浑善达克沙地南缘晚更新世黄土-古土壤序列的高分辨率磁性气候记录

浑善达克沙地处于季风边缘区，目前对该区域末次间冰期以来的古气候演化历史及其动力学机制尚不清楚。文章以浑善达克沙地南缘山间低洼地带北沟剖面总厚度4.72 m（未见底）的黄土-古土壤序列为研究对象，开展了光释光测年和精细的岩石磁学、环境磁学、地球化学和漫反射光谱分析。结果表明：1）该剖面记录了末次间冰期以来完整的风成堆积历史，并通过东亚夏季风指标（Rb/Sr及χ_ARM/χ）与深海氧同位素曲线的对比建立了13万年来该黄土-古土壤序列精细的年龄模式；2）在黄土高原上常规的用来指示成壤强度和夏季风强度的磁参数并不完全适用于干旱-半干旱地区（400 mm等降水线附近）的风成黄土，而地球化学指标在指示季风边缘区的地球化学风化过程上表现出一定的合理性，北沟剖面复杂的磁信号可能是对末次冰期季风边缘区显著减弱的夏季风的阈值响应；3）在末次间冰期东亚夏季风可以延伸到内蒙古中东部，北沟剖面记录了浑善达克沙地南缘末次间冰期以来区域古季风的演化历史。

Magnetoclimatological patterns of a Late Pleistocene loess-paleosol sequence in the southern Hunshandake Sandy Land

Because of spatial scarcity of reliable sedimentary archives, paleoclimatic history in eastern Inner Mongolia since the last interglacial remains unclear. Although extensive lakes, eolian sands, and soils within and surrounding the Hunshandake Sandy Land(HSL), located in the current northern margin of the East Asian summer monsoon(EASM), provide ideal paleoarchives for retrieving paleoenvironmental variations in the arid and semi-arid eastern Inner Mongolia, these records are commonly confined to the last deglaciation, hampering a complete understanding of the exact climatic fluctuations on glacial-interglacial time scales. Recently, we found a complete Late Pleistocene loess-paleosol sequence with a total thickness of 4.72 m in southern extremity of the HSL. This unique record provides an excellent opportunity to reconstruct regional climatic variations in the monsoonal margin. We therefore carried out detailed rock magnetic, geochemical and diffuse reflectance spectrometer analyses of this section. The 4.72 m-thick Beigou section(42.40°N, 115.71°E) was sampled at 4 cm intervals, and a total of 118 bulk samples were collected. The mass-specific magnetic susceptibility (χ) was measured using an AGICO MFK 1-FB Kappabridge at frequencies of 976 Hz, 3904 Hz, and 15616 Hz, respectively. Both anhysteretic remanent magnetization(ARM) and saturation isothermal remanent magnetization(SIRM) were measured using a 2G 755-4K cryogenic magnetometer. DRS were performed by a Cary 5000 UV-vis-IR spectrophotometer. Concentrations of major and trace elements were determined by PW4400 X-ray Fluorescence spectrometer and ICP-MS-PE300D, respectively. We constructed the depth-age model based on linear interpolation of three OSL dates and detailed correlations between the variations of our Rb/Sr ratio, a faithful proxy of EASM strength, with the LR04 benthic δ18O stack. This compelling land-ocean resemblance suggests that the sedimentation of the Beigou loess commenced since the last interglacial. During the last interglacial, our magnetic record clearly defines three sub-paleosol layers of S1-1, S1-2 and S1-3 with enhanced pedogenesis corresponding to MIS 5a, MIS 5c and MIS 5e, and all concentration dependent magnetic parameters(e.g.,χ, ARM and SIRM) exhibit concordant variations with higher values reflecting stronger pedogenesis, similar with those of typical loess in the hinterland of the Chinese Loess Plateau(CLP). Contrary to higher S-0.3 values of paleosol on the CLP, a case of lower S-0.3 values in paleosol is found here, suggesting that higher proportions of pedogenesis-derived high-coercivity magnetic minerals are formed during the interglacial. During the last glacial, the commonly-used grain-size dependent magnetic parameters(e.g.,χfd, ARM and χARM/χ), which have been successfully adopted to denote EASM variations, show muted variations that are less correlative with loess records from the CLP, reflecting the much weaker influence of EASM on the monsoon-marginal region. By contrast, Rb/Sr and Ba/Sr records exhibit broad comparability with summer precipitation variations and the grain size record from the western CLP, Chinese speleothem δ18O record, the ice-core δ18O record in Greenland, and the northern hemisphere summer insolation, demonstrating that eolian deposits in the southern HSL have the good potential in recording sub-orbital EASM variations as well as several millennial-scale cooling events, regardless of limited sedimentation rates. Interestingly, during the last glacial, both SIRM and Saturation magnetization(Ms) display high-amplitude variations with higher values, and are positively correlative with the variations of Zr/Rb ratio, a robust geochemical indictor of East Asian winter monsoon(EAWM), reflecting a strong control of winter vigor on magnetic signals dominated by lithological ferrimagnetic minerals. The consistently high S-0.3 values during the last glacial also suggest the general dominance of coarse-grained ferrimagnetic minerals. We also found that the climatic conditions during the last interglacial-glacial transition are characterized by gradual retreat of EASM but stepwise propagation of EAWM, indicating an anti-phased pattern of EASM and EAWM during this climatic transition. In conclusion, our combined mineral magnetic and geochemical results demonstrate that during the last interglacial the front of the EASM can extend to middle-eastern Inner Mongolia, consistent with modern observations of climate change. A negative correlation between SIRMs and ZR/Rb during the last interglacial but a positive correlation during last glacial suggest that SIRMs are controlled by pedogenesis during interglacial and by wind vigor during glacial, respectively. We attribute this complex magnetic record to a threshold response to the East Asian monsoon variations in eastern Inner Mongolia.