invariant and changing transitional field configurations in a sequence of geomagnetic reversals in the past few years the increasing number of detailed records of geomagnetic transitions has provided new and important information about the behaviour of the earth s magnetic field during a reversal surprisingly however no attempt has been made to obtain information about the evolution of the transitional field characteristics over a long period of time we report here the results of a palaeomagnetic study of four different geomagnetic reversals three of which are successive recorded at different stratigraphical levels of two upper tortonian marine clay sections in western crete greece this sequence represents 1 5 myr of geomagnetic field history the results obtained suggest that the process of reversal from either starting polarity may remain invariant for a long period of time then change rather abruptly between successive reversals

paleomagnetic and 40ar 39ar geochronologic data bearing on the structural evolution of the silver peak extensional complex west central nevada the silver peak extensional complex located in the silver peak range of west central nevada is a displacement transfer system linking the furnace creek fish lake valley fault system and transcurrent faults of the central walker lane late neogene northwest directed motion of an upper plate composed of lower paleozoic sedimentary rocks and late tertiary volcanic and volcaniclastic strata exhumed a lower plate assemblage of metamorphic tectonites with proterozoic and mesozoic protoliths paleomagnetic investigation of miocene pliocene pyroclastic and sedimentary rocks of the upper plate and miocene mafic dikes in the lower plate reveals modest horizontal axis tilting northwest side up and vertical axis rotation clockwise within the extensional complex eight to ten samples from each of 123 sites were demagnetized 95 sites yielded interpretable results dual polarity results from one population of mafic dikes in the lower plate assemblage indicate moderate northwest side up tilting declination d 329 inclination i 37 95 4 3 number n 30 sites in situ 95 the confidence limit for the calculated mean direction expressed as an angular radius from the calculated mean direction some dikes yield exclusively normal polarity results that are interpreted to indicate modest clockwise vertical axis rotation d 021 i 57 95 4 3 n 19 sites in situ concurrent with uplift of the lower plate rocks and nine sites yield magnetization directions that are north directed with positive inclinations of moderate steepness similar to an expected miocene field late miocene pyroclastic rocks in the upper plate yield normal polarity magnetizations suggestive of moderate clockwise vertical axis rotation d 032 i 53 95 8 8 n 10 sites the apparent clockwise rotation is unlikely to result from incomplete sampling of the geomagnetic field because the overall dispersion of the vgp virtual geomagnetic pole positions is high for the latitude of the site location middle miocene sedimentary rocks probably were remagnetized shortly after deposition of eight 40ar 39ar determinations from mafic dikes in the lower plate five groundmass concentrates yield saddle shaped age spectra and one separate provided a plateau date of low confidence isochron analysis reveals that all six groundmass concentrates contain excess ar if rapid cooling and ar retention below 250 c are assumed the preferred age estimate for mafic intrusions is provided by isochron dates and suggests emplacement between 12 and 10 5 ma the 40ar 39ar age spectrum data are consistent with existing fission track cooling and k ar isotopic age information from lower plate granitic rocks and indicate rapid cooling of the lower plate assemblage from well above 300 c to 100 c between 13 and 5 ma rapid cooling may explain the overall distribution of paleomagnetic results from lower plate intrusions such that the earliest acquired magnetizations reflect both northwest side up tilt and clockwise rotation and the younger magnetizations reflect northwest side up tilt overall the paleomagnetic data from the silver peak extensional complex are interpreted to suggest that vertical axis rotation of crustal scale blocks associated with displacement transfer in the central walker lane may play an integral part in accommodating strain within a continental displacement transfer system

dune formation on the humid tropical sector of the north queensland coast australia several previous attempts have been made to explain the apparent poor development of coastal dunes in the humid tropics in terms of lack of wind energy failure of sand supply to the shoreline excessive climatic wetness salt crust formation on beaches and the character of tropical back beach vegetation however recent published reports indicate that coastal dune occurrences are more common in the humid tropics than was formerly thought throwing suspicion on the idea that environmental conditions militate against dune formation in these areas as a whole evidence from the humid tropical sector of the north queensland coast suggests that the poor development of dunes in this area primarily reflects poor sediment sorting in the beach and nearshore zone and low wind energy at the shoreline due to the nature of the coastal orientation and physiography in relation to the prevailing southeasterly winds these limiting factors are not unique to humid tropical climates

the use of magnetic field excursions in stratigraphy abstract the use of magnetic field excursions in stratigraphy is difficult primarily because the excursion field is complex and not dominantly dipolar in contrast with a reversal which is a global event an excursion can be evidenced at one location but not another although this does not by itself rule out the use of excursions in stratigraphy it does limit the geographic area over which they may be correlated we recommend somewhat conservatively that excursions can be used to correlate between sedimentary cores separated by angular distances of less than 30 on earth s surface correlation between cores separated by more than 45 should not be attempted

relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

desert loess versus glacial loess quartz silt formation source areas and sediment pathways in the formation of loess deposits abstract pathways are presented which show the proposed sequence of events involved in the formation of loess deposits both hypothetical deposits and those of central china hungary nigeria and tunisia these pathways illustrate the potential role of and inter relationships between a variety of silt generating mechanisms in the formation of loess using these pathways the existence of desert loess is discussed and it is concluded that this depends on how the terms glacial loess and desert loess are applied if desert loess is taken either to signify nonglacial mechanisms of silt production or to characterise the environment from which aeolian entrainment took place then desert loess is a very real and widely distributed deposit however if the term glacial loess is used to refer to the global climatic regime under which the loess deposit has formed rather than the geomorphic mechanisms responsible for silt generation then desert loess would not be an appropriate term for the classification of loess deposits it is suggested that classifying loess deposits with reference to the global climatic regime under which they accumulated would facilitate a greater understanding of the role of environmental conditions at all stages in the formation of loess deposits

a detailed record of paleomagnetic field change from searles lake california 2 the gauss matuyama polarity reversal this new study of the gauss matuyama transition from searles lake california conjoined with other records from the western united states provides interesting insights into the structure of the reversing magnetic field the present study employs improved measurement and data reduction techniques multiple parallel strings of samples and a finer sampling interval than was used in the original study by liddicoat 1982 particularly crucial to this investigation was the use of overprint directions to reconstruct declinations required because the core was rotary drilled the results of this technique were corroborated by employing an independent method that uses anisotropy of magnetic susceptibility to resolve a sediment fabric the fabric facilitated the alignment of core segments the new record reveals that the main swing of the transition occurs over a significantly shorter time span than was found in the original study in addition it brings out several small scale variations that were absent in the old record some of which take the form of relatively rapid jumps in direction that punctuate more steadily varying changes this alternating steady and rapid field change is similar to behavior observed in volcanic records which argues that such behavior is not merely an artifact of episodic volcanism the searles lake record is strongly nonzonal and is defined in the northern hemisphere by a swath of virtual geomagnetic poles vgps stretching from northern eurasia to west africa and to the northwest pacific glen et al 1994 showed that a collection spanning 15 myr of western north american transition and excursion records displays this same pattern indicating that the vgp swath is a persistent feature of the transitional field in addition the compilation reveals that the swath extends into the southern hemisphere outlining a region marked by an absence of poles that is centered on the indian ocean the fact that this pattern is offset from a similar one seen in global compilations suggests that the persistent fields have a significant nondipolar component seven additional records are now available making the western north america data set perhaps the finest regional set of high resolution records consisting of both igneous and sedimentary records the new records which provide an important test of the existence of the vgp pattern strongly support the findings that reveal the presence of persistent long term 15 myr nondipolar transitional fields

long lived transitional states of the geomagnetic field and the two dynamo families palaeomagnetic data from lavas produced at the hawaii and society islands hotspots suggest that one or more specific long lived field configurations recur during successive polarity reversals spanning perhaps several million years the apparent quasistationarity of these states and the relative placement of the two recording sites offer a rare opportunity to explore the fundamental geometrical properties of transitional fields

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

quaternary environmental change in equatorial regions with particular reference to vegetation history a bibliography abstract it was thought for a long time that equatorial regions had remained virtually unchanged at least since the tertiary period it is only from the late tertiary that we can talk of the present disposition of land and sea about the equator our teaching has supposed that these regions were the hearth from which general spread to the remainder of the planet s surface by gradual adaptation and speciation this warm humid zone has been seen as a refuge for species unable to maintain temperature ranges in the face of stresses and fluctuations to north and south the diversity of species in rain forest environments was assumed to reflect their essential stability so indeed it may but not in quite the manner supposed originally by ecologists the variety of species present in the various regions of rain forest are greatly different with african exhibiting a relatively poor list compared with south america this as it turns out may well be a function of disturbance during the last 25 years and gathering momentum within the last decade a variety of studies have demonstrated that the concept of equatorial biological stasis is no longer tenable evidence for equatorial environmental changes is derived from theoretical as well as empirical studies regarding the former various attempts have been made to model the earth s atmosphere at different periods of the pleistocene and there is now fairly good agreement about the consequences for equatorial regions of a general global cooling namely a critical drying at lower elevations the empirical evidence is derived on the one hand from stratigraphic studies of peats and lake muds and also the analysis of ocean cores it is also derived from studies of the geographical ranges of plant and animal species

the character of the field during geomagnetic reversals it is well established that the geomagnetic field which is predominantly dipolar at the earth s surface has alternated between normal and reverse polarity through geologic time figure 1 evidence of this behavior is preserved in the ancient magnetism of rocks the remanence of radio metrically dated lava flows for example shows that the field has reversed at least 23 times over the past 5 million years figure 2 reversals are not periodic it is apparent in figure 2 that lengths of recent polarity intervals range from lo4 to lo6 years and that the intervals follow a very irregular pattern this behavior contrasts sharply with that of the sun s field which switches polarity every 1 1 years paleomagnetic observations discussed in more detail below show that a geomagnetic reversal takes thousands of years to complete what happens to the geomagnetic field during the transitional period does the entire field just rotate from one orientation to the other does the field disappear

long term geometry of the geomagnetic field for the last five million years an updated secular variation database lee 1983 assembled more than two thousand paleomagnetic directional data from lava flows in 65 sampling sites with ages spanning the last 5 million years constable 1992 recently suggested that the virtual geomagnetic poles vgp derived from this data base have been preferentially located within two antipodal bands of longitude we have reanalyzed and updated the lee data base yielding a total of 3179 data from 86 distinct sites neither the total data set nor various subsets of the data unambiguously show any large and clear maxima in the longitude distribution of vgps the shape of the common site longitude distribution is far more striking with a pronounced and robust minimum close to the common site longitude and secondary maxima about 120 o away from it the former first order feature is equivalent to the far sided effect discovered by wilson 1970 and corresponds to a persistent axial quadrupole amounting to about 5 of the axial dipole as previously found by several authors the latter second order feature could correspond to the vgp biasing effect described by egbert 1992 although its observed amplitude appears larger than predicted part of this effect an asymmetry in the maxima may also correspond to wilson s 1971 right handed effect given only a small axial quadrupolar component and the very uneven actual site distribution the expected longitude distribution of vgps can be calculated this is found to be in good agreement with the paleomagnetic observations i e rather flat with a moderate maximum near 120oe this study emphasizes problems related to the unsatisfactory distribution of sites the amplitude and significance of other terms beyond the dominant axial dipole and significant axial quadrupole contribution remain to be assessed

dipolar reversal states of the geomagnetic field and core mantle dynamics palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years these long lived states provide a way to explain directional rebounds aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole the polar orientations correlate with near radial flux concentrations recognizable when today s field is stripped of its axial dipole and with lower mantle seismic anomalies suggesting a tie to deep earth dynamics

ultrahigh resolution marine magnetic anomaly profiles a record of continuous paleointensity variations a distinctive pattern of small scale marine magnetic anomalies 25 100 nt amplitude 8 25 km wavelength tiny wiggles is superimposed on the more generally recognized seafloor spreading pattern between anomalies 24 and 27 in the indian ocean by normalizing and stacking multiple profiles it is demonstrated that this pattern of tiny wiggles is a high resolution recording of paleodipole field behavior between chrons c24 and c27 the pattern of tiny wiggles between anomalies 26 and 27 is compared to an ultrafast spreading 82 mm yr half rate profile from the southeast pacific where a similar signal is observed confirming the paleodipole field origin of the anomalies two basic models are considered in which the tiny wiggles are attributed either to short polarity intervals or to paleointensity fluctuations we conclude that tiny wiggles are most likely caused by paleointensity fluctuations of the dipole field and are a ubiquitous background signal to most fast spreading magnetic profiles the implications of this study are that 1 tiny wiggles may provide information on the temporal evolution of the geomagnetic dynamo 2 the small scale anomalies observed in the jurassic quiet zones may be due to paleointensity fluctuations 3 tiny wiggles are potential time markers in large regions of uniform crustal polarity such as the cretaceous quiet zones and 4 much of the variance in anomaly profiles normally attributed to crustal eraplacement processes particularly at fast and ultrafast spreading rates is actually due to intensity variations in the paleomagnetic field

abrupt post glacial climate events in west asia and north africa monsoon domains regions beyond the present or past penetration of the indian and african monsoons have experienced several large and abrupt climatic fluctuations over the past 13 14c kyr pollen and lake records from west asia western tibet and rajasthan east africa ethiopia and west africa western sahara sahel and subequatorial africa were selected on the basis of chronological control sensitivity of both site and environmental indicators to climate change the continuity of the record and interdisciplinary control of the palaeoclimatic interpretation conditions wetter than those of today prevailed during the early mid holocene period but major dry spells are recorded at all sites during the intervals 11 0 9 5 kyr bp 8 7 kyr bp and 3 4 kyr bp several records also suggest dry events of minor amplitude around 6 kyr bp potential boundary forcings of insolation and sea surface and tropical land surface conditions are discussed the solar radiation accounts for the general envelop of the post glacial monsoon fluctuations but explains neither the timing nor the amplitude of the short term changes in spite of apparent covariation between fluctuations in sea surface conditions in the north atlantic and the monsoon record no direct mechanism could be found relating the intensity of the oceanic thermohaline conveyor belt to the monsoon strength changes in tropical land surface conditions soil moisture negative feedback and changes in ch4 production from wetlands provide a more satisfactory hypothesis for explaining abrupt reversal events

geomorphology water table and soil relationships in holocene beach ridges in southern sierra leone summary the coast of southern sierra leone is fringed by a wave built beach ridge barrier characterized by young weakly developed soils composed of medium to coarse quartz sand these littoral deposits have been more or less extensively modified by geomorphic processes resulting from sea level oscillations and possibly climatic fluctuations soil heterogeneity in these young beach deposits results from spatial and temporal variations in water table level such variations occur in response to geomorphic variables such as topography beach ridge dissection or progradation and swale incision

lower mantle convection and geomagnetism the statistical properties of recent marine magnetic anomaly time scales covering the period from 165 ma to the present are analyzed via the methods presented in the companion paper mcfadden this issue the data no longer support the hypothesis that during the last 86 ma there were two periods separated by a discontinuity at about 45 ma when the properties were stationary instead there is strong evidence that the rate of reversals decreased in an approximately linear manner with time from 165 ma until it reached zero resulting in the cretaceous normal polarity interval at about 86 ma ago the process of reversals began again and the rate of reversals increased linearly with time there is a slight indication that the rate of reversals reached a maximum about 10 ma ago and is again decreasing analysis of the reversal sequences shows there is no good evidence for the concept of differences in the relative stabilities of the two polarity states nor need such a concept be invoked to explain long intervals without reversals such as the cretaceous normal polarity interval quite clearly there has been some form of thermal perturbation at the base of the mantle with a characteristic time of the order of 108 years our interpretations require convection deep within the mantle and suggest that the temperature at the base of the mantle attains a maximum during long magnetic quiet intervals

middle pleistocene magnetostratigraphy and susceptibility stratigraphy data from a carbonate aeolian system mallorca western mediterranean abstract this study shows that successions of pleistocene carbonate aeolian deposits can be placed successfully in a geochronologic framework using magnetostratigraphic and susceptibility stratigraphic analysis supplemented by luminescence dating studies of wave cut platforms and biostratigraphic evidence the investigated aeolian system covers a significant part of southernmost mallorca and is exposed in impressive coastal cliff sections at the study site at els bancals the aeolian system has a maximum thickness of 16 m and is composed of alternating dark red colluvial deposits and greyish red aeolian dune and sand sheet deposits forming seven cyclostratigraphic units each cyclostratigraphic unit represents landscape stabilisation colluviation and soil formation followed by dunefield development when marine carbonate sand was transported far inland by westerly or north westerly winds the aeolian system is located on top of a wave cut marine platform 12 14 m a s l this platform probably formed during a sea level highstand in marine isotope stage mis 11 427 364 ka and renewed marine activity probably later in mis 11 is indicated by the formation of beach deposits two sections at els bancals were sampled for a paleomagnetic study additional samples were taken to detect variations in magnetic susceptibility ms the characteristic remanent magnetisation has been recovered for the most part of the succession in spite of diagenetic overprinting there is evidence for two probably three reversal polarity excursions possible connected to the levantine cr1 and cr0 biwa iii episodes if this correlation is correct the sampled succession represents a time interval in the middle pleistocene between ca 410 and ca 260 ka this age estimate is supported by the ms study and by luminescence dates of 333 70 ka aeolianite from lower part of the succession and 275 23 ka aeolianite from the top of the succession the nature of the succession suggests deposition during alternating warm and moist colluvial deposition soil formation and cold dry and windy conditions dunefield formation the susceptibility signal can be correlated with the insolation signal at 65 n suggesting that environmental variation on mallorca was linked to orbitally forced climate change and it seems that aeolian activity and dunefield formation were linked to glacial or stadial periods

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

precise k ar 40ar 39ar rb sr and u pb mineral ages from the 27 5 ma fish canyon tuff reference standard the accuracy of ages measured using the 40ar 39ar technique is affected by uncertainties in the age of radiation fluence monitor minerals at present there is lack of agreement about the ages of certain minerals used as fluence monitors the accuracy of the age of a standard may be improved if the age can be measured using different decay schemes this has been done by measuring ages on minerals from the oligocene fish canyon tuff fct using the k ar 40ar 39ar rb sr and u pb methods k ar and 40ar 39ar total fusion ages of sanidine biotite and hornblende yielded a mean age of 27 57 0 36 ma the weighted mean 40ar 39ar plateau age of sanidine and biotite is 27 57 0 18 ma a biotite feldspar rb sr isochron yielded an age of 27 44 0 16 ma the u pb data for zircon are complex because of the presence of precambrian zircons and inheritance of radiogenic pb zircons with 207pb 235u 0 4 yielded a discordia line with a lower concordia intercept of 27 52 0 09 ma evaluation of the combined data suggests that the best age for fct is 27 51 ma

the distribution of carbonate eolianite abstract carbonate eolianite forms distinctive coastal landforms on many mid latitude continents and islands numerous studies of extensive continental and smaller island deposits in both hemispheres have shown the eolianites are predominantly pleistocene in age the coorong coastal plain of south australia contains the world s longest record of eolianite deposition revealing that major phases of carbonate accumulation occurred during interglacial and some interstadial sea level highstands throughout the quaternary similarly the well studied eolianite islands of bermuda and the bahamas record a close match between dune accretion and sea level highstands since the middle pleistocene several other eolianite deposits appear to have formed at least partially during glacial periods the glacial age deposits are eolian reworked relict shelf or coastal sediments or remnants of former highly mobile coastal dunes sourced from low sea level shorelines the distribution and bioclastic composition of large scale eolianite deposits suggests carbonate production on shallow shelves and banks is the ultimate control on the formation of eolianite the bulk of the carbonate comprising those eolianites is heterozoan being produced where there is low terrigenous sediment input in dominantly temperate cool water and subtropical carbonate provinces quaternary cyclical movements of sea level have also controlled the form and rate of coastal carbonate deposition with repeated pulses of sedimentation during highstands forming laterally coalesced vertically stacked or composite dune successions major unconformities within the deposits are marked by paleosols which record hiatuses in coastal sedimentation in several regions major eolianite units overlie coastal deposits of terrigenous sediment those successions suggest that the trend towards aridity and reduction in the supply of terrigenous sediment to shelves during the quaternary has enhanced the production of carbonate and the deposition of eolianite

revisit and comparative analysis of the typical graphite deposits in the paleoproterozoic khondalite series western north china craton implications for genesis depositional environment and prospecting potential abstract the phaneritic graphite deposits in the western north china craton can be delineated into an e w trending belt which is consistent with the paleoproterozoic khondalite series to interpret why graphite is not ubiquitous among the paleoproterozoic supracrustal rocks we revisit and comparatively analyze the typical graphite deposits in the paleoproterozoic khondalite series employing sequence stratigraphy chronology tectonic deformation and metamorphism for the khondalite series and carbon isotope analysis for graphite our renewed profile of the western graphite belt wgb reveals the following 1 a greater abundance of organic matter was trapped within the lower part of the khondalite series than in the middle and upper parts consequently all the known deposits are restricted to the lower part of the khondalite series 2 as the protolithic basin evolved from a passive continental margin into an active continental margin subduction related magmatism provided the influx of heat needed to transform the organic matter into amorphous graphite 3 continental collision induced high grade metamorphism and associated magmatism finally reworked the original amorphous graphite into the present day phaneritic graphite deposits

a new geomagnetic polarity time scale for the late cretaceous and cenozoic we have constructed a magnetic polarity time scale for the late cretaceous and cenozoic based on an analysis of marine magnetic profiles from the world s ocean basins this is the first time since heirtzler et al 1968 published their time scale that the relative widths of the magnetic polarity intervals for the entire late cretaceous and cenozoic have been systematically determined from magnetic profiles a composite geomagnetic polarity sequence was derived based primarily on data from the south atlantic anomaly spacings in the south atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles fine scale information was derived from magnetic profiles on faster spreading ridges in the pacific and indian oceans and inserted into the south ariantic sequence based on the assumption that spreading rates in the south atlantic were smoothly varying but not necessarily constant a time scale was generated by using a spline function to fit a set of nine age calibration points

history of earth s magnetic field and possible connections to core mantle boundary processes the core mantle boundary cmb sets boundary conditions for processes occurring within the core thus the history of the geomagnetic field is intimately connected with the history of the cmb information about one can often provide information about the other it is a simple matter to separate sources of external origin from those of internal origin but there is no unique way to specify the location of the internal sources indirect arguments suggest that sources within earth s outer core strongly dominate the field for spherical harmonic terms with degree less than about 14 and that crustal sources play an increasing role in harmonics with higher degrees building on the pioneering studies of roberts and scott and of backus in the 1960s the velocity field of the core fluid at the top of the outer core can be estimated from geomagnetic secular variation data however even with appropriate assumptions about location of the magnetic field source there is a serious nonuniqueness in inversion of the secular variation data to velocity field because magnetic field lines are not changed by movement of conductive fluid parallel to those field lines additional assumptions about the flow are therefore required sometimes leading to quite different estimates in the pattern of fluid flow the resulting velocity field estimates appear to be sensitive to conditions e g small lateral variations in temperature at the cmb consideration of mantle dynamics suggests that large changes in the cmb conditions probably occur on a 10 to 100 m y timescale secular variations with periods shorter than a million years but longer than several years almost certainly originate from processes operating in the outer core unfortunately there is not yet consensus as to what those processes are longer period variations in the paleomagnetic record including nonstationarities in the rates of magnetic field reversals in paleointensities and in paleosecular variation may reflect changes in cmb conditions because these boundary conditions are controlled primarily by mantle dynamics there have been several speculations regarding causal links between changes in earth s lithosphere and changes in earth s magnetic field finally the evidence linking lateral variations at the cmb to perceived systematics in polarity transition data is found to be intriguing but insufficient

isotopic evidence for involvement of c0 2 bearing magmas in granulite formation it has been suggested1 2 that co2 bearing magmas play an important part in the formation of granulites these magmas crystallize directly into granulites and also expel fluids which promote the development of granulite facies mineral assemblages in adjacent country rocks direct evidence for carbonic fluids remains elusive however as granulite facies aureoles adjacent to charnockitic intrusives1 may result either from the influx of carbonic fluids derived from the intrusives or simply from extraction of water into a vapour absent melt here we report results of a detailed study of carbon isotope compositions of graphite associated with a charnockite dyke from the ponmudi quarry of south india higher graphite abundances and an anomalously heavy carbon isotopic composition 13cgr 8 1 0 at the dyke margins indicate that externally derived co2 rich fluids were transported into the ponmudi paragneisses by the intrusive and precipitated nearly quantitatively as graphite on encountering reducing country rocks the occurrence of large orthopyroxene crystals along the dyke margins suggests that these fluids also served to dehydrate the country rocks this dyke may thus be representative of the widespread transport of carbonic fluids in this terrain by felsic magmas

chenier plain development in northern sierra leone west africa abstract the coast of northern sierra leone between the scarcies and mellacoree estuaries forms a prograding muddy holocene plain comprising two separate as yet undated clusters of sand cheniers the general coastal morpho stratigraphy and available data on regional climatic and sealevel history suggest that alternations between muddy progradation and chenier cluster formation have been controlled by a probable conjunction of several factors including the local geomorphic framework a climatically induced change in mud supply and relative sealevel oscillations since the middle holocene the inner chenier cluster was apparently elaborated under conditions of efficient wave action over a deep largely unfilled mellacoree estuary and at a time of reduced mud availability it peters out seaward probably in response to several factors including an increase in mud supply from the upland a negative sealevel pulse resulting in higher rates of mud export from accreted inner tidal flats increased muddy sedimentation over a shallower nearshore zone and attenuated wave action as a result of energy capture by accreted estuarine shoals the outer cluster which partly fringes the coast was formed in a mayor embayment representing the terminus of longshore sand drift it is suggested that its formation may have been favoured by a higher sealevel resulting in efficient winnowing of sands from subtidal muds a greater propensity for accretion of tidal flats landwards and consequently a lower mud supply to the foreshore although the timing of muddy progradation and chenier formation has not been determined the foregoing interpretation suggests a complex sequence of events reflecting as much the influence of local morphodynamic factors as external factors

do virtual geomagnetic poles follow preferred paths during geomagnetic reversals virtual geomagnetic poles vgps recorded in sediments during reversals of the earth s magnetic field show an apparent preference for two antipodal sectors of longitude1 3 not only in records of the same reversal from different sites but also in records of different reversals if preferred bands really have persisted from one reversal to the next this would imply that the mantle exerts a significant control over the reversal process4 here we analyse the available database of reversal records from the past 12 myr using a statistical test specifically designed to test the hypothesis of two preferred antipodal longitudinal bands our analysis shows that the records taken as a group of independent observations do show an overall preference for two antipodal longitudinal bands however the site longitudes are also strongly grouped5 and a comparison of the transitional vgp longitudes with site longitudes shows an unlikely grouping under the hypothesis of a genuine geographical preference for transitional vgps we conclude that it is premature to accept the hypothesis of mantle control over the core during geomagnetic reversals

the steens mountain oregon geomagnetic polarity transition 3 its regional significance study of the variations of direction and intensity of the geomagnetic field as recorded by the miocene lava flows on steens mountain southeastern oregon has resulted in a detailed description of total field behavior during a reversal in polarity in addition to information about the polarity reversal itself the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15 5 ma in order to test the feasibility of using this record as a means of correlation in this part of the western united states comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the steens basalt or rocks of equivalent age despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes convincing similarities among some of the records are evident the steens basalt paleomagnetic record does indeed have potential as a correlation tool during this time of widespread basaltic volcanism additionally paleomagnetic data from flows that were sampled in detail yield a middle miocene paleomagnetic pole at 88 3 n 209 0 95 6 3 for the high lava plains of oregon this pole position is statistically indistinguishable from the earth s rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted data from selected sites within the coeval part of the columbia river basalt group yield a paleomagnetic pole at 88 7 n 171 6 e 95 4 0 the columbia river basalt group pole is statistically indistinguishable from either the rotational axis or from the high lava plains pole these findings indicate no post 20 ma differential rotation between south eastern washington and south central oregon in contrast to previous interpretations

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

paleomagnetism and the nature of the geodynamo records of direct observations of the earth s magnetic field cover less than a ten millionth ofthe known lifetime of the field thus our knowledge of several geomagnetic phenomena critical to our understanding of the geodynamo must come firom the paleomagnetic record a combination of substantial advances during the past decade or so both in dynamo theory previously the domain of the mathematician and in paleomagnetism previously the domain of the geologist has led to provocative models of the earth s magnetic field and a better understanding of the geodynamo

a relationship between dunes fire and climate recorded in the holocene deposits of quebec during the past 5 000 yr bp the tropics have experienced an increase in aridity and aeolian activity in both maritime and continental areas1 4 in contrast the onset and subsequent development of post hypsithermal climates in northern latitudes were primarily conducive to global cooling it is suggested here that aeolian activity in cold environments represents a direct response to this long term cooling trend that began after 6 000 5 000 yr bp at the end of the climatic optimum5 9 this study establishes the holocene chronology of sand dunes near the outer forest limit in northern quebec fig 1 i show here that fires initiate the aeolian activity and that the fire dune relationship is typical of cooling period on this basis a palaeoclimatic reconstruction since the climatic optimum is proposed

reversal records in marine marls and delayed acquisition of remanent magnetization records of geomagnetic reversals frozen into the magnetic components of sediments provide a means to study the time dependent behaviour of the geomagnetic field during polarity transitions although sedimentary records have the advantage of being readily available and continuous the process by which they acquire a remanent magnetization is still not fully understood1 magnetites which lose their magnetization at relatively high temperatures 580 c are generally considered to carry the primary remanence of the sediment that is to record the palaeomagnetic direction here we describe two reversed to normal transitional records from marine marls in which this high temperature component shows a delayed remanence acquisition relative to a lower temperature component in these samples therefore the high temperature component does not reflect geo magnetic changes during the reversal at least for marine marls detailed palaeomagnetic rock magnetic and geochemical studies are apparently necessary to judge the validity of reversal records

anisotropy of magnetic susceptibility data bearing on the transport direction of mid tertiary regional ignimbrites candelaria hills area west central nevada in west central nevada the oligocene candelaria pyroclastic sequence reaches a local thickness of up to 1 3 km in what has been referred to as the candelaria trough but more generally the accumulation of ash flow tuffs and related volcanic rocks is less than 300 m thick complete to near complete outcrops are scattered over about 3200 km2 in the candelaria hills and surrounding ranges of the southern walker lane structural zone three regionally extensive compound cooling units within the overall sequence 25 8 ma metallic city 24 1 ma belleville and 23 7 ma candelaria junction tuffs have distinguishing characteristics and are the focus of study at 106 sites anisotropy of magnetic susceptibility ams data provide an estimate of transport direction of each tuff inferred transport directions based on the ams data are corrected for a modest clockwise yet variable magnitude vertical axis rotation that affected these rocks in late miocene to pliocene time as revealed by paleomagnetic studies the ams data show a somewhat orderly pattern of magnetic fabrics that we interpret to define unique transport directions for the metallic city and candelaria junction tuffs the low susceptibility and degree of anisotropy of the belleville tuff limits our interpretation from this pyroclastic deposit the metallic city and candelaria junction tuffs typically show gentle south southeast and southeast dipping magnetic fabric imbrication respectively and very gently plunging magnetic lineations these ams fabric elements indicate the tuffs were transported to the north northwest and northwest respectively the ams fabric data from the metallic city and candelaria junction tuffs suggest relatively unrestricted flow during emplacement evidence across the 3 200 km2 area to support more regionally controlled channelized flow into and or flow along the east northeast west southwest axis of the candelaria trough is lacking the ignimbrites clearly filled a topographic depression inferred to have formed concurrent with early localized basin and range extension during pyroclastic emplacement but based on the uniformity of ams fabric data we infer that the depression quickly filled and did not hinder flow across the region unrecognized eruptive centers for the three ignimbrites may lie buried beneath neogene basin fill sediments south southeast of the candelaria hills or concealed below younger deposits farther southeast into the palmetto mountains

astronomical calibration age for the oligocene miocene boundary the stratotype section for the base of the miocene is at a reversed below to normal above magnetic transition that is claimed to represent magnetic chron c6cn 2n o deep sea drilling project dsdp site 522 is the only location we are aware of that unambiguously records the three normal events of c6cn we have quantitatively determined the range of the short lived nannofossil sphenolithus delphix and the lower limit of s disbelemnos in dsdp holes 522 and 522a in order to calibrate their precise relationship to the magnetostratigraphy and to confirm the completeness of the record at this site astronomical tuning of ocean drilling program odp sites 926 928 and 929 shows that s disbelemnos appears at 22 67 ma and that the entire range of s delphix is from about 22 98 ma to 23 24 ma using these ages linear interpolation in dsdp site 522 suggests that the age of c6cn 2n o and of the oligocene miocene boundary is 22 92 0 04 ma our value conservatively expressed as 22 9 0 1 ma is 0 9 m y younger than the currently accepted age of the oligocene miocene boundary and of c6cn 2n o which was assigned an age of 23 8 ma based on an estimate of 23 8 1 ma for the oligocene miocene boundary the bulk sediment carbon isotope data from dsdp site 522 is correlated to the record from benthic foraminifera at odp site 929 to refine the calibration of magnetic reversals from c6cn 1n o to c7n 2n o at dsdp site 522 on the astronomical time scale

holocene deltaic sequence in the saloum estuary senegal abstract the 2250 km2 saloum delta south of dakar senegal consists of a holocene sedimentary sequence deposited during five evolutionary stages submerged postglacial ria funnel like estuary barred estuary cuspate delta and reverse estuary the passage from one stage to another was marked by the deposition of sedimentary units consisting of shelly sands silty or sandy clays and an extensive thick green mud muds in the lower strata dating between 6130 and 3500 yr b p were deposited on a subtidal mudflat while fine sands and silty clays characterize intertidal deposits a drastic change in sedimentation patterns resulting in the accumulation of a series of beach barriers likely was associated with a major climatic change the regional climate was then very dry and silt and sand were trapped in the water behind the barriers in two stages from 1990 to 1510 and 1020 to 600 yr b p each event was accompanied by construction of shell middens that demonstrate the presence of humans who exploited newly available shallows and banks relative sea level did not change significantly between 6000 yr b p and the present morphological and sedimentological changes observed in the late holocene sequences were apparently induced by a constraining climatic background

revisions to the age of the brunhes matuyama boundary and the pleistocene geomagnetic polarity timescale the currently referenced age of 730 ka for the brunhes matuyama b m geomagnetic polarity boundary mankinen and dalrymple 1979 is based largely on k ar and paleomagnetic data from postcollapse rhyolites in the valles caldera new mexico doell and dalrymple 1966 doell et al 1968 dating of sanidine from valles rhyolites by the 40a 39ar laser fusion technique indicates that samples critical to defining the b m boundary are up to 90 ka older than previously thought the new data combined with earlier results yield an age for the b m boundary of 780 10 ka 40ar 39ar ages from other samples suggest the jaramillo normal polarity subchron extended from 1010 to 915 ka

persistent features of polarity transition records from western north america we report on a high resolution transition record from searles valley california that appears not to be hampered by problems of smoothing or recording breakdown and unlike many other sedimentary records defines a path unconfined in longitude a prominent feature of the record is the steady oscillation of the pole along a swath stretching from west africa across north eurasia to the northwest pacific the endpoints of the swath are associated with a change between steady and rapid field variation consistent with results for an older transition from a nearby volcanic record the transition culminates with poles migrating from west africa to south america midway between the two earlier proposed longitudinal bands this path indicates the reversing field has either a wider spectrum of behavior than recently suggested or is described by a more complicated field geometry than one dominated by an equatorial dipole or strong low order nondipole component from a comparison with existing records from the same general geographic area we address the significance of the swath the lack of longitudinal confinement patterns of steady and rapid field changes and the temporal persistence of these features in relation to the geometry of the reversing field

active displacement transfer and differential block motion within the central walker lane western great basin velocities determined for 50 global positioning system sites within the central walker lane indicate differential motion among tectonic blocks forming a boundary zone between the great basin extensional province and the sierra nevada the velocity field is related to displacement transfer from the owens valley and furnace creek fault systems of east ern california to transtensional structures of the walker lane and extensional faults of the central nevada seismic belt block boundaries are sharp and appear to be inherited from pre tertiary crustal structure the block geometries exert strong influence on dif ferential displacements concentrated along boundaries as belts of divergent transcurrent and convergent motion the aggregate velocity accounts for about 25 of the relative motion between the pacific and north american plates about 5 mm yr of the motion is localized along the eastern margin of the sierra nevada whereas about 10 mm yr is stepped 100 km east along a belt of east northeast trending transtensional faults that merge with northwest trending transcurrent structures of the walker lane about 6 mm yr of the velocity field is transferred to north northeast trending extensional faults of the central nevada seismic belt the heterogeneous distribution of motions is consistent with partitioning of a regional velocity field formed by westward extension and n408w directed shear

mineralization and wall rock alteration at the bogala graphite deposit bulathkohupitiya sri lanka the graphite mineralization in the bulathkohupitiya area was studied with an emphasis on the bogala deposit vein type mineralization occurs in the zones of deep seated fractures in supracrustal stratiform metasedimentary rocks resulting from the differential rise of subcrustal migmatites the mineralization at bogala comprises several veins of graphite and a large vein of syenite pegmatite mineralogical and geochemical study of the wall rocks reveals a well zoned alteration halo about the deposit chemical zoning in the altered wall rock and the relict mineral replacement textures indicate deep seated hydrothermal activity formation of vein graphite by decarbonation of carbonates and reduction of co 2 formed during the decarbonation reaction is indicated by the available carbon isotope data chemical zoning in the alteration halo may be a useful aid in the exploitation of known graphite deposits

transitional field clusters from uppermost oligocene volcanic rocks in the central walker lane western nevada abstract three sections of the candelaria hills volcanic sequence west central nevada appear to have recorded parts of two transitional field records or reversal excursions paleomagnetic data and 40 ar 39 ar laser fusion sanidine age estimates for pyroclastic rocks and associated flows show that these rocks recorded the unusual field behavior at about 25 7 ma and about 23 8 ma fifteen sites yield northeast declination moderate to shallow negative inclination mean directions and 16 sites yield west to southwest declination moderate negative inclination directions both populations of site mean directions representing a total of 12 independent eruptive units are highly discordant to a time averaged late tertiary field direction and neither can be explained by a geologically reasonable magnitude of vertical axis rotation virtual paleomagnetic poles vgps estimated from the directional data lie at low to intermediate latitudes 29 of the 31 flows at intermediate latitudes

the pan african event in east antarctica a view from sri lanka and the mozambique belt abstract late pan african 580 550 ma high grade deformation of the sri lankan basement gave rise to a fold and thrust belt the resulting axial planes to upright folds and earlier stretching lineations define a large scale syntaxial bend this syntaxis is attributed to e w shortening and simultaneous sinistral strike slip along a n s trending transcurrent zone located west of sri lanka prior to gondwana break up the resulting geometry is consistent with a corner effect brun and burg 1982 produced by westward indenting of the basement underneath eastern sri lanka during collision in gondwana reconstructions the sinistral strike slip zone fits into the late kinematics of the mozambique belt shackleton 1986 these new structural data constrain the position of sri lanka in gondwana and favour reconstructions in which sri lanka is located near the lutzow holm bay in east antarctica a review of data from adjacent gondwana fragments suggests that the tectonometamorphic evolution of the sri lankan basement is closely related to the interplay between two orogenic belts the mozambique belt and the lutzow rayner belt it is argued that a late event of n s shortening gondwana coordinates in the lutzow rayner belt can be attributed to a pan african tectonometamorphic event in this interpretation the lutzow rayner belt developed in part simultaneously with the mozambique belt reworking an older granulitic basement in the rayner complex

absence of preferred longitude sectors for poles from volcanic records of geomagnetic reversals according to a recent compilation of sedimentary records from the past 12 myr the reversing geomagnetic field displays a marked long term longitudinal organization which seems to be correlated with the thermal structure of the lower mantle1 however the crucial palaeomagnetic observation that intermediate virtual geomagnetic poles are preferentially confined to longitudes over the americas or antipodal to this sector2 3 might be an artefact caused by distortion and smoothing of the geomagnetic signal by sediments4 6 here we present a compilation of 400 intermediate poles from 121 volcanic records of excursions and reversals less than 16 myr old sampled at various longitudes and latitudes which does not confirm previous inferences1 and reinforces doubts4 6 about the ability of sediments to properly record a rapidly varying field in contrast to virtual poles from excursions those from reversals are uniformly distributed in longitude which indicates that the reversing field is statistically axisymmetrical thus we conclude that there is no evidence for control of transitional fields by the temperature distribution in the lowermost mantle

inclination anomalies from indian ocean sediments and the possibility of a standing nondipole field we report magnetic inclinations measured in deep sea sediments of the equatorial indian ocean which record the behavior of a nondipole component of the time averaged geomagnetic field during the plio pleistocene 0 5 ma the magnitude of the nondipole effect recorded in these sediments appears to depend on polarity state with inclinations showing departures from geocentric axial dipole directions which are small 2 o during normal polarity and larger 5 o during reverse polarity times the overall nondipole effect observed here is consistent with prior spherical harmonic estimates of the paleomagnetic field the polarity bias found agrees in both sense and magnitude with earlier reports of polarity asymmetry in the low degree zonal harmonic fields the presence of this asymmetry supports previous suggestions of the existence of a standing component of the nondipole field which does not invert during reversals of the main field we explore whether the standing field so indicated may have influenced paleomagnetic directions recorded during polarity transitions at other equatorial sites

holocene changes in tropical wind intensity and rainfall evidence from southeast ghana abstract remnants of a fixed aeolian dune ridge occur along the southeast coast of ghana just behind the present shoreline aeolian sands also cover extensive areas of the accra plains no dunes are present here the sands mainly occurring as sheets which blanket an early holocene landscape the sediments are of mid holocene age and were deposited during the interval 4500 b p 3800 yr b p when the southwesterly winds were stronger than they are at present and much of tropical africa seems to have been subject to marked aridity the onset of drier windier conditions around 4500 yr b p brought to an end the more equable climates than had characterized much of west africa during the earlier holocene aridity intensified winds and desert expansion between 4500 and 3800 yr b p parallel environmental conditions in tropical continental areas at the height of the late pleistocene glaciation

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

late pleistocene geomagnetic excursion in icelandic lavas confirmation of the laschamp excursion in 1980 kristjansson and gudmundsson 1 reported a late glacial geomagnetic excursion in three hills in the reykjanes peninsula iceland with shallow negative inclinations and westerly declinations they named it the skalamaelifell excursion more extensive field work has identified the same excursional paleomagnetic direction declination 258 inclination 15 at four additional outcrops in a 10 10 km area in the reykjanes peninsula the excursion lavas are olivine tholeiites with similar petrography and chemical compositions paleointensity determinations by the thellier method average 4 2 0 2 t for 8 samples more than an order of magnitude weaker than the present geomagnetic field in iceland together these results suggest extrusion of the excursion lavas in a very brief span of time probably less than a few hundred years kar dating of the excursion lavas gives a mean age for 19 determinations of 42 9 7 8ka 2 compilation of thirty kar ages of the laschamp and olby flows by three laboratories yield a new age for the laschamp excursion in france of 46 6 2 4ka 2 the age of the excursion in southwestern iceland is statistically indistinguishable from the laschamp excursion at the 95 confidence level and both have very low paleointensities therefore we suggest that the laschamp and olby flows in france and the skalamaelifell units of iceland recorded essentially the same geomagnetic excursion differences in the virtual paleomagnetic poles vgps of these excursions may be due to 1 the probable non dipole character of the geomagnetic field during the excursion 2 rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in france and iceland and or 3 crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

a high resolution palaeoclimatic record for the last 27 500 years in tropical west africa from the carbon and nitrogen isotopic composition of lacustrine organic matter variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of lake bosumtwi ghana reflect climatically induced changes to the lake and the catchment flora cores spanning the last 27 5 kyr of sedimentation in the lake show large oscillations in 13corg pdb and 15norg air the late pleistocene record is particularly detailed showing changes of ca 20 in carbon and ca 10 in nitrogen isotopic compositions these variations are of complex origin although different in magnitude major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the last glacial maximum the allerod younger dryas period also seems to have been marked by generally dry conditions in this part of tropical west africa nitrogen isotopic evidence suggests that during the period 9 2 3 2 kyr the lake had an extremely stable water column probably due to the absence of a windy or cool dry season or both regular circulation of the water column recommenced during the late holocene and has persisted until the present day

quaternary aeolian dust processes and sediments in the australian region abstract aeolian dust processes have played an important role in the evolution of the australian landscape yet little is known of the processes and their sediments two major dust transport pathways are described based upon paleo dune patterns and present day dust storms and related processes these dust pathways have been fed by alluvium deposited in internally draining river basins during the period leading up to and including the late pleistocene aridity rates of dust entrainment transport and deposition were significantly higher than at present the ocean sediments of the tasman sea the pacific and indian oceans received massive amounts of aeolian sediment dust contributions to some australian soils have also been considerable

deformation of a high grade gondwana fragment sri lanka abstract the sri lankan gondwana fragment is a high grade gneiss terrain consisting of at least three crustal units namely the wanni complex wc the highland complex hc and the vijayan complex vc which were juxtaposed during the pan african collision at least six phases of ductile deformation were recognized in the smallest gondwana fragment sri lanka there is microscopic and mesoscopic evidence for a first deformation d 1 which occurred during prograde metamorphism but before the main granulite facies metamorphism this first deformation is represented by straight to curved and crenulated inclusion trails in garnet porphyroblasts microfolds in some metapelites and quartzo feldspathic gneisses and by a s 1 foliation preserved in crenulation folds and some rootless intrafolial folds d 2 has two main stages of which the second phase is the most intense and dominant deformation which produced most part of the major composite and transposed compositional layering as well as the prominent stretching lineation l 2 the first stage of d 2 may have been responsible for the formation of a crenulation cleavage of s 1 as seen in garnet porphyroblasts in some metapelites the compositional layering s 2 in metapelites and metasemipelites appears to have formed through the development of a crenulation cleavage f 2 folds are mesoscopic isoclinal rootless and occur as intrafolial folds and their limbs are highly attenuated major part of the deformation during the latter stage of d 2 was non coaxial many minor and large scale recumbent isoclinal folds f 3 were produced during d 3 both d 2 and d 3 were coeval with the peak conditions of granulite facies metamorphism d 4 produced very large gentle nearly e w upright folds probably during the beginning of uplift of the entire high grade assemblage from deeper crustal levels still under granulite facies conditions d 5 is the second strongest deformation and was responsible for the large scale upright folds f 5 which control the present map pattern of sri lanka the superimposition of d 5 folds on d 4 folds gave rise to a large scale type i fold interference pattern of ramsay 1967 d 6 deformation caused local refolding of the f 5 folds and was accompanied by amphibolite facies conditions during uplift the deformation represented by the majority of vein type structures such as patchy charnockite veins cordierite bearing veins and graphite veins clearly post date d 5 it is argued that the wc the hc and the vc in sri lanka may be three distinctly different proterozoic crustal units brought into contact by tectonism in late proterozoic at two different stages

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

deuterium excess in an east antarctic ice core suggests higher relative humidity at the oceanic surface during the last glacial maximum a continuous deuterium excess profile d d 8 18o was obtained from the dome c ice core spanning 32 000 yr it exhibits significant variations over this period in particular d was 4 5 lower around 18 000 yr bp than over the past 7 500 yr interpreted as reflecting higher relative humidity estimated 90 over the oceanic areas providing moisture for antarctic precipitation

brief reversed polarity interval during the cretaceous normal polarity superchron a striking feature of the geomagnetic time scale is the 34 m y period of normal polarity field bias known as the cretaceous normal polarity superchron new paleomagnetic data from three deep sea drilling project cores sites 317a 402a and 463 indicate that a brief interval of reversed polarity occurred during the superchron this reversed polarity interval may help refine past motions of the pacific plate and aid resolution of the statistical structure of the geomagnetic reversal chronology

the relative stabilities of the reverse and normal polarity states of the earth s magnetic field recent analyses of the geomagnetic reversal sequence have led to different conclusions regarding the important question of whether there is a discernible difference between the properties of the two polarity states the main differences between the two most recent studies are the statistical analyses and the possibility of an additional 57 reversal events in the cenozoic these additional events occur predominantly during reverse polarity time but it is unlikely that all of them represent true reversal events nevertheless the question of the relative stabilities of the polarity states is examined in detail both for the case when all 57 events are included in the reversal chronology and when they are all excluded it is found that there is not a discernible difference between the stabilities of the two polarity states in either case inclusion of these short events does however change the structure of the non stationarity in reversal rate but still allows a smooth non stationarity only 7 of the 57 short events are pre 38 ma but the evidence suggests that this is a real geomagnetic phenomenon rather than degradation of the magnetic recording or a bias in observation this could be tested by detailed magnetostratigraphic and oceanic magnetic surveys of the paleogene and late cretaceous overall it would appear that the present geomagnetic polarity timescale for 0 160 ma is probably a very good representation of the actual history and that different timescales and additional events now represent only changes in detail

millennial scale instability in the geomagnetic field prior to the matuyama brunhes reversal changes in the earth s magnetic field have global significance that reach from the outer core extending out to the uppermost atmosphere paleomagnetic records derived from sedimentary and volcanic sequences provide important insights into the geodynamo processes that govern the largest geomagnetic changes polarity reversals but dating uncertainties have hindered progress in this understanding here we report a paleomagnetic record from multiple lava flows on tahiti that bracket the matuyama brunhes m b polarity reversal 771 thousand years ago our high precision 40 ar 39 ar ages constrain several rapid and short lived changes in field orientation up to 33 000 years prior to the m b reversal these changes are similar to ones identified in other less well dated lava flows in maui chile and la palma that occurred during an extended period of reduced field strength recorded in sediments we use a simple stochastic model to show that these rapid polarity changes are highly attenuated in sediment records with low sedimentation rates this prolonged 33 000 year period of reduced field strength and increased geomagnetic instability supports models that show frequent centennial to millennial scale polarity changes in the presence of a strongly weakened dipole field

paleomagnetism of the late cretaceous early tertiary north central montana alkalic province paleomagnetic results have been obtained from the late cretaceous early tertiary igneous complexes of the north central montana alkalic province data from 94 sites in eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82 0 n 170 2 e a95 3 5 k 18 6 while 36 sites in paleocene intrusions yield a paleomagnetic pole at 81 8 n 181 4 e a95 5 4 k 20 2 these poles differ by only 1 6 and are not significantly different statistically the 130 site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander apw during the period of magnetization of the igneous centers postmagnetization structural complications in these rocks are minimal the presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration the northcentral montana data together with other early tertiary cretaceous and mid tertiary paleomagnetic results require modification of our earlier apw chronology diehl et al 1980 it now appears that apw relative to north america since the early cretaceous consists of a polar still stand during much of the cretaceous 120 75 m y b p a period of rapid movement in latest cretaceous time 75 65 m y b p and a period of slow polar movement thereafter the onset of this period of rapid apw correlates well with a major change in plate motions at 80 m y and the beginning of the laramide orogeny

the nature and origin of the aeolian mantles of central northern nigeria abstract evidence is presented here to show that the upland soils of central northern nigeria are formed in aeolian mantles deposited by two aeolian processes aeolian suspension or dust deposition and aeolian saltation or dune sand deposition the soils to the south and west of kano are largely the product of harmattan dust deposition whereas the northern soils appear to originate from dune sand migration from the quaternary dune fields to the east of the study area the spatial pattern of particle size populations or quanta and quartz grain roundness characteristics within the mantles and dune sands indicates that considerable mixing of dune sands and dust has occurred in the formation of these mantles

a quantitative comparison of two paleomagnetic records of the cobb mountain subchron from north atlantic deep sea sediments we present a new paleomagnetic record of the cobb mountain subchron obtained from deep sea sediments cored at ocean drilling program site 647 in the southern labrador sea the details of the transitional field behavior documented by this record appear to be very similar to those recorded in a previously published record of this subchron obtained at deep sea drilling project dsdp site 609 in the north atlantic clement and kent 1987 we used a quantitative correlation technique martinson et al 1982 to establish statistically the degree of similarity between these two records and thereby constrain the spatial variability in these transitional fields the error in the alignments is reduced significantly by aligning records of virtual geomagnetic pole positions rather than directions indicating that these records document such large scale changes in the fields that we can not distinguish them from dipolar changes given the proximity of these two sites these replicate records of the cobb mountain subchron provide evidence that deep sea sediments are capable of providing high resolution records of geomagnetic field behavior a reexamination of the sequence of polarity transitions recorded at dsdp site 609 in light of these results suggests the presence of two preferred transitional field configurations the field appears to change from one configuration to the other for several reversals and then back to the original configuration suggesting that a geographical influence on the reversal process persists through this sequence the variability in these reversal records provides insights into the response of the geodynamo to this geographical influence

vein graphite deposits geological settings origin and economic significance graphite deposits result from the metamorphism of sedimentary rocks rich in carbonaceous matter or from precipitation from carbon bearing fluids or melts the latter process forms vein deposits which are structurally controlled and usually occur in granulites or igneous rocks the origin of carbon the mechanisms of transport and the factors controlling graphite deposition are discussed in relation to their geological settings carbon in granulite hosted graphite veins derives from sublithospheric sources or from decarbonation reactions of carbonate bearing lithologies and it is transported mainly in co2 rich fluids from which it can precipitate graphite precipitation can occur by cooling water removal by retrograde hydration reactions or reduction when the co2 rich fluid passes through relatively low fo2 rocks in igneous settings carbon is derived from assimilation of crustal materials rich in organic matter which causes immiscibility and the formation of carbon rich fluids or melts carbon in these igneous hosted deposits is transported as co2 and or ch4 and eventually precipitates as graphite by cooling and or by hydration reactions affecting the host rock independently of the geological setting vein graphite is characterized by its high purity and crystallinity which are required for applications in advanced technologies in addition recent discovery of highly crystalline graphite precipitation from carbon bearing fluids at moderate temperatures in vein deposits might provide an alternative method for the manufacture of synthetic graphite suitable for these new applications

dune ages in the sand deserts of the southern sahara and sahel in this paper we aim to document the history of aeolian processes within the southern sahara as part of the inqua dune atlas we review available luminescence ages for sand dunes across the southern sahara to develop an improved understanding of the dune chronology on a regional basis and attempt to correlate periods of sand accumulation this was achieved by analysing dune age by country as well as by latitude and longitude the results show a very patchy spatial distribution of dune ages with large gaps that encompass some of the largest sand seas despite these gaps some related patterns in dune morphology and stratigraphy appear to be consistent between northern nigeria and southern mali where older linear dunes are distinct from younger late holocene transverse and barchanoid dunes elsewhere in mauretania linear dunes with different orientations appear to have accumulated at different times most likely in response to changes in atmospheric circulation regional climatic changes are identified where dunes are transgressed by lake deposits within endorehic basins we identify four locations where dune accumulation is terminated by lacustrine transgressions two of which in lake chad and the bodele depression occur shortly after the last glacial maximum lgm the third example at gobiero in niger occurred later in the early holocene around 8 4 ka and a fourth marks a later transgression of palaeolake megachad after 4 7 ka larger scale latitudinal and longitudinal distributions in dune ages across the southern sahara do not show any consistent patterns though this may be due to the small sample size relative to the study area in addition local variations in external controls such as wind regime rainfall vegetation and sand supply need to be considered sometimes on a site by site basis limiting the analysis to dune ages determined using the single aliquot regenerative dose sar protocol indicates a lack of dune preservation during the lgm and the younger dryas times associated with increased dust input to the oceans which is assumed to indicate increased aeolian activity the sar dune dates suggest that preservation of dunes at the onset of succeeding humid intervals is an important component of the dune record the most striking examples of this phenomenon occur where dunes are preserved within endorehic basins by lacustrine transgressions

geodynamo energy source constraints from palaeomagnetic data abstract analyses of palaeo geomagnetic data can provide surprisingly strong constraints upon models of the geodynamo in particular it is shown here from an analysis of the geomagnetic reversal sequence that long term variation in reversal rate does not occur as a result of change in any inherent inhibition or encouragement in the reversal process instead there must be some physical process that actually triggers reversals and this process must derive from an energy source effectively independent of the energy source powering the main geomagnetic field from this two very simple generalised models referred to as the hot blob and cold blob models are suggested data from individual reversal transitions are used to discriminate between these two models the preferred model is that the geodynamo is driven primarily by convection related to freezing of the outer core at the inner core outer core boundary cold blobs descending from the core mantle interface are identified as the instabilities that trigger reversals

paleomagnetic assessment of oroflexural deformation in west central nevada and significance for emplacement of allochthonous assemblages abrupt westerly deflections of north south trending facies boundaries the isopleth of 87sr 86sr 0 076 and arcuate structural trends of paleozoic mesozoic and cenozoic age in the west central basin and range province have been explained by 1 post early jurassic oroflexural folding and shear 2 the existence of a pre late jurassic resistant block along the western continental margin possibly representing late precambrian breakup and 3 regional flexure mina deflection prior to intrusion of late cretaceous plutons which crosscut flexure related structural trends paleomagnetic data gathered from late cretaceous plutons and remagnetized metasedimentary rocks along and north of the northern margin of the zone of east west structural trends are not in support of significant oroflexural folding and attending major clockwise rotation since late cretaceous time the data water canyon pluton d 1 i 57 95 4 k 9 n 132 gunmetal pluton d 354 i 60 95 6 k 21 n 29 east garfield hills pluton d 349 i 74 95 11 k 14 n 12 la panta pluton d 22 i 66 95 10 k 16 n 13 gillis canyon pluton d 354 i 50 95 4 k 20 n 50 and folded units of the excelsior mountains d 309 i 79 95 7 k 12 n 37 are dispersed from expected cretaceous directions for the mina deflection region but they do not conform to a model of major systematic regional clockwise rotation the generally small deflections of unit means from expected directions could be explained by irregular components of late cenozoic tilting and or rotation the paleomagnetic data do not deny the possibility of pre late cretaceous oroclinal bending nevertheless regional structural analysis complimented by the paleomagnetic results indicate that the mina deflection is in all likelihood a primary feature whose geometry reflects the late precambrian fragmentation of western north america structures related to the accretion of allochthonous assemblages between middle devonian and cretaceous time in this part of the cordillera also reflect the primary nature of this feature

asymmetry in the reversal rate before and after the cretaceous normal polarity superchron abstract the geomagnetic reversal chronology shows a decreasing reversal rate from about 160 ma to about 118 ma a cessation in the reversal process the cretaceous normal polarity superchron until about 83 ma and then an increasing until recent times visually the rate of decrease in leading into the superchron appears larger than the rate of increase in following the superchron statistical tests developed here confirm the visual perception of an asymmetry in the rate of change in this asymmetry could be caused by a relatively rapid breakdown of d with thermal energy being advected away followed by a slower thickening of the layer through thermal conduction however in the absence of good thermal models as to how different core mantle boundary conditions affect the geodynamo there are probably many apparently viable speculations as to the origin of the observed rate asymmetry

geomagnetic field stability reversal events and excursions abstract it has been suggested that there are several reverse subchrons in the brunhes and this suggestion is gaining some acceptance however such a suggestion has fundanmental implications for the reversal process because it implies that the reverse polarity state has been substantially less stable than the normal polarity state during the brunhes chron analysis of the latest polarity reversal timescale confirms that there is no reason to reject the hypothesis of a common stability for the two polarity states the cretaceous superchron appears to be a time when the reversal rate slowed to the point where the reversal process actually halted so does not indicate any difference in the stability of the two polarity states after examining the data we found no convincing evidence for even a single reversal event during the brunhes and we conclude that the only reversal excursions that are sufficiently well documented for the brunhes are the laschamp and the blake further we note that the question of whether of large excursions represent aborted reversals is unresolved so excursion data should not at this stage be grouped with transition polarity data finally we note that most of the time the nearly axially symmetric normal and reverse polarity states appear to be remarkably stable

gold graphite association in granulite terrains implications for ore genesis abstract the gold occurrences of sri lanka are closely associated with the graphite deposits and are genetically related the quartz graphite margins of the vein graphite are shown to be metal accumulating the movement of co 2 rich fluids formation of graphite and the concentration of au are closely related whereas the co 2 rich fluids may well be the carbon carriers in the formation of graphite the original source of co 2 is still debated hydrothermal solutions when in contact with the graphite tend to activate the carbon thereby producing sites for the enrichment of gold the genetic link between the graphite veins and gold deposition has useful implications in the investigation of ore genesis in high grade metamorphic terrains and the exploration for gold

self reversal of chemical remanent magnetization on the transformation of maghemite to haematite summary self reversed chemical remanent magnetization crm has been observed in haematite formed on heating maghemite which has been produced by the dehydration of acicular crystals of synthetic lepidocrocite 1 to 2 pm in length our experimental evidence suggests that self reversal of haematite remanence only occurs when the parent maghemite is still blocked at the temperature of its transformation to haematite when the transformation temperature is above the blocking temperature of the parent maghemite and it is unblocked the resulting haematite remanence is normally magnetized it is suggested that the strong dependence on remanent state supports exchange control of the self reversal process we propose that the self reversal is probably a general feature of the maghemite to haematite transition and the significance of the source lepidocrocite in our experiments is that it produces maghemite of a suitable grain size so that much of it remains blocked at the elevated temperatures required to make the transformation to haematite occur over the short time scale of the laboratory experiments in nature transformation of maghemite to haematite will occur at much lower temperatures due to the much longer time scales involved and self reversal of the resulting crm may occur over a much less restricted grain size range of parent maghemite