patterns of deposition from experimental turbidity currents with reversing buoyancy turbidity currents are turbulent sediment laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep water abyssal plains because of the natural stratification of the oceans and or fresh water river inputs to the source area the interstitial fluid within which the particles are suspended will often be less dense than the deep water ambient fluid consequently a turbidity current may initially be denser than the ambient sea water and propagate as a ground hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water when this occurs all or part of the turbidity current lofts to form a buoyant sediment laden cloud from which further deposition occurs deposition from such lofting turbidity currents containing a mixture of fine and coarse sediment suspended in light interstitial fluid is explored through analogue laboratory experiments complemented by theoretical analysis using a box and cloud model particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit a range of beds can be deposited by bimodal lofting turbidity currents lofting may encourage the formation of tabular beds with a rapid pinch out rather than the gradually tapering beds more typical of waning turbidity currents lofting may also decouple the fates of the finer and coarser sediment depending on the initial flow composition the coarse fraction can be deposited prior to or during buoyancy reversal while the fine fraction can be swept upwards and away by the lofting cloud an important feature of the results is the non uniqueness of the deposit architecture different initial current compositions can generate deposits with very similar bed profiles and grading characteristics highlighting the difficulty of reconstructing the nature of the parent flow from field data it is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep water massive sands thinly bedded turbidite sequences and linked debrites depending on the parent flow and its subsequent development for example a lofting flow may lead to a well sorted largely ungraded or weakly graded bed if the fines are transported away by the cloud however a poorly sorted largely ungraded region may form if during buoyancy reversal high local concentrations and associated hindered settling effects develop at the base of the cloud

the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

time averaged paleomagnetic field and secular variation predictions from dynamo solutions based on lower mantle seismic tomography abstract we compare three dynamo solutions incorporating laterally varying boundary heat flux with paleomagnetic models and data the boundary condition is defined by the d seismic shear wave velocity and the three solutions have boundary anomalies with different amplitudes the generated fields appear to divide into a stationary boundary locked part and a time varying part with persistent centres of activity both parts contribute to the time average a very long averaging time can be needed for nearly locked solutions but a rough time average that remains within the threshold set by the accuracy of paleomagnetic data is achieved in a few diffusion times the locked part dominates for larger amplitude boundary anomalies in previous work the locked field was shown to have strong similarities with the modern geomagnetic field previous dynamo solutions that were not locked to the boundary show similarities with our solutions with weak boundary forcing the axisymmetric time average has small g 2 0 and larger g 3 0 components and peaks in inclination anomaly in high latitudes associated with the locked field and low latitudes associated with the time average of the time varying fields the non axisymmetric time average displays a striking longitudinal variation in inclination anomaly with a large negative anomaly in the pacific region in agreement with observations none of the dominant geomagnetic coefficients are axisymmetric and g 2 0 negligible in all three models secular variation is concentrated in equatorial latitudes as in some recent paleomagnetic models the locked field agrees with the inclination difference found between hawai i and reunion in agreement with paleomagnetic averages the locked field agrees with the paleomagnetic time average rather better than the fields with less boundary variations we conclude that because the locked field agrees with the modern field as well as some aspects of the long term time average the geomagnetic field spends a considerable time in the present four lobe configuration it is not a coincidence that the present field resembles the time average longitudinal variations are likely to be at least as important as latitudinal variations in the paleomagnetic time average this presents a challenge for dynamo theory since a move to more geophysically realistic parameters would appear to destroy the locked solutions

a model of correlated episodicity in magnetic field reversals climate and mass extinctions correlated periodicities of mass extinctions climate magnetic field reversals and other geological phenomena may best be explained by cycles of activity within the mantle and core we propose that these cycles are due to variations in the thickness of the thermal boundary layer at the base of the mantle as the layer alternates between two phases of activity in the quiescent phase little flow occurs and the layer thickens with time by thermal diffusion the active phase begins when the thickening layer becomes dynamically unstable hot material erupts from the layer causing it to become thinner as the layer thins the rate of energy supply to the geodynamo and hence its reversal frequency increases the hot material rises to the surface where it causes widespread basaltic volcanic eruptions these eruptions release large amounts of co 2 and sulfates which have a pronounced effect on the climate and biota

palaeointensity results for historic lavas from mt etna using microwave demagnetization remagnetization in a modified thellier type experiment summary the bane of conventional thellier type palaeointensity experiments is the thermal alteration of the samples during experimentation however high frequency microwaves can be used to stimulate the magnetic minerals directly thus eliminating the need for the bulk sample to be heated walton et al 1993 the microwave technique has been successfully applied to ceramics shaw et al 1996 1999 and here we present results for historic lavas from mt etna sicily 20 samples were randomly selected from 10 diverent historic flows dating from 1843 to 1983 hysteresis parameters were monitored as a check for alteration during experimentation and minimal alteration was found rock magnetic analysis and af plus microwave demagnetizations were carried out on all samples and microwave intensity analysis was carried out on those samples that were stable to microwave demagnetization 16 in total with one exception all samples gave high quality intensity results two or three subsamples from each sample were investigated and a weighted average calculated to give a single estimate of the field when the intensity values were compared with the sv model based on direct observatory measurements bloxham gubbins 1985 it was found that those samples that contained a high multidomain md component gave intensity values around 20 per cent lower than expected this can be explained by the cooling rate evect dodson mcclelland brown 1980 and or the presence of an md component mcclelland et al 1996 those samples with the lowest numbers of md grains gave field values which within error were the same as those for the model or slightly too high this is compatible with the cooling rate evect

deep water massive sands facies processes and channel geometry in the numidian flysch sicily abstract the sedimentology of the numidian flysch in northern sicily has been little studied in the last twenty years with most works concentrating on the structural complexities of the area rather than the facies characteristics the numidian basin was intensively deformed during its complex tectonic history and the basin deposits have become segmented into different structural units the oligo miocene aged flysch comprises predominantly quartzarenitic sandstones and interbedded mudstones both of turbiditic affinities and most likely derived from a nw african source the sediments are well exposed along the coast and in dry river valleys in northern sicily and provide excellent examples of deep water massive sands i e very thick 4 25 m units of structureless sandstone associated with turbidites and related facies the overall characteristics of the examined sections indicate that the sandstones were deposited within both isolated channels and larger channel complexes that fed across a mud dominated slope the more proximal sections e g ponte finale and pollina have a high proportion of pebble conglomerates shale clast conglomerates and very thick bedded structureless pebbly sandstones deposited by high density turbidity currents debris flows and slumping other sections either more distal or with a more uniform sand rich source e g contrada di romano are dominated by very thick bedded and amalgamated structureless sandstones with extensive zones of water escape structures the massive sands are thought to originate from the gradual aggradation of sediment beneath steady or near steady flows with rapid deposition from final stage modified grain flows forming the water escape features

archeointensity in northeast brazil over the past five centuries abstract this study presents the first archeointensity results from northeast brazil obtained from 14 groups of architectural brick fragments sampled in the city of salvador bahia state 13 s 38 5 w and dated between the middle of the xvith century and the beginning of the xixth century the dating is ascertained by historical documents complemented by archeological constraints yielding in all cases age uncertainties of less than 50 years analyses were carried out using two experimental protocols 1 the zero field in field version of the classical thellier and thellier method as proposed by coe tt zi including partial thermoremanent magnetization ptrm and ptrm tail checks and 2 the triaxe procedure involving continuous high temperature magnetization measurements both trm anisotropy and cooling rate effects were taken into account for the intensity determinations the cooling rate effect was further explored for the tt zi protocol using three increasing slow cooling times 5 h 10 h and 25 h between 450 c and room temperature following archeological constraints the slowest cooling time was retained in our study yielding decreases of the raw intensity values by 4 to 14 for each fragment a mean intensity was computed and retained only when the data obtained from all specimens between 2 and 6 satisfied a coherence test at 5 a total of 57 fragments 183 specimens was considered for the computations of site mean intensity values with derived standard deviations of less than 8 of the corresponding means when separately computed using the two experimental techniques the site mean intensity values always agree to within 5 a good consistency is observed between intensity values of similar or close ages which strengthen their reliability our data principally show a significant and continuous decrease in geomagnetic field intensity in northeast brazil between the first half of the xviith century and the xxth century one result dated to the second half of the xvith century further suggests that the geomagnetic field intensity reached a maximum around 1600 ad this evolution is in good agreement with that expected in the city of salvador from the available global geomagnetic field models however the accuracy of these models appears less well constrained between 1550 ad and 1650 ad

archaeomagnetic results from southern italy and their bearing on geomagnetic secular variation archaeodirectional results from kilns and other baked structures in southern italy are presented they are generally compatible with the much larger data sets from france and bulgaria in particular a summary of all the results associated with the well known eruption of vesuvius that destroyed pompeii n 9 d 355 i 5 8 95 1 5 provides a reliable archaeomagnetic anchor point supporting the french and bulgarian master curves it is extremely well constrained in time and it comprises independent studies carried out in four different countries furthermore it is derived from a diverse set of features agreement amongst which argues strongly against significant perturbations due to magnetic refraction structural disturbance or depositional shallowing in terms of geomagnetic secular variation we interpret the western european archaeomagnetic data summarized here in terms of an open loop caused by westward drift followed by an inclination low spanning the first few centuries ce representing the signal of a static flux pulse that reaches a maximum magnetic moment of a few percent of the earth s main central dipole 2005 elsevier b v all rights reserved

paleosecular variations of the geomagnetic field during the holocene from eastern asia abstract high resolution paleomagnetic secular variation psv records bear great information of dynamics processes of the earth s geomagnetic field and can be further used for inter profile correlation and for dating sediments however effects of changes in the depositional environment on psv records have not been fully determined this study constructed holocene psv records for the gravity piston core zsqd34 obtained from the northern south china sea rock magnetic and transmission electron microscope tem results indicate that single sd and psd domain magnetites are the main carrier of the natural remanent magnetization comparable to the records derived from the freshwater lakes and the modeling results we observed that direction curves from these two environments of contrasting salinity content are rather consistent the direction curves are independent of the constructed salinity however the gradual increasing trend of relative intensity since about 5 kyr might be related to the decreasing sea surface salinity furthermore on the centennial and millennial time scale the relative intensity and salinity show some positive relation suggesting a potential contribution of salinity to the paleomagnetic relative intensity recording processes

formation of beach rock at siesta key florida and its influence on barrier island development seaward dipping strata of carbonate cemented shell debris located along the coast of siesta key on the gulf coast of the florida peninsula have long been interpreted to be beachrock equivalent in age to the pleistocene anastasia formation stage 5e of the east coast of florida detailed examination of thin sections along with radiometric dating and isotopic analyses demonstrates clearly that this is a holocene deposit that is not beachrock but was lithified in a meteoric environment whole rock dates dates from shells only and from cement only demonstrate that these beach deposits were in place by at least 1800 yr bp and might have been there as long ago as 4300 yr bp this means that some type of barrier island was in place at that time previous investigations have depicted siesta key as having a maximum age of 3000 yr with these deposits being located about 2 km landward of the beach deposits this suggests that the beach deposits might have been the site of the original position of siesta key these data also indicate that sea level must have been near its present position at the time that these foreshore beach deposits were deposited sometime between 1800 and 4300 yr ago this scenario indicates that sea level along this coastal reach probably reached its present level at least about 2000 yr ago

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

the effect of thermal core mantle interactions on the palaeomagnetic secular variation we compare a numerical dynamo model with a model of the palaeosecular variation and find that although there is good agreement in the meridional distribution of the secular variation the amplitude of secular variation in the dynamo model is too small increasing the rayleigh number does little to increase the amplitude of the secular variation while it increases the axial dipole component of the field to geophysically unreasonably large values however by introducing lateral variations in heat flux across the core mantle boundary into the dynamo model we are able to match the amplitude of the secular variation in addition we find that the dynamo model then exhibits persistently anomalous behaviour beneath the pacific region

elasto gravitational deformation relative gravity changes and earth dynamics summary we consider the elasto gravitational deformation of an earth model with fluid parts core ocean atmosphere a love number formalism is used in order to express the global deformation induced by volume potentials tides variations in the earth s rotation or boundary conditions in pressure transverse stress load at the interfaces between the fluid layers and the solid mantle the induced disturbances in gravity are expressed with the help of generalized gravimetric factors and special attention is paid to resonance processes various applications related to global earth dynamics are developed and numerically quantified the surface gravity changes caused either by a variation in the direction of the earth s rotational axis polar motion or in the rotation rate length of the day are compared with the lunisolar gravimetric tides the loading effect due to the oceans is found to increase the oceanless gravimetric factor by about 2 5 per cent two simplified approaches are proposed to model the gravity perturbations originating in the fluid core e g due to buoyancy core modes it is shown that a degree 2 planetary scale pressure field acting at the core mantle boundary of about 80 pa is able to induce a 10 ngal surface signal similarly gravity contributions from the atmosphere or oceans are derived using outer pressure load or spheroidal shear stress gravimetric factors a degree 2 surface pressure and transverse traction of 100 pa causes a gravity change of 0 27 and 0 14 pgal respectively finally we point out that in addition to the solar gravitational tide there are several other contributions rotational meteorological appearing in the annual gravity signal by different gravimetric factors we therefore conclude that high precision relative gravimetry can provide useful information not only about the earth s physical properties but also about its dynamical behaviour

longitudinally confined geomagnetic reversal paths from non dipolar transition fields it has long been thought that conditions at the boundary between the core and mantle influence the earth s magnetic field but the supporting evidence is rather indirect1 3 recent palaeomagnetic results suggesting that there are persistent preferred longitudinal paths for the virtual geomagnetic pole vgp during reversals4 would provide the first direct evidence of the solid mantle s influence on the core although their statistical significance has been disputed5 6 the results are potentially exciting because the preferred paths lie close to the pacific rim where the present geomagnetic secular variation changes character2 7 here we present a simple model based on an extension of a previous theory8 that produces reversals with vgp paths confined within relatively narrow longitude bands despite the transition field having a substantially non dipolar structure thus although longitude bias of the vgp paths is definitive evidence for core mantle interaction simple vgp paths are not evidence of near dipolar transition fields

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

recent north magnetic pole acceleration towards siberia caused by flux lobe elongation the wandering of earth s north magnetic pole the location where the magnetic field points vertically downwards has long been a topic of scientific fascination since the first in situ measurements in 1831 of its location in the canadian arctic the pole has drifted inexorably towards siberia accelerating between 1990 and 2005 from its historic speed of 0 15 km yr 1 to its present speed of 50 60 km yr 1 in late october 2017 the north magnetic pole crossed the international date line passing within 390 km of the geographic pole and is now moving southwards here we show that over the last two decades the position of the north magnetic pole has been largely determined by two large scale lobes of negative magnetic flux on the core mantle boundary under canada and siberia localized modelling shows that elongation of the canadian lobe probably caused by an alteration in the pattern of core flow between 1970 and 1999 substantially weakened its signature on earth s surface causing the pole to accelerate towards siberia a range of simple models that capture this process indicate that over the next decade the north magnetic pole will continue on its current trajectory travelling a further 390 660 km towards siberia observation based modelling suggests that recent acceleration of earth s north magnetic pole towards siberia can be linked to elongation of a lobe of negative magnetic flux at the core mantle boundary beneath canada

the ages of fractures in the eye dashwa pluton atikokan canada ages of fractures in the eye dashwa granite pluton north of atikokan ontario are estimated from analyses of their filling materials pegmatite aplite hornblendeporphyry epidote chlorite gypsum diabase iron oxides carbonate and clays the analyses include radiometric age determinations of filling materials comparison of crystallization temperatures of filling materials with the expected temperature of the pluton through time and paragenetic relations derived from textures the major fractures primarily epidote and chlorite filled were produced by penetrative deformation during cooling immediately after intrusion of the pluton from 2 6 ga to 2 4 ga they transected the pluton and largely defined the present overall fracture network less severe penetrative deformation occurred about 1 1 ga when a few diabase dikes and small gypsum fractures developed a second source of fracturing appears to have existed through most of geological time and it reopened segments of old fractures within about 400 m of the present erosional surface it is unknown whether the near surface or supergene fractures formed more or less continuously through time or in distinct episodes such as during glacial periods open fractures which are most common in the near surface layer tend to transmit groundwater but fracture sealing due to the accumulation of filling materials seems to occur with time underground excavations such as nuclear fuel waste disposal vaults would be less susceptible to the incursion of groundwater if they were located below the near surface layer

high fidelity paleointensity determination from historic volcanoes in japan 1 providing high fidelity paleointensity estimates is pivotal to identify temporal fluctuation of ancient geomagnetic field despite abundance of available paleointensity data in japan only dozen of them satisfy modern standards with systematic alteration checks high fidelity paleointensity estimates were obtained from historic andesitic lavas collected from mt aso ojodake 700 bc mt kirishima iwoyama ad 1768 ohachi ad 1235 and mt sakurajima an ei ad 1779 nabeyama ad 764 766 variation of geomagnetic field intensity is distinctively different from the prediction of global models arch3k 1 cals3k 3 cals3k 4 cals7k 2 and cals10k 1b for the past 4000 years in japan the compilation of high fidelity thellier data set in japan showed two obvious high intensities at ad 590 765 and ad 1330 1435 and one low intensity at 700 bc these time intervals with anomalous high low intensities are nearly identical to three of the four potential archeomagnetic jerks recognized from the european archeomagnetic data implying that the archeomagnetic jerks were global or at least northern hemispheric features to improve the poor representation of regional geomagnetic field variation with respect to the global model prediction more high fidelity paleointensity determinations are required in east asia

ams 14c ages and petrological features for solidified fractures with carbonates at coastal outcrops of yakushima island japan this paper describes the petrological features and the ages of rock fractures filled mainly with carbonates at coastal outcrops of yakushima island japan microscopic observation and geochemical analysis were used to investigate the petrological features and the compositions of the fracture fillings in addition ams 14c dating was also performed to estimate the ages of them microscopic study indicated that the fracture fillings contain not only cementing materials but also lithic fragments from host rock and bioclasts sem observation showed that the cements exhibit the characteristic textures of beachrocks the cements were identified as mg calcite from xrd analysis it was also observed from geochemical analysis that there were at least two stages of precipitation within the fractures in addition ams 14c dating for seventeen samples of fracture fillings showed that the ages range from 2 460 to 5 130 years bp which is found to be younger than that of the uplifted coral at yakushima island approximately 5 300 5 600 years bp the results suggest that the fractures were solidified after the coral was uplifted

on the use of horizontal components of magnetic field in determining core motions summary geomagnetic secular variation is caused by flow of fluid in the earth s core if electrical diffusion can be ignored field lines are frozen to the core fluid and can be used as tracers for the flow at the top of the free stream which lies beneath the diffusive boundary layer in this paper we assume the flux is frozen in and investigate the jump in the horizontal field across the boundary layer theoretical studies indicate that such a jump will be small about 1000 nt we test the hypothesis that the jump is zero within the errors of measurement of the magnetic field integral constraints which are conserved if the horizontal components of field at the base of the mantle can be equated to the components at the top of the free stream are derived and computed using seven main field models from epochs 1935 5 to 1980 0 these main field models have been forced to satisfy all the conditions of frozen flux theory the integral constraints are found to be constant within their expected errors and we conclude that the observations are consistent with zero jump in horizontal field across the boundary layer this result implies that use can be made of the horizontal components of field in determining core flow we attempt to compute the flow along null flux curves which is not determined at all by the radial component of the field for epoch 1970 the result is very noisy because of its reliance on gradients of the secular variation at the core mantle boundary which are poorly determined we do not attempt to compute reasonable core flows consistent with these observations although presumably many such flows exist because they would be so poorly constrained by observation

simultaneous stochastic inversion for geomagnetic main field and secular variation 2 1820 1980 we apply the method of simultaneous stochastic inversion for the geomagnetic main field and secular variation to data from the period 1820 1980 producing two time dependent field models one for the period 1900 1980 the other for 1820 1900 we introduce several adaptations of our original formalism of the inverse problem we choose to represent the time dependency of the field by an expansion in chebychev polynomials we apply an alternative regularization condition in the time domain and we adopt a different method from that used in our earlier study for solving the resulting system of equations whereas our earlier study used only twentieth century observatory data we consider almost all the available data from the time interval 1820 1980 we examine the effects of the crustal field on our observations and find that existing methods of accounting for these fields as sources of random noise are inadequate in two circumstances the first is sequences of measurements made at one particular site for example at permanent magnetic observatories in which case the crustal field produces temporal correlation of the errors in the observations we devise a method for accounting for these correlations the other is measurements made at satellite altitude the crustal field producing spatial correlation of the errors we show how using a very simple statistical model of the crustal field these correlations can be calculated rather than attempting to account directly for these correlations which would require without further assumptions massive computation we instead limit our data selection to points sufficiently well separated in space that the effect of the crustal field is minimal we are able to fit the data to within our estimates of the noise in the data yet the resulting models have lower power in the secular variation than previously postulated by some authors many of the features in the field at the core mantle boundary described in recent work by bloxham and gubbins are seen most clearly indeed this time varying representation of the field reinforces many of their earlier conclusions

toroidal flow in the outer core and the thermal regime below the core mantle boundary numerical calculations of fluid dynamos are used to constrain the probable origin of toroidal flows in the outer core azimuthal drift of the core fluid relative to the mantle is the sum of three components 1 uniform differential rotation of the core relative to the mantle produced by electromagnetic torques on the mantle 2 cylindrical flow driven by electromagnetic torques in the core and 3 buoyantly driven thermal wind flow produced by lateral variations of temperature and composition dynamo calculations indicate that electromagnetic torques on the mantle contribute to long term drift but are not likely to be its primary cause zonal flow driven by electromagnetic torques in the core and thermal wind flows driven by zonal temperature gradients are both sufficiently energetic to account for azimuthal flows with angular velocities of 0 1 0 5 yr 1 as inferred from frozen flux analyses of secular variation

geomagnetic secular variation recorded by sediments deposited during the cretaceous normal superchron at low latitude abstract paleosecular variation of the earth s magnetic field reflects the convective vigor of the geodynamo to investigate the energy regime of the geodynamo during the cretaceous normal superchron we collected 534 samples for a paleosecular variation study from a 1400 m long paleontologically well described section in northern peru thermal demagnetization isolates stable magnetization directions carried by greigite arguments are equivocal whether this remanence is syn diagenetic acquired during the cretaceous normal superchron or a secondary overprint acquired during a chron of solely normal polarity in the upper cenozoic yet pre bruhnes 800 kyr we explore the ramifications on the s value which quantifies paleosecular variation that arises from directional analysis sun compass correction bedding correction sampling frequency outlying directions and different recording media the sum of these affects can readily raise the s value by more than 20 s values from northern peru are indistinguishable from other s values for the cretaceous normal superchron as well as those for the last 5 ma summing over all the potential uncertainties we come to the pessimistic conclusion that the s value is an unsuitable parameter to constrain geodynamo models alternatively no statistical difference in paleosecular variation exists during much of the cretaceous normal superchron and during the last 5 ma

holocene sea level change and hydro isostasy along the west coast of kyushu japan abstract mid to late holocene sea level variations have been obtained at sites along the west coast of kyushu japan sea level observations for this relatively stable area are very important for evaluating the crustal tilting associated with the hydro isostatic adjustment due to the last deglaciation and for examining the cause of underwater jomon sites submerged archeological sites during the jomon period of mid holocene typically observed in this region for these purposes we conducted systematic boring samplings at sites from goto island to tamana along the latitude of about 33 n then we performed diatom assemblage and pyrite sulfur analyses of these sediment samples and 14 c datings for intertidal shells in order to get the sea level variations for these sites observations at tsumizu faced to oomura bay with tidal range of 0 9 m show a sea level high stand of 1 m at about 5500 yr b p years before present followed by smoothly falling to the present level evidences for sea level oscillation of greater than 50 cm have not been found here relative sea levels at about 5000 yr b p are however 2 m at goto island 125 km west of tsumizu and 2 m at tamana 50 km east of tsumizu thus observations at sites along the traverse from goto island to tamana are indicative of crustal tilting of about 3 4 m for the distance of 175 km which seems to be consistent with the prediction caused by the hydro isostatic adjustment

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

deformation of the core mantle boundary induced by spherical shell compressible convection we derive the single mode mean field equations for spherical shell convection with variable viscosity and compressibility included core mantle boundary cmb topography produced by whole mantle convection is computed for various rheologies and different thermal boundary conditions at cmb results for long wavelength undulations show the extreme sensitivity of these signatures to rheological and equation of state parameters in order to obtain excess ellipticity of about 1 km the activation energy and volume in the lower mantle should be low and the temperatures at cmb around 4 500 k in accord with recent experimental findings

the myth of the pacific dipole window the angular dispersion of virtual geomagnetic poles vgp from lava flows is often cited as having been anomalously low in the pacific during the brunhes epoch because the dispersion from hawaiian lavas is said to be much lower than measured elsewhere this led to the concept of the pacific dipole window or pacific non dipole low because lavas tend to be erupted in bursts of activity many of the hawaiian data are serially correlated and thus cannot necessarily be treated as independent observations geochronological controls as are available for hawaii have been used in a previous analysis to thin the data to try to avoid repeated sampling of the same geomagnetic field unfortunately in that analysis the angular dispersion of vgps was calculated about the mean vgp instead of about the spin axis and thus underestimated the dispersion we have therefore reanalyzed the relevant data using the following criteria 1 only those lavas with 95 10 have been considered 2 a latitude dependent cut off angle is used to eliminate those vectors that are not part of the normal secular variation 3 we have developed a statistical method to rationalise those flows that have repeatedly sampled the same geomagnetic field vector because suitable geochronological controls are rarely available application of this new method to the hawaiian data shows excellent agreement with the results from using geochronological controls where possible we have therefore included this method in our overall analysis as expected the resulting global data are compatible with a fisher distribution about the spin axis brunhes age data from hawaii n 96 independent measurements give a vgp angular dispersion about the spin axis of sf 12 4 and for the pacific region as a whole sf 12 5 n 190 between latitudes 15 and 30 north or south these values are the same as sf 12 4 n 160 calculated for the rest of the world in the same latitude range this clearly demonstrates that the hypothesis of the pacific dipole window may confidently be rejected

the geomagnetic field over the past 5 million years summary the modern geomagnetic field is usually expressed as a spherical harmonic expansion although the palaeomagnetic record is very incomplete in both space and time sufficient data are available from a span of ages to generate time averaged spherical harmonic field models with many degrees of freedom here three data sets are considered directional measurements from lavas inclination measurements from ocean sediments and intensity measurements from lavas individual data are analysed as well as siteaverages using the same methods that have been developed for the modern field to give models for the past 5 myr the normal polarity field model has an axial dipole intensity similar to that of the modern day field whilst the equatorial dipole component is very much smaller the field is not axisymmetric but shows flux concentrations at the core s surface under canada and siberia similar to those observed in the field over historical timescales tests on synthetic data show that it is unlikely that these similarities result from the overprinting of the palaeomagnetic field due to inadequate cleaning of the samples the reverse polarity field model does not show such obvious features but this may be due to the sparsity of the data the patterns observed in the normal polarity field with persistent features in the northern hemisphere and a smooth southern hemisphere could be explained if the present pattern of secular variation is typical of the past several million years this would reveal itself as large variations over time in the direction of the magnetic vector in regions of high secular variation with relatively little change over quieter regions however we have been unable to find any evidence for a geographical pattern of secular variation in the data

tangentially geostrophic flow at the core mantle boundary compatible with the observed geomagnetic secular variation the large scale component of the flow abstract we present a method for determining the large scale component of a tangentially geostrophic flow beneath the core mantle boundary compatible with magnetic secular variation observations we use a tangentially geostrophic basis to ensure the geostrophy of the motion the fit of the secular variation sv generated by the motion to the observed sv in fact sv models is adequate taking into account the existing error level as in any horizontal geostrophic motion the flow is expressed as a sum of two independent tangentially geostrophic flows a zonal component which is toroidal and a non zonal component which is directly linked with the motions deeper in core the flow derived for the recent epoch 1970 1985 presents interesting symmetry properties the non zonal velocities are the same at two antipodal points while the zonal velocities are the same at two points symmetrical about the equator the non zonal component of the flow is more vigorous than the zonal one the consoidal ingredient though weaker than the toroidal ingredient is essential and indicates strong vertical motion at depth in low latitude areas the sv is actually compatible with a geostrophic motion at the core mantle boundary and appears to be mainly due to the action of the non zonal component of the flow

coastal progradation and very early diagenesis of ultramafic sands as a result of rubble discharge from asbestos excavations northern corsica western mediterranean abstract the western coast of cape corse in northern corsica is a remarkable area by virtue of its geological biological and landscape diversity asbestos extraction between 1948 and 1965 however at canari mine has modified the natural rocky escarpments cutting the mountain into steps and has altered the coastal and marine zones from punta di canella to farinole albo harbour was filled with rubble discharging directly to the sea and the coast prograded 400 m after 4 years between 1954 and 1958 artificial shores of up to 300 m in width appeared at the bottom of existing steep cliffs over a distance of more than 5 km between 1948 and 1973 the posidonia seagrass was buried by the introduction of debris since june 1965 the mine is closed and coastal modification results from material movement from north to south as an exception campana beach appeared in the northern part of the discharge area in 1973 the sediment on nonza beach already displays cemented slabs that attest to very early diagenesis this particular environment of ultramafic sands and pebbles appear to have produced alkaline conditions and probably enhances carbonate precipitation

emergence and evolution of the south atlantic anomaly revealed by the new paleomagnetic reconstruction shawq2k abstract the south atlantic anomaly saa is one of the most outstanding features of the present geomagnetic field thus a good knowledge of the saa is clue for a better understanding of the dynamical behavior of the geomagnetic field to achieve this goal paleomagnetic data are crucial since they provide the unique way to investigate past changes in the earth s magnetic field here we present a new global geomagnetic field reconstruction the shawq2k model which is based on a critical revision of the global archeomagnetic and volcanic dataset the new model provides an improved description of the geomagnetic field during the last 2 millennia and yields surprising outcomes about the emergence and development of the saa it shows that the reversed flux patch observed at the core mantle boundary and linked to the saa emerged in the southern hemisphere from at least 950 ad this patch moved westward from the equator to southern latitudes being clearly isolated after 1400 ad in addition since 1550 ad a second reversed flux patch moving northeastward is observed in the north atlantic the new data now available for the southern hemisphere coming from africa and south america together with the use of an appropriated weighting scheme in the modeling process have improved our understanding of past geomagnetic field behavior and showed new evolutionary aspects of the saa

paleomagnetic tomography of the core mantle boundary abstract analysis of updated global paleomagnetic data indicates that over the last 5 million years the normal and reverse polarity states are discernibly different this difference which can be characterized by a higher ratio of the axial quadrupole to dipole field for the reverse polarity relative to the normal polarity is attributed to lateral variations in temperature or composition at or near the core mantle boundary although several mechanisms are considered thermal electric effects associated with core mantle topography and or with chemical variations in d the seismic layer approximately 150 km thick at the base of the mantle are judged to be the most likely sources of the observed polarity asymmetry this asymmetry should not necessarily be expected to give rise to systematic trends in polarity transitions

geomagnetic polarity reversals a connection with secular variation and core mantle interaction geomagnetic polarity reversal remains one of nature s most enigmatic phenomena dynamo theory admits solutions in pairs with reversed magnetic fields b and b but detailed calculations are required to understand how the field can change sign theory also admits separate solutions with different symmetry across the equatorial plane the symmetric es quadrupole and the antisymmetric ea dipole solutions which may be important in the reversal process and which offer a simple framework for interpreting small paleomagnetic data sets ordinary secular variation leads to very large changes in the magnetic field over several centuries and could easily develop into full reversal the theory of secular variation which is relatively well developed may therefore help in understanding reversals other clues to geomagnetic reversals come from the sun whose magnetic field reverses every 11 years paleomagnetic data show the earth s magnetic field reverses every million years or so with each transition taking about a thousand years during which the intensity may fall by as much as 1 order of magnitude reversal frequency undergoes a modulation on the long timescale 107 years of mantle convection and there have been two long intervals in the past with no reversals such behavior is typical of a highly nonlinear dynamical system but the very long timescale of changes in reversal frequency and its close proximity to the overturn time of mantle convection suggests some control of the dynamo by the mantle short term phenomena such as change in the length of day and secular variation have been studied extensively for evidence of core mantle interactions and we may draw on this body of evidence in order to understand long term effects three physical mechanisms have been proposed topographic electromagnetic and thermal with the last two being most significant for long term effects symmetries allow the dynamo to generate an ea field with the major component a dipole but lateral variations on the core mantle boundary may lead to magnetic fields with no symmetry reflecting the structure of the boundary anomalies changes in reversal frequency on the mantle convection timescale could arise either from changes in total heat flux from the core to the mantle or from instabilities associated with lateral variations at the core mantle boundary neither mechanism is well understood but the former involves significant changes to the earth s overall heat budget whereas the latter must always arise as a natural consequence of deep mantle convection recent measurements of transition fields show pole paths that lie close to two preferred longitudes near 90 w and 90 e if substantiated the result would provide the first definitive evidence of long term mantle control of the geomagnetic field further evidence suggests that the geomagnetic pole during stable polarity also lies along these two longitudes and that magnetic flux at the core surface tends to concentrate along the same longitudes as does the present field a simple theory is proposed relating changes in the core field to apparent transition paths measured at the earth s surface the model shows that longitude confinement of the transition paths can occur for quite complicated core fields and that surface intensities can drop by 1 order of magnitude on average simply because of the reduction in length scale of the transitional field simple transition paths may be an indication of some organization of flux at the core surface but not of large scale or small amplitude core fields

complex seismic anisotropy in the top of the earth s inner core beneath africa 1 we report complex seismic anisotropy in the top 80 km of the earth s inner core beneath africa the anisotropy in the top 80 km of the inner core is constrained using differential travel times amplitude ratios and waveforms of the pkikp pkikp phases sampling africa along various directions the differential pkikp pkikp time residuals relative to the preliminary reference earth model prem along the polar paths are larger than those along the equatorial paths by 0 1 4 s indicating the presence of seismic anisotropy in the top 80 km of the inner core furthermore the observations along the polar paths show complex regional variations beneath africa the differential pkikp pkikp travel time residuals vary from 1 2 s beneath eastern africa to 0 1 s beneath central africa and to 0 2 to 0 8 s beneath western africa a correlation between small pkikp pkikp amplitude ratios and large differential pkikp pkikp travel time residuals is observed the waveform data are spatially binned into six groups to constrain the regional dependence of velocity and attenuation anisotropy in the top 80 km of the inner core overall the seismic data can be explained by an isotropic upper inner core uic overlying an anisotropic lower inner core lic in the top 80 km of the inner core across africa the thickness of the isotropic uic varies from 0 to 50 km and the p velocity transition from the isotropic uic to the anisotropic lic is sharp with velocity increases laterally varying from 1 6 to 2 2 the attenuation structure along the polar paths has a q value of 600 for the isotropic uic and q values varying from 150 to 400 for the anisotropic lic the complex seismic anisotropy in the top of the inner core is found in a region where a rapid change of the inner core boundary icb between 1993 and 2003 was discovered wen 2006 and may be explained by complex alignments of iron crystals resulting from a localized anomalous solidification of the inner core

stable isotopic composition of carbonate cemented recent beachrock along the mediterranean and the red sea coasts of egypt abstract recent beachrocks on the mediterranean and the red sea coasts of egypt have been studies in the mediterranean beachrock skeletal particles mainly molluscan fragments are the most abundant grain type comprising approximately 60 of the framework grains intraclasts make up the remaining 40 in contrast the red sea beachrocks are dominantly composed of igneous and metamorphic crystalline basement rock fragments that range in dimension from large boulders in proximal parts of the fan to gravels sands and silts in the distal parts high energy conditions as well as the presence of relatively coarse high porous and permeable sediments ensure that adequate volumes of supersaturated water are able to move through the pore system to accomplish the cementation process textural study light and sem microscope on samples from both localities reveals that high mg calcite cements occur as thick to thin bladed isopachous crusts which sometimes occlude all pore spaces aragonite cements are also present as thin circumgranular acicular crusts however some samples along the red sea coast contain high mg calcite displaying scalenohedral crystal forms the isotopic composition of these beachrock cements generally show high values of 18 o and 13 c in the mediterranean beachrock cements the 18 o and 13 c values range from 0 4 to 1 2 and from 1 0 to 2 1 pdb respectively whereas the 18 o and 13 c values of red sea beachrock cements range from 0 1 to 1 2 and from 2 1 to 4 5 pdb respectively the oxygen and carbon isotopes together with the texture and mineralogical composition of these cements are good indicators of the conditions of primary cementation and reflect the gross of stable isotopic and chemical composition of marine water

beachrocks from the island of la palma canary islands spain beachrocks on la palma island developed on platform forming lavas of the cumbre vieja volcano some of these lavas are related to the 1585 puerto naos 1677 and 1971 echentive eruptions radiocarbon dating of the charco verde beachrock gives a conventional age of 33 330 8 490 bp while that at playa chica beach gives a calibrated age of 14 940 8 525 bp the beachrocks up to 1 5 m thick and some tens of metres wide consist of several decimetre thick horizons dipping 2 15 seaward petrographically they can be classified as rudstones and arenites with volcanic clasts as their main component the original porosity of the beachrocks was intergranular and occasionally intragranular and was partially occluded by cementation and locally by internal sediments the main cements are fibrous aragonite and micrite high magnesium calcite hmc spar aragonite peloidal hmc and microbotryoidal hmc are scarce the elemental geochemistry of these cements is consistent with a marine origin whereas the isotopic geochemistry indicates precipitation from marine waters slightly modified by meteoric waters the evolution of beach deposits and especially the beachrocks in la palma island follows three stages 1 beach deposition 2 beachrock formation and 3 beach retrogradation and or erosion the studied beachrocks prompt us to make some important considerations 1 the mean tidal range in the canary islands has not varied over the last thousand years 2 the position of the beachrocks at the present day sea level would require a combination of eustatic and isostatic movements to keep the sea level stable at the present level over the last thousand years 3 volcanic activity supplies the sediment that forms the beaches 4 a dry warm climate with a very low rainfall below 250 mm year and a high insolation rate 6 11 h day favours and favoured cement precipitation and beachrock formation by increasing the water temperature in the intertidal zone and in the inner part of the beaches 5 the presence of beachrocks in the la palma beaches prevents the total disappearance of the beaches 2003 elsevier science b v all rights reserved

beachrock occurrence characteristics formation mechanisms and impacts beachrocks are hard coastal sedimentary formations consisting of various beach sediments lithified through the precipitation of carbonate cements the objectives of this contribution are to a collate and review information on the reported occurrences characteristics and formation mechanisms of beachrocks and b consider their impacts on the coastal zone the analysis of the available information has shown that a beachrock formation is a global and diachronic phenomenon and b the great majority of beachrocks are found in tropical subtropical and low temperate latitude microtidal coasts the cementing agents of beachrocks are composed predominantly of the metastable carbonate phases high magnesian calcite hmc and aragonite ar appearing in a diverse crystalline morphology it has been suggested that cement precipitation in the coastal environment is controlled by i the physicochemical conditions ii the presence of organic compounds and microbes iii the magnitude and distribution of the wave energy along the coast and iv the textural characteristics of the constituent sediments various theories have been proposed to explain beachrock formation itself linking the phenomenon to either physicochemical or biological processes these theories however do not seem to be of universal validity and acceptance as each is able to explain only some of the reported occurrences the presence of beachrocks appears to affect beach morphodynamics by i locking the beach profile ii modifying the nearshore hydrodynamics iii changing the porous character of the beach and thus its response to wave forcing and iv differential bed erosion at the margins of the beachrock outcrops that can alter significantly the long and particularly the cross shore sediment transport therefore although relict submerged beachrock outcrops may provide some coastal protection by reducing the wave energy impinging onto the coastline modern beachrocks may promote offshore loss of unconsolidated beach sediments and buried beachrock outcropping finally the presence of beachrocks may have also significant ecological impacts as the indigenous mobile substrate fauna and flora of the beach is replaced by hard substrate benthic assemblages which are commonly arranged in hydrodynamically controlled zones

on the analysis of paleomagnetic secular variation modern methods of geomagnetic field analysis can be applied to paleomagnetic data provided account is taken of the relatively sparse and inaccurate nature of paleomagnetic results paleomagnetism tends to produce data sets dominated by inclination measurements which do not provide a unique field model as an example two magnetic fields are found with closely similar inclinations but completely different declinations and strengths directional measurements are difficult to deal with because they are nonlinearly related to the field model the equations are simplified by linearizing the field about an axial dipole the approximation is adequate for paleomagnetic data and can be used to show that the vgp scatter function is not an appropriate estimate of the non axial dipole field even when averaged over longitude four assertions made in the paleomagnetic literature are shown to be correct within this linearization but are not true in general in particular equatorial symmetry of the vgp scatter functions for the separate symmetric quadrupole field holds within the approximation the virtual geomagnetic pole vgp scatter functions for the modern geomagnetic field has strong north south asymmetry but historical field models show it to be a relatively recent feature the vgp scatter function for the symmetric part of the field sq is dominated by the equatorial dipole and is approximately equal to the angular distance between the geomagnetic and geographic poles that for the antisymmetric part of the field sd is dominated by geomagnetic coefficients of degree 2 and order 1 surprisingly the axisymmetric quadrupole and octupole terms contribute very little highlighting the dangers of modeling the paleomagnetic field without regard to departures from axial symmetry

formation exposure and evolution of a high latitude beachrock in the intertidal zone of the corrubedo complex ria de arousa galicia nw spain abstract the presence of beachrocks is noted on the galician shoreline at 41 n a latitude at which this phenomenon is rarely found the present study focuses on the petrological characterization and sedimentological evolution of a beachrock that was exhumed during an exceptional storm in the winter of 2001 in the intertidal zone of a high energy beach the cemented material comprises medium to coarse mixed siliciclastic carbonate sands unlike tropical beachrocks the cement consists of epitaxial low mg calcite presenting varied morphologies and textures acicular and bladed coatings and pore linings pore fillings of sparitic calcite and meniscus style cements seawater dominantly wave action plus marine and meteoric mixed waters are invoked as the main generative fluids beachrock formation and evolution are approached from a morphodynamic viewpoint considering four progressive stages 1 initial cementation in the intertidal zone 2 exposure and modelling by wave action 3 colonization and hardening and 4 disintegration preservation and burial finally the morphodynamic significance climatic implications and occurrence of this type of cementation along the north and northwest shoreline of spain are discussed

mapping s velocity heterogeneities in the d region from smks differential travel times abstract seismic velocity heterogeneities in the d region are studied using differential travel time residuals between smks waves previous studies indicate that regional variations in the propagation times of these waves are not due to the propagation through the liquid core furthermore they are only weakly influenced by the topography of the core mantle boundary cmb thus the differential travel time anomalies are interpreted as the result of lower mantle heterogeneity we compiled all the available published data and added new observations of travel times from geoscope stations we thus obtained 807 s2ks sks and 209 s3ks s2ks differential times differential travel time residuals with respect to the mean earth model prem preliminary reference earth model are corrected for ellipticity and for mantle heterogeneity down to 2590 km depth 300 km above the cmb using the 3 dimensional s wave velocity model sh12 wm13 this model gives a 10 variance reduction of the residuals the remaining variance of residuals is higher for s2ks sks than for s3ks s2ks which is consistent with a d origin for the heterogeneities the data are inverted for regional variations using a least squares analysis assuming a single layer d structure 300 km thick the earth is divided in 150 equal area sectors about 115 of which are sampled by four rays or more we performed inversions using 18 different geographic grids and averaged the results so as to minimize the artefacts induced by imposing artificial block boundaries the velocity heterogeneity map though affected by the poor sampling of the oceans reveals significant long wavelength structure with velocity perturbations exceeding 3 with respect to a reference velocity which is slightly lower than the prem one this heterogeneity map is consistent with the results from most of previous studies global as well as regional the location of some of the high velocity anomalies coincides with places where subducted slab material may have accumulated at the cmb these high velocity heterogeneities are also generally correlated with places where a discontinuous velocity increase of several per cent has been detected at the top of d

cryptic intertidal microbialites in beachrock heron island great barrier reef implications for the origin of microcrystalline beachrock cement microbialites with microcolumnar growth morphology occur on the surfaces of solution enlarged crypts in modern beachrock an heron island great barrier reef australia the microbialites consist predominantly of felted aragonite but blocky mg calcite and fringes of acicular aragonite also occur a variety of microbial bodies and filaments occur within and on the surface of the microbialites although most heron beachrock is cemented by isopachous fringes of acicular aragonite irregular micritic cement is common adjacent to the microbialites the micritic cement consists predominantly of felted aragonite and contains microbial filaments and organic matter similar to those in the microbialites both the microbialite and the micritic cement are interpreted to be biologically induced although physico chemical models including co2 degassing and evaporation are commonly held to be sufficient to promote the formation of beachrock the similarity of heron cryptic beachrock microbialite with micritic beachrock cements supports the contention that some beachrock cements are biologically induced additionally the primary difference between fringes of acicular aragonite and the felts of aragonite in microbialites appears to be that the former nucleated on a well defined surface of organic matter whereas the latter nucleated within a framework of organic matter that contained irregularly disposed nucleation sites hence aragonite fringe cements may also be localized and or induced by films of organic matter c 1999 elsevier science b v all rights reserved

secular variation in the geographic location of the dip equator the geographic location of the dip equator on the globe has been determined for several epochs between 1550 and 1990 a d it is shown that two regions near 165 e and 285 e are devoid of migratory changes whereas between 300 and 0 e there is a large northward movement the period between 1945 and 1990 is marked by a nearly linear change in the latitude of the dip equator while earlier epochs show more complex trends consistent with the earlier observations of the movement of the magnetic center towards or away from the centre of the earth before and after 1800 a d the secular curves in certain longitude zones indicate a clear change in pattern the observed secular variations in the vertical cornponent at several low latitude stations are generally in agreement with the direction of the movement of the dip equator examples of unexpected behaviour are highlighted the eccentric dipole model appears to be adequate to delineate the dip equator for any epoch

holocene paleomagnetic secular variation records from the western equatorial pacific ocean abstract we have carried out detailed rock magnetic and paleomagnetic studies on a suite of four deep sea sediment cores from indonesia and the philippine islands which form a transect from 6 3 n to 10 6 s the cores have average sediment accumulation rates of 35 65 cm kyr that all yield high resolution paleomagnetic secular variation psv records the psv records contain 44 correlatable inclination and declination features that corroborate the individual psv records and indicate that they are all continuous with no significant stratigraphic gaps thirty four ams radiocarbon dates provide a high resolution time stratigraphic framework for the psv records which extend from 0 18 000 yrs bp these are the first high resolution holocene psv records ever recovered from anywhere on earth within 17 of the equator almost 1 3 of the earth s surface area statistical analysis of the holocene psv records indicates that they have i anomalies of 5 to 9 consistent with 5 ma averaged results the strong similarity in i anomaly pattern of holocene and longer term 5 ma averaged psv data suggest that the dynamo process causing the anomaly is largely described by holocene psv by contrast the holocene psv records have vgp angular dispersions that are significantly lower than 5 ma averaged psv data yet significantly higher than psv data averaged over only the last few thousand years this difference indicates that whatever dynamo process causes the overall directional variability of the earth s magnetic field is not totally described by recent last few thousand year psv or even by late quaternary last 20 000 yrs psv

cementation patterns and genetic implications of holocene beachrocks from northeastern brazil abstract this paper describes the complex patterns of diagenesis in holocene beachrock ledges located along both the eastern and northern coasts of the state of rio grande do norte northeastern brazil these beachrocks show highly variable diagenetic processes fabrics and textures carbonate cements are exclusively composed by high mg calcite and occur in seven main morphologies 1 cryptocrystalline coatings 2 isopachous prismatic rims 3 equant spar 4 cryptocrystalline or micritic pore filling 5 pseudo peloidal aggregates 6 radial aggregates 7 isolated and randomly oriented clumps of scalenohedral crystals other diagenetic features include a carbonate siliciclastic mixed sediment b infiltrated micrite c authigenic smectitic clay minerals d organic films e chalcedony pore filling cement f vadose silty internal sediment the texture and composition of the cements are indicative of precipitation in the active marine phreatic zone the presence of vadose features and planktonic foraminifers within the infiltrated materials suggest that they were introduced in the backshore sands by muddy waters dislocated from the shelf during storms a general succession of diagenetic phases can be recognized comprising the precipitation of authigenic smectite micritic coatings radial aggregates isopachous rims of prismatic crystals or equant spar cryptocrystalline and pseudo peloidal pore filling cements and vadose infiltration of micritic carbonate siliciclastic mixed sediments or silty materials the absence of organic structures such as microbial bacteria or fungi filaments and bodies within the cements suggests that the mechanism behind beachrock cementation is essentially inorganic most probably the evaporation of entrapped seawater in response to the dry climatic conditions

first u pb detrital zircon ages from numidian sandstones in southern apennines italy evidences of african provenance abstract two samples of quartz rich sandstones collected in the numidian flysch of southern apennines italy have been studied to highlight the provenance of detritus using radiometric dating by la icp ms of detrital zircons and to compare the obtained ages with those of the betic and maghrebian chains the provenance of quartzose detritus from european or african plates is still debated in these chains accordingly the ages of the detrital zircons can contribute significantly to discriminate the origin of the quartzose supply the u pb zircon ages n 47 vary from 3047 13 ma mesoarchean to 516 19 ma cambrian the predominance of paleo proteozoic ages 2500 1600 ma and the lack of hercynian and alpine ones suggest a provenance of the numidian supply from north african cratonic areas during the early middle langhian when the numidian successions of southern apennines were deposited in addition a cluster of ages at 773 24 ma and 668 12 ma in one sample and at 664 17 ma in the other sample calculated on zircon domains with magmatic zoning testify to an important contribution from neo proterozoic granitic rocks widely outcropping in the north african craton the age data on detrital zircons from numidian sandstones in southern apennines overlap those found in the numidian sandstones widespread in the betic cordillera and in the maghrebian chain from south spain to sicily this suggests that the entire depositional zone in which numidian flysch deposited was fed from a southerly source represented by the african craton where archean proterozoic and cambrian rocks widely crop out from the atlantic coast to the hoggar and tibesti massifs finally it must be outlined that a meso archean zircon age 3047 ma has been found in the numidian flysch of the southern apennines whereas in the numidian flysch of the maghrebian chain zircons older than paleo proterozoic 1840 ma have not yet been found

stable regions in the earth s liquid core summary slow cooling of the whole earth can be responsible for the convection in the core that is required to generate the magnetic field previous studies have assumed the cooling rate to be high enough for the whole core to convect here we study the effects of a low rate of cooling by assuming the temperature at the base of the mantle to remain constant with an initially entirely molten adiabatic core we argue that in such a situation convection would stop at the top of the core and calculate the consequent thermal evolution a stable density stratified layer grows downwards from the core mantle boundary reaching a thickness of 100 1000 km in a few thousands of millions of years there is some geomagnetic evidence to support belief in the existence of such a stable layer

mantle convection and core mantle boundary topography explanations and implications abstract recent seismic inferences of the topography on the core mantle boundary cmb constitute a new and potentially important constraint on the dynamics of the thermal convective circulation in the earth s mantle we employ viscous flow models for the mantle to predict the flow induced deflections of the cmb that are expected on the basis of seismic tomographic inferences of internal mantle density heterogeneity the good agreement between our theoretical predictions and the seismically inferred cmb deflections of morelli and dziewonski 1987 suggests that the latter may indeed be good approximations to the dynamically induced cmb topography the amplitude of the seismically inferred cmb topography may be construed to provide a direct constraint on the temperature derivative of seismic p wave velocity in the lower mantle and we determine a value for this parameter in the course of fitting the data that differs significantly from the value determined by laboratory measurements as appropriate for upper mantle phases the seismically observed topography on the cmb is also shown to provide evidence favouring the whole mantle convection hypothesis and in this connection we argue that deep penetration of subducted lithospheric slabs into the lower mantle is likely to be responsible for controlling many of the features of this topography

fault and fluid interaction in a rifted margin integrated study of calcite sealed fault related structures southern corinth margin this work integrates microstructural petrologic microthermometric and geochemical analysis of calcite sealed fault related structures of the major faults of the southern margin of the corinth rift greece it draws attention to the main deformational processes and fault fluid inter action in fault zones along the whole margin data analysed come from the compact breccia located immediately adjacent to major fault planes samples were collected at different positions alongside different faults juxtaposing synrift continental deposits hanging wall against the prerift pindus limestones footwall this implies hectometric fault displacements and that the fault zones have recorded a relatively long evolution of deformation our study identifies four main defor mation features common to all studied faults calcite cemented breccia extensional veins shear veins and fault slip surfaces the integrated data analysis shows that deformation structures and fault rocks that coexist in the same outcrop formed at different depths and times during inferred upward fault propagation the style of deformation evolved from distributed brecciation to loca lized slip surfaces and from a closed system with fluid rock equilibrium during brecciation to a more open one with limited influx of meteoric water during extensional and shear fracturing this implies relative uplift of the footwall during fault propagation and fault interaction with different water aquifers as the fault propagates upwards the results demonstrate similar fault evolution and fault fluid interaction through the entire rifted margin and during the whole rifting period

on mantle induced heat flow variations at the inner core boundary abstract seismic shear wave velocity variations in the earth s lower mantle have led to the inference that fluid motion in the core can be controlled by the mantle non axisymmetric features of the observed geomagnetic field as well as fields produced in dynamo calculations support this viewpoint however it is not well understood how the mantle modified flow pattern can affect heat flow as far down as the inner core surface in this paper we study the possible effect of lower mantle variations on the heat flux distribution at the earth s inner core boundary icb as the lateral variation at the core mantle boundary cmb is increased in a geodynamo model strong narrow downwelling jets are produced which impinge on the lower boundary giving rise to regions of intense cooling separated by broad regions where warm fluid can rise relatively slowly significant heat flow into the inner core which might lead to melting beneath africa and the pacific is more likely to occur in a partially locked solution rather than a quasi stationary tightly locked solution we also report a dynamo solution where the computed maximum heat flux variation at the core mantle boundary is one order of magnitude higher than the mean flux although this regime does not have heat flowing into the inner core varying the magnitude of the cmb anomaly can potentially make the heat flux negative over a large area of the inner core surface

dynamo waves and palaeomagnetic secular variation summary a dynamic model is proposed for large amplitude geomagnetic secular variation with periodicity in the range of 1000 3000yr thermal convection in the lower mantle imposes a spatially variable heat flux at the core mantle boundary if the total heat loss from the core exceeds approximately 3 x 10 2w then both i a convective layer and ii large scale quasigeostrophic shear flows are likely to exist beneath the core mantle boundary the combined effects of convection and shear at the top of the outer core can generate three dimensionally propagating magnetic field disturbances called cuw dynamo waves an analysis is made of the surface magnetic field disturbances due to dynamo wave propagation in the outer core in the presence of turbulent convection and an east west shear flow nonaxisymmetric disturbances exhibit the following characteristics i dynamo waves propagate in each hemisphere with a poleward or equatorward component depending on the sign of the a effect ii both eastward and westward propagation is allowed althcugh westward propagation is preferred the apparent velocity at the earth s surface is the sum of propagation and drift velocities in the core iii dynamo wave disturbances are transient with exponential growth and decay phases iv traces of inclination versus declination of individual fourier components obey runcorn s circularity rule a comparison is made with lake sediment secular variation records from central north america measured by lund banerjee the band passed time series and the major inclination declination loops can be modelled from 2600 to 10 000 yr bp by a single dynamo wave periodic in 2400 yr propagating under the site along an azimuth of 345 between 0 and 2600 yr bp the record can be modelled by addition of a wave with 1200yr period propagating along an azimuth of 30 this indicates that the palaeomagnetic secular variation record in north america to 10 000 yr may be dominated by poleward propagating wave like disturbances

paleomagnetic secular variation and relative paleointensity during the holocene in south china huguangyan maar lake revisited the scarcity of reliable paleomagnetic secular variation psv records from east asia especially from low latitude regions impedes better understanding of global psv mechanisms here we report on a radiocarbon dated holocene psv record from a composite 6 7 m long core collected from the high sedimentation rate huguangyan maar lake hml in subtropical tropical south china rock magnetic results demonstrate that the natural remanent magnetization resides in single domain magnetite alternating field demagnetization experiments at 1 cm spacing on u channel samples reveal six distinct inclination highs at 7 500 bce 5 100 bce 4 600 bce 3 600 3 400 bce 1 600 1 200 bce and 600 800 ce three inclination lows at 4 800 bce 600 300 bce and 1 000 1 300 ce and three eastward declination trends at 3 600 3 200 bce 2 600 2 400 bce and 400 bce to 200 ce the similarity between the hml psv record and other independently dated records from east asia and geomagnetic field models corroborates the robustness of our age model and holocene psv record strikingly centennial to millennial scale psv features of the hml are comparable within age uncertainties with other holocene records from europe north america and canada suggesting that such directional patterns are likely to be hemispheric in scale although relative paleointensity data of hml are affected by environmental factors e g organic matter diagenesis the record still provides a regionally important new psv reference curve whose conspicuous features may serve as stratigraphic markers for east asian paleorecords

isotopic and morphological features of fracture calcite from granitic rocks of the tono area japan a promising palaeohydrogeological tool this study aimed to develop a methodology for assessing the hydrochemical evolution of a groundwater system using fracture filling and fracture lining calcite fracture calcite in deep to ca 1000 m granitic rocks of the tono area central japan was investigated by optical and electron microscopy and chemical and isotopic analysis coupled with geological evidence these new data imply 3 main origins for the waters that precipitated calcite 1 relatively high temperature hydrothermal solutions precipitating calcite distinguished by 18osmow from 3 to ca 10 and with 13cpdb from ca 18 to 7 2 seawater probably partly of miocene age which precipitated calcite distinguished by 13cpdb of ca 0 and 18osmow ca 20 3 fresh water with a variable 13cpdb composition but which precipitated calcite distinguished by 13cpdb that was significantly ca 17 data for 14c suggest that at least some of the fresh water calcite formed within the last 50 ka the present day hydrogeological regime in the tono area is also dominated by fresh groundwater however the marine calcite of probable miocene age found at depth has shown no evidence for dissolution and many different calcite crystal forms have been preserved studies of other groundwater systems have correlated similar crystallographic variations with variations in the salinity of coexisting groundwaters when this correlation is applied to the tono observations the calcite crystal forms imply a similar range of groundwater salinity to that inferred from the isotopic data thus the present study suggests that even in presently low salinity groundwater systems calcite morphological variations may record the changing salinity of coexisting groundwaters it is suggested that calcite morphological data coupled with isotopic data could provide a powerful palaeohydrogeological tool in such circumstances

secular variation in numerical geodynamo models with lateral variations of boundary heat flow we study magnetic field variations in numerical models of the geodynamo with convection driven by nonuniform heat flow imposed at the outer boundary we concentrate on cases with a boundary heat flow pattern derived from seismic anomalies in the lower mantle at a rayleigh number of about 100 times critical with respect to the onset of convection the magnetic field is dominated by the axial dipole component and has a similar spectral distribution as earth s historical magnetic field on the core mantle boundary cmb the time scales of variation of the low order gauss coefficients in the model agree within a factor of two with observed values we have determined the averaging time interval needed to delineate deviations from the axial dipole field caused by the boundary heterogeneity an average over 2000 years the archeomagnetic time scale is barely sufficient to reveal the long term nondipole field the model shows reduced scatter in virtual geomagnetic pole positions vgps in the central pacific consistent with the weak secular variation observed in the historical field longitudinal drift of magnetic field structures is episodic and differs between regions westward magnetic drift is most pronounced beneath the atlantic in our model although frozen flux advection by the large scale flow is generally insufficient to explain the magnetic drift rates there are some exceptions in particular equatorial flux spot pairs produced by expulsion of toroidal magnetic field are rapidly advected westward in localized equatorial jets which we interpret as thermal winds

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

core mantle interactions abstract recent studies of the magnetic field at the core mantle boundary have revealed fixed sites of either static magnetic features or persistent secular variation this suggests part of the magnetic field behaviour is controlled by the mantle the most plausible mechanism for core mantle interaction is thermal coupling although topography may also be significant the magnetic sites coincide with anomalies in lower mantle seismic velocity as determined from tomography and density as determined by flow models of mantle convection constrained by tomography and the geoid some magnetic features coincide with subduction zones particularly those near indonesia they may be caused by bumps on the core mantle boundary beneath trenches paleomagnetic pole positions suggest the magnetic behaviour has persisted for at least 5 myr as would be expected if it were controlled from the mantle

persistent high paleosecular variation activity in southern hemisphere for at least 10 000 years abstract direct observations of the geomagnetic field show that secular variation is strong in the atlantic hemisphere and comparatively reduced in the pacific region the dipole has been decaying since at least 1840 ad driven by growth and migration of reverse flux patches in the southern hemisphere we investigate whether anything like this modern pattern of geomagnetic secular variation persists and can be detected in global paleomagnetic field models synthesis of results from two new time varying spherical harmonic models shows that geographically distinct geomagnetic secular variation extends to at least 10 000 bp the models use the same database but differ in methodology leading to some regional differences in results consistent large scale surface features include strong average fields in the northern hemisphere and weaker fields with greater overall variability in the south longitudinal structure is present with weaker average fields in the western pacific than in the east and prominent negative inclination anomalies extending beneath indonesia across africa and to brazil but weaker anomalies in the central pacific marginally positive inclination anomalies occur west of the americas paleosecular variation activity peaks at high southern latitudes and there is a pattern of reduced activity at equatorial and mid latitudes beneath the pacific although the dipole has exhibited both growth and decay over the interval 0 10 000 bp our results show that geomagnetic paleosecular variation is preferentially focused in similar geographic regions to secular variation seen in the modern field

seafloor spreading anomalies in the magsat field of the north atlantic we show that a simple magnetization model for the crust of the north atlantic basin will explain the major anomalies of the magsat intermediate wavelength magnetic field over the basin our seafloor spreading model incorporates anomalous skewness and enhanced magnetization near the spreading axis and at the cretaceous quiet zones the anomalies over the young oceanic crust indicate a decrease in susceptibility away from the spreading axis the enhanced magnetization over the cretaceous quiet zone could be due to an increased main field intensity or a change in the spreading center processes during the cretaceous though similar observations are not readily apparent for other basins the north atlantic correlation indicates that an investigation of the remanent magnetization of the ocean crust could provide a standard for the evaluation of the intermediate wavelength magnetic field the models presented in this paper demonstrate that a sizable percentage of the intermediate wavelength energy in the magsat field is derived from sources within the earth s crust it also demonstrates that remanent magnetization is an important source of the magsat field over oceanic areas we seek to demonstrate in this paper that although components of the main field are likely to contribute to the intermediate wavelength scalar field we can correlate the anomaly field over the north atlantic to the well studied thermal remanent magnetization of the oceanic crust with a minor component of induced magnetization associated with younger oceanic crust

character of holocene paleomagnetic secular variation in the tangent cylinder evidence from the chukchi sea abstract we have carried out a paleomagnetic study on three deep sea cores from the chukchi sea 72 n in order to characterize the holocene paleomagnetic secular variation psv in this high latitude region the chukchi sea lies within the geomagnetic field tangent cylinder and psv variability in this region might be expected to have a different pattern than psv at sites located outside the tangent cylinder at lower latitudes we have recovered correlatable directional psv records and relative paleointensity records from all three cores 15 radiocarbon dates were used to develop a chronostratigraphy for the psv records these records constitute the highest resolution full vector psv records ever recovered from such high latitudes we have compared our results with other previous studies from the region and find that our overall psv is consistent with these other studies although there are sometimes age differences up to 1000 years between correlatable psv features our statistical psv characteristics indicate that field variability vgp angular dispersion is lower than in regions just south of the chukchi sea and outside the tangent cylinder but our records are probably not long enough to completely characterize psv however our results are consistent with the only other published vgp angular dispersion results from inside the tangent cylinder antarctica 79 s

holocene cemented beach deposits in belize abstract two types of cemented beach deposits occur on reef islands off the coast of belize these are 1 intertidal beachrock that is dominantly cemented by marine aragonite and high magnesium calcite cements and 2 supratidal cayrock that is cemented mainly by vadose low magnesium calcite cements besides differences in position relative to present sea level and resulting early diagenesic features beachrock and cayrock can be distinguished on the basis of differences in composition texture geographical position and age whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments cayrock is enriched in benthic foraminifera intertidal beachrock is moderately to well sorted and well cemented while supratidal cayrock is very well sorted poorly cemented and friable beachrock occurs preferentially on windward beaches of sand shingle gays on the middle and southern barrier reefs and on the isolated platforms glovers and lighthouse reefs cayrock only occurs on larger mangrove sand gays of the isolated platforms turneffe islands lighthouse reef and the northern barrier reef 14c dating of ten whole rock and mollusk shell samples produced calibrated dates between ad 345 and ad 1435 for beachrock and between bc 1085 and ad 1190 for cayrock the large scale distribution of beachrock in belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution only on windward sides of cays that are close to the reef crest where large amounts of seawater flush the beaches considerable amounts of cements can be precipitated to produce beachrock cayrock forms due to cementation in the vadose zone and is only preserved on larger stable mangrove sand cays

fof2 long term trend linked to earth s magnetic field secular variation at a station under the northern crest of the equatorial ionization anomaly long term trend of the critical frequency of the f 2 ionospheric region f o f 2 at phu thuy station 21 03 n 105 96 e vietnam located under the northern crest of the equatorial ionization anomaly eia is studied annual mean data are analyzed at 04 lt and 12 lt for the period 1962 2002 using monthly median values and monthly mean values during magnetically quiet days am 20 in both cases we obtain similar trends at 4 lt and 12 lt which we interpret as an absence of geomagnetic activity effect over trends the positive trends obtained are not consistent with the negative values expected from greenhouse gases effect at this layer of the upper atmosphere the increasing trend observed at 12 lt is qualitatively in agreement with the expected effect of the secular displacement of the dip equator over the eia latitudinal profile at 04 lt when the eia is absent the positive trend is in qualitative agreement with the secular variation of the earth s magnetic field inclination i and the consequent increase of the sin i cos i factor at the corresponding location

calcite fracture fillings as indicators of paleohydrology at laxemar at the aspo hard rock laboratory southern sweden isotopic compositions of c 13c o 18o and sr 87sr were determined for calcite fracture fillings in the crystalline rock penetrated by a 1 6 km drill hole at laxemar near the aspo hard rock laboratory ahrl in southern sweden these calcites precipitated from groundwater some time in the past and their 13c 18o and 87sr values reflect those of the source waters the present day groundwater system is hydrochemically stratified with highly saline water underlying more shallow brackish and fresh water the origin of this stratified system is probably related to past glaciations although the ultimate origin of the deep highly saline water is still problematical none of the calcite fracture fillings sampled below 900 m could have precipitated from any of the present day ground waters which in view of the glacial history of the region is not surprising however several shallow calcite fracture fillings are formed by precipitation from the present day groundwater coupled variations in 13c 18o and 87sr isotopes at depths in excess of 900 m suggest that these isotope systems in calcite are recording a time dependent evolution of groundwater composition

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

a refined age calibrated paleosecular variation and relative paleointensity stack for the nw barents sea implication for geomagnetic field behavior during the holocene abstract reconstruction of paleomagnetic secular variation psv of the geomagnetic field is fundamental both to assess geodynamo models and to obtain age constraints for rocks sediments and archaeological material we present refined age calibrated holocene psv and relative paleointensity rpi stack curves derived from arctic marine sediments northwestern barents sea the holocene sections of four sedimentary cores were correlated on the basis of paleomagnetic trends and age models and stacked the resultant composite psv and rpi holocene records nbs stack and the reconstructed holocene virtual geomagnetic pole vgp path were evaluated in comparison with the most recent paleomagnetic stack curves and geomagnetic field models the data indicate that during the holocene time the vgps moved within the superficial projection of the inner core tangent cylinder with the exception of short time intervals around 5600 and 3200 cal yr bp when vgps extended to lower latitudes these deviations might reflect regional geomagnetic features such as persistent geomagnetic flux lobes at core mantle boundary our data confirm that the large vgp shift observed around 5600 cal yr bp is the result of an increased radial magnetic field at the core mantle boundary over north america whilst the vgp shift around 3200 cal yr bp represents a major swing to middle latitudes toward the middle east and might be associated to a regional high paleointensity peak known as levantine iron age anomaly liaa

pangea in permian to jurassic time the apparent polar wander paths apwps for the northern and southern continents have been redetermined using revised time scales and data when plotted on the conventional pangea pangea a the paths are distinct but essentially linear and parallel from late carboniferous to late permian time both paths then undergo a sharp change in direction and converge so as to overlap in early to middle jurassic time if the permo triassic magnetic field was an axial dipole then paleomagnetic data show that pangea was mobile during most and possibly all of the mid triassic to mid jurassic interval because of the uncertainties in pole ages and positions and errors in measurements the new apwps are consistent with a range of pangeas in particular the data are compatible with two paleomagnetic pangeas a2 and a new class of pangea here named the d pangeas these resemble morel and irving s 1981 pangea b and the c pangeas of smith et al 1981 unlike the a2 pangeas the b c and d pangeas all require large 2000 km relative motion between the northern and southern continents geological data require the transition from an earlier pangea to pangea a to take place along a transform zone here named the pangea transform zone or ptz the ptz is considered to be technically analogous to the great glen fault of scotland four types of possible ptz are examined passing through either the western or eastern end of the brunswick magnetic anomaly of eastern north america or north and south of iberia the geological data suggest that those pangeas that require large displacements on ptzs that pass east of the brunswick magnetic anomaly are improbable b and c pangeas are snapshots giving precise paleomagnetic reconstructions d pangeas fit long stretches of the apwps as well as possible a2 pangeas fit the apwps moderately well but also require the geological displacement to be smaller the differences in displacement between the d and a2 pangeas reflect geometrical relationships between euler poles ptz curvature and the apwps the general geological evidence suggests that d pangeas are less probable than a2 pangeas but that they cannot be rejected transition from a2 to a is most likely to have occurred via a sequence of rotations about different poles rather than a single rotation in detail the conventional pangea requires modification africa must be broken into at least two fragments along the benue trough and its northern continuation northwest africa then fits more tightly into northern south america which modifies the apwps and reduces the gap between them by including the effects of continental extension and using detailed geological data the gap can be reduced to one in which the a63 circles touch or overlap non dipole fields or an expanding earth need not be invoked to explain the mismatch of the apwps on the conventional pangea the finite rotation required for the older pangea to pangea a is very close to van der voo and french s 1974 single rotation for a2 to a but is actually the sum of three smaller rotations that reflect the modification to pangea a continental extension and the postulated ptz displacement of the possible ptzs a ptz parallel to the west african continental magnetic anomaly with a displacement of about 170 km gives a better fit of this anomaly to the east coast magnetic anomaly of north america further refinements to pangea a2 will depend on a better knowledge of stretching factors on the passive continental margins of the modified pangea a and incorporation of rotations due to major faults such as the south atlas fault in morocco

archeomagnetic secular variation from korea implication for the occurrence of global archeomagnetic jerks abstract an archeomagnetic paleosecular variation psv was first defined in korea using baked materials collected from 26 kilns or hearths with ages ranging from 1100 bc to ad 1790 variations of geomagnetic declination and inclination from the korean peninsula are distinctively different from the prediction of a global model cals3k 3 or cals7k 2 for the past 3500 yr in particular a distinctive offset in magnitude and phase is noticeable between the observations and predictions at 745 bc ad 300 and ad 1400 1700 a bi plot of magnetic declination versus inclination displays three cusps at the corresponding time intervals these time intervals are nearly identical to or at least overlap with three of the four potential archeomagnetic jerks suggested by gallet et al 2003 from the european archeomagnetic data a comparison of the psv curves for neighboring countries regions revealed that european archeomagnetic jerks at 800 bc ad 200 ad 800 and ad 1400 were all preserved in east asia suggesting that the archeomagnetic jerks were global or at least northern hemispheric features

climatic reconstructions for the northeast atlantic region ad 1685 1700 a new source of evidence from naval logbooks this paper draws on a newly developed source of data in the form of ships logbooks to cast a detailed light on the weather and climate of a critical period in recent climatic history the decades of the 1680s and 1690s possibly the coldest of the last 1000 years logbooks of the period provide daily data for wind force direction precipitation and general weather conditions these were abstracted and aggregated into monthly statistics these data indicate the highly episodic character of air flow patterns at this time with marked changes of phase between high and low zonality in the westerlies the data also provide evidence of the stormy character of the period particularly with respect to summer conditions comparisons are made with contemporary and independently derived instrumental and proxy series the data source is confirmed as providing a valuable addition to climatic sources and is additionally important because it is based on regular daily observations and made at sea an area very poorly r

a holocene paleosecular variation record from 14c dated volcanic rocks in western north america 1 a paleosecular variation psv curve for western north america is presented on the basis of 94 virtual geomagnetic poles vgps from dated volcanic rocks sampled at 446 sites approximately 60 of the paleomagnetic database has been previously published a curve defined by spherical smoothed splines is fitted to the vgps ranked by the quality of the age determinations where the data density is highest between 3690 and 30 years before present b p a d 1950 between 7800 and 7050 years b p and between 14 060 and 12 700 years b p the younger segments of the curve derived from volcanic rocks are similar but less complex than other high resolution psv curves derived from lacustrine sediments particularly the record at fish lake oregon the psv record from lava flows psvl however is perhaps more reliable in its general shape and chronology because of the higher fidelity of volcanic rocks as magnetic field recorders and because of the greater density of 14c dates the new psvl record provides a partial holocene master curve for western north america and will be of particular value in dating geological and archeological materials using paleomagnetic directions