late ordovician turinian chatfieldian carbon isotope excursions and their stratigraphic and paleoceanographic significance five positive carbon isotope excursions are reported from platteville decorah strata in the upper mississippi valley all occur in subtidal carbonate strata and are recognized in the mifflin grand detour quimbys mill spechts ferry and guttenberg intervals the positive carbon isotope excursions are developed in a platteville decorah succession in which background 13c values increase upward from about 2 at the base to about 0 vienna pee dee belemnite vpdb at the top a regional north south 13c gradient with lighter values to the north and heavier values to the south is also noted peak excursion 13c values of up to 2 75 are reported from the quimbys mill excursion and up to 2 6 from the guttenberg excursion although there are considerable local changes in the magnitudes of these events the quimbys mill spechts ferry and guttenberg carbon isotope excursions occur in units that are bounded by submarine disconformities and completely starve out in deeper more offshore areas closely spaced chemostratigraphic profiles of these sculpted pyrite impregnated hardground surfaces show that they are associated with very abrupt centimeter scale negative 13c shifts of up to several per mil possibly resulting from the local diagenetic effects of incursions of euxinic bottom waters during marine flooding events

magnetic field modelling and interpretation of the himalayan tibetan plateau and adjoining north indian plains abstract a prominent magnetic low is seen over the himalayan tibetan plateau and the northern indian shield region in satellite anomaly data here we model the sources for the observed magnetic low in the region the induced magnetic field is modelled based on a crustal magnetisation model computed in a geographical information system with horizontal layers containing susceptibility derived from the geology and a tectonic map of the region and global seismic crustal thickness model the modelling results indicate that the upper crust of the himalayan tibetan plateau is magnetised while much of the middle crust and the lower crust appears non magnetic thermal modelling corroborates the predictions placing the curie isotherm in the southern tibetan plateau no deeper than 10 km the results suggest that the sources of the magnetic low observed in the region are likely to be a thin magnetised crust because of the elevated curie isotherm in this region the indian shield region is believed to extend up to the foothills of the himalayas in the north but most of the shield is obscured by a thick sediment cover of the gangetic plains to the north the magnetic field modelling suggests a weakly magnetic crust in the region indicating either the indian shield may not extend up to the base of the himalayas or the lower crust in the region has been modified

geomagnetic secular variation in the indian ocean abstract annual repeat stations in australia and in south africa show that secular variation may change rapidly and over short geographical distances in the indian ocean area satellite models show large secular variations in the center of the indian ocean where there are few island geomagnetic observatories this paper investigates marine geomagnetic measurements to see if they give more information about secular variations in this area between 1960 and 1988 there were more than 70 port to port cruises with ships towing proton precession magnetometers in the indian ocean change in field intensity from one cruise to another provides new information about the secular variation in this part of the world several methods were investigated to determine this change from the ship s data observing the change on closely parallel or crossing tracks provides an estimate of this change these results indicate that there are short time and distance scales of secular variation in the indian ocean which have not been accounted for in geomagnetic field models

global vector and scalar magsat magnetic anomaly maps empirical and analytical techniques for modeling ionospheric fields in magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies this task has been accomplished for equatorial x z and b component and polar z and b component measurements the techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data processing techniques developed during the last decade data processing techniques include retention of common signal in a correlation analysis of adjacent passes analysis of field differences between dawn and dusk data at points where their orbits cross and retention of common signal in a covariant spherical harmonic analysis procedure results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere vector components as well as scalar

scalar magnetic anomaly maps of earth derived from pogo and magsat data a new pogo scalar magnetic anomaly map at 400 km altitude is presented which consists of spherical harmonics of degree 15 60 on the basis of the common features of this map with two new magsat anomaly maps dawn and dusk two scalar magnetic anomaly maps of the earth are presented using two selection criteria with different levels of stringency these selection criteria suppress the noncrustal components of the original maps by different amounts the more stringent selection criteria seek to eliminate as much contamination as possible at the expense of suppressing some anomaly signal this map is represented by spherical harmonics of degree 15 60 the less stringent selection criteria seek to retain as much crustal signal as possible at the expense of also retaining some contaminating fields this map is represented by spherical harmonics of degree 15 65 the resulting two maps are highly correlated with degree correlation coefficients greater than 0 8

widespread late devonian marine anoxia in eastern north america a case study of the kettle point formation black shale southwestern ontario1 the kettle point formation of southwestern ontario consists of intervals of organic rich interlaminated black shale interbedded with organic poor greyish green mudstones and rare red beds separate

observing system simulation experiments in geomagnetic data assimilation 1 observing system simulation experiments osses have been carried out for a geomagnetic data assimilation system the purpose of these experiments is to demonstrate how new algorithms for assimilating geomagnetic observations into a geodynamo model can be evaluated a nature run is carried out to establish a true evolution of the earth s core state from which synthetic surface geomagnetic field observations are created these observations are then assimilated into the geodynamo model using an optimal interpolation oi scheme model error is simulated by using a different rayleigh number in the nature and model runs because the true core state evolution is known completely the assimilation results can be evaluated in terms of any state variable and at any point in the computational domain in this work we focus on the poloidal observed and toroidal unobserved components of the magnetic field throughout the outer core experiments are carried out using observations of different degrees and varying forecast error correlation length scales we also investigate the impact of model error on the assimilation and on the accuracy of geomagnetic forecasts assimilation runs lasting about 90 of the magnetic free decay time show a positive impact on both components of magnetic field deep within the outer core forecasts of the surface magnetic field show much lower error growth indicating that the initial condition used for the forecasts has been substantially improved through assimilation

a comprehensive model of the quiet time near earth magnetic field phase 3 summary the near earth magnetic field is caused by sources in the earth s core ionosphere magnetosphere lithosphere and from coupling currents between the ionosphere and the magnetosphere and between hemispheres traditionally the main field low degree internal field and magnetospheric field have been modelled simultaneously with fields from other sources being modelled separately such a scheme however can introduce spurious features especially when the spatial and temporal scales of the fields overlap a new model designated cm3 comprehensive model phase 3 is the third in a series of efforts to coestimate fields from all of these sources this model has been derived from quiet time magsat and pogo satellite and observatory hourly means measurements for the period 1960 1985 it represents a significant advance in the treatment of the aforementioned field sources over previous attempts and includes an accounting for main field influences on the magnetosphere main field and solar activity influences on the ionosphere seasonal influences on the coupling currents a priori characterization of the influence of the ionosphere and the magnetosphere on earth induced fields and an explicit parametrization and estimation of the lithospheric field the result is a model that describes well the 591 432 data with 16 594 parameters implying a data to parameter ratio of 36 which is larger than several popular field models

synfolding and prefolding magnetizations in the upper devonian catskill formation of eastern pennsylvania implications for the tectonic history of acadia the upper devonian catskill formation was sampled for paleomagnetic study in east central pennsylvania 41on 76ow in one area the dominant component of magnetization se is revealed over a broad spectrum of demagnetization temperatures ranging to at least 660oc a conventional fold test is positive at the 99 confidence level however statistical analysis of dispersion with incremental bedding tilt correction shows a significant peak in the precision parameter after about 3 4 unfolding the magnetization is therefore secondary with a mean direction of d 166 6 o i 1 8 o the

thelodont and other agnathan distributions as tests of lower palaeozoic continental reconstructions abstract syntheses of palaeomagnetic and palaeontological data indicate that the distribution of the siluro devonian agnathans particulary the thelodonts were strongly influenced by palaeolatitude and the availability of direct land connections the great spread of thelodonts in the lower devonian suggests that either this dependence on direct land connections became less important for the turinia fauna which seems unlikely or that the dispersal occurred as a result of temporary land bridge connections the cessation of such bridges at the end of the devonian causing an isolation of the fauna in australasia which later became extinct because of climatic changes caused by migration of the continental blocks into different latitudes or by direct competition such dispersal and extinction patterns appear to be a fore runner and directly analogous to the dispersion and partial extinction of the marsupial fauna in cretaceous tertiary times if such a model is valid then it also places constraints on the life habitats of the agnathans

geochemistry of reduced gas related to serpentinization of the zambales ophiolite philippines abstract methane hydrogen gas seeps with mantle like c and noble gas isotopic characteristics issue from partially serpentinized ultramafic rocks in the zambales ophiolite philippines new measurements of noble gas and 14c isotope abundances rock mixed volatile equilibrium calculations and previous chemical and isotopic data suggest that these reduced gases are products of periodotite hydration the gas seeps are produced in rock dominated zones of serpentinization and similar gases may be ubiquitous in ultramafic terranes undergoing serpentinization

new example of devonian predatory boreholes and the influence of brachiopod spines on predator success three abundant devonian frasnian brachiopod species from the lime creek fm of iowa usa douvillina arcuata devonoproductus walcotti and pseudoatrypa devoniana contain boreholes pooled drilling frequency is 10 6 the boreholes are small usually 1 0 mm diameter cylindrical and drilled perpendicular to the shell surface the borings appear to be type a boreholes as described by ausich and gurrola 1979 j paleontol 53 335 344 based on multiple criteria including statistically significant stereotypy with respect to valve preference site on valve and size class of the prey the boreholes are inferred to be predatory or parasitic in origin for each species the predator preferred to attack the valve closest to the substrate regardless of whether that was the pedicle or brachial valve boreholes were concentrated over the muscle field on each of the three species the success percentage percent of complete holes was statistically lower 2 p 0 04 for the spinose devonoproductus walcotti 33 33 than for the non spinose but otherwise morphologically similar douvillina arcuata 54 22 in spite of the fact that d walcotti has a thinner shell this is the first quantitative evidence for an anti predatory function for brachiopod spines

local time stacking of geomagnetic solar daily variations 1 regular geomagnetic solar daily variations the sq signal is used in electromagnetic induction studies to investigate the electrical conductivity structure of earth s upper mantle traditionally sq induction studies employ observatory magnetic data from a few quiet days in which the overprint on the regular sq ionospheric current vortices from various irregular disturbances is minimal active days are typically discarded a procedure which results in a gappy time series that is difficult to analyze furthermore the quiet day selection is largely arbitrary since daily geomagnetic activity levels range continuously from very quiet to very active this fact precludes the establishment of rigorous quiet active thresholds the average local time variation on active days resembles typical quiet day variations except for a multiplicative scaling factor a multiyear stack of yearlong geomagnetic time series aligned with respect to local time incorporates regular active days while greatly attenuating the effects of irregular geomagnetic disturbances external and internal current systems that rise and fall in step with the sun s transit across the sky above an observatory contribute coherently to the local time stack all other current systems are incoherent and tend to stack out an exact 1 year long stack preserves the seasonal variation the resulting stacked time series contains no gaps is dominated by regular solar daily variations and can be used for improved sq induction studies and comprehensive geomagnetic field modeling hourly geomagnetic values from niemegk boulder mbour and meanook observatories between 1970 and 2002 along with the ap index are used in this study

the near earth magnetic field from magnetospheric and quiet day ionospheric sources and how it is modeled abstract early attempts to model fields from sources in the magnetosphere during magnetically quiet times were unsuccessful this is attributed to the presence of fields from lithospheric sources and to deficiencies in the data distribution the availability of data from the magsat satellite allowed definitive determination of the first degree field from magnetospheric sources temporal variation of those fields on time scales of hours to a few years was implemented by use of a proxy function thought to mimic that variation the dst index was chosen as the proxy function extension of determination of the first degree magnetospheric field to earlier times namely to 1900 is accomplished first by use of the annual average of the aa index as a proxy function and then by use of cubic b splines as temporal basis functions the model first degree field is well correlated with the annual sunspot number both in its general level changes and in its detailed year to year variation the correlation coefficient of the year to year variation of the first degree zero order external term with the annual sunspot number is 0 77 when the external term lags the sunspot number by 2 years another extension of the results made possible by the magsat data is simultaneous determination of a model of the fields from the magnetosphere the ionosphere and internal to the earth in this case data from the magsat and pogo satellites and magnetic observatory hourly mean and annual mean data were analyzed together though still in an experimental stage these models are shown to fit all the data closely and to be able to readily isolate the fields according as the source is in the magnetosphere ionosphere or earth

the missing dimension in magsat and pogo anomaly studies the development of a comprehensive model of the near earth magnetic field in terms of sources in the core ionosphere and magnetosphere has allowed the isolation of heretofore unknown north south trending anomalies of lithospheric origin previously along track filtering of satellite magnetic field data had been necessary in order to remove unmodeled magnetospheric and ionospheric magnetic fields these fields change between adjacent passes by tens of nanoteslas and have wavelengths which overlap the crustal spectrum an example from australia illustrates the additional information that can be extracted from the magsat data set when this missing dimension is included a north south trending low covers most of eastern australia in agreement with the aeromagnetic map of australia this feature is seen in both the dawn and dusk subsets collected from magsat the center of this low is roughly coincident with the eromanga basin but extends for almost 2000 km in a n s direction the low corresponds to a region of almost featureless magnetic field on the aeromagnetic map it is also consistent with an elevated geotherm previously proposed from studies of xenoliths in southeastern australia filtering the new map in a north south direction reproduces the previous best estimate of the australian satellite anomaly field we expect that this comprehensive model will provide a new window into the magnetic field of the earth s lithosphere the model continues to be enhanced

cm5 a pre swarm comprehensive geomagnetic field model derived from over 12 yr of champ orsted sac c and observatory data a comprehensive magnetic field model named cm5 has been derived from champ rsted and sac c satellite and observatory hourly means data from 2000 august to 2013 january using the swarm level 2 comprehensive inversion ci algorithm swarm is a recently launched constellation of three satellites to map the earth s magnetic field the ci technique includes several interesting features such as the bias mitigation scheme known as selective infinite variance weighting sivw a new treatment for attitude error in satellite vector measurements and the inclusion of 3 d conductivity for ionospheric induction sivw has allowed for a much improved lithospheric field recovery over cm4 by exploiting champ along track difference data yielding resolution levels up to spherical harmonic degree 107 and has allowed for the successful extraction of the oceanic m2 tidal magnetic field from quiet nightside data the 3 d induction now captures anomalous solar quiet features in coastal observatory daily records cm5 provides a satisfactory continuous description of the major magnetic fields in the near earth region over this time span and its lithospheric ionospheric and oceanic m2 tidal constituents may be used as validation tools for future swarm level 2 products coming from the ci algorithm and other dedicated product algorithms

extracting ocean generated tidal magnetic signals from swarm data through satellite gradiometry ocean generated magnetic field models of the principal lunar m2 and the larger lunar elliptic n2 semidiurnal tidal constituents were estimated through a comprehensive inversion of the first 20 5 months of magnetic measurements from european space agency s esa swarm satellite constellation mission while the constellation provides important north south along track gradiometry information it is the unique low spacecraft pair that allows for east west cross track gradiometry this latter type is crucial in delivering an m2 estimate of similar quality with that derived from over 10 years of champ satellite data but over a shorter interval at higher altitude and during more magnetically disturbed conditions recovered n2 contains nonoceanic signal but is highly correlated with theoretical models in regions of maximum oceanic amplitude thus satellite magnetic gradiometry may eventually enable the monitoring of ocean electrodynamic properties at temporal resolutions of 1 to 2 years which may have important implications for the inference of ocean temperature and salinity

can the thermal thickness of the continental lithosphere be estimated from magsat data abstract the long wavelength magnetic anomaly field derived from magsat data is modelled by large scale lateral magnetization variations in the continental lithosphere of europe central and southeast asia a simple model with six domains with uniform magnetization is shown to be consistent with this long wavelength anomaly field in this model all major alpine mountain belts are associated with magnetization lows and proterozoic cratons with magnetization highs in particular a significant contrast of magnetization is associated with the tornquist tesseyre line the structural boundary between the paleozoic western part of europe and the proterozoic eastern domain it is thus proposed that the magnetization contrasts and their associated long wavelength magnetic anomalies may be attributed to differences of tectono thermal age of the lithosphere the main magnetization contrasts displayed by the model are then shown to be associated with seismic velocity contrasts this observation supports the hypothesis that magnetization variations are generated by magnetic sources distributed over the whole upper lithosphere down to the depth of the 600 c isotherm to support this conclusion it is shown that the magnetic thickness predicted by the magnetization model obtained is in good agreement with the depth of the 600 c deduced from heat flow measurements

simulations of magnetic fields generated by the antarctic circumpolar current at satellite altitude can geomagnetic measurements be used to monitor the flow 1 with a volume transport of 134 106 m3 s at the drake passage the antarctic circumpolar current acc is the strongest ocean current in the interest of estimating the secondary magnetic fields generated by the magnetohydrodynamic interaction of this flow with earth s main field we compare numerical results for the magnetic fields obtained using flow from three different ocean general circulation models these simulations all expect detectable ocean signals in the magnetic records at ground and satellite altitude 400 km the variability of this contribution is highly correlated with the acc transport a very important variable for climate studies observed magnetic fields could then be used in principle to derive an index of variability of the acc however given its small amplitude compared with other magnetic contributions extracting the ocean s signal from observations remains a challenge at this time

global magnetic anomaly maps derived from pogo spacecraft data abstract magnetic anomaly maps which are reduced to common elevation and to the pole are derived using data from the pogo spacecraft the maps are global and are presented in both equatorial and polar projections reduction is accomplished using the equivalent source technique including principal components analysis to deal with instabilities at and near the geomagnetic equator data reduction methods are discussed and average anomaly maps are presented together with their standard error such maps are derived for four distinct local time regions and intercompared in order to estimate the remaining effects of fields from ionospheric and magnetospheric sources

observatory crustal magnetic biases during magsat and orsted satellite missions 1 computing main field models derived from magsat and orsted satellite magnetic measurements the crustal influence on satellite data can be treated as random noise satellite data can thus be used in conjunction with magnetic observatory measurements to isolate the non core field at the observatory locations crustal biases for the horizontal northward x eastward y and vertical downward z components are computed for all magnetic observatories where data are available for magsat 1979 1980 and orsted 1999 2000 epochs to study their correlation over twenty years for a set of observatories installed after 1979 new crustal biases are computed

spherical harmonic analysis of geomagnetic tides 1964 1965 the mathematical forms chosen for analyses of s q and l by seventeen different authors are reviewed on the basis of the review and on consideration of the ionospheric dynamo theory and the hough function structure of wind velocities in the upper atmosphere a mathematical model is chosen that includes two more terms than the recent analysis by malin 1973 hourly mean values from 130 magnetic observatories for the i q s ys international quiet solar years 1964 and 1965 were prepared in machine readable form and analysed the solar and lunar transient magnetic variations were evaluated together with the lunar elliptic magnetic tide and the seasonal change at one and two cycles per year for all magnetic tides spherical harmonic analyses were made of the phase law tides and the smaller partial tides altogether 94 different spherical harmonic analyses were completed for a total of ten different magnetic tides two of the ten tides are the solar and lunar magnetic tides usually denoted s and l respectively the results are tabulated in a form that distinguishes between eastward and westward moving terms which is suitable for the evaluation of phase angle differences and amplitude ratios between internal and external parts malin s ocean dynamo calculation has been applied to the lunar and lunar elliptic magnetic tides the spherical harmonic coefficients are compared in some detail with those of malin 1973 for years of high sunspot activity the magnetic tidal potential associated with a given atmospheric tidal mode is derived theoretically and used to obtain an estimate of the hough function components of the solar and lunar semi diurnal atmospheric tides at ionospheric levels from the corresponding magnetic tidal potential

electrical conductivity in the earth s mantle inferred from champ satellite measurements i data processing and 1 d inversion summary a novel time domain approach to the global electromagnetic induction problem is applied to vector magnetometer data observed by the champ satellite data recorded during 11 storm events in 2001 2003 are processed track by track yielding time series of spherical harmonic coefficients the data are then interpreted in terms of 1 d layered electrical conductivity models the inversion is performed by full search of model parametric space which yields sensitivity of misfit with respect to conductivities of layers and positions of interfaces in the upper 50 km the inversion solidly recovers a conductive layer corresponding to averaged surface conductance the conductivity of the lower mantle is established at 6 s m 1 assuming the upper lower mantle interface is fixed at the seismic based 670 km boundary however the satellite data favour the models with a large jump around 1000 km to unrealistic conductivity values exceeding 10 3 sm 1 the resolution of the method in the resistive upper mantle sandwiched between conductive crust and lower mantle is poor nevertheless an upper bound of 0 01 s m 1 is suggested by the data a conductivity increase in the transition zone is not

weak influences of the earth s magnetic field on ocean circulation 1 in this work models for the earth s magnetic field and global ocean circulation are combined to produce the first quantitative estimates of the electromagnetic lorentz forces which couple abyssal ocean layers with the energetic surface layers under present conditions these coupling forces are very weak typical time scales for the transfer of momentum to the lower layers are about three thousand years but they are worth describing quantitatively because of the following 1 the coupling rapidly increases in proportion with the square of the earth s magnetic field strength 2 this process is intrinsically efficient at accelerating slow or stagnant abyssal ocean layers 3 these time scales while long are similar to those of rapid climate variability which are expected to be associated with past variability in ocean circulation overturning and heat transport

late middle ordovician environmental change and extinction harbinger of the late ordovician or continuation of cambrian patterns positive excursions in carbon isotope compositions of carbonate 3 and organic carbon 4 6 from the late middle ordovician middle caradocian of the midcontinent and the eastern united states indicate widespread increases in productivity and rates of organic carbon burial that may have drawn down atmospheric p co 2 precipitating global cooling although not necessarily ice sheet formation these climatic changes were associated with regional orogenic uplift a relative rise in sea level changes in epeiric sea circulation patterns and carbonate platform destruction that led to regional extinction of marine benthos the combination of sea level rise changing ocean circulation and extinction in the middle caradocian is similar to the suite of environmental changes described for cambrian biomere boundaries suggesting shared causes for these events in contrast middle caradocian environmental changes are markedly different from the environmental patterns associated with the late ordovician mass extinction despite the evidence for long term cooling from the middle to the late ordovician

models for circulation patterns in epicontinental seas applied to paleozoic facies of north america craton circulation in epicontinental seas develops in response to thermohaline wind driven and tidal motions in various climatic geographic and bathymetric settings mass balance relations in a seaway are established by the relative rates of outflow inflow freshwater and ocean water and evaporation which constrain two contrasting styles of epicontinental circulation quasietuarine circulation qec is established where rates of freshwater influx exceeds evaporation surface outflow from a seaway with qec typically is balanced by inflow of oceanic water at depth a density stratification can be established below the limits of vertical mixing a bottom layer of oxygen poor or anoxic water may develop in a seaway with or without an entrance sill if the bottom waters are derived from the oxygen minimum zone of the open ocean or if organic productivity exceeds the limits of aerobic respiration upwelling may be promoted in specific epicontinental settings with qec antiestuarine circulation aec is developed in arid climates where evaporation exceeds freshwater influx evaporative concentration in the surface layer causes oxygenated water to sink bottom outflow of denser more saline water is balanced by surface inflow from the open ocean a sill or barrier may inhibit bottom circulation in seas with aec permitting development of a basal brine layer stagnation in the brine layer may lead to oxygen depletion seas with aec commonly are characterized by extensive carbonate evaporite facies geographic and stratigraphic syntheses of lithofacies and biofacies provide the empirical basis for interpretations of seaway dynamics in ancient epicontinental settings examples from the upper ordovician maquoketa formation and upper devonian lime creek sweetland creek formations of cratonic north america are interpreted in the context of stratified qec patterns in each case oxygen poor to anoxic waters appear to have impinged along the basin slope below the margin of an oxygenated carbonate shelf upwelling along the shelf margin in the maquoketa example helps explain the distribution of phosphatic sediments a stratified aec pattern is interpreted for the gower formation silurian of eastern iowa a barrier to bottom circulation at the mouth of an embayment in the epicontinental sea resulted in a basal layer of hypersaline water devoid of shelly benthos surface water salinities increased progresively shoreward within the embayment but benthic organisms flourished where carbonate mounds built upward into the oxygenated layer

the global continent ocean magnetization contrast abstract a schematic global model for the distribution of magnetization in the lithosphere is devised this model is characterized by a uniform stepwise change of radially integrated susceptibility at continent ocean boundaries and an axial dipolar inducing field a spherical harmonic expansion of the magnetic field produced by this source distribution is computed then the standard method of truncation at degree n 13 is used to determine the respective contribution of this field to the main field and to the lithospheric anomaly field the latter is shown to display significant anomalies near continental margins as well as within continents and oceanic basins the energy density spectrum of this field is shown to be very similar to the spectrum of the magsat derived lithospheric anomaly field furthermore several features of this field are shown to account for unexplained anomalies of the magsat field this supports the hypothesis that such an abrupt magnetization variation may occur at several continental margins and contribute to the signal in the magsat derived long wavelength anomaly maps this hypothesis provides an order of magnitude estimate for the magnetization difference between oceanic and continental lithospheres finally it is argued that the synthetic anomaly field produced by this model provides an upper bound to the energy density spectrum of the lithospheric field at low harmonic degrees and concluded that the lithospheric field does not contribute significantly to the earth s field below degree 13

magpysv a python package for processing and denoising geomagnetic observatory data magpysv a python package for processing and denoising geomagnetic observatory data measurements obtained at ground based geomagnetic observatories are crucial to our understanding of secular variation sv of the geomagnetic field and permit investigations of earth s deep interior observatory monthly means are widely used for this purpose but are highly sensitive to measurement errors due to their high temporal resolution and also suffer from significant external magnetic field contamination these noise sources often obscure fine scale details required for studying rapid observed features such as geomagnetic jerks many current processing methods rely on piecemeal closed source codes or are performed by hand on an ad hoc basis this requires much time effort and detailed knowledge of observatory data processing and hampers efforts to reproduce the datasets underlying many published results the aim of this work is to provide a consistent means of generating high resolution sv time series with baseline jumps outliers and external contamination removed we present magpysv a python package designed to process and denoise observatory hourly means distributed by the world data centre for geomagnetism at the british geological survey edinburgh this package allows the user to obtain the dataset in wdc format from bgs servers and keep it up to date by replacing preliminary data with the most definitive version available it produces time series of the x y and z components of the field and sv at the desired frequency typically monthly means and applies corrections for all documented baseline jumps optionally the user may exclude data using the ap index which removes effects from documented geomagnetic storms robust statistics are used to identify and remove outliers the software extends the denoising methods of wardinski holme 2011 gji and brown et al 2013 pepi which use the covariance matrix of the residual between the observed sv and that predicted by a global field model to create and remove a proxy for external field signal from the data this extension creates a single covariance matrix for all observatories of interest combined and applies the external field correction to all locations simultaneously finally we present a denoised sequence of data produced by magpysv and discuss its application to geomagnetic jerks

observing geomagnetic induction in magnetic satellite measurements and associated implications for mantle conductivity 1 currents induced in earth by temporal variations in the external magnetic field have long been used to probe mantle electrical conductivity but almost exclusively from sparsely distributed land observatories satellite borne magnetometers such as flown on magsat orsted and champ offer the prospect of improved spatial coverage the approach we have taken is to isolate induction by harmonic dst disturbance storm time excitation of the magnetospheric ring current in satellite magnetic measurements this is done by removing the magnetic contributions of the main core magnetic field the crustal magnetic field and ionospheric fields cause of the daily variation using sabaka et al s 2000 2002 cmp3 comprehensive model the dst signal is then clearly evident in the midlatitude satellite passes lower than 50 degrees geomagnetic latitude at higher latitudes auroral and field aligned currents contaminate the data we fit the internal and external components of the dst signal for each equatorial pass exploiting the fact that the geometry for the internal and external components is different for the azimuthal and radial vector components the resulting timeseries of internal and external field variations shows that the dst signals for the dawn passes are half those of the dusk passes the sum of equatorial external and internal components of the field averaged over dawn and dusk passes provides an excellent estimate for the dst index and may in fact be superior when used as a proxy for the purposes of removing induced and magnetospheric fields from satellite magnetic data we call this estimate satellite dst cross spectral analysis of the internal and external timeseries shows both greater power and higher coherence in the dusk data we processed the transfer function between internal and external dusk timeseries to provide globally averaged frequency dependent impedances that agree well with independently derived estimates we estimate earth s radial electrical conductivity structure from these impedances using standard regularized inversion techniques a near surface conductor is required of thickness less than 10 km with a conductivity thickness product almost exactly that of an average earth ocean inversions suggest that an increase in conductivity at 440 km depth predicted by recent laboratory measurements on high pressure phases of olivine is not favored by the data although as in previous studies the 670 km discontinuity between the upper and lower mantle is associated with a two orders of magnitude jump in conductivity a new feature in our inversions is a further increase in lower mantle conductivity at a depth of 1300 km a global map of the internal induced component of the magnetic field provides a qualitative estimate of three dimensional 3 d variations in earth electrical conductivity demonstrating graphically that the satellite data are responsive to lateral variations in electrical conductivity caused by the continents and oceans

palaeozoic oolitic ironstones on the north american craton abstract more than 40 mostly minor palaeozoic oolitic ironstones oi accumulated on low latitude cratonic north america almost entirely in usa a few middle cambrian oi among the oldest anywhere were deposited on the western cratonic shelf of usa widely scattered late cambrian ones developed on the southwestern southeastern and northeastern flanks of the transcontinental arch middle ordovician oi were deposited on the cratonic interior and on the southern and southeastern cratonic margins in latest ordovician time neda oi spread across north central usa east of the arch and south of the laurentian upland minor ones developed in the southern part of the taconian foreland basin after major orogeny early and middle silurian clinton oi flourished throughout the foreland basin in eastern usa producing the largest oi deposit on the north american craton minor early late silurian oi accumulated in the central part of the basin middle and late devonian oi in the acadian foreland basin in northeastern cratonic usa developed progressively westward of the encroaching catskill delta an isolated middle devonian one accumulated in southwestern cratonic usa and latest devonian ones in north central usa east of the arch during this palaeozoic episode sites of oi deposition shifted eastward across the craton most of the oi were deposited during a hiatus in normal sedimentation accumulating in the upper part of shoaling upward sequences some early palaeozoic ones occur in glauconitic siliciclastic carbonate facies and contain mostly spherical hematitic ooids these suggest derivation from iron rich soils developed on glauconitic deposits oi in wholly siliciclastic facies containing distorted chamositic ooids with cores of mud peloids were common in later palaeozoic time these oi like many other phanerozoic ones suggest a synsedimentary to early diagenetic origin

sea level forcing of carbon isotope excursions in epeiric seas implications for chemostratigraphy five new 13c excursions are reported from the upper ordovician galena group of iowa the 13ccarb and 13corg trends are compared to sea level trends based on conventional sedimentological and novel chemostratigraphic end methods it is found that rising 13c values occur during rising sea level peak magnitude 13c values are coincident with inferred peak flooding and declining 13c values occur with declining sea levels the relationship to sea level is explained by a simple model of episodic flooding of the galena platform by cool oxygen depleted nutrient rich bottom waters originating from the sebree trough which was itself connected to the nearby iapetus ocean flooding of the inner galena shelf during sea level transgressions caused local changes in the carbon isotope balance of inner shelf waters due to 1 locally increased primary productivity and organic carbon burial 2 locally increased exchange of 13c enriched dissolved inorganic carbon dic with surrounding epeiric sea water masse

small scale features in the earth s magnetic field observed by magsat a spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of magsat data global maps representing the contribution due to terms of the expansion above n 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar even in polar regions an expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high order field structure at a constant altitude than do component maps derived independently the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 008 2 50 payment must accompany order

new global vector magnetic anomaly maps derived from magsat data new global maps of the vector magnetic anomaly field of earth are derived from magsat data improvement on earlier published vector and scalar maps is achieved with the following procedure 1 the noise level of vector measurements associated with the discontinuities of the in flight sensor attitude determination is reduced by selecting the data 2 a monthly model of the main field is computed to account for secular variation using only quiet day data 3 the time dependence of the ring current effect is taken into account using hourly values of the dst index based on ground obseivations 4 sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity these indices provide a better discrimination of quiet data than the global kp index usually used for that purpose 5 a time and longitude dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes anomaly maps of x and z components are obtained using selected measurements within the reduced altitude range 350 425 km these maps are compared with earlier magsat anomaly maps and their respective resolutions are discussed these maps evidence large anomalies of lithospheric origin within the oceanic domain in particular the z map displays a characteristic pattern of long wavelength anomalies in these areas it is first shown that some of these anomalies particularly in the north atlantic and indian oceans may be correlated with the age of the lithosphere then a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced this suggests that long wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor topography

new evidence of a meridional current system in the equatorial ionosphere an interesting variation in the geomagnetic d component has been found by analysis of the magsat data characteristics of this variation are that it appears everyday on the low latitude duskside and is antisymmetric about the dip equator negative in the northern hemisphere that the magnitude from 5 to 25 nt depends upon the geographic longitude and altitude and that no corresponding change can be seen on the ground magnetic data the change suggests a meridional current system in the equatorial ionosphere produced in association with the electrojet current along the dip equator this effect should be taken into account in the study of geomagnetic anomaly of internal origin with the magsat data

a study of spectral methods of estimating the depth to the bottom of magnetic sources from near surface magnetic anomaly data summary based on a critical evaluation of several different spectral magnetic depth determination techniques on areally large synthetic layered and random magnetization models we recommend the following considerations in the usage of the methods as necessary prerequisites to successful bottom depth determinations 1 using windows with sufficient width to ascertain that the response of the deepest magnetic layer is captured and by verifying the spectra and computing the depth estimates with the largest possible windows 300 500 km 2 avoiding filtering to remove arbitrary regional fields accomplished by compiling magnetic anomalies derived from modern spherical harmonic degree 13 earth s main field models e g recent international geomagnetic reference field models igrf or comprehensive models cm 3 ascertaining the near circularity of the autocorrelation function to avoid analysing biased spectra containing strong anomaly trends and 4 avoid determining the slopes from the exponential low wavenumber part of the spectra in the cases of layered magnetization we also describe the details of the new spectral peak forward modelling method and discuss the conditions under which the method can lead to useful results we found that despite all these precautions in some cases the results can still be erroneous and therefore we recommend a critical evaluation of the results by modelling heat flow and taking into account seismic information on the crustal and lithospheric thicknesses and seismic velocities wherever possible in the southcentral us east of the rockies where the surface heat flow ranges between 40 and 65 mw m 2 w e obtained the magnetic bottom depth of 40 10 km using the approach of the forward modelling of the spectral peak this range is similar to the seismically derived crustal thickness of 45 50 km suggesting therefore that the entire crust may be magnetic in this region because of the uncertainties in the various heat flow contributing parameters such as the variations in thermal conductivity radiogenic heat and hydraulic regime we could not constrain the lithospheric thickness beyond an estimate ranging approximately from 100 to 200 km

the electrical conductivity of the mantle beneath europe derived from c responses from 3 to 720 hr summary the c response which connects the magnetic vertical component and the horizontal gradient of the horizontal components of electromagnetic fluctuations forms the basis for estimating the conductivity depth profile of the earth this paper describes new estimates of the c response obtained from observatory hourly mean values the z y method is applied which means that the vertical component is used locally whereas the horizontal components are used globally by expansion into terms of spherical harmonics a special evort is made to obtain unbiased estimates of c when applied to 90 months of hourly mean values from about 90 observatories the method yields consistent results for european observatories in the entire period range from 3 to 720 hr and for two diverent source mechanisms s and d st a good description of the source structure for individual time segments is essential this was achieved by a separate spherical harmonic analysis of the data for each month for d st or each day for s and by estimating a large number 120 of expansion coeycients the results are interpreted by means of 1 d conductivity models which show that the upper mantle has remarkably little structure with a monotonic decrease of resistivity from 100 v m near z 200 km to 0 7 v m below z 1000 km

local controls on carbon cycling in the ordovician midcontinent region of north america with implications for carbon isotope secular curves abstract the carbon isotope record from ancient epicontinental seas may contain much more of a local scale carbon cycling signal than is generally appreciated a unique opportunity exists to examine this issue in the case of the late ordovician mohawkian sea of eastern laurentia where the millbrig k bentonite stratigraphic framework has been used to delineate a time slice at 454 ma extending over 1 500 000 km2 of the eastern united states across the time slice carbonate and organic carbon 13c vary by 4 5 and 7 5 respectively a spatial variation that is as large as temporal secular changes in epeiric sea 13c that have been reported in the past these new data are considered in the context of geographic variations in lithological biological and other geochemical sediment characteristics collectively these sediment properties distinguish regions of the mohawkian sea which likely differed in terms of the nature and relative importance of carbon cycling processes water column depth and structure and barriers to free exchange of water across the mohawkian sea may have been overarching factors in the development of these regions raising the possibility that changes in circulation patterns such as those caused by sea level change played a role in driving secular carbon isotope excursions by changing the rate of exchange of dissolved inorganic carbon between water masses if the observed effects of local carbon cycling on the distribution of mohawkian sea 13c were commonplace in ancient epicontinental marine environments it would imply that local scale carbon cycling may have left a nontrivial imprint on epeiric sea records of secular variations in 13c in addition to the imprint left by changes in the global carbon cycle this may have contributed to the broad scatter in 13c values observed in the paleozoic portion of the global carbon isotope secular curve

statistical treatment of crustal magnetization summary we present a general statistical theory for relating magnetic fields to the magnetization of their source region when the geometry of the source is spherical and the magnetization is a realization of a stationary isotropic random process for the case of gaussian statistics considered here the second order statistics are sufficient to determine the process uniquely observed high degree coefficients from a spherical harmonic expansion of the field appear to be consistent with this model we find simple models of crustal magnetization that are compatible with observational constraints namely the values of the magnetic power spectra believed to be derived from the crust and the total crustal power any statistical model of the crust can be used to pre whiten observations prior to modelling the core field in a way described previously by jackson

variation of spherical harmonic power as a function of harmonic order for earth s core and crustal magnetic field and for mars crustal field 1 the variation of power of the spherical harmonics of earth s magnetic field as a function of degree is well known at earth s surface the power decreases as a function of degree until the crustal component dominates at the core surface the spectrum is flat apart from the dipole term and each degree contributes about equally to the power however as a function of order the power tends to decrease for high harmonic orders these represent sources at low geographic latitudes and so a field in which high order power is low represents one in which low latitude sources are weaker than high latitude sources thus the explanation for the reduction in power for high order harmonics in earth s core field is that in the core low latitude sources of the nondipole field are less intense than high latitude sources earth s crustal field is represented by harmonic degrees greater than 15 and also shows the power falling off as a function of order implying that there is an increase in the intensity of magnetic anomalies from equator to poles this can be explained if the magnetizations have been produced by a dipole field aligned along the spin axis because such a field is twice as strong at the poles as at the equator for the martian field there is no central dipole source but there are strong crustal sources that were created when mars had a planetary dynamo and presumably an axial dipolar field martian harmonics show the same pattern of variation as a function of order as those on earth one explanation for this is that these crustal sources were magnetized with a dipolar field aligned along the present spin axis implying that there has not been much martian polar wander since the crust was magnetized components 8239 words 15 figures 1 table

a time dependent model of the earth s magnetic field and its secular variation for the period 1980 2000 1 this study presents an investigation and description of the secular variation of the earth s magnetic field between 1980 and 2000 a time dependent model c3fm continuous covariant constrained endpoints field model of the main field and its secular variation between 1980 and 2000 is developed with gauss coefficients expanded in time on a basis of cubic b splines this model is constrained to fit field models from high quality vector measurements of magsat in 1980 and orsted in 2000 and to fit both magnetic observatory and repeat station secular variation estimates for the period in between these secular variation estimates first time derivatives are derived from observatories monthly or annual means and repeat station data in order to reduce the contributions of crustal noise annual and semiannual variation on average the model input consists secular variation estimates of the x y and z components at 130 locations per month treatment of covariance between the different components allows a higher temporal sensitivity of the model due to the exclusion of some external field variation the model is computed up to degree and order 15 the model is a useful extension of the hitherto existing time dependent description of the secular variation gufm which describes the secular variation until 1990 it reveals a short term secular variation on subdecadal timescale and has a higher spatial resolution than previously resolved the model is also valuable to test the frozen flux hypothesis and to link features of the radial field at the core mantle boundary to the geodynamo

scarcity of the c30 sterane biomarker 24 n propylcholestane in lower paleozoic marine paleoenvironments abstract 24 n propylcholestane 24 npc a c30 sterane compound derived from sterol precursors which are the major sterol constituents of modern pelagophyte microalgae occurs in certain neoproterozoic rocks and oils and throughout the phanerozoic rock record this broad distribution leads 24 npc to be widely considered a reliable indicator of open to partially restricted marine depositional conditions for source rocks and oils here we report two significant hiatuses in the occurrences of 24 npc in the lower paleozoic marine rock record the first in the middle late cambrian and the second in the late ordovician early silurian transition for a range of lithofacies carbonates and siliciclastic rocks organic carbon contents both organic lean and organic rich and paleoceanographic environments shelf and deeper water marine settings and observed offshore of two paleocontinents laurentia and baltica the ordovician silurian gap is at least 9 million years and possibly up to 20 million years in duration robust older occurrences of 24 npc steranes in some neoproterozoic rocks and oils suggest that oceanographic conditions in our intervals of lower paleozoic time were unfavorable for the proliferation of pelagophyte algae as phytoplankton caution should therefore be applied when interpreting a lacustrine versus marine depositional environmental setting for source rocks and oils in these intervals of early paleozoic time using lipid biomarker assemblages

high resolution carbon isotope δ13ccarb chemostratigraphy of the lower permian from the u s midcontinent checking the pulse of the late paleozoic ice age high resolution stable isotope chemostratigraphy 13 c carb and 18 o from the asselian and sakmarian stages cisuralian series permian system was carried out on the newby 2 28r core from stevens county sw kansas the cyclothemic strata of the council grove and chase groups sampled in this study record the rapid rise and fall of sea level due to the expansion and contraction of glacial mass during the early permian in the waning phases of the late paleozoic ice age lithologies interpreted to be deposited during higher sea level stand consistently show elevated 13 c carb values when compared with shallower water parts of each cycle and show as much as a 6 0 shift in 13 c carb within single cycles the 13 c carb values recorded here mirror changes in sea level throughout the sampled interval indicating a close correspondence between at least the local carbon cycle of the u s midcontinent and anadarko basin with global ice sheet dynamics and eustatic sea level as has been seen in previous studies however this basin contains generally lower absolute values of 13 c carb compared to other basins such as those in russia moscow basin china yangtze platform and nevada great basin the data presented here represent the first high resolution analysis of carbon cycle dynamics during the early permian and it remains to be seen if the heartbeat of the late paleozoic ice age preserved in this region is recognizable globally

stratigraphic completeness of carbonate dominated records from continental interiors versus continental margins stratigraphic thinning occurs via condensation and omission at multiple scales abstract over phanerozoic time scales stratigraphic records from continental interiors or cratons are dramatically thinner and are generally assumed to be relatively incomplete with more numerous and longer duration hiatuses compared to records from more rapidly subsiding continental margins however this assumption need not be true for the shorter time scales i e several 106 years and shorter over which accumulation of the preserved stratigraphic record on the continental interior actually takes place this study compares carbonate dominated sedimentary records from the devonian continental interior iowa and continental margin nevada to evaluate whether the continental interior record is 1 miniaturized i e thinner but equally complete 2 comparable in thickness and quality where the sedimentary record is preserved but certain portions of the record are notably absent i e omitted or truncated or 3 so invariably preserved that equivalent stratigraphic packages cannot be recognized compared to the continental margin record this study finds that fourth to fifth order cycles in iowa are on average approximately half as thick and half as numerous compared to the continental margin of nevada moreover the stratigraphic record on the continental margin of nevada is dominated by cycles that are composed of tidal flat and shallow subtidal lithofacies capped by simple flooding surfaces and exhibit catch up or keep up depositional styles the continental interior record in iowa is dominated by cycles that are composed of subtidal lithofacies capped by a range of bounding surfaces and exhibit catch down or give up depositional styles these findings suggest that the thin record in iowa is largely a result of low sedimentation rates associated with suppressed carbonate production in this epeiric sea setting rather than minimal subsidence rates and an increased potential for subaerial exposure and associated loss of record

the near earth magnetic field at 1980 determined from magsat data data from the magsat spacecraft for november 1979 through april 1980 and from 91 magnetic observatories for 1978 through 1982 are used to derive a spherical harmonic model of the earth s main magnetic field and its secular variation at epoch 1980 0 the model is called gsfc 12 83 constant coefficients are determined through degree and order 13 and secular variation coefficients through degree and order 10 the first degree external terms and corresponding induced internal terms are given as a function of dst preliminary modeling using separate data sets at dawn and dusk local time showed that the dusk data contain a substantial field contribution from the equatorial electrojet current the final data set was therefore selected first from dawn data and then augmented by dusk data to achieve a good geographic data distribution for each of three time periods 1 november december 1979 2 january february 1980 and 3 march april 1980 a correction for the effects of the equatorial electrojet was applied to the dusk data utilized the solution included calculation of fixed biases or anomalies for the observatory data although similar in many respects gsfc 12 83 differs from international geomagnetic reference field 1980 by 3 6 nt in the g10 term and shows a slightly negative b in the northern polar region as well as other differences in secular variation pattern

fluctuations in epeiric sea carbonate production during silurian positive carbon isotope excursions a review of proposed paleoceanographic models positive excursions in the carbon isotopic ratio of marine carbonates 13ccarb are commonly attributed to an increase in the burial ratio of organic versus carbonate carbon in the global oceans regardless of whether this increased deposition of organic carbon was oceanographically or biologically induced the fact that high carbon isotope values are most often attributed to the sequestration and burial of organic carbon remains the notion that high 13ccarb values should be the result of organic rich deposition in epeiric sea settings is not surprising considering that the overwhelming majority of organic carbon burial in the modern oceans occurs in continental shelf and margin settings what is surprising is that major positive carbon isotope excursions during the silurian are recorded in clean carbonate successions with little to no appreciable black shale deposition in epeiric seas in fact silurian 13ccarb excursions occurred during intervals of prolific reef development and an overall expansion of carbonate platform environments globally this discrepancy has sparked considerable debate among silurian carbon isotope specialists but has offered a valuable opportunity to re examine the intricate connections among nutrients primary productivity carbonate sedimentology and the global carbon cycle in the lower paleozoic here we consider several sedimentological and geochemical features of one of these silurian positive carbon isotope 13ccarb excursions and discuss the broader implications of current silurian 13ccarb models to the interpretation of lower paleozoic carbonate sequences

the tice event perturbation of carbon nitrogen cycles during the mid tournaisian early carboniferous greenhouse icehouse transition abstract carbonate carbon 13ccarb and bulk nitrogen 15n isotopic variation during the mid tournaisian was analyzed in the malanbian and long an sections of south china their c isotope profiles document a large positive excursion herein termed the mid tournaisian carbon isotope excursion tice during the siphonodella isosticha conodont zone although its magnitude differed between the two sections 6 at malanbian versus 3 at long an the tice event coincided with sedimentologic and oxygen isotopic evidence of climatic cooling and glaciation during the mid tournaisian it was probably triggered by an increase in organic carbon burial rates linked to changes in global ocean circulation the study sections also document a large positive shift in 15n from 1 7 to 4 2 at malanbian and from 1 5 to 3 8 at long an the n isotope shift shows no termination within the study sections and is likely to mark the onset of an extended interval of 15n enriched marine nitrate that lasted for the duration of the late paleozoic ice age its initiation coincided with tice and thus may have been linked to ocean circulation changes that resulted in intensified upwelling and an increase in water column denitrification the continuation of the n isotope shift over millions of years may have been linked to glacio eustatic fall and a long term shift in the locus of denitrification from continental shelf sediments to continent margin oxygen minimum zones the tice event thus marks the onset of sustained continental glaciation during the late paleozoic ice age

tracking geomagnetic impulses at the core mantle boundary abstract the main magnetic field of the earth is generated and maintained by convective motions in the fluid outer core temporal field variations occur on an impressive variety of scales ranging from months to millions of years among these the most intriguing features are geomagnetic impulses or jerks v courtillot j ducruix and j l le mouel sur une acceleration recente de la variation seculaire du champ magnetique terrestre acad sci paris c r d287 1978 1095 1098 j l le mouel j ducruix and c ha duyen geophys res lett 5 1983 369 1 2 here we investigate these very rapid events at the surface of the earth s core using a global model designed for use at this depth we identify two regions of very active variations where wave like structures propagate the geometrical characteristics of these wave like motions provide new constraints on the intricate dynamics of the core

continuous global geomagnetic field models for the past 3000 years abstract several global geomagnetic field models exist for recent decades but due to limited data availability models for several centuries to millennia are rare we present a continuous spherical harmonic model for almost 3 millennia from 1000 b c to 1800 a d based on a dataset of directional archaeo and paleomagnetic data and axial dipole constraints the model named continuous archaeomagnetic and lake sediment geomagnetic model for the last 3k years cals3k 1 can be used to predict both the field and secular variation comparisons and tests with synthetic data lead to the conclusion that cals3k 1 gives a good general large scale representation of the geomagnetic field but lacks small scale structure due to the limited resolution of the sparse dataset in future applications the model can be used for comparisons with additional new data for that time span for better resolved regions the agreement of data with cals3k 1 will provide an idea about the general compatibility of the data with the field and secular variation in that region of the world for poorly covered regions and time intervals we hope to iteratively improve the model by comparisons with and inclusion of new data animations and additional snapshot plots of model predictions as well as the model coefficients and a fortran code to evaluate them for any time can be accessed under http www mahi ucsd edu cathy holocene holocene html the whole package is also stored in the earthref digital archive at http www earthref org

genesis of unusual lithologies associated with the late middle devonian taghanic biocrisis in the type taghanic succession of new york state and pennsylvania abstract the middle devonian late middle givetian tully formation in new york state is the type regional expression of the global taghanic biocrisis it is marked by anomalous development of massive to rhythmically bedded micritic limestone deposits within a vastly thicker acadian foreland basin clastic succession lower medial tully carbonates record the incursion of tropical old world realm owr taxa tully fauna and the outage of the long standing higher diversity cooler water endemic eastern americas realm ear biota hamilton fauna succeeding tully strata record a return of the hamilton fauna followed by its demise due both to replacement by the lower diversity cosmopolitan genesee fauna and by the overspread of widespread anoxic bottom conditions coupled with the onset of drastically increased sediment input following tully group deposition massive micritic limestones yielding modest diversity tully fauna assemblages are particularly characteristic of the lower medial tully succession on the western new york tully platform these are coeval with nearly barren ribbon limestone and siltstone deposits in the adjacent new berlin east central new york and central pennsylvania basins these depocenters trapped terrigenous sediment and helped to create sediment starved conditions on the platform anomalous tully carbonate deposition is believed to be coincident with strong super estuarine water mass stratification in offshore tully shelf and basin settings where a warmer denser more saline water mass flowed northward shoreward across the study area beneath a counter flow of oxygenated river influenced surface water dysoxic conditions would have developed below the pycnocline favoring the tully fauna incursion precipitation of carbonate mud and possibly promoting the formation of ooidal chamosite at least five thin 1 30 cm thick mappable beds of black ooidal chamosite are observed in taghanic shelf margin slope and basin deposits these beds are typically intensely bioturbated variably rich in associated siderite contain corroded low diversity fossil assemblages and are regionally associated with condensed lag deposits above unconformities work by others suggests that chamosite formation occurs under minimally reducing post oxic interstitial conditions which are followed in turn by more strongly reducing conditions which allow for variable siderite and pyrite formation terrestrial pedogenic alteration of silicates by evolving land plant communities under warm humid conditions led to an increased supply of kaolinite as well as iron oxides and hydroxides that served as precursors to chamosite formation in the new berlin basin the transgressive top hamilton base taghanic succession is marked by a succession of diastems associated with basin deepening and upward increasing chamosite occurrence in dysoxic facies occurrence of ooidal chamosite in upper medial tully platform deposits smyrna bed may reflect water mass stratification and extreme sediment starvation at the shelf margin eastward spectral transformation of sparsely fossiliferous smyrna ooidal chamosite in the western new berlin basin into richly fossiliferous non chamositic equivalent neritic deposits east of that basin suggests that the counter flow of oxygen and nutrient rich lower salinity water had blocked the denser restricted water layer from advancing over the eastern tully clastic shelf closer to shore

spatiotemporal sampling of sq induced geomagnetic responses at leo satellite altitude for a radially conductive earth summary a new simulation tool is introduced for extracting earth conductivity information from geomagnetic satellites in low earth orbit leo such as champ swarm and potential follow on missions theoretical reconstruction of spherical harmonic spectra of global universal time ut geomagnetic field maps is analysed an idealized regular solar daily variation field is assumed along with its induced counterpart from a radially stratified earth solar quiet sq spectra can be reliably reconstructed from discrete time series of measurements sampled daily within a fixed ut window a single leo satellite should be in orbit for at least 1 yr under quiet time conditions to ensure accurate spectral reconstruction of an sq geomagnetic spectrum the length of the daily sampling window and random day to day variability in the sq source strength have only minor effects on spectral reconstruction the shape of the spectra and spectral ratios are independent of radial mantle conductivity further research employing 3 d forward modelling of induction in a laterally heterogeneous earth with more realistic and complete external source descriptions is advocated the new simulation tool should prove valuable to planners of future multisatellite geomagnetic missions as well as scientists interested in analysing and interpreting satellite induction signals

measurements and uncertainties of the occurrence time of the 1969 1978 1991 and 1999 geomagnetic jerks geomagnetic jerks are rapid time variations of the magnetic field at the earth s surface that are thought to be of primarily internal origin jerks are relevant for studies of the earth interior they likely give information on core dynamics and possibly on mantle electrical conductivity in such studies a precise determination of the jerk occurrence time and its error bar at each observatory is required we analyze the most well known global jerks 1969 1978 and 1991 and a possible local jerk in 1999 considering all three components of the magnetic field x y and z different data sets are investigated annual means 12 month running averages of observatory monthly means in rotated geomagnetic dipole coordinates and data representing the core field contribution synthesized from the cm4 time dependent field model the secular variation in each component of the field around the time of a jerk was modeled by two straight line segments using both least squares and 1 norm methods the 1969 1978 and 1991 jerks were globally detected while the 1999 event was only locally identified using this simple method enables us to calculate error bars in the jerk occurrence times and to quantify their nonsimultaneous behavior we find that our error bars are not in general symmetric about the mean occurrence time and that the mean errors on the x and z components of 1 7 years and 1 5 years are larger than that of 1 1 years on the y component generally the error bars were found to be larger in the southern hemisphere observatories our results are necessary prerequisites for further studies of the inverse problem that attempt to determine mantle electrical conductivity from variations in jerk occurrence times

ordovician metre scale cycles implications for climate and eustatic fluctuations in the central appalachians during a global greenhouse non glacial to glacial transition abstract metre scale shallowing upward cycles in ordovician carbonates of the central appalachian basin record climate and eustatic fluctuations during a transition from early ordovician global greenhouse to late ordovician icehouse conditions peritidal facies and shale abundance suggest a long term trend in this area from semi arid early ordovician to more humid middle to early late ordovician conditions with a return to semi arid conditions during the late ordovician the climatic fluctuations were most likely produced by tectonics uplift and erosion related to taconic orogenesis plate motion of north america and the areal extent of water covering the shelf peritidal cyclic facies indicate that high frequency relative sea level fluctuations were of small amplitude 20 m relative sea level fluctuations that decreased into the later ordovician if these sea level changes are eustatic then the increased amplitude may mark the early initiation of continental glaciation on gondwana in the late middle ordovician followed by waning of ice sheets prior to the latest ordovician glaciation

geomagnetic jerks in the polar regions 1 the occurrence of geomagnetic jerks over the arctic and antarctic regions is here investigated maps of geomagnetic secular acceleration over the polar regions are produced from the cm4 and chaos models and the occurrence of geomagnetic jerks is associated with jumps in secular acceleration the obtained results confirm that in antarctica geomagnetic jerks systematically follow geomagnetic jerks in the arctic region with a time delay from one to three years evidence is found of an abrupt change in secular acceleration in both polar regions around 1985 suggesting that the 1985 local jerk could actually be a worldwide event combining our results with the results previously obtained on the occurrence of a geomagnetic jerk at low mid latitudes around 2003 we support the hypothesis of a global extension of an event occurred at the beginning of the 21st century

a new approach to directly determine the secular variation from magnetic satellite observations 1 observatory monthly means provide an excellent opportunity to study the temporal changes of the geomagnetic field at a given location unfortunately determination of the global pattern of changes using observatory data is hampered by their uneven distribution satellite data provide excellent global coverage but the spacecraft movement makes direct comparisons of satellite and observatory data difficult to investigate short period secular variation in observatory and satellite data we developed an approach to extract satellite monthly means for virtual observatories at 400 km altitude using champ magnetic measurements comparison of these virtual observatory monthly means with the corresponding ground values shows a remarkably well correlated signal at time scales of months to years which is beyond the temporal resolution limit of recent global models here we describe this newly developed approach its validation and discuss how it can be used to understand short period changes of the recent geomagnetic field

source rock potential of evaporitic environment examination of modern saline lakes solar evaporation ponds and lagoons shows that the evaporitic environment can be very productive of organic matter few species survive in the brines but those that do commonly exist in great profusion in a marine evaporitic embayment the flow of surface currents is persistently toward regions of highest salinity so that there is a continual supply and concentration of nutrients prolific growth of phytoplankton may be similar to that in areas of upwelling in modern oceans only carbonates precipitate in the mesosaline part 4 to 12 salinity of such an evaporitic environment and no great dilution of organic matter by clastic or biogenic sediments occurs because stratification of brine may occur and reducing conditions may be a sociated with the bottom waters much of the organic matter produced can be preserved upon maturation the result may be a rich carbonate source rock frequently unrecognized in the geologic column in the middle east mesosaline conditions are known to have occurred many times from the triassic to cretaceous and may be responsible for the vast reserves of petroleum in the area evaporitic conditions may also have played a part in the petroleum productivity of many other areas including the michigan and paradox basins

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

extending comprehensive models of the earth s magnetic field with orsted and champ data summary a new model of the quiet time near earth magnetic field has been derived using a comprehensive approach which includes not only pogo and magsat satellite data but also data from the orsted and champ satellites the resulting model shows great improvement over its predecessors in terms of completeness of sources time span and noise reduction in parameters with its well separated fields and extended time domain of 1960 to mid 2002 the model is able to detect the known sequence of geomagnetic jerks within this frame and gives evidence for an event of interest around 1997 because all sources are coestimated in a comprehensive approach intriguing north south features typically filtered out with other methods are being discovered in the lithospheric representation of the model such as the s atlantic spreading ridge and andean subduction zone lineations in addition this lithospheric field exhibits significantly less noise than previous models as a result of improved data selection the f region currents through which the satellites pass are now treated as lying within meridional planes as opposed to being purely radial results are consistent with those found previously for magsat but an analysis at orsted altitude shows exciting evidence that the meridional currents associated with the equatorial electrojet likely close beneath the satellite besides the model a new analysis technique has been developed to infer the portion of a model parameter state resolved by a particular data subset this has proven very useful in diagnosing the cause of peculiar artefacts in the magsat vector data which seem to suggest the presence of a small misalignment bias in the vector magnetometer

geomagnetic secular variation we analyse over 175000 magnetic observations from an interval spanning 1695 1980 to produce a sequence of maps of the magnetic field at the core mantle boundary we find that even the earlier data enable us to determine reliable maps we produce these maps at approximately 60 year intervals through the eighteenth and nineteenth centuries and at 10 year intervals in the twentieth century this span of maps is long enough to render straightforward the distinction between static and drifting features in the field we observe that some features show no sign whatsoever of drift over the entire 285 year time interval although others drift westwards in particular we observe that the secular variation is very low beneath the pacific ocean but beneath southern africa and the south atlantic ocean we observe rapid secular variation we interpret the morphology of the static field in terms of a simple model of the dynamo and conjecture that interactions between the core and the mantle are an important element of the process as part of the static field we identify four main concentrations of flux two in each hemisphere at high latitudes these features largely account for the earth s axial dipole moment we find unequivocal evidence that magnetic flux has not remained frozen over the time span of our models much of the diffusive behaviour that we identify is associated with the formation of a pair of flux spots a core spot beneath southern africa early in this century nevertheless we are able to construct maps that satisfy a set of necessary conditions for frozen flux and use these maps to construct maps of the core surface fluid flow based on the steady flow hypothesis although we find no strong evidence against the steady flow hypothesis we do find some grounds on which to doubt the validity of the flow maps

the historical development of the magnetic method in exploration the magnetic method perhaps the oldest of geophysical exploration techniques blossomed after the advent of airborne surveys in world war ii with improvements in instrumentation navigation and platform compensation it is now possible to map the entire crustal section at a variety of scales from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale methods of data filtering display and interpretation have also advanced especially with the availability of low cost high performance personal computers and color raster graphics the magnetic method is the primary exploration tool in the search for minerals in other arenas the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications such as locating intrasedimentary faults defining subtle lithologic contacts mapping salt domes in weakly magnetic sediments and better defining targets through 3d inversion these new applications have increased the method s utility in all realms of exploration in the search for minerals oil and gas geothermal resources and groundwater and for a variety of other purposes such as natural hazards assessment mapping impact structures and engineering and environmental studies

the cm4 model prediction of ground variation of the geomagnetic diurnal field away from quiet time we analyse the geomagnetic diurnal variation for days away from quiet time to see how well the comprehensive model cm4 can reasonably predict ground variation of the diurnal field to do this we compared ground observatory hourly means to predictions given by the cm4 model for days away from quiet time our results show that away from quiet time the cm4 model is producing more reasonable predictions than expected despite the lack of active data in the original model dataset however the cm4 model is not doing so well predicting short term features during period of rapid variations especially for the x component of the geomagnetic diurnal variation field when comparing the different modelled diurnal variation field maps of the cm4 model and the observatory data our results show that the model to data fitness increases as we increase the spherical harmonic degrees from our results we could see that the inability of the cm4 model to accurately predict the geomagnetic diurnal field for days away from quiet time during time of rapid variations may be due to the fact that the external field descriptions included in the cm4 model could not sufficiently explain the field contributions for days away from quiet time this is seen in the low coherence agreement and correlation in the comparison and cross correlation coefficient between the x component of the observatory data and the dst index which allows cm4 model to respond to active conditions outside of the original geomagnetic activity remit

band limited global scalar magnetic anomaly map of the earth derived from magsat data the magnetometer satellite magsat data are separated into two distinct sets the dawn and the dusk sets the two sets are analyzed independently because of the dawn dusk asymmetry of the external magnetic field components of the data two scalar magnetic anomaly maps the dawn and the dusk maps are derived using the spherical harmonic expressions of the respective data set the significant correlation of the two maps in the harmonic range of 18 41 indicates the repeatability of the magsat data in these harmonics the shortest repeatable wavelength is about 920 km a band limited global scalar magnetic anomaly map is derived at about 410 km altitude by arithmetic averaging of the dawn and the dusk maps the map is then downward continued to about 10 km altitude the downward continued magnetic anomalies are relatively more localized the main features of the anomalies are as follows 1 continents have relatively stronger magnetic anomalies than oceans especially the younger oceans 2 many circum pacific subduction zones have magnetic signatures and 3 there is no overall correlation between anomalies and major shields

signal from noise in geomagnetic field modelling denoising data for secular variation studies summary we analyse the residuals between a continuous geomagnetic field model of the internal field c3fm and observed secular variation a large part of the residual variations correlate closely with the dst index suggesting an origin from unmodelled external field variations removal of this signal enhances the resolution of fine scale detail in secular variation this is useful in considering the phenomenology of geomagnetic jerks the residual variations between different observatories show even better correlation suggesting the possibility of the construction of a proxy for the dst index notable cross correlation is also seen between the residuals and dst index with a lag of about 55 months

heat flow pattern at the chicxulub impact crater northern yucatan mexico abstract along an east west profile crossing the chicxulub impact structure in northern yucatan mexico curie depths were obtained from statistical spectral analysis of a grid of aeromagnetic data 256 km wide and 600 km long these depths were corrected for flight height and depth to the sea floor to determine the geothermal gradient assuming a temperature of 580 c for the curie temperature heat flow was then calculated from the geothermal gradients using a value of 2 67 w m k for the mean crustal thermal conductivity the results show a conspicuous heat flow high above on the impact basin in this location the heat flow is 80 mw m 2 approximately available offshore estimates of the depth to the crustal magnetic source bases on the northern yucatan platform and onshore heat flow determination on 8 shallow bore holes and in a 1511 m deep one support the existence of this major high heat flow anomaly associated with the impact crater this high heat flow might be related to the impact through 1 an uplift of the crystalline basement rocks in the center of the crater and 2 impact induced radioactive element concentration into the crust below the impact structure higher thermal conductivities at the lower crust might also play a key role available seismological and thermal property data are compatible with these mechanisms