structural anatomy of the kushtagi schist belt dharwar craton south india an example of archaean transpression abstract the nw se trending kushtagi schist belt in the eastern dharwar craton records the tectonic consequences of a transpression regime it was formed due to late archaean oblique convergence in the dharwar craton this relatively unknown schist belt is characterized by two major episodes of deformation the dominant d 1 deformation forming tight to isoclinal non cylindrical folds and a regional axial planar schistosity is a consequence of transpression the transpression related sinistral shear regime is followed by development of a conjugate set of f 2 folds and s 2 cleavages that are superimposed on the d 1 structures the maximum shortening direction associated with the f 2 folds is nw se which suggests longitudinal compression subsequent to transpression riedel shear fractures were developed in the sinistral shear regime and dolerite dykes were emplaced along the conjugate riedel shear fractures intrusive granite bodies within and outside the schist belt were emplaced in the late phase of d 1 transpression

flow and suspended sediment division at two highly asymmetric bifurcations in a river delta implications for channel stability the division of sediment at river bifurcations results from the complex interaction between three dimensional flow planform and channel bed morphology as well as the heterogeneity of the bed material sediment division processes cannot be incorporated in their full complexity in scale experiments and are difficult to reproduce with numerical models field measurements are thus necessary to advance our understanding of those processes in river deltas however such measurements are rare we present measurements of the flow and sediment division at two tidally influenced bifurcations of the kapuas river a large sand bedded suspended load dominated river in west kalimantan indonesia at both bifurcations a smaller channel branches off from the side of the main river which makes the planform strongly asymmetric the planform of both bifurcations has been stable at least since the end of the nineteenth century when the region was mapped for the first time based on our measurements we explore possible factors that stabilize the bifurcations we measure the flow velocities with a boat mounted acoustic velocity profiler and determine the sediment concentration from acoustic backscatter calibrated against water samples the side branch of the first bifurcation receives a proportionally lower fraction of sediment than water in contrast the side branch at the second bifurcation receives a proportionally higher fraction of sediment than water a comparison of flow velocity and suspended sand concentration indicates that the bed material sorting strongly influences the division of sediment in particular at one of the bifurcations which is situated in a meander bend

sr ca u ca and δ18o records in recent massive corals from bermuda relationships with sea surface temperature abstract high resolution records of sr ca u ca ratios and 18o have been obtained in two recent colonies of massive corals diploria labyrinthiformis from bermuda the three geochemical proxies display regular seasonal variations and are well correlated with each other however some important discrepancies are observed between the two colonies the average seasonal variations of the three geochemical proxies are lower by almost 50 in one of the profiles where the seasonal oscillation also displays a strong asymmetry with narrower summer maxima different calculations are discussed for calibrating sr ca and 18o with sea surface temperatures sst we show that the method using only temperature minima and maxima is slightly more accurate and is also more reliable our results from bermuda corals confirm that the temperature dependency of sr ca u ca and 18o is species dependent as previously shown by others and that vital effects are clearly involved in the geochemical incorporation of trace elements in the coral skeleton finally a simple model involving seasonal variation of the growth rate and a kinetic fractionation related to growth rate is presented to explain the differences between the two bermuda colonies it is shown that these factors may prevent corals from recording the complete temperature seasonality and could be the cause for the discrepancies observed between profiles resulting in significant biases of the sst reconstructions

morphological adaptation of river channels to vegetation establishment a laboratory study while the scientific community has long recognized that alluvial rivers are the product of interactions between flowing water and bed material transport it is increasingly evident that vegetation mediates these interactions and influences the stream channel characteristics in a novel set of mobile bed laboratory experiments with variable discharge we demonstrate that vegetation colonization affects bank erosion rates channel shape channel sinuosity and bar pattern our analyses compared the morphological evolution of channels with initially steady bars considering the following three scenarios 1 channel without vegetation 2 channel with vegetation added to the floodplains and 3 channel with vegetation added to both the floodplains and the bar surfaces that emerge at low flows absence of vegetation produced the widest and shallowest channel with the lowest sinuosity floodplain vegetation in the second scenario reduced bank erosion and resulted in a deeper and more sinuous channel with shorter bars in the third scenario vegetation establishment on emerging bar surfaces intensified erosion on the opposing bank enlarging the amplitude of bends enhanced sedimentation on vegetated bar areas increased both bar elevation and bar length compared to the second scenario the results show that the colonization of bar surfaces by plants creates the conditions for new floodplain and island formation fostering channel meandering and anabranching finally our experiments emphasize the role of alternating high and low flows on the morphological development of streams mediated by vegetation

corroborated rainfall records from aragonitic stalagmites abstract sr and ba levels of two proximal aragonite speleothems from cold air cave south africa are examined at m resolution a compositional map derived from parallel secondary ion mass spectrometry linescans indicates heterogeneity perpendicular to the stalagmite growth axis precluding the use of single linescans as a climatic proxy technique a 40 yr averaged trace element record for the cave produced from multiple parallel linescans on both stalagmites is compared with regional and local climatic rainfall temperature data for 1955 1996 this period includes two extensive drought episodes there is poor correlation between trace elements and annual mean temperature droughts correspond to minima in trace element ratios and peaks correspond to annual maxima in rainfall the onset and termination of droughts are notable inflexions however in detail annual rainfall magnitude does not correlate directly with trace element ratios indicating that kinetic factors notably variations in speleothem growth rate play a key role in trace element uptake by aragonite averaged long data sets constrained by chronological control have potential in the study of sub decadal precipitation patterns

simulating bank erosion over an extended natural sinuous river reach using a universal slope stability algorithm coupled with a morphodynamic model meandering river channels are often associated with cohesive banks yet only a few river modelling packages include geotechnical and plant effects existing packages are solely compatible with single threaded channels require a specific mesh structure derive lateral migration rates from hydraulic properties determine stability based on friction angle rely on nonphysical assumptions to describe cutoffs or exclude floodplain processes and vegetation in this paper we evaluate the accuracy of a new geotechnical module that was developed and coupled with telemac mascaret to address these limitations innovatively the newly developed module relies on a fully configurable universal genetic algorithm with tournament selection that permits it 1 to assess geotechnical stability along potentially unstable slope profiles intersecting liquid solid boundaries and 2 to predict the shape and extent of slump blocks while considering mechanical plant effects bank hydrology and the hydrostatic pressure caused by flow the profiles of unstable banks are altered while ensuring mass conservation importantly the new stability module is independent of mesh structure and can operate efficiently along multithreaded channels cutoffs and islands data collected along a 1 5 km long reach of the semialluvial medway creek canada over a period of 3 5 years are used to evaluate the capacity of the coupled model to accurately predict bank retreat in meandering river channels and to evaluate the extent to which the new model can be applied to a natural river reach located in a complex environment our results indicate that key geotechnical parameters can indeed be adjusted to fit observations even with a minimal calibration effort and that the model correctly identifies the location of the most severely eroded bank regions the combined use of genetic and spatial analysis algorithms in particular for the evaluation of geotechnical stability independently of the hydrodynamic mesh permits the consideration of biophysical conditions for an extended river reach with complex bank geometries with only a minor increase in run time further improvements with respect to plant representation could assist scientists in better understanding channel floodplain interactions and in evaluating channel designs in river management projects

sediment supply as a driver of river meandering and floodplain evolution in the amazon basin the role of externally imposed sediment supplies on the evolution of meandering rivers and their floodplains is poorly understood despite analytical advances in our physical understanding of river meandering1 2 the amazon river basin hosts tributaries that are largely unaffected by engineering controls and hold a range of sediment loads allowing us to explore the influence that sediment supply has on river evolution here we calculate average annual rates of meander migration within 20 reaches in the amazon basin from landsat imagery spanning 1985 2013 we find that rivers with high sediment loads experience annual migration rates that are higher than those of rivers with lower sediment loads meander cutoff also occurs more frequently along rivers with higher sediment loads differences in meander migration and cutoff rates between the study reaches are not explained by differences in channel slope or river discharge because faster meander migration and higher cutoff rates lead to increased sediment storage space in the resulting oxbows we suggest that sediment supply modulates the reshaping of floodplain environments by meandering rivers we conclude that imposed sediment loads influence planform changes in lowland rivers across the amazon

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

geochemistry of the mesozoic bedded cherts of central baja california vizcaino cedros san benito implications for paleogeographic reconstruction of an old oceanic basin abstract in central baja california vizcaino peninsula and cedros and san benito islands two distinct radiolarian bedded chert sequences of late triassic and late jurassic lowermost cretaceous age can be differentiated on lithostratigraphic and geochemical criteria these bedded chert sequences are part of the conformable sedimentary cover of more or less dismembered ophiolites which are overthrusted by the san andres cedros volcanic arc system of middle late jurassic age major and trace elements permit paleogeographic zonation of the late jurassic lowermost cretaceous radiolarites lying conformably upon ophiolites considered as fragments of an oceanic basin floor which developed westward of the san andres volcanic arc progressive accretion of this oceanic basin floor along the continental margin is supported by the fact that the more distal radiolarian chert sequences belong to the lowermost structural units of this area

numerical analysis of river channel processes with bank erosion this paper presents a numerical analysis of river channel processes with bank erosion the model can be used for investigating both bed deformation and bankline shifting in 2d plan form the basic equations are used in a moving boundary fitted coordinate system and a new formulation of nonequilibrium sediment transport is introduced to reproduce the channel processes the model was applied to examine the morphological behavior of experimental channels temporal changes in the plan form in a meandering channel can be classified into two patterns meander developing and meander straightening comparison of the observed and calculated results indicates that the model is applicable to both channel changes under various hydraulic conditions on the basis of the numerical findings the paper clarifies the influence of hydraulic variables on the location of bank erosion and bed scouring the model also was used to investigate the effect of alternate bars on bank erosion and to investigate the development of channel meandering from an initially straight channel

geochemical characteristics of archean cherts and other sedimentary rocks in the pilbara block western australia evidence for archean seawater enriched in hydrothermally derived iron and silica abstract geochemical and mineralogical characteristics of archean sedimentary rocks such as cherts banded iron formations bifs shales and sandstones collected from the pilbara block western australia were studied lithologic features of various types of chert occurring within the pilbara supergroup are closely related to their chemical compositions red and brown bands of banded cherts generally show a higher fe 2 o 3 total iron as fe 2 o 3 concentration than do other cherts whereas grey greenish grey and pale yellowish grey cherts are characteristic of relatively higher concentrations of al and ti white cherts are generally composed almost exclusively of sio 2 geochemical characteristics of the cherts suggest that they were precipitated from hydrothermal solutions evidence for the hydrothermal origin includes 1 low mno fe 2 o 3 values 2 low concentrations of heavy metals 3 positive euanomalies 4 low co zn and ni zn values and 5 high concentrations of mn and fe in carbonates positive correlations between fe and heavy metals such as ni and zn imply the intermittent precipitation of fe oxides hydroxides along with these heavy metals in the ancient ocean cherts except for grey greenish grey and pale yellowish grey cherts are highly depleted in detrital materials this may have been caused by rapid precipitation of hydrothermally derived fe and si the archean pilbara ocean is believed to have been enriched in fe and si derived from hydrothermal activity

ph control on oxygen isotopic composition of symbiotic corals abstract boron carbon and oxygen isotopic compositions were determined at the micrometre scale by high resolution ion microprobe in a sample of modern coral massive hermatypic coral porites lutea the ion probe data show for b and o much larger isotopic variations at the micrometre scale than those measured at the millimetre scale by conventional techniques 18opdb values range from 10 6 0 9 to 0 2 0 5 and 11b values range from 18 6 1 5 to 30 6 1 6 by contrast 13c values show the same range of variations from 4 6 0 65 to 2 2 0 67 at the micrometre and millimetre scales the range of 11b values indicates that significant ph variations from 7 1 to 9 0 are present at the sites of calcification the largest 18o variations correspond to the highest 11b values i e to the highest phs this measurement of ph allows modelling the oxygen isotopic fractionation occurring during aragonite precipitation taking into account the rate of o isotopic equilibrium between dissolved carbonate species h2co3 hco3 and co32 and water via the two reactions of hydration and hydroxylation the full range of 18o values measured at the micrometre scale can be modelled for residence times of dissolved carbonates in the calcifying fluid ranging between 1 h and at maximum 12 h the ph controls the 18o of the growing carbonate through the relative fractions of dissolved carbonate species and through the kinetics of their isotopic equilibration with water via hydration and hydroxylation the so called vital effect systematically observed for 18o in corals can thus be understood as representing an average of rapid ph variations due to coral biology during coral growth selectively measuring 18o values in the zones of coral skeletons that have low 11b values i e formed at low ph should significantly improve the quality of palaeoclimatic reconstructions based on 18o values

paleomagnetic study of 1765 ma dyke swarm from the singhbhum craton implications to the paleogeography of india abstract we report the first key paleopole as a result of paleomagnetic study on a precisely dated 1765 3 1 0 ma wnw ese trending dyke swarm from singhbhum craton this pole has been used in this study to propose the paleogeographic reconstruction of india with baltica craton and north china craton incremental alternating field af and thermal demagnetization isolated high coercivity components with north to north westerly declination and shallow negative inclination from 9 sampling sites which are representing different individual dykes the primary origin of the chrm is supported by the positive baked contact test the wnw ese trending dykes yield a mean paleomagnetic direction with a declination 329 2 and an inclination 22 8 k 31 6 95 9 3 the positive bake contact test proves the primary nature of remanence the pole position of singhbhum craton at 1765 ma is 45 n 311 e dp 5 2 and dm 9 9 paleogeographic reconstruction at ca 1770 ma supported by geological tectonic and metallogenic evidences indicate that the baltica craton and india linkage can be stable for at least 370 ma 1770 1400 ma there is also reasonable evidence in support of india north china craton spatial proximity at 1770 ma

rare earth elements in river waters we measured rare earth element ree concentrations in river waters to characterize the suspended and dissolved river flux of the ree to the oceans the ree pattern of river suspended materials is sensitive to drainage basin geology a positive correlation is observed between la yb ratios and nd model ages for the rivers studied major rivers have light ree enriched patterns relative to the north american shale composite nasc with la yb n 1 6 2 7 river water dissolved material 0 2 m is heavy ree enriched relative to suspended material and the most pronounced negative ce anomalies occur in rivers of high ph light ree concentrations vary by approximately 3 orders of magnitude and are inversely related to ph and major cation concentrations from these data we estimate that typical major river runoff has heavy ree depleted suspended material with la yb n 1 9 we conclude that the terrigenous input to the oceans from major rivers is heavy ree depleted relative to shales from the available data average river water dissolved material appears to be heavy ree enriched with la yb n 0 4 estuarine removal processes lower the dissolved ree river flux by approximately 60 and result in a flux that is more heavy ree enriched with la yb n 0 2 calculated oceanic residence times with respect to river input range from 2300 to 21 000 years are shortest for ce and greatest for the heavy ree and la such long residence times may suggest the presence of additional sources of ree in seawater

morphological modelling of rivers with erodible banks a bank erosion mechanism and provisions to account for the associated planform changes and input of bank erosion products are added to a two dimensional depth averaged model of river morphology the model is applied to a reach of the meandering gravel bed river oh e eger in the former state of czechoslovakia the agreement with observations is poor but this can be ascribed to shortcomings in the flow and bed topography submodels rather than to shortcomings in the bank erosion submodel better results are expected when a three dimensional flow model equations for sediment mixtures and a bank accretion mechanism are included this inclusion will have to be preceded by fundamental research on bank accretion mechanisms and on hiding and exposure effects in the relationship for the influence of gravity pull on sediment transport direction 1998 john wiley sons ltd

archean banded iron formations of india abstract archean banded iron formations bifs are important lithologies for understanding the early earth processes greenstone belts in the four archean cratonic nucleii of peninsular india namely singhbhum bastar dharwar and bundelkhand host bif units that can be important candidates for case studies the bifs in these greenstone successions represent both algoma and superior type associations the oldest record comes from the paleoarchean southern eastern and western iron group of the singhbhum craton and the sargur supracrustals of the dharwar craton mesoarchean and neoarchean greenstone belts in the dharwar and bundelkhand cratons record a number of cycles of bif deposition in a spectrum of depositional settings from manganiferous arenite carbonate shelf associations to deep water volcanics and graywacke associations the bifs from the neoarchean bailadila group in the bastar craton were deposited in siliciclastic shelf association compositional variations in major trace and ree patterns have been evaluated from published data high grade iron ores are mainly mined from the bifs and locally mn ores occur in association as well microbial carbonates associated with some of the paleoarchean as well as neoarchean iron manganese formations suggest early life activities the lithological association of the archean iron formations have been explained in terms of both active margin and passive margin depositional settings bifs in association with manganiferous formations as old as paleoarchean time provide an opportunity to probe into the redox stratification of oceanic hydrosphere and in turn the oxygenation of early atmosphere

numerical study on the effects of floodplain vegetation on river planform style the effects of floodplain vegetation on river planform have been investigated for a medium sized river using a 2d morphodynamic model with submodels for flow resistance and plant colonization the flow resistance was divided into a resistance exerted by the soil and a resistance exerted by the plants in this way it was possible to reproduce both the decrease in bed shear stress reducing the sediment transport capacity of the flow within the plants and the increase in hydraulic resistance reducing the flow velocities colonization by plants was obtained by instantaneously assigning vegetation to the areas that became dry at low water stages this colonization presents a step forward in the modelling of bank accretion bank erosion was related to bed degradation at adjacent wet cells bank advance and retreat were reproduced as drying and wetting of the computational cells at the channel margins the model was applied to a hypothetical case with the same characteristics as the allier river france the river was allowed to develop its own geometry starting from a straight uniform channel different vegetation densities produced different planforms with bare floodplains the river always developed a braided planform even if the discharge was constant and below bankfull with the highest vegetation density grass the flow concentrated in a single channel and formed incipient meanders lower vegetation density pioneer vegetation led to a transitional planform with a low degree of braiding and distinguishable incipient meanders the results comply with flume experiments and field observations reported in the literature

coupling fluvial hydraulic models to predict gravel transport in spatially variable flows this study investigated spatial temporal variations of shear stress and bed load transport at three gravel bed river reaches of the williams fork river colorado a two dimensional flow model was used to compute spatial distributions of shear stress for four discharge levels between one third of bankfull qbf and qbf results indicate that mean values are highly variable among sites however the properties of the mean normalized distributions of are similar across sites for all flows the distributions of are then used with a transport function to compute bed load transport rates of individual grain size fractions probability distributions of the instantaneous unit width transport rates qb indicate that most of the bed load is transported through small portions of the bed with high the mean normalized probability distributions of qb are different among sites for all flows except at qbf when the distributions overlap we also find that the grain size distribution gsd of the bed load adjusts with discharge to resemble the grain size distribution of the subsurface at qbf we extend these results to 13 locations in the basin using the mean normalized distributions of shear stress and measured subsurface grain sizes to compute bed load transport rates at qbf we found a remarkably similar shape of the qb distribution among sites highlighting the basin wide balance between flow forces and gsd at qbf and the potential to predict sediment flux at the watershed scale

the influence of riparian vegetation on stream width eastern pennsylvania usa we surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches while deposition is localized on active floodplains on the insides of bends our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches two dimensionless parameters and e explain our observations represents the influence of grassy vegetation on rates of active floodplain deposition it is 5 times higher in nonforested reaches than in forested reaches e is proportional to rates of cutbank migration it is 3 times higher in nonforested reaches than in forested reaches differences in width between forested and nonforested reaches are proportional to e in forested reaches channels are wide with banks that are difficult to erode dense tree roots create a low value of e and the channel migrates slowly e is high however because is very low shade from trees inhibits the growth of grass on active floodplains in nonforested reaches channels are narrow with banks that are easy to erode e is high and the channel migrates rapidly e is low however due to a very large value of grass grows readily on nonforested convex bank floodplains thus differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active floodplains implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetation mediated rates of deposition on active flood plains these results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and floodplain depositional processes rather than focusing on these processes in isolation from one another

characteristics of intermittent turbulent structures for river bank undercut depth increment abstract the complex interplay between the resistive forces of the bank material and reynolds shear stress of the turbulent flow field control the fluvial bank erosion in depth knowledge on the interactions of the flow form at the river bank is a prerequisite to understand erosion mechanism and quantification of the erosion rate measurement at 1 5 m from the bank face as reported in the literature revealed that turbulent lateral flux of stream wise momentum propagated towards the bank face that was held responsible for the removal of material from the bank face further experimental results from the literature depict that the stream wise and lateral fluctuating velocity components deploy a random forcing effect on the river bank face which may be one of the primary agents for sediment particle entrainment from the bank face it is gestated from these previous studies that the momentum flux and turbulent structures of the flow field has a great impact on the river bank erosion nevertheless the previous field and experimental studies do not focus on the flow turbulence and sediment interaction that extracts information on the detailed temporal features of sediment entrainment process thus the present study was carried out to understand the coupled dynamics of the effect of intermittent flow structures on the intermittent removal of river bank materials and for advancing the knowledge on river bank erosion process the present experimental study focuses on the evaluation of the turbulent flow characteristics when the undercut mechanism initiates in a cohesive river bank based on the results it is evident that moderate scale intermittent stream wise flow structures probably imposes a periodic loading on the cohesive aggregates of the bank face this accelerates the entrainment of aggregates from the bank face the study also reveals that the undercut increment is a small scale intermittent process findings of the present study may be helpful for the improved design of bank protection measures

the influence of kinetics on the oxygen isotope composition of calcium carbonate paleotemperature reconstructions rely on knowledge of the equilibrium separation of oxygen isotopes between aqueous solution and calcium carbonate although oxygen isotope separation is expected on theoretical grounds the temperature dependence remains uncertain because other factors such as slow exchange of isotopes between dissolved co2 species and water can obscure the temperature signal this is problematic for crystal growth experiments on laboratory timescales and for interpreting the oxygen isotope composition of crystals formed in natural settings we present results from experiments in which inorganic calcite is precipitated in the presence of 0 25 m dissolved bovine carbonic anhydrase ca the presence of dissolved ca accelerates oxygen isotope equilibration between the dissolved carbon species co2 h2co3 hco3 co32 and water thereby eliminating this source of isotopic disequilibrium during calcite growth the experimental results allow us to isolate for the first time kinetic oxygen isotope effects occurring at the calcite water interface we present a framework of ion by ion growth of calcite that reconciles our new measurements with measurements of natural cave calcites that are the best candidate for having precipitated under near equilibrium conditions our findings suggest that isotopic equilibrium between calcite and water is unlikely to have been established in laboratory experiments or in many natural settings the use of ca in carbonate precipitation experiments offers new opportunities to refine oxygen isotope based geothermometers and to interrogate environmental variables other than temperature that influence calcite growth rates

effects of uncertain topographic input data on two dimensional flow modeling in a gravel bed river 1 many applications in river research and management rely upon two dimensional 2d numerical models to characterize flow fields assess habitat conditions and evaluate channel stability predictions from such models are potentially highly uncertain due to the uncertainty associated with the topographic data provided as input this study used a spatial stochastic simulation strategy to examine the effects of topographic uncertainty on flow modeling many equally likely bed elevation realizations for a simple meander bend were generated and propagated through a typical 2d model to produce distributions of water surface elevation depth velocity and boundary shear stress at each node of the model s computational grid ensemble summary statistics were used to characterize the uncertainty associated with these predictions and to examine the spatial structure of this uncertainty in relation to channel morphology simulations conditioned to different data configurations indicated that model predictions became increasingly uncertain as the spacing between surveyed cross sections increased model sensitivity to topographic uncertainty was greater for base flow conditions than for a higher subbankfull flow 75 of bankfull discharge the degree of sensitivity also varied spatially throughout the bend with the greatest uncertainty occurring over the point bar where the flow field was influenced by topographic steering effects uncertain topography can therefore introduce significant uncertainty to analyses of habitat suitability and bed mobility based on flow model output in the presence of such uncertainty the results of these studies are most appropriately represented in probabilistic terms using distributions of model predictions derived from a series of topographic realizations

a morphology independent methodology for quantifying planview river change and characteristics from remotely sensed imagery abstract remotely sensed imagery of rivers has long served as a means for characterizing channel properties and detection of planview change in the last decade the dramatic increase in the availability of satellite imagery and processing tools has created the potential to greatly expand the spatial and temporal scale of our understanding of river morphology and dynamics to date the majority of gis and automated analyses of planview changes in rivers from remotely sensed data has been developed for single threaded meandering river systems these methods have limited applicability to many of the earth s rivers with complex multi channel planforms here we present the methodologies of a set of analysis algorithms collectively called spatially continuous riverbank erosion and accretion measurements scream scream analyzes planview river metrics regardless of river morphology these algorithms quantify both the erosion and accretion rates of riverbanks from binary masks of channels generated from imagery acquired at two time periods additionally the program quantifies the area of change between river channels and the surrounding floodplain and area of islands lost or formed between these two time periods to examine variations in erosion rates in relation to local channel attributes and make rate comparisons between river systems of varying sizes the program determines channel widths and bank curvature at every bank pixel scream was developed and tested on rivers with diverse and complex planform morphologies in imagery acquired from a range of observational platforms with varying spatial resolutions validation and verification of scream generated metrics against manual measurements show no significant measurement errors in determination of channel width erosion and bank aspects scream has the potential to provide data for both the quantitative examination of the controls on erosion rates and for the comparison of these rates across river systems ranging broadly in size and planform morphology

iron in precambrian rocks implications for the global oxygen budget of the ancient earth abstract banded iron formations bif are prominent in sediments older than 2 ga however little is known about the absolute abundance of bif in archean and early proterozoic sediments and the source of the fe is still somewhat uncertain also unknown is the role that fe may have played in the maintenance of low oxygen pressures in the archean and early proterozoic atmosphere an analysis of the chemical composition of precambrian rocks provides some insight into the role of fe in precambrian geochemical cycles the fe content of igneous rocks is well correlated with their ti content plots of fe vs ti in precambrian sandstones and graywackes fall very close to the igneous rock trend plots of fe vs ti in precambrian shales also follow this trend but show a definite scatter toward an excess of fe phanerozoic shales and sandstones lie essentially on the igneous rock trend and show surprisingly little scatter mn ti relations show a stronger indication of precambrian mn loss perhaps due to weathering under a less oxidizing early atmosphere these data show that fe was neither substantially added to nor significantly redistributed in archean and early proterozoic sediments enough hydrothermal fe was added to these sediments to increase the average fe content of shales by at most a factor of 2 this enrichment would probably not have greatly affected the near surface redox cycle or atmospheric oxygen levels continued redistribution of fe and mixing with weathered igneous rocks during the recycling of precambrian sediments account for the excellent correlation of fe with ti in phanerozoic shales and for the similarity between their fe ti ratio and that of igneous rocks

rare earth element geochemistry of some archean iron formations north of lake superior ontario rare earth element ree compositions of iron formation from two archean terrains in western ontario have been determined in order to assess the possible influence of hydrothermal activity on the ree patterns of chemical sediments one terrain is characterized by sulfide facies iron formation in association with volcanic flows and volcaniclastic sediments whereas the other is dominated by oxide facies iron formation intercalated within submarine fan clastic sediments mineral separates of chert magnetite and pyrite from the iron formations have low ree concentrations 20 30 ppm and display moderate to strong positive eu anomalies relative to archean shale the positive anomalies and lack of negative ce anomalies indicate that archean sea water from which iron formation locally precipitated was reduced although to varying degrees the ree patterns of mineral separates from a given locality are almost identical but the patterns for various localities differ in detail a number of iron formation

a new paradigm for δ18o in coral skeleton oxygen isotope fractionation response to biological kinetic effects abstract this study investigated the relative impact of temperature and light on oxygen isotope fractionation in the aragonite of the symbiotic coral acropora sp which was cultured under controlled conditions aragonite was randomly sampled and the oxygen isotopic composition 18o was analyzed at the micrometer scale measurements were collected from six cultured colonies using a factorial design of three temperatures 22 c 25 c and 28 c and two light intensities 200 and 400 mol photon m 2 s 1 observation of the newly formed skeletons revealed fibrous morphology the 18o distribution following a bin width of 0 25 in histograms was bimodal because the colonies were cultured under constant environmental conditions we suggest this bimodality in isotope distribution was caused by the diurnal cycle of 12 h light and 12 h dark that is necessary for coral health and to mimic natural conditions we assume that the two modal values correspond to the two fractionation end members present in daytime and nighttime fibers then we investigated the precipitation mechanism for each in contrast to a common assumption that the two fractionations are due to differences in ph created by more or less efficient ca2 pumping we demonstrate that the ph at the calcification site cannot explain the isotopic discrepancy between daytime and nighttime and conclude that aragonite precipitation does not obey classical thermodynamic rules we suggest that secretion of proteins by the calicoblastic layer different following the diurnal cycle can induce distinct biological kinetic processes we highlight the interplay of carbonic anhydrase ca and organic molecules to accelerate and facilitate the crystallization of coral aragonite only governed by coral metabolism

geology geochemistry and tectonic setting of archaean greywackes from karnataka nucleus india more than 80 of the area covered by the chitradurga and shimoga schist belts 3 0 2 6 ga of karnataka nucleus india is made up of greywackes feldspathic lithic and quartzose greywackes are deposited respectively along the western northern and the eastern margin of the chitradurga schist belt greywackes of the shimoga schist belt show a large compositional scatter between feldspathic and lithic types compositional differences indicate variable provenance for the different localities tonalite trondhjemite gneisses ttg mafic ultramafic schists basic intermediate acid volcanic rocks sedimentary rocks like banded iron formation quartzites and cherts and k granites in minor quantity were exposed in the source area volcanic debris was also contributed from contemporaneous volcanism in the basin greywackes in this region are invariably interbedded with fine grained biotite rich phyllites and schists marked compositional differences are observed between the greywackes and the interbedded phyllites the greywackes have k2o na2o 1 whereas phyllites show large variation in k2o na2o ratios ranging from 1 5 to 15 0 high cr ni content of the greywackes and phyllites of the chitradurga belt are indicative of an important contribution from ultramafic rocks light ree enrichment and negative eu anomalies are particularly pronounced in the mudpacks of the greywackes represented by phyllite the ree and other chemical data such as la th ti zr and k2 mgo ratios indicate that these greywackes were supplied from a juvenile continental nucleus and were laid down on a simatic and converging active continental margin an archaean miniplate model with low angle subduction and sedimentary basins of moderate size is proposed to explain the geochemical and tectonic features of the schist belts

predicting the geomorphological responses of gravel bed rivers to flow and sediment source perturbations at the watershed scale an application in an alpine watershed predicting morphological channel changes using physically based models requires extended data for the description of the river channel and for hydrological and sedimentological inputs at the watershed scale these data are usually scarce and such a refined modeling is typically difficult to build a simpler modeling of the morphological impacts due to the changes in the principal drivers that control channel shape and dynamics is more adaptable in this study we focused on the morphological responses of gravel bed rivers to flow and sediment source perturbation at watershed scale the aim is to develop and test a tool capable of semi quantitatively predicting the morphological river response at the watershed scale due to a set of spatially distributed perturbations the model considers flow regime q and sediment supply s as the two main factors controlling the fluvial morphology in alluvial rivers two indicators have been proposed to evaluate the alteration on q and s and they are illustrated as vectors on each reach of the river network the magnitude of the vectors corresponds to the intensity of the perturbation and its direction represents the changing trend that nine selected morphological variables bed elevation slope width depth wetted area width to depth ratio d50 terrace formation and colonization of vegetation are likely to follow from an initial state the trends or trajectories of changes were assessed based on empirical relations case studies and conceptual models this method was applied to the isere watershed 5700 km2 at grenoble france a river that hosts large and complex hydropower plant systems constructed during 50s 70s the predictions over 23 river reaches and eight variables were evaluated in the range where the model was capable of predicting the morphological evolution of the river system its performance was verified and in the majority of the cases the results were coherent with field surveys and previous observations the results indicate that this is a complex problem which needs more careful consideration of constraints that are difficult to assess such as simultaneous and different sources of perturbations hypotheses of initial dynamic equilibrium and sediment supply quantification

effect of sediment transport boundary conditions on the numerical modeling of bed morphodynamics abstractexperimental sediment transport and river morphologic studies in laboratory flumes can use two sediment supply methods an imposed feed at the upstream end or a recirculation of sediment f

mineralogy and geochemistry of archaean greenstone belt hosted mn formations and deposits of the dharwar craton redox potential of proto oceans

petrology and geochemistry of the 2 9ga itilliarsuk banded iron formation and associated supracrustal rocks west greenland source characteristics and depositional environment abstract here we present new field petrographic and geochemical data from the 2 9 ga itilliarsuk banded iron formation bif and associated lithologies within the itilliarsuk supracrustal belt south eastern nussuuaq west greenland the supracrustals represent a volcanic sedimentary sequence which rests unconformably on a basement of tonalite trondhjemite granodiorite ttg lithologies felsic metagreywackes meta semipelites and thinly bedded ferruginous shales were identified intercalated with the itilliarsuk bif other associated rocks include metapelites acidic metavolcanics and metagabbroic sills the supracrustals have experienced amphibolite facies metamorphism which has resulted in complete resetting of the u pb system with an apparent age of 1895 48 ma this tectono metamorphic event corresponds well with the paleoproterozoic rinkian orogeny known from this region the itilliarsuk oxide facies bif has been divided into two segments on the basis of major and trace elements chemistry a shaley bif with a strong clastic component and a more chemically pure bif the shaley bif contains high terrigenous influx as reflected by elevated al 2 o 3 up to 12 wt tio 2 high field strength elements hfse and transition metals the chemically pure bif is characterised by alternating high iron 68 wt and high silica 64 wt bands with low total rare earths and yttrium rey al 2 o 3 tio 2 and hfse contents suggesting a low detrital component the least altered bands of the bif record diagnostic archaean seawater features with post archaean average shale paas normalised positive la and eu anomalies enrichment in heavy rare earth elements hree relative to light rare earth elements lree pr yb paas t dm model ages of 3 23 2 85 ga and initial nd nd i values in the range of 0 43 to 4 05 average of 1 35 in contrast the chemically pure fe rich bands yield t dm model ages of 3 61 3 22 ga and nd i values from 2 87 to 0 09 average of 1 29 the associated supracrustal rocks in the study area have significantly higher positive nd i values the 143 nd 144 nd in the itilliarsuk bif therefore contrasts world bifs by exhibiting radiogenic positive nd i values in shallow seawaters where the rey s were controlled by a local depleted continental crust whereas the negative nd i values found in the iron rich layers suggest that the submarine hydrothermal source was influenced by an enriched mantle possibly an older subcontinental lithospheric segment the felsic metagreywackes are immature first cycle sio 2 al 2 o 3 4 4 la yb 1 chon metasediments with affinities to ttg suites primarily extrusives whereas the meta semipelites and metapelites contain a larger mafic contribution with higher content of fe 2 o 3 mgo cr ni and hrees this suggests that the bif was deposited in a highly unstable basin presumably in a palaeo continental slope or outer shelf environment with frequent fluctuations of epiclastic and volcanogenic sediments derived from adjacent bimodal sources the t dm model ages and the use of th sc zr and la th sc tectonic discrimination plots indicate that the metasediments were sourced from a juvenile ocean island arc setting

modulation of outer bank erosion by slump blocks disentangling the protective and destructive role of failed material on the three dimensional flow structure the three dimensional flow field near the banks of alluvial channels is the primary factor controlling rates of bank erosion although submerged slump blocks and associated large scale bank roughness elements have both previously been proposed to divert flow away from the bank direct observations of the interaction between eroded bank material and the 3 d flow field are lacking here we use observations from multibeam echo sounding terrestrial laser scanning and acoustic doppler current profiling to quantify for the first time the influence of submerged slump blocks on the near bank flow field in contrast to previous research emphasizing their influence on flow diversion away from the bank we show that slump blocks may also deflect flow onto the bank thereby increasing local shear stresses and rates of erosion we use our measurements to propose a conceptual model for how submerged slump blocks interact with the flow field to modulate bank erosion

upstream perturbation and floodplain formation effects on chute cutoff dominated meandering river pattern and dynamics a sustained dynamic inflow perturbation and bar floodplain conversion are considered crucial to dynamic meandering past experiments one dimensional modelling and linear theory have demonstrated that the initiation and persistence of dynamic meandering require a periodic transverse motion of the inflow however it remains unknown whether the period of the inflow perturbation affects self formed meander dynamics here we numerically study the effect of the inflow perturbation period on the development and meander dynamics of a chute cutoff dominated river which requires two dimensional modelling with vegetation forming floodplain on bars we extended the morphodynamic model nays2d with growth and mortality rules of vegetation to allow for meandering we tested the effect of a transversely migrating inflow boundary by varying the perturbation period between runs over an order of magnitude around typical modelled meander periods following the cutoff cascade after initial meander formation from a straight channel all runs with sufficient vegetation show series of growing meanders terminated by chute cutoffs this generates an intricate channel belt topography with point bar complexes truncated by chutes oxbow lakes and scroll bar related vegetation age patterns the sinuosity braiding index and meander period which emerge from the inherent biomorphological feedback loops are unrelated to the inflow perturbation period although the spin up to dynamic equilibrium takes a longer time and distance for weak and absent inflow perturbations this explains why in previous experimental studies dynamic meandering was only accomplished with a sustained upstream perturbation in flumes that were short relative to the meander wavelength our modelling of self formed meander patterns is evidence that scroll bar dominated and chute cutoff dominated meanders develop from downstream convecting instabilities this insight extends to many more fluvial estuarine and coastal systems in morphological models and experiments which require sustained dynamic perturbations to form complex patterns and develop natural dynamics 2019 the authors earth surface processes and landforms published by john wiley sons ltd

gold mineralisation and alteration of penakacherla schist belt india constraints on archaean subduction and fluid processes abstract in the dharwar craton southern india gold deposits are found mostly along the six arcuate shear zones passing through late archaean greenstone belts 2 7 ga one such shear zone complex extends for about 400 km within and along the ramagiri hungund schist belt the penakacherla sector of this shear zone is excellently exposed enabling a detailed investigation of synorogenic gold mineralisation and its relationship to associated hydrothermal alteration metamorphism and deformation under ne sw compression associated with archaean subduction processes converted mafic volcanic rocks into amphibolites and intermediate to felsic volcanic rocks into quartz mica schists continued compression generated a 50 100 m wide shear zone complex consisting of mafic phyllonites advection of hydrothermal fluids through this shear zone and reaction between fluids and the mafic phyllonites resulted in a silicified k metasomatic assemblage mainly consisting of chlorite amphibole k mica plagioclase ankerite quartz fe oxides pyrite chalcopyrite and arsenopyrite networks of quartz and carbonate veinlets a few millimeters to a few centimeters thick formed along the foliation planes giving rise to microscopic alteration envelope in which individual veinlet systems are merged into one another to form a composite alteration system gold is found within these quartz veinlets mafic phyllonites and at their mutual contacts hydrothermal fluids have modified the primary major minor trace and lree compositions of host rocks such that their mutual behaviour became non systematic some hfse and hree also show minor mobility but the overall ree pattern generally resembles that of the precursor mafic volcanic rocks mass and volume loss gain by si and ca has made significant impact on al ti and zr abundances which are generally immobile during hydrothermal alteration however element pairs such as zr hf v sc and nb ta maintain primary inter element ratios although their absolute abundances are drastically diluted similarly ree in highly silicified and carbonatised samples are reduced but patterns remain similar to those of relatively least altered mafic phyllonites with la n yb n between 1 and 3 in some samples lree enrichment is observed elevating in la n yb n from 3 to 11 pathfinder elements and base metals such as as cd cu pb zn and sb have been added along with the au and ag 13 c of carbon varies from 16 to 21 suggesting a biogenic origin whereas coexisting pyrite 34 s ranges from 1 to 3 pointing towards the involvement of magmatic or average crustal sulphur overall concentrations of k rb sr ba nb ta ti cs cr co v y and sc and many of the ratios such as k rb la sc la yb indicate that metamorphism devolatilisation and dehydration of an oceanic subducting slab might have partially contributed the mineralising fluids and generated the alteration assemblage observed in the host rocks fluid sources were mantle and greenstone belt dehydration and devolatilisation generating observed compositional and alteration diversity

numerical simulation of relatively wide shallow channels with erodible banks a two dimensional numerical model was developed to simulate relatively wide shallow rivers with an erodible bed and banks composed of well sorted sandy materials a moving boundary fitted coordinate system was used to calculate water flow bed change and bank erosion the cubic interpolated pseudoparticle method was used to calculate flow which introduced little numerical diffusion the sediment transport equation for the streamline and transverse transport was used to estimate bed and bank evolution over time while considering the secondary flow bank erosion was simulated when the gradient in the cross sectional direction of the banks was steeper than the submerged angle of repose because of bed erosion near the banks the numerical model reproduced the features of central bars well such as bar growth channel widening due to divergence of the flow around the bars scour holes at the lee of the bars and the increase of bar size with time these features were in accordance with the observations for laboratory experiments it also reproduced the features of braided rivers such as the generation of new channels and abandonment of old channels the bifurcation and confluence of channels and the lateral migration of the channels the model showed that the sediment discharge rate fluctuated with time one of the dynamic features observed in braided channels

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

palaeomagnetic geochronological and geochemical study of mesoproterozoic lakhna dykes in the bastar craton india implications for the mesoproterozoic supercontinent abstract palaeomagnetic analysis of the lakhna dykes bastar craton india yields a palaeopole at 36 6 n 132 8 e dp 12 4 dm 15 9 and the u pb zircon age obtained from one of the rhyolitic dykes is 1466 4 2 6 ma mswd 0 21 concordia age based on two analyses with identical pb u ages similar to previously published u pb ages major and trace element analyses of the lakhna dykes show shoshonitic and high k calc alkaline affinities consistent with a subduction related characteristics suggesting an active continental margin setting this is in keeping with the palaeo to mesoproterozoic tectonic environments in the eastern indian margin the new 1460 ma indian palaeopole was used to test possible palaeopositions of india within the mesoproterozoic supercontinent columbia of the four palaeomagnetically permissible reconstructions juxtaposing western india against south west baltica is geologically the most reliably constrained and best fitting model our preferred reconstruction implies a long palaeo to mesoproterozoic accretionary orogen stretching from south eastern laurentia through south western baltica to south eastern india breakup of india and baltica probably occurred in the late mesoproterozoic but additional constraints are needed

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

petrography mineralogy and geochemistry of the late eocene oolitic ironstones of the jebel ank southern tunisian atlas abstract the oolitic ironstones ore deposit of jebel ank central tunisia is a simply folded stratiform ore body of about 2 5 8 m thickness located in the upper part of the epicontinental souar formation late eocene and is covered by the continental segui formation mio pliocene the deposit contains about 20 mt of ore with an average grade of 50 fe generally oolitic iron deposition occurs in shallow water lagoonal environments the jebel ank deposit lies between two regional disconformities late eocene and miocene and is evidence of a transitional stage at the end of regional regression before renewed transgression the footwall of the oolitic iron ore bearing bed consists of a fine grained sandstone bed 10 20 cm thick pinching out laterally westward into green clays the hanging wall is composed of thin bedded limestone and clay alternations 2 3 5 m thick iron occurs in the form of cryptocrystalline goethite with limited al fe substitution the goethite contains around 48 fe 5 al and up to 1 5 p jarosite alunite and manganese minerals cryptomelane psilomelane and manjiorite are supergene secondary minerals probably related to descending surface fluids these manganese minerals occur as accessory minerals with the goethite and are most abundant at the lowermost part of the succession showing varied morphologies local cement space filling and free centimeter sized nodules fe oolites in the deposit are similar to those documented in many other oolitic ironstone deposits the dominant fe oolite type 90 has a concentrically laminated cortex with no nucleus the nuclei of the oolites that do have a nucleus are most commonly detrital quartz grains major elements in high grade samples fe 2 o 3 65 vary within a limited range and show higher concentrations of sio 2 average 7 85 and al 2 o 3 average 5 1 with minor tio 2 mno mgo na 2 o k 2 o and so 3 less than 1 paas normalized trace elements of bulk samples and fe oolite generally show similar behavior both are enriched in v co ni mo as zn and y and are depleted in cu rb zr nb ba and hf anomalous v cr ni zn and ree y are correlated with goethite paas normalized ree y patterns of both bulk samples and fe oolite show slight hree enrichment positive ce with negative y anomalies the mineralogy goethite and cryptomelane along with the geochemistry si vs al as cu mo pb v zn vs ni co binary plots zn ni co triangular diagram ree y content and patterns and ce ce vs nd and ce ce vs y n ho n binary plots of the studied oolitic ironstone are congruent with a hydrogenetic type while two possible sources of iron for jebel ank ironstone can be proposed i submarine weathering of glauconite rich sandstone and ii detrital iron from adjacent continental hinterland the later is the more plausible source of iron based on paleogeographic setting the occurrence of fine sandstone underlying the iron level occurrence of mn ores in the lower part of the fe ores succession high phosphorous zinc ree y concentrations and y ho ratios and low la ce ratios

correlation of boron isotopic composition with ultrastructure in the deep sea coral lophelia pertusa implications for biomineralization and paleo ph using the crpg cnrs cameca 1270 ion microprobe facility we have measured boron isotopic compositions b 11 b 10 in different ultrastructural components of the deep sea aragonitic scleractinian coral lophelia pertusa we observe a systematic difference in b isotopic composition between the early mineralization zone emz and adjacent fibrous skeleton in emz the measured delta 11 b values are consistently low fibrous aragonite is characterized by systematically higher delta 11 b values but also displays b isotopic heterogeneity associated with specific growth bands in the calyx wall the magnitude of the observed b isotopic variations cannot be explained by changes in environmental conditions and is likely caused by biological processes involved in the biomineralization of new skeleton i e vital effects the observed b isotopic variations are opposite to the predictions of geochemical models for vital effects these models are based on the idea that stable isotopic fractionations including c and o in coral skeleton are driven by changes in ph of the fluid from which the skeleton is presumed to precipitate our data indicate that ph variations are not responsible for the observed stable isotopic fractionations geochemical models therefore do not provide an adequate framework within which to understand coral skeletal formation without a better understanding of these processes the use of b isotopic composition to reconstruct paleo ph variations in the oceans must be considered problematic at least as far as lophelia pertusa is concerned

numerical simulation of large scale bed load particle tracer advection dispersion in rivers with free bars asymptotic characteristics of the transport of bed load tracer particles in rivers have been described by advection dispersion equations here we perform numerical simulations designed to study the role of free bars and more specifically single row alternate bars on streamwise tracer particle dispersion in treating the conservation of tracer particle mass we use two alternative formulations for the exner equation of sediment mass conservation the flux based formulation in which bed elevation varies with the divergence of the bed load transport rate and the entrainment based formulation in which bed elevation changes with the net deposition rate under the condition of no net bed aggradation degradation a 1 d flux based deterministic model that does not describe free bars yields no streamwise dispersion the entrainment based 1 d formulation on the other hand models stochasticity via the probability density function pdf of particle step length and as a result does show tracer dispersion when the formulation is generalized to 2 d to include free alternate bars however both models yield almost identical asymptotic advection dispersion characteristics in which streamwise dispersion is dominated by randomness inherent in free bar morphodynamics this randomness can result in a heavy tailed pdf of waiting time in addition migrating bars may constrain the travel distance through temporary burial causing a thin tailed pdf of travel distance the superdiffusive character of streamwise particle dispersion predicted by the model is attributable to the interaction of these two effects

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

morphodynamics of a bedrock alluvial meander bend that incises as it migrates outward approximate solution of permanent form in meandering rivers cut into bedrock erosion across a channel cross section can be strongly asymmetric at a meander apex deep undercutting of the outer bank can result in the formation of a hanging cliff which may drive hillslope failure whereas the inner bank adjoins a slip off slope that connects to the hillslope itself here we propose a physically based model for predicting channel planform migration and incision point bar and slip off slope formation bedrock abrasion the spatial distribution of alluvial cover and adaptation of channel width in a mixed bedrock alluvial channel we simplify the analysis by considering a numerical model of steady uniform bend flow satisfying cyclic boundary conditions thus in our analysis sediment supply i e the total volume of alluvium in the system is conserved in our numerical simulations the migration rate of the outer bank is a specified parameter our simulations demonstrate the existence of an approximate state of dynamic equilibrium corresponding to a near solution of permanent form in which a bend of constant curvature width cross sectional shape and alluvial cover distribution migrates diagonally downward at constant speed leaving a bedrock equivalent of a point bar on the inside of the bend channel width is set internally by the processes of migration and incision we find that equilibrium width increases with increasing sediment supply but is insensitive to outer bank migration rate the slope of the bedrock point bar varies inversely with both outer bank migration rate and sediment supply although the migration rate of the outer bank is externally imposed here we discuss a model modification that would allow lateral side wall abrasion to be treated in a manner similar to the process of bedrock incision this article is protected by copyright all rights reserved

an overview of biomineralization processes and the problem of the vital effect biomineralization links soft organic tissues which are compositionally akin to the atmosphere and oceans with the hard materials of the solid earth it provides organisms with skeletons and shells while they are alive and when they die these are deposited as sediment in environments from river plains to the deep ocean floor it is also these hard resistant products of life which are mainly responsible for the earth s fossil record consequently biomineralization involves biologists chemists and geologists in interdisciplinary studies at one of the interfaces between earth and life leadbeater and riding 1986 biomineralization refers to the processes by which organisms form minerals the control exerted by many organisms over mineral formation distinguishes these processes from abiotic mineralization the latter was the primary focus of earth scientists over the last century but the emergence of biogeochemistry and the urgency of understanding the past and future evolution of the earth are moving biological mineralization to the forefront of various fields of science including the earth sciences the growth in biogeochemistry has led to a number of new exciting research areas where the distinctions between the biological chemical and earth sciences disciplines melt away of the wonderful topics that are receiving renewed attention the study of biomineral formation is perhaps the most fascinating truly at the interface of earth and life biomineralization is a discipline that is certain to see major advancements as a new generation of scientists brings cross disciplinary training and new experimental and computational methods to the most daunting problems it is however by no means a new field the first book on biomineralization was published in 1924 in german by w j schmidt schmidt 1924 and the subject has continued to intrigue a dedicated community of scientists for many years until the early 1980s the field was known as

geochemistry of the neoarchaean high mg basalts boninites and adakites from the kushtagi hungund greenstone belt of the eastern dharwar craton edc implications for the tectonic setting abstract volcanic rocks of the kushtagi hungund greenstone belt from the eastern dharwar craton edc show extreme diversity due to the presence of high mg high fe high mg fe basalts high mg dacites and andesites boninitic affinities and sodic plagioclase quartz rich felsic rocks adakites with na2o 4 6 wt the adakites drummond and defant 1990 martin 1999 and high mg dacites andesites are being reported for the first time from greenstone belts of the dharwar craton trace elements in these different types of volcanic rocks have distinct characteristics for example the basalts have 1 al2o3 tio2 16 pm value of 17 2 la yb n 1 3 3 low order negative anomalies of nb nb zr zr hf hf and ti ti 4 zr nb 4 10 and 5 ce nb 3 4 two samples of andesites having mg enrichment and al2o3 tio2 ratios of 31 35 are interpreted to have boninitic affinities other dacites and andesites are characterized by 1 high mg 8 10 wt 2 high al2o3 tio2 22 27 3 zr nb 8 14 4 la yb n 2 7 5 negative nb nb zr zr hf hf and ti ti anomalies and 6 sc lu 67 172 the adakites are characterized by 1 a narrow range of sio2 67 70 wt 2 enriched na2o 4 6 wt with very high na2o k2o 2 5 7 7 3 low mg 0 38 0 48 4 high la yb n 13 31 with fractionated patterns no eu anomaly and gd yb n 2 4 5 very high al2o3 tio2 50 80 and 6 enriched ni cr and co total 50 ppm these adakites resemble 3 0 ga tonalite trondhjemite gneisses of the dharwar craton which are interpreted as products of partial melting of the basaltic slab the available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting

morphological and compositional changes in the skeletons of new coral recruits reared in acidified seawater insights into the biomineralization response to ocean acidification 1 we reared primary polyps new recruits of the common atlantic golf ball coral favia fragum for 8 days at 25 c in seawater with aragonite saturation states ranging from ambient 3 71 to strongly undersaturated 0 22 aragonite was accreted by all corals even those reared in strongly undersaturated seawater however significant delays in both the initiation of calcification and subsequent growth of the primary corallite occurred in corals reared in treatment tanks relative to those grown at ambient conditions in addition we observed progressive changes in the size shape orientation and composition of the aragonite crystals used to build the skeleton with increasing acidification densely packed bundles of fine aragonite needles gave way to a disordered aggregate of highly faceted rhombs the sr ca ratios of the crystals measured by sims ion microprobe increased by 13 and mg ca ratios decreased by 45 by comparing these variations in elemental ratios with results from rayleigh fractionation calculations we show that the observed changes in crystal morphology and composition are consistent with a 80 decrease in the amount of aragonite precipitated by the corals from each batch of calcifying fluid this suggests that the saturation state of fluid within the isolated calcifying compartment while maintained by the coral at levels well above that of the external seawater decreased systematically and significantly as the saturation state of the external seawater decreased the inability of the corals in acidified treatments to achieve the levels of calcifying fluid supersaturation that drive rapid crystal growth could reflect a limit in the amount of energy available for the proton pumping required for calcification if so then the future impact of ocean acidification on tropical coral ecosystems may depend on the ability of individuals or species to overcome this limitation and achieve the levels of calcifying fluid supersaturation required to ensure rapid growth

geology geochemistry and genesis of bif of kushtagi schist belt archaean dharwar craton india the banded iron formation bif of the kushtagi schist belt dharwar craton is interbedded with metavolcanics the oxide fades cherty al2o3 2 bifs show large scale variations in their major and trace elements abundance cherty banded iron formation cbif is depleted in al2o3 tio2 zr hf and other trace elements like cr ni co rb sr v y and ree in comparison to shaley banded iron formation sbif depleted ree positive eu anomalies and the flat to hree enriched pattern of cbif indicate that fe and sio2 for these bifs were added to ambient ocean water by hydrothermal solutions at the amor vent sites it is inferred that the higher amount of hydrothermal fluid flux with a higher exit temperature provided enormous quantities of iron and silica fine grained sedimentation in the basin gave rise to the observed variability in the composition of bif during transgression a wave base was raised up consequently deposition of cbif became possible whereas during the regressive stage these chemical sediments were buried by and or mixed with the terrigenous sediments resulting in deposition of sbif and interbedded shales volcaniclastic activity within the basin appears to have contributed significantly to the composition of some sbif and shales the hydrothermal exhalative hypothesis combined with the archaean miniplate model explains most of the chemical features of the bifs of greenstone belts

channel adjustments to changing discharges powder river montana sixteen years of annual surveys reveal how powder river responds to varying discharges during 1978 the second largest recorded daily mean discharge occurred cutbank migration bed degradation net bank erosion and overbank deposition all contributed to increase the channel area at 12 cross sections by an average of 62 during the ensuing years the channel area decreased as sediment was stored in low lying benches adjacent to the active bed of the channel the survey data indicate that the balance between bank erosion and deposition varies with discharge in years when the annual maximum daily mean discharge is 3 s a flow of 60 m 3 s has a recurrence interval of 1 1 yr bank erosion and deposition are approximately equal in years when the annual maximum daily mean discharge is between 60 and 150 m 3 s a discharge of 150 m 3 s has a recurrence interval of 2 7 yr bank deposition exceeds bank erosion and the channel contracts often by developing benches in years with higher discharges the channel expands through net bank erosion these results demonstrate that the channel of powder river is influenced by a wide variety of formative discharges powder river9s recent history of expansion and contraction and the development of prominent benches cannot be explained by equilibrium models based on a single channel forming discharge

formation and dynamics of vegetated fluvial landforms follow the biogeomorphological succession model in a channelized river feedback between hydrogeomorphological processes and riparian plants drives landscape dynamics and vegetation succession in river corridors we describe the consequences of biogeomorphological feedback on the formation and dynamics of vegetated fluvial landforms based on observations from the channelized isere river in france the channel was laterally confined with embankments and mostly straightened from the beginning of the 1970s to the end of the 1990s alternate bars were progressively but heavily colonized by vegetation this context presented an exceptional opportunity to analyse temporal adjustments between fluvial landforms and vegetation succession from bare gravel bars to mature upland forest as the consequence of biogeomorphological interactions based on a gis analysis of aerial photographs between 1948 and 1996 we show that the spatiotemporal organization of vegetated bars within the river channel observed in 1996 resulted from a bioconstruction and biostabilization effect of vegetation and interactions between bars of varying age size and mobility field measurements in 1996 reflected how a strong positive feedback between sedimentary dynamics and riparian vegetation succession resulted in the construction of the vegetated bars a highly significant statistical association of geomorphological and vegetation variables rv of co inertia analysis 0 41 p 0 001 explained 95 of the variability in just one axis supporting the existence of very strong feedback between geomorphological changes i e the transformation of small bare alternate bars to fluvial landforms covered by mature upland forest and vegetation succession such dynamics reflect the fluvial biogeomorphological successions model as described by the authors earlier 2020 john wiley sons ltd

stable isotope vital effects in coccolith calcite uncertainties about the origin of the many disequilibrium or vital effects in a variety of calcifying organisms and whether these effects are constant or variable have hampered paleoceanographic application of carbon and oxygen isotopic ratios unraveling the source of these effects will improve paleoceanographic applications and may provide new information on changes in cell physiology and ecology culture of eight species of coccolithophorids a dominant marine phytoplankton group reveals a 5 array of disequilibrium or vital effects in both the carbon and oxygen isotopic composition of coccolith calcite in moderate light and nutrient replete cultures oxygen isotopic fractionation and carbon isotopic fractionation correlates directly with cell division rates and correlates inversely with cell size across a range of species however when growth rates of a single species are increased or decreased by higher or lower light levels 18o is relatively invariant likewise growth rate variations as a function of temperature do not influence coccolith 18o the slope of the 18o vs temperature relation in cultures of both gephyrocapsa oceanica and helicosphaera carteri is the same as for abiogenic carbonates this suggests a constant species specific isotopic fractionation which does not vary with cell physiology the constancy of vital effects suggests that coccolith stable isotopes will provide reliable phase for paleoceanographic reconstruction of temperature and seawater chemistry as long as monospecific fractions are analyzed or changes in nannofossil assemblages are accounted for with species specific correction factors we suspect that the cell size and its constraints on the rate of co2 diffusion relative to c fixation may be the first order influence on coccolith stable isotope vital effects a quantitative model of this process may provide important constraints on mechanisms of carbon acquisition of coccolithophorids in both modern and extinct species

europium redox equilibria in aqueous solution thermochemical data and recent advances in theoretical aqueous solution chemistry enable prediction of the relative stabilities of aqueous eu2 and eu3 over a wide range of temperatures and pressures at low temperatures near earth surface conditions the aqueous geochemistry of europium should be dominated by the trivalent state except possibly in the most reducing alkaline pore waters of anoxic marine sediments however at temperatures greater than about 250 c and elevated pressures divalent europium should predominate even with significant amounts of complexing trivalent europium is not stable at elevated temperatures and pressures consequently under most hydrothermal and metamorphic conditions europium in aqueous solution should be divalent at intermediate temperatures around 100 c significant activities of both eu2 and eu3 and related complexes can occur in aqueous solutions depending on the oxidation state and the ph of the solutions and the activities of potential ligands such as sulfate carbonate and chloride the predicted stability of divalent europium in aqueous solution at elevated temperatures is consistent with the large positive europium anomalies in rare earth element patterns of high temperature barites of hydrothermal and metamorphic origin reported by guichard et al 5 and with the observed depletion of europium as a result of high temperature sericitization of feldspar bearing assemblages discovered by alderton et al 1 it is suggested that significant fractionation of europium relative to the other rare earth elements may take place during high temperature hydrothermal alteration processes such as in the mid oceanic ridge systems because of the expected stability of divalent europium during these processes

numerical simulation of river meandering with self evolving banks 1 in this study the natural process of river meandering is captured in a computational model that considers the effects of bank erosion the process of land accretion along the inner banks of meander bends and the formation of channel cutoffs the methodology for predicting bank erosion explicitly includes a submodel treating the formation and eventual removal of slump blocks the accretion of bank material on the inner bank is modeled by defining the time scale over which areas that are originally channel become land channel cutoff formation is treated relatively simply by recomputing the channel alignment at a single model time step when migrating banks meet the model is used to compute meandering processes in both steady and unsteady flows the key features of this new model are the ability a to describe bank depositional and bank erosional responses separately b to couple them to bed morphodynamics and thus c to describe coevolving river width and sinuosity two cases of steady flow are considered one with a larger discharge i e bankfull and one with a smaller discharge i e low flow in the former case the shear stress is well above the critical shear stress but in the latter case it is initially below it in at least one case of constant discharge the planform pattern can develop some sinuosity but the pattern appears to deviate somewhat from that observed in natural meandering channels for the case of unsteady flow discharge variation is modeled in the simplest possible manner by cyclically alternating the two discharges used in the steady flow computations this model produces a rich pattern of meander planform evolution that is consistent with that observed in natural rivers also the relationship between the meandering evolution and the return time scale of floods is investigated by the model under the several unsteady flow patterns the results indicate that meandering planforms have different shapes depending on the values of these two scales in predicting meander evolution it is important to consider the ratio of these two time scales in addition to such factors as bank erosion slump block formation and decay bar accretion and cutoff formation which are also included in the model

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

trace rare earth element and oxygen isotopic systematics for the genesis of banded iron formations evidence from kushtagi schist belt archaean dharwar craton india abstract the ree distribution pattern shapes free 5 92 and 27 87 ppm in chert and hematite respectively positive eu anomalies eu eu 3 25 and 2 1 and la n yb n ratios 0 75 and 0 87 of the chert and hematite layers of the oxide facies bifs of kushtagi schist belt indicate that these constituents were derived from hydrothermal solutions generated and emplaced at an archaean ocean ridge however their oxygen isotopic composition have been modified by mixing with the ambient ocean water and diagenetic reactions 18o values 13 9 17 4 c and 8 6 to 3 3 in chert and hematite respectively yield 150 250 c temperature in the diagenetic and lowest greenschist facies field the mixing model of hydrothermal solutions with ocean water of 1 1000 ratio will not significantly change the oxygen isotope ratio of the ambient ocean water therefore while the trace element data have retained hydrothermal signatures oxygen isotopic signatures are imprinted by diagenetic processes

compositional variability in a cold water scleractinian lophelia pertusa new insights into vital effects we analyzed sr ca and mg ca ratios in the thecal wall of lophelia pertusa a cold water coral using sims ion microprobe techniques the wall grows by simultaneous upward extension and outward thickening compositional variability displays similar trends along the upward and outward growth axes sr ca and mg ca ratios oscillate systematically and inversely the sensitivity of lophelia sr ca ratios to the annual temperature cycle 0 18 mmol mol 1 c is twice as strong as that exhibited by tropical reef corals and four times as strong as the temperature dependence of sr ca ratios of abiogenic aragonites precipitated experimentally from seawater a comparison of the skeletal composition of lophelia with results from precipitation calculations carried out using experimentally determined partition coefficients suggests that both temperature dependent element partitioning and seasonal changes in the mass fraction of aragonite precipitated from the calcifying fluid influence the composition of lophelia skeleton results from calculations that combine these effects reproduce both the exaggerated amplitude of the sr ca and mg ca oscillations and the inverse relationship between sr ca and mg ca ratios

heavy metal and sulfur transport during subcritical and supercritical hydrothermal alteration of basalt influence of fluid pressure and basalt composition and crystallinity crystalline basalt diabase and basalt glass have been reacted with a na ca k cl fluid of seawater ionic strength at 350 425 c 375 400 bars pressure and fluid rock mass ratios of 0 5 1 0 to assess the role of temperature basalt diabase chemistry and texture on heavy metal and sulfur mobility during hydrothermal alteration alteration of basalt diabase is characterized by cation fixation and hydrolysis reactions which show increased reaction progress with increasing temperature at constant pressure correspondingly ph in a series of 400 bar experiments ranges from 4 8 to 2 7 at 350 and 425 c respectively and is typically lower for alteration of a sio2 rich crystalline basalt than for other rock types due in part to relatively high sio2 concentrations in solution high sio2 concentrations stabilize hydrous na and ca rich alteration phases causing ph to decrease according to reactions such as 3 0 caal2si2o8 1 0 ca 2 0 h2o 2 0 ca2al3si3o12 oh 2 0 h phases experimentally produced include mixed layer chlorite smectite ca rich amphibole and clinozoisite clinozoisite was identified as a replacement product of plagioclase from diabase solution interaction experiments in direct response to h production dissolved fe mn and h2s concentrations increase dramatically for early stage reaction h2s typically exceeds fe and mn however at 425 c and after long term reaction at 400 c h2s is lost from solution apparently in response to pyrite replacement of oxide and silicate phases pyrrhotite formed at temperatures 375 c whereas magnetite was identified in all run products except from basalt glass alteration cu and zn concentrations in solution are not simple functions of ph these metals achieve greatest solubility in fluids from experiments at 375 400 c except when basalt glass is used as a reactant the relatively low concentrations of these species in solution during basalt glass reaction may be due to adsorption by fine grained alteration phases

ree fe variations in hydrothermal sediments implications for the ree content of seawater seafloor hydrothermal vent solutions exhibit rare earth element ree enrichments ranging between one to three orders of magnitude greater than average seawater to assess the impact of these hydrothermal inputs on ocean chemistry we have examined the behavior of rees for hydrothermal sediments collected adjacent to two pacific spreading ridge sites the east pacific rise at 19 s and the southern juan de fuca ridge at 4s n in general the ree fe ratios for both proximal and distal hydrothermal sediments are greater than vent solutions by a factor of 2 to 500 and these ratios increase with increasing distance away from the ridge axis an evaluation of these results in the context of previous models of ree behavior indicates that in fact seawater experiences a net depletion in rees as a result of hydrothermal activity this is due primarily to the large scavenging capacity of iron oxyhydroxides which precipitate from these solutions such an interpretation explains why the ree content of seawater collected in the vicinity of hydrothermal vents is anomalously lower than normal seawater sampled from a comparable depth

origin and depositional environment of ordovician stratiform iron mineralization from zamora nw iberian peninsula the ordovician stratiform iron deposits at zamora nw iberia are arranged in several levels ranging between 0 2 and 1 5 m in thickness which are interstratified in the upper member of the pielgo quartzites formation arenig the sandy nature and trace fossils corresponding to the ichnogenus cruziana and daedalus together with major and trace element contents of this formation suggest an inter and subtidal shallow marine depositional environment which on a global scale formed part of a broad shelf situated in the northern margin of the gondwana continent the iron mineralization displays a foliated and banded structure due to the alternation of quartzitic phosphatic apatite chamositic chamositic biotitic and ferriferous magnetite and hematite beds they have high tio2 ta sc v nb co zn and y contents the magnetite contains unusually large amounts of tio2 v cr and ni there is also a clear depletion in eu and the eu sm cn ratio is 1 the chamosite contains high concentrations of cr and v these results suggest that iron was supplied from the weathering of a continental source in combination with volcanic activity such as within basin basic volcanism or the presence of basic volcanic rocks in the exposed land the physicochemical conditions of iron mineral crystallization calculated from chamosite compositions are the following log f o2 38 8 to 30 7 log f s2 13 2 to 9 5 and t 200 to 330 c these results together with the 18o value 2 of the magnetite suggest that chamosite and magnetite were crystallized during later diagenesis and early low grade metamorphism under redox conditions below the magnetite hematite buffer

dynamic single thread channels maintained by the interaction of flow and vegetation most rivers on earth today flow as a single channel in some cases with occasional islands and follow a more or less sinuous course however single thread channels have proven difficult to reproduce and study experimentally experimental self formed channels tend to widen and subdivide leading to a braided pattern cohesive sediment has been the main mechanism studied for stabilizing banks and producing a single thread channel we show how laboratory experiments using vegetation to stabilize banks can organize the flow and convert the planform morphology from braided to single thread our experimental strategy a repeated cycle of short periods of high water discharge alternating with longer periods of low discharge accompanied by plant seeding and growth leads to the evolution of a dynamic self maintaining single thread channel with well defined banks and floodplain by eliminating weak flow paths the vegetation corrals the water into a single dominant channel until the reduction in total wetted width leads to a new self organized state in which the flow removes vegetated area as fast as it is produced the new channel is deeper and has a broader distribution of depths than the braided one with channel size adjusted to carry almost all the flood flow the resulting system maintains a dynamic steady state via similar mechanisms to those that operate in meandering channels in the field specifically erosion at the outside of bends bend growth and bar development our methodology provides a basis for experimental development of self sustaining high amplitude meanders and has applications for river management and basic research purposes

the role of a transcrustal shear zone in orogenic gold mineralization at the ajjanahalli mine dharwar craton south india the ajjanahalli gold mine is spatially associated with a late archean craton scale shear zone in the eastern chitradurga greenstone belt of the dharwar craton india gold mineralization is hosted by an 100 m wide antiform in a banded iron formation original magnetite and siderite are replaced by a peak metamorphic alteration assemblage of chlorite stilpnomelane minnesotaite sericite ankerite arsenopyrite pyrite pyrrhotite and gold at ca 300 to 350 c elements enriched in the banded iron formation include ca mg c s au as bi cu sb zn pb se ag and te whereas in the wall rocks as cu zn bi ag and au are only slightly enriched strontium correlates with cao mgo co2 and as which indicates cogenetic formation of arsenopyrite and mg ca carbonates the greater extent of alteration in the fe rich banded iron formation layers than in the wall rock reflects the greater reactivity of the banded iron formation layers the ore fluids as interpreted from their isotopic composition 18o 6 5 8 5 initial 87sr 86sr 0 7068 0 7078 formed by metamorphic devolatilization of deeper levels of the chitradurga greenstone belt arsenopyrite chalcopyrite and pyrrhotite have 34s values within a narrow range between 2 1 and 2 7 per mil consistent with a sulfur source in chitradurga greenstone belt lithologies based on spatial and temporal relationships between mineralization local structure development and sinistral strike slip deformation in the shear zone at the eastern contact of the chitradurga greenstone belt we suggest that the ajjanahalli gold mineralization formed by fluid infiltration into a low strain area within the first order structure the ore fluids were transported along this shear zone into relatively shallow crustal levels during lateral terrane accretion and a change from thrust to transcurrent tectonics based on this model of fluid flow exploration should focus on similar low strain areas or potentially connected higher order splays of the first order shear zone

effect of clay fraction on turbulence characteristics of flow near an eroded bank abstract stream bank erosion in the alluvial channel is a matter of great concern particularly for the irretrievable loss of fertile land this in effect has direct impact on the agriculture environment economy and social issues at large a series of laboratory experiments were carry out to study the turbulence structure over eroded bank topography modulation of mean turbulent flow characteristics and the benefaction of turbulent bursting structures to the total reynolds shear stress due to the changed geometry of the eroded bank wall are described for three different clay fraction variations a side looking micro acoustic doppler velocimeter adv was used to record the detailed high frequency velocity data near the bank wall to understand the coupled dynamics of flow and erosion on sediment water interface the information on clay and fine sand content of natural river bank was obtained from field observation results from the present study indicate that higher turbulence intensities and reynolds shear stress have prominent role in the removal of bank materials from the bank wall and continues the dislodgement of material from the bank face until mass failure quadrant analysis to the random velocity fluctuation in the vicinity of eroded stream bank shows that downstream bank material movement is strongly correlated to the presence of sweep events probability density function pdf kinetic energy fluxes and coefficients of skewness and kurtosis of velocity fluctuations show significant changes with the change in clay fraction of the bank material composition though the velocity spectral densities are in general observed to be similar for all the three different bank undercut depths for different clay fraction of bank however spectral energy shows signature of higher energy peak for lower clay fraction of the bank material