| Summary: | Research Doctorate - Doctor of Philosphy (PhD) The climate evolution of the southern sub-equatorial tropics during marine isotope stage (MIS)5a/b and the Holocene is explored using geochemical tracers from speleothems on Flores island, Indonesia. Oxygen isotope measurements from two precisely-dated stalagmites reveal that the Australian-Indonesian monsoon increased during the Younger Dryas (YD) cooling event, when Atlantic meridional overturning circulation was relatively weak. Monsoon precipitation intensified even more rapidly from 11 to 7 ka ago, when the Indonesian continental shelf was flooded by global sea-level rise. Analysis of oxygen (δ18O) and hydrogen (δD) isotope ratios from speleothem fluid inclusions shows that inclusion-δ18O values vary in phase with speleothem calcite δ18O during the Holocene, confirming that calcite δ18O primarily reflects variations in the δ18O of meteoric rainfall. Cave drip-water temperatures, reconstructed from coupled measurements of δ18O in speleothem calcite and fluid inclusions, remained relatively constant through the Holocene but were significantly cooler during the YD, consistent with the high northern latitudes. To help confirm the stable isotope records, trace elements were used to reconstruct the position of the austral summer inter-tropical convergence zone and east Indonesian rainfall variability during the Holocene. Mg/Ca and Sr/Ca ratios correlate significantly with one another, and with δ18O and δ13C, throughout the record suggesting that the trace element ratios were dominated by prior calcite precipitation, a process whereby degassing in the vadose zone during periods of low recharge causes deposition of calcite and disproportionate loss of Ca2+ ions (relative to Mg2+ and Sr2+) ‘upstream’ of the stalagmite. Comparison of speleothem δ18O time-series from Flores and Borneo shows that they vary in unison for much of the Holocene. However, there is an exception during the mid-Holocene when a distinct anomaly in δ18O in the Borneo record, possibly caused by a change in the circulation of the Australian-Indonesian summer monsoon (AISM) in response to a period of positive IOD-like conditions in the eastern Indian Ocean, occurred between the two regions. A stalagmite reconstruction of Indo-Pacific climate through the interval 84 - 91 ka shows that the lower-frequency oxygen isotope trend indicates that the AISM was largely controlled by local summer insolation during this time, while the carbon isotopes show a pattern that is closer linked with northern polar-latitude ice-core records. Most notably, an abrupt decrease in the temperature-controlled δ13C values at the MIS 5a/b transition occurs in parallel with GIS 21 in the GISP2 δ18O and CH4 records highlighting the strong connection between the IPWP and North Atlantic during the last glacial period.
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