silicon isotope fractionation during silica precipitation from hot spring waters evidence from the geysir geothermal field iceland this study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the geysir geothermal area iceland a setting where sinter deposits are actively formed the 30si values of dissolved silica measured in the spring water and sampling sites along outflowing streams covering a temperature range between 20 and 100 c were relatively constant around 0 2 whereas the 30si signatures of associated opaline sinters from the streambeds were between 0 1 and 4 0 becoming progressively more negative in the downstream parts of the aprons here the deposited sinters represent some of the most 30si depleted abiotically produced terrestrial materials documented to date compared to the data reported for icelandic basalts considered to be the source of the silicon the 30si values of the fluids and sinter deposits are higher and lower respectively the resulting values for apparent solid water isotope fractionation 30sisolid water decreased with decreasing temperature from ca 0 7 at 80 c to 3 7 at 20 c locally down to 4 4 this temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid however the apparent fractionation magnitudes observed in the field are ca 2 more negative and thus significantly larger we infer that solid water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature but is essentially a function of the precipitation rate such that the fractionation factor decreases with increasing rate as an important corollary the effective fractionation behavior during precipitation of silica from saturated solutions is a system dependent feature which should be taken into account when using silicon isotopes for paleo environmental reconstructions

the unique geomorphology and physical properties of the vestalia terra plateau abstract we produced a geologic map of the av 9 numisia quadrangle of asteroid vesta using dawn spacecraft data to serve as a tool to understand the geologic relations of surface features in this region these features include the plateau vestalia terra a hill named brumalia tholus and an unusual dark ribbon material crossing the majority of the map area stratigraphic relations suggest that vestalia terra is one of the oldest features on vesta despite a model crater age date similar to that of much of the surface of the asteroid cornelia numisia and drusilla craters reveal bright and dark material in their walls and both cornelia and numisia have smooth and pitted terrains on their floors suggestive of the release of volatiles during or shortly after the impacts that formed these craters cornelia fabia and teia craters have extensive bright ejecta lobes while diogenitic material has been identified in association with the bright teia and fabia ejecta hydroxyl has been detected in the dark material within cornelia numisia and drusilla three large pit crater chains appear in the map area with an orientation similar to the equatorial troughs that cut the majority of vesta analysis of these features has led to several interpretations of the geological history of the region vestalia terra appears to be mechanically stronger than the rest of vesta brumalia tholus may be the surface representation of a dike fed laccolith the dark ribbon feature is proposed to represent a long runout ejecta flow from drusilla crater

imprint of the rheasilvia impact on vesta geologic mapping of quadrangles gegania and lucaria abstract we produced two 1 250 000 scale geologic maps of the adjacent quadrangles av 6 gegania and av 7 lucaria located in the equatorial region of 4 vesta 0 144 e 22 s to 22 n the mapping is based on clear and color filter images of the framing camera fc onboard the dawn spacecraft which has captured the entire illuminated surface of vesta with high spatial resolution up to 20 m pixel and on a digital terrain model derived from fc imagery besides the geologic mapping itself a secondary purpose of this work is to investigate one of the most prominent morphological features on vesta namely the aggregation of several giant equatorial troughs termed the divalia fossae most probably formed during the rheasilvia impact near vesta s south pole the up to 465 km long and 22 km wide troughs show height differences of up to 5 km between adjacent troughs and ridges another imprint of the rheasilvia impact is the 350 km long and 250 km wide swath of ejecta crossing quadrangle av 6 gegania this lobe shows a distinct appearance in fc color ratios and a high albedo in fc images indicating a mineralogical similarity to material typically found within the rheasilvia basin in particular composed of diogenite rich howardites almost the entire northern half of the mapping area shows the oldest surface being dominated by upper crustal basaltic material to the south increasingly younger formations related to the rheasilvia impact occur either indicated by the troughs formed by rheasilvia or by the rheasilvia ejecta itself only medium sized impact craters with diameters less than 22 km occur within the two mapped quadrangles some of the craters exhibit ejecta blankets and or distinctly dark or bright ejecta material in ejecta rays outside and exposures within the crater and mass wasting deposits down crater slopes forming the youngest surfaces

subsurface failure in spherical bodies a formation scenario for linear troughs on vesta s surface abstract many asteroids in the solar system exhibit unusual linear features on their surface the dawn mission recently observed two sets of linear features on the surface of the asteroid 4 vesta geologic observations indicate that these features are related to the two large impact basins at the south pole of vesta though no specific mechanism of origin has been determined further the orientation of the features is offset from the center of the basins experimental and numerical results reveal that the offset angle is a natural consequence of oblique impacts into a spherical target here we demonstrate that a set of shear planes develops in the subsurface of the body opposite to the point of first contact these subsurface failure zones then propagate to the surface under combined tensile shear stress fields after the impact to create sets of approximately linear faults on the surface comparison between the orientation of damage structures in the laboratory and failure regions within vesta can be used to constrain impact parameters e g the approximate impact point and likely impact trajectory

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

diagenetic fractionation of ge and si in reducing sediments the missing ge sink and a possible mechanism to cause glacial interglacial variations in oceanic ge si the average ge si ratio in the ocean is determined by the budgets for each of these elements previous budget formulations have assumed that the only important sink for both elements is burial as opal based on studies of the si cycle and the close oceanic coupling observed between inorganic ge and si distributions however these budgets implied two paradoxes 1 hydrothermal flow through ocean ridges is smaller than predicted by other tracers and 2 the lower ge si ratio of opal deposited during glacial times compared to that deposited during interglacial times required enhanced weathering during cooler drier climates both paradoxes could be resolved if a significant sedimentary sink for ge other than opal burial could be identified and the objective of this study was to search for one two pore water profiles collected in equatorial pacific sediments show that ge and si behave similarly in the upper 10 cm of sediment indicating no evidence for a significant non opal sink for ge in oxic sediments by contrast profiles in several cores from the california margin demonstrate that in reducing sediments ge diagenesis is poorly coupled to si diagenesis significant ge removal is evident both downcore and sometimes in the near surface benthic flux chamber measurements at three continental slope stations all with an oxic layer less than 1 cm thick and large iron gradients in near surface pore waters showed that 55 9 of the ge released by opal dissolution is sequestered however at two locations with anoxic sediments but little pore water fe 2 in the upper 2 cm flux measurements indicated little fractionation from the oceanic ratio during diagenesis implicating the importance of iron for fractionating ge from si during diagenesis if the ge sequestration observed in the iron rich ca margin sediments is typical of all slope sediments using a depth range of 200 1000 m then the ge sink is sufficient to bring the hydrothermal budget based on ge into concurrence with that based on other tracers the temporal variation in oceanic ge si could be explained if ge and si inputs remain constant and the effective diagenetic fractionation of ge increases by a factor of 2 3 during glacial times increased fractionation would require that glacial periods are characterized by increased opal dissolution in iron rich reducing sediments this could be caused by 1 thinning of the oxygenated sediment layer in response to decreased bottom water oxygen concentrations or increased rain of organics to the sea floor 2 increased rain of iron rich detrital sediments in areas receiving high opal rain 3 increased rain of opal to sediments in margin areas if the oceanic ge si ratio reflects increased rain of diatom opal or organic carbon in margin areas during glacial periods it may indicate an increase in the efficiency of the biological pump for co2 during glacial times

geologic mapping of ejecta deposits in oppia quadrangle asteroid 4 vesta abstract oppia quadrangle av 10 288 360 e 22 is a junction of key geologic features that preserve a rough history of asteroid 4 vesta and serves as a case study of using geologic mapping to define a relative geologic timescale clear filter images stereo derived topography slope maps and multispectral color ratio images from the framing camera on nasa s dawn spacecraft served as basemaps to create a geologic map and investigate the spatial and temporal relationships of the local stratigraphy geologic mapping reveals the oldest map unit within av 10 is the cratered highlands terrain which possibly represents original crustal material on vesta that was then excavated by one or more impacts to form the basin feralia planitia saturnalia fossae and divalia fossae ridge and trough terrains intersect the wall of feralia planitia indicating that this impact basin is older than both the veneneia and rheasilvia impact structures representing pre veneneian crustal material two of the youngest geologic features in av 10 are lepida 45 km diameter and oppia 40 km diameter impact craters that formed on the northern and southern wall of feralia planitia and each cross cuts a trough terrain the ejecta blanket of oppia is mapped as dark mantle material because it appears dark orange in the framing camera clementine type color ratio image and has a diffuse gradational contact distributed to the south across the rim of rheasilvia mapping of surface material that appears light orange in color in the framing camera clementine type color ratio image as light mantle material supports previous interpretations of an impact ejecta origin some light mantle deposits are easily traced to nearby source craters but other deposits may represent distal ejecta deposits emplaced 5 crater radii away in a microgravity environment

mechanisms controlling the silicon isotopic compositions of river waters it has been proposed that silicon si isotopes are fractionated during weathering and biological activity leading to heavy dissolved riverine compositions in this study the first seasonal variations of stable isotope compositions of dissolved riverine si are reported and compared with concomitant changes in water chemistry four different rivers in switzerland were sampled between march 2004 and july 2005 the unique high resolution multi collector icp ms nu1700 has been used to provide simultaneous interference free measurements of 28si 29si and 30si abundances with an average limiting precision of 0 04 on 30si this precision facilitates the clarification of small temporal variations in isotope composition the average of all the data for the 40 samples is 30si 0 84 0 19 1 sd despite significant differences in catchment lithologies biomass climate total dissolved solids and weathering fluxes the averaged isotopic composition of dissolved si in each river is strikingly similar with means of 0 70 0 12 for the birs 0 95 0 22 for the saane 0 93 0 12 for the ticino and 0 79 0 19 for the verzasca however the 30si undergoes seasonal variations of up to 0 6 comparisons between 30si and physico chemical parameters such as the concentration of dissolved si and other cations the discharge of the rivers and the resulting weathering fluxes permits an understanding of the processes that control the si budget and the fate of dissolved si within these rivers the main mechanism controlling the si isotope composition of the mountainous verzasca river appears to be a two component mixing between the seepage of soil ground waters with heavier si produced by clay formation and superficial runoff associated with lighter si during high discharge events a biologically mediated fractionation can be excluded in this particular river system the other rivers display increasing complexity with increases in the proportion of forested and cultivated landscapes as well as carbonate rocks in the catchment in these instances it is impossible to resolve the extent of the isotopic fractionation and contributed flux of si contributed by biological processes as opposed to abiotic weathering the presence of seasonal variations in si isotope composition in mountainous rivers provides evidence that extreme changes in climate affect the overall composition of dissolved si delivered to the oceans the oceanic si isotope composition is very sensitive to even small changes in the riverine si isotope composition and this parameter appears to be more critical than plausible changes in the si flux therefore concurrent changes in weathering style may need to be considered when using the si isotopic compositions of diatoms sponges and radiolaria as paleoproductivity proxies

the influence of recent major crater impacts on the surrounding surfaces of 21 lutetia we present 3 d simulations of impacts into asteroid 21 lutetia the subject of a fly by by the european space agency s rosetta mission to comet 67p churyumov gerasimenko using a 3 d shape model of the asteroid impacts of sizes sufficient to reproduce the observed craters in lutetia s north polar crater cluster npcc as observed by the osiris experiment have been simulated using the smoothed particle hydrodynamics technique the asteroid itself has been modelled both as a homogeneous body and as a body with an iron core crater erasure in the vicinity of the npcc has been observed by osiris the results show that this erasure has most probably been caused by ejecta deposition following the impact of a 2 3 km diameter projectile impacting at a velocity of 5 km s 1 or an impact with similar energy this would produce a crater of roughly 34 km in diameter comparable to the largest and oldest member of the npcc erasure of craters via the shock associated with such an impact is shown to be less significant and does not reproduce the observed spatial distribution of erased craters or ghost craters time series of the surface velocity fields resulting from the simulated impacts are also presented it is suggested that the surface velocity field and velocity shear may play a role in the generation of lineaments our model calculations show that the velocity field lines around 50 s after impact exhibit a reasonable qualitative correlation with the orientation of lineaments observed on the entire visible surface of lutetia it is also shown that incorporation of a core of 25 30 km in diameter does not modify the velocity field evolution with time and as such the presence or otherwise of such a core cannot be inferred from lineament observations if this concept for their formation is valid

dissolution of biogenic silica from land to ocean role of salinity and ph the dissolution rates of diatom frustules phytoliths two diatomaceous lake sediments a siliceous ooze from the southern ocean a diatomite deposit and a synthetic amorphous silica were measured in flow through reactors supplied with either seawater or freshwater although the rates varied by more than one order of magnitude among the different siliceous materials for any given solid the rate was systematically higher in seawater than freshwater on average by a factor of five flow through reactor experiments with the diatom frustules and synthetic silica using mixtures of freshwater and seawater indicated that most of the rate increase occurred for seawater fractions between 0 and 50 the observed rate enhancement is attributed to the higher ph of seawater and the catalytic effect of seawater cations on the hydrolysis of siloxane bonds at the silica surface because of their abundance in seawater na and mg2 are mainly responsible for the salinity induced rate increase the large difference in dissolution kinetics between freshwater and seawater helps explain the very efficient recycling of biogenic silica in marine environments compared with freshwater lakes enhanced dissolution at the land ocean transition of biogenic silica produced by terrestrial plants and freshwater diatoms may represent a significant but largely overlooked source of nutrient silicon for estuarine and nearshore marine ecosystems

vesta s shape and morphology vesta s surface is characterized by abundant impact craters some with preserved ejecta blankets large troughs extending around the equatorial region enigmatic dark material and widespread mass wasting but as yet an absence of volcanic features abundant steep slopes indicate that impact generated surface regolith is underlain by bedrock dawn observations confirm the large impact basin rheasilvia at vesta s south pole and reveal evidence for an earlier underlying large basin veneneia vesta s geology displays morphological features characteristic of the moon and terrestrial planets as well as those of other asteroids underscoring vesta s unique role as a transitional solar system body

role of increased marine silica input on paleo pco2 levels changing the supply of silica to the ocean may alter pco2 levels the increase in dust delivered to the ocean during glacial times increased the availability of silica for biological uptake the increased silica levels shifted species composition diatom populations increased and coccolith populations decreased decreasing the population of coccoliths decreased the flux of calcite to the sediments which in turn lowered pco2 levels enough to explain the glacial interglacial pco2 transition furthermore the contemporary increase in dust delivered to the ocean s mixed layer may be removing significant amounts of carbon dioxide from the atmosphere at present to set the stage this silica hypothesis is compared with the iron fertilization and nitrogen fixation hypotheses

eucritic crust remnants and the effect of in falling hydrous carbonaceous chondrites characterizing the composition of vesta s marcia region abstract the equatorial marcia quadrangle region is characterized by the large relatively young impact craters marcia and calpurnia and their surrounding dark ejecta field a hill with a dark rayed crater named aricia tholus and an unusual diffuse material surrounding the impact crater octavia the spectral analysis indicates that while this region is relatively uniform in the pyroxene band centers it instead shows large differences in pyroxene band depths and reflectance a large variation of reflectance is seen in the quadrangle bright and dark materials are present as diffuse material and as concentrated spots and outcrops moreover oh signature is pervasive in the quadrangle with a few exceptions the region especially the marcia ejecta field is characterized by spectra showing the 2 m band shifted at long wavelengths this is commonly associated with eucritic material believed to have crystallized as lava on vesta s surface or within relatively shallow level dikes and plutons thus suggesting that this region is a remnant of the old vestan basaltic crust however other characteristics of the spectra do not fully fit the eucritic composition indicating an alternative explanation for the band center distribution including the presence of carbonaceous chondritic material mixed with the native vestan pyroxene the detailed mineralogical analysis of the marcia quadrangle indicates that this quadrangle is the result of the mixture of the vestan endogenic minerals with the exogenic carbonaceous chondrites the stratigraphic units around marcia clearly show the bright uncontaminated material interlaced and mixed with the dark material that contains a strong oh signature only few small areas can be considered as representative of the old vestan original material

vesta s north pole quadrangle av 1 albana geologic map and the nature of the south polar basin antipodes as part of systematic global mapping of vesta using data returned by the dawn spacecraft we have produced a geologic map of the north pole quadrangle av 1 albana extensive seasonal shadows were present in the north polar region at the time of the dawn observations limiting the ability to map morphological features and employ color or spectral data for determination of composition the major recognizable units present include ancient cratered highlands and younger crater related units undivided ejecta and mass wasting material on crater floors the antipode of vesta s large southern impact basins rheasilvia and veneneia lie within or near the av 1 quadrangle therefore it is of particular interest to search for evidence of features of the kind that are found at basin antipodes on other planetary bodies albedo markings known as lunar swirls are correlated with basin antipodes and the presence of crustal magnetic anomalies on the moon but lighting conditions preclude recognition of such albedo features in images of the antipode of vesta s rheasilvia basin hilly and lineated terrain found at the antipodes of large basins on the moon and mercury is not present at the rheasilvia or veneneia antipodes we have identified small scale linear depressions that may be related to increased fracturing in the rheasilvia and veneneia antipodal areas consistent with impact induced stresses buczkowski d et al 2012b analysis of the large scale troughs on vesta and correlation to a model of giant impact into

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 lithologic heterogeneity of asteroid 4 vesta abstract high precision spectrophotometry of 4 vesta the third largest asteroid was used to establish the surface composition of this body and to investigate mineralogical variations across its surface the average surface of vesta is analogous to howardite and or polymict eucrite assemblages regolith derived members of the hed meteorite suite which consist of a eucrite matrix containing differing amounts of a diogenite component color and or spectral changes which exhibit a consistent relationship to rotational phasing are evident in 16 of 18 studies of vesta dating back to 1929 including the present work after elimination of possible sources of spurious spectral or color changes it is concluded that these variations arise from hemispheric variations in surface materials and hence provide a means of spatially resolving subhemispheric compositional units on the surface of vesta the background surface of vesta is a relatively dark howardite or polymict eucrite pyroxene plagioclase assemblage with several compositionally distinct bright regions clustered in one hemisphere viewed around the maximum in vesta s lightcurve these include what appears to be an olivine bearing unit suggested name leslie formation located near vesta s equator which probably represents an impact basin and or its associated ejecta that penetrated through the basaltic crust other high albedo compositional units including an apparently low calcium eucrite region and several diogenite pyroxenite regions at least one located near the southern pole may be smaller shallower impact basins by analogy to the eucrite meteorites which represent surface flows or shallow intrusions and which constitute the major component of the regolith derived howardites and polymict eucrites we conclude that the howardite polymict eucrite units represent a regolith gardened original surface of vesta it is probable that the low albedo of the background surface on vesta is due to an age related darkening effect similar to that inferred from the galileo images of gaspra and ida this mechanism is consistent with the correlation of absorption feature intensity with the lightcurve vesta appears to have an old eucritic surface darkened with age and represented among the meteorites by the regolith derived howardites and polymict eucrites on which several impacts on one hemisphere have exposed fresher brighter diogenite and olivine bearing material based on qualitative analysis of the mineralogical variations as a function of rotation a generalized lithologic map of vesta was produced there is reasonable control on the longitude of lithologic features but little control on their latitudes except where specifically noted in producing this map discrete circular features were used on the plausible assumption that impacts have been the most important geologic process on this surface for most of the age of the solar system the shape of the features has thus been assumed rather than derived and is subject to future revision as data improve

dawn completes its mission at 4 vesta the dawn mission was designed to test our hypothesis about the origin and evolution of the early solar system by visiting the largest differentiated basaltic asteroid 4 vesta believed to be a survivor from the earliest times of rocky body formation observations from orbit show that vesta is the parent body of the howardite eucrite diogenite meteorites vesta has an iron core and a eucritic diogenitic crust its surface is characterized by abundant impact craters but with no evident volcanic features it has two ancient impact basins in the southern hemisphere that are associated with circum planetary troughs the northern hemisphere is the more heavily cratered and contains the oldest terrains the surface of vesta is diverse with north south and east west dichotomies in the eucrite to diogenite ratio its surface contains both very bright and very dark material and its color varies strongly from region to region both the mineralogical and the elemental compositions agree with that expected for the hed parent body significant oh or h may be present in the upper crust and the presence of pits in fresh craters is consistent with the devolatilization of the surface after a collision either brought to or tapped a source of water on vesta the presence of dark material on the surface of vesta suggests efficient transport pathways for organic material and the mixing of the dark material with the more pristine pyroxene explains the varying albedo across the surface vesta has proven to be a reliable witness to the formation of the solar system

evidence for the interior evolution of ceres from geologic analysis of fractures ceres is the largest asteroid belt object and the dawn spacecraft observed ceres since 2015 dawn observed two morphologically distinct linear features on ceres s surface secondary crater chains and pit chains pit chains provide unique insights into ceres s interior evolution we interpret pit chains called the samhain catenae as the surface expression of subsurface fractures using the pit chains spacings we estimate that the localized thickness of ceres s fractured outer layer is approximately 58 km at least 14 km greater than the global average we hypothesize that extensional stresses induced by a region of upwelling material arising from convection diapirism formed the samhain catenae we derive characteristics for this upwelling material which can be used as constraints in future interior modeling studies for example its predicted location coincides with hanami planum a high elevation region with a negative residual gravity anomaly which may be surficial evidence for this proposed region of upwelling material

weathering and the global carbon cycle geomorphological perspectives abstract the links between the weathering of silicate minerals the global carbon cycle and climate change are complex and involve a whole range of often inter related tectonic climatic geomorphological and biological factors silicate weathering rates are known to have varied through geological time and increasingly sophisticated models now link such variation to the long term carbon cycle and climate change in order to improve the modelling further one key research need is for more and better datasets on weathering and the factors that control it as a contribution to this we review the nature of the weathering carbon cycle system the sources of evidence used to understand it and the challenges that remain in quantifying the feedbacks between weathering and the global carbon cycle we propose that increased understanding of geomorphological diversity of landscapes and processes and especially the dynamic relationships between weathering and erosion will aid clearer portrayal of the links between weathering and the long term carbon cycle

weathering dust and biocycling effects on soil silicon isotope ratios abstract silicon isotope ratios 30si of bulk mineral materials in soil integrate effects from both silicon sources and processing here we report 30si values from a climate gradient of hawaiian soils developed on 170 ka basalt and relate them to patterns of soil chemistry and mineralogy the results demonstrate informative relationships between the mass fraction of soil si depletion and 30si in upper 1 m deep soil horizons generally correlate with the isotope fractionation predicted by a study of dissolved si in basalt watershed rivers and driven by preferential 28si removal from the dissolved phase during precipitation in contrast after correcting for the influence of dust secondary mineral si depletion and 30si values in shallow

phobos grooves and impact craters a stereographic analysis abstract phobos parallel grooves were first observed on viking images 38 years ago and since then they have been greatly debated leading to several formation hypotheses nevertheless none of them have been favoured and widely accepted in this work we provide a different approach assuming that phobos grooves can be the expression of fracture planes and deriving their spatial distribution and orientation on 3d reconstructions we point out that any origin related only to craters at phobos surface should be ruled out since the majority of the grooves is unrelated to any craters now present at its surface this raises the intriguing possibility that such grooves if expression of fracture planes are remnant features of an ancient parent body from which phobos could have originated such scenario has never been considered for phobos though this origin was already proposed for the formation of 433 eros grooves buczkowski d l barnouin jha o s prockter l m 2008 icarus 193 39 if this idea holds true the observed groove distribution could be explained as the result of possible major impacts on the larger parent body which were inherited by the phobos shard

a first look at the distribution of the stable isotopes of silicon in natural waters the first data on the distribution of the stable isotopes of silicon in marine and freshwater systems are reported both marine and riverine 30si are more positive than 30si of igneous rocks suggesting isotopic fractionation during weathering and clay formation and or biomineralization the 30si value for dissolved silicic acid from several ocean basins is 1 1 0 3 n 69 more positive values are found in surface waters due to discrimination against 30si during biogenic opal formation by diatoms the deep pacific ocean is depleted in 30si relative to the deep atlantic by roughly 0 4 30si of riverine dissolved silicon averages 0 8 0 3 n 8 the biologic fractionation of silicon isotopes coupled with effects associated with thermohaline circulation appears to play a significant role in controlling the distribution of silicon isotopes in the sea the average 30si value of inputs of silicon to the ocean most likely falls between 0 3 and 0 9 and that of the outputs falls between 0 9 to 1 9 probably in the neighborhood of 1 0

the geology of the marcia quadrangle of asteroid vesta assessing the effects of large young craters abstract we used dawn spacecraft data to identify and delineate geological units and landforms in the marcia quadrangle of vesta as a means to assess the role of the large relatively young impact craters marcia 63 km diam and calpurnia 53 km diam and their surrounding ejecta field on the local geology we also investigated a local topographic high with a dark rayed crater named aricia tholus and the impact crater octavia that is surrounded by a distinctive diffuse mantle crater counts and stratigraphic relations suggest that marcia is the youngest large crater on vesta in which a putative impact melt on the crater floor ranges in age between 40 and 60 ma depending upon choice of chronology system and marcia s ejecta blanket ranges in age between 120 and 390 ma depending upon choice of chronology system we interpret the geologic units in and around marcia crater to mark a major vestan time stratigraphic event and that the marcia formation is one of the geologically youngest formations on vesta marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor the smooth unit we interpret as evidence of flow of impact melts and for the pitted terrain release of volatiles during or after the impact process the distinctive dark ejecta surrounding craters marcia and calpurnia is enriched in oh or h bearing phases and has a variable morphology suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite rich material aricia tholus which was originally interpreted as a putative vestan volcanic edifice based on lower resolution observations appears to be a fragment of an ancient impact basin rim topped by a dark rayed impact crater octavia crater has a cratering model formation age of 280 990 ma based on counts of its ejecta field depending upon choice of chronology system and its ejecta field is the second oldest unit in this quadrangle the relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older rheasilvia and veneneia impact basins and vesta s ancient crust preserved on vestalia terra

combining multi scale geophysical techniques for robust hydro structural characterisation in catchments underlain by hard rock in post glacial regions summary accurate conceptual models of groundwater systems are essential for correct interpretation of monitoring data in catchment studies in surface water dominated hard rock regions modern ground and surface water monitoring programmes often have very high resolution chemical meteorological and hydrological observations but lack an equivalent emphasis on the subsurface environment the properties of which exert a strong control on flow pathways and interactions with surface waters the reasons for this disparity are the complexity of the system and the difficulty in accurately characterising the subsurface except locally at outcrops or in boreholes this is particularly the case in maritime north western europe where a legacy of glacial activity combined with large areas underlain by heterogeneous igneous and metamorphic bedrock make the structure and weathering of bedrock difficult to map or model traditional approaches which seek to extrapolate information from borehole to field scale are of limited application in these environments due to the high degree of spatial heterogeneity here we apply an integrative and multi scale approach optimising and combining standard geophysical techniques to generate a three dimensional geological conceptual model of the subsurface in a catchment in ne ireland available airborne lidar electromagnetic and magnetic data sets were analysed for the region at field scale surface geophysical methods including electrical resistivity tomography seismic refraction ground penetrating radar and magnetic surveys were used and combined with field mapping of outcrops and borehole testing the study demonstrates how combined interpretation of multiple methods at a range of scales produces robust three dimensional conceptual models and a stronger basis for interpreting groundwater and surface water monitoring data

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

plant induced weathering of a basaltic rock experimental evidence abstract the active role of higher plants in the weathering of silicate minerals and rocks is still a question for debate the present work aimed at providing experimental evidence of the important role of a range of crop plants in such processes in order to quantitatively assess the possible effect of these diverse plant species on the weathering of a basaltic rock two laboratory experiments were carried out at room temperature these compared the amounts of elements released from basalt when leached with a dilute salt solution in the presence or absence of crop plants grown for up to 36 days for si ca mg and na plants resulted in an increase in the release rate by a factor ranging from 1 to 5 in most cases ca and na seemed to be preferentially released relative to other elements suggesting that plagioclase dissolved faster than the other constituents of the studied basalt negligible amounts of fe were released in the absence of plants as a consequence of the neutral ph and atmospheric po2 that were maintained in the leaching solution however the amounts of fe released from basalt in the presence of plants were up to 100 to 500 fold larger than in the absence of plants for banana and maize the kinetics of dissolution of basalt in the absence of plants showed a constantly decreasing release rate over the whole duration of the experiment 36 days no steady state value was reached both in the absence and presence of banana plants however in the latter case the rates remained at a high initial level over a longer period of time up to 15 days before starting to decrease for fe the maximum rate of release was reached beyond 4 days and this rate remained high up to 22 days of growth of banana the possible mechanisms responsible for this enhanced release of elements from basalt in the presence of plants are discussed although these mechanisms need to be elucidated the present results clearly show that higher plants can considerably affect the kinetics of dissolution of basalt rock therefore they need to be taken into account when assessing the biogeochemical cycles of elements that are major nutrients for plants such as ca mg and k but also micronutrients such as fe and nonessential elements such as si and na

composition of the northern regions of vesta analyzed by the dawn mission abstract the surface composition of the northern regions of vesta observed by the dawn spacecraft offers the possibility to test several hypotheses related to impact related processes we used mostly imaging spectrometry in the visible and near infrared to assess the distribution of mafic lithologies hydrated components and albedo properties and use the link with howardite eucrite and diogenite meteorites heds to investigate the origin of those materials we established that rheasilvia ejecta reached part of the northern regions and have a diogenitic rich composition characteristic of the lower crust investigations of the antipodes of the two major impact basins rheasilvia and veneneia did not reveal any correlation between geographic location geological features and the surface composition the northern wall of mamilia crater which is one of the freshest craters above 22 n contains relatively pure eucritic rich diogenitic rich and dark hydrated materials which are representative of the rest of the northern regions and most of vesta with the exception of an olivine like component found in bellicia crater by ammannito et al ammannito e et al 2013a nature 504 7478 122 125 we determined that similar types of materials are found in various proportions over a large region including bellicia arruntia and pomponia craters and their origin does not seem to be related to rheasilvia ejecta these materials are hydrated which could indicate an exogenous origin and not as dark as expected for carbonaceous chondrites which likely compose the majority of dark hydrated materials on vesta spectral mixture analysis reveals that mixtures of pyroxenes hypersthene pigeonite and diopside could offer an alternative interpretation to olivine in this area

compositional evidence of magmatic activity on vesta like the earth and other terrestrial planets the asteroid vesta has a basaltic crust and a large iron core its surface is dominated by impact craters of all sizes and has tectonic features the presence of basaltic compositions olivine howardite eucrite diogenite meteorites and models of vesta s formation suggests that volcanic and or magmatic activity could have occurred on vesta a global search for lobate structures did not find unequivocal evidence of volcanic features nevertheless several morphological properties of brumalia tholus on vestalia terra suggest that this topographic high most likely formed as a magmatic intrusion the presence of more orthopyroxene rich material relative to surrounding terrain in the ejecta of teia a fresh impact crater on the northern face of brumalia tholus supports the hypothesis of magmatic intrusions on vesta

mass balance modeling of mineral weathering rates and co2 consumption in the forested metabasaltic hauver branch watershed catoctin mountain maryland usa mineral weathering rates and a forest macronutrient uptake stoichiometry were determined for the forested metabasaltic hauver branch watershed in north central maryland usa previous studies of hauver branch have had an insufficient number of analytes to permit determination of rates of all the minerals involved in chemical weathering including biomass more equations in the mass balance matrix were added using existing mineralogic information the stoichiometry of a deciduous biomass term was determined using multi year weekly to biweekly stream water chemistry for a nearby watershed which drains relatively unreactive quartzite bedrock at hauver branch calcite hosts 38 mol of the calcium ion ca2 contained in weathering minerals but its weathering provides 90 of the stream water ca2 this occurs in a landscape with a regolith residence time of more than several ka kiloannum previous studies indicate that such old regolith does not typically contain dissolving calcite that affects stream ca2 na ratios the relatively high calcite dissolution rate likely reflects dissolution of calcite in fractures of the deep critical zone of the carbon dioxide co2 consumed by mineral weathering calcite is responsible for approximately 27 with the silicate weathering consumption rate far exceeding that of the global average the chemical weathering of mafic terrains in decaying orogens thus may be capable of influencing global geochemical cycles and therefore climate on geological timescales based on carbon balance calculations atmospheric derived sulfuric acid is responsible for approximately 22 of the mineral weathering occurring in the watershed our results suggest that rising air temperatures driven by global warming and resulting in higher precipitation will cause the rate of chemical weathering in the hauver branch watershed to increase until a threshold temperature is reached beyond the threshold temperature increased recharge would produce a shallower groundwater table and reduced chemical weathering rates copyright 2013 john wiley sons ltd

olivine or impact melt nature of the orange material on vesta from dawn abstract nasa s dawn mission observed a great variety of colored terrains on asteroid 4 vesta during its survey with the framing camera fc here we present a detailed study of the orange material on vesta which was first observed in color ratio images obtained by the fc and presents a red spectral slope the orange material deposits can be classified into three types a diffuse ejecta deposited by recent medium size impact craters such as oppia b lobate patches with well defined edges nicknamed pumpkin patches and c ejecta rays from fresh looking impact craters the location of the orange diffuse ejecta from oppia corresponds to the olivine spot nicknamed leslie feature first identified by gaffey gaffey m j 1997 icarus 127 130 157 from ground based spectral observations the distribution of the orange material in the fc mosaic is concentrated on the equatorial region and almost exclusively outside the rheasilvia basin our in depth analysis of the composition of this material uses complementary observations from fc the visible and infrared spectrometer vir and the gamma ray and neutron detector grand several possible options for the composition of the orange material are investigated including cumulate eucrite layer exposed during impact metal delivered by impactor olivine orthopyroxene mixture and impact melt based on our analysis the orange material on vesta is unlikely to be metal or olivine originally proposed by gaffey gaffey m j 1997 icarus 127 130 157 analysis of the elemental composition of oppia ejecta blanket with grand suggests that its orange material has 25 cumulate eucrite component in a howarditic mixture whereas two other craters with orange material in their ejecta octavia and arruntia show no sign of cumulate eucrites morphology and topography of the orange material in oppia and octavia ejecta and orange patches suggests an impact melt origin a majority of the orange patches appear to be related to the formation of the rheasilvia basin combining the interpretations from the topography geomorphology color and spectral parameters and elemental abundances the most probable analog for the orange material on vesta is impact melt

a unified nomenclature for tectonic structures on the surface of enceladus abstract enceladus has experienced widespread and diverse tectonic deformation the diversity and nonuniform distribution of structures visible on the surface attests to a complicated and perhaps long lived tectonic history currently no fundamental classification scheme based on morphology exists for the tectonic structures on enceladus which limits analysis and discussion of tectonic structures and regional and global tectonic histories characterization and classification of structures on enceladus is an important first step in understanding its tectonic history here we propose a global morphologic i e nongenetic classification scheme for tectonic structures on enceladus five classes of tectonic structures on enceladus have been identified based on images and limited topographic data troughs scarps chasmata ridges and bands morphological variation of structures exists within these classes reinforcing the complicated tectonic history of this small body

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

geomorphological evidence for transient water flow on vesta abstract vesta the second most massive asteroid has long been perceived as anhydrous recent studies suggesting the presence of hydrated minerals and past subsurface water have challenged this long standing perception yet direct geologic indications of water activity on vesta s surface were unexpected herein we show evidence that transient water flowed on the surface in a debris flow like process and left distinctive geomorphologic features based on detailed analysis of highest resolution 20 m pixel images obtained by the dawn spacecraft we identify a class of locally occurring interconnected and curvilinear gully networks on the walls of young

terrestrial paleorecords of ge si cycling derived from lake diatoms abstract modern river studies support the generalization that surface waters in regions undergoing extensive chemical weathering have elevated dissolved germanium silicon ge si ratios compared to regions with less extensive chemical weathering thought to be the result of ge fractionation in 2 1 clays temporal variations in ge si observed in marine diatoms have thus been linked to past global trends in terrestrial weathering however this relationship has not been adequately ground truthed by a terrestrial based paleoclimate study utilizing ge si ratios in lake diatoms to this end a sediment core was extracted from dry lake el 2763 m in the headwaters of the santa ana river of southern california in july 1996 the dry lake drainage basin is comprised of biotite muscovite gneiss and granite with sparse pine forests and relatively high relief the core had a basal ams 14c age of 8 350 60 ybp we successfully separated diatom samples large enough for cleaning dissolution and chemical analysis without contamination by detrital materials confirmed by trace metal analyses two dissolved opal samples were analyzed for ge and si concentrations along with modern water samples collected from waters within the drainage basin diatoms obtained from 8000 year old sediments near the bottom of the core had an opal ge si of 0 79 10 6 mol mol a composite sample of diatoms deposited within the past 100 years yielded a significantly lower ge si of 0 34 10 6 analysis of ge si was also performed from a variety of other materials in the dry lake basin including stream and lake water unweathered bedrock soils and mineral separates together with sedimentologic records from the lake sediments and ge si recorded in other basin materials it appears that the high ge si values recorded in the 8000 year opal sample were the result of preferential early weathering of high ge si biotite and muscovite minerals from the slopes thus the detailed examination of ge si cycling in this isolated basin indicates that factors other than clay mineral transformations may drive ge si paleorecords in some settings

gravity field expansion in ellipsoidal harmonic and polyhedral internal representations applied to vesta abstract a 20th degree ellipsoidal harmonic gravity field of vesta is determined by processing radiometric doppler and range data from the dawn mission the gravity field shows sensitivity up to degree 18 and the coefficients are globally determined on average to degree 15 gravity anomalies are mapped to the brillouin ellipsoid 304 289 247 km which is a substantially closer fit to the surface than the reference ellipsoid 290 290 265 km used to map the conventional spherical harmonic series especially near the poles two models of internal structure are subsequently explored in which density variations are permitted in the uppermost layer i e crust in order to explain vesta s local gravitational signature these models include the case of a two layer model with an average crustal thickness of 55 5 km and a three layer model with an average crustal thickness of 22 4 km for both two layer and three layer scenarios the bouguer gravity anomaly is minimized for a crustal density of 2970 kg m 3 the remaining bouguer anomalies can be explained by lateral crustal density variation of 2310 3440 kg m 3 and 2660 3240 kg m 3 for the 22 4 km and 55 5 km crustal thickness models respectively the general trend of the estimated lateral crustal densities for the two cases is very similar with a wider range for the 22 4 km case due to a thinner crust this indicates that a thick crust e g 55 5 km would be more favorable for minimizing the range of lateral crustal density variations consideration of independent geochemical and petrological constraints suggests that a three layer model is a more appropriate representation of vesta s internal structure despite the fact that two layer models provide a satisfactory fit to gravity data alone in detail it is found that densities derived from gravity data assuming three layer models and those derived from the howardite eucrite diogenite meteorites and estimates of plausible bulk vesta composition show an excellent level of mutual consistency

basins fractures and volcanoes global cartography and topography of pluto from new horizons abstract the 2015 new horizons flyby has produced the first high resolution maps of morphology and topography of pluto and charon the most distant objects so mapped global integrated mosaics of pluto were produced using both lorri framing camera and mvic line scan camera data showing the best resolution data obtained for all areas of the illuminated surface 78 of the body a unique feature of the pluto imaging data set is the observation of terrains illuminated only by light scattered from atmospheric haze allowing us to map terrains in the southern hemisphere that would otherwise have been in darkness mvic 4 color data were combined with the panchromatic map to produce full color global maps digital elevation models dems over 42 of pluto were produced using combinations of mvic hemispheric scans and lorri mosaics from which slopes at scales of 1 km can be determined pluto can be divided into regions each with distinct topographic signatures corresponding with major physiographic terrain types large areas of pluto are comprised of low relief moderately cratered plains units deeply pitted and glaciated plains east of sputnik planitia are elevated 0 7 km the most dominant topographic feature on pluto is the 1200 by 2000 km wide depression enclosing the bright sputnik planitia ice sheet the surface of which is 2 5 to 3 5 km deep relative to the rim and 2 km deep relative to the mean radius the partial ring of steep sided massifs several of which are more than 5 km high along the western margins of sputnik planitia produce some of the locally highest and steepest relief on pluto with slopes of 40 50 the second major topographic feature is a complex eroded ridge trough system 300 400 km wide and at least 3200 km long extending north to south along the 155 meridian this enormous structure has several kilometers of relief it may predate the large impact event forming the basin though some post sputnik planitia deformation is evident the large depressed partially walled plain hyecho palus lies due southwest of sputnik planitia near the center of hyecho palus lie the circular constructional edifices wright and piccard montes wright mons rises 4 5 km above these plains with a central depression 4 5 km deep whereas piccard mons best observed in haze light rises 5 5 km above the plains but has a bowl shaped central depression 5 5 km below the plains for a total relief of up to 11 km the greatest observed on pluto both of these features are interpreted as constructional volcanic in nature additional prominent topographic features include a 2 3 km high and 600 km wide dome centered on the illuminated iau pole and the amoeboidal plateaus of bladed terrains in the equatorial region which rise 2 5 km above local terrains and are the highest standing geologic units on the encounter hemisphere the mean elevations in the integrated dem for the two radio occultation areas are consistent with the 5 6 km difference in elevation as determined independently by the radio experiment and a limb profile near the egress point confirms the presence of elevated bladed terrains in that area local relief of 3 5 km at massifs troughs and pits supports conclusions that the icy shell of pluto is relatively rigid numerous examples of topographic control of ice or frost deposition occur across pluto including the distinct coloration of the polar dome the elevated terrains of eastern tombaugh regio and along the ridge trough system where ridge tops and fossae rims are covered in different ices than at lower elevations the topographic hypsogram of pluto s encounter hemisphere is strongly bimodal due to the large sputnik planitia depression otherwise the topographic signature of pluto is controlled by deviations from the otherwise dominant low plains including elevated bladed terrain plateaus and the depressed volcanic province including wright and piccard montes

weathering plants and the long term carbon cycle the chemical weathering of ca mg silicate rocks is the principal process whereby co2 is removed from the atmosphere on long or multimillion year time scales at present vascular land plants exert an important influence on weathering by recycling water and by bringing about the secretion of acids and the buildup of high levels of co2 in soils before the spread of vascular plants on the continents during the mid paleozoic chemical weathering must have been achieved by higher levels of atmospheric co2 and or by the intercession of primitive land biota such as lichens and cyanobacteria from examination of several pre vascular weathering scenarios it is concluded that in order to prohibit unreasonably high levels of atmospheric co2 weathering during the precambrian and early paleozoic must have taken place under essentially closed system abiotic conditions linear feedback or via strong regulation by primitive biota preliminary examination of modern weathering by lichens over tens to thousands of years militates against the latter possibility however much more work needs to be done on the quantitative effect of both primitive biota and vascular plants on the rate of weathering and their response to changes in atmospheric co2 before anything more definitive can be said

silica cycling over geologic time the earth s long term silica cycle is intimately linked to weathering rates and biogenic uptake changes in weathering rates and the retention of silica on land have altered silica availability in the oceans for hundreds of millions of years

the effect of land plants on weathering rates of silicate minerals abstract land plants and their associated microbiota directly affect silicate mineral weathering in several ways by generation of chelating ligands by modifying ph through production of co2 or organic acids and by altering the physical properties of a soil particularly the exposed surface areas of minerals and the residence time of water in laboratory experiments far from equilibrium 1 mm oxalate a strong chelator of al has a negligible effect on the dissolution rate of alkali feldspars but some effect on calcic feldspars and olivine by analogy to oxalate the overall effect of organic ligands on the weathering rate of silicate minerals in nature is likely to be small except perhaps in microenvironments adjacent to roots and fungal hyphae the effect of ph on silicate mineral dissolution rate depends on ph below ph 4 5 the rate increases with decreasing ph in the circumneutral region the rate is ph independent and at ph values above around 8 the rate increases with increasing ph vegetation should thus cause an increase in weathering rate through the ph effect only where the ph is below 4 5 as an overall generalization the effect of plants on weathering rate through changes in soil solution chemistry is probably small for granitic rocks it may be greater for more mafic rocks it is the release of ca and mg from mafic rocks that has the greatest influence on the global co2 budget the effect of changes in soil physical properties on weathering rate can be major by binding fine particles plants can greatly increase weathering rates in areas of high physical erosion where erosion rates are lower the effect of plants is less clear on long timescales plants may decrease chemical weathering by binding secondary products and isolating unweathered minerals from meteoric water a major unknown in estimating the effect of the advent of land plants on weathering rates is the nature thickness particle size distribution permeability of the regolith on the pre silurian continents the indirect effect of vegetation through changing the regional distribution of precipitation may be as important as the direct effects

antipodal terrains created by the rheasilvia basin forming impact on asteroid 4 vesta 1 the rheasilvia impact on asteroid 4 vesta may have been sufficiently large to create disrupted terrains at the impact antipode this paper investigates the amount of deformation expected at the rheasilvia antipode using numerical models of sufficient resolution to directly observe terrain modification and material displacements following the arrival of impact stresses we find that the magnitude and mode of deformation expected at the impact antipode is strongly dependent on both the sound speed and porosity of vesta s mantle as well as the strength of the vestan core in the case of low mantle porosities and high core strengths we predict the existence of a topographic high a peak caused by the collection of spalled and uplifted material at the antipode observations by nasa s dawn spacecraft cannot provide definite evidence that large amounts of deformation occurred at the rheasilvia antipode largely due to the presence of younger large impact craters in the region however a deficiency of small craters near the antipodal point suggests that some degree of deformation did occur

detections and geologic context of local enrichments in olivine on vesta with vir dawn data the magmatism characterizing the early history of the asteroid vesta has long been investigated with the mafic and ultramafic meteorites howardite eucrite and diogenite hed the lack of geologic context for the meteorites however has limited its understanding here we use the visible to near ir vir orbital observations of vesta s surface to detect relative enrichments in olivine and to study the associated geologic features because the near ir signature of olivine on vesta s surface is subtle relative to the widespread pyroxene absorption bands a method was developed to distinguish olivine enrichments from admixture of pyroxenes with high fe2 m1 dark material and potential fe bearing glass relative enrichment of olivine 50 60 vol is found in 2 5 km wide morphologically fresh areas our global survey reveals a dozen of these areas clustering in the eastern hemisphere of vesta the hemispherical coincidence with a widespread low enrichment in diogenite like pyroxene suggests the presence of a distinct compositional terrain on the central mound of the rheasilvia impact basin no olivine enrichment was found suggesting the absence of an olivine dominated mantle above the basin s excavation depth or alternatively a low amount of olivine homogeneously mixed with diogenite like pyroxenes rare olivine enriched areas in close proximity to diogenite like pyroxene are found as part of material ejected by the rheasilvia impact such cooccurrence is reminiscent of local ultramafic lithologies within the crust the possible formation of such lithologies on vesta is supported by some hed meteorites dominated by olivine and orthopyroxene

geologic map of the northern hemisphere of vesta based on dawn framing camera fc images abstract the dawn framing camera fc has imaged the northern hemisphere of the asteroid 4 vesta at high spatial resolution and coverage this study represents the first investigation of the overall geology of the northern hemisphere 22 90 n quadrangles av 1 2 3 4 and 5 using these unique dawn mission observations we have compiled a morphologic map and performed crater size frequency distribution csfd measurements to date the geologic units the hemisphere is characterized by a heavily cratered surface with a few highly subdued basins up to 200 km in diameter the most widespread unit is a plateau cratered highland unit similar to although of lower elevation than the equatorial vestalia terra plateau large scale troughs and ridges have regionally affected the surface between 180 e and 270 e these tectonic features are well developed and related to the south pole veneneia impact saturnalia fossae trough unit elsewhere on the hemisphere they are rare and subdued saturnalia fossae cratered unit in these pre rheasilvia units we observed an unexpectedly high frequency of impact craters up to 10 km in diameter whose formation could in part be related to the rheasilvia basin forming event the rheasilvia impact has potentially affected the northern hemisphere also with s n small scale lineations but without covering it with an ejecta blanket post rheasilvia impact craters are small

biogenic silica a neglected component of the coupled global continental biogeochemical cycles of carbon and silicon on geological time scales 106 years the global geochemical cycles of carbon and silicon are coupled by the drawdown of atmospheric co2 through chemical weathering of ca and mg silicate minerals in continental rocks rivers transport the soluble products of weathering cations alkalinity and silicic acid to the oceans where they are utilized by marine ecosystems on decadal to glacial interglacial time scales however large biotic fluxes and storages of si within terrestrial and freshwater ecosystems need to be taken into account recent studies have emphasized the importance of si accumulating plants which deposit significant amounts of amorphous hydrated silica in their tissues as opal phytoliths these include grasses sedges palms some temperate deciduous trees and conifers and many tropical hardwoods landscapes dominated by accumulator plants such as tropical rainforests grasslands herbaceous wetlands and bamboo forests act as silica factories important silica bioengineers in freshwater ecosystems comprise diatoms sponges and chrysophytes this paper reviews the biological role of si in higher plants the impact of vegetation on rates of chemical weathering the fluxes of si through catchment ecosystems lakes and rivers and the potential contribution of new geochemical and isotopic tracers to si biogeochemistry multiproxy investigations of lake sediments will provide novel insights into past variations in si biocycling from terrestrial to aquatic realms on 10 106 year time scales copyright 2008 john wiley sons ltd

orbital forcing of glacial interglacial variations in chemical weathering and silicon cycling within the upper white nile basin east africa stable isotope and biomarker evidence from lakes victoria and edward on quaternary time scales the global biogeochemical cycle of silicon is interlocked with the carbon cycle through biotic enhancement of silicate weathering and uptake of dissolved silica by vascular plants and aquatic microalgae notably diatoms for which si is an essential nutrient large tropical river systems dominate the export of si from the continents to the oceans here we investigate variations in si cycling in the upper white nile basin over the last 15 ka using sediment cores from lakes victoria and edward coupled measurements of stable o and si isotopes on diatom separates were used to reconstruct past changes in lake hydrology and si cycling while the abundances of lipid biomarkers characteristic of terrestrial emergent higher plants submerged floating aquatic macrophytes and freshwater algae document past ecosystem changes during the late glacial to mid holocene 15 5 5 ka bp orbital forcing greatly enhanced monsoon rainfall forest cover and chemical weathering riverine inputs of dissolved silica from the lake catchments exceeded aquatic demand and may also have had lower si isotope values since 5 5 ka bp increasingly dry climates and more open vegetation reinforced by the spread of agricultural cropland over the last 3 4 ka have reduced dissolved silica inputs into the lakes centennial to millennial scale dry episodes are also evident in the isotopic records and merit further investigation

mineralogical and spectral analysis of vesta s gegania and lucaria quadrangles and comparative analysis of their key features abstract this work is aimed at developing and interpreting infrared albedo pyroxene and oh band depths and pyroxene band center maps of vesta s gegania and lucaria quadrangles obtained from data provided by the visible and infrared vir mapper spectrometer on board nasa s dawn spacecraft the gegania and lucaria quadrangles span latitudes from 22 s to 22 n and longitudes from 0 e to 144 e the mineralogical and spectral maps identify two large scale units on this area of vesta which extend eastwards and westward of about 60 e respectively the two regions are not associated to large scale geological units which have a latitudinal distribution rather than longitudinal but are defined by different contents of carbonaceous chondrites cc the eastern region poor in ccs is brighter and oh depleted whereas the western one rich in ccs is darker and oh enriched a detailed analysis of the small scale units in these quadrangles is also performed almost all the units show the typical correspondence between high albedo deep pyroxene bands short band centers and absence of oh and vice versa only a few exceptions occur such as the ejecta from the aelia crater where dark and bright materials are intimately mixed the most characteristic features of these quadrangles are the equatorial troughs and the lucaria tholus the equatorial troughs consist of graben i e a depression limited by two conjugate faults the graben do not present their own spectral signatures but spectral parameters similar to their surroundings in agreement to their structural origin this is observed also in graben outside the gegania and lucaria quadrangles however it is possible to observe other structural features such as tectonic grooves characterized by a changing composition and hence an albedo variation this result is confirmed not only by mineralogical maps of vesta but also by analyzing the virtis rosetta observations of lutetia the albedo change is instead a typical behavior of geomorphic grooves finally ridges are characterized by a bluer color and in some cases shorter band centers than their surroundings suggesting that they are composed of fresher materials we also performed a comparative analysis between the three tholi of vesta i e lucaria which gives the name to its quadrangle aricia in the marcia quadrangle and brumalia numisia quadrangle whereas brumalia tholus is a young magmatic intrusion the absence of diogenites the low albedo and the orientation of aricia and lucaria tholi suggest that they are older features which are covered by dark materials and therefore experienced a different geological history than brumalia

historical adjustments by walker river to lake level fall over a tectonically tilted half graben floor walker lake basin nevada abstract historical records of lake and river adjustments in the tectonically active north south elongated walker lake extensional basin of west central nevada provide important insight to the style and rate that rivers react to tectonic tilt in half grabens the northern part of this basin historically containing walker lake is now occupied by the south flowing walker river walker lake presently restricted to the central basin sector is a perennial water body 30 m deep sustained by and forming the terminus of walker river both the river and lake have a strongly asymmetric distribution located preferentially near the active walker lake fault bounding the western basin margin walker lake has withdrawn from the northern basin sector since 1882 due to a 45 m drop in lake level caused by human diversion of the river upstream this withdrawal has forced walker river to incrementally lengthen and to sequentially reposition its delta along the retreating northern lake margin twelve deltas have been deposited since 1882 in response to these changes the initiation of eight of the twelve post 1882 deltas was associated with channel avulsion and four with channel lengthening ten of the twelve channel and delta relocation events entailed significant westward lateral movements toward the walker lake fault illustrating the dominating influence of the basin floor tectonic tilt on river adjustments the two eastward shifts were triggered intrinsically by avulsions resulting from erosion of the outer bank of channel meanders the three phases of progressive westward shifting of the river towards the walker lake fault occurred over periods of 18 6 and 16 years respectively documenting the rapid response time of walker river to basin floor tectonic tilt despite the countering effects of the intrinsic avulsions and hindrances to lateral migration caused by the muddy substrate of the former lake bottom deposits within which the river is incised

biogenic silica in wetlands and their relationship with soil and groundwater biogeochemistry in the southeastern of buenos aires province argentina although phytoliths constitute part of the wetland suspended load there are few studies focused on the quantification of them in the biogenic silica bsi pool so the aim of this paper is both to determine bsi content diatoms and phytoliths and its relationship with dissolved silica in surface waters and the influence of soil and groundwater si biogeochemistry in los padres wetland buenos aires province argentina in the basin of the los padres wetland dissolved silica dsi concentration is near 840 232 mol l and 211 83 275 92 mol l in groundwaters and surface waters respectively bsi represents an 5 6 22 1 of the total suspension material and 8 34 of the total mineralogical components of the wetland bottom sediments dsi and bsi vary seasonally with highest bsi content diatoms specifically during the spring summer in correlation to the lowest dsi concentration dsi 660 917 5 mol l and phytolith 3 35 5 84 concentrations in the inflow stream are higher than in the wetland and its outflow stream 19 1 113 mol l 0 45 3 2 respectively probably due to the high phytolith content in soils the high silica concentration in the soil solution and the groundwater inflow diatom content 5 16 8 in the wetland and its outflow stream is higher than in the inflow stream 0 45 1 97 controlling dsi in this system the understanding of the groundwater surface water interaction in an area is a significant element for determining the different components and the role that they play on the local biogeochemical cycle of si

harmonic and statistical analyses of the gravity and topography of vesta abstract we examine the gravity and topography of the asteroid 4 vesta as recently revealed by the dawn mission the observed gravity is highly correlated with the observed topography and suggests little lateral variation in density the variance spectra of both gravity and topography follow power laws which are very similar to those seen for the moon mars venus and earth a significant way in which vesta differs from these larger silicate bodies is that both gravity and topography are significantly anisotropic with more north south variation than east west variation rapid rotation plausibly contributes to this anisotropy but only at harmonic degree two the remainder of the anisotropy appears related to the large impacts which formed the rheasilvia and veneneia basins we note that as usual gravitational inverse problems are non unique while the observed gravity and topography of vesta do not preclude existence of a metallic core they certainly do not require it

geomorphology and structural geology of saturnalia fossae and adjacent structures in the northern hemisphere of vesta vesta is a unique intermediate class of rocky body in the solar system between terrestrial planets and small asteroids because of its size average radius of 263 km and differentiation with a crust mantle and core vesta s low surface gravity 0 25 m s2 has led to the continual absence of a protective atmosphere and consequently impact cratering and impact related processes are prevalent previous work has shown that the formation of the rheasilvia impact basin induced the equatorial divalia fossae whereas the formation of the veneneia impact basin induced the northern saturnalia fossae expanding upon this earlier work we conducted photogeologic mapping of the saturnalia fossae adjacent structures and geomorphic units in two of vesta s northern quadrangles caparronia and domitia our work indicates that impact processes created and or modified all mapped structures and geomorphic units the mapped units ordered from oldest to youngest age based mainly on cross cutting relationships are 1 vestalia terra unit 2 cratered highlands unit 3 saturnalia fossae trough unit 4 saturnalia fossae cratered unit 5 undifferentiated ejecta unit 6 dark lobate unit 7 dark crater ray unit and 8 lobate crater unit the saturnalia fossae consist of five separate structures saturnalia fossa a is the largest maximum width of 43 km and is interpreted as a graben whereas saturnalia fossa b e are smaller maximum width of 15 km and are interpreted as half grabens formed by synthetic faults smaller second order structures maximum width of 1 km are distinguished from the saturnalia fossae a first order structure by the use of the general descriptive term adjacent structures which encompasses minor ridges grooves and crater chains for classification purposes the general descriptive term minor ridges characterizes ridges that are not part of the saturnalia fossae and are an order of magnitude smaller maximum width of 1 km vs maximum width of 43 km shear deformation resulting from the large scale diameter of 100 km rheasilvia impact is proposed to form minor ridges 2 km to 25 km in length which are interpreted as the surface expression of thrust faults as well as grooves 3 km to 25 km in length and pit crater chains 1 km to 25 km in length which are interpreted as the surface expression of extension fractures and or dilational normal faults secondary crater material ejected from small scale and medium scale impacts diameters of 100 km are interpreted to form ejecta ray systems of grooves and crater chains by bouncing and scouring across the surface furthermore seismic shaking also resulting from small scale and medium scale impacts is interpreted to form minor ridges because seismic shaking induces flow of regolith which subsequently accumulates as minor ridges that are roughly parallel to the regional slope in this work we expand upon the link between impact processes and structural features on vesta by presenting findings of a photogeologic structural mapping study which highlights how impact cratering and impact related processes are expressed on this unique intermediate solar system body

biogenic silica storage in soils the role of plants in soil development has become of crucial interest in quantifying global weathering processes on the volcanic reunion island indian ocean a 15 cm thick phytolith rich horizon biogenic opal a developed at the expense of trachytic ashes between 3820 85 and 335 90 yr b p then 97 138 t km 2 yr of sio 2 were biogeochemically recycled from the weathering of parent rocks through a past forest of bamboos nastus borbonicus this rate is in the same range as the present chemical weathering rates of silica in the area considered to be among the highest in the world these results demonstrate that the storage of biogenic silica in soils may be significant and may retard the output of silica to rivers and ocean

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

plant impact on the biogeochemical cycle of silicon and related weathering processes abstract the contribution of plants to the biogeochemical cycle of si and related weathering processes was studied in an equatorial rainforest ecosystem congo where the biologic turnover of si is high 58 to 76 kg ha y litterfall leaves a soil profile and groundwaters were analysed phytoliths and organic matter have a similar distribution with depth in the soil profile the model of a bicompartmental distribution of organic matter is applied to phytolith distribution and shows that about 92 of the biogenic silica input is rapidly recycled while about 8 of the biogenic silica input supplies a stable pool of phytoliths with a lower turnover reprecipitation of silica was observed at the base of the soil profile indicating a local geochemical environment that is oversaturated with respect to amorphous silica a balance in biogeochemical cycle of si requires that the vegetation absorb dissolved silicon released from weathering of minerals which otherwise would be available for mineral neoformation or export from the profile towards regional drainages plant uptake of si increases the chemical weathering rate without increasing the denudation rate this study shows that the uptake storage and release of si by the vegetation have to be taken into account when using dissolved si for tracing chemical weathering dynamics

implications for the history of cenozoic opal deposition from a quantitative model in addition to being a time of enhanced crustal activity and a warmer climate the eocene also saw prolonged periods of silica accumulation on the ocean floor the peak of this accumulation occurred at approximately 50 ma creating horizon ac a layer of silica rich sediments spanning approximately 4 my horizon ac is comparatively isolated in time from known silicic acid addition processes leading mcgowran 1989 mcgowran b 1989 silica burp in the eocene ocean geology 17 pp 857 860 to suggest a novel mechanism the silica burp hypothesis that centres on the temporal decoupling of silicic acid supply and burial by climatic variation here we examine this hypothesis using a quantitative biogeochemical model of the silicon cycle our results unequivocally show that mcgowran s hypothesis is unable to account for horizon ac the model s residence time and ocean capacity for silicic acid are insufficient to permit the degree of temporal decoupling proposed by mcgowran inverting the model s results suggests that instead the sediment record may be a useful proxy for silicic acid additions to the ocean

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

pit chains on enceladus signal the recent tectonic dissection of the ancient cratered terrains abstract enceladus is the first outer solar system body on which pit chains have been positively identified we map the global distribution of pit chains and show that pit chains are among the youngest tectonic features on enceladus s surface concentrated in the cratered plains centered on enceladus s saturnian and anti saturnian hemispheres pit chains on enceladus are interpreted as the surface expressions of subsurface dilational fractures underlying a cover of unconsolidated material which we infer to be a geologically young cover of loose regolith that mantles the surface of enceladus a widespread layer of regolith may act to insulate the surface which has implications for the thermal state of enceladus s ice shell the widespread distribution of pit chains across the cratered plains indicates that this ancient surface has recently been tectonically active

ge si ratios indicating hydrothermal and sulfide weathering input to rivers of the eastern tibetan plateau and mt baekdu abstract concentrations of dissolved silicon in river waters reflect a complex interplay among chemical weathering of primary silicate minerals formation and weathering of secondary clay minerals hydrothermal input and biological cycling formation and dissolution of opal phytoliths and growth of diatoms we applied the ge si ratio to assess the different sources of dissolved si in rivers hailing from the eastern tibetan plateau the salween mekong chang jiang yangtze hong red and huang he yellow and from mt baekdu the duman elevated riverine ge si ratios were observed in arid regions with high geothermal activity in the salween chang jiang and mt baekdu streams in the huang he and hong river basins geothermal influence was not as pronounced but weathering of sulfide and coal bearing minerals may be responsible for the high ge si ratios in rivers where inputs from hydrothermal and sulfide weathering are minimal our data mostly fall in the weathering limited regime of high riverine si concentrations and low ge si ratios

river fluxes of dissolved silica to the ocean were higher during glacials ge si in diatoms rivers and oceans two centric marine diatom species thalassiosira oceanica and thalassiosira antarctica were grown in batch cultures to determine the incorporation of germanium ge and silicon si into siliceous shells opal the results were modeled as ge si isotope fractionation during exponential growth diatoms take up and incorporate ge si from solution without major discrimination against ge during stationary phase growth near silica limitation the antarctic species t antarctica discriminates slightly against ge but integrated ge si opal produced over the latter portion of the growth cycle is indistinguishable from the initial solution ratio these results confirm experiments using radioactive 68ge that showed absence of fractionation during diatom silica uptake azam and volcani 1981 in contrast to two box ocean models that invoke 50 ge discrimination by diatoms to explain the observed excess surface ocean germanium concentration murnane and stallard 1988 froelich et al 1989 and late pleistocene ocean sediment ge si opal records mortlock et al 1991 runs of a 10 box ocean ge and si model pandora with 50 discrimination reproduce the excess surface ocean ge but introduces curvature into the deep ocean ge versus si relationship that is not observed in the oceans thus 50 fractionation is not supported by either cultures or models if diatoms do not fractionate ge si then late pleistocene ge si opal variations in piston cores are caused not by changes in local biosiliceous production and silica utilization mortlock et al 1991 but rather by whole ocean changes in ge si seawater the marine ge si opal record of the last 450 kyr can be modeled as transient oceanic responses to instantaneous continental climate transitions consistent with the chemical weathering model of murnane and stallard 1990 glacial periods are characterized by lower continental weathering intensity lower ge si riv and two fold higher dissolved silica river fluxes marine ge si opal records thus contain a history of ocean silica chemistry that reflect rapid global changes in continental weathering

large scale troughs on vesta a signature of planetary tectonics abstract images of vesta taken by the dawn spacecraft reveal large scale linear structural features on the surface of the asteroid we evaluate the morphology of the vesta structures to determine what processes caused them to form and what implications this has for the history of vesta as a planetary body the dimensions and shape of these features suggest that they are graben similar to those observed on terrestrial planets not fractures or grooves such as are found on smaller asteroids as graben their vertical displacement versus length relationship could be evaluated to describe and interpret the evolution of the component faults linear structures are commonly observed on smaller asteroids and their formation has been tied to impact events while the orientation of the large scale vesta structures does imply that their formation is related to the impact events that formed the rheasilvia and veneneia basins their size and morphology is greatly different from impact formed fractures on the smaller bodies this is consistent with new analyses that suggest that vesta is fully differentiated with a mantle and core we suggest that impact into a differentiated asteroid such as vesta could result in graben while grooves and fractures would form on undifferentiated asteroids

color and albedo heterogeneity of vesta from dawn multispectral images 0 44 to 0 98 m of asteroid 4 vesta obtained by the dawn framing cameras reveal global color variations that uncover and help understand the north south hemispherical dichotomy the signature of deep lithologies excavated during the formation of the rheasilvia basin on the south pole has been preserved on the surface color variations band depth spectral slope and eucrite diogenite abundance clearly correlate with distinct compositional units vesta displays the greatest variation of geometric albedo 0 10 to 0 67 of any asteroid yet observed four distinct color units are recognized that chronicle processes including impact excavation mass wasting and space weathering that shaped the asteroid s surface vesta s color and photometric diversity are indicative of its status as a preserved differentiated protoplanet

biomineralization and global biogeochemical cycles biological activity is a dominant force shaping the chemical structure and evolution of the earth surface environment the presence of an oxygenated atmosphere hydrosphere surrounding an otherwise highly reducing solid earth is the most striking consequence of the rise of life on earth biological evolution and the functioning of ecosystems in turn are to a large degree conditioned by geophysical and geological processes understanding the interactions between organisms and their abiotic environment and the resulting coupled evolution of the biosphere and geosphere is a central theme of research in biogeology biogeochemists contribute to this understanding by studying the transformations and transport of chemical substrates and products of biological activity in the environment biogeochemical cycles provide a general framework in which geochemists organize their knowledge and interpret their data the cycle of a given element or substance maps out the rates of transformation in and transport fluxes between adjoining environmental reservoirs the temporal and spatial scales of interest dictate the selection of reservoirs and processes included in the cycle typically the need for a detailed representation of biological process rates and ecosystem structure decreases as the spatial and temporal time scales considered increase much progress has been made in the development of global scale models of biogeochemical cycles although these models are based on fairly simple representations of the biosphere and hydrosphere they account for the large scale changes in the composition redox state and biological productivity of the earth surface environment that have occurred over geological time since the cambrian explosion mineralized body parts have been secreted in large quantities by biota because calcium carbonate silica and calcium phosphate are the main mineral phases constituting these hard parts biomineralization plays an important role in the global biogeochemical cycles of carbon calcium silicon and phosphorus the chapter starts with introducing the basic concepts of

chondritic models of 4 vesta implications for geochemical and geophysical properties simple mass balance and thermodynamic constraints are used to illustrate the potential geochemical and geophysical diversity of a fully differentiated vesta sized parent body with a eucrite crust e g core size and density crustal thickness the results of this analysis are then combined with data from the howardite eucrite diogenite hed meteorites and the dawn mission to constrain vesta s bulk composition twelve chondritic compositions are considered comprising seven carbonaceous three ordinary and two enstatite chondrite groups our analysis excludes ci and ll compositions as plausible vesta analogs as these are predicted to have a negative metal fraction second the melts thermodynamic calculator is used to show that the enstatite chondrites the cv ck and l groups cannot produce juvinas like liquids and that even for the other groups depletion in sodium is necessary to produce liquids of appropriate silica content this conclusion is consistent with the documented volatile poor nature of eucrites furthermore carbonaceous chondrites are predicted to have a mantle too rich in olivine to produce typical howardites and to have fe mn ratios generally well in excess of those of the heds on the other hand an na depleted h chondrite bulk composition is capable of producing juvinas like liquids has a mantle rich enough in pyroxene to produce abundant howardite diogenite and has a fe mn ratio compatible with eucrites in addition its predicted bulk silicate density is within 100 kg m 3 of solutions constrained by data of the dawn mission however oxidation state and oxygen isotopes are not perfectly reproduced and it is deduced that bulk vesta may contain approximately 25 of a cm like component values for the bulk silicate composition of vesta and a preliminary phase diagram are proposed

high resolution shape model of ceres from stereophotoclinometry using dawn imaging data abstract we present a high resolution global shape model of ceres determined using the stereophotoclinometry technique developed at the jet propulsion laboratory by processing dawn s framing camera data acquired during approach to post low altitude mapping orbit lamo phases of the mission a total of about 38 000 images were processed with pixel resolutions ranging from 35 6 km pixel to 35 m pixel and the final global shape model was produced with 100 m grid spacing the final spc derived topography was computed relative to the 482 km 482 km 446 km mean ellipsoid which ranges from 7 3 km to 9 5 km for the purpose of validation we performed various error analyses to assess and quantify realistic uncertainties in the derived topography such as dividing the data into different subsets and re computing the entire topography based on these studies we show that the average total height error of the final global topography model is 10 2 m and 88 9 of the surface has the total height error below 20 m we also provide improved estimates of several physical parameters of ceres the resulting gm estimate is 62 62905 0 00035 km3 s2 or the mass value of 938 392 0 005 1018 kg the volume estimate is 434 13 0 50 106 km3 with a volumetric mean radius of 469 72 km combined with the mass estimate the resulting bulk density is 2161 6 2 5 kg m3 other improved parameters include the pole right ascension 0 291 42763 0 0002 pole declination 0 66 76033 0 0002 and prime meridian and rotation rate of w0 170 309 0 011 and dw dt 952 1532635 0 000002 deg day respectively also for geophysical and geological studies we provide spherical harmonic coefficients and a gravitational slope map derived from the global shape model

the chronostratigraphy of protoplanet vesta in this paper we present a time stratigraphic scheme and geologic time scale for the protoplanet vesta based on global geologic mapping and other analyses of nasa dawn spacecraft data complemented by insights gained from laboratory studies of howardite eucrite diogenite hed meteorites and geophysical modeling on the basis of prominent impact structures and their associated deposits we propose a time scale for vesta that consists of four geologic time periods pre veneneian veneneian rheasilvian and marcian the pre veneneian period covers the time from the formation of vesta up to the veneneia impact event from 4 6 ga to 2 1 ga using the asteroid flux derived chronology system or from 4 6 ga to 3 7 ga under the lunar derived chronology system the veneneian period covers the time span between the veneneia and rheasilvia impact events from 2 1 to 1 ga asteroid flux derived chronology or from 3 7 to 3 5 ga lunar derived chronology respectively the rheasilvian period covers the time span between the rheasilvia and marcia impact events and the marcian period covers the time between the marcia impact event until the present the age of the marcia impact is still uncertain but our current best estimates from crater counts of the ejecta blanket suggest an age between 120 and 390 ma depending upon choice of chronology system used regardless the marcia impact represents the youngest major geologic event on vesta our proposed four period geologic time scale for vesta is to a first order comparable to those developed for other airless terrestrial bodies

terrestrial ecosystems and the global biogeochemical silica cycle 1 most research on the global si cycle has focused nearly exclusively on weathering or the oceanic si cycle and has not explored the complexity of the terrestrial biogeochemical cycle the global biogeochemical si cycle is of great interest because of its impact on global co2 concentrations through the combined processes of weathering of silicate minerals and transfer of co2 from the atmosphere to the lithosphere a sizable pool of si is contained as accumulations of amorphous silica or biogenic silica bsi in living tissues of growing plants known as phytoliths and after decomposition of organic material as remains in the soil the annual fixation of phytolith silica ranges from 60 200 tmol yr 1 and rivals that fixed in the oceanic biogeochemical cycle 240 tmol yr 1 internal recycling of the phytolith pool is intense with riverine fluxes of dissolved silicate to the oceans buffered by the terrestrial biogeochemical si cycle challenging the ability of weathering models to predict rates of weathering and consequently changes in global climate consideration must be given to the influence of the terrestrial bsi pool on variations in the global biogeochemical si cycle over geologic time and the influence man has had on modifying both the terrestrial and aquatic biogeochemical cycles