sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

comparison of magnetic hysteresis parameters of unremagnetized and remagnetized limestones for white magnetite bearing mesozoic pelagic limestones from italy which carry a primary magnetization the values of mrs ms and hcr hc generally lie in the pseudo single domain psd field of the day et al 1977 plot the logarithmic plot of mrs ms against hcr hc gives a straight line r 0 814 with slope and intercept close to the empirical mixing line of parry 1982 for single domain sd and multidomain md magnetite for one of the white pelagic limestone formations maiolica formation samples with hysteresis ratios closer to the md field display increased paramagnetic susceptibility and are from the upper part of the formation characterized by increased detrital clay we therefore associate the increased md magnetite with increased detrital influx for pinkish and reddish varieties of the italian pelagic limestones the presence of hematite is manifest by high saturation fields a wide range of hcr hc and wasp waisted hysteresis loops attributed to the mixing of magnetite and high coercivity authigenic hematite the hysteresis ratios for a collection of paleozoic and mesozoic remagnetized magnetite bearing limestones from britain nevada alaska and the appalachians lie mainly outside the psd field and appear to follow a power law trend following jackson et al 1993 the high values of hcr hc and the characteristically wasp waisted hysteresis loops can be interpreted in terms of a fine grained subspherical high coercivity sd magnetite mixed with a high proportion of superparamagnetic magnetite the slope and intercept of the power law relationship for mrs ms and hcr hc in the remagnetized limestones are distinct from those observed for the italian limestones and may provide a means of fingerprinting magnetite of primary as opposed to diagenetic origin

benthic fluxes and porewater concentration profiles of dissolved organic carbon in sediments from the north carolina continental slope abstract numerous studies of marine environments show that dissolved organic carbon doc concentrations in sediments are typically tenfold higher than in the overlying water large concentration gradients near the sediment water interface suggest that there may be a significant flux of organic carbon from sediments to the water column furthermore accumulation of doc in the porewater may influence the burial and preservation of organic matter by promoting geopolymerization and or adsorption reactions we measured doc concentration profiles for porewater collected by centrifugation and sipping and benthic fluxes with in situ and shipboard chambers at two sites on the north carolina continental slope to better understand the controls on porewater doc concentrations and quantify sediment water exchange rates we also measured a suite of sediment properties e g sediment accumulation and bioturbation rates organic carbon content and mineral surface area that allow us to examine the relationship between porewater doc concentrations and organic carbon preservation sediment depth distributions of doc from a downslope transect 300 1000 m water depth follow a trend consistent with other porewater constituents co2 and so42 and a tracer of modern fine grained sediment fallout pu suggesting that doc levels are regulated by organic matter remineralization however remineralization rates appear to be relatively uniform across the sediment transect a simple diagenetic model illustrates that variations in doc profiles at this site may be due to differences in the depth of the active remineralization zone which in turn is largely controlled by the intensity of bioturbation comparison of porewater doc concentrations organic carbon burial efficiency and organic matter sorption suggest that doc levels are not a major factor in promoting organic matter preservation or loading on grain surfaces the doc benthic fluxes are difficult to detect but suggest that only 2 of the dissolved organic carbon escapes remineralization in the sediments by transport across the sediment water interface

evaluating the timing of hydrocarbon generation in the devonian duvernay formation paleomagnetic rock magnetic and geochemical evidence abstract the physical and chemical changes associated with the thermal maturation of organic rich shale have affected the paleomagnetic and rock magnetic characteristics of the devonian duvernay formation in the western canada sedimentary basin this formation has several lithofacies that correspond to deposition in platform slope and deeper water settings under varied redox conditions shale laminated mudstone and some massive mudstone facies show evidence of magnetic changes associated with maturation but wackestone packstone and some massive mudstone facies appear to be unaffected by the process rock magnetic evidence suggests that thermal maturation induces a change in the magnetization carrier from magnetite and hematite to solely magnetite the packstone and wackestone facies commonly show a reversed characteristic magnetization with a paleopole at 194 e 70 n a95 13 2 of late cretaceous age shale and laminated mudstone facies in immature areas of the basin have inclination only characteristic remanent magnetization chrm means that range from 55 to 67 c requiring a pre cretaceous magnetization age shale and laminated mudstone facies in mature areas of the basin have a much steeper chrm in direction ranging from 77 to 83 c their very steep nature suggests that step demagnetization has not completely removed a drilling induced remanence in some wells

palaeomagnetism and magnetic anisotropy of late neoproterozoic strata south australia implications for the palaeolatitude of late cryogenian glaciation cap carbonate and the ediacaran system abstract the ediacaran global stratotype section and point gssp is defined near the base of the nuccaleena formation the cap carbonate overlying the terminal cryogenian glaciogenic elatina formation at a basin wide sequence boundary in the adelaide geosyncline south australia previous palaeomagnetic data for 205 specimens from the elatina formation yielded a structurally corrected mean declination d 208 3 and inclination i 12 9 95 4 2 palaeolatitude 6 5 2 2 and pole position at latitude p 47 3 s longitude p 359 3 e dp 2 1 and dm 4 2 new palaeomagnetic data for 141 specimens from the nuccaleena formation yield a structurally corrected mean direction of d 208 3 i 34 9 95 3 4 19 2 2 3 2 2 and pole position at p 32 3 s p 350 8 e dp 2 2 and dm 3 9 there is a substantial difference in inclination between the elatina and nuccaleena formations despite their juxtaposition positive fold tests for both formations seem to rule out remagnetisation of one or both formations fold tests on tepee like structures in the nuccaleena formation accord with the magnetisation having been acquired at or near the time the structures formed during early diagenesis magnetisation therefore variably lags deposition causing the three identified magnetostratigraphic horizons to appear highly bedding transgressive whereas local correlations may seem satisfactory regional correlations are not nevertheless the number and duration of polarity chrons at various localities appear valid implying that the nuccaleena formation accumulated over an interval of 10 5 years the question of compaction related inclination shallowing was addressed by examining the anisotropy of remanence using a superconducting 12 t magnet the effective flattening factors f determined were nuccaleena dolostones n 4 f 0 98 0 02 elatina tidal rhythmites n 6 f 0 97 0 01 elatina arenites n 9 f 0 92 0 02 the mean elatina inclination is corrected to 14 0 palaeolatitude of 7 1 and the mean nuccaleena inclination to 35 4 palaeolatitude of 19 6 inclination shallowing also was investigated using the elongation inclination method the distributions of directions for the elatina were somewhat skewed with respect to bedding possibly invalidating use of this method nevertheless for the elatina the method yields an inclination of 19 palaeolatitude of 10 for the nuccaleena the corrected inclination is 41 9 palaeolatitude of 24 these corrections do not materially alter the near equatorial palaeolatitude for the elatina formation which is supported by data for other neoproterozoic strata in southern australia however the substantial increase in palaeolatitude between the late cryogenian and the early ediacaran is maintained the sequence boundary below the nuccaleena formation is a disconformity to low angle unconformity that in places cuts deeply into the 1500 m thick late cryogenian glaciogenic succession and may represent a significant time gap marked by rapid polar wander such a hiatus may be an important factor in the juxtaposition of glacial deposits and cap carbonates

regional patterns of magnetite authigenesis in the appalachian basin implications for the mechanism of late paleozoic remagnetization we report paleomagnetic and rock magnetic data from sedimentary carbonates of the appalachian basin that have implications for the mechanism of widespread late paleozoic remagnetization in this rock type sampling was accomplished mostly along two across basin transects one in devonian carbonates of new york state and the other in ordovician and mississippian carbonates of tennessee and alabama paleomagnetic investigations of the new york devonian carbonates along a transect from albany to buffalo indicate that these rocks were completely remagnetized in alleghenian time rock magnetic studies show that magnetite concentration varies in a regular fashion along the transect with a local maximum near syracuse and decreasing gradually to the east and west this pattern shows a striking correlation with the degree of diagenetic alteration of clay minerals in a devonian bentonite horizon which took place during alleghenian time we conclude that most of the magnetite present in these carbonates is of authigenic origin and that clay mineral alteration and magnetite authigenesis were coeval late paleozoic events that were controlled by the same diagenetic factors thermoviscous remagnetization processes cannot be ruled out in this setting in light of the observed degree of thermal maturity however our results suggest that chemical factors played a critical role in remagnetization since they allow us to infer that magnetite authigenesis and remagnetization are about the same age we propose a geochemical model for magnetite authigenesis wherein the iron is derived from detrital smectites during diagenetic illitization which is triggered by the introduction of potassium rich brines results from the tennessee transect from nashville to chattanooga show a very different pattern paleomagnetic studies indicate that late paleozoic remagnetization has affected ordovician carbonates of the nashville dome paleozoic carbonates in the overthrust belt near chattanooga also carry the late paleozoic remagnetization however mississippian carbonates between the dome and the overthrust belt are very weakly magnetized and show no evidence for late paleozoic remagnetization samples show only a present field magnetization or at one locality in northwestern alabama a dual polarity magnetization of probable mississippian age rock magnetic studies indicate that higher concentrations of magnetite are present in the remagnetized rocks of the nashville dome and the overthrust belt than in the nonremagnetized mississippian carbonates we therefore conclude that magnetite authigenesis and remagnetization are related events in this setting and that the remagnetization must be due to chemical processes the presence of a remagnetization shadow in the younger rocks now exposed between the fold thrust belt and the flexural arch is attributed to westward flow of magnetite forming aqueous fluids through deep sub carboniferous aquifers during alleghenian time upward migration of the fluids through the mississippian carbonates may have been prevented by the devono mississippian chattanooga shale an impermeable stratum that is present throughout the region

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

surface deformation and coherence measurements of kilauea volcano hawaii from sir c radar interferometry the shuttle imaging radar c x synthetic aperture radar sir c x sar radar on board the space shuttle endeavor imaged kilauea volcano hawaii in april and october 1994 for the purpose of measuring active surface deformation by the methods of repeat pass differential radar interferometry observations at 24 cm l band and 5 6 cm c band wavelengths were reduced to interferograms showing apparent surface deformation over the 6 month interval and over a succession of 1 day intervals in october a statistically significant local phase signature in the 6 month interferogram is coincident with the pu u o o lava vent interpreted as deformation the signal implies centimeter scale deflation in an area several kilometers wide surrounding the vent peak deflation is roughly 14 cm if the deformation is purely vertical centered southward of the pu u o o caldera delays in the radar signal phase induced by atmospheric refractivity anomalies introduce spurious apparent deformation signatures at the level of 12 cm peak to peak in the radar line of sight direction though the phase observations are suggestive of the wide area deformation measured by global positioning system gps methods the atmospheric effects are large enough to limit the interpretation of the result it is difficult to characterize centimeter scale deformations spatially distributed over tens of kilometers using differential interferometry without supporting simultaneous spatially distributed measurements of refractivity along the radar line of sight studies of the interferometric correlation of images acquired at different times show that l band is far superior to c band in the vegetated areas even when the observations are separated by only 1 day these results imply longer wavelength instruments are more appropriate for studying surfaces by repeat pass observations

a primary magnetization fingerprint from the cretaceous laytonville limestone further evidence for rapid oceanic plate velocities paleomagnetic data from the cretaceous 101 88 ma laytonville limestone of northern california are of geodynamic interest because they suggest a rate of absolute plate motion 15 cm yr that exceeds those of all present day plates such findings assume that a primary magnetization is preserved in these accreted pelagic limestone outcrops this assumption is supported by a trend in the paleomagnetic inclination data that matches in magnitude and sign the expected sense of motion predicted by comparing the mean paleolatitude value with the north american apparent polar wander path the pervasiveness of remagnetizations seen in shallow water carbonates of north america and elsewhere however raises questions as to whether the laytonville limestone is remagnetized regardless of tests indicating a primary magnetization the remagnetized carbonates have a distinctive wasp waisted magnetic hysteresis signature thought to reflect a bimodal distribution of magnetite grain sizes this signature can be quantified by the relationship between saturation remanence mr saturation magnetization ms coercivity hc and the coercivity of remanence hcr mr ms 0 89 hcr hc 06 this power law is compared with reference data from pacific deep sea carbonates deep sea drilling project sites 167 317 and 463 ocean drilling program site 806 and tethyan pelagic carbonates valle del mis section southern alps which together define a different curve mr ms 0 53 hcr hc 1 0 the laytonville limestone falls in two categories on the basis of unblocking temperature spectra resistance to alternating field demagnetization and behavior during the acquisition of isothermal remanent magnetization red laytonville limestone is thought to contain pigmentary hematite in addition to magnetite hysteresis curves from red laytonville limestone are wasp waisted but because such curves also typify magnetite hematite mixtures these data do not provide an unambiguous test for remagnetization some white laytonville limestone however appears to contain negligible amounts of hematite and can be directly compared with the remagnetized carbonates hysteresis parameters derived from white samples are indistinguishable from those of the reference pelagic limestones such samples have a stratigraphic significance occurring where paleolatitude data indicate an equatorial crossing these samples may have depleted pigmentary hematite due to increased magnetic dissolution as the limestone entered the equatorial belt of high productivity the lack of a remagnetization fingerprint in these white samples together with paleomagnetic lithologic and paleontologic data form a consistent data set supporting the primary nature of the laytonville limestone magnetization and rapid oceanic plate velocities a small plate size coupled with the unusually vigorous mantle plume volcanism of the cretaceous pacific basin could have combined to reduce the effectiveness of asthenospheric drag accounting for the rapid motion

satellite radar interferometry for monitoring ice sheet motion application to an antarctic ice stream satellite radar interferometry sri provides a sensitive means of monitoring the flow velocities and grounding line positions of ice streams which are indicators of response of the ice sheets to climatic change or internal instability the detection limit is about 1 5 millimeters for vertical motions and about 4 millimeters for horizontal motions in the radar beam direction the grounding line detected by tidal motions where the ice goes afloat can be mapped at a resolution of 0 5 kilometer the sri velocities and grounding line of the rutford ice stream antarctica agree fairly well with earlier ground based data the combined use of sri and other satellite methods is expected to provide data that will enhance the understanding of ice stream mechanics and help make possible the prediction of ice sheet behavior

paleoenvironmental reconstruction of the ediacaran araras platform western brazil from the sedimentary and trace metals record ocean and atmosphere oxidation in the ediacaran period paved the way for the dawn of animals in an attempt to better document the record of seawater redox state at the onset of the ediacaran we performed a paleoenvironmental study of post marinoan 635 ma carbonate strata from the southeastern margin of the amazon craton western brazil five sections were sampled along the araras carbonate platform outcrop based facies analysis complemented by petrographic description of representative samples was performed on these sections seven facies associations fa were recognized four fa are encountered in the inner shelf from the basal glacio marine deposits of the puga formation to shallow and moderately deep platform facies which are systematically covered by deeper caco3 over saturated facies five fa occur on the outer shelf including storm wave influenced facies below storm wave base facies and slope intraformational breccia these facies associations indicate a transgressive systems tract over the carbonate platform trace metals u mo zn pb cd cu ni v and al concentrations pyrite abundance and total organic carbon toc contents in these sections are generally low they nonetheless present from base to top of the sections significant stratigraphic variations which can be traced along the platform with successive enrichments of i pb and zn ii u and iii both u and mo in the thin marl levels containing the highest amount of organic carbon and pyrite 0 4 and 1 9 respectively we interpret this succession to record the progressive evolution of sediment pore waters from oxic to more reducing conditions driven by an increase in sedimentary organic matter accumulation the lack of evidence for persistent sulphide mineralization and associated enrichments in mo suggests that sulfidic conditions were only achieved in sediment pore waters in the aftermath of the marinoan glaciation the water column must then have been essentially oxic on the araras platform with anoxia only sporadically reaching the sediment water column interface in the deepest parts of the platform

low latitude and multiple geomagnetic reversals in the neoproterozoic puga cap carbonate amazon craton palaeomagnetic study of the carbonates that ubiquitously cap glacial deposits may constrain the latitudinal extent of neoproterozoic glaciations and the duration of the greenhouse recovery we present the first palaeomagnetic data on the neoproterozoic cap carbonates covering the amazon craton which are folded along the paraguay belt samples collected at deformed beds along the paraguay belt present a single polarity secondary magnetization acquired by the end of the brasiliano orogeny 540 520 ma in the cratonic area a dual polarity component was isolated in dolostones at the base of the sequence the presence of a stratabound reversal stratigraphy along with high unblocking temperatures strongly suggest that this magnetization is primary this result implies a low palaeolatitude 22 6 5 for the amazon block just after deposition of puga diamictites in addition the presence of multiple reversals across the first 20 m of the cap carbonate sequence suggests that their sedimentation must have spanned hundreds of thousands of years at least

microfacies diagenesis and hydrocarbon potential of the neoproterozoic cap carbonate of the southern amazon craton abstract the low latitude marinoan glaciation 635 ma record in the southern amazon craton has been one of the most important sites with strong evidence for the neoproterozoic snowball earth hypothesis in central brazil the 50 m thick carbonate succession of the post marinoan cap carbonate exposed in the tangara da serra region central brazil represents a complementary section of the classical occurrence of the puga cap carbonate in mirassol d oeste region that overlies glaciogenic deposits marking the well preserved cryogenian ediacaran boundary in south america the 8 m thick cap dolostone that represents the mirassol d oeste formation consists of pinkish microcrystalline peloidal dolomudstone exhibiting even parallel to low angle lamination locally tubestone and megaripple bedded dolopackstone the cap dolostone is succeeded upsection by the cap cementstone limestone basal guia formation comprising laminated lime mudstone rich in silt sized terrigenous grains quartz and subordinately bituminous shale dolomitic marls intraformational breccias and abundant calcite crystal fans and crusts crystal fans occur in isolated and decimetre scale arrays and are considered aragonite pseudomorphs primary precipitation of dolomite crystal fans and micrite and the formation of tubestone are related to depositional processes the cap dolostone was affected by the neomorphism of the dolomicrite development of vug and intercrystalline porosity euhedral dolomite precipitation synsedimentary fracturing calcite and quartz precipitation chemical compaction fe oxide substitution and precipitation the diagenetic processes of the cap cementstone limestone were pyrite precipitation calcite cementation neomorphism of micrite and crystal fan partial crystal fan dissolution dolomitization chemical compaction fracturing ferrous dolomite and calcite precipitation pyrite pseudomorph and micrite replacement with fe oxide and replacement of rhombohedral dolomite by mn oxides hydrocarbons are rare and its migration represents the last diagenetic event being found 1 in fractures and interpeloidal vug and intercrystalline pores 2 with low permeability and connectivity of the cap dolostone and 2 in intercrystalline pores fractures and rare pores in crystal fans of the cap cementstone limestone during the post marinoan sea level rise occurred the establishment of a shallow to moderately deep microbially induced dolomitic platform locally influenced by storm action the progressive transgression led the implantation of a deep anoxic and caco3 oversaturated aragonitic platform the similarities of cap carbonate microfacies among the post marinoan succession worldwide reflects an extraordinary and non recurring global precipitation event in geological history in contrast besides the majority of cap carbonate have the same textural and compositional framework the diagenetic history seems to be regional and completely dependent on the tectonic thermal and burial history of the depositional basins where they were deposited

nature and origin of magnetic minerals within the middle jurassic shallow water carbonate rocks of the paris basin france implications for magnetostratigraphic dating summary the middle and upper jurassic bathonian oxfordian shallow water carbonate rocks from the paris basin france consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurface despite a preliminary positive study these deposits have been considered to be largely remagnetized rochette private communication and hence not amenable to palaeomagnetic dating to establish their magnetic mineralogy and test this remagnetization hypothesis we have used an integrated investigation combining petrographic geochemical rock magnetic and palaeomagnetic measurements on samples extracted from five cores from the paris basin and from outcrops in burgundy magnetic minerals in the bathonian oxfordian carbonates include 1 primary biogenic single domain magnetite and detrital multi domain ti magnetite and their oxidized form maghemite 2 authigenic spheres of magnetite probably related to hydrocarbons and 3 goethite either restricted to ferruginous ooid layers or resulting from surficial alteration notably replacement of pyrite framboids rock magnetic experiments carried out on 68 samples reveal hcr hc and mrs ms ratios ranging from 1 88 to 5 58 and 0 017 to 0 314 respectively these values are clearly distinct from diagnostic values for a chemical remagnetization pyrrhotite was not identified within these sediments moreover the average hcr hc ratio of 3 14 is significantly different from the value of 1 333 for natural pyrrhotite dekkers 1988 these results have a direct implication for the preservation of the primary magnetization consequently these deposits are selectively amenable for magnetostratigraphic dating and possible regional correlations

chemostratigraphy of the neoproterozoic mirassol d oeste cap dolostones mato grosso brazil an alternative model for marinoan cap dolostone formation abstract we have conducted a detailed study of the neoproterozoic mirassol d oeste cap dolostones that overlay the glacial diamictites of the puga formation 635 ma amazon craton brazil in order to understand the formation of these post glacial dolostones petrographic features indicate that the dolostones are primary to early diagenetic in origin and precipitated in a moderately shallow water platform corresponding to a carbonate ramp during transgressive conditions major and trace element contents as well as c and o isotopic signatures are consistent with an anoxic sediment influenced by sulphate reducing bacteria such an environment is known to provide favourable conditions for the precipitation of dolomite as observed nowadays in modern hypersaline lagoons isotopic compositions of tube like structures suggest local upward fluid seepage from the underlying cap dolostone our data concur with geochemical data from other neoproterozoic cap dolostones to support a microbially mediated model in specific environmental conditions for the formation of these unusual deposits worldwide

detrital remanent magnetization in haematite bearing neoproterozoic puga cap dolostone amazon craton a rock magnetic and sem study summary neoproterozoic cap dolostones of the araras group mato grosso brazil deposited on top of glacial diamictites puga formation 630 ma yielded a dual polarity component providing al ow palaeolatitude for the amazon craton because palaeomagnetic negative directions are very close to the present day dipolar field a rock magnetic and sem study was conducted in order to check the primary nature of the remanent magnetization and to identify the nature and origin of the remanence carriers results show that detrital specular haematite grain size 10 m is the main magnetic carrier in the araras dolostones and was likely inherited from the underlying puga diamictites in some sites a mixture of haematite and magnetite carries the remanence of both polarities the primary nature of the magnetization is supported by the detrital character of the magnetic carriers in addition the remanence anisotropy is weak pointing to the lack of significant shallowing due to compaction these results reinforce the low palaeolatitude of the amazon craton at the time of the marinoan ice age

ferro and antiferromagnetism of ultrafine grained hematite aluminum substituted hematite here referred to as al hm is an important magnetic mineral for paleo and environmental magnetism however the magnetic properties of nanosized al hm are poorly known in this study a series of al hm samples fourteen samples and their nonsubstituted counterparts referred to as pure hm seven samples were synthesized with particle sizes ranging from 14 to 124 nm with decreasing particle size coercive force bc magnetic remanence mr acquired in a 5 t field at room temperature and the maximum blocking temperature tb of al hm descend gradually tb drops abruptly from 800 to 200 k at 17 2 3 7 nm this size defines the superparamagnetic sp single domain sd threshold of al hm which is significantly lower than that of pure hm 27 5 1 5 nm the antiferromagnetic high field susceptibility anti and the saturation magnetization ms for the weak ferromagnetic component in hematite are both correlated negatively with temperature between 20 and 300 k and grain size in the vicinity of the sp threshold for very small grains below 16 17 nm tb increases with increasing particle size however beyond this size it descends with increasing particle size up to 19 nm before increasing with grain size again these trends are attributed to the competition of surface anisotropy and bulk anisotropy elastic anisotropy this new study substantially improves our understanding of the complex magnetic properties of fine grained al hm and pure hm in natural sample which is significant for the paleoenvironmental and climatic studies of natural samples e g red beds soils

rapid deformation of the south flank of kilauea volcano hawaii the south flank of kilauea volcano has experienced two large magnitude m 7 2 and m 6 1 earthquakes in the past two decades global positioning system measurements conducted between 1990 and 1993 reveal seaward displacements of kilauea9s central south flank at rates of up to about 10 centimeters per year in contrast the northern side of the volcano and the distal ends of the south flank did not displace significantly the observations can be explained by slip on a low angle fault beneath the south flank combined with dilation deep within kilauea9s rift system both at rates of at least 15 centimeters per year

remagnetization in bituminous limestones of the neoproterozoic araras group amazon craton hydrocarbon maturation burial diagenesis or both 1 neoproterozoic carbonates of the araras group exhibit two distinct magnetic components across the same carbonate succession in a cross section between the amazon craton and the paraguay fold belt pink dolostones of the mirassol d oeste formation carry a dual polarity primary component whereas black bituminous limestones of the guia formation yield a secondary postfolding component magnetic signatures of the guia limestones such as high anhysteretic remanence magnetization saturation isothermal remanence magnetization ratios high frequency dependent magnetic susceptibility and contradictory lowrie fuller and cisowski tests are typical of remagnetized carbonates unblocking temperatures suggest that the stable high temperature remanence is carried by both pyrrhotite and magnetite for which an authigenic origin is suggested by scanning electron microscope observations the different magnetic properties noted between dolostones with or without bitumen and between dolostones and limestones in the same metamorphic conditions lead to the hypothesis that the amount of hydrocarbon as well as the lithology influence nucleation of authigenic magnetic minerals in these rocks presence of magnetite pseudoframboids and euhedral iron sulphide crystals occurring in fracture and voids are in favor of a chemical remanence crm the presence of pyrrhotite as one of the main carriers of crm in these rocks and its association with bitumen in fractures is probably related to an epigenetic enrichment of sulfur due to hydrocarbon seepage however hydrocarbon maturation solely could not explain the differences of the magnetic mineralogy observed in the craton and the fold belt enhanced magnetite formation in the thrust and fold belt is interpreted to be the result of higher temperatures leading to stronger diagenesis of clay minerals

deflation of mount etna monitored by spaceborne radar interferometry ground based measurements of volcano deformation can be used to assess eruptive hazard but require the costly and often hazardous installation and maintenance of an instrument network here we show that spaceborne radar interferometry which has already shown its utility in mapping earthquake related deformation1 can be used to monitor long term volcano deformation two families of synthetic aperture radar images acquired from ascending and descending orbits by the satellite ers 1 and looking at mount etna from opposite sides cover the time period from 17 may 1992 to 24 october 1993 and include the second half of etna s most recent eruption despite artefacts of the interferometric technique we can observe a volcano wide deflation which is an expected consequence of the eruption but which had not previously been appreciated we quantify it using a simple model based on the change of pressure in a sphere located in an elastic half space the modelled deformation increases linearly with time until the end of the eruption our results show that it will be possible to use this technique to detect the inflation of volcanic edifices that usually precedes eruptions

scanning and transmission electron microscope observations of magnetite and other iron phases in ordovician carbonates from east tennessee previous paleomagnetic observations for the carbonates of the lower ordovician knox group have indicated that ancient magnetizations in these rocks are of the same age as the late paleozoic alleghenian orogeny rock magnetic properties strongly suggest magnetite as the carrier of the magnetization but the textural and crystalline characteristics sizes morphologies and mineral associations of these magnetites are poorly known we have examined magnetic extracts and iron oxides in thin sections with scanning sem and scanning transmission stem electron microscope techniques to determine whether the observed iron oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers several different forms of magnetite in limestones and dolomites which in places are host to mississippi valley type deposits are documented by imaging and energy dispersive analysis using sem and stem by x ray diffraction and electron diffraction patterns using stem the magnetite is either spherical with a dimpled surface or nonspherical in the form of void filling single grains or grain aggregates most of the iron oxides have the composition of pure end member magnetite but occasional titanomagnetite and hematite including rare zincian hematite have been observed only in limestone wherever found in thin section nonspherical magnetites occur in association with secondary dolomite potassium feldspar illite and quartz some iron oxides have in fact inclusions of k feldspar and quartz some of the magnetite spherical and nonspherical is polycrystalline this implies that the larger observed grains may consist of single domains or pseudo single domains this provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single domain but interacting subgrains on the basis of remanent coercivities and blocking temperatures we interpret the pure end member magnetite to be authigenic having formed at approximately the same time as the k feldspars which in nearby areas have yielded late paleozoic radioisotopic ages 278 322 ma the knox carbonates therefore are inferred to carry a chemical remanent magnetization iron rich clays or original iron titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution precipitation processes these processes require rock fluid interactions which are thought to be related to migrating connate brines during the alleghenian orogeny

erratum to identification of tsunami induced deposits using numerical modeling and rock magnetism techniques a study case of the 1755 lisbon tsunami in algarve portugal physics of the earth and planetary interiors volume 182 issues 3 4 pages 187 198 october 2010 abstract storm and tsunami deposits are generated by similar depositional mechanisms making their discrimination hard to establish using classic sedimentologic methods here we propose an original approach to identify tsunami induced deposits by combining numerical simulation and rock magnetism to test our method we investigate the tsunami deposit of the boca do rio estuary generated by the 1755 earthquake in lisbon which is well described in the literature we first test the 1755 tsunami scenario using a numerical inundation model to provide physical parameters for the tsunami wave then we use concentration ms sirm and grain size arm arm b1 2 arm sirm sensitive magnetic proxies coupled with sem microscopy to unravel the magnetic mineralogy of the tsunami induced deposit and its associated depositional mechanisms in order to study the connection between the tsunami deposit and the different sedimentologic units present in the estuary magnetic data were processed by multivariate statistical analyses our numerical simulation show a large inundation of the estuary with flow depths varying from 0 5 to 6 m and run up of 7 m magnetic data show a dominance of paramagnetic minerals quartz mixed with lesser amount of ferromagnetic minerals namely titanomagnetite and titanohematite both of a detrital origin and reworked from the underlying units multivariate statistical analyses indicate a better connection between the tsunami induced deposit and a mixture of units c and d all these results point to a scenario where the energy released by the tsunami wave was strong enough to overtop and erode important amount of sand from the littoral dune and mixed it with reworked materials from underlying layers at least 1 m in depth the method tested here represents an original and promising tool to identify tsunami induced deposits in similar embayed beach environments

early dolomitization of platform carbonates and the preservation of magnetic polarity results from a combination of techniques are presented to evaluate the nature of magnetization in shallow water platform carbonates which have undergone recrystallization during early calcification and dolomitization magnetic grain separates coercivity spectra modified lowrie fuller tests magnetization efficiency and magnetostratigraphic constraints indicate that the ultrafine grained magnetite is preserved during early burial geochemical regimes inversion from aragonite high magnesium calcite to low magnesium calcite and even pervasive dolomitization these single domain crystals are thought to occur as interacting multigrain clusters some of which may exceed 1 m in diameter these large clusters may help prohibit magnetic reorientation during diagenesis furthermore during both fabric preserving and fabric destructive dolomitization the ultrafine scale replacement process restricts reorientation of the clusters thus preserving depositional or early postdeposition magnetic orientation this early dolomitization matrix stabilization may even help protect and extend the subsurface lifespan of the original polarity

paleoclimatic implications of magnetic susceptibility in late pliocene quaternary sediments from deep drilling core sg 1 in the western qaidam basin ne tibetan plateau 1 lake sediments are important archives of paleoclimate change and erosion history a 938 5 m long core sg 1 of lacustrine sediments dated at 2 77 ma to 0 1 ma was obtained from the western qaidam basin in the northeastern tibetan plateau consisting of dark grayish mudstone and grayish siltstone intercalated with salts and fine sandstones in the upper part magnetic susceptibility data combined with detailed rock magnetic properties were analyzed for revealing the significance of ferro i magnetic concentration for past changes of climate and erosion mass specific susceptibility shows a striking cyclic and long term variation samples with high values are dominated by magnetite and maghemite with pseudo single domain properties in contrast samples with low values contain maghemite from single domain to multidomain and additionally a significant fraction of hematite the driving mechanism of variation can be explained by three alternative models 1 different source regions with alternations of wind and cryoclastic erosion in a wider hinterland dry cold climate and surface runoff erosion from a narrower area more humid climate and 2 and 3 low temperature oxidation occurring either in the lake sediments dry climate or in the catchment area during weathering more humid climate trends of match with changes in sedimention rates and are roughly synchronous with the deep sea 18o record on a glacial interglacial timescale therefore the concentration of magnetic minerals in the western qaidam basin sediments is likely controlled by both tectonic influence and paleoenvironmental changes but can be best interpreted by alternations and trends of dry cold and more humid periods due to asian drying and global cooling

magnetostratigraphic dating of shallow water carbonates from mururoa atoll french polynesia implications for global eustasy magnetostratigraphic results are reported from a core penetrating the top 100 m of mururoa atoll french polynesia the sequence of polarity reversals based on relative up down direction in the unoriented core is correlated to the geomagnetic polarity time scale from the gauss early pliocene through the brunhes chron late pleistocene holocene the polarity sequence is matched with the help of an early brunhes radiometric date in a nearby core and the projected level of gephyrocapsa oceanica mid brunhes from an adjacent inclined core rock magnetic tests and extraction of magnetic components suggest a mixture of single and multi domain magnetite or maghemite carries the magnetic remanence newly refined atoll chronostratigraphy indicates a punctuated growth history and variable sedimentation rate of the reefal rim magnetostratigraphically dated periods of low accumulation correspond well with the increased frequency of subaerial exposure horizons these new data suggest that third order scale 1 m y sea level events are recorded in reefal deposits on moderate to slowly subsiding atolls mururoa and the bahamian island of san salvador have similar accumulation curves suggesting a global eustatic control

accuracy of topographic maps derived from ers 1 interferometric radar an interferometric radar technique for topographic mapping of surfaces promises a high resolution approach to the generation of digital elevation models the authors present analyses of data collected by the synthetic aperture radar instrument on board the ers 1 satellite on successive orbits use of a single satellite in a nearly repeating orbit is attractive for reducing cost and spaceborne hardware complexity also it permits inference of changes in the surface from the correlation properties of the radar echoes the data have been reduced to correlation maps and digital elevation models the correlation maps show that temporal correlation decreases significantly with time but not necessarily at a constant well defined rate likely depending on environmental factors when correlation among passes remains high however it is possible to form digital elevation models analyses of noise expected in ers 1 interferometric data collected over alaska and the southwestern united states indicate that maps with relative errors less than 5 m rms are possible in some regions however orbit uncertainties imply that tie points are required in order to reduce absolute height errors to a similar magnitude the authors find that about 6 tie points per 40 spl times 40 km scene with 5 m rms or better height accuracy are needed to keep systematic map height errors below 5 m rms the performance of the ers 1 radar system for topographic applications though useful for a variety of regional and local discipline studies may be improved with respect to temporal decorrelation errors and absolute height acuity by modifying the orbit repeat period and incorporating precise orbit determination techniques the resulting implementation will meet many but not all objectives of a global mapping mission

a method to study alterations of magnetic minerals during thermal demagnetization applied to a fine grained marine marl trubi formation sicily summary the method presented is based on monitoring changes in the coercivity spectrum of an isothermal remanent magnetization irm during stepwise thermal demagnetization the procedure allows one to determine alteration temperatures and the coercivities and blocking temperatures of the magnetic minerals involved in the alterations it distinguishes between thermal decay due to unblocking of the remanence and demagnetization due to alterations of magnetic minerals as a result the initial magnetic mineralogy can also be determined the method is demonstrated with samples of fine grained marine marls from the pliocene trubi formation in sicily italy single domain sd magnetite is the dominant remanence carrier in this sediment a first alteration during heating to only 145 c is accompanied by a considerable reduction of coercivities higher than 0 1 t this alteration is thought to be due to the reduction of stress in superficially maghemitized sd magnetite grains between 390 and 480 c grains with magnetite like properties are produced most likely by oxidation of pyrite in the sediment at about 560 c the breakdown of iron bearing silicates also produces magnetic grains the magnetic grains formed during both alteration processes have low blocking temperatures demagnetization above 510 c is not only due to unblocking of the irm alteration of magnetite to haematite proves to be an important demagnetization mechanism

atmospheric limitations to repeat track radar interferometry in its recent radar imaging mission the shuttle imaging radar satellite sir c devoted three days to repeat track interferometry we have analyzed the data from a test site in the mojave desert of california although good topography 10 m on 21 m postings was obtained most of the error was caused by turbulent water vapor in the lower atmosphere spatial structure of 6 km and all smaller sizes was observed the rms one way time delay was found to be 0 24 cm essentially identical results were obtained at two wavelengths 24 and 5 7 cm

mapping small elevation changes over large areas differential radar interferometry a technique that uses synthetic aperture radar sar images to measure very small 1 cm or less surface motions with good resolution 10 m over large swaths 50 km is presented along with experimental results the method could be used for accurate measurements of many geophysical phenomena including swelling and buckling in fault zones residual displacements from seismic events and prevolcanic swelling the method is based on sar interferometry where two images are made of a scene by simultaneously flying two physically separated antennas then the phases of corresponding pixels are differenced and altitude formation is deduced from some simple computation and image rectification it is also possible to use one antenna flown twice over the same scene then if the second flight exactly duplicates the track of the first an interesting possibility occurs there would be no phase changes between the images at all unless there was a physical change in the scene such as ground swelling that would alter the distance from some resolution element to the antenna since the phase changes all occur at the short carrier wavelength the basic limitation on sensitivity is only the phase noise in the system when the two imaging passes are made from flight tracks that are separated which is the case with the seasat images used here it is no longer possible to distinguish surface changes from the parallax caused by topography however with some additional computation a third image made at some other baseline may be used to remove the topography and leave only the surface changes this method was applied using seasat data to an imaging site in imperial valley california where motion effects were observed that were ascribed to the expansion of water absorbing clays phase change images of this area are shown along with associated ground truth about the presence of water problems with the technique are explored along with a discussion of future experimental possibilities on upcoming sar missions like earth observing system eos earth resources satellite ers 1 sir c and the venus imaging radar magellan

rock magnetism of remagnetized paleozoic carbonates low temperature behavior and susceptibility characteristics we have conducted a new set of rock magnetic experiments on samples of remagnetized paleozoic carbonates of eastern north america these experiments were designed to investigate the origin of the unusual hysteresis behavior of these rocks by evaluating 1 the importance of ferrimagnetic pyrrhotite as a remanence carrier and 2 the sources of low field susceptibility low temperature measurements of saturation isothermal remanent magnetization sirm indicate that the pyrrhotite magnetic transition at 32 k is absent in the trenton and onondaga limestones of new york this transition is observed but poorly expressed in the knox group of east tennessee room temperature sirms cooled to 10 k in zero field lose a small fraction of their intensity between 30 and 35 k samples from all three formations show a broad peak in irm intensity at about 200 k which is typical of pyrrhotite in the trenton and onondaga samples the ferrimagnetic component of low field susceptibility is significantly larger than the ratio mrs hcr and is thus probably due dominantly to magnetite in some of the knox samples the reverse is true suggesting an important pyrrhotite contribution for all of the samples the ferrimagnetic susceptibility normalized by the saturation magnetization is anomalously high about a factor of 5 or 10 higher than the typical value for magnetite we believe that this indicates a very substantial contribution from superparamagnetic particles strong frequency dependence of susceptibility and very high ratios of anhysteretic to saturation remanence confirm the importance of ultrafine particles spanning the superparamagnetic single domain boundary all three of these chemically remagnetized carbonates units exhibit the following properties which have not previously been found together for any rock or synthetic analog and which therefore appear to constitute a diagnostic set of rockmagnetic criteria for recognizing chemically remagnetized rocks mrs ms 0 89 hcr hc 0 6 k ms 50 m a arm sirm 20 ka mrs hcr 50

fast or slow melting of the marinoan snowball earth the cap dolostone record article i nfo the end of the neoproterozoic era is punctuated by two global glacial events marked by the presence of glacial deposits overlaid by cap carbonates duration of glacial intervals is now consistently constrained to 3 12 million years but the duration of the post glacial transition is more controversial due to the uncertainty in cap dolostone sedimentation rates indeed the presence of several stratabound magnetic reversals in brazilian cap dolostones recently questioned the short sedimentation duration a few thousand years at most that was initially suggested for these rocks here we present new detailed magnetostratigraphic data of the mirassol d oeste cap dolostones mato grosso brazil and bomb spike calibrated ams 14 c data of microbial mats from the lagoa vermelha rio de janeiro brazil we also compile sedimentary isotopic and microbiological data from post marinoan outcrops and or recent depositional analogues in order to discuss the deposition rate of marinoan cap dolostones and to infer an estimation of the deglaciation duration in the snowball earth aftermath taken together the various data point to a sedimentation duration in the range of a few 10 5 years

scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early paleozoic carbonates from missouri this study was supported by nsf ear 90 05075 and 92 19616 institute for rock magnetism irm contribution 9303 the irm is funded by the keck foundation the national science foundation and the university of minnesota

neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites abstract neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large cryogenian glaciations several thin dolomite units that have features resembling cap dolomites occur within the cryogenian ediacaran succession of the adelaide geosyncline while each of these dolomite units has distinctive features they also share many similarities with cap dolomites including stratigraphic thickness ranging from several centimetres to several metres impure dolomicrite composition graded beds with a sharp top that become more dolomite rich upwards and evidence of volume expansion in the form of sheet cracks tepees or other soft sediment deformation structures the carbon isotopic compositions for these thin dolomite horizons have values ranging from 5 to 2 per mil vpdb which overlap those commonly found in both sturtian and marinoan cap dolomites the generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds volume expansion structures are a product of this near surface dolomicrite precipitation brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited marine carbonate source for the precipitating dolomite we suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of cryogenian early ediacaran oceans and caused by alkalinity producing anoxic bacterial reactions when sedimentation rates slow sufficiently e g during transgressions thin dolomicrite horizons are produced in this hypothesis cap dolomites may largely be the result of sediment starvation combined with a possible increase in carbonate saturation during and following the post glacial transgressions associated with the sturtian and marinoan glacial events this scenario explains most of the sedimentological features of these unusual carbonates the widespread development of such authigenic carbonate would have profound implications for the interpretation of precambrian marine carbon isotopic compositions

a carbon isotope challenge to the snowball earth the snowball earth hypothesis contends that most of the planet was covered with thick ice for millions of years about three quarters of a billion years ago a dramatic warming event thought to have been fuelled by high atmospheric carbon dioxide levels eventually defrosted earth a new isotopic analysis of rocks that were laid down after the initial glaciation challenges the idea of a hard snowball earth the data are consistent with a rather low partial pressure of atmospheric co2 not the orders of magnitude above current levels that would provide the extreme warming necessary for escape from snowball earth this supports the idea that the initial glaciation was much less severe than has been proposed the snowball earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the neoproterozoic era in a self enhanced glaciation caused by the high albedo of the ice covered planet in a hard snowball picture the subsequent rapid unfreezing resulted from an ultra greenhouse event attributed to the buildup of volcanic carbon dioxide co2 during glaciation1 high partial pressures of atmospheric co2 from 20 000 to 90 000 p p m v in the aftermath of the marinoan glaciation 635 myr ago have been inferred from both boron and triple oxygen isotopes2 3 these values are 50 to 225 times higher than present day levels here we re evaluate these estimates using paired carbon isotopic data for carbonate layers that cap neoproterozoic glacial deposits and are considered to record post glacial sea level rise1 the new data reported here for brazilian cap carbonates together with previous ones for time equivalent units4 5 6 7 8 provide estimates lower than 3 200 p p m v and possibly as low as the current value of 400 p p m v our new constraint and our re interpretation of the boron and triple oxygen isotope data provide a completely different picture of the late neoproterozoic environment with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard snowball earth

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

surface displacement of the 17 may 1993 eureka valley california earthquake observed by sar interferometry satellite synthetic aperture radar sar interferometry shows that the magnitude 6 1 eureka valley earthquake of 17 may 1993 produced an elongated subsidence basin oriented north northwest parallel to the trend defined by the aftershock distribution whereas the source mechanism of the earthquake implies a north northeast striking normal fault the 3 millimeter accuracy of the radar observed displacement map over short spatial scales allowed identification of the main surface rupture associated with the event these observations suggest that the rupture began at depth and propagated diagonally upward and southward on a west dipping north northeast fault plane reactivating the largest escarpment in the saline range

on the derivation of coseismic displacement fields using differential radar interferometry the landers earthquake we present a map of the coseismic displacement field resulting from the landers california june 28 1992 earthquake derived using data acquired from an orbiting high resolution radar system we achieve results more accurate than previous space studies and similar in accuracy to those obtained by conventional field survey techniques data from the ers 1 synthetic aperture radar instrument acquired in april july and august 1992 are used to generate a high resolution wide area map of the displacements the data represent the motion in the direction of the radar line of sight to centimeter level precision of each 30 m resolution element in a 113 km by 90 km image our coseismic displacement contour map gives a lobed pattern consistent with theoretical models of the displacement field from the earthquake fine structure observed as displacement tiling in regions several kilometers from the fault appears to be the result of local surface fracturing comparison of these data with global positioning system and electronic distance measurement survey data yield a correlation of 0 96 thus the radar measurements are a means to extend the point measurements acquired by traditional techniques to an area map format the technique we use is 1 more automatic 2 more precise and 3 better validated than previous similar applications of differential radar interferometry since we require only remotely sensed satellite data with no additional requirements for ancillary information the technique is well suited for global seismic monitoring and analysis

siliciclastic prelude to elatina nuccaleena deglaciation lithostratigraphy and rock magnetism of the base of the ediacaran system

sem stem observation of magnetic minerals in presumably unremagnetized paleozoic carbonates from indiana and alabama abstract the silurian wabash formation in indiana and the mississippian pride mountain formation in alabama appear not to have been affected by a late paleozoic remagnetization event in an attempt to characterize the magnetic mineralogy in these presumably unremagnetized carbonates and in order to compare their magnetic mineralogy to that of remagnetized carbonates scanning and scanning transmission microscope sem stem observations and rock magnetic investigations were carried out it is possible to recognize differences in magnetic mineralogy in the unremagnetized carbonate from that in remagnetized carbonates 1 1 iron oxides associated with iron sulfides are hematite in this study as a result of replacement of pyrite instead of magnetite as was found elsewhere 2 2 occurrences of large euhedral pure iron oxides of secondary origin are common in the unremagnetized carbonates 3 3 a rare occurrence of fine grained single crystal magnetite capable of carrying a remanence in the unremagnetized carbonates is noticeable as compared to the abundance of such grains in the remagnetized carbonates although the abundance of the fine grained magnetite grains in remagnetized carbonates is inferred to be a diagnostic factor to distinguish the remagnetized from the unremagnetized carbonates this clarifies only the carriers in the remagnetized rocks and leaves the question of the carriers in unremagnetized limestones unresolved to a large extent the lack of remagnetization is commonly attributed to a restricted amount of fluid influx into the rocks for the wabash and the pride mountain formations this may also be true early cementation has significantly reduced the porosity and permeability in the wabash formation in indiana whereas the presence of the impermeable chattanooga shale may have protected the mississippian pride mountain formation in alabama

timing of diagenetic haematite growth in red pelagic limestones from gubbio italy abstract magnetite haematite and to a minor extent maghaemite are recognised in the cretaceous and paleocene red pelagic limestones at gubbio the magnetite is detrital or biological whereas the haematite grew during diagenesis from a goethitic precursor thermal and af demagnetization of samples collected from close to reversal boundaries indicate that the various magnetization components do not record the polarity reversal at exactly the same stratigraphic level in the few tens of centimetersbelow a recorded geomagnetic reversal defined by the magnetite magnetization some of the haematite grains are magnetized in the post reversal field the blocking temperature spectra of this haematite fraction with post reversal magnetization are found to shift toward higher temperatures as the reversal boundary is approached the blocking temperature spectra reflect the grain size spectra of the haematite which we interpret as arising by the continual nucleation of grains down to a certain burial depth where the conditions are no longer conducive to further haematite growth the depth below reversal boundaries to which haematite with post reversal magnetization can occur is estimated to be about 60 cm after compaction and is equivalent to a time of about 105 years for these particular sediments a detailed study of the magnetization components at reversal boundaries indicates that the first diagenetic growth of haematite through the single domain critical volume occurs prior to the mechanical fixation of the detrital or biological magnetite subsequently the diagenetic haematite grains do not rotate in response to the ambient geomagnetic field polarity as easily as the magnetite because of their occurrence as pigmentary coatings on larger non magnetic grains

the rock magnetic fingerprint of chemical remagnetization in midcontinental paleozoic carbonates supported by nsf ear 90 05075 contribution number 9203 of the institute for rock magnetism the irm is supported by grants from the keck foundation the national science foundation and the university of minnesota

topographic mapping from interferometric synthetic aperture radar observations interferometric synthetic aperture radar observations provide a means for obtaining high resolution topographic terrain maps from data acquired simultaneously at two slightly displaced antennas calculation of the three dimensional coordinates of all the points in a radar image can be made from the combination of along track slant range and interferometer fringe measurements thus the result compensates for the layover exhibited by conventional radar maps and removes the false targets generated by multiple signal paths to a given object in the scene we have derived a topographic map of a portion of the san francisco bay area utilizing data that were recorded by a radar system mounted on a nasa cv990 aircraft and processed by a general purpose digital computer this map displays the height above sea level of a region approximately 11 km by 10 km in size sampled on an 11 m pixel grid uncertainties in the estimated height result from imprecise knowledge of the observational geometry radar bandwidth limitations and finite signal to noise ratios for our system each factor contributed approximately 3 4 m to a relative rms error of about 6 m additional global errors of the order of 10 m can result from inaccurate monitoring of aircraft attitude the statistical variation of the height measurements from the ocean portion of the map which we can presume to be quite flat is 2 10 m rms which is consistent with the theoretical estimate comparison of our results with u s geological survey contour maps indicates a high degree of correlation between the two sets of altitude data

identification of tsunami induced deposits using numerical modeling and rock magnetism techniques a study case of the 1755 lisbon tsunami in algarve portugal abstract storm and tsunami deposits are generated by similar depositional mechanisms making their discrimination hard to establish using classic sedimentologic methods here we propose an original approach to identify tsunami induced deposits by combining numerical simulation and rock magnetism to test our method we investigate the tsunami deposit of the boca do rio estuary generated by the 1755 earthquake in lisbon which is well described in the literature we first test the 1755 tsunami scenario using a numerical inundation model to provide physical parameters for the tsunami wave then we use concentration ms sirm and grain size arm arm b1 2 arm sirm sensitive magnetic proxies coupled with sem microscopy to unravel the magnetic mineralogy of the tsunami induced deposit and its associated depositional mechanisms in order to study the connection between the tsunami deposit and the different sedimentologic units present in the estuary magnetic data were processed by multivariate statistical analyses our numerical simulation show a large inundation of the estuary with flow depths varying from 0 5 to 6 m and run up of 7 m magnetic data show a dominance of paramagnetic minerals quartz mixed with lesser amount of ferromagnetic minerals namely titanomagnetite and titanohematite both of a detrital origin and reworked from the underlying units multivariate statistical analyses indicate a better connection between the tsunami induced deposit and a mixture of units c and d all these results point to a scenario where the energy released by the tsunami wave was strong enough to overtop and erode important amount of sand from the littoral dune and mixed it with reworked materials from underlying layers at least 1 m in depth the method tested here represents an original and promising tool to identify tsunami induced deposits in similar embayed beach environments

current challenges and future developments in magnetic fabric research abstract magnetic fabrics are used extensively as proxies for mineral fabrics and to correct paleomagnetic data and have been proposed to quantify pore fabrics today s understanding and interpretation of magnetic anisotropy benefits from decades of carefully observing anisotropic mineral and rock properties establishing and testing empirical relationships between magnetic and mineral fabrics and developing models to quantify aspects of anisotropy each advance was preceded by some puzzling observations i e data that could not be explained based on the models available at the time e g oblique or inverse fabrics or lithology dependent anisotropy strain relationships these observations led to numerous experimental and numerical techniques designed to characterize magnetic fabrics and determine their origin despite the successful application of magnetic fabrics in many structural and tectonic problems there are still phenomena and measurements that cannot be explained based on today s magnetic fabric theory with the purpose of fostering future development i will touch on three main areas where i see challenges in magnetic fabric research 1 experimental characterization of magnetic anisotropy including the non uniqueness of reported tensors and non linearity 2 anisotropy modelling and defining the carrier minerals and origin of the magnetic fabric 3 selection of adequate sets of anisotropy tensors to correct paleomagnetic data the description of the challenges presented here will hopefully help define directions for future research inform those that are new to anisotropy and advance our field

soft sediment deformation at the base of the neoproterozoic puga cap carbonate southwestern amazon craton brazil confirmation of rapid icehouse to greenhouse transition in snowball earth stratigraphic and isotopic data identify the lower 45 m of the araras group on the southwest margin of the amazon craton as a neoproterozoic platform cap carbonate deposited below wave base upon varanger glacial diamictites of the puga formation the basal beds consist of moderately deep water pinkish dolomudstone with stratiform to wavy fenestral microbialites locally cut by tube like structures and fenestral nonmicrobial planar laminites with tepee like features above the basal carbonates are deep water bituminous lime mudstones with alternating thin calcite crusts and lime mudstone laminae commonly disrupted by calcite crystal fans pseudomorphs after aragonite the basal contact of the puga cap exhibits soft sediment deformational structures principally load casts that are here attributed to rebound induced seismicity acting upon both recently deposited carbonate sediments and underlying unconsolidated diamictite these features constitute the first clearly recognized sedimentological evidence for the rapid change from icehouse to greenhouse conditions as postulated in the snowball earth model of neoproterozoic glaciation

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

thermal effects of marine sedimentation in hydrothermally active areas summary a hydrothermally active ocean basin vigorous hydrothermal circulation in highly permeable basement rocks maintains a nearly constant temperature at the base of the overlying accumulating sediment section to investigate the thermal effects of sedimentation in such geological settings we have developed a simple onedimensional finite element model and applied it to cases in the northeast pacific the model accounts for differential motion of fluids and sediment grains during compaction and can be used with any porosity depth function results demonstrate clearly that the constant basal temperature of an accumulating sediment section maintained by convective heat transfer in the basement causes the section to remain thermally near steady state for even very high rates of accumulation particularly when compared to conditions estimated for a section where heat is transported in the basement by conduction a 10 kyr period of thermal recovery due to the highly diminished sediment supply during the post pleistocene further reduces the thermal effects of sedimentation by a significant amount only in rare cases where rates of accumulation exceed 10 mm yr l and sediment thicknesses exceed 1 km are the sea floor heat flow and temperatures at depth diminished significantly an example is found in middle valley of the juan de fuca ridge in a part of which over 2 km of sediment has accumulated in the past 200 kyr even in this extreme case the heat flow is estimated to be lower than that of the steady state by about only 15 per cent while rates of accumulation are also high in other parts of middle valley and in many other hydrothermally active areas such as guayamas basin escanaba trough and the eastern flank of the juan de fuca ridge these rates and the accumulated sediment thicknesses are found to be insufficient to cause appreciable thermal anomalies