Loess Section Research Articles

Variations of the Intensity of the Siberian High During the Last Glacial Revealed by the Sorting Coefficient of Loess‐Paleosol Deposits in Eastern Central Asia

AbstractEastern Central Asia has an arid environment and experiences frequent dust storms. Modern observations indicate that the dust storm frequency in eastern Central Asia is modulated mainly by the intensity of the Siberian High (SH). However, a lack of knowledge about the process influencing the variation of the SH limits our ability to understand the genesis of dust storms and their predictions. In this study, we present data on heavy mineral assemblages and sediment grain size in a loess section in eastern Central Asia, spanning the last glacial. Combined with previously published results, we propose that the sorting coefficient in loess‐paleosol sequences is a more sensitive proxy for wind intensity—and thus for the intensity of the SH—than the commonly used proxies of the mean grain size and grain size fractions, which are also influenced by changes in dust source and vegetation cover. Our sorting coefficient series clearly reveals Heinrich events, as well as the prominent ∼5 kyr Bond cycle. Larger sorting coefficients correspond to more depleted Sanbao Cave stalagmite δ18O values, revealing an antiphase relationship between the intensity of the SH and the East Asian Summer Monsoon during the last glacial. The sorting coefficient series also support the significant influence of Arctic sea‐ice cover on the intensity of the SH.

Paleoclimates inform on a weakening and amplitude-reduced East Asian winter monsoon in the warming future

AbstractThe East Asian winter monsoon (EAWM) has significant impacts on the weather and climate, and subsequently on the economy and society, in East Asia during boreal winters, and its projection into the future is scientifically and practically significant. However, projections relying on geological EAWM reconstructions, which can compensate for instrumental record limitations, are still lacking and urgently needed. It is more promising to conduct prediction under the background of not only instrumental but also geological changes in the EAWM. We used grain-size records from four high-resolution, chronologically well-defined loess sections on the Chinese Loess Plateau to represent past EAWM intensity and its amplitudes. Our results show that the EAWM is weaker and has lower amplitudes during warm periods than during cold stages at various time scales. Moreover, instrumental records reveal that the EAWM shows a weak level and reduced interannual amplitudes after the mid-1980s under the context of global warming. We propose that the EAWM will experience long-term weakening and reduced (e.g., interannual) amplitudes under 21st century global-warming scenarios.

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

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

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.

The myth of ‘Bohunician soil’: A re-evaluation of the MIS 3 palaeosol record at the Brno-Bohunice site (Czechia)

Several important loess sections containing marine isotope stage 3 palaeosols have been discovered in the area of the Bohunice district of the city of Brno. Most of them were previously examined mainly from an archaeological point of view in relation to the Middle/Upper Palaeolithic transition, and their palaeopedological records were not studied in detail. The term ‘Bohunician soil’ was introduced during the initial archaeological research of the Brno-Bohunice site and has since been used to refer to palaeosols containing Bohunician artefacts in the surrounding region without any clear definition of what the term actually means. A newly exposed loess section (Brno-Bohunice 2018) and a preserved section from the last archaeological research (Brno-Bohunice 2002) provide an opportunity to revise the Brno-Bohunice palaeosol record and to assess the reasonability of the term ‘Bohunician soil’. We present a comprehensive multiproxy evaluation of soil development over the period of 60–30 ka BP, based on a combination of soil micromorphology and physical and geochemical proxies. The oldest recorded soil horizon was identified below the originally recognized ‘Lower palaeosol’ in the 2002 section whereas in the 2018 section the oldest horizon is of colluvial origin. We newly classify the ‘Lower palaeosol’ as a Cambisol and Tundra gley and interpret the ‘Upper palaeosol’ as a Regosol. Our new division and reassessment of dating results shows that the majority of the Bohunician artefacts found mostly at the transition between the Cambisol and Tundra gley were not in their original stratigraphic position. They were probably lifted by freezing-thawing processes from an earlier position between the soliflucted soil horizon and the Cambisol. It is likely that even in the first excavated contexts of so-called Bohunician soil, artefacts were not necessarily found in their original position. The term therefore appears to have no relevance.

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.

Jet transitions caused multiple abrupt droughts in the Asian summer monsoon margin during Holocene times

The Asian Summer Monsoon (ASM) margin is vulnerable to climate change and a distinct boundary of population and archeological sites exists along the margin. Although lots of studies have been undertaken along the margin, the Holocene history of climate variation is highly debated; for example, whether the climatic optimum occurred in the early Holocene or middle Holocene. In addition, moisture variations on millennial to centennial timescales remain unclear. In this study, we reconstruct moisture variation at the modern junction of the ASM and the Westerlies based on a well-dated (13.3–0.5 ka) loess section in northwest China. On an orbital timescale, moisture variations generally follow changes in insolation with the longest and most intense humid period from 11.3 ka to 8.7 ka, supporting the concept of an early Holocene climatic optimum in the ASM margin. On millennial to centennial timescales, four sequences of long-term gradual wetting and subsequent abrupt drought are noted, i.e., thousands of years were required to reach a favorable environment in the ASM margin, while in only a few hundred years of drought would occur subsequently. Terminations of humid periods correspond to the occurrences of cold events in the North Atlantic realm, indicating an important role for the Atlantic Meridional Overturning Circulation (AMOC) on millennial to centennial timescales. We suggest solar insolation and the AMOC as the dominant factors on orbital timescale and millennial to centennial timescales, respectively. Both factors modulate moisture variations in the ASM margin by influencing the interplay between the ASM and the Westerlies.

Records of Organic Carbon Isotopic Composition and Its Paleoenvironmental Implications in Shengshan Island Loess Deposition in the East China Sea during the Last Glacial Period

Organic carbon isotopic composition (δ13Corg) in loess deposits is an important indicator of terrestrial paleovegetation, and it has been widely used for paleoenvironmental reconstruction in aeolian sediments around the world. However, little research has been done on the variation and paleoenvironmental implication of δ13Corg from loess deposits on Shengshan Island, East China Sea, during the last glacial period (LG). In this research, we present optically stimulated luminescence (OSL) ages, total organic carbon (TOC) data and δ13Corg records of the loess section at Chenqianshan (CQS) on Shengshan Island. Additionally, to study the effectiveness of δ13Corg in documenting paleoenvironmental changes, magnetic susceptibilities and diffuse reflectance spectra were surveyed. TOC concentration for the CQS loess section ranged from 0.11% to 0.47%, and the δ13Corg composition of the CQS loess section varied between −20.80‰ and −24.56‰ during the LG. The average value of C4 abundance was 21.31%. TOC, δ13Corg, χfd, and Hm/(Hm + Gt) curves for the CQS loess section showed similar patterns. The results of our study indicated that the vegetation of the CQS loess deposit was mainly C3/C4 mixed vegetation, and C3 vegetation was the most important vegetation. The comparison between the δ13Corg curve for the CQS section and other existing δ13Corg records of the loess sections from central and northern China showed similar trends and their vegetation succession exhibited synchronous change during the LG. Based on a comparison of the δ13Corg record, C4 abundance and χfd of the CQS section and other global geological records, it was concluded that the mutual effects of precipitation and temperature caused the change of paleovegetation in loess deposits on islands in the East China Sea during the LG.

Anthropogenic Influences on Soil Erosion since the Late Holocene and Contrasting Regional Sustainability in China

A multi-disciplinary investigation of loess sections in the southeast and northwest of the Chinese Loess Plateau (CLP) and a sediment core drilled in Zhejiang Province of southeast China was conducted. Discrepancies among grain size distribution, rock magnetic properties, geochemical characteristics and chroma features, and up-section weakening relation between various proxies in the Sanmenxia loess section were found. The results were compared with those of the Baicaoyuan loess section in the northwest of the CLP and the sediment records across the plateau and elsewhere. It was suggested that human impacts began to increase soil erosion on the CLP since the middle Holocene. In addition to the increased soil erosion being decoupled from drying climate after 4 ka, renewed intensification of soil erosion was suggested to occur within the interval of 1.5–2.5 ka as a result of enhanced human activities. The two detected increases in human-induced soil erosion on the CLP are consistent with the human-driven land use changes or human–land interactions at national or regional scales, including the anthropogenic influences on the changes in the sediment load of the Yellow River. In contrast, no human impacts overwhelming hydroclimate control of soil erosion was revealed in the Beihuqiao cores, Zhejiang. The population growth during the past 2400 years showed a relative decreasing trend on CLP and a relative increasing trend in Zhejiang. It is indicated that anthropogenic factors have played a key role in modulating the Earth’s surface environment. In particular, ecologically fragile areas, such as the CLP, would be much more susceptible to human disturbance and climate change. The current serious land degradation on the CLP mainly results from the negative feedback between human–land interactions. Regional heterogeneity should be taken into account for sustainable development.

Siberian High Modulated Suborbital‐Scale Dust Accumulation Changes Over the Past 30 ka in the Eastern Yili Basin, Central Asia

AbstractLoess contributes considerably to understanding the interactions between dust cycles and climate change. However, systematic and regional‐scale reconstructions of variations in the dust mass accumulation rate (MAR) using Central Asian loess are lacking. Based on the high‐resolution luminescence dating of 10 loess sections, we present a stacked suborbital‐scale (multi‐millennial‐scale herein) dust MAR record over the past 30 ka in the eastern Yili Basin, Central Asia, which shows obvious advantages at representing basin‐scale dust accumulation changes when compared with that based on individual sites. The stacked results in the eastern Yili Basin indicate that the MAR level remained high during the Last Glacial Maximum, followed by a dramatically decreasing trend throughout the last deglaciation, and exhibited a slight decrease during the early Holocene and early middle Holocene, followed by a clear increasing trend toward the late Holocene. We propose that changes in the Siberian High intensity dominantly modulated the aforementioned MAR variations. These results effectively link dust accumulation to the Siberian High, imply a possibly weakened dust activity in the Yili Basin at a long‐term scale under future global warming scenarios, and also provide a significant reference for dust accumulation reconstruction in other basins in Central Asia.

Testing post-IR-IRSL dating on Armenian loess-palaeosol sections against independent age control

Loess-palaeosol sections (LPS) in northern Armenia exhibit at least three alternating loess and pedocomplex units, indicative of an environment changing between erosional/depositional and stable conditions. The LPS they are located in a central key position between loess in Europe, central Asia and the Russian/Ukrainian Plains. Since an accurate chronology is crucial for any palaeoenvironmental reconstruction and correlation with other proxy records, we establish a post-IR-IRSL chronology for two loess sections in the Sevkar loess area and compare it with independent age control. This is mainly based on an 40 Ar/ 39 Ar age of a tephra encountered in the L-2 loess layer from the penultimate glacial, which is dated to 194.0 ± 8.5 ka. Further checks on the accuracy are provided by the expected stratigraphy and comparison with other palaeoenvironmental records. The comparison indicates that a fading correction is necessary for the older samples within the L-2 loess layer but when correcting for fading with fading rates measured in the laboratory, the ages are clearly overestimated. Therefore, it is tentatively suggested using a reduced fading rate by subtracting a fixed percentage of the measured fading rate as surrogate for a laboratory artefact. In our study, we reduce all fading rates by 1%/decade and yield fading corrected pIRIR ages which are in agreement with the independent age control. The resulting chronology reaches down to at least MIS8 and shows that loess deposition in northern Armenia occurred during global cold phases, whereas global warm phases are associated with landscape stability and soil formation.

Evaluation of geochemical proxies and radiocarbon data from a loess record of the Upper Palaeolithic site Kammern-Grubgraben, Lower Austria

Abstract. Two loess sections from the Upper Palaeolithic site of Kammern-Grubgraben (Lower Austria) were analysed to test geochemical proxies, as well as radiocarbon data of different components, for their reliability and consistency in an archaeological context. Only a reliable basal age (28.9–27.8 ka cal BP) was obtained from charcoal fragments derived from a tundra gley underlying the archaeological horizons and assigned to Greenland Interstadials 3 or 4. Grain size, organic and inorganic geochemistry, and stable isotopes of the fine organic fraction (δ13Corg) and of rhizoconcretions (δ13C, δ18O) were analysed to provide information on palaeoenvironmental conditions. Low-resolution geochemical and sedimentological analyses document a humidity-related variability, while δ13Corg values indicate predominant C3 vegetation. High-resolution elemental variations derived from X-ray fluorescence scanning exhibit increasing Ca and decreasing Fe and Ti values, indicating drier conditions towards the top. Secondary pedogenic carbonate concretions provide post-sedimentary (Holocene) ages and are not suitable for assessing climate and environmental changes for the Palaeolithic.

Revisiting Late Pleistocene Loess–Paleosol Sequences in the Azov Sea Region of Russia: Chronostratigraphy and Paleoenvironmental Record

Loess-paleosol sequences are the most intensively studied terrestrial archives used for the reconstruction of Late Pleistocene environmental and climatic changes in the Azov Sea region, southwest Russia. Here we present a refined chronostratigraphy and a multiproxy record of Late Pleistocene environmental dynamics of the most complete and representative loess–paleosol sequences (Beglitsa and Chumbur-Kosa sections) from the Azov Sea region. We propose a new chronostratigraphy following the Chinese and Danubean loess stratigraphic models that refines the subdivision of the Last Interglacial paleosol (S1) complex in two Azov Sea sites, resolve the uncertainty of the stratigraphic position of the weakly developed paleosol (L1SSm) in Beglitsa section, and allow for direct correlation of the Azov Sea sections with those in the Danube Basin and the Chinese Loess Plateau. More importantly, it may serve as a basis for better constraining local and regional chronostratigraphic correlations, and facilitate the interpretation of climatic connections and possible forcing mechanisms responsible for the climatic trends in the region. In addition, a general succession of environmental dynamics is reconstructed from these two vital sections, which is broadly consistent with other loess records in the Dnieper Lowland and Lower Danube Basin, demonstrating similar climatic trends at Glacial–Interglacial time scales. Furthermore, our results have important implications for the chronostratigraphic representativeness of Beglitsa as a key regional loess section and for the reconstruction of the temporal and spatial evolution of Late Pleistocene climate in the Azov Sea region.

Age and source of coastal loess in Shandong Peninsula, Bohai Sea, China: Implications for dust aggradation in respond to sea-level change

Coastal loess is a unique and intriguing eolian sediment on Earth, but understanding its formation time, provenance, composition and the geomorphic processes that resulted in its accumulation is very limited compared to its inland counterpart. In this paper, we focused on a loess section at Xiazhupan (XZP) Village, Penglai City in Shandong Peninsula, the only known coastal loess bluffs preserved in China. The coastal loess here not only has characteristics that are common to loess terrains such as being composed of predominantly silt-sized particles and the tendency to stand in vertical exposures, but it also reportedly preserves planktonic foraminifera, sizable pebbles and volcanic glass. In addition to its peculiar near-shore locality, these features make it more unusual compared to other loess deposits. Systematic optically stimulated luminescence (OSL), infrared stimulated luminescence (IRSL and pIRIR) and radiocarbon dating reveal it was deposited mostly during 60–20 ka, Marine Isotope Stage (MIS) 4-2, which correlates to the deposition of the Malan loess (L1) in the Chinese Loess Plateau. High-resolution grain size data indicate that the coastal loess is relatively coarse-grained, suggesting a source terrain that is at least partially proximal. Given its proximal source and the geochronological evidence, this coastal loess was sourced from the exposed continental shelf during the sea-level lowstand of the last glacial period, and this hypothesis is further supported by a core collected from the Bohai Sea in which silt was present and would have been subaerially exposed during MIS 4-2. In addition, this source area of exposed continental shelf is comparable to coastal loess in Europe. The coastal loess in Shandong can be regarded as a special type of desert or dryland loess, rather than glacial loess. Finally, future sea level rise will likely increase the erosion potential of these vulnerable coastal loess bluffs, making it urgent to study this special landform.

Quaternary sediment sources and loess transport pathways in the Black Sea - Caspian Sea region identified by detrital zircon U-Pb geochronology

Constraining the controls on the distribution of sediment at a continental scale is a critical step in understanding long-term landscape and climate evolution. In particular, understanding of the role of rivers in wider sediment routing and impacts on aeolian loess formation on a continental scale remains limited. Extensive Quaternary loess deposits are present on the East European Plain and in the Black Sea - Caspian Sea region and are associated with major rivers draining numerous surrounding cratonic and orogenic hinterland areas. Coupled with this, complex changes in local and global sea level have affected the extent and drainage of the Caspian Sea and the Black Sea, and Quaternary glaciations have impinged on the northern margin of the East European Plain. This suggests that sediment routing and loess formation may show complex patterns and controls. Here, we apply UPb dating of detrital zircons from fluvial, marine and aeolian (dominantly loess) sedimentary records on the East European Plain and in the Black Sea - Caspian Sea region. This shows a strong control of large rivers on the distribution of sediments at a continental scale in the region, through long-distance transport of several 1000 km, sourced from continental and mountain glacier areas prior to marine or atmospheric reworking and transportation. Strong spatial variability in zircon UPb data from loess deposits on the East European Plain reveals multiple diverse sources to the different individual loess sections, whereas no significant temporal variability in loess source is detected during the Late Pleistocene of the Lower Volga loess in South Russia. While the sediment supply from glacial areas via rivers plays an important role for the provenance of East European Plain loess deposits, our data indicate that the stark spatial diversity in loess provenance on the East European Plain is often driven by the input of multiple local sources. Similar to the loess, marine sediments from different basins of the Black Sea and the Caspian Sea also show significant spatial variability. This variability is controlled by the bathymetry of the seas, leading to sedimentary intermixing by sea currents within, but not between different separated sea basins. A direct comparison of marine and aeolian sediments at the same depositional site suggests that although loess and marine sediments are both dominantly sourced from river sediments containing far travelled sedimentary material, local sources play a more important role in many loess deposits.

Undetection of Australasian microtektites in the Chinese Loess Plateau

Australasian strewn field of tektites and microtektites, formed by a large impact event at ~0.788 Ma, is the youngest and largest known strewn field on Earth. The location of its source crater is still unconfirmed after nearly 60 years of exploration. The geographic boundary and the spatial density of tektites and microtektites within this strewn field are the most important information for the search of the potential parent crater. It has been noticed that the northern boundary of this strewn field may be extended further than currently sketched, as possible microtektites have been reported, but not confirmed in the Chinese Loess Plateau (i.e., CLP). The potential microtektites reported in CLP were used as a stratigraphic marker for Quaternary Science, and their existence may indicate a larger impact event that formed this strewn field, redirecting the search area for the source crater. In the past four years, we systematically searched for Australasian microtektites in the Chinese Loess Plateau, and this study is a final report for this project. Referring to the magnetic susceptibility and local deposition rates of loess materials, we sampled nineteen loess sections that cover deposition ages of ~0.788 Ma. The sections are distributed at eight different locations across CLP, and potential Australasian microtektites were reported in some of these sections. At each section, we performed continuous sampling with depths, and each sample was ~1500 g in weight. The total section area of the sampled sections is much larger than that of deep ocean drill cores where Australasian microtektites were searched. We processed over four hundred kilograms of samples and discovered twenty glass microspherules that have spherical and rotational shapes. These microspherules are scattered at different depths of the sections, and their water content is as low as that of typical microtektites. However, both the content of major elements and detailed surface morphology of the microspherules are different from those of Australasian microtektites, but consistent with those of manmade soda-lime glasses and coal fly ashes. This project concludes no discovery of Australasian microtektites, suggesting that microtektites are not widespread stratigraphic markers in CLP. The result further concurs that microtektites in this strewn field were preferentially deposited at the downrange of the impactor trajectory. While microtektites may have temporarily escaped the atmosphere during this impact event, subsequent atmospheric reentry and lateral transportation in the upper atmosphere did not deposit microtektites towards the uprange.

Holocene aeolian dust accumulation rates across the Chinese Loess Plateau

The Chinese Loess Plateau (CLP) is a major region of Quaternary aeolian dust deposition. However, the temporal and spatial variations of Holocene aeolian dust accumulation rate (DAR) across the CLP have not been systematically evaluated. Here, we present a synthesis of Holocene DARs across the CLP based on 36 well-dated loess sections. The results show that the DAR was relatively high during the early Holocene, lowest during the mid-Holocene, and increased substantially during the late Holocene. These trends are in good agreement with the temporal variations of the East Asian summer monsoon (EASM). We conclude that the dominant factor controlling dust storm activity across the CLP during the Holocene was EASM precipitation, rather than the East Asian winter monsoon (EAWM). However, during intervals of a strong EAWM there was a significant positive correlation between the DAR in the CLP and dust deposition in Greenland, suggesting that a strong EAWM may have elevated Asian desert dust to high altitudes, enabling it to be entrained by the westerly jet and transported for long distances. Anthropogenic influences may be superimposed on EASM-modulated dust activity. Against the same background of EASM precipitation, the DAR after 4 kyr BP was significantly higher than previously, which may be related to the intensification of human activities in the dust source areas. Notably, several intervals of high DAR during the past 2000 years were not accompanied by decreases in EASM precipitation, and instead they correspond to significant population increases in the dust source areas. • Holocene dust accumulation rates across the Chinese Loess Plateau were assessed. • EASM dominated Holocene dust storm activity across the Chinese Loess Plateau. • Anthropogenic impacts on dust activity can be traced back to at least 4 kyr BP.

Establishing a Luminescence-Based Chronostratigraphy for the Last Glacial-Interglacial Cycle of the Loess-Palaeosol Sequence Achajur (Armenia)

The loess-palaeosol section Achajur is part of the Sevkar loess area in north-eastern Armenia and comprises three prominent pedocomplexes with intercalated loess units, probably spanning at least three glacial-interglacial cycles. With its central position between the Black Sea and the Caspian Sea, the Sevkar loess area serves as important bridge between loess sections in southeastern Europe and Central Asia. Establishing a numerical chronology of the Achajur loess section enables correlation with other loess-palaeosol sections and a palaeoenvironmental reconstruction of the region. In this paper, we focus on the chronology of the last glacial-interglacial cycle using luminescence dating on fine grain (4–11 µm) material. We compare two post-IR-IRSL protocols on polymineral fine grain using different stimulation temperatures, in order to evaluate the influence of fading and potential unbleachable residuals on our samples. For a backup of the younger ages (<50 ka), we also applied a single aliquot protocol on the quartz fine grain fraction. The results indicate that the upper loess unit was deposited during MIS3 and 4, while MIS2 ages are not present. The underlying pedocomplex and loess unit fall into MIS5 and MIS6, respectively. This confirms that the loess was deposited during dry phases and pedocomplexes formed during wetter phases, as is also shown by comparison with other palaeoenvironmental proxies from the area.

Rapid Northwestward Extension of the East Asian Summer Monsoon Since the Last Deglaciation: Evidence From the Mollusk Record

The magnitude and rate of the expansion of the East Asian summer monsoon (EASM) rain belt under future climatic warming are unclear. Appropriate ecological proxy data may provide an improved understanding of the spatial extension of the EASM during past warming intervals. We reconstructed the spatiotemporal pattern of the extension of the EASM since the Last Glacial Maximum (LGM), using six well-dated mollusk fossil sequences from Chinese loess sections located on the modern northern edge of the EASM. The abundance of typical dominant mollusk species indicative of EASM intensity shows a delayed response, from ∼17 ka in the southeastern sections to ∼9 ka in the northwestern sections, during the last deglacial warming. Isoline plots based on a mollusk data synthesis show that the mollusk EASM indicators have a northeast–southwest zonal distribution for both the present-day, the cold LGM, and the warm mid-Holocene, which is consistent with the spatial pattern of modern precipitation. The resulting estimated expansion rate of EASM intensity accelerated during ∼12–9 ka (∼50 km/ka), which corresponds to the early Holocene interval of rapid climatic warming, a northwestward shift of ∼150 km compared to today. This implies that the northern fringe of the EASM in northern China will become wetter in the coming century, under moderate warming scenarios.

Seasonality of Response to Millennial‐Scale Climate Events of the Last Glacial: Evidence From Loess Records Over Mid‐Latitude Asia

AbstractAlthough difficult to resolve, seasonality is an important element in considering paleoenvironmental records and simulated results, and it is therefore critical to address this shortcoming in order to develop more accurate reconstructions of past climate. In this study, climatic proxies with seasonal implications (magnetic susceptibility and mean grain size) are analyzed for several high‐resolution loess sections from mid‐latitude Asia using the Ensemble Empirical Mode Decomposition (EEMD) to detect their response to millennial‐scale oscillations during the last glaciation. In so doing, we are capable to estimate the amplitude and relative contribution of the reconstructed climate components. Combined with an analysis of modern processes, magnetic susceptibility (χ) can be interpreted as representing summer and spring precipitation, while mean grain size (Mz) can be interpreted as representing spring and summer dust activity in the Chinese Loess Plateau (CLP) and southern central Asia (SCA), respectively. Our results show that the spring and summer signals are clearly correlated with Heinrich events, but that the spring signal is more prominent than the summer signal during Dansgaard‐Oeschger (D‐O) oscillations of the last glacial. These results are consistent with modeled simulations. It is proposed that weakening or complete shutdown of AMOC influences the response of seasonal signals to abrupt climate events. The study highlights the need for further high‐resolution climate proxies with robust seasonality indicators in order to develop a deeper understanding of the response of mid‐latitude Asia to rapid climate events.