The loess-paleosoil sequences in Chinese loess plateau are the excellent archives for studying the magnetic polarity history of the geomagnetic field and reconstructing the paleoprecipitations and paleotemperatures of the north hemisphere in the Quaternary. Their environmental rock magnetic properties recorded the environmental and climatic information since 2.5 million years before the present continuously and sensitively. Especially, being an excellent climatic proxy, magnetic susceptibility has been extensively applied. However, the actual mechanism of susceptibility enhancement remains debatable. Currently, the viewpoint that pedogenesis plays a key role in enhancing magnetic susceptibility of paleosoils has been generally accepted. Some functions of magnetic susceptibilities established by different methods may help to reconstruct the paleoprecipitations and paleotemperatures. Pedogenic magnetite is generally responsible for susceptibility enhancement. Maghemite, meanwhile, is an important contributor to the enhanced susceptibilities of paleosoils. The grain size of magnetic minerals can substantially influence the magnetic susceptibilities too. Additionally, the anisotropy of magnetic susceptibility may be in use of the reconstruction of Quaternary wind field.

中国黄土分布广泛、沉积连续性好,是进行磁性地层研究和重建古环境、恢复古气候的理想材料。中国黄土高原黄土—古土壤序列的环境岩石磁学性质详细地记录了2.5 Ma BP以来古气候和古环境变化的信息,尤其是黄土和古土壤的磁化率作为很好的气候代用指标已得到广泛应用。古土壤磁化率增强成因机制一直在争论中,其中,与气候变化关系密切的成壤作用对古土壤磁化率增强起主导作用的观点已被普遍接受,目前利用不同方法建立了磁化率气候转换函数,从而使重建古降水量和古温度成为可能。

Abstract

In moderately to highly strained sandstones, both the long axis of the bedding-parallel finite-strain ellipse, as calculated by the normalized Fry method, and the projection of the long axis of the AMS ellipsoid on the plane of bedding, align well with local “structural grain” (trends of cleavage, folds, and faults). This relationship implies that results of both 2D Fry and AMS analyses represent the local layer-parallel tectonic strain component. Do both methods provide comparable results for very low-strain sandstone (e.g., <5%)? To address this question, Fry and AMS analyses were conducted in very low-strain sandstone from two localities in the Appalachian foreland fold–thrust belt: near Rosendale in New York and the Lackawanna synclinorium of Pennsylvania. We compared the map projections of both bedding-parallel Fry ellipses and AMS ellipsoids to the local structural grain. In both study areas, projections of the long axis of Fry strain ellipses do not cluster in a direction parallel to structural grain, whereas the projection of the long axes of AMS ellipsoids do cluster closely to structural grain. This observation implies that in very low-strain sandstone, AMS analysis provides a more sensitive “quick” indicator of tectonic fabric than does normalized Fry analysis.

The expansion of inland Asia deserts has considerably influenced the environmental, social and economic activities in Asia. Aridification of inland Asia, especially timing of the initiation of Asian desertification, is a contentious topic in paleoclimatology. Late Cenozoic eolian loess-red clay sequences on the Chinese Loess Plateau, which possess abundant paleoclimatic and paleo-environmental information, can be regarded as an indicator of inland Asia desertification. Here we present a detailed magnetostratigraphic investigation of a new red clay sequence about 654 m in Zhuanglang located at the western Chinese Loess Plateau. Sedimentological, geochemical, mineralogical, and quartz morphological lines of evidence show that the red clay is of eolian origin. Magnetostratigraphic correlations indicate that this core sequence spans from 25.6 to 4.8 Ma, and typical eolian red clay appears as early as 25 Ma. This extends the lower limit of the red clay on the Chinese Loess Plateau from the previously thought early Miocene back into the late Oligocene. This new red clay record further implies that the inland Asia desertification was initiated at least by the late Oligocene. This sequence provides a unique high-resolution geological record for understanding the inland Asia desertification process since the late Oligocene.

The climatic change since the Last Glacial Maximum (LGM) period was reconstructed based on the analysis of geochemical characteristics of the aeolian deposits in Gonghe Basin, northeastern Qinghai-Tibetan Plateau, coupled with the ¹⁴C and OSL chronology. The cold and wet climate dominated the region before 21 ka BP, and the climate became very cold and dry in 21~15.82 ka BP, corresponding to the LGM period. From 15.82 to 9.5 ka BP, the regional climate tended to be warm and dry in general. During this period, the cold and dry condition occurred in 14.5~13.6 ka BP and 11.9-9.5 ka BP, implying the Oldest Dryas and Younger Dryas phases, while the relatively warm climate was found in 15.82~14.5 ka BP and 13.6~11.9 ka BP (BøllingAllerød period, B/A period). The warm and humid degree obviously increased between 9.5 and 7.2 ka BP, marking the relatively warm and wet condition. The cold-dry and warm-humid climate fluctuated in 7.2~5.1 ka BP. In 5.1~2.7 ka BP, the degree of warm and wet is relatively stable but it tended to be cold with high moisture since 2.7 ka BP. These changes are coincident with the paleoclimatic records in Qinghai-Tibetan Plateau, reflecting higher consistency in climatic change between Gonghe Basin and Qinghai-Tibetan Plateau.

通过对共和盆地东部风成沉积的地球化学分析，并结合¹⁴C和OSL年代，重建了区域末次盛冰期以来气候变化过程。21 ka BP之前气候寒冷偏湿，21~15.82 ka BP 为末次盛冰期（Last Glacial Maximum，LGM），气候极为寒冷干燥；15.82~9.5 ka BP 气候转暖且偏干，其中14.5~13.6 ka BP 和11.9~9.5 ka BP 气候明显冷干，分别为老仙女木时期（Oldest Dryas，OD）和新仙女木时期（Younger Dryas，YD），而15.82~14.5 ka BP和13.6~11.9 ka BP （Bølling-Allerød暖期, B/A）相对温暖；9.5~7.2 ka BP 暖湿程度明显提高，7.2~5.1 ka BP 气候波动频繁，相对冷干和相对暖湿多次更替。5.1~2.7 ka BP 暖湿程度基本稳定，之后气候趋于寒冷但湿度明显较大。这些气候变化过程与青藏高原大量的古气候信息记录具有良好的一致性，表明共和盆地气候变化与青藏高原气候变化的高度一致性。

Earlier studies on two sections (QA-Ⅰ and QA-Ⅱ) in the Qin′an County from the western Loess Plateau demonstrated that loess deposits started by 22MaB.P., indicating the onset of Asian desertification and winter monsoon circulation by the Early Miocene time. Another Miocene loess section, Miziwan section with a thickness of 164.4m, 30km in east to the QA-Ⅰ section, is geomagnetically dated in this study.

文章对西部黄土高原糜子湾黄土-古土壤剖面进行了磁性地层学研究。该剖面厚度为164.4m,记录了15个正磁极性段和15个负磁极性段,可与标准极性表中从C5r.3r到C5En之间的极性带对比。根据极性界限用平均沉积速率外推,得到该剖面顶界年龄为约11.6MaB.P.,底界年龄为约18.5MaB.P.。糜子湾剖面位于已发表的QAⅠ中新世黄土剖面以东约30km,两剖面相同时代的地层在岩性地层结构和磁化率曲线波动上具有高度的一致性,表明中新世黄土-古土壤序列与第四纪黄土-古土壤序列一样,地层在空间上有很好的可对比性。文章的结果为从空间角度研究中新世环境变化行为提供了新剖面的年代地层框架。

 The study of the variations of C₃/C₄ relative abundance in terrestrial ecosystem since the Late Miocene has been  the “hotspot” of international community of geosciences during the past decades. In the Chinese Loess Plateau, several profiles have been studied to get the knowledge of past C₃/C₄ variations since the Late Miocene in this area. However, stable carbon isotopic data of pedogenic carbonate from this area have many inconsistencies in different aspects, such as: the beginning time of C₄ expansion, the process of C₄ expansion, the consistency of regional and global C₄ expansion, and spatial gradient of C₄ relative abundance and the main driven factor of C₄ expansion in this area. This may reveal the internal unreliability of carbon isotopic composition of pedogenic carbonate as an indicator of past C₃/C₄ variations in this area. At the mean time, carbon isotopes of animal fossil from Linxia Basin of this area are remarkably different with carbon isotopes of pedogenic carbonate, which indicated that C₄ plants became a significant fraction of local ecosystemm, which only existed in the Quaternary. Considering the reliability and timescale resolution of proxy indicator, we suggest that studies on organic matters should be reinforced in ongoing works in the future, in order to make the fact and driven mechanics of terrestrial C₃/C₄variations since the Late Miocene in the Chinese Loess Plateau clear.

晚中新世以来陆地生态系统中C₃/C₄植物相对丰度的演化历史是国际地学界关心的热点问题。我国黄土高原地区也有数个剖面开展了相关的研究，然而基于土壤碳酸盐碳同位素的研究结果在C₄植物扩张开始的时间、过程、区域和全球一致性、C₄植物相对丰度的空间梯度等主要的地质历史事实以及该区域C₄植物扩张的主要驱动因素等方面的认识均存在较大的差异，这可能表明了代用指标本身存在的问题。而来自动物化石的碳同位素研究结果与碳酸盐碳同位素研究结果同样差异比较大，认为直到第四纪时期，C₄植物才成为区域植被当中一个重要组分。从代用指标可靠性以及时间分辨率2个方面考虑，建议在后续的研究工作中，大力加强有机质碳同位素方面的研究，进一步查明黄土高原地区晚中新世以来C₄植物扩张的历史和可能的驱动机制。

On the basis of a newly-constructed record of magnetic susceptibility (SUS) and the depositional rate change of eolian loess-red clay sequences in the last 7.2 Ma BP from the Loess Plateau, together with a comparison of a record of @18O values from the equatorial East Pacific Ocean and eolian Quartz flux variations from the North Pacific Ocean, the evolutionary process of the Late Cenozoic Great Glaciation in the Northern Hemisphere can be divided into three stages: the arrival stage around 7 .2-3 .4 Ma BP, the initial stage at about 3 .4-2.6 Ma BP, and the Great Ice Age since 2.6 Ma BP. The evolution of the East Asian monsoon is characterized by paired winter and summer monsoons , and it is basically composed of the initial stage of weak winter and summer monsoons, the transitional stage of simultaneous increase in intensity of winter and summer monsoons, and the prevailing stage of strong winter and weak summer monsoons, or weak winter and strong summer monsoons. The Late Cenozoic global tectonic uplift, particularly the Qinghai-Xizang Plateau uplift and the associated C0₂ concentration variation, controls the cooling processes of the onset of Great Glaciation and the long-term changes of East Asian monsoom climate in the Northern Hemisphere to a large extent. The accelerating uplift of the Qinghai-Xizang Plateau between 3.4 and 2.6 Ma BP provided an important driving force to global climiatic change.