laser step heating 40ar39ar age spectra from early archean 3 5 ga barberton greenstone belt sediments a technique for detecting cryptic tectono thermal events abstract samples from sediments of the fig tree group located in the central part of the circa 3 2 to 3 5 ga barberton greenstone belt bgb have been analyzed by the 40 ar 39 ar laser step heating technique this technique has enabled previously cryptic thermal overprints to be detected in various sedimentary units which include reworked chemical sediments barite clastic sediments sandstone shale and one stromatolite in most cases the apparent age plateaux can be identified by corresponding ca k and cl k ratio plots as belonging to separate mineral phases most of the samples exhibit simple ar diffusion loss during the low temperature part of the experiments while occasionally showing excess ar or possible 39ar recoil effects the various sediments analyzed show evidence for distinct overprinting between 2025 and 2090 ma one barite sample gave t0 original blocking age 2673 3 ma 1 which is close to a calculated model ar diffusion loss age for the same sample of 2688 3 ma 1 this age is interpreted as representing final granitoid activity adjacent to the bgb and or craton scale tectonism associated with the limpopo orogeny two samples gave t0 ages of 2350 2400 ma which may reflect increased thermal gradients associated with the formation of the thick transvaal sedimentary basin that may once have covered the bgb the dominant apparent age plateaux together with modelled diffusive ar loss ages of 2025 2090 ma are thought to represent regional thermal anomalies related to large scale tectono thermal activity in the kaapvaal craton of which the bushveld complex which covers a surface area of 60 000 km2 and formation of the vredefort structure are obvious manifestations the strong thermal overprinting recorded by the sediments has effectively removed any ancient atmosphere i e 40 ar 36 ar ratios

rb sr and sm nd isotope geochemistry and chronology of cherts from the onverwacht group 3 5 ae south africa we report rb sr and sm nd isotopic analyses of chert samples from the geluk subgroup previously called the mafic to felsic unit mfu of the onverwacht group in south africa these archean cherts have the same characteristic rb and sr concentrations as phanerozoic cherts with high rb sr ratios the dense massive essentially low carbonate cherts from the onverwacht group show a very large range of 87 rb 86 sr ratios from 0 859 to 7 99 one sample of chert leached in acid yielded 87 rb 86 sr 7 04 the data define a very good correlation line in the 87 rb 87 sr evolution diagram which corresponds to an age of 2119 44 m y with an initial 87 rb 86 sr ratio of 0 72246 16 exchange of sr in the cherts with the associated carbonate appears very limited the correlation line is interpreted as reflecting the age of rehomogenization of the sr in the protolith and the recrystallization of these cherts due to circulating hydrothermal fluids during regional metamorphism about 1 4 ae after deposition of the onverwacht group it may also correspond to the closure of the rb sr system in the cherts when tectonic activity stopped in the region 2200 m y the initial 87 rb 86 sr ratio obtained for the cherts requires the protolith which provided the sr to have rb sr 1 the sm nd systematics do not provide any strict chronologic information but appear to reflect an ancient source age 3 5 ae the nd at 3 54 ae for the cherts are grouped around zero i e close to the initial value observed for volcanics of the same age and provide no evidence for the presence of abundant crustal material older than 3 5 ae

high archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3 5 ga swaziland supergroup south africa new and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3 5 3 2 ga swaziland supergroup barberton greenstone belt south africa the low 1 8 o values of these cherts indicate extremely high ocean temperatures of 55 85 c previously the large depletion in 1 8 o shown by all barberton cherts relative to their phanerozoic counterparts has been attributed to low 1 8 o in archean oceans chert formation during late diagenesis wholesale loss of 1 8 o during alteration and or regional silicification of sediments around hot springs these alternative explanations are not compatible with the new results cherts in the onverwacht group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying fig tree and moodies groups demonstrating that the chert 8 0 o values were fixed prior to archean uplift and erosion which started at 3 26 ga the maximum 1 8 o value in barberton cherts 22 is lower than the minimum values 23 in phanerozoic bedded cherts precluding late diagenesis as the explanation of the overall low 1 8 o values regional metamorphic hydrothermal or long term resetting of original 1 8 o values is also precluded by preservation of 1 8 o values across different metamorphic grades and by systematic 1 8 o differences among interbedded chert types stratigraphic units and conglomerate clasts the 7 1 8 o variation of these archean cherts is similar to that of phanerozoic deep sea cherts formed when opal converted to microquartz during burial but the actual archean values are 10 lower marine opal was apparently converted to microquartz during burial to depths of 1 km cherts with 1 8 o 15 reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and or hydrothermal activity cherts with higher 1 8 o values formed during early diagenesis and indicate an extremely hot archean ocean and surface environment

isotopic composition of hydrothermal precipitates from the mariana trough hydrothermal silica chimneys from the axial region of the mariana trough were investigated for their oxygen isotopic composition the opal ct samples revealed delta 18o values between 28 5 and 31 2 per mille sign this range of delta values indicates precipitation temperatures between 31 5 and 57 2 c minimum and maximum temperatures using different equations describing the silica water equilibrium and assuming a delta 18o value of 0 per mille sign for the hydrothermal fluid two coexisting with opal ct nontronite samples have an isotopic composition of 26 3 and 26 6 per mille sign a formation temperature near 20 c results for this nontronite it is inferred that both mineralogical phases opal ct and nontronite are not cogenetic as they did not precipitate at the same temperature from the same solution

skin friction behind isolated hemispheres and the formation of obstacle marks we have measured the mean magnitude and direction and the rms fluctuation intensity of the skin friction behind plain hemispherical obstacles and behind hemispheres with tapered artificial tails one and four obstacle heights long downstream of reattachment the mean skin friction magnitude is about 20 greater than its free stream value along the centre line and comparably reduced to either side of it the horizontal divergence of the skin friction vector field is positive divergent along the centre line and negative convergent to either side of it neither of these conditions favours development of a ridge longer than the separation length along the centre line the development of ridges or tails many obstacle heights long commonly observed in nature requires considerable modification of the simple sediment free wakes we have studied

inferred oxygen isotope profile of archaean oceanic crust onverwacht group south africa whole rock oxygen isotope data from a suite of mafic and ultramafic samples from the onverwacht group the basal unit of the archaean greenstone succession of the barberton mountain land south africa show a range of values from 18o 3 to 18o 14 a range which coincides with those of phanerozoic ophiolites and oceanic crust when the samples are arranged in an inferred ophiolitic pseudostratigraphy from basal serpentinized ultramafic cumulates through altered mafic extrusives they have an oxygen isotope distribution profile which is indistinguishable from that of phanerozoic ophiolites the distinctive isotopic profiles and secondary mineral assemblages in phanerozoic ophiolites are caused by hydrothermal interaction between seawater 18o 0 and oceanic crust 18o 5 8 the existence of a similar isotopic and metamorphic profile in the onverwacht group argues strongly that these rocks were hydrothermally altered by an archaean ocean whose isotopic composition was indistinguishable from modern sea water

origin and timing of the metasomatic silicification of an early archean komatiite sequence barberton mountain land south africa abstract stratiform units of pervasively silicified early archean ultramafic rocks occur near the boundary between the onverwacht and fig tree groups in the barberton mountain land republic of south africa the origin of these units has been variously ascribed to early archean subaerial weathering or submarine alteration to cataclastic metamorphism and to the alteration of silicic tuffs an exceptionally well exposed example of such an alteration sequence along the skokohla river valley in the central barberton mountain land was studied to determine the nature of the precursor rocks and the timing and nature of the alteration processes well preserved ghosts of spinifex and culmulate textured olivines and pyroxenes establish the komatiitic ancestry of the skokohla rocks rock units of several distinctive lithofacies can be correlated over the entire 1 5 km strike of the sequence and were derived from the metasomatic conversion of a series of ultramafic flow units to rocks dominated by quartz dioctohedral mica and chlorite the original ultramafics have been strongly enriched in si k rb and ba and depleted in fe mg ca na mn sr zn and ni the elements al ti p cr zr y and nb were immobile alteration apparently occurred soon after extrusive activity and before any significant tectonic deformation it is most probable that alteration resulted from the dynamic hydrothermal upwelling of fluids through original fracture porosity in the ultramafic flows

three dimensional simulation of scour inducing flow at bridge piers presently available formulas for estimating the maximum depth of scour near bridge piers often lead to unreliable results a fully three dimensional hydrodynamic model is therefore used in this study to simulate the flow occurring at the base of a cylindrical bridge pier within a scour hole the results of the numerical simulation are compared with laboratory observations by melville and raudkivi 1977 quantitative and qualitative agreement between the studies is quite good discrepancies between the results of the two studies are of an order that may be attributed to choices in numerical model parameters the simulations may be supplemented by lagrangian particle tracking to estimate the depth of the equilibrium scour condition

hydrothermal sulfide and oxide deposits on seamounts near 21 n east pacific rise hydrothermal deposits were sampled by submersible from the calderas and summit benches of two seamounts 20 and 30 km west of the east pacific rise at 21 n the deposits document a wide variety of off axis hydrothermal activity including samples from the interiors of seafloor sulfide deposits in contrast to most mid ocean ridge deposits sulfide deposits on green seamount are rich in si fe and cu are poor in zn and contain abundant quartz sulfide chimneys grew by quenching of hydrothermal fluids and deposition of walls of colloform pyrite marcasite zn sulfide and minor galena whereas coarse grained pyrite and chalcopyrite were deposited at higher temperatures 250 c in chimney interiors as chimney walls grew outward into sea water increased temperatures in the inner walls resulted in recrystallization to pyrite and deposition of pyrite and chalcopyrite in pore space cooling in the outer portions of chimneys caused deposition of opal at temperatures of about 80 170 c as chimneys coalesced and opal rich material was heated in the interiors of sulfide edifices opal was recrystallized to chalcedony and quartz faulting has exposed the interiors of sulfide deposits where further recrystallization and deposition of chalcopyrite and quartz occurred at temperatures of 230 320 c clogging of chimneys changes in flow paths and cooling of the deposits led to the deposition of late mixed layer illite smectite iron sulfides fe poor sphalerite opal and barite the sulfide deposits formed over a time span of at least 140 000 to 70 000 yr ago probably coincident with caldera and pit crater formation the ages of the sulfide deposits provide constraints on the last volcanic and tectonic activity and they also suggest that it took a maximum of 260 000 yr for the seamount to reach its present height red seamount is presently hydrothermally active and deposits consisting predominantly of amorphous fe oxyhydroxide are forming on small pillow cones in the caldera at temperatures of 10 15 c hydrothermal nontronite deposits capped by mn oxide crusts cover much of the southern summit bench of red seamount and formed at temperatures of around 30 c iron rich talc and sulfide grains in nontronite provide evidence of former high temperature activity and suggest that a sulfide deposit is present nearby

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

early archean 3 2 ga fe oxide rich hydrothermal discharge vents in the barberton greenstone belt south africa ironstone pods fe 2 o 3 72 5 97 2 wt interpreted to have formed by early archean sea floor related hydrothermal activity are described from the ca 3 5 3 1 ga barberton greenstone belt south africa most of the pods are elliptical in shape and have their longest dimensions subparallel to the local stratigraphy they rest on silicified ultramafic rocks and are overlain by ferruginous shales silicified sand stones other coarse clastics and barite horizons the ironstone pods grade along strike into laminated iron oxide facies banded iron formation that is inferred to represent periodic discharge of iron oxide and silica rich flocculates from the pods the massive texture of the iron stone pods and their lack of internal sedimentary features suggest that they formed directly on the sea floor relict hydrothermal discharge chimney structures have been recognized honeycomb like cavities and possible fluid flow channel textures attest to primary porosity the iron stone pods are dominated by massive locally coarse grained specular hematite and goethite with lesser amounts of quartz and an x ray amorphous fe ai bearing silicate the pods are distinct from typical massive sulfide gossans in total trace element concentrations including precious and base metals indicating a different origin in contrast the ironstone pods have similarities in major oxide and trace metal concentrations to iron oxide deposits presently forming in part from low temperature hydrothermal fluids on the sea floor for example red seamount iron oxide deposit fluid inclusion studies on quartz show dominant type i primary fluid inclusions with salinities of 4 7 to 15 8 wt nacl equiv and homogenization temperatures t h of 90 to 150 c no evidence for boiling is seen type i fluid inclusions are most likely dominated by nacl cacl 2 h 2 o although their measured eutectic temperatures are consistent with the presence of fecl 2 these inclusions represent the end member hydrothermal fluid more saline 24 29 6 wt cacl 2 equiv lower temperature t h 33 109 c type ii and ila fluid inclusions represent pulses of a hydrothermal fluid of nacl cacl 2 fecl 2 or mgcl 2 h 2 o or possibly fecl 3 bearing composition mixing with ambient sea water in the case of type ii inclusions the various fluid types are interpreted to be indicative of intermediate compositions of an evaporitic brine the lowest salinity measurement for a type i inclusion of 3 1 wt nacl equiv provides a constraint on the salinity of unenriched by evaporation sea water the t h data give a minimum trapping pressure of 7 bars which equates to a minimum sea water depth of 60 m the lack of precise pressure estimates has not enabled calculation of maximum water depths above the pods quartz 18 o analyses average 17 3 0 6 1 n 5 calculated 18 o h 2 o for two samples are 1 4 and 1 2 respectively these estimates are within error of the postulated barberton early archean sea water value of 0 quartz hematite goethite mineral pairs yield temperatures inconsistent with fluid inclusion t h data suggesting isotopic disequilibrium

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

numerical investigation of flow and scour around a vertical circular cylinder flow and scour around a vertical cylinder exposed to current are investigated by using a three dimensional numerical model based on incompressible reynolds averaged navier stokes equations the mod

cherts of the barberton greenstone belt interpreted as products of submarine exhalative activity cherts studied from the southern barberton greenstone belt are identified as replacement products of various pillow lavas pyroclastic deposits and sedimentary deposits silica filled fracture systems within the cherts have been found to be associated with this silicification process these systems include 1 silica filled fractures 2 silica vein stockworks 3 silica cemented breccias and 4 pseudoconglomerate bodies some of the silica filled fracture systems can be shown to have formed soon after sedimentation silica vein stockworks can be seen to develop laterally into silica cemented breccias the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate the cherts are interpreted as silicified hemi pelagic sediments that overlie pillow lavas some of which are now also cherts cells of convective sea water interacted with oceanic crust to produce silica rich fluids these fluids silicified lava and sediments on re entering the near surface env

three dimensional calculation of scour around cylinders the flow of water around a cylinder was calculated by a numerical model the model used a finite volume method to solve the nontransient navier stokes equations for three dimensions on a general nonorthogonal grid the k e turbulence model was used to solve the reynolds stress term the flow field gave the shear stress on the boundaries which was used to calculate sediment concentration for the bed elements the diffusion convection equation for the sediment concentration was solved and continuity for the sediment concentration gave changes in the bed elevation using the modified bed a new flow field was calculated and also a new sediment concentration this sequence was repeated until the depth of the calculated scour hole was identical to the depth of the scour hole from a physical model study the rest of the geometry of the resulting scour hole compared well with the measured scour hole from the experimental study also other flow characteristics given by the numerical model were found to be in ag

hydrothermal silica chimney fields in the galapagos spreading center at 86 w silica chimneys were discovered in 1985 at 86 w in the rift valley of the galapagos spreading center at 2600 m depth cauliflower garden the inactive chimneys lack any sulfides and consist almost entirely of amorphous silica up to 96 wt sio2 opal a fe and mn oxides are minor constituents oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32 c 29 9 18o and 42 c 27 8 18o th udating reveals a maximum age of 1440 300y amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid seawater mixture assuming equilibrium with quartz at 500 bars initial fluid temperatures of more than 175 c i e a concentration of 182 ppm sio2 were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40 c and 260 bars if the silica chimneys originate from the same or a similar fluid as higher temperature 300 c sulfide silica precipitates found nearby i e 2 5 km away then subsurface deposition of sulfides may have occurred

numerical simulation of scour and backfilling processes around a circular pile in waves abstract this study continues the investigation of flow and scour around a vertical pile reported by roulund et al 2005 flow and scour backfilling around a vertical pile exposed to waves are investigated by using a three dimensional numerical model based on incompressible reynolds averaged navier stokes equations the model incorporates 1 k turbulence closure 2 vortex shedding processes 3 sediment transport both bed and suspended load as well as 4 bed morphology the numerical simulations are carried out for a selected set of test conditions of the laboratory experiments of sumer et al 1997 2013a and the numerical results are compared with those of the latter experiments the simulations are carried out for two kinds of beds rigid bed and sediment bed the rigid bed simulations indicate that the vortex shedding for waves around the pile occurs in a one cell fashion with a uniform shedding frequency over the height of the cylinder unlike the case for steady current where a two cell structure prevails the rigid bed simulations further show that the horseshoe vortex flow also undergoes substantial changes in waves the amplification of the bed shear stress around the pile including the areas under the horseshoe vortex and the lee wake region is obtained for various values of the keulegan carpenter number the principal parameter governing the flow around the pile in waves the present model incorporated with the morphology component is applied to several scenarios of scour and backfilling around a pile exposed to waves in the backfilling simulations the initial scour hole is generated either by a steady current or by waves the present simulations indicate that the scour and backfilling in waves are solely governed by the lee wake flow in agreement with observations the numerical model has proven successful in predicting the backfilling of scour holes exposed to waves the results of the numerical tests indicate that the equilibrium depth of scour holes is the same for both the scour and the backfilling for a given keulegan carpenter number in full agreement with observations

numerical model for local scour under offshore pipelines a numerical model based on potential flow theory is proposed for simulating the equilibrium scour hole formed by unidirectional flow underneath offshore pipelines the model employs a finite difference method to solve the laplace equation in terms of velocity potential in a curvilinear coordinate system a boundary adjustment technique based on the newton raphson method is used to calculate the free boundary formed by the eroded seabed by means of the equilibrium of all forces acting on a sediment particle on a sloping bed because the solution of flow field and adjustment of the seabed topography are carried out in an iterative manner the model takes into account the interactions between the flow pipe and the seabed the comparison of the present model with empirical formulas on the prediction of the maximum scour depth indicates that the present model is useful for approximate estimation of scour depth at a pipeline on the seabed for the case of clear water scour

local scour upstream of a slit weir ordinary differential equation based model under steady and unsteady flow conditions abstracttime varying scouring upstream of a sediment release gate during a flood needs to be accurately predicted for reducing sedimentation around a water intake this paper presents an ordinary differential equation based model for computing time variation of scoured volume and maximum scour depth upstream of a slit weir for steady and unsteady flow the model is based on the sediment continuity equation and sediment entrainment volume from a scour hole there are some hypotheses in the model and these hypotheses were examined by carrying out 1 45 runs of scour inception tests 2 36 runs of numerical tests using computational fluid dynamics and 3 33 runs of physical scour tests measuring time variation of scoured topography the model was validated with the scour tests for steady and unsteady flow conducted in the present study in addition to those reported in different research the model was found to agree well with the data of time variation of the scoured volume and maximum scour depth allo

gliding and overthrust nappe tectonics in the barberton greenstone belt abstract regional recumbent folds inverted stratigraphy nappes and olistostromes are described from the southern part of the 3 3 3 5 ga barberton greenstone belt overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness occurred over minimum distances of 86 km more ductile and coherent units were overfolded up to at least 2 km during their emplacement the glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns driven by heat sources from igneous intrusions the upwelling areas of the geothermal convection cells were sites of mud pools and hydrothermal vents which may mark the trailing edges pull aparts of the overthrust units progressive silica and carbonate precipitation due to decreasing temperatures within the zones of fluid migration distant from the areas of high heat flow probably acted as built in braking systems below the travelling slabs active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt the recognition of nappe and overthrust tectonics in the barberton belt processes which may have been commonplace in archaean terrains necessitates a re evaluation of the stratigraphy of this belt

the 18o 16o ratio of 2 billion year old seawater inferred from ancient oceanic crust an oxygen isotope profile of the 2 billion year old purtuniq ophiolite overlaps with similar profiles of younger ophiolites and the modern oceanic crust this overlap implies i that there was a similar style of seawater ocean crust interaction during the past 2 billion years ii that the oxygen isotope composition of early proterozoic seawater was similar to the modern value iii that early proterozoic sea floor spreading rates were similar to or greater than average modern rates and iv that early proterozoic carbonate rocks and cherts with low 18o 16o ratios do not reflect global scale 18o depletion of early proterozoic oceans

influence of boulder concentration on turbulence and sediment transport in open channel flow over submerged boulders in this paper the effects of boulder concentration on hydrodynamics and local and reach averaged sediment transport properties with a flow over submerged boulder arrays are investigated four numerical simulations are performed in which the boulders streamwise spacings are varied statistics of near bed velocity reynolds shear stresses and turbulent events are collected and used to predict bed load transport rates the results demonstrate that the presence of boulders at various interboulder spacings altered the flow field in their vicinity causing 1 flow deceleration wake formation and vortex shedding 2 enhanced outward and inward interaction turbulence events downstream of the boulders and 3 a redistribution of the local bed shear stress around the boulder consisting of pockets of high and low bed shear stresses the spatial variety of the predicted bed load transport rate qs based on local bed shear stress is visualized and is shown to depend greatly on the boulder concentration quantitative bed load transport calculations demonstrate that the reach averaged bed load transport rate may be overestimated by up to 25 times when including the form drag generated shear stress of the immobile boulders in the chosen bed load formula further the reach averaged bed load transport rate may be underestimated by 11 if the local variability of the bed shear stress is not accounted for finally it is shown that for the small spaced boulder array the bed load transport rates should no longer be predicted using a normal distribution with standard deviation of the shear stress distribution

hydromorphological processes at submerged solitary boulder obstacles in streams abstract a series of 51 experiments was conducted in a laboratory flume to investigate hydromorphological processes at submerged solitary boulder like obstacles as part of a programme to validate the experimental results five sites with obstacle marks at large boulders were surveyed in two gravel bed ephemeral streams located in eastern spain three hydromorphological system states i iii were observed each exhibiting characteristic dynamic interactions between hydraulic and sedimentary processes i wake vortex domination and absent scouring ii wake vortex domination and downstream scouring iii horseshoe vortex domination frontal scouring and ridge like sediment accumulation at the lee side the latter system state iii comprises typical fluvial obstacle marks in mobile alluvial beds obstacles may tilt upstream into the scour hole when frontal scour depths exceed 0 6 times obstacle length favouring the rapid achievement of steady state conditions state iii field and experimentally modelled obstacle marks are related to each other when frontal scour depth width and length as well as ridge width are used as morphometric variables an analytical model was adapted and successfully applied for determination of these variables at the steady state condition the model was also used to assess the critical threshold for the formation of obstacle marks it integrates mean flow velocity water depth obstacle height and width obstacle shape median grain size relative to obstacle width and grain size distribution dynamic interactions between hydraulic processes and sediment exchange around obstacles together with well defined boundaries between evolutionary states critical thresholds suggest a self organised scale invariant mechanism driving the generation of obstacle marks

modelling of the oxygen isotope evolution of seawater implications for the climate interpretation of the δ18o of marine sediments it is often argued that the 18o value of oceanic water was maintained close to 0 for hundreds of millions of years as a consequence of oxygen isotope exchange between oceanic crust and seawater however for several decades the interpretation of the biosedimentary oxygen isotope record has conflicted with the igneous record because with increasing age a general trend of decreasing 18o values about 6 is observed in most carbonates cherts and phosphates especially for the paleozoic and early mesozoic we developed a dynamical model of seawater crust interaction that computes the 18o value in these two reservoirs as function of time this model takes into account the continuous production of crust at oceanic ridges its expansion rate the permeability profile with space and time the mineralogical mode of the crust and the kinetics of oxygen isotope exchange between rock forming minerals and seawater the model indicates that the 18o value of seawater may vary by 2 with a time response ranging from 5 to 50 ma for expansion rates of 1 to 10 cm a 1 the variation of 2 is fixed by both integrated water rock ratio and closure time of the seawater crust system by sediments variations in the oxygen isotope ratio of seawater through time have important implications for the interpretation of the systematically low 18o values of pre jurassic marine sediments according to our model marine paleotemperatures could be up to 10 c lower than those expected when applying the classical hypothesis of an ice free ocean with a 18o value of 1

sm nd rb sr pb pb dating of carbonates from the early archaean barberton greenstone belt south africa evidence for post depositional isotopic resetting at low temperature abstract two silicic carbonate sequences occurring in the onverwacht group og and the overlying fig tree group ftg of the early archaean barberton greenstone belt south africa contain silicified dolomites with primary structures and textures they occur with volcanic rocks dated by single zircons the associated onverwacht felsic volcanics have an age of 3440 3416 ma whereas the fig tree volcanics are 3225 ma old therefore the depositional age of both sequences is between 3440 and 3225 ma pb isotopic data for different parts of a single og whole rock sample form a linear array corresponding to a pb pb age of 2632 99 ma sm nd and rb sr isotopes were analysed in four whole rock samples of the ftg silicified carbonates and form an sm nd array corresponding to an age of 2612 84 ma with initial end 8 1 0 5 and an rb sr array of 2743 36 ma with initial 87 sr 86 sr 0 7013 0 0003 these whole rock samples have rb sr ratios of 1 and unusually high rb concentrations for carbonates 100 ppm which derive from processes accompanying silicification all three isotopic systems of whole rocks indicate ages of 2 7 ga much younger than the depositional age of the successions the agreement provides strong evidence that an alteration event reset all of the isotopic systems sm nd as well as rb sr and u pb we suggest that the ages represent the timing of silicification and sericitization of the carbonates by large scale fluid movement at an apparent temperature of 200 c

numerical modeling of flow and scour below a pipeline in currents part ii scour simulation abstract a vertical two dimensional 2d numerical model for time dependent local scour below offshore pipelines subject to unidirectional steady flow is developed the governing equations for the flow and sediment transport are solved by using finite difference method in a general curvilinear coordinate system the performance of two turbulence models the standard k model and smagorinsky subgrid scale sgs model on modeling time dependent scour processes is examined both suspended load and bed load are considered in the scour model the suspended load model is verified against two channel sediment transport cases the change of bed level is calculated from the continuity equation of total sediment transport a new time marching scheme and a sand slide scheme are proposed for the scour calculation it is found that the proposed time marching scheme and sand slide model work well for both clear water and live bed scour situations and the standard k turbulence closure is more preferable than the sgs model in the 2d scour model developed in this study

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

numerical and experimental investigation of flow and scour around a circular pile the flow around a vertical circular pile exposed to a steady current is studied numerically and experimentally the numerical model is a three dimensional model the model validation was achieved against new experimental data which include two component laser doppler anemometry lda flow measurements and the hot film bed shear stress measurements and reported in the present paper and the data of others and a k w turbulence model was used for closure the model does not have a free surface facility and therefore is applicable only to cases where the froude number is small fr 0 0 2 the flow model was used to study the horseshoe vortex and lee wake vortex flow processes around the pile the influence on the horseshoe vortex of three parameters namely the boundary layer thickness the reynolds number and the bed roughness was investigated in the latter investigation the steady solution of the model was chosen a study of the influence of the unsteady solution on the previously mentioned flow processes was also carried out the ranges of the parameters covered in the numerical simulations are the boundary layer thickness to pile diameter ratio is varied from 2 x 10 2 to 10 2 the pile reynolds number from 10 2 to 2 10 6 and the pile diameter to roughness ratio from 2 to about 10 3 the amplification of the bed shear stress around the pile including the areas under the horseshoe vortex and the lee wake region was obtained for various values of the previously mentioned parameters the steady state flow model was coupled with a morphologic model to calculate scour around a vertical circular pile exposed to a steady current in the case of non cohesive sediment the morphologic model includes i a two dimensional bed load sediment transport description and ii a description of surface layer sand slides for bed slopes exceeding the angle of repose the results show that the present numerical simulation captures all the main features of the scour process the equilibrium scour depth obtained from the simulation agrees well with the experiments for the upstream scour hole some discrepancy up to 30 was observed however for the downstream scour hole the calculations show that the amplification of the bed shear stress around the pile in the equilibrium state of the scour process is reduced considerably with respect to that experienced at the initial stage where the bed is plane

numerical and experimental investigation of flow and scour around a half buried sphere abstract the paper describes the results of a numerical and experimental investigation of flow and scour around a half buried sphere exposed to a steady current hot film bed shear stress and laser doppler anemometer measurements were made with a half sphere mounted on the smooth bed in an open channel the hydrodynamic model is a 3 d general purpose n s flow solver the k omega sst turbulence model was used for closure the flow model was used to study the horseshoe vortex and lee wake vortex flow processes around the sphere the flow model was coupled with a morphologic model to calculate scour around the half buried sphere in currents the morphologic model includes a sediment transport description and a description of surface layer sand slides for bed slopes exceeding the angle of repose the sediment transport description includes for the first time the effect of externally generated turbulence induced by the horseshoe vortex flow and the lee wake flow processes on sediment transport the results show that the scour depth increases and time scale decreases when the effect of externally generated turbulence is incorporated in the calculations empirical expressions representing the numerically obtained data on the equilibrium scour depth and the time scale are presented the results show that the equilibrium maximum scour depth in the live bed regime can be approximated by 0 5 d in which d is the sphere diameter

influence of turbulence on bed load sediment transport this paper summarizes the results of an experimental study on the influence of an external turbulence field on the bed load sediment transport in an open channel the external turbulence was generated by 1 a horizontal pipe placed halfway through the depth ih 2 a series of grids with a clearance of about one third of the depth from the bed and extending over a finite length of the flume and 3 a series of grids with a clearance in the range 0 1 1 0 ih from the bed but extending over the entire length of the flume two kinds of experiments were conducted plane bed experiments and ripple covered bed experiments in the former case the flow in the presence of the turbulence generator was adjusted so that the mean bed shear stress was the same as in the case without the turbulence generator in order to single out the effect of the external turbulence on the sediment transport in the ripple covered bed case the mean and turbulence quantities of the streamwise component of the velocity were measured and the shields parameter due to skin friction was determined the shields parameter together with the rms value of the streamwise velocity fluctuations was correlated with the sediment transport rate the sediment transport increases markedly with increasing turbulence level

the impact of coarse grain protrusion on near bed hydrodynamics in steep rivers sediment is often transported over immobile cobbles and boulders previous studies of such conditions have observed that the entrainment rate of the mobile sediment strongly depends on the level of protrusion of the immobile grains here experiments are conducted in a laboratory flume in order to quantify how different levels of protrusion of large aggregates above a fixed fine sediment bed modeled as a patch of hemispheres modify the local hydrodynamics near the fine sediment bed five protrusion levels defined by p k r 0 20 40 60 80 were investigated where k is the protrusion height and r the radius for small protrusion p 20 enhanced shear stress and turbulence intensity on the fine sediment bed is observed as the mixing layer generated at the hemisphere top impacts the fine sediment bed moreover sweep events generated near the top of the hemispheres reach the fine sediment bed for large protrusions p 60 the mixing layer generated near the top of the hemispheres does not reach the fine grains with the consequence that the shear stress drops the remaining turbulence near the fine sediment bed although enhanced by the wakes generated by the hemispherical caps is quasi isotropic the transition between these two distinct near bed flow regimes is found to be around p 40 corresponding to the protrusion levels observed by grams and wilcock 2014 https doi org 10 1002 2013jf002925 above which the erosion of fine sediment ceases

edge scour at scour protections around piles in the marine environment laboratory and field investigation abstract when building offshore wind turbines with monopile foundations scour protection typically is placed to avoid scouring of the soil close to the monopile an important aspect is that the scour protection itself causes erosion inflicted by the local increase in current and or wave velocities and in turn increased bed shear stresses scour of the edge material alongside the scour protection may cause deformations and failure of the scour protection of offshore wind turbine foundations this can reduce the stability of the stone layer and cause exposure of cables running between the monopiles where they go from buried to the transition piece on the foundation although much information is available on the design of scour protection systems around monopiles little is known on the mechanisms causing edge scour and the equilibrium stages of the edge scour process in steady current waves and combined waves and current this paper presents an extensive experimental campaign to explain the edge scour process in current and combined irregular waves and current as well as tidal current the three dimensional flow field around the pile and scour protection is resolved by particle image velocimetry and bed shear stress measurements showing a local increase in the flow velocities and bed shear stresses leading to increased sediment transport and scour the governing process in steady current is a pair of symmetrical counter rotating vortices emerging in the near bed region in the wake of the pile and scour protection causing a significant downstream scour hole it is found that the equilibrium scour hole depth and length scales with the pile diameter and the ratio between the thickness and the width of the scour protection in the second part of the present paper the results from the experimental campaign are compared with the edge scour experienced in practice outlined by a survey program of the offshore wind park egmond aan zee and a published field investigation of scroby sands owf by whitehouse et al 2011

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation

nodular chert from the arbuckle group slick hills sw oklahoma a combined field petrographic and isotopic study nodular chert from the middle and upper arbuckle group early ordovician in the slick hills sw oklahoma was formed by selective replacement of grainstones burrow fillings algal structures and evaporite nodules chert nodules are dominantly microquartz with minor fibrous quartz both quartzine and chalcedony megaquartz and microflamboyant quartz lepisphere textures of an opal ct precursor are preserved in many especially in finely crystalline chert nodules the 18o values of microquartz chert range from 23 4 to 28 80 00 smow significantly lower than those of cenozoic and mesozoic microquartz chert formed both in the deep sea and from near surface sea water the 18o values of chert decrease with increasing quartz crystal size silicification in the arbuckle group occurred during early diagenesis with the timing constrained by the relative temporal relationships among silicification burial compaction and early dolomite stabilization silica for initial chert nucleation may have been derived from both dissolution of sponge spicules and silica enriched sea water chert nucleation appears to have been controlled by the porosity permeability and organic matter content of precursor sediments this conclusion is based on the fact that chert selectively replaced both porous grainstones and burrows and algal structures enriched in organic matter growth of chert probably occurred by a maturation process from opal a to opal ct to quartz as indicated by the presence of opal ct precursor textures in many chert nodules although field and petrographic evidence argues for an early marine origin for chert in the arbuckle group the light 18o values are inconsistent with this origin meteoric resetting of the 18o values of the chert during exposure of the carbonate platform best explains the light 18o values because i the 18o values of chert nodules decrease with decreasing 18o values of host limestones and ii chert nodules from early dolomite which underwent more extensive meteoric modification than associated limestones have lighter 18o values than chert nodules from limestones increasing recrystallization of chert nodules by meteoric water resulted in progressive 18o depletion and quartz crystal enlargement

an analysis of low level jets in the greater ross ice shelf region based on numerical simulations abstract an analysis of the presence and location of low level jets lljs across the ross ice shelf region in antarctica is presented based on the analysis of archived output from the real time antarctic mesoscale prediction system amps the method of self organizing maps soms is used to objectively identify different patterns in column averaged wind speed over the approximately lowest 1200 m of the atmosphere as an identifier to the location of lljs the results indicate three primary lljs in the region the largest and most dominant llj is along the transantarctic mountains by the siple coast and the southern end of the ross ice shelf the second llj extends from the base of byrd glacier and curves to the north passing by the eastern extremes of ross island the third llj extends from the base of reeves glacier and curves to the north across the western ross sea a strong seasonality is observed in the frequency and intensity of the lljs with the highest values for wind speed and the size of the

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

fluid heterogeneity during granulite facies metamorphism in the adirondacks stable isotope evidence the preservation of premetamorphic whole rock oxygen isotope ratios in adirondack metasediments shows that neither these rocks nor adjacent anorthosites and gneisses have been penetrated by large amounts of externally derived hot co2 h2o fluids during granulite facies metamorphism this conclusion is supported by calculations of the effect of fluid volatilization and exchange and is also independently supported by petrologic and phase equilibria considerations the data suggest that these rocks were not an open system during metamorphism that fluid rock ratios were in many instances between 0 0 and 0 1 that externally derived fluids as well as fluids derived by metamorphic volatilization rose along localized channels and were not pervasive and thus that no single generalization can be applied to metamorphic fluid conditions in the adirondacks

numerical modeling of flow and scour at pipelines a numerical model for the description of fluid flow and suspended and bed load sediment transport is presented density effects are included in the momentum reynolds equations and in the turbulence kappa and epsilon equations changes in bed levels are calculated from sediment continuity and the finite element grid is adapted to the geometry the reynolds equations and the transport equation for suspended sediment are solved numerically using a taylor galerkin finite element method the flow at a surface mounted cylinder in a steady flow is predicted in good agreement with experiments periodic vortex shedding from a cylinder placed above a rigid bed is predicted in good agreement with laboratory experiments provided that sufficiently detailed grids approximately 5 000 nodes are used scour calculations are performed for a cylinder in a steady flow with its underside placed at the level of the original flat bed predicted scour at a pipeline in steady flow is in good agreement with laboratory measurements reported in the literature

growth history of a hydrothermal silica chimney from the mariana backarc spreading center southwest pacific 18 13 n abstract a hydrothermal field consisting of inactive silica chimneys was investigated in 1988 at 18 13 n in the median rift valley of the mariana trough backarc spreading centre topless tower that was reported to be active 3 years previously a detailed mineralogical and geochemical study of a 0 75 m long and 0 61 m wide silica chimney revealed its growth history the inactive chimney consists almost entirely of amorphous silica up to 86 wt opal a and barite a cross section indicates two ancient fluid channels infilled by pyrite barite and marcasite the outer rim of the chimney shows alteration oxidation features of up to 30 fe2o3 the distribution patterns of fe mn ba cu and zn reveal the structure of the chimney the alteration effects in the outer rim by mn and fe enrichments and the two fluid channels by ba cu and zn enrichments oxygen isotope data indicate that the formation of the silica chimney occurred at temperatures ranging between 41 and 50 c although the occurrence of pyrite barite marcasite and quartz in the fluid channels suggests a higher growth temperature for the inner part 230 c 230 th 234 u dating indicates an age of 7500 yr for the chimney in agreement with 226ra and 210pb results amorphous silica solubility relationships suggest that the silica chimney was formed by conductive cooling as well as by mixing of hydrothermal fluids and seawater a second stage of high temperature 230 280 c hydrothermal discharge occurred that resulted in the formation of 226ra rich sr and ba bearing silica with quartz and pyrite