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

first cycle quartz arenites in the orinoco river basin venezuela and colombia modern first cycle quartz arenites are forming in the orinoco drainage basin by at least two distinct mechanisms common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur in the llanos andean foreland basin extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains in lowland portions of the guayana shield long soil mineral residence times are produced by very low erosion and transport rates resulting from low relief and tectonic quiescence both weathering intensity and the duration of weathering are important in the modification of sand composition first cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences furthermore although tectonic setting is of great importance in determining sand composition modifications during transport and deposition can overprint the effects of the tecto

modern sands of south america composition provenance and global significance standard petrographic methods were used to study 604 modern sands from south america of which 351 came from rivers and 253 from beaches in spite of the wide geomorphic contrasts these sands belong to only three great families 1 an immature andean family of lithic arenites rich in volcanic and metamorphic grains that covers about 30 of south america 2 a cratonic association rich in quartz that covers about 62 of the continent and 3 a transitional molasse association which occurs between these two and covers only about 8 of the continent the grand arithmetic average q f rf ratio for the entire continent is 60 11 29 and the area weighted carbonate free average qa fa rfa is 68 10 22 a superior continental estimate and the first ever for an entire continent

provenance shallow to deep diagenesis and chemical mass balance in supermature arenites and pelites ordovician simpson group oklahoma and kansas u s a abstract this study considering original detrital compositions and diagenetic alterations relates sandstone and shale mineral and chemical composition to provenance and mass balance between the sedimentary assemblage and its provenance mature quartz rich sandstones and siltstones with clay rich shales in cores at depths of 60 m 5 2 km and burial temperatures of 150 c from over 50 wells were utilized in this work these provide the material for analysis of compositions and alterations in coastal and shallow marine clastic sedimentary rocks of the middle ordovician simpson group in oklahoma and kansas u s a mineral and geochemical data were obtained by optical petrography x ray diffraction scanning electron microscopy electron microprobe and whole rock x ray fluorescence and icp ms analyses results of the study show significant progressive diagenetic changes with increased burial depth and temperature mineralogically and chemically less altered rocks are comparable to modern quartz rich sands and kaolinitic muds in siliciclastic sediments developed during humid tropical weathering in crystalline basement shield provenance in africa and south america by analogy to modern tropical cases muds composed of major kaolinite with minor smectite and illite at deposition were diagenetically transformed to illite rich rocks by possible microbial action at surface to shallow 20 in sandstone buried 3 km during deep burial high temperature 110 c calcite cement from deep basin sources was precipitated in sandstones geochemical mass balance estimates physical and chemical evidence suggest quartz cement was sourced from within clastic rocks of the simpson group with minor external contribution of sio2 na is in low abundance due to loss in solution during weathering in the provenance rare earth elements ree have typical abundances and distributions in shales with enrichment in light ree littoral environment siltstones and matrix rich sandstones display enrichment of heavy ree in phosphatic brachiopod shell fragments

weathering processes and the composition of inorganic material transported through the orinoco river system venezuela and colombia abstract the composition of river borne material in the orinoco river system is related primarily to erosion regime which in turn is related to tectonic setting especially notable is the contrast between material derived from tectonically active mountain belts and that from stable cratonic regions for a particular morpho tectonic region the compositional suites of suspended sediment bed material overback deposits and dissolved phases are fairly uniform are are typically distinct from whose of other regions for each region a consistent set of chemical weathering reactions can be formulated to explain the composition of dissolved and solid loads in developing these formulations erosion on slopes and storage of solids in soils and alluvial sediments are important considerations compositionally verymature sediment is derived from areas of thick soils where erosion is transport limited and from areas where sediments are stored for extended periods of time in alluvial deposits compositionally immature sediments are derived from tectonically active mountain belts where erosion is weathering limited weathering limited erosion also is important in the elevated parts of the guayana shield within areas of sleep topography compared to the mountain belts sediments derived from elevated parts of the shield are more mature a greater degree of chemical weathering seems to be needed to erode the rock types typical of the shield the major element chemistry and mineral composition of sediment delivered by the orinoco river to the ocean are controlled by rivers that have their headwaters in mountain belts and cross the llanos a region of alluvial plains within the foreland basin the composition of sediments in rivers that drain the shield seems to be established primarily at the site of soil formation whereas for rivers that drain the mountain belts additional weathering occurs during s episodes of storage on alluvial plains as sediments are transported across the llanos to the main stem of the orinoco after mixing into the main stem there seems to be little subsequent alteration of sediment

the taiwan ryukyu subduction collision complex folding of a viscoelastic slab and the double seismic zone 1 the termination of the ryukyu trench against eurasia and the oblique subduction of the philippine sea plate create a subduction collision complex offshore taiwan which has not previously been elucidated in detail we combine traveltime data from the seismic networks in taiwan and japan to better illuminate how the subducting ryukyu slab deforms in this subduction collision zone more than 5000 events recorded by both networks were relocated with the double difference method using an optimal regional one dimensional velocity model the offshore seismicity indicates that the double seismic zone with a gap of 15 20 km exists in the subducting slab in the depth range of 40 80 km focal mechanisms suggest that the double seismic zone is caused by east west compression resulting from oblique convergence the improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the ryukyu trench in the depth range 50 100 km the anomalous curvature together with the focal mechanisms suggests that the slab folds against the eurasian lithosphere and that this deformation cannot be fully elastic we model this deformation mode as the developing instability of a viscoelastic maxwell layer embedded in a viscous medium the characteristic wavelength of the instability i e 250 km is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along strike elastic membrane strain as small as 0 01 or more than 3 orders of magnitude higher if 5 elastic strain is allowed

crustal deformation and block kinematics in transition from collision to subduction global positioning system measurements in northern taiwan 1995 2005 1 we present global positioning system gps measurements for the period 1995 2005 at 125 campaign surveyed sites in northern taiwan based on elastic rotating block modeling analyses derived from the gps data we describe the transitional tectonics from arc continent luzon chinese collision to the converging ryukyu trench subduction and back arc opening along the chinese continental margin station velocities relative to station s01r in the chinese stable continental margin were estimated from coordinate time series of each station by using the weighted least squares technique we found two distinct deformation patterns in two geological areas which are basically separated by the surface projection of the nw trending boundary of the subducting philippine sea plate across northern taiwan 1 a waning collision area to the west and 2 a transition zone to the east in the waning collision area the horizontal velocity field shows vectors of 0 3 7 3 mm yr toward the nw in the foothills and the hsuehshan range of northwestern taiwan the tectonic blocks represent a significant nw se internal contraction along with a small block rotation rate 3 0 myr the transition zone can be further divided into an outer range and inner range with distinguishing rotation rates and deformation behaviors in the outer range of the transition zone velocities of 1 0 7 8 mm yr from south to north rotating from 008 to 143 is found in the northernmost foothills and the hsuehshan range the tectonic blocks within the outer range are characterized by a coherent rotation low internal strain rate of 0 10 strain yr with an angular velocity of 5 1 myr where the euler pole is located near its southeastern boundary in the inner range of transition zone a larger clockwise rotation from west to east with horizontal velocities of 9 3 41 2 mm yr from 053 to 146 are found in the northernmost central range the tectonic blocks of the inner range reveal a remarkable nw se internal extension with an ultrarapid clockwise rotation 47 3 myr where the euler pole is near the southern boundary of the range close to the collision comer with the colliding luzon arc the trench roll back together with back arc opening are interpreted to be substantially superposed on the arc continent collision induced rotation in the transition zone with particular regard to the inner range of the northeast taiwan mountain belt

regional controls on geomorphology hydrology and ecosystem integrity in the orinoco delta venezuela interacting river discharge tidal oscillation and tropical rainfall across the 22 000 km2 orinoco delta plain support diverse fresh and brackish water ecosystems to develop environmental baseline information for this largely unpopulated region we evaluate major coastal plain shallow marine and river systems of northeastern south america which serves to identify principal sources and controls of water and sediment flow into through and out of the orinoco delta the regional analysis includes a summary of the geology hydrodynamics sediment dynamics and geomorphic characteristics of the orinoco drainage basin river and delta system because the amazon river is a major source of sediment deposited along the orinoco coast we summarize amazon water and sediment input to the northeastern south american littoral zone we investigate sediment dynamics and geomorphology of the guiana coast where marine processes and holocene history are similar to the orinoco coast major factors controlling orinoco delta water and sediment dynamics include the pronounced annual flood discharge the uneven distribution of water and sediment discharge across the delta plain discharge of large volumes of water with low sediment concentrations through the rio grande and araguao distributaries water and sediment dynamics associated with the guayana littoral current along the northeastern south american coast inflow of large volumes of amazon sediment to the orinoco coast development of a fresh water plume seaward of boca grande disruption of the guayana current by trinidad boca de serpientes and gulf of paria and the constriction at boca de serpientes

arkose subarkose quartz sand and associated muds derived from felsic plutonic rocks in glacial to tropical humid climates abstract in the range of climates from high latitude glacial to equatorial tropical the mineral and chemical compositions of over 600 modern nonmarine first cycle sands and muds produced from felsic crystalline basement provenance in various tectonic settings were studied under glacial conditions sediments are developed by mostly mechanical weathering and thus are mineralogically and chemically similar to their source rocks humid tropical climates yield quartz rich sands kaolinite rich muds along with si mg ca na and k in solution between these extremes a wide range of detrital sediment compositions are developed in arid to wet climates in arid climate and semi arid climate sands feldspar is commonly more abundant than quartz and plagioclase k feldspar except in the cases of granite source rocks the muds are feldspathic with smectite dominant in the clays with increasing temperature and moisture in temperate to subtropical climates mafic silicates and feldspars are destroyed so that the sands range from arkoses with plagioclase as the major feldspar to subarkoses with dominant k feldspar through this range of climate muds become more kaolinitic and thus more aluminous with increasingly severe weathering typically feldspathic sands are associated with feldspathic muds with decreasing feldspar in sands there is less feldspar in muds with increasingly intense chemical weathering rare earth elements ree are more fractionated into muds typically sands contain 150 ppm ree with much of the ree in heavy minerals abundance differences are greatest between humid tropical sands and muds in many samples the ree are mostly in monazite allanite sphene and apatite more intensely weathered samples contain more xenotime and zircon evidenced by yb enrichment

assessing the role of climate on uranium and lithium isotope behaviour in rivers draining a basaltic terrain this study presents uranium u and lithium li isotope and major and trace element data for the dissolved load suspended particles and bedload from rivers draining dominantly basaltic catchments on the island of sao miguel in the azores archipelago uranium activity ratios are at secular equilibrium in the bedload but are higher for suspended material suggesting sorption of uranium from the solution onto particle surfaces the 234u 238u of the riverine dissolved phase varies between 1 02 and 1 86 and lies on a mixing trend between the values expected from chemical weathering of basalt high u 234u 238u at secular equilibrium and an endmember with low u and high 234u 238u which probably reflects recoil effects associated with physical weathering and or soil formation lithium isotope ratios are consistently lighter in suspended material than the corresponding bedload suggesting preferential retention of 6li in the suspended load in turn the 7li of the dissolved load is always isotopically heavier ranging from 5 9 to 36 2 although the lightest values are affected by hydrothermal input the lithium isotopic composition of the dissolved load reflects the balance of primary mineral dissolution to secondary mineral formation comparison of the behaviour of u and li isotopes between sao miguel iceland and other basaltic terrains suggests that whilst u isotopes are dominated by weathering regimes dependent on factors such as weathering rates temperature runoff and climate neither weathering intensity nor climate exerts a direct influence on li isotope behaviour rather the formation of secondary minerals which is indirectly controlled by climate weathering primary mineralogy and biology appears to play the dominant role

hydrochemistry of the narmada and tapti rivers india the chemistry of major ions and elements in narmada and tapti river water was studied based on the monitoring of the drainage basins for monsoon and post monsoon seasons in 2004 the results show the influence of rock weathering on the concentration of major ions in the surface water there is a significant contribution from the catchment geologies in the major constituents of water the total dissolved solids tds concentrations are reflecting the signature of weathering of rocks in the catchment areas in the narmada river hco 3 ca and mg are dominant ions whereas in the tapti river the ions dominating major compositions are hco 3 na and mg the high concentration of cl and so 4 2 is a signature of anthropogenic inputs in addition to the contribution of discharge from springs and or groundwater the high concentration of no 3 can exclusively be attributed to the anthropogenic activities the average concentration of dissolved silica being higher than the indian and world average is reflecting the contribution of silicate weathering the present study gives the silicate and carbonate weathering rates and the corresponding co consumption rates considering the contribution of carbonate rocks present in the selected river basins of the deccan basaltic province the mean silicate weathering rates during the investigation period for the narmada and tapti rivers show lower values but the corresponding mean co consumption rates give higher values compared with earlier estimated rates by dessert et al 2001 earth and planetary science letters 188 459 for the study period the mean carbonate weathering rates and the associated co consumption rates for the narmada and tapti rivers are 12 67 t km year and 7 32 t km year and 0 36 x 10 mol km year and 0 22 x 10 mol km year respectively

carbon flux and c s isotopic characteristics of river waters from a karstic and a granitic terrain in the yangtze river system abstract we analyzed dissolved major elements 13 c dic and 34 s so4 in river waters from a karstic and a granitic terrain to examine weathering processes and the influence of basin geology on water quality and associated riverine alkalinity transport results show that waters from the karstic terrain have higher total dissolved solids tds major ion concentrations and 13 c dic values but lower k na and si concentration and 34 s so4 values than that from the granitic terrain the compositions of major ions and isotopes indicate that the solutes of the granitic terrain are controlled by processes of silicate and carbonate dissolution by carbonic acid however carbonate dissolution by both carbonic and sulfuric acid are dominant processes in karstic terrain total erosion rates dissolved inorganic carbon dic flux and co 2 uptake were 28 0 t km 2 yr 1 2 9 10 5 mol km 2 yr 1 and 2 3 10 5 mol km 2 yr 1 for the granitic terrain and 70 2 t km 2 yr 1 11 9 10 5 mol km 2 yr 1 and 6 4 10 5 mol km 2 yr 1 for the karstic terrain respectively these results indicated the karstic area plays an important role in uptake of co 2 and dic flux along the yangzte river basin our results also revealed that co 2 uptake due to carbonate chemical weathering in both karstic and granitic terrain is very high and provide as much or more dic flux than that from silicate chemical weathering

major element chemistry of the changjiang yangtze river the chemistry of major elements ca mg na k hco3 so4 cl and si in the river water of the changjiang yangtze river was studied based on continuously monitored data at 191 stations in the drainage basin for the period 1958 1990 the results show that the total dissolved solid tds concentration of the changjiang varies over an order of magnitude throughout the basin 49 7 518 1 mg l with a medium tds concentration of 205 9 mg l about three times the global average in contrast the tds at a given main channel station varies only slightly in different seasons with a variation factor less than 2 0 despite a substantial water dilution in the summer flood season the major element chemistry of the changjiang is mainly controlled by rock weathering with the anion hco3 and the cation ca dominating the major ion composition due to the abundance of carbonate rocks in the basin a persistently increasing trend has been observed in the concentrations of so4 and to a lesser extent cl in the changjiang a signature of considerable anthropogenic impacts e g acid deposition flux calculations at datong the most downstream main channel station without tidal influence indicate that the changjiang transports ca 154 106 tons year of tds to the sea second only to the amazon in the world

lithium isotopes as a probe of weathering processes orinoco river abstract lithium isotopes have the potential to be effective tracers of weathering processes due to their large relative mass difference and therefore fractionation in this study an attempt is made to fill a major gap in the knowledge of li isotope fractionation during continental weathering and of the mechanisms involved finally the relationship between the suspended and dissolved material is made on a basin wide scale the orinoco basin provides a clear contrast in reaction limited and transport limited weathering regimes that has already been documented by a comprehensive study on its fluvial geochemistry edmond et al geochim cosmochim acta 60 1996 2949 2976 edmond et al geochim cosmochim acta 59 1995 3301 3325 conspicuous in our new results is the difference in 6li of the dissolved load between the andean 30 to 22 and shield 22 to 7 tributaries while the 6li of the suspended load is similar between the two to a first approximation during superficial weathering in high relief tectonically active terrains the dissolved load is high in li and isotopically heavy more negative 6li whereas in stable shield regions the concentrations are low and isotopically light in proportion to the increasing degree of weathering

origin of the southern okinawa trough volcanism from detailed seismic tomography 1 magmatism associated with subducting plate edges or slab tears has been suggested in the southern okinawa trough the cross back arc volcanic trail which consists of a cluster of about 70 seamounts is located above a ryukyu slab tear lying along the 123 3 e meridian in november 2003 more than 3300 earthquakes recorded in this area by 15 ocean bottom seismometers and surrounding land stations during a period of 12 days were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a mantle inflow characterized by low vp and vs and high vp vs passing through the slab tear is imaged the fluid and or melt component is rising obliquely from the slab tear in the directions of the cross back arc volcanic trail the northern slope of the southern okinawa trough and to north of iriomote island the asthenospheric intake is also imaged by an inclined chip like high vp vs and low vp and vs body dipping northerly which might be linked to the slab retreat west of the slab tear most of the earthquakes are located around low vp and vs and high vp vs bodies which suggests that the seismicity is related to magmatic and or fluid activities east of it earthquakes are concentrated in an area characterized by high vp and vs velocities and low vp vs suggesting that the magma chamber is absent beneath the axial part of the trough and that normal faulting is the main factor controlling the seismicity

the apure river geochemistry of major and selected trace elements in an orinoco river tributary coming from the andes venezuela the analysis of physicochemical variables and selected dissolved elements was performed on the apure river waters for 15 months the variables ph alkalinity dissolved o2 conductivity and na ca mg and cd concentrations showed maximum values during low water whereas k si fe al mn zn cu cr and dissolved organic carbon doc showed maximum concentrations during rising and high water five important factors were found to control the amount and temporal variability of the dissolved elements lithology hydrology vegetation floodplain processes redox conditions and organic complexation weathering of silicates carbonates and evaporites in the andes provides most of the proportion of na ca mg and hco3 to waters the temporal variability of these ions is controlled by a dilution process although si can be taken up by the biomass si and k can be leached from the floodplain by weathering of clays microbial decay of the submerged plants in the floodplain during the inundation periods provides doc and k to river waters and changes the redox conditions in water the changing redox conditions control the solubility of mn zn and fe dissolved mn is a function of ph dependent redox process whereas zn solubility is controlled by scavenging of zn during the oxidation of mn2 to mno2 positive relationships between al fe cu cr and doc suggest that these elements are complexed by organic colloids generated in the floodplain moreover the binding capacity of fe with doc increases under reducing conditions although cd seems to be provided by weathering in the andes several processes can affect the mobility of cd during transport copyright 2010 john wiley sons ltd

the detrital record of cretaceous to pliocene sandstones across the ne south american margin the cretaceous to pliocene erosional history of the ne south american margin depicts the increasing influence of the uplifting caribbean mountain belt petrographic analyses of 59 cretaceous to late pliocene sandstone samples derived from across the islands of trinidad and barbados demonstrate systematic mineralogical changes whereby early cretaceous arkosic arenites gave way to mature quartz arenites by the late cretaceous and caribbean mountain derived lithic rich arenites during the neogene changes in the framework compositions are accompanied by changes in textural attributes with caribbean mountain detritus being notably finer grained than cratonic input silica cements predominate in quartzose sandstones and sericitic and clay cements within litharenites the changing mineralogical compositions record the gradual deformation of the northern south american passive margin and initiation of the andean caribbean mountain foreland basin system with incipient orogenic detritus evident from the latest eocene and unambiguous orogenic input by the late oligocene the preponderance of finer grain sizes within deltaic pliocene sandstones records the later fill of an established foredeep with sediments sourced primarily from erosion of the andean hinterland these interpretations agree with heavy mineral compositional changes and the timing of uplift from geo and thermo chronological data across the margin supplementary material qualitative descriptions sample coordinates and raw point counts are available at https doi org 10 6084 m9 figshare c 3659381

comparison of ba ca and as freshwater proxies a multi species core top study on planktonic foraminifera from the vicinity of the orinoco river mouth past river run off is an important measure for the continental hydrological cycle and the assessment of freshwater input into the ocean however paleosalinity reconstructions applying different proxies in parallel often show offsets between the respective methods here we compare the established foraminiferal ba ca and view the mathml source salinity proxies for their capability to record the highly seasonal orinoco freshwater plume in the eastern caribbean for this purpose we obtained a data set comprising ba ca and view the mathml source determined on multiple species of planktonic foraminifera from core tops distributed around the orinoco river mouth our findings indicate that interpretations based on either proxy could lead to different conclusions in particular ba ca and view the mathml source diverge in their spatial distribution due to different governing factors apparently the orinoco freshwater plume is best tracked by ba ca ratios of g ruber pink and sensu lato morphotypes while view the mathml source based on the same species is more related to the local precipitation evaporation balance overprinting the riverine freshwater contribution other shallow dwelling species g sacculifer o universa show a muted response to the freshwater discharge most likely due to their ecological and habitat preferences extremely high ba ca ratios recorded by g ruber are attributed to ba2 desorption from suspended matter derived from the orinoco samples taken most proximal to the freshwater source do not show pronounced ba ca or view the mathml source anomalies here the suspension loaded freshwater lid developing during maximum discharge suppresses foraminiferal populations both proxies are therefore biased towards dry season conditions at these sites when surface salinity is only minimally reduced

okinawa trough backarc basin early tectonic and magmatic evolution the okinawa trough lying between japan and taiwan is a backarc basin formed by extension within the continental lithosphere behind the ryukyu trench arc system stress directions associated with the two last extensional phases in the southwestern okinawa trough have been deduced from a comparison with analog modeling the direction of extension is n 150 o for the pleistocene phase of extension 2 0 1 ma and n 170 o for the late pleistocene holocene phase of extension 0 1 0 ma the present day ryukyu volcanic arc a narrow continuous feature extending from japan to taiwan is located on the eastern side of the okinawa trough 80 100 km above the wadati benioff zone the minimum depth for emplacement of arc magmatism scarce present day backarc volcanism appears in the middle and southern okinawa trough within linear en echelon bathymetric depressions a n045 o oriented seamount volcanic chain cuts across obliquely the southwestern okinawa trough and lies in the direct line of extension of the gagua ridge a n s linear volcanic feature of the philippine sea plate associated with this extension of the gagua ridge a large reentrant located at the base of the ryukyu prism the uplift of part of the nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross backarc volcanism could be tied to the subduction of the gagua ridge located there at a depth of 80 100 km beneath the backarc basin a second area of anomalous volcanism has been identified in the middle okinawa trough in the ene extension of the daito ridge a wnw ese 400 km long volcanic feature of the philippine sea plate we suggest that the gagua and daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere allowing magmas with arc affinities to erupt at the seafloor this excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts the okinawa trough is consequently still in an early stage of evolving from arc type to backarc activity

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

relocation of the 2006 pingtung earthquake sequence and seismotectonics in southern taiwan article i nfo we combine the arrival time data from eleven ocean bottom seismometers obs and from permanent stations of the central weather bureau seismic network cwbsn and the taiwan strong motion instrumentation program tsmip to improve the three dimensional vp and vp vs structures in the taiwan region relocate hypocenters of earthquakes in southern taiwan including the 2006 pingtung earthquake sequence and determine the focal mechanisms of some of the larger events our tomography result suggests that in the region south of taiwan the eurasian continental lithosphere is being subducted beneath the oceanic philippine sea plate leading to a region of low p wave speed with seismicity on top of a relatively high p wave speed zone the hypocentral distribution of the pingtung earthquake sequence indicates that the first event in the initial two largest shocks was a normal faulting quake caused by the bending of the subducting lithosphere the second event perhaps triggered by the first one was a strike slip earthquake

lithologic controls on osmium isotopes in the rio orinoco abstract the seawater 187 os 188 os ratio has been increasing over the last 50 ma and at present is 1 06 reflecting that about 80 of os in the oceans is derived from the continents that provide os with an average 187 os 188 os ratio of about 1 26 whether the past variations in seawater os isotopes record changes in the bulk os flux with an average isotopic composition or the changes in the average 187 os 188 os ratio of the continental inputs or a combination of these two pathways is not clear the fundamental importance of lithology in controlling continental weathering rates and dissolved ion yields has been well established for the major cations it is dramatically manifested in the sr isotopes of rivers that are affected by selective dissolution of carbonates accordingly for os isotopes it has been proposed that disproportionate contributions of os with extremely high 187 os 188 os ratio come from weathering of organic rich black shales here we examine the effect of the weathering of specific lithologies on osmium isotopic composition and concentration in water samples of the orinoco river basin a major river basin draining both black shales exposed in the northern andes and granites of the precambrian guyana shield the northern andes was uplifted 17 ma which corresponds to the time of a dramatic rise in 187 os 188 os ratio the osmium isotopic composition and concentrations vary considerably in the tributaries and the main stem of the orinoco the highest os concentrations are observed in the rivers draining the andes however waters draining the andes are not highly radiogenic 187 os 188 os 187 os 188 os ratio of seawater during the last 17 million years

cross channel mixing and its effect on sedimentation in the orinoco river at high discharge cross channel mixing of water in the orinoco river main stem requires several days and up to 500 km of downriver transit for completion during mixing substantial amounts of 0 063 mm sediment are lost from suspension the water closest to the left bank appears to lose the most probably by deposition onto the floodplain this occurs in two ways first sediment laden left bank tributaries draining the andes such as the apure deposit sediment in distributary systems following impoundment by the orinoco backwater second when sediment laden waters from the andes actually join the orinoco the slow nature of cross channel mixing causes the water and sediment to be confined more to the left side of the channel significant sediment loss or storage probably occurs only near flood stage since sedimentation occurs preferentially on the left andean foreland basin bank the orinoco remains stably situated on the border of the foreland basin with the guayana shield

geochemistry of the amazon 1 precipitation chemistry and the marine contribution to the dissolved load at the time of peak discharge analyses of precipitation and surface water are used to estimate the fluxes of marine cyclic salts through that part of the amazon river system draining past obidos 80 of the basin at the time of peak discharge in june amazon precipitation chemistry can be devided into two principal components marine and terrestrial the marine component determined from analyses of marine rain consists of na k mg ca and cl in approximately sea salt proportions with s doubly enriched the excess sulfur is probably derived from gas phase inputs the terrestrial component makes an important contribution of k ca s and n much of which can be related to biological emissions the emission of reduced sulfur in the marine and terrestrial environment and nitrogen in the terrestrial environment is responsible for a natural acid rain in the amazon region with a ph from 4 7 to 5 7 this is about one tenth the acidity of polluted urban rain the chloride content of lowland rivers which drain regions lacking significant geologic sources of chloride shows a systematic decrease in chloride with increasing distance from the ocean this trend is used to define the cyclic salt background for amazonian surface waters cyclic salts in general make only a minor contribution relative to terrestrial inputs to the chemistry of amazon basin rivers even those draining intensely weathered terrains an estimated 17 6 cl 6 9 na 1 3 mg 3 6 s 0 4 k and 0 1 ca of the dissolved load at obidos during peak discharge is cyclic

hydrochemical variations of a tropical mountain river system in a rain shadow region of the southern western ghats kerala india abstract river water chemistry of pambar river basin prb draining a rain shadow region of the southern western ghats india with granite gneiss and hornblende biotite gneiss lithology was monitored for three sampling seasons such as monsoon mon post monsoon pom and pre monsoon prm to ascertain the spatio temporal trends in hydrochemistry in prb upstream and downstream areas have differing climate i e tropical wet dry humid upstream while semi arid downstream and land use plantations and farmland dominate the upstream while pristine forest environment covers the downstream the hydrochemical attributes except ph and k exhibit distinct temporal variation mainly due to monsoon driven climatic seasonality relative abundance of cations between upstream and downstream samples of prb shows noticeable differences in that the upstream samples follow the order of abundance ca2 mg2 na k while the downstream samples are in the order na mg2 ca2 k ca2 mg2 na k si na k cl na and hco3 ca2 ratios suggest multiple sources processes controlling hydrochemistry e g atmospheric supply silicate weathering dissolution of carbonate minerals and soil evaporites as well as anthropogenic inputs domestic and farm plantation residues even though weathering of silicate and carbonate minerals is the major hydrochemical driver in both upstream and downstream portions of prb gibbs diagram and scatter plot of mg2 na vs mg2 ca2 imply the importance of evaporation in the downstream hydrochemistry piper diagram and partial pressure of co2 pco2 values suggest that a groundwater dominated discharge exerts a significant control on the downstream hydrochemistry irrespective of sampling season although spatial variability of rainfall in prb shows a linear downstream decreasing trend the best fit model for the dissolved load suggests that the downstream hydrochemical variability in prb i e an increasing trend follows a power function f x ax k this study suggests that climate has a significant role in the spatio temporal variability of hydrochemistry in prb

suspended sediments of the modern amazon and orinoco rivers abstract the amazon and orinoco rivers are massive transcontinental conveyance systems for suspended sediment they derive about 90 of their sediment from the andes that support their western headwaters transport it for thousands of kilometers across the breadth of the continent and deposit it in the coastal zones of the atlantic at their points of maximum suspended sediment discharge the amazon transports an average of 1100 1300 10 6 tons per year and the orinoco transports about 150 10 6 tons per year relations of sediment discharge to water discharge are complicated by unusual patterns of seasonal storage and remobilization increased storage and reduced transport of sediment in the middle orinoco during periods of peak water discharge and storage of suspended sediment in the lower amazon during rising discharge and resuspension during falling discharge spatial distributions of suspended sediment in cross sections of both rivers are typically heterogeneous not only in the vertical sense but also in the lateral the cross channel mixing of tributary inputs into the mainstem waters is a slow process that requires several hundred kilometers of downriver transport to complete considerable fine grained sediment is exchanged between rivers and floodplains by the combination of overbank deposition and bank erosion

present weathering rates in a humid tropical watershed nsimi south cameroon the study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks common for the temperate zone has not yet been widely applied to the tropics especially humid areas this paper presents an updated database for a six year period for the small experimental watershed of the mengong brook in the humid tropics nsimi south cameroon this watershed is developed on precambrian granitoids north congo shield and consists of two convexo concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter mineralogical and geochemical investigations were carried out in the protolith the saprolite the hillside lateritic soils and the swamp hydromorphic soils biomass chemical analyses were done for the representative species of the swamp vegetation the groundwater was analysed from the parent rock saprolite weathering front to the upper fringe in the hillside and swamp system the chemistry of the wet atmospheric and throughfall deposits and the mengong waters was monitored in the nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter these organic colloids contribute significantly to the mobilization and transfer of fe al zr ti and th in the uppermost first meter of the swamp regolith when the organic colloid content is low i e in the hillside groundwater th and zr concentrations are extremely low 3 pmol l icp ms detection limits strongly insoluble secondary thorianite tho 2 and primary zircon zrsio 4 crystals control their mobilization respectively this finding thus justifies the potential use of both these elements as inert elements for isoelement mass balance calculations pertaining to the hillside regolith chloride can not be used as a conservative tracer of hydrological processes and chemical weathering in this watershed biogenic recycling significantly influences the low cl input fluxes sodium is a good tracer of chemical weathering in the watershed the sodium solute flux corrected from cyclic salt input was used to assess the chemical weathering rate even though low 2 8 mm kyr the chemical weathering rate predominates over the mechanical weathering rate 1 9 mm kyr compared to the rio icacos watershed the most studied tropical site the chemical weathering fluxes of silica and sodium in the mengong are 16 and 40 times lower respectively this is not only related to the protective role of the regolith thick in both cases but also to differences in the hydrological functioning this is to be taken into account in the calculations of the carbon cycle balance for large surfaces like that of the tropical forest ecosystems on a stable shield at the global level

improved seismic tomography offshore northeastern taiwan implications for subduction and collision processes between taiwan and the southernmost ryukyu summary to improve the resolution in imaging the 3 d vp and vp vs structures in the taiwan region especially offshore eastern taiwan we combine the arrival times from eleven ocean bottom seismometers obss and from seismic stations of the japan meteorological agency jma with those from the permanent stations of taiwan central weather bureau seismic network cwbsn and taiwan strong motion instrumentation program tsmip by doing so we have obtained a new 3 d tomographic model for taiwan and its surrounding regions with a better resolution especially in the area offshore northeastern taiwan we also used this new tomography model to relocate the hypocentres of the earthquakes in northeastern taiwan and determined the focal mechanisms of relatively large events our tomography results indicate that in the region northeast of taiwan the subducting oceanic philippine sea plate beneath the eurasian continental lithosphere is characterized by a high vp layer surrounded by lower vp areas this p wave velocity characteristics of the subducting slab provides a better constraint on the geometry of the subduction interface especially in its shallower portion in the hangingwall eurasian block above the subduction interface a vertically elongated high vp vs body appears to originate from the interface at depths between 100 and 140 km we suggest that it represents the partially melted materials that are related to the magmatic activity of the ryukyu volcanic arc

study of the erosion rates in the upper maracuja basin quadrilatero ferrifero mg brazil by the in situ produced cosmogenic 10be method the present work quantifies the erosive processes in the two main substrates schists phyllites and granites gneisses of the upper maracuja basin in the quadrilatero ferrifero mg brazil a region of semi humid tropical climate two measuring methods of concentration were used i in situ produced 10be in quartz veins surface erosion rates and ii 10be in fluvial sediments basin erosion rates the results confirm that i erosion tends to be more aggressive close to the headwaters than in the lower parts of the basin and ii the region is now affected by dissection copyright 2006 john wiley sons ltd