coupled ice ocean dynamics in the marginal ice zones upwelling downwelling and eddy generation this study is aimed at modeling mesoscale processes such as upwelling downwelling and ice edge eddies in the marginal ice zones a two dimensional coupled ice ocean model is used for the study the ice model is coupled to the reduced gravity ocean model through interfacial stresses the parameters of the ocean model were chosen so that the dynamics would be nonlinear the model was tested by studying the dynamics of upwelling winds parallel to the ice edge with the ice on the right produce upwelling because the air ice momentum flux is much greater than air ocean momentum flux thus the ekman transport is greater under the ice than in the open water the stability of the upwelling and downwelling jets is discussed the downwelling jet is found to be far more unstable than the upwelling jet because the upwelling jet is stabilized by the divergence a constant wind field exerted on a varying ice cover will generate vorticity leading to enhanced upwelling downwelling regions i e wind forced vortices steepening and strengthening of vortices are provided by the nonlinear terms when forcing is time varying the advection terms will also redistribute the vorticity the wind reversals will separate the vortices from the ice edge so that the upwelling enhancements are pushed to the open ocean and the downwelling enhancements are pushed underneath the ice

sub ice topography and meteorite finds near the allan hills and the near western ice field victoria land antarctica we have investigated the glaciological causes of large meteorite concentrations on blue ice fields west and southwest of the allan hills in antarctica a sub ice topography map for the area was prepared from data of a radio echo sounding survey the map reveals a mesa type paleosurface it was formed prior to and modified by glacial processes during the initial stage of glaciation of antarctica ice flow toward mawson glacier north of the allan hills is largely confined to a n s trending depression between the allan hills ice field and the near western ice field blue ice at the margins of the ice stream flows over the mesas on both sides of this depression meteorites entrained in blue ice are uncovered by sublimation and ablation we present arguments showing that currently almost all of the blue ice flowing into the allan hills ice field is sublimated leaving meteorites on the ice surface we present evidence that windblown meteorites are trapped by snow bridges across crevasses resulting in a near surface meteorite concentration near the ice ridges of the allan hills ice field nevertheless most meteorites are exposed for only a short period of time to the atmosphere before they are blown by the wind across the ice toward the western foot of the allan hills from where they are slowly carried northward to mawson glacier and the ross sea

ice dynamics of the allan hills meteorite concentration sites revealed by satellite aperture radar interferometry the ice flow conditions of a 100 x 100 km area of victoria land antarctica were analyzed with the synthetic aperture radar sar technique the area includes a number of meteorite concentration sites in particular the allan hills ice fields regional ice flow velocities around the mid western and near western ice fields and the allan hills main ice field are shown to be 2 5 m yr 1 these sites are located on a horseshoe shaped area that bounds an area characterized by higher ice flow velocities of up to 5 m yr 1 meteorite find locations on the elephant moraine are located in this high ice flow area the sar derived digital elevation model dem shows atypical low surface slopes for antarctic conditions which are the cause for the slow ice movements numerous ice rises in the area are interpreted to cap sub ice obstacles which were formed by tectonic processes in the past the ice rises are considered to represent temporary features which develop only during warm stages when the regional ice stand is lowered ice depressions which develop in warm stages on the lee side of ice rises may act as the sites of temporary build up of meteorite concentrations which turn inoperative during cold stages when the regional ice level rises and the ice rises disappear based on a simplified ice flow model we argue that the regional ice flow in cold stages is reduced by a factor of at least 3

la purisima volcanic field baja california sur mexico miocene to quaternary volcanism related to subduction and opening of an asthenospheric window geological mapping and geochemical analyses combined with 40 k 40 ar ages for lavas from the late miocene to quaternary la purisimavolcanicfield bajacaliforniasur provideevidenceforfivevolcanicevents these inturn mayreflectplateinteractionsin the region the oldest event event 1 prior to 11 ma corresponds to the emission of normal to k rich calc alkaline lavas exposed as large mesas in the eastern part of the studied area and as pyroclastic breccias and volcaniclastic sediments to the west it is associated with the end of the comonduarc activity resulting from subduction of the farallon and guadalupe plates between 10 6 and 8 8 ma event 2 magnesian andesites and tholeiites were emplaced at 5 5 ma event 3 and 2 5 ma event 4 small volumes of magnesian andesites erupted in the central and southern parts of the volcanic field finally between 1 2 ma and holocene event 5 numerous basaltic and magnesian andesitic fissural and central emissions resulted in the formation of strombolian cones and associated lava flows mainly distributed within a nnw sse trending graben located se of the town of la purisima magmatic events 2 to 5 occurred well after the supposed end of the subduction event their geochemical characteristics are still typical of subduction modified sources and possibly indicate partial melting of hot slab and formation of an asthenospheric window due to a slab rupture event which followed ridge trench collision prior to the continental breakup of the gulf of california extensional province d 2005 elsevier b v all rights reserved

noble gases in h chondrites and potential differences between antarctic and non antarctic meteorites abstract thirty one antarctic h chondrites from the allan hills have been analyzed for their noble gases seven of the specimens contain solar gases thirteen have a cosmic ray exposure age around 8 ma both fractions are as observed for h chondrites from the rest of the world scrutiny of the data for paired samples yields a minimum number of 17 independent falls with 6 falls required for the 8 ma peak of exposure age the distribution of exposure ages as well as that of radiogenic 4 he and 40 ar yield no evidence that h chondrites from the allan hills region represent a mixture of populations that is different from that brought to moderate latitudes of the earth in more recent times

on b be ratios in the mexican volcanic belt boron and beryllium concentrations were measured in a diverse suite of well characterized rocks from the mexican volcanic belt mvb low b and high be result in relatively low b be ratios in the mvb compared to other arcs nevertheless b systematics resemble those of other arcs and provide insights into mantle processes in the mvb b enrichment depends first on magma type and second on edifice type and location b be values are highest 5 15 in andesites and dacites erupted from calcalkaline strato volcanoes located along the volcanic front such as volcan colima and v san juan rocks from strato volcanoes located behind the volcanic front generally have lower values 1 5 b be values are also elevated in differentiated members of rock suites that show evidence for significant crustal assimilation in the westernmost mvb west of the michoacan guanajuato volcanic field mgvf cindercone ejecta including basalts lamprophyres and basanites contain low b be values 5 the lamprophyres and basanites have very low b be despite high ba ce and other common measures of subduction signature in the mgvf where cinder cones occur exclusively b be values in primitive calc alkaline basalts are distinctly higher than those from alkaline basalts 3 8 vs 1 3 indicating that high b be is a mantle derived feature and not an artifact of crustal assimilation comparison among various elemental ratios indicates that cs and u show enrichment patterns similar to b all are enriched in calc alkaline rocks but not in lamprophyres or basanites in contrast ba k and sr are enriched in both calc alkaline rocks and the lamprophyres and basanites multi stage processes and differing melting mechanisms are inferred to explain the variable characteristics of mvb volcanic rocks first slab derived fluids rich in fluid mobile elements including b and ba infiltrate the mantle wedge these fluids cause fluid fluxed melting that produces calc alkaline magmas enriched in all fluid mobile elements these lavas erupt from large volcanic front strato volcanoes the slab derived fluids also metasomatize portions of the mantle wedge producing phlogopite and or amphibole these phases have high partition coefficients for ba sr k but may have low partition coefficients for b cs u accordingly subduction zone metasomatism produces a mantle wedge enriched in ba sr k but not necessarily in b cs u decompression melting of this type of metasomatized mantle will consume the hydrous phases and produce magmas such as lamprophyres or basanites with high ba ce sr la and k la but low b be cs la and u la this interpretation implies two types of subduction zone signatures one involving enriched ba sr k elements that have longer residence times in the mantle wedge and another involving enriched b cs u which all partition so strongly into fluids or melts that they have short residence times in the mantle and are only enriched in magmas generated by fluid fluxed melting or that have assimilated crustal material assimilation of granites and crustal rocks can also enrich differentiated lavas in b

wave propagation in the marginal ice zone model predictions and comparisons with buoy and synthetic aperture radar data in this paper the ocean wave dispersion relation and viscous attenuation by a sea ice cover are studied for waves propagating into the marginal ice zone miz the derivation of the dispersion relation and the viscous attenuation by an ice sheet are discussed for waves under flexure and pack compression in the miz the flexure effect is important for short waves for a fixed wave period the changes in wavelength and group velocity as a function of ice thickness are significant in turn the exponential wave attenuation rate shows a rollover at short wave periods whereby the rapid increase in wave attenuation rate with decreasing wave period slows down or even turns into a decrease the labrador ice margin experiment limex conducted on the miz off the east coast of newfoundland canada in march 1987 provides us with aircraft synthetic aperture radar sar imagery wave buoy and ice property data on the basis of the wave number spectrum from the sar data and the concurrent wave frequency spectrum from the ocean buoy data and accelerometer data on the ice during limex 87 the dispersion relation has been estimated and compared with a model wave energy attenuation rates are estimated from sar data and the ice motion package data which were deployed at the ice edge and into the ice pack and compared with the model the model data comparisons are reasonably good for the ice conditions observed during limex 87 some previously reported data of wave attenuation in the miz are revisited for model comparison

chemical studies of h chondrites 5 temporal variations of sources we report 36cl 301 kyr half life data obtained by accelerator mass spectrometry allowing nominal terrestrial ages to be determined for 39 antarctic h4 6 chondrites for which contents of volatile trace elements are known the compositional difference between these antarctic meteorites and 58 non antarctic falls increases with terrestrial age and using multivariate statistical techniques becomes highly significant for antarctic samples with ages 50 kyr the compositional difference is inconsistent with trivial causes such as weathering and seems to reflect differences in thermal histories of parent sources temporal source variations for the h chondrite flux on earth thus exist not only on a short term 40 years basis dodd et al 1993 but also on a long term 50 kyr basis

the breakup of a meteorite parent body and the delivery of meteorites to earth whether many of the 10 000 meteorites collected in the antarctic are unlike those failing elsewhere is contentious the antarctic h chondrites one of the major classes of stony meteorites include a number of individuals with higher induced thermoluminescence peak temperatures than observed among non antarctic h chondrites the proportion of such individuals decreases with the mean terrestrial age of the meteorites at the various ice fields these h chondrites have cosmic ray exposure ages of about 8 million years experienced little cosmic ray shielding and suffered rapid postmetamorphic cooling breakup of the h chondrite parent body 8 million years ago may have produced two types of material with different size distributions and thermal histories the smaller objects reached earth more rapidly through more rapid orbital evolution

the natural thermoluminescence of meteorites 4 ordinary chondrites at the lewis cliff ice field natural thermoluminescence tl measurements have been made on 302 meteorites from the vicinity of the lewis cliff in the beardmore region of antarctica the data provide information on terrestrial age and unusual radiation and thermal histories which in turn are helpful in identifying fragments of a single fall and in understanding ice sheet movements and the mechanisms by which meteorite concentration occurs at this site the present data with data for induced tl class find location hand specimen descriptions and mineral composition have enabled 70 of the present samples to be assigned to 27 groups of paired meteorites with between 5 and 2 meteorites in each group the distribution of meteorites on the ice the shape of the fields of paired meteorites and trends in the natural tl data indicate that there is a western component to the movement of the ice at this location as well as the previously supposed movement to the north this western vector probably explains the concentration of meteorites along the western edge of the ice tongue meteorites at the northern end of the tongue the lower ice tongue and meteorite moraine to the east have relatively high natural tl and therefore young terrestrial ages while those on the upper tongue show a broad range of ages including a great many large ages details of the meteorite concentration are different on the upper and lower tongue and it might be that these two parts of the ice sheet are unrelated these new natural tl data identify several recent falls and several meteorites which probably had unusually small perihelia immediately prior to capture by the earth

distribution of terrestrial age and petrologic type of meteorites from western libya abstract a group of 54 meteorites have been recovered from daraj western libya after assessment of pairing of samples using petrologic criteria 14 c terrestrial ages were obtained on 13 samples selected from 9 different fall events eleven of the ages range from 3500 to 7600 years with only two samples having ages in excess of 10 000 years the cut off in ages may be related to the timing of climatic changes in the hammadah al hamra

ocean ice interaction in the marginal ice zone using synthetic aperture radar imagery ocean ice interaction processes in the marginal ice zone miz by wind waves and mesoscale features such as up downwelling and eddies are studied using earth remote sensing satellite ers 1 synthetic aperture radar sar images and an ocean ice interaction model a sequence of seven sar images of the miz in the chukchi sea with 3 or 6 days interval are investigated for ice edge advance retreat simultaneous current measurements from the northeast chukchi sea as well as the barrow wind record are used to interpret the miz dynamics sar spectra of waves in ice and ocean waves in the bering and chukchi sea are compared for the study of wave propagation and dominant sar imaging mechanism by using the sar observed ice edge configuration and wind and wave field in the chukchi sea as inputs a numerical simulation has been performed with the ocean ice interaction model after 3 days of wind and wave forcing the resulting ice edge configuration eddy formation and flow velocity field are shown to be consistent with sar observations

the frontier mountain meteorite trap antarctica the frontier mountain blue ice field is an important antarctic meteorite trap which has yielded 472 meteorite specimens since its discovery in 1984 remote sensing analyses and field campaigns from 1993 to 1999 have furnished new glaciological data on ice flow ice thickness bedrock topography ice ablation and surface mass transport by wind along with detailed descriptions of the field situation at the trap this solid set of data combined with an updated meteorite distribution map and terrestrial ages available from literature allows us to better describe the nature of the concentration mechanism in particular we observe that the meteorite trap forms in a blue ice field 1 located upstream of an absolute and a shallow sub ice barriers 2 characterized by compressive ice flow with horizontal velocities decreasing from 100 to 10 cm year on approaching the obstacle 3 undergoing mean ablation rates of 6 5 cm year 4 nourished by a limited snow accumulation zone extending 20 km upstream of the blue ice area we also draw the following conclusions 1 the origin of the meteorite trap can be explained according to the present day glaciological situation 2 the meteorite concentration develops according to the general principles of the ice flow model 3 the accumulation model can be described as stagnant ice or slow moving ice against an absolute and submerged barriers according to the descriptive schemes present in literature 4 the frontier mountain ice field is an effective trap for meteorites weighing more than 200 g for smaller masses the combination of wind and glacial drift may remove meteorites in less than a few tens of thousands of years 5 although the activation age of the frontier mountain trap is not yet constrained we infer that one of the most important findsites may be as old as 50 ka predating the last glacial maximum

cirque infills in the khibiny mountains kola peninsula russia palaeoglaciological interpretations and modern analogues in east antarctica we report here on cirque infills mapped in the khibiny mountains kola peninsula russia cirque infills are morainic deposits located near the headwalls of valleys and cirques their location and shape often with concave margins towards the valley side indicate that they were deposited by ice flowing up valley into the mountains rather than by local glaciers we suggest that they formed during the last deglaciation when khibiny was a nunatak and fennoscandian ice sheet lobes extended into valleys and cirques of the massif the formation of cirque infills is probably more related to ice sheet dynamic factors occurring when the ice margin retreated from the cirques than to climate driven interruption in the ice marginal retreat glacial conditions similar to those prevalent when the khibiny cirque infills were formed occur today in antarctica where the ice sheets engulf nunatak ranges in heimefrontfjella antarctica the formation of supraglacial moraines at the head of cirques are linked to blue ice conditions indicating locally low accumulation rates a dry continental climate and sublimation dominated ablation we suggest that these antarctic moraines are modern analogues of cirque infills on the kola peninsula and possibly that the cirque infills may be used as palaeoenvironmental indicators copyright 2007 john wiley sons ltd

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

mesoscale eddies in the fram strait marginal ice zone during the 1983 and 1984 marginal ice zone experiments during the summer marginal ice zone experiment in fram strait in 1983 and 1984 fourteen mesoscale eddies in both deep and shallow water were studied between 78 o and 81on sampling combined satellite and aircraft remote sensing observations conductivity temperature depth observations drift of surface and subsurface floats and current meter measurements typical scales of these eddies were 20 40 km rotation was mainly cyclonic with a maximum speed in several cases subsurface of up to 40 cm s observations further suggest that the eddy lifetime was at least 20 to 30 days five generation sources are suggested for these eddies several of the eddies were topographically trapped while others primarily formed by combined baroclinic and barotropic instability moved as much as 10 15 km d with the mean current the vorticity balance in the nontrapped eddies is dominated by the stretching of isopycnals accompanied by a change in the radial shear in the most completely observed eddy south of 79on the available potential energy exceeded the kinetic energy by a factor of 2 quantitative estimates suggest that the abundance of these eddies enhances the ice edge melt up to 1 2 km d

application of aerosondes to high resolution observations of sea surface temperature over barrow canyon 1 continuous observation of sea surface temperature sst were obtained for a period of 27 hours over barrow canyon on 20 21 september 2002 using a small robotic aircraft called the aerosonde these observations demonstrated significant spatial and temporal variations in the sst at scales that would be very difficult to measure using mooring and satellite observations over a region of 25 km2 sst values varied from 1 6 c a sharp sst front with a temperature gradient of 3 c was observed over the northwestern shelf of barrow canyon which moved northward during the observation period relatively cold water was transported from the southwest into the region current speeds were estimated using the time series of observations this analysis demonstrates new technology for oceanographic observations and supports previous evidence that the flow through barrow canyon has a strongly sheared flow into the arctic ocean

paired renazzo type cr carbonaceous chondrites from the sahara ten chondrites with chemical and mineralogical similarities to the carbonaceous chondrite renazzo were recovered at two locations of the sahara acfer 059 087 097 114 139 186 187 209 270 and el djouf 001 although the el djouf location is more than 500 km away from the acfer location all samples appear to result from a single fall based on chemical and petrographic similarities and supported by light element stable isotope geochemistry noble gas record and similar 26al contents the acfer el djouf meteorite is classified as a cr renazzo type carbonaceous chondrite this group presently comprises three non antarctic members al rais renazzo acfer el djouf and five antarctic meteorites the major lithological components of the acfer el djouf meteorite are large chondrules up to 1 cm in size mean diameter 1 0 0 6 mm chondrule and mineral fragments ca al rich inclusions feni metal about 8 10 vol and dark inclusions embedded in a fine grained fragment bearing groundmass mineral compositions of the ten acfer el djouf samples are similar to those of other cr chondrites most of the ca al rich inclusions are below 300 m in size and rich in melilite and spinel in some cais the rare phase caal4o7 is dominant fo rich cr bearing olivine fa0 4 and enstatite fs0 4 are the major phases of the chondrite the meteorite is mildly shocked with a shock stage of s2 indicating a peak shock pressure of 5 10 gpa for the bulk meteorite the oxygen isotopic compositions and carbon and nitrogen stable isotope geochemistry of the acfer el djouf samples are very similar to those of the other cr type chondrites the major element composition of the acfer el djouf meteorite is indistinguishable from cr chondrites when compared to renazzo the acfer el djouf samples however have systematically lower contents of the moderately volatile elements zn ga as au sb and se and the highly volatile elements br c and n this is thought to reflect primary differences between renazzo and the acfer el djouf meteorite

noble gases and the history of jilin meteorite abstract potassium and noble gases have been determined in more than twenty specimens from the largest known stone meteorite the h5 chondrite jilin thirteen specimens came from the surface of the present main mass the remainder from various locations in the strewn field the average k content is 802 ppm 29 samples from 23 specimens maximum deviations from the mean are 7 and 11 whole rock gas retention ages of different specimens are distinctly different they vary between 2 22 and 3 90 ae for40ar 40k and between 0 44 and 2 0 ae for4he u th severe losses of4he and40ar up to 95 and 80 respectively must have occurred up to less than 440 ma ago they cannot have happened during the fall of the meteorite however differences in concentration of cosmic ray produced3he 21ne and38arme by factors of five six and seven respectively reflect a complex irradiation history they are compatible with a short 4 irradiation and an extended one in 2 geometry at shallow depth top samples were most probably located near the transition maximum of nuclear active particles at around 15 cm depth they definitely cannot have been buried deeper than 4 m 3he 21ne ratios in bulk samples are lowered by diffusion losses of3he 25 61 while22ne 21ne ratios appear to be unaffected 22ne 21ne values range between 1 060 and 1 086 with a mean of 1 069 which is at variance with predictions for the particular irradiation conditions of jilin low36ar 38ar ratios down to 0 553 in clean metal samples are interpreted as the combined effect of large size and the transient lowering of this ratio because of a sudden increase in production rates upon going from 2 to 4 irradiation

climate archives from 90 to 250 ka in horizontal and vertical ice cores from the allan hills blue ice area antarctica abstract terrestrial meteorite ages indicate that some ice at the allan hills blue ice area ah bia may be as old as 2 2 ma as such ice from the ah bia could potentially be used to extend the ice core record of paleoclimate beyond 800 ka we collected samples from 5 to 10 cm depth along a 5 km transect through the main icefield and drilled a 225 m ice core s27 at the midpoint of the transect to develop the climate archive of the ah bia stable water isotope measurements d of the surface chips and of ice core s27 yield comparable signals indicating that the climate record has not been significantly altered in the surface ice measurements of 40 ar atm and 18 o atm taken from ice core s27 and eight additional shallow ice cores constrain the age of the ice to approximately 90 250 ka our findings provide a framework around which future investigations of potentially older ice in the ah bia could be based

the natural thermoluminescence of meteorites 7 ordinary chondrites from the elephant moraine region antarctica we report natural and induced thermoluminescence tl measurements for meteorites from the elephant moraine region 76 17 s 157 20 e of antarctica we use our data to identify fragmented meteorites i e pairings our dataset of 107 samples represents at most 73 separate meteorite falls pairing groups are generally confined to single icefields or to adjacent icefields but a small proportion cross widely separated icefields in the region suggesting that the fields can be considered as a single unit meteorites from this region have high natural tl levels which indicates that they have small terrestrial surface exposure ages 12 500 years there do not appear to be significant differences in natural tl levels and hence surface exposure ages between individual blue icefields in the region the proportion of reheated meteorites from the elephant moraine region is similar to that of other antarctic sites and modern falls consistent with the uniformity of the meteoritic flux in this regard an unusual subset of h chondrites with high induced tl peak temperatures is absent among the data for meteorites collected in the elephant moraine region which stresses their similarity to modern falls we suggest that the elephant moraine icefields formed through shallow ablation of the ice unlike the allan hills sites to the south lateral transport is probably less important relative to the infall of meteorites in concentrating meteorites on these icefields

coastal ocean fronts and eddies imaged with ers 1 synthetic aperture radar ers 1 c band synthetic aperture radar sar data were collected during the norwegian continental shelf experiment norcsex both in november 1991 during the ers 1 commissioning phase and at different seasons in 1992 and 1993 characteristic sar image expressions are observed in relation to perturbation of the surface current short wave interaction across the norwegian coastal current front for winds less than 10 12 rn s 1 in situ measurements document the existence of alternating zones of convergence and divergence coexisting with a strong near surface current shear of nearly 4f where f is the coriolis parameter across a distance of a few kilometers under calm to moderate winds i e 4 7 rn s 1 characteristic expressions of upper ocean circulation features also include the manifestation of eddies through the presence of surface film which damps the bragg waves comparison of a near coincident national oceanic and atmospheric administration advanced very high resolution radiometer image and an ers 1 sar image supports the interpretation that surface current fronts are imaged by sar in combination with an sar image simulation model the relative quantitative importance of shear convergence and divergence along the front is examined although the model formulation is simple and the absolute magnitude of the perturbations is uncertain the study shows that the sar images can sometimes be used to interpret frontal dynamics including growth and decay of meanders

advance of east antarctic outlet glaciers during the hypsithermal implications for the volume state of the antarctic ice sheet under global warming we present the first circum east antarctic chronology for the holocene based on 17 radiocarbon dates generated by the accelerator method marine sediments from around east antarctica contain a consistent high resolution record of terrigenous ice proximal and biogenic open marine sedimentation during holocene time this record demonstrates that biogenic sedimentation beneath the open marine environment on the continental shelf has been restricted to approximately the past 4 ka whereas a period of terrigenous sedimentation related to grounding line advance of ice tongues and ice shelves took place between 7 and 4 ka an earlier period of open marine biogenic sedimentation conditions following the late pleistocene glacial maximum is recognized from the prydz bay ocean drilling program record between 10 7 and 7 3 ka clearly the response of outlet systems along the periphery of the east antarctic ice sheet during the mid holocene was expansion this may have been a direct consequence of climate warming during an antarctic hypsithermal temperature accumulation relations for the antarctic indicate that warming will cause a significant increase in accumulation rather than in ablation models that predict a positive mass balance growth of the antarctic ice sheet under global warming are supported by the mid holocene data presented herein

mid late pleistocene glacial evolution in the grove mountains east antarctica constraints from cosmogenic 10be surface exposure dating of glacial erratic cobbles abstract glacial histories from the east antarctic ice sheet eais provide keys to understanding correlations between the eais and global climate they are especially helpful in the assessment of global sea level change and as a means of quantifying the magnitude of past glacial activity and the rate at which ice responded to climate change given the significance of eais glacial histories it is imperative that more glacial chronologic data for this region be obtained especially for the mid to late pleistocene we report cosmogenic 10 be surface exposure dating results from glacially transported cobbles embedded in blue ice moraine material at mount harding the grove mountains eais forty exotic cobbles sampled along two profiles a and b on this blue ice moraine present apparent exposure ages ranging from 7 2 to 542 2 ka we explore this scattered dataset by using principal component analysis pca to identify statistically significant trends in the data we identify a correlation between exposure age and distance of the cobbles from mount harding in profile a cobbles further from mount harding yield older exposure ages than those that are relatively close in profile b cobbles closer to mount harding are found to have relatively older exposure ages in term of glacial history we suggest that the direction of ice flow changed during the period from 60 to 200 ka and that multiple glacial fluctuations occurred in the mid late pleistocene

allan hills 88019 an antarctic h chondrite with a very long terrestrial age we have measured the concentrations of the cosmogenic radionuclides loge 26al and 36cl half lives 1 51 ma 716 ka and 300 ka respectively in two different laboratories by accelerator mass spec trometry ams techniques as well as concentrations and isotopic compositions of stable he ne and ar in the antarctic h chondrite allan hills alh 88019 in addition nuclear track densities were measured from these results it is concluded that the meteoroid alh 880 19 had a preatmospheric radius of 20 5 cm and a shielding depth for the analyzed samples of between 4 and 8 cm using calculated and experimentally deter mined production rates of cosmogenic nuclides an exposure age of 40 ma is obtained from cosmogenic 21ne and 38ar the extremely low concentrations of radionuclides are explained by a very long terrestrial age for this meteorite of 2 f 0 4 ma a similarly long terrestrial age was found so far only for the antarctic l chon drite lewis cliff lew 86360 such long ages establish one boundary condition for the history of mete orites in antarctica

the flux of meteorites to the earth and weathering in hot desert ordinary chondrite finds abstract the abundance of ferric iron produced by weathering in ordinary chondrites collected from hot desert regions is related to their terrestrial age enabling a variety of processes involved in meteorite weathering to be deduced fe0 fe2 fe3 content was measured quantitatively by 57fe mossbauer spectroscopy it is observed that meteorites from different regions are oxidised at different rates we also propose the tentative hypothesis that one source of scatter in total oxidation versus terrestrial age for a region is climatic change a mechanism to explain the variable response to climatic conditions involves the high initial porosity of samples being reduced as ferric oxides and other alteration products are produced reliable recovery data from a meteorite accumulation site roosevelt county new mexico usa and a quantitative measure of weathering and terrestrial age permit an estimate of the flux of meteorites to the earth s surface having established a decay constant for meteorites and accounting for the area of searches and pairing we estimate an accumulation rate of 159 falls of mass greater than 20 g per 106 km2 per yr this calculation is distinct from but in broad agreement with previous estimates from camera observations suggesting that the accumulation rate of meteorite falls to the earth s surface has remained essentially unchanged over the last 50 000 years at between 59 and 159 falls of mass exceeding 20 g per 106 km2 per yr the error for both methods is approximately a factor of 2

evidence of rapid cenozoic uplift of the shoulder escarpment of the cenozoic west antarctic rift system and a speculation on possible climate forcing the cenzoic west antarctic rift system characterized by cenozoic bimodal alkalic volcanic rocks extends over a largely ice covered area from the ross sea nearly to the bellingshausen sea it is bounded on one side by a spectacular 4 to 5 km high rift shoulder scarp maximum bedrock relief 5 to 7 km from northern victoria land queen maud mountains to the ellsworth whitmore horlick mountains jurassic tholeiites crop out with the late cenozoic volcanic rocks along the section of the transantarctic mountains from northern victoria land to the horlick mountains the cenozoic rift shoulder diverges here from the jurassic tholeiite trend and the tholeiites are exposed discontinuously along the lower elevation 1 2 km section of the transantarctic mountains to the weddell sea various lines of evidence no one of which is independently conclusive lead the authors as others have also suggested to interpret the following the transantarctic mountains part of the rift shoulder and probably the entire shoulder has been rising since about 60 ma at episodic rates of approximately 1 km m y most recently since mid pliocene time rather than continuously at the mean rate of 100m m y uplift rates vary along the scarp which is cut by transverse faults the authors speculate that this uplift may have climatically forcedmore the advance of the antarctic ice sheet since the most recent warm period they suggest a possible synergistic relation between episodic tectonism mountain uplift and volcanism in the cenozoic west antarctic rift system and waxing and waning of the antarctic ice sheet beginning about earliest oligocene time less

effect of cenozoic ice sheet fluctuations in antarctica on the stratigraphic signature of the neogene stratigraphic successions from the gulf of mexico offshore alabama northeast java indonesia ross sea antarctica and several other continental margins have been examined all are characterized by very similar neogene stratal geometries in seismic profiles and well log cross sections from these areas a large mid oligocene i e latest early oligocene earliest late oligocene basinward shift in coastal onlap is followed by a major early miocene transgression and aggradation which is in turn followed by early middle miocene transgressions and late middle and late miocene progradational episodes the succession culminates in plio pleistocene high frequency progradations and transgressions the interregional character of the neogene stratal signature and its similarity to the stratal geometry found in seismic data from the ross sea continental shelf antarctica suggest that the neogene stratal signature is a manifestation of glacioeustatic fluctuations a review of the literature and an analysis of recently acquired and published data indicate that the first major ice sheet grounding event in the ross sea occurred during middle to late oligoce time the ross sea is the repository for ice flowing from a major portion of the continental interior thus the glacial record of the ross sea should serve as a gage of ice volume changes on the continent that were large enough to influence global eustasy the ice advance onto the ross sea continental shelf during middle to late oligocene time may have been the result of a decrease in the rate of shelf subsidence as rifting in the ross sea slowed or ceased advance of the ice sheet resulted in widespread erosion of the continental shelf and shelf overdeepening it is hypothesized that metastable marine based ice sheets have waxed and waned on the antarctic continental shelf since at least the oligocene and that the these waxing and waning events were responsible for the development of a global neogene stratigraphic signature

the natural thermoluminescence of meteorites 5 ordinary chondrites at the allan hills ice fields natural thermoluminescence tl data have been obtained for 167 ordinary chondrites from the ice fields in the vicinity of the allan hills in victoria land antarctica in order to investigate their thermal and radiation history pairing terrestrial age and concentration mechanisms using fairly conservative criteria including natural and induced tl find location and petrographic data the 167 meteorite fragments are thought to represent a maximum of 129 separate meteorites natural tl values for meteorites from the main ice field are fairly low typically 5 30 krad indicative of terrestrial ages of 400 ka while the farwestern field shows a spread with many values 30 80 krad suggestive of 150 ka terrestrial ages there appear to be trends in tl levels within individual ice fields which are suggestive of directions of ice movement at these sites during the period of meteorite concentration these directions seem to be confirmed by the orientations of elongation preserved in meteorite pairing groups the proportion of meteorites with very low natural tl levels 5 krad at each field is comparable to that observed at the lewis cliff site and for modern non antarctic falls and is also similar to the fraction of small perihelia 0 85 au orbits calculated from fireball and fall observations induced tl data for meteorites from the allan hills confirm trends observed for meteorites collected during the 1977 1978 and 1978 1979 field seasons which show that a select group of h chondrites from the antarctic experienced a different extraterrestrial thermal history to that of non antarctic h chondrites

current response to wind in the chukchi sea a regional coastal upwelling event a cruise was conducted on the noaa ship oceanographer in august and september 1986 in the chukchi sea four short term moorings were instrumented with sediment traps and aanderaa rcm4 current meters the currents at the three moorings near the alaskan coast indicate a reversal of the normal northeastward flow to southwestward this reversal was produced by wind which had begun to blow from the east northeast at up to 4 2 m s 1 the temperature time series indicated that upwelling colder water was moving upward near the coast the nearshore mooring had the largest amplitude variation of currents and the largest temperature variation the amplitude of the reversal decreased offshore the station farthest from the coast was near the ice edge and on the other side of barrow canyon and a subsea bank the flow there was consistently toward the east and is only poorly related to the wind water mass analysis was conducted using two techniques the first was a traditional ts diagram method and the second was a cluster analysis on ts pairs for the surface and near bottom waters based on the shapes of the ts curves and their positions on the ts diagrams a map of the water masses was constructed a cluster analysis was performed on the surface ts pairs from each station and separately for the bottom ts pairs for both of these techniques the line separating the groupings follows the temperature contours 5 at the surface and 4 at the bottom and the bottom salinity contours 32 5 psu

alkaline and calc alkaline lavas near los volcanes jalisco mexico geochemical diversity and its significance in volcanic arcs a remarkably diverse suite of lavas erupted during the late pliocene at the volcanic front of the western mexican volcanic belt near the town of los volcanes jalisco this region is much closer to the middle america trench than the main axis of quaternary andesite dacite stratovolcanoes and volcanism occurred in a complex tectonic regime involving both subduction of the young rivera plate and transverse crustal extension of the jalisco structural block the variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the mexican volcanic belt other alkaline varieties intermediate between these extremes include absarokites trachybasalts and trachyandesites phlogopite amphibole and apatite are common phenocryst phases whole rock compositions show a wide range of alkali contents e g k2o of 1 0 8 6 wt and typically contain 5 wt mgo mgo ni v and cr show little systematic decrease with increasing sio2 suggesting that these lavas have evolved from primitive mantle derived magmas with a wide range of sio2 contents strong enrichments in incompatible trace elements are observed in all of the lavas sr 700 5100 ppm ba 470 4800 ce 22 325 ppm zr 90 700 ppm as is the relative enrichment of large ion lithophile ba sr rb and light rare earth elements la ce over the high field strength elements ti zr which is typical of magmas in volcanic arcs this enrichment pattern suggests that these magmas come from source regions which contain incompatible element rich phases such as phlogopite amphibole and apatite the petrological and geochemical features of the lavas which occur in the los volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins the complex tectonic regime in western mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub arc mantle

oceanographic conditions in the marginal ice zone north of svalbard in early fall 1979 with an emphasis on mesoscale processes during september october 1979 the norwegian remote sensing experiment was carried out in the marginal ice zone north of svalbard convergence of the ice cover is correlated with along ice edge winds with the ice to the right while divergence occurs during office winds or calm conditions a wind driven ice edge jet is observed wind driven upwelling of the pycnocline of up to 7 m was present along the ice edge during a 10 to 15 m s easterly wind event the upwelling is due to ekman divergence at the ice edge caused by higher wind stress over ice than over open water the ice edge meanders with a scale of 20 40 km and sheds eddies with a scale of 5 to 15 km into the open water this scale is of the same order as the rossby radius of deformation eddies with the same scale are also seen in the conductivity temperature and depth observations conditions during the experiment were such that barotropic instabilities could have generated these eddies

terrestrial ages pairing and concentration mechanism of antarctic chondrites from frontier mountain northern victoria land we report concentrations of cosmogenic 10be 26al 36cl and 41ca in the metal phase of 26 ordinary chondrites from frontier mountain fro antarctica as well as cosmogenic 14c in eight and noble gases in four bulk samples thirteen out of 14 selected h chondrites belong to two previously identified pairing groups fro 90001 and fro 90174 with terrestrial ages of 40 and 100 kyr respectively the fro 90174 shower is a heterogeneous h3 6 chondrite breccia that probably includes more than 300 individual fragments explaining the high h l chondrite ratio 3 8 at frontier mountain the geographic distribution of 19 fragments of this shower constrains ice fluctuations over the past 50 100 kyr to less than 40 m supporting the stability of the meteorite trap over the last glacial cycle the second h chondrite pairing group fro 90001 is much smaller and its geographic distribution is mainly controlled by wind transport most l chondrites are younger than 50 kyr except for the fro 93009 01172 pair which has a terrestrial age of 500 kyr these two old l chondrites represent the only surviving members of a large shower with a similar preatmospheric radius 80 cm as the fro 90174 shower the find locations of these two paired l chondrite fragments on opposite sides of frontier mountain confirm the general glaciological model in which the two ice flows passing both ends of the mountain are derived from the same source area on the plateau the 50 fro meteorites analyzed so far represent 21 different falls the terrestrial ages range from 6 kyr to 500 kyr supporting the earlier proposed concentration mechanism

volcanism and tectonism in western mexico a contrast of style and substance the jalisco block of southwestern mexico records the initial stages of continental rifting superimposed on a convergent continental margin pliocene quaternary eruptions in north northwest trending graben systems within the block produced extraordinarily diverse lava types including minette leucitite absarokite and andesite interspersed with these are several basaltic shield volcanoes that have compositional similarities to lavas from oceanic islands evidence of miocene basin and range extension which is seen around much of the gulf of california is found only along the northern margin of the jalisco block large volumes of alkali basalt were also erupted during the late miocene at several locations along this northern margin we propose that pliocene quaternary extension and associated volcanism within the confines of the jalisco block are consequences of complex plate boundary reorganizations that are causing the block to rift away from the mexican mainland

accumulation mechanisms and the weathering of antarctic equilibrated ordinary chondrites induced thermoluminescence tl is used to quantitatively evaluate the degree of weathering of meteorites found in antarctica we find a weak correlation between tl sensitivity and descriptions of weathering in hand specimens the highly weathered meteorites having lower tl sensitivity than unweathered meteorites analysis of samples taken throughout large meteorites shows that the heterogeneity in tl sensitivity within meteorite finds is not large relative to the range exhibited by different weathered meteorites the tl sensitivity values can be restored by minimal acid washing suggesting the lower tl sensitivities of weathered meteorites reflects thin weathering rims on mineral grains or coating of these grains by iron oxides produced by hydration and oxidation of metal and sulfides small meteorites may tend to be more highly weathered than large meteorites at the allan hills ice fields we find that meteorite fragments 150 g may take up to 300 000 years to reach the highest degrees of weathering while meteorites 150 g require 40 000 years however at other fields local environmental conditions and variability in terrestrial history are more important in determining weathering than size alone weathering correlates poorly with surface exposure duration presumably because weathering occurs primarily during interglacial periods the allan hills locality has served as a fairly stable surface over the last 100 000 years or so and has efficiently preserved both small and large meteorites meteorites from lewis cliff however have experienced extensive weathering probably because of increased surface melt water from nearby outcrops meteorites from the elephant moraine locality tend to exhibit only minor degrees of weathering but small meteorites are less weathered than large meteorites which we suggest is due to the loss of small meteorites by aeolian transport

update on terrestrial ages of antarctic meteorites abstract cosmic ray produced 36 cl half life 3 01 10 5 years has been measured in 90 antarctic meteorites by accelerator mass spectrometry the terrestrial ages of the meteorites were calculated from the results after excluding possible paired objects 138 terrestrial ages from 18 different locations are available from 14 c 81 kr and 36 cl measurements for application to antarctic meteorite and glaciological studies the terrestrial ages of allan hills meteorites vary from 2000 years to 1 million years and are clearly longer than those of yamato meteorites and other antarctic meteorites the oldest allan hills meteorites were found close to the eastern edge and in the southeast of the main icefield among all antarctic meteorites measured to date only l and ll chondrites have terrestrial ages older than 370 000 years

do microplates in subduction zones leave a geological record active microplate boundaries in ocean continent subduction zones may induce deformation of the overlying plate and spatial or geochemical variations in the volcanic arc we discuss two modern cases the first is the south gorda juan de fuca plate boundary in the cascadia subduction zone where there is little or no effect on the overriding plate and the oceanic plate takes up much of the deformation the second case is the cocos rivera plate boundary in the middle america trench where the overlying colima graben contains substantial deformation in a zone extending from the trench to the volcanic arc and the sub duction related volcanism is spatially and geochemically complex we apply these observations to boundaries of the arguello monterey guadalupe and magdalena microplates which existed in the subduction zone west of baja california at various times from 20 to 12 5 ma the past positions of these boundaries relative to baja california are constrained by global plate reconstructions closure of the gulf of california and an estimate of extension in the mexican basin and range province existing regional mapping and our additional reconnaissance mapping show that paleocene to eocene fluvial and marine sedimentary rocks south of ensenada along the western baja california peninsula and eastward to the mid miocene volcanic arc are undeformed limited available data reveal no major spatial or geochemical variations in the mid miocene volcanic arc that might correlate with the past positions of the microplate boundaries thus these microplate boundaries had little to no effect on the overriding continental plate the nature of guadalupe and magdalena interactions with north america may have been closer to the south gorda juan de fuca example with possible internal deformation of the microplates the monterey and arguello microplates may have behaved like the modern explorer plate with largely strike slip motion relative to north america during their last stages of existence tectonic patterns similar to these examples may be expected from other plate boundaries where a plate is fragmenting as it enters a subduction zone e g the aluk plate in the trench beneath west antarctica in early tertiary time whether these microplates subsequently become attached to the overriding continental plate or to a larger oceanic plate and whether this causes deformation in the region of the former subduction zone may depend on the velocities of the nearby major plates and the relative orientations of the microplate boundaries

antarctic and non antarctic meteorites form different populations compared with non antarctic falls antarctic meteorites include many of hitherto rare unique or unknown type1 such as lunar and putative martian samples2 4 attention has not previously been paid to the more common sorts of antarctic meteorites and here we focus on the most frequently encountered h5 chondrites we find that compositional differences between antarctic and non antarctic specimens of this class especially those involving thermally sensitive trace and ultra trace elements are so substantial that it seems extremely doubtful that the two sample populations are derived from the same parent population this implies that the contemporary falls less than 200 years old and the substantially older on average 300 000 years for victoria land antarctic meteorites either differ in genetic history or sample different extraterrestrial parent populations or both

feedback between ice flow barotropic flow and baroclinic flow in the presence of bottom topography coupling between externally driven barotropic flow and locally driven baroclinic flow in the presence of an ice cover and topography is studied the topography is a necessary ingredient in this coupling this study shows that the observed mesoscale activity of the ice edge seen in satellite imagery does not necessarily reflect the mesoscale baroclinic activity in the ocean besides oceanic eddies the ice cover can trace the topographic changes via the coupling to the barotropic flow a two layer ocean model coupled to an ice model is constructed to simulate an ice ocean system with a varying bottom topography in the absence of wind forcing the ice cover reflects the externally driven barotropic ocean response especially topographically forced taylor columns by forming ice streamers or meanders some of these features propagate advected with the ocean currents as a whole downstream creating an image of eddy propagation even though there is no baroclinic structure underneath when downwelling favorable winds ice on the left from the wind direction are turned on in addition to this background barotropic flow the ekman flow in the ocean toward the open ocean will enhance the meandering of the ice edge due to the barotropic flow during upwelling favorable winds the ice edge stays rather compact including the case when a strong baroclinic cyclonic vortex is developing beneath an ice meander supported by the taylor column in the ocean

airborne synthetic aperture radar observations and simulations for waves in ice the canada centre for remote sensing cv 580 aircraft collected c band synthetic aperture radar sar data over the marginal ice zone off the east coast of newfoundland during the labrador ice margin experiment limex in march 1989 one component of the limex 89 program was the study of ocean waves penetrating the marginal ice zone in this paper we consider nearly coincidental observations of waves in ice by airborne sar and wave induced ice motion measurements we explain the wave patterns observed in the sar imagery and the corresponding sar image spectra in terms of sar wave imaging models these include the well known tilt cross section modulation linear quasi linear and nonlinear velocity bunching forward mapping models fmms and the assertion that the concept of coherence time limitation applies differently to the cases of waves in ice and open water we modify the concept of the scene coherence time to include two parts first a decorrelation time deduced from the inherent azimuth cutoff in the nonlinear velocity bunching fmm and second the intrinsic scene coherence time which is a measure of the time scale over which an open water bragg scattering patch retains its phase structure either of these coherence time scales could dominate the sar image formation process depending upon the environmental conditions the wave spectrum and the wind speed for example these two coherence time scales are independently estimated based upon a quasi linear velocity bunching fmm applied to some of the limex 89 observations observed sar image spectra and forward mapped ice motion package spectra are favorably compared

steep subduction geometry of the rivera plate beneath the jalisco block in western mexico the morphology of the rivera plate subducted beneath the jalisco block in western mexico is determined from accurately located hypocenters of locally recorded microearthquakes and from earthquakes with magnitude mb 4 5 recorded at teleseismic distances the hypocenters of these latter earthquakes are relocated and for five of them the focal depth is constrained by the inversion of long period body waves the wadati benioff zone inferred from these data indicates a steep subduction of the rivera plate that resembles the geometry of subduction of the cocos plate beneath central america it is however very different from the shallower and almost subhorizontal subduction of the cocos plate observed in southern mexico southeast of this region the rivera plate is comparable to the juan de fuca plate in terms of the small areal extent young seafloor age low relative velocity and low teleseismic activity in the subduction zone this study shows that the dip of both the juan de fuca and rivera plates are similar once they are decoupled from the overriding continental crust the downgoing rivera plate initially starts with a dip of 10 down to a depth of 20 km and then increases gradually to a constant dip of 50 below a depth of 40 km intermediate depth seismicity is low in this zone associated with the subduction of the slow 2 cm yr and young 9 m y rivera plate the maximum depth extent of earthquakes observed in the rivera subduction zone is about 130 km the andesitic calc alkaline colima volcano appears to be directly related to the subduction of the rivera plate to the nw of this volcano the observed quaternary volcanism in the jalisco block which is parallel to the trench may also be explained by the subduction of the rivera plate

on the glaciological meteorological and climatological significance of antarctic blue ice areas this paper presents the current state of our knowledge about antarctic blue ice areas scattered widely over the antarctic continent yet covering only approximately 1 of its surface area blue ice areas have some peculiar characteristics that make them of particular interest for planetary geologists glaciologists meteorologists and climatologists for instance blue ice areas are subject to net ablation mainly by sublimation since any accumulation is easily swept off their smooth surfaces by winds moreover they are significantly warmer than their surroundings and the wind over them is generally more gusty an important feature of blue ice is its relatively low albedo 0 56 compared with that of snow 0 80 blue ice is probably considerably smoother than snow in the aerodynamic sense with roughness lengths as low as 3 10 6 m which indicates that blue ice is probably the only permanent natural surface over which the air flow is mainly in the aerodynamic smooth regime further since it is likely that very old ice is exposed at the surface of some blue ice areas this ice would be of great interest for paleoclimatic studies it has also been suggested that blue ice areas are sensitive to climate change this would make them useful as antarctic climate indicators because they can easily be monitored by satellite finally this paper addresses several gaps in our knowledge about blue ice areas and gives suggestions for future blue ice area research

late tertiary history of the antarctic ice sheet evidence from the dry valleys data from the dry valleys antarctica suggest that outlet glaciers of a local ice cap carved primary valley systems on both flanks of the transantarctic mountains while coeval alpine glaciers eroded exposed intervalley mountain ranges subsequently a thick ice sheet overrode the transantarctic mountains at least twice flowing northeastward across major pre existing valleys the youngest overriding episode postdated and the older episode antedated middle to early late miocene time we think that an extensive ice sheet covered east and west antarctica during overriding episodes

new insights into the subducting oceanic crust in the middle american trench off western mexico 17 19 n abstract we present new multibeam bathymetric and multichannel seismic data on the middle american trench between 17 and 19 n these data show north of the present tip of the east pacific rise unsedimented seafloor with an abyssal hill fabric bare of sediments at the outer wall of the trench 18 30 n two multichannel seismic profiles orthogonal to the middle american trench display clear oceanic crust extending more than 20 km beneath the overriding plate these seismic images suggest that the subducting oceanic crust along the middle american trench is younger than previously proposed this result therefore implies a new location for the east pacific rise termination in relation to the trench the subduction of this young oceanic crust off western mexico provides new constraints on the deformation and magmatism of the western mexican mainland

wave current interaction study in the gulf of alaska for detection of eddies by synthetic aperture radar high resolution ers i synthetic aperture radar sar images are used to detect a mesoscale eddy such features limit dispersal of pollock larvae and therefore likely influence recruitment of fish in the gulf of alaska during high sea states and high winds the direct surface signature of the eddy was not clearly visible but the wave refraction in the eddy area was observed the rays of the wave field are traced out directly from the sar image the ray pattern gives information on the refraction pattern and on the relative variation of the wave energy along a ray through wave current interaction these observations are simulated by a ray tracing model which incorporates a surface current field associated with the eddy the numerical results of the model show that the waves are refracted and diverge in the eddy field with energy density decreasing the model data comparison for each ray shows the model predictions are in good agreement with the sar data

chemical studies of h chondrites ii weathering effects in the victoria land antarctic population and comparison of two antarctic populations with non antarctic falls abstract we report rnaa data for 14 siderophile lithophile and chalcophile volatile mobile trace elements ag au bi cd co cs ga in sb se te tl u zn in interior portions of 45 different h4 6 chondrites 49 samples from victoria land antarctica and 5 h5 chondrites from the yamato mts antarctica relative to h5 chondrites of weathering types a and b all elements are depleted 10 at statistically significant levels in extensively weathered types b c and c samples probably by leaching on the ice sheet surface contents of 8 elements in extensively weathered samples may provide a qualitative ranking for terrestrial surface residence chondrites of weathering types a and b seem compositionally uncompromised and as useful as contemporary falls for trace element studies when data distributions for these 14 trace elements in non antarctic h chondrite falls and unpaired samples from victoria land and from the yamato mts queen maud land are compared statistically numerous significant differences are apparent concentrations of 8 elements differ significantly in the victoria land and non antarctic h5 chondrite populations essentially the same compositional differences and 53 mn content and shock history differences appear when h4 6 populations are compared contents of 8 elements differ when queen maud land and victoria land populations are compared and 5 for the queen maud land non antarctic comparison these and other differences give ample cause to doubt that the various sample populations derive from the same parent population the observed differences do not reflect weathering chance or other trivial causes a preterrestrial source must be responsible the least unlikely of these involves a temporal variation in the source regions from which meteoroids derive

theory of synthetic aperture radar ocean imaging a marsen view this paper reviews basic synthetic aperture radar sar theory of ocean wave imaging mechanisms using both known work and recent experimental and theoretical results from the marine remote sensing marsen experiment several viewpoints that have contributed to the field are drawn together in a general analysis of the backscatter statistics of a moving sea surface a common focus for different scattering models is provided by the mean image impulse response function which is shown to be identical to the spatially varying frequency variance spectrum of the local complex reflectivity coefficient from the analysis has emerged a more complete view of the sar imaging phenomenon than has been previously available a new generalized imaging model is proposed

the acambay graben active intraarc extension in the trans mexican volcanic belt mexico the trans mexican volcanic belt is an active volcanic arc related to subduction along the middle america trench the central part of the belt is being deformed by the chapala tula fault zone an approximately 450 km long and 50 km wide zone of active extension the volcanic arc and the arc parallel chapala tula fault zone are superposed nearly perpendicularly on the preexisting stress and deformation province of the mexican basin and range the acambay graben about 40 km long and 15 km wide is located approximately 100 km northwest of mexico city and is one of the major troughs within the chapala tula fault zone the border faults of the acambay graben acambay tixmadeje in the north and pastores in the south are separated in the west by stepovers from range bounding faults of similar orientation epitacio huerta in the north and venta de bravo in the south the stepovers occur at the intersection of these faults with an older system of basin and range faults an early stage right lateral component of motion along the venta de bravo and pastores faults is inferred on a map scale from a left stepping en echelon array of normal fault segments the divergence of the en echelon segments from the general fault trend decreases gradually from west to east suggesting that the early extension was rotational the present relative displacement along the southern margin of the system on the other hand results in a left lateral strike slip component this is documented on a map scale from extension structures at left stepovers and on an outcrop scale from fault striations indicating left oblique slip the striations measured at the northern system margin indicate nearly pure extensional dip slip without a consistent lateral displacement component this is supported on a map scale by the structure of the right stepover between the acambay tixmadeje and epitacio huerta faults which shows no evidence of local extension or shortening the divergence between the present directions of motion at the southern and northern margins of the extended zone can be explained by a minor rotational deformation component with the pole of rotation being located to the east of the zone of deformation this could explain why no active extension has been observed to the east of the chapala tula fault zone in the eastern part of the trans mexican volcanic belt during the ms 6 9 acambay earthquake of november 19 1912 surface rupture occurred along both margins of the graben at the base of multiple event scarps along the acambay tixmadeje fault the coseismic rupture is 41 km long the vertical offset increases gradually from the eastern end of the surface rupture to its center where it is with 50 cm at a maximum furthermore the change in the vertical surface offset along the fault is approximately proportional to the change in height of the acambay tixmadeje multiple event fault scarp the easternmost part of the ground rupture passes through a plain and not at the base of a multiple event scarp as farther west it may therefore correspond partly to an increase in length of the acambay tixmadeje fault during the 1912 earthquake the slip rate along the southern border of the acambay graben can be estimated from the displacement and age of a basalt flow for which we have obtained a 40ar 39ar age of 0 4 0 1 ma this basalt may be displaced up to 15 m by the pastores fault which indicates a middle late quaternary slip rate of 0 04 mm yr furthermore based on a coseismic surface rupture of approximately 20 cm along this fault in the 1912 earthquake we estimate a recurrence interval of 5000 years for major earthquakes along the faults of the acambay graben

influence of glacial isostatic adjustment on tide gauge measurements of secular sea level change modem tide gage records of secular sea level change are strongly contaminated by the ongoing influence of glacial isostatic adjustment in this paper we employ an accurate high resolution model of the glacial isostatic adjustment process to filter this signal from time series consisting of annual mean heights observed on over 500 tide gages from which records of duration longer than 10 years are available these records are all contained in the archive of the permanent service for mean sea level at bidston in the united kingdom prior to removal of the isostatic adjustment contamination from these records the secular sea level change signal is seen to exhibit both rising and falling levels depending upon geographic location with regions of falling level corresponding to those that were ice covered during the last glaciation event of the present ice age after filtering the geographic heterogeneity in rates is considerably reduced so much so that a globally coherent component consisting of a rise of sea level at a rate of 2 4 0 9 mm yr 1 is revealed this could conceivably be driven by the greenhouse effect although no particular mechanism has been unambiguously identified by our analyses we investigate the impact of a number of different variants of the analysis procedure upon the inferred global signal

a model of the glacial retreat of upper rennick glacier victoria land antarctica the morphology of the lichen hills in the upper section of rennick glacier victoria land antarctica is summarised as follows a a top surface on the volcanic rocks with scattered erratic blocks b an exhumed kukri peneplain sculptured with roches moutonnees with striae and crescentic gouges on which lie moraines and patches of drift of mainly volcanic rocks c a granitic bedrock eroded by glaciers into sharp peaks and cirques on top of which there is a glacial drift attributable to ancient blue ice areas higher than those observed at present and which may be correlated with the terra nova drift late wisconsin d various holocene ice cored moraines that are pushed to the lee side of the nunataks arid are often banded in strips of different lithology the bands of the holocene moraines are related to the rock complexes that became exposed from the ice during the lowering of the surface of the glacier analysis of the lithology and pattern of the supraglacial debris as well as of the blue ice areas allows us to construct a depositional model for the moraines and to relate the glacial drift to blue ice areas existing since the last glacial maximum lgm the proposed model shows the different stages of recession of upper rennick glacier that are also valid for similar situations observed in northern victoria land a surface lowering of upper rennick glacier of several hundred metres shows that significant changes have occurred at the pacific edge of the east antarctic ice sheet since the lgm

new 40ar 39ar ages and geochemistry of late carboniferous early permian lamprophyres and related volcanic rocks in the saxothuringian zone of the variscan orogen germany

geochemical and isotope data from the acatlan volcanic field western trans mexican volcanic belt origin and evolution abstract the quaternary acatlan volcanic field avf is located at the western edge of the trans mexican volcanic belt tmvb this region is related to the subduction of the pacific cocos and rivera plates beneath the north american plate since the late miocene avf rocks are products of pleistocene volcanic activity and include lava flows domes erupted basaltic andesite trachyandesite trachydacite and rhyolite of calc alkaline affinity most rocks show depletion in high field strength elements and enrichment in large ion lithophile elements and light rare earth elements as is typical for magmas in subduction related volcanic arcs 87sr 86sr values range from 0 70361 to 0 70412 while nd values vary from 2 3 to 5 2 sr nd isotopic data plot along the mantle array on the other hand lead isotope compositions 206pb 204pb 18 62 18 75 207pb 204pb 15 57 15 64 and 208pb 204pb 38 37 38 67 give evidence for combined influences of the upper mantle fluxes derived from subducted sediments and the upper continental crust involved in magma genesis at avf additionally 18o whole rock analyses range from 6 35 in black pumice to 10 9 in white pumice of the acatlan ignimbrite a fairly good correlation is displayed between sr as well as o isotopes and sio2 emphasizing the effects of crustal contamination compositional and isotopic data suggest that the different avf series derived from distinct parental magmas which were generated by partial melting of a heterogeneous mantle source

the eruptive history of the tequila volcanic field western mexico ages volumes and relative proportions of lava types the eruptive history of the tequila volcanic field 1600 km 2 in the western trans mexican volcanic belt is based on 40 ar 39 ar chronology and volume esti mates for eruptive units younger than 1 ma ages are reported for 49 volcanic units including volcn tequila an andesitic stratovolcano and peripheral domes flows and scoria cones volumes of volcanic units 1 ma were obtained with the aid of field mapping ortho aerial pho tographs digital elevation models dems and arcgis software between 1120 and 200 kyrs ago a bimodal distribution of rhyolite 35 km 3 and high ti basalt 39 km 3 dominated the volcanic field between 685 and 225 kyrs ago less than 3 km 3 of andesite and dacite erupted from more than 15 isolated vents these lavas are crystal poor and show little evidence of storage in an upper crustal chamber approximately 200 kyr ago 31 km 3 of andesite erupted to form the stratocone of volcn tequila the phenocryst assemblage of these la vas suggests storage within a chamber at 2 3 km depth after a hiatus of 110 kyrs 15 km 3 of andesite erupted along the w and se flanks of volcn tequila at 90 ka most likely from a second discrete magma chamber lo cated at 5 6 km depth the youngest volcanic feature 60 ka is the small andesitic volcano cerro tomasillo 2 km 3 over the last 1 myr a total of 128 22 km 3 of

wave effects on ocean ice interaction in the marginal ice zone the effects of wave train on ice ocean interaction in the marginal ice zone are studied through numerical modeling a coupled two dimensional ice ocean model has been developed to include wave effects and wind stress for the predictions of ice edge dynamics the sea ice model is coupled to the reduced gravity ocean model through interfacial stresses the main dynamic balance in the ice momentum is between water ice stress wind stress and wave radiation stresses by considering the exchange of momentum between waves and ice pack through radiation stress for decaying waves a parametric study of the effects of wave stress and wind stress on ice edge dynamics has been performed the numerical results show significant effects from wave action the ice edge is sharper and ice edge meanders form in the marginal ice zone owing to forcing by wave action and refraction of swell system after a couple of days upwelling at the ice edge and eddy formation can be enhanced by the nonlinear effects of wave action wave action sharpens the ice edge and can produce ice meandering which enhances local ekman pumping and pycnocline anomalies the resulting ice concentration pycnocline changes and flow velocity field are shown to be consistent with previous observations

motion of the rivera plate since 10 ma relative to the pacific and north american plates and the mantle abstract to better understand the influence of rivera plate kinematics on the geodynamic evolution of western mexico we use more than 1400 crossings of seafloor spreading magnetic lineations along the pacific rivera rise and northern mathematician ridge to solve for rotations of the rivera plate relative to the underlying mantle and the pacific and north american plates at 14 times since 9 9 ma our comparison of magnetic anomaly crossings from the undeformed pacific plate to their counterparts on the rivera plate indicates that significant areas of the rivera plate have deformed since 9 9 ma dextral shear along the southern edge of the plate from 3 3 2 2 ma during a regional plate boundary reorganization deformed the rivera plate farther into its interior than previously recognized in addition seafloor located north of two rupture zones within the rivera plate sutured to north america after 1 5 ma anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the rivera plate finite rotations that best reconstruct pacific plate anomaly crossings onto their undeformed counterparts on the rivera plate yield stage spreading rates that decrease gradually by 10 between 10 and 3 6 ma decrease rapidly by 20 after 3 6 ma and recover after 1 ma the slowdown in pacific rivera seafloor spreading at 3 6 ma coincided with the onset of dextral shear across the then incipient southern boundary of the rivera plate with the pacific plate the available evidence indicates that the rivera plate has been an independent microplate since at least 10 ma contrary to published assertions that it fragmented from the cocos plate at 5 ma motion of the rivera plate relative to north america has changed significantly since 10 ma in concert with significant changes in pacific rivera motion a significant and robust feature of rivera north america motion not previously recognized is the cessation of margin normal convergence and thus subduction from 2 6 to 1 0 ma along the entire plate boundary followed by a resumption of trench normal subduction along the southern half of the rivera north america plate boundary after 1 0 ma motion of the rivera plate relative to the underlying mantle since 10 ma has oscillated between periods of landward motion and seaward motion the evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion

concentrating antarctic meteorites on blue ice fields the frontier mountain meteorite trap the collection of meteorites in antarctica has greatly stimulated advancement in the field of meteoritics by providing the community with significant numbers of rare and unique meteorites types and by yielding large numbers of meteorites that sample older infall epochs grady et al 1998 the majority of antarctic meteorites are found on blue ice fields where they are thought to be concentrated by wind and glacial drift cf cassidy et al 1992 the basic ice flow model describes the concentration of meteorites by the stagnation or slowing of ice as it moves against a barrier located in a zone with low snow accumulation however our limited knowledge of the details of the actual concentration mechanisms prevents establishing firm conclusions concerning the past meteorite flux from the antarctic record zolensky 1998 the terrestrial ages of antarctic meteorites indicate that their concentration occurs on time scales of tens to hundreds of thousands of years nishiizumi et al 1989 it is a challenge to measure a mechanism that operates so slowly and since such time scales can span more than one glacial epoch one cannot assume that the snow accumulation rates ice velocities and directions etc that are measured today are representative of those extant over the age of the trap testing the basic ice flow model therefore requires the careful measurement of meteorite locations glacialogical ice flow data ice thicknesses bedrock and surface topology ice ablation and snow accumulation rates and mass transport by wind over an extended period of time in a location where these quantities can be interpreted in the context of past glacialogical history

arctic sea ice drift from wavelet analysis of nscat and special sensor microwave imager data wavelet analysis of nasa scatterometer nscat backscatter and dmsp special sensor microwave imager ssm i radiance data can be used to obtain daily sea ice drift information for the arctic region this technique provides improved spatial coverage over the existing array of arctic ocean buoys and better temporal resolution over techniques utilizing data from satellite synthetic aperture radars comparisons with ice motion derived from ocean buoys give good quantitative agreement both comparison results from nscat and ssm i are compatible and the results from nscat can definitely complement that from ssm i when there are cloud or surface effects then three sea ice drift daily results from nscat ssm i and buoy data can be merged as a composite map by some data fusion techniques the ice flow streamlines are highly correlated with surface air pressure contours examples of derived ice drift maps in december 1996 illustrate large scale circulation reversals over a period of 4 days these calibrated validated results indicate that nscat ssm i merged daily ice motions are suitably accurate to identify and closely locate sea ice processes and to improve our current knowledge of sea ice drift and related processes through the data assimilation of ocean ice numerical model

climate variability over the last two millennia in the north american monsoon region recorded in laminated lake sediments from laguna de juanacatlan mexico high resolution titanium ti data obtained using an itrax xrf core scanner from a laminated sediment core from the laguna de juanacatlan western central mexico yield a unique high resolution record of runoff precipitation change for the last 2000 years in the absence of reliable long term meteorological records comparison of the ti data with information from the rich spanish colonial period archives and the post independence period confirms that ti is a proxy for runoff this interpretation is supported by comparison with other high resolution archives from the surrounding region primarily tree rings and other lake sediment sequences the juanacatlan ti record is therefore a proxy for summer monsoonal rainfall the record provides new evidence from the pacific margin of tropical north america of the occurrence of dry conditions through much of the classic period c ad 300 900 and wetter conditions during the later medieval period c ad 1200 1350 the period commonly known as the little ice

meteorites as surface exposure time markers on the blue ice fields of antarctica episodic ice flow in victoria land over the last 300 000 years abstract meteorites have been discovered in large numbers on certain blue ice fields in the antarctic it is possible to estimate how long these meteorites have been on earth using cosmogenic radionuclide abundances and as discussed in this paper how long they have been exposed on the ice surface using natural thermoluminescence tl levels in this paper we discuss two methods to determine surface exposure ages for antarctic meteorite finds one method assumes an initial tl level and average surface exposure temperature while the other method uses multiple samples from a single meteorite and assumes a temperature gradient across the meteorite the former method does not work for approximately 15 of meteorites which were heated prior to earth impact these data suggest that no meteorite in the current study was exposed on the ice surface for more than 300 000 years which may imply that the blue ice fields in question were not stable platforms prior to this point in time or that a rapid pulse in ice thickness cleared the field of meteorites steps in surface exposure ages suggest that accumulation of meteorites on the ice fields has been episodic over the last 300 000 years although these episodes have been of lesser magnitude than that at about 300 000 years ago

meteorites constrain the age of antarctic ice at the frontier mountain blue ice field northern victoria land abstract we show that meteorites can provide chronological constraints upon the age of the ice cropping out at the frontier mountain meteorite trap antarctica when their terrestrial age is placed in a glaciological context amongst the over 700 meteorites found so far frontier mountain fro 84001 99028 93005 and 93054 were most likely not wind drifted across the ice field since their masses 772 1665 g are much heavier than the local 200 g wind transport threshold the four meteorites were found along a stretch of ice where a representative section of the frontier mountain blue ice crops out based on the bedding of englacial tephra layers the structure of the ice along the section appears to be essentially an up glacier dipping monocline the 14 c terrestrial age of fro 8401 99028 and 93005 are 13 2 21 3 and 27 2 ky respectively the 41 ca 36 cl age of fro 93054 is 40 10 ky the terrestrial ages of the four meteorites increase from the top to the bottom layers of the monocline this geographic distribution is best explained by delivery of meteorites at the ice surface through the ice flow model i e englacial transport from the snow accumulation zone and exhumation in the blue ice area through ablation rather than direct fall since the effect of ablation in decoupling terrestrial ages of meteorites and the age of the ice on which they sit must have been minor most likely 7 ky based on the local ice dynamics we conclude that the age of the bulk of the ice body currently under ablation at frontier mountain is up to 50 ky old this result has implications on both the meteorite concentrations mechanism at frontier mountain and the regional ice dynamics

lewis cliff 86360 an antarctic l chondrite with a terrestrial age of 2 35 million years we measured the concentrations of the cosmogenic radionuclides 10be half life 1 51 106 a 26al 7 05 105 a and 36cl 3 01 105 a in lewis cliff lew 86360 an l chondrite from the lewis cliff stranding area east antarctica in addition the concentrations and isotopic compositions of he ne and ar were measured the combined results yield a terrestrial age of 2 35 0 15 ma only one other stony meteorite with a similar terrestrial age 2 ma is known from the allan hills stranding area alh 88019 whereas all previously dated stony meteorites from antarctica are younger than 1 ma we argue that lew 86360 spent most of its terrestrial residence time deep inside the ice near the base of the glacier where ice flow rates are much lower than at the surface the terrestrial ages of lew 86360 and alh 88019 are consistent with existing hypotheses concerning the stability and persistence of the east antarctic ice sheet

evolution of the east antarctic ice sheet a numerical study of thermo mechanical response patterns with changing climate an efficient numerical ice sheet model including time dependence and full thermo mechanical coupling has been developed in order to investigate the thermal regime and overall configuration of a polar ice sheet with respect to changing environmental conditions from basic sensitivity experiments in which a schematic east antarctic ice sheet is forced with a typical glacial interglacial climatic shift it is found that i the mutual interaction of temperature and deformation has a stabilizing effect on its steady state configuration ii in the transient mode this climatic transition initially leads to increased ice thickness due to enhanced accumulation after which this trend is reversed due to a warmer base timescales for this reversal are of the order of 103 years in marginal zones and of 104 years in interior regions iii horizontal heat advection plays a major role in damping possible runaway behaviour due to the dissipation strain rate feed back suggesting that creep instability is a rather unlikely candidate to initiate surging of the east antarctic ice sheet the model is then applied to four east antarctic flow lines only the flow line passing through wilkes land appears to be vulnerable to widespread basal melting due to enhanced basal warming following climatic warming time dependent modelling of the vostok flow line indicates that the vostok station area has risen about 95 m since the beginning of the present interglacial due to thermomechanical effects which is of particular interest in interpreting the palaeoclimatic signal of the ice core obtained there

a model of the antarctic ice sheet numerical modelling of ice sheets and glaciers has become a useful tool in glaciological research a model described here deals with the vertical mean ice velocity is time dependent computes bedrock adjustment and uses an empirical diagnostic relationship to derive the distribution of ice thickness in ice shelves the rate of snowfall and ice snow melt depends on the prescribed sea level temperature surface slope elevation and distance to open water the model is able to reproduce the major features of the antarctic ice sheet when it is run to a steady state for present climatic conditions the main difference with the present ice sheet is that the shallow parts of the weddell sea become covered by grounded ice

meteorite mass distributions and differences between antarctic and non antarctic meteorites meteorite mass distributions provide important insight into real and apparent differences between antarctic and non antarctic meteorites antarctic meteorites are typically smaller than non antarctic meteorites and represent a different portion of the mass distribution of infalling meteorites this results from the different methods employed to acquire the collections and from the much smaller effective collecting area for antarctic meteorites comparisons of the proportions and mass distributions of h and l chondrites for different ice fields suggest that previously reported high h to l ratio among antarctic meteorites compared to witnessed falls results from an unrecognized h5 shower fall that covered the allan hills main and near western ice fields in the relatively recent past the probable presence of unrecognized shower falls among the antarctic meteorites means that population statistics even corrected for presently recognized pairing cannot be used to support the suggestion by dennison et al 1986 of a temporal variation in the mixture of meteorites arriving on earth

catch a falling star meteorites and old ice a model for the process of meteorite concentration in blue ice regions of the antarctic ice sheet is proposed based on data from near the allan hills and the assumptions that both meteorite influx and glacial flow have been constant the meteorite influx is calculated to be 60 x 10 6 kilogram per square kilometer per year and the age of the exposed ice to be 0 to 600 000 years varying with distance from the allan hills these results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites providing support for the assumption of steady flow and meteorite influx this may be the oldest sequence of ice in stratigraphic order yet discovered and the results imply that this part of the east antarctic ice sheet has been approximately steady during this time interval