Multi-proxy Evidence of Australian Summer Monsoon Variability During the Holocene: Links to the East-Asian Monsoon and the North Atlantic

The Australian summer monsoon (ASM) is the dominant factor controlling rainfall variability and terrestrial productivity in northern Australia and the Indonesian archipelago. Understanding the mechanisms that influence its variability over different time-scales, and their teleconnections with other parts of the global climate system, has proven difficult because we lack high-resolution, precisely dated records of past monsoon behaviour. Linkages between the tropics and North Atlantic have been well documented north of the equator, but the degree to which these teleconnection patterns extend into the southern sub-equatorial tropics and their effects on the ASM are undocumented. We present a precisely dated, high-resolution oxygen isotope and trace element record of ASM variability from stalagmites located on Flores (east Indonesia) over the period 13 kyr B.P. to present. The multi-proxy records are constrained by over 30 TIMS and MC-ICP-MS U-series ages. The δ18O profile displays a gradual intensification of the ASM through the Holocene, which is in phase with precipitation changes in southern Brazil but antiphased with East Asian monsoon (EAM) intensity. The low frequency trend in the oxygen isotopes tracks changes in southern hemisphere summer insolation at 25° S located directly over the heat-low region of the Australian continent. Superimposed upon the δ18O trend are multi-decadal to centennial scale increased ASM events that occur concurrently (within dating errors) with periods of decreased EAM intensity and North Atlantic ice-rafting events. Thus, late-Pleistocene/Holocene cold events in the North Atlantic, related to reductions in the Atlantic meridional overturning circulation and variations in solar output, were associated with a southward migration of the ITCZ. While precessional forcing appears to be the dominant driver of ASM circulation over orbital time-scales, the high synchroneity between the Flores isotope variations and titanium (Ti) content of Chinese lake sediments suggests that the Asian winter monsoon (AWM) may have a more dominant influence at the secular scale. The Ti content of the lake sediment, which is used as a proxy for the strength of AWM winds, shows a strong inverse relationship with the detrended δ18O (Δδ18O) record, whereby periods of strong (weak) AWM intensity correspond with intervals of high (low) ASM precipitation. This result confirms previous suggestions that the Australian monsoon is largely controlled by a 'push-pull' relationship between the Australian-East Asian monsoon systems. Overall, changes in ASM circulation during the Holocene reflect a combination of precession-controlled variations in external radiative forcing as well as internal climate dynamics associated with North Atlantic circulation and strength of the AWM winds.