spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

a soft solid surface on titan as revealed by the huygens surface science package the surface of saturn s largest satellite titan is largely obscured by an optically thick atmospheric haze and so its nature has been the subject of considerable speculation and discussion1 the huygens probe entered titan s atmosphere on 14 january 2005 and descended to the surface using a parachute system2 here we report measurements made just above and on the surface of titan by the huygens surface science package3 4 acoustic sounding over the last 90 m above the surface reveals a relatively smooth but not completely flat surface surrounding the landing site penetrometry and accelerometry measurements during the probe impact event reveal that the surface was neither hard like solid ice nor very compressible like a blanket of fluffy aerosol rather the huygens probe landed on a relatively soft solid surface whose properties are analogous to wet clay lightly packed snow and wet or dry sand the probe settled gradually by a few millimetres after landing

paleomagnetic arguments for block rotations along the arakapas fault cyprus paleomagnetic data from the sheeted dike complex north of the arakapas fault reveal significant internal rotations the north south trending dikes have remanent magnetization directions close to the well known westerly paleomagnetic direction of the troodos massif whereas the east west trending dikes near the arakapas fault have northerly remanent magnetization directions the paleomagnetic data are in agreement with the apparent clockwise rotation of the dikes contrary to recent geological analyses which postulate that the pattern of dike trends is due to dike injection into a sigmoidal stress field along a sinistral transform fault the paleomagnetic results indicate clockwise rotations about vertical axes in response to a dextral shear in addition to the rotations around vertical axes small rotations around subhorizontal axes are inferred from the paleomagnetic data for tilted dikes thus these data reinforce the vertical intrusion hypothesis

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

toward an optimal geomagnetic field intensity determination technique 1 paleointensity determinations based on double heating techniques in field zero field cooling zero field in field cooling and two in field steps with opposite laboratory fields are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates to investigate this premise we have developed a simple mathematical model we find that both the zero field first and in field first methods have a strong angular dependence on the laboratory field parallel orthogonal and anti parallel while the two in field steps method is independent of the direction of the laboratory produced field contrary to common practice each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met even with marginal 10 tails of partial thermoremanence our calculations suggest that the zero field first method with the laboratory produced field anti parallel to the natural remanence nrm is the most robust paleointensity determination technique when the intensity of the lab induced field is smaller than ancient field however the zero field first method with the laboratory field parallel to the nrm is the optimum approach when the intensity of the lab induced field is larger than the ancient field by far the best approach however is to alternatethe infield zerofield iz steps with zerofield infield zi steps

noise in the quiet zone abstract we have carried out a detailed paleomagnetic investigation of two stratigraphically overlapping sections from the scaglia bianca formation 85 89 5 ma in the umbria marche area in central italy sampling was conducted over 32 m and 7 m intervals at la roccaccia and furlo respectively after af cleaning the majority of specimens show the expected normal magnetic field orientation however a number of specimens are directionally anomalous some of these deviant specimens are accompanied by apparent spikes or dips in normalized intensity a detailed investigation of rock magnetics shows that most of these deviations are not a sign of excursionary geomagnetic field behavior but rather correspond to specimens with distinct rock magnetic characteristics and are therefore rock magnetic noise such specimens should not be interpreted as records of the geomagnetic field our experience suggests that detailed rock magnetic and magnetic fabric analysis should be done on all anomalous directions prior to interpreting them as geomagnetic field behavior after elimination of rock magnetic noise in the scaglia bianca data sets there is a high degree of agreement in direction and to a lesser extent relative intensity between correlative portions of the two sections we therefore offer this data set as a robust record of geomagnetic field behavior during the 4 5 myr interval represented by the la roccaccia section a statistical analysis of the relative intensity observations suggests that this period of the cretaceous normal superchron is characterized by a normalized variability in paleointensity standard deviation about 28 of the mean value that is significantly lower than seen during the oligocene over intervals in which reversals or tiny wiggles occur typically about 50 the directional stability results in virtual geomagnetic pole dispersion compatible with that found in volcanic rocks from around the same latitude and ranging in age from 80 to 110 ma

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

solubility of the hydrated mg carbonates nesquehonite and dypingite from 5 to 35 c implications for co2 storage and the relative stability of mg carbonates abstract hydrated mg carbonate minerals form during the weathering of ultramafic rocks and can be used to store atmospheric co2 to help combat greenhouse gas fueled climate change optimization of engineered co2 storage and prediction of the composition and stability of mg carbonate phase assemblages in natural and engineered ultramafic environments requires knowledge of the solubility of hydrated mg carbonate phases and the transformation pathways between these metastable phases in this study we evaluate the solubility of nesquehonite mgco3 3h2o and dypingite mg5 co3 4 oh 2 5 or 8 h2o and the transformation from nesquehonite to dypingite between 5 c and 35 c using constant temperature batch reactor experiments the logarithm of the solubility product of nesquehonite was determined to be 5 03 0 13 5 27 0 15 and 5 34 0 04 at 5 c 25 c and 35 c respectively the logarithm of the solubility product of dypingite was determined to be 34 95 0 58 and 36 04 0 31 at 25 c and 35 c respectively with eight waters of hydration this is the first reported dypingite solubility product the transformation from nesquehonite to dypingite was temperature dependent and was complete within 57 days at 25 c and 20 days at 35 c but did not occur during experiments of 59 days at 5 c this phase transformation appeared to occur via a dissolution reprecipitation mechanism external nesquehonite crystal morphology was partially maintained during the phase transformation at 25 c but was eradicated at 35 c together our results facilitate the improved evaluation of mg carbonate mineral precipitation in natural and engineered ultramafic mineral weathering systems that sequester co2 and for the first time allow assessment of the saturation state of dypingite in aqueous solutions

cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows 1 cooling rate effects on the intensity of thermoremanent magnetization trm have been well documented in ceramics in that case laboratory cooling is generally more rapid than the initial cooling leading to an overestimate of the paleofield by 5 10 in thellier type paleointensity experiments the reverse scenario however has never been tested we examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass sbg samples from 13 sites at 17 30 18 30 s on the east pacific rise absolute cooling rates determined by relaxation geospeedometry at five of these sites range from 10 to 330 c min 1 at the glass transition 650 c over the dominant range of remanence blocking temperatures 200 400 c the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the thellier experiment these results suggest that while the cooling rate effect might introduce some within site scatter it should not result in a systematic bias in paleointensity from sbg paleointensity estimates from the 15 sites range from 29 to 59 t with an average standard error of 1 t comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent indistinguishable from present day and the oldest is at least 500 and probably several thousand years old these age estimates are consistent with available radiometric ages and geologic observations

structure of a fossil ridge transform intersection in the troodos ophiolite recent palaeomagnetic studies of sheeted dykes adjacent to the east west trending southern troodos transform fault of cyprus have demonstrated significant clockwise block rotations which are most easily interpreted as being due to drag induced by dextral strike slip faulting along the transform1 6 we show here that such rotations have not occurred along the entire length of the transform instead to the west of the areas previously studied the sheeted dykes have been subjected to simple tilting only indicating normal faulting but no strike slip the changeover between these two domains of differing structural styles near the village of mandria is approximately in line with the centre of the solea graben a north south trending structure within the troodos massif which has previously been interpreted as a fossil ridge axis7 we suggest that the mandria area represents a fossilized ridge transform intersection and present the results of a detailed field investigation of the area together with preliminary palaeomagnetic data to determine the relationship between tectonism and magmatism at such plate boundary zones we use our findings to test a recent theoretical model8 of deformation processes at ridge transform intersections and show that distributed deformation due to block rotation is largely confined to the ridge transform corner itself

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

geomagnetic intensity variations over the past 780 kyr obtained from near seafloor magnetic anomalies knowledge of past variations in the intensity of the earth s magnetic field provides an important constraint on models of the geodynamo a record of absolute palaeointensity for the past 50 kyr has been compiled from archaeomagnetic and volcanic materials and relative palaeointensities over the past 800 kyr have been obtained from sedimentary sequences but a long term record of geomagnetic intensity should also be carried by the thermoremanence of the oceanic crust here we show that near seafloor magnetic anomalies recorded over the southern east pacific rise are well correlated with independent estimates of geomagnetic intensity during the past 780 kyr moreover the pattern of absolute palaeointensity of seafloor glass samples from the same area agrees with the well documented dipole intensity pattern for the past 50 kyr a comparison of palaeointensities derived from seafloor glass samples with global intensity variations thus allows us to estimate the ages of surficial lava flows in this region the record of geomagnetic intensity preserved in the oceanic crust should provide a higher time resolution record of crustal accretion processes at mid ocean ridges than has previously been obtainable

dipole strength and variation of the time averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals 1 single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field while the silicate host limits natural and experimentally induced alteration here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 myr an interval of moderate reversal frequency during the paleogene the nonreversing field of the cretaceous normal polarity superchron and a period of very high reversal occurrence during the late jurassic an inverse relationship between reversal rate and intensity is supported furthermore the strength of the time averaged reversing field is more variable than that of the nonreversing field these observations are consistent with an active lower mantle shaping core mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years

cretaceous geomagnetic paleointensities thellier experiments on pillow lavas and submarine basaltic glass from the ontong java plateau 1 we present new thellier paleointensity results on pillow lavas and pillow rim submarine basaltic glass sbg from 120 ma basalts erupted on the ontong java plateau all pillow lavas have suffered low temperature alteration and are unsuitable for thellier paleointensity experiments in contrast pillow rim sbg is virtually impermeable to seawater and therefore has suffered little or no low temperature alteration out of 21 selected sbg samples with magnetic moment larger than 10 10 am2 18 samples from 12 chilled sbg margins yielded paleointensity estimates of good technical quality rock magnetic measurements show the primary carrier of remanence is low ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite as suggested by a recent study magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400 440 c which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature we suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength for several pillow margins two to three samples were measured and in such cases the data agree well altogether we obtain reliable estimates from 12 sbg pillow rims yielding a mean paleointensity of 27 1 13 4 t and a corresponding virtual axial dipole moment of 5 8 2 8 1022 am2 this value is slightly higher than whole rock data from similar aged subaerial lava flows the paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the cretaceous normal superchron

source of tiny wiggles in chron c5 a comparison of sedimentary relative intensity and marine magnetic anomalies 1 in addition to the well established pattern of polarity reversals short wavelength fluctuations are often present in both sea surface data tiny wiggles and near bottom anomaly data while a high degree of correlation between different geographical regions suggests a geomagnetic origin for some of these wiggles anomaly data alone cannot uniquely determine whether they represent short reversals or paleointensity variations independent evidence from another geomagnetic recording medium such as deep sea sediments is required to determine the true nature of the tiny wiggles we present such independent evidence in the form of sedimentary relative paleointensity from chron c5 we make the first comparison between a sedimentary relative paleointensity record odp site 887 at 54 n 148 w and deep tow marine magnetic anomaly data 43 n 131 w bowers et al 2001 for chron c5 the sediment cores are densely sampled at 2 5 kyr resolution the inclination record shows no evidence for reverse intervals within the 1 myr long normal chron c5n 2n rock magnetic measurements suggest that the primary magnetic carrier is pseudo single domain magnetite we choose a partial anhysteretic magnetization parm as our preferred normalizer and the resulting relative paleointensity record is used as input to a forward model of crustal magnetization we then compare the results of this model with the stacked deep tow anomaly records the two records show a significant degree of correlation suggesting that the tiny wiggles in the marine magnetic anomalies are likely produced by paleointensity variations an analysis of our sampling density suggests that if any reverse intervals exist at this site they are likely to be 5 kyr in duration furthermore we suggest that reverse intervals during chron c5n 2n documented in other locations are unlikely to be global

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the mid atlantic ridge south of the kane fracture zone mark area 1 the remanent magnetization of gabbroic material of the mid atlantic ridge south of the kane fracture zone mark area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment the remanence of these gabbroic samples is often complex with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales tens of centimeters to a few meters moreover several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components given the nominal age 1 ma of the crust we suggest that this pattern of normal reversed normal polarity most plausibly reflects emplacement and or cooling through three successive polarity intervals jaramillo normal polarity interval 1 07 0 99 ma a portion of the matuyama reversed polarity interval 0 99 0 78 ma and the brunhes normal polarity interval 0 78 ma to present a small number of samples with three well defined magnetization components have magnetic characteristics compatible with a remanence carried by fine grained possibly single domain magnetite laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the jaramillo matuyama 0 99 ma and matuyama brunhes 0 78 ma polarity transitions together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as 350 475 c for a minimum of 0 21 m y after emplacement we suggest that continued injection of liquid in the form of sills or small magma bodies over a broad region half width of 3 km is responsible for this slow cooling in addition inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole suggesting that little if any resolvable tilt occurred during uplift of these rocks to the seafloor

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

troodos extrusive series of cyprus a comparison with oceanic crust the extrusive section on the northern flank of the troodos ophiolite is 1 000 to 1 500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia some pillowed units which average 100 to 200 m in thickness vary systematically from lava tubes up to 6 m in diameter at the base through common pillows less than 1 m in diameter to small pillows and extrusive breccias at the top these assemblages appear to be individual volcanoes suggesting episodic volcanic accretion the upper 100 to 200 m of the section is pervasively altered and oxidized but fresh volcanic glass is preserved throughout most of the extrusive section the lavas are subdivided on geochemical evidence into a lower differentiated andesite to dacitic andesite series resembling evolved island arc tholeiites and an upper series of picrite mafic basalt and basaltic andesite very low in tio2 k2o p2o5 nb rb and ba the highly depleted basalt and basaltic andesite resemble lavas from ocean crust near island arcs lateral continuity of lithologic units interpreted as individual volcanoes and the relatively constant dips of the extrusive rocks suggest that tilting and rotation have not played major roles in the accretion process the occurrence of fractionated lavas at the base and primitive lavas at the top suggests a broad zone of accretion located above a subduction zone

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

a view into the cretaceous geomagnetic field from analysis of gabbros and submarine glasses the nature of the geomagnetic field during the cretaceous normal polarity superchron cns has been a matter of debate for several decades numerical geodynamo simulations predict higher intensities but comparable variability during times of few reversals than times with frequent reversals published geomagnetic paleointensity data from the cns are highly scattered suggesting that additional studies are required here we present new paleointensity results from 18 sites collected from the lower oceanic crust of the troodos ophiolite cyprus 92 1 ma old together with recently published data from the troodos upper crust we obtain three independent paleointensity time series these sequences reveal quasi cyclic variations of intensities about a mean value of 54 20 z am 2 providing insight into the fluctuating nature of the cretaceous magnetic field our data suggest the cns field was both weaker and more variable than predicted by geodynamo simulations the large amplitudes of these variations may explain the wide range of dipole moments previously determined from the cns 2007 elsevier b v all rights reserved