the gisp ice core record of volcanism since 7000 b c two technical comments are presented on an article by zielinski concerning the gisp ice core record of volcanic activity in 7000 bc including a detailed record of the volcanic contribution to sulfate concentrations the authors describe concerns about 1 correct c14 dating for known eruptions correlated to calendar years and 2 the need for unambiguous volcanic source identifications 24 refs 1 tab

drastic climatic shift at 2 8ma as recorded in eolian deposits of china and its implications for redefining the pliocene pleistocene boundary there has been a long running debate over the position of the pliocene pleistocene boundary i e 1 8 ma versus 2 6 ma this study presents litho and pedo stratigraphic characteristics grain size magnetic susceptibility and dust sedimentation rates for red clay loess sequences over northern china in order to assess the drastic climatic event s recorded in the quasi continuous eolian deposits and to readdress the position of the pliocene pleistocene boundary in complete chinese loess sequences 33 soil loess couplets have been identified and labeled with si li system traditionally the base of chinese loess has been designated at the bottom of loess unit l33 however a typical loess layer within the uppermost red clay is identified and formally designated here as l34 and the soil unit between l33 and l34 as s33 which lowers the basal age of chinese loess from 2 6 ma to w2 8 ma although the neogene red clay and the overlying quaternary loess are both of windblown origin they have quite different pedogenic characteristics the loess consists of alternating reddish soil and yellowish loess layers while the red clay is composed of soils with much redder colour and carbonate nodule layers in general the red clay has been subjected to stronger pedogenic processes than the overlying loess and can be regarded as an extremely thick soil complex the alternation of loess and soils reflects large scale oscillations between glacial and interglacial conditions whereas the well developed red clay deposition indicates a relatively stable climate condition at w2 8 ma shortly before the gauss matuyama boundary the eolian sequences show a sedimentary transition from the well weathered red clay to the alternating loess and paleosol deposits accompanied by coarser particle size increased dust accumulation rates and change of magnetic susceptibility signature these indicate a large climatic shift from long lasting warm humid conditions to large amplitude cold dry and warm humid fluctuations around 2 8 ma and a dramatic increase in aridity over the dust source region in contrast such a drastic event is not identified around the top of the olduvai event or at any other time of the past 7 7 ma as the gauss matuyama boundary offers a sound foundation for global time correlation and chronostratigraphic classification these results still support the proposal of ics and inqua that the conventional pliocene pleistocene boundary should be lowered from 1 8 ma to 2 6 ma

10be evidence for delayed acquisition of remanent magnetization in marine sediments implication for a new age for the matuyama brunhes boundary abstract fluxes of the meteoric cosmogenic radionuclide 10 be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations the variability in the 10 be flux can be used to synchronize ice cores as well as marine sediments by comparison with the relative paleointensity variations of the geomagnetic field however lock in of the paleomagnetic signal at some depth below the sediment water interface in marine sediments through acquisition of a post depositional remanent magnetization pdrm adds uncertainty to synchronization despite the long history of such studies the magnitude of the pdrm lock in depth remains controversial in this article we present clear evidence for a downward offset of the paleointensity minimum relative to the 10 be flux anomaly at the matuyama brunhes m b geomagnetic polarity boundary which we interpret to result from a 15 cm pdrm lock in depth this lock in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments and the age of the m b boundary should be revised to ca 10 kyr younger which is consistent with a younger ice core derived age of 770 6 ka 2 this cosmogenic age tuning strategy will contribute to refining paleomagnetic based age models for marine sediments and identifying of lead lag relationships for global abrupt environmental changes

why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

a scottish speleothem record of the h 3 eruption or human impact a comment on baker smart barnes edwards and farrant studies of a stalagmite sample from sutherland scotland have identified a period of enhanced growth that lasted for four years and has been dated to 1135 130 bc baker et al 1995 this episode is unique within this sample and has not been observed elsewhere the authors correlate it with the icelandic volcanic eruption at 1021 130 100 bc that produced the hekla 3 h 3 tephra there is however no direct evidence for a causal relationship between volcanic activity in general or the h 3 eruption in particular and the growth patterns of the stalagmite as an alternative to the volcanic explanation of enhanced growth we suggest that the speleothem could reflect environmental changes associated with woodland decline and the spread of blanket peat

magnetostratigraphical dating of loess deposits in china the age of chinese loess deposits has long been disputed biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the pleistocene the new palaeomagnetic measurements reported here on a bore hole section near lochuan shaanxi province suggest a clearly defined magnetic polarity zonation which extends below the olduvai subchron a late pliocene age of 2 4 myr is assigned to the oldest loess sediments measured the intensity variations of natural remanent magnetization nrm and low field magnetic susceptibility are indicative of climatic changes during loess deposition

the distinct morphological and petrological features of shock melt veins in the suizhou l6 chondrite the morphology and petrology of distinct melt veins in the suizhou l6 chondrite have been investigated using scanning electron microscopy electron microprobe analyses and raman spectroscopy synchrotron energy dispersive diffraction and transmission electron microscopy it is found that the melt veins in the suizhou meteorite morphologically are the simplest straightest and thinnest among all shock veins known from meteorites at first glance these veins look like fine fractures but petrologically they are solid melt veins of chondritic composition and consist of fully crystalline materials of two distinct lithological assemblages with no glassy material remaining the suizhou melt veins contain the most abundant high pressure mineral species when compared with all other veins known in chondrites thus these veins in suizhou are classified as shock veins all rock forming and almost all accessory minerals in the suizhou shock veins have been transformed to their high pressure polymorphs and no fragments of the precursor minerals remain in the veins among the 11 high pressure mineral phases identified in the suizhou veins three are new high pressure minerals namely tuite after whitlockite xieite and the cf phase after chromite on the basis of transformation of plagioclase into maskelynite it is estimated that the suizhou meteorite experienced shock pressures andmore shock temperatures up to 22 gpa and 1000 c respectively shearing and friction along shock veins raised the temperature up to 1900 2000 c and the pressure up to 24 gpa within the veins hence phase transition and crystallization of high pressure minerals took place only in the suizhou shock veins fast cooling of the extremely thin shock veins is regarded as the main reason that up to 11 shock induced high pressure mineral phases could be preserved in these veins less

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

reconciling astrochronological and 40ar 39ar ages for the matuyama brunhes boundary and late matuyama chron when five matuyama brunhes m b boundary records from the north atlantic are placed on isotope age models produced by correlation of the 18o record directly or indirectly to an ice volume model the m b boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773 1 ka standard deviation 0 4 kyr 7 kyr younger than the presently accepted astrochronological age for this polarity reversal 780 781 ka two recently proposed revisions of the age of the 40ar 39ar fish canyon sanidine fcs standard to 28 201 0 046 ma and 28 305 0 036 ma would adjust 40ar 39ar ages applicable to the m b boundary and other reversals and excursions back to 1 2 ma to ages older than the new astrochronological ages by 8 24 kyr the variables used to construct the ice volume models cannot account for the discrepancy the fcs standard age that best fits the astrochronological ages is 27 93 ma which is within the uncertainty associated with the commonly used value of 28 02 0 16 ma but younger than the recently proposed fcs ages the edc2 and edc3 age models in the dome c antarctic ice core yield ages of 771 7 ka and 766 4 ka respectively for the 10be flux peak that denotes the paleointensity minimum at the reversal boundary implying that the edc2 rather than edc3 age model is consistent with the observations from marine sediments at least close to the m b boundary

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

magnetostratigraphy of chinese loess paleosol sequences abstract as one of the longest and most continuously deposited terrestrial sedimentary archives in the world chinese loess paleosol sequences record paleoclimatic and paleomagnetic variations at a range of time scales magnetostratigraphic studies provide a first order chronological framework for chinese loess sequences in this review we highlight recent developments in loess magnetostratigraphy including pedostratigraphy based on magnetic susceptibility variations we highlight progress in understanding the mechanisms by which the natural remanent magnetization nrm is acquired and discuss in detail the fidelity of paleomagnetic recording in loess records including the recording of magnetic polarity reversals excursions and relative paleointensity variations finally we discuss future prospects for studies of loess nrm

new holocene tephras and a proxy climate record from a blanket mire in northern skye scotland four holocene tephras of icelandic origin have been identified and geochemically characterised from a water shedding blanket peat sequence on the trotternish ridge isle of skye scotland geochemical characterisation of the shards propose the glen garry tephra to be present a tephra layer of hekla origin incorporating shards from hekla 4 as well as two new tephras dated by interpolation from a radiocarbon based chronology to ca 830 cal yr bp and ca 2340 cal yr bp the new historic tephra has an ambiguous geochemistry and therefore has not been correlated with other known icelandic historic tephras the new prehistoric tephra is suggested as originating from the snaefellsj okull volcano in northwest iceland and forms an important stratigraphical marker in this holocene sequence a proxy climate record has been derived from humification analyses of the peat which compares well with other regional palaeoclimatic reconstructions as well as enabling correlations based on tephra geochemical linkages between sites and climatic records at precise times in the past

petrogenesis of complex veins in the chantonnay l6f chondrite abstract two cross cutting veins in the chantonnay l6f chondrite illustrate different patterns of fractionation of total chondritic shock melts the earlier vein which is dark colored and bears abundant host rock xenoliths is strongly reduced and sodium poor relative to the bulk meteorite it resembles and may be cogenetic with melt pockets in chantonnay the later vein which is lighter colored and somewhat vesicular lacks evidence of either na loss or reduction but shows modest internal differentiation its metal and total iron contents 26 5 wt are higher than normal for l group chondrites the trend of chemical fractionation recorded in the earlier chantonnay vein resembles that reported for chondrules in ordinary chondrites suggesting that chemical variations among chondrules in part reflect variations among their parental shock melts

the blake geomagnetic polarity episode recorded in chinese loess a geomagnetic anomaly consisting of three clearly defined periods of reverse polarity separated by two short intervals of normal polarity has been documented in a loess section near xining western china on the basis of previous studies of chinese loess stratigraphy we correlate this geomagnetic polarity episode with the blake event while more research is needed to improve our understanding of the exact process of remanence acquisition of loess deposits this study shows that loess may provide records of fast geomagnetic events and lends further support to the global nature of the blake geomagnetic event

revisiting the stratigraphic position of the matuyama brunhes geomagnetic polarity boundary in chinese loess article i nfo it is commonly accepted that the climate records in chinese loess can be correlated well with that in marine sediments however discrepancies for the stratigraphic position of paleomagnetic polarity reversal boundary seriously restrict an accurate teleconnection between these two archives for example the exact stratigraphic position of the whole matuyama brunhes mb transitional polarity zone remains uncertain in chinese loess in this study an accurate transitional zone of the mb reversal from the mangshan profile southeast margin of the chinese loess plateau clp was statistically determined using multiple subsets of parallel samples by integrating results from the central clp the whole mb transitional zone is consistently located across the pedostratigraphic and climatostratigraphic transitional zones between s8 and l8 over a wide region of the clp this conclusion further supports that the paleosol unit s8 should be teleconnected to the marine oxygen isotope stage 19 rather than 21 and thus unambiguously supplies an accurate age control in constructing a new loess time scale

multiple rapid polarity swings during the matuyama brunhes transition from two high resolution loess paleosol records 1 two high resolution magnetic records of the matuyama brunhes m b transition have been obtained from xifeng and baoji loess paleosol sequences in the chinese loess plateau the results of stepwise thermal and alternating field af demagnetizations confirm that both treatments are effective in isolating characteristic remanent magnetization chrm from specimens of loess paleosol sediments which suffer from weak pedogenesis and that af demagnetization clearly fails to isolate chrm from specimens of paleosol sediments which have undergone strong pedogenesis on the basis of virtual geomagnetic pole latitudes crossing the virtual geomagnetic equator the m b transition involves at least 15 rapid polarity swings rock magnetic experiments and anisotropy of low field magnetic susceptibility show that high frequency polarity swings during the m b transition cannot be attributed to rock magnetic and or sedimentary causes extrapolating a constant accumulation rate between the jaramillo matuyama and m b boundary ages the m b transition interval of both the xifeng and baoji sections is estimated to have a duration of more than 10 kyr durations of each short polarity interval are estimated to range from about 0 3 to 5 6 kyr our new results combined with previous paleomagnetic observations from lavas and marine sediments suggest that multiple rapid polarity swings occurred during the m b transition

rock magnetic proxies of climate change in the loess palaeosol sequences of the western loess plateau of china summary results of the first detailed study of the climate proxy record in the loess palaeosol sequence at xining one of the few palaeoclimate sites in the currently arid western loess plateau of china illustrate the importance of making many types of rockmagnetic measurements other than susceptibility a multiparameter approach yielded confirmation that here as elsewhere in the loess plateau the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite nevertheless magnetic enhancement was caused not exclusively by changes in relative grain size but also by variations in concentration and mineralogy of the magnetic fraction the effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence respectively the resulting signal was ascribed more confidently to variation in magnetic grain size which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility variations in magnetic mineralogy were also determined to constrain interpretations further the data were then used to discuss climate history at xining finally results from xining were compared with other western sites and contrasted with eastern sites in summary 1 data is presented from a new loess plateau site which also appears to yield a global climate signal 2 a demonstration is made of a more rock magnetically robust way to separate concentration composition and grain size controls on susceptibility and other magnetic parameters and 3 models are provided for inter regional comparisons of palaeoclimate proxy records

a new astronomical timescale for the loess deposits of northern china here we present a refined timescale for the entire sequence of quaternary chinese loess which relies upon the correlation of detailed monsoon records to the astronomical solution of laskar 1990 and the oceanic odp677 18o record of shackleton et al 1990 the chronological scheme considers in detail the relative structures of the palaeoclimatic and palaeomagnetic records to produce an accurate timescale that is consistent with the current understanding of loess depositional and post depositional processes analysis of this chronological framework demonstrates downward displacement of the palaeomagnetic horizons with respect to the climatic record

stacked 2 6 ma grain size record from the chinese loess based on five sections and correlation with the deep sea δ18o record 1 this study aims to establish a stacked climate record of the quaternary period from the chinese loess sequence and to address the forcing mechanisms for the regional climate history of the loess plateau by correlating the stacked record with a composite 18o record in deep sea sediments a total of 18 352 samples were obtained from five loess sections located at baoji lingtai jingchuan puxian and pingliang in the southern and middle loess plateau these yielded high resolution grain size records between section correlation of these shows that although small depositional hiatuses are present in places within a single section most parts of the sections display near continuous dust deposition throughout the quaternary the grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the earth s orbit the grain size records plotted on their orbital timescales were then averaged to form a stacked loess grain size time series termed the chiloparts record this resolves most of the orbital timescale paleoclimate events buried in the loess soil sequences of the southern and middle loess plateau and can be used as a regional archive of the pleistocene climate history in the loess plateau comparison of the chiloparts record with a composite marine 18o record shows that for the past 1 8 ma the loess paleosol record can be correlated almost cycle by cycle with the marine record several discrepancies in the climatic events between the two records have also been identified implying that regional forcing mechanisms may have played a part in the climatic evolution of the chinese loess plateau

new data on speleothem deposition and palaeoclimate in britain over the last forty thousand years 230th 234u dates are presented for 26 speleothems from three areas in britain assynt n w scotland n w yorkshire mendip hills somerset these dates suggest that speleothem growth was widespread but rare in the period from 40 to 26 ka absent from 26 to 15 ka and abundant from 15 ka to the present the absence of speleothems between 26 and 15 ka is attributed to glaciation in yorkshire and assynt and tentatively to continuous permafrost development in the mendip hills the occurrence of speleothems before 26 ka places restrictions on the extent of any ice sheet which may have been present in scotland and indicates that n w scotland was not subjected to continuous permafrost at that time

seven tephra isochrones in scotland this paper reports the presence and geochemical characteristics of seven separate tephras in scotland that fell in ad 1510 and about 450 bp 2100 bp 3600 bp 3830 bp 5600 bp and around 6000 bp the results come from 14 peat bogs spread throughout the highlands and islands where 26 tephra deposits have been characterized using over 400 grain discrete electron probe micro analysis firstly tephras are identified and defined by major element characteristics at a reference site secondly tephras are linked between sites on the basis of geochemistry and dated informal names based on british sites are suggested for tephras not yet linked to precise sources in iceland despite some apparent variation in colour all analysed grains in these seven deposits are silicic or intermediate in composition sio2 57 this phenomenon does not appear to be an artifact of preservation for two reasons first small 40 m basic glass shards in peat bogs in northern iceland are found to have undergone minimal alte

reliability of the natural remanent magnetization recorded in chinese loess 1 chinese loess paleosol sequences undoubtedly have recorded geomagnetic events both polarity reversals and excursions however the fidelity of the rapid paleomagnetic field oscillations during a polarity reversal remains uncertain to test the reliability and consistency of the natural remanent magnetization records in chinese loess 10 subsets of parallel samples across the matuyama brunhes mb reversal boundary were obtained from the luochuan region in the hinterland of the chinese loess plateau our paleomagnetic results show diversified virtual geomagnetic poles vgps during the mb transition but consistent vgps outside of the transitional zone the anisotropy of magnetic susceptibility and rock magnetism results indicate that the sampled interval is rather uniform and undisturbed the discrepancies of the characteristic remanent magnetization within the mb transition are probably due to the low efficiency in aligning magnetic grains mainly pseudo single domain magnetite associated with the low field intensity nevertheless the stratigraphic location of the mb boundary can be confidently defined therefore we conclude that chinese loess paleosol sequences can record geomagnetic reversal events but the morphology within the polarity transition is rather questionable

a magnetostratigraphic reassessment of correlation between chinese loess and marine oxygen isotope records over the last 1 1 ma abstract we present a magnetic polarity based paleoclimatic correlation between chinese loess and marine oxygen isotope stage mis records over the last 1 1 ma this revised land ocean comparison indicates that inconsistent occurrence of the matuyama brunhes boundary which lies in mis 19 in marine sediments and occurs either at the base lower part of loess l8 or the uppermost part of soil s8 in chinese loess is due to regional and or local climate variability across the chinese loess plateau and does not need to be attributed to lock in effects of loess remanence acquisition associated with pedogenesis this result implies that the geomagnetic polarity boundaries in chinese loess in general are reliable chronostratigraphic time markers therefore astronomically tuned timescales for chinese loess that presume a downward shift of polarity boundaries due to a time lag in remanence acquisition may have overestimated ages of certain loess paleosol units

shock induced black veins and organic compounds in ordinary chondrites abstract a detailed study of barbotan h5 and charsonville h6 and a partial study of aa pervomaisky l6 indicates that shock induced black veins differ from the host part of chondrites by the following peculiarities 1 1 smaller quantities of na and quite possible fe ni co and in some cases ir 2 2 the presence of a major portion of glass and melted structures of troilite metal mixture 3 3 different composition of metal troilite ca poor pyroxene feldspar and some grains of olivine 4 4 the presence of organic compounds organic matter in h chondrites is identical and corresponds to paraffins and in l chondrite to aliphatic hydrocarbons and carbonyl and n containing compounds their presence in black veins may be related to two possible sources 1 1 contamination or 2 2 cosmic origin the character of distribution and disposition of organic compounds in the close pores of veins and their chemical interrelation with chemical group of chondrites allows speculation about a cosmic origin of organic matter related to shock metamorphism

chemical magnetization in chinese loess the relationship between the intensity of characteristic remanent magnetization chrm and pedogenesis is examined in the luochuan section the results show a consistent link between the intensity values of remanent magnetization after every thermal demagnetization step and magnetic susceptiblity and other indexes of pedogenesis moreover the intensity of chrm is also much higher in paleosols than in loess these results suggest that chemical remanent magnetization crm is the main mode of nrm acquisition in paleosols and is linked to pedogenesis pedogenic processes overprint the magnetic signal to a certain depth in the underlying loess so the interpretation of the high resolution paleomagnetic records from paleosols requires a degree of caution

astronomical calibration of the matuyama brunhes boundary consequences for magnetic remanence acquisition in marine carbonates and the asian loess sequences we have compiled 19 records from marine carbonate cores in which the matuyama brunhes boundary mbb has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes by correlation of the 18o data to a timescale based on astronomical forcing we estimate astronomical ages for each of the mbb horizons in all but one record the mbb occurs within stage 19 most magnetostratigraphic sections in asian loess place the mbb within a loess interval since loess deposition is presumed to be associated with glacial intervals loess horizons should correspond to even numbered oxygen isotope stages a glacial age for the mbb is at odds with the results presented here which firmly place the mbb within interglacial stage 19 inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed the mean of the stage 19 age estimates for the mbb is 777 9 1 8 n 18 inclusion of the single stage 20 age results in a mean of 778 8 2 5 n 19 the astronomical age estimate of the mbb compares favorably with an unweighted mean of 778 2 3 5 n 10 from a compilation of 40ar 39ar results of transitional lava flows combining the two independent data sets yields a grand mean of 778 0 1 7 n 28 the new compilation shows virtually no trend in placement of the mbb within isotope stage 19 as a function of sediment accumulation rate we interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment water interface separating the cores into two geographic regions an indo pacific caribbean ipc group and an atlantic group results in a significant difference in the position of the mid point of the reversal with respect to the astronomical time scale the data presented here suggest a difference of several thousand years between the two regions this observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest systematic differences in remanence acquisition or by genuine differences in the timing of the directional changes between the two regions

influence of atlantic meridional overturning circulation on the east asian winter monsoon east asian summer monsoon precipitation varied on millennial timescales during the last glacial period sediment records and climate modelling suggest that the winter monsoon was also affected by millenial scale variability and that the abrupt changes were driven by changes in the strength of the atlantic meridional overturning circulation

a lock in model for the complex matuyama brunhes boundary record of the loess palaeosol sequence at lingtai central chinese loess plateau summary in most marine sedimentary records the matuyama brunhes boundary mbb has been found in interglacial oxygen isotope stage 19 in the magnetostratigraphic records of most chinese loess palaeosol profiles the mbb is located in loess layer l8 which was deposited during a glacial period the mbb at lingtai central chinese loess plateau also occurs in l8 and is characterized by multiple polarity flips the natural remanent magnetization is mainly carried by two coexisting components the higher coercivity harder component dominates in loess layers and is thought to be of detrital origin the lower coercivity softer component prevails in palaeosols and was most probably formed in situ by bio chemical processes a lock in model for the lingtai mbb record has been developed by extending the lithologically controlled pdrm model of bleil von dobeneck 1999 it assumes two lock in zones the nrm of the magnetically harder component is physically locked by consolidation shortly after loess deposition whereas the softer component is formed at greater depth by pedogenesis and acquires a chemical remanent magnetization of younger age at polarity boundaries grains carrying reversed and normal directions may therefore occur together within a single horizon the model uses arm coercivity spectra to estimate the relative contributions of the two components it is able to explain the observed rapid multiple polarity flips and low magnetization intensities as well as the stratigraphic shift of the lingtai mbb with respect to the marine records

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

impacts of post depositional processes on rapid monsoon signals recorded by the last glacial loess deposits of northern china abstract rapid east asian monsoon oscillations recorded by chinese loess are thought to be dynamically linked to north atlantic climate however few efforts have been made to assess the effects of post depositional processes e g surface mixing and pedogenesis on loess paleoclimatic records here a detailed optically stimulated luminescence dating of a thick loess sequence from the western loess plateau is presented offering a reliable chronology for last glacial deposits magnetic susceptibility and mean grain size records from three loess paleosol sequences along a northwest southeast transect are investigated to evaluate impacts of post depositional processes on these loess based proxy records our results indicate that 1 loess sequences developed within the flat tableland of the central and western loess plateau are nearly continuous during the last glaciation and 2 post depositional processes have distinct impacts on rapid monsoon signals recorded in loess sequences from different regions in the central loess plateau rapid monsoon signals have been attenuated to various degrees depending on the sedimentation rate and pedogenic intensity in the northwestern loess plateau however due to high sedimentation rate and relatively weak pedogenesis high resolution grain size oscillations reliably record rapid monsoon changes and can be well correlated to rapid climate changes recorded in the greenland ice core and hulu cave stalagmite

the matuyama brunhes polarity reversal in four chinese loess records high fidelity recording of geomagnetic field behavior or a less than reliable chronostratigraphic marker abstract the paleomagnetic recording fidelity of chinese loess has been a matter of long term dispute to help shed light on the acquisition retention mechanisms of natural remanent magnetization in loess we report on high sampling resolution paleomagnetic constraints on the position of the matuyama brunhes boundary mbb in four typical loess sections in different marginal areas of the chinese loess plateau clp each of which has experienced different east asian winter summer monsoon strengths and pedogenic intensities detailed thermal demagnetization experiments for 1850 specimens reveal that none of the high accumulation rate loess with the weakest pedogenesis at caotan western clp the massive pedogenesis loess characterized by extensive development of thick calcareous nodule layers at jixian se clp and the moderately pedogenized loess at fanshan ne clp have preserved high fidelity geomagnetic records across the mbbs jiuzhoutai western clp one of the thickest loess profiles in the world contains the thickest and hitherto the most complex mbb that is too extensive to be regarded as rapid fluctuations of geomagnetic field during the m b reversal at jixian a predominance of pedogenesis induced chemical remanent magnetization overprinting has heavily obscured the original remanence the significant downward displacement of the mbb at fanshan and even its total absence at caotan together with extensive remagnetization of the coarse grained upper sandy loess layer l9 in both sections further suggest that the processes of remanence acquisition retention of chinese loess in the marginal areas of the clp are rather complicated thus interpretations of the fine structure of geomagnetic transitional field behavior and excursions especially in the matuyama chron should be regarded with caution

thin tephra layers in peat revealed by x radiography x radiography has been found to provide a rapid economical non destructive method of locating very thin fine grained tephra of icelandic origin in scottish peats tephra layers form time parallel marker horizons or isochrones that are of great archaeological significance because they cover large areas offer precise chronological control for associated material and occur at times of significant cultural and environmental changes not all of the discrete deposits of recent volcanic fallout in scotland are sufficiently dense to show clearly on x radiographs but those which do provide particularly clear easily located marker horizons with many applications in palaeoenvironmental studies the tephra layer in caithness produced by fallout from a c 4000 by eruption of hekla provides a particularly good example of the use of x radiography to locate volcanic ash the method also reveals other structures of archaeological and palaeoenvironmental significance butler 1992 journal of archaeological science 19 151 161

magnetic susceptibility evidence of monsoon variation on the loess plateau of central china during the last 130 000 years the magnetic susceptibility of loess and paleosols in central china represents a proxy climate index closely related to past changes of precipitation and vegetation and thus to summer monsoon intensity time series of magnetic susceptibility constructed for three loess paleosol sequences in the southern part of the chinese loess plateau document the history of summer monsoon variation during the last 130 000 yr they correlate closely with the oxygen isotope record of stages 1 to 5 in deep sea sediments soils were forming during intervals of strong summer monsoon whereas loess units were deposited at times of reduced monsoon intensity the chinese loess paleosol sequence can thus be viewed as a proxy record of asian monsoon variability extending over the last 2 5 myr

timing of the brunhes matuyama magnetic polarity reversal in chinese loess using 10be in chinese loess the brunhes matuyama b m geomagnetic reversal occurs 25 k y prior to the age found in marine sediments this offset has been attributed by some to post depositional magnetic overprinting of loess while others have argued it is due to errors in the loess time scale here we solve this long standing debate by exploiting a new method to extract reproducible records of geomagnetic field intensity from loess with 10 be a proxy for global average geomagnetic field intensity and using it to show that a pronounced minimum in field intensity a requirement for dipole field reversal is recorded in two separate loess records at ca 780 3 kyr b p this timing is synchronous with the b m reversal timing seen in marine records verifying the standard loess time scale as correct but it is 25 k y younger than the age depth of the magnetic polarity reversal recorded in these same chinese loess sediments demonstrating that loess magnetic overprinting has occurred

geomagnetic episodes of the last 1 2 myr recorded in chinese loess 1 the matuyama brunhes m b boundary termination of jaramillo normal subchron j m as well as three young geomagnetic excursions has been revealed in the weinan loess section zhu et al 1994 1998 1999 recently we carried out further paleomagnetic directional studies on the same section and recovered the onset of the jaramillo normal subchron m j and four geomagnetic excursions two in the brunhes and two in the late matuyama the fact that all seven excursions found in this section are in loess units and expected excursions in paleosols are not found suggests that short lived geomagnetic signals could be only recorded in loess units and may have been fully smoothed or wiped out in paleosol units by strong pedogenesis constrained by a new astronomical timescale hld00 ages of the geomagnetic excursions and reversal boundaries have been estimated we tentatively correlated these excursions with previous results indicating that a large lock in model may not be correct for the whole loess profile although some apparently big positive age discrepancies were observed for the main boundaries

misleading positions of geomagnetic reversal boundaries in eurasian loess and implications for correlation between continental and marine sedimentary sequences abstract the matuyama brunhes boundary mbb where the earth s magnetic field changed to its present orientation is one of the most frequently used time markers in continental quaternary stratigraphy although magnetic signals have been successfully recovered from loess deposits and an apparently coherent magnetostratigraphy seems to be in place a close examination of the published loess records from northern mid latitudes reveals significant uncertainty and variability for the position of the mbb in asia and part of central europe the mbb has been found in loess a glacial deposit this is in contrast to the well established palaeomagnetic records in deep sea sediments where the mbb is located in the interglacial oxygen isotope stage 19 this discrepancy has caused serious confusion in stratigraphic correlation between loess and marine sequences here we examine the causes of the discrepancy between the two sedimentary environments in their records of the mbb our approach includes a comparison of the mbb in loess and other continental records and an evaluation of the stratigraphic significance of the microtektites found around the mbb based on the occurrence of the mbb in an interglacial stage in lake and continental margin sequences and the correlative stratigraphic positions of the microtektites in marine sediments and loess we demonstrate that the measured positions of the mbb in loess are erroneous and misleading and its true occurrence was during the time of formation of the palaeosol corresponding to oxygen isotope stage 19 the most plausible cause for the apparent offset i e the mbb being found in the underlying loess instead is that the acquisition of remanent magnetisation in loess occurs at a certain depth substantially below the land surface the degree of such displacement is dependent on the lithology and structure of the deposits formed prior to the reversal and the time delay is estimated to be in the order of 103 104 years we show that other geomagnetic boundaries such as those of jaramillo and cobb mountain in loess have been similarly displaced though to a varying extent we suggest that the remanence acquisition in loess is closely related to the post depositional evolution in the structure of loessic materials which is in part influenced by regional climate conditions our conclusions on the displaced remanence acquisition have implications for loess chronology magnetostratigraphy as well as the geomagnetic interpretation of detailed magnetic signatures from loess

a 9000 year old ash bed on the faroe islands abstract radiocarbon datings of the saksunarvatn ash bed on the faroe islands indicate an age of 9000 9100 yr b p the saksunarvatn ash bed differs geochemically from both ash zone 1 in the north atlantic and the vedde ash bed of norway and the norwegian sea all mentioned ashes are assumed to originate from iceland the vedde ash has been dated at 10 600 60 yr b p consequently the saksunarvatn and vedde ash beds provide an opportunity for precise dating of events around the pleistocene holocene boundary in marine cores especially from the region where the two plumes overlap

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

the hekla 3 volcanic eruption recorded in a scottish speleothem recent research has demonstrated the presence of annual luminescent growth banding in a stalagmite from sutherland n w scotland baker smart edwards and richards 1993 the banding record of this sample contains a short duration increase in annual band width indicating a doubling of growth rate for a period of four years predicted growth rates derived from calcite precipitation kinetics are compared with observed growth rate variability to demonstrate that such a doubling of growth rate would not be expected within the probable range of normal climate variation the growth rate peak is dated to 1135 130 bc by thermal ionization mass spectrometric 238u 234 u 230th dating not inconsistent with the timing of the hekla 3 volcanic eruption this event may explain the observed variation in growth rate and provide a new source of evidence for interpretation of the effects of large explosive eruptions

timing and lock in effect of the laschamp geomagnetic excursion in chinese loess 1 the laschamp geomagnetic excursion lge at 41 ka is a critical age control for hemispherical comparison of paleoclimate records from various kinds of geological archives e g ice core marine and lake sediments the timing of the lge in chinese loess however remains poorly constrained due to the lack of a reliable chronology and the complex acquisition processes of natural remanent magnetization here we systematically compare the paleomagnetic results of three optically stimulated luminescence dated loess sequences on the chinese loess plateau our results indicate that the timing of the lge in the northwestern loess plateau is slightly older than the absolute radiometric age determination and the timing inferred from greenland ice core 10be flux and marine i e relative paleointensity and authogenic 10be 9be stack records but younger than the counterparts in the central loess plateau we attribute the different timing of the lge in the three loess sections to a progressive southeastward increase in the lock in depth caused by the combined effects of postdepositional processes e g surface mixing chemical weathering and lock in delay on the remanence acquisition process we conclude that caution is needed to use the lge in chinese loess as a reliable tie event for high resolution chronological correlation to marine and ice core records only if the potential lock in effect can be precisely determined

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

in situ remagnetization experiments of loess on the chinese loess plateau evidence for localized post depositional remanent magnetization 1 inconsistent stratigraphic positions of reversal boundaries and spurious paleomagnetic variations associated with polarity reversals recorded by loess paleosols on the chinese loess plateau clp have in general been attributed to inferred delayed acquisition of magnetization and high frequency variations of the geomagnetic field respectively these conclusions are partly based on an incomplete knowledge of the magnetization acquisition process of loess recent laboratory experiments which demonstrate that dry deposited loess acquires a stable remanent magnetization after its first wetting precipitation resulted in significant inclination errors that are not generally encountered in chinese loess laboratory experiments only remotely mimic natural conditions and we have therefore performed in situ experiments at two sites on the clp separated by some 650 km to elucidate whether variations in natural conditions can impose secondary remanent magnetization components in loess l1 the experiments were performed by relocating blocks volume 5 10 3 m3 of last interglacial loess after being turned upside down inside loess exposures inverted blocks were retrieved and analyzed after 8 and 17 months respectively thermal demagnetization reveals that partial remagnetization has occurred within different parts of the loess blocks this secondary magnetization which is characterized by blocking temperatures that overlap with those of the primary component to above 450 c cannot be attributed to a viscous remanent magnetization and is therefore proposed to represent local and partial realignment of magnetic grains we tentatively propose that physical unlocking and realignment of magnetic grains may be assisted by pressure release caused by differential thermal expansion of the different mineral grains in loess localized remagnetization may account for a number of reported spurious paleomagnetic results from clp

origin of rapidly solidified metal troilite grains in chondrites and iron meteorites inclusions of troilite and metallic fe ni 0 2 4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites cooling rates in the interval 1400 950 c calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10 7 to 104 c sec in 8 of these chondrites which are breccias containing some normal slow cooled metal grains the inclusions solidified before they were incorporated into the breccias their cooling rates of 1 300 c sec indicate cooling by radiation or by conduction in contact with cold silicate or hot silicate volumes only 6 40 mm in size this is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body by impact and were not formed at depth from an internally derived melt in ramsdorf rose city and shaw which show extensive reheating to 1000 c fe fes textures in melted areas are coarser and indicate cooling rates of 10 1 to 10 4 c sec during solidification this metal may have solidified inside hot silicate volumes that were 10 300 cm in size as shaw and rose city are breccias of unmelted and melted material their melted metal did not necessarily cool through 1000 c within a few m of the surface shock melted fine grained irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of 105 c sec their kamacite and taenite compositions may result from annealing at 250 c of metallic glass or exceedingly fine grained quench products

towards an orbital time scale for chinese loess deposits abstract in this study we analyzed the grain size of the baoji loess soil sequence at 10 cm intervals results show that grain size variations are very sensitive to loess soil alternations with loess units being much coarser than soils such a change in loess soil grains is interpreted as the result of the glacial interglacial cyclic variations in the intensity of the winter monsoon winds out of siberia the grain size record is thus employed as a proxy indicator of the winter monsoon circulation and tuned to the orbital records calculated recently by berger and loutre 1991 under the control of major magnetic reversals the tuning is independent of any correlation with 18o signals in the deep sea sediments the resulting grain size time scale is tightly constrained as suggested by the following facts 1 the filtered obliquity and precession components from the grain size data on the orbital time scale closely match the theoretical orbital records 2 ages of the major magnetic reversals estimated from the grain size time scale are in good agreement with the k ar dated ages 3 there is close coherence between the baoji grain size time series and the orbital variations at the orbital frequency bands over the entire 0 2 5 ma period and 4 the grain size record on the orbital time scale shows a close similarity to the orbitally tuned dsdp site 607 and odp site 677 18o records examination of time dependent characteristics of the grain size record suggests that there are two major shifts of dominant periodicities in the long term monsoonal cycles one occurring at about 0 8 ma bp and the other around 1 7 ma bp

two geomagnetic excursions during the brunhes chron recorded in chinese loess palaeosol sediments summary detailed palaeomagnetic integrated with rock magnetic studies have been carried out on a loess palaeosol sequence in baoji shaanxi province southern chinese loess plateau for most samples stepwise thermal demagnetization revealed two well defined magnetization components a low temperature component ltc which was isolated between 100 and 200 c is consistent with the present geomagnetic field direction a high temperature component htc which was isolated between 250 and 620 680 c shows normal reversed or transitional polarities our new magnetostatigraphy revealed two distinct geomagnetic excursions recorded in loess unit of l5 and palaeosol unit of s7 respectively and the matuyama brunhes m b polarity boundary in loess unit of l8 rock magnetic experiments demonstrated that the specimens from the excursion zones have the same magnetic properties as those from the brunhes normal or matuyama reversed polarity zones measurements of anisotropy of magnetic susceptibility ams showed that the sediments have primary sedimentary fabrics based on the palaeoclimatological and magnetostratigraphic age models the middle brunhes excursion in loess l5 is dated at 413 433 ka and the early brunhes excursion is estimated to occur 23 33 ka after the m b reversal comparing with previously reported geomagnetic excursions in the brunhes chron the middle brunhes excursion l5 is likely global for the early brunhes excursion s7 we need further studies to examine its global occurrence

petrological evidence for shock melting of carbonates in the martian meteorite alh84001 the meteorite alh84001 a shocked igneous rock of probable martian origin contains chemically and isotopically heterogeneous carbonate globules associated with which are organic and inorganic structures that have been interpreted as possible fossil remains of ancient martian biota a critical assumption underlying this suggestion is that the carbonates formed from low temperature fluids penetrating the cracks and voids of the host rock here we report petrological studies of alh84001 which investigate the effects of shock on the various mineralogical components of the rock we find that carbonate plagioclase and silica were melted and partly redistributed by the same shock event responsible for the intense local crushing of pyroxene in the meteorite texture and compositional data show that during the period of shock decompression monomineralic melts were injected into pyroxene fractures that were subsequently cooled and resealed within seconds our results therefore suggest that the carbonates in alh84001 could not have formed at low temperatures but instead crystallized from shock melted material this conclusion weakens significantly the arguments that these carbonates could host the fossilized remnants of biogenic activity

a precursor to the matuyama brunhes reversal in chinese loess and its palaeomagnetic and stratigraphic significance summary we report high resolution palaeomagnetic results across the lower part of s8 in the luochuan area northwest of china using parallel subsets samples a palaeomagnetic anomaly with low palaeointensity and disordered magnetic direction was identified 21 ka prior to the matuyama brunhes mb polarity reversal and is consistent with the mb precursor observed from marine sediments and lava flows this is the first convincing report of the mb precursor in terrestrial material chinese loess which attests the global feature of the mb precursor constrained by the stratigraphic position of both the mb polarity reversal and the mb precursor the loess unit l8 and palaeosol unit s8 of the chinese loess sequence are undoubtedly tied to the marine isotope stages 18 and 19 respectively this correlation is critical for an accurate understanding of the correspondences of palaeomagnetic records between the chinese loess and marine sediments

record of volcanism since 7000 b c from the gisp2 greenland ice core and implications for the volcano climate system sulfate concentrations from continuous biyearly sampling of the gisp2 greenland ice core provide a record of potential climate forcing volcanism since 7000 b c although 85 percent of the events recorded over the last 2000 years were matched to documented volcanic eruptions only about 30 percent of the events from 1 to 7000 b c were matched to such events several historic eruptions may have been greater sulfur producers than previously thought there are three times as many events from 5000 to 7000 b c as over the last two millennia with sulfate deposition equal to or up to five times that of the largest known historical eruptions this increased volcanism in the early holocene may have contributed to climatic cooling

astronomical calibration of loess paleosol deposits at luochuan central chinese loess plateau abstract the 140 m loess paleosol profile at luochuan in the central chinese loess plateau was sampled at 5 cm intervals in loess units and at 3 cm in paleosol units in order to obtain a high resolution climatic record covering the past 2 5 million years all samples were measured for magnetic susceptibility which is regarded as a good proxy index of the east asian summer monsoon strength on the basis of the astronomical theory of pleistocene climatic change an age model of the luochuan loess paleosol sequence was developed by tuning the magnetic susceptibility record to time series of insolation changes the results show that the ages of the boundaries between the malan and lishi and lishi and wucheng loess formations are 71 and 1320 kyr bp respectively the onset of loess accumulation is at 2470 kyr bp our age model was tested by comparing the orbitally derived ages with absolute age determinations of magnetic reversals and cross spectrum analyzing with solar radiation variations for summer at 65 n these indicate that the calibration provides a reliable time scale for the luochuan loess paleosol deposit

subaerial and subaqueous deposition of loess experimental assessment of detrital remanent magnetization in chinese loess abstract loess and intercalated paleosols on the chinese loess plateau clp are the most extensive and continuous terrestrial archive of geomagnetic and paleoclimate variations covering the last 2 6 ma a first order time frame has been established for these sequences using paleomagnetic polarity boundaries major inconsistencies between the positioning of the matuyama brunhes polarity boundary in chinese loess paleosol sequences and marine records previously accounted for by assuming magnetization lock in depths in loess of up to 2 3 m have recently been resolved using a paleoclimatic indicator insensitive to pedogenesis that does require any major post depositional acquisition of a remanent magnetization this resolved controversy accentuates the lack of an adequate understanding of how chinese loess acquires and retains a stable remanent magnetization we report on laboratory re deposition experiments with subaerial and subaqueous simulations of natural loess to elucidate factors controlling the acquisition of magnetization in loess our work complements and extends recently published results from comparable laboratory experiments by zhao and roberts 2010 our major finding is that a stable magnetization is acquired and retained after initial wetting of dry deposited loess with no significant time lag in general accord with recent findings

age of matuyama brunhes boundary constrained by u pb zircon dating of a widespread tephra abstractthe youngest geomagnetic polarity reversal the matuyama brunhes boundary mbb provides an important datum plane for sediments ice cores and lavas its frequently cited age of 780 ka is based on orbital tuning of marine sedimentary records and is supported by 40 ar 39 ar dating of hawaiian lavas using recent age calibrations challenging this age how ever are reports of younger astrochronological ages based on oxygen isotope stratigraphy of high sedimentation rate marine records and cosmogenic nuclides in marine sediments and an antarctic ice core here we present a u pb zircon age of 772 7 7 2 ka from a marine deposited tephra just below the mbb in a forearc basin in japan u pb dating has a distinct advantage over 40 ar 39 ar dating in that it is relatively free from assumptions regarding stan dardization and decay constants this u pb zircon age coupled with a newly obtained oxygen isotope chronology yields an mbb age of 770 2 7 3 ka our mbb age is consistent with those based on the latest orbitally tuned marine sediment records and on an antarctic ice core we provide the first direct comparison between orbital tuning u pb dating and magnetostratig raphy for the mbb fulfilling a key requirement in calibrating the geological time scale introduction

a new correlation between chinese loess and deep sea δ18o records since the middle pleistocene abstract the quaternary chinese loess is one of the most continuous terrestrial sediments and preserves excellent palaeoclimate records at different timescales the conventional land ocean palaeoclimatic comparison has been established by correlation between magnetic susceptibility of the chinese loess and deep sea 18 o records during the 1980s however such a correlation results in long standing debates on the lock in depth of the palaeomagnetic polarity boundaries in chinese loess in this study the lock in depth of palaeomagnetic signals for loess

the matuyama brunhes and upper jaramillo transitions recorded in a loess section at weinan north central china abstract the paleomagnetic records of the matuyama brunhes mb and upper jaramillo uj polarity reversals have been obtained from a loess section at weinan shaanxi province in china an investigation of the rock magnetic properties using stepwise irm acquisition hysteresis loops thermomagnetic analysis and x ray diffraction on magnetic extracts identifies pseudo single domain magnetite as the carrier of the main fraction of the remanence associated with maghemite and hematite the total duration of the directional change based upon independent loess accumulation rates is about 5000 yr for the mb reversal and about 3200 yr for the uj reversal these values are consistent with previous estimates obtained from marine or lacustrine sediments apart from a few intermediate points observed on north america mb and in the pacific uj the transitional virtual geomagnetic pole vgp paths are situated in two longitudinal bands over eastern asia and australia mb and the americas uj the bulk of the uj record and the part of the mb record over the americas are not consistent with the hypothesis of a record arising from smoothing of pre and post transitional directions 1 a reappraisal of the statistical approach of mcfadden et al 2 shows that the longitudinal distribution of the records of reversals is consistent with the hypothesis of genuine geographically preferred sectors for the paths independent of site longitude with a probability higher than 0 45 when the two weinan records are considered thus these results obtained from wind blown loess sustain the hypothesis of two preferential longitudinal bands for reversing transitional vgps suggested from the study of marine and lacustrine sediments

impact melt products of chondritic material a large variety of objects in chondritic meteorites formed as a result of impact melting shock veins metal troilite mixtures metal and sulfide nodules melt pockets vugs agglutinates glassy and crystalline spherules and numerous types of melt rock clasts the type of object produced in an impact event is related to the interaction of the shock waves with the particular target rock impact melting has also affected large portions of several chondrite breccias many of these contain vugs and patches of silicate glass some workers have suggested that various iron meteorites including groups iab iiicd and iie as well as several ungrouped irons were formed from individual impact melt pools in chondritic regoliths the eagle station trio pallasites and several ungrouped meteorites bencubbin weatherford enon acapulco and allan hills a77081 may have formed from impact melted chondritic material it is also possible that the parent magmas of ureilites were derived from impact melted metal rich carbonaceous chondrite material

shock melted material in the krymka ll3 1 chondrite behavior of the opaque minerals six large millimeter to centimeter size regions of one specimen of the krymka ll3 1 ordinary chondrite show evidence of having been completely or nearly completely shock melted in situ a phenomenon rarely observed in primitive chondrites the shock pressure nominally in the range of 75 90 gpa could only have been 30 35 gpa in a porous material like fine grained matrix the melted regions have an igneous texture and their silicates are zoned and unequilibrated large metal troilite intergrowths formed in these regions the metal has a nickel content corresponding to martensite and the troilite contains up to 4 2 wt nickel melting must have been very short and cooling very fast 100 c h at high temperature the metal contains up to 0 7 wt phosphorus abundant chromite crystals and sodium iron phosphate glass globules are found in troilite the differences in composition between the opaque phases found in the melted regions and those generally observed in unmetamorphosed chondrules are assigned to melting under closed system conditions surprisingly high co concentrations up to 13 wt were found in some metal grains in or at the periphery of melted regions they likely resulted from sulfurization of metal by sulfur vapor produced during the shock after solidification at least one other shock led to mechanical effects in the melted regions

post depositional remanent magnetization lock in and the location of the matuyama brunhes geomagnetic reversal boundary in marine and chinese loess sequences article i nfo pdrm lock in loess paleosol bioturbation disturbs detrital magnetic particles after deposition so accurate recording of the ancient geomagnetic field in bioturbated sediments is widely attributed to acquisition of a post depositional remanent magnetization pdrm whereby the geomagnetic field exerts a torque on a magnetic particle and aligns it with the field after the final mixing event experienced by the particle the relationship between the matuyama brunhes boundary mbb and oxygen isotope age tie points in marine sediments has been widely used to determine the depth at which the paleomagnetic signal is locked in however such analyses can be badly affected by age discrepancies among different paleoclimatic proxies and by varying isotopic compositions of seawater in different locations and from the presence of different water masses at different depths at the same location it is therefore necessary to separately compare paleomagnetic data with respect to either benthic or planktonic foraminiferal oxygen isotope records for sites from the same water mass to avoid inadvertently introducing age differences to the analysis when the global data set is subjected to such a rigorous analysis few reliable data remain for the mbb using two complementary approaches we estimate that the mbb is on average shifted 20 cm below its true position in marine sediments this offset is the sum of the thickness of the bioturbated surface mixed layer which is possibly dominant and the pdrm lock in depth there is also controversy concerning observed differences in the position of the mbb relative to paleoclimatic proxies in marine sediments and chinese loess deposits for the chinese loess quartz grain size is insensitive to pedogenic alteration and is a useful parameter for determining the true position of the mbb with respect to paleoclimatic boundaries we conclude that the mbb occurs late in marine oxygen isotope stage 19 and in the upper part of chinese paleosol unit s8 rather than at the mid or lower part of loess unit l8 our results require adjustment of the generally accepted positions for the mbb and resolve a longstanding chronological conundrum for marine and chinese loess sequences

paleoclimatic significance of the temperature dependent susceptibility of holocene loess along a nw se transect in the chinese loess plateau we have conducted mineral magnetic investigations of samples of modern dune sand pristine loess and present day loess along a nw se transect from the northern to the central loess plateau in china the methods used include the temperature dependence of susceptibility tds before and after citrate bicarbonate dithionite cbd treatment as well as x ray diffraction xrd analysis the xrd analyses demonstrate that magnetite and hematite both exist in the chinese loess paleosol sequence and its modem source area but the tds measurements show that magnetite is the predominant contributor to magnetic susceptibility maghemite is present in the pristine loess and the present day loess due to pedogenesis the pedogenic processes that produce the maghemite are closely linked to paleoclimate and for this transect precipitation appears to be the most important climatic variable we suggest that tds can be used as a useful method for the analysis of pedogenesis and climatic change

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

remagnetization of quaternary eolian deposits a case study from se chinese loess plateau 1 the loess paleosol succession in the southeastern margin of the chinese loess plateau retains high resolution archives of sedimentary and environmental change in this study we report a detailed paleomagnetic and rock magnetic investigation of loess paleosol sequences in the sanmenxia area spanning the last 1 1 myr the results demonstrate that the matuyama brunhes boundary occurs at the top of soil s8 and the upper and lower boundaries of the jaramillo normal subchron are encountered at the top of soil s10 and loess l13 respectively in agreement with the classic luochuan section loess l9 also referred to as the upper sand layer is the coarsest loess unit over the last 1 1 myr and carries an imperfect normal polarity magnetization although it is fully separated by a short reversed polarity interval this thick normal polarity zone is too extensive to represent excursions like kamikatsura and or santa rosa and is concluded to represent an artifact rather than a record of geomagnetic field directions precise multiparameter environmental magnetic determination reveals that the short reversed polarity interval within the middle of this thick normal polarity zone exactly corresponds to a thin pedogenic horizon detailed rock magnetic analyses show that there are no intrinsic differences in magnetomineralogical properties between this pedogenic interval and other parts of l9 although the composition of magnetically soft components magnetite maghemite in the short pedogenic interval is relatively higher than that of other parts of l9 thus it is proposed that extensive remagnetization of parts of l9 most likely reflects a lithology dependent process and the dramatic coarsening of particle size across l9 is the fundamental reason for pervasive remagnetization our magnetostratigraphic results strongly imply that we need to reconsider the remanence recording fidelity of coarse loess l9 and probably l15 and care should be taken to interpret excursions in future paleomagnetic investigations of the chinese loess

depth of post depositional remanence acquisition in deep sea sediments a case study of the brunhes matuyama reversal and oxygen isotopic stage 19 1 although post depositional magnetization pdrm in deep sea sediments appears to be acquired during the earliest stages of sediment compaction the natural variability of the pdrm lock in depth in deep sea sediments is poorly understood and as yet unquantified here we consider variations in the relative stratigraphic positions of oxygen isotopic interglacial stage 19 1 and the brunhes matuyama b m chronozone reversal for eight deep sea sediment cores results from a similar study of the displacement between a widespread microtektite layer and the b m boundary are also included 1 the pdrm lock in depth and the temporal relationships between the b m and stage 19 1 datums can be determined from the offsets between the paleomagnetic and the isotopic and microtektite stratigraphies for cores with sedimentation rates greater than 1 cm kyr a the depth offset between the paleomagnetic and isotopic datums is a linear function of sedimentation rate a simple model r 2 0 77 demonstrates that 1 pdrm acquisition occurs 16 cm below the sediment surface and 2 the b m reversal occurs 6 kyr 4 2 kyr after the stage 19 1 datum and the stage 19 1 datum occurs 9 kyr 4 3 kyr after the deposition of the australasian microtektite strewnfield this example argues against simple geophysical models linking geomagnetic field reversals to climate change or impact events the b m boundary is anomalously deep 30 50 cm in very low accumulation rate sediments 1 cm kyr 1 and this may reflect the unusual physical properties of these sediments a review of the geotechnical literature suggests that very low accumulation rate sediments have sufficient time to develop enhanced interparticle rigidity structural strength which inhibits early compaction and hence pdrm acquisition

temperature dependence of magnetic susceptibility in an argon environment implications for pedogenesis of chinese loess palaeosols summary temperature dependence of magnetic susceptibility t has been widely used to determine changes in mineralogy of natural samples during heat treatment we carried out integrated rock magnetic experiments to interpret the t curves of the chinese loess palaeosols in argon we used both raw materials and heated samples in addition we also investigated the magnetic properties of magnetic extracts and residues to quantify contributions from each fraction to the bulk magnetic properties for the heating curves the susceptibility loss 30 per cent between 300 400 c is caused by the inversion from pedogenic fine grained maghemite to haematite suggesting that the susceptibility loss can be used as a new concentration index of the pedogenic fine grained superparamagnetic sp particles in the chinese loess palaeosols unlike the warming curves the cooling curves are dominated by newly formed fine grained magnetites with a dominant size of 35 nm the onset for the new production of these fine grained magnetic particles occurs at 400 c it is interesting that the room temperature magnetic susceptibility ph of the samples heated after a 700 c run is independent of the degree of pedogenesis and saturates at approximately 33 35 10 7 m3 kg 1 indicating that the susceptibility enhancement is controlled only by the reduction of fe bearing aeolian minerals during heating it appears that the 700 c thermal treatment in argon could be in some sense an analogue to the pedogenic processes thus we predict that 33 35 10 7 m3 kg 1 is the maximum susceptibility that pedogenesis can generate for the last interglacial palaeosol unit s1 in practice ph would be useful to quantify the aeolian inputs to the chinese loess plateau

latest olduvai short lived reversal episodes recorded in chinese loess 1 a duplicate high resolution magnetic record was obtained from two parallel loess sections separated by about 300 m near baoji city southern chinese loess plateau this reveals at least eight short lived reversed polarity episodes in the uppermost part of the olduvai normal polarity subchronozone rock magnetic experiments and anisotropy of low field magnetic susceptibility confirm that the multiple occurrence of short lived reversals is not due to rock magnetic and or sedimentary causes assuming a constant accumulation rate within the olduvai subchron the short episode zone was estimated to span about 24 30 ka and 24 31 ka for the two sections respectively the duration of each short episode ranges from about 0 3 ka to 2 1 ka the present results together with previous worldwide observations show that the occurrence of numerous short reversals is peculiar to the latest olduvai subchron the geodynamo may have been in an anomalous state at that time

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

icelandic volcanic ash and the mid holocene scots pine pinus sylvestris pollen decline in northern scotland a volcanic ash layer in peat from northern scotland has been identified and coincides exactly with an abrupt decline in pinus sylvestris l scots pine pollen frequencies this provides an isochrone time equivalent marker horizon with which to investigate the timing of the holocene pine decline furthermore two possible causes of the southward shift of the range of pinus in scotland c 4000 bp are suggested a direct effect of acid pollution by chemicals produced by the eruption of hekla h 4 or a volcanically induced climatic perturbation these possibilities have wider implications for the influence of volcanism on postglacial environmental change

remagnetization mechanism and a new age model for l9 in chinese loess chinese loess during the quaternary is important for both paleomagnetic and paleoclimatic studies although magnetostratigraphy has been used for the initial loess timescale paleomagnetic records in loess still remain in debates such as magnetic polarity anomalies in l9 which is a thick and coarse sandy horizon as a typical marker bed to better understand the magnetization mechanism of these anomalies a new age model for l9 was first constructed based on a new correlation between loess and marine d 18 o which proposed that l9 should be correlated to marine oxygen stage mis 20 24 not 22 24 as previous suggested using combined grain size and environmental magnetism parameters an extended climatic optimum corresponding to mis 21 can be identified in middle of l9 a remagnetization mechanism of vis cous remanent magnetization overprinting on characteristic remanent magnetization chrm carriers mainly large pseudo single domain and multidomain magnetite particles associated with the chaotic detrital remanent magnetization caused by low efficiency alignment of chrm carriers along with the ambient field during deposition is proposed to interpret the magnetic anomalies this study provides new insights into the magnetization mechanism of the chinese loess and the new age model for l9 helps to accurately refine the finer scale correlation of paleoclimatic records between the chinese loess and marine sediments

paleoenvironmental significance of the magnetic fabrics in chinese loess paleosols since the last interglacial 130 ka abstract we compare new anisotropy of magnetic susceptibility ams data with previously determined paleoclimatically sensitive parameters for a loess profile at lingtai central loess plateau china to unravel factors that control loess ams magnetic lineations are clustered along ne sw and nw se orientations during cold and warm periods respectively which probably corresponded to paleowind directions of both winter and summer monsoons anomalously steep magnetic foliations coincidentally occur at the paleoclimatic stage boundaries indicating sediment disturbance lineation strength appears to be sensitive to millennial scale rapid paleoclimatic fluctuations generally the winter monsoon produces a lower degree of lineation than the summer monsoon furthermore by studying lineation subsets for the cold and warm periods we find that the cold interval lineation strengths co vary with grain size over the last 70 ka during the last interglacial about 74 128 ka the lineation and declination of the susceptibility maximum dec k max are sensitive to rapid fluctuations in winter and summer monsoons long term trends of magnetic foliation and anisotropy degree replicate grain size variations with superimposed short term peaks related to post depositional compaction as suggested by density the ams shape parameter t indicates oblate fabrics these new findings will not only improve our understandings of loess ams but also expand the possibilities of paleoclimatic studies in the chinese loess

delayed build up of arctic ice sheets during 400 000 year minima in insolation variability an east asian winter monsoon proxy record using grain size variations in chinese loess over the past 900 000 years shows that for up to 20 000 years after the interglacials at 400 000 year intervals the weak monsoon winds maintain a mild non glacial climate at high northern latitudes qingzhen hao and colleagues present a record of grain size variations in the chinese loess plateau a proxy of variations in the strength of the east asian winter monsoon and show that relatively high insolation at 400 000 year intervals held off the inception of the subsequent ice age for about 20 000 years relative to other glacial inceptions mechanistically the extension of warm conditions was probably linked to a weak siberian high pressure system associated with a delayed build up of northern ice and snow the authors speculate that these observations imply that arctic climate may remain in non glacial mode for more than 40 000 years even in the absence of anthropogenic greenhouse gases knowledge of the past variability of climate at high northern latitudes during astronomical analogues of the present interglacial1 may help to inform our understanding of future climate change unfortunately long term continuous records of ice sheet variability in the northern hemisphere only are scarce because records of benthic 18o content represent an integrated signal of changes in ice volume in both polar regions2 however variations in northern hemisphere ice sheets influence the siberian high3 an atmospheric pressure system so variations in the east asian winter monsoon eawm as recorded in the aeolian dust deposits on the chinese loess plateau can serve as a useful proxy of arctic climate variability before the ice core record begins here we present an eawm proxy record using grain size variations in two parallel loess sections representative of sequences across the whole of the chinese loess plateau over the past 900 000 years the results show that during periods of low eccentricity and precessional variability at approximately 400 000 year intervals the grain size inferred intensity of the eawm remains weak for up to 20 000 years after the end of the interglacial episode of high summer monsoon activity and strong pedogenesis in contrast there is a rapid increase in the eawm after the end of most other interglacials we conclude that for both the 400 000 year interglacials the weak eawm winds maintain a mild non glacial climate at high northern latitudes for much longer than expected from the conventional loess and marine oxygen isotope records during these times the less severe summer insolation minima at 65 n ref 4 would have suppressed ice and snow accumulation leading to a weak siberian high and consequently weak eawm winds

majorite olivine high ca pyroxene assemblage in the shock melt veins of pervomaisky l6 chondrite high pressure minerals majorite pyrope garnet and jadeite were found in the pervomaisky l6 ordinary chondrite majorite pyrope 79 mol majorite was observed within the fine grained silicate matrix of a shock melt vein smv coexisting with olivine and high ca pyroxene this is the first report of a garnet olivine high ca pyroxene assemblage that crystallized from the melt in the smv matrix of meteorite p t conditions of the formation of the smv matrix with olivine fragments are 13 5 15 0 gpa and 1750 2150 c the lowest parameters among all known majorite bearing h l chondrites the estimated conditions include the olivine olivine ringwoodite phase boundary and there is a possibility that observed olivine is the result of wadsleyite ringwoodite back transformation during a cooling and decompression stage in the framework of this hypothesis we discuss the problem of survival of the high pressure phases at the post shock stage in the meteorites and propose two possible p t paths 1 the high pressure mineral is transformed to a low pressure one during adiabatic decompression above the critical temperature of direct transformation and 2 quenching below the critical temperature of direct transformation within the stability field of the high pressure phase and further decompression the aggregates with plagioclase composition ab 81 1 an 14 9 or 4 1 occur in host rock fragments near or inside of the smv and have a radial concentric spherulite like microstructure previously described in the novosibirsk meteorite and that is very similar to the texture of tissintite in the tissint martian meteorite it is likely that jadeite is related to crystallization of the smv and could have formed from albitic feldspar plagioclase melt at 13 5 15 0 gpa and 2000 c

how does chinese loess become magnetized despite nearly three decades of paleomagnetic research on the extensive chinese loess deposits a convincing explanation has yet to be developed for how chinese loess becomes magnetized to address this problem we conducted re deposition experiments using weakly consolidated holocene chinese loess that was disaggregated in the laboratory and compare our results with published paleomagnetic data we simulated a depositional remanent magnetization drm associated with dry deposition of eolian sediments and a post depositional remanent magnetization pdrm in which the sediment was water saturated after deposition the simulated drm faithfully records the declination of the applied field but with systematic inclination flattening addition of minor water slightly improves recording of the applied field but inclination flattening persists reliable recording of the applied field occurs for pdrm simulation in water saturated sediment our synthesis of paleomagnetic data from chinese loess indicates that time averaged paleomagnetic directions are often indistinguishable from the expected geocentric axial dipole gad field but in many cases inclinations are shallower than for a gad field we conclude that the chinese loess is magnetized by a combination of drm and pdrm mechanisms with water content providing the dominant control on which mechanism aligns the detrital mineral fraction where pedogenesis causes neoformation of magnetic minerals an additional chemical remanent magnetization crm will occur the magnetization of chinese loess therefore appears to be controlled by a complex time varying combination of drm pdrm and crm mechanisms

apparent timing and duration of the matuyama brunhes geomagnetic reversal in chinese loess the matuyama brunhes mb geomagnetic reversal in chinese loess has been studied extensively as an important boundary for land ocean stratigraphic and paleoclimatic correlations however the apparent timing and duration of the mb boundary remain controversial in chinese loess deposits due to its inconsistent stratigraphic position and the uncertain chronologies here we synthesized high resolution paleomagnetic records from four loess sequences in the central chinese loess plateau and synchronized the loess paleosol chronology by matching the grain size variations to orbitally tuned grain size time series the synthesized paleomagnetic results reveal consistent features of the mb transition in chinese loess including the stratigraphic position l 8 s 8 transition timing 808 826 ka duration 14 16 ka and rapid directional oscillations compared with the mb transition in marine records 770 775 ka the timing of the mb transition is relatively older and longer in chinese loess due to a complex interplay between different remanence acquisition mechanisms which occurred during the course of postdepositional physical and chemical processes

geomagnetic excursions recorded in chinese loess in the last 70 000 years detailed paleomagnetic investigations conducted on the malan loess l1 at a well dated section weinan in the chinese loess plateau revealed two distinct anomalous directional intervals adi accompanied by lower relative paleointensity rock magnetic properties and anisotropy of magnetic susceptibility ams of samples taken from sedimentary horizons carrying the anomalous directions are similar to those taken from outside the anomalous directional intervals thus the anomalous directions are interpreted as geomagnetic excursions based on 14c and tl data the excursions occurred at 27 1 26 0 and 46 8 37 4 kyr b p the ages are consistent with the ages assigned to the mono lake excursion mle and laschamp excursion le respectively the result indicates that the mle and le were independent geomagnetic excursions the morphology of le at this section suggests that the le might be an aborted geomagnetic reversal

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

can relative paleointensities be determined from the normalized magnetization of the wind blown loess of china although the wind blown loess and interbedded paleosol sequence in the loess plateau of china has been extensively studied and regarded as one of the most complete terrestrial records of both the geomagnetic field and climatic changes during the quaternary little attention has been paid to it as a source of relative paleointensity in this study we examine the malan loess l1 the last glacial sediments in the lingtai section to determine whether loess is able to reliably record the relative paleointensity toward that end both the conventional normalizing method as well as the pseudo thellier method tauxe et al 1995 were used in conjunction with the examination of the rock magnetic properties and natural remanent magnetization nrm rock magnetic properties of l1 show the uniformity in terms of magnetic mineralogy and grain size suggesting that they may be suitable for relative paleointensity studies normalized remanences derived from l1 show highs between both 72 66 ka and 62 51 ka and lows near 63 ka 42 ka and between 20 and 10 ka comparable results using different normalization parameters suggest that large amplitude variation of normalized magnetization may reflect the intensity variation of the geomagnetic field however comparisons of the normalized magnetization with contemporaneous marine records show some dissimilarities in particular the intensity low at 20 10 ka corresponding to the last glacial maximum is not compatible with the sint 800 composite record on the other hand spectral coherence analyses of middle and high frequency components of the normalized magnetizations suggest some climatic influence thus this apparent discrepancy may be explained by climatic changes in the loess plateau and the resulting effect on the nrm

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