tidally generated turbulence over the knight inlet sill very high turbulent dissipation rates above 5 1024 wk g 21 were observed in the nonlinear internal lee waves that form each tide over a sill in knight inlet british columbia this turbulence was due to both shear instabilities and the jumplike adjustment of the wave to background flow conditions away from the sill turbulent dissipation was significantly lower 5 1027 to 5 1028 wk g 21 energy removed from the barotropic tide was estimated using a pair of tide gauges a peak of 20 mw occurred during spring tide approximately twothirds of the barotropic energy loss radiated away as internal waves while the remaining one third was lost to processes near the sill observed dissipation in the water column does not account for the near sill losses but energy lost to vortex shedding and near bottom turbulence though not measured could be large enough to close the energy budget

topographic amplitude dependence of internal wave generation by tidal forcing over idealized three dimensional topography 1 this numerical study using the princeton ocean model investigates the conversion of energy from an eastward propagating surface tide to the internal wave field as an oscillating tidal current flows over and around idealized three dimensional gaussian seamounts in particular the relationship between the energy flux in the internal wave field and the height of the topography is explored using a range of horizontal aspect ratios in agreement with theory the energy flux is found to increase as the square of the topographic amplitude ho for subcritical topography for supercritical topographies the functional relationship is more complex aspect ratio dependent it increases more rapidly than ho2 for near critical topography for larger circular seamounts the energy flux increases much more slowly with the rate of increase dropping with amplitude as the seamounts are elongated in the cross tidal wave north south direction the energy flux increases more rapidly with height and for a two dimensional ridge increases as approximately ho2 2 the directional dependence of the energy flux also depends on the aspect ratio and amplitude of the seamount for circular seamounts energy flux has its maximum in directions between 20 and 45 counterclockwise from the east west axis northern hemisphere for elongated seamounts the energy flux is directed in the east west direction

deep sea fluid and sediment dynamics influence of hill to seamount scale seafloor topography abstract deep sea sediments play a central role in a wide range of subject areas a number of important controls on the formation of sedimentary deposits have been studied however to date the impact of submarine landscape geometry as a possible control has received comparatively little attention this seems to be particularly true for intermediate scale topographic features such as abyssal hills knolls and seamounts that can be found in many regions of the global seafloor recent estimates suggest that in the deep open oceans away from continental margins there might be as many as 25 106 abyssal hills knolls and seamounts despite this large number very little is known about how they influence environmental complexity and patchiness biogeochemical fluxes and the formation of sedimentary records this paper reviews the currently known types of fluid flow interactions with abyssal hills knolls and seamounts that could potentially influence the way sediments are formed the main types of relevant flow components are quasi steady to eddying background flow internal lee and near inertial waves barotropic and baroclinic tides and seamount trapped waves previous studies looking into systematic links between fluid dynamics and sediments at hills knolls and seamounts are reviewed finally a case study is presented which aims to combine our current knowledge and investigate whether a given combination of recent fluid flow components leaves a detectable imprint in the recent sediments on and around a short seamount the main conclusions and implications are as follows 1 topographically generated flow field geometries that are composed of a number of different prevailing fluid flow components can be reflected and detected in properties of the underlying sediments 2 tidal and other higher frequency lee wave near inertial components of deep ocean currents can be essential for locally driving total current velocities across threshold values for non deposition erosion resuspension of freshly deposited deep sea sediments moreover there is evidence suggesting that not only maximum current speeds but also intensities of higher frequency tidal and or near inertial current direction variability might control sediment dynamics and sediment formation this relativises the view that current speed is the main or even only controlling factor for sediment dynamics and sediment formation 3 when it comes to the reconstruction of paleo flows these findings imply that certain sedimentary records may well reveal more about variability in the higher frequency flow components than about variability in the basin scale net flow component that often is the focus of paleoceanographic studies 4 single core paleo records from hill seamount or similarly controlled sediment deposits may be biased due to the asymmetry of flow fields around these topographic features to arrive at unbiased paleo records for non fluid dynamic parameters the influence of the flow field geometry would have to be removed from the record first 5 it seems the mechanistic understanding of hill and seamount related flow topography interactions and their links to sediment dynamics is approaching a level that may a facilitate improved interpretation of topographically controlled sedimentary paleo records b help fill in the knowledge gap that exists for functional deep sea biodiversity at intermediate space scales and c improve predictive capabilities for exploration of economically relevant iron manganese fe mn crusts on seamounts

rising social complexity agricultural intensification and the earliest rice paddies on the loess plateau of northern china geoarchaeological studies of landscapes immediately adjacent to archaeological sites can contribute information on the direct impact of small scale societies on their associated landscapes this direct connection allows us to understand aspects of the motivations economic decision making and agricultural strategies and how they affected local site catchments the origin and spreading of farming communities onto the loess plateau of northern china provides a good example of this we recorded sediment profiles that were immediately adjacent to the site of huizui in the yiluo river basin here we identified evidence for human land use beginning with the early mid holocene deposits which are consistent with stable hillslope soils indicating that the first mixed forager millet farmers of the peiligang neolithic had a very light ecological footprint on the landscape this is in contrast to the later middle neolithic yangshao period farmers sediments from the yangshao period revealed paleolandscape and phytolith evidence for the earliest neolithic paddy farming well outside of the natural habitat of wild rice in addition to evidence for massive deforestation and soil erosion a 15 m deep sediment sequence containing sets of gravels beginning ca 7200 cal bp and gleyed soils dating from ca 6600 cal bp contained rice phytoliths archaeological waste suggesting manuring and micromorphological data indicating trampling these signs of intensive landscape management go hand in hand with rapidly increasing social complexity

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

experiments on the generation of internal waves over continental shelf topography experiments were performed to examine the generation of internal waves by a barotropic tide forcing a continuously stratified fluid over idealized continental shelf slope topography a range of responses was observed including the generation of both internal wave beams and boundary layer boluses primarily dependent on the values of both the reynolds number and the topographic steepness parameter the formation of beams required a critical bottom slope whilst for bolus formation a large vertical fluid excursion was necessary a bolus formed when the non dimensional vertical excursion parameter hn w 0 3 2 here h is the vertical excursion n is the buoyancy frequency and w 0 is the near bottom vertical velocity associated with the local depth averaged velocity we simplified the classification of the observed flow regimes using a generation parameter g defined as the ratio of a reynolds number to the topographic steepness parameter the estimated flow regime boundaries were for g g g g 400 there was no bolus observed we estimated that approximately 4 of the barotropic energy was converted to baroclinic energy when beams were generated

thermal constraints on the early history of the h chondrite parent body reconsidered abstract reconstructions of the early thermal history of the h chondrite parent body have focused on two competing hypotheses the first posits an undisturbed thermal evolution in which the degree of metamorphism increases with depth yielding an onion shell structure the second posits an early fragmentation reassembly event that interrupted this orderly cooling process here we test these hypotheses by collecting a large number of previously published closure age and cooling rate data and comparing them to a suite of numerical models of thermal evolution in an idealized parent body we find that the onion shell hypothesis when applied to a parent body of radius 75 130 km with a thermally insulating regolith is able to explain 20 of the 21 closure age data and 62 of the 71 cooling rates furthermore six of the eight meteorites for which multiple data at different temperatures are available can be accounted for by onion shell thermal histories we therefore conclude that no catastrophic disruption of the h chondrite parent body occurred during its early thermal history the relatively small number of data not explained by the onion shell hypothesis may indicate the formation of impact craters on the parent body which while large enough to excavate all petrologic types were small enough to leave the parent body largely intact impact events fulfilling these requirements would likely have produced transient crater diameters at least 30 of the parent body diameter

internal tides generated on a corrugated continental slope part ii along slope barotropic forcing recent measurements in a region of continental slope characterized by ridges and valleys running up and down the slope reveal interesting high mode structure in the tidal band velocity signals with enhanced mixing above the corrugations in order to understand these observations numerical simulations of the internal tide generation in this region of topography were performed here the focus is on the response of the flow to along slope barotropic tidal forcing for small amplitude barotropic forcing internal waves are generated over the continental slope that propagate toward the ocean surface and toward shallower water when higher amplitude forcing is combined with large amplitude corrugations the flow is locally supercritical downstream of ridges and transient internal hydraulic jumps result as the flow relaxes each half tidal period these jumps are released as internal wave packets which propagate up into the thermocline the internal hydraulic jumps are a source of mixing in the valleys while the small scale shears associated with the internal waves could lead to mixing higher up the water column over the forcing range considered the response is dominated by slightly higher harmonics of the tidal forcing frequency than predicted by existing analytic theories

coarse grained chondrule rims in type 3 chondrites abstract relatively coarse grained rims occur around all types of chondrules in type 3 carbonaceous and ordinary chondrites those in h l ll3 chondrites are composed primarily of olivine and low ca pyroxene those in cv3 chondrites contain much less low ca pyroxene average grain sizes range from 4 m in h l ll3 chondrites to 10 m in cv3 chondrites such rims surround 50 10 and 1 of chondrules in cv3 h l ll3 and co3 chondrites respectively but are rare 1 around cv3 ca al rich inclusions rim thicknesses average 150 m in h l ll3 chondrites and 400 m in cv3 chondrites the rims in h l ll3 chondrites are composed of material very similar to that which comprises darkzoned chondrules and recrysiallized matrix dark zoned chondrules and coarse grained rims probably formed in the solar nebula from clumps of opaque matrix material heated to sub solidus to sub liquidus temperatures during chondrule formation mechanisms capable of completely melting some material while only sintering other material require steep thermal gradients suitable processes are lightning reconnecting magnetic field lines and possibly aerodynamic drag heating cv chondrites may have formed in a region where the chondrule formation mechanism was less efficient probably at greater solar distances than the ordinary chondrites the lesser efficiency of heating could be responsible for the greater abundance of coarse grained rims around cv chondrules alternatively cv chondrules may have suffered fewer particle collisions prior to agglomeration

grain growth in partially molten olivine aggregates grain growth rates in partially molten olivine aggregates were determined experimentally at a pressure of 1 gpa the aim of the experiments with fine grained starting material solution gelation derived fo90 olivine with a grain size of 1 m with 2 and 4 wt added basaltic glass was to determine growth rates during steady state grain growth a series of experiments was conducted at a fixed temperature of 1 350 c for durations from 2 to 700 h and a second series at fixed duration 168 h at temperatures from 1 250 to 1 450 c the resulting activation energy e 390 kj mol and growth exponent n 4 3 are comparable to melt free results nichols and mackwell 1991 the observed grain size distributions are broader than those predicted for normal grain growth or ostwald ripening and most similar to the log normal distribution the observed melt distribution is influenced by the continuous rearrangement of neighboring grains during growth the relatively large growth exponent observed in this study and by nichols and mackwell 1991 where secondary phases are present suggests that grain growth in the upper mantle is slower than predicted by karato 1989

grain growth and inclusion formation in partially molten carbonate rocks to learn more about the kinetics and mechanisms of coarsening and melt inclusion formation we investigated the effects of melt content viscosity and topology on the microstructural evolution of partially molten and melt free calcite aggregates synthetic marbles with eutectic melts were produced by annealing mixtures of calcite and either calcium hydroxide or lithium carbonate for up to 80 h at a confining pressure of 300 mpa and temperatures of 973 1 023 k the melts produced in the two systems are expected to differ significantly in viscosity generally coarsening rates decrease with increasing melt fraction probably because the diffusion length across melt pockets increases analysis of grain shapes in the samples with about 40 melt indicated that coarsening was accommodated by agglomeration in the samples of the calcium lithium carbonate system in the calcium carbonate hydroxide system classical ostwald ripening occurred for melt contents about 10 and below melt filled pores are either dropped from or dragged along with migrating grain boundaries depending on the pore size and the grain boundary curvature these data can be used to constrain the conditions where fluid or melt inclusions form under natural conditions combining our results and previous studies illustrates a systematic relation between the grain boundary mobility in calcite aggregates and the diffusion kinetics associated with second phases residing on the grain boundaries in particular boundaries with no porosity are most mobile those boundaries dragging melt filled pores are slower those with gas filled pores are slower yet and those containing solid phases are slowest or may even be motionless

energetics of barotropic and baroclinic tides in the monterey bay area abstracta detailed energy analysis of the barotropic and baroclinic m2 tides in the monterey bay area is performed the authors first derive a theoretical framework for analyzing internal tide energetics based on the complete form of the barotropic and baroclinic energy equations which include the full nonlinear and nonhydrostatic energy flux contributions as well as an improved evaluation of the available potential energy this approach is implemented in the stanford unstructured nonhydrostatic terrain following adaptive navier stokes simulator suntans results from three dimensional high resolution suntans simulations are analyzed to estimate the tidal energy partitioning among generation radiation and dissipation a 200 km 230 km domain including all typical topographic features in this region is used to represent the monterey bay area of the 152 mw energy lost from the barotropic tide approximately 133 mw 88 is converted into baroclinic energy through internal tide generation and 42 56

preliminary simulations of internal waves and mixing generated by finite amplitude tidal flow over isolated topography much recent observational evidence suggests that energy from the barotropic tides can be used for mixing in the deep ocean here the process of internal tide generation and dissipation by tidal flow over an isolated gaussian topography is examined using two dimensional numerical simulations employing the mitgcm four different topographies are considered for five different amplitudes of barotropic forcing thereby allowing a variety of combinations of key nondimensional parameters while much recent attention has focused on the role of relative topographic steepness and height in modifying the rate of conversion of energy from barotropic to baroclinic modes here attention is focused on parameters dependent on the flow amplitude for narrow topography large amplitude forcing gives rise to baroclinic responses at higher harmonics of the forcing frequency tall narrow topographies are found to be the most conducive to mixing dissipation rates in these calculations are most efficient for the narrowest topography

an experimental study of ostwald ripening of olivine and plagioclase in silicate melts implications for the growth and size of crystals in magmas we carried out an experimental study to characterize the kinetics of ostwald ripening in the forsterite basalt system and in the plagioclase an65 andesite system eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution csd with time t the experiments were performed in a 1 atmosphere quench furnace at 1 250 c for plagioclase and 1 300 c for olivine very contrasted coarsening kinetics were observed in the two series in the plagioclase series the mean grain size increased as log t from 3 m to only 8 7 m in 336 h the kinetic law in log t means that ostwald ripening was rate limited by surface nucleation at plagioclase liquid interfaces in the olivine series the mean grain size increased as t1 3 from 3 m to 23 2 m in 496 h a kinetic law in t1 3 is expected when ostwald ripening is rate limited either by diffusion in the liquid or by grain growth dissolution controlled by a screw dislocation mechanism the shape of olivine csds in particular their positive skewness indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism not by diffusion as the degrees of undercooling t or supersaturation involved in ostwald ripening are essentially 1 c the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers we extrapolated our experimental data to geological time scales to estimate the effect of ostwald ripening on the size of crystals in magmas in the case of plagioclase ostwald ripening is only efficient for mean grain sizes of a few microns to 20 m even for a time scale of 105 years it can however result in a significant decrease of the number of small crystals per unit volume and contribute to the development of convex upwards csds for olivine the mean grain size increases from 2 3 m to 70 m in 1 year and 700 m in 103 years a mean grain size of 3 mm is reached in 105 years accordingly the rate of grain size dependent processes such as compaction of olivine rich cumulates or melt extraction from partially molten peridotites may significantly be enhanced by textural coarsening

tidal conversion by supercritical topography calculations are presented of the rate of energy conversion of the barotropic tide into internal gravity waves above topography on the ocean floor the ocean is treated as infinitely deep and the topography consists of periodic obstructions a green function method is used to construct the scattered wavefield the calculations extend the previous results of balmforth et al for subcritical topography wherein waves propagate along rays whose slopes exceed that of the topography everywhere by allowing the obstacles to be arbitrarily steep or supercritical so waves propagate at shallower angles than the topographic slopes and are scattered both up and down a complicated pattern is found for the dependence of energy conversion on e the ratio of maximum topographic slope to wave slope and the ratio of obstacle amplitude and separation this results from a sequence of constructive and destructive interferences between scattered waves that has implications for computing tidal conversion rates for the global ocean

olivine grain growth in partially molten fe ni s a proxy for the genesis of pallasite meteorites abstract the origin of pallasites has been the focus of a number of recent studies yet their formation process remains elusive while the mechanism leading to the genesis of the sub group termed mixed type pallasites containing polygonal rounded and fragmental olivines simultaneously is unclear here we test the hypothesis of mixing of olivine fragments with fe ni s after a non destructive impact followed by annealing employing both experimental analogues and numerical models the experimental series evidenced that the addition of sulfur to olivine fe ni accelerates olivine grain growth though the growth rate is reduced when fe ni s is not fully molten this is shown to be the consequence of competing growth of olivine and fe ni grains numerical models satisfying available formation constraints from natural samples indicate that planetesimals with radii 200 km are favorable for the genesis of rounded olivine bearing pallasites by annealing of fragments in partially molten fe ni s moreover early mixing in the planetesimal can form regions containing olivine grains with different grain sizes that could explain the formation of mixed type pallasites

arecibo s thermospheric gravity waves and the case for an ocean source 1 wave like disturbances in electron density ne have been observed in the thermosphere above arecibo observatory puerto rico throughout its 45 year history however only recently has it become evident that these waves are continuously present in the arecibo thermosphere the wave characteristics are fairly constant between day and night and from season to season high resolution electron density measurements obtained by applying the coded long pulse radar technique to photoelectron enhanced langmuir waves are presented these new observations strongly suggest that the perturbations in electron density are the result of internal acoustic gravity waves agws propagating through the arecibo thermosphere the agws appear to be broadbanded in wave number space the downward phase trajectories of ne ne between 400 and 120 km combined with the low horizontal phase velocities obtained from airglow measurements support the idea that the agws are not ducted but rather are locally produced in addition the altitudes at which major peaks in ne ne are observed follow theoretical estimates for nonducted waves the nominal period of the agws is 60 min at 250 km altitude but periods of 20 min are also evident at lower attitudes classic sources of agws do not appear to be consistent with the arecibo observations of a continuous flux of background agws ray tracing of the agws combined with 630 0 nm airglow observations point to a source location in the atlantic ocean that is roughly 2100 km east northeast of arecibo internal ocean waves generated in response to the internal tide at the mid atlantic ridge are the most likely source of arecibo s thermospheric agws

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

3d particle size distributions from 2d observations stereology for natural applications abstract we have developed a general formulation for stereological analysis of particle distributions which is applicable to any particle size or size distribution not limited to log normal unimodal etc we have applied numerical techniques to define intersection probability distributions for any shapes previously only known for spheres and quantified the errors involved in using spherical coefficients for various non spherical particles this stereological technique is based on knowing the probability distribution of random cross sections through various particles so that small circle cross sections can be subtracted from an observed population to provide the 3d size distribution of particles the results indicate that the most important parameter controlling calculated size distribution is particle aspect ratio for a distribution of particles with a specific aspect ratio or range of aspect ratios variations of particle form spherical vs cubic rectangular vs elliptical do not alter the results so the technique can be applied to a range of particle shapes applications can be made in a petrology volcanology and other fields only a few of which can also be treated using expensive x ray techniques

internal wave breaking at concave and convex continental slopes abstract internal wave reflection from a sloping topographic boundary may lead to enhanced shear if the topographic angle to the horizontal is close to that of the internal wave group velocity vector previous analytic studies have suggested that shear enhancement is reduced at concave slopes as compared with convex and planar slopes near the critical angle here the internal wave reflection from concave and convex slopes that pass through the critical angle is investigated numerically using the nonhydrostatic massachusetts institute of technology general circulation model mitgcm overturning shear instability and resultant mixing are examined results are compared with simulations of wave reflection from planar slopes with angles greater than less than and equal to the critical angle in contrast to the analytic predictions no reduction in mixing is found for the concave slope as compared with the other slopes in all cases stratification is eroded in a band above the slope bounded at its outer

tidal influence on particulate organic carbon export fluxes around a tall seamount as tall seamounts may be stepping stones for dispersion and migration of deep open ocean fauna an improved understanding of the productivity at and food supply to such systems needs to be formed here the 234th 238u approach for tracing settling particulate matter was applied to senghor seamount a tall sub marine mountain near the tropical cape verde archipelago in order to elucidate the effects of topographically influenced physical flow regimes on the export flux of particulate organic carbon poc from the near surface topmost 100 m into deeper waters the comparison of a suitable reference site and the seamount sites revealed that poc export at the seamount sites was 2 4 times higher than at the reference site for three out of five seamount sites the calculated poc export fluxes are likely to be underestimates if this is taken into account it can be concluded that poc export fluxes increase while the passing waters are advected around and over the seamount with the highest export fluxes occurring on the downstream side of the seamount this supports the view that biogeochemical and biological effects of tall seamounts in surface ocean waters might be strongest at some downstream distance from rather than centred around the seamount summit based on measured vessel mounted adcp and modelled regional flow field aviso internal tides at senghor mitgcm flow dynamics it is proposed that tidally generated internal waves result in a screen of increased rates of energy dissipation that runs across the seamount and leads to a combination of two factors that caused the increased poc export above the seamount 1 sudden increased upward transport of nutrients into the euphotic zone driving brief pulses of primary production of new particulate matter followed by the particles export into deeper waters and 2 pulses of increased shear driven aggregation of smaller slower settling into larger faster settling particles this study shows that under certain conditions there can be an effect of a tall seamount on aspects of surface ocean biogeochemistry with tidal dynamics playing a prominent role it is speculated that these effects can control the spatiotemporal distribution of magnitude and nutritional quality of the flux of food particles to the benthic and benthic pelagic communities at and near tall seamounts

generation of internal waves in the deep ocean by tides 1 a direct computation of the tidal generation of internal waves over the global ocean is presented it is based on linear wave theory and high resolution data for the bottom topography the geographical distribution of the energy flux from tides to internal waves is determined with a spatial resolution of a few kilometers the total flux over the area with a depth greater than 500 m is found to be 1 2 tw the greatest uncertainties of the computation are due to unresolved topography and to nonlinear effects caused by supercritical bottom slope

formation of large metal nodules in ordinary chondrites literature data show that many large 2 30 mm metallic fe ni nodules in ordinary chondrites are depleted in refractory siderophiles e g re os ir ru pt and rh and cu relative to fine grained metal but are not depleted in normally refractory w and mo common siderophiles e g ni and co and volatile siderophiles e g au as and sb it was recently proposed that metamorphic diffusion could form the metal nodules and that partitioning of siderophile elements between kamacite and taenite could account for the nodules unusual compositions however the nodules have the same compositional trends as metal nodules and veins in shocked meteorites that clearly formed by impact related processes if large metal nodules formed during metamorphism type 6 chondrites should contain abundant large nodules instead the mean metal grain size in h6 chondrites is 0 12 0 15 mm far smaller than any of the nodules it seems likely that the metal nodules formed by impact vaporization oxidation of w and mo to form volatile oxides concomitant partial oxidation of re and os rapid fractional condensation of refractory siderophiles transport of the residual vapor condensation of cu s and se into vugs and fractures to form sulfide and condensation of the remaining siderophiles to form metal nodules and veins during vapor cooling and condensation or during subsequent parent body annealing w mo re and os oxides were reduced

morphological analysis of olivine grains annealed in an iron nickel matrix experimental constraints on the origin of pallasites and on the thermal history of their parent bodies two types of pallasites can be distinguished on the basis of the grain shape of olivine rounded or angular it has been suggested that these two types of textures resulted from different degrees of annealing at high temperature in the parent body in order to characterize the kinetics of rounding of olivine grains in an fe ni matrix we carried out a series of annealing experiments using a mixture of olivine and fe ni powder we were able to reproduce at a miniature scale the range of textures in pallasites the rate of rounding was rapid enough to be observed and measured at the scale of a few micrometers to 20 m even though the experiments were performed below the solidus of the fe ni metal for instance grains 14 m in diameter became nearly spherical within 7 days at 1400 oc for the morphological analysis of olivine grains we used two independent techniques the critical diameter method and the gaussian diffusion resample method a new technique specially developed for our study both techniques indicate that the rounding time scale is proportional to the cube of the grain size and that morphological adjustments in our experiments occurred by volume diffusion in the olivine lattice not by surface diffusion along the olivine metal boundaries we used our experimental data to estimate the time scales required for the development of olivine metal textures in natural pallasites we determined that small scale rounding of olivine grains in a solid metal matrix can be produced within relatively short time intervals 100 years to produce rounded olivine grains 0 1 mm in radius at 1300 1400oc

a mechanism for local dissipation of internal tides generated at rough topography abstract fine and micro structure observations indicate that turbulent mixing is enhanced within o 1 km above rough topography enhanced mixing is associated with internal wave breaking and in many regions of the ocean has been linked to the breaking and dissipation of internal tides the generation and dissipation of internal tides are explored in this study using a high resolution two dimensional nonhydrostatic numerical model which explicitly resolves the instabilities leading to wave breaking configured in an idealized domain with a realistic multiscale topography and flow characteristics the control simulation chosen to represent the brazil basin region produces a vertical profile of energy dissipation and temporal characteristics of finescale motions that are consistent with observations results suggest that a significant fraction of mixing in the bottom o 1 km of the ocean is sustained by the transfer of energy from the large scale internal tides to smaller scale internal waves by nonlin

grain growth in the forsterite diopside system abstract grain growth experiments in dunite clinopyroxenite and wehrlites with various forsterite diopside ratios were performed to investigate the effect of modal composition on grain growth kinetics in the earth s uppermost mantle the experiments were conducted using a piston cylinder apparatus at 1200 c and 1 2 gpa for 2 763 h under dry conditions normal grain growth ngg occurred in dunite clinopyroxenite and relatively forsterite poor wehrlites 70 vol of forsterite grain growth rates of forsterite and diopside in relatively forsterite poor wehrlites were much slower than those in dunite and clinopyroxenite in the forsterite rich wehrlites 80 vol of forsterite ngg of diopside and abnormal grain growth agg of forsterite occurred the growth rate of diopside was significantly slower than that in clinopyroxenite while the growth rate of forsterite by agg was found to be similar to that by ngg in dunite the presence of 20 vol diopside had the effect of inhibiting the forsterite grain growth during the initial stage resulting in agg of forsterite thus overtaking the growth rate in dunite our experimental results suggest that there would be a significant difference in grain growth rate and consequently in effective viscosity between olivine rich peridotites depleted mantle and relatively olivine poor peridotites fertile mantle in the case of grain size sensitive creep variation of mean grain sizes in the upper mantle would result not only from differences in temperature and phase assemblage but also from the variation of modal compositions

fast grain growth of olivine in liquid fe s and the formation of pallasites with rounded olivine grains abstract despite their relatively simple mineralogical composition olivine fe ni metal fes pyroxene the origin of pallasite meteorites remains debated it has been suggested that catastrophic mixing of olivine fragments with fe ni s followed by various degrees of annealing could explain pallasites bearing solely or prevalently fragmented or rounded olivines in order to verify this hypothesis and to quantify the grain growth rate of olivine in a liquid metal matrix we performed a series of annealing experiments on natural olivine plus synthetic fe s mixtures the best explanation for the observed olivine grain size distributions gsd of the experiments are dominant ostwald ripening for small grains followed by random grain boundary migration for larger grains our results indicate that olivine grain growth in molten fe s is significantly faster than in solid sulphur free metal we used the experimentally determined grain growth law to model the coarsening of olivine surrounded by fe s melt in a 100 600 km radius planetesimal in this model an impact is responsible for the mixing of olivine and fe ni s numerical models suggest that annealing at depths of up to 50 km allow for i average grain sizes consistent with the observed rounded olivine in pallasites ii a remnant magnetisation of fe ni olivine inclusions as measured in natural pallasites and iii for the metallographic cooling rates derived from fe ni in pallasites this conclusion is valid even if the impact occurs several millions of years after the differentiation of the target body was completed

temperature dependent grain growth of forsterite nickel mixtures implications for grain growth in two phase systems and applications to the h chondrite parent body abstract grain growth experiments in the system forsterite fo nickel ni have been performed on two analogue mixtures of ordinary chondrites with volume of fo ni 95 5 and 80 20 these two mixtures have been studied at temperatures of 1390 c and 1340 c at an oxygen fugacity f o 2 three orders of magnitude below the ni nio buffer for durations between 2 h and 10 days microstructures and grain size distributions show that grain growth is normal and that for durations 10 h the zener relation is verified i e the ratio of fo and ni grain size is independent of time comparison with results previously obtained at 1440 c shows a similar grain growth exponent n 5 for both phases consistent with growth of forsterite by grain boundary migration limited by the growth rate of nickel the details of size distribution frequencies and the value of grain growth exponent indicate that the nickel grains which pin forsterite grain boundaries grow by diffusion along one dimensional paths i e along forsterite triple junctions the derived activation energies for nickel and forsterite are 235 33 kj mol and 400 48 kj mol respectively within the framework of the zener relation this unexpected difference of activation energy is shown to be related to temperature dependent variations in the ratio of ni and fo grain size that are consistent with observed variations in fo ni fo dihedral angle these data thus indicate that the presence of all phases should be taken into account when considering the activation energy of growth rate of individual phases as an application the experimentally derived growth law for metal has been used in conjunction with temperature time paths taken from models of the thermal history of the h chondrite parent body to estimate the grain size evolution of metal in h chondrites a remarkably self consistent picture emerges from experimentally derived grain growth laws textural data of metal grains in well characterised h chondrite samples and geochemically constrained temperature time paths providing the potential to use textural data of metal as a window into the thermal history of chondritic samples

internal tide reflection and turbulent mixing on the continental slope abstract observations of turbulence internal waves and subinertial flow were made over a steep corrugated continental slope off virginia during may june 1998 at semidiurnal frequencies a convergence of low mode onshore energy flux is approximately balanced by a divergence of high wavenumber offshore energy flux this conversion occurs in a region where the continental slope is nearly critical with respect to the semidiurnal tide it is suggested that elevated near bottom mixing k 10 3 m2 s 1 observed offshore of the supercritical continental slope arises from the reflection of a remotely generated low mode m2 internal tide based on the observed turbulent kinetic energy dissipation rate the high wavenumber internal tide decays on time scales o 1 day no evidence for internal lee wave generation by flow over the slope s corrugations or internal tide generation at the shelf break was found at this site

idealized solutions for the energy balance of the finescale internal wave field abstract recent fine and microstructure observations indicate enhanced finescale shear and strain in conjunction with bottom intensified turbulent dissipation above rough bathymetry in the brazil basin such observations implicate the bottom boundary as an energy source for the finescale internal wave field simple analytical and numerical solutions to an equation governing the spatial and temporal evolution of the finescale wave field are described here the governing equation implicitly treats the effects of wave breaking on the vertical propagation of internal waves through a flux representation of nonlinear transports associated with internal wave wave interactions these solutions identify the rate of dissipation of turbulent kinetic energy with downscale energy transports at high vertical wavenumber resulting in an estimate of dissipation versus depth the sensitivity of the turbulent dissipation depth profile to various environmental parameters is examined observed dissipation profiles and shear

tidally driven processes leading to near field turbulence in a channel at the crest of the mendocino escarpment abstractin situ observations of tidally driven turbulence were obtained in a small channel that transects the crest of the mendocino ridge a site of mixed diurnal and semidiurnal tides diurnal tides are subinertial at this latitude and once per day a trapped tide leads to large flows through the channel giving rise to tidal excursion lengths comparable to the width of the ridge crest during these times energetic turbulence is observed in the channel with overturns spanning almost half of the full water depth a high resolution nonhydrostatic 2 5 dimensional simulation is used to interpret the observations in terms of the advection of a breaking tidal lee wave that extends from the ridge crest to the surface and the subsequent development of a hydraulic jump on the flanks of the ridge modeled dissipation rates show that turbulence is strongest on the flanks of the ridge and that local dissipation accounts for 28 of the energy converted from the barotropic tide into baroclinic motion

internal tide generation by seamounts ridges and islands a numerical investigation is made into the generation of semidiurnal internal tides by tidal flow over steep topographic features in the deep ocean a fully three dimensional free surface nonlinear hydrostatic model the princeton ocean model is used with real stratification representative tidal forcing and gaussian shaped seamounts ridges and islands for the topography the efficiencies of the different topographies in extracting energy from the barotropic tide and in generating an internal tide are considered each topography produces an internal tide characterized by signals propagating away from the feature as beams that follow internal wave characteristic paths the strength of the signals however varies markedly for different topographies this is largely attributed to the way in which the barotropic tide interacts with the three dimensional topography internal wave generation requires a significant vertical displacement of stratified water by the barotropic tide as it flows over topography it is found that for symmetric seamounts and islands the barotropic flow tends to go around the feature producing only a weak internal tide when elongated into a ridge barotropic flow is forced across isobaths generating an energetic internal tide changing the horizontal aspect ratio of the topography from 1 1 a seamount to 3 1 a ridge increases the resulting baroclinic energy flux by nearly an order of magnitude the slope of the topography relative to that of the internal wave characteristics is also shown to be important with strongest generation occurring at regions of critical or supercritical topographic slope

grain growth in forsterite nickel mixtures analogues of small parent bodies during early accretion abstract we report an experimental study on the grain growth kinetics and microstructural evolution of olivine metal systems 32 experiments have been performed on four different mixtures of forsterite fo and nickel ni respectively in vol 95 5 80 20 30 70 and 10 90 all experiments were carried out at high temperature 1440 c at oxygen fugacity fo 2 3 log units below the ni nio buffer and for durations between 2 h and 21 days in aggregates containing isolated forsterite grains in a nickel matrix fo ni 10 90 forsterite did not grow and grain growth of nickel was abnormal in all other cases features of normal grain growth were observed and the data were fit to the classic grain growth law d n d 0 n kt for all these aggregates grain growth exponents are in the range 4 0 n nichols and mackwell 1991 for porous olivine aggregates both ni and pores apparently behaving as mobile minor phases extrapolation to time scales relevant to the formation of ordinary chondrite parent bodies shows that mean grain sizes reached fo and ni are of the same order of magnitude 100 m to 1 mm as those observed in several types of meteorites iron rich phases kamacite taenite and or troilite in several h chondrites are shown to present normal grain growth features and ngsd shapes similar to those in our experimental samples where nickel grains are not interconnected i e fo ni 95 5 and 80 20 the grain growth mechanism for the iron rich phases in these meteorites may thus also occur by coalescence rate limited by grain boundary migration of olivine

an estimate of tidal energy lost to turbulence at the hawaiian ridge abstract an integrated analysis of turbulence observations from four unique instrument platforms obtained over the hawaiian ridge leads to an assessment of the vertical cross ridge and along ridge structure of turbulence dissipation rate and diffusivity the diffusivity near the seafloor was on average 15 times that in the midwater column at 1000 m depth the diffusivity atop the ridge was 30 times that 10 km off the ridge decreasing to background oceanic values by 60 km a weak factor of 2 spring neap variation in dissipation was observed the observations also suggest a kinematic relationship between the energy in the semidiurnal internal tide e and the depth integrated dissipation d such that d e1 0 5 at sites along the ridge this kinematic relationship is supported by combining a simple knife edge model to estimate internal tide generation with wave wave interaction time scales to estimate dissipation the along ridge kinematic relationship and the observed vertical and cross ridge st