mineral magnetic record of late quaternary climatic changes in a high alpine lake the magnetic properties of a sediment core from a high altitude lake in the swiss alps were compared with palynological and geochemical data to link climatic and mineral magnetic variations according to pollen data the sediments extend from the present to the younger dryas i e they cover more than 10 000 years of environmental change in the alps the major change in magnetic properties corresponds to the climatic warming of the early holocene high coercivity magnetic minerals that characterize the late glacial period almost disappeared during the holocene and the concentration of ferrimagnetic minerals increased sharply the contribution of superparamagnetic grains also decreased in the holocene sediments similar variations in sp content and coercivity of smaller magnitude are found in the holocene and are interpreted to represent minor climatic variations comparison with the historical record of the last 1000 years confirms this interpretation the magnetic mineralogy the superparamagnetic contents and the irm intensity in the coarse grained late glacial sediments are similar to those measured in the catchment bedrock this indicates a detrital origin the different properties and the higher concentration of magnetic minerals in the holocene sediments are due to authigenic phases magnetic properties provide a high resolution record of climatic change they are sensitive even to small variations that are not recorded in the pollen or loi data magnetic parameters show fine scale variation and constitute a valuable supplement to conventional climatic indicators 1999 elsevier science b v all rights reserved

rock magnetic criteria for the detection of biogenic magnetite abstract we report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron reducing bacteria magnetotactic bacterial mtb strains ms1 mv1 and mv2 and dissimilatory iron reducing bacterium strain gs 15 grown in pure cultures were used in this study our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain sd particles of magnetite from mtbs the most useful magnetic property measurements include 1 acquisition and demagnetization of isothermal remanent magnetization irm using static pulse and alternating fields 2 acquisition of anhysteretic remanent magnetization arm and 3 thermal dependence of low temperature 20 k saturation irm after cooling in zero field zfc or in a 2 5 t field fc from 300 k however potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero field and field cooled sirms on warming through the verwey transition t 100 k intact chains of unoxidized magnetite magnetosomes have ratios of fc zfc greater than 2 where the parameter is a measure of the amount of remanence lost by warming through the verwey transition disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the fc zfc ratio to around 1 similar to values observed for some inorganic magnetite maghemite greigite and gs 15 particles numerical simulations of fc zfc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the fc zfc parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole sediment samples the strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made compared to techniques such as electron microscopy

geologic controls on transgressive regressive cycles in the upper pictured cliffs sandstone and coal geometry in the lower fruitland formation northern san juan basin new mexico and colorado three upper pictured cliffs sandstone tongues in the northern part of the san juan basin record high frequency transgressive episodes during the late cretaceous and are inferred to have been caused by eustatic sea level rise coincident with differential subsidence outcrop and subsurface studies show that each tongue is an amalgamated barrier strand plain unit up to 100 ft 30 m thick successive upper pictured cliffs tongues display an imbricate relation and are offset basinward reflecting net shoreline progradation northeastward upper pictured cliffs barrier strand plain sandstones underlie and bound thickest fruitland coal seams on the seaward side controls on fruitland coal seam thickness and continuity are a function of local facies distribution in a coastal plain setting shoreline positions related to transgressive regressive cycles and basin subsidence during periods of relative sea level rise the pictured cliffs shoreline was temporarily stabilized allowing thick coastal plain peats to accumulate although some coal seams in the lower fruitland tongue override abandoned pictured cliffs shoreline deposits many pinch out against them differences in the degree of continuity of these coal seams relative to coeval shoreline sandstones are attributed to either differential subsidence in the northern part of the basin multiple episodes of sea level rise local variations in accommodation and progradation stabilization of the shoreline by aggrading peat deposits or a combination of these factors fruitland coalbed methane resources and productivity are partly controlled by coal seam thickness other important factors include thermal maturity fracturing and overpressuring the dominant production trend occurs in the northern part of the basin and is oriented northwestward coinciding with the greatest fruitland net coal thickness similar relationships between trends of thick coal seams of coeval origin with stacked shoreface sandstones exist in other western interior basins in the united states and serve as models for coalbed methane exploration in other basins of the world

regional scale strain variations in banded iron formations of eastern india results from anisotropy of magnetic susceptibility studies anisotropy of magnetic susceptibility ams data are used as a tool to determine strain variations in different parts of the banded iron formations bifs of the bonai synclinorium eastern india ams data of 88 cylindrical cores drilled from 29 samples collected from the limb and hinge parts of mesoscopic scale folds as well as different parts of the entire synclinorium are presented it is found that the samples from limbs of small scale folds and also from limbs of the regional scale synclinorium have higher degrees of anisotropy than the hinges this is inferred to indicate that the limbs accommodated higher strain than the hinges ams orientation data are analysed in conjunction with field data it is concluded that the magnetic fabric developed in the limbs as well as hinges of the bifs of the study area is related to deformation and is not a manifestation of sedimentary fabric

magnetic properties of a soil from southeastern brazil in the presence of magnetic biomineralization by social insects abstract it is known that the migratory ant pachycondyla marginata and its prey the termite neocapritermes opacus synthesize magnetic minerals which may contribute to the magnetic characteristics of soil a comparative study of the magnetic properties of the minerals found in those species as well as their nests and surrounding soil aimed at investigating the contribution of the biogenic minerals to the magnetization of the soil partially oxidized magnetite t v 120 k was found in the pachycondyla marginata as shown by the verwey transition in the zfc fc and rtsirm curves low temperature remanence measurements of the neocapritermes opacus indicated the presence of maghemite and or titanomaghemite while partially oxidized pseudo single domain magnetites and or titanomaghemite were recognized in the soil and insect nests based on the similarity in the magnetic characteristic of the samples it is concluded that there is significant influence of biogenic mineralization upon the pedogenic contribution

geochemical and sedimentological control of the magnetic properties of hemipelagic sediments owing to their high sedimentation rates and wide areal extent hemipelagic sediments along continental borderlands are potentially important high resolution recorders of geomagnetic secular variation to assess this possibility we studied suboxic hemipelagic muds from the oregon continental slope and anoxic laminated diatomaceous oozes from the gulf of california these sediments were rapidly deposited with average sedimentation rates of 121 and 135 cm kyr respectively bulk sediment sedimentological and geochemical analyses indicate that the two areas represent contrasting depositional regimes the remanence in both environments resides in fine grained magnetite particles although minor amounts of hematite were observed in the topmost gulf of california sediments despite first order differences the sediments from both areas show similar downcore patterns of systematic increases in solid sulfur mainly as pyrite dramatic decreases in natural nrm anhysteretic and isothermal remanent magnetization intensities and accompanying shifts in the magnetic stabilities these changes are consistent with reduction and dissolution of the ferrimagnetic iron oxides with depth due to early diagenesis of organic matter the magnetic grain size distribution first appears to rapidly coarsen downcore as the smallest and most abundant grains are removed then slowly grows finer as the remaining particles dissolve no evidence was observed for authigenic formation of magnetite provided sufficient concentration and stability of the detrital magnetic particles paleomagnetic directions can survive this dissolution diagenesis however relative paleointensity determinations based on normalization of nrm by concentration in related parameters will give erroneous estimates of paleofield behavior in such diagenetically altered sediments

water infiltration underneath single ring permeameters and hydraulic conductivity determination summary a single ring permeameter is suitable for measurement of soil hydraulic conductivity at a large field scale because of its simple structure easy operation portability small amount of recharging water and particularly because of its variable radius the authors established a simple single ring permeameter which consists of a hollow cylinder that is simply inserted into the top soil based on the theories of the philip and nestingen methods for single ring permeameter tests this paper presents new equations for describing 1 d infiltration within the inserted depth of the single ring infiltrometer and 3 d flow beyond the depth under both gravity and soil suction forces the soil hydraulic parameters of hydraulic conductivity and soil suction were optimized based on the declines of the ponded water depth within the single ring cylinder the new method was validated using numerical simulations of the declines of the water column within the cylinder through the variably saturated flow modflow vsf modflow the new method was then successfully applied in laboratory troughs and on a hillslope in huanjiang county northwest guangxi in southwest china the results demonstrate that the hydraulic conductivity determined by the new method is more accurate than those from the 1 d green ampt method and the 3 d nestingen method

falling head ponded infiltration with evaporation comment reference ecol article 1994 002doi 10 1016 0022 1694 94 90013 2 url http www elsevier com wps find journaldescription cws home 503343 description description record created on 2005 12 09 modified on 2016 11 26

magnetotactic bacteria and their magnetofossils in sediments abstract living magnetotactic bacteria from freshwater environments show under natural and laboratory conditions a great variety of morphological appearances their magnetosomes vary in number shape and size one species of bacteria yields up to 1000 magnetosomes per cell individual particles reach a size of up to 200 nm as a rule they form elongated chains in bacteria which are held under laboratory conditions loop shaped arrangements and nearly unordered clusters are also found during the laboratory experiments different species of bacteria periodically dominated freshwater samples always exhibited a great variety of different magnetosomes on the other hand samples from marine environment clearly showed the preponderance of one type of particle these observations may reflect specific properties of the physico chemical milieu in which the bacteria grew at the end of our experiments north and south seeking bacteria co existed in roughly equal quantities quaternary unconsolidated sediments from the ammersee bavaria and the antarctic quaternary to tertiary deep sea sediments from the atlantic and pacific and jurassic limestones from the sonnwendgebirge tyrol austria were analyzed for fossil magnetosomes we found that fossil magnetosomes were morphologically similar to those from living bacteria but seemed to be corroded in some cases the observed differences in quantity size and shape of magnetosomes from various sediments may be influenced by variations in bacterial viability and particle stability at different sites but are not determined by geographical latitude

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

practical estimates of field saturated hydraulic conductivity of bedrock outcrops using a modified bottomless bucket method 1 the bottomless bucket bb approach nimmo et al 2009a is a cost effective method for rapidly characterizing field saturated hydraulic conductivitykfsof soils and alluvial deposits this practical approach is of particular value for quantifying infiltration rates in remote areas with limited accessibility a similar approach for bedrock outcrops is also of great value for improving quantitative understanding of infiltration and recharge in rugged terrain we develop a simple modification to the bb method for application to bedrock outcrops which uses a nontoxic quick drying silicone gel to seal the bb to the bedrock these modifications to the field method require only minor changes to the analytical solution for calculatingkfs on soils we investigate the reproducibility of the method with laboratory experiments on a previously studied calcarenite rock and conduct a sensitivity analysis to quantify uncertainty in our predictions we apply the bb method on both bedrock and soil for sites on pahute mesa which is located in a remote area of the nevada national security site the bedrock bb tests may require monitoring over several hours to days depending on infiltration rates which necessitates a cover to prevent evaporative losses our field and laboratory results compare well to kfs values inferred from independent reports which suggests the modified bb method can provide useful estimates and facilitate simple hypothesis testing the ease with which the bedrock bb method can be deployed should facilitate more rapid in situ data collection than is possible with alternative methods for quantitative characterization of infiltration into bedrock

magnetic measurements used to identify paleoxidation fronts in deep sea sediments from the madeira abyssal plain we describe the results of rock magnetic measurements of a sediment core from the madeira abyssal plain which contains multiple turbidites previous studies of trace element e g u concentration and observations of sediment color changes have shown that turbidite emplacement has resulted in reductive diagenesis and the formation of paleoxidation fronts we show that variations in sediment lithology and postdepositional sedimentary processes can be characterized very sensitively using magnetic measurements low concentrations of magnetic minerals occur in turbidites with a high organic content reflecting the destruction of magnetic material via the process of reductive diagenesis in addition fossil redox fronts are readily identified by rock magnetic properties

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

palaeomagnetic and rock magnetic study of a deeply exposed continental section in the charnockite belt of southern india implications to crustal magnetisation and palaeoproterozoic continental nucleii abstract this study has investigated palaeomagnetism and magnetic properties of a tract of continental basement in south india metamorphosed to granulite facies close to the archaean proterozoic boundary 2600 ma at former depths of 20 35 km the metamorphism is characterised by extensive development of charnockite produced by flushing of co2 rich fluids through older crust rock magnetic studies thermomagnetic high and low temperature susceptibility hysteresis and irm show that very low ti metamorphic magnetite is the ubiquitous ferromagnet in all rock types it has predominantly multidomain characteristics but is of high magnetic stability probably due to exsolution precipitation and microfracture filling producing anisotropic grain shapes magnetic susceptibility shows little increase with temperature falling to zero at 550 c implying that induced magnetisation drops to a weak paramagnetic effect near this isotherm susceptibilities and intensities of magnetisation increase from amphibolite to granulite facies and by an order of magnitude within charnockites in the lower dehydrated crust induced magnetisations are able to explain 50 of observed magnetic anomalies of deep origin intensities of magnetisation and qn values are high especially in charnockite where metasomatism has resulted in new magnetite formation and it is concluded this facies is able to explain observed magnetic anomalies provided that viscous remnant magnetisation is enhanced at depth to contribute the remaining magnetisation the magnitude of anisotropy of magnetic susceptibility ams is also increased by charnockite formation as are degrees of lineation and foliation however directions of ams in these anhydrous rocks are essentially random and reflect the static nature of the charnockite metasomatism no appreciable phanerozoic overprinting is evident in this terrane and the dominant feature of the palaeomagnetic record is a west to north shallow swathe of component directions with a minor antiparallel sequence between means summarised by a1 d i 280 8 a2 d i 319 11 and a3 d i 1 10 this swathe is interpreted as an uplift related cooling acquisition following charnockite formation at 2600 ma and a remanence age of 2600 2400 ma is implied by the absence of most of the swathe in early palaeoproterozoic and younger dyke swarms cutting the region a second group of component directions b mean d i 238 87 is similar to magnetisations found in dykes linked to an age of 2370 ma and interpreted as somewhat younger than the a swathe we argue that the entire magnetic record was acquired before final basement uplift prior to 2000 1800 ma collective late archaean early proterozoic palaeopoles from india define a low to high latitude movement of the shield at 2600 2300 ma which is replicated in contemporaneous results from the other ancient gondwana nucleii of southern africa and australia the cratonic nucleii of these three shields have the oldest laterally extensive supracrustal covers and comprise the core of the protocontinent of ur the 2900 2200 ma palaeopoles are consistent with close proximities and subsequent movements between them were apparently limited because continental proximities but not configurations were similar within the palaeozoic supercontinent of gondwana

formation of single domain magnetite by a thermophilic bacterium magnetite is a common product of bacterial iron reduction and may serve as a potential physical indicator of biological activity in geological settings here the authors report the formation of single domain magnetite under laboratory conditions by a thermophilic fermentative bacterial strain tor 39 that was isolated from the deep subsurface time course analyses were performed at 65 c to study the effect of bacterial activity on solution chemistry and magnetite formation during the growth of tor 39 run products were examined by transmission electron microscopy magnetite particles formed exclusively outside of bacterial cells and exhibited octahedral shapes having relatively equal length and width 15 difference tiny magnetite particles 12 nm nucleated between 10 and 11 h of incubation and increased to average lengths of 55 4 26 8 nm after 24 h of incubation between 24 h and 22 d of incubation magnetite particles maintained average lengths of 56 2 24 8 nm based on size constraints greater than 85 of the particles observed fell within the magnetic single domain little to no magnetite was detected in abiotic controls at 65 or 95 c or in tor 39 cultures whose activity was suppressed unlike mesophilic iron reducing bacteria e g gs 15 tor 39 produced crystals having shapes and sizesmore similar to some particles produced intracellularly by magnetotactic bacteria thus the single domain magnetite produced by thermophiles such as tor 39 may represent a heretofore unrecognized biological contribution to natural remanent magnetization in sedimentary basins and other geothermal environments less

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

evidence for a relationship between hydrocarbons and authigenic magnetite establishing a relationship between hydrocarbon migration and the precipitation of authigenic magnetite in sedimentary rocks is of significant interest with respect to 1 elucidating mechanisms for remagnetization and establishing the origin of secondary magnetizations residing in magnetite 2 developing a method to date hydrocarbon migration events by determining the time of remanence acquisition by palaeomagnetic methods and 3 evaluating whether searching for anomalous concentrations of diagenetic magnetic minerals and or aeromagnetic anomalies is justified as a relatively inexpensive exploration tool the direct association of hydrocarbon migration and the precipitation of authigenic magnetite has however not been established in this paper we report palaeomagnetic rock magnetic petrographic and geochemical results of a study of samples from permian spelebthems and gilsonite found in the ordovician arbuckle group in southern oklahoma the results indicate that there is a genetic relationship between hydrocarbon migration and the precipitation of authigenic magnetite

magnetic hysteresis properties of greigite fe3s4 and a new occurrence in holocene sediments from swedish lappland a new occurrence of ferrimagnetic greigite fe3s4 is reported in freshwater holocene sediments from gaskkamus gorsajavri in swedish lappland the presence of greigite was inferred from the mineral magnetic analysis of sediment cores the greigite bearing sediments possess a high saturation isothermal remanent magnetisation to susceptibility sirm ratio a characteristic shared by other natural sediments known to contain greigite the hysteresis properties measured using a vibrating sample magnetometer of natural greigite extracted from lake sediments show much higher saturation coercivity b0 c and mrsms ratio values approximately 50 mt and 0 5 respectively than synthetic samples of greigite approximately 12 mt and 0 2 the results suggest that synthetic greigite samples contain crystals ranging from small non remanence carrying superparamagnetic grains to large multi domain grains in contrast the natural samples probably consist predominantly of pseudo single domain and single domain crystals capable of carrying a chemical remanent magnetisation however the unstable nature of greigite under laboratory conditions creates difficulty in characterising its physical attributes

a palaeomagnetic study of charnockites from madras block southern granulite terrain india abstract the south indian craton is composed of low grade and high grade metamorphic rocks across different tectonic blocks between the moyar bhavani and palghat cauvery shear zones and an elongated belt of eastern margin of the peninsular shield the madras block north of the moyar bhavani shear zone which evolved throughout the precambrian period mainly consists of high grade metamorphic rocks in order to constrain the evolution of the charnockitic region of the pallavaram area in the madras block we have undertaken palaeomagnetic investigation at 12 sites chrm directions in 61 oriented block samples were investigated by alternating field af and thermal demagnetization titanomagnetite in cation deficient cd and multi domain md states is the remanence carrier the samples exhibit a chrm with reverse magnetization of d m 148 1 i m 48 6 k 22 2 95 9 0 and a palaeomagnetic pole at 37 5 n 295 6 e dp dm 7 8 11 8 this pole plots at a late archaean location on the indian apparent polar wander path apwp suggesting an age of magnetization in the pallavaram charnockites as 2600 ma the nearby st thomas mount charnockites indicate a period of emplacement at 1650 ma mesoproterozoic thus the results of madras block granulites also reveal crustal evolution similar to those in the eastern ghats belt with identical palaeopoles from both the areas

superposed deformation fabrics in the precambrian metabasic rocks of the iron ore group singhbhum craton eastern india evidences from anisotropy of magnetic susceptibility studies abstract anisotropy of magnetic susceptibility ams data have been utilized to build up the multiple deformation sequences and strain variation from an apparently undeformed metabasic lava sequence of the basal part of the paleo mesoarchean greenstone succession of the iron ore group rocks in the western iron ore basin singhbhum craton eastern india the basal lava crops out bordering the regional horseshoe shaped outcrop the horseshoe synclinorium structure of the overlying bif rocks ams data of 892 cylindrical specimens drilled from 83 samples collected from different structural domains within the synclinorium structure are presented in the analysis ams orientation data have been analyzed in the light of superposed fold type 1 interference fabrics developed within bif as a result of successive deformation episodes d1 and d2 it is found that the lava samples ubiquitously present two consistent magnetic fabrics homogeneous over smaller sub areas which have been successfully correlated with the d1 and d2 deformation fabrics from the bif rocks moreover a regional scale variation in intensity and nature of strain is noticed from the ams data which have been utilized to decipher folding mechanism as well as the changing deformation pattern during successive folding events

the nature and chronostratigraphy of quaternary pyroclastic accumulations from lake barombi mbo west cameroon abstract the k ar dating of a limburgitic flow indicates that the barombi mbo maar was active a million years ago the morphology of the maar and the unusual high k 2 o na 2 o ratio of the limburgite suggest either that this age is too old or that this flow is not the last activity of the maar twenty three meters of sediments were cored from the bottom core bm6 of the 110 m deep lake which currently fills the maar they are composed of clayey and sideritic laminae with interstratified ash layers in the lower half of the core these ashes composed of slightly vesiculated sideromelane with olivine and labradorite phenocrysts have been dated between 21 000 and 11 000 yr b p they show no alteration and correspond to aerial fallout of alkaline sodic basaltic affinity which is common in the area but are distinct from the mafic magmas which generated the maar the origin of the tilting and uplifting of a 1 5 m sedimentary column in the lower part of the core remains controversial holocene age pyroclastites are rare and consist of a mixture of granulitic xenocrysts and typically transitional tachylite similar to volcanics from the northeast manengouba magnetite microspherules occurring in holocene layers have no volcanic equivalent they could be related to biogenic or pedogenetic processes induced by more humid climatic conditions

comment on variable head ponded infiltration under constant or variable rainfall by j r philip keywords time equation reference ecol article 1996 002doi 10 1029 95wr03645 url http www agu org journals wr record created on 2005 12 09 modified on 2016 08 08

the late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron abstract the geochemistry of late stage diagenetic replacement of pyrite by magnetite was investigated by laboratory experiments of pyrite dissolution and magnetite precipitation involving organically complexed ferric iron the design of the experiments is based on the hypothesized geochemical conditions for in situ diagenesis in the pennsylvanian age belden formation an organic rich carbonate in northwestern colorado with pyrite grains rimmed with magnetite that contains a cretaceous paleopole position the geochemistry is applicable to both deep and near surface reactions involving pyrite magnetite and organic matter and bears upon the geologic interpretation of secondary chemical remanent magnetizations crm aqueous solutions of ferric ligand complexes and pyrite were heated 60 c for 60 days in the absence of light and at low partial pressures of oxygen additional experiments with each ligand were performed both in the presence of bentonite and without ferric iron ligand only raising the ph to 8 5 produced a ferrous hydroxide precipitate and subsequent heating 90 c resulted in the formation of magnetite ferric complexes of oxalate salicylate and acetylacetonate resulted in pyrite dissolution and magnetite formation the activity of strongly reducing ligands e g catechol or unsaturated fe iii exchange capacity particularly in experiments with bentonite or with solutions of uncomplexed ligands can result in an effective reduction of ferric iron concentration and prevent pyrite dissolution and or interfere with the crystallization of magnetite the bentonite acted to adsorb dissolved species and also underwent direct dissolution especially by the oxalate ligands although these reactions could occur at any depth a continuing supply of molecular oxygen more available in near surface settings would result in the formation of hematite or the production of acidic sulfate conditions whereas at greater depths or restricted porosity the redox gradation is halted at magnetite the results of this study thus provide a likely geochemical mechanism for the replacement of pyrite by magnetite during the latter stages of diagenesis

infiltration into fractured bedrock experiment performed on fractured ash flow tuff water was released from a 3 4m 2 infiltration plot divided into 12 square subplots with a head of 0 04 m over a period of 800 days this experiment revealed peculiar infiltration patterns not amenable to current infiltration models which were originally developed for infiltration into soils over a short duration in particular we observed that in part of the infiltration plot the infiltration rate abruptly increased a few weeks into the infiltration tests we suggest that these anomalies result from increases in fracture permeability during infiltration which may be caused by swelling of clay fillings and or erosion of infill debris interaction of the infiltration water with subsurface natural cavities lithophysal cavities could also contribute to such anomalies this paper provides a conceptual model that partly describes the observed infiltration patterns in fractured rock and highlights some of the pitfalls associated with direct extension of soil infiltration models to fractured rock over a long period

biogenic magnetite in stromatolites i occurrence in modern sedimentary environments abstract in this paper we report the occurrence of biogenic ultra fine grained single domain magnetite in both marine and non marine modern stromatolitic environments magnetotactic bacteria were found associated with the microbial communities involved in the deposition of laminated sediments at laguna figueroa baja california mexico and carbonate stromatolitic nodules at sugarloaf key fl u s a these bacteria and the ultra fine grained magnetite they produce have a profound effect on the magnetic properties of the sediments the presence of this single domain magnetite was detected using rock magnetic methods while the morphology was identified by transmission electron microscopy examination by rock magnetic methods of microbial mats and laminated sediments from solar lake sinai guerrero negro baja california mexico and shark bay australia as well as stromatolites from shark bay and clifton lake australia bacalar lake and cuatro cienegas mexico and walker lake u s a indicate these localities contain single domain magnetite biogenic magnetites of presumed bacterial origin have been extracted from shark bay mats walker lake stromatolites and bahama carbonates these findings suggest that bacterial magnetites can be recognized as the tiniest trace fossil and may be found preserved in fossil stromatolites and laminated sediments

magnetotactic bacteria and single domain magnetite in hemipelagic sediments remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time scale study geomagnetic reversals and secular variation and measure the rates of seafloor spreading although these sediments may contain different magnetic minerals magnetite is the most commonly observed and magnetically stable phase and its size shape and post depositional fate affect the magnetic remanence of the sediments biogenic magnetites are single domain with a high natural magnetic remanence nrm and have been suggested as a significant source of magnetic remanence in marine sediments we have studied surface sediments from the santa barbara basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra fine grained single domain magnetite using a novel extraction technique transmission electron microscopy and squid magnetometry we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments

electron optical characterization of bacterial magnetite abstract magnetic grains isolated from magnetococcoid bacterial cells were studied by means of transmission electron microscopy electron diffraction and electron microprobe analysis observed in situ the magnetic grains are each surrounded by an organic membrane and are usually found in a random array although chains are also seen electron diffraction confirms the magnetite mineralogy and provides additional evidence in favor of vacancies in the structure electron microprobe analysis shows the magnetite to be slightly titaniferous electron microscopy indicates that the grains rather than being flake shaped are parallelepiped crystals with a mean length of 99 3 8 7nm a mean width of 62 3 6 1nm yielding a width to length ratio of 0 63 these data support the contention that the magnetic bacterial grains are single domain crystals capable of producing a natural remanent magnetization in sediments

rock magnetic and geochemical record in a sediment core from the eastern arabian sea diagenetic and environmental implications during the late quaternary abstract rock magnetic concentration grain size and mineralogy parameters together with organic carbon calcium carbonate redox sensitive elements 18 o of globigerinoides ruber and radiocarbon dating were carried out on a 445 cm long sediment core collected at 1380 m depth off mangalore southwestern margin of india the top 290 cm sediments of the core correspond to the last 18 kabp the 18 o and magnetic records exhibit major events at 16 kabp 14 5 kabp 11 5 kabp and 9 8 8 6 kabp related to start and intensity of the summer monsoon and climate change and are synchronous with that of the western arabian sea and north atlantic the sediments with high magnetic susceptibility correlate with high sedimentation rates the sediments are dominated by fine grained magnetite but intervals of 1 2 3 8 kabp and 10 13 5 kabp were subjected to diagenetic changes resulting in the dissolution of fine grained magnetites and enrichment of redox sensitive trace elements cu ni zn v mo and u the sediments between 290 cm and 445 cm correspond to 18 27 kabp and are characterized by distinct decrease in magnetic concentration grain size and mineralogy parameters high organic carbon low concentrations of redox sensitive trace elements and abundant pyritized tubules the reductive diagenetic conditions indicated by rock magnetic properties are in contrast with the weak sub oxic conditions revealed by low concentrations of trace elements in the sediments the seasonal organic matter flux produced during the winter monsoon and moderate sedimentation rates favoured reductive diagenesis in the sediments at and below the last glacial maximum lgm intermittent bioturbation however allowed oxidants to penetrate into the sediments remobilized redox sensitive trace elements into the water column and modified the primary geochemical signal of the sedimentary environment

use of anisotropy to determine the origin of characteristic remanence in the siwalik red beds of northern pakistan it is often difficult or impossible to determine the origin of the characteristic remanent magnetization of red beds from the bulk remanence alone however anisotropy of remanence or susceptibility is strongly controlled by the statistical alignment of hematite grains this in turn may reflect the development of the magnetic fabric of the sediment over time so the shape of the anisotropy ellipsoid may provide clues to the origin of remanence in this work we make a study of the anisotropy of magnetic susceptibility ams in siwalik red beds of miocene age from northern pakistan comparison of the results with detailed petrographic studies and other information suggests that advanced soil development leads to the destruction of primary fabrics and often with it a coherent magnetization furthermore it should be possible to use ams fabric information to quantify the degree of pedogenesis in these miocene soils we attempted to determine the anisotropy of isothermal remanence air but found ams to be the technique of choice because of apparent changes in coercivity during air experiments we interpret the air data as resulting from metastable domains in hematite grains which change domain state during the air experiment

palaeomagnetic study of archaean banded hematite jasper rocks from the singhbhum orissa craton india abstract the banded hematite jasper formation within the iron ore supergroup of the singhbhum craton in eastern india comprises fine alternating layers of jasper and specularite it was deposited at 3000 ma and deformed by a mobile episode at 2700 ma hematite pigment 10 m is chiefly responsible for red to brown rhythmic bands in the hematite jasper facies although thermomagnetic study also shows that minor amounts 1 2 of magnetite are present palaeomagnetic study identifies a dual polarity remanence resident in hematite d i 283 60 95 12 which predates deformation studies of the fabric of magnetic susceptibility and rock magnetic results suggest a diagenetic origin for this magnetisation with the hematite formed from oxidation of primary magnetite the palaeopole 32 e 24 n dp dm 14 18 records the earliest post metamorphic magnetisation event in the orissa craton a minimum apparent polar wander motion of the orissa singhbhum craton of through 80 is identified during late archaean times 2900 2600 ma

falling head ponded infiltration it is remarkable that the green ampt or delta function solution for falling head ponded infiltration takes precisely the same functional form as that for constant head infiltration only the values of the constants change falling head infiltration experiments are thus as amenable to green ampt analysis as more complicated constant head experiments

origin of the sedimentary magnetic record at ocean drilling program sites on the owen ridge western arabian sea the 3 2 m y whole core magnetic susceptibility record obtained during ocean drilling program leg 117 from the owen ridge in the western arabian sea represents one of the most convincing demonstrations of the ability of rock magnetic measurements to yield paleoceanographically significant information the salient features of this record are that 1 it correlates strongly with variations in the concentration and flux of eolian dust 2 it is driven strongly at earth orbital periodicities and 3 there is a significant change in the spectral character at 2 4 m y which may reflect the effect of the initiation of major northern hemisphere glaciation on aridity cycles within the eolian source areas in view of its potential paleoclimatic significance we have examined the origin of the rock magnetic signal from the owen ridge in more detail we find that despite the strong relationship throughout the record between magnetic susceptibility and percent terrigenous content there has been significant postdepositional alteration of the magnetic minerals via the process of reductive diagenesis we ascribe two steplike shifts in magnetic properties in the uppermost part of the section to this process the first occurs at about 1 5 m depth and is characterized by a loss of fine grained ferrimagnetic material the second stepshift in magnetic properties occurs at a depth of 6 8 m and is characterized by changes in rock magnetic parameters which indicate significant loss of both low coercivity magnetite and high coercivity hematite and or goethite magnetic mineral components scanning electron microscope and energy dispersive xray analyses show that the bulk of the coarse grained strongly magnetic fraction is detrital titanomagnetite and that there is a significant reduction in the fe ti ratio of these grains across the lower diagenetic front this suggests that titanomagnetite grains with higher ti contents may be more resistant to the process of reductive diagenesis and this may provide a mechanism for the relative stability of the ferrimagnetic fraction below the second diagenetic front

authigenic magnetite formation in suboxic marine sediments the resolution and reliability of magnetostratigraphy and reconstructed time series of geomagnetic field behaviour depend critically on where remanence is acquired and fixed in the sediment column and whether the magnetization is altered chemically after deposition if authigenic magnetic minerals are formed at depth or if the magnetic carriers are changed after deposition then the nature and timing of magnetic events can be affected by depth offsets in remanence acquisition and mixing of detrital and authigenic signals using palaeo and rock magnetic and sediment geochemical analyses we have studied how early diagenesis affects the magnetic properties of suboxic hemipelagic sediments here we report evidence of the formation of fine grained authigenic magnetites near the commonly observed brown tan green colour boundary which marks the transition from fe oxidizing to fe reducing conditions we propose that biogenic magnetite produced by magnetotactic bacteria forms as part of the microbially mediated sequence of reactions involved in the oxidation of organic matter the magnetite is created as a metabolic by product of the microorganisms use of iron redox transitions as a source of energy active magnetite formation appears to be restricted to a zone between the levels of nitrate and iron reduction

biogenic magnetite and the magnetization of sediments biogenic magnetites are produced through the reduction of ferric iron by both biologically induced extracellular and biologically controlled intracellular processes with few exceptions all are ultra fine grained single domain magnetite biogenic magnetites formed by magnetotactic bacteria biologically controlled have been shown to contribute significantly to the natural remanent magnetization of carbonates and limestones hemipelagic and deep sea marine sediments the input into sediments of ultra fine grained magnetite produced by dissimilatory iron reducing bacteria biologically induced has yet to be firmly established but may be even more significant whether either type of authigenic biomagnetite is preserved is determined by postdepositional factors including oxidation reduction by substitution and dissolution unconsolidated and lithified sediments can be screened for putative biogenic magnetite by rock magnetic techniques it is not generally appreciated that magnetotactic bacteria and their magnetofossils can be identified by the unusual and in some cases unique morphologies size range and composition of the magnetite crystals magnetite produced by dissimilatory iron reducing bacteria have a distinctive morphology and size range but it is currently controversial as to whether these can be distinguished from certain chemically precipitated magnetites the presence of dissimilatory iron reducing bacteria however can be detected using microbiological techniques and sediment geochemistry biogenic magnetites are trace fossils and potentially useful environmental indicators and are considered to have significant input to the magnetization of most sediments both modern and ancient

power series solution for falling head ponded infiltration with evaporation 1 assumptions for many of the derived analytical infiltration models are either a constant infiltration rate or a constant head at the soil surface for many realistic infiltration situations a falling head boundary condition is more appropriate philip showed that both the wetting profiles and the cumulated infiltration in one dimensional infiltration into homogenous soil with constant head at the upper boundary can be written as a power series in t presently it is shown that a power series solution can be applied for falling head infiltration with evaporation when comparing with the constant head solution the falling head solution differs only from the second term i e the sorptivity remains unchanged a result which can be applied to falling head infiltrometer technique the effect of a constant evaporation rate in the falling head scenario appears from the third term in the presented numerical examples the retention model by van genuchten and the hydraulic conductivity model by mualem are applied by comparing the wetting profiles obtained with a fem solution it is shown that the discrepancies between the models are very small also the estimation of the time at which the pond is emptied is concordant the infiltration model is compared with a model by parlange and colleagues and a simple infiltrometer method for determining the saturated hydraulic conductivity is proposed

magnetite dissolution and neocrystallization during cleavage formation paleomagnetic study of the martinsburg formation lehigh gap pennsylvania paleomagnetic study of the shale to slate transition in the ordovician martinsburg formation at lehigh gap pennsylvania reveals that the remanence of the relatively undeformed shales predates cleavage formation and that the slates were remagnetized during cleavage development the shales display two components of remanence component 1 c1 was the only magnetization direction in the majority of the shales and the first removed component in 10 shale specimens component 2 c2 is observed as a single component remanence with a low 300 400 c unblocking temperature range in samples from one shale station and as a second removed component in 10 shale specimens the mean directions are declination inclination 347 04 346 47 in situ tilt corrected for c1 and 80 63 135 40 for c2 the pencil slates and well cleaved slates both display single component remanences the mean directions are 198 12 201 26 for the pencil slates and 199 24 225 54 for the well cleaved slates using the relationship between cleavage development and remagnetization in the slates the timing of cleavage development and folding and the regional geology we can constrain the relative ages of the remanences as follows 1 the shale c1 direction is most likely prefolding and represents a primary upper ordovician direction 2 the slate directions are younger in age than the shale directions and are probably of permo carboniferous alleghenian age these results show that remagnetization occurred during development of slaty cleavage in these rocks we interpret the mechanism for this remagnetization to have been strain induced dissolution and neocrystallization of magnetite the additional difference in the characteristic directions of the pencil slates and well cleaved slates may be attributed to strain reorientation of the pencil slate remanence after new growth of magnetite the unknown temporal relationship between dissolution and neocrystallization and strain precludes any quantitative attempt to correct remanence for strain in these samples our study shows that paleomagnetic studies of deformed sediments must consider the possibility of dissolution and neocrystallization of remanence carriers in addition to strain induced rotation

comment on falling head ponded infiltration by j r philip reference ecol article 1995 001doi 10 1029 94wr02565 url http www agu org record created on 2005 12 09 modified on 2016 08 08

geology and geothermal origin of grant canyon and bacon flat oil fields railroad valley nevada eastern nevada s grant canyon and bacon flat oil fields show strong evidence of formation in a still active moderate temperature geothermal system modern manifestations of this system include unusually elevated oil reservoir temperature at shallow depth 116 122 c at 1 1 1 6 km and dilute na hco3 cl thermal waters directly associated with hot oil hydrogen and oxygen isotopic compositions indicate that these thermal waters are meteoric in origin but were probably recharged prior to the holocene before 10 ka the waters apparently ascended to oil reservoir elevations after deep heating in response to the normal regional thermal gradient there is no evidence for a modern magmatic heat source the beginning of oil reservoir evolution at both fields is recorded by late stage fracture filling quartz in the vuggy brecciated paleozoic dolostone reservoir rocks oil and aqueous solutions were trapped as fluid inclusions in the quartz at temperatures comparable to those now prevailing in the reservoirs apparent salinities of the aqueous inclusions closely match the actual concentrations of current oil field waters and the quartz has the oxygen isotopic composition predicted for its crystallization from these waters at contemporary temperatures present day and fluid inclusion temperatures define essentially coincident isothermal profiles through and beneath the oil reservoir interval a phenomenon consistent with near constant convective heat transfer since inception of the eothermal system textural and mineralogic clues indicate that hot waters circulating in this system also increased porosity by dissolving carbonate minerals and helped seal reservoir margins by precipitating silica and kaolin more importantly the rising thermal waters may have aided oil transport and accelerated source rock maturation through an increase in the shallow 3 km local thermal budget along with the aforementioned fluid inclusion and isotopic evidence radiometrically dated life spans for numerous extinct geothermal systems epithermal ore deposits in the basin and range make it likely that the grant canyon bacon flat system and associated oil reservoirs are no older than 2 5 ma

elongated prismatic magnetite crystals in alh84001 carbonate globules potential martian magnetofossils using transmission electron microscopy tem we have analyzed magnetite fe 3o 4 crystals acid extracted from carbonate globules in martian meteorite alh84001 we studied 594 magnetites from alh84001 and grouped them into three populations on the basis of morphology 389 were irregularly shaped 164 were elongated prisms and 41 were whisker like as a possible terrestrial analog for the alh84001 elongated prisms we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain mv 1 by tem again we examined 206 magnetites recovered from strain mv 1 cells natural darwinian selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment in this study we describe six properties of magnetite produced by biologically controlled mechanisms e g magnetotactic bacteria properties that collectively are not observed in any known population of inorganic magnetites these criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories of the alh84001 magnetites that we have examined the elongated prismatic magnetite particles 27 of the total are indistinguishable from the mv 1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals

falling head ponded infiltration in the nonlinear limit the green and ampt infiltration solution represents only an extreme example of behavior within a larger class of very nonlinear delta function diffusivity soils the mathematical analysis of these soils is greatly simplified by the existence of a sharp wetting front below the soil surface solutions for more realistic delta function soil models have recently been presented for infiltration under surface saturation without ponding after general formulation of the problem solutions for a full suite of delta function soils are derived for ponded surface water depleted by infiltration exact expressions for the cumulative infiltration as a function of time or the drainage time as a function of the initial ponded depth may take implicit or parametric forms and are supplemented by simple asymptotic expressions valid for small times and small and large initial ponded depths as with surface saturation without ponding the green ampt model overestimates the effect of the soil hydraulic conductivity at the opposing extreme a low conductivity model is identified that also takes a very simple mathematical form and appears to be more accurate than the green ampt model for larger ponded depths between these two the nonlinear limit of gardner s soil is recommended as a physically valid first approximation relative discrepancies between different soil models are observed to reach a maximum for intermediate values of the dimensionless initial ponded depth and in general are smaller than for surface saturation without ponding

microstructures in a banded iron formation gua mine india this paper provides a detailed documentation of microstructures developed in the banded iron formation bif of gua mine located in the bonai synclinorium eastern india where the rocks have been subjected to three deformations d 1 to d 3 folded iron ores quartz strain fringes around rigid core objects and folded iron ore layers and refracted quartz veins are described from samples taken from d 2 folds in the banded iron formation orientations of microstructures are compared with mesoscopic structures to interpret the generations of ore minerals planar structures and the time relationship between deformation and development of different microstructures the mechanism of d 2 folding is worked out and its bearing on microstructure development is discussed the d 2 folds are inferred to have developed by a combination of tangential longitudinal strain in the competent layer flexural flow in the incompetent layers and flexural slip at the interface between layers of differing competence homogeneous flattening strain superposed the earlier strain which led to modification of the folds in the competent layer from class 1b to 1c this strain is quantified and is found to be higher in the limb than the hinge of a fold diffusive mass transfer by solution and bulging dynamic recrystallization in quartz are inferred as the dominant deformation processes during folding moreover based on comparison with published deformation microstructure maps the microstructures of the present study are estimated to have developed between 300 and 350 c temperatures at a strain rate of 10 14 10 12 s 1 which are geologically realistic conditions for naturally deformed rocks

a stacked late quaternary fluvio periglacial sequence from the axe valley southern england with implications for landscape evolution and palaeolithic archaeology the current model of mid latitude late quaternary terrace sequences is that they are uplift driven but climatically controlled terrace staircases relating to both regional scale crustal and tectonic factors and palaeohydrological variations forced by quasi cyclic climatic conditions in the 100 k world post mid pleistocene transition this model appears to hold for the majority of the river valleys draining into the english channel which exhibit 8 15 terrace levels over approximately 60 100 m of altitudinal elevation however one valley the axe has only one major morphological terrace and has long been regarded as anomalous this paper uses both conventional and novel stratigraphical methods digital granulometry and terrestrial laser scanning to show that this terrace is a stacked sedimentary sequence of 20 30 m thickness with a quasi continuous i e with hiatuses pulsed record of fluvial and periglacial sedimentation over at least the last 300 400 k yrs as determined principally by osl dating of the upper two thirds of the sequence since uplift has been regional there is no evidence of anomalous neotectonics and climatic history must be comparable to the adjacent catchments both of which have staircase sequences a catchment specific mechanism is required the axe is the only valley in north west europe incised entirely into the near horizontally bedded chert crypto crystalline quartz and sand rich lower cretaceous rocks creating a buried valley mapping of the valley slopes has identified many large landslide scars associated with past and present springs it is proposed that these are thaw slump scars and represent large hill slope failures caused by vauclausian water pressures and hydraulic fracturing of the chert during rapid permafrost melting a simple 1d model of this thermokarstic process is used to explore this mechanism and it is proposed that the resultant anomalously high input of chert and sand into the valley during terminations caused pulsed aggradation until the last termination it is also proposed that interglacial and interstadial incision may have been prevented by the over sized and interlocking nature of the sub angular chert clasts until the lateglacial when confinement of the river overcame this immobility threshold one result of this hydrogeologically mediated valley evolution was to provide a sequence of proximal palaeolithic archaeology over two mis cycles this study demonstrates that uplift tectonics and climate alone do not fully determine quaternary valley evolution and that lithological and hydrogeological conditions are a fundamental cause of variation in terrestrial quaternary records and landform evolution

palaeomagnetism of sukinda chromites and their geological implications summary thirty eight oriented samples were collected from seven sites over the precambrian sukinda chromite ore between 85 43 e 21 01 n and 85 50 e 21 03 n which is in the form of a plunging syncline on isolating the primary component of magnetization and affecting the necessary tectonic correction four sites a b c f were found to possess a reverse magnetic polarity d 210 i 22 with k 122 and 95 6 and one site e was found to bear a normal polarity d 49 i 16 with k 474 and 95 2 the remaining two sites d and g were rejected the former because of scatter in directions and the latter due to overprinting by a dominant secondary magnetization induced by lateritization on comparison of vgps evaluated for these chromite deposits with those of other precambrian rocks almost of a similar age it is seen that these chromites are probably older than the gwalior traps whose age is 1830 200 ma from the rock magnetic and microscopic examination it is deduced that the massive chromites of the sukinda valley have not been affected magnetically to any great extent by serpentinization though their host rocks are serpentinized ultramafics consistent directions of remanent magnetism point to the fact that these deposits crystallized in fairly stable conditions characteristic of the stratiform type of chromite deposits therefore leading us to infer the sukinda chromites to be of that type these palaeomagnetic data when subjected to the fold test show the primary remanence to pre date the tectonic disturbance and hence prove these chromites to have consolidated prior to the regional tectonic activity of the area

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

magnetostratigraphic dating of shallow water carbonates from san salvador bahamas magnetostratigraphic results are reported here from a sequence of late neogene quaternary shallow water carbonate sediments from a continuous core drilled on the island of san salvador bahamas on the basis of the remanent magnetism of 136 samples from a 91 m measured section of core the polarity sequence can be correlated with the magnetic polarity time scale from the gilbert chron early pliocene through the late brunhes chron late pleistocene holocene magnetic polarities were determined on the basis of relative up down direction in the unoriented core extraction studies of the magnetic particles reveal the presence of single domain crystals of magnetite resembling those produced by the magnetotactic bacteria and algae the sequence of reversals provides a minimum of six new major chronostratigraphic markers for the pliocene pleistocene of the bahamas it confirms and refines the local timing of both the lithologic change from skeletal to nonskeletal sediments and the disappearance of coral and molluscan species from the bahamas as upper late pliocene between 2 6 and 2 7 ma that the primary magnetic remanence is preserved in shallow water carbonates including replacement dolomites suggests that this technique could be used to date similar tertiary and possibly even older carbonate sequences the establishment of a reliable magnetostratigraphy provides refined dating of shallow water carbonates and regional faunal appearances or disappearances sediment accumulation rates subsidence and depositional events

magnetofossils the magnetization of sediments and the evolution of magnetite biomineralization magnetite fe 3o 4 is one of the most stable carriers of natural remanent magnetization nrm in sedimentary rocks and paleomagnetic studies of magnetite bearing sediments such as deep sea cores and pelagic limestones have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales despite this important role there is as yet a very poor understanding of how ultrafine grained 0 1 m magnetite is formed transported and preserved in marine environments a major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite formed by magnetotactic bacteria and or other magnetite precipitating organisms is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks since these magnetite particles are of biogenic origin they are termed properly magnetofossils kirschvink chang 1984

evidence for bacterial palaeoecological origin of mineral magnetic cycles in oxic and sub oxic tasman sea sediments abstract sediments from the tasman basin in the southwest pacific ocean show a cyclic pattern of variation in mineral magnetic properties including susceptibility and remanence over the brunhes chron the magnetic pattern is dominated by fluctuations in the concentration of a single domain ferrimagnetic component identified as bacterial magnetofossils by tem investigation of the magnetic fraction the intervals of low bacterial magnetite concentration occurring in glacial periods are accompanied by a change of the bacterial magnetosome morphology assemblage interpreted as recording a change in bacterial species assemblage there is no evidence of iron or sulphate reducing conditions during low bacterial magnetite intervals concentrations of bacterial magnetite are low when oxygen concentrations in surface sediment pore waters dropped either as the result of variation in bottom water oxygen concentration or sedimentation rate the variations in the magnetic signal are therefore interpreted as resulting primarily from bacterial palaeoecological control rather than dissolution or alteration the bacteria in this deep sea environment are apparently aerophilic and grow near the sediment surface within the oxic zone implying that magnetic remanence is also acquired near the surface but up to tens of centimetres below where the oxic zone is very thick

approximate analysis of falling head lined borehole permeameter the falling head lined borehole permeameter consists of a snugly fitting tube extending to the base of a vertical borehole in unsaturated soil water is suddenly introduced into the tube to standard depth and the subsequent time course of water depth is observed soil water data in the amazon basin has been secured with this instrument we develop an approximate analysis based on a green ampt model with the flow treated as a gravity perturbed spherically symmetrical pressure capillarity driven flow

biogenic magnetite in stromatolites ii occurrence in ancient sedimentary environments abstract in this paper we report the discovery of fossil bacterial single domain magnetite particles in ancient stromatolites the biogenicity of the crystals was determined by the following criteria 1 distinctive morphology and habit 2 composition and 3 environment of deposition stromatolites ranging in age from the middle archean to pleistocene composed of both carbonate and chert were analyzed for the presence of single domain magnetite using rock magnetic methods the granulometry and composition of the ultra fine grained magnetite crystals extracted were determined by transmission electron microscopy and electron diffraction the oldest magnetofossils were extracted from stromatolitic chert of the gunflint iron formation which is approximately 2000 ma old the implications of these findings and the potential uses of fossil bacterial magnetite in studies of the evolution of biomineralization and prokaryotic metabolic processes paleomagnetism and as an indicator of ancient oxygen levels are discussed bacterial magnetite represents the oldest evidence of biomineralization yet discovered in the fossil record

diagenesis of magnetic minerals in recent haemipelagic sediments rapidly deposited sediments from marine and lake environments are being used increasingly to study decadal to millennial fluctuations in the earth s magnetic field the objectives are to gain more fundamental understanding of the geodynamo and to establish a new dating technique for sediments while lacustrine sections are generally restricted to temperate latitude glacial lakes 20 000 yr old rapidly deposited marine sediments along continental margins potentially offer continuous high resolution yet long term records of geomagnetic secular variation however to interpret the sedimentary magnetic record accurately geochemical processes that affect the reliability of the magnetic signal must be understood we now report downcore magnetic profiles from undisturbed kasten cores taken in rapidly deposited laminated sediments from the gulf of california and in bioturbated haemipelagic muds on the oregon continental slope which give apparently reliable directions but show dramatic decreases in the intensities of natural nrm and artificial arm irm remanences with depth downcore porewater and solid sulphur analyses show concave down decreases in porewater sulphate and systematic increases in pyrite and metastable monosulphides the maximum curvature of the sulphide profile occurs directly below the high magnetization zone combined with other compositional and mineralogical analyses these data suggest that due to oxidative decomposition of organic matter magnetites and other iron oxides become progressively reduced and subsequently sulphidized and pyritized with depth iron reduction seems to occur before sulphide formation changes in magnetic stability parameters are consistent with selective dissolution of the finer sized grains causing downcore coarsening of the magnetic fraction

a record of holocene climate in the mineral magnetic record of alpine lakes sagistalsee and hinterburgsee the effect of climate variations on the magnetic properties of sediments from two small alpine lakes in the bernese alps switzerland was investigated although relatively similar in water volume and general environment sa gistalsee and hinterburgsee have different catchment vegetation and lithology due to their different elevation and geographic locations the climatic signal in the mineral magnetic parameters of the sediments was validated and interpreted with the aid of palynological and geochemical data the sediment age was determined with several accelerated mass spectrometry 14 c dates the results from the different lakes compare well on a common time scale and show that the influence of local conditions on the magnetic record is rather small a climatic influence on the mineral magnetism has been recognized at both studied sites this is interpreted as a consequence of the production of an authigenic mineral with particularly uniform magnetic properties during the warmer stages and the influx of heterogeneous detrital magnetic minerals during the colder stages the clearest climate record is found in sediments with no organic matter whereas the anoxic conditions found in highly organic sediment can completely obliterate the climate signal recorded in the sediment magnetic properties fl 2001 elsevier science b v all rights reserved

ultrafine grained magnetite in deep sea sediments possible bacterial magnetofossils a new extraction technique now permits ultrafine magnetite crystals to be separated from a variety of deep sea sediments morphologic characterization of these particles with transmission electron microscopy reveals the presence of several distinct crystal types some of which closely resemble those formed by the magnetotactic bacteria the apparently biogenic magnetite particles are of single domain size and dominate the population in calcareous deep sea sediments bacterially precipitated magnetite may therefore be a major source of the stable magnetic remanence in some marine sediments these objects possibly constitute the smallest mineral fossils yet recovered from the sedimentary record

bacterial magnetite in lake sediments late glacial to holocene climate and sedimentary changes in northern norway magnetic properties of sediments retrieved from a small lake in northern norway spanning the last f14500 cal years have been analyzed in detail the high resolution core with its ability to detect sub centurial variability records the rapid retreat of the fennoscandian ice sheet fis in northern lyngen and subsequently the establishment of a stable allerod to younger dryas watershed from early holocene and to the present magnetotactic bacteria mb producing stable single domain ssb magnetite become the dominating magnetic carrier the small low relief catchment area with its quartzite bedrock scattered till and vegetated surface generates a negligible contribution of detrital magnetic minerals this allows for the opportunity to follow the presence and changes of the magnetosomes during the last 10000 years a strong positive statistical relationship r 2 0 83 between the anhysteretic remanent magnetisation susceptibility varm and loss on ignition loi suggests an environmental control over the bacteria magnitude quart int 88 2002 13 that most likely is related to summer climate a comparison between the magnetic mineralogy as evident from the s ratio jirm100mt jsirm3t and the reconstructed summer temperatures from a nearby lake shows a close covariation between cooler summers and harder s ratios for a series of climatic events intervariability of loi data and other proxies such as august sea surface temperatures sst from the nordic sea suggests land ocean connections during converging phases of cooler climate where the most prominent in phase periods are approximately 1 6500 5000 2 3500 2400 and 3 1700 1000 cal years bp the most significant change in the holocene magnetic signal is initiated at f3700 cal years bp which we refer to here as the holocene turnover d 2004 elsevier b v all rights reserved

coupled effects of canal lining and multi layered soil structure on canal seepage and soil water dynamics ponding tests were conducted in the shiyang river basin in northwest china to assess canal leakage characteristics four anti seepage constructions concrete lining pebble lining clay lining plus compacted canal bed compacted canal bed only were performed on four canal sections which were situated in multi layered soils the canal sections were tested using a two stage approach first a stable water level was maintained followed by a stage where the water level in the canal section was permitted to drop the canal seepage rate and the soil water content near the canal bed were monitored during the stable and dropping stages soil texture bulk density and hydraulic conductivity were tested in each canal section and soil layer double ring infiltration tests were performed to investigate infiltration behaviour from the canal sections the saturated unsaturated flow model hydrus 2d was applied to simulate canal seepage and the local soil water response the simulation results compared well with the monitored data indicating that the model can reliably simulate canal seepage under these complex soil structures and different canal liners both experimental results and numerical modelling show that the clay lining plus compacted canal bed provides the best anti seepage performance followed by compacted canal bed only then pebble and concrete lining simulation results also predicted that the soil water content was discontinuous at the interface of distinct soil layers and that the range and form of wetting front varied greatly in the four canal sections with a larger wetted area for the more permeable canals simulations were performed to study the sensitivity on canal seepage of to the permeability of each soil lining layer and the lining effect on canal seepage the results confirmed by double ring infiltration tests indicated that the canal lining is not the only dominant factor affecting canal seepage the soil permeability can also influence the seepage greatly especially where there exists a low permeability layer e g compacted soil layer close to the canal