Linkages between the Australian-Indonesian and East Asian summer monsoon systems during the Younger Dryas cold event

This thesis investigates centennial-scale fluctuations in the strength of the Australian-Indonesian and East Asian summer monsoons during the Younger Dryas cold reversal 12900 to 11700 years ago (12.9 to 11.7 kyr B.P.). A stalagmite from Liang Luar Cave, Flores, Southern Indonesia (LR06-B1), was utilised to produce a high-resolution reconstruction of the Australian-Indonesian summer monsoon throughout this interval. Oxygen and carbon stable isotope (8180 and 813C) variations in the stalagmite calcite were used to construct this record, and were reinforced by magnesium-to-calcium (Mg/Ca) trace element ratios. Previous work at the site showed that oxygen isotope variations are a reliable proxy for intensity of this monsoon system and that the local monsoon intensified during the YD (Griffiths et al. 2009) when the Atlantic meridional overturning circulation decreased in strength (McManus et al 2004). This was most likely caused by enhanced winter monsoon outflow from Asia, and an associated southward migration of the intertropical convergence zone (Yancheva et al. 2007). In this new record, these results are not only replicated (confirming that the YD was a wet but cool period in Flores) but also demonstrate the existence of three prominent submillennial episodes during the middle part of the YD. These represent episodes of local monsoon weakening. The Liang Luar monsoon reconstruction was compared with the contemporaneous East Asian summer monsoon records from Kulishu and Hulu Caves (China) to explore possible teleconnections between the two monsoon regions. It was found that the three isotopic peaks of monsoon weakening in Flores are synchronous with episodes of resurgent Asian summer monsoon activity, which in turn have been linked to centennial-scale temperature reversals observed during the mid-YD in the NGRIP ice core record (Greenland). However, a slower transition into the YD was observed in the cave records, which lagged changes in Greenland temperature. These results suggest that, even at sub-millennial scales, the influence of climate changes in the high latitudes of the North Atlantic extend into the southern equatorial tropics. Potential explanations of the centennial variability evident in the LR06-B1 8180 record are the influence of solar variability and/or ocean circulation changes on temperature in the high northern-latitudes. These are propagated to the study site by the presence of cross-equatorial atmospheric linkages between the two monsoon systems