magnetic susceptibility of soil an evaluation of conflicting theories using a national data set magnetic susceptibility values for topsoils across england arc combined with data for soil type geochemistry and concentrations of magnetotactic bacteria in order to evaluate different theories for explaining soil magnetism strongly magnetic soils in unpolluted areas are found over weakly magnetic substrates and are dominated by ultrafine superparamagnetic grains magnetotactic bacteria are present in insufficient concentrations to account for strongly magnetic soils and crop burning is discounted as a major factor a small number of samples show high values associated with either airborne magnetic particulates from pollution or residual primary ferrimagnetic minerals from igneous substrates the results are used to construct a new mechanism for the formation of secondary ferrimagnetic minerals that links abiological weathering and biological fermentation processes the fundamental driving force in the mechanism is fe supply which may be linked to climate observed causative associations between climate and the magnetic susceptibility of loess palaeosol sequences are supported by the findings

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

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

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

quantitative extraction and analysis of carriers of magnetization in sediments we have applied an integrated procedure for quantitative magnetic mineral extraction based on the separation method of petersen von dobeneck vali 1986 to a range of sediment types to examine the efficiency and representative nature of the extraction process carriers of magnetization have been identified by rock magnetic measurements microscopy and x ray diffraction quantification of the extraction efficiencies is achieved by before and after extraction rock magnetic measurements susceptibility anhysteretic and isothermal remanences these magnetic measurements show that our modified extraction method extracts large proportions of the magnetization carriers in a range of sediment types e g over 75 per cent for magnetite dominated sediments the extraction efficiency is dependent on the sample magnetic mineralogy and whether the magnetic grains occur as discrete grains or as inclusions within host grains susceptibility extraction efficiencies are strongly dependent on whether the susceptibility is of paramagnetic or ferrimagnetic origin the amount of material recovered in the extract shows some inverse correlation with the density of the sediment suspension used during extraction in terms of the mineralogies extracted we identify a diverse and complex range of mineral assemblages all sizes of discrete grains of magnetite are extracted including single domain and superparamagnetic grains and chains of bacterial magnetite other commonly extracted iron and titanium oxides are haematite and ilmenite ferrimagnetic chromites and sulphides were also obtained from some samples considerable amounts of quartz and feldspar are extracted due to the presence of magnetic inclusions within these diamagnetic host grains in the deep sea sediments we examined feldspars constitute a large proportion of the extracts but are significantly less abundant in other sediments where quartz is dominant a wide variety of paramagnetic minerals was identified in the extracts including pyroxenes amphiboles chlorites micas mg cr spinels garnets ti oxides apatites tourmaline and zircon many of which contain ferrimagnetic inclusions possibly less than 0 1 m in grain size dissolution of ultrafine grains of magnetite during pre extraction carbonate dissolution as suggested by sun jackson 1994 does not occur in our samples

paleoclimatic significance of the mineral magnetic record of the chinese loess and paleosols the origins of the magnetic susceptibility variations of the chinese loess and paleosols are explored by scanning and transmission electron microscopy of magnetic extracts and by magnetic modeling of magnetic hysteresis data to provide quantified estimates of the major magnetic components microscopy identifies several distinct size and shape characteristics in the magnetic carriers lithogenic magnetites intact and abraded dominate the coarse grained magnetic fraction the smallest of the coarse grains is 2 i m the remaining magnetic materal is ultrafine in size with two types of magnetite particles present type a particles strongly resemble soil magnetites produced by inorganic precipitation type b particles which occur rarely are probably bacterial in origin quantitative modeling of these magnetic assemblages shows that over 90 of the susceptibility variations is accounted for by the superparamagnetic magnetite component compared to the loess units the paleosols are richer in magnetite particularly of superparamagnetic size and have a threefold higher ratio of magnetite to hematite we identify pedogenic formation of magnetite as the major contributor to the loess magnetic record matching this record against other paleoclimatic records we find an extremely high correlation with the standard 18o record the chinese loess sequences record a very high resolution magnetic stratigraphy directly related to changing climate

a late quaternary magnetic record of tunisian loess and its climatic significance magnetic measurements for a 10 m loess palaeosol sequence from the matmata plateau in southern tunisia show significant down profile variations in concentrations of pedogenic ferrimagnetic minerals the sequence appears to correlate with regional and global records of climate change over the past 70 kyr there is strong evidence from comparisons of mineral magnetic properties in modern soils and palaeosols that the sequence holds a proxy record of winter precipitation

normalized magnetic parameters and their applicability to paleomagnetism and environmental magnetism using simple theory one can show that normalized magnetic parameters i e one primary magnetic measurement divided by a second are not independent of the concentration of magnetic minerals in an assemblage except in a few special cases in the common situation when the number of minerals in an assemblage is unknown these parameters can only be used to show a lack of change in domain size or mineralogy

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

magnetic properties derived from a loess section at the northern piedmont of tianshan mountains xinjiang china and their paleoenvironmental significance

characterisation of soils by mineral magnetic measurements processes of iron mineral authigenesis diagenesis and dissolution in interaction with primary inputs of iron minerals act to produce vertical differentiation of soil magnetic properties thus magnetic iron oxides in common with other iron forms may both respond to and reflect soil forming processes investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance in several areas of environmental study for example the observed magnetic variation can be used to discriminate between individual soils and soil horizons for the purposes of soil surveying sediment tagging and tracing and empirical modelling of source sediment linkages this paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance the effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters including magnetic susceptibility frequency dependent susceptibility and anhysteretic and saturation remanences contrary to current pedological thought but in accord with other magnetically based studies e g mullins the presence of magnetite maghemite typically of superparamagnetic single domain grain size within soils has been found to be an extremely widespread phenomenon its contemporary neoformation within the soil environment is inferred however the input of artificially generated ferrimagnetic material is indicated for one of the magnetically enhanced soils examined here and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic processes of magnetic enhancement are inferred from the data presented discrimination of industrially derived magnetic particles appears possible on the basis of their very low levels of frequency dependent susceptibility fd low anhysteretic remanence arm and dissociation from the finer clay sized not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons within gleyed waterlogged soils processes of magnetite dissolution appear to be grain size specific preferential reduction of those grains of ultrafine superparamagnetic and fine single domain magnetic grain size is indicated where soil forms a major contributor to catchment sediment loads this has significance in the area of source sediment modelling soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments further only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record

vegetation and seasonality shifts during the late quaternary deduced from 13c 12c ratios of grazers at equus cave south africa abstract season of rainfall can be a major influence on the relative proportions of c3 and c4 grasses in a landscape since their relative distribution is essentially constrained by temperatures during the growing season cool growing seasons favor c3 grasses whereas warm ones favor c4 grasses 13c 12c ratios in tooth enamel carbonate of grazers from the site of equus cave northern cape province south africa were used to determine vegetation and rainfall seasonality shifts during the late quaternary the results show shifts in the relative proportion of c3 and c4 grasses during the latter stages of the pleistocene and domination by c4 grasses in the holocene the data show that the northwestern cape did not fall within a winter rainfall zone during this period as has been postulated in recent climate models instead a more complex situation is suggested in which at least two periods of enhanced winter rainfall occurred within a predominantly summer rainfall regime

secondary ferrimagnetic minerals in welsh soils a comparison of mineral magnetic detection methods and implications for mineral formation summary a range of low temperature and isothermal magnetic measurements are used to identify the secondary ferrimagnetic mineral sfm grain sizes in 10 representative soil samples from wales a comparison of percentage frequency dependent susceptibility fd percentage and low temperature remanence measurements shows that they are sensitive to different ranges of superparamagnetic sp grains the relative distributions of sp grains and stable single domain ssd grains are similar in nine of the samples typical distributions for soils dominated by sfms are 20 30 per cent ssd and 70 80 per cent sp multidomain md grains were not detected in the samples studied there is evidence that some soils contain significant numbers of ultrafine sp grains 0 010 m that are not detected by low temperature remanence measurements at 20 k and which will have the effect of depressing values of low field susceptibility lf and fd percentage a mixing model suggests that fd percentage may be used semi quantitatively to estimate the proportion of sp grains in a sample the positively skewed grain size distributions strongly suggest a mechanism of sfm formation that is driven by processes at the 10 8 m scale thus supporting weathering and fermentation as controlling processes rather than the degradation of ssd bacterial magnetosomes and primary minerals

transmission electron microscope and rock magnetic investigations of remanence carriers in a precambrian metadolerite a precambrian metadolerite dyke has two distinct types of remanence carriers those with medium high coercivities unblocking fields of 20 120 mt and those with low coercivities unblocking fields of 15 mt optical examination reveals numerous submicron probably opaque inclusions in the plagioclase feldspar and also large opaque grains consisting of coarse oxidation exsolution intergrowths of magnetite and ilmenite all opaque phases have been examined using transmission electron microscopy together with microanalysis and electron diffraction the submicron inclusions in the plagioclase are titanomagnetites 0 x 0 14 with a size range between about 0 01 and 0 5 m and axial ratios between 1 equidimensional and about 0 3 many of these inclusions fall in the single domain field but some are probably pseudo single domain the large opaque grains contain almost pure magnetite and ilmenite and show no fine scale exsolution the magnetite regions of the intergrowths are of multidomain size and reveal multidomain structure under lorentz electron microscopy there are also some primary ilmenites containing very fine exsolved haematite and there are very fine plates of ilmenite and very elongate needles of magnetite within the augite experiments on artificial samples containing very carefully prepared separates of plagioclase and large opaque grains show that the pure plagioclase acquires a remanence with unblocking fields of 20 140 mt and blocking temperatures of 390 590 c and the large opaque grains acquire a remanence with unblocking fields of less than 15 mt but a wide range of blocking temperatures up to about 570 c it is concluded that the medium high coercivity component of remanence in the rock is carried largely or possibly entirely by the submicron magnetites within the plagioclase and that the low coercivity component is carried largely or entirely by the multidomain magnetites in the large opaques the contribution of the magnetite needles in the augite is uncertain as the rock does not contain any detectable component of remanence with the extremely high coercivities expected from their very elongate shape

quantitative estimates of palaeoprecipitation at xifeng in the loess plateau of china abstract using10be recent studies heller et al 1993 beer et al 1993 have tried to reconstruct palaeoclimatic records by separating the secondary pedogenic and primary aeolian detrital contributions to the magnetic susceptibility of chinese loess here we use a technique which monitors the temperature dependence of magnetic remanence to estimate the magnetisation and relative contribution of the superparamagnetic sp single domain sd and multidomain md ferrimagnetic components in the xifeng loess section on the assumption that the sp ferrimagnetic component is dominantly of pedogenic origin the magnetisation of this component has also been determined for samples taken from the topmost palaeosol s0 at six sites over the loess plateau and directly correlated with present day precipitation at these sites this correlation is then used to estimate quantitatively palaeoprecipitation for the topmost 16 m of the xifeng section which covers about the last 140 000 years the data indicate a considerably reduced precipitation during the glacial loess forming periods and a significantly higher level of precipitation than today during the last interglacial period

development of magnetic soil from ferroan dolomite an occurrence of maghemitic soil in association with ferroan dolomite in minas gerais is characterized by x ray diffraction magnetization measurements and mossbauer spectroscopy altered layers on outliers of dolomite which has composition ca1 04mg0 83fe0 11mn0 02 co3 2 indicate the weathering sequence ferroan dolomite calcite poorly crystallized ferric hydroxide maghemite hematite goethite the spontaneous magnetization of the maghemitic soil 65 wt fe2o3 is 8 8 jt 1kg 1 the magnetization of the soil maghemite itself is estimated to be 25 5 jt 1kg 1 which is greatly reduced from the value for pure maghemite because of isomorphous substitution

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

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

toward the discrimination of fine grained ferrimagnets by magnetic measurements in lake and near shore marine sediments the magnetic properties of low field susceptibility frequency dependent susceptibility fd and susceptibility of anhysteretic remanent magnetisation arm are used to define sets of natural soil and sediment samples within which by the criteria of king et al 1982 and maher 1988 the modal ferrimagnetic grain diameter is less than 0 07 0 1 m within this sample set two distinct groups with low and high values respectively for the quotients arm and arm fd are defined the first group includes sediment samples from sites where published studies propose a detrital origin for the fine grained ferrimagnetic content where catchment samples are available for comparison they fall within the same envelope as do clays from palaeosol samples within chinese loess this envelope of low quotient values also overlaps with the values for the fine grained synthetic magnetite samples within maher s 1988 new mt series the high quotient envelope of values includes sediments from the adriatic sea and clays from saltmarsh and shallow water marine sediments in nw britain in these cases no catchment source is postulated for the fine grained magnetite the size range of magnetite in this set appears to be almost exclusively stable single domain ssd and a bacterial origin seems likely sample sets from estuarine environments between river inflow sites and the open sea show intermediate values which in the case of the potomac are ordered by distance down river although at this stage magnetic measurements alone cannot discriminate between bacterial and fine grained detrital ferrimagnets in sediments they hold out some promise of doing so provided the distinctions proposed here can be substantiated by transmission electron microscopy