paleomagnetic reconstruction of the global geomagnetic field evolution during the matuyama brunhes transition iterative bayesian inversion and independent verification abstract the earth s magnetic field changed its polarity from the last reversed into today s normal state approximately 780 000 years ago while before and after this so called matuyama brunhes reversal the earth magnetic field was essentially an axial dipole the details of its transitional structure are still largely unknown here a bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data this is achieved by minimizing the total variational power at the core mantle boundary during the transition under paleomagnetic constraints the validity of the inversion technique is proved in two ways first by inverting synthetic data sets from a modeled reversal here it is possible to reliably reconstruct the gauss coefficients even from noisy records second by iteratively combining four geographically distributed high quality paleomagnetic records of the matuyama brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model the obtained spatio temporal reversal scenario successfully predicts most independent matuyama brunhes transitional records therefore the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field it is found that radial magnetic flux patches form at the equator and move polewards during the transition our model indicates an increase of non dipolar energy prior to the last reversal and a non dipolar dominance during the transition thus the character and information of surface geomagnetic field records is strongly site dependent the reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration different types of directional variations of the surface geomagnetic field continuous or abrupt are found during the transition two preferred longitudinal bands along the americas and east asia are not predicted for uniformly distributed sampling locations on the globe similar to geodynamo models with cmb heatflux derived from present day lower mantle heterogeneities a preference of transitional vgps for the pacific hemisphere is found the paleomagnetic duration of reversals shows not only a latitudinal but also a longitudinal variation even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe the described bayesian inversion technique can easily be applied to other high quality full vector reversal records also its extension to inversion of secular variation and excursion data is straightforward

why are geomagnetic excursions not always recorded in sediments constraints from post depositional remanent magnetization lock in modelling abstract many geomagnetic excursions have been documented during the brunhes chron however high resolution paleomagnetic studies of sediments often provide evidence of few if any excursions we have investigated the reasons for this observation by modelling the post depositional remanent magnetization pdrm lock in process a cubic lock in function produces more rapid lock in compared to linear and exponential functions proposed in the literature we therefore used a cubic function to model best case scenarios for the quality of the paleomagnetic record when a high frequency geomagnetic input signal is convolved with the sediment lock in function for a wide range of sedimentation rates even for a lock in depth of 10 cm where 95 of the pdrm is locked in 5 cm below the surface mixed layer an input signal containing abundant excursions with 1 kyr duration is smoothed so drastically at low sedimentation rates 1 3 cm kyr that no excursions are recorded excursions are further attenuated by increasing the lock in depth as well as by sampling at discrete stratigraphic intervals the pdrm process acts as a low pass filter for high frequency secular variation signals even at moderate sedimentation rates 10 cm kyr we have also modelled the effects of modulation of sedimentation rate by a climatic forcing function e g insolation even though the average sedimentation rate might be substantial pdrm recording becomes much less reliable in intervals where sedimentation rates are lowest in these cases some excursions are reliably recorded in detail while others may not be recorded at all our modelling suggests that in order to consistently detect the presence of geomagnetic excursions it is ideal to work with sediments that maintain minimum sedimentation rates above 10 cm kyr if the normal spectrum of geomagnetic field behaviour contains abundant excursions failure to document excursions in many apparently high resolution analyses probably results from pdrm lock in within relatively slowly deposited sediments or to unrecognized intervals of slow sedimentation in environments with higher average sedimentation rates

lower pliocene polarity transitions from the suva marl fiji we present a paleomagnetic record of the lower cochiti polarity transition and new results from the lower and upper nunivak transitions obtained from the lower pliocene suva marl fiji 18 s 178 e this formation has yielded a well defined magnetostratigraphy that combined with 40ar 39ar ages from interbedded tuff beds provides excellent chronostratigraphic control clement et al 1997 we previously presented the upper cochiti and the upper and lower nunivak polarity transition records from this unit clement et al 1995 1996 new results were obtained from oriented block samples spanning these reversals each of these reversals was sampled at duplicate sections separated by 4 to 8 km the agreement between the records obtained from different sections suggests that the paleomagnetic records are not artifacts of local sedimentary disturbances or random degrees of incompletely removed overprints instead the similarities in these and other records of these same reversals from different types of paleomagnetic recorders suggest that they provide information about the behavior of the geomagnetic field as it reverses

radar imagery of mercury s putative polar ice 1999 2005 arecibo results abstract we present an updated survey of mercury s putative polar ice deposits based on high resolution 1 5 km imaging with the upgraded arecibo s band radar during 1999 2005 the north pole has now been imaged over a full range of longitude aspects making it possible to distinguish ice free areas from radar shadowed areas and thus better map the distribution of radar bright ice the new imagery of the south pole though derived from only a single pair of dates in 2005 improves on the pre upgrade arecibo imagery and reveals many additional ice features some medium size craters located within 3 of the north pole show near complete ice coverage over their floors central peaks and southern interior rim walls and little or no ice on their northern rim walls while one large 90 km crater at 85 n shows a sharp ice cutoff line running across its central floor all of this is consistent with the estimated polar extent of permanent shading from direct sunlight some craters show ice in regions that though permanently shaded should be too warm to maintain unprotected surface ice owing to indirect heating by reflected and reradiated sunlight however the ice distribution in these craters is in good agreement with models invoking insulation by a thin dust mantle comparisons with goldstone x band radar imagery indicate a wavelength dependence that could be consistent with such a dust mantle more than a dozen small ice features have been found at latitudes between 67 and 75 all of this low latitude ice is probably sheltered in or under steep pole facing crater rim walls although since most is located in the mariner unimaged hemisphere confirmation must await imaging by the messenger orbiter these low latitude features are concentrated toward the cold longitudes possibly indicating a thermal segregation effect governed by indirect heating the radar imagery places the corrected locations of the north and south poles at 7 w 88 35 n and 90 w 88 7 s respectively on the original mariner based maps

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

mercury radar imaging evidence for polar ice the first unambiguous full disk radar mapping of mercury at 3 5 centimeter wavelength with the goldstone 70 meter antenna transmitting and 26 antennas of the very large array receiving has provided evidence for the presence of polar ice the radar experiments conducted on 8 and 23 august 1991 were designed to image the half of mercury not photographed by mariner 10 the orbital geometry allowed viewing beyond the north pole of mercury a highly reflective region was clearly visible on the north pole during both experiments this polar region has areas in which the circular polarization ratio pt was 1 0 to 1 4 values 0 1 are typical for terrestrial planets such high values of have hitherto been observed in radar observations only from icy regions of mars and icy outer planet satellites

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

mercury full disk radar images and the detection and stability of ice at the north pole the first full disk radar images of mercury were obtained on august 8 and 23 1991 these images were constructed using the very large array vla in socorro new mexico to receive and map radar flux at 3 5 cm x band which was continuously transmitted from the 70 m jet propulsion laboratory deep space network antenna at goldstone california approximately 77 of the surface was imaged at resolutions as good as 150 km about half of the hemisphere photographed by mariner 10 was imaged as well as most of the hemisphere which has not previously been photographed at the time of the observations the north pole was visible and the feature with the highest same sense ss circular reflectivity in the images is near the nominal polar position the peak ss reflectivity of this feature was 7 9 and the circular polarization ratio throughout much of the feature is 1 our best estimate of the size of the feature is that its diameter is 350 km we interpret the feature to be indicative of the presence of ices because of the signal strength and polarization characteristics the ices must be very clean and were thus probably deposited in a relatively short time period the most likely place to find these ices is in any permanently shadowed areas in the polar regions which would be very cold in comparison to absolute reflectivities of other icy bodies and regions in the solar system the reflectivity of the north polar feature is slightly depressed this is probably because the ices do not fill an observational resolution cell i e there is incomplete areal coverage or are covered by a layer of dust or soil which absorbs some of the incoming and outgoing radar energy or both a covering layer would also protect any ices from erosion by energetic sources other prominent features on the unphotographed hemisphere correspond to positions where atmospheric sodium enhancements have been measured from earth this may indicate that these locations are large basins similar to the caloris basin where an atmospheric potassium enhancement has been measured which may be the result of increased degassing in the disrupted surface and near surface direct comparison of the radar echoes from our features with those from the caloris basin are hard to make as caloris was not imaged by us in a favorable geometry features on the photographed hemisphere are smaller and mostly associated with craters and crater complexes most notably the kuiper crater and its environs

stability of polar frosts in spherical bowl shaped craters on the moon mercury and mars following svitek martian water frost control of global distribution by small scale processes ph d thesis california institute of technology 1992 analytic solutions are presented for the effective albedo the effective emissivity and the radiative equilibrium temperature in the shadowed portions of a spherical bowl shaped crater the model assumes that the surface is a lambert scatterer with visual albedo and infrared emissivity each independent of wavelength across their respective spectral ranges absorption emission and multiple scattering from the walls of the crater are treated rigorously to all orders for airless bodies whose surfaces are in radiative equilibrium all shadowed portions of any individual crater have the same temperature whose value depends on four quantities the insolation product of the solar constant and the sine of the solar elevation angle the depth diameter ratio of the crater the visual albedo and the infrared emissivity as long as the crater is deep enough to have shadows the lowest temperatures are for the shallowest crater those with the smallest depth diameter ratio the model is applied first to the moon and mercury using a depth diameter ratio of 0 2 which is typical of the lunar highlands according to pike geophys res lett 1 291 294 1974 in impact and explosion cratering roddy et al eds pp 489 509 pergamon new york 1977 for mercury and the moon temperatures in shadows in polar craters are below 102 k so the sublimation rate of water ice calculated according to the model of watson et al j geophys res 66 3033 3015 1961 is less than 1 cm per byr the latitudinal extent of the cold zone on the moon is greater than that on mercury although temperatures at the poles of the two planets are similar the other application is to polar frosts on mars illuminated water frosts in radiative equilibrium grow rougher because the average temperature of a depression is greater than that of flat ground subliming co 2 frosts which are always at the same temperature grow rougher at low solar elevation angles because the heat flux absorbed by a depression is greater than that for a flat surface at high insolation rates high sun near perihelion the average heat flux to a depression is less than for a flat surface the latter evaporates faster which makes the average surface smoother and leads to a high average albedo this behavior helps explain the fact that the south co 2 cap which receives its greatest insolation near perihelion has a higher effective albedo and therefore can survive the summer whereas the north co 2 cap has a lower effective albedo and disappears each year around summer solstice

analyzing the ages of south polar craters on the moon implications for the sources and evolution of surface water ice abstract surface water ice in the permanently shadowed polar regions of the moon has a patchy surficial distribution and is not found within all available cold trapping areas to date it is not well understood when the ice was delivered which has important implications for the surficial characteristics of the ice as well as for possible delivery mechanisms here we present absolute model ages for 20 south polar craters that host surface water ice providing maximum estimates of the ages of surface ice contained within these craters we quantify the amount of available cold trapping surface area that is occupied by water ice in order to examine the relationship between the patchiness of ice within each crater and the age of each host crater the majority of surface ice is contained in old craters 3 1 gyr where the majority of cold trapping area on the pole exists the ice is these ancient craters is very patchy in surficial distribution occupying siegler et al 2016 if true polar wander did occur on the moon then the ages of ice bearing craters presented here set an upper limit for the age of post true polar wander hydrogen emplacement of 4 1 0 1 gyr

simulated geomagnetic reversals and preferred virtual geomagnetic pole paths summary the question of whether virtual geomagnetic poles vgps recorded during reversals and excursions show a longitudinal preference is a controversial one amongst palaeomagnetists one possible mechanism for such vgp clustering is the heterogeneity of heat flux at the core mantle boundary cmb we use 3 d convection driven numerical dynamo models with imposed non uniform cmb heat flow that show stochastic reversals of the dipole field we calculate transitional vgps for a large number of token sites at the earth s surface in a model with a simple heat flux variation given by a y22 harmonic the vgp density maps for individual reversals differ substantially from each other but the vgps have a tendency to fall around a longitude of high heat flow the mean vgp density for many reversals and excursions shows a statistically significant correlation with the heat flow in a model with an imposed heat flux pattern derived from seismic tomography we find maxima of the mean vgp density at american and east asian longitudes roughly consistent with the vgp paths seen in several palaeomagnetic studies we find that low latitude regions of high heat flow are centres of magnetic activity where intense magnetic flux bundles are generated they contribute to the equatorial dipole component and bias its orientation in longitude during reversals the equatorial dipole part is not necessarily dominant at the earth s surface but is strong enough to explain the longitudinal preference of vgps as seen from different sites

effects of orbital evolution on lunar ice stability 1 many regions near the lunar poles are currently cold enough that surface water ice would be stable against sublimation losses for billions of years however most of these environments are currently too cold to efficiently drive ice downward by thermal diffusion leaving impact burial as the primary means of protection from surface loss processes in this respect most of the present near surface thermal environments on the moon may actually be quite poor traps for water ice this was not always the case long term orbital changes have dramatically altered the lunar polar thermal environment we develop a simple model of the evolution of the lunar orbit and spin axis to examine the thermal environments available for volatile deposition and retention in the past our calculations show that some early lunar polar environments were in the right temperature regime to have collected subsurface ice if a supply were available however a high obliquity period which occurred when the moon was at about half its present distance from the earth would either have driven this ice out into space or deep into the lunar subsurface since that time as the lunar obliquity has slowly decreased to its present value environments have undergone their own thermal evolution and each of the current cold traps experienced a period when they were most efficient at thermally burying ice we examine the thermal history of a lunar polar crater to provide a framework for examining other processes effecting volatiles in the moon s near surface cold traps

high resolution magnetostratigraphy and radio isotope dating of early pleistocene lake sediments from southern armenia abstract the pleistocene geology of armenia is dominated by widespread occurrence of sediments recording recurring rapid and drastic changes of the environmental conditions during at least the last 2 million years these sediments predominantly diatomites contain a huge variety of various fossil remains allowing the reconstruction of flora fauna as well as the climatic conditions especially during dispersal of early man into eurasia 1 8 ma radio isotope dating of pumice levels as well as a detailed magnetostratigraphic study have been carried out in syunik province near the town of sisian southern armenia in order to establish a temporal correlation of all major outcrops of paleo lake sediments and to provide a timeframe for paleoenvironmental studies within this lake sediment succession a total of 455 oriented drill cores was sampled at six large and four short sections with a sampling resolution between 5 and 20 cm detailed paleomagnetic experiments reveal the presence of a characteristic remanent magnetization chrm pointing either to the north and down or south and up a positive reversal test suggests the primary character of this magnetization component combining the normal and reverse polarity directions results in an overall sample mean direction of d 354 8 i 51 5 with a fisherian precision parameter k of 14 6 and the associated 95 of 2 9 combining demagnetization data of 169 samples in addition ash layers were sampled for 40ar 39ar geochronology at five of the sections studied paleomagnetically and three additional small outcrops the resulting ages range between 1 31 ma and 1 08 ma with two samples yielding 1 65 and 1 90 ma the latter are characterized by extremely low abundances of radiogenic 40ar suggesting that the mineral system has not been closed since the time of eruption leading to substantial loss in ar integration of these radio isotope dates with the polarity data suggests that the normal polarities can be linked to the jaramillo subchron within the matuyama reversed polarity chron based on the longest interval of normal polarity identified at ashotavan 2 a lower limit for the sedimentation rate of 37 cm ky can be estimated therefore we infer the duration of lake sedimentation as 350 ky starting at 1 4 ma

detection of noninteracting single domain particles using first order reversal curve diagrams we present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles unisd in rocks and sediments the method is based on high resolution measurements of first order reversal curves forcs unisd particles have a unique forc signature that can be used to isolate their contribution among other magnetic components this signature has a narrow ridge along the hc axis of the forc diagram called the central ridge which is proportional to the switching field distribution of the particles therefore the central ridge is directly comparable with other magnetic measurements such as remanent magnetization curves with the advantage of being fully selective to sd particles rather than other magnetic components this selectivity is unmatched by other magnetic unmixing methods and offers useful applications ranging from characterization of sd particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments

late collision rhyolitic volcanism in the north eastern part of the armenian highland abstract plio pleistocene acidic volcanism of the northeastern part of the armenian highland is related to a continental collision zone as a result of convergence between eurasia and arabia lithosphere plates the development of volcanism is divided into three stages of 10 17 4 5 7 5 and 0 1 2 8 ma a crustal origin model is proposed to explain the geochemical and petrogenetic aspects of the acidic volcanism formation spatially separated ba rich and rb rich geochemical types of rhyolites were revealed indicating more evolved and primitive rhyolitic magma types respectively the variation interval of the primary 87sr 86sr ratio was found to be from 0 70438 to 0 70636 age and lateral variation of geochemical and isotopic parameters were determined by differences of initial substrates and degrees of their partial melting the mechanisms of rhyolitic magma evolution were the remelting of heterogeneous sialitic materials and low pressure fractional crystallization in isolated magmatic chambers these facts confirm the model of crustal anatexis origin of the eutectic melts the eutectic rhyolitic magma was generated by an open system melting in the presence of deep horizon mobile k rb fluids initiated by mantle resources

persistently illuminated regions at the lunar poles ideal sites for future exploration abstract the lunar reconnaissance orbiter camera lroc provides multi temporal and high resolution imaging of the north and south polar regions these images delimit illuminated areas from those in shadow and are used to analyze the illumination environment of the polar regions over the course of a lunar year the wide angle camera wac provides repeat imaging of the north and south pole at a frequency of roughly 2 h with a ground sampling distance of 100 m the lroc narrow angle camera nac acquires images with a ground sampling distance of 0 5 2 0 m providing the means to construct high resolution maps that reveal illuminated terrain under varying lighting conditions with the multi temporal coverage provided by the wac and the high resolution images from the nac the lroc dataset enables a more comprehensive analysis of the illumination conditions near the lunar poles than any previous image based dataset furthermore these images are used to validate previously published numerical models that simulate polar illumination conditions from our analysis of the lroc images we identified localized regions where the lunar surface remains illuminated for nearly 94 of the year with the longest eclipsed period lasting only 43 h we also identified small illuminated peaks tens of meters across in areas previously modeled to be in shadow together the wac and nac dataset provide direct high resolution observations of the actual surface illumination environment of potential exploration sites near the lunar poles

craters hosting radar bright deposits in mercury s north polar region areas of persistent shadow determined from messenger images 1 radar bright features near mercury s poles were discovered in earth based radar images and proposed to be water ice present in permanently shadowed areas images from messenger s one year primary orbital mission provide the first nearly complete view of mercury s north polar region as well as multiple images of the surface under a range of illumination conditions we find that radar bright features near mercury s north pole are associated with locations persistently shadowed in messenger images within 10 of the pole almost all craters larger than 10 km in diameter host radar bright deposits there are several craters located near mercury s north pole with sufficiently large diameters to enable long lived water ice to be thermally stable at the surface within regions of permanent shadow craters located farther south also host radar bright deposits and show a preference for cold pole longitudes thermal models suggest that a thin insulating layer is required to cover these deposits if the radar bright material consists predominantly of long lived water ice many small 10 km diameter and low latitude extending southward to 66 n craters host radar bright material and water ice may not be thermally stable in these craters for 1 gy even beneath an insulating layer the correlation of radar bright features with persistently shadowed areas is consistent with the deposits being composed of water ice and future thermal modeling of small and low latitude craters has the potential to further constrain the nature source and timing of emplacement of the radar bright material

morphometry of impact craters on mercury from messenger altimetry and imaging abstract data acquired by the mercury laser altimeter and the mercury dual imaging system on the messenger spacecraft in orbit about mercury provide a means to measure the geometry of many of the impact craters in mercury s northern hemisphere in detail for the first time the combination of topographic and imaging data permit a systematic evaluation of impact crater morphometry on mercury a new calculation of the diameter d t at which craters transition with increasing diameter from simple to complex forms and an exploration of the role of target properties and impact velocity on final crater size and shape measurements of impact crater depth on mercury confirm results from previous studies with the exception that the depths of large complex craters are typically shallower at a given diameter than reported from mariner 10 data secondary craters on mercury are generally shallower than primary craters of the same diameter no significant differences are observed between the depths of craters within heavily cratered terrain and those of craters within smooth plains the morphological attributes of craters that reflect the transition from simple to complex craters do not appear at the same diameter instead flat floors first appear with increasing diameter in craters at the smallest diameters followed with increasing diameter by reduced crater depth and rim height and then collapse and terracing of crater walls differences reported by others in d t between mercury and mars despite the similar surface gravitational acceleration on the two bodies are confirmed in this study the variations in d t between mercury and mars cannot be adequately attributed to differences in either surface properties or mean projectile velocity

thermal stability of volatiles in the north polar region of mercury thermal models for the north polar region of mercury calculated from topographic measurements made by the mercury surface space environment geochemistry and ranging messenger spacecraft show that the spatial distribution of regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice messenger measurements of near infrared surface reflectance indicate bright surfaces in the coldest areas where water ice is predicted to be stable at the surface and dark surfaces within and surrounding warmer areas where water ice is predicted to be stable only in the near subsurface we propose that the dark surface layer is a sublimation lag deposit that may be rich in impact derived organic material

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals

non axisymmetric behaviour of olduvai and jaramillo polarity transitions recorded in north central pacific deep sea sediments the causes of the reversals of the earth s magnetic field can be investigated by studying the palaeomagnetic record preserved in rocks a proper documentation of reversal phenomena should yield valuable information about the generation of the field deep sea sediments provide a globally and temporally distributed set of polarity transition records our studies2 4 of the olduvai 1 88 1 72 myr 1 and jaramillo 0 94 0 88 myr 1 subchrons using deep sea sediments showed that the time averaged behaviour of the geomagnetic field during these reversals proved to be reliably recorded independent of the latitude and longitude of the sample site here we test some phenomenological transition models and present directional data virtual geomagnetic poles vgps and related directions in rotated declination inclination d i space5 using two records of the olduvai reverse normal reverse r n r and two records of the jaramillo r n r polarity transitions these data are better explained by the flooding rather than the standing field model but they also indicate a degree of asymmetry of the transitional field

age constraints of mercury s polar deposits suggest recent delivery of ice abstract surface ice at the poles of mercury appears as several m thick deposits that are composed of nearly pure water we provide new age estimates of the surfaces of mercury s polar deposits from combined analyses of poisson statistics and direct observations of crater densities within permanently shadowed radar bright regions imaged by the messenger spacecraft these age estimates conservatively suggest that ice was delivered to mercury within the last 330 myr the geologically young ages suggest that the surfaces have been recently refreshed and this may be accomplished by the delivery of ice in a young impactor or impactors a single recent impactor is more consistent with the relative purity of the ice as suggested by the earth based radar observations in contrast to ice on mercury observations of the lunar poles are suggestive of a highly patchy distribution of surface frost the patchiness of lunar polar deposits is consistent with long exposure times to the space weathering environment given enough time the polar deposits on mercury may age into a more heterogeneous spatial distribution similar to that on the moon

a combined paleomagnetic dating investigation of the upper jaramillo transition from a volcanic section at tenerife canary islands abstract a coupled paleomagnetic dating investigation has been conducted on a sequence of 25 successive lava flows emplaced during the upper transition of the jaramillo subchron in tenerife canary islands this sequence is located along the western wall of the guimar collapse scar in the south central part of the island nine flows distributed throughout this sequence were dated using unspiked k ar and 40 ar 39 ar methods they bracket the section between 1009 22 ka and 971 21 ka 2 a first group of 8 flows at the bottom of the sequence is characterized by normal polarity with paleointensity values of the order of present day field intensity in the canary islands the virtual geomagnetic poles vgp of these 8 flows describe a short loop at high latitudes seven overlying flows are transitional in directions and dated between 991 14 ka and 1002 11 ka consistently with published ages of the upper jaramillo reversal this second group of flows is characterized by low paleointensity values around 8 12 t that are less than 30 of the present dipole value in tenerife the vgps of the first two transitional flows lie over northeastern pacific whereas the five following transitional flows have all negative inclinations and their vgps lie initially over east antarctica then describe a northward loop almost reaching new zealand the final group of ten flows yield intensities varying between 20 and 35 t and vgps close to the southern pole with two of them describing a small amplitude second loop to southeastern pacific assuming a constant extrusion rate as a very first approximation the distribution of the obtained ages suggests a duration of 7 6 5 6 ka for the transitional interval the obtained transitional positions of vgps are consistent with the path reported for the same reversal from north atlantic sediments but are different from the only other volcanic record from tahiti the intensity low characterizing the transitional interval remains the best tie point centered at 996 7 ka 2 relative to 28 02 ma fc sanidine

areas of permanent shadow in mercury s south polar region ascertained by messenger orbital imaging 1 radar bright features near mercury s poles have been postulated to be deposits of water ice trapped in cold permanently shadowed interiors of impact craters from its orbit about mercury messenger repeatedly imaged the planet s south polar region over one mercury solar day providing a complete view of the terrain near the south pole and enabling the identification of areas of permanent shadow larger in horizontal extent than approximately 4 km in mercury s south polar region all radar bright features correspond to areas of permanent shadow application of previous thermal models suggests that the radar bright deposits in mercury s south polar cold traps are in locations consistent with a composition dominated by water ice provided that some manner of insulation such as a thin layer of regolith covers many of the deposits

extent age and resurfacing history of the northern smooth plains on mercury from messenger observations abstract messenger orbital images show that the north polar region of mercury contains smooth plains that occupy 7 of the planetary surface area within the northern smooth plains nsp we identify two crater populations those superposed on the nsp post plains and those partially or entirely embayed buried the existence of the second of these populations is clear evidence for volcanic resurfacing the post plains crater population reveals that the nsp do not exhibit statistically distinguishable subunits on the basis of crater size frequency distributions nor do measures of the areal density of impact craters reveal volcanically resurfaced regions within the nsp these results suggest that the most recent outpouring of volcanic material resurfaced the majority of the region and that this volcanic flooding emplaced the nsp over a relatively short interval of geologic time perhaps 100 my or less stratigraphic embayment relationships within the buried crater population including partial crater flooding and the presence of smaller embayed craters within the filled interiors of larger craters and basins indicate that a minimum of two episodes of volcanic resurfacing occurred from the inferred rim heights of embayed craters we estimate the nsp to be regionally 0 7 1 8 km thick with a minimum volume of volcanic material of 4 10 6 to 10 7 km 3 because of the uncertainty in the impact flux at mercury the absolute model age of the post plains volcanism could be either 3 7 or 2 5 ga depending on the chronology applied

the thermal stability of water ice at the poles of mercury recent radar observations of mercury have revealed the presence of anomalous radar reflectivity and polarization features near its north and south poles thermal model calculations show that despite mercury s proximity to the sun the temperatures of flat low reflectivity surfaces at mercury s poles are not expected to exceed 167 kelvin the locations of the anomalous polar radar features appear to be correlated with the locations of large high latitude impact craters maximum surface temperatures in the permanently shadowed regions of these craters are expected to be significantly colder as low as 60 kelvin in the largest craters these results are consistent with the presence of water ice because at temperatures lower than 112 kelvin water ice should be stable to evaporation over time scales of billions of years

can heterogeneous core mantle electromagnetic coupling control geomagnetic reversals abstract an analytical model is developed for the electromagnetic torques exerted on the mantle by the poloidal magnetic field of the core interacting with a laterally heterogeneous conducting layer at the base of the mantle torques due to changes in both orientation and intensity of a dipolar poloidal field are included contrary to earlier suggestions our calculations predict that the trajectory of the magnetic pole during a polarity reversal is not strongly affected by heterogeneity in mantle electrical conductivity

iron biogeochemistry in holocene palaeo and actual salt marshes in coastal areas of the pampean plain argentina in salt marshes the hydrodynamics and the availability of iron organic matter and sulphate influence the formation and or dissolution of iron sulfides and iron oxyhydroxides therefore they constitute key factors affecting the iron biogeochemical processes in these environments the aim of this work is to evaluate the physico chemical and mineralogical variations associated to iron biogeochemistry in palaeo and actual salt marshes in the area of influence of the mar chiquita coastal lagoon pampean plain argentina in soils of exhumed palaeo marshes the iron contents are 56 95 mol g 1 whereas these contents decrease to 36 75 mol g 1 in actual marsh soils the presence of framboidal and poliframboidal pyrites associated with gypsum barite calcite halite and iron oxyhydroxides defines the conditions of the pedosedimentary sequences of the holocene paleomarshes sequences of pyrite formation sulfidization and degradation sulfuricization were observed these processes were evidenced by a sequential extraction reflecting that the largest proportion of iron is in the form of crystalline iron oxides 28 76 and lepidocrocite 6 16 while the proportion associated with ferrihydrite and pyrite is low 0 9 and 1 17 respectively these facts could be partly explained by the complex redox processes characteristic of these environments such as aeration generated by the rhizosphere and intense bioturbation by invertebrates these iron biomineralizations have been useful because they allow paleoenvironmental interpretations and characterization of paleomarshes and environmental inferences related to the management of actual salt marshes

radar images of mars full disk images of mars have been obtained with the use of the very large array vla to map the radar reflected flux density the transmitter system was the 70 m antenna of the deep space network at goldstone california the surface of mars was illuminated with continuous wave radiation at a wavelength of 3 5 cm the reflected energy was mapped in individual 12 minute snapshots with the vla in its largest configuration fringe spacings as small as 67 km were obtained the images reveal near surface features including a region in the tharsis volcano area over 2000 km in east west extent that displayed no echo to the very low level of the radar system noise the feature called stealth is interpreted as a deposit of dust or ash with a density less than about 0 5 gram per cubic centimeter and free of rocks larger than 1 cm across the deposit must be several meters thick and may be much deeper the strongest reflecting geological feature was the south polar ice cap which was reduced in size to the residual south polar ice cap at the season of observation the cap image is interpreted as arising from nearly pure co 2 or h 2o ice with a small amount of martian dust less than 2 percent by volume and a depth greater than 2 to 5 m only one anomalous reflecting feature was identified outside of the tharsis region although the elysium region was poorly sampled in this experiment and the north pole was not visible from earth

pleistocene ignimbrites of western armenia paleomagnetic and magnetic anisotropy constraints on flow direction and stratigraphy abstract voluminous early to middle pleistocene pyroclastic flow deposits were generated from mount aragats aragats volcanic province avp one of the largest volcanoes in the arabia eurasia continental collision zone here we report paleomagnetic results in combination with anisotropy of magnetic susceptibility ams analysis from 31 outcrops belonging to six ignimbrite units of the western part of the avp covering an area of more than 5000 km2 paleomagnetic directions analysis of samples from 26 outcrops of the three younger ignimbrite units yields a well defined normal polarity direction of d 339 2 i 55 7 k 278 4 and 95 1 7 the virtual coincidence of the paleomagnetic directions seems to point to an emplacement of the units within a restricted time of similar geomagnetic secular variation i e few hundred years however new isotope 40ar 39ar ages of the three units suggest two separated events at 0 65 and 0 75 ma we cannot fully exclude the possibility of similar paleomagnetic directions at 0 65 and 0 75 ma or the occurrence of a widespread remagnetization event at 0 65 ma but both scenarios are highly unlikely in contrast to that five sites from the three older units all show reversed magnetic polarity which implies an age of emplacement older than the last magnetic polarity reversal at 780 ka this agrees with recent 40ar 39ar ages of the three older units at 0 93 ma the paleomagnetic scatter of the older units is high compared to the younger ones even if paleomagnetic directions and radiometric age are similar available data cannot unequivocally indicate if the three older units were generated from the same or three different eruptions the high quality ams data result in three different types of fabric lineated foliated and undetermined that are all commonly observed in the studied pyroclastic deposits accordingly we use lineation tilt of foliation or both for the determination of paleoflow direction results reveal a radial flow pattern surrounding mt aragats which is however highly influenced by topographic features such as valleys even in proximity of the eruption center

the role of the earth s mantle in controlling the frequency of geomagnetic reversals a series of computer simulations of the earth s dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magnetic field generation in the fluid core eight different patterns of heat flux from the core to the mantle are imposed over the core mantle boundary spontaneous magnetic dipole reversals and excursions occur in seven of these cases although sometimes the field only reverses in the outer part of the core and then quickly reverses back the results suggest correlations among the frequency of reversals the duration over which the reversals occur the magnetic field intensity and the secular variation the case with uniform heat flux at the core mantle boundary appears most earth like this result suggests that variations in heat flux at the core mantle boundary of the earth are smaller than previously thought possibly because seismic velocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin or because of enhanced heat flux in the mantle s zones of ultra low seismic velocity

spline analysis of holocene sediment magnetic records uncertainty estimates for field modeling 1 sediment and archeomagnetic data spanning the holocene enable us to reconstruct the evolution of the geomagnetic field on time scales of centuries to millennia in global field modeling the reliability of data is taken into account by weighting according to uncertainty estimates uncertainties in sediment magnetic records arise from 1 imperfections in the paleomagnetic recording processes 2 coring and sub sampling methods 3 adopted averaging procedures and 4 uncertainties in the age depth models we take a step toward improved uncertainty estimates by performing a comprehensive statistical analysis of the available global database of holocene magnetic records smoothing spline models that capture the robust aspects of individual records are derived this involves a cross validation approach based on an absolute deviation measure of misfit to determine the smoothing parameter for each spline model together with the use of a minimum smoothing time derived from the sedimentation rate and assumed lock in depth departures from the spline models provide information concerning the random variability in each record temporal resolution analysis reveals that 50 of the records have smoothing times between 80 and 250 years we also perform comparisons among the sediment magnetic records and archeomagnetic data as well as with predictions from the global historical and archeomagnetic field models combining these approaches we arrive at individual uncertainty estimates for each sediment record these range from 2 5 to 11 2 median 5 9 interquartile range 5 4 to 7 2 for inclination 4 1 to 46 9 median 13 4 interquartile range 11 4 to 18 9 for relative declination and 0 59 to 1 32 median 0 93 interquartile range 0 86 to 1 01 for standardized relative paleointensity these values suggest that uncertainties may have been underestimated in previous studies no compelling evidence for systematic inclination shallowing is obtained from the analysis of the available database of holocene sediment magnetic records the analysis highlights the importance of collecting oriented cores publishing and archiving unprocessed raw paleosecular variation determinations and presenting a detailed chronology so that changes in the sedimentation rate can be assessed with regard to future field models workers should consider rejection of anomalous cores through comparisons to other sources and ensure that realistically large uncertainties are allocated to high latitude declination records

the last two geomagnetic polarity reversals recorded in high deposition rate sediment drifts high resolution records of geomagnetic polarity transitions are rare owing to the difficulty of preserving in the geological record details of a process with a duration of only a few thousand years lavas can record the instantaneous field on cooling but lava records are themselves discontinuous and therefore polarity transition fields are seldom captured sedimentary records are more continuous but typical deep sea sedimentation rates of a few cm kyr 1are too low to record transition fields adequately here we report transition records from north atlantic drift deposits with high sedimentation rates 12 cm kyr 1 at the ocean drilling program sites 983 and 984 virtual geomagnetic polar vgp paths are similar within and between sites for the last two geomagnetic reversals attesting to a memory effect spanning back to back reversals the complex vgp paths feature two vgp clusters one located in northeastern asia and the other in the south atlantic ocean off south america and provide a link between the longitudinally constrained transitional vgps often recorded in sediments1 2 which represent highly smoothed renditions of transition fields and the scattered transitional vgps3 and vgp clusters4 observed in lava records

imaging mercury s polar deposits during messenger s low altitude campaign images obtained during messenger s low altitude campaign in the final year of the mission provide the highest spatial resolution views of mercury s polar deposits images for distinct areas of permanent shadow within 35 north polar craters were successfully captured during the campaign all of these regions of permanent shadow were found to have low reflectance surfaces with well defined boundaries additionally brightness variations across the deposits correlate with variations in the biannual maximum surface temperature across the permanently shadowed regions supporting the conclusion that multiple volatile organic compounds are contained in mercury s polar deposits in addition to water ice a recent large impact event or ongoing bombardment by micrometeoroids could deliver water as well as many volatile organic compounds to mercury either scenario is consistent with the distinctive reflectance properties and well defined boundaries of mercury s polar deposits and the presence of volatiles in all available cold traps

near surface ice on mercury and the moon a topographic thermal model abstract a thermal model that can be easily adapted to craters of arbitrary shape is developed and applied to high latitude impact craters on mercury and the moon chao meng fu crater at 87 5 l on mercury an unnamed bowl shaped crater at 86 7 l on mercury and peary crater at 88 6 l on the moon for an assumed input topography and grid of surface elements the model computes for each element the irradiation from direct insolation and reflected and emitted radiation from other elements taking into account shadowing by the walls of the crater partial obscuration of the solar disk near the poles and the diurnal orbital and seasonal cycles temperatures are computed over the surface grid as functions of depth and time from the surface to a specified depth and over the pertinent astronomical cycles including the effects of direct and indirect surface irradiation infrared radiation heat conduction and interior heating vapor fluxes and ice recession times are computed as functions of ice depth over the surface grid temperatures profiles vapor fluxes and ice recession times were computed for flat surfaces not associated with craters near the poles of mercury and the moon it was found that water ice could have existed throughout geologic time within the maximum radar detection depth of recent observation of mercury j k harmon and m a slade 1992 science 258 640 643 poleward of 87 88 l on mercury and poleward of 73 l on the moon for chao meng fu crater it was found that 40 of the crater floor is permanently shadowed from direct solar insolation while the remainder of the crater floor is periodically illuminated by a partially obscured sun temperatures at the upper levels of the south wall can slightly exceed 550 k surface temperatures in the permanently shadowed region of the crater floor are under 130 k which could have allowed water ice to exist throughout geologic time within the radar detection depth of recent observation of mercury for the small bowl shaped crater on mercury it was found that most of the crater is permanently shadowed from direct solar radiation except for a narrow semicircular band bordering the north rim however temperatures in the permanently shadowed region periodically reach a maximum near 315 k due to efficient heating of the small crater by thermal emission and reflection from the small sunlit region which periodically reaches temperatures exceeding 630 k water ice could not have existed throughout geologic time anywhere in this crater within the radar detection depth for peary crater on the moon the entire crater floor is permanently shadowed from direct solar insolation with maximum temperatures under 120 k the upper level of the north wall periodically reaches a maximum temperature near 310 k the low temperatures on the crater floor would have allowed water ice to exist near the surface throughout geologic time provided that the moon s obliquity was always as low as it is at present

sampling bias in vgp longitudes i derive probability densities for virtual geomag netic pole vgp longitudes for a general statistical model of local magnetic field variations i show that even for very simple statistically homogeneous models of secular varia tion the distribution of vgp longitudes is peaked 90 o away from the sampling longitude thus when sites are distri buted unevenly a non uniform overall distribution of vgp longitudes is to be expected analysis of the recent geomag netic field indicates that this bias can be significant particu laxly when random errors in paleomagnetic data are allowed for it is possible that some of the deviations from unifor mity seen in recent compilations of paleomagnetic reversal and secular variation data are a statistical artifact resulting from the distortion of the vgp transformation and the non uniformity of paleomagnetic sampling sites

high resolution radar imaging of mercury s north pole abstract the recently upgraded arecibo s band 12 6 cm radar was used to make delay doppler images of mercury s north polar region where earlier observations had shown strong echoes from putative ice deposits in craters the image resolution of 1 5 3 km is a substantial improvement over the 15 km resolution of the older arecibo images j k harmon et al 1994 nature 369 213 215 the new observations confirm all the original polar features and reveal many additional features including several at latitudes as low as 72 75 n and several from craters less than 10 km in diameter all of the new features located on the mariner imaged side of the planet can be matched with known craters or other shaded areas we find the north pole to be located 65 km from the original mariner based pole and 15 km from the new mariner based pole of m s robinson et al 1999 j geophys res 104 30 847 30 852 the improved resolution reveals fine structure in the radar features and their respective host craters including radar shadowing highlighting by central peaks and rim walls rim terracing and preferential concentration of radar bright deposits in shaded southern floor areas the radar features high brightness circular polarization inversion c 1 25 and confinement to regions permanently shaded from direct sunlight are all consistent with volume scattering from a cold trapped volatile such as clean water ice the sizes and locations of most of the features show good agreement with the thermal model of a r vasavada d a paige and s e wood 1999 icarus 141 179 193 for insulated buried water ice although the problems of explaining radar features in small craters and the rapid burial required at lower latitudes suggest that other factors may be suppressing ice loss after emplacement

radar mapping of mercury full disk images and polar anomalies a random code technique has been used at arecibo to obtain delay doppler radar images of the full disk of mercury anomalously bright features were found at the north and south poles the north polar feature is oblong 4 by 8 and offset from the pole the smaller south polar feature is mostly confined to the floor of the crater chao meng fu the polar locations and radar properties of these features indicate that they may be produced by volume scattering in ice the images also reveal a variety of more subdued reflectivity features ranging in size from hundreds to thousands of kilometers some of these appear to have an impact origin

possible links between long term geomagnetic variations and whole mantle convection processes the earth s internal magnetic field varies on timescales of months to billions of years the field is generated by convection in the liquid outer core which in turn is influenced by the heat flowing from the core into the base of the overlying mantle much of the magnetic field s variation is thought to be stochastic but over very long timescales this variability may be related to changes in heat flow associated with mantle convection processes over the past 500 myr correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late mesozoic era beginning about 180 myr ago simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability such as occurred between the middle jurassic and middle cretaceous periods may have been triggered by a decrease in core mantle boundary heat flow either globally or in equatorial regions this decrease in heat flow could have been linked to reduced mantle plume head production at the core mantle boundary an episode of true polar wander or a combination of the two

demise of the rapid field change hypothesis at steens mountain the crucial role of continuous thermal demagnetization abstract rapid continuous thermal demagnetization in a triaxe vibrating sample magnetometer disproves an earlier hypothesis that an episode of extraordinarily fast field change is recorded in the remanent magnetization of a single lava flow that cooled during a geomagnetic polarity reversal the original evidence for this hypothesis was the distinctive pattern of variation in direction with vertical position after stepwise demagnetization to 500 c but continuous thermal demagnetization now proves that this flow actually carries consistent high temperature remanence parallel to that of the flow immediately below why does continuous thermal demagnetization succeed when conventional thermal and alternating field demagnetization fail to isolate this direction detailed investigation of the demagnetization trajectories and underlying rock magnetic properties suggest that normal polarity secondary remanent magnetization acquired both at room temperature in today s polarity chron and during modest reheating in a normal field during cooling of the overlying flow is responsible for the misleading pattern of directions alteration during ordinary thermal demagnetization evidently promotes the overprint to higher blocking temperatures so that it masks the primary component whereas the rapid heating 40 c min during continuous demagnetization is fast enough to demagnetize the normal overprint before this masking can happen

estimating slip rates and recurrence intervals for strong earthquakes along an intracontinental fault example of the pambak sevan sunik fault armenia north of the arabian plate active tectonics is characterised by both n s compression and e w extension associated with strike slip faults the pambak sevan sunik fault pssf zone in armenia is one of the major active structures of the region the fault is comprised of four main segments and it displays morphological evidence for dextral movement during the holocene however no large earthquake m 7 has occurred in the northern or central parts of the fault during the last 2000 years we undertook a geomorphological and paleoseismological investigation along the pambak sevan sunik fault with the aim of estimating the long term slip rate and recurrence interval of strong earthquakes trenches were excavated at three sites detailed studies of trench cross sections and dating radiocarbon and ceramics show three faulting events that occurred in the vanadzor artanih segment fioletovo and semionovka areas whereas a single event took place in the artanish sunik segment khonarhasar area in both areas we estimated the average slip rate using i the offset of rivers along the vanadzor artanish segment 2 24 0 96 mm year over an interval of 120 300 ka and ii the offset of volcanic cones along the artanish sunik segment 0 53 0 04 mm year over an interval of 1 4 ma these results suggest that a greater slip rate characterises the vanadzor artanish segment fioletovo site from the artanish sunik khonarhasar site segment division of the pambak sevan sunik fault zone into two main branches east of the artanish peninsula sevan lake could explain the difference in slip rate in addition to its segmented seismic behaviour the pambak sevan sunik fault is a well documented example of a fault that generates strong earthquakes with long recurrence time intervals about 3000 4000 years d 2001 elsevier science b v all rights reserved

lower mantle superplume growth excites geomagnetic reversals abstract seismic images of the lower mantle reveal two large scale low shear wave velocity provinces beneath africa and the pacific that are variously interpreted as superplumes plume clusters or piles of dense mantle material associated with the d layer here we show that time variations in the height of these structures produce variations in heat flux across the core mantle boundary that can control the rate at which geomagnetic polarity reversals occur superplume growth increases the mean core mantle boundary heat flux and its lateral heterogeneity thereby stimulating polarity reversals whereas superplume collapse decreases the mean core mantle boundary heat flux and its lateral heterogeneity inhibiting polarity reversals our results suggest that the long stable polarity geomagnetic superchrons such as occurred in the cretaceous permian and earlier in the geologic record were initiated and terminated by the collapse and growth of lower mantle superplumes respectively

high resolution 10be and paleomagnetic recording of the last polarity reversal in the chiba composite section age and dynamics of the matuyama brunhes transition we present new magnetic direction and relative paleointensity and beryllium isotope 10 be 9 be and 10 be 9 be ratio results covering the last geomagnetic reversal i e the matuyama brunhes transition mbt from the chiba composite section cbcs east central japan the very high sedimentation rates 90 cm ka of the studied site a candidate site for the global boundary stratotype section and point gssp of the lower middle pleistocene boundary allow the acquisition of a dataset of exceptional resolution coupled cosmogenic 10 be and magnetic results measured on the same samples demonstrate that the magnetization acquisition conserved the timing of rapid geomagnetic features allowing accurate paleomagnetic interpretations a polarity switch ps capturing most of the angular deviation occurring between 771 9 and 773 9 ka was followed by a directional instability rebound ic1 until 768 5 ka this sequence with 5 4 ka duration was characterized by a weak dipole field as low as 2 3 0 3 10 22 am 2 four rapid episodes of directional instabilities preceding and following the ps ic1 phase completed the reversal sequence the asymmetry observed between the long term dipole decay and sharp recovery and rapid oscillations 1 ka demonstrate the complex transitional field behavior during dipole moment low before reestablishment of the full polarity state our observations reinforce the fact that most reversal records do not integrate the full field behavior associated with the geodynamo action although this poses problems for understanding the underlying physical processes that produce reversals it does not hamper stratigraphic correlations among most geologic records

early pleistocene climate cycles in continental deposits of the lesser caucasus of armenia inferred from palynology magnetostratigraphy and 40ar 39ar dating plio pleistocene diatomitic sequences in the shamb paleo lake south armenia lesser caucasus offer a rare opportunity to give new insights on the paleo climate of western asia we present an integrated palynological 40ar 39ar geochronologic and magnetostratigraphic study for the most complete section in the sedimentary deposits of the shamb paleo lake 40ar 39ar dating of two volcaniclastic layers provided ages of 1 24 0 03 and 1 16 0 02 ma 2 magnetostratigraphic data show that the entire shamb section is of reversed polarity which combined with 40ar 39ar dating suggests that the entire section correlates with part of the matuyama period 1 785 1 070 ma pollen assemblages and macroremains diversity clearly show an alternation of glacial and interglacial phases age calibrations and accumulation rate extrapolation allow a direct correlation of climate changes with the global isotopic curve and show that the shamb section probably ranges from approximately 1 300 to 1 080 ma marine isotopic stages 40 to 31 the vegetation of the lesser caucasus developed in a mosaic pattern in a pleistocene continental mostly arid climate comparable to the present day climate the observed vegetation changes record a dominant climate response to the obliquity orbital parameter but the influence of precession could not be established from the shamb data pollen and macroflora both indicate that glacial periods were cold and dry and that interglacials were warm with local humidity the early pleistocene climatic model for western asia is thus similar to the climatic model for the mediterranean area

bright and dark polar deposits on mercury evidence for surface volatiles measurements of surface reflectance of permanently shadowed areas near mercury s north pole reveal regions of anomalously dark and bright deposits at 1064 nanometer wavelength these reflectance anomalies are concentrated on poleward facing slopes and are spatially collocated with areas of high radar backscatter postulated to be the result of near surface water ice correlation of observed reflectance with modeled temperatures indicates that the optically bright regions are consistent with surface water ice whereas dark regions are consistent with a surface layer of complex organic material that likely overlies buried ice and provides thermal insulation impacts of comets or volatile rich asteroids could have provided both dark and bright deposits

investigating mercury s south polar deposits arecibo radar observations and high resolution determination of illumination conditions there is strong evidence that mercury s polar deposits are water ice hosted in permanently shadowed regions in this study we present new arecibo radar observations of mercury s south pole which reveal numerous radar bright deposits and substantially increase the radar imaging coverage we also use images from messenger s full mission to determine the illumination conditions of mercury s south polar region at the same spatial resolution as the north polar region enabling comparisons between the two poles the area of radar bright deposits in mercury s south is roughly double that found in the north consistent with the larger permanently shadowed area in the older cratered terrain at the south relative to the younger smooth plains at the north radar bright features are strongly associated with regions of permanent shadow at both poles consistent with water ice being the dominant component of the deposits however both of mercury s polar regions show that roughly 50 of permanently shadowed regions lack radar bright deposits despite some of these locations having thermal environments that are conducive to the presence of water ice the observed uneven distribution of water ice among mercury s polar cold traps may suggest that the source of mercury s water ice was not a steady regular process but rather that the source was an episodic event such as a recent large impact on the innermost planet

mantle control of the geodynamo consequences of top down regulation the mantle global circulation including deep subduction and lower mantle superplumes exerts first order controls on the evolution of the core the history of the geodynamo and the structure of the geomagnetic field mantle global circulation models that include realistic plate motions deep subduction and compositional heterogeneity similar to the observed large low seismic velocity provinces in the lower mantle predict that the present day global average heat flux at the core mantle boundary cmb exceeds 85 mw m 2 this is sufficient to drive the present day geodynamo by thermochemical convection and implies a very young inner core with inner core nucleation between 400 and 1100 ma the mantle global circulation also generates spatially heterogeneous heat flux at the cmb with peak to peak lateral variations exceeding 100 mw m 2 such extreme lateral variability in cmb heat flux in conjunction with the high thermal conductivity of the core implies that the liquid outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle but thermally stable beneath the large low seismic velocity provinces numerical dynamo simulations show how this pattern of heterogeneous boundary heat flux affects flow in the outer core producing localized circulation patterns beneath the cmb tied to the mantle heterogeneity and long lived deviations from axial symmetry in the geomagnetic field

near surface temperatures on mercury and the moon and the stability of polar ice deposits abstract in order to assess the thermal stability of polar ice deposits we present model calculated temperatures of flat surfaces and surfaces within bowl shaped and flat floored polar impact craters on mercury and the moon our model includes appropriate insolation cycles realistic crater shapes multiple scattering of sunlight and infrared radiation and depth and temperature dependent regolith thermophysical properties unshaded water ice deposits on the surface of either body are rapidly lost to thermal sublimation a subsurface water ice deposit is stable within 2 latitude of the moon s poles meter thick water ice deposits require billions of years to sublime if located in the permanently shaded portions of flat floored craters within 10 latitude of the poles of mercury and 13 latitude of the poles of the moon results for craters associated with radar features on mercury are consistent with the presence of stable water ice deposits if a thin regolith layer thermally insulates deposits at lower latitudes and within smaller craters a regolith cover would also reduce losses from diffusion ion sputtering impact vaporization and h ly and is implied independently by the radar observations permanently shaded areas near the moon s poles are generally colder than those near mercury s poles but the moon s obliquity history its orbit through earth s magnetospheric tail and its radar opaque regolith may limit the volume and radar detectability of ice deposits there

illumination conditions of the lunar polar regions using lola topography abstract we use high resolution altimetry data obtained by the lunar orbiter laser altimeter instrument onboard the lunar reconnaissance orbiter to characterize present illumination conditions in the polar regions of the moon compared to previous studies both the spatial and temporal extent of the simulations are increased significantly as well as the coverage fill ratio of the topographic maps used thanks to the 28 hz firing rate of the five beam instrument we determine the horizon elevation in a number of directions based on 240 m resolution polar digital elevation models reaching down to 75 latitude the illumination of both polar regions extending to 80 can be calculated for any geometry from those horizon longitudinal profiles we validated our modeling with recent lunar reconnaissance orbiter wide angle camera images we assessed the extent of permanently shadowed regions psrs defined as areas that never receive direct solar illumination and obtained total areas generally larger than previous studies 12 866 and 16 055 km2 in the north and south respectively we extended our direct illumination model to account for singly scattered light and found that every psr does receive some amount of scattered light during the year we conducted simulations over long periods several 18 6 years lunar precession cycles with a high temporal resolution 6 h and identified the most illuminated locations in the vicinity of both poles because of the importance of those sites for exploration and engineering considerations we characterized their illumination more precisely over the near future every year a location near the shackleton crater rim in the south polar region is sunlit continuously for 240 days and its longest continuous period in total darkness is about 1 5 days for some locations small height gains 10 m can dramatically improve their average illumination and reduce the night duration rendering some of those particularly attractive energy wise as possible sites for near continuous sources of solar power

the thickness of radar bright deposits in mercury s northern hemisphere from individual mercury laser altimeter tracks the discovery of mercury s radar bright deposits has expanded our understanding of volatiles in the solar system key to deciphering the history and origin of the radar bright deposits is an estimate of the volume of radar bright material that in turn requires a measure of the average thickness of the deposits in this study we investigate changes in topography across radar bright deposits hosted in flat floored complex craters using individual edited mercury laser altimeter mla tracks we compare the difference in heights of radar bright regions and non radar bright regions of the crater floor and the difference of similarly sized and located regions in non radar bright craters and show that the two populations cannot be distinguished the similarity of topography in these two sets of craters allows an upper limit of 15 m to be placed on the thickness of the radar bright deposits

illumination conditions at the lunar south pole recent remote sensing data strongly suggest that deposits of ice occur in permanently shadowed regions at the lunar poles clementine by providing the first contiguous coverage of the moon has enabled the production of an illumination map of the south pole this map shows the extent of the areas of permanent darkness during winter in this region as well as identifying places on the lunar surface that receive illumination for more than 50 of the lunar day the permanently dark areas are prime candidates for locations of ice deposits while regions that are illuminated for anomalous periods are possible sites for lunar bases in this paper we study in detail the illumination history during a lunar day of some of these sites while we find no area that receives permanent illumination we do find two areas only 10 km apart which collectively receive sunlight for over 98 of the time

authigenic kaolinite and associated pyrite in chalk of the cretaceous niobrara formation eastern colorado abstract cores from the smoky hill chalk member of the cretaceous niobrara formation have several zones containing authigenic kaolinite as spherical moldic polycrystalline aggregates that occur within single or multichambered foraminiferal tests and are commonly associated with framboidal pyrite such kaolinite is inferred to result from volcanic ash deposited during chalk sedimentation shortly after burial a colloidal aluminous gel or solution formed from the unstable ash and moved into organic rich foraminiferal tests where sulfate reducing bacteria created a favorable microenvironment for the simultaneous crystallization of kaolinite and pyrite

images of surface volatiles in mercury s polar craters acquired by the messenger spacecraft images acquired by nasa s messenger spacecraft have revealed the morphology of frozen volatiles in mercury s permanently shadowed polar craters and provide insight into the mode of emplacement and evolution of the polar deposits the images show extensive spatially continuous regions with distinctive reflectance properties a site within prokofiev crater identified as containing widespread surface water ice exhibits a cratered texture that resembles the neighboring sunlit surface except for its uniformly higher reflectance indicating that the surficial ice was emplaced after formation of the underlying craters in areas where water ice is inferred to be present but covered by a thin layer of dark organic rich volatile material regions with uniformly lower reflectance extend to the edges of the shadowed areas and terminate with sharp boundaries the sharp boundaries indicate that the volatile deposits at mercury s poles are geologically young relative to the time scale for lateral mixing by impacts and either are restored at the surface through an ongoing process or were delivered to the planet recently

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

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

upper and lower jaramillo polarity transitions recorded in iodp expedition 303 north atlantic sediments implications for transitional field geometry sediments collected during the integrated ocean drilling program iodp expedition 303 in the north atlantic provide records of polarity transitions and geomagnetic excursions at a high resolution here we investigate polarity transitions at the upper and lower boundaries of the jaramillo subchronozone at sites u1305 u1304 and u1306 the sediments carry strong natural remanent magnetizations nrm with median destructive fields consistent with magnetite as the dominant magnetic carrier both polarity transitions are characterized by low values of relative paleointensity the u1305 record of the lower jaramillo reversal exhibits a marked cluster of virtual geomagnetic poles vgps over southern south america and a secondary accumulation in the region of ne asia north pacific the main south american vgp cluster is also visible at site u1304 which documents a less complex pattern possibly because of a higher degree of smoothing records of the upper jaramillo polarity transition document a vgp loop over the americas followed by north to south motion including a secondary vgp accumulation near india the similarity between several records of the upper jaramillo transition obtained at various sites in the north atlantic is a strong indication that geomagnetic field changes have been faithfully captured results suggest that a transverse possibly dipolar field component has fluctuated during these polarity reversals and that these fluctuations combined with a reduced axial dipole component yielded the observed field at the earth s surface during the polarity transitions the lower jaramillo transition also exhibits vgp clusters in the vicinity of south america and eastern asia but these clusters are shifted relative to the upper jaramillo vgp clusters implying a memory exerted by the upper mantle on polarity transition geometry

statistical assessment of preferred transitional vgp longitudes based on palaeomagnetic lava data summary we examine the statistical significance of a recent report indicating that palaeomagnetic lava data recording reversals and excursions over the past 20 myr yield transitional virtual geomagnetic poles vgps tending to fall along american and asian longitudes using a bootstrap analysis of spherical densitygrams we find that this result is neither a sensitive function of nor heavily dependent upon particular transitional records previous analyses used 2 and kuiper tests of longitudinal uniformity of transitional vgps however the bootstrap analysis presented here takes such statistical tests one step further showing specifically that american and asian longitudes are preferred with significance at about the 95 per cent confidence level furthermore provided the appropriate polarity convention is used we confirm that icelandic transitional vgps tend to fall along asian longitudes these results are indicative of non dipolar transitional fields whose structure is governed by some form of core mantle coupling

planetary science constant illumination at the lunar north pole thinking of a property on the moon the northern rim of peary crater could be the place to be an illumination map of the moon s north pole based on images obtained by the clementine spacecraft reveals areas around the peary crater that are in permanent sunlight these are prime locations for lunar outposts as well as abundant solar energy temperatures are relatively benign and nearby regions in permanent shadow may conveniently contain water ice images returned by the spacecraft clementine have been used to produce a quantitative illumination map of the north pole of the moon revealing the percentage of time that points on the surface are illuminated during the lunar day we have used this map to identify areas that are constantly illuminated during a lunar day in summer and which may therefore be in permanent sunlight all are located on the northern rim of peary crater close to the north pole permanently sunlit areas represent prime locations for lunar outpost sites as they have abundant solar energy are relatively benign thermally when compared with equatorial regions and are close to permanently shadowed regions that may contain water ice

radar mapping of mercury s polar anomalies ground based radar observations of mercury have revealed unusually strong highly depolarized echoes from the north1 2 and south2 poles these anomalous echoes have been cited as evidence of polar ice deposits1 5 thermal studies3 5 suggest that the permanently shaded floors of large polar craters are cold enough to preserve water ice in a stable state over aeons in spite of mercury s proximity to the sun here we present high resolution radar maps of mercury s polar regions derived from delay doppler measurements we have resolved the north and south polar anomalies into numerous crater sized features and we have been able to identify the source craters for many of these features after making small corrections to the pole positions on the mariner 10 images the coincidence with crater locations together with other properties of the radar features are consistent with the polar ice hypothesis

arecibo radar imagery of mars ii chryse xanthe polar caps and other regions abstract we conclude our radar imaging survey of mars which maps spatial variations in depolarized radar reflectivity using arecibo s band 12 6 cm observations from 2005 2012 whereas our earlier paper harmon et al 2012 arecibo radar imagery of mars the major volcanic provinces icarus 220 990 1030 covered the volcanic regions of tharsis elysium and amazonis this paper includes non volcanic regions where hydrologic and impact processes can be the dominant resurfacing agents affecting radar backscatter many of the more prominent and interesting radar bright features outside the major volcanic provinces are located in and around chryse planitia and xanthe terra these features are identified with a basin in northeast lunae planum containing the combined deposits from maja vallis and ganges catena outflows channel outwash plains in western and southern chryse basin plateaus bordering chasma chaos zones where surface modification may have resulted from hydrologic action associated with incipient chaos formation and some bright ejecta craters in chryse basin of a type otherwise rare on mars dark halo craters have also been identified in chryse and elsewhere that are similar to those seen in the volcanic provinces although the cratered highlands are relatively radar bland they do exhibit some bright depolarized features these include eroded crater rims several unusual ejecta flows and impact melts and terrain softened plains the rims of large impact basins hellas argyre isidis show a variety of radar bright features provisionally identified with massif slopes erosion sediments eroded pyroclastics impact melts and glacial deposits the interiors of these basins are largely radar dark which is consistent with coverage by rock free sediments tempe terra and acheron fossae show bright features possibly associated with rift volcanism or eroded tectonic structures and northwest tempe terra shows one very bright feature associated with glacial or other ice processes in the dichotomy boundary region the first delay doppler images of the radar bright features from the north and south polar icecaps are presented both poles show the circular polarization inversion and high reflectivity characteristic of coherent volume backscatter from relatively clean ice the south polar feature is primarily backscatter from the residual co 2 icecap with a lesser contribution from the polar layered deposits whose finite optical depth probably accounts for the feature s strong s x band wavelength dependence conversely the north polar radar feature appears to be mostly backscatter from the h 2 o ice rich polar layered deposits rather than from the thin residual h 2 o cap the north polar region shows additional radar bright features from korolev crater and a few other outlying circumpolar ice deposits

detailed jaramillo field reversals recorded in lake sediments from armenia lower mantle influence on the magnetic field revisited abstract while it is well established that the earth s magnetic field is generated by a self sustaining dynamo that reversed its polarity at irregular intervals in the geological past the very mechanisms causing field reversals remain obscure paleomagnetic reconstructions of polarity transitions have been essential for physically constraining the underlying mechanisms in terms of time scale but thus far remain ambiguous with regard to the transitional field geometry here we present new paleomagnetic records from a rapidly deposited lacustrine sediment sequence with extraordinarily stable paleomagnetic signals which has captured in unprecedented detail the bottom reverse to normal r n and top normal to reverse n r transitions of the jaramillo subchron at 1 072 ma and at 0 988 ma the obtained virtual geomagnetic pole vgp path indicates an oscillatory transitional field behavior with four abrupt transequatorial precursory jumps across the pacific the distribution of vgp positions indicates regions of preferred occurrence our results are in agreement with previously proposed bands of transitional vgp occurrence over the americas and australia northwest pacific additionally our vgp positions seem to avoid large low shear velocity provinces llsvps above the core mantle boundary cmb thus our data supports the idea that the transitional field geometry is controlled by heat flux heterogeneities at the cmb linked to llsvps

comparison of areas in shadow from imaging and altimetry in the north polar region of mercury and implications for polar ice deposits earth based radar observations and results from the messenger mission have provided strong evidence that permanently shadowed regions near mercury s poles host deposits of water ice messenger s complete orbital image and topographic datasets enable mercury s surface to be observed and modeled under an extensive range of illumination conditions the shadowed regions of mercury s north polar region from 65 n to 90 n were mapped by analyzing mercury dual imaging system mdis images and by modeling illumination with mercury laser altimeter mla topographic data the two independent methods produced strong agreement in identifying shadowed areas all large radar bright deposits those hosted within impact craters 6 km in diameter collocate with regions of shadow identified by both methods however only 46 of the persistently shadowed areas determined from images and 43 of the permanently shadowed areas derived from altimetry host radar bright materials some sizable regions of shadow that do not host radar bright deposits experience thermal conditions similar to those that do the shadowed craters that lack radar bright materials show a relation with longitude that is not related to the thermal environment suggesting that the earth based radar observations of these locations may have been limited by viewing geometry but it is also possible that water ice in these locations is insulated by anomalously thick lag deposits or that these shadowed regions do not host water ice

mercury s global color mosaic an update from messenger s orbital observations abstract we report an update to the photometric correction used to produce global color mosaics of mercury derived from an analysis of photometric observations acquired during the orbital phase of messenger s primary mission comparisons between versions of the color mosaic produced with photometric corrections derived from flyby and orbital data indicate that areas imaged at high incidence and emission angles 50 are better standardized to a common illumination and viewing geometry with the orbit derived corrections seams between images taken at very different illumination geometries however are still present at visually detectable levels further improvements to the photometric correction await updates to the radiometric calibration that will enable data retrieval over a larger range of photometric angles

alternating field demagnetization single domain like memory and the lowrie fuller test of multidomain magnetite grains 0 6 356 μm 1 a fundamental question in paleomagnetism is how magnetite grains much larger than single domain size preserve stable remanence over millions of years in an effort to answer this question we measured alternating field af demagnetization of thermoremanent magnetization trm and saturation isothermal remanent magnetization sirm before and after low temperature demagnetization ltd ltd zero field cycling through 120 k unpins or nucleates domain walls reducing the remanence of multidomain grains we used two sets of sized crushed magnetites 0 6 1 3 6 9 14 20 110 and 135 m one set unannealed and the other annealed and a set of hydrothermally grown magnetites 0 8 6 3 25 64 94 and 356 m for all sizes trm and sirm memories after ltd are much harder to af demagnetize than the original remanences af demagnetization curves after ltd are flat for the first 10 mt such initial plateaus are one hallmark of single domain behavior in high stress unannealed grains after ltd response depends on grain size larger grains demagnetizing more easily than smaller ones in low stress annealed and hydrothermal magnetites after ltd response is almost independent of grain size over nearly 3 decades in grain size this size independent behavior could be due to grains in metastable single domain states in the smaller sized samples but in 100 m grains there must be a source of single domain like af behavior within the multidomain grains themselves we propose an ad hoc model in which ltd triggers domain wall unpinning and nucleation events up to a coercivity threshold producing the observed initial plateaus in af demagnetization curves of trm and sirm memories the size independent demagnetization behavior of memory in hydrothermal and annealed magnetites is ascribed to nucleation events above the threshold level and the additional size dependent af demagnetization of memory in high stress unannealed grains is explained by wall unpinning from strong stress centers in both cases the af properties merely mimic single domain behavior although ltd memory has single domain like af curves and erased remanence has multidomain like curves in grains of all sizes the lowrie and fuller 1971 test still works for the annealed samples for memory erased fraction and pre ltd remanence alike trm is more stable than sirm in fine grains 1 m less stable in large grains 135 m and of comparable stability in medium size grains 9 and 20 m