magnetic parameter variations in the chaona loess paleosol sequences in the central chinese loess plateau and their significance for the middle pleistocene climate transition abstract a high resolution rock magnetic investigation was performed on the chaona quaternary loess paleosol sequences in the central chinese loess plateau based on a newly developed independent unturned time scale and magnetic records we reconstructed the history of the east asia monsoons during the last 3 ma and explored the middle pleistocene climate transition mpt rock magnetic results show that the loess layers are characterized by relatively high coercivity and remanent coercivity lower magnetic susceptibility ms and that the paleosol layers are characterized by relatively high ms saturation magnetization and remanent saturation magnetization spectrum analyses indicate that there are various periods in addition to orbital periodicities according to the onset and stable appearance of 100 kyr period we consider that the mpt recorded in this section began at 1 26 ma and was completed by 0 53 ma which differs from previous investigations based on orbitally tuned time scales the forcing mechanism for the mpt was more complicated than just the orbital forcing we conclude that the rapid uplift of the tibetan plateau may have played an important role in the shift of periodicities during the middle pleistocene

paleoclimatic significance of magnetic properties on the red clay underlying the loess and paleosols in china abstract the red clay unit of northern china is a widespread miocene pliocene 7 2 2 5 ma aeolian deposit that underlies much of the quaternary loess paleosol sequence in the central loess plateau it comprises a sequence of highly developed soils and interbedded layers of less weathered loess like material here referred to as reddish loess in common with the loess paleosol alternations of the overlying quaternary deposits the soil reddish loess alternations are interpreted to represent climatic fluctuations in this case between warm humid and relatively dry cool conditions however while magnetic susceptibility variations in the quaternary deposits provide a good proxy for pedogenic intensity in the red clay the relationship is less clear and magnetic susceptibility data are in conflict with other measures of pedogenesis in an attempt to resolve this issue we have investigated a red clay section and overlying loess paleosol sequence at xifeng and have supplemented these data with samples from the red clay at lingtai our study indicates that the red clay and loess paleosol sequence have a common magnetic mineralogy comprising magnetite maghemite and hematite and possibly goethite and that both sequences show a good correlation between magnetic susceptibility and the magnitude of the superparamagnetic sp component this implies that susceptibility enhancement in the red clay is strongly related to the magnitude of the sp content a component that is widely recognised as having a pedogenic origin in quaternary deposits further support for a correlation between pedogenesis and magnetic susceptibility in the red clay is provided by a good correlation between magnetic susceptibility and the rb sr ratio an independent weathering index however differences in the magnetic mineralogy of the two units are also evident the minimum magnetic susceptibility of the red clay is less than half the value of the overlying quaternary deposits at locations such as xifeng and lingtai while the maximum sp content is relatively larger than for the quaternary deposits while we cannot discount the possibility that the former difference is a reflection of different parent materials for the two units climate may also be a contributory factor in explaining these differences precipitation is a major control on magnetic enhancement in paleosols maher and thompson 1995 quat res 44 383 391 liu et al 1995 palaeogeogr palaeoclimatol palaeoecol 113 243 248 but soil anoxia and iron oxide dissolution occur if moisture exceeds some critical level soil moisture is therefore a critical factor in determining whether the correlation between pedogenesis and magnetic susceptibility in loess deposits is positive central loess plateau china magnetic enhancement negative alaska siberia magnetic destruction or uncertain pakistan argentina new zealand cycles of enhancement and destruction while our results indicate a generally positive correlation between pedogenesis and magnetic susceptibility in the red clay they also imply that a more thorough paleoclimatic interpretation can only be achieved using complementary but independent techniques more work is required to deconvolve the climatic record of the red clay but the reward may be the extension of the record of paleomonsoon evolution back into the pliocene and late miocene

rock magnetic record of late neogene red clay sediments from the chinese loess plateau and its implications for east asian monsoon evolution abstract the quaternary loess paleosol and underlying neogene red clay sequences on the chinese loess plateau clp contain a remarkable continental record of past east asian monsoon changes and aridification of the asian interior while monsoon variability during the quaternary is reasonably well understood the evolution and dynamics of the monsoon during the neogene are still debated the rock magnetism of aeolian sediments is a well established tool for reconstructing the palaeoclimatic history of asia here we present a rock magnetic record from late neogene red clay sediments of the chaona section in the central clp the results indicate that the main magnetic minerals in the red clay are magnetite maghemite hematite and goethite similar to the overlying quaternary loess palaeosol sequences the high resolution rock magnetic record combined with the results of other proxies demonstrate a stepwise intensification of the summer monsoon and a progressive enhancement of the winter monsoon in east asia during the late neogene based on the results the climatic evolution of east asia can be divided into three intervals interval iii from 8 1 to 6 8 ma is characterized by a cool dry climate with a relatively weak east asian summer monsoon easm and east asian winter monsoon eawm interval ii from 6 8 to 4 3 ma is characterized by cool warm cycles and interval i from 4 3 to 2 8 2 6 ma was warm and humid and within which the easm strengthened rapidly and the eawm fluctuated considerably with an increasing trend we infer that before 4 3 ma the variations of both the easm and eawm were closely related to global cooling and that the intensified easm during the late pliocene was primarily caused by tectonic events including the gradual closure of the panama seaway and the uplift of the tibetan plateau rather than by global cooling

mineral magnetic studies of the vermiculated red soils in southeast china and their paleoclimatic significance abstract vermiculated red soil the main pedostratigraphical unit of the red soil sequences widespread in subtropical and tropical china has experienced a higher degree of pedogenesis than loess paleosols and the underlying red clays in the loess plateau of northern china previous studies suggested that magnetic properties of the vermiculated units of the red soils are significantly degraded compared to those of the non vermiculated counterparts the mechanism for this magnetic decrease however has not been well determined in order to resolve this problem this study has investigated two red soil sequences of different origin in southeast china by integrating environmental and mineral magnetic research the results show that the magnetic depletion in the vermiculated units was probably caused by both dissolution of magnetic minerals in the white veins and transformation of highly magnetic maghemite to weakly magnetic hematite it appears that stronger pedogenesis favors the transformation of more maghemite to hematite and magnetic properties associated with the relative concentration of maghemite to hematite can be used as climate indicator in the vermiculated red soils the variation trends of the magnetic parameters of the vermiculated red soils prior to 0 85 0 9 ma suggest a strengthening of the east asian summer monsoon the reverse trend of these parameters after 0 85 0 9 ma may correlate with a significant strengthening of the east asian winter monsoon since 0 85 ma

ac magnetic susceptibility studies of chinese red clay sediments between 4 8 and 4 1 ma paleoceanographic and paleoclimatic implications 1 recent studies show that measuring ac magnetic susceptibility with in phase and quadrature components versus temperature provides an effective way to map the grain size distribution of pedogenic maghemite applying this method we mapped the pedogenic maghemite grain size distribution of samples of the red clay sediments deposited on the chinese loess plateau clp during the interval 4 8 4 1 ma during which the validity of magnetic susceptibility as an east asian summer monsoon precipitation intensity proxy has been questioned the results show that the pedogenic maghemite grain size distribution is the same as in the overlying loess paleosol sequence and in the other intervals of the red clay sequence suggesting similar pedogenesis mechanisms and marked precipitation seasonality on the clp between 4 8 and 4 1 ma compared with the other intervals we argue that the seasonal position shift of the polar westerlies produces the required precipitation seasonality and thus produces a uniform pedogenic grain size distribution during this time interval reconstructing paleo rainfall on the central clp on the basis of surface calibration results suggests that the intensification of the east asian summer monsoon might be a key trigger for the subsequent significant ocean and cryosphere adjustments during the pliocene

enhancement mechanisms of magnetic susceptibility in the chinese red clay sequence 1 little is known about the mechanisms of magnetic susceptibility enhancement for the red clay sequence on the chinese loess plateau clp in comparison to the overlying loess paleosol sequence here we present a rock magnetic study of the red clay sediments from the central clp our results show that frequency dependence of fd lf hf where lf and hf are measured at 470 hz and 4700 hz respectively lf and susceptibility of anhysteretic remanent magnetization arm are linearly correlated within the red clay sequence this linear correlation indicates that the pedogenic magnetic minerals of the red clay have a rather uniform grain size distribution as in the loess paleosol sequence and the grain size is independent of the degree of pedogenesis nevertheless red clay sediments are slightly more enriched in superparamagnetic magnetic particles than the overlying loess paleosol sediments as indicated by the higher slope of the regression line between fd and lf

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

provenance control on chemical weathering index of fluvio lacustrine sediments evidence from the qaidam basin ne tibetan plateau carbon dioxide drawdown resulting from enhanced chemical weathering during orogenesis has been invoked to explain late cenozoic global cooling establishing chemical weathering records from the india asia collision zone is important to test this hypothesis because uplift of the tibetan plateau is thought to be responsible for cenozoic cooling however proxies for the intensity of chemical weathering can be affected by additional factors such as sediment grain size and provenance here we report major element compositions and calculated chemical weathering intensity records of three size fractions 0 5 5 20 20 63 mu m from the dahonggou section of the qaidam basin in the northeastern tibetan plateau and compare those records with published provenance data from the same section results show that the indices of the fine 0 5 mu m fraction vary in coordination with provenance shifts but variations of the 5 20 and 20 63 mu m fractions are less affected by provenance variations comparison with upper continental crust reveals that some labile elements are not leached but instead are enriched in the fine fraction indicating that it does not faithfully record chemical weathering intensity in addition weathering can result in clay mineral transformation instead of elemental variations complicating the relationship between element based parameters and weathering intensity this work suggests that changes in sediment provenance must be accounted for when inferring variations in chemical weathering intensity on the basis of element based weathering intensity indices of the clay fraction

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

link between benthic oxygen isotopes and magnetic susceptibility in the red clay sequence on the chinese loess plateau 1 recent rock magnetic work on the red clay sequence on the chinese loess plateau clp convincingly demonstrates that the enhancement mechanisms of low frequency magnetic susceptibility i e measured at 470 hz lf in the red clay sequence are similar to those in the loess paleosol sequence therefore lf in the red clay sequence should indicate precipitation intensity received by the clp as is the case in the overlying loess paleosol sequence based on this result we compared lf in the red clay sequence with benthic oxygen isotope records we infer that the primary precipitation source on the clp varies over time in three phases during 8 1 4 5 ma the east asian summer monsoon dominates during 4 5 4 ma the polar westerlies dominate during 4 0 ma the east asian summer monsoon dominates we attribute these precipitation source shifts on the clp to the closure of the panama seaway around 4 5 ma and the tibetan uplift during the interval 4 2 6 ma

the importance of solar insolation on the temperature variations for the past 110 kyr on the chinese loess plateau abstract the late quaternary temperature history on the chinese loess plateau clp is little known due to the absence of suitable paleothermometers here we present a quantitative temperature record for the last 110 kyr from lantian county southern clp based on the distributions of bacterial tetraethers lantian loess temperature variations display significant correlation with the northern hemisphere insolation 35 n as well as the cave monsoon records at the precession band we attribute such correlation to both the latent heat release from the east asian summer monsoon and the direct influence of sun s sensible heat over the semi arid landscape the long term cooling during the marine isotope stage mis 3 and the modest temperature drop of 5 c from 79 to 65 ka are best explained by interactions among multiple climate forcings including insolation atmospheric co 2 concentration global ice volume and regional glacier activities the cold and modestly wet climate during mis 3 may have promoted maximum regional glacial advances whereas the minimal temperature during local lgm reduced moisture transfer and led to glacial recession the unusually early deglacial warming at 22 5 ka highlights the importance of regional climate records to decipher the complex continental climate dynamics

clay mineralogy of the upper miocene pliocene red clay from the central chinese loess plateau and its paleoclimate implications abstract study of clay mineral is one of effective methods for reconstruction of the paleoclimate and paleoenvironment here we represent clay mineral records from an upper miocene pliocene red clay succession in the central chinese loess plateau clp on the basis of the existing age model we employ x ray diffraction and scanning electron microscopy to carry out clay mineral analyses the results indicated that illite was the dominant clay mineral and the contents of smectite chlorite and kaolinite were minor from the analysis of contents of clay minerals illite crystallinity illite 5 a 10 a smectite illite chlorite ratio magnetic susceptibility summer monsoon intensity and deep sea 18o values the paleoclimatic evolution between 7 5 and 2 6 ma can be divided into three intervals interval i from 7 5 to 6 9 ma it was cool and dry interval ii from 6 9 to 4 1 ma was cool warm oscillations interval iii from 4 1 to 2 6 ma was significantly warm and humid stage of the seasonal climate conditions the proxy records display a large amplitude transition in the later interval iii from 2 8 to 2 6 ma which implied an abrupt climate and environment variability through comparison the local and global paleoclimatic records we found that clay minerals reflect regional paleoclimatic change superimposing global climatic records during the late miocene pliocene period we conclude that the regional tectonic activity and global cooling had affected east asia monsoon which was characterized by the different temperature and precipitation in the central clp

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

environmental magnetic study of a xeralf chronosequence in northwestern spain indications for pedogenesis article i nfo magnetic enhancement of a and b horizons during soil development is a common phenomenon to better understand the exact mechanism for the magnetic enhancement especially the mineral transformation pathways systematic rock magnetic studies were conducted on a xeralf chronosequence in northwestern spain a positive correlation was found between the average grain size and the concentration of pedogenic maghemite particles with the exception of some a horizons in which multiple factors seem to influence the nature and concentration of neoformed ferrimagnets it is argued that the interaction between the positively charged iron oxides and the anionic ligands present in the solution of these acidic soils plays an important role in this respect the time trends of the average grain size of pedogenic ferrimagnets and the strong correlation between the concentrations of hematite and maghemite in the magnetically enhanced horizons a and b are consistent with the hypothesis of a gradual formation of maghemite later converted into hematite via a precursor most likely ferrihydrite therefore this study provides strong evidence for the dynamic evolution of magnetic minerals upon pedogenesis

variation of rb sr ratios in the loess paleosol sequences of central china during the last 130 000 years and their implications for monsoon paleoclimatology abstract rb sr and magnetic susceptibility have been measured in the last interglacial glacial loess profiles at luochuan and huanxian central china a high degree of similarity between the parameters in both profiles suggests that variations of rb sr ratios in the sequence can be regarded as an indicator of east asian summer monsoon strength matching the rb sr record with the specmap 18o curve suggests that the rb sr ratio responds sensitively to changes of the east asian monsoon induced by global ice volume variation

magnetic enhancement is linked to and precedes hematite formation in aerobic soil 1 soil formation usually increases magnetic susceptibility most often by increasing the concentrations of magnetite and maghemite which are two ferrimagnetic iron oxides here we provide evidence that magnetic enhancement in aerobic soils not affected by detrital magnetic inputs or thermal transformation of other iron oxides is mostly due to the formation of maghemite which is later transformed into hematite the iron oxide that gives red color to soil we show that the maghemite hematite ratio is influenced by the particular environment and the degree of soil development so it constitutes an effective tool for paleoenvironmental and planetary studies

magnetic enhancement and palaeoclimate study of a loess palaeosol couplet across the loess plateau of china summary the origin of the milankovitch palaeoclimatic signal so dramatically recorded in the chinese loess remains uncertain at least five hypotheses have been put forward to explain the well documented enhancement of magnetic susceptibility in palaeosols which formed when conditions were warmer and wetter we report here a magnetic study of a prominent loess soil couplet l4 s3 at three widely separated sites where present day mean annual rainfall and temperature range from 350mm to 700mm and from 6 c to 13 c the properties determined are laboratory remanences isothermal irm anhysteretic arm viscous vrm including viscous decay room temperature susceptibility and thermomagnetic behaviour at fields beyond 100 mt the irm spectra are all essentially identical both in shape and in absolute magnitude suggesting a uniform magnetic component widespread across the plateau subtracting the signal of this component isolates a second component the amount of which varies by an order of magnitude between sites the magnetic properties of this latter material are remarkably similar to those reported for bacterial magnetite in geological sediments and modern soils viscous decay of vrm takes place in two distinct stages one of which falls to zero in 10min we attribute magnetic enhancement which is the source of the palaeoclimatic signal to in situ pedogenetic formation of magnetite which we characterize in our interpretation as two separate forms of bacterial magnetofossils the most magnetic sample requires a mass fraction of about half of one per cent of this biogenic material

rock magnetic record of core sg 3 since 1 ma in the western qaidam basin and its paleoclimate implications for the ne tibetan plateau abstract quaternary saline lake deposits are well formed and widely distributed in the qaidam basin on the northeastern tibetan plateau nw china studying these lacustrine salt mineral deposits can improve our understanding of tectonic deformation asian interior aridification and monsoon climate associated with global cooling and tibetan plateau uplift previous paleoclimate records in the qaidam basin are mostly documented by geochemical proxies pollen spectra and sedimentary characteristics however supporting evidence from the rock magnetic index of sediments with salt deposits in the basin has not been well studied here we present a continuous high resolution 1 0 myr rock magnetic record from the well dated drill core sg 3 for typical saline lake deposits in the western qaidam basin the results show that deposits with low magnetic susceptibility are caused by low contents of magnetic minerals with single domain sd grains while those with high have relatively high contents of magnetic minerals with sd and pseudo single domain psd grains in full consideration of the sedimentary environment weathering and erosion associated with the global climate since the middle pleistocene we propose that variations in the closed lake are mainly influenced by erosion intensity and precipitation by combining the changes with pollen and grain size records we infer that the drying process of central asia since 1 ma with intensified aridity at 0 6 ma and 0 13 ma was primarily induced by global cooling

characteristics and genesis of maghemite in chinese loess and paleosols mechanism for magnetic susceptibility enhancement in paleosols abstract morphological characteristics and microstructures of magnetic minerals extracted from chinese loess and paleosols were investigated using powder x ray diffraction xrd and high resolution transmission electron microscopy hrtem our results indicate that maghemite in loess paleosol sequences was transformed from magnetite through oxidation of magnetite maghemite transformed from eolian magnetite during chemical weathering has low angle grain boundaries among maghemite nano crystals some nano crystalline maghemites with nanoporous texture resulted from microbe induced precipitation of magnetite or transformation of poorly crystalline ferric fe oxy hydroxides in presence of fe reducing bacteria aggregates of euhedral maghemite nano crystals were transformed from magnetite magnetosomes both microbe induced nanoporous magnetite and microbe produced magnetite magnetosomes are directly related to microbial activities and pedogenesis of the paleosols it is proposed that the formation of nano crystalline maghemite with superparamagnetic property in paleosol results in the enhancement of magnetic susceptibility although the total amount weight percent of magnetic minerals in both paleosol and loess units is similar our results also show that nano crystalline and nanoporous magnetite grains prefer to transform into maghemite in semi arid soil environments instead of hematite although hematite is a thermodynamically stable phase this result also indicates that a decrease in crystal size will increase stability of maghemite it is also inferred that surface energy of maghemite is lower than that of hematite

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

magnetic mineralogy of soils across the russian steppe climatic dependence of pedogenic magnetite formation formation of ferrimagnets in well drained buffered unpolluted soils appears to be related to climate and especially rainfall if robust this magnetism rainfall couple can be used to estimate past rainfall from buried soils particularly the multiple soils of the quaternary loess soil sequences of central asia however dispute exists regarding the role of climate vs dust flux for the magnetic properties of modern loessic soils here we examine the mineralogical basis of the magnetism rainfall link for a climate transect across the loess mantled russian steppe where critically dust accumulation is minimal at the present day magnetic and independent mineralogical analyses identify in situ formation of ferrimagnets in these grassland soils increased ferrimagnetic concentrations are associated with higher annual rainfall xrd and electron microscopy show the soil formed ferrimagnets are ultrafine grained 50 nm and pure ferrimagnetic contributions to ma ssbauer spectra range from 17 in the parent loess to 42 for a subsoil sample from the highest rainfall area total iron content varies little but the systematic magnetic increases are accompanied by decreased fe2 content reflecting increased silicate weathering for this region parent materials are loessial deposits topography is rolling to flat and duration of soil formation effectively constant the variations in soil magnetic properties thus predominantly reflect climate and its co variant organic activity statistical analysis identifies strongest relationships between rainfall and magnetic susceptibility and anhysteretic remanence this magnetic response correlates with that of the modern soils across the chinese loess plateau such correlation suggests that the rainfall component of the climate system not dust flux is a key influence on soil magnetic properties in both these regions

how long does iron oxide dissolution and transformation require under water logged conditions a perspective from agricultural activity abstract previous studies have revealed that magnetic enhancement caused by the pedogenic process often occurs in aerobic soils experiencing low to moderate rainfall while magnetic reduction caused by iron oxide dissolution often occurs in anaerobic soils experiencing high rainfall to explore the time required for iron oxide dissolution when the soil conditions transform from aerobic to anaerobic a typical weathering crust profile was selected the results show the complete process of magnetic enhancement and reduction and that the former occurred in aerobic soil due to the newborn magnetic minerals during the pedogenic process while the latter occurred under water logged conditions caused by rice cultivation with a history of approximately 200 years therefore it can be inferred that the time required for the significant dissolution of iron oxides is probably shorter than 200 years which is longer than that determined by indoor experiments but much shorter than the theoretical weathering model

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

spatial and altitudinal variations in the magnetic properties of eolian deposits in the northern tibetan plateau and its adjacent regions implications for delineating the climatic boundary abstract delineating the climatic boundary i e of precipitation and temperature in the northern tibetan plateau and along its margins is important for understanding the dynamics of global atmospheric circulation and the processes of dust production in central asia to date however meteorological data are scarce for this vast region which limits our understanding of regional climatic patterns and past climatic changes in eurasia in the present study we use a synthesis of both new and published rock magnetic data from the late pliocene the last glacial interglacial cycle and modern eolian sediments to identify the climatic boundary in the northern tibetan plateau and its adjacent areas the results demonstrate that eolian deposits under different environmental scenarios exhibit a clear contrast in the type and grain size distribution of magnetic minerals with the transition from humid to arid climatic regimes a gradual shift from magnetic minerals of predominantly pedogenic origin to predominantly detrital origin can be observed in the surface soils the spatial and altitudinal variations in the magnetic properties of surface soils reveal a distinct boundary of pedogenic intensity in the eastern qilian mountains the western pamir plateau and the northern tianshan mountains the inferred variations in pedogenic intensity combined with analysis of the available meteorological data demonstrate that the pedogenic boundary represents a critical climatic boundary between sub humid to semi arid and arid regions in the northern tibetan plateau and the adjacent regions stronger pedogenesis occurs in surface soils at higher altitudes on the windward side of the major mountains of the northern tibetan plateau supporting the conclusion that precipitation rather than temperature exerts the dominant effect on the magnetic enhancement of surface soils in addition a comparison of several late pliocene pleistocene and last glacial interglacial loess red clay sequences suggests that the modern spatial pattern of the climatic gradients in the northeastern tibetan plateau and the adjacent regions has been maintained since at least the late pliocene or early pleistocene moreover the results demonstrate that during the warm and humid interglacial periods a steepened rainfall gradient occurred in the northern tibetan plateau and its adjacent regions in the future detailed rock magnetic investigations of eolian deposits or surface soils could provide new insights into the dynamics of atmospheric circulation and the natural boundary conditions for paleoclimate modeling in the northern tibetan plateau

a quantitative model of magnetic enhancement in loessic soils we present a quantitative model for the climatic dependence of magnetic enhancement in loessic soils the model is based on the widely accepted hypothesis that ultrafine magnetite precipitates during alternating wetting and drying cycles in the soil micropores the rate at which this occurs depends on the frequency of drying wetting cycles and on the average moisture of the soil both parameters are estimated using a statistical model for the soil water balance that depends on frequency and intensity of rainfall events and on water loss by evapotranspiration monthly climatic tables are used to calculate the average soil moisture and the rate of pedogenic magnetite production which is proportional to a new parameter called magnetite enhancement proxy mep our model is tested by comparing mep calculated for known present day climates with the magnetic enhancement of modern soils the magnetic enhancement factor defined as the ratio between a given magnetic enhancement parameter and mep is expected to be a site independent constant we show that magnetic enhancement differences between soils from the chinese loess plateau and from midwestern u s are explained by our model which yields similar magnetic enhancement factors for the two regions our model is also successful in predicting the mean annual rainfall threshold above which magnetic enhancement declines in a given type of climate

half precessional cycles recorded in chinese loess response to low latitude insolation forcing during the last interglaciation abstract chinese loess offers favourable conditions for identifying evidence for high resolution climatic changes previous studies have examined orbital scale climatic cycles and the other shorter time scale climatic oscillations similar to the recorded glacial instability in deep sea cores in this study we focus on examining the high resolution palaeoclimatic records in the northwestern loess plateau where the dust accumulation rate is high and the loess soil sequences are highly sensitive to the shifting pattern of the east asian monsoon system our results of magnetic susceptibility and particle size analysis from the northwestern loess plateau reveal well defined half precessional cycles during the last interglaciation culminating at approximately 122 000 111 000 100 000 88 000 and 79 000 yr these variations are a direct response to the low latitude forcing through its modulation on the east asian summer monsoon in this context the east asian summer monsoon has played an important role in conveying climatic signals from tropical pacific to the middle latitude areas

astronomical timescale and palaeoclimatic implication of stacked 3 6 myr monsoon records from the chinese loess plateau abstract magnetic susceptibility and grain size records from two continuous red clay and loess palaeosol sequences on the chinese loess plateau have been generated to investigate the evolution and variability of the east asian monsoon eam during the late pliocene and pleistocene tuning the grain size records to orbital obliquity and precession yields an improved astronomical timescale for the loess palaeosol sequence 0 2 6 ma and an extended age model for the upper red clay formation 2 6 3 6 ma this timescale indicates older ages for a number of the magnetic polarity boundaries consistent with lock in depth offsets in loess sequences good site to site correlation enables generation of stacked grain size and susceptibility records spanning the last 3 6 myr these records indicate that monsoon evolution since the late pliocene can be subdivided into three phases 0 1 25 1 25 2 72 and 2 72 3 4 ma each phase is characterized by unique amplitude and frequency characteristics for both summer monsoon magnetic susceptibility and winter monsoon grain size spectral analyses of the stacked monsoon proxies indicate that characteristics of both the summer and winter monsoons are dominated mainly by variance in the eccentricity 410 and 100 kyr obliquity 41 kyr and precession 23 and 19 kyr bands over the past 3 4 myr implying a non linear response of the long term eam evolution to orbital and glacial forcing

an integrated study of the grain size dependent magnetic mineralogy of the chinese loess paleosol and its environmental significance 1 to investigate the grain size dependent properties of the magnetic minerals in chinese loess paleosol samples touxiangdao xining qinghai province china magnetic extracts were divided into two size fractions by gravitational settling on the basis of hysteresis measurements thermal demagnetization of low temperature saturation isothermal remanent magnetization and nonmagnetic studies sem and xrd we identified magnetic phases both in the grain size fractions of the magnetic extracts and in the less magnetic residues to provide more accurate and complete descriptions of all the magnetic components in the bulk natural samples the results show that the oxidation degree nonstoichiometry of magnetic minerals is strongly affected by both grain size and the paleoclimatic environment in which they were deposited and altered in ascending order of the oxidation degree of our samples we find 1 loess coarse particles lc are multidomain md magnetite with slight oxidation 2 paleosol coarse pc particles are also md magnetite but with a higher oxidation degree compared to lc 3 loess fine lf particles are pseudo single domain psd magnetite with a high oxidation degree and 4 paleosol fine pf particles are psd maghemite single domain sd and superparamagnetic sp maghemite mainly stay in the residues further thermomagnetic analysis of pf psd maghemite revealed that this natural maghemite has a curie temperature identical to that of magnetite and that the conversion efficiency of transformation from maghemite to hematite is only about 50 after a 700 c heating cooling cycle these new results identify the sources of multicomponent nrm in chinese loess sequences as well as clarify the paleoenviromental and paleoclimatic controls on the remanence components

long lived continent ocean interaction in the early proterozoic ungava orogen northern quebec canada the early proterozoic ungava orogen contains evidence for the development of a longlived 80 m y and presumably wide ocean that was subsequently destroyed during plate convergence leading to arc continent collision a magmatic arc was built on 2 00 ga oceanic crust above a north dipping subduction zone between about 1 90 and 1 86 ga development of a south verging continental thrust belt containing units associated with prior rifting of the archean superior province basement probably records impingement of the upper plate against the underthrust continental margin at ca 1 87 ga subduction of the superior province continental crust is inferred to have led to a reversal in subduction polarity a younger south dipping subduction zone is suggested by a suite of 1 85 1 83 ga quartz diorite to granite plutons that intrude the upper plate oceanic crust and the older arc the arc continent collision resulted in a minimum of 65 km of tectonic overlap of the composite arc9s plutonic core on the autochthonous footwall basement

a high resolution record of holocene rainfall variations from the western chinese loess plateau antiphase behaviour of the african indian and east asian summer monsoons tropical monsoons are key mechanisms for transfer of heat and moisture to higher latitudes here we present a high resolution terrestrial proxy summer monsoon record for the southeast asian monsoon from a rapidly accumulating holocene loess soil sequence in the western chinese loess plateau we use magnetic and clastic grain size proxies to make quantitative estimates of holocene rainfall and identify variations in winter monsoon intensity our record reveals cyclical millennial and multimillennial rainfall changes as with the northwest african southwest asian monsoon records a short arid interval at 12 5 to 11 5 ka bp the younger dryas and subsequent summer monsoon intensification are recorded however at 6 ka bp the southeast asian summer monsoon weakened when the northwest african southwest asian monsoons strengthened and then from 5 ka bp intensified when northwest africa southwest asia became dry these antiphase monsoonal relationships may reflect competition between sea surface temperature changes and solar forcing the intensity of the southeast asian winter monsoon intensified from 9 ka bp onwards varying in its phase relationships with summer monsoon intensity after 2 25 ka bp extreme climatic instability is indicated by both climate proxies

correlation of the magnetic susceptibility stratigraphy of chinese loess and the marine oxygen isotope record chronological and palaeoclimatic implications abstract the thick deposits of wind blown silt of the loess plateau in central china constitute arguably the most important terrestrial record of the late pliocene and pleistocene climate magnetic susceptibility stratigraphy has proven extremely useful for quantifying the palaeoclimatic record of chinese loess with intervals of high magnetic susceptibility corresponding to warm humid interglacials and intervals of low susceptibility to cooler drier glacials several workers have noted that the magnetic susceptibility stratigraphy strongly resembles the marine oxygen isotope record for which a high resolution chronology is available from tuning to the frequencies of earth orbital parameters attempts have been made to correlate the two records statistically and we use this approach to generate a new magnetic susceptibility time series for the central chinese loess plateau our results suggest that the two records are sufficiently similar over the interval ca 0 1 5 my bp for this approach to yield a useful age model for loess accumulation spectral analysis of the resulting magnetic susceptibility time series clearly shows that the growth of 100 ky variance which occurs in the marine oxygen isotope record after about 1 my bp also occurs synchronously in the magnetic susceptibility record reinforcing recent suggestions of a strong linkage between the high latitude northern hemisphere and the asian climate we also show that a significant improvement in the strength of the loess magnetic susceptibility marine oxygen isotope correlation occurs after about 1 5 my bp which corresponds to a significant increase in the rate of sediment accumulation across the loess plateau

late neogene rock magnetic record of climatic variation from chinese eolian sediments related to uplift of the tibetan plateau abstract the late cenozoic red clay loess sequence in the chinese loess plateau provides a high resolution record of paleoclimatic and tectonic changes related to the phased uplift of the tibetan plateau detailed magnetostratigraphy of a recently recovered 125 m thick sediment sequence from the central chinese loess plateau shows that the red clay began to accumulate at ca 8 1 ma implying a marked drying in the interior of asian as well as the initial establishment of the east asian paleomonsoon regime prior to 3 8 ma the ferromagnetic mineral concentrations as revealed by both magnetic susceptibility and s ratios exhibit a cyclic pattern superimposed upon a linear trend this pattern collapsed immediately after 3 8 ma suggesting a change in source area this in turn may have resulted from the reorganization of atmospheric circulation and the onset of the modern asian monsoon probably associated with the accelerated uplift of the tibetan plateau to a certain threshold elevation

tibetan uplift intensified the 400 k y signal in paleoclimate records at 4 ma knowledge of when and how a specific climatic threshold is crossed is one key to an understanding of the evolution of earth s climate system we have observed a simultaneous intensification of the similar to 400 k y cycle in both proxy records for global ice volume and for east asian monsoon precipitation on the chinese loess plateau at ca 4 ma indicating that important climatic thresholds were crossed the 400 k y climatic cycles can be substantially intensified by clipped responses to insolation we argue that tibetan uplift accelerated at ca 4 ma and triggered such responses the weaker orbital band climatic signal during the interval 4 3 6 ma indicates that the thresholds were only crossed slightly

new insights into magnetic enhancement mechanism in chinese paleosols article i nfo magneticsusceptibilityof chineseloess paleosolsequenceshasbeenextensivelyusedasareliablepaleoclimatic proxy for reconstruction of the asian summer monsoon system and its enhancement is usually attributed to neoformation of fine grained ferrimagnetic particles via pedogenesis but there is little understanding of either the formation process of such magnetic particles or their actual form and grain sizes to investigate these problems scanning electron microscope observations were conducted on magnetic extracts from pristine loess and strongly pedogenicpaleosol samples from xifeng onthe chineseloess plateau both magneticextracts commonly contained coarse grained detrital magnetite with grain size ranging from a few mt o several tens of m however the magnetic extract from the mature paleosol sample additionally included coarse silicate parti cles with ultrafine ferrimagnetic inclusions while that from the pristine loess sample did not the size of the ferrimagnetic inclusions mainly ranged from tens to hundreds of nanometers the temperature dependent susceptibility and hysteresis loops of the paleosol sample combined with the stratigraphic variations of rock magnetic parameters showed that the pedogenic maghemite inclusions should be a potential contributor to magnetic enhancements in chinese paleosols the fine grained ferrimagnetic inclusions have been pedogenically formed by weathering of coarse grained fe bearing silicate minerals our proposed magnetic enhancement mechanism is consistent with the accepted rock magnetic result that magnetic enhancement in chinese paleosols is caused by pedogenic maghemite grains in superparamagnetic and single domain size regions the presence of ultrafine ferrimagnetic grains as inclusions within coarse silicate particles is also consis tent with han and jiang s 1999 significant experimental result that magnetic enhancement in chinese paleosols is mainly contributed by the 0 5 m sediment fractions

six million years of magnetic grain size records reveal that temperature and precipitation were decoupled on the chinese loess plateau during 4 5 2 6 ma abstract magnetic grain size variations have been used as sensitive paleoclimate proxies to investigate the evolution of the east asian summer monsoon but their relationship with temperature and precipitation is not entirely clear here we find that two magnetic grain size proxy records arm lf and arm sirm where arm lf and sirm are anhysteretic remanent magnetization susceptibility magnetic susceptibility measured at 470 hz and saturation isothermal remanent magnetization respectively of chinese loess and red clay sediments co vary during the last 6 ma except between 4 5 and 2 6 ma when these two records had opposite trends we attribute this disparate behavior to the different responses of arm lf and arm sirm to temperature and precipitation during 4 5 2 6 ma when temperature and precipitation on the chinese loess plateau were decoupled a comparison of the loess and red clay arm lf and arm sirm records with the global ice volume proxy records reveals that arm lf is more sensitive to temperature variations than arm sirm the results suggest that temperature on the chinese loess plateau had a cooling trend from 4 5 to 2 6 ma whereas rainfall tended to increase our studies demonstrate that joint analysis of loess arm lf and arm sirm records can reveal paleoclimatic information that cannot be revealed by a single parameter

bulk particle size distribution and magnetic properties of particle sized fractions from loess and paleosol samples in central asia recent studies demonstrate that particle size separation based on gravitational settling and detailed rock magnetic measurements of the resulting fractionated samples constitutes an effective approach to evaluating the relative contributions of pedogenic and detrital components in the loess and paleosol sequences on the chinese loess plateau so far however similar work has not been undertaken on the loess deposits in central asia in this paper 17 loess and paleosol samples from three representative loess sections in central asia were separated into four grain size fractions and then systematic rock magnetic measurements were made on the fractions our results demonstrate that the content of the 75 m exhibits the minimum values of arm and sirm demonstrating that the concentrations of ferrimagnetic grains are not positively correlated with the bulk particle size in the central asian loess deposits

spatial and temporal reconstructions of changes in the asian palaeomonsoon a new mineral magnetic approach abstract a new quantitative approach is proposed for estimating palaeoprecipitation across the chinese loess plateau at present there is a strong rainfall gradient across the plateau from 300 mm yr in the near desert conditions in the northwest to over 750 mm yr in the southeast just 700 km distant we find that the concentration of ferrimagnetic iron oxide minerals in nine modern soil types represented by 37 individual soil profiles is strongly correlated with this contemporary rainfall gradient the ferrimagnetic concentration rises along this gradient from 0 01 in the northwest to over 0 2 in the southeast the nine modern soil types have been used in the construction of a rainfall vs magnetic susceptibility least squares regression climofunction past variations of loess soil iron oxide content are easily established through magnetic susceptibility measurements and so can be used to reconstruct the rainfall of former interglacial and glacial periods the physical and pedological basis of the rainfall vs susceptibility relationship is discussed and potential limitations of our rainfall reconstruction method are explored our palaeoclimate reconstructions indicate dramatic changes in rainfall due to variations in the structure of the asian monsoon the rainfall variations are about four times greater than has been suggested for this region by atmospheric general circulation modelling our data indicate increased rainfall throughout central china both in interglacial periods and in the early holocene the increases in monsoonal rain were particularly pronounced at our westernmost sites adjacent to the northeastern edge of the tibetan plateau in contrast for glacial periods a reduction in rainfall is found across the whole loess area with the greatest decreases in the southeast

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

rock magnetic constraints for the mid miocene climatic optimum from a high resolution sedimentary sequence of the northwestern qaidam basin ne tibetan plateau abstract the thick sequence of cenozoic deposits in the qaidam basin of the ne tibetan plateau has great potential for understanding the tectonic and climatic evolution of the region in this study we performed detailed rock magnetic analyses of a well dated 20 7 11 2 ma upper cenozoic sedimentary sequence from the huatugou htg section in the nw qaidam basin the magnetic susceptibility of the sequence is relatively low before 17 4 ma high during 17 4 14 5 ma and lower after 14 5 ma again this pattern is consistent with the occurrence of the mid miocene climatic optimum mmco during 17 4 14 5 ma with global climatic cooling thereafter measurements of multiple rock magnetic parameters indicate a peak in the concentration of fine grained magnetic minerals during 17 4 14 5 ma this feature demonstrates that the enhancement of the magnetic susceptibility is due to the pedogenic production of superparamagnetic sp and single domain sd magnetite suggesting the occurrence of relatively high effective precipitation and high temperature in the qaidam basin at this time comparing with regional and global climatic records the warm and humid climatic conditions coincide with the mmco implying that global climate changes were responsible for the climatic evolution in the qaidam basin throughout the mmco however after 14 5 ma the rapid uplift of the northern tibetan plateau caused it to reach a critical elevation which blocked the moisture supply to the qaidam basin and may also have contributed to the observed climatic changes

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

geochemical and sm nd isotopic study of amphibolites in the cathaysia block southeastern china evidence for an extremely depleted mantle in the paleoproterozoic abstract geochemical and sm nd isotopic results are reported for late paleoproterozoic mafic amphibolites from sw zhejiang and nw fujian parts of the cathaysia block of se china two suites of contemporaneous amphibolites are distinct in their geochemical characteristics group 1 samples from nw fujian have chemical compositions of transitional and alkali basalts show lree enriched patterns and plot mainly in the field of within plate basalt on a number of trace element discrimination diagrams group 2 rocks from sw zhejiang have tholeiitic compositions and are characterized by flat to lree depleted patterns and fall into the morb field all the amphibolite samples have high end t values of 5 6 to 8 5 t 1766 19 ma a positive correlation between end t and nb th suggests possible mixing of a mantle derived magma and a crustal component with the least contaminated samples having very high end t values 8 8 5 and nb th ratios of 13 the geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto oceanic basin e g the gulf of tadjoura the exceptionally high end t values of up to 8 5 for some of the amphibolites suggest the presence of a time integrated extremely depleted mantle source beneath cathaysia during the paleoproterozic

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

lithologic and rock magnetic evidence for the mid miocene climatic optimum recorded in the sedimentary archive of the xining basin ne tibetan plateau abstract thick cenozoic deposits in the xining basin provide great potential for understanding the tectonic and climatic evolution of the ne tibetan plateau however the middle miocene climatic optimum mmco in this basin has not been reported up until now due to the poor exposure of the uppermost strata covering this interval in this study a 194 m deep core of flood plain to channel flood deposits was obtained from the tashan flats above the xiejia section in the xining basin magnetostratigraphic dating of the core determines its age at ca 18 5 13 5 ma lithological and rock magnetic analyses demonstrate that the most prominent changes recorded in the present study occurred between 17 and 14 ma during this period the sedimentary sequence mostly consists of gray white or bluish gray claystone and muddy siltstone intercalated with some thin red claystone the rock magnetic parameters show a large variability with higher values in thin red claystone beds and lower values in gray white or bluish gray layers these features demonstrate that due to a warm and wet climate extensive rivers had developed in the mid miocene and that a reducing sedimentary environment prevailed these favorable climatic conditions coincide with the mmco as based on global deep sea deposit records indicating that this warm event exerted a major impact on regional climate change in the ne tibetan plateau during the mid miocene

the type specimens middle cambrian of the trace fossil archaeonossa fenton and fenton the original description of archaeonossa from beds considered early cambrian but now dated as early middle cambrian included at least two unrelated forms of trace fossils the ichnogenus and the type ichnospecies are redefined a lectotype is designated and the type material is reiilustrated along with topotype material from yoho national park british columbia although this trace fossil was originally attributed to movement by a gastropod it is concluded on the basis of restudy that the type occurrence should not be used as evidence of activity by gastropoda

grain growth and transformation of pedogenic magnetic particles in red ferralsols soil magnetism depends on the mineral composition and grain size distribution of magnetic particles the accumulation of magnetic particles with different grain sizes has been found in the laboratory to precede the formation of hematite hm which competes with goethite gt in natural systems to explore changes in the correlation between magnetic particles and hm during pedogenesis we focus on three representative red ferrasol profiles with comparable total concentrations of iron oxides but different hm hm gt ratios from 0 14 to 0 74 superparamagnetic particles and single domain particles commonly become enriched as the concentration of hm increases in the profiles however the accumulation rates of the particles decrease with hm hm gt along the profiles we attributed these changes to the simultaneous grain growth of pedogenic magnetic particles and their partial transformation into hm once a large amount of pedogenic magnetic particles have accumulated in hematitic soils

iron geochemistry of loess and red clay deposits in the chinese loess plateau and implications for long term asian monsoon evolution in the last 7 0 ma abstract recently some groups of authors have demonstrated that the tertiary red clay deposits underlying the pleistocene loess paleosol sequence in the chinese loess plateau are wind blown in origin thus extending the eolian record in the plateau from 2 6 ma back to about 7 0 ma and providing a good opportunity to reconstruct long term monsoon changes as magnetic susceptibility the widely used paleomonsoon proxy of loess deposits is problematic in reconstructing the monsoon history recorded in the red clay the development of other proxies is urgently needed in this study we analyzed the ratios of citrate bicarbonate dithionite cbd extractable fe2o3 to total fe2o3 concentrations in four loess sections along a north south transect of the loess plateau and in the lingtai loess red clay sequence of 305 m thick the fe2o3 ratio in the loess sections clearly shows higher values in soil units than in loess horizons and a southward increase in this ratio is also remarkable along the transect these are in good agreement with the temporal spatial variations in the east asia summer monsoon intensity identified by other monsoon proxies and soil development observations the fe2o3 ratio thus attests to being a good proxy indicator of the summer monsoon evolution the lingtai fe2o3 ratio record shows high values over three time intervals 4 8 4 1 ma 3 4 2 6 ma and during the interglacial periods of the past 0 8 ma the increase in summer monsoon intensity over the three intervals also coincides with the well developed soil characteristics it is therefore concluded that the east asia summer monsoon has experienced a non linear evolution since the late miocene the occurrence of a monsoon prime at about 4 8 4 1 ma implies that the tibetan plateau one of the most important boundary conditions in maintaining the monsoon circulation must have been uplifted to a critical height at that time

palaeoclimatic records of the loess palaeosol sequences of the chinese loess plateau whether during past climate stages or into a progressively warming world changes in precipitation constitute a key component of climatic change quantitative proxies for palaeo precipitation are relatively rare the magnetic properties of the windblown loess units and interbedded palaeosols of the famous chinese loess plateau provide key palaeo precipitation data for this populous monsoon dominated region the loess palaeosol sediments record rainfall totals directly complementing the oxygen isotope records of chinese speleothems these isotopic records predominantly reflect moisture source and hence large scale atmospheric circulation changes the two major asian monsoon systems appear to display antiphase behaviour dominance of the indian summer monsoon system seems associated with minimum precession maximum northern hemisphere summer heating dominance of the east asian summer monsoons with maximum precession at 2 8 ma more intense development of the east asian winter monsoon initiated major increases in dust deposition rates and formation of relatively unweathered loess layers glacial stage loess units then interleaved with interglacial interstadial stage palaeosols throughout the quaternary period decoupling of the loess palaeosol rainfall records from the chinese cave records of moisture source shows that the indian and east asian monsoon winds were continuously driven by precessional forcing while summer monsoonal rainfall was greatly suppressed during cool glacial stages the timing of these east asian climatic transitions the subsequent intensification of northern hemisphere glaciations and the association between monsoon circulation changes and north atlantic temperatures indicates a possibly leading global role for these monsoonal changes via alterations in the poleward distribution of heat and moisture

new insights into the palaeoclimatic interpretation of the temperature dependence of the magnetic susceptibility and magnetization of mid late pleistocene loess palaeosols in central asia and the chinese loess plateau the temperature dependence of magnetic susceptibility chi t curves and magnetization m t curves has been used as a routine rock magnetic tool to characterize the magnetic mineralogy and magnetic granulometry of chinese loess palaeosols however palaeoclimatic interpretation of these thermomagnetic analyses remains controversial in the present study total organic carbon toc thermomagnetic and low temperature magnetic experiments on mid late pleistocene loess palaeosols in central asia and the chinese loess plateau clp have been conducted we found that the m t cooling curves at room temperature were mostly lower than the corresponding heating curves whereas for the chi t analyses the cooling curves at room temperature were always much higher than the heating curves low temperature magnetic measurements demonstrated that a large amount of superparamagnetic ferrimagnetic particles were produced during the thermal treatment and resulted in the aforementioned differences this finding further indicated that the use of the m t curves to estimate the relative content of maghemite in the loess palaeosols from the clp was problematic in addition a positive correlation exists between the toc and the frequency dependent susceptibility chi fd in the clp suggesting that stronger pedogenesis would result in the simultaneous increase in the content of both maghemite and organic matter consequently the parameters delta chi 1 representing the relative content of pedogenic maghemite delta chi 2 inverted left perpendicular chi ph chi inverted right perpendicular delta chi 1 and chi ph related to the organic matter concentration which can be calculated from the chi t analyses can potentially be used as new indicators of pedogenesis and palaeoclimate in central asia and the clp

pedostratigraphy and paleomagnetism of a 7 0 ma eolian loess red clay sequence at lingtai loess plateau north central china and the implications for paleomonsoon evolution abstract a 305 m thick loess red clay sequence was discovered recently at lingtai which is located in the middle part of the chinese loess plateau it consists of a complete pleistocene loess soil sequence with a thickness of about 175 m and 130 m of tertiary red clay deposits the red clay sequence at lingtai is the thickest one presently known in the loess plateau field observations show that the lingtai red clay contains over 110 couplets of pedogenic b horizons and horizontal carbonate nodule horizons and that it can be regarded as an extremely thick soil complex paleomagnetic studies of 680 samples suggest that the basal age of the lingtai loess red clay sequence is about 7 05 ma grain size analysis of samples taken at 3 3 cm intervals indicates that the red clay has the same sedimentological characteristics as those observed in the loess thus suggesting a wind blown origin of the tertiary red clay and continuous atmospheric dust deposition in the loess plateau during the last 7 05 ma the pedogenic characteristics of the paleosols within the pleistocene loess and the b horizons in the red clay suggest that the east asia summer monsoon in the latest miocene may have already been slightly stronger than that during the holocene this implies that at about 7 5 ma bp the tibetan plateau could have been uplifted to a critical height in maintaining the east asia summer monsoon system observations of the loess red clay sequence also suggest that the long term changes in the east asia summer monsoon strength have been nonlinear since the latest miocene

consistent grain size distribution of pedogenic maghemite of surface soils and miocene loessic soils on the chinese loess plateau abstract there is agreement that ultra ne maghemite grains 100nm are responsible for themagnetic enhancement of chinese loess recent studies show that grain size distribution of ultra nepedogenic maghemite grains within the aeolian sequences on the central chinese loess plateaudeposited in the last 8 ma is consistent however whether the observed grain size distribution ofultra ne pedogenic maghemite grains can be observed in the western chinese loess plateau andmodernsoilsisnotentirelyclear herewe ndthatyoungsurfacesoilsamplesacrossthechineseloessplateau and early neogene loessic soils from the western chinese loess plateau also show consistentgrain size distribution with that of ultra ne pedogenic maghemite grains on the central chinese loessplateau the fact that young surface soil samples show apparent magnetic enhancement suggests thatthe ultra ne pedogenic maghemite grains derive from oxidation of ultra ne magnetite grains copy right 2009 john wiley sons ltd keywords loess plateau magnetic susceptibility arm miocene loess maghemite

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

holocene loess accumulation and soil development at the western edge of the chinese loess plateau implications for magnetic proxies of palaeorainfall a high resolution holocene sequence of loess palaeosols and incipient soils at the western edge of the chinese loess plateau dated usingoptically stimulated luminescence techniques is used to identify the respective influence of dust accumulation rate time and climate on soil magnetic properties dust deposition and soil formation have both been quasi continuous through the holocene at this site the soils are thus accretionary in nature the degree of soil development as indicated both by geochemical and magnetic properties which correlate strongly varied through the holocene compared with the less weathered loess units each palaeosol is enriched in nitrogen and organic carbon depleted of carbonate due to leaching and displays higher values of pedogenic magnetic susceptibility frequency dependent susceptibility remanences and magnetisation magnetic grain size indicators show that the pedogenic ferrimagnets are ultrafine of single domain and superparamagnetic dimensions sediment accumulation rates were lowest from b12 to 2 5 ka providing ample time 100 s of years for weathering and soil formation to proceed yet pedogenesis through this interval was relatively weak conversely soil development was stronger during later intervals when loess accumulation rates were higher and soil forming intervals were correspondingly shorter in terms of jenny s 1941 soil forming equation accumulation rate and thus time sediment source parent material and topography show no systematic variation between the loess and the palaeosols therefore climate seems to be the key soil forming factor which has controlled the geochemical and magnetic properties of these holocene palaeosols r 2002 elsevier science ltd all rights reserved

rock magnetic and grain size evidence for intensified asian atmospheric circulation since 800 000 years b p related to tibetan uplift abstract paleomagnetic rock magnetic and grain size studies of a thick loess sequence in the west qin ling mountain range show that loess deposition there began about 800 ka the data reveal a progressively increasing coarse grain size fraction upwards into the holocene the averages of these coarse size fractions are higher than in the central loess plateau which was apparently farther from the source area and slightly lower than those of the western loess plateau and the eastern tibetan plateau which were therefore closer to the source area the coarsening and source area location suggest 1 that asian air circulation may have changed and intensified at about 800 ka resulting in dust deposition in west qin ling 2 that dust carrying winds were driven not only by the asian winter monsoon but included also the westerlies and a winter monsoon caused by the tibetan plateau high and 3 that intensification of all these air circulation systems continues to the present increased elevation of the tibetan plateau so that it reached into the cryosphere by about 800 ka and a subsequent persistent uplift of the plateau may have been the mechanisms to trigger a change and intensify the air circulation system moreover this circulation shift and intensification simultaneous with a shift in milankovitch periodicity may have contributed to large global climate changes such as the 15 increase in global ice volume at ca 800 ka

palaeoenvironmental and chronological constraints on the early pleistocene mammal fauna from loess deposits in the linxia basin ne tibetan plateau abstract the longdan mammal fauna from the central part of linxia basin ne tibetan plateau is the first early pleistocene fauna in china in which the fossils are derived loess deposits and it provides an excellent opportunity to document mammalian and environmental evolution in asia however the precise age and palaeoenvironmental setting of the fauna are controversial due to the poor exposure of the outcrop section in the present study a 105 m long drill core was obtained from longdan village and used for detailed magnetostratigraphic dating the results demonstrate that the late pliocene pleistocene loess deposits in the longdan section deposited since ca 3 ma and that the longdan fauna has an age range of 2 5 2 2 ma in addition the results of lithological and rock magnetic analyses demonstrate that paleosols are weakly developed throughout the whole core and that in the lower and middle parts the core the magnetic susceptibility and its frequency dependence are relatively low and uniform these observations combined with the ecological characteristics of the longdan fauna indicate that during the early pleistocene the climate in the longdan area and even in the linxia basin was sub humid and that the aeolian dust was frequently subjected to post depositional reworking by water

warm fjords and vegetated landscapes in early pliocene east antarctica abstract the response of the antarctic ice sheets to future warming is uncertain the ipcc are predicting minimal melt from antarctica while others suggest increased meltwater contributions are possible the pliocene period 5 333 ma to 2 58 ma may provide insights into future ice sheet response because atmospheric co2 concentrations were similar to today 350 450 ppmv and the earth surface was between 2 c and 4 c warmer than the preindustrial conditions geological records indicate that antarctica s ice sheets were smaller and more dynamic at this time and many sea level estimates require meltwater input from the greenland west wais and east antarctic ice sheets eais however only a few records exist proximal to the antarctic ice sheet which allow for reconstruction of the pliocene climate state we present a multiproxy climate reconstruction from a sedimentary succession that was deposited in an ancient fjord within the transantarctic mountains covering discrete intervals between the early pliocene and the late pleistocene in contrast to modern frigid conditions our records indicate sea surface temperatures of about 5 6 c at c 4 1 ma the presence of a plant community at the fjord margins and evidence of soil formation simulations of potential vegetation cover in the pliocene indicate our reconstruction is most compatible with a complete collapse of the wais and a large scale retreat of the eais from the subglacial basins with atmospheric co2 levels of less than 450 ppmv our study indicates that under present day atmospheric co2 conditions in the early pliocene the antarctic ice sheets retreated significantly understanding the mechanisms driving this large scale ice sheet retreat would enable us to assess whether current atmospheric co2 concentrations will lead to the same ice sheet configuration once the earth system has come to a new equilibrium state

preliminary magnetostratigraphy of the red clay underlying the loess sequence at baoji china magnetic remanence and bulk susceptibility measurements are reported from 93 samples spanning the uppermost 27m of the red clay underlying the loess sequence at baoji shaanxi province china if a new previously undetected short about 20ka excursion or reversed interval near 3 3ma be accepted then a reasonable fit to the standard time scale emerges a linear accumulation rate of 1 5cm ka is implied and the correlation coefficient is 0 992 no significant hiatus occurs between the red clay and the overlying loess so the 27m investigated reach back to about 4 3ma fourier analysis of the bulk susceptibility data yields a strong peak near 400ka in agreement with astronomical calculations of the orbital parameters responsible for climatic forcing

correlation between climate events in the north atlantic and china during the last glaciation episodes of massive iceberg release heinrich events 1 3 into the north atlantic ocean during the last glaciation were associated with recurring episodes of unusually cold north atlantic surface water bond cycles 4 and cold air temperatures over greenland dansgaard oeschger events 5 6 four of the youngest of these cold events have also been reported in climate records from sites outside the north atlantic region7 but until now the entire suite has been identified only in north atlantic marine sediments greenland ice core records and tentatively in french lake sediments8 here we examine grain size data from chinese loess and intercalated accretionary palaeosols of last glacial age for evidence of similar climate signals remote from the north atlantic region we see grain size maxima with ages that match those of the last six heinrich events which we interpret as an indication of the changing strength of the east asian winter monsoon which largely controls the transport and deposition of central asian aeolian dust thus it seems that these heinrich events have left their signature in the chinese loess record this is consistent with simulations of the glacial climate9 which imply that the climates of the north atlantic and china were linked by the effect of westerly winds