Western Chinese Loess Plateau Research Articles

Contrasting orbital rhythms preserved in loess grain-size records across the Chinese Loess Plateau

The loess-paleosol sequences on the Chinese Loess Plateau (CLP) are among the best terrestrial archives for recording orbital-scale global paleoenvironmental and East Asian monsoon changes during the Quaternary Period. Dust provenance and climate patterns vary across the CLP due to its vast size. However, whether available climate proxies were influenced by varying signals from the different dust source areas remains unknown. Here we present time series analysis results of high-resolution grain-size records from four loess sections (Jingbian, Xifeng, Baoji, and Weinan sections) spanning a north to south transect in the eastern CLP across the past 0.7 Ma. By comparison with data from the previously reported Luochuan section in the eastern CLP, and the Gulang, Menyuan, Jingyuan, Lanzhou, and Linxia sections in the western CLP, it is revealed that the dominant orbital signal in grain size variations in the eastern CLP is the ∼100-kyr ice-age cycle, with precession only very weakly expressed. By contrast, western CLP records exhibit both ∼100-kyr and ∼ 20-kyr precession cycles. We show that this contrasting orbital patterns between the eastern and western CLP are likely to be influenced by the climate signals from the respective source regions. We propose that the grain size variations in the western CLP not only contain the ∼100-kyr ice-age related winter monsoon cycles but also precession cycles related to the mid-latitude Westerlies and the Tibetan Plateau. The grain size variations in the eastern CLP are, by contrast, mainly influenced by ∼100-kyr ice-age cycle-regulated winter monsoon changes. The spatial diversity of periodicity in loess grain-size records from the CLP suggests that caution should be taken when discussing the periodicities of loess records based on any single site.

Quaternary provenance changes in the loess deposits of the western Chinese Loess Plateau: Insights from U-Pb age spectra of detrital zircon

The western region of the Chinese Loess Plateau (CLP) on the western side of the Liupan Mountains connects the Tibetan Plateau with the deserts of the northern arid zones. This region is key for understanding of the coupled relationship between tectonic uplift and surface processes (erosion and climate) on the Tibetan Plateau; however, the variation of Quaternary wind-dust sources in the region remains uncertain, confounding analysis. In this paper, we conduct a source tracing investigation using detrital zircon U-Pb dating of 12 loess layers from the Huining aeolian sedimentary sequence of the western CLP, reconstruct the history of provenance changes during the Quaternary Period, and further explore driving mechanisms. Results show that aeolian dust from the western CLP is mainly from the northeastern margin of the Tibetan Plateau (NTP). At 1.8–1.66 Ma, the main source region was the NTP, which corresponds to the Episode C uplift event of the Tibetan Plateau. The dominant source area transitioned to the Gobi Altay Mountains (GAMs) around 1.5–1.4 Ma. This shift suggests that the tectonic uplift of the Tibetan Plateau had entered a period of stasis following the conclusion of Episode C, leading to increased transport of detrital material from the GAMs by the East Asian Winter Monsoon. After 1.24 Ma, the dominant source region abruptly changed from the GAMs to the NTP and lasted until 0.04 Ma. The topographic fluctuations caused by the Kunhuang Movement appears to have been the main reason for the sudden source change. Based on our results and evidence from previous studies, the source changes that occurred during the Quaternary Period were in response to a combination of tectonic events and climate change. Comparison of zircon age spectra between the eastern and western CLP further support the idea that the Plateau Winter Monsoon was the main transport system for aeolian dust in the western CLP.

Paleoclimatic significance of the organic carbon isotopes of the Tajikistan loess in arid Central Asia, on orbital timescales since the last interglacial

During the past few decades, the stable carbon isotope composition of bulk organic matter (δ13Corg) in loess sediments has been increasingly used for reconstructing changes in the terrestrial vegetation and climate in several regions, including Europe, Xinjiang (the eastern part of arid central Asia, ACA), and the Chinese Loess Plateau (CLP). However, previous research indicates significant differences in the climatic significance of δ13Corg between the loess-paleosol sequences of Europe and the western and eastern ACA, compared to the loess of the central and southern CLP in monsoonal Asia. The δ13Corg values of loess-paleosol sequences in western and eastern ACA on different timescales exhibit consistent patterns of variation, suggesting that loess δ13Corg in these areas may be influenced by similar climatic factors and have similar mechanisms. However, the lack of detailed δ13Corg studies in central ACA limits our understanding of their applicability to long loess sequences within this region. This limitation not only constrains our understanding of the paleoclimatic significance of loess δ13Corg in central ACA, but it also affects the practical application of loess δ13Corg as a reliable paleoclimate proxy in this region. Consequently, it impedes a comprehensive understanding and in-depth analysis of loess δ13Corg across the mid-latitude Eurasian continent. To address this deficiency, we conducted a δ13Corg study of the Darai Kalon (DK) loess section, in southern Tajikistan, central ACA, since the last interglacial, with the aim of exploring the environmental significance of loess δ13Corg and the origin of the climatic record of this section, on orbital timescales. The results show that the ecosystem in southern Tajikistan, and throughout ACA, was dominated by C3 vegetation since the last interglacial. Loess δ13Corg records can be used as a precipitation proxy in ACA, at least since the last interglacial. However, regional comparisons revealed negative correlations between loess δ13Corg values and pedogenic intensity in Europe, ACA, and the western CLP, which is the opposite to the positive correlations observed in the central and southern CLP. Comparison of the paleoclimate record of the DK section with those of loess-paleosol sequences from monsoonal Asia indicated that warm, humid conditions occurred during past interglacials (MIS5, MIS1) and interstadials (MIS3), and cold, arid conditions occurred during the glacial/stadial periods (MIS4, MIS2) in both ACA and monsoonal Asia. We propose that changes in Northern Hemisphere insolation were responsible for this in-phase pattern of climate change between these two regions on orbital timescales since the last interglacial.

Robust land surface temperature record for north China over the past 21,000 years.

Numerous proxy reconstructions have provided general insight into late Quaternary East Asian Monsoon variability. However, challenges persist in precisely assessing absolute temperature impacts on proxy variations. Here, we use two independent paleothermometers, based on bacterial membrane lipids and clumped isotopes of snail shells, in the same section of the western Chinese Loess Plateau to establish a robust land surface temperature record spanning the past approximately 21,000 years. Our independent temperature records consistently reveal (i) similar land surface temperatures between the Last Glacial Maximum and late Holocene and (ii) a gradual cooling Holocene, which contrasts with the climate model predictions. We propose that changes in soil moisture availability over the deglaciation modulates the land surface temperature recorded by the proxies. A land surface energy partitioning model confirms this mechanism, suggesting that effects of soil moisture availability should be properly considered when comparing proxy records with climate model outputs.

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.

Thermoluminescence dating of gypsum in loess deposits

Gypsum deposits within loess deposits are an important indicator of weathering processes and climate change. We conducted a thermoluminescence (TL) dating study of the gypsum from loess deposits of the Jiuzhoutai section in the western Chinese Loess Plateau. Determination of the equivalent doses (De) of gypsum samples was conducted using several protocols, including single-aliquot regenerative-dose (SAR-TL), multiple-aliquot additive dose (MAAD-TL), and Isothermal TL of SAR (SAR-ITL). The TL glow peak at 280 °C was targeted because of its reported stable signals. The test dose check, estimation of sensitivity changes, and the dose recovery test were applied to assess the applicability of the different protocols, and the characteristics of the thermoluminescence signals for gypsum were evaluated. The De results for the SAR-TL, MAAD-TL, and SAR-ITL protocols were within the error ranges. Among the three protocols, SAR-ITL did not involve high-temperature heating and it provided a more accurate De value, and thus it was used for De determinations for gypsum dating. With a dose rate of 1.74 ± 0.07 Gy/ka, the minimum TL age of the gypsum veins was ∼182 ± 7 ka. U-series dating was also applied to the same samples for comparison. However, the extremely low ratio of 230Th/232Th (≤4 × 10−6) of the gypsum veins deposited within the loess resulted in a large error of the U-series dates. We conclude that TL dating is more appropriate for the dating of gypsum in loess deposits.

Goethite in the Red Clay sequences on the western Chinese Loess Plateau and its responses to the middle Miocene Climate Transition

The middle Miocene Climate Transition (MMCT, ca. 14 Ma), with a sharp drop in temperature, is a key transition period in the Earth’s climatic and biotic evolution. However, our knowledge of how arid-monsoon environments were coupled during the middle Miocene Climate transition remains limited, due to the lack of robust proxies from well-dated sedimentary sequences covering the MMCT in monsoon areas. Goethite is an abundant iron oxide in soils, which is sensitive to persistent wet and cold environments. However, aluminium (Al) substitution is ubiquitous in the pedogenic process, and the Al substitution for goethite in the Red Clay sequence is unclear. In this study, we combined the rock magnetic and DRS methods to assess the Al substitution level of goethite and quantify the goethite content in the Zhuanglang (ZL) Red Clay deposits from the western Chinese Loess Plateau. Our results show that Al substitution in the ZL Red Clay sequence is relatively stable, facilitating the quantification of goethite using this approach. Goethite content displays an increasing trend from approximately 0.2% at 18 Ma to 0.5% at 14 Ma, and then gradually decreased towards the late Miocene in the ZL Red Clay deposits. The maximum goethite production at around 14 Ma indicates a cold and wet climate condition during MMCO at this location. Together with other palaeoenvironmental proxies of this sequences and other sedimentary sequences in the Tianshui Basin, we suggest that climate conditions during the MMCT in the monsoonal western CLP were relatively cold and wet, which was different from the cold and dry climate in the central Asian. Global cooling and the uplift of the Tibetan Plateau might be predominantly responsible for the cold and wet climate during the MMCT.

Variability of East Asian Summer Monsoon precipitation during the Early Pleistocene based on pollen analysis from the western Loess Plateau, China

Understanding Early Pleistocene variability in precipitation associated with the East Asian Summer Monsoon (EASM) may help improve understanding of the effects of future global warming on this system. In this study, a quantitative EASM precipitation record with ∼2.7 kyr resolution was reconstructed for the Early Pleistocene (∼2.2–1.7 Ma) based on a pollen record from a loess-paleosol sequence in Lanzhou Basin, on the western Chinese Loess Plateau (CLP). The results show that the average annual precipitation (Pann) value was ∼505 mm, almost ∼1.6 times as much as today (∼310 mm), implying much stronger EASM intensity from ∼2.2–1.7 Ma. The EASM experienced a period of stronger intensity from ∼2.2 to 2.07 Ma, with Pann up to ∼525 mm and vegetation dominated by forest-steppe, as well as a period of weaker intensity between ∼2.07 and 1.99 Ma with lower average Pann (∼481 mm) and steppe dominated vegetation. A gradual intensification followed from ∼1.99 to 1.7 Ma, with increasing Pann (∼503 mm) accompanied by a significant increase in arboreal abundance. Our analysis suggests that the west to east temperature gradients across the Equatorial Pacific region influenced the Early Pleistocene EASM evolution over geological timescales. This finding differs from the traditional view that high-latitude ice sheet fluctuations were the primary driving force of Quaternary EASM. On orbital timescales, the Pann exhibits a dominant ∼41 kyr obliquity periodicity between ∼2.2–1.99 Ma and a ∼18 kyr precession periodicity between ∼1.99–1.7 Ma, indicating that Northern Hemisphere ice sheet and low-latitude solar insolation were the dominant drivers of EASM precipitation cycles in these two periods.

Isotopic and magnetic proxies are good indicators of millennial-scale variability of the East Asian monsoon

Past East Asian Monsoon variability has been reconstructed using oxygen isotopes of cave speleothems as well as proxy indicators from Chinese loess sequences. However, where the speleothem record is dominated by precession cycles, loess magnetic susceptibility primarily shows a glacial-interglacial pattern. Here we generate a ~ 130,000 years high resolution record of plant wax hydrogen isotopes from a loess section on the western Chinese Loess Plateau that can directly be compared with both speleothem oxygen isotopes and with magnetic susceptibility from the same section. We find that variations in our plant wax hydrogen isotope record follow the precessional pattern of the speleothem record as opposed to the glacial-interglacial changes in magnetic susceptibility. We propose that hydrogen isotopes mainly record precipitation during the growing season, whereas magnetic susceptibility represents an annual climate signal, including precipitation and temperature. Our findings imply that summer vs annual climate variability is driven by distinct orbital forcings.

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.

Co-evolution of orbital-to-millennial East Asian monsoon and westerlies variability over the past 150 ka

Understanding the evolution and interaction of the mid-latitude westerlies and East Asian monsoon (EAM) is crucial for comprehending Asian climate dynamics. While abundant evidence from loess, lake and speleothem records in arid Central Asia (ACA) and East Asia has revealed the interplay between the mid-latitude westerlies and the EAM on orbital and millennial scales, most of these records only cover the period since the last glacial maximum. Thus, the trade-off relationship between the mid-latitude westerlies and the EAM on glacial-interglacial to millennial timescale since the last interglacial period remains unclear. In this study, we present 150 ka-long grain-size and organic carbon records from two loess sections in the western Chinese Loess Plateau (CLP) and Yili Basin to assess the wind intensity and moisture variability between East Asia and ACA at glacial-interglacial to millennial timescales. Our results indicate that the East Asian winter monsoon (EAWM) and westerly wind exhibit synchronous glacial-interglacial and precession-scale fluctuations. However, the moisture exhibits consistent variations between East Asia and ACA on glacial-interglacial timescale, whereas inconsistent pattern at precessional timescale. The moisture variations in ACA lagged those in the East Asia by 2 kyr during marine isotope stages (MIS)5. Millennial-scale wind and moisture variability in East Asia and ACA are broadly correlated with cooling events in the North Atlantic. Based on the modeling results of the TraCE-21 ka, we suggest that the wind and moisture variations in East Asia and ACA are controlled by the combined effects of ice sheets, insolation, and meltwater flux from orbital to millennial timescales. Therefore, our study provides new insights into the complex interplay between the EAM and mid-latitude westerlies, which can contribute to a better understanding of Asian climate dynamics.

Mineral magnetic variation of the minle loess/palaeosol sequence of the late glacial to holocene period in the northeastern Tibetan Plateau

SUMMARY Located at the modern junction of the Asian Summer Monsoon and the Westerlies, the northeastern Tibetan Plateau and western Chinese Loess Plateau are in distinct climatic setting from the middle and eastern Chinese Loess Plateau. Besides, Holocene climate variations near the Asian Summer Monsoon margin are highly debated with different models proposed. We carried out environmental magnetic analyses on the Minle loess section, with continuous deposition between 13.3–0.5 ka, located near this margin. Results show that the magnetic susceptibilities of palaeosols are dominantly contributed by superparamagnetic (SP) and single-domain (SD) maghemite and/or magnetite produced by pedogenesis. Limited changes in the content of haematite indicate that dust origin hardly influenced the magnetic mineral variations. The section is divided into four substages according to the changes in magnetic indices and provides a full scene of Holocene climatic evolutions. During the coldest and driest Late Glacial (13.3–11.7 ka), it displays the weakest pedogenesis, as suggested by the lowest SP and SD concentration. The strongest pedogenesis in the Minle section produced higher magnetic susceptibility than the loess records in the Chinese Loess Plateau during the early Holocene Optimum (11.7–8.1 ka). The subsequently declined summer insolation, equatorward migrated Westerlies and the weakened Asian Summer Monsoon led to weak pedogenesis during the middle Holocene (8.1–5.3 ka). Stronger pedogenesis during the late Holocene (5.3–0.5 ka) than the middle Holocene might be due to lower evapotranspiration from weaker insolation and/or to the enhanced winter Westerlies. We attribute the spatiotemporal differences in climatic records of the Chinese loess to precipitation, effective humidity and aeolian geomorphological processes.

Rapid retreat of the East Asian summer monsoon on the western Chinese loess Plateau during the middle to late Holocene and its environmental and societal implications

Spatiotemporal changes in the Holocene climate of the arid region of China were principally caused by shifts in the intensity and location of the East Asian summer monsoon (EASM). However, our knowledge of these EASM variations is limited, especially since the middle Holocene. We investigated two aeolian sedimentary sections (the Linxia/LX and Gulang/GL sections) on the western Chinese Loess Plateau (CLP), to explore the variations in the intensity and location of the EASM and their impact on the regional environment and human cultural development during the middle to late Holocene. The longitudes of these two loess sections are similar, but the GL section is located on the northern edge of the CLP, in the eastern Hexi Corridor, and it is situated further north than the LX section. The chronologies of both sections are based on accelerator mass spectrometry 14C dating, and environmental magnetic parameters and grain size end-members were used to reconstruct their moisture histories. The results suggest that the climate at the LX section was relatively dry before 3.5 ka, after which it became wetter. In contrast, at the GL section, the climate was also relatively dry before 3.5 ka, with the wettest conditions occurring during 3.5–2 ka, but subsequently there was an abrupt shift to a dry climate. We suggest that these spatiotemporal patterns reflect the westward movement and rapid southward retreat of the EASM and the linked monsoon rainfall belt. TraCE-21 ka climate simulation results support our inference. We also found that, compared with other ancient cultures in the eastern Hexi Corrido, sites of the Shajing culture, dated to ∼2.4–2.8 ka, are predominantly located in areas that are arid at the present-day, and relatively distant from rivers. We suggest that this distribution pattern reflected the occurrence of a humid climate during the period of the Shajing culture, thus demonstrating the impact of the EASM on cultural development in this region.

Weathering Intensity Response to Climate Change on Decadal Scales: A Record of Rb/Sr Ratios from Chaonaqiu Lake Sediments, Western Chinese Loess Plateau

The Rb/Sr ratio of lake sediments has been widely adopted as an indicator of weathering intensity in studies of past climate change, but the geochemical significance of this ratio varies with timescale. Here, we present Rb/Sr data for the past 300 years for sediments collected from Chaonaqiu Lake in the Liupan Mountains of the western Chinese Loess Plateau as a decadal-scale record of weathering intensity. To validate the application of this weathering proxy, we correlated the record with those of other major elements, rock-forming minerals, and paleoclimatic proxies. We found that Rb/Sr ratios are influenced mainly by Sr activity within the lake catchment (where Sr is likely sourced from albite). In addition, higher (lower) Rb/Sr ratios of bulk sediments from Chaonaqiu Lake are correlated with lower (higher) fractions of terrigenous detritus (SiO2, Ti, K2O, Al2O3, and Na2O). These indicate that the Rb/Sr ratios of bulk sediments in Chaonaqiu Lake are closely linked to terrigenous detritus input on decadal scales and also correlate well with TOC (a precipitation indicator) and other high-resolution paleoclimate records (e.g., tree rings and drought/flood index) in neighboring regions, with higher (lower) Rb/Sr ratios corresponding to more (less) precipitation. Lake bulk sediment Rb/Sr ratios are dominated by the input of terrigenous detritus over decadal timescales. Our data show that physical and chemical weathering in the Chaonaqiu Lake watershed have opposing influences on Rb/Sr ratios of bulk sediment, competing to dominate these ratios of lake sediments over different timescales, with ratios reflecting the relative importance of the two types of weathering.

Seasonal climatic instability in the western Chinese Loess Plateau during Marine Isotope Stages 12–10

Because of similar astronomical background, Marine Isotope Stage (MIS) 11 is viewed as an analogue of the Holocene, but the evolution of seasonal climatic instability during MIS 11 has not been well investigated. Here we present a time series of land-snail eggs—a recently-developed proxy of seasonal cooling events—from the Chinese Loess Plateau (CLP) to investigate seasonal climatic instability during MIS 11 and adjacent glacials. Due to the impact of low temperatures on egg hatching, egg-abundance peaks document seasonal cooling events. A total of five egg-abundance peaks were recorded in the CLP during MIS 12, MIS 11 and MIS 10. Three peaks are strong and occur close to glacial inception or interglacial-to-glacial transition; two weaker peaks occur during MIS11. These peaks imply seasonal climatic instability intensifies mainly during glacial initiation or transition. All these events correspond to ice-sheet growth and the loss of ice-rafted debris at high northern latitudes. Moreover, they occurred at the minima of local spring insolation during the MIS 12 and MIS 10 glacials, but at the maxima during the MIS 11 interglacial. This may contribute to the difference in the intensity of seasonal cooling events between low-eccentricity glacials and interglacials. Our results provide new evidence for understanding low-eccentricity interglacial–glacial evolution.

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.

Tectonic Forcing of the Extreme Aridification of the East Asian Interior at Around 900 ka–Insights From the Spatially Inconsistent Magnetization of Chinese Loess

AbstractThe upper sandy loess unit L9 on the Chinese Loess Plateau (CLP) corresponds to marine isotope stages 22–24, and it represents aeolian deposition under conditions of extreme aridification. However, the forcing mechanism remains controversial. Numerous paleomagnetic studies in the eastern CLP show that the coarsest part of L9 is remagnetized and has a normal geomagnetic polarity. However, our results show that in loess sections in the western CLP the coarsest part of L9 records a primary reverse polarity. This spatially inconsistent magnetization pattern originates mainly from the different magnetic carriers of the characteristic remanent magnetization (hematite in the western CLP and magnetite in the eastern CLP), which suggests a different dust provenance between the western and eastern CLP. We ascribe this spatial contrast in dust provenance to the episodic uplift of the northeastern Tibetan Plateau, which also led to the extreme aridification of the East Asian interior at ∼900 ka.

Neogene global climate change and East Asian dust sources: Combined rutile geochemistry and zircon U-Pb analysis from the northern Chinese Loess Plateau

During the late Neogene, global climate underwent a long-term transition to cooler climates that culminated in the Quaternary icehouse conditions. In the East Asian terrestrial realm, atmospheric dustiness increased, and the volume of aeolian dust deposits expanded significantly. The Neogene Red Clay deposits, located on the Chinese Loess Plateau (CLP), provide an exceptional geologic archive to investigate pre-Quaternary climates and environments. Constraining the provenance of the Red Clay is crucial for unravelling the links between late Neogene Central-East Asian climate, tectonics and desertification. However, Red Clay provenance is highly debated and data are scarce. In this study, we have used a multi-proxy approach at a high sampling resolution to study the provenance of the c. 7–2.6 Ma Baode Red Clay. Our data consist of joint detrital zircon U-Pb ages and detrital rutile trace element geochemistry of the Baode Red Clay, and of a massive rutile geochemistry dataset from 14 potential dust source areas. The data indicate that the Mio-Pliocene and Plio-Pleistocene global climate transitions were coupled with gradual dust provenance shifts. We propose these shifts indicate the intensification of East Asian winter monsoon and/or enhanced Central-East Asian drying as a response to global cooling. We also identify temporary Pliocene provenance changes that interrupt the long-term winter monsoon -controlled dust transport at c. 4 Ma and c. 3.5 Ma. Several indicators point to dust transport by the westerly jet at c. 4 Ma, possibly caused by a shift in the jet position as a response to global cooling, and/or enhanced wet deposition caused by intensified summer monsoon. Alternatively, the role of the paleo-Yellow River should be investigated further. We infer the 3.5 Ma provenance signal was caused by increased Yellow River -transported material from eroding Northeastern Tibetan Plateau and/or western CLP. Implications of our results not only shed light on the links between late Cenozoic global climate change and Central-East Asian dust cycle, but also provide detailed information to further investigate the late Neogene regional geomorphology and its effects on dust emission, transport, and deposition. Our combined rutile-zircon analysis also verifies that a multi-proxy single-grain approach that targets different types of primary source rocks is needed for reliable provenance analysis of the CLP dust.

Dominant precessional forcing of the East Asian summer monsoon since 260 ka

Abstract One of the most perplexing problems in paleoclimate research is how orbital cyclicities force East Asian summer monsoon (EASM) precipitation variation over the middle to late Quaternary. Chinese loess records suggest that EASM precipitation was dominated by 100 k.y. cycles controlled by Northern Hemisphere ice sheet forcing. In contrast, speleothem records suggest that EASM precipitation was dominated by 23 k.y. cycles caused by Northern Hemisphere summer insolation forcing. In order to resolve this inconsistency, we present high-resolution paleoclimate records from Xijin drill cores on the western Chinese Loess Plateau for the past 260 k.y., the rough upper limit of luminescence dating. Magnetic susceptibility (χ) shows clear 23 k.y. precessional cycles over interglacials but has constant low values over glacials. This is interpreted as indicating a lack of pedogenesis, such that χ cannot record EASM precipitation variations, rather than an absence of EASM variation itself. To circumvent this issue, we use inversed sand content as an alternative proxy for EASM precipitation over glacials and splice this with the interglacial logarithmic χ from Xijin drill cores. This new record reveals dominant 23 k.y. cycles over both interglacials and glacials, consistent with speleothem δ18O data and dominant insolation forcing. These findings allow a consistent understanding of EASM variability and forcing mechanisms from both loess and speleothem archives, resolving one of the largest debates in past monsoon research. These results challenge suggestions of high-latitude ice sheet forcing of the EASM based on slowly accumulated loess records from the central Loess Plateau.

Provenance of Quaternary aeolian silts in western China and its spatial difference with source of the Yellow River sediments

Quaternary aeolian silts in the Chinese Loess Plateau (CLP) has long been regarded as a product of sand ablation during the process of transportation originally from sandy deserts of northern China and Mongolian Gobi deserts driven by winter monsoon. Recently, this traditional idea was challenged by a new idea that aeolian silts in the CLP were emitted from fluvial sediments in the middle reaches of the Yellow River which was originally transported from the Tibetan Plateau ascribed to intensified Asian summer monsoon. We systematically determined the origin of Quaternary aeolian silts in a 318-m-thick loess/palaeosol sequence by using U–Pb age spectrum of detrital zircon grains, and compared it with Quaternary fluvial sediments on the Yellow River terraces in Lanzhou, western CLP. The aeolian silts were originated from Qilian Mountains in Northeastern Tibetan Plateau since 1.4 Ma with exception in the intervals of 1.02–0.93 Ma, ∼0.75 Ma and ∼0.06 Ma when aeolian silts were originated from Gobi-Altai Mountains. In contrast, Quaternary fluvial sediments of the Yellow River were mainly originated from Songpan block south to the Qilian Mountains. We propose that the Quaternary aeolian silts in western CLP were emitted and transported by winter wind circulation from piedmonts of the Qilian Mountains more than by the Yellow River from the Songpan block to the south. This study promotes the recognition in terms of the source of the Quaternary aeolian silts in western China.