late miocene coesite eclogite exhumed in the woodlark rift late miocene pliocene eclogites were exhumed in the woodlark rift of eastern papua new guinea an actively extending region west of the woodlark basin seafl oor spreading center we report the discovery of coesite in late miocene eclogite from the lower plate of one of the d entrecasteaux islands metamorphic core complexes within the woodlark rift zircon crys tallization temperatures 650 675 c and 238 u 206 pb age ca 8 ma and rutile thermometry 695 743 c combined with garnet pyroxene thermometry 600 760 c and garnet pyroxene phengite barometry 18 27 kbar indicate that the coesite eclogite was exhumed from mantle depths 90 km to the earth s surface at plate tectonic rates cm yr 1 this late miocene coesite eclogite is the youngest exhumed ultrahigh pressure uhp rock on earth and its preservation ahead of the westward propagating seafl oor spreading center forces reevaluation of models for uhp exhumation as well as the geologic and tectonic evolution of the woodlark rift

an approximation for the xu white velocity model in 1995 s xu and r e white described a method for estimating compressional and shear wave velocities of shaley sandstones from porosity and shale content their model was able to predict the effect of increasing clay content on compressional wave velocity observed in laboratory measurements a key step in the xu white method estimates dry rock bulk and shear moduli for the sand shale mixture this step is performed numerically by applying the differential effective medium method to the kuster toksoz equations for ellipsoidal pores this step is computationally intensive using reasonable assumptions about dry rock elastic properties this step can be replaced with a set of approximations for dry rock bulk and shear moduli numerical experiments show an extremely close match between velocities obtained with these approximations and velocities computed with the differential effective medium method these approximations simplify the application of the xu white method and make the method computationally more efficient they also provide insight into the xu white method for example these approximations show how the xu white model is related to the critical porosity model

origin of eclogite bearing domed layered metamorphic complexes core complexes in the d entrecasteaux islands papua new guinea compositionally layered metamorphic rocks of the d entrecasteaux islands papua new guinea are folded into domes and antiforms bounded by faults parallel to metamorphic layering and foliation the structures are broadly similar to the metamorphic core complexes of western north america lenses of ultramafic rock lie on the bounding faults and the same faults have served as loci for quaternary andesitic volcanic activity metamorphic grade in the northern islands goodenough and fergusson is amphibolite facies with pockets of eclogite fergusson island only and granulite and is greenschist facies in the southern island normanby in all three islands there is a characteristic tectonostratigraphic sequence fmu sequence from felsic metamorphic rocks at base or internally through mafic metamorphic rocks to ultramafic rocks at top or externally the association of metamorphic and ultramafic rocks apparently developed in a north dipping paleogene subduction system and was exhumed to upper crustal level in the oligocene early miocene possibly by reversal of movement on faults in the former subduction system vigorous uplift and development of domes and antiforms in the pliocene was triggered by westward propagation of the woodlark basin spreading ridge and was accompanied by rifting rift related magmatism rapid erosion and deposition of coarse sediment in the adjacent trobriand basin

timing of uhp exhumation and rock fabric development in gneiss domes containing the world s youngest eclogite facies rocks southeastern papua new guinea the youngest known ultrahigh pressure uhp rocks in the world occur in the woodlark rift of southeastern papua new guinea since their crystallization in the late miocene to early pliocene these eclogite facies rocks have been rapidly exhumed from mantle depths to the surface and today they remain in the still active geodynamic setting that caused this exhumation for this reason the rocks provide an excellent opportunity to study rates and processes of u hp exhumation we present new rb sr results from 12 rock samples from eclogite bearing gneiss domes in the d entrecasteaux islands and use those results to examine the time lag between u hp metamorphism and later ductile thinning penetrative fabric development and accompanying metamorphic retrogression at amphibolite facies conditions during their exhumation a rb sr age for a sample of mafic eclogite with no preserved coesite from the core zone of the mailolo gneiss dome fergusson island provides a new estimate of the timing of hp metamorphism 5 6 1 6 ma the strongly deformed quartzofeldspathic and granitic gneisses 90 95 by volume that enclose variably retrogressed relict blocks of mafic eclogite 5 10 by volume yield rb sr isochron ages from 4 4 to 2 4 ma for the uhp bearing gneisses of mailolo dome previously published u pb ages on zircon and our rb sr isochron ages are consistent with a mean time lag of 2 2 1 5 ma 95 c i for passage of the rock between eclogite and amphibolite facies conditions new thermobarometric data indicate that the main syn exhumational foliation developed at amphibolite facies conditions of 630 665 c and 12 1 14 4 kbar these pressure estimates indicate that the lower crust of the woodlark rift was unusually thick 40 km at the time of the amphibolite facies overprint possibly as a result of accumulation and underplating of uhp derived material from below our data imply a minimum unroofing rate of 10 7 mm year 1 95 c i for the u hp body from minimum hp depths 73 7 km to lower crustal depths this minimum unroofing rate reinforces previous inferences that the exhumation from the mantle to the surface of the gneiss domes in the d entrecasteaux islands took place at plate tectonic rates on the basis of previous structural studies and the new thermobarometry we attribute the high cm year 1 exhumation to diapiric ascent of the partially molten terrane from mantle depths with a secondary contribution from pure shear thinning of the terrane after its arrival in the crust

problems and possible solutions concerning radiocarbon dating of surface marine sediments ross sea antarctica abstract radiocarbon accelerator mass spectrometric ams dates on the acid insoluble fraction from 38 core tops from the western ross sea antarctica are used to address these questions 1 what are the apparent ages of sediments at or close to the present sediment water interface 2 is there a statistically significant pattern to the spatial distribution of core top ages and 3 is there a correction factor that can be applied to these age determinations to obtain the best possible holocene downcore chronologies ages of core top sediments range from 2000 to 21 000 14c yr b p some old core top dates are from piston cores and probably represent the loss of sediment during the coring process but some core top samples 6000 14c yr b p may represent little or no holocene deposition four possible sources of variability in dates 6000 14c yr b p n 28 are associated with 1 different sample preparation methods 2 different sediment recovery systems 3 different geographic regions and 4 within sample lateral age variability statistical analysis on an a posteriori design indicates that geographic area is the major cause of variability there is a difference in mean surface sediment age of nearly 2000 yr between sites in the western ross sea and sites east of ross bank in south central ross sea the systematic variability in surface age between areas may be attributed to a variable sediment accumulation rates sar surface age is inversely related to sar b differences in the percentage of reworked dead carbon between each area and or c differences in the co2 exchange between the ocean and the atmosphere

microstructures and quartz lattice preferred orientations in the eclogite bearing migmatitic gneisses of the d entrecasteaux islands papua new guinea 1 lattice preferred orientations lpo of quartz in gneiss domes of the d entrecasteaux islands woodlark rift shed insight into exhumation of the world s youngest 5 7 ma eclogite bearing terrane at cm yr rates we focus on deformation that affected the terrane as it transited between lower crustal depths and the surface including 1 grain scale deformation mechanisms and 2 style of flow and mode of emplacement of the domes electron backscatter diffraction was used to analyze microstructure and lpos of 37 quartzofeldspathic gneiss samples that enclose meter scale mafic blocks preserving original eclogite facies assemblages during exhumation of the ultrahigh pressure uhp terrane gneisses were retrogressed in the amphibolite facies at lower crustal depths the lpos change from dome cores to carapaces consistent with decreasing deformational temperatures in the relatively chilled outer carapaces of the domes the quartz lpos consist of mostly crossed girdle c axis patterns with some cleft girdle and small circle lpos and record dislocation creep accommodated by mixed slip in the cores of the migmatitic domes a chessboard pattern of subgrains is common and quartz lpos primarily record prism c slip probably at 630 c other microstructures indicate recovery by high temperature grain boundary migration grain boundary mobility was anisotropic leading to strong grain shape fabrics oblique to foliation but not obviously relatable to shear sense evidence for melt present deformation is abundant and microstructures including partially dissolved feldspar grains indicate some deformation by fluid assisted grain boundary diffusion creep lpos in carapace rocks are symmetrical recording flow that was dominantly coaxial we interpret the gneiss domes to have been emplaced into the rift as partially molten diapirs

sedimentation associated with antarctic peninsula ice shelves implications for palaeoenvironmental reconstructions of glacimarine sediments abstract recent disintegration of a number of antarctic peninsula ice shelves has given us a unique opportunity to investigate sub ice shelf sediments we characterize three sediment facies associations of two antarctic peninsula ice shelves larsen a and larsen inlet subglacial facies consist mainly of basal till diamicton with high shear strengths deposited under a grounded ice sheet progressive upward decrease in shear strength reflects a gradual decrease in the confining vertical effective pressure of grounded ice during till deposition proximal ice shelf glacimarine facies diamicton gravel rich and sand rich facies gravelly mud dropstone mud and sandy muds were deposited by sub ice shelf rain out bottom current activity and sediment gravity flows following decoupling of grounded ice from the sea floor distal ice shelf glacimarine and or open marine facies comprise terrigenous and diatom bearing bioturbated muds and gravelly muds that contain limited ice rafted debris these accumulated after recession of the grounding line to the coast with coarse grained surface sediments possibly documenting most recent ice shelf break up antarctic peninsula ice shelf sediments are more heterogeneous than ice shelf facies deposited elsewhere in antarctica cold polar antarctic ice shelves can be differentiated from temperate and sub polar marine terminated glacial sedimentary systems by the dominance of coarse grained proximal ice shelf glacimarine facies and an absence of subaqueous outwash meltwater sediment facies

radiocarbon constraints on antarctic peninsula ice sheet retreat following the last glacial maximum lgm abstract we present marine sedimentologic and radiocarbon data for the timing of retreat of the largely marine based antarctic peninsula ice sheet since the last glacial maximum lgm our findings indicate minimum estimates of deglaciation between 18 000 and 9000 calibrated years before present cal yr bp roughly in phase with the northern hemisphere deglaciation and eustatic sea level rise our findings show this retreat occurred progressively from the outer middle and inner continental shelf regions as well as progressively from the north to the south retreat initiated on the outer shelf of the northern peninsula by 18 000 cal yr bp and continued southward by 14 000 cal yr bp on the outer shelf of marguerite bay several thousand years earlier than estimated by numeric models while individual cores yield estimates of glacial retreat that may vary up to 1100 years we note steps in the data occur at 14 000 and possibly 11 000 cal yr bp coincidental to rapidly rising eustatic sea level including the well documented melt water pulses mwp 1a and 1b these data support the hypothesis that rapidly rising sea level is associated with marine ice sheet destabilization although additional dates are necessary to substantiate this finding this study highlights problems with radiocarbon dating acid insoluble organic aio matter in proximal lateglacial sediments as well as the need for more accurate dating techniques

how large are departures from lithostatic pressure constraints from host inclusion elasticity minerals trapped as inclusions within other host minerals will develop non lithostatic pressures during both prograde and retrograde metamorphism because of the differences between the thermoelastic properties of the host and inclusion phases there is only a single possible path in p t space the entrapment isomeke along which no residual pressure would be developed in a host inclusion system non lithostatic pressures are developed in inclusions as a result of the external pressure and temperature deviating from the isomeke that passes through the entrapment conditions with modern equation of state and elasticity data for minerals now available it is possible to perform precise calculations of the isomekes for mineral pairs these show that isomeke lines are not straight lines in p t space at metamorphic conditions we show that silicate inclusions in silicate hosts tend to have flat isomekes with small values of because of the small range of thermal expansion coefficients of silicate minerals as a consequence the general behaviour under decompression is for soft silicate inclusions in stiffer hosts to develop excess pressures whereas a stiff silicate inclusion in a softer matrix will experience lower pressures than lithostatic pressure the opposite effects occur for compression after entrapment on the prograde path the excess pressures in inclusions including allowance for mutual elastic relaxation of the host and inclusion are most easily calculated by using the isomeke as a basis analysis of the simplest possible model of a host inclusion system indicates that deviations from lithostatic pressure in excess of 1 gpa can be readily produced in quartz inclusions within garnet in metamorphic rocks for softer host minerals such as feldspar the pressure deviations are smaller because of greater elastic relaxation of the host the maximum pressure deviation from lithostatic pressure in the host phase around the inclusion is one third of the pressure deviation in the inclusion routines for performing these calculations have been added to the eosfit7c software package

environmental magnetism of antarctic late pleistocene sediments and interhemispheric correlation of climatic events abstract recent developments in paleomagnetism and environmental magnetism provide new tools for the detailed correlation of climatically induced magnetic mineralogy changes in sedimentary sequences studies of these changes contribute to the reconstruction of climate history for the glacial interglacial cycles of the late pleistocene and to the delineation of the range of natural variability for global climate during the past hundred thousands years here we show that sharp coercivity minima observed in fine grained sediments from the continental rise of the western antarctic peninsula correlate to the major rapid cooling events of the northern atlantic heinrich layers we interpret such an environmental magnetic signal in terms of variations in deep sea diagenetic processes of sulfide formation which reflect changes in the input of detrital organic matter controlled by sea ice extent with the inherent uncertainties in age controls the sedimentary paleoclimatic markers of the two hemispheres are almost contemporaneous but interhemispheric time lags or leads of the order of 1 2 kyr such as those recently reported from the greenland and antarctic ice cores are also compatible with the data

a simple method of predicting s wave velocity prediction of shear wave velocity plays an important role in seismic modeling amplitude analysis with offset and other exploration applications this paper presents a method for predicting s wave velocity from the p wave velocity on the basis of the moduli of dry rock elastic velocities of water saturated sediments at low frequencies can be predicted from the moduli of dry rock by using gassmann s equation hence if the moduli of dry rock can be estimated from p wave velocities then s wave velocities easily can be predicted from the moduli dry rock bulk modulus can be related to the shear modulus through a compaction constant the numerical results indicate that the predicted s wave velocities for consolidated and unconsolidated sediments agree well with measured velocities if differential pressure is greater than approximately 5 mpa an advantage of this method is that there are no adjustable parameters to be chosen such as the pore aspect ratios required in some other methods the predicted s wave velocity depends only on the measured p wave velocity and porosity

evolution of host inclusion systems a visco elastic model a visco elastic model is suggested to recover the p t t paths of spherically symmetrical host inclusion systems from residual pressure patterns for a coesite garnet aragonite garnet and diamond garnet system with partial transition of high pressure polymorphs to low pressure ones the model predicts that the exhumation history of ultra high pressure metamorphic rocks from depths below 90 km began with pressure decrease to 1 gpa 40 km depth while the temperature remained high 800 c then the system cooled and decompressed to ambient conditions non spherical inclusion shapes in the 3 d model lead to heterogeneities in residual pressure and shear stress distributions as well as to significant gradients of shear stress which may affect the kinetics and thermodynamics of phase transformation in the inclusions

deglacial seasonal and sub seasonal diatom record from palmer deep antarctica the antarctic peninsula is one of the most sensitive regions of antarctica to climate change here ecological and cryospheric systems respond rapidly to climate fluctuations a 4 4m thick laminated diatom ooze deposited during the last deglaciation is examined from a marine sediment core odp site 1098 recovered from basin i palmer deep western antarctic peninsula this deglacial laminated interval was deposited directly over a glaciomarine diamict hence during a globally recognised period of rapid climate change the ultra high resolution deglacial record is analysed using sem backscattered electron imagery and secondary electron imagery laminated to thinly bedded orange brown diatom ooze near monogeneric hyalochaete chaetoceros spp resting spores alternates with blue grey terrigenous sediments open water diatom species these discrete laminae are interpreted as austral spring and summer signals respectively with negligible winter deposition sub seasonal sub laminae are observed repeatedly through the summer laminae suggesting variations in shelf waters throughout the summer tidal cycles high storm intensities and or intrusion of circumpolar deep water onto the continental shelf introduced conditions which enhanced specific species productivity through the season

deglacial ocean and climate seasonality in laminated diatom sediments mac robertson shelf antarctica the palaeoceanography and climate history of the east antarctic margin eam are less well understood than those of west antarctica yet the eam plays an important role in deep ocean circulation and the global ocean system and has likely done so in the past deglacial age marine sediments from the eam provide clues about its past role during this critical period of rapid climate change several deep basins across the eam such as iceberg alley 67 s 63 e on the mac robertson shelf mrs accommodate thick marine sequences that archive the deglaciation in the form of diatom rich continuously laminated varved sediments these laminated sediments are pristinely preserved and contain seasonal and long term information on the cryospheric and palaeoceanographic changes associated with the rapid retreat of the glacial ice sheet across the mrs we present results of microfabric analysis of the lower 2 m of deglacial varves from jumbo piston core jpc43b iceberg alley backscattered electron imagery bsei of polished thin sections and scanning electron microscope secondary electron imagery sei of lamina parallel fracture surfaces are used to analyze the varves one hundred and ninety two laminations are investigated and their nature and temporal significance are discussed in terms of seasonal deposition and cyclicity of diatom species our high resolution palaeodata record exceptionally high diatom production and silica flux associated with the retreat of the east antarctic ice sheet and seasonal sea ice changes along the eam this information is invaluable for assessing cryospheric oceanographic variation and therefore the local and regional response to this period of rapid climate change varves are made up of lamina couplets comprising i thickly laminated to thinly bedded orange orange brown very pure diatom ooze dominated by hyalochaete chaetoceros spp vegetative cells and resting spores and ii brown blue grey terrigenous angular quartz sand silt and clay with an abundant mixed diatom flora the colour variation between these two types of lamination is striking using floristic and textural information we interpret the diatom oozes as spring flux and the terrigenous laminae as summer flux each couplet pair represents one annual cycle and reflects seasonal changes in nutrient availability and stratification associated with the cyclical advance and retreat of seasonal sea ice the diatom oozes can reach up to 7 5 cm in thickness indicating enormous silica flux to the sea floor associated with ice sheet retreat

age assignment of a diatomaceous ooze deposited in the western amundsen sea embayment after the last glacial maximum reliable dating of glaciomarine sediments deposited on the antarctic shelf since the last glacial maximum lgm is challenging because of the rarity of calcareous micro fossils and the recycling of fossil organic matter consequently radiocarbon 14 c ages of the acid insoluble organic fraction aio of the sediments bear uncertainties that are difficult to quantify here we present the results of three different methods to date a sedimentary unit consisting of diatomaceous ooze and diatomaceous mud that was deposited following the last deglaciation at five core sites on the inner shelf in the westernamundsen sea west antarctica in three coresconventional 14 c dating of the aio in bulk samples yielded age reversals down core but at all sites the aio 14 c ages obtained from diatomaceous ooze within the diatom rich unit yielded similar uncorrected 14 c ages between 13517 56 and 11543 47 years before present a bp correction of these ages by subtracting the core top ages which probably reflect present day deposition as indicated by 210 pb dating of the sediment surface at one core site yielded ages between ca 10500 and 8400 cal a bp correction of the aio ages of the diatomaceous ooze by only subtracting the marine reservoir effect mre of 1300 a indicated deposition of the diatom rich sediments between 14100 and 11900 cal a bp most of these ages are consistent with age constraints between 13 0 and 8 0ka for the diatom rich unit which we obtained by correlating the relative palaeomagnetic intensity rpi records of three of the sediment cores with global and regional reference curves as a third dating technique we applied conventional radiocarbon dating of the aio included in acid cleaned diatom hard parts extracted from the diatomaceous ooze this method yielded uncorrected 14 c ages of only 5111 38 and 5106 38 a bp respectively we reject these young ages because they are likely to be overprinted by the adsorption of modern atmospheric carbon dioxide onto the surfaces of the diatom hard parts prior to sample graphitisation and combustion for 14 c dating the deposition of the diatom rich unit in the western amundsen sea suggests deglaciation of the inner shelf before ca 13ka bp the deposition of diatomaceous oozes elsewhere on the antarctic shelf around the same time however seems to be coincidental rather than directly related copyright 2009 john wiley sons ltd

subglacial bedforms reveal complex basal regime in a zone of paleo ice stream convergence amundsen sea embayment west antarctica the flow of ice streams which account for most discharge from large ice sheets is controlled by processes operating at the ice stream bed data from modern ice stream beds are difficult to obtain but where ice advanced onto continental shelves during glacial periods extensive areas of the former bed can be imaged using modern swath sonar tools we present new multibeam swath bathymetry data analyzed alongside sparse preexisting data from the amundsen sea embayment the compilation is the most extensive continuous area of multi beam data coverage yet obtained on the inner continental shelf of antarctica the data reveal streamlined subglacial bedforms that define a zone of paleo ice stream convergence but in contrast to previous models do not show a simple downflow progression of bedform types along paleo ice stream troughs we interpret high spatial variability of bedforms as indicating a complex mechanical and hydrodynamic regime at the former ice stream beds consistent with observations from some modern ice streams we conclude that care must be taken when using bedforms to infer paleo ice stream velocities

quartz in garnet and ti in quartz thermobarometry methodology and first application to a quartzofeldspathic gneiss from eastern papua new guinea

ice shelf history from petrographic and foraminiferal evidence northeast antarctic peninsula a detailed record of late pleistocene deglaciation followed by mid holocene ice shelf breakup and late holocene re growth is contained in continental shelf sediments in the northern larsen area northeast antarctic peninsula the zero age of core tops is confirmed by new and published 210pb profiles and 70 accelerator mass spectrometer ams 14c dates on bulk organic carbon define sedimentation rates of 7 6 92 cm ka the varied geology in the local ice drainage basins facilitates the use of ice rafted debris ird provenance in determining the presence or absence of ice shelves all inshore cores contain an interval of non local ird in the post glacial section demonstrating widespread ice shelf breakup in the mid holocene both breakup and re growth may have taken centuries and there are no widespread debris layers associated with breakup cores beyond and up to 30 km inside the historical ice shelf limit exhibit a varied ird provenance throughout the holocene suggesting the maximum ice shelf limit may date only from the little ice age benthic foraminiferal assemblages are related to water masses and position on the continental shelf and have been modified by taphonomic processes nevertheless we discern a deglaciation signal in prince gustav channel of a calcareous spike in predominantly agglutinated assemblages and this is repeated at the time of mid holocene ice shelf breakup the inferred mid holocene warm period occurred later in the northern larsen area than on the west coast of the antarctic peninsula

geomagnetic field intensity north atlantic deep water circulation and atmospheric δ14c during the last 50 kyr we present simulated records of past changes in the atmospheric 14c for the last 50 kyr due to changes in geomagnetic field intensity and in the strength of the north atlantic deep water nadw a new geomagnetic record was used largely based on the napis 75 record laj et al phil trans r soc london a 358 2000 1009 1025 which has been extended for the 0 20 kyr interval using archeomagnetic and volcanic data past changes of the nadw were derived from a mineral magnetic study of the cores used in the construction of napis 75 two box models of different complexity 4 and 17 boxes were used to simulate the carbon cycle calculated records of 14c are consistent with experimental determinations for the last 24 kyr for older ages the records calculated with variable oceanic circulation conditions reach values as high as 600 with an average of 500 between 20 and 40 kyr with maxima around 21 30 and 38 kyr gisp2 age model while low values are observed prior to 42 kyr although large inconsistencies in experimental data preclude precise comparison the average record simulated with the 17 box model is overall consistent with the icelandic sea record voelker et al radiocarbon 40 1998 517 534 42 2000 437 452 except for the extremely high peak observed in this record at 40 5 kyr on the other hand the results recently reported from a stalagmite recovered from a submerged cave in the bahamas beck et al science 292 2001 2453 2458 are inconsistent with all our model simulations in the 20 45 kyr interval the improved geomagnetic record combined with the new nadw profile allows us to give a modeled evaluation of the relative contribution of these factors to changes in atmospheric 14c the average simulation provides a first order modeled correction for conventional radiocarbon ages older than 25 kyr for which no calibration curve is available as yet

esr dating of fossil shells in the lutzow holm bay region east antarctica abstract the authors investigate esr dating of in situ fossil aragonitic shells from the emerged marine sediments in the lutzow holm bay region east antarctica to compare esr ages with ams 14c dates and to discuss late pleistocene chronology from the esr ages the ams 14c ages of late pleistocene for the samples 33 43 ka seem to be underestimated and to be much closer to the marine isotope stage mis 4 5 or mis 6 7 rather than to mis 3 our results suggest that further dating studies as well as 14c dating should be made in the study area by using various techniques

elastic wave velocity characterization of gas hydrate bearing fractured reservoirs in a permafrost area of the qilian mountain northwest china abstract there are two types of gas hydrate bearing reservoirs in the permafrost area of qilian mountain most of the gas hydrates occur mainly in the fractured mudstone reservoirs and rarely in the pores of the sandstone reservoirs in this study for the acoustic velocity characterization of the fractured gas hydrate reservoirs of the qilian mountain permafrost area some mudstone core samples were collected for physical rock experiments such as the acoustic experiment and the porosity and permeability experiment an acoustic velocity numerical simulation of gas hydrate reservoirs was performed according to the biot theory and the differential effective medium theory with the conditions of multiple gas hydrate occurrence models including the suspension model the semi cementation model and the cementation model and considering both infinite and penny shaped cracks fracture porosity was added to the core samples that only contain matrix porosity with fracture porosity ranging from 0 01 to 5 the variation laws between acoustic velocity with fractured porosity and hydrate saturation are obtained 1 in the case of an infinite crack if the fractured porosity is 0 01 1 the p wave velocity decreases rapidly in the case of the three occurrence models if the fractured porosity is higher than 1 the acoustic velocity decreases gradually if the crack shape is a penny shaped crack the p wave velocity decreases almost linearly with increasing fracture porosity 2 if the hydrate occurrence model is the suspension model the p wave velocity increases slightly with increasing hydrate saturation if the occurrence model is the semi cementation model or the cementation model when the gas hydrate saturation of the infinite crack ranges from 0 to 80 the acoustic velocity increases approximately linearly whereas when the gas hydrate saturation ranges from 80 to 100 the velocity increases rapidly if the crack is a penny shaped crack the velocity increases almost linearly with increasing gas hydrate saturation from 0 to 100 3 it is found that the fractured gas hydrate reservoirs of the qilian mountain permafrost area contain both penny shaped and infinite cracks of which the infinite crack is the main crack shape the gas hydrate occurrence in the qilian mountain permafrost area mainly follows the suspension model this has significance for the seismic exploration and log evaluation of gas hydrate bearing fractured reservoirs in the permafrost area of the qilian mountain in studying the acoustic velocity characterization the crack shapes and occurrence models of gas hydrate reservoirs in the study area

zircon and quartz inclusions in garnet used for complementary raman thermobarometry application to the holsnoy eclogite bergen arcs western norway mineral inclusions are common and have been widely used to investigate complex geological history when a rock undergoes cooling and decompression after the entrapment of an inclusion into a host mineral residual pressure may develop within the inclusion because of the differences in thermal expansivity and compressibility between the inclusion and host by combining laser raman spectroscopy and experimental data relating hydrostatic pressure and raman shift it is possible to estimate the entrapment pressure temperature p t conditions using an isotropic elastic model in this study we report raman spectroscopic data on both zircon and quartz inclusions in garnet host from the holsnoy eclogite bergen arcs norway averaged residual pressures based on different raman peaks for zircon and quartz inclusions are obtained to be ca 0 6 gpa and ca 0 65 gpa respectively using the equation of state for zircon and quartz the entrapment p t conditions are constrained to be 1 7 1 9 gpa 680 760 c consistent with previous estimates based on phase equilibria heating cooling experiments are performed on an entrapped zircon inclusion a clear trend is found between the residual zircon inclusion pressure and the externally controlled temperature we show that the residual zircon inclusion pressure sealed in garnet host is very sensitive to the entrapment temperature and can be used as a raman thermometer the effects of laser heating and the thermo elastic anisotropy of zircon inclusion are quantified and discussed

diatom evidence for late holocene climatic events in granite harbor antarctica downcore diatom data from a deep basin in granite harbor antarctica provide new high resolution information concerning changes in oceanographic and depositional processes that have occurred during approximately the last 1250 years the influence of global scale climatic change during the time of the medieval warm period and possibly the little ice age is revealed by long term changes in floral assemblages as recorded by changes in the relative concentration of nitzschia cylindrus the diatom data suggest that granite harbor experienced less annual ice and pack ice cover during both these periods possibly because of higher temperatures during the medieval warm period and stronger offshore winds during the little ice age the sedimentary record reveals several distinct short lived events in which nearly monospecific blooms of corethron criophilum and chaetoceros spp occurred possibly the result of strong stabilization of the upper water column and were most likely a response to regional rather than global conditions although short lived these events may play a significant role in the delivery of organic carbon and biogenic silica to the seafloor

titaniq deformed experimental deformation of out of equilibrium quartz porphyroclasts abstract the role of deformation on the equilibration of ti concentrations in quartz was investigated by running experiments on out of equilibrium quartz crystals embedded as porphyroclasts within quartz aggregates quartz crystals were first grown from a hydrostatic fluid to set the initial ti content crystals grown at temperatures above 925 c and below 875 c the deformation temperature 900 c contain ti contents 60 ppm higher or lower than the equilibrium solubility these out of equilibrium quartz crystals were then deformed as porphyroclasts by embedding them in a matrix composed of either dry or wet quartz to induce two end member deformation regimes high stress or low stress respectively in high stress experiments quartz porphyroclasts contain deformation bands and undulose extinction with some grains showing evidence for grain boundary bulging blg recrystallization in low stress experiments some grains deformed to high strains with minimal evidence for dynamic recrystallization resulting in ribbon grains whereas others recrystallized by dislocation creep processes showing microstructural evidence for subgrain rotation sgr and grain boundary migration gbm recrystallization by comparing quartz microstructures with patterns of ti re distribution in variably deformed and recrystallized porphyroclasts we find that grains deformed to high strain with minimal recrystallization show little to no evidence for re equilibration whereas grains that recrystallized via grain boundary processes have equilibrated ti contents that reflect the pressure temperature conditions of deformation

magnetite dissolution in siliceous sediments 1 magnetite dissolution and consequent loss of magnetization is widely observed in reducing sedimentary environments where the decrease in eh ph values with depth is driven by bacterially mediated degradation of organic carbon we have observed low magnetizations in sediments with elevated pore water silica concentrations that arise from diagenesis of biogenic silica and or silicic volcanic ash these depletions in magnetization are greater than can be accounted for by dilution with magnetite poor sediments and suggest that postdepositional destruction of magnetite has occurred biosiliceous sediments usually also contain elevated concentrations of organic carbon which makes it difficult to separate organic carbon related magnetite dissolution from other possible mechanisms for magnetite dissolution however the extent of magnetite dissolution in the sedimentary sequences that we have studied is not obviously related to the redox state of the environment this suggests that other mechanisms might have given rise to magnetite dissolution in these siliceous sediments thermodynamic calculations indicate that magnetite is unstable under conditions of elevated dissolved silica concentrations and appropriate eh ph conditions and predict that magnetite will break down to produce iron bearing smectite a survey of magnetic susceptibility and pore water geochemical data from widely distributed ocean drilling program sites supports this observed link between high dissolved silica concentrations and low magnetic susceptibilities this observed link also holds for environments with low biogenic silica productivity and low organic carbon content but with high interstitial silica concentrations due to dissolution of silicic volcanic ashes dissolution of magnetite is therefore predicted to be a common feature of siliceous sedimentary environments

diatom distribution in southern ocean surface sediments atlantic sector implications for paleoenvironmental reconstructions abstract a study of 230 surface sediment samples collected in the atlantic sector of the southern ocean between the southernmost weddell sea and the subtropical zone documents the modern distribution of diatoms revealing patterns of paleoenvironmental significance estimations of diatom valves per gram dry sediment display numbers of 50 200 106 in the zone of high opal burial located between the mean position of the winter sea ice edge and the polar front and maximum values of greater than 200 106 in the near shore sedimentary basins off the antarctic peninsula lowest diatom concentrations and assemblages strongly affected by dissolution were encountered in the weddell basin despite alteration of the diatom assemblages prior to their incorporation into the sediment record the biogeographic distribution and the abundance pattern of most of the 35 studied diatom species shows a close relationship with the surface hydrography water temperature these relationships can be used to estimate past surface water temperatures based on statistical treatments of the assemblages or on simple relations of species occurrences in the geological record another close link occurs between the distribution of sea ice and sea ice related diatoms

climate evolution at the last deglaciation the role of the southern ocean abstract two sediment sequences recovered close to and south of the present polar front 50 53 s in the atlantic sector of the southern ocean were analysed in order to evaluate the environmental evolution of the southern ocean surface over the last deglaciation and the holocene our reconstruction is based on radiocarbon dated records of diatom based sea surface temperature and sea ice proxies and planktic foraminiferal stable isotopes the onset of the deglacial warming and gradual southward retreat of the winter sea ice field between 18 and 17 cal kyr bp occurrence of a thermal reversal centered at ca 13 cal kyr bp antarctic cold reversal or acr and early holocene occurrence of the climatic optimum well correlate with climatic reconstructions available from the indian southern ocean and from antarctic ice cores time correspondence of southern ocean warming and heinrich event 1 in the north atlantic is compatible with the transmission of the climate signal from the northern to the southern hemisphere through the bipolar seesaw our data support modeling results suggesting that the northern hemisphere bolling warming and turn on of the north atlantic deep water formation are triggered by gradual warming and sea ice retreat in the southern ocean meltwater shedding into the southern ocean associated with the acr may maintain northern hemisphere warming during the allerod the development of sea surface warming and sea ice retreat is compatible with a southern ocean control on the atmospheric co2 increase during the deglaciation during the early holocene 9 7 cal kyr bp our southern core records renewed surface ocean cooling and northward readvance of the winter sea ice field such early holocene cooling is common to cores from the high latitude atlantic sector and might be brought about by expansion of the weddell gyre circulation at that time

tectonics of the new guinea region the new guinea region evolved within the obliquely and rapidly converging australian and pacific plate boundary zone it is arguably one of the most tectonically complex regions of the world and its geodynamic evolution involved microplate formation and rotation lithospheric rupture to form ocean basins arc continent collision subduction polarity reversal collisional orogenesis ophiolite obduction and exhumation of ultra high pressure metamorphic rocks we describe the major onshore and offshore tectonic and geologic components including plate boundaries seismicity faults and magmatism and we integrate these with emerging ideas about mantle dynamics to evaluate the cenozoic tectonic evolution of new guinea future research opportunities to resolve the mantle structure beneath new guinea will enable mantle dynamics to be linked to lithospheric and surface processes virtually all plate tectonic and mantle processes have been active in the new guinea region throughout the cenozoic and as such

laminations from the palmer deep a diatom based interpretation 1 highly laminated holocene age diatomaceous sediments are characteristic of the palmer deep western antarctic peninsula ocean drilling program odp leg 178 site 1098 from 10 000 years b p to the present laminations are comprised of several groups of diatoms chaetoceros resting spores the dominant laminae former result from intense spring blooms rhizosolenia proboscia thalassiothrix and corethron are common also the consequence of summer production marked by a well stratified water column each lamination represents a single productivity event but laminations are not necessarily annual high concentrations of a subpolar form of eucampia antarctica in laminations between 9000 and 6700 years b p suggest early holocene warmth a consequence of southward intrusion of more subpolar waters the glacial interglacial transition is distinguished by pairs of laminae most likely deposited annually laminations with an overwhelming dominance of chaetoceros resting spores alternate with more terrigenous laminae representing alternation of intense spring blooms with more mixed deposition during the summer proximity to retreating glacial ice results in the supply of silt and sand that provides a marker bed between successive blooms

lu hf garnet geochronology applied to plate boundary zones insights from the u hp terrane exhumed within the woodlark rift abstract high pressure and ultra high pressure u hp metamorphic rocks occur in many of the world s major orogenic belts suggesting that subduction of continental lithosphere is a geologically important process despite the widespread occurrence of these rocks relatively little is known about the timescales associated with u hp metamorphism this is because most u hp terranes are tectonically overprinted and juxtaposed against rocks with a different history an exception to this are the late miocene u hp metamorphic rocks found in active metamorphic core complexes mcc in the woodlark rift of southeastern papua new guinea this region provides a rare opportunity to study the garnet lu hf isotopic record of u hp metamorphism in a terrane that is not tectonically overprinted in order to constrain the timing of garnet growth relative to the history of u hp metamorphism and the evolution of the woodlark rift lu hf ages were determined in conjunction with measurements of lu and major element zoning for garnets from three metamorphic rocks garnets from the three samples yielded different ages that instead of recording the spatial and temporal evolution associated with a single metamorphic event provide information on the timing of three separate plate boundary events the youngest lu hf age determined was 7 1 0 7 ma for garnets in a late miocene coesite eclogite the age is interpreted to record the time when a garnet bearing partial melt of the mantle crystallized within subducted continental lithosphere at u hp conditions the young lu hf age from the coesite eclogite is in contrast to a 68 3 6 ma lu hf age obtained on large 1 2 cm garnet porphyroblasts from within the pleistocene amphibolite facies shear zone carapace bounding exposures of u hp rocks in the d entrecasteaux islands this older age records the growth of garnet in response to continental subduction and ophiolite obduction in the region north and east of australia during late mesozoic early cenozoic time in order to relate the u hp metamorphic rocks to the larger scale tectonic evolution of the woodlark rift a third lu hf age of 11 2 2 1 ma was determined for dynamically recrystallized garnet from the southeastern margin of the rift while the tectonic implications of the lu hf age from the southeastern rifted margin are not fully known this result suggests that the eastern and western parts of the woodlark rift might have a different metamorphic history the garnet lu hf results demonstrate that prior to rifting the woodlark rift underwent a complex history of metamorphism involving garnet growth and recrystallization the ability of the lu hf system to date late miocene garnet growth and at the same time preserve latest cretaceous garnets within pleistocene shear zones in the woodlark rift demonstrate that lu hf garnet geochronology is an essential component of investigations where the primary objective is documenting and understanding the full range of plate boundary processes recorded in metamorphic minerals

magnitude and geographic variation of the radiocarbon content in antarctic marine life implications for reservoir corrections in radiocarbon dating abstract radiocarbon age estimates on marine derined materials from the antarctic appear old in comparison with the terrestrial radiocarbon timescale as a result of significantly depleted 14 c concentrations in antarctic seas we review measurements of 14 c concentrations in modern antarctic marine life from south georgia the shetland islands and antarctic peninsula and suggest correction factors required to relate radiocarbon measurements on fossil marine materials from these areas to the conventional radiocarbon timescale for dates based on shells seaweeds sealbones and penguin bones the minimum correction factor for south georgia is ca 750 years and for the south shetland islands and antarctic peninsula it may be up to 1250 1300 years for whalebones a minimum correction of 1000 years may apply over a wide geographic area for comparison we also review radiocarbon measurements on marine species and seawater from other sectors of the antarctic in high latitudes correction factors of ca 1400 years are generally applicable to shell seal and penguin samples

ultrasonic velocity porosity relationships for sandstone analogs made from fused glass beads using fused glass beads we have constructed a suite of clean sandstone analogs with porosities ranging from about 1 to 43 percent to test the applicability of various composite medium theories that model elastic properties we measured p and s wave velocities in dry and saturated cases for our synthetic sandstones and compared the observations to theoretical predictions of the hashin shtrikman bounds a differential effective medium approach and a self consistent theory known as the coherent potential approximation the self consistent theory fits the observed velocities in these sandstone analogs because it allows both grains and pores to remain connected over a wide range of porosities this behavior occurs because this theory treats grains and pores symmetrically without requiring a single background host material and it also allows the composite medium to become disconnected at a finite porosity in contrast the differential effective medium theory and the hashin shtrikman upper bound overest

developing sediment geochronologies for high latitude continental shelf deposits a radiochemical approach during january and february of 1986 47 box cores and 55 piston cores were collected from the bransfield strait gerlache strait and marguerite bay antarctica apparent sediment accumulation rates established for 100 yr time scales with excess 210pb chronologies range from 0 08 to 0 34 cm yr in the bransfield strait 0 15 to 0 51 cm yr in gerlache strait and from 0 10 to 0 27 cm yr in marguerite bay for the antarctic peninsula continental shelf 137cs appears to have a limited usefulness in assessing the effects of deep sediment mixing by benthic fauna on seabed excess 210pb profiles because the observed activities for the regions examined are close to counter detection limits conventional 14c ages of surface sediments from the peninsula region range from 1370 to 2820 yr the spatial variations in the conventional 14c surface ages are thought to be influenced predominantly by inputs of older eroded sediment from lateral transport current winnowing mass wasting and or a variable carbon source on a 1000 yr time scale 14c measurements of five piston cores from the bransfield strait yielded sediment accumulation rates ranging from 0 06 to 0 49 cm yr these data support the assertion by nelson 1988 that the excess 210pb sediment accumulation rates range of 0 08 to 0 34 cm yr for this region represent true rates the two sediment accumulation rates of 0 01 and 0 08 cm yr for the gerlache strait are significantly lower than the observed apparent 210pb accumulation rates mean 0 27 0 12 cm yr for this region likewise two piston cores from marguerite bay yielded 14c sediment accumulation rates of 0 02 and 0 05 cm yr which also are considerably lower than the excess 210pb apparent accumulation rates mean 0 17 0 06 cm yr for this area the one dimensional advection diffusion equation of goldberg and koide geochim cosmochim acta vol 51 1962 was solved simulatneously with 210pb and 14c data for bc18 and bc113 to resolved the effects of sediment mixing and accumulation in bransfield strait and marguerite bay box core samples respectively this model indicates that 210pb and 14c sediment accumulation rates agree within a factor of 2 in the bransfield strait whereas in marguerite bay the apparent 210pb accumulation rates may be as much as an order of magnitude too high in the southern high latitude environments of the antarctic peninsula radiocarbon measurements in the seabed are a better approach than 210pb for establishing accurate rates of sediment accumulation the 14c chronologies when coupled with 210pb data provide more reliable assessments of biological deep mixing and sediment accumulation rates than do excess 210pb data coupled with measurements of bomb produced 137cs the sediment accumulation rates for both 100 yr and 1000 yr time scales tend to decrease southward from the bransfield strait to marguerite bay which is consistent with glacial marine sedimentation processes as modulated by climate for the western antarctic peninsula seabed measurements of 210pb 137cs and 14c also provides valuable insights on water circulation and upwelling atmospheric inputs and sediment depositional processes for the antarctic peninsula region

postglacial marine diatom record of the palmer deep antarctic peninsula odp leg 178 site 1098 1 total diatom abundance 1 late pleistocene to holocene climate variability in the antarctic peninsula is described from ocean drilling program odp leg 178 site 1098 palmer deep using total diatom abundance valves per gram dry sediment as a paleoproductivity proxy diatom frustules are absent in the diamicton at the base of the core which was deposited prior to age calibrated circa 13 2 ka immediately above the diamicton frustules are in low abundance as rare fragments and become increasingly more abundant and well preserved up core a rapid and major increase in productivity after circa 12 8 ka is evident from distinctly laminated sediments that overlie the diamicton the strongly laminated section was deposited during a deglaciation phase that followed the retreat of grounded ice after the last glacial maximum diatom productivity decreased during a climatic cooling and or change in meltwater supply until circa 9 0 ka between 9 0 and 4 4 ka productivity increased steadily again with diatom abundance maxima reached during a mid holocene climatic optimum the optimum is interrupted by a brief cold event of 1000 years duration 6 0 5 0 ka after 4 4 ka productivity generally decreased and the last 500 years have experienced the lowest diatom productivity of the postglacial period the palmer deep sediment is compared to ice core and lake sediment records from other regions around the antarctic peninsula these diverse data sets confirm that the antarctic peninsula has been a region of rapid and variable climate fluctuation since the last glacial maximum

divergent plate motion drives rapid exhumation of ultra high pressure rocks abstract exhumation of ultra high pressure u hp rocks by upper plate divergent motion above an unbroken slab first proposed in the western alps has never been tested by numerical methods we present 2d thermo mechanical models incorporating subduction of a thinned continental margin beneath either a continental or oceanic upper plate followed by upper plate divergent motion away from the lower plate results demonstrate how divergent plate motion may trigger rapid exhumation of large volumes of u hp rocks directly to the earth s surface without the need for significant overburden removal by erosion model exhumation paths are fully consistent with natural examples for a wide range of upper plate divergence rates exhumation rates are systematically higher than the divergent rate imposed to the upper plate and the modeled size of exhumed u hp domes is invariant for different rates of upper plate divergence major variations are instead predicted at depth for differing model scenarios as larger amounts of divergent motion may allow mantle wedge exhumation to shallow depth under the exhuming domes the transient temperature increase due to ascent of mantle wedge material in the subduction channel has a limited effect on exhumed continental u hp rocks already at the surface we test two examples the cenozoic u hp terranes of the western alps continental upper plate and eastern papua new guinea oceanic upper plate the good fit between model predictions and the geologic record in these terranes encourages the application of these models globally to pre cenozoic u hp terranes where the geologic record of exhumation is only partly preserved

recent sediment remolding on a deep shelf ross sea implications for radiocarbon dating of antarctic marine sediments abstract coarse bioclastic rich sands have been widely reported from the banks of the antarctic continental shelf but their origin is still poorly known we report on a suite of coarse sediments recovered from the top of the mawson bank in the northwestern ross sea radiocarbon ages of biogenic calcite for modern and apparently late pleistocene deposits range from 1085 45 to 20 895 250 yr b p discovery of soft tissue ascidian preserved as an incrustation on a pebble at 2 m depth indicates aggregation of the sediment within several months or a year of core recovery radiocarbon ages of acid insoluble organic matter aiom are less than those of the foraminifera calcite the aiom ages are also reversed in sequence indicating reworking of the sediment during deposition these observations and a review of recently published literature suggest that much of the bank top sediment in antarctica is presently undergoing remobilization under the influence of strong currents and or icebergs even under interglacical high stand sea levels these observations point out the need for careful integrated studies on high latitude marine sediment cores before resultant ages alone are used as the foundation for paleoglacial reconstructions

deep water sedimentary environments of the northwestern weddell sea and south sandwich islands antarctica abstract two transects have been sampled using short cores multi and box seabed photography video sequences and sediment profile images across the northwestern weddell sea and south sandwich forearc antarctica a total of 12 core stations were examined for sediment structure texture and composition to determine their depositional history four of the core stations from the weddell continental slope abyssal plain and south sandwich forearc and trench were further examined for trace metal analysis ba u mn and pb for sediment source and palaeoproductivity and 210pb for age profiles and depth of mixing by bioturbation the 3055 m station at the base of the weddell continental slope recovered laminated muddy silty contourites with a surface layer of hemipelagites with abundant ice rafted debris possibly originating from the 1995 larsen a and 2002 larsen b ice shelf disintegration this is interpreted as a moderate energy depositional environment influenced by weddell sea bottom water along slope current reworking sediment sediment sources to this station include the direct input of clastic material from ice with variation in sediment sources from in situ current winnowing and along slope transport reflected in the mn li and ba li ratios the 4760 m station on the weddell abyssal plain recovered hemipelagites interpreted as a low energy depositional environment with sediment supplied both from direct ice rafting and pelagic settling during productive seasonally ice free periods the 210pb profiles indicate a sedimentation rate of 0 024 cm year 1 with the 206 207pb isotope ratios suggesting a principle sediment source from ice rafting the 2292 and 6326 m stations from the slope and floor of the south sandwich forearc recovered pelagic and turbiditic sediments this is interpreted as a high energy down slope depositional environment with high surface biogenic productivity and sediment supply leading to localised down slope mass movements on the slope and floor high levels of mn li and elevated u li ratios demonstrate a strong concentration of mn in organic poor sediment the mn redoxcline is present at 10 cm depth the high ba li 500 ppm in the forearc floor core reflects some limited enhanced productivity source sediment for the forearc may in part be derived from the northwest weddell abyssal plain based on the 206 207pb isotope ratios reflecting transport by weddell sea bottom water

ice sheet extent of the antarctic peninsula region during the last glacial maximum lgm insights from glacial geomorphology we present marine geological and geophysical data for the antarctic peninsula that call for a larger ice sheet reconstruction during the last glacial maximum lgm than suggested by previous studies such glacial reconstructions are important for quantifying post lgm sea level rise and providing boundary conditions for general circulation models megascale glacial lineations on the continental shelf surrounding antarctica provide documentation for grounded ice that was streaming swath bathymetry data nbp02 01 reveal lineations in each of the major glacial troughs except for smith trough which features grooves and bedrock drumlins 0 5 3 km spacing 1 20 elongation ratios on crystalline bedrock we place the maximum extent of ice at the seaward limit of the lineations at or near megascale glacial lineations imaged in this study and other areas surrounding antarctica including the ross sea pine island bay have a consistent morphology with elongation ratios of 80 1 and spacings of 200 600 m mode of 300 m in contrast lineations bundles canals et al 2000 in the gerlache boyd trough exhibit larger spacing 1 5 km and the upstream portions 7 km are carved directly into bedrock rather than till as suggested by previously unpublished air gun 120 hz seismic data these features have yet to be dated however they are believed to have been active during the lgm radiocarbon dates in glaciomarine sediments foraminifera and organic matter from seven glacial troughs indicate that megascale glacial lineations were formed during the lgm other geomorphic features such as the grounding zone wedge in the northwestern weddell sea formed when the ice sheet was retreating the initial retreat of the grounded ice from the outer shelf occurred by 18 500 cal yr b p the inner shelf was mostly ice free by 13 000 cal yr b p this is significantly earlier than numerical models which suggest antarctic deglaciation began at 12 000 cal yr b p this also suggests the peninsula area contributed to global sea level rise associated with meltwater pulse mwp 1a

antarctic sediment chronology by programmed temperature pyrolysis methodology and data treatment 1 we report a detailed programmed temperature pyrolysis combustion methodology for radiocarbon 14c dating of antarctic sub ice shelf sediments the method targets the autochthonous organic component in sediments that contain a distribution of acid insoluble organic components from several sources of different ages the approach has improved sediment chronology in organic rich sediments proximal to antarctic ice shelves by yielding maximum age constraints significantly younger than bulk radiocarbon dates from the same sediment horizons the method proves adequate in determining isotope ratios of the pre aged carbon end member however the isotopic compositions of the low temperature measurements indicate that no samples completely avoided mixing with some proportion of pre aged organic material dating the unresolved but desired young end member must rely on indirect methods but a simple mixing model cannot be developed without knowledge of the sedimentation rate or comparable constraints a mathematical approach allowing for multiple mixing components yields a maximum likelihood age a first order approximation of the relative proportion of the autochthonous component and the temperature at which allochthonous carbon begins to volatilize and mix with the autochthonous component it is likely that our estimation of the cutoff temperature will be improved with knowledge of the pyrolysis kinetics of the major components chronology is improved relative to bulk acid insoluble organic material ages from nine temperature interval dates down to two but incorporation of inherently more pre aged carbon in the first division becomes more apparent with fewer and larger temperature intervals

geothermobarometric history of subduction recorded by quartz inclusions in garnet 1 burial histories of subduction zone rocks are often difficult to accurately constrain owing to a lack of robust mineral geobarometers applicable to high pressure mineral assemblages knowledge of the depth histories of subduction is however required for our understanding of global geochemical cycles subduction related seismicity and the evolution of destructive tectonic boundaries the high spatial resolution of quartz inclusion geobarometry can be used to determine pressure evolution during metamorphic growth of individual garnet crystals quartz inclusions in garnet from sifnos greece preserve such a record of the pressure of garnet growth allowing detailed reconstruction of the metamorphic evolution of these rocks pressure dependent raman spectra of quartz inclusions were combined with elastic modeling to infer the conditions at which they were trapped during garnet growth all measured inclusions suggest that garnet growth occurred between 19 and 20 5 kbars with little evidence for significant pressure variation during the garnet growth interval which is interpreted to record 100 c of heating coupled with thermometry and geochronology these results show that early cold burial was followed by a phase of rapid heating which immediately preceded exhumation garnet growth occurred primarily during this heating phase

postglacial marine diatom record of the palmer deep antarctic peninsula odp leg 178 site 1098 2 diatom assemblages 1 the distribution of late pleistocene to holocene diatom assemblages from ocean drilling program odp leg 178 site 1098 palmer deep is described and compared using multivariate statistical methods cluster analysis and analysis of variance five assemblages are identified from site 1098 which are correlated with the climatic zones identified previously in the palmer deep domack et al 2001 a thalassiosira diatom assemblage characterizes the deglaciation phase 13 2 11 0 ka and was deposited during glacial retreat at the end of the regional last glacial maximum a well preserved rhizosolenia assemblage was deposited during a climatic reversal 11 0 9 0 ka and indicates periods of seasonally open water with diatom blooms an open water fragilariopsis kerguelensis assemblage characterizes the mid holocene climatic optimum 9 0 3 7 ka in the neoglacial late holocene 3 7 ka climatic cooling increased sea ice cover and increased wind are inferred from a cocconeis assemblage during the climatic optimum neoglacial transition 4 4 1 8 ka laminae characterized by corethron criophilum were preserved it is uncertain whether the laminae represent short lived interannual bloom events with high settling rates or have been deposited over longer time periods

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

preservation efficiencies and accumulation rates for biogenic silica and organic c n and p in high latitude sediments the ross sea rates of biogenic sediment accumulation biogenic silica and organic c n and p and pore water flux have been established for a variety of depositional environments in the ross sea on the basis of 14c measurements in kasten cores sediment accumulation rates ranged from 250 cm kyr 1 in the coastal basin of granite harbor to 1 2 cm kyr 1 in the shelf and slope environments of the northern and eastern ross sea burial of biogenic material was most rapid in the southwestern ross sea where biogenic silica accumulation rates ranged from 2 to 31 g cm 2 kyr 1 and organic carbon accumulation rates ranged from 0 05 to 1 4 g cm 2 kyr 1 in the northern and eastern ross sea biogenic silica and organic carbon accumulation rates typically equaled 0 02 0 03 g sio2 cm 2 kyr 1 and 0 002 0 004 g c cm 2 kyr 1 flux core measurements were used to estimate seabed regeneration rates for biogenic silica organic carbon and phosphorus pore water fluxes in general showed much less variability across the ross sea than did the biogenic accumulation rates pore water silicate fluxes in the study area ranged from 0 6 to 5 3 g sio2 cm 2 kyr 1 whereas carbon fluxes ranged from 0 1 to 1 2 g c cm 2 kyr 1 and phosphate fluxes varied from 0 006 to 0 012 g p cm 2 kyr 1 seabed preservation efficiencies were calculated for biogenic silica organic carbon and phosphorus by combining the solid phase and pore water data the seabed preservation efficiencies for biogenic silica 1 86 were greater than for organic carbon 1 71 at all nine stations examined the preferential preservation of biogenic silica relative to organic carbon also was apparent in ross sea sediments because the biogenic silica organic carbon ratio in the material buried in the seabed generally was 2 times greater than the ratio in sediment particles arriving at the sediment water interface sediment accumulation rate correlated strongly with both the biogenic silica and the organic carbon preservation efficiency data p preservation efficiencies remained relatively high 24 65 even when the accumulation rates were low 1 2 cm kyr 1 because of the near zero phosphate fluxes out of the seabed of the total p in the seabed 0 04 0 09 wt approximately 25 existed in the form of organic p the total amount of biogenic silica accumulating on the ross sea shelf is 2 3 1012 g sio2 yr 1 which is approximately an order of magnitude less than the ledford hoffman et al 1986 estimate that was based on 210pb chronologies biogenic silica accumulation rates in the southern central and western ross sea increased during the mid and late holocene reaching their maximum values during the past 500 to 1000 years

thermoba raman try calibration of spectroscopic barometers and thermometers for mineral inclusions abstract mineral inclusions may develop residual pressures p incl during cooling and exhumation because of differential expansion and compression relative to the host entrapping mineral shifts to raman spectra as determined using laser raman microspectroscopy permit inference of p incl knowing the pressure temperature p t dependence of molar volumes from thermodynamic databases and the shear modulus of the host phase entrapment p at assumed t or entrapment t at assumed p can be calculated for a measured p incl this concept is developed theoretically to fit simplified empirical expressions of entrapment p t p incl or t p p incl for a wide range of common mineral host inclusion pairs typical residuals are quartz diamond graphite can preclude recovery of entrapment p s except for very high p s or rapid exhumation

assessment of the reliability of elastic geobarometry with quartz inclusions abstract elastic geobarometry makes use of the contrast in elastic proprieties between host and inclusion crystals to determine the entrapment conditions of the inclusions from the residual stress and strain measured in the inclusion when its host is at ambient conditions the theoretical basis has been developed extensively in the past few years but an experimental validation of the method is still required we performed two syntheses experiments with quartz inclusions in pure almandine garnet at eclogitic conditions experiment labelled alm 1 with synthesis performed at p 3 0 gpa and t 775 c and alm 2 at p 2 5 gpa and t 800 c all the experiments have been carried out in a piston cylinder press isolated fully enclosed quartz inclusions in the recovered garnets have been then measured using micro raman spectroscopy all fully buried inclusions exhibit raman peaks at higher frequencies and wavenumbers than those obtained from quartz crystals at ambient pressure if these peak shifts are interpreted as a remnant pressures by use of hydrostatic calibrations of the raman shifts of quartz with pressure the remnant pressures show a large spread in values and lead to significant errors in back calculated entrapment pressures of up to 1 4 gpa for inclusions synthesised at 3 0 gpa these results confirm that quartz inclusions trapped inside garnet are not subject to hydrostatic pressure we therefore used the phonon mode gruneisen tensors of quartz to calculate the full strain state of each inclusion from which the full anisotropic stress state can be calculated by using the elastic properties of quartz the mean residual remnant stress of the inclusions determined in this way show a much smaller spread in values entrapment pressures calculated from this mean stress with the isotropic model for host inclusion systems differ from the known experimental values by

thermochronologic constraints on the tectonic evolution of active metamorphic core complexes d entrecasteaux islands papua new guinea metamorphic core complexes in the d entrecasteaux islands papua new guinea formed as the result of active extension at the western end of the propagating woodlark basin spreading center domes of high grade metamorphic rocks i e amphibolites eclogites and migmatites intruded by large granodiorite bodies comprise the lower plate of the d entrecasteaux metamorphic core complexes the domes are transected by kilometer scale shear zones a thermochronologic study of the d entrecasteaux islands utilizing k ar 40ar 39ar and fission track techniques has documented the unroofing history of these active metamorphic core complexes gneisses in the cores of the domes cooled rapidly 100 c m y as indicated by hornblende and biotite 40ar 39ar apparent ages of 2 7 to 3 0 ma and 1 6 to 1 7 ma respectively and apatite fission track ages of 0 4 to 0 9 ma 40ar 39ar apparent ages on white mica biotite and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling in some cases 500 c m y and suggest shear zones were active from 4 0 to 3 5 ma and 1 9 to 1 4 ma in general 40ar 39ar mineral ages for retrogressed core zone gneisses shear zone gneisses and granodiorites are 2 0 to 3 0 ma amphiboles 1 5 to 1 7 ma muscovites and 1 4 to 1 8 ma biotites and 1 0 to 2 0 ma k feldspars apatite fission track ages from core zone gneisses shear zone gneisses and granodiorites range from 0 4 to 1 0 ma thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes the granodiorite bodies associated with the d entrecasteaux islands domes represent syn kinematically emplaced granitoids intruded into an area of active continental extension

pre bomb radiocarbon and the reservoir correction for calcareous marine species in the southern ocean four antarctic marine mollusc shells which were collected alive between 1917 and 1940 were analyzed by accelerator mass spectrometry to provide the first pre bomb radiocarbon measurements of biogenic carbonates from the southern ocean after correcting for the impact of fossil fuel combustion suess effect radiocarbon activities of the pre bomb shells averaged 149 8 10 4 in contrast the 14c values for post bomb molluscs echinoderms brachiopods and foraminifera averaged 96 1 25 2 these biogenic carbonate 14c values are nearly identical to pre bomb estimates 148 to 152 and post bomb measurements 98 4 22 0 of the surface waters in the southern ocean average radiocarbon ages of the biogenic carbonates before and after 1950 1303 84 years and 811 205 years respectively along with those from seals and penguins indicate that the antarctic marine radiocarbon reservoir has decreased in age by nearly 500 years during the second half of the 20th century marine species and seawater measurements firmly place the radiocarbon reservoir correction at 1300 100 years for calcareous marine fossils which are widespread abundant and well preserved organic materials for interpreting ice sheet climate and sea level impacts on the antarctic marine ecosystem during the holocene

exhumation of high pressure metamorphic rocks during crustal extension in the d entrecasteaux region papua new guinea the d entrecasteaux islands of eastern papua new guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea floor spreading in the west woodlark basin the complexes are defined by mountainous domes 2500 m high of fault bounded high grade metamorphic rocks including eclogite facies intruded by 2 4 ma granodiorite plutons garnet clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730 900 c the jadeite component of omphacite indicates minimum pressure of 21 kbar suggesting depths of 70 km the metamorphic rocks have undergone widespread retrogression to amphibolite facies retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes p t conditions in the early stages of shear zone activity determined using the garnet biotite exchange thermometer and the gasp and grips barometers were 570 730 c and 7 11 kbar a second phase of re equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons one re equilibrated gneiss indicated maximum temperature of 730 c at estimated pressures of approximately 4 kbar this late high temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes both closely coincide spatially with high temperature metamorphic rocks the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement these observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the d entrecasteaux islands metamorphic core complexes

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

a 1 3 myr palaeoceanographic record from the continental margin off dronning maud land antarctica a 12 5 m long core was retrieved from the continental margin off dronning maud land antarctica magnetostratigraphy stable isotopes 14 c accelerator mass spectrometer and amino acid analyses indicate a continuous sediment record going back 1 3 myr comparison of caco 3 results with those from odp site 1089 and an index of north atlantic deep water nadw influence in surface waters indicate that nadw upwelled along the antarctic continental margin during the whole of this period the midpleistocene transition 1 0 0 6 ma was accompanied by an apparent decline in the nadw influence and was followed by extended carbonate dissolution during the interglacials of marine isotope stages mis 13 and 11 less extensive periods of dissolution occur at the end of the interglacials younger than mis 11 while interglacial dissolution is characteristic of the pacific and indian oceans the carbon isotopes return to pre transition values indicative of renewed nadw upwelling the concentration of ice rafted debris may reflect changes in the relative rate of interglacial sedimentation it is speculated that the high ice rafted debris ird concentrations during interglacials younger than 400 kyr may be due to a reduced relative sedimentation rate of other interglacial components whereas the low concentrations during interglacials before the mid pleistocene transition may be due to a higher relative sedimentation rate of these

relationships between compressional wave and shear wave velocities in clastic silicate rocks new velocity data in addition to literature data derived from sonic log seismic and laboratory measurements are analyzed for clastic silicate rocks these data demonstrate simple systematic relationships between compressional and shear wave velocities for water saturated clastic silicate rocks shear wave veloci ty is approximately linearly related to compressional wave velocity and the compressional to shear velocity ratio decreases with increasing compressional velocity laboratory data for dry sandstones indicate a nearly constant compressional to shear velocity ratio with rigidity approximately equal to bulk modulus ideal models for regular packings of spheres and cracked solids exhibit behavior similar to the observed water saturated and dry trends for dry rigidity equal to dry bulk modulus gassmann s equations predict velocities in close agreement with data from the water saturated

uranium series and radiocarbon geochronology of deep sea corals implications for southern ocean ventilation rates and the oceanic carbon cycle we present new uranium series and radiocarbon measurements for deep sea corals from the southern ocean these data are used to reconstruct ventilation ages both at present and at the end of the last glacial period approximately 16 500 years ago we apply an improved two component mixing approach to correct uranium series dates for contaminant thorium and protactinium present in oxide coatings calculated seawater radiocarbon values for contemporary samples decrease with depth in the water column and agree with direct seawater radiocarbon measurements for this area this indicates that deep sea corals can accurately record seawater radiocarbon distributions two of three glacial samples experienced open system uranium series systematics however a third sample from the drake passage yields concordant thorium and protactinium dates as well as seawater values for initial 234u 238u this coral yields a ventilation age that is approximately 20 40 greater than modern values for its location this increase is consistent with published deep sea coral and calibrated planktonic benthic foraminifera radiocarbon data suggesting that the glacial oceans as a whole may have been substantially less ventilated presumably due to decreased formation of north atlantic deep water an overall decrease in oceanic mixing rates could have contributed to lower dissolved carbon in surface ocean water and lower atmospheric pco2 during the past glacial period

determining the amount of overstepping required to nucleate garnet during barrovian regional metamorphism connecticut valley synclinorium the novel method of inclusion barometry coupled with calculation of the required affinity for garnet nucleation is applied to three samples from the previously well characterized connecticut valley synclinorium in central vermont raman shifts for quartz inclusions record a range of maximum peak shifts of the quartz 464 cm 1 peak from 2 4 3 0 cm 1 temperature of garnet nucleation was constrained by calculating mineral assemblage diagrams in the mnnckfmasht system and plotting the intersection of quartz inclusion in garnet barometry quig with zr in rutile thermometry utilizing the intersection of zr in rutile thermometry with quig barometry garnet nucleation is inferred to have occurred within a p t range of 8 6 9 5 kbar and 560 575 c these p t conditions for garnet nucleation are significantly higher than calculated equilibrium garnet in isograds for the three samples affinities for garnet nucleation were calculated as the difference between the free energy of a fictive garnet composition based on the matrix assemblage and the free energy of the nucleated garnet the calculated nucleation affinity varied from 300 600 kj mol o for st ky grade samples these results suggest that the assumption that metamorphism proceeds as a sequence of near equilibrium conditions cannot in general be made for regional metamorphic terranes this body of work agrees with numerous recent studies showing that garnet producing reactions must be overstepped in order to for garnet to nucleate this article is protected by copyright all rights reserved

quantifying the opal belt in the atlantic and southeast pacific sector of the southern ocean by means of 230th normalization 1 a set of 114 samples from the sediment surface of the atlantic eastern pacific and western indian sectors of the southern ocean has been analyzed for 230th and biogenic silica maps of opal content th normalized mass flux and th normalized biogenic opal flux into the sediment have been derived significant differences in sedimentation patterns between the regions can be detected the mean bulk vertical fluxes integrated into the sediment in the open southern ocean are found in a narrow range from 2 9 g m 2 yr 1 eastern weddell gyre to 15 8 g m 2 yr 1 indian sector setting upper and lower limits to the vertically received fraction of open ocean sediments the silica flux to sediments of the atlantic sector of the southern ocean is found to be 4 2 1 4 1011 mol yr 1 just one half of the last estimate this adjustment represents 6 of the output term in the global marine silica budget

chronology of the palmer deep site antarctic peninsula a holocene palaeoenvironmental reference for the circum antarctic palmer deep sediment cores are used to produce the first high resolution continuous late pleistocene to holocene time series from the antarctic marine system the sedimentary record is dated using accelerator mass spectrometer radiocarbon methods on acid insoluble organic matter and foraminiferal calcite fifty four radiocarbon analyses are utilized in the dating which provides a calibrated timescale back to 13 ka bp reliability of resultant ages on organic matter is assured because duplicates produce a standard deviation from the surface age of less than laboratory error i e 50 years in addition surface organic matter ages at the site are in excellent agreement with living calcite ages at the accepted reservoir age of 1260 years for the antarctic peninsula spectral analyses of the magnetic susceptibility record against the age model reveal unusually strong periodicity in the 400 200 and 50 70 year frequency bands similar to other high resolution records from the holocene but so far unique f

the sediment infill of subglacial meltwater channels on the west antarctic continental shelf subglacial meltwater plays a significant yet poorly understood role in the dynamics of the antarctic ice sheets here we present new swath bathymetry from the western amundsen sea embayment west antarctica showing meltwater channels eroded into acoustic basement their morphological characteristics and size are consistent with incision by subglacial meltwater to understand how and when these channels formed we have investigated the infill of three channels diamictons deposited beneath or proximal to an expanded grounded west antarctic ice sheet are present in two of the channels and these are overlain by glaciomarine sediments deposited after deglaciation the sediment core from the third channel recovered a turbidite sequence also deposited after the last deglaciation the presence of deformation till at one core site and the absence of typical meltwater deposits e g sorted sands and gravels in all three cores suggest that channel incision pre dates overriding by fast flowing grounded ice during the last glacial period given the overall scale of the channels and their incision into bedrock it is likely that the channels formed over multiple glaciations possibly since the miocene and have been reoccupied on several occasions this also implies that the channels have survived numerous advances and retreats of grounded ice

variations in the geomagnetic dipole moment during the holocene and the past 50 kyr all absolute paleointensity data published in peer reviewed journals were recently compiled in the geomacia50 database based on the information in geomagia50 we reconstruct variations in the geomagnetic dipole moment over the past 50 kyr with a focus on the holocene period a running window approach is used to determine the axial dipole moment that provides the optimal least squares fit to the paleointensity data whereas associated error estimates are constrained using a bootstrap procedure we subsequently compare the reconstruction from this study with previous reconstructions of the geomagnetic dipole moment including those based on cosmogenic radionuclides be 10 and c 14 this comparison generally lends support to the axial dipole moments obtained in this study our reconstruction shows that the evolution of the dipole moment was highly dynamic and the recently observed rates of change 5 per century do not appear unique we observe no apparent link between the occurrence of archeomagnetic jerks and changes in the geomagnetic dipole moment suggesting that archeomagnetic jerks most likely represent drastic changes in the orientation of the geomagnetic dipole axis or periods characterized by large secular variation of the non dipole field this study also shows that the holocene geomagnetic dipole moment was high compared to that of the preceding similar to 40 kyr and that similar to 4 10 22 am 2 appears to represent a critical threshold below which geomagnetic excursions and reversals occur c 2008 elsevier b v all rights reserved less

quartz in garnet inclusion barometry under fire reducing uncertainty from model estimates quartz inclusions in garnet are suitable for barometry because quartz is highly compressible relative to garnet and the garnet host can maintain large stress differences generated as pressure temperature conditions change however experimental validation of the quartz in garnet approach has been limited raising questions concerning the accuracy of calculated entrapment pressure here we test the results of quartz in garnet barometry by conducting in situ raman analysis of natural inclusions over a range of temperatures 175 to 600 c we assess the temperature t dependence of inclusion pressure p incl at 1 bar and compare calculated entrapment pressures derived from measurements at different temperatures experiments used two quartz standards oriented c and and fully encapsulated quartz inclusions from three different terranes retaining different p incl at room t 444 296 and 755 mpa the stretched quartz inclusion p incl 0 had the greatest increase in p incl 264 mpa during heating from 25 to 500 c whereas the high p inclusions underwent less change 138 mpa in p incl over the same t interval the greater t sensitivity of inclusions with low p incl reflects the greater thermal expansivity of quartz near the to quartz transition while measured p incl t trends are consistent with predictions numerical models tend to overestimate p incl at elevated t and calculated entrapment pressures show an unrealistic dependence on reference t raman spectroscopic measurements conducted in situ at elevated t provide optimal results in addition we have recalibrated the thermal portion of the numerical method based on the present results and provide new empirical expressions for improved quartz in garnet barometry

modulus ratio of dry rock based on differential effective medium theory there are two points of view with regard to the relationship between the dry framework modulus ratio of bulk to shear modulus and porosity one view assumed by most empirical models such as the critical porosity model is that the dry framework modulus ratio is independent of porosity however this assumption is sometimes in disagreement with experiments another view supported by the differential effective medium dem theory is that the dry framework modulus ratio depends on porosity ordinary differential equations ode for bulk and shear moduli are coupled and there is no analytical formula for the modulus ratio of dry porous rock by assuming that the difference between the polarization factors of dry inclusions for bulk and shear modulus within the ode has an almost linear relationship with the effective modulus ratio an analytical solution for the dry rock modulus ratio is derived from the ode the above linear relationship characterized by intercept a and gradient b is confirmed by the numerical results calculated from the analytical formulas for the three specific pore shapes spherical pores needle shaped pores and penny shaped cracks then the validity of this assumption is tested by integrating the full dem equation numerically the analytical approximation gives a good estimate of the numerical results for the cases of the three given pore shapes the dry frame modulus ratio is monotonic with respect to porosity and its monotonicity depends on the mineral grain modulus ratio and the pore shape the analytical formula can be simplified further to some popular models such as the critical porosity model by setting constants a and b to specific values predictions of the analytical formula compare favorably with real and synthetic sandstone data and granite data measured in the laboratory

permeability and acoustic velocity controlling factors determined from x ray tomography images of carbonate rocks carbonate reservoir rocks exhibit a great variability in texture that directly impacts petrophysical parameters many exhibit bi and multimodal pore networks with pores ranging from less than 1 m to several millimeters in diameter furthermore many pore systems are too large to be captured by routine core analysis and well logs average total porosity over different volumes consequently prediction of carbonate properties from seismic data and log interpretation is still a challenge in particular amplitude versus offset classification systems developed for clastic rocks which are dominated by connected intergranular unimodal pore networks are not applicable to carbonate rocks pore geometrical parameters derived from digital image analysis dia of thin sections were recently used to improve the coefficient of determination of velocity and permeability versus porosity although this substantially improved the coefficient of determination no spatial information of the pore space was considered because dia parameters were obtained from two dimensional analyses here we propose a methodology to link local and global pore space parameters obtained from three dimensional 3 d images to experimental physical properties of carbonate rocks to improve p wave velocity and permeability predictions results show that applying a combination of porosity microporosity and 3 d geometrical parameters to p wave velocity significantly improves the adjusted coefficient of determination from 0 490 to 0 962 a substantial improvement is also observed in permeability prediction from 0 668 to 0 948 both results can be interpreted to reflect a pore geometrical control and pore size control on p wave velocity and permeability

holocene paleomagnetic records from the st lawrence estuary eastern canada centennial to millennial scale geomagnetic modulation of cosmogenic isotopes two long holocene piston cores md99 2220 and md99 2221 were raised from the st lawrence estuary eastern canada because of the expanded holocene sediment sequence this location provides a u channel based paleomagnetic study supported by an accelerator mass spectrometry ams 14c chronology rock magnetic and sedimentological data indicates that these sediments provide a paleomagnetic directional secular variation psv and relative paleointensity rpi proxy records for the last 8500 cal bp sedimentation rates vary from 1 2 to 4 2 m kyr comparison of inclination and declination features with other north american holocene psv records are generally consistent and temporal offsets are within chronological uncertainties the normalized natural remanent magnetization intensity record a rpi proxy from the postglacial sediments of core 2220 compares favorably with north american and european holocene rpi records at millennial and even some centennial timescales comparisons between core 2220 rpi proxy and the 10be flux record from the greenland summit gisp2 ice core finkel and nishiizumi j geophys res 102 1997 26699 26706 and a 14c production rate record bond et al science 294 2001 2130 2136 suggest that geomagnetic modulation may control the millennial and even some centennial scale variability within cosmogenic isotope records this implies that the core 2220 rpi record reflects changes in global scale geomagnetic field at these timescales

holocene history of the larsen a ice shelf constrained by geomagnetic paleointensity dating a sedimentary record collected from beneath the former lar sen a ice shelf reveals the holocene history of the larsen a re gion the record begins with the transition from grounded ice to a floating ice shelf completed by 10 7 6 0 5 ka and ends with the modern recession the record contains several late holocene dia tomaceous ooze layers that suggest proximity to productive open water events radiocarbon ages obtained from these sediments were complicated by the presence of detrital and reworked carbon we have eliminated these complications and constructed a chro nology for the larsen a ice shelf history via tuning of the geo magnetic field paleointensity record with a reference curve this approach provides chronological control to sediment sequences that lack appropriate material for radiocarbon dating geomag netic paleointensity features with wavelengths of 2 3 k y can be recognized and interhemispherically correlated illustrating the po tential to use geomagnetic paleointensity variations as a global cor relation and dating tool at sub milankovitch time scales

post glacial seasonal diatom record of the mertz glacier polynya east antarctica an ultra high resolution post glacial laminated sediment record from mertz ninnis trough east antarctic margin eam has been analysed using sem backscattered electron imagery secondary electron imagery and quantitative diatom abundance laminations are classified using visually dominant diatom species and terrigenous content four biogenic diatom ooze laminae types one diatom bearing terrigenous lamina type and one diatom bearing terrigenous sub lamina type have been identified diatom ooze lamina types comprise near monogeneric hyalochaete chaetoceros spp resting spore laminae laminae characterised by corethron pennatum laminae characterised by rhizosolenia spp and mixed diatom assemblage laminae diatom bearing terrigenous lamina and sub lamina types comprise mixed diatom assemblage terrigenous laminae and sub laminae characterised by porosira glacialis resting spores formation of each of these lamina types is controlled by seasonal changes in nutrients oceanographic regimes and the mertz glacier polynya dynamics

the reconstruction of late quaternary antarctic sea ice distribution the use of diatoms as a proxy for sea ice the study of annual diatom transfer from the southern ocean surface to the sea floor using time series sediment traps combined with the mapping of diatom assemblages from surface sediments results in the definition of a proxy for past variations of sea ice extent the application and robustness of the proposed proxy are demonstrated by the study of late pleistocene sediment cores recovered from different environmental regimes in the southern ocean relative amounts of the sea ice indicator diatoms fragilariopsis cylindrus and fragilariopsis curta of 3 in diatom assemblages preserved in sediments are representative of the presence of seasonal winter sea ice summer sea ice occurrence is signalled by a distinct drop in biogenic sedimentation rate as a result of strongly reduced biogenic export in the presence 3 of diatom assemblage of the deep temperature diatom fragilariopsis obliquecostata the spatial and temporal distribution pattern of the sea ice proxy indicates significant sea ice variations during marine isotope stages 1 and 5 in the present antarctic zone of the southern ocean during glacial maxima the antarctic winter sea ice was extended into the present polar front zone while the antarctic summer sea ice was most probably expanded into the area of the present winter sea ice edge during the short climatic optima at the onset of interglacial periods the winter sea ice extent was distinctly reduced

the pressure path of solid inclusions in minerals the retention of coesite inclusions during uplift abstract an inclusion model allows calculation of the stresses in and around minerals included in other minerals during changes in pressure and temperature the equations are applied to illustrate cooling and uplift histories of quarzo feldspathic rocks from 500 c and various pressures to ambient conditions even in the absence of pore fluid pressure microfractures may open at external pressures of 200 400 mpa and temperatures of 200 400 c due to differential volume changes of the constituent minerals coesite included in garnet cannot have formed during progressive metamorphism from quartz at lithostatic pressures below the coesite stability field because of differential volume changes coesite inclusions are captured by their host minerals at ultra high pressures and they persist to lower pressures because of the large volume increase occurring at the coesite to quartz transition the pi t path followed by the sio2 inclusion traces the boundary between the stability fields of coesite and quartz until radial fractures develop in the host at low external pressure

226ra in barite absolute dating of holocene southern ocean sediments and reconstruction of sea surface reservoir ages absolute dating of marine sediments is necessary to achieve reliable correlations of paleoclimate records the 2 2 6 ra decay in barite separated from a deep sea core of the southern ocean was used to determine absolute ages for the holocene epoch comparison with 1 4 c ages measured on planktic foraminifers provides the first past sea surface reservoir ages in the antarctic zone of the southern ocean throughout the middle to late holocene our results indicate a reservoir age of ca 1100 yr comparable to modern estimates a significantly higher reservoir age ca 1900 yr is obtained for the early holocene which suggests a major change in the southern ocean circulation at that time