fault controlled soil co2 degassing and shallow magma bodies summit and lower east rift of kilauea volcano hawaii 1997 soil co2 flux measurements were carried out along traverses across mapped faults and eruptive fissures on the summit and the lower east rift zone of kilauea volcano anomalous levels of soil degassing were found for 44 of the tectonic structures and 47 of the eruptive fissures intercepted by the surveyed profiles this result contrasts with what was recently observed on mt etna where most of the surveyed faults were associated with anomalous soil degassing the difference is probably related to the differences in the state of activity at the time when soil gas measurements were made kilauea was erupting whereas mt etna was quiescent although in a pre eruptive stage unlike mt etna flank degassing on kilauea is restricted to the tectonic and volcanic structures directly connected to the magma reservoir feeding the ongoing east rift eruption or in areas of the lower east rift where other shallow likely independent reservoirs are postulated anomalous soil degassing was also found in areas without surface evidence of faults thus suggesting the possibility of previously unknown structures

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

field dependent postdepositional grain realignment in the pdrm process experimental evidence and implications the field dependence of postdepositional remanent magnetization pdrm has been investigated by two component pdrm acquisition experiments in deviating magnetic fields of different strengths the experiments have been performed on samples consisting of crushed natural magnetite grains in pseudo single domain psd states embedded 0 1 by weight in a slowly curing epoxy araldite the increasing viscosity with time of the epoxy is thought to mimic consolidation processes in sediments which gradually inhibits magnetic grain realignment resulting pdrm directions are partly intermediate between final and initial field directions which are unresolvable by alternating field demagnetization to 60 mt the results suggest that partial pdrm acquired in initial low fields are modified by subsequent exposure to stronger fields this may be due to alignment of grains not affected by the initial field or by remobilization of grains previously aligned in the weaker initial field the suggested field dependent pdrm smoothing may prevent improvement of paleomagnetic records by deconvolution procedures and may imply that longitudinally confined paleomagnetic records in sediments of transitional polar paths during reversals may partly be artifacts of the pdrm recording process

matuyama brunhes polarity transition near bishop california summary exposed pleistocene lake sediments at three localities separated by as much as six kilometres near bishop california contain the change from reverse to normal palaeomagnetic polarity that occurred about 0 73 million years ago the matuyama brunhes m b transition a decrease in the relative field strength is used to define the length of the transition the width of that decrease exceeds by about one fifth the width where the palaeomagnetic directions show the switch in polarity because some of the usually six subsamples in a measured horizon in the central part of the transition have normal polarity while others have intermediate or reverse polarity the behaviour of the palaeomagnetic field in that region cannot be determined accurately there is however a short reversal within the upper quarter of the transition at one locality that can be documented at least 30 cm of normal polarity underlie reverse polarity at three sites spread over a distance of 150m taken as a whole the palaeomagnetic data for the full transition are a warning that some rapid short reversals within the transition that are reported in the literature might be caused by inaccurate recording of the palaeomagnetic field the palaeomagnetic polarity 17m beneath the m b transition at one locality places a minimum age of about 1 0 myr on the glass mountain d ash bed that is found in east central and southern california

deep magma body beneath the summit and rift zones of kilauea volcano hawaii a magnitude 7 2 earthquake in 1975 caused the south flank of kilauea volcano hawaii to move seaward in response to slippage along a deep fault since then a large part of the volcano s edifice has been adjusting to this perturbation the summit of kilauea extended at a rate of 0 26 meter per year until 1983 the south flank uplifted more than 0 5 meter and the axes of both the volcano s rift zones extended and subsided the summit continues to subside these ground surface motions have been remarkably steady and much more widespread than those caused by either recurrent inflation and deflation of the summit magma chamber or the episodic propagation of dikes into the rift zones kilauea s magmatic system is therefore probably deeper and more extensive than previously thought the summit and both rift zones may be underlain by a thick near vertical dike like magma system at a depth of 3 to 9 kilometers

heat flux through sea ice in the western weddell sea convective and conductive transfer processes the heat flux through the snow and sea ice cover and at the ice ocean interface were calculated at five sites in the western weddell sea during autumn and early winter 1992 the ocean heat flux averaged 7 2 w m2 from late february to early june and average ice air heat flux in the second year floes depended on the depth of the snow cover and ranged from 9 to 17 0 8 w m2 in late february three of the five sites had an ice surface which was depressed below sea level resulting at two of the sites in a partially flooded snow cover and a slush layer at the snow ice interface as this slush layer froze to form snow ice the dense brine which was rejected flowed out through brine drainage channels and was replaced by lower salinity nutrient rich seawater from the ocean upper layer we estimate that about half of the second year ice in the region was covered with this slush layer early in the winter as the slush layer froze over a 2 to 3 week period the convection within the ice transported salt from the ice to the upper ocean and increased total heat flux through the overlying ice and snow cover on an area wide basis approximately 10 cm of snow ice growth occurred within second year pack ice primarily during a 2 to 3 week period in february and march this ice growth near the surface of the ice provides a salt flux to the upper ocean equivalent to 5 cm of ice growth despite the thick about 1 m ice cover in addition to the ice growth in the small area less than 5 open water regions

the multiple inverse method applied to meso scale faults in mid quaternary fore arc sediments near the triple trench junction off central japan the multiple inverse method for fault slip data is applied to meso scale faults observed in mid quaternary fore arc sediments near the triple trench junction oa the boso peninsula japan data from the otadai umegase and kokumoto formations were processed and three stresses were obtained as significant solutions vertical axial compression and triaxial stresses with the s3 axis in wnw ese and nne ssw directions the triaxial stresses were determined from the otadai and umegase data however the wnw ese tensile stress is not detected from the youngest kokumoto suggesting that the stress is older than the formation the area was subject to a wnw ese tensile stress 01 2 1 0 ma but tensile direction changed to a ne sw trend thereafter the succession was simultaneous with a tectonic event in the landward slope of the sagami trough suggesting that the subduction of the philippine sea plate aaected the stresses in the overriding plate the older tensile stress was probably a manifestation of the gravitational collapse of the hayama mineoka ridge which was growing parallel to the trough the inferred stress history is concordant with the variation of plate convergence at the trough 2000 elsevier science ltd all rights reserved

waters associated with an active basaltic volcano kilauea hawaii variation in solute sources 1973 1991 chemical and isotopic analyses of samples collected from a december 1962 m deep research borehole at the summit of kilauea volcano provide unique time series data for composition of waters in the uppermost part of its hydrothermal system these waters have a distinctive geochemical signature a very low proportion of chloride relative to other anions compared with other hawaiian waters thermal 30 c or nonthermal 30 c and with most thermal waters of the world isotope data demonstrate that the borehole waters are of essentially meteoric origin with minimal magmatic input the water chemistry exhibits marked temporal variations including pronounced short term days to weeks effects of rainfall dilution and longer term months to years decline of total solutes the 1973 1974 samples are na sulfate dominant but samples collected after july 1975 are mg ca bicarbonate dominant this compositional shift probably abrupt was associated with an increase in the partial pressure of co2 p co2 related to volcanic degassing of co2 accompanying a large eruption december 31 1974 and associated intense seismicity following the initial sharp increase the p co2 then decreased approaching pre eruption values in april 1976 beginning in mid 1975 solute concentrations of the borehole waters decreased substantially from 45 meq l to 25 meq l in only eight months by 1991 total solute concentrations were 17 meq l this decline in solutes cannot be attributed to rainfall dilution and is inferred to reflect the decreasing availability with time of the easily leachable salts of alkali metals and sulfate which originated in sublimates and fumarolic encrustations in fractures and cavities of rocks along the hydrologic flow paths the overall chemistry of the summit borehole waters is largely determined by hydrolysis reactions associated with normal weathering of host tholeiitic basalts on a geologic time scale despite short term perturbations in composition caused by rainfall dilution or volcanic activity

polarity transitions and excursions of the geomagnetic field the nature of geomagnetic field behavior during polarity transitions is one of the most hotly debated issues in modern geophysics dynamo action in the earth s fluid outer core is known to be responsible for generating the earth s main magnetic field however the mechanism for polarity transition is still unknown polarity transitions are a fundamentally important property of the geomagnetic field but because they have not taken place in historic time it is necessary to turn to the paleomagnetic record to understand the mechanism by which they occur

high resolution diatom record of paleoceanographic variations across the early middle pleistocene boundary in the chiba section central japan abstract we conducted diatom analyses of a core from the pleistocene marine sequence drilled at the chiba section central japan to reveal detailed paleoceanographic variations across the early middle pleistocene boundary diatom valves are most abundant at 4 5 m below the matuyama brunhes magnetic polarity boundary followed by the second highest abundance at 18 6 m above the boundary which is consistent with the sea level changes inferred from planktonic 18 o data the highest abundance is correlated with the marine isotope stage mis 19 3 sea level highstand and the second highest with the mis 19 1 highstand the abundance of the extinct species actinocyclus ingens is quite low around these sea level highstands whereas it is greatest at the core depth with the minimum diatom total valves which is correlated to the mis 19 2 lowstand cold water diatom species that are characteristic of the oyashio current are dominant but coexist with warm water species of the kuroshio current below a core depth just postdating mis 19 2 above this depth warm diatom species of the kuroshio current become dominant the change in dominant diatoms suggests that the kuroshio front shifted northward just after mis 19 2 and remained in the northern area until at least early mis 18 this shift caused a rapid increase in sea surface temperature by 4 5 c within a sediment thickness of 1 3 m representing 300 years

the sea ice thickness distribution in the northwestern weddell sea we present new data on distribution of snow and sea ice thicknesses in the northwestern weddell sea the data were obtained through direct measurements along 19 profiles each approximately 100 m long on 17 different floes located between 54 46 w and 59 64 s the overall probability density functions pdfs for ice thicknesses reflect the complex mixture of first second and multi year ice to be expected in the outflowing branch of the weddell gyre further differentiation of the data reveals four distinct thickness classes which reflect differences in the formation and subsequent histories of the ice encountered these classes i iv represent strongly deformed first year ice less deformed first and second year ice and deformed second or multi year ice respectively each of the classes is characterized by a specific set of quantities related to ice texture and surface snow characteristics and by distinct pdfs for snow and ice thicknesses in addition geometric surface and bottom roughness characteristics differ significantly for each of the floe classes

palaeomagnetic records of the brunhes matuyama polarity transition from odp leg 124 celebes and sulu seas summary palaeomagnetic records of the brunhes matuyama geomagnetic polarity transition were obtained from deep sea sediments of odp leg 124 in the celebes and sulu seas advanced piston core apc samples with high magnetization intensities 2 200 ma m1 and high sedimentation rates 8 4 10 cm kyr1 were recovered from this cruise rock magnetic measurements revealed the carrier of the remanence to be nearly pure magnetite in the pseudo single domain range pass through measurements at intervals of 5 mm on apc cores across the brunhes matuyama polarity transition for holes 767b 769a and 769b were deconvolved with the magnetometer sensor response using the abic minimizing method discrete samples were also taken from the polarity transition zones and subjected to either thermal or alternating field stepwise demagnetization the results were generally consistent with the pass through data after the deconvolution results from the three holes are in good agreement particularly those from the two holes 100 m apart at site 769 the transitional vgp paths from these two holes show two small loops near new zealand before the equator is crossed the vgps continue to swing eastwards to the north atlantic and then move to the northeastern margin of the pacific ocean the positions of the vgp loops obtained from site 769 are dierent from the vgp clusters obtained from both the known volcanic records and the sediment records at the boso peninsula and the north atlantic sites with high sedimentation rates such a dierence may imply the predominance of the non dipole field during the transition relatively stable mid high northern latitude vgps are recognized on all three cores just after the reversal lasting about 4000 years the plot of relative intensity versus vgp latitude for the sediment records from the north atlantic dsdp hole 609b shows a remarkable similarity with our records similar patterns were also obtained for the plot of the palaeointensity versus vgp latitude for the la palma volcanic lava records these results may suggest the existence of a metastable state of the geodynamo producing a zonal component just after the brunhes matuyama boundary which may have played a role in the change of the field intensity

low pressure water assisted anatexis of basic dykes in a contact metamorphic aureole fuerteventura canary islands oxygen isotope evidence for a meteoric fluid origin migmatites produced by low pressure anatexis of basic dykes are found in a contact metamorphic aureole around a pyroxenite gabbro intrusion px2 on fuerteventura dykes outside and inside the aureole record interaction with meteoric water with low or negative 18o whole rock values 0 2 to 3 4 decreasing towards the contact recrystallised plagioclase diopside biotite and oxides from within the aureole show a similar evolution with lowest 18o values 2 8 4 2 4 4 and 7 6 respectively in the migmatite zone close to the intrusion relict clinopyroxene phenocrysts preserved in all dykes retain typically magmatic 18o values up to the anatectic zone where the values are lower and more heterogeneous low 18o values decreasing towards the intrusion can be ascribed to the advection of meteoric water during magma emplacement with increasing fluid rock ratios higher dyke intensities towards the intrusion acting as fluid pathways and higher temperatures promoting increasing exchange during recrystallisation

reactivation of map scale faults in response to changes in crustal stress examples from boso peninsula japan abstract we have investigated the reactivation of quaternary faults near the trench trench trench type triple junction off boso peninsula central japan on the pacific side of the boso peninsula psbp region map scale nne ssw trending faults developed during 1 2 0 7 ma although these faults are inactive at present we observed multiple and overlapped slickenlines on parts of map scale fault planes in the psbp region these slickenlines can be classified into two different directions which we consider to be the reactivation of the map scale faults in response to a change in crustal stress after 1 ma based on angular misfits between the predicted and observed slip directions we successfully compared the older and younger slip directions to dip slip normal faulting and oblique sinistral normal faulting respectively the reactivation of the map scale faults in response to a relatively short 105 yrs stress change has been verified based on the direct evidences on the map scale fault planes

large scale sea ice drift and deformation comparison between models and observations in the western weddell sea during 1992 statistical comparisons between numerical sea ice models and an observed large scale strain array in the western weddell sea during 1992 are used to evaluate the performance of three of the more generally utilized sea ice rheology formulations results show that sea ice velocity is reproduced with relatively high accuracy 90 coherence 80 normalized cross correlation in models having high quality atmospheric forcing fields e g the european centre for medium range weather forecasts on the other hand temporal and spatial variability of the velocity field as exemplified by progressive vector plots and ice deformation respectively are reproduced less accurately coherence and normalized cross correlation 50 in terms of model sensitivity this means that deformation and temporal variability are more discriminating in terms of elucidating specifics about the constitutive relation and mechanical properties of sea ice on a large scale for example inclusion of both compressive and shear stresses is important in attaining a proper probability distribution of deformation relative to observations additional analysis shows that adjustments to specific model parameters improve the model results for either drift or select deformation components but no best solution could be found given the models examined here results suggest that inclusion of more physically based processes such as subdaily tidal and inertial oscillations reconsideration of the boundary layer formulation and consideration of anisotropy may be necessary to include in next generation sea ice models especially those that are intended for coupling with high resolution eddy resolving ocean models

a database for the matuyama brunhes magnetic reversal abstract we present a paleomagnetic database designated mbd97 comprising sedimentary and lava data for the matuyama brunhes reversal transition field using criteria regarding suitability of the rock samples measurement methods and temporal resolution of the data we consider 62 different studies of the matuyama brunhes reversal of these only 11 satisfy our selection criteria four lava sites one loess site three shallow marine sedimentary sites and three deep ocean sedimentary sites we discuss our reasons for accepting or disguarding each candidate data set we investigate the distribution of transitional virtual geomagnetic poles vgps they fall along american and asian australian longitudes the spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar the directional transition of the matuyama brunhes reversal took 2300 5000 years during which time the surface intensity fell to 10 20 of its pre and post transitional value we do not find any systematic bias due to site distribution or inclination shallowing although the errors in the data do vary over the duration of the reversal mbd97 is a small database but it has a fairly good geographic distribution and is suitable for studies of the matuyama brunhes transition nonetheless it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis the data in mbd97 are listed

multiple rapid polarity swings during the matuyama brunhes transition from two high resolution loess paleosol records 1 two high resolution magnetic records of the matuyama brunhes m b transition have been obtained from xifeng and baoji loess paleosol sequences in the chinese loess plateau the results of stepwise thermal and alternating field af demagnetizations confirm that both treatments are effective in isolating characteristic remanent magnetization chrm from specimens of loess paleosol sediments which suffer from weak pedogenesis and that af demagnetization clearly fails to isolate chrm from specimens of paleosol sediments which have undergone strong pedogenesis on the basis of virtual geomagnetic pole latitudes crossing the virtual geomagnetic equator the m b transition involves at least 15 rapid polarity swings rock magnetic experiments and anisotropy of low field magnetic susceptibility show that high frequency polarity swings during the m b transition cannot be attributed to rock magnetic and or sedimentary causes extrapolating a constant accumulation rate between the jaramillo matuyama and m b boundary ages the m b transition interval of both the xifeng and baoji sections is estimated to have a duration of more than 10 kyr durations of each short polarity interval are estimated to range from about 0 3 to 5 6 kyr our new results combined with previous paleomagnetic observations from lavas and marine sediments suggest that multiple rapid polarity swings occurred during the m b transition

an integrated chemical and stable isotope model of the origin of midocean ridge hot spring systems chemical and isotopic changes accompanying seawater basalt interaction in axial midocean ridge hydrothermal systems are modeled with the aid of chemical equilibria and mass transfer computer programs incorporating provision for addition and subtraction of a wide range of reactant and product minerals as well as cation and oxygen and hydrogen isotopic exchange equilibria the models involve stepwise introduction of fresh basalt into progressively modified seawater at discrete temperature intervals from 100 to 350 c with an overall water rock ratio of about 0 5 being constrained by an assumed 18 o h 2o at 350 c of 2 0 per mil h craig personal communication 1984 this is a realistic model because 1 the grade of hydrothermal metamorphism increases sharply downward in the oceanic crust 2 the water rock ratio is high 50 at low temperatures and low 0 5 at high temperatures and 3 it allows for back reaction of earlier formed minerals during the course of reaction progress the results closely match the major element chemistry von damm et al 1985 and isotopic compositions craig et al 1980 of the hydrothermal solutions presently emanating from vents at 21 n on the east pacific rise the calculated solution chemistry for example correctly predicts complete loss of mg and so 4 and substantial increases in si and fe however discrepancies exist in the predicted ph 5 5 versus 3 5 measured and state of saturation of the solution with respect to greenschist facies minerals the calculated d h 2o is 2 6 per mil in excellent agreement with analytical determinations the calculated chemical mineralogic and isotopic changes in the rocks are also in good accord with observations on altered basalts dredged from midocean ridges humphris and thompson 1978 stakes and o neil 1982 as well as with data from ophiolites gregory and taylor 1981 predicted alteration products include anhydrite and clay minerals at low temperatures and a typical albite epidote chlorite tremolite greenschist assemblage at 350 c the models demand that the major portion of the water rock interaction occur at temperatures of 300 350 c interaction at temperatures below approximately 250 c results in negative 18 o h 2o shifts contrary to the observed positive 18 o values of the fluids exiting at midocean ridge vents hydrogen isotope fractionation curves by suzuoki and epstein 1976 lambert and epstein 1980 and liu and epstein 1984 among others are compatible with the model and require d h 2o to increase at all temperatures as a result of seawater basalt interaction

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

origin and history of hydrothermal fluids of the reykjanes and krafla geothermal fields iceland the reykjanes and krafla geothermal fields are both examples of active high temperature systems and show similar assemblages of alteration minerals but the fluid at reykjanes is dominantly sea water whereas that at krafla is meteoric oxygen isotope analyses of surface rock and of drill chip samples from different depths are presented together with results for the krafla fluid which is close to local precipitation 18o 11 9 d 86 8

the matuyama brunhes transition recorded from lake tecopa sediments california abstract the paleomagnetic record of the matuyama brunhes reversal in the sediment of ancient lake tecopa provided the first evidence that the geomagnetic field during transition is not dipolar we resampled the section to increase the time resolution of the record our new data show that the characteristic direction could not be completely isolated by af demagnetization the transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non dipolar remains the vgp paths lying close to the site longitude is consistent with an axisymmetric transition field

a marker tephra bed close to the lower middle pleistocene boundary distribution of the ontake byakubi tephra bed in central japan abstract tephrochronology is an exceptionally important tool in the precise regional correlation of early and middle pleistocene sedimentary strata in japan the present study reveals that the yukawa tephra 5 yut5 derived from the older ontake volcano the nezumigawa nzg and mitamitajima mtj tephras of the ina bazin and the byakubi e tephra byk e of the boso peninsula are the same tephra on the basis of their lithofacies bulk grain composition mafic mineral composition major element composition of hornblende and stratigraphic relationships with the dated tephras we propose to call the series of tephras correlated with byk e the ontake byakubi tephra bed on byk tephra following the naming convention in which the tephra name consists of the names of the source volcano and the type location the matuyama brunhes chronozone boundary occurs just above byk e in the type section of the kokumoto formation in the kazusa group which is a candidate global boundary stratotype section and point gssp for the lower boundary of the middle pleistocene subseries therefore on byk tephra becomes a critically important marker tephra bed for the early middle pleistocene boundary in central japan the present study indicates that the major element composition of hornblende can be a useful tool for identification and correlation of strongly weathered tephra layers such as nzg and mtj in which all the volcanic glass shards have been altered

magnetostratigraphy of lower pleistocene banyoles palaeolake carbonate sediments from catalonia ne spain evidence for relocation of the cobb mountain sub chron the banyoles palaeolake carbonate sediments cored at bobila ordis hole bo iv 52 15 m long carry a weak natural remanent magnetization mean nrm intensity 0 59 0 54 mam 1 of reversed magnetic polarity except for a shea zone of normal polarity close to the top 16 7 14 05 m the nrm is probably carried by magnetite as well as by greigite fe3s4 in some zones a chronostratigraphic constraint based on the presence of rodent teeth belonging to the lower pleistocene coinciding climatic events duration of the deposit estimated by cyclostratigraphy as well as sedimentation rates suggests that the short normal polarity zone can be correlated to the cobb mountain sub chron 1 19 ma this event is recorded at the oxygen isotope stage 36 35 transition in marine sediment cores in core bo iv the normal polarity zone probably belongs to the end of an interglacial period as indicated by a cool and humid climate inferred from conifer pollen grains there is thus an apparent time difference between the oxygen isotope and pollen signal which is tentatively attributed to a significantly larger post depositional lag in the acquisition of the remanent magnetization in marine sediments relative to the lacustrine sediment in question the cobb mountain sub chron might therefore have actually occurred at the end of the interglacial isotope stage 35

paleoclimatic and paleoceanographic records through marine isotope stage 19 at the chiba composite section central japan a key reference for the early middle pleistocene subseries boundary abstract marine isotope stage mis 19 is an important analogue for the present interglacial because of its similar orbital configuration especially the phasing of the obliquity maximum to precession minimum however sedimentary records suitable for capturing both terrestrial and marine environmental changes are limited and thus the climatic forcing mechanisms for mis 19 are still largely unknown the chiba composite section east central japanese archipelago is a continuous and expanded marine sedimentary succession well suited to capture terrestrial and marine environmental changes through mis 19 in this study a detailed oxygen isotope chronology is established from late mis 20 to early mis 18 supported by a u pb zircon age and the presence of the matuyama brunhes boundary new pollen marine microfossil and planktonic foraminiferal 18o and mg ca paleotemperature records reveal the complex interplay of climatic influences our pollen data suggest that the duration of full interglacial conditions during mis 19 extends from 785 0 to 775 1 ka 9 9 kyr which offers an important natural baseline in predicting the duration of the present interglacial a younger dryas type cooling event is present during termination ix suggesting that such events are linked to this orbital configuration millennial to multi millennial scale variations in our 18o and mg ca records imply that the subarctic front fluctuated in the northwestern pacific ocean during late mis 19 probably in response to east asian winter monsoon variability the climatic setting at this time appears to be related to less severe summer insolation minima at 65 n and or high winter insolation at 50 n our records do not support a recently hypothesized direct coupling between variations in the geomagnetic field intensity and global regional climate change our highly resolved paleoclimatic and paleoceanographic records coupled with a well defined matuyama brunhes boundary 772 9 ka duration 1 9 kyr establish the chiba composite section as an exceptional climatic and chronological reference section for the early middle pleistocene boundary

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

a magnetization process of sediments laboratory experiments on post depositional remanent magnetization summary palaeomagnetic studies require a theory of magnetization mechanism of sediments and a method of estimating magnetic field intensity from their remanences this paper establishes a physical basis for the generation of the remanence in deep sea and lake sediments experimentally redeposition experiments have been carried out under centrifugal force in weak magnetic fields the centrifuging method produces post depositional remanent magnetization post drm in the compacted sediment and its remanence and susceptibility are compatible with those of natural sediments and reconstituted materials of other redepositional experiments three properties of the post drm have been deduced from the experiments 1 the efficiency of acquisition of post drm decreases with increase in density during the compaction process 2 the total post drm is equal to the sum of the partial post drm addition law and 3 time is not a substantial factor for alignment of the magnetic particles these results lead to the conclusions that the magnetic particles do not rotate steadily but in a series of steps and that the density change is the crucial factor giving rise to the post drm a mathematical formula representing the remanence record in sediments is proposed on the basis of the experimental results and the model the principal equation is expressed as an integral of the product of three parameters over time when sediments have been compacted the field intensity variation characteristic function of the sediment and the time derivative of the density change

18o concentrations in sea ice of the weddell sea antarctica we present data on ice texture salinity and delta o 18 obtained from identical sections of ice cores during the winter weddell sea project 1986 on rv polarstern from july through august 1986 in the longitude range between 5 degrees w and 7 degrees e we find no uniquely definable relationship between delta o 18 values and ice texture in a particular section however most of the snow ice as well as some sections of frazil ice ar found to have negative delta o 18 concentrations this is due to varying degrees of admixtures of meteoric ice snow and sea water during formation of snow ice in contrast to common assumptions our results seem to indicate that a snow cover contributes positively to sea ice growth rather than slowing down the overall growth rate based on a simple model we have estimated the contributions of meteoric ice mean of 3 3 and the combined meteoric ice sea water fraction a minimum of 7 6 to the total ice thickness for the majority of the sampled floes although this is only a moderate contribution to the overall mass balance in the absence of congelation growth it nevertheless enhances ice growth in general this hypothesis is dependently supported by our snow and ice thickness data wadhams and others 1987 which demonstrate that the depression of the snow ice interface below the water line i e a negative freeboard and the formation of snow ice is a common occurrence in the weddell sea therefore we hypothesize that the major part of the observed apparent increase in ice thickness between our inbound and outbound tracks of wwsp 86 may not be derived from regular thermodynamically driven congelation growth but rather from the snow ice component in floes of the weddell sea

seasonality of southern ocean sea ice the sea ice cover of the southern ocean undergoes a rapid decrease from mid november to mid january the atmosphere to ocean heat flux is insufficient to account for this melting even in the presence of the high percentage of open water characteristic of the southern ocean sea ice it is estimated that sea air heat exchange in the 60 to 70 s zone can account for roughly 50 of the required spring heating the remainder must be supplied by the relatively warm deep water residing below the southern ocean pycnocline deep to surface water heat flux is accomplished by upwelling of the pycnocline due to the regional ekman divergence of the surface layer and by cross pycnocline mixing it is estimated that the required vertical mixing coefficient for the pycnocline is 1 5 cm2 s this value is considered as realistic in view of the relatively weak pycnocline which is a consequence of low levels of fresh water input to the surface layer it is suggested that the magnitude of fresh water input is a major factor in determining the degree of seasonality of southern ocean sea ice

evidence for the kamikatsura and santa rosa excursions recorded in eolian deposits from the southern chinese loess plateau 1 a detailed paleomagnetic investigation of a loess paleosol sequence in baoji southern chinese loess plateau reveals two distinct geomagnetic excursions accompanied by inferred relative paleointensity minima in the late matuyama chron rock magnetic experiments demonstrate that the sediments of the excursion intervals in loess unit l9 have the same magnetic properties as those from surrounding normal and reversed polarity intervals assuming constant sedimentation rate between the jaramillo matuyama and matuyama brunhes magnetic polarity boundaries the midpoint ages and durations for these two excursions are estimated at 0 89 ma and 16 kyr and 0 92 ma and 7 kyr respectively in addition these two events are separated by an interval with fully reversed polarity that had a duration of 16 kyr these estimates are supported by paleoclimatologically inferred age constraints the two excursions are probably correlated with the kamikatsura and santa rosa excursions respectively

remagnetization in lava flows recording pretransitional directions basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of la palma are characterized by overlapping components with very high unblocking temperatures a medium high temperature mht component with normal polarity is resistant up to 500 550 c while in most cases the high temperature ht primary reverse component cannot be isolated below 520 c the same situation with opposite polarities is observed also in a lava flow associated with the onset of the upper reunion reversal in the gamarri sequence in ethiopia the presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling the direction of mht is either close or similar to the direction of the ht component of the overlying flows which indicates that remagnetization occurred shortly after emplacement rock magnetic studies microscopic observations microprobe analyses and remagnetization experiments have been carried out to investigate the origin of mht the rock magnetic parameters do not exhibit specific anomalies the unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling the most plausible scenario is that baking by the overlying flows was accompanied by low temperature oxidation of titanomagnetite to cation deficient titanomagnetite this description also includes the presence of cracks fractures and other heterogeneities of the lava which induce variability between parallel profiles similarities with characteristics observed at steens mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution

high resolution stratigraphy across the early middle pleistocene boundary from a core of the kokumoto formation at tabuchi chiba prefecture japan abstract we present a detailed stratigraphy of the lithology sedimentary facies magnetic susceptibility and chemical composition of a 54 m thick core sequence drilled adjacent to the chiba section a candidate for the global boundary stratotype section and point of the early middle pleistocene boundary siltstones rich in trace fossils are dominant throughout the core except for the lowermost sand beds the byakubi tephra is divided into three units of which the uppermost 39 54 39 49 m core depth is identified as a primary bed showing evidence of fallout the preliminary matuyama brunhes magnetic polarity boundary lies 1 m above the top of the fallout unit maximum and minimum susceptibility axes for anisotropy of magnetic susceptibility have shallow 73 inclinations respectively throughout the core the results suggest that sedimentary structures formed in calm environments remain undisturbed in the sediment clustered maximum axis directions indicate the influence of a persistent nw se contour current the homogeneous depositional environment for the siltstone interval is supported by relatively smooth variations in magnetic susceptibility and chemical composition stratigraphic correlation with published oxygen isotope data indicates that the depositional age of the core may extend from the end of marine oxygen isotope stage mis 20 to just after mid mis 19 from which an average sedimentation rate of 2 2 5 m ky is estimated these results suggest that the chiba section has a high potential to provide an excellent stratotype for the early middle pleistocene boundary

temperature variations in lake ice in central alaska usa abstract in winter 2002 03 and 2003 04 thermistors were installed in the ice on two shallow pondsin central alaska in order to obtain data on ice temperatures and their response to increasing anddecreasing air temperatures and flooding and snow ice formation snow depth and density and icethickness were also measured in order to understand how they affected and were affected by icetemperature variability the lowest ice temperature 15 5 8 c and steepest temperature gradient 39 8 8 cm 1 occurred during a 9week period in autumn when there was no snow on the ice withsnow on the ice temperature gradients were more typically in the range 20 to 5 8 cm 1 average icetemperatures were lower during the warmer first winter and higher during the cooler second winterbecause of differences in the depth and duration of the snow cover isothermal ice near the freezingpoint resulted from flooding and snow ice formation and brief episodes of warm weather with freezingrain under these circumstances congelation ice growth at the bottom of the ice cover was interrupted even reversed it is suggested that the patterns in temperatures brought about by the snow ice formationand rain events may become more prevalent due to the increase in frequency of these events in centralalaska if temperature and precipitation change as predicted by arctic climate models

the formation and morphology of ice stalactites observed under deforming lead ice during the leadex main field experiment held in april 1992 in the alaskan beaufort sea a number of large ice stalactites were observed growing under young lead ice formation of the stalactites was associated with rafting of the thin highly saline ice the rafting caused the brine to drain rapidly from the ice at a temperature well below the freezing point of the surrounding water which in turn caused ice to form in a hollow cylinder around the brine plume within a 15 h period after the rafting event the stalactites which were located approximately 10 m apart in a line along the up wind edge of a 150 m wide lead had grown to a length of 2 m a detailed structural analysis of the upper part of one of these stalactites revealed that the interior channel down which the brine flowed was bounded by a zone of frazil ice that developed into a shell of columnar ice the growth of the columnar ice was directed radially outward and the c axes of these crystals were oriented perpendicular to their growth direction development of the stalactites illustrates the impact ice deformation can have on the process of brine rejection in freezing leads and potentially on the thermohaline structure of the upper ocean in ther immediate vicinity of the lead

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

hydrogeology of the hawaii scientific drilling project borehole kp 1 2 groundwater geochemistry and regional flow patterns a series of downhole and surface water samples were taken from the i km deep kp 1 borehole located on the eastern flank of the island of hawaii early samples from depths of more than 700 m showed salinities nearly equivalent to seawater but having anomalous cation concentrations that are attributed to ion exchange between formation fluids and residual drilling mud clays later deep samples found only minor variations from seawater cation chemistry that are consistent with low temperature weathering of basalts delta 18 o values are equivalent to seawater values and are consistent with this interpretation carbon 14 activities of dissolved inorganic carbonate indicate a water age ranging from 5890 to 7170 years b p and fluid transport rates of 1 8 to 2 2 m yr fluid samples from perforations at 310 m in the borehole demonstrate that a freshwater aquifer is present at the mauna kea mauna loa interface borehole resistivity logs indicate that it is similar to 200 m thick although it ha s not yet been possible to obtain samples of the freshwater zone without contamination from the deep saline fluids the chloride concentrations of the low salinity zone are estimated using a mixing enthalpy calculation to be less than 100 mg l light stable isotope data indicatemore that the fresh water at 320 m is derived from recharge entering the island at an average elevation of 2000 m inferred c 14 activities of the dissolved bicarbonate in the freshwater zone indicate an average calibrated age of 2200 years b p and an average fluid velocity of at least 14 m yr a regional water flow model is proposed that suggests that the fresh water found at the 320 m depth is derived from rainfall recharge from the middle elevations of mauna kea volcano this rainfall is channeled beneath the mauna loa lavas by the thick soil layer separating the two volcanoes a second shallow fresh to brackish water zone derived from mauna loa recharge is also inferred to exist below the carbonate formation that underlies the shallow basal lens the results of our preliminary study of the groundwater system below the kp 1 drill site demonstrate that intervolcano and interflow aquicludes can have a substantial impact on water circulation and discharge from young island volcanoes less

high resolution record of geomagnetic secular variation from late pleistocene lake lisan sediments paleo dead sea we measured geomagnetic secular variation in lake lisan sediments paleo dead sea more than 1500 oriented samples were collected from a 27 3 m section of alternating aragonite and detritus laminae in the dead sea basin ranging in age from 67 to 32 ka the natural remanent magnetization nrm is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic the nrm is very stable and was acquired several hundred years after deposition the mean direction of 878 horizons is dd 005o id 45o 95d 1o d 22 we observed three modes of directional geomagnetic variation as a function of and by inference time very rapid inter sample changes slow variation in mean direction and inclination shallowing of about 1o m the overall rate of change in direction is 0 57 0 57o year not significantly different from zero for about 83 of the record the rate of change is less than 1o year and comparable to historical values high rates of change are observed more frequently in the lisan than in historical records and peak rates are up to ten times faster a smoothed curve resulting in a maximum rate of change of 0 66o year and a mean 0 10 0 10o year may be a more realistic representation of the field behavior no reverse nrms were observed but geomagnetic field excursions may be present where the vgps deviate by more than 40o from the geographic north at about 52 and 41 ka the latter may represent the laschamp event 1998 elsevier science b v all rights reserved

high resolution record of the matuyama brunhes transition constrains the age of javanese homo erectus in the sangiran dome indonesia a detailed paleomagnetic study conducted in the sangiran area java has provided a reliable age constraint on hominid fossil bearing formations a reverse to normal polarity transition marks a 7 m thick section across the upper tuff in the bapang formation the transition has three short reversal episodes and is overlain by a thick normal polarity magnetozone that was fission track dated to the brunhes chron this pattern closely resembles another high resolution matuyama brunhes mb transition record in an osaka bay marine core in the sangiran sediments four successive transitional polarity fields lie just below the presumed main mb boundary their virtual geomagnetic poles cluster in the western south pacific partly overlapping the transitional virtual geomagnetic poles from hawaiian and canary islands lavas which have a mean 40ar 39ar age of 776 2 ka thus the polarity transition is unambiguously the mb boundary a revised correlation of tuff layers in the bapang formation reveals that the hominid last occurrence and the tektite level in the sangiran area are nearly coincident just below the upper middle tuff which underlies the mb transition the stratigraphic relationship of the tektite level to the mb transition in the sangiran area is consistent with deep sea core data that show that the meteorite impact preceded the mb reversal by about 12 ka the mb boundary currently defines the uppermost horizon yielding homo erectus fossils in the sangiran area

a coupled sea ice mixed layer pycnocline model for the weddell sea a dynamic thermodynamic sea ice model is coupled to a one dimensional model of the oceanic mixed layer and pycnocline and is applied to the weddell sea this model prognostically determines the vertical oceanic heat flux from the mixed layer dynamics in contrast to earlier sea ice modeling where the oceanic heat flux was prescribed in addition to the standard simulation polynya and paleoclimate experiments were performed to investigate the effects of sea ice dynamics furthermore the mixed layer pycnocline model is compared to the original kraus turner approach

a link between geomagnetic reversals and events and glaciations abstract the apparent duration of geomagnetic polarity events in arctic ocean sediments is much longer than in sediments from lower latitudes in fact while the remanence of brunhes age sediment cores from the yermak plateau at 82 n is fully reversed for 30 of their lengths 1 the events often evade detection in many other continuously deposited sediments for example the laschamp event is absent in an otherwise high resolution record of secular variation from lac du bouchet 2 which is located near the laschamp volcanics where the event was first detected very short event durations of a few hundred years at the most have been suggested before 2 3 because sedimentation rates in the arctic ocean were increased during glaciations the exaggerated proportion of reverse polarities in sediments from high latitudes suggests a link between glaciation and field reversals this suggestion is supported by magnetostratigraphic results obtained from thick loess paleosol sequences in china 4 these demonstrate that all polarity boundaries separating chrons and subchrons since the gauss matuyama field reversal have been recorded in loess and thus during periods of cold climate although conflicting evidence exists for some boundaries furthermore the ages of 22 events and chron boundaries have been compared with the oxygen isotope record 5 thought to represent global ice volume all events and reversals younger than 2 6 ma may have occurred during periods of global cooling or during cold stages however some ages are still too poorly dated for a definite correlation climatic signals also exist in the two longest relative paleointensity records 6 7 but these are suspected to be caused by climatically driven variations in the rock magnetic parameters a mechanism for field reversals may be the acceleration of the earth s rotation caused by lowering of the sea level during glaciations the short duration of events also implies that the geomagnetic field can reverse an order of magnitude faster than commonly assumed

sedimentary processes and depositional environments of a continuous marine succession across the lower middle pleistocene boundary kokumoto formation kazusa group central japan abstract the chiba composite section of the kokumoto formation in central japan is a remarkable thick marine succession and is a candidate for formal recognition as the global boundary stratotype section and point gssp for the base of the middle pleistocene stage although there have been numerous studies of the kokumoto formation the sedimentology of the formation particularly in the chiba composite section has been unclear we use field observations and laboratory analyses to elucidate the sedimentary processes and depositional environments of the formation the chiba composite section includes the matuyama brunhes paleomagnetic reversal which is the primary guide for the lower middle pleistocene boundary along with multiple tephra deposits that provide isochronous correlational tie points one bed byk e also known as ontake byakubi tephra immediately underlies the paleomagnetic reversal and hence provides a useful widespread marker for the boundary the predominant silty beds of the chiba composite section are intensely bioturbated and lack evidence of episodic deposition such as slumps or muddy turbidites however minor sandy beds intercalated within the silty section particularly in its lower part are either 1 fining upward sequences with faint planar or ripple laminations and an erosional base or 2 sequences containing abundant mud clasts in a poorly sorted sand matrix with a sharp base and an irregular top surface the silty beds are interpreted to be hemipelagite formed by deposition of fine grained suspended material under stable and calm bottom water conditions the sandy deposits are interpreted to be sediment gravity flow deposits i e turbidites and debrites these sedimentary processes and the trace fossil assemblage indicate that the chiba composite section was formed in a continental slope environment the depositional environments of the chiba composite section together with tephrostratigraphy make it suitable for high resolution studies on the paleoenvironment paleoclimate and paleoceanography of the northwest pacific ocean across the lower middle pleistocene boundary it is therefore a suitable candidate to become the gssp for the middle pleistocene

an oxygen isotopic profile through the upper kilometer of the oceanic crust dsdp hole 504b dsdp hole 504b is the deepest basement hole in the oceanic crust penetrating through a 571 5 m pillow section a 209 m lithologic transition zone and 295 m into a sheeted dike complex an oxygen isotopic profile through the upper crust at site 504 is similar to that in many ophiolite complexes where the extrusive section is enriched in18o relative to unaltered basalts and the dike section is variably depleted and enriched basalts in the pillow section at site 504 have 18o values generally ranging from 6 1 to 8 5 smow mean 7 0 although minor zeolite rich samples range up to 12 7 rocks depleted in18o appear abruptly at 624 m sub basement in the lithologic transition from 100 pillows to 100 dikes coinciding with the appearance of greenschist facies minerals in the rocks whole rock values range to as low as 3 6 but the mean values for the lithologic transition zone and dike section are 5 8 and 5 4 respectively oxygen and carbon isotopic data for secondary vein minerals combined with the whole rock data provide evidence for the former presence of two distinct circulation systems separated by a relatively sharp boundary at the top of the lithologic transition zone the pillow section reacted with seawater at low temperatures near 0 c up to a maximum of around 150 c and relatively high water rock mass ratios 10 100 water rock ratios were greater and conditions were more oxidizing during submarine weathering of the uppermost 320 m than deeper in the pillow section the transition zone and dikes were altered at much higher temperatures up to about 350 c and generally low water rock mass ratios 1 and hydrothermal fluids probably contained mantle derived co2 mixing of axial hydrothermal fluids upwelling through the dike section with cooler seawater circulating in the overlying pillow section resulted in a steep temperature gradient 2 5 c m across a 70 m interval at the top of the lithologic transition zone progressive reaction during axial hydrothermal metamorphism and later off axis alteration led to the formation of albite and ca zeolite rich alteration halos around fractures this enhanced the effects of cooling and18o enrichment of fluids resulting in local increases in 18o of rocks which had been previously depleted in18o during prior axial metamorphism

thin and thinner sea ice mass balance measurements during sheba 1 as part of a large interdisciplinary study of the surface heat budget of the arctic ocean sheba we installed more than 135 ice thickness gauges to determine the sea ice mass balance while installing these gauges during the fall of 1997 we found that much of the multiyear ice cover was only 1 m thick considerably thinner than expected over the course of the yearlong field experiment we monitored the mass balance for a wide variety of ice types including first year ice ponded ice unponded ice multiyear ice hummocks new ridges and old ridges initial ice thicknesses for these sites ranged from 0 3 to 8 m and snow depths varied from a few centimeters to more than a meter however for all of their differences and variety these thickness gauges sites shared a common trait at every site there was a net thinning of the ice during the sheba year the thin ice found in october 1997 was even thinner in october 1998 the annual cycle of ice thickness was also similar at all sites there was a steady increase in thickness through the winter that gradually tapered off in the spring this was followed by a steep drop off in thickness during summer melt and another tapering in late summer and early fall as freeze up began maximum surface melting was in july while bottom ablation peaked in august combining results from the sites we found an average winter growth of 0 51 m and a summer melt of 1 26 m which consisted of 0 64 m of surface melt and 0 62 m of bottom melt there was a weak trend for thicker ice to have less winter growth and greater net loss for the year however ice growth was also impacted by the snow depth considerable variability was observed between sites in both accretion and ablation the total accretion during the 9 month growth season ranged from zero for thick ridged ice to more than a meter for young ice ponds tended to have a large amount of surface melting while ridges had considerable bottom ablation

climate change loess and palaeosols proxy measures and resolution in north china the loess plateau of north china is a key region for the study of records of quaternary climatic change various techniques to abstract climatically sensitive data were first applied to the classic loess palaeosol site at luochuan and current views on climatic change in asia have been strongly moulded by results from this central part of the loess plateau data including the magnetic susceptibility granulometry mineralogy geochemistry micromorphology fossil content and inorganic and organic carbon content of asian loess palaeosol series have been used singly and in combination as surrogate measures of palaeoclimates to infer climatic changes with particular reference to changes in the asian monsoon some of this work is reviewed and the status of a number of parameters as climatic proxies is discussed some lithostratigraphic sequences from which proxy measures of climate have been derived are incomplete as shown by erosion surfaces water laminated zones and truncated palaeosol profiles while others provide records of relatively low resolution most descriptions of quaternary climatic changes based on studies of the loess emphasize alternations in the relative dominance of warm and moist v cool dry and dust laden monsoons some recent studies of the much higher resolution loess palaeosol successions on the western margins of the loess plateau have suggested that such changes may sometimes be out of phase with the inference that the current climate model is too simple means of testing this hypothesis are proposed

high resolution 10be and paleomagnetic recording of the last polarity reversal in the chiba composite section age and dynamics of the matuyama brunhes transition we present new magnetic direction and relative paleointensity and beryllium isotope 10 be 9 be and 10 be 9 be ratio results covering the last geomagnetic reversal i e the matuyama brunhes transition mbt from the chiba composite section cbcs east central japan the very high sedimentation rates 90 cm ka of the studied site a candidate site for the global boundary stratotype section and point gssp of the lower middle pleistocene boundary allow the acquisition of a dataset of exceptional resolution coupled cosmogenic 10 be and magnetic results measured on the same samples demonstrate that the magnetization acquisition conserved the timing of rapid geomagnetic features allowing accurate paleomagnetic interpretations a polarity switch ps capturing most of the angular deviation occurring between 771 9 and 773 9 ka was followed by a directional instability rebound ic1 until 768 5 ka this sequence with 5 4 ka duration was characterized by a weak dipole field as low as 2 3 0 3 10 22 am 2 four rapid episodes of directional instabilities preceding and following the ps ic1 phase completed the reversal sequence the asymmetry observed between the long term dipole decay and sharp recovery and rapid oscillations 1 ka demonstrate the complex transitional field behavior during dipole moment low before reestablishment of the full polarity state our observations reinforce the fact that most reversal records do not integrate the full field behavior associated with the geodynamo action although this poses problems for understanding the underlying physical processes that produce reversals it does not hamper stratigraphic correlations among most geologic records

correlation of widespread tephra deposits based on paleomagnetic directions link between a volcanic field and sedimentary sequences in japan abstract magnetic measurements were made of the yabakei and the imaichi pyroclastic flow deposits of the late matuyama chron in central kyushu and their correlative co ignimbrite ashes the pink and the azuki tephra deposits in the kinki district and the ku6c and the o7 tephra deposits in boso peninsula site mean magnetic directions of the imaichi deposit are all of reversed polarity with declinations deflected slightly to the east these directions are consistent with those of the azuki and the ku6c ashes magnetizations of the yabakei the pink and the o7 ashes are also consistent with one another all are characterized by shallow inclinations about 30 of normal polarity with slightly westerly declinations the identical paleomagnetic directions of these widespread tephra confirm correlations that have been proposed mainly on the basis of glass and mineral chemistry and provide a tephrochronological linkage between the source volcanic area and the lacustrine or marine sequences over distances of at least 1000 km this linkage allows us to correlate the oxygen isotope record of the kazusa group in boso peninsula with the alternating marine non marine lithology of the osaka group in the kinki district radiometric dates from the yabakei and the imaichi pyroclastic flow deposits are concordant with the astronomically calibrated geomagnetic polarity time scale

a long term geomagnetic excursion from plio pleistocene sediments in java a paleomagnetic record from the plio pleistocene sediments in sangiran central java represents a long period change of the geomagnetic field and reveals an existence of large scale swing of declination between the olduvai and jaramillo events the field directions in reverse polarity show a gradual clockwise swing to almost west pointing directions followed by a rapid return to south pointing directions the geomagnetic behavior has been well duplicated from the plio pleistocene sediments in mojokerto 200 km west of sangiran at both sites the declinations have maximum deflection more than 70 from the axial dipole field direction and the inclinations remain low during the swing the full duration of the swing recorded over sections about 40 m thick at both sites was estimated about 130 000 years and a zone of most deviated fields 50 000 years assuming a constant sedimentation rate the phenomenon was regarded as an excursion neither geomagnetic excursion nor geomagnetic event corresponding to the entire duration of the excursion has been reported at present although only the final stage a duration of most deviated fields may be associated with the geomagnetic events reported from north america and europe an upward pointing eccentric radial dipole model rather than a downward pointing one seems plausible to explain the geomagnetic behavior during the excursion the former becomes one possible source for the inclination anomalies in the time averaged fields of the plio pleistocene times in the equatorial areas a short sequence of the upper olduvai transitional fields obtained from mojokerto exhibits a vgp path confined to a band of longitude 150 w 20 inconsistent with those from other sites on the globe this suggests that a dipolar component was not dominant globally in the upper olduvai transitional fields

a record of the blake event during the last interglacial paleosol in the western loess plateau of china abstract a high resolution record of the blake event has been obtained from a 40 m well dug in a loess section at jiuzhoutai in the city of lanzhou in the western loess plateau of china the paleomagnetic signal is principally carried by magnetite both in loess and paleosols the blake event is located at the boundary between paleosol s1 c of the last interglacial equivalent to mis 5e in deep sea records and loess l2 2 mis 5d and is characterized by a triple feature consisting of two short reversed intervals separated by a short normal interval the directional changes are abrupt as previously revealed in other records and suggest geocentric dipole field behavior before after and during the event but not necessarily during n r or r n transitions the age and duration of the event are estimated as between 119 97 and 114 47 kyr bp and 5 5 kyr respectively from thermoluminescence dating and astronomically tuned climate stratigraphy based on high resolution magnetic susceptibility

geomagnetic poles over the past 10 000 years locations of the geomagnetic pole over the past 10 000 years have been calculated by averaging the vgp positions obtained from paleomagnetic data the distribution of the geomagnetic pole was elongated to the direction parallel to the meridian of 45 and 225 longitude and westward movement of the pole was predominant throughout this period the time sequence of the polar motion can be divided into three intervals the intervals between ca 10 000 b p and ca 7000 b p between ca 7000 b p and ca 3700 b p and between ca 3700 b p and the present during the period between ca 7000 and ca 3700 b p the range of the movement of the geomagnetic pole was limited within 5 degrees around the geographical pole before and after this period the movement was very active fluctuating over 10 degrees the results of the last 2000 years show good consistency with the geomagnetic pole calculated from archaeomagnetic data by merrill and mcelhinny 1983

heat flux at the ice ocean interface the role of the oceanic heat flux fw in the sea ice energy and mass balance is discussed and a method is described for determining fw from measurements of the temperature and thickness of sea ice results obtained using this method and data collected in the fall of 1988 during a drift in the eastern arctic northeast of fram strait are presented estimates of fw ranged from 0 to 37 w m 2 and varied in both time and space the gradual variation over the course of the drift reflects transformation of the west spitsbergen current into arctic water masses a significant intersite variability on horizontal scales of 10 to 100 m is ascribed to the interaction between under ice topographic features and turbulent energy and mass transfer in the boundary layer in addition there is a variability in the ice growth rate on horizontal scales of 0 1 to 1 m which is ascribed to the instability of the ice ocean phase boundary in the presence of turbulent flow these observations have implications for modeling and future experiments

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

detailed paleomagnetic records during the brunhes matuyama geomagnetic reversal and a direct determination of depth lag for magnetization in marine sediments abstract detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the brunhes matuyama geomagnetic reversal outcropping in the boso peninsula central japan the records are based on three continuous sediment cores from three sections within a distance of 14 km the stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the brunhes matuyama reversal the sedimentation rate according to the oxygen isotope stratigraphy for the three sections decreases eastward and is 3 70 2 20 and 1 83 m ka 1 the depth lag of magnetization in the sediments was calculated directly as 42 cm based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole vgp paths for the three sections vgp movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability the last swing of the vgp from the southern hemisphere to the northern hemisphere in the brunhes matuyama geomagnetic reversal took only 38 yr intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one fifth for about 5 4 ka including the geomagnetic reversal the movement of the vgp longitude is 0 36 yr 1 westward and this westward drift can be related to the rotation of the non dipole field

fluid flow and water rock interaction in the east rift zone of kilauea volcano hawaii the east rift zone of kilauea volcano in hawaii represents a major area of geothermal activity fluid inclusion and stable isotope analyses of secondary hydrothermal minerals in core samples from three scientific observation holes soh drilled into the rift zone indicate that the geothermal system is dominated by meteoric waters to depths of as much as 1500 m below sea level calculated 18o and d values for fluids on the north side of the rift zone indicate that the deep meteoric fluids may be derived from precipitation on the upper slopes of mauna loa volcano in the interior of the rift zone recharge is dominated by seawater mixed with local meteoric water water rock ratios in the rift area are approximately 2 but strongly 18o enriched fluids in the deeper parts of the soh 2 and soh 4 drill holes on the north side of the rift indicate that the fluids underwent extensive interaction with rocks prior to reaching this part of the rift zone marine carbonates at the subaerial to submarine transition between 1700 and 1780 m depth in soh 4 have not fully equilibrated with the fluids suggesting that the onset of hydrothermal activity in this area was relatively recent 2000 years this may represent increased volcanic activity along the rift after the end of the ai la au phase of eruptive activity at the kilauea summit approximately 1000 years ago or it may reflect progressive evolution of the hydrothermal system in response to southward migration of intrusive activity within the rift

roughness of weddell sea ice and estimates of the air ice drag coefficient the roughness of a sheet of sea ice encodes its deformational history and determines its aerodynamic coupling with the overlying air and underlying water here we report snow surface ice surface and ice underside roughness computed from 47 surface elevation profiles collected during a transect of the weddell sea the roughness for each surface parameterized as the standard deviation of the surface elevation segregates according to whether or not a floe has been deformed deformed ice has greater roughness than undeformed ice regardless of deformational history the underside roughness is almost always greater than the snow surface and ice surface roughnesses which are nearly equal roughness spectra for all three surfaces and for both deformed and undeformed ice roll off roughly as k 1 when the wavenumber k is between 0 1 and 3 rad m 1 the snow surface and underside spectra roll off somewhat faster than k 1 and the ice surface spectra roll off somewhat slower than k 1 both top and underside arctic ice roughness spectra on the other hand have been reported to roll off faster than k 2 we speculate that the excess spectral intensity at high wavenumbers in the antarctic ice surface spectra results from the small scale roughness that the ice sheet had on consolidation this excess high wavenumber spectral intensity persists in the ice surface spectra of second year ice evidently once formed the ice surface remains unchanged on the microscale until the entire ice sheet melts with a remote measurement of roughness we should be able to decide whether an ice floe is deformed or undeformed our spectral analysis hints that remote sensing may also be able to differentiate between first year and second year ice from the snow surface spectra we compute a roughness scale that parameterizes the air ice momentum coupling and lets us estimate the neutral stability drag coefficient referenced to a height of 10 m cdn10 typical cdn10 values are 1 1 1 4 10 3 over undeformed ice and 1 3 1 8 10 3 over deformed ice

latest olduvai short lived reversal episodes recorded in chinese loess 1 a duplicate high resolution magnetic record was obtained from two parallel loess sections separated by about 300 m near baoji city southern chinese loess plateau this reveals at least eight short lived reversed polarity episodes in the uppermost part of the olduvai normal polarity subchronozone rock magnetic experiments and anisotropy of low field magnetic susceptibility confirm that the multiple occurrence of short lived reversals is not due to rock magnetic and or sedimentary causes assuming a constant accumulation rate within the olduvai subchron the short episode zone was estimated to span about 24 30 ka and 24 31 ka for the two sections respectively the duration of each short episode ranges from about 0 3 ka to 2 1 ka the present results together with previous worldwide observations show that the occurrence of numerous short reversals is peculiar to the latest olduvai subchron the geodynamo may have been in an anomalous state at that time

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

aeromagnetic study of the island of hawaii an aeromagnetic study of the island of hawaii provides new insight on magnetic properties of subsurface rock and geologic structure on a regional scale spectral depth analysis delineates two shallow magnetic zones each roughly 1 5 km thick lying at a depth of 1 km one zone of unknown origin lies in the center of the island and correlates with a regional magnetic high the other zone coincides with pronounced magnetic lows paralleling kilauea s active east rift zone these magnetic lows probably depict rocks chemically altered by hydrothermal fluids in which titanomagnetite has been destroyed analysis of magnetic terrain effects indicates that magnetization also decreases with depth within mauna kea and mauna loa we estimate that magnetization is reduced by about half at a depth of 1 km the magnetic method is particularly useful in delineating the lateral extent of local shield structures such as rifts summit calderas pit craters and vent fissures rifts possess characteristic magnetic patterns primarily long wavelength linear magnetic low zones we propose alteration processes reduce magnetizations along the flanks of rifts on the other hand along young rifts e g kilauea s east rift zone short wavelength magnetic anomalies probably reflect slowly cooled unaltered intrusions altered rock may also produce magnetic lows that help define buried summit calderas

models of hawaiian volcano growth and plume structure implications of results from the hawaii scientific drilling project the shapes of typical hawaiian volcanoes are simply parameterized and a relationship is derived for the dependence of lava accumulation rates on volcano volume and volumetric growth rate the dependence of lava accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the hawaiian plume with the eruption rate determined from a specified radial dependence of magma generation in the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit the timescale of volcano growth is t 2 r plate where r is the radius of the melting zone of the circular plume and plate is the velocity of the pacific plate the growth progress of a volcano can be described by a dimensionless time t t plate 2r where t 0 is chosen to be the start of volcano growth and t 1 approximates the end of shield growth using a melt generation rate for the whole plume of 0 2 km 3 yr a plume diameter of 50 km and a plate velocity of 10 cm yr we calculate that the lifetime of a typical volcano is 1000 kyr for a volcano that traverses the axis of the plume the standard dimensions are a volume of 57 000 km 3 a summit thickness of 18 km a summit elevation of 3 6 km and a basal radius of 60 km the volcano first breaches the sea surface at t 0 22 when it has attained only 5 of its eventual volume 80 of the volume accumulates between t 0 3 and t 0 7 typical lava accumulation rates start out over 50 m kyr in the earliest stages of growth from the seafloor and level out at 35 m kyr from t 0 05 until t 0 4 from t 0 4 to t 0 9 the submarine lava accumulation rates decrease almost linearly from 35 m kyr to 0 subaerial accumulation rates are about 30 lower the lava accumulation rate is a good indicator of volcano age a volcano that passes over the plume at a distance 0 4r off to the side of the plume axis is predicted to have a volume of about 60 of the standard volcano a lifetime about 8 shorter and lava accumulation rates about 15 20 smaller the depth age data for mauna kea lavas cored by the hawaii scientific drilling project are a good fit to the model parameters used given that mauna kea appears to have crossed the plume about 15 20 km off axis the lifetime of mauna kea is estimated to be 920 kyr mauna loa is predicted to be at a stage corresponding to t 0 8 kilauea is at t 0 6 and loihi is at t 0 16 the model also allows the subsurface structure of the volcanoes the interfaces between lavas from different volcanoes to be modeled radial geochemical structure in the plume may be blurred in the lavas because the volcanoes capture magma from a sizeable cross sectional area of the plume this inference is qualitatively born out by available isotopic data the model predicts that new hawaiian volcanoes are typically initiated on the seafloor near the base of the next older volcano but generally off the older volcano s flank

numerical simulation of the weddell sea pack ice to investigate the role of ice dynamics on the seasonal cycle of antarctic sea ice a series of 2 year simulations of the weddell sea pack ice are carried out the simulations employ the dynamic thermodynamic model developed by hibler 1979 and use a 1 day time step on an 18 15 grid with a resolution of 222 km daily atmospheric data from 1979 are used to drive the simulations with atmospheric temperature fields modified to reflect buoy observations the simulations yield a seasonal cycle of ice with maximum and minimum extents close to that observed except for portions of the western weddell the advance of the ice is found to be primarily thermodynamic in nature while the rapid decay depends critically on the presence of both leads and lateral ice advection the average fraction of open water is substantial even in winter and varies from a minimum of 10 in september to a maximum of 35 in march these values are in general agreement with estimates from satellite microwave data in early summer the ice motion causes a residual tongue of ice to extend eastward from the antarctic peninsula in agreement with observations mean ice thicknesses are consistent with observations and vary from about 3 m in the perennial ice in the western weddell to 1 m in first year ice in the eastern weddell possible feedback between the ice extent and atmosphere are investigated by simulations with the surface air temperatures constrained to below freezing values in the presence of ice these coupled simulations yield a more realistic rapid decay in summer with dynamics included but an unrealistically small decay without dynamics simulated ice drift results yield mean drift rates of about 5 km day in good agreement with buoy drift observations but with slightly inadequate northward transport in the western weddell near the ice edge the drift rates are relatively insensitive to the ice strength near the coast however lower strengths are found to yield a decrease in northward drift rates

stratigraphy of the kazusa group boso peninsula an expanded and highly resolved marine sedimentary record from the lower and middle pleistocene of central japan major climatic reorganizations including changes in the nature of glacial interglacial cycles through the pleistocene are a key issue for improving the understanding of earth s climate system highly resolved marine sedimentary records are essential to reconstruct the details of these past climatic changes and investigate the mechanisms responsible for them the kazusa group located in the central part of the japanese island chain is well exposed and contains a remarkably continuous and thick deep and shallow water marine sedimentary succession this group also contains well preserved marine microfossils pollen paleomagnetic reversal events geochemical signatures and a large number of tephra beds these features allow us to establish a robust chronological and stratigraphic framework for the kazusa group and provide a rare opportunity to study oceanic and terrestrial climatic and environmental changes at high resolution especially through the lower and lower middle pleistocene in the boso peninsula chiba prefecture the kazusa group is deeply incised yielding spectacular river cut exposures the matuyama brunhes paleomagnetic reversal is observed immediately above the widespread byakubi e byk e tephra in the chiba composite section within the kokumoto formation kazusa group because the matuyama brunhes boundary customarily serves as the primary guide for the lower middle pleistocene subseries boundary the chiba composite section is considered an excellent candidate for its global boundary stratotype section and point gssp this study reviews the published data for the kazusa group hitherto almost exclusively in japanese in order to place the chiba composite section within its broader depositional and chronostratigraphic context

southern ocean winter mixed layer austral winter 1986 observations from the polarstern along the greenwich meridian from the ice edge to the antarctic margin show the mixed layer beneath the winter sea ice cover to be significantly depressed in oxygen saturation incorporation of weddell deep water wdw into the winter mixed layer responsible for this undersaturation also introduces heat and salinity into the surface layer which strongly influences the mixed layer sea air exchanges and sea ice formation processes the total wdw transfer into the mixed layer averages 45 myr 1 implying a residence time for the surface water of 2 5 years the associated winter heat flux is 41 w m 2 which limits ice thickness to about 0 55 m agreeing quite well with observations the air temperatures during the cruise are just sufficient to remove the wdw heat input in the presence of observed ice thickness and concentration this suggests that the sea ice cover and wdw heat input into the mixed layer are in approximate balance by midwinter the annual heat flux from wdw to the surface layer and hence into the atmosphere is estimated as 16 w m 2 extrapolation of the greenwich meridian wdw entrainment value to the full circumpolar 60 70 s belt yields total up welling of 24 106 m3 s 1 similar extrapolation of the heat flux value gives a circumpolar total of 2 8 1014 w as a consequence of circulation topography interaction the maud rise water column stands out as an anomaly relative to the surrounding region with a significantly more saline and dense mixed layer below the mixed layer the water column over the crest of the rise is identical to that over the flanks if the latter water column is upwelled by 400 m this uplifting is believed to be a response of the upstream flow encountering the rise increased upstream flow would be expected to increase maud rise upwelling and the dependent salinity density of the mixed layer slight increases in the mixed layer density could trigger a convective mode and generation of a polynya it is hypothesized that spin up of the weddell gyre s barotropic circulation induced by an increase of the regional wind stress curl would enhance the probability of polynya development over maud rise

thermohaline stratification below the southern ocean sea ice the end of winter stratification within the cold cyclonic trough of the weddell gyre near 60 s between 5 e and the greenwich meridian is resolved with the mikhail somov data set the temperature maximum of the weddell deep water wdw is for the most part less than 0 5 c but warmer cells of wdw are found these warm wdw cells have temperature salinity and oxygen properties similar to the wdw characteristic of the weddell gyre inflow which is situated to the southeast of the mikhail somov study region the warm wdw cells are accompanied by domes in the pycnocline of 40 m amplitude over the surrounding pycnocline while deeper isopycnals are depressed anticyclonic shear below the 27 83 isopycnal within the warm wdw cells is compensated by the cyclonic shear associated with the pycnocline dome the pycnocline domes are exposed to about 50 greater entrainment by the turbulently active winter mixed layer relative to the regional entrainment rate this entrainment can significantly erode the warm cells in a single winter season introducing excess heat and salt into the mixed layer while the heat is lost to the atmosphere the excess salt is not necessarily compensated by increased fresh water introduction it is hypothesized that the warm wdw cells within the weddell gyre trough are derived from instability within the frontal zone which extends from maud rise to the northeast separating the weddell warm regime from the cold regime greater than normal injection of warm wdw cells into the weddell gyre trough would increase the surface salinity which would tend to destabilize the pycnocline increasing the probability of deep convection and polynya events

on the importance of leads in sea ice to the energy balance and ice formation in the weddell sea for a considerable coverage the energy balance of and ice formation by leads in sea ice in the weddell sea are evaluated on the basis of data obtained from drifting buoys for the winter periods from 1986 to 1994 and by using a kinematic thermodynamic sea ice model the net heat flux is defined as the sum total of radiative and turbulent fluxes for thin ice the net turbulent flux is 3 4 times the net radiative flux the contribution of the net heat flux through open and refrozen leads to the total net heat flux through sea ice is twice as large as the area contribution of open and refrozen leads to the total area covered with sea ice in the eastern and central parts of the weddell sea leads contribute some 30 to the total energy flux from the ocean to the atmosphere this flux increases from 10 15 mbox w m 2 in the eastern and central weddell sea regions to 30 mbox w m 2 in the western part of the weddell sea where leads contribute more than 80 of the total net energy transfer the increase is mainly due to the colder and windier atmosphere in connection with the higher variability of the ice motion in the diurnal andsemidiurnal band in the western weddell sea the contribution of leads in winter ice formation exceeds 50 in the whole of the weddell sea monthly area weighted ice growth is 10 15 cm in the east and up to 30 mbox cm over the western continental shelf region in the western part of the weddell sea tidal and inertial motions in the diurnal and semidiurnal bands enhance lead formation and thus contribute 7 to total net heat flux 12 to ice formation and 23 to the salt mass released during ice growth the results are used to assess quantitatively the importance of leads to the interaction of the ocean the sea ice and the atmosphere

chronostratigraphy of the pliocene pleistocene boundary in forearc basin fill on the pacific side of central japan constraints on the spatial distribution of an unconformity resulting from a widespread tectonic event the upper ikego formation pliocene and the urago formation pliocene to lower pleistocene have been examined for their lithostratigraphy calcareous nannofossil biostratigraphy and magnetostratigraphy both formations are forearc basin fills exposed on the miura peninsula on the pacific side of central japan the boundary between these formations has been previously treated as the western extension of the kurotaki unconformity which was considered to be related to widespread tectonic change at ca 3 ma in the japanese forearc region however the presence of the unconformity on the miura peninsula remains unclear due to sparse lithostratigraphic and chronostratigraphic data around the boundary between the both formations in the study area the ikego formation consists mainly of sandy mudstone and the urago formation of sandstone and muddy sandstone both formations contain numerous tuff beds key tuff beds and the last appearance of discoaster tamalis have been identified in the upper ikego formation and the lower urago formation allowing these formations to be considered conformable the following continuous stratigraphic markers were identified in ascending order top of the mammoth subchronozone 3 21 ma base 3 13 ma and top 3 05 ma of the kaena subchronozone last appearance of discoaster tamalis 2 76 ma the widespread tuff bed kgp ca 2 5 ma and the last appearance of discoaster pentaradiatus 2 41 ma sedimentation rates are almost constant among these age controlled horizons which indicates continuous sedimentation from 3 2 to 2 4 ma the results indicate that the kazusa group conformably overlies the miura group on the miura peninsula which provides a well established continuous stratigraphic section for analyses of regional paleoenvironmental and tectonic evolution in the japanese forearc region

apparent timing and duration of the matuyama brunhes geomagnetic reversal in chinese loess the matuyama brunhes mb geomagnetic reversal in chinese loess has been studied extensively as an important boundary for land ocean stratigraphic and paleoclimatic correlations however the apparent timing and duration of the mb boundary remain controversial in chinese loess deposits due to its inconsistent stratigraphic position and the uncertain chronologies here we synthesized high resolution paleomagnetic records from four loess sequences in the central chinese loess plateau and synchronized the loess paleosol chronology by matching the grain size variations to orbitally tuned grain size time series the synthesized paleomagnetic results reveal consistent features of the mb transition in chinese loess including the stratigraphic position l 8 s 8 transition timing 808 826 ka duration 14 16 ka and rapid directional oscillations compared with the mb transition in marine records 770 775 ka the timing of the mb transition is relatively older and longer in chinese loess due to a complex interplay between different remanence acquisition mechanisms which occurred during the course of postdepositional physical and chemical processes

sea ice thickness retrieval algorithms based on in situ surface elevation and thickness values for application to altimetry 1 in situ measurements of sea ice thickness i snow depth s and snow freeboard fsn from drilling profile lines from 15 cruises into the southern ocean antarctica were analyzed i was calculated from in situ fsn and s using an isostatic approach i was also directly estimated from fsn as can be obtained from laser altimetry the root mean square difference rmsd between observed and calculated i reduces and the correlation between fsn and i increases substantially when 1 using values averaged over the survey lines 50 m instead of single drill hole measurements 1 m and 2 treating positive and negative sea ice freeboard fi separately for small fi however s approximates fsn pointing toward an isostatic balance also between s and i our linear regression analysis between the in situ measurements suggests a direct conversion of fsn into i using a region specific set of equations rmsd values are similar to those obtained employing isostatic balance models and treating positive and negative fi separately however more data would have been needed to obtain significant differences between most of the various models suggested still our new approach gives a viable alternative for antarctic i retrieval from altimetric measurements of fsn alone correlation between in situ observations of fsn and s is high rmsd between observed and calculated s is small this suggests estimation of s from altimetric fsn measurements such s has an estimated precision of 5 cm and is neither affected by snow wetness or grain size nor limited to s 50 cm

paleomagnetic record over the past 35 000 years of a sediment core from off shikoku southwest japan the paleomagnetism of a sediment core kt89 18 p4 recovered from the sea floor off southwest japan was studied to investigate geomagnetic field variation over the past 35 000 years alternating fields demagnetization revealed that the remanent magnetization of the unconsolidated sediments was stable and not affected by any secondary overprints 14c datings were made on 21 horizons using a tandetron accelerator mass spectrometer providing a detailed time scale to the core the inclination record agrees with that from lake biwa kawai et al 1975 and supports the existence of the geomagnetic excursions which had been found in the lake biwa core the ages of the excursions are estimated to be during ca 32 ky b p ca 28 ky b p and ca 26 ky b p ca 24 ky b p

millennial to submillennial scale features of the matuyama brunhes geomagnetic polarity transition from osaka bay southwestern japan 1 a high resolution magnetic record of the matuyama brunhes mb reversal transition was obtained from homogeneous marine clay in a 1700 m core from osaka bay the transition stretches over a core length of 6 4 m and yields four short reversal episodes two predate the main mb boundary mbb and two postdate it we made diatom analyses of sediments to estimate relative sea level changes and constructed an age model by correlation with the astronomically calibrated marine oxygen isotope curve the age model shows the short reversal episodes with durations of 1 kyr occurred at 4 5 kyr intervals within stage 19 and the mbb occurred about 5 kyr after the sea level highstand corresponding to isotopic event 19 3 it also shows the paleointensity minimum of 15 20 of the postreversal mean occurred at about 6 kyr before the mbb and another minimum at about 1 kyr after four virtual reverse to normal polarity boundaries found in this study suggest different ages for the mbb could be observed in lava flow studies some millennial to submillennial scale transition features found in this study are compatible with the previous numerical simulations of geomagnetic field reversal