mercury s global contraction much greater than earlier estimates observations of compressional structures on mercury have fallen short of accommodating the global contraction that is required owing to cooling of the planet s interior mapping of folds and faults across mercury s surface using messenger spacecraft images reveals deformation consistent with a planet that has contracted radially as much as seven kilometres over its history

beagle rupes evidence for a basal decollement of regional extent in mercury s lithosphere abstract thanks to its location at low latitude and close to the terminator in the outbound view of mercury obtained during messenger s first fly by the beagle rupes lobate scarp on mercury has been particularly clearly imaged this enables us to interpret it as a component of a linked fault system consisting of a frontal scarp terminated by transpressive lateral ramps the terrain bounded by these surface manifestations of faulting is the hanging wall block of a thrust sheet and must be underlain by a basal decollement a detachment horizon constituting the fault zone at depth the decollement must extend a minimum of 150 km eastwards from the frontal scarp and at least 400 km if displacement is transferred to features interpreted as out of sequence thrusts and offset lateral ramps that appear to continue the linked fault system to the east the depth of the basal decollement could be controlled by crustal stratigraphy or by rheological change within or at the base of the lithosphere previous interpretations of mercurian lobate scarps regard their thrusts as uniformly dipping and dying out at depth lacking lateral ramps and any extensive detachment horizon anticipated improvements in image resolution and lighting geometry should make it possible to document what percentage of lobate scarps share the beagle rupes style of tectonics

status of the geopotential substantial progress has been made in modeling the geopotential field since the last reporting period anderle 1979a models of the gravity potential show improvement across the entire gravitational spectrum with resolution down to 1 haif wavelength major sources of data utilized for advancing our knowledge of the geopotential field consist of satellite laser ranging smith 1982 wilson 1979 linder 1981 which contributed to the long wavelength portion of the spectrum satellite altimetry geos 3 1979 seasat 1981 and improved surface gravity anomalies rapp 1981b both of which contributed to the intermediate and short wavelength features of the spectrum uniform global distribution of data in combination solutions is generally achieved by using altimeter data over the ocean and terrestrial gravity anomalies over land

interpretation of aeromagnetic data across the central crystalline shield area of nigeria summary a residual map of the total magnetic field above 25 000 nt base is presented for a portion of the central crystalline shield area of nigeria and overlapping small portions of the chad basin and the benue rift 8 30 12 oo lat and 7 10 30 long the map based on a dataset digitized from recently released aeromagnetic sheets of nigeria leads to four results 1 a magnetic boundary evident on the map separates the younger granite complexes into two groups the groups are petrologically different and the boundary may be a fault line with uplift to the south 2 south of the boundary the map is dominated by a system of sub parallel anomalies striking ne sw possibly representing major tectonic trends and a set of fractures through which the younger granite complexes were intruded the trend of the system parallels the benue rift and lineaments in the oceanic crust off west africa 3 negative magnetic anomalies lie over most of the known ring complexes and over some suspected buried ring complexes and other intrusions 4 2 and 3 d modelling shows that the larger complexes extend to 12 km depth and the smaller ones to 6 km they have nearly vertical sides and magnetization contrasts range from 0 3 to 0 5 a m i

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

structural interpretation of thrust fault related landforms on mercury using earth analogue fault models abstract the yakima fold province has often been referenced as an earth analogue to thrust fault related landforms on many solar system bodies including mars the moon and mercury the asymmetric ridges of deformed flood basalts in this region evoke the morphology and topographic expression of the landforms observed in the smooth plains units of mercury observed similarities have supported the interpretation that thrust faults and folds comprise both sets of landforms however this is an incomplete analogy recent modeling efforts have revealed that the terrestrial landforms developed over millions of years with vertical uplift associated with each landform accumulating on multiple faults and accommodated by large folds these studies highlight the important role that folding plays in accommodating crustal shortening work presented here uses the structural geometries and folding styles from the yakima fold province as a framework for constructing ten three dimensional models of thrust fault related landforms in smooth plains units near suisei planitia on mercury which traditionally would have been classified as wrinkle ridges based on appearance and some aspects of morphology forward modeling of topography revealed that multiple faults and folds are necessary to reproduce complex topographic features folding was found to account for 50 of vertical uplift associated with the landforms and without considering folding shortening accommodated by the landforms was underestimated by 61 to 88 studies which use thrust fault related landforms as proxies for stress and strain must therefore consider the effect of folding on these estimates and their associated effect on tectonic and thermal histories

east west faults due to planetary contraction abstract contraction expansion and despinning have been common in the past evolution of solar system bodies these processes deform the lithosphere until it breaks along faults their characteristic tectonic patterns have thus been sought for on all planets and large satellites with an ancient surface while the search for despinning tectonics has not been conclusive there is good observational evidence on several bodies for the global faulting pattern associated with contraction or expansion though the pattern is seldom isotropic as predicted the cause of the non random orientation of the faults has been attributed either to regional stresses or to the combined action of contraction expansion with another deformation despinning tidal deformation reorientation another cause of the mismatch may be the neglect of the lithospheric thinning at the equator or at the poles due either to latitudinal variation in solar insolation or to localized tidal dissipation using thin elastic shells with variable thickness i show that the equatorial thinning of the lithosphere transforms the homogeneous and isotropic fault pattern caused by contraction expansion into a pattern of faults striking east west preferably formed in the equatorial region by contrast lithospheric thickness variations only weakly affect the despinning faulting pattern consisting of equatorial strike slip faults and polar normal faults if contraction is added to despinning the despinning pattern first shifts to thrust faults striking north south and then to thrust faults striking east west if the lithosphere is thinner at the poles the tectonic pattern caused by contraction expansion consists of faults striking north south i start by predicting the main characteristics of the stress pattern with symmetry arguments i further prove that the solutions for contraction and despinning are dual if the inverse elastic thickness is limited to harmonic degree two making it easy to determine fault orientation for combined contraction and despinning i give two methods for solving the equations of elasticity one numerical and the other semi analytical the latter method yields explicit formulas for stresses as expansions in legendre polynomials about the solution for constant shell thickness though i only discuss the cases of a lithosphere thinner at the equator or at the poles the method is applicable for any latitudinal variation of the lithospheric thickness on iapetus contraction or expansion on a lithosphere thinner at the equator explains the location and orientation of the equatorial ridge on mercury the combination of contraction and despinning makes possible the existence of zonal provinces of thrust faults differing in orientation north south or east west which may be relevant to the orientation of lobate scarps

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

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

limits on the brittle strength of planetary lithospheres undergoing global contraction the degree and depth of fracturing of the lithospheres of mars mercury and the moon remain poorly known it is these two properties however that govern the mechanical behavior of a planetary lithosphere considering the lithosphere as a cohesive rock mass that consists of small and large blocky interlocked rock fragments as opposed to an intact body or a body entirely lacking cohesion provides insight into the effect of lithospheric fracturing on tectonic processes on these bodies characterization of the near surface lithospheric brittle strength that incorporates the degree of fracturing is necessary for a realistic assessment of the lithospheric response to global contraction resulting from interior secular cooling such an assessment shows that all of these bodies could have accommodated substantial amounts of global contraction prior to the formation of thrust fault related landforms in fact their lithospheres were sufficiently strong so as to experience changes in radius of as much as 2 2 0 4 km mars 2 1 0 4 km mercury and 1 4 0 3 km the moon prior to the onset of widespread thrust faulting these values imply that the process of global contraction begins before any evidence of it is established in the geologic record requiring an earlier onset and for mars and mercury a faster initial strain rate of global contraction than previously thought such results add a heretofore unrecognized component of planetary radial decrease with implications for timing and strain rate to studies of global contraction on mars mercury and the moon in particular and to planetary bodies in general

a rock mechanical assessment of mercury s global tectonic fabric abstract mercury s global tectonic history is thought to have been dominated by two major processes tidal despinning and global contraction each process is expected to have produced a distinctive global stress field and resultant fault pattern thousands of thrust fault related landforms documented on mercury can be attributed to global contraction but no global signature of tidal despinning has been conclusively documented because global contraction operated throughout an extended portion of mercury s geologic history any tidal despinning pattern either would have formed together with global contraction or would have been modified by global contraction after despinning was complete here we reassess global fracture patterns predicted to result from tidal despinning and from a combination of tidal despinning and global contraction we specifically make use of rock strength and deformability parameters appropriate for mercury s fractured lithosphere results indicate that a tidal despinning pattern would consist only of a global set of opening mode fractures joints in the upper part of the lithosphere whereas the combination of tidal despinning and global contraction would have produced a global population of thrust faults with no preferred orientations in the polar regions but with an increasing preference for north south orientations toward the equator if an equatorial bulge from an early state of rapid spin was supported by mercury s lithosphere two end member scenarios for the timing and duration of these two processes may be considered in one tidal despinning predated global contraction in the other tidal despinning and global contraction overlapped in time we test the predictions of both scenarios against the distribution and orientations of mercury s tectonic landforms the global pattern of thrust faults is generally consistent with predictions for the scenario under which tidal despinning and global contraction temporally overlapped

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

duration of activity on lobate scarp thrust faults on mercury lobate scarps landforms interpreted as the surface manifestation of thrust faults are widely distributed across mercury and preserve a record of its history of crustal deformation their formation is primarily attributed to the accommodation of horizontal shortening of mercury s lithosphere in response to cooling and contraction of the planet s interior analyses of images acquired by the mariner 10 and mercury surface space environment geochemistry and ranging messenger spacecraft during flybys of mercury showed that thrust faults were active at least as far back in time as near the end of emplacement of the largest expanses of smooth plains however the full temporal extent of thrust fault activity on mercury particularly the duration of this activity following smooth plains emplacement remained poorly constrained orbital images from the messenger spacecraft reveal previously unrecognized stratigraphic relations between lobate scarps and impact craters of differing ages and degradation states analysis of these stratigraphic relations indicates that contraction has been a widespread and long lived process on the surface of mercury thrust fault activity had initiated by a time near the end of the late heavy bombardment of the inner solar system and continued through much or all of mercury s subsequent history such deformation likely resulted from the continuing secular cooling of mercury s interior

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

distribution of large scale contractional tectonic landforms on mercury implications for the origin of global stresses the surface of mercury is dominated by contractional tectonic landforms that are evidence of global scale crustal deformation using messenger orbital high incidence angle imaging and topographic data large scale lobate thrust fault scarps have been mapped globally the spatial distribution and areal density of the contractional landforms are not uniform concentrations occur in longitudinal bands and between the north and south hemispheres their orientations are generally north south at low latitude to midlatitude and east west at high latitudes the spatial distribution and distribution of orientations of these large scale contractional features suggest that planet wide contraction due to interior cooling cannot be the sole source of global stresses the nonrandom orientations are best explained by a combination of stresses from global contraction and tidal despinning combined with an equator to pole variation in lithospheric thickness while the nonuniform areal density of the contractional features may indicate the influence of mantle downwelling or heterogeneities in lithospheric strength

a preliminary study of satellite derived gravity and magnetic anomalies of india and contiguous regions abstract the residual satellite gravity anomaly map in and around the indian region has been prepared these residual anomalies are interpreted for crustal inhomogeneities using the magsat anomalies as one of the constraints while a strong correlation between these anomalies is observed over the dharwar aravalli marwar and singhbhum regions inverse correlation prevails over the eastern ghats and cambay basin the positive gravity and magnetisation strong correlation over the stable blocks of the peninsula is related to the alternating layers of basic and ultrabasic materials the positive gravity and low magnetisation poor correlation over the eastern ghats and cambay basin is interpreted as thinning of the crust the residual gravity map identifies the signatures of west central indian lineament and the submarine topographic features ridges in the indian ocean such as the chagos laccadive 85 e and ninety east ridges which are not so obvious in the unprocessed anomaly map at ground level an attempt is also made to explain the anomaly over the indo ganga brahmaputra basin using the ground and 400 km level satellite gravity anomaly maps

spherical earth gravity and magnetic anomaly analysis by equivalent point source inversion abstract to facilitate geologic interpretation of satellite elevation potential field data analysis techniques are developed and verified in the spherical domain that are commensurate with conventional flat earth methods of potential field interpretation a powerful approach to the spherical earth problem relates potential field anomalies to a distribution of equivalent point sources by least squares matrix inversion linear transformations of the equivalent source field lead to corresponding geoidal anomalies pseudo anomalies vector anomaly components spatial derivatives continuations and differential magnetic pole reductions a number of examples using 1 averaged surface free air gravity anomalies and pogo satellite magnetometer data for the united states mexico and central america illustrate the capabilities of the method

global magnetic anomaly maps derived from pogo spacecraft data abstract magnetic anomaly maps which are reduced to common elevation and to the pole are derived using data from the pogo spacecraft the maps are global and are presented in both equatorial and polar projections reduction is accomplished using the equivalent source technique including principal components analysis to deal with instabilities at and near the geomagnetic equator data reduction methods are discussed and average anomaly maps are presented together with their standard error such maps are derived for four distinct local time regions and intercompared in order to estimate the remaining effects of fields from ionospheric and magnetospheric sources

strength of faults on mars from mola topography the stresses which must be maintained on faults bounding the rift topography at tempe fossae the north tempe rift ntr and valles marineris vm on mars are estimated using a simple elastic model and topographic data from the mars orbiter laser altimeter mola the absence of rift flank uplift at the ntr is consistent with an elastic thickness te of 20 km or greater at the time of rift formation the maximum resolved shear stresses on bounding faults due to this topography do not therefore exceed 20 mpa similar to the inferred strength of terrestrial faults elastic thickness estimates at vm are mostly around 50 km or greater therefore for canyon widths of 400 km the bounding faults of vm if present must be able to withstand stresses of up to approximately 100 mpa however if the fault controlled sections of the canyons do not exceed 150 km in width as suggested by geomorphological analysis the fault strength required is only 20 mpa although the maximum resolved shear stresses required to support the topography at vm may need to be greater than the stresses which terrestrial faults can support at least some faults on mars are no stronger than similar features on earth this observation is consistent with the existence of liquid water in the shallow subsurface of mars at the time the faults were active on venus plate tectonics is probably prevented by the frictional resistance to motion across strong faults on mars it is more likely that the large thickness of the elastic layer of the lithosphere and the possible positive buoyancy of the crust are responsible for the observed lack of plate tectonics c 2002 elsevier science usa

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

regional north american gravity and magnetic anomaly correlations summary correlation of gravity and magnetic anomalies combined with other geological and geophysical data is useful for enhancing the quality of geological interpretation of potential anomaly fields maps produced by equivalent point source inversion are used to investigate visual spatial correlations of surface free air gravity and pogo satellite magnetic anomalies and regional heat flow and tectonic data for north america and adjacent marine areas a quantitative analysis of regional potential field anomaly correlations at satellite elevations is also considered utilizing poisson s theorem in a moving window linear regression between derivatives of the anomalous gravity and magnetic fields an inverse relationship is observed between long wavelength gravity and magnetic anomalies over continental terrain negative gravity and positive magnetic anomalies are areas characterized by relatively thick crust and high magnetization an example is a prominent magnetic high which corresponds to a trend of gravity minima extending from the anadarko basin to the cincinnati arch negative magnetic and positive gravity anomalies characterize thinner crust and regions of higher heat flow such as the cordillera of north and central america and specifically the yellowstone geothermal region although gravity and magnetic anomalies over oceanic areas show poor correlation the sign of the statistical correlation generally is positive 1 crustal correlation considerations the utility of long wavelength gravity anomalies covering several degrees of surface area for geological analysis is well recognized magnetic anomalies which have significant energy in wavelengths of several hundred kilometres also have been identified as originating from within the earth s crust and perhaps the uppermost mantle the geological interpretation of these regional anomalies is hindered by the effect of anomaly superposition and source ambiguity which is inherent to the analysis of potential fields an approach to minimizing these limitations especially in continental terrains is to evaluate the correlation between anomalous gravity and magnetic fields the basis of correlation analysis is the hypothesis commonly validated in continental areas that variations in lithology and physical properties of the crystalline crust are reflected

satellite magnetic anomalies and the middle america trench abstract quantitative modeling of the middle america trench satellite elevation crustal magnetic anomaly using seismic epicenter data to constrain the slab geometry and a magnetization representing induced and viscous contributions similar to that found for the aleutian arc slab agrees well with reduced to pole magsat data incorporating minor contributions from crustal sources in mexico and central america produces only minor improvement in the agreement between the modeled and observed anomaly supporting the idea that it is the slab which is the major source body for the middle america trench anomaly

lithospheric magnetic property contrasts within the south american plate derived from damped least squares inversion of satellite magnetic data abstract a ridge regression or damped least squares method has been used in a large scale equivalent source inversion of south american magsat data the optimum damping parameter is selected based on percent increase in the sum of squared residual of the observed and the calculated anomaly and on the variance and the range of the solution for a series of damping parameters solutions obtained by the damping parameter selected in this manner are numerically stable and geologically meaningful consequently these solutions are used in various applications such as transformation into effective magnetic susceptibility variation altitude normalization differential reduction to pole and downward continuation of the data set the spacing of equivalent sources and the source observation distance are shown to introduce significant errors in the estimation of the physical property variation from equivalent source solutions for the large source spacing 1 commonly in use the correction for these geometric factors is essential in estimating the physical property variation from the equivalent source solutions a continental and an oceanic profile have been analyzed from the south american maps derived from the above techniques the continental profile transects a variety of geologic tectonic features including shields aulacogens basins and fold belts near the north coast of south america along the profile there is a good correlation between the radially polarized magsat magnetic anomalies and other available geologic and geophysical information in addition the variation in effective susceptibility obtained from the damped equivalent source inversion is consistent with our present knowledge of continental lithospheric susceptibilities expected from the geologic tectonic provinces along the profile the oceanic profile extends from the trailing margin of south america through the cretaceous normal polarity rocks of the south atlantic ocean the variations in the magnetic properties along this profile can be broadly correlated with the depth anomaly variation indicating a possible genetic relationship two conceptual models requiring undepleted mantle intrusions under the oceanic uplifts appear possible in the first model the high magnetization can be accounted for by a weakly magnetic upper mantle below the uplifts in the second model very highly magnetic rocks are required in the crust of these uplifted regions to account for the observed anomaly values

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

despinning plus global contraction and the orientation of lobate scarps on mercury predictions for messenger abstract observations by the mariner 10 spacecraft suggest that the lobate scarps on mercury which have been interpreted to record at most 1 2 km of radial contraction of the planet after the end of the late heavy bombardment possess a global preferred n s orientation but lack a strong latitudinal dependence on their surface expression here we reexamine the idea that a decrease in the planetary rotation rate despinning coupled with global contraction of at least 3 5 5 km prior to the end of late heavy bombardment resulted in global n s oriented thrust faults the surface expression of these faults is assumed to have been erased by the end of the late heavy bombardment and the faults were subsequently reactivated by later global contraction producing generally n s oriented thrust faults from an isotropic stress field we use the estimate of 3 5 5 km contraction prior to 4 ga as an additional constraint to thermomechanical simulations of the evolution of mercury finding that a wide range of models are consistent with this observation the fact that a wide range of states are consistent with the contraction of mercury prior to the end of late heavy bombardment but only a restricted set of states are consistent with the at most 1 2 km of subsequent contraction bolsters the idea that there may be hidden strain on mercury features unseen by mariner 10 but likely visible to the messenger spacecraft

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

late movement of basin edge lobate scarps on mercury basin edge lobate scarps are a sub type of tectonic shortening structure on the surface of mercury that have formed at the edge of volcanic units that fill or partly fill impact basins we have performed a global survey of these features and find that they are widespread in basins across the planet we obtained model ages from crater size frequency distribution analysis for a subset of our surveyed basins for both the smooth plains infill and for the last resolvable tectonic activity on the associated basin edge scarps our results indicate that some of these lobate scarps were still accumulating strain in the late mansurian approximately 1 ga from a photogeological assessment we find that the orientations of these basin edge lobate scarps are similar to those reported for the global population of lobate scarps in earlier studies appearing to align north south at low latitudes and east west at higher latitudes however reassessing these landforms orientation with artificially illuminated topographic data does not allow us to rule out the effect of illumination bias we propose that these landforms the result of crustal shortening in response to global contraction formed along the interface between the basin floor and the smooth plains unit which acted as a mechanical discontinuity along which shortening strains were concentrated

the tectonics of mercury the view after messenger s first flyby article i nfo during its first flyby of mercury messenger imaged many tectonic landforms most of which are contractional in nature and include lobate scarps high relief ridges and wrinkle ridges wrinkle ridges are found on the smooth plains that partially fill the interior and surround the exterior of the caloris basin and also on smooth plains that fill the interiors of smaller impact basins and larger craters messenger revealed a radial graben complex pantheon fossae nearlyco centered with the caloris basin pantheon fossae and a network of mostly basin concentric graben intheouterportionsofthecalorisbasininteriorformapatternofextensionnotseenelsewhereonmercury the first clear example of extensional faults outside of the caloris basin has been documented on smooth plains inside the peak ring of the relatively young raditladi basin a map of the distribution of tectonic landforms imaged by messenger and mariner 10 shows that lobate scarps are the most widespread type of feature estimates of the horizontal shortening associated with lobate scarps that crosscut and overthrust small impact craters imaged by messenger range from 1 to 3 km previously unrecognized lobate scarps detected in areas imaged by mariner 10 indicate thatearlierestimatesof contractional strainarelow anewestimate of theaverage arealcontractionalstrain 0 06 accommodated byscarps is at leastonethirdgreater thancomparableprevious estimates andcorresponds to a decrease in mercury s radius of at least 0 8 km since the end of heavy impact bombardment of the inner solar system these figures are lower bounds because of the likelihood that not all lobate scarps have been identified even in areas imaged to date crosscutting and embayment relations revealed by messenger suggest that lobate scarps formed before the end of smooth plains emplacement and continued to be active after the emplacement of the youngest smooth plains deposits relatively recent activity on lobate scarps may be the expression of slow but continuous radial contraction that accompanied cooling of mercury s interior and the growth of the planet s solid inner core published by elsevier b v

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

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

thrust faults and the global contraction of mercury 1 lobate scarps on mercury are the surface expression of thrust faults and are generally thought to be the result of global contraction our analysis indicates that these thrust faults are not uniformly distributed and exhibit non random patterns on the imaged hemisphere the greatest area normalized cumulative length of the faults occurs in the southern hemisphere below 50 s the thrust faults in this region dip to the north indicating a broad zone where the crust has been displaced to the south widespread thrust faulting may not have initiated before the emplacement of calorian smooth plains well after the emplacement of pre tolstojan intercrater plains our observations suggest that regional scale tectonic stresses on mercury dominated over stresses resulting from global contraction

delineation of canadian sedimentary basins from magsat data abstract a crustal scalar magnetic anomaly map of canada and the northern united states is derived using data collected from the magsat satellite the anomalies are correlated to geological features basins associated with failed arms of old rifts have high magnetic anomalies the rocky mountains the appalachian mountains the suture zone of the grenville province modern hotspots and ocean ridges have low magnetic anomalies

a preliminary assessment of the igrf 1990 for mexico abstract a preliminary assessment of the igrf 1990 in mexico is carried out by comparing geomagnetic elements measured throughout mexico and igrf derived geomagnetic values mean differences of 10 6 nt 24 4 nt and 0 6 are obtained for total intensity horizontal component and declination rms deviations of 206 nt 116 nt and 18 7 respectively the differences correlate with magsat derived magnetic anomalies e g they are associated with crustal sources we conclude that the igrf provides a satisfactory geomagnetic reference field for mexico

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

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

accommodation of lithospheric shortening on mercury from altimetric profiles of ridges and lobate scarps measured during messenger flybys 1 and 2 the mercury laser altimeter on the nasa messenger mission has ranged to several ridges and lobate scarps during two equatorial flybys of the planet mercury the tectonic features sampled like others documented by spacecraft imaging and earth based radar are spatially isolated and have vertical relief in excess of 1 km the profiles also indicate that the faulting associated with their formation penetrated to tens of kilometers depth into the lithosphere and accommodated substantial shortening to gain insight into the mechanism s of strain accommodation across these structures we perform analytical and numerical modeling of representative dynamic localization mechanisms we find that ductile localization due to shear heating is not favored given our current understanding of thermal gradients and shallow thermal structure of mercury at the time of ridge and scarp formation and is likely to be of secondary importance at best brittle localization associated with loss of resistance during fault development or with velocity weakening during sliding on mature faults is weakly localizing but permits slip to accumulate over geological time scales the range of shallow thermal gradients that produce isolated faults rather than distributed fault sets under the assumption of modest fault weakening is consistent with previous models for mercury s early global thermal history to be consistent with strain rates predicted from thermal history models and the amount of shortening required to account for the underlying large offset faults ridges and scarps on mercury likely developed over geologically substantial time spans

topography of lobate scarps on mercury new constraints on the planet s contraction lobate scarps are landforms on mercury that appear to have formed by thrust faulting and are thought to reflect global contraction due to cooling of the planet s interior topographic data for 10 mercurian lobate scarps derived from mariner 10 images using photoclinometry and digital stereoanalysis indicate a range in maximum relief of 0 1 to 1 5 km and a range in horizontal shortening of 0 3 to 3 2 km assuming fault plane dips of 25 previous estimates of cumulative compressional strain recorded in the lobate scarps suggest a decrease in mercury s radius of 1 to 2 km our estimate of compressional strain based on these new topographic measurements is 0 056 25 this suggests that mercury s radius decreased by 1 km

stratigraphy of the upper ordovician upper vaureal and ellis bay formations eastern anticosti island quebec marked facies changes occur in late ordovician strata assigned to the uppermost vaureal and ellis bay formations ashgill rawtheyan hirnantian on anticosti island quebec western anticosti features shales and carbonates whereas outcrops along the eastern coast contain prominent discontinuous mixed siliciclastic carbonate units detailed section measurement along the northeast coast allows for the first time accurate definition of seven new members within this uninterrupted sequence sands present in the upper vaureal and lower ellis bay formations in the east appear to have deterred the growth of muddy bottom brachiopod communities comparable to those in the western and central regions of anticosti sand units within the upper vaureal formation contain 1 m diameter colonies of paleofavosites coeval small coral patch reefs are found in the central part of the island where sands are absent the uppermost ellis bay formation of northeast anticosti is marked by a shallow subtidal coral algal onco

on the origin of graben and ridges within and near volcanically buried craters and basins in mercury s northern plains 1 images of mercury s northern volcanic plains taken by the messenger spacecraft reveal a large number of buried impact craters and basins discernible by wrinkle ridge rings that overlie their rims many of these ghost craters and basins contain interior graben of diverse widths and orientations here we use finite element models to test a variety of mechanisms for the formation of these graben and ridges results show that graben are best explained by cooling of large thicknesses of flood lavas within the craters and basins conservation of surface area during cooling induces the required extensional stress state in contrast the development of wrinkle ridge rings is best explained as the result of cooling and contraction of mercury s interior during which a reduction in mercury s surface area led to a compressional state of stress the critical factor in determining where large graben form is the thickness of the youngest cooling unit the topmost sequence of lavas that cooled coevally a thicker cooling unit leads to a deeper initiation of normal faulting wider graben floors consistent with observations the widest graben are predicted to occur where pooled lavas were thickest and no graben are predicted within generally thinner plains outside of major craters observed concentrically oriented graben can be explained by variations in the thickness of the youngest cooling unit in contrast none of the basin uplift mechanisms considered including isostatic response to crater topography inward flow of the lower crust or exterior loading by volcanic plains can account for concentrically oriented graben

magsat scalar anomaly map of south america a scalar magnetic anomaly map for south america and adjacent marine areas was prepared from magsat data preparation of the map posed problems notably in separating external field and crustal anomalies and in reducing data to a common altitude external fields are manifested in a long wavelength ring current effect a medium wavelength equatorial electrojet and short wavelength noise the ring current is corrected through use of a standard ring current equation augmented by further wavelength filtering because the electrojet is confined primarily to dusk profiles its effect is minimized by drawing the data set from dawn profiles only the noise is reduced by selecting profiles from quiet periods k 2 the optimum filter cutoff level is determined by a statistical regression technique to be approximately 50 degrees wavelength when wavelengths longer than this are rejected resultant profiles from redundant satellite orbits correlate well with differences due primarily to satellite altitude the

a closer examination of the reduction of satellite magnetometer data for geological studies global and regional studies of the data abstracted from the pogo satellites have demonstrated the utility of satellite magnetometer measurements in geological studies however detailed examination of these data reveals that there exists a considerable amount of noise resulting from external fields and data reduction techniques such noise inhibits more detailed studies with the pogo data and could similarly affect the utility of data from the magsat mission correlation of pogo satellite data with ground observatory records and geomagnetic indices was employed to identify the existence of external field effects and document the data reduction problems significant effects of the sq current system including the equatorial electrojet are shown to be still in the data despite the local time screening such effects are totally in agreement with theoretical models of this current system and should be considered in magsat data reduction as the theoretical model predicts that the sq current may at times have significant effects even at the dawn dusk local times of the magsat orbit the state of inflation and deflation of the magnetosphere is vividly portrayed in the pogo measurements and is well correlated with the dst index the ring current correction applied to the pogo data was seen to be of varying quality depending upon the state of the magnetosphere and the length of satellite track data reduced these effects combined with the irregular distribution of data from the individual pogo satellites can result in pseudo anomalies as evidenced by a comparison of satellite data reduced in a fashion to minimize them with those previously published over the same area

faulted craters as indicators for thrust motions on mercury craters cross cut by faults are used as markers to obtain fault geometric and kinematic properties assumingthattheshapeofthesecraterswasoriginallycircular itispossibletomeasure the horizontal and vertical components of fault displacement as well as the slip trend by applying trigonometric relations slip plunge displacement magnitude fault true dip and fault rake can be derived from the observed values an example application of this method on craters faulted by lobate scarps on mercury shows that most of these inferred reverse faults have moderate oblique slip trends moreover the derived dips of thrusts vary over a wide range of angles some preliminary results in terms of fault rake compared with fault dip strike and latitude are pre sented together with a pilot study to test and discriminate global tectonic models suggested for the evolution of mercury the possibility of estimating quantitative fault parameters through remotely sensed data provides significant assistance in the structural characterization of faults on planetary surfaces

late ordovician early silurian ashgillian llandovery sea level curve derived from conodont community analysis anticosti island quebec abstract earlier workers have established sea level curves for the ashgillian llandovery sequence on anticosti island quebec canada based on sedimentology and on brachiopod and reef communities the complete anticosti sequence has been collected for conodonts to provide a database of over 77 600 conodonts derived from 272 samples through the 800 1100 m sequence this paper uses a statistical analysis of the database to establish the pattern of conodont communities and their interpreted water depths this allows the establishment of a eustatic sea level curve for the sequence this curve is in general agreement with those of the earlier workers but is more detailed and better constrained stratigraphically given the large sampling program the curve reflects more oscillating sea level changes for the whole sequence than those constructed earlier and different water depths for certain intervals from those established earlier the curve is valuable given that the anticosti sequence is one of the most complete in the world for this interval and its divergence from global eustatic curves for this interval is noted

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

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

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

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

long wavelength topography on mercury is not from folding of the lithosphere abstract previous work suggested that the lithosphere of mercury could undergo folding in response to global contraction and indeed observations from the messenger mission revealed several regions where long wavelength topography is present here we test via finite element simulations that use a more realistic rheological model than that earlier work lithospheric folding as a formation mechanism for long wavelength topography on mercury from interior secular cooling over the last 3 8 gyr this radial contraction has been estimated from geological observations to be less than 10 km which translates into small amounts of horizontal shortening of

an interpretation of magnetic signatures of aulacogens and cratons in africa and south america abstract magnetic susceptibility anomalies associated with the lithosphere in africa and south america show that the lithosphere underlying sedimentary basins within aulacogens which originated during the rupture of the gondwana continent have lower magnetic susceptibility than the lithosphere underlying the cratons this can be explained by the demagnetization effects of high temperature asthenospheric material possibly intruded into the aulacogens during the rupture thermal evolution of different intrusion models implies that a thermal demagnetization may last for about 300 ma b there is no close relationship between the early subsidence rate of a basin and the magnetization of the underlying lithosphere c surface heat flow anomalies do not necessarily correlate with the magnetic susceptibility anomalies and d once an intrusion of more magnetic material cools to the ambient temperature the total magnetic susceptibility of the lithosphere may increase high magnetic susceptibilities associated with sedimentary basins over old crustal rifts can be explained by this phenomenon

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

intermediate wavelength magnetic anomalies over the central pacific we have extracted the intermediate wavelength magnetic anomaly field from the marine survey data of the central pacific in the band pass of 4000 400 km spherical harmonic degree 10 100 our technique minimizes the effects of external field sources secular variation and strike aliasing the objective of this paper is to demonstrate this capability compare the derived sea surface field to the equivalent magsat data set and demonstrate that anomalies observed in both fields are correctable to geologic features within the oceanic lithosphere as expected the sea surface field exhibits a strong spatial correlation to and a better resolution than the magsat field however the magsat field also displays a diminished resolution for wavelengths less than 1900 km when compared to the upward continued sea surface data two reasons are likely for this diminished resolution 1 differences in the resolution of the spherical harmonic reference fields and 2 an increased noise level in the magsat data at the shorter wavelengths other sources of error are also considered we also demonstrate that the remanent magnetization of the central pacific seamounts produces negative magnetic anomalies which are observed at satellite altitude other sources including the thickened crust of the oceanic plateaus and regional petrologic variations can be associated with intermediate wavelength anomalies observed in both the magsat and the sea surface fields

a satellite magnetic model of northeastern south american aulacogens magnetic modeling of the amazon river and takatu aulacogens in northeastern south america illustrate the utility of satellite magnetic data in characterizing the properties and structure of the lithosphere specifically reduction of preliminary magsat scalar magnetic anomaly data to an equivalent condition of vertical polarization shows a general correlation between magnetic anomaly minima and the amazon river and takatu aulacogens surface gravity data demonstrate a correlative positive anomaly spherical earth modeling of the magnetic data indicates a less magnetic crust associated with the aulacogens which is compatible with previous studies over the mississippi river aulacogen and rio grande rift in north america

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

tectonic patterns of shortening landforms in mercury s northern smooth plains abstract mercury s northern smooth plains are volcanically emplaced units characterized by ghost craters volcanically buried impact basins and thrust fault related landforms we analyzed the thrust fault related landforms traditionally categorized as lobate scarps and wrinkle ridges within the northern plains in order to describe trends in how these landforms are organized and oriented and what style of deformation either thin or thick skinned their map patterns represent our analysis also establishes geologic constraints for which global processes may have produced stresses contributing to these tectonic patterns we mapped 4853 thrust fault related landforms in the northern plains at a map scale of 1 1 000 000 using three mercury surface space environment geochemistry and ranging messenger global monochrome mosaics these landforms described as curvi linear asymmetric ridges frequently occur in complex geometrical arrangements that are interpreted to share similar structural characteristics we called these arrangements compound landforms like prior studies we observed thrust faults to follow rims of buried craters we also observed 1 sigmoidal rises bounded by fault related landforms 2 v shaped rises composed of two landforms terminating at a single sharp point 3 broad arcuate rises of nearly equal width 4 parallel evenly spaced ridges and 5 landforms showing alternation in direction of tectonic transport along strike respectively we interpreted these landforms as transpressional uplifts faults with sharply juxtaposing ramps pop up structures fold and thrust belts and antithetic fault intersections by comparison with earth analogues and patterns produced in numerical and physical models our results suggest that deformation in the nsps is thin skinned orientation analysis showed that the northernmost landforms 90 70 n were predominantly oriented east west while most of the landforms between 50 and 30 n were oriented north south variations in orientation with latitude indicate that the growth of thrust fault related landforms was influenced by sources of stress other than global contraction if reorientation of the pole due to the formation of the caloris basin impact did occur the pattern of fault orientations indicates that geologic processes producing the pattern operated after reorientation

thermal evolution of mercury as constrained by messenger observations 1 orbital observations of mercury by the mercury surface space environment geochemistry and ranging messenger spacecraft provide new constraints on that planet s thermal and interior evolution specifically messenger observations have constrained the rate of radiogenic heat production via measurement of uranium thorium and potassium at the surface and identified a range of surface compositions consistent with high temperature high degree partial melts of the mantle additionally messenger data have placed new limits on the spatial and temporal variation in volcanic and tectonic activity and enabled determination that the planet s core is larger than previously estimated because mercury s mantle layer is also thinner than previously thought this result gives greater likelihood to the possibility that mantle convection is marginally supercritical or even that the mantle is not convecting we simulate mantle convection and magma generation within mercury s mantle under two dimensional axisymmetry and a broad range of conditions to understand the implications of messenger observations for the thermal evolution of the planet these models demonstrate that mantle convection can persist in such a thin mantle for a substantial portion of mercury s history and often to the present as long as the mantle is thicker than 300 km we also find that magma generation in mercury s convecting mantle is capable of producing widespread magmas by large degree partial melting consistent with messenger observations of the planet s surface chemistry and geology

regional magnetic and gravity anomalies of south america preliminary satellite scalar magnetic anomaly data magsat reduced to vertical polarization and long wavelength pass filtered free air gravity anomaly data of south and central america are compared to major tectonic features a number of correlations are observed but these must be generalized because of the preliminary nature of the geophysical data and the inherent petrophysical variations within tectonic features statistical analysis of the magnetic data reveals that south and central america are more magnetic and magnetically more variable than adjacent marine areas more obvious correlations exist between magnetic anomalies and tectonic elements of the continents than in the case of oceanic areas no obvious correlations occur between the tectonic features of the atlantic ocean including the mid atlantic ridge and magnetic anomalies the continental shields generally are more magnetic than adjacent basins oceans and orogenic belts in contrast the major aulacogens are characterized by negative magnetic anomalies positive free air gravity anomalies are related to the andean foldbelt but the relationship of this feature to magnetic anomalies is much less obvious however along the west coast of south america the magnetic anomalies of the pacific ocean are separated from those of the eastern platforms by north to northwest trending anomalies south of the equator along the foldbelt gravity maxima are related to magnetic minima a relationship analogous to the situation observed in the rocky mountain cordillera north of the equator in columbia gravity and magnetic maxima roughly correlate along the foldbelt