A 150,000-year lacustrine record of the Indo-Australian monsoon from northern Australia

Abstract

Nearly two thirds of the world's population depend on monsoon rainfall, with monsoon failure and extreme precipitation affecting societies for millennia. Monsoon hydroclimate is predicted to change as the climate warms, albeit with uncertain regional trajectories. Multiple glacial-interglacial terrestrial records of east Asian monsoon variability exist, but there are no terrestrial records of equivalent length from the coupled Indo-Australian monsoon at its southern limit — Australia. We present a continuous 150,000-year lacustrine record of monsoon dynamics from the core monsoon region of northern Australia based on the proportion of dryland tree pollen in the total dryland pollen spectra and the hydrogen isotope composition of long chain n-alkanes. We show that rainfall at the site depends strongly on sea level, which changes proximity of the coast to the site by 320 km over the last glacial-interglacial cycle. Long-term trends in rainfall are broadly anti-phased with the east Asian monsoon modulated by coastal proximity. The record also contains multiple, short intervals (∼2 to < 10,000 years) of large changes in tree cover (from 5 to 95 % tree pollen over 3000 years in one instance). Changes in tree cover are frequently but not always, accompanied by synchronous large changes in the other hydroclimate proxies. While these wetter periods cannot be easily ascribed to orbitally induced changes in insolation or coastal proximity, they are correlated with most Heinrich events. This relationship implies that strong asymmetry in inter-hemispheric monsoon rainfall might be one outcome of the current weakening in the strength of the Atlantic meridional overturning circulation, through a reduction in oceanic heat transfer from the Southern to the Northern Hemisphere.