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

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

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

palacomagnetic and rock magnetic investigation of late pleistocene loess deposits in belgium abstract the results of a palaeomagnetic and rock magnetic investigation of two loess profiles of late pleistocene age in tongrinne tg and rocourt rc in belgium are given we examined the possibility of using this type of sediment which is strictly speaking eolian in origin to gain information about the dynamic behaviour of the ancient geomagnetic field the influence of geological processes especially those known to occur in periglacial environments were examined a field test was applied to the upturned strata near a fossil frost wedge of weichselian age in tongrinne to demonstrate that at least some large directional changes found in periglacial sediments and which could be mistaken for excursions cannot be attributed to the earth s magnetic field but are due to mechanical disturbances the drm arm method and the requirements for uniformity by levi et al to obtain relative geomagnetic palaeointensities were tested on those loess deposits which contain a large silt fraction

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

paleoclimatic evolution of holocene loess and discussion of the sensitivity of magnetic susceptibility and median diameter climate changes are closely related to peoples livelihoods holocene climate changes are therefore particularly important to humans it is known that eolian loess deposits in china are one of the best geological records for paleoclimatic change as it is the top layer of loess sequence holocene loess may be easily disturbed by human activities for that reason it has not been studied thoroughly recently a continuously deposited high resolution and well preserved holocene loess section has been found in baicaoyuan huining county gansu province located in the northwest edge of the chinese loess plateau the magnetic and grain size signals of the section reflect holocene paleoclimate variations and are comparable to the stalagmite oxygen isotope records of sanbao and hulu caves they consistently record the general evolution process of holocene paleoclimate in eastern asia during the period 14 2 12 8 ka bp last glacial maximum turned into holocene transition phase during 12 8 11 5 ka bp the climate was cold and dry dominated by the younger dryas between 11 5 and 10 5 ka bp the climate rapidly turned warm and wet and 10 5 5 2 ka bp was the holocene optimum period during which some relatively dry cold events could be observed since 5 2 ka bp the climate slowly became dry and cold in summary the climate showed a general drying trend from the early holocene to the late holocene which was controlled by variation of solar radiation compared with magnetic susceptibility the median diameter curve fluctuates frequently some climate events were recorded by the median diameter curve but not observed in the magnetic susceptibility curve it is suggested that the grain size parameter is more sensitive to paleoclimate changes than susceptibility at the baicaoyuan section during the holocene

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

the chinese loess in xifeng i the primary study on magnetostratigraphy of a loess profile in xifeng area gansu province summary 1848 specimens were collected systematically from about 900 horizons in the loess profile of xifeng gansu province china the magnetostratigraphy is established on the basis of the palaeomagnetic results after thermal cleaning usually at 300 c this result has been found to be generally consistent with previous work in luochuan the susceptibility varies regularly with the appearance of loess and palaeosol in the profile and thus could be used as a criterion to classify the loess and palaeosol sequence

are chinese loess deposits essentially continuous 1 we conducted a paleomagnetic examination of the last glacial loess of three representative profiles along an east west transect in the central loess plateau in order to assess the continuity of chinese loess the results show that the xifeng and luochuan profiles record the laschamp geomagnetic excursion but with different morphologies together with the published results from weinan southern plateau our results suggest that sedimentation of the last glacial loess in the central southern plateau was continuous at the time scale equivalent to the duration of the laschamp excursion 2 kyr but probably episodic at finer time scales 2 kyr no geomagnetic excursion was found at the yichuan profile near the yellow river valley where loess accumulation may be strongly affected by local environmental changes and thus may have been discontinuous both site location and time scale therefore need to be considered when considering continuity of chinese loess

magnetic stratigraphy constraints on the matuyama brunhes boundary recorded in a loess section at the southern margin of chinese loess plateau although the matuyama brunhes boundary mbb in the chinese loess plateau clp is very important in reliably correlating quaternary loess with other sediments in the world particularly with marine and polar ice cores its exact stratigraphic position remains controversial previous investigations usually placed the mbb between paleosol unit s8 and loess unit l8 in various locations to better understand the spatial differences in the mbb position a high resolution paleomagnetic study was conducted in a loess section of the lantian basin at the southern margin of the clp the results show that the mbb is situated in the middle of the relatively weak paleosol unit s7 consistent with a recent report on the mbb based on a be 10 study from the xifeng and luochuan loess sections of the central clp however the regional anomalously low magnetic susceptibility in paleosols s7 and s8 indicates that it is more reliable to determine the paleoclimate boundaries between loess and paleosol horizons of this segment with median grain size then the mbb in the yushan section can be correlated with the bottom of paleosol s7 corresponding to the older part of interglacial marine isotope stage 19 this result temporally reconciles the striking discrepancy of the position of the mbb recorded in between loess and other typical sedimentary sequences and further confirms that the stratigraphic position of the mbb could spatially vary to a certain extent due to regional sedimentary or paleoclimatic conditions in the marginal areas of the clp in the yushan section the high frequency variations of paleomagnetic directions during a long period of similar to 31 ka before the mbb however could not be attributed to a genuine response to the true geomagnetic behaviour moreover the climate offset defined by the magnetic susceptibility and median grain size of the section can be preliminarily attributed to the regional geology and paleoenvironment background a multiproxy based stratigraphic division is considered very necessary when paleomagnetic and climatic boundaries are defined exactly in a specific area of the southern clp

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

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

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

sedimentation and diagenesis of chinese loess implications for the preservation of continuous high resolution climate records chinese loess has been extensively utilized to produce continuous and high resolution climate records of the late cenozoic such work assumes uninterrupted loess deposition and limited diagenesis here closely spaced optically stimulated luminescence osl dates are used to characterize the holocene and late pleistocene sedimentation histories of three sites across a nw se transect of the chinese loess plateau the results suggest that sedimentation is episodic at subglacial interglacial time scales with rates rapidly varying within units and between sites unconformities noneolian deposition and mixing of sediments also appear to be common existing understanding of loess deposition therefore requires reexamination while previous reconstructions of rapid climate change not dated using absolute methods should be regarded with caution loess deposits may still yield detailed climate records from specific high sedimentation rate strata and evidence for rapid climate change may yet be obtainable by targeting these units through absolute dating the rapid changes in sedimentation presented here indicate the east asian monsoon has the capacity to vary on millennial scales

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

compaction and inclination shallowing in deep sea sediments from the pacific ocean progressive downcore shallowing of the remanent inclination has been observed in a 120 m section of marine sediments at deep sea drilling project site 578 in the northwest pacific representing the past 6 5 m y near the top of the section the average inclination corresponds to the expected geocentric axial dipole value of 53 but shallows down section by about 6 to 8 northward translation of the pacific plate accounts for only about a quarter of the inclination shallowing moreover no inclination shallowing was observed at neighboring site 576 which has considerably lower sedimentation rates and the last 5 m y are represented by a 26 m sedimentary section we correlate the inclination shallowing at site 578 with an average decrease in porosity of 3 4 the porosities in the top 26 m at site 576 are slightly higher than at site 578 and show no definite trend downhole we interpret these results to suggest that both the downhole inclination shallowing and decrease of porosity in site 578 are caused by sediment compaction compaction does not play a significant role for the section at site 576 due to its much shorter length

compaction induced inclination shallowing of the post depositional remanent magnetization in a synthetic sediment summary a synthetic sediment comprised of kaolinite distilled water and either equidimensional or acicular magnetite was given a post depositional remanent magnetization pdrm by being stirred in a magnetic field this sediment was compacted under pressures which varied continuously from 0 to 0 14 mpa in a water tank consolidometer and to higher pressure steps 2 53 mpa in a standard soil consolidometer compaction took place in the same magnetic field in which the sample was given its pdrm the compaction caused shallowing of the sample s magnetic inclination this shallowing was found to be a function of the sample s initial magnetic inclination and the degree of sample compaction tan i 1 aav tan lo where ir is the remanent inclination after compaction av is the volume change i is the initial magnetic inclination and a is an empirically derived constant the data show a maximum inclination shallowing of 12 for an initial inclination of 54 in good agreement with the maximum inclination shallowing predicted by the above equation we propose an electrostatic mechanism to be the cause of the inclination shallowing in this model positively charged magnetite grains adhere to the surface of negatively charged clay grains with their long dimension parallel to the clay grain s surface as the clay grains become reorientated due to compaction the easy axes of magnetization are rotated away from the axis of compression alternating field demagnetization data reveal that our samples have shallower characteristic magnetizations than their post compaction nrms implying that the smaller higher coercivity grains are most affected by the compaction process these data support our model since a surface charge mechanism for inclination shallowing would predict that the smallest magnetic grains would be preferentially affected

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

detrital magnetizations from redeposition experiments of different natural sediments abstract we carried out several experiments with carbonate rich and clay rich sediments that were redeposited in plastic cubes or in 1 meter long cylindrical tubes with the aim of investigating the sensitivity of the depositional remanent magnetization drm to various environmental and physical parameters in contrast to previous studies we did not observe any difference in the drm acquired with or without saline water for either kind of sediment taking advantage of a gelatin that fixes the position of particles in suspension we were able to measure the magnetization of suspended sediments within water filled columns and thus to test the degree of alignment of magnetic grains the magnetization of clay rich sediments was not far from saturation but the magnetization was considerably reduced after deflocculation because the large flocs that favored alignment of magnetic grains were destroyed by the deflocculant similarly the large flocs found at the bottom of the deposition tubes also reveal an efficient magnetic alignment in contrast the weak magnetization of the carbonate rich sediments with small flocs yielded accurate and well grouped magnetization directions that were not sensitive to deflocculation only a fraction of magnetic grains was mechanically oriented by the field in this case because of the weak net magnetic moments of the grains embedded within small flocs the drm was constrained by redeposition in plastic cubes performed without gelatin and with different field intensities the drm intensity of the carbonate rich sediments is linearly related to field strength which attests to their suitability for studies of relative paleointensity tests performed with mud from several stratigraphic levels in different marine sediment cores failed to reveal a significant influence of carbonate content on drm which suggests a relatively minor role of lithological changes on records of relative paleointensity

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

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

paleomagnetic and mineral magnetic investigation of the baicaoyuan loess paleosol sequence of the western chinese loess plateau over the last glacial interglacial cycle and its geological implications closely spaced paleomagnetic and mineral magnetic studies were conducted on the baicaoyuan bcy loess paleosol sequence since the last interglacial in the western chinese loess plateau the multiproxy records of mineral magnetic parameters clearly indicate loess paleosol alternations with the fine structure of pedostratigraphy over the last glacial interglacial cycle however the paleomagnetic results reveal the absence of any short lived geomagnetic excursions in the bcy section this absence could be interpreted by two alternative mechanisms such as magnetic overprinting and discontinuous sedimentation the absence of the mono lake and laschamp excursions in the loess unit l1 is probably due to the viscous overprinting of the remanence carried by coarse grained magnetite of eolian origin and or to the episodic nature of loess accumulation during the last glacial period however discontinuous sediment accumulation may more probably lead to the absence of the blake excursion in the last interglacial soil s1 as well as of the mono lake and laschamp excursions in the last glacial loess l1 at the bcy section together with previously published paleomagnetic results from other sections over the loess plateau the findings probably further indicate the discontinuous nature of loess accumulation in the western loess plateau where sedimentation of the last interglacial soil may be episodic at the timescale equivalent to the duration of the blake geomagnetic excursion 4 6 ka much longer than the timescale for episodic loess sedimentation in the last glacial loess of both the western and eastern loess plateau due to the reduced sediment accumulation rate compared to that during the last glacial period thus sediments with relatively high sedimentation rate in some parts of the western loess plateau do not consistently guarantee the faithful recording of high frequency geomagnetic variations

inclination flattening and the geocentric axial dipole hypothesis abstract william gilbert first articulated what has come to be known as the geocentric axial dipole hypothesis the gad hypothesis is the principle on which paleogeographic reconstructions rely to constrain paleolatitude for decades there have been calls for permanent non dipole contributions to the time averaged field recently these have demanded large contributions of the axial octupole which if valid would call into question the general utility of the gad hypothesis in the process of geological recording of the geomagnetic field earth filters distort the directions many processes for example sedimentary inclination flattening and random tilting can lead to a net shallowing of the observed direction therefore inclinations that are shallower than expected from gad can be explained by recording biases northward transport or non dipole geomagnetic fields using paleomagnetic data from the last 5 million years from well constrained lava flow data allows the construction of a statistical geomagnetic field model such a model can predict not only the average expected direction for a given latitude but also the shape of the distribution of directions produced by secular variation the elongation of predicted directions varies as a function of latitude from significantly elongate in the up down direction at the equator to circularly symmetric at the poles sedimentary inclination flattening also works in a predictable manner producing elongations that are stretched side to side and the degree of flattening depending on the inclination of the applied field and a flattening factor f the twin tools of the predicted elongation inclination relationship characteristic of the geomagnetic field for the past 5 million years and the distortion of the directions predicted from sedimentary inclination flattening allows us to find the flattening factor that yields corrected directions with an elongation and average inclination consistent with the statistical field model the method can be tested using sediments deposited in a known field application of the elongation inclination correction method to two magnetostratigraphic data sets from red beds in asia and pakistan brings the inclinations into agreement with those predicted from modern gps measurements and from global paleomagnetic data there appears to be no compelling reason at this time to abandon the geocentric dipole hypothesis which has provided such an excellent working model for so long

evaporation and recondensation of sodium in semarkona type ii chondrules abstract we have investigated the na distributions in semarkona type ii chondrules by electron microprobe analyzing olivine and melt inclusions in it mesostasis and bulk chondrule to see whether they indicate interactions with an ambient gas during chondrule formation sodium concentrations of bulk chondrule liquids melt inclusions and mesostases can be explained to a first approximation by fractional crystallization of olivine pyroxene the most primitive olivine cores in each chondrule are mostly between fa8 and fa13 with 0 0022 0 0069 0 0013 wt na2o type iia chondrule olivines have consistently higher na contents than olivines in type iiab chondrules we used the dependence of olivine liquid na partitioning on feo in olivine as a measure of equilibration extreme olivine rim compositions are fa35 and 0 03 wt na2o and are close to being in equilibrium with the mesostasis glass olivine cores compared with the bulk chondrule compositions particularly in iia chondrules show very high apparent dna indicating disequilibrium and suggesting that chondrule initial melts were more na rich than present chondrule bulk compositions the apparent dna values correlate with the na concentrations of the olivine but not with concentrations in the bulk melt we use equilibrium dna to find the na content of the true parent liquid and estimate that type iia chondrules lost more than half their na and recondensation was incomplete whereas type iiab chondrules recovered most of theirs in their mesostases glass inclusions in olivine have lower na than expected from fractionation of bulk composition liquids and mesostases have higher na than expected in calculated daughter liquids formed by fractional crystallization alone these observations also require open system behavior of chondrules specifically evaporation of na before formation of melt inclusions followed by recondensation of na in mesostases within this record of evaporation followed by recondensation there is no indication of a stage with zero na in the chondrules which is predicted by models for shock wave cooling at canonical nebular pressures suggesting high pt the high na concentrations in olivine and mesostases indicate very high pna while chondrules were molten this may be explained by local very high particle densities where type ii chondrules formed the high pt pna and number densities of chondrules implied suggest formation in debris clouds after protoplanetary collisions as an alternative to formation after passage of shock waves through large particle rich clumps in the disk encounters of partially molten chondrules should have been frequent in these dense swarms however in many ordinary chondrites like semarkona cluster chondrites compound chondrules are not abundant but instead chondrules aggregated into clusters chondrule melting cooling and clustering in dense swarms contributed to rapid accretion possibly after collision by fallback on the grandparent body and by reaccretion as a new body downrange

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

age and palaeoclimatic significance of the loess of lanzhou north china extending in a broad arc that abuts the sandy non gobi deserts the loess plateau of northern china1 3 is one of the most massive accumulations of loess in the world the loess sequence is typically characterized by an alternation of silty or sandy loess with more clay rich palaeosols these alternations in conjunction with their enclosed faunas and distinctive mineralogies have been interpreted as reflecting pleistocene glacial interglacial cycles3 8 because it holds implications for the climatic and anthropological history of china9 10 the definition of a reliable chronological framework for loess deposition is of great interest recent palaeomagnetic studies4 5 have indicated that loess deposition in shaanxi province commenced 2 4 myr ago to assess the synchrony of loess accumulation across the loess plateau we have dated a 330 m thick loess sequence near lanzhou gansu province the magnetostrati graphical results reported here indicate that the base of this loess succession dates from 1 3 my r ago this young age in comparison to the shaanxi sequence is attributed to uplift along the northern fringe of the tibetan plateau11 that precluded early pleistocene loess preservation in this mountainous region palaeosols in the basal loess occur on average once every 25 kyr suggesting that climates conducive to soil forming events may have been modulated by orbital precession in the early pleistocene

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

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

a long term increasing aridification and cooling trend at the chinese loess plateau during the pliocene the development of asian aridification and cooling is a research hotspot nowadays loess red clay sequence in the chinese loss plateau provides an excellent terrestrial archive of such paleoclimatic evolution this paper applied the magnetic unmixing technique to isolate the magnetic components of the red clay sequence for the first time the jiaxian section in the north edge of chinese loess plateau is sensitive to paleoclimate changes because it is in the transitional zone of the monsoon and non monsoon area systematic magnetic measurements and xrd analysis consistently indicate that the magnetic components of red clay are predominantly magnetite maghemite and hematite the strongly developed paleosols exhibit high content of soft magnetic minerals higher ms and mrs and lower bc and bcr while the weakly developed paleosols and loess samples display the opposite features with increasing alternating field the contributions of soft components to sirmx mt are cleaned gradually sirm is the saturation isothermal remanent magnetization sirmx mt represents the residual sirm after alternating field demagnetization at x mt the significant contributor to sirm100 mt is hematite an indication of the intensity of chemical weathering variations in the ratios of sirm100 mt sirm sirm100 mt sirm60 mt and sirm100 mt sirm30 mt document a long term up section decreasing trend during the last 5 2 6 ma with similar trends to the long term up section increase in 18o values of marine records decreased hematite content indicates weakened chemical weathering which further implies an increasing aridification and cooling and a gradually weakened summer paleomonsoon mainly in the depositional area chinese loess plateau in some loess layers where hematite is mainly of aeolian origin the mineral magnetic signals may indicate aridification and cooling in the asian interior source region this long term aridification and cooling trend is probably attributed to the comprehensive activity of ongoing global cooling progressive uplift of the tibetan plateau and changes in land sea distribution

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

reinterpreting climate proxy records from late quaternary chinese loess a detailed osl investigation abstract numerous authors have utilised physical properties of chinese loess and red clay deposits to develop apparently detailed and continuous past climate records from the miocene into the holocene many of these studies have further suggested that the principal climatic agent responsible for the aeolian emplacement and diagenesis of chinese loess the east asian monsoon has fluctuated rapidly on millennial to sub millennial timescales in concert with dramatic changes in the north atlantic dansgaard oeschger cycles and heinrich events and the western pacific el nino southern oscillation much of this evidence is based on reconstructions and age models that are tied to assumptions concerning the nature of loess sedimentation and diagenesis for example the belief that loess sedimentation can be viewed as essentially continuous some authors have however cast doubt on these assumptions and suggest that the application of radiometric techniques may be required to determine their validity recent studies utilising optically stimulated luminescence osl methods have reinforced these doubts and here osl dates obtained at 10 cm intervals from three sites along a transect across the chinese loess plateau have been used in combination with climate proxy evidence to test the existing assumptions that underpin many palaeoclimatic reconstructions from loess in this way the first time continuous and independently dated late quaternary climate reconstruction is developed from loess the data indicate that sedimentation is episodic and that once emplaced loess is prone to pedogenic disturbance diagenetic modification and in some cases erosion the relationships between proxies and sedimentation rates are also assessed and climatic interpretations based on different age models compared the implications of these findings for reconstructions of climate from loess are explored and comparisons are made between the developed palaeoclimate records and evidence from ice and ocean cores this exercise also highlights important information concerning the relative influence of forcing mechanisms behind east asian monsoon change over the late quaternary

grain size of quartz as an indicator of winter monsoon strength on the loess plateau of central china during the last 130 000 yr the chinese loess paleosol sequence constitutes an important record of variations in asian monsoon climate over the past 2 4 myr magnetic susceptibility of loess and paleosols has been used as a proxy for summer monsoon intensity while median grain size has been regarded as a measure of the strength of winter monsoon winds that were responsible for most of the dust transport however median grain size is only an approximate index of winter monsoon strength because both paleosols and loess have been modified to various degrees by weathering processes that have produced pedogenic clay the quartz component of loess and paleosols is largely unaffected by weathering processes and therefore constitutes a more reliable proxy index of monsoon wind strength median grain size qmd and maximum grain size qmax values of monomineralic quartz isolated from the loess paleosol section at luochuan in the central loess plateau are characterized by two main intervals during the last ca 130 000 yr when these parameters were significantly greater than 9 and 85 m respectively and three main intervals when they were lower the data imply that the winter monsoon weakened during the intervals with low qmd and qmax values which coincide with marine oxygen isotope stages 5 3 and 1 and was strongest ca 67 000 and 20 000 yr ago during isotope stages 4 and 2 however both quartz grainsize records display second order high frequency high amplitude variations which are lacking in the magnetic susceptibility record that imply rapid and significant changes in atmospheric conditions that affect dust transport and deposition

the acquisition of post depositional detrital remanent magnetization in a variety of natural sediments summary post depositional detrital remanent magnetization pdrm is the primary means whereby many sediments acquire their palaeomagnetic signal we have studied the acquisition of this magnetization in a variety of natural sediments our technique involves determining the magnetic direction recorded by a sediment as a function of the water content present in the sediment when the sediment experiences a change in the direction of the applied magnetic field most of the sediments used in this study were collected wet from natural environments and were preserved in their original state until they were used in the experiments grain sizes were measured by the settling tube method which led to the determination of the clay silt and sand fractions in each sediment isothermal remanent magnetization acquisition studies indicated that the predominant magnetic carriers were magnetite in the pdrm acquisition studies two distinct modes of behaviour were found for sediments with a sand content less than 60 per cent the original direction of magnetization was preserved regardless of the water content such behaviour is not consistent with a theoretical model which assumes that at high water contents the magnetic carriers remain mobile within fluid filled voids and hence are able to realign along a new magnetic field direction for sediments with a sand content in excess of 60 remagnetization along a new magnetic field direction occurred as expected provided the sediments were sufficiently wet studies of natural sediments and corresponding samples of dried and reconstituted sediments have demonstrated that the magnetic characterization of a sediment can be reliably determined even for older desiccated sediments

inter profile correlation of the chinese loess paleosol sequences during marine oxygen isotope stage 5 and indications of pedogenesis abstract low field magnetic susceptibility has been widely used to determine the pedostratigraphy of the chinese loess paleosol sequences however uncertainties remain in correlating between the loess magnetic susceptibility and the marine oxygen isotope records because susceptibility variations are affected by both global and local paleoclimatic changes to provide a more sound paleoclimatic interpretation of magnetic susceptibility variations age models across marine oxygen isotope stage mis 5 for the jiuzhoutai jzt and yuanbao yb sections western chinese loess plateau were constructed through an integrated approach by linking the major pedostratigraphic boundaries of the loess profiles to the specmap oxygen isotope curve and by correlating relative magnetic paleointensity records with both the sint800 global paleointensity stack from marine sediments and 36cl records from the grip ice core results indicate good correlation of sirm60 mt a residual remanence of saturation isothermal remanent magnetization after a 60 mt alternating field demagnetization variations between these two sites which agree well with fluctuations in subtropical atlantic sea surface temperatures all cooling events recorded by ice core and atlantic marine sediments within mis5 have counterparts in sirm60 mt sirm60 mt is partially controlled by the degree of low temperature oxidation which is strongly temperature dependent however strong pedogenesis can decrease sirm60 mt due to further oxidation of partially oxidized magnetites above some critical points therefore we propose that sirm60 mt is best suited to record paleotemperature changes in loess profiles from the western chinese loess plateau where pedogenesis is the weakest furthermore by inter profile correlation between the yb and jzt sections we note that the seemingly uniform sub paleosol unit with a broad susceptibility peak previously assigned to mis5c between 34 4 and 37 4 m in the yb profile actually consists of two independent units lower part of s1l1 mis5b and s1s2 mis5c this indicates that susceptibility values can be strongly affected by local factors e g mainly precipitation therefore beside the simplistic traditional paleoclimatic interpretation of variations in loess susceptibility involving only cold dry and warm humid scenarios cold humid and warm dry scenarios should also be considered

magnetostratigraphy of the baoji loess paleosol section in the north central china loess plateau the baoji section is judged to be the most complete accessible quaternary loess paleosol section in the north central china loess plateau it is located 5 km north of the city of baoji it contains 37 paleosols and was recently named the type pedostratigraphic section the pedostratigraphic units geosols of the baoji section can be correlated throughout the loess plateau by a combination of magnetostratigraphy and character position and association of pedostratigraphic units magnetic properties were determined from 483 samples taken from 159 m of loess that overlies the red clay formation all major reversals that occurred in the last 2 5 ma were recorded the brunhes matuyama boundary occurs at a depth of 58 5 m within loess unit l8 the jaramillo subchron between 72 0 m in loess unit l10 and 76 4 m in paleosol s11 the olduvai subchron between 125 5 m in loess unit l25 and 131 5 m in loess unit l27 and the gauss matuyama boundary at 157 8 m within loess unit l33 just above the contact with the red clay formation this is the only known site where all major magnetic reversals are clearly defined in distinct field identifiable loess soil units in one section the magnetostratigraphy of the baoji section is thus recommended as the type section for the loess plateau

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

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

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

magnetic properties of sediments from major rivers aeolian dust loess soil and desert in china abstract the continent of china delivers huge terrigenous sediments to the east asian marginal seas and northwest pacific ocean by riverine and aeolian inputs which exerts a great impact on marine sedimentation primary productivity and biogeochemical cycle in this study magnetic properties of the sediments from the changjiang and huanghe river systems were investigated in order to provide potential provenance tracers besides the top soil from the loess plateau taklimakan desert sand and dust storm particles were comparatively studied to reveal the controls of magnetic properties in various depositional environments the changjiang sediment is characterized by the highest concentration of magnetic minerals and the largest variations for most magnetic parameters while the huanghe sediment has much lower content of magnetic minerals the loess soil sediment yields the highest frequency dependent susceptibility fd while the aeolian dust has similar magnetic susceptibility but higher saturation isothermal remnant magnetization and lower anhysteretic remnant magnetization arm the desert sand has the lowest values of all magnetic parameters indicating the lowest ferrimagnetic mineral concentration with coarser grain size the magnetic properties of the changjiang sediments are primarily determined by the diversity of lithology in its large drainage basin while the sediment grain size basically accounts for the variation of magnetic parameters in the huanghe sediment the gradual increase of fd towards the lower reaches suggests it may potentially indicate grain size fractionation in the catchments although some magnetic parameters can apparently discriminate the origins of fluvial and aeolian sediments reliable magnetic proxy for distinguishing sediment origins in marine environment can only be established if hydrodynamic differentiation and post depositional diagenetic alteration of magnetic minerals are fully understood

on the interpretation of chinese loess as a paleoclimate indicator the records of wind blown dust i e loess in china are some of the most important terrestrial records of past climate changes stretching back over the last 10 ma in the paleoclimate literature intervals of increased dust generation have been almost always interpreted as being associated with more intense or prolonged wintertime conditions here it is shown that in accordance with modern observations dust outbreaks in asia are predominantly springtime phenomena during spring frequent cyclogenesis in the lee of the mongolian altai and the passage of strong cold fronts produce the intense windstorms that loft and entrain dust into the air the meteorology governing such outbreaks is likely robust in past climates contrary to the common paleoclimate presumption it is actually the breakdown of the siberian high that permits the dust producing windstorms to occur the importance of cold fronts in generating such windstorms suggests that cooling of high latitude climate during the miocene or during glacial intervals might play a significant role in the signal recorded in the loess deposits the unique springtime factors that generate dust storms are an example of why the common partitioning of asian climate into a winter and summer monsoon is oversimplified and can be misleading

asynchronous records of brunhes matuyama reversal in marine sediments and chinese loess review and discussion abstract geomagnetic polarity reversals are generally considered to occur synchronously around the world and are commonly used as time markers to correlate events between different sediment archives they also play an indispensable role for understanding the mechanisms which generate the earth s geomagnetic field nevertheless in the case of the most recent reversal the brunhes matuyama b m reversal 780 ka comparison of paleomagnetic studies in chinese loess paleosol sequences versus marine cores reveal a marked discrepancy in timing of this event this timing offset has led to uncertainties in correlating the chinese loess paleosol sequences with other global climate records this paper briefly reviews the history of study on the b m reversal both in marine and chinese loess sediments and reviews several contentious explanations of the aforementioned b m boundary offset finally the most recent progress of paleogeomagnetic field variation studies using cosmogenic 10 be in chinese loess is introduced with the purpose of suggesting a potential way to address this b m boundary timing offset issue

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

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

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

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

the impact of glaciation river discharge and sea level change on late quaternary environments in the southwestern kara sea sedimentary records from the southwestern kara sea were investigated to better understand the extent of the last glaciation on the eurasian arctic shelf sea level change and history of the ob and yenisey river discharge sediment core and seismic reflection data indicate that the quaternary depositional sequence in the southwestern kara sea consists of glacial glaciomarine and marine sedimentary units glaciogenic sediments in the deep novaya zemlya trough are presumably related to the last glacial maximum lgm whereas further east they may represent an earlier glaciation thus it is inferred that the southeastern margin of the lgm barents kara ice sheet was contained in the southwestern kara sea east of the novaya zemlya trough changes in mineralogical foraminiferal and stable isotopic composition of sediment cores indicate that riverine discharge strongly influenced sedimentary and biotic environments in the study area during the late weichselian and early holocene until ca 9 ka consistent with lowered sea levels subsequent proxy records reflect minor changes in the holocene hydrographic regime generally characterized by reduced riverine inputs