mechanical and geological effects of impact cratering on ida asteroids respond to impact stresses differently from either laboratory specimens or large planets gravity is typically so small that seismic disturbances of a few cm s 1can devastate unconsolidated topography yet the presence of regolith and the likelihood that many asteroids are gravitational assemblages tell us that gravity cannot generally be ignored we use numerical models for impact fracture in solids to examine the initial stage of crater formation on asteroid 243 ida up to the cessation of fracture and the establishment of the cratering flow at this stage we can infer final crater diameters but not profiles we find that a modified strength scaling applies for craters up to a few 100 m in diameter forming in rock subject to ida s gravity and that gravity controls all craters larger than 1 km bright annuli around a number of intermediate craters may be the result of low velocity surface disturbances rather than bright proximal ejecta deposits we also consider large impactors to which ida presents a curved finite target surface with irregular gravity these can excavate asymmetrical concavities stresses from large events can refocus and cause fracture far from the crater using the shape of ida as a basis for 3d hydrocode simulations we show that impact genesis of the vienna regio concavity can cause fracture in pola regio where grooves are observed in spacecraft images other simulations indicate that the formation of the 10 km crater azzurra might have reopened these fractures which may account for their fresh appearance this mechanism of groove formation requires an interior which coherently transmits elastic stress while this precludes a classic rubble pile asteroid it does allow well joined fault planes and welded blocks or pores smaller than the stress pulse

the shape of 433 eros from the near shoemaker laser rangefinder measurements from the near earth asteroid rendezvous near shoemaker laser rangefinder nlr indicate that asteroid 433 eros is a consolidated body with a complex shape dominated by collisions the offset between the asteroid s center of mass and center of figure indicates a small deviation from a homogeneous internal structure that is most simply explained by variations in mechanical structure regional scale relief and slope distributions show evidence for control of topography by a competent substrate impact crater morphology is influenced by both gravity and structural control small scale topography reveals ridges and grooves that may be generated by impact related fracturing

the significance of the theropod collections of the royal tyrrell museum of palaeontology to our understanding of late cretaceous theropod diversity1 significant specimens of theropod dinosaurs were part of the collections that were transferred to the royal tyrrell museum of palaeontology when it split off from the provincial museum of alberta in 1981 collecting activity of the institution increased dramatically in the period leading up to the opening of the building and displays in 1985 and resulted in the recovery and preparation of many fine theropod skeletons new specimens have been added to the collection every year since the museum opened several mostly small taxa are only represented by isolated bones partial skeletons and or teeth theropod specimens also include footprints coprolites eggs and feathers in amber although theropods are relatively rare in comparison with herbivorous dinosaurs the tyrrell has managed to build one of the finest research collections of late cretaceous forms thirty seven species of theropods in the tyrrell are currently accepted as being valid of which 30 are from the upper cretaceous deposits of alberta

global structure of asteroid 433 eros 1 models of asteroid formation and evolution suggest a variety of internal structures ranging from gravitationally bound accumulations of debris to fully rigid monolithic bodies the near shoemaker spacecraft has provided detailed imaging and gravity data relevant to the characteristics of structural features and global mass properties a long ridge coplanar with a degraded trough ridge system and an approximately parallel surface facet suggest exploitation of a nearly global fabric by both compressive and extensional failure at significantly different times

application of tandem accelerator mass spectrometer dating to late pleistocene holocene sediments of the east antarctic continental shelf abstract glacial recession from the antarctic continental shelf is recorded by glacial marine diamictons sands and overlying siliceous oozes in order to clarify the chronology for this sequence use was made of the university of arizona tandem accelerator mass spectrometer tams for 14 c dating small samples of benthic and planktonic foraminifera were selectively removed from diamictons graded sands and surface sediments which were recovered from the wilkes land continental shelf and slope east antarctica organic carbon was also utilized as a source for tams dating of the siliceous oozes and muds uncorrected ages varied from 14 260 140 to 3230 200 yr b p carbon fixed by phytoplankton and foraminifera is strongly influenced by old glacial derived co 2 thus reservoir corrections of up to 5500 yr are needed for the 14 c dates iceberg turbation reworks foraminifera so that dates from resulting deposits diamictons are interpreted as maximum ages the consistency of corrected ages from the shelf along with the sedimentologic interpretation suggests a rather recent recession perhaps mid holocene for this portion of the east antarctic ice sheet further application of the tams method should help clarify other problems concerning the late quaternary glacial history of antarctica

small scale topography of 25143 itokawa from the hayabusa laser altimeter abstract the surface topography of asteroid 25143 itokawa is explored using the light detection and ranging instrument lidar the data confirm the presence of a rough highland and a smooth lowland the highland is dominated by boulders but also possesses topography associated with surface lineaments and broad surface facets the boulders ensure that the roughness of the highlands over short distances is typically greater relative to most surfaces on 433 eros over larger distances itokawa is always smoother than eros possibly because of its smaller size and weak rubble pile structure the lowlands of itokawa are very smooth and are typically devoid of boulders some transitional regions midway between the highlands and lowlands also exist in these areas craters that retain their regolith fill possess flat floors and resemble ponds seen on 433 eros analyses of surface elevation imagery and a quantitative measure of surface roughness are consistent with regolith flowing downhill from the highlands to fill in the low areas of itokawa probably covering up any pre existing rough terrain using this interpretation we find a minimum 2 3 0 4 m thick layer of regolith in the lowlands which if spread evenly across the entire asteroid corresponds to a 42 1 cm thick layer it is very difficult to generate this amount of regolith with the population of craters seen on itokawa however an itokawa composed of several large masses may have retained this regolith during its formation the presence of such large masses could account for the observed lineaments and what appear to be exposures of bedrock on the largest steep slope observed

penecontemporaneous metamorphism fragmentation and reassembly of ordinary chondrite parent bodies the occurrence and proportions of petrologic types among ordinary chondrites have been explained by internal heating of three h l ll parent asteroids of 100 km radius e g miyamoto et al 1982 this conventional onion shell model predicts an inverse relation between petrologic type and metallographic cooling rate but none has been observed scott and rajan 1981 devised a metamorphosed planetesimal model to explain this discrepancy whereby metamorphism occurs in planetesimals a few kilometers in radius which then accrete to form r 100 km parent bodies metallographic cooling rates are then controlled by burial depth thermal and collisional constraints on the metamorphosed planetesimal model are examined here and the model is found to be applicable only to highly insulating 26al rich planetesimals that can accrete intact without being shattered by impact or to shattered planetesimals whose fragments are not widely dispersed over the target surface an alternative model is presented here in which onion shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled if reassembly times are short days then metallographic cooling rates would be determined by burial depth in the reaccreted parent body this model unlike previous ones can explain both coherent and incoherent cooling rates of breccia clasts by collisions during or after metamorphism respectively

the global surface roughness of 433 eros from the near laser rangefinder abstract we performed the first global surface roughness assessment of the asteroid 433 eros at baselines horizontal distances of 4 200 m we measured surface roughness using the root mean square rms deviation over a variety of baselines after first detrending the height to remove long wavelength slope effects the global surface roughness of eros is found to be self affine at all baselines investigated the surface roughness is statistically correlated with crater density at baselines of 100 200 m and boulders at a baseline of 5 m no global spatial statistical correlation was found for baselines of 4 200 and mapped tectonic lineaments ponds slope or geopotential elevation the surface roughness of the crater shoemaker charlois regio is controlled by the interplay of a high boulder density producing higher surface roughness values at small baselines and low crater density lowering surface roughness values at long baselines we estimated the mobile regolith thickness regolith that moves around and infills topography to be 0 2 6 2 m from the difference in the surface roughness values at the baseline of 4 m furthermore we find that the change in rms deviation as a function of baseline compares favorably with the moon and differs significantly from existing results for rubble pile asteroid itokawa

images of asteroid 21 lutetia a remnant planetesimal from the early solar system images obtained by the optical spectroscopic and infrared remote imaging system osiris cameras onboard the rosetta spacecraft reveal that asteroid 21 lutetia has a complex geology and one of the highest asteroid densities measured so far 3 4 0 3 grams per cubic centimeter the north pole region is covered by a thick layer of regolith which is seen to flow in major landslides associated with albedo variation its geologically complex surface ancient surface age and high density suggest that lutetia is most likely a primordial planetesimal this contrasts with smaller asteroids visited by previous spacecraft which are probably shattered bodies fragments of larger parents or reaccumulated rubble piles

surface lithofacies biofacies and diatom diversity patterns as models for delineation of climatic change in the southeast atlantic ocean abstract distribution of diatom species in surface sediments of the southeast atlantic ocean is regulated by present day oceanographic and hydrodynamic processes five assemblages vectors defined by factor vector analysis reflect different environments and conditions assemblage a is a high diversity flora associated with the nutrient rich relatively cold waters south of the polar front in the diatom ooze belt occurrence of this flora on maud rise and vicinity may reflect periodic occurrence of a polynya in that area assemblage b is a reworked assemblage that dominates the southern portion of the study area productivity there is low reflecting sea ice cover during most of the year the area north of the polar front is dominated by assemblage c whose characteristic species reflect the relatively warm subantarctic surface water winnowing and frustule breakage have altered assemblage d found in three isolated samples by removing relatively delicate forms leaving lag deposits of more robust species assemblage e is a low diversity stress flora reflecting unstable unpredictable environments along the polar front antarctic slope and the northern boundary of winter sea ice these sites are characterized by the sinking of cold water downcore analysis of cores lying adjacent to the polar front and the diatom ooze pelagic clay boundary show evidence for past climatic variation the low trophic level occupied by diatoms and the subsequent sensitivity of these organisms to abiotic environmental parameters such as light quality make relative diatom abundance a useful tool for monitoring fluctuations of winter sea ice and the temperature changes responsible for these fluctuations the position of the polar front has migrated at least three times within the last 0 015 m y b p within the last 0 3 m y b p warm maxima have occurred at approximately 0 0 0 015 0 125 and 0 3 m y b p

a previously undescribed caenagnathid mandible from the late campanian of alberta and insights into the diet of chirostenotes pergracilis dinosauria oviraptorosauria until recently caenagnathids were a family of oviraptorosaurs represented only by fragmentary material as such caenagnathid biology has never been studied in depth a well preserved mandible provides new information on the anatomy and dietary habits of chirostenotes the mandible is edentulous has a completely fused symphysis with sharp occlusal margins and complex lingual surfaces finite element analysis shows that the lingual ridges are reinforced this suggests that they had a function in food processing these and other features suggest adaptations for an efficient shearing mechanism and the overall morphology is poorly adapted for durophagous behaviour comparisons with three groups with convergently similar mandibles especially dicynodonts indicate caenagnathids were capable of handling an herbivorous diet here an omnivorous diet is proposed for chirostenotes including folivory and small prey

macroporosity and grain density of rubble pile asteroid 162173 ryugu rubble pile asteroids such as 162173 ryugu have large bulk porosities which are believed to result from void spaces in between the constituent boulders macroporosity as well as void spaces wit

modern distribution of diatoms in sediments from the george v coast antarctica abstract currently little information exists concerning high frequency climate change in the antarctic data that are essential to gaining a more complete understanding of global climate change sediments from the george v coast continental margin potentially contain a high resolution record of the late holocene the relatively high rate of sedimentation 3 mm year and the presence of laminated intervals which contain undisturbed sequences make these sediments valuable for future downcore work in order to utilize diatom assemblages as proxies of paleoclimatic and paleoceanographic parameters a preliminary study of the surface sediment distribution of diatoms from the george v coast continental margin was completed nitzschia curta van heurck hasle and nitzschia kerguelensis o meara hasle present great potential for tracking the holocene retreat of ice across the continental shelf of this region concentrations of n curta an ice related form decreases in an offshore direction whereas n kerguelensis an oceanic species associated with circumpolar flow displays an increase in sediments offshore the genus thalassiosira cleve displays a clear association with areas of open water primary production as has been noted in others parts of the antarctic downcore variability in relative abundance of this genus will be useful in understanding the history of sea ice versus open water production on the george v coast shelf in contrast the distribution of chaetoceros ehrenberg resting spores appears to be more complex but may be related to the position of the stationary ice edge associated with the annual minimal extent of sea ice finally an understanding of the downcore variability of thalassiothrix antarctica schimper ex karsten a species which is very abundant in the laminated sections of the sediment cores will have to rely on studies of its distribution in other areas of the antarctic as it is sparsely distributed along the george v coast today

hydrogen and major element concentrations on 433 eros evidence for an l or ll chondrite like surface composition a reanalysis of near x ray gamma ray spectrometer xgrs data provides robust evidence that the elemental composition of the near earth asteroid 433 eros is consistent with the l and ll ordinary chondrites these results facilitated the use of the gamma ray measurements to produce the first in situ measurement of hydrogen concentrations on an asteroid the measured value 1100 1600 700 ppm is consistent with hydrogen concentrations measured in l and ll chondrite meteorite falls gamma ray derived abundances of hydrogen and potassium show no evidence for depletion of volatiles relative to ordinary chondrites suggesting that the sulfur depletion observed in x ray data is a surficial effect consistent with a space weathering origin the newfound agreement between the x ray gamma ray and spectral data suggests that the near landing site a ponded regolith deposit has an elemental composition that is indistinguishable from the mean surface this observation argues against a pond formation process that segregates metals from silicates and instead suggests that the differences observed in reflectance spectra between the ponds and bulk eros are due to grain size differences resulting from granular sorting of ponded material

spectral properties and rotational spectral heterogeneity of 433 eros composite spectra of different rotational aspects of the s asteroid 433 eros were assembled from previous studies covering the wavelength range 0 33 2 5 m these data allow us to measure significant attributes of eros s spectrum its mafic mineral absorptions and the degree and nature of compositional heterogeneity of its surface we also evaluated previous s asteroid taxonomies using independent methods and from this test determined that the taxonomy of gaffey et al 1993 provides a sound basis for relating eros to other s asteroids on average eros resembles the s iv asteroids which bear the closest resemblance to ordinary chondrites in mafic mineralogy however eros also exhibits distinct rotational spectral variations comparable to the greatest yet recognized on an s asteroid one side of the asteroid has a shorter wavelength 1 m absorption and a stronger 2 m absorption than the global average and is interpreted to be richer in pyroxene the opposite face has a longer wavelength 1 m absorption and a weaker 2 m absorption and is interpreted to be richer in olivine the pyroxene rich side coincides with a facet like face identified in radar studies and the olivine rich side coincides with a convex shaped face each side of the asteroid exhibits spectral affinities with a different spectral class of s asteroids demonstrating that such subdivisions of s asteroids represent different parts of a compositional continuum high spatial resolution measurement of the mineralogic and elemental compositions and especially the geologic settings of these different materials by the near earth asteroid rendezvous mission will provide a great advance in the understanding of the origin and evolution of a candidate ordinary chondrite parent body

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

diatom distribution and paleoceanographic reconstruction in the southern ocean present and last glacial maximum abstract diatom assemblage and preservational data are used to reconstruct paleoceanographic conditions at the last glacial maximum 18 000 yrs bp in the southern ocean from these data the following points can be made about the last glacial maximum in this region 1 contraction and slight northern shift of the belt of well preserved diatoms appears to be related to the northward shift of late spring early summer sea ice cover 2 presence of open ocean though poorly preserved diatom assemblages to the south of this belt strongly suggest that during many summers large areas of the southern ocean were ice free 3 the distribution of thenitzschia kerguelensis factor both in surface sediments and at the last glacial maximum indicates that the gyre systems particularly the weddell gyre were intensified during glacial times 4 although data are sparse in the higher latitude south atlantic there is an indication that the weddell polynya also existed during glacial times although it was shifted a few degrees northward

near at eros imaging and spectral results eros is a very elongated 34 kilometers by 11 kilometers by 11 kilometers asteroid most of the surface of which is saturated with craters smaller than 1 kilometer in diameter the largest crater is 5 5 kilometers across but there is a 10 kilometer saddle like depression with attributes of a large degraded crater surface lineations both grooves and ridges are prominent on eros some probably exploit planes of weakness produced by collisions on eros and or its parent body ejecta blocks 30 to 100 meters across are abundant but not uniformly distributed over the surface albedo variations are restricted to the inner walls of certain craters and may be related to downslope movement of regolith on scales of 200 meters to 1 kilometer eros is more bland in terms of color variations than gaspra or ida spectra 800 to 2500 nanometers are consistent with an ordinary chondrite composition for which the measured mean density of 2 67 0 1 grams per cubic centimeter implies internal porosities ranging from about 10 to 30 percent

the small inner satellites of saturn shapes structures and some implications abstract properties of the small inner satellites of saturn are updated using final cassini image data the equatorial ridges on pan and atlas are morphologically structurally and gravitationally distinct from their cores and represent distinct additions to rather than modifications of the precursor bodies exposures of lineated substrates on pandora and epimetheus show a relationship between regolith grooves and structures in underlying bedrock structures exposed on epimetheus suggest formation at depth including filling or annealing of fractures these structures and sharp bedrock regolith boundaries indicate a history far more complex than simple rubble pile assembly these exposures of bedrock also indicate past removal of the expected regolith from large fractions of these object s surfaces late in their cratering records

the first records of elmisaurus saurischia theropoda from north america the discovery of a fused tarsometatarsus in the upper cretaceous campanian strata of dinosaur provincial park shows that elmisaurus previously known only from mongolia also lived in north ameri

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

the southern ocean surface between marine isotope stages 6 and 5d shape and timing of climate changes summer sea surface temperature ssst and sea ice extent obtained at millennial to centennial resolution are presented for seven cores recovered in the atlantic and western indian antarctic zone from termination ii and the last interglacial s s i e marine isotope substage mis 5e the diatom based ssst estimates show that during the late mis 6 intensified export of cold waters from the weddell sea resulted in sssts close to 0 c in the central and eastern atlantic sector due to northward expansion of the winter sea ice edge by 3 5 latitude the sea ice seasonality was reduced such conditions have great potential for efficient year round reduction in air sea gas exchange and glacial co2 draw down the warming at termination ii started at 132 131 ka marked by rapid retreat of the sea ice edge the ssst increased within a few thousands of years to reach maximum interglacial summer temperatures ranging between 2 5 and 3 5 c in the earliest mis 5e 128 125 ka besides a distinct cold reversal around 129 5 ka the warming was punctuated by short lived temperature rebounds with amplitudes of 1 1 5 c occurring at 200 300 yr frequency thus in the range of the periodicity of solar input increased chaetoceros spp abundance suggests meltwater events as the primary cause of the coolings the ssst optimum characterised by strongly enhanced biogenic sedimentation rates ended at ca 125 ka with the full re establishment of the global conveyor belt and beginning of the northern hemisphere climatic optimum at this time sssts decreased stepwise and stabilised on values slightly warmer than today showing in the following period of mis 5e only low amplitude oscillations during mis 5d a northward expansion of the sea ice field took place but sssts only slightly colder than modern ones and a weakened meridional temperature gradient indicate that the southern ocean did not fall in a true glacial mode the tight correlation between southern ocean ssst and winter sea ice records and vostok atmospheric temperature and co2 records support the idea that the antarctic winter sea ice and southern ocean temperature exert a primary control on the atmospheric ocean co2 exchanges and hence on global climate reduction of the antarctic sea ice field and increase of ssst at the onset of termination ii could have been triggered by precessional changes influencing southern high latitude summer insolation positive feedback mechanisms reduction of surface albedo and release of co2 into the atmosphere and changes in global ocean circulation affected by a collapse or strong reduction in north atlantic deep water nadw production could have induced further reinforcement of the southern ocean warming

boulder size and shape distributions on asteroid ryugu abstract in 2018 the japanese spacecraft hayabusa2 arrived at the small asteroid ryugu the surface of this c type asteroid is covered with numerous boulders whose size and shape distributions are investigated in this study using a few hundred optical navigation camera onc images with a pixel scale of approximately 0 65 m we focus on boulders greater than 5 m in diameter smaller boulders are also considered using five arbitrarily chosen onc close up images with pixel scales ranging from 0 7 to 6 cm across the entire surface area 2 7 km2 of ryugu nearly 4400 boulders larger than 5 m were identified boulders appear to be uniformly distributed across the entire surface with some slight differences in latitude and longitude at 50 km 2 the number density of boulders larger than 20 m is twice as large as on asteroid itokawa or bennu the apparent shapes of ryugu s boulders resemble laboratory impact fragments with larger boulders being more elongated the ratio of the total volume of boulders larger than 5 m to the total excavated volume of craters larger than 20 m on ryugu can be estimated to be 94 which is comparatively high these observations strongly support the hypothesis that most boulders found on ryugu resulted from the catastrophic disruption of ryugu s larger parent body as described in previous papers watanabe et al 2019 sugita et al 2019 the cumulative size distribution of boulders larger than 5 m has a power index of 2 65 0 05 which is comparatively shallow compared with other asteroids visited by spacecraft for boulders smaller than 4 m the power index is even shallower and ranges from 1 65 0 05 to 2 01 0 06 this particularly shallow power index implies that some boulders are buried in ryugu s regolith based on our observations we suggest that boulders near the equator might have been buried by the migration of finer material and as a result the number density of boulders larger than 5 m in the equatorial region is lower than at higher latitudes

geology of 243 ida abstract the surface of 243 ida is dominated by the effects of impacts no complex crater morphologies are observed a complete range of crater degradation states is present which also reveals optical maturation of the surface darkening and reddening of materials with increasing exposure age regions of bright material associated with the freshest craters might be ballistically emplaced deposits or the result of seismic disturbance of loosely bound surface materials diameter depth ratios for fresh craters on ida are 1 6 5 similar to gaspra results but greater than the 1 5 ratios common on other rocky bodies contributing causes include rim degradation by whole body ringing relatively thin ejecta blankets around crater rims or an extended strength gradient in near surface materials due to low gravitational self packing grooves probably represent expressions in surface debris of reactivated fractures in the deeper interior isolated positive relief features as large as 150 m are probably ejecta blocks related to large impacts evidence for the presence of debris on the surface includes resolved ejecta blocks mass wasting scars contrasts in color and albedo of fresh crater materials and albedo streaks oriented down local slopes color data indicate relatively uniform calcium abundance in pyroxenes and constant pyroxene olivine ratio a large relatively blue unit across the northern polar area is probably related to regolith processes involving ejecta from azzurra rather than representing internal compositional heterogeneity a small number of bluer brighter craters are randomly distributed across the surface unlike on gaspra where these features are concentrated along ridges this implies that debris on ida is less mobile and or consistently thicker than on gaspra estimates of the average depth of mobile materials derived from chute depths 20 60 m grooves 30 m and shallowing of the largest degraded craters 20 50 m minimum 100 m maximum suggest a thickness of potentially mobile materials of 50 m and a typical thickness for the debris layer of 50 100 m

the biogeography of major diatom taxa in southern ocean sediments 2 open ocean related species diatoms from 228 southern ocean core top sediment samples were examined to determine the geographic distributions of 32 major diatom species taxa preserved in the sediments of three zonally distinct regions sea ice open ocean and the tropical subtropical in the first of three papers 14 species taxa occurring in the region where sea ice covers the ocean surface on an annual basis are geographically documented comparisons are drawn between the diatom abundances on the sea floor sea ice parameters annual duration and concentration in february and september and february sea surface temperature such parameters are commonly used in reconstructing past oceanographic conditions in the sea ice and open ocean zones analysis of the geographic patterns and sea surface parameter correlations reveals species specific distributions regulated primarily by sea ice coverage and sea surface temperature which support the use of diatom remains for the estimation of these past seasurface environmental parameters comparison with reliable accounts of the 14 species from the sediments or plankton also provides the first glimpses into species specific ecology and habitat linkages d 2005 elsevier b v all rights reserved

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

meteoritic and other constraints on the internal structure and impact history of small asteroids studies of the internal structure of asteroids which are crucial for understanding their impact history and for hazard mitigation appear to be in conflict for the s type asteroids eros gaspra and ida spacecraft images and geophysical data show that they are fractured coherent bodies whereas models of catastrophic asteroidal impacts family and satellite formation and studies of asteroid spin rates and other diverse properties of asteroids and planetary craters suggest that such asteroids are gravitationally bound aggregates of rubble these conflicting views may be reconciled if 10 50 km s type asteroids formed as rubble piles but were later consolidated into coherent bodies many meteorites are breccias that testify to a long history of impact fragmentation and consolidation by alteration metamorphism igneous and impact processes ordinary chondrites which are the best analogs for s asteroids are commonly breccias some may have formed in cratering events but many appear to have formed during disruption and reaccretion of their parent asteroids some breccias were lithified during metamorphism and a few were lithified by injected impact melt but most are regolith and fragmental breccias that were lithified by mild or moderate shock like their lunar analogs shock experiments show that porous chondritic powders can be consolidated during mild shock by small amounts of silicate melt that glues grains together and by friction and pressure welding of silicate and metallic fe ni grains we suggest that the same processes that converted impact debris into meteorite breccias also consolidated asteroidal rubble internal voids would be partly filled with regolith by impact induced seismic shaking consolidation of this material beneath large craters would lithify asteroidal rubble to form a more coherent body fractures on ida that were created by antipodal impacts and are concentrated in and near large craters and small positive gravity anomalies associated with the psyche and himeros craters on eros are consistent with this concept spin data suggest that smaller asteroids 0 6 6 km in size are unconsolidated rubble piles c type asteroids which are more porous than s types and their analogs the volatile rich carbonaceous chondrites were probably not lithified by shock

drainage pits in cohesionless materials implications for the surface of phobos viking orbiter images show grooves and chains of pits crossing the surface of phobos many of which converge toward the large crater stickney or its antipode although it has been proposed that the pits and grooves are chains of secondary craters their morphology and geometric relations suggest that they are the surface traces of fractures in the underlying solid body of phobos several models have been proposed to explain the pits of which the most plausible are gas venting and drainage of regolith into open fractures the latter mechanism is best supported by the image data and is the mechanism studied in this investigation drainage pits and fissures are modeled experimentally by using two rigid substrate plates placed edge to edge and covered by uniform thicknesses of dry fragmental debris simulated regolith fracture extension is simulated by drawing the plates apart allowing drainage of regolith into the newly created void a typical drainage experiment begins with a shallow depression on the surface of the regolith above the open fissure increased drainage causes local drainage pits to form continued drainage causes the pits to coalesce forming a cuspate groove the resulting experimental patterns of pits and grooves have pronounced similarities to those observed on phobos characteristics such as lack of raised rims linearity of grooves and chains of pits uniform spacing of pits and progression from discrete pits to cuspate grooves are the same in the experiments and on phobos in contrast gas venting pits occur in irregular chains and have raised rims these experiments thus indicate that the phobos grooves and pits formed as drainage structures the pit spacing in an experiment is measured at the time that the maximum number of pits forms prior to groove development the average pit spacing is compared to the regolith thickness for each material regression line fits indicate that the average spacing of drainage pits in unconsolidated noncohesive regolith is nearly equal to the thickness of regolith and appears to gbe independent of the angle of repose within the resolution of our experiments this provides a simple means of estimating regolith thickness where drainage pits are present on phobos two locations differing by 90 degrees in longitude have average pit spacings that suggest regolith thicknesses of 290 and 300 m suggesting that large areas of phobos have a nearly uniform regolith thickness of approximately 300 m

small scale topography of 433 eros from laser altimetry and imaging abstract the near laser rangefinder nlr obtained more than 16 million range returns from asteroid 433 eros we present the first results from analyses of topographic profiles interpreted with the aid of simultaneous boresighted images obtained by the near multispectral imager msi the location of the nlr boresight relative to that of msi is determined by detailed correlations of ranging data and simultaneous images including cases where the laser boresight slewed off and on the limb of the asteroid and cases where the laser illuminated a boulder close to the time of an image in the data presented the precision of the range measurements is about 1 m with the minimum spot diameter under 5 m and successive spots are contiguous or overlapping elevation on the irregular object eros is given with respect to the gravitational and centrifugal potential landslides in craters are characterized possible crater benches are identified examples of infilled craters are presented these observations suggest a depth of unconsolidated regolith which is subject to sliding of typically a few tens of meters an example of structurally controlled cratering is presented examples of tectonic features are described surface roughness on eros is approximately self affine from scales of a few meters to hundreds of meters a comparison of fractal statistics shows that eros is extremely rough on observed scales when compared to terrestrial a a lava on submeter scales and undisturbed lunar regolith on subcentimeter scales

a new hybrid framework for simulating hypervelocity asteroid impacts and gravitational reaccumulation abstract we present a hybrid approach for simulating hypervelocity impacts onto asteroids the overall system response is separated into two stages based on their different characteristic timescales first the short timescale fragmentation phase is simulated using a modified version of the tonge ramesh material model implemented in a material point method framework then a consistent hand off to an n body gravity code is formulated to execute the long timescale gravitational reaccumulation calculation we demonstrate this hybrid approach by considering the 5 km s head on impact of a 1 21 km diameter basalt impactor on a 25 km diameter target asteroid the impact event resulted in the fragmentation but not complete disruption of the entire target a granular core is observed at the end of the fragmentation simulations which acts as a gravity well over which reaccumulation occurs in the n body simulations our results suggest that disruption thresholds for rocky asteroids are higher when energy dissipating mechanisms such as granular flow and pore collapse are included

diatom flux in mcmurdo sound antarctica abstract quantitative floral analyses have been performed on sea ice sediment trap and surface sediment samples collected from six sites in mcmurdo sound antarctica although diatom abundances in the sea ice reach 108 109 cells m 2 maximum diatom fluxes between october december 1984 as measured in the sediment trap samples are only 105 107 individuals m 2 day 1 the cumulative flux of diatoms for this three month period is approximately 107 109 individuals m 2 possibly an order of magnitude less than the sea ice abundances implying that diatom and opal flux must significantly increase when the sea ice melts and releases those particles previously trapped in the ice five species dominate the sea ice assemblage amphiprora sp pleurosigma sp nitzschia stellata pinnularia quadratarea andnitzschia curta these species are also common in the water column along withthalassiosira spp a genus rarely found within the sea ice within the upper 250 m of the water column at a site in granite harbor diatom flux decreases between 47 79 from 34 to 220 m opal flux however decreases by only 13 40 over this same interval indicating that dissolution of thinly silicified diatom frustules has occurred at mid water depths increases in absolute diatom flux of two species in particular nitzschia curta andthalassiosira spp may indicate their transport from other areas at all sites the greatest increase in diatom flux occurs just above the sea floor resuspension of diatom tests and or lateral advection creates a near bottom nepheloid layer in which significant modification of the diatom assemblage occurs a combination of preferential dissolution of those forms which dominate the sea ice and dilution of the assemblage with resuspended and or advected diatoms representative of many year averages of the flora may be responsible for the production of a sediment assemblage primarily made up ofthalassiosira spp nitzschia curta and other robust forms ofnitzschia

the density and porosity of meteorites from the vatican collection abstract we have developed a nondestructive technique to measure the densities and porosities of large 50 g to several kilogram meteorite samples and have begun a systematic measurement program starting with meteorites from the vatican observatory collection our technique utilizes a modified archimedean method 40 m glass spheres substituting for the fluid to determine bulk density and an unusually large he pycnometer to determine grain density from these the porosity is calculated we report here our first results for 89 samples nearly doubling the published database on porosities of meteorite hand samples including porosities of 35 different meteorites these include several types not previously reported eight iron meteorites six stony iron meteorites an enstatite chondrite the ci carbonaceous chondrite orgueil and the shergottite nakhlatite chassignite snc meteorite nakhla

porosity and density of ordinary chondrites clues to the formation of friable and porous ordinary chondrites densities and porosities of meteorites are physical properties that can be used to infer characteristics of asteroid interiors we report density and porosity measurements of 42 pieces of 30 ordinary chondrites and provide a quantification of the errors of the gas pycnometer method used in this study based on our measurements we find that no significant correlation exists between porosity and petrologic grade chemical group sample mass bulk and grain density or shock level to investigate variations in porosity and density between pieces of a meteorite we examined stones from two showers holbrook and pultusk examination of nine samples of holbrook suggests relative homogeneity in porosity and density between pieces of this shower measurements of three samples of pultusk show homogeneity in bulk density in contrast to wilkison and robinson 2000 a study that reported significant variations in bulk density between 11 samples of pultusk finally examination of two friable ordinary chondrites bjurbole and allegan reveal variability in friability and porosity among pieces of the same fall we suggest that friable ordinary chondrites may have formed in a regolith or fault zone of an asteroid

near s flyby of 253 mathilde images of a c asteroid on 27 june 1997 the near earth asteroid rendezvous near spacecraft flew within 1212 kilometers of asteroid 253 mathilde mathilde is an irregular heavily cratered body measuring 66 kilometers by 48 kilometers by 46 kilometers the asteroid s surface is dark estimated albedo between 0 035 and 0 050 and similar in color to some cm carbonaceous chondrites no albedo or color variations were detected the volume derived from the images and the mass from doppler tracking of the spacecraft yield a mean density of 1 3 0 2 grams per cubic centimeter about half that of cm chondrites indicating a porous interior structure

new tools for the reconstruction of pleistocene antarctic sea ice abstract based on the quantitative analysis of diatom assemblages preserved in 274 surface sediment samples recovered in the pacific atlantic and western indian sectors of the southern ocean we have defined a new reference database for quantitative estimation of late middle pleistocene antarctic sea ice fields using the transfer function technique the detrended canonical analysis dca of the diatom data set points to a unimodal distribution of the diatom assemblages canonical correspondence analysis cca indicates that not only winter sea ice wsi but also summer sea surface temperature ssst represent the most prominent environmental variables that control the spatial species distribution to test the applicability of transfer functions for sea ice reconstruction in terms of concentration and occurrence probability we applied four different methods the imbrie and kipp method ikm the modern analog technique mat weighted averaging wa and weighted averaging partial least squares wapls using logarithm transformed diatom data and satellite derived 1981 2010 sea ice data as a reference the best performance for ikm results was obtained using a subset of 172 samples with 28 diatom taxa taxa groups quadratic regression and a three factor model ikm d172 28 3q resulting in root mean square errors of prediction rmsep of 7 27 and 11 4 for wsi and summer sea ice ssi concentration respectively mat estimates were calculated with different numbers of analogs 4 6 using a 274 sample 28 taxa reference data set mat d274 28 4an 6an resulting in rmsep ranging from 5 52 4an to 5 91 6an for wsi as well as 8 93 4an to 9 05 6an for ssi wa and wapls performed less well with the d274 data set compared to mat achieving wsi concentration rmsep of 9 91 with wa and 11 29 with wapls recommending the use of ikm and mat the application of ikm and mat to surface sediment data revealed strong relations to the satellite derived winter and summer sea ice field sea ice reconstructions performed on an atlantic and a pacific southern ocean sediment core both documenting sea ice variability over the past 150 000 yr mis 1 mis 6 resulted in similar glacial interglacial trends of ikm and mat based sea ice estimates on the average however ikm estimates display smaller wsi and slightly higher ssi concentration and probability at lower variability in comparison with mat this pattern is a result of different estimation techniques with integration of wsi and ssi signals in one single factor assemblage by applying ikm and selecting specific single samples thus keeping close to the original diatom database and included variability by mat in contrast to the estimation of wsi reconstructions of past ssi variability remain weaker combined with diatom based estimates the abundance and flux pattern of biogenic opal represents an additional indication for the wsi and ssi extent

biogenic facies in the antarctic glacimarine environment basis for a polar glacimarine summary abstract the association of glacial deposits diamicts or tillites with biogenic and or chemically derived sedimentary rocks in the ancient record is one which continues to challenge interpretation an analogous association can be found on portions of the east antarctic continental shelf where relict glacial and glacimarine diamictons are overlain by a varied and complex suite of modern biogenic deposits carbonate bioclastic material can be found in two distinct zones and its deposition may have been contemporaneous with partial glacial expansion onto the shelf a nearshore carbonate facies consists of in situ and partially reworked barnacles bryozoans ostracods pelycepods gastropods planktonic and benthonic foraminifera a siliceous biogenic component is intermixed within the shallow water 500 m siliceous facies of pelagic hemipelagic and sediment gravity flow origin is found within inner shelf depressions and consists predominantly of diatom frustules and radiolaria these accumulations of siliceous ooze and mud clearly post date glacial recession from the shelf and contain laminated cross laminated and massive interbeds which are accumulating at rates of 0 3 cm yr the extreme polar climate inhibits meltwater production and terrigenous influx into marine basins this allows for the accumulation of biogenic rich material which is also favored by the high seasonal productivity inherent in the south polar oceans ancient analogs to these modern deposits exist and their association with glacimarine strata could be used to infer polar climatic settings and glacial retreat that is related to eustatic sea level rather than regional warming

sea ice diatom contributions to holocene nutrient utilization in east antarctica combined high resolution holocene 30sidiat and 13cdiat paleorecords are presented from theseasonal ice zone east antarctica both data sets re ect periods of increased nutrient utilization by diatomsduring the hypsithermal period circa 7800 to 3500 calendar years cal years b p coincident with a higherabundance of open water diatom species fragilariopsis kerguelensis increased biogenic silica productivity bsi and higher regional summer temperatures the neoglacial period after circa 3500 cal years b p isre ected by an increase in sea ice indicative species fragilariopsis curta and fragilariopsis cylindrus upto50 along with a decrease in bsi and 13cdiat 18 to 23 however over this period 30sidiatdata show an increasing trend to some of the highest values in the holocene record average of 0 43 competing hypotheses are discussed to account for the decoupling trend in utilization proxies including ironfertilization species dependent fractionation effects and diatom habitats based on mass balance calculations we highlight that diatom species derived from the semi enclosed sea ice environment may have a confoundingeffect upon 30sidowncorecompositions of the seasonal sea ice zone a diatom composition with approximately28 of biogenic silica derived from the sea ice environment diat si can account for the increased averagecompo sition of 30sidiatduring the neoglacial these data highlight the signi cant role sea ice diatoms can playwith relation to their export in sediment records which has implications on productivity reconstructions fromthe seasonal ice zone

metallic minerals thermal histories and parent bodies of some xenolithic ordinary chondrite meteorites we have studied metal grains in the hosts and lithic fragments of widely differing petrologic types in four xenolithic chondrftes using reflected light microscopy and electron probe analysis in weston and fayetteville which both contain solar flare tracks and solar wind gases kamacite taenite and tetrataenite ordered feni and troilite show a variety of textures on a wood plot of central ni content vs dimension taenite analyses scatter as if metal grains cooled at rates of 10 1000 and 1 100 k myr respectively through 700 k although metal in an h6 clast in fayetteville plots coherently with a cooling rate of 50 k myr we propose that metal grains cooled at these rates in chondritic clasts at different locations before host and clasts were compacted and were not subsequently heated above 650 k we predict a similar history for all gas rich ordinary chondrites by contrast metallic minerals throughout bhola and mezo madaras show more uniform textures and plot coherently giving cooling rates in the range 750 to 600 k of 0 1 and 1 k myr respectively we conclude that host and xenoliths in both chondrites were slowly cooled after compaction thus clasts in these chondrites experienced peak metamorphic temperatures and slow cooling through 700 k in different environments according to the conventional onion shell model for h l or ll chondrite parent bodies material of petrologic types 3 5 was arranged in successive shells around a type 6 core prior to catastrophic collisions which mixed all types intimately but if peak metamorphic temperatures were reached during not after accretion as seems plausible maximum metamorphism may have occurred in planetesimals 10 km in radius cooling through 700 k may then have occurred in larger bodies that accreted from these planetesimals iron meteorites mesosiderites and some achondrites may also have experienced melting in planetesimals and slow cooling in larger bodies

quaternary surface water temperature estimations new diatom transfer functions for the southern ocean abstract here we present an improvement of paleotemperature reconstructions for the southern ocean by combining new diatom data from the pacific sector with published atlantic and indian sector reference data the statistical analysis of 336 surface sediment samples recovered from a wide area of southern ocean environments defines a supra regional reference data set for quantitative summer sea surface temperature ssst estimations in situ temperature measurements covering the time span from approx 1900 to 1991 were used as reference instead of more recent time series of satellite derived data possibly biased by ocean warming different transfer function tf models for the imbrie and kipp method ikm the modern analog technique mat weighted averaging wa and weighted averaging partial least squares wapls were tested best performance for ikm was obtained using the d336 set with 29 diatom taxa and three factors resulting in root mean square error of prediction rmsep of 0 833 c for ssst mat estimates were best with six analogs resulting in the lowest rmsep of 0 812 c wapls applied to d336 resulted in a rmsep of 0 782 c wa performed less well expressed by a rmsep of 0 974 c furthermore two subsets for the atlantic d151 and the pacific sectors d107 were applied with ikm to test for the advantages of localized tfs ikm d151 and ikm d107 performed comparably good as ikm d336 with rmsep of 0 71 c and 0 68 c respectively application of the augmented reference data sets on two pleistocene sediment records from the atlantic ps1768 8 and pacific ps58 271 1 sectors led to the best performance of ikm with d336 expressed by high overall communalities 0 75 and fewer ps1768 8 to no ps58 271 1 no analogs compared to the regional data sets proving ikm d336 to deal better with higher assemblage variability ssst estimates for both cores exhibit similar glacial interglacial patterns for all four applied d336 based tf methods with the best concordance between ikm and wapls

collisional evolution of the asteroid belt abstract a new synthesis of asteroid collisional evolution is motivated by the question of whether most asteroids larger than 1 km size are strengthless gravitational aggregates rubble piles near found eros not to be a rubble pile but a shattered collisional fragment with a through going fracture system and an average of about 20 m regolith cover of four asteroids visited by spacecraft none appears likely to be a rubble pile except perhaps mathilde nevertheless current understanding of asteroid collisions and size dependent strength and the observed distribution of rotation rates versus size have led to a theoretical consensus that many or most asteroids larger than 1 km should be rubble piles is eros the best observed asteroid highly unusual because it is not a rubble pile is mathilde if it is a rubble pile like most asteroids what would be expected for the small asteroid itokawa the muses c sample return target an asteroid size distribution is synthesized from the minor planet center listing and results of the sloan digital sky survey an infrared space observatory survey the small main belt asteroid spectroscopic survey and the infrared astronomical satellite survey a new picture emerges of asteroid collisional evolution in which the well known dohnanyi result that the size distribution tends toward a self similar form with a 2 5 index power law is overturned because of scale dependent collision physics survival of a basaltic crust on vesta can be accommodated together with formation of many exposed metal cores the lifetimes against destruction are estimated as 3 gyr at the size of eros 10 gyr at ten times that size and 40 gyr at the size of vesta eros as a shattered collisional fragment is not highly unusual the new picture reveals the new possibility of a transition size in the collisional state where asteroids below 5 km size would be primarily collisional breakup fragments whereas much larger asteroids are mostly eroded or shattered survivors of collisions in this case well defined families would be found in asteroids larger than about 5 km size but for smaller asteroids families may no longer be readily separated from a background population moreover the measured boulder size distribution on eros is re interpreted as a sample of impactor size distributions in the asteroid belt the regolith on eros may result largely from the last giant impact and the same may be true of itokawa in which case about a meter of regolith would be expected there even a small asteroid like itokawa may be a shattered object with regolith cover

osteology and relationships of chirostenotes pergracilis saurischia theropoda from the judith river oldman formation of alberta canada a partial skeleton of a small theropod found in the judith river formation of dinosaur provincial park alberta shows that chirostenotes pergracilis is the same animal as macrophalangia canadensi

sedimentation of biogenic siliceous particles in antarctic waters from the atlantic sector abstract sediment traps were deployed in the drake passage bransfield strait and west of south orkney islands powell basin during december 1980 january 1981 december 1983 and between march and december 1983 respectively most of the trapped material is biogenic opal except in the lower half of the water column in bransfield strait where large amounts of resuspended aluminosilicates and quartz grains were present frustules and skeletons of siliceous microorganisms diatoms silicoflagellates radiolarians chrysophycean cysts fragments and moults of crustaceans and tests of foraminifera were found quantitatively diatoms are the dominant constituent of the trapped biogenic material alteration of diatom assemblages in the water column is due to mechanical breakdown by grazing zooplankton it mainly affects large frustules e g corethron criophilum castracane in the uppermost part of the water column dissolution of frustules occurs mostly at the sediment water interface and leads to the enrichment of strongly silicified valves e g nitzschia kerguelensis o meara hasle thalassiosira antarctica comber resting spores at the bransfield strait site a large part of biogenic opal was incorporated into fecal pellets of krill and copepods the bulk of pellet content consists of fragmented diatom frustules 1 10 m in size most intact valves found in the sediments have settled through the water column by means other than fecal pellet transport e g settling as solitary particles or incorporated into or attached to marine snow or large amorphous aggregates

the global effects of impact induced seismic activity on fractured asteroid surface morphology abstract impact induced seismic vibrations have long been suspected of being an important surface modification process on small satellites and asteroids in this study we use a series of linked seismic and geomorphic models to investigate the process in detail we begin by developing a basic theory for the propagation of seismic energy in a highly fractured asteroid and we use this theory to model the global vibrations experienced on the surface of an asteroid following an impact these synthetic seismograms are then applied to a model of regolith resting on a slope and the resulting downslope motion is computed for a full range of impactor sizes next this computed downslope regolith flow is used in a morphological model of impact crater degradation and erasure showing how topographic erosion accumulates as a function of time and the number of impacts finally these results are applied in a stochastic cratering model for the surface of an eros like body same volume and surface area as the asteroid with craters formed by impacts and then erased by the effects of superposing craters ejecta coverage and seismic shakedown this simulation shows good agreement with the observed 433 eros cratering record at a main belt exposure age of 400 200 myr including the observed paucity of small craters the lowered equilibrium numbers loss rate production rate for craters less than 100 m in diameter is a direct result of seismic erasure which requires less than a meter of mobilized regolith to reproduce the near observations this study also points to an upper limit on asteroid size for experiencing global surface modifying seismic effects from individual impacts of about 70 100 km depending upon asteroid seismic properties larger asteroids will experience only localized regional seismic effects from individual impacts

deglacial environments in eastern prydz bay east antarctica abstract a high resolution continuous record of diatom census counts and diatom specific biomarkers in sediment core nbp0101 jpc24 allows assessment of oceanographic and environmental conditions in eastern prydz bay during the deglaciation 11 100 9000 cal yr bp at decadal timescale our study improves previous snapshots investigations based on resin embedded thin sections and presents a new proxy that compliments the diatom census counts our results suggest that the ice sheet retreat over the core site is dated at 11 100 cal yr bp setting the onset of local deglaciation and subsequent open marine conditions the glacial retreat in prydz bay is due to global warming initiated at 18 cal ka bp and the regional development of the prydz bay cyclonic gyre our results further demonstrate that the deglaciation in eastern prydz bay can be separated in four phases the first between 11 100 and 10 900 cal yr bp when the ice shelf was proximal and sea ice was almost perennial the second and the third phases between 10 900 10 400 cal yr bp and 10 400 9900 cal yr bp respectively when the ice shelf retreated and seasonal sea ice cycle consequently developed promoting warmer water to pump into the bay within the gyre which in turn forced the ice shelf recession and the yearly sea ice cycle establishment and the fourth between 9900 and 9000 cal yr bp when holocene condition were set with a recurrent seasonal sea ice cycle and a well established prydz bay gyre

biogenic sedimentation in mcmurdo sound antarctica surface sediments from eastern mcmurdo sound and fjord basins of the victoria land coast are enriched in organic carbon and opal at depths below 600 m opal contents commonly exceed 30 organic carbon contents average 1 5 and are as high as 3 5 opal and organic carbon are supplied by local production within and below sea ice and by advection from open water areas of the southwestern ross sea the distribution of organic carbon and opal at the sea floor is consistent with cyclonic clockwise water circulation in mcmurdo sound advective transport from the ross sea supplies biogenic sediment to eastern and northern mcmurdo sound the southwestern shelf is bathed by waters derived in part from beneath the ross ice shelf which transport very little allochthonous carbon the supply of biogenic debris in southwestern mcmurdo sound is further curtailed by sea ice conditions e g more prevalent multi year sea ice which reduces photosynthesis and the absence of summer basal melting and ice breakout which restricts the flux of sea ice and open water production to the sea floor consistent with this hypothesis organic carbon fluxes measured via sediment trapping beneath fast ice are one to two orders of magnitude higher in eastern versus western mcmurdo sound pronounced cross sound gradients in shallow water benthic biomass and species diversity have previously been attributed to east west variations in productivity our surface sediment data suggest that similar or even more dramatic contrasts in benthic community structure may exist in the deeper water areas of mcmurdo sound pervasive resuspension is documented by pronounced near bottom flux increases measured by sediment trapping resuspension events which may be more common during the austral winter act to winnow and transport biogenic debris and fine terrigenous sediment from shallow regions 400 m to protected shelf basins despite oxidative loss within the water column the accumulation rate of organic carbon in the deep basins of mcmurdo sound averages 45 mg c m 2 day 1 more than an order of magnitude higher than the world average organic carbon flux to continental margins and equivalent to accumulation rates observed in many anoxic settings preservation efficiency of opal during transport and deposition appears anomalously high and the most opal rich sediments of mcmurdo sound are depleted in organic carbon relative to mid and low latitude siliceous deposits despite this observation if similar patterns of biogenic sedimentation exist on many other parts of the shelf the antarctic continental margin is an important sink for sedimentary organic carbon as has been previously suggested for the silica system

surface expressions of structural features on eros abstract near shoemaker imaging of the asteroid eros has revealed a variety of surface forms that probably reflect underlying structures they range from global to local in scale the most prominent positive topographic feature is a long ridge rahe dorsum which extends over 18 km and has the morphology of a thrust fault two large ancient trough systems are also present grooves are ubiquitous commonly occur in suborthogonal sets and have linear or scalloped outlines the grooves on eros are similar in morphology and scale to those on other small bodies such as gaspra ida and phobos and are likely to have a similar mode of origin namely drainage of regolith into underlying fractures models that use groove dimensions as indicators of regolith thickness imply depths of up to several tens of meters many craters have been subjected to structural control resulting in squared outlines we find no evidence that specific groove sets are associated with individual impact events however evidence exists that preexisting structures have been reactivated by later impact activity the large scale of rahe dorsum and many grooves on eros s surface suggests that the asteroid is a largely coherent but fractured body

eros shape topography and slope processes stereogrammetric measurement of the shape of eros using images obtained by near s multispectral imager provides a survey of the major topographic features and slope processes on this asteroid this curved asteroid has radii ranging from 3 1 to 17 7 km and a volume of 2535 20 km3 the center of figure is within 52 m of the center of mass provided by the navigation team this minimal difference suggests that there are only modest variations in density or porosity within the asteroid three large depressions 10 8 and 5 3 km across represent different stages of degradation of large impact craters slopes on horizontal scales of 300 m are nearly all less than 35 although locally scarps are much steeper the area distribution of slopes is similar to those on ida phobos and deimos regions that have slopes greater than 25 have distinct brighter markings and have fewer large ejecta blocks than do flatter areas the albedo patterns that suggest downslope transport of regolith have sharper boundaries than those on phobos deimos and gaspra the morphology of the albedo patterns their lack of discrete sources and their concentration on steeper slopes suggest transport mechanisms different from those on the previously well observed small bodies perhaps due to a reduced relative effectiveness of impact gardening on eros regolith is also transported in talus cones and in connected sinuous paths extending as much as 2 km with some evident as relatively darker material talus material in at least one area is a discrete superposed unit a feature not resolved on other small bodies flat floored craters that apparently contain ponded material also suggest discrete units that are not well mixed by impacts

diatom assemblages in surface sediments of the northwestern weddell sea antarctic peninsula a study of 42 surface sediment samples from the northwestern weddell sea documents the relationship between the modern distribution of diatom assemblages and present oceanographic conditions in the region three diatom assemblages were identified using r mode principal components analysis and cluster analysis while q mode cluster analysis was used to distinguish two geographically distinct biofacies based on the relative abundance of diatom species the biofacies of the former larsen a ice shelf lais is more closely associated with a sea ice diatom assemblage while the biofacies of the prince gustav channel northern peninsula pgc np is associated with two open coastal diatom assemblages this division is consistent with variance observed in the current oceanographic setting whereby the lais is generally characterized by a longer seasonal duration of sea ice coverage and lower primary productivity relative to the pgc np which is marked by a shorter seasonal duration of sea ice coverage and greater surface water productivity this relationship is further reflected in the distribution of the paleoenvironmentally significant diatom species fragilariopsis curta thalassiosira antarctica and chaetoceros subg hyalochaete spp as well as in absolute abundance data specifically f curta and the morphological form t antarctica t1 are representative of greater seasonal duration of sea ice coverage and lower productivity chaetoceros subg hyalochaete spp and the morphological form t antarctica t2 are associated with decreased seasonal duration of sea ice cover and greater surface water productivity a setting that is further reflected in high absolute diatom abundances these findings provide a basis for future paleoceanographic work in the region especially as studies of surface sediment diatom assemblages are vital to establishing a link between downcore diatom assemblages and those presently found in the water column

the geology of 433 eros the global high resolution imaging of asteroid 433 eros by the near earth asteroid rendezvous near shoemaker spacecraft has made it possible to develop the first comprehensive picture of the geology of a small s type asteroid eros displays a variety of surface features and evidence of a substantial regolith large scale facets grooves and ridges indicate the presence of at least one global planar structure directional and superposition relations of smaller structural features suggest that fracturing has occurred throughout the object as with other small objects impact craters dominate the overall shape as well as the small scale topography of eros depth diameter ratios of craters on eros average 0 13 but the freshest craters approach lunar values of 0 2 ejecta block production from craters is highly variable the majority of large blocks appear to have originated from one 7 6 km crater shoemaker the interior morphology of craters does not reveal the influence of discrete mechanical boundaries at depth in the manner of craters formed on lunar mare regolith and on some parts of phobos this lack of mechanical boundaries and the abundant evidence of regolith in nearly every high resolution image suggests a gradation in the porosity and fracturing with depth the density of small craters is deficient at sizes below 200 m relative to predicted slopes of empirical saturation this characteristic which is also found on parts of phobos and lunar highland areas probably results from the efficient obliteration of small craters on a body with significant topographic slopes and a thick regolith eros displays a variety of regolith features such as debris aprons fine grained ponded deposits talus cones and bright and dark streamers on steep slopes indicative of efficient downslope movement of regolith these processes serve to mix materials in the upper loose fragmental portion of the asteroid regolith in the instance of ponded materials and crater wall deposits there is evidence of processes that segregate finer materials into discrete deposits the near observations have shown us that surface processes on small asteroids can be very complex and result in a wide variety of morphologic features and landforms that today seem exotic future missions to comets and asteroids will surely reveal still as yet unseen processes as well as give context to those discovered by the near shoemaker spacecraft

microfossil distributions in modern amundsen sea sediments abstract acquisition of 20 piston cores from the amundsen sea continental shelf provides material for the first determinations of modern and paleo distributions of microfossils including planktonic and benthic foraminifera diatoms and radiolaria in this region microfossil analyses of filtered sea water and sea ice samples allow comparisons of the biocoenosis with the thanatocoenosis in the sediments in all three biotic provinces are distinguished 1 the outer shelf region north of thurston island is characterized by high abundances of planktonic and calcareous benthic foraminifera low diatom abundances in sediments here despite high abundances in the sea ice may reflect prevention of deposition by ocean currents 2 the southernmost province in pine island bay is marked by very low abundances of all microfossil groups possibly because the floating terminus of pine island glacier has retreated from this area only during the last 100 years in that case the polynya that exists today in this region during some austral summers is so recent a feature that little or no biogenic sediment has had an opportunity to accumulate beneath it 3 the eastern margin province situated between the pine island bay and outer margin provinces along the eastern coast of the amundsen sea is characterized by high abundances of diatoms and arenaceous benthic foraminifera which probably record the presence of the amundsen sea polynya in this area during the last 1000 to 2500 years dominance of the benthic fauna in this area by arenaceous foraminifera is a function of the great depth from which most cores in this area were retrieved 500 924 m below the ccd

introduction to the special issue commemorating the 30th anniversary of the royal tyrrell museum of palaeontology with a summary of the museum s early history and its research contributions located just west of drumheller in southern alberta canada the royal tyrrell museum of palaeontology rtmp is nestled among outcrops of the upper cretaceous horseshoe canyon formation exposed along the red deer river valley fig 1 since it officially opened on 25 september 19851 the museum has become one of alberta s must see tourist attractions a significant contributor to the local economy and earned an international reputation for the quality of its palaeontological collections and the research conducted by its scientists the rtmp is unique within canada in being the only government supported museum dedicated to palaeontology and it is one of the few such institutions in the world fig 1 the royal tyrrell museum of palaeontology a front of museum and main entrance circa 2010 looking westwards and with horizontal beds of the upper cretaceous horseshoe canyon formation exposed in the walls of the red deer river valley in the distance b aerial photograph oblique angle and looking to the north northeast the museum sits on top of the number 7 coal seam sensu gibson 1977 and at the base of the horsethief member of the horseshoe canyon formation d a eberth pers comm 2015 that stratigraphic position correlates with an age of about 71 5 ma i e late campanian and lies within the upper part of the edmontosaurus regalis pachyrhinosaurus canadensis dinosaur assemblage zone see eberth et al 2013 fig 6 c the iconic running theropod logo that until recently graced the exterior of the museum below a life sized tyrannosaurid sculpture by brian cooley one of many such sculptures by cooley that have been featured at the museum since its opening in 1985 and with the education wing opened in 2003 as the atco tyrrell learning centre in the background d the cretaceous alberta life sized diorama opened in 2007 an example of a rtmp exhibit that relies heavily on discoveries and interpretations by museum researchers in this case work done in the albertosaurus bonebed and at other localities throughout the horseshoe canyon formation all photographs courtesy of the royal tyrrell museum of palaeontology this special issue of the canadian journal of earth sciences presents a collection of papers commemorating the rtmp s 30th or pearl anniversary our rationale for doing a 30th anniversary volume was two fold first it is an opportunity to showcase the breadth of research conducted by current and former museum scientists as well as by some colleagues having a history of collaboration with the rtmp the 13 invited papers included in this volume are a mix of review and original research papers all thematically focused on the geology and palaeontology of alberta to varying degrees each contribution is the result of work supported by the rtmp and relies on specimens housed in the museum s collections second the 30th anniversary is an opportunity to reflect on the rtmp s history and its research program as is typical for most institutions the rtmp has done little to formally document its own history this year likely will be the last major milestone for which the original generation of museum staff are still working and available to share their recollections about the museum fieldwork and research already deaths and departures have winnowed away part of the original

recent diatom record of mcmurdo sound antarctica implications for history of sea ice extent analyses of diatom assemblages within surface sediments from mcmurdo sound antarctica document the regional circulation patterns in this large embayment in the southwestern ross sea thalassiosira spp indicative of water column primary productivity is most common in eastern and portions of northwestern mcmurdo sound as a result of its advection into these regions from areas of open water primary productivity surface sediments from southwestern mcmurdo sound are composed mainly of nitzschia curta a small pennate form commonly found both as a member of the sea ice microbial community as well as a dominant component in ice edge blooms in the ross sea the northward advection of oligotrophic water into the western sound from beneath the mcmurdo ice shelf results in the dominance of an autochthonous flora in the southwestern sound sediments deposited during the past 500 years also have been dominated by thalassiosira spp and nitzschia curta thalassiosira spp concentrations reached a maximum between 1600 and 1875 a d corresponding to the time of the little ice age more persistent winds may have been responsible for more prevalent polynyas at that time resulting in greater amounts of open water primary productivity and subsequent higher percentages of thalassiosira spp increased areal extent and or duration of coastal polynyas during the little ice age suggests that within the southwestern ross sea the production of high salinity shelf water and hence antarctic bottom water may have been greater at that time

433 eros lineaments global mapping and analysis using images and laser ranging data from the near shoemaker mission we map lineaments on the surface of eros in order to investigate the relationship between surface morphology and interior structure several sets of lineations are clearly related to visible impact craters while others suggest that different parts of the asteroid may have undergone different stress histories some of these sets infer internal structure at least on a local level this structure may derive from eros parent body and suggest although largely coherent eros interior may have portions that have not undergone a common history

an estimate of eros s porosity and implications for internal structure abstract earth based spectral measurements and near shoemaker magnetometer x ray and near infrared spectrometer data are all consistent with eros having a bulk composition and mineralogy similar to ordinary chondrite meteorites oc by comparing the bulk density of 433 eros 2 67 0 03 g cm 3 with that of ocs 3 40 g cm 3 we estimate the total porosity of the asteroid to be 21 33 macro or structural porosity best estimated to be 20 is constrained to be between 6 and 33 we conclude that eros is a heavily fractured body but we find no evidence that it was ever catastrophically disrupted and reaccumulated into a rubble pile

original structures and fragmentation and reassembly histories of asteroids evidence from meteorites abstract if chondritic meteorites were internally heated after accretion had ended then the hottest material would have been buried the deepest and should have cooled the slowest if this is correct there ought to be a correlation between cooling rate and petrographic type a measure of the extent to which chondrites were metamorphosed i e heated published and new cooling rates derived from the compositions of metallic iron nickel grains do not display this correlation implying either that chondrite parent asteroids never had onion shell structures or that bodies with onion shell structures were broken up and reassembled prior to cooling to below 500 c the temperature at which cooling rate information is recorded in metallic iron nickel chondritic regolith breccias formed from materials that resided on the surfaces of their parent asteroids metallic iron nickel grains in h and l chondrite regolith breccias indicate that the breccia constituents cooled at rates ranging from 1 to 1000 k myr based on thermal calculations these cooling rates suggest that the materials spread out on the surfaces of h and l chondrite parent asteroids originated at depths ranging from about one kilometer to several tens of kilometers craters deep enough to excavate tens of kilometers cannot form on typical asteroidal bodies only 100 to 300 km in diameter without disrupting them therefore it appears that at least some asteroids namely the parent bodies of h and l chondrites were disrupted after cooling to below 300 c and then reassembled to create surfaces containing rocks that originated at a wide range of depths these results support theoretical calculations suggesting that many asteroids were broken up and subsequently reassembled into gravitationally bound rubble piles

stenonychosaurus inequalis saurischia theropoda from the judith river oldman formation of alberta new findings on metatarsal structure a partial foot of stenonychosaurus from the judith river oldman formation dinosaur provincial park alberta shows features in the metatarsus that have not previously been recognized these include compression of metatarsal ii and exclusion of the proximal half of metatarsal iii from the anterior dorsal surface of the metatarsus exclusion from the anterior surface of metatarsal iii to this extent is unprecedented among theropods and further underscores the distinctiveness of the sauromithoididae

diatom assemblages from coastal settings of west antarctica abstract diatom assemblages from 33 core tops collected along a 2500 km transect of the northern antarctic peninsula and pine island bay areas were examined for their use as paleoenvironmental proxies environmental affinities of assemblages were interpreted within a broad geographical context and a wide range of ecological conditions four major assemblages were identified three pelagic assemblages related to different sea ice duration sst and other conditions affecting primary production and one benthic and epiphytic dominated assemblage typical of near shore shallow water habitats chaetoceros resting spores are an important component of diatom assemblages their high concentrations may reflect increased primary production and are associated with low total assemblage diversities the lowest diversities are observed at sites with increased resting spore percentages along the nw coast of the antarctic peninsula sedimentation rates strongly influence total diatom concentrations by contrast relative abundances of chaetoceros resting spores remain mostly unaffected by terrigenous flux and may serve as a reliable proxy for primary production this broad sampling of antarctic coastal environments fills a gap in our knowledge of phytoplankton communities in polar regions and helps establish the framework for reconstructing paleoenvironments from the paleorecord

diatom evidence of hydrographic changes and ice conditions in igaliku fjord south greenland during the past 1500 years biostratigraphic diatom analyses were carried out on two sediment cores from igaliku fjord south greenland in order to reconstruct hydrographic and climatic fluctuations during the late holocene ad 550 to present pronounced differences in hydrographic setting of the coring sites provide palaeoenvironmental information of both local and more regional character five diatom zones were identified from the subfossil diatom records each describing a hydrographic setting two major climate periods the mediaeval warm period ad 800 1250 and the little ice age ad 1580 1850 that were separated by a variable transitional period were recognized prior to these periods a cold and presumably moist climate regime prevailed between ad 500 and 700 cooling events were identified during the mediaeval warm period in the period ad 960 to 1140 the diatom data demonstrate that marked hydrographic changes occurred in the course of a relatively short timespan beginning at about ad 1300 and culminating around

the non trivial problem of meteorite pairing pairing is the procedure of identifying fragments of a single meteorite fall that were separated during atmospheric passage or during terrestrial history by establishing the similarity of two or more meteorite fragments we argue that pairing is governed by two principles that only a single mismatch of properties is required to refute a proposed pairing and that virtually all pairings bear some degree of uncertainty using data distributions for modern falls we take a probability approach to estimate degrees of certainty associated with proposed pairings emphasizing the importance of unusual features for new pairing criteria or new analytical additions to old criteria the degree of variation within individual meteorites must be delineated and the degree of variation within meteorite classes must be quantified criteria for pairing can be divided into 1 parent body history indicators 2 meteoroid space history indicators and 3 terrestrial history indicators included in these categories are 11 specific criteria including petrographic textures mineralogy and mineral composition terrestrial age estimates cosmic ray exposure ages and natural thermoluminescence tl levels not all criteria are applicable to all meteorite types about 2275 pairings suggested in the literature have been subjected to this analysis many literature pairings especially those involving common meteorite types bear large uncertainties due to lack of data

distribution patterns of diatom surface sediment assemblages in the laptev sea arctic ocean abstract the modern diatom distribution in the laptev sea arctic ocean was investigated in 89 surface sediment samples diatom concentrations are relatively low showing values between 0 01 10 6 and 6 7 10 6 valves per gram dry sediment based on a factor analysis using seventeen taxa or taxa groups five diatom surface sediment assemblages can be defined the ice diatom assemblage of the central region of the laptev sea the chaetoceros assemblage of the eastern and southeastern shelf the thalassiosira antarctica assemblage of the continental slope and deep sea the freshwater diatom assemblage in the vicinity of river mouths and deltas and the thalassiosira nordenskioeldii assemblage which shows a patchy occurrence on the central laptev sea shelf the distribution pattern of diatom assemblages in surface sediment is significantly related to oceanographic conditions of surface water masses the main factors controlling the distribution of diatoms in the laptev sea are the riverine freshwater input during the summer which strongly affects the salinity conditions and the sea ice extent furthermore the composition of the thalassiosira antarctica assemblage of the continental slope is largely influenced by dissolution and lateral transport processes

surface winds over the antarctic continent a review surface winds over antarctica often classified as katabatic are intimately related to the orientation and steepness of the underlying ice terrain such flows result primarily from the strong radiational cooling of the sloping ice surface which creates a downslope directed horizontal pressure gradient force it is possible to diagnose the time averaged near surface streamline patterns of cold air drainage currents using recently compiled detailed antarctic ice topography maps and appropriate estimates of the strength of the surface temperature inversion the resulting streamlines agree with available observations and illustrate the high degree of nonuniformity of gravity driven flows off the ice sheets in certain interior sections negatively buoyant air from a large catchment area drains through a restricted section of coastline such confluence zones represent regions of enhanced supplies of cold air and are shown to be related to anomalously strong katabatic winds implications for sea ice formation are discussed

impact produced seismic shaking and regolith growth on asteroids 433 eros 2867 steins and 25143 itokawa abstract of the several near earth and main belt asteroids visited by spacecraft to date three display a paucity of small craters and an enhanced number of smoothed and degraded craters 433 eros which has a deficit of craters 100 m in diameter 2867 steins which has a deficit of craters 500 m in diameter and 25143 itokawa which has a deficit of craters 100 m in diameter the purpose of this work was to investigate and model topographic modification and crater erasure due to impact induced seismic shaking as well as impact driven regolith production and loss on these asteroid surfaces to perform this study we utilized the numerical three dimensional cratered terrain evolution model ctem initially presented in j e richardson icarus 204 2009 which received a small body sb specific update for this work sbctem simulations of the surface of 433 eros correctly reproduce its observed cratering record for craters up to 3 km in diameter at a minimum main belt exposure age of 225 75 myr using a very weak rock target strength of 0 5 5 0 mpa and producing a mean regolith depth of 80 20 m which agrees with published estimates of an actual regolith layer tens of meters in depth sbctem simulations of the surface of 2867 steins correctly reproduce its heavily softened cratering record at a minimum main belt exposure age of 175 25 myr using a weak rock target strength of 2 5 5 0 mpa extending this simulation produces a mean regolith depth of 155 5 m at a minimum main belt exposure age of 475 25 myr in good agreement with the 145 35 m actual mean regolith depth estimated from two chains of drainage pit craters on the surface with both crater density and regolith depth in an equilibrium state sbctem simulations of the surface of 25143 itokawa correctly reproduce its extremely sparse cratering record at a minimum main belt exposure age of 20 5 myr using a soft soil target strength of 10 20 kpa and producing a mean regolith overturn depth of 5 0 1 0 m given that itokawa is composed entirely of regolith and boulders these simulations demonstrate the efficacy of impact induced seismic shaking to erase small craters on the surface of asteroids 25 km mean diameter correctly reproducing their observed cratering records while using a main belt impactor population and standard crater scaling relationships these simulations also show that the observed regolith layer on asteroids in the 5 50 km size range can be generated entirely through impact cratering processes and indicate that regolith depth can be used as a additional constraint on the surface age of a given asteroid

dissolution and preservation of antarctic diatoms and the effect on sediment thanatocoenoses abstract comparison of southern ocean diatom populations from i surface ocean production ii underlying antarctic sediments and iii laboratory dissolution experiments demonstrates that dissolution can account for the temporal and spatial variations in sedimentary diatom assemblages observed in southern ocean sediments increasing dissolution causes relative depletions in n kerguelensis k enrichments in t lentiginosa l and slight enrichments in e antarctica a this reflects the relative susceptibility to dissolution of the three species that dominate antarctic sediments we have devised a preservation index for the southern ocean based on the ratio k k l to estimate relative extents of dissolution and applied it to natural assemblages holocene southern ocean sediments display increasing opal preservation toward higher latitudes but south of the antarctic polar front preservation decreases in the order well preserved se indian s atlantic sw indian se pacific poorly preserved dissolution also accounts for the pattern of diatom assemblages in the last glacial maximum lgm sediments of the indian and pacific sectors but in the atlantic increased e antarctica abundances at lgm must have resulted from an increase in surface ocean production of this species holocene and lgm diatoms in atlantic and pacific sector sediments are equally well preserved but in the indian sectors holocene sediments are better preserved than those of lgm age paleoceanographic and paleoclimatic transfer functions derived from factor analyses of variations in the sedimentary abundances of these three diatoms have ignored the effects of differential dissolution on thanatocoenosis and thus should be interpreted with caution

the surface windfield over the antarctic ice sheets the intense radiative cooling of air over the ice slopes of antarctica generates a surface wind regime that is strongly controlled by topography and plays a key role in determining the behaviour of the atmosphere and ocean in high southern latitudes1 7 resultant surface winds are intimately linked to the orientation of the ice terrain fig 1 and display the highest degree of persistence found on earth the close coupling between wind and topography allows estimation of the former if the latter is known with some precision here we report on time averaged near surface airflow over the antarctic continent during winter diagnosed from a recent accurate synthesis of terrain slopes and from estimates of the lower atmospheric temperature structure the simulated drainage pat tern exhibits strong spatial variability with the airflow concentrated into several zones near the coastal margin these confluence regions are responsible for strong persistent katabatic winds over downstream coastal stretches and are indicative of zones of greatest katabatic potential

a model for impact induced lineament formation and porosity growth on eros abstract we investigate the impact history of the near earth asteroid nea eros 433 using a new material model for brittle materials such as rocks where initial flaw distributions within the rock are explicitly defined to match what is known about flaw size distributions in rocks these simulations are limited to the initial impact phase of the crater formation process and use a very crude approximation for the effect of the gravitational overburden pressure given these approximations our simulations of this numerical approximation of eros suggest that the current observed bulk porosity of about 25 could be consistent with the porosity generated by the formation of the three largest craters observed on eros indicating that eros could have started as an intact shard from a prior impact event further we investigate the consequences of two possible internal flaw distributions for the asteroid a strong flaw distribution with shorter crack lengths that are more difficult to activate during cratering and a weak flaw distribution with longer flaws the strong distribution produces localized deformation regions lineaments that are resolved by the simulations while the weak distribution does not produce resolved localized features for either distribution of internal flaws the initial impact assumed to be the himeros forming impact shatters but does not disrupt the body implying that simulations of asteroid mitigation approaches should assume that asteroids will behave like rubble piles subsequent impact events activate linear features created by prior impacts but only change the orientation of the lineament structure near the impact site