cambrian grand cycles a northern appalachian perspective large scale sedimentary cycles or grand cycles are the most obvious features of the port au port group in southwestern newfoundland each cycle is interpreted to represent deposition in outer platform muddy tidal flats lower shaly half cycle and ooid shoal complexes upper carbonate half cycle unpredictable metre scale assemblages of oolite and carbonate laminite constituting the carbonate half cycles are attributed to frequent migration of the shoal complexes in response to hydrographic factors such as tidal fluctuations and storms predictable metre scale shallowing upward cycles of parted limestone and shale in the shaly half cycles were probably controlled mainly by variable rates of carbonate sedimentation a eustatic mechanism however cannot be discounted comparison with other north american grand cycles suggests a eustatic mechanism for grand cycle formation possibly involving variations in the rate of sea level rise the interpreted sequence of formation established on the basis of the port au port group involves 1 a rapid sea level rise which flooded the platform with shoreward derived siliciclastic muds and resulted in the development of muddy tidal flats and 2 the gradual development of ooid shoal complexes which encroached onto the tidal flats with decreasing rates of sea level rise in contrast to western north american examples newfoundland grand cycles are composed almost entirely of peritidal sediments they are also thinner and have a smaller areal extent than do those in the west these differences may be due to such factors as narrower shelves slower rates of sedimentation and lower amplitudes of sea level rise on the present northeastern margin of the north american craton than on the western margin

arenigian early ordovician sea level history and the response of conodont communities western newfoundland four cluster analyses were performed which recognized 17 conodont communities in the arenigian lower ordovician of western newfoundland the analyses include 69 598 identifiable conodont specimens recovered from 153 conodont bearing samples from four stratigraphical sections representing the environmental settings of the platform upper proximal slope lower proximal slope and distal slope the distribution of conodont communities along the platform to slope environmental gradient shows that sea level changes simultaneously affected the development and replacement of the conodont communities in the different facies the pattern of change in conodont communities allows an interpretation of sea level change that is correlated precisely into the detailed graptolite biozonation a gradual transgression lasted most of tetragraptus approximatus zone time which was followed by a brief regression a transgression regression cycle occurred in the t akzharensis zone time a major transgression caused a highst

the road river group of northern yukon canada early paleozoic deep water sedimentation within the great american carbonate bank cambrian devonian sedimentary rocks of the northern canadian cordillera record both the establishment and demise of the great american carbonate bank a widespread carbonate platform system that fr

dolomitization of the lower ordovician aguathuna formation carbonates port au port peninsula western newfoundland canada implications for a hydrocarbon reservoir the lower ordovician aguathuna formation 100 m thick is formed of shallow marine carbonates which constitute the uppermost part of the st george group of western newfoundland sedimentation wa

dolomitization and isotope stratigraphy of the vazante formation sao francisco basin brazil the vazante formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the vazante carbonates dolomites can be classified based on petrography and geochemistry into four generations ranging in crystal size between 4 m almost micritic and 3 mm and occurring as both replacements and cements the sr ca molar ratios calculated for the dolomitizing fluid 0 0006 0 0138 suggest a contribution from a non marine possibly meteoric water component the 18 oa nd 13 c values vary from 0 1 to 14 3 pdb and 0 2 to 2 3 pdb respectively fluid inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 c a mixing zone model of dolomitization for dolomites i iii satisfies the constraints from elemental chemistry 18 o 87 sr 86 sr and fluid inclusions the petrographic and chemical criteria of the latest generation dolomite iv reflect conditions of deep burial environment at temperatures above 120 130 c the 13 c values show only small variations at the base of the formation followed by a major negative plunge 4 at the top this plunge correlated with the sturtian glacial phase in contrast to c isotopes only few samples may have retained their near primary 87 sr 86 sr values of 0 70614 0 70734 the lowest 87 sr 86 sr value 0 70614 from fibrous cement in the upper part of the formation correlates with the negative 13 c shift and is also consistent with the sr isotope signature proposed for the glacial sturtian seawater the 34 s values obtained from sulfates trapped in carbonates range between 10 8 and 16 9 with a jump to 21 3 in the overlying formation all isotope signals are within the range suggested for the early neoproterozoic seawater but the precambrian baseline is poorly known and additional work is required to confirm this tentative assignment 2001 elsevier science b v all rights reserved

the geologically recent giant impact basins at vesta s south pole since 17 july 2011 nasa s spacecraft dawn has been orbiting the asteroid vesta the second most massive and the third largest asteroid in the solar system see the cover russell et al p 684 1 use dawn s observations to confirm that vesta is a small differentiated planetary body with an inner core and represents a surviving proto planet from the earliest epoch of solar system formation vesta is also confirmed as the source of the howardite eucrite diogenite hed meteorites jaumann et al p 687 2 report on the asteroid s overall geometry and topography based on global surface mapping vesta s surface is dominated by numerous impact craters and large troughs around the equatorial region marchi et al p 690 3 report on vesta s complex cratering history and constrain the age of some of its major regions based on crater counts schenk et al p 694 4 describe two giant impact basins located at the asteroid s south pole both basins are young and excavated enough amounts of material to form the vestoids a group of asteroids with a composition similar to that of vesta and hed meteorites de sanctis et al p 697 5 present the mineralogical characterization of vesta based on data obtained by dawn s visual and infrared spectrometer revealing that this asteroid underwent a complex magmatic evolution that led to a differentiated crust and mantle the global color variations detailed by reddy et al p 700 6 are unlike those of any other asteroid observed so far and are also indicative of a preserved differentiated proto planet 1 lookup doi 10 1126 science 1219381 2 lookup doi 10 1126 science 1219122 3 lookup doi 10 1126 science 1218757 4 lookup doi 10 1126 science 1223272 5 lookup doi 10 1126 science 1219270 6 lookup doi 10 1126 science 1219088

introduction to advances in low temperature geochemistry diagenesis and seawater and climate evolution through the earth s history a tribute to jan veizer this special issue of the canadian journal of earth science which is entitled advances in low temperature geochemistry diagenesis seawater and climate evolution through the earth s history a special tribute to j veizer is in honor of professor dr jan veizer who is the distinguished university professor emeritus of earth sciences at the university of ottawa and former chair of sedimentary and isotope geology institute for geology mineralogy und geophysics of ruhr university bochum germany

vesta surface thermal properties map the first ever regional thermal properties map of vesta has been derived from the temperatures retrieved by infrared data by the mission dawn the low average value of thermal inertia 30 10 j m 2 s 0 5 k 1 indicates a surface covered by a fine regolith a range of thermal inertia values suggesting terrains with different physical properties has been determined the lower thermal inertia of the regions north of the equator suggests that they are covered by an older more processed surface a few specific areas have higher than average thermal inertia values indicative of a more compact material the highest thermal inertia value has been determined on the marcia crater known for its pitted terrain and the presence of hydroxyl in the ejecta our results suggest that this type of terrain can be the result of soil compaction following the degassing of a local subsurface reservoir of volatiles

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

elastoviscoplastic relaxation of impact crater topography with application to ganymede and callisto 1 we use an elastoviscoplastic rheological model to study the long term relaxation of impact crater topography on ganymede and callisto we employ a complete set of rheological parameters for ductile creep in ice ih and implement a new shallower initial shape model under high heat flow conditions stresses are relieved in deeper less viscous material and the remainder concentrates as flexural bending stresses in a viscously stiffer near surface layer essentially a mechanical lithosphere forms that focuses stresses but with uplifts and stress levels that are always greater than those obtained for a purely viscous half space we discuss how classic elastic flexure and purely viscous relaxation are special cases of this more generalized rheological model stress concentration in the flexing i e bending layer can exceed the plastic yield strength of ice for high enough heat flows weakening the layer somewhat and increasing the total amount of relaxation these plastic strains tend to be limited to small magnitudes 1 mm permits both retention and relaxation of topography at the lower and higher ends respectively of the anticipated heat flow range consideration of relaxed craters in marius regio and of the palimpsests and penepalimpsests suggests ancient heat flows in excess of 10 mw m 2 consistent with other estimates

carbon isotope stratigraphy of the uppermost cambrian in eastern laurentia implications for global correlation the 13 c profile from an interval of the martin point section in western newfoundland canada spans the upper furongian uppermost cambrian the interval 90 m is a part of the green point formation of the cow head group and consists of the martin point lower and the broom point upper members it is formed of slope marine carbonates alternating with shales rhythmites and conglomeratic interbeds the preservation of the investigated micritic carbonates was meticulously evaluated by multiple petrographic and geochemical screening tools the 13 c and 18 o values 0 5 0 8 vpdb and 7 1 0 3 vpdb respectively exhibit insignificant correlation r 2 0 002 and similarly the correlation of 13 c values with their sr and mn counterparts which supports the preservation of at least near primary 13 c signatures that can be utilized to construct a reliable high resolution carbon isotope profile for global correlations the 13 c profile exhibits two main negative excursions a lower broad excursion 3 that reaches its maximum at 70 m below the martin point broom point members boundary and an upper narrow excursion 2 5 immediately below the same boundary the lower excursion can be correlated with the global latest furongian herb event toce which is also recognized in the c isotope profile of the gssp boundary section at green point whereas the upper excursion matches with that of the cambrian ordovician boundary in the same section the peak of the herb 13 c excursion is correlated with positive shifts on the th u and ni profiles redox and productivity proxies

stratigraphie framework for the cambrian ordovician rift and passive margin successions from southern quebec to western newfoundland the taconian humber zone stretches from western newfoundland to southern quebec the early cambrian slope succession in newfoundland is found in the curling group whereas in quebec various units were deposited during that first time slice biostratigraphic data allow correlation of the curling group with the labrador group in newfoundland and with the newly time constrained slope succession in quebec the end of the rift drift transition is marked by a sea level lowstand at the end of the early cambrian the middle cambrian to latest early ordovician passive margin history recorded five cyclic sea level fluctuations three of these cycles are recorded in the shallow marine middle to late cambrian platform port au port group and slope sediments preserved in the cow head and northern head groups in newfoundland the biostratigraphic information assists correlation with cambrian passive margin units in quebec major sea level lowstands are recognized along the continental margin in early middle late camb

vesta s pinaria region original basaltic achondrite material derived from mixing upper and lower crust abstract analysis of data from the dawn mission shows that the pinaria region of vesta spanning a portion of the rim of the rheasilvia basin is bright and anhydrous reflectance spectra absorption band centers and their variations cover the range of pyroxenes from diogenite rich to howardite and eucrite compositions with no evidence of olivine in this region by examining band centers and depths of the floor walls and rims of six major craters in the region we find a lane of diogenite rich material next to howardite eucrite material that does not follow the local topography the source of this material is not clear and is probably ejecta from post rheasilvia impacts material of a howardite eucrite composition originating from beyond the rheasilvia basin is evident on the western edge of the region overall the pinaria region exposes the complete range of basaltic achondrite parent body material with little evidence of contamination of non basaltic achondrite material with both high reflectance and low abundance of hydrated material this region of vesta may be considered the pinaria desert

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

carbon isotope stratigraphy of the lower ordovician succession in northeast greenland implications for correlations with st george group in western newfoundland canada and beyond article i nfo the lower ordovician sequence on the ella o island in northeast n e greenland is a thick shallow marine platform carbonate sequence 1415 m thick and constitutes the major part of the kong oscar fjord group it consists from bottom to top of the antiklinalbugt septemberso and cape weber formations which are believed to be respectively coeval with the watts bight boat harbour and catoche formations of the st george group in western newfoundland canada samples were collected from outcrops at high resolution intervals and micritic materials were extracted by microdrilling after screening their petrographic and geochemical criteria to evaluate the degree of preservation the 13 c and 18 o values of well preserved micrite microsamples range from 5 2 to 0 5 vpdb and from 10 3 to 6 5 vpdb respectively the 13

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

geochemistry of modern brachiopods applications and implications for oceanography and paleoceanography abstract modern rhynchonellida terebratulida thecideida and a few inarticulated brachiopods were collected live or soon after death at 30 locations from depths of 2 3940 m from the equator to high latitudes covering the atlantic pacific indian caribbean and mediterranean oceans seas well preserved fibers of the secondary layer combined with sr 450 1928 mg kg mn 1 199 mg kg and fe 1 610 mg kg contents confirm with few exceptions the excellent preservation of the modern brachiopod material evaluation of the stable isotope compositions was conducted on 276 specimens and on 374 published samples for a total database of 650 samples from 80 locations it revealed that oxygen isotope incorporation into shell calcite secondary layer of modern articulated brachiopods in most cases occurs in equilibrium with the ambient water and as such reflects surrounding environmental conditions thecideida as a group shallow water and low latitude incorporate oxygen isotopes 3 04 to 0 35 into shell calcite in equilibrium with their ambient water a large majority of genera and species of the terebratulida shallow water and low mid latitude also incorporate oxygen 2 30 to 3 48 into shell calcite in isotopic equilibrium with their surrounding water in contrast equilibrium incorporation of oxygen isotopes into shell calcite by the rhynchonellida is uncertain but a vital effect probably applies to the craniida the question of equilibrium disequilibrium incorporation of carbon isotopes into shell calcite of modern brachiopods remains unresolved at this time isotope compositions range up to 4 5 5 6 and 1 5 for 13 c and up to 5 5 5 9 and 0 3 for 18 o for shallow water low mid and high latitude brachiopods respectively strontium isotopes of modern brachiopods as expected show no significant variation with location latitude water depth temperature salinity and general water conditions once local habitat and ecological influences of fossil brachiopods from similar latitudes are considered their isotopic elemental values should be valuable diagenetic indicators and important oceanographic chemostratigraphic proxies

viscous relaxation of ganymede s impact craters constraints on heat flux abstract measurement of crater depths in ganymede s dark terrain have revealed substantial numbers of unusually shallow craters indicative of viscous relaxation see companion paper singer k n schenk p m bland m t mckinnon w b 2017 relaxed impact craters on ganymede regional variations and high heat flow icarus submitted these viscously relaxed craters provide insight into the thermal history of the dark terrain the rate of relaxation depends on the size of the crater and the thermal structure of the lithosphere here we use finite element simulations of crater relaxation to constrain the heat flux within the dark terrain when relaxation occurred we show that the degree of viscous relaxation observed cannot be achieved through radiogenic heating alone even if all of the relaxed craters are ancient and experienced the high radiogenic fluxes present early in the satellite s history for craters with diameter 10 km heat fluxes of 40 50 mw m 2 can reproduce the observed crater depths but only if the fluxes are sustained for 1 gyr these craters can also be explained by shorter lived heat pulses with magnitudes of 100 mw m 2 and timescales of 10 100 myr at small crater diameters 4 km the observed shallow depths are difficult to achieve even when heat fluxes as high as 150 mw m 2 are sustained for 1 gyr the extreme thermal conditions required to viscously relax small craters may indicate that mechanisms other than viscous relaxation such as topographic degradation are also in play at small crater diameters the timing of the relaxation event s is poorly constrained due to the sparsity of adequate topographic information though it likely occurred in ganymede s middle history neither recently nor shortly after satellite formation the consistency between the timing and magnitude of the heat fluxes derived here and those inferred from other tectonic features suggests that a single event caused both ganymede s tectonic deformation and its crater relaxation future observations should permit more robust determination of the relative timing of the heating event that caused viscous relaxation

post hunnebergian ordovician carbon isotope trend in baltoscandia its environmental implications and some similarities with that of nevada abstract carbon isotope changes during post hunnebergian ordovician time were investigated as the corresponding sequence of rocks is stratigraphically nearly complete in baltoscandia an attempt was made to use it to elaborate the general pattern of carbon isotope changes in most of the ordovician complications caused by several local hiatuses condensed sections and facies changes were mitigated by the study of overlapping sections a total of 326 new whole rock analyses and published data from 18 drill cores and 4 outcrops in estonia latvia and sweden were considered the following positive carbon isotope events were observed 1 the mid darriwilian excursion peak 13c value 1 9 in the aseri stage 2 the mid caradoc excursion 2 2 in the uppermost part of the keila stage 3 the first late caradoc excursion 2 3 in the lower part of the rakvere stage 4 the second late caradoc excursion 2 4 in the upper part of the nabala stage 5 the early ashgill excursion 2 5 in the lowermost part of the pirgu stage 6 the mid ashgill excursion 2 0 in the upper part of the pirgu stage 7 the widely known large hirnantian excursion in estonia the peak value reaches 6 7 in the porkuni stage the study interval comprises a long 19 ma period characterized by low magnitude carbon isotope changes and a following much briefer 12 ma interval with often medium scale shifts and a major event at the end the latter interval reflects onset in the mid caradoc of a period of variable climate alternation of arid and humid episodes which ended with a serious cooling and glaciation the baltoscandian and north american nevada 13c trends are in general lines very similar providing good prospects for global chemostratigraphical correlation and understanding of role of oceanic processes in carbon cycling

the violent collisional history of asteroid 4 vesta vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids the dawn spacecraft extensively imaged vesta s surface revealing a collision dominated history results show that vesta s cratering record has a strong north south dichotomy vesta s northern heavily cratered terrains retain much of their earliest history the southern hemisphere was reset however by two major collisions in more recent times we estimate that the youngest of these impact structures about 500 kilometers across formed about 1 billion years ago in agreement with estimates of vesta asteroid family age based on dynamical and collisional constraints supporting the notion that the vesta asteroid family was formed during this event

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

high resolution c isotope chemostratigraphy of the uppermost cambrian stage stage 10 in south china implications for defining the base of stage 10 and palaeoenvironmental change the wa9ergang section in south china has been proposed as a potential global stratotype section and point gssp for the base of stage 10 the uppermost stage of the cambrian system in this study high resolution c isotopic compositions are reported and we identified three large negative 13 c excursions namely n1 n2 and n3 at wa9ergang the n1 is located just above the first appearance datum fad of lotagnostus americanus corresponding to the possible base of the proconodontus posterocostatus conodont zone the n2 was identified within the micragnostus chuishuensis trilobite zone and the proconodontus muelleri conodont zone the n3 is located in the lowermost part of the leiagnostus cf bexelli archaeuloma taoyuanense trilobite zone or eoconodontus conodont zone the n1 and n2 can be correlated with the negative 13 c excursions preceding the top of cambrian carbon isotope excursion toce observed globally the n3 can be correlated with the toce or the hellnmaria red tops boundary herb event the inter basinal correlation of n1 and l americanus strongly supports that the base of stage 10 may be best defined by the fad of l americanus we also used a box model to quantitatively explore the genesis of the negative 13 c excursions from south china our numerical simulations suggest that weathering of the organic rich sediments on the platform probably driven by intermittent sea level fall and or the oxygenation of the dissolved organic carbon doc reservoir in seawater may have contributed to the generation of the negative 13 c excursions observed in the stage 10 at wa9ergang in south china

evaluating the relationship between the carbon and sulfur cycles in the later cambrian ocean an example from the port au port group western newfoundland canada abstract we present a high resolution 34 s sulfate and pyrite and 13 c carbonate record from the middle upper cambrian port au port group a mixed carbonate siliciclastic succession exposed in western newfoundland canada the results illustrate systematic 34 s sulfate shifts of 15 over relatively short stratigraphic intervals 10 m likely 34 s sulfate pyrite ca 23 and a generally positive coupling between changes in 13 c carbonate and 34 s sulfate together these results indicate that middle to late cambrian sulfate concentrations were low and that the sulfate reservoir was more sensitive to change than it was in either terminal neoproterozoic or cenozoic oceans however a simple carbon c and sulfur s isotope box model of the late cambrian ocean illustrates that low sulfate concentrations alone fail to account for the 15 34 s sulfate shifts recognized in port au port strata such large shifts can be generated only if fluctuating oceanic redox is invoked marine anoxia forces reduced c s burial and elevated 34 s driving larger 34 s changes per mole of organic carbon buried the conclusion that later cambrian oceans featured both low sulfate levels and widespread subsurface anoxia supports hypotheses that link fluctuating marine redox conditions in the delayed recovery of skeletal animals and metazoan reefs from late early cambrian extinction

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

modeling complexly magnetized two dimensional bodies of arbitrary shape a method has been devised for the forward computation of magnetic anomalies due to two dimensional 2 d polygonal bodies with heterogeneously directed magnetization the calculations are based on the equivalent line source approach wherein the source is subdivided into discrete elements that vary spatially in their magnetic properties this equivalent dipole line method provides a fast and convenient means of representing and computing magnetic anomalies for bodies possessing complexly varying magnitude and direction of magnetization the algorithm has been tested and applied to several generalized cases to verify the accuracy of the computation the technique has also been used to model observed aeromagnetic anomalies associated with the structurally deformed remanently magnetized keweenawan volcanic rocks in eastern lake superior this method is also easily adapted to the calculation of anomalies due to two and one half dimensional 2 5 d and three dimensional 3 d heterogeneously magnetized sources

high resolution chemostratigraphy of the cambrian ordovician gssp enhanced global correlation tool abstract the green point formation of the cow head group in western newfoundland canada represents the global stratotype section and point gssp for the cambrian ordovician boundary on laurentia the formation consists of the martin point lower and the broom point upper members which constitute a thick 170 m deep subtidal to slope marine carbonate sequence preservation of the micritic carbonates of the green point formation was evaluated by multiple petrographic and geochemical screening tools the 13 c and 18 o values of near primary micrites range from 4 7 to 1 7 vpdb and from 8 7 to 5 5 vpdb respectively with no significant correlation r 2 0 002 similarly the 13 c values show no significant correlation with mn sr ratios or total ree contents which suggest that the investigated carbonates retain their near primary 13 c signatures that can be utilized to construct a high resolution carbon isotope profile for the gssp the 13 c carb profile about the gssp trends generally positive into bed 22 of the green point formation the top of the c intermedius zone marks the onset of a negative 13 c carb shift to about 6 0 culminating in the upper part of the zone subsequently the 13 c carb profile trends positive with a double switch back about the boundary point and then continues on to the highest value in the uppermost part of bed 23 in addition the ce ce values approximately between 0 8 and 1 0 through the gssp suggest generally dysoxic waters before during and immediately after the boundary the high resolution chemostratigraphic results curve and trends covering the gssp should facilitate correlation of the cambrian ordovician boundary sections sequences from other locations

impact excavation on asteroid 4 vesta hubble space telescope results hubble space telescope images of asteroid 4 vesta obtained during the favorable 1996 apparition show an impact crater 460 kilometers in diameter near the south pole color measurements within the 13 kilometer deep crater are consistent with excavation deep into a high calcium pyroxene rich crust or olivine upper mantle about 1 percent of vesta was excavated by the crater formation event a volume sufficient to account for the family of small vesta like asteroids that extends to dynamical source regions for meteorites this crater may be the site of origin for the howardite eucrite and diogenite classes of basaltic achondrite meteorites

stony meteorite thermal properties and their relationship with meteorite chemical and physical states abstract in our ongoing survey of meteorite physical properties we have to date measured the thermal conductivity for seventeen stony meteorites at temperatures ranging from 5 k to 300 k here we report new results for nine ordinary chondrites one enstatite chondrite and the basaltic achondrites frankfort howardite and los angeles shergottite we find that thermal conductivity is significantly lower than would be expected from averaging the laboratory conductivities of their constituent minerals with a dependence on temperature different from the expected conductivity of pure minerals in addition we find a linear relationship between the inverse of the porosity of the samples measured and their thermal conductivity regardless of meteorite composition or type we conclude that thermal conductivity is controlled by the presence of shock induced microcracks within the meteorites which provide a barrier to the transmission of thermal energy via phonons in contrast to conductivity our first measurement of heat capacity as a function of temperature on los angeles suggests that heat capacity is primarily a function of oxide composition and is not strongly affected by the physical state of the sample

high resolution isotope stratigraphy of the lower ordovician st george group of western newfoundland canada implications for global correlation the lower ordovician st george group of western newfoundland consists mainly of shallow marine platform carbonates 500 m thick it is formed from bottom to top of the watts bight boat harbour catoche and aguathuna formations the top boundary of the group is marked by the regional st george unconformity outcrops and a few cores from western newfoundland were sampled at high resolution and the extracted micritic materials were investigated for their petrographic and geochemical criteria to evaluate their degree of preservation the d13c and d18o values of wellpreserved micrite microsamples range from 4 2 to 0 vpdb and from 11 3 to 2 9 vpdb respectively the d13ccarb profile of the st george group carbonates reveals several negative shifts which vary between 2 and 3 and are generally associated with unconformities disconformities or thin shale interbeds thus reflecting the effect of or link with significant sea level changes the st george unconformity is associated with a negative d13ccarb shift 2 on the profile and correlated with major lowstand around the end of arenig on the local sea level reconstruction and also on those from the baltic region and central australia thus suggesting that the st george group unconformity might have likely had an eustatic component that contributed to the development enhancement of the paleomargin other similar d13ccarb shifts have been recorded on the st george profile but it is hard to evaluate their global extension due to the low resolution of the documented global lower ordovician tremadoc middle arenig d13ccarb profile

evolution of a lower paleozoic continental margin carbonate platform northern canadian appalachians the western margin of the northern appalachian orogene and adjacent craton is composed of a lower paleozoic low latitude carbonate platform which originally lay along the northern margin of iapetus ocean parts of the platform interior are now exposed in quebec much lies beneath the gulf of st lawrence and the outer shelf and deep water deposits crop out in western newfoundland the shelf break and upper slope are nowhere exposed but their nature has been determined from numerous clasts in sediment gravity flows redeposited on the lower slope and now stacked in allochthonous thrust complexes four separate phases recording platform growth and demise can be differentiated reflecting the interplay between tectonics eustasy and the evolving lower paleozoic biota phase 1 preplatform shelf reflects initial siliciclastic deposition on rifted crystalline basement followed by a short phase of carbonate sedimentation dominated by archeocyathan buildups and ooid shoals that was terminated by offlap of thick quartzarenites phase 2 narrow high energy platform is characterized by mixed siliciclastic carbonate peritidal sedimentation throughout manifest as three grand cycles contemporaneous deep water sediments comprise basal welded conglomerates overlain by quartzose carbonate turbidites phase 3 wide low energy platform is an onlap package of muddy fossiliferous subtidal and peritidal carbonates arranged inmore the form of two unconformity bounded megacycles the adjacent deep water slope was a narrow belt of debris flows and wide carbonate shale apron deposited in the lower part of an oxygen minimum zone phase 4 foundered platform documents the initial uplift faulting subsidence and fragmentation of the platform in a sequence of peritidal to subtidal to deep water carbonates reflecting the initial stages of the taconic orogeny less

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

time dependent flexure of the lithospheres on the icy satellites of jupiter and saturn abstract previous analyses into flexural deformation on the icy satellites of jupiter and saturn have assumed static elastic lithospheres viscous creep within the lithosphere however can cause evolution over time here we apply a finite element model that employs a time dependent elastic viscous plastic rheology in order to investigate flexure on icy satellites factors that affect this time dependent response are those that control creep rates surface temperature heat flow and grain size our results show that surface temperature is by far the dominant factor at higher surface temperatures 100 130 k the evolution of the deformation is such that the thickness of a modeled elastic lithosphere could vary by up to an order of magnitude depending on the time scale over which the deformation occurred because the flexure observed on icy satellites generally indicates transient high heat flow events our results indicate that the duration of the heat pulse is an important factor for the icy worlds of jupiter and saturn static models of lithospheric flexure should be used with caution

olivine in an unexpected location on vesta s surface although olivine was expected to occur within the deep south pole basins of asteroid vesta which are thought to be excavated mantle rocks spectral data from nasa s dawn spacecraft show that it instead occurs as near surface materials in vesta s northern hemisphere between july 2011 and september 2012 nasa s dawn spacecraft was in orbit around the asteroid vesta in this paper dawn s visible and infrared mapping spectrometer vir team presents a surprising finding the signature of olivine on the asteroid s surface olivine is a major component of the mantle of differentiated bodies including earth vesta is a large asteroid large enough to have differentiated into an earth like layered structure and the expectation was that olivine would be found within vesta s deep south pole basins thought to be excavated mantle rocks yet the spectroscopic data reveal olivine rich material close to the surface in the northern hemisphere an understanding of the differentiation processes that have occurred on vesta will be invaluable as a window on the primordial solar system but these latest findings show that vesta s evolutionary history is more complicated than was thought olivine is a major component of the mantle of differentiated bodies including earth howardite eucrite and diogenite hed meteorites represent regolith basaltic crust lower crust and possibly ultramafic mantle samples of asteroid vesta which is the lone surviving large differentiated basaltic rocky protoplanet in the solar system1 only a few of these meteorites the orthopyroxene rich diogenites contain olivine typically with a concentration of less than 25 per cent by volume2 olivine was tentatively identified on vesta3 4 on the basis of spectral and colour data but other observations did not confirm its presence5 here we report that olivine is indeed present locally on vesta s surface but that unexpectedly it has not been found within the deep south pole basins which are thought to be excavated mantle rocks6 7 8 instead it occurs as near surface materials in the northern hemisphere unlike the meteorites the olivine rich more than 50 per cent by volume material is not associated with diogenite but seems to be mixed with howardite the most common7 9 surface material olivine is exposed in crater walls and in ejecta scattered diffusely over a broad area the size of the olivine exposures and the absence of associated diogenite favour a mantle source but the exposures are located far from the deep impact basins the amount and distribution of observed olivine rich material suggest a complex evolutionary history for vesta

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

on the possibility of viscoelastic deformation of the large south polar craters and true polar wander on the asteroid vesta the asteroid vesta located within the inner asteroid belt is a differentiated body with a prominent rotational bulge nasa s dawn mission revealed the presence of two large relatively shallow impact craters in the south polar region one with a high standing central peak the shallowness and prominent central peak are reminiscent of large craters on some icy satellites that may have experienced strong topographic relaxation the location of these basins near the south pole is also unusual and suggests true polar wander which requires relaxation of the rotational bulge thus we use the finite element method and a viscoelastic rheology to examine the feasibility of relaxation processes operating on vesta given the plausible thermal state of vesta by the decay of long lived radioactive elements we find that the lithosphere is not compliant enough to allow strong relaxation of the large south polar craters and thus the peculiar morphology is possibly a product of the formation of these large basins at a planetary scale additionally the asteroid has not been warm enough to permit the relaxation of the rotational bulge consequently these craters both happened to form near the south pole as unlikely as that is

carbon isotope stratigraphy of the furongian berry head formation port au port group and tremadocian watts bight formation st george group western newfoundland and the correlative significance carbon isotope stratigraphy of the furongian stage 10 upper cambrian and tremadocian lowermost ordovician reveals distinct variations from the carbonates of the berry head and watts bight form

determination of both chemical and stable isotope composition in milligramme size carbonate samples abstract isotopic analysis of carbonates involves dissolution of the sample in 100 phosphoric acid analysis of the acid after reaction using inductive coupled plasma source spectrometry gives the chemical composition of the same sample molar fractions of cations in carbonate minerals can be determined to about 0 1 for ca 0 04 for mg 0 06 for fe 60 ppm for mn and 90 ppm for sr even the minor carbonate component in shales can be analyzed if a correction for leaching of clay minerals is made by reference to the al content of the solution however loss in accuracy ensues because of the uncertainty in the correction

the interactions of sea level change terrigenous sediment influx and carbonate productivity as controls on upper cambrian grand cycles of western newfoundland canada the upper cambrian stratal record in western newfoundland is complex and reflects the interplay and feedback between 1 changes in third order sea level 2 episodic incursion of terrigenous fines onto the paleoshelf and 3 productivity levels of the ancient carbonate factory mixed carbonate and terrigenous clastic rocks of western newfoundland are configured in large scale third order stratal patterns called grand cycles such cycles are tens to hundreds of meters in stratigraphic thickness and appear to span millions of years grand cycles are divided here into ribbon half cycles composed principally of ribbon rock and oolitic half cycles dominated by oolite stratigraphic distribution of shale in these rocks however is not everywhere coincident with large and small scale patterns in carbonate lithofacies and it suggests that the incursion of terrigenous clay and silt onto the ancient platform must be evaluated separately in part from the dynamics of carbonate accumulation areal and stratigraphic distribution of rock cycles record shifts in facies belts on the ancient platform presumably in response to third order changes in accommodation potential meter scale cycles in carbonate lithofacies are of two types peritidal cycles that fine upward and shelf cycles that coarsen upward thick successions of shelf cycles composed of ribbon rock are interpreted as highstand deposits and are separated by a sequence boundary marked by peritidal cycles from lowstand deposits comprising thick successions of ootite shelf cycles the former mid grand cycle transitions of previous workers are thereby reinterpreted in this study as sequence boundaries the long term position of sea level and thus the degree of platform inundation influenced the health of the platform carbonate factory during times of low sea level the platform was bathed in water more saline than open ocean sea water and it had a robust non skeletal carbonate factory that was resilient to poisoning by episodic influx of terrigenous fines onto the shelf in contrast the carbonate factory during highstands was less resilient to terrigenous poisoning and was susceptible to localized drowning by the reduction in the rate of carbonate sediment production the thick shale units are therefore interpreted to represent the coincidence of and interaction between 1 incursion of terrigenous fines onto the ancient platform and 2 a weakened highstand carbonate factory unable to recover from this terrigenous poisoning

global characteristics of porosity and density stratification within the lunar crust from grail gravity and lunar orbiter laser altimeter topography data the gravity recovery and interior laboratory grail mission is providing unprecedentedly high resolution gravity data the gravity signal in relation to topography decreases from 100 km to 30 km wavelength equivalent to a uniform crustal density of 2450 kg m3 that is 100 kg m3 smaller than the density required at 100 km to explain such frequency dependent behavior we introduce rock compaction models under lithostatic pressure that yield radially stratified porosity and thus density and examine the depth extent of porosity our modeling and analysis support the assertion that the crustal density must vary from surface to deep crust by up to 500 kg m3 we found that the surface density of megaregolith is around 2400 kg m3 with an initial porosity of 10 20 and this porosity is eliminated at 10 20 km depth due to lithostatic overburden pressure our stratified density models provide improved fits to both grail primary and extended mission data

carbon isotope record of late cambrian to early ordovician carbonates of the argentine precordillera a more than 2000 m thick cambrian ordovician carbonate platform succession developed on the exotic argentine precordillera terrane during rifting from laurentia and drifting towards gondwana on base of these carbonates a carbon isotope curve could be developed for the cambrian ordovician we measured 13ccarb and 13corg values on bulk rocks selected components and diagenetic cements whereas the carbon isotope signals of intertidal and supratidal rocks are altered by diagenesis most subtidal carbonates exhibit primary marine values we can report reliable curves for the middle late cambrian transition and for the latest cambrian to earliest middle ordovician the 13c curve matches well with the published data from global cambrian ordovician boundary sections excursions and shifts in the carbon isotope curve correspond to events in sequence stratigraphy this indicates the interdependence on sea level which rules the productivity and or preservation of organic carbon and therefore the partition between corg and ccarb burial

studying lower crustal flow beneath mead basin implications for the thermal history and rheology of venus abstract mead the largest crater on venus has low topographic relief at the surface and at the crust mantle boundary due to high surface temperatures viscous deformation could play an important role in crustal structure using the finite element method we simulate the long term viscoelastic deformation of mead crater and investigate the role of lower crustal flow in the evolution of the surface and subsurface topography we examine the thermal states that allow this evolution to occur and determine the background heat flux our study constrains the background heat flux in the vicinity of mead basin to 55 90 mw m 2 this surface heat flow is generally higher than the average venusian global values suggested by recent thermal models in addition by applying hydrous and anhydrous creep rheological parameters we demonstrate that the venus s interior is rheologically dry and that the crust near mead could be relatively high in plagioclase

impact basin relaxation on rhea and iapetus and relation to past heat flow abstract evidence for relaxation of impact crater topography has been observed on many icy satellites including those of saturn and the magnitude of relaxation can be related to past heat flow e g moore j m schenk p m bruesch l s asphaug e mckinnon w b 2004 icarus 171 421 443 dombard a j mckinnon w b 2006 j geophys res 111 e01001 http dx doi org 10 1029 2005je002445 we use new global digital elevation models of the surfaces of rhea and iapetus generated from cassini data to obtain crater depth diameter data for both satellites and topographic profiles of large basins on each in addition to the factor of three lower amplitude of global topography on rhea compared to iapetus we show that basins on iapetus 100 km in diameter show little relaxation compared to similar sized basins on rhea because of the similar gravities of rhea and iapetus we show that iapetus basin morphologies can be used to represent the initial unrelaxed morphologies of the rhea basins and we use topographic profiles taken across selected basins to model heat flow on both satellites we find that iapetus has only experienced radiogenic heat flow since formation whereas rhea must have experienced heat flow reaching a few tens of mw m 2 although this heat flow need only be sustained for as little as several million years in order to achieve the observed relaxation magnitudes rhea experienced a different thermal history from iapetus which we consider to be primarily related to their different formation mechanisms and locations within the saturnian system a recent model for the formation of saturn s mid sized icy satellites interior to and including rhea charnoz s et al 2011 icarus 216 535 550 describes how rhea s orbit would have expanded outwards after its accretion from a giant primordial ring which would have instigated early heating through rapid despinning and tidal interaction with saturn and other satellites rhea s basins would therefore be required to have formed within the first few tens of myr of rhea s formation in order to relax due of this heating and if so may provide an important anchor point for saturn system chronology none of these heating mechanisms are viable for iapetus in its isolated position far from saturn and as such it has remained dynamically inert since formation confirming conclusions based on thermal modeling of iapetus interior rapid and complete relaxation and subsequent erosion by bombardment of a first generation of large basins on rhea is regarded as an explanation for the lower counts of large basins on rhea relative to iapetus and the overall lower amplitude of topography on rhea compared to iapetus

viscoelastic evolution of lunar multiring basins 1 we investigate the evolution of multiring basins on the moon the orbital geophysical and geochemical data collected by lunar prospector allow three distinct groups of basins to be identified group 1 basins show well preserved topography a bull s eye gravity signature and little mare basalt fill group 2 basins contain large amounts of mare basalt and show mare dominated gravity and topography signatures group 3 basins are characterized by low amplitude topographic relief and weak gravity anomalies while the crustal structures of group 1 and 2 basins appear to date from the collapse of the transient cavity group 3 basins are highly degraded and show very little crustal thinning using a viscoelastic model we explore the evolution of the lunar multiring basins with the goal of explaining the degraded condition of group 3 basins our results show that while viscous relaxation is a plausible explanation for the state of group 3 farside basins requiring heat flux 25 40 mw m 2 the thin nearside crust renders subsolidus ductile flow in the crust unlikely the existence of many degraded basins on the nearside points to a weakened and possibly partially molten lower crust that south pole aitken appears not to have relaxed suggests that similar conditions did not exist there and that the nearside was hotter than the farside early on this is likely a consequence of the nearside farside crustal thickness dichotomy which would have caused the farside to reach full crystallization first leaving the nearside with the remaining liquid and an excess of heat producing elements

isotope stratigraphy of the lapa formation sao francisco basin brazil implications for late neoproterozoic glacial events in south america the lapa formation is a thick carbonate sequence 900 m that constitutes the upper part of the vazante group on the s francisco craton brazil it conformably overlies a previously unrecognized glacial diamictite unit of poorly constrained age the sequence above the glacial unconformity consists predominantly of organic rich shale subtidal rhythmic dolomicrites and microbialaminites and intertidal stromatolites four boreholes spanning different depositional settings were sampled at highresolution and investigated for their petrographic and chemical criteria to evaluate their degree of preservation the 13 c and 18 o values of well preserved lapa carbonate microsamples range from 8 2 to 3 3 vpdb and from 13 6 to 0 9 vpdb respectively each of the 13 c profiles of the investigated cores reveals two strong negative excursions of up to 8 an event in post glacial dolomicrites immediately above the glaciogenic unit and a 10 m interval of organic rich shale and a second near the top of the sequence associated with a shale interval based on the observation of dropstones and sedimentary iron formation in the underlying diamictite as well as the distinguishable carbon isotope trends the lapa formation is considered as a cap carbonate lithofacies the age of the lapa formation is presently unknown but the least radiogenic 87 sr 86 sr value 0 7068 associated with a negative carbon isotope excursion matches that from the rasthof formation in namiba on the congo craton which is radiometrically constrained to be younger than ca 750 ma

the late cambrian spice δ13c event and the sauk ii sauk iii regression new evidence from laurentian basins in utah iowa and newfoundland carbon isotope data from upper cambrian sections in three laurentian basins in northern utah central iowa and western newfoundland record a large positive d 13 c excursion spice event of up to 1 5 peak d 13 c ratios are well dated by trilobite collections to the middle of the steptoean stage dunderbergia zone and occur during maximum regression associated with formation of the sauk ii sauk iii subsequence boundary on the north american craton max imum regression was marked by an influx of quartz sand into carbon ate platform settings in all three widely separated basins in northern utah this quartz sand formed a thick sequence known as the worm creek quartzite which marks a conspicuous interruption of carbonate deposition during the middle to late cambrian in the region in west ern newfoundland the thickness of the quartz sand unit is much re duced but still marks a brief shutdown of the carbonate factory that is unique to the cambrian shelf succession of the area in the central iowa area of the cratonic interior an upward shallowing carbonate succession culminates in cross stratified trilobite grainstones at the peak of the spice in dunderbergia zone time and the lowest point on the relative sea level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface although it is difficult to determine precisely the departure from baseline d 13 c that marks the beginning of the spice excursion in the stratigraphic successions analyzed our results are consistent with a rise and subsequent fall in d 13 c tracking a major regressive transgressive event recorded across northern laurentia the correlation of a major d 13 c excursion with regression is similar to that described for the late ordovician for which the pattern has been attributed to either in creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic matter containing silicates at high latitudes or high productivity and organic carbon burial driven by oceanic overturn the lack of known steptoean age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic carbon burial was the likely cause of the spice event we suggest that increased weathering and erosion rates during relative sea level fall sauk ii iii increased the burial fraction of organic carbon in an expanded region of fine grained siliciclastic deposits in shelf and upper slope environments during the steptoean

estimates of martian crustal thickness from viscous relaxation of topography isostatically compensated crustal thickness variations and associated topographic contrasts at the surface of a planet result in lateral pressure gradients which may cause the lower crust to flow and reduce the relief areas of thicker crust are generally associated with more rapid relaxation of topography on mars topographic features such as impact basins and the hemispheric dichotomy have survived for 4 gyr we use a finite difference representation of depth dependent non newtonian lower crustal flow to investigate how topography decays with time for a dry diabase rheology total radiogenic concentrations 80 of terrestrial values and crustal radiogenic concentrations similar to terrestrial basalts we find that an upper bound on the mean planetary crustal thickness is 100 km in the probably unrealistic case where all the radiogenic elements are in the crust this maximum crustal thickness can be increased to 115 km the main uncertainty in these results is the total radiogenic abundances on mars comparing our results with the observed shape of the crustal dichotomy provides no evidence that this slope is primarily the result of lower crustal flow both hellas and the dichotomy are isostatically compensated if the mechanism is airy isostasy then the lower bound on mean crustal thickness is 30 km crustal thicknesses of 30 100 km on mars can be produced by mid ocean ridge spreading at potential temperatures of 1350 1600 c however for such crustal thicknesses the lithosphere is likely to be positively buoyant making subduction difficult

two dimensional numerical modeling of the rheasilvia impact formation 1 we numerically modeled the formation of rheasilvia crater an enormous impact basin centered on asteroid 4 vesta s south pole using a trial and error method our models were adjusted to produce the best possible fit to rheasilvia s size and shape as observed during the vesta orbital stage of the dawn mission the final model yields estimates of the shock wave decay escaped material volume depth of excavation and other relevant characteristics to the extent allowed by the two dimensional axially symmetric approximation of the simplified arbitrary lagrangian eulerian hydrocode our model results permit interpretation of the dawn data on vesta s shape topographic crater profiles and the origin of the vestoid asteroid family as escaped ejecta from the rheasilvia crater

calculating the topography of a differentiated vesta 1 previous examination of the topography of a homogenous vesta revealed an immense seemingly abnormal impact crater near the south pole of the asteroid the crater appeared anomalously shallow with an extremely tall central peak suggesting an unusual formation or subsequent modification conversely vesta is almost certainly differentiated and we incorporate this notion to reexamine its topography we find that the central peak is well below the crater rim and that the overall depth of the crater increases giving it a more ordinary profile the large density contrast between the metallic core and the overlying silicate layers has the strongest control on the topography as previously discussed the location of the crater at the south pole suggests polar wander necessitating a mechanical structure that allows reorientation of vesta s large rotational bulge yet preserves the crater s shape a possibly restrictive scenario nasa s dawn spacecraft may observe tectonics consistent with polar wander

carbonate platform to foreland basin revised stratigraphy of the table head group middle ordovician western newfoundland carbonates of the table head group and associated strata were deposited along the western margin of a foreland basin during initial stages of the taconian orogeny and record collapse and cannibalization of a long lived carbonate platform the stratigraphy of klappa opalinski and james is here revised to reflect better understanding of lithologic units within this complex tectono stratigraphic assemblage the table head group now contains only three formations the table point and table cove formations as originally defined and the cape cormorant formation redefined and restricted to western port au port peninsula black cove formation shales are removed from the table head distinctive conglomerates once placed in the cape cormorant are now recognized as separate units within the overlying flysch and called the daniel s harbour member the foreland basin developed in three stages 1 fragmentation uplift and erosion of the platform and subsequent deposition of shallow water limestones table point

stable isotopes of carbon as an invaluable stratigraphic tool an example from the cambrian of the northern appalachians usa this study illustrates the usefulness of carbon isotopes in stratigraphic interpretations of poorly fossiliferous strata beyond the resolution possible by biostratigraphy the upper cambrian strata of the lower gorge formation in northwestern vermont have been interpreted as highly condensed continental slope deposits with a hiatus bracketed by the trilobite fauna of the dunderbergia zone steptoean and the saukia zone upper sunwaptan this interpretation was based on information from two thin fossiliferous intervals the occurrence of one of them being unconfirmed in a recent study carbon isotopes provide a means for testing this interpretation because marine carbonate rocks deposited during the steptoean age of the late cambrian record a large global positive carbon isotope excursion 13 c values of up to 5 relative to the vienna peedee belemnite standard if the proposed age for the lowermost gorge formation is correct these strata should record the start of the excursion and the hiatus should produce an abrupt termination in the record of the excursion near maximum values the determined 13 c values 0 57 to 0 39 indicate that the excursion is not recorded at this locality the results question the age for these strata and suggest that the hiatus is greater than previously recognized encompassing most of the steptoean this major sedimentary hiatus lends supporting evidence for a significant eustatic or pan laurentian sea level event during the steptoean perturbation in the global cycling of carbon

using the viscoelastic relaxation of large impact craters to study the thermal history of mars we simulate the long term deformation of martian craters and investigate the role of lower crustal flow in the evolution of surface and subsurface topography using the finite element method and a viscoelastic rheological model we model the deformation of more than 30 large craters and quasi circular depressions qcds in the diameter range of 200 500 km in both the northern lowlands and southern highlands we determine the most appropriate background heat fluxes that produce the current topography beneath the impacts at the crust mantle boundary ranges from 40 to 90 mw m 2 our study shows that a higher background heat flux leads to more relaxation at the surface and subsurface by applying various viscous creep parameters for hydrous and anhydrous rheologies we demonstrate that mars s interior is wet to a certain degree which is consistent with other estimates since craters and qcds are distributed fairly equally on the surface of the red planet this study provides a less regionally biased picture of the thermal history of early mars than in previous studies based on our results the ancient average background heat flux in the northern lowlands was higher than that of the southern highlands which could indicate that whatever process formed the crustal dichotomy had a thermal signature at least through the middle noachian

climatic and oceanographic isotopic signals from the carbonate rock record and their preservation stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases terrestrial carbonates meteoric cements calcretes and speleothems similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover primary environmental isotopic signals may be obscured by the effects of post depositional diagenetic alteration cementation and replacement reactions can take place in a wide range of diagenetic environments the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting together with subsequent changes in relative sea level and burial history carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration water rock interaction and fluid mixing may produce non linear distributions attempts to determine long term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow water limestones must include an assessment of the diagenetic history of the materials analysed petrographic examination using conventional microscopy backed up where appropriate by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary environmental isotopic signals recent research has shown that these techniques can be successfully employed in both phanerozoic and precambrian sediments

facies specific calcitic and bimineralic ooids from middle and upper cambrian platform carbonates western newfoundland canada abstract oolites in the port au port group consist of intercalated gray oolite deposited on the shallow subtidal mobile fringe of ooid sand shoals and brown oolite which accumulated on intertidal sand flats along the shoal crests gray oolite is composed of finely and coarsely preserved radial ooids that were precipitated and deposited in situ and whose fabric was controlled in large part by suspension versus bedload transport brown oolite is composed of concentric ooids or superficial radial ooids that were either formed and deposited in situ or swept onto the sand flats from the mobile fringe the present fabric of the ooids is the result of protracted burial diagenesis cortices of finely and coarsely preserved radial ooids excluding radial concentric ones and superficial radial ooids were mg calcite originally they changed to calcite still retaining fine fabric and subsequently to a variety of cortical fabrics during shallow burial and altered to blocky fabrics during deep burial concentric ooids and radial concentric ooids were bimineralic originally composed of aragonite now micrite and blocky calcite and mg calcite now radial fibrous calcite primary aragonitic laminae were partially or completely dissolved and crushed during shallow burial resulting voids were filled with blocky calcite during deep burial correlation of original ooid mineralogy fabric and environment of formation indicates that local physical conditions such as turbulence influenced ooid formation in cambrian time this correlation is absent in modern deposits suggesting that more widespread chemical and organic controls now prevail

fabric retentive non luminescent brachiopods as indicators of original delta 13 c and delta 18 o composition a test abstract fabric retentive non luminescent helderbergian brachiopods from open marine deposits analogous to brachiopods used in other studies to determine paleo oceanic stable isotopic composition exhibit depleted 13c values immediately below the erosional contact and more enriched values of 13c at greater stratigraphic distances below this surface in addition 13c values of these brachiopods display a marked shift across the erosional surface and values above it are essentially the same as those 30 m or more beneath the surface complementing these trends whole rock analyses metastable high agnesian calcite constituents echinoderms as well as early equant calcite cement also exhibit an increased depletion trend in 13c values as the helderberg oriskany contact is approached from below fabric retentive luminescent as well as obviously recrystallized brachiopods display stable isotopic values similar to those of fabric retentive non luminescent brachiopods thus best preserved fabric retentive non luminescent brachiopods exhibit values similar to obviously recrystallized brachiopods diagenetically altered metastable constituents echinoderms stable early meteoric cements and whole rock samples helderberg strata of new york state were affected by early meteoric diagenesis during a siegenian exposure event prior to deposition of the overlying marine oriskany sandstone meteoric diagenesis during this exposure gave rise to a characteristic carbon isotopic depletion trend approaching the helderberg oriskany contact all of the skeletal constituents underwent isotopic exchange during early diagenetic exposure to meteoric waters consequently the practice of using fabric retentive non luminescent brachiopods as indicators of their original chemistry and by extension paleo oceanic chemistry is questioned

simultaneous changes in carbon isotopes sea level and conodont biozones within the cambrian ordovician boundary interval at black mountain australia carbon isotope profiles of marine carbonate units within the cambrian ordovician boundary section at black mountain western queensland australia reveal cyclical changes in 13c within the georgina basin during the boundary interval minimum values in the 13c profile are found in strata bearing lithostratigraphic evidence for sea level drop and also correlate strongly with conodont assemblage zone boundaries these relations indicate that direct mechanistic links existed between carbon cycles changes in sea level and conodont evolution

isotope signatures associated with early meteoric diagenesis the environments in which carbonate diagenesis proceeds have been documented in previous studies of holocene and late pleistocene sediments and limestones on barbados west indies variations in the carbon and oxygen isotopic composition of limestones produced during early freshwater diagenesis have been observed in this study to occur in specific patterns six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework 1 the subaerial exposure surface is characterized by strongly 12c enriched limestones 13c compositions of underlying limestones grow progressively heavier with increasing depth 2 the subaerial exposure surface may also be marked by slight 18o enrichment 3 an abrupt shift in 18o values may differentiate sediments above the exposure surface from those below 4 sediments altered in the marine meteoric mixing zone may be characterized by positive covariance between their 18o and 13c compositions 5 the vadose phreatic boundary may be marked by a sharp increase in 13c values in the seaward portions of a fresh groundwater system and 6 samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of 18o and a wide range of 13c compositions analysis of samples from five palaeozoic and mesozoic formations which contained petrographic evidence of early freshwater diagenesis showed that isotope patterns similar to those observed in barbados limestones have been preserved in rocks as old as mississippian these isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks they might be used to identify limestones diagenetically altered in meteoric environments to identify mixing zone cements and dolomites and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations

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

sr c and o isotope geochemistry of ordovician brachiopods a major isotopic event around the middle late ordovician transition here we present sr c and o isotope curves for ordovician marine calcite based on analyses of 206 calcitic brachiopods from 10 localities worldwide these are the first ordovician wide isotope curves that can be placed within the newly emerging global biostratigraphic framework a total of 182 brachiopods were selected for c and o isotope analysis and 122 were selected for sr isotope analysis seawater 87sr 86sr decreased from 0 7090 to 0 7078 during the ordovician with a major quite rapid fall around the middle late ordovician transition most probably caused by a combination of low continental erosion rates and increased submarine hydrothermal exchange rates mean 18o values increase from 10 to 3 through the ordovician with an additional short lived increase of 2 to 3 during the latest ordovician due to glaciation although diagenetic alteration may have lowered 18o in some samples particularly those from the lower ordovician maximum 18o values which are less likely to be altered increase by more than 3 through the ordovician in both our data and literature data we consider that this long term rise in calcite 18o records the effect of decreasing tropical seawater temperatures across the middle late ordovician transition superimposed on seawater 18o that was steadily increasing from 3 standard mean ocean water smow by contrast 13c variation seems to have been relatively modest during most of the ordovician with the exception of the globally documented but short lived latest ordovician 13c excursion up to 7 nevertheless an underlying trend in mean 13c can be discerned changing from moderately negative values in the early ordovician to moderately positive values by the latest ordovician these new isotopic data confirm a major reorganization of ocean chemistry and the surface environment around 465 to 455 ma the juxtaposition of the greatest recorded swings in phanerozoic seawater 87sr 86sr and 18o at the same time as one of the largest marine transgressions in phanerozoic earth history suggests a causal link between tectonic and climatic change and emphasizes an endogenic control on the o isotope budget during the early paleozoic better isotopic and biostratigraphic constraints are still required if we are to understand the true significance of these changes we recommend that future work on ordovician isotope stratigraphy focus on this outstanding middle late ordovician event

cryptalgal metazoan bioherms of early ordovician age in the st george group western newfoundland bioherms are common in the st george group a sequence of shallow water carbonate rocks deposited on the western continental shelf of iapetus ocean they range from small heads and metre sized mounds to extensive banks and complexes many metres thick and hundreds of metres in lateral extent the cores of these bioherms are principally composed of thrombolites unlaminated branching columnar stromatolites structures quite distinct from laminated stromatolites which are common in intertidal beds associated with thrombolites is a diverse fauna of burrowing invertebrates trilobites nautiloids pelmatozoans brachiopods gastropods rostroconchs and archaeoscyphiid sponges on the basis of framework building components three main bioherm types are distinguished 1 thrombolite mounds 2 thrombolite lichenaria or sponge mounds and 3 thrombolite lichenaria renalcis reef complexes the framework of the last is the most complex with abundant cavities and a demonstrably uneven growth surface of thrombolites corals and free standing renalcis heads walls and roofs some cavities were active sediment conduits while others were protected their roofs draped with renalcis and their walls coated by cryptalgal laminites these bioherms possess the attributes of shallow water ecologic reefs they span a critical time gap in the development of reefs the transition period from algal dominated bioherms of the precambrian and cambrian to the metazoan dominated bioherms of the middle ordovician and remaining phanerozoic

a magmatic loading model for coronae on venus 1 models for the formation of coronae quasi circular volcanotectonic features on venus must explain four critical characteristics coronae display 1 a wide range of diameters 2 complex varied topography 3 fracture annuli and 4 sometimes extensive volcanism previous models have difficulty simultaneously satisfying all four constraints on the basis of observations and interpretations of features on venus and earth and experiments in geophysical fluid dynamics we propose that coronae form in response to magmatic loading of the crust over zones of partial melting at the tops of thermally buoyant heads of transient mantle plumes that impinge on the base of the thermal lithosphere by tying corona formation to a melt zone and not directly to an impinging upwelling our conceptual model can account for the wide range in corona diameters lateral crustal flow facilitated by magmatic heating may lead to central depressions characteristic of many coronae a thermomechanical finite element simulation with a dissipating thermal anomaly leads to crustal thinning and predicts an elevation profile consistent with one of the more populous topographic classes of coronae deformation concentrated at the transition from thicker to thinner lithosphere above the thermal anomaly yields a narrow annulus of enhanced differential stress consistent with formation of fracture annuli estimates of excess heat in an upwelling exceed the thermal energy in the melt that comprises the corona band pass filtered maps of the topography and gravity fields in the beta atla themis region are consistent with upwellings now impinging on the lithosphere beneath seven coronae previously inferred to be active

efficient early global relaxation of asteroid vesta abstract the asteroid vesta is a differentiated planetesimal from the accretion phase of solar system formation although its present day shape is dominated by a non hydrostatic fossil equatorial bulge and two large mostly unrelaxed impact basins vesta may have been able to approach hydrostatic equilibrium during a brief early period of intense interior heating we use a finite element viscoplastic flow model coupled to a 1d conductive cooling model to calculate the expected rate of relaxation throughout vesta s early history we find that given sufficient non hydrostaticity the early elastic lithosphere of vesta experienced extensive brittle failure due to self gravity thereby allowing relaxation to a more hydrostatic figure soon after its accretion vesta reached a closely hydrostatic figure with 40 200 my after formation depending on the assumed depth of megaregolith the veneneia and rheasilvia giant impacts which generated most non hydrostatic topography must have therefore occurred 40 200 my after formation based on crater retention ages topography and relation to known impact generated features we identify a large region in the northern hemisphere that likely represents relic hydrostatic terrain from early vesta the long wavelength figure of this terrain suggests that before the two late giant impacts vesta had a rotation period of 5 02 h 6 3 faster than present while its spin axis was offset by 3 0 from that of the present the evolution of vesta s figure shows that the hydrostaticity of small bodies depends strongly on its age and specific impact history and that a single body may embody both hydrostatic and non hydrostatic terrains and epochs

the structure of the asteroid 4 vesta as revealed by models of planet scale collisions three dimensional simulations of the global evolution of asteroid 4 vesta under two overlapping planet scale collisions closely reproduce its observed shape but expected large areas of olivine rich rocks and pure diogenites are not observed on the surface possibly implying that the outer 100 km is composed mainly of a basaltic crust eucrites with ultramafic intrusions diogenites nasa s dawn mission to the asteroid 4 vesta revealed some unexpected surface features including a huge crater named veneneia underlying the even larger crater rheasilvia that was formed about a billion years ago the juxtaposition of these two impact basins provides an opportunity to develop an accurate model of vesta s topography martin jutzi et al report three dimensional simulations of vesta s global evolution through two successive planet scale collisions their model closely reproduces vesta s observed shape and provides a firm basis for the interpretation of the surface mineralogy and geology of vesta and by extension other small solar system bodies asteroid 4 vesta seems to be a major intact protoplanet with a surface composition similar to that of the hed howardite eucrite diogenite meteorites1 2 3 4 the southern hemisphere is dominated by a giant impact scar5 but previous impact models6 7 8 have failed to reproduce the observed topography the recent discovery that vesta s southern hemisphere is dominated by two overlapping basins9 provides an opportunity to model vesta s topography more accurately here we report three dimensional simulations of vesta s global evolution under two overlapping planet scale collisions we closely reproduce its observed shape and provide maps of impact excavation and ejecta deposition spiral patterns observed in the younger basin rheasilvia9 about one billion years old10 are attributed to coriolis forces during crater collapse surface materials exposed in the north come from a depth of about 20 kilometres according to our models whereas materials exposed inside the southern double excavation come from depths of about 60 100 kilometres if vesta began as a layered completely differentiated protoplanet then our model predicts large areas of pure diogenites and olivine rich rocks these are not seen11 12 13 possibly implying that the outer 100 kilometres or so of vesta is composed mainly of a basaltic crust eucrites with ultramafic intrusions diogenites