puzzling distribution of the fossil and living genus hiatella bivalvia abstract the bivalve hiatella has been present for the last 150 ma and is consistently linked to cool temperate and polar regions its origin can be traced back to at least the jurassic with earliest known records being temperate forms that occurred on the northern side of the tethys the subsequent fossil record suggests two possible alternative migration routes the more plausible one is that hiatella invaded the southern seas from the atlantic by the eocene from there it migrated northward to eastern asia with first valid north pacific records in the far east region of russia japan and alaska from the oligocene to miocene a second explanation is that during the mesozoic hiatella penetrated into the circumpolar zone and reached the boreal pacific province before the arctic became isolated in the late cretaceous 60 ma due to the formation of beringia as its subsequent oldest records in the north pacific are from the oligocene to miocene hiatella probably evolved during the paleogene in the north pacific and migrated to southern latitudes antarctica southern south america new zealand and australia as connecting seaways formed in the trans pacific route in any case the north pacific lineage has been separated from the circum atlantic lineage of hiatella since at least the opening of the bering strait 3 0 3 5 ma during the late pliocene when it probably participated in the trans arctic migration during the late pliocene pleistocene hiatella evolved adopting its present widespread cold temperate distribution in both hemispheres although best developed in the polar regions of the northern hemisphere hiatella probably achieved extinction resistance by virtue of broad geographic distributions associated with dispersal facilities and great plasticity in its habitat and mode of life

application of k ϵ turbulence models to enclosed basins the role of internal seiches 1 a numerical model was developed for the prediction of the density stratification of lakes and reservoirs it combines a buoyancy extended k model with a seiche excitation and damping model to predict the diffusivity below the surface mixed layer the model was applied to predict the seasonal development of temperature stratification and turbulent diffusivity in two medium sized lakes over time periods ranging from 3 weeks to 2 years depending on the type of boundary condition for temperature two or three model parameters were optimized to calibrate the model the agreement between the simulated and the observed temperature distributions is excellent in particular if lake surface temperatures were prescribed as surface boundary condition instead of temperature gradients derived from heat fluxes comparison of different model variants revealed that inclusion of horizontal pressure gradients and or stability functions is not required to provide good agreement between model results and data with the aid of uncertainty analysis it is shown that the depth of the mixed surface layer during the stratified period could be predicted accurately within 1 m the sensitivity of the model to several parameters is discussed index terms 4211 oceanography general benthic boundary layers 4255 oceanography general numerical modeling 4568 oceanography physical turbulence diffusion and mixing processes keywords lake turbulence model seiche stratification simulation turbulence kinetic energy

three dimensional modeling of negatively buoyant flow in diverging channels diverging channels also known as diffusers represent common natural and industrial outlets to lakes reservoirs and rivers if the outflow in a diffuser has a larger density than the ambient water the inflow may plunge and form a density underflow in this paper a three dimensional numerical study was conducted to gain insight into the mechanism of negatively buoyant flows in diffusers with a sloping bottom of particular interest is the formation of separated flows such as wall jet and free jet flows various cases of plunging and the associated density current in a diffuser with different divergence angles and inflow densimetric froude numbers are considered the model successfully simulates the formation of attached flow wall jets and free jets in a negatively buoyant environment the onset evolution and stabilization of a stall and the subsequent development of a wall jet in a negatively buoyant flow are investigated in detail computed results also show favorable agreement with some published experimental data on density current generated by the plunging of cold water in ambient warm water in a diverging channel

subcritical junction flow the main flow features of subcritical junction flows are explored based on an extended hydraulic model study the flow conditions chosen resulted in flows that are governed by the froude similarity law so called simple junctions with junction angles of 30 60 and 90 were tested the main emphasis was made in determining the characteristics of the lateral flow and the flow contraction in the tailwater branch further expressions for the momentum correction coefficients the lateral wall pressure force and the ratio of flow depths in the lateral and upstream branches were provided a rational approach for the momentum contribution of the lateral branch is presented and applied for the prediction of the backwater effect across a simple junction the complex flow pattern is further documented with selected photographs such that a rather complete description of junction flow is now available

bed morphology and sedimentology at the confluence of unequal depth channels abstract spatial and temporal variations in the bed geometry and bed material size of a sand bed river confluence with unequal depth channels were monitored during a sequence of floods additionally two other sand bed confluences were surveyed to test the replicability of these observations the confluences studied here have only one avalanche face which corresponds to the front of a tributary mouth bar in the shallower channel and do not exhibit a marked scour zone three distinct morpho sedimentological zones are present 1 an area around the upstream corner of the junction where the sediments are generally finer than the mean 2 a maximum depth zone with coarser than average particles and 3 a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream changes in relative discharge between the two channels had little effect on the grain size of the downstream junction corner bar but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar the downstream junction corner bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels the contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone

spinifex biogeochemical expressions of buried gold mineralisation the great mineral exploration penetrator of transported regolith a major challenge for mineral explorers is to efficiently detect mineralisation beneath the weathered cover that extends across the landscape determination of the elemental composition of plants biogeochemical exploration can aid in the detection of buried ore deposits due to their root penetration through the weathered cover at the coyote prospect in western australia a range of plant species were sampled traversing a buried au orebody here it is shown that the soft spinifex triodia pungens accumulated important pathfinder elements related to the mineralisation which produced a multi element surficial expression of the underlying ore deposit spinifex grasses are one of the most widespread grasses over the australian continent and have vertical root systems that can extend for many 10s of metres through sedimentary cover and interact with buried mineral deposits and alteration zones in the underlying substrate spinifex biogeochemistry therefore has potential to be a low cost low environmental impact sampling medium for mineral exploration programs across large parts of arid australia the examination of similar grasses from other semi arid and arid regions from around the world therefore also warrants further investigation

the space time relationship of taxonomic diversity and morphological disparity in the middle jurassic ammonite radiation the middle jurassic ammonite radiation from the late aalenian to the end of the mid bathonian is traced using combined analyses of morphological disparity and taxonomic diversity the global signals of disparity and diversity are compared these signals are then broken down by paleogeographical provinces to detect any heterogeneity in the radiation an examination of the global signals reveals three biodiversity crises discordances between signals where morphological disparity grows while taxonomic diversity declines the subdivision of the signals indicates the radiation was heterogeneous between provinces the global signal is an aggregate of signals from each province the three biological crises have different paleogeographical signatures the first is visible in a few provinces only while the other two are visible in most provinces first order crises can be distinguished from second order ones by studying the number of paleogeographical provinces affected

assessing the record and causes of late triassic extinctions accelerated biotic turnover during the late triassic has led to the perception of an end triassic mass extinction event now regarded as one of the big five extinctions close examination of the fossil record reveals that many groups thought to be affected severely by this event such as ammonoids bivalves and conodonts instead were in decline throughout the late triassic and that other groups were relatively unaffected or subject to only regional effects explanations for the biotic turnover have included both gradualistic and catastrophic mechanisms regression during the rhaetian with consequent habitat loss is compatible with the disappearance of some marine faunal groups but may be regional not global in scale and cannot explain apparent synchronous decline in the terrestrial realm gradual widespread aridification of the pangaean supercontinent could explain a decline in terrestrial diversity during the late triassic although evidence for an impact precisely at the boundary is lacking the presence of impact structures with late triassic ages suggests the possibility of bolide impact induced environmental degradation prior to the end triassic widespread eruptions of flood basalts of the central atlantic magmatic province camp were synchronous with or slightly postdate the system boundary emissions of co2 and so2 during these eruptions were substantial but the contradictory evidence for the environmental effects of outgassing of these lavas remains to be resolved a substantial excursion in the marine carbon isotope record of both carbonate and organic matter suggests a significant disturbance of the global carbon cycle at the system boundary release of methane hydrates from seafloor sediments is a possible cause for this isotope excursion although the triggering mechanism and climatic effects of such a release remain uncertain d 2003 elsevier b v all rights reserved

causes of rapid mixing at a junction of two large rivers rio parana and rio paraguay argentina 1 airborne and satellite observations show that when large rivers join they can take hundreds of kilometers to mix completely but on occasion may mix very rapidly application of established semitheoretical analyses shows that long mixing lengths should be expected the first measurements of mixing processes at a large river junction rio parana and rio paraguay argentina combined width 2 8 km are presented at two occasions first when they mix in 400 km and second when mixing is complete in only 8 km downstream of the junction for the case of slower mixing at a point surveys showed that mixing driven by turbulent shear associated with a near vertical shear layer was restricted to close to the junction to 0 272 multiples of the postconfluence width downstream transect surveys showed penetration of more turbid water from the rio paraguay underneath the rio parana but this was insufficient to promote more rapid mixing there was no clear channel scale circulation present and slow mixing was compounded by reverse topographic forcing on the mainstream rio parana side of the river this kept more turbid water on the rio paraguay side of the river close to the bed in the case of rapid mixing we found clear channel scale circulation the momentum ratio between the combining flows reinforced the effects of the discordance in bed height between the tributaries at the confluence and allowed penetration of more turbid rio paraguay water further across the channel width deeper within the flow the importance of the interaction between momentum ratio and bed morphology at channel junctions makes mixing rates at the confluence dependent upon basin scale hydrological response which is more likely to differ between large confluent rivers than small rivers as a result of the different climatic topographic zones that they may capture

tritium as an indicator of recharge and dispersion in a groundwater system in central ontario the detailed distribution of tritium 3h in the recharge area of a shallow unconfined sand aquifer near sturgeon falls ontario is described at this forested shallow water table site bomb tritium has penetrated uniformly to a depth of 8 12 m indicating recharge of 15 cm year which is 16 of precipitation the zone of bomb tritium contains 3h concentrations of from 16 to 269 tritium unit tu whereas much lower values 1 tu are observed in groundwaters recharged only a few years prior to 1953 the year when significant fallout of bomb tritium began tritium distribution is accurately simulated using one and two dimensional models when the ottawa record is used for the post 1953 tritium input function when dispersion is low and when a prebomb tritium input value of 3 tu is used this prebomb value is slightly lower than that suggested by kaufman and libby 1954 based on precipitation and surface water samples from the early 1950s the simulations indicate and field data corroborate that in 1986 3h levels of more than 100 tu occur only in groundwaters recharged between 1957 and 1971 while levels in excess of 200 tu occur only in groundwaters recharged during 1961 1967 the 1960s tritium peak is observed within a narrow distinct depth zone at all locations along the flow section investigated

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

marine biodiversity through the late cenomanian early turonian palaeoceanographic controls and sequence stratigraphic biases changes in the marine macro and microfauna sedimentary geochemistry and surface water palaeoproductivity through the last 500 000 years of the cenomanian and first 300 000 years of the turonian are documented these are based on the succession at eastbourne the thickest and most complete section through the late cenomanian and early turonian in the anglo paris basin two levels of rapid faunal and geochemical change are identified one coincident with a significant increase in siliciclastic input at the base of the plenus marls member and the other with a marked drop in surface water productivity near the top of the same unit faunal change is demonstrated to be largely a pattern of immigration emigration rather than true extinction and our sequence stratigraphical analysis shows that it was coincident with major sea level changes no evidence is found to support the hypothesis that reduced bottom water oxygenation developed and was responsible for extinctions amongst the benthos in mid shelf environments the onset of pure chalk facies is interpreted to mark the breakdown of shelf break fronts and the spread of oligotrophic oceanic waters over much of the continental shelf initiated by rising sea level the cenomanian turonian event far from recording a mass extinction of shelf fauna is most probably an artifact caused by a significant switch in the nature of the surviving sedimentary record as a result of a major but perfectly ordinary oceanographic change

palaeobiogeography and the ordovician and mesozoic cenozoic biotic radiations curves of taxonomic diversity through geological time consistently show major evolutionary radiations during the ordovician period and from the mid mesozoic to the present day both intervals were characterized by marked biotic provincialism reflecting episodes of major continental break up and global tectonism and their later histories featured steep global climatic gradients the ordovician radiation can be recognized at a wide spectrum of taxonomic levels from species to class and the biogeographical patterns associated with the radiation of individual clades reflect a complex combination of plate distribution tectonic activity sedimentary environment sea level rise and ultimately glaciation the true scale of the mid mesozoic cenozoic biotic radiation is currently a topic of intense debate but there is no doubt that it affected plants and animals in both the marine and terrestrial realms the role of land bridges and ocean gateways in controlling the for mation of biodiversity patterns has been a persistent theme in mesozoic cenozoic bio geography and a complex set of neogene tectonic events probably aided the development of both latitudinal and longitudinal provinces during the cenozoic the present volume highlights some of the successes across a spectrum of approaches to unravelling the ordovician and mesozoic cenozoic radiations within the context of palaeobiogeography

vertical carbon 14 profiles for resolving spatial variability in recharge in arid environments summary groundwater age tracers are often measured to help constrain estimates of groundwater recharge especially in arid environments where other methods are unsuitable however multiple processes can influence the shape of vertical tracer profiles in an aquifer including 1 variation in tracer input concentrations from the unsaturated zone 2 the role of diffusion in transporting tracer into the aquifer when fluxes are low and 3 spatial variability in recharge this study demonstrates the influence of spatially variable recharge and spatially variable carbon 14 14 c activities in the unsaturated zone on vertical 14 c profiles in groundwater through groundwater flow and solute transport modelling we demonstrate that recharge estimated from single point measurements of 14 c may be wrong more than an order of magnitude when unsaturated zone 14 c activities and recharge vary spatially we then present a case study from the ti tree basin in arid central australia where detailed profiles of 14 c activity in unsaturated zone gas and groundwater have been measured and spatial variability in unsaturated zone 14 c is observed ranging from 54 to 106 pmc above the watertable through modelling our data we show that when unsaturated zone 14 c activities are known measurement of the 14 c profile can help constrain estimates of recharge and its spatial variability this approach improves our understanding of groundwater flow in the ti tree basin by showing mountain front recharge to be an important mechanism

the permo triassic extinction the end permian mass extinction brought the palaeozoic great experiment in marine life to a close during an interval of intense climatic tectonic and geochemical change improved knowledge of latest permian faunas coupled with recent advances in isotopic studies and biostratigraphy have greatly enhanced our understanding of the events of 250 million years ago and have begun to provide answers to many questions about the causes of extinction

reply to discussion of imran et al on the orientation of helical flow in curved channels by corney et al sedimentology 53 249 257

a unique assemblage of epibenthic sessile suspension feeders with archaic features in the high antarctic we suggest that the epibenthic communities of passive suspension feeders that dominate some high antarctic seafloors present unique archaic features that are the result of long isolation together with the effects of environmental features including reduced terrestrial runoff and favourable feeding conditions these features probably originated during the late cretaceous when the high antarctic environment started to become different from the surrounding oceans modern antarctic communities are thus composed of a mixture of palaeozoic elements taxa that migrated from the deep ocean during interglacial periods and a component of fauna that evolved from common gondwana cretaceous ancestors we explore this hypothesis by revisiting the palaeoecological history of antarctic marine benthic communities and exploring the abiotic and biotic factors involved in their evolution including changes in oceanic circulation and production plankton communities the development of glaciation restricted sedimentation isolation life histories and the lack of large predators the conditions favouring the retention of apparently archaic features in the antarctic marine fauna remain to be fully elucidated but high antarctic communities are clearly unique and deserve special conservation

palaeoenvironmental trends in the history of trace fossils tracks trails and burrows made by organisms are commonly preserved in phanerozoic sedimentary rocks these biogenic sedimentary structures or trace fossils have been most intensively studied from rocks deposited in marine environments and have proven to be extremely useful in palaeoenvironmental reconstruc tions our systematic analysis of occurrences in the stratigraphic record of two of the most common trace fossils zoophycos and ophiomorpha indicates that occurrences for each show an environmentally linked directional pattern through time zoophycos is commonly reported from palaeozoic strata deposited from nearshore to deep sea environments whereas in post palaeozoic stratait is generally reported only from rocks deposited in deeper water environments ophiomorpha first appeared in permian shallowwater environments and was common in deep sea fan environments by the cretaceous and has been distributed from nearshore to deep sea environments since then these onshore offshore trends which are similar to those previously reported for phanerozoic benthic macroinvertebrate assemblages and individual higher taxa require integration into future palaeoenvironmental reconstructions that use trace fossils

secondary currents in a curved stratified estuarine channel this paper presents a study of secondary circulation in a curved stratified channel in northern san francisco bay over a 12 5 hour tidal cycle secondary currents were strong at times varying by up to 35 cm s from top to bottom but relatively transient as the balance between centrifugal and lateral baroclinic forcing changed over time the short travel time around the bend did not allow a steady state balance to develop between centrifugal and lateral baroclinic forcing during the flood tide the confluence of two streams with different velocities produced a strong lateral gradient in streamwise velocity as a result lateral advection was a significant term in the streamwise momentum balance having the same order of magnitude as the barotropic and baroclinic pressure gradients and the frictional terms during the first part of the ebb secondary currents were induced by lateral baroclinic forcing the direction of the secondary circulation reversed later in the ebb as the baroclinic forcing became weaker than the centrifugal acceleration the gradient richardson number showed that stratification was stable over most of the tidal cycle decreasing the importance of friction and allowing secondary currents to persist

constraining spatial variability in recharge and discharge in an arid environment through modeling carbon 14 with improved boundary conditions carbon 14 14c has been widely used to estimate groundwater recharge rates in arid regions and is increasingly being used as a tool to assist numerical model calibration however lack of knowledge on 14c inputs to groundwater potentially limits its reliability for constraining spatial variability in recharge in this study we use direct measurements of 14c in the unsaturated zone to develop a 14c input map for a regional scale unconfined aquifer in the ti tree basin in central australia the map is used as a boundary condition for a 3 d groundwater flow and solute transport model for the basin the model is calibrated to both groundwater 14c activity and groundwater level and calibration is achieved by varying recharge rates in 18 hydrogeological zones we test the sensitivity of the calibration to both the 14c boundary condition and the number or recharge zones used the calibrated recharge rates help resolve the conceptual model for the basin and demonstrate that spatially distributed discharge through evapotranspiration is an important part of the water balance this approach demonstrates the importance of boundary conditions for 14c transport modeling 14c input activity for improving estimates of spatial variability in recharge and discharge

co2 dynamics in the amargosa desert fluxes and isotopic speciation in a deep unsaturated zone 1 natural unsaturated zone gas profiles at the u s geological survey s amargosa desert research site near beatty nevada reveal the presence of two physically and isotopically distinct co2 sources one shallow and one deep the shallow source derives from seasonally variable autotrophic and heterotrophic respiration in the root zone scanning electron micrograph results indicate that at least part of the deep co2 source is associated with calcite precipitation at the 110 m deep water table we use a geochemical gas diffusion model to explore processes of co2 production and behavior in the unsaturated zone the individual isotopic species 12co2 13co2 and 14co2 are treated as separate chemical components that diffuse and react independently steady state model solutions constrained by the measured 13c in co2 and 14c in co2 profiles indicate that the shallow co2 source from root and microbial respiration composes 97 of the annual average total co2 production at this arid site despite the small contribution from deep co2 production amounting to 0 1 mol m 2 yr 1 upward diffusion from depth strongly influences the distribution of co2 and carbon isotopes in the deep unsaturated zone in addition to diffusion from deep co2 production 14c exchange with a sorbed co2 phase is indicated by the modeled 14c profiles confirming previous work the new model of carbon isotopic profiles provides a quantitative approach for evaluating fluxes of carbon under natural conditions in deep unsaturated zones

mixing and circulation at the confluence of two rivers entering a meandering reservoir 1 a field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows ebro and segre with distinct physical chemical characteristics as they join at the upstream end of ribarroja reservoir in northern spain given the short average residence time of water in the reservoir the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam in winter inflows have similar densities 0 o 10 5 and their spatial distribution is largely determined by inertial forces and in particular by the discharge ratio downstream of the confluence both rivers flow side by side and largely unmixed over long distances in summer with 0 of o 10 3 the flow fields at the confluence are largely controlled by buoyancy forces atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms leading to localized high mixing rates mixing in general though is weak at this time of the year in fall and early winter density differences are largely controlled by conductivity differences between the incoming flows the warmer ebro water with larger thermal inertia flows beneath the colder segre water the spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong likely as a result of mixed water being denser than either of the incoming flows

mixing processes in the amazon river at the confluences of the negro and solimoes rivers encontro das aguas manaus brazil the confluence of the rivers negro and solimoes gives birth to the amazon river near manaus brazil at their confluence these two rivers with their very different physical and geochemical characteristics provide an interesting example of the mixing of waters along a reach of approximately one hundred kilometres the purpose of this article is to describe and explain the spatial configuration of the solimoes negro rivers mixing zone based on a systematic sampling survey carried out in september 1997 using a special sampler and an acoustic doppler current profiler acoustic doppler current profiler the waters of the negro river differ from those of the solimoes river in their lower velocity 0 3 vs 1 m s 1 conductivity 8 vs 80 s cm 1 at 25 c turbidity 5 vs 80 ntu ph 5 5 vs 7 0 and higher temperatures by 1 c due to their higher density the waters of the solimoes river slide under those of the negro river and consequently negro river waters are located at the surface close to the left bank and solimoes river waters are located at the bottom close to the right bank the contact between the waters of the two rivers changes from a clearly defined vertical boundary to a diffuse horizontal boundary as they move downstream the complete mixing process takes more than 30 h and 100 km copyright 2009 john wiley sons ltd

three dimensional tidal flow around headlands field measurements of tidal flow around a headland indicate secondary circulation induced by flow curvature the secondary flow defined to be the flow in the plane normal to the direction of the vertically averaged current is directed toward the headland near the bottom and seaward near the surface consistent with theoretical predictions the strength of the secondary flow varies from 5 to 15 of the streamwise flow it is strongest when the water column is stratified because of both the enhanced shear of the streamwise flow and the reduced frictional damping of the secondary flow the transverse exchange accomplished by the secondary flow significantly influences the structure of the streamwise flow causing a broadening of the transverse shear and changing the horizontal and vertical distributions of momentum it may also have a significant influence on horizontal dispersion in the vicinity of headlands

the importance of aspiration and channel curvature in producing strong vertical mixing over a sill on the basis of observations of the time dependent tidally forced flow over a long sill we find that aspiration and channel curvature set the flow structure and condition the flow to allow intense vertical mixing aspiration reduces the potential energy of the water column by thinning it while maintaining its density contrast channel curvature induces a cross channel circulation that can rapidly overturn a stratified flow eighteen along channel sections of density velocity and dissipation rate of turbulent kinetic energy were collected in and around the tacoma narrows of puget sound a site suspected of driving a strong vertical circulation in adjoining main basin rapid inflow to the narrows on flood from one channel of a triple junction reduces dynamic pressure allowing dense water from below sill depth to be uplifted or aspirated into the narrows we estimate water from below 150 m 3 times the sill depth is drawn into the narrows on a 3 m flood tide once in the narrows the flow remains stratified until it passes a 50 bend where a strong secondary circulation overturns the 50 m deep water column and generates intense turbulent mixing cross channel velocities of up to 0 4 m s 1 are observed and maximum values of exceed 10 3 w kg 1 upon leaving the sill stratification is reestablished and turbulence decays a similar set of sequences occurs on ebb except that the outflow bypasses the flood inflow channel and instead discharges into colvos passage the third branch of the triple junction colvos passage ultimately discharges the ebb effluent back into main basin enhancing the impact of mixing at the narrows by discharging the mixed product far from the source scaling of the cross channel momentum equation suggests that below a threshold value of along channel velocity stratification should suppress secondary circulation for a given vertical shear radius of curvature and channel width above the threshold velocity the magnitude of the cross channel velocity is roughly consistent with predictions for unstratified flow we estimate the maximum effective eddy diffusivity that aspiration and mixing in the narrows can produce in main basin to be 10 3 m2 s 1

carbon dioxide respiration in the deep vadose zone implications for groundwater age dating in the deep vadose zone at the dalmeny site subsurface gas samples were collected and analyzed for co2 and the 13c and 14c ratios of that co2 high concentrations of co2 depleted in 14c near the water table necessitate the use of an open system model of calcite dissolution to match observed dissolved inorganic carbon 14c ratios just below the water table groundwater age dating models assuming closed system calcite dissolution predict incorrect groundwater age dates at the dalmeny site these results and our field observations suggest that such errors may generally occur where deep vadose generation of nonmodern co2 is not accounted for in groundwater age dating exercises

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

palaeoatmospheres in the phanerozoic except in the recent geological past palaeoatmospheric composition cannot be measured but only inferred from palaeontological or geochemical observations the likely controls on atmospheric composition are reviewed and the role of organisms stressed as monitors recorders motors and moderators of palaeoatmospheric changes whereas there is a good general case for changes in po 2 and pco 2 during phanerozoic time proposals for timing and mechanism in the earlier phanerozoic are speculative at present it seems probable that the late palaeozoic spread of land plants raised o 2 and reduced co 2 levels in the atmosphere the end cretaceous crisis had a transient impact on the atmosphere whose general constancy through geological time is one of the most remarkable features of earth history

late cretaceous and paroxysmal cretaceous tertiary extinctions the various geological signatures at cretaceous tertiary time including iridium and other associated elements microspherules and shock deformation features are compatible with the suggestion that the transition is marked by a period of intense volcanism the volatile emissions from this volcanism would lead to acid rain reduction in the alkalinity and ph of the surface ocean global atmospheric temperature changes and ozone layer depletion these environmental effects coupled with those related to the major sea level regression of the late cretaceous provide the framework for an explanation of the selective nature of the observed extinction record

hydraulic geometry and changes in flow velocity at a river confluence with coarse bed material a survey of flows was conducted at a river confluence with coarse bed material bridges were installed on both tributaries at the confluence and farther downstream on the receiving stream at these stations flow velocities were measured over a dense grid for seven conditions ranging from very low flows to the bankfull stage hydraulic geometry relationships established at all four stations revealed that flow is accelerated through the confluence as stage rises at bankfull discharge average velocity is 1 6 times higher at the confluence than on either tributary flow acceleration occurs at and above intermediate flow stages and is concentrated at the centre of a linear pool located at the confluence the development of a zone of high shear stress is also associated with the cell of high flow velocity flow acceleration is dissipated at the exit of the pool where water surges over boulder ribs the acceleration is not related to the development of flow separation zones as observed by best and reid 1984 for wide junction angles nor is it explained by the reduction of the friction exerted by the banks acceleration is associated with the plan geometry of the confluence with the lateral slopes which permit water to converge and with a reduction in grain roughness at the confluence owing to the curvature of the tributary and to the acute angle of entry relative power losses through the confluence decrease with increasing stages

time averaged flow structure in the central region of a stream confluence previous process oriented field studies of stream confluences have focused mainly on fluvial dynamics at or immediately downstream of the location where the confluent flows enter the downstream channel this study examines in detail the spatial evolution of the time averaged downstream velocity cross stream velocity and temperature fields between the junction apex where the flows initially meet and the entrance to the downstream channel a well defined vertically oriented mixing interface exists within this portion of the confluence suggesting that lateral mixing of the incoming flows is limited the downstream velocity field near the junction apex is characterized by two high velocity cores separated by an intervening region of low velocity or recirculating fluid in the downstream direction the high velocity cores move inwards towards the mixing interface and high velocity fluid progressively extends downwards into a zone of scour resulting in an increase in flow velocity in the centre of the confluence the cross stream velocity field is dominated by flow convergence but also includes a component associated with a consistent pattern of secondary circulation this pattern is characterized by two surface convergent helical cells one on each side of the mixing interface the helical cells appear to be the mechanism by which high momentum fluid near the surface is advected downwards into the zone of scour for transport ineffective flows the dimensions and intensities of the cells are controlled by the momentum ratio of the confluent streams and by the extant bed morphology within the confluence although the flow structure of formative events was not measured directly in this study documented patterns of erosion and deposition within the central region of the confluence suggest that these events are dynamically similar to the measured flows except for the fact that formative flows are not constrained by but can reshape the bed morphology the results of this investigation are consistent with and augment previous findings on time averaged flow structure in the downstream portion of the confluence 1998 john wiley sons ltd

effect of bed morphology on flow mixing length at river confluences mixing processes at river confluences have an important bearing on problems such as pollutant dispersal and the management of river systems but remain poorly understood previous studies1 11 have indicated that flow mixing downstream of a confluence is a slow process typically completed at distances greater than 100 times the channel width bed morphology is now emerging as a critical factor flume experiments have shown that the width to depth ratios12 and height discordance13 14 between the confluent channels should influence mixing rates12 14 but the magnitude of these effects in real rivers is not known here we report measure ments from three river confluences of moderate size which show that bed discordance can markedly increase mixing rates for the rivers studied mixing is complete at distances five to ten times shorter than those reported previously

trilobite diversity patterns in the middle cambrian of southwestern europe a comparative study abstract this paper describes the pattern of trilobite diversity throughout the middle cambrian in two fossiliferous basins of the western gondwana margin the iberian chains northeastern spain and the montagne noire southern france the documented fluctuations of species diversity allow us to recognize 1 a substantial extinction event recorded in latest early cambrian times named valdemiedes event which separates the culmination of a widespread decline of trilobites and the stepwise immigration of cosmopolitan invaders 2 a major trilobite radiation occurring in the earliest leonian and culminating across the caesaraugustian languedocian transition in which a peak in diversity of trilobites and carpoids was close to a major flooding surface 3 a major reduction of trilobite taxa across the early middle languedocian transition related to a well documented regressional trend and 4 a second immigration of trilobite fauna in the late languedocian which coincided with trangressional pulses and the establishment of suitable shaly substrates in addition we summarize the known trilobite occurrences from the middle cambrian of southwestern europe maps of the distribution of some relevant taxa genera and species over the western gondwana margin are documented

numerical investigation of plunging density current when a buoyant inflow of higher density enters a reservoir it sinks below the ambient water and forms an underflow downstream of the plunge point the flow becomes progressively diluted due to the fluid entrainment the entrainment rate is strongly dependent on the richardson number and reaches a constant value well downstream of the plunge point this study is concerned with the analysis of the plunging phenomenon and the determination of the entrainment a k e model including buoyancy effects both in a sloping and a diverging channel is used to reproduce the main flow characteristics a relation between the depth at the plunge point in a channel of constant width and in a diverging channel is established and theoretical results for the calculation of the dense layer thickness are provided the latter indicates that the spreading rate of the dense layer in a diverging channel is a function of both the entrainment rate and the channel width the predictions of the plunge line location are in agreemen

open to closed system transition traced through the tdic isotopic signature at the aquifer recharge stage implications for groundwater 14c dating 14c dating models are limited when considering recent groundwater for which the carbon isotopic signature of the total dissolved inorganic carbon tdic is mainly acquired in the unsaturated zone reducing the uncertainties of dating thus implies a better identification of the processes controlling the carbon isotopic composition of the tdic during groundwater recharge geochemical interactions between gas water and carbonates in the unsaturated zone were investigated for two aquifers the carbonate free fontainebleau sands and carbonate bearing astian sands france in order to identify the respective roles of co2 and carbonates on the carbon isotopic signatures of the tdic this analysis is usually approached using open or closed system terms under fully open system conditions the seasonality of the 13c values in the soil co2 can lead to important uncertainties regarding the so called initial 14c activity used in 14c correction models in a carbonate bearing unsaturated zone such as in the astian aquifer we show that an approach based on fully open or closed system conditions is not appropriate although the chemical saturation between water and calcite occurs rapidly within the first metre of the unsaturated zone the carbon isotopic contents 13c of the co2 and the tdic evolve downward impacted by the dissolution precipitation of the carbonates in this study we propose a numerical approach to describe this evolution the 13c and the a14c radiocarbon activity of the tdic at the base of the carbonate bearing unsaturated zone depends on i the 13c and the a14c of the tdic in the soil determined by the soil co2 ii the water s residence time in the unsaturated zone and iii the carbonate precipitation dissolution fluxes in this type of situation the carbonate 13c a14c evolutions indicate the presence of secondary calcite and permit the calculation of its accretion flux equal to 4 5 0 5 10 9molgrock 1yr 1 more generally for other sites under temperate climate and with similar properties to the astian sands site this approach allows for a reliable determination of the carbon isotopic composition at the base of the unsaturated zone as the indispensable input function data of the carbon cycle into the aquifer

turbulent mixing in the amazon river the isotopic memory of confluences rivers continuously discharge dissolved material to the oceans dissolved compounds partially result from water rock interactions which produce a large range of water chemical and isotopic compositions these waters are collected by rivers that are commonly assumed to be well mixed with regard to their different tributaries as a result of turbulent dispersion in this paper we test this hypothesis on the solimoes river at manacapuru the largest tributary of the amazon river by analyzing the sodium concentration and strontium isotopic composition of river water on a transverse section at different depths high precision measurements reveal lateral heterogeneities this reflects poor mixing between two main river masses that have distinct chemical and isotopic signatures a hundred kilometers downstream from their confluence the solimoes mainstream and the purus river using sodium concentration data the transverse dispersion coefficient is estimated for the studied solimoes reach the earth s largest river on which such an estimate now exists and is found to be 1 8 0 2 m2 s comparison with previously reported data highlights the potential role of bed morphology and islands in the efficiency of lateral mixing in large rivers we finally demonstrate that the characteristic length of lateral mixing downstream from confluences in large rivers is at least of several tens of kilometers

factors affecting carbon 14 activity of unsaturated zone co2 and implications for groundwater dating summary unsaturated zone processes may influence the carbon 14 14 c activity of infiltrating groundwater and thus introduce error in derived groundwater residence times however unsaturated zone 14 c activities are rarely measured and there is little understanding of how they may vary spatially in a groundwater basin in this study we measured 14 c activity in unsaturated zone gas at five sites with different watertable depths 8 2 31 5 m in the arid ti tree basin central australia we observed a relatively uniform decrease in 14 c activity of unsaturated zone gas with depth at most sites with variation in unsaturated zone depths leading to variation in 14 c activities directly above the watertable at each site ranging from 54 to 106 percent modern carbon pmc through modelling we show that the profiles are influenced by co 2 production at different depths from sources with different isotopic ratios including production of modern co 2 in the root zone and production of old co 2 above the watertable scenario modelling showed that these processes are independent of recharge when recharge is low 0 10 mm y 1 but that higher recharge rates 100 mm y 1 result in more advective transport of atmospheric co 2 to the watertable the variation in 14 c above the watertable was more sensitive to watertable depth and shallow and deep co 2 production rates these findings offer insight into how unsaturated zone 14 c activities may vary spatially and provide guidance as to when 14 c depletion in unsaturated zone co 2 may become important for groundwater dating particularly in arid settings

permian marine paleoecology and its implications for large scale decoupling of brachiopod and bivalve abundance and diversity during the lopingian late permian abstract the transition during the permian triassic interval from brachiopod rich benthic marine assemblages to communities with diverse molluscs has been well constrained by measures of global taxonomic richness but its ecological context has not been studied using quantitative relative abundance data new relative abundance data from early middle and late permian silicified fossil assemblages from offshore shelf carbonate environments indicate that molluscs increased significantly in relative abundance from 0 8 in the middle permian guadalupian to 65 4 in the late permian lopingian these numerical changes are accompanied by increases in the abundance of infaunal bivalves and motile gastropods in the late permian although this ecological change was coincident with the end guadalupian extinction a new global diversity compilation indicates that the overall genus level severity of that biotic crisis was substantially lower than previous estimates the pronounced ecological shift was instead concurrent with the onset of deep marine anoxic or euxinic conditions around the guadalupian lopingian boundary suggesting that increased environmental variability in proximity to the deep water mass may have given eurytopic molluscs a competitive advantage over more stenotopic brachiopods however the lack of global changes in relative genus richness implies dramatic decoupling of global taxonomic and local ecological processes the pronounced increase in molluscan abundance and alpha diversity in the silicified assemblages contrasts with their unchanged global taxonomic richness and may also have resulted from increased environmental stress in offshore shelf environments these results demonstrate that the ecological transition from the brachiopods to molluscs was more complex than inferred from global measures of taxonomic richness and imply that initial stages of permo triassic environmental deterioration may have been responsible for the decoupling of local ecological from global taxonomic processes during the dramatic late permian rise of the modern fauna

flow and particle paths at a natural river confluence with coarse bed material abstract an investigation of flow and particle trajectories was conducted at a gravel bed confluence where the junction angle is gradually reduced from 60 to 20 as the tributary joins the main channel a shallow and elongated scour zone is present at the mouth of the tributary three flow stages ranging from 1 7 to 1 2 bankfull were recorded flow velocity vectors were measured from five bridges installed across the channels sediment transport paths were determined by following the progression of seeded marked particles through the confluence from may to october 1986 the results show that flow structure is responsive to stages at lower stages less than 1 3 bankfull flow velocity vectors are controlled by local bed topography and specifically by the steeper slopes around the scour zone at higher stages velocity vectors become aligned with the plan geometry of the confluence for all stages surface flow velocities are higher in the scour zone bed shear stress computed from the law of the wall for the deepest flow is higher at the bottom of the scour zone compared to its sides head and exit the trajectories of particles seeded on the tributary pass through the scour zone particles coming from the main channel displayed a dual pattern first moving laterally towards the scour zone early in the season then following paths that are consistent with the plan geometry of the confluence this shift in pattern reflects an increase in the peak discharges during the survey period these results highlight the role of stage in controlling the flow dynamics and particle paths at the confluence