the present day formation of apatite in mexican continental margin sediments abstract results of pore water and sediment analyses from the western mexican continental margin strongly suggest the present day formation of apatite the interstitial water phosphate and fluoride profiles indicate chemical removal at a depth which corresponds to a large maximum in the phosphorus content of the sediments apatite is identified within this maximum via x ray diffraction but is elsewhere undetectable in the core radioisotopic thorium uranium and radium data support the conclusion that this deposit is modern the present day depositional environment is consistent with those reported by other workers for phosphorite formation with the exception that pore water magnesium is not depleted below its seawater value

saqqar a 34 km diameter impact structure in saudi arabia here we present the first proof of an impact origin for the saqqar circular structure in northwestern saudi arabia neville et al 2014 with an apparent diameter of 34 km centered at 29 35 n 38 42 e the structure is formed in cambrian devonian siliciclastics and is unconformably overlain by undeformed cretaceous and paleogene sediments the age of impact is not well constrained and lies somewhere between 410 and 70 ma the subsurface structure is constrained by 2 d reflection seismic profiles and six drilled wells first order structural features are a central uplift that rises approximately 2 km above regional datums surrounded by a ring syncline the crater rim is defined by circumferential normal faults the central uplift and ring syncline correspond to a bouguer gravity high and an annular ring like low respectively the wells were drilled within the central uplift the deepest among them exceed 2 km depth sandstone core samples from these wells show abundant indicators of a shock metamorphic overprint planar deformation features pdfs were measured with orientations along 0001 103 and less frequently along 101 and 104 planar fractures pfs predominantly occur along 0001 and 101 and are locally associated with feather features ffs in addition some shocked feldspar grains and strongly deformed mica flakes were found the recorded shock pressure ranges between 5 and 15 gpa the preserved level of shock and the absence of an allochthonous crater fill suggest that saqqar was eroded by 1 2 km between the devonian and maastrichtian the documentation of unequivocal shock features proves the formation of the saqqar structure by a hypervelocity impact event

bias corrected population size distribution and impact hazard for the near earth objects abstract utilizing the largest available data sets for the observed taxonomic binzel et al 2004 icarus 170 259 294 and albedo delbo et al 2003 icarus 166 116 130 distributions of the near earth object population we model the bias corrected population diameter limited fractional abundances of the taxonomic complexes are a 0 2 c 10 d 17 o 0 5 q 14 r 0 1 s 22 u 0 4 v 1 x 34 in a diameter limited sample 30 of the neo population has jovian tisserand parameter less than 3 where the d types and x types dominate the large contribution from the x types is surprising and highlights the need to better understand this group with more albedo measurements combining the c d and x complexes into a dark group and the others into a bright group yields a debiased dark to bright ratio of 1 6 overall the bias corrected mean albedo for the neo population is 0 14 0 02 for which an h magnitude of 17 8 0 1 translates to a diameter of 1 km in close agreement with morbidelli et al 2002 icarus 158 2 329 342 coupling this bias corrected taxonomic and albedo model with the h magnitude dependent size distribution of stuart 2001 science 294 1691 1693 yields a diameter distribution with 1090 180 neos with diameters larger than 1 km as of 2004 june the spaceguard survey has discovered 56 of the neos larger than 1 km using our size distribution model and orbital distribution of stuart 2001 science 294 1691 1693 we calculate the frequency of impacts into the earth and the moon globally destructive collisions 10 21 j of asteroids 1 km or larger strike the earth once every 0 60 0 1 myr on average regionally destructive collisions with impact energy greater than 4 10 18 j 200 m diameter strike the earth every 56 000 6000 yr collisions in the range of the tunguska event 4 8 10 16 j occur every 2000 3000 yr these values represent the average time between randomly spaced impacts actual impacts could occur more or less closely spaced solely by chance as a verification of these impact rates the crater production function of shoemaker et al 1990 geological society of american special paper 247 has been updated by combining this new population model with a crater formation model to find that the observed crater production function on both the earth and moon agrees with the rate of crater production expected from the current population of neos

the bow city structure southern alberta canada the deep roots of a complex impact structure geological and geophysical evidence is presented for a newly discovered probable remnant complex impact structure the structure located near bow city southern alberta has no obvious morphological expression at surface the geometry of the structure in the shallow subsurface mapped using downhole geophysical well logs is a semicircular structural depression approximately 8 km in diameter with a semicircular uplifted central region detailed subsurface mapping revealed evidence of localized duplication of stratigraphic section in the central uplift area and omission of strata within the surrounding annular region field mapping of outcrop confirmed an inlier of older rocks present within the center of the structure evidence of deformation along the eastern margin of the central uplift includes thrust faulting folding and steeply dipping bedding normal faults were mapped along the northern margin of the annular region isopach maps reveal that structural thickening and thinning were accommodated primarily within the belly river group evidence from legacy 2 d seismic data is consistent with the subsurface mapping and reveals additional insight into the geometry of the structure including a series of listric normal faults in the annular region and complex faulting within the central uplift the absence of any ejecta blanket breccia suevite or melt sheet based on available data is consistent with the bow city structure being the remnant of a deeply eroded complex impact structure accordingly the bow city structure may provide rare access and insight into zones of deformation remaining beneath an excavated transient crater in stratified siliciclastic target rocks

the flux of small near earth objects colliding with the earth asteroids with diameters smaller than approximately 50 100 m that collide with the earth usually do not hit the ground as a single body rather they detonate in the atmosphere these small objects can still cause considerable damage such as occurred near tunguska siberia in 1908 the flux of small bodies is poorly constrained however in part because ground based observational searches pursue strategies that lead them preferentially to find larger objects a tunguska class event the energy of which we take to be equivalent to 10 megatons of tnt was previously estimated to occur every 200 300 years with the largest annual airburst calculated to be approximately 20 kilotons kton tnt equivalent ref 4 here we report satellite records of bolide detonations in the atmosphere over the past 8 5 years we find that the flux of objects in the 1 10 m size range has the same power law distribution as bodies with diameters 50 m from this we estimate that the earth is hit on average annually by an object with approximately 5 kton equivalent energy and that tunguska like events occur about once every 1 000 years

zircon u pb ages and lu hf isotope systematics from late tectonic granites geita greenstone belt implications for crustal growth of the tanzania craton granite plutons that intruded south of lake victoria and north of the geita greenstone belt have geochemical characteristics similar to high k granites when compared to the late high k granites from the musoma mara region they have lower sio2 content and higher tio2 al2o3 cao na2o p2o5 mgo and feot they also show higher values for v sr zr ba and hf concentrations all samples display high total ree abundances ree 139 393 and weak to moderate eu depletion eu eu 0 25 0 77 their chondrite normalized pattern indicates that the light ree are moderately fractionated 3 03 lan smn 6 24 whereas the heavy ree are weakly fractionated 1 63 gdn ybn 2 80 the granite plutons are generally undeformed with 207pb 206pb ages between 2660 and 2620 ma and intruded after the main ductile deformation that affected the geita greenstone belt the 176hf 177hf ratios for zircons from all samples are identical within error suggesting evolution from an isotopically uniform reservoir the hf values plot along the chur evolution line indicating that the granite represents juvenile crustal melts derived from greenstone material available zircon ages from the tanzania craton suggest that crustal growth occurred in three distinct periods 2850 2800 ma 2770 2730 ma and 2700 2620 ma with the dominant period of crustal growth around 2700 ma zircon ages from the sukumaland and musoma mara greenstone belts indicate that the greenstone sequences formed during the same three periods of crustal growth

genesis of phosphorite crusts off peru abstract authigenic phosphorite crusts from the shelf off peru 9 40 s to 13 30 s consist of a facies with phosphatic coated grains covered by younger phosphatic laminite the crusts are composed of carbonate fluorapatite which probably formed via an amorphous precursor close to the sediment water interface as indicated by low f p2o5 ratios sr and ca isotopes as well as rare earth element patterns agreeing with seawater dominated fluids small negative ce anomalies and u enrichment in the laminite suggest suboxic conditions close to the sediment water interface during its formation increased contents of chalcophilic elements and abundant sulfide minerals in the facies with phosphatic coated grains as well as in the laminite denote sulfate reduction and consequently point to episodical development of anoxic conditions during phosphogenesis the peruvian phosphorites formed episodically over an extended period of time lasting from middle miocene to pleistocene individual phosphatic coated grains show a succession of phosphatic layers with varying contents of organic matter and sulfide rich phosphatic layers coated grains supposedly formed as a result of episodic suspension caused by high turbulence and shifting redox conditions episodic anoxia in the pore water induced pyritization in the outermost carbonate fluorapatite layer phosphatic coated grains were later transported to the place of crust formation where subsequent laminite formation was favored under lower energy conditions a similar succession of phosphatic layers with varying contents of organic matter and sulfide rich layers in the laminite suggests a formation mechanism analogous to that of individual coated grains

age and origin of the lowlands of mars one of the many significant findings of the mars global surveyor mission is the presence of hundreds of quasi circular depressions qcds observed from high resolution mola topography data their presence has recently been interpreted to reflect a northern lowlands that archive some of the earliest recorded rocks on mars mostly below a veneer of hesperian and amazonian materials here we analyze these data coupled with a recent synthesis of geologic geophysical geomorphic topographic and magnetic information such analysis allows us to suggest a potential plate tectonic phase during the recorded early into middle noachian martian history that transitioned into a monoplate world with episodic magmatic driven activity persisting to present this working hypothesis is based on 1 the observation that the basement of the northern plains is younger than the basement of the southern highlands but older than the material exposures of the cratered highlands suggesting different formational ages for each one of the three geologic time units 2 the observation that parts of the very ancient highland s crust are highly magnetized thus suggesting that most if not all of the formation of the lowlands basement postdates the shut off of the martian dynamo some 4 gyr ago and so allowing hundreds of millions of years for the shaping of the buried lowlands consequently the role of endogenic processes in the earliest geological evolution of mars early perhaps into middle noachian requires reconsideration since mola topographic and mgs magnetic data afford a temporal window sufficient for very early non primordial shaping of the northern lowlands basement

history of ice rafting and water mass evolution at the northern siberian continental margin laptev sea during late glacial and holocene times the late glacial and holocene history of environmental changes in the western laptev sea is reconstructed on the basis of continuous records of ice rafted debris ird and atlantic water indicative subpolar planktic foraminifers and benthic foraminifer cassidulina neoteretis from the two ams14c dated sediment cores one from the upper continental slope and another from the khatanga paleoriver valley on the outer shelf since seasonal sea ice cover is a permanent feature in this high arctic region the prominent ird rich intervals in the oldest 15 4 ka and youngest 7 2 ka sediment units are attributed to iceberg rafting rock types like phyllites are used as indicators of sediment material supplied by icebergs from local ice caps of the severnaya zemlya archipelago authigenic concretions of vivianite and rhodochrosite found in the basal ird rich core section on the slope point to strong water stratification likely caused by meltwater inputs the oldest late glacial ird peaks provide evidence for iceberg production by the local ice caps on severnaya zemlya subsequent meltwater influence is tentatively related to the freshwater release from a retreating barents kara ice sheet in the northeastern kara sea already prior to 15 4 ka atlantic derived waters intermittently reached the western laptev sea continental slope probably facilitated by upwelling in coastal polynyas while subsurface atlantic derived waters were constantly present at the continental slope between 15 4 and 12 ka the disappearance of c neoteretis together with reduced abundance of subpolar planktic foraminifers after 12 ka suggests establishment of a freshened shelf water mass on the flooded outer shelf which affected the water mass structure on the continental slope after 7 ka climate cooling and enhanced atlantic derived water inflow caused re growth of ice caps on severnaya zemlya leading to a recurrence of ird peaks of this renewed ird input are centered at 7 2 6 4 5 4 3 and 2 ka implying that this millennial scale variability in the ird input might be related to an overregional changes in atmospheric circulation patterns

investigating the architecture of the paganica fault 2009 mw 6 1 earthquake central italy by integrating high resolution multiscale refraction tomography and detailed geological mapping this article has been accepted for publication in geophysical journal internationa 2017 published by oxford university press on behalf of the royal astronomical society all rights reserved

carbon and phosphorus burial associated with modern phosphorite formation phosphorite formation plays a significant but poorly constrained role in the marine c and p cycles this report concerns a site of modern phosphorite formation on the upper continental slope off southern baja california mexico sta 6 380 m water depth 23 41 n 111 05 w at this site a series of six narrow 1 3 cm partially indurated phosphorite layers have formed authigenically during the last 15 000 years within a surficial section of slowly but steadily accumulating 3 8 10 3 cm year 1 organic rich mud a poorly oxygenated benthic environment has prevailed at the study site since approximately 8100 year b p probably helping to produce and preserve the pristine phosphorite layers the mean p burial flux 1 1 mol cm 2 year 1 recorded by these deposits agrees closely with the downward diffusive p flux 1 3 mol cm 2 year 1 attributed to active francolite growth such a coincidence implies that the phosphorite layers could have formed entirely under present conditions however a much larger upward diffusive p flux 4 2 mol cm 2 year 1 directed out of the sediment limits the efficiency of ongoing phosphorite formation and the most recently deposited sediment does not contain enough labile p to maintain the overall p regeneration rate indefinitely attendant burial fluxes of inorganic and especially organic c 7 7 and 6 6 mol cm 2 year 1 respectively do not accord with a depositional model of coastal upwelling suggesting that an alternative mechanism possibly fe redox cycling controls porewater p concentrations and hence phosphorite formation a comparison with previously published p burial fluxes for a wide variety of other modern marine settings reveals that phosphorite formation in general does not disproportionately affect the global p burial flux from these data we tentatively estimate a minimum global p burial flux of approximately 70 109 mol year 1 with almost 80 occurring on continental margins

structural controls on mud volcano vent distributions examples from azerbaijan and lusi east java structural mapping nearest neighbour and two point azimuth statistical analysis of mud volcano vent distributions from nine examples in azerbaijan and the lusi mud volcano in east java are described distributions are non random forming alignments subparallel to faults within anticlines ring faults conjugate faults and detachment faults this finding confirms a spatial relationship and supports a model for subsurface flow along these features as well as showing fractionation at depth as fracture and fault orientations are related to structures such as anticlines and the in situ stress state they are therefore predictable we use vent distributions in azerbaijan where the structural geology is well constrained to propose what controls the distribution of 169 vents at the lusi mud volcano this mud volcano system shows evidence for initial eruptions along a ne sw trend parallel to the watukosek fault changing to eruptions that follow east west trends subparallel to regional fold axes our analysis indicates that regions east and west of the lusi mud volcano are more likely to be affected by new vents than those to the north and south owing to probable onset of elongate caldera collapse within a 10 km diameter of the central vent

the marine phosphorus cycle phosphorus introduced to the ocean must ultimately be removed if a steady state is to be maintained over geologic time aeolian input is insignificant and there is evidently no hydrothermal contribution rivers dominate p input to the ocean the fluvial p flux is poorly quantified due to the interactions of two factors 1 man s influence on river nutrient fluxes and 2 inherent complexities in understanding the biological and inorganic associations of p in estuarine and coastal waters our best estimate of the natural or pre man p input flux is about 10 x 10 sup 9 moles p cm sup 2 ocean surface year of the identified flux of phosphorus to the sediments about 90 percent is in the form of biogenic debris or its regeneration products the burial of phosphatic fish debris is negligible p removal by direct reaction of phosphate in seawater with sea floor basalts during mid ocean ridge hydrothermal circulation is insignificant p removal by processes such as sorption on clay surfaces or formation of authigenic fe and al phosphate phases is presently unquantified and remains a serious uncertainty in the phosphorus cycle the identified sources and sinks approximately balance yielding a residence time for p in the oceans of about 10 sup 5 yrs

forcing of wet phases in southeast africa over the past 17 000 years a continuous record of hydrologic variability for the past 17 000 years at the mouth of the zambezi river shows that hydrologic conditions in southeast africa were controlled by variations in local insolation and migrations of the intertropical convergence zone rather than by indian ocean temperature records of hydroclimatological variability in southern africa since the last glacial maximum have produced a conflicting picture of the relative influences of events in the remote northern hemisphere and the more local indian ocean this paper presents a continuous record of hydrological variability for the past 17 000 years from marine sediment core drilled off the mouth of the zambezi river the record shows that hydrological conditions in the region were influenced by variations in local insolation and migrations of the intertropical convergence zone which were probably driven by events in the northern hemisphere rather than the indian ocean intense debate persists about the climatic mechanisms governing hydrologic changes in tropical and subtropical southeast africa since the last glacial maximum about 20 000 years ago in particular the relative importance of atmospheric and oceanic processes is not firmly established1 2 3 4 5 southward shifts of the intertropical convergence zone itcz driven by high latitude climate changes have been suggested as a primary forcing2 3 whereas other studies infer a predominant influence of indian ocean sea surface temperatures on regional rainfall changes4 5 to address this question a continuous record representing an integrated signal of regional climate variability is required but has until now been missing here we show that remote atmospheric forcing by cold events in the northern high latitudes appears to have been the main driver of hydro climatology in southeast africa during rapid climate changes over the past 17 000 years our results are based on a reconstruction of precipitation and river discharge changes as recorded in a marine sediment core off the mouth of the zambezi river near the southern boundary of the modern seasonal itcz migration indian ocean sea surface temperatures did not exert a primary control over southeast african hydrologic variability instead phases of high precipitation and terrestrial discharge occurred when the itcz was forced southwards during northern hemisphere cold events such as heinrich stadial 1 around 16 000 years ago and the younger dryas around 12 000 years ago or when local summer insolation was high in the late holocene that is during the past 4 000 years

reconnaissance survey of the duolun ring structure in inner mongolia not an impact structure the duolun basin which is located in inner mongolia china has been proposed to be an impact structure with an apparent rim diameter of about 70 or even 170 km the designation as an impact structure was based on its nearly circular topography consisting of an annular moat that surrounds an inner hummocky region and the widespread occurrences of various igneous rocks polymict breccias and deformed crustal rocks critical shock metamorphic evidence is not available to support the impact hypothesis we conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure we collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations especially those sharing similar appearances with impact breccias suevites impact melt rocks and shatter cones field investigation together with thin section examination discovered that the suspected impact melt rocks are actually early cretaceous and late jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions and the interpreted impact glass is typical volcanic glass petrographic analyses of all the samples reveal no indications for shock metamorphic overprint all these lines of evidence suggest that the duolun basin was not formed through impact cratering the structural deformation and spatial distribution pattern of the igneous rocks suggest that the duolun basin is most likely a jurassic cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin followed by resurgent doming of the central area

phanerozoic transgressions and regressions on the continents a quantitative approach based on areas flooded by the sea and areas of marine and continental deposition

downsizing the mjolnir impact structure barents sea norway abstract stratigraphic relationships indicate that the submarine mjolnir impact structure in the sw barents sea is of cretaceous age 142 ma and the impact palaeolatitude is estimated at 56 n though the crater has been intensively studied its dimensions at depth are uncertain density and magnetic susceptibility measurements of core samples are presented and used to interpret a newly available aeromagnetic survey and gravity data and to model the potential field anomaly data of the area jointly so as to constrain better the signature of the crater forward modelling suggests that the crater diameter is about 20 km and thus needs downscaling to about half of the earlier proposed size

volcanosedimentary paleoenvironments of ediacaran fossils in newfoundland a new perspective on paleoenvironments of ediacaran fossils of the upper conception group newfoundland comes from geochemical and sedimentological study of volcanic tuffs and sedimentary rocks tuffs in the conception group have major and trace element compositions and u pb ages comparable with those of source volcanics on the nearby burin and bonavista peninsulas and the islands of st pierre and miquelon loss of silica and alkalies in some ashes indicates weathering on land not marine diagenesis volcanic crystal and lapilli tuffs fail to show grading and have lapilli and highly vesicular scoria scattered in a fine grained matrix and so they were deposited on land not in water these as well as block and ash flows volcanic spindle bombs and degassing features are evidence of eruptions from nearby subaerial volcanic edifices the fossiliferous conception group accumulated within a forearc basin formed on continental crust inboard of the holyrood horst and uplifted as part of an ancient subduction complex or accreted terrane like analogous forearc basins in oregon washington southern chile and japan the conception group includes not only marine bay turbidites but also a variety of intertidal and terrestrial tsunamites seismites tempestites and paleosols traditional marine turbidite models explain deposition of the mall bay lower drook and lower briscal formations of the conception group but the gaskiers upper drook upper briscal and mistaken point formations were deposited in coastal plains and intertidal zones paleoenvironments of vendobiont fossils preserved in life position in newfoundland were terrestrial to marginal marine not deep sea

hyperspectral imaging of drill core from the steen river impact structure canada implications for hydrothermal activity and formation of suevite like breccias

reconstruction of the evolution of deep fluids in light oil reservoirs in the central tarim basin by using pvt simulation and basin modeling abstract to better understand the fluid phase state and its evolution in a deep environment an integrated pvt simulation and basin modeling method for two light oil reservoirs in the central tarim basin was proposed and applied the fluid phase models were built using pvt simulation technology after successfully recovering the fluid compositions the envelopes of fluid from either the awatage formation 2a for well zs1 or the wusongar formation 1w for well zs5 show the order from cricondenbar pm critical point cp to cricondentherm tm suggesting a single liquid phase for the present fluids in the cambrian subsalt dolomite reservoirs the result of this study indicates that the two reservoirs did not experience phase changes phase differentiation or oil cracking and maintained a universal state of the liquid phase over time by combining the evolutionary history of temperature and pressure derived from basin modeling the fluid density and viscosity experienced relatively complicated fluctuation processes and the fluctuations of fluid viscosity over geologic history are much more apparent than the variations in fluid density the density and viscosity of the awatage formation 2a for well zs1 are found to be lower than those of the wusongar formation 1w for well zs5 the results reveal that factors such as good trap conditions a stable tectonic background a low paleo geothermal gradient 1 6 2 8 c 100 m and the weak effect of thermal sulfate reduction tsr altogether act on the early accumulated light oil and gas that has been preserved as a single liquid phase over time these results are significant for future explorations of large scale liquid petroleum in cambrian subsalt dolomite reservoirs of the central tarim basin

the trace element geochemistry of clinopyroxenes from pyroxenites in the lewisian of nw scotland insights into light rare earth element mobility during granulite facies metamorphism clinopyroxenes from layered pyroxenites and from pyroxenite pods in felsic gneisses of the lewisian granulite complex nw scotland have distinctive chemistries suggestive of different origins clinopyroxenes in the layered pyroxenites crystallised from mafic melts in a magma chamber located in the middle to shallow crust whereas clinopyroxenes in pods in the felsic gneisses crystallised from the tonalitic protolith to the felsic gneisses in detail clinopyroxenes in the layered pyroxenites are variably enriched in the light ree inversion modelling shows that this is not a primary feature inherited from their parent magmas rather selective light rare earth element enrichment took place through reaction with a felsic melt generated by the localised partial melting of the hornblende pyroxenites during granulite facies metamorphism published isotopic evidence suggests that the light ree mobilisation took place at ca 2 7 ga about 200 ma after the time of crust formation this observation provides an explanation for the scattered pattern of whole rock isochron ages from the lewsian granulites

phosphorus dynamics around the sulphate methane transition in continental margin sediments authigenic apatite and fe ii phosphates abstract the formation of authigenic phosphate minerals in marine sediments is an important process for the burial and long term storage of the bio essential nutrient phosphorus p in this context we report the composition of pore waters bulk sediments and the speciation of p in four sediment cores recovered on the continental margins off the amazon rio de la plata and zambezi rivers here pronounced sulphate methane transitions smts occur between 4 5 and 6 5 m sediment depth where sulphate is consumed by the anaerobic oxidation of methane and free hydrogen sulphide builds up in the pore waters this leads to the reductive dissolution of primary fe oxyhydr oxides feox by hydrogen sulphide and the subsequent liberation of feox adsorbed phosphate into pore waters at the smt the released phosphate builds up to significant concentrations making it available for the precipitation of authigenic minerals within and below the smt using a sequential p extraction we find consistently high contributions of carbonate fluorapatite cfa to the total p pool within the smt where it likely precipitates due to high local phosphate concentrations high alkalinity and abundant dissolved ca in the pore waters phreeqc calculations confirm these results with highest saturation states with respect to authigenic apatite calculated at all smts cfa authigenesis however is insufficient to completely consume pore water phosphate leading to diffusive loss of phosphate from the smt while the upward decrease in phosphate above the smt is relatively gentle the significantly steeper phosphate gradient into underlying sediments suggests the formation of another authigenic mineral phase sequential p extraction and phreeqc results clearly show that the dominant authigenic phase below the smt is not cfa instead geochemical conditions below the sulphidic zone at all four sites are favourable for the precipitation of fe ii phosphate minerals e g vivianite these conditions are summarized as the absence of sulphate and free hydrogen sulphide but the presence of fe 2 and phosphate in pore waters and low calcium carbonate contents in the sediment while we did not directly detect or quantify fe ii phosphate minerals fe bound p clearly increases in the sediments below the sulphidic zones at all studied sites and this is where highest pore water saturation states with respect to vivianite are calculated with phreeqc in addition geochemical inverse models performed in phreeqc show that vivianite formation is an important geochemical process controlling the observed patterns of dissolved p and fe ph and redox potential of the pore waters analysed below the smt we therefore argue that fe ii phosphate minerals are formed in these ocean margin sediments on a large scale and pore water and sediment data indicate that this fe bound p largely originates from feox adsorbed phosphate liberated within the sulphidic smt this study suggests that under specific but not unusual depositional conditions smts are hotspots for biogeochemical p cycling in marginal marine sediments worldwide with cfa precipitating within the smt and fe ii phosphates forming below it it also adds to the growing evidence that fe ii phosphates may contribute significantly to the long term burial of reactive p phases in fe rich methanic marine sediments and thus act as a previously underestimated sedimentary p sink

the geochronology of large igneous provinces terrestrial impact craters and their relationship to mass extinctions on earth the relationship between sudden catastrophic events and sudden mass extinctions on earth has attracted a great deal of research effort including measurements of the absolute and relative ages of these events the result has been a rapid improvement in the precision and accuracy of radiometric dates for large igneous provinces lips for which there is now a consensus on the peak eruption times over the last 250 ma in parallel there has been a more gradual improvement in the radiometric dates for craters formed by hypervelocity impacts hvis of asteroids and comets with earth the absolute ages of the largest craters are known to within a few million years but the ages of many smaller craters are poorly constrained in addition whereas relatively few lips have formed in the last 250 ma at least one hvi crater larger than 20 km diameter has formed on earth every million years only 25 such craters have been discovered so coincidence between the ages of hvi crater formation and ages of mass extinctions is related as much to crater preservation as to coincidence of the events the difference in quality of geochronology between hvis and lips is the result of differing availability of samples lips cover large areas and present an array of basalt samples suitable for dating whereas hvi craters are often poorly preserved and present few datable samples the radiometric data for preserved hvis and lips are now sufficiently precise and accurate for us to be confident that there is no one to one correlation between either lips or hvis and global mass extinctions of life on earth however there is a close association between a significant number of lips and sudden environmental change and a very close association between at least one massive hvi crater formation and a mass extinction further there is no apparent correlation between hvis and lips and thus no reason to suggest a causative relationship finally the radiometric dating of hvi craters indicates several clusters in the geological record but the correlation with sudden environmental change is no better than for large craters

hailar crater a possible impact structure in inner mongolia china abstract hailar crater a probable impact structure is a circular depression about 300 m diameter in inner mongolia northeast china with broad elevated rims the present rim to floor depth is 8 20 m regional geological background and geomorphological comparison suggest that this feature is likely not formed by surface processes such as salt diapir karst aeolian glacial or volcanic activity its unique occurrence in this region and well preserved morphology are most consistent with it being a cenozoic impact crater two field expeditions in 2016 and 2017 investigated the origin of this structure recognizing that 1 no additional craters were identified around hailar crater in the centimeter scale digital topography models that were constructed using a drone imaging system and stereo photogrammetry 2 no bedrock exposures are visible within or adjacent to the crater because of thick regolith coverage and only small pieces of angular unconsolidated rocks are present on the crater wall and the gently sloped crater rim suggesting recent energetic formation of the crater 3 most samples collected from the crater have identical lithology and petrographic characteristics with the background terrain but some crater samples contain more abundant clasts and silicate hydrothermal veins indicating that rocks from depths have been exposed by the crater 4 no shock metamorphic features were found in the samples after thin section examinations and 5 a systematic sample survey and iron detector scan within and outside of the crater found no iron rich meteorites larger than 2 cm in size in a depth of 30 cm although no conclusive evidence for an impact origin is found yet hailar crater was most likely formed by an impact based on its unique occurrence and comparative geomorphologic study we suggest that drilling in the crater center is required to verify the impact origin where hypothesized melt bearing impactites may be encountered

ediacaran gaskiers glaciation of newfoundland reconsidered newly discovered palaeosols in the gaskiers formation of newfoundland provide a new perspective on its palaeoenvironment and are evidence that it was not a deep submarine tillite nor an anoxic deep sea floor nor a cap carbonate from extreme geochemical perturbation of the global ocean discovery of oxidized palaeosols and re evaluation of sedimentary facies and stratigraphic relationships now suggest reinterpretation as a coastal plain with glacial moraines in the forearc basin of a continental volcanic arc like modern japan ediacaran palaeosols of the gaskiers formation lack large root traces but show soil profile differentiation clay enrichment and depletion of alkali and alkaline earth elements at the expense of feldspar and rock fragments and diagnostic soil structures blocky peds argillans sepic plasmic fabric these palaeosols are evidence of a humid temperate climate and marked marine regression accompanying the gaskiers glaciation of the early ediacaran 580 ma geochemical weathering trends in the palaeosols especially phosphorus depletion are characteristic of biologically active soils ediacaran microbial earth ecoystems may have been responsible for filamentous disruption of bedding in the palaeosols

the phanerozoic impact cratering rate evidence from the farside of the moon the relatively recent 1 b y flux of asteroids and comets forming large craters on the earth and moon may be accurately recorded by craters with bright rays on the moon s farside many previously unknown farside rayed craters are clearly distinguished in the low phase angle images returned by the clementine spacecraft some large rayed craters on the lunar nearside are probably significantly older than 1 ga rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity most of the farside crust has a more homogeneous composition and only immature rays are visible the size frequency distribution of farside rayed craters is similar to that measured for eratosthenian craters up to 3 2 b y at diameters larger than 15 km the areal density of farside rayed craters matches that of a corrected tabulation of nearside copernican craters hence the presence of bright rays due to immature soils around large craters provides a consistent time stratigraphic basis for defining the base of the copernican system the density of large craters less than 3 2 b y old is 3 2 times higher than that of large farside rayed craters alone this observation can be interpreted in two ways 1 the average cratering rate has been constant over the past 3 2 b y and the base of the copernican is 1 ga or 2 the cratering rate has increased in recent geologic time and the base of the copernican is less than 1 ga we favor the latter interpretation because the rays of copernicus 800 850 m y old appear to be very close to optical maturity suggesting that the average copernican cratering rate was 35 higher than the average eratosthenian rate other lines of evidence for an increase in the phanerozoic 545 ga cratering rate are 1 the densities of small craters superimposed on copernicus and apollo landing sites 2 the rates estimated from well dated terrestrial craters 120 m y and from present day astronomical observations and 3 the proterozoic rate suggested by the crater record of australia the hypothesis most consistent with several key observations is that the cratering rate has increased by 2x during the past 300 m y

sedimentation patterns off the zambezi river over the last 20 000 years marine sediments from continental margins provide high resolution archives of marine and continental climate in particular near large river mouths the zambezi is one of the largest rivers in east africa discharging large amounts of fine grained sediments onto the western margin of the mozambique channel accurate reconstructions of past variations in river discharge can be retrieved from the depositional history of riverine sediments in this study sedimentation patterns along the mozambique margin are inferred from a series of sediment cores spanning the last 20 kyr these cores were retrieved off the zambezi mouth from the shelf and from various depths at the continental slope to provide detailed information on the depositional history of this region high resolution x ray fluorescence and magnetic susceptibility core scanning measurements as well as grain size distributions of the lithogenic sediment fraction and organic matter and carbonate content analyses resolve centennial millennial timescale variations in sediment transport and deposition along the mozambique margin largest changes in lithogenic sediment deposition coincide with the flooding of the mozambique shelf due to deglacial sea level rise and changes in oceanic currents fine grained mainly zambezi sediments were no longer primarily deposited south of the zambezi mouth but increasingly deposited onto the mozambique shelf and transported northwards over the shelf before spilling over to the slope in addition flooding of the shelf initiated remobilization and down slope transport of coarse grained winnowed sediments the heinrich event 1 and younger dryas are clearly indicated as events of increased fine grained sediment deposition along the mozambique margin possibly linked to increased riverine runoff from the zambezi catchment which is in harmony with a southward shift of the intertropical convergence zone itcz and associated african rainfall belts causing wetter conditions this study illustrates that interpreting marine and continental climate proxy records in the vicinity of river deltas in terms of climate change requires proper consideration of sea level and oceanic circulation induced effects on the transport and deposition patterns of fine grained sediments at continental slopes

the first large meteorite impact structure discovered in the middle east jebel waqf as suwwan jordan triggered by re evaluation of a 1960s report on the regional geology of the northeastern border region of jordan and following landsat satellite image investigation a 5 5 km diameter complex circular structure was discovered in the central eastern region of the kingdom of jordan initial ground truthing revealed complex geological structures involving upper cretaceous and paleogene strata and including a prominent outer rim rising up to 60 m above the surrounding plain an intermediate ring of up to 20 m elevation within a ring syncline and a central zone of stratigraphically uplifted sedimentary strata characterized by intense macroscopic folding and faulting widespread cataclasis and locally mesoscopic cataclasis deformation ten sites with shatter cone development in fine grained sandstone or limestone have been mapped to date mostly in the outer parts of the central uplifted area this finding confirms that the jebel waqf as suwwan structure was formed as the result of the impact of an extraterrestrial projectile search for impact diagnostic micro deformation has been rather unsuccessful only 1 quartz grain with both planar deformation features and planar fractures has been detected in a sandstone sample to date the overall majority of the approximately 70 samples investigated by micropetrographic analysis consist of extremely fine grained chert siltstone or marly limestone cataclasis is widespread in chert and limestone also on the micro scale considering the severely limited amount of characteristic impact microdeformation and the stratigraphic situation within the central uplift it is likely that a relatively deep level of the central uplift is currently exposed the extensive drainage demonstrated for this region supports the conclusion that this impact structure could be quite deeply eroded especially as its geology involves some relatively soft lithologies marls limestones the age of this impact event is at present poorly constrained at post middle to lower eocene

crustal inhomogeneities in the northern north sea from potential field modeling inherited structure and serpentinites abstract a new crustal model for the northern north sea was developed by gravity and magnetic modeling along the deep seismic line nsdp84 1 utilizing vertical gradients allowed distinguishing between shallow and deep crustal sources the upper crust is characterized by low magnetic susceptibilities and low densities which is typical for felsic rocks a new finding was that the deep crust below the western viking graben and the east shetland basin is the source of high magnetic anomalies combined with low gravity anomalies which was interpreted to represent rocks with very high magnetic susceptibilities and low to intermediate densities such rock parameters may indicate serpentinites but intermediate intrusives or a combination of both is also possible honoring the string of three near equidistant magnetic maxima which follow the trend of the nne ssw striking east shetland basin in the map plane it is suggested that this area is part of an island arc of the iapetus ocean which has been assembled during the collision between laurentia and baltica in late silurian times partly serpentinized peridotites and intermediate intrusives will relate in such a model to slab dehydration of the subducting oceanic plate below the island arc these inherited or synorogenic serpentinites are expected to persist in the geothermal regime of the caledonian orogeny to a depth of at least 50 km increased heat flow by later rift phases will have caused metamorphism of the remaining serpentinites to meta peridotites at depth below the present day moho fluid release related to dehydration of the serpentinites may have triggered further serpentinization of the inherited partly serpentinized rocks at shallower depth an alternative origin for the suggested serpentinites valid only for the area under the western part of the viking graben may be synrift serpentinization due to the heavy faulting during the jurassic rift phase

the hazard of near earth asteroid impacts on earth near earth asteroids neas have struck the earth throughout its existence during epochs when life was gaining a foothold f4 ga the impact rate was thousands of times what it is today even during the phanerozoic the numbers of neas guarantee that there were other impacts possibly larger than the chicxulub event which was responsible for the cretaceous tertiary extinctions astronomers have found over 2500 neas of all sizes including well over half of the estimated 1100 neas 1 km diameter neas are mostly collisional fragments from the inner half of the asteroid belt and range in composition from porous carbonaceous chondrite like to metallic nearly one fifth of them have satellites or are double bodies when the international telescopic spaceguard survey which has a goal of discovering 90 of neas 1 km diameter is completed perhaps as early as 2008 nearly half of the remaining impact hazard will be from land or ocean impacts by bodies 70 600 m diameter comets are expected to contribute only about 1 of the total risk the consequences of impacts for civilization are potentially enormous but impacts are so rare that worldwide mortality from impacts will have dropped to only about 150 per year averaged over very long durations after the spaceguard goal has presumably ruled out near term impacts by 90 of the most dangerous ones that is in the mid range between very serious causes of death disease auto accidents and minor but frightening ones like shark attacks differences in perception concerning this rather newly recognized hazard dominate evaluation of its significance the most likely type of impact events we face are hyped or misinterpreted predicted impacts or near misses involving small neas d 2004 elsevier b v all rights reserved

3d seismic reflection mapping of the silverpit multi ringed crater north sea the first comprehensive mapping of the silverpit multi ringed crater is presented based on five adjoining 3d seismic surveys that together provide 78 216 km2 areal coverage of the structure with spatial and depth sampling of 10 m the top cretaceous surface shows an excavated crater 3 km in diameter surrounded by 10 15 concentric ring structures out to a radius of 9 4 km seismic sequence stratigraphy and drilled wells indicate the lower age bound is a disconformity spanning 74 54 ma the upper bound is eocene termination of concentric ring fault growth ca 45 ma the inner rings are extensional faults defining rotated fault blocks that would be considered part of the crater in an impact structure interpretation giving a final impact crater diameter of 8 km the outer rings show a surprising variation in style from graben in the west to compressional structures in the east this may be explained by postcratering gravity spreading on easterly dipping intra cretaceous detachments within the crater the top cretaceous seismic reflector is disrupted and blocky but defines a central uplift with a slight crestal depression below this uplift the prominent base cretaceous seismic reflector shows jurassic strata rising into a cone 300 m high and 1 km wide characterized by radial faults this central uplift within the crater may structurally balance the inner ring fault blocks via a slumping model but we also identify low seismic velocity in the uplifted jurassic so fracturing may have contributed to growth of the central peak at a radius of 2 5 km base cretaceous shows a dish shaped depression that we interpret as marking either the extent of slumping toward the crater at this structural level or the limit of shockwave induced porosity collapse seismic reflectors above the top cretaceous excavated crater show differential compaction during the eocene accommodated by drape structure plus two conical faults that define narrow ring graben and represent the final phase of crater related tectonics

a new particulate mn fe p shuttle at the redoxcline of anoxic basins abstract pelagic redoxclines of anoxic basins and deeps form the suboxic transition between oxygenated surface and anoxic or even sulfidic bottom waters intense element cycling favoured by elevated microbial activity causes steep gradients of physico chemical parameters nutrients and redox sensitive trace metals this study presents a conceptual model for authigenic particle formation at pelagic redoxclines which is based on the tight coupling of mn fe and p cycles besides the well known occurrence of mn oxides textural sem edx and geochemical icp oes icp ms analyses of particles from the redoxclines of the black sea and the baltic sea gotland basin landsort deep evidence the existence of earlier postulated fe oxyhydroxo phosphates and emphasize mixed phases consisting of mn oxides and fe oxyhydroxo phosphates as a new solid species most of the analyzed particles are star shaped of about 5 m in size and occur as single particles or aggregates without any morphological differences between mn oxides fe oxyhydroxo phosphates and mixed phases throughout the redoxcline these minerals show a general succession with maximum abundance of mn oxides above the redoxcline followed by mixed phases and almost pure fe phosphates within and below the redoxcline respectively molar fe p ratios of single particles argue against the formation of known pure fe phosphates like vivianite or strengite at the lower end of the redox transition zone but are consistent with recent experimental findings for colloidal p bearing hydrous ferric oxides moreover morphological similarities suggest the formation of irregular fe oxyhydroxo coatings due to oxidation of upward diffusing fe 2 by oxygen and stepwise replacement of mn iv by fe iii on sinking mno x particles followed by immediate adsorption or even co precipitation of phosphate batch type experiments using biogenic mno x particles demonstrate the efficient potential of fe 2 oxidation by sinking mno x particles when entering sulfidic waters mno x particles are progressively reduced leading to an increasing relative abundance of fe and p rich particles in deeper parts of the water column these particles are also reductively dissolved thereby releasing fe 2 and phosphate to the water column this mn fe p shuttle likely affects phosphate transport throughout the water column and thus impacts primary production at least over longer time scales furthermore the particulate mn fe p shuttle must have played an important role for the cycling of p and certain trace metals in ancient ocean basins e g during certain periods of cretaceous black shale formation and should be considered in future mass balances and modeling approaches dealing with oxic anoxic interfaces of aquatic ecosystems

impact origin of the newporte structure williston basin north dakota the newporte field is located just south of the united states canada border in renville county north dakota in the north central portion of the williston basin integration of seismic well log and core data supports the interpretation of an impact origin for the newporte structure the structure involves both precambrian basement and lower paleozoic sedimentary units oil and gas production began in 1977 from brecciated basement rocks along the rim of the 3 2 km diameter circular structure both well logs and seismic data were used to determine thickness changes of sedimentary units overlying the structure resulting isopach maps reveal a circular bowl shaped feature with a recognizable rim microscopic shock metamorphic features in quartz and feldspar are visible in basement clasts that form a mixed breccia with cambrian deadwood sandstone within the western rim of the structure a late cambrian early ordovician age is suggested for the structure because of the presence of flat lying deadwood sandstone overlying mixed basement sandstone breccia along portions of the rim identification of the newporte structure as an impact crater adds to the gro ing base of evidence revealing the relevance of impact craters to petroleum exploration

tectono eustatic controls on reservoir compartmentalisation and quality an example from the upper miocene of the san joaquin basin california abstract systematic changes in stratigraphic architecture and reservoir quality of upper miocene stevens sandstones at north and south coles levee fields san joaquin basin reflect tectono eustatic controls on deposition two depositional sequences 330 500 m thick are present in the coles levee stevens the upper sequence contains sandstones deposited by sediment gravity flows in water depths of 300 500 m backstepping retrogradational reservoir compartments in this sequence overlie vertically stacked to prograding compartments although the backsteps are the better reservoirs with higher permeabilities and porosities the vertically stacked to prograding compartments are volumetrically more important curiously the upper stevens section is sand rich 60 80 ss on structural highs that were also palaeotopographic highs thicker sections with less and characterise the flanks these sand distributions are anomalous since sediment gravity flows preferentially deposit in bathymetric lows apparently the flanks of stevens palaeohighs subsided faster than the crests consequently accomodation increases were greatest on the flanks and resulted in aggradation with preservation of section less accommodation on the crests resulted in progradation with erosion and reworking through lobe switching processes some sand reached all parts of the depocenter subsequent progradation with erosion of shale interbeds and attendant reworking by density currents created sand rich sections in slowly subsiding areas crests concurrent aggradation preserved shale interbeds and created shale dominated sections in rapidly subsiding areas flanks greater compaction of thicker shale dominated flanks equalised thicknesses of flank and crest sections thus the reservoir architecture represents the integrated effects of compensation lobe switching accommodation subsidence sea level and compaction

how will we recognize buried impact craters in terrestrial sedimentary basins criteria that are deemed critical for recognition of terrestrial impact craters include features from hand specimens such as shock metamorphic structures although these criteria have been selected because they are unique physical products of bolide impact strict adherence to these criteria means that we cannot positively identify buried astroblemes that are tomographically imaged on three dimensional seismic data but are undrilled because many of earth9s deeply buried structures may never be drilled because of prohibitive cost and lack of commercial incentive a large proportion of as yet undiscovered terrestrial astroblemes including the best preserved examples may remain unclassified and be excluded from impact structure research a framework for the identification of impact structures imaged on three dimensional seismic data is proposed on the basis of geometrical criteria plus additional image quality and preservation criteria that assign a level of confidence to the assessment

impact origin of the avak structure arctic alaska and genesis of the barrow gas fields geophysical and subsurface geologic data suggest that the avak structure which underlies the arctic coastal plain 12 km southeast of barrow alaska is a hypervelocity meteorite or comet impact structure the structure is a roughly circular area of uplifted chaotically deformed upper triassic to lower cretaceous sedimentary rocks 8 km in diameter that is bounded by a ring of anastomosing inwardly dipping listric normal faults 12 km in diameter a zone of gently outward dipping sedimentary country rocks forms a discontinuous ring of rim anticlines within the peripheral ring of normal faults beyond these anticlines the sedimentary rocks are almost flat lying basement consists of strongly deformed ordovician and silurian argillite density and acoustic impedance con rasts between the argillite and the overlying strata produce gravity and seismic reflection signatures that define a ring of anticlines around the disturbed zone and a structural high surrounded by an annular structural low at its center in the adjacent barrow gas fields the tops of the informally named neocomian pebble shale unit and the gas producing lower jurassic barrow sand local usage lie at average subsea depths of 488 m and 670 m respectively in the avak 1 well drilled on the central high the pebble shale and the barrow sand lie near the surface documenting more than 500 m of relative uplift at the high the cores in this well have steep dips 30 90 degrees mixed breccia with franklinian argillite clasts 10 and 90 m above basement quartz grains with shock mosaicism and multiple sets of shock lamellae oriented concussion fractures in sand size quartz grains and shatter cones resembling those found in the peripheral zones of well documented impact structures in addition above background levels o fractured quartz grains in barrow sand were found as far as 19 km beyond the rim of the avak structure data concerning the age of the avak structure are not definitive if submarine landslide deposits in the upper part of the aptian and albian torok formation in the subsurface 200 km to the east were triggered by the avak event then the avak meteorite struck a submerged marine shelf about 100 or 5 ma however the impact features found at avak shatter cones concussion fractures shock lamellae and shock mosaicism in quartz grains and widespread cataclasis characterize the distal zones of meteorite impact structures fused rocks plastic deformation and shock metamorphic minerals found in more proximal zones of impact structures are apparently missing these observations and the lack of avak ejecta in cuttings and cores from the torok formation and nanushuk group albian to middle cenomanian in surrounding test wells indicate that the impact event postdated these beds in this case the avak meteorite struck a late cretaceous or tertiary marine shelf or coastal plain between the cenomanian ca 95 ma and deposition of the basal beds of the overlying late pliocene and quaternary gubik formation ca 3 ma

exploring the continental shelf for low geological risk nuclear waste repository sites using petroleum industry databases a uk case study abstract site selection for deep disposal of intermediate and high level radioactive waste in the uk to date has not taken full advantage of oil industry databases that cover offshore areas the shallow water area of the uk continental shelf is twice that of the onshore area and due to variations in regional geology many geological settings occur that are not found onshore the geology and hydrogeology of the uk continental shelf contrast in many respects with that of onshore areas the geology of the north sea in particular is constrained by over 3000 deep 2 km exploration wells 50 000 km 2 of 3d seismic data and several million line kilometres of 2d seismic data vastly more than the coverage of data constraining the deep geology onshore combined with detailed studies of oil and gas fields this data has led to a greater technical understanding of many aspects of offshore geology relative to that onshore these data represent an original investment of 20bn by oil and gas companies but are now in the public domain these databases are used here to compare onshore and offshore settings in terms of criteria such as structural geology fault activity and uplift subsidence clear differences emerge such as risk of seismicity exhumation and the variety of hydrogeological systems the distribution and geometry of candidate repository sites throughout the north sea salt structures shale basins sealed sandstone reservoirs and basement highs are illustrated several lines of petroleum related research have yielded lessons that may be useful in assessing deep repository locations such as migration of hydrocarbons through shale seals the exceptional combination of seismic data and well control means that offshore areas on the continental shelf and the north sea in particular can now be thoroughly explored in this context and contribute candidate sites into a nationwide portfolio of deep geological disposal options in the event of site selection being pursued in the uk an example framework for this process is presented here

collapse infilling and postimpact deformation at the mjolnir impact structure barents sea a dense composite grid of high resolution single channel shallow multichannel and conventional multichannel seismic reflection profiles defines in detail the structures and stratigraphy associated with impact induced tectonics and the postimpact evolution of the mjolnir impact structure barents sea clear seismic images of the well preserved 40 km diameter structure reveal that the primary impact induced deformation is bounded on top by discernible structural relief at this stratigraphic level the structure exhibits a distinct radial zonation pattern composed of a 12 km wide complex outer zone including a marginal fault zone and a modestly elevated ring a 4 km wide annular depression and an uplifted central high 8 km in diameter key features of the mjolnir structure are similar to the majority of large terrestrial complex craters sharp boundary faults form an 150 m high near circular rim wall and separate highly deformed strata within the crater from intact platform strata a 45 180 m thick layer of disturbed and incoherent seismic reflectivity caused by the impact is confined by prominent fault blocks and the postimpact strata the seismic data also provide evidence of crater influenced sedimentation and extensive secondary postimpact deformation expressed by structural reactivation and differential subsidence reconstruction of the original crater relief reveals a subtle structure that has average depth variations of only 30 40 m when compared to the current morphology of the crater floor this demonstrates substantial enhancement of structural features by postimpact burial and deformation we infer that impact in a shallow marine sedimentary basin led to an atypically shallow crater depth resulting from gravitational collapse on an apparent low angle decollement at the crater periphery and considerable infilling of the crater the extensive postimpact deformation is closely related to the instability and lateral changes in physical properties within the impact affected rock volume triggered by prograding postimpact sediments

earth impact effects program a web based computer program for calculating the regional environmental consequences of a meteoroid impact on earth we have developed a web based program for quickly estimating the regional environmental consequences of a comet or asteroid impact on earth www lpl arizona edu impacteffects this paper details the observations assumptions and equations upon which the program is based it describes our approach to quantifying the principal impact processes that might affect the people buildings and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions the program requires six inputs impactor diameter impactor density impact velocity before atmospheric entry impact angle the distance from the impact at which the environmental effects are to be calculated and the target type sedimentary rock crystalline rock or a water layer above rock the program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse the thermal radiation emitted by the impact generated vapor plume fireball and the intensity of seismic shaking the program also approximates various dimensions of the impact crater and ejecta deposit as well as estimating the severity of the air blast in both crater forming and airburst impacts we illustrate the utility of our program by examining the predicted environmental consequences across the united states of hypothetical impact scenarios occurring in los angeles we find that the most wide reaching environmental consequence is seismic shaking both ejecta deposit thickness and air blast pressure decay much more rapidly with distance than with seismic ground motion close to the impact site the most devastating effect is from thermal radiation however the curvature of the earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon

efficient disruption of small asteroids by earth s atmosphere accurate modelling of the interaction between the atmosphere and an incoming bolide is a complex task but crucial to determining the fraction of small asteroids that actually hit the earth s surface most semi analytical approaches have simplified the problem by considering the impactor as a strengthless liquid like object pancake models1 2 but recently a more realistic model has been developed that calculates motion aerodynamic loading and ablation for each separate particle or fragment in a disrupted impactor3 4 here we report the results of a large number of simulations in which we use both models to develop a statistical picture of atmosphere bolide interaction for iron and stony objects with initial diameters up to 1 km we show that the separated fragments model predicts the total atmospheric disruption of much larger stony bodies than previously thought in addition our data set of 1 000 simulated impacts combined with the known pre atmospheric flux of asteroids with diameters less than 1 km5 6 7 8 9 10 11 12 elucidates the flux of small bolides at the earth s surface we estimate that bodies 220 m in diameter will impact every 170 000 years

revised wonoka isotopic anomaly in south australia and late ediacaran mass extinction the global late ediacaran shuram wonoka carbon isotope anomaly has been regarded as the largest and longest known isotopic anomaly in the ocean assuming that all ediacaran carbonate is marine disregarding carbonate in south australia shown here to be palaeosol or palaeokarst the synchronous marine organic carbonate excursion is only 8 for 13 c organic and 6 for 13 c carbonate and lasted less than a million years this revised magnitude and duration is comparable with perturbations across the permian triassic boundary and correlative with a global late ediacaran acritarch mass extinction like permian triassic isotopic excursions the revised organic carbonate wonoka excursion may also have been a greenhouse palaeoclimatic warm spike which terminated valley incision and glacioeustatic drawdown during the mid ediacaran fauquier glaciation and preceded chill of the late ediacaran billy springs glaciation supplementary material measured sections and tables of mineral and grain size proportions major element and stable isotope analyses are available at www geolsoc org uk sup18756

the rubble pile asteroid itokawa as observed by hayabusa during the interval from september through early december 2005 the hayabusa spacecraft was in close proximity to near earth asteroid 25143 itokawa and a variety of data were taken on its shape mass and surface topography as well as its mineralogic and elemental abundances the asteroid9s orthogonal axes are 535 294 and 209 meters the mass is 3 51 1010 kilograms and the estimated bulk density is 1 9 0 13 grams per cubic centimeter the correspondence between the smooth areas on the surface muses sea and sagamihara and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting itokawa is considered to be a rubble pile body because of its low bulk density high porosity boulder rich appearance and shape the existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re agglomeration into a rubble pile object

estimates of yet to find impact crater population on earth abstract one hundred and seventy six craters on earth are currently accepted as resulting from bolide impact this list is growing at a rate of approximately two craters per year because of new discoveries and revised interpretations here we take the subset of craters most relevant to sedimentary basin exploration and utilization phanerozoic kilometre scale craters and tackle the question of how many remain to be found in phanerozoic strata on earth a surprising range in predictions of crater numbers is obtained from published crater population functions reaching two orders of magnitude variation for craters in the range 1 5 km diameter this range in predictions is largely due to variations in treatment of impactor breakup in the atmosphere also required is quantification of the total area and timespan of phanerozoic sediment currently preserved on earth an estimate of 8 87 10 15 km 2 a is derived here together with a relatively conservative crater population function this yields a prediction of 845 craters subtracting the 131 known phanerozoic craters larger than 1 km leaves a predicted population yet to find of 714 of which 228 are larger than 2 5 km uncertainties subsets of this population and spatial distribution are also considered

circular geological structures outcropping in the sedimentary basins of saudi arabia abstract a number of circular geological structures outcrop in the sedimentary basins of saudi arabia several are reviewed here to assess their mode of origin they are unrelated to each other are easily seen on aerial imagery and their origins are assessed here on the basis of new fieldwork and reflection seismic data the structures range in size from hundreds of meters to several kilometers in diameter jabal rayah 28 39 n 37 11 30 e is 5 km in diameter with ring faults and anomalously steep dips in siluro devonian strata reflection seismic data demonstrate that underlying strata are structureless and jabal rayah is interpreted as a probable impact structure ash shutbah 21 37 08 n 45 39 21 e is 2 3 km in diameter and consists of a zone of concentric sub horizontal folds in mesozoic carbonates surrounding a central area of disharmonic steeply plunging folds and a mass of stratigraphically anomalous sandstone there are dissolution structures in the vicinity and although this is a viable explanation the uniquely large and complex nature of ash shutbah suggests an alternative origin by impact reflection seismic data show that underlying strata in the vicinity of ash shutbah are structureless a province of at least 100 circular structures occurs in northeast saudi arabia aerial imagery and reflection seismic data shows that these structures are a mixture of bioherms and dissolution collapse features the kidan crater 22 06 53 n 53 26 38 e is a subcircular feature approximately 100 m in diameter exposed in quaternary sabkha between large sand dunes in the rub al khali field investigation reveals an unfaulted bowl with a slightly fractured floor in sabkha facies 3d reflection seismic data shows a small fault underlying the crater leading to a fluid escape interpretation for the depression

a comparative study of iron and manganese diagenesis in continental slope and deep sea basin sediments off uruguay sw atlantic pore water and solid phase from surface sediments of the continental slope off uruguay and from the argentine basin southwestern atlantic were investigated geochemically to ascribe characteristic early diagenetic reactions of iron and manganese solid phase iron speciation was determined by extractions as well as by mossbauer spectroscopy both methods showed good agreement 6 deviation for total fe speciation the proportion of easy reducible iron oxyhydroxide relative to total fe oxides decreased from the continental slope to the deep sea which is attributed to an increase in crystallinity during transport as well as to a general decrease of iron mobilization the product of iron reoxidation is fe oxyhydroxide which made up less than 5 of total fe in addition to this fraction a proportion of smectite bound iron was found to be redox reactive this fraction made up to 10 of total fe in sediments of the argentine basin and was quantitatively extracted by 1 n hcl the redox reactive fe ii fraction of smectite was almost completely reoxidized within 24 h under air atmosphere and may therefore considerably contribute to iron redox cycling if bioturbation occurs in the case of the slope sediments we found concurrent iron and manganese release to pore water it is not clear whether this is caused by dissimilatory iron and manganese reduction at the same depth or dissimilatory iron reduction alone inducing mn iv reduction by abiotic reaction with released fe2 the argentine basin sediment showed a significant manganese solid phase enrichment above the denitrification depth despite the absence of a distinct pore water gradient of mn this implies a recent termination of manganese mobilization and thus a non steady state situation with respect to sedimentation or to organic carbon burial rate

the effect of the oceans on the terrestrial crater size frequency distribution insight from numerical modeling on earth oceanic impacts are twice as likely to occur as continental impacts yet the effect of the oceans has not been previously considered when estimating the terrestrial crater size frequency distribution despite recent progress in understanding the qualitative and quantitative effect of a water layer on the impact process through novel laboratory experiments detailed numerical modeling and interpretation of geological and geophysical data no definitive relationship between impactor properties water depth and final crater diameter exists in this paper we determine the relationship between final and transient crater diameter and the ratio of water depth to impactor diameter using the results of numerical impact models this relationship applies for normal incidence impacts of stoney asteroids into water covered crystalline oceanic crust at a velocity of 15 km s 1 we use these relationships to construct the first estimates of terrestrial crater size frequency distributions over the last 100 million years that take into account the depth area distribution of oceans on earth we find that the oceans reduce the number of craters smaller than 1 km in diameter by about two thirds the number of craters 30 km in diameter by about one third and that for craters larger than 100 km in diameter the oceans have little effect above a diameter of 12 km more craters occur on the ocean floor than on land below this diameter more craters form on land than in the oceans we also estimate that there have been in the region of 150 impact events in the last 100 million years that formed an impact related resurge feature or disturbance on the seafloor instead of a crater

vivianite is a major sink for phosphorus in methanogenic coastal surface sediments abstract studies of authigenic phosphorus p minerals in marine sediments typically focus on authigenic carbonate fluorapatite which is considered to be the major sink for p in marine sediments and can easily be semi quantitatively extracted with the sedex sequential extraction method the role of other potentially important authigenic p phases such as the reduced iron fe phosphate mineral vivianite fe ii 3 po4 8h2o has so far largely been ignored in marine systems this is in part likely due to the fact that the sedex method does not distinguish between vivianite and p associated with fe oxides here we show that vivianite can be quantified in marine sediments by combining the sedex method with microscopic and spectroscopic techniques such as micro x ray fluorescence xrf elemental mapping of resin embedded sediments as well as scanning electron microscope energy dispersive spectroscopy sem eds and powder x ray diffraction xrd we further demonstrate that resin embedding of vertically intact sediment sub cores enables the use of synchrotron based microanalysis x ray absorption near edge structure xanes spectroscopy to differentiate between different p burial phases in aquatic sediments our results reveal that vivianite represents a major burial sink for p below a shallow sulfate methane transition zone in bothnian sea sediments accounting for 40 50 of total p burial we further show that anaerobic oxidation of methane aom drives a sink switching from fe oxide bound p to vivianite by driving the release of both phosphate aom with sulfate and fe oxides and ferrous fe aom with fe oxides to the pore water allowing supersaturation with respect to vivianite to be reached the vivianite in the sediment contains significant amounts of manganese 4 8 wt similar to vivianite obtained from freshwater sediments our results indicate that methane dynamics play a key role in providing conditions that allow for vivianite authigenesis in coastal surface sediments we suggest that vivianite may act as an important burial sink for p in brackish coastal environments worldwide

carboniferous mantle derived felsic intrusion in the chinese altai nw china implications for geodynamic change of the accretionary orogenic belt abstract this paper reports whole rock sr nd and zircon u pb and hf isotopic compositions as well as major and trace element data for the granodiorites and their mafic microgranular enclaves mmes in the tuokesalei area northwestern chinese altai the granodiorites are characterized by peraluminous compositions a cnk 1 01 1 45 with relatively high sio 2 64 4 to 70 9 wt and low mgo 0 75 to 1 97 wt contents these rocks possess low total rare earth element ree contents 67 187 ppm with weakly negative eu anomalies and mantle like whole rock nd sr isotopic compositions e nd t 2 5 to 4 5 87 sr 86 sr i 0 7038 to 0 7048 and zircon e hf t values 5 93 to 12 9 the mmes contain abundant magmatic amphiboles and needle like apatites have low sio 2 51 7 to 54 3 wt and high mgo 2 86 to 3 75 wt and ree contents 162 298 ppm and are characterized by moderately negative eu anomalies eu eu 0 40 0 60 and slightly depleted to enriched lree la nd n 0 74 1 41 and flat hree gd yb n 1 16 1 39 patterns the mmes have whole rock nd sr isotopic compositions e nd t 2 6 to 2 7 87 sr 86 sr i 0 7034 0 7050 similar to those of the host granodiorites their similarities in crystallization age mineral assemblage and isotopic composition document that the mmes may represent an early crystallized phase which was broken into pieces and brought to the shallow level during the host magma ascent and emplacement their precursor magmas were probably derived from an oceanic lithospheric mantle which was characterized by pronounced nd ta depletions mineral compositions of mmes suggest that amphibole was a major early fractional mineral which effectively differentiated the mantle derived basaltic liquids to silica rich melts zircon u pb dating yielded an age of 313 5 ma for the host granodiorites and 313 13 ma for their mmes the geochemical data of this study records an important episode of magmatism in an environment transitional from a subduction related regime to a post tectonic extension results of this study suggest that the accretionary orogenic process did not completely terminate until the late carboniferous to permian in this region

nonuniform cratering of the terrestrial planets article history we estimate the impact flux and cratering rate as a function of latitude on the terrestrial planets using a model distribution of planet crossing asteroids and comets bottke w f morbidelli a jedicke r petit j m levison h f michel p metcalfe t s 2002 icarus 156 399 433 after determining the planetary impact probabilities as a function of the relative encounter velocity and encounter inclination the impact positions are calculated analytically assuming the projectiles follow hyperbolic paths during the encounter phase as the source of projectiles is not isotropic latitudinal variations of the impact flux are predicted the calculated ratio between the pole and equator is 1 05 for mercury 1 00 for venus 0 96 for the earth 0 90 for the moon and 1 14 for mars over its long term obliquity variation history by taking into account the latitudinal dependence of the impact velocity and impact angle and by using a crater scaling law that depends on the vertical component of the impact velocity the latitudinal variations of the cratering rate the number of craters with a given size formed per unit time and unit area is in general enhanced with respect to the equator the polar cratering rate is about 30 larger on mars and 10 on mercury whereas it is 10 less on the earth and 20 less on the moon the cratering rate is found to be uniform on venus the relative global impact fluxes on mercury venus the earth and mars are calculated with respect to the moon and we find values of 1 9 1 8 1 6 and 2 8 respectively our results show that the relative shape of the crater size frequency distribution does not noticeably depend upon latitude for any of the terrestrial bodies in this study nevertheless by neglecting the expected latitudinal variations of the cratering rate systematic errors of 20 30 in the age of planetary surfaces could exist between equatorial and polar regions when using the crater chronology method