multiphase u pb geochronology of sintered breccias from the steen river impact structure canada mixed target considerations for a jurassic cretaceous boundary event abstract in situ u pb geochronology has been conducted using laser ablation inductively coupled mass spectrometry la icp ms on shocked and thermally metamorphosed apatite titanite and zircon grains from the steen river impact structure canada the dated relict mineral phases occur within impact melt bearing breccias that underwent post deposition sintering at 450 c t

estimation of sampling efficiency of the big spring number eight bsne sampler at different heights based on sand particle size in the taklimakan desert abstract the big spring number eight bsne sampler is an important and widely used instrument in studies of aeolian sand transport its sampling efficiency is an important parameter and varies with particle size however little is known about the variations in sampling efficiency caused by the differences in particle size with height the variations in particle size with height from 10 to 200 cm were obtained from a field experiment at four sites in the taklimakan desert from may 2010 to april 2011 the variation in the sampling efficiency of the bsne sampler with particle size was estimated based on previous studies and a wind tunnel experiment it was found that the average particle size decreased with height above ground from 10 to 200 cm at three experimental sites while at the other site it decreased from 10 to 15 cm and then slightly increased the percentage distribution of particle sizes with height was similar for sizes below 63 m and between 125 and 200 m at all experimental sites but there were differences in the percentage of size classes between 63 and 125 m and above 200 m the sampling efficiencies ranged from 66 27 to 58 98 64 97 to 57 47 56 04 to 52 76 and 72 07 to 58 9 at the four sites respectively with an average of 64 84 to 57 03 at heights from 10 to 200 cm after correcting for sampling efficiency the amount of aeolian sand that passed through a section of 100 cm width 200 cm height increased by 44 5 52 1 78 8 and 26 4 at the four sites respectively with an average of 50 5 these results indicate that the sampling efficiency of the device needs to be considered when making observations across a gradient of particle sizes

gas hydrates clathrates causing pore water freshening and oxygen isotope fractionation in deep water sedimentary sections of terrigenous continental margins the occurrence of gas hydrates in deep water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep sea drilling results on the pacific continental slope off guatemala gas hydrates were brought up for the first time from two holes 497 498a drilled during leg 67 of the dsdp in water depths of 2360 and 5500 m respectively the hydrates occur in organic matter rich pleistocene to miocene terrigenous sediments in the hydrate bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10 20 m subbottom depth pore waters at the bottom of the holes near 400 m subbottom have as little as half the chlorinity of seawater i e 9 similar but less pronounced trends were observed during previous legs of the dsdp in other hydrate prone segments of the continental margins where recharge of fresh water from the continent can be excluded e g leg 11 the crystallization of hydrates like ice excludes salt ions from the crystal structure during burial the dissolved salts are separated from the solids subsidence results in a downward motion of the solids including hydrates relative to the pore fluids thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation the volume of the latter decreases downhole thus causing downward decreasing salinity chlorinity hydrate formation is responsible for oxygen isotope fractionation with18o enrichment in the hydrate explaining increasingly more positive 18o values in the pore fluids recovered after hydrate dissociation with depth

calcareous nannofossil zonation of the jurassic cretaceous boundary interval and correlation with the geomagnetic polarity timescale abstract calcareous nannofossil stratigraphy has been investigated in six european land sections and at two deep sea drilling project sites 391c and 534a ranging from the upper kimmeridgian to the lower valanginian most of the sections contain a continuous record of the jurassic cretaceous boundary interval a time of rapid increase in nannofossil abundance and diversity and allow development of a revised nannofossil zonation scheme for this period numerous nannofossil lineages have been studied in detail and the results help to increase potential stratigraphic resolution in this interval the proposed scheme consists of six zones which are units recognizable in all sections regardless of nannofossil preservation and eleven subzones determinable in all but the most poorly preserved material thirty eight additional biohorizons are proposed based on the stratigraphy of the two dsdp sites these can be accurately established only in moderately well preserved material but spotty occurrences of these taxa in other sections may be stratigraphically useful most of the sections studied have been investigated magnetostratigraphically and the results of this study allow correlation between nannofossil zonation and the geomagnetic polarity time scale magnetostratigraphy indicates that many nannofossil events can be quite precisely determined and are not significantly time transgressive finally the magnetostratigraphic definitions of two stage boundaries are refined the kimmeridgian tithonian and berriasian valanginian boundaries are placed within chrons cm22n and cm15n respectively two new genera faviconus and umbria and seven new species are described umbria granulosa rhagodiscus nebulosus cretarhabdus octofenestratus faviconus multicolumnatus nannoconus infans nannoconus compressus and nannoconus wintereri eight subspecies and three taxonomic emendations are also described

near bed mass flux profiles in aeolian sand transport high resolution measurements in a wind tunnel vertical profiles of the streamwise mass flux of blown sand in the near bed 17 mm region are analysed from high resolution measurements made using an optical sensor in a wind tunnel this analysis is complemented by detailed measurements of mass flux and mean velocity profiles throughout the boundary layer depth 0 17 m using passive chambered sand traps of small dimensions and armoured thermal anemometers respectively the data permit a preliminary analysis of the relations between the observed forms of the profiles of near bed fluid stress and horizontal mass flux within a carefully conditioned boundary layer profiles of mass flux density are found to be characterized by three regions of differing gradient with transitions at about 2 mm and 19 mm above the bed the exponential decay of mass flux with height is confirmed for elevations above 19 mm and when plotted as a function of u 2 g a parameter of mean vertical trajectory height in saltation the gradient of mass flux in this region scales with the wake corrected friction velocity u where u 0 30 m s 1 a separate near bed region of more intense transport below 19 mm is identified which carries 80 per cent of the total mass flux this region is evident in some previous field and wind tunnel data but not in profiles simulated by numerical models ventilated passive sand traps underestimate mass flux in this region by 37 per cent at slow or moderate wind speeds a third significant region below 2 mm is observed these regions are likely to be related to grain populations in successive saltation low energy ejections and intermittent bed contact respectively optical measurements reveal locally high grain concentrations at some elevations below 5 mm these heights scale with transport rate mass flux gradient and wind speed copyright 1999 john wiley sons ltd

the upper jurassic kimmeridgian fluvial aeolian systems of the southern neuquen basin argentina abstract the kimmeridgian quebrada del sapo formation in the southernmost neuquen basin in argentina represents a succession up to 40 m thick of coarse to fine grained fluvial deposits overlain by aeolian deposits these fluvial aeolian deposits reflect a significant palaeogeographic change in the basin and are related to a major tectonically enhanced relative sea level fall the fluvial section is dominated by braided channel fine grained ephemeral and sheetflood deposits aeolian facies are dominated by dune deposits with minor sandsheet and interdune units changes in the nature of both fluvial and aeolian sedimentation within the studied area suggest a regional variability of accommodation sediment supply conditions the regional changes of the aeolian succession likely reflect different relative positions within a major erg in the upwind margin of the erg a shallow water table promoted water lain sedimentation in interdune areas whereas in the central parts of the erg dry sediment accumulation took place above the regional water table level the vertical transition observed in the quebrada del sapo formation from fluvial to aeolian deposits may be the result of a local climatic change to drier conditions due to the development of a climatic barrier imposed by growth of a magmatic arc to the west alternatively the vertical transition could be related to a lowering of the water table associated with the compartmentalization of the basin during a period of low sea level

dissociation of oceanic methane hydrate as a cause of the carbon isotope excursion at the end of the paleocene isotopic records across the latest paleocene thermal maximum lptm indicate that bottom water temperature increased by more than 4 c during a brief time interval 104 years of the latest paleocene 55 6 ma there also was a coeval 2 to 3 excursion in the 13c of the ocean atmosphere inorganic carbon reservoir given the large mass of this reservoir a rapid 13c shift of this magnitude is difficult to explain within the context of conventional hypotheses for changing the mean carbon isotope composition of the ocean and atmosphere however a direct consequence of warming bottom water temperature from 11 to 15 c over 104 years would be a significant change in sediment thermal gradients and dissociation of oceanic ch4 hydrate at locations with intermediate water depths in terms of the present day oceanic ch4 hydrate reservoir thermal dissociation of oceanic ch4 hydrate during the lptm could have released greater than 1 1 to 2 1 1018 g of carbon with a 13c of approximately 60 the release and subsequent oxidation of this amount of carbon is sufficient to explain a 2 to 3 excursion in 13c across the lptm fate of ch4 in oceanic hydrates must be considered in developing models of the climatic and paleoceanographic regimes that operated during the lptm

age of the barremian aptian boundary and onset of the cretaceous normal superchron abstract the age assigned to the boundary of the barremian and aptian stages remains one of the most poorly constrained post pangean stratigraphic boundaries the lack of a global boundary stratotype section and point gssp for the stage boundary has hampered efforts to calibrate the absolute age of the cretaceous period in the geological time scale the current definition of the barremian aptian boundary also approximates the onset of magnetic polarity chron m0r the end of this chron denotes the start of the cretaceous normal polarity superchron that is of fundamental importance for plate reconstructions currently there is up to 5 discrepancy in the age estimates of the barremian aptian boundary ca 126 121 ma and the start of the cretaceous normal superchron here we review available geochronological information from the late barremian and early aptian stages collected from the pacific ocean china california the ontong java nui large igneous province and the high arctic large igneous province by utilizing only robust geochronological data including u pb and recalibrated 40ar 39ar ages from sites with magnetic polarity information and or paleontological constraints we calculate a best estimate of between 123 8 and 121 8 ma for the barremian aptian boundary and the onset of chron m0r at 2 confidence using estimates of the duration of chron m0r 0 49 0 10 myr 2 we conservatively compute the start of the cretaceous normal superchron to between 123 4 and 121 2 ma 2 using an age of 83 07 0 15 ma 2 for the end of the cretaceous normal superchron the duration of the superchron is also constrained to between 38 0 and 40 5 myr 2 these age ranges for the barremian aptian boundary the onset of the cretaceous normal superchron and the duration of the superchron currently provide the best estimates until a gssp is formally ratified

3d ejection behavior of different sized particles in the grain bed collision process abstract the impact ejection process on a mixed grain size bed with granular packing was simulated with the discrete element method in order to understand the interaction between different sized grains in natural aeolian sand transport in this model the granular bed was formed by settling the randomly generated two sized particles under gravity and then a foreign particle was shot onto the granular bed at different speeds and angles the recorded speed direction and number of the ejected particles were then analyzed it was found that the probability distributions of the ejection speed and angle for different sized particles are all identical to those for the single size grain bed collision process the mean ejection speeds of different sized particles are nearly equal and the mean ejection angles of different sized particles are all equal to a constant of 60 the average number of each size of ejected particles grows linearly with the increasing impact speed but remains invariant for various impact angles moreover the smaller particles are preferentially ejected and the ratio between the mean numbers of different sized particles is independent of both the impact speed and angle additionally the ejected particles were found to move in a 3d space they become distributed symmetrically around the incident plane and jump not only forward but also backward these results are critical to understanding the grain size induced inhomogeneity in aeolian sand transport

jurassic cretaceous boundary age and mid ocean ridge type mantle source for shatsky rise basalt sills cored in lowermost berriasian sediments on shatsky rise in the northwest pacific yield a mean 4 0 ar 3 9 ar incremental heating age of 144 6 0 8 ma providing a minimum estimate on the age of the jurassic cretaceous boundary the largest part of the rise was emplaced at rates similar to those of several continental flood basalts the rise formed at an oceanic triple junction and the sills as well as lavas from two dredge sites exhibit an ocean ridge type nd pb sr isotope signature this signature is provisionally consistent with either a perisphere or meteorite impact model but is not predicted by the plume head model of plateau formation however no model can yet be rejected given the very sparse sampling of shatsky rise

interhemispheric radio astrochronological calibration of the time scales from the andean and the tethyan areas in the valanginian hauterivian early cretaceous an integrated radio astrochronological framework of the agrio formation in the andean neuquen basin of west central argentina provides new constraints on the age and the duration of the late valanginian through hauterivian stratigraphic interval a ca id tims u pb age of 126 97 0 04 0 07 0 15 ma is presented here from the upper hauterivian agua de la mula member of the agrio formation biostratigraphic data from ammonoids and calcareous nannofossils and this high precision new radioisotopic age together with three former ones from the same agrio formation are combined with new astrochronological data in the andes these are correlated with modern cyclostratigraphic studies in the classical sections of the mediterranean province of the tethys supporting detailed interhemispheric correlations for the early cretaceous we also provide new 13c data from the agrio formation which are compared with records from the classic tethyan sections according to our calibration the minimum in the values in the mid hauterivian appears to be synchronous and thus another important stratigraphic marker for global correlation a new duration of 5 21 0 08 myr is calculated for the hauterivian stage starting at 131 29 0 19 ma and ending at 126 08 0 19 ma the difference between the duration of the hauterivian in gts2016 and in this study is 1 32 myr while the base and top of the gts2016 hauterivian differ respectively by 3 40 and 4 69 myr

refining the jurassic cretaceous boundary re os geochronology and depositional environment of upper jurassic shales from the norwegian sea abstract limited chronological information for upper jurassic rocks hampers precise age assignment of the jurassic cretaceous j k boundary this study presents re os geochronology rock eval pyrolysis and geochemical data for upper jurassic rocks in the nordland vii area offshore northern norway four intervals of organic rich shale were investigated two from the krill and one from the alge member of the hekkingen formation and one from the underlying masnykan formation drilled cores 6814 04 u 01 and 6814 04 u 02 rock eval pyrolysis data indicate immature organic matter with mixed type ii iii oil gas potential in these units among the four intervals re os isochroneity prevails only for the hekkingen formation samples data from the shale interval in the otherwise sand rich masnykan formation deposited in a shoreface to foreshore marine environment are not isochronous non isochroneity of the samples from relatively proximal setting is attributable to non conservative elemental desorption of os from sediments to dissolved phase and isotopic exchange of os at this depositional site the re os isochron age for the alge member 153 2 7 3 ma model 3 mswd 29 n 9 is consistent with its biostratigraphic kimmeridgian age 152 1 157 3 ma a re os isochron for the lower part of the krill member yields an age of 144 5 1 4 ma model 3 mswd 3 7 n 10 with an initial 187os 188os ratio of 0 552 0 007 given the interval s lower tithonian biostratigraphic age the re os age suggests that the j k boundary must be a few myr younger than its currently assigned age of 145 0 ma 2017 02 time scale www stratigraphy org a re os model age for shales from the upper part of the krill member places an upper age limit for the j k boundary at 142 2 ma hence the j k boundary is between 142 ma and 144 5 ma fe and s concentrations for the hekkingen formation show strong correlation and the s fe slope 1 19 mimics the stoichiometric s fe ratio for pyrite 1 15 a fe s toc ternary diagram for these samples points to an iron limiting sulphidic depositional environment anoxic to sulphidic depositional conditions for the hekkingen shales are also evident from relatively higher authigenic enrichment of mo compared to u unlike the hekkingen formation shales from the more sandy masnykan formation have lower s fe ratios and lower enrichment factors for redox sensitive elements indicating relatively oxygenated marine conditions during deposition

the saltations of different sized particles in aeolian sand transport abstract wind tunnel experiments were performed in order to investigate the effect of mixing on the aeolian transport of sands with different grain sizes two types of sand with different grain size distributions and an equal mass binary mixture of these sands were used comparing the gradients of their measured mass flux profiles and some published profiles for mixed sand transport with those for nearly uniform sand transport it was found that for both of these types of sand bed the negative gradient of the mass flux profile on a log linear plot varies with the mass averaged grain size of the sand bed according to the same power law hence it can be deduced that during the aeolian transport of mixed sand beds the mean vertical ejection speeds of different sized grains are nearly identical to that for the transport of monospecific sized sand with the same mass averaged grain size theoretical analysis was undertaken to explore the characteristics of ejections of different sized grains during the aeolian transport of sand with mixed size grains it is proposed that the mean ejection angles and the mean ejection speeds for sand grains of different sizes are nearly identical and are equivalent to that for monospecific sized sand with the same mass averaged grain size it was also evident that the ratio between the transport rate of each grain size group expressed as the fraction in the whole transport rate and its mass fraction in the mixed size sand is a combined consequence of both the wind effect on the mean saltation distance of different sized grains and the effect of the mass fraction of each grain size group in the original mixture on its ejection potential responding to an impact of a saltating sand grain

experimental investigation of mass flux and transport rate of different size particles in mixed sand transport by wind abstract sand transport by wind in arid regions is a geomorphic process affected by the grain size distribution knowledge of the transport behavior of each size group in mixed sand transport is important for understanding the saltation dynamics in dune migration this paper focuses on the characteristics of mass flux mass concentration horizontal velocity and transport rate of different particle size groups and the relation to their mass proportion in air experiments were carried out to analyze these relations in a wind tunnel by particle tracking velocimetry the mass flux of different size particles usually decays exponentially with height for larger size groups and deviates from exponential decay for smaller size groups the trend of mass concentration of different size groups with height is similar to the mass flux the mean horizontal velocity of different size groups generally increases with increasing height and decreases with the increase of particle size at the same height with increasing height the ratio of mass flux of each size group to the total mass flux increases for the smaller size group but decreases for the larger size group and the transition diameter of particles is about 0 14 mm for the 0 16 mm sand sample and about 0 21 0 27 mm for the 0 24 mm sand sample for the smaller size group the ratio of the mass weighted average horizontal velocity of each size group to that of all particle sizes is 1 0 and the ratio of the transport rate of each size group to the total transport rate is usually larger than its mass proportion in air this is the opposite for the larger size groups the maximum deviation of mass flux proportion of each size group from its mass proportion in air is about 15 which is determined by the mass weighted average horizontal velocity of each size group

lower cretaceous calcareous nannoplankton from the neuquen basin argentina abstract we present nannofossil analyses from five new sections of the neuquen basin which together provide a relatively continuous stratigraphic succession ranging from tithonian to hauterivian vaca muerta to agrio formations the sections provide an opportunity to study nannofossils from a relatively little studied area which nevertheless represents an important marginal basin and a relict of the single largest mesozoic marine habitat the pacific ocean our aims were firstly to generate biostratigraphic data in order to test existing ammonite based age models and ultimately to improve stratigraphic correlations between the argentinian sections and global chronostratigraphic standards and secondly to collect palaeogeographic information in order to improve our understanding of mesozoic nannoplankton distributions and ecology the sections yielded a continuous nannofossil succession with assemblages of moderate preservation abundance and diversity although many standard biostratigraphic index species of both boreal and tethyan affinities were absent we were able to use age diagnostic taxa to provide a relatively robust stratigraphic framework the first and last occurrences of clepsilithus maculosus cruciellipsis cuvillieri eiffellithus primus eiffellithus windii eiffellithus striatus lithraphidites bollii and nannoconus ligius throughout the sections there is remarkably good agreement between the correlation suggested by the nannofossils based largely on globally distributed species and ammonites which are largely endemic to the basin the vaca muerta and upper agrio member correlations are identical or within one or two ammonite zones the lower agrio member correlations are similarly close but the occurrence of wide canalled nannoconids suggests slightly younger latest valanginian to early hauterivian ages for much of the member the neuquen nannofossil assemblages are rather low in diversity and suggest that the basin supported a restricted nannoflora dominated by shelf adapted taxa watznaueria micrantholithus nannoconus biogeographically distinctive taxa whether tethyan or boreal austral are limited to a few rare and sporadically distributed examples but two new species of crucibiscutum may prove to be austral in affinity the presence of nannoconids consistently from the late valanginian indicates the presence of a good shallow marine connection with tethys via the hispanic corridor seaway but their low abundance and rather sporadic occurrence suggests they were probably at the limits of their ecological ranges four new nannofossil species are described crucibiscutum trilensis crucibiscutum neuquenensis calculites suturus and nannoconus quadricanalis

decoding the mediterranean salinity crisis this historical narrative traces the steps to unravel over a span of 40 years an extraordinary event in which 5 of the dissolved salt of the oceans of the world was extracted in a fraction of a million years to form a deposit more than 1 million km 3 in volume a buried abyssal salt layer was identified with reflection profiling and sampled during the deep sea drilling project leg 13 the dolomite gypsum anhydrite and halite in the drill cores paint a surprising picture of a mediterranean desert lying more than 3 km below the atlantic ocean with brine pools that shrank and expanded by the evaporative power of the sun the desert drowned suddenly when the gibraltar barrier gave way the explanation of deep basin shallow water desiccation and the notion of a catastrophic zanclean flood had a mixed reception however the hypothesis became broadly accepted following subsequent drilling expeditions nevertheless as experts examined the evaporate facies and sequences of equivalent age in the terrestrial outcrops weaknesses appeared in the concept of repeated flooding and drying to account for the magnitude of the deposits the rosetta stone used to decipher conflicting interpretations turns out not to be the deposits but the erosion surfaces that enclose them these surfaces and their detritus formed in response to the drop in base level during evaporative drawdown extend to the basin floor evaporative drawdown began halfway through the salinity crisis when influx from the atlantic no longer kept up with evaporation prior to that time a million years passed as a sea of brine concentrated towards halite precipitation during the later part of this interval more than 14 cyclic beds of gypsum accumulated along shallow margins modulated by orbital forcing the thick salt on the deep seabed precipitated in just the next few cycles when drawdown commenced and the brine volume shrank upon closure of the atlantic spillway the remnants of the briny sea transformed into salt pans and endorheic lakes fed from watersheds of eurasia and africa the revised model of evaporative concentration now has shallow margin shallow water and deep basin deep water precursors to desiccation

grain size characteristics of sediment transported during dust storms abstract the grain size characteristics of sediment transported by wind in creep saltation and suspension during 15 dust storms in the slims river valley yukon territory were investigated grain size analysis of the transported sediment indicated that the suspended sediment is significantly finer and better sorted than the sediment transported in either surface creep or saltation the data also show that during dust storms the mean particle size of the suspended sediment decreases as a power function of height the grain size distributions of the sediment transported in suspension and surface creep are characterized by strong positive skewness which results from the selective removal of surface particles small enough to be transported by the wind at a given velocity in contrast the grai size distribution of the saltation samples are negatively skewed which is thought to result from the selective removal of the finer particles into suspension from those particles initially lifted from the surface into the air stream thus the proportion of sediment initially entrained into the air stream which returns to the surface in saltation will lack the fine grains removed in suspension and will therefore tend to be less positively or negatively skewed

the ammonite sequence in the agrio formation lower cretaceous neuquen basin argentina the agrio formation of the neuquen basin argentina contains an extensive sequence of ammonite faunas most of which are monogeneric detailed collecting through 15 sections across the basin has facilitated a major revision of the ammonite zonation formerly embracing four broad zones the agrio formation is now divided into nine zones the lowest four of which are divided into a total of 11 subzones the new zonation provides a standard against which other south american faunas can be compared the degree of subdivision now achieved is comparable to that for the standard sequences of the west mediterranean region although it is not possible to correlate the two regions in detail the occurrence of some widely distributed genera olcostephanus karakaschiceras oosterella spitidiscus and crioceratites at well defined levels in the neuquen basin provides some crucial links thus the approximate positions of the lower upper valanginian valanginian hauterivian and lower upper hauterivian boundaries can be determined

steady state saltation in air coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain surface impact process experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced including a diminution of the sub saltation layer mean wind speed as the friction speed increases in this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free stream shear stresses and the grain mass flux increases with friction speed uf less rapidly than uf3

the harmonious character in equilibrium aeolian transport on mixed sand bed wind tunnel experiments were carried out to measure the spatial distribution in equilibrium transport of four types of mixed sand the flux profiles of each grain size group were calculated it is found that the vertical distribution of mean grain size has a close relation with the grain size distribution in the sand bed in a log linear plot the flux profiles of main grain size groups are all straight lines and their slopes are nearly equal it is also found that the ratio of transport rate of each size group to the whole transport rate is directly proportional to the mass ratio of each size group in the sand bed and the proportion value is only dependent on the grain size this harmonious law is applicable to all four types of mixed sand used in the experiment

a mesozoic time scale we present an integrated geomagnetic polarity and stratigraphic time scale for the triassic jurassic and cretaceous periods of the mesozoic era with age estimates and uncertainty limits for stage boundaries the time scale uses a suite of 324 radiomenc dates including high resolution 40 ar 39 ar age estimates this framework involves the observed ties between 1 radiometric dates biozones and stage boundaries and 2 between biozones and magnetic reversals on the seafloor and in sediments interpolation techniques include maximum likelihood estimation smoothing cubic spline fitting and magnetochronology

new constraints on the jurassic cretaceous boundary in the high andes using high precision u pb data abstract the jurassic cretaceous j k boundary is poorly constrained and is the only phanerozoic system boundary that lacks an internationally accepted reference stratigraphic section gssp precise radio isotopic u pb data are unavailable for the earliest stage of the cretaceous the berriasian the age of the jurassic cretaceous boundary was based on several assumptions including the relative duration of ammonite zones the constant spreading rates of magnetic anomalies and the extrapolation of rb sr or k ar isotopic data this paper discusses a site in an andean basin of western gondwana showing the j k boundary interval with geographically widespread nannofossil markers which are here uniquely combined with precise radiometric dates the recent finding of a sequence of marine black shales in the high andes of argentina interbedded with ash fall tuffs provides important constraints on this boundary this succession bears calcareous nannofossils and ammonites which allow correlation with well established tethyan floras and faunas in the northern hemisphere the tithonian berriasian transition in the andes was recognized on the basis of ammonite zones and nannofossil bioevents for the first time in the southern hemisphere the new ages obtained are 137 9 0 9 ma by sensitive high resolution ion microprobe shrimp and 139 55 0 09 0 18 ma by chemical abrasion isotope dilution thermal ionization mass spectrometry tims near the base of the berriasian these new ages can be interpreted in two different ways the first alternative would indicate that the present geological time table is correct and the fossil levels should be late berriasian the second alternative is that the j k boundary is 5 ma younger than the recently published geological time scale the authors support the last alternative and propose that the j k boundary should be close to 140 ma

changes in grain size of sand in transport over a foredune suspended sand is sampled at several heights and positions on a beach and foredune providing detailed insight into the vertical and horizontal variation in sand content in the air during landward transport grain size analysis is used to study the changes in grain size distribution during landward transport mean grain size and sorting decrease during transport changes in textural parameters follow a gradual and regular path when the sediment is transported into the foredune sediment trapped on the seaward slope at a height of 30 50 cm above the surface closely resembles the sediment trapped landward which implies that changes in the direction of transport are related to vertical changes within the sediment transport profiles the movement of sand over the vegetated foredunes is induced by turbulent forces created by the air flowing across the vegetation and the foredune leading to a change from saltation on the beach to modified saltation and suspension on the foredune small grains are lifted higher and fall more slowly than coarse grains and therefore are transported further landward resulting in a gradual decrease in grain size of the landward deposited sediment

saltating particles over flat beds measurements of ejection and impact velocities trajectory lengths and maximum rise heights of sand grains median diameters 118 and 188 m in saltation over a flat sand bed in a wind tunnel have been obtained from the digitization of multiple image photo graphs the mean angle of ejection is found to be about 30 o from the horizontal rather than 90 o with mean vertical ejection velocity of about 2u where u is the friction velocity trajectories of saltating grains have been computed using the measurements of the initial ejection velocities and the mean velocity profile of the air flow

berriasian early cretaceous radiometric ages from the grindstone creek section sacramento valley california abstract the grindstone creek section glenn county northern california is a sequence of hemipelagic mudstone siltstone and sandstone interbedded with concretionary limestone and a few thin tuffs and bentonites two tuffs have been collected from a narrow interval of this sequence and subjected to mineralogical and isotopic analyses uzpb isotopic analyses of zircon fractions from these volcanic horizons indicate an age of 137 1 1 6 0 6 ma a detailed investigation has been conducted on the calcareous nannofossil stratigraphy of this section based on numerous samples with moderately preserved assemblages the nannoflora is largely of tethyan affinity and allows direct correlation with the berriasian stratotype section with sections with published magnetostratigraphies and with a dsdp site drilled between known magnetic anomalies the dated tuffs lie in the lower part of the upper berriasiancretarhabdus angustiforatus zone assipetra infracretacea subzone and within the narrow range ofrhagodiscus nebulosus at three different sections this subzone can be correlated with m sequence polarity zones m16 and m16n an independent magnetostratigraphic correlation is provided at dsdp site 387 drilled between anomalies m15 and m16 where basal sediments containr nebulosus buchia collected within a meter of the lower tuff lie within theb uncitoides zone which is berriasian in age the upper tuff level which occurs 65 m above the lower tuff is situated within the overlyingb pacifica zone this zone had previously been correlated with the early valanginian but is clearly also partly of berriasian age based on nannofossil stratigraphy our results allow an estimate of the age of the berriasian valanginian and jurassic cretaceous boundaries of 135 1 ma and 141 1 ma respectively and these fall within the range of but differ significantiy from several published time scales

changes in the saltation flux following a step change in macro roughness author s gillies john a etyemezian vicken nikolich george nickling william g kok jasper f

from alluvial fan to playa an upper jurassic ephemeral fluvial system neuquen basin argentina abstract the upper jurassic fill of the back arc neuquen basin includes a lowstand wedge known as the tordillo formation the studied deposits crop out along a n s oriented belt that runs parallel to the andean magmatic arc they are limited to the south by the east west oriented positive structure of the huincul arch formed as a result of upper jurassic tectonic inversion the tordillo deposits were formed in an arid fluvial dominated system characterised by systematic downstream changes in architectural style a gravelly and sandy bedload fluvial system is recognised in the southern upstream sector the reduced thickness and the coarse grain size suggest steep gradients excess of bedload supply and a low subsidence rate thicker and finer grained deposits prevail to the north and northeast they were formed under arid conditions in a wadi sand flat playa fluvial system this distal facies association indicates increased accommodation owing to high rates of subsidence relative to coarse siliciclastic sedimentation rates these low gradient deposits are characterised by cyclic alternations of mud dominated and sand dominated packages interpreted as high and low accommodation systems tracts the overall fining upward stacking pattern of the tordillo formation suggests a change towards higher accommodation rates this is accompanied by frequent development of soil horizons and darker primary and reworked pyroclastic deposits these attributes indicate a stronger explosive volcanic activity associated with increased precipitation and high water table emplacement towards the end of the tordillo lowstand wedge

magnetostratigraphy nannofossil stratigraphy and apparent polar wander for adria africa in the jurassic cretaceous boundary interval article i nfo existing correlations of early cretaceous nannofossil events to polarity zones in italian pelagic limestones are the basis for correlation of polarity chrons to geologic stages and hence calibration of the early cretaceous geomagnetic polarity timescale here we present correlations of nannofossil events to polarity chrons for the late jurassic and jurassic cretaceous boundary interval from the southern alps making the case for revisions of current geologic timescales in this interval the jurassic cretaceous and kimmeridgian tithonian boundaries can be usefully defined at the onsets of polarity chrons cm18r and cm22r respectively the oxfordian kimmeridgian boundary can be assigned to cm25 an apparent polar wander path apwp for the southern alps derived from and time calibrated by bio magnetostratigraphic data is similar to and provides a proxy for the african apwp in the absence of adequate african data of this age the southern alpine apwp is characterized by a cusp hairpin at 30 n 270 e at a time close to the oxfordian kimmeridgian boundary polarity chron cm25 154 ma rapid apwp from high latitude n60 n in the preceding middle jurassic bajocian oxfordian interval coincident with the onset of central atlantic ligurian oceanic rifting and a return to high latitudes during the cretaceous early jurassic poles at high latitudes n60 n are evident for sinemurian to toarcian aalenian time 175 195 ma an apwp for this time interval with this level of temporal calibration cannot be constructed using data from africa itself or from other atlantic bordering continents by rotation to african coordinates

sedimentary and diagenetic markers of the restriction in a marine basin the lorca basin se spain during the messinian abstract the lorca basin southeastern spain is part of a chain of small marginal neogene basins located in the structurally active betic area the upper miocene messinian sequence is composed of a thick diatomite bearing series tripoli unit overlain by the main evaporites analogous to the classical succession that records the main events during the salinity crisis in the mediterranean region the shallow restricted conditions of this region amplified the sedimentary responses to local and global forcings an integrated approach using sedimentology micropalaeontology stable isotope geochemistry and organic geochemistry has been applied to the tortonian messinian succession of the lorca basin in order to obtain a continuous record of the environmental changes the sediments record two major events which affected the whole mediterranean 1 high levels of productivity that led to the formation of the diatomite bearing deposits in the early messinian tripoli and 2 the messinian salinity crisis with its two major stages represented by the halite and gypsum units both mainly precipitated from marine derived brines the rapid reflooding of the mediterranean by normal marine waters at the base of the pliocene did not reach the lorca basin nor other basins of this part of the betic area instead continental sediments were deposited as a consequence of the regional uplift of se iberia which started close to the messinian pliocene boundary the most prominent feature of this basin concerns the record of its restriction by the time of the deposition of the tripoli unit which led to intercalations of precursor evaporitic layers consisting of ca sulphate deposited in sub aqueous and sabkha conditions interbedded with diatomites this alternation of evaporites and diatomites proves that the lorca basin was periodically restricted and reflooded by marine waters a possible cause for this being relative sea level fluctuations in the mediterranean this strengthens evidence of diachronism that suggests that the onset of the first messinian evaporitic deposition was not synchronous but was dependent on bathymetry and local tectonics high productivity during the early messinian in this basin is demonstrated by the thick deposits of diatomites however stagnation episodes may have occurred during this interval as suggested by the preservation of high amounts of organic matter organic rich shales and the extent of bacterial sulphate reduction which apparently occurred during early diagenesis the formation of organo sulphur compounds replacement of sulphates by carbonates and the high levels of elemental sulphur are by products of diagenetic processes occurring in a restricted hypersaline environment

gas hydrate dissociation in the lorca basin se spain during the mediterranean messinian salinity crisis abstract the tortonian marls which were deposited in the lorca basin se spain prior to the messinian tripoli formation contain discontinuous levels of indurated nodules and massive layers up to a few meters thick of diagenetic dolomite the oxygen and carbon isotopic compositions of these dolomites indicate that the carbonate diagenesis occurred mostly in 13c rich and 18o rich to 18o poor solutions the vertical distribution of the values of these dolomites is interpreted as the result of the past occurrence of gas hydrate reservoirs where the methane source probably originated from the organic rich marls we postulate that the gas hydrates contained in the tortonian marls were destabilized during the water level drops related to the messinian salinity crisis associated sedimentary structures include numerous syn sedimentary deformations and brecciation which are the expression of important sedimentary instabilities which could have been generated during the gas hydrates decomposition

tectonic control of the oil rich large igneous carbonate salt province of the south atlantic rift abstract the early cretaceous south atlantic magmatic province samp which includes the parana etendeka lip produced about 8 million km 3 of tholeiitic basalt and diabase over an area of 4 million km 2 huge pre salt oil reserves discovered in 2007 by petrobras in non marine carbonates are estimated at more than 45 billion barrels here we show the close causal relationship of the southward increasing width of the wedge shaped south atlantic rift with the similarly southward increase in igneous activity in the thicknesses of non marine carbonate and salt and in the size of oil reserves all controlled mainly by south america s early clockwise rotation away from africa about a pole in its northeast large diabase dike swarms transversal to the rift witness to south america s rotation that opened in its wake the southward widening south atlantic rift westward increasing pressure on the equatorial margin by south america s clockwise rotation forced open the benue trough and created pre late aptian folds in the demerara plateau and in brazil s solimoes upper amazonas basin prerift and synrift volcanic activity increases southward culminating in the parana etendeka lip and in the offshore volcanic sdrss that continue southward to the cape basin berriasian valanginian rift sediments deposited from about 145 ma 10 ma before the flood basalts of the parana etendeka lip the largest transversal dike swarm continued in the proto walvis ridge that separated the central south atlantic endorheic rift basin from the sea in the south erosion and leaching of basalts supplied ca mg and sio 2 to the endorheic basin for the deposition of non marine carbonates and authigenic clays basalt flows intercalated with carbonates nearly until salt deposition about 113 ma hypogenic leaching of carbonates by mantle derived co 2 created optimal reservoirs supergiant oil deposits occur where the widest endorheic basin and the volcanic province overlap

the flux profile of a blowing sand cloud a wind tunnel investigation abstract the flux profile of a blowing sand cloud or the variation of blown sand flux with height is the reflection of blown sand particles that move in different trajectories and also the basis for checking drifting sand here we report the wind tunnel results of systematic tests of the flux profiles of different sized sands at different free stream wind velocities the results reveal that within the 60 cm near surface layer the decay of blown sand flux with height can be expressed by an exponential function q h a exp h b where q h is the blown sand transport rate at height h a and b are parameters that vary with wind velocity and sand size the significance of coefficient a and b in the function is defined a represents the transport rate in true creep and b implies the relative decay rate with height of the blown sand transport rate the true creep fraction the ratio of the sand transported on the surface h 0 to the total transport varies widely decreasing with both sand size and wind speed the flux profiles are converted to straight lines by plotting sand transport rate q h on a log scale the slope of the straight lines that represents the relative decay rate with height of sand transport rate decreases with an increase in free stream wind velocity and sand grain size implying that relatively more of the blown sand is transported to greater heights as grain size and wind speed increase the average saltating height represented by the height where 50 of the cumulative flux percentage occurs increases with both wind speed and grain size implying that saltation becomes more intense as grain size and or wind velocity increase

on the rate of aeolian sand transport a simple explicit formula for the transport rate of windblown sand is derived based on physical reasoning the curve relating the dimensionless transport rate to the dimensionless friction speed exhibits the empirically well established peak in fact the theory developed in the paper can explain the peaked shape of this curve three empirically determined coefficients in the formula can be given a physical interpretation the formula is shown to fit transport rates measured in wind tunnel experiments well except for very coarse sand formulae for the wind profile in the saltation layer and for the grain dislodgement rate in dependence on the friction speed are obtained as part of the theory

evidence of precessional and eccentricity orbital cycles in a tithonian source rock the mid outer carbonate ramp of the vaca muerta formation northern neuquen basin argentina basin to mid ramp cyclic facies of the tithonian vaca muerta formation are exposed in the loncoche creek section of the neuquen basin mendoza province argentina this unit is characterized by a decimeter scale rhythmic alternation of marls shales and limestones and extends from the lower tithonian to the upper berriasian cyclostratigraphic studies based on a detailed facies analysis allowed the identification of cyclic patterns with frequencies within the milankovitch band according to biostratigraphic data the dominant cycle in the studied section has a period of 20 k y which correlates with the earth s axis precession element spectral analysis based on a series of compacted and decompacted cycle thickness identified a subordinate frequency of about 90 to 120 k y which we interpret as the modulation of the precessional cycle caused by the earth s orbital eccentricity the strength of the precession signal together with the absence of a well defined cyclicity attributable to the obliquity orbital cycle i e 40 k y is in agreement with previous data from the northern hemisphere

the impact of calcareous nannofossils on the pelagic carbonate accumulation across the jurassic cretaceous boundary abstract calcareous nannofossils were important producers of pelagic carbonates in mesozoic oceans in order to better understand the origin of mesozoic pelagic carbonates we studied upper jurassic to lower cretaceous sediments in the central atlantic ocean dsdp sites 105 367 534a with respect to their content of calcareous nannofossils the interval under investigation is characterized by a significant increase in the deposition of carbonate rich sediments a trend that goes along with the rapid radiation of calcareous nannofossils the assemblage composition and the size variation of common taxa were analyzed and subsequently the contribution of this phytoplankton group to the pelagic carbonate accumulation was calculated results reveal two long term changes of the nannofossil assemblage composition 1 the early tithonian coccolith assemblages are of low diversity watznaueria spp cyclagelosphaera spp zeugrhabdotus spp whilst the mid to late tithonian assemblage is dominated by nannoliths conusphaera mexicana polycostella beckmannii nannoconus spp and large sized watznaueria 2 the early berriasian is characterized by a shift from the late tithonian nannolith rich assemblage to a highly diverse coccolith assemblage morphometric studies of the placolith genus watznaueria show for all three dsdp sites large sized forms of this genus in the mid and late tithonian followed by a decrease of up to 2 m for the mean size in the earliest berriasian at dsdp site 105 the studied nannolith taxa show an increase in size during the mid tithonian to berriasian interval the records of both nannofossil carbonate estimates and the measured bulk rock carbonate reveal two periods of increase in the nannofossil carbonate record of dsdp site 105 a first significant increase of the carbonate accumulation is observed in the mid and late tithonian probably caused by mass occurrences of strongly calcified taxa c mexicana p beckmannii nannoconus spp watznaueria cf manivitae this interval is here named nannofossil calcification event nce the second carbonate maximum in the late berriasian is related to a rise in absolute abundances of nannofossils this peak is amplified by an overall increase of the sedimentation rate the calculated accumulation rates of nannofossils nannofossil carbonate and bulk rock carbonate for the late berriasian are on the same scale as values from recent ocean surface sediments a comparison of nannofossil carbonate values with the bulk rock carbonate content shows that on average only 27 of the total carbonate can be explained by our nannofossil carbonate estimates this discrepancy is most likely caused by the high amount of unidentifiable micrite and fragments of calcareous nannofossils other factors contributing to the error are possible inaccuracies in the determination of absolute abundances and nannofossil volume calculations the nce occurs during a long term sea level fall dry climate and presumed low pco 2 levels a decline in abundance of strongly calcified nannofossils coincides with the opening of the pacific atlantic seaway via central america which may have had a substantial impact on the palaeoceanographic situation in the central atlantic ocean these changes are here considered to have at least partly caused the shifts in abundance size and assemblage composition of calcareous nannofossils observed across the jurassic cretaceous boundary interval

isotopic compositions of diagenetic dolomites in the tortonian marls of the western mediterranean margins evidence of past gas hydrate formation and dissociation abstract previous studies in the lorca basin s e spain sediment geol 2002 have shown that the diagenetic dolomitic concretions from the tortonian marls are related to the past occurrence of gas hydrates which were rapidly dissociated when the sea level dropped during the messinian salinity crisis these conclusions were based on the oxygen and carbon isotopic compositions of these dolomites which indicated that the carbonate diagenesis occurred either in 13 c rich 18 o poor solutions as it is typical in gas hydrate systems or in 13 c poor 18 o rich solutions generated during gas hydrate decomposition we have recently discovered similar dolomitic nodules in other tortonian marly successions from s e spain and northern morocco the oxygen and carbon isotopic compositions of the diagenetic dolomites from these areas in spain and morocco are very similar to those from the lorca basin this demonstrates that the dolomitic nodules from the tortonian marls of the betic and rifian basins were all related to the same diagenetic environments where pore fluids were modified by gas hydrate formation dissociation processes these overall observations in s e spain and northern morocco suggest that the dissociation of gas hydrates during the messinian salinity crisis was not a local phenomenon but corresponded to a major event which affected both sides of the western mediterranean margins it might have been initiated during the first major restriction step that started around 6 7 ma in the mediterranean sea and probably climaxed at 5 9 ma when the mediterranean sea level dropped by more than 1000 m during the salt deposition stage

field evaluation of some sand transport models fluorescent tracer procedures originally developed for research on fluvial sediment movement were used to monitor the movement of three commercially purchased sands in a natural dune environment results were compared with estimates of transport rates from three theoretical models estimates from models by bagnold 1941 and hsu 1971 were larger by as much as one order of magnitude than the rates observed in the tracer study the model of a a kadib 1965 provided closer correspondence to observed transport rates for medium sand mean diam 0 653 mm but underestimated rates for coarse sand mean diam 0 992 mm

in situ zircon u pb and lu hf isotope systematic on magmatic rocks insights on the crustal evolution of the neoproterozoic goias magmatic arc brasilia belt central brazil abstract the mara rosa arc constitutes the northern portion of the goias magmatic arc an important tectonic component of the brasilia belt that represents the result of the convergence of the amazonian and san francisco cratons between 900 and 600 ma new in situ zircon u pb and lu hf isotope data by la mc icp ms on representative magmatic rocks permitted to characterize three neoproterozoic magmatic events in the mara rosa arc zircons from a metavolcanic rock yielded a u pb crystallization age of 916 5 ma with positive ehf values 8 to 12 suggesting that the host magma derived from a depleted mantle two orthogneisses showed heterogeneous zircon populations the youngest zircons from the two samples gave u pb crystallization ages of 792 8 ma and 811 7 ma respectively the abundant inherited zircons with meso to paleoproterozoic u pb ages showed for both samples negative ehf and old tdm values suggesting an important contribution of paleoproterozoic to archean crust in the petrogenesis of the host magmas zircons from post orogenic qz diorite yielded a crystallization age of 638 4 ma with a variation in ehf values 1 to 8 suggesting that the host magma involved both a juvenile and a recycled older crustal components

jurassic and cretaceous marine connections between the southeast pacific and tethys abstract the affinities of the ammonoid faunas from central west and southern south america are used to analyze the evolution of seaways and oceanic connections with tethys during the jurassic and cretaceous during the early and middle jurassic marine connection with eastern tethys was around the australian margin of gondwana from the pliensbachian onwards connection with western tethys occurred through the sporadically opened hispanic corridor in central america in late jurassic time the break up of gondwanaland was initiated and a seaway began to open between south america africa and antarctica connection of the eastern pacific with tethyan waters through southern africa was sporadic and was still mainly around australia and along the now open central american seaway throughout the early cretaceous the opening of the south atlantic proceeded from south to north and the first marine connection between the southern and northern atlantic took place in albian time a definite marine connection between the opened south atlantic and eastern tethys with faunal migration routes was established in late hauterivian barremian time and became fully open in the aptian albian successive stages in the opening of a south african seaway between tethys and the southeast pacific show an asymmetric development to the west and east of south africa during late cretaceous time oceanic circulation continued between the south atlantic and eastern tethys and true oceanic conditions developed between the southern and northern atlantic the antarctic peninsula was still located in a position adjacent to south america preventing open circulation between the south atlantic and pacific oceans

a steady state model of wind blown sand transport a model for steady state eolian saltation is presented in which computation of sand grain trajectories in the wind and of the wind velocity profile modified by saltating grain drag forces is combined with experimental data on grain bed collisions in an iterative simulation scheme which resembles the time development of natural saltation qualitative features of eolian saltation are reproduced in the simulations including a height in the saltating layer at which the wind velocity remains roughly constant with changing free stream friction velocity and an approximate separation of the saltating population into successively hopping grains and reptating or creeping grains the numerical solutions give rise to more general arguments in support of the conclusions that fluid entrainment of grains is unimportant in steady state saltation that the mass flux profile obeys roughly an inverse power law dependence on height and that the vertically integrated mass flux dependence on the free stream friction velocity

cyclostratigraphy of an orbitally driven tithonian valanginian carbonate ramp succession southern mendoza argentina implications for the jurassic cretaceous boundary in the neuquen basin we conducted detailed sedimentological sequence stratigraphic and cyclostratigraphic analyses on four sections of the vaca muerta formation this unit is characterized by a decimetre scale rhythmic alternation of marls and limestones it consists of five facies associations which represent basin to middle carbonate ramp deposits the rhythmic vertical organization shows a well ordered hierarchy of cycles where elementary cycles bundles of cycles and superbundles within the milankovitch frequency band are recognized cyclostratigraphic data allowed us to build a floating orbital scale for the lower tithonian lower valanginian interval in the neuquen basin orbital calibration of these sections is consistent with riccardi s biostratigraphic scheme which places the jurassic cretaceous boundary within the substeueroceras koeneni ammonite zone equivalent to durangites spp jacobi grandis and initial occitanica standard zones

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

electrification of saltating particles in wind blown sand experiment and theory 1 the electrification of sand particles is a common phenomenon in wind blown sand movement the charge to mass ratio of particles is an important parameter to characterize the property of charged particles however understanding of the charge to mass ratio remains open and significant discrepancies exist between previous contact electrification models and measurements this work focuses on the charge to mass ratio of saltating particles in wind blown sand experiments were carried out in a wind tunnel using sand particles with the mean diameter d 160 m the motions of the saltating particles in the air flow were analyzed by particle image velocimetry and the charge to mass ratios were determined by measuring both the charge and mass of saltating sand particles collected in a faraday cup the measured results show that the charge to mass ratio increases exponentially with the height above the surface and the vertical profiles of the charge to mass ratios can be reproduced well by considering the multiple collisions process between saltating particles and sand bed in addition the results suggest that the charge to mass ratio is determined by the number of collisions impacting velocity and sand grain diameter combined

aeolian particle flux profiles and transport unsteadiness vertical profiles of aeolian sediment flux are commonly modeled as an exponential decay of particle mass transport with height above the surface data from field and wind tunnel studies provide empirical support for this parameterization although a large degree of variation in the precise shape of the vertical flux profile has been reported this paper explores the potential influence of wind unsteadiness and time varying intensity of transport on the geometry slope curvature of aeolian particle flux profiles field evidence from a complex foredune environment demonstrates that i the time series of wind and sediment particle flux are often extremely variable with periods of intense transport referred to herein as sediment flurries separated by periods of weak or no transport ii sediment flurries contribute the majority of transport in a minority of the time iii the structure of a flurry includes a ramp up phase lasting a few seconds a core phase lasting a few seconds to many tens of seconds and a ramp down phase lasting a few seconds during which the system relaxes to a background low intensity transport state and iv conditional averaging of flux profiles for flurry and nonflurry periods reveals differences between the geometry of the mean profiles and hence the transport states that produce them these results caution against the indiscriminate reliance on regression statistics derived from time averaged sediment flux profiles especially those with significant flurry and nonflurry periods when calibrating or assessing the validity of steady state models of aeolian saltation

filling the gap new precise early cretaceous radioisotopic ages from the andes two tuffs in the lower cretaceous agrio formation neuquen basin provided u pb zircon radioisotopic ages of 129 09 0 16 ma and 127 42 0 15 ma both horizons are well constrained biostratigraphically by ammonites and nannofossils and can be correlated with the standard sequence of the mediterranean province the lower horizon is very close to the base of the upper hauterivian and the upper horizon to the hauterivian barremian boundary indicating that the former lies at c 129 5 ma and the latter at c 127 ma these new radioisotopic ages fill a gap of over 8 million years in the numerical calibration of the current global early cretaceous geological time scale

mid eocene giant slope failure sedimentary melanges in the ligurian accretionary wedge nw italy and relationships with tectonics global climate change and the dissociation of gas hydrates upper lutetian bartonian sedimentary melanges corresponding to ancient mud rich submarine mass transport deposits are widely distributed over an area c 300 km long and tens of kilometres wide along the exhumed outer part of the external ligurian accretionary wedge in the northern apennines the occurrence of methane derived carbonate concretions septarians in a specific tectonostratigraphic position below these sedimentary melanges allows us to document the relationships among a significant period of regional scale slope failure climate change the early and mid eocene optimum stages the dissociation of gas hydrates and accretionary tectonics during the ligurian tectonic phase early mid lutetian the distribution of septarians at the core of thrust related anticlines suggests that the dissociation of gas hydrates was triggered by accretionary tectonics rather than climate change the different ages of slope failure emplacement and the formation of the septarians support the view that the dissociation of gas hydrates was not the most important trigger for slope failure the latter occurred during a tectonic quiescence stage associated with a regressive depositional trend and probably minor residual tectonic pulses which followed the ligurian tectonic phase favouring the dynamic re equilibrium of the external ligurian accretionary wedge our findings provide useful information for a better understanding of the factors controlling giant slope failure events in modern accretionary settings where they may cause tsunamis

aeolian vertical mass flux profiles above dry and moist sandy beach surfaces abstract the vertical distribution of aeolian mass flux was investigated in a natural beach environment field experiments conducted on the beach of the eba barrier southern baltic coast poland measured the sand transport rate and the vertical mass flux distribution above dry rippled sand and a moist flat sandy surface the experiments were intended to show the changes in the vertical distribution of sand with changing wind speed all the data represent saturated flux conditions sand transport was measured using 0 5 m high vertically segmented passive sand traps while the wind speed and direction were monitored at 1 m elevation the obtained dataset comprises 65 measurements on dry surfaces and 51 measurements on moist sandy surfaces the sand transport rate above the moist surface was up to 90 higher than above the dry surface for wind speeds of 7 11 m s but higher velocities gave smaller differences between the surfaces the saltation layer was thicker above the moist surface than above the dry surface all the vertical sand flux profiles are best described by exponential decay functions analysis of the normalised flux profiles grouped by wind velocity shows that the fitted curves are less inclined for moist surfaces than dry surfaces moreover the regression coefficients depict a marked trend in which the intercept decreases and the slope increases with increasing wind speed this indicates that more sand is transported at higher elevations above the bed and less at lower elevations the proportion of total transport seems to be independent of wind speed at elevations of approximately 35 mm and 50 mm above the dry and moist surfaces respectively differences between the measured and exponential fit values of mass flux are particularly distinct close to the bed where the exponential fit either over or under predicts the measured values over predictions occur in weaker winds up to 6 7 m s whereas under predictions become more pronounced as the wind becomes stronger and when the layer in which the under prediction occurs thickens the under predictions are particularly obvious above the moist surface it is not clear whether this phenomenon is due to technique measurement error or to another source the proportion of creep is often considered to be responsible for the difference from the exponential fit which represents the saltation mode of aeolian transport this proportion is difficult to determine because the mass flux measured by the lowermost chamber of the sand trap used in the experiments represents sand transported in both saltation and creep or and reptation

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com