Desert Loess Research Articles

East Asian monsoon variations in the loess–desert transitional zone (northern China) during the past 14 ka and their comparison with TraCE21K simulation results

Abstract The Holocene is a critical period for understanding the East Asian monsoon system (EAM) over long timescales, but high-precision dating and high-resolution records from the Holocene epoch at monsoonal margins of East Asia are lacking. Here, on the basis of closely spaced radiocarbon and optically stimulated luminescence dating results obtained from a typical loess–paleosol sequence on the northern Chinese Loess Plateau (CLP), we provide an independent age-based, high-resolution depositional record of East Asian summer (EASM) and winter monsoons (EAWM) variations over the past ~14 ka. We find that both the EASM and EAWM simultaneously strengthened sometime during the Holocene optimum (~7–5 ka BP), with greater seasonality, and weakened during the Late Holocene. These findings are counterintuitive to our understanding of the EAM variations based on loess records at suborbital scales during interglacial periods, providing an alternative scenario of the monsoon system evolution. We postulate that high-latitude forcing and surface feedbacks, such as vegetation change, have modulated the EAM variations during the Holocene warmth.

Mechanisms and Empirical Analysis of the Impact of Soil and Water Conservation on the Livelihood and Well-Being of Farmer Households: A Case Study in Desert–Loess Transition Zone of China

Climate change brings great uncertainty to the sustainable livelihood of farmers. Soil and water conservation measures are the key measures to adapt to climate change, and studying their effects is of great significance to formulating and adjusting future work. Based on the analysis framework of sustainable livelihood, this study constructed a path model to analyze the influence path among soil and water conservation, farmers’ livelihood and well-being from the perspective of model integration and discussed the mechanism of the effect of soil and water conservation well-being. The results show that (1) soil and water conservation has a significant effect on both the livelihood and well-being of farmer households. Soil and water conservation has a positive effect on farmer households’ livelihood capital, and farmers who participated in soil and water conservation prefer to engage in agricultural activities, with a cumulative effect of livelihood capital and livelihood strategies dependence. However, the direct effect of soil and water well-being is not significant, but only in terms of farmers’ security and health. (2) Through the “livelihood capital accumulation mechanism”, “livelihood strategies dependence mechanism” and “livelihood chain mechanism”, soil and water conservation affects the basic material needs, safety and health, freedom of choice and movement of farmers’ well-being. (3) In order to further promote soil and water conservation measures, relevant policy makers can indirectly enhance the soil and water conservation well-being effect by optimizing the livelihood portfolio of farmers, thus attracting the extensive participation of farmers. This study provides analytical ideas for exploring the role of the relationship between soil and water conservation, livelihood and well-being, and offers suggestions for increasing the participation of farmers in soil and water conservation.

Quaternary influx of proximal coarse-grained dust altered circum-Mediterranean soil productivity and impacted early human culture

AbstractThe carbonate mountainous landscape around most of the Mediterranean is karstic, is almost barren, and has thin soils. Erosion of preexisting thicker soils is a common hypothesis used to explain this bare terrain. An alternative hypothesis is that in the Mediterranean region, thin soils are attributed to long-distance transport of very fine, silty clay dust, resulting in low mass accumulation rates. Even if accreted over millennia, such dust cannot produce thick, highly productive soils. A pronounced anomaly in the Mediterranean is the thick, more productive soil of the semiarid southern Levant (SL). These soils contain order-of-magnitude coarser grains than the characteristic thin soils in the Mediterranean and a high proportion (>70%) of coarse silt quartz sourced from the nearby Sinai-Negev erg, the primary contributor of the Negev loess. This proximal intense dust supply produced greatly thicker soils. However, influx of coarse silt quartz loess is a geologically recent phenomenon in the SL. Pre-loess (i.e., older than 200 ka, pre-coarse-silt influx) SL soils are much finer and were generated by long-distance dust from the Sahara and Arabia like most other Mediterranean soils. Thus, we hypothesize that the geologically recent Negev Desert loess interval caused a drastic change in mountainous soil properties within the SL, enriching the Levant’s ecology and affecting early human development. The high amounts of coarse silt deposited on the landscape have contributed to the unique sustainable agriculture in the SL, which assisted in transforming the Levant into “the land of milk and honey” and a cradle of civilizations.

On the formation of desert loess

AbstractSequences of quartz-rich coarse (20−63 μm) silt occur in many low- and midlatitude unglaciated arid and semiarid areas and have been termed “desert loess.” The processes by which these deposits are generated have been debated for decades. All hypotheses to explain their origin seek to provide mechanisms for the generation of silt-sized material without glacial grinding, which is the main process involved in the production of coarse silt at high latitudes. Possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones, mechanical weathering of silicate rocks, and abrasion of sand grains in active dune environments during intense transport events. Examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and/or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt. Improvements in the characterization of desert loess particle size, mineralogy, and geochemistry are needed, however, to identify sources and sinks of coarse silt, especially when combined with climatic back-trajectory analysis. Properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations.

Mountain loess or desert loess? New insight of the sources of Asian atmospheric dust based on mineral magnetic characterization of surface sediments in NW China

Abstract The arid basins in NW China are one of the most important sources of Asian atmospheric dust. Numerous studies have been conducted of dust flux and its physical and chemical characteristics in terrestrial and marine sediments in East Asia. However, knowledge of the exact sources of the emitted dust is comparatively limited. In this study, we investigated the magnetic characteristics of 541 surface samples collected in NW China. Based on the application of cluster analysis, we propose that the silt in arid basins is sourced from the surrounding high mountains, and that rivers play an important role in transporting the material, whereas sand dunes are only a relatively limited store of silt. Comparison of the results with the magnetic characteristics of loess samples from the Chinese Loess Plateau (CLP) indicates that the slopes of the Kunlun-Alkin-Qilian Mountains System and the surrounding fluvial plains are the major sources of silty dust in NW China, and that the contribution of sediment from sand dunes has been overestimated. In addition, our results indicate that magnetic fingerprinting can be used to trace the pathway of dust transport, and that the loess deposits in central Asia and the CLP are more accurately described as ‘mountain loess’ rather than ‘desert loess’.

Desert loess: a selection of relevant topics

Abstract In discussions on loess, two types are often demarcated: glacial loess and desert loess. The origin of the idea of desert loess appears to lie with V.A. Obruchev who observed wind-carried silt on the Potanin expedition to Central Asia in 1895. It might be considered that desert loess would be defined as loess associated with deserts but it came to be thought of as loess produced in deserts. This led to some controversy as no mechanism for producing silt particles in deserts was readily available. Bruce Butler in Australia in particular cast doubt on the existence of desert-made loess. Butler indicated loess-like deposits in Australia which he called Parna; these are very like loess but the silt sized particles are actually clay mineral agglomerates of silt size- formed in dry lake regions. At the heart of the desert loess discussion is the problem of producing loess material in deserts. It has been suggested that there are no realistic mechanisms for forming large amounts of loess dust but there is a possibility that sand grain impact may produce particle shattering and lead to the formation of quartz silt. This would appear to be a reasonable mechanism for the African deposits of desert loess, but possibly inadequate for the huge deposits in China and Central Asia. The desert loess in China and Central Asia is loess associated with a desert. The material is formed in cold, high country and carried by rivers to the vicinity of deserts. It progresses then from deserts to loess deposit. Adobe ground may be defined as desert loess. Adobe occurs on the fringe of deserts, notably in the Sahelian region of Africa, and in SW USA. The use of adobe in construction represents the major utilization of desert loess in a social context. More understanding of adobe is required, in particular with respect to the adobe reaction, the low order chemical reaction which provides modest cementitious properties, and can be likened to the pozzolanic reactions in hydrating cement systems. The location of loess and loess-like ground on the peripheries of deserts is aided by the observation of the nesting sites of bee-eater birds. These birds have a determined preference for loess ground to dig their nesting tunnels; the presence of nest tunnels suggests the occurrence of desert loess, in desert fringe regions. We seek amalgamation and contrast: ten main topics are considered: words and terms, particles, parna, geotechnical, adobe, people, birds, Africa, Central Asia, Mars. The aim is some large generalizations which will benefit all aspects of desert loess investigation.

The first loess map and related topics: contributions by twenty significant women loess scholars

AbstractWomen scholars have contributed to the study of loess. Charlotte Hibbert made the first map of loess distribution and there were many subsequent achievements. The twenty people selected represent a subjective choice; there is no overall biobibliographical metric that can be applied. The twenty are mss: Hibbert, Owen, Swineford, Conea, Marković-Marjanović, Fotakieva, Ivanova, Davin, Coude-Gaussen, Wintle, Matviishina, Gerasimenko, Billard, Grabowska-Olszewska, Sycheva, Mavlyanova, Rendell,Łanczont, Klukanova, Morozova. The INQUA Loess Commission provides a convenient temporal divide, and also a divide in style of loess investigation: early studies pre-Commission; Loess Commission activities; modern scientific loess research post-Commission (after 2003).TopicsHypotheses of loess formation, aeolian vs. fluvial, aeolian vs. in-situ, desert loess, Danubian loess, INQUA Loess Commission, Dating techniques, hydroconsolidation, loessification, Ukraine, Russia, Central Asia, loess structure.The paper by Jelena Marković-Marjanović in the 1961 INQUA Loess Symposium was one of the key pioneering documents in the development of loess stratigraphy; it should be more widely recognized.

Middle to late Pleistocene shift in eolian silts contribution into Mediterranean soils at the fringe of the Negev loess, Israel

A common spatial feature within loess deposits worldwide is a downwind decrease in thickness and grain size, trends that are powerful tools for reconstructing paleowinds and past atmospheric circulation. Although such trends have been identified, there is limited knowledge of similar trends farther downwind from the loess region, where eolian influx can influence soil formation and hydrological processes. To examine these impacts we studied Quaternary sequences in prehistoric sites in Jerusalem, located only ∼50 km downwind from the edge of the Negev loess. All sequences are composed of two units separated by an unconformity. The lower unit is of middle Pleistocene age, it is composed of unimodal clay to silty clay dust deposits with chert clasts and Lower-Middle Paleolithic artifacts. A non-deposition interval characterized the middle to late Pleistocene transition, when dust accumulation rates were low. The upper unit age is late Pleistocene to Holocene; it is composed of bimodal silty clay to silty clay loam. Quartz, K-feldspars, and plagioclase contents together with the location and amplitude of grain-size coarse mode increase up-sequence. The addition of coarse silts to the upper unit of the sequences was coeval with episodes of loess accretion in the Negev. These silts were generated most likely by eolian abrasion of sand grains in the upwind dunes. Similar to the Negev, the addition of silts resulted in burial of the drainage network and reduced runoff and soil erosion rates. We stress the importance of desert loess in determining soil composition and surficial hydrology in wetter areas located in adjacent downwind regions.

A model of the ground surface temperature for micrometeorological analysis

Micrometeorological models at various scales require ground surface temperature, which may not always be measured in sufficient spatial or temporal detail. There is thus a need for a model that can calculate the surface temperature using only widely available weather data, thermal properties of the ground, and surface properties. The vegetated/permeable surface energy balance (VP-SEB) model introduced here requires no a priori knowledge of soil temperature or moisture at any depth. It combines a two-layer characterization of the soil column following the heat conservation law with a sinusoidal function to estimate deep soil temperature, and a simplified procedure for calculating moisture content. A physically based solution is used for each of the energy balance components allowing VP-SEB to be highly portable. VP-SEB was tested using field data measuring bare loess desert soil in dry weather and following rain events. Modeled hourly surface temperature correlated well with the measured data (r 2 = 0.95 for a whole year), with a root-mean-square error of 2.77 K. The model was used to generate input for a pedestrian thermal comfort study using the Index of Thermal Stress (ITS). The simulation shows that the thermal stress on a pedestrian standing in the sun on a fully paved surface, which may be over 500 W on a warm summer day, may be as much as 100 W lower on a grass surface exposed to the same meteorological conditions.

Heinrich events recorded in a loess–paleosol sequence from Hexigten, Inner Mongolia

We describe the last glaciation climatic history Marine Isotope Stage (MIS, 2–4) from 66.7 ka to 14.5 ka in Hexigten, northeast Inner Mongolia, North China. The climate of the region experienced frequent and significant fluctuations between dry-cold and less dry-cold during the late MIS4. The climate was generally warm and humid during early MIS3 (MIS3c) and late MIS3 (MIS3a), whereas it was cold and dry in middle MIS3 (MIS3b) and during MIS2. In this study, the cold and dry conditions were correlated with a stronger East Asian winter monsoon and strong dune activity; whereas, warm and humid conditions were related to a stronger East Asian summer monsoon (EASM) and weak dune activity. This study establishes six distinct dry and cold intervals during the last glacial period (66.7–14.5 ka) based on optically stimulated luminescence data, multi-proxies record (magnetic susceptibility, grain size analysis, Rb/Sr, SiO2/TiO2) and chemical index of alteration (CIA). The last glacial period may be correlated with Heinrich events 1 to 6 which were further confirmed by comparison with the Hulu cave stalagmites and Greenland ice core records. It is concluded that the study area was substantially affected by the EASM, as compared with the loess–desert transition zone of the Chinese Loess Plateau, especially in MIS3c and suggested that the East Asian monsoon played a pivotal role in the last glacial period climate and dune activity.

New perspectives on the Li isotopic composition of the upper continental crust and its weathering signature

Lithium isotopes are increasingly used to trace both present-day and past weathering processes at the surface of the Earth, and could potentially be used to evaluate the average degree of past weathering recorded by the upper continental crust (UCC). Yet the previous estimate of average δ7Li of the UCC has a rather large uncertainty, hindering the use of Li isotopes for this purpose. New δ7Li for desert and periglacial loess deposits (windblown dust) from several parts of the world (Europe, Argentina, China and Tajikistan) demonstrate that the former are more homogeneous than the latter, and may therefore serve as excellent proxies of the average composition of large tracts of the UCC. The Li isotopic compositions and concentrations of desert loess samples are controlled by eolian sorting that can be quantified by a binary mixing between a weathered, fine-grained end-member, dominated by phyllosilicates and having low δ7Li, and an unweathered, coarse-grained end-member, that is a mixture of quartz and plagioclase having higher δ7Li. We use correlations between insoluble elements (REE, Nd/Hf and Fe2O3/SiO2), Li concentrations (henceforth referred as [Li]), and δ7Li to estimate a new, more precise, average Li isotopic composition and concentration for the UCC: [Li]=30.5±3.6(2σ) ppm, and δLi7=+0.6±0.6(2σ). The δ7Li for desert loess deposits is anti-correlated with the chemical index of alteration (CIA). Using this relationship, along with our average δ7Li, we infer that (1) the present-day CIA of the average UCC is 61−2+4(2σ), higher than the common reference value of 53, and (2) the average proportion of chemically weathered components is as high as 37−10+17(2σ)% at the surface of the Earth.

China: Materials for a loess landscape

For various reasons it was thought that material for the great Chinese loess deposits originated in the deserts of the north—and the idea of a ‘desert’ origin was widely accepted. But Butler, in Australia in 1956, cast doubt on the actual existence of desert loess and this led to considerable discussion. One facet of the argument (advocated by Smalley and Vita-Finzi, 1968) proposed that there were no desert specific mechanisms which could produce the large amounts of loess material observed. Applying this idea to the Chinese loess was particularly appropriate because of the huge extent and thickness of the deposits. How was this vast amount of loess material produced?Smalley and Krinsley (1978) proposed a sequence of events that could lead to the formation of the Chinese loess. This required that the loess be a mountain loess (eventually defined by Smalley and Derbyshire, 1990)—the material was made in the mountains to the west; and the Yellow River had a role to play in bringing it to the loess deposit region; and that loess material in desert regions was in a state of transit.The 1978 model has been proved to be true by a whole sequence of ingenious and intricate experiments. In particular the analysis of zircons has produced data allowing particle sources to be identified. The zircon particle proves to be a key component of the loess landscape; but the mode particle remains the silt-sized quartz particle.The problem running in parallel with the ‘how did the material form?’ question is the puzzle of why loess material has such a restricted size range. What controls operate on the formation of loess material? In the quartz particles it appears that a combination and interaction of two events produce a size control. The high–low quartz transformation introduces tensile stresses; the eutectic sizing of the quartz particles in the original granite also constrains the level of stress development. The stress levels produced cause a particle size product of around coarse silt size.

Active sand seas and the formation of desert loess

Loess is a widespread eolian deposit dominated by coarse silt and very fine sand quartz grains (20–80 μm) that serve as an important archive of information on Quaternary climate change. Despite the intensive research during the past century on desert loess formation, its origin remains poorly understood and is still a fundamental problem in sedimentology. The ongoing debate is focused on the first stage of loess formation – the production of coarse silt quartz grains. Although laboratory experiments indicate the potential of dunes with their abundant quartz sand grains as a primary source for generating coarse silt grains, this concept has been generally rejected because field-based evidence of abrasion from sand grains (eolian abrasion) is scarce. Here we have adopted a global view of the sand sea - loess association and examine in detail several well-known late Pleistocene loess regions in different subtropical deserts (Africa, The Middle East and Arabia). This analysis demonstrates that all these loess regions are located downwind of adjacent sand seas. Together with evidence of mineralogical similarity between the loess and the sand dunes and the contemporaneous deposition of the loess and movement of the dunes, these observations suggest that sand seas are an important source for desert loess. Because there is only limited silt storage in sand dunes, we postulate that the silt grains comprising most of the loess are not reworked from the dunes but are generated probably through active eolian abrasion of the sand grains under past climates characterized by intensified winds. As a result, the role of sand dunes and eolian abrasion in the formation of desert loess may be more important than previously thought.

Abrasion-derived sediments under intensified winds at the latest Pleistocene leading edge of the advancing Sinai–Negev erg

Quaternary desert loess and sandstone–loessite relationships in the geological record raise questions regarding causes and mechanisms of silt formation and accretion. In the northern Sinai–Negev desert carbonate terrain, only sand abrasion in active erg could have produced the large quantities of quartzo-feldspathic silts constituting the late Quaternary northwestern Negev loess. In the continuum of source (medium to fine sand of dunes) to sink (silts in loess) the very fine sand is unaccounted for in the record. This weakens the sand abrasion model of silt formation as a global process. Here, we demonstrate that, as predicted by experiments, abrasion by advancing dunes generated large quantities of very fine sand (60–110 μm) deposited within the dune field and in close proximity downwind. This very fine sand was generated 13–11 ka, possibly synchronous with the Younger Dryas under gusty sand/dust storms in the southeastern Mediterranean and specifically in the northern Sinai–Negev erg. These very fine sands were washed down slope and filled small basins blocked by the advancing dunes; outside these sampling basins it is difficult to identify these sands as a distinct product. We conclude that ergs are mega-grinders of sand into very fine sand and silt under windy Quaternary and ancient aeolian desert environments.

Sulfur Isotope Ratio of Sulfates in Desert Sand and Loess in Arid and Semi-arid Regions of China

中国の乾燥・半乾燥地域に分布する沙漠砂・レスに含まれる硫酸塩の硫黄同位体比の分析を行った。その結果，タクラマカン沙漠とその周辺地域の硫黄同位体比は＋4.08～＋29.5‰の範囲であり，沙漠内に比べ，周辺山岳地帯で採取された試料が高い同位体比を示す傾向にあった。ジュンガル盆地―トルファン盆地は＋3.59～＋9.05‰の範囲であり，トルファン盆地に比べてジュンガル盆地が低い同位体比を示していた。また，この地域で採取された蒸発岩は＋10.45～＋10.98‰と高い同位体比を示した。チャイダム盆地―ムウス沙漠は＋4.18～＋22.0‰の範囲であり，中国東北部ナイマン周辺では＋8.80‰，－0.66‰の二つの結果が得られた。

10Be in desert sands, falling dust and loess in China

Abstract Cosmogenic 10Be is produced in the atmosphere, and deposits onto the surface of the earth mainly through wet precipitation and dust. Based on the analysis of 10Be in Chinese loess, we believe that 10Be in loess is composed of two components: locally precipitated atmospheric 10Be, and windblown 10Be adsorbed on the surface of silt grains. On the Loess Plateau, 10Be concentrations in loess and paleosol range from (1.4 to 2.8) × 108 atoms/g and (2.7 to 4.5) × 108 atoms/g, respectively. To investigate the sources of 10Be in loess, we measured 10Be in sand grains from deserts in western China and falling dust from the deposition regions. The results show that the 10Be concentrations in sand and dust are (1.1–5.1) × 107 atoms/g and (1.3–2.8) × 108 atoms/g, respectively. Loess and paleosol on the Loess Plateau both contain inherited 10Be adsorbed on silt grains from dust; most of the windblown deposited loess materials do not directly come from the Gobi and other sand deserts, but mainly from the loess–desert transitional zones, which are characterized by silt and dust holding areas.

Molluscs as evidence for a late Pleistocene and early Holocene humid period in the southern coastal desert of Peru (14.5°S)

The southern Peruvian coastal desert around Palpa, southern Peru (14.5°S) is currently characterized by hyper-arid conditions. However, the presence of two species of molluscs ( Scutalus, Pupoides) and desert-loess deposits indicates the past development of semi-desert and grassland ecosystems caused by a displacement of the eastern desert margin due to hydrological changes. Radiocarbon dating shows that the transition to a semi-arid climate in the southern Peruvian coastal desert took place during the Greenland interstadial 1, ∼ 13.5 cal ka BP. At the beginning of the Holocene, the mollusc fauna vanished due to increasing humidity and the development of grasslands. Dust particles were fixed by the grasses, as indicated by abundant Poaceae phytoliths, and desert loess was formed. The humid period we observe here is out of phase with the palaeoenvironmental records from the Titicaca region, which indicates dry conditions at that time. This paper offers a new idea for this contradiction: an orbitally driven meridional shift of the Bolivian high might have altered the moisture supply across the Andes.

A sequence of fluvial and aeolian deposits (desert loess) and palaeosoils covering the last 60 ka in the Opuwo basin (Kaokoland/Kunene Region, Namibia) based on luminescence dating

Arid and semi-arid areas are lacking in fossils for the reconstruction of the Quaternary climate. Therefore, sediments from ephemeral rivers are a welcome climatic archive in Namibia. Restrictions concerning the palaeoclimatic evidence lie in the fact that the sediments examined usually reflect the climate of the upper reaches of the rivers rather than the local climate. The upper reaches may belong to a different climatic zone or (damp) altitudinal zone of vegetation. In order to rule out such allochthonous influences in the study of the climatic history of the escarpments between the edge of the Namib Desert and the savannah, research is limited to sediments in basins and valley floors which are not subject to such influences. Research in the Opuwo basin has extended knowledge of the recent Quaternary climatic history of northern Namibia into the period since ca. 60 ka. The sedimentation of the basin floor pediments began with the deposition of fluvial sands. From ca. 55 ka, desert loess was blown in the course of aridification, but this was washed away from the pediments towards the centre of the basin by ca. 45 ka. Surface runoff was sometimes strong enough to transport gravels and sands. Thus geomorphological activity in this period also differed from the drier period which set in around 45 ka. It was characterised by the exclusive sedimentation of desert loess, even on the pediments at the basin's rim. There are a total of three cambic regosol soil formation phases. However, as there are no indications of new clay formation or eluviation the climate can only have been slightly more humid during their formation.

Dating methods and geomorphic evidence of palaeoenvironmental changes at the eastern margin of the South Peruvian coastal desert (14°30′S) before and during the Little Ice Age

The Atacama Desert is known to be one of the driest places on earth. However, in the study area between Ica and Nasca, South Peru (75–76°W, 14–15°S), palaeoclimate proxies show several semi-arid phases during the Holocene. A more humid period during the early Holocene favoured an open grass land, which allowed the accumulation of desert loess [Eitel, B., Hecht, S., Mächtle, B., Schukraft, G., Kadereit, A., Wagner, G., Kromer, B., Unkel, I., Reindel, M., 2005. Geoarchaeological evidence from desert loess in the Nazca-Palpa Region, Southern Peru: palaeoenvironmental changes and their impact on pre-Columbian cultures. Archaeometry, 47, 137–158.]. The Paracas and Nasca cultures existing in that region between the 9th century BC and 7th century AD had propitious conditions to evolve and to settle close to the rivers fed by the monsoonal precipitation in the Cordillera Occidental. Culminating aridity after 600 AD might have caused the collapse of the Nasca civilization. The onset of semi-arid climate towards the end of the Late Intermediate period (∼14th century AD), allowed a reoccupation of the area. Here, we present 14C and OSL data from sediments of river terraces, a debris flow and loess snails ( Scutalus sp.), indicating appreciable geomorphic activity between 1390 and 1714 cal AD. This phase is isochronous to the Little Ice Age period on the northern hemisphere and to an increase in the ice accumulation on the Quelccaya glacier in the Peruvian Altiplano [Thompson, L.G., Mosley-Thompson, E., Bolzan, J.F., Koci, B.R., 1985. A 1500-year record of tropical precipitation in ice cores from the quelccaya ice cap, Peru. Science, 229, 971–973.].

Orbital- and millennial-scale variations in Asian dust transport path to the Japan Sea

Orbital- and millennial-scale variations in the Asian monsoon and its probable association with the Dansgaard–Oeschger (D–O) Cycles have been demonstrated by previous studies. However, the origin and nature of such variations are poorly understood. The Japan Sea is located down wind of the arid areas of the Eurasian continent, and thus receives significant amounts of aeolian dust. Consequently, the hemipelagic sediments of the Japan Sea are expected to record a continuous aeolian dust accumulation, which may provide information about the past variations in Asian monsoon. Grain size, flux, and provenance of aeolian dust in the sediments were examined using two sediment cores obtained from the northern and southern parts of the Japan Sea in order to understand the spatial variability of Asian monsoon. Since the silt fraction of the detrital materials in hemipelagic sediments of the Japan Sea is composed predominantly of aeolian dust, the grain size, flux, and provenance of the silt fraction were examined. The Taklimakan Desert–Loess Plateau and Siberia–Northeast China areas were identified as possible source areas of aeolian dust in the sediments of the Japan Sea based on the Electron Spin Resonance (ESR) signal intensity and crystallinity of quartz. Moreover, the relative contribution of aeolian dust from each source area was estimated based on the ESR signal intensity and crystallinity of quartz. Aeolian dust fluxes from the two source areas at the southern site were then estimated. Grain size and flux of eolian dust at the northern site, and fluxes of aeolian dust from two source areas at the southern site show orbital-scale variations in harmony with the insolation change at 30° N in June. These results indicate orbital-scale changes in the extent of the arid area and/or frequency of dust storms in the Taklimakan Desert–Loess Plateau and Siberia–Northeast China areas, and suggest that southward (northward) shifts of the westerly jet axis occurred together with either intensified (weakened) winter monsoon, decreased (increased) winter precipitation in the Siberia–Northeast China area, or southeastward (northwestward) shifts of summer monsoon limit during periods of smaller (larger) insolation at 30° N in June. In addition to these orbital-scale variations, millennial-scale variations possibly associated with the D–O Cycles are observed in the grain size record of aeolian dust at the two sites. Based on analogy with orbital-scale variations, these millennial-scale variations are considered to reflect millennial-scale changes in the position of the westerly jet axis together with the changes either in winter monsoon intensity, winter precipitation, or position of summer monsoon limit. Our results suggest N–S oscillations in the westerly jet and summer monsoon limit, and changes in the intensity of the winter monsoon on orbital and millennial timescales.