中更新世气候转型早期黄土高原间冰期降雨增加的岩石磁学记录【封面文章】

Rock magnetic records of increased rainfall during the interglacial period on the Loess Plateau in the early Middle Pleistocene transition【Cover】

关键词:  季风降雨  黄土高原  岩石磁学  环境磁学

Key words:  monsoon rainfall  Chinese Loess Plateau  rock magnetism  environmental magnetism

摘要:

东亚夏季风是全球气候系统的重要组成部分，其在全球变暖背景下的演化对未来气候预测具有重要指示意义。中更新世气候转型期是探讨东亚夏季风动力学机制的关键时期。以黄土高原南部白鹿原黄土-古土壤钻孔岩心为研究对象，通过岩石磁学和环境磁学方法，重建过去1.6 Ma以来东亚夏季风降水的变化历史。岩石磁学结果表明，白鹿原岩心的磁学性质由细颗粒磁铁矿/磁赤铁矿主导，其磁化率能够较好地反映该区域降水量的变化。岩心的低频磁化率记录显示，自约1.3 Ma起，间冰期的东亚夏季风降水显著增强，标志着中更新世气候转型初期东亚季风系统的显著转变。该变化特征亦在黄土高原其他剖面中有所体现，表明中更新世气候转型伴随着整个黄土高原范围内夏季风降水格局的重塑。研究认为，此次中更新世气候转型初期夏季风增强可能与热带海洋表面温度的变化以及青藏高原的构造隆升密切相关。

Abstract:

Background, aim, and scope The East Asian summer monsoon (EASM) is a critical component of the global climate system, transporting heat and moisture from warm tropical oceans to the continental interior of East Asia and influencing the lives of more than 1.5 billion people. The Middle Pleistocene transition (MPT) was marked by a fundamental shift in the pacing of Earth’s glacial cycles from about 40 ka to 100 ka. Chinese Loess Plateau (CLP) is of particular interest to Quaternary geologists because it hosts a vast expanse of eolian loess-paleosol sediments, which has been regarded as one of the most complete and reliable terrestrial archives of paleoclimatic and environmental change. In typical Chinese loess-paleosol sequence, red paleosols consistently exhibit higher low-frequency magnetic susceptibility (χ _lf) than yellow loess layers and χ _lf is widely used as a proxy for summer monsoon intensity. Multiple χ _lf records from the CLP show that the EASM precipitation shifted from 40 ka to 100 ka cyclicity across the MPT. In contrast to previous studies, we focus on interglacial EASM variability across the MPT by analyzing new environmental and magnetic records from the Bailuyuan loess-paleosol core on the southern CLP. Materials and methods Samples were extracted at 5 cm intervals from the drilled Bailuyuan loess-paleosol core. Each sample was dried at low temperature in a and then ground with an agate mortar. Approximately 10 g of powdered material from each sample was placed into a standard paleomagnetic sample cube (2 cm side length) for magnetic susceptibility measurements. χ _lf was then measured on all 1702 unoriented samples using an AGICO MFK1-FA rotating Kappabridge magnetic susceptibility meter. Based on the observed variations in magnetic susceptibility and lithology, nine representative samples from different stratigraphic layers were selected for detailed rock magnetic measurements. These included temperature-dependent susceptibility (χ—T) curves, hysteresis loops, isothermal remanent magnetization (IRM) acquisition curves, low-temperature saturation isothermal remanent magnetization (SIRM) curves, and first order reversal curve (FORC) diagrams. Results Environmental magnetic records reveal that χ _lf increased sharply around 1.3 Ma, decreased around 0.93 Ma, and subsequently rose again at approximately 0.53 Ma. The mean χ _lf value is 4.58×10⁻⁷ m³·kg⁻¹. Rock magnetic analyses show that the magnetic properties of the Bailuyuan loess-paleosol sequence are dominated by fine-grained magnetite/maghemite population, ranging from superparamagnetic (SP)/single-domain (SD) to small pseudo-single domain (PSD) (<30 nm to about 1000 nm). Discussion Rock magnetic analyses of the Bailuyuan loess-paleosol core support the classical use of χ _lf as a reliable proxy for reconstructing EASM precipitation on the CLP. The χ _lf record from Bailuyuan core exhibits clear glacial-interglacial variability matching the global marine benthic δ ¹⁸O record over the past 1.6 Ma, consistent with other χ _lf records from the CLP. It is characterized by a notable transition in periodicity from about 40 ka to 100 ka across the MPT. Importantly, our results reveal a stepwise increase in interglacial χ _lf values around 1.3 Ma, in addition to a similar rise previously identified near about 0.5 Ma. This pattern suggests a phased intensification of interglacial EASM precipitation beginning at about 1.3  Ma, in contrast to coeval global cooling. Comparable trends are also discernible in other χ _lf records from the CLP, reinforcing the regional significance of this shift. The inconsistency between global temperature/ice volume changes and enhanced interglacial monsoon precipitation during the early MPT suggests that global cooling cannot adequately explain the observed monsoonal trends. We propose that the intensification of the meridional and zonal sea surface temperature (SST) gradient in the western Pacific, along with the uplift of the Qinghai-Xizang Plateau during this period, likely contributed to the strengthening of the EASM in the early phase of the MPT. Conclusions The rock magnetic results indicate that the magnetic properties of the Bailuyuan loess-paleosol core are dominated by fine-grained magnetite/maghemite, affirming that χ _lf record reliably reflects precipitation variability on the CLP. The χ _lf record spanning the past 1.6  Ma reveals a pronounced increase in interglacial EASM precipitation at the onset of the MPT, approximately 1.3 Ma. This transition is also clearly evident in other sections across the CLP suggesting that the MPT was accompanied by large-scale changes in EASM precipitation across the region. We attribute this monsoonal shift to changes in tropical sea surface temperature and the tectonic uplift of the Qinghai-Xizang Plateau. Recommendations and perspectives Under-standing the evolutionary mechanism of the EASM on the orbital timescales requires comprehensive consideration of the interactions among multiple climatic factors. Incorporating more diverse perspectives and higher-resolution geological records is essential for more accurately reconstructing EASM precipitation history.