climate change during the early paleogene in the bogota basin colombia inferred from paleosol carbon isotope stratigraphy major oxides and environmental magnetism abstract paleosols in the bogota basin colombia record an increase in chemical weathering across the paleocene eocene p e transition rock magnetic properties and major element geochemistry exhibit an abrupt change in an interval identified as including the p e boundary as established by previously published biostratigraphy and a u pb date on volcanic zircons 56 2 1 6 ma during the stratigraphic interval that contains the p e transition magnetic susceptibility increases significantly with an order of magnitude higher concentration of magnetite maghemite and hematite the preponderance of pure stoichiometric magnetite in the paleosols of this interval indicates that the increase in magnetic susceptibility is due to changes in the rate of pedogenesis rather than an increase in the erosion of nearby volcanic rocks which would contribute titanomagnetite pedogenic structures preserved within the paleosols a lack of iron oxides as cement friability of the sandstones and previously published thermochronologic data are not consistent with burial diagenesis as an explanation for the origin of the magnetic mineral assemblage these enhanced pedogenesis and related chemical weathering is also confirmed by increases in the concentration of fe 2 o 3 and al 2 o 3 and loss on ignition values an increase in mean precipitation across this interval is inferred by the lack of carbonate nodules and a decrease in sio 2 in paleosols thus we hypothesize that there was an intensification of chemical weathering during the interval that contains the p e transition

magnetic minerals as recorders of weathering diagenesis and paleoclimate a core outcrop comparison of paleocene eocene paleosols in the bighorn basin wy usa abstract magnetic minerals in paleosols hold important clues to the environmental conditions in which the original soil formed however efforts to quantify parameters such as mean annual precipitation map using magnetic properties are still in their infancy here we test the idea that diagenetic processes and surficial weathering affect the magnetic minerals preserved in paleosols particularly in pre quaternary systems that have received far less attention compared to more recent soils and paleosols we evaluate the magnetic properties of non loessic paleosols across the paleocene eocene thermal maximum a short term global warming episode that occurred at 55 5 ma in the bighorn basin wy we compare data from nine paleosol layers sampled from outcrop each of which has been exposed to surficial weathering to the equivalent paleosols sampled from drill core all of which are preserved below a pervasive surficial weathering front and are presumed to be unweathered comparisons reveal an increase in magnetization in outcrops compared with core equivalents which is principally driven by secondary hematite production authigenic hematite production in outcrops presents a complication for goethite hematite based paleoprecipitation proxies where estimates will be biased toward drier climate regimes the occurrence of low coercivity minerals is more consistent between core and outcrop however we propose an alteration process for pedogenic magnetite that is observed in both core and outcrop where pedogenic magnetite becomes progressively oxidized leading to higher mean coercivities and broader coercivity distributions compared to modern pedogenic magnetite this combination of diagenetic processes and surface weathering influences the magnetic properties of paleosols despite these changes magnetic enhancement ratios from b horizons correlate with independent map estimates from geochemical proxies which suggests that paleoprecipitation information is preserved future work should continue to address these complications by developing useful protocols that isolate the magnetic properties that are most resistant to alteration and remain strong indicators of map and climate

climatic control on magnetic mineralogy during the late mis 6 early mis 3 in lake chalco central mexico abstract sediments from lake chalco in central mexico spanning from ca 150 to 35 ka ago provide evidence of paleoclimatic variability in the north american tropics associated with the end of marine isotopic stage mis 6 the transition to the last interglacial mis 5 5 ca 130 115 ka ago and part of the last glacial mis 5 4 to early mis 3 115 to 35 ka ago we applied a multiproxy approach based on the analysis of mineral magnetism diatom assemblages and major elements geochemistry the reconstructed paleoenvironmental history identify the end of the globally cool mis 6 as wetter than present with high lake level and a subsequent change to drier climates at the onset of the last interglacial ca 130 ka large amplitude changes in most of the analyzed parameters from ca 130 to 74 ka are approximately coincident with mis 5 130 71 ka the amplitude of these changes decreases in mis 4 71 57 ka and the early part of mis 3 57 35 ka we proposed that the inferred climatic oscillations follow insolation variations during mis 6 and part of mis 5 150 88 ka low summer and spring insolation and lower seasonality inhibited evaporation and favored high lake levels conversely maxima in spring and summer insolation promoted dry conditions and low lake levels the major wet cold glacial and dry warm interglacial relationship found in lake titicaca bolivia and lake chalco records shows the sensitivity of high altitude tropical sites to climatic variability

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

basin wide magnetostratigraphic framework for the bighorn basin wyoming new paleomagnetic data from six different sections in the bighorn basin are combined with previously published results to construct a basin wide magnetostratigraphic framework geomagnetic polarity reversals between chrons c26r c26n c25r c25n c24r and c24n have been identifi ed among multiple stratigraphic sections in different parts of the basin using the new magnetostratigraphic framework paleontological paleobotanical and isotopic information from these varied locations in the basin can now be correlated and compared to similar records from elsewhere in the world these new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as biohorizon b which is slightly below the chron c24r c24n boundary close to the position of the elmo isotope excursion in marine records backstripping analysis using these new magnetostratigraphic data helps defi ne the time transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

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

magnetic mineral assemblages in soils and paleosols as the basis for paleoprecipitation proxies a review of magnetic methods and challenges abstract magnetic iron oxide minerals principally magnetite maghemite hematite and goethite are formed in well drained soils in response to a suite of physical chemical and biological factors despite a wide range of complexity in the pedogenic processes that lead to magnetic mineral formation dissolution and transformation there are well documented empirical relationships between various magnetic mineral assemblages in soils with environmental and climatic conditions recently there has been an increase in the number of quantitative magnetic paleoprecipitation proxies that have been developed and there is great potential for magnetic methods to be used in the geologic record to develop reconstructions of past climates magnetic paleoprecipitation proxies have been widely utilized in quaternary or younger loess paleosol systems however they have yet to be utilized in the pre quaternary fossil record future studies of magnetic mineralogy of soils and paleosols should aim to explore non loessic modern soils and pre quaternary paleosols with more focus on understanding the interaction between magnetic mineral assemblages and soil moisture applications of existing and novel magnetic paleoprecipitation proxies in the fossil record should prove to be a valuable resource for paleoclimatologists

a new method in mineral magnetism for the separation of weak antiferromagnetic signal from a strong ferrimagnetic background 1 this paper describes a new magnetic method for determining the contribution of weakly magnetic hematite against a strongly magnetic e g magnetite background the method involves measurements of mfr a residual or final remanence after hysteretic demagnetization of saturated isothermal remanent magnetization in terms of sensitivity and precision our proposed method is superior to the currently popular approach known as the hirm method compared to the latter mfr has no error amplification caused by subtraction of terms the method utilizes the available software for hysteretic demagnetization function on an automated vibrating sample magnetometer vsm this in turn automatically decreases some of the disturbing effects of the strong magnetic background e g magnetic viscosity initially non zero offset of hirm for partially oxidized magnetite etc

direct estimates of pedogenic magnetite as a tool to reconstruct past climates from buried soils 1 variations in magnetic properties of buried soils can be used to reconstruct past climatic conditions during paleosol formation most methods however are based on comparisons between the magnetically enriched upper soil horizons and the magnetically unaltered parent material in thin loess paleosol sequences such a comparison can be problematic because all horizons soil and underlying loess may be affected to varying degrees by pedogenesis we propose two direct estimates of pedogenic magnetite based on the analysis of anhysteretic remanent magnetization ratios carm isothermal remanent magnetization and coercivity distributions these estimates are independent of any information regarding the parent material and are possible if pedogenic minerals have similar magnetic properties throughout the study region this condition seems to be met throughout the midwestern united states and a few loessic soils elsewhere the remanence carrying part of pedogenic magnetite is composed of single domain particles with consistent well constrained magnetic properties these particles are extremely well dispersed in the soil matrix as indicated by the absence of noticeable magnetostatic interaction effects our analyses of over 70 modern loessic soil profiles demonstrate that the abundance of pedogenic magnetite correlates well with modern climate and that the method is suited for reconstruction of past climates from paleosols

preserved pedogenic mineral magnetic signature pedogenesis and paleoclimate change pennsylvanian roca shale virgilian asselian central kansas usa abstract this study focuses on the field characteristics and mineral magnetism of four stacked paleosols in the upper pennsylvanian virgilian asselian roca shale exposed near manhattan kansas usa the roca shale paleosols pi at base p4 at top include a protosol p1 argillic calcisol p2 calcic argillisol p3 and calcic vertisol p4 soil climatic changes during the development of each soil profile are suggested by cross cutting relationships and superposition observations including 1 clasts of a lower vertic part are incorporated in an upper calcic horizon and 2 carbonate glaebules have argillans covered by hematitic ferrans decreasing carbonate content more strongly developed of vertic properties and argillans and more mature profiles in successive paleosols are consistent with an interpretation of a longer term soil climate shift from more arid to more seasonal or humid during development of the four paleosols several observations are consistent with a preserved pedogenic mineral magnetic signature in particular 1 each type of soil horizon b bt bk be and c within and among paleosols contains consistent magnetic susceptibility natural remanent magnetization nrm anhysteretic remanent magnetization arm and isothermal remanent magnetization irm signatures different from other horizons 2 similar redox trends are indicated by magnetic mineralogy and uranium concentrations 3 observed magnetic character and uranium concentrations are compatible with those expected in gleyed and oxidized intervals as defined by field observations 4 pedogenic carbonate glaebules and adjacent pedogenically altered siliciclastic strata preserve the same trends and 5 paleosols contain single domain sd magnetite and some maghemite similar to many modern soils comparison with other paleozoic vertisols indicates that the roca shale paleosols are characterized by more complex pedogenic and mineral magnetic attributes this variability in the roca shale paleosols is probably related to glacial interglacial climatic variability during pedogenesis the results of this study show that coupled field observation and mineral magnetic characterization of other pre quaternary paleosols potentially will yield significant paleoclimatic and pedogenic information

two massive rapid releases of carbon during the onset of the palaeocene eocene thermal maximum the release of massive amounts of carbon led to abrupt warming 55 5 million years ago an analysis of soil carbonates shows two distinct carbon injections at the event onset each releasing over 0 9 petagrams of carbon per year over hundreds to thousands of years

transient drying during the paleocene eocene thermal maximum petm analysis of paleosols in the bighorn basin wyoming abstract the paleocene eocene thermal maximum petm was a transient episode of global warming that occurred at the paleocene eocene boundary the event is marked by a sharp decline in carbon isotope values that is globally recognized from marine and continental sections the negative isotope excursion has been attributed to the release of methane hydrates from oceanic sediments although studies concur that temperatures rose during the petm changes in precipitation are not well understood alluvial paleosols in the bighorn basin that span the petm interval contain a continuous and highly resolved record of climate including information on precipitation they show a significant but transient decrease in precipitation at the onset of the petm but a gradual return to pre petm levels by the end of the interval the paleosols also show additional although less dramatic wet dry cycles within the petm interval that may correspond to precessional cycles that have been identified in the marine record of the petm this study counters interpretations of increased precipitation for wyoming at this time and shows the importance of detailed case studies of continental strata to test climatic generalizations and models that have been developed for petm precipitation patterns

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

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

how abundant is pedogenic magnetite abundance and grain size estimates for loessic soils based on rock magnetic analyses 1 the upper soil horizons of many modern and ancient soils are enriched in fine grained pedogenic ferrimagnetic minerals we use three grain size and concentration dependent proxies anhysteretic remanent magnetization isothermal remanent magnetization ratios coercivity spectra derived from alternating field demagnetization of saturation isothermal remanent magnetization and hysteresis properties to quantify the abundance and grain size of the pedogenic magnetic component our analyses of modern loessic soils from the midwestern united states show that relatively small additions 2 10 vol of the total ferrimagnetic component of fine grained coarse superparamagnetic to fine pseudosingle domain magnetite or maghemite are sufficient to explain the changes in concentration and grain size dependent properties observed in the upper soil horizons furthermore the pedogenic components of all studied sites display a narrow range of magnetic properties which argues for a common origin of these particles over a wide range of climatic conditions

a new paleoprecipitation proxy based on soil magnetic properties implications for expanding paleoclimate reconstructions description of precipitation patterns and changes in the hydrological cycle during periods of past global change is crucial for providing an understanding of terrestrial climate systems and for predicting impacts of future climate change such as shifting water availability while a number of proxies and climofunctions exist for reconstructing paleoprecipitation using paleosols all of the available tools for reconstructing paleoprecipitation are either limited to certain precipitation ranges effective only for low precipitation regimes e g depth to bk chemical index of alteration cia k or are relevant only to a limited range of paleosols single pedotype relationships e g calcium magnesium index calmag here we measure the acquisition of isothermal remanent magnetization in b horizons of modern soils to quantify the ratio of pedogenic magnetic minerals goethite and hematite and we use the relationship between these soil magnetic properties and measured climatic variables at each soil site to derive a new quantitative proxy for precipitation by compiling both literature derived and measured goethite hematite g h ratios and mean annual precipitation estimates for a global suite of modern soils n 70 we describe a strong linear relationship r 2 0 96 between the g h ratios of soil b horizons and mean annual precipitation that can be used to estimate paleoprecipitation values for a wide range of climatic regimes 100 3300 mm yr 1 and soil types inceptisols alfisols ultisols oxisols mollisols aridisols spodosols we tested the new climofunction using paleosols from the early eocene of wyoming which show that estimates based on g h ratios compare favorably to and expand upon previously published estimates based on paleosol data

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

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

magnetite originating from bonfires in a brazilian prehistoric anthrosol a micro raman approach abstract soils within archaeological sites may show magnetic properties due to the presence of magnetic minerals such as magnetite and maghemite which form during deposition of the soil or due to local fires the similarities in the crystallographic structures of magnetite and maghemite lead to difficulties in the identification of these mineral phases this work aims to study the genesis of fe nodules at an archaeological site in a cave in quartzite with a very low fe content and no spinel group minerals the guiding hypotheses of the research are as follows i the abundant fe nodules in all the studied stratigraphic layers are of pedogenic origin and were generated by local bonfires and ii these fe nodules are composed of magnetite maghemite and hematite and the conversion of magnetite to maghemite indicates pedogenic processes micromorphological analysis and raman spectroscopy were used for mineral phase determination the results confirm the in situ pedological nature of the fe nodules associated with anthropogenic fires in the cave

high field remanence properties of synthetic and natural submicrometre haematites and goethites significance for environmental contexts haematite and goethite are significant magnetic components both of marine and terrestrial sediments variable magnetic behaviour in haematite and goethite has been reported reflecting variations in grain size crystal defects nonstoichiometry and mode of formation here we provide new data first for a range of synthetic haematite and goethite powders of known grain size and second for a variety of haematite and goethite bearing natural samples including red bed samples and modern and fossil soils based on the synthetic data we identify two high field parameters which may be of value in identifying and characterizing the high coercivity components of natural environmental samples even when these are dominated magnetically by the presence of trace concentrations of ferrimagnets h indicates the proportion of the room temperature rt remanence acquired in fields from 2 tesla t up to 7 t and hcool indicates the increase or decrease in high field remanence upon cooling in zero field to 77 k lt used in tandem these parameters can differentiate between goethite dominated and haematite dominated samples and indicate differences in haematite grain size h values for the synthetic haematites vary between 27 and 38 with some apparent grain size dependence and for the goethites between 84 and 92 hcool values range from 130 to 157 for the goethites and 75 to 95 for those haematites of grain size 100 nm

astronomical climate control on paleosol stacking patterns in the upper paleocene lower eocene willwood formation bighorn basin wyoming the willwood formation of the bighorn basin wyoming usa is a thick succession of upper paleocene and lower eocene fluvial floodplain sandstones and mudstones reddish paleosols formed on the floodplain mudstones alternate rhythmically on various scales with heterolithic intervals of small channel sandstones and mudstones showing weak pedogenesis spectral analysis of redness in the willwood successions at polecat bench and red butte reveals significant spectral peaks corresponding to cycle thicknesses of 8 and 3 m the 8 m cycle reflects distinct clusters of 3 5 paleosols age constraints show that the period of this cycle closely matches the 21 k y climatic precession cycle the 3 m cycle corresponds to individual paleosols with a period of 7 8 k y this period is similar to millennial scale sub milankovitch cycles found in marine and lacustrine successions of pliocene pleistocene age precession and millennial scale climate variations probably affected paleosol development through cyclic changes from predominantly overbank to predominantly channel avulsion deposition with the latter periodically halting soil formation because of high sediment accumulation a new age model was developed for the paleocene eocene carbon isotope excursion cie at polecat bench based on the precessional origin of paleosol clusters the main body of the cie spans 5 5 precession cycles or 115 k y and the recovery tail of the cie spans 2 precession cycles or 42 k y this outcome is consistent with and independently confirms recent estimates of cie duration based on deep sea cores

alluvial response to the paleocene eocene thermal maximum climatic event polecat bench wyoming u s a abstract the stratigraphic interval spanning the paleocene eocene thermal maximum in the northern bighorn basin wyoming shows changes in the alluvial record that can be tied to a high resolution climate record the complexity and stratigraphic spacing of paleosols change through the study section comparison to the climate record reconstructed from paleosols indicates the changes correspond to paleocene eocene thermal maximum climatic fluctuations in particular the middle of the section contains thick welded paleosols and thin avulsion deposits that link to times of well drained floodplains and lower mean annual precipitation stratigraphic intervals below and above the main part of the paleocene eocene thermal maximum interval correspond to times of less well drained floodplains and higher mean annual precipitation these strata contain thinner paleosols and thick avulsion deposits differences in paleosol complexity and spacing suggest that sediment flux to the depositional site varied in response to precipitation fluctuations associated with the paleocene eocene thermal maximum welded paleosols and thin avulsion deposits indicate reduced floodplain accretion during deposition of the middle of the paleocene eocene thermal maximum intervals with widely spaced paleosols indicate more rapid accretion we hypothesize that drier episodes associated with warming caused reduced vegetation in source areas and promoted erosion and increased sediment yield because precipitation was reduced much of that sediment was stored in upstream reaches of the fluvial system rather than moving to the depositional basin welded paleosols formed because of diminished sediment supply to the basin with a return to wetter conditions during the recovery phase of the paleocene eocene thermal maximum upstream water flux increased stored sediment moved to the basin and vertically spaced thinner paleosols developed the results demonstrate how vertical sections of alluvial paleosols can provide information on how climate fluctuated through time and how the fluvial system responded to climate change

rainfall dependent transformations of iron oxides in a tropical saprolite transect of hainan island south china spectral and magnetic measurements 1 the iron oxide content of soils and sediments controlled by weathering and pedogenesis is generally considered a reasonable indicator of climate previous studies in temperate zones have established a positive correlation in aerobic soils between ferrimagnets and low to moderate rainfall the correlation seems to be reversed under extreme climates with high rainfall here we present a transect of saprolitic soils from hainan island south china with high rainfall 1440 2020 mm yr little temperature variation 23 24 c and extreme weathering along this transect we observed that both hematite concentration and magnetic susceptibility decrease with increasing rainfall whereas goethite concentration displays a large increase however there is no systematic trend in the total amount of iron oxides related to chemical weathering intensity along the transect goethite is the favored mineral phase of iron oxide with increasing rainfall and accumulates at the expense of hematite and maghemite through the dominance of rainfall driven processes these pedogenic processes coincide with the fundamentals of previous nonmonotonic models of hematite magnetic susceptibility and rainfall control of an inflection point this study also verifies a common genetic relationship between hematite and pedogenic ferrimagnets across a wide climate range a conceptual model considering both rainfall and temperature is proposed to help interpret the mechanism of ferrimagnet formation and changes in the rainfall inflection point

magnetic susceptibility of early paleozoic and precambrian paleosols abstract paleosols formed by weathering and plant colonization during breaks in eolian deposition of quaternary loesses are marked by enhanced magnetic susceptibility these paleosols can be recognized in the field from their soil structure and soil horizons but the most obvious and diagnostic feature of paleosols are root traces for paleosols of early paleozoic and precambrian age pre dating the evolution of rooted land plants root traces cannot be used as a diagnostic field criterion for recognizing paleosols surficial enhancement of magnetic susceptibility can be useful confirmation that some precambrian and early paleozoic rocks were paleosols unfortunately magnetic susceptibility enhancement of some paleosol profiles is compromised by burial gleization which reduces overall susceptibility values but especially susceptibility at the surface of the profile and produces a marked covariance of magnetic susceptibility with total iron content groundwater and surface water gleization during soil formation also depletes susceptibility over a wide range of iron content to give relatively flat depth functions of susceptibility in contrast well drained soils and paleosols have pronounced surficial susceptibility enhancement over a narrow range of iron content magnetic susceptibility peaks are useful supporting evidence for paleosols entirely red and oxidized but not for paleosols with common green gray reduction spots abundant organic matter iron manganese nodules or other evidence of gleization all known paleozoic and precambrian paleosols have susceptibility an order of magnitude less than in comparable quaternary paleosols and soils so that quaternary magnetic susceptibility climofunctions and chronofunctions cannot yet be applied to paleozoic and precambrian paleosols although their susceptibility values are comparable with those of modern gleyed soils paleozoic and precambrian paleosols maintain a depth function of surficial enhancement of susceptibility as well as red color deeply penetrating cracks and other features of well drained soils burial recrystallization of fine grained magnetite and maghemite may have compromised the magnitude of their susceptibility peaks alternatively they may not have been as biologically active in susceptibility enhancing microbes as modern soils

rock magnetic properties of a soil developed on an alluvial deposit at buttermilk creek texas usa 1 the evolution of magnetization within a floodplain soil begins with initial deposition of magnetic particles during sedimentation and continues via subsequent alteration and growth of iron bearing compounds by pedogenic and biologic processes measurements of soil magnetic properties capture information about the developmental history of the soil and are a convenient method by which to investigate environmental change and pedogenesis using a range of magnetic measurements a comprehensive scenario for soil development was constructed for floodplain sediments at the debra l friedkin site an important archeological site near buttermilk creek texas floodplain deposits have traditionally been avoided for soil magnetism studies because it is thought that the episodic input of sediment would form soils characterized by discrete sedimentary units rather than a continuous record of pedogenesis we demonstrate that alluvial deposits can sometimes carry a straightforwardly interpretable magnetic signal similar to those typically seen in loess deposits smooth variation of rock magnetic parameters as a function of depth also leads us to conclude that the soil at this site is largely undisturbed and that the age of lithic artifacts found within the soil may be interpreted within stratigraphic context

paleohydrologic response to continental warming during the paleocene eocene thermal maximum bighorn basin wyoming abstract geologically rapid global warming occurred during the paleocene eocene thermal maximum petm 56 ma several studies have argued that important changes occurred in the hydrological cycle during the petm but results have been inconsistent ranging from global increases in humidity to drier conditions changes in paleosols during the petm in the southeastern bighorn basin document major drying during the body of the event paleosol changes also suggest transitional episodes of climate change that both preceded and followed the petm qualitative semi quantitative and fully quantitative analyses of a 70 m thick interval of paleosols provide a high resolution record of changes in soil moisture and precipitation those changes are compared to changes in temperature determined from 18o values of tooth enamel from the mammal coryphodon a distinct shift to drier soils occurred just prior to the petm a conclusion that is consistent with previous observations that warming began before the onset of the negative carbon isotope excursion associated with the petm paleosols show a progressive drying trend into the lower part of the petm and become even drier in the upper part of the body of the petm purple red paleosols that appear during the recovery phase of the petm indicate wetter soils although they are better drained than paleosols below the onset the purple red paleosols continue for 15 m above the recovery and indicate that wetter soil conditions persisted after the recovery it is not clear whether changes in the paleosols that preceded and followed the petm reflect global forcing factors like orbital cycles or release of carbon that lacks an isotopic label however such mechanisms would provide a unifying explanation for shifts seen in continental and marine environments

examining the spatial consistency of palaeosol proxies implications for palaeoclimatic and palaeoenvironmental reconstructions in terrestrial sedimentary basins terrestrial sediments are a significant archive of past conditions and the critical zone in particular is important for understanding past and ongoing interactions between the atmosphere biosphere and lithosphere understanding the details of a palaeolandscape or palaeocatena record can greatly improve palaeoenvironmental descriptions and palaeoclimate reconstructions during crucial time periods in the past this article presents multi proxy data including palaeosol descriptions major element geochemistry magnetic composition and stable isotope geochemistry from an early eocene palaeosol transect which show distinct trends in palaeosol features and geochemistry across the landscape particularly in terms of the differences between a horizon and b horizon samples a horizon geochemical and physical features are highly variable and reflect other not strictly pedogenic controls whereas b horizon composition is extremely consistent given that most palaeoclimatic or palaeoenvironmental proxies based on palaeosols use b horizon features these results highlight the robustness of those proxies at the same time the difference between a horizons and b horizons and the variability within a horizons of the same palaeosol in different geomorphic settings underscores the importance of palaeosol description detailed sampling and a multi proxy approach to produce both precise and accurate reconstructions of past environments and climates using terrestrial sedimentary records

evaluating the use of weathering indices for determining mean annual precipitation in the ancient stratigraphic record abstract different quantitative methods have been developed to determine paleo rainfall using paleosols of particular interest are methods that use bulk geochemistry to calculate a weathering index and from that mean annual precipitation twenty three paleosols that formed during a time of significant climate change the paleocene eocene thermal maximum petm were analyzed to evaluate the relative merits of two geochemical proxies that have been developed for estimating paleoprecipitation one depends on the chemical index of alteration without potash cia k and the other uses the calmag weathering index which depends on cao and mgo the paleosols are located in the paleogene fort union and willwood formations in the bighorn basin wy morphologic differences among the paleosols indicate that they formed under varied soil moisture conditions a morphology index was developed from pedogenic features that are sensitive to soil drainage and thus to rainfall to assess that variability quantitative mean annual precipitation map values were calculated for each paleosol using the cia k index and the calmag index which was developed specifically for vertisols those results were compared to the morphology index for 23 paleosols the results indicate that increases and decreases in map calculated from both methods correlate relatively well to changes in the soil morphology index however the results from the calmag weathering index show a stronger correlation the results suggest that the calmag proxy provides a more robust map estimate for vertisols in addition to comparing map estimates to the soil morphology index we compared the map values to paleorainfall estimates determined from paleofloras at the same stratigraphic levels as three of the paleosols that comparison confirms that the calmag method is reliable for reconstructing map from ancient vertisols the results also show that in the bighorn basin vertisols with a b horizon

characteristics and formation mechanism of pedogenic hematite in quaternary chinese loess and paleosols abstract hematite is an important iron oxide mineral in loess paleosol sequences in northwest china samples containing hematite and related minerals in the loess and paleosol units from chinese loess plateau were investigated using powder x ray diffraction xrd scanning electron microscopy sem and high resolution transmission microscopy hrtem the results show that there are at least three genetic types of hematite in quaternary loess paleosol sequences of northern china 1 weathering products of fe bearing silicate minerals such as chlorite and biotite that precipitated as aggregates of hematite nano crystals on mineral surfaces 2 oxidation products of eolian magnetite grains with hematite nano crystals closely associated with magnetite 3 coarse silt sized hematite grains present in the fresh loess hematite with a nanoporous texture indicates that dehydration of iron hydroxide ferrihydrite is the main mechanism for the formation of hematite coating during chemical weathering of fe bearing silicates it is proposed that nano phase hematite is primarily responsible for the red color of the paleosol units paleosol layers are also characterized by higher magnetic susceptibility and increased oxidation of eolian magnetite it is proposed that the major contribution to magnetic susceptibility is from the nano phase magnetite that may be related to microbial activity this research demonstrates that the study of mineral characteristics formation mechanisms and the relationship between hematite other iron oxides and fe bearing silicate minerals helps to understand better the geochemical process in loess both before and after dust deposition in addition our study helps to explain the correlation between magnetic susceptibility and paleoclimate in northern china during the quaternary

magnetic properties of modern soils and quaternary loessic paleosols paleoclimatic implications the magnetic properties of paleosols developed in quaternary sequences of loess have been used for stratigraphic definition correlation with other terrestrial and deep sea sequences and paleoclimatic paleorainfall reconstruction in some loess paleosol sequences including those of the chinese loess plateau tajikistan and the czech republic maxima in magnetic susceptibility values correspond with the paleosol horizons and minima with the least weathered loess layers in other loess soil sequences including those of siberia alaska and argentina the relationship is completely opposite with susceptibility minima associated with the most developed paleosols to account for these opposite relationships the respective roles of 1 magnetic enhancement and 2 magnetic depletion and or dilution in determining soil magnetic properties are investigated for a range of modern soil types most magnetic enhancement is seen in the upper horizons of well drained cambisols absence or loss of magnetic iron oxides is apparent in acid podsol profiles and waterlogged soils for the cambisol profiles significant correlation is found between susceptibility and organic carbon cation exchange capacity and clay content the mineralogy morphology and grain size of soil magnetic carriers extracted from three modern enhanced soils and a paleosol from the chinese loess plateau are also identified by independent petrographic means microscopy and x ray diffraction of magnetic extracts magnetite and maghemite of ultrafine grain size from 0 4 to 0 001 i m are the major contributors to the magnetically enhanced soils weathering can concentrate detrital magnetic grains especially in the fine silt size fractions of soils the magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site specific basis taking into account the possibilities of pedogenic enhancement pedogenic dilution or depletion and allochthonous inputs of magnetic minerals excessively arid wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets well drained intermittently wet dry soils with reasonable buffering capacity and a substrate source of fe show most magnetic enhancement for those soils which favour magnetic enhancement processes correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall the almost unique pedogenic system in the chinese loess plateau where variation in soil forming factors other than climate is reduced to a global minimum allows use of the paleo susceptibility values as proxy paleorainfall values to identify the mechanism of any link between magnetic properties for example susceptibility and climate change the mineralogy and grain size of soil magnetic carriers requires detailed investigation given the prospect of 1 quantitative paleorainfall records from continental loess records 2 predictions of present and future climate change and 3 the value of paleoclimate data for testing of numerical climate models such magnetic investigations can contribute significantly to our understanding of past and future environmental change

significance of different modes of rhizolith preservation to interpreting paleoenvironmental and paleohydrologic settings examples from paleogene paleosols bighorn basin wyoming u s a abstract different modes of preservation of root traces rhizoliths provide information on soil moisture regimes in alluvial paleosols in the paleogene fort union and willwood formations of the bighorn basin wyoming u s a this paper links different styles of rhizolith preservation to paleosols whose other pedogenic attributes provide information on ancient soil drainage consequently the paleodrainage significance of different rhizolith preservation patterns is established more confidently rhizoliths consisting of elongate gray mottles rhizohaloes with red rims are common in relatively well drained red paleosols the gray zones are depleted of iron fe whereas the color of the rims indicates hematite accumulation these features are typical of surface water gley processes that caused fe and manganese mn to move from the root channel outward to the soil matrix calcareous rhizocretions either calcareous tubular concretions or micro accumulations of carbonate within gray rhizotubules are also common in moderately well drained red paleosols more poorly drained purple paleosols also have rhizoliths consisting of fe depletion zones however the rhizohaloes are surrounded by yellow brown goethite rims rather than red hematitic rims in many paleosols the red and yellow brown accumulation rims were partly removed by continued depletion producing abundant irregular shaped mottles the nature of the rhizohalo and the color of the fe oxide rim provide information about the intensity of gleying that a paleosol underwent the poorly drained paleosols also show rhizoliths preserved as goethite accumulations contained within gray depletion zones indicating movement of reduced fe from the matrix towards the root channel probably as a result of groundwater gley processes the goethite accumulations are intermingled locally with lignite representing preservation of the root organic material some rhizoliths are preserved as tube shaped concentrations of small 0 1 0 2 mm diameter black spheres whose color indicates a mix of mn and fe oxides these are most common in paleosols with low chroma gray matrix colors indicating very poor drainage the very poorly drained paleosols also show rhizoliths preserved in jarosite which is an oxidation product of pyrite careful observational and geochemical analysis of rhizoliths which are common in continental deposits can help produce a clearer and more thorough interpretation of ancient drainage conditions information about degree of ancient soil wetness or moisture is important for understanding past climatic conditions and for reconstructing terrestrial paleolandscapes

what do the hirm and s ratio really measure in environmental magnetism the hard isothermal remanent magnetization hirm and the s ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations respectively of antiferromagnetic minerals hematite and goethite in mineral mixtures we demonstrate that synthetic al substituted hematite and goethite exhibit a wide range of coercivities which significantly influences the hirm and s ratio these parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite goethite to circumvent this problem we propose a new parameter the l ratio which is the ratio of two remanences after alternating field af demagnetization of an irm imparted in a 1 t field with a peak af of 100 mt and 300 mt irmaf 300mt irmaf 100mt these parameters are easily measured using modern vibrating sample or alternating gradient magnetometers changes in hirm only reflect changes in the absolute concentration of hematite and or goethite if the l ratio is relatively constant conversely l ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite goethite corresponding hirm and s ratio variations should be interpreted with caution in such cases the l ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols for example the hirm is equivalent to irmaf 300mt likewise 0 5 sirm irm 100mt where irm 100mt represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back field of 100 mt is equivalent to irmaf 100mt the hirm 0 5 sirm irm 100mt ratio is therefore a suitable substitute for the l ratio when measurements are made with a long core magnetometer the newly proposed l ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism the hirm and s ratio

long term changes in precipitation recorded by magnetic minerals in speleothems speleothems are important paleoclimate archives researchers typically compile measurements of stable isotopic ratios dated using high precision u th radiometric techniques to reconstruct regional and global climate magnetic material incorporated within speleothems can provide an independent means of connecting large scale climatic changes with their impact on more localized processes in soils overlying cave systems under certain environmental conditions pedogenic processes can produce magnetite nanoparticles enhancement of pedogenic magnetite in soil profiles depends strongly on local precipitation pedogenic magnetite can be subsequently transferred via drip waters into underlying cave systems and incorporated into speleothems as they grow here we employ high resolution magnetic methods to analyze a well dated stalagmite from buckeye creek cave west virginia usa and find that changes in magnetite concentration follow both changes in stable isotopes measured in the same stalagmite and global climate proxies we interpret the changes in magnetite concentration as reflecting variations in local pedogenic processes controlled by changes in regional precipitation this record demonstrates how magnetic measurements on speleothems can constrain interpretations of speleothem climate proxies

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

analysis of the field dependence of remanent magnetization curves 1 a new method to calculate and analyze coercivity distributions of measured acquisition demagnetization curves of remanent magnetization is presented the acquisition demagnetization curves are linearized by rescaling both the field and the magnetization axes an appropriate filtering of the linearized curves efficiently removes measurement errors prior to evaluating the coercivity distributions the filtered coercivity distributions are modeled using a set of generalized probability density functions in order to estimate the contributions of different magnetic components an error estimation is calculated for these functions with analytical and numerical methods in order to evaluate whether the model is significantly different from the measured data three sediment samples from baldeggersee switzerland and three samples of urban atmospheric particulate matter pm have been analyzed using this method it is found that the coercivity distributions of some of the magnetic components show significant and consistent deviations from a logarithmic gaussian function large deviations are found also in the coercivity distributions of theoretical af demagnetization curves of single domain and multidomain particles constraints in the shape of model functions affect the identification and quantification of magnetic components from remanent magnetization curves and should be avoided as far as possible the generalized probability density function presented in this paper is suitable for appropriate modeling of gaussian and a large number of non gaussian coercivity distributions