Western Chinese Loess Plateau Research Articles

Optically stimulated luminescence ages for human occupation during the penultimate glaciation in the western Loess Plateau of China

ABSTRACTThe chronology of few Palaeolithic sites in the Loess Plateau of China has been well investigated for the Middle Pleistocene. In this study, the Yangshang Palaeolithic site with significant archaeological remains, located in Gansu Province of the western Chinese Loess Plateau, was dated using blue‐stimulated optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) techniques on eight medium‐grained (45–63 µm) quartz and polymineral samples through the Palaeolithic sequence. Except for one sample from the upper cultural layer (55 ± 3 ka), age estimates obtained by quartz OSL dating exceed its upper datable limit, and so an elevated temperature post‐IR IRSL SAR protocol was used on 45–63 μm polymineral grains to derive a numerical chronology for the site. The luminescence ages of seven samples from six cultural layers lie between 104 ± 5 and 220 ± 11 ka; for the three main cultural layers the results varied from 149 ± 9 to 186 ± 10 ka, consistent with the presently observed stratigraphy. The dating results shown that early hominins may have occupied this region in the western Loess Plateau of China during Marine Isotope Stage (MIS) 7, early glacial MIS 6 and early interglacial MIS 5, reflecting their ability to adapt to variable environments.

A warmer but drier Marine Isotope Stage 11 during the past 650 ka as revealed by the thickest loess on the western Chinese Loess Plateau

Marine Isotope Stage 11 (MIS 11; ca. 423–362 ka) is generally considered to be the best analogue for the present interglacial (Holocene), and investigation of it will improve our understanding of current climate variability and assist in predictions of future climate change. However, many recent studies primarily focus on the structure and duration of MIS 11. Little research has focused on climate warmth and stability recorded in the Chinese loess-paleosol sequences (LPS) during the S4 paleosol formation (equivalent to MIS 11). On the basis of previous work, this study presents a high-resolution record (ca. 75 a/cm) that spans from MIS 1 to MIS 15, as preserved in the thickest known Jingyuan loess section on the western Chinese Loess Plateau (CLP). This LPS is almost 165 m thick and was sampled from the upper part of L6 to the modern soil at 2-cm depth intervals. Measurements of magnetic susceptibility, mean grain size and >63 µm particle content, carbonate content, total organic carbon, and soil color of samples were made to reconstruct the paleoclimate variation, and a grain-size age model was used to constrain the chronological framework. The primary results show that a generally warm-humid climate dominated the S4 paleosol development, but the climate condition was extremely unstable during the whole of MIS 11. Two obviously different climate regimes controlled the MIS 11 climate variation: the early part of MIS 11 was extremely warm and stable, but the latter part was relatively cool (non-glacial) and unstable. This climate pattern was consistent with records on the central CLP and wavelet analysis suggested that it was forced by the 65°N insolation variability modulated by a quasi-100-ka cycle. In addition, a multi-proxy comparative study on the climate conditions during S0 to S5 paleosol development indicates that the period of S4 development might be the warmest interglacial of the past 650 ka. However, the climate condition during S4 development was not the most humid episode as recorded in Xifeng and Luochuan loess sections on the central CLP. On the contrary, it was drier than both the MIS 15 and the present interglacial on the western CLP, which is somewhat similar to the present climate pattern on the central CLP.

Magnetostratigraphic age and monsoonal evolution recorded by the thickest Quaternary loess deposit of the Lanzhou region, western Chinese Loess Plateau

The loess-paleosol sequences of the Chinese Loess Plateau (CLP) are major paleoclimatic archives which document the evolution of the East Asian Monsoon (EAM) and changes in the Northern Hemisphere ice sheets during the Quaternary glacial-interglacial cycles. However, the mechanisms regulating the trend of EAM variations on a tectonic scale are unclear. The loess deposits of the western CLP, which have a close relationship with tectonics and climate, are much better-suited to exploring these mechanisms than those of the central CLP. However, studies of long-term EAM evolution from the western CLP have been hindered by the lack of long, accurately-dated sequences with high sediment accumulation rates. Here, we address this problem via high resolution magnetostratigraphic, magnetic susceptibility and grain-size analyses of a 416.2 m-long drill core located at Xijin Village, near Lanzhou. Paleomagnetic dating indicates that the basal age of the Xijin loess is ∼2.2 Ma. The χ and grain-size records reveal that the East Asian Summer Monsoon (EASM) and East Asian Winter Monsoon (EAWM) strengthened synchronously at ∼1.24 Ma. Subsequently, during interglacial periods, the EASM began to penetrate, and then dominate, in the Lanzhou region. This was followed by two stepwise uptrends, commencing at ∼0.87 and ∼0.62 Ma, which resulted in an increasingly moist interglacial climate in the region. We suggest that the uplift of the Tibetan Plateau was largely responsible for these three stepwise enhancements of the EASM. Overall, however, the long-term trend of strengthening in EAWM in the area may have been primarily caused by long-term global cooling from the Late Pliocene onwards.

Variations in the provenance of the late Neogene Red Clay deposits in northern China

The voluminous loess-Red Clay deposits in northern China forming part of the Chinese Loess Plateau (CLP) are valuable terrestrial archives of climatic evolution for the late Cenozoic Era. Fundamental in reconstructing the late Miocene and Pliocene wind patterns and aridification history is a detailed knowledge of the provenance of these deposits. This paper provides end member modelling of bulk grain-size distributions and U–Pb dating of detrital zircons for three distant Red Clay sequences in the northeastern (Baode), southern (Lantian) and western (Dongwan) CLP. Data show that these different sections each display a distinctive compositional structure indicating variable depositional processes, but they also share two significant zircon age populations of 200–300 Ma and 400–500 Ma. While the Permian–Triassic (200–300 Ma) group accounts for a larger proportion of zircons' ages in the northeastern (NE) CLP, the Ordovician–Silurian (400–500 Ma) component is dominant in the southern and western CLP. It is suggested that the Red Clay in the southern and western CLP was mainly derived from the Northern Tibetan Plateau (NTP) and the Taklimakan desert by low-level westerly winds. Samples of the NE CLP show an increased signature of sediments transported by near-surface northwesterly winds from the broad area of the Central Asian Orogen Belt (CAOB). This spatial transport and deposition pattern is supported by the results from the backtrace trajectory modelling of the dominant dust transport pathways in the CLP. It is noted that the Red Clay sample of around 3.6 Ma obtained from the NE CLP shows increased detrital contributions from its west, possibly indicating an intensified westerly wind strength and/or aridity of the NTP and Taklimakan desert due to the uplift of the Tibetan Plateau and Tianshan Mountains in the Pliocene. The onset of enhanced drainage of the Yellow River caused by the increased denudation of the NETP since 3.6 Ma could also have contributed to this.

Geochemical evidence for seasonal variations in potential loess sources of the western Chinese Loess Plateau

This paper aims to characterize seasonal variations in the potential sources of loess deposited on the western Chinese Loess Plateau (CLP). Fallout was sampled every five days between March 2013 and August 2014 at Jiuzhoutai, the site with the largest loess deposits in northwest China. A total of 45 macro and trace elements, as well as rare earth elements, were analyzed in the samples. The results show that, at present, the potential sources of loess are mixed with pollutants. After exclusion of the pollutants, principal component analysis (PCA) showed significant seasonal variations in the potential loess sources on the western CLP. During the spring, summer–autumn, and winter periods, there are possibly six, four, and two potential loess sources, respectively, with crustal provenance at Jiuzhoutai. No significant differences in fallout amounts were found on the western CLP during periods with and without dust events, which suggests that the frequency of dust events may not be an effective index for deposition rates in areas of loess formation. The relatively high proportion of coarser material (>50 μm in diameter) in the fallout indicates that at least part of the loess in the fallout samples originated from adjacent deserts and associated systems, and that both dry and wet deposition contributed equally to the fallout. Based on the significant seasonal and spatial variations in the loess sources, combined with variations in the link between the different particle size classes and the geochemical characteristics of potential components of the loess on the western CLP, the use of the geochemical characteristics of loess stratigraphic sequences as a proxy for regional paleoclimatic and environmental reconstructions should be carefully appraised.

Miocene climate change on the Chinese Loess Plateau: Possible links to the growth of the northern Tibetan Plateau and global cooling

AbstractThe evolution of the Asian monsoon‐arid environmental system during the Cenozoic was closely related to the growth of the Himalayan‐Tibetan Plateau and global climate change. However, due to inconsistencies in paleoclimatic reconstructions and to various constraints on the timing of the growth of the Tibetan Plateau, the relative impacts of regional uplift and global cooling on Asian climate change remain controversial. Here we investigate the mineralogical composition of a Miocene Red Clay deposit on the western Chinese Loess Plateau in order to infer changes in chemical weathering and monsoon intensity. Variations of four mineralogical ratios (chlorite/quartz, illite/quartz, calcite/quartz, and protodolomite/quartz) reveal that the summer monsoon intensity was relatively strong during the early Miocene (23.5–18.5 Ma), weakened gradually until ∼9.5 Ma, and strengthened again in the late Miocene. We synthesized previously published thermochronological data from the northeastern Tibetan Plateau and surrounding mountains, and the results suggest that two phases of the rapid growth of northern Tibet occurred around 24–17 and 13–7 Ma. Comparison of paleoclimatic proxies and thermochronological data suggests that the gradual weakening of the summer monsoon intensity from 18.5 to 9.5 Ma paralleled global cooling, whereas two intervals of strengthened monsoon in the early and late Miocene were possibly related to the rapid growth of northern Tibet. Our combination of paleoenvironmental proxies and thermochronological data reveals possible links between Miocene Asian monsoon evolution, phased growth of the Tibetan Plateau, and global climate change, and confirms the interconnection of geodynamic and atmospheric processes in the geological past.

East Asian monsoon evolution during the Eemian, as recorded in the western Chinese Loess Plateau

We present a high resolution environmental magnetic record of the East Asian summer monsoon spanning paleosol S1, corresponding to Marine Oxygen Isotope (MIS) 5, from the Jiuzhoutai (JZT) loess section in Lanzhou, on the northwestern edge of the Chinese Loess Plateau. We use a chronology based on a sediment grain-size age model frequently used for Chinese loess. The Eemian is dated to 127.37 – 121.31 ka BP and can be divided into three sub-stages based on variations in summer monsoon intensity: early Eemian (127.37 – 125.67 ka BP) with an intermediate summer monsoon intensity; mid Eemian (125.67 – 124.24 ka BP) with weakest summer monsoon intensity, and late Eemian (124.24 – 121.31 ka BP) with the strongest summer monsoon intensity. Contrasting, the winter monsoon weakened at the beginning of MIS5e, reached its weakest intensity during the late Eemian (∼121 ka BP), and then commenced an irreversible strengthening trend. Furthermore, the Eemian exhibits a similar climatic trend to the Holocene optimum, since both are divided into two parts by an intervening arid interval and the most humid period occurs in the secondary pedogenic stage. Therefore we suggest that the climate evolution during Eemian and later may provide useful information about climate forecast in the future.

Stable isotopic investigations of modern and charred foxtail millet and the implications for environmental archaeological reconstruction in the western Chinese Loess Plateau

Stable isotopic analysis of carbon and nitrogen in human and faunal remains has been widely used to reconstruct prehistoric diets and environmental changes. Isotopic analysis of plant remains allows for a more extensive consideration of paleodiets and can potentially provide information about the environment in which the crops were grown. This paper reports the results of δ13C and δ15N analyses performed on modern and charred archaeological foxtail millet samples collected from the western part of the Chinese Loess Plateau. The δ13C mean value of modern samples is lower than that of ancient samples. There is a significant difference between grain and leaf δ15N values. These results challenge the standard assumption in isotope studies that the nitrogen isotope signals of the different part of plants consumed by humans and animals are the same. The 3–5‰ difference between human and animal δ15N values is always regarded as an indicator of whether human diets contained considerable animal protein. The difference between grain and leaf δ15N values makes this assumption problematic in a foxtail millet-dominated society.

Deciphering magnetoclimatological patterns of late Early to early Middle Pleistocene loess–paleosol sequences in the western Chinese Loess Plateau

A detailed environmental magnetic investigation in conjunction with geochemical, bulk grain size and redness analyses of late Early to early Middle Pleistocene loess–paleosol sequences at the Jiuzhoutai and Caotan loess sections has been carried out, aimed to explore the reliability of magnetic parameters as paleoclimatic proxies in the western Chinese Loess Plateau (CLP). The results show that magnetic enhancement of early- to mid-Pleistocene loess–paleosol sequences at both sites is not solely due to the neoformation of ultrafine magnetic particles through pedogenesis, but in the case of several loess layers (e.g., L8 and L9) is more attributed to the significant input of coarse-grained magnetic minerals associated with the advance of desert margin and/or with significant changes in local sediment input. Since the magnetoclimatological signals studied here are obviously controlled by both pedogenesis and wind vigor, measurements of magnetic susceptibility (χ) and saturation isothermal remanent magnetization (SIRM), both of which are responsive to both ultrafine pedogenic magnetic particles and eolian coarse-grained pseudo-single domain/multi-domain magnetic particles, could not provide unambiguous interpretation of the magnetic response to the East Asian summer monsoon intensity. In contrast, anhysteretic remanent magnetization (ARM) and the percentage of frequency-dependent magnetic susceptibility (χfd%), which are notably responsive to ultrafine magnetic particles, can aptly trace changes in the concentration of pedogenic magnetic particles and can therefore be used as reliable proxies of pedogenic intensity. The magnetic grain size dependent proxies, e.g., ARM/χ and ARM/Ms (Ms, saturation magnetization), geochemical indices (e.g., chemical index of alteration, Zr/Rb and Al/Si ratios), and bulk grain size records all exhibit concordant variations and are more straightforward proxies for addressing the East Asian monsoon variability. Therefore the combination of multi-parameter mineral magnetic, geochemical and grain size analyses is very useful for distinguishing, delimiting and correlating loess sequences, and for accurately deciphering their paleoclimatic signals embedded in the weakly weathered loess. As the classical precipitation-driven pedogenic model for magnetic susceptibility enhancement is not entirely applicable to the early- to mid-Pleistocene loess developed in the semi-arid western CLP, considerable caution should also be warranted in using magnetic susceptibility measurements alone in the quantitative reconstruction of paleoprecipitation.

Astronomical and glacial forcing of East Asian summer monsoon variability

The dynamics of glacial–interglacial monsoon variability can be attributed to orbitally induced changes in summer insolation and internal boundary conditions. However, the relative impacts of astronomical and internal factors on East Asian summer monsoon variability remain controversial. Here we combine proxy data and model results to evaluate the response of East Asian summer monsoon change to these forcings. δ13C of loess carbonate, a sensitive summer monsoon proxy from the western Chinese Loess Plateau, demonstrates coexistence of distinct 100-, 41- and 23-ka periods, in contrast to precession-dominated speleothem δ18O records in South China. Model results indicate that insolation, ice and CO2 have distinct impacts on summer precipitation changes in East Asia, whereas their relative impacts are spatially different, with a relatively stronger insolation effect in south China and a more dominant ice/CO2 influence in north China. Combined proxy data and model results indicate that East Asian summer monsoon variability was induced by integrated effects of summer insolation and changing boundary conditions (e.g., ice sheets and CO2 concentration). Our proxy-model comparison further suggests that gradual weakening of the summer monsoon related to slowly decreasing summer insolation at astronomical timescales will be likely overwhelmed by the projected ongoing anthropogenic CO2 emissions.

The Matuyama–Brunhes polarity reversal in four Chinese loess records: high-fidelity recording of geomagnetic field behavior or a less than reliable chronostratigraphic marker?

The paleomagnetic recording fidelity of Chinese loess has been a matter of long-term dispute. To help shed light on the acquisition/retention mechanisms of natural remanent magnetization in loess, we report on high-sampling-resolution paleomagnetic constraints on the position of the Matuyama–Brunhes Boundary (MBB) in four typical loess sections in different marginal areas of the Chinese Loess Plateau (CLP), each of which has experienced different East Asian winter/summer monsoon strengths and pedogenic intensities. Detailed thermal demagnetization experiments for 1850 specimens reveal that none of the high-accumulation-rate loess with the weakest pedogenesis at Caotan (western CLP), the massive Pedogenesis loess characterized by extensive development of thick calcareous nodule layers at Jixian (SE CLP), and the moderately pedogenized loess at Fanshan (NE CLP) have preserved high-fidelity geomagnetic records across the MBBs. Jiuzhoutai (western CLP), one of the thickest loess profiles in the world, contains the thickest, and hitherto the most complex MBB that is too extensive to be regarded as rapid fluctuations of geomagnetic field during the M/B reversal. At Jixian, a predominance of pedogenesis-induced chemical remanent magnetization overprinting has heavily obscured the original remanence. The significant downward displacement of the MBB at Fanshan and even its total absence at Caotan, together with extensive remagnetization of the coarse-grained ‘upper sandy loess layer’ (L9) in both sections, further suggest that the processes of remanence acquisition/retention of Chinese loess in the marginal areas of the CLP are rather complicated. Thus, interpretations of the fine structure of geomagnetic transitional field behavior and ‘excursions’, especially in the Matuyama Chron, should be regarded with caution.

Vegetation and climate changes in the western Chinese Loess Plateau since the Last Glacial Maximum

Pollen analysis was conducted for loess deposits from three sites in the western Chinese Loess Plateau, i.e. the loess area west of the Liupan Mountains. Results show that during the Last Glacial Maximum (LGM), in-situ vegetation was dominated by Artemisia and some drought–tolerant species such as Echinops-type, Chenopodiaceae, Nitraria, and Ephedra, while coniferous forest (mainly Picea) flourished in nearby river valleys. During the Holocene Optimum, Picea almost disappeared, and Echinops-type, Chenopodiaceae, Nitraria and Ephedra decreased; vegetation was characterized by Artemisia, Taraxacum-type, Polygonaceae and Leguminosae, implying the climate was warmer and wetter than during the LGM. During the late Holocene, Chenopodiaceae, indicator of human–managed habitats, increased in the study area, indicating enhanced human activity.The climate was warmer and more humid in the loess areas east of the Liupan Mountains than in the west during both the LGM and Holocene Optimum. Likewise, a significant difference in specific plant types was observed between the east and west since the LGM. During the LGM, Pinus and some broadleaf trees emerged, but no Picea forest grew, while in the west, vegetation was characterized by desert shrub and desert steppe in situ, and by dark coniferous forests (mainly Picea) in nearby river valleys. During the Holocene Optimum, treeline advanced upward as a result of increased temperature. Picea thus withdrew from the western loess areas. Therefore, temperature is the major factor controlling the growth of Picea in the Chinese Loess Plateau.

A theoretical model on the relation between wind speed and grain size in dust transportation and its paleoclimatic implications

In studies of paleoclimate and paleoenvironment based on loess-palaeosol deposits, the rigorous relations between climate elements and the properties of loessic sediment are scarce because the dynamic processes controlled by the related physical laws are often ignored or arbitrarily simplified. In the current study, the horizontal distance of a suspended dust particle is re-estimated using classical mechanics and the relation between wind speed and the grain size of primary loess is then established. The actual application of this new theoretical relation is given by an example of Duowa profile, western Chinese Loess Plateau. The numerical results imply that the Qaidam Basin could be a likely dust source of the Chinese Loess Plateau.

Reliability of radiocarbon dating on various fractions of loess-soil sequence for Dadiwan section in the western Chinese Loess Plateau

The accurate radiocarbon dating of loess-soil sequences plays an essential role in the reconstruction of the environmental and climatic changes in continental settings during the last glaciation and Holocene. However, our knowledge about the reliability of radiocarbon ages of various fractions of soil and loess samples is still insufficient. Here, we present our study results on radiocarbon ages based on bulk organic matter, humin fraction, and carbonate of samples collected from a loess-paleosol section in the western Chinese Loess Plateau. We compare these observations with the optically stimulated luminescence ages and charcoal radiocarbon ages to evaluate the reliability of these fractions. We observed that the radiocarbon ages of humin fraction are very close to those of charcoal and are consistent with the optically stimulated luminescence ages within the experimental errors. We observed a significant deviation in the radiocarbon ages of carbonate and bulk organic matter from those of charcoal and optically stimulated luminescence ages, likely due to the dilution of these fractions during the pedogenetic process. Our results reveal that, except for charcoal, the humin fraction may yield reliable 14C ages for the Chinese loess-soil sequence.

Late miocene-pliocene paleoclimatic evolution documented by terrestrial mollusk populations in the western Chinese Loess Plateau.

The Neogene eolian deposits in the Chinese Loess Plateau (CLP) are one of the most useful continental deposits for understanding climatic changes. To decipher Late Neogene paleoclimatic changes in the CLP, we present a terrestrial mollusk record spanning the time interval between 7.1 and 3.5 Ma from the western CLP. The results indicate four stages of paleoclimatic evolution: From 7.1 to 6.2 Ma, cold and dry climatic conditions prevailed as evidenced by high values of the total number of cold-aridiphilous (CA) mollusk species and by low values of all of the thermo-humidiphilous (TH) mollusk indices. From 6.2 to 5.4 Ma, the climate remained cold and dry but was not quite as dry as during the preceding phase, as indicated by the dominance of CA mollusks and more TH species and individuals. From 5.4 to 4.4 Ma, a warm and moist climate prevailed, as indicated by high values of the TH species and individuals and by the sparsity of CA species and individuals. From 4.4 to 3.5 Ma, all of the CA indices increased significantly and maintained high values; all of the TH indices exhibit high values from 4.4 to 4.0 Ma, an abrupt decrease from 4.0 Ma and a further increase from 3.7 Ma. The CA species of Cathaica pulveraticula, Cathaica schensiensis, and Pupopsis retrodens are only identified in this stage, indicating that the CA species were diversified and that the climate was becoming drier. Moreover, the CA mollusk group exhibits considerable diversity from 7.1 to 5.4 Ma when a cold, dry climate prevailed; whereas the diversity of the TH group was high during the relatively warm, wet interval from 5.4 to 4.4 Ma. This indicates that variations in the diversity of the CA and TH mollusk groups were closely related to climatic changes during the Late Miocene to Pliocene.

Automated SEM–EDS heavy mineral analysis reveals no provenance shift between glacial loess and interglacial paleosol on the Chinese Loess Plateau

Understanding provenance of glacial loess and interbedded interglacial paleosol deposits on the Chinese Loess Plateau (CLP) is important to understanding paleo-atmospheric circulation patterns. However, great debate exists about whether Chinese loess and paleosol deposits have similar provenance both spatially and temporally. Here we report heavy mineral assemblage results of two sets of loess–paleosol couplets (L1 and S1; L4 and S4) from western CLP based on scanning electron microscopy with linked energy dispersive spectrometers (SEM–EDS) analysis (QEMSCAN@). The results demonstrate no apparent provenance shift on the CLP between glacial and interglacial periods. Heavy mineral assemblage comparison between the western and the central CLP reveals no provenance shift spatially, either. Finally, a comparison of heavy mineral assemblage between loess–paleosol and potential source regions reveals that the western Mu Us desert, the Badain Jaran desert, the Ulan Buh desert as well as the Yellow river are likely key dust contributors for the CLP. In contrast, the northeastern Mu Us desert are less likely key dust sources for the CLP. The ultimate provenance history of the CLP relies no doubt on integrated provenance studies. However, due to its rapidity and robustness, integrated provenance analysis should incorporate the QEMSCAN-based heavy mineral analysis.

How prehistoric humans use plant resources to adapt to environmental change: A case study in the western Chinese Loess Plateau during Qijia Period

Fossilized charcoals can document how prehistoric humans used wood resources. Here, we present a quantitative result of charcoal fossil records, compiled with archaeobotanical data of Qijia Culture (4.3–3.8 cal. kyr BP) in the western Chinese Loess Plateau, to study how prehistoric people used plant resources and adapted to their environment. The charcoal records showed that Quercus, Ulmus, Pyrus, Acer, Picea, Prunus, and Bambusoideae were abundant during the Qijia Period, implying a warmer and wetter environment at that time. It also clearly showed that Qijia people had a strong preference to select woods in their daily life. Quercus was the most favorable wood, and thus, it was found in all samples. Many fruit trees planted in the Chinese Loess Plateau today were found in charcoal records. These fruit trees were, during the Qijia Period, at least in the process of being domesticated. Qijia people expanded the scope of using plant resources, and many new crops appeared during that time. These results provide a new insight into economic management and human adaption strategies in the Neolithic Age.

High-resolution summer precipitation variations in the western Chinese Loess Plateau during the last glacial

We present a summer precipitation reconstruction for the last glacial (LG) on the western edge of the Chinese Loess Plateau (CLP) using a well-dated organic carbon isotopic dataset together with an independent modern process study results. Our results demonstrate that summer precipitation variations in the CLP during the LG were broadly correlated to the intensity of the Asian summer monsoon (ASM) as recorded by stalagmite oxygen isotopes from southern China. During the last deglaciation, the onset of the increase in temperatures at high latitudes in the Northern Hemisphere and decline in the intensity of the East Asia winter monsoon in mid latitudes was earlier than the increase in ASM intensity and our reconstructed summer precipitation in the western CLP. Quantitative reconstruction of a single paleoclimatic factor provides new insights and opportunities for further understanding of the paleoclimatic variations in monsoonal East Asia and their relation to the global climatic system.

Quartz OSL chronology and dust accumulation rate changes since the Last Glacial at Weinan on the southeastern Chinese Loess Plateau

The fine‐grained (4–11 μm) quartz Optically Stimulated Luminescence (OSL) dating technique was applied to the Weinan section on the southeastern Chinese Loess Plateau (CLP) with a high luminescence sampling resolution (10‐ and 20‐cm intervals). Fifty‐eight OSL ages, spanning c. 1–74 ka, were obtained for the 10‐m loess–palaeosol sequences. The reliability of the OSL dating and the constructed chronology was confirmed by comparing the OSL ages with independent dates from other studies and by correlation with palaeoclimatic time series. The closely spaced OSL ages at Weinan indicate that the mean dust accumulation rate (DAR) of L1–1 (MIS 2, 7.0±0.9 cm ka−1) is lower than those of L1–3 (MIS 4, 19.1±6.1 cm ka−1) and L1–2 (MIS 3, 16.0±0.7 cm ka−1) and that the mean DAR c. 30–20 ka ago (11.4±2.4 cm ka−1) is higher than that c. 20–10 ka ago (3.0±0.1 cm ka−1) in L1–1. The subsequent calculation of the mass accumulation rate (MAR) indicates that the MAR of L1–1 (107 g m−2 a−1) is much lower than those of L1–2 (247 g m−2 a−1) and L1–3 (307 g m−2 a−1). By comparing the mean DAR results during the Last Glacial at Weinan with those at a further six sites from other studies, we observed that the mean DAR of L1–2 is higher (lower) than that of L1–1 on the eastern (western) CLP, and that the mean DAR during MIS 2 has an evident transition from high to low at c. 20 ka on the entire CLP. Possible mechanisms for the above mean DAR changes at orbital and sub‐orbital time scales are presented.

Tetraether biomarker records from a loess-paleosol sequence in the western Chinese Loess Plateau

The ubiquitous occurrence of glycerol dialkyl glycerol tetraethers (GDGTs) in soils and their ability to record temperature and environmental changes offer the prospect of independently reconstructing continental paleotemperature and paleoenvironment from the loess-paleosol sequences (LPS) from the Chinese Loess Plateau (CLP). In this study we present records of GDGT-derived proxies for the last 70 kyr from the Yuanbao LPS, western CLP. Temperature record reconstructed from the cyclization and methylation index of branched tetraethers (MBT-CBT) displays that the onset of deglacial warming at ~20 kyr before present (BP) precedes the strengthening of summer monsoon at ~15 kyr BP, which is in agreement in timing with previous MBT-CBT temperature records from the southeastern CLP. The maximal deglacial warming of ~10°C is slightly higher than those in the southeastern CLP, perhaps due to the higher latitude and farther inland of the study site. The Branched and Isoprenoid Tetraether (BIT) index shows higher values (0.87–0.96 range, 0.93 average) in the glacial loess and lower values (0.76–0.91 range, 0.83 average) in the Holocene paleosols, with a steady decreasing trend since the early Holocene. The decreasing trend could suggest enhanced Thaumarchaeota relative to GDGT producing bacteria activity since the early Holocene, but other possibilities, such as preferential degradation of isoprenoid GDGTs or upward increase in living archaea relative to bacteria in the paleosol profile, cannot be fully excluded. Our results thus demonstrate the need of future study on microbial community structure in soil column and differential degradation of GDGT molecules.