Stalagmite oxygen-isotope (18O/16O) records from China and Borneo have revealed changes in Asian monsoon rainfall over glacial-interglacial cycles (e.g. Wang et al. 2008, Cheng et al. 2010, Meckler et al. 2012), yet little is known about orbital- and millennial-scale climate change in the ‘southern half’ of the Australasian monsoon domain. To fill this gap, we aim to build stalagmite 18O/16O records for the seasonal monsoon rainfall belt of south-central Indonesia. We have completed four expeditions to Liang Luar cave on the island of Flores (Figure 1) and are currently analysing 18O/16O and carbon-isotope ratios (13C/12C) in stalagmites with U-series ages extending to ~90,000 yBP.
The new Flores 18O/16O records for ~90,000 to 35,000 yBP (analysed in 2012) serve to complete the first high-resolution, absolute-dated Late Pleistocene history of rainfall variability across the entire Australasian monsoon system. There is clear (but non-linear) antiphasing of the Flores and China (Hulu/Sanbao caves) stalagmite 18O/16O records on precession time-scales over the last ~90,000 years (Figure 2). A strong synchronous climate shift marks the onset of Marine Isotope Stage 3 ~60,000 yBP (drier Flores, wetter China) and heralds the driest 30,000-year interval on Flores. A distinct monsoon rainfall maximum on Flores ~21,000 yBP suggests the intertropical convergence zone (ITCZ) moved southward during the Last Glacial Maximum in response to the southern hemisphere summer insolation maximum at that time (Ayliffe et al., submitted).
Interestingly, the largest 13C/12C anomaly for the last ~90,000 years on Flores begins at ~70,000 yBP in the absence of any clear climate forcing (record not shown). The ~4,000-year-long 13C/12C signal is under investigation, but probably reflects catastrophic vegetation collapse in the aftermath of a massive volcanic eruption (See Scroxton et al. 2012 RSES Research Highlight).
Targeted U-series dating of the new Flores stalagmite 18O/16O record is in progress, but it already shows that Australasian monsoon rainfall and climate change in the North Atlantic region are inextricably linked on millennial timescales (e.g. Griffiths et al. 2009, Lewis et al. 2011). For example, cooling in the North Atlantic region during Heinrich Event 1 (~16,000 yBP) and the Younger Dryas (~12,000 yBP) correlates with a southward shift of the Australasian ITCZ and increased rainfall in Flores. There are still small gaps in the Flores record around Heinrich events 2 and 3, but a similar antiphased monsoon response is evident around Heinrich events 4, 5 and 6 (~38,000 yBP, ~48,000 yBP, ~61,000 yBP), and during other less distinctive intervals.
Our findings indicate that millennial-scale changes in ITCZ positioning in tropical Australasia, through their influence on large-scale oceanic-atmospheric circulation, could have played a key role in the rise of atmospheric CO2 and global warming that ultimately led to the demise of the last ice age, as summarised by Denton et al. (2010) and others.
This research is supported by Australian Research Council Discovery grants DP0663274 to M.G., J.-x.,Z., R.D. and W.H. and DP1095673 to M.G., R.D., J.H., W.H., L.E. and H.C.
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