tem study of in situ organic matter on continental margins occurrence and the monolayer hypothesis abstract tem photomicrographs show that organic matter in the continental margin sediments that we have examined is primarily patchy in distribution and occurs as 1 discrete discontinuous blebs and micro blebs of differentiated and undifferentiated protoplasm 2 bacterial cells and associated muco polysaccharide networks and 3 localized smears generally associated with clay minerals and domain junctions in clay rich flocs not as thin uniform grain coatings or as infillings of nicks or etch pits on sediment grains with high degrees of surface roughness this occurrence contradicts recent assertions that most organic matter in continental margins occurs as monolayer or monolayer equivalent coatings on mineral grains and inorganic bioclastic particles our results are based on tem photomicrographs of samples from the northern california continental slope that fall within the monolayer equivalent envelope defined by mayer 1994 marine snow from the overlying nepheloid layer and sediments from eckernforde bay in the baltic sea these samples were prepared specifically to preserve and image in situ sediment fabric and organic constituents textural evidence suggests that low permeabilities caused by the interaction between the organic inorganic and biologic components of the clay size fraction play a major role in determining organic matter preservation on continental margins

organic molecules formed in a primordial womb primordial organic molecules may have evolved in seafloor hydrothermal systems by mineral catalysis most organic compounds decompose in 300 c vent fluid however we show that smectite type clays can protect and promote development of diverse organic compounds that may be precursors to biomolecules smectite provides a safe haven for the synthesis of organic molecules essentially like a primordial womb our experiments simulated seafloor hydrothermal conditions 300 c 100 mpa and reacted common clays montmorillonite saponite illite with dilute methanol as a source of c montmorillonite reacts under these conditions to illite while the other clays do not change we observed increased organic synthesis over time with montmorillonite during mineralogical reaction approaching equilibrium smectite contracts and organic molecules are expelled organic compounds unstable in hot fluid where the smectite reacts may survive in cooler waters outside the vent

sediment redistribution versus paleoproductivity change weddell sea margin sediment stratigraphy and biogenic particle flux of the last 250 000 years deduced from 230thex 10be and biogenic barium profiles high resolution 230thex and 10be and biogenic barium profiles were measured at three sediment gravity cores length 605 850 cm from the weddell sea continental margin applying the 230thex dating method average sedimentation rates of 3 cm kyr for the two cores from the south orkney slope and of 2 4 cm kyr for the core from the eastern weddell sea were determined and compared to 18o and lithostratigraphic results strong variations in the radionuclide concentrations in the sediments resembling the glacial interglacial pattern of the 18o stratigraphy and the 10be stratigraphy of high northern latitudes were used for establishing a chronostratigraphy biogenic ba shows a pattern similar to the radionuclide profiles suggesting that both records were influenced by increased paleoproductivity at the beginning of the interglacials however 230thex0 fluxes 0 stands for initial exceeding production by up to a factor of 4 suggest that sediment redistribution processes linked to variations in bottom water current velocity played the major role in controlling the radionuclide and biogenic barium deposition during isotope stages 5e and 1 the correction for sediment focusing makes the true vertical paleoproductivity rates deduced from the fluxes of proxy tracers like biogenic barium much lower than previously estimated very low 230thex0 concentrations and fluxes during isotope stage 6 were probably caused by rapid deposition of older resedimented material delivered to the weddell sea continental slopes by the grounded ice shelves and contemporaneous erosion of particles originating from the water column

new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 moore g f taira a klaus a becker l boeckel b cragg b a dean a fergusson c l henry p hirano s hisamitsu t et al 2001 new insights into deformation and fluid flow processes in the nankai trough accretionary prism results of ocean drilling program leg 190 geochemistry geophysics geosystems 2 article no 2001gc000166

sediment deformation and hydrogeology of the nankai trough accretionary prism synthesis of shipboard results of odp leg 131 the main objective of leg 131 was to provide data on the deformational processes and associated hydrogeology of the nankai prism toe drilling succeeded for the first time in the history of ocean drilling in penetrating the complete sedimentary sequence to basaltic basement reaching 1327 mbsf metres below seafloor with good core recovery 55 excellent correlation of the lithology and structure including the frontal thrust and the decollement with seismic reflection images was also determined bedding dips faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to nw se shortening as expected from the plate tectonic convergence vector below the decollement no significant deformation features were observed indicating that the decollement is a sharp discontinuity in stress transmission physical properties data show major discontinuities at the decollement notably an increase in porosity below the later this may indicate excess pore pressure in the subducted section and decollement zone a less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature attempts to make downhole measurements were severely hampered by unstable hole conditions but useful constraints have been placed on the thermal regime and some calibration of laboratory physical properties toin situ conditions has been provided andin situ stress and pore pressure were measured in the uppermost sediments evidence of channelized fluid flows is inconclusive no sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found thermal measurements are not significantly different from those predicted by a purely conductive heat flow model a signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events we conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site

quaternary history of the antarctic circumpolar current evidence from the scotia sea within the scotia sea the axis of the antarctic circumpolar current acc is geographically confined and sediments therefore contain a record of palaeo flow speed uncomplicated by acc axis migration we outline holocene and last glacial maximum lgm current controlled sedimentation using data from 3 5 khz profiles cores and current meter moorings geophysical surveys show areas of erosion and deposition controlled by neogene basement topography deposition occurs in mounded sediment drifts or flatter areas where 500 1000 m of sediment overlies acoustic basement 3 5 khz profiles show parallel continuous sub bottom reflectors with highest sedimentation rates in the centre of the drifts and reflectors converging towards marginal zones of non deposition locally on the flanks of continental blocks e g south georgia downslope processes are dominant the absence of mudwaves on the sediment drifts may result from the unsteadiness of acc flow a core transect from the acc axis south to the boundary with the weddell gyre shows a southward decrease in biogenic content controlled by the polar front and the spring sea ice edge both these features lay farther north at lgm the cores have been dated by relative abundance of the radiolarian cycladophora davisiana and by changes in the biogenic ba content a palaeoproductivity indicator sedimentation rates range from 3 to 17 cm ka the grain size of holocene sediments shows a coarsening trend from south to north consistent with strongest bottom current flow near the acc axis though interpretation is complicated by the presence of biogenic grains year long current meter records indicate mean speeds from 7 cm s in the south to 12 cm s in the north with benthic storm frequency increasing northwards lgm sediments are predominantly terrigenous and show a clearer northward coarsening trend with well sorted silts in the northern scotia sea assuming a constant terrigenous source this implies stronger acc flow at the lgm contrasting with weaker weddell gyre flow deduced from earlier work

the oxygen isotope record in murchison and other carbonaceous chondrites the ranges of 18o and 17o in components of the murchison c2 chondrite exceed those in all other meteorites analyzed previous authors have proposed that c2 chondrites are the products of aqueous alteration of anhydrous silicates a model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes the minimum number two of initial isotopic reservoirs is assumed two major stages of reservoir interaction are required one at high temperature to produce the16o mixing line observed for the anhydrous minerals and another at low temperature to produce the matrix minerals the isotopic compositions severely constrain the conditions of the low temperature process requiring temperatures 44 extension of the model to the data on c1 chondrites requires aqueous alteration in a warmer wetter environment

evidence for a weak and overpressured decollement beneath sediment dominated accretionary prisms we present structural data from several underthrust sequences in southwest alaska that suggest that the deformation regime beneath accretionary prisms is characterized by a subvertical 1 and regionally extensive layer parallel shearing these results contrast markedly with the results obtained from the more shallow parts of accretionary prisms and fold and thrust belts where 1 plunges gently seaward or toward the deformation front on the basis of mohr circle diagrams we propose that these different stress directions are consistent only with a model in which the two regimes are separated by a very weak decollement and calculate coefficient of friction values that range from 0 25 to 0 08 these values are significantly lower than coefficient of friction values calculated for the basal decollement in the taiwan fold and thrust belt e g 0 2 barr and dahlen 1989a and they are also lower than laboratory friction measurements for most rocks e g 0 6 0 8 byerlee 1978 consequently these low values suggest the presence of rocks with unusual material properties e g montmorillonite logan and rauenzahn 1987 or anomalous fluid pressures along the decollement on the basis of the occurrence of shale diapirs the distribution of early veins and the absence of montmorillonite in the underthrust sequences we propose that there was an anomalously high fluid pressure along the decollement fault instead of there being such a low coefficient of friction these results emphasize the importance of dewatering and fluid flow in controlling the position of the decollement which in turn determines how the accretionary prism grows in essence it is the development of a weak overpressured decollement within a thick sediment pile that allows plate convergence to be partitioned into offscraping and underthrusting regimes

compaction in the nankai and barbados accretionary prisms new insights from logging while drilling data 1 sediment deformation changes in physical properties and fluid migration can be studied under well defined conditions in accretionary prisms our investigations focused on the accretionary prisms of barbados caribbean and nankai japan to interpret the effects of compaction on different lithologies during accretion the interpretation is based on very good quality logging while drilling data statistical data analysis is used to determine sediment composition and its specific physical properties finally the sediments are decompacted to determine the initial compaction before accretion and thus to quantify both the lateral and the vertical compaction components this investigation shows that different lithologies undergo similar compaction and porosity reduction the porosity differences of selected sediment layers between adjacent sites are significantly lower in nankai than in barbados which implies higher fluid expulsion in nankai decompaction of these sediment layers proves that the lateral compaction component is higher than the vertical compaction component

the oxygen isotopic composition of olivine and pyroxene from ci chondrites the ci chondrites are taken to represent average solar system material based on the similarity of their elemental compositions to that of the solar photosphere however their oxygen isotope geochemistry is dominated by secondary minerals that formed during aqueous alteration on the ci parent body precursors to this alteration namely olivine and pyroxene are extremely rare in ci chondrites precluding previous measurements of their oxygen isotopic composition we report ion microprobe analyses of oxygen isotopes in single olivine and pyroxene grains separated from ci chondrites orgueil and ivuna the ci chondrite olivine and pyroxene grains most likely represent liberated chondrule phenocrysts based on petrographic chemical and isotopic evidence consistent with crystallization from a melt the oxygen isotope data form an array that falls nearly along the carbonaceous chondrite 16o mixing line with 18o values ranging from 9 3 to 12 3 o and 17o from 11 3 o to 7 8 o consistent with nebular processes being the source of the oxygen isotopic compositions the degree of 16o enrichment in orgueil olivines is negatively correlated with feo content but the exact nature and timing of the process that introduced this variation remains unknown the pyroxene oxygen isotopic compositions are similar to those of olivines with 5 mol fayalite the oxygen isotopic analyses of the olivine and pyroxene in ci chondrites have been used to revise previous models for the isotopic evolution of ci materials our data require more complete gas solid equilibration in the nebula and constrain the initial aqueous fluids on the ci parent body to have lower 17o values than previously postulated the refined model indicates that the temperature of aqueous activity on the ci parent body was no higher than 50 c and the fluid rock ratio was significantly less than previously estimated even prior to alteration and formation of secondary minerals the ci chondrites were the most 16o depleted carbonaceous chondrites and thus the solids originally contained in the ci chondrites are the most equilibrated nebular materials represented in the carbonaceous chondrites the data suggest the oxygen isotopic composition of average solar system to be approximately equivalent to average terrestrial oxygen as recorded in the compositions of terrestrial and lunar basalts

light element geochemistry of the tagish lake ci2 chondrite comparison with ci1 and cm2 meteorites we have studied the carbon and nitrogen stable isotope geochemistry of a small pristine sample of the tagish lake carbonaceous chondrite by high resolution stepped combustion mass spectrometry and compared the results with data from the orgueil ci1 eet 83334 cm1 and murchison cm2 chondrites the small chip of tagish lake analysed herein had a higher carbon abundance 5 81 wt than any other chondrite and a nitrogen content 1220 ppm between that of ci1 and cm2 chondrites owing to the heterogeneous nature of the meteorite the measured carbon abundance might be artificially high the carbon inventory and whole rock carbon isotopic composition 13c 24 4 of the chip was dominated by 13c enriched carbon from the decomposition of carbonates between 1 29 wt and 2 69 wt 13c 67 and 18o 35 in the proportions 4 1 dolomite to calcite in addition to carbonates tagish lake contains organic carbon 2 6 wt 13c 9 1033 ppm n 15n 77 a level intermediate between ci and cm chondrites around 2 of the organic material is thermally labile and solvent soluble a further 18 of the organic species are liberated by acid hydrolysis tagish lake also contains a complement of presolar grains it has a higher nanodiamond abundance 3650 4330 ppm than other carbonaceous chondrites along with 8 ppm silicon carbide whilst carbon and nitrogen isotope geochemistry is not diagnostic the data are consistent with classification of tagish lake as a ci2 chondrite

high resolution holocene climate record from maxwell bay south shetland islands antarctica the highest resolution holocene sediment core from the antarctic peninsula to date was collected during the first shaldril cruise nbp0502 drilling yielded a 108 2 m long core 87 recovery site nbp0502 1b from maxwell bay south shetland islands this high resolution sediment record comes from a region that is currently experiencing dramatic climate change and associated glacial retreat such records can help to constrain the nature of past climate change and causal mechanisms and to provide a context for evaluating current climate change and its impacts the base of the drill site sampled till and or proximal glacimarine sediments resting directly on bedrock glacimarine suspension deposits composed of dark greenish gray silty mud with variable diatom abundance and scattered very fine sand laminations make up the majority of the sedimentary section detailed sedimentological and geochemical analyses including magnetic susceptibility total organic carbon toc content carbon and nitrogen isotopic composition pebble content and biogenic silica content allow subdivision of the glacimarine section into nine units and seismic facies analyses resulted in the identification of six distinct seismic units we used 29 radiocarbon ages to construct an age model and calculate sedimentation rates that vary by two orders of magnitude from 0 7 mm a to 30 mm a radiocarbon ages from glacimarine sediments just above the till date back to between 14 1 and 14 8 ka thus ice was grounded in the fjord during the last glacial maximum and eroded older sediments from the fjord following initial retreat of grounded ice from maxwell bay the fjord was covered by a permanent floating ice canopy probably an ice tongue the highest sedimentation rate corresponds to an interval that contains abundant sand laminations and gravelly mud intervals and likely represents a melt out phase or period of rapid glacial retreat from 10 1 ka to 8 2 ka there is no evidence for an early holocene climatic reversal as recorded farther south at the palmer deep drill site minimum sea ice cover and warm water conditions occurred between 8 2 and 5 9 ka from 5 9 to 2 6 ka there was a gradual cooling and more extensive sea ice cover in the bay after 2 6 ka the climate varied slightly causing only subtle variation in glacier grounding lines there is no compelling evidence for a little ice age readvance in maxwell bay the current warming and associated glacial response in the northern antarctic peninsula appears to be unprecedented in its synchroneity and widespread impact

elevated fluid pressure and extreme mechanical weakness of a plate boundary thrust nankai trough subduction zone pore fluid pressure is a key parameter governing both the shear strength of faults and the conditions for seismic and tsunamigenic slip on subduction zone megathrusts however quantitative constraints on this parameter based on in situ data in these or any faults have proven elusive using seismic interval velocities derived from a three dimensional seismic reflection survey we compute porosity fluid pressure and effective stress at the plate interface in the nankai trough subduction zone offshore southwestern japan to 20 km down the fault dip using well constrained locally calibrated empirical relationships between velocity porosity and effective stress we show that the fault and immediately subjacent footwall are nearly undrained implying that the subduction megathrust slips under a shear stress of

underthrust sediments fluid migration paths and mud volcanoes associated with the accretionary wedge off costa rica middle america trench styles of deformation and tectonic responses resulting from the convergence of oceanic and continental plates are strongly dependent on fluids in the sediments we estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the middle america trench near costa rica reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion within 4 km of the deformation front where overburden is 800 m more than half of the total water in the pore spaces may have been expelled the rate of fluid transfer from the underthrust section is highest in the zone 0 4 km arcward of the deformation front the clarity of the reflection associated with the decollement is exceptional and shows a phase reversal relative to the seafloor this high amplitude phase reversed reflection ends about 4 km arcward of the deformation front suggesting a rapid reduction in the density and or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50 m and extent of perhaps 500 m 1000 m a reflection traced beneath a seafloor mud volcano crosses through the accretionary wedge slope cover boundary and may mark a fluid conduit locally high amplitude reflections associated with the base of slope cover are coincident with closure of structure and indicate local traps to upward organized or diffusive fluid flow some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section near the trench where fluid expulsion rates are highest water may exit to the seafloor seaward of the downslope pinch out of the slope section

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

abrupt cooling of antarctic surface waters and sea ice expansion in the south atlantic sector of the southern ocean at 5000 cal yr b p antarctic surface waters were warm and ice free between 10 000 and 5000 cal yr b p as judged from ice rafted debris and microfossils in a piston core at 53 s in the south atlantic this evidence shows that about 5000 cal yr b p sea surface temperatures cooled sea ice advanced and the delivery of ice rafted detritus ird to the subantarctic south atlantic increased abruptly these changes mark the end of the hypsithermal and onset of neoglacial conditions they coincide with an early neoglacial advance of mountain glaciers in south america and new zealand between 5400 and 4900 cal yr b p rapid middle holocene climate changes inferred from the taylor dome ice core antarctica cooling and increased ird in the north atlantic and the end of the african humid period the near synchrony and abruptness of all these climate changes suggest links among the tropics and both poles that involved nonlinear response to gradual changes in northern hemisphere insolation sea ice expansion in the southern ocean may have provided positive feedback that hastened the end of the hypsithermal and african humid periods in the middle holocene

amino acids in the tagish lake meteorite high performance liquid chromatography hplc based amino acid analysis of a tagish lake meteorite sample recovered three months after the meteorite fell to earth have revealed that the amino acid composition of tagish lake is strikingly different from that of the cm and ci carbonaceous chondrites we found that the tagish lake meteorite contains only trace levels of amino acids total abundance 880 parts per billion ppb which is much lower than the total abundance of amino acids in the ci orgueil 4 100 ppb and the cm murchison 16 900 ppb because most of the same amino acids found in the tagish lake meteorite are also present in the tagish lake ice melt water we conclude that the amino acids detected in the meteorite are terrestrial contamination we found that the exposure of a sample of murchison to cold water lead to a substantial reduction over a period of several weeks in the amount of amino acids that are not strongly bound to the meteorite matrix however strongly bound amino acids that are extracted by direct hcl hydrolysis are not affected by the leaching process thus even if there had been leaching of amino acids from our tagish lake meteorite sample during its three month residence in tagish lake ice and melt water a murchison type abundance of endogenous amino acids in the meteorite would have still been readily detectable the low amino acid content of tagish lake indicates that this meteorite originated from a different type of parent body than the cm s and ci s the parent body was apparently devoid of the reagents such as aldehyldes ketones hcn and ammonia needed for the effective abiotic synthesis of amino acids based on reflectance spectral measurements tagish lake has been associated with p or d type asteroids if the tagish lake meteorite was indeed derived from these types of parent bodies our understanding of these primitive asteroids needs to be re evaluated with respect to their potential inventory of biologically important organic compounds

the fall recovery orbit and composition of the tagish lake meteorite a new type of carbonaceous chondrite the preatmospheric mass of the tagish lake meteoroid was about 200 000 kilograms its calculated orbit indicates affinity to the apollo asteroids with a semimajor axis in the middle of the asteroid belt consistent with a linkage to low albedo c d and p type asteroids the mineralogy oxygen isotope and bulk chemical composition of recovered samples of the tagish lake meteorite are intermediate between cm and ci meteorites these data suggest that the tagish lake meteorite may be one of the most primitive solar system materials yet studied

decollement processes at the nankai accretionary margin southeast japan propagation deformation and dewatering the decollement zone expressed on seismic profiles and observed in drill cores from the nankai accretionary margin off the southeast coast of japan reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the decollement the extension of this fault zone well in front of the tectonic deformation front and the absence of features indicative of precursory shear for example folded sediments shear bands and penetrative mineral fabrics these characteristics suggest that the mode of formation and evolution of this decollement zone may be unique from that of most thrust faults we propose that the decollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures to test this possibility physical property measurements and clay mineral fabrics were obtained for several samples from the nankai decollement zone using computed tomography and x ray texture methods our findings suggest that deformation within the decollement zone is partitioned into a volumetric component preserved as reduced porosities within coherent fragments and a localized shear component evidenced by mineral preferred orientations along discrete slip surfaces we suggest that the reduced porosities result from the destruction of cementation in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures the nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward and the weakened protodecollement subsequently accommodates shear displacements along discrete fractures

on the fate of the antarctic slope front and the origin of the weddell front 1 data from the deep ocean ventilation through antarctic intermediate layers dovetail and antarctic large scale box analysis and the role of the scotia sea albatross projects are combined in the region of the antarctic peninsula and the south scotia ridge to determine the paths and transports associated with the antarctic coastal current the antarctic slope front and the weddell front the antarctic coastal current flows over the antarctic continental shelf and is not tied to a particular isobath it transports 1 sv westward to the bransfield strait its subsequent course is uncertain but we speculate that it may continue southwest along the western side of the antarctic peninsula close to the coast the antarctic slope front present almost all the way around antarctica tied to the continental slope has not previously been mapped after it reaches the tip of the antarctic peninsula we show that the frontal jet splits into two branches in the northwest powell basin where the isobaths diverge the shoreward portion of the water associated with the front lying above isobaths shallower than 1500 m is able to cross the south scotia ridge and head northwestward and retains many of the properties of the antarctic slope front it is associated with a transport of 7 sv the portion of the front constrained to lie above isobaths deeper than 1500 m becomes the weddell front this takes a path around the northern powell basin and south of the south orkney islands of the 13 sv that circulates around the southern flank of the south orkney plateau 8 sv of weddell sea deep water and warm deep water leaves the front to continue west the remaining shallow and intermediate water retroflects and returns south associated with the weddell front through the orkney passage approximately 5 7 sv is transported eastward associated with the weddell front tied closely to the southern flanks of the bruce and discovery banks above the 2000 2500 m isobaths and forming the southern boundary of the weddell scotia confluence

the fall and recovery of the tagish lake meteorite the tagish lake c2 ungrouped carbonaceous chondrite fall of january 18 2000 delivered 10 kg of one of the most primitive and physically weak meteorites yet studied in this paper we report the detailed circumstances of the fall and the recovery of all documented tagish lake fragments from a strewnfield at least 16 km long and 3 to 4 km wide nearly 1 kg of pristine meteorites were collected one week after the fall before new snow covered the strewnfield the majority of the recovered mass was collected during the spring melt ground eyewitnesses and a variety of instrument recorded observations of the tagish lake fireball provide a refined estimate of the fireball trajectory from its calculated orbit and its similarity to the remotely sensed properties of the d and p class asteroids the tagish lake carbonaceous chondrite apparently represents these outer belt asteroids the cosmogenic nuclide results and modeled production indicate a prefall radius of 2 1 2 4 m corresponding to 60 90 tons consistent with the observed fireball energy release the bulk oxygen isotope compositions plot just below the terrestrial fractionation line tfl following a trend similar to the cm meteorite mixing line the bulk density of the tagish lake material 1 64 0 02 g cm 3 is the same within uncertainty as the total bulk densities of several c class and especially d and p class asteroids the high microporosity of tagish lake samples 40 provides an obvious candidate material for the composition of low bulk density primitive asteroids

holocene paleoclimate change in the antarctic peninsula evidence from the diatom sedimentary and geochemical record abstract holocene marine deposition in lallemand fjord antarctic peninsula is reinterpreted using statistical analyses cluster analysis analysis of variance nonmetric multidimensional scaling and multiple regression to compare diatom assemblages and the primary sedimentological proxies the assemblages have been deposited in a variable sea ice zone over the last ca 10 500 yr bp in response to a climate change in the late pleistocene early holocene 10 580 7890 yr bp a sea ice diatom assemblage was deposited in the presence of a retreating ice shelf at the head of the fjord in the mid holocene 7890 3850 yr bp an open water assemblage was deposited and sea ice cover was at a minimum we associate the assemblage with climatic warming which characterizes much of the antarctic peninsula during this time a second sea ice assemblage different from that deposited in the early holocene has been deposited in lallemand fjord since the late holocene

climate changes in the south orkney plateau during the last 8600 years climatic and oceanographic changes in the south orkney plateau western antarctica during the last 8600 years are reconstructed from a 525 cm long gravity core based on sedimentological geochemical and diatom analyses the core sediments are composed mostly of light greyish olive diatomaceous silt and mud with a few diatom ooze laminae in the basal part the core can be divided at 350 cm into two units 4800 cal yr bp the lower unit is characterized by variable total organic carbon toc content and higher caco3 content and the upper unit is characterized by higher toc and lower caco3 the content of biogenic silica varies similar to toc content in the lower unit but does not increase in the upper unit despite increased toc the variations in the organic matter composition and the amount of carbonate suggest that scotia sea water had been dominant in the study area prior to 4800 cal yr bp warmer conditions during the middle holocene are also supported by the fewer sea ice diatom taxa and a more su

relationship between magnetic susceptibility and sediment grain size since the last glacial period in the southern ocean off the northern antarctic peninsula linkages between the cryosphere and atmospheric circulation abstract magnetic susceptibility ms values in scotia sea sediments showed strong correlations to ice core non sea salt ca2 concentration dust input which emphasizes the role of atmospheric circulation in the southern ocean as a result the correlation between these values was suggested as a powerful tool for age reconstruction of marine sediments in the southern ocean however controls on ms variation in scotia sea sediments are not clear in this study we documented records of grain size ms values 10 6 cgs g of bulk sediments and ms values of sand sized 63 m coarse silt sized 16 63 m and fine sediment fractions

high pore pressure generation in sediments in front of the barbados ridge complex dsdp data for the barbados ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches we model the evolution of pore pressure by using a finite element procedure three consecutive stages are recognized 1 in the oceanic basin sedimentation rate is low there is little excess pore pressure 2 approaching the trench but before being incorporated into the accretionary prism sediments are subjected to increasing tectonic compression as well as greater sediment overburden thus high pore pressure can be created an important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone 3 arcward of the trench sediments above the decollement are offscraped while those beneath it are subducted fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth this facilitates the formation of mud volcanoes even oceanward of the deformation front

antarctic peninsula ice sheet evolution during the cenozoic era the antarctic peninsula region is currently undergoing rapid environmental change resulting in the thinning acceleration and recession of glaciers and the sequential collapse of ice shelves it is important to view these changes in the context of long term palaeoenvironmental complexity and to understand the key processes controlling ice sheet growth and recession in addition numerical ice sheet models require detailed geological data for tuning and testing therefore this paper systematically and holistically reviews published geological evidence for antarctic peninsula ice sheet variability for each key locality throughout the cenozoic and brings together the prevailing consensus of the extent character and behaviour of the glaciations of the antarctic peninsula region major contributions include a downloadable database of 186 terrestrial and marine calibrated dates an original reconstruction of the lgm ice sheet and a new series of isochrones detailing ice sheet retreat following the lgm glaciation of antarctica was initiated around the eocene oligocene transition in east antarctica palaeogene records of antarctic peninsula glaciation are primarily restricted to king george island where glacigenic sediments provide a record of early east antarctic glaciations but with modification of far travelled erratics by local south shetland island ice caps evidence for neogene glaciation is derived primarily from king george island and james ross island where glaciovolcanic strata indicate that ice thicknesses reached 500 850 m during glacials this suggests that the antarctic peninsula ice sheet draped rather than drowned the topography marine geophysical investigations indicate multiple ice sheet advances during this time seismic profiling of continental shelf slope deposits indicates up to ten large advances of the antarctic peninsula ice sheet during the early pleistocene when the ice sheet was dominated by 40 kyr cycles glacials became more pronounced reaching the continental shelf edge and of longer duration during the middle pleistocene during the late pleistocene repeated glacials reached the shelf edge but ice shelves inhibited iceberg rafting the last glacial maximum lgm occurred at 18 ka bp after which transitional glaciomarine sediments on the continental shelf indicate ice sheet retreat the continental shelf contains large bathymetric troughs which were repeatedly occupied by large ice streams during pleistocene glaciations retreat after the lgm was episodic in the weddell sea with multiple readvances and changes in ice flow direction but rapid in the bellingshausen sea the late holocene epoch was characterised by repeated fluctuations in palaeoenvironmental conditions with associated glacial readvances however this has been subsumed by rapid warming and ice shelf collapse during the twentieth century

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

late quaternary glacial history of the south orkney plateau antarctica abstract piston cores from the south orkney plateau penetrated overcompacted diamictons in water depths of up to 250 m detailed textural and petrological analyses of these diamictons indicate that they are basal tills seismic records from the plateau show a widespread surface of glacial erosion and provide additional evidence of an ice cap grounded to a depth of 250 m piston cores from the slope of the plateau penetrated diatomaceous muds resting directly on poorly sorted muds with very little to no biogenic material the ice rafted debris in these glacial marine sediments is composed almost exclusively of material derived from the south orkney islands this implies deposition beneath an ice shelf as opposed to iceberg rafting in contrast diatomaceous muds contain relatively abundant exotic iceberg rafted material and reflect a glacial maritime setting similar to that of today the sharp contact separating diatomaceous surface sediments from basal tills and sub ice shelf deposits indicates that the ice cap and ice shelf retreated from the plateau rapidly radiocarbon dates for diatomaceous muds from a glacial trough on the plateau indicate that the ice cap and ice shelf retreated from the plateau prior to 6000 to 7000 years ago the homogeneity of surficial diatomaceous sediments suggests that sea ice conditions over the plateau have not changed radically since that time

critical porosity a key to relating physical properties to porosity in rocks discovering accurate relations between porosity and seismic wave velocities in porous rocks has been an important topic of rock physics for decades such relations if they actually exist are critical when seismic and or sonic data are to be used to infer porosity and porosity related properties e g permeability strength saturation in situ these inferences became especially important in recent years with the rapid development of seismic and sonic methods which are applied to the characterization of heterogeneous reservoirs and recovery monitoring

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

consolidation patterns during initiation and evolution of a plate boundary decollement zone northern barbados accretionary prism borehole logs from the northern barbados accretionary prism show that the plate boundary decollement initiates in a low density radiolarian claystone with continued thrusting the decollement zone consolidates but in a patchy manner the logs calibrate a three dimensional seismic reflection image of the decollement zone and indicate which portions are of low density and enriched in fluid and which portions have consolidated the seismic image demonstrates that an underconsolidated patch of the decollement zone connects to a fluid rich conduit extending down the decollement surface fluid migration up this conduit probably supports the open pore structure in the underconsolidated patch

deformation and fluid pressure variation during initiation and evolution of the plate boundary decollement zone in the nankai accretionary prism 1 the plate boundary decollement zone in the muroto region of the nankai accretionary prism records deformation and consolidation histories that have been affected by temporal changes in fluid pressure microstructural observations and chemical analysis demonstrate that the decollement zone initiated in an interval of porous clayey sediments characterized by cementation due to intergranular bonding of authigenic clays crosscutting relations of microstructures indicate that the decollement zone records two compactive deformations the early compactive deformation involved destruction of porous cemented structure probably caused by fluid pressure fluctuation the late compactive deformation was characterized by clay particle rotation and porosity collapse along the sets of slip surfaces resulting in zones of preferred orientation of clay particles these compactive deformations led to significantly higher bulk densities within the decollement zone compared to the compaction trend of the overlying prism sediments elevated fluid pressure following compactive deformations induced an overconsolidated state within the decollement zone with fluid filled dilatant fractures bulk density abruptly decreases at the top of the underthrust sediments but there is no microstructural evidence for cementation fluids in the dilated fractures and underconsolidated underthrust sediments are potential sources for the elevated fluid pressure in and below the decollement zone resulting in mechanical decoupling of the accretionary prism from underthrust sediments the fault fluid interactions in the muroto region may be applicable to other convergent plate margins where high temperature associated with the subduction of a spreading ridge or hot young oceanic crust enhances diagenesis and cementation

particle size dependent magnetic properties of scotia sea sediments since the last glacial maximum glacial ice sheet discharge controlling magnetic proxies abstract the strong glacial interglacial similarity between the magnetic susceptibility ms of southern ocean sediments and antarctic ice core dust records has often been used to reconstruct southern hemisphere atmospheric variability although evaluation of various magnetic properties is essential for identifying the magnetic carriers linked to sedimentological variation detailed magnetic studies are not sufficient in the scotia sea here we investigate the bulk and particle size dependent magnetic properties of scotia sea sediments over the past 22 kyr to determine the main sediment transport mechanism driving bulk magnetic proxies including ms in bulk sediments ms is highest during the last glacial period and is accompanied by an increase in the concentration and grain size of ferrimagnetic and antiferromagnetic minerals for magnetic mineral assemblages coarse detrital magnetite is dominant of three particle size fractions 63 16 63 and 63 m throughout the last deglaciation indicating a close linkage between their input mechanisms thus the sediment particles ranging from sand to coarse silt which control the bulk glacial magnetic proxies are most plausibly transported by iceberg rafted debris ird as hematite is relatively concentrated in the sand fraction the hematite contribution in the bulk sediment can highlight ird related magnetic signals rather than magnetite the bulk hematite contribution simultaneously varies with the deglacial influx of coarse ird particles 1 mm in scotia sea sediments although their glacial inconsistency possibly suggests a different ird input mechanism during the advancement and retreat of the ice sheet consequently the glacial increase in the bulk magnetic concentration indicates vigorous iceberg calving activity in the scotia sea and further suggests the coupled cryosphere atmosphere system

terrestrial and submarine evidence for the extent and timing of the last glacial maximum and the onset of deglaciation on the maritime antarctic and sub antarctic islands this paper is the maritime and sub antarctic contribution to the scientific committee for antarctic research scar past antarctic ice sheet dynamics pais community antarctic ice sheet reconstruction the overarching aim for all sectors of antarctica was to reconstruct the last glacial maximum lgm ice sheet extent and thickness and map the subsequent deglaciation in a series of 5000 year time slices however our review of the literature found surprisingly few high quality chronological constraints on changing glacier extents on these timescales in the maritime and sub antarctic sector therefore in this paper we focus on an assessment of the terrestrial and offshore evidence for the lgm ice extent establishing minimum ages for the onset of deglaciation and separating evidence of deglaciation from lgm limits from those associated with later holocene glacier fluctuations evidence included geomorphological descriptions of glacial landscapes radiocarbon dated basal peat and lake sediment deposits cosmogenic isotope ages of glacial features and molecular biological data we propose a classification of the glacial history of the maritime and sub antarctic islands based on this assembled evidence these include type i islands which accumulated little or no lgm ice type ii islands with a limited lgm ice extent but evidence of extensive earlier continental shelf glaciations type iii seamounts and volcanoes unlikely to have accumulated significant lgm ice cover type iv islands on shallow shelves with both terrestrial and submarine evidence of lgm and or earlier ice expansion type v islands north of the antarctic polar front with terrestrial evidence of lgm ice expansion and type vi islands with no data finally we review the climatological and geomorphological settings that separate the glaciological history of the islands within this classification scheme

organic matter preservation on continental slopes importance of mineralogy and surface area abstract theoretical considerations calculations and data reported by keil et al 1994a were used to assess the recent hypotheses that mineral surface roughness controls the specific surface area of continental margin sediments and that the matrix linked organic carbon content of continental margin sediments occurs as a monolayer or monolayer equivalent coating of organic compounds on the surfaces of detrital minerals and other detrital nonorganic grains mayer 1994 results of our analysis indicate that it is not the surface roughness of the terrigenous detrital framework grains that controls the specific surface area of most continental margin sediments even in the sand and silt fractions but rather the presence of nonspherical high surface area to volume particles primarily clays but also oxy hydroxides and ultra structured nonorganic bioclasts such as diatom frustules analysis of the mineralogy organic carbon content surface area and in situ microfabric of continental margin sediments off california in the mendocino and san luis obispo areas and off the state of washington indicate a strong correlation between the amount of matrix linked organic matter present and the suite of clay minerals data indicate that organic carbon appears to be preferentially sequestered in smectite rich sediments compared to those whose clay fractions are dominated by chlorite we suggest that this association is a function of differences in the site density and chemistry of the clays and differences in their flocculation behavior our data also indicate that organic carbon preservation on the three california transects all of which have a suboxic oxygen minimum zone that impinges on the sea floor is not significantly influenced by differences in bottom water oxygen concentration and carbon stable isotope data suggest no preferential preservation of continental organic matter over that of marine origin

fluid flow along a decollement layer a model applied to the 16 n section of the barbados accretionary wedge a seismic section across the whole barbados accretionary complex at 16 n recently published by westbrook et al 1988 shows that the reflection marking the decollement fades between 40 and 60 km arcward of the deformation front but is present again further inland this interruption appears to be related to a thrust cutting through the wedge one dimensional steady state models of water flow along the decollement suggest a mechanism responsible for decollement disruption a local increase of the slope either of the wedge or of the decollement causes the subducted layer to compact and locally increases the friction along the decollement if the thrust is used as a conduit for fluids to escape from the decollement the friction along the decollement toeward of the thrust is further increased the model also shows that the permeability of the decollement should be of the order of 10 14 m2 but may be higher near the deformation front for short periods if fluid expulsion mainly occurs in transient bursts associated with hydraulic fracturing as a consequence of the non linearity of the compaction equation pore pressure at depth in the decollement depends on the water flux on the lithostatic pressure gradient along the decollement and on the parameters from which the physical properties of the sediment are calculated however it is almost independent on the pore pressure below the toe

palaeolimnological evidence for marked holocene environmental changes on signy island antarctica lake sediment cores from heywood and sombre lakes on signy island south orkney islands antarctica have yielded a conformable radiocarbon chronology for the holocene and a high resolution record of environmental change the lakes share a common climate and geology but have distinct catchments this provides an opportunity for using lake sediments to differentiate between local within lake catchment events and those at a regional scale analyses of various biological and physical remains from the lakes suggest that both catchments have undergone considerable changes during the last 5700 years macrofossils moss and crustacean remains are more abundant in the late holocene being associated with a period of high sediment accumulation which is related to diatom evidence for more nutrient rich conditions at the sites this is inter preted as a response to a holocene climate optimum at c 3800 1300 14 c yr bp the record is consistent with other lake ice and ocean core studies although the climate optimum appears to have persisted for a longer period at signy island

matrix mineralogy of the orgueil ci carbonaceous chondrite a petrographic and transmission electron microscopic study of the orgueil ci carbonaceous chondrite shows that the matrix consists mainly of fe bearing mg rich serpentine and smectite saponite as well as a poorly crystallized fe rich material that contains minor variable s and ni the fe rich material is probably ferrihydrite a ferric hydroxide the ferrihydrite occurs in small particles 80 a in diameter and the s and ni are probably adsorbed on its surfaces the serpentine and saponite occur in poorly crystallized fine crystallites fine phyllosilicates that are intimately associated with the ferrihydrite coarsely crystallized phyllosilicates coarse phyllosilicates occur in clusters relatively free of ferrihydrite the ferrihydrite is a major matrix constitutent and probably accounts for most of the fe3 as well as the superparamagnetic fraction in the orgueil matrix the textures suggest that the fine phyllosilicates resulted from alteration of the coarse phyllosilicates the alteration probably involved a substantial amount of water and coincided with the period of sulfate vein formation during the aqueous alteration magnetite grains disaggregated from framboids and fe ni sulfides were replaced by ferrihydrite simultaneously aqueous solutions reacted with the coarse phyllosilicates and precipitated ferrihydrite producing intimate pervasive mixtures with the fine phyllosilicates the matrix mineralogy of the orgueil chondrite suggests that orgueil experienced considerably different alteration conditions from the cm chondrites the mineralogical and petrological differences between ci and cm chondrites can not simply be explained as the result of different degrees of aqueous alteration

extensive underthrusting of undeformed sediment beneath the accretionary complex of the lesser antilles subduction zone along the eastern margin of the caribbean plate which overrides the atlantic oceanic lithosphere at 20 km myr 1 in an easterly direction1 east of the lesser antilles island arc lies a wide accretionary complex barbados ridge complex 260 km wide at the latitude of barbados and 20 km thick beneath barbados3 4 it increases in width and thickness from north to south2 fig 1 the style of deformation varies considerably along the eastern margin of the complex2 3 5 11 and is related to the thickness and type of sediment on the atlantic ocean floor a common feature is that a proportion of undeformed sediment from the ocean floor is brought beneath the deformed sediments of the accretionary complex and in some areas a clearly defined seismic reflector separates the deformed and undeformed sediments which has been interpreted as a decollement surface5 12 13 a seismic section from an investigation of the accretionary complex conducted from rss discovery in 1980 using 12 channel seismic reflection and long range side scan sonar gloria shows undeformed sedimentary rocks carried horizontally 74 km beneath deformed rocks forming the accretionary wedge we report here that the low shear stresses required on the decollement between the deformed and undeformed rocks are made possible by porewater at pressures close to that of the lithostatic load

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

prebiotic carbon in clays from orgueil and ivuna ci and tagish lake c2 ungrouped meteorites transmission electron microscopic tem and electron energy loss spectroscopic eels study of the ivuna and orgueil ci and tagish lake c2 ungrouped carbonaceous chondrite meteorites shows two types of c clay assemblages the first is coarser grained to 1 m clay flakes that show an intense o k edge from the silicate together with a prominent c k edge but without discrete c particles nitrogen is common in some clay flakes individual orgueil and tagish lake meteorite clay flakes contain up to 6 and 8 at c respectively the c k edge spectra from the clays show fine structure revealing aromatic aliphatic carboxylic and carbonate c the eels data shows that this c is intercalated with the clay flakes the second c clay association occurs as poorly crystalline to amorphous material occurring as nanometer aggregates of c clay and fe o rich material some aggregates are dominated by carbonaceous particles that are structurally and chemically similar to the acid insoluble organic matter the c k edge shape from this c resembles that of amorphous c but lacking the distinct peaks corresponding to aliphatic carboxylic and carbonate c groups nanodiamonds are locally abundant in some carbonaceous particles the abundance of c in the clays suggest that molecular speciation in the carbonaceous chondrites is partly determined by the effects of aqueous processing on the meteorite parent bodies and that clays played an important role this intricate c clay association lends credence to the proposal that minerals were important in the prebiotic chemical evolution of the early solar system

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

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

abundance of smectite and the location of a plate boundary fault barbados accretionary prism isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics the frontal decollement zone of the northern barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy fluid flow chemical interactions and sediment physical properties affect strain localization this plate boundary fault is centered at a lithologic contact between miocene claystone and fine grained oligocene turbidites the fault zone is nearly 40 m thick at ocean drilling program site 948 and its upper part passes through smectite rich deposits a sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement there is a consistent increase in percent smectite with distance above the base of the decollement but the top of the decollement is poorly defined by clay mineralogy the intrinsic mechanical weakness of strata with abundant smectite group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the atlantic abyssal plain a second inherited parameter is the local abundance of radiolarians which contribute to higher than normal porosities sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic the principal cause of excess pore pressure seems to be updip fluid advection in theory however decreases in pore fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals the imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions if fresher pore water migrates to the propagating tip of the decollement its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more the location and evolution of the decollement therefore are controlled by a complicated interplay of static factors inherited from the abyssal atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry

protection of organic matter by mineral matrix in a cenomanian black shale abstract three types of pathways degradation recondensation natural sulphurization and selective preservation are commonly considered for the formation of kerogen dispersed in sedimentary rocks a fourth pathway has been recently put forward however from studies on recent marine sediments the so called sorptive protection mechanism this pathway is based on the adsorption of otherwise labile organic compounds onto minerals thus preventing their diagenetic degradation and promoting their subsequent condensation into kerogen the main results of the present study are derived from a combination of microscopic and pyrolytic methods applied on a cenomanian kerogen they provide i evidence on an ancient material for a crucial role of the mineral matrix both in organic matter om preservation during kerogen formation and in kerogen stability once formed ii indications that the dominant protective process likely involves physical protection by minerals resulting from alternation of organic and clay nanolayers of approximately 100 nm in thickness rather than om adsorption as molecular monolayers and iii observations of the relatively poor stability of an isolated kerogen contrary to the inertness commonly assumed for fossil macromolecular organic matter

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

nanosized carbon rich grains in carbonaceous chondrite meteorites abstract macromolecular carbonaceous material is common in primitive meteorites little information exists on its form and composition and there are no published data on nanometer scale chemical and structural variations transmission electron microscopy tem studies of cm meteorites reveal abundant previously unrecognized nanosized carbonaceous grains they have a high aromatic component as revealed by electron energy loss spectroscopy eels with up to 20 at substituted by s n and o they occur as discrete hollow and solid nanospheres and sparse nanotubes the grains exhibit considerable variations in composition size morphology and abundance among meteorites and may represent materials from multiple reservoirs

friction experiments on saturated sediments and their implications for the stress state of the nankai and barbados subduction thrusts the nankai and barbados forearcs have low stress subduction thrusts the sediments entering the subduction zone and namely the material in the decollement zones have been well characterized by numerous deep sea drilling legs and studies of the recovered cores nankai has high heat flow and significant amounts of illite while barbados is a smectite dominated system based on results from ring shear 2 mpa normal stress and direct shear 30 mpa tests on marine sediments and mineral standards this translates into a residual frictional resistance of r 0 25 and r 0 11 in clay horizons respectively such values agree with theoretical estimates from critical wedge theory nankai b 0 16 0 26 and barbados b 0 06 0 09 and fault spacing geometries from seismic profiles nankai b 0 12 0 23 and barbados b 0 11 0 19 maximum pore pressure ratios of 0 85 and 0 73 for nankai and barbados respectively allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only 10 mpa or less in the frontal 50 km of the decollement zone respectively our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone with the first probably dominating the shallow decollement shear velocity stepping tests show that the clay dominated gouges strengthen velocity irrespective of the clay mineralogy and hence suggest that clay transformation does not affect the onset of seismogenesis

acoustic and mechanical properties of nankai accretionary prism core samples we studied undeformed sediment and accreted strata recently recovered by ocean drilling program integrated ocean drilling program odp iodp drilling in nankai trough convergent margin to unravel the changes in physical properties from initial deposition to incipient deformation we have derived acoustic vp and mechanical uniaxial poroelastic compliance compaction amplitude properties of samples from various drill sites along the muroto odp 1173 and kii transects iodp c0001 c0002 c0006 and c0007 from isotropic loading tests where confining and pore pressure were independently applied we quantified the dependence of vp on both effective peff and confining pc pressure which can be used to correct atmospheric pressure measurements of vp experimental vp obtained on core samples extrapolated to in situ conditions are slightly higher than logging derived velocities which can be attributed either to velocity dispersion or to the effect of large scale faults and weak zones on waves with longer wavelength in the high porosity 30 60 tested sediments velocities are controlled at first order by porosity and not by lithology which is in agreement with our static measurements of drained framework incompressibility much smaller than fluid incompressibility rather than framework incompressibility shear modulus is probably the second order control on vp accounting for most of the difference between actual vp and the prediction by wood s 1941 suspension model we also quantified the mechanical state of nankai samples in terms of anisotropy diagenesis and consolidation both acoustic and mechanical parameters reveal similar values in vertical and horizontal directions attesting to the very low anisotropy of the tested material when considering the porous samples of the upper shikoku basin sediments site 1173 as examples of diagenetically cemented material several mechanical and acoustic attributes appeared as reliable experimental indicators of the presence of intergrain cementation we also detected incipient cementation in samples from iodp site c0001 accretionary prism unit in terms of consolidation we distinguished two classes of material response shallow deformable samples and deep hardly deformable ones based on the amount of compaction upon application of a peff large with respect to the inferred in situ value with a transition that might be related to a critical porosity

a 13 200 year history of century to millennial scale paleoenvironmental change magnetically recorded in the palmer deep western antarctic peninsula abstract a 13 200 yr record of magnetic parameters from the palmer deep western antarctic peninsula records a sequence of five distinct shifts in glacimarine sedimentation coupled with century scale variations in paleoproductivity the five major shifts are manifested as abrupt order of magnitude changes in low field magnetic susceptibility accompanied by changes in magnetic particle size and mineralogy the late holocene 3 4 0 ka the early holocene 11 5 9 ka and the last glacial maximum prior to 13 2 ka are intervals of strong low field magnetic susceptibility and are characterized by multi domain md magnetite md magnetite is associated with zones of abundant gravel grains and is interpreted here as an indicator of material transported as ice rafted debris deglaciation 13 2 11 5 ka and the middle holocene 9 3 4 ka were times of enhanced productivity the middle holocene marks the onset of century scale productivity cycles seen in high field magnetic susceptibility which is responding to variations in biogenic silica deglaciation and the middle holocene interval contain pseudo single domain magnetite and titanomagnetite respectively these observations are surprising given the abundance of coarse md magnetite available in local source rocks the magnetic mineral assemblage in the deglaciation interval however can be explained by density sorting in meltwater plumes during the middle holocene the magnetic mineral assemblage suggests the reduction or cessation of locally derived terrigenous sediment and by inference the reduction or cessation of iceberg generation the early holocene middle holocene shift in terrigenous sedimentation may be responding in part to sea level which controls the position of the grounding line the palmer deep late holocene interval coincides with the neoglacial period a time of glacier re advances worldwide

solid state cp mas 13 c nmr of the insoluble organic matter of the orgueil and murchison meteorites quantitative study solid state cp mas 13c nuclear magnetic resonance nmr study of the insoluble organic fraction isolated by hf hcl treatment under an inert atmosphere from the orgueil and murchison carbonaceous meteorites is reported based on these spectra eight different types of carbons aro and ali linked ch3 ch2 aliphatic c linked to heteroelements protonated and non protonated aromatic c carboxyls and carbonyls were identified their relative amounts were determined and theoretical nmr derived h c atomic ratios were calculated comparison of these h c ratios with those obtained from elemental analysis revealed that a large part of the carbons of the insoluble fraction of both meteorites are not detected by cp mas 13c nmr taking into account these undetected carbons the relative abundance of aromatic carbons as of the total carbons was calculated as between 69 and 78 in orgueil and 61 and 67 in murchison and thus shown to be much higher than previously thought in addition the nmr data allowed to calculate the ratio of non protonated and protonated aromatic carbons based on previous studies on chemical and thermal degradation the present nmr data were interpreted as reflecting the occurrence of highly substituted rather small aromatic moieties in these insoluble organic materials along with ftir data nmr results also pointed to a relatively high branching level in the aliphatic chains especially for murchison according to the above results the insoluble organic matter in these meteorites did not suffer a marked thermal maturation during the late hydrothermalism of the parent body its synthesis involved a statistical combination of all the possible bonds between ch3 ch2 and ch radicals producing both the aliphatic and the aromatic moieties this model explains not only the chemical features of the chondritic insoluble organic material but also the typical ftir signature of the interstellar medium therefore organic material found in the interstellar medium and that incorporated in the most primitive objects of the solar system seem to share a common organosynthesis

rock magnetic carriers of century scale susceptibility cycles in glacial marine sediments from the palmer deep antarctic peninsula abstract a 4200 year long magnetic susceptibility profile from the palmer deep records century and millennial scale variability in paleoenvironmental conditions along the western antarctic peninsula the susceptibility profile shows regularly spaced highs and lows over the last 3360 years b p then drops to uniformly low values these features are formed by the variable dilution of terrigenous material with biogenic sediments and shifts in magnetic mineralogy high susceptibility intervals are characterized by multi domain md pure magnetite low susceptibility intervals are characterized by pseudo single domain psd magnetite and titanomagnetite prior to 3360 years b p the magnetic mineral assemblage is dominated by fine psd titanium rich titanomagnetite and superparamagnetic particles suggesting a change in sediment provenance and or sediment transport processes the absence of md magnetite suggests a cessation in locally derived ice rated debris this horizon represents the transition from the mid holocene climatic optimum to the neoglacial period marked by an increase in glacial sedimentation

generation of high pore pressures in accretionary prisms inferences from the barbados subduction complex recent geological and geophysical exploration of several accretionary complexes reveals that pore pressures inside them are much higher than the hydrostatic values indirect inferences further suggest that such high pore pressures may be present inside most accretionary complexes in this study the mechanisms for generating excess pore pressures is investigated by focusing on the barbados ridge complex which is an end member of sediment rich accretionary prisms modeling is carried out using a two dimensional finite element procedure with constraints provided by the following data deep sea drilling project results on stratigraphy and lithology seismic results on sediment structure and thickness regional geology on tectonic history and laboratory measurements on mechanical properties of sediments coupling among the equations for fluid flow heat transfer and sediment deformation is made throughout the computation the results of the modeling predict that the principal decollement beneath the barbados ridge complex is a zone of high pore pressure high porosity and reversal in seismic velocity these characteristics may be responsible for the mechanical weakness of the decollement which allows continuous subduction of sediments below it and for its seismic signatures results of the modeling further suggest that tectonic overburden may be the most important mechanism for the generation of high pore pressures inside accretionary prisms the direct effect of tectonic compression is of secondary importance and the effect due to temperature increase with subduction is of little significance thrust faulting near the toe of accretionary prisms can disturb pore pressure patterns and produce localized highs rich sediment supply low permeability high subduction angle and high subduction velocity are all favorable factors in creating high pore pressures inside accretionary prisms

distribution of chaetoceros resting spores in modern peri antarctic sediments abstract one hundred and sixty five surface sediment samples from the southern ocean were examined for distribution and relative abundance of chaetoceros resting spores the contribution of resting spores to the total diatom assemblage ranges from 0 in the subantarctic zone to 95 in the antarctic peninsula sector on the basis of both absolute and relative abundances four biogeographic zones are distinguished 1 the antarctic peninsula sector 2 the embayment systems ross sea and weddell sea 3 the continental shelf zone water depth 2000 m chaetoceros resting spores abundance reaches up to 900 10 6 valves g of dry sediment in the gerlache strait southwest of the antarctic peninsula the hydrology of this region is characterized by an intense stratification of the water column due to sea ice meltwater inputs continental glacial runoffs and thermal warming of the surface water layer the availability of nutrients the lack of vertical mixing in those surface waters having low salinity 2 4 c is thought to be the main pre condition for development of large chaetoceros species blooms we propose that increased relative abundances of chaetoceros resting spores in fossil diatom assemblages from the southern ocean can therefore be used as tracers of water column stratification due to glacial melt water

the estimation of diffuse strains in the toe of the western nankai accretionary prism a kinematic solution arcward changes in the vertical spacing of seismically defined stratigraphic reflectors and sediment porosities determined from drill cores and seismic interval velocities suggest a compact kinematic solution for estimating diffuse deformation within accretionary prism toes the method involves the solution of the lagrangian form of the conservation of mass equation for the two dimensional finite deformation gradient field within discrete deforming domains we apply this technique to the unfaulted protothrust zone ptz along a western transect across the nankai accretionary prism in the vicinity of deep sea drilling project sites 582 and 583 the results yield horizontal shortening estimates of about 1 7 km or about 20 this shortening appears to be accommodated through different strain responses depending on the consolidation state of the sediments at the time of accretion shallow sediments respond to horizontal shortening dominantly by expelling pore fluids producing volume strains up to 30 while deeper sediments accommodate horizontal shortening dominantly through vertical extension reaching 70 in the footwall of the frontal thrust horizontal shear strains within the ptz are not well resolved by this solution due to uncertainties in the porosity distribution but they generally appear to be quite small

contamination by sesquiterpenoid derivatives in the orgueil carbonaceous chondrite the free organic components of the orgueil carbonaceous chondrite have been the subject of much controversy several proposals for their source have been put forward including extraterrestrial abiotic processes extraterrestrial life and terrestrial contamination in this study we have analysed the free components to assess their distribution and possible sources compounds suggestive of terrestrial contamination by essential plant oil derivatives are evident their distribution suggests a source such as cleaning products disinfectant and air freshener where they are present in refined form this study strongly highlights the need for extreme care when interpreting organic data from meteorites with long curational histories future meteorites falls and extraterrestrial sample returns should be stored so as to avoid being compromised by similar terrestrial substances

mineralogy and texture of fe ni sulfides in ci1 chondrites clues to the extent of aqueous alteration on the ci1 parent body to better understand the role of aqueous alteration on the ci1 parent body we have analyzed the texture composition and mineral associations of iron nickel sulfides in four of the five known ci1 chondrites the most commonly occurring sulfide present in the ci1 chondrites is the iron deficient fe ni sulfide pyrrhotite fe ni 1 xs that has a composition close to that of stoichiometric troilite fes three of the ci1s alais ivuna and tonk also contain pentlandite fe ni 9s8 although pentlandite is a rare phase in ivuna cubanite cufe2s3 was found in both alais and ivuna in this study although it has also been reported in orgueil macdougall and kerridge 1977 the pyrrhotite grains in all four chondrites form hexagonal rectangular or irregular shapes and show no evidence of ni or co zoning the pyrrhotite grains in orgueil and ivuna are in general smaller and show more corrosions or embayments than those in alais or tonk we suggest that the precursor sulfide present in the ci1 chondrites was troilite which during brecciation and oxidation on the parent body at a temperature of 100 c or less converted the troilite to magnetite and pyrrhotite with pentlandite inclusions subsequently continued alteration on the parent body removed pentlandite partially from alais tonk and ivuna completely from orgueil leaving behind pyrrhotite with spaces corrosions where the pentlandite had been ni derived from the pentlandite was incorporated into ferrihydrite onto the surface of which the ni na sulfate ni bloedite formed based on the size and abundant corrosions within pyrrhotite grains combined with observations from other authors we conclude that orgueil and ivuna have undergone a greater degree of alteration than alais and tonk further work is needed to assess the conditions under which pentlandite would be dissolved preferentially to pyrrhotite as the study of terrestrial literature indicates that the latter mineral is preferentially removed

modal mineralogy of carbonaceous chondrites by x ray diffraction and mossbauer spectroscopy carbonaceous chondrites are among the most analyzed geological materials on earth however despite this attention and unlike most terrestrial rocks little is known on the abundance of individual phases within them here we show how a combination of several novel x ray diffraction xrd techniques including a high brightness x ray microsource and mssbauer spectroscopy allows a complete modal mineralogy to be ascertained from even the most highly unequilibrated fine grained chondrites for all minerals of abundance 1 wt knowledge of the modal mineralogy of a sample also allows us to calculate grain density we analyzed allende murchison tagish lake and orgueil based on our modal data the grain density estimates for allende murchison and orgueil are close to literature values in the case of tagish lake there is no published grain density although a bulk density measurement does exist taking our estimate of grain density and the measured bulk density we calculate an exceptionally high porosity of 41 for this meteorite similar to some chondritic idps and in line with a porosity calculated from an entry model for the tagish lake fireball although it is an oxidized cv magnetite is present in allende at a level of 0 5 wt or 0 3 vol a result that is substantiated by several other instrumental studies this may be an oxidized meteorite but that oxidation is not manifested in abundant magnetite in addition we note appreciable fayalitic olivine in orgueil detected by both xrd and mossbauer we employed microsource xrd to look at heterogeneity in mineral abundance in orgueil and found substantial variation with phyllosilicates varying inversely with olivine the data suggest that orgueil was initially composed primarily of anhydrous materials which have been partially but not completely altered although the data are preliminary comparison between our xrd modal assessment bulk chemistry grain density and mossbauer data suggests that our estimates of mineral abundance are robust the advent of microsource xrd allows similar modal data to be acquired from samples as small as a few hundred micrograms

strain decoupling across the decollement of the barbados accretionary prism the interrelation between deformation styles and behavior of fluids in accretionary prisms is under debate particularly the possibility that overpressuring within the basal decollement may enable mechanical decoupling of the prism from the subducting material anisotropy of magnetic susceptibility ams data from sediments spanning the basal decollement of the barbados accretionary prism show a striking progression across this structure that strongly supports the hypothesis that it is markedly overpressured in the accretionary prism above the decollement the minimum ams axes are subhorizontal and oriented nearly east west whereas the maximum ams axes are oriented nearly north south and shallowly inclined at the top of the decollement the minimum ams axes orientations abruptly change to nearly vertical this orientation is maintained throughout the decollement and in the underthrust sediments below the ams orientations in the prism sediments above the decollement are consistent with lateral shortening due to regional tectonic stress as the minimum axes generally parallel the convergence vector of the subducting south american plate and the maximum axes are trench parallel because the orientations of the ams axes in deformed sediments usually parallel the orientations of the principal strains the ams results indicate that the incremental strain state in the barbados prism is one dominated by subhorizontal shortening in contrast the ams axes within and below the decollement are consistent with a strain state dominated by vertical shortening compaction this abrupt change in ams orientations at the top of the decollement at site 948 is a direct manifestation of mechanical decoupling of the off scraped prism sediments from the underthrust sediments the decoupling horizon occurs at the top of the decollement zone coinciding with the location of flowing high pressure fluids

brecciation and chemical heterogeneities of ci chondrites abstract fragments in the size range from 40 m to several hundred m in the ci chondrites orgueil ivuna alais and tonk show a wide range of chemical compositions with variations in major elements such as iron 10 4 42 4 wt feo silicon 12 7 42 2 wt sio 2 and sulfur 1 01 15 8 wt so 3 but also important minor elements such as phosphorous up to 5 2 wt p 2 o 5 or calcium up to 6 6 wt cao these variations are the result of the varying mineralogical compositions of these fragments the distribution of phyllosilicates magnetites and possibly ferrihydrite in particular control the abundances of these elements high ree contents up to 150 times the solar abundances were observed in phosphates while matrix and sulfates are ree depleted the studied 113 fragments were subdivided into eight lithologies with similar mineralogical and thus chemical properties the most common is the cga lithology consisting of a groundmass of mg rich coarse grained phyllosilicates and varying abundances of inclusions such as magnetite the second most abundant lithology is the fga lithology consisting of a groundmass of fine grained fe rich phyllosilicates a rare but important lithology consists of fragments with high contents of phosphates and other minerals the proposed model for the evolution of these lithologies is based on a closed system alteration where mineralogical differences in the lithologies reflect heterogeneities in the starting material comparison of our results with literature data indicates a general similarity of the four ci chondrites analyzed further comparison of bulk analyses suggests that the mass threshold for chemical heterogeneities in ci chondrite samples is smaller than 1 2 g

coupled organic synthesis and mineral alteration on meteorite parent bodies the hypothesis that the soluble fraction of the organic compounds present in carbonaceous chondrite meteorites was formed during aqueous alteration of the parent body was tested with masstransfer reaction path calculations in these calculations we start with likely compositions of the original parent body and asteroidal fluids that are far from thermodynamic equilibrium and metastable and stable equilibrium constraints are imposed as the total gibbs free energy of the parent body environment is minimized the results of these calculations suggest that the classes of soluble organic compounds present in carbonaceous chondrite meteorites could have formed during relatively low temperature aqueous alteration of the meteorite parent body or bodies the main controls on the potential for synthesis and transformation of organic compounds were the oxidation state of the rock fluid system the bulk composition of that system and the temperatures that were achieved during the alteration event or events it also appears that the alteration mineral assemblages were influenced by the presence of soluble organic compounds and reaction among them

antarctic sea ice extent during the holocene reconstructed from inland ice core evidence 1 interannual fluctuations in the antarctic sea ice extent sie affect the global albedo and heat balance and they may affect global co2 levels we found that the annual trend of sodium ion fluxes in the snowpack in the dome fuji region of east antarctica can serve as a proxy for the present day sie in the southern indian ocean and weddell sea sectors we then used sodium ion profiles obtained from the dome fuji ice core to reconstruct the sie during the holocene we observed that 1 the beginnings of both the climatic optimum 12 0 ka bp and neoglaciation 5 4 ka bp in the southern ocean indicated by siliceous microfossil records are consistent with decreases 11 8 ka bp and increases 5 4 ka bp in the sodium ion fluxes in the dome fuji ice core and 2 the interannual fluctuation in the sie during some periods of the neoglaciation was greater than the interannual fluctuation during the climatic optimum

major nutrients and dissolved oxygen as indicators of the frontal zones in the atlantic sector of the southern ocean 1 the chemical tracers of the main frontal zones of the atlantic sector of the southern ocean are considered before the beginning of the spring bloom frontal zones are distinguished by lateral gradients of dissolved oxygen phosphate nitrate and silicate during the spring bloom the smoothing of nutrient concentrations on both sides of the fronts weakens lateral gradients of chemical properties the position of surface gradients of nutrients within the subtropical frontal zone stfz does not coincide with the location of temperature and salinity gradients as a result fronts in this region have a stepped character the best chemical indicator of the northern stfz front is the dissolved oxygen gradient which coincides with the temperature and salinity gradients the southern boundary of the stfz is distinguished by the gradient of nitrate the chemical criterion for identifying the subantarctic front is the gradient of oxygen which ranges from 0 5 to 4 0 mol kg 1 per km the polar front is identified by the gradient of silicate 0 56 to 2 78 m per km at the surface the weddell scotia confluence wsc is distinguished not by the temperature and salinity but by chemical parameters the best year round criterion is the lateral gradient of silicate to phosphate atomic ratio which ranges from 25 to 35 other markers of the wsc are the gradients of silicate at the surface oxygen at the upper boundary of the circumpolar deep water and the depths of its location

carbonates in ci chondrites clues to parent body evolution all ci chondrites are regolith breccias consisting of various types of chemically and mineralogically distinct mineral and lithic fragments or units in the ci chondrite ivuna for example four different lithological units were identified and are referred to as lithology i ii iii and iv so far lithologies iii and iv have been identified in orgueil as well it appears that at least ivuna and orgueil consist of the same basic lithologies in different proportions carbonates in cis occur as individual grains within such lithic units or exist as large fragments between them carbonate fragments are remnants of former carbonate veins and in contrast to individual carbonate grains are not genetically linked to lithological units four different types of carbonates dolomite breunnerite calcite siderite occur in cis and they constitute on average 5 vol of each studied section in this study carbonates in the ci chondrites orgueil ivuna alais and tonk were studied petrographically mineralologically and chemically the results clearly indicate that in contrast to most previous studies compositional differences exist between dolomites within and among ci chondrites from these differences it can be derived that 1 several episodes of alteration occurred on the ci parent body 2 physicochemical conditions during carbonate formation must have been different among cis and 3 ci carbonates obviously were formed at low temperatures in equilibrium with surrounding fluid s while local compositional changes in fluid s on a micrometer to millimeter scale as reflected by dolomite compositions within ci chondrites were most likely controlled by the availability of ca2 mg2 fe2 and especially mn2 ions in the aqueous solutions more widespread compositional changes on a meter or even larger scale were controlled by variable ph eh co2 partial pressure and especially temperature conditions as reflected by the compositional variability of dolomites among cis

compaction and dewatering processes of the oceanic sediments in the costa rica and barbados subduction zones estimates from in situ physical property measurements abstract during ocean drilling program legs 170 and 171a logging while drilling lwd tools were deployed on the costa rica and barbados subduction margins high quality density resistivity and natural ray logs were acquired across the decollement zones on both margins based on a new method of layer by layer correlation of the logs changes in thickness and volume between incoming and subducted or accreted sediments are determined with 15 m resolution and 1 accuracy the change in sediment thickness and volume generally decreases with depth however this change strongly depends on the lithology siliceous layers such as diatomaceous and radiolarian clay tend to be fluid bearing and the stratigraphic position of such zones is a critical factor in the fate of the subducted sediment section on the costa rica margin the sediment section on the cocos plate is underthrust intact beneath the toe of the caribbean plate with no frontal offscraping where a siliceous fluid bearing zone is present only in the upper part of the section on the barbados margin a layer of radiolarian clay exists providing a narrow zone of mechanical weakness and anomalously high dewatering in the middle of the sediment section this layer divides the sediments that are subducted from those that are accreted accreted and subducted sediments show different compaction styles accreted sediments are characterized by rapid compaction with vertical thickening whereas subducted sediments are characterized by slow compaction with vertical flattening the vertical thickening of the accreted sediments is due to horizontal tectonic compaction and contributes to the vertical thickening of the accretionary prism as a whole in the early stages of deformation dewatering flux is calculated by the volume change of the sediment sequences across the trench the dewatering flux computed from the lwd data provides an estimate of the minimum fluid flux in the subduction zone and is significantly greater than the flux estimated from laboratory experiments because of meter scale fluid conduits which influence the downhole logs but not the centimeter scale sample measurements

holocene climate variability in antarctica based on 11 ice core isotopic records a comparison is made of the holocene records obtained from water isotope measurements along 11 ice cores from coastal and central sites in east antarctica vostok dome b plateau remote komsomolskaia dome c taylor dome dominion range d47 km105 and law dome and west antarctica byrd with temporal resolution from 20 to 50 yr the long term trends possibly reflect local ice sheet elevation fluctuations superimposed on common climatic fluctuations all the records confirm the widespread antarctic early holocene optimum between 11 500 and 9000 yr in the ross sea sector a secondary optimum is identified between 7000 and 5000 yr whereas all eastern antarctic sites show a late optimum between 6000 and 3000 yr superimposed on the long time trend all the records exhibit 9 aperiodic millennial scale oscillations climatic optima show a reduced pacing between warm events typically 800 yr whereas cooler periods are associated with less frequent warm events pacing 1200 yr

compositional and fluid pressure controls on the state of stress on the nankai subduction thrust a weak plate boundary abstract we show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the nankai subduction plate boundary off southwest japan ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients r at stress levels between 1 0 and 40 mpa the direct shear r values for purified smectite are 0 14 0 02 for illite 0 25 0 01 and for chlorite 0 26 0 02 for point load velocities of 0 0001 mm s these clay minerals dominate the nankai subduction decollement zone illite plus quartz is mechanically important in the altered incoming muroto section and the predicted decollement r should lie between 0 2 and 0 32 this low residual strength together with elevated fluid pressure limits shear stresses to below 4 mpa within the frontal 50 km of the subduction system consistent with the low wedge taper in this region a higher wedge taper off the ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in r are predicted to occur as a result of variations in smectite and total clay content these variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite however our data suggest the low r values of the clay rich decollement still limit shear stresses to between 17 and 29 mpa within the frontal 50 km of the wedge consistent with other estimates of plate boundary weakness indeed we propose that it should be expected that subduction plate boundaries like nankai will be weak because of the intrinsic presence of clay rich faults and moderate fluid overpressures our data do not support the hypothesis that the smectite to illite reaction directly controls the onset of seismogenic behavior deep in the nankai system because there is already a mechanical dominance of illite rather than smectite in the shallow decollement zone and we find all the clay phases tend to velocity strengthen however temperature activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up dip limit of the seismogenic zone

the role of compaction contrasts in sediments in decollement initiation in an accretionary prism abstract to understand how decollements develop into the pristine sedimentary succession entering subduction zones we have performed mechanical tests on samples from the sediment column entering the nankai accretionary prism japan odp site 1173 both poroelastic compliance and plastic shrinkage upon application of a large effective pressure sharply decrease with depth in a 100 m thick domain in the upper section of the lower shikoku basin unit i e in a domain stratigraphically close to the actual location of the decollement near the toe of the prism these property contrasts provide a potential explanation for the outward migration of the decollement into the incoming sediments when approaching the deformation front a given material particle is affected by an increase in stress which has a component of vertical loading due to the deposition of overburden trench sediment and also a component of lateral compression transmitted from the accretionary wedge depending on its initial mechanical state the amount of lateral shortening in the incoming nankai sediment column varies with depth and causes horizontal velocity gradients that concentrate into the mechanical transition zone upper section of the lower shikoku basin at appx 450 550 m depth into which the decollement eventually propagates future work has to assess the role of this plastic deformation relative to other governing factors such as friction coefficient and excess pore pressure both at nankai and along other active margins

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

paleoceanographic changes in the southern ocean off elephant island since the last glacial period links between surface water productivity nutrient utilization bottom water currents and ice rafted debris abstract to understand past changes in ocean cryosphere interactions in the southern ocean off the antarctic peninsula multi proxy analyses of three sediment cores located off elephant island were used to reconstruct changes in paleoproductivity nutrient utilization bottom current intensity and iceberg calving since the last glacial period the glacial period was characterized by low surface water productivity with high nutrient utilization indicating surface water stratification during the deglaciation surface water productivity increased with decreasing nutrient utilization implying that the increase is associated with increased nutrient supply from the subsurface water by enhancing antarctic circumpolar current acc influence as fronts migrate southward with warming abundant occurrence of grains 1 mm during the deglacial period indicates rapid ice sheet retreat with large scale melting and calving during the glacial period however coarse silt fine sand sized fraction represented ice rafted debris ird the different ird grain size characteristics are thought to be related to the ird source material characteristics regardless of ird input the running downcore correlation 5 to 9 point between sortable silt mean grain size and percentage showed that sediments are well sorted by bottom current however the cross plot of them showed different temporal relationships sediments were sorted by the acc and southwestward flowing bottom current along with southward migration of fronts and the acc southwestward flowing bottom current influence diminished whereas the acc influence increased particularly from 7 ka our results indicate that the sedimentary processes in the scotia sea largely depend on the regional interactions between the ocean and the cryosphere

first survey of antarctic sub ice shelf sediments reveals mid holocene ice shelf retreat the retreat of five small antarctic peninsula ice shelves in the late 20th century has been related to regional possibly anthropogenic climate warming we use the record of ice rafted debris ird in cores to show that the prince gustav channel ice shelf also retreated in mid holocene time early and late holocene age sediments contain ird derived entirely from local ice drainage basins which fed the section of ice shelf covering each site core top and mid holocene 5 2 ka sediments include a wider variety of rock types recording the drift of far traveled icebergs which implies seasonally open water at the sites the period when the prince gustav ice shelf was absent corresponds to regional climate warming deduced from other paleoenvironmental records we infer that the recent decay cannot be viewed as an unequivocal indicator of anthropogenic climate perturbation

clay mineral organic matter relationships in the early solar system as the solar system formed it inherited and perpetuated a rich organic chemistry the molecular products of which are preserved in ancient extraterrestrial objects such as carbonaceous chondrites these organic rich meteorites provide a valuable and tangible record of the chemical steps taken towards the origin of life in the early solar system chondritic organic matter is present in the inorganic meteorite matrix which in the cm and ci chondrites contains evidence of alteration by liquid water on the parent asteroid an unanswered and fundamental question is to what extent did the organic matter and inorganic products of aqueous alteration interact or display interdependence we have used an organic labelling technique to reveal that the meteoritic organic matter is strongly associated with clay minerals this association suggests that clay minerals may have had an important trapping and possibly catalytic role in chemical evolution in the early solar system prior to the origin of life on the early earth

consolidation state and strength of underthrust sediments and evolution of the decollement at the nankai accretionary margin results of uniaxial reconsolidation experiments 1 uniaxial reconsolidation experiments conducted on ocean drilling program drill cores along the muroto transect of the nankai accretionary margin demonstrate complex yield and postyield behavior and provide evidence for enhanced strengths within sediments beneath the decollement zone tests were conducted on samples collected from similar stratigraphic levels below the decollement and its seaward projection consolidation state of the samples increased landward and with depth and was tracked closely by sediment yield stress for all but one of the samples the sediments however exhibited substantial postyield strength up to 2 8 times the predicted in situ effective vertical stress beneath the protothrust zone this enhanced strength results from diagenesis that leads to matrix cementation during stable effective stress conditions within the underthrust section the close correspondence between yield stress and predicted in situ effective stress suggests that despite the cemented state the sediment matrix remains sensitive to in situ stress conditions the low yield stress of one sample collected within 40 m of the decollement fault at site 808 may reflect diagenesis under reduced effective stress conditions due to postconsolidation increases in pore pressure along the decollement due to their cemented state the strong underthrust sediments resist decollement downcutting beneath the toe of the prism leading to preferential incorporation of weaker continuously deforming accreted sediments during shear seismogenic slip along the decollement at depth may create stresses in excess of sediment strength causing shear failure and rapid release of trapped pore fluids feeding high pore fluid pressures along the decollement zone