High precision TIMS U-Th disequilibrium dating and C, O, Sr isotope-based multi-proxy palaeoclimatic study of Speleothems in Australia

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,nfluvial deposits and sand dunes. The difficulty in the interpretation of such records is the lack ofnprecise age constraints, particularly for those beyond the dating range of 14C. Speleothems (cavendeposits) are important palaeoclimatic archives for the terrestrial environment, and readilyndatable by the recently established high-precision thermal ionization mass spectrometric (TIMS)n234U/230Th method. Compared with many other traditional terrestrial and marine climatic records,nspeleothems can provide relatively continuous, higher resolution records. n The principal aim of this research is to employ the unique research facilities in RadiogenicnIsotope and Geochemistry Laboratory, the University of Queensland, to obtain high-resolution,nwell dated, and relatively long and continuous multi-proxy climatic records from Australiannspeleothems. For this purpose, a total of twelve speleothem samples collected from five cavesnfrom North Queensland and Tasmania were analysed for high-precision TIMS U-Th ages andnhigh-resolution C, O and Sr isotopic and trace elements. The detailed results have been reportednin four independent chapters of this thesis. n Four stalagmites from the Chillagoe Caves, North Queensland, were dated between 18 to 6nthousand years (ka) before present. Based on combined d18O, d13C, (234U/238U)o, U and growthnrate records of a speleothem, the climatic patterns and trends reconstructed for the tropicalnQueensland during the last deglaciation period are consistent with records of pollen sequence,nlake level, charcoal level and floods from the North Atherton Tableland and the CarpentarianBasin (Kershaw, 1986; 1994; Harrison, 1994; Crowley et al., 1990; Nott et al., 1996; Nanson etnal., 1991). However, the new speleothem record is continuous, characterized by high temporalnresolution on a calendar-year time scale. Such a high-resolution continuous record enablesnidentification of significant cold and dry reversals between 14.5 and 10.5 ka following by rapidnphase transition to great wetness, which is well correlated with both Antarctic ice-core andntropical ocean coral SST records, which is previously untenable by other methods.n n 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 Tasmanianexperienced a continuing climatic amelioration and a relatively stable environment from 9.5 ka ton8.1 ka. The temperature rose abruptly at 8.1 ka and remained higher than the present day from 8.1nka to 6.8 ka. The qHolocene Optimumq with wettest condition and highest forest cover, wasnreached during 7.5 to 7.0 ka. The drying episode started at 7.0 ka, followed by a dramatic dryingnand cooling stage that occurred between 6.2 and 5.1 ka. The paleoenvironmental record ofnspeleothems correlates well with local pollen and lake level evidence. n The speleothem growth record over the last 160ka from Tasmania based on 48 TIMS U-Th datesnin this study and 41 previously reported a-counting dates indicates that high speleothem growthnoccurred at 7 ka, 12 ka, 82 ka, 101 ka, 110 ka, 117 ka, and 127 ka, while low growth occurrednduring Marine isotope stages 3 and 4 (20 to 77ka). The curve of speleothem growth frequencyncorresponds closely with the fluctuations in the SPECMAP d18O curve and compares closely withnratio-curve of herbs to woody taxa at Lake Selina during the interglacial and interstadials, whilenin direct contrast with that from Naracoorte Cave in Southern Australia, where the highnspeleothem growth frequency occurred mainly during cool interstadials and stadials, whereasninterglacials and warm interstadials, as well as glacial maxima, were comparatively low.n n Sixteen TIMS U-Th ages for a stalagmite from Newdegate Cave in southern Tasmania define anhigh-resolution record of precipitation between 100 and 155 ka and provides the first terrestrialnconstraints on the timing and duration of the Last Interglacial in the Southern Hemisphere. Thenfastest stalagmite growth between 129.2p1.6 and 122.1p2.0 ka (~61.5 mm/ka) corresponds thenearly warm stage of the Last Interglacial. This record coincides with a time of prolific coralngrowth from Western Australia. The growth rate between 129.2 and 142.2 ka (5.9/ka) was lowernthan 142.2 and 154.5 ka (18.7mm/ka), implying drier conditions during the PenultimatenDeglaciation, despite rising temperature and sea-level. The initial (234U/238U) ratios are alsonintimately correlated with growth rates, suggesting initial (234U/238U) is a potential paleo-precipitationnindicator, C, O stable isotope records do not show systematic changes between thenglacial and interglacial cycles and are complex to interpret.n n In summary, this study demonstrates that speleothems contain valuable climatic proxies that cannbe 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 globalnscales. The results show that during interglacial and Holocene periods, climatic trends and eventsnin Tasmania are synchronous on regional and global scales, evidenced by the timing and durationnof the highest Last Interglacial precipitation in the Newdegate record and the timing andncharacteristics of ~8200-yr-old global climatic event recorded m the Lynds Cave stalagmite. Onnthe other hand, during the deglacial period, the climatic trends and oscillations revealed in thenChillagoe record are more analogous to those in the Antarctic ice-core records, rather than in thenGreenland ice-cores in the Northern Hemisphere. In terms of intensity, the ~8200-yr-old qcoolingneventq is much weaker than the 11.5-12.8-ka Younger Dryas (YD) cold reversal. However, nonconclusive evidence has never been reported for the presence of YD signals in the SouthernnHemisphere. Because of this, we speculate that climatic forcing during glacial-deglacial periodsnmight be different from that during the interglacial periods, and such a difference might benrelated to the waxing and waning of qSouthwest Pacific Warm Poolq in the tropics vs, those ofnthe continental icesheets in high-latitude regions.n