directions and intensities of the earth s magnetic field during a reversal results from the permo triassic siberian trap basalts russia abstract an extensive palaeomagnetic study was carried out on an approximately 250 ma old reversed to normal transition of the earth s magnetic field emf recorded in 86 volcanic lava flows of the siberian trap basalts north siberia russia in addition to the investigation of the directional behaviour of the field 700 specimens a total of 298 specimens was subjected to thellier type palaeointensity determinations adding several modifications to the original thellier experiment such as tests for md tails and the additivity of partial thermoremanent magnetisation yields highly reliable palaeointensity estimates of the late permian early triassic emf transitional directions of the emf were obtained from 20 flows during the reversal a clustering of the virtual geomagnetic poles vgps is observed 15 flows palaeointensity estimates suggest that this feature is not an artifact due to rapid flow emplacement since the directional cluster is associated with a well defined increase in palaeointensity from 6 to 13 t subsequently the next vgps move towards the pole position of normal polarity departing in a sudden movement from normal polarity the vgps form a second directional cluster comprising the results of 14 flows this feature is interpreted as a post transitional excursion but lacks the characteristic intensity variation recorded during the first transitional cluster the rest of the section 41 flows is of normal polarity the characteristic features of this reversal low intensities and directional clustering during the reversal and an excursion shortly after the reversal were also observed in records of polarity transitions of younger age this suggests that the underlying reversal processes were similar the mean virtual dipole moment calculated for the stable normal part of the studied section yields a rather low value of 2 2 0 9 1022 am2 these findings confirm that the mesozoic dipole low extends at least down to the late permian early triassic the geomagnetic vgp dispersion was calculated for the stable normal part of the section and yields values similar to those for the last 5 ma considering that the intensity of the emf was considerably higher in this time interval than in the early triassic a direct relationship between intensity and secular variation seems unlikely

low latitude and southern hemisphere origin of anisian triassic bedded chert in the inuyama area mino terrane central japan paleomagnetic and paleontologic studies of middle triassic to earliest jurassic radiolarian bedded chert in the inuyama area southern mino terrane of central japan have shown that the chert was deposited in an equatorial region with a paleolatitude between 10 n and 10 s and in particular the lower to middle anisian chert depositional basin was situated in the southern hemisphere the chert has four characteristic components of remanent magnetization the highest blocking temperature component carried by hematite shows positive fold and reversal tests and is regarded as the primary remanent magnetization acquired just after deposition the other three components are interpreted as a present field overprint and two postfolding remagnetizations the absolute tilt corrected inclinations of the primary component are less than 20 for all the samples of middle triassic to earliest jurassic age with the mean inclination of the lower to middle anisian chert being 11 0 4 3 paleomagnetic polarity reversals observed for the lower to middle anisian chert are well correlated with those of the european triassic sections under paleontologic age constraints using radiolarians and conodonts provided that the anisian chert in inuyama was deposited in the southern hemisphere this conclusion illuminates the paleogeographic evolution of the chert bearing accretionary complexes not only in japan but along the eastern margin of asia as well

calibration of magnetic granulometric trends in oceanic basalts abstract the validity of magnetic granulometric estimates relies heavily on the ability to distinguish ultrafine particles from coarser grains for example populations with dominantly superparamagnetic sp or multidomain md grains both are characterized by low remanence and coercivity and distinguishing these endmembers may provide valuable clues to the origin of magnetization in the intervening stable single domain sd size range the natural grain size variations associated with variable cooling rates in submarine lavas provide a rare opportunity for examining progressive changes in average magnetic grain size from sp sd mixtures in submarine basaltic glass to sd md mixtures in flow interiors based on microanalysis and rock magnetic measurements on pillow basalt samples dredged from the flanks of the mid atlantic ridge ages

self reversal of magnetization in oceanic submarine basalts studied with xmcd 1 in oceanic basalts self reversal of magnetization can be produced during extreme low temperature oxidation of titanomagnetite by ionic reordering which leads to neel n type magnetism titanomaghemites showing n type reversal below room temperature were found in submarine basalts recovered during ocean drilling program odp leg 197 in order to better understand the mechanism of self reversal we carried out x ray magnetic circular dichroism xmcd at fe k edge at room temperature and low temperature on such a titanomaghemite sample as well as on pure magnetite and maghemite samples we found that the xmcd spectrum of the n type titanomaghemite at 20 k is a mirror image of the xmcd spectrum at 300 k which shows that the octahedral and tetrahedral subnetworks reverse in this process ligand field multiplet calculations of xmcd at fe k edge help identify the contributions of the different elements in the measured xmcd spectra this mechanism could also cause self reversal above room temperature which has important consequences for the reliability of paleomagnetic measurements

magnetic hysteresis properties of synthetic monodomain titanomaghemites titanomagnetites of near stoichiometric composition fe 2 4 al o 4 0 0 1 and 0 2 were oxidized in air to form compositions in the corresponding titanomaghemite system the magnetic hysteresis properties saturation magnetization isothermal remanence coercive force initial susceptibility were determined as a function of degree of oxidation in some cases being measured at elevated temperatures the titanomagnetites of the submarine crust are believed to become progressively maghemitized and the measured properties are related to the way in which the intensity and stability of an initially acquired thermoremanence will be modified during submarine weathering the results from the present monodomain synthetic analogues suggest that the intensity of remanence would fall by up to 50 during weathering this fall in intensity might be increased by a fraction of the grains becoming superparamagnetic although this was not so with the grain size range of the synthetics the stability of remanence would at first be increased by maghemitization and subsequently fall the rise in susceptibility with maghemitization would lead to an enhancement of the component of magnetization induced in an ambient field although the study probably simulates the compositions produced during submarine weathering the changes in microstructure e g due to grain cracking or heterogeneities in larger grains which may occur in nature are probably not reproduced in the synthetic analogues

inversion of titanomaghemite in oceanic basalt during heating abstract we studied the change of magnetic behaviour upon laboratory heating of altered mid ocean ridge basalt morb samples in the age range of 16 35 ma to determine the influence of titanomaghemite inversion on the thermal demagnetisation of natural remanent magnetisation nrm of these basalts morb samples were heated to successively higher temperatures and at the same time the temperature dependence of either saturation magnetisation or nrm was monitored continuously after each heating step hysteresis loops and remanent magnetisation curves between 10 k and room temperature were measured with this procedure it is shown that the dominant magnetic remanence carrier in our morb samples is cation deficient titanomaghemite moreover it is demonstrated that the titanomaghemite is gradually changing to a ti poor titanomagnetite as the final inversion product during inversion both the curie temperature as well as the maximum unblocking temperature of the nrm are gradually increasing we show that the paradox of unblocking temperatures above the curie temperatures often observed for altered morbs is an artefact of this gradual heating induced inversion process

well documented travel history of mesozoic pelagic chert in japan from remote ocean to subduction zone the mino tanba belt in southwest japan a segment of the cordilleran type orogenic chain of jurassic east asia is composed mainly of a middle upper jurassic subduction accretion complex in which triassic and lower jurassic bedded radiolarian cherts occur as large allochthonous units structurally interlayered with middle upper jurassic clastic rocks high resolution microfossil conodont and radiolaria research has identified very low average sedimentation rates of about 0 5 g cm 1000 yr in the chert units similar to those of modern pelagic sediments accumulated in open ocean environments judging from the low average sedimentation rate high purity of biogenic silica long duration of continuous deposition 50 m y and wide along strike extent 1000 km the bedded radiolarian cherts in the mino tanba belt are best understood as ancient pelagic sediments that accumulated in open ocean environments accordingly the alleged origin in smaller marginal basins is untenable upward lithologic change from bedded chert to overlying siliceous mudstone in the uppermost portion of chert sequences suggests the gradual landward approach of the oceanic plate toward a trench the tectonic interlayering of these cherts and coarse grained terrigenous elastics is a secondary feature that was added through duplexing underplating in the subduction zone on the basis of the primary stratigraphy and field occurrence of triassic bedded chert in the mino tanba belt newly proposed are an idealized oceanic plate stratigraphy and a generalized travel history of a cordilleran type bedded chert from its birth at a mid oceanic ridge to its demise at a subduction zone

saturation magnetostriction and its low temperature variation inferred for natural titanomaghemites implications for internal stress control of coercivity in oceanic basalts summary highly oxidized titanomaghemite in oceanic basalts often carries remanent magnetization of high coercivity stability helping preserve the oceanic magnetic anomaly pattern we study the source of this high coercivity in four oceanic basalts from odp sites 238 572d 470a and 556 containing highly oxidized titanomaghemite titanium content parameter x 0 55 and oxidation parameter z 0 9 on average most of the titanomaghemite is likely in singledomain grains with uniaxial anisotropy because the ratio of saturation remanence j rs to saturation magnetization j s approaches 0 50 j rs j s 0 46 on average we show that the uniaxial anisotropy is very likely magnetostrictively controlled through internal stresses i in the titanomaghemite grains this allows us to use a novel indirect method to estimate the saturation magnetostriction s of the titanomaghemite a saturation remanence j rs is given along the axis of a cylindrical sample of each basalt then a small compression is applied

n type magnetism at cryogenic temperatures in oceanic basalt abstract here we report on a self reversal of magnetization at cryogenic temperatures 25 60 k due to n type thermomagnetic behavior in samples of cretaceous mid oceanic ridge basalt recovered from pacific deep sea drilling project site 166 the magnetic carrier is highly oxidized single to pseudo single domain titanomaghemite with a composition 0 57 x 0 66 0 8 z 0 9 consistent with model predictions of ionic reordering the observation of n type magnetization provides further insight into the compositional field in which ionic reordering and self reversal can occur in oceanic basalt specifically our observations support the original models of verhoogen verhoogen j 1956 ionic reordering and self reversal of magnetization in impure magnetites j geophys res 61 201 209 verhoogen j 1962 oxidation of iron titanium oxides in igneous rocks j geol 70 168 181 and o reilly and banerjee o reilly w banerjee s k 1966 oxidation of titanomagnetites and self reversal nature 211 5044 26 28 indicating that self reversal above and below room temperature is limited to relatively high titanium contents x 0 6 and high oxidation states

geophysical and petrophysical study of an iron oxide copper gold deposit in northern sweden abstract a geophysical petrophysical study has been performed in an area wsw of the city of kiruna northern sweden the sub regional tectonic setting is dominated by two important shear zones which define the boundary of a granitic body many cu fe occurrences are located in proximity of faults related to these major deformation zones particular attention has been given to the tjarrojakka iron oxide copper gold iocg deposit here the bedrock is characterised by intermediate to mafic meta volcanics metamorphosed intermediate to mafic dykes and gabbroic dioritic intrusions of svecofennian ages 1 96 1 75 ga the major cu and fe occurrences are hosted by the meta andesites the aim of the study is to put the deposits into a tectonic framework and test existing hypotheses for their occurrences glacial deposits cover almost the entire area leading to a scarcity of outcrops and inferring that geophysical data are fundamental for geological understanding in addition to this petrophysical analysis is vital for the interpretation of geophysical data gravity airborne magnetics and radiometrics very low frequency and for the definition of geophysical signatures of the deposits the anisotropy of magnetic susceptibility ams was also studied for the tectonic analysis more than 150 oriented samples were collected in a number of outcrops along a profile intersecting the major structures in the tjarrojakka area from the airborne magnetic data two major linear features are interpreted as deformation zones the strike of these deformation zones is approximately nw se and e w respectively the same trends have been defined from other geophysical data such as airborne vlf and ground gravity data a third important structural trend striking sw ne has been defined by k th data and ground magnetic data very good agreement has been found between geophysical lineaments and ams directions magnetic foliations determined by ams measurements confirm the existence of three major trends in the study area sw ne e w and nw se the major fe orebody shows approximately a sw ne strike direction as defined from ground magnetic data this is parallel to the strike of magnetic foliation determined in outcrops 1 km nw of the deposit the epigenetic nature of the cu and fe occurrences in tjarrojakka and their spatial relationship with deformation zones suggest a connection between the formation of the deposits and a tectonic event a later tectonic episode resulted in e w trending deformation in the central area affecting the orebodies themselves other probable compressive deformations have been indicated from petrophysical and geophysical analyses thermomagnetic measurements indicate that fe oxides ti magnetite are common in the area while fe sulphides are almost absent multi domain magnetite has been identified as the most common fe oxide in different rock types while an unstable magnetic mineral has been detected in metamorphosed volcanics a good spatial correlation has been observed between cu deposits and high k th values from radiometric data values that are expressions of potassic alteration

unraveling the magnetic carriers of igneous cores from the atlantic pacific and the southern indian oceans with rock magnetic characterization previous rock magnetic investigations of oceanic basement samples have been mainly restricted by single drill holes of dredge collections in this study we present new and published data on the rock magnetic properties of igneous rock samples recovered during seven ocean drilling program odp legs in the atlantic pacific and the southern indian oceans the recovered igneous rocks from these legs are mainly basaltic flows both submarine and subaerial diabase sills and serpentinized peridotites with approximate ages of 140 000 years 10 12 34 69 100 108 and 110 121 ma which offers an opportunity to investigate magnetic properties and mineral changes attending alteration of basement rocks over a wide range of ages from variable tectonic settings titanomagnetite and titanomaghemite are commonly present in igneous rock samples of the ontong java plateau the kerguelen plateau broken ridge the eastern equatorial pacific the middle atlantic ridge trans atlantic geotraverse area and the newfoundland iberia rifted margins for basaltic rock samples three general groups a b and c respectively can be divided in terms of rock magnetic properties samples from group a have a single phase of ti poor titanomagnetite with curie temperatures ranging between 480 and 580 degrees c and exhibit a strong verwey transition in the vicinity of 110 k basalts from this group are most likely good paleomagnetic recorders and probably have preserved original and stable magnetic remanences group b is mainly observed in pillow lavas and is characterized by a curie temperature of 260 280 degrees c which is typical of low temperature oxidized titanomaghemite or titanium rich titanomagnetite the low temperature curves for group b do not show the verwey transition group c has more than one curie temperature which suggests the presence of multiple magnetic phases the thermomagnetic signature indicates the inversion of titanomaghemite to a strongly magnetized magnetite as shown by the irreversible cooling curves for the serpentinized peridotites recovered from both sides of the newfoundland iberia rifted margins results from low temperature measurements show that titano magnetites are present in the dark colored peridotites with a strong verwey transition in the vicinity of 110 k and with field and frequency dependent susceptibility curves that resemble those of synthetic tmo in contrast to the magnetic properties observed from the dark colored peridotites the low temperature curves for the yellow brown colored peridotites did not show any verwey transition thermomagnetic analysis also failed to show evidence for titanomagnetites the remanent magnetization is carried by a thermally unstable mineral that breaks down at about 420 degrees c probably maghemite the magnetic signatures of the serpentinized peridotites recovered from both sides of the newfoundland iberia rift appear not in conflict with the notion that conjugate margins will have generally similar crustal structure and evolution history our rock magnetic data indicate that the differences in the rock magnetic properties of basaltic rocks are mainly a function of mineralogy and alteration there is no apparent coincidence between the age of the rocks and the degree of low temperature alteration as suggested by the presence of nearly unoxidized titanomagnetite in the cretaceous aged basalts and by an examination of curie temperature versus sample age that does not unambiguously show a positive relationship between the degree of low temperature oxidation and crust ages c 206 elsevier b v all rights reserved less

influence of the changing lunar orbit on the astronomical frequencies of pre quaternary insolation patterns the sensitivity of the frequencies of the earth s orbital elements involved in the astronomical theory of paleoclimates obliquity and climatic precession to the earth moon distance and consequently to the length of the day and to the dynamical ellipticity of the earth is investigated for the last billions of years but the new frequencies have been computed for the last 440 million years only a time interval over which data and a model of the earth were available moreover we consider only the impact of the changing precessional parameter assuming that the fundamental frequencies of the planetary motion are steady at the geological time scale under such hypotheses the shortening of the earth moon distance and of the length of the day back in time induces a shortening of the fundamental astronomical periods for obliquity and precession a shortening that is larger for longer periods

micromagnetics of paleomagnetically significant mineral grains with complex morphology 1 micromagnetic calculations relevant to paleomagnetism have generally focused on ideal shapes such as spheres and cubes however oxide grains that occur naturally in rocks often have irregular morphologies highly skeletal dendritic forms being particularly common to investigate the potential effects of complex grain morphology on magnetic stability we have carried out two parallel sets of calculations the first is based on randomly distorted 30 120 nm magnetite spheres we use a three dimensional finite element boundary integral micromagnetic model able to generate suitable morphologies unrestricted by the regular cell structure required by finite difference models the second model consists of a 300 nm magnetite octahedron from which most of the material has been removed to leave a small octahedral crystallite at each of the six apices connected by an orthogonal framework of thin rods the results obtained imply that micromagnetic models currently provide no unambiguous evidence that morphological complexities endow magnetite nanoparticles with enhanced coercivity

self reversed magnetization carried by titanomaghemite in oceanic basalts abstract self reversal the remarkable ability of some rocks to acquire a magnetization antiparallel to the earth s magnetic field was of great interest in the 1950s and 1960s but it was subsequently found to be rare limited to a few hemoilmenite bearing igneous rocks here we show through thermal demagnetization experiments that pillow basalts from the northwestern pacific ocean lacking hemoilmenite contain two nearly antiparallel components of natural remanent magnetization partial thermoremanent magnetization experiments indicate that the component isolated at lower temperatures is a self reversal of the primary magnetization the self reversal is carried by titanomaghemite formed by low temperature oxidation because maghemitization of oceanic crust is ubiquitous self reversals may be an important factor contributing to the decrease with age in marine magnetic anomaly amplitudes observed for crust formed during the last 30 million years

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

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

volcanism and deep ocean acidification across the end triassic extinction event abstract the end triassic extinction event marks one of the big five mass extinction events of the phanerozoic the ultimate cause of the extinction is considered to be volcanic activity at the central atlantic magmatic province camp yet the underlying nature of global environmental changes that accompanied the biotic turnover remains elusive here we present chemical and mineralogical studies across the end triassic extinction level of the deep sea chert sequence inuyama japan depleted hematite content normalized by terrigenous material predated the end triassic extinction level with significant rock color change from brick red to purple which is consistent with the rock magnetic records of hematite reported this suggests the loss of authigenic hematite possibly due to the acidification of bottom water and the underlying sediment pore water this timing is consistent with the initial eruption of camp volcanism suggesting a catastrophic release of greenhouse gases as a cause of deep ocean acidification across the end triassic extinction interval mgo al2o3 fe2o3 al2o3 and al2o3 sio2 increased with change in color from purple to dusty red this trend became close to those of weathered camp basalts in arid area implying that it became the considerable source of aeolian dust in cherts after the end triassic extinction event these temporal relations support the synchrony among the initial eruption of camp deep ocean acidification and the end triassic extinction similar rock color changes of cherts might have potential information for the volcanisms and deep ocean acidification in other geologic events

oceanic basalt continuous thermal demagnetization curves summary the palaeomagnetic standard technique of stepwise thermal demagnetization std long regarded as unreliable for oceanic basalts that have undergone low temperature alteration has recently been applied in a number of studies to characterize the natural remanent magnetization nrm of such rocks in order to better understand std data of oceanic basalts and to possibly identify the magnetominerals that are carrying the nrm we have carried out a number of continuous and std experiments on seven oceanic basalt samples during continuous thermal demagnetization ctd a sample is heated to a certain temperature and its nrm is measured during heating and subsequent cooling even when ctd reveals only titanomaghemite unblocking at 400 c as the remanence carrier std behaviour can be very complex and unblocking is observed at temperatures of up to 500 c and higher ctd also allowed to identify a partial or full self reversal of nrm due to interaction between two types of magnetominerals in one sample the higher degree of maghemitization of smaller titanomaghemite grains with respect to larger ones which are less efficient in carrying the remanence was seen for three samples by a shift of 80 c between the strong field thermomagnetic curve and the nrm measured at elevated temperature in several cases the identification of the nrm carrying magnetomineral was not possible from ctd data due to the ambiguity of curie temperatures in the titanomagnetite titanomaghemite system

comment on detecting uniaxial single domain grains with a modified irm technique by r mitra l tauxe and j s gee summary in a recent article mitra et al 2011 propose a modified irm technique to identify the symmetry of magnetic anisotropy in single domain particle ensembles they apply this technique to support an earlier suggestion that single domain grains in young mid ocean ridge basalts morb exhibit multiaxial anisotropy here it is shown that the design of their measurement is flawed in that they do not take into account that the outcome essentially depends on the initial demagnetization state of the sample before the experiment and on the coercivity distribution of the sample because all morb specimens measured by mitra et al 2011 carried their original nrm which closely resembles a thermally demagnetized state their measurements first of all reflect the coercivity distributions and domain states of the samples and contain little or no information about the symmetry of the magnetic anisotropy all arguments previously put forward in favour of a dominant uniaxial anisotropy in morb are therefore still valid

curie temperatures of synthetic titanomagnetites in the fe ti o system effects of composition crystal chemistry and thermomagnetic methods 1 the present study is aimed at improving the calibration of the compositional dependence of the curie temperature tc of titanomagnetite tmt on the basis of temperature dependent magnetic susceptibility c t curves measured on synthetic tmts in the fe ti o system in order to assess the possible influence of high temperature cation vacancies onto the tc values we have synthesized two types of assemblages in subsolidus conditions at 1 bar 1100c and 1300c under controlled oxygen fugacity conditions tmts synthesized in equilibrium with ilmenite hematitess ilmss are expected to have the highest vacancy concentrations those in equilibrium with wustite wus the lowest the composition and homogeneity of the synthetic tmts were carefully checked with a scanning electron microscope sem and an electron microprobe emp tc was determined from c t curves using a kappabridge and for comparison from ms t curves measured with a variable field translation balance our data set shows systematically higher tc values for tmt coexisting with ilmss than for tmt coexisting with wus most c t curves are nonreversible whereby the largest dtc 40 k concern tmt ilmss o f intermediate compositions synthesized at 1300c nonreversibility is interpreted as reflecting cation reordering in tmt during the high temperature c t measurements tc values obtained from ms t curves are higher than those obtained from the c t curves whereby the difference regularly increases up to 40 k with increasing ti content up to xusp 0 6 our new calibration curves are suitable to retrieve tmt compositions in basalts that were rapidly cooled and not oxidized by deuteric or hydrothermal fluids citation lattard d r engelmann a kontny and u sauerzapf 2006 curie temperatures of synthetic titanomagnetites in the fe ti o system effects of composition crystal chemistry and thermomagnetic methods j geophys res 111 b12s28

high resolution stratigraphy and zircon u pb geochronology of the middle triassic buchenstein formation dolomites northern italy precession forcing of hemipelagic carbonate sedimentation and calibration of the anisian ladinian boundary interval orbitally forced cyclic variations in sedimentary sequences provide evidence for short term fluctuations of earth climate and a tool for high resolution timescale calibration we here present stratigraphic and geochronological evidence for precession forcing in middle triassic hemipelagic limestones of the buchenstein formation dolomites northern italy high resolution stratigraphy of several correlative sections of the buchenstein formation documents a coherent cycle pattern isotope dilution thermal ionization mass spectrometry zircon u pb geochronology of tuffs bracketing the cyclic interval reveals an average cycle duration of 18 5 2 1 kyr consistent with a shorter climatic precession cycle in the middle triassic compared with today this suggests a predominantly precession controlled climate in this low latitude setting of the western tethys and allows high precision calibration of the anisian ladinian boundary interval from integrating cyclostratigraphic and u pb geochronological constraints our best estimate for the age of the anisian ladinian boundary is 241 464 0 064 0 097 0 28 ma we also provide precise estimates for lithostratigraphic boundaries biostratigraphic markers and magnetic reversals within the boundary interval stratigraphic intervals with elevated sedimentation rate record a sub milankovitch signal that may be equivalent to patterns in adjacent carbonate platforms such as the latemar platform the origin of this sub milankovitch signal remains unknown but highlights the potential to investigate shorter term climatic variations in mesozoic sedimentary sequences supplementary material isotope dilution thermal ionization mass spectrometry u pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables as well as the results of the bayesian age modelling are available at https doi org 10 6084 m9 figshare c 3861817

paleomagnetism of triassic and jurassic red bedded chert of the inuyama area central japan a paleomagnetic study was conducted on triassic and jurassic red bedded chert from the inuyama area central japan paleomagnetic specimens were collected from single chert beds at nine sites within mesoscopic 1 5 m folds whose ages were determined by radiolarian fossils progressive thermal demagnetization experiments were conducted and three of the four magnetization components provided tectonic implications component d unblocking temperature of 560 680 c from six sites passes the fold test with the mesoscopic folds paleolatitude was estimated as 12 3 15 6 n or s 19 0 10 5 n or s and 25 7 16 7 n or s for 237 ma 223 5 ma and 210 ma respectively in the case of northern latitude a northward migration of 5 6 9 cm yr is estimated between 237 ma and 210 ma with no significant latitudinal migration from 210 ma to 160 ma component c unblocking temperature of 350 530 c has normal polarity and is thought to have been acquired as a chemical remanent magnetization carried by magnetite which was acquired as synfolding magnetization with the sakahogi synform the magnetization is considered to have been acquired through heating in the accretionary wedge during the cretaceous normal superchron 120 5 83 5 ma component b unblocking temperature of 200 350 c has reversed polarity and is thought to have been acquired as a thermoviscous remanent magnetization after the bending of the sakahogi synform the most probable period of the magnetization acquisition is between c27r and c24r 58 63 ma after the emplacement of late cretaceous granitic batholiths

approach to saturation analysis of hysteresis measurements in rock magnetism and evidence for stress dominated magnetic anisotropy in young mid ocean ridge basalt abstract young mid ocean ridge basalts contain titanomagnetite crystals of varying size and composition many studies of their hysteresis properties have found m rs m s ratios considerably above the theoretical limit of 0 5 for uniaxial single domain particles since titanomagnetite is a cubic mineral high m rs m s could occur due to cubic anisotropy which allows for m rs m s values up to 0 866 on the other hand titanomagnetites with high ti content possess extremely large magnetostriction constants already slight internal stress easily outweighs cubic anisotropy and enforces uniaxial behavior are high m rs m s ratios now a proof for very low internal stress no on the contrary previous studies on synthetic titanomagnetite with high m rs m s show that after annealing this ratio decreases a possible explanation is that insufficient saturation of the hysteresis loop used to infer m rs m s leads to underestimation of m s here a systematic experimental study on a young mid ocean ridge basalt using fields of up to 7 t demonstrates that indeed the m rs m s ratio of the single domain fraction does not significantly deviate from the theoretical value of 0 5 for uniaxial anisotropy it is further estimated that internal stress above 200 mpa is necessary to explain the observed hysteresis behavior a value which is consistent with recent independent approximations on the other hand theoretical loops for cubic minerals do not fit the observed data in order to assess the validity of m s determinations from hysteresis measurements an improved method to evaluate the approach to saturation behavior of hysteresis loops is developed it allows to recognize insufficiently saturated loops and thereby helps to avoid misinterpretation of standard m rs m s measurements when high fields are not accessible

effect of maghemization on the magnetic properties of nonstoichiometric pseudo single domain magnetite particles the effect of maghemization on the magnetic properties of magnetite fe3o4 grains in the pseudo single domain psd size range is investigated as a function of annealing temperature x ray diffraction and transmission electron microscopy confirm the precursor grains as fe3o4 ranging from 150 to 250 nm in diameter whilst mossbauer spectrometry suggests the grains are initially near stoichiometric the fe3o4 grains are heated to increasing reaction temperatures of 120 220 c to investigate their oxidation to maghemite fe2o3 high angle annular dark field imaging and localized electron energy loss spectroscopy reveal slightly oxidized fe3o4 grains heated to 140 c exhibit higher oxygen content at the surface off axis electron holography allows for construction of magnetic induction maps of individual fe3o4 and fe2o3 grains revealing their psd vortex nature which is supported by magnetic hysteresis measurements including first order reversal curve analysis the coercivity of the grains is shown to increase with reaction temperature up to 180 c but subsequently decreases after heating above 200 c this magnetic behavior is attributed to the growth of a fe2o3 shell with magnetic properties distinct from the fe3o4 core it is suggested there is exchange coupling between these separate components that results in a vortex state with reduced vorticity once fully oxidized to fe2o3 the domain states revert back to vortices with slightly reduced coercivity it is argued that due to a core shell coupling mechanism during maghemization the directional magnetic information will still be correct however the intensity information will not be retained

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

late cretaceous orbitally paced carbon isotope stratigraphy from the bottaccione gorge italy abstract a refined astronomical tuning of the upper albian lower campanian is proposed from the bottaccione reference section gubbio central italy long term eccentricity cycles filtered from a high resolution 13c record were tuned to the highly stable 405 kyr cycles of the new la2010 astronomical solution for the earth s orbital elements the achieved orbital tuning provides a new precise and accurate age model for dating biostratigraphic magnetostratigraphic and carbon isotope events through a 23 myr long record cycles of 8 0 4 7 3 4 and 2 4 myr modulate the entire 13c record thus extending their detection from the cenozoic to 100 ma and represent primary and stable long term oscillation modes of earth s climate ocean system although an ultimate driver of these long term periodicities is lacking we speculate that specifically the periodicity at 4 7 myr represents a homologue of the present eccentricity grand cycles evolved by the chaotic behaviour of solar system planets during the mesozoic the long term periodicities potentially reflect an unexplored expression of the low frequency response of the carbon cycle to global biogeochemical dynamics of major nutrients particularly phosphorus associated with modulation of inputs to the ocean in turn triggered by high order marine transgressions and formation of highly productive shelf seas this very long term eccentricity control modulated by periodic low energy cycles is suggested to play a crucial role in carbon cycling controlling a chain of climate sensitive global biogeochemical processes on the earth finally these grand cycles provide a potential tool for geological correlation and provide a robust constraint for accurate calculation of the orbital evolution of the solar system

the stardalur magnetic anomaly revisited new insights into a complex cooling and alteration history abstract this study provides new rock magnetic and magneto mineralogical data including mossbauer spectroscopy of basaltic drill cores from the stardalur volcanic complex iceland in order to better understand the strong magnetic anomaly which is caused by an extraordinary high natural remanent magnetization nrm nrm and magnetic susceptibility display a positive linear correlation r 2 0 81 and reach very high values up to 121 a m and 148 10 3 si although a curie temperature of 580 c and a verwey transition at about 160 c is indicative of magnetite t heating experiments in argon and air atmosphere and thermal demagnetization measurements of nrm revealed a slight cation deficiency according to induced remanent magnetization experiments the remanence is carried solely by this low coercive phase minor titanomaghemite with a t c at about 340 c only occurs in samples with larger oxide grains 20 80 m high vesicle abundances and the exsolution texture of fe ti oxides suggest subaerial extrusion of the lava a high oxygen fugacity probably above the nno buffer and a low ti ti fe ratio of the basaltic melt are suggested as a precondition for high concentration of magnetic minerals and therefore high primary trm during high temperature oxidation ilmenite exsolution lamellae developed in titanomagnetite and symplectic magnetite pyroxene formed by the breakdown of olivine this secondary magnetite grown at temperatures above the curie temperature increases the primary trm early stage hydrothermal alteration below about 375 c led to maghemitization of titano magnetite clearly indicated by shrinkage cracks and irreversible t curves during later stage hydrothermal alteration nrm intensity increased slightly due to the growth of secondary magnetite at lower temperatures about 250 300 c this hydrothermally formed magnetite acquired only a low crm but increased magnetic susceptibility significantly according to our results it is suggested that hydrothermal alteration does not necessarily lower remanent magnetization but contributes to an increase in magnetization the interplay of the three factors melt composition small grain sizes of secondary magnetite due to decomposition of silicates and new formation under hydrothermal conditions caused the strong magnetic anomaly at the surface

the effect of encapsulated fine grain sediment and test morphology on the resistance of planktonic foraminifera to dissolution the tests of planktonic foraminifera recovered from deep sea sediment are commonly observed to encapsulate fine grain carbonate sediment within their chambers in sediment below the lysocline the interstitial water within the chambers may not be as corrosive as the seawater in contact with the outer surface of the test due to slow continuous dissolution of the encapsulated carbonate as a consequence the pore walls of the foram dissolve more slowly than the outer surface of the test using published dissolution rate measurements for foraminifera and deep sea sediment the effect of diffusional reduction of pore wall dissolution was quantitatively estimated with a one dimensional model for the steady state condition where diffusional flux out of the foraminifer s pores is balanced by the dissolution flux from the encapsulated fines and pore walls the diffusional effect is found to principally depend on the structural parametric ratiotwdt f wheretw is the wall thickness dt the test diameter andf is the test porosity in the case of adult planktonic foraminifera the ratio of the pore wall to outer surface dissolution flux is predicted to vary between 60 for the thin walled porous species and 30 for thick walled tests incorporation of the predicted pore to outer surface flux ratios into the morphologic index equation of adelseck 1978 results in a very good prediction 73 of the variation of the solution index of berger 1975 obtained from ranking species counts from core tops a simple empirical equation which may be useful for prediction of the resistance of extinct microfossils was found as follows r tw 1 aw as ap 74 27log twdt f is the measured ratio of pore wall area to outer surface of the test andtw dt f is in units of 104 m2

low temperature oxidation in morb of titanomagnetite to titanomaghemite a gradual process with implications for marine magnetic anomaly amplitudes mid ocean ridge basalt morb samples varying in age from recent to jurassic were selected for electron microscopic and rock magnetic studies our observations indicate that the degree of oxidation of titanomagnetite in morb increases only gradually with sample age the titanomagnetite in recent morb 100 ka both electron microscopic observations and rock magnetic data support the notion that low temperature oxidation of titanomagnetite to titanomaghemite in morb is a gradual process moreover the rate of maghemitization is controlled by many factors on both macroscopic and microscopic scales including regional oceanic crustal structures lithological features grain size and surrounding matrices pillow lavas and the tops of massive flows tend to have higher degrees of oxidation than the interiors of massive flows owing to higher porosity and permeability of the former in contrast interstitial glass can protect fine titanomagnetite grains from alteration the natural remanent magnetization nrm intensity of morb varies as a function of age magnetic granulometry concentration of magnetic materials and the degree of alteration fine grained morb samples typically have higher nrm intensity the nrm intensity appears to decrease substantially with increasing degree of maghemitization and hence with age the envelope of the smoothed magnetic anomaly amplitudes resembles the change in nrm intensity for morb samples of the last 30 million years but the underlying assumption that the highest degree of maghemitization observed in the samples of a given age is entirely responsible for these intensity changes is not supported maghemitization of the micrometer sized and larger grains is probably only partly responsible whereas a significant contribution to nrm intensities is inferred from those submicrometer sized titanomagnetite grains that remained protected from oxidation by the surrounding matrix of interstitial glass

alteration and self reversal in oceanic basalts 1 we report on magnetic properties and compositions of titanomaghemites from moderately to highly oxidized basalts recovered at nine drilling sites in the pacific ocean these new data together with similar data sets allow us to test compositional fields of titanomaghemite that could carry self reversed components of natural remanent magnetization above room temperature these compositions have been proposed on the basis of theoretical considerations of cation distributions during ionic reordering associated with low temperature oxidation maghemitization the data indicate that very high oxidation states z 0 9 and relatively high ti contents x 0 6 are needed to produce natural self reversed components this compositional range is much more limited than predicted the severe restrictions on compositions and oxidation state strongly suggest that self reversed magnetizations should not be a general feature of the magnetization carried by oceanic basalt instead the formation of self reversed components may signify unusual conditions such as extreme fluid flow and iron removal which could result in the required oxidation states while maintaining stability of the cation deficient titanomaghemite lattice structure

consequences of the maghemitization on the magnetic properties of submarine basalts synthesis of previous works and results concerning basement rocks from mainly d s d p legs 51 and 52 abstract this study attempts to point out the effects of low temperature oxidation on the magnetic properties of oceanic basalts to evidence the maghemitization effect in terms of chemical change alone we tried to keep constant both the concentration and the grain size of the magnetic minerals this study bears essentially on basaltic samples drilled at site 417a and 417d during the d s d p legs 51 and 52 but also includes other available data the concentration changes were neutralized either by counting the magnetic oxides with a microscope or by normalizing a magnetic parameter to another one which similarly varies upon maghemitization for the grain size we selected magnetic criteria that seem the best to represent the magnetic domain state configuration of the titanomagnetite thus we chose to work with constant ranges of both h cr h c and j rs j s values the magnetic minerals at site 417a and 417d are titanomagnetites displaying varying maghemitization degrees but whose original composition varies little around x 0 55 including a roughly constant amount of minor elements al 0 08 and mg 0 04 ions per formula unit therefore it was assumed that the curie temperature rise reliably represents the maghemitization increase the main results are the following ones the saturation induced magnetization j s continuously decreases by a factor of two to three during maghemitization consequently this process can actually be considered to be the main cause of the decrease in the magnetic anomalies amplitude when going away from the sea floor spreading axes the other magnetic parameters are grain size dependent and appear to evolve in a roughly opposite way in the fine grained sd psd and in the coarse grained psd md basalts the initial susceptibility first slightly decreases then increases in the former case while continuously decreasing in the latter the evolution of the coercive force parameters is opposite to the susceptibility s in both groups these results are in general good agreement with theoretical predictions and for the fine grained basalts with results obtained from synthetic substituted sd titanomaghemites however contrary to expectations we did not find any change in the magnetic viscosity with maghemitization it seems rather that the viscosity is mainly controlled by the magnetic domain structure and microstructures in the titanomagnetites being generally negligible in the fine grained basalts pillows and significant in the doleritic basalts this study shows that even if maghemitization slightly alters the reliability of fine grained basalts as palaeomagnetic recorders these rocks still remain the main source of the oceanic magnetic anomalies alternatively due to their significant magnetic viscosity the doleritic basalts contribute little to the anomaly pattern whatever their maghemitization degree

thermally induced inversion of al substituted titanomagnetite in basalts evidence for partial self reversal 1 we report properties of laboratory induced thermoremanence combined with detailed rock magnetic experiments on basaltic samples which contain aluminum substituted titanomagnetite fe2 24ti0 64al0 12o4 prior to thellier coe paleointensity experiments specimens were first demagnetized using an alternating field with a peak field of 90 mt and were then imparted an anhysteretic remanent magnetization arm to represent a natural remanent magnetization nrm results show that the partial thermal remanent magnetization ptrm gain and arm remaining are linearly correlated up to 300 c between 300 and 460 c specimens acquire thermoremanence with a direction antiparallel to the external field direction leading to intensity decreases this coincides with the onset of the mineral transformation from al titanomagnetite to a magnetic phase with higher tc our results support that phase interaction with distinct tc values in partially oxidized titanomagnetite can produce partial self reversal thermoremanence in rocks

documenting the reliability and utility of the γ method as applied to cyclic sections using forward modeling abstract the problem of determining if cyclic sections which are older than 5 m y in age were deposited in response to orbital control is discussed gamma analysis a method which uses the assumption that effective accumulation rates are facies dependent to minimize differences in cycle duration is presented as a possible solution to this problem while the assumptions of analysis are generally not met by geologic data the effective accumulation rates estimated may be instrumental in recovering a spectrum which reveals orbital control we address the problem of assessing the reliability of results by performing analyses on three cyclic time series generated by a forward model our results show that exact accumulation rates are not likely to be obtained by the method however the accumulation rates obtained markedly improve the time scale resulting in a spectrum from which one could distinguish orbital forcing if present only when spectra of cyclic data show predicted orbital peaks can analysis be considered to yield effective accumulation rates which are more nearly like the true effective accumulation rates than the assumption of a single effective accumulation rate for all facies

magnetic state of 10 40 ma old ocean basalts and its implications for natural remanent magnetization abstract the natural remanent magnetization nrm of ocean basalts giving rise to the pattern of marine magnetic anomalies is known to be of comparatively low intensity for about 20 ma old oceanic crust the aim of this study is to detect possible peculiarities in the rock magnetic properties of ocean basalts of this age and to establish a link between magnetomineralogy rock magnetic parameters and the low nrm intensity ocean basalts covering ages from 0 7 to 135 ma were selected for rock magnetic experiments and their room temperature hysteresis parameters curie temperature and temperature dependence of saturation magnetization ms t was determined and complemented by reflected light microscopy the majority of samples is magnetically dominated by titanomagnetite and titanomaghemite with increasing oxidation state with age for these a strong dependence of hysteresis parameters on the age of the samples is found the samples have a minimum in saturation magnetization and a maximum in magnetic stability in the age interval ranging from approximately 10 to 40 ma coinciding with the age interval of low nrm intensity the observed change in saturation magnetization is in the same order as that for the nrm intensity a further peculiarity of the titanomaghemites from this age interval is the shape of their ms t curves which display a maximum above room temperature neel p type and sometimes a self reversal of magnetization below room temperature neel n type these special rock magnetic properties can be explained by titanomagnetite low temperature oxidation and highly oxidized titanomaghemites in the age interval 10 40 ma a corresponding measurement of the nrm at elevated temperature allows to identify a maximum in nrm intensity above room temperature for the samples in that age interval this provides evidence that the nrm is equally carried by titanomaghemites and that the low nrm intensities for about 20 ma old ocean basalts are caused consequently by the low saturation magnetization of these titanomaghemites

crustal magnetization in the south atlantic from inversion of magnetic anomalies a method for inverting magnetic anomalies in the presence of topography parker and huestis 1974 is used to analyze and describe crustal magnetization along 12 profiles in the south atlantic the profiles are approximately 2000 km long and are almost perpendicular to the trend of the magnetic anomalies a bandpass filter is used to remove long wavelength anomalies unassociated with seafloor spreading processes a filter that passes only wavelengths between 200 and 3 km produces a magnetization that approximates the positive and negative aspects of the geomagnetic reversal sequence the positive aspects originating from normally magnetized portions of the crustal source and negative aspects originating from reversely magnetized portions of the crustal source a 6 km thick crust is assumed in order to produce magnetizations consistent with dredged oceanic rocks a long wavelength pattern is evident in the magnetization solutions the highest magnetizations are associated with the ridge crest 1 35 1 74 a m within the western section of the study area a sharp decrease in magnetization occurs within 100 km of the ridge crest 4 5 ma a minimum in magnetization is reached at 400 km from the axis or at 18 ma 0 51 0 57 a m the magnetization then gradually increases out to 1100 km from the ridge axis or 50 ma 0 91 0 93 a m and dramatically decreases to 1900 km or 86 ma 0 34 0 43 a m within the eastern section of the study area a sharp decrease in magnetization occurs within the first 100 km of the ridge crest 5 ma a minimum in the magnetization occurs at approximately 400 km from the ridge axis or at 20 ma 0 54 0 56 a m the magnetization then gradually increases out to 1200 km from the ridge axis or to 60 ma 1 15 1 24 a m an overall pronounced decrease occurs to 1900 km from the ridge axis or to 95 ma 0 36 0 40 a m in order to explain this behavior the field intensity may have changed with a time scale of several millions of years alternatively the higher magnetizations at 50 60 ma could be explained by a uniform intensity of magnetization in a thicker crust

insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at odp iodp site 1256 from a cluster analysis of rock magnetic properties we analyze magnetic properties from ocean drilling program odp integrated odp iodp hole 1256d 6 44 1 n 91 56 1 w on the cocos plate in 15 2 ma oceanic crust generated by superfast seafloor spreading the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting fuzzy c means cluster analysis and nonlinear mapping are utilized to study down hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization the coercive force the ratio of the remanent coercive force and coercive force the low field magnetic susceptibility and the curie temperature to evaluate the effects of magmatic and hydrothermal processes on magnetic properties a statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas which differ from two distinct clusters mainly representing the dikes and gabbros extensive alteration can obliterate magnetic property differences between lavas dikes and gabbros the imprint of thermochemical alteration on the iron titanium oxides is only partially related to the porosity of the rocks thus the analysis complements interpretation based on electrofacies analysis all clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and or the presence of intergrowths the upper part of the extrusive sequence the granoblastic dikes and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization

single domain and superparamagnetic titanomagnetite with variable ti content in young ocean floor basalts no evidence for rapid alteration high resolution electron microscope studies have been carried out on zero age new flow basalts from the juan de fuca ridge and on young 20 ka basalts from the axis of the east pacific rise at 12 n such data lead to characterization of the magnetic minerals especially those of smaller grain size which have been hypothesized by kent and gee to have undergone grain size dependent alteration in addition to larger titanomagnetite grains abundant submicrometer titanomagnetite has been observed in globules within a glassy matrix these grains likely to be single domain sd or superparamagnetic are associated with apatite uncommon pyrrhotite and residual glass the submicrometer titanomagnetite grains have a wide compositional range 0 x 0 8 where x is the fraction of ulvospinel component whereas the larger multi domain md sized titanomagnetite grains have a narrow composition range of approximately x 0 6 this variability in ti content provides a ready explanation for the thermal rock magnetic properties observed by kent and gee and eliminates the need to invoke extremely rapid 20 ka alteration of these young basalts

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

absolute paleointensities and paleodirections of miocene and pliocene lavas from fernando de noronha brazil abstract forty four volcanic units were sampled on the equatorial atlantic island of fernando de noronha brazil twenty seven sites are from the 1 8 3 3 myr old quixaba formation and the others are from the approximately 8 12 myr old remedios formation analyses of paleodirections yielded normal and reverse polarities in both formations with virtual geomagnetic poles vgps and vgp dispersion s f of vgp lat 86 9 vgp long 136 3 s f 11 2 for the quixaba formation and vgp lat 86 8 vgp long 196 7 s f 15 5 for the remedios formation successful paleointensity determinations were obtained for 9 of the 27 sampled units in the quixaba formation a modified thellier method was used that includes checks to monitor alterations and repetition of demagnetization steps to detect multidomain tails of partial thermoremanent magnetizations ptrm and magnetization stabilization processes furthermore the additivity of ptrms was monitored during this modified technique of the thellier experiment these additivity checks provide a quality test for correcting the paleointensity determination for magnetomineralogical changes which occur during laboratory treatment such correction methods were used for analyzing approximately 50 of the determinations and lead to a reduction of the within site scatter the mean paleointensity result of 29 3 t corresponding to a virtual axial dipole moment vadm of 7 5 10 22 a m 2 yields approximately the present day field strength mean values of the paleodirections for volcanic units of reversed and normal polarity are closer to a dipolar field configuration than previously observed today s geomagnetic field in the equatorial atlantic shows a significant dipole anomaly our results indicate that this anomaly was far less pronounced in the geological past

dual polarity directions in basaltic andesitic dykes reversal record or self reversed magnetization summary rock magnetic analyses were performed on basaltic andesitic dyke samples from the lhasa block which were previously used for palaeomagnetic investigations of the india asia collision the aim of the study is to prove whether antipodal directions separated in these samples represent different polarities of the earth s magnetic field or a self reversal magnetization reflected light microscopy domain pattern observations and energy dispersive x ray analyses revealed two generations of titanomagnetite tm grains one consisting of largesized 20 m to more than 100 m and inhomogeneous ti poor tm and another with small smaller than 10 m and rather homogeneous ti rich tm grains partial thermoremant magnetization experiments and reflected light microscopy show that magnetostatic or superexchange interaction between these two phases is unlikely temperature dependence of saturation magnetization and low temperature curves of isothermal remanent magnetization disprove a possible n type behaviour of the samples a self reversal due to ionic reordering can be ruled out as it requires a high degree of oxidation of the ti rich tms which was not observed in summary a self reversal magnetization is very unlikely in the studied dyke samples and therefore the observed antipodal directions most probably represent a record of different polarity epochs of the earth s magnetic field this conclusion implies that the time of remanence acquisition in the studied dykes was sufficiently long to average out paleosecular variation supporting the significance and reliability of the palaeomagnetic results

long term variations in magnetization intensity with crustal age in the northeast pacific atlantic and southeast indian oceans the magnetization intensity of oceanic crust in the northeast pacific atlantic and southeast indian oceans was calculated from marine magnetic anomaly data to investigate long term variations in magnetization intensity with crustal age the magnetization intensity was derived by two dimensional inversion of the upward continued marine magnetic anomalies the principal magnetization intensity variations versus age are summarized as follows 1 in all the oceans a high magnetization intensity exists at the ridge crest followed by a sharp decrease within 8 my and a minimum is reached at 8 20 ma 2 in the northeast pacific and atlantic oceans the magnetization intensity gradually increases up to at least 70 ma this variation pattern is consistent with the previous results of nrm measurements of drilled rocks and inversion of marine magnetic anomalies in the south atlantic

astronomical cycle origin of bedded chert a middle triassic bedded chert sequence inuyama japan abstract astronomical forcing is one of the main drivers of climate change and astronomical cyclicity recorded in sediments provides a clue to understand the dynamics of the global climate system bedded cherts consist of rhythmic alternations of chert and shale beds although previous studies have hypothesized that the origin of bedded chert is related to astronomical cycles e g fischer 1976 hori et al 1993 conclusive proof remains elusive to explore this possibility we established a continuous high resolution lithostratigraphy of middle triassic bedded chert in central japan the average duration of each chert shale couplet is 20 kyr similar to that of the precession cycle spectral analysis of a bed number series of thickness variations in chert beds was performed assuming that each chert shale couplet represents a 20 kyr precession cycle the results reveal cycles involving approximately 200 20 5 and 2 3 beds corresponding to periodicities of approximately 4000 400 100 and 40 60 kyr respectively by further assuming that the 20 bed cycle represents a 405 kyr eccentricity cycle of constant and stable periodicity we converted the bed number series to a time series spectral analysis of the time series revealed distinct periodicities of 3600 117 97 and 38 kyr in addition to 405 kyr besides 3600 kyr these periodicities agree well with the 120 95 and 37 kyr periodicities for eccentricity cycles and the obliquity cycle during the triassic moreover we detected amplitude modulation of the approximately 100 kyr cycle of thickness variations in chert beds with a 405 kyr periodicity which may correspond to amplitude modulation of 100 kyr climatic precession cycle with the 405 kyr periodicity the approximately 3600 kyr periodicity described above and 1800 kyr periodicity manifested as the amplitude modulation of the 405 kyr cycle are correlated to present day long term eccentricity cycles of 2400 and 4800 kyr evolved by chaotic behavior of solar planets collectively these similarities in the periodicities of dominant cycles their hierarchy and the nature of amplitude modulation of cycles in chert bed thickness with those of astronomical cycles strongly support the hypothesis that the sedimentary rhythm of bedded chert is paced by astronomical cycles thereby indicating the potential of bedded chert as a template for a mesozoic cyclostratigraphy

diagenetic formation of bedded chert implications from a rock magnetic study of siliceous precursor sediments abstract bedded chert is a distinctive rock type characterized by a rhythmic alternation of quartz rich chert beds and clay rich interbeds the origin of the lithologic variations is not well understood a rock magnetic study of a sequence of alternating bands of radiolarian rich and clay rich bands recovered by odp leg 129 which is considered to be the best known analog for the bedded chert precursor sediments shows that the relative abundances and characteristics of magnetic phases are similar in both sediment types with only a minor 1 5 times on average increase in bulk concentration in clay rich bands comparison with magnetic properties of the diagenetically mature chert shale couplets suggests that siliceous precursor sediments undergo strong differential diagenetic modification a simple silica transfer model with dissolution of silica in proto shale donor beds and deposition in proto chert receiver beds indicate that the known features of the bedded chert sequences such as the difference in thickness cosmic dust content between chert and shale can be reproduced in case of donor beds having initially lower clay higher biogenic silica content compared to receiver beds while contrary to the current depositional model explaining chert beds formation by episodic radiolarian bloom on a background of slow accumulation of clays an association of proto shale beds with high productivity episodes is supported by an independent paleoproductivity proxy their higher al ti values and is in full agreement with the known silica diagenesis trends particularly with the retarding influence of detrital minerals on silica transformations the impetus for diagenetic silica segregation is provided by initial variability in clay content within a siliceous sedimentary succession silica dissolution starts in clay poor intervals when diffusing silica encounters clay rich beds adsorption to clay lowers dissolved silica content below the saturation level of the higher solubility phases and leads to deposition of low solubility quartz early deposition of quartz cement prevents significant compaction of chert layers while the loss of silica in donor beds leads to their compaction and concomitant increase in non reactive clay and cosmic dust particles severe distortion of primary environmental signals during differential diagenesis questions the use of bedded chert sequences for cyclostratigraphic studies

paleomagnetism of red cherts a case study in the inuyama area central japan this study is an attempt to isolate and identify the primary magnetization in red cherts red chert samples from a middle triassic sequence in the inuyama area 35 4 n 137 0 e central japan have a multicomponent magnetization which is delineated by thermal demagnetization two of the components have high coercivity and high blocking temperatures one is carried by hematite and the other by magnetite the former was concluded to be the primary magnetization for two reasons 1 it predates intraformational folding and 2 it records geomagnetic reversals the other component which accounts for the dominant part of natural remanent magnetization was probably acquired at a time long after the deposition two paleomagnetic results were drawn from the middle triassic red chert sequence 1 the mean inclination of the samples in the sequence was 1 4 6 8 therefore the paleolatitude amounts to 0 7 3 4 in contrast to the paleolatitude expected from the paleopole of eurasian continent which is about 70 and 2 the magnetostratigraphy of this sequence indicates that two geomagnetic epochs in the middle triassic are considerably longer more than several million years than the average of those in the cenozoic

amplitude variation of marine magnetic anomalies to determine the large scale amplitude variation of marine magnetic anomalies 16 profiles have been analysed 8 across the mid atlantic ridge 5 across the east pacific rise and 3 across the indian ocean the profiles were normalized to the age of the anomalies and finally transformed to the same latitude of 45 both ridges show qualitatively the same trend with the central anomaly over the spreading axis clearly dominating the flanking anomalies gradually decrease in amplitude until a minimum is reached over an age range from 10 to 30 ma beyond this minimum the anomaly amplitudes progressively increase again to another maximum near the onset of the mesozoic magnetic anomalies there exists however a distinct difference in the absolute value of the magnetic anomalies for the two oceans analysed the observed amplitude variation can well be explained by a corresponding variation of the natural remanent magnetization of the upper 1000 meters of oceanic basalts as measured on individual samples from the deep sea drilling project model calculations lead us to propose a 3 layer model for the sources of marine magnetic anomalies an uppermost layer of 1000 m basalt corresponding more or less to seismic layer 2 with age dependent magnetization and two layers corresponding to seismic layers 3a and 3b with age independent magnetization the most likely cause for the age dependency of magnetization of the uppermost layer of oceanic basalts is gradual low temperature oxidation of the magnetic minerals contained in the rocks

paleoclimate proxies for cyclostratigraphy comparative analysis using a lower triassic marine section in south china abstract multiple proxies for paleoclimatic and paleoenvironmental change within sedimentary sequences have been developed but understanding their relationships and their relative strengths or weaknesses are required to access a wealth of untapped paleoclimate information we propose two main criteria for multiple proxy datasets to estimate their individual signal to noise levels and to decipher the relationship between multiple proxies first for proxies affected by similar processes then their time dependent changes should be similar second a paleoclimate proxy that is more sensitive to an external forcing such as astronomical forcing is more useful than those that do not have such a response we introduce two methods to evaluate these criteria 1 hierarchical cluster analysis hca probes the relationships among multiple proxies based on similarities of their oscillation patterns and 2 power decomposition analysis pda tests proxy sensitivity to external climate forcing we evaluate 16 high resolution paleoclimate proxies in detail for their applications in paleoclimatology from the marine lower triassic daye formation at the daxiakou section in the three gorges region of south china which has carbonate claystone cycles deposited on an outer ramp the proxies are spectral gamma ray gamma ray intensity potassium uranium thorium thorium uranium and thorium potassium rock color lightness l redness a and yellowness b magnetic susceptibility measured in both laboratory and outcrop anhysteretic remanent magnetization arm lithologic rank simplified lithologic rank and non carbonate fraction carbonate thickness and couplet thickness hierarchical cluster analysis hca gathers these proxies into groups likely affected by the same process the arm and thorium uranium proxies seem to reflect the hinterland weathering process during the early triassic gamma ray potassium uranium thorium magnetic susceptibility and non carbonate fraction proxies refer to terrestrial input the l a and lithologic rank proxies indicate the productivity the redox state and the relative sea level respectively at this section this case study contributes to the understanding of the sensitivity of these types of proxies within marine strata for deep time paleoclimate and astronomical tuned time scales

a 70 million year astronomical time scale for the deep sea bedded chert sequence inuyama japan implications for triassic jurassic geochronology abstract the astronomical time scale ats has provided high resolution geochronology however the early mesozoic ats is still under construction partly due to the lack of continuous pelagic sequences of the early mesozoic here we present ca 70 myr long ats constructed from the early mesozoic deep sea bedded chert sequence exposed in the inuyama area central japan the sedimentary rhythms of bedded chert display a full range of climatic precession related cycles 20 kyr cycle as a chert shale couplet and 100 405 2000 to 4000 and 10 000 kyr cycles as chert bed thickness variation the newly established ats inuyama ats is tuned by 405 kyr eccentricity cycle and is anchored at the end triassic radiolarian extinction level as 201 4 0 2 ma this inuyama ats gives ages consistent with the radiometric ages projected to the inuyama deep sea sequence using biostratigraphy and carbon isotope stratigraphy the inuyama ats provides the age constraints for the triassic and jurassic stage boundaries which support the long norian option of muttoni et al 2004 because the deep sea bedded chert sequence covers a long time interval before the cretaceous the ats for the bedded chert will serve as a template for the astrochronology of mesozoic and older ages

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

astronomical forcing of a middle permian chert sequence in chaohu south china abstract astronomical forcing has been shown to be a fundamental driver of climate change through geological time pelagic bedded cherts deposited in mesozoic ocean basins with chert mudstone cycles have been shown to contain the imprint of milankovitch astronomical climate forcing in the chaohu region south china we studied a middle permian radiolarian chert sequence gufeng formation with chert mudstone couplets reminiscent of the mesozoic cherts but deposited on a continental shelf spectral analysis of lithologic bed thickness data from two sections of this chert sequence reveals that 13 cm to 20 cm chert mudstone cycles in the stratigraphic domain match theoretical 32 kyr middle permian obliquity cycling together with a hierarchy of other cycles with 12 cm 9 cm 7 cm 6 6 cm and 5 4 cm wavelengths tuning the 13 cm to 20 cm stratigraphic cycles to earth s obliquity cycle periodicity indicates that the cm scale cycles are precession scale variations with a strong 400 kyr amplitude modulation tuning to theoretical precession terms provides further support for the astronomical forcing of the chert sequence we propose that monsoon controlled upwelling contributed to the development of the chert mudstone cycles a seasonal monsoon controlled by astronomical forcing i e insolation influenced the intensity of upwelling stronger upwelling increased radiolarian productivity in the surface ocean increasing silica deposition glacio eustatic oscillations from ice sheet dynamics in southern gondwana modulated terrigenous mud flux to the basin the two processes jointly contributed to the astronomical rhythms of these tropical chert mudstone sequences which are characterized by comparably strong obliquity and precession responses subsequent diagenesis distorted the chert and mudstone layering but not enough to destroy the original stratigraphic patterns the resulting astronomical time scale ats assumes a roadian wordian boundary age of 268 8 ma for the onset of the first chert layer at the base of the sequence and ends at 264 1 ma for a total duration of 4 7 myr

the intensity of the earth s magnetic field over the past 160 million years in contrast to our detailed knowledge of the directional behaviour of the earth s magnetic field during geological and historical times1 2 data constraining the past intensity of the field remain relatively scarce this is mainly due to the difficulty in obtaining reliable palaeointensity measurements a problem that is intrinsic to the geological materials which record the earth s magnetic field although the palaeointensity database has grown modestly over recent years3 4 5 these data are restricted to a few geographical locations and more than one third of the data record the field over only the past 5 myr the most recent database5 covering the time interval from 5 to 160 myr contains only about 100 palaeointensity measurements here we present 21 new data points from the interval 5 160 myr obtained from submarine basalt glasses collected from locations throughout the world s oceans whereas previous estimates for the average dipole moment were comparable to that of the earth s present field6 the new data suggest an average dipole moment of 4 2 2 3 1022 a m2 or approximately half the present magnetic field intensity this lower average value should provide an important constraint for future efforts to model the convective processes in the earth s core which have been responsible for generating the magnetic field

self reversal of remanent magnetization in basalts due to partially oxidized titanomagnetites summary a range of basaltic samples from olby france and vogelsberg germany displaying the phenomenon of partial and complete self reversal was studied in order to decipher the physical mechanism responsible for self reversed magnetic remanence in basalts microscopic observations and rock magnetic measurements show titanomagnetite to be the carrier of remanence due to low temperature oxidation the titanomagnetites are present in two magnetic phases forming close side by side phase assemblages the ore grains consist of a non oxidized magnetically soft titanomagnetite part with curie temperatures between 140 and 300 c and a magnetically hard low temperature oxidized part with curie temperatures between 410 and 590 c magnetic force microscopy observations present evidence that the oxidation process does not only influence the curie temperatures and thus the blocking temperature spectrum but also the domain configuration during acquisition of a thermoremanence the two phases are magnetically coupled leading to a remanent magnetization of the low curie temperature phase which is antiparallel to the applied external magnetic field computational modelling of the remanence acquisition process explains the coupling of the two phases by magnetostatic interaction as the observed partial oxidation of ore grains is not uncommon in subaerial basalts partial and complete self reversal is probably a frequently occurring phenomenon nevertheless it remains unnoticed in most cases as it cannot be detected by the standard stepwise thermal demagnetization technique despite the complex remanence properties our investigations indicate that the upper part of the blocking temperature spectrum still carries reliable palaeodirectional information

on the compositional field of self reversing titanomaghemite constraints from deep sea drilling project site 307 1 in 1956 verhoogen proposed that ionic reordering during low temperature oxidation of titanomagnetite could result in self reversal of natural remanent magnetization later in 1966 o reilly and banerjee concluded that 1 the compositional field of self reversing titanomaghemite was restricted to very high oxidation states and 2 that high oxidation temperature 250 c would be required to reach these oxidation levels which would in turn be inconsistent with stability of the cation deficient structure of titanomaghemite in nature however an example of the process has been reported in some altered pillow basalt lavas of late cretaceous age from detroit seamount ocean drilling program odp sites 883 1203 and 1204 in the northwestern pacific ocean here we present new paleomagnetic rock magnetic and compositional data from mid ocean ridge tholeiitic basalts of late jurassic age collected at deep sea drilling project dsdp site 307 although severely oxidized the dsdp site 307 basalt samples lack the clear self reversed magnetizations seen in some samples from detroit seamount these data constrain the compositional field of self reversing titanomaghemite to extremely high oxidation states z 0 95 consistent with the first conclusion of o reilly and banerjee the comparison of results from odp sites 883 1203 and 1204 on detroit seamount and those from the dsdp site 307 basalts suggests that the presence of self reversed magnetizations may signify unusually vigorous and sustained fluid flow that resulted in very efficient removal of fe from titanomaghemite producing the extremely high oxidation states required for self reversal

magnetization of the oceanic crust thermoremanent magnetization of chemical remanent magnetization we propose a model in which chemical remanent magnetization crm acquired within the first 20 m y of crustal evolution may account for 80 of the bulk natural remanent magnetization of older basalts the crm of the crust is acquired as the original thermoremanent magnetization trm is lost through low temperature alteration the crm intensity and direction are controlled by the posternplacement polarity history this model explains several independent observations concerning the magnetization of the oceanic crust the model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern it also explains the decay of magnetization away from the spreading axis and the enhanced magnetization of the cretaceous quiet zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust the model also explains discrepancies in the anomaly skewness parameter observed for anomalies of cretaceous age further our study indicates varying rates of trm decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer

advances in characterizing the cyclostratigraphy of binary chert mudstone lithologic successions permian roadian lower capitanian chaohu lower yangtze south china abstract successions of mesozoic and paleozoic radiolarian bedded cherts display a binary cyclic hierarchy of cm scale chert and mudstone couplets some of these couplet successions contain an apparent imprint of milankovitch cycles in which the couplets may reflect changes in silica deposition related to milankovitch forcing for example high insolation for chert deposition and low insolation for mudstone the challenge has been how to represent such binary lithologic series in a format that is suitable for signal processing to search for milankovitch cycles here we present an analysis of chert mudstone binary series by considering in lieu of a boxcar linear interpolation a midpoint triangle linear interpolation the procedure is demonstrated first on synthetic data then on the binary chert mudstone stratigraphic series of the permian guadalupian roadian lower capitanian gufeng formation chaohu lower yangtze south china we assign 1 to the midpoint of each mudstone bed and 1 to the midpoint of each chert bed then linearly interpolate from midpoint to midpoint along the succession time frequency analysis of the midpoint triangle series highlights a previously unnoticed stratigraphic alternation of obliquity and precession band power finally we present a revised astrochronology for the gufeng succession that is anchored at 270 ma at the base of the jinogondolella nankingensis conodont zone base of roadian stage and ending at 264 6 ma in the undifferentiated follicucullus scholasticus ruzhencivisponus uralicas radiolarian zone mid capitanian stage for a duration of 5 4 myr

paleomagnetism of the jurassic transantarctic mountains revisited evidence for large dispersion of apparent polar wander within less than 3myr the transantarctic mountains predominantly consist of jurassic continental flood basalts kirkpatrick and sills ferrar emplaced during the earliest phase of the break up of pangea published ages based on a variety of geochronological methods all agree rather well and cluster around 180 ma suggesting emplacement during a rather short time interval of not more than 3 myr paleomagnetic studies mostly carried out between the 60s and 90s of the last century however yield two well defined but significantly different groups of paleomagnetic pole positions plotting either in intermediate latitudes a or at latitudes exceeding 60 degrees s b pole positions belonging to group a are generally interpreted to be of primary origin whereas the significance of group b poles remains unclear here we report new data from the kirkpatrick basalts of northern victoria land antarctica where 157 oriented paleomagnetic samples were taken covering about 800 m of stratigraphy and 23 volcanic flows in gair mesa 73 4666 degrees s 162 8666 degrees e after removal of a steep magnetic overprint with rather low coercivities straight linear segments of exclusively normal polarity trending toward the origin of projection are identified in 151 samples from 22 sites maximum unblocking temperatures do not exceed 580 degrees c and maximum coercivities not 60 mt the resulting mean virtual geomagnetic pole vgp for the gair mesa plots at 66 4 degrees s 227 7 degrees e alpha 95 of 6 2 degrees k 25 7 comparable to group b poles reflecting light microscopy reveals that the magnetic inventory of all samples analyzed is dominated by magnetite showing shrinking cracks and broad ilmenite lamellae the latter being diagnostic for high temperature oxidation analysis of the distribution of the 22 pole positions obtained here shows that secular variation has successfully been averaged s b 15 9 degrees suggesting that the rocks studied here have recorded the time averaged geomagnetic field during early mid jurassic times based on these new results we postulate that both clusters a and b of pole positions reflect primary magnetizations and that the kirkpatrick basalts of gair mesa have been emplaced in a rather short normal polarity time interval of 3 myr if our reasoning is correct the apparent polar wander path for the latest early jurassic might be more complex than previously thought whether this complexity is evidence for massif true polar wander remains speculative c 2015 international association for gondwana research published by elsevier b v all rights reserved

the hysteresis properties of multidomain magnetite and titanomagnetite titanomaghemite in mid ocean ridge basalts abstract hysteresis measurements have been carried out on a suite of ocean floor basalts with ages ranging from quaternary to cretaceous approximately linear yet separate relationships between coercivity bc and the ratio of saturation remanence saturation magnetization mrs ms are observed for massive doleritic basalts with low ti magnetite and for pillow basalts with multi domain titanomagnetites with x 0 6 even when the morb has undergone low temperature oxidation resulting in titanomaghemite the parameters are still distinguishable although offset from the trend for unoxidized multidomain titanomagnetite the parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types this plot provides a previously underutilized and non destructive method to detect the presence of low titanium magnetite in igneous rocks notably morb

source and sink of fluid in pelagic siliceous sediments along a cold subduction plate boundary abstract subduction zones where old oceanic plate underthrusting occurs are characterized by thick pelagic sediments originating from planktonic ooze as well as cold thermal conditions for a better understanding of dehydration from pelagic sediments and fluid behavior which would play a key role in controlling the dynamics in the shallow portion of the subduction zone as observed in the 2011 tohoku earthquake and tsunami we investigate cherts in a jurassic accretionary complex in japan the microstructure and microchemistry of these cherts indicate dissolution of sio 2 from a pressure solution seam and precipitation of sio 2 to the white chert layer which would act as a fluid conduit the amount of water necessary to precipitate sio 2 in the white chert is 10 2 times larger than that produced by compaction and silica clay diagenesis other fluid sources such as hydrated oceanic crust or oceanic mantle are necessary to account for this discrepancy in the fluid budget a large amount of external fluid likely contributed to rising pore pressure along cold plate boundaries

paleomagnetic calibration of milankovitch cyclicity in lower cretaceous sediments abstract bedded pelagic limestones of early to mid cretaceous age may record cyclic climate variability driven by changes in the earth s orbit milankovitch cycles if properly calibrated the lithologic cycles could provide sedimentary chronometry at far shorter time scales than biostratigraphy and magnetic stratigraphy the lower cretaceous maiolica formation of northern and central italy displays rythmic bedding in sections whose magnetic reversal sequence is clearly correlated to the upper m series marine anomalies the magnetic reversals divide measured sections into 0 5 2 myr increments in which sedimentation rate assuming continous deposition can be measured to 15 25 accuracy paleomagnetically estimated sedimentation rates over a number of polarity zones establish with one exception a narrow range in periods of the carbonate cycles carbonate couplets have an estimated mean period of 23 5 kyr and prominent modulations in bedding thickness bundling have a mean period of 117 kyr kent and gradstein 1985 time scale the close match of sedimentary periodicities to orbital repeat times implies that the carbonate cycles reflect a combination of professional and eccentricity climatic forcing comparison of measured sections from different locations shows that bed to bed correlations are possible regionally cosistent with the milankovitch hypothesis episodes of barremian black shale deposition occur in the troughs of the short eccentricity cycle as roughly one half of the carbonate marl couplet or about 10 kyr duration it is possible to estimate both the timing relative to the top of reversed chron m 0 and duration of a period of unusual organic carbon rich deposition selli horizon in the early part of the cretaceous long normal magnetic polarity interval by tracing the sedimentary cycles up from the barremian strata

new perspectives on orbitally forced stratigraphy this survey of the current status of research into earth s orbitally forced paleoclimatic record summarizes recent developments in the theory of earth s orbital parameters and reviews how various techniques of data collection and analysis have fared in the search and recovery of orbital signals in ancient stratigraphy the emerging significance of the quasi periodicity of earth s orbital variations as a prin cipal tool in the analysis of orbitally forced stratigraphy is discussed in detail five case studies are presented that illustrate new directions in research a time series analysis of discontinuous strata b measurement of ultra high resolution stratigraphic signals c new perspectives on the 100 kyr pleistocene glaciation problem d strati graphic evidence for solar system resonance modes and e evaluating phanerozoic length of day from orbitally forced stratigraphy the study of how earth s orbital parameters came to be recorded in ancient stra tigraphy has recently moved into a new phase of scientific inquiry and discovery notable advances in celestial mechanics and in the analysis of stratigraphic data have given rise to new ways of solving old problems and have uncovered new problems for the first time new tools have facilitated the recovery of extended high resolution orbital signals from stratigraphy advanced analytical techniques have now been fully incorporated into the field an updated theory of earth s orbital parameters provides an accurate ephemeris for the past 16 million years for precision correlation between earth s orbital parameters and orbitally forced stratigraphy some orbital eccentricity modes can be used for orbital stratal cal ibrations to several hundred million years ago these breakthroughs have transformed the study of orbitally forced stratigra phy into an applied field that addresses long standing problems in disciplines as far ranging as geochronology geodynamics and astrodynamics orbital signals provide the framework for a high resolution orbital chronostratigraphy as far back

milankovitch and sub milankovitch cycles of the early triassic daye formation south china and their geochronological and paleoclimatic implications article the mass extinction at the end of permian was followed by a prolonged recovery process with multiple phases of devastation restoration of marine ecosystems in early triassic the time framework for the early triassic geological biological and geochemical events is traditionally established by conodont biostratigra phy but the absolute duration of conodont biozones are not well constrained in this study a rock magnetic cyclostratigraphy based on high resolution analysis 2440 samples of magnetic susceptibility ms and anhysteretic remanent magnetization arm intensity variations was developed for the 55 1 m thick early triassic lower daye formation at the daxiakou section hubei province in south china the lower daye formation shows exceptionally well preserved lithological cycles with alternating thinly bedded mud stone marls and limestone which are closely tracked by the ms and arm variations power spectral wavelet and amplitude modulation am analyses of the arm and ms series reveal strong evidence for the presence of milankovitch to sub milankovitch frequencies dominated by precession index signal and 4 5 ka cycles cy cles expressed by variations in ms and arm were likely controlled by changes in the input of fine grained detrital magnetite which in turn may have been driven by astronomically induced changes in monsoon in tensity in the equatorial eastern paleotethys during the early triassic greenhouse period on the basis of the 100 ka tuning results the astronomically constrained duration of the induan stage is 1 16 ma with the griesbachian and dienerian substages of 490 ka and 670 ka respectively the new astro nomical time scale also provides time constraints for the conodont and bivalve biozones and the carbonate carbon isotope 13 c records of the lower triassic daye formation time constraints for the conodont bio zones include 34 ka for hindeodus parvus 2 4 ka forisarcicella staschei i isarcica 366 ka for neogondolella planata ng carinata 66 ka for neogondolella discreta 255 ka for neospathodus kummeli and 415 ka for neospathodus dieneri the duration for the negative 13 c shift from high 13