Central Chinese Loess Plateau Research Articles

Multiple luminescence dating on heated materials at the nanzuo archaeological site, central Chinese Loess Plateau

Nanzuo is a key Neolithic archaeological site in the central Chinese Loess Plateau (CCLP). The site is important for understanding the origin and development of the Chinese civilization, yet, it lacks a robust chronological framework. The current work, for the first time, dates two different types of heated materials (pottery and burnt soil) from a profile near the main excavation of the Nanzuo archaeological site using various luminescence dating techniques. These techniques included single grain (SG) quartz OSL and K-feldspar post-infrared infrared-stimulated (pIRIR) dating for burnt soil samples and pottery, and pIRIR dating for pottery slices using the rock surface luminescence technique (RSL). The reliability of luminescence dating was investigated by comparing SG quartz, K-feldspar and pottery slice ages with radiocarbon (14C) ages obtained from the same cultural layer. The results show that the SG quartz OSL and K-feldspar pIRIR225 age estimates of the two coarse potteries are consistent with the 14C ages, however, the age estimates of the pottery slice (NZ22-CPS150) are significantly younger, which is primarily explained by underestimation in equivalent doses (De). Moreover, it is noted that while there is a considerable variation in the De of coarse pottery slices with depth, no significant correlation is observed between K- content and De. Heterogeneous K content was revealed by μ-XRF scanning on coarse pottery slices. Conversely, fine pottery displayed homogeneous K-content distribution and age estimates consistent with the 14C ages. Thus fine pottery slices are suggested to be more suitable for pIRIR dating relative to coarse pottery slices. The underestimation of De values in mixed minerals is observed as the primary reason for underestimated slice ages compared to SG K-feldspar, with grain size of K-feldspar and K content variation playing only a minor role. Therefore, by integrating the 14C ages with the luminescence ages of the pottery, it is inferred that human occupation of the Nanzuo site occurred around 5100 to 4500 years ago, which aligns with the 14C ages obtained from the main section of the Nanzuo site. The current work further concludes that the single-grain luminescence dating method is reliable for pottery. The study recommends to prioritize materials characterized by relatively uniform potassium (K) content for pIRIR dating using pottery slices.

Orbital- and millennial-scale Asian winter monsoon variability across the Pliocene–Pleistocene glacial intensification

Intensification of northern hemisphere glaciation (iNHG), ~2.7 million years ago (Ma), led to establishment of the Pleistocene to present-day bipolar icehouse state. Here we document evolution of orbital- and millennial-scale Asian winter monsoon (AWM) variability across the iNHG using a palaeomagnetically dated centennial-resolution grain size record between 3.6 and 1.9 Ma from a previously undescribed loess-palaeosol/red clay section on the central Chinese Loess Plateau. We find that the late Pliocene–early Pleistocene AWM was characterized by combined 41-kyr and ~100-kyr cycles, in response to ice volume and atmospheric CO2 forcing. Northern hemisphere ice sheet expansion, which was accompanied by an atmospheric CO2 concentration decline, substantially increased glacial AWM intensity and its orbitally oscillating amplitudes across the iNHG. Superposed on orbital variability, we find that millennial AWM intensity fluctuations persisted during both the warmer (higher-CO2) late Pliocene and colder (lower-CO2) early Pleistocene, in response to both external astronomical forcing and internal climate dynamics.

Unusual weakening trend of the East Asian winter monsoon during MIS 8 revealed by Chinese loess deposits and its implications for ice age dynamics

A fundamental aspect of marine benthic foraminiferal δ18O records of the last one million years is their “saw-tooth” pattern, characterized by gradual cooling to full glacial conditions followed by very rapid glacial terminations. Understanding the mechanism of this pattern is crucial for understanding ice age dynamics. We investigated the climatic trend within each glacial of the last 880 kyr by comparing the signal of the East Asian winter monsoon (EAWM) with global terrestrial and marine paleoclimate records. Our new grain size record from the Huining loess-paleosol section in the western Chinese Loess Plateau, together with published climate records from the Yimaguan and Luochuan sections in the central Chinese Loess Plateau, demonstrates a strengthening trend of the EAWM within most glacials, which largely mirrors the “saw-tooth” pattern of the benthic foraminiferal δ18O record. However, the EAWM record during Marine Isotope Stage (MIS) 8 shows an unusual weakening trend which is the exception over the last 880 kyr. Based on the linkages between Arctic ice sheets, the Siberian High Pressure system, and the EAWM, we propose that the weakening trend of the EAWM throughout MIS 8 reflects the overall shrinking of Arctic ice sheets towards the glacial termination, in contrast to the increasing trend of the benthic foraminiferal δ18O values. This conclusion is supported by a compilation of global paleoclimate records suggesting that MIS 8 was a unique glacial period over the last 900 kyr, characterized by a mild climate in middle to high northern latitudes and a cold climate in high southern latitudes, during late MIS 8. The inferred changes in Arctic ice sheets would alone cause a decreasing trend in benthic foraminiferal δ18O values during MIS 8, which is the opposite to the observed trend. We suggest that an increased ice volume in high southern latitudes led to the cooling of Antarctic Bottom Water and hence to a decrease in ocean bottom water temperature with an effect to increase benthic foraminiferal δ18O during MIS 8 as observed. Our results highlight the complexity of using the benthic δ18O signal alone as a paleoclimate proxy, and that MIS 8 is an important time window for better understanding glacial dynamics driven by changes in the polar regions of both hemispheres.

The centennial-resolution loess δDwax record indicates summer precipitation variations in the marginal region of the East Asian monsoon during the last deglaciation

Precipitation isotopic composition variations that are directly recorded in geological samples play an important role in understanding past hydroclimatic changes. In arid regions, the hydrogen isotopes of leaf wax (δDwax) in loess sections can provide records of summer precipitation isotopes. However, previous studies have primarily focused on the orbital scale, and there was still a lack of clear understanding of variations in precipitation isotopes on shorter timescales and their implications for climate change. In this study, we report centennial-resolution loess δDwax records from the central Chinese Loess Plateau (CLP) from the last deglaciation to the early Holocene. We compared the loess optically stimulated luminescence chronology and U-Th ages conveyed by linking stalagmite δ18O and loess δDwax to validate the δDwax method as a potential tool for assessing loess chronology on the suborbital scale. Our findings also indicated that regional precipitation isotopes experienced a depletion of approximately 24‰ for δD and around 3.2‰ for δ18O during the last deglaciation. The overall precipitation isotope amplitude in the central CLP was similar to that of typical East Asian monsoon regions and lower than that of the Indian monsoon and westerly regions. However, the precipitation isotope oscillations on the millennial scale in the central CLP were significantly weaker than those in eastern to central China. The finding affirmed that although the summer precipitation in the CLP was primarily influenced by the East Asian monsoon system at the orbital scale, the northern boundary of the summer rain belt remained a distance away from the interior of the CLP during the last deglaciation. As a result, the short-term oscillations of atmospheric circulation were not strongly reflected in the precipitation isotopes of the central CLP. This result highlighted spatial and temporal differences in precipitation isotope variability between the dominant and marginal regions of the East Asian monsoon system.

Transformation of lakebed into a major soluble-salt-bearing dust source for Asia since the mid-pleistocene

Loess in the Chinese Loess Plateau (CLP) preserves abundant information about the dust cycle over Asia. However, the variations of loess source and composition over time remain controversial. Soluble salt is an important dust component and is linked to the landforms and environments associated with dust mobilization, transport and deposition. Based on an investigation of the soluble salts of two loess sequences from the central and western CLP, we show that the soluble salts of Chinese loess mainly stepwise increased at ∼1 and ∼ 0.5 Ma. Several lines of evidence indicated that the increased supply of soluble salts mainly originated from dust input, suggesting the enhanced salinity of Asian dust. The enhanced soluble-salt-bearing dust was driven by lakebed evolution in the potential source areas, as confirmed by abundant geomorphological and sedimentological evidence. The Mid-Pleistocene Transition (associated with Tibetan Plateau uplift) induced aridity of the potential source areas (PSA) and Mid-Brunhes Transition triggered intensifying East Asian summer monsoon are responsible for the stepwise lakebed evolution in the PSA at ∼1 and ∼ 0.5 Ma, respectively, which in turn acted as an important soluble-salt-bearing dust reservoir. These findings provide new evidence for understanding the evolution of regional dust emissions and their possible environmental effects in the past and future.

Eccentricity-paced geomagnetic field and monsoon rainfall variations over the last 870 kyr

Whether there are links between geomagnetic field and Earth's orbital parameters remains unclear. Synchronous reconstructions of parallel long-term quantitative geomagnetic field and climate change records are rare. Here, we present 10Be-derived changes of both geomagnetic field and Asian monsoon (AM) rainfall over the last 870kyr from the Xifeng loess-paleosol sequence on the central Chinese Loess Plateau. The10BeGM flux (a proxy for geomagnetic field-induced 10Be production rate) reveals 13consecutive geomagnetic excursions in the Brunhes chron, which are synchronized with the global records, providing key time markers for Chinese loess-paleosol sequences. The10Be-derived rainfall exhibits distinct ~100 kyr glacial-interglacial cycles, and superimposed precessional (~23 kyr) cycles that match with those in Chinese speleothem δ18O record. We find that changes in the geomagnetic field and AM rainfall share a common ~100 kyr cyclicity, implying a likely eccentricity modulation of both the geomagnetic field and climate.

Proximal Supply in Loess Deposition of the Chinese Loess Plateau

AbstractPrevious studies suggest that the northeastern Tibetan Plateau has been the major source area for the Chinese Loess Plateau (CLP); this contribution was used to prove that the Yellow River played a dominant role in transporting the materials from the northeastern Tibetan Plateau, making them available for the CLP. Accordingly, the proximal North China Craton is considered to have increased supply potential of dust to the CLP further eastward. However, the origin of the eastern CLP and the relationship between loess and proximal river sediments have never been systematically studied, which prevents us from working out the surface process between source and sink. In this study, we conducted U–Pb dating and analyses of the grain morphologies of detrital zircons and heavy mineral assemblages on the upper Quaternary loess and riverbed sediments on the eastern and central CLP. We also built a data set (n = 48,161) of the zircon U–Pb ages of the Quaternary loess, deserts, and potential source areas. Our results show that most loess samples on the eastern CLP were strongly affected by the North China Craton. The dust supply ability of rivers to loess is very limited. Along the dust transport path, the proximal mountains, rivers and loess can all provide dust for loess deposition. Our findings emphasize the important role proximal supply has played in the evolution of the CLP since at least the late Neogene, for example, the northeastern Tibetan Plateau for the western and central CLP, and the North China Craton for the eastern CLP.

Ice Volume and Insolation Forcing of Abrupt Strengthening of East Asian Winter Monsoon During Glacial Inceptions

AbstractIt is generally accepted that the glacial‐interglacial variations of the East Asian winter monsoon (EAWM) are controlled by the volume of the Northern Hemisphere ice sheets (NHIS), but the fact that they exhibit different evolution patterns during glacial inceptions is often overlooked. By generating an independent chronology framework and integrating multi‐proxy records from the loess sections on the central Chinese Loess Plateau, here we show that the rapid intensifying of the EAWM during glacial inceptions reflects millennial perturbations in the large‐scale atmospheric circulation in East Asia in response to insolation‐triggered abrupt North Atlantic cooling. This climate teleconnection between North Atlantic and East Asia is found to be particularly effective only when the NHIS reaches a critical large size. Our integrated multi‐proxy records thus highlight the key role of ice volume in modulating the response of the EAWM to insolation‐triggered North Atlantic cooling during the interglacial‐glacial transitions.

100 k.y. pacing of the East Asian summer monsoon over the past five glacial cycles inferred from land snails

Abstract The effects of orbital forcing on the East Asian summer monsoon (EASM) after the mid-Pleistocene transition are controversial. Chinese cave δ18O records only show low-latitude ∼20 k.y. cycles, while pedogenic proxy records from Chinese loess are dominated by highlatitude 100 k.y. cycles. This discrepancy may result from the multicomponent origin of proxies, particularly for pedogenic signals in loess deposits, where the primary climatic signals are modified by pedogenic smoothing, leaching, and changes in sedimentation rate, and the latter are also being forced by 100 k.y. cycles. We present an EASM record spanning the past 470 k.y. from the central Chinese Loess Plateau based on the δ13C values of land snail shells (δ13Cshell), which eliminates the influence of the above processes and exclusively records the local past EASM precipitation. The δ13Cshell record is dominated by the 100 k.y. cycle, with more depleted values during interglacials compared to glacials. At the end of marine isotope stage (MIS) 11, δ13Cshell-based precipitation remained at an interglacial level following the MIS 11 super-interglacial climate in the Northern Hemisphere, although a glacial period had commenced on a global basis. Overall, our δ13Cshell record is highly coupled with high-northern-latitude ice-volume variations, possibly supporting the high-latitude forcing of the EASM.

Stratigraphy and chronology of two newly discovered Early Pleistocene Palaeolithic sites in the Hanjiang River Valley, Central China

Numerous open air Palaeolithic and hominin fossil sites have been discovered in the Qinling Mountain Range (QMR) in central China. However, a small number have been confirmed as dating to the Early Pleistocene. The present study introduces stratigraphic and chronological studies of the newly discovered Guanmenyan Palaeolithic site, Danjiangkou Basin, and Yuelianghu Palaeolithic site, Yunxian Basin along the Hanjiang River Valley, in the southern QMR. The artefacts recovered from Guanmenyan and Yuelianghu are consistent with Early Palaeolithic assemblages found at other localities in the region. Based on magnetostratigraphy and correlation with the loess-palaeosol sequence from the central Chinese Loess Plateau, our dating results show both open-air sites can be dated to the Early Pleistocene. Guanmenyan is dated ∼0.787–0.819 Ma (L8), while the upper and lower Palaeolithic cultural layers of Yuelianghu are dated ∼0.819–0.865 Ma (S8) and ∼ 0.943–989 Ma (S9), respectively. Thus Guanmenyan and Yuelianghu help to fill a chronological gap in the Palaeolithic record of Hanjiang River Valley and establish the QMR as a major region documenting this important period of hominin evolution in China.

Quartz luminescence sensitivity variation in the Chinese loess deposits: the potential role of wildfires

AbstractThe sensitivity of optically stimulated luminescence (OSL) of quartz grains has been applied to trace the provenances of sediments in various environments. Here, we studied the OSL sensitivity of quartz grains in four sections (Jingyuan, Huanxian, Xifeng and Luochuan) along a west–east transect on the Chinese Loess Plateau (CLP). The quartz OSL sensitivity in the palaeosol units is higher overall than that in the loess units, and it shows an increasing trend from the west CLP to the east CLP. In palaeosol units, it increases by a factor of ~20 along the west–east transect; while in loess units it increases by a smaller degree of 3–5. The change of provenances and the change of distances between the provenances and the loess sections cannot fully account for the large degree of sensitivity variation between the loess and palaeosol units in the central and eastern CLP. We propose that wildfires may have played an important role in enhancing the quartz OSL sensitivity in the palaeosol units in the central and eastern CLP. Quartz OSL sensitivity has the potential to be used as a proxy for palaeowildfires.

An absolutely dated record of climate change over the last three glacial–interglacial cycles from Chinese loess deposits

Abstract Chinese loess-paleosol sequences have long been regarded as the continental counterpart of deep-sea sediments in terms of their record of the Quaternary glacial–interglacial cycles. However, absolute chronologies for loess-paleosol sequences older than ca. 130 ka on the Chinese Loess Plateau are scarce. We conducted the first high-resolution luminescence dating, extending back to 350 ka, of the classical Luochuan loess section of the central Chinese Loess Plateau. Bacon age-depth modeling of the luminescence ages was used to obtain an age framework. The results indicate that on orbital timescales, loess accumulation at Luochuan was continuous over the last three glacial–interglacial cycles. The new age framework resolves discrepancies between orbitally tuned age models at several boundaries and provides new evidence supporting the reassignment of the Marine Isotope Stage 8/9 boundary age from 300 ka to ca. 280 ka. Our results also suggest that published astronomical timescales for the loess deposits of the Chinese Loess Plateau over the last ~2.6 m.y. are probably broadly reliable, as are the related regional and global climatic interpretations.

Genesis of Loess Particles on the Chinese Loess Plateau

AbstractThe sources of the sediments in distinct areas of the Chinese Loess Plateau (CLP) are still disputed, starting with the particle formation in the original desert areas, as well as the neighboring river systems. The formation of loess particles in the southern CLP was investigated using Scanning Electron Microscopy (SEM) and combined material modal composite analysis. Here, we determined the mineralogy in the Duanjiapo (DJP) section on the southernmost CLP and the last glacial‐interglacial samples on the central and north CLP. The results reveal that the samples plot along the N‐S transect, which is very likely the result of differential weathering. The SEM observations of the internal textures of soil aggregates from DJP suggest that precipitation mediated dissolution led to the formation of the fine particle fraction (5–10 μm). The most dissolution was observed for the hornblende particles. The further evolution of these smaller particles followed a decrease (2.6–0 Ma) in the grain size up section. Our record of the changes in and trend of the hornblende concentrations suggests an imbalance in the budget of the hornblende influx via sub‐aerosol wind transport versus the mineral dissolution, which provides robust evidence for a sediment provenance in the deserts north and west of CLP. Understanding the sediment source is critical to trace Asian monsoon evolution. The hornblende record suggests a coeval change in the trends of the summer monsoon and winter monsoon, and this contemporaneous change may not support the recently proposed Pacific Walker Circulation mechanism in the interpretation of Asian monsoon.

Multiproxy records of temperature, precipitation and vegetation on the central Chinese Loess Plateau over the past 200,000 years

The strength of the East Asian Summer Monsoon (EASM) and associated moisture availability have been linked to the spatial distribution of occurrence of C3 and C4 vegetation on the Chinese Loess Plateau (CLP). Variations in the stable carbon isotopic composition of organic matter in loess-paleosol sequences from several locations on the CLP indicate that vegetation has shifted from mainly C3 plants during cool, dry glacials to more C4 plants during warm, wet interglacials. Although increased temperatures generally lead to an expansion of C4 vegetation, increased humidity has an opposite effect, as does increasing pCO2, leaving the exact driver(s) of vegetation change, and thus EASM strength, elusive. Here we reconstruct continuous, directly comparable records of air temperature, monsoon precipitation, and vegetation type over the past 200,000 years based on branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids, plant leaf waxes and their isotopic stable carbon and hydrogen isotopic composition (δ13Cwax and δ2Hwax, respectively) preserved at Lingtai on the central CLP. BrGDGT-based temperatures vary between 12 and 21 °C over glacial-interglacial cycles, and consistently lead changes in loess proxies (grain size, magnetic susceptibility), as also observed elsewhere on the CLP. Variations in δ13Cwax are only minor (<2‰) and indicate that C3 plants have continuously dominated at Lingtai, in contrast to a nearby section where C4 plants flourished during interglacials. This difference can be explained by the site elevation (∼1300 m above sea level), resulting in air temperatures too low for widespread C4 vegetation occurrence. Instead, variations in the average chain length of the leaf waxes coinciding with large shifts in δ2Hwax (∼40‰) during glacial-interglacial transitions suggest that changes in rainfall source and seasonality may have pushed C3 woody vegetation present during glacials to more non-woody C3 vegetation during interglacials, as a result of differences in respective water-use efficiency. Our multiproxy records thus indicate that subtle changes in the C3 species composition can be used to reveal variations in EASM precipitation dynamics in elevated areas where temperature exerts a first order control on the vegetation type.

Relative pollen productivities of major woody plant taxa in deciduous broadleaved forest in the Ziwuling Mountains of the central Chinese Loess Plateau

Accurate estimates of relative pollen productivity (RPP) values are fundamental to the use of fossil pollen records to quantitatively reconstruct paleovegetation. Based on a pollen dataset from moss polster and related vegetation data for 34 sites in the central part of the Ziwuling Mountains (central Chinese Loess Plateau, with a temperate–warm temperate climate), we estimated the relevant source area of pollen (RSAP) and the RPP values of 8 major woody plant taxa in deciduous broadleaved forest, using the Extended R-Value (ERV) model. The best results were obtained with ERV sub-model 2 and the inverse distance (1/d) method to distance weight the vegetation data. The RSAP was estimated to be 855 m and the rank order of the RPP values for the 8 major woody plant taxa is as follows: Betula (2.083 ± 0.091), Syringa (1.367 ± 0.087), Quercus (1 ± 0, the reference taxon), Pinus (0.791 ± 0.036), Ulmus (0.643 ± 0.062), Carpinus (0.621 ± 0.122), Malus (0.035 ± 0.015), and Acer (0.035 ± 0.025). Repeatability tests using the leave-one-out and K-fold methods showed that the RPP values of all 8 major woody plant taxa have a good reproducibility, which indicates that the RPP values are consistent in the study area. Comparison of the RPP values of the major plant taxa from deciduous broadleaved forest in temperate and warm temperate regions shows that the RPP values for the same pollen taxon may differ significantly between different areas. For example, the RPP for Pinus in areas with dominantly natural vegetation is much lower than for areas where the vegetation has been substantially altered by human activity. We suggest that differences in vegetation/landscape characteristics (including community structure and species composition/abundance) are the major causes of the difference. Therefore, changes in the regional landscape should be considered when choosing suitable RPP values for quantitative paleovegetation reconstruction.

Hydraulic modeling of water flow in the thick vadose zone under precipitation

Data from field monitoring and artificial rainfall experiments suggest that the thick vadose zone can be divided into two sub-zones based on soil water variation, namely the active and steady zones. The soil water content of the top active zone (2–5 m depth) is sensitive to precipitation and evaporation and dominated by transient water flow. Soil water content of the underlying steady zone remains constant over time and there is a steady flow under the force of gravity. However, since the transition from transient flow to steady flow is difficult to observe in nature, the physical mechanism of this transition remains poorly understood. This study establishes a hydraulic model to visually demonstrate water flow in the entire vadose zone under multiple infiltration events. The model comprises of a series vertically aligned water tanks, each with a small outlet at the bottom, and each representing a soil unit. The water level in a tank represents the water content and the related permeability of the soil unit. The results of an experiment conducted with the model clearly show that transient flow in the upper active zone will transfer to steady flow. A zoomed out data with an annual rainfall record at a site in the central Chinese Loess Plateau is applied in the model to simulate the water content and the flow state of the vertical profile, and the results are in accordance with in-situ monitoring data. The outcomes of this study suggest that although water content in the steady zone remains unchanged, there is a constant steady flow seeping downward through the zone, acting as a typical source of groundwater recharge in the loess region.

Asynchronized erosion effects due to climate and human activities on the central Chinese Loess Plateau during the Anthropocene and its implications for future soil and water management

AbstractVariations and regional differences in land surface processes during the Anthropocene have directly impacted local economic development and social stability. Nevertheless, little is known about the historical erosion effects and human–environment interactions at annual and decadal scales on the central Chinese Loess Plateau (CLP) over the past 200 years, when human activities and annual sediment discharge both reached historical peaks. Located in the northcentral hilly–gully and southcentral plateau–gully regions, respectively, the Jingbian and Heshui landslide‐dammed reservoirs were ideal settings to reconstruct the erosion flux and precipitation over A.D. 1850–2000 due to vertical stacking of interannual couplets. During this period, the mean annual specific sediment yield reached 1.17 × 104 t/(km2 a) in the Jingbian catchment (precipitation threshold of 30 mm), while it reached 1.13 × 104 t/(km2 a) in the Heshui catchment (precipitation threshold of 40 mm) according to sediment yield with frustum and extrapolation of historical precipitation. Interannual El Niño–Southern Oscillation (ENSO) events were deemed a primary climatic factor inducing soil erosion over the central CLP. This should remind decision‐makers that it is necessary to prevent large‐scale floods and severe erosion before future ENSO events. Changes in population, policies, and human activities also affected land surface processes on the central CLP during 1850–2000. In addition, the results revealed that abandoned farmlands in the southcentral regions with 600 mm and more precipitation had a strong ecological resilience, while this was weak in the northcentral drier region. These findings imply that future ecological restoration of the central CLP should consider the ecological resilience of farmlands that have been recently abandoned due to urbanization. These novel results provide new insights into regional soil and water management under China's development agenda of ‘Ecological Conservation and High‐quality Development of the Yellow River Basin’.

Climate influence on zinc isotope variations in a loess–paleosol sequence of the Chinese Loess Plateau

Zinc (Zn) is an essential micronutrient, and its isotopes are increasingly applied to understand trace metal biogeochemical cycling. Atmospheric deposition from anthropogenic or natural origins can perturbate the nutrient supply globally. However, our understanding on Zn isotope variations in natural mineral dust and during dust–climate interaction is still rare. This study presents Zn isotope data and leaching experiments with ammonium acetate buffering solution (pH = 5) on a loess–paleosol profile (S0–L3) from the Yimaguan section in the central Chinese Loess Plateau. The loess–paleosol sequence records the contributions from Asian mineral dust accumulation and chemical weathering, and the response to glacial–interglacial climate changes. Bulk loess–paleosol samples display limited Zn/Al (3.5 × 10-4 – 4.5 × 10-4) and δ66Zn variations (0.17–0.30‰). The mean δ66Zn of paleosol (0.19 ± 0.01‰, 2SD/√n, n = 11) is slightly lower than that of loess (0.24 ± 0.01‰, 2SD/√n, n = 13). Correlations with magnetic susceptibility and grain size distribution suggest that bulk δ66Zn variations between loess and paleosol are not sensitive to glacial–interglacial source fluctuations or wind sorting, but could result from loess pedogenesis.The leachable Zn fractions have high δ66Zn values (0.80–1.29‰) and display good positive correlations with Ca, Sr, Mg, Fe and Mn (r > 0.7), suggesting that carbonates are enriched in heavy Zn isotopes. Although Zn contents in carbonates are low (<3%) and dissolution of carbonates alone cannot explain the bulk Zn/Al variations (∼7% of Zn loss), significant carbonate weathering and biotic activities during loess pedogenesis could modify soil conditions (pH) and provide dissolved bicarbonate ions and organic ligands, which may facilitate the transport of heavy Zn isotopes by soil liquids. Conversely, the bulk δ66Zn of loess developed during glacial times were not modified by the low degree of chemical weathering, and thus the mean value of 0.24‰ for loess can be regarded as the average composition of the upper continental crust.

Geochemical characteristics of Holocene loess-paleosol sequences in central Chinese Loess Plateau and their implications for East Asian monsoon evolution

The geochemical composition and magnetic susceptibility (MS) of three sections (Weinan, Jingchuan and Jingbian) in the central Chinese Loess Plateau, located along a north-south climate gradient, were analyzed at high resolution in order to reconstruct changes in weathering intensity and hence Asian monsoon evolution during the Holocene. The results show distinct differences in element composition between the loess and paleosol layer, as well as spatial differences which reflect changes in mean annual temperature and annual precipitation along the climatic gradient. The measured major element compositions, combined with the results of previous research, indicate that the provenance of the Holocene loess-paleosol sequences in the central Chinese Loess Plateau was constant over time and space. The degree of chemical weathering in the study area is relatively weak and at an intermediate weathering stage. Based on a magnetic susceptibility age model, the East Asian summer monsoon changes recorded by the loess-paleosol sequences show a distinct two-stage pattern of evolution, with a gradual increase during 13.1–8.5 ka and a gradual decrease thereafter. Comparison of the results with climate records from elsewhere indicates that the East Asian monsoon evolution was mainly controlled by changes in Northern Hemisphere summer insolation and ice sheets.

Decoupling of the detrital linkage between proximal dunefields and early and middle Pleistocene accumulation in the Chinese Loess Plateau: evidence from the Badain Jaran and Tengger sandy deserts

The dunefields upwind of the Chinese Loess Plateau (CLP) are widely regarded as the major source of the sediments of the CLP. To test this inference, we conducted a source tracing study using U–Pb age spectra of detrital zircons from the central Badain Jaran Desert (BJD). The results reveal that since 1.1 Ma the sediments in the central BJD have originated predominantly from the Gobi-Altay Mountains (GAMs), with a dominant Northern Tibetan Plateau (NTP) provenance at ∼0.7 Ma and at the present. Together with a previous study of the neighboring Tengger Desert (TD), our results support the conclusion that during 2.35–0.62 Ma the central parts of the proximal dunefields of the BJD and TD acted as reservoirs of sediments deflated mainly from strata with a GAMs-origin, with the addition of NTP-derived sediments after 0.36 Ma. Comparison of the original provenance areas of sediments in the central parts of the BJD and TD with the loess deposits in the central and southern CLP indicates that the proximal dunefields of the BJD and TD were not the dominant sources of dust transported to the CLP during early to middle Quaternary. Based on our results, combined with evidence from previous studies, we propose that NTP-derived sediments dominated accumulation in the CLP for most of the Quaternary, while during 2.35–0.62 Ma the proximal dunefields, including the BJD and TD, acted only as reservoirs of sediments deflated and eroded predominantly from the GAMs. Essentially, these sandy deserts functioned as transfer stations, which, after ∼0.36 Ma, received increased amounts of NTP-derived sediments and subsequently supplied silt to downwind areas, including the CLP.