Indo-Australian Summer Monsoon activity in northern Australia during the past glacial cycle: A flood record of hydroclimate-mediated landscape response to changing runoff

Understanding of the Indo-Australian Summer Monsoon's (IASM) response to climate forcings under glacial conditions is limited by a significant knowledge gap regarding the hydroclimate in northern Australia during the last glacial. We present a continuous geochemical record from the eastern Kimberley, Australia, which spans ~65 ka to the Last Glacial Maximum (LGM). The record comes from the floodplain of the Bullo River, which is currently directly impacted by the IASM, and represents variations in the hydroclimate of its 2000 km 2 catchment. Results show that the hydroclimate during the glacial lacked the prolonged and high-amplitude changes that marked the hydroclimate following the LGM. Flooding was less frequent or of reduced magnitude than during the deglacial and early Holocene. While there was an absence of substantial long-term change, there is evidence of a subtle shift in mid Marine Isotope Stage 3 to less frequent or lower magnitude flooding and longer instances of dry conditions in the catchment. The low temporal resolution of these results means that centennial or shorter scale periods of enhanced IASM activity cannot be ruled out. These results extend our understanding of the long-term hydroclimate of monsoonal Australia during the glacial, which has, to date, largely been inferred from discontinuous geomorphic records. • The hydroclimate of northwestern Australia between 65 ka and the LGM lacked the prolonged and high-amplitude changes which marked the hydroclimate in the deglacial and Holocene. • Moisture availability and wet season precipitation in northwestern Australia was reduced between MIS 4 and the LGM. • Wet season precipitation and moisture availability reduced to their lowest levels in the record during mid MIS 3 and these conditions persisted into MIS 2.