Variations in the sensitivity of the optically stimulated luminescence from quartz in the Xifeng section: A spatiotemporal comparison across the Chinese Loess Plateau since the last interglaciation

The sensitivity of the optically stimulated luminescence (OSL) from quartz is thought to be mainly influenced by source rock (crystallization temperature and quartz defect types) and depositional history (irradiation/bleaching cycles). In this study, the OSL sensitivity of quartz of different grain sizes from loess (L1) and paleosol (S1) layers at the Xifeng section of the Chinese Loess Plateau was analyzed. The OSL sensitivity of the samples from the Xifeng section shows a distinct difference between glacial and interglacial cycles, which is consistent with the patterns observed from magnetic susceptibility (MS) and grain sizes. The highest OSL sensitivity is observed in S1, which is twelve times the lowest sensitivity observed in L1. Moreover, the contribution of the fast component of quartz OSL from the paleosol accounts for more than 90% of the total OSL signal. In contrast, the middle and slow components contribute more in loess, and a significant variation among different aliquots was observed.Comparison of the data obtained from loess sections from different regions, including Xifeng, Luochuan, Jingyuan and Shimao, show significant differences in the magnitudes of OSL sensitivity but a similar pattern between glacial and interglacial periods. The results of this study suggest that, except for the wildfire model proposed to interpret the high OSL sensitivity in paleosol (Zhang, 2018), the shift of wind pattern of westerly wind (i.e., transporting “dim” source from North Tibetan Plateau and northwesterly wind (i.e., transporting “bright” source from Alxa Arid Lands and the subsequent mixture from the different sources may partially contribute to the different OSL sensitivities observed for Xifeng and Luochuan. In contrast, the OSL sensitivity of Shimao samples from L1 was affected by the input from the Mu Us Desert, and those of Jingyuan may affected by the contribution from the Yellow River.     

六盘山东麓晚更新世以来黄土粒度指示的物源特征及演变

在宁夏长城塬完整的晚更新世-全新世黄土-古土壤剖面(长城塬剖面)进行高分辨率采样,对125个样品的粒度数据做了粒度端元分析,并对各端元组分含量在地层深度尺度上做了小波分析。结果认为：端元分析法可以把代表4种不同动力的沉积端元分离出来。端元1可能代表了西风带控制下的由高空气流搬运的远源粉尘物质;端元2可能代表了沉积物沉积后在东亚夏季风作用下的风化以及成壤改造作用;端元3可能代表了后期的淋溶作用动力特征;端元4可能代表了东亚冬季风作用下典型的风成黄土粗颗粒组分特征。根据小波分析认为各端元组分含量在不同的深度上均存在着准周期变化,指示了相对应的时间尺度上古季风变化具有准周期性的特征,同时也说明了沉积剖面一直处于较为连续沉积状态,比较完整地记载了各沉积动力过程。各端元组分在不同的时间尺度上具有不同的准周期性特征。     

黄土碳酸盐碳同位素环境意义讨论

黄土碳酸盐碳同位素广泛应用于第四纪气候环境变化的研究中,以往研究中多利用钙结核、次生碳酸盐或成壤碳酸盐,认为其反映了C4植物的丰度.黄土高原碳酸盐碳同位素表现为黄土层高,古土壤层中低,即黄土层中C4植物丰度高于古土壤层.然而,这样的结果和黄土有机碳同位素得到的结果矛盾,有机碳同位素的结果表明温度对C4植物的分布起到了决...     

河南晚更新世—全新世黄土记录的环境变化及物源特征

基于河南新郑县的一套完整晚更新世—全新世黄土古土壤剖面(格大张剖面),应用端元分析模型,对75个沉积样品的粒度分析数据进行分析,并对各端元组分在地层深度对应的时间尺度上做了小波变换。结果认为,从格大张剖面沉积物粒级组分中可以分离出三个沉积端元,端元1可能指示了东亚夏季风作用动力特征,夏季风的强弱引起的温湿变化造成相应时期内沉积物的古土壤化的强弱;端元2可能指示了东亚冬季风作用下沙尘暴沉积动力作用特征,代表了典型风成黄土的粗颗粒组分特征;端元3粒级组分更粗,代表了更强搬运动力条件,应属于东北风搬运黄泛平原粗颗粒而来的近源沉积物,距今3100 a以来尤为显著。根据小波分析结果,认为各端元组分在不同的时间尺度上具有周期性特点。端元分析法在指示沉积物沉积动力环境和物源特征上具有很好的效果,各端元组合特征能够敏感地反映出沉积动力组合特征,很好地反映晚更新世以来各动力变化特征。采用小波分析方法对识别各端元组的周期性特征以及根据目前状态推断未来可能的趋势具有参考意义。     

Vertical variations of luminescence sensitivity of quartz grains from loess/paleosol of Luochuan section in the central Chinese Loess Plateau since the last interglacial

The provenance of loess in Chinese Loess Plateau, including origin, transport pathways and source areas, has long been one of the most important questions. In this study, the vertical variations of the luminescence sensitivity of quartz grains from the central Chinese Loess Plateau were investigated by using optically stimulated luminescence (OSL) techniques. Our results indicate that the luminescence sensitivity of quartz grains of paleosols can be much higher than that of loess beds. In addition, the quartz grains from the loess-paleosol sequence exhibit a temporal trend in the strength of luminescence sensitivity, characterized by higher values in soils and lower values in loess beds. The OSL sensitivity of quartz grains of the loess-paleosol sequence also shows very similar trend to the magnetic susceptibility and particle size fluctuations, implying that the luminescence sensitivity might be climatic dependent. The possible factors affecting the variations of luminescence sensitivity were discussed including particle size, natural radioactivity, and the provenance of eolian deposits. We suggest that the temporal variations of luminescence sensitivity can be attributed to the retreat-advance of deserts, the different contributions of glacial origin quartz particles associated with mountain processes, and wind patterns during glacial/interglacial cycles. Therefore, the secular variations of luminescence sensitivity of quartz grains are ultimately influenced by past climatic change through its controlling on sediment provenance changes.     

Red Clay Sediment in the Central Chinese Loess Plateau and Its Implication for the Uplift of the Tibetan Plateau

Introduction A set of reddish clay-silt-sized sediments named red clay underlying the Quaternary loess-paleosol sequence widely distribute in the Chinese Loess Plateau. The thickness of the red clay sediments ranges from decades of meters to over 100 m (Evan et al.1991,Mo and Derbyshire 1991, SUN et al. 1997&1998, DING et al. 1999, GUO et al. 2001, QIANG et al. 2001). Previous studies show that not only loess-paleosol (e.g.LIU et al.1985,AN et al.1990,DING et al.1992),but also red …     

高分辨率洛川剖面黄土磁性地层学

洛川剖面作为经典的黄土剖面,磁极性地层界线的具体位置还不清楚.通过对洛川黄土-古土壤序列以10 cm间隔的古地磁研究,厘定了磁极性界线的具体位置：（1）MB (Matuyama-Brunhes) 界线位于53 m处,S8的上部.（2）Jaramillo位于67.8~72.5 m,上界在S₁₀的上部,其下界位于L₁₃的顶部.（3）Olduvai位于107.2~114.2 m,上界位于S₂₅的顶部,相当于WS₂的上部,其下界位于L₂₉的上部,相当于WL₂的上部.4）并未检测到Matuyama的底界,可能位于红粘土内.     

Variability of carbonate pedofeatures in a loess-paleosol sequence and their use for paleoreconstructions

Carbonate pedofeatures were studied in details in a loess-paleosol pedocomplex near Kursk, in the central part of the European Plain. The soils studied included a modern Chernozem and five Pleistocene paleosols. Carbonate morphologies and distributions were described in the field. Various morphologies were sampled for further studies including micromorphology in thin sections and submicroscopic studies under a scanning electron microscope. The complex approach of investigation at macro-, micro-, and submicroscopic levels found that most of carbonate pedofeatures are secondary and multiphase. Many of them are related to cryogenic features. The general trend was of decreasing carbonate content in the older paleosols except for increased content in the 1St and 3rd paleosols. Most of carbonate morphologies are similar in the modern soil and paleosols, but pseudomycelium was found only in the modern Chernozem, while soft masses and impregnations are associated with the paleosols. Detailed study on carbonate attributes revealed their specific features in the modern soil and in paleosols. Veins have dissolution features under the modern environment and seem to reprecipitate in form of pseudomycelium. Coatings in paleosols at a depth of the Ist paleosol include specific fine tubes. Powdery soft masses and impregnations in the paleosols are strongly related to cryogenic fissures and paleopermafrost level. Under SEM they consist of tiny crystals, much smaller than in regular soft masses. Several types of hard concretions were identified：rounded, irregular and platy. Small concretions in the modern Chernozem have similar morphology and similar 14C-age as the rounded concretions of the upper paleosols. They are expected to represent the same generation of concretions. Platy concretions fill the bottom part of the large fissures. The 14C-age of hard concretions varies from 3310±80 yrs. B.P in the Ist paleosol up to 20 400 yrs. B.P. in the 3^rd. As a result we found the reflection of several wet-dry and cold-warm stages in carbonate morphologies and distributions. High variability of carbonates at macro-, micro- and submicroscopic levels indicates several generations and multiphase formation of carbonate pedofeatures in the loess-paleosol pedocomplex. Carbonate pedofeatures in the examined pedocomplex are more informative for the study of late Pleistocene and Holocene climatic cycles rather than characterization of a single paleosol.     

Luminescence chronology of late pleistocene loess-paleosol and tephra sequences near Fairbanks, Alaska

Thermoluminescence (TL) and infrared-stimulated luminescence (IRSL) sediment-dating methods have been applied to paleosol- and tephra-bearing loess sequences younger than marine oxygen isotope stage (MIS) 7 at three important sites. TL ages indicate the development of significant paleosols ∼75,000 and ∼30,000 yr ago in the loess sequence at the Gold Hill site. Relatively minor soil development occurred ∼70,000 and ∼48,000 yr ago. Like the ∼75,000-yr-old soil, the 30,000-yr-old soil is apparently of global extent, and consistent in timing with inferred warm intervals elsewhere (e.g., Greenland, Europe, western and central China). At Birch Hill, replicate TL dating of primary loess combined with two earlier TL results from the same site, and with an earlier mean fission-track-glass-shard age of 140,000 ± 10,000 yr for the associated Old Crow tephra, yield a more precise numeric age of 142,300 ± 6600 yr for this Alaska/Yukon chronostratigraphic marker ash bed. Three of the TL ages at the Halfway House site are difficult to interpret, but combined with other evidence, they indicate: (1) the upper 5-6 m of loess from Halfway House is not part of the Gold Hill Loess (equivalent to pre-MIS 5 age) as long thought by T.L. Péwé, but rather is much younger; (2) the regionally significant variegated tephra, found in the Fairbanks and Klondike areas and previously thought to be older than MIS 5, has an age of 77,800 ± 4100 yr (late MIS 5).     

Holocene climate changes in westerly-dominated areas of central Asia: Evidence from optical dating of two loess sections in Tianshan Mountain,China

Optical dating was applied to two loess-paleosol sections (Lujiaowan and Shuixigou) from the northern piedmont of Tianshan Mountain, Xinjiang province, China. The two sections are over 200 km apart and have a similar depositional sequence, which consists of two paleosol layers embedded by one loess layer. Two difficulties were met in optical dating. First, because the sections are located on the slope of the mountain, it was found that some cliff debris, with coarse grains (>200 μm), were mixed with the eolian sediments by rainfall, especially in the paleosol layers. Second, the optically stimulated luminescence (OSL) signals of quartz grains from the deposits were too dim to obtain a reliable equivalent dose (D_e). The 63–90 μm K-feldspar grains were separated to decrease the debris portion, and they yielded bright infrared stimulated luminescence (IRSL) signals. A multiple-elevated-temperature post-IR IRSL (MET-pIRIR) procedure was applied to determine D_e. Comparing the optical dating ages of the two sections, the Lujiaowan (LJW) and Shuixigou (SXG) sections recorded almost the same depositional process during the Holocene. The ages of the two loess layers (2.44–3.38 ka at LJW; 2.47–4.36 ka at SXG) suggested that one drought event happened widely in this westerly dominated area. The same drought event 2.5–3.5 ka ago also happened in the Chinese Loess Plateau (CLP), where the summer monsoon dominated. However, the paleosol development period (6.6–4 ka) in the study area was distinguished from the monsoon dominated area (8–4 ka), which suggests an arid early Holocene in the westerly area.