Loess Deposits Research Articles

Loss of loess in the geological record due to poor preservation

AbstractLoess deposits are widespread in the Quaternary, but relatively rare in older geological records. This disparity is commonly linked to the unique climate conditions of the Quaternary, but those cannot fully explain the scarcity of loess in older records. Instead, we propose that the poor preservation of loess also plays an essential role. To test this hypothesis, we assess the preservation potential of loess by quantifying its modern‐day distribution in active sedimentary basins. This analysis shows that on the global scale only 20% of loess occurs in basins of which the majority is in a foreland setting. This could be due to nearby silt‐producing mountains and the effects of rain shadow aridity. The other 80% is ultimately either eroded or reworked and therefore poorly preserved in the long term. This conclusion implies that loess deposits may have been more common in pre‐Quaternary periods, despite being less abundant in the geological record.

Thorium isotope evidence for glacial–interglacial dust storminess and productivity in the North Pacific gyre

The flux of dust deposition to the ocean and its impact on marine biological productivity are outstanding issues of the Earth’s climate system. The North Pacific gyre (NPG) is the Earth’s largest ecosystem and is important for marine nitrogen fixation, which is sensitive to Asian dust-regulated surface nutrient stoichiometry. Meanwhile, it is one of the least studied regions in the global ocean during the late Pleistocene, because of the extremely slow sediment accumulation and lack of biological material preservation. 232Th and 230Th concentrations recorded by hydrogenetic ferromanganese crusts in the deep NPG provide a novel and quantitative proxy of dust deposition and export productivity in this region. For this purpose, we have established an in situ U–Th isotope analytical technique on hydrogenetic ferromanganese crusts collected from the deep NPG. Iron, Mn, and Co of our samples have also been analyzed to constrain the impact of dust dissolution on the Fe geochemical cycle. Remarkably, variability of the 232Th-based dust flux over the last ∼900 ky exhibits a striking resemblance to grain-size records of the Chinese loess deposits and sea-level reconstructions but clearly decouples from the global temperature evolution. On orbital time scales, dust storms over the NPG are apparently sensitive to effects related to continental ice sheet sizes but decoupled from major global cooling during the early glaciation. While our 232Th data indicate a 2- to 3-fold variability of glacial–interglacial dust flux, scavenging of Fe, Mn, and Co in the deep NPG is much less variable through the late Pleistocene. We suggest that variable dust fluxes have a limited impact on the inventory of dissolved Fe in the deep waters of NPG, even though the scavenging of Fe has likely been regulated by glacial–interglacial paleoceanography. The paired 230Th-scavenging-based biological productivity tracks gyre sea surface temperature rather than dust flux during the last glacial cycle. We observe no correlation between dust flux and export productivity in the NPG, indicating that dust-derived Fe is not the productivity-limiting factor on orbital time scales in this region. Instead, these results imply that changes in gyre mixing and mode water formation might have led to increased supply of phosphorous and thus enhanced glacial biological carbon sequestration, with potentially stronger nitrogen fixation under higher eolian input.

A loess-paleosol record of climate and vegetation change during the past 27,000 years from South-East of the Caspian Sea, Iran

Loess and paleosols are valuable terrestrial archives of past climate change in arid and semiarid Asia, yet most of the available records are located in central Asia and China, with only few records from western Asia. Here we report on a loess-paleosol sequence retrieved from the Gorgan Plain Southeast of the Caspian Sea that extends back to ca. 27,000 years ago. By combining palynological and micromorphological data with granulometry and magnetic properties in different horizons, we infer past changes in soil formation and vegetation composition, and the related climatic conditions. In order to set up a more reliable chronological framework, we applied luminescence and radiocarbon dating. Our results suggest a dry and cold glacial period characterized by steppe vegetation and loess deposition until about 24,000 years ago. Climate conditions during the Last Glacial Maximum (19–23,000 years ago) were relatively wet followed by a drying trend towards the end of the deglaciation. Our record suggests a relatively wet early Holocene but remains inconclusive for the late Holocene due to concurrent anthropogenic impact. We argue that changes in wintertime temperature and precipitation have been decisive in shaping the vegetation and soil formation in our study area during the Holocene, whereas changes in the position of the westerlies together with sea-level fluctuations of the Caspian Sea controlled the regional climate during the last glacial period.

The patchwork loess of Central Asia: Implications for interpreting aeolian dynamics and past climate circulation in piedmont regions

ABSTRACTReconstruction of mass accumulation rates (MARs) in loess deposits are widely used for interpreting long‐term aeolian transport and climate dynamics in terrestrial environments. However, these interpretations are often driven by a preponderance of reconstructions from individual or selected sites, which can bias our understanding of past climate, especially in the absence of other proxy information. Recent studies on MARs from multiple loess sites in Arid Central Asia (ACA) reveal disparities in the timing of peaks in accumulation between sites, as well as asynchronies with loess flux in the Chinese Loess Plateau (CLP). We investigate this issue by (1) dating five new sites from the western Ili Basin, therefore extending the spatial cover of loess chronologies across ACA and (2) combining that with MARs from >30 sites across ACA and the CLP over the last 60 ka. Our results indicate spatio‐temporal inhomogeneity in the timing and rate of loess deposition across the ACA, and highlight the importance of interrogating local and regional influences on dust supply and transport. Our synthesis of MARs from ACA and the CLP suggests that the timing of peak dust flux as an indicator of large‐scale climate dynamics is best derived from an aggregate of sites; this removes site‐specific bias where local processes or topographic settings outweigh the climate signature.

Dust and loess as archives and agents of climate and climate change in the late Paleozoic Earth system

Abstract Palaeo-loess and silty aeolian-marine strata are well recognized across the Carboniferous–Permian of equatorial Pangaea. Aeolian-transported dust and loess appear in the Late Devonian in the west, are common by the Late Carboniferous, and predominate across equatorial Pangaea by the Permian. The thickest loess deposits in Earth history – in excess of 1000 m – date from this time, and archive unusually dusty equatorial conditions, especially compared to the dearth of equatorial dust in the Cenozoic. Loess archives a confluence of silt generation, aeolian emission and transport, and ultimate accumulation in dust traps that included ephemerally wet surfaces and epeiric seas. Orogenic belts sourced the silt, and mountain glaciation may have exacerbated voluminous silt production, but remains controversial. In western Pangaea, large rivers transported silt westward, and floodplain deflation supplied silt for loess and dust. Expansion of dust deposition in Late Pennsylvanian time records aridification that progressed across Pangaea, from west to east. Contemporaneous volcanism may have created acidic atmospheric conditions to enhance nutrient reactivity of dusts, affecting Earth's carbon cycle. The late Paleozoic was Earth's largest and most long-lived dust bowl, and this dust represents both an archive and agent of climate and climate change.

Experimental Investigation of Helical Pile Performance for Loess Deposits Improvement

Collapsible loess is classified as problematic soil found on most continents. The loess immediately collapses under stresses induced by axial compressive loading or the soil weight when exposed to moisture. In Iran, vast areas of the Golestan province are covered with Aeolian loess deposits. This paper investigates whether the simultaneous effect of inundation and pre-loading in helical pile installation is operative and effective in improving Golestan loess. The effect of installation conditions on loess collapsibility is studied via two main approaches, i.e., field investigations and physical modeling. The experimental studies include the installation of ten single and triple helix piles in the Amirkabir University of Technology Frustum Confining Vessel, i.e., FCV-AUT. Also, ten single and double helix piles with embedment depth of 3 to 6 m and a helix diameter of 250 mm were installed in Golestan province, and full-scale pile static loading tests were performed. The results indicated that the simultaneous application of crowd loading and water pressure during the installation of helical piles results in collapsibility potential reduction due to the soil treatment. Moreover, high or low crowd loads accompanied by water pressure during pile installation bring about similar load-displacement behavior of model piles installed in FCV-AUT and full-scale helical piles. Eventually, it was illustrated that the applied improvement technique is effective on loess soils since it uses the least amount of material, machinery, and equipment while attaining the highest possible bearing capacity ratio regarding sustainability.

Variation in the properties of chernozems in the Priob Plateau within the boundaries of the key site “Volodarka” (Altai Territory, Russia)

The aim of the study. To assess spatial heterogeneity of the chemical properties of the chernozems of the Near-Ob Plateau within the boundaries of the key site “Volodarka” and explain the reasons that determined the variation of the studied soil properties. Location and time of the study. Steppe zone of the northeastern part of the Near-Ob Plateau within the boundaries of the key site “Volodarka” (52°41-42' N and 83°38'E); the results of the long-term research (2007–2019) are discussed. Objects of the study. Chernozems that formed under the dry-steppe vegetation on loess deposits, as well as on different horizons of paleosols of Middle Pleistocene origin that reached surface. Methodology. Soil samples were collected to a depth of 40 cm in a continuous column at an increment of 2 to 10 cm, taking into account the visible boundaries of genetic horizons. To characterize soil chemical properties, the methods commonly used in Russian soil science studies were used. Specifically, soil organic carbon content was determined by dichromate digestion method. Results. The studied properties of chernozems, currently functioning in the same environmental conditions of the key area located on the territory of the Altai Territory, showed significant variation, as can be judged by comparing their means and coefficients of variation. The greatest fluctuations in all soil properties in all soil profiles were observed, as a rule, in the 10–20 or 20–40 cm layers, whereas in the 0–10 cm layer they changed less significantly. It was revealed that the degree of humification and humus reserves, as well as nitrogen reserves and the nitrogen enrichment of humic substances in the 0-40 cm layer, can be regarded as significant; they displayed the greatest variation in the 20–40 cm layer. Conclusions. The heterogeneity of the main properties of chernozems located in the same natural conditions of the Priob Plateau within the boundaries of the key site “Volodarka”, even within 0-40 cm layer, is most likely due to the currently present features of ancient pedogenesis. Those features do not correspond to the combination of modern soil formation conditions of the dry-steppe type: it is confirmed by the presence and extent of clearly visible Middle Pleistocene pedocomplexes in the coastal outcrops of the Ob River. Different horizons of those pedocomplexes reached the surface and had been overlayed by products of modern soil formation in layers of different thicknesses. The combination of heterogeneity of soil profiles in terms of properties, position in the coastal outcrop and their visual differentiation allowed us to conclude that the variation in the compositional characteristics and properties of the studied soils resulted from the history of the territory development, which determined the surfacing of the buried paleosols. The knowledge of the heterogeneity of soil profiles makes it possible to use the studied soil properties and characteristics of their composition for monitoring the state of modern soils, substantiating forecasts of their behavior under changing ecological conditions, as well as for identifying features of ancient pedogenesis in order to reconstruct the paleonatural environment.

Chronostratigraphy of the Periglacial Loess-Paleosol Sequence in Zaprężyn, SW Poland

Abstract Loess-paleosol sequences (LPS) are an exceptional source of palaeoclimate data constituting an indirect record of changing environmental and climatic conditions that prevailed during loess deposition and early diagenesis. The Zaprężyn LPS (17°11′52″E, 51°14′44″N) is situated in the southern morphological edge of the Trzebnica Hills. This sequence provides an opportunity to reconstruct past climate conditions in this part of SW Poland. In this work, we used two sets of optically stimulated luminescence (OSL) dating from the Gliwice and Bayreuth laboratory to establish a chronological framework for this profile. The results of 14C accelerator mass spectrometry (AMS) dating were also used. The dating results are accompanied by detailed analyses of grain-size distribution and colour. The applied methodology allowed us to distinguish four litho-pedotratigraphic units: modern soil S0, L1LL1 loess unit, L1LL2 loess unit and fossil S1 soil. The OSL dating results indicate an unconformity representing a time interval >30 ka between the L1LL1 and L1LL2 units. This unconformity explains the lack of a L1SS1 soil unit. In Zaprężyn LPS, the presence of the ice wedge pseudomorph in L1LL2 loess unit was recorded. Ice wedge pseudomorph approves the presens the permafrost conditions during the Lower Plenivistulian in Trzebnica Hills.

New Pleistocene vertebrate assemblages from the Villány Hills (SW Hungary): Siklós and Palkonya

The Villány Hills in SW Hungary have the richest archive of Pliocene–Quaternary vertebrate faunas in the Pannonian Basin, mostly in karstic cavities. Here we present three new sites that extend the list of Pleistocene vertebrate locations for the area and add information to the evolution history of the region. In the northern part of the Siklós quarry, bone breccia was found coming from fissures in Jurassic or Cretaceous limestones. Its lithofacies and fossil content are similar to those of other well-known Plio-Pleistocene karst infills of the region. As it contained mostly snake vertebrae, its age could not be constrained precisely. In the southern part of the same quarry, two vertical shaft s were discovered, which are unusual in several respects. Th ey formed in a Middle Triassic dolomite succession, a rock type generally not prone to karstification. Th ey might have been created by gravitational deformation of the relatively steep slope, probably at diff erent times. One of them was closed from above and contained fl owstones precipitated during the late Middle Pleistocene, during the late Rissian MIS7 interglacial. The other one was filled from above with loess, rock fragments and remains of large mammals – Equus cf. ferus, Bos primigenius and Coelodonta antiquitatis –, possibly between 140–40 ka, during one of the stadials of the Weichselian or the latest Saalian. The site shows that fossil-bearing cavities could also form in lithologies not favourable for karstification, which then trapped fossils in a similar way karstic cavities do. In contrast with the previous two and with most of the other known vertebrate sites of the Villány Hills, the Palkonya outcrop is not a karst cavity fill but was deposited on the (palaeo)surface. Bison sp., possibly B. schoetensacki remains were found between the Triassic basement and Quaternary slope sediments, and within the latter succession. The Bison bones are probably Middle Pleistocene or late Early Pleistocene, older than ~300 ka. The overlying slope sediments originate from the reworking of various older deposits. Th ey were covered with loess in the Weichselian (~22 ka ago), then again with slope deposits. The abundance of bones in and around the outcrop suggests that this site acted as a fossil trap as well. Bones probably enriched in the sediments during reworking of older deposits. In cold periods, loess deposition decreased (subdued) the relief through infilling the depressions. With 21 figures and 3 tables.

Formation and evolution of the Asian landscape during the Cenozoic

<p>Asia has a unique set of landscapes, notably characterized by the highest plateau in the world, the strongest monsoon circulation, and an immense arid region. When and how this landscape was formed have long been investigated, but explanations remain elusive. In this paper, through a synthesis of both terrestrial and marginal marine sedimentary records and numerical simulation analyses, we propose that tectonic uplift/deformation and global cooling during the Cenozoic have controlled the evolution of Asian topography and climate, respectively, and, the Asian landscape. The Asian landscape has undergone three main stages of evolution, <i>viz</i>: development of low-relief topography and warm-dry climate during the Eocene (~50-34 Ma); emergence of high relief topography in the southwest, along with a diverse range of landforms elsewhere, associated with a warm monsoon-arid climate coupling system during the Oligocene to early Pliocene (34-3.0 Ma); and development of the typical Asian summer and winter monsoon circulation and present landscape since the late Pliocene, accompanied by significant global cooling. Our analyses reveal that the distribution of river catchments, flora, fauna and soils are controlled by tectonic uplift/deformation and palaeoclimate changes. Stepwise evolution of the Asian landscape since ~50 Ma was driven by tectonic deformation and global temperature changes, each of which may play different role(s) in landscape evolution. For instance, the India-Asia collision played a vital role in the formation of the Asian topographic configuration in the middle Eocene, which generated regional atmospheric circulation. On the other hand, global cooling during the late Pliocene played an important role in generating the modern-like Asian landscape including the rapid accumulation of loess deposits, strengthening of the Asian monsoon circulation, and the evolution of modern topography. To summarize, we broadly propose a three-stage history of landscape evolution in Asia during the Cenozoic. The detailed processes of Asian landscape evolution and the specific roles of tectonism and global temperature in controlling Asian landscapes and palaeoclimate evolution need to be further investigated.</p>

Birth of the Taklamakan Desert: When and How?

<abstract> <p>The Taklimakan Desert is one of the largest deserts and sand seas in the world and plays a critical role in the global aerosol (eolian dust) system, impacting both climate and ecosystems on continents and over oceans. Geological evidence suggests that much of the western part of the Tarim Basin, which is now covered by the vast sandy desert, was once the eastern fringe of the Paratethys Sea. After the retreat of the sea, the area transformed into a fluvial system before becoming an mountain front alluvial fan system due to the uplift of the northern Tibetan Plateau. Deserts developed during this period, as sand dunes and eolian loess deposits were formed and buried to become part of the sedimentary sequence. Dating of these sequences suggests that the Taklimakan Desert, in a similar form to what we observe today, came into existence no later than 25 million years ago. Taklimakan Desert was formed as a response to a combination of widespread regional aridification due to the rain shadow effect and increased erosion in the surrounding mountain fronts, both of which are closely linked to the tectonic uplift of the Tibetan-Pamir Plateau and Tian Shan, which had reached a climatically sensitive threshold at this time.</p> </abstract>

Analysis of the structural and environmental impacts of hydrophilic ZSM-5 molecular sieve on loess

Foundation treatment is an essential pre-work for the building construction. Recently, a lot of attention has been paid to loess stabilizers, due to the wide distribution of loess deposits. In this work, ZSM-5 molecular sieve is firstly applied as a loess stabilizer, with the aim of investigating its stabilizing and environmental performance. The unconfined compression strength (USC) and shear strength envelope of hydrophilic ZSM-5 molecular sieve stabilized loess (ZSL) samples with different ZSM-5 molecular sieve contents are measured, showing that the strength of ZSL samples is significantly enhanced. The reasons for the changes in mechanical properties are explained by a microstructural interpretation considering different aspects of ZSL sample microstructure, including cementation strength, mineral analysis, morphological, pore, and elemental composition. Based on the experimental evidence of the changes induced by stabilization on microstructure and mechanical properties, two categories are then defined in terms of ZMS-5 content, and a schematic diagram of the stabilization mechanism is provided. The environmental impacts of the loess stabilization process achieving similar shear strength with ZSM-5 molecular sieve, lime, and cement are figured out and compared by life cycle assessment. The research proves that the stabilization method with ZSM-5 molecular sieve is characterized a better environmental performance in terms of greenhouse gas emission. By analyzing the contribution of every material used in the production of ZSM-5 molecular sieve, a more environmentally friendly and effective production strategy has been finally proposed, which can promote the diffusion of ZSM-5 molecular sieve in the market of loess foundation stabilization.

Partitioned Soil Water Balance and Its Link With Water Uptake Strategy Under Apple Trees in the Loess‐Covered Region

AbstractIt is important, but challenging, to partition soil water balance (SWB) to understand the impacts of afforestation on soil hydrological processes. This study will investigate the partitioning of SWB by combining stable and radioactive water isotopes to quantify the effects of afforestation, and analyze the mechanism by which SWB changes through identifying the water uptake strategies of apple trees of 18 and 26 years old (A18 and A26). Compared to the reference farmland, apple orchards significantly increased evapotranspiration (ET) by 5%–10%, which in turn decreased soil water storage and deep drainage by 5%–14% and 50%–95%, respectively. Further, the partitioning of ET showed that apple tree planting increased transpiration by 15%–28% but decreased evaporation by 17%–30%. The above change in SWB appeared to be closely related to plant water uptake strategies. The apple trees shifted their water source from shallow (0–2 m) to deep soils (below 2 m), utilizing approximately 62% of deep soil water in the late growing season. In particular, 23% of source water may come from soil water older than 50 years. The older apple trees tended to extract more water from deeper soils (45% for A26 vs. 38% for A18). Therefore, the soil water deficit was the cumulative effects of root water uptake. The methods for SWB partitioning provides technical support for similar studies, and the findings are helpful to better understand the hydrological processes in the thick loess deposition.

The Likhvin (=Holsteinian, =Hoxnian) small mammal faunas of Europe (MIS 11) with reference to the easternmost Likhvin small mammal locality Rybnaya Sloboda (Volga basin, Russia)

Numerous small mammal localities assigned to the beginning of the Late Middle Pleistocene (Holstein, Likhvin, Hoxne Interglacial) were discovered in Europe during the last years. The stratigraphical position of a number of these faunas and the regional differences are discussed in the paper. The new Likhvin small mammal locality Rybnaya Sloboda (Lower Kama basin, Volga basin, Russia) is the easternmost site of this age in Europe. The rich fauna was discovered in fluvial deposits of the 3rd terrace (III) of the Kama River; sediments that are covered by loess deposits with a series of paleosols. The Rybnaya Sloboda fauna includes 16 species of insectivores, lagomorphs and rodents. Remains of the archaic water vole Arvicola cantianus and the Middle Pleistocene steppe lemming Lagurus transiens dominate the fossils assemblage. The evolutional level of the Arvicolinae remains permits the assignment to the Gun'kovian small mammal assemblage and a correlation with the Likhvin Interglacial (MIS 11) (∼424–374 ka BP). The ecological features of the small mammal assemblage indicate a forest-steppe landscape in the lower Kama basin during MIS 11. The comparison with, and the analysis of numerous European Likhvin (= Holsteinian, = Hoxnian) small mammal faunas indicate a geographical biozonation that shows similarities with the biozonation reconstructed for the Eemian (= Mikulino, MIS 5e) Interglacial.

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.

Depositional conditions of the Upper Younger Loess during the Last Glacial Maximum in central and eastern Europe

This paper is a summary of the results of research on the accumulation conditions of the Upper Younger Loess (LMg) in Poland and Bug loess (bg) in Ukraine from the maximum stage (MIS 2) of the Vistulian (Weichselian) Glaciation in central and eastern Europe. These studies included an analysis of the morphological (topographic) situation of the loess cover, its grain size and heavy mineral composition, the preserved structures of loess sedimentation as well as mollusc and pollen analyses of this loess. They revealed that the accumulation of Upper Younger Loess (UYL) might have been more dependent on the prevailing moisture conditions than previously thought. These conditions could have been caused by cold air masses from an ice sheet and warm air masses from the Mediterranean Sea and Atlantic coming together in the Carpathians and the Holy Cross Mountains and favouring the formation of dust storms and precipitation. In this process, a loading of loess dust (formed from local rocks weathering in periglacial conditions) by atmospheric moisture particles was especially significant. The moist substrate not only favoured the periodic development of vegetation and molluscs but also enabled the interception of dust and the accumulation of an increasingly thick loess cover. Westerly and south-westerly winds predominated in the UYL as indicated by the topographic position of loess patches and the mineral composition of the studied loess. Periodically an increased air circulation from the east and northeast occurred.

Stability classification probability model of loess deposits based on MCS-Cloud.

The stability classification of loess deposits around tunnels is a vital prerequisite for safe construction in underground environment. Due to the fuzziness and randomness of loess physical and mechanical parameters, the stability prediction of loess deposits shows uncertainty. Existing loess deposit stability classification models rarely consider the uncertainty of influencing factors. A novel classification probability model of loess deposits is proposed for the above problems based on Monte Carlo simulation and multi-dimensional normal cloud (MCS-Cloud). Specifically, five loess parameters, including water content, cohesion, internal friction angle, elastic modulus, and Poisson ratio, were selected as predictors for the stability level of loess deposits. The weights of the predictors were obtained through 50 test samples. After acquiring the numerical characteristics of the normal cloud, the stability level can be comprehensively evaluated with the weighted multi-dimensional normal cloud model. The classification model was applied to the loess tunnel in Yan'an, China. The prediction results are in good agreement with practical engineering, denoting the rationality of the weighted multi-dimensional normal cloud. Finally, the stability classification of loess deposits was discussed from the perspective of uncertainty analysis with the application of MCS. Results proved that the MCS-Cloud model is feasible for classifying the stability of loess deposits surrounding tunnels. The obtained classification probability can be used for quantitative risk assessment of loess tunnels.

Provenance variations of the loess deposits in the East Asian monsoon boundary zone, Northeast China: Response to the variations of climate and wind regimes

Provenance studies are critical for the paleoclimatic reconstruction of loess. However, very little information is available about the provenance variations of the loess in the monsoon boundary zone, limiting our understanding of aeolian dust dynamics, the evolution of atmospheric circulation and driving mechanisms on the glacial-interglacial timescale. To this end, the integration of grain size end-member modeling, machine learning, sediment fingerprinting and wavelet transform analysis is performed for the Harbin loess in Northeast China to understand the long-term provenance variations, atmospheric circulation evolutions and forcing mechanisms. The results show the Harbin loess comprises three grain size end-members, EM1 (7.81 μm), EM2 (27.2 μm) and EM3 (54.41 μm), and they correspond to the background of atmospheric dust, the intensity of the southwesterly and of the East Asian Winter Monsoon (EAWM), respectively, indicating the three transport dynamics of dust delivered to this region. The quantitative source reconstruction indicates that the loess experienced the dramatic source variations, characterized by an abrupt termination of the dust contribution from the Hulun Buir Sandy Land at ∼ 250 ka, and by variations in the relative contribution of the dust sources (i.e., Songnen Sandy Land, Horqin Sandy Land, Onqin Daga Sandy Land, and to a lesser extent, Hulun Buir Sandy Land) during the glacial-interglacial cycles. The comparisons between the indicators, e.g., EM2, the deep-sea oxygen isotopes and the rate of dust accumulation in the North Pacific, combined with wavelet transform analysis indicating a dramatic shift of orbital cycles from the significant 100-kyr and 41-kyr cycles to a weakened 100-kyr cycle, suggest that the weakened EAWM induced by both reduced the Northern Hemisphere ice volume and summer solar radiation, accounted for a termination of the Hulun Buir Sandy Land as the loess source. However, the change in wind regimes due to the glacial-interglacial cycles, and climate change (e.g., enhanced aridification) are the dominant forcing for the variations in the relative contribution of the dust sources to the Harbin loess.

Irreversible intensification of the East Asian winter monsoon and aridity in Asian interior linked to the uplifted Tibetan Plateau: Evidence from magnetoclimatology of the Xining loess since 1.3 Ma

The impact of the uplifted Tibetan Plateau on the East Asian monsoon (EAM) evolution is not an uncontroversial topic. Disentangling complex paleoclimatic signals derived from solar insolation, global ice volume, and the rise of Tibetan Plateau in the EAM domain is challenging because of the paucity of continuous Quaternary sedimentary records shed from the Tibetan Plateau. The 260-m-thick Xining loess deposits in the northeastern Tibetan Plateau (NETP) with ultra-high average accumulation rate of ∼23 cm/ka during 0.2–1.3 Ma provide an excellent opportunity to delineate the long-term influence of the uplifted Tibetan Plateau on paleoclimatic variations in the western margin of the EAM. High-resolution grain size and mineral magnetic analyses for this unique record reveal a much more significant long-term increasing trend of the East Asian winter monsoon strength and aridification compared with those of typical loess records on the Chinese Loess Plateau, confirming that the uplifted Tibetan Plateau may play an important part in enhancing the EAM circulation and aridity of Asian interior under global cooling. It is further proposed that the negative correlation between physical erosion and chemical weathering in the NETP does not contradict the prevailing uplift-weathering hypothesis, but probably suggests a strong climate-silicate-weathering feedback in this tectonically active region. This result provides robust evidence that the existence of the NETP and adjacent topography exerts a profound impact on the irreversible intensification of Asian interior aridity.

Millennial-timescale quantitative estimates of climate dynamics in central Europe from earthworm calcite granules in loess deposits

Ice core and marine archives provide detailed quantitative records of last glacial climate changes, whereas comparable terrestrial records from the mid-latitudes remain scarce. Here we quantify warm season land-surface temperatures and precipitation over millennial timescales for central Europe for the period spanning 45,000–22,000 years before present that derive from two temporally overlapping loess-palaeosol-sequences, dated at high resolution by radiocarbon on earthworm calcite granules. Interstadial temperatures were 1–4 °C warmer than stadial climate, a temperature difference which is strongly attenuated compared to Greenland records. We show that climate in the Rhine Valley was significantly cooler during the warm season and overall drier with annual precipitation values reduced by up to 70% compared to the present day. We combine quantitative estimates with mesoscale wind and moisture transport modelling demonstrating that this region was dominated by westerlies and thereby inextricably linked to North Atlantic climate forcing, although ameliorated.