Sensitivity of Australian monsoon to changing climate boundary conditions across the Mid-Pleistocene Transition
Final Report Abstract
The Australian Monsoon (AM) subsystem is a highly sensitive monitor of tropical hydroclimate variability, due to its location at the southern edge of the largest amplitude seasonal swing of the Intertropical Convergence Zone within the large-scale Asian-Australian monsoon system. However, the sensitivity of the AM to changing climate boundary conditions such as ice volume and greenhouse gas concentrations and the interhemispheric coupling with other monsoonal subsystems remain highly enigmatic due to our limited understanding of its past variability. During IODP Expedition 363, an extended (sedimentation rate: ~10 cm/kyr), undisturbed Pleistocene hemipelagic sediment succession was retrieved for the first time off NW Australia (Site U1483, 13°5.24ʹS, 121°48.25ʹE, water depth: 1733 m). This carbonate- and clay-rich sequence provides an ideal archive to monitor the intensity and variability of the AM through different meanstates of Earth’s climate and to better constrain its sensitivity to changes in radiative forcing. Our project focused on the Middle Pleistocene Transition (MPT, ~1.2-0.6 Ma), which registered fundamental changes, as Earth’s climate transitioned from an overall warmer phase with a reduced interhemispherical thermal contrast to a high-amplitude, quasi-periodic (~100 kyr) mode of glacialinterglacial variability. We used X-ray fluorescence (XRF) scanner derived terrigenous runoff (elemental ratios and accumulation rates) and paleoproductivity/carbon flux and bottom water oxygenation records (chlorins from spectrophotometry and redox-sensitive elements from XRF-scanning and spectral gamma ray measurements) to investigate the primary drivers of monsoonal climate evolution between 1.6 and 0.4 Ma. In combination with a high-resolution benthic oxygen isotope stratigraphy, the U1483 records of precipitation (terrigenous runoff during austral summer) and monsoonal wind (driving upwelling and productivity during austral winter) shed light on the timing of major reorganizations in Southern Hemisphere tropical climate, linkages of high- and lowlatitude climate change and the interhemispheric coupling of the AM with other monsoon subsystems. Our results show that monsoonal precipitation and runoff primarily responded to precessional insolation forcing until ~0.95 Ma, but exhibited heightened sensitivity to ice volume and pCO2 related feedbacks following intensification of glacial-interglacial cycles. Our records further suggest that monsoon variability at the precessional band was closely tied to the thermal evolution of the West Pacific Warm Pool and strength of the Walker circulation over the past ~1.6 Myr. By contrast, winter monsoon proxy records consistently tracked glacial-interglacial variability, reflecting changing rhythms in polar ice fluctuations and Hadley circulation strength. We conclude that the Australian Monsoon underwent a major re-organization across the MPT and that extratropical feedbacks played a key role in driving short- and long-term variability. These findings raise the possibility that future pCO2 increase and global warming will strengthen the wet summer monsoon and weaken Southern Hemisphere Trade winds.
Publications
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Data report: revised composite depth scale and splice for IODP Expedition 363 Site U1483. Proceedings of the International Ocean Discovery Program (c(2020, 6, 18)). International Ocean Discovery Program.
Gong, L.; Lübbers, J.; Beil, S. & Holbourn, A.E.
