observations of currents and water properties in the amazon frontal zone measurements of currents and water properties were obtained at various locations in the frontal zone of the amazon river outflow plume during four cruises in 1989 1991 as part of a multidisciplinary amazon shelf sediment study a salinity front resembling a salt wedge is found approximately 150 km seaward of the river mouth it continues northwestward more than 400 km along the shelf between the 10 and 20 m isobaths tidal currents which are oriented principally in the cross shelf direction reach 200 cm s 1 during spring tides and drop to less than 80 cm s 1 during neaps in the vicinity of the river mouth the nontidal flow is strongly sheared with near surface speeds of up to 100 cm s 1 and weak near bottom velocities the direction of the near surface flow is generally northwestward except near the river mouth where river discharge leads to a significant offshore component of flow the strong velocity shears are stabilized by strong stratification by salinity and by highly concentrated suspended sediment or fluid mud near the seabed variations in stratification and frontal position are due principally to spring neap variations in tidal mixing and seasonal variations in runoff

subaqueous delta of the ganges brahmaputra river system abstract the ganges brahmaputra is among the world s three largest river systems in terms of sediment load but until now no high resolution seismic data have been obtained to document the nature of the sediment deposit seaward of the rivers mouths the other two amazon huanghe discharge into energetic coastal environments and form subaqueous deltas with characteristic clinoform stratigraphy high resolution seismic reflection profiles of the bengal shelf reveal similar stratigraphy topset beds dip gently 0 036 and diverge offshore more steeply dipping foreset beds 0 190 converge farther seaward and relatively thin gently dipping bottomset beds 0 022 extend across the outer shelf overlying an erosional surface presumed to be of late pleistocene age sediment accumulation rates are highest in the foreset region 5 cm year and reduced in the bottomset region

lagrangian flow observations of the amazon river discharge into the north atlantic nine satellite tracked drifters with near surface drogues were launched in or near the mouth of the amazon river on the north brazil shelf during 1989 1991 at four different stages of river discharge the drifters initially moved along shelf toward the northwest over the north brazil shelf with mean speeds varying from 41 to 128 cm s and a maximum along shelf velocity of 197 cm s many drifters experienced strong acceleration near 2 n between the 5 m and 20 m isobaths caused in part by the sharp reduction in cross shelf cross sectional area near the shore perpendicular cabo norte shoal the drifter trajectories showed strong cross shelf tidal variability in the flow field near the amazon river mouth and the largest low frequency along shelf variability was located over the open shelf just north of cabo norte good visual correlation between drifter along shelf velocity and along shelf wind stress supports the dynamical model of lentz this issue that subtidal fluctuations in near surface along shelf currents in the amazon river plume are strongly wind driven drifters left the north brazil shelf after about 5 to 30 days three drifters moved northwestward parallel to the northeast coast of south america crossed the shelf break near 8 n and moved into deeper north atlantic water where they began looping in anticyclonic eddies often called retroflection eddies or loopers with periods of approximately 11 days as they moved northwestward to the caribbean sea four drifters initially moved along shelf toward the northwest and then offshore between 3 n and 7 n where they followed the retroflection of the north brazil current only one drifter entered the zonal current system that characterizes the equatorial atlantic circulation

elemental redistribution in tieschitz and the origin of white matrix two types of mesostasis coexist within some porphyritic chondrules in tieschitz one type is smooth the other confined to chondrule margins is blocky on a 5 10 m scale mesostases in one porphyritic olivine pyroxene pop chondrule and one porphyritic olivine po chondrule were analysed by scanning electron microscopy sem and energy dispersive x ray spectrometry eds as was white matrix nearby mesostases in the po chondrule and in four others were analysed by ion probe pyroxene phenocrysts or dendrites extend across contacts between smooth and blocky mesostasis with no compositional change relative to smooth mesostasis blocky mesostasis is enriched in al alkalis ba f and cl but depleted in si fe and ca white matrix fills channels between the chondrules it is physically and chemically similar to blocky mesostasis but three ion probe analyses indicate that unlike the mesostases it is poor in sc and has variable and fractionated rare earth elements rees smooth mesostasis is interpreted as solidified primary chondrule liquid whereas blocky mesostasis is its alteration product or less likely a precipitate replacing smooth mesostasis leached out by aqueous fluid white matrix may have formed by secondary alteration or replacement of mesostases that had been expelled from chondrules during accretion or as a precipitate filling interchondrule voids iron may have been lost from the bulk meteorite but most other elements merely underwent internal redistribution disturbed isotopic systems indicate that aqueous fluid may have been active on the tieschitz parent body only 2 ga ago if correct this would be the first evidence that an ordinary chondrite parent body underwent internal reprocessing significantly later than 4 5 ga ago

non steady state 210pb flux and the use of 228ra 226ra as a geochronometer on the amazon continental shelf the recovery of quasi cyclic 210pb profiles from the foreset region of the amazon subaqueous delta has raised questions concerning the non steady state flux of 210pb to this region of the shelf radioisotopic profiles 210pb 234th 228ra 226ra 137cs from seven kasten cores along with the analysis of grain size organic carbon and clay sized mineralogy are used to investigate the time scale and the controlling mechanisms of the 210pb cyclicity a common assumption used in 210pb geochronology is that the initial specific activity is constant with time an assumption that is invalid in this region variations of particle affinity for 210pb have been shown to affect the initial specific activity reaching the seabed in various other studies however investigations of grain size organic carbon and mineralogy have revealed no relationship with the fluctuating 210pb activity in this area a new geochronometer 228ra 226ra is used to determine sediment accumulation rates over a 30 year time scale which range from 10 to 60 cm yr in the study area spectral analysis of the 210pb cyclicity has revealed that the major component of 210pb cyclicity has a frequency of approximately 2 3 years in addition components of the 210pb cyclicity occur on annual and semi annual frequencies the controlling mechanism for the 210pb cyclicity on these time scales appears to be the seaward transport and accumulation of low activity fluid muds from the topset region

rates of flood plain sedimentation on the central amazon river field measurements of water and sediment discharge and floodplain topography numerical simulations of hydrology and landsat data on sediment concentrations were combined to estimate rates of transport and the fate of water and sediment in a 200 km reach of the central amazon river results from field data collected during the annual flood in july 1986 indicate that sediment was transported to the flood plain at rates ranging from 3 to 5 t d 1 m 1 with percentage deposition 80 yielding vertical rates of 0 8 to 1 6 cm day comparable rates were calculated from geographic information system overlays of numerically simulated specific water discharge and land sat derived sediment concentrations rates for july 31 1977 include 4 to 18 t d 1 m 1 of transport onto the flood plain and an average of 64 deposition at vertical rates from 0 4 to 3 3 cm d a 1 m higher flow on august 2 1989 yielded on average higher rates of transport onto the flood plain 8 to 15 t d 1 m 1 but lower percentage 46 deposition implying vertical rates between 0 3 and 2 9 cm d

from gullies to mountain belts a review of sediment budgets at various scales this paper reviews the state of the art in the concept as well as in the application of sediment budgets in sedimentary research sediments are a product of mass dispersal at the earth surface and take part in global cycles sediment budgets aim at quantifying this mass transfer based on the principle of mass conservation and are the key to determine ancient fluxes of solid matter at the earth surface this involves fundamental questions about the interplay of uplift climate and denudation in mountain belts and transfer of sediments from the continents to the oceans as well as applied issues such as soil and gully erosion reservoir siltation and coastal protection first after introducing basic concepts relevant scales and methodologies the different components of quaternary routing systems from erosion in headwaters river systems glacial and paraglacial systems lakes deltas estuaries coasts shelves epicontinental seas and deep sea fans are discussed in terms of their sediment budget most suitable are sedimentologically closed or semi closed depositional environments e g alluvial fans lakes deltas and deep sea fans in a second step the dynamics of passive active and collisional tectonic settings and sediment budgets in related sedimentary basins are explored and new concepts of sediment portioning at large geodynamic scales are introduced ancient routing systems are more or less incomplete and may be intensively fragmented or destroyed in active tectonic settings in terms of sedimentary basin types rifts intracontinental and epicontinental settings are preferred objects of sediment budgets because of their persistence and relatively simple overall sedimentary architecture however closing basins such as foreland forearc retroarc piggy back and wedge top basins may provide excellent snapshots of orogenic sediment fluxes in a third step the large long lived routing systems of the amazon the ganges brahmaputra and the rhine are reviewed for each system estimates of either sediment volumes mass or sediment fluxes of continental and marine subsystems have been compiled in order to receive a complete routing in terms of mass conservation for specific time periods since the late glacial maximum as well as the cenozoic following lessons can be taken from these case studies i depositional centers and fluxes show strong shifts in space and time and call for caution when simply looking at subsystems ii the response times of these large systems are within the milankovich time interval thus lower than predicted from diffusion models iii cyclic routing of sediments in continental basins is much more dominated by climate human control than by eustacy and iv at long time scales ultimate sinks win over intermittent storage it is concluded from this review that the quantitative understanding of global sediment cycling over historic and geologic time and its response to allogenic forcing is still in its infancy and further research is needed towards a holistic view of sediment routing systems at various temporal and spatial scales and their coupling with global biogeochemical cycles this includes i to better determine response times of large routing systems by linking quaternary with cenozoic sediment budgets and continental with marine sub systems ii to combine advanced provenance techniques with sediment budgets in order to reconstruct ancient systems iii to study sediment partitioning at the basin scale iv to reconcile continental supply dominated sequence stratigraphy with the eustatic dominated marine concept and iv to account for non actualism of ancient systems with respect to their erosion and transport mode in particular during glaciations and pronounced arid intervals glacial and eolian sediment routing may cross over hydrologic boundaries of drainage basins thus challenging the principle of mass conservation

separation of tidal and subtidal currents in ship mounted acoustic doppler current profiler observations a simple method is developed to analyze current measurements obtained from a moving platform the need for this is motivated by the now common use of the ship mounted acoustic doppler current profiler adcp to acquire absolute velocity data during an oceanographic survey of a given region the full potential of this new measurement technique is severely hindered when the presence of high frequency phenomena e g tidal or inertial motions prevents a clear visualization of the lower frequency current structure of interest our analysis method is based on a spatial interpolation scheme using arbitrary functions that allows for the tidal current time variability which then permits the tide induced motions to be subtracted from the adcp data to yield the subtidal current field the method also allows nearby moored and drifter current measurements if available to be combined with the shipboard adcp observations in a single analysis to obtain the best description of the tidal and subtidal currents to illustrate this method we present results from the analysis of adcp data taken during oceanographic surveys in two different continental shelf regions the east china sea and the amazon shelf a 5 day conductivity temperature depth ctd and adcp survey was made in the east china sea near the mouth of the yellow sea during january 1986 there the currents were essentially barotropic and dominated by the semidiurnal tide the adcp derived cotidal chart for the m2 12 42 hours component agrees well with existing charts derived empirically from sea level observations or from regional numerical models the adcp derived steady flow is also consistent with the observed density field and indicates little flow in or out of the yellow sea and a transport of about 1 sv toward the tsushima strait two ctd adcp surveys lasting 21 and 23 days were conducted over the amazon shelf during march and may 1990 simultaneously moored current observations were also obtained at three locations along a cross shelf array 200 km north of the amazon river mouth for a common period of at least 2 months february 12 to april 13 over the shelf tidal and subtidal currents were comparable in magnitude our analysis indicates that the tidal currents were essentially barotropic semidiurnal and polarized in the cross shelf direction increasing in magnitude toward the inner shelf where current values of more than 2 m s are common the adcp derived steady currents were aligned in the along shelf direction and strongly influenced by the north brazil current nbc during both adcp surveys the northwestward flowing nbc was transporting more than 2 sv over the shelf at depths shallower than 100 m

anaerobic methane oxidation on the amazon shelf anaerobic methane oxidation on the amazon shelf is strongly controlled by dynamic physical sedimentation processes rapidly accumulating physically reworked deltaic sediments characteristic of much of the shelf typically support what appear to be low rates of steady state anaerobic methane oxidation at depths of 5 8 m below the sediment water interface methane oxidation in these cases is responsible for 10 m methane charged sediment directly to seawater in this nonsteady state situation methane is a major source of recently produced co2 and an important reductant for sulfate these observations suggest that authigenic sedimentary carbonates derived from anaerobic methane oxidation may sometimes reflect physically enhanced nonsteady state exposure of methane to sulfate in otherwise biogeochemically unreactive deposits the concentration profiles of ch4 so 4 and co2 in the eroded deposit were reproduced by a coupled reaction transport model this area of the shelf was reexposed to seawater approximately 5 10 years ago based on the model results and the assumption that the erosion of the deposit occurred as a single event that has now ceased the necessary second order rate constant for anaerobic methane oxidation was 0 1 mm 1 d 1

origin of increased terrigenous supply to the ne south american continental margin during heinrich stadial 1 and the younger dryas we investigate the redistribution of terrigenous materials in the northeastern ne south american continental margin during slowdown events of the atlantic meridional overturning circulation amoc the compilation of stratigraphic data from 108 marine sediment cores collected across the western tropical atlantic shows an extreme rise in sedimentation rates off the parnaiba river mouth about 2 s during heinrich stadial 1 hs1 18 15 ka sediment core geob16206 1 raised offshore the parnaiba river mouth documents relatively constant 143nd 144nd values expressed as end 0 throughout the last 30 ka whereas the homogeneous end 0 data support the input of fluvial sediments by the parnaiba river from the same source area directly onshore the increases in fe ca al si and rb sr during hs1 indicate a marked intensification of fluvial erosion in the parnaiba river drainage basin in contrast the end 0 values from sediment core geob16224 1 collected off french guiana about 7 n suggest amazon sourced materials within the last 30 ka we attribute the extremely high volume of terrigenous sediments deposited offshore the parnaiba river mouth during hs1 to i an enhanced precipitation in the catchment region and ii a reduced north brazil current which are both associated with a weakened amoc

lowermost amazon river morphology and shallow seismic characteristics abstract the riverbed morphology of the lowermost amazon river was characterized by high resolution acoustic profiling in 21 areas this technique has been used for the first time in this area and allowed the discovery of a variety of reflectors which delineate areas either of sediment transport or of deposition on an older river bottom records obtained with the pinger system made it possible to distinguish three main kinds of reflectors both surface and sub bottom 1 flat 2 wavy and 3 irregular reflection patterns which can be interpreted as strata deposited in a variable mostly high energy setting or reworked structures of initially flat or wavy strata that have been changed mapping of these different reflectors shows that they are not randomly distributed the constant association of a given reflector with a particular morphological setting is indicative of its lithological significance supported by riverbed sampling and erosional depositional processes depositional areas are characterized by transparent units consisting of soft silt overlying a highly reflective sub bottom related to old substrates alter do chao or barreiras formation this may result from inundation and incision caused by quaternary sea level changes previously eroded areas are covered by a wavy layer consisting of very fine to fine sand unit fs which can change locally to medium coarse sand unit ms reflectors are more abundant downstream a minimum of 3 units were indicated by seismic records and could be related to sea level fluctuations and changes in the river discharge moreover the detection of fault scarps interpreted to result from neotectonic activity shows that neotectonic features can be important locally and should not be ignored as a possible reason for modern river instability the dynamic setting of the lowermost amazon area results in patterns of deposition and erosion probably controlled by complex interactions of fluvial and oceanic processes the effect of the gurupa arch may result in a zone of sediment bypass in the central part and a zone of increased deposition in the distal downstream part

an introduction to the geological significance of sediment transport and accumulation on the amazon continental shelf in order to understand the formative processes and resulting stratigraphy on the amazon shelf and adjacent shoreline of amapa geological investigations were undertaken as part of a multidisciplinary amazon shelf sediment study amasseds the design of the study provided results of a multifaceted nature integrated with observations in other disciplines focused on sedimentary processes representative of fluctuating conditions on several time scales and broadly distributed on the shelf including regions not investigated in the past on short time scales most muddy sediment is transported northwest of the river mouth from there it moves seaward as fluid mud to cause rapid accumulation of foreset beds and moves northwestward to prograde the northernmost amapa shoreline and to supply sediment to the guianas fluid muds cover the shelf as far northward as 3 5 n and allow strong tides to propagate to shore where tidal currents cause most of the amapa shoreline to undergo erosion today averaged over decades and centuries about half of the amazon sediment discharge accumulates on the adjacent shelf and another significant fraction one sixth leads to accretion of the northernmost amapa shoreline and northwestward bypassing of sediment the remaining sediment is hypothesized to be trapped in deltaplain deposits of the lower amazon river system the foreset region and shoreline represent the two common loci of sediment accumulation that alternate their predominance on time scales of millennia and lead to a two stage progradation of coastal plain and subaqueous deltaic deposits this muddy regressive sedimentation during high sea level is replaced by formation of erosional sand layers during low sea level and transgressive conditions future research in the study area should address important considerations that were delineated by the present study including mechanisms of shoreline accretion the holocene history recorded in topset and coastal plain strata the role in local sedimentation played by the large shoal extending from cabo norte and the entrapment of amazon sediment by the delta plain

measurements in the bottom boundary layer on the amazon subaqueous delta abstract an instrumented bottom tripod geoprobe recorded flow and suspended sediment data in the bottom boundary layer above the lower foresets of the amazon subaqueous delta in 65 m mean water depth in february 1990 after about two weeks of operation the apparent seafloor at the tripod site rapidly elevated over a 14 hour period by about 44 cm this sudden change which was detected by an acoustic altimeter and which caused the loss of signals from the lowermost geoprobe current and optical sensors is thought to have been caused by the incursion of a dense bottom layer of fluid mud that migrated downslope from shallower sections of the foresets the fluid mud migration across the outer part of the foresets if a repetitive and occasional process in this region could be a major mechanism for episodic seaward growth of the delta current velocity profiles are used to estimate shear velocities u and roughness lengths zo during the first two weeks of measurements the extremely energetic semi diurnal tidal motion required correction of the estimates of u and zo for acceleration average values of u and zo for the two week period were 1 7 cm s and 0 3 cm respectively the effects of strong stratification due to flocculated near bottom suspended sediment on the estimates of u are significant for the lower values of u u

distribution of fluid muds on the amazon continental shelf abstract a comprehensive study of suspended sediment transport on the amazon shelf was conducted as part of the amasseds project a multidisciplinary amazon shelf sediment study to understand the link between physical processes and the fate of amazon river sediment a small instrumented tripod was used to measure the fluid flow and suspended sediment characteristics throughout the water column on varying time scales e g semidiurnal fortnightly seasonal dense nearbed suspensions or fluid muds suspended sediment concentration 10 g l on the order of a few meters in thickness were found on the inner and middle shelf covering an area ranging from 5 700 km2 during falling and low river discharge to 10 000 km2 during rising and high discharge fluid muds generally observed in the region of bottom salinity fronts extend across the topset beds to the foreset beds during rising and high discharge salinity and temperature anomalies frequently observed in fluid muds imply that often the muds do not result from erosion and resuspension from the seabed but from trapping processes at the bottom salinity front and transport across the shelf the vertical structure of fluid mud varies in space and tune variability close to the river mouth is predominantly fortnightly related to the location and structure of the bottom salinity front on the open shelf the variability appears to be related to seasonal differences in river and sediment discharge and the location of the bottom salinity front the total suspended sediment inventory on the shelf during the season of high discharge is approximately equal to the annual sediment discharge from the river 1 2 109 tons yr and 90 of that inventory is within fluid mud suspended sediment transport on the shelf is dominated by fluid mud

river tributaries as sediment sinks processes operating where the tapajos and xingu rivers meet the amazon tidal river the amazon river is the largest fluvial source of freshwater and sediment to the global ocean and has the longest tidally influenced reach in the world two major rivers the tapajos and xingu enter the amazon along its tidal reach however unlike most fluvial confluences these are not one way conduits through which water and sediment flow downstream toward the sea the drowned river valleys rias at the confluences of the tapajos and xingu with the amazon river experience water level fluctuations associated not only with the seasonal rise and fall of the river network but also with semidiurnal tides that propagate as far as 800 km up the amazon river superimposed seasonal and tidal forcing distinct sediment and temperature signatures of amazon and tributary waters and antecedent geomorphology combine to create mainstem tributary confluences that act as sediment traps rather than sources of sediment hydrodynamic measurements are combined with data from sediment cores to determine the distribution of tributary derived and amazon derived sediment within the ria basins characterize the sediment transport mechanisms within the confluence areas and estimate rates of sediment accumulation within both rias the tapajos and xingu ria basins trap the majority of the sediment carried by the tributaries themselves in addition to ca 20 mt yr 1 of sediment sourced from the amazon river these findings have implications for the interpretation of stratigraphy associated with incised valley systems such as those that dominated the transfer of sediment to the oceans during lowstands in sea level this article is protected by copyright all rights reserved

stratigraphic evidence of changes in amazon shelf sedimentation during the late holocene abstract late holocene sedimentation on the amazon shelf was investigated using a combination of seismic sedimentological and geochronological methods piston cores collected in the subaqueous delta region during a multidisciplinary amazon shelf sediment study amasseds penetrated a regionally traceable 1 25 10 4 km 2 seismic discontinuity which corresponds to a dense relict mud reflector overlain by acoustically transparent sediment distinct offsets in profiles of wet bulk density p wave velocity and 210 pb activity are noted in cores at the reflector horizon unconformity signifying a past change in sedimentation conditions on the shelf bulk sediment 14 c dating indicates net depositional conditions for the subaqueous delta from 1800 yr b p to 700 yr b p and a net erosional phase from about 700 yr b p to 100 yr ago sedimentological evidence of the erosional phase includes lag deposits and burrows present at the unconformity surface in piston cores the deposits above and below the unconformity are similar with respect to grain size parameters and sedimentary structures suggesting that environmental conditions during the two depositional periods were similar according to 14 c dates of coastal deposits the shoreline of amapa state brazil served as a major depocenter of amazon mud during the erosional phase on the shelf 210 pb geochronology indicates that the most recent phase of sediment accumulation in the study area commenced about 100 yr ago resulting in rapid progradation of topset deposits and burial of relict strata in the present environment oceanographic and meteorologic processes cause along shelf variations in sediment dispersal and depositional patterns and about 1400 km 2 of the erosional relict deposits are still exposed at the seabed the short time scales over which amazon shelf strata respond to changes in the physical environment preclude the influence of relative sea level cyclicity on recent sedimentation patterns during the late holocene sedimentation and strata formation on the amazon shelf are linked to fluvial oceanographic and meteorologic processes that affect sediment supply deposition and erosion

channel floodplain geomorphology along the solimoes amazon river brazil across the cratonic landscape of brazil the solimoes amazon river transports to its delta plain 1240 mt of suspended sediment derived from andean erosion and reworks another 3200 mt of floodplain sediments distribution of these sediments has resulted in a variable along stream pattern of geomorphology the upstream reaches are characterized by sediment erosion in the main channel and deposition in floodplain channels that are an order of magnitude smaller in discharge than the main channel sediment deposition in and migration of the floodplain channels erases oxbow lakes of the main channel and yields an intricate scroll bar topography that forms the boundaries of hundreds of long narrow lakes in contrast downstream reaches are characterized by channels restricted by stabilizing long term levee building and floodplain construction dominated by overbank deposition overbank deposition buries the scroll bar topography resulting in a flat floodplain covered by a patchwork of large more equant shallow lakes on the basis of estimated rates of recycling of floodplain sediments the modern floodplain of the brazilian amazon could have been recycled in 5000 yr and is recycled more rapidly in the upstream than the downstream reaches the cratonic interior is interrupted by structural arches that bound intracratonic basins four of these arches cross the valley of the main river system at intervals of several hundred kilometres and impart a tectonic imprint on the channel floodplain geomorphology at this spatial scale structural arches appear to exert a primary influence by promoting entrenchment of the river as it passes through zones of deformation thus restricting channel movement for example as the river crosses the purus arch the valley narrows to 20 km compared to an average of 45 km the water surface gradient decreases sediment is deposited and yet the rate of channel migration is negligible hence the effect of the arches is to create a landscape where on the spatial scale of hundreds of kilometres the river is confined and entrenched in its valley is straight and is relatively immobile local valley tilting apparently unrelated to the arch structures also imprints the geomorphology in particular a tilted valley in the upstream reaches appears to have caused avulsions which have left behind the only large scale oxbow type features on the brazilian amazon river floodplain

the m2 tide on the amazon shelf as part of a multidisciplinary amazon shelf sediment study amasseds moored and shipboard current measurements made over the amazon shelf during 1990 1991 have been analyzed to determine the dominant semidiurnal tidal constituent the m2 these results have been combined with coastal sea level data from within the amazon and para rivers the adjacent shelf and with satellite derived tidal elevation data from off the shelf to provide a more complete description of the m2 tide in this complex river shelf system near the amazon river mouth the m2 tide propagates across the shelf and through the mouth as a damped progressive wave with its amplitude decreasing and phase increasing upriver over the adjacent shelf north of cabo norte the m2 tide approaches a damped standing wave with large amplitudes greater than 1 5 m near the coast due to near resonance within the coastal embayment formed by the cabo norte shoal to the south and cabo cassipore to the north the observed m2 tidal currents are nearly rectilinear and oriented primarily across the local isobaths comparisons between tidal observations in both the north channel and the cabo norte cabo cassipore embayment and a simple variable width channel tidal model indicate that 1 most of the m2 tidal energy dissipation occurs over the mid and inner shelf in water depths less than 20 m and 2 fluid muds found there cause a significant reduction of order 50 in the effective bottom friction felt by the m2 tide the approximate resonant period of the cabo norte cabo cassipore embayment is 11 9 hours and at resonance the average energy dissipation per forcing period is roughly 2 2 times the average mechanical energy in the embayment this damping rate is large enough that the tidal amplification is rather insensitive to forcing frequency so that the response of the embayment to forcing over the semidiurnal band should be essentially the same the vertical structure of the m2 tidal current is examined at one outer shelf site located in 65 m water depth the observed semimajor axis increases logarithmically with height above bottom within the lowest 1 2 m and reaches a maximum in excess of 0 5 m s at approximately 11 m above bottom the mean ellipticity is small less than 0 1 and positive indicating clockwise rotation of a nearly rectilinear current and the semimajor axis is oriented within 10 of the local cross isobath direction the m2 phase increases with height above bottom with flood at the bottom leading flood at the surface by about 1 hour a simple local homogeneous tidal model with time and space dependent eddy viscosity simulates the observed near bottom velocity reasonably well however the model suggests that stratification above the lowest few meters may significantly affect the tidal boundary layer structure at this site the m2 energy flux onto the amazon shelf and into the amazon and para rivers has been estimated using current and surface elevation data and the best fit variable width channel model results the net m2 energy flux into the mouths of the amazon and para rivers is 0 47 1010w and 0 19 1010w respectively a net m2 energy flux of about 3 3 1010w occurs onto the shelf between the north channel of the amazon river and cabo cassipore this stretch of the amazon shelf accounts for about 1 3 of the global dissipation of the m2 tide

exchanges of sediment between the flood plain and channel of the amazon river in brazil sediment transport through the brazilian sector of the amazon river valley a distance of 2010 km involves exchanges between the channel and the flood plain that in each direction exceed the annual flux of sediment out of the river at obidos 1200 mt yr 1 the exchanges occur through bank erosion bar deposition settling from diffuse overbank flow and sedimentation in flood plain channels we estimated the magnitude of these exchanges for each of 10 reaches of the valley and combined them with calculations of sediment transport into and out of the reaches based on sediment sampling and flow records to define a sediment budget for each reach residuals in the sediment budget of a reach include errors of estimation and erosion or deposition within the channel the annual supply of sediment entering the channel from bank erosion was estimated to average 1570 mt yr 1 1 3 the obidos flux and the amount transferred from channel transport to the bars 380 mt yr 1 and the flood plain 460 mt yr 1 in channelized flow 1230 mt yr 1 in diffuse overbank flow totaled 2070 mt yr 1 1 7 the obidos flux thus deposition on the bars and flood plain exceeded bank erosion by 500 mt yr 1 over a 10 16 yr period sampling and calculation of sediment loads in the channel indicate a net accumulation in the valley floor of approximately 200 mt yr 1 over 16 yr crudely validating the process based calculations of the sediment budget which in turn illuminate the physical controls on each exchange process another 300 400 mt yr 1 are deposited in a delta plain downstream of obidos the components of the sediment budget reflect hydrologic characteristics of the valley floor and geomorphic characteristics of the channel and flood plain which in turn are influenced by tectonic features of the amazon structural trough

carbon sedimentation at lago grande de curuai a floodplain lake in the low amazon region insights into sedimentation rates we studied recent organic carbon sedimentation in an amazonian floodplain using sediment trap samples and one core sediment traps were placed in several lakes of the lago grande de curuai floodplain during different water level phases the end of the period of water level reduction falling water and the start of the period of water level increase rising water vertical seasonal fluxes of settling particles sp are very difficult to estimate and interpret especially during falling water periods when the concentration of suspended matter in the water column is greater than during rising water periods probably due to resuspension processes vertical fluxes of settling particles sp varied between ca 4 and 2000 g m 2 day 1 fluxes of organic carbon ranged between 5 50 g c m 2 day 1 during the falling water and between 0 041 4 g c m 2 day 1 during the rising water the core is characterised by two laminated sedimentological units sediments in the upper unit which generally coarsens upwards are interpreted as a progradation sequence of the sand and silt flat at the entrance of an inflow channel where the core was collected this unit is composed of three sub units a superficial layer reworked by bioturbation a lower sub unit characterised by horizontal and sub horizontal laminations of fine sands and silts and a basal sub unit composed of horizontal laminations of fine silts and clays the lower unit is characterised by horizontal and sub horizontal laminations of fine sands and silts and corresponds to the top of an older accumulating sequence these changes show that the sedimentary environment is very dynamic at the centennial scale sedimentation rates generally ranged between 0 42 and 1 34 cm year 1 and were estimated to range from 0 56 to 1 35 cm year 1 after normalisation for clay sized particles 4 m floodplain sediment accumulation generally varies between 0 42 1 16 and 1 10 g m 2 year 1 within the first second and third sub units respectively in the second unit the non supported 210pb date is close to the detection limit carbon content is relatively low around 1 sediment weight through the core although mean carbon accumulation rates are very high generally 100 g m 2 year 1 and peaking at 250 g m 2 year 1 these results suggest that amazon floodplain lakes may constitute an important sink for organic carbon

seabed dynamics of the inner amazon continental shelf temporal and spatial variability of surficial strata the continental shelf southeast and northwest of the amazon river mouth is overlain by a sediment layer of about a meter in thickness characterized by 210pb profiles consisting of one or more segments with nearly uniform excess activity uniform 210pb activity profiles in shelf environments commonly are a result of intense biological reworking of sediments however biological activity is minimal in this area hence this surficial layer sl is a result either of physical reworking of the sediments or of rapid possibly episodic deposition of sediment factors controlling the formation of the sl were examined during the amasseds project between august 1989 and october 1991 primarily using 210pb t1 2 22 3 yr activity profiles and x radiography the thickness and character of the sl were assessed at four locations reoccupied during four seasonal periods corresponding to various riverine and oceanographic conditions e g discharge stage wind stress one of these stations was reoccupied during semidiurnal and fortnightly periods additionally replicate sediment cores were obtained at stations in order to assess small scale spatial variability of geochemical and sedimentological characteristics in the seabed the sl thicknesses in replicate sediment cores obtained at each site were found to vary within 5 cm normalized thickness of the sl decreased by an average of 17 cm between august 1989 and february 1990 a period of increasing wind stress during falling to rising river discharge the sl thickness subsequently increased slightly 8 cm from february to may 1990 and increased again 27 cm between may 1990 and october 1991 the maximum sl thicknesses were observed in october a period of low wind stress and minimal river input suggesting that the observed seasonal changes are dictated by oceanographic conditions as well as by sediment discharge variations in sl thickness are reflected by changes in suspended sediment mostly as fluid muds as the highest suspended sediment inventories and most extensive fluid muds are observed during the period when sl thickness is at a minimum although a significant sl was present during all of the sampling periods changes in the character of 210pb profiles at two of the stations suggest that substantial perhaps complete removal and redeposition of the sl occurred some time between february and may preserved strata i e below the sl is characterized by greater evidence of bioturbation and increased sand content but otherwise is similar to that of the sl long term preservation of strata is thought to occur through episodic suspension and redeposition of the sl with net accumulation of decimeter thick hiatus bounded layers

structure and dynamics of fluid muds on the amazon continental shelf thick layers of fluid mud occur in strong tidal flows over the inner portion of the amazon continental shelf in regions of strong salinity fronts associated with the plume discharged from the amazon river detailed shipboard profile measurements obtained in this region during a multidisciplinary amazon shelf sediment study amasseds provide an unprecedented opportunity to examine the structure and dynamics of fluid muds under natural conditions the analysis focuses on flows in which the motion is fully turbulent and suspended sediment dominates the stratification under these conditions a comparison of measurements and one dimensional model calculations indicates that vertical transport is controlled by suppression of turbulent mixing at gradient richardson numbers near 1 4 this constraint produces a distinctive vertical structure and leads to an upper bound on the total amount of suspended sediment that may be carried in a turbulent suspension by a tidal flow

late holocene evolution of the mahakam delta east kalimantan indonesia abstract the late holocene mahakam delta located along the tropical eastern shore of kalimantan indonesia is considered to be a textbook example of a mixed tide fluvial dominated delta system the delta prograded about 60 km during the past 5000 years which led to the development of a distinct network of distributary and tidal channels wave action is low due the limited fetch in the narrow strait of makassar mahakam river discharge is about a quarter of the mississippi river discharge and is characterized by absence of flood surges therefore natural levees crevasse splays and avulsions are absent in the delta plain for the past four decennia both modern and ancient mahakam delta deposits have been studied in detail in order to better understand subsurface miocene and tertiary mahakam deposits which host large volumes of hydrocarbons this study focuses on the dynamics and stratal patterns of delta plain delta front platform deposits and suspended sediments due to the predominance of semi diurnal tides and the associated flow reversals depositional patterns are highly variable which has resulted in the formation of characteristic sand mud couplets the distribution of the sand mud couplets found in this study differs from previously proposed conceptual models they are limited to the fluvial domain and form in the distributary channels lateral channel bar or at the fluvial dominated delta front platform which flanks the mouth bar deposits in offshore direction the sand mud couplets which formed as delta front platform and lateral channel bar deposits are similar and can only be identified based on their 14 c age the sand content decreases significantly towards the tidal dominated areas due to limitation in transport capacity turbidity measurements taken in front of the river mouth also show rapid settlement of river plume sediments some 22 new ams 14 c dates show that late holocene sea level history resembles the eustatic sea level curve giving a first approximation of the late holocene sea level history for east kalimantan the dates suggest that the progradational delta system evolved under conditions of slowly rising sea level which compares well to the eustatic sea level curve in addition calculated averaged deposition rates of the sand mud couplets indicate that deposition is driven by the spring neap tide cycles instead of the daily tidal cycle

flexural deformation due to amazon fan loading a feedback mechanism affecting sediment delivery to margins the uplift associated with the peripheral bulge engendered by sediment loads such as the amazon fan may provide a potential tectonic feedback mechanism that affects both coastal and fluvial depositional processes for example topographic and geologic maps of the brazilian margin document the existence of a drainage divide affecting coastal river networks that is spatially coincident with the location of the peripheral bulge predicted by our flexural modeling of the fan load while regions in close proximity to the fan load are depressed regions farther from the load are lifted i e peripheral bulge by 25 50 m which is sufficient to cause subaerial exposure of large portions of the shelf and modification of existing fluvial drainage networks furthermore our modeling results suggest that the development of the peripheral bulge may have deflected the sediment laden waters of the amazon river progressively southward through time this prediction is consistent with the existence of canyons off the amapa coast north of the present day amazon canyon and with the distribution of amazon fan sediments both of which suggest that the proto amazon river delivered sediment farther north along the coast however changing the location of sediment input into a basin also changes the shape of the cumulative sediment load and thus the resultant deformation this feedback between sediment loading and the peripheral bulge uplift modifies the spatial and temporal history of sediment delivery to the margin and provides a viable way to change base level without having to invoke glacial eustasy the efficiency of this feedback is dependent on the flexural wavelength and thus the strength of the lithosphere at the time of loading and the relative position of the sediment load with respect to the continental margin

the deltaic nature of amazon shelf sedimentation despite the annual discharge of more than a billion tons of sediment by the amazon river the sedimentary environment near the river mouth has little subaerial expression and thus does not meet the classic definition of a delta the river mouth however is not an estuary either these observations raise a major question as to what type of sedimentary environment the amazon river mouth represents seismic stratigraphy has been examined on the continental shelf at the mouth of the amazon river using high frequency 3 5 khz seismic records from about 6 000 km of ship track these records demonstrate three regions 1 the amazon river has built a subaqueous feature which stretches for hundreds of kilometres offshore and alongshore from its mouth the feature is prograding seaward and accreting upward and it contains fine scale stratification typical of classic deltas the feature forming at the mouth of the amazon is a subaqueous delta it differs from classic deltas primarily in its lack of subaerial expression subaqueous deltas such as the amazon represent the general case of a major river entering an energetic oceanic regime

carbonate formation by anaerobic oxidation of methane evidence from lipid biomarker and fossil 16s rdna carbonate chimneys and other carbonate structures occur widespread in the gulf of cadiz and probably reflect the presence of cold seeps and associated release of methane in the geological past possibly in the early pleistocene but it is unclear under what conditions and by which processes these carbonates were formed we studied a fossil methane related carbonate crust collected from the kidd mud volcano in the gulf concentrations of microbial lipids their stable carbon isotope composition sequences of fossil 16s rrna genes of anaerobic methanotrophic archaea in combination with mineralogical and carbon and oxygen isotopic composition of carbonate were obtained for seven different horizons of the crust this combination of organic and inorganic geochemical techniques with molecular ecological methods gave a consistent view on processes resulting in the formation of the crust and indicated that it took place in two phases and in a downward direction archaeal lipid biomarkers and fossil 16s rrna gene sequence data revealed the dominance of archaeal anme 2 group and elevated methane partial pressures during the formation of the top part of the crust the lower part of the carbonate was likely formed in an environment with reduced methane fluxes as revealed by the dominance of fossil remains of anme 1 archaea the combination of these methods can be used as an effective tool to reconstruct in unprecedented detail the palaeo biogeochemical processes resulting in the formation of carbonate fabrics this interdisciplinary strategy may also be applied for other fossil methane derived carbonates generating new concepts and knowledge about past methane related carbonate systems

near bottom suspended sediment variability caused by storms and near inertial internal waves on the ebro mid continental shelf nw mediterranean abstract near bottom currents and suspended sediment concentrations ssc recorded on the ebro continental shelf at 60 m depth during the transition between fair and stormy weather have revealed some of the major processes controlling the sediment re suspension and transport at mid shelf depths in this nw mediterranean area storm waves were the main contributor to the transport of sediment although they only resuspended surface sediments during extreme wave conditions h s 4 5 m and t s 11 s during the recording period the net suspended sediment flux along the shelf was one order of magnitude higher than the net suspended sediment flux across the shelf and the resultant transport was towards the sw and seaward fluctuations of the near bottom ssc during fair weather showed a periodicity around 17 h and were related to the occurrence of near inertial internal waves caused by wind pulses during storm events the energy propagation of these internal waves reached critical angles at the mid and outer shelf which could favor the development and maintenance of the bottom nepheloid layer on the ebro continental shelf as well as on other mediterranean continental shelves

luminescence of quartz and feldspar fingerprints provenance and correlates with the source area denudation in the amazon river basin the amazon region hosts the world s largest watershed spanning from high elevation andean terrains to lowland cratonic shield areas in tropical south america this study explores variations in opti

neogene and quaternary development of the neotropical rain forest the forest refugia hypothesis and a literature overview abstract the upheaval of the northern andes in miocene and pliocene time changed the drainage system in northern south america significantly and caused the present day rain forest areas of choco and the lower magdalena valley became separated from amazonas plant diversity may have reached the highest level in the miocene or pliocene and excessive present day phytodiversity may be regarded as a legacy of the tertiary rather than an evolutionary product of the quaternary in the quaternary strong temperature oscillations related to the series of ice ages were superposed on the late tertiary forest dynamics which included river displacement and latitudinal migrations of the equatorial rain belt caloric equator with the rhythm of the precession cycle of orbital climate forcing the hypothesis that claims a permanent rain forest cover all over the amazon basin during the last glacial is in contrast with the forest refugia hypothesis which accepts replacement of rain forest by savanna or savanna forest during dry climatic intervals both scenarios have been evidenced by pollen records in this paper it is suggested that both hypotheses are not necessarily conflicting and apparently did occur in different parts of the amazon basin and in different periods depending on the climatological constraints a compilation of the most important literature concerning the vegetational climatic and environmental history of the rain forest areas of amazonas and choco and surrounding dry ecosystems has been included

kinetic stability of a melted iron globule during chondrule formation i non rotating model we have investigated the kinematics of the separation of iron globules from chondrules during chondrule formation a simple model which assumes that the system has no angular momentum was used to calculate the energy of a system with an iron globule and a chondrule the energies of three different states were calculated 1 a melted iron globule fully embedded in a melted chondrule 2 a melted iron globule on the surface of a melted chondrule and 3 a melted iron globule being separated from a melted chondrule we also calculated the lowest energy shape for a melted iron globule on the surface of a melted chondrule and compared our result with the shapes of four natural samples of chondrules and iron globules in thin sections the shapes were calculated using an assumed value for the interface energy between the four couples of melted chondrules and the iron globules and agree well with the natural shapes of chondrules and iron globules the results of our calculations show that the iron globules of these four samples would be strongly bound to the surface of the melted chondrule during chondrule formation and separation would be difficult if the iron globules had been on the surface of precursors of these chondrules our results also show that if these iron globules were initially inside and transported to the surface of the melted chondrule most of them would be ejected from the inside to outside because of surface tension forces as long as the energy losses due to viscous dissipation when the globules pass through the surface of melted chondrules were sufficiently small although further improvement of the model is required our results demonstrate that this ejection process may be responsible for the depletion of siderophile elements in natural chondrules

the amazon river plume during amasseds subtidal current variability and the importance of wind forcing current variability over the north brazilian shelf particularly within the amazon plume is characterized using observations from a moored array deployed about 300 km north of the amazon river mouth from february to june 1990 as part of a multidisciplinary amazon shelf sediment study amasseds the moored array consisted of a transect perpendicular to the coast with inner shelf 18 m depth and midshelf 65 m moorings and a third mooring near the shelf break in 100 m of water to our knowledge the amasseds moored array includes the first long term 2 months moored observations made within the amazon plume the current variability is dominated by two components semidiurnal cross shelf currents with peak velocities of 50 to 200 cm s and vertically sheared subtidal timescales days to weeks along shelf currents this study focuses on the subtidal flow within the amazon plume which is strong and variable with along shelf currents ranging from 50 cm s to over 150 cm s the variability in the along shelf current within the plume is wind driven the relatively weak 0 5 dyn cm2 along shelf wind stresses in this region drive strong along shelf current variability because the amazon plume is thin and the interfacial drag is weak however in the absence of an along shelf wind stress the plume flow is northwestward at 40 to 80 cm s indicating that direct wind forcing does not account for the mean northwestward flow of the amazon plume in the cross plume direction there is a tendency for the onshore wind stress due to the persistent trade winds to balance the typically offshore buoyancy force and the coriolis force associated with the wind driven along shelf flow thus the wind stress is a key factor influencing variability in both the flow and cross shelf structure of the amazon plume in the vicinity of the moored array

variations of sediment texture on the amazon continental shelf abstract grain size distribution on the continental shelf adjacent to the amazon river reveals three detrital sedimentary deposits an outer shelf sand deposit 1 5 3 0 well sorted negatively skewed an inner shelf mud deposit 6 9 poorly sorted positively skewed and a corridor extending from the river mouth across the inner shelf in which mud is interbedded with sand modern mud covers most of the inner shelf and surface sediment does not reveal along shelf or across shelf sorting of discrete particles size sorting of particle aggregates floccules fecal pellets may occur at the time of deposition but is difficult to evaluate a lack of shoreward fining reverse grading c ntradicts the sedimentation model of gibbs 1976 the model most consistent with textural observations is that of a modern subaqueous delta prograding over a relict basal sand the outer shelf sand modern sand in surface sediment is restricted to the corridor extending from the river mouth across the inner shelf ambient physical processes are not competent to transport this sand along shelf subsurface sand layers within the inner shelf mud deposit i e outside the corridor are similar in texture to surficial sand in the corridor of interbedded mud and sand these observations may suggest that the location and or size of the corridor has changed with time

new insights to high resolution stratigraphy on the amazon continental shelf abstract the surficial stratigraphy upper 20 m of the amazon subaqueous delta has been well documented but the high resolution record of underlying strata was unknown prior to this study a geopulse seismic system was utilized to obtain new information about the preservation potential of sedimentary deposits on the amazon shelf widespread distribution of methane gas was documented on the inner shelf and prevented deep penetration of the seismic signal on the outer shelf high resolution