interpreting temporal patterns in the late quaternary dust flux from asia to the north pacific abstract a comparative study of the literature on the late quaternary accumulation of aeolian dust in china japan and the pacific ocean shows three different temporal patterns of dust signals the main dust sources in asia the deserts and desert margins contain large amounts of wind suspensible allochthonous material commonly an increase in dust accumulation is taken as an indicator of increased aridity in the interior of asia differences among the signals of asian desert dust accumulation in different depositional areas indicate that this interpretation requires critical evaluation the dust supply of the desert regions was maximal during the pleistocene glacial maxima the widespread assumption that these stages were the most arid periods is not necessarily valid additional factors likely to lead to temporal and regional differences in dust accumulation are 1 eustatic sea level changes associated with changes in the amount of exposed shelf material and changes in the extent of sea ice affecting atmospheric changes 2 the frequency of meteorological situations allowing long range dust transport 3 the strength of the dust transporting winds 4 changes in the wet deposition rate and 5 the presence of a dust trapping vegetation cover temporal variations in dustiness in different depositional areas are the result of specific combinations of these factors simplified assumptions concerning the reasons for dust variability may lead to incomplete or even erroneous palaeoecological conclusions

dust a diagnostic of the hydrologic cycle during the last glacial maximum dust concentrations in ice of the last glacial maximum lgm are high in ice cores from greenland and antarctica the magnitude of the enhancements can be explained if the strength of the hydrologic cycle during the lgm was about half of that at present this notion is consistent with a large decrease 5 c in ocean temperature during the lgm as recently deduced from measurements of strontium and calcium in corals

penetration of atlantic westerly winds into central and east asia abstract the geographical position of the convergence zone where the western atlantic climate system meets the asian winter monsoonal system depends on their relative strength these individual strengths are determined by the intensities of the air pressure gradients over the north atlantic and the siberian high pressure cell respectively since the factors that forced those gradients varied throughout glacial interglacial cycles the location of the convergence zone should have moved accordingly both the grain size characteristics of wind blown sediments and climate modelling prove that the atlantic influence extended remarkably far eastward at times the northeastern tibetan plateau was at the cross road of western and monsoonal influences during the last glacial this region was dominated by an atlantic signal with millennial scale climatic oscillations that are similar to the north atlantic marine and greenland ice core records in contrast that same region was only under monsoonal influence during the last interglacial it implies that the atlantic winds penetrated more vigorously eastward in full glacial conditions than in interglacial conditions

the paleoclimatic record provided by eolian deposition in the deep sea the geologic history of wind the mineral component of pelagic sediment is brought to the deep sea by transport in the wind extraction and analysis of this dust allows estimation of the past aridity of the eolian source region via flux determinations and of the intensity of the transporting winds from grain size data these two parameters the grain size and mass flux of dust vary independently there are three significant sources of modern dust eastern and central asia northwest africa and arabia all in the northern hemisphere as the rainfall associated with the intertropical convergence zone is an effective barrier to southerly transport of dust the northern hemisphere is an order of magnitude more dusty than is the southern an asymmetry that has characterized most of the cenozoic eolian flux data show that most of the northern hemisphere was more arid during glacial maxima with 3 to 5 times as much dust transported during glacial stages than during interglacials only northwestern south america varied in the opposite sense the periodicity of quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the milankovitch cycles of orbital variability wind intensities vary on a shorter time scale than the general 100 kyr cycles of glaciation and general aridity eolian grain sizes display forcing at precessional 19 and 23 kyr tilt 41 kyr and at approximately 30 kyr periodicities as a result the generalization that winds are uniformly stronger during glacial times is not valid whole cenozoic records show that the largest change in dust flux an order of magnitude increase occurred in the northern hemisphere and reflects continental drying associated with the late pliocene onset of northern hemisphere glaciation southern hemisphere eolian records show no sign of paleoclimatic changes in the late pliocene the most important change in cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the paleocene eocene boundary before then latest cretaceous and paleocene winds were essentially as strong as those of the late cenozoic this shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 ma

cyclic magnetite dissolution in pleistocene sediments of the abyssal northwest pacific ocean evidence for glacial oxygen depletion and carbon trapping the carbonate free abyss of the north pacific defies most paleoceanographic proxy methods and hence remains a blank spot in ocean and climate history paleomagnetic and rock magnetic geochemical and sedimentological methods were combined to date and analyze seven middle to late pleistocene northwest pacific sediment cores from water depths of 5100 to 5700 m besides largely coherent tephra layers the most striking features of these records are nearly magnetite free zones corresponding to glacial marine isotope stages miss 22 12 10 8 6 and 2 magnetite depletion is correlated with organic carbon and quartz content and anticorrelated with biogenic barite and opal content within interglacial sections and mid pleistocene transition glacial stages mis 20 18 16 and 14 magnetite fractions of detrital volcanic and bacterial origin are all well preserved such alternating successions of magnetic iron mineral preservation and depletion are known from sapropel marl cycles which accumulated under periodically changing bottom water oxygen and redox conditions in the open central northwest pacific ocean the only conceivable mechanism to cause such abrupt change is a modified glacial bottom water circulation during all major glaciations since mis 12 oxygen depleted antarctic bottom water aabw sourced bottom water seems to have crept into the abyssal northwest pacific below 5000 m depth thereby changing redox conditions in the sediment trapping and preserving dissolved and particulate organic matter and in consequence reducing and dissolving both biogenic and detrital magnetite at deglaciation a downward progressing oxidation front apparently remineralized and released these sedimentary carbon reservoirs without replenishing the magnetite losses

continental erosion and the cenozoic rise of marine diatoms marine diatoms are silica precipitating microalgae that account for over half of organic carbon burial in marine sediments and thus they play a key role in the global carbon cycle their evolutionary expansion during the cenozoic era 66 ma to present has been associated with a superior competitive ability for silicic acid relative to other siliceous plankton such as radiolarians which evolved by reducing the weight of their silica test here we use a mathematical model in which diatoms and radiolarians compete for silicic acid to show that the observed reduction in the weight of radiolarian tests is insufficient to explain the rise of diatoms using the lithium isotope record of seawater as a proxy of silicate rock weathering and erosion we calculate changes in the input flux of silicic acid to the oceans our results indicate that the long term massive erosion of continental silicates was critical to the subsequent success of diatoms in marine ecosystems over the last 40 my and suggest an increase in the strength and efficiency of the oceanic biological pump over this period

eolian quartz flux to lake biwa central japan over the past 145 000 years abstract eolian quartz flux eqf g cm 2 103yr 1 to lake biwa central japan provides direct information on variations of the east asian winter monsoon lake biwa sediments spanning the past ca 145 000 yr are characterized by two main periods when eqf values were significantly greater than 5 50 g cm 2 103yr 1 and two main intervals during which eqf values were lower two periods with eqf values 5 50 g cm 2 103yr 1occurred from ca 145 000 to 125 000 and 73 000 to 13 000 yr b p while times of lower eqf values occurred from ca 125 000 to 73 000 yr b p and around ca 5500 yr b p between ca 125 000 and 73 000 yr b p three minimum eqf values and two intervening peaks of slightly higher eqf values are recorded eqf increased markedly from ca 73 000 to 13 000 yr b p whereas between ca 53 000 and 20 000 yr b p the values recorded were relatively lower than those recorded during either the preceding or the subsequent episodes the data imply that the east asian winter monsoon strengthened during the periods when eqf values were high and weakened during the intervals with low eqf values the eqf record of lake biwa can be correlated with the grain size record of the quartz fraction in chinese loess and with the specmap marine 18o record however the eqf record apparently lags ca 5000 yr behind the loess and 18o records during stage 6 5 and 2 1 transitions and ca 10 000 yr during stage 5 4 transition these apparent lags could be due to problems with the chronology alternatively they may imply that the eolian quartz flux depended more on the extent of dust source regions than on wind intensity during these transitions

quantification of the contribution of pedogenic magnetic minerals to magnetic susceptibility of loess and paleosols on chinese loess plateau paleoclimatic implications 1 this study investigates the grain size dependent properties of magnetic minerals in chinese loess samples with an aim to better understand the contribution of pedogenesis to magnetic susceptibility enhancement of the paleosols samples of loess and paleosol were divided into two fractions magnetic extracts and residues by extraction in a weak magnetic field both the raw samples of loess and paleosol i e before magnetic extraction and the samples of residues after magnetic extraction were then separated into five grain size fractions by gravitational settling and centrifugation low field magnetic susceptibility ms and the frequency dependent ms of these samples were measured and samples treated with citrate bicarbonate dithionite cbd to remove the iron oxides were also analyzed for loess samples coarse grains 2 m contribute more than 50 of the magnetic signal the 0 3 m fraction considered to be of pedogenic origin contributes 22 before magnetic extraction or 36 after magnetic extraction of the magnetic signal in loess a different pattern is found in the paleosol samples for these samples the 0 3 m fraction contributes 60 before magnetic extraction or 71 after extraction to the bulk ms in addition 91 of the magnetic signal in paleosol s5 is cbd extractable the cbd technique is able to extract pedogenic superfine grained magnetic particles and coarse dust magnetic particles these results confirm that pedogenesis leads to enhanced ms and helps to quantify the multiple origins of the bulk magnetic material that defines loess and paleosol ms this approach provides a reliable estimate of the contribution of pedogenic magnetic materials to ms our study indicates that multiple methods should be employed in analyzing paleoclimatic changes recorded by ms in the chinese loess paleosol sequences

grain size of fine grained windblown sediment a powerful proxy for process identification abstract dust transport by the wind is not a uniform process but may occur in different modes according to source area conditions and transport height and distance subsequently these differences are expressed in terms of grain size and fluxes of the aeolian deposits transport distances may vary from several tens of meters to thousands of kilometers while the height accordingly may vary from meters to more than thousand meters a relation with grain size distributions may be established on the base of type occurrences of different loess facies three main loess populations and several subpopulations of primary windblown origin may be defined according to their grain size dominated by fine sand to very coarse silt silt and very fine silt to clay respectively each of them reflects a specific aeolian process and transport conditions it follows that the grain size distribution of a loess deposit is an excellent proxy for the reconstruction of aeolian processes and wind circulation patterns apart from primary pure wind deposition loess may also be affected by secondary post depositional processes examples are settling of loess particles in a lacustrine setting and reworking by rivers or surface runoff although the primary loess characteristics are maintained reworking processes leave also their imprint in the grain size distribution as a useful tool for secondary process identification

late pleistocene continental climate and oceanic variability recorded in northwest pacific sediments core v21 146 provides a continuous record of northwest pacific pelagic sedimentation spanning the past 530 000 years downcore variations of 18o from benthic foraminiferal calcite were correlated to the specmap record to provide an age model for late pleistocene paleoclimatic and paleoceanographic variation fluxes of caco3 and eolian material have been determined by combining information about linear sedimentation rates bulk densities and component percentages the mass accumulation rate mar of caco3 varies from 58 to 1435 mg cm kyr 1 and exhibits the pacific ocean pattern of higher accumulation associated with glacial periods for the younger portion of the record however the pattern of caco3 accumulation appears to shift during the mid brunhes so that prior to 350 000 years ago it exhibits an opposite pattern of greater accumulation during interglacial periods the flux of eolian dust is a proxy measure of the source area aridity and correlates well to the loess soil stratigraphy in china dust fluxes range from 43 to 718 mg cm kyr 1 and are greater by a factor of about 4 during glacial times cross spectral analysis of the eolian flux and oxygen isotope records shows an inphase and coherent relationship at each of the major orbital periodicities of 100 41 and 19 kyr eolian grain sizes generally used as a proxy record of wind intensity range from 8 4 2 9 m to 6 2 13 6 m a change in grain size variability occurs near about 300 ka such that the older portion is characterized by lower frequency and higher amplitude fluctuations than the younger portion power spectra for the eolian grain size record show dominant peaks at 100 and 50 kyr and a broad 25 35 kyr peak but coherency to the oxygen isotope curve at only the 100 and 33 kyr periodicities the phase relationship for eccentricity between the eolian grain size and 18o records suggests that at the 100 kyr frequency the smallest grain sizes are associated with glacial conditions

the role of the indian summer monsoon and the mid latitude westerlies in himalayan glaciation review and speculative discussion new dates for late quaternary glaciations in the himalayas show that during the last glacial cycle glaciations were not synchronous throughout the region rather in some areas glaciers reached their maxima at the global glacial maximum of c 18 20 ka bp whereas in others glaciers were most extensive at c 60 30 ka bp comparison of these data with palaeoclimatic records from adjacent regions suggest that on millennial timescales himalayan glacier fluctuations are controlled by variations in both the south asian monsoon and the mid latitude westerlies

contrasting dust supply patterns across the north western chinese loess plateau during the last glacial interglacial cycle loess grain size distributions of four loess paleosol sequences located on a west to east transect from the north eastern tibetan plateau tp to the north western chinese loess plateau clp are unmixed with the end member modelling algorithm emma the unmixing results indicate that the loess is a mixture of three components representing very fine sandy coarse silty and medium silty loess the proportional contributions of these loess components in conjunction with loess mass accumulation rate estimates reveal that during the last glacial interglacial cycle two contrasting dust supply patterns were active over the north eastern tp and the north western clp a constant supply of medium silty loess and an episodic supply of coarse silty loess and fine sandy loess the variable input of the two coarse dust components is the main cause for the variation in grain size patterns and mass accumulation rates between the studied sites alluvial fans and river floodplains in the intra mountainous basins of the tibetan plateau and the deserts of northern china and inner mongolia are the main dust source areas for loess deposits of the north eastern tp and the clp respectively the sensitivity of these dust source areas to climate variations determines the timing of dust transport and deposition in general high dust fluxes are recorded during the last glacial period marine isotope stages mis 2 4 and low dust fluxes during the last interglacial mis 5 the loess sections in the two regions tp clp show contrasting dust flux patterns during mis 3 the records from the clp show a relatively low dust input in this period most likely related to increased humidity in the northern dust sources deserts where an increased vegetation cover minimized dust deflation in contrast to this the tibetan plateau records appear to show increased and highly variable dust fluxes for mis 3 alternating periods of increased humidity and aridity on the tibetan plateau during mis 3 have resulted in an overall increase in fluvial alluvial activity hence the enhanced availability of exposed fine grained fluvial alluvial sediments on the tibetan plateau facilitated dust entrainment which resulted in an increased dust supply towards the tibetan plateau sites during this period

grain size characterization of reworked fine grained aeolian deposits abstract after a previous review of the grain size characteristics of in situ primary fine grained aeolian deposits reworked secondary aeolian deposits as modified in lacustrine environments and by alluvial and pedogenic processes are discussed in this paper as a reference the grain size characteristics of primary loess deposits are shortly described commonly pedogenesis and weathering of primary loess may lead to clay neoformation and thus to an enrichment in grain diameters of 4 8 m a size which is comparable to the fine background loess remarkably the modal grain size values of primary loess are preserved after re deposition in lakes and floodplains but secondary lacustrine settings show a very characteristic admixture with a clayey population of 1 2 5 m diameter due to the process of settling in standing water similarly alluvial settings show often an addition with coarse grained sediment supplied by previously eroded sediment however floodplain settings show also often the presence of pools and other depressions which behave similarly to lacustrine environments as a result alluvial secondary loess sediments are characterized by the poorest grain size sorting when compared with the other secondary loess and primary loess despite the characteristic texture of each of these deposits grain size characteristics of the described individual sediment categories are not always fully diagnostic and thus grain size analysis should be complemented by other information as sedimentary structures and fauna or flora to reliably reconstruct the sedimentary processes and environments

landscape evolution and development of eolian modified unconsolidated gravel surfaces and yardangs in the hami basin china abstract unconsolidated gravel surfaces alluvial terraces and yardangs are widespread in the turpan hami topographical depression of nw china these landscape features provide an opportunity to investigate the wind erosion history and landscape evolution of the hami basin along with anomalously high near surface wind speeds these features suggest that the hami basin was an important source of mineral dust during the pleistocene we present the results of an integrated geomorphologic stratigraphic and chronologic study on the genesis of unconsolidated gravel surfaces and yardang fields in the hami basin mobilization of silt to sand sized fractions from the surfaces of the quaternary alluvial sediments occurred under generally dry conditions leaving unconsolidated gravels to organize into inflationary type desert pavement surfaces type 1 and deflationary type desert pavements surfaces type 2 we propose that the pleistocene alluvial terraces of the hami basin formed principally during wetter intervals which are assumed to correspond with interglacial periods the coarse grained alluvium atop terraces protected underlying strata from wind erosion the bedrock then began to be sculpted into yardangs after the removal of the coarse grained armoring surfaces with mostly sub horizontal bedding the geometries of yardangs was controlled by lithology wind and water erosion based on uav 3d models optically stimulated luminescence osl and electron spin resonance esr ages we find an erosion rate of 0 23 mm yr since 200 ka this combined with widespread uneroded strata armored by unconsolidated gravel surfaces indicates that the hami basin had considerable dust emission potential in the past

making the material the formation of silt sized primary mineral particles for loess deposits abstract in 1911 e e free wrote it may be well to point out that there are really two problems of the loess a problem of the origin of material and a problem of deposition the problem of the origin of material has been relatively neglected in comparison to problems of deposition of the formation of deposits and latterly in comparison to studies of loess stratigraphy if we are to understand the true nature of loess this fascinating material we need to be aware of all the significant events in the processing of the particles which form a deposit a large amount of silt sized primary mineral material has to be produced to make a loess deposit loess is a quaternary phenomenon and we see the entire process being completed in quaternary times some powerful quaternary forces are able to produce very large amounts of granular material glacial grinding vigorously supported by smalley 1966a provides an obvious source for some loess material but the vast deposits of north china and central asia have no obvious glacial connection however they are adjacent to high asia a region of vast tectonic activity and an enormous release of terrestrial energy in the last several million years high asia is a large producer of loess silt much of which finds its way south to make the ganges delta but large amounts are moved north and east to make huge loess deposits africa presents a problem recent work e g coude gaussen 1987 suggests that there is a region of peri saharan loess but earlier observations e g penck indicate quite clearly that the sahara region is free of loess rathjens 1928 in his study of tripolitania indicated the presence of loess like deposits and described their nature but he did point out that 90 of the granular material was greater than 100 pm in diameter which would remove it from a strict loess definition on the soil taxonomy map of africa there is a belt of soil d3 which could include considerable loess deposits it stretches from the matmata loess in tunisia across northern libya to the western part of the egyptian coastal region if d3 contains loess this points to a genuine desert loess particle producing mechanism operating in the sahara

sr nd isotopic and ree geochemical constraints on the provenance of fine grained sands in the ordos deserts north central china surface sands and sandstones in the ordos region alluvial sands from the surrounding mountains and fluvial sands in the yellow river are systemically collected for granulometric mineralogical geochemical and isotopic analyses fine grained fractions are small for the ordos desert sands as well as alluvial sands of the surrounding mountains whereas they are abundant for fluvial sands of the yellow river quartz feldspar and muscovite are common minerals but their relative proportions are variable for various sediments ree geochemical compositions are roughly identical but isotopic compositions particularly end 0 values of fine grained sands vary significantly in the ordos deserts based on the integrated sr nd isotopic data the possible source areas are isotopically divided into four regions for fine grained sands in the ordos deserts region a the helan yin mountain ranges region b the ordos sandstone areas region d the tarim basin the alxa plateau the hexi corridor the northern qinghai xizang plateau and the yellow river drainage and region e the jungger basin and the southern mongolia through geochemical and particularly isotopic comparisons in combination with expert meteorological and geographical knowledge fine grained sands in the ordos deserts originated both from weathering of local sandstones and from blowing of fine sediments in region d the input from region d is subordinate to the local source for fine grained sands in the ordos deserts contributions from weathering of local sandstones might be roughly homogeneous from the west to the east of the ordos region as well as the input of fine grained sediments from remote areas such as the tarim basin the qinghai xizang plateau and the alxa plateau geographical nd isotopic variations of fine grained sands in the ordos deserts might be caused by the input from adjacent places such as the yellow river drainage due to the near surface winds

gravel deposits on the margins of the qinghai xizang plateau and their environmental significance abstract the thick deposits of quaternary gravels found along the margins of the qinghai xizang tibet plateau have been studied for decades here we concentrate on two sediment series widely developed along the kunlun tian shan and qilian mountains the xiyu formation a molassic series of lower pleistocene age and the middle and upper pleistocene gobi gravels these two series with their contained fossil assemblages throw light on the question of the mean altitudes and uplift rates of the tibetan plateau during the quaternary and on the related problem of the vigour and penetration of advected summer monsoonal moisture into central asia the sedimentary series on the northern piedmonts of the tibetan plateau considered here challege the view that plateau uplift accelerated in the upper pleistocene they indicate that the plateau had reached mean altitudes in the range 4000 5000 m perhaps by the end of the lower pleistocene and certainly by the middle pleistocene this diminished the incursion of southerly monsoonal moisture and so imposed arid and semi arid conditions across much of a vast region north of the plateau which among other effects precluded widespread mountain glaciation in the lower pleistocene and general ice sheet glaciation of the tibetan plateau as a whole

aeolian sediments on the north eastern tibetan plateau abstract aeolian sediments on the tibetan plateau are an important archive of palaeoclimatic information this study presents a detailed analysis of sediments from the donggi cona catchment on the north eastern part of the tibetan plateau long and short distance sediment transport leads to a complex pattern of aeolian sediment deposition that depends on climatic changes as well as on the availability of sediments based on the largest dataset of osl datings 51 from a single catchment on the tibetan plateau so far different periods of increased sediment transport have been reconstructed increased aeolian deposition in this high elevation environment started in the early holocene with the accumulation of sands from around 10 5 to 7 ka loess sediments have been preserved from a period from 10 5 to 7 5 ka both archives are related to the strengthening of the asian summer monsoons characterized by wetter and warmer climate this change in climate supported the trapping of aeolian sediments under full monsoon conditions from around 9 ka onwards fluvial processes resulted in erosion of the aeolian archives and the formation of colluvial sediments until 6 ka a dry and cooler climate resulted in the reactivation of dune sands from 3 ka to present possibly in combination with stronger human influence aeolian sediments on the tibetan plateau therefore indicate two different climatic modes during the early holocene wetter conditions were favourable to retain aeolian sediments the reactivation of sediment in the late holocene due to small scale disturbances in the vegetation cover points to a cooler and drier climate

quaternary fluctuations in the northern hemisphere trade winds and westerlies the mass accumulation rate and grain size of the total colian component isolated from pelagic sediment in two north pacific cores piston core kk75 02 under the prevailing westerlies and deep sea drilling project site 503 beneath the trade winds have been used to evaluate changes in the intensity of atmospheric circulation and source area aridity over the past 700 000 yr the eolian grain size a direct indicator of wind intensity fluctuates at periodicities similar to those calculated for the earth s orbital parameters of precession obliquity and eccentricity both sites display greater variability in wind intensity prior to 250 000 yr ago eolian accumulation rates an indicator of source area aridity fluctuate at periodicities similar to those of the glacial interglacial cycles lower eolian accumulation rates during colder glacial times most likely reflect increased glacial age humidity at central asian and central american source areas

snowline elevation and eolian dust flux in the japanese islands during isotope stages 2 and 4 abstract changes in snowline elevation and dust flux clearly contrast the paleoenvironments of the japanese islands during isotope stages 2 and 4 a lower snowline elevation and a smaller dust flux during isotope stage 4 suggests a climate with a cold and dry summer dominated by winter monsoonal precipitation between 70 and 55 ka a higher snowline elevation and a larger dust flux during isotope stage 2 indicates a climate with a cold and very dry summer with a minimized winter monsoon precipitation especially around 18 ka a drier climate during stage 2 can be explained by 1 a lowering of sea surface temperature in the japan sea by blocking the warm tsushima current during a sea level lowering 2 the extension of sea ice over the northern part of the japan sea and the sea of okhotsk and 3 weakening of the summer monsoon and the southern migration of the polar front the increase in dust flux in both stages 2 and 4 suggests an intensification of westerlies and winter monsoon during these stages a bigger dust flux in stage 2 than stage 4 reflects 1 stronger westerlies and winter monsoon and 2 the emergence of continental shelf which became an important dust source

mineralogical and geochemical trends across the loess plateau north china abstract the mineralogy and major element geochemistry of malan loess the uppermost loess layer were studied along two transects from the baiyu mountains in the north to the wei river lowlands 350 km to the south the mineralogy of the coarse silt fine silt and clay fractions is generally uniform across the loess plateau however in the coarse silt fraction there is a decrease in the weight percentage of heavy minerals and in the hornblende epidote ratio southwards also in the clay fraction mica contents increase slightly southwards and smectite increases northwards along one transect the major element geochemistry of malan loess shows several clear trends silicon sodium and magnesium generally decrease from north to south while aluminium iron potassium titanium and manganese generally increase the observed mineralogical and chemical patterns appear to be mainly related to the changing particle size from sandy loess in the north to clayey loess in the south slightly higher potassium and phosphorus contents in the clayey loess suggest that this loess has a greater natural fertility overall the results confirm particle size evidence that malan loess was derived from a desert source area to the north and northwest of the loess plateau

dust transport and the question of desert loess formation although contemporary dust storms are frequent in arid and semi arid areas desert loess deposits are poorly developed much of the world s loess occurs in mid latitude areas which experienced glaciation during the pleistocene ocean core evidence indicates that dust transport from sub tropical deserts increased during cold stages of the pleistocene but loess formed only on certain desert margins for reasons which have not been fully explained this paper re examines the mechanisms of dust transport and deposition and the circumstances leading to the accumulation of thick loess typical loess is composed mainly of medium silt grains which are transported in short term suspension a few metres above the ground significant thicknesses of loess form only when dust is trapped within a limited area often relatively close to the source dust particles finer than 20 m are transported mainly in long term suspension over a greater height range and may be widely dispersed the availability of silt and the frequency magnitude and direction of dust transporting winds are important factors governing the potential for loess formation but the existence of a suitable dust trap is particularly important traps may be formed by topographic obstacles areas of moist ground or vegetated surfaces vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid latitudes during the pleistocene dust blown during glacial periods from certain deserts notably in sinai soviet central asia and china accumulated as loess in neighbouring semi arid regions on the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps during most of the holocene net dust deposition rates in all desert marginal areas have been too low for significant loess accumulation this is mainly due to a reduction in silt availability and a tendency towards landscape stability reported dust storm frequencies during the past 50 years over estimate the longer term holocene dust flux due to the effects of human activities much modern dust owes its origin to erosion of cultivated soils in semi arid areas and is finer than typical loess

major asian aeolian inputs indicated by the mineralogy of aerosols and sediments in the western north pacific recent atmospheric chemistry studies have shown that substantial quantities of soil material are being transported out of asia and across large regions of the north pacific1 2 in the central north pacific the estimated1 6 12 1012 g yr 1 and measured2 20 1012 g yr 1 deposition rate of mineral aerosols could account for a substantial fraction of the non biogenic portion of deep sea sediments in this region where sedimentologists have previously estimated that 75 95 3 4 of the surface sediment is derived from atmospheric dust fallout the quantitative importance of wind transported aeolian material to sedimentation in the north pacific would be more firmly established if it could be shown that the composition of the present day soil aerosol is similar to that of mineral particles in the underlying sediments which have been deposited over a period of thousands of years here we compare the compositions of soil aerosols and sediments collected from the western north pacific and find that their mineralogies are identical except for one mineral this supports the hypothesis that the sediments are heavily impacted by aeolian sources

late quaternary records of the atmospheric input of eolian dust to the center of the chinese loess plateau abstract the concentrations of 15 elements and mineral grain size distributions were determined for late quaternary loess sediments from the chinese loess plateau particle size distributions of the loess were approximately log normal for each of three modes i e 0 04 to 1 m diameter 2 to 20 m and 20 to 200 m that integrate the transport and deposition of eolian dust the mass particle size distributions of typical crustal elements si ca fe ti k mn and v in paleosol and loess samples show patterns similar to those of mineral aerosols but higher concentrations occur in the submicrometer sediment particles absolute principal component analysis of the elemental data suggests that the loess can be described in terms of synsedimentary and postdepositional concentration and subvertical migration of caco3 and various elements through the deposit a variety of geochemical indicators in the loess are apparently sensitive to asian monsoon variations these include the mass ratio of course to fine particles the concentration ratios of certain trace elements the elemental fluxes associated with eolian dust and the bulk concentration of ca in the sediments

a direct link between the china loess and marine δ 18 o records aeolian flux to the north pacific most studies of quaternary climates conducted during the past 150 years have focused on glacial sequences and for the past 25 years marine sediments have provided significant further understanding of glacial and interglacial palaeoclimates an important goal of palaeoclimatology however to link the well known detailed continental records to marine records of similar resolution has yet to be realized the classic loess sequences of china provide an apparently continuous record of continental climate over the past 2 4 million years1 2 the oxygen isotope 18o timescale developed from the marine record3 gives us a global chrono stratigraphy of late pleistocene climates previously it has been suggested in studies of the aeolian component in pelagic sediments that it may be possible to link these two records directly by finding a suitable core from the north west pacific downwind from china4 5 here we report the results of such an effort for core v21 146 fig 1 and tie the last 500 000 years of the chinese loess soil sequence directly to the 18o timescale and in so doing refine the dating of regional loess and soil horizons

late quaternary aeolian dust input variability on the chinese loess plateau inferences from unmixing of loess grain size records abstract the grain size distributions gsds of loess paleosol sequences may provide valuable information on provenance transport and paleoclimate variability and consequently a wide range of methods to extract genetic information from loess gsds has been proposed here it is shown that a genetically meaningful decomposition of a series of late quaternary loess grain size records extending across the chinese loess plateau can be accomplished with the end member modelling algorithm emma the unmixing results in conjunction with loess accumulation rate estimates reveal that two contrasting dust supply patterns were active over the loess plateau during the last glacial interglacial cycle i a background sedimentation pattern that was dominant during interglacial periods especially over the central and southern parts of the loess plateau is reflected by the constant flux of the fine grained loess component ii an episodic highly variable dust input pattern that was dominant during glacial periods throughout the loess plateau and noticeable during interglacial periods mainly over the northern loess plateau and almost disappearing over the southern loess plateau is reflected in the admixture of two coarse grained loess components a genetic interpretation and the paleoclimatic significance of the mixing model are provided by comparison of the modelled end members with modern dust samples in terms of their gsd and flux rates

timing and provenance of loess in the sichuan basin southwestern china abstract loess like sediments locally called the chengdu clay are widely distributed in the sichuan basin at the eastern margin of the tibetan plateau tp southwest china geomorphology sedimentology pedology grain size distributions and quartz oxygen isotopic ratios collectively demonstrate the aeolian origin of the chengdu clay and its main source area on the tp evidence also supports its transport by the westerlies and the tibetan winter monsoon the magnetostratigraphy of the huagai loess section of the chengdu clay on the highest river terrace of the anchang he river indicates that the sediments began accumulating at about 825 ka bp this suggests the establishment of a circulation system similar to the present day system related to the rapid uplift of the tp at that time

geomorphological and palaeoclimate dynamics recorded by the formation of aeolian archives on the tibetan plateau abstract aeolian sediments are an important archive for palaeoclimate reconstructions on the tibetan plateau the accumulation of aeolian sediments and the formation of palaeosols are used as a proxy for dry and wet phases respectively during the last decade the number of osl optically stimulated luminescence ages of aeolian sediments has been rapidly increasing this study summarises the results from more than 300 individual osl ages despite the widespread occurrence of aeolian sediments on the tibetan plateau nearly all osl samples are located on the northeastern and the southern part in most regions the strongest accumulation occurred during the late glacial and in the early holocene this coincides with the strengthening of the asian summer monsoon on the tibetan plateau and wetter climate conditions after the relatively dry glacial times the development of a sufficient vegetation cover acting as a sediment trap seems to be an important requirement for the preservation of aeolian archives short dry periods like the younger dryas led to a more enhanced aeolian accumulation without destruction of previously formed archives in some regions the formation of palaeosols occurred mainly during the wet early and mid holocene but the number of ages is relatively small in contrast a second phase of aeolian accumulation during the late holocene was related to the weakening of the asian summer monsoon and drier climate conditions this indicates that the reaction of aeolian accumulation on the tibetan plateau depends on the previous climate state a comparison between the osl ages from aeolian sands and loess deposits yields a different timing of deposition of these two sediment types while the sand deposits in different basins resemble wet and dry phases loess deposits show no common signal between the analysed areas this indicates an influence of local factors such as varying source areas and erosive processes on the slopes in general the aeolian archives in the high mountain environment are governed by numerous factors besides the climatic influence

dust deposits on la graciosa island canary islands spain texture mineralogy and a case study of recent dust plume transport abstract la graciosa volcanic island evolved in the late pleistocene holocene it is situated along the northeast of the canarian archipelago with a subaerial surface of 27 km2 this island is located close to the western sahara being 80 km from the north western african continental shelf and 145 km from the coast the island supports a very small permanent population the climate is coastal arid 116 mm precipitation per annum regular occurrences of saharan dust plumes about 30 of the year with a dust accumulation rate of 20 g 2 year 1 produce sedimentation of dust on the island of ca 540 t year 1 wind blown sediments cover a large area more than 52 of the otherwise volcanic island forming sandy beaches sand sheets nebkhas and other aeolian deposits re distributed dust deposits often occur on the leeward sides of inter volcano areas inter dune areas and in endorheic sediment traps many of these deposits are edaphized and carbonatized textural analysis of the dust deposit samples shows a variable proportion of silt 80 20 clay 19 9 and fine to very fine sand 71 4 the general mineralogical composition of these sediments as measured by xrd and sem eds is calcite 26 illite 26 quartz 11 augite 10 aragonite 6 anorthite 8 kaolinite 5 and montmorillonite 3 the mineralogy in different size fractions is very consistent for example quartz content decreases from 15 in the coarse silt fraction to 5 in the fine sand a mineral and grain size comparison with airborne dust collected on gran canaria island was undertaken close similarities were found in the two sample sets such closely matched characteristics point to a similar origin for both airborne dust and dust deposits in line with the saharan plume dust that regularly traverses this archipelago it is unusual to find so much quartz in the fine sand fraction of these aeolian dust deposits individual quartz grains with an intermediate axial length of 160 m were identified by sem eds previous investigators have found similar giant particles in long range transported aeolian dust middleton et al 2001 in contrast to the classic model for gravity settling of airborne dust particles the debate on this subject remains open for discussion we present evidence of long range wind transported large mineral particles and a 2004 case study of uplift wind velocity vs plume dust generation as well as a transport efficiency model that can explain the existence of fine sand in the dust deposits on la graciosa island and in the airborne samplers on gran canaria island recent saharan dust shows that about 10 of similar coarse grained particulate matter is also present

dust sources and deposition during the last glacial maximum and current climate a comparison of model results with paleodata from ice cores and marine sediments mineral dust aerosols in the atmosphere have the potential to affect the global climate by influencing the radiative balance of the atmosphere and the supply of micronutrients to the ocean ice and marine sediment cores indicate that dust deposition from the atmosphere was at some locations 2 20 times greater during glacial periods raising the possibility that mineral aerosols might have contributed to climate change on glacial interglacial time scales to address this question we have used linked terrestrial biosphere dust source and atmospheric transport models to simulate the dust cycle in the atmosphere for current and last glacial maximum lgm climates we obtain a 2 5 fold higher dust loading in the entire atmosphere and a twenty fold higher loading in high latitudes in lgm relative to present comparisons to a compilation of atmospheric dust deposition flux estimates for lgm and present in marine sediment and ice cores show that the simulated flux ratios are broadly in agreement with observations differences suggest where further improvements in the simple dust model could be made the simulated increase in high latitude dustiness depends on the expansion of unvegetated areas especially in the high latitudes and in central asia caused by a combination of increased aridity and low atmospheric co2 the existence of these dust source areas at the lgm is supported by pollen data and loess distribution in the northern continents these results point to a role for vegetation feedbacks including climate effects and physiological effects of low co2 in modulating the atmospheric distribution of dust

pleistocene loess in the humid subtropical forest zone of east asia 1 loess deposits in asia have been used as indicators of palaeoclimate because they are usually found bordering deserts this paper reports extensive and thick deposits of loess in tropical southwest china between latitudes 18 and 23 30 n which is 1300 km south of known and extensively researched loess deposits in north china the present climate of the reported loess areas is hot and humid with mean annual rainfall of 1000 2000 mm and vegetation of subtropical evergreen broadleaf forest this compares with less than 400 mm rainfall and vegetation of semi desert steppe in areas of current loess accumulation on desert margins in north china the source area of the loess which is dated by optical luminescence to the late pleistocene from 90 222 ka is thought to be the exposed east asian shelf which was up to 140 m below present sea level during quaternary arid phases recent research on the nature of the shelf environment and the relatively large particle size of the loess suggests a local origin the reported loess is not interbedded with palaeosols and small amounts of soil cover the loess compared with well developed soils in loess in semiarid regions in north china this is explained by elimination of the supply source by sea level rise following each arid phase as continued dust supply appears necessary for soil to form this preliminary report of loess in southwest china conflicts with palynological evidence and suggests that recent reconstructions of pleistocene aridity in east asia may be conservative

late pleistocene evolution of salt lakes in the qaidam basin qinghai province china abstract the qaidam basin is a tectonically controlled depression at 2700 m altitude on the northern margin of the qinghai xizang tibet plateau rimmed by the high aljun qilian ranges to the north and kunlun peaks to the south rising above 5000 m the basin floor experiences hyper arid conditions with evaporation precipitation ratios exceeding 100 1 periglacial and glacial conditions prevail on montane peaks with permafrost above 3800 m on the basin floor small waterbodies lie on extensive halite salt flats on the largest the qarhan salt flat 5800 km2 in extent the small lakes dabuxun and taijinier represent contracted remnants of a once expansive water body throughout neogene time a succession of clastic sediments indicates lake depo centre migration from nw to se towards late pliocene evaporitic conditions developed with formation of gypsum halite celestite mirabilite and bloedite continued deformation through pleistocene time has developed a series of nw se trending domes and troughs this setting has continued to exert a controlling influence on basin and brine evolution with progressive development through sulfate to chloride brines as dry conditions intensified in late pleistocene expanded lake conditions qarhan mega lake corresponding to near the 2800 m contour indicate the existence of past climates much more humid than those of today a drilling program has defined the lateral and vertical extent of evaporitic halite deposits freshwater clastics and shell deposits which predate the halite phase indicate humid lacustral conditions until after 40 000 30 000 yr b p desiccation from about 25 000 yr until about 9000 yr b p resulted in deposition of halite to 60 m thick with thin parting muds representing brief returns to more humid environments high lake shorelines with aragonitic ooids surrounding the dry playa kunteyi and high level ostracod rich beach and near shore deposits on xiao qaidam attest to the presence of expanded late pleistocene lacustral conditions on lakes other than qarhan preceding the final evaporite stage clastic sediments associated with the halite are dominated by loessial silts equivalent to the late pleistocene malan deposits of central china simultaneous halite and loess deposition reflect the dry conditions that developed in the qaidam basin during the latter part of the last glaciation and which have persisted ever since the final evolution of brines has resulted in development of extensive potassium rich brines and salts with the mineral carnallite providing the basis for a large potash extraction industry near golmud the hydrologic history documented by the analyses of lake sediments reflects the interaction of tectonic deformation and climatic change in the development of brines to the potash stage

the dry holocene megathermal in inner mongolia the paleoclimate since 14 kyr bp 14c age was reconstructed based on a 16 22 m long sediment core collected from lake yanhaizi a saline lake located near the northern limit of the east asian summer monsoon in inner mongolia coarse sediments were deposited there during a shrinkage phase of the lake when sand dunes reactivated these sediments have low organic carbon contents but high maturity indices indicating that they were deposited in an arid environment by contrast based on high organic contents and low maturity indices fine sediments were deposited during periods of high lake stand in a humid environment it was in general dry between 8 0 and 4 3 kyr bp the above dry and wet phases are consistent with those recovered from the arid semiarid transition zone elsewhere but are unlike the widely perceived humid holocene megathermal reported in east china and the newly reconstructed record in the alpine retreat lake in taiwan the discrepancy may be due to a relative insensitivity to humidity changes in these two areas since they have both been under the total influence of the summer monsoon on the other hand much enhanced evaporation over higher monsoon precipitation at lake yanhaizi reduces the effective humidity in the warm climate near the northern boundary of the summer monsoon this also accounts for the fact the high temperature holocene megathermal as revealed in the okinawa trough and the northern south china sea is correlated to the dry phases at lake yanhaizi conversely the 4 2 kyr bp coldest period in the holocene corresponds to a wet phase at lake yanhaizi

late quaternary dust flow on the chinese loess plateau abstract the dust flux of over 25 g cm2 103yr was reached in the central part of the chinese loess plateau during the last glacial maximum this is more than three times higher than during the last interglacial and early glacial time the grain size of the pleniglacial dust reached a peak shortly after 20 000 years ago and testifies to the occurrence of strong winds and frequent dust storms

variations in rates of accumulation of tropospheric fine quartz in tephra loess and associated paleosols since the last interglacial stage tokachi plain northern japan and paleoclimatic inferences abstract terrestrial cover bed sequences comprising loess tephra and associated paleosols are collectively called weathered volcanic ash soils in japan widely distributed these deposits have provided effective records for reconstructions of climatic changes related to glacial interglacial cycles the rates of accumulation of fine material and fine quartz

the silicon isotope composition of ethmodiscus rex laminated diatom mats from the tropical west pacific implications for silicate cycling during the last glacial maximum the cause of massive blooms of ethmodiscus rex laminated diatom mats ldms in the eastern philippine sea eps during the last glacial maximum lgm remains uncertain in order to better understand the mechanism of formation of e rex ldms from the perspective of dissolved silicon dsi utilization we determined the silicon isotopic composition of single e rex diatom frustules 30sie rex from two sediment cores in the parece vela basin of the eps in the study cores 30sie rex varies from 1 23 to 0 83 average 1 04 a range that is atypical of marine diatom 30si and that corresponds to the lower limit of reported diatom 30si values of any age a binary mixing model upwelled silicon versus eolian silicon accounting for silicon isotopic fractionation during dsi uptake by diatoms was constructed the binary mixing model demonstrates that e rex dominantly utilized dsi from eolian sources i e asian dust with only minor contributions from upwelled seawater sources i e advected from subantarctic mode water antarctic intermediate water or north pacific intermediate water e rex utilized only 24 of available dsi indicating that surface waters of the eps were eutrophic with respect to silicon during the lgm our results suggest that giant diatoms did not always use a buoyancy strategy to obtain nutrients from the deep nutrient pool thus revising previously proposed models for the formation of e rex ldms

lake evolution during the past 30 000 years in china and its implications for environmental change abstract taking china as a whole lake expansion occurred 500 b c 0 a d 650 950 a d and 1250 1650 a d over the past 3000 yr the expansions were mainly due to climatic fluctuations and correspond to frequent flood reports three regional characteristics of lake development have been recognized 1 fluctuations of near coastal lakes and those in other regions as well were out of phase during 500 b c 500 a d which possibly resulted from out of phase fluctuations of climate and sea level on 10 to 100 yr time scale 2 as the lower yellow river channel migrated southward the lakes in the south of the north china plain formed and reached their largest sizes much later than those in the north 3 for the lakes in the middle and lower yangtze river valley the higher a lake basin lies above a neighboring river and the further the lake is from the river s mouth in general the younger the lake is through land reclamation the ancient chinese have changed most of the lakes greatly however human activities were also constrained in certain ways by the natural changes of the lakes reclamation usually was banned and farmland was abandoned to repair reservoirs while lakes were expanding

dry periods on the ne tibetan plateau during the late quaternary abstract environmental conditions on the tibetan plateau are changing rapidly this study reconstructs late quaternary environmental changes on the northeastern tibetan plateau based on sand deposits the study area is located close to the present maximum extent of the east asian summer monsoon and is likely to register shifts in monsoon extent furthermore different sediment deposits are available that allow reconstruction of sediment pathways between the different archives sediment deposition in the catchment is dominated by deposition of sand sandy deposits in the form of dune fields and sand sheets are widespread on the northeastern tibetan plateau and are an important archive for paleoclimatic reconstruction this study shows that sand deposits on the northeastern tibetan plateau are local phenomena that still bear the sedimentological and geochemical fingerprint of their source materials reconstruction of transport processes in the local sediment cascade shows that local sand deposits are the main source of tibetan mountain loess six long sections were sampled and analysed for their geochemical and sedimentological characteristics showing that both sand and loess deposits have a local source a special focus was placed on the manganese concentration and the related environmental implications in several sections an increase in mn concentration from 318 44 to 878 294 ppm was found independent of concentrations in fe and organic compounds since the increase in mn is caused by solid mn3 and mn4 alone it is interpreted as increased dust input the synchronicity with climatic variations on the northeastern tibetan plateau confirms the climatic interpretation of the variations in mn sand and dust transport both took place around 11 ka followed by a period of fixation or even erosion of sediments although increased sand transport in the catchment already started around 3 ka the increased dust input did not take place before 0 7 ka therefore the mn concentrations are interpreted as an indication of increased dust transport during the very late holocene related to colder and drier circumstances during that time the use of manganese as a proxy for dust is a valuable addition to the toolbox of paleoclimate reconstruction since it is a direct proxy of dust in deposits however further validation and testing should be performed before general acceptation of mn as a paleoclimate proxy in terrestrial sediments

geomorphological investigations and luminescence dating in the southern part of the khangay and the valley of the gobi lakes central mongolia geomorphological investigations in the catchment area of the baydragiyn river along the southern slope of the khangay and in the valley of the gobi lakes in western mongolia provide evidence for late quaternary glaciations and lake level changes thermoluminescence tl and infrared optically stimulated luminescence irsl dating of aeolian and colluvial sediments from the central khangay place the sediments that overlie fluvial and glaciofluvial terraces in the holocene an age of 21 ka is determined for a sand deposit overlying the terrace that is related to the last glacial ice margin lacustrine sediments from higher beach lines in the valley of the gobi lakes provide evidence for a slightly more humid period around 1 5 ka and a larger extent of the lakes in the early holocene at about 8 5 ka as also reported from other parts of central asia remnants of lacustrine sediments buried by alluvial gravel and indicating a huge palaeolake in the basin of the orog nuur however date to the early stage of the last glaciation around 70 ka copyright 2001 john wiley sons ltd

asian provenance of glacial dust stage 2 in the greenland ice sheet project 2 ice core summit greenland samples of dust from the greenland ice sheet project 2 gisp2 ice core summit greenland dated within marine isotope stage 2 between 23 340 and 26 180 calendar years b p around the time of the coldest local last glacial temperatures have been analyzed to determine their provenance to accomplish this we have compared them with approximately coeval aeolian sediments mostly loesses sampled in possible source areas psas from around the northern hemisphere the 5 m grain size fraction of these samples was analyzed on the basis that it corresponds to the atmospheric dust component of that time and locale which was sufficiently fine grained to be transported over long distances on the basis of comparison of the clay mineralogy and sr nd and pb isotope composition with ice dust and psas and assuming that we have sampled the most important psas we have determined that the probable source area of these gisp2 dusts was in eastern asia the dust was not derived from either the midcontinental united states or the sahara two more proximal areas that have been suggested as potential sources based on atmospheric circulation modeling except for a brief period during an interstadial when dust transport was exceptionally low for glacial times and had a mineralogical composition indicative of a slightly more southern provenance the source area of the dust did not change significantly during times of variably higher fluxes of dust with larger mean grain size or lower fluxes of dust with smaller mean grain size this includes the high dust period that correlates with the heinrich 2 period of major iceberg discharge into the north atlantic variable wind strengths must therefore be invoked to account for these abrupt and significant changes in dust flux and grain size

contribution of inherent organic carbon to the bulk δ13c signal in loess deposits from the arid western chinese loess plateau the isotopic composition of total organic carbon toc in loess deposits has been widely used to reconstruct palaeovegetation and palaeoclimate for terrestrial ecosystems the possible significance of the distribution and contribution of organic carbon from dust source regions to loess toc remains unclear but may potentially affect interpretation of stable carbon isotope distributions in loess records with respect to local vegetation two loess profiles from arid areas in china with sparse vegetation coverage have been investigated after removing the surface loess affected by recent farming we detected a narrow range of carbon isotope 13c values from 21 4 to 20 in the lanzhou area and from 21 5 to 19 7 in the xining area the values are inconsistent with that of the established vegetation in these areas where c3 plants are dominant in contrast the 13c values of the loess toc closely resemble those from dust sources together with data from loess toc and magnetic susceptibility we demonstrate that the isotopic composition of loess toc in arid areas was largely controlled by the carbon isotope composition of the dust source with only a minor contribution from local native vegetation the results suggest that background carbon isotopic signals should be taken into account when interpreting carbon isotope compositions of loess deposits in arid areas

the history and nature of wind erosion in deserts recently the importance of wind erosion has been reevaluated many low angle surfaces developed on susceptible materials possess closed depressions pans and these often have a distinctive morphology and lunette dunes on their lee sides it has also become apparent that in drylands there are extensive areas of stone pavements some of which have been molded by deflation however it is now recognized that other horizontal sorting processes have played a role especially movement of fines by overland flow vertical sorting associated with wetting and drying salt heave bioturbation frost action and dust accretion have also played a role in hyperarid areas especially with unidirectional winds bedrock outcrops and old lake beds have been molded to give aerodynamic forms called yardangs the study of dust storms by analysis of climatological data and remote sensing has revealed the importance of deflation especially in hyperarid areas with centripetal drainage deflation hot spots such as the bo

mineral magnetic variation of the jingbian loess paleosol sequence in the northern loess plateau of china implications for quaternary development of asian aridification and cooling a high resolution mineral magnetic investigation has been carried out on the jingbian loess paleosol sequence at the northern extremity of the chinese loess plateau results show that the magnetic assemblage is dominated by large pseudo single domain and multidomain like magnetite with associated maghemite and hematite variations in the ratios of sirm100mt sirm sirm100mt sirm30mt and sirm100mt sirm60mt sirm is the saturation isothermal remanent magnetization sirmnmt represents the residual sirm after an n mt alternating field demagnetization have been used to document regional paleoclimate change in the asian interior by correlating the mineral magnetic record with the composite 18o record in deep sea sediments the long term up section decreasing trend in those ratios in both loess and paleosol units has been attributed to a long term decrease in the relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema respectively which reveals a long term decreasing trend in chemical weathering intensity in both glacial stage source region the gobi and deserts in northwestern china and interglacial stage depositional area the loess plateau region we further relate this long timescale variation to long term increasing aridification and cooling during both glacial extrema in the dust source region and interglacial extrema in the depositional area over the quaternary period changes in those ratios are most likely due to quaternary aridification and cooling driven by ongoing global cooling expansion of the arctic ice sheet and progressive uplift of the himalayan tibetan complex during this period

mechanisms of loess sized quartz silt production and their relative effectiveness laboratory simulations the production of loess sized quartz silt is frequently ascribed to glacial grinding mechanisms in this study a series of laboratory simulations was set up the purpose of which was to demonstrate that a range of geomorphic mechanisms are capable of producing silt under laboratory conditions the geomorphic mechanisms investigated were aeolian abrasion fluvial comminution glacial grinding salt weathering and frost weathering the results confirmed that under laboratory conditions all of these mechanisms were capable of producing silt the relative effectiveness of these mechanisms at producing silt under laboratory conditions was then calculated in order to provide a starting point for a consideration of their potential effectiveness within the natural environment these results indicate that fluvial and aeolian activity are highly effective at producing quartz silt over short time periods in a dust storm of four days duration 287 g of silt could potentially be produced from every kilogram of sand entrained while fluvial comminution in a turbulent fluvial environment could potentially produce 900 g of silt from every kilogram of sand entrained within 32 h over longer periods of time the glacial grinding and weathering mechanisms represent more continuous and hence more effective silt producing mechanisms although it is realised that the relative importance of each silt producing mechanism will differ spatially and temporally due to differences in climate relief geology and geomorphic history the findings from this work do have implications for the genesis of loess deposits they demonstrate that glacial and cold weathering processes are not solely responsible for fine particle formation and that efficient silt producing mechanisms can operate within a variety of environments under suitable conditions weathering together with episodic contributions from aeolian and fluvial mechanisms could be responsible for a considerable proportion of global quartz silt production

late pleistocene and holocene aeolian dust deposition in north china and the northwest pacific ocean abstract rates of dust deposition in the north pacific and the central loess plateau of china show a poor correlation during the last 30 000 yr although in both cases the dust was derived from deserts in northern china loess accumulation rates were high at the time of the last glacial maximum while dust flux to the north pacific was low at this time maximum dust flux to the north pacific is recorded in the mid holocene but holocene rates of loess accumulation have been low these differences are shown to reflect the different nature of the wind conditions responsible for dust transport to the loess plateau and the north pacific dust transport responsible for formation of the loess occurred mainly in the lower 1 5 km of the atmosphere and was accomplished by cold low level northwesterly winds associated with cold waves emanating from the mongolian anticyclone at present such conditions occur quite frequently in winter but during the coldest phases of the pleistocene they probably also occurred in the spring and summer long range dust transport from northern china to the north pacific requires that the dust is lifted to high levels above 3 km where it can be incorporated into the strong mid latitude westerlies strong vertical motions of the atmosphere occur most frequently in the spring when the mongolian anticyclone begins to weaken and frontal depressions move across the deserts of northern china such frontal depressions were probably more frequent during the climatic optimum of the mid holocene and during previous interglacials changes in continental aridity have probably played a minor role in controlling the magnitude of the dust flux to the north pacific

asian aridity and the zonal westerlies late pleistocene and holocene record of eolian deposition in the northwest pacific ocean rea d k and leinen m 1988 asian aridity and the zonal westerlies late pleistocene and holocene record of eolian deposition in the northwest pacific ocean palaeogeogr palaeoclimatol palaeoecol 66 1 8 a 30 000 year record of eolian deposition in the northwestern pacific ocean provides a history of the aridity of the asian source region and information on the changing latitude and intensity of the zonal westerlies the position of the maximum in eolian flux to the deep sea remained unchanged at 38 to 40 n during this entire time period but the dust flux record of aridity is greatest 6000 years ago at the holocene climatic optimum eolian grainsize data indicate that the northern margin of the westerly jet stream retreated northward during the holocene to a poleward extreme at about 6000 years ago and then moved back toward the equator the change in wind intensity across this margin was continually reduced throughout the holocene these sorts of data enhance our understanding of the behavior of the atmosphere during times of climate change and can be used to test computer generated models of past climatic conditions

evolution of the norwegian current and the scandinavian ice sheets during the past 2 6 m y evidence from odp leg 104 biogenic carbonate and terrigenous records abstract records of biogenic and terrigenous components have been obtained from the interval corresponding to the last 2 6 m y of odp sites 643 and 644 in order to reconstruct surface and deep water regimes in the norwegian sea surface water regimes record long lasting moderate glacial conditions during the interval 2 6 1 0 ma small intrusions of atlantic water episodically penetrated into the norwegian sea forming a narrow tongue along the eastern margin which is documented at site 644 the polar front was most probably situated between the site 644 and 643 locations on the outer voring plateau during these time intervals deep water regimes reflect long term persistent corrosive bottom waters most probably due to a weakly undersaturated water column and a low rate of carbonate shell production in surface waters deep water production in the norwegian greenland sea may have operated in a different way e g brine formation during winter sea ice growth bottom waters were oxygenated throughout the entire period and deep water was exchanged persistently with the north atlantic increased glacial interglacial environmental contrasts are documented reflecting a strengthening of the norwegian current and intensified glaciations on the surrounding land masses during the interval 1 0 0 6 ma during this time a major shift in the mode of deep water production occurred the onset of large amplitudes in glacial interglacial environmental conditions with maximum contrasts in surface water regimes different modes of deep water production and intensified exchange with the north atlantic marks the last 0 6 ma a broad development of the norwegian current is observed during peak interglacials while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions

late quaternary environments in the gobi desert of mongolia vegetation hydrological and palaeoclimate evolution abstract considerable efforts have been devoted to decipher the late quaternary moisture and thermal evolution of arid central asia however disparate interpretations still exist concerning different proxies the spatial and temporal heterogeneities have inhibited a holistic understanding of general patterns and underlying mechanisms to address these issues two parallel cores onw i 6 00 m onw ii 13 35 m were retrieved in the gobi desert of mongolia from lake orog nuur multidisciplinary investigations including geomorphological mapping radiocarbon dating sedimentological palynological and ostracod analyses enabled us to gain a comprehensive dataset for vegetation development and hydrological variability over the last 45 kyr higher lake levels during the marine isotope stage mis 3 and early mis 2 35 24 kyr were probably caused by increased precipitation the sharp transition of termination i 11 kyr is indicated by sedimentological palynological and ostracod data during the late pleistocene the lower area of the orog nuur catchment was dominated by artemisia steppe and gradually altered to chenopodiaceae desert steppe in the holocene the early holocene is also characterized by a relatively humid environment the humid pulses during the mis 3 and the early holocene were also recorded in other archives and are possibly the trait of a larger scale phenomenon in arid central asia four major harsh climatic periods were documented in the core at 43 kyr 36 kyr during the global last glacial maximum and the younger dryas as playa phases reduced westerlies moisture transport and a retreated east asian summer monsoon influence probably caused those dry phases in the gobi desert of mongolia

late pleistocene paleoclimatology of the central equatorial pacific a quantitative record of eolian and carbonate deposition detailed records of vso 6i3c percentage and mass accumulation rate of caco and eolian percentage mass accumulation rate and grainsize generated for core rcl l 210 from the equatorial pacific reveal the timing of paleoclimatic events over the past 950 000 yr the caco percentage record shows the standard pacific correlation of high caco content with glacial periods but displays a marked change of character about 490 000 yr ago with older stages showing much less variability the carbonate mass flux record however does not show such a noticeable change sedimentation rates vary from about 0 5 to 3 0 cm l000 yr and during the past 490 000 yr sections with enhanced sedimentation rates correspond to periods of high caco percentage eolian mass accumulation rates an indication of the aridity of the source region are usually higher during glacial times eolian grainsize an indication of the intensity of atmospheric circulation generally fluctuates at a higher frequency than the loo ooo yr glacial cycle the mid brunhes climatic event centered at 300 000 yr ago appears as a 50 000 yr interval of low intensity and reduced variability of atmospheric circulation furthermore the nature of this entire record changes then with the younger portion indicating less variation in wind intensity than the older part of the record the late matuyama increase in amplitude of paleoclimatic signals begins 875 000 yr ago in the eolian record 25 000 yr before the 6i80 and caco percentage amplitude increases about

paleoenvironmental implications of the insoluble microparticle record in the gisp2 greenland ice core during the rapidly changing climate of the pleistocene holocene transition oscillations in the time series of insoluble microparticle characteristics between 0 7 and 11 0 m in the greenland ice sheet project 2 gisp2 ice core reflect changes in environmental conditions in the northern hemisphere from 10 500 to 14 000 yr ago elevated values in microparticle number and mass especially during the younger dryas are related to northern hemisphere aridity and the subsequent increase in dust available for long range transport to greenland this scenario occurs with the colder climatic conditions that result from a more expanded spatially and temporally polar vortex peaks in mean grain size based on number mean number diameter are a proxy for increased strength in zonal winds westerlies highs in mean number diameter in the earlier part of the record often coincide with number and mass peaks reflecting the increased temperature and pressure gradients with an expanded polar vortex highs in mean grain size based on mass mean mass diameter reflect greater deposition of the coarser size fraction and thus are a proxy for increased storminess associated with better developed synoptic scale pressure systems in the northernmost atlantic region peaks in mean mass diameter often lead these other parameters by 100 200 yr suggesting an increase in storminess with the initial southward migration of the mean position of the polar front prior to full development of a more expanded polar vortex the general decline in number and mass trends decreased aridity with a contracting polar vortex together with increasing mean number diameter trends strengthening zonal winds following the maxima in the early younger dryas suggest an expansion of mid latitude circulation systems subtropical highs thereby maintaining latitudinal temperature and pressure gradients increased variability in mean mass diameter during the warm preboreal compared to the colder younger dryas may be a function of the greater seasonality during warmer climatic periods and thus more frequent storms associated with higher frequency oscillations in the position of the polar front with changing seasons and increased interannual variability in climate