Contrasting Drought Episodes across the Mid-Brunhes Event in Northwest China

Drought events seriously impact our society, yet the occurrence and evolution of extreme drought events, and the underlying mechanisms for them, remain largely unknown in ancient times. Here, we present a suite of novel microbial lipid-based proxy records from the Chinese Loess Plateau (CLP), spanning the past 700 thousand years (ka), which reveal two contrasting drought regimes associated with distinct climatic backgrounds before and after the Mid-Brunhes Event (MBE, ~430 ka). By measuring the relative abundance of archaeal isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) vs. bacterial branched GDGTs (brGDGTs), i.e., the Ri/b ratio, and BIT (Branched and Isoprenoid Tetraethers), all diagnostics of drought events, we find a significant shift in both the timing and magnitude of drought episodes across the MBE. Droughts identified during glacial-interglacial transitions were much larger in amplitude after the MBE than before, consistent with a transition from generally colder to warmer interglacial conditions reconstructed from bacterial brGDGTs. This may result from the increased air temperature and atmospheric CO2 concentrations, together with vegetation-wildfire feedbacks. In addition, distinct climate systems and feedbacks appear to have modulated drought dynamics across the MBE: prior to the transition, droughts were more closely linked to weakening of the Indian summer monsoon, and were amplified by vegetation physiological effects under declining atmospheric CO2, while after the MBE, tropical Pacific Sea surface temperature (SST) variability and ENSO (El Niño-Southern Oscillation)-like conditions exerted stronger control, with amplification by the CO2 radiative effect during periods of rapid warming. The shift after the MBE to higher temperatures and climate sensitivity somewhat analogous to the current global warming, which, in the context of projected hydrological changes in the populous region of Northern China, suggests more droughts are likely to occur in the future, accompanied by enhanced short-timescale hydroclimatic variability.干旱事件对人类社会造成严重冲击，但古时段极端干旱事件的发生演化过程与内在机制仍鲜为人知。本研究基于中国黄土高原（Chinese Loess Plateau, CLP）一套全新的微生物脂类代用记录，时间跨度达70万年，揭示了中布容事件（Mid-Brunhes Event, MBE，约430 ka）前后两种截然不同的干旱型态及其对应的独特气候背景。通过测定古菌类异戊二烯甘油二烷基甘油四醚（archaeal isoprenoid glycerol dialkyl glycerol tetraethers, isoGDGTs）与细菌支链GDGT（bacterial branched GDGTs, brGDGTs）的相对丰度比值（即Ri/b比值）以及BIT（支链与类异戊二烯四醚）指数——二者均为干旱事件的诊断指标——研究团队发现，MBE前后干旱事件的发生时间与强度均发生了显著转变。冰期-间冰期过渡阶段的干旱事件在MBE后的振幅远大于此前，这与基于细菌brGDGTs重建的气候背景从普遍偏冷转为暖间冰期的结果一致。该转变可能源于气温升高、大气CO₂浓度上升，以及植被-野火反馈作用。此外，不同的气候系统与反馈机制调控了MBE前后的干旱动力学特征：在MBE转型之前，干旱与印度夏季风的减弱关联更为紧密，并在大气CO₂下降期间通过植被生理效应被放大；而在MBE之后，热带太平洋海表温度（Sea surface temperature, SST）变率与类厄尔尼诺-南方涛动（El Niño-Southern Oscillation, ENSO）状态的调控作用更强，并在快速增温时段通过CO₂辐射效应被进一步放大。MBE后向更高温度与气候敏感性的转型，与当前全球变暖存在一定相似性；结合人口稠密的华北地区未来水文变化的预测结果，这意味着未来干旱事件发生概率或将上升，同时伴随短时尺度水文气候变率的增强。