a 100 kyr periodicity in the flux of extraterrestrial 3he to the sea floor most of the helium 3 in oceanic sediments comes from interplanetary dust particles idps and can therefore be used to infer the accretion rate of dust to the earth through time 3he records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of years probably owing to cometary and asteroidal break up events muller and macdonald have proposed that periodic changes in this accretion rate due to a previously unrecognized 100 kyr periodicity in the earth s orbital inclination might account for the prominence of this frequency in climate records of the past million years here we report variations in the 3 he flux to the sea floor that support this idea we find that the flux recorded in rapidly accumulating quaternary sediments from the mid atlantic ridge oscillates with a period of about 100 kyr we cannot yet say however whether the 100 kyr climate cycle is a consequence of a cause of or an effect independent of these periodic changes

wind effects on a front over the continental shelf slope a continuously stratified two dimensional vertical and seaward model is used to study the effects of alongshore wind on a front between lighter water over the continental shelf and heavier water in the offshore ocean the model produces three components of alongshore flows one component associated with the horizontal density front and two wind driven components one near the coast and one over the shelf break the cross shore circulation has two near surface features a seaward component resulting from the offshore density gradient and a shoreward seaward component resulting from the right hand left hand alongshore wind stress which has the coast to the right left facing downstream these two components contribute to the maintenance of the front with no wind low density water over the shelf tends to move toward the offshore ocean associated with the near surface seaward flow resulting in flatter isopycnal surfaces a moderate right hand alongshore wind suppresses the density driven surface flow and constrains the shelf water while a strong wind in the same direction pushes the front shoreward and makes isopycnal surfaces steeper resulting in increased exchange of the shelf water a left hand alongshore wind contributes to more rapid frontal relaxation and a seaward shift in its position these results are applied to the labrador shelf

absolute plate velocities from seismic anisotropy importance of correlated errors the errors in plate motion azimuths inferred from shear wave splitting beneath any one tectonic plate are shown to be correlated with the errors of other azimuths from the same plate to account for these correlations we adopt a two tier analysis first find the pole of rotation and confidence limits for each plate individually second solve for the best fit to these poles while constraining relative plate angular velocities to consistency with the morvel relative plate angular velocities our preferred set of angular velocities sks morvel is determined from the poles from eight plates weighted proportionally to the root mean square velocity of each plate sks morvel indicates that eight plates amur antarctica caribbean eurasia lwandle somalia sundaland and yangtze have angular velocities that differ insignificantly from zero the net rotation of the lithosphere is 0 25 0 11 ma 1 95 confidence limits right handed about 57 1 s 68 6 e the within plate dispersion of seismic anisotropy for oceanic lithosphere 19 2 differs insignificantly from that for continental lithosphere 21 6 the between plate dispersion however is significantly smaller for oceanic lithosphere 7 4 than for continental lithosphere 14 7 two of the slowest moving plates antarctica vrms 4 mm a 1 29 and eurasia vrms 3 mm a 1 33 have two of the largest within plate dispersions which may indicate that a plate must move faster than 5 mm a 1 to result in seismic anisotropy useful for estimating plate motion the tendency of observed azimuths on the arabia plate to be counterclockwise of plate motion may provide information about the direction and amplitude of superposed asthenospheric flow or about anisotropy in the lithospheric mantle

flux and size fractionation of 3he in interplanetary dust from antarctic ice core samples article i nfo accretion of extraterrestrial material to earth is of interest for a variety of reasons including as a possible driver of long or short term climate change and as a record of solar system events preserved in the geological record 3 he is highly enriched in extraterrestrial material and provides a useful tracer of its input into sedimentary archives previous work showed that polar ice could be a suitable archive for studying variations in extraterrestrial input additional measurements reported here confirm that the late quaternary 3 he flux derived from antarctic ice samples is similar to 3 he fluxes determined from marine sediments the mean flux from nine replicate 1 kg ice samples from the vostok ice core site 112 115 m depth age of 3800 years is 1 25 0 37 10 12 cm 3 stp cm 2 ka 1 mean 2se the large range for the 9 replicates is probably due to the small number of interplanetary dust particles idps present and illustrates that large ice samples are required for precise constraints on temporal variations in the 3 he flux size fraction experiments show that the majority of the 3 he flux is delivered by particles in the 5 10 m size range consistent with the hypothesis

shallow mantle anisotropy beneath the juan de fuca plate the anisotropic fabric of the oceanic mantle lithosphere is often assumed to parallel paleo relative plate motion rpm however we find evidence that this assumption is invalid beneath the juan de fuca jdf plate using travel times of seismic energy propagating through the topmost mantle we find that the fast direction of p wave propagation is rotated 18 3 counterclockwise to the paleo spreading direction and strikes between pacific jdf relative and jdf absolute plate motion apm the mean mantle velocity is 7 85 0 02 km s with 4 6 0 4 anisotropy synthesis of the plate averaged pn anisotropy signal with measurements of pn anisotropy beneath the jdf ridge and sks splits across the jdf plate suggest that the anisotropic structure of the topmost mantle continues to evolve away from the spreading center to more closely align with apm we infer that the oceanic mantle lithosphere may record the influence of both paleo rpm and paleo apm

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

search for the origin of exotic helium in deep sea sediments merrihue1 first observed helium with a very high 3he 4he isotope ratio 10 4 here termed exotic helium in a magnetic separate of a deep ocean sediment and attributed it to input of extraterrestrial material krylov et al 2 subsequently reported the existence of exotic he in several deep sea sediments while tilles3 measured argon isotope ratios in magnetic separates of two ocean sediments observing that the 3he 4he ratio is inversely correlated with the sedimentation rate ozima et al 4 concluded that the exotic he is carried by interplanetary dust particles idps we have now examined other deep sea sediments to characterize the exotic he and to identify its carrier and have confirmed that the exotic he resides mainly in a strongly magnetic fraction stepwise degassing experiments suggest that the exotic he consists of a single component the carrier of the he is likely to be chondritic idps the higher level of exotic he in magnetic fractions of sediments can be explained by secondarily produced magnetite in idps as a result of thermal shock during atmospheric entry

absolute plate motions relative to deep mantle plumes abstract advances in whole waveform seismic tomography have revealed the presence of broad mantle plumes rooted at the base of the earth s mantle beneath major hotspots hotspot tracks associated with these deep mantle plumes provide ideal constraints for inverting absolute plate motions as well as testing the fixed hotspot hypothesis in this paper 27 observed hotspot trends associated with 24 deep mantle plumes are used together with the morvel model for relative plate motions to determine an absolute plate motion model in terms of a maximum likelihood optimization for angular data fitting combined with an outlier data detection procedure based on statistical tests the obtained t25m model fits 25 observed trends of globally distributed hotspot tracks to the statistically required level while the other two hotspot trend data comores on somalia and iceland on eurasia are identified as outliers which are significantly incompatible with other data for most hotspots with rate data available t25m predicts plate velocities significantly lower than the observed rates of hotspot volcanic migration which cannot be fully explained by biased errors in observed rate data instead the apparent hotspot motions derived by subtracting the observed hotspot migration velocities from the t25m plate velocities exhibit a combined pattern of being opposite to plate velocities and moving towards mid ocean ridges the newly estimated net rotation of the lithosphere is statistically compatible with three recent estimates but differs significantly from 30 of 33 prior estimates

changes in seismic anisotropy shed light on the nature of the gutenberg discontinuity the boundary between the lithosphere and asthenosphere is associated with a platewide high seismic velocity lid overlying lowered velocities consistent with thermal models seismic body waves also intermittently detect a sharp velocity reduction at similar depths the gutenberg g discontinuity which cannot be explained by temperature alone we compared an anisotropic tomography model with detections of the g to evaluate their context and relation to the lithosphere asthenosphere boundary lab we find that the g is primarily associated with vertical changes in azimuthal anisotropy and lies above a thermally controlled lab implying that the two are not equivalent interfaces the origin of the g is a result of frozen in lithospheric structures regional compositional variations of the mantle or dynamically perturbed lab

new insights into the carrier phase s of extraterrestrial 3he in geologically old sediments to better understand the composition characteristics of helium diffusion and size distribution of interplanetary dust particles idps responsible for the long term retention of extraterrestrial 3he we carried out leaching stepped heating and sieving experiments on pelagic clays that varied in age from 0 5 ma to 90 myr the leaching experiments suggest that the host phase s of 3he in geologically old sediments are neither organic matter nor refractory phases such as diamond graphite al2o3 and sic but are consistent with extraterrestrial silicates fe ni sulfides and possibly magnetite stepped heating experiments demonstrate that the 3he release profiles from the magnetic and non magnetic components of the pelagic clays are remarkably similar because helium diffusion is likely to be controlled by mineral chemistry and structure the stepped heating results suggest a single carrier that may be magnetite or more probably a phase associated with magnetite furthermore the stepped outgassing experiments indicate that about 20 of the 3he will be lost through diffusion at seafloor temperatures after 50 myrs while sedimentary rocks exposed on the earth s surface for the same amount of time would lose up to 60 the absolute magnitude of the 3he loss is however likely to depend upon the 3he concentration profile within the idps which is not well known contrary to previous suggestions that micrometeorites in the size range of 50 100 m in diameter are responsible for the extraterrestrial 3he in geologically old sediments stuart f m harrop p j knott s turner g 1999 laser extraction of helium isotopes from antarctic micrometeorites source of he and implications for the flux of extraterrestrial 3he flux to earth geochim cosmochim acta63 2653 2665 our sieving experiment demonstrates that at most 20 of the 3he is carried by particles greater than 50 m in diameter the size distribution of the 3he bearing particles implies that extraterrestrial 3he in sediments record the idp flux rather than the micrometeorite flux

on the fluxes and fates of 3he accreted by the earth with extraterrestrial particles abstract the influx of extraterrestrial et matter on the earth results in appreciable fluxes of 3 he originating from several different processes we have analyzed the published data on the observed concentrations of 3 he in interplanetary dust particles idps sampled in the stratosphere in polar ice samples and in meteorites this information considered together with production of secondary fragments during the atmospheric entry of meteoroids allows us to make plausible estimates of fluxes of the extra terrestrial fluxes of the two principal 3 he types accreted along with the extraterrestrial materials i solar wind sw and solar energetic particle sep 3 he implanted in et particles and ii 3 he produced in the et matrix by interactions of galactic cosmic ray gcr particles an appreciable fraction of 3 he brought to the earth by et particles is expected to be lost from the particles during ablation and fragmentation of the particles during their transit through the atmosphere by comparing the measured 3 he fluxes in two terrestrial archives polar ice and marine sediments and based on supplementary data 3 he 4 he and 20 ne 22 ne ratios we have attempted to determine the relative fluxes of solar sw and sep and gcr 3 he that are preserved in them our analyses establish the fact that the reported 3 he concentrations in particles in relatively small size polar ice samples from greenland and antarctica are primarily due to the solar 3 he accreted via idps of 2 5 m radii scavenged in the polar ice we predict that if larger ice samples were analyzed greater contributions would be found from gcr 3 he present in fragments of meteoroids produced during their entry through the atmosphere it follows therefore that long term fluxes of idps can be estimated from studies of 3 he in relatively small samples of polar ice sheets one observes appreciable concentrations of 3 he in marine sediments extensive observational data are available for sediments in the atlantic and pacific oceans we show that besides idps there is an important source which is an important contributor to the flux of 3 he in marine sediments this is the gcr produced 3 he in meteoroids of sizes up to 1 m we compare the measured 3 he fluxes in marine sediments with those predicted from solar 3 he in idps and from gcr produced 3 he in secondary fragments of meteoroids of sizes up to 1 m produced during their transit through the atmosphere our studies allow us to reach plausible conclusions regarding the predominant source of 3 he in marine sediments

on the possible influence of extraterrestrial volatiles on earth s climate and the origin of the oceans abstract a consideration of observational and circumstantial evidence suggests that earth may be subject to high influx rates 1011 1012 kg yr of extraterrestrial sourced volatile elements carbon hydrogen oxygen nitrogen derived from comets or other primitive solar system material the total extraterrestrial influx rate may be four to five orders of magnitude greater than previously thought large enough to account for today s total near surface inventories of water and carbon the possibility of high rates of extraterrestrial volatile accretion suggests a new climatic paradigm wherein earth s surface temperature is influenced by conflicting internal and external processes a variable influx of volatile elements tends to warm the earth while terrestrial processes cool the planet by absorbing these gasses at a more uniform rate variations in extraterrestrial influx rates may explain the variation of sea level and mean global temperature over geologic time as well as some types of climate change the occurrence of the pleistocene ice ages and the asymmetry of the phanerozoic climate record sudden warmings slow coolings the extraterrestrial influx rate may also act as the pacemaker of terrestrial evolution at times leading to mass extinctions through climatic shifts induced by changes in accretion rates with concomitant disruptions of the carbon and nitrogen cycles life on earth may be balanced precariously between cosmic processes which deliver an intermittent stream of life sustaining volatiles from the outer solar system or beyond and biological and tectonic processes which remove these same volatiles from the atmosphere by sequestering water and carbon in the crust and mantle

extra terrestrial noble gases in deep sea sediments abstract magnetic fractions separated from four deep sea sediments from the pacific ocean were analysed for elemental abundances and isotopic ratios of noble gases by stepwise heating a 3 he 4 he ratio of 1 8 0 4 10 4 was observed in most samples in the high temperature fractions in all the samples 20 ne 22 ne ratios are constant 11 6 0 6 in most temperature fractions these ratios are different from he a ne a and he b ne b and appear to come from a unique component the 40 ar 36 ar ratios in high temperature fractions are lower than the atmospheric ratio indicating the existence of extra terrestrial ar while the 38 ar 36 ar ratios are indistinguishable from atmospheric planetary and solar ar indicating negligible spallogenic ar the isotopic ratios of kr and xe are indistinguishable from atmospheric ratios from the 20 ne 22 ne ratios and from thermal release patterns of he we conclude that the extra terrestrial noble gases are implanted solar flare particles the energy range of the solar flares is estimated from the relation between the grain size of idps interplanetary dust particles and the penetration depth of particles impinging on to a grain which gives less than 1 mev n the uniformity of isotopic ratios of he and ne can be explained by a grain size sieving effect of idps during atmospheric entry and by the subsequent diffusion loss of shallowly implanted solar wind noble gases

the contribution by interplanetary dust to noble gases in the atmosphere of mars interplanetary dust particles accumulate significant concentrations of noble gases implanted by the solar wind and solar energetic particles while they are in space when these dust particles enter the atmosphere of a planet they are heated and decelerated depositing some of the implanted noble gases directly into the atmosphere and if the particle survives atmospheric entry emplacing the remainder on the surface of the planet where it can be released by particle decomposition or by episodic surface heating because of the low mass of the atmosphere of mars compared to the atmospheres of venus and earth the atmosphere of mars is more severely influenced by the addition of noble gases by interplanetary dust the contribution of noble gases to mars has been modeled using estimates of the interplanetary dust flux at mars and measurements of the noble gas contents of interplanetary dust collected from the atmosphere of the earth over the past 3 6 b y interplanetary dust particles have contributed quantities of 3he 4he 20ne and 22ne comparable to the current total atmospheric inventories of these isotopes in the present era the rates of addition of 3he 20ne 22ne 36ar and 38ar to the atmosphere of mars by interplanetary dust range from about 2 to 20 of the modeled rates of addition by planetary outgassing pepin 1994 krasnopolsky and gladstone 1996 and the isotopic compositions of the he and ne added by interplanetary dust are distinctly different from those assumed for planetary outgassing 3he 4he 2 8 10 4 in the interplanetary dust nier and schlutter 1992 versus almost pure 4he for outgassing of u and th decay products and 20ne 22ne 11 8 in the interplanetary dust niert 1994 versus 20ne 22ne 13 7 assumed for planetary outgassing in pepin s 1994 model since the actual outgassing rate of mars in the present era is not well constrained by observations the contributions of ne and ar to the atmosphere of mars by the interplanetary dust must serve at least as a lower limit on the current sources of these noble gases

size fractionation reproducibility and provenance of helium isotopes in north equatorial pacific pelagic clays helium isotope compositions were measured in late quaternary pelagic clays from the north equatorial pacific where terrestrial and extra terrestrial end member components are identified and quantified in different grain size populations the results provide important constraints on the nature of interplanetary dust particles idps in sedimentary environments and on their application as constant flux proxies in paleoceanographic studies approximately 92 95 of the extra terrestrial 3 he 3 heet is concentrated in particles smaller than 20 mm within this grain size group 44 of 3 heet is concentrated in the 20 10 and 10 2 mm fractions while more than 50 of the total 3 heet is found in fine particles smaller than 2 mm the peak concentration of 3 heet in the very fine particle size population explains a consistently better reproducibility of 3 heet in sediments compared to model predictions that is related to the relatively homogenous distribution of clay size particles in sediments compared to coarser particles especially in deep sea sediments and ice cores the 3 he 4 he ratios are highest in the coarser than 20 mm fractions peaking between 63 and 38 mm 1 7871 13 10 4 compared to the bulk and the o20 mm fractions 3 2072 62 10 5 and 2 1170 88 10 5 respectively the isotopic composition of helium in each of the size fractions reflects a mixture between a pure extraterrestrial end member and a terrigenous component which is itself comprised of varying mixtures of volcanic material and eolian material associated with the fine grain size fraction of loess deposits in south east asia poor 3 heet reproducibility observed here and elsewhere inhibiting its application as a constant flux proxy stems from the presence of a small number of aliquots displaying significantly higher 3 heet contents compared to the average of their reference group these helium rich particles hrps are probably characterized by a o20 mm diameter and carry significant amounts of extraterrestrial 3 he and 4 he careful rejection of such aliquots improves the signal reproducibility by more than two fold

a 57 ma pacific plate palaeomagnetic pole determined from a skewness analysis of crossings of marine magnetic anomaly 25r summary an increase in the accuracy and age resolution of the apparent polar wander path of the pacific plate could be important for testing reconstructions that relate the motion of pacific basin plates to other plates for testing if hotspots in different ocean basins are stationary relative to one another and for estimating the motion of hotspots relative to the spin axis with these goals in mind herein we investigate how accurately a palaeomagnetic pole can be estimated from skewness analysis of many crossings of a single magnetic anomaly on the pacific plate apparent effective remanent inclinations of the sea floor magnetization were estimated from the skewnesses of 132 useful out of 149 total crossings of anomaly 25r 56 5 57 8 ma distributed over a distance of more than 11000 km across the pacific plate these estimates were inverted to obtain a best fitting palaeomagnetic pole latitude pole longitude and anomalous skewness for this single reversed polarity chron the best fitting model gives a pole of 78 2 n 4 8 e with a 95 per cent confidence ellipse having a 6 4 major semi axis oriented 93 clockwise of north and a 4 1 minor semi axis anomalous skewness is 16 2 4 6 95 per cent confidence limits we also investigated the effect of the dependence of anomalous skewness on spreading rate by correcting our data using an empirical model the pole obtained from the inversion of this alternative data set lies a statistically insignificant 0 6 from the pole obtained using no correction that a pole with usefully compact confidence limits and a narrowly resolved precisely estimated age can be so determined suggests that an accurate apparent polar wander path with a fine age resolution can be determined for the pacific plate by applying the same approach to the shapes of other marine magnetic anomalies comparison of our chron 25r pole with other pacific palaeomagnetic and palaeoequatorial sediment facies data indicates that the pacific plate remained nearly stationary relative to the spin axis during the eocene 0 05 myr 1 0 28 myr 1 but probably moved rapidly northward during the paleocene 0 83 myr 1 0 46 myr 1 comparison of these data to latitudes of dated volcanic edifices along the hawaiian emperor chain indicates that the hawaiian hotspot drifted southward by 10 2 3 4 95 per cent confidence limits since 57 ma but only by 1 7 1 9 since 39 ma which gives a southward displacement of 8 5 3 9 95 per cent confidence limits between 57 and 39 ma corresponding to a rate of southward motion of 52 24mm yr 1 incorporation of realistic uncertainties of volcano ages would increase these uncertainties considerably however we also examined the distance between the crossings of anomalies 25 and 27 on all the profiles we analysed along the palaeo pacific farallon boundary these distances are inconsistent with the joint hypotheses of symmetric spreading and single pacific and farallon plates between 62 and 56 ma indicating that the evidence for a single pacific plate in early tertiary time is not as compelling as it had previously seemed

bounds on geologically current rates of motion of groups of hot spots it is widely believed that groups of hot spots in different regions of the world are in relative motion at rates of 10 to 30 mm a 1 or more here we present a new method for analyzing geologically current motion between groups of hot spots beneath different plates in an inversion of 56 globally distributed equally weighted trends of hot spot tracks the dispersion is dominated by differences in trend between different plates rather than differences within plates nonetheless the rate of hot spot motion perpendicular to the direction of absolute plate motion vperp differs significantly from zero for only 3 of 10 plates and then by merely 0 3 to 1 4 mm a 1 the global mean upper bound on vperp is 3 2 2 7 mm a 1 therefore hot spots move slowly and can be used to define a global reference frame for plate motions

origin of azimuthal seismic anisotropy in oceanic plates and mantle abstract seismic anisotropy is ubiquitous in the earth s mantle but strongest in its thermo mechanical boundary layers azimuthal anisotropy in the oceanic lithosphere and asthenosphere can be imaged by surface waves and should be particularly straightforward to relate to well understood plate kinematics and large scale mantle flow however previous studies have come to mixed conclusions as to the depth extent of the applicability of paleo spreading and mantle flow models of anisotropy and no simple globally valid relationships exist here we show that lattice preferred orientation lpo inferred from mantle flow computations produces a plausible global background model for asthenospheric anisotropy underneath oceanic lithosphere the same is not true for absolute plate motion apm models a 200 km thick layer where the flow model lpo matches observations from tomography lies just below the 1200 c isotherm of a half space cooling model indicating strong temperature dependence of the processes that control the development of azimuthal anisotropy we infer that the depth extent of shear and hence the thickness of a relatively strong oceanic lithosphere can be mapped this way these findings for the background model and ocean basin specific deviations from the half space cooling pattern are found in all of the three recent and independent tomographic models considered further exploration of deviations from the background model may be useful for general studies of oceanic plate formation and dynamics as well as regional scale tectonic analyses

relative hotspot motions versus true polar wander abstract the fixity of hotspots and mantle plume locations has long been axiomatic if the assumption of fixed hotspots is granted absolute plate motions and movements of the spin axis with respect to the hotspot framework defined by some as true polar wander tpw can be determined however this assumption can be tested by paleomagnetic data and such tests are gradually raising some doubts about the fixity of hotspots the result is that discrepancies between cretaceous and tertiary hotspot and paleomagnetic reference frames are now beginning to be interpreted as the result of plume drift within a convective mantle in the indo atlantic hotspots have remained relatively stationary with respect to the spin axis for the last 95 million yr however the pacific hotspots notably hawaii appear to have undergone large scale southward drift with respect to the spin axis during the early tertiary global paleomagnetic data do not indicate that any tpw occurred during the late cretaceous or tertiary although the early cretaceous paleomagnetic and hotspot frames for the indo atlantic realm can be interpreted as slow tpw direct estimates of paleolatitude and hotspot motion in particular the kerguelen hotspot challenge tpw as a global phenomenon at present we consider that the large early cretaceous discrepancy between hotspot and paleomagnetic data is best explained by southward drift of the atlantic hotspots prior to 95 ma

orbital changes and climate at the 41 000 period of orbital tilt summer insolation forces a lagged response in northern ice sheets this delayed ice signal is rapidly transferred to nearby northern oceans and landmasses by atmospheric dynamics these ice driven responses lead to late phased changes in atmospheric co2 that provide positive feedback to the ice sheets and also project late 41 k forcing across the tropics and the southern hemisphere responses in austral regions are also influenced by a fast response to summer insolation forcing at high southern latitudes at the 22 000 year precession period northern summer insolation again forces a lagged ice sheet response but with muted transfers to proximal regions and no subsequent effect on atmospheric co2 most 22 000 year greenhouse gas responses have the early phase of july insolation july forcing of monsoonal and boreal wetlands explains the early ch4 response the slightly later 22 k co2 response originates in the southern hemisphere the early 22 k ch4 and co2 responses add to insolation forcing of the ice sheets the dominant 100 000 year response of ice sheets is not externally forced nor does it result from internal resonance internal forcing appears to play at most a minor role the origin of this signal lies mainly in internal feedbacks co2 and ice albedo that drive the gradual build up of large ice sheets and then their rapid destruction ice melting during terminations is initiated by uniquely coincident forcing from insolation and greenhouse gases at the periods of tilt and precession

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

diffusion of cosmogenic3he in olivine and quartz implications for surface exposure dating abstract thein situ production of3he in surface rocks by cosmic ray induced nuclear reactions offers an important geochronological tool to evaluate helium loss problems in this technique cosmogenic3he diffusivities were measured in quartz and olivine by incremental heating at 150 600 c arrhenius temperature dependences were observed in both minerals with similar activation energies ea 25 4 kcal mole logd0 3 7 0 9 in olivine andea 25 2 0 9 logd0 0 2 0 4 in quartz and imply very low diffusivities when extrapolated to environmental temperatures less than 10 18 cm2 s in quartz and 10 22 cm2 s in olivine at 20 c these low diffusivities suggest helium loss will not significantly affect cosmogenic helium exposure dating for time scales on the order of 106 years in quartz and 109 years in olivine provided large 2 mm sample grains are used exposure ages obtained with smaller grains need to be corrected for diffusive helium loss and equations are provided for this purpose cosmogenic3he diffuses more than 100 times faster than trapped magmatic3he in olivine or radiogenic4he in quartz the mechanisms responsible for these differences have yet to be determined but may involve both trapping processes at mineral defects and enhanced mobility associated with spallation produced crystal damage therefore it is possible that helium diffusivities may depend on a sample s cosmic ray exposure or radiogenic production history whatever the precise mechanism the higher mobility of3he suggests that incremental heating will be useful in separating cosmogenic helium from magmatic or radiogenic helium in future studies comparison of the diffusivity results with total cosmogenic3he contents of different quartz size fractions for an antarctic quartzite suggests that he loss may have occurred more rapidly than predicted by the laboratory measurements nonetheless retention of cosmogenic3he is quite high equivalent to more than a million years of exposure and cosmogenic3he geochronology is feasible in this and other quartz containing rocks

a comparative study of accumulation rates derived by he and th isotope analysis of marine sediments abstract we present a detailed down core analysis of helium isotope ratios and concentrations for bulk sediments from the central equatorial pacific that span the last two glacial interglacial cycles measured 3 he 4 he ratios range from 1 0 10 5 to 2 1 10 4 or 7 4 to 149 times the atmospheric ratio the 3he from interplanetary dust particles idps constitutes virtually all of the 3he measured within the sediment because carbonate accumulation rates are high in the equatorial pacific the measured 3he concentrations are lower than have been measured elsewhere and range from 4 7 10 13 to 3 0 10 12 cm3stp g 1 if the cosmic dust 3he flux is constant with time sediment mass accumulation rates can be determined from the 3he concentration in sediments the excess 230th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in situ decay of 234u in seawater to first order initial excess 230th activities correlate with 3he concentrations within this core based on the 230th results we estimate the 3he flux to the earth s surface as 9 6 2 0 10 16 cm3stp cm 2 a 1 if this flux has remained constant over extended periods of time it can be used to determine sediment accumulation rates beyond the 230th range 300 000 yr

high 3 he 4 he ratio in ocean sediments two decades ago merrihue1 reported 3he 4he ratios of 10 4 in ferromagnetic separates from a pacific deep ocean red clay and concluded that the high ratio is due to extraterrestrial debris amounting to 1 of the sediment a decade later krylov et al 2 compiled 3he 4he isotopic data on ocean sediments measured in the soviet union and observed that the 3he 4he ratio is generally higher in pelagic sediments where the sedimentation rate is lower they suggested that the high 3he 4he ratio was attributable to extraterrestrial materials which were concentrated in slowly accumulating ocean floor however these important discoveries were almost completely neglected until we re examined the problem we have measured 39 sediments from 12 different sites 10 sites from the western to central pacific and two sites from the atlantic ocean we find 3he 4he ratios 5 10 5 for six sites well above the values generally observed in common terrestrial materials the very high 3he 4he ratio in the sediments is probably due to input of extraterrestrial materials input of stratospheric dust of 1 p p m which corresponds to a fallout rate of 2 000 tons per year can explain the observation

plug flow in the earth s asthenosphere abstract recent seismic observations focused on mantle flow below the pacific plate indicate the presence of two shear layers in the earth s asthenosphere this is difficult to explain under the classic assumption of asthenosphere flow driven by plate shear from above we present numerical mantle convection experiments that show how a power law rheology together with dynamic pressure gradients can generate an asthenosphere flow profile with a near constant velocity central region bounded above and below by concentrated shear layers a configuration referred to as plug flow the experiments show that as the power law dependence of asthenosphere viscosity is increased from 1 to 3 maximum asthenosphere velocities can surpass lithosphere velocity the wavelength of mantle convection increases and asthenosphere flow transitions from a linear profile couette flow to a plug flow configuration experiments in a 3d spherical domain also show a rotation of velocity vectors from the lithosphere to the asthenosphere consistent with seismic observations global mantle flow remains of whole mantle convection type with plate and asthenosphere flow away from a mid ocean ridge balanced by broader return flow in the lower mantle our results are in line with theoretical scalings that mapped the conditions under which asthenosphere flow can provide an added plate driving force as opposed to the more classic assumption that asthenosphere flow is associated with a plate resisting force

cosmogenic 26al in the atmosphere and the prospect of a 26al 10be chronometer to date old ice abstract cosmogenic radionuclides in the one million year half life range offer unique possibilities for age determinations in geophysics in measurements where the radioactive decay is being utilized as a clock uncertainties in age determinations may be reduced if the ratio of two radioisotopes with different half lives can be used as a chronometer in this work we investigate the atomic ratio of atmospheric 26al t1 2 0 717 ma to 10be t1 2 1 386 ma measured with accelerator mass spectrometry ams and its potential as a chronometer for dating old ice the 26al 10be ratio decreases with an effective half life of t1 2 26al 10be 1 49 ma for its application as a chronometer the atmospheric 26al 10be ratio has to be well characterized however the properties of atmospheric 26al have been understood only poorly so far at the vera ams facility of the university of vienna a first systematic study of the global variations of the 26al 10be ratio in the atmosphere and in surface firn has been carried out and pilot measurements of the 26al 10be ratio in deep antarctic ice have been performed our results indicate that this ratio is globally constant to within 5 in the atmosphere and in surface firn with a mean value of 1 89 10 3 the data also suggest that non atmospheric sources of 26al such as extraterrestrial in situ produced or re suspended 26al do not contribute significantly to the observed 26al 10be ratio in addition atmospheric mixing seems to exert only a minor influence in a first application of the method 26al 10be ratios were measured in chips collected in connection with the drilling of the lowest part of an ice core 2250 to 2760 m in dronning maud land antarctica surprisingly variable 26al 10be ratios ranging between 0 5 and up to 2 times the atmospheric ratio were found at different locations in this deep ice core while the cause for the ratios higher than atmospheric remains unexplained so far the ratios lower than atmospheric may be caused by radioactive decay allowing a first dating attempt using the 26al 10be ratio thus at an ice depth of 2760 m an approximate date of 6 7 2 6 105 years was established

terrigenous helium in deep sea sediments abstract we have measured the isotope ratios of helium contained in various terrigenous materials that contribute to deep sea sediments these materials include ice rafted debris from the north atlantic chinese loess and sediment collected at or near the mouths of three large rivers the amazon the ganges and the yangtze we observe terrigenous 3he 4he ratios that vary from 1 8 10 9 to 4 6 10 7 i e values that are higher than the theoretically derived range of 10 9 to 10 8 atlantic and pacific deep sea sediment 3he 4he ratios can be explained by mixing of helium from similar extraterrestrial but different terrigenous sources terrigenous sources for north atlantic and north pacific sediments are characterized by 3he and 4he contents that are higher and 3he 4he ratios that are lower than those for central and eastern equatorial pacific sediments this is consistent with the supply to the north atlantic and north pacific oceans of older cratonic continental material that contains high amounts of in situ produced nucleogenic and radiogenic helium terrigenous material transported to central and eastern equatorial pacific sediments contains lower amounts of 3he and 4he and higher 3he 4he ratios indicative of supply from a more juvenile andean source in the equatorial atlantic core v31 135 we have used previously published 230th data to determine an extraterrestrial 3he flux of 1 16 0 15 10 12 cm3stp cm 2 ka 1 within error of that previously determined in sediments from the equatorial pacific ocean 0 78 0 29 10 13 cm3stp cm 2 ka 1 marcantonio et al 1996

predicting trench and plate motion from the dynamics of a strong slab the motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle these models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation here using a recent compilation of a global data set we found that the motion of trenches either advancing or retreating with respect to upper plates scales with their corresponding subducting plates motion based on simple experimental tests we found that subduction of strong slabs inside the upper mantle correctly predicts these kinematic relationships we deduce that the motion of the trenches represent the surface manifestation of the resistance encountered by the subducting lithosphere to bend and penetrate within the upper mantle

cenozoic variations in the flux of interplanetary dust recorded by 3he in a deep sea sediment helium 3 concentrations and 3he 4he ratios in modern pelagic sediments are known to be far in excess of terrestrial values as a result of micrometeorite fallout here i report that extraterrestrial helium is easily detected in a pelagic clay core dating back more than 70 myr the remarkable preservation of the extraterrestrial signature arises from high retention of 3he within interplanetary dust particles coupled with loss of radiogenic 4he from terrestrial mineral grains the core provides a continuous record of the fallout of extraterrestrial helium for the cenozoic era this record suggests that there have been significant variations in the influx of interplanetary dust through time probably related to asteroidal breakup events or the passage of comets through the inner solar system the results also show 3he to be a far more sensitive tracer of the interplanetary dust flux than is iridium

chemical studies of h chondrites 8 on contemporary meteoroid streams using date and time of fall and petrographic classification as criteria many equilibrated h chondrites that fell during september and october from 1812 to the present form four significant clusters denoted as cluster 2 through cluster 5 on day year plots using radiochemical neutron activation analysis we determined 15 trace elements u au co sb ga rb ag se cs te zn cd bi tl and in ordered by increasing putative volatility during nebular condensation in 27 members of these four clusters we used model dependent and model independent multivariate statistical techniques to compare contents of the 10 most volatile elements separately in the four clusters with those of a 33 member suite of random h chondrite falls from 1773 to 1970 the clusters 2 and 5 suites that fell in september 1880 1991 and october 1919 1984 respectively each of which is represented by 10 h chondrite falls are not compositionally distinguishable from the suite of random falls however the 17 member combined suite of clusters 3 and 4 chondrites that fell during september october 1812 1992 proves compositionally distinguishable from random falls at moderate to strong significance levels of 0 01 0 001 this 17 member suite is less readily distinguished from random falls than are the previously reported suite of cluster 1 falls may 1855 1895 or antarctic h chondrites with nominal terrestrial ages 50 kyr each of which is highly significant at 0 001 levels all suites are genomict and exhibit a range of cosmic ray exposure ages with a plurality having 6 8 ma ages inconclusive results are obtained in the cases of clusters 2 and 5 however three h chondrite suites clusters 1 3 and 4 distinguishable from the random background by one property time of fall are also distinguishable by another contents of volatile trace elements or thermal history temporal change of h chondrite sources sampled by earth are indicated by these data

cenozoic latitudinal shift of the hawaiian hotspot and its implications for true polar wander abstract pacific plate equatorial sediment facies provide estimates of the northward motion of the pacific plate that are independent of paleomagnetic data and hotspot tracks analyses of equatorial sediment facies consistently indicate less northward motion than analyses of the dated volcanic edifices of the hawaiian emperor chain the discrepancy is largest 60 70 ma b p the 60 to 70 ma equatorial sediment facies data agree with recent paleomagnetic results from deep sea drilling on suiko seamount 1 and from a northern pacific piston core 2 equatorial sediment facies data and paleomagnetic data combined with k ar age dates along the emperor chain 3 indicate a position of the spin axis at 65 ma b p of 82 n 205 e in the reference frame in which the pacific ocean hotspots are fixed this pole agrees well with the position of the spin axis in the reference frame in which the atlantic ocean hotspots and the indian ocean hotspots are fixed 4 5 supporting the joint hypotheses that 1 the pacific ocean hotspots are fixed with respect to the hotspots in other oceans 2 the hotspots have shifted coherently with respect to the spin axis and 3 the time average of the earth s magnetic field 65 ma b p was an axial geocentric dipole global neogene paleomagnetic data suggest that a shift of the mantle relative to the spin axis has been occurring during the neogene in the same direction as the shift between 65 ma b p and the present all data are consistent with a model in which the hotspots and by inference the mantle have shifted with respect to the spin axis about a fixed euler pole at a constant rate of rotation for the last 65 ma

the potential of crinoids as u th sm he thermochronometers abstract u th sm he dating of crinoid fossils from late paleozoic rocks from a variety of sites in north america suggest that this material can be used as a useful low t thermochronometer under certain conditions available data suggest that common he is present in crinoids and at the low production rates typical of calcite this common he will be sufficient to produce spurious ages in crinoid samples of any age heating of samples to temperatures in excess of the opening temperature will expel all he common and radiogenic with all subsequent he accumulation being exclusively radiogenic such samples have potential for being robust thermochronometers sensitive to temperatures in the range of 60 110 c without common he analysts can take advantage of the large size of crinoid columnals and produce reliable ages even when young miocene or with low production rates eu 0 1 ppm samples not subjected to post depositional heating above the opening temperature burial to depths of 2 to 4 km depending on geothermal gradient sedimentation rate and residence time at peak t will likely retain common he and be unsuitable for thermochronologic studies

the late cenozoic uplift climate change paradox the geologic evidence for worldwide uplift of mountain ranges in the neogene is ambiguous estimates of paleoelevation vary according to whether they are based on the characteristics of fossil floras on the masses and grain sizes of eroded sediments or on calculations of increased thickness of the lithosphere as a result of faulting detrital erosion rates can be increased both by increased relief in the drainage basin and by a change to more seasonal rainfall patterns the geologic record provides no clear answer to the question whether uplift caused the climatic deterioration of the neogene or whether the changing climate affected the erosion system in such a way as to create an illusion of uplift we suggest that the spread of c4 plants in the late miocene may have altered both the erosion and climate systems these changes are responsible for the apparent contradictions between data supporting uplift and those supporting high elevations in the past

olivine settling in cosmic spherules during atmospheric deceleration an indicator of the orbital eccentricity of interplanetary dust a new type of cosmic spherule is reported with textures suggesting settling of olivine during atmospheric deceleration numerical simulations of entry heating reveal that relict forsterite which survives melting can settle over the 1 2 s of flight at high entry angles and entry velocities up to 16 km s 1 enhanced crystallisation of phenocrysts by heterogeneous nucleation on accumulated relict forsterites is the most likely origin of the observed cumulate textures in cosmic spherules such textures in cosmic spherules reveal interplanetary dust with higher encounter velocity with the earth that correspond to orbital eccentricities 0 3 the relative abundance of cumulate spherules suggests 14 of ordinary chondrite related s iv type asteroid dust over the last 800 kyr had relatively high orbital eccentricity owing to secular and planetary perturbations the textures of cosmic spherules collected from sediments can therefore be used to trace dust orbital variations with time which may influence terrestrial climate

pacific plate apparent polar wander hot spot fixity and true polar wander during the formation of the hawaiian island and seamount chain from an analysis of the skewness of magnetic anomaly 20r 44 ma

temporal variation of 3he 4he ratio recorded in deep sea sediment cores he isotopic composition was measured for 72 deep sea sediments from 16 sites in the pacific and atlantic oceans and the japan sea 3he content ranges from less than 10 12 cm3 stp g to about 6 10 11 cm3 stp g and 3he 4he from about 10 8 to 1 67 10 4 the he isotopic data lie fairly well on a mixing curve in a 3he 4he 3he diagram where the end members of the mixing curve are extraterrestrial he in interplanetary dust particles idp s and terrestrial he in ocean sediments this confirms the view that there are significant amounts of extraterrestrial he in ocean sediments from global fallout of idp s into deep sea sediments 3he flux estimated for two long cores gives a value of about 1 5 1 0 10 15 cm3 stp cm2 yr over a time span of 40 m y which corresponds to an annual fallout rate of idp s size 10 m of about 400 t in one of the cores we observe a 3he spike which is associated with anomalously high ir content 1 86 0 56 ppb this may be due to meteorite debris

an alternative age model for the paleocene eocene thermal maximum using extraterrestrial 3he a continuous age model for the brief climate excursion at the paleocene eocene boundary has been constructed by assuming a constant flux of extraterrestrial 3he 3he et to the seafloor 3he et measurements from odp site 690 provide quantitative evidence for the rapid onset few kyr and short duration 120 kyr of global warming and of the associated disturbance to the earth s surficial carbon budget at this time these observations support astronomically calibrated age models indicating extremely rapid release of isotopically light carbon possibly from seafloor methane hydrate as the proximal cause of the event however the 3he et technique indicates a previously unrecognized and extreme increase in sedimentation rate coincident with the return of climate proxies to pre event values the 3he et based age model thus suggests a far more rapid recovery from the climatic perturbation than previously proposed or predicted on the basis of the modern carbon cycle and so may indicate additional or accelerated mechanisms of carbon removal from the ocean atmosphere system during this period 3he et was also measured at odp site 1051 to test the validity of the site 690 chronology comparison of these data sets seems to require removal of several tens of kyr of sediment within the climatic excursion at site 1051 an observation consistent with sediment structures and previous age modeling efforts the site 1051 age model shows a 30 kyr period in which climate proxies return toward pre event values after which they remain invariant for 80 kyr if this rise represents the recovery interval identified at site 690 then the 3he et based age models of the two sites are in good agreement however alternative interpretations are possible and work on less disrupted sites is required to evaluate the reliability of the proposed new chronology of the climate excursion regardless of these details this work shows that the 3he et technique can provide useful independent evidence for the development and testing of astronomically calibrated age models

laser extraction of helium isotopes from antarctic micrometeorites source of he and implications for the flux of extraterrestrial 3he to earth helium isotopes have been measured in 45 putative micrometeorites in the size range 50 to 400 m that were extracted from antarctic ice forty of the antarctic micrometeorites amms are irregular in shape and display varying degrees of partial melting whereas five are spherical and thought to represent completely melted particles of the 30 50 to 100 m suspected micrometeorites only 8 released neither 3he or 4he sixteen released 3he and 4he 0 004 3 5 10 3 cm3 stp g having 3he 4he in the range 1 5 10 4 to 4 1 10 4 six particles released only 3he yielding minimum 3he 4he of 1 6 to 10 6 10 4 5 of which fall in the range of values for amms that had measurable 3he and 4he only 2 of the 15 100 to 400 m amms released measurable 4he 4 44 10 7 cc g 1 these have 3he 4he of 6 10 4 and 20 10 4 respectively significantly higher than the majority of the ratios of 50 to 100 m amms implanted solar energetic particles are the dominant source of he in the 50 to 100 m amms with small contributions of solar wind and or spallogenic he present in 10 particles the absence of sw he is likely due to the loss of the outermost layers of the micrometeorites by ablation during atmospheric entry and abrasion during residence in ice and sampling spallation he is most important in the 100 to 400 m amms consistent with their smaller surface area to volume ratio maximum spallation 3he exposure ages of six particles are close to that predicted from poynting robertson spiral in times of asteroidal micrometeorites the two terrestrial particles identified by chemical composition and mineralogy released only 4he yielding radiogenic he isotope compositions an inverse relationship between particle sizes and solar he abundance is explained by he loss during entry helium loss is likely controlled by phyllosilicate breakdown where present and diffusion controlled bubble growth and rupture rather than melting the 50 to 100 m micrometeorites deliver 8 10 8 cm3 stp 3he g to earth this is more than 100 times less than transported by 20 m particles 2 10 5 cm3 stp 3he g but because 50 to 100 m micrometeorites deliver approximately 20 times more extraterrestrial matter to earth than 20 m particles the larger particles may contribute 5 of the total flux of extraterrestrial 3he to terrestrial sediments the mean he concentrations of the 50 to 100 m amms antarctic micrometeorites and the five spherules 2 1 10 9 cm3 stp 3he g 1 overlap those of the magnetic fraction of deep sea sediments 0 8 68 10 9 cm3 stp g a combination of this material could provide the extraterrestrial 3he measured in ancient deep sea sediments without calling upon the survival of friable 20 m chondritic interplanetary dust particles for tens of millions of years

an assessment of the meteoritic contribution to the martian soil the addition of meteoritic material to the mars soils should perturb their chemical compositions as has been detected for soils on the moon anders et al 1973 and sediments on earth kyte and wasson 1986 using the measured mass influx at earth and estimates of the mars earth flux ratio we estimate the continuous planet wide meteoritic mass influx on mars to be between 2700 and 59 000 t yr if distributed uniformly into a soil with a mean planetary production rate of 1 m b y consistent with radar estimates of the soil depth overlaying a bouldered terrain in the tharsis region christensen 1986 our estimated mass influx would produce a meteoritic concentration in the mars soil ranging from 2 to 29 by mass analysis of the viking x ray fluorescence data indicates that the mars soil composition is inconsistent with typical basaltic rock fragments but can be fit by a mixture of 60 basaltic rock fragments and 40 meteoritic material clark and baird 1979 the meteoritic influx we calculate is sufficient to provide most or all of the material required by the clark and baird 1979 model particles in the mass range from 10 7 to 10 3 g about 60 200 m in diameter contribute 80 of the total mass flux of meteoritic material in the 10 13 to 106 g mass range at earth hughes 1978 on earth atmospheric entry all but the smallest particles generally 50 m in diameter in the 10 7 to 10 3 g mass range are heated sufficiently to melt or vaporize mars because of its lower escape velocity and larger atmospheric scale height is a much more favorable site for unmelted survival of micrometeorites on atmospheric deceleration we calculate that a significant fraction of particles throughout the 60 1200 m diameter range will survive mars atmospheric entry unmelted thus returned mars soils may offer a resource for sampling micrometeorites in a size range which is not collectable in unaltered form at earth

accretion of interplanetary dust in polar ice measurements of helium isotopes in particles separated from polar ice demonstrate that extraterrestrial he dominates the he flux at the gisp2 greenland and vostok antarctica ice core sites replicate measurements of late holocene ice samples yield he fluxes of 0 62 0 27 10 12 cm stp cm 2 ka 1 gisp2 and 0 77 0 25 10 12 cm stp cm 2 ka 1 vostok similar to results from marine sediments these are the first detailed measurements of he in particles from ice core samples and they demonstrate the utility of the ice core record for evaluating the temporal history of the extraterrestrial dust flux results from vostok samples from 1096 1403 m depth 75 97 ka b p are similar to the late holocene data with the exception of two highly anomalous results from 1307 m the latter probably indicate the presence of rare large or gas rich extraterrestrial particles

does interplanetary dust control 100 kyr glacial cycles the cause of the 100 kyr glacial interglacial cycles during the past 800 kyr is one of the fundamental puzzles in paleoclimatology the widely accepted milankovitch theory relating earth s climate cycles to variations in insolation caused by periodic changes in orbital parameters has difficulties to explain the predominant 100 kyr rhythm although earth s eccentricity varies with a period of 100 kyr the resulting change in insolation is too small to produce the corresponding climate cycle by direct forcing imbrie et al paleoceanography 8 1993 699 in order to solve the 100 kyr problem muller and macdonald nature 377 1995 107 science 277 1997a 215 proc nat acad sci usa 94 1997b 8329 proposed an alternative orbital but non milankovitch mechanism attributing the glacial cycles to regular variations in the accretion of interplanetary dust particles idp caused by 100 kyr cycles in the orbital inclination of the earth to test this controversial hypothesis we study the idp accumulation in deep sea sediments from a period in the early pleistocene we find apparent 41 kyr cycles but no 100 kyr periodicity in the idp accumulation rate as there is no known mechanism to produce 41 kyr cycles in idp supply from space we conclude that the 41 kyr cycles are caused by the dynamics of sediment accumulation and that changes in the idp flux do not drive the pleistocene glacial cycles r 2004 elsevier ltd all rights reserved

revised and synthetic apparent polar wander paths of the african eurasian north american and indian plates and true polar wander since 200 ma we have reviewed paleomagnetic data available for the eurasian african north american and indian plates over the last 200 ma selection criteria are those generally accepted with an emphasis on evidence for lack of remagnetization accurate dating and proper structural analysis this results in 23 35 51 and 2 poles for eurasia africa north america and india respectively we believe that this limited set of higher quality data is more likely to reveal key features of apparent polar wander apw paths than averaging of larger data sets involving less stringent selection criteria we propose and describe revised apw paths but more importantly we next use relative motion models to transfer all data in a common reference frame we find good agreement between transferred data when they are averaged separately for each plate in independent 20 ma windows this is a check on consistency of paleomagnetic data kinematic models and of the geocentric dipole hypothesis transferred data from all plates are averaged in 20 ma windows to generate synthetic apw paths for all plates studied these synthetic paths are in agreement with the original revised apw paths that use only data from a single plate moreover both geographic and time resolution are improved spatial confidence intervals are of the order of 5 the synthetic paths display interesting features such as a previously ill recognized apw loop for eurasia paleomagnetic and hotspot apw are next compared and a determination of true polar wander tpw is derived we find significant tpw amounting to over 20 in the last 200 ma tpw appears to be episodic with a standstill between 180 and 110 ma there is general agreement with a previous study of livermore et al 1984 the tpw standstill appears to correlate with a time of decreasing reversal frequency ending with the cretaceous long normal superchron other periods of fast tpw would seem to correspond to increasing reversal frequency however it is suggested that the major tpw hairpin at 50 ma might correspond to the collision of india with eurasia tentative correlations with core or mantle indicators can be understood as a result of couplings between the core mantle and lithosphere we believe primarily related to episodic evolution of the d layer

a fixed sublithospheric source for the late neogene track of the yellowstone hotspot implications of the heise and picabo volcanic fields the heise and picabo volcanic fields of eastern idaho are part of the more extensive time transgressive yellowstone snake river plain hotspot track calderas associated with these two silicic volcanic fields are buried under 1 to 3 km of younger basalt so their locations and eruption record histories have been based on analysis of silicic units along the margins of the eastern snake river plain along with some limited geophysical data a 1 5 km borehole penetrating through basalt into underlying silicic rocks provides new data we used to reassess caldera locations and the timing of eruptions of these volcanic fields using these new caldera locations we calculate an extension adjusted rate of 2 35 cm yr for the north american plate over the last 6 66 m y and a velocity of 2 30 cm yr over the 10 27 m y recalculation of a previously determined plate velocity based migration of the deformation field surrounding the eastern snake river plain yields an extension adjusted rate of 2 38 0 21 cm yr these migration rates all fall within the previously published range of north american plate velocities of 2 2 0 8 cm yr 2 4 cm yr and 2 68 0 78 cm yr based on a global hot spot reference frame the consistency of these rates suggest that over the last 10 m y the yellowstone hot spot is fixed with respect to the motion of the north american plate and therefore consistent with a classical deep sourced hotspot model

geochronology of some canarian dike swarms contribution to the volcano tectonic evolution of the archipelago we present new conventional k ar and 39ar 40ar age data and geometric observations on dike swarms which intrude the islands of la gomera tenerife and fuerteventura canary archipelago the younger dated dikes of la gomera 5 4 5 2 ma were emplaced by a radial fissural structure independently superimposed on an older one dated between 10 5 and 7 0 ma the dated dike swarms of both the anaga and teno peninsula of tenerife are contemporaneous 5 8 3 6 ma while ages of 0 43 ma were measured on two dikes intruding the pedro gil ridge the dikes of the basal complex of fuerteventura give well defined 39ar 40ar plateaus when they are situated far away from basic and ultrabasic plutons and disturbed 39ar 40ar age spectra when they are situated in the vicinity of the plutons these disturbances are probably due to a thermal metamorphism the results of both the conventional k ar and 39ar 40ar stepwise heating methods show the existence of an important magmatic phase between 24 and 17 ma the dike swarms of the basal complex are partly contemporaneous with other dike systems intruding the basaltic series i of fuster et al 1968a this phase seems to correspond to the final stages of activity on the main nne ssw volcano tectonic axis following this a volcanic edifice was built on the nw part of jandia peninsula mainly between 15 4 and 14 2 ma the similarity of the ages obtained on dikes and lava flows shows the importance of fissural volcanism in these islands this study shows that dating of dike swarms reveals the existence of major eruptive phases an important part of the products of which have been eroded brief episodes of fissural volcanic activity sometimes probably as short as 200 000 years may be correlated with the migration of the lithosphere over a fixed hot spot the north south 3 5 ma old present and the nne ssw 24 17 ma old oriented dike swarms of la palma staudigel et al in prep and fuerteventura may represent the regional paleostress orientation plane 1 2 which affected the lithosphere during their intrusion although other phenomena may control the orientation of dikes these directions are concordant with the known contemporaneous nw african tectonics

high resolution estimates of nubia somalia plate motion since 20 ma from reconstructions of the southwest indian ridge red sea and gulf of aden

the influence of wind stress and river runoff on a shelf sea front abstract runoff from land gives rise to the formation of a front jet system separating fresher coastal waters from more saline waters offshore over some continental shelves notably in the mid atlantic bight in winter three first order characteristics of such fronts are the anchor depth h where the front intersects the bottom the stretch l i e the distance between its surface and bottom intersections and the buoyancy contrast b across the front the three characteristics may be determined from the cross front property transfer rate the along front momentum balance and the turbulence energy dissipation rate key independent variables are the horizontal buoyancy transport bx the friction velocity u and direction of the wind and the coriolis parameters f simple approximate formulae connecting dependent to independent variables arewhere m and are constants of order 300 1 and 0 1 respectively the entrainment rate across the front is u2 f 1 cos which is small compared to ekman transpo

the accretion rate of extraterrestrial 3he based on oceanic 230th flux and the relation to os isotope variation over the past 200 000 years in an indian ocean core abstract in the eastern equatorial indian ocean the flux of extraterrestrial 3 he a proxy of interplanetary dust particles idps has been relatively constant over the past 200 ka the flux is equal to 1 1 0 4 10 12 cm 3 stp cm 2 ka 1 a value obtained using the xs 230 th profiling method variations in mass accumulation rates mars derived assuming a constant extraterrestrial 3 he flux have a 40 ka periodicity similar to that observed in the 18 o derived mars this frequency is similar to that of the earth s obliquity measured 187 os 188 os ratios are less radiogenic than present day seawater 0 49 0 98 reflecting the mixing of os derived from extraterrestrial terrigenous and hydrogenous sources when coupled with he data measured on the same samples os isotope data yield important information about the terrigenous component supplied to the eastern equatorial indian ocean the amount of os in the sample derived from the extraterrestrial component can be deduced with the help of the helium systematics once corrected for the extraterrestrial component of os os isotope signatures in conjunction with the 4 he concentrations suggest a supply of terrigenous material from indonesian ultramafic and himalayan crustal sources that clearly varies through time

a comparison of multiple proxies for export production in the equatorial pacific 1 several commonly used paleoproductivity proxies were compared to evaluate the validity of assumptions and limitations associated with each proxy export production fluxes cexport over glacial interglacial timescales were calculated from previously developed proxy specific algorithms at ttn013 pc72 and ttn013 pc114 both located in the equatorial pacific comparison of data from the same core intervals yields conflicting results despite calibrations based on the same core top samples the periodicity of marine barite excess ba and excess al based cexport records is similar the relative magnitude of cexport when calculated using excess ba and algorithms based on sediment trap data is significantly different than records based on core top calibrations particularly during glacial intervals at both sites bulk sedimentary al ti and ba ti ratios covary however these ratios do not correspond with the downcore records of their respective excess concentrations and their accumulation rates or with contemporaneous records based on marine barite excess ba and excess al accumulation although downcore records based on sediment mass accumulation rates may be compromised by sediment focusing this process cannot explain all the differences observed among the various data sets presented here this implies that some or all of these proxies do not exclusively respond to changes in export production the contradictions among these data highlight the importance of addressing inconsistencies among paleoproxies and re examining assumptions imbedded in proxy fundamentals prior to applying paleoproductivity proxies and interpreting paleoceanographic records

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

climate driven glacial interglacial variations in the osmium isotope composition of seawater recorded by planktic foraminifera abstract this study presents osmium os isotope and elemental data for cleaned planktic foraminifera authigenic fe mn oxyhydroxides and pelagic carbonate host sediments from odp site 758 in the southernmost reaches of the bay of bengal the os in the bulk sediments appears to be dominantly hydrogeneous sourced by carbonate and fe mn oxyhydroxide but variations in this particular core are controlled by the presence of volcanic ash fe mn oxyhydroxide leachates of the bulk sediments from holocene samples also yield an os isotope composition close to that of seawater but the record diverges from that of foraminifera at a depth corresponding to the oxic post oxic boundary suggesting diagenetic mobilization of os at depths below this holocene planktic foraminifera cleaned using oxidative reductive techniques also give os isotope compositions indistinguishable from modern seawater but the record obtained for the past 150 kyr shows strong covaraitions of 187 os 188 os with both the local and global oxygen isotope record with less radiogenic os isotope compositions during glacial intervals these results indicate that foraminifera provide a robust record of seawater os isotope compositions and comparison of the data obtained here with records from the other major oceans demonstrate global changes in 187 os 188 os over this time interval while the covariation with oxygen isotopes suggest a process controlling the os isotope composition that is in phase with global climate cycles global excursions to relatively unradiogenic 187 os 188 os during glacial intervals are consistent with decreased input of radiogenic continental material reflecting cooler temperatures and reduced continental runoff modelling indicates that the shift to unradiogenic values during glacial intervals could be caused by an 30 decrease in the global river flux with an 5 change in river composition if the residence time of os in the oceans is 5 ka then the post glacial recovery to present day seawater values is consistent with a corresponding increase in the river flux of around 30 however if the residence time of os is closer to 40 ka as is suggested by the global river flux then this demands either significant changes in both the riverine os flux and composition of around 40 and 30 respectively that closely follow the oxygen isotope record or else a short lived post glacial pulse of weathering some 75 greater than the steady state flux in either case these results clearly indicate that climatic changes affect both the flux and composition of weathered material delivered to the oceans on glacial interglacial timescales

noble gas compositions of antarctic micrometeorites collected at the dome fuji station in 1996 and 1997 the noble gases he ne ar kr and xe were measured in 27 individual antarctic micrometeorites amms in the size range 60 to 250 m that were collected at the dome fuji station eleven of the amms were collected in 1996 f96 series and 16 were collected in 1997 f97 series one of the f97 amms is a totally melted spherule whereas all other particles are irregular in shape noble gases were extracted using a nd yag continuous wave laser with an output power of 2 5 3 5 w for 5 min most particles released measurable amounts of noble gases 3he 4he ratios are determined for 26 amms 0 85 9 65 10 4 solar energetic particles sep are the dominant source of helium in most amms rather than solar wind sw and cosmogenic he three samples had higher 3he 4he ratios compared to that of sw showing the presence of spallogenic 3he the ne isotopic composition of most amms resembled that of sep as in the case of helium spallogenic 21ne was detected in three samples two of which had extremely long cosmic ray exposure ages 100 ma calculated by assuming solar cosmic ray scr galactic cosmic ray gcr production these two particles may have come to earth directly from the kuiper belt most amms had negligible amounts of cosmogenic 21 ne and exposure ages of 1 ma 40ar 36ar ratios for all particles 3 9 289 were lower than that of the terrestrial atmosphere 296 indicating an extraterrestrial origin of part of the ar with a very low 40ar 36ar ratio plus some atmospheric contamination indeed 40ar 36ar ratios for the amms are higher than sw sep and q ar values which is explained by the presence of atmospheric 40ar the average 38ar 36ar ratio of 24 amms 0 194 is slightly higher than the value of atmospheric or q ar suggesting the presence of sep ar which has a relatively high 38ar 36ar ratio according to the elemental compositions of the heavy noble gases dome fuji amms can be classified into three groups chondritic eight particles air affected nine particles and solar affected eight particles the eight amms classified as chondritic preserve the heavy noble gas composition of primordial trapped component due to lack of atmospheric adsorption and solar implantation the average of 129xe 132xe ratio for the 16 amms not affected by atmospheric contamination 1 05 corresponds to the values in matrices of carbonaceous chondrites 1 04 one amm f96dk038 has high 129xe 132xe in excess of this ratio our results imply that most dome fuji amms originally had chondritic heavy noble gas compositions and carbonaceous chondrite like objects are appropriate candidate sources for most amms

accretion rate of extraterrestrial matter iridium deposited 33 to 67 million years ago iridium measured in 149 samples of a continuous 9 meter section of pacific abyssal clay covering the time span 33 to 67 million years ago shows a well defined peak only at the cretaceous tertiary boundary in the rest of the section iridium ranges from a minimum concentration near 0 35 nanograms per gram in the paleocene to a maximum near 1 7 in the eocene between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years correction for terrestrial iridium leads to an extraterrestrial flux of9 3 nanograms of iridium per square centimeter per million years and an estimated annual global influx of 78 billion grams of chondritic matter consistent with recent estimates of the influx of dust meteorites and crater producing bodies with masses ranging from 10 13 to 1018 grams combining the recent flux of objects ranging in mass from 106 to 107 grams with the flux of 1014 to 1015 gram objects indicates that the number of objects is equal to 0 54 divided by the radius in kilometers to the 2 1 power periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years the predicted iridium maxima more than 30 times background are not present in the intervals associated with the cretaceous tertiary boundary or the tektiteproducing late eocene events

change in motion of pacific plate at 5 myr bp alignments of volcanoes such as the hawaiian seamount chain record the passage of oceanic plates over mantle plumes or hot spots and changes in the orientation of a seamount chain may indicate changes in the direction of motion of the plate within the hotspot frame of reference1 this hypothesis can be tested for the pacific plate by comparing data from the hawaiian volcanoes with tectonic movements inferred from the geology of adjacent plates we have now found that the motion of the pacific plate changed to a more northerly direction 5 myr ago probably as a result of the detachment of a piece of pacific sea floor which had been subducted beneath the fiji plateau during the tertiary

paleomagnetic evidence for motion of the hawaiian hotspot during formation of the emperor seamounts the bend in the hawaiian emperor chain is the best example of a change in plate motion recorded in a fixed hotspot frame of reference alternatively the bend might record primarily differences in motion of the hawaiian hotspot relative to the pacific lithosphere new paleomagnetic data from the emperor chain support the latter view althouth the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain the best available paleomagnetic data suggest pacific hotspots may have moved at rates comparable to those of lithospheric plates 30 mm yr 1 in late cretaceous to early tertiary times 81 43 ma if correct this requires a major change in how we view mantle dynamics and the history of plate motions in the early to mid cretaceous 128 95 ma hotspots in the atlantic moved at similar rates these episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion such as the past 5 m y

extraterrestrial accretion from the gisp2 ice core the rate of extraterrestrial accretion for particles in the size range 0 45 m to 20 m was determined from dust concentrates extracted from greenland ice sheet project 2 gisp2 ice core samples using instrumental neutron activation analysis inaa we determined the iridium ir content of the dust following a core specific correction for terrestrial ir and assuming a chondritic ir abundance of 500 ppb we measure an average accretion rate for 0 45 m to 20 m particles over the entire earth of 0 22 0 11 109 g yr kton yr for 317 years of ice through the interval 6 to 20 ka this is consistent with the interplanetary dust accretion rate of 0 17 0 08 x 109 g yr that we derive from published 3he data for the gisp2 core accounting for particles that are larger and smaller than those detected by or experiment our best estimate of the total accretion rate including particle sizes up to about 4 cm in diameter is 2 5 109 g yr the uncertainty in this estimate is dominated by statistical fluctuations in the number of particles expected to end up in the ice core and not by measurement error based on monte carlo simulations we estimate the upper limit for total extraterrestrial accretion to earth of 6 25 109 g yr 95 confidence level this accretion rate is consistent with some estimates from micrometeorite concentrations in polar ice estimates from ground based radar studies and with accretion estimates of 3he bearing interplanetary dust particles assuming that 3he is correlated with particle surface area it is however lower than estimates based on platinum group element studies of marine sediments the conflict may indicate systematic errors with either the marine or the non marine samples departures from the assumed particle spectrum of grun and coauthors or time variable accretion rates with the early holocene period being characterized by low levels of accretion

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

paleolatitude of the hawaiian hot spot since 48 ma evidence for a mid cenozoic true polar stillstand followed by late cenozoic true polar wander coincident with northern hemisphere glaciation

historical and recent evidence of scotian shelf water on southern georges bank abstract historical data from 1912 1987 indicate that low salinity less than 32 0 psu near surface water occurs occasionally on southern georges bank during may while none occurs during april composite monthly plots of historical near surface salinity and estimated advection rates show that the southwestern scotian shelf is the immediate upstream source of this low salinity water optimally interpolated satellite derived sst and hydrographic data reveal that very cold less than 2 0 c low salinity less than 32 0 psu scotian shelf water ssw initially located south of the 200 m isobath off southern georges bank in early march 1992 moved north onto southern georges bank during april 1992 ssw was not as extensive during the same period in 1993 as evidence by significantly higher temperatures and salinities these differences show large interannual variability in the transport and or properties of ssw flowing onto georges bank lower higher salinities measured during spring 1992 1993 on southern georges bank are consistent with higher lower st lawrence river discharge noted during spring 1991 1992 and the nine month lag between annual discharge maxima from rivers located upstream and minimum salinity at cape sable in february however comparisons between historical occurrences of low salinity less than 32 0 psu ssw on southern georges bank noted for may 1966 1971 and 1978 and cumulative st lawrence river discharge from the spring prior to each occurrence show no relationship this suggests that the occurrence of low salinity water on southern georges bank is not directly related to variations in upstream river discharge although an idealized model of wind forcing on a bank trapped density front shows that near surface flow is related to superposition of an ekman layer and the along bank gyral circulation evidence from late winter spring 1992 and 1993 on georges bank shows that except for on bank movement of the shelf slope front during late spring 1992 simple ekman theory does not explain on bank movement of the ssw plume during 1992 nor the immobile nature of the shelf slope front during late winter 1992 and late winter spring 1993 surface heat flux and satellite derived sst from georges bank and the scotian shelf together with a slab mixed layer model allows estimation of the advective heat flux and transport of ssw flowing onto georges bank from the scotian shelf during late winter spring 1992 mean advective heat flux onto georges bank during march may 1992 is 14 5 mw cm 2 with an estimated error of 3 5 mw cm 2 demonstrating that advection of cold ssw onto georges bank is required mean velocity and transport of ssw flowing onto georges bank during march may 1992 are 13 4 cm s 1 and 0 21 sv assuming a ssw depth width of 40 m 40 km demonstrating that flow of ssw onto georges bank was robust during the period at this time the dynamics that control the flow of ssw across northeast channel are unknown

variations in the osmium isotope composition of sea water over the past 200 000 years the first osmium isotope data are presented for marine sediments spanning the past 200 000 years two full glacial cycles and establish that there have been ocean wide variations in sea water osmium isotope composition over this time while the osmium isotope composition of sea water has been relatively invariant over much of this interval 187os 186os of 8 5 to 8 6 excursions to lower isotope ratios 187os 186os 8 1 occur late in the last two glacial periods at 20 ka and 160 ka these are times of high global aridity low global temperature and maximum ice extent while various interpretations are possible a likely explanation is that the low sea water ratios reflect reduced chemical weathering of the continents during times of extreme cold and aridity there is no evidence that there is a net enhancement of chemical weathering during periods of continental glaciation the response of ocean composition to changes in input flux constrain the oceanic residence time of osmium to be less than 12 000 years

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr