computational fluid dynamics as a tool for investigating separated flow in river bends the detailed three dimensional structure of the flow patterns in river bends with separated flow and factors controlling the existence and characteristics of these flow patterns are unclear at present it is shown here firstly how a computational fluid dynamics cfd program may be used to reproduce the qualitative features of the mean flow in a real river bend to allow testing of the model s capabilities it is then shown how the cfd code may be used to construct hypothetical channel bends which allow the experimentation necessary to investigate the controls on the extent of the separated flow

origin of chromite in mafic ultramafic hosted hydrothermal massive sulfides from the main uralian fault south urals russia abstract mafic ultramafic hosted hydrothermal fe cu ni co sulfide ores from the main uralian fault zone mufz south urals ivanovka and ishkinino ore fields contain a relatively large up to 3 proportion of chromite this association is common for magmatic fe ni cu sulfides but definitely unusual for hydrothermal sulfides textural morphological and compositional data are used here to gain an insight into the origin and significance of this unusual chromite sulfide association the studied chromites occur both as broken fragments and as euhedral or subhedral crystals which are included in the sulfides or scattered in their talc chlorite saponite quartz carbonate matrix they are characterized by high cr cr al ratios 0 58 0 85 and range in composition from magnesiochromite to chromite sensu stricto textural morphological and compositional features as well as the occurrence of relatively high silica low ti low k melt inclusions in some of the crystals indicate that the ore associated chromites i are a mixed population of grains derived from mafic ultramafic mantle and crustal magmatic rocks and mantle peridotite melting residua ii have no genetic relation with the host sulfides and iii represent relicts derived from the hydrothermally altered country rocks the compositions of the chromites and of the melt inclusions denote a clear supra subduction zone signature the melts parent to the cumulitic chromites had an arc tholeiitic to possibly boninitic affinity these data suggest that the host mafic ultramafic complexes formed in an early arc or forearc setting and do not represent obducted portions of morb oceanic lithosphere hence contrary to previous interpretations the associated massive sulfides could not originate on a mid ocean ridge but rather in an early arc or forearc environment given the relatively short life of the western uralian arc system the most probable time window for sulfide ore deposition is confined to early to middle devonian time

detrital and metamorphic 40 ar 39 ar ages from muscovite and whole rock samples meguma supergroup southern nova scotia 40ar 39ar dates from eight muscovite separates and two whole rock samples of slate siltstone and sandstone provide detrital metamorphic and post metamorphic overprinting ages for the low grade metamorphic rocks of the meguma supergroup in the mahone bay region of southern nova scotia variably reset detrital muscovite ages of 600 550 ma agree with previously obtained u pb dates from detrital zircon separates of foliation parallel muscovite yield precise 40ar 39ar ages of 395 388 ma petrographic and metamorphic data indicate that these are metamorphic mica growth ages the range overlaps with the younger limit of previously reported whole rock ages 410 380 ma but is more tightly constrained an apparent inconsistency between previous 40ar 39ar whole rock ages as old as 410 ma and regional deformation that affected sedimentary rocks as young as 385 ma has been resolved by the younger ages reported here and by recent adjustments to the early paleozoic time scale which suggest that the lochkovian to

silurian graptolite biostratigraphy in the welsh basin silurian graptolite zones are of markedly shorter duration than those of the ordovician averaging 0 8 ma several can be sub divided and further refinement will undoubtedly follow a revision of many of the graptolite species and their stratigraphic range locally integration of palaeontological and sedimentological studies facilitates correlation a premilinary subdivision of the crispus and griestoniensis zones in central wales is made

new constraint on the division of cambrian time a u pb zircon age of 530 7 0 9 ma for a volcanic ash in the lower placentian series of the avalonian terrane in southern new brunswick provides a precise point for calibration of early cambrian biologic evolution the ash is from the middle rusophycus avalonensis zone in saint john new brunswick this date is on subtrilobitic lower cambrian strata that may be approximately correlative with the sub tommotian manykaian stage of siberia and the lowest meishucunian stage of the south china platform this age when combined with published geochronologic data from the cambrian suggests that the manykaian stage may have lasted approximately 13 m y and that the tommotian may have been considerably shorter

genesis and evolution of the fumaroles of vulcano aeolian islands italy a geochemical model a geochemical model explaining the presence of fumaroles having different gas composition and temperature at the top of the crater and along the northeastern coast of vulcano island is proposed a pressurized biphase liquid vapor reservoir at the depth of about 2 km is hypothesized energy and mass balance sheets controlp t conditions in the system p t must vary along a boiling curve of brine as liquid is present the co2 content in the steam is governed by the thermodynamic properties of the fluids in the h2 nacl co2 system on the assumption that oxygen fugacity in the system is between the hm fmq oxygen buffers observed so2 h2s co2 co co ch4 ratios in the fumarolic gases at the fossa crater appear in equilibrium with a temperature higher than that observed such as may exist at depth the more reduced gas phases present on the sea side may result from re equilibrium processes in shallower aquifers the suggested model would help in monitoring changes in volcanic activity by analyzing fumarolic gases

helium 3 anomalies and crust mantle interaction in italy abstract the distribution of primordial 3he in the crust of italy has been investigated through a survey of groundwaters hot springs exploration and production geothermal wells the mantle derived component varies from 0 to 60 and reaches a maximum at vulcano in the eolian islands 3he anomalies occur where the conductive heat flow is relatively high but the relationships between heat flow and anomaly are uncertain the distribution of 3he in the main reservoir and cap rock to the larderello geothermal system shows that the mantle derived 3he varies from 5 to 40 of total he and reaches a maximum in fluids extracted from a depth of ca 3 km within the basement bomb produced tritiogenic 3he appears to make a near negligible contribution well 107 at larderello has been monitored over a period of 17 months and the 3 he 4 he ratio r varies from 1 4 to 1 6 times the atmospheric ratio ra whilst he ne varies from 9 to 160 a model is proposed for the larderello geothermal system whereby mantle derived heat and volatiles are advected to shallow 6 8 km depths in association with melts and released into the main geothermal reservoir at larderello in a non uniform manner possibly associated with fracturing isotopic and elemental equilibrium of gaseous species c o h appears to have taken place in the basement to the main reservoir from where fluid with r ra 3 2 has been extracted

calibrating the devonian time scale a synthesis of u pb id tims ages and conodont stratigraphy the recalibrated devonian time scale represents an important improvement of devonian chronology regarding two aspects firstly a data set of 13 biostratigraphically well bracketed u pb id tims zircon and monazite ages constitutes the framework of the scale secondly approximately time linear biostratigraphic scales have been used for the interpolation between the isotopic ages the new construction thus represents a biochronometric time scale which allows the assignment of numeric ages not only to stage boundaries but also to each biozone boundary the method of interpolation applied also enables the projection of the geochronological error onto the time scale according to the newly calibrated scale the devonian lasted 57 4 5 7 ma from 418 1 3 0 to 360 7 2 7 ma this is compared to previously published time scales the longest time span ever calculated for this period the age interpolations for the eight stage boundaries are famennian tournaisian devonian carboniferous 360 7 2 7 ma frasnian famennian 376 1 3 6 ma givetian frasnian 383 7 3 1 ma eifelian givetian 388 1 2 6 ma emsian eifelian 391 9 3 4 ma pragian emsian 409 1 3 8 ma lochkovian pragian 412 3 3 5 ma pridolian lochkovian silurian devonian 418 1 3 0 ma full size table table options view in workspace download as csv the duration of the middle devonian is calibrated here as quite short with 8 2 ma and the emsian and the famennian are the longest stages with interpolated durations of 17 2 and 15 4 ma respectively together all devonian stages contain 57 conodont zones including subzones according to the standard conodont zonation giving a mean duration of about 1 ma in the highly resolved part of the time scale mid eifelian to the devonian carboniferous boundary zonal resolution averages 0 6 ma in contrast the lowest resolution is shown in the early emsian to mid eifelian interval with zonal durations of up to 5 5 ma serotinus zone

computational fluid dynamics modelling of boundary roughness in gravel bed rivers an investigation of the effects of random variability in bed elevation results from a series of numerical simulations of two dimensional open channel flow conducted using the computational fluid dynamics cfd code fluent are compared with data quantifying the mean and turbulent characteristics of open channel flow over two contrasting gravel beds boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra grid scale roughness elements e g particle clusters and small bedforms results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed typically at y h 0 1 0 3 this is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed use of the random elevation model to represent supra grid scale roughness also allows a reduction in the height of the near bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness despite these benefits the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution increasing the horizontal mesh resolution results in an increase in the near bed velocity gradient and turbulent kinetic energy effectively roughening the bed varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles but results in substantial changes to the shape of the turbulent kinetic energy profile these findings have significant implications for the application of cfd within natural gravel bed channels particularly with regard to issues of topographic data collection roughness parameterization and the derivation of mesh independent solutions copyright 2001 john wiley sons ltd

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

modelling hydrodynamics in the rio parana argentina an evaluation and inter comparison of reduced complexity and physics based models applied to a large sand bed river abstract depth averaged velocities and unit discharges within a 30 km reach of one of the world s largest rivers the rio parana argentina were simulated using three hydrodynamic models with different process representations a reduced complexity rc model that neglects most of the physics governing fluid flow a two dimensional model based on the shallow water equations and a three dimensional model based on the reynolds averaged navier stokes equations flow characteristics simulated using all three models were compared with data obtained by acoustic doppler current profiler surveys at four cross sections within the study reach this analysis demonstrates that surprisingly the performance of the rc model is generally equal to and in some instances better than that of the physics based models in terms of the statistical agreement between simulated and measured flow properties in addition in contrast to previous applications of rc models the present study demonstrates that the rc model can successfully predict measured flow velocities the strong performance of the rc model reflects in part the simplicity of the depth averaged mean flow patterns within the study reach and the dominant role of channel scale topographic features in controlling the flow dynamics moreover the very low water surface slopes that typify large sand bed rivers enable flow depths to be estimated reliably in the rc model using a simple fixed lid planar water surface approximation this approach overcomes a major problem encountered in the application of rc models in environments characterised by shallow flows and steep bed gradients the rc model is four orders of magnitude faster than the physics based models when performing steady state hydrodynamic calculations however the iterative nature of the rc model calculations implies a reduction in computational efficiency relative to some other rc models a further implication of this is that if used to simulate channel morphodynamics the present rc model may offer only a marginal advantage in terms of computational efficiency over approaches based on the shallow water equations these observations illustrate the trade off between model realism and efficiency that is a key consideration in rc modelling moreover this outcome highlights a need to rethink the use of rc morphodynamic models in fluvial geomorphology and to move away from existing grid based approaches such as the popular cellular automata ca models that remain essentially reductionist in nature in the case of the world s largest sand bed rivers this might be achieved by implementing the rc model outlined here as one element within a hierarchical modelling framework that would enable computationally efficient simulation of the morphodynamics of large rivers over millennial time scales

isotopic composition of natural thermal discharges on vulcano island southern italy this paper reviews the isotopic 18o16o and d h ratios reported in the literature for phreatic waters in the plain of vulcano island off sicily and condensates from high temperature fumaroles at the top of the summit crater the vulcano phreatic waters have a rather high average annual temperature greater than 30 dgc but do not seem to be heated directly by mixture with condensates of fumarolic steam they seem to originate from rainwater and to undergo high evaporation particularly in summer owing to high conductive heat flow in the island by comparing the 18o16o and d h ratios with cl as hcl content it is estimated that 75 of the steam from the high temperature fumaroles comes from seawater and 25 from low salinity waters the latter however do not seem to come directly from the shallow aquifer of vulcano because the steam from the fumaroles does not contain radiogenic tritium this mixing model and the lack of steam condensation in the vulcano aquifer implies indirectly that there are no steam losses during the rise of deep fluids and that some observed variations in the outlet temperatures without accompanying variations in the composition of the associated gas phase may be due to increased or decreased extent of fracturation in the volcanic edifice as a result for instance of tectonic movements which cause more or less vaporization of low salinity fresh water into the volcanic ducts as well as faster or slower outflow of deep gases

application of a roughness length representation to parameterize energy loss in 3 d numerical simulations of large rivers 1 recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers however measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach length scales that can be monitored similar constraints exist for computational fluid dynamics cfd studies of large rivers requiring maximization of mesh or grid cell dimensions and implying a reduction in the representation of bedform roughness elements that are of the order of a model grid cell or less even if they are represented in available topographic data these subgrid elements must be parameterized and this paper applies and considers the impact of roughness length treatments that include the effect of bed roughness due to unmeasured topography cfd predictions were found to be sensitive to the roughness length specification model optimization was based on acoustic doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths this proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness length treatments however the general spatial flow patterns are effectively predicted by the model changes in roughness length were shown to have a major impact upon flow routing at the channel scale the results also indicate an absence of secondary flow circulation cells in the reached studied and suggest simpler two dimensional models may have great utility in the investigation of flow within large rivers

the numerical age of the devonian carboniferous boundary the shrimp ion microprobe has been used to date zircons in a 1 cm thick bentonite located in the hasselbachtal auxiliary global stratotype section through the devonian carboniferous boundary in germany and a tuff located at a similar biostratigraphic level in australia multiple replicate analyses have yielded indistinguishable ages and indicate 353 2 4 0 ma 2 as the age of the boundary obtaining an age for the boundary in the german reference section depended on the ability of shrimp to date a small sample and to target small areas of structurally complex zircons future refinements of the age of this boundary may depend on re analysis of this uniquely placed bentonite and this highlights the value of defining biostratigraphic stratotypes in the vicinity of known dateable horizons

the durations of silurian graptolite zones recently published radiometric data show the duration of the silurian period to be approximately 32 ma significantly longer than previous estimates radiometric and graphic correlation studies demonstrate considerable variation in the durations of the four silurian epochs approximately 13 ma for the llandovery 4 ma for the wenlock 5 ma for the ludlow and 10 ma for the p idoli using this new geochronological data figures for average graptolite zone duration are presented for each epoch ranging from as little as 0 44 ma in the wenlock to 1 43 ma in the p idoli the figures provide a refined subdivision of the silurian timescale of value to studies which use silurian graptolite zones to calibrate geological processes

geothermal resources within carbonate reservoirs in western sicily italy a review abstract low to medium temperature fluid reservoirs hosted in carbonate rocks are some of the most promising and unknown geothermal systems western sicily is considered a key exploration area this paper illustrate a multidisciplinary and integrated review of the existing geological geochemical and geophysical data mainly acquired during oil and gas explorations since the 1950s specifically re analyzed for geothermal purposes has led to understanding the western sicily geothermal system as a whole and to reconstructing the modalities and particular features of the deep fluid circulation within the regional reservoir the data review suggests the presence of wide groundwater flow systems in the reservoir beneath impervious cap rocks we identified the main recharge areas reconstructed the temperature distribution at depth recognized zones of convective geothermal flow and depicted the main geothermal fluid flow paths within the reservoir we believe that our reconstruction of geothermal fluid circulation is an example of the general behavior of low to medium enthalpy geothermal systems hosted in carbonate units on a regional scale due to the recent technological developments of binary plants these systems have become more profitable not only for geothermal direct uses but also for power production

evolution of fumarolic gases boundary conditions set by measured parameters case study at vulcano italy physical chemical and isotopic parameters were measured in fumaroles at the vulcano crater and in drowned fumaroles near the beach the data were used to define boundary conditions for possible conceptual models of the system crater fumaroles time variations of co2 and so2 concentrations indicate mixing of saline gas rich water with local fresh water cl br ratios of 300 400 favour sea water as a major source for cl brand part of the water in the fumaroles cl concentrations and d values revealed independently amixing of 0 75 sea water with 0 25 local freshwaterin furmarole f 5 during september 1982 patterns of parameter correlation and mass balances reveal that co2 s nh3 and b originate from sources other than sea water the co2 value of 13c 2 o favours at least partial origin from decomposition of sedimentary rocks rather than mantle derived material radiogenic4he 1 3 lo 3 ccstp g water and radiogenic40ar 10 6 10 4 ccstp g water are observed 4he 40ar radiogenic 1 2 well in the range of values observed in geothermal systems drowned fumaroles strongly bubbling gas at a pond and at the beachappears to have the same origin and initial compositionas the crater fumaroles 2 km away the fumarolic gas is modified by depletion of the reactive gases caused by dissolution in shallow water atmospheric ne ar kr and xe are addeden route some radiogenic he and ar are maintained the vulcano system seems to be strongly influenced by the contribution of sea water and decomposition of sedimentary rocks evidence of magmatic contributions is mainly derived from heat

an appraisal of the velocity reversal hypothesis for stable pool riffle sequences in the river severn england the velocity reversal hypothesis is the linchpin for a number of recent conceptual models pertaining to sediment sorting and the maintenance of pool riffle sequences in gravel bedded streams the literature in support of the hypothesis together with published adverse criticism is reviewed it is concluded that convincing evidence for the ubiquitous occurrence of such a reversal in a range of channel geometries is currently unavailable continuity considerations indicate that riffles need to be considerably wider than pools for a reversal in the mean velocity to occur under conditions of subcritical flow high stage and stable morphology these observations are substantiated by a detailed study of the hydraulic geometry of stable pool riffle sequences in the river severn england neither the sectionally averaged velocity nor the near bed shear velocity is sensibly greater in the pools than over the riffles during bankfull or near bankfull flow instead a tendency towards equalization of the values of average hydraulic variables is noted as discharge increases a detailed investigation of the three dimensional character of the flow is required to demonstrate whether the entrainment forces within pools can locally exceed those over neighbouring riffles unusual behaviour of the energy gradient over riffles during moderate discharges is related to backwater effects as mediated by the spacing of the riffles the hydraulic data are used to comment on the stability of the test reaches in the context of the development of the river severn in the vicinity of shrewsbury

the detroit river group is middle devonian discussion on early devonian age of the detroit river group inferred from arctic stromatoporoids in their paper on the age of the detroit river group the authors e c prosh and c w stearn suggest that the occurrence of two to four species of stromatoporoids in the detroit river which are also known to occur in well dated lower devonian arctic formations outweighs the evidence of conodonts and goniatites that is currently used to date the detroit river as mostly or entirely middle devonian the conodont and goniatite evidence is much stronger than indicated by the authors and too strong to be set aside in response to their new data correlation difficulties are attributable to the known endemism of the eastern north american early and middle devonian faunas and to environmental differences

timing of acadian deformation in northeastern maine 40ar 39ar ages determined for 24 mineral separates from the pleasant lake hunt ridge and cochrane lake plutons bear on the timing of the main phase of the acadian orogeny in northern maine six hornblende samples and one clinopyroxene sample have only minor disturbances in their release spectra the increments with concordant ages form overlapping plateaus at 403 4 ma many of the biotite samples have discordant release spectra the average of their total gas ages is 400 6 ma calculated closure temperatures for the hornblendes and biotites are 550 c and approximately 350 c respectively the difference between closure temperatures and concordancy in ages demonstrates that the plutons cooled very rapidly upon emplacement therefore the 40ar 39ar ages date both the time of argon retention in the minerals and the time of emplacement of the plutons the best estimate for their time of emplacement is the average of the combined ages of the micas hornblendes and clinopyroxene 402 6 ma the plutons are undeformed and crosscut upright open acadian folds in the carys mills smyrna mills and madrid formations the age of the pleasant lake hunt ridge and cochrane lake plutons therefore places a lower limit of 402 6 ma on the time of acadian folding in northern maine

the absolute age of the eifelian tioga ash bed pennsylvania the tioga ash bed bed b of the zeigler pit union county pennsylvania lies in the polygnathus costatus costatus zone of the eifelian stage of the middle devonian series the 27 5 cm thick bed contains igneous zircon and monazite crystals that have been dated by precise u pb techniques zircons are characterized by inherited pb that occurs as barely perceptible interior regions enriched in inclusions the zircons are discordant and do not reveal the igneous age of the bed in contrast high quality monazite crystals lack any evidence of inheritance or loss of pb and multiple analyses have overlapping 207 pb 235 u ages of 390 0 pm 0 5 ma the monazite analyses are concordant or very slightly reversely discordant the consistency of the monazite data provides a new precise estimate of age for the lower eifelian

stratigraphy and eruptive history of the late devonian mount pleasant caldera complex canadian appalachians stratigraphic petrographic and geochemical evidence indicate that the volcano sedimentary rocks of the late devonian piskahegan group located in the northern appalachians of southwestern new brunswick represent the eroded remnants of a large epicontinental caldera complex this complex the mount pleasant caldera is one of few recognizable pre cenozoic calderas and is divisible into exocaldera intracaldera and late caldera fill sequences the intracaldera sequence comprises four formations that crop out in a triangular shaped area and includes thick ash flow tuffs thick sedimentary breccias that dip inward and stocks of intermediate to felsic composition that intrude the volcanic pile or are localized along caldera margin faults the exocaldera sequence contains ash flow tuffs mafic lavas alluvial redbeds and porphyritic felsic lavas that comprise five formations the late caldera fill sequence contains rocks that are similar to those of the outflow facies and comprises two formations and two minor intrusive units geochemical and mineralogical data support the stratigraphic subdivision and indicate that the basaltic rocks are mantle derived and have intraplate chemical affinities the andesites were probably derived from basaltic magma by fractional crystallization and assimilation of crustal material the various felsic units are related by episodes of fractional crystallization in a high level zoned magma chamber fractionation was repeatedly interrupted by eruption of material from the roof zone such that seven stages of caldera development have been identified the genesis of the caldera is related to a period of lithospheric thinning that followed the acadian orogeny in the northern appalachians

new uppermost cambrian u pb date from avalonian wales and age of the cambrian ordovician boundary a crystal rich volcaniclastic sandstone in the lower peltura scarabaeoideszone at ogof ddu near criccieth north wales yields a u pb zircon age of 491 1 ma this late late cambrian date indicates a remarkably young age for the cambrian ordovician boundary whose age must be less than 491 ma hence the revised duration of the post placentian trilobite bearing cambrian indicates that local trilobite zonations allow a biostratigraphic resolution comparable to that provided by ordovician graptolites and mesozoic ammonites

chronometric calibration of mid ordovician to tournaisian conodont zones a compilation from recent graphic correlation and isotope studies three available graphic correlation analyses are used to calibrate mid palaeozoic conodont zonations sweet s scheme for the mid to upper ordovician kleffner s for the mid to upper silurian and murphy berry s for the lower and middle lower devonian the scheme of sweet is scaled by applying the high precision u pb zircon date of tucker and others for the rocklandian and linked with that of kleffner by scaling the graptolite sequence of the ordovician silurian global stratotype section to fit two similarly derived dates from this sequence the top of kleffner s scheme all of murphy berry s as well as standard zones to the frasnian are calibrated by using tie points of the latest cambridge bp time scale gts 89 however the recent microbeam zircon date by claoue long and others for the hasselbachtal devonian carboniferous auxiliary stratotype is used to calibrate the standard famennian zones also the similarly derived but preliminary determination reported by roberts and others from the isismurra formation of new south wales is tentatively taken as the top of the tournaisian and so used to calibrate tournaisian zones despite the considerable extrapolation required to compile these schemes and their inherent errors the resultant time scale closely agrees with other dates of tucker and others from the llanvirn as well as the gts 89 homerian gorstian tie point this suggests that stratigraphic methods can be usefully applied to geochronometry the llandovery appears to have lasted longer 16 m y than usually envisaged and the ordovician silurian boundary may need to be lowered to approximately 443 5 ma certainly chrons varied widely in duration and further stratigraphic studies to estimate their relative durations as well as high resolution dating for their calibration will be crucial to more accurate biochronometries

slope induced changes in channel character along a gravel bed stream the allt dubhaig scotland changes in channel character along a small river in the scottish highlands are described using measurements in seven reaches over a 3 km length with no significant tributaries but a decline in slope from 0 02 to o ooo15 because of local baselevel control this decline in slope is associated with rapid downstream fining of the gravel bed followed by an abrupt transition to a sand bed the channel pattern alters progressively rather than abruptly in the sequence 1 near braided 2 meandering with active point bar chutes 3 meandering with active outer bank talweg 4 stable equiwidth sinuous the changes in channel pattern and hydraulic geometry are predicted better by rational approaches based on critical shear stress or other physical cdncepts than by purely empirical discriminant or trend equations measurements in five reaches confirm a downstream decrease in shear stress and the amount and calibre of bedload it is argued that the downstream changes in channel character in this stream are induced by profile concavity inherited from deglacial conditions are typical of many streams in mountainous areas and can be understood in terms of slope induced changes in hydraulic properties

implications of spore evidence for late devonian age of the piskahegan group southwestern new brunswick the predominantly volcanic piskahegan group has commonly been considered early carboniferous based on its stratigraphic position however spores recently discovered in the carrow formation an alluvial fan deposit in the exocaldera facies indicate that most if not all of the group is of late devonian late famennian age the spore assemblage includes several species reported previously from ireland belgium and eastern europe some of them apparently restricted to the southern parts of the old red sandstone continent in late devonian time comparison of records of earliest occurrences suggests that the incoming of some species was diachronous volcanic rocks of the piskahegan group are coeval with post acadian tin tungsten bearing granites elsewhere in new brunswick and are considered the surface expression of plutonism that resulted from acadian continental collison

numerical simulation of hydrodynamics and bank erosion in a river bend we present an integrated analysis of bank erosion in a high curvature bend of the gravel bed cecina river central italy our analysis combines a model of fluvial bank erosion with groundwater flow and bank stability analyses to account for the influence of hydraulic erosion on mass failure processes the key novel aspect being that the fluvial erosion model is parameterized using outputs from detailed hydrodynamic simulations the results identify two mechanisms that explain how most bank retreat usually occurs after rather than during flood peaks first in the high curvature bend investigated here the maximum flow velocity core migrates away from the outer bank as flow discharge increases reducing sidewall boundary shear stress and fluvial erosion at peak flow stages second bank failure episodes are triggered by combinations of pore water and hydrostatic confining pressures induced in the period between the drawdown and rising phases of multipeaked flow events

numerical simulation of three dimensional flow hydraulics in a braided channel results are presented from a numerical simulation of three dimensional flow hydraulics around a mid channel bar carried out using the fluent uns computational fluid dynamics cfd software package fluent uns solves the three dimensional reynolds averaged form of the navier stokes equations turbulence closure is achieved using a rng k model simulated flow velocities are compared with measured two dimensional velocities downstream and cross stream obtained using an electromagnetic current meter ecm the results of the simulation are qualitatively consistent with the flow structures observed in the field quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two r 0 88 and a mean difference of 0 09 m s 1 deviations between simulated and measured velocities may be identified that are both random and systematic the former may reflect a number of factors including subgrid scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ecm orientation model mesh configuration roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities these results illustrate the potential for using cfd software to simulate flow hydraulics in natural channels with complex configurations they also highlight the need for detailed spatially distributed datasets of three dimensional flow variables to establish the accuracy and applicability of cfd software copyright 1999 john wiley sons ltd

organic and inorganic geochemistry of low temperature gas discharges at the baia di levante beach vulcano island italy abstract discharge from subaereal and submarine gas vents of the baia di levante beach gases from the vulcano island were sampled for major and trace gas components in may and november 1995 chemical compositions and equilibrium calculations suggest three different groups of co 2 rich gas emissions depending on their distance from the la fossa crater 1 gas vents close to the faraglione area are characterised by high h 2 s contents high calculated equilibrium temperatures based on inorganic species and relatively high proportion of alkene compounds 2 gas vents close to vulcanello are characterised by low calculated equilibrium temperatures and low amounts of alkenes and 3 pontile sample has the highest equilibrium co 2 pressure up to 68 bars which may account for the observed absence of benzene the relative large variability of h 2 s in the baia di levante beach gas discharge may be attributed to either different interactions between iron sulphides and weakly acid waters or catalytic effect of elemental sulphur on the de hydrogenation of cyclo hexane thermodynamic calculations suggest that the main inorganic species and ch 4 may have re equilibrated at relatively shallow depth 10 200 m b s l and 30 600 m b s l for a lithostatic and hydrostatic pressure respectively the slow kinetics of reactions in the c n h 2 n c n h 2 n 2 systems with respect to that of ch 4 co co 2 may explain the observed propene propane ratios which can only be reached at reaction temperatures of 300 350 c this low speed of reactions can also explain the observed disequilibrium of c1 c4 alkanes

the application of computational fluid dynamics to natural river channels three dimensional versus two dimensional approaches this paper addresses the recent trend in fluvial geomorphology towards using computational fluid dynamics to explore the adjustment between flow processes sediment transport and river channel morphology it aims to evaluate the extent to which three dimensional 3d models improve predictive ability and prediction utility compared to two dimensional 2d applications this is achieved through comparing the predictions of both 3d and 2d models with high quality field data identical boundary conditions obtained from a confluence within a gravel bed river system with high relative roughness are defined for each model evaluation of the 3d model suggests that there is a fundamental limitation upon model predictive ability due to problems of specifying topographic complexity however comparison with the 2d model shows that the 3d model has a higher predictive ability particularly if the 2d model is not corrected for the effects on flow structure of secondary circulation further the 3d model provides more reliable estimates of bed shear stress and other more useful information such as the three dimensional flow field important for mixing processes this suggests that there is significant merit in the move towards 3d models but that research is required to incorporate methods developed in other fields for dealing with boundary condition uncertainties

helium isotopes and tectonics in southern italy geodynamic evolution of southern italy can be understood within the framework of the mediterranean alpine system subduction of a plate along the sicily calabrian forearc under the tyrrhenian sea has been suggested by many geophysicists although it is not yet confirmed and remains somewhat controversial helium isotope ratios provide useful information on the geotectonic structure of the region the authors report here the sup 3 h sup 4 he ratios of terrestrial gas samples from southern italy the observed sup 3 he sup 4 he ratios are relatively high in the eolian volcanic arc region and low in the other areas dichotomous explanations are presented firstly volcanic arc forearc hypothesis suggests the subduction along the sicily calabrian forearc secondly horizontal transport hypothesis is described based on the relationship between the ratios and radial distance from the recent spreading basin in southern tyrrhenian sea