, lake levels, fluvial deposits and sand dunes. The difficulty in the interpretation of such records is the lack of precise age constraints, particularly for those beyond the dating range of 14C. Speleothems (cave deposits) are important palaeoclimatic archives for the terrestrial environment, and readily datable by the recently established high-precision thermal ionization mass spectrometric (TIMS) 234U/230Th method. Compared with many other traditional terrestrial and marine climatic records, speleothems can provide relatively continuous, higher resolution records.
The principal aim of this research is to employ the unique research facilities in Radiogenic Isotope and Geochemistry Laboratory, the University of Queensland, to obtain high-resolution, well dated, and relatively long and continuous multi-proxy climatic records from Australian speleothems. For this purpose, a total of twelve speleothem samples collected from five caves from North Queensland and Tasmania were analysed for high-precision TIMS U-Th ages and high-resolution C, O and Sr isotopic and trace elements. The detailed results have been reported in four independent chapters of this thesis.
Four stalagmites from the Chillagoe Caves, North Queensland, were dated between 18 to 6 thousand years (ka) before present. Based on combined δ18O, δ13C, (234U/238U)o, U and growth rate records of a speleothem, the climatic patterns and trends reconstructed for the tropical Queensland during the last deglaciation period are consistent with records of pollen sequence, lake level, charcoal level and floods from the North Atherton Tableland and the Carpentaria Basin (Kershaw, 1986; 1994; Harrison, 1994; Crowley et al., 1990; Nott et al., 1996; Nanson et al., 1991). However, the new speleothem record is continuous, characterized by high temporal resolution on a calendar-year time scale. Such a high-resolution continuous record enables identification of significant cold and dry reversals between 14.5 and 10.5 ka following by rapid phase transition to great wetness, which is well correlated with both Antarctic ice-core and tropical ocean coral SST records, which is previously untenable by other methods.
A 1300-mm long stalagmite in Lynds Cave, central-northern Tasmania was dated between 9.5 and 5.1 ka. Its C, O isotopic compositions and growth rates of the stalagmite suggest that Tasmania experienced a continuing climatic amelioration and a relatively stable environment from 9.5 ka to 8.1 ka. The temperature rose abruptly at 8.1 ka and remained higher than the present day from 8.1 ka to 6.8 ka. The "Holocene Optimum" with wettest condition and highest forest cover, was reached during 7.5 to 7.0 ka. The drying episode started at 7.0 ka, followed by a dramatic drying and cooling stage that occurred between 6.2 and 5.1 ka. The paleoenvironmental record of speleothems correlates well with local pollen and lake level evidence.
The speleothem growth record over the last 160ka from Tasmania based on 48 TIMS U-Th dates in this study and 41 previously reported a-counting dates indicates that high speleothem growth occurred at 7 ka, 12 ka, 82 ka, 101 ka, 110 ka, 117 ka, and 127 ka, while low growth occurred during Marine isotope stages 3 and 4 (20 to 77ka). The curve of speleothem growth frequency corresponds closely with the fluctuations in the SPECMAP δ18O curve and compares closely with ratio-curve of herbs to woody taxa at Lake Selina during the interglacial and interstadials, while in direct contrast with that from Naracoorte Cave in Southern Australia, where the high speleothem growth frequency occurred mainly during cool interstadials and stadials, whereas interglacials and warm interstadials, as well as glacial maxima, were comparatively low.
Sixteen TIMS U-Th ages for a stalagmite from Newdegate Cave in southern Tasmania define a high-resolution record of precipitation between 100 and 155 ka and provides the first terrestrial constraints on the timing and duration of the Last Interglacial in the Southern Hemisphere. The fastest stalagmite growth between 129.2±1.6 and 122.1±2.0 ka (~61.5 mm/ka) corresponds the early warm stage of the Last Interglacial. This record coincides with a time of prolific coral growth from Western Australia. The growth rate between 129.2 and 142.2 ka (5.9/ka) was lower than 142.2 and 154.5 ka (18.7mm/ka), implying drier conditions during the Penultimate Deglaciation, despite rising temperature and sea-level. The initial (234U/238U) ratios are also intimately correlated with growth rates, suggesting initial (234U/238U) is a potential paleo-precipitation indicator, C, O stable isotope records do not show systematic changes between the glacial and interglacial cycles and are complex to interpret.
In summary, this study demonstrates that speleothems contain valuable climatic proxies that can be precisely dated, providing a unique opportunity for reconstruction of continental climatic records that can be compared with regional marine records as well as on hemispheric to global scales. The results show that during interglacial and Holocene periods, climatic trends and events in Tasmania are synchronous on regional and global scales, evidenced by the timing and duration of the highest Last Interglacial precipitation in the Newdegate record and the timing and characteristics of ~8200-yr-old global climatic event recorded m the Lynds Cave stalagmite. On the other hand, during the deglacial period, the climatic trends and oscillations revealed in the Chillagoe record are more analogous to those in the Antarctic ice-core records, rather than in the Greenland ice-cores in the Northern Hemisphere. In terms of intensity, the ~8200-yr-old "cooling event" is much weaker than the 11.5-12.8-ka Younger Dryas (YD) cold reversal. However, no conclusive evidence has never been reported for the presence of YD signals in the Southern Hemisphere. Because of this, we speculate that climatic forcing during glacial-deglacial periods might be different from that during the interglacial periods, and such a difference might be related to the waxing and waning of "Southwest Pacific Warm Pool" in the tropics vs, those of the continental icesheets in high-latitude regions.