mangrove forest evolution in a sediment rich estuarine system opportunists or agents of geomorphic change the majority of the world s mangrove forests occur on mostly mineral sediments of fluvial origin two perspectives exist on the biogeomorphic development of these forests i e that mangroves are opportunistic with forest development primarily driven by physical processes or alternatively that biophysical feedbacks strongly influence sedimentation and resulting geomorphology on the firth of thames coast new zealand we evaluate these two possible scenarios for sediment accumulation and forest development using high resolution sedimentary records and a detailed chronology of mangrove forest avicennia marina development since the 1950s cores were collected along a shore normal transect of known elevation relative to mean sea level msl activities for lead 210 pb 210 caesium 137 cs 137 and beryllium 7 be 7 and sediment properties were analysed with pb 210 sediment accumulation rates sars compensated for deep subsidence similar to 8 mm yr 1 used as a proxy for elevation gain at least four phases of forest development since the 1950s are recognized an old growth forest developed by the late 1970s with more recent seaward forest expansion thereafter excess 210pb profiles from the old growth forest exhibit relatively low sars near the top 7 12 mm yr 1 and bottom 10 22 mm yr 1 of cores separated by an interval of higher sars 33 100 mm yr 1 a general trend of increasing sar over time characterizes the recent forest biogeomorphic evolution of the system is more complex than simple mudflat accretion progradation and mangrove forest expansion surface elevation gain in the old growth forest displays an asymptotic trajectory with a secondary depocentre developing on the seaward mudflat from the mid 1970s two to ten fold increases in 210pb sars are unambiguously large and occurred years to decades before seedling recruitment demonstrating that mangroves do not measurably enhance sedimentation over annual to decadal timescales this suggests that mangrove forest development is largely dependent on physical processes with forests occupying mudflats once they reach a suitable elevation in the intertidal copyright c 2015 john wiley sons ltd

depositional facies of a mud shoreface in suriname south america a mud analogue to sandy shallow marine deposits abstract immense volumes of argillaceous muds are being deposited along the world s longest continuous mud coastline which faces the open equatorial atlantic not only are these muds deposited in an environment normally associated with sand deposits but just as in sandstones sedimentary structures within these muds record the dynamic processes that deposited them this study investigates a 100 km long portion of the suriname coast utilizing cores bathymetric surveys and field observations from nearshore and innershelf environments using terminology borrowed from sand coastlines the innershelf nearshore of suriname is divided into transition zone shoreface foreshore and backshore the transition zone consists of a mix of offshore sands and shoreface muds shoreface and foreshore are subdivided into bank and interbank zones these zones are transitory because mud banks which make up the bank zones migrate westwardly mud banks are up to 5 m high 50 to 60 km long 10 to 20 km wide and oriented at oblique angles to the coastline these banks generally consist of fluid muds which are so fluid that they interact with surface waves causing them to be altered and damped banks are attached to the shoreline and commonly produce progradation of the coastline substrate within the interbank zone is more consolidat d than within the mud banks because of lack of wave dampening wave energies are relatively high within the interbank zone producing erosion of the shoreface and shoreline sand beaches are generally formed within the erosional interbank shoreline the vertical sedimentary sequence within the shoreface muds is a record of bank migration and other sedimentary processes present wave and current energies are recorded by scour and fill features discontinuity features micro cross laminations lenticular laminations and parallel to subparallel laminations laminations appear to be created by boundary layer shearing within fluid muds which are above wave base and made mobile by wave generated shore parallel currents large scale evidence for migration of the mud banks is westward dipping acoustical reflectors within the banks these reflectors which are over a kilometer apart mark the boundaries of annual migration events biogenic structures are generally absent within the fluid bank muds but are present within the more consolid ted interbank zone and associated with discontinuity features which are buried interbank surfaces migration and vertical stacking of mud banks are recorded by vertical sequences of massive and laminated muds which were deposited by mud banks and discontinuity features which were formed by interbank zones the coastal deposits of suriname are a mud analogue to nearshore and innershelf sand deposits transition zones of both mud and sand systems reflect a transition from offshore to shoreface processes and sediments both mud and sand shoreface foreshore systems reflect sediment input and wave current and tidal processes that act on them mud banks of suriname resemble linear sand ridges on the continental shelf of the eastern u s a in shape oblique orientation to the coastline and orientation with respect to dominant direction of transport processes the sequence of massive and laminated muds with discontinuity features resembles laminated to burrowed sequences found in ancient and recent nearshore and shallow marine sands

the amazon river plume during amasseds spatial characteristics and salinity variability the amazon river discharge forms a plume of low salinity water that extends offshore and northwestward over the north brazilian shelf observations acquired as part of a multidisciplinary amazon shelf sediment study amasseds are used to characterize the spatial structure and temporal variability of the amazon plume four shipboard conductivity temperature depth ctd surveys spanning the shelf from 1 s to 5 n during rising march 1990 maximum may 1990 falling august 1989 and minimum november 1991 discharge show the amazon plume is typically 3 to 10 m thick and 80 to over 200 km wide northwest of the river mouth the plume is often characterized by a wedge of low salinity water adjacent to the coast and a separate tongue of low salinity water extending offshore over the middle to outer shelf a bottom front separating the low salinity plume water from oceanic water is consistently located between the 10 and 20 m isobaths a moored array deployed about 300 km northwest of the river mouth from february to june 1990 included inner and midshelf moorings in 18 and 65 m of water on which temperature conductivity measurements were made the moored observations reveal salinity variations within the amazon plume of over 10 psu on timescales of days to weeks this variability includes intermittent events in which plume water pools up in the vicinity of the river mouth and the plume width can exceed 200 km these accumulation events are apparently due to wind events with a southeastward component which impede or block the normally northwestward freshwater transport the resulting bulges in the plume are then released northwestward when the wind reverses volume budgets indicate the amazon plume entrains roughly twice the river discharge between the river mouth at the equator and 3 n estimates of gradient richardson numbers from the moorings suggest entrainment due to the strong semidiurnal tidal currents occurs where the plume intersects the bottom and over the outer portion of the plume where salinities approach oceanic values

evaluation of recent tectonomagmatic discrimination diagrams and their application to the origin of basic magmas in southern mexico and central america discrimination diagrams to decipher tectonic settings have been in use for nearly 40 years although old diagrams have been extensively used the recent ones based on discriminant functions of ratio variables with or without log transformation proposed during 2004 2010 for the discrimination of four tectonic settings of island arc continental rift ocean island and mid ocean ridge were newly evaluated to show their high success rates of 57 3 100 and 58 5 100 for major element and immobile element based diagrams respectively for the continental arc of the andes evaluated for its similarity to island arc these four sets of diagrams showed success rates of 62 1 83 8 these four sets of five diagrams per set were therefore used to infer tectonic setting of the mexican volcanic belt mvb los tuxtlas volcanic field ltvf and central american volcanic arc cava using this approach the mvb especially its western central and eastern parts and the ltvf of southern mexico show a dominantly continental rift setting and the cava shows an arc setting the west central part of the mvb is consistent with dual tectonics of arc and rift these results confirm the application of an unusual mantle upwelling rift model for the mexican on land volcanism whereas the conventional plate tectonic subduction model seems to be applicable for the cava from guatemala to north western costa rica

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

light elements and li isotopes across the northern portion of the central american subduction zone 1 quaternary mafic rocks have been erupted from volcanoes both on and behind the volcanic front in southeastern guatemala and western el salvador behind the front volcanism extends almost continuously some 110 km from the front b and li contents as well as b nb and li yb ratios are generally higher in the mafic volcanic rocks of the volcanic front and lower in those behind the front beryllium concentrations show the opposite relationships with generally higher values behind the front in addition many behind the front samples have lower 7li 2 3 versus 4 6 for the volcanic front guatemalan volcanic rocks in general have lower 7li than those found along the remainder of the central american margin the light element variations across northern central america clearly indicate that the volcanic front receives a much more pronounced fluid contribution from dehydrating subducting cocos lithosphere than the behind the front region this dehydration pulse occurs at slab depths between 85 and 105 km and selective enrichments of b cs and cl in volcanic rocks of the volcanic front suggest that the source of this pulse is dehydrating serpentinite in addition the dehydration pulse from subducted serpentinite is likely superimposed on a continuous dehydration signal provided by subducting oceanic crust which continues behind the volcanic front to slab depths 200 km the enhanced fluid contributions at the front produce higher degrees of wedge melting thus explaining the lower be contents of front samples there are two possible explanations of the lower 7li of many behind the front volcanic rocks from southeastern guatemala western el salvador selective loss of 7li during progressive dehydration of subducting cocos lithosphere or diffusive fractionation accompanying late stage selective crustal contamination since the slightly lower 7li of guatemalan volcanic rocks in general is consistent with a minor crustal overprint the second explanation is favored and could be relevant to the few subduction zones where lower li isotopic values characterize back arc regions light element variation across southeastern guatemala and western el salvador is consistent with the operation of decompression melting behind the volcanic front be zr ratios across the northern portion of the central american subduction zone may indicate some mobility of be

distribution and sources of bulk organic matter om on a tropical intertidal mud bank in french guiana from elemental and isotopic proxies the mobile french guiana coast is a shoreface region downdrift of the amazon river where enormous quantities of inorganic and organic materials are exchanged with the atlantic ocean the rapid accumulation of these materials forms highly unstable shore attached mud banks which can be temporally emerged and then rapidly colonized and stabilized by microphytobenthos and opportunistic mangroves i e avicennia germinans mud banks are preferential sites for the accumulation and significant remineralization of organic matter om due to intense erosion deposition cycles and potential biological colonization the distribution and sources of bulk sedimentary om were characterized by elemental and isotopic analyses of four sediment cores together with samples from three potential om sources mangrove plants suspended particulate matter spm and microphytobenthos all collected from the landward face of the macouria mud bank french guiana total organic carbon toc and nitrogen tn concentrations in the sediment cores showed that om sources were characterized by spatio temporal variations in this mud bank the relative contributions of mangrove plants spm and microphytobenthos were estimated using a three end member mixing model based on tn toc atomic ratios and 13c values sedimentary om is mostly controlled by spm associated with variable amounts of om derived from mangrove plants and microphytobenthos these variations could be explained by topography and bed elevation which decrease submersion time and increase desiccation higher contributions of microphytobenthos are associated with black om rich laminae identified in sedimentary cores and linked to temporal emersion phases of the mud bank which favor the growth of benthic microalgae this result is confirmed by the calculation of the average sediment accumulation rate around 36 7 14 8 cm yr 1 taking into account the emersion of the study site every spring tide a fortnightly cycle this value is within the range of the previous results from other mud banks in french guiana but is more than ten times greater than values generally obtained in other coastal contexts

sediment transport in the yellow sea and east china sea abstract eight survey cruises in different seasons have been conducted in the yellow sea ys and east china sea ecs during the period from 2000 to 2008 suspended sediment concentration ssc and hydrological data were collected during each cruise data analysis showed that total suspended sediment mass was approximately 0 18 109 tons in the surveyed area during spring and autumn seasons highly turbid waters were found in the shallow waters between the subei coast the changjiang estuary and the zhejiang coast with seasonal variations the investigation provided convincing evidence that suspended sediments were transported from the subei coast and the southern entrance of the yellow sea trough to the central ys muddy patches were found at the sea bed when tidal currents were too weak to erode the bottom sediments and water column stratification and eddies provided favorable conditions for trapping the fine grained suspended sediments the investigation indicated that the different front may have different effect on sediment transport during different seasons the front between the yellow sea cold water and the coastal water in the subei coast appeared to trap the high ssc sediments on the shallow water side of the front during summer the front along the zhejiang coast seemed to prevent suspended sediments from transport across the front due to circulation in the front zone and upwelling along the zhejiang coast during winter sediments were likely transported across the front to the central continental shelf of the ecs with the help of the downwelling and lateral ekman transport driven by monsoon winds

palaeoenvironments palaeogeography and physiography of a large shallow muddy ramp late cenomanian turonian kaskapau formation western canada foreland basin the kaskapau formation spans late cenomanian to middle turonian time and was deposited on a low gradient shallow storm dominated muddy ramp dense well log control coupled with exposure on both proximal and distal margins of the basin allows mapping of sedimentary facies over about 35 000 km2 the studied portion of the kaskapau formation is a mudstone dominated wedge that thins from 700 m in the proximal foredeep to 50 m near the forebulge about 300 km distant regional flooding surfaces permit mapping of 28 allomembers each of which represent an average of ca 125 kyr more than 200 km from shore calcareous silty claystone predominates whereas 100 to 200 km offshore mudstone and siltstone predominate from about 30 to 100 km offshore centimetre bedded very fine sandstone and mudstone record along shelf sse directed storm generated geostrophic flows five to thirty kilometres from shore decimetre bedded hummocky cross stratified fine sandstone and mudstone record strongly oscillatory wave dominated flows whereas some gutter casts indicate shore oblique apparently mostly unidirectional geostrophic flows nearshore facies are dominated by swaley cross stratified or intensely bioturbated clean fine sandstone interpreted as recording respectively areas strongly and weakly affected by discharge from distributary mouths shoreface sandstones grade locally into river mouth conglomerates and sandstones including conglomerate channel fills up to 15 m thick locally brackish lagoonal shelly mudstones are present on the extreme western margin of the basin there is no evidence for clinoform stratification which indicates that the kaskapau sea floor had extremely low relief lacked a shelf slope break and was probably nowhere more than a few tens of metres deep the absence of clinoforms probably indicates a long term balance between rates of accommodation and sediment supply mud is interpreted to have been transported 250 km offshore in a sea bed nepheloid layer repeatedly re suspended by storms fine grained sediment accumulated up to a mud accommodation envelope perhaps only 20 to 40 m deep continuous re working of the sea floor by storms ensured that excess sediment was redistributed away from areas that had filled to the accommodation envelope being deposited in areas of higher accommodation further down the transport path the facies distributions and stratal geometry of the kaskapau shelf strongly suggest that sedimentary facies especially grain size were related to distance from shore not to water depth as a result the 100 to 300 m depth interpreted from calcareous claystone facies for the more central parts of the interior seaway might be a significant overestimate

the source of central american lavas inferences from geochemical inverse modeling the rate earth element chemistry of a large suite of samples from the central american volcanic front has been determined to understand their petrogenesis different segments of the volcanic front are compared on the basis of their calculated source rare earth patterns as deduced from inverse modeling the results yield a range in the extent of light rare earth enrichment of the source as well as in source mineralogy moderateto strong lree enrichment and high modal garnet contents are observed for the sources of both guatemala and central costa rica whereas slight lree depletion and little or no garnet occurs in the nicaraguan source region although distinct source regions beneath each central american segment are permitted by the modeling it is more likely that the mantle wedge is broadly homogeneous but locally heterogeneous small volume relatively enriched garnet bearing veins surrounded by a matrix of isotopically depleted mantle periodotite could exist throughout the mantle wedge apparently distinct sources occur due to the variation in partial melting beneath the different segments controlled by the amount of subduction generated flux per unit wedge volume which in turn is a function of the dip of the subducted lithosphere

depositional setting of the 2 1 ga francevillian macrobiota gabon rapid mud settling in a shallow basin swept by high density sand flows the depositional setting of the 2 1 ga fill to the franceville basin of gabon is important for understanding the habitat energy and availability of light and oxygen and taphonomy of recently discovered early macro organisms buried in black shales in unit fb the available data bearing on the stratigraphy and sedimentology of unit fb provide new insight into processes acting on the palaeo seafloor the shales are interpreted to have formed as fluid mud deposits interstratified with structureless sands the latter poubara sandstones were emplaced during a forced regression during the terminal infill of fault bounded sub basins following a stage characterised by a ferruginous to anoxic water column the structureless sandstones were deposited from high density gravity currents along with a locally strong bottom oscillation of the water column tuft structures preserved in cyanobacterial mats together with the position of the macro organisms at the top of the sandstone beds within associated black shales point to a water depth of less than 80 m the relative sea level fall that drove deposition of the poubara sandstones controlled the rise of a phototrophic ecosystem and also possibly favoured the supply of oxygen and nutrients via density flows this article is protected by copyright all rights reserved

a hybrid 3 d electromagnetic method for induction detection of hydraulic fractures through a tilted cased borehole in planar stratified media as one of the most important nondestructive characterization techniques electromagnetic em methods can be used in the subsurface fracture detection especially for hydraulic fracture evaluation in unconventional petroleum exploration and development the multiscale nature of long but extremely thin 3 d fractures is difficult for conventional em modeling methods such as the finite element method fem in numerical simulation the problem becomes even more challenging when the effects of tilted borehole casing and planar stratified media need to be considered so far modeling a tilted borehole in layered media is still a major challenge for conventional methods in this paper we present the hybrid numerical mode matching method with the stabilized biconjugate gradient fast fourier transform method as the forward modeling algorithm that can efficiently model 3 d fractures in planar stratified media with a cased borehole environment numerical results validate the accuracy of the hybrid forward method and show orders of magnitude higher efficiency of this forward solver than the fem

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

behaviour and taphonomic loss in foraminiferal assemblages of mangrove swamps of french guiana abstract fifty three surface sediment samples were collected in january 1999 july 2000 and december 2000 on mud flats and in mangrove swamps of french guiana sampling stations were selected to provide information on different environmental conditions on the mud flats and in the mangrove forest thirteen samples were also collected at a depth of about 8 12 cm in the sediment additional samples were collected from three long cores in the channel of the kaw river the main parameter controlling the distribution of recent foraminiferal assemblages is the seasonally variable relative influence of coastal water of reduced salinity and low ph low ca river water areas under the influence of coastal water are dominated by calcareous species mainly ammonia tepida and cribroelphidium spp even in mangrove swamps during the dry season river water influences almost all the study area during the rainy season and leads to the dominance of agglutinated species with the nature of the assemblages depending on local environmental characteristics on the mud flats tests are typically well preserved in the 10 cm below the surface that is the most important taphonomically active zone for foraminifera however owing to intense bioturbation the taphonomic processes in the mangrove swamp may result in the total destruction of the microfauna including agglutinated tests this study shows that the preservation of thanatocoenoses in mangrove swamp environments of french guiana only occurs in exceptional circumstances when they are preserved the remaining assemblages even if modified by taphonomic processes give valuable information for paleoenvironmental reconstructions in this environment

boron geochemistry of the central american volcanic arc constraints on the genesis of subduction related magmas boron contents were measured in representative quaternary lavas from the central american volcanic arc to evaluate along strike variations in subduction processes despite the significant range in b concentrations 2 37 ppm in the mafic lavas b la ratios vary in a systematic fashion along the arc higher values 1 are typical between guatemala and northern costa rica whereas low values most 0 5 typify central costa rica and western panama b la is highly correlated with 10be 9be r2 0 94 excluding one sample and appears to be a useful indicator of subduction contributions to the magma sources if enrichments of both b and 10be are proportional to the flux of subducted sediment along strike variations in b la suggest at least a twofold variation in this flux with maximum values below western nicaragua and minimum values below costa rica and western panama where the cocos ridge is being subducted these data may also reflect significant differences in thermal state of the descending slab which in turn differentially affects release patterns of fluids and fluid mobile trace elements and possibly melting processes beneath different parts of the arc the following scenario is suggested to explain the geochemical results beneath the northwestern part of the arc steep subduction of older relatively cold slab favors more efficient subduction of fluid components to depths beneath the volcanic front the released fluids carry fluid mobile elements to the overlying mantle which upon melting produces calcalkalic magmas shallow subduction of warmer slab beneath the southeastern part of the arc favors shallow release of fluids and limits fluid related metasomatism of sub arc mantle beneath the volcanic front under such conditions b la and ba la ratios in the sub arc mantle vary little from values seen in oceanic island basalts magmas in this part of the arc nevertheless display the highest la yb and lowest ba la and b la ratios which are consistent with prior light rare earth element enrichment in the source significantly lower degrees of melting or a combination thereof because some of the largest volcanoes occur in costa rica and magma flux there is nearly an order of magnitude higher than elsewhere in the arc source enrichment is considered to be the more plausible explanation it is proposed that quaternary magma production below costa rica involved lithospheric sources containing trapped or stored melt components but this enrichment process is unlikely to have involved typical arc magmas or subduction fluids because we see no b enrichment

fluidization of mud in estuaries the south alligator river located in the northern territory australia is a macrotidal estuary with suspended sediment concentration values reaching 10 g 1 1 in september 1986 in the dry season the estuary was well mixed in temperature and salinity while the vertical gradients in suspended sediment concentration were small at flood tides for most of the ebb tide duration a lutocline separated a clear upper layer from an extremely turbid bottom layer both layers being of comparable thickness the tidal evolution of the suspended sediment concentration is consistent with that computed by a numerical model based on the equation of conservation of mass of suspended sediment in this model sediment is entrained from the bottom and mixed vertically upward by eddy diffusion but through a richardson number dependence sediment induced buoyancy effects inhibit vertical mixing the final depth of the turbid layer can be readily estimated analytically as a result of a balance between the rate of kinetic energy input and the buoyancy flux determined by the particle fall velocity the presence of a lutocline helps form mud banks on the inner side of a meander

coastal dynamics and chenier sands in french guiana abstract the holocene coastal plain of french guiana is an open chenier plain the shoreline is characterized by vast migrating shoreface attached mudbanks cheniers develop within the interbank zone these sandbodies which rest on the demerara marine clays have also been observed in the inner parts of the coastal plain this paper deals with the coastal area between mahuri and the maroni rivers in french guiana an overview of the present state of knowledge regarding the coastal research in this area is given and based on new evidence the problem of the provenance of the sandy accumulations will be discussed and further defined

material exchange between the continental shelf and mangrove fringed coasts with special reference to the amazon guianas coast abstract mangrove fringed coasts are a major coastal environment in the tropics that exchange enormous quantities of inorganic and organic materials and dissolved ions with the adjacent ocean in unique ways the 1 600 km long guiana coast from the amazon river mouth to the orinoco the world s most extensive example is exposed to significant oceanic wave and tidal energy but is almost exclusively composed of mud deposits the huge alluvial discharge of the amazon advected along the inner continental shelf is reworked into 20 25 shore attached and mobile mud banks 10 40 km in length frequent and deep resuspension created by the energetic conditions results in fluid mud suspensions 10 400 g 1 1 that dewater upon re deposition resulting in extreme shoreline progradation rates up to hundreds of metres per year that are equally rapidly mobilized by wave attack biological assemblages mangroves foraminifera diatoms etc and biogeochemical conditions are strongly controlled by the intense recycling of sediments on the shoreface differences in sedimentary ecological and biogeochemical processes are striking when this coast is compared with mangrove fringed deltaic and estuarine coasts elsewhere including those of west africa and southeast asia

millennial to secular time scale impacts of climate and sea level changes on mangroves from the doce river delta southeastern brazil this work integrates geomorphological sedimentological and palynological data with radiocarbon dating as well as 13c 15n and c n from sedimentary organic matter to provide a model of mangrove dynamics during the evolution of a wave dominated delta in southeastern brazil mangrove dynamics are analyzed within the context of millennial and secular climatic and sea level changes tidal flats positioned at highest limit of the intertidal zone along the edge of a lagoon sheltered by beach ridges were occupied by wetlands represented by mangrove and herbaceous vegetation during the mid holocene high sea level after considering the relative sea level fall and relatively higher fluvial sediment discharge during the last 6350 years progradation took place along this shoreline resulting in extensive beach ridge deposits that overlie transgressive muds this process led to loss of mangrove area similar dynamics were repeated at 3043 cal yr bp although in a relatively more distal i e seaward posi

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

the composition of sedimentary organic matter in relation to the dynamic features of a mangrove fringed coast in french guiana the sedimentary organic matter content of a series of 2 m deep cores was examined in relation to the evolution of mangrove forest on the basis of geochemical analyses and optical observations avicennia dominated forest deposits developing along the highly dynamic coastline of french guiana were collected in five stations based on stage of forest evolution the sedimentary organic matter in the upper sediment of the youngest mangrove swamp is mainly derived from algal mats with low carbon nitrogen ratios c n ratio from 6 to 8 and typical greyish amorphous organic flakes as observed in optical studies indeed rare young avicennia trees are present and effectively geochemical parameters do not give evidence of a litter made up of higher plant debris these rare debris being probably exported by the tides a slight increase with depth in the first decimetres of both total organic carbon toc content and c n ratio results from the development of the radial cable root system of the pioneer avicennia germinans early diagenetic conditions of this young forest are rather controlled by dominant suboxic processes as suggested by high eh values range 200 400 mv and local anoxic processes occurrence of pyrite in micro environments this is mainly due to the oxygen available by roots and crab bioturbation the organic content of the senescent mangrove sediment is mainly derived from higher plant debris in the uppermost 30 cm as indicated by relatively high c n ratios and the predominance of ligno cellulosic debris the strong decrease in hydrogen index values results from the degradation of the higher plant debris losing hydrogen bounds through decay processes moderately acidic ph values low ehs and the presence of pyrite framboids point towards the reducing decay processes in surficial layers of the senescent mangrove mediated by sulphate reducing bacteria whatever the stage of evolution of the forest the geochemical characteristics of the sediment below 30 cm are those of the shoreface one with opaque refractory debris derived from the amazon river detritus discharge the sediment collected from dead mangrove forests subsequently recolonized by pioneer mangroves contains organic markers which predate recolonization recording previous phases of erosion and accretion

sediment exchange between amazon mudbanks and shore fringing mangroves in french guiana time series remote sensing data sediment cores and water column profiles of fluid properties and suspended sediment concentration were utilized to examine the mechanisms and volumes of sediment exchange between the mangrove fringe and the coastal zone in french guiana remote sensing estimates of sediment yield from mangrove shoreline erosion by wave attack during the 30 year periods when offshore amazon mudbanks are not adjacent to a 30 40 km long section of the shoreline are 6 23 million tons year 1 7be 137cs and 210pb inventories in sediment cores from inner 5 m water depth mudbank areas suggest the mangrove erosion occurs primarily during the january april period of high wave energy and is responsible for the bulk of sediment deposition on the leading edge of the mudbank remote sensing estimates of shoreline accretion associated with onshore flux of sediment from a mudbank are also large 8 26 million tons year 1 radiotracer generated strata age relationships indicate that the lower intertidal zone of the leading depositional edge of the mudbank is a relict surface suggesting the offshore mudbank is decoupled from the shoreline water column profiles and radiochemical signatures point to fluid mud suspensions 10 400 g l 1 as the primary mechanism for delivering sediment across the relict intertidal zone to create the accretion at the shoreline if the estimates of sediment volume involved in shoreline accretion and erosion in the studied area are extrapolated along the entire 1400 km long guianas coastline possessing offshore mudbanks the annual exchange of sediment to and from the mangrove fringe is at least equivalent to the average input of new sediment from the amazon

southward migration of the austral limit of mangroves in south america abstract temperature influences the global distribution of mangroves and global warming may be causing a poleward mangrove expansion sedimentary features pollen and isotopes data from six sediment cores as well as 14c datings indicated a marine transgression during the holocene and it contributed to the expansion of tidal flats occupied by saltmarshes environmental conditions suitable for mangroves development occurred on the study site during the holocene but according to 210pb and 14c dating the establishment of mangroves mainly represented by laguncularia trees only began between 1957 and 1986 ad on the studied tidal flats spatial temporal analysis based on satellite and drone images revealed a mangrove expansion of 10 ha in the study area between 2003 96 1 ha and 2019 106 1 ha nowadays in the study area saltmarshes mainly characterized by spartina and acrostichum are sharing tidal flats with mangroves represented by laguncularia 5 m tall and avicennia 11 m tall probably the absence of mangroves during the holocene followed by their establishment and expansion during the anthropocene in the subtropical zone is associated with a migration of the austral mangrove limit into the temperate zone caused by the gradual increase in winter temperatures this process may be related to a poleward mangrove migration since the late holocene caused by a natural holocene global warming however the industrial era warming must have intensified the mangrove expansion into temperate zones

half a century of dynamic coastal change affecting mangrove shorelines of french guiana a case study based on remote sensing data analyses and field surveys the mobile mud banks several kilometres wide and about 30 km long which form the sedimentary environment of the coast of the guianas are a consequence of the huge particulate discharge of the amazon these mud banks shift towards the northwest influenced by the combined action of accretion and erosion a process also affected by periodic variability because of this movement the coastline is unstable and continuously changing such changes determine the structure and composition of mangrove forests the only type of vegetation adapted to this dynamic environment the objectives of this study were to identify coastal changes that took place over the last 50 years and to relate them to natural processes of turnover and replenishment of mangrove forests these objectives have been achieved through a combination of remote sensing techniques aerial photographs and spot satellite images and field surveys in the area of the sinnamary estuary french guiana ground data were collected in representative mangrove forest stands chosen as a function of their growth stages and their structural features from pioneer and young stages to adult mixed and declining formations the coastline changes and the mangrove dynamics over the 1951 1999 period are analyzed through production of synthetic digital maps showing an alternation of net accretion 1951 1966 and erosion periods 1966 1991 followed by the present accretion phase based on this structural functional and historic information a global scenario of mangrove forest dynamics is proposed including a model of forest development forest gap processes and sedimentological dynamics the results of this research are discussed within the context of regional coastal amazonian area and global climate d 2004 elsevier b v all rights reserved

nejapa tephra the youngest c 1 ka bp highly explosive hydroclastic eruption in western managua nicaragua nejapa maar 2 5 1 4 km c 120 m deep the largest maar along the 15 km long holocene nejapa miraflores lineament nml is the source vent of the youngest relatively widespread basaltic tholeiitic tephra blanket nejapa tephra nt in western central nicaragua as shown by isopachs and isopleths rausch and schmincke 2008 the nt covers an area of 10 km2 in w nw managua the minimum total magma volume erupted is estimated as 0 09 km3 juvenile dominantly slightly vesicular 20 40 vol basically tachylitic cauliflower shaped lapilli with an average density of 2 1 g cm3 make up 90 vol of the deposit while lithoclasts comprise 10 vol except proximally this the paucity of fine grained tuffs and the dominant plane parallel bedding all suggest fragmentation by shallow interaction of a rising magma starting to vesiculate and fragment pyroclastically with external water the complex particles so generated erupted in moderately high eruption columns at least 7 10 km and were dominantly deposited as dry to damp warm to cool fallout minor surge transport is inferred from fine grained locally cross bedded tephra beds chiefly north of nejapa and just west of asososca maars synvolcanic faulting along the nml is inferred faults in the study area indicate that activation of the n s trending nejapa miraflores fault nmf representing the western flank of managua graben preceded deposition of nt and underlying masaya tuff c 1 8 ka bp chiltepe pumice c 1 9 ka bp and masaya triple layer 2 1 ka bp the nt deposit is underlain regionally by a paleosol and topped by a soil the basal paleosol contains pottery sherds made by the usulutan negative technique during the late formative period 700 bce 300 ce 2 7 1 7 ka bp the soil overlying nt contains pottery related to the ometepe technique dated as between 1350 and 1550 ce 650 450 a bp these and the radiocarbon dates of the pottery bearing paleosols 1245 125 and 535 110 a bp obtained by pardo et al 2008 indicate that nejapa maar erupted between c 1 2 and 0 6 ka bp future eruptions in this area of similar magnitude eruptive and transport mechanisms would represent a major hazard and risk to the densely populated western suburbs of managua a city expanding rapidly westward assuming a similar eruption scenario poor quality roofs common in nicaragua would be prone to collapse up to 12 km peripheral to nejapa maar or another close by eruptive site and buildings at a distance of up to 500 m are likely to be severely affected in view of the past frequency of eruptions along the nml further eruptions are likely to occur in the near future

brachyuran crab community structure and associated sediment reworking activities in pioneer and young mangroves of french guiana south america this study in french guiana evaluates the changes of crab assemblages and their bioturbation activities between mangrove early stages pioneer and young mangrove and within stages by taking their spatial heterogeneity tidal channels flat areas pools into account the results show differences in crab assemblage structure between and within the early stages of mangrove in relation to microhabitat and sediment characteristics the sediment reworking rates are a function of the biomass or density of particular species ucides cordatus uca cumulanta and burrower functional groups crab species or functional interactions mediate changes in sediment reworking rates suggesting the need to consider entire benthic communities rather than single species this study suggests that the role of the micro habitat in determining the biologically induced sediment reworking rates depends on the age of the mangrove feeding activity results in a sediment turnover of 11 7 9 7 g dw m 2 day 1 and 6 8 3 0 g dw m 2 day 1 in the pioneer and young mangroves respectively burrow maintenance excavates 40 5 7 4 g dw m 2 day 1 and 251 3 419 7 g dw m 2 day 1 in the pioneer and young mangroves respectively upscaling to the studied area sinnamary estuary 6 km 2 shows that 500 tons day 1 and 20 tons day 1 of sediments could be excavated and pelletized respectively during the spring tides of the dry season thus biological sediment reworking would greatly contribute to the sedimentary dynamics of the guianese mangroves under amazonian influence c 2016 elsevier ltd all rights reserved

radium isotopes in coastal waters on the amazon shelf abstract radium isotope distributions on the continental shelf adjacent to the amazon river are a complex function of salinity the river and seawater endmembers for each radium isotope are low data from intermediate salinities plot as a highly scattered region above the conservative mixing line connecting the river and ocean endmembers spatial and temporal variability in sediment mixing and water residence times cause the scatter in the radium isotope data we have used the distributions of 226 ra 228 ra and 224 ra collected during four cruises representing different conditions on the shelf to construct inventories of radium due to nonconservative additions from sediments using estimates of water residence times we calculate fluxes of these radium isotopes from the amazon system the amount of fresh river sediment entering the system each year is adequate to support the nonconservative flux of 226 ra from the shelf sediments however the ultimate release of 226 ra from the sediments requires weeks to months to support the nonconservative flux of 228 ra from this system sediments of the 1 2 m thick surface mixed layer must be resuspended every two years the resulting flux supports 15 of the 228 ra entering the surface atlantic ocean release of 224 ra from suspended sediments primarily fluid mud is more than adequate to support the inventory of 224 ra on the amazon shelf comparing the amasseds low discharge cruise november 1991 to a low discharge sampling in december 1982 revealed no significant changes in the 226 ra and 228 ra distributions

coupling between sedimentary dynamics early diagenetic processes and biogeochemical cycling in the amazon guianas mobile mud belt coastal french guiana coastal french guiana lies within the vast mobile mudbelt that characterizes much of the s american inner shelf downdrift of the amazon river approximately 15 20 of the amazon sediment outfall moves nw along the guianas in part as a series of 20 25 coastal mud waves seasonal studies of a representative migrating mudwave located between kourou and sinnamary french guiana demonstrate that diagenetic properties and biogeochemical cycling are closely coupled to sedimentary dynamics and the refluxing of reduced components between the seabed coastal facies and well oxygenated overlying water an unsteady two zone diagenetic regime is typically present in the shallow subtidal and low intertidal regions a surficial layer usually 0 5 1 m thick is nonsulfidic highly reactive and dominated by fe mn cycling this suboxic layer unconformably overlies more consolidated sediment in which net so42 reduction is often observed and contracts shoreward toward the mangrove fringe as sediments stabilize and become bioturbated the relatively stable intertidal flats are also sites of significant benthic primary production despite suboxic conditions over extensive intervals anaerobic remineralization rates are high co2 0 2 0 6 mm day 1 and often show little evidence of attenuation with depth integrated co2 fluxes are 20 235 mmol m 2 day 1 wet dry season average 120 far exceeding diffusive o2 fluxes into the bottom high reactivity 0 08 day 1 and low c n remineralization release ratios 5 3 of decomposing material imply fresh substrate of marine origin most reduced solid phase fe is present as nonpyritic authigenic fe minerals 20 fe ii is in pyrite and low sedimentary c s ratios 6 8 reflect the generally nonsulfidic conditions a broad range of tracers e g 234th t1 2 24 days 210pb t1 2 22 years seasonal cl profiles and nonsteady state diagenetic models of pore water concentrations and oxidant reductant relationships demonstrate that the upper 0 1 1 m of deposits are reworked and exchanged with overlying water on timescales of 10 days to seasonally thus the seafloor acts as a massive suboxic batch reactor entraining and processing reactive marine plankton regenerating fe mn oxides exchanging metabolites and nutrients with the oxygenated water column and generating suites of nonsulfidic authigenic minerals the apparent paradoxical properties of high remineralization rate low reductant content corg 630 mol g 1 and dominance of suboxic metal cycling are a direct consequence of frequent entrainment of highly reactive organic substrate and simultaneous regeneration of the major fe oxidants in oxygenated water the amazon guianas mobile mudbelt is a zone of extraordinarily intense sedimentary and biogeochemical recycling greatly exceeding stable coastal systems such as salt marshes in material exchange with the sea

effects of sea level rise and climatic changes on mangroves from southwestern littoral of puerto rico during the middle and late holocene abstract we evaluated the response to sea level rise and climatic changes of the northeastern caribbean by establishing a palaeoenvironmental reconstruction of the flamenco lagoon located in the coastal side of the guanica dry forest a mab unesco reserve in the semi arid southwestern coast of puerto rico 17 57 10 31 n 66 50 39 30 w the reconstruction was based on pollen records sediments analysis 13 c 15 n c n data and ams 14 c dating of one sediment core the geology of the area is rocky calcareous karst with rocky outcrops and canyons with intermittent streams that move water during periods of high run off conditions the present lagoon is surrounded by rocky outcrops where dry forest vegetation is present on the terrestrial sides and sand dunes on the coastal side we propose that the studied lagoon developed after the stabilization of sea level rise in the region following three main stages 1 before 5400 cal yr bp sediments were accumulated according to small channels dynamics and trees shrubs and herbs were present the sedimentary organic matter was mainly sourced from terrestrial c 3 and c 4 plants 2 between 5400 and 4400 cal yr bp tidal flats were established and mangroves mainly represented by rhizophora trees migrated landward with sea level rise and established in the area and the sedimentary organic matter was sourced from terrestrial c 3 plants 3 around 4400 cal yr bp sea level was near the stabilization the endorheic minibasin was closed when a coastal sand dune was established as a result of drier climatic conditions mangroves were eliminated from the system as a result of the closure of the area by sand dunes and increased salinity in water due to high evaporative demand of the climate the pollen dominance of herbaceous and shrubby vegetation prevails until present throughout the lagoon development sedimentary organic matter was sourced from c 3 and c 4 plants as well as phytoplankton organic matter the closure of the lagoon and the drier conditions prevalent in the region during the late holocene which lasts until the present day and increased the contribution of salinity tolerant phytoplankton that characterizes the current conditions of the flamenco lagoon

origin of amazon mudbanks along the northeastern coast of south america abstract seismic profiles sediment cores and water column measurements were collected along the northeastern coast of brazil to examine the origin of mudbanks in the amazon coastal mud belt these 10 60 km long shore attached features previously had been observed to migrate along the 1200 km coast of the guianas in response to wave forcing chirp 3 5 khz seismic profiles of the shoreface and inner shelf located two mudbanks updrift of the previous eastern limit in french guiana 210 pb geochronology shows that these two banks are migrating to the northwest over a relict mud surface in 5 20 m water depth the mudbanks are 3 4 m thick and are translating over a modern shoreface mud wedge deposited by previous mudbank passage in

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

the influence of the trade winds on the coastal development of the guianas at various scale levels a synthesis abstract the coastal development of the guianas is dominated by extensive shoreface attached and alongshore westward migrating mudbanks the behaviour of these mudbanks is mainly determined by a combination of the large supply of amazon borne silt and clay and the trade wind generated wave action in the front zone of the mudbanks fluid mud is formed as a result the incoming waves are strongly attenuated and deposition is favoured at their rear side compacted clay deposits are eroded by waves the mudbanks migrating in this manner are the driving mechanism in the development of the smallest coastal unit which consists of a mudbank and the eastward adjacent interbank area eisma et al neth j sea res 28 1991 181 has demonstrated that the frequency of the northeast trade wind which blows towards the coast of the guianas in the windy season january till april shows a systematic change in time for the period 1953 1986 there is evidence that the frequency of winds coming from directions between 50 and 80 roughly east northeast increases the same applies to the wind velocity from that direction in the last 10 years a reversal of these trends appears to occur for a more or less east west directed coastal section e g the coastline of suriname a more easterly direction of the northeast trade wind results in an increased longshore wave energy flux in the coastal waters this causes an increase in length of the mudbanks at the detriment of the usually eroding interbank parts of the coast this is evidenced by the large scale coastal development during the related period demonstrating an increasing length of the mudbanks and consequently an increasing net accretion it is hypothesised that this development will reverse if the new trends in wind direction and velocity develop further at the geological scale it is considered that a coincidence of a systematic increase of more northerly directed wind frequencies and a fall in sea level are responsible for the two erosive interruptions in the generally accreting coastal plain during the young holocene period

numerical modeling of eccentered lwd borehole sensors in dipping and fully anisotropic earth formations logging while drilling lwd borehole sensors are used to provide real time resistivity data of adjacent earth formations for hydrocarbon exploration this allows for a proactive adjustment of the dipping angle and azimuth direction of the drill and hence geosteering capabilities the analysis of borehole eccentricity effects on lwd sensor response in full 3 3 anisotropic earth formations is important for correct data interpretation in deviated or horizontal wells in this paper we present a cylindrical grid finite difference time domain model to tackle this problem the grid is aligned to the sensor axis to avoid staircasing error in the sensor geometry but in general misaligned to the eccentered borehole formation interface a locally conformal discretization is used to compute effective conductivity tensors of partially filled grid cells at those interfaces involving an isotropic medium borehole and a full 3 3 anisotropic medium in general dipped earth formation the numerical model is used to compute the response of eccentered lwd sensors in layered earth formations with anisotropic dipping beds

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

tide induced mixing in the amazon frontal zone the freshwater discharge from the amazon river produces a bottom salinity front on the amazon shelf that extends many hundreds of kilometers along the shelf between the 10 and 20 m isobaths strong cross shelf tidal currents cause advective excursions of the front of more than 20 km and supply the major source of kinetic energy for mixing on the shelf this mixing is manifested in spring neap variations in the frontal structure and position with weaker stratification and seaward migration of the front during periods of strong tidal currents and stronger stratification and landward migration during neap tides mixing appears to be driven by shear instability based on local estimates of gradient richardson number and bulk estimates of the water column stability maximum mixing occurs during ebb when vertical shear is greatest suspended sediment reaches such high concentrations in the frontal zone that it makes a significant contribution to the stratification thus increasing the stability of the flow and reducing the effectiveness of tide induced mixing

measurements of high concentration suspended sediments using the optical backscatterance sensor abstract an optical backscatterance sensor obs was used to measure extremely high concentration suspended sediments on the amazon continental shelf as part of the amasseds project a multidisciplinary amazon shelf sediment study using a pumping system for obtaining water samples to calibrate the sensor in situ the obs output was observed to increase approximately linearly for concentrations 36 g l the obs response was nearly continuous from 0 to 320 g l making it possible to obtain continuous suspended sediment concentration profiles through fluid muds

modeling low frequency electrode type resistivity tools in invaded thin beds the authors formulate and implement a numerical mode matching nmm method to model electrode type resistivity tools in invaded thin beds the authors derive the low frequency approximation of the maxwell s equations to obtain the partial differential equation for the potential field the new nmm program is validated by comparing the numerical results with those obtained from other dc programs it is found that this new program is much faster than the program using the finite element method fem and hence is useful for routine interpretation of resistivity logs and for inversion

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

unsteady diagenetic processes and sulfur biogeochemistry in tropical deltaic muds implications for oceanic isotope cycles and the sedimentary record abstract sedimentary s cycling is usually conceptualized and interpreted within the context of steadily accreting 1 d transport reaction regimes unsteady processes however are common in many sedimentary systems and can result in dramatically different s reaction balances and diagenetic products than steady conditions globally important common examples include tropical deltaic topset and inner shelf muds such as those extending from the amazon river 1600 km along the guianas coast of south america these deposits are characterized by episodic reworking of the surface seabed over vertical depths of 0 1 3 m reworked surface sediments act as unsteady suboxic batch reactors unconformably overlying relict anoxic often methanic deposits and have diagenetic properties largely decoupled from net accumulation of sediment despite well oxygenated water and an abundant reactive organic matter supply physical disturbance inhibits macrofauna and benthic communities are dominated by microbial biomass across immense areas in the surficial suboxic layer molecular biological analyses tracer experiments sediment c s fe compositions and 34s 18o of pore water so 4 2 indicate close coupling of anaerobic c s and fe cycles 18o so 4 2 can increase by 2 3 during anaerobic recycling without net change in 34s so 4 2 demonstrating so 4 2 reduction coupled to complete anaerobic reoxidation to so 4 2 and a 18o so 4 2 reduction reoxidation fractionation factor 12 summed magnitudes s reoxidation must be coupled to fe oxide reduction contributing to high dissolved fe2 1 mm and fe mobilization export the reworking of amazon guianas shelf muds alone may isotopically alter 18o so 4 2 equivalent in mass to 25 of the annual riverine delivery of so 4 2 to the global ocean unsteady conditions result in preservation of unusually heavy 34s isotopic compositions of residual cr reducible s ranging from 0 to 30 in physically reworked deposits in contrast bioturbated facies adjacent to physically reworked regions accumulate isotopically light s 34s to 20 in otherwise similar decomposition regimes the isotopic patterns of both physically and biologically reworked regions can be simulated with simple diagenetic models heavy s isotopic signatures are largely a consequence of unsteady diffusion and progressive anaerobic burndown into underlying deposits whereas isotopically depleted bioturbated deposits predominantly reflect biogenic diffusive scaling and isotopic distillation diffusive pumping associated with reoxidation in burrow walls immediately adjacent to reduced zones the s isotopic transition from unsteady physically controlled regions of the amazon delta moving laterally into bioturbated facies mimics the transition of s isotopic patterns temporally in the geologic record during the rise of bioturbation no special role for s disproportionation is required to explain these differences the potential role of unsteady suboxic diagenesis and dynamic reworking of sediments has been largely ignored in models of the evolution of surficial elemental cycling and interpretations of the geologic record

geochemical characterization of surface waters and groundwater resources in the managua area nicaragua central america abstract this paper reports new geochemical data on dissolved major and minor constituents in surface waters and ground waters collected in the managua region nicaragua and provides a preliminary characterization of the hydrogeochemical processes governing the natural water evolution in this area the peculiar geological features of the study site an active tectonic region nicaragua depression characterized by active volcanism and thermalism combined with significant anthropogenic pressure contribute to a complex evolution of water chemistry which results from the simultaneous action of several geochemical processes such as evaporation rock leaching mixing with saline brines of natural or anthropogenic origin the effect of active thermalism on both surface waters e g saline volcanic lakes and groundwaters as a result of mixing with variable proportions of hyper saline geothermal na cl brines e g momotombo geothermal plant accounts for the high salinities and high concentrations of many environmentally relevant trace elements as b fe and mn in the waters at the same time the active extensional tectonics of the managua area favour the interaction with acidic reduced thermal fluids followed by extensive leaching of the host rock and the groundwater release of toxic metals e g ni cu the significant pollution in the area deriving principally from urban and industrial waste water probably also contributes to the aquatic cycling of many trace elements which attain concentrations above the who recommended limits for the elements ni 40 g l and cu 10 g l limiting the potential utilisation of lake xolotlan for nearby managua

vertical eigenstate method for simulation of induction and mwd resistivity sensors a method for computing responses of induction or mwd measurement while drilling resistivity sensors in layered formations with invasion zones is presented the geometry is assumed to have an axial symmetry resulting in a two dimensional problem the solution of the wave equation is expressed by a summation of eigenstates in the vertical direction in each cylindrical region the eigenmodes in each region are found numerically and the variation of fields in the horizontal direction is expressed in terms of analytical functions computed results are used to simulate logging tool responses in invaded formations it is found that the apparent resistivity reading is a complicated function of the resistivities of the invaded zone that of the uninvaded zone bed thickness and invasion diameter since no simple rule can be used to describe the characteristics of the investigation depth of the tool the use of a computer simulation software such as the one developed in this work would be very useful to a log analyst in log interpretation

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

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

sediment dynamics in the lower mekong river transition from tidal river to estuary a better understanding of flow and sediment dynamics in the lowermost portions of large tropical rivers is essential to constraining estimates of worldwide sediment delivery to the ocean flow velocity salinity and suspended sediment concentration were measured for 25 h at three cross sections in the tidal song hau distributary of the mekong river vietnam two campaigns took place during comparatively high seasonal and low seasonal discharge and estuarine conditions varied dramatically between them the system transitioned from a tidal river with ephemeral presence of a salt wedge during high flow to a partially mixed estuary during low flow the changing freshwater input sediment sources and estuarine characteristics resulted in seaward sediment export during high flow and landward import during low flow the dinh an channel of the song hau distributary exported sediment to the coast at a rate of about 1 t s 1 during high flow and imported sediment in a spatially varying manner at approximately 0 3 t s 1 during low flow scaling these values results in a yearly mekong sediment discharge estimate about 65 smaller than a generally accepted estimate of 110 mt yr 1 although the limited temporal and spatial nature of this study implies a relatively high degree of uncertainty for the new estimate fluvial advection of sediment was primarily responsible for the high flow sediment export exchange flow and tidal processes including local resuspension were principally responsible for the low flow import the resulting bed sediment grain size was coarser and more variable during high flow and finer during low and the residual flow patterns support the maintenance of mid channel islands

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

physical processes and fine grained sediment dynamics coast of surinam south america abstract the prograding holocene mud wedge between the amazon and orinoco rivers in the trade wind belt of northeastern south america provides a modern day example of muds accumulating under moderate wave energy conditions gigantic shore attached mudbanks 10 km 20 km composed partly of thixotropic fluid mud gel front this coast every 30 60 km to form a buffer to wave attack and a temporary storage for fine grained sediments this mesotidal coast tide range 2 0 m with gentle offshore slope 0 0006 allows the exposure twice a day of extensive tidal flat deposits which are backed by mangrove swamps on a well developed chenier plain complex field experiments were conducted in surinam du ing 1975 and 1977 to provide new information on process form relationships in this interesting but unusual muddy environment simultaneous measurements of waves currents tide elevation suspended sediment concentration and variations in mud density show that soft intertidal and subtidal muds are suspended at both tide and wave frequency suspended sediment concentrations typically exceed 1 000 mg l at the surface as incoming solitary like waves partially disperse fluid mud into overlying water on a falling or rising tide redeposition of mud may occur near time of high tide the strong attenuation of shallow water waves by these muds provides conditions that are favorable for further sedimentation high concentrations of suspended fluid mud together with solitary like waves from the northeast throughout the year can lead to extraordinarily high net sediment transport rates in the nearshore zone calculations based on solitary wave theory and on data obtained from this study indicate that 15 65 106 m3 of mud can move along shore each year without involving breaking waves the concept of radiation stress and a nearshore circulation cell or bedload transport farther offshore outside the zone of wave dominance wind driven currents and the guiana current combine to transport muds to the northwest consistent with the observed direction of mudflat migration

decadal scale morphological evolution of a muddy open coast analysis of the long term morphological evolution of a muddy open coast sediment transport suggests a growth of mudbanks migrating along the coast cross shore exchange of sediment is equally significant as longshore transport mudbank and shoreline erosion accretion trends are reversed

shallow open system evolution of basaltic magma beneath a subduction zone volcano the masaya caldera complex nicaragua abstract lavas and pyroclastic material from the masaya caldera complex have a number of distinctive geochemical features relative compositional homogeneity low al2o3 and high feo contents a tholeitic differentiation trend and elevated large ion lithophile lil element concentrations e g ba 800 ppm on cmas projections their compositions always fall on or near low pressure cotectics in addition the basalts of masaya have unusually high 87sr 86sr and 10be masaya has exhibited medium term compositional cycles best exhibited by the sawtoothed changes in tio2 and feo mgo there are also a number of longer term compositional changes which are abrupt and generally coincide with caldera formation many of the geochemical characteristics of masaya coupled with a number of volcanological observations indicate masaya is underlain by a large shallow open system magma chamber perhaps on the order of 10 km3 in size although fractional crystallization is a significant magmatic process in masaya s open system chamber magma mixing contamination is equally important magma mixing is necessary to explain the discontinuous stratigraphic changes in magma composition observed at masaya and crustal contamination is necessary to explain their generally elevated 87sr 86sr and lil element concentrations two components therefore have been admixed into the magma chamber of masaya a lil poor basaltic component such has been erupted from the nearby nejapa and granada cinder cones and a lil rich acidic component such has been erupted from the nearby calderas of apoyo and apoyeque admixtures of the former have dominated admixtures of the latter ended with caldera formation ironically open system behavior has exerted fundamental control on the maintenance of relative compositional homogeneity

powerful unconfined turbidity current captured by cabled observatory on the fraser river delta slope british columbia canada turbidity currents are one of the main sediment transport processes on earth yet are notoriously difficult to monitor directly this article presents the first direct and high bandwidth observation of a turbidity current using a cabled sea floor observatory on 5 june 2012 a platform on ocean networks canada located in 107 m of water on the fraser river delta slope was displaced downslope and severed from its data cable the platform weighed ca 1000 kg in water the event took place during high river discharge high tides and rapid sediment accumulation on adjacent upslope areas of the sea floor data recorded as it tumbled downslope allow a reconstruction of the flow which is inferred to have been an unconfined turbidity current lines of evidence indicate that the flow came in as a bed hugging wedge and built up to between 1 m and 4 m in height as the head passed through comparison with laboratory data suggest that the flow was initially supercritical while the adjacent slope offset to the north clearly exhibits change over an annual resurvey period the bathymetry directly at the event location show no resolvable change over a period from seven months before the event to one month after sediment cores collected after the event were pervasively biototurbated and they contain no obvious deposit connected with this event the remarkable aspects of this research follow the flow was powerful enough to carry a 1 tonne platform and sever a heavily armoured cable the current occurred on the unchannelized open slope this powerful event failed to cause discernible seabed elevation change the flow was triggered by tidal conditions the event was detected by a purpose designed cabled observatory thus providing high bandwidth data and also alerting researchers in real time to mount follow on investigations

an efficient solution for the response of electrical well logging tools in a complex environment a symmetrical form of the solution for an electrical source in a multibed well logging environment is derived the method uses local reflection and transmission operators of a single bed boundary and a general recursive algorithm to derive generalized reflection and transmission operators using this method the computation time scales linearly as n where n is the number of beds in the environment a computer program was developed to implement the solution the program is robust and generates accurate results from 20 khz to 25 mhz

understanding spatio temporal barrier dynamics through the use of multiple shoreline proxies abstract at the coast risk arises where and when static human developments are situated within dynamic surroundings barrier islands are often sites of heightened coastal risk since they frequently support substantial human populations and undergo extensive morphological change owing to their low lying form and persistence in energetic hydrodynamic and meteorological conditions using the mixed sand gravel barrier of blakeney point this study argues that to avoid an only partial understanding of coastal zone processes it is necessary to make use of multiple shoreline proxies capturing processes operating both at different timescales and different cross shore positions here five shoreline proxies were extracted from three data sources shoreline error was quantified and compared to observed shoreline change rates to establish proxy specific appropriate timescales for shoreline change analysis the map derived mean high water line at blakeney point revealed landward retreat of 0 61 m a 1 over the past 130 years with a shift from drift towards swash alignment of the barrier since 1981 over the past 24 years the high water line ridge line and vegetation line reveal proxy specific response to management regime change the termination of barrier reprofiling of the eastern section of the barrier has resulted in increased sediment release to the downdrift barrier terminus buffering retreat there at the expense of the updrift section the vegetation line represents an effective proxy for storm driven overwash with maximum shoreline retreat during surge events of 172 m illustrating a strong event driven component to barrier morphodynamics by comparison to the other proxies the lidar light detection and ranging derived mean high water line offers relatively limited insights into barrier dynamics emphasising the importance of multi proxy approaches in the face of technological advance we demonstrate the continued importance of critical attention towards the dependencies that exist between shoreline proxy selection and the processes that can be observed as a result

dynamics of a fringe mangrove forest detected by landsat images in the mekong river delta vietnam mangrove forests dominate many tropical coastlines and are one of the most bio diverse and productive environments on earth however little is known of the large scale dynamics of mangrove canopies and how they colonize intertidal areas here we focus on a fringe mangrove forest located in the mekong river delta vietnam a fast prograding shoreline where mangroves are encroaching tidal flats the spatial and temporal evolution of the mangrove canopy is studied using a time series of landsat images spanning two decades as well as shuttle radar topography mission srtm elevation data our results show that fast mangrove expansion is followed by an increase in normalized difference vegetation index ndvi in the newly established canopy we observe three different dynamics of the mangrove fringe in the southwest part of the fringe near a deltaic distributary where the fringe boundary is linear the canopy expands uniformly on the tidal flats with a high colonization rate and high ndvi values in the northeast part of the fringe near another distributary the canopy expands at a much lower rate with low ndvi values in the fringe center far from the river mouths the fringe boundary is highly irregular and mangroves expansion in characterized by sparse vegetated patches displaying low ndvi values we ascribe these different dynamics to wave action and southwest longshore transport triggered by energetic northeasterly monsoons during winter we further link the large scale dynamics of the fringe to small scale physical disturbances waves erosion and deposition that might prevent the establishment of mangrove seedlings based on these results we include mangrove encroachment in an already published conceptual model of progradation of the mekong river delta we conclude that high ndvi values and a constantly linear vegetation water interface are indicative of stable mangrove canopies undergoing fast expansion probably triggered by sediment availability at the shore our results can be applied more generally to mangrove forests growing in minerogenic and high tidal range environments with high sediment inputs copyright 2016 john wiley sons ltd

studies of a mangrove basin tuff crater new zealand i mangrove biomass and production of detritus abstract the mangrove avicennia marina var resinifera in a tidally flooded explosion crater tuff crater near the southern latitudinal limit of mangroves in new zealand adopts two distinct growth forms taller tree like mangroves up to 4 m tall along the banks of the tidal creek and low stunted shrub mangroves less than 1 m tall on the mudflats twelve trees were felled and on the basis of a biomass height relationship for the taller trees and a biomass canopy width relationship for the lower above ground biomass excluding pneumatophores was estimated average above ground biomass for the taller mangrove was estimated to be 104 1 t ha 1 and for the lower 6 8 t ha 1 while the value for the taller mangroves is similar to figures reported for more complex tropical mangroves the fact that 94 of the basin is covered by low generally sparse mangroves means that total biomass for the basin is estimated to be 153 t an average of only 7 6 t ha 1 litter fall beneath the taller mangroves is estimated as 7 6 2 5 t ha 1 a 1 and beneath the lower mangroves 3 3 0 5 t ha 1 a 1 the value for the taller mangroves is similar to that reported from mangroves in many other parts of the world but because of the extensive low sparse mangroves the total for the basin is estimated as 53 7 t a 1 an average rate of 2 7 t ha 1 a 1 a very low rate of litter fall when compared with elsewhere decomposition of mangrove leaves occurs relatively rapidly with leaves losing half their dry weight in 10 weeks and then continuing to degrade but at a slower rate substrate sediment samples contain high organic matter content and although some organic matter appears to be exported via the tidal creek a proportion of the detrital production is evidently recycled in situ

migration of mud banks and changing coastline in french guiana abstract the french guiana coast is characterized by migrating mud banks which act not only as markers of hydrodynamic and climatic processes but also have an important impact on coastal evolution the role of mud banks was documented by vertical aerial photographs taken in 1979 1982 and 1984 these allow the delineation of the intertidal parts of the banks at low tide and at identical tidal ranges the following results were obtained 1 six mud banks separated by interbank eroding areas are present along the french guiana coast the intertidal parts of three of these banks are cut by river mouths 2 between 1979 and 1984 the intertidal parts of the banks migrated to the west northwest along the coast at rates ranging from 320 to 1220 m yr mean rate 900 m yr 3 in the same period the extent of the intertidal areas colonized by new mangroves 60 km 2 was nearly the same as that of the areas bearing older mangroves 58 km 2 that were lost by erosion while the nature of the surface covered by the intertidal parts of the banks remained nearly the same in spite of the migration of these banks these results suggest the existence of hydrodynamic circulation cells linked to large coastal mud banks along northeastern south america additionally historical data 1955 1972 show long term variations in the velocity of mud bank migration

enhancement of atmospheric radiation by an aerosol layer the presence of a stratospheric haze layer may produce increases in both the actinic flux and the irradiance below this layer such haze layers result from the injection of aerosol forming material into the stratosphere by volcanic eruptions simple heuristic arguments show that the increase in flux below the haze layer relative to a clear sky case is a consequence of photon trapping we explore the magnitude of these flux perturbations as a function of aerosol properties and illumination conditions with a new radiative transfer model that can accurately compute fluxes in an inhomogeneous atmosphere with nonconservative scatterers having arbitrary phase function one calculated consequence of the el chichon volcanic eruption is an increase in the midday surface actinic flux at 20 n latitude summer by as much as 45 at 2900 a this increase in flux in the uv b wavelength range was caused entirely by aerosol scattering without any reduction in the overhead ozone column

three dimensional finite volume analysis of directional resistivity logging sensors a finite volume fv technique is introduced to model directional resistivity logging sensors that employ tilted coil antennas in 3 d anisotropic earth formations the algorithm is based on a coupled scalar vector potential formulation on a 3 d cylindrical staggered grid the choice of discretization enforces the correct continuity equations across material boundaries avoids spurious modes and leads to a better conditioned system matrix in order to model situations where the sensor tool axis does not coincide with the borehole axis eccentered tools a locally conformal fv scheme is also implemented the proposed fv formulation is validated against pseudoanalytical solutions in cases where the latter is applicable showing very good agreement and utilized to investigate the effects of the drill collar borehole anisotropy and azimuthal rotations on the response of directional resistivity sensors

evolution of progradation erosion along the french guiana mangrove coast a comparison of mapped shorelines since the 18th century with holocene data the problems in mapping the mangrove shoreline of french guiana are reviewed the present variability of the coastline due to the shifting mudbanks with avicennia mangroves on their shoreface attached upper fringe is extended to the historical period two and a half centuries ago by means of ancient maps and nautical charts in selected sites around the sinnamary river and the mana river estuaries cayenne peninsula the earliest reported shoreline before 1765 compared with other 18th and 19th century maps is relatively stable demonstrating that erosion repeatedly counterbalanced progradation on a centennial time scale this data contrasts with the 20th century initiation of a sedimentary regime that results in very high local progradation rates an examination of the entire holocene sediment record in preserved in french guiana including palynological evidence suggests that most of the holocene progradation may have occurred by different accretion processes than have formed the historical and recent mudbanks settled by avicennia mangals

the amazon influenced muddy coast of south america a review of mud bank shoreline interactions the 1500 km long coast of south america between the amazon and the orinoco river mouths is the world s muddiest this is due to the huge suspended sediment discharge of the amazon river 106 754 tons yr 1 9 part of which is transported alongshore as mud banks mud bank formation is controlled by the physical oceanography of the continental shelf seaward of the amazon river mouth an initial seafloor storage area for much of the suspended sediment discharged from the river in this area rapid and sustained fluid mud concentration and trapping are associated with fresh water salt water interaction and estuarine front activity on the shelf due to the enormous amazon water discharge ca 173 000 m3 s 1 at obidos 900 km upstream of the mouth fluid mud is transported shoreward and then along the coasts of the guianas by a complex interaction of wave and tidal forcing and wind generated coastal currents the mud banks which may number up to 15 or more at any time are up to 5 m thick 10 to 60 km long and 20 to 30 km wide and each may contain the equivalent mass of the annual mud supply of the amazon as the banks migrate alongshore their interaction with waves results in complex and markedly fluctuating shorelines that are associated with space and time varying depositional bank phases and erosional inter bank phases bank zones are protected from wave attack as a result of wave energy dampening by mud and undergo significant albeit temporary coastal accretion accompanied by rapid mangrove colonization the dampening of waves in bank areas as they propagate onshore is accompanied by the shoreward recycling of mud commonly in the form of individual mud bars these bars progressively undergo desiccation and consolidation and thus constitute a major pathway for rapid and massive colonization by mangroves erosion by waves propagating across relatively mud deficient shoreface zones in inter bank areas can lead to muddy shoreline retreat rates of tens of metres to several kilometres over a few months to a few years accompanied by massive removal of mangroves notwithstanding the higher incident wave energy on inter bank shores inter bank shorefaces are permanently muddy due to the pervasive influence of the amazon muddy discharge inter bank and transitional bank to inter bank phases are associated with both periodic sandy chenier formation and extreme forms of rotation of rare headland bound sandy beaches the high mud supply from the amazon has been the overarching geological control on the quaternary evolution of the northeastern south american coast having led to the growth of a muddy shelf clinoform at the mouth of the amazon and more or less important progradation throughout this coast net progradation reflects an imbalance in favour of deposition during each mud bank inter bank cycle the high mud supply has presumably blanketed shelf sand deposited by smaller rivers during eustatic lowstand phases the shelf clinoform structure at the mouth of the amazon and the muddy coastal progradation throughout the coast of the guianas and into venezuela provide analogues of the geological record on muddy shorefaces

trade wind waves and mud dynamics on the french guiana coast south america input from era 40 wave data and field investigations the south american coast between brazil and venezuela is affected by longshore migrating mud banks derived from the fine grained amazon sediment discharge onshore mud migration prevails over shallow bank areas alternating alongshore with deeper inter bank areas the transport on the inner shelf and attachment to the shoreline of this migrating mud has been attributed mainly to wind waves however the lack of in situ data on waves hampers understanding of the relationship between waves and mud dynamics a 44 yr record 1960 2004 of the era 40 wave dataset generated by the european centre for medium range weather forecasts ecmwf was used in conjunction with field investigations in french guiana to define both event scale and longer term patterns of mud mobilisation induced by waves the ratio h03 t2 combining wave height h and period t and the angle of wave incidence were singled out as the most relevant parameters for describing wave forcing typical bank and inter bank profiles and corresponding mud densities and a 3 month record of changes in the thickness of the fluid mud layer in an estuarine navigation channel were monitored by echo sounding from october 2002 to january 2003 an 80 day record of bed level changes in the intertidal zone was obtained from august to november 2004 using a pressure transducer the results on the wave regime of french guiana confirm a distinctly seasonal pattern and highlight an increase in h03 t2 over the 44 yr period related to an increase in trade wind velocities determined from corresponding trends in atlantic wind pseudo stress off the south american coast wave forcing over bank areas leads to the liquefaction of a 1 3 m thick layer of mud that is transported onshore and alongshore by the longshore component of wave energy the episodic nature of high wave energy events generally results in the formation of mud bar features from the shoreward mobilisation of gel like fluid mud the effect of waves on mud is particularly marked following long periods of low energy and especially at the onset of the high wave energy season october to may when even moderate wave energy events can lead to significant mobilisation of mud significant phases of increased wave energy are attended by higher long term annual rates of longshore mud bank migration but the correlation is rather poor between the wave forcing parameter h03 t2 and migration rates because stronger wave forcing is generally associated with low angles of wave incidence this suggests a complementary role of other hydrodynamic mechanisms such as geostrophic and tidal currents in longshore mud bank migration

rates and mechanisms of shoreface progradation and retreat downdrift of the amazon river mouth field surveys of the 350 km shoreline adjacent to the amazon river mouth reveal three distinct types erosional mud accretionary sand and accretionary mud formation of these zones is controlled by the delivery of amazon suspended sediment mediated by the hydrodynamic regime erosional mud shorelines extend from rio araguari near the amazon river mouth northwestward 280 km to 3 5 n near cabo cassipore shoreface 5 m water depth retreat annually yields 1 4 106 tons of fine grained sediment deposited during an earlier phase of shoreface progradation sandbodies up to 5 m thick overlie erosional mud shorefaces for 10 30 km downdrift of small rivers the sand is supplied from these rivers and released by shoreline retreat of the river mouth areas amazon river sand is restricted in the coastal zone from its mouth north to the cabo norte shoal area where it mixes with sand carried by rio araguari from the guiana highlands north of 3 5 n mud aggradation and progradation is taking place on underconsolidated low gradient tidal flats backed by mangrove swamps 210pb and 14c geochronology of vibracores from the mudflats indicate that sediment accumulation is rapid 0 24 2 0 cm yr landward of the 2 m isobath supplied from a thick 50 150 cm seasonal surface layer shoreface progradation is episodic separated by decadal hiatuses fine grained suspended sediment flux from the amazon and minor amounts of sand and mud from the local rivers supply sediment to the mudflats shore normal tidal currents and solitary waves rework the surface mud layer preferentially transporting available sand landward into the mangrove fringe and producing very fine grained accumulation on the tidal flat 10 12 mean grain size lateral accretion of features 10 100 km long termed mudcapes produces tens of kilometers of seaward coastal plain addition along the northern coast similar features are identified downdrift along the guianas coast as far as the orinoco river 1600 km the northern amapa shoreface deposits are a locus of modern sediment accumulation which progrades over subaqueous deltaic strata

modeling of the dielectric logging tool at high frequencies applications and results the high frequency dielectric logging tool is useful in electromagnetic well logging because by measuring the dielectric constants of rock formations at high frequencies 1 ghz the water saturation of rocks can be inferred without knowing the water salinity in the rocks the results of a theoretical model presented in a previous paper by the author are reported ibid vol 26 p 382 7 1988 use is made of the theoretical model to study the behavior of such a tool across geological sedimentary beds the effect of using different polarizations on the measurement is considered along with the standoff and mudcake effects that reflect on the depth of investigation of such a tool the use of borehole compensation modes in the measurements is also considered it has been found that the te polarization is more robust than the tm polarization in terms of mudcake and standoff effects

hyperpycnal sediment discharge from semiarid southern california rivers implications for coastal sediment budgets southern california rivers discharge hyperpycnal river density greater than ocean density concentrations of suspended sediment 40 g l according to buoyancy theory during flood events mostly during el nino southern oscillation enso conditions because hyperpycnal river discharge commonly occurs during brief periods hours to occasionally days mean daily flow statistics often do not reveal the magnitude of these events hyperpycnal events are particularly important in rivers draining the transverse range and account for 75 of the cumulative sediment load discharged by the santa clara river over the past 50 yr these events are highly pulsed totaling only 30 days 0 15 of the total 50 yr period observations of the fate of sediment discharge although rare are consistent with hyperpycnal river dynamics and the high likelihood of turbidity currents during these events we suggest that much of the sediment load initially bypasses the littoral circulation cells and is directly deposited on the adjacent continental shelf thus potentially representing a loss of immediate beach sand supply during particularly exceptional events 100 yr recurrence intervals flood underflows may extend past the shelf and escape to offshore basins

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

an instrument system for monitoring and sampling suspended sediment in the benthic boundary layer abstract an instrument system has been constructed that can monitor and sample suspended sediment distributions in the benthic boundary layer it consists of miniature nephelometers and suspended sediment samplers placed within one meter of the seabed the system is capable of continuously monitoring suspended sediment profiles at eight levels between 14 and 100 cm above the seabed and collecting suspended sediment samples at four levels 20 50 70 and 100 cm at three times during a deployment period the suspended sediment system is designed to fit onto the instrumented tripod geoprobe which contains four electromagnetic current meters pressure sensor bottom stereo camera two temperature sensors transmissometer and a savonius rotor current meter sensor operation data recording and sediment sampling events are synchronized thus detailed measurements of the near bottom flow conditions are made concurrently with suspended sediment measurements the combined system has been used in sediment transporting environments within san francisco bay california and puget sound washington

the behavior of particle reactive tracers in a high turbidity environment 234th and 210pb on the amazon continental shelf excess 234th and 210pb seabed inventories were measured in cores collected from the amazon continental shelf to examine particle scavenging and seabed dynamics typical excess 210pb inventories range from 100 to 300 dpm cm 2 and the total excess 210pb inventory for the amazon shelf was determined to be 2 7 1017 dpm the 210pb measurements indicate that particle reactive species are scavenged not only from the amazon river but also from the lateral advection of offshore water in order to sustain the 210pb inventories the volume of water supplied by the lateral advection from offshore must be approximately five to ten times the water discharge of the amazon river this lateral advection supplies about 67 of the total excess 210pb to the amazon continental shelf with relatively small contributions from riverine input 31 atmospheric fallout 2 3 and in situ production 0 1 the 234th inventories were measured on four cruises which occurred during periods of differing river discharge wind stress and flow rates of the north brazil current the 234th excess seabed inventories show large spatial and seasonal variability with a range from 0 to 22 dpm cm 2 ratios of predicted 234th inventories over observed 234th inventories indicate that a large portion of the inventories on the inner and mid shelf occur in the fluid mud layer or the suspended sediments the 234th excess seabed inventories are controlled by resuspension cycling the inventories between the water column and the seabed 210pb and 234th excess seabed inventories are compared directly using corrections for the different time scales and offshore concentrations this approach indicates that for most of the shelf the inventories of the shorter term tracer 234th are less than predicted by the inventories of the longer term tracer 210pb there are two explanations for this trend the first is that a larger portion of the 234th inventory occurs in the fluid muds or the water column relative to 210pb the second is that the supply of offshore water scavenging efficiency and or deposition have been lower over the two year study period relative to the last one hundred years

mobile mud dynamics in the east china sea elucidated using 210pb 137cs 7be and 234th as tracers mobile mud mm which has fine grain size distribution 90 clay silt and 2 5 in south inshore indicates that the sediment focusing resulted in the increased mass flux the residence time of mm is estimated as 3 6 years both by mass balance of mm and 210pbex in mm

quantification of refractory organic material in amazon mudbanks of the french guiana coast abstract sub oxic overturning mudbanks originating in the amazon outlet and traveling northwestward along the coast of south america store 3 5 tg organic carbon on the amazon shelf annually the predominance of old soil carbon burial associated with the sediment has been supported by relatively old bulk radiocarbon ages 2200 3200 14 c y reported but this amount has yet to be quantified for this dynamic setting here we employ ramped pyrolysis radiocarbon dating to mudbank sediments from three cores to further constrain the amount of refractory sedimentary organic carbon present using radiocarbon age as a proxy for reactivity we find that at least 36 of this organic carbon is pre aged refractory terrestrial organic carbon likely sourced from terrestrial soils and averaging ages of 16 100 300 14 c y this implies significant preservation of pre aged terrestrial soil material in this marine environment despite the mobility and oxygen availability in the muds concurrently an average of 77 of this organic material is aged over 1000 14 c y consistent with previous estimates that the majority of organic carbon buried along the shelf is refractory analyses of extracted lignin phenols and terrestrial humic acids also support this preservation of refractory terrestrial material these data indicate that sediment discharged from the amazon river may preserve refractory terrestrial carbon in marine sediments at a minimum of 3 1 tg c y which is more efficient than originally thought

lead isotope composition of central american volcanoes influence of the galapagos plume 1 lead isotopic analyses of lavas from central america both along and behind the volcanic arc help to clarify source components in the mantle wedge analysis of previous pb isotopic data had implied that little or no marine sediment lead was added to the central american source region as all samples fell within the morb field in contrast to other information e g ba la 10be 87sr 86sr that indicated a high subduction component the data presented here include several analyses of local marine sediment showing it to be exceptionally unradiogenic in pb and thus permitting high sediment contributions to the mantle source region without significant changes in pb isotopes combined pb nd and pb sr isotopic diagrams clearly illustrate the influence of crustal contamination for samples from guatemala and honduras and of subducted sediment for all lavas of the volcanic front samples collected behind the volcanic front are derived from mixing between enriched and depleted mantle sources and in central costa rica extending to the back arc overlap pb sr and nd isotope values for both cocos island and some components of the galapagos hot spot the restricted geographical occurrence of the enriched mantle signature in central america coupled with the persistence of the signal well into the back arc region imply that these lavas are sampling galapagos plume influenced mantle the presence of this plume component beneath southern central america and extending to the northeast beneath the caribbean confirms a galapagos hot spot origin for this part of caribbean plate

internal tides as a major process in amazon continental shelf fine sediment transport abstract the description of hydrodynamics associated with the extensive reef system on the shelf break adjacent to the amazon river is still a challenge for ocean sciences despite the discharge of more than one billion tons of cohesive sediment per year the outer continental shelf of the world s largest river presents very low concentrations of suspended sediment near the bottom and an absence of modern fine sediment deposits nearly one hundred kilometers before the shelf break the offshore limit of the subaqueous delta consists of a sigmoidal clinoform standing between 40 and 70 m in depth a depositional feature that cannot be explained solely by estuarine like gravitational circulation this paper aims to test the hypothesis that internal tides have a major role in the control of offshore fine sediment transport for that we implement a set of tridimensional non hydrostatic and high resolution up to 2 m vertical and 2 km horizontal delft3d models the experiments showed that even disregarding river plume buoyancy wind drag superficial waves and ocean currents the exclusive interaction between barotropic tidal currents bathymetry and the stratification structure of the ocean is capable of generating asymmetrical current patterns compatible with modern deposition the maximum shelf slope and the relative depth between the outer shelf and the pycnocline represent the main factors influencing the generation and shoreward propagation of internal tides over time spring neap cycles are eventually capable of reverting cross shore subtidal transport tendencies while seasonal variability in ocean stratification modulates the intensity of baroclinic processes

the spatial and temporal distribution of grain size breaks in turbidites grain size breaks are surfaces where abrupt changes in grain size occur vertically within deposits grain size breaks are common features in turbidites around the world including ancient and modern systems despite their widespread occurrence grain size breaks have been regarded as exceptional and not included within idealized models of turbidity current deposition this study uses ca 100 shallow sediment cores from the moroccan turbidite system to map out five turbidite beds for distances in excess of 2000 km the vertical and spatial distributions of grain size breaks within these beds are examined five different types of grain size break are found type i in proximal areas between coarse sand and finer grained structureless sand type ii in proximal areas between inversely graded sand overlain by finer sand type iii in proximal areas between sand overlain by ripple cross laminated finer sand type iv throughout the system between clean sand and mud and type v in distal areas between mud rich debrite sand and mud this article interprets types i and v as being generated by sharp vertical concentration boundaries controlled by sediment and clay concentrations within the flows whilst types ii and iii are interpreted as products of spatial temporal fluctuations in flow capacity type iv are interpreted as the product of fluid mud layers which hinder the settling of non cohesive grains and bypasses them down slope decelerating suspensions with sufficient clay will always form cohesive layers near to bed promoting the generation of type iv grain size breaks this may explain why type iv grain size breaks are widespread in all five turbidites examined and are commonplace within turbidite sequences studied elsewhere therefore type iv grain size breaks should be understood as the norm not the exception and regarded as a typical feature within turbidite beds