surface wind response to oceanic fronts 1 the response of surface winds to ocean fronts characterized by sharp gradients in both sea surface temperature sst and ocean currents was analyzed using scatterometer nscat and quikscat wind data and gulf stream path positions in conjunction with simulations made with the pennsylvania state university psu national center for atmospheric research ncar mesoscale model mm5 all match ups between each scatterometer pass and the gulf stream path were visually examined and only those for which the wind field was free of atmospheric fronts or large curvature over a reasonably straight segment of the gulf stream were selected ten match ups met these criteria for the period studied from 16 september 1996 to 29 june 1997 for nscat and from 24 july 1999 to 31 december 2000 for quikscat changes in the modeled surface wind field across the front in each of the ten cases agree well with changes in the observed winds our findings suggest that the perturbation pressure gradient resulting from the thermal forcing by the front accounts for the decrease in wind speed when moving from warm to cold water and the increase observed in the converse in the cases examined the adjustment of the surface wind to the front occurred as a result of the vertical motion induced by horizontal divergence convergence and advection in the marine atmospheric boundary layer mabl the dynamical forcing associated with strong surface currents is also shown to modify scatterometer derived winds finally the numerical simulations suggest that the dynamical and thermal effects are very nearly additive

impact crater degradation on venusian plains in venusian plains impact craters without extensive low backscatter ejecta deposits are more likely to have low backscatter floors be embayed by volcanic deposits and exhibit fractures as compared to craters with extensive low backscatter ejecta we interpret these trends as evidence of ongoing degradation of low backscatter ejecta by aeolian activity weathering and volcanism using a crater age sequence based on extent of preservation of low backscatter ejecta together with monte carlo simulations we find that tectonic activity has extended over a longer time period than volcanism

major paleozoic land plant evolutionary bio events abstract adaptive radiations and extinctions affecting tracheophytes the only group of land plants with a well documented fossil record occur during time intervals distinct from those affecting marine invertibrates this distinctive pattern is recognizable at both era and period levels discordance between tracheophyte appearance diversification and extinction and contemporary marine bio events has been reflected in the recognition of planetary floras palaeophytic mesophytic cenophytic the counterparts of marine invertibrate based eras of the planerozoic paleozoic mesozoic cenozoic some have recognized significant levels of land plant evolution epochs within these planetary floras with the discovery in the past 20 years of a major group of middle ordovician lower silurian post algal pre tracheophytic land plants the concept of the palaeophytic and its epochs must be redefined here the redefined palaeophytic extends from the mid permian downward into the mid ordovician and three evolutionary land floras are recognized eoembryophytic mid ordovician to pre latest early silurian eotracheophytic latest early silurian through early gedinnian and eutracheophytic late gedinnian to mid permian the silurian devonian boundary stressed in prevous palaeophytic subdivisions is considered of limited importance as a major benchmark in land plant evolution a newly defined era the epeirophytic a continental counterpart of kryshtofovich s marine thalassophytic is recognized with an upper boundary at the mid ordovician and extending down indefinitely into the precambrian one major evolutionary flora the phycomycophytic first recognized by kryshtofovich for the thalassophytic is recognized for the continental habitat as well

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

lipid biomarker and stable isotopic profiles through early middle ordovician carbonates from spitsbergen norway abstract one of the most dramatic episodes of sustained diversification of marine ecosystems in earth history took place during the early to middle ordovician period changes in climate oceanographic conditions and trophic structure are hypothesised to have been major drivers of these biotic events but relatively little is known about the composition and stability of marine microbial communities controlling biogeochemical cycles at the base of the food chain this study examines well preserved carbonate rich strata spanning the tremadocian through upper dapingian stages from the oslobreen group in spitsbergen norway abundant bacterial lipid markers elevated hopane sterane ratios average 4 8 maximum of 13 1 detection of chlorobi markers in organic rich strata and bulk nitrogen isotopes 15ntotal averaging 0 to 1 for the open marine facies suggest episodes of water column redox stratification and that primary production was likely limited by fixed nitrogen availability in the photic zone near absence of the c30 sterane marine algal biomarker 24 n propylcholestane 24 npc in most samples supports and extends the previously observed hiatus of 24 npc in early paleozoic late cambrian to early silurian marine environments very high abundances of 3 methylhopanes average 9 9 maximum of 16 8 extends this biomarker characteristic to early ordovician strata for the first time and may reflect enhanced and sustained marine methane cycling during this interval of fluctuating climatic and low sulfate marine conditions olenid trilobite fossils are prominent in strata deposited during an interval of marine transgression with biomarker evidence for episodic euxinia anoxia extending into the photic zone of the water column

climatic and oceanic changes during the middle late ordovician transition in the tarim basin nw china and implications for the great ordovician biodiversification event abstract the middle late ordovician transition darriwilian to sandbian age witnessed a major pulse of the great ordovician biodiversification event gobe and distinctive oceanic geochemical fluctuations such as coeval negative c and sr isotope excursions in this study investigations into geochemical variations notably the hg abundance or hg toc have been carried upon the organic rich black shale of the middle upper ordovician saergan formation to unravel the causes of this pulse based on these data three phases were identified phase 1 0 to 3 m is characterized by rising hg toc up to 138 ppb wt and ti al values as well as high ciacorr corrected chemical index of alteration values 68 9 72 3 with negligible enrichment of redox sensitive elements rse and nutrient elements e g u 5 2 ppm v 153 ppm mo 1 8 ppm p2o5 0 2 suggesting intensified volcanism which could have emitted significant amounts of greenhouse gases thereby leading to climate warming in contrast phase 2 3 to 11 m is characterized by decreasing hg toc and ti al ratios relatively low though slightly fluctuating ciacorr values generally depleted in rse except moderately enriched u up to 14 6 ppm and increased p al and ba al ratios implying weakening volcanic activity and subsequent climate cooling and the potential for improved seawater ventilation as a result of oceanic upwelling phase 3 sandbian age 11 13 m witnessed continuous decrease in hg toc ratio an increase in ti al and ciacorr values fairly low values of rse enrichment and p al and ba al ratios indicating recurrent climate warming and the potential for slowed oceanic circulation and attenuated upwelling of nutrient rich deep waters onto the shallow shelf these changes could have diminished bioproductivity and organic output onto the seafloor this study offers insights into volcanic climatic oceanic interactions during a major pulse of the gobe around the middle late ordovician transition while black shales were extensively deposited

surface modification of venus as inferred from magellan observations of plains in sedna planitia clear stratigraphic relations can be discerned among volcanic flow units young flows exhibit sar specific cross section values similar to fresh terrestrial basalt flows whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows total degradation of 1 m depth over 0 6 b y is inferred for the sedna area from radar signatures impact crater abundances and ejecta superposition relations with respect to volcanic flow units analyses of parabolic ejecta deposits associated with the crater stuart imply that the material is typically centimeters in thickness a relatively small fraction 10 of venusian impact craters exhibit prominent parabolic ejecta deposits these craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years the inferred dispersal rate 10 3 m yr is too low to produce the degradation of flows at sedna planitia and it is concluded that the dominant flow modification process is in situ weathering in addition elevation dependent weathering is inferred in western ovda regio where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations the inferred rate of generation of high reflection coefficient materials is no more than 10 2 m yr based on the inability of aeolian activity to cover high reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high reflectivity surfaces at high elevations surface modification rates on venus are orders of magnitude lower than on earth venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on mars but are comparable to the estimated rate of weathering and erosion of martian bedrock low surface modification rates imply that it will be possible to determine regional scale age variations on venus based on the degree of preservation of volcanic landforms and microwave signatures

crater associated dark diffuse features on venus properties of surficial deposits and their evolution 1 in order to assess the nature of crater associated radar dark diffuse features ddfs on venus and to understand their formation and evolution we analyzed magellan radar roughness emissivity and reflectivity data in the vicinity of craters accompanied by these features following others we assumed that ddfs are deposits mantles of ejected material emplaced during formation of the impact crater the majority of radar dark parabolas the youngest ddfs are characterized by a smooth mantle atmosphere interface having low root mean square rms slopes on scales of 1 100 m as derived from magellan altimeter data older ddfs also often have areas with low rms slopes suggesting that the mantle rms slopes can be preserved for geologically long periods of time some parabolas and older ddfs have asymmetric small scale decimeter scale relief that is interpreted to be dunes that formed as a result of eolian processes this implies that the mantle material is mobile and can saltate under the influence of wind action on average aging of these features is accompanied by a decrease of mantle material dielectric permittivity the most efficient mechanism for parabola degradation seems to be the removal of mantle material from the site of initial deposition by subsequent winds we found a few examples of features that could be interpreted to be the result of in situ modification of the primary mantle

searching for travertines calcretes and speleothems in deep time processes appearances predictions and the impact of plants common models for modern calcite precipitation in and around caves soils springs and streams involve co 2 supplied by thick high p co 2 biogenic soils which were probably thin or non existent before vascular plants indeed plant influenced chemical weathering might have caused accelerated terrestrial carbonate production from the devonian onwards however terrestrial carbonates have also been documented from the archaean proterozoic cambrian ordovician and silurian mechanisms which could have caused non marine carbonates to precipitate without organic rich soils are described and some geological events likely to have influenced non marine carbonate precipitation up to the origin of vascular plants are highlighted as organisms have evolved so have the petrographic characteristics of non marine carbonates some examples of this are also given here

biostratigraphic significance of lower paleozoic microfaunas from eastern canada chitinozoan studies recently carried out in the appalachian forelands and st lawrence platform national geoscience mapping program natmap project have confirmed the regional biostratigraphic value of a number of chitinozoan species and led to a better documentation of their stratigraphic and geographic distribution in eastern canada the typical darriwilian microfaunas first described from the table head group of western newfoundland and containing conochitina chydaea are now recognised in the riviere ouelle formation at les mechins gaspe peninsula in the upper ordovician successions of the st lawrence platform at neuville and in the charlevoix area quebec conochitina primitiva is indicative of the multidens pre americanus graptolite zonal range hercochitina duplicitas of the americanus zone and hercochitina spinetum and acanthochitina cancellata characterize the ruedemanni lower spiniferus zonal interval the occurrence of cyathochitina vaurealensis and hercochitina crickmayi in turbidite d

uranium thorium and potassium in the venusian rocks at the landing sites of vega 1 and 2 concentrations of natural radioactive elements in venusian rocks at the landing sites of vega 1 and vega 2 were determined by gamma ray spectrometers on the landers as follows vega 1 k 0 45 0 22 u 0 64 0 47 10 4 th 1 5 1 2 10 4 vega 2 k 0 40 0 20 u 0 68 0 39 10 4 th 2 0 1 0 10 4 the abundances of natural radioactive elements in the investigated rocks are similar to that of basic rocks tholeiitic basalts and gabbros of the earth s crust the same conclusion about the type of the rocks at the landing site of vega 2 was made from the xrf experiment

woce section p1w in the sea of okhotsk 1 oceanographic data description we describe the results of a world ocean circulation experiment woce survey line p1w from the open pacific ocean through the bussol strait and across the sea of okhotsk observations are presented of distributions of temperature and salinity dissolved oxygen and nutrients and some derived quantities discussion centers on the distribution of the sea of okhotsk component of the north pacific intermediate water mass and its properties the sea of okhotsk has been suggested as the only location where deep convection may occur within the north pacific or its marginal seas our analysis suggests that the surface 26 85 has recently been ventilated and arguments might be made to suggest that ventilation has occurred to 27 05 this is discussed here but detailed evidence is covered in a companion paper analyzing cfc observations the okhotsk component of the north pacific intermediate water extends to substantially greater depths and densities and so some other process is needed to create the denser water mass

woce line p1w in the sea of okhotsk 2 cfcs and the formation rate of intermediate water from august 30 to september 21 1993 temperature salinity nutrient and cfc data were collected along a section starting in the pacific ocean near bussol strait in the kuril islands crossing the basin and shelf region of the sea of okhotsk and ending near the coastal town of okhotsk at the northwest corner of the sea of okhotsk world ocean circulation experiment woce line p1w water on the continental shelf had potential density as great as 27 0 with high cfc concentrations and low temperature and salinity the sea of okhotsk intermediate water soiw ranging from 26 8 to 27 4 had uniform water properties along isopycnal surfaces that were distinctly different from either the north pacific intermediate water npiw or the shelf water found at the same density since the soiw was colder and fresher than the npiw we postulate that soiw is formed by sinking plumes and diapycnal mixing shelf derived water sdw as dense as 27 0 is formed on the shelf during the winter by cooling and brine rejection during ice formation prevailing winds move the sdw off the shelf where it sinks to an equilibrium depth and subsequently mixes isopycnally with north pacific water npw flowing into the sea of okhotsk through the kuril islands the observations on the shelf suggest that sdw directly modifies soiw to 27 0 using potential temperature salinity s and cfc data we calibrated a simple box model to determine the input of sdw into the soiw model results required a minimum of 0 9 sv 1 sv 10 6 m 3 s 1 of sdw in the formation of soiw and produced a minimum export of 2 7 sv of soiw from the sea of okhotsk our minimum calculated export of soiw into the north pacific ocean makes the sea of okhotsk a significant source of the salinity minimum present in the npiw below 27 2 the box model failed because the cfc concentrations of the sdw were too high this suggests that sdw directly ventilates the soiw to a maximum of 27 2 the unique properties of the soiw below 27 2 must be the result of diapycnal mixing in the sea of okhotsk and isopycnal mixing with inflowing npw

the penetration of tritium into the tropical pacific abstract the persistence of subsurface tritium maxima coincident with the equatorial currents is used to show that advection along isopycnals by the mean wind driven circulation is the dominant process in the at most 14 year time scale for the penetration of high northern latitude water to the equator above 26 2 sigma theta ventilation of the equatorial pacific thermocline from the north contrasts sharply with the equatorial atlantic thermocline which is ventilated from the south the most striking manifestation of the north pacific circulation is evidenced by a tritium maximum and salinity minimum at the equator between 145 and 125 w located above 25 6 sigma theta it shows that regardless of time of sampling the eastern sol central equator has received the moat high latitude water probably as a consequence of recirculation by the equatorial currents between the same meridians there is a tritium maximum on and north of the equator at the surface which is interpreted as an expression of upwelling

pyroclastic flow deposits on venus as indicators of renewed magmatic activity radar bright deposits on venus that have diffuse margins suggest eruptions that distribute debris over large areas due to ground hugging flows from plume collapse we examine deposits in eastern eistla western eistla phoebe and dione regiones using magellan data and earth based radar maps the radar bright units have no marginal lobes or other features consistent with viscous flow their morphology radar echo strength polarization properties and microwave emissivity are consistent with mantling deposits comprised of few cm or larger clasts this debris traveled downhill up to 100 km on modest slopes and blanketed lava flows and tectonic features to depths of tens of cm to a few meters over areas up to 40x103 km2 there is evidence for ongoing removal and exhumation of previously buried terrain a newly identified occurrence is associated with a ridge belt south of ushas mons we also note radar bright streaks of coarse material west of rona chasma that reflect the last traces of a deposit mobilized by winds from the formation of mirabeau crater if the radar bright units originate by collapse of eruption columns with coarse fragmental material entrained and fluidized by hot gases then their extent suggests large erupted volatile co2 or h2o amounts we propose that these deposits reflect the early stage of renewed magmatic activity with volatile rich disrupted magma escaping through vents in fractured regions of the upper crust rapidly eroding under venus surface conditions or buried by subsequent eruptions these markers of recently renewed activity have disappeared from older regions

first record of rellimia leclercq bonamo aneurophytales from gondwana with comments on the earliest lignophytes

caradocian land plant microfossils from libya the oldest paleontologically well dated spore tetrads and cuti clelike sheets of cells have been recovered from beds of caradoc mid ordovician age from boreholes in the murzuk murzuq and rhadames ghad mis basins of western libya these microfossils together with late llandovery early wenlock vascular plant megafossils lycophytes and psilophyton known from libya lead us to conclude that land plants including vascular plants probably had a long pre silurian record extending at least into the basal caradocian additonal evidence suggests that this record does not extend as far back as the earlier late cambrian dresbachian

paired isotope records of carbonate and organic matter from the middle ordovician of argentina intrabasinal variation and effects of the marine chemocline abstract we investigate the expression of the middle darriwilian isotope carbon excursion mdice across marine shelf environments in the argentine precordillera previous work identified the mdice in the las chacritas formation in argentina but did not recognize the expression of the mdice in time equivalent strata of the deeper water las aguaditas formation albanesi et al 2013 palaeogeography palaeoclimatology palaeoecology v 398 p 48 66 recent biostratigraphic investigations of these units have for the first time provided the opportunity for high resolution correlation which suggest that the mdice or at least the initiation of the mdice should be observed in both the las chacritas and las aguaditas formations here we present new paired carbon isotope data of carbonate and organic carbon from the las chacritas and las aguaditas formations we identify a 2 positive shift in the isotopic composition of marine carbonate in the las chacritas formation whereas values abruptly fall to

rapid thermohaline transition in the pacific western subarctic and oyashio fresh core eddies seven years of conductivity temperature depth data enable direct computations of volume transport and a description of the temporal changes of water properties in the western subarctic pacific the cooling of the western subarctic surface layer is shown through the observation of the thermohaline transition which occured in the western subarctic during 1990 1997 its main consequences were changes of the oyashio path and restratification of the western subarctic water the example presented here uses data obtained during the canada russia international north pacific ocean climate study as well as the russian project subarctic to show that a warming and ventilation of the midlayers of the western subarctic 200 1000 dbar occurred because of the influx of warm subtropical waters to the oyashio and sea of okhotsk and to the weak coastal oyashio before the transition this influx appears to have reversed in the early 1990s now subarctic intermediate waters displace the remnants of subtropical waters that penetrated to the sea of okhotsk through the soya strait and create strong stratification in the oyashio area this transition appears to have begun during the reinforcement in the coastal oyashio

magellan observations of extended impact crater related features on the surface of venus a total of 57 parabolic shaped and 9 approximately circular extended impact crater related features have been found in magellan synthetic aperture radar sar and thermal emissivity data covering 92 of the surface of venus the parabolic features are with seven exceptions oriented e w with the apex to the east and the impact crater located just west of the apex they were first identified in the surface emissivity data derived from magellan radiometry measurements but the great majority are only clearly visible in the sar imagery the overall sizes of both the parabolic and circular features range from several hundred to about two thousand kilometers and are loosely correlated with the diameters of the parent craters the floors of almost all these craters have high specific radar backscatter cross sections i e they are bright in the sar imagery relative to their surroundings and tend to have low emissivities approximately one third of the impact craters with diameters 15 km appear to have bright floors and about half of these have an associated parabolic feature which can be observed in the sar or emissivity data no features have been found which overlie the parabolic features indicating that they are among the youngest features on the surface of the planet this suggests that radar bright floors characterize the freshest impact craters and that modification processes subsequently darken their radar signature a model for the formation of the parabolic features is developed based on the injection of small particles into the upper atmosphere at the time of impact and their transport to the west by the e w zonal winds fitting of a small perturbation scattering model to the measured average scattering law for the parabolic features placed an upper limit of about 0 6 cm on the wavelength scale 12 6 cm surface roughness and hence of 1 to 2 cm on the largest particle sizes of interest fallout times from 50 km in the venus atmosphere for particles of this size are about 2 hours allowing westerly drifts of several hundred kilometers for zonal winds of 50 to 100 m s 1 measurements of the change in backscatter cross section of features overlaid by these extended ejecta deposits are consistent with deposit depths of a few centimeters to 1 or 2m

sensitivity of the epeiric sea carbon isotope record to local scale carbon cycle processes tales from the mohawkian sea ancient marine carbon isotope records derived from epeiric sea carbonates can bear a strong influence of local scale carbon cycling processes across the late ordovician mohawkian sea of eastern laurentia the carbon isotope compositions d 13 c of contemporaneous sediments range from 2x in the interior of the sea to 2 5x along the margin here we use a box model to explore the relative importance of carbon fluxes within the mohawkian sea from a mass balance perspective in the model the mohawkian sea carbon reservoir is coupled to but distinct from the ocean and atmosphere carbon reservoirs at initial steady state conditions the residence time of carbon in the mohawkian sea is less than 10 years we find that to produce model d 13 c values representative of the range in d 13 c across the mohawkian sea we must divide the sea into an inner region and an outer region and limit dissolved inorganic carbon dic exchange between the two to 15 of the initial steady state unrestricted exchange rate it is also necessary to supply low d 13 c carbon 29x to the inner region of the sea at a rate of 17 4 10 12 mol c yr 1 the required flux of low d 13 c carbon could not have been sustained over the long term by remineralization of organic carbon stored in sediments of the inner region of the mohawkian sea and the geographic d 13 c pattern is unlikely to have been generated by upwelling along the sebree trough rather the low d 13 c flux might have been derived from bryophytedominated terrestrial ecosystems suggesting an important role for terrestrial carbon cycling prior to the advent of vascular plants aninterestingfeatureofthemodelisthattheisotopiccompositionsofboththeepeiric seaandthelargerocean atmospheresystem are sensitive to changes in exchange rates within the epeiric sea this has important implications for interpreting global d 13 c

density and morphology of impact craters on tessera terrain venus densities of impact craters on tessera which is complex ridged terrain of tectonic origin and on the remainder of the planet which is mostly volcanic plains were studied using magellan images for about 96 of the surface of venus the density of large d 16 km impact craters on tessera is higher by a factor of about 1 4 than on the remainder of the planet this means that the tessera crater retention age is larger than the age of the plains this is in agreement with the well known fact that tessera is embayed by the surrounding volcanic plains the density of small d 16 km impact craters on tessera is lower than on the remainder of the planet evidently an observational bias caused by a difficulty in recognizing the small craters on rough tessera terrain the absence of recognizable tectonic deformation in most of the large on tessera craters means that during the period of crater emplacement most of the studied tesserae were tectonically stable and did not undergo noticeable degree of deformation

a dynamical interpretation of the tritium maximum in the central equatorial pacific abstract the tropical tritium distribution between 1974 and 1981 is characterized by a maximum along the equator centered between 125 and 145 w it signifies that this region has received the maximum input of high northern latitude water a dynamical interpretation of the maximum shows that it can be explained by the strength of the sverdrup circulation in the central equatorial pacific where there is a strong zonal convergence in the north equatorial countercurrent necc and a strong meridional geostrophic flow towards the equator

radar properties as clues to relative ages of ridge belts and plains on venus 1 an ongoing issue regarding the crustal evolution of venus is whether or not certain features and processes occurred in the same sequence and at the same time globally this paper seeks to test one aspect of directional models for the geologic history the inference that all ridge belts are older than regional plains this test involves traditional geologic analysis but also involves the use of radar properties in a unique way we use a simplified model for the ridge shape and an estimate of the diffuse scattering behavior of the venus surface to determine the true backscatter coefficient of the slope of the ridge tilted away from the radar and ratio this value to the echoes from nearby plains units in general ridge forming terrain in these belts appears to have greater inferred backscatter coefficients and thus decimeter scale roughness than the regional plains plains units closely adjacent to many ridge belts have higher backscatter than plains at some distance which we infer is generally due to aeolian deposition of debris characterized by centimeter scale microdunes or ripples where the belt topography interferes with wind patterns the rougher apparent surface texture within the belt forming terrain is not consistent with the gentle folding that forms the ridges and so is likely a result of weathering or tectonic processes operating prior to belt deformation we thus conclude that the material incorporated in the ridge belts we have studied predates the surrounding regional plains

impact craters on venus initial analysis from magellan magellan radar images of 15 percent of the planet show 135 craters of probable impact origin craters more than 15 km across tend to contain central peaks multiple central peaks and peak rings many craters smaller than 15 km exhibit multiple floors or appear in clusters these phenomena are attributed to atmospheric breakup of incoming meteoroids additionally the atmosphere appears to have prevented the formation of primary impact craters smaller than about 3 km and produced a deficiency in the number of craters smaller than about 25 km across ejecta is found at greater distances than that predicted by simple ballistic emplacement and the distal ends of some ejecta deposits are lobate these characteristics may represent surface flows of material initially entrained in the atmosphere many craters are surrounded by zones of low radar albedo whose origin may have been deformation of the surface by the shock or pressure wave associated with the incoming meteoroid craters are absent from several large areas such as a 5 million square kilometer region around sappho patera where the most likely explanation for the dearth of craters is volcanic resurfacing there is apparently a spectrum of surface ages on venus ranging approximately from 0 to 800 million years and therefore venus must be a geologically active planet

reefs during the multiple crises towards the ordovician silurian boundary anticosti island eastern canada and worldwide multiple latest ordovician rawtheyan hirnantian glaciations in central africa with concomitant global sea level lowstands and cooler restricted equatorial carbonate shelves and ramps interrupted by warmer interstadial highstands had a dramatic global impact on the tropical shallow water reef ecosystem and carbonate production with the ordovician silurian boundary strata on anticosti island as a global standard for a carbonate shelf ramp setting the latest ordovician and earliest silurian reveal three reef phases ended by three extinctions the first extinction towards the end of the rawtheyan affected the last richmondian type reefs vaureal formation mill bay member the second extinction was less pronounced ending with reefs at the base of the prinsta member ellis bay formation interpreted as the top of the normalograptus extraordinarius graptolite subzone the third and most severe extinction phase capped the laframboise patch reef complex ellis bay formation at the top of the no

mixing in warm core rings of the kuroshio water mass evolution and ventilation in warm core rings of the kuroshio is discussed using hydrographic t s data remote sensing and tritium measurements the kuroshio warm core ring a3 detached from the kuroshio in 1986 migrated northwest along the japan and kuril trenches and reached the southern kuril islands in 1989 this ring is characterized by weak stratification to a depth of about 700 m in its upper layer the ring displayed a pronounced thermostad and a high tritium concentration in its lower portion however temperature and salinity were density compensated on the basis of tritium data analyses the thermostad water was shown to originate in intermediate layers of the kuroshio south of the subarctic front this implies that the warm core rings are a major factor influencing the tritium transport in this region and that they are an important link for water exchange between the subtropical and subpolar gyres in the north pacific the tritium data confirm the idea about the origin of the thermostad water of anticyclonic ring from intermediate layers of the kuroshio therefore a mechanism must exist in the kuroshio frontal zone which selectively draws water from the kuroshio to the rings and acts to ventilate the subtropical gyre it is also noted that during the ring cooling period a surface layer of low tritium content was formed above the ring

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

late ordovician early silurian cryptospore occurrences on anticosti island ile d anticosti quebec canada residues from hirnantian upper ordovician and aeronian lower silurian strata of anticosti island quebec canada have yielded the oldest occurrence of terrestrial based palynomorphs cryptospores from eastern north america a low abundance low diversity cryptospore assemblage containing specimens of pseudodyadospora velatitetras and alete monads has been recovered from the velleda member of the ellis bay formation upper ordovician hirnantian the gun river formation lower silurian aeronian contains a low abundance low diversity assemblage composed of specimens of velatitetras laevolcancis and alete monads the occurrence of these assemblages provides evidence for early land plants in this part of north america during the late ordovician and early silurian and documents the existence of a terrestrial based flora during and after the saharan late ordovician glaciation

wind interaction with falling ejecta origin of the parabolic features on venus unusual parabolic features associated with impact craters have been observed by magellan on venus a strong correlation exists between the orientation of the features and the zonal winds on venus we propose a quantitative model in which the parabolic features are produced by the interaction of the zonal winds with material ejected ballistically from the impact crater as the ejecta particles fall through the atmosphere the winds transport them downwind from their entry point smaller particles being transported a greater distance since the ejecta distribution is initially axially symmetric and smaller particles are thrown farther from the crater the winds blow the particles on the upwind side back upon one another leading to a pile up of material on the downwind side the winds disperse the ejecta particles and no pile up occurs the resulting thickness distribution on the venusian surface matches the observed parabolic features closely the dual parabolic feature associated with the crater carson is also explained by this model

equilibrium resurfacing of venus results from new monte carlo modeling and implications for venus surface histories abstract venus impact crater population imposes two observational constraints that must be met by possible model surface histories 1 near random spatial distribution of 975 craters and 2 few obviously modified impact craters catastrophic resurfacing obviously meets these constraints but equilibrium resurfacing histories require a balance between crater distribution and modification to be viable equilibrium resurfacing scenarios with small incremental resurfacing areas meet constraint 1 but not 2 whereas those with large incremental resurfacing areas meet constraint 2 but not 1 results of monte carlo modeling of equilibrium resurfacing strom et al 1994 is widely cited as support for catastrophic resurfacing hypotheses and as evidence against hypotheses of equilibrium resurfacing however the monte carlo models did not consider intermediate size incremental resurfacing areas nor did they consider histories in which the era of impact crater formation outlasts an era of equilibrium resurfacing we construct three suites of monte carlo experiments that examine incremental resurfacing areas not previously considered 5 1 0 7 and 0 1 and that vary the duration of resurfacing relative to impact crater formation time 1 1 suite a 5 6 suite b and 2 3 suite c we test the model results against the two impact crater constraints several experiments met both constraints the shorter the time period of equilibrium resurfacing or the longer the time of crater formation following the cessation of equilibrium resurfacing the larger the possible areas of incremental resurfacing that satisfy both constraints equilibrium resurfacing is statistically viable for suite a at 0 1 suite b at 0 1 and suite c for 1 0 7 and 0 1 areas of incremental resurfacing

paired δ13ccarb and δ13corg records of upper ordovician sandbian katian carbonates in north america and china implications for paleoceanographic change late sandbian early katian marine carbonates from two sections in north america and one in south china were analyzed for paired 13ccarb and 13corg and revealed similar 13ccarb but varying trends in 13corg stratigraphy while all sections recorded a well known positive 13ccarb shift these 13corg records are the first through the guttenberg 13ccarb excursion gice in china oklahoma and west virginia in south china a positive excursion in 13corg is associated with the latter part of the positive 13ccarb excursion and closely resembles trends from iowa and pennsylvania that have been interpreted to reflect a lowering of atmospheric pco2 levels the 13corg trends from oklahoma and west virginia differ significantly from what is observed in china this likely indicates that local changes in nutrient cycling and phytoplankton growth rates were the dominant control on 12c fractionation in oklahoma and west virginia masking a possible global signal of lowered atmospheric pco2 levels observed in china

ages of mare basalts on the lunar nearside the chronology of lunar volcanism is based on radiometric ages determined from apollo and luna landing site samples regional stratigraphic relationships and crater degradation and size frequency distribution data for units largely defined prior to the end of the apollo program here we report on new crater size frequency distribution data for 139 spectrally and morphologically defined basalt units which are exposed in six nearside impact basins australe tranquillitatis humboldtianum humorum serenitatis and imbrium crater size frequency distribution measurements are a statistically robust and accurate method to derive absolute model ages of unsampled regions of the moon compared to crater degradation ages crater size frequency ages performed on spectrally defined units offer significant improvements in accuracy our investigation showed that 1 in the investigated basins lunar volcanism was active for at least 1 5 2 0 b y starting at about 3 9 4 0 b y and ceasing at 2 0 b y 2 most basalts erupted during the late imbrian period at about 3 6 3 8 b y 3 significantly fewer basalts were emplaced during the eratosthenian period 4 basalts of copernican age were not found in any of the investigated basins 5 lunar basin filling volcanism probably started within 100 m y after the formation of the individual basins we also assessed the relationship between impact basin age and the history of mare basalt emplacement in these basins we found that 1 in all pre nectarian basins australe and tranquillitatis as well as in the humboldtianum basin which is of nectarian age the distribution of surface ages is clearly dominated by only a single peak in the number of erupted units at 3 6 3 8 b y 2 in the younger basins humorum serenitatis and imbrium a second peak at 3 3 3 5 b y is observed 3 basalt eruptions younger than 2 6 b y occur only intermittently and 4 in the youngest basins serenitatis and imbrium we see an extended period of active basin filling volcanism 1 5 1 6 b y which is 500 m y longer than in the australe and humorum and even 1 0 b y longer than in tranquillitatis and humboldtianum

venus surface thermal emission at 1 μm in virtis imaging observations evidence for variation of crust and mantle differentiation conditions 1 the venus express spacecraft images the nightside thermal emissions using the visible and infrared thermal imaging spectrometer virtis at 1 02 1 10 and 1 18 m thermal emission from the surface is observed the signal is attenuated by scattering and absorption in the dense atmosphere the measured flux at the top of the atmosphere is positively correlated with surface temperature and surface emissivity the surface temperature of venus is relatively well constrained as being mainly a function of altitude with a gradient lesser or equal to the adiabatic lapse rate this study examines the correlation of virtis images showing a signal of the surface at 1 02 m with viewing geometry stray sunlight cloud opacity and topography and applies semiempirical relations to remove their influence the remaining contrast can be either ascribed to surface emissivity or unexpected temperature variations temperature variations due to active volcanism are unlikely to be persistent over the time of observations therefore the mosaic of all processed images is here interpreted in terms of surface emissivity variation the emissivity variation found is correlated with geomorphological features established from magellan synthetic aperture radar images it is generally lower at tessera terrain some but not all volcanic edifices show increased emissivity large lava flows in the lada terra lavinia planitia region also show an increased thermal emission this might indicate a more felsic surface composition of tessera highlands and large scale extrusive volcanism of ultramafic composition

oxygen and carbon isotopes and event stratigraphy near the ordovician silurian boundary anticosti island quebec the growth and destruction of continental ice sheets in north africa and adjacent parts of gondwana during the hirnantian stage had profound influence on the sedimentary style of mixed carbonate siliciclastic sequences on anticosti island quebec canada these strata deposited in a tropical mid shelf setting show direct evidence of local sea level changes of up to 40 m after correction for tectonic and load induced subsidence this indicates maximum global glacio eustatic changes of 32 to 36 m this implies maximum ice volume of about 12 to 14 106 km3 and related changes in the oxygen isotopic composition of global sea water amounting to as little as 0 5 to 0 6 given that the oxygen isotope ratios in syn glacial samples from the anticosti sequence range from 8 9 to 2 8 18o it is not possible to clearly distinguish the effects of ice cap development from temperature and diagenetic controlled variation the strong negative shift in 13c values at the top of the ellis bay formation may indicate a pronounced increase in atmospheric co2 coincident with destruction of the late ordovician ice caps

virtis emissivity of alpha regio venus with implications for tessera composition abstract the composition of venus tessera terrain is unknown the visible and infrared thermal imaging spectrometer virtis aboard venus express vex collects data that yields the surface emissivity at 1 m which contains information convolving a number of surface properties including composition we examine the variation of emissivity in the vicinity of alpha regio which is the largest exposure of tessera terrain imaged by virtis we find that the emissivity of alpha regio tessera is lower than adjacent plains materials and the deposits and flows of eve corona both of which have previously been interpreted to be basaltic the emissivity of the bulk of alpha is also lower than its western boundary which is interpreted to comprise plains structurally deformed to the same degree as tessera terrain this suggests that the lower emissivity of alpha is independent of structural elements macroscale roughness or local sedimentation processes and is due to material properties like composition or grain size the deviation of the emissivity of alpha from that of the plains for which a bulk basaltic composition is well supported corresponds to a significant difference in rock type or surface mineral assemblage the 1 m emissivity of alpha is consistent with rocks with low ferrous iron content this includes felsic igneous rocks like granitoids that form under either water rich or water poor conditions a water rich origin would require both a hydrosphere and a plate recycling mechanism and thus be limited to the lifetime of surface water on venus alternatively granitoids could form via the differentiation of basaltic melts the production of all tessera terrain by this mechanism would require the accumulation and preservation of felsic melts from a volume of mafic magma that exceeds what is preserved in the currently observed plains both mechanisms of granitoid formation would require that tessera terrain be formed prior to the emplacement of the plains consistent with their stratigraphic position anorthosites also satisfy the emissivity signature and can form from copious amounts of partial melting of a mafic source low emissivity values are also consistent with carbonates sulfates phyllosilicates and their dehydration products which may have formed via weathering of basalts under conditions of higher atmospheric p h2o all of these hypotheses suggest the mineralogy of alpha tessera records an extinct era of venus history and is a key target for future exploration

aliphatic and aromatic biomarkers from gondwanan sediments of late ordovician to early devonian age an early terrestrialization approach abstract twenty one core samples of late ordovician to early devonian age from sections in southern tunisia north african platform gondwana and containing marine and terrestrial organic matter with microbial input were investigated to link the aliphatic and aromatic hydrocarbon distributions with the terrestrial and marine palynomorph content e g acritarchs prasinophytes chitinozoans cryptospores and trilete spores in addition to several biomarkers of algal bacterial origin long chain n alkanes that might be derived from land plants as well as the terrestrial diterpane norabietane were found several land plant derived biomarkers such as retene cadalene simonellite tetrahydroretene and c19 isohexylalkyl naphthalene were observed in the aromatic fractions while these terrestrial biomarkers could be clearly recognized in the middle silurian lower devonian samples their presence in the upper ordovician lower silurian sediments is more doubtful because of much lower relative abundances the land plant biomarkers show a fairly good correlation with the occurrence and abundance of cryptospores and trilete spores derived from bryophytes and tracheophytes which covered the emerged areas around the ghadamis basin during the silurian and devonian the early tracheophytes e g cooksonia lycophytes and zosterophylls are therefore suggested as a new terrestrial source for most of the saturated and aromatic biomarkers found in sediments of middle silurian to early devonian age

tectonic evaluation of alpha regio highland and environs venus abstract systematic geologic mapping of an area on venus provides geologic unit and morphologic information about the area that was evaluated to provide a simple geologic history in the study area a large complexly deformed highland alpha regio was found to be among the oldest material units but its relation to other nearby complexly deformed units was indeterminable to better delineate the complex geologic and tectonic history of the area the authors expanded on the initial geologic map assessment by including tectonic unit mapping of the alpha regio highland and geophysical modeling of the highland to assess the possible effect of its formation on surrounding geologic units tectonic unit mapping identified five discrete periods of tectonic activity involved in the formation of the highland additionally the tectonic assessment showed that late stage extensional features within the highland were related to a regional tectonic event that occurred after the formation of the highland geophysical models of highland formation by both upwelling and downwelling were assessed and a downwelling model was applied to the alpha regio highland to calculate stresses related to its formation results of the modeling suggest that tectonic features mapped outside of the highland were not caused by stresses induced by the formation of alpha regio

recent volcanic resurfacing of venusian craters ejecta from impact craters on venus are a major source of fine grained materials across the planet and crater spatial distribution has been studied as a guide to the relative age and resurfacing rates of large regions of particular interest is the potential intersection of distal crater deposits and tesserae highly deformed landforms that may be the oldest materials on venus the composition of tesserae is unknown but is key to understanding whether water played a role in crustal differentiation thus tesserae are ideal sites for future landed missions to identify possible felsic materials but the short lifespan of surface landers means that efforts must be made to avoid contaminating surface materials from craters here we develop a method to detect distal crater ejecta on tessera terrain across venus using nasa magellan radar data our results show that fine grained ejecta are unevenly distributed in the tesserae with respect to nearby craters many tesserae within a few hundred kilometers of plains craters do not have evidence for thick 5 10 cm mantling material indicating that eolian or mass wasting processes have moved the debris off the highland ridge slopes at sudenitsa tesserae within the young beta atla themis region we observe a radar signature of mantling debris but there is no apparent source crater to which this material can be traced we infer that the source crater has been resurfaced by volcanic activity within the past 80 m y and suggest that similar fine grained ejecta deposits may have built up over time in other tesserae across venus

recent hotspot volcanism on venus from virtis emissivity data the questions of whether venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change nine hotspots areas analogous to hawaii with volcanism broad topographic rises and large positive gravity anomalies suggesting mantle plumes at depth have been identified as possibly active this study used variations in the thermal emissivity of the surface observed by the visible and infrared thermal imaging spectrometer on the european space agency s venus express spacecraft to identify compositional differences in lava flows at three hotspots the anomalies are interpreted as a lack of surface weathering we estimate the flows to be younger than 2 5 million years and probably much younger about 250 000 years or less indicating that venus is actively resurfacing

evidence for crater ejecta on venus tessera terrain from earth based radar images abstract we combine earth based radar maps of venus from the 1988 and 2012 inferior conjunctions which had similar viewing geometries processing of both datasets with better image focusing and co registration techniques and summing over multiple looks yields maps with 1 2 km spatial resolution and improved signal to noise ratio especially in the weaker same sense circular sc polarization the sc maps are unique to earth based observations and offer a different view of surface properties from orbital mapping using same sense linear hh or vv polarization highland or tessera terrains on venus which may retain a record of crustal differentiation and processes occurring prior to the loss of water are of great interest for future spacecraft landings the earth based radar images reveal multiple examples of tessera mantling by impact parabolas or haloes and can extend mapping of locally thick material from magellan data by revealing thinner deposits over much larger areas of particular interest is an ejecta deposit from stuart crater that we infer to mantle much of eastern alpha regio some radar dark tessera occurrences may indicate sediments that are trapped for longer periods than in the plains we suggest that such radar information is important for interpretation of orbital infrared data and selection of future tessera landing sites

revisiting the great ordovician diversification of land plants recent data and perspectives abstract recent molecular clock data suggest with high probability a cambrian origin of embryophyta also called land plants indicating that their terrestrialization most probably started about 500 ma the fossil record of the cambrian explosion was limited to marine organisms and not visible in the plant fossil record the most significant changes in early land plant evolution occurred during the ordovician for instance the earliest bryophyte like cryptospores and the oldest fragments of the earliest land plants are from the middle and late ordovician respectively organic geochemistry studies on biomarker compositions hint at a transition from green algae to land plants during the great ordovician biodiversification event gobe the colonization of the terrestrial realms by land plants clearly had an impact on marine ecosystems interactions between the terrestrial and marine biospheres have been proposed and the radiation of land plants potentially impacted on co2 and o2 concentrations and on global climate in addition the shift of strontium isotopes during the ordovician is probably linked to changing terrestrial landscapes affected by the first massive land invasion of eukaryotic terrestrial life the land plants seem unaffected by the first global mass extinction at the end of the ordovician that eliminated many marine invertebrate taxa

recent variability in the pacific western subarctic boundary currents and sea of okhotsk abstract the objective of this research is to describe physical processes which are the cause of the recent variability of the pacific western subarctic waters rapid thermohaline changes have occurred within the oyashio and kamchatka current during the last decade this variability has included a warming of the kamchatka current warm intermediate layer but a cooling and freshening of the upper layer in the oyashio and sea of okhotsk the example presented here uses data obtained during the canada russia inpoc and woce projects as well as the new russian studies with high resolution station grid the possible physical mechanism that generated the upper layer freshening during the thermohaline transition is examined major components of the fresh water budget of the okhotsk sea are considered in order to describe the dramatic changes in salinity which have recently occurred in the pacific subarctic significant changes in precipitation and other fresh water inputs are demonstrated it is suggested that upper layer of the oyashio and kamchatka current became cooler and fresher because of the export of cold fresher waters from the bering and okhotsk seas these waters from the marginal seas have cooled the bottom of the halocline reducing evaporation and acting as a feedback that has kept the upper layer of the western subarctic boundary currents fresh it is also shown that the outflow of the cold sea of okhotsk water has changed its path during this recent thermohaline transition

new evidence for land plants from the lower middle ordovician of saudi arabia macerations of middle ordovician llanvirnian shales from saudi arabia yield an assemblage of spores of probable land plants cryptospores acritarchs and chitinozoa the production of sporopollenin containing sporelike tetrads is considered a fundamental character of the embryophytes because no extant algae produce spores of this type no trilete spores were found at this horizon reinforcing previous assertions that obligate meiotic tetrads predate the earliest trilete spores sporomorph tetrads and dyads in conjunction with cuticlelike fragments were probably derived from terrestrial plants at a bryophyte grade although there are reports of possibly older cryptospores the hanadir assemblage described herein clearly establishes their presence by llanvirnian time

an organic geochemical perspective on terrestrialization the colonization of land required new strategies for safe gamete diaspore dispersal and to cope with desiccation harmful radiation fire and gravity accordingly the morphology behaviour and physiology of the organisms changed here we explore to what extent physiological adaptations reflected in the molecular content of the sediments add to our understanding of the terrestrialization many compounds considered characteristic of land organisms do not provide valuable information from the fossil record since 1 they were not preserved 2 they occur or correspond to substances that evolved prior to the terrestrialization e g cutan vs algaenan cellulose or 3 they have been changed diagenetically and or catagenetically the latter leads to geo macro molecules without a chemical fingerprint relating them to their original bio macro molecules despite sometimes excellent morphological preservation of the organic remains nevertheless some molecular markers and their stable isotopes provide independent information on the terrestrialization process the odd predominance of n alkane surface waxes is a feature already apparent in early land plants and could with caution be used as such furthermore fossil terpenoids and their derivatives are valuable for reconstructing the evolution of major plant groups the radiation of the phenylpropanoid pathway with for example sporopollenin and lignin seems to be closely related to the evolution of land plants as for other disciplines occupied with unravelling past life and environment organic geochemistry relies heavily on the paradigm that the present provides a key to the past for organic geochemistry the biosynthetic pathways of living organisms provide such a key the biosynthetic differences between organisms provide insight into the evolution of biosynthetic pathways and this can be applied to and calibrated against the fossil record evolution also implies adaptation and by linking species ecology to their biochemistry the adaptive value of the biosynthetic pathways and the biomolecules produced may be resolved this biosynthetic link to environment can also be applied to the past for instance the notion that oxygen is required in steroid and non hopanoid triterpenoid synthesis implies that analysis of steroids in ancient sediments may help to unravel the early evolution of the atmosphere summons et al 2006 analogous to the oxygenation of the earth s atmosphere terrestrialization also required major adaptations of the terrestrializing organisms to the living conditions on land the conquest of land required the development or strengthening of supportive structures such as skeletons stems and roots to withstand earth s gravity without the support of water and to resist wind it also demanded water saving strategies such as arthropod and plant cuticles vertebrate skin and cork to survive low humidity environments and sometimes fire as well as the development of water conducting tissues in plants roots tracheids finally the higher exposure to harmful radiation required sunscreens such as pigments aromatic and other substances as uv filters whereas the endoand exoskeletal adaptations are intrinsically based on the formation of large biopolymers water saving and uv protection may be also realized by means of smaller molecules our working hypothesis is that the major changes in biosynthetic pathways which accompanied the terrestrialization of organisms are reflected in the molecular composition of the organic matter present in palaeozoic sedimentary rocks for instance a minor change in the biosynthesis of sporopollenin may have led to lignin in return we can expect the changes in composition of the organic matter om in palaeozoic rocks to help determine following evolutionary trends the question being asked is to what extent is our working hypothesis valid and what does this provide us with to understand terrestrialization we restrict ourselves to primary producers and arthropods since they have a rich fossil record both organic geochemically and as microand macroscopic remains we exclude other terrestrial heterotrophic life forms worms snails vertebrates etc since they do not usually leave a chemically characteristic signature in the sediments from vecoli m clement g meyer berthaud b eds the terrestrialization process modelling complex interactions at the biosphere geosphere interface geological society london special publications 339 11 36 doi 10 1144 sp339 3 0305 8719 10 15 00 the geological society of london 2010 organic matter analysis lipids and

impact crater related surficial deposits on venus multipolarization radar observations with arecibo 1 the distribution of surficial deposits in the vicinity of impact craters on venus was studied using measurements of the polarization properties of the reflected radar wave subsurface scattering of an incident circularly polarized radar signal results in a linearly polarized component in the radar echo due to the differing transmission coefficients at a smooth at wavelength scales atmosphere surface boundary for the horizontal h and vertical v linearly polarized components of the incident wave arecibo 12 6 cm wavelength radar observations in 1999 and 2001 provided images of the surface of venus in the full stokes polarization parameters of the reflected echo from which images in the degree of linear polarization were derived these images show that substantial areas of sedna guinevere and lavinia planitias return a radar echo with a significant degree of linear polarization indicating that mantling deposits may be relatively widespread on the plains of venus the areas showing linear polarization enhancements are strongly correlated with topographic features primarily impact craters dome fields and windblown deposits including dune fields a strong linearly polarized echo component 10 40 linear polarization is found from regions near 45 impact craters including 5 parabolas these linear polarization features typically correspond to diffuse higher backscatter cross section bright regions in magellan images the linearly polarized component in these regions is attributed to subsurface echoes from a mantled substrate or from buried rocks

local and global perspectives on carbon and nitrogen cycling during the hirnantian glaciation abstract the impact of climate and ocean circulation changes on marine carbon and nitrogen cycling was investigated for a period covering the late ordovician hirnantian glaciation three sections were studied two from nevada one proximal and one distal and one from the yukon the opportunity to compare 13 c carb 13 c org and 13 c grap profiles between ocean and epeiric platform settings enabled a more thorough assessment of environmental and diagenetic factors influencing the preservation of seawater 13 c values at each stratigraphic section using the best preserved sedimentary components from the two nevada sections a lowstand sedimentary gradient of 4 was found between platform and ocean settings during the hirnantian glaciation gradients of similar magnitude have been reported from hirnantian platforms in the canadian arctic and baltic basins taking the outboard sections as the most representative of the ocean carbon pool a mean positive 13 c shift of just 2 7 0 4 is calculated which is significantly smaller than the 5 7 positive shifts recorded in epeiric platform settings the discrepancy is attributed to the effects of local carbon cycling in tropical and subtropical epeiric seas it is hypothesized that increased carbonate weathering caused 13 c values of local carbon weathering fluxes to increase which in turn caused seawater 13 c values to increase in basin proximal settings in addition nutrient dust fluxes may have contributed to increased levels of photosynthetic activity in proximal settings driving sedimentary 13 c values even higher much higher than sediment deposited in outboard settings where carbon exchanges with the surface ocean were less restrictive a 15 n profile reconstructed from deposits of the distal vinini creek section records a 1 5 positive shift through the hirnantian glacial interval interpreted as a major late ordovician upwelling zone high organic carbon 1 wt and high hydrogen indices 400 600 make the vinini creek section a suitable candidate for the preservation of original sedimentary 15 n records to explain the results a conceptual model of ocean nitrogen cycling under greenhouse climate conditions is presented that highlights the role of cyanobacteria as both primary producers and nitrogen fixers it is hypothesized that in greenhouse climates the fixed nitrogen inventory of the abyssal ocean was low due to efficient denitrification in the ocean s expansive oxygen minimum zones this caused algal productivity to become strongly dependent on continuous supplies of fixed nitrogen from cyanobacteria in the photic zone yielding sedimentary 15 n values between 0 and 2 by contrast the cooler icehouse climate of the hirnantian led to increased ocean ventilation greater partitioning of atmospheric oxygen into downwelling surface waters and oxygen minimum zone shrinkage as a result fixed nitrogen levels climbed in the ocean s abyssal depths due to the lower denitrification rates this in turn allowed nitrogen with high 15 n values to recycle back into the photic zone driving sedimentary 15 n values higher during the hirnantian glaciation

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

tessera terrain on venus a survey of the global distribution characteristics and relation to surrounding units from magellan data the tessera terrain on venus comprised of areas of high radar backscatter complex deformation patterns relative to other units and topography standing higher than surrounding plains covers 35 33 106 km2 about 8 of the surface of venus and is nonrandomly distributed being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30 s individual tessera occurrences range in area from the lower limits of our measurements about 200 km2 up to the largest tessera ovda with an area of about 8 6 106 km2 or about 2 of the surface area of venus the size frequency distribution of tessera patches is strongly unimodal and skewed toward smaller sizes reflecting the great abundance of small tessera fragments modes of occurrence include 1 large clusters e g aphrodite terra and ishtar terra 2 arc like segments which may extend for thousands of kilometers and are either concave inward toward the major tessera cluster development or away from it 3 areas where tesserae are rare or absent which occur both as low lying plains e g guinevere planitia and as elevated regions e g atla regio tessera terrain has a bimodal elevation frequency distribution with the main peak at about 0 1 km and an additional peak at about 3 km above mean planetary radius in terms of number of occurrences however tesserae do not display a correlation with elevation at the global scale since small tessera patches commonly occupy low lying regions although tessera exhibit a range of gravity signatures many occurrences are interpreted to represent relatively shallow crustal levels of compensation tessera boundaries include type i sinuous embayed dominated by adjacent lava plains embaying tessera massifs 73 of total tesserae boundaries and type ii linear tectonic only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial tessera formation occurrence of broad slopes of post tessera embayed plains away from tessera boundaries suggests that regional tilting occurred subsequent to final tessera deformation in some places several lines of evidence suggest the possibility that a widespread tessera like basement comprising at least 55 of the surface of venus is buried under a cover of lava plains a few hundred meters to as much as 2 4 km thick a wide variety of deformational structures and patterns is observed within the tessera including those representing extension compression shear and transpression in some cases the apparently complex patterns can be resolved into single event kinematic interpretations involving noncoaxial deformation e g itzpapalotl while in other cases polyphase deformation is more likely e g central ovda and thetis where relations can be determined stratigraphically earliest deformation within the tessera is primarily related to crustal shortening and compression phase i followed by pervasive extensional deformation commonly oriented normal to the strike of phase i features generally along the same principal stress direction phase ii evidence also exists for the contemporaneous formation of these distinctive deformation patterns lava plains within and adjacent to the tessera embay both of these fabrics but sometimes overlap in time with phase ii extensional deformation and with regional tilting tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed an observation consistent with many other mapping studies we see no evidence for transitional stages between tessera and volcanic rises and or lowlands that might represent a long term sequence of upwelling or downwelling followed by crustal deformation and tessera formation no impact craters deformed by phase i deformation have yet been observed on tessera suggesting that phase i tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before 300 500 m y ago however the starting time and thus duration of tessera formation is unknown on the basis of the very small number of on tessera craters deformed by phase ii extensional deformation this period probably did not last more than several tens of millions of years after the cessation of phase i little observable deformation of the tessera terrain appears to have taken place in the last several hundred million years during which time the vast volcanic plains were emplaced although tilting of early plains along some tessera margins is observed building on the global synthesis presented here future analyses of individual tessera occurrences will provide the detailed descriptions kinematic interpretations and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for tessera formation

the early land plants from the armorican massif sedimentological and palynological considerations on age and environment the ch teaupanne unit belongs to the south armorican domain of the armorican massif france which is part of the variscan belt this unit includes two lower devonian plant levels and one of them corresponds to the basal member of the chalonnes formation a sedimentological and palaeontological analysis of these fossiliferous deposits from the ch teaupanne quarry montjean loire maine et loire france is presented here for the first time the age determination based on palynology indicates that the locality records the earliest occurrence of plant megafossils in the armorican massif their presence suggests an emergence event that has never been described before our study highlights the promising potential of the basal member of the chalonnes formation to aid in understanding these occurrences and provides new insights into the history of the variscan belt

carbon isotopes support the presence of extensive land floras pre dating the origin of vascular plants abstract multiple lines of evidence indicate that earth s land masses became green some 2 7 ga ago about 1 billion years after the advent of life about 2 2 billion years later land plants abruptly appear in the fossil record and diversify marking the onset of ecologically complex terrestrial communities that persist to the present day given this long history of land colonization surprisingly few studies report direct fossil evidence of emergent vegetation prior to the continuous record of life on land that starts in the mid silurian ca 420 425 ma ago here we compare stable carbon isotope signatures of fossils from seven ordovician silurian 450 420 ma old appalachian biotas with signatures of coeval marine organic matter and with stable carbon isotope values predicted for ordovician and silurian liverworts bryocarb model the comparisons support a terrestrial origin for fossils in six of the biotas analyzed and indicate that some of the fossils represent bryophyte grade plants our results demonstrate that extensive land floras pre dated the advent of vascular plants by at least 25 ma the appalachian fossils represent the oldest direct evidence of widespread colonization of continents these findings provide a new search image for macrofossil assemblages that contain the earliest stages of land plant evolution we anticipate they will fuel renewed efforts to search for direct fossil evidence to track the origin of land plants and eukaryotic life on continents further back in geologic time

paleozoic lake faunas establishing aquatic life on land abstract lakes are important archives for continental records of paleoenvironmental as well as paleoclimatic change they also record a unique macroevolutionary pattern that occurred when faunas invaded the continental realm in order to document that pattern we compiled a database of over one hundred and six lake basins from the neoproterozoic to permian looking at this critical time in lacustrine evolution several patterns emerge for the neoproterozoic through silurian time there is a paucity of documented lake deposits and lake faunal records however the records that are available indicate that lakes were oligotrophic and their nutrient cycling regimes primitive it is not until the establishment of land plants in the silurian that lakes began to respond with higher diversities and more complex physical and chemical conditions during the devonian carboniferous periods diversity was on the rise as trophic structure became more complex most lakes of the devonian and carboniferous formed along continental margins or in tectonic basins with occasional connection to the marine realm the faunas from these types of lakes were commonly comprised of mixed marine and freshwater elements and were far more diverse than other more inland lakes this estuary effect created a gateway or filter through which faunas invaded the continental realm the early history of lake faunas is one of opportunity and amelioration the feedback loops created by the establishment of vascular plants altered the nutrient cycle on land and in lakes all trophic levels were established early but became increasingly complex throughout the paleozoic as roles changed and faunal elements became established groups invading the continents via the estuary effect did so numerous times before establishing themselves permanently this was linked with the episodic re establishment in marine freshwater connections along these continental margins