trace fossils and substrates of the terminal proterozoic cambrian transition implications for the record of early bilaterians and sediment mixing the trace fossil record is important in determining the timing of the appearance of bilaterian animals a conservative estimate puts this time at 555 million years ago the preservational potential of traces made close to the sediment water interface is crucial to detecting early benthic activity our studies on earliest cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the phanerozoic which can be attributed both directly and indirectly to the low levels of sediment mixing the low levels of sediment mixing meant that thin event beds were preserved the shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment water interface increasing the likelihood of recording shallow tier trace fossils in muddy sediments overall trace fossils can provide a sound record of the onset of bilaterian benthic activity

a concentrated outbreak of tornadoes downbursts and microbursts and implications regarding vortex classification abstract a remarkable case of severe weather occurred near springfield illinois on 6 august 1977 aerial and ground surveys revealed that 17 cyclonic vortices an anticyclonic vortex 10 downbursts and 19 microbursts occurred in a limited 20 km 40 km area associated with a bow shaped radar echo about half of the vortices appeared to have occurred along a gust front some of the other appear to have occurred within the circulation of a mesocyclone accompanying the bow echo but these vortices seem to have developed specifically in response to localized boundary layer vorticity generation associated with horizontal and vertical wind shears on the periphery of microbursts some of these vortices and other destructive vortices in the literature do not qualify as tornadoes as defined in the glossary of meteorology a moon pragmatic definition of a tornado is suggested

observations of a colorado tornado part ii combined photogrammetric and doppler radar analysis abstract an integrated analysis of photographic and doppler radar observations of a tornadic storm during the convection initiation and downburst experiment cinde is presented high resolution single doppler radar measurements are combined with cloud photogrammetry to reveal the detailed structural relationship of the hook echo and the misocyclone with visual features of the tornado vertical cross sections of dual doppler winds in the plane of the photographs were also examined to determine the complex motions within and surrounding the vortex the tornado was found to be within a weak echo hole of the hook echo the hole progressed upward above cloud base as the tornado matured an annulus of higher reflectivity that formed a book echo is hypothesized to have been composed of sparse but large raindrops the airflow fields suggest that vortex breakdown and axial downdrafts were present near the ground at early stages shortly after the tornado became visible later axial upward flow dominated at all le

late precambrian pennatulids sea pens from the eastern yangtze gorge china paracharnia gen nov abstract pennatulids preserved as membranous carbonised compressions from the shibantan member dengying formation sinian system in the eastern yangtze gorge hubei province represent the first genuine discovery of the late precambrian ediacara type metazoan remains in china the specimens were initially referred to charnia ford 1958 and described as c dengyingensis by ding and chen in 1981 a re examination of the type material and comparing it with other late precambrian frond like forms particularly those from the ediacara assemblage of south australia and from the charnwood assemblage of england indicate that this chinese species represents a distinct genus paracharnia gen nov its characteristic polyp leaves individually free standing on distinct leaf stalks along the lateral sides of the median stem resemble those of many living sea pens represented by pennatula and contribute a significant feature to the precambrian charniidae glaessner 1979 its close association with abundant macroscopic algal remains of vendotaenia sp and its proximity to the overlying basal cambrian small shelly fossil assemblages in the same succession emphasise its palaeontological and biostratigraphic significance

bank accretion and the development of vegetated depositional surfaces along modified alluvial channels this paper describes the recovery of stable bank form and development of vegetated depositional surfaces along the banks of channelized west tennessee streams most perennial streams in west tennessee were straightened and dredged since the turn of the century patterns of fluvial ecological responses to channelization have previously been described by a six stage model dendrogeomorphic tree ring techniques allowed the determination of location timing amount and rate of bank sediment deposition channel cross sections and ecological analyses made at 101 locations along 12 streams encompassing bends and straight reaches show that channel and bank processes initially react vertically to channelization through downcutting a depositional surface forms on banks once bed degradation and heightened bank mass wasting processes have eased or slowed the formation of this depositional surface marks the beginning of bank recovery from channelization dominating lateral processes characteristic of stable or natural channels return during the formation and expansion of the depositional surface suggesting a relation with thalweg deflection point bar development and meanderloop extension characteristic woody riparian vegetation begins to grow as this depositional surface develops and becomes part of the process and form of restabilizing banks the depositional surface initially forms low on the bank and tends to maintain a slope of about 24 mean accretion rates ranges from 5 9 cm yr on inside bends to 0 cm yr on most outside bends straight reaches have a mean accretion rate of 4 2 cm yr the relatively stable convex upward depositional surface expands and ultimately attaches to the flood plain the time required for the recovery process to reach equilibrium averaged about 50 years indicative pioneer speccies of woody riparian vegetation include black willow river birch silver maple and boxelder stem densities generally decrease with time after and initial flush of about 160 stems per 100 m2 together bank accretion and vegetative regrowth appear to be the most important environmental processes involved in channel bank recovery from channelization or rejuvenation

early cambrian ocean anoxia in south china the cause of the most marked changes in the evolution of life which define the first order stratigraphic boundary between the precambrian and the phanerozoic eon remains enigmatic and a highly topical subject of debate a global ocean anoxic event triggered by large scale hydrogen sulphide h 2 s release to surface waters has been suggested by wille et al on the basis of two data sets from south china and oman to explain the fundamental biological changes across the precambrian cambrian pc c boundary here we report a new precise shrimp u pb zircon age of 532 3 0 7 million years myr ago fig 1 for a volcanic ash bed in the critical unit that reflects the ocean anoxic event the lowermost black shale sequence of the niutitang formation in the guizhou province south china this age is significantly younger than the precise pc c boundary age of 542 0 0 3 myr ago approximately 10 myr younger than the extinction of the ediacaran fauna and thus challenging the view of a major ocean anoxic event having been responsible for the major changes in the direction of evolution at the pc c boundary

using magnetic resonance imaging and petrographic techniques to understand the textural attributes and porosity distribution in macaronichnus burrowed sandstone abstract magnetic resonance images are paired with petrographic data to evaluate the textural characteristics of rocks dominated by macaronichnus segregatis a trace fossil that is commonly associated with rocks deposited in shallow marginal marine sedimentary environments mri techniques used revealed the three dimensional geometry of the trace fossil burrows are typically horizontal and in plan view range between straight sinuous meandering and spiral geometries changes in burrow morphology may be related to population density and patchy resource distribution the pairing of mri and petrographic data helped map the distribution of porosity in the burrowed rock because mri images represent complex composites of nuclear spin density and mr relaxation times each of which is related to pore size stronger mr signals must be calibrated to known porous zones by integrating petrographic data with mr data the complex distribution of porosity and its relationship to the matrix show that this fabric represents a dual porosity permeability system and may affect the resource reservoir or aquifer quality of similarly burrowed sedimentary rocks future research should elaborate upon the porosity permeability model for this and similar fabrics

assemblage palaeoecology of the ediacara biota the unabridged edition fossils of the ediacara biota offer our earliest insight into diverse macroscopic life on this planet in particular given the diversity and range of exquisite soft bodied preservation the potential for unraveling aspects of the paleobiology and paleoecology is great clearly however there can be a taphonomic overprint that dictates how complete the assemblage is new diversity data including richness and evenness from south australia is compared to previously published data from newfoundland and the white sea and is within the range of values for both modern and phanerozoic assemblages however missing from our current understanding of ediacaran ecosystems is a full description and interpretation of the many problematica structures and organic surfaces

climate and ephemeral stream processes twentieth century geomorphology and alluvial stratigraphy of the little colorado river arizona during the first 40 years of the twentieth century erosion was the dominant geomorphic process affecting the morphology of the little colorado river channel the discharge regimen was one of frequent large floods and high annual discharge that created a wide sandy channel free of vegetation in the 1940s and early 1950s average annual precipitation declined reducing annual discharge to about 57 of that of the preceding period as well as reducing the frequency of large floods the channel adjusted to the new hydrologic regimen by reducing its width parts of the channel were frequently dry and riparian vegetation primarily nonnative salt cedar became established on the higher channel surfaces precipitation and discharge thereafter increased and aggradation by overbank deposition was the primary geomorphic process as indicated by accretion of 2 to 5 m of flood plain alluvium between 1952 and 1978 events of 1980 however suggest that the flood plain has ceased to accrete although climate has not fluctuated the flood plain has probably reached a critical height above the channel beyond which further accretion is unlikely under the existing discharge regimen the recent history of the little colorado broadly suggests that flood plain development was initiated by climatically induced hydrologic fluctuations flood plain deposits in the stratigraphic column of such ephemeral streams may record repeated adjustments to altered hydrologic conditions

the lagrange tornado during vortex2 part i photogrammetric analysis of the tornado combined with single doppler radar data abstractthis study presents a single doppler radar analysis combined with cloud photography of the lagrange wyoming tornado on 5 june 2009 in an attempt to relate the radar observed hook echo weak echo hole weh and rotational couplet to the visual characteristics of the tornado the tornado was rated ef2 the circulation at low levels went through two intensification periods based on azimuthal shear measurements the first intensification was followed by the appearance of a brief funnel cloud the second intensification was coincident with the appearance of a second funnel cloud that remained in contact with the ground until the tornado dissipated a deep weh rapidly formed within the hook echo after damaging wind was identified at the ground and before the appearance of a funnel cloud the echo pattern through the hook echo on 5 june undergoes a dramatic evolution initially the minimum radar reflectivities are near the surface 15 dbz and the weh does not suggest a tapered structure near the gro

rapid scan polarimetric doppler radar observations of tornadogenesis and tornado dissipation in a tornadic supercell the el reno oklahoma storm of 24 may 2011 abstracton 24 may 2011 a mobile rapid scan x band polarimetric doppler radar raxpol collected data on a supercell as it produced two tornadoes near el reno oklahoma the first tornado rated an ef 3 was documented from intensification to decay and the genesis and intensification of a second tornado that was rated an ef 5 was subsequently also documented the objective of this study is to examine the spatiotemporal evolution of the rotation associated with the tornadoes i as the first tornado weakened to subtornadic intensity and ii as the second tornado formed and intensified it is found that weakening did not occur monotonically the transition from tornadic to subtornadic intensity over the depth of the radar volume 4 km occurred in less than 30 s but this behavior is contingent upon the threshold for doppler shear used to define the tornado similarly the onset of a tornadic strength doppler velocity couplet occurred within a 30 s period over all elevations additionally the evolutio

observed and numerically simulated structure of a mature supercell thunderstorm abstract through the interactive use of doppler radar analyses and a three dimensional numerical storm simulation the detailed structure of a supercell tornadic storm is analyzed this storm named the del city storm occurred in central oklahoma on 20 may 1977 the storm exhibits certain important features which are essential to maintaining its longevity and which promote the storm s transition to its tornadic phase these features are strongly influenced by the rotational character of the storm separates the precipitation from the updraft and which orients the resulting downdrafts to which reinforce low level convergence along the gust front and sustain the storm analyses of air parcel and rain trajectories within the storm provide a detailed visualization of this internal structure these trajectories reveal that air parcels rising through the cyclonically rotating updraft actually turn anticyclonically with height owing to the influence of the storm relative environmental wind field downdraft traj

use of radiometric cs 137 pb 210 geomorphic and stratigraphic techniques to date recent oxbow sediments in the rio puerco drainage grants uranium region new mexico in the absence of historic geochemical baseline data for the grants uranium region environmental changes resulting from uranium mine mill activities can be determined only by indirect methods a methodology for determining the age of recent sediments in streams draining the region has been established based on combined geomorphic stratigraphic and radiometric dating techniques because clayrich sediments retain possible radionuclides and heavy metals derived from mineralization and mined sources sample sites which contain fine grained deposits that both predate and postdate mine mill activity were located in abandoned channel segments oxbows of major streams draining the eastern grants uranium region aerial photographs and derivative maps taken between 1935 and 1971 provided the historical and geomorphic documentation of approximate dates of oxbow formation and ages of alluvial fills in the abandoned channel segments pits were dug at these oxbow sites to determine stratigraphy and composition of the deposits samples collected from pit walls and auger holes below the pits were subjected to radiometric analysis by gamma ray spectrometry for the artificial radionuclide cs 137 and the natural radionuclide pb 210 as well as other u 238 and th 232 daughters because of the dynamic nature of the system absolute dating with cs 137 was not possible but samples could be dated as either pre or post 1950 the 1950 date is important because it marked the beginning of the uranium exploitation in the region the pb 210 dating was not possible because background pb 210 was very high relative to fallout pb 210

accuracy of post bomb 137cs and 14c in dating fluvial deposits abstract the accuracy and precision of 137cs and 14c for dating post 1950 alluvial deposits were evaluated for deposits from known floods on two rivers in arizona the presence of 137cs reliably indicates that deposition occurred after intensive above ground nuclear testing was initiated around 1950 there was a positive correlation between the measured level of 137cs activity and the clay content of the sediments although 137cs was detected even in sandy flood sediments with low clay content 137cs is a valuable dating tool in arid environments where organic materials for 14c or tree ring dating are scarce and observational records are limited the 14c activity measured in different types of fine organic detritus yielded dates within 1 to 8 yr of a 1980 flood deposit and the accuracy was species dependent however undifferentiated mixtures of fine organic materials from several post bomb deposits of various ages repeatedly yielded dates between 1958 and 1962 and detrital charcoal yielded a date range of 1676 1939 in semiarid environments the residence time of most types of organic debris precludes accurate annual resolution of post bomb 14c dates

dating of geomorphological processes using dendrogeomorphological methods abstract the object of this study was to develop methods for investigating recent geomorphodynamics of aggradation processes by dendrogeomorphological analyses of the stems and the roots of partially buried spruces every accumulation process causes growth reactions in the affected trees which is reflected in the growth rings analysis of the growth rings enables us to determine the time and the magnitude of burial yet the critical burial depth which when exceeded causes the death of the tree is 1 6 to 1 9 m the burial of a spruce stem causes the sprouting of adventitious root horizons a single event may cause simultaneous sprouting of up to six root horizons of which the uppermost horizon usually is the strongest one the dating of the onset of sprouting of the oldest adventitious roots among each root system enables one to establish absolute dates of debris flow events with an accuracy 0 to 3 years yet this method is very time consuming the simplest dating method proved to be the determination of burial events by the onset of a pronounced reduction of increment in the stem wood of the affected trees with only one core to be taken of each tree this method therefore allows a spatiotemporal reconstruction of aggradation processes the information recorded on the stem cross section of stem by the growth ring pattern lessens with increasing burial depth though

vortex lines within low level mesocyclones obtained from pseudo dual doppler radar observations abstract vortex lines passing through the low level mesocyclone regions of six supercell thunderstorms three nontornadic three tornadic are computed from pseudo dual doppler airborne radar observations obtained during the verification of the origins of rotation in tornadoes experiment vortex in every case at least some of the vortex lines emanating from the low level mesocyclones form arches that is they extend vertically from the cyclonic vorticity maximum then turn horizontally usually toward the south or southwest and descend into a broad region of anticyclonic vertical vorticity this region of anticyclonic vorticity is the same one that has been observed almost invariably to accompany the cyclonic vorticity maximum associated with the low level mesocyclone the vorticity couplet straddles the hook echo of the supercell thunderstorm the arching of the vortex lines and the orientation of the vorticity vector along the vortex line arches compared to the orientation of the ambient barotrop

ichnofabric analysis of the shallow marine nukhul formation miocene suez rift egypt implications for depositional processes and sequence stratigraphic evolution abstract the shallow marine early syn rift miocene nukhul formation suez rift egypt is highly bioturbated and allows relationships between changes in trace fossils and ichnofabrics within a shallow marine depositional system to be documented and placed in a high resolution sequence stratigraphic framework seven ichnofabrics are present in a succession of interfingering calcareous mudstones and calcarenites forming coarsening up units of up to 30 m thick bounded by marine flooding surfaces the units grade upwards from a basal mudstone package with bed parallel concretions and a planolites chondrites ichnofabric offshore through a coarsening up succession of alternating calcarenites and mudstones with thalassinoides mottled sediment offshore transition ophiomorpha irregulaire lower shoreface ophiomorpha nodosa thalassinoides lower middle shoreface thalassinoides taenidium middle shoreface and o nodosa upper shoreface ichnofabrics gastrochaenolites hardground ichnofabric is separate as it is not genetically related to the other ichnofabrics ichnofabric development is primarily controlled by depositional environment e g bottom water oxygenation sediment type food abundance and energy level which control substrate colonisation sedimentation rate marine flooding surfaces are generally well cemented and marked by distinctive epifaunal and infaunal colonisation and can be traced out from proximal to distal settings over distances of 5 km the epifaunal colonisation in proximal settings consists of abundant oysters and corals with the substrate below marine flooding surfaces containing abundant thalassinoides and ophiomorpha isp abundance and diversity of epifauna and trace fossils and burrow size decreases distally into the basin in the most distal settings epifaunal colonisation is absent and only planolites and chondrites colonise the basinal mudstone marine flooding surfaces in the most distal settings are poorly cemented but are marked by carbonate concretions 10 15 cm below the surface

a new modular palaeopascichnid fossil curviacus ediacaranus new genus and species from the ediacaran dengying formation in the yangtze gorges area of south china non biomineralizing ediacaran macrofossils are rare in carbonate facies but they offer valuable information about their three dimensional internal anatomy and can broaden our view about their taphonomy and palaeoecology in this study we report a new ediacaran fossil curviacus ediacaranus new genus and species from bituminous limestone of the shibantan member of the dengying formation in the yangtze gorges area of south china curviacus is reconstructed as a benthic modular organism consisting of serially arranged and crescent shaped chambers the chambers are confined by chamber walls that are replicated by calcispars and are filled by micritic sediments such modular body construction is broadly similar to the co occurring yangtziramulus zhangii and other ediacaran modular fossils such as palaeopascichnus the preservation style of curviacus is similar to yangtziramulus although the phylogenetic affinities of both genera remain unresolved the new fossil adds to the diversity of ediacaran modular organisms

a study of the tornadic region within a supercell thunderstorm abstract the transition of a supercell thunderstorm into its tornadic phase is investigated through high resolution numerical cloud model simulations initiated within the interior portion of a previously simulated mature supercell storm with the enhanced grid resolution the low level cyclonic vorticity increases dramatically and the gust front rapidly occludes as small scale downdrafts develop in the vicinity of the low level center of circulation as the occlusion progresses a ring of high vorticity air surrounds the circulation center and could be conducive to multiple vortex tornado formation numerous features of the simulated transition bear resemblance to those observed in tornadic storms in the model simulation the large low level vorticity is generated through the tilting and intense stretching of air from the inflow side of the storm this vertical vorticity is derived from the horizontal vorticity of the environmental shear and also from horizontal vorticity generated solenoidally as low l

finescale single and dual doppler analysis of tornado intensification maintenance and dissipation in the orleans nebraska supercell abstract finescale single and dual doppler observations are used to diagnose the three dimensional structure of the wind field surrounding a tornado that occurred near the town of orleans nebraska on 22 may 2004 the evolution of the vorticity and divergence fields and other structures near the tornado are documented in the lowest kilometer changes in tornado intensity are compared to the position of the tornado relative to primary and secondary gust fronts circulation on scales of a few kilometers surrounding the tornado remains relatively constant during the analysis period which spans the intensifying and mature periods of the tornado s life cycle stretching of vertical vorticity and tilting of horizontal vorticity are diagnosed but the latter is near or below the threshold of detectability in this analysis during the observation period in the analyzed domain low level circulation within 500 m of the tornado increased several minutes before vortex relative and ground relative near surface wind

simulation and analysis of tornado development and decay within a three dimensional supercell thunderstorm abstract a three dimensional numerical simulation using a two way interactive nested grid is to study tornado genesis within a supercell during a 40 minute period two tornadoes grow and decay within the storm s mesocyclone the tornadoes have life spans of approximately 10 minutes maximum ground relative surface wind speeds exceed 60 m s 1 during both tornadoes and horizontal pressure gradients reach 18 hpa km 1 during the second tornado comparison of the simulated storm evolution with doppler and field observations of supercells and tornadoes shows many similar features vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm tornadogenesis is initiated when mesocyclone rotation increase above cloud base the increased rotation generates lower pressure in the mesocyclone increasing the upward pressure gradient forces the upwar

the san angelo texas supercell of 31 may 1995 visual observations and tornadogenesis abstract airborne radar analysis combined with cloud photogrammetry of a supercell that formed on 31 may 1995 near san angelo texas is presented the superposition of the doppler wind syntheses with the cloud pictures provides a unique view of the relationship between the low level mesocyclone and the developing lowered cloud base the mesocyclone accompanying the storm went through two periods of intensification the end of the first period of intensification did not lead to the formation of a tornado an occlusion downdraft formed but was primarily driven by negative buoyancy rather than a downward directed perturbation pressure gradient recent studies have suggested that cold downdrafts inhibit the formation of tornadoes the present study supports this hypothesis and provides the first quantitative view of the vertical structure of this downdraft the mesocyclone evolved through a second period of intensification that produced a weak tornado that was visually apparent from the aircraft no occlusio

the kellerville tornado during vortex damage survey and doppler radar analyses abstract a detailed aerial and ground survey of a long track 50 km f5 tornado is presented the survey revealed that the tornado exhibited unusual nonlinear movements at two different locations one portion of the track was associated with a pronounced sinusoidal pattern while another location was characterized by a cusplike pattern for the first time high resolution dual doppler wind measurements can be used to evaluate mechanisms for such deviations from a linear tornado path the analyses of data collected with electra doppler radar eldora suggest that these departures are trochoidal marks produced as the tornado was revolving within the larger scale mesocyclone retrieved perturbation pressures indicate that the mesocyclone departed significantly from a cyclostrophically balanced state during these deviations the maximum vorticity associated with the mesocyclone at low levels was shown to be an unreliable indicator of the tornado s intensity vertical cross sections of wind vertical vorticity

kinematic and polarimetric radar observations of the 10 may 2010 moore choctaw oklahoma tornadic debris signature abstracttornadoes are capable of lofting large pieces of debris that present irregular shapes near random orientations and a wide range of dielectric constants to polarimetric radars the unique polarimetric signature associated with lofted debris is called the tornadic debris signature tds while ties between tds characteristics and tornado and storm scale kinematic processes have been speculated upon or investigated using photogrammetry and single doppler analyses little work has been done to document the three dimensional wind field associated with the tds data collected by the oklahoma city oklahoma ktlx and norman oklahoma koun wsr 88d s band radars as well as the university of oklahoma s ou advanced radar research center s polarimetric radar for innovations in meteorology and engineering ou prime c band radar are used to construct single and dual doppler analyses of a tornadic supercell that produced an ef4 tornado near the towns of moore and choctaw oklahoma on 10 may 2010 thi

observations on the origins of rotation the newcastle tornado during vortex 94 abstract observations of a strong f3 tornado near newcastle texas on 29 may 1994 during the verification of the origins of rotation in tornadoes experiment vortex are presented the visual characteristics and intensity of the tornado were revealed by a photogrammmetric analysis of pictures taken by chase teams and by a detailed damage survey the tornado developed from a low level shear feature along the flanking line of a supercell vortex stretching of this feature to tornadic intensity occurred under the influence of an intense updraft from a rapidly growing storm along the flanking line no apparent midlevel mesocyclone accompanied this tornado in contrast the supercell was characterized by a well defined mid and low level mesocyclone however no tornado was observed by ground chase teams the implications of these findings on the current understanding of tornadogenesis are discussed

orbisiana linearis from the early ediacaran lantian formation of south china and its taphonomic and ecological implications abstract orbisiana is an ediacaran fossil characterized by linearly arranged circular or cylindrical units the genus was first described from the late ediacaran redkino series in the moscow syncline of russia where it is preserved as pyritic molds in siltstone here we report orbisiana linearis originally described as seirisphaera lineare from black shale of the early ediacaran lantian formation in south china unlike the type species o simplex o linearis is preserved as carbonaceous compressions on the bedding surface with rare specimens preserved three dimensionally allowing morphological characterization using x ray microct techniques morphological paleoecological and taphonomic analysis suggests that o linearis was a chain like modular organism characterized by uniserially arranged millimeter sized and tangentially contacting circular or cylindrical units ontogenetic growth of o linearis was likely achieved through both accretionary addition of new modules and inflational expansion of existing units and it appears that inflational growth was determinate although its phylogenetic affinity remains uncertain o linearis was likely a procumbent epibenthic or semi endobenthic organism that lived on the seawater sediment interface or partly buried in sediments o linearis in the lantian formation was preserved in situ although some specimens are disarticulated and many are tectonically deformed orbisiana joins a growing list of ediacaran genera that have a remarkable range of stratigraphic environmental ecological and taphonomic distribution

single and dual doppler analysis of a tornadic vortex and surrounding storm scale flow in the crowell texas supercell of 30 april 2000 abstract fine resolution single and dual doppler data were collected in the tornadic region of a supercell storm intercepted by two doppler on wheels radars on 30 april 2000 near crowell texas eleven dual doppler analyses characterize the 2d and 3d near surface wind fields associated with a tornado during a 13 min period an interesting evolution of the low level rotation is observed initially concentric tornado 500 m wide and tornado cyclone 2 km wide radar velocity couplets make a transition into a solitary intermediate sized 750 m wide circulation that widens and makes a further transition into a two celled multiple vortex structure with an asymmetric distribution of vertical vorticity the asymmetry and eventual disruption of the multiple vortex structure may have been partially controlled by locally strong outflow winds that affect the convergence fields in its vicinity a smaller 500 m wide tornado embedded in a broad area of rotation is subsequently observed the dual doppler wi

the origin of decoupled carbonate and organic carbon isotope signatures in the early cambrian ca 542 520 ma yangtze platform abstract the early cambrian ca 542 520 ma strata in south china record two prominent negative carbonate carbon isotope 13ccarb excursions of early nemakit daldynian n d and early tommotian ages across each of these excursions carbonate and organic carbon isotopes 13ccarb and 13corg are strongly decoupled regional correlation across a shelf to basin transect shows lateral heterogeneity of 13corg during the early middle n d but more homogenized 13corg values across the basin during the late n d and tommotian the temporal and lateral variations in 13corg suggest that decoupled 13ccarb and 13corg across the n d 13ccarb excursion were possibly caused by diagenetic alteration of organic matter and or amplification of detrital organic carbon isotope signature in low toc carbonates in contrast decoupled 13ccarb and 13corg of the upper n d and tommotian were likely resulted from chemoautotrophic methanotrophic biomass contribution to toc in organic rich black shale and carbonates the decoupled 13ccarb 13corg pattern from the lower n d strata ca 542 ma shows striking similarities with those from the basal ca 635 ma and upper ca 551 ma doushantuo formation in all three cases decoupled 13ccarb 13corg are seen in organic poor carbonates toc 0 1 and coupled 13ccarb 13corg occur in organic rich black shale and carbonates at the end of the negative 13ccarb excursion these similarities suggest that the shift from decoupled to coupled 13ccarb 13corg has no causal link with the terminal oxidation of a large oceanic doc reservoir given the pervasive anoxia euxinia in ediacaran early cambrian oceans local doc rich environments may have been common but a large oceanic doc reservoir capable of buffering the 13c of marine organic matter requires independent evidence

diagnosed three dimensional axisymmetric structure of the mulhall tornado on 3 may 1999 on 3 may 1999 an unusually large tornado that caused f4 level damage and killed several people was intercepted by the doppler on wheels dow mobile radar near mulhall oklahoma from a range of 4 to 9 km resulting in high resolution volumetric data every 55 s up to 1 5 km altitude over a period of 14 min for the first time the evolution and three dimensional structure of a tornado were deduced using the ground based velocity track display gbvtd technique after the circulation center was determined the tangential wind and radial wind were derived from the gbvtd technique at each radius and height in addition the axisymmetric vertical velocity angular momentum vorticity and perturbation pressure were deduced from the tangential and radial wind fields this study focuses on the axisymmetric aspects of this tornado the primary circulation of the mulhall tornado consisted of an 84 m s 1 peak axisymmetric tangential wind with the radius of maximum wind rmw ranging from 500 to 1000 m the secondary circulation exhibited a two cell structure characterized by a central downdraft surrounded by an annular updraft near the rmw the calculated maximum pressure deficit from a 3 km radius to the tornado center at 50 m altitude was 80 hpa the maximum vorticity during the first 8 min of observation was located inside the rmw away from the tornado center this vorticity profile satisfied the necessary condition of barotropic instability as the tornado weakened afterward the vorticity monotonically increased toward the center the computed swirl ratios were between 2 and 6 consistent with the observed multiple vortex radar signatures and the vorticity pattern swirl ratios were generally smaller during the weakening phase

trace fossil preservation and the early evolution of animals abstract the trace fossil record is an important element in discussions of the timing of appearance of bilaterian animals a conservative approach does not extend this record beyond about 560 555 ma crucial to the utility of trace fossils in detecting early benthic activity is the preservational potential of traces made close to the sediment water interface our studies on the earliest cambrian sediments suggest that shallow tiers were preserved to a greater extent than typical for most of the phanerozoic this can be attributed both directly and indirectly to the low levels of sediment mixing the low levels of sediment mixing meant that thin event beds were preserved the shallow depth of sediment mixing also meant that muddy sediments were firm close to the sediment water interface increasing the likelihood of recording shallow tier trace fossils in muddy sediments preservation of surficial trace fossils in this type of setting remains problematic but the above factors suggest that also these can be expected to have left a reasonable record overall the trace fossil record can be expected to provide a sound record of the onset of bilaterian benthic activity the lack of convincing trace fossils significantly before the cambrian supports models of late appearance of macroscopic benthic bilaterians

the role of vegetation and bed level fluctuations in the process of channel narrowing abstract a catastrophic flood ire 1965 on plum creek a perennial sandbed stream in the western great plains removed most of the bottomland vegetation and transformed the single thalweg stream into a wider braided channel following eight years of further widening associated with minor high flows a process of channel narrowing began in 1973 narrowing continues today the history of channel narrowing was reconstructed by counting the annual rings of 129 trees and shrubs along a 5 km reach of plum creek near louviers colorado sixty three of these plants were excavated in order to determine the age and elevation of the germination point the reconstructed record of channel change was verified from historical aerial photographs and then compared to sediment stratigraphy and records of discharge and bed elevation from a streamflow gaging station in the study reach channel narrowing at plum creel occurs in two ways first during periods of high flow sand and fine gravel are delivered to the channel temporarily raising the general bed level subsequently several years of uninterrupted low flows incise a narrower channel second during years of low flow vegetation becomes established on the subaerial part of the present channel bed in both cases surfaces stabilize as a result of vegetation growth and vertical accretion of sediment

forcing mechanisms for an internal rear flank downdraft momentum surge in the 18 may 2010 dumas texas supercell abstractthe forcing and origins of an internal rear flank downdraft rfd momentum surge observed by the second verification of the origin of rotation in tornadoes experiment vortex2 within a supercell occurring near dumas texas on 18 may 2010 is assessed through ensemble kalman filter enkf storm scale analyses enkf analyses are produced every 2 min from mobile doppler velocity data collected by the doppler on wheels and shared mobile atmospheric research and teaching radars as well as radial velocity and reflectivity data from the kama amarillo texas wsr 88d enkf analyses are found to reproduce the structure and evolution of an internal rfd momentum surge observed in independent mobile doppler radar observations pressure retrievals of enkf analyses reveal that the low level rfd outflow structure is primarily determined through nonlinear dynamic perturbation pressure gradient forcing horizontal acceleration into a trough of low perturbation pressure between the low level mesocyclone and mesoa

beyond the stony veil reconstructing the earth s earliest large animal traces via computed tomography x ray imaging abstract trace fossils are superb lines of evidence for examining the ancient biologic world because they offer an opportunity to infer behavioral ecology of organisms however traces can be difficult to parse from their matrix which leads to the loss of important morphological and behavioral data this is especially true for the earliest marine animal traces from the ediacaran period 635 541 ma which are usually small

retrieved pressure field and its influence on the propagation of a supercell thunderstorm an analysis of the observed propagation of the garden city supercell storm based on data collected by an airborne doppler radar is presented the environmental wind profile was characterized by a straight hodograph wind syntheses were used to retrieve the perturbation pressure fields in order to isolate the important dynamical processes controlling storm movement the pressure perturbations produced by the linear and nonlinear effects were comparable in magnitude the rightward bias in storm motion was primarily a result of the forcing produced by the vertical gradients of perturbation pressure this finding is consistent with the results based on numerical simulations the vertical gradients of the linear perturbation pressure were important in determining storm motion early in its life cycle subsequently the nonlinear effects dominated a decomposition of the nonlinear forcing into contributions from cyclostrophically balanced flow and nonlinear shear was also presented this partitioning revealed that both the forcing produced by the mesocyclone and the horizontal shear that was baroclinically produced by the gradient of buoyancy along the flanks of the updraft were important effects to consider

trace fossil evidence for ediacaran bilaterian animals with complex behaviors abstract distinguishing ediacaran trace fossils from tubular body fossils can be a challenge and several ediacaran fossils previously interpreted as animal traces have been shown to be tubular body fossils nonetheless true ediacaran trace fossils are present but they are relatively few and morphologically simple dominated by horizontal trails and shallow burrows such simple morphologies have been taken as evidence for a modest behavioral complexity and limited geobiological impact of animal bioturbators before the cambrian explosion here we report three types of trace fossils horizontal tunnels surface tracks trails and vertical traces from the latest ediacaran dengying formation 551 541 ma in the yangtze gorges area of south china cross cutting tunnels and the presence of scratch marks indicate that these traces are unlikely tubular body fossils instead these three types of traces likely represent animal activities related to under mat feeding epibenthic locomotion and temporary dwelling respectively we show that these three types of traces were constructed by the same bilaterian animals that had moderately complex interactions with microbial mats to exploit nutrients and o 2 resources these animals heralded a new age in ecosystem engineering animal sediment interaction and biogeochemical cycling

stable carbon isotopes of sedimentary kerogens and carbonaceous macrofossils from the ediacaran miaohe member in south china implications for stratigraphic correlation and sources of sedimentary organic carbon abstract ediacaran stratigraphic correlation depends on the integration of paleontological and 13ccarb data however these data often come from different lithologies e g shales vs carbonates making it difficult to directly link biostratigraphic and chemostratigraphic markers ediacaran successions in south china consist of mixed carbonates and shales offering an opportunity for integrated biostratigraphic and chemostratigraphic investigations black shales of the ediacaran miaohe member in the yangtze gorges area contain macroalgae and ediacara type macrofossils both preserved as carbonaceous compressions the miaohe member has been traditionally regarded as an equivalent to member iv of the uppermost doushantuo formation that records part of the shuram negative 13ccarb excursion in south china thus the miaohe member may have the potential to clarify the stratigraphic relationship between ediacara type fossils and the shuram excursion recently however the miaohe member has been partially correlated with the shibantan member of the dengying formation the a correlation or partially with member iv the z correlation these correlations have different implications for the relationship between the miaohe member and the shuram excursion here we test these two correlations using 13c data of sedimentary kerogen or total organic carbon 13corgtoc taking advantage of the distinct 13corgtoc values between the shibantan member and member iv our data show that 13corgtoc of the miaohe member is more similar to that of member iv than to shibantan member providing tentative support for the z correlation in addition we also analyzed 13corg of individual carbonaceous compression fossils 13corgtaxon which have lower carbon isotope values than toc the data suggest that oxygenic photosynthesizers e g macroalgae cyanobacteria and eukaryotic phytoplankton were not the only contributors to sedimentary toc methylotrophs photoautotrophs and chemoautotrophs living in anoxic waters or at the oxycline may have fixed recycled co2 derived from organic carbon remineralization thus contributing a significant amount of 13c depleted organic carbon to sedimentary kerogen in the miaohe member

numerical simulation of thunderstorm outflow dynamics part i outflow sensitivity experiments and turbulence dynamics abstract in this first paper of a two part series a two dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows by focusing only on the outflow and using essentially inviscid equations and high spatial resolution we are able to explicitly represent important physical processes such as turbulent mixing to simplify interpretation of the results the model atmosphere used in all experiments is calm and dry adiabatic this approach allows us to establish basic characteristics of modeled outflows in simple physical settings and provides a foundation for future studies using more realistic environments all simulated outflows are initialized by prescribing a controlled horizontal flux of cold air into the model domain through a lateral boundary in a series of sensitivity tests we examine three parameters of the cold air source region 1 the vertical temperature deficit profile 2 the magnitude of the temperature deficit and 3 the cold air depth by hol

taphonomic study of ediacaran organic walled fossils confirms the importance of clay minerals and pyrite in burgess shale type preservation burgess shale type bst macrofossils and organic walled microfossils are preserved as carbonaceous compressions and may share similar taphonomic processes previous taphonomic investigations of carbonaceous compressions have primarily focused on the microchemistry of cambrian bst fossils but comparative data from organic walled microfossils were not available to address these issues we analyzed two organic walled taxa from the yangtze gorges area of the south china block chuaria an acritarch and vendotaenia a ribbon shaped fossil their abundance offers the opportunity for destructive microanalysis including petrography electron microscopy and elemental mapping on both bedding planes and in cross sections our data suggest that chuaria preservation is remarkably similar to bst fossils in that its vesicle walls are closely associated with clay minerals in addition like many bst macrofossils chuaria and vendotaenia are also closely associated with pyrite chuaria vesicles are often filled with framboidal pyrite and vendotaenia fossils are associated with sulfate partly derived from pyrite oxidation the comparative taphonomy of chuaria and vendotaenia and bst macrofossils indicates that the preservation of organic walled acritarchs may be aided by clay and pyrite mineralization and that these abundant microfossils may serve as proxies for uncovering in more detail the taphonomic histories of bst preservation

initiation of convective storms at radar observed boundary layer convergence lines abstract the origin of 653 convective storms occurring over a 5000 km2 area immediately east of the colorado rocky mountains from 18 may to 15 august 1984 was examined seventy nine percent of the 418 storms that initiated within the study area occurred in close proximity to radar observed boundary layer convergence lines this percentage increased to 95 when only the more intense storms 60 dbze were considered colliding convergence lines initiated new storms or intensified existing storms in 71 of the cases a new storm took a median time of 24 min to grow to 30 dbz following line collision the convergence lines ranged in length between ten and several hundred kilometers both radar and mesonet stations indicated that the primary convergence was concentrated in a zone 0 5 to 5 km in width these lines were characterized on doppler radar as thin lines of enhanced reflectivity between 0 and 20 dbze and as a line of strong radial or azimuthal gradient in doppler velocity these lines were observed ev

the garden city kansas storm during vortex 95 part ii the wall cloud and tornado abstract on 16 may 1995 a supercell storm produced an f1 tornado near garden city kansas during vortex verification of the origins of rotation in tornadoes experiment this event provided the first opportunity to synthesize data collected by a new airborne radar platform called eldora electra doppler radar developed by the national center for atmospheric research the evolution of the low and midlevel mesocyclone was presented in a companion paper in this paper the sequence of events that triggered tornadogenesis within the mesocyclone circulation is shown the occlusion downdraft documented in part i appears to promote vortex breakdown resulting in the generation of multiple vorticity centers it is one of these centers that intensifies into the garden city tornado the structural relationship between the observed wall cloud and the radar reflectivity associated with the developing hook echo and the kinematic wind field is also presented although the wall cloud appears to be uniform there

low level mesocyclonic concentration by nonaxisymmetric transport part i supercell and mesocyclone evolution an idealized simulation of a supercell using the regional atmospheric modeling system rams was able to produce a low level mesocyclone near the intersection of the forward and rear flank downdrafts the creation of the low level mesocyclone is similar to previous studies after 3600 s the low level mesocyclone underwent a period of rapid intensification during which its form changed from an elongated patch to a compact center this transition was also accompanied by a sudden decrease in pressure to 12 mb below that of the neighboring flow and was found to occur even in the absence of nested grids it is shown that the stage of strong intensification does not begin aloft as in the dynamic pipe effect and then descend to the surface rather the vortex is initiated near the surface and then builds upward the process is completed in 5 min and the final vortex can be clearly distinguished from the larger scale mesocyclone at the cloud base the reduction of pressure can be explained as a consequence of the evacuation of mass in the horizontal convergence equation this is in contrast to axisymmetric models of vortex intensification which generally rely on the evacuation of mass in the vertical divergence equation in the latter cases a positive horizontal convergence tendency is what initiates the concentrated vortex however nondivergent models prove that vorticity concentration can occur in the absence of any horizontal convergence here the concentration is associated with a negative horizontal convergence tendency

interactions between ediacaran animals and microbial mats insights from lamonte trevallis a new trace fossil from the dengying formation of south china abstract a new ichnogenus and ichnospecies lamonte trevallis is formally described from the shibantan member limestone of the upper ediacaran dengying formation yangtze gorges area south china it is characterized by horizontal tunnels connected with short vertical burrows and surface trails the horizontal burrows are elliptical or bilobed in transverse cross section preserved in full relief and filled with carbonate intraclasts micrites as well as calcite and silica cements they occur exclusively in silty crinkled and microlaminated layers that are interpreted as amalgamated cyanobacterial microbial mats no burrows have been found in intraclastic layers adjacent to the microlaminated layers the vertical traces are filled with the same material as the burrows but they typically project through the crinkled microlaminae and are exposed on the bedding surface the surface tracks are always preserved in negative epirelief or positive hyporelief and consist of two parallel series of either sharp scratch marks or small knobs the burrow infill has 18ocarb and 13ccarb values distinct from but intermediate between microlaminated and intraclastic layers consistent with petrographic observation that burrow infill consists of a mixture of early carbonate cements intraclasts and micrites bedding plane bioturbation intensity 20 40 measured as percentage of bedding plane area covered by l trevallis traces is comparable to similar measurements in pre trilobite cambrian carbonates the exclusive occurrence of l trevallis within microbial mats may have both taphonomic and ecological significance these mats may have provided firm substrates and localized geochemical conditions that contributed to the structural integrity of the burrows and they may have also facilitated early diagenetic cementation of burrow infill thus facilitating burrow preservation the close association of these burrows with microbial mats implies that the trace producers actively mined cyanobacterial mats to exploit oxygen or nutrient resources the trace makers of l trevallis were better able to utilize the resources around them than many other ediacaran trace makers and provide an ichnological record of a flourishing benthic ecology in late ediacaran oceans at the dawn of the agronomic revolution

dual doppler analysis of winds and vorticity budget terms near a tornado abstract three dimensional dual doppler observations with unprecedented finescale spatial and temporal resolution are used to characterize the vector wind field and vorticity generation terms in and near a weak short lived tornado the beam widths of the two doppler on wheels dow mobile radars at the range of the tornado are 250 m with gate lengths of 75 m resulting in a resolution of less than 107 m3 one of the dows collected data during the 240 s prior to the formation of the tornado enabling examination of the genesis process a single set of volumetric scans suitable for dual doppler analyses were completed by both dows permitting the calculation of vertical and horizontal vorticity divergence and stretching and tilting terms in the vorticity budget of the large but weak tornado and its surroundings but no local tendency terms analyses of the dual doppler vector wind fields document for the first time in a supercellular tornado revealed several structures expected to be associated with

three dimensional microct analysis of the ediacara fossil pteridinium simplex sheds new light on its ecology and phylogenetic affinity abstract ediacara fossils often exhibit enigmatic taphonomy that complicates morphological characterization and ecological and phylogenetic interpretation such is the case with pteridinium simplex from the late ediacaran aar member in southern namibia p simplex is often preserved as three dimensional 3d casts and molds in coarse grained quartzites making detailed morphological characterization difficult in addition p simplex is often transported distorted and embedded in gutter fills or channel deposits further obscuring its morphologies by utilizing microfocus x ray computed tomography microct techniques we are able to trace individual specimens and their vanes in order to digitally restore the 3d morphology of this enigmatic fossil our analysis shows that p simplex has a very flexible integument that can be bent folded twisted stretched and torn indicating a certain degree of elasticity in the analyzed specimens we find no evidence for vane identity change or penetrative growth that were previously used as evidence to support a fully endobenthic lifestyle of p simplex instead evidence is consistent with the traditional interpretation of a semi endobenthic or epibenthic lifestyle this interpretation needs to be further tested through microct analysis of p simplex specimens preserved in situ rather than transported ones the elastic integument of p simplex is inconsistent with a phylogenetic affinity with xenophyophore protists instead its physical property is consistent with the presence of collagen chitin and cellulose an inference that would provide constraints on the phylogenetic affinity of p simplex

an investigation of the goshen county wyoming tornadic supercell of 5 june 2009 using enkf assimilation of mobile mesonet and radar observations collected during vortex2 part i experiment design and verification of the enkf analyses abstracthigh resolution doppler radar velocities and in situ surface observations collected in a tornadic supercell on 5 june 2009 during the second verification of the origins of rotation in tornadoes experiment vortex2 are assimilated into a simulated convective storm using an ensemble kalman filter enkf a series of enkf experiments using a 1 km horizontal model grid spacing demonstrates the sensitivity of the cold pool and kinematic structure of the storm to the assimilation of these observations and to different model microphysics parameterizations an experiment is performed using a finer grid spacing 500 m and the most optimal data assimilation and model configurations from the sensitivity tests to produce a realistically evolving storm analyses from this experiment are verified against dual doppler and in situ observations and are evaluated for their potential to confidently evaluate mesocyclone scale processes in the storm using trajectory analysis and calculations of lagrangian vorticity

ontogeny of a flood plain the ontogeny of five flood plain segments is described for a period of 18 yr following a major flood in 1978 on the powder river in southeastern montana the flood plains developed on relatively elevated sand and gravel deposits left within the channel by the 1978 flood in cross section the flood plains resemble benches with well developed natural levees flood plain growth occurred as sediment was draped onto preexisting surfaces in layers of sand and mud a few centimeters to decimeters thick resulting in some lateral but mostly vertical accretion annual and biannual measurements indicated that as the flood plain segments grew upward the annual rate of vertical accretion decreased as the partial duration recurrence interval for the threshold or bankfull discharge increased from 0 16 to 1 3 yr it is clear that a constant recurrence interval for overbank flow cannot be meaningfully assigned to this type of flood plain ontogeny these flood plains did not grow on migrating point bars and vertical accretion at least initially occurred within the channel rather than across the valley flat during extensive overbank flows sediments of these flood plains define narrow elongated stratigraphic units that border the active channel and onlap older flood plain deposits these characteristics are considerably different from those of many facies models for meandering river deposits facies similar to those described in this paper are likely to be preserved thereby providing important evidence in the geologic record for episodes of periodic channel expansion by ancient rivers

protracted development of bioturbation through the early palaeozoic era mobile organisms first appeared in the fossil record prior to the precambrian cambrian transition sediment textures indicate that the degree of sediment mixing by animal activity remained low for 120 million years following the transition

the multiple vortex structure of the el reno oklahoma tornado on 31 may 2013 abstractthis study documents the formation and evolution of secondary vortices associated within a large violent tornado in oklahoma based on data from a close range mobile polarimetric rapid s

channel narrowing by vertical accretion along the green river near green river utah the green river is the longest tributary of the colorado river near the town of green river utah the green river narrowed in two discrete phases during the twentieth century the first phase of narrowing decreased average width by about 5 and occurred between about 1930 and 1940 when the magnitude of 2 yr flood mean annual discharge and effective discharge decreased by about 30 28 and 37 respectively during this first phase of narrowing saltcedar tamarisk spp an invading non native tree began to establish itself in the study area but botanists of that time did not describe the tree as abundant channel width was stable in the 1940s and 1950s even though saltcedar were becoming already abundant on the river9s banks further narrowing of an additional 14 occurred after 1959 this latest period of narrowing began following three successive years when the magnitude of floods was less than the present 1 5 yr recurrence flood and when saltcedar were already abundant along the river the deposits that comprise the banks of the narrowing green river are composed of the suspended load of the river and these alluvial deposits are characterized by horizontal layers which indicate that they formed by vertical accretion a mechanism is proposed to explain the coarsening upward sequence of beds found in these vertically accreted deposits these changes in the channel of the green river are based on analysis of more than 2600 discharge measurements made by the u s geological survey resurvey of an abandoned measurement site matches of historical ground level photography and analysis of historical aerial photography within a geographic information system we have developed analytical techniques that permit analysis of width data from u s geological survey discharge measurements where gaging cross sections have adjustable beds and banks these techniques allow the spatially rich but temporally poor data from aerial photographs to be supplemented with gaging station data which add great detail about the timing and actual processes of channel narrowing that cannot be determined from aerial photographs alone such an analytical strategy provides a more complete record of historical channel adjustment than can be obtained by other means

dating floodplain sediments using tree ring response to burial floodplain sediments can be dated precisely based on the change in anatomy of tree rings upon burial when a stem of tamarisk tamarix ramosissima or sandbar willow salix exigua is buried subsequent annual rings in the buried section resemble the rings of roots rings become narrower vessels within the rings become larger and transitions between rings become less distinct we combined observations of these changes with tree ring counts to determine the year of deposition of sedimentary beds exposed in a 150 m long trench across the floodplain of the rio puerco a rapidly filling arroyo in new mexico this method reliably dated most beds thicker than about 30 cm to within a year of deposition floodplain aggradation rates varied dramatically through time and space sediment deposition was mostly limited to brief overbank flows occurring every few years the most rapid deposition occurred on channel margin levees which migrated laterally during channel narrowing at the decadal timescale the cross section average sediment deposition rate was steady but there was a shift in the spatial pattern of deposition in the 1980s from 1936 to 1986 sediment deposition occurred by channel narrowing with little change in elevation of the thalweg after 1986 sediment deposition occurred by vertical aggradation from 1936 to 2000 about 27 per cent of the arroyo cross section filled with sediment the rate of filling from 1962 to 2000 was 0 8 vertical m decade or 85 m2 decade published in 2005 by john wiley sons ltd

hook echoes and rear flank downdrafts a review abstract nearly 50 years of observations of hook echoes and their associated rear flank downdrafts rfds are reviewed relevant theoretical and numerical simulation results also are discussed for over 20 years the hook echo and rfd have been hypothesized to be critical in the tornadogenesis process yet direct observations within hook echoes and rfds have been relatively scarce furthermore the role of the hook echo and rfd in tornadogenesis remains poorly understood despite many strong similarities between simulated and observed storms some possibly important observations within hook echoes and rfds have not been reproduced in three dimensional numerical models

precambrian cambrian trace fossils from the yangtze platform south china and the early evolution of bilaterian lifestyles abstract siliciclastic carbonaceous and phosphoritic shallow marine sediments from several neoproterozoic ediacaran and precambrian cambrian pc c boundary sections on the yangtze platform south china contain remarkable trace fossil assemblages the present paper provides a general overview on the distribution and the possible palaeobiological and palaeoenvironmental significance of these trace fossil assemblages from selected pc c boundary sections on the passive yangtze platform margins comparative investigations of the typology of neoproterozoic ediacaran ichnofaunas as well as of the related sedimentary environments accompanying the ichnoassemblages demonstrate in general correspondence with other studies worldwide that the neoproterozoic ichnofaunas are mainly composed of slightly curved to straight and unbranched horizontal traces which are usually related to the occurrence of shallow marine softgrounds sealed by microbial mats rare traces show ichnomorphological patterns that indicate developments toward a more effective horizontal exploitation of the feeding ground during the late neoproterozoic and earliest cambrian the majority of these traces appear to represent undermat miners with simple feeding strategies during the earliest phanerozoic early meishucunian stage equivalent to the nemakit daldynian stage of siberia the oxic shallow water environments along the ancient continental passive margins of the yangtze platform were populated by large as yet unknown arthropods which produced rusophycus type trace fossils the late precambrian to early phanerozoic sediments of the yangtze platform generally provide excellently preserved trace fossil records which together with the presence of index fossils ssf from the nemakit daldynian stage provide a more detailed insight into the development of pc c palaeoenvironments and the evolution of lifestyles among early bilaterian metazoans a preliminary comparison of the neoproterozic biotas occurring on the yangtze platform and in other time equivalent ediacaran habitats demonstrates general similarities but also some striking differences probably due to palaeoenvironmental variations the data presented here from south china are critically discussed in the light of current palichnological approaches and theories

geobiology of a palaeoecosystem with ediacara type fossils the shibantan member dengying formation south china abstract despite the importance of palaeoecosystems with ediacara type fossils for the early evolution of metazoans only little is known about the interplay of geological and biological processes in these environments the reason is that sedimentary structures biogenic structures and bio geochemical signatures e g hydrocarbon biomarkers are commonly not well preserved due to the predominance of volcanic and siliciclastic lithologies the shibantan member dengying formation south china is one of only few carbonate settings with ediacara type organisms worldwide and its lithology promises an excellent preservation of sedimentary facies and bio geochemical signatures here we provide the first comprehensive geobiological characterisation of the shibantan member in order to reconstruct the interplay between sedimentary and bio geochemical processes and to assess the microbial activities in the palaeoecosystem with ediacara type fossils facies analyses revealed that carbonate and organic matter were autochthonously formed by bio geochemical processes linked to microbial mats however the material was frequently reworked and re deposited within the same setting i e para autochthonous as evidenced by small scale hummocky cross stratification erosional contacts lenticular bedding and load casts negative ce anomalies ce ce and low v cr ratios demonstrate that molecular o 2 was present in the water column whereas characteristic ni co v v ni and v sc ratios suggest the contemporaneous presence of sub to anoxic water taken together these observations imply a temporarily stratified water body frequently mixed and ventilated by storms 13 c enrichments in the shibantan carbonates 13 c 3 29 to 3 98 vpdb together with 13 c depletions of syngenetic n alkanes cleaved from the extraction residue using catalytic hydropyrolysis hypy 13 c 31 7 to 36 3 vpdb could indicate a significant withdrawal of 12 c by primary producers that thrived within the mats at the same time sulphurised biomarkers in the bitumen and hypy pyrolysate hint at organic matter decomposition and concomitant sulphide production by sulphate reducing bacteria when oxygen was available at the sediment water interface due to mixing by storms sulphide oxidising bacteria were possibly temporarily favoured the results demonstrated that palaeoenvironmental conditions dynamically changed through a complex interplay of biogenic and abiogenic processes

direct surface thermodynamic observations within the rear flank downdrafts of nontornadic and tornadic supercells abstract despite the long surmised importance of the hook echo and rear flank downdraft rfd in tornadogenesis only a paucity of direct observations have been obtained at the surface within hook echoes and rfds in this paper in situ surface observations within hook echoes and rfds are analyzed these mobile mesonet data have unprecedented horizontal spatial resolution and were obtained from the verifications of the origins of rotation in tornadoes experiment vortex and additional field experiments conducted since the conclusion of vortex the surface thermodynamic characteristics of hook echoes and rfds associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and rfds are favorable or unfavorable for tornadogenesis tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy potential buoyancy as measured by the convective available potential energy and equivalent potential temperature in the r

non supercell tornadoes abstract analyses of tornadoes that are not associated with supercells are presented the database for this study was collected during cinde convention initiation and downburst experiment a field project operated during the summer of 1987 in colorado a total of 27 visual vortices were studied they appeared to form as shear instabilities along radar detected convergence lines the circulations initiated at low levels generally in the absence of precipitation echo subsequently as these vortices propagated along the convergence line they appeared to strengthen to tornadic intensity when they became colocated with the updraft of a rapidly developing storm it is hypothesized that vortex stretching is responsible for intensifying the initial rotation although these tornadoes were weaker than those accompanied by strong midlevel mesocyclones estimates of their strength suggest damage capability as high as f2 the implications for operational radars to detect these types of phenomena were identified the

tornado maintenance investigated with high resolution dual doppler and enkf analysis abstractdual doppler wind synthesis and ensemble kalman filter analyses produced by assimilating doppler on wheels velocity data collected in four tornadic supercells are examined in order to further understand the maintenance of tornadoes although tornado scale features are not resolved in these analyses larger scale processes involved with tornado maintenance are well represented the longest lived tornado is maintained underneath the midlevel updraft within a zone of low level horizontal convergence along a rear flank gust front for a considerable time and dissipates when horizontally displaced from the midlevel updraft the shortest lived tornado resides in a similar zone of low level convergence briefly but dissipates underneath the location of the midlevel updraft when the updraft becomes tilted and low level convergence is displaced several kilometers from the tornado this suggests that a location beneath the midlevel updraft is not always a sufficient condition for tornado maintenance particu

absence of caesium 137 from recent sediments in eastern australia indications of catchment processes summary sediment and soil containing the isotope caesium 137 137 cs are generally understood to have been sub aerially exposed within the last 35 to 40 years the horizon of first appearance of 137 cs in sedimentary deposits in australia is equated with the mid 1950s the time of first appearance of the isotope in australia some sediments however which are known to have been deposited in alluvial cutoffs since the mid 1950s do not contain the isotope this is interpreted as resulting from the high magnitude of the events which entrained and deposited the sediment the sediment was eroded from sub surface sites and deposited and buried rapidly thereby preventing the adsorption of the isotope the data indicate that 137 cs should be used very cautiously as a dating tool in settings where a good knowledge of the fluvial and sedimentological events accompanying the emplacement of the sediments is not available

the garden city kansas storm during vortex 95 part i overview of the storm s life cycle and mesocyclogenesis abstract analysis of a supercell storm that produced an f1 tornado near garden city kansas is presented this event provided one of the first opportunities to synthesize data collected by a new airborne radar platform called eldora electra doppler radar developed by the national center for atmospheric research the early stages of development of the midlevel mesocyclone and the entire evolution of the low level mesocyclone are captured over a 70 min period the low level mesocyclone began as an incipient shallow circulation along a synoptic scale trough the circulation intensified and grew in depth via vortex stretching under the influence of a strong updraft as this rotation built up from the boundary layer it initially remained separate and distinct from the midlevel mesocyclone subsequently the two mesocyclones merge to produce a single column of rotation 4 5 km in diameter an occlusion downdraft develops within the mesocyclone circulation during the last passes by the storm signaling the beg

a tale of three taphonomic modes the ediacaran fossil flabellophyton preserved in limestone black shale and sandstone abstract ediacaran macrofossils are typically preserved in three taphonomic modes casts molds in siliciclastic rocks casts molds in carbonate rocks and carbonaceous compressions in black shales only a few taxa are known to be preserved in more than one of these taphonomic modes flabellophyton is a genus that has been previously reported from lower ediacaran black shale of the lantian formation 635 551 ma in south china and upper ediacaran sandstone of the ediacara member 560 550 ma in south australia here we report flabellophyton from upper ediacaran limestone of the shibantan member of the dengying formation 551 539 ma in south china making flabellophyton the only genus that occurs in all three taphonomic modes we also provide a systematic description of flabellophyton based on material from the lantian and dengying formations in south china recognizing three morphospecies f lantianense f typicum sp nov and f obesum sp nov flabellophyton is reconstructed as an erect epibenthic marine organism attached to sandy carbonate and muddy substrates its phylogenetic affinity remains ambiguous though it was historically interpreted as an algal fossil the wide environmental and stratigraphic distribution of flabellophyton allow comparative taphonomic and paleoecological analysis taphonomic analysis of flabellophyton indicates that multiple taphonomic pathways can facilitate the preservation of ediacaran macrofossils as a window into ediacaran paleoecology flabellophyton and other ediacaran fossils played a crucial role in the construction of epibenthic communities in ediacaran oceans and helps to understand the ecological migration and evolutionary expansion from deeper to shallower oceans during the ediacaran period

dual doppler and single doppler analysis of a tornadic storm undergoing mergers and repeated tornadogenesis abstract dual doppler observations with unprecedented finescale spatial and temporal resolution are used to characterize the vector wind field in and near a tornado occurring near kiefer oklahoma on 26 may 1997 analyses of the dual doppler vector wind fields document in detail for the first time several structures associated with the tornado a proximate updraft region a rear flank downdraft wrapping around the tornado a double gust front structure occluding near the tornado and a region of enhanced vorticity separated from the tornado that may have been associated with cyclic tornadogenesis the analyses are compared to conceptual and computer models of tornadic storms a subsequent tornadogenesis was observed with radar every 18 s providing a finescale temporal view of the genesis process the genesis process was complex and the evolution of tornado intensity parameters was not monotonic in time low level rotation contracted and intensified then broadened then contracted and intensified a se

when the worm turned concordance of early cambrian ichnofabric and trace fossil record in siliciclastic rocks of south australia studies of sedimentary fabrics and trace fossils across the precambrian cambrian interval in south australia demonstrate that the appearance of treptichnus pedum which defines the base of the cambrian also marks the initial development of preservable infaunal activity there is a significant shift in the amount and nature of bioturbation at this time these results support the broad scale utility of t pedum as the indicator of the base of the cambrian

reply to comment on trace fossil evidence for ediacaran bilaterian animals with complex behaviors precambrian res 224 2013 690 701 state key laboratory of paleobiology and stratigraphy nanjing institute of geology and paleontology chinese academy of sciences nanjing 210008 china department of geosciences virginia polytechnic institute and state university blacksburg va 24061 usa hunan institute of geological survey changsha hunan 410116 china department of geological sciences university of missouri columbia mo 65211 usa

mobile doppler radar observations of a tornado in a supercell near bassett nebraska on 5 june 1999 part i tornadogenesis abstract this two part paper details an analysis of high resolution wind and reflectivity data collected by a mobile w band doppler radar the dataset captures the near surface life history of a tornado in a supercell in north central nebraska on 5 june 1999 the formation of the tornado vortex near the ground is described from a sequence of sector scans ranging from 30 s intervals prior to tornadogenesis to 10 15 s intervals during much of the lifetime of the tornado cyclonic vortices of 100 200 m width were found along a bow shaped line of enhanced radar reflectivity at what appears to have been the leading edge of a rear flank gust front at the time of tornadogenesis one of these vortices was located just ahead of the nose of the bow shaped radar echo and a jet which were embedded within a larger scale cyclone at other times small scale cyclonic vortices coexisted with the tornado along an arc shaped line extending to its north and northeast but did not appear to interact with the tornado the

near surface vortex intensification through corner flow collapse abstract results are presented from a large set of large eddy simulations of a class of unsteady vortex evolution that may sometimes play a role in tornadogenesis or tornado variability beginning with a high swirl parent vortex with an excess of low swirl flow through the surface corner core region perturbation of the low swirl near surface inflow at a large radius can trigger a subsequent dynamic corner flow collapse producing dramatic near surface intensification relative to conditions aloft of an order of magnitude or more in velocity scale this paper presents a more detailed treatment of the physics and simulation of corner flow collapse expanding upon the presentation given in a companion paper treating near surface vortex intensification more generally for both steady and unsteady conditions the basic scaling of the onset intensification structure and duration of the phenomenon is explored as a function of some of the dominant physical parameters involved a dimensionless rate of change of

rates of sediment supply to arroyos from upland erosion determined using in situ produced cosmogenic 10be and 26al using 10be and 26al measured in sediment and bedrock we quantify rates of upland erosion and sediment supply to a small basin in northwestern new mexico this and many other similar basins in the southwestern united states have been affected by cycles of arroyo incision and backfilling several times in the past few millennia the sediment generation 275 65 g m 2 yr 1 and bedrock equivalent lowering rates 102 24 m myr 1 we determine are sufficient to support at least three arroyo cycles in the past 3 000 years consistent with rates calculated from a physical sediment budget within the basin and regional rates determined using other techniques nuclide concentrations measured in different sediment sources and reservoirs suggest that the arroyo is a good spatial and temporal integrator of sediment and associated nuclide concentrations from throughout the basin that the basin is in steady state and that nuclide concentration is independent of sediment grain size differences between nuclide concentrations measured in sediment sources and reservoirs reflect sediment residence times and indicate that subcolluvial bedrock weathering on hillslopes supplies more sediment to the basin than erosion of exposed bedrock

episodic sediment accumulation on amazonian flood plains influenced by el nino southern oscillation continental scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains the spatial extent and depths of such deposits have been described and flood plain accumulation has been determined at decadal timescales but it has not been possible to identify discrete events or to resolve deposition on near annual timescales here we analyse 210 pb activity profiles from sediment cores taken in the pristine beni and mamore river basins which together comprise 720 000 km2 of the amazon basin to investigate sediment accumulation patterns in the andean amazonian foreland we find that in most locations sediment stratigraphy is dominated by discrete packages of sediments of uniform age which are typically 20 80 cm thick with system wide recurrence intervals of about 8 yr indicating relatively rare episodic deposition events ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with la nina events which debouch extraordinary volumes of sediments from the andes we conclude that transient processes driven by the el nino southern oscillation cycle control the formation of the bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon nutrients and pollutants to the amazon main stem

the stratigraphic complexity of the middle ediacaran carbon isotopic record in the yangtze gorges area south china and its implications for the age and chemostratigraphic significance of the shuram excursion abstract the middle ediacaran shuram excursion represents the most pronounced negative carbon isotopic shift in earth history and has been considered as evidence for a profound disturbance to the global carbon cycle and proposed as a key chemostratigraphic marker for ediacaran stratigraphic subdivision and global correlation previous study has revealed a pronounced negative 13c shift en3 in the upper doushantuo formation of south china which has been interpreted as an equivalent of the shuram excursion detailed 13c investigation of multiple sections of the ediacaran doushantuo formation around the huangling anticline western hubei province south china indicates that the 13c variation in the upper doushantuo formation is more complex than previously reported in the western region of the huangling anticline a moderately positive 13c excursion here termed the diaoyapo positive excursion bisects two strongly negative 13c excursions here termed the jiuqunao negative excursion and the younger miaohe negative excursion with a 551 ma ash bed capping the miaohe negative excursion in the central region there is only one negative 13c excursion whereas in the eastern region the negative 13c excursion is absent the variable stratigraphic expression of these 13c excursions is most parsimoniously interpreted as reflecting local facies variation diagenesis and or a cryptic unconformity the more complete but more complex 13c chemostratigraphic record in the western region of huangling anticline implies that depending on how this record is correlated with the shuram excursion the latter may be significantly older than 551 ma if it does represent a global chemostratigraphic marker