the oldest known metriorhynchid crocodylian from the middle jurassic of north eastern italy neptunidraco ammoniticus gen et sp nov abstract metriorhynchidae is a clade of marine adapted crocodilians known from several middle jurassic early cretaceous specimens collected predominantly in south america and europe but poorly known in the northern margin of gondwana the portomaggiore crocodile is the most complete specimen of an italian metriorhynchid to date it consists of a partial skeleton that has been provisionally referred to an unnamed species of late jurassic metriorhynchus or geosaurus the specimen is preserved in the reddish nodular limestone of the rosso ammonitico veronese formation bajocian tithonian new data on microfossil associations constrain the age of the metriorhynchid to the late bajocian earliest bathonian on the basis of cranial synapomorphies the portomaggiore crocodile falls as the closest sister taxon of the late jurassic early cretaceous geosaurines and is referred to neptunidraco ammoniticus gen et sp nov it is unique among middle jurassic metriorhynchids in showing an incipient streamlining of the skull shared with late jurassic and cretaceous taxa since neptunidraco is the oldest known member of metriorhynchidae its phylogenetic position supports the hypothesis that the timing of the initial metriorhynchid and geosaurine diversifications should start in the bajocian

gravity gradiometry resurfaces the red october was heading southwest on gorshkov s railroad his speed was thirteen knots installed in the keel was a highly sensitive device called a gradiometer essentially two large lead weights separated by a space of one hundred yards a laser computer system measured the space between the weights down to a fraction of an angstrom distortion of that distance or lateral movement of the weights indicated variations in the local gravitational field with careful use of gravitometers he could plot the vessel s location to within a hundred meters

high noble metal concentrations in a late pliocene sediment a 2 3 myr old layer in a sediment from the antarctic ocean contains ir and au at levels comparable with those at the cretaceous tertiary boundary a sizable fraction of the noble metals is contained in vesicular millimetre sized poly mineralic grains that closely resemble ablation debris from chondritic meteorites and there is little doubt that the noble metals resulted from the accretion of a large extraterrestrial object no massive extinctions or other evidence of environmental stress seem to be associated with this accretionary event

the impact of salt fingering on the thermohaline circulation under mixed boundary conditions abstract the impact of salt fingers on the thermohaline circulation in a single hemisphere basin under mixed boundary conditions is investigated through scaling analysis and numerical experiments by assuming that the internal density ratio is determined by the surface horizontal density ratio the effect of a double diffusive parameterization on the vertical diffusivity of density k is estimated salt fingers reduce the magnitude of k with the extent of the reduction dependent on the magnitude of the density ratio the reduced diapycnal mixing leads to a diminished thermohaline circulation and modifies the stability criteria for the thermal haline mode transition under mixed boundary conditions quasi equilibrium numerical experiments with the gfdl modular ocean model produce results consistent with the scaling analysis in the reduction of the magnitude of the thermohaline circulation and the change in the critical freshwater forcing required for the existence of the stable thermal mode sensitivity

double diffusion in the mediterranean sea observation and parameterization of salt finger convection 1 for the mediterranean sea a large set of historical conductivity temperature depth casts has been investigated for the occurrence of double diffusive vertical mixing due to salt finger convection all casts were screened in terms of the density ratio r providing probability distributions of r for the upper 1000 dbar range of eight geographical areas vertical profiles of the salt finger driven diffusivities ks and kt of salinity and temperature have been evaluated from the statistics of r and a presumed mixing law and analytic expressions are provided which can be used to parameterize the effects of salt finger mixing in ocean circulation models of the mediterranean in most areas the diffusivity profiles exhibit an absolute maximum below the core of the levantine intermediate water toward the surface the diffusivities decrease rapidly while the downward decay is less pronounced a different behavior is found in the tyrrhenian and adriatic seas where the diffusivities remain high also at greater depth

numerical simulation of the interannual variability of the mediterranean sea upper ocean circulation numerical simulations reveal that variations in wind stress and heat fluxes can induce significant interannual fluctuations in the circulation of the upper layers of the mediterranean from january 1980 to november 1988 the atmosphere shows changes in the structure and magnitude of the surface winds and in the air temperatures which induce modifications in the upper ocean structure and currents the model prediction of the interannual fluctuations of the sicily strait baroclinic westward volume transport is in agreement with observations and the variability is explained as a function of the wind curl forcing in the region the current anomalies persist for many months after a winter atmospheric anomalous disturbance has occurred over the basin the eastern mediterranean basin is the area where the interannual ocean response is most pronounced

an iridium anomaly in the middle lower jurassic of the venetian region northern italy a significant iridium enrichment has been found in a jurassic marine sequence about 180 million years old outcropping in the alps of the venetian region the maximum iridium concentration of 3 2 0 2 ng g 1 occurs in a brown crust 2 3 mm thick containing essentially iron hydroxide this crust characterized by a nearly total absence of detritic components lies on top of upper lias limestones and is overlaid with bajocian bathonian limestones the origin of the ir anomaly is not clearly understood yet but could be explained by either a drop in the sedimentation rate resulting in the concentration within a thin sediment layer of the normally infalling cosmic dust and or by an increase of cosmic material infall following an asteroid or comet impact s

effective diffusivities within oceanic thermohaline staircases a scale for the thickness of layers in regular diffusive type thermohaline staircases derived from dimensional analysis is found to collapse oceanic data combining this scale with laboratory derived double diffusive flux laws we formulate effective diffusivities for salt heat and density the diffusivities depend on the turner number r but are independent of the brunt vaisala frequency for r near 1 the diffusivities for salt and heat are approximately equal 10 4 m2 s 1 they decrease roughly as r 4 and r 2 respectively over the oceanic range 1 r 10

evidence from crater ages for periodic impacts on the earth recent evidence has indicated that the impact of a comet or asteroid may have been responsible for mass extinction at the ends of both the cretaceous1 and the eocene2 4 quantitative analysis by raup and sepkoski5 showed that mass extinctions occur with a 26 myr period similar to the period seen in qualitative pelagic records by fischer and arthur6 to account for the possibility of periodic comet showers davis et al 7 proposed that such showers could be triggered by an unseen solar companion star as it passes through perihelion on a moderately eccentric orbit to test a prediction implicit in this model we examined records of large impact craters on the earth we report here that most of the craters occur in a 28 4 myr cycle within measurement errors this period and its phase are the same as those found in the fossil mass extinctions the probability that such agreement is accidental is 1 in 103

feasibility of using full tensor gradient ftg data for detection of local lateral density contrasts during reservoir monitoring summary three dimensional full tensor gradiometry 3 d ftg acquires ultrasensitive measurements of the earth s vector gravity gradient field departures from simple weakening of the field in the vertical direction are due to subsurface variations in density we have undertaken a numerical examination of the feasibility of using this system for detecting time variations in local lateral density contrasts in subsurface layers during reservoir production monitoring our gravity modelling focuses on the value added by taking account of the horizontal components of gravity gradient in imaging local targets we have studied the sensitivity of these components to model and acquisition parameters iterative regularized inversion algorithms that can explain the behaviour of a hydrocarbon reservoir in terms of time lapse density changes have been described these algorithms are based on the homotopy method that utilizes the solution space topology to stabilize the inversion we have introduced the maximum compactness of anomalous sources along one or more directions this allows the use of a priori information natural to time lapse monitoring about the arbitrary shape and spatial locations of the sources the time evolving density contrast within the local target zone is recovered by means of an iterative procedure the treatment of either correlated or random noise fits naturally into this procedure the final scheme has been illustrated by several examples using synthetic ftg data generated from various density distributions of practical interest we have considered the effect of model and data uncertainties on the inversion solution it appears that the iterative process is effective numerically stable and rapidly convergent in the presence of both random and structural noise results show that it is relatively insensitive to the choice of starting model appropriate applications include direct monitoring of gas oil contact and temperature front expansion during steam injection in heavy oil reservoirs at shallow or moderate depths

extraterrestrial impact events the record in the rocks and the stratigraphic column abstract the known terrestrial impact record is a biased sample of a much larger population of impact events the biases are due to the modifying effects of terrestrial geologic processes coupled with incomplete searches for impact structures and impact related materials terrestrial impact structures have the same basic forms as impact craters on the other planets of the inner solar system but because of post impact modification by terrestrial geologic process are recognised by the occurrence of shock metamorphic effects in some cases siderophile anomalies have been identified in impact lithologies and have been used to estimate the composition of the impacting body similar shock metamorphic effects and a siderophile anomaly in k t boundary materials are indicative of a major impact event which has been correlated with the formation of the chicxulub structure mexico evidence of a small number of other impacts occur in the stratigraphic record most commonly as tektite or microtektite horizons in some cases they are known to be accompanied by geochemical anomalies in other cases a number of ir anomalies have been reported in the stratigraphic record but there is no confirmatory evidence that they are due to impact the majority of known impact events in the stratigraphic record are from relatively recent geologic time logic dictates however that many more impacts must be recorded in terrestrial sediments and model calculations indicate that relatively small impacts d 20 km have the potential to cause atmospheric blow out and thus global dispersion of some of the impact products geochemical detection however of such events may not be easy in some cases because of relatively small absolute signals against the background of the daily infall of cosmic material in addition non chondritic bodies may result in no appreciable geochemical anomaly in view of this any claim to a geochemical signature of impact in the stratigraphic record should be accompanied by a physical search for impact materials although in the case of impacts into oceanic crust this too will be difficult given the k t experience however and the fact that large scale impact on earth is a natural consequence of the character of the solar system the potential of impacts to provide local and global marker horizons can not be ignored similarly the fact that impacts may have the potential to result in short term biologic or climatic excursions can not be dismissed arbitrarily when considering the causes of such phenomena as stable isotope anomalies in the stratigraphic record

trace element patterns at a non marine cretaceous tertiary boundary at the fossil pollen defined cretaceous tertiary boundary in the raton basin of new mexico and colorado an iridium abundance anomaly and excess scandium titanium and chromium are associated with a thin ash or dust fallout bed now kaolinitic clay that was preserved in freshwater coal swamps

a search for iridium in the deccan traps and inter traps it has been suggested that flood basalts in the deccan india might be associated with events at the cretaceous tertiary boundary ktb a search for iridium in 47 samples from lava flows and inter trap sediments in the deccan yields negative results concentrations are not statistically different from zero with a minimum detection level on the order of 0 1 ng g 1 ppb this does not help to constrain the debate on the internal vs external origin of ktb boundary events since for instance ir could have been outgassed from the magma and original concentrations of only 0 05 ng g 1 prior to outgassing are sufficient to account for the worldwide ir excess mass at the ktb

generalized compact gravity inversion extending the compact gravity inversion technique by incorporating a priori information about the maximum compactness of the anomalous sources along several axes provides versatility thus the method may also incorporate information about limits in the axes lengths or greater concentration of mass along one or more directions the judicious combination of different constraints on the anomalous mass distribution allows the introduction of several kinds of a priori information about the arbitrary shape of the sources this method is particularly applicable to constant linear density sources such as mineralizations along faults and intruded sills dikes and laccoliths in a sedimentary basin the correct source density must be known with a maximum uncertainty of 40 percent otherwise the inversion produces thicker bodies for densities smaller than the true value and vice versa because of the limitations of the inverse gravity problem the proposed technique requires an empirical technique to analyze the sensitivity of solutions to uncertainties in the a priori information the proposed technique is based on a finite number of acceptable solutions presumably representative of the ambiguity region by using standard statistical techniques each parameter is assigned a coefficient measuring its uncertainty the known hematite and magnetite ore body shape in the vicinity of iron mountain mo was reproduced quite well using this inversion technique

recent changes in deep water formation and spreading in the eastern mediterranean sea a review abstract observations of the last decade testify that the characteristics of the deep thermohaline circulation in the eastern mediterranean sea have changed thoroughly the source of the most dense waters of the basin has moved from the adriatic to the aegean sea this new source has proved to be very efficient since the estimated formation rate for the period 1989 95 was more than 1 sv about three times more than the dense water formation rate of the adriatic sea these new waters with hydrological characteristics that are warmer and more saline have replaced almost 20 of the older deep waters of the basin and have uplifted the deep isopycnals by about 500 m this major event can be attributed to important meteorological anomalies in the eastern mediterranean and to changes of circulation patterns the extended dry period of 1988 93 and the exceptionally cold winters of 1987 and 1992 93 created favorable conditions for increased formation of dense water in the aegean furthermore changes in the circulation patterns in the intermediate water range levantine intermediate water liw and cretan intermediate water themselves possibly linked to meteorological anomalies appear to have played an important role in the redistribution of salt as a result of an interruption to exchanges between the ionian and levantine basin the salinities in the latter started to rise high salinity waters were diverted into the aegean malanotte rizzoli p manca b b ribera dacala m theocharis a 1998 the eastern mediterranean in the 80s and in the 90s the big transition emerged from the poem bc observational evidence rapport du commission international de la mer medittanee 35 174 175 and the westward transport of liw was reduced an additional effect of the deep water discharge from the aegean and the resulting uplifting of mid depth waters was to lower salinities in the liw layer this effect is most strongly felt in the ionian sea a 3 d primitive equation numerical model for the eastern mediterranean with a 20 km grid size is used to simulate the observed changes and understand the basic mechanisms which caused them under appropriate atmospheric forcing the model successfully reproduces the main characteristics of the transient these results indicate that the observed changes can be at least partially explained as a response of the eastern mediterranean and more specifically of the aegean to atmospheric forcing variability

the distribution and geochemistry of platinum group elements as event markers in the phanerozoic abstract the discovery of the platinum group element pge enrichment in the cretaceous tertiary k t boundary impact layer popularized the use of pges as geochemical event markers and prompted an explosion of interest in these elements whose geochemical behavior is poorly understood instead of concentrating on the more easily analyzed pges e g iridium many studies now include the entire group analytical methods have been improved to the point where small samples 5 g can be analyzed confidently at the sub ppb level allowing the determination of the distribution of pges in the oceans atmosphere and biosphere the presence or absence of a pge anomaly is often used as a criterion for determining whether an event horizon is impact related however to date only the k t boundary yields a distinct pge anomaly with positive interelement correlations and solar interelement ratios terrestrial sources such as volcanic emissions are also being investigated as potential contributors to pge anomalies at other phanerozoic event horizons primary signatures e g interelement ratios characteristic of the pge source may be altered during various high and or low temperature processes for example fractionation during impact melting vaporization condensation remobilization redistribution and diagenesis can obscure original signatures and make determination of the pge source difficult future research might be directed towards establishing the behavior of the pges during such processes

iridium anomaly in the upper devonian of the canning basin western australia a moderate iridium anomaly about 20 times the local background has been found in upper devonian rocks in the canning basin it occurs at or near the frasnian famennian boundary which is known to be associated with a major massextinction event of global extent the anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions its occurrence suggests a causal link with some form of meteoroid impact moreover carbon isotope data indicate that a large reduction in biomass could have occurred at this level however the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium frutexites iridium platinum iron manganese cobalt arsenic antimony and cerium are preferentially concentrated in filaments of this organism with concentrations ranging from two to five times that of the matrix it is possible that frutexites extracted these elements directly from seawater without the need for their derivation from an extraterrestrial source

strangelove ocean before the cambrian explosion the palaeozoic and mesozoic eras were terminated by faunal changes involving mass extinction of the old and explosive evolution of the new fauna but the fossil record shows only a cambrian explosion at the end of the precambrian stanley speculated that the explosion was only possible after the ubiquitous algae community had been largely eliminated1 ecological niches were thus liberated for explosive evolution if the cambrian explosion were preceded by a mass mortality or by a mass extinction such an event should leave a record in the form of geochemical anomalies we have undertaken a search for geochemical anomalies at the precambrian cambrian contact we report here the discovery of a sharp negative carbon isotope shift in the carbonate of a clay immediately above a marker in the precambrian cambrian boundary the china c marker and interpret this signal as evidence of sudden decrease in fertility before the cambrian explosion of invertebrate evolution the discovery suggests that the precambrian cambrian boundary might be defined by an event marker at a palaeontologically correlative horizon

the steepbank crosswell seismic project reservoir definition and evaluation of steamflood technology in alberta tar sands two crosswell seismic surveys were conducted at chevron canada s oil sand lease 49 steepbank located about 60 km northeast of fort mcmurray alberta this is the site of the hasdrive commercial pilot project in which steam is circulated through a horizontal well in order to preheat the tar sand prior to conventional steam injection the objective of the crosswell survey was to define the geology and to monitor the movement of the injected steam as part of the hasdrive evaluation the first survey was conducted 25 september 6 october 1991 just prior to steam injection the second was acquired 7 14 january 1992 after 72 days of continuous steam injection

circulation and hydrography of the levantine basin results of poem coordinated experiments 1985 1986 abstract a brief review of the meteorological setting hydrography and the circulation in the levantine basin of the eastern mediterranean is given the recent high resolution data obtained in poem coordinated experiments of 1985 1986 are then used to optimally estimate the circulation in the basin in two different seasons and to describe the water mass distributions some of the features observed during the experiments support the historical knowledge on the locations of sub basin scale gyres and the general circulation in addition to which some new features are established details of the circulation such as the intensity the multiple scales and the three dimensional structure of the various vortices and the central levantine basin current are displayed extensively a variety of sub basin meso and sub mesoscale vortices occur with highly assymmetric baroclinic vertical structures some eddies split into multiple centres with depth interpreted as indicating possible coalescences a number of long lived eddies were persistent in both surveys the sub surface atlantic water aw is advected by and entrapped within the eddy field the levantine intermediate water liw at intermediate depths is shown to be maintained throughout the year in the northeastern sector of the levantine basin and along the periphery of the rhodes gyre the levantine intermediate water also has a patchy distribution owing to the advection and trapping by the eddy field which it helps to generate through adjustment processes

an iridium anomaly in the ludlow bone bed from the upper silurian england the ludlow bone bed in the upper silurian of the welsh borderland shows an anomalously high concentration of iridium 0 49 ppb compared with background 0 040 ppb ir considering the overall major and trace element pattern and the mineralogy of the bone bed it appears that the bulk of their has precipitated from sea water and is not primarily related to an asteroid impact event a secondary relation of the ir to such an event however cannot be excluded the profound sedimentological similarity skeletal sands and hummocky cross stratification between the ir carrying storm deposit at the cretaceous tertiary boundary at brazos river texas and the lbb and overlying sediments may indicate such a relation

eigenvector analysis of gravity gradient tensor to locate geologic bodies we have developed a new method to locate geologic bodies using the gravity gradient tensor the eigenvectors of the symmetric gravity gradient tensor can be used to estimate the position of the source body as well as its strike direction for a given measurement point the eigenvector corresponding to the maximum eigenvalue points approximately toward the center of mass of the causative body for a collection of measurement points a robust least squares procedure is used to estimate the source point as the point that has the smallest sum of square distances to the lines defined by the eigenvectors and the measurement positions it s assumed that the maximum of the first vertical derivative of the vertical component of gravity vector gzz is approximately located above the center of mass observation points enclosed in a square window centered at the maximum of gzz are used to estimate the source location by increasing the size of the window the number of eigenvectors used in the robust least squares and

the tunguska explosion of 1908 discovery of meteoritic debris near the explosion site and at the south pole submillimeter sized metallic spheres extracted from soil in the tunguska region of central siberia contain noble metals in cosmic proportions the trace element composition and geographical distribution of these spheres suggest that they are from the 30 june 1908 tunguska explosion and not meteoritic ablation products falling continuously on the earth debris from this explosion was also discovered in a south pole ice core this discovery indicates that the tunguska object exploded in the atmosphere with subsequent stratospheric injection and transport of the debris the celestial body that exploded over tunguska weighed more than 7 million tons was more than 0 16 kilometer in diameter and may well have been a stony meteorite this discovery offers a new precision time marker in polar ice strata for the year 1909 the steady state influx of cosmic matter at the south pole is estimated to be 1 8 x 10 8grams per square centimeter per year which corresponds to a global influx of 4 x l05tons per year

abundance variation of iridium and trace elements at the permian triassic boundary at shangsi in china despite recent searches for anomalous iridium ir concentrations at the permian triassic boundary p tr of marine sediments1 3 no ir above a level of 0 5 p p b parts per 109 has been found detailed event stratigraphical research has revealed a comparatively high platinum group enrichment and geochemical anomalies of trace elements in the p tr boundary clay at meishan changxing county zhejiang province china4 we report here an investigation of ir and trace element abundances in the p tr boundary layer of shangsi section guangyuan county sichuan province fig 1 an ir value of 2 p p b detected in the boundary layer supports the occurrence of an extraterrestrial event at the end of the permian

the role of straits and channels in understanding the characteristics of mediterranean circulation abstract straits in the mediterranean sea form an important network from which one can determine the characteristics of the water exchange between all the constituent sub basins this includes the definition of water masses and water transport and their time variability from 1994 all the major straits in the mediterranean sea gibraltar sicily otranto balearic sea straits cretan arc straits and corsica were subject to long term observations as part of various research projects besides adding new elements to the knowledge of internal strait conditions the data sets collected allow us to propose a fairly consistent representation of the mediterranean circulation and budgets in key points within the basin the amplitude of the annual water transport measured at these straits was about 1 sv and it appears to be modulated by a significant low frequency and seasonal variability for the first time a seasonal component was identified at gibraltar thus raising new questions on the actual state of the mediterranean also the very likely existence of a significant interannual component was documented in the corsica channel this component was found to be related to the interannual variability of the north atlantic oscillation the observations in the cretan arc straits have provided a more comprehensive representation of the recent changes in the eastern mediterranean thermohaline cell it is noteworthy that the effects of these changes have been observed both in the otranto and sicily straits and are now affecting the adjacent sea regions the presence of a stream of modified atlantic water in the balearic sea channels indicates that part of the atlantic inflow may be diverted directly into the northern region of the western mediterranean finally data gathered in the sardinia channel indicate that the central mediterranean region plays a critical role in controlling exchanges between the eastern and the western mediterranean while it is emphasized that the tyrrhenian sea area plays a role in strongly modifying some of the water masses that contribute to the large scale basin circulation their mixing creates new water types which modify the currently known pattern and composition of the mediterranean circulation

winter circulation and convection in the antalya basin eastern mediterranean abstract from an oceanographic survey of the antalya basin in february 1997 the following horizontal circulation pattern was found the asia minor current amc was detached from the turkish coast flowing to the southwest the cilician current was present and feeding the amc the amc exhibited meander wavelengths of about 100 km and phase speed circa 10 cm s 1 a train of cyclonic eddies was located at the southern flank of the amc the central part of the antalya basin was occupied by the antalya anticyclone a second anticyclone farther west was identified as the anaximander anticyclone from a comparison with previous surveys it is concluded that the circulation of the antalya basin is controlled by the position of the amc the circulation is cyclonic if the amc flow path is confined to the coast and anticyclonic if the path is distant from the coast in the latter case the anticyclonic flow may force the cilician current to reverse its direction a nearly homogeneous pool of levantine surface water

seismic time lapse surprise at teal south that little neighbor reservoir is leaking why perform time lapse seismic monitoring is it to verify the reservoir model no we should conduct time lapse seismic surveys in order to find out what is incorrect in the reservoir model in a way similar to the production history matching familiar to reservoir engineers as they look for improvements to the model this being the case it is difficult to determine in advance of monitoring just what it is we should be monitoring thus surveys designed specifically to test one feature of a reservoir model may be missing other important features in this paper we present a set of very surprising results from the teal south time lapse multicomponent 4 d 4 c study in eugene island block 354 in the gulf of mexico we will show that time lapse seismic observations have revealed that an undrilled reservoir near a producing reservoir is exhibiting time lapse changes consistent with expansion of a free gas phase and that this implies that oil is being lost through the spill point never to be recovered ev

the extraterrestrial component in marine sediments description and interpretation the influx of extraterrestrial matter to the earth is dominated by two size fractions sub millimeter interplanetary dust and impacting asteroids and comets over geologic time the major contribution of extraterrestrial matter is from the largest impactors interplanetary dust is largely vaporized by atmospheric entry but some surviving material is found in sediments as cosmic spherules impacting asteroids and comets can produce fallout layers either regionally or globally which may contain a significant fraction of meteoritic material as well as shock metamorphosed and shock melted terrestrial material iridium is a sensitive tracer of extraterrestrial matter although high concentrations of ir in marine sediments probably have an extraterrestrial origin they do not necessarily indicate the presence of a major impact event of the thousands of impact horizons which must be resolvable in the sedimentary record to date only seven probable impact horizons have been identified in the entire phanerozoic the marine geochemistry and occurrence of ir is still poorly understood but the thousands of ir analyses performed in the last several years have demonstrated that the global occurrence high ir 10 ng g concentrations in cretaceous tertiary boundary sediments is a truly anomalous phenomenon

a perspective on the evidence for periodic cometary impacts on earth abstract recent suggestions of periodic cometary showers coincident with mass extinctions have prompted a review of the terrestrial cratering record from the point of view of completeness of search and crater retention the record used to argue for statistical periodicities is incomplete and may not be representative estimates of crater ages are of variable accuracy and precision and in some cases different dating methods yield discordant results it is possible to define through time series analysis a number of periodicities of different magnitude 29 m y 21 m y 18 5 m y 13 5 m y and phase depending on which craters are considered the most representative sample of the record arguing against periodic cometary showers is the observation that estimates of the average cratering rate in recent time and observations on apollo bodies are equivalent in addition siderophile element data from impact melt rocks suggest a variely of projectile types were responsible for relatively large terrestrial craters given these observations it is concluded that statements regarding periodic cometary showers are not categorical and require evidence beyond arguments based on time series analyses of an incomplete cratering record

iridium and other platinum group elements as geochemical markers in sedimentary environments iridium anomalies are commonly used as indicators of extraterrestrial processes which caused global extinctions however platinum group element pge anomalies may result from several different processes both syn and postdepositional acting separately or jointly the sources and processes of enrichment involved are extraterrestrial asteroid comet cosmic dust impact ejecta volcanogenic pge rich condensates precipitation from seawater low sedimentation rate anoxic conditions microbial concentration dissolution reprecipitation exhalative hydrothermal leaching transport and precipitation at a redox boundary geochemical characteristics of pge and accompanying elements in sediments can be used to identify the contributions of some specific sources and processes 1 extraterrestrial chondritic ru ir and os ir ratios low 187os 186os ratio high ir compared to pt pd and au 2 volcanogenic high iridium and se as sb and in contents 3 exhalative hydrothermal high au pd and pt relatively low ir contents 4 precipitation from seawater high pt and low pd contents also high cd and ag contents under anoxic conditions and 5 reduction from intermediate and low temperature solutions high pge and cu or mo u ni contents

accretion rate of extraterrestrial matter iridium deposited 33 to 67 million years ago iridium measured in 149 samples of a continuous 9 meter section of pacific abyssal clay covering the time span 33 to 67 million years ago shows a well defined peak only at the cretaceous tertiary boundary in the rest of the section iridium ranges from a minimum concentration near 0 35 nanograms per gram in the paleocene to a maximum near 1 7 in the eocene between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years correction for terrestrial iridium leads to an extraterrestrial flux of9 3 nanograms of iridium per square centimeter per million years and an estimated annual global influx of 78 billion grams of chondritic matter consistent with recent estimates of the influx of dust meteorites and crater producing bodies with masses ranging from 10 13 to 1018 grams combining the recent flux of objects ranging in mass from 106 to 107 grams with the flux of 1014 to 1015 gram objects indicates that the number of objects is equal to 0 54 divided by the radius in kilometers to the 2 1 power periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years the predicted iridium maxima more than 30 times background are not present in the intervals associated with the cretaceous tertiary boundary or the tektiteproducing late eocene events

the general circulation of the mediterranean sea from a 100 year simulation results from a 120 year simulation of the mediterranean sea with a 1 4 19 level eddy permitting cox model with monthly varying wind and buoyancy forcing are presented mean and seasonal currents are examined along with water distributions on isopycnals detailed equilibrated thermohaline water pathways broadly consistent with observed hydrographic data are determined the most important factor permitting a long model run is low vertical mixing which maintains levantine intermediate water liw above the level of the sicily sill so that it can reach the western basin a new feature is tyrrhenian deep water which is formed after 40 years by deep water transfer from the balearic basin south of sardinia a water mass analysis reveals the modification of water properties in t s space through the mixing processes that take place at straits and through the action of mesoscale eddies the surface heat and freshwater fluxes derived a posteriori are in very good agreement with observations showing that basin average fluxes are closely tied to the generation of a realistic thermohaline circulation a complete analysis of surface and strait fluxes demonstrates that equilibrium conditions have been reached a study of the heat budget in the intermediate water layer at 280 m depth reveals that the dominant diabatic terms are at localized convection sites suggesting that at this depth the thermocline is under advective rather than diffusive control the eddy and mean flow heat transports balance to a large degree away from these sites with a negligible role for diffusion

three dimensional gravity inversion using simulated annealing constraints on the diapiric roots of allochthonous salt structures in the northern continental slope of the gulf of mexico large oil and gas reservoirs are often found beneath sheetlike allochthonous salt structures that are laterally extensive some of these salt structures retain their diapiric feeders or roots beneath them these hidden roots are difficult to image seismically in this study we develop a method to locate and constrain the geometry of such roots through 3 d inverse modeling of the gravity anomalies observed over the salt structures this inversion method utilizes a priori information such as the upper surface topography of the salt which can be delineated by a limited coverage of 2 d seismic data the sediment compaction curve in the region and the continuity of the salt body the inversion computation is based on the simulated annealing sa global optimization algorithm the sa based gravity inversion has some advantages over the approach based on damped least squares inversion it is computationally efficient can solve underdetermined inverse problems can more easily implement complex a priori information and does not introduce smoothing effects in the final density structure model we test this inversion method using synthetic gravity data for a type of salt geometry that is common among the allochthonous salt structures in the gulf of mexico and show that it is highly effective in constraining the diapiric root we also show that carrying out multiple inversion runs helps reduce the uncertainty in the final density model

unmelted meteoritic debris in the late pliocene iridium anomaly evidence for the ocean impact of a nonchondritic asteroid ir bearing particles have been recovered from 2 piston cores in the antarctic basin in the southeastern pacific in core e13 3 the particles closely correspond to the late pliocene ir anomaly and have a fluence of 100 mg cm 2 in core e13 4 120 km to the southwest the particle fluence is 4 mg cm 2 particles with diameters from 0 5 to 4 mm contain at least 35 of the ir in this horizon three types of particles have been identified 1 vesicular 2 basaltic and 3 metal the vesicular particles appear to be shock melted debris derived from the oceanic impact of a howarditic asteroid containing a minor metal component these particles have recrystallized from a melt and impact into the ocean has resulted in the incorporation of na k cl and radiogenic sr from the ocean water target the basaltic clasts appear to be unmelted fragments of the original asteroid which may have separated from the main body prior to impact combined vesicular and basaltic particles are believed to have formed by collisions in the debris cloud estimates of the diameter of the projectile range from 100 to 500 m by many orders of magnitude this is the most massive achondrite sampled by a single meteorite fall

fundamentals of seismic rock physics during the past 50 years or so tremendous progress has been made in studying physical properties of rocks and minerals in relation to seismic exploration and earthquake seismology during this period many theories have been developed and many experiments have been carried out some of these theories and experimental results have played important roles in advancing earth sciences and exploration technologies this tutorial paper attempts to summarize some of these results

modeling the dispersal of levantine intermediate water and its role in mediterranean deep water formation this paper demonstrates the importance of levantine intermediate water liw in the deep water formation process in the mediterranean using the modular ocean general circulation model at 0 25 resolution 19 vertical levels over the entire mediterranean with an open gibraltar strait liw formation is strongly prescribed in the rhodes gyre region by haney 1971 relaxation while in other regions surface salinity relaxation is much reduced by applying the mixed thermohaline surface boundary conditions isopycnal diagnostics are used to trace water mass movements and volume fluxes are monitored at straits low viscosity and diffusion are used to permit baroclinic eddies to play a role in water mass dispersal the overall water budget is measured by an average flux at gibraltar of 0 8 sv of which 0 7 sv is exchanged with the eastern basin at sicily liw density around 28 95 spreads rapidly after formation throughout the entire levantine due to baroclinic eddies toward the west liw accumulates in the northern and central ionian with some entering the adriatic through otranto and some mixing southward in eddies and exiting to the western mediterranean through sicily liw is converted to deep water in the south adriatic at an average rate of 0 4 sv water exchange through the otranto strait appears to be buoyancy driven with a strong bias to the end of winter march april while at sicily the exchange has a strong symmetric seasonal cycle with maximum transport of 1 1 sv in december indicating the effects of wind driving liw pathways in the west are complex and variable in the tyrrhenian intermediate water becomes uniform on isopycnal surfaces due to eddy stirring west of sardinia two liw boundary currents are formed in the balearic basin one flows northward up the west coast of sardinia and corsica and one westward along the northern african coast the northward current is consistent with observations while the westward current is intermittent for the first 10 years often breaking up into eddies which enter the basin interior some observations of high salinity waters near the african coast may support this interpretation liw retains a subsurface salinity maximum of 38 4 38 5 practical salinity units psu when reaching the northwestern mediterranean contrasting with surface waters fresher than 38 0 psu west mediterranean deep water is formed below 1500 m depth with climatological characteristics when it is mixed and cooled during winter convection in lions gyre

currents water masses eddies and jets in the mediterranean levantine basin abstract hydrographic measurements in the southeastern levantine basin are analyzed and the climatological water masses of the region and their seasonal variations are identified we observe the formation of the salty and warm levantine surface water layer lsw we characterize the subsurface atlantic water layer aw and we describe the properties of the thermocline waters called levantine intermediate waters liw the baroclinic dynamical modes are computed for the climatological stratification parameters the empirical orthogonal function eof analysis of the vertical shear profiles shows that considerable energy is contained in the second eof at the thermocline and deep levels maps of the baroclinic streamfunction field referred to 700 meters are displayed 16 instantaneous flow field realizations show an intense mesoscale eddy file never revealed before in the region the space scales of the eddies are about 100 km and a smaller scale 60 70 km variability is also evident the eddies are pres