relative paleointensity in sediments a pseudo thellier approach we present a method for normalizing sedimentary records for estimating relative paleointensity of the geomagnetic field similar to that successfully used to obtain absolute paleointensity from thermally blocked remanences it has the advantages that it is more effective in removing unwanted viscous remanence thereby improving agreement among various records and that it allows the estimation of the uncertainty in the relative paleointensity calculated

giant magnetofossils and hyperthermal events abstract magnetotactic bacteria biomineralize magnetic minerals with precisely controlled size morphology and stoichiometry these cosmopolitan bacteria are widely observed in aquatic environments if preserved after burial the inorganic remains of magnetotactic bacteria act as magnetofossils that record ancient geomagnetic field variations they also have potential to provide paleoenvironmental information in contrast to conventional magnetofossils giant magnetofossils most likely produced by eukaryotic organisms have only been reported once before from paleocene eocene thermal maximum petm 55 8 ma sediments on the new jersey coastal plain here using transmission electron microscopic observations we present evidence for abundant giant magnetofossils including previously reported elongated prisms and spindles and new giant bullet shaped magnetite crystals in the southern ocean near antarctica not only during the petm but also shortly before and after the petm moreover we have discovered giant bullet shaped magnetite crystals from the equatorial indian ocean during the mid eocene climatic optimum 40 ma our results indicate a more widespread geographic environmental and temporal distribution of giant magnetofossils in the geological record with a link to hyperthermal events enhanced global weathering during hyperthermals and expanded suboxic diagenetic environments probably provided more bioavailable iron that enabled biomineralization of giant magnetofossils our micromagnetic modelling indicates the presence of magnetic multi domain i e not ideal for navigation and single domain i e ideal for navigation structures in the giant magnetite particles depending on their size morphology and spatial arrangement different giant magnetite crystal morphologies appear to have had different biological functions including magnetotaxis and other non navigational purposes our observations suggest that hyperthermals provided ideal conditions for giant magnetofossils and that these organisms were globally distributed much more work is needed to understand the interplay between magnetofossil morphology climate nutrient availability and environmental variability

finite faults and inverse theory with applications to the 1979 imperial valley earthquake abstract using a representation theorem from elastodynamics subsurface slip on a known fault is formulated as the solution to an inverse problem in which recorded surface ground motion is the data two methods of solution are presented the least squares method which minimizes the squared differences between theory and data and the constrained least squares method which simultaneously maintains a set of linear inequalities instabilities in the solution are effectively eliminated in both methods and the sensitivity of the solution to small changes in the data is quantitatively stated the inversion methodology is applied to 77 components of near field ground acceleration recorded during the 15 october 1979 imperial valley earthquake the faulting is constrained to propagate bilaterally away from the epicenter at an average velocity of 90 per cent of the shear wave speed on a vertical fault plane extending from the surface to 10 km depth inequality constraints are used to keep the faulting sequence physically reasonable by maintaining right lateral motion and positive slip velocity the preferred solution is stable and provides a good fit to the data it is also realistic and consistent with observed surface offsets and independent estimates of seismic moment

crustal structure of the diablo and gabilan ranges central california a reinterpretation of existing data the compressional wave velocity structure of the crust of the coast ranges of central california was modeled from seismic refraction data reported previously for both the diablo range east of the san andreas fault and the gabilan range west of the fault two alternative velocity models were derived for each range by iterative two dimensional ray tracing each pair of models shows a sedimentary layer and an underlying crust composed of three or four layers for the diablo range the velocities in the sediments range from 2 9 to 4 8 km sec and the depth to basement varies from 0 2 km within the central diablo range to a maximum of 3 4 km north of the livermore valley in the gabilan range the velocities in the sediments overlying the granitic basement are higher north of the gabilan range 3 6 to 4 6 km sec than to the south 2 2 to 3 8 km sec and the depth to basement both north and south of the range is less than 2 km below the sediments and fractured near surface material a resolvable difference in the crustal velocity structure on opposite sides of the san andreas fault indicates compositional differences at depth the upper crust has an average velocity of 5 7 km sec at depths between 3 and 16 3 km in the diablo range and an average velocity of 6 1 km sec at depths between 2 and 9 1 km in the gabilan range the lower crust has an average velocity of 6 9 km sec at depths between 16 3 and 30 km in the diablo range and an average velocity of 6 5 km sec at depths between 10 and 24 km in the gabilan range the models also show that the depth to moho differs by several kilometers between the two ranges in the diablo range models the crust mantle boundary becomes shallower from south to north rising from a depth of 30 to 26 km in the gabilan range models the moho is at a depth of 24 to 26 km depending on the velocity assumed at the base of the crust laboratory measurements of rock velocities and the mapped surface geology allow us to interpret the velocity models in terms of crustal composition we conclude that the diablo range probably consists largely of metagraywacke to a depth of 16 3 km and gabbroic material below this depth the crust of the gabilan range probably consists of granitic material to a depth of 9 1 km and gneissic material below this depth franciscan rocks are not regionally present in the crust of the gabilan range if the gneissic lower crust consists of the same rocks as are found at the surface in the gabilan range then the presumed large scale lateral motion of the salinian block has taken place at or below the crust mantle boundary rather than along a mid crustal slip plane

cyclic magnetite dissolution in pleistocene sediments of the abyssal northwest pacific ocean evidence for glacial oxygen depletion and carbon trapping the carbonate free abyss of the north pacific defies most paleoceanographic proxy methods and hence remains a blank spot in ocean and climate history paleomagnetic and rock magnetic geochemical and sedimentological methods were combined to date and analyze seven middle to late pleistocene northwest pacific sediment cores from water depths of 5100 to 5700 m besides largely coherent tephra layers the most striking features of these records are nearly magnetite free zones corresponding to glacial marine isotope stages miss 22 12 10 8 6 and 2 magnetite depletion is correlated with organic carbon and quartz content and anticorrelated with biogenic barite and opal content within interglacial sections and mid pleistocene transition glacial stages mis 20 18 16 and 14 magnetite fractions of detrital volcanic and bacterial origin are all well preserved such alternating successions of magnetic iron mineral preservation and depletion are known from sapropel marl cycles which accumulated under periodically changing bottom water oxygen and redox conditions in the open central northwest pacific ocean the only conceivable mechanism to cause such abrupt change is a modified glacial bottom water circulation during all major glaciations since mis 12 oxygen depleted antarctic bottom water aabw sourced bottom water seems to have crept into the abyssal northwest pacific below 5000 m depth thereby changing redox conditions in the sediment trapping and preserving dissolved and particulate organic matter and in consequence reducing and dissolving both biogenic and detrital magnetite at deglaciation a downward progressing oxidation front apparently remineralized and released these sedimentary carbon reservoirs without replenishing the magnetite losses

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

estimating superparamagnetism in marine sediments with the time dependency of coercivity of remanence superparamagnetic grains ultrafine 50 nm particles that do not retain a magnetic remanence at room temperature can form through several biogeochemical pathways in marine sediments knowing the distribution of such particles therefore provides a means of better understanding the past diagenetic history of sediments and evaluating their potential record of environmental change herein we employ magnetic hysteresis measurements in a new way to determine the abundance of superparamagnetic sp grains we start with a phenomenological model that describes a dependency of coercivity of remanence hcr with measurement time as a function of superparamagnetic grain content calculated time dependencies of hcr agree with experimental data suggesting that the model can be used to derive quantitative sp estimates when applied to pelagic sediments of the western equatorial pacific ocean ocean drilling program site 805c the time dependencies of hcr reveal a 25 30 sp increase starting just above the modern iron redox boundary irb next we compare our approach with a previous method that detects sp grains through the thermal demagnetization of saturation remanence acquired at very low temperature 20 k we find this method fails to record the sp changes detected using the time dependency of hcr and suggest that the cause is a sensitivity of the low temperature data to small differences in magnetic mineralogy that accompany diagenesis specifically differential maghemitization causes varying suppression of the remanence changes associated with the verwey transition at 120 k the removal of maghemite shells on magnetic grains below the irb results in greater remanence changes at the verwey transition and a corresponding sp underestimate using the low temperature method in addition some iron oxyhydroxides paramagnetic at room temperature can become ferromagnetic at very low temperatures resulting in an sp overestimate in oxic sediments above the irb the increase of sp content in the zone of magnetic mineral reduction observed with the time dependency of hcr may be related to the breakdown of larger magnetic grains and the activity of dissimilatory iron reducing bacteria superparamagnetism measured by the time dependency of hcr may be useful for tracking the history of these biogeochemical processes in marine sediments

relative geomagnetic field intensity across the jaramillo subchron in sediments from the california margin ocean drilling program leg 167 two relative paleointensity records of the jaramillo subchron were obtained during ocean drilling program leg 167 at sites 1010 and 1021 on the california margin the two sites are 1300 km apart and differ in sediment composition and deposition rate the demagnetization of the natural remanent magnetizations nrm of discrete samples revealed a vertical drilling overprint at both sites which could be removed by alternating field demagnetization at 20 mt for the sediments from site 1010 this was confirmed by a positive reversal test on the characteristic remanent magnetization of the discrete samples across the jaramillo subchron rock magnetic criteria for determining relative paleointensities are fulfilled for the samples from site 1010 because of variations in the rockmagnetic properties two short intervals had to be excluded from relative paleointensity determinations at site 1021 normalization of the nrm at several demagnetization steps with various magnetic parameters yielded similar results only for site 1010 and contributions from viscous remanent magnetization could not be detected in the sediments from this site using the pseudo thellier method of tauxe et al 1995 the relative paleointensity data of site 1021 show variations of up to 50 between normalizations with different magnetic parameters and therefore had to be excluded from further analysis several fine scale intensity features are detectable in the record of hole 1010c displaying a maximum in relative paleointensity shortly above the lower jaramillo transition a prominent local minimum in the middle of the subchron and a gradual decrease toward the upper jaramillo reversal a comparison of this record from the california margin with several published records of relative paleointensity across the jaramillo subchron with different sedimentation rates and sediment composition shows reasonable agreement between the paleointensity patterns but differences in amplitudes this suggests an influence of postdepositional magnetization processes which were not removed during normalization

environmental magnetic implications of magnetofossil occurrence during the middle eocene climatic optimum meco in pelagic sediments from the equatorial indian ocean abstract magnetic properties of pelagic marine sediments that record the middle eocene climatic optimum meco at 40 ma provide information about major environmental changes the main variations observed during this transient warming event reflect a bacterial magnetofossil signal but the cause of the linkage between bacterial production and climate remains unclear we present an environmental magnetic study of middle eocene deep sea sediments from the northern edge of madingley rise ocean drilling program hole 711a equatorial indian ocean to investigate the origin of the increased magnetic mineral concentration concomitant with subchron c18n 2n which corresponds to the meco interval in odp hole 711a this magnetic mineral peak also coincides with a change in lithofacies from calcareous nannofossils to radiolarian ooze and a slight increase in clay concentration magnetite is the main magnetic mineral in the meco sediments which occurs as magnetically non interacting single domain biogenic particles the increased magnetic mineral concentration across the meco event is likely to have been caused by increased eolian iron fertilization this is interpreted to have given rise to increased surface ocean productivity where increased delivery of iron and nutrients to the seafloor enhanced magnetotactic bacterial populations during the meco event

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

the age and duration of the matuyama brunhes transition from new k ar data from la palma canary islands and revisited 40ar 39ar ages abstract paleomagnetic investigations conducted on lava sequences from la palma canary islands yielded several independent records of the matuyama brunhes transition mbt seven k ar ages of flows sampled across the mbt suggest a duration between 0 and 11 kyr and provide a weighted mean age of 786 3 ka analytical uncertainty only for the transition this value is significantly older than the previously admitted age of 779 2 ka derived from volcanic or sedimentary sequences when the 1 uncertainty on the calibration standard used in the present study is considered our age for the transition recorded at la palma becomes 786 8 ka we have recalculated the 40ar 39ar determinations from the previous studies using recent revisited ages for 40ar 39ar standards this yields a global dataset of 23 k ar and 40ar 39ar determinations our best estimate for the mbt age derived from this compilation is 789 2 ka analytical error or 789 8 ka total error within this 8 kyr uncertainty which could be considered as a minimum estimate this age remains compatible with recent determinations made by the astronomical time scale however the present study suggests that better control of the lock in depth as well as better quantification of uncertainties involved in the tuning of 18o records to orbital forcing models are needed to improve further the astronomical polarity time scale derived from sedimentary records in addition absolute dating by k ar and 40ar 39ar will gain in accuracy when uncertainty determinations on standards will be significantly improved

sawtooth variation in magnetic intensity profiles and delayed acquisition of magnetization in deep sea cores abstract recent records of normalised intensity of natural remanent magnetization nrm obtained from deep sea sediments document an asymmetrical saw tooth pattern associated with geomagnetic reversals this observation may reflect a fundamental aspect of the geodynamo time evolution or alternatively delay in acquisition of the nrm it is demonstrated here that a simple model in which a large fraction of the magnetic grains acquire nrm at deposition time while remaining grains reorientate or acquire magnetization after deposition is able to produce a saw tooth profile strikingly similar to that observed interestingly this mechanism of nrm acquisition does not damp out high frequency oscillations of the field such as those observed in the deep sea records exhibiting the saw tooth profile it provides an accurate record of field directions except for periods of very low field intensity

long term secular variation in geomagnetic field inclination during brunhes chron recorded in sediment cores from ontong java plateau abstract a paleomagnetic study has been conducted on three gravity cores obtained from the ontong java plateau ojp in the western equatorial pacific in order to investigate long term secular variations during the brunhes chron the cores consist of nanofossil foraminiferal ooze and covers the last 500 000 800 000 years with average sedimentation rates of 6 9 m m y downcore changes of magnetic concentration represented by magnetic susceptibility are a factor of six or less and variations in magnetic grain size and mineralogy are estimated to be small from the hysteresis parameters ratio of anhysteretic remanent magnetization arm to saturation isothermal remanent magnetization sirm and s ratio these results indicate that the sediments are rock magnetically homogeneous in general and are hence suitable for relative paleointensity estimation i chose sirm as a normalizer for relative paleointensity the three cores could be precisely correlated with each other using relative paleointensity variations depth to age conversion was carried out by tying to the sint 800 paleointensity stack guyodo and valet 1999 the ages are consistent with the oxygen isotope stratigraphy available for one of the three cores inclination variations were obtained after alternating field demagnetization large inclination anomaly about 7 in average was observed as has been suggested to occur in the western equatorial pacific by some studies the inclination records of the three cores were integrated into a single record based on the correlation by the relative paleointensity long term inclination variations of the order of 10 000 100 000 years were recognized the inclinations tend to deflect toward negative when relative paleointensity is low the variations are similar in general to the record from the west caroline basin yamazaki and oda 2002 which supports their model that long term inclination variation is controlled by relative strength of the geocentric axial dipole gad and persistent non dipole components

variations in relative abundances of goethite and hematite in bengal fan sediments climatic vs diagenetic signals abstract relative abundances of goethite and hematite in marine sediments have been increasingly used in reflectance spectrometry studies to infer the precipitation regime at the source area of detrital sediments this interpretation is contingent on the assumption that the ratio of these minerals is not modified by diagenetic processes our rock magnetic study of the 10 5 4 0 ma interval of the bengal fan sedimentary sequence indicates that this is not always the case we demonstrate that a statistical analysis of isothermal remanent magnetization acquisition curves provides an adequate estimate of the content of goethite and hematite in sediments we also identified distinctive biogenic and detrital components likely titano magnetite or maghemite in the studied samples the sediments of the bengal fan contain a significant change in relative abundance of goethite and hematite at 7 ma which is approximately coeval with a postulated climatic turnover in the sediment source region however systematic variations in the detrital and biogenic components as well as an increase in the total organic carbon content and the appearance of pyrite concretions at the same stratigraphic level suggest that this change in magnetic mineral content reflects a change in the degree of diagenetic alteration of the initial detrital assemblage rather than a climatic signal when assigning climatic interpretations to changes in the relative abundance of goethite and hematite in marine sediments in future studies the possibility of diagenetic modification should be evaluated with rock magnetic methods alteration of ancient sediments can be detected and the severity of alteration can be estimated based on the presence and grain size variation of the ferrimagnetic fraction magnetite maghemite which is more sensitive to early reductive dissolution than high coercivity minerals goethite and hematite

magnetostratigraphy and relative palaeointensity of late neogene sediments at odp leg 167 site 1010 off baja california sediments recovered from the most seaward site of the baja transect of the california margin odp leg 167 yielded a high fidelity magnetostratigraphic record spanning the last 6 myr using a long core cryogenic magnetometer we made nrm measurements of 59 u channel samples which cover the interval from 0 to 78 5 mcd metres composite depth of the spliced section of holes 1010c and 1010e stepwise af demagnetization between 20 and 60 mt showed that the remanence is essentially composed of a single stable component which was not affected by a coring induced magnetization the investigated section can be divided into at least 25 polarity intervals which are assigned to the chrons from c3a n2 to the brunhes c1n the sedimentation rate was nearly constant at about 13 m myr 1 except for a reduction to 9 m myr 1 between 4 and 5 ma arm and irm measurements of the u channels revealed that the sediments below 23 mcd exhibit sharp drops in magnetic concentration at several horizons but that the upper part has a relatively uniform magnetic concentration containing magnetite of pseudo single domain size the upper interval yielded consistent relative intensity records when normalized with arm irm and shipboard susceptibility data several peaks in the relative intensity variations correspond to drops in the s ratio in particular at 0 1 t or to anomalous peaks in the irm and magnetic susceptibility profiles suggesting that the normalized intensity record is partly influenced by episodic changes in the flux of magnetic grains the normalized intensity for the intervals younger than 1 2 ma shows similarities with published palaeointensity records from the equatorial pacific including results from odp leg 138 in particular the intensity minima at 0 05 0 11 0 18 and 0 29 ma and those associated with the onset and termination of the jaramillo subchron are well correlated

biogeochemical remanent magnetization in pelagic sediments of the western equatorial pacific ocean paleomagnetic data from pelagic sediments are an important source of information on the history of the geomagnetic field however fundamental uncertainties about how these sediments acquire their magnetizations remain here we address whether biogenic magnetite formed at depth within the sediments can contribute to remanent magnetizations in sediments of the western equatorial pacific ocean odp site 805 biogenic magnetite found near the fe redox boundary carries a natural remanent magnetization because the production of biogenic magnetite is tied to the geochemical zonation of the sediments we call this magnetization a biogeochemical remanence these data indicate that some pelagic sediments can continue to acquire remanence significantly after the post depositional lock in of detrital magnetic grains

saw toothed pattern of relative paleointensity records and cumulative viscous remanence several studies of relative paleointensity data derived from marine sediments spanning the last 4 myr display an asymmetrical saw toothed pattern polarity reversals of the geomagnetic field are associated with low points in paleofield intensity preceded by a long term progressive decay of the field and followed by a rapid post transitional recovery 1 3 since similar behavior is observed in far flung sites whose rock magnetic records are different it is argued that the world wide saw toothing represents geomagnetic field behavior 3 we present an alternative explanation calling on the effect of hard viscous remanence

a new proxy for bottom water ventilation in the eastern mediterranean based on diagenetically controlled magnetic properties of sapropel bearing sediments magnetic properties of eastern mediterranean sediments recovered during leg 160 of the ocean drilling program odp provide insight into non steady state diagenetic reactions associated with accumulation and degradation of organic matter in sapropels according to their magnetic properties sapropels can be classified as one of three types that correspond to increasingly anoxic conditions at the time of sapropel formation a combination of magnetic and geochemical data suggests a causal relationship that enables determination of the relative role of bottom water ventilation versus productivity in the resulting diagenetic stage reached for the three types of sapropels it appears that increased productivity is a prerequisite for sapropel formation but once organic matter is available in sufficient amounts variable efficiencies in bottom water ventilation are more important for modulating the diagenetic context in which different types of sapropels formed magnetic properties are more sensitive to variations in bottom water ventilation than to productivity and can be used to establish relative variations in bottom water ventilation both at and after periods of sapropel formation magnetic results and the distribution and type of sapropels at odp site 966 eratosthenes seamount between 2 3 and 4 0 ma suggest that bottom water ventilation was modulated by the orbital eccentricity component with ventilation being restricted during 400 kyr eccentricity maxima and enhanced during eccentricity minima enhanced ventilation during eccentricity minima as indicated by magnetic data is consistent with the occurrence of red intervals at site 966 and at other eastern mediterranean sites drilled during odp leg 160 and also with astronomically modulated variations in caco3 content found in mediterranean land sections this suggests that variations in bottom water ventilation modulated by 400 kyr eccentricity cycles operated at a basin wide scale

fossil bacterial magnetite in deep sea sediments from the south atlantic ocean the magnetostratigraphy of deep sea sediment cores has proven to be a powerful and high resolution method of dating and comparing sedimentary records in contrast to this widespread application relatively few attempts have been made to analyse and identify the mineralogy grain size and origin of the carrier of the remanent magnetization1 2 it is widely assumed that dominant ferrimagnetic minerals in deep sea sediments are of lithogenic origin that is that they derive from oceanic or continental detritus volcanic ashfalls micrometeorites hydrothermal precipitation or chemical mineralization processes a very different view was taken by kirschvink and lowenstam3 who suggested that fossil remnants of magnetite mineralizing organisms in particular magnetotactic bacteria might play an important part in the magnetization of sediments deposited in certain detritus poor aquatic systems we have used rock magnetic diagnostic methods to characterize the magnetic phases in deep sea sediments from the south atlantic these phases were then extracted and studied with the electron microscope our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from quarternary to eocene different types of bacterial magnetosomes were detected most of them occurring in chains of identical particles and nearly all of them falling into the theoretically determined4 size range of single domain magnetitite

magnetotactic bacterial abundance in pelagic marine environments is limited by organic carbon flux and availability of dissolved iron article i nfo article history magnetotactic bacteria intracellularly biomineralize magnetite of an ideal grain size for recording palaeomagnetic signals however bacterial magnetite has only been reported in a few pre quaternary records because progressive burial into anoxic diagenetic environments causes its dissolution deep sea carbonate sequences provide optimal environments for preserving bacterial magnetite due to low rates of organic carbon burial and expanded pore water redox zonations such sequences often do not become anoxic for tens to hundreds of metres below the seafloor nevertheless the biogeochemical factors that control magnetotactic bacterial populations in such settings are not well known we document the preservation of bacterial magnetite which dominates the palaeomagnetic signal throughout eocene pelagic carbonates from the southern kerguelen plateau southern ocean we provide evidence that iron fertilization associated with increased aeolian dust flux resulted in surface water eutrophication in the late eocene that controlled bacterial magnetite abundance via export of organic carbon to the seafloor increased flux of aeolian iron bearing phases also delivered iron to the seafloor some of which became bioavailable through iron reduction our results suggest that magnetotactic bacterial populations in pelagic settings depend crucially on particulate iron and organic carbon delivery to the seafloor

detrital remanent magnetization viscosity theory of the lock in zone post depositional remanent magnetization pdrm becomes permanent within a lock in zone whose apparent time delay depth and time span thickness are related to the characteristic time constants of compaction and magnetic grain rotation two simple models of the pdrm lock in zone are described using yaskawa s idea of effective sedimentary viscosity and mooney s empirical relation for the viscosity of a concentrated suspension of spheres the impulse response of the lock in zone determines the fidelity of the paleomagnetic recording

magnetotactic bacterial response to antarctic dust supply during the palaeocene eocene thermal maximum distinct magnetic properties of marine sediments that record the palaeocene eocene thermal maximum petm have been suggested to be due to a bacterial magnetofossil signal that is linked to enhanced weathering conditions during the petm we document the dominance of bacterial magnetite in deep sea sediments from southern kerguelen plateau ocean drilling program hole 738c southern ocean not only during the petm but also before and after the thermal event this occurrence of magnetofossils throughout the petm indicates that the occurrence of bacterial magnetosomes is not due to a preservation effect instead we suggest that it is due to sustained mild iron reducing conditions that dissolved the most labile aeolian derived iron which favoured continued magnetotactic bacterial activity without being strong enough to dissolve the less reactive magnetite and haematite enhanced aeolian haematite abundances at the beginning of the petm indicate drier conditions on the neighbouring antarctic continent at those times our results provide evidence that iron fertilisation by aeolian dust was the main limiting factor that conditioned proliferation of magnetotactic bacteria in the deep sea at the southern kerguelen plateau with the exception of two short periods of rapidly changing palaeoenvironmental conditions at the onset and termination of the petm increased iron supply from aeolian dust that enhanced oceanic primary productivity and subsequent delivery of organic carbon to the seafloor along with mild iron reducing diagenetic conditions seem to have been necessary to provide the iron needed for magnetite biomineralization by magnetotactic bacteria to drive their marked increase in abundance in the studied petm record from southern kerguelen plateau our analyses of a deep sea petm record from hole 1051b at blake nose atlantic ocean failed to identify magnetofossils despite evidence for the occurrence of magnetite and haematite of probable aeolian origin contrasting magnetic properties at these petm sections indicate that further work is needed to understand the palaeoenvironmental and diagenetic factors whose interactions lead to production and preservation of magnetofossils in deep sea sediments

non steady state magnetic mineral reduction chemical lock in and delayed remanence acquisition in pelagic sediments abstract magnetic hysteresis data collected from pelagic sediment sites in the western equatorial pacific ocean define a progressive deepening of modern redox boundaries with decreased organic carbon input paleomagnetic data from the site with the highest organic carbon shows marked changes in the natural remanent magnetization at the modern fe redox boundary suggesting a type of chemical lock in process in which some magnetic grains form in situ above the boundary and freely orient in the field the depth of chemical lock in suggests paleomagnetic data will be smoothed on time scales ranging from less than 40 kyr to more than 425 kyr the higher values are compatible with models calling for a delayed remanence acquisition to account for the saw tooth pattern observed in some sedimentary paleointensity records

a precursor to the matuyama brunhes transition field instability as recorded in pelagic sediments abstract the period some 20 25 kyr just prior to the most recent generally recognized geomagnetic field polarity transition the matuyama to brunhes reversal appears to be marked by significant geomagnetic variability manifested as pronounced oscillations in intensity we compare several previously published paleomagnetic records with new high resolution paleomagnetic data obtained from five pelagic marine sites north atlantic dsdp hole 609b equatorial atlantic odp hole 665a and western equatorial pacific odp holes 803b 804c and 805b using standard rock magnetic normalization for all of the samples as well as a thellier thellier method on the sediments of hole 804c we consistently find a decrease in paleointensity dip of 1 approximately 15 kyr prior to the matuyama to brunhes transition in the five new records as well as in the previously published records despite sedimentation accumulation rates sar that range from 11 cm kyr to 1 cm kyr these sequences yield paleointensity curves that are broadly similar in form even at the lowest sars the intensity of the pre reversal low dip1 appears to be of the same magnitude as that of the transition itself dip2 of 1 in some of the records a directional excursion to nearly full normal polarity accompanies dip1 and remains after alternating field af and or thermal demagnetization whereas in other records the directional changes vanish with demagnetization and appear to be caused by overprinting a viscous remanent magnetization vrm contribution to nrm was identified in two of the records and until removed by af or thermal demagnetization was found to blur the double dip nature of the paleointensity profiles into an apparent single dip and also resulted in an apparent but erroneous sawtooth like post transitional sudden increase in paleointensity after appropriate normalization the magnitude of the post transitional recovery was much reduced the magnetic directions of three of the new records after cleaning and adjusting the stable declinations to either 0 normal or 180 reverse map to vgp positions lying in the pacific the directional variations however are far less consistent than the intensity variations the confirmed global existence of this dip so closely preceding a major reversal invites questions about its relation to the reversal itself the apparent normal character of this interval can also present problems for magnetostratigraphical interpretations based on coarse or incomplete sampling by mimicking the brunhes matuyama reversal some 15 kyr earlier in the section than its true location

early diagenesis in an organic rich turbidite and pelagic clay sequence from the cape verde abyssal plain ne atlantic magnetic and geochemical signals abstract processes of early diagenesis in sediments from the cape verde abyssal plain are investigated using the complimentary techniques of environmental magnetism and solid phase geochemistry the sequence captured by a 2 m long box core comprises an organic rich turbidite derived from the nw african continental margin emplaced ca 330 ka bp overlain by brown pelagic clay diagenesis in the pelagic clay mainly involves slow in situ oxidation of the iron oxide phases present which mossbauer effect data suggest are detrital haematite and magnetite possibly maghaemitised in the turbidite diagenesis has proceeded in two stages the first involving suboxic reductive processes following depletion of porewater o2 and the second involving oxidative processes associated with the slow descent of an oxidation front through the bed due to diffusion of o2 from the overlying seawater carbonate free magnetic parameters interparametric quotients and remanence acquisition demagnetisation and coercivity measurements of the core reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains like titano magnetite has occurred within the turbidite during suboxic diagenesis and confirm previous suggestions that reductive dissolution of such components is a grain size selective process however the higher oxidation state fe3 canted antiferromagnetic iron oxide grains like haematite and goethite are much less susceptible to reductive diagenesis remobilisation of transition metal ions from below the oxidation front in the turbidite has led to authigenic precipitation of ferrimagnetic fe mn micronodules in a distinct layer immediately overlying the front and above this a further interval of mn enriched diagenetic laminae however within the suboxic organic rich zone of the turbidite well below the oxidation front there is clear evidence for the presence of a significant concentration of fine single domain ferrimagnetic iron oxide grains which are most probably associated with a population of live magnetotactic bacteria between this zone and the oxidation front there is a transitional interval in which the number of live magnetotactic bacteria declines rapidly and fossil magnetosomes in the sediment are being reductively dissolved progressively up sequence these results are broadly similar though differing in some respects to magnetic and geochemical studies of hemipelagic and high productivity pelagic sediments in which mn and fe redox boundaries have been recognised and further illustrate the value of environmental magnetic parameters for characterising early diagenetic processes in sediments of this kind

environmental magnetism of pleistocene sediments in the north pacific and ontong java plateau temporal variations of detrital and biogenic components 1 an environmental magnetic study was conducted to investigate temporal variations of the sources of magnetic minerals in the pacific ocean using sediment cores of pleistocene age the proportion of interacting i to noninteracting n i single domain components was estimated from first order reversal curve diagrams and the proportion of middle m to low l coercivity components was determined from decomposition of isothermal remanent magnetization irm acquisition curves the i and m components are interpreted to be carried by terrigenous maghemite while the n i and l components represent biogenic magnetite an inverse correlation between the ratio of anhysteretic remanent magnetization susceptibility arm to saturation irm sirm and the i n i ratio was confirmed for sediments across wide expanses in the pacific ocean this implies that arm sirm can be used to estimate the relative abundances of biogenic and terrigenous components in sediments in the north pacific the relative abundance of the i and m components increases in glacial periods variations of arm sirm and s ratio s 0 1t resemble each other and decrease in the same time intervals these variations reflect increases of terrigenous input in glacial periods as eolian dust on the ontong java plateau in the western equatorial pacific on the other hand minima in arm sirm and s 0 1t occur at glacial to interglacial transitions and the relative abundance of the i and m components increases in these periods terrigenous material in this region is considered to be transported mainly from the new guinea and solomon islands by the equatorial undercurrent the higher proportion of the terrigenous component at glacial to interglacial transitions is coeval with carbonate preservation maxima reported from the ontong java plateau suggesting a linkage between the two

modeling the rupture process of the 2003 september 25 tokachi oki hokkaido earthquake using 1 hz gps data received 10 september 2004 accepted 3 october 2004 published 3 november 2004 1 high rategpshasthepotentialtorecoverbothdynamic and static displacements accurately we analyze 1 hz gps data recorded during the 2003 tokachi oki earthquake the 1 hz gps displacement waveforms show good agreement with integrated accelerometer records except for low frequency noise that are inherently present in integrated seismic records the gps waveforms were inverted to model the spatio temporal evolution of the fault slip during the rupture the slip is found to propagate downdip in the subduction zone with largest moment release 50 km northwest of the hypocenter the region of largest slip agrees in general with traditional seismic studies indicating that 1 hz gps can be used for finite fault studies the 1 hz gps slip model shows clearer contrast with afterslip distributions than those inferred from strong motion data possibly because 1 hz gps is more sensitive to cumulative slip distribution index terms 1294 geodesy and gravity instruments and techniques 7209 seismology earthquake dynamics and mechanics 7212 seismology earthquake ground motions and engineering citation miyazaki s k m larson k choi k hikima k koketsu p bodin j haase g emore and a yamagiwa 2004 modeling the rupture process of the 2003 september 25 tokachi oki hokkaido earthquake using 1 hz gps data geophys res lett 31 l21603 doi 10 1029 2004gl021457

magnetic mineral diagenesis abstract reduction oxidation redox reactions occur during burial because sediments contain reactive mixtures of oxidised and reduced components diagenetic chemical reactions represent the approach of all sedimentary components toward equilibrium and control the long term stability of sedimentary iron bearing minerals magnetic minerals are sensitive indicators of sedimentary redox conditions and of changes in these conditions through time with diagenetic effects ranging from subtle to pervasive despite the importance of magnetic mineral diagenesis in paleomagnetism rock magnetism and environmental magnetism and the usefulness of these subjects in the earth and environmental sciences there is no systematic single published treatment of magnetic mineral diagenesis this paper is an attempt to provide such a treatment for the full range of diagenetic environments magnetic mineral diagenesis during early burial is driven largely by chemical changes associated with organic matter degradation in a succession of environments that range from oxic to nitrogenous to manganiferous to ferruginous to sulphidic to methanic where the free energy yielded by different oxidants decreases progressively in each environment in oxic environments the most important diagenetic processes involve surface oxidation of detrital minerals and precipitation of fe3 bearing minerals from solution in ferruginous environments the most reactive detrital and authigenic iron oxides undergo dissolution often mediated by dissimilatory iron reducing bacteria which releases fe2 that becomes available for other reactions the fe2 in solution can diffuse upward where it is oxidised to form new authigenic iron oxyhydr oxide minerals or it can become bioavailable to enable magnetotactic bacteria to biomineralise magnetite generally at the base of the overlying nitrogenous zone alternatively dissimilatory iron reducing bacteria can produce extracellular magnetite within ferruginous environments in sulphidic environments iron bearing detrital mineral assemblages undergo more radical alteration hydrogen sulphide which is a byproduct of bacterial sulphate reduction or of anaerobic oxidation of methane reacts with the fe2 released from iron mineral dissolution or directly with solid iron oxyhydr oxide minerals to form iron sulphide minerals mackinawite greigite and pyrite authigenic growth of ferrimagnetic greigite has important implications for paleomagnetic recording secondary iron sulphide formation can also occur as a result of anaerobic oxidation of methane methane migration through sediments in association with biogenic or thermogenic methane production or in association with gas hydrate dissociation can disrupt the diagenetic steady state and give rise to greigite and monoclinic pyrrhotite formation that remagnetises sediments most of the above described diagenetic processes occur below 50 c with continuing burial above 50 c but at sub metamorphic temperatures magnetic minerals can undergo further thermally induced chemical changes that give rise to a wide range of mineralogical transformations that affect the magnetic record of the host sediment these changes include remagnetisations magnetic analysis can provide much valuable information concerning diagenesis in environmental processes the range of processes discussed in this paper should assist researchers in analysing sediment magnetic properties for which the assessment of diagenetic effects has become a necessary component

rock magnetic characterization of early redoxomorphic diagenesis in turbiditic sediments from the madeira abyssal plain rock magnetic measurements of two sediment cores from the madeira abyssal plain map north atlantic are used to investigate post depositional changes in the concentration grain size and composition of magnetic minerals in the sediments that have occurred within organic rich turbidite horizons the changes are associated with an initial stage of suboxic reductive diagenesis following depletion of porewater o2 and a later stage of oxidative diagenesis associated with the slow descent of an oxidation front through the sediment as a result of diffusion of o2 from the overlying sea water the turbidites are of late quaternary age 18o stages 1 3 and derive both from different sites on the nw african continental margin and from the flanks of the canary islands thus the turbidites are variable compositionally especially in terms of carbonate detrital magnetic mineral and organic carbon content diagenetic changes in these sediments have been identified using solid phase geochemical data u mn corg and caco3 reported previously in more than one study rock magnetic parameters of the sediments when expressed on a carbonate free basis reveal that significant depletion of detrital ferrimagnetic iron fe2 fe3 oxide grains has occurred within organic rich turbidites during redoxomorphic diagenesis normalized quotients of magnetic parameters also show that reductive diagenesis is a ferrimagnetic grain size selective process but it has a minimal effect on the canted antiferromagnetic fe3 oxides in the sediment such components if present therefore become relatively enriched in magnetic assemblages as the ferrimagnetic grains are dissolved progressively and bulk magnetic concentration is thus depleted there is clear evidence in both cores for the existence of ultrafine ferrimagnetic grains at depth within the suboxic zone of the organic rich turbidites beneath both active and fossil oxidation fronts these grains are most probably associated with populations of live magnetotactic bacteria which commonly inhabit such organic rich horizons and play a part in the chain of bacterially mediated reactions normally associated with suboxic diagenesis these results show that simple and rapid rock magnetic techniques can be used to characterize early diagenetic processes involving iron phases in deep sea sediments at least as effectively as more laborious time consuming and sample destructive geochemical measurements

natural remanent magnetization acquisition in bioturbated sediment general theory and implications for relative paleointensity reconstructions we present a general theory for the acquisition of natural remanent magnetizations nrm in sediment under the influence of a magnetic torques b randomizing torques and c torques resulting from interaction forces dynamic equilibrium between a and b in the water column and at the sediment water interface generates a detrital remanent magnetization drm while much stronger randomizing torques may be provided by bioturbation inside the mixed layer these generate a so called mixed remanent magnetization mrm which is stabilized by mechanical interaction forces during the time required to cross the surface mixed layer drm is lost and mrm is acquired at a rate that depends on bioturbation intensity both processes are governed by a mrm lock in function the final nrm intensity is controlled mainly by a single parameter that is defined as the product of rotational diffusion and mixed layer thickness divided by sedimentation rate this parameter defines three regimes 1 slow mixing 10 with mrm acquisition and full drm randomization and 3 intermediate mixing because the acquisition efficiency of drm is larger than that of mrm nrm intensity is particularly sensitive to in case of mixed regimes generating variable nrm acquisition efficiencies this model explains 1 lock in delays that can be matched with empirical reconstructions from paleomagnetic records 2 the existence of small lock in depths that lead to drm preservation 3 specific nrm acquisition efficiencies of magnetofossil rich sediments and 4 some relative paleointensity artifacts

recovering permanent displacements from seismic records of the june 9 1994 bolivia deep earthquake 1 ground displacements of the 1994 deep bolivia earthquake are recovered from six on scale velocity records of a temporary array located 600 km south of the epicenter i first removed the instrument sts 2 response using a time domain deconvolution to correct for baseline drift in the deconvolutions i used two separate windows of the trace one before and one after the transient motion from the earthquake and fit them simultaneously with a 4th order polynomial the amount of permanent displacement is included in the post transient signal window and is determined together with the polynomial coefficients by a least squares inversion the polynomial is then subtracted from the displacement to remove the drift i found that this technique produces robust displacement waveforms as demonstrated by similar results at all stations the obtained vertical permanent subsidences range from 5 5 to 6 4 mm which agree well with predictions by using normal mode summation

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

low temperature magnetic properties of pelagic sediments ocean drilling program site 805c tracers of maghemitization and magnetic mineral reduction magnetic properties of pelagic sediments from the western equatorial pacific ocean drilling program odp site 805c have been measured at low temperatures to investigate changes in magnetic mineralogy and the distribution of bacterial magnetite with depth a saturation isothermal remanent magnetization mrs was given to the samples by applying a strong magnetic field 2 5 t at 20 k after cooling both in the absence zero field cooling zfc and presence of a 2 5 t magnetic field field cooling fc the thermal demagnetization of both mrszfc and mrsfc was measured from 20 to 300 k no pronounced verwey transition was observed in the data from samples shallower than the iron redox boundary below the boundary the verwey transition is visible for all samples studied we interpret this behavior as reflecting low temperature oxidation maghemitization of primary magnetite and the subsequent dissolution of maghemite coatings below the iron redox boundary the low temperature oxidation may occur both at the water sediment interface and deeper in the suboxic zone of sediments as a result of bacteria mediated processes including the reduction of nitrate compounds mrszfc and mrsfc thermal demagnetization curves of all samples diverge for the entire temperature range measured the divergence may be caused by the low temperature stabilization of spontaneous magnetization vectors along with exchange interaction in the magnetite maghemite system together maghemitization and magnetic reduction diagenesis appear to play a dominant role in controlling the low temperature properties of the sediment studied as a result low temperature magnetic studies may provide more information on differences in reduction diagenesis than on the distribution of bacterial magnetite in natural sediments the changes in effective oxidation state below the iron redox boundary detected by low temperature methods may be useful in the study of some paleoenvironmental processes

temporal trends of magnetic dissolution in the pelagic realm gauging paleoproductivity abstract magnetic hysteresis data from the western equatorial pacific suggest that the preservation of magnetic remanence in pelagic carbonates is controlled by paleoproductivity hysteresis excursions 100 k y intervals are linked here to greater magnetic dissolution associated with increased glacial organic carbon supply changes in the redox boundary coupled to glacial terminations result in anomalous magnetic features which are preserved in the sedimentary record these high frequency variations are superimposed on a longer term signal of magnetic dissolution which may reflect decreased productivity since the miocene

asian monsoon modulation of nonsteady state diagenesis in hemipelagic marine sediments offshore of japan we have identifiedmillennial scale variations inmagneticmineral diagenesis from pacific ocean sediments offshore of japan that we correlate with changes in organic carbon burial that were likely driven by asian monsoon fluctuations the correlation was determined by identifying offsets between the positions of fossil diagenetic fronts and climatically induced variations in organic carbon burial inferred frommagnetic and geochemical analyses episodes of intense monsoon activity and attendant sedimentmagneticmineral diagenesis also appear to correlate with heinrich events which supports the existence of climatic telecommunications between asia and the north atlantic region several lines of evidence support our conclusions 1 fluctuations in down coremagnetic properties and diagenetic pyrite precipitation are approximately coeval 2 localized stratigraphic intervals with relatively strongermagneticmineral dissolution are linked to enhanced sedimentary organic carbon contents that gave rise to nonsteady state diagenesis 3 down core variations in elemental s content provide a proxy for nonsteady state diagenesis that correlate with key records of asianmonsoon variations and 4 relict titanomagnetite that is preserved as inclusions within silicate particles rather than secondary authigenic phases e g greigite dominates the strongly diagenetically altered sediment intervals and are protected against sulfidic dissolution we suggest that such millennial scale environmentalmodulation of nonsteady state diagenesis that creates a temporal diagenetic filter and relict magneticmineral signatures is likely to be common in organic rich hemipelagic sedimentary settingswith rapidly varying depositional conditions ourwork also demonstrates the usefulness of magnetic mineral inclusions for recording important environmentalmagnetic signals

detecting missing beats in the mediterranean climate rhythm from magnetic identification of oxidized sapropels ocean drilling program leg 160 eastern mediterranean sapropels are organic rich sediments whose formation is related to variations in the earth s orbit they are therefore important for reconstructing past climatic variations and for producing astronomically tuned geological timescales previous studies have suggested that the distinctive magnetic properties of sapropels which result from non steady state diagenetic reactions related to degradation of organic matter might be used for identifying sapropels that have escaped visual identification after being completely erased during post depositional oxidation we present a high resolution multi proxy magnetic geochemical and paleontological data set from selected intervals of ocean drilling program sites 966 and 967 our results demonstrate that magnetic properties can be unambiguously used for identifying oxidized sapropels and also for determining whether suspected intervals actually correspond to oxidized sapropels because they enable detection of the former presence of organic matter and of climatic and oceanographic conditions suitable for sapropel formation systematic application of high resolution magnetic analyses to future coring efforts in the eastern mediterranean should allow determination of the original distribution of sapropels through long sedimentary sequences which will improve our knowledge of paleoceanographic and paleoclimatic conditions that led to their formation

high rate real time gps network at parkfield utility for detecting fault slip and seismic displacements 1 a network of 13 continuous gps stations near parkfield california has been converted from 30 second to 1 second sampling with positions of the stations estimated in real time relative to a master station most stations are near the trace of the san andreas fault which exhibits creep the noise spectra of the instantaneous 1 hz positions show flicker noise at high frequencies and change to frequency independence at low frequencies the change in character occurs between 6 to 8 hours our analysis indicates that 1 second sampled gps can estimate horizontal displacements of order 6 mm at the 99 confidence level from a few seconds to a few hours high frequency gps can augment existing measurements in capturing large creep events and postseismic slip that would exceed the range of existing creepmeters and can detect large seismic displacements

enhanced primary productivity and magnetotactic bacterial production in response to middle eocene warming in the neo tethys ocean abstract earth s climate experienced a warming event known as the middle eocene climatic optimum meco at 40 ma which was an abrupt reversal of a long term eocene cooling trend this event is characterized in the deep southern atlantic pacific and indian oceans by a distinct negative 18 o excursion over 500 kyr we report results of high resolution paleontological geochemical and rock magnetic investigations of the neo tethyan monte cagnero mca section northeastern apennines italy which can be correlated on the basis of magneto and biostratigraphic results to the meco event recorded in deep sea sections in the mca section an interval with a relative increase in eutrophic nannofossil taxa and decreased abundances of oligotrophic taxa spans the culmination of the meco warming and its aftermath and coincides with a positive carbon isotope excursion and a peak in magnetite and hematite goethite concentration the magnetite peak reflects the appearance of putative magnetofossils while the hematite goethite apex is attributed to an enhanced detrital mineral contribution likely as aeolian dust transported from the continent adjacent to the neo tethys ocean during a drier more seasonal climate during the peak meco warming based on our new geochemical paleontological and magnetic records the meco warming peak and its immediate aftermath are interpreted as a period of high primary productivity sea surface iron fertilization is inferred to have stimulated high phytoplankton productivity increasing organic carbon export to the seafloor and promoting enhanced biomineralization of magnetotactic bacteria which are preserved as putative magnetofossils during the warmest periods of the meco event in the mca section together with previous studies our work reinforces the connection between hyperthermal climatic events and the occurrence or increased abundance of putative magnetofossils in the sedimentary record

magnetostatic interactions in deep sea sediments inferred from first order reversal curve diagrams implications for relative paleointensity normalization 1 understanding magnetostatic interactions in sediments is important for paleomagnetism and rock magnetism magnetostatic interactions are known to affect significantly the efficiency of anhysteretic remanent magnetization arm acquisition and hence the ratio of arm susceptibility arm to saturation isothermal remanent magnetization sirm a magnetic grain size proxy widely used for environmental applications can be controlled by magnetostatic interactions relative paleointensity estimations may also be influenced by magnetostatic interactions because arm is often used as a normalizer to correct for the difference in magnetizability of sediments in this study characterization of magnetic grains in deep sea sediments from the north pacific ocean was conducted using first order reversal curve forc diagrams and irm acquisition curves the forc diagrams indicate that the magnetic grains consist mainly of a noninteracting single domain sd component and an interacting sd component and additionally of a multidomain md component the relative abundances of these components were semiquantitatively estimated by curve fitting of cross sections along a line parallel to the axis of local interaction fields hu and through a peak in the coercivity hc three components were fitted assuming a gaussian distribution of interaction fields hu the irm acquisition curves can be described by two dominant components assuming a log gaussian distribution the mean coercivities of the two components are 40 and 100 mt respectively it is estimated that the former component is carried by biogenic magnetite and roughly corresponds to the noninteracting sd component derived from the forc diagrams and that the latter component is carried by eolian maghemite corresponding to the interacting sd component the arm sirm ratio decreases with increasing concentration of the interacting sd component this implies that the arm sirm ratio is significantly affected by magnetostatic interactions and that it does not necessarily reflect magnetic grain size the effect of magnetostatic interactions on natural remanent magnetization acquisition in sediments is not well understood but if it is similarly sensitive to the sirm normalization by arm in relative paleointensity estimations would overcompensate for the concentration of magnetic grains and cause the significant coherence between the normalized intensity and the normalizer that has been reported in the literature

magnetic characterization of holocene sedimentation in the south atlantic surface sediment samples representative for the tropical and subtropical south atlantic 15 n to 40 s were investigated by isothermal magnetic methods to delineate magnetic mineral distribution patterns and to identify their predominant holocene climatic and oceanographic controls individual parameters reveal distinct yet frequently overlapping regional sedimentation characteristics a probabilistic fuzzy c means cluster analysis was applied to five concentration independent magnetic properties assessing magnetite to hematite ratios and diagnostic of bulk and fine particle magnetite grain size and coercivity spectra the resultant 10 cluster structures establish an oceanwide magnetic sediment classification scheme tracing the major terrigenous eolian and fluvial fluxes authigenic biogenic magnetite accumulation in high productivity areas transport by ocean current systems and effects of bottom water velocity on depositional regimes distinct dissimilarities in magnetic mineral inventories between the eastern and western basins of the south atlantic reflect prominent contrasts of both oceanic and continental influences

diagenetic alteration of magnetic signals by anaerobic oxidation of methane related to a change in sedimentation rate geochemical and rock magnetic investigations of sediments from three sites on the continental margin off argentina and uruguay were carried out to study diagenetic alteration of iron minerals driven by anaerobic oxidation of methane aom the western argentine basin represents a suitable sedimentary environment to study nonsteady state processes because it is characterized by highly dynamic depositional conditions mineralogic and bulk solid phase data document that the sediment mainly consists of terrigenous material with high contents of iron minerals as a typical feature of these deposits distinct minima in magnetic susceptibility are observed pore water data reveal that these minima in susceptibility coincide with the current depth of the sulfate methane transition smt where hs is generated by the process of aom the released hs reacts with the abundant iron oxyhydr oxides resulting in the precipitation of iron sulfides accompanied by a nearly complete loss of magnetic susceptibility modeling of geochemical data suggest that the magnetic record in this area is highly influenced by a drastic change in mean sedimentation rate sr which occurred during the pleistocene holocene transition we assume that the strong decrease in mean sr encountered during this glacial interglacial transition induced a fixation of the smt at a specific depth the stagnation has obviously enhanced diagenetic dissolution of iron oxyhydr oxides within a distinct sediment interval this assumption was further substantiated by numerical modeling in which the mean sr was decreased from 100 cm kyr 1 during glacial times to 5 cm kyr 1 in the holocene and the methane flux from below was fixed to a constant value to obtain the observed geochemical and magnetic patterns the smt must remain at a fixed position for 9000 yrs this calculated value closely correlates to the timing of the pleistocene holocene transition the results of the model show additionally that a constant high mean sr would cause a concave up profile of pore water sulfate under steady state conditions copyright 2005 elsevier ltd

using 1 hz gps data to measure deformations caused by the denali fault earthquake the 3 november 2002 moment magnitude 7 9 denali fault earthquake generated large permanent surface displacements in alaska and large amplitude surface waves throughout western north america we find good agreement between strong ground motion records integrated to displacement and 1 hertz global positioning system gps position estimates collected 140 kilometers from the earthquake epicenter one hertz gps receivers also detected seismic surface waves 750 to 3800 kilometers from the epicenter whereas these waves saturated many of the seismicinstruments in the same region high frequency gps increases the dynamic range and frequency bandwidth of ground motion observations providing another tool for studying earthquake processes

anomalous directions of the natural remanent magnetization in late pleistocene marine sediments from the coast of mauritania west africa abstract two closely located deep sea cores taken along the coast of mauritania contain zones of anomalous directions of the natural remanent magnetization nrm which fall mainly within the potential time spans of the published excursions events e e of the earth magnetic field for the last 220 kyr in two cases even reversed polarity is seen in all cases in which anomalous directions are detected the sediments show a strong change in magnetic properties such as natural and anhysteretic remanent magnetization and low field susceptibility changes of the magnetic parameters and anomalous remanence directions coincide with rapid changes of stable oxygen isotope ratios i e correspond to isotope stage or substage boundaries and thus correspond to glacial interglacial transitions the good accordance of ages of the observed anomalous directions with previously identified e e may be partially caused by the large uncertainty in dating of many of these previously determined short e e therefore it is concluded that climatically controlled changes of the ferromagnetic mineralogy with related remagnetization and mineral interactions cause the anomalous nrm directions on the other hand if one considers the correlation between anomalous directions climatic changes and published e e to be significant one is led to postulate either that both earth field related directional changes of the nrm and climatically induced mineralogical changes are caused by a common mechanism or that changes in climate with changing sea level and changing ocean circulation triggered the e e of the geomagnetic field

a first order correction to minimize environmental influence in sedimentary records of relative paleointensity of the geomagnetic field we propose a simple method to minimize the residual environmental influence which may exist in paleointensity records obtained from sediments it consists of adding or subtracting a fraction of the normalizer arm or irm to the normalized intensity record so that the corrected record exhibits fluctuations uncorrelated with those of the normalizer this method was tested with paleointensity records obtained from two cores taken at midlatitude in the south indian ocean cores eastward of the kerguelen plateau for each core correction increases the correlation between the three normalized intensity records nrm nrm arm and nrm irm it also reduces the coherence between paleointensity records and bulk magnetic parameters used as normalizers although it has to be used with caution this correction may improve paleointensity records by reducing nongeomagnetic influence it can also be used to detect intervals where paleointensity may be unreliable i e intervals where large differences between uncorrected and corrected records are observed before and after correction

iron oxide formation in the active oxidation front above sapropel s1 in the eastern mediterranean sea as derived from low temperature magnetism low temperature magnetic properties of eastern mediterranean sediments from a box core have been investigated this box core contains the present day oxic suboxic boundary that is situated at the top of the relic of the youngest sapropel sl the upper half of sapropel s1 has been oxidized and fe oxides have precipitated in the oxidized sapropel zero field cooling zfc and field cooling fc saturation remanent magnetization m r induced in a field of 2 5 t at 20 k after cooling from 300 k with or without the 2 5 t field was measured during warming to 300 k an m r imparted at room temperature rtsirm was cycled to 20 k as well the difference between zfc and fc curves around the verwey transition as estimated by the parameter f c z f c suggests the presence of magnetosomes in the oxidized part of the sapropel we propose a new parameter d m 1 5 0 3 0 0 k r f c m 1 5 0 1 3 0 0 k r z f c m 3 0 0 k r f c where m 1 5 0 3 0 0 k r z f c represents the average remanence between 150 and 300 k to estimate the difference between zfc and fc curves above the verwey transition this is interpreted as being indicative of the number of defects and the extent of partial maghemitization at the oxic suboxic boundary where fe oxides actively precipitate the magnetite appears to be least maghemitized as inferred from low values for d further upward in the oxidized sapropel in older precipitates the magnetite is slightly more maghemitized just below the sapropel maghemitization is most pronounced in addition the initial slope of zfc and fc curves indicates that small grains or grain coatings that are superparamagnetic at room temperature sp are enriched at the oxic suboxic boundary higher in the oxidized sapropel the relative contribution of sp grains decreases presumably because they age to larger grains

searching for single domain magnetite in the pseudo single domain sedimentary haystack implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations 1 magnetic hysteresis measurements of sediments have resulted in widespread reporting of pseudo single domain like magnetic properties in contrast the ideal single domain sd properties that would be expected to be responsible for high quality paleomagnetic records are rare determining whether sd particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment we apply a range of such techniques and find that sd particles are much more common than has been reported in the literature and that magnetite magnetofossils the inorganic remains of magnetotactic bacteria are widely preserved at depth in a range of sediment types including biogenic pelagic carbonates lacustrine and marine clays and possibly even in glaci marine sediments thus instead of being rarely preserved in the geological record we find that magnetofossils are widespread this observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments magnetofossils also have grain sizes that are substantially smaller than the 1 15 m size range for which there is reasonable empirical support for relative paleointensity studies the different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies this issue needs much closer attention despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years much remains to be understood about how they become magnetized

relative geomagnetic paleointensity of the brunhes chron and the matuyama brunhes precursor as recorded in sediment core from wilkes land basin antarctica abstract paleomagnetic and rock magnetic investigation was performed on the 35 m long md03 2595 cado coring adelie diatom oozes piston core recovered on the continental rise of the wilkes land basin east antarctica analysis of the characteristic remanent magnetization chrm inclination record indicates a normal magnetic polarity for the uppermost 34 m of the sequence and a distinctive abrupt polarity change at the bottom of the core this polarity change which spans a 27 cm thick stratigraphic interval represents a detailed record of the matuyama brunhes m b transition and it is preceded by a sharp oscillation in paleomagnetic directions that may correlate to the m b precursor event paleomagnetic measurements enable reconstruction of geomagnetic relative paleointensity rpi variations and a high resolution age model was established by correlating the cado rpi curve to the available global reference rpi stack indicating that the studied sequence reaches back to ca 800 ka with an average sedimentation rate of 4 4 cm ka orbital periodicities 100 ka and 41 ka were found in the chrm inclination record and a significant coherence of chrm inclination and rpi record around 100 ka suggests that long term geomagnetic secular variation in inclination is controlled by changes in the relative strength of the geocentric axial dipole and persistent non dipole components moreover even if the relatively homogeneous rock magnetic parameters and lithofacies throughout the recovered sequence indicates a substantial stability of the east antarctic ice sheet during the middle and late pleistocene influence of the 100 ka and 41 ka orbital periodicities has been detected in some rock magnetic parameters indicating subtle variations in the concentration and grain size of the magnetic minerals linked to orbital forcing of the global climate

alteration of magnetic mineralogy at the sulfate methane transition analysis of sediments from the argentine continental slope on the argentine continental slope off the rio de la plata estuary the sulfate methane transition smt has been encountered at shallow depths of a few meters below the seafloor at around this horizon where sulfate diffusing downward from the bottom water is met and reduced by methane rising from deeper in the sediment column intense alteration affects the detrital magnetic mineral assemblage less than 10 of the dominant primary low coercivity ferrimagnetic titano magnetite remains after alteration in the upper part of the suboxic environment underlying the iron redox boundary which is located at a depth of 0 1 m approximately 60 of the finer grained detrital fraction is already dissolved while the high coercivity minerals are relatively unaffected in the suboxic environment large portions 40 are diagenetically dissolved in the sulfidic smt zone nevertheless the characteristics of the magnetic residue are entirely controlled by a high coercivity mineral assemblage unlike common observations that diagenetic alteration produces coarser magnetic grain sizes in suboxic milieus a distinct overall fining is found in the sulfidic zone different factors should contribute to this effect scanning electron microscope analysis combined with x ray microanalysis identified fine grained titano magnetite preserved as inclusions in silicates and between high ti titanohematite lamellae and possibly of prime importance a comprehensive fragmentation of larger grains in the course of maghemitization the only secondary iron sulfide mineral detected is pyrite which is present as clusters of euhedral crystals or directly replaces titano magnetite the thermomagnetic measurements did not provide evidence for the presence of ferrimagnetic sulfides such as greigite different from other studies reporting a marked magnetic enhancement at around the smt due to the precipitation and preservation of such metastable ferrimagnetic sulfides a complete pyritization process will cause a distinct magnetic depletion like in the present case 2005 elsevier b v all rights reserved

normalized remanence in sediments of the western equatorial pacific relative paleointensity of the geomagnetic field piston cores from the ontong java plateau collected during the eurydice expedition have been used to estimate regional geomagnetic paleointensity rock magnetic measurements of the sediments show that 1 the remanence is carried by stable pseudo single domain magnetite 2 the grain sizes and concentrations are relatively uniform with respect to the normalization technique and 3 susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field we therefore have measured the remanent intensity both natural and demagnetized to 15 mt and susceptibility of archive quarters of three cores at centimeter intervals using an automated through bore magnetometer a further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores one of our cores displays high coherence over a broad range of frequencies while for two others coherence is insignificantly different from zero at the 95 level of confidence oxygen isotopic records paleontological datum levels and the brunhes matuyama boundary provide an independent temporal framework for inter core comparison when the two cores which pass our acceptance criteria are placed on a common time scale the coherence between the normalized intensity records is significantly different from zero at well above the 95 level of confidence suggesting some common input signal over the same band of frequencies furthermore there is a great deal of similarity between the records from the ontong java plateau and a record from core rc10 167 taken some 3000 km to the north the distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows 0 5 ma with a slightly smaller standard deviation the inter core agreement within a single region the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years over the period 0 350 ka this feature is however lacking for the period 350 700 ka

relative paleointensity of the geomagnetic field during brunhes chron recorded in north pacific deep sea sediment cores orbital influence abstract relative paleointensity records of the geomagnetic field during the brunhes chron were obtained from two sediment cores in the north pacific near the hess rise the cores taken below the carbonate compensation depth ccd showed an oxidized environment and accompany little paleoclimatically induced lithological changes homogeneity of magnetic mineralogy and magnetic grain size which are prerequisites for sedimentary paleointensity were thoroughly examined using various techniques including low temperature magnetometry room temperature hysteresis parameters and s ratio isothermal remanent magnetization irm was used as a normalizer of the natural remanent magnetization nrm the appropriateness of the normalization was checked by the absence of correlation between the normalized intensity nrm irm and the normalizer irm the relative paleointensity records obtained are generally similar to the datasets from the east and central equatorial pacific spectral analysis showed significant power at the orbital eccentricity 100 kyr frequency in the relative intensity record but not for the normalizer this suggests orbital influence on the geomagnetic field intensity but the possibility of paleoclimatic contamination through rock magnetic changes should be examined further using cores from different sedimentary regime

a 17 000 yr paleomagnetic secular variation record from the southeast alaskan margin regional and global correlations high resolution sedimentary records on two cores from the gulf of alaska margin allow development of a 17 400 yr reconstruction of paleomagnetic secular variation psv general agreement between the two records on their independent chronologies confirms that local psv is recorded demonstrating that such archives notwithstanding complexities due to variable sedimentary regimes deposition rates and diagenetic conditions provide meaningful information on past changes of the geomagnetic field comparisons with other independently dated sedimentary paleomagnetic records from the ne pacific indicate largely coherent inclination records that in combination create a ne pacific sedimentary inclination anomaly stack nepsias capturing the common signal over an area spanning 30 longitude and latitude from alaska through oregon to hawaii comparisons of nepsias with high quality declination records from the northern north atlantic nna show that negative shallow inclination anomalies in nepsias are associated with eastward nna declinations while positive steep inclination anomalies in nepsias are associated with westward nna declinations comparison of these directional records to regional geomagnetic intensities over the past 3000 yrs in north america and back nearly 8000 yrs in the euro mediterranean region are consistent with a driving mechanism of oscillations in the relative strength of the north american and euro mediterranean flux lobes the persistence of these dynamics through the holocene implicates a long lived organizing structure likely imposed on the geomagnetic field by the lower mantle and or inner core these observations underscore a fundamental connection between directional psv in the north pacific with that of the north atlantic supporting the potential for long distance correlation of directional psv as a chronostratigraphic tool

depth distribution of magnetofossils in near surface sediments from the blake bahama outer ridge western north atlantic ocean determined by low temperature magnetism 1 fe oxide and fe sulfide trace minerals in sediments and sedimentary rocks provide proxy records of biogeochemical processes record past variations in the geomagnetic field and can serve as proxies for climatic variations an important class of these fe oxides is produced by bacteria magnetic particles produced by magnetotactic bacteria have been proposed as a primary recorder of the geomagnetic field in many terrestrial marine sediments and have also been suggested to represent fossil evidence of life on the planet mars to better understand their distribution and preservation in the sediment column and their relationship to other biochemical processes we present rock magnetic data that document the occurrence and abundance of fossil biogenic magnetite magnetofossils in marine sediments from the blake bahama outer ridge magnetic hysteresis and low temperature magnetism both indicate that the occurrence of magnetofossils is closely linked to the depth of the modern fe redox boundary within the sediment column and that a fraction of the magnetic minerals in the sediment column above the fe redox boundary are in the form of intact and relatively unaltered chains of nanophase magnetite crystals below the fe redox boundary the abundance of these magnetofossils is markedly decreased the important conclusions of this work are to demonstrate that nondestructive rock magnetic methods can be used to successfully document the occurrence and relative abundance of magnetofossils in geologic materials

diagenetic sensitivity of paleoenvironmental proxies a rock magnetic study of australian continental margin sediments a rock magnetic study of the upper 40 m of the ocean drilling program site 133 820a recovered at the outer edge of the northeastern australian continental margin shows that downcore variations in magnetic parameters are diagenetically driven and correlate with the changes in global sea level we identified intervals enriched in single domain sd magnetite in the studied section unlike previous studies that postulated a detrital source we show that the bulk of the sd fraction is biologically produced and is likely to be authigenic the abundance of sd magnetite thus cannot be used as an indicator for provenance or sediment transport mechanisms the biogenic magnetite was preserved during the high sedimentation rate periods likely due to a short residence time in the corrosive zone of active iron reduction the presence of the biogenic magnetite thus can be used as an indicator for a low degree of reductive dissolution more advanced dissolution during the periods of slow sedimentation coincident with sea level highstands resulted in significant changes in the composition of the detrital assemblage particularly in relative abundances of different mineral phases the overall stability of the detrital magnetic minerals toward dissolution varies in the studied section as hematite magnetite goethite due to the higher stability of hematite reductive diagenesis in general will lead to the lowering of the goethite hematite g h ratio which can be mistaken for an increased aridity in the sedimentary source areas in the conventional interpretation for this proxy the sensitivity of the g h proxy to diagenesis should be taken into account in paleoenvironmental studies

detection of live magnetotactic bacteria in south atlantic deep sea sediments abstract since they were first descrbed in 1975 1 magnetotactic bacteria have been observed in a wide variety of environments including soils 2 rivers freshwater lakes and coastal marine sediments 3 4 and in one case at a water depth of about 600 m in a hemipelagic sediment 5 here we report the first systematic findings of magnetotactic bacteria in pelagic and hemipelagic sediments of the eastern south atlantic different morphologies were identified cocci spirilla vibrionic and rod shaped forms at water depths to about 3000 m on the african continental margin between the equator and 30 s and on the walvis ridge in a pelagic environment about 1400 km off the coast no actively swimming magnetotactic bacteria could be detected in deeper waters magnetotactic bacteria were observed only in the upper 10 cm of the sediments with the highest concentrations always found in a well defined narrow subsurface layer the depth of this layer is dependent on the input and turnover rates of organic matter in the sediment measurements of oxygen and nitrate concentrations 6 7 provide an assignment of depth distributions of magnetotactic bacteria to the stratification of terminal electron acceptors in the sediment the data show clearly that the majority of marine magnetotactic bacteria live in the anaerobic zone in contrast to former assumptions that they may inhabit microaerobic layers 8 11

high resolution rock magnetic cyclostratigraphy in an eocene flysch spanish pyrenees 1 a rock magnetic cyclostratigraphy based on anhysteretic remanent magnetization arm intensity variations was developed for the eocene arguis formation in the spanish pyrenees the arguis formation was sampled for arm cyclostratigraphy rock magnetic and paleomagnetic analyses rock magnetic measurements indicate that the dominant magnetic mineral controlling the arm cyclostratigraphy is depositional magnetite using thermal demagnetization a detailed magnetostratigraphy was developed for the arguis to provide an absolute time framework for the arm cyclostratigraphy the magnetostratigraphy is carried by a combination of depositional magnetite and secondary iron sulfides spectral analysis of the magnetostratigraphically scaled arm time series reveals the presence of significant milankovitch frequencies including eccentricity obliquity and precession the arm time series was tuned to the orbital eccentricity model for the eocene coherency analysis indicates that the eccentricity tuned arm is in phase with october november insolation for this site s paleolatitude varying amounts of terrigenous input delivered by a fluvial source at orbital time scales during the rainy season and diluted by a relatively constant input of marine carbonate is the most likely explanation for the cyclicity of the arm record the absolute age resolution of the arm chronostratigraphy is 1 or less based on an age offset with the reference chronology that was used for the eocene the offset in age between the two chronologies may be due in part to the uncertainty in the chron ages for this part of the eocene the relative age resolution within the arm chronostratigraphy is much better

diagenetic alteration of magnetic minerals in labrador sea sediments iodp sites u1305 u1306 and u1307 1 in order to reveal the potential effects of early diagenesis on magnetic minerals in deep sea sediments we studied early diagenetic zones and magnetic mineral characteristics of lower pliocene hemipelagic sediment samples from iodp sites u1305 u1306 and u1307 on eirik drift labrador sea all samples analyzed were unlithified silty clay sediments recovered by a piston corer from depths down to 200 meters composite depth mcd based on shipboard interstitial water geochemistry we divided the sediment column from each site into six early diagenetic zones magnetite fe3o4 was present at all analyzed depths whereas maghemite fe2o3 was found only above the iron reduction zone we attribute this to associated changes in interstitial redox conditions which induced preferential dissolution of maghemitized surfaces on magnetite grains mineral magnetic results indicate a general down hole change in mean grain size of magnetic minerals at site u1307 which has relatively low organic carbon contents the diagenetic zones occur at greater depths than at the other studies sites this suggests that interstitial oxygen levels at this site remained high enough to degrade organic matter through oxic bacterial activity and that detrital magnetic minerals have been preserved even at depth

signatures and significance of aeolian fluvial bacterial and diagenetic magnetic mineral fractions in late quaternary marine sediments off gambia nw africa 1 two gravity cores retrieved off nw africa at the border of arid and subtropical environments geob 13602 1 and geob 13601 4 were analyzed to extract records of late quaternary climate change and sediment export we apply end member em unmixing to 350 acquisition curves of isothermal remanent magnetization irm our approach enables to discriminate rock magnetic signatures of aeolian and fluvial material to determine biomineralization and reductive diagenesis based on the occurrence of pedogenically formed magnetic minerals in the fluvial and aeolian ems we can infer that goethite formed in favor to hematite in more humid climate zones the diagenetic em dominates in the lower parts of the cores and within a thin near surface layer probably representing the modern fe2 fe3 redox boundary up to 60 of the irm signal is allocated to a biogenic em underlining the importance of bacterial magnetite even in siliciclastic sediments magnetosomes are found well preserved over most of the record indicating suboxic conditions temporal variations of the aeolian and fluvial ems appear to faithfully reproduce and support trends of dry and humid conditions on the continent the proportion of aeolian to fluvial material was dramatically higher during heinrich stadials especially during heinrich stadial 1 dust export from the arabian asian corridor appears to vary contemporaneous to increased dust fluxes at the continental margin of nw africa emphasizing that meltwater discharge in the north atlantic had an enormous impact on atmospheric dynamics

a biogenic origin for anomalous fine grained magnetic material at the paleocene eocene boundary at wilson lake new jersey 1 the paleocene eocene thermal maximum which occurred 55 5 ma was caused by a massive release of carbon as indicated by an 3 negative carbon isotope excursion recorded in the marine atmospheric and terrestrial reservoirs one suggested source for the carbon a cometary impactor is based on the sudden appearance and high concentration of single domain sd magnetite in paleocene eocene p e boundary cores from the north atlantic continental margin we evaluate the potential sources of sd magnetite at the p e boundary by presenting new magnetic hysteresis low temperature magnetic remanence and transmission electron microscopy data from the north atlantic coastal ocean our results show a similar increase in sd material but demonstrate that the magnetic material has a biogenic origin these findings indicate that the high concentrations of sd magnetite immediately above the p e boundary are the result of unusual accumulations and or preservation of magnetotactic bacteria such bacteria typically occupy the oxic anoxic transition zone near the sediment water interface or in the water column the high abundances of sd magnetite in sediments from across the shelf may be an artifact of nonsteady state redox conditions and exceptional preservation of sd magnetite it may also indicate that the oxic anoxic redox boundary shifted into the water column the latter explanation implies transient eutrophy of the coastal ocean in this region most likely due to seasonally enhanced runoff and increased stratification and nutrient loading

spatial and temporal distribution of slip for the 1992 landers california earthquake we have determined a source rupture model for the 1992 landers earthquake m w 7 2 compatible with multiple data sets spanning a frequency range from zero to 0 5 hz geodetic survey displacements near field and regional strong motions broadband teleseismic waveforms and surface offset measurements have been used explicitly to constrain both the spatial and temporal slip variations along the model fault surface our fault parameterization involves a variable slip multiple segment finite fault model which treats the diverse data sets in a self consistent manner allowing them to be inverted both independently and in unison the high quality data available for the landers earthquake provide an unprecedented opportunity for direct comparison of rupture models determined from independent data sets that sample both a wide frequency range and a diverse spatial station orientation with respect to the earthquake slip and radiation pattern in all models consistent features include the following 1 similar overall dislocation patterns and amplitudes with seismic moments of 7 to 8 10 26 dyne cm seismic potency of 2 3 to 2 7 km 3 2 very heterogeneous unilateral strike slip distributed over a fault length of 65 km and over a width of at least 15 km though slip is limited to shallower regions in some areas 3 a total rupture duration of 24 sec and an average rupture velocity of 2 7 km sec and 4 substantial variations of slip with depth relative to measured surface offsets the extended rupture length and duration of the landers earthquake also allowed imaging of the propagating rupture front with better resolution than for those of prior shorter duration strike slip events our imaging allows visualization of the rupture evolution including local differences in slip durations and variations in rupture velocity rupture velocity decreases markedly at shallow depths as well as near regions of slip transfer from one fault segment to the next as rupture propagates northwestward along the multiply segmented fault length the rupture front slows as it reaches the northern limit of the johnson valley landers faults where slip is transferred to the southern homestead valley fault an abrupt acceleration is apparent following the transfer this process is repeated and is more pronounced as slip is again passed from the northern homestead valley fault to the emerson fault although the largest surface offsets were observed at the northern end of the rupture our modeling indicates that substantial rupture was also relatively shallow less than 10 km in this region

maghemite to magnetite reduction across the fe redox boundary in a sediment core from the ontong java plateau influence on relative palaeointensity estimation and environmental magnetic application summary during reduction diagenesis production of dissolved fe2 by reduction of ferric oxide starts at the fe redox boundary the associated magnetic property changes may influence palaeomagnetic and environmental magnetic records of marine sediments however this has not been evaluated thoroughly in this study using a gravity core from the ontong java plateau we document in detail rock magnetic changes across the fe redox boundary and investigate their influence on relative palaeointensity estimation and on a magnetic proxy for the proportion of terrigenous biogenic magnetic minerals the magnetic mineral assemblage above the fe redox boundary is characterized by a component with a mean coercivity of 100 mt in isothermal remanent magnetization irm component analyses and low s ratios s 0 1t low temperature irm measurements and mossbauer spectroscopy indicate that the degree of maghemitization is higher above the fe redox boundary from these observations we conclude that reduction of maghemite to magnetite occurs at the fe redox boundary and we infer that a maghemite skin on magnetic grains is lost across the boundary relative palaeointensity variations obtained by normalizing nrm intensity with sirm agree well with regional and global palaeointensity stacks which suggests that relative palaeointensity estimation is not significantly affected by the fe redox boundary temporal variations of the ratio of anhysteretic remanent magnetization susceptibility and saturation irm arm sirm coincide with the regional pattern across the ontong java plateau it is also possible to estimate variations in the proportion of terrigenous to biogenic components using the arm sirm ratio across the fe redox boundary

magnetofossils the magnetization of sediments and the evolution of magnetite biomineralization magnetite fe 3o 4 is one of the most stable carriers of natural remanent magnetization nrm in sedimentary rocks and paleomagnetic studies of magnetite bearing sediments such as deep sea cores and pelagic limestones have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales despite this important role there is as yet a very poor understanding of how ultrafine grained 0 1 m magnetite is formed transported and preserved in marine environments a major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite formed by magnetotactic bacteria and or other magnetite precipitating organisms is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks since these magnetite particles are of biogenic origin they are termed properly magnetofossils kirschvink chang 1984

slip history of the 2003 san simeon earthquake constrained by combining 1 hz gps strong motion and teleseismic data 1 the slip history of the 2003 san simeon earthquake is constrained by combining strong motion and teleseismic data along with gps static offsets and 1 hz gps observations comparisons of a 1 hz gps time series and a co located strong motion data are in very good agreement demonstrating a new application of gps the inversion results for this event indicate that the rupture initiated at a depth of 8 5 km and propagated southeastwards with a speed 3 0 km sec with rake vectors forming a fan structure around the hypocenter we obtained a peak slip of 2 8 m and total seismic moment of 6 2 10 18 nm we interpret the slip distribution as indicating that the hanging wall rotates relative to the footwall around the hypocenter in a sense that appears consistent with the shape of the mapped fault trace index terms 1242 geodesy and gravity seismic deformations 7205 7212 seismology earthquake ground motions and engineering 8123 tectonophysics dynamics seismotectonics citation ji c k m larson y tan k w hudnut and k choi 2004 slip history of the 2003 san simeon earthquake constrained by combining 1 hz gps strong motion and teleseismic data geophys res lett 31 l17608 doi 10 1029 2004gl020448

superparamagnetism and reduction diagenesis in pelagic sediments enhancement or depletion superparamagnetic grains ultra fine d 0 05 m particles unable to retain a remanence at room temperature have come under increased scrutiny as indicators of the diagenetic and authigenic history of sedimentary rocks in marine sediments fine grained magnetite with large surface area to volume is thought to be preferentially removed during magnetic mineral reduction on the basis of new magnetic hysteresis low temperature and geochemical studies of pelagic sediments from the western equatorial pacific ocean it is proposed that reduction processes increase rather than deplete the ultra fine magnetic grain population at the modern fe redox boundary enhanced superparamagnetism coincides with a coarsening of remanence carrying grains superparamagnetism also tracks proposed temporal changes in magnetite reduction caused by climatically driven fluctuations in organic carbon corg supply together with hysteresis characteristics changes in superparamagnetism may help the identification of paleointensity artifacts resulting from non steady state magnetic mineral reduction

coercivity analysis of magnetic phases in sapropel s1 related to variations in redox conditions including an investigation of the s ratio 1 abstract the most recent sapropel s1 in the eastern mediterranean has been extensively investigated with geochemical and rock magnetic techniques different redox conditions prevailed in different zones of the sediment through time the oxidized sapropel zone is particularly interesting because earlier studies indicated that new magnetic material was formed including possible magnetosomes here we utilize component analyses of isothermal remanent magnetization irm acquisition curves and the analysis of first order reversal curves forc to further investigate the magnetic mineralogy in the entire box core abc26 the original input of eolian dust consisted of both magnetite and hematite in the oxidized sapropel and in the active oxidation zone an additional magnetite component is present this magnetite component has a higher coercivity than the eolian magnetite and a very small coercivity dispersion suggesting a narrow grain size distribution this is a strong indication that magnetosomes which are formed in the active oxidation zone are the magnetic carriers of this coercivity fraction forc diagrams support these findings the s ratio is forwardly modeled for mixed magnetic mineralogies with varying coercivity distributions to explain the down core s ratio behavior close to 1 in the top sediment and in the oxidized sapropel a drop in values in the active oxidation zone and again close to 1 in the syn sapropel the observed s ratio pattern in the oxidized sapropel and in the active oxidation zone can be explained by the recovered irm components whereas both oxidized sapropel and active oxidation zone contain the extra magnetite component their s ratios are different because of the differences in coercivity characteristics coercivity and dispersion of the two magnetites in these zones thus in these zones the s ratio does not reflect variations in the relative contributions of hematite to magnetite but variations in the characteristics of the individual magnetite assemblages also in the rest of the core the s ratio depends on the coercivity characteristics of the magnetite component rather than on the relative contributions of high versus low coercivity minerals the s ratio thus appears to be an unsuitable parameter to describe variations in the magnetic mineralogy especially when more than two components are present therefore the classical interpretation of the s ratio should be treated with caution

magnetic properties and geochemistry of the active oxidation front and the youngest sapropel in the eastern mediterranean sea summary magnetic properties irm arm xin s ratio at 0 3 t room temperature rt hysteresis and thermomagnetic curves and geochemical data fe s mn al ti organic c were studied in two eastern mediterranean boxcores abc26 and bc19 at a resolution of 3 5 mm the boxcores contain sapropel s1 9 6 kyr bp at a few decimetres below seafloor the magnetic fraction consists predominantly of single domain sd to pseudo singledomain psd magnetite in the entire cores the original input of magnetic grains comes from two sources aeolian dust both cores and volcanic ash from the minoan eruption of santorini core bc19 only non steady state diagenesis has changed the magnetic mineralogy considerably in these alternating organic rich organic poor sediments during deposition of sapropel s1 reductive diagenesis and pyritization in and just below the sapropel caused lower magnetic intensities coarser magnetic grain sizes and partial maghemitization in thermomagnetic curves two types of pyrite can be identified one oxidizes below 450 uc and the other above 450 uc the higher oxidation temperature is predominantly found below the sapropel this may be related to the microtexture of pyrite which is euhedral below sapropels and mainly framboidal within sapropels since the end of sapropel deposition a downward moving oxidation front has oxidized the upper half c 5 cm of the sapropel the oxidized part of the sapropel is enriched in diagenetically formed fe oxides with relatively high coercivity and arm the maximum coercivity is found in a distinct layer between the present day mn and fe redox boundaries at the top of the unoxidized sapropel the freshly precipitated fe oxides in this centimetre thick layer contain a mixture of superparamagnetic sp grains and high coercivity sd magnetite higher in the oxidized zone the freshly precipitated fe oxides have aged into generally slightly lower coercivity sd grains with relatively high arm in addition to the diagenetic formation of fe oxides at the top of the sapropel formation of a ferrimagnetic fe monosulphide may have occurred within the sapropel during later stages of diagenesis which may have enhanced the arm signal in the organic rich interval in particular

single magnetic domains in magnetotactic bacteria magnetotactic bacteria construct an internal permanent magnetic dipole based on single magnetic domain particles of magnetite or greigite the organisms exert a high degree of control over the size and morphology of the particles this may be relevant in distinguishing biogenic from nonbiogenic iron mineral particles

magnetic properties of pelagic marine carbonates abstract pelagic carbonates are deposited far from continents usually at water depths of 3000 6000 m at rates below 10 cm kyr and are a globally important sediment type recent advances with recognition of widespread preservation of biogenic magnetite the inorganic remains of magnetotactic bacteria have fundamentally changed our understanding of the magnetic properties of pelagic carbonates we review evidence for the magnetic minerals typically preserved in pelagic carbonates the effects of magnetic mineral diagenesis on paleomagnetic and environmental magnetic records of pelagic carbonates and what magnetic properties can tell us about the open ocean environments in which pelagic carbonates are deposited we also discuss briefly late diagenetic remagnetisations recorded by some carbonates despite recent advances in our knowledge of these phenomena much remains undiscovered we are only at early stages of understanding how biogenic magnetite gives rise to paleomagnetic signals in sediments and whether it carries a poorly understood biogeochemical remanent magnetisation recently developed techniques have potential for testing how different magnetotactic bacterial species which produce different magnetite morphologies respond to changing nutrient and oxygenation conditions future work needs to test whether it is possible to develop proxies for ancient nutrient conditions from well calibrated modern magnetotactic bacterial occurrences a tantalizing link between giant magnetofossils and paleogene hyperthermal events needs to be tested much remains to be learned about the relationship between climate and the organisms that biomineralised these large and novel magnetite morphologies rather than being a well worn subject that has been studied for over 60 years the magnetic properties of pelagic carbonates hold many secrets that await discovery

relative paleointensity record during the last 800 ka from the equatorial indian ocean implication for relationship between inclination and intensity variations 1 paleomagnetic and rock magnetic investigations were carried out on three sediment cores taken from the ninetyeast ridge the eastern equatorial indian ocean the cores are 4 1 10 2 m long and cover the last 270 790 ka with average sedimentation rates of 1 3 1 9 cm ka generally stable demagnetization behaviors and relatively uniform rock magnetic characteristics allowed us to reconstruct relative paleointensity records nrm30mt irm30mt for each core the relative paleointensity records showed a beneficial agreement with each other except for the uppermost parts of the cores and patterns similar to the sint 800 paleointensity stack and other relative paleointensity records spectral analyses of the relative paleointensity records revealed a dominant periodicity at about 100 ka but not for the normalizer and other rock magnetic proxies these results suggested that the 100 ka periodicity in the paleointensity records during the brunhes chron would not be caused by sedimentological effects the mean inclination values of the three cores were close to the expected hypothetical geocentric axial dipole gad direction which implies that 5 10 of negative inclination anomaly i is centered at the western equatorial pacific ocean and not extended to the eastern equatorial indian ocean a comparison between the relative paleointensity variation and i amplitude between the eastern equatorial indian ocean and the western equatorial pacific ocean suggests that changes of the gad intensity control the relative contribution of the persistent nondipole components and produces the inclination variation

relative palaeointensity records from the ontong java plateau summary rock magnetic palaeomagnetic and oxygen isotopic results are presented from core rndb 75p which was recovered from the ontong java plateau ojp a high degree of uniformity in magnetic properties characterized by relatively small changes in concentration and grain size in the upper 4 m of the core combined with a lack of coherence between the normalized remanence and rock magnetic data suggests that the natural remanence normalized by saturation remanence reflects variations in relative palaeointensity of the geomagnetic field the record from rndb 75p replicates other ontong java records spanning the last 400ka and extends the record back to some 700ka spectral analysis of the ontong java record suggests periodic behaviour in the relative palaeointensity record with a dominant period of between 30 and 40ka which appears not be be an artefact of lithologic variability this dominant period lies between functions describing climatic precession and obliquity changes in the earth s orbit comparison of the normalized remanence record with astronomical precession 26 ka period however is much more favorable none the less tuning the palaeointensity record to that of astronomical precession appears inconsistent with existing isotopic age constraints derived from the specmap time scale based on these data we must choose between assuming that the earth s orbit controlled ice volume inherent in the specmap time scale and assuming that the earth s magnetic field is driven by astronomical precession the former assumption has a substantial theoretical and observational base and we prefer to interpret the data presented here as suggesting that the earth s orbit has not played a detectable role in the modulation of the magnetic field plots of saturation remanence and magnetic susceptibility are very sensitive to quite subtle changes in magnetic grain size a slight shift within the pseudo singledomain grain size range toward the multidomain or superparamagnetic field was detected at about 4m in rndb 75p this change in grain size may reflect a diagenetic alteration of the magnetite such as dissolution and may be related to the phenomenon responsible for the loss of magnetic remanence at depth detected in other cores from the region

a faulting model for the 1979 imperial valley earthquake by comparing synthetic particle velocities with the near source strong motion data we have constructed by trial and error a faulting model for the 1979 imperial valley earthquake the calculation of the synthetic seismograms takes into account the vertical inhomogeneity of the elastic parameters in the imperial valley and the spatial variation of the slip rate parameters on the fault plane the independent slip rate parameters are 1 the strike slip rate amplitude 2 the dip slip rate amplitude 3 the duration that slip rate is nonzero the rise time of the slip function and 4 the rupture time which determines when the slip rate is initiated our faulting model has the following principal features 1 faulting occurred on the imperial fault and on the brawley fault rupture on the brawley fault being triggered by rupture on the imperial fault 2 the imperial fault is a plane 35 km long and 13 km wide with a strike of 323 measured clockwise from north and a dip of 80 ne the brawley fault is a 10 km long and 8 km wide plane with a strike of 360 and a dip of 90 3 faulting on the imperial fault is primarily right lateral strike slip with a small component of normal dip slip in the sediments at its northern end the larger strike slip rates are generally confined between depths of 5 and 13 km with maximum values of about 1 0 m s the duration varies on the fault with a maximum of 1 9 s which is considerably shorter than the total time for the rupture to take place 4 the rupture velocity on the imperial fault is highly variable locally it exceeds the shear wave velocity and in one instance the compressional wave velocity the average rupture velocity though is less than the shear wave velocity 5 although the slip on the brawley fault contributes only about 4 of the total moment it greatly affects the ground motion at nearby stations 6 the total seismic moment is 6 7 1018 n m where the imperial fault contributes 6 4 1018 n m and the brawley fault contributes 2 7 1017 n m in the process of trying almost 300 faulting models we found that given the elastic parameters of the medium the synthetic seismograms were most sensitive to the specification of the rupture time although the slip rate amplitudes are linearly related to the data the rupture time and the duration are not the parameterization of the nonlinear variables has a strong effect on the generation of synthetic seismograms from a finite fault

holocene millennial centennial scale multiproxy cyclicity in temperate eastern australian estuary sediments we have undertaken a comparative study of down core variation in multiproxy palaeoclimate data magnetic susceptibility calcium carbonate content and total organic carbon from two coastal water bodies myall and tuggerah lakes in temperate eastern australia to identify local regional and global forcing factors within holocene estuarine sediments the two lakes lie within the same temperate climate zone adjacent to the tasman sea but are not part of the same catchment and drain different geological provinces one is essentially a freshwater coastal lake whereas the other is a brackish back barrier lagoon despite these differences data from two sites in each of the two lakes have allowed us to investigate and compare cyclicity in otherwise uniform single facies sediments within the frequency range of 200 2000 years limited by the sedimentation rate within the lakes and our sample requirements we have auto and cross correlated strong periodicities at 360 years 500 530 years 270 290 years 420 450 years and 210 years and subordinate periods of 650 years 1200 1400 years and 1800 years our thesis is that climate is the only regionally available mechanism available to control common millennial and centennial scale cyclicity in these sediments given the geographical and other differences however regional climate may not be the dominant effect at any single time and either location within the range of frequency spectral peaks we have identified several fall within known long term periodical fluctuations of sun spot activity however feedback loops associated with short term orbital variation such as dansgaard oeschger cycles and the relationship between these and palaeo enso variation are also possible contributors copyright 2005 john wiley sons ltd

a geomagnetic paleointensity record of 0 6 to 3 2 ma from sediments in the western equatorial pacific and remanent magnetization lock in depth

detection of arbitrarily large dynamic ground motions with a dense high rate gps network 1 we describe the detection of teleseismic surface waves from the 3 november 2002 mw 7 9 denali fault earthquake in alaska with a dense network of 1 hz gps stations in southern california about 3900 km from the event relative horizontal displacements with amplitudes in excess of 15 mm and duration of 700 seconds agree with integrated velocities recorded by nearby broadband seismometers with an rms difference of 2 3 mm the displacements are derived from independent 1 hz instantaneous positions demonstrating that a gps network can provide direct measurements of arbitrarily large dynamic and static ground horizontal displacements at periods longer than 1 s and amplitudes above 2 mm with an inherent precision signal to noise that improves indefinitely with amplitude without clipping and in real time high rate real time gps networks can enhance earthquake detection and seismic risk mitigation and support other applications such as intelligent transportation and civil infrastructure monitoring

paleomagnetic reconstruction of the global geomagnetic field evolution during the matuyama brunhes transition iterative bayesian inversion and independent verification abstract the earth s magnetic field changed its polarity from the last reversed into today s normal state approximately 780 000 years ago while before and after this so called matuyama brunhes reversal the earth magnetic field was essentially an axial dipole the details of its transitional structure are still largely unknown here a bayesian inversion method is developed to reconstruct the spherical harmonic expansion of this transitional field from paleomagnetic data this is achieved by minimizing the total variational power at the core mantle boundary during the transition under paleomagnetic constraints the validity of the inversion technique is proved in two ways first by inverting synthetic data sets from a modeled reversal here it is possible to reliably reconstruct the gauss coefficients even from noisy records second by iteratively combining four geographically distributed high quality paleomagnetic records of the matuyama brunhes reversal into a single geometric reversal scenario without assuming an a priori common age model the obtained spatio temporal reversal scenario successfully predicts most independent matuyama brunhes transitional records therefore the obtained global reconstruction based on paleomagnetic data invites to compare the inferred transitional field structure with results from numerical geodynamo models regarding the morphology of the transitional field it is found that radial magnetic flux patches form at the equator and move polewards during the transition our model indicates an increase of non dipolar energy prior to the last reversal and a non dipolar dominance during the transition thus the character and information of surface geomagnetic field records is strongly site dependent the reconstruction also offers new answers to the question of existence of preferred longitudinal bands during the transition and to the problem of reversal duration different types of directional variations of the surface geomagnetic field continuous or abrupt are found during the transition two preferred longitudinal bands along the americas and east asia are not predicted for uniformly distributed sampling locations on the globe similar to geodynamo models with cmb heatflux derived from present day lower mantle heterogeneities a preference of transitional vgps for the pacific hemisphere is found the paleomagnetic duration of reversals shows not only a latitudinal but also a longitudinal variation even the paleomagnetically determined age of the reversal varies significantly between different sites on the globe the described bayesian inversion technique can easily be applied to other high quality full vector reversal records also its extension to inversion of secular variation and excursion data is straightforward

slip history and dynamic implications of the 1999 chi chi taiwan earthquake we investigate the rupture process of the 1999 chi chi taiwan earthquake using extensive near source observations including three component velocity waveforms at 36 strong motion stations and 119 gps measurements a three plane fault geometry derived from our previous inversion using only static data ji et al 2001 is applied the slip amplitude rake angle rupture initiation time and risetime function are inverted simultaneously with a recently developed finite fault inverse method that combines a wavelet transform approach with a simulated annealing algorithm ji et al 2002b the inversion results are validated by the forward prediction of an independent data set the teleseismic p and sh ground velocities with notable agreement the results show that the total seismic moment release of this earthquake is 2 7 10 20 n m and that most of the slip occurred in a triangular shaped asperity involving two fault segments which is consistent with our previous static inversion the rupture front propagates with an average rupture velocity of 2 0 km s 1 and the average slip duration risetime is 7 2 s several interesting observations related to the temporal evolution of the chi chi earthquake are also investigated including 1 the strong effect of the sinuous fault plane of the chelungpu fault on spatial and temporal variations in slip history 2 the intersection of fault 1 and fault 2 not being a strong impediment to the rupture propagation and 3 the observation that the peak slip velocity near the surface is in general higher than on the deeper portion of the fault plane as predicted by dynamic modeling

latest quaternary rock magnetic record of climatic and oceanic change tanner basin california borderland a high resolution record of sediment magnetic properties and stable oxygen isotopes has been determined from marine isotope stage mis 6 to the holocene 170 ka to present day for hemipelagic sediments site 1014 of ocean drilling program leg 167 from a subbasin west of the tanner basin off the coast of southern california the spacing between samples is 10 cm the sedimentation rate in the top 20 m of the sequence was determined to be 12 cm k y we have taken the intensity of anhysteretic remanent magnetization arm as a measure of the concentration of magnetic minerals the quantity of magnetic material increased during transitions from warm to cool periods this change is inferred to have resulted from increased shelf erosion during lower sea levels during interglacials and possibly interstadials a reduction occurred in the quantity of magnetic material in the sediment and magnetic susceptibility therefore dropped this decrease seems to have resulted from dilution of the magnetic fraction due to increased biogenic deposition during these warm intervals the ratio of arm to arm after 15 mt alternating field demagnetization arm arm 15 is used as a coercivity indicator the magnetic properties of the sediments from the subbasin west of the tanner basin are largely determined by two climatically controlled fluxes of magnetic material large and magnetically softer titanohematite grains were deposited during times of decreasing sea level when the concentration of magnetic particles was elevated small magnetite grains 2 m control the magnetic properties in sediments deposited during transgressions and times of high sea levels when detrital titanohematite content was low the arm arm 15 ratio leads the 18o curve by 0 3 0 8 m this offset between the rock magnetic coercivity parameter and intervals of climatic change represents evidence for authigenic growth of magnetite grains near the fe redox boundary the fe redox boundary appears to have been at 0 8 m depth during mis 3 and at 0 3 m during mis 5 authigenic growth of magnetite is associated with rapid climate events as recorded by planktonic foraminiferal assemblage shifts these relationships may have resulted from changes in oxygen concentrations in intermediate depth waters associated with the climate shifts therefore the ratio of arm arm 15 is suggested to be a new indicator proxy of past changes in oxygen levels of intermediate water