the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

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

tectonic evolution of nw iberia during the paleozoic inferred from the geochemical record of detrital rocks in the cantabrian zone the cantabrian zone of nw iberia preserves a voluminous almost continuous sedimentary sequence that ranges in age from ediacaran to early permian its tectonic setting is controversial and recent hypotheses include i passive margin deposition along the northern margin of gondwana ii an active continental margin or iii the margin of a drifting ribbon continent and iv a combination of the three aforementioned possibilities in this paper we present geochemical data from 21 samples taken in detrital rocks in the cantabrian zone from ediacaran to early permian age the results together with previously published detrital zircon ages from these strata allow a comprehensive analysis of changing tectonic scenarios and provenance through time collectively these data indicate that this portion of iberia was an active margin during the ediacaran that evolved to become part of the passive margin of gondwana at least from ordovician to late devonian times changes in geochemistry zircon age distribution and sm nd isotopes during the carboniferous are attributed to the far field effects of the onset of the collision between gondwana and laurussia and related processes such as recycling of older sedimentary sequences as well as the involvement of the rocks formed during the variscan orogeny and oroclinal buckling latest carboniferous and permian show a more juvenile sm nd and higher values of illite cristallinity that may support the hypothesis of lithospheric foundering after oroclinal buckling

a rock and paleomagnetic study of a holocene lava flow in central mexico abstract magnetic measurements of the tres cruces lava flow ca 8500 years bp central mexico show the presence of two remanence carriers a ti rich titanomagnetite with a curie temperature between 350 and 400 c and a ti poor magnetite with a curie temperature close to 580 c magnetic changes after heating indicate that the titanomagnetite exsolves into magnetite and ilmenite when the sample is heated to 580 c paleointensity estimates with the thellier and thellier method thellier e thellier o 1959 sur l intensite du champ magnetique terrestre dans le passe historique et geologique ann geophysique 15 285 376 were only successful up to temperatures of 350 to 400 c this temperature corresponds with the curie temperature of the titanomagnetite which is probably pseudo single or multi domain therefore the paleointensities should be interpreted with caution the magnetic composition changes after 580 c heating may explain the large variations in previous paleointensity determinations for the tres cruces rocks gonzalez s sherwood g bohnel h schnepp e 1997 palaeosecular variation in central mexico over the last 30 000 years the record from lavas geophys j int 130 201 219 using the shaw method shaw j 1974 a new method of determining the magnitude of the palaeomagnetic field application to five historic lavas and five archaeological samples geophys j r astr soc 39 133 141

investigating the reasons for the failure of palaeointensity experiments a study on historical lava flows from mt etna italy summary a refined palaeointensity experiment accompanied by rock magnetic studies has been carried out on six lava flows from 1910 and 1928 from mt etna the purpose of the study was to try to understand why these very young basaltic flows are generally unable to provide a correct estimate of the magnitude of the ambient magnetic field during flow cooling susceptibility versus temperature curves and ore microscopic studies show that 3 types of magnetic minerals phases h m and l are present in these flows some samples containing a single largely dominant magnetic phase while others contain a mixture of several phases phase h i s a thermally stable near magnetite phase resulting from titanomagnetite oxyexsolution phase l is a thermally stable titanomagnetite with a curie temperature of approximately 200 c phase m is a titanomagnetite phase of curie temperature between 450 490 c which is unstable at temperatures above 400 c in addition to the usual reliability checks of the thellier method nrm trm linearity ptrm checks our palaeointensity experiments included additional heating allowing determination of the md or psd sd character of each ptrm and determination of crm or transdomain remanences possibly acquired during heating from the 28 samples studied 20 provide a linear nrm trm plot over about 1 4 or more of total nrm however only six of them all containing near magnetite as a single phase display positive ptrm checks nevertheless these six samples yield a mean palaeointensity of about 52 t which exceeds the real field palaeomagnitude 42 t by some 25 per cent the reasons for this almost total failure of palaeointensity experiments are diverse for samples with a dominant l phase ptrms present a behaviour typical of large md grains with as much as 1 3 of remanence with unblocking temperatures exceeding the blocking range no crm is acquired yet a remanence does develop during heating in a field followed by cooling in zero field we suggest that this remanence is a transdomain remanence resulting from domain rearrangements these two observations are in conflict with some of the basic requirements of the thellier method phase m seems chemically stable up to 400 c but ptrm changes start at lower temperatures this thermal instability and the development of a significant transdomain remanence seem to be the causes of the failure of palaeointensity experiments the reason for the rather large and quite unexpected error in the average palaeointensity provided by the samples containing only a near magnetite phase h may lie in the fact that the low to medium temperature ptrms which represent a significant fraction of the total trm seem to be carried by small md particles in conclusion several modifications of the thellier method are proposed

sedimentary evolution of a palaeozoic basin and ridge system the middle and upper devonian of the ahnet and mouydir algerian sahara the ahnet and mouydir regions of southern algeria are part of one of the world s largest almost undeformed exposures of palaeozoic rocks which exemplify a hitherto poorly known early variscan development of a devonian basin and ridge system this area includes a series of intracratonic basins along the northern margin of the west african craton which consists from w to e of the reggane basin azel matti ridge ahnet basin foum belrem ridge and mouydir basin the depositional and palaeogeographic interpretation is based on 71 sections in this region which for the first time were biostratigraphically calibrated by means of conodonts goniatites and brachiopods the structural evolution during devonian times was probably controlled by reactivation of ancient n s to nw se running faults in the precambrian basement which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf a hiatus during emsian times indicates widespread emergence during this interval the entire area was flooded during the earliest eifelian when the first vestiges of the azel matti ridge become evident by stratigraphic condensation the palaeogeographic differentiation is most apparent during the givetian when a shoal with reduced carbonate sedimentation was established on the azel matti ridge passing towards the west and east into basinal environments of the reggane and ahnet basins respectively the foum belrem ridge is distinguished by increased subsidence during the early givetian and by revived uplift during the late givetian in the mouydir basin further east up to 1000 m of shales were deposited during the givetian the early frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites these lithologies occur repeatedly already during the middle devonian and document intermittent anoxic conditions the basin and ridge topography is levelled by the shallowing up sequence of up to 1400 m thick upper frasnian and famennian shales which grade into a deltaic sequence of uppermost famennian tournaisian sandstones the up to now only vaguely discriminated lithostratigraphic formations of the devonian have been biostratigraphically defined in suitable type sections

a reliable absolute palaeointensity determination obtained from a non ideal recorder reliable measurements of the intensity of the palaeomagnetic field are notoriously difficult to obtain the approach generally taken is to produce multiple estimates per rock unit and assume that those which meet certain minimum standards of technical quality will combine to produce an accurate mean however here using results produced from a package of 20th century basaltic lava flows from mount etna in sicily we demonstrate that this approach can fail in this case the application of typical sets of selection criteria actually introduces bias into the mean determination that we measure we demonstrate that this is caused by two types of non ideal behaviour acting in combination the first is a result of the multidomain grains that the samples contain and the second is caused by differences in the natural and laboratory cooling rates we discuss means of avoiding these sources of error in future palaeointensity studies performed on ancient rocks we also develop a new more general reliability criterion which is effective here and which we argue should be applied wherever possible in future palaeointensity studies in conjunction with standard criteria it requires two distinct types of materials which both produce some good quality palaeointensity measurements and uses their range of overlap to constrain the true palaeointensity both the application of this criterion and general reliability considerations require that future palaeointensity studies should measure many samples per cooling unit and that these should be as diverse in terms of their rock magnetic properties and cooling histories as possible

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

burial of terrestrial organic matter in marine sediments a re assessment 1 calculations based on recent observations indicate that approximately one third of the organic matter presently being buried in marine sediments may be of terrestrial origin with the majority of this terrestrial organic matter tom burial occurring in muddy deltaic sediments these calculations further suggest that the remineralization of terrestrial organic matter in the oceans is also much less efficient than that of marine organic matter these two underappreciated observations have important implications in terms of our understanding of the controls on the global carbon cycle from a paleoceanographic perspective the results presented here also suggest that changes in tom burial on glacial interglacial timescales have the potential to impact the global carbon cycle i e atmospheric co2 levels

paleointensity determination magnetic domain structure and selection criteria this special section is focused on the impact of kittel s theory of magnetic domains on rock magnetism and this paper is focused on the implications for paleointensity determinations absolute paleointensity determination is based on specific properties of thermoremanent magnetizations trms mainly the linear dependence of a trm s intensity on the inducing field and the additivity and independence of partial trms these properties are exhibited by populations of noninteracting single domain sd grains but not by populations of multidomain grains the situation is more complex in the pseudo single domain population the most common one in natural rocks usable for paleointensity studies which sometimes exhibit sd like behavior but not always if the experiments were restricted to sd rocks it would not be possible to find enough outcrops to have the spatial and temporal resolution needed for interpretation of the data in terms of geodynamo however as paleointensity determination is very time consuming a preselection of suitable samples is necessary a new procedure will be proposed using a direct test of partial trm s properties with a fully automated experiment fast to implement

a new quality check for absolute palaeointensities of the earth magnetic field absolute palaeointensity determinations of recent lavas from mt etna have been carried out using the koenigsberger thellier thellier ktt method but with two different data presentations the first which is the standard method compares the natural remanent magnetization nrm demagnetized and the thermoremanent magnetization trm acquired in successive temperature intervals cumulatively the second compares the nrm and trm in the successive intervals in an incremental manner the latter method appears to distinguish aberrant behaviour better than the classical method such aberrant behaviour can be related to changes in hysteresis ratio parameters saturation remanent magnetization to saturation magnetization and remanent coercive force to coercive force which are reflected by displacements in the day et al 1977 plot these changes in hysteresis parameters are caused by irreversible changes in domain state or transformations of magnetic phases brought about by the thermal cycling the changes in hysteresis parameters serve as a useful quality factor for absolute palaeointensity determinations

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

palaeointensity just at the onset of the cretaceous normal superchron abstract a combined palaeomagnetic and geochronologic study has been conducted on a basaltic lava sequence of five flows at jianchang in liaoning province northeastern china radiometric 40 ar 39 ar dating indicates that the volcanism occurred at about 119 ma within the marine anomaly c34n in cretaceous normal superchron cns rock magnetic investigations show that pseudo single domain psd or a mixture of single domain and multi domain grain titanium poor titanomagnetite is dominant in the studied lava flows both stepwise thermal and alternating field demagnetizations isolate the well defined normal characteristic remanent magnetization chrm in five independent lava flows with a mean direction of d i 11 1 57 2 95 5 7 palaeointensity was determined using the coe modified thellier method with systematic partial thermoremanent magnetization ptrm checks all together 61 samples from 286 samples with positive ptrm and tail checks yield palaeointensities ranging between 10 1 and 73 6 t to further check the effects of thermal alteration on the palaeointensity results a new ratio m rs 480 c m rs 25 c where m rs 25 c and m rs 480 c are the room temperature saturation isothermal remanent magnetizations for the raw sample and the product after a 480 c thermal treatment respectively is proposed results show that the ratio changes significantly even for those samples with positive ptrm check we further put forward that palaeointensity results are acceptable with ratio between 0 9 and 1 1 with this new criteria 17 samples show consistent palaeointensity results with an average of 20 0 3 2 t this value corresponds to the virtual dipole magnetic moments vdm of 3 5 0 6 10 22 am 2 which is about half of the value of present field this finding combined with other available results suggests that magnetic field of the earth just at the onset of the cns is characterized by a weak palaeointensity

analysis of an updated paleointensity database q pi pint for 65 200 ma implications for the long term history of dipole moment through the mesozoic

culture based investigations of key microbial functional groups in gas hydrate bearing sediments of the krishna godavari basin in offshore india abstract results of national gas hydrate program 02 nghp 02 expedition have confirmed the resource potential of sand hosted gas hydrate accumulations in the deep offshore of krishna godavari basin kg basin nghp 02 has identified two types of gas hydrate accumulations one is thick layer type units developed in sand rich facies at depths of a few 100 m below sea floor mbsf and the other fracture type unit of variable thickness at shallow depths fracture type and layer type gas hydrates were observed in areas b c and e in the kg basin as defined under nghp 02 the importance of each type of gas hydrate occurrence as discovered during nghp 02 has led to the need for microbial profiling different classes of microbes anaerobic fermentative microbes afm methanotrophs and sulphate reducing bacteria srb were studied to understand the microbial system for biogenic gas generation in sedimentary sequences encountered in eight core holes 4 in area b 3 in area c and 1 in area e as established during nghp 02 this study reveals the availability of highest concentration of methane generating microbes that are closely associated with high total organic carbon toc content in the identified gas hydrate bearing sedimentary units cored in each borehole the presence of microbial consortia for methane generation through both processes of reduction of co2 and fermentation of formate acetate has been confirmed stratigraphic correlation between drill sites have also been generated to better understand the spatial relationship of various microbial processes an observation from the study is that the entire late miocene age sediments has potential to generate biogenic gas in holes nghp 02 05b 17b 22b and 23b late to early pleistocene age sediments are showing methane generating activity in holes nghp 02 01c 08b and 09b while early pleistocene to late pliocene age sediments contains methane generating microbes in holes nghp 02 05b 17b 19b 22b and 23b the existence of methane generating biogenic microbial system in areas b c and e of kg basin has been confirmed

geomagnetic intensity and inclination variations at hawaii for the past 98 kyr from core soh 4 big island a new study and a comparison with existing contemporary data abstract reliable thellier thellier paleointensity determinations were obtained from 321 out of 766 examined samples from the upper 200 units 675 m of core soh 4 drilled in the kilauea volcano the main trend of the soh 4 record is a marked decrease from values higher than present in the last 4 kyr to values about 40 smaller from about 20 kyr to the bottom of the sampled interval three main lows coinciding with shallow or negative inclinations observed in the record were identified with an excursion previously documented in hawaii and the mono lake and laschamps events for the last 15 kyr the soh 4 intensities are largely in agreement with the worldwide distribution of volcanic data in the 15 75 kyr interval the general lack of volcanic data makes comparison with the soh 4 data difficult the soh 4 record is similar to a composite record of paleointensity largely based on a recent north atlantic sedimentary paleointensity stack napis 75 the soh 4 record is also generally consistent with a record of geomagnetic field intensity obtained from a study of 36 cl in the grip ice core calculated with the assumption that the production rate of 36 cl is controlled only by the geomagnetic field one pronounced discrepancy between the different records is observed around 50 kyr bp when the sedimentary and cosmogenic based records document high intensities which are not observed in soh 4 these high intensities are also absent in a recent record based on the analyses of sea floor magnetic anomalies the histogram of vadms obtained from soh 4 is different from that obtained from the sedimentary or 36 cl based records and is more skewed than the distribution given by kono and tanaka for the last 2 5 million years

a comparison of a quasi perpendicular method of absolute palaeointensity determination with other thermal and microwave techniques abstract we describe test and explain the relative effectiveness of six thermal and microwave techniques of absolute palaeointensity determination applied to lava samples from mount etna sicily that acquired their thermoremanence trm in a known geomagnetic field the application of a single alternating field demagnetisation treatment prior to each measurement in a standard thellier experiment is demonstrated to be beneficial increasing the quality of the results and making the analysis simpler a new quasi perpendicular qp method dramatically improves the linearity of arai plots by reducing or altogether eliminating non ideal effects due to multidomain md grains two broad types of sample were encountered by this study and the qp method is the only one capable of producing accurate measurements from them both its ease of use speed and increased reliability should allow it to replace standard double heating methods in studies such as these where laboratory alteration can be detected by other means than ptrm checks the results of experiments performed using a microwave palaeointensity system provide further proof that the microwave de magnetisation process is equivalent to conventional thermal excitation they also demonstrate that microwave experiments are subject to the same risks as conventional experiments when performed on samples containing md grains the behaviour of the samples in both the thermal and microwave experiments is in good agreement with a recently proposed model of md trm which is based on first order symmetry of the remagnetisation process

magnetic hysteresis monitoring of cretaceous submarine basaltic glass during thellier paleointensity experiments evidence for alteration and attendant low field bias on the basis of studies of holocene samples submarine basaltic glass sbg is thought to be an ideal paleointensity recorder because it contains unaltered single domain magnetic inclusions that yield thellier paleointensity data of exceptional quality to be useful as a recorder of the long term geomagnetic field older sbg must retain these optimal properties here we examine this issue through rock magnetic and transmission electron microscope tem analyses of cretaceous sbg recovered at ocean drilling program site 1203 northwestern pacific ocean these sbg samples have very low natural remanent magnetization intensities nrm 2 g and tem analyses indicate a correspondingly low concentration of crystalline inclusions thellier experiments on samples with the strongest nrm intensity 5 10 11 am 2 show a rapid acquisition of thermoremanent magnetization trm with respect to nrm demagnetization taken at face value this behavior implies magnetization in a very weak 17 t ambient field but monitoring of magnetic hysteresis properties during the thellier experiments on subsamples of the sbg samples used for paleointensity determinations indicates systematic variations in values over the same temperature range where the rapid trm acquisition is observed a similar change in properties during heating is observed on monitor sbg specimens using low temperature data with progressive heatings the verwey transition becomes more distinct we suggest that these experimental data record the partial melting and neocrystallization of magnetic grains in sbg during the thermal treatments required by the thellier method resulting in paleointensity values biased to low values we further propose that this process is pronounced in cretaceous and jurassic sbg relative to holocene sbg because devitrification on geologic time scales i e tens of millions of years lowers the transition temperature at which the neocrystallization can commence magnetic hysteresis monitoring may provide a straightforward means of detecting the formation of new magnetic inclusions in sbg during thellier experiments

experimental reassessment of the shaw paleointensity method using laboratory induced thermal remanent magnetization 1 artificial thermoremanent magnetization imposed on natural basalt samples was used to check the reliability of shaw s palaeointensity method shaw 1974 kono 1978 rolph and shaw 1985 and to give an indication of the accuracy of paleointensity determinations first thermal remanence was induced in all samples 50 in total in a known field 50 0 1 t this remanence was regarded as the original natural remanent magnetization nrm the samples were divided in to three subsets a b and c and intensity experiments were performed each subset of samples used different thermal remanent magnetization trm acquisition parameters rock magnetic properties on 14 representative subsamples were also measured both before and after the trm acquisition steps to monitor possible magnetochemical alteration it has been demonstrated that intensity estimates by the shaw method give values close to the expected value 50 5 t in all three sets of basalt samples the modified method with rolph and shaw 1985 correction i e using ratios of anhysteretic remanent magnetization given before the trm step arm1 and after arm2 is proved to be a powerful approach to correct the trm capacity changes that result from magnetochemical alteration this is especially so for sample set c where the samples were heated up to 700 c and then were held at this temperature for 40 min during trm acquisition after correction 45 of set c samples gave acceptable results however some samples with strong magnetochemical alteration sometimes invalidated the arm1 arm2 ratio correction and gave wrong intensity estimates monitoring rock magnetic properties is therefore very important when applying the shaw method

phanerozoic oolitic ironstones geologic record and facies model oolitic ironstonesl composed predominantly of ooidsl of ferric oxide and chamosite accumulated throughout much of phanerozoic time figure 1 and some of them have been important ore deposits they are built of two very common products of weathering iron oxide and clay but they constitute a distinctive and subordinate sedimentary facies for which there is no simple modem analogue as a result considerable disagreement has attended more than 125 years of discussion about their origin early analyses of these rocks for example the minette are of france focused mainly on their chemical and mineral composition and on processes of diagenesis more recent work has described their sedimentary textures and structures as well as their general facies and depositional environments ad eleye 1973 brookfield 1973 corbin 1980 hallam 1975 karpov et a11967

an evaluation of the influence of the experimental cooling rate along with other thermomagnetic effects to explain anomalously low palaeointensities obtained for historic lavas of mt etna italy methodological aspects in obtaining reliable absolute palaeointensity estimates have attracted renewed attention in recent years obtaining a reliable palaeointensity from lavas however still is notoriously difficult in many cases lavas have been shown to be a non ideal recorder of the palaeofield here we assess the influence of various thermomagnetic effects on the outcome of palaeointensity experiments on a suite of recent lavas from mt etna several palaeointensity studies have targeted recent flows of mt etna before often palaeointensity underestimates were found we apply both thellier and multispecimen style palaeointensity experiments and obtain estimates generally 20 35 per cent below the expected reference value in line with previous studies we explored subtle chemical alteration viscous magnetic behaviour and differences between the natural and laboratory cooling rates used during the palaeointensity experiments as possible causes for the observed underestimates of the palaeofield for the latter a furnace with controlled programmable cooling rates was constructed it can cool samples very slowly with rates that are in the same order of magnitude as natural cooling rates typical of mt etna lava flows 10 k day both viscous magnetic behaviour and differences in cooling rate appear to influence the outcome of the palaeointensity experiments however the magnitude of the changes observed is not sufficient to explain the observed underestimates we therefore hypothesize that the observed underestimates may be primarily explained by transdomain processes occurring during the heating steps of the palaeointensity experiments

seismic characterization of hydrates in faulted fine grained sediments of krishna godavari basin full waveform inversion in fine grained faulted sediments both stratigraphic and fault induced structural variations can simultaneously determine the gas hydrate distribution insights into hydrate distribution can be obtained from p wave velocity vp and attenuation qp 1 character of the gas hydrate stability zone ghsz in this paper we apply frequency domain full waveform inversion fwi to surface towed 2d multichannel seismic data from the krishna godavari kg basin india to image the fine scale 100 x 30 m vp and qp 1 variations within the ghsz we validate the inverted vp model by reconciling it with a sonic log from a nearby 250 m well the vp model shows a patchy distribution of hydrate away from the faults dominated parts of the profile hydrates demonstrate stratigraphic control which appears to be permeability driven the qp 1 model suggests that attenuation is relatively suppressed in hydratesbearing sediments elevated attenuation in non hydrate bearing sediments could be driven by the apparent pore fluid immiscibility at seismic wavelengths the vp and the qp 1 models also suggest that fault zones within the ghsz can be hydrate or free gas rich depending on the relative supply of free gas and water from below the ghsz

in search of high fidelity geomagnetic paleointensities a comparison of single plagioclase crystal and whole rock thellier thellier analyses the strength of earth s magnetic field on timescales greater than a million years is known primarily from thellier thellier analyses of igneous rocks in the thellier thellier method the natural remanent magnetization nrm is compared with a thermoremanent magnetization trm imparted during heating in a known laboratory field unfortunately the multiple heating steps required by the method often result in alteration of magnetic minerals in clay bearing groundmass of whole rock samples this alteration may result in the creation of new magnetic minerals that could affect trm acquisition analyses of single plagioclase crystals which contain magnetic inclusions that retain paleofield values may offer a way to retrieve paleointensity data that is less affected by alteration we test this hypothesis through a study of whole rocks and single plagioclase crystals from a mid cretaceous 113 115 ma basalt flow of the rajmahal traps northeastern india magnetic hysteresis and thermal demagnetization data from plagioclase crystals and whole rock samples are similar suggesting a common magnetic mineralogy however hysteresis data from heated whole rock samples indicate that a fine grained magnetic phase has formed this effect was either absent or much less pronounced in the single plagioclase crystals tested paleointensity data from these crystals suggest a paleofield value of 65 1 5 3 t n 15 data from whole rock samples overlap these values but are on average lower we attribute the difference in this study to the preferential growth of new magnetic minerals in the whole rock samples resulting in an enhanced acquisition of trm and shallowing of the slope of nrm lost versus trm gained curves these data suggest that subtle alteration effects may bias thellier thellier results from some whole rock samples toward low paleofield values in these cases the single plagioclase crystal approach may yield more reliable estimates of geomagnetic paleointensity

metallogenesis of cratonic oolitic ironstone deposits in the bled el mass azzel matti ahnet and mouydir basins central sahara algeria the bled el mass azzel matti ahnet and mouydir areas are located in the northwest of the touareg shield central sahara algeria within the devonian sedimentary formations nine oolitic ironstone occurrences of exid type extensive ironstone deposition are interbedded

sedimentologic framework of late devonian oolitic iron formation shatti valley west central libya abstract late devonian paralic to shallow marine shatti formation exposed along the northern margin of murzuk basin in west central libya is in the upper part of an essentially continuous paleozoic detrital succession iron bearing shatti deposits a maximum of 90 m thick comprise five coarsening upward regressive sequences that record intermittent detrital influx along a low energy coast quartz arenite sandbodies include long breaker bars on a chenier coast thin storm generated littoral sheet sands wave dominated delta front sands thin crevasse splays and minor deeply scoured channel fill processes and products along the shatti shoreline ranged from those characteristic of the low energy shelf of southwestern louisiana to those of fluvial wave interaction on the rhone delta shatti seq ences are marked by the absence of large well winnowed shoreface sandbodies and other evidences of major transgressive nearshore sedimentation and also of any associated platform carbonate facies as many as 10 beds of ferric oxide chamosite ooids are present in the iron bearing section most of the thicker oolitic ironstones overlie coarsening upward deltaic sequences where they accumulated during episodes of delta abandonment some interfinger laterally with sheeted shoreline sandbodies the ooids were formed on low energy mudflats and then concentrated in thick nearshore bars by local currents interruptions in accretion produced burrowed ferruginous horizons when building ceased the ooid bars were deeply penetrated by large vertical burrowers commonly an ooid bar was succeeded by or graded laterally into a thin ferruginous phosphatic intraclast lag deposit that developed when relatively higher coastal energy of storms or renewed transgression winnowed the sediment star ed mudflats then the oolite and intraformation conglomerate were buried under marine mud that accumulated during the succeeding transgression

an integrated rock magnetic approach to the selection or rejection of ancient basalt samples for palaeointensity experiments abstract the rock magnetic properties of basalts are often useful indicators of suitability for use in palaeointensity experiments the normal approach to sample selection is based on the use of a limited amount of rock magnetic data possibly from only a single rock magnetic technique this approach may result in the exclusion of otherwise suitable samples or the inclusion of unsuitable samples this study stresses the importance of analysing results from a number of rock magnetic techniques before selecting or rejecting samples for use in palaeointensity experiments a new integrated rock magnetic method of sample selection or rejection is proposed which uses the combined results from thermal thermomagnetic and high temperature susceptibility and non thermal low temperature susceptibility hysteresis and alternating field demagnetisation rock magnetic techniques to define sets of rock magnetic behaviour for precambrian basalts with a mixed grain size of pure magnetite the behaviour of all samples in each set during both thellier and shaw palaeointensity experiments is categorised allowing a set of selection or rejection criteria to be defined this new approach to sample selection or rejection for palaeointensity experiments has value for basalts of all ages provided the sample collection is large enough and all available rock magnetic techniques are used

a change from warm humid to cold dry climate conditions recorded in lower barremian clay dominated continental successions from the se iberian chain ne spain abstract a combined facies and clay mineralogy analysis of the continental succession recorded in the lower part of the blesa formation early barremian oliete subbasin ne spain allowed us to specify the palaeoclimatic and palaeoenvironmental conditions under which this sedimentary record originated six outcrops including clays clays marls and dm thick interbedded palustrine lacustrine limestone levels were logged and sampled for x ray diffraction and optical and electron microscopy analysis the lower levels consist of red clays with abundant ferruginous pisoids 1 5 mm in size and macropisoids 5 mm whereas the intermediate and upper levels are ochre and violet clays marls with few or no pisoids the pisoids normally consist of a nucleus and a cortex made up of several layers some pisoids have a fragment of a previous pisoid acting as a nucleus indicating local re working processes both the nucleus and the cortex are formed by hematite goethite variable contents of kaolinite and minor proportions of ilmenite their texture and mineralogy show an in situ growth interbedded limestone beds within the clay and clay marl dominated levels are formed by micrite microsparite and sparite these limestones are cemented by kaolinite and they also have pisoids indicating the occurrence of edaphic processes the red clays have the lowest calcite content and the highest hematite anatase diaspore goethite and kaolinite content by contrast the ochre and violet clays marls have higher illite content smectite is only detected in the ochre clays marls this mineral association is characteristic of lateritic palaeosols formed under warm and humid conditions kaolinite and smectite textures indicate that they are authigenic whereas illite may be of detrital origin the upward decrease in kaolinite hematite anatase diaspore and goethite content along with the increase in quartz content and the presence of smectite and illite reflect a change from warm and humid conditions to drier and colder conditions during the early barremian in the studied area

magnetic field intensity study of the 1960 kilauea lava flow hawaii using the microwave palaeointensity technique summary it is extremely valuable to study historic lava flows where the geomagnetic field at their time of extrusion is well known in this study two vertical sections 16 m apart have been sampled from the approximately 1 m thick 1960 kilauea lava flow hawaii variations are seen in the rock magnetic and palaeomagnetic properties between and within the two sections indicating that there are small scale lateral and vertical variations in the lava flow the two sections showed dierent responses to microwave palaeointensity analysis section h6001 generally gave ideal linear behaviour on plots of natural remanent magnetization nrm lost against microwave induced thermoremanent magnetization t m rm gained whilst the majority of samples from h6002 showed anomalous two slope behaviour when all plots were interpreted by taking the best fitting line through all points the flow mean intensity for h6001 was 31 6 3 6 mt and that for h6002 was 37 1 6 4 mt compared with the expected intensity of 36 mt additional historic flows need to be studied in order to ascertain whether this behaviour is typical of all lava and whether it is best to always interpret nrm lost t m rm gained plots by taking the line of best fit regardless of shape

bulk magnetic domain stability controls paleointensity fidelity nonideal nonsingle domain magnetic grains are ubiquitous in rocks however they can have a detrimental impact on the fidelity of paleomagnetic records in particular the determination of ancient magnetic field strength paleointensity a key means of understanding the evolution of the earliest geodynamo and the formation of the solar system as a consequence great effort has been expended to link rock magnetic behavior to paleointensity results but with little quantitative success using the most comprehensive rock magnetic and paleointensity data compilations we quantify a stability trend in hysteresis data that characterizes the bulk domain stability bds of the magnetic carriers in a paleomagnetic specimen this trend is evident in both geological and archeological materials that are typically used to obtain paleointensity data and is therefore pervasive throughout most paleomagnetic studies comparing this trend to paleointensity data from both laboratory and historical experiments reveals a quantitative relationship between bds and paleointensity behavior specimens that have lower bds values display higher curvature on the paleointensity analysis plot which leads to more inaccurate results in field quantification of bds therefore reflects low field bulk remanence stability rapid hysteresis measurements can be used to provide a powerful quantitative method for preselecting paleointensity specimens and postanalyzing previous studies further improving our ability to select high fidelity recordings of ancient magnetic fields bds analyses will enhance our ability to understand the evolution of the geodynamo and can help in understanding many fundamental earth and planetary science questions that remain shrouded in controversy

les mineralisations ferriferes du sahara algerien le gisement de fer oolithique de mecheri abdelaziz bassin de tindouf le gisement de fer oolithique de mecheri abdelaziz famennien terminal du bassin de tindouf presente des caracteres morphologiques et sedimentologiques qui permettent de penser que la mineralisation s est mise en place dans des edifices deltaiques progradants