magnetic mineralogy of pelagic limestones summary due to their extremely small size and low concentration the carriers of magnetization in limestones are usually identified through indirect rock magnetic methods this paper describes a method for extracting tiny ferromagnetic grains from consolidated calcareous rocks and summarizes a scanning electron microscope sem investigation of magnetic phases from several palaeomagnetically important pelagic limestones details of surface characteristics and elemental composition are readily accessible with the sem and complement normal rock magnetic experiments in giving a more complete assessment of the major ferromagnetic phases their origin and the timing and acquisition of magnetization the results confirm many indications of the indirect methods and introduce several important new features in addition to detrital titanomagnetite diagenetic titanohaematite and iron sulphides there is a significant contribution by authigenic microcrystalline haematite detrital cosmic spherules chromian spinels and skeletal titanomaghaemite

shear in the lower crust during extension not so pure and simple abstract as lower crustal reflectivity appears to be related to extension it is sensible to consider the role of the lower crust in the extension of the lithosphere and in particular the way the lower crust deforms the occurrence of transparent zones wrapped around by reflective bands can be interpreted as low strain regions enclosed within anastomosing shear zones suggesting that deformation in the lower crust is by the relative movements of low strain lozenges along acoustically reflective high strain zones although this deformation can easily accommodate bulk pure shear it can also develop during bulk simple shear a component of subhorizontal simple shear within the lower crust is necessary if deformation in the mantle is localised into shear zones with the geometry and distribution imaged on birps profiles the lower crust may be the critical link in the lithosphere transferring deformation between the relatively strong upper crust and upper mantle and being strongly sheared in the process the form of this shearing is a direct consequence of the way the entire lithosphere extends

upper pleistocene palaeoenvironmental change in eastern spain new pollen analytical data from cova beneito alicante abstract the results of a palynological investigation of the upper palaeolithic interval from the beneito cave southeastern spain are herewith considered in the context of results from a previous report on the underlying deposits two new percentage and concentration pollen diagrams with good correspondence are presented the upper palaeolithic sequence is dominated by poaceae chenopodiaceae and artemisia whereas arboreal pollen are less important mesophilous trees and mediterranean taxa show a constant presence but in low proportions the new data add support to the view that interpleniglacial development of mediterranean taxa was important in eastern spain while they were considerably reduced during upper pleniglacial times emphasis is placed on aridity as a limiting factor in influencing major vegetation changes in the area two additional topics are also presented they deal with the extrapolation of north european interstadials to mediterranean spain and the palaeoecological value of archaeopalynological records

magnetite authigenesis and diagenetic paleotemperatures across the northern appalachian basin the concentration of magnetite in the lower and middle devonian helderberg and onondaga carbonate formations varies in a distinct pattern along an east west profile across new york state magnetite concentrations are uniformly low in the western half of the profile increase sharply eastward to a peak value near syracuse and decrease farther east the pattern strongly resembles previously reported variations in the extent of clay mineral diagenesis that reflect differences in paleotemperatures along the profile previous paleomagnetic studies have documented that the magnetite carries a well defined magnetization of pennsylvanian permian age and magnetite authigenesis is therefore no younger than late paleozoic we attribute the correlation between high magnetite concentration and high illite content to temperature dependent diagenesis triggered by orogenic fluids the large proportion of secondary magnetite indicates that over most of the area the mechanism of late paleozoic remagnetization was principally chemical thermal effects played a less direct role in the remagnetization of the strata by controlling the extent of the chemical processes that resulted in overprinting of the original remanence

late paleozoic remagnetization in limestones of the craven basin northern england and the rock magnetic fingerprint of remagnetized sedimentary carbonates samples of lower carboniferous chatburn and pendleside limestones collected from 27 sites in the craven basin northern england carry a syn hercynian remagnetization residing in magnetite the paleomagnetic directions are all of reversed polarity consistent with a kiaman superchron remagnetization the partially 62 tilt corrected mean direction has the minimum correlation statistic implying a synfolding magnetization the resulting pole appears to be late carboniferous in age and there is no indication of any magnetization component that predates the remagnetization event the magnetization appears to be of the same type as the synfolding remagnetizations documented in several appalachian carbonate units jackson 1990 reported that remagnetized appalachian carbonates have distinctive hysteresis properties and we have obtained similar hysteresis loops from remagnetized paleozoic carbonates from the craven basin england the great basin nevada alaska and new york state the hysteresis loops are all wasp waisted and yield highly anomalous hysteresis ratios compared to data from synthetic or typical magnetite bearing samples in contrast unremagnetized limestones from italy show hysteresis behavior which is more typical for magnetite bearing rocks channell and mccabe this issue these results suggest that remagnetization in magnetite bearing carbonates can be readily detected on the basis of hysteresis properties prior to a full scale paleomagnetic investigation

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

paleolatitudes of the tibetan himalaya from primary and secondary magnetizations of jurassic to lower cretaceous sedimentary rocks the tibetan himalaya represents the northernmost continental unit of the indian plate that collided with asia in the cenozoic paleomagnetic studies on the tibetan himalaya can help constrain the dimension and paleogeography of greater india the indian plate lithosphere that subducted and underthrusted below asia after initial collision here we present a paleomagnetic investigation of a jurassic limestones and lower cretaceous volcaniclastic sandstones section of the tibetan himalaya the limestones yielded positive fold test showing a prefolding origin of the isolated remanent magnetizations detailed paleomagnetic analyses rock magnetic tests end member modeling of acquisition curves of isothermal remanent magnetization and petrographic investigation reveal that the magnetic carrier of the jurassic limestones is authigenic mag netite whereas the dominant magnetic carrier of the lower cretaceous volcaniclastic sandstones is detrital magnetite our observations lead us to conclude that the jurassic limestones record a prefolding remagnetiza tion whereas the lower cretaceous volcaniclastic sandstones retain a primary remanence the volcaniclastic sandstones yield an early cretaceous paleolatitude of 55 5 s 52 5 s 58 6 s for the tibetan himalaya suggesting it was part of the indian continent at that time the size of greater india during jurassic time cannot be estimated from these limestones instead a paleolatitude of the tibetan himalaya of 23 8 s 21 8 s 26 1 s during the remagnetization process is suggested it is likely that the remagnetization caused by the oxidation of early diagenetic pyrite to magnetite was induced during 103 83 or 77 67 ma the inferred paleolatitudes at these two time intervals imply very different tectonic consequences for the tibetan himalaya

comparison of magnetic hysteresis parameters of unremagnetized and remagnetized limestones for white magnetite bearing mesozoic pelagic limestones from italy which carry a primary magnetization the values of mrs ms and hcr hc generally lie in the pseudo single domain psd field of the day et al 1977 plot the logarithmic plot of mrs ms against hcr hc gives a straight line r 0 814 with slope and intercept close to the empirical mixing line of parry 1982 for single domain sd and multidomain md magnetite for one of the white pelagic limestone formations maiolica formation samples with hysteresis ratios closer to the md field display increased paramagnetic susceptibility and are from the upper part of the formation characterized by increased detrital clay we therefore associate the increased md magnetite with increased detrital influx for pinkish and reddish varieties of the italian pelagic limestones the presence of hematite is manifest by high saturation fields a wide range of hcr hc and wasp waisted hysteresis loops attributed to the mixing of magnetite and high coercivity authigenic hematite the hysteresis ratios for a collection of paleozoic and mesozoic remagnetized magnetite bearing limestones from britain nevada alaska and the appalachians lie mainly outside the psd field and appear to follow a power law trend following jackson et al 1993 the high values of hcr hc and the characteristically wasp waisted hysteresis loops can be interpreted in terms of a fine grained subspherical high coercivity sd magnetite mixed with a high proportion of superparamagnetic magnetite the slope and intercept of the power law relationship for mrs ms and hcr hc in the remagnetized limestones are distinct from those observed for the italian limestones and may provide a means of fingerprinting magnetite of primary as opposed to diagenetic origin

following the last neanderthals mammal tracks in late pleistocene coastal dunes of gibraltar s iberian peninsula abstract the aeolian pleistocene deposits recorded in the rock of gibraltar s iberian peninsula present a significant sedimentary thickness that fills the wide sea caves and covers its steep slopes in catalan bay these deposits register the development of cliff foot and rampant dunes that form a 35 sand ramp 1 km long and up to 300 m in height formed by moderately to well sorted sands slightly carbonated and weakly cemented this morphosedimentary evidence is associated with mis 5 2 sea level fluctuations its internal structure shows lamination sets of 2 3 m in thickness and planar and trough cross bedding in these beds five morphotypes of mammalian footprints have been recognized in vertical cross sections and convex hyporeliefs most of them present a moderate degree of conservation due to the poor cementation of the sands these icnites are related to the activity of terrestrial mammals possibly belonging to homininae proboscidea artiodactila and carnivora the affinity between the footprints and their tracemakers has been established in the basis of their physical parameters the shape of the footprints their comparison with similar footprints and the correlation with the fossil fauna of vertebrates described for the late pleistocene in gibraltar these findings represent the first palaeoichnological record in the aeolian sediments of gibraltar and indirectly corroborate the occupation of the coastal dune landscape by several terrestrial mammals during the late pleistocene in addition they help reconstructing the animal behaviour and interaction with the extensive dune systems that developed at the foot of the rock slopes the age of the tracks has been established within the mis 2 during the probable middle upper palaeolithic transition in the climate refuge of gibraltar

concurrence of folding and remagnetization events in the monterrey salient ne mexico abstract carbonate formations from many locations reveal magnetizations that are secondary attaching a numerical age to these remagnetizations would provide information on orogenic processes to contribute to the effort of assigning an age to remagnetization events lower cretaceous formations from the monterrey salient in northeast mexico were sampled individual fold tests reveal eight site pair synfolding remagnetizations set a of eocene age the remanence is a chemical remanent magnetization crm carried by magnetite the magnetization of set b is possibly due to an early carbonate alteration that produced a remanence significantly older than folding a complexity of set b is that the site pairs reveal k versus unfolding percentage diagrams with maximum unfolding peaks at 100 to 125 an explanation of multiple deformation phases with an earlier horizontal folding phase around a non plunging horizontal axis and a later vertical axis rotation is suggested for the past 100 peaks of the b sites the latter deformation rotation about a vertical axes is revealed by a declination strike test in set a sites and site means of the full collection that deviate counterclockwise from the reference declination by combining synfolding results with published 40ar 39ar illite ages of folding fitz diaz et al 2016 remagnetization ages in set a sites of 48 52 ma are obtained the magnetization in set b sites is likely older though not well constrained interestingly prior results from the central sierra madre oriental to the south show two remagnetization events late cretaceous and early eocene in age in a succession from w to e nemkin et al 2015 the age of folding and remagnetization acquisition in set a sites along the monterrey salient are concurrent with the frontal remagnetized folds in the central sierra madre oriental thus the timing of major remagnetization in the monterrey salient occurred in the eocene

regional patterns of magnetite authigenesis in the appalachian basin implications for the mechanism of late paleozoic remagnetization we report paleomagnetic and rock magnetic data from sedimentary carbonates of the appalachian basin that have implications for the mechanism of widespread late paleozoic remagnetization in this rock type sampling was accomplished mostly along two across basin transects one in devonian carbonates of new york state and the other in ordovician and mississippian carbonates of tennessee and alabama paleomagnetic investigations of the new york devonian carbonates along a transect from albany to buffalo indicate that these rocks were completely remagnetized in alleghenian time rock magnetic studies show that magnetite concentration varies in a regular fashion along the transect with a local maximum near syracuse and decreasing gradually to the east and west this pattern shows a striking correlation with the degree of diagenetic alteration of clay minerals in a devonian bentonite horizon which took place during alleghenian time we conclude that most of the magnetite present in these carbonates is of authigenic origin and that clay mineral alteration and magnetite authigenesis were coeval late paleozoic events that were controlled by the same diagenetic factors thermoviscous remagnetization processes cannot be ruled out in this setting in light of the observed degree of thermal maturity however our results suggest that chemical factors played a critical role in remagnetization since they allow us to infer that magnetite authigenesis and remagnetization are about the same age we propose a geochemical model for magnetite authigenesis wherein the iron is derived from detrital smectites during diagenetic illitization which is triggered by the introduction of potassium rich brines results from the tennessee transect from nashville to chattanooga show a very different pattern paleomagnetic studies indicate that late paleozoic remagnetization has affected ordovician carbonates of the nashville dome paleozoic carbonates in the overthrust belt near chattanooga also carry the late paleozoic remagnetization however mississippian carbonates between the dome and the overthrust belt are very weakly magnetized and show no evidence for late paleozoic remagnetization samples show only a present field magnetization or at one locality in northwestern alabama a dual polarity magnetization of probable mississippian age rock magnetic studies indicate that higher concentrations of magnetite are present in the remagnetized rocks of the nashville dome and the overthrust belt than in the nonremagnetized mississippian carbonates we therefore conclude that magnetite authigenesis and remagnetization are related events in this setting and that the remagnetization must be due to chemical processes the presence of a remagnetization shadow in the younger rocks now exposed between the fold thrust belt and the flexural arch is attributed to westward flow of magnetite forming aqueous fluids through deep sub carboniferous aquifers during alleghenian time upward migration of the fluids through the mississippian carbonates may have been prevented by the devono mississippian chattanooga shale an impermeable stratum that is present throughout the region

paleomagnetism of the upper silurian and lower devonian carbonates of new york state evidence for secondary magnetizations residing in magnetite abstract paleomagnetic directions for the upper silurian and lower devonian carbonates of the helderberg escarpment new york state differ from expected late silurian and early devonian directions for cratonic north america the mean direction d 165 i 10 paleopole at 50 n 129 e is similar to late carboniferous and early permian results negative fold tests and a lack of reversals suggest that the magnetization is secondary however low coercivities low blocking temperatures the thermomagnetic curves tc near 570 c and the acquisition of isothermal remanent magnetizations all suggest that the remanence is carried by magnetite if a detrital origin of these magnetites is assumed the secondary nature of the remanence would argue for thermal resetting as a result of deep burial of the rocks however no evidence for such thermal resetting is seen in the alteration of conodonts more likely perhaps is a chemical or thermochemical origin of the remanence this would require the magnetites to be authigenic

kinematics of orocline tightening in the core of an arc paleomagnetic analysis of the ponga unit cantabrian arc northern spain 1 paleomagnetic and structural analyses of the western european variscan belt wevb suggest that the most viable kinematic model for variscan deformation in northern iberia is oroclinal bending of an originally linear belt in a two stage tectonic history this history represents two regional compression phases e w in the late carboniferous and n s in the permian both in present day coordinates which resulted in the refolding about steeply plunging axes of initially north south trending thrusts and folds in the hinge zone and oroclinal tightening due to vertical axis rotation of the belt s limbs however the orocline model has yet to be critically tested in the wevb s core this study reports new paleomagnetic rock magnetic and structural data from the inner core of the wevb in order to test opposing kinematic models for the well documented fault and fold interference structures formed by late stage variscan deformation and to better understand the overall development of the wevb arc map scale structural features within the wevb core have a highly sinuous geometry characterized by transverse and thrust parallel fold systems formed by fault bend folding over footwall ramps the intersections of these two fold systems produce steeply plunging interference folds which are best exposed in the ponga thrust unit a total of 67 paleomagnetic sites were collected in the barcaliente formation of the ponga unit with emphasis placed on detailed spatial coverage of individual structural domains a multicomponent paleomagnetic remanence was measured which is interpreted to be composed of two components a low unblocking temperature recent viscous magnetization and a high unblocking temperature ancient magnetization that was acquired in the latest stephanian to early permian after initial d1 thrusting and folding bur prior to major secondary rotation rock magnetic experiments show that the characteristic remanence magnetization is carried by secondary authigenic pseudosingle domain magnetite these paleomagnetic results are used to determine the kinematics and geometry of postmagnetization deformation by restoring in situ magnetic vectors back to a defined reference direction on the basis of the restored ponga unit geometry the steeply plunging interference folds found in the wevb core are best described by a combination of secondary buckling of frontal ramp parallel hanging wall anticlines and modification of d1 lateral oblique ramps as frontal ramps during late stage north south shortening this combination of structural modification of the inner core of the wevb was necessary to accommodate oroclinal bending during the final amalgamation of pangea in the late paleozoic

regional trends in the timing of alleghanian remagnetization in the appalachians pole positions related to remagnetized components isolated in appalachian limestone and redbed rock units range over about 60 m y of the permian carboniferous apparent polar wander path for north america apparent ages of remagnetization are older in the southern appalachians and younger to the north if the remagnetizations are associated with fluids expelled during the alleghany orogeny then the apparent remagnetization age trend could describe the timing of thrust sheet emplacement

replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism limestones are an important source of palaeomagnetic data with magnetite the dominant carrier of the magnetization because the magnetite is present in low concentrations however it has been directly observed only rarely and in trace amounts in acid treated separates such residues are composed primarily of spheres the ultimate origin of which has been enigmatic7 yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite which has been inferred to carry either a chemical or a viscous remanent magnetization here we present electron microscope data obtained from a sequence of samples of onondaga limestone from an east west traverse across northern new york which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite remagnetization is thus caused by chemical rather than viscous processes this chemical remanent magnetization is compatible with a fluid mediated event occurring on a regional scale induced by tectonic stress of alleghenian late palaeozoic age

magnetic hysteresis in young mid ocean ridge basalts dominant cubic anisotropy magnetic hysteresis data from young mid ocean ridge basalts include samples with saturation remanence to saturation magnetization mrs ms ratios greater than 0 5 the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy under the usual assumption of dominant uniaxial anisotropy the narrow single domain grain size distribution implied by these high mrs ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains dominant cubic anisotropy provides a plausible explanation for the high mrs ms ratios and if generally valid requires reinterpretation of granulometric and domain state inferences made from hysteresis data

diapiric uplift of an mis 3 marine deposit in sw spain implications for late pleistocene sea level reconstruction and palaeogeography of the strait of gibraltar abstract in the bay of cadiz sw spain an upper pleistocene beach deposit 31 5 ka bp has been recognised at about 1 3 m above m s l the deposit is affected by a set of joints and fractures filled by calcretes and other subaerial sediments dated at 19 9 ka bp deformation and uplift of this level is related to the moderate activity of a diapiric structure the resulting uplift produced local emersion of the deposit and a transition from marine to continental conditions during the late quaternary the deformational style and tectonic location of the deposit argue against strong vertical motion regional comparisons between this diapir and other similar and coeval structures near the zone suggest a vertical uplift of about 25 m therefore between 30 and 20 ka bp the sea level can be supposed to have been placed near to its present day position probably less than 30 m below these results confirm other regional data indicating that during mis 3 several relative sea level rises took place reaching heights of only several tens of metres below the present m s l the palaeogeographical implications of these results include the existing controversy about the possible crossing of the strait of gibraltar by neanderthals between ca 40 and 30 ka the palaeogeographical reconstruction of the strait for this period suggests that its width and depth were very similar to the present ones

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates a rock magnetic study hysteresis measurements lowrie fuller tests cisowski tests and low temperature demagnetization experiments on samples of the knox dolomite trenton limestone and onondaga limestone indicate that the remanence in these remagnetized carbonate units resides principally in single domain magnetite in contrast to previous interpretations volumetrically however a significant fraction of the magnetite is coarser grained as previous scanning electron microscopic work on magnetic extracts has shown these conclusions are based mainly on the characteristic wasp waisted shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity both of which clearly indicate a bimodal distribution of coercivities grain sizes the magnetization is controlled mainly by cubic magnetocrystalline anisotropy consistent with the spheroidal particles commonly observed in extracts hysteresis trends show that especially the finest particles lack shape anisotropy in view of these conclusions the two components of natural remanence in these rocks are interpreted as 1 a late paleozoic kiaman chemical remanence carried by single domain diagenetic magnetite and 2 a cenozoic thermoviscous overprint residing principally in coarser pseudosingle domain and multidomain magnetite also of late paleozoic diagenetic origin

effects of tidal amplitude on intertidal resource availability and dispersal pressure in prehistoric human coastal populations the mediterranean atlantic transition abstract in this paper i argue that there is a growing body of evidence supporting an increasingly central position of coastal environments in human evolution and dispersals rather than as merely peripheral habitats eustatic fluctuations during glacial cycles have meant that most prehistoric coastlines are now underwater and lack of evidence to date of a close relationship between people and the coast can be most plausibly ascribed to the limited studies so far on submerged sites coastal environments provide high diversity in food resources consisting of multiple ecotones in close proximity which reduces the need to forage widely one of the richest and most easily exploited coastal resources by human populations living on the coast are molluscs from marine rocky intertidal communities which recent evidence has highlighted as important as far back as the middle palaeolithic however the density of these resources is limited by a number of factors and this varies geographically one of the main large scale factors limiting rocky intertidal mollusc densities is tidal amplitude beyond which smaller scale local factors such as exposure to wave action and shore aspect further affect species distributions the area around the strait of gibraltar is used as a case study of an area which is affected by large variations in tidal amplitudes thus allowing for quantitative comparisons between taxonomically and climatically similar regions shorelines along the mediterranean coast with reduced tidal amplitudes exhibit compressed zonations and harbour fewer macro mollusc individuals with the reverse being the case along the atlantic coast which has significantly larger tides data from middle and upper palaeolithic sites along the strait are used to establish harvested species and present day data are used to model the potential distributions and associated variables such as calorific returns of key food species an optimal foraging model is used to explore the effects of differential levels of resource availability along these coastlines results suggest that the nature and variability of the coastline has a direct influence on the tempo and mode of subsistence strategy followed and whether certain strategies can be adopted at all the possible implications of these biogeographical distributions for foraging economics dispersals and social structures in homo are discussed

late paleozoic remagnetization of the trenton limestone samples of the middle ordovician trenton limestone were collected from 8 sites in quebec ontario and new york state for paleomagnetic study stepwise thermal demagnetization reveals a very well defined characteristic magnetization that has a corresponding paleomagnetic pole at 53 0 n 126 9 e k 97 7 a 95 2 7 this pole is not at all close to other ordovician poles from cratonic north america but is near late carboniferous and early permian ones indicating that the trenton was remagnetized in the late paleozoic an earlier paleomagnetic investigation of the trenton gave a pole that did not appear to be late paleozoic in age however we demonstrate that this earlier study failed to isolate the ancient characteristic magnetization rock magnetic experiments and analyses of magnetic extracts show that the carrier of magnetization is fe3o4 that does not contain ti or other elements in solid solution that would indicate a detrital source the magnetite and its associated remanence appear to have a diagenetic origin

remagnetization by basinal fluids testing the hypothesis in the viola limestone southern oklahoma migration of orogenic or basinal fluids is a recently invoked mechanism to explain the widespread presence of late paleozoic secondary magnetizations in the rocks of north america paleomagnetic and geochemical results from the ordovician viola limestone in southern oklahoma are evaluated to assess the role of basinal fluids in leading to secondary magnetizations in the unit the viola limestone contains what we interpret to be a pervasive pennsylvanian synfolding magnetization residing in magnetite and a localized permian magnetization which resides in hematite and occurs in alteration zones around mineralized veins both secondary magnetizations are interpreted as chemical remanent magnetizations crm based on low burial temperatures and the presence of authigenic magnetic phases the relative proportion of the permian crm in hematite gradually decreases whereas the magnetite crm increases with distance from the veins fluid inclusion and sr isotope studies indicate that the vein mineralization calcite with mississippi valley type oxides and sulfides precipitated from basinal fluids which were warm saline and radiogenic the radiogenic 87sr 86sr values of the limestones in the alteration zones and the fact that there is more significant alteration closer to the veins suggests that the basinal fluids were also responsible for alteration in the limestones the coincidence of the geochemical and remagnetization trends suggests that the permian crm dates the migration of basinal fluids through the veins geochemical results from the viola limestone containing the pervasive crm indicate that it is relatively unaltered with no evidence for basinal fluids the lack of evidence for basinal fluids suggests that other mechanisms for the origin of the pervasive crm need to be tested the results of this study indicate that flow of basinal fluids was focused in veins and only locally altered the host limestone

climate forcing and neanderthal extinction in southern iberia insights from a multiproxy marine record abstract paleoclimate records from the western mediterranean have been used to further understand the role of climatic changes in the replacement of archaic human populations inhabiting south iberia marine sediments from the balearic basin odp site 975 was analysed at high resolution to obtain both geochemical and mineralogical data these data were compared with climate records from nearby areas baexcces was used to characterize marine productivity and then related to climatic variability since variations in productivity were the consequence of climatic oscillations climate productivity events have been established sedimentary regime primary marine productivity and oxygen conditions at the time of population replacement were reconstructed by means of a multiproxy approach climatic oceanographic variations correlate well with homo spatial and occupational patterns in southern iberia it was found that low ventilation u th high river supply mg al low aridity zr al and low values of baexcess coefficient of variation may be linked with neanderthal hospitable conditions we attempt to support recent findings which claim that neanderthals populations continued to inhabit southern iberia between 30 and 28 ky cal bp and that this persistence was due to the specific characteristics of south iberian climatic refugia comparisons of our data with other marine and continental records appear to indicate that conditions in south iberia were highly inhospitable at 24 ky cal bp thus it is proposed that the final disappearance of neanderthals in this region could be linked with these extreme conditions

hysteresis properties of magnetic spherules and whole rock specimens from some paleozoic platform carbonate rocks it is widely established that many paleozoic carbonate rocks were remagnetized during the late paleozoic kiaman reversed superchron yet the paleomagnetic recorders of this event have remained elusive magnetic spherules have been candidates for this role because they are probably authigenic therefore we have studied the hysteresis properties of individual magnetic spherules from two formations the remagnetized onondaga devonian new york and the effectively unremagnetized wabash silurian indiana also we have compared the hysteresis properties of spherules to those of remagnetized limestones from the onondaga trenton ordovician new york and helderberg devonian new york formations and from the unremagnetized wabash formation the spherules studied here vary from about 20 m to over 100 m in diameter nevertheless on the basis of hysteresis behavior this spherule population spans the full range of domain states from truly multidomain md to pseudo single domain psd to single domain sd because these spherules are so large it is remarkable that more than half display the highly psd like behavior typical of many remagnetized carbonate rocks equally as remarkable is the lack of dependence on spherule size of various hysteresis properties such as coercive force hc remanent coercive force hcr the ratio of saturation remanence to saturation moment mrs ms and the ratio of remanent coercive force to coercive force hcr hc these findings dispel the preconception that large spherules are too magnetically soft to be stable carriers of ancient magnetization surprisingly onondaga and wabash spherules yield virtually identical suites of hysteresis properties on the one hand this high degree of similarity could indicate that spherules are irrelevant to remagnetization more intriguing however is that this similarity could mean that the wabash formation although effectively unremagnetized nevertheless contains magnetic mineralogical traces of a regional remagnetization event it is significant that for these spherules a bilogarithmic plot of mrs ms versus hcr hc does not define a simple linear trend instead mrs ms increases more rapidly with decreasing hcr hc for spherules in the psd range than in the md range when a power law is fit solely to the hysteresis ratios of psd like to sd like spherules extrapolation to the sd limit gives mrs ms 0 75 similarly a power law fit to the hysteresis ratios obtained here from remagnetized carbonate rocks yields mrs ms 0 72 at the sd boundary these results could indicate that a large percentage of sd like spherules like the sd carriers in remagnetized carbonate rocks are governed by magnetocrystalline anisotropy finally the present data point to values of hc near 350 oe both for sd like spherules and for the sd grains thought to carry remanence in remagnetized carbonates clearly psd and sd like spherules share several key rock magnetic properties with remagnetized carbonate rocks thus spherules still are likely candidates for carrying the remagnetization signal

increased seasonality in the western mediterranean during the last glacial from limpet shell geochemistry the seasonal cycle is a fundamental aspect of climate with a significant influence on mean climate and on human societies assessing seasonality in different climate states is therefore important but outside the tropics very few palaeoclimate records with seasonal resolution exist and there are currently no glacial age seasonal resolution sea surface temperature sst records at mid to high latitudes here we show that both mg ca and oxygen isotope 18o ratios in modern limpet patella shells record the seasonal range of sst in the western mediterranean a region particularly susceptible to seasonal change analysis of a suite of fossil limpet shells from gibraltar shows that sst seasonality was greater during the last glacial by 2 c as a result of greater winter cooling these extra tropical seasonal resolution sst records for the last glacial suggest that the presence of large ice sheets in the northern hemisphere enhances winter cooling this result also indicates that seasonality in the mediterranean is not well represented in most palaeoclimate models which typically show little change in seasonal amplitude and provides a new test for the accuracy of climate models

a palaeoenvironmental study in semi arid southeastern spain the palynological and sedimentological sequence at perneras cave lorca murcia abstract results are presented of palynological and sedimentological research at the palaeolithic site of perneras cave coastal murcia southeastern spain sedimentological features point to a warm somewhat humid phase corresponding with the lowermost part of the mousterian sequence which was followed by a climatic deterioration which lasted throughout the later mousterian and upper palaeolithic the palynological study described here barely allows palaeoclimatological inferences to be drawn but it does highlight interesting paleobotanical data namely the presence of ibero maghrebian and mediterranean elements which suggest that the site was an important refuge for thermophilous taxa the reliability of the interpretation of the pollen assemblages is assessed in terms of the incorporation preservation and distribution of pollen grains into the profile

chronology of the late pleistocene archaeological sequence at vanguard cave gibraltar insights from quartz single and multiple grain luminescence dating abstract vanguard cave is an archaeological site located on the shoreline of the rock of gibraltar at the south western extreme of the iberian peninsula it is part of a limestone cave system facing the adjacent governor s beach on the south eastern coast of gibraltar and has been filled to the roof with more than 17 m of sedimentary deposits due to its long stratified sequence comprising rich palaeoenvironmental and faunal records as well as multiple palaeolithic occupation layers vanguard cave provides valuable information for our understanding of human behaviour and dispersal across south eastern iberia in general and particularly about the strategic role of the promontory of gibraltar for past human populations the development of a reliable absolute chronology for the sedimentary sequence at vanguard cave is therefore of great importance in this context in this study we applied optically stimulated luminescence dating to sand sized quartz grains from the uppermost 4 m of the vanguard cave deposits as well as from the hyaena cave sediments a small niche adjacent to the main cave chamber we use single grain and multiple grain dating to clarify the depositional history of the sedimentary sequence as well as to assess the reliability of the two dating approaches and their potential for future chronological studies at the site the single grain and multi grain ages are consistent with one another and indicate no partial bleaching or post depositional mixing of the sediments our results suggest that vanguard cave experienced continuous sediment accumulation probably since the last sea level highstand in mis 5 and was completely filled by 43 ka mis 3 the eastern side of the rock of gibraltar faced an exposed coastal shelf covered with savannah and an active coastal dune system during mis 3 our results indicate that similar environmental conditions are likely to have persisted also during mis 4 and large parts of mis 5

scanning and transmission electron microscope observations of magnetite and other iron phases in ordovician carbonates from east tennessee previous paleomagnetic observations for the carbonates of the lower ordovician knox group have indicated that ancient magnetizations in these rocks are of the same age as the late paleozoic alleghenian orogeny rock magnetic properties strongly suggest magnetite as the carrier of the magnetization but the textural and crystalline characteristics sizes morphologies and mineral associations of these magnetites are poorly known we have examined magnetic extracts and iron oxides in thin sections with scanning sem and scanning transmission stem electron microscope techniques to determine whether the observed iron oxide grain textures match the rock magnetic properties and paleomagnetic inferences about the mode of formation of the magnetic carriers several different forms of magnetite in limestones and dolomites which in places are host to mississippi valley type deposits are documented by imaging and energy dispersive analysis using sem and stem by x ray diffraction and electron diffraction patterns using stem the magnetite is either spherical with a dimpled surface or nonspherical in the form of void filling single grains or grain aggregates most of the iron oxides have the composition of pure end member magnetite but occasional titanomagnetite and hematite including rare zincian hematite have been observed only in limestone wherever found in thin section nonspherical magnetites occur in association with secondary dolomite potassium feldspar illite and quartz some iron oxides have in fact inclusions of k feldspar and quartz some of the magnetite spherical and nonspherical is polycrystalline this implies that the larger observed grains may consist of single domains or pseudo single domains this provides an explanation of the observed rock magnetic properties that apparently reflect the presence of single domain but interacting subgrains on the basis of remanent coercivities and blocking temperatures we interpret the pure end member magnetite to be authigenic having formed at approximately the same time as the k feldspars which in nearby areas have yielded late paleozoic radioisotopic ages 278 322 ma the knox carbonates therefore are inferred to carry a chemical remanent magnetization iron rich clays or original iron titanium oxides in the carbonates may have been the source materials for at least some of the secondary magnetite as it formed through complete dissolution precipitation processes these processes require rock fluid interactions which are thought to be related to migrating connate brines during the alleghenian orogeny

a middle late silurian paleomagnetic pole from limestone reefs of the wabash formation indiana u s a a middle late silurian paleomagnetic pole has been obtained from reefal limestones in north central indiana eleven of 25 collected sites yielded well clustered characteristic directions after stepwise alternating field demagnetization one additional site gave the same characteristic direction after great circle analysis these 12 site level directions were used to calculate the mean paleomagnetic pole reef flank beds dip steeply about 38 away from reef centers field and petrographic studies demonstrate that part of this dip is a primary depositional dip and that part is due to a tilt caused by early postdepositional differential compaction of underlying mudstone beds present attitudes of geopetal surfaces indicate that the postdepositional component of the observed bedding dip is about 12 a 12 tilt correction to paleomagnetic directions from oppositely dipping flank beds brings the mean directions into significantly better agreement indicating that the magnetization was acquired early the presence of both normal and reversed polarities also suggests early acquisition of the characteristic remanence the mean of the site paleomagnetic poles with sites from steeply inclined flank beds corrected for 12 of tilt is 16 8 n 124 9 e k 74 a95 5 1 in good agreement with the middle silurian rose hill pole for north america

marine mollusc exploitation as evidenced by the gorham s cave gibraltar excavations 1998 2005 the middle upper palaeolithic transition abstract there is increasing evidence that humans have exploited intertidal and shallow water species for much longer than has been previously considered and certainly not restricted to anatomically modern humans amh one of the principal reasons for the lack of evidence up till recently has been a lack of consideration for the temporal and spatial backdrop to such activities throughout human evolution in particular related to changes in sea level during the pleistocene bailey et al 2008 this study reports on the marine molluscs excavated from gorham s cave between 1998 and 2005 focussing in particular between levels iii and iv corresponding to the upper amh and middle palaeolithic neanderthals respectively given that gorham s cave was never more than approximately 2 km away from the coastline it still preserves evidence of exploitation of marine molluscs for food by neanderthals and in this article the data obtained are compared across the middle upper palaeolithic transition the results obtained suggest a high degree of consistency in the mode of marine mollusc exploitation between levels and comparisons with extant communities supports the contention that marine molluscs were exploited in direct proportion to their relative abundance and accessibility patterns in shell size distributions for some of the main species exploited are discussed as are possible anthropic valve selection and the marine climatic signals that can be extracted from such data the main difference that emerges between upper and middle palaeolithic levels was a lack of evidence of collection for decoration in middle palaeolithic levels but even here the relatively small size of the level iv sample precluded totally excluding this possibility based only on absence of evidence

further studies of carbonate remagnetization in the northern appalachian basin paleomagnetic and rock magnetic studies of sedimentary carbonates in the northern appalachian basin demonstrate that the late paleozoic remagnetization in this area is probably related to authigenesis of single domain magnetite fifteen sites within middle devonian carbonates were sampled along a transect extending from albany new york to the algonquin arch in southeastern ontario paleomagnetic studies show that all sites except the two westernmost sites on the algonquin arch were remagnetized during alleghenian time the very weakly magnetic unremagnetized sites retain a dual polarity magnetization of probable devonian age in the remagnetized sites both the size of the kiaman components and magnetite concentration estimated from low field susceptibility x are highest in the central part of the transect and lower on either side x ray powder diffraction analyses demonstrate that k feldspar is also most abundant in the middle of the transect with lower concentrations to the west and east we suggest that formation of authigenic magnetite and k feldspar may be related in this setting sites with the largest kiaman components have the highest unblocking temperatures and the largest ratios of anhysteretic remanent magnetization arm to bulk susceptibility arm x indicating that these sites have the greatest amounts of single domain magnetite several remagnetized sites exhibit northerly shallow magnetic components which may be kiaman normal directions we conclude that remagnetization along the new york ontario transect is a chemical remanent magnetization resulting primarily from authigenesis of single domain magnetite we present a model in which pulses of heated fluids triggered magnetite formation scanning electron microscope observation of magnetic extracts from 14 sites show that most of the resolvable magnetite grains are multidomain grains of irregular shape many sites also contain a small percentage of magnetic spheres and or euhedral magnetite particles a majority of the magnetite grains contain fe as the only cation but many also contain minor amounts of other cations including various combinations of cr ni ti zn mn si al ca and mg single domain magnetite is not resolvable with our observational techniques but its presence is inferred from rock magnetic tests much of this magnetite may be of authigenic origin

radiocarbon dating of marine samples from gulf of cadiz the reservoir effect abstract the 14 c content of marine shells can be used as a proxy for past coastal upwelling intensity the iberian coast of the gulf of cadiz can be divided in two regions with different oceanographic conditions the southern portuguese coast with an active upwelling influenced by the northeastern atlantic circulation and the southwestern andalusian coast where due to its configuration a wind driven coastal upwelling is absent because these oceanographic conditions r a parameter that quantifies the marine reservoir effect has positive values with a weighted mean of 65 20 14 c yr southern portuguese coast or negative values with a weighted mean of 135 20 14 c yr andalusian coast nevertheless between 4400 bp and 4000 bp r values are strongly positive suggesting a very strong upwelling existed in the entire region of the gulf of cadiz this does not reflect wind driven processes but probably an eastward extension of the azores front which is characterized by locally intense upwelling along the azores current that penetrated into the gulf of cadiz the study highlights the importance of using appropriate calibration methods when developing 14 c chronologies from the gulf of cadiz as well as the potential of r for reconstructing palaeoceanographic conditions

environmental conditions and geomorphologic changes during the middle upper paleolithic in the southern iberian peninsula article i nfo this study utilizes geomorphology marine sediment data environmental reconstructions and the gorham s cave occupational record during the middle to upper paleolithic transition to illustrate the impacts of climate changes on human population dynamics in the western mediterranean geomorphologic evolution has been dated and appears to be driven primarily by coastal dune systems sea level changes and seismo tectonic evolution continental and marine records are well correlated and used to interpret the gorham s cave sequence specific focus is given to the three hiatus sections found in gorham s cave during heinrich periods 4 3 and 2 these time intervals are compared with a wide range of regional geomorphologic climatic paleoseismic faunal and archeological records our data compilations indicate that climatic and local geo morphologic changes explain the homo sapiens spp occupational hiatuses during heinrich periods 4 and 3 the last hiatus corresponds to the replacement of homo neanderthalensis by h sapiens records of dated cave openings slope breccias and stalactite falls suggest that marked geomorphologic changes seismic activ ity and ecological perturbations occurred during the period when homo replacement took place

hydrothermal fluid controlled remagnetization of sedimentary rocks in korea tectonic importance of pervasive tertiary remagnetization abstract in order to better understand the tectonic environment of the korean peninsula since cretaceous a paleomagnetic study was carried out on the jinan basin located in southwestern korea from the demagnetization of 597 samples we found that remanence is carried by pyrrhotite and or magnetite black siltstone and hematite red siltstone which is a common magnetic mineral assemblage of the rocks remagnetized in early tertiary in korea all the rock units recorded identical ancient geomagnetic field directions in particular paleodirections were better clustered in geographic coordinates indicating a secondary origin the presence of abundant silicates as inclusions within fe oxides and fe sulfides confirms the authigenic origin of the magnetic carriers precipitated by fluid mediated chemical processes compilation of the korean cretaceous and tertiary poles indicates that the jinan basin was remagnetized in the early tertiary age in addition comparison of the korean cretaceous and tertiary poles with those from adjacent major terrains including china and siberia reveals that a dominant feature of tectonic motions in korea was a vertical axis rotation sense of vertical axis rotation was time dependent with clear distinction between clockwise rotation in cretaceous and counter clockwise rotation in early tertiary such differences in the mode of vertical axis rotation might be caused by the activities of basin bounding strike slip faults in korea possibly driven by the changes of subduction regime of the kula pacific plate in early tertiary

remagnetizations and thrusting in the idaho wyoming overthrust belt a study on the relationship between deformation and paleomagnetism was carried out in the frontal segment of the idaho wyoming overthrust belt samples were taken from the jurassic twin creek unit of the most easterly thrust the prospect thrust for paleomagnetic study detailed analysis of the paleomagnetic data showed a synfolding remagnetization with a paleopole latitude longitude 83 n 286 e located on the tertiary segment of the north american apparent polar wander path apwp the age of the paleopole roughly coincides with the age of thrusting on the prospect thrust as determined by synorogenic deposits the magnetization was found to reside in magnetite spherules previous data from the absaroka and darby thrust sheets that lie to the west of the prospect thrust were reevaluated and they also indicate a synfolding age for the remagnetization the corresponding paleopole 64 n 162 e lies on the late mesozoic segment of the north american apwp for these more interior thrust sheets the apparent age of the remagnetization is older than that for the exterior prospect thrust sheet but again roughly corresponds with the inferred age of thrusting for the sampled thrusts we conclude that remagnetization in the idaho wyoming overthrust belt is temporally related to deformation and we suggest that authigenic magnetite growth occurred during discrete fluid pulses that were concurrent with thrusting and folding events

insights into the mechanism for orogen related carbonate remagnetization from growth of authigenic fe oxide a scanning electron microscopy and rock magnetic study of devonian carbonates from northern spain 1 a rock magnetic and sem study of devonian carbonates from the cantabria asturias region northern spain was undertaken to further our understanding of the pervasive remagnetization of carbonate rocks during the late paleozoic and the mechanism by which these remagnetizations occur these rocks contain three ancient late paleozoic magnetizations the rock magnetic properties of mineral extracts were compared with those of whole rock chips and nonmagnetic residue to deduce magnetic carrier s and grain sizes hysteresis measurements for rock chips show typical wasp waisted loops whereas extract shows typical pseudosingle domain like psd unrestricted loops within all sites there is a noticeable contribution of superparamagnetic sp grains seen in hysteresis properties and low temperature magnetization measurements of whole rock chips whereas a trend away from a strong sp contribution is seen when hysteresis properties of whole rock are compared with those of residue and extract consequently our extraction process predictably removes sp grains while preserving the characteristic fraction of remanence carrying material which behaves like a typical mixture of single domain sd and psd magnetite paradoxically the typical fingerprint of remagnetized carbonates as seen in the whole rock data seems to be a response to abundant sp grains associated with the acquisition of chemical remanent magnetizations crm and not the actual remanence carrying population itself scanning electron microscopy sem observations of magnetic extract reveal abundant authigenic fe oxides characterized as either 10 100 m ni free spherules or individual 0 1 10 m euhedral grains sem observations of thin sections reveal abundant evidence of fluid flow driven chemical reactions that resulted in formation of new fe oxide such reactions occurred along cracks and grain boundaries and within void space and are associated with fe rich clay and calcite dolomite reactions or as oxidation of fe sulfide framboids together the sem observations and rock magnetic experiments reveal that the three late paleozoic remagnetizations experienced by cantabria asturias paleozoic carbonates are crms facilitated by the presence of fluids activated during late paleozoic variscan deformation

the relief canyon gold deposit nevada a mineralized solution breccia the relief canyon gold deposit in the humboldt range of western nevada is a low grade high tonnage orebody of tertiary or younger age the host rocks include limestones of the triassic cane spring formation which are overlain by shales of the triassic grass valley formation the rocks were folded and metamorphosed to greenschist grade during jurassic and cretaceous regional tectonic activity mesozoic thrusting may have occurred along the shale limestone contact but evidence has been obscured by later hydrothermal activity the sedimentary rocks were nominally offset along several late tertiary normal faults related to uplift of the range the upper part of the cane spring formation is composed of a poorly sorted breccia composed of limestone clasts with a clay matrix irregular pockets within this zone are filled with clay to pebble sized fragments derived from the grass valley shale the enclosing limestone beds were tilted moderately to the southwest during mesozoic deformation whereas bedding within these pockets is generally horizontal indicating post tilting deposition of the sediments the sediments show graded bedding and other sedimentary features that indicate deposition from flowing water thermally mature carbon derived from the limestone is also concentrated in small pockets in the matrix the breccia unit is likely the product of low temperature solution brecciation ground water dissolved much of the limestone directly beneath the shales progressively creating irregular cavities and the breccia sediments derived from the overlying grass valley shale were fiuvially deposited as a matrix to the developing solution breccia episodic pulses of hydrothermal fluids were introduced along faults and possibly mixed with the ground water in the breccia zone initially jasperoids formed along the faults but later hydrothermal pulses introduced gold silica and fluorine into both the early jasperoids and the unconsolidated cave fill sediments to form the orebody continued solution related brecciation chaotically disrupted the gold deposit gold fluorite pyrite silver calcite and fine grained silica are the principal hydrothermal minerals in the deposit gold was deposited as micron sized flakes of native gold and rarely as electrum during a relatively late stage of silicification of the jasperoids the carbon rich zones and the clay rich matrix of the breccia fluorite was deposited with and later than the gold in the jasperoids and it in part replaced the clay rich breccia matrix antimony arsenic mercury and thallium are directly associated with gold in the orebody the deposit formed at a relatively shallow depth on the basis of fluid inclusion data late stage hydrothermal fluids related to gold and fluorite deposition were extremely dilute and had temperatures near 200 degrees c the fluid inclusions in fluorite show no evidence for boiling but porous crackle breccias in the jasperoids suggest that hydrobrecciation took place

late paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east tennessee evidence for a post ore chemical event thermal demagnetization studies on samples from mineralized and unmineralized rocks in the lower ordovician knox group of east tennessee reveal a mean direction of d i 157 18 95 7 7 k 52 9 after 60 unfolding combining stable endpoint and remagnetization circle data although not significant at the 95 confidence level the fold test is suggestive of a synfolding alleghenian age of the magnetization the magnetization cannot be postfolding i e permian or younger because the in situ directions do not resemble those for post carboniferous times the magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the early permian but unlike poles for pre late carboniferous times characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test the best estimate for the paleopole resulting from this study falls at 40 n 126 e within the late carboniferous segment of the north american paleozoic apparent polar wander path thus the magnetization is interpreted as synfolding and secondary due to widespread kiaman remagnetization blocking temperatures of up to 520 c and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres secondary dolomites have the same magnetic characteristics as the primary limestones and it is inferred that similar magnetite spheres are the carriers of their magnetization because this magnetization and therefore its carrier occurs in sphalerite and secondary dolomite it must be coeval with or younger than the ore which itself is thought to be of middle to late paleozoic age it is argued that the magnetization was acquired during growth of magnetite associated with a late paleozoic chemical event

rock magnetism of remagnetized paleozoic carbonates low temperature behavior and susceptibility characteristics we have conducted a new set of rock magnetic experiments on samples of remagnetized paleozoic carbonates of eastern north america these experiments were designed to investigate the origin of the unusual hysteresis behavior of these rocks by evaluating 1 the importance of ferrimagnetic pyrrhotite as a remanence carrier and 2 the sources of low field susceptibility low temperature measurements of saturation isothermal remanent magnetization sirm indicate that the pyrrhotite magnetic transition at 32 k is absent in the trenton and onondaga limestones of new york this transition is observed but poorly expressed in the knox group of east tennessee room temperature sirms cooled to 10 k in zero field lose a small fraction of their intensity between 30 and 35 k samples from all three formations show a broad peak in irm intensity at about 200 k which is typical of pyrrhotite in the trenton and onondaga samples the ferrimagnetic component of low field susceptibility is significantly larger than the ratio mrs hcr and is thus probably due dominantly to magnetite in some of the knox samples the reverse is true suggesting an important pyrrhotite contribution for all of the samples the ferrimagnetic susceptibility normalized by the saturation magnetization is anomalously high about a factor of 5 or 10 higher than the typical value for magnetite we believe that this indicates a very substantial contribution from superparamagnetic particles strong frequency dependence of susceptibility and very high ratios of anhysteretic to saturation remanence confirm the importance of ultrafine particles spanning the superparamagnetic single domain boundary all three of these chemically remagnetized carbonates units exhibit the following properties which have not previously been found together for any rock or synthetic analog and which therefore appear to constitute a diagnostic set of rockmagnetic criteria for recognizing chemically remagnetized rocks mrs ms 0 89 hcr hc 0 6 k ms 50 m a arm sirm 20 ka mrs hcr 50

structural development during flysch basin collapse the fencemaker allochthon east range nevada abstract during mesozoic deformation in north central nevada a thick succession of triassic siliciclastic and hemipelagic carbonate strata that were deposited in a deep water basin shortened against and overrode penecontemporaneous shallow water carbonates and interbedded siliciclastics at the eastern margin of the basin initial rheologic contrasts developed between lithologic units during shallow pretectonic burial localized displacements within fine grained poorly cemented units and along lithologic boundaries changes in theology during low pressure and low temperature deformation led to more brittle conditions possibly due to fluid expulsion and subsequent displacements truncated bedding structures in triassic strata suggest that the w facing slope of the shelf to basin transition localized thrusting collapse of the basinal strata against strata of the basin margin and formation of a thrust at the shelf to basin transition demonstrate the ability of stratigraphic declivities and associated rheologic contrasts to buttress deformation and localize thrusting

scanning electron microscopy and rock magnetic studies of magnetic carriers in remagnetized early paleozoic carbonates from missouri this study was supported by nsf ear 90 05075 and 92 19616 institute for rock magnetism irm contribution 9303 the irm is funded by the keck foundation the national science foundation and the university of minnesota

triassic flysch of the fencemaker allochthon east range nevada fan facies and provenance the fencemaker allochthon a regionally extensive tectonic feature at the outer edge of the mesozoic north american foreland contains upper triassic flysch deposited in a deep water basin that occupied much the same geographic tract as does the present allochthon polyphase deformation in the allochthon has been sequentially removed to determine the pre tectonic position of strata rocks of the allochthon in the east range of north central nevada include calcareous and siliciclastic strata transported by varied sub aqueous processes thirteen facies are recognized in the allochthon vertical facies sequences are indicative of deposition on middle and inner portions of a submarine fan during retrogradational and progradational cycles flysch deposition began in the early norian and continued through the middle norian with northward migration of major siliciclastic conduits penecontemporaneous shelf strata compose the autochthon to the fencemaker thrust the eastern margin of the bivergent fencemaker allochthon and record early norian deltaic and middle norian post deltaic shelf sedimentation the fencemaker thrust was localized by the shelf to basin transition sediments in both the allochthon and the autochthon were derived primarily from the triassic craton far to the east with addition from local sites regional mesozoic unroofing in north central nevada probably occurred after late triassic time

abrupt temperature changes in the western mediterranean over the past 250 000 years a continuous high resolution western mediterranean sea surface temperature sst alkenone record spanning the past 250 000 years shows that abrupt changes were more common at warming than at cooling during marine isotope stage mis 6 sst oscillated following a stadial interstadial pattern but at lower intensities and rates of change than in the dansgaard oeschger events of mis 3 some of the most prominent events occurred over miss 5 and 7 after prolonged warm periods of high stability climate during the whole period was predominantly maintained in interglacial interstadial conditions whereas the duration of stadials was much shorter

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

theory and application of the day plot mrs ms versus hcr hc 2 application to data for rocks sediments and soils 1 new theoretical curves relating the hysteresis parameters mrs ms and hcr hc for single domain sd superparamagnetic sp pseudo single domain psd and multidomain md grains and their mixtures are applied to published data for natural materials the day plot of mrs ms versus hcr hc has been used to crudely classify samples into box like sd psd and md or sometimes incorrectly md sp regions with arbitrary boundaries new type curves for md psd sd md and sd sp grains and mixtures permit more subtle and precise modeling the predicted md trend and its junction with the psd trend are observed in two data sets for magnetite spherules from carbonate rocks and for temperature varying hysteresis results spanning the verwey transition the latter data are the basis of a suggested new method for pinpointing the psd md threshold size selected data for pottery clays soils and paleosols generally follow sd md type curves and indicate intermediate size psd magnetite with narrow to broad size distributions a lake sediment section with known grain size progression tracks in the predicted sense along the sd md trend selected data for glaciomarine and pelagic sediments are also generally compatible with sd md trends examples of remagnetized carbonate rocks submarine basaltic glasses and glassy rims of pillow basalts all follow predicted sp sd or sp psd mixing curves with a large range in volume fraction of sp grains 0 75 but a narrow range of sp particle sizes 10 2 nm larger sp grains spanning the range to sd size 25 30 nm are absent for unknown reasons oceanic dolerites gabbros and serpentinized peridotites in some cases fall in a novel region of the day plot parallel to but below magnetite sd md mixing curves

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

characteristics of magnetic carriers responsible for late paleozoic remagnetization in carbonate strata of the mid continent u s a magnetic analysis of carbonate strata of the mid continent region of the united states indicates that the late paleozoic remagnetization already recognized in the appalachians also affects almost the entire mid continent magnetic intensity is regionally variable with more intense magnetizations occurring on intracratonic arches magnetization is carried predominately by magnetite which often occurs in tiny hollow spheroids composed of well formed octahedral microcrysts spheroid chemistry and morphology suggests that the magnetite is authigenic composed of iron extracted from pre existing pyrite or clay the timing of the remagnetization the character of the magnetite and the regional variation in magnetic intensity support the idea that the remagnetization is a diagenetic consequence of tectonically driven brine migration

neotectonics and shoreline history of the rock of gibraltar southern iberia abstract several sets of staircased quaternary marine deposits can be observed along the gibraltar coast ranging from 1 to 210 m above the present mean sea level geomorphological mapping establishes from the relationship between shore scree and dune sedimentary formations five main morphotectonic steps on the rock marine terraces between 1 and 25 m 30 60 m 80 130 m 180 210 m and above each terrace level and its slope aeolian linked sediments is backed by a steep relict sea cliff margin so forming a composite cliff the most recent coastal landforms and sediments are associated with the last 250 ka linked to oxygen isotope stages ois 1 3 5 and 7 these landforms determine a morphosedimentary highstand lowstand sequence with several staircased and offlapped episodes comprising a major morphotectonic step a well developed palaeocliff usually separates the highstand marine terraces of ois 9 from those of ois 7 the gibraltar mean tectonic uplift value of 0 05 0 01 mm yr is maintained from 200 ka to the present before this at least to 250 ka the mean uplift rate was higher 0 33 0 05 mm yr possibly compatible with major tectonic events in response to a nnw sse compressive stress field between africa and iberia

sem stem observation of magnetic minerals in presumably unremagnetized paleozoic carbonates from indiana and alabama abstract the silurian wabash formation in indiana and the mississippian pride mountain formation in alabama appear not to have been affected by a late paleozoic remagnetization event in an attempt to characterize the magnetic mineralogy in these presumably unremagnetized carbonates and in order to compare their magnetic mineralogy to that of remagnetized carbonates scanning and scanning transmission microscope sem stem observations and rock magnetic investigations were carried out it is possible to recognize differences in magnetic mineralogy in the unremagnetized carbonate from that in remagnetized carbonates 1 1 iron oxides associated with iron sulfides are hematite in this study as a result of replacement of pyrite instead of magnetite as was found elsewhere 2 2 occurrences of large euhedral pure iron oxides of secondary origin are common in the unremagnetized carbonates 3 3 a rare occurrence of fine grained single crystal magnetite capable of carrying a remanence in the unremagnetized carbonates is noticeable as compared to the abundance of such grains in the remagnetized carbonates although the abundance of the fine grained magnetite grains in remagnetized carbonates is inferred to be a diagnostic factor to distinguish the remagnetized from the unremagnetized carbonates this clarifies only the carriers in the remagnetized rocks and leaves the question of the carriers in unremagnetized limestones unresolved to a large extent the lack of remagnetization is commonly attributed to a restricted amount of fluid influx into the rocks for the wabash and the pride mountain formations this may also be true early cementation has significantly reduced the porosity and permeability in the wabash formation in indiana whereas the presence of the impermeable chattanooga shale may have protected the mississippian pride mountain formation in alabama

palaeoenvironmental and palaeoclimatic proxies of the gorham s cave small mammal sequence gibraltar southern iberia abstract gorham s cave is located in the british territory of gibraltar in the southernmost end of the iberian peninsula the cave was discovered in 1907 and first excavated in the 1950s by john waechter of the institute of archaeology in london new excavations which started in 1997 have exposed 18 m of human occupation in the cave spanning the late pleistocene as well as including brief phoenician carthaginian and neolithic occupations the late pleistocene levels consist of two upper palaeolithic occupations attributed to the solutrean and magdalenian technocomplexes level iii and which are dated by ams radiocarbon to between 18 000 and 10 000 years ago the underlying mousterian layer level iv dated by ams radiocarbon to between 23 000 and 33 000 years ago is separated from level iii by an archaeologically sterile layer which spans some 4000 years this paper presents previously unpublished palaeoenvironmental and paleoclimatic reconstructions of gorham s cave during these pleistocene occupations using the small mammal assemblage the small mammal assemblage at gorham s cave comprises of at least 12 species 4 insectivores crocidura russula sorex gr coronatus araneus sorex minutus and talpa occidentalis 3 chiropters myotis myotis myotis nattereri and miniopterus schreibersii and 5 rodents microtus iberomys cabrerae microtus terricola duodecimcostatus arvicola sapidus apodemus sylvaticus and eliomys quercinus the presence of these small mammal species indicates that the landscape surrounding the rock of gibraltar was predominantly an open habitat with the presence of woodland and water stream meadows as well as the presence of larger bodies of water these results are then compared with pollen and charcoal analysis as well as other faunal proxies such as the herpetofauna bird and large mammal assemblages providing an accurate reconstruction of the climatic and environmental conditions that prevailed during the late pleistocene in the southernmost of the iberian peninsula

a palaeoecological study in the western mediterranean area the upper pleistocene pollen record from cova beneito alicante spain abstract results are presented of palynological analyses of pleistocene deposits at cova beneito an archaeological site on the southern slopes of the sierra de benicadell alicante province se spain 650 m a s l with a wsw orientation the chronology is supported by radiocarbon dating archaeological evolution previous local sequences and recourse to several wider correlations for the first time in the region a warming event is defined in the mid last glacial stage during this event vegetation took on a mediterranean character with sclerophyllous elements such as quercus olea phillyrea rhamnus helianthemum etc the rest of the sequence shows a predominance of pinus within the ap phases of steppe like character and an aridity crisis immediately after the climate improvement due to local conditions forest cover was never very great

geologic evolution of the betic cordilleras in the western mediterranean miocene to the present abstract the betic cordilleras were formed during the eo meso and neo alpine evolution of the western mediterranean area during the neo alpine stage the northward subduction of the african plate took place and the algerian provencal basin opened with the creation of oceanic floor associated space problems produced the expulsion and extensive stretching of the internal zones to the west which in turn disrupted and deformed the external zones of the betic and rif cordilleras these occurrences essentially took place in the burdigalian and continued with diminished intensity into the middle miocene following the suturing of the transcurrent contact between the internal and external zones important n60 90 e and nw se faults occurred the betic cordilleras are characterized as having been deformed by regional horizontal shortening compression oriented wnw ese or nw se which gradually rotated to the nnw sse in the late miocene when its most important intramontane neogene basins were formed the alboran basin with a thinned continental crust was formed as a western prolongation of the opening of the algerian provencal basin with its basic features already apparent in the burdigalian the volcanism present in the southeast of the betic cordilleras and in northeast morocco crossing the alboran basin is mainly late miocene in age it is linked to fracturing and exhibits chemistry related to the different thickness of the crust

coastal uplift in continental collision plate boundaries data from the last interglacial marine terraces of the gibraltar strait area south spain abstract the spatial height analyses of deposits belonging to isotopic substages 5e and 5c indicate the occurrence of differential uplift in the gibraltar strait area maximum mean uplift rates do not reach values of 0 2 mm year displaying a gentle deceleration from 0 15 to 0 10 mm year for the last 128 ka maximum estimated rates are recorded in the central sector of the gibraltar strait the combined analysis of the accumulated vertical movements before and after iss 5c indicates that differential uplift is accommodated along ne sw sinistral and nw se dextral mostly inherited strike slip fault systems separating coastal segments of contrasting uplifting subsiding behaviour the occurrence of inversions in the relative vertical offsets along some of these faults arises when this combined analysis is performed a model of asymmetric pure shear defined by these two fault systems generated in response to the roughly n s convergence between the african and eurasian plates can explain the recorded uplift patterns in this transpressive setting generated by the africa eurasia continental collision the recent convergence 4 mm year is mostly released by shear processes and subsidiary e w horizontal extension whilst the recorded coastal uplift only represents a low percentage of the recent convergence rates

origin of magnetite responsible for remagnetization of early paleozoic limestones of new york state scanning electron microscope and scanning transmission electron microscope observations of thin sections and separates of the devonian onondaga and helderberg limestones and ordovician trenton limestone in new york state allow us to identify three types of magnetite pseudoframboids nonspherical magnetite and fine grained magnetite magnetite was observed replacing pyrite occurring as crystals in spherical pseudoframboids or with nonspherical shapes fine grained magnetite consisting of aggregates of one or more rounded single crystals approximately 2000 a 200 run in diameter could not be observed in thin sections of new york carbonates due to its small size but was found in magnetic extracts hysteresis measurements of magnetic extracts verify that fine grained magnetite is capable of carrying remanent magnetizations however pseudoframboidal magnetite and nonspherical magnetite are polycrystalline and consist of assemblages of single to pseudosingle domain sized crystals that also can be carriers of the remagnetization these data in combination with observations from other localities collectively imply that the alleghenian remagnetization is carried by the fine grained magnetite although pseudoframboidal and nonspherical magnetite may also contribute thus the alleghenian remagnetization is a chemical remanent magnetization due to authigenic magnetite crystallization of magnetite was mediated by fluids with dissolution and crystallization activated by stress during the alleghenian orogeny the origin of such fluids is unknown but they may have originated through crustal scale fluid migration tectonically induced brine migration due to emplacement of thrust sheets is preferred over a meteoric source of the fluids

calciclastic submarine fans an integrated overview abstract calciclastic submarine fans are rare in the stratigraphic record and no bona fide present day analogue has been described to date possibly because of that and although calciclastic submarine fans have long intrigued deep water carbonate sedimentologists they have largely been overlooked by the academic and industrial communities to fill this gap we have compiled and critically reviewed the existing sedimentological literature on calciclastic submarine fans thus offering an updated view of this type of carbonate slope sedimentary system calciclastic submarine fans range in length from just a few to more than 100 km three different types can be distinguished 1 coarse grained small sized most calciclastic submarine fans were formed on low angle slopes and were sourced from distally steepened carbonate ramps subjected to high energy currents under these conditions shallow water loose grainy sediments were transferred to the ramp slope and eventually funnelled into the submarine fan by sediment gravity flows these conditions seem to have been more easily met on leeward margins in which the formation of reefs was hampered by cool waters nutrient enrichment or oligophoty another circumstance that contributes to the transfer of shallow water sediments to the distal ramp slope is a low sea level forcing the carbonate factory closer to the slope break and destabilizing sediments by increased pore water pressure however the most important factor controlling the development of calciclastic submarine fans was the existence of an efficient funnelling mechanism forcing sediment gravity flows to merge downslope and build up a point sourced sedimentary accumulation in most cases this occurred through a major slope depression associated with tectonic structures an inherited topography or large scale mass failures

undrowning a lost world the marine isotope stage 3 landscape of gibraltar abstract the rock of gibraltar at the south western extreme of the iberian peninsula and 21 km from the north african coast is a 6 km long limestone peninsula which was inhabited by neanderthals from mis 5e until the end of mis 3 a total of 8 sites either with neanderthal fossils or their mousterian lithic technology have been discovered on the rock two gorham s and vanguard caves are the subject of ongoing research these caves are currently at sea level but during mis 3 faced an emerged coastal shelf with the shoreline as far as 5 km away at times they hold a unique archive of fauna and flora in the form of fossils charcoal and pollen helping environmental reconstruction of now submerged shelf landscapes in addition geological and geomorphological features a 300 metre dune complex elevated aeolian deposits raised beaches scree speleothems complement the biotic picture the work is further complemented by a study of the ecology of the species recorded at the site using present day observations the species composition in this fossil record closely matches the present day fauna and vegetation of the donana national park sw spain a mosaic of pine groves coastal dunes shrubland and seasonal wetlands and currently the richest reserve in terms of biodiversity in the iberian peninsula located only 100 km to the northwest from gibraltar all this information permits for the first time the quantification of the vegetation structure of the ancient coastal plain and the modelling of the spatio temporal dynamics of the mis 3 coastal shelf off gibraltar

links between marine and atmospheric processes oscillating on a millennial time scale a multi proxy study of the last 50 000 yr from the alboran sea western mediterranean sea abstract the large dataset obtained from the extensive study of images core md95 2043 recovered from the alboran sea western mediterranean shows the periodicities and phase relationships of oceanographic and atmospheric processes on a millennial time scale the 1470 yr cycle is the most significant with the exception of the records reflecting climatic or environmental changes on land which show statistically significant 3300 and 8000 frequency bands the investigation of these core records on a millennial scale resolution allows us to establish the evolution of oceanographic and atmospheric mechanisms that influenced the western mediterranean region in the course of the dansgaard oeschger cycles accordingly possible land sea interactions can be identified and situated in the context of the temporal succession of the different climatic processes for instance saharan dust supply from northern africa appears to lead high latitude climate changes suggesting that low latitude feedback processes were involved in forcing the millennial climatic variability in the westernmost mediterranean

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

magnetic properties of marine limestones numerous paleomagnetic investigations have been carried out on limestones but their rock magnetic properties have often been neglected in this review geological and rock magnetic factors which determine whether marine limestones are suitable for paleomagnetic study are summarized and laboratory techniques for the identification of the magnetic mineralogy are evaluated the conditions under which pelagic limestones form are especially favorable for the acquisition of a stable primary natural remanence nrm probably by a postdepositional alignment process the effectiveness and timing of acquisition of this remanence are influenced by the thoroughness of bioturbation other remanence components can be acquired during diagenesis in the soft sediment right after deposition or much later even in the indurated limestone the magnetic mineralogy of a limestone is difficult to describe optically because of the small grain size and low concentration magnetic techniques are more convenient direct analysis of extracted magnetic minerals is handicapped by the difficulty of obtaining a representative extract a particularly useful technique involves combined observations of isothermal remanent magnetization irm acquisition in strong fields up to at least 4 t and the subsequent destruction of this irm during stepwise or continuous thermal demagnetization the magnetic minerals most commonly identified by these techniques are magnetite goethite hematite and maghemite these magnetic minerals occur in different combinations in the marine limestones of central europe and the mediterranean realm pyrite is common in certain limestones where it can be the precursor of an unwelcome late goethite pyrrhotite may be responsible for unstable magnetization in the swiss helvetic limestones but otherwise is rare thermal demagnetization of some limestones causes changes of susceptibility and coercivity at quite low temperatures 300 400 c due to the breakdown of a metastable mineral such as maghemite goethite or pyrrhotite at high temperatures above 500 c pronounced changes of magnetic mineralogy occur in all limestones because of the growth of new magnetite this necessitates great care in interpretation of thermal demagnetization of high blocking temperature components and often results in observational difficulties due to viscous remanent vrm behavior of the newly formed magnetite vrm is a widespread component of nrm in many limestones but it can usually be removed effectively by standard magnetic cleaning techniques the susceptibility serves as a simple control of mineralogical change during heating and can serve as an indicator of lithologic variation of magnetic mineral concentration and type in magnetostratigraphic profiles the susceptibility is anisotropic and shows a classical depositional pattern of the distribution of principal axes however the probable postdepositional mechanism of nrm acquisition especially in bioturbated sediments implies that the anisotropy pattern results from postdepositional compaction of the sediment

recent coastal evolution of the donana national park sw spain abstract since the last holocene sea level rise about 6900 bp a series of depositional littoral landforms has been generated at the outlet of the guadalquivir river with progradation along the predominant longshore drift towards the east the first coastal progradation occurred between 6900 and 4500 bp the donana and perhaps la algaida spits both associated with the oldest and highest marshland in the donana national park are assumed to have been developed at an early stage originally the guadalquivir estuary was wider and deeper than now and its environment was mainly marine the oldest littoral formations have been dated as ca 4735 bp they show erosional events and indicate the breaking up of earlier spit barriers to form inlets the marine environment became increasingly dominant with heavy erosion of cliffs and a retreating coastline this period was followed by another sedimentary cycle 4200 2600 bp that surrounded the earlier eroded barriers the size of the estuary decreased due to the increasing marsh deposits and a fluvial environment was born about 2600 years ago progradation gave way to a new period of intense erosion the resulting morphology of littoral strands and erosional surfaces permitted the return to a marine environment the shoreline again retreated from 2300 bp coastal progradation has prevailed with an erosional interruption at 1000 bp the present day outlet of the guadalquivir is an estuarine delta of inactive marshland the donana national park the dominant environment is fluvial

abrupt climatic changes during the last glaciation based on pollen analysis of the abric romani catalonia spain the abric romani continental pollen record provides evidence of abrupt climate variations during the last glaciation the pollen record reveals a pattern of high frequency variations similar to that in ice cores as shown by oxygen isotopes or dust content analyses of 14 travertine samples yield coherent u series ages in correct stratigraphic order ranging from 40 000 to 70 000 yr according to changes in the composition and structure of the vegetation five paleoclimatic phases have been differentiated the oldest ranging from 70 200 to 65 500 yr ago records thermophilous taxa which correspond to the last warm events of isotopic stage 5 the next phase from 65 500 to 56 800 yr records a cold humid climate that correlates with isotope stage 4 during the third phase from 56 800 to 49 500 yr relatively warm and cold events alternate the fourth phase from 49 500 to 46 200 yr records a cold dry climate the fifth phase ranging from 46 200 to 40 800 yr records a milder phase that is similar in appearance to the beginning of the holocene

rock magnetism of remagnetized carbonate rocks another look authigenic formation of fine grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks authigenic magnetite grains dominantly in the super paramagnetic and stable single domain size range also give rise to distinctiverock magnetic prop erties now commonly used as a fingerprint of remagnetization we re examine the basis of this association in terms of magnetic mineralogy and particle size distribution in remagnetized carbon ates having these characteristic rock magnetic properties including wasp waisted hysteresis loops high ratios of anhysteretic remanence to saturation remanence and frequency dependent susceptibility new measurements on samples from the helderberg group allow us to quantify the proportions of superparamagnetic stable single domain and larger grains and to evaluate the mineralogical composition of the remanence carriers the dominant magnetic phase is magnetite like with sufficient impurity to completely suppress the verwey transition particle sizes are extremely fine approximately 75 of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single domain although it has been pro posed that the single domain magnetite in these remagnetized carbonates lacks shape anisotropy and is therefore controlled by cubic magnetocrystalline anisotropy we have found strong exper imental evidence that cubic anisotropy is not an important underlying factor in the rock magnetic signature of chemical remagnetization the usefulness of a palaeomagnetic remanence is directly related to the accuracy with which its age is known e g van der voo 1990 primary remanence acquiredduringorverysoonafterrockformation pro videsdirectinformationonpalaeofieldorientationand strengthatthattime thepossibilityofpartialorcom plete remagnetization at a later time complicates palaeomagneticinterpretation ashaslongbeenrecog nized e g graham 1949 every robust palaeomag netic study must therefore include some effort to constrain the ages of identified components of the natural remanent magnetization nrm relative dating with respect to sedimentary processes struc tural tilting or cross cutting relationships is the basis of the classical geometric palaeomagnetic tests fold test and conglomerate test graham 1949 baked contact test everitt clegg 1962 unconformity test kirschvink 1978 the special circumstances required for application of these tests are often not available however or positive test results may providerelativelylooseconstraintsallowingmagneti zation ages that significantly post date the age of the rock formation as a result other evidence although generally more indirect becomes essential in evaluating the age origin and significance of nrm components such evidence commonly includes petrographic observations isotopic andgeo chemical data and rock magnetic characterization by their very existence magnetic overprints provide evidence of some process or event of

are magnetite spherules capable of carrying stable magnetizations to determine if magnetite spherules extracted from ancient carbonate rocks can carry ancient remanent magnetizations electron microscope observations and magnetic data have been obtained for more than 100 individual spherules extracted from the jurassic twin creek limestone wyoming and the mississippian leadville limestone colorado two categories of surface textures smooth and patterned correlate with different hysteresis parameters spherules with patterned surfaces group a have ratios of saturation remanent magnetization to saturation magnetization msr ms 3 most spherules with smooth surfaces group b have 0 1 msr ms 0 3 and hcr hc 3 the patterned surfaces consist of 1 parallel dendritic nets with units 1 m wide 2 platelets with widths of 1 to 3 m 3 mosaic surface units with diameters between 2 and 5 m each of these surface elements consists of a single crystal as verified by selected area electron diffraction patterns showing sharp spots whereas the spherules as a whole show multicrystalline patterns by x ray diffraction the msr ms and hc values of group a increase with decrease in grain size these group a magnetite spherules are undoubtedly multi domain in character and are unlikely to carry stable ancient remanences the smooth surfaced group b spherules are not very abundant 8 of the entire population and have pseudo single domain psd or multi domain md magnetic properties they may carry ancient remanences although we infer that single domain sd non spherical submicrometer magnetite is the principal carrier of remanence in most carbonates