temperature effects on creep of tuff and its time dependent damage analysis abstract an investigation and explanation are given of temperature effects on creep of tuff creep tests have been conducted at room temperature and at elevated temperature 204 c in order to model the creep behavior of tuff a new time dependent damage model and two definitions of stress intensity factor are proposed the results of experiments and theoretical analysis show that the creep strain of tuff increases with increasing temperature under the same loading condition and that the stress intensity factor is not only a function of stress states but also of temperature

origin of maghaemite in chinese loess deposits aeolian or pedogenic maghaemite and magnetite common ferrimagnetic minerals in surficial soils have physical properties which are similar except for the thermal instability of the former utilising this exception a study of thermomagnetic properties has been made to systematically separate these two magnetic components in loess and palaeosol samples from the upper parts of the luochuan section of the central chinese loess plateau this investigation has shown that although maghaemite thermally unstable component occurs in greater amounts in palaeosols ms 0 0187 am2 kg 1 1 1 than in loess units ms 0 0156 am2 kg 1 1 1 in relative terms its contribution to the ferrimagnetic component of magnetisation is higher in the loess units ms 32 2 2 0 than the soils ms 23 5 3 7 such an observation suggests that much of the maghaemite component originates in the aeolian source area with some addition of pedogenic maghaemite occurring in the palaeosols

rock magnetism iron oxide mineralogy and geochemistry of quaternary red earth in central china and their paleopedogenic implication abstract this paper presents a detailed investigation on the rock magnetism iron oxide mineralogy and geochemistry of a quaternary red earth qre in central china the magnetic profile of qre showed a characteristic vertical variation pattern the uniform red clay urc section 0 2 2 m of the profile had a higher magnetic susceptibility while vermiculated red clay vrc section 2 6 m had a very low 8 m 3 kg 1 rock magnetism and differential xrd analysis revealed that magnetite maghemite and hematite were the main magnetic carriers in the urc section the major elements fe 2 o 3 and al 2 o 3 of qre increased with depth while the sio 2 content decreased with depth indicating a leaching of the mobile elements and an enrichment of the immobile elements in comparison with the urc the vrc has a greater depletion in the content of na mg k and mg indicating a stronger weathering intensity the dithionite citrate bicarbonate dcb extracted iron fed the ratio of fed to total iron oxide fe fet and the weathering indices of qre confirmed that the vrc had experienced a stronger pedogenesis than the urc strong pedogenesis in the vrc resulted in a transformation of the highly magnetic maghemite to the weakly magnetic hematite which was responsible for the weak magnetism and hard magnetic behavior of the vrc our studies suggest that the vrc had experienced a hot climate with a dry season following one that was warm and moister magnetic reduction of the vrc section resulting from the transformation of highly magnetic maghemite to weakly magnetic hematite appeared to be an important paleopedogenic process in the qre we concluded that the magnetic parameters hard isothermal remanent magnetization hirm and sirm of qre can be used for the analysis of paleopedogenic processes

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

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

magnetism of loess palaeosol sequences recent developments abstract investigation of the global loess record primarily as a palaeoclimatic archive continues unabated one important aspect of this research effort is the determination of the magnetic properties of the loess itself and of the palaeosols developed within it in earlier work these have provided a record of major long term past climatic changes however recent studies are now revealing sub milankovitch signals with teleconnections from the chinese loess plateau clp to the greenland and antarctic ice caps as well as to climate related events in sediments recovered from the north atlantic these high resolution profiles cover to the last 130 000 years however progress is also being made in extending the record down below the loess sensu stricto of the clp into the so called red clay so that the aeolian record can now be carried back beyond 7 ma bp several authors have attempted to obtain quantitative estimates of palaeoprecipitation by comparing magnetic susceptibility of palaeosols with values for modern soils although the broad picture is qualitatively clear wetter interglacials dryer glacials the actual quantitative values provided by the various techniques differ significantly in the classic chinese sites palaeosols are always magnetically enhanced due to in situ pedogenic processes however evidence is now emerging to indicate that this is not the only magnetoclimatological model in cases such as siberia where wind vigour dominates over pedogenesis the opposite pattern is found with magnetic susceptibility minima in palaeosols this also occurs in situations where the sediments are water logged for significant amounts of time leading to gleying and concomitant loss of magnetic minerals the use of magnetic remanence as a geomagnetic polarity clock requires caution early interpretations were acceptable for establishing broad chronological control however recent scrutiny suggests that remanence lock in may be delayed by as much as 20 000 years furthermore synthetic magnetostratigraphies modelled on the basis of realistic sediment characteristics indicate that there are limits to the amount of geomagnetic detail likely to be resolved this seems to be the case for the blake event which manifests itself in a very variable way at different sites in the clp

spatial variations of magnetic susceptibility of chinese loess for the last 600 kyr implications for monsoon evolution 1 we examine spatial variations in magnetic susceptibility ms over the loess plateau in china based on 50 sections in order to identify spatial changes in monsoon climate at key glacial interglacial and interstadial intervals for the last 600 kyr the results indicate strong coherence between ms variations during the interglacial periods and present day precipitation and temperature patterns this suggests that the strength of the summer monsoon had a dominant influence on the ms signals in soils through modulating pedogenic intensity the distribution of ms during glacial periods is characterized by weak s n gradients and rough w e zonal patterns indicating a negligible effect of the summer monsoon interstadial patterns are intermediate between the glacial and interglacial ones interglacial patterns for the last 600 kyr are essentially similar suggesting that the climate regime during these periods has not undergone significant changes and that the east asian summer monsoon has remained the main moisture carrier our estimates of the relative amplitudes of climate oscillations during these time slices are consistent with earlier paleoclimate studies these data associated with the available susceptibility based climofunctions may be used to estimate spatial changes of paleorainfall and paleotemperature for these key periods and hence to test climate models

signature of magnetic enhancement in a loessic soil in nebraska united states of america our study of a loessic soil profile from east central nebraska shows that the a horizons of the modern soil are characterized by higher concentrations of fine grained b0 1 am magnetic minerals this pedogenic magnetic component leads to higher values of concentration dependent parameters such as magnetic susceptibility v isothermal remanent magnetization irm and anhysteretic remanent magnetization arm combined with increases in frequency dependent susceptibility vfd and arm irm ratios hysteresis properties are relatively insensitive towards the presence of this pedogenic magnetic component the magnetic properties of the soil profile are dominated by ferrimagnetic magnetite or maghemite analyses of bsoftq irm sirm and bhardq irm hirm however do show that approximately 80 90 of the remanence carrying magnetic component exists in the form of hematite or goethite and that the magnetically enhanced horizons are enriched in both ferri and antiferromagnetic minerals the pedogenic magnetic component is most likely caused by the conversion of paramagnetic iron bearing minerals to ferriand antiferromagnetic minerals soil compaction lessivage or decalcification cannot explain the observed magnetic soil properties magnetic analyses of loess paleosol sequences from the midwestern united states may yield valuable information about regional variability of paleoclimate based on the fine grained nature of the pedogenic magnetic component we expect grain size dependent proxies arm arm irm vfd to yield better paleoclimatic information than low field magnetic susceptibility used in previous analyses d 2004 elsevier b v all rights reserved

partly pedogenic origin of magnetic variations in chinese loess quaternary sequences of interbedded aeolian silts loess and buried soils palaeosols potentially provide one of the best terrestrial records of past climates1 5 magnetic susceptibility variations in loess and palaeosol sequences from china are strongly correlated with climate induced fluctuations of oxygen isotope ratios in deep sea sediments2 4 6 8 as an explanation of this correlation it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6 8 it has also been suggested that such a model involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7 here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units this suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non magnetic loess is inadequate our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols

quantitative estimates of pedogenic ferromagnetic mineral formation in chinese loess and palaeoclimatic implications abstract the quasi continuously deposited loess sediments of the central chinese loess plateau constitute one of the most complete and long lasting records of quaternary palaeoclimate on land much of this record has been uncovered by measurement and interpretation of the magnetic properties of the loess the polarity of natural remanent magnetization has provided absolute age dates by correlation with the geomagnetic polarity time scale magnetic low field susceptibility has been used to reconstruct the cyclic changes of pleistocene palaeoclimates although the origin of the signal is still poorly understood from comparison with the 10 be concentration throughout the sequence we are able to determine the contributions to susceptibility from inherited and authigenic sources for the last 130 ky since the production of authigenic magnetite is directly related to regional precipitation annual palaeoprecipitation rates can be calculated palaeoprecipitation rates similar to present day rates are derived for oxygen isotope stage 5 including the last interglacial in contrast cold periods corresponding to oxygen isotope stages 2 and 4 experienced negligible precipitation the beginning and end of the time interval represented by oxygen isotope stage 3 however was characterized by short episodes when precipitation exceeded the holocene average

review of recent developments in mineral magnetism of the chinese loess mineral magnetism of the chinese loess has been investigated for more than 20 years although there is a consensus that the neoformation of fine grained maghemite particles in the superparamagnetic sp and single domain sd grain size regions accounts for the magnetic enhancement in the chinese paleosols quantitative retrieval of paleoclimatic signals in terms of rock magnetic proxies is still a subject of debate the ambiguities arise from the inherent complexities of magnetic proxies as well as the multiple factors that control the pedogenic processes therefore a better description of the magnetic assemblage including its mineralogy grain size distribution and stoichiometry of two distinct origins pedogenic and eolian can help us better understand mechanisms behind variations in magnetic proxies at different timescales in order to link them to the paleoclimatic processes this review focuses on recent developments in loess magnetism and carefully evaluates merits and limitations of rock magnetic proxies furthermore several currently unsolved problems are addressed

rock magnetism and geochemistry of two plio pleistocene chinese loess palaeosol sequences implications for quantitative palaeoprecipitation reconstruction abstract stratigraphic and spatial variations in the magnetic properties of chinese loess are widely accepted as reflecting changes in the intensity of soil forming processes over time and space recently several workers have used statistical correlations between the magnetic susceptibility and related parameters of modern loess plateau soils and modern climate to generate transfer functions to reconstruct past temporal and spatial trends in precipitation here we try to assess the viability of this approach by conducting magnetic and geochemical measurements of two long 2 5 my loess palaeosol sequences from different locations on the modern precipitation gradient across the loess plateau one site duanjiapo is located on the southern humid margin of the plateau and the other site luochuan is located in a drier modern environment some 160 km further north our results indicate discrepancies between magnetic and geochemical indicators of weathering and soil forming intensity and that any attempt to predict palaeoprecipitation at duanjiapo using published approaches would be significantly in error overall our results indicate a complex response of magnetic mineral transformations to climate including the production of both ferrimagnetic and antiferromagnetic minerals within the same climatic interval together with the destruction of magnetic material within some of the palaeosols therefore we suggest considerable caution should be exercised in generating quantitative climatic reconstructions from a single magnetic parameter

separation of local signals from the regional paleomonsoon record of the chinese loess plateau a rock magnetic approach we propose a method based on thermal unblocking of low temperature saturation remanent magnetization for a quantitative estimation of the superparamagnetic cullity 1972 fraction size d 30 nm of magnetite produced by pedogenesis in the chinese loess plateau liu 1988 we applied this method to the proxy climatic records of the last 130 ka from two sites 250 km apart but separated by the mountain range liupan shan xifeng to the east 35 7 n 107 6 e and baicaoyuan to the west 36 2 n 105 0 e currently have humid and arid microclimates respectively as expected the superparamagnetic fraction increases during known warm temperature intervals at each site furthermore the more humid site clearly has higher overall superparamagnetic fractions during most of the last 130 ka however during the period 5 to 10 ka ago the relative humidity at both sites was the same within experimental errors bulk grain size evidence confirms the magnetic data and we suggest that the present easterly summer monsoon in china came from a more southerly direction during this time to flow parallel to liupan shan resulting in very similar summer humidity at xifeng and baicaoyuan

pleistocene chemical weathering history of asian arid and semi arid regions recorded in loess deposits of china and tajikistan quaternary loess soil sequences in china and tajikistan are valuable archives of the chemical weathering history for arid and semiarid regions of asia the development of new proxies independent of grain size are vital to reconstruct the long term chemical weathering history of eolian deposits in this study we analyze major elemental and rb sr and ba concentrations of decarbonated residue from representative loess and soil units along a north south transect on the chinese loess plateau and compare these concentrations among different grain size fractions results show that most of the elemental abundances and ratios vary considerably among different grain size fractions for both loess and paleosols indicating transport driven compositional differentiation of minerals during subaerial transport however cao na2o mgo tio2 ratios show little variation among different size fractions for the loess soil transect loess soil sequences at lingtai northern china and chashmanigar southern tajikistan for the past 1 50 1 77 ma show lower cao na2o mgo tio2 ratios in soil units than in adjacent loess horizons in good agreement with greater weathering intensity in soils than in loess units the cao na2o mgo tio2 ratio of the decarbonated residue is therefore a reliable proxy for chemical weathering of loess deposits that is independent of grain size the cao na2o mgo tio2 ratios of the lingtai and chashmanigar sections display a relatively regular oscillation within a narrow range in the early pleistocene and a rapid increase in average values and in variance from 0 85 0 6 ma to the present indicating lowered chemical weathering intensity in both the dust source regions and the depositional areas this event may be causally related to the expansion of northern hemisphere ice and or the regional tectonic uplift of high mountains in asia since the mid pleistocene

paleorainfall reconstructions from pedogenic magnetic susceptibility variations in the chinese loess and paleosols the rock magnetic properties of the chinese loess and paleosols constitute a unique and sensitive record of east asian paleoclimate through the quaternary period systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal which can be easily and rapidly measured at many sites across the loess plateau variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages as well as glaciations and interglaciations past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations the susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the loess plateau our rainfall reconstructions identify enhanced summer monsoonal activity in the chinese loess plateau region in the early holocene and the last interglaciation in the presently semiarid western area of the plateau annual precipitation in interglacial times was up to 80 higher than at present in the more humid southern and eastern areas values were up to 20 higher than today s levels during the last glaciation precipitation decreased across the entire plateau typically by 25 the relationship between pedogenic susceptibility climate and weathering age was examined over the northern hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate rather than soil age is the predominant factor that controls pedogenic susceptibility enhancement in loess soils

distribution provenance and onset of the xiashu loess in southeast china with paleoclimatic implications abstract loess deposits are important archives of past climate change in subtropical china where long term terrestrial records are scarce however only a few long term records with reliable dating were reported in jiangsu province where loess deposits in southeastern china were first discovered and designated as the xiashu loess moreover the provenance of the xiashu loess is still controversial this study presents the magnetostratigraphy and detrital zircon u pb age results of loess deposits at zhoujiashan in nanjing jiangsu province to date this section with basal age of approximately 0 88 ma may be the oldest loess deposits discovered in southeastern china based on the detrital zircon u pb ages of the zhoujiashan section together with published results of geochemistry geomorphology sedimentology and meteorology we propose that the xiashu loess has two mixed sources local materials derived from the yangtze river basin including exposed river lake beds floodplains and mountains served as a major contributor the distal eolian dust that originated from the arid areas of north china including deserts piedmont alluvial fans and drylands played a minor role in the formation process of the xiashu loess the nearly synchronous onset age approximately 0 88 0 85 ma of the xiashu loess was attributed to a regional environmental event during the middle pleistocene transition

east asian summer monsoon intensity inferred from iron oxide mineralogy in the xiashu loess in southern china abstract the xiashu loess in comparison to the well studied loess sequences in the chinese loess plateau clp provides a good opportunity for studying east asian monsoon variations from a southern china perspective here we present a study of the iron oxide mineralogy of the xiashu loess using integrated geochemical and diffuse reflectance spectroscopy drs measurements as well as magnetic data our results show that the free iron oxide fe d to total iron fe t ratio fe d fe t hematite hm to goethite gt ratio hm gt and saturation isothermal remanent magnetization sirm to magnetic susceptibility ratio sirm all indicate particularly strong summer monsoons during the formation of paleosols ps5 and ps4 equivalent to marine isotope stage 13 and 11 respectively however magnetic susceptibility and fe d fe t are not consistently reliable indicators of summer monsoon intensity for the whole section our results indicate that a multi proxy approach can give a more reliable summer monsoon intensity reconstruction the summer monsoon shows a cooling trend and a declining of precipitation from 0 5 to 0 3 ma after which it becomes warmer and wetter towards the top of paleosol ps1 equivalent to mis 5 however ps1 was formed under a relatively cooler temperature and wetter soil conditions in comparison to ps5 and ps4 such supra orbital variations in the east asian summer monsoon superimposed on the effects of glacial interglacial cycles in southern china are also reflected in the 0 4 0 5 ma cycle of marine carbon isotopes in the global ocean possibly indicating a strong link between terrestrial weathering and the global carbon cycle

an integrated rock magnetic and geochemical approach to loess paleosol sequences from bohemia and moravia czech republic implications for the upper pleistocene paleoenvironment in central europe abstract two central european loess paleosol sequences mis 6 2 from central bohemia zem chy and southern moravia dobsice situated in the important climatic transition zone between the oceanic atlantic and continental climate regimes were investigated for rock magnetism geochemistry grain size and micromorphology in both sites the magnetic profile shows good agreement with the standard model of magnetic enhancement in paleosols as established in the chinese loess the magnetic signal is dominated by the presence of fine grained magnetite and maghemite formed in situ and controlled by the intensity of soil formation magnetic proxies corresponded to proxies of chemical weathering and leaching intensity such as the chemical index of alteration cia the cation exchange capacity cec and the rb sr ratio correlation coefficient r 0 6 0 93 during the whole lower weichselian mis 5d 5a and in general also during the pleniglacial mis 4 2 nevertheless for the eemian period mis 5e this relationship was not always valid either due to weak magnetic enhancement at dobsice or even weak depletion of the magnetic minerals with soil formation at zem chy the presented data demonstrate that magnetic signals can be overprinted more easily than chemical proxies during later diagenetic events and are hence not in all cases reliable for deciphering paleoclimate changes during all periods of the studied time interval rock magnetic geochemical and micromorphological results suggest drier conditions in southern moravia compared to central bohemia during the weichselian glacial probably due to the more continental climate in southern moravia

grain size distribution of pedogenic magnetic particles in chinese loess paleosols 1 variations in magnetic parameters of the long chinese loess paleosol sequences reflect paleoclimatic fluctuations however there remain ambiguities whether the magnetic proxies are controlled by the concentration or by the distribution of magnetic grain sizes we systematically investigated the frequency dependence of low field magnetic susceptibility fd and the decay rate dr dependence of anhysteretic remanence magnetization arm susceptibility arm dr for a set of representative loess paleosol samples from the yuanbao section western chinese loess plateau after removing the effect of aeolian coarse grained magnetites both fd and arm dr are nearly independent of the degree of pedogenesis indicating a presence of fine grained magnetic particles ranging from superparamagnetic to single domain with an almost constant grain size distribution therefore the magnetic enhancement of magnetic susceptibility or arm is caused dominantly by changes in the concentration of these fine grained pedogenic grains

chemical weathering of the loess deposits in the lower changjiang valley china and paleoclimatic implications abstract loess deposits xiashu loess are widely distributed in the lower changjiang valley east china and are considered to be of eolian origin with a similar provenance to the well documented loess of north central china the xiashu loess is characterized by strong dissolution of carbonate and considerable weathering of some silicate minerals especially plagioclase chemical mobility of ca sr na mg was observed in the xiashu loess sediments whilst most transition metals are relatively enriched chemical index of variation cia and element ratios of k na and k ca record several paleoclimate events in the lower changjiang valley during the mid late quaternary corresponding with isotopic records in deep sea sediments east china might be governed by stronger summer monsoons than north central china during interglacial periods of the mid late quaternary which may somewhat obscure the record of winter monsoons during glacial periods

an empirical strength criterion for heterogeneous tuff abstract an empirical failure criterion is formulated by expressing the second invariant of stress deviation at failure as a function of the first invariant of stress key parameter and volume strength data from apache leap tuff specimens are analyzed as a demonstration example the density variable included as a key parameter for this tuff minimizes the effect of heterogeneity caused by nonuniform distribution of pores mineralogy inclusions welding and grain bonding the criterion incorporates all principal stresses and the scale effect and hence allows predicting the strength toward in situ conditions the proposed derivation improves the correlations between test results and failure envelope for the heterogeneous tuff the criterion formulated from the uniaxial and triaxial test data adequately predicts the strengths for biaxial compression tests

holocene environmental change inferred from the loess palaeosol sequences adjacent to the floodplain of the yellow river china abstract chinese loess deposits are generally considered to be the product of dust storms and dust falls from the central asia arid zones that were transported across china by the northwesterly continental monsoon in contrast the zhengzhou loess found southeast of the loess plateau adjacent to the floodplain of the yellow river records a different eolian regime and dust source the zhengzhou loess was investigated by field observations measurements of magnetic susceptibility particle size distribution loss on ignition caco3 and chemical contents both field observations and the laboratory results indicate that during the last glacial the zhengzhou loess was supplied by two different eolian regimes and dust sources one was from the fresh flood deposits of the yellow river driven by the northeast winds from the low lying floodplain and the other was from the dust storms and dust falls that traveled across the loess plateau driven by the northwesterly continental monsoon from the central asian arid lands the early holocene 11 500 8500 a bp was a transition during the change in eolian regime and dust source because of the weakened northwesterly monsoon along with the global climatic amelioration following the retreat of the northwesterly monsoon from the onset of the mid holocene climatic optimum at 8500 a bp dust supply from the drifting sand zone on the yellow river floodplain became dominant because of the intensified strength of the northeast winds from the bohai sea from 3100 a bp onwards climatic aridity and extensive human disturbance have resulted in intensive eolian processes causing the incursion of the drifting sand into the zhengzhou loess zone these results show that loess accumulation is more complex than traditionally assumed the origin of loess deposits elsewhere outside the loess plateau may be related to dust sources derived from alluvial sediments of major river systems

magnetism of chinese loess deposits summary the age of chinese loess deposits has long been disputed biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the pleistocene and ascertained a maximum loess age of about 1 2 myr a new collection of nearly 500 samples from a natural outcrop and a borehole section near lochuan lat 35 8 n long 109 2 e shaanxi province has been dated by magnetic stratigraphy thermal cleaning of the natural remanent magnetization nrm removes a strong secondary component of viscous origin along the present geomagnetic field which resides largely in magnetite the characteristic nrm component is due to haematite which is thought to be of chemical origin rhythmical intensity variations of nrm and initial susceptibility depend on the loess lithology and may reflect climatic changes during loess deposition the palaeomagnetic results are consistent between the two sections and yield a clearly defined magnetic polarity zonation the brunhes matuyama boundary and the jaramillo subchron have been positively identified in both outcrops at exactly the same stratigraphic level the olduvai subchron has been found in the borehole section which records the entire loess sequence most probably the formation of chinese loess began shortly after the matuyama gauss polarity transition therefore a late pliocene age of about 2 4 myr is assigned to the oldest loess sediments measured

effect of citrate bicarbonate dithionite treatment on fine grained magnetite and maghemite mineral magnetic properties of soils and parent materials have been interpreted in terms of paleoclimate and rates of soil formation but it is important to understand which minerals contribute to the mineral magnetic signal citrate bicarbonate dithionite cbd treatment has been used to determine the amounts of fine grained often pedogenic ferrimagnetic minerals relative to coarse grained often inherited magnetic minerals however questions have been raised about the effect of particle size on the efficacy of cbd in dissolving magnetite and maghemite grains in this paper we use magnetic susceptibility and its frequency dependence and the low temperature behavior of a saturation isothermal remanent magnetization to track the dissolution of carefully sized magnetite grains we found that the standard cbd procedure dissolves fine magnetite particles ca 1 etm essentially intact thin oxidized coatings presumably maghemite are also dissolved by the cbd procedure these results support previous interpretations that the cbd procedure can be used to distinguish between pedogenic and lithogenic magnetic grains assuming that most pedogenic magnetic grains are sufficiently small ca 1 im these results also show that the standard procedure is too harsh to differentiate between 1 grm grains of magnetite and maghemite a modified cbd extraction that uses half as much dithionite reduces the magnetic susceptibility of 1 jam magnetite grains by only 10 this method may be useful in distinguishing between magnetite and maghemite grains in this size range

toward establishing a maritime proxy record of the east asian summer monsoons for the late quaternary abstract an astronomically tuned late quaternary planktic foraminiferal 18o record of site md972142 12 41 33 n 119 27 90 e 1557 m water depth in the southeastern south china sea was established the difference in 18o between md972142 and odp792 of the sulu sea is regarded as a maritime proxy of the summer monsoon intensity over the south china sea and southeast asia the profile of this maritime proxy matches well with the summer monsoon index obtained from the terrestrial record of louchuan central chinese loess plateau the amplified planktic 18o signals of the south china sea relative to the sulu sea record are partly caused by the changing intensities of the east asian monsoons at the glacial interglacial time scale throughout the late quaternary

rock magnetic characterization of early redoxomorphic diagenesis in turbiditic sediments from the madeira abyssal plain rock magnetic measurements of two sediment cores from the madeira abyssal plain map north atlantic are used to investigate post depositional changes in the concentration grain size and composition of magnetic minerals in the sediments that have occurred within organic rich turbidite horizons the changes are associated with an initial stage of suboxic reductive diagenesis following depletion of porewater o2 and a later stage of oxidative diagenesis associated with the slow descent of an oxidation front through the sediment as a result of diffusion of o2 from the overlying sea water the turbidites are of late quaternary age 18o stages 1 3 and derive both from different sites on the nw african continental margin and from the flanks of the canary islands thus the turbidites are variable compositionally especially in terms of carbonate detrital magnetic mineral and organic carbon content diagenetic changes in these sediments have been identified using solid phase geochemical data u mn corg and caco3 reported previously in more than one study rock magnetic parameters of the sediments when expressed on a carbonate free basis reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains has occurred within organic rich turbidites during redoxomorphic diagenesis normalized quotients of magnetic parameters also show that reductive diagenesis is a ferrimagnetic grain size selective process but it has a minimal effect on the canted antiferromagnetic fe3 oxides in the sediment such components if present therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are dissolved progressively and bulk magnetic concentration is thus depleted there is clear evidence in both cores for the existence of ultrafine ferrimagnetic grains at depth within the suboxic zone of the organic rich turbidites beneath both active and fossil oxidation fronts these grains are most probably associated with populations of live magnetotactic bacteria which commonly inhabit such organic rich horizons and play a part in the chain of bacterially mediated reactions normally associated with suboxic diagenesis these results show that simple and rapid rock magnetic techniques can be used to characterize early diagenetic processes involving iron phases in deep sea sediments at least as effectively as more laborious time consuming and sample destructive geochemical measurements

magnetic properties and geochemistry of the xiashu loess in the present subtropical area of china and their implications for pedogenic intensity variations in the magnetic susceptibility of chinese loess have been used to indicate pedogenic intensity and therefore past climate across the chinese loess plateau clp however the relationship between climate and magnetic properties has not been fully resolved and therefore a study of loess sections distributed over a wider climatic region is timely in this study we have investigated a 22 1 m loess section named the xiashu loess at dagang china by integrating both rock magnetic and geochemical proxies this loess profile is situated in a modern subtropical climate region with annual mean precipitation and temperature of 1100 mm and 15 4 c respectively and therefore has experienced a stronger weathering intensity due to its lower latitude higher temperature and rainfall geochemical and magnetic evidence indicate the same source area for the xiashu loess as for loess in northern china the magnetic mineral assemblage of the xiashu loess is dominated by a higher concentration of superparamagnetic sp single domain sd magnetite maghemite grains in strongly weathered paleosols than in the loess layers consistent with the production of fine grained ferrimagnetic minerals during pedogenic processes nevertheless compared to loess in the clp the xiashu loess shows 1 reduced contrasts in magnetic susceptibility hard isothermal remanent magnetization hirm and remanence of coercivity bcr between loess and paleosols units which indicates that the loess units have undergone stronger pedogenic alterations 2 lower peak values of hirm and in paleosols in addition the ratios sirm arm sirm where sirm and arm are saturation isothermal remanent magnetization and the susceptibility of anhysteretic remanent magnetization respectively and bcr are found to be significantly correlated with the quotient cao na2o mgo tio2 a geochemical indicator of pedogenic intensity overall it seems that pedogenic processes in the xiashu loess lead to a relative increase in fine grained ferrimagnetic sp sd grains but depletion of anti ferromagnetic minerals magnetic and geochemical proxies indicate strengthened east asia summer monsoon during marine isotope stages 13 11 and 5 which is in good agreement with the records from the clp and the south china sea 2007 elsevier b v all rights reserved

non magnetic indicators of pedogenesis related to loess magnetic enhancement and depletion examples from the czech republic and southern siberia abstract we carried out a multiparameter investigation for non magnetic proxies of the intensity of pedogenesis in loess paleosol sections from two different climate settings dolni v stonice central europe and krasnogorskoye southern siberia the work was performed in order to clarify the problem of two opposite scenarios of magnetic susceptibility ms patterns in these two profiles the chinese model with magnetically enhanced paleosols and the siberian alaskan model with magnetically depleted paleosols the age constraints on the krasnogorskoye section were inferred from an independent correlation with the well dated dolni v stonice section based on ms and sediment color cie l and with the ms log from lake baikal the pedologic description of the loess paleosol sections and their correlation was considerably improved by the use of vis spectral proxies cie l reflectance in the red band their weathering intensity can be effectively characterized by diffuse reflectance spectroscopy drs and geochemical cec and rb sr proxies whereas the ms is strongly dependent on the type of soil chernozems vs brown soils the geochemical proxies in particular cec proved to be relatively independent of the soil type these parameters are less affected by region specific conditions than the ms signal the non magnetic proxies indicate relatively dissimilar climatic trends at the two sites during the last glacial interglacial cycle a long distance correlation based solely on the ms signal can be adversely affected by switching between the chinese and the siberian alaskan magnetic modes this having been observed at the krasnogorskoye section in general no single universal proxy of weathering or pedogenesis intensity would be applicable across broad regions or considerable climatic gradients paleoclimatologic interpretations of large regions should always be based on a carefully designed multi proxy approach

magnetostratigraphy and environmental magnetism in a sedimentary sequence of miramar buenos aires argentina abstract a sedimentary sequence called fogones de la costa located at 38 17 35 s 57 50 50 w on the coastal cliffs south of miramar city was studied from geological paleomagnetic and rock magnetic viewpoints the stratigraphic column shows normal and reverse polarity records associated with chrons and subchrons of magnetopolarity the younger sediments were assigned to the brunhes polarity chron 1 9 ma on the basis of these data the sequence should span from the lower pleistocene to the holocene and may be correlated although partially with the geological formations of san andres miramar and arroyo seco described in coastal cliffs north of miramar the model of behavior of concentration parameters ms and mr is in agreement with that determined in other loess sections studied in northern buenos aires province the values of the concentration of magnetic parameters decrease with increasing pedogenesis and weathering of the original material loess and loess like this is a behavioral feature independent of the carrier agent wind or water the medium silt fraction is dominant over the other grain sizes in that fraction the amount of magnetic minerals increases significantly conversely the layers corresponding to paleochannels and paleolagoons show a decrease of magnetic minerals the medium silt fraction also decreases and the very fine silts and clays increase

lack of correlation between paleoprecipitation and magnetic susceptibility of chinese loess paleosol sequences we have conducted a detailed mineral magnetic study of loess unit 8 l8 and paleosol unit 8 s8 from three localities jingbian yichuan and duanjiapo along a n s transect in the chinese loess plateau as expected the low field magnetic susceptibility has higher values in s8 and lower values in l8 similarly superparamagnetic particle concentrations increase in s8 with increasingly humid cli mates along the n s transect which suggests that pedogenic magnetic enhancement is related to climate however in s8 at duanjiapo is so low that there is no correlation between and the degree of pedogenesis in this loess paleosol cycle it therefore appears that is not consistently the most suit able indicator of paleoclimate in chinese loess paleosol se quences our results indicate that there are complexities in the mineral magnetic response to pedogenesis which could invalidate interpretation of the record of chinese loess paleosol sequences in terms of paleoprecipitation

climatic thresholds for pedogenic iron oxides under aerobic conditions processes and their significance in paleoclimate reconstruction abstract iron oxides are widely distributed across the surface of the earth as a result of the aerobic weathering of primary fe bearing minerals pedogenic iron oxides which consist mainly of hematite hm goethite gt maghemite mgh are often concentrated synchronously in aerobic soils under low to moderate rainfall regimes magnetic susceptibility and redness which respectively reflect the content of mgh and hm in soils are considered reasonable pedogenic and climatic indicators in soil taxonomy and paleorainfall reconstruction however under high rainfall regimes the grain growth of mgh and transformation to hm combined with the prior formation of gt under conditions of high relative humidity rh can result in magnetic reduction and dramatic yellowing of soils and sediments which explains the existence of rainfall thresholds for mgh and hm at a large scale even before the pedogenic environment turns anaerobic in order to capture the rainfall thresholds for mgh and hm occurring under aerobic conditions we explored a tropical transect across a granitic region where the soil color turned from red to yellow under a wide rainfall range of 900 2200 mm yr and a corresponding mean annual rh range of 77 85 we observed a lower rainfall threshold of 1500 mm yr and a corresponding rh 80 for mgh and hm along this transect as well as a higher rainfall threshold of 1700 mm yr and a corresponding rh of 81 for gt and total pedogenic iron oxides citrate bicarbonate dithionite extractable fe fe d cross referencing with comparable studies in temperate and subtropical regions we noted that the rainfall or rh thresholds for fe d and hm or mgh likewise increase with temperature moreover the different thresholds for total and individual iron oxide phase indicates that a negative correlation between chemical weathering intensity and redness or in sediment sequences can occur under the prevalent climate regime just between their thresholds finally we developed an integrated model to interpret the sequential formation of rainfall thresholds for different pedogenic iron oxides in soils and sediments with conditions from aerobic to anaerobic

the effect of weathering on the grain size distribution of red soils in south eastern china and its climatic implications abstract particle size analysis is a useful way to determine the source and deposition of sediments however there are inconsistencies when this method is used to constrain the origin of the red soils in south eastern china to address this problem we performed a detailed grain size analysis of two red soil sequences in xuancheng and qiliting located along the lower reaches of the yangtze river by comparing their particle size characteristics with those of the loess on the loess plateau in northern china we found that the aeolian samples plot in a particular zone in the c m grain size of the cumulative 1 versus median grain size plot and cluster nearer the lower left corner of the plot as their degree of weathering increases the grain size features suggest that the onset of large scale aeolian deposition along the lower reaches of the yangtze river occurred at approximately 0 8 ma although both sedimentary sorting and post depositional weathering control the grain size variations in the deposits the extremely strong weathering due to the humid warm climate along the lower reaches of the yangtze river primarily modified the grain size distributions of the primary red soil deposits strong weathering increased the very fine silt 2 5 m fraction and decreased the coarse 63 m fraction we also found that certain grain size parameters of the red soils varied with the weathering intensity which can be used as indicators of palaeoclimate variations the grain size variations in both the qiliting and xuancheng sequences suggest that the mid pleistocene climate transition mpt may have affected the lower reaches of the yangtze river at 0 9 ma

magnetic grain size distribution of the enhanced component in the loess palaeosol sequences in the western loess plateau of china summary rock magnetic measurements of chinese loess palaeosol samples from the beiyuan section in linxia city suggest the presence of two magnetic components a background component and an enhanced component the magnetic properties of the enhanced component suggest strong grain size control which is in contrast with variable grain size distribution in loess palaeosol from the central loess plateau chemically formed magnetic grains do not fulfil the requirements in the case of the western loess plateau because they may show shifts in the grain size distribution the difference in climate between the western and central loess plateau may lead to different origins and different preservation conditions of the pedogenic magnetite

grain size magnetic susceptibility and geochemical characteristics of the loess in the chaohu lake basin implications for the origin palaeoclimatic change and provenance abstract rare studies on the aeolian deposit located in north bank of the yantgze river are documented recently it is found in the field investigations and in bore sections that the loess in the chaohu lake basin has the largest thickness of over 40 m in this study the probability cumulative curves frequency distribution the grain size distributions and the discriminant function of the grain size suggest that the loess in the chaohu lake basin is of eolian origin the magnetic susceptibility curves of the loess in the basin coincide perfectly with those of the loess in the northern china and the marine isotope stages mis and show that paleoclimatic cycles and sub cycles were documented since l3 during middle late pleistocene in the basin the ms curve of paleosol s1 paleosol s2 and loess l3 in the basin coincide perfectly with mis5 mis 7 and mis 8 respectively the good correspondence indicates that the loess in the basin has given a sensitive response to the globe paleoclimatic change since l3 on the other hand the climate changes in some stages recorded by the loess has regional characteristics obviously which might be the result of the dual effect of globe climate changes and east asia monsoon climate changes the result of geochemical characteristics suggests that the loess in the basin has undergone moderate to strong chemical weathering most elements are mobilized during chemical weathering na and ca of the loess are markedly lost and the removal of k is also evident and chemical weathering doesn t evidently turn into the si removal stage the chemical weathering of the loess is more intensive than that of the loess deposits in northwestern china and the upper reaches of the yantgze river the intensive chemical weathering has been documented by the loess might be related to strong monsoon climate in chaohu lake basin the provenance of the loess also differs from that in northern china and is discussed firstly with the lithofacies palaeogeography the well developed alluvial lacustrine deposits in the huaihe floodplain seem to be the major source materials of the loess

three large shifts in east asian monsoon circulation indicated by loess paleosol sequences in china and late cenozoic deposits in japan abstract correlations between loess paleosol sequences in china and late cenozoic deposits in japan suggest that the east asian monsoon circulation experienced three large shifts during the late cenozoic about 2 5 ma ago the onset of loess accumulation in north central china accompanied by a dramatic increase in eolian dust flux to the sea of japan marked a pronounced development of the east asian winter monsoon winds since ca 1 2 to 1 1 ma ago the obvious alternation of loess and paleosols in the chinese loess sequence with increased coarse grain size fraction in the loess units and higher than average magnetic susceptibility values in the paleosols and the repeated occurrence of interstratified marine clay layers in the osaka group of central japan indicate that the east asian monsoon circulation strengthened and the contrast between the summer and winter monsoons increased the paleosols of the chinese loess sequence were more strongly developed from ca 550 ka ago and loess started to accumulate in areas of the mid lower reaches of the yangtze river and in lake biwa of central japan from ca 450 ka ago reflecting more vigorous summer and winter monsoon circulations since ca 550 to 450 ka ago these large changes in east asian monsoon circulation were not only related to the changing solar radiation global glacial interglacial cycles and the changing configuration of land and sea but were also directly controlled by the uplift of the tibetan plateau

the history and variability of the east asian paleomonsoon climate abstract changes in the east asian paleomonsoon reflect interactions between the global atmosphere ocean land and ice systems and are also an expression of their combined effect within the boundary conditions imposed by the east asian continent and solar radiation the history of the east asian monsoon is an alternation between dominance by the dry cold winter and warm humid summer monsoons high resolution eolian sequences preserved in the chinese loess plateau reveal that the east asian monsoon may have commenced at least 7 2 ma ago they also provide evidence indicating that the pulsed uplift of the tibetan plateau at about 3 4 and 7 2 ma may have played an important role in inducing climate change the palaeoclimatic records of the last glacial cycle show high frequency variability on time scales of 1000 year or even shorter and instability of the east asian paleomonsoon system the high frequency variability could be due to a non linear response to orbital forcing or a result of the coupling processes between different components of the global system cold air activity in northern high latitudes the trans equatorial air streams from the southern hemisphere and possibly enso may have played an important role in east asian monsoon variability the synchroneity of all the palaeoclimatic events along the polar equator polar pep transect is still an open question correlation of limited palaeoclimatic records for the last 30 kyr obtained from east asia and australia suggests that the trans equatorial air streams driven by the monsoon and trade winds may have had an influence on opposite hemispheres

non stationary and nonlinear visco elastic shear creep model for shale abstract based on visco elastic shear creep experimental results of shale with four various shear stress levels a new non stationary and nonlinear visco elastic shear creep model is put forward first stationary linear visco elastic shear creep model is used to identify the creep parameters by the discrimination criterion of the square of correlation coefficient r 2 and sum of least error square q in stationary linear visco elastic shear creep model the visco elastic shear modulus g 2 of shale decreases but the visco coefficient 1 increases with increasing time which has distinct time scale effect then by constructing the relation between stationary creep parameters g 2 1 and time a non stationary visco elastic shear creep model is put forward to describe visco elastic shear creep deformation behavior of shale the non stationary shear creep model has a better agreement with the experimental data than does the stationary shear creep model but fails in predicting long term deformation characteristics therefore by introducing a time index a nonlinear visco elastic shear creep model is constructed which can predict better for long term deformation behavior of shale in the end based on the relation between nonlinear creep parameters and shear stress the shear creep deformation with all kinds of shear stress levels is predicted approximately in the range of 0 45 mpa 1 45 mpa

deep sea pollen from the south china sea pleistocene indicators of east asian monsoon abstract a high resolution pollen record sampling interval averages 820 years has been obtained from odp site 1144 water depth 2037 m northern south china sea the 504 m sequence in composition length covers the last 1 03 million years according to micropaleontological and isotopic stratigraphy the pollen assemblages are characterized by high proportions of pinus and herb pollen and by their frequent alternations based on these alternations 29 pollen zones have been recognized that are closely correlated to the marine oxygen isotope stages mis 1 29 pinus dominant pollen zones correspond to interglacial periods with lighter 18o values while herb marked ones relate to the heavier 18o stages assigned to glacials judging from the pollen data the exposed northern continental shelf of the south china sea during the glacials was covered by grassland and the extensive northern shelf has formed only since mis 6 ca 150 ka probably as a result of tectonic subsidence tree pollen influx values are indicative of winter monsoon which began to intensify 600 ka ago the summer monsoon variations can be approximated by the fern percentage within the total pollen and spore abundance and the result shows high values in general occurring at interglacials with the maxima at mis 15 5e and 1 the relatively high fern percentage with smaller amplitude in variations before 600 ka might suggest more stable humid conditions before the intensification of winter monsoon

summer monsoon variations over the last 1 2 ma from the weathering of loess soil sequences in china the loess soil sequence in northern china contains a near continuous record of quaternary paleoclimate magnetic susceptibility and grain size have so far been the only proxies available to address the long term changes of the east asian paleomonsoon extending back to more than one million years in this study the ratio of cbd citrate bicarbonate dithionite extractable free fe2o3 fed a measure of iron liberated by chemical weathering versus the total fe2o3 available fet was measured on samples at 10 cm intervals taken from two loess sections deposited over the last 1 2 ma variations of this index are highly consistent with other pedological indicators but in addition provide a quantitative measurement of the degree of pedogenesis in the loess plateau since chemical weathering in the region mainly depends upon summer precipitation and temperature weathering intensity primarily reflects changes in the east asian summer monsoon the new proxy has been used to document a series of summer monsoon changes of global significance which are not necessarily recorded by magnetic susceptibility

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

pedogenesis and paleoclimate interpretation of the magnetic susceptibility record of chinese loess paleosol sequences in the chinese loess paleosol sequences from luochuan haanxi province china variations in the magnetic susceptibility have been accepted as an excellent proxy for paleoclimate and the standard interpretation is that climatic processes have enhanced the magnetic susceptibility of the paleosols beyond the base line level of the loess by using mineral magnetic properties and a soil chemistry extraction procedure we show that most of the magnetic susceptibility signal in both the paleosols and the loess is due to pedogenesis in effect loess deposition and pedogenesis are competing processes at all times and the presence of a paleosol simply indicates that the latter process was predominant these results imply that any interpretation of the paleoclimate record of loess paleosol sequences must recognize the pedogenic nature of the magnetic susceptibility signal and that the focus of future research must be on the critical role of climate in the development of the paleosols

mineral magnetic variation of the jiaodao chinese loess paleosol sequence and its bearing on long term climatic variability 1 to retrieve reliable long term paleoclimatic signals from the chinese loess paleosol sequences deposited over the past 2 6 myr multiparameter mineral magnetic investigations have been conducted on the jiaodao section on the chinese central loess plateau first the lithogenic magnetic components and the pedogenic overprinting were chemically separated by the citrate bicarbonate dithionite cbd method the magnetic properties of the least pedogenically altered loess units and post cbd loess paleosols are dominated by coarse grained lithogenic magnetite increasing degree of pedogenesis increases both the concentration and grain size of pedogenic magnetite maghemite long term changes of the pre cbd magnetic properties suggest a long term decrease in summer monsoon intensity and a long term increase in winter monsoon intensity from the late pliocene to the late pleistocene long term variations of the post cbd magnetic properties suggest an increase in both concentration and grain size of coarse grained lithogenic psd md magnetite grains and hence indicate a long term increase in winter monsoon intensity over that period the coercivity decrease of the lithogenic component for the loess unit l15 may be related to the accelerated uplift of the tibetan plateau and its adjacent regions during the period of l15 deposition from paleosol s5 to loess l1 higher values of coercivity of the lithogenic component occur in typical loess and paleosols and lower values are found at the loess paleosol transition these coercivity variations are thought to be related to significant changes in the source region as well as dust deposition in the loess plateau caused by changes in ice snow or vegetational cover these new results lead to a better understanding of the relationships between the effects of tectonic paleoclimatic and paleoenvironmental processes in northwestern china

geochemistry of the xining xifeng and jixian sections loess plateau of china eolian dust provenance and paleosol evolution during the last 140 ka abstract loess and paleosol samples from three distant sections xining xifeng jixian of the chinese loess plateau were analyzed for chemical and nd sr isotopic compositions in order to obtain information about the paleoclimatic variation during the last 140 ka these three sections represent three different climatic conditions from arid in the west xining to more humid in the east jixian across the loess plateau isotopic analyses show that all the three sections display a restricted range of 143 nd 144 nd ratios or e nd 10 0 5 indicating the dominance of relatively young and uniform upper crustal sources for the eolian dust the loess and paleosol samples have typical upper crustal 87 sr 86 sr ratios 0 714 0 718 and their variation can partly be attributed to different carbonate silicate ratios in the samples ree data from the three sections are indistinguishable from each other they show typical upper crust patterns characterized by la yb n 10 and eu eu 0 65 the high degree of chemical and isotopic homogeneity suggests that the dust source region must have undergone multiple recycling and thorough sedimentary mixing processes various chemical indicators have been used to monitor the pedogenetic intensity in these sections although results for the jixian section agree reasonably well with the known climatic conditions on the loess plateau the other two sections yield results not in agreement with some well accepted proxies like magnetic susceptibility or grain size distribution some chemical proxy indicators established in our previous work on the luochuan section such as the systematic u depletion and negative ce anomalies in ree patterns in soils are not clearly recorded in the present case the discrepancy between different climatic proxies could be related to the effective soil water budget temperature as well as the rate of development of each proxy

magnetostratigraphy of the qiliting section se china and its implication for geochronology of the red soil sequences in southern china summary the red soil sequences in southern and southeastern china provide important terrestrial archives of palaeoclimate and palaeoenvironment and are important sources of palaeolithic settlements and therefore early human occupation here we present detailed palaeomagnetic investigation coupled with mineral magnetic and petrographic analyses of a red soil sequence at qiliting southeastern china the sediments contain both strongly magnetic minerals such as maghemite and magnetite and weakly magnetic minerals such as haematite and possibly goethite maghemite and haematite were identified as the main carriers of the natural remnant magnetisations only haematite dominates the characteristic remnant magnetisations palaeomagnetic findings suggest that the qiliting sedimentary sequence recorded the early brunhes chron and the late matuyama chron including the jaramillo subchron our finding coupled with previously published magnetostratigraphy establishes that the red soil sequences in southern china began to accumulate no later than the jaramillo age i e 1 0 ma it is further suggested that the capabilities of recording magnetic polarity stratigraphy in chinese red soil sequences are variable between areas because of different climate regimes

mineral magnetic record of the chinese loess and paleosols variations in the magnetic susceptiblity of the chinese loess and its interbedded paleosols correlate strikingly well with the deep sea oxygen isotope record low susceptibility values are found for the loess layers and high values are found for the soil horizons two interpretative models have been advanced to account for these magnetic variations both of these models discount any significant in situ formation of magnetite during soil forming periods instead they infer relative concentration of detrital or atmospheric magnetite at these times we critically examine the assumptions made in these models and on the basis of some new mineral magnetic data and fromalized magnetic flux calculations offer an alternative explanation of the loess magnetic record our model identifies pedogenic formation of magnetite as the major contributor to the high magnetic susceptibility of the paleosols the formation and preservation of this pedogenic magnetite is dependent on soil froming conditions and hence reflects the regional climate

geochemical evidence for provenance diversity of loess in southern china and its implications for glacial aridification of the northern subtropical region abstract the thick xiashu loess in southern china is distributed not only in the yangtze river valley but also and most extensively in the region between the qinling mountains and the lower reaches of the yangtze river however there are few studies of the provenance and climatic implications of the xiashu loess in this latter region here we present the results of a provenance study of last glacial loess samples from twenty two typical xiashu loess sections using the major and trace element composition as a provenance indicator seventeen of the sites are located to the north of the yangtze river and our primary aim was to determine the provenance of the xiashu loess within this region our results indicate that the alluvial plain located mainly in the present drainage area of the huai river was the primary dust source for the xiashu loess in this region the occurrence of multiple local dust sources within this alluvial plain suggested by their geochemical heterogeneity indicates that the alluvial plain experienced intensified aridity which caused it to become a dust source for local loess deposits in northern subtropical china during the last glacial our study provides strong evidence to challenge the long held view that the occurrence of xiashu loess resulted from the southward incursion of the clp loess deposits sourced from deserts of the asian interior and it confirms the substantial role of aridification of the present humid northern subtropical region for dust emission to the climate system during the last glacial

climatic interpretation of the luochuan and lingtai loess sections china based on changing iron oxide mineralogy and magnetic susceptibility abstract hematite and goethite estimated from diffuse reflectance spectra and magnetic susceptibility ms have been determined from both the luochuan and lingtai loess sections of the loess plateau of central china changing concentrations of these iron oxides and ms were used to determine paleoclimatic change based on a model that relates pedogenic production of hematite and magnetite to precipitation we propose two precipitation phases on the loess plateau during the last 2 6 ma from the wucheng red clay boundary up to paleosol s5 mis 13 precipitation on the loess plateau was moderate probably not frequently exceeding 450 mm yr and highly seasonal from s5 to the present precipitation during interglacial times has gradually increased to the present levels of 600 650 mm yr for the localities examined throughout the pleistocene spanning both glacial and interglacial conditions hematite to goethite ratios display a consistent decrease this decrease suggests that precipitation on the loess plateau has been getting less seasonal and or has been occurring during times when temperature was lower in addition to these long term changes it also appears that iron oxide records from the loess plateau exhibit 400 500 ka cycles from the wucheng red clay boundary to s5 these 400 500 ka cycles may be analogous to similar length cycles recorded in 13 c marine records in the south china sea and may be linked through weathering and silica production as a result of the asian monsoon

palaeoclimatic and sedimentary history from magnetic susceptibility of loess in china the magnetostratigraphic age of the oldest loess sediments in the central chinese loess plateau investigated so far has recently been placed at ca 2 4 million years b p thus glaciations in central asia which are believed to provide the detritus for the formation of chinese loess commenced at the same time as the first ice rafted debris sank into the ocean basins of the northern hemisphere shackleton et al 1984 lithologic palaeontologic geochemical and especially rock magnetic variations throughout the loess column are indicative of palaeoclimatic fluctuations during loess deposition the variations of magnetic low field susceptibility in loess closely resemble those of the marine oxygen isotope record shackleton and opdyke 1976 and establish for the first time a continuous record of the pleistocene glacial and interglacial stages in a continental section spectral analysis of the rock magnetic time series indicates slow apparent sedimentation during interglacial periods which are characterized mainly by calcium dissolution and related compaction of the soils variable aeolic sediment influx as well as erosion and or polygenetic soil formation may also strongly influence the sedimentary history

magnetic proxy climate results from the duanjiapo loess section southernmost extremity of the chinese loess plateau we report mineral magnetic results from a 7 5 m loess sequence 150 samples from the southernmost extremity of the chinese loess plateau which includes the last two glacial cycles in this area the loess sediments experienced particularly intense weathering processes the magnetic assemblage is dominated by a mixture of pseudo single domain psd and multidomain md magnetite with associated superparamagnetic sp grains of either magnetite or maghemite in the paleosols and weathered loess horizons all the rock magnetic parameters fluctuate in parallel with marine sediment 18o data over the last 150 kyr thus reflecting changing global paleoclimatic conditions this relationship is also supported by the evidence of milankovitch cycles in the magnetic susceptibility record paleorainfall estimates when compared with other studies from the chinese loess plateau underline the more humid character of this region during the last 130 kyr

the magnetic properties of particle sized samples from the luo chuan loess section evidence for pedogenesis abstract measurements of magnetic susceptibility and of laboratory remanences have been carried out on particle size fractions of loess and palaeosol samples from luo chuan china they show that the sub micron clay fraction of the palaeosol is greatly enriched in fine grained

magnetic granulometric and geochemical characterizations of loess sections in the eastern arid central asia implication for paleoenvironmental interpretations abstract loess deposits in the arid central asia aca contain sensitive messages concerning the evolution of the westerlies and the interior aridification of asia in this study we conducted a combined study of geophysical and geochemical characterization on loess sections in the eastern aca to discuss possible paleoclimate development since the late pleistocene era results show that the aca loess chiefly exhibited similar geophysical and geochemical compositions with loess deposits in other regions like the chinese loess plateau there are various accumulation rates that would have been responsible for the depositional difference within the aca loess profiles in addition there are three magnetic responses to the bulk grains and geochemical proxies in this study at the bottom segment of the section magnetic enhancement was associated with coarse grain sizes and water leaching magnetic enhancement responded to stronger pedogenesis and moderate paleo wind intensity in the uppermost parts moreover there is a transitional segment in the middle part of the section characterized by inordinately detrital inputs and weak pedogenic contributions thus it may be better to evaluate the magnetic climatology of the aca loess through different depositional stages furthermore this study generally reveals that the paleoclimate pattern in the aca was probably characterized by relatively low moisture during warm intervals and vice versa in the middle and bottom parts of the profiles but relatively enhanced moisture was indicated during the holocene period further studies are still necessary to obtain reliable proxies and chronology for a detailed paleoclimate interpretation of the aca loess

breaking into the plate a 7 6 mw fracture zone earthquake adjacent to the central indian ridge 1 on 15 july 2003 an extremely large 7 6 mw strike slip earthquake initiated on or near the end of an active transform along the northern central indian ridge the event propagated away from the plate boundary along the typically inactive fracture zone with a sense of slip that opposes the active transform slip direction seismically and hydroacoustically determined aftershock locations delineate a 210 25 km long mainshock rupture the seismic moment and rupture dimensions imply a stress drop of 4 5 6 5 mpa and a mean slip of 3 0 0 5 m the largest aftershock 5 6 nw occurred on the active portion of a neighboring transform at a distance of 160 km where mainshock induced static stress changes are predicted to promote failure near axis fracture zone earthquakes may promote and inhibit ridge parallel diking along different spreading segments perhaps contributing to inter segment variability in the rate and asymmetry of spreading

coupled hydromechanical modeling of rock fractures under normal stress in this paper a nonlinear poromechanical model is developed for a single rock fracture under normal stress the fracture is represented by a set of voids and the progressive fracture displacement

separation of pedogenic and lithogenic components of magnetic susceptibility in the chinese loess palaeosol sequence as determined by the cbd procedure and a mixing analysis summary magnetic susceptibility variations in the chinese loess palaeosol sequences have been used extensively for palaeoclimatic interpretations the magnetic signal of these sequences must be divided into lithogenic and pedogenic components because the palaeoclimatic record is primarily reflected in the pedogenic component in this paper we compare two methods for separating the pedogenic and lithogenic components of the magnetic susceptibility signal the citrate bicarbonate dithionite cbd extraction procedure and a mixing analysis both methods yield good estimates of the pedogenic component especially for the palaeosols the cbd procedure underestimates the lithogenic component and overestimates the pedogenic component the magnitude of this eect is moderately high in loess layers but almost negligible in palaeosols the mixing model overestimates the lithogenic component and underestimates the pedogenic component both methods can be adjusted to yield better estimates of both components the lithogenic susceptibility as determined by either method suggests that palaeoclimatic interpretations based only on total susceptibility will be in error and that a single estimate of the average lithogenic susceptibility is not an accurate basis for adjusting the total susceptibility a long term decline in lithogenic susceptibility with depth in the section suggests more intense or prolonged periods of weathering associated with the formation of the older palaeosols the cbd procedure provides the most comprehensive information on the magnitude of the components and magnetic mineralogy of loess and palaeosols however the mixing analysis provides a sensitive rapid and easily applied alternative to the cbd procedure a combination of the two approaches provides the most powerful and perhaps the most accurate way of separating the magnetic susceptibility components