chemical and physical denudation in the amazon river basin we present major and trace element data on the suspended and dissolved phases of the amazon river and its main tributaries the sr isotopic composition of the dissolved load is also reported special attention is paid to the abundances of ree and to their fractionation between the dissolved and suspended phase the rivers of the amazon basin are among the richest in dissolved ree and are similar to the rivers of the congo system however a greater range of fractionation between lree and hree is reported here at a global scale the rivers have intermediate patterns between those of the congo system and those of high ph rivers such as the indus and mississippi rivers only few elements rb u ba k na sr and ca are mobilized by silicate weathering these elements are strongly depleted in the suspended phase with respect to upper continental crust in the dissolved load these elements are controlled by atmospheric inputs and the weathering of the main lithologies we propose a model based on mass budget equations that allow the proportions derived from the different sources to be calculated as a consequence silicate carbonate and evaporite weathering rates can be estimated as well as the consumption of co2 by weathering of each of these lithologies physical weathering rates can be estimated by two complementary approaches on the one hand the multi year average of suspended sediments yields can be used to estimate physical denudation on the other hand we have developed a steady state model of erosion that allows us to calculate physical erosion rates on the basis of the dissolved load of rivers a mean crustal composition is assumed in this model for the rock sources of the drainage basins comparison of the rates predicted by the model to the observed rates shows good agreement for the lowland rivers but a strong discrepancy for the rivers derived from the andes andean rivers solimoes madeira and amazon have observed sediment yields much greater than those predicted according to the steady state model of chemical and physical weathering two interpretations can account for this inconsistency the first is that these rivers are not in steady state and hence that the soils are being destroyed the second requires that the local continental crust is different from the average continental crust of taylor and mclennan and contains a large proportion of sedimentary rocks using the measured sediment yields and assuming a steady state we can estimate the amount of sediment recycling for each drainage basin for the amazon at santarem we find that at least 25 of the mass of the upper continental crust of the amazon drainage basin is constituted of recycled material

propagation velocity of a natural hydraulic fracture in a poroelastic medium the propagation rate of a natural hydraulic fracture is limited by the rate that fluid flows from the saturated rock into the void space created by fracture growth unlike induced hydraulic fractures natural hydraulic fractures can not be modeled by specifying rates of fracture growth or fluid flow into the fracture as boundary conditions numerical solutions of the governing equations for natural hydraulic fracture growth in a poroelastic medium indicate that growth rates are primarily controlled by the hydraulic conductivity the storage sp and the initial flaw length 2 o computationally more efficient models which partially and completely decouple material stresses from fluid pressures give similar results results for several rock types indicate that although the rate of fracture propagation is limited by fluid flow fracture growth still accelerates results are generalized in dimensionless plots of fracture length versus time for various values of the dimensionless parameter 1 gsp where g is the shear modulus and is the drained poisson s ratio

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

variational phase field modeling of hydraulic fracture interaction with natural fractures and application to enhanced geothermal systems in every tight formation reservoir natural fractures play an important role for mass and energy transport and stress distribution enhanced geothermal systems egs make no exception and stimulation aims at increasing the reservoir permeability to enhance fluid circulation and heat transport egs development relies upon the complex task of predicting accurate hydraulic fracture propagation pathway by taking into account reservoir heterogeneities and natural or preexisting fractures in this contribution we employ the variational phase field method which handles hydraulic fracture initiation propagation and interaction with natural fractures and is tested under varying conditions of rock mechanical properties and natural fractures distributions we run bidimensional finite element simulations employing the open source software opengeosys and apply the model to simulate realistic stimulation scenarios each one built from field data and considering complex natural fracture geometries in the order of a thousand of fractures key mechanical properties are derived from laboratory measurements on samples obtained in the field simulations results confirm the fundamental role played by natural fractures in stimulation s predictions which is essential for developing successful egs projects

onshore geologic storage of co2 the possibility that substantial quantities of co2 can be injected into subsurface porous rock formations has been investigated sufficiently to show that pore space available to contain the co2 is abundant multiple rock types and physical mechanisms can be used to trap the co2 indefinitely with careful site selection and operations leakage to the near surface region can be avoided the next step is to test these injection processes at the scale of a large power plant

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

grain size control of river suspended sediment geochemistry clues from amazon river depth profiles residual solid products of erosion display a wide range of size density shape mineralogy and chemical composition and are hydrodynamically sorted in large river channels during their transport we characterize the chemical and isotopic variability of river sediments of the amazon basin collected at different water depths as a function of grain size absolute chemical concentrations and sr and nd isotopic ratios greatly varies along channel depth the al si ratio tightly linked to grain size distribution systematically decreases with depth mostly reflecting dilution by quartz minerals a double normalization diagram is proposed to correct from dilution effects elements define fan shaped patterns and can be classified in three different groups with respect to hydrodynamic sorting during transport in the amazon 1 poorly sorted insoluble elements like al fe th and rees 2 well sorted insoluble elements like zr and ti mainly carried by heavy minerals and 3 alkali na to cs and alkali earth elements mg to ba for which a large variety of patterns is observed related for alkali to their variable affinity for phyllosilicates sr isotopes show that the amazon river at the mouth is stratified the madeira and solimoes derived sediments being preferentially transported near the channel surface and at depth respectively the comparison between the solimoes and madeira rivers shows how the interplay between grain sorting weathering and crustal composition controls the composition of the suspended river sediments

comparison of geomechanical deformation induced by megatonne scale co2 storage at sleipner weyburn and in salah geological storage of co2 that has been captured at large point source emitters represents a key potential method for reduction of anthropogenic greenhouse gas emissions however this technology will only be viable if it can be guaranteed that injected co2 will remain trapped in the subsurface for thousands of years or more a significant issue for storage security is the geomechanical response of the reservoir concerns have been raised that geomechanical deformation induced by co2 injection will create or reactivate fracture networks in the sealing caprocks providing a pathway for co2 leakage in this paper we examine three large scale sites where co2 is injected at rates of 1 megatonne y or more sleipner weyburn and in salah we compare and contrast the observed geomechanical behavior of each site with particular focus on the risks to storage security posed by geomechanical deformation at sleipner the large high permeability storage aquifer has experienced little pore pressure increase over 15 y of injection implying little possibility of geomechanical deformation at weyburn 45 y of oil production has depleted pore pressures before increases associated with co2 injection the long history of the field has led to complicated sometimes nonintuitive geomechanical deformation at in salah injection into the water leg of a gas reservoir has increased pore pressures leading to uplift and substantial microseismic activity the differences in the geomechanical responses of these sites emphasize the need for systematic geomechanical appraisal before injection in any potential storage site

modeling suggests that oblique extension facilitates rifting and continental break up 1 in many cases the initial stage of continental break up was and is associated with oblique rifting that includes break up in the southern and equatorial atlantic separation from eastern and western gondwana as well as many recent rift systems like gulf of california ethiopia rift and dead sea fault using a simple analytic mechanical model and advanced numerical thermomechanical modeling techniques we investigate the influence of oblique extension on the required tectonic force in a three dimensional setting while magmatic processes have been already suggested to affect rift evolution we show that additional mechanisms emerge due to the three dimensionality of an extensional system focusing on non magmatic rift settings we find that oblique extension significantly facilitates the rift process this is due to the fact that oblique deformation requires less force in order to reach the plastic yield limit than rift perpendicular extension the model shows that in the case of two competing non magmatic rifts with one perpendicular and one oblique to the direction of extension but otherwise having identical properties the oblique rift zone is mechanically preferred and thus attracts more strain

chemical weathering and atmospheric soil co2 uptake in the andean and foreland amazon basins abstract this study is a geochemical investigation of the andean and foreland basins of the amazon river at high spatial and time resolution carried out within the framework of the hybam research program hydro geodynamics of the amazon basin monthly sampling was carried out at 27 gauging stations located in the upper tributaries of the amazon basin from north to south the napo maranon ucayali madre de dios beni and mamore rivers the aim of this paper is to estimate the present day chemical weathering rate cwr as well as the flux of co 2 consumption from total and silicate weathering in the andes and foreland amazon basins and to discuss their distribution as a function of geomorphic and structural parameters based on the forward method the napo and other ecuadorian basins present high silicate weathering rates in comparison with the other basins we confirm that the maranon and ucayali rivers control the amazon hydrochemistry due to the presence of salt rocks and carbonates in these basins the madre de dios beni and mamore basins do not contribute much to the amazon dissolved load this north to south cwr gradient can be explained by the combination of decreasing weatherable lithology surface and decreasing runoff rates from the north to the south the foreland part of the basins or mountain lowland transition accounts for nearly the same proportion of the amazon silicate chemical weathering and carbonate chemical weathering fluxes as the andean part this result demonstrates the importance of the sediment accumulation areas in the amazon basin weathering budget and can be explained by the occurrence of a higher temperature the deposition of fresh sediments from andean erosion and a higher sediment residence time than in the upper part of the basin with a total co 2 consumption rate of 744 10 3 moles km year 1 and a silicate co 2 consumption rate of 300 10 3 moles km year 1 the upper amazon river andes foreland part is the most intense part of the amazon basin in terms of atmospheric co 2 consumption by weathering processes it is an important co 2 sink by weathering processes but accounts for only somewhat more than half of the co 2 consumption by silicate weathering of the amazon basin this result points out the importance of the lowland part of the basin in the inorganic c silicate budget the upper amazon accounts for 2 4 of the world s silicate co 2 consumption which is the same proportion as for the southern and southern east himalaya and tibetan plateau

spatial and temporal evolution of injected co2 at the sleipner field north sea time lapse three dimensional 3d seismic surveys have imaged an accumulation of injected co 2 adjacent to the sleipner field in the north sea basin the changing pattern of reflectivity suggests that co 2 accumulates within a series of interbedded sandstones and mudstones beneath a thick caprock of mudstone nine reflective horizons within the reservoir have been mapped on six surveys acquired between 1999 and 2008 these horizons have roughly elliptical planforms with eccentricities ranging between two and four in the top half of the reservoir horizon areas grow linearly with time in the bottom half horizon areas initially grow linearly for about eight years and then progressively shrink the central portions of deeper reflective horizons dim with time amplitude analysis of horizons above within and below the reservoir show that this dimming is not solely caused by acoustic attenuation instead it is partly attributable to co 2 migration and or co 2 dissemination which reduce the impedance contrast between sandstone and mudstone layers growth characteristics and permeability constraints suggest that each horizon grows by lateral spreading of a gravity current this model is corroborated by the temporal pattern of horizon velocity pushdown beneath the reservoir horizon shrinkage may occur if the distal edge of a co 2 filled layer penetrates the overlying mudstone if the buoyant plume draws co 2 upward or if the effective permeability of deeper mudstone layers increases once interstitial brine has been expelled topographic control is evident at later times and produces elliptical planforms especially toward the top of the reservoir our results show that quantitative mapping and analysis of time lapse seismic surveys yield fluid dynamical insights which are testable shedding light on the general problem of co 2 sequestration

crustal structure of the ocean continent transition at flemish cap seismic refraction results 1 we conducted a seismic refraction experiment across flemish cap and into the deep basin east of newfoundland canada and developed a velocity model for the crust and mantle from forward and inverse modeling of data from 25 ocean bottom seismometers and dense air gun shots the continental crust at flemish cap is 30 km thick and is divided into three layers with p wave velocities of 6 0 6 7 km s across the southeast flemish cap margin the continental crust thins over a 90 km wide zone to only 1 2 km the ocean continent boundary is near the base of flemish cap and is marked by a fault between thinned continental crust and 3 km thick crust with velocities of 4 7 7 0 km s interpreted as crust from magma starved oceanic accretion this thin crust continues seaward for 55 km and thins locally to 1 5 km below a sediment cover 1 9 3 1 km s oceanic layer 2 4 7 4 9 km s is 1 5 km thick while layer 3 6 9 km s seems to disappear in the thinnest segment of the oceanic crust at the seawardmost end of the line the crust thickens to 6 km mantle with velocities of 7 6 8 0 km s underlies both the thin continental and thin oceanic crust in an 80 km wide zone a gradual downward increase to normal mantle velocities is interpreted to reflect decreasing degree of serpentinization with depth normal mantle velocities of 8 0 km s are observed 6 km below basement there are major differences compared to the conjugate galicia bank margin which has a wide zone of extended continental crust more faulting and prominent detachment faults crust formed by seafloor spreading appears symmetric however with 30 km wide zones of oceanic crust accreted on both margins beginning about 4 5 m y before formation of magnetic anomaly m0 118 ma

the crustal structure across the transform continental margin off ghana eastern equatorial atlantic forward modeling of a suite of wide angle seismic lines across the transform continental margin at the eastern end of the romanche fracture zone off ghana has shown the transition from continental to oceanic crust to be confined to a narrow 6 to 11 km wide zone located at the foot of the steep continental slope the structure of the adjacent oceanic and continental crusts has also been resolved these results are confirmed and enhanced by gravity and magnetic models the crust of the ocean continent transition zone is characterized by high velocities 5 8 7 3 km s 1 a high density 3 10 mg m 3 and high magnetizations 1 1 1 25 a m 1 this rules out a purely continental origin for the zone and suggests that it may be formed of basic igneous rocks intruded when the hot oceanic spreading center migrated along the margin the oceanic crust within 70 km of the margin is abnormally thin compared to normal atlantic oceanic crust and shows an average thickness of just 4 4 km we suggest that the region of abnormally thin oceanic crust is the result of a reduced magma supply due to a combination of closely spaced fracture zones and the conductive loss of heat from the upwelling oceanic mantle in small basins surrounded on at least three sides by cold continental lithosphere the preferred seismic model also shows a new fracture zone 30 km southeast of the ocean continent transition which is characterized by low velocities in the upper crust a lens shaped layer with velocities of 7 2 7 4 km s 1 at the base of the crust and a crustal thickness of just 3 4 km the continental crust appears largely unaffected by the proximity of the adjacent oceanic crust there is no evidence for underplating of the continental crust adjacent to the ocean continent transition zone

pre breakup continental extension in east gondwanaland and the early opening of the eastern indian ocean abstract extension of continental crust by up to 360 km on a north northeast azimuth in the great australian bight occurred prior to the middle cretaceous 96 ma onset of seafloor spreading between australia and antarctica this large amount of continental extension is constrained to the late jurassic 160 ma middle cretaceous interval about a pole estimated to lie about 40 southeastward of southeastern australia using bathymetric data combined with seismic and magnetic determinations of the continent ocean boundaries off australia india and antarctica we determine a revised fit of east gondwanaland prior to continental extension and at various stages thereafter in the first stage from 160 to 132 5 ma m11 india australia rotated from antarctica with continental crustal extension between australia and antarctica and largely transform motion between the coromandel coast margin of india and the kron prins olav kyst margin of antarctica in the second stage from 132 5 to 96 ma india rotated northwestward away from australia antarctica producing m10 and younger magnetic anomalies along the western margin of australia continental extension was greatest between australia and antarctica during this stage the newly determined 132 5 96 ma rotation between india and australia provides an improved fit between the orientation of observed m10 m0 magnetic anomalies and the southward decrease in spreading rate off the western australian margin together with bathymetry the new rotation parameters lead to identification of the abandoned 96 ma spreading ridge along the lost dutchmen and dirck hartog ridges the naturaliste fracture zone is found to lie nearly parallel to the early cretaceous transform direction between india and antarctica suggesting that slow seafloor spreading required between australia and antarctica to accommodate the continental extension further east occurred between naturaliste fracture zone and the m4 m0 anomalies adjacent to the naturaliste plateau the revised fit of india in east gondwanaland reduces the eastward extent of greater india to 95 e but does not change its extent as far north as the cape range fracture zone the new pole determined for the early cretaceous rotation between india and australia antarctica lies within 15 of the southern tip of india significantly closer than in previous determinations thereby reducing the amount of seafloor spreading generated between africa and antarctica by the india antarctica rotation and thus offering a possible solution to the perceived spreading rate problems between m11 and m0 in the somali mozambique and antarctic basins

a rouse based method to integrate the chemical composition of river sediments application to the ganga basin 1 the ganga river is one of the main conveyors of sediments produced by himalayan erosion determining the flux of elements transported through the system is essential to understand the dynamics of the basin this is hampered by the chemical heterogeneity of sediments observed both in the water column and under variable hydrodynamic conditions using acoustic doppler current profiler adcp acquisitions with sediment depth profile sampling of the ganga in bangladesh we build a simple model to derive the annual flux and grain size distributions of the sediments the model shows that ca 390 30 mt of sediments are transported on average each year through the ganga at haring bridge bangladesh modeled average sediment grain size parameters d 50 and d 84 are 27 4 and 123 9 mm respectively grain size parameters are used to infer average chemical compositions of the sediments owing to a strong grain size chemical composition relation the integrated sediment flux is characterized by low al si and fe si ratios that are close to those inferred for the himalayan crust this implies that only limited sequestration occurs in the gangetic floodplain the stored sediment flux is estimated to c a 10 of the initial himalayan sediment flux by geochemical mass balance the associated globally averaged sedimentation rates in the floodplain are found to be ca 0 08 mm yr and yield average himalayan erosion rate of ca 0 9 mm yr this study stresses the need to carefully address the average composition of river sediments before solving large scale geochemical budgets citation lupker m c france lanord j lave j bouchez v galy f metivier j gaillardet b lartiges and j l mugnier 2011 a rouse based method to integrate the chemical composition of river sediments application to the ganga basin

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

the geochemical filter of large river confluences abstract this study reports the behaviour and fate of major and trace elements in solid and solution in the encontro das aguas mixing zone in the amazonian basin this area results from the confluence of acidic organic rich rio negro and andean sediment rich rio solimoes the differences existing between these two water masses are likely to modify the partitioning of elements between solid and solution or to induce losses by physical or chemical removal to account for lateral and vertical heterogeneities in the river sections the fluxes of elements in dissolved and solid loads were estimated as accurately as possible using an interpolation technique a general loss of suspended particulate matter spm of 23 is measured 80 km after the confluence but the various elements and their associated hosted minerals are not affected in a similar way sispm caspm naspm or srspm mostly associated with coarse phases quartz and ca na feldspars are preferentially lost refractory elements cospm crspm tispm alspm nispm other alkali a and alkali earth ae elements kspm mgspm baspm fespm or organic carbon oc are lost to a lesser extent due to their occurrence in finer fractions such as clays smectite illite or kaolinite oxides and particulate organic matter pom in solution a ae and cudiss mostly originating from the rio solimoes behave conservatively by contrast refractory elements coming mostly from the rio negro exhibit important losses from 10 for doc to 60 for codiss changes in ph and ionic strength during the mixing may induce a sorption of these elements to particles supplied by the rio solimoes a clustering and or a settling of nanokaolinites phases or a sorption of aldiss fediss codiss mndiss or zndiss by dissolved organic matter dom the differential elemental settling at a river confluence could thus induce a phase shift between element supply and export out of a watershed the physical and chemical processes reported here for a river confluence could be applied in other mixing zones such as estuaries or mining exhausts

zinc and copper behaviour at the soil river interface new insights by zn and cu isotopes in the organic rich rio negro basin abstract the complex behaviour of zn and cu at the soil river interface was investigated in soil and riverine water samples from the rio negro basin a secondary tributary of the rio amazonas using their stable isotope compositions this acidic and organic river drains two types of intensely weathered terrains podzols in its upstream part and lateritic soils downstream bulk soil particles suspended particulate matter spm as well as colloidal fractions were sampled across the whole basin during low and high water stages in the basin zn and cu are mostly exported from lateritic soils and transported by organic colloids where significant losses are observed in the downstream part of the river the use of 66zn and 65cu measurements reveals distinct stories for these two metals in suspended sediments and colloids in the colloids the constant 66zncoll across the basin is induced by the same weak association mode between zn and organic ligands regardless of the origin of the water by contrast in spm the speciation of zn and thus 66znspm differ according to the type of drained soils zn is associated with organic complexes in particles exported with water draining podzol whereas zn2 is incorporated in the structure of the remaining kaolinite clays in lateritic output the stronger reactivity of cu than zn with organic ligands induces its complete complexation copper is controlled by refractory particulate organic matter pom and by reactive colloidal organic matter the latter being enriched in 65cu due to stronger binding interactions than in pom while the cu content remains constant in the upstream part of the rio negro downstream the decrease of spm and colloidal cu fluxes is associated with a constant 65cuspm and with an increase of 65cucoll at the rio negro outlet geochemical mass balance modelling based on spm cu and zn fluxes in spm and their associated isotopic signatures confirms distinct host phases for zn and cu and identifies the most probable places where losses of these two metals occur in colloids the observed cu isotope fractionation from 0 24 to 0 45 superimposed on the significant cucoll loss is assumed to result from a new isotopic equilibrium in a low velocity and high productivity zone cu rich colloids enriched in 63cu aggregate and settle down whereas the remaining heavy cu is partially complexed on strong organic ligands secreted by phytoplankton forming new cu colloids

sequential faulting explains the asymmetry and extension discrepancy of conjugate margins rifted continental margins are the sites that record the thinning and break up of continents to form new oceans cesar ranero and marta perez gussinye use seismic imaging to accurately measure fault extension at the conjugate west iberia and newfoundland margins and compare this to crustal thinning they use these observations to create a balanced kinematic model of rifting that resolves the extension discrepancy in which crustal thinning appears to be greater than the extension caused by brittle faulting by producing both fault controlled crustal thinning which progresses from a rift basin to the asymmetric structure and extreme thinning of conjugate rifted margins these authors use seismic imaging to accurately measure fault extension at the conjugate west iberia and newfoundland margins and compare this with crustal thinning they use these observations to create a balanced kinematic model of rifting that resolves the extension discrepancy where crustal thinning seems to be greater than the extension caused by brittle faulting during early extension cold continental lithosphere thins and subsides creating rift basins if extension continues to final break up the split and greatly thinned plates subside deep below sea level to form a conjugate pair of rifted margins although basins and margins are ubiquitous structures the deformation processes leading from moderately extended basins to highly stretched margins are unclear as studies consistently report that crustal thinning is greater than extension caused by brittle faulting1 2 3 4 this extension discrepancy might arise from differential stretching of brittle and ductile crustal layers2 but that does not readily explain the typical asymmetric structure of conjugate margins5 6 in cross section one margin displays gradual thinning accompanied by large faults and the conjugate margin displays abrupt thinning but smaller scale faulting5 whole crust detachments active from early in the rifting could in theory create both thinning and asymmetry1 but are mechanically problematical furthermore the extension discrepancy occurs at both conjugate margins leading to the apparent contradiction that both seem to be upper plates to a detachment fault7 8 alternative models propose that much brittle extension is undetected because of seismic imaging limitations caused either by subseismic resolution faulting9 invisible deformation along top basement 100 km scale detachments8 or the structural complexity of cross cutting arrays of faults3 here we use depth migrated seismic images to accurately measure fault extension and compare it with crustal thinning the observations are used to create a balanced kinematic model of rifting that resolves the extension discrepancy by producing both fault controlled crustal thinning which progresses from a rift basin to the asymmetric structure and extreme thinning of conjugate rifted margins contrary to current wisdom the observations support the idea that thinning is to a first degree explained by simple andersonian faulting that is unambiguously visible in seismic data

fluid driven multiple fracture growth from a permeable bedding plane intersected by an ascending hydraulic fracture 1 in bedded sedimentary rocks the energy for spontaneous growth of multiple vertical fractures from a bedding plane may be provided by an overpressurized sublayer fracture that connects a fluid source to the bedding plane in this paper using our coupled deformation and flow model we study the processes and mechanisms involved in the formation and interaction of closely space fractures from preexisting flaws or starter fractures located along the bedding plane fracture growth from multiple flaws can be convergent parallel or divergent depending on the factors like contrasts in moduli and far field stresses flaw sizes and locations and initial bed conductivity fluid viscosity and injection rate as well as time the results presented here have been obtained for conditions where fluid viscous dissipation is dominant in contrast to other results available in literature based on uniform pressure assumption equivalent to use of an inviscid fluid it is demonstrated that the earlier a hydraulic fracture starts to extend the more likely it is to become the primary fracture in a system of closely spaced fractures the fracture closest to the fluid source typically grows faster as a result of a higher pressure level because viscous dissipation results in a decrease in pressure with distance from the fluid source but its development does not completely inhibit the growth of other hydraulic fractures simultaneous growth of closely spaced fractures is supported by the local stress and energetic analyses and the fracture distance can be very small their length to spacing ratio is accordingly much larger than that predicted previously under certain circumstances a longer and more permeable fracture may grow to a greater extent than a shorter fracture closer to the fluid source which may grow toward and merge with the longer fracture to create fracture clusters adjacent to a bedding plane

a quantitative analysis of transtensional margin width abstract continental rifted margins show variations between a few hundred to almost a thousand kilometres in their conjugated widths from the relatively undisturbed continent to the oceanic crust analogue and numerical modelling results suggest that the conjugated width of rifted margins may have a relationship to their obliquity of divergence with narrower margins occurring for higher obliquity we here test this prediction by analysing the obliquity and rift width for 26 segments of transtensional conjugate rifted margins in the atlantic and indian oceans we use the plate reconstruction software gplates www gplates org for different plate rotation models to estimate the direction and magnitude of rifting from the initial phases of continental rifting until breakup our rift width corresponds to the distance between the onshore maximum topography and the last identified continental crust we find a weak positive correlation between the obliquity of rifting and rift width highly oblique margins are narrower than orthogonal margins as expected from analogue and numerical models we find no relationships between rift obliquities and rift duration nor the presence or absence of large igneous provinces lips

clay mineral composition of river sediments in the amazon basin abstract clay minerals are important in evaluating the maturity of suspended sediments weathering intensity and source area however there are processes that can change the mineral assemblage such as river transportation deposition remobilization and tributary inputs in terms of water discharge and sediment yield the amazon is one of the largest rivers in the world most of the suspended sediments come from the andes crossing the lowlands before reaching the ocean this study measures the spatial distribution of clay mineral assemblages over the entire amazon basin the results obtained show the main features of the amazon river main stem and larger tributaries from their sources to their confluence clay mineral composition highlights the evolution of the madeira and maranon solimoes river which start in the andes with high illite chlorite content downstream smectite contents increase moreover all shield tributaries show high kaolinite content the lower amazon river is characterized by relative high smectite content different from the andean sources the clay mineral results show that suspended sediments of the amazon river have three main sources 1 the andes mountains 2 the amazon shields and 3 the piedmont basins especially the pastaza alluvial megafan and the fitzcarrald arch basin lateral bank erosion plays also a significant role by the introduction of more mature sediments into the river enriched in smectite

iron isotope systematics in estuaries the case of north river massachusetts usa abstract recent studies have suggested that rivers may present an isotopically light fe source to the oceans since the input of dissolved iron from river water is generally controlled by flocculation processes that occur during estuarine mixing it is important to investigate potential fractionation of fe isotopes during this process in this study we investigate the influence of the flocculation of fe rich colloids on the iron isotope composition of pristine estuarine waters and suspended particles the samples were collected along a salinity gradient from the fresh water to the ocean in the north river estuary ma usa estuarine samples were filtered at 0 22 m and the iron isotope composition of the two fractions dissolved and particles were analyzed using high resolution mc icp ms after chemical purification dissolved iron results show positive 56 fe values with an average of 0 43 0 04 relative to the irmm 14 standard and do not display any relationships with salinity or with percentage of colloid flocculation the iron isotopic composition of the particles suspended in fresh water is characterized by more negative 56 fe values than for dissolved fe and correlate with the percentage of fe flocculation particulate 56 fe values vary from 0 09 at no flocculation to 0 1 at the flocculation maximum which reflect mixing effects between river borne particles lithogenic particles derived from coastal seawaters and newly precipitated colloids since the process of flocculation produces minimal fe isotope fractionation in the dissolved fe pool we suggest that the pristine iron isotope composition of fresh water is preserved during estuarine mixing and that the value of the global riverine source into the ocean can be identified from the fresh water values however this study also suggests that 56 fe composition of rivers can also be characterized by more positive 56 fe values up to 0 3 relative to the crust than previously reported in order to improve our current understanding of the oceanic iron isotope cycling further work is now required to determine the processes controlling the fractionation of fe isotopes during continental run off

probabilistic geomechanical analysis of compartmentalization at the snohvit co2 sequestration project pressure buildup caused by large scale co2 injection is a key concern during a carbon sequestration project large overpressures can compromise seal integrity reactivate faults and induce seismicity furthermore pressure buildup is directly related with storage capacity in this work we study the geomechanical response to co2 injection at snohvit to understand the potential for fault reactivation leakage and contamination of the producing interval through bounding faults furthermore we evaluate the potential contribution of a structural component to reservoir compartmentalization we combine simplified analytical models based on critically stressed fracture theory and a mohr coulomb failure criterion with a rigorous sensitivity analysis large stress uncertainties are present and reflected in the modeling results it was found that under the most likely stress state the faults are fairly stable and caprock hydrofracturing would be expected before fault reactivation in most of the analyzed cases the critical pressure perturbation needed for reactivation is above 13 mpa which was the limiting pressure increase before reaching the fracture pressure faults were found to be 20 less stable when considering variations in shmax orientation in those cases fault reactivation could be expected before caprock failure if injection continued however if the pressure increase did reach the critical values for seal failure estimated under the worst case and least likely stress state no indication of such failure can be observed in the measured pressure response finally the potential role of a structural component in the compartmentalization and fluid migration is difficult to assess due to the stress state uncertainty

status of co2 storage in deep saline aquifers with emphasis on modeling approaches and practical simulations carbon capture and storage ccs is the only viable technology to mitigate carbon emissions while allowing continued large scale use of fossil fuels the storage part of ccs involves injection of carbon dioxide captured from large stationary sources into deep geological formations deep saline aquifers have the largest identified storage potential with estimated storage capacity sufficient to store emissions from large stationary sources for at least a century this makes ccs a potentially important bridging technology in the transition to carbon free energy sources injection of co2 into deep saline aquifers leads to a multicomponent multiphase flow system in which geomechanics geochemistry and nonisothermal effects may be important while the general system can be highly complex and involve many coupled nonlinear partial differential equations the underlying physics can sometimes lead to important simplifications for example the large density difference between injected co2 and brine may lead to relatively fast buoyant segregation making an assumption of vertical equilibrium reasonable such simplifying assumptions lead to a range of simplified governing equations whose solutions have provided significant practical insights into system behavior including improved estimates of storage capacity easy to compute estimates of co2 spatial migration and pressure response and quantitative estimates of leakage risk when these modeling studies are coupled with observations from well characterized injection operations understanding of the overall system behavior is enhanced significantly this improved understanding shows that while economic and policy challenges remain co2 storage in deep saline aquifers appears to be a viable technology and can contribute substantially to climate change solutions

geological co2 sequestration in multi compartment reservoirs geomechanical challenges 1 sequestration of large amounts of co2 within deep underground reservoirs has been proposed as a potential approach for reducing atmospheric emissions of greenhouse gases a co2 sequestration project should address the associated environmental and safety issues and in this respect the importance of geomechanics has recently been widely recognized geomechanics is even more important when fluid injection is planned in faulted reservoirs how much co2 can be safely injected into multi compartment reservoirs are geomechanical constraints more restrictive than flow dynamic constraints these and other questions are addressed in the present study using a three dimensional finite element interface element geomechanical model we simulate the possible mechanical failure in both the injected formation and the caprock the fault thrust reactivation and the ground surface displacement in a faulted reservoir of the offshore northern italy where seismic surveys provided an accurate characterization of the faulted geological structure based on reliable petrophysical geomechanical properties from well logs and pore overpressure as predicted by a fluid dynamic model the results show that the injection of 1 106 ton a of co2 may be performed over a few years only thereafter part of the injected formation fails by shear stress a number of parametric scenarios are investigated to address the major uncertainties on the geomechanical response to co2 injection the modeling outcome suggests that shear failure and faults thrusts reactivation can occur much before attaining the hydraulic fracturing pressure hence representing two major constraints for a safe and permanent containment

full fit palinspastic reconstruction of the conjugate australian antarctic margins 1 despite decades of study the prerift configuration and early rifting history between australia and antarctica is not well established the plate boundary system during the cretaceous includes the evolving kerguelen broken ridge large igneous province in the west as well as the conjugate passive and transform margin segments of the australian and antarctic continents previous rigid plate reconstruction models have highlighted the difficulty in satisfying all the available observations within a single coherent reconstruction history we investigate a range of scenarios for the early rifting history of these plates by developing a deforming plate model for this conjugate margin pair potential field data are used to define the boundaries of stretched continental crust on a regional scale integrating crustal thickness along tectonic flow lines provides an estimate of the prerift location of the continental plate boundary we then use the prerift plate boundary positions along with additional constraints from geological structures and large igneous provinces within the same australian and antarctic plate system to compute full fit poles of rotation for australia relative to antarctica our preferred model implies that the leeuwin and vincennes fracture zones are conjugate features within gondwana but that the direction of initial opening between australia and antarctica does not follow the orientation of these features rather the geometry of these features is likely related to the earlier rifting of india away from australia antarctica previous full fit reconstructions based on qualitative estimates of continental margin overlaps generally yield a tighter fit than our preferred reconstruction based on palinspastic margin restoration

hafnium and neodymium isotopes and rey distribution in the truly dissolved nanoparticulate colloidal and suspended loads of rivers in the amazon basin brazil abstract radiogenic isotopes in river sediments and river waters have been widely used in provenance studies as these samples naturally integrate the geology chemistry of the entire catchment while the hf and nd isotope systems are coupled during igneous processes they are decoupled during supergene processes at the earth s surface which is reflected by the isotope composition of riverine sediments we present the first data for both hf and nd isotope compositions of the dissolved 0 2 m filtrates rich in nanoparticles and colloids npcs and the truly dissolved 1 kda ultrafiltrates load of rivers hafnium and nd isotope compositions and concentrations of the rare earths and yttrium rey and hf were determined for suspended particles 0 2 m as well as for the dissolved and the truly dissolved load of the rio solimoes the amazon s largest tributary draining the andes and of the rio negro an organic npc and particle rich river draining the rainforest of northern amazonia we also analyzed the nd isotope compositions of suspended sediments and 0 2 m filtered water samples from the amazon river and its tributaries rio tapajos rio xingu and rio jari our novel results clearly show that the decoupling of the hf and nd isotope systems is related to incongruent weathering processes on the continent as this decoupling can already be observed in the different hf and nd pools i e in the particulate the npc dominated dissolved and the truly dissolved load of rivers in the rio negro and rio solimoes a strong particle size dependent difference in hf isotope composition is observed values of e hf become more radiogenic as filter poresize decreases which can be related to the density and size dependent distribution of hf rich minerals e g zircons and their absence from the truly dissolved pool in contrast the nd isotope composition of amazonian river waters reflects that of their catchment geology tributaries draining the precambrian brazilian and guyana shields show very unradiogenic e nd values of 19 to 25 in their dissolved load whereas the rio solimoes draining the andes yields a more radiogenic e nd signal of only 7 the dissolved nd isotope composition of the amazon is dominated by its andean tributaries and averages at 8 although nd isotope compositions are thought to not being fractionated by earth surface processes significant differences of 1 3 1 9 e units can be observed between the dissolved and suspended loads in the amazon river and its main tributary the rio solimoes in these rivers the dissolved load is more radiogenic than the suspended sediment which is likely due to incongruent weathering and related mineral sorting in the andean headwaters in contrast the organic rich and mineral poor shield rivers do not show a difference between the truly dissolved dissolved and suspended load as the nd in all these pools is controlled by surface and solution complexation and hence isotopically homogenized by continuous exchange and re equilibration

multi element chemistry of some amazonian waters and soils abstract the rivers flowing through the amazon basin are both physically and chemically heterogeneous through detailed geochemical analyses variability is shown to be primarily controlled by both substrate lithology in the source region and soil geochemistry in the erosional regime the solute rich waters of the rio solimoes reflect the drainage from the andes mountains and the fertility of the varzea in contrast the solute deficient waters of the rio negro reflect the infertility of the lateritic and podsolitic terrains of the central amazon rivers flowing through the precambrian shield have an intermediate composition such observations suggest that it is possible to classify the chemical composition of amazonian rivers according to the soil types in their catchment regions and vice versa these findings have profound implications for using water chemistry as an indicator of the agricultural and mineral potential of a region results from this study also confirm that precipitation is a significant source of major cations and trace metals to solute deficient river systems such as the rio negro

late cretaceous magmatism in madagascar palaeomagnetic evidence for a stationary marion hotspot late cretaceous basaltic volcanics in the morondava basin sw madagascar possess high quality and pre fold palaeomagnetic data declination 353 5 inclination 54 8 95 2 4 the palaeomagnetic data are all of normal magnetic polarity and remanence acquisition is linked to the terminal stages of the cretaceous normal superchron 83 ma this is sustained by an 40ar 39ar age of 83 6 1 6 ma from one of the tested basaltic flows a precise u pb zircon baddeleyite age from northeast madagascar demonstrates magmatism at least back to 91 6 0 3 ma thus reliable isotope ages for the madagascar cretaceous igneous province span a range of 8 million years late cretaceous palaeomagnetic data obtained from volcanics and dolerites all over madagascar are directionally concordant and the combined palaeomagnetic pole latitude 68 5 n longitude 230 3 e a95 5 5 n 8 studies sampling age range ca 84 90 ma represents one of the best late cretaceous poles for the former gondwanan elements the collective palaeomagnetic data yield a palaeolatitude of 45 3 s 5 3 4 7 for the proposed focal point of the marion plume volcan de l androy southeast madagascar during the late cretaceous this is in perfect agreement with hotspot controlled reconstructions that place the marion hotspot 46 0 s beneath southeast madagascar during the late cretaceous setting true polar wander aside hotspot movements in the indian ocean do not appear to exceed ca 0 75 cm yr and the marion hotspot appears stationary within the resolution power of palaeomagnetic data

lithium isotopes in large rivers reveal the cannibalistic nature of modern continental weathering and erosion abstract the erosion of major mountain ranges is thought to be largely cannibalistic recycling sediments that were deposited in the ocean or on the continents prior to mountain uplift despite this recognition it has not yet been possible to quantify the amount of recycled material that is presently transported by rivers to the ocean here we have analyzed the li content and isotope composition li 7 of suspended sediments sampled along river depth profiles and bed sands in three of the largest earth s river systems amazon mackenzie and ganga brahmaputra rivers the li 7 values of river sediments transported by these rivers range from 5 3 to 3 6 and decrease with sediment grain size we interpret these variations as reflecting a mixture of unweathered rock fragments preferentially transported at depth in the coarse fraction and present day weathering products preferentially transported at the surface in the finest fraction only the finest surface sediments contain the complementary reservoir of li solubilized by water rock interactions within the watersheds li isotopes also show that river bed sands can be interpreted as a mixture between unweathered fragments of igneous and sedimentary rocks a mass budget approach based on li isotopes li al and na al ratios solved by an inverse method allows us to estimate that for the large rivers analyzed here the part of solid weathering products formed by present day weathering reactions and transported to the ocean do not exceed 35 li isotopes also show that the sediments transported by the amazon mackenzie and ganga brahmaputra river systems are mostly sourced from sedimentary rocks 60 rather than igneous rocks this study shows that li isotopes in the river particulate load are a good proxy for quantifying both the erosional rock sources and the fingerprint of present day weathering processes overall li isotopes in river sediments confirm the cannibalistic nature of erosion and weathering

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

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

how important is it to integrate riverine suspended sediment chemical composition with depth clues from amazon river depth profiles abstract the vertical variability in mineralogical chemical and isotopic compositions observed in large river suspended sediments calls for a depth integration of this variability to accurately determine riverine geochemical fluxes in this paper we present a method to determine depth integrated chemical particulate fluxes of large rivers based on river sampling along depth profiles and applied to the amazon basin lowland tributaries the suspended particulate matter spm concentration data from depth profiles is modeled for a number of individual grain size fractions using the rouse model which allows to predict the grain size distribution of suspended sediment throughout the whole river cross section then using 1 the relationship between grain size distribution and the al si ratio 2 relationships between the al si ratio and the chemical concentrations the chemical composition of river sediment is predicted throughout the river cross section and integrated to yield the depth integrated chemical particulate flux for a number of chemical elements e g si al fe na rees for elements such as al fe rees th the depth integrated flux is around twice as high as the one calculated from river surface sample characteristics for na and si the depth integrated flux is three times higher than the surface estimate due to the enrichment of albite and quartz at the bottom of the river depth integrated 87sr 86sr composition of suspended sediment also predictable using this method differs by more than 10 3 from the surface sample composition finally potential implications of depth integrated estimates of amazon sediment chemistry are explored depth integration of particulate 87sr 86sr isotopic ratios is necessary for a reliable use of sr isotopes as a provenance tracer the concept of steady state weathering of a large river basin is revisited using depth integrated sediment composition this analysis shows that in the amazon basin river the previously observed discrepancy between 1 weathering intensities of channel surface sediment and 2 silicate derived dissolved fluxes is only slightly accounted for by the vertical variability of suspended sediment weathering intensities this observation confirms that most large rivers basins are not eroding at steady state

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

hydromechanical modeling of nonplanar three dimensional fracture propagation using an iteratively coupled approach accurate numerical modeling of fracture propagation and deflection in porous media is important in the development of geo resources to this end we propose a novel modeling framework to simulate n

insights into iron sources and pathways in the amazon river provided by isotopic and spectroscopic studies abstract the present study investigated the weathering and transport mechanisms of fe in the amazon river a particular emphasis was placed on fe partitioning speciation and isotopic fractionation in the contrasting waters of the solimoes and negro rivers and their mixing zone at the beginning of the amazon river samples collected in the end member rivers and thirteen sites distributed throughout the mixing zone were processed through frontal vacuum filtration and tangential flow ultrafiltration to separate the different suspended solid fractions i e particulate p 0 45 m and p 0 22 m colloidal 0 22 m c 5 kda and truly dissolved elements td 57 fe values relative to irmm 14 while the particulate fractions yielded light isotopic compositions of 0 344 for p 0 22 m and 0 104 for p 0 45 m fractions the mineral particulate rich waters of the solimoes river had dissolved and colloidal fractions with light isotopic composition 0 532 and 0 176 respectively whereas the particulate fractions yielded 57 fe values close to those of the continental crust i e 0 029 for p 0 22 m and 0 028 for p 0 45 m ten kilometers downstream from the negro and solimoes junction the concentrations of colloidal and dissolved fe species deviate markedly from conservative mixing a maximum fe loss of 43 g l i e 50 of the dissolved and colloidal fe is observed 110 km downstream from the rivers junction the contrasting negro and solimoes rivers isotopic compositions along the pore sized water fractions is attributable to the biogeochemical processes involving different types of upland soils and parental materials for instance the isotopic composition of colloidal and dissolved fe from the negro river are consistent with fe oxidation and complexation mechanisms at the interface between waterlogged podzols and river networks as supported by strong organo fe complexes signals observed by epr conversely the particulate and colloidal fractions from the solimoes river have 57 fe consistent with strong mechanical erosion in the andean cordillera and upland soils as evidenced by high concentrations of fe 3 oxides sensu lato measured by epr the massive dissolved and colloidal fe removal is associated with the evolution of the physical and chemical composition of the waters i e ionic strength during mixing which influences organo fe 3 and fe 3 oxyhydroxides stability several models are discussed to explain fe non conservative behavior including dissociation of organo fe complexes and the subsequent formation of solid fe 3 oxyhydroxides and semiquinone free radicals as evidenced by epr spectra demonstrating that organo fe signals decrease as fe 3 oxyhydroxides and free radicals signals increase as in estuarine regions the mechanisms involving fe transfer and loss in the mixing zone has a negligible effect on the bulk water fe isotopic composition this result suggests that a tropical basin similar to the amazon river basin delivers to the ocean waters with an fe isotopic composition similar to that of the earth s continental crust

composition and trace element geochemistry of detrital clay and heavy mineral suites of the lowermost amazon river a provenance study abstract compositions of clays and heavy minerals as well as trace element geochemistry of the lowermost amazon sediments reflect source regions and the intense chemical weathering conditions illite and montmorillonite indicate mountainous sources under physical weathering conditions whereas kaolinite is related to the soil composition under humid tropical conditions where chemical weathering predominates the suite of unstable heavy minerals on the other hand reflects some sediment transport from the andes more supply from tributaries draining the basic and metamorphic rocks from the southeastern guiana shield and strongest influence of the upstream amazon and solimoes areas these source signals are damped by transitional storage and intense lateral sediment exchange between channel and floodplain the brazilian shield contribution via the xingu river seems to be of minor importance although the alter do chao and the barreiras formations which are products of erosion of the shields supply suites of stable heavy minerals that reflect recycling rare earth and trace element data from the clay and heavy mineral fractions provide additional clues to the source areas in the downstream direction superimposed upon provenance of sediments from an undifferentiated arc the andes is provenance from strongly weathered continental crust the precambrian shields as well as sedimentary recycling small scale provenance can be delineated by the most immobile elements heavy rare earth elements in the heavy mineral fraction increase downstream toward estreitos to the para river

fate of particulate copper and zinc isotopes at the solimoes negro river confluence amazon basin brazil abstract the behaviour and fate of copper cu and zinc zn at river confluences is poorly understood although chemical and physical processes during mixing of compositionally different tributaries might condition metal availability and fluxes to the ocean to identify and quantify the effect of such processes in river mixing zones particulate cu and zn isotope signatures 65cuspm and 66znspm were measured along cross sections and vertical profiles at the largest river confluence in the world the encontro das aguas mixing zone of the amazon river where the organic rich negro river meets the sediment rich solimoes river the negro river suspended sediments with highly variable cu and zn concentrations as well as 65cuspm and 66znspm are mostly influenced by organic matter and by the lateritic cover of the watershed the solimoes river suspended sediments more homogeneous in cu and zn concentration and isotope composition across the river section reflect the signature of weathered silicate rich sediments derived from the andes the solimoes river supplies the majority of the suspended cu and zn to the amazon river and despite important flux losses across the confluence 35 for cu and 27 for zn 65cuspm and 66znspm show a conservative behaviour during the mixing in the dissolved load cu concentrations and 65cudiss mostly supplied by the solimoes river behave conservatively whereas zn derived mainly from the negro river suffers an important loss 58 that can be attributed to zn adsorption onto the suspended sediments from the solimoes river this transfer does not induce a significant 66znspm shift in the amazon river suspended sediments therefore cu and zn isotope ratios in the suspended sediments behave conservatively through this confluence which mixes two very chemically contrasted rivers our findings thus suggest that the riverine isotopic information on the sources of particulate cu and zn is preserved during tributary mixing