Sensitivity of Australian monsoon to changing climate boundary conditions across the Mid-Pleistocene Transition

澳大利亚季风（AM）子系统是一个高度敏感的热带水文气候变化的监测，由于其位置在最大幅度的季节性摆动的热带辐合带的大尺度亚洲-澳大利亚季风系统的南部边缘。然而，由于我们对其过去变化的了解有限，AM对变化的气候边界条件（如冰量和温室气体浓度）以及与其他季风子系统的半球间耦合的敏感性仍然非常神秘。在IODP考察期间，在澳大利亚西北部海域（地点U1483，13°5.24东经，121°48.25东经，水深：1733 m）首次获得了一个扩展的（沉积速率：~10 cm/kyr）、未受扰动的更新世半远洋沉积物序列。这种富含碳酸盐和粘土的序列提供了一个理想的档案，以监测AM的强度和变化，通过不同的平均状态的地球气候，并更好地限制其敏感性的变化，辐射强迫。我们的项目将集中在中更新世过渡期（MPT，~1.2-0.6 Ma），其中记录了根本性的变化，因为地球的气候从一个整体温暖的阶段，减少半球间的热对比过渡到一个高振幅，准周期（~100 kyr）模式的冰川-间冰期变化。我们将比较X射线荧光（XRF）扫描仪得出的陆源径流（元素比和积累率）和古生产力记录（XRF扫描仪得出的生物硅，碳积累率和碳同位素梯度）的温度和冰量的变化（来自有孔虫氧同位素和Mg/Ca）。结合高分辨率的底栖氧同位素地层学，（在南方夏季改变盐度和陆源径流）和季风风（在南半球冬季驱动上升流和生产力）将揭示南半球热带气候主要重组的时间，高纬度和低纬度气候变化的联系以及AM与其他季风子系统的半球间耦合。特别是，我们将检验以下假设：1）以全年降雨为特征的潮湿条件在温暖的“41 kyr世界”中占主导地位，而降雨季节性随着高振幅的冰川-间冰期~100 kyr周期的出现而增加; 2）主要气候转变在~1.6，~1.2，~0.9和~0.6 Ma之前和整个MPT与AM动力学（降水强度和季节性以及与季风风相关的上层海洋对流混合）的变化相吻合。

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 an IODP Expedition, 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 mean-states of Earth’s climate and to better constrain its sensitivity to changes in radiative forcing. Our project will focus 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 glacial-interglacial variability. We will compare X-ray fluorescence (XRF) scanner derived terrigenous runoff (elemental ratios and accumulation rates) and paleoproductivity records (XRF-scanner derived biogenic silicon, carbon accumulation rates and carbon isotope gradients) in relation to temperature and ice volume variations (derived from foraminiferal oxygen isotopes and Mg/Ca). In combination with a high-resolution benthic oxygen isotope stratigraphy, the U1483 records of precipitation (altering salinity and terrigenous runoff during austral summer) and monsoonal wind (driving upwelling and productivity during austral winter) will shed light on the timing of major re-organizations in Southern Hemisphere tropical climate, linkages of high- and low-latitude climate change and the interhemispheric coupling of the AM with other monsoon subsystems. In particular, we will test the hypotheses that 1) humid conditions, characterized by year-round rainfall, prevailed in the warmer "41 kyr world", whereas rainfall seasonality increased with the emergence of high-amplitude, glacial-interglacial ~100 kyr cycles and that 2) major climate transitions at ~1.6, ~1.2, ~0.9 and ~0.6 Ma prior to and across the MPT coincided with changes in AM dynamics (intensity and seasonality of precipitation and of upper ocean convective mixing related to monsoonal winds).