modeling basin scale internal waves in a stratified lake basin scale internal waves provide the driving forces for vertical and horizontal fluxes in a stratified lake below the wind mixed layer thus correct modeling of lake mixing and transport requires accurate modeling of basin scale internal waves examining this capability with a hydrostatic z coordinate three dimensional 3d numerical model at coarse grid resolutions is the focus of this paper it is demonstrated that capturing the correct thermocline forcing with a 3d mixed layer model for surface dynamics results in a good representation of low frequency internal wave dynamics the 3d estuary and lake computer model elcom is applied to modeling lake kinneret israel and is compared with field data under summer stratification conditions to identify and illustrate the spatial structure of the lowest mode basin scale kelvin and poincarewaves that provide the largest two peaks in the internal wave energy spectra the model solves the unsteady reynolds averaged navier stokes equations using a semi implicit method similar to the momentum solution in the trim code with the addition of quadratic euler lagrange dis cretization scalar e g temperature transport using a conservative flux limited approach and elimination of vertical diffusion terms in the governing equations a detailed description is provided of turbulence closure for the vertical reynolds stress terms and vertical turbulent transport using a 3d mixed layer model parameterized on wind and shear energy fluxes instead of the convential eddy viscosity diffusivity assumption this approach gives a good representation of the depth of the mixed layer at coarse vertical grid resolutions that allows the internal waves to be energized correctly at the basin scale wind stresses surface heating and density currents form the driving energy fluxes of a stratified lake the basin scale energy flux from the wind is of particular interest because of its dominant role in setting the thermocline in motion which in the absence of inflows and outflows is the primary energy store for transport and mixing below the wind mixed layer thus modeling the basin scale internal wave behavior is an a priori requirement to modeling and quantifying the flux paths of nutrients in a stratified lake imberger 1994 this paper takes a first step in this direction by analyzing our ability to model basin scale internal waves that are seen in lake kinneret israel energy flux path in a stratified lake energy flux through a stratified lake has a fundamental dependence on forced and free baroclinic motions the wind imparts both momentum and turbulent kinetic energy tke to the water in the sur face layer the tke distributes momentum vertically in the

reconstructing the sedimentation history of the bengal delta plain by means of geochemical and stable isotopic data abstract the purpose of this study is to examine the sedimentation history of the central floodplain area of the bengal delta plain in west bengal india sediments from two boreholes were analyzed regarding lithology geochemistry and the stable isotopic composition of embedded organic matter different lithofacies were distinguished that reflect frequent changes in the prevailing sedimentary depositional environment of the study area the lowest facies comprises poorly sorted fluvial sediments composed of fine gravel to clay pointing at high transport energy and intense relocation processes this facies is considered to belong to an early holocene lowstand systems tract that followed the last glacial maximum fine to medium sands above it mark a gradual change towards a transgressive systems tract upwards increasing proportions of silt and the stable isotopic composition of embedded organic matter both indicate a gradual change from fluvial channel infill sediments towards more estuarine and marine influenced deposits youngest sediments are composed of clayey and silty overbank deposits of the hooghly river that have formed a vast low relief delta floodplain close to the surface small concretions of secondary mn oxides and fe oxyhydr oxides occur and mark the fluctuation range of the unsaturated zone these concretions are accompanied by relatively high contents of trace elements such as zn ni cu and as to sum up the outcomes of this study provide new insights into the complex sedimentation history of the barely investigated central floodplain area of west bengal

the initial response of a stratified lake to a surface shear stress laboratory experiments are used to study the initial response of a stratified fluid to the action of a wind stress the experiments are described in the context of a parameterization scheme that quantifies the strength of the applied stress relative to the bulk stability of the fluid and also the duration of the wind stress relative to the periods of the waves generated by the stress this study concentrates on the first fundamental internal wave period in experiments where the fluid is considered to have upwelled i e the stratified region of the fluid reaches the surface at the upwind endwall the majority of the experiments use three layer initial density profiles as an approximation to a continuously stratified water column a linear model using normal modes proved successful prior to the commencement of upwelling and this enabled an estimate to be made of the time at which upwelling occurred at this point the wave development ceased and the flows developed via entrainment mechanisms consideration of the energy budget showed that little of the input energy was stored in the system the initial mixing efficiency defined as the ratio of the mean potential energy gained to the energy imparted by the belt never exceeded 30 peak efficiency occurred when the surface stress was just sufficient to bring the interfacial region to the surface

the cause of internal outflow surges in a high resolution simulation of the 8 may 2003 oklahoma city tornadic supercell abstracta high resolution simulation of the 8 may 2003 oklahoma tornadic supercell is analyzed to determine the origin of internal outflow surges within the low level cold pool the analyzed simulation has 50 m horizontal grid spacing and is quadruply nested within larger lower resolution domains that were initialized via three dimensional variational data assimilation 3dvar of radar and other observations the high resolution simulation produces two tornadoes that track in close proximity to the observed tornado on 8 may 2003 the authors previous study determined that an internal outflow surge instigated tornadogenesis for the first tornado in this simulation but the cause of this internal outflow surge was unclear in this study the vertical momentum equation is analyzed along backward trajectories that are initialized within the tornado triggering internal outflow surge the analysis reveals that the internal outflow surge is forced by the dynamic part of the vertical pressure gradient further ex

sequence stratigraphy and the evolution of the ganges brahmaputra delta complex 1 a seismic stratigraphic analysis of 2600 km of data covering part of the ganges brahmaputra delta has shown that it evolved in three stages 1 the proto ganges delta developed immediately after the breakup of gondwana at approximately 126 ma the indian plate then moved rapidly northward ultimately crossing more than 50 degrees of latitude because the catchment area was small sedimentation was relatively restricted and the early delta consists of relatively small scale prograding clastic bodies alternating with mixed carbonate clastic and carbonate platform assemblages 2 the transitional delta developed following a major eustatic sea level low at 49 5 ma initially sedimentation was relatively restricted but at about 40 ma the flux of water and clastic sedime ts dramatically increased and the delta grew rapidly with the development of major prograding clastic depositional sequences abundant evidence of lobe switching suggests that the delta was stream dominated at that time the abrupt change in the morphology of the delta appears to be a direct response to the collision of the indian plate with the eurasian plate and the rise of the himalayas the delta remained stream dominated some time after the major plate collision and until the bay of bengal became a well developed deep embayment at about 21 ma the bengal fan began to accumulate during this interval as turbidity currents carried the increased sediment load into deeper water 3 the modern delta began to appear in its present form following a major eustatic sea level low and erosio at 10 5 ma by this time the delta had become tidally dominated and had prograded well beyond the study area into the now deeply embayed bay of bengal

strong sea forcing and warmer winter during solar minima 2765 yr b p recorded in the growth bands of crassostrea sp from the confluence of river ganges eastern india abstract long term variation of solar activity plays a key role in controlling climatic oscillations during glacial interglacial cycles the records of such climatic shifts can be retrieved from sedimentary archives in overbank deposits found in the estuary regions of major rivers in the tropics which are fed by glaciers in this study we have shown the effect of solar variability on regional climate by altering the river discharge and incursion of warm water pool into the region adjoining bay of bengal during seasonal dry period the incremental growth bands present in the modern day meretrix sp and late holocene crassostrea sp were examined for reconstruction of temperature and water composition at the head bay region of the river ganges the conventional c14 techniques on fossil oyster yielded age of 2765 130 yr b p which coincides with a solar minima analysis of clumped isotope thermometry on the growth bands provided temperature estimates for the growth of shells the temperature estimates for the modern shell suggesting range of values showed a range between 13 and 42 c close to the observed temperatures recorded in the climatological data while the fossil shell had a range of values between 22 and 38 c the 18o measured in the aragonite together with the estimated temperature were used to deduce the water composition during growth at equilibrium condition the water 18o varied between 4 8 and 1 2 for the modern sample close to the observed values of water measured near this locality while the range in water composition inferred for the paleo samples was from 2 37 to 0 82 suggesting a stronger influence of sea water throughout the year the results are consistent with the argument of infiltration of water from neighbouring warm water pool into the estuary a similar approach can be extended to evaluate the effects of climate variability due to differential action of river discharge into the sea at seasonal time scales based on available mollusc shells in the sedimentary successions from the region

vortex lines within low level mesocyclones obtained from pseudo dual doppler radar observations abstract vortex lines passing through the low level mesocyclone regions of six supercell thunderstorms three nontornadic three tornadic are computed from pseudo dual doppler airborne radar observations obtained during the verification of the origins of rotation in tornadoes experiment vortex in every case at least some of the vortex lines emanating from the low level mesocyclones form arches that is they extend vertically from the cyclonic vorticity maximum then turn horizontally usually toward the south or southwest and descend into a broad region of anticyclonic vertical vorticity this region of anticyclonic vorticity is the same one that has been observed almost invariably to accompany the cyclonic vorticity maximum associated with the low level mesocyclone the vorticity couplet straddles the hook echo of the supercell thunderstorm the arching of the vortex lines and the orientation of the vorticity vector along the vortex line arches compared to the orientation of the ambient barotrop

vertical propagation of lakewide internal waves 1 internal waves with diurnal period dominated velocities measured by an acoustic doppler profiler adp in a small lake main basin 3000 m by 400 m by 18 m adp profiles and an along lake temperature section indicate that the observed waves like seiches had horizontal wavelengths exceeding the metalimnion length however unlike non dissipative seiches the observed waves propagated vertically carrying energy to the lakebed where waves were absorbed rather than being strongly reflected this absorption is predicted by a standard parameterization of boundary layer dissipation the absence of upward propagating energy precludes seiche resonance limits focusing of waves toward attractors and suggests that hypolimnion dissipation was limited by the supply of downward propagating energy vertical wavelengths were less than the lake depth simplified calculations suggest that vertically propagating waves as opposed to vertically standing seiches are most likely where vertical wavelengths are short near bed stratification is strong and lakes are short and deep

modeling earth deformation from monsoonal flooding in bangladesh using hydrographic gps and gravity recovery and climate experiment grace data 1 the ganges brahmaputra and meghna rivers converge in bangladesh with an annual discharge second to the amazon most of the flow occurs during the summer monsoon causing widespread flooding the impounded water represents a large surface load whose effects can be observed in gravity recovery and climate experiment grace and gps data bangladesh is at the center of the second largest seasonal anomaly in the grace gravity field reflecting water storage in southeast asia eighteen continuous gps stations in bangladesh record seasonal vertical motions up to 6 cm that inversely correlate to river level we use 304 river gages to compute water height surfaces with a digital elevation model to separate surface water from groundwater porosity of 20 was used to estimate groundwater mass and calculate the water load results show 100 gt of water are stored in bangladesh 7 5 of annual discharge but can reach 150 gt during extreme events the calculated water mass agrees with monthly grace water mass equivalents from bangladesh within statistical limits we compute the deformation due to this water load on an elastic half space and we vary young s modulus to fit gps data from our two most continuous records the water loading can account for 50 of the variance in the gps data the best fitting young s modulus is 117 124 gpa for dhak and 133 135 gpa for sust although the upper bound is not well constrained these estimates lie between sediment 30 75 gpa and mantle 190 gpa values indicating that response to loading is sensitive to structure throughout the lithosphere and is not absorbed by the weak sediments

cretaceous eocene sedimentation tectonism and biofacies in the bengal basin india abstract the bengal basin located in the eastern part of the indian subcontinent is one of the thickest sedimentary basins in the world the basin started downwarping in the early cretaceous and the sedimentation has since been continuous from an initial transgressive marine phase lasting almost up to end of the eocene the basin has evolved through a subsequent regressive marine and a later continental deltaic phase in the absence of detailed information available from the deeper basinal parts an analysis of the sedimentary and tectonic history and biofacies of the regions forming the western and northwestern shelves of the bengal basin pertaining to the initial phase of its evolution is attempted this is achieved through the synthesis of all the geological information available on the basis of recent exploration and research conducted on the sequences developed in west bengal meghalaya mikir hills and assam india the subsidence of the bengal basin was the result of differential adjustments of the crystalline basement along certain well established trends and was initiated soon after the gondwana tectonism sedimentation and volcanicity the earlier sediments were not typically marine but were more paralic to estuarine in nature the entire bengal basin except the area now comprising assam came under the influence of the first regional marine transgression during the late campanian maestrichtian the coarse clastic sediments were deposited under shallow coastal marine conditions a more widespread marine transgression took place during the early middle eocene which covered almost the entire shelf area including assam during this transgression the sediments belonging to the orthoquartzite carbonate association were formed in the shelf the sea started receding in the late eocene during which the entire bengal shelf came under the influence of heavy unloading of fine clastics the top of this fine clastic shaly sequence is an important sedimentary datum in the bengal shelf which marks the end of the first phase in the evolutionary history of the basin

provenance and depositional variability of the thin mud facies in the lower ganges brahmaputra delta west bengal sundarbans india the sundarbans is one of the largest coastal wetland sites in the world and covers an area of approximately one million hectares of the western delta of the ganges and brahmaputra g b rivers located across bangladesh and india since the late holocene the western delta has not been directly fluvially sourced due to the ganges shift to the east present day bangladesh the depositional facies thin mud facies of the late holocene abandoned western region the sundarbans is derived from dominant estuary tidal dynamics however the provenance of the associated tmf sedimentation in this far western zone indian sundarbans per se is as yet equivocal in this study sediment cores from the indian sundarbans saptamukhi thakuran estuary were closely examined for grain size distributions gsds mineralogy through x ray diffraction xrd and geochemistry with x ray fluorescence xrf the tmf in the west bengal sundarbans has been determined to show intensively weathered terrestrial sediment derived principally from the ganges alluvial plain gap there is a predominance of quartz mica and clay minerals with quartz interpreted as a product of low relief tropical weathering sourced via the g b rivers draining the himalayas lithofacies interpreted through gsd analysis of the tmf is indicative of a muddy tidal flat environment with aggradation and a general fining up trend between the adjacent estuaries the sediment provenance indicates a continuing g b sediment source which moves westward along the bay of bengal from the active delta front and is then reworked over the far western abandoned delta by tidal estuarine forcing

turbulent mixing induced by nonlinear internal waves in mono lake california describing the spatial and temporal occurrence of turbulent mixing and the mechanisms of turbulence production are fundamental problems in physical limnology and oceanography here we present results from mono lake california that illustrate pronounced steepening of the pycnocline upon cessation of strong wind forcing and the enhanced turbulence resulting from this nonlinearity three events occurred with wind speed of 10 m s21 upwind the upper pycnocline upwelled but on relaxation of the wind it downwelled 6 10 m and compressed the similarity of wave amplitude during compression to mixed layer depth and lake numbers 4 were indicative of significant nonlinearity energy in the internal wave field increased by two orders of magnitude but 50 of the energy was lost within 24 h the largest energy losses occurred at low frequencies rather than via an energy flux from low to moderate and high frequencies eddy diffusivities computed over 6 d intervals using the heat budget method averaged 1026 m2 s21 in the energized pycnocline but were five to 10 times higher during the initial upwelling and downwelling periods microstructure casts taken 1 2 d after the first event revealed that the percentage of the pycnocline that was turbulent varied with bottom slope being 80 turbulent where bottom slopes were up to 10 6 0 where bottom slopes were 3 and less than 7 where bottom slopes were 0 1 the threshold for turbulent transport was exceeded only 10 of the time and only where bottom slopes were highest

geochemical evolution of the tertiary succession of the nw shelf bengal basin bangladesh implications for provenance paleoweathering and himalayan erosion abstract whole rock major trace and rare earth element ree compositions of paleogene to neogene sedimentary rocks of the nw shelf succession province 1 of bangladesh contain a record of interaction of the india and asian plates himalayan tectonism and climatic development analyses of 66 sandstones and mudrocks from the tertiary succession of bangladesh were made to examine provenance source weathering and the influence of paleoclimate and tectonism the sediments display linear geochemical trends due to quartz dilution and varying quartz clay ratios produced by hydrodynamic sorting chondrite normalized ree patterns for both sandstones and mudrocks from different groups are similar to upper continental crust with moderate to high lree enrichment lithotypes within formations average lan ybn 5 31 11 41 and marked negative eu anomalies eu eu 0 51 0 69 based on geochemical criteria the succession can be divided into three parts jaintia barail surma and dupi tila very high silica contents in jaintia tura sandstones and high chemical index of alteration cia indices in kopili mudrocks fe shales suggest derivation from a deeply weathered and stable cratonic source india the tura sandstones are interpreted as first cycle quartz arenites produced while the indian plate drifted across equatorial regions during the paleocene eocene thermal maximum petm the barail surma and dupi tila sediments were derived from a felsic orogen the himalaya the barail surma sediments were mainly derived from the trans himalayan batholith and associated granitoids with significant contribution from the lesser himalaya mafic input is also evident probably from intraoceanic arc material within the himalaya barail mudrocks have uniformly high cia values 92 95 suggesting intense steady state weathering of their proto himalayan source and warm and humid climate in contrast cia values of surma mudrocks range from 66 to 93 average 84 suggesting non steady state weathering related to active uplift in the himalaya the dupi tila sediments were derived from a more felsic lesser himalaya source with significant contribution from the trans himalaya and very little or no ophiolitic or arc material dupi tila mudrocks have cia ratios of 62 99 average 72 also indicating non steady state weathering in the rising himalayan source geochemical compositions of the nw shelf sediments are comparable to coeval successions in the surma basin province 2 of bangladesh and the siwaliks india indicating similar source evolution of the indian monsoon and associated high precipitation caused intense chemical weathering of the surma and dupi tila source despite rapid uplift the surma group thus bears the signature of evolution of the asian monsoon in the bengal basin at 21 ma simultaneous with the development of the east asian monsoon this supports proposals that both monsoon systems developed at the same time

influence of water and sediment supply on the long term evolution of alluvial fans and deltas statistical characterization of basin filling sedimentation patterns 1 the temporal and spatial variability of sedimentation resulting from sediment storage and release and the lateral mobility of sediment transporting flows imparts fundamental patterns into the stratigraphic record recent studies show that paleoenvironmental allogenic signals preserved in stratigraphy may be contaminated by internally generated autogenic sedimentation patterns however it is unclear how the magnitude of autogenic dynamics is related to allogenic forcings utilizing statistical methods we quantify basin filling trends in three laboratory experiments where input water and sediment flux were varied we use the compensation index and compensation time scale to estimate the strength of compensation defined here as the tendency to fill topographic lows faster than would result from random deposit stacking and to estimate the time scales over which autogenic processes operate in the experiments topography of channelized deltas formed by noncohesive sediment was monitored in a basin experiencing temporally and spatially uniform relative subsidence each experiment resulted in construction of a stratigraphic package in excess of 25 channel depths thick we find that compensation strength in the experiments is not influenced by absolute magnitudes of water and sediment flux but does increase as a function of the ratio of water to sediment flux a compensation time scale defined as the maximum depth of a system s channels divided by the long term deposition rate accurately defines the maximum time scale at which autogenic dynamics occur in all experiments when applied to field scale systems we predict that autogenic dynamics occur out to time scales between 5 and 150 kyr

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

mass accumulation rates in asia during the cenozoic summary this work establishes estimates of mass accumulation rates in 18 mostly offshore sedimentary basins in asia since the beginning of the cenozoic 66 ma the estimates were derived from isopach maps cross sections and drill holes or stratigraphic columns assuming regional similarity of the strata average solid phase volumes and accumulation rates were calculated for nine epochs approximately corresponding to geological periods palaeocene 66 58 ma eocene 58 37 ma oligocene 37 30 and 30 24 ma miocene 24 17 17 11 and 11 5 ma pliocene 5 2 ma and quaternary 2 0 ma these rates shed new light on the geological history of asia since the onset of the collision of india with asia 50 ma the overall average accumulation rates curve for asian sedimentary basins since the beginning of the tertiary shows an exponential form with slow accumulation rates less than 0 5 106 km3 myr 1 until the beginning of the oligocene more than 15 myr after the onset of the collision from the oligocene onwards rates increase quickly in an exponential manner reaching their maximum values in the quaternary more than 1 5 106 km3 myr 1 from these observations we suggest that extrusion and crustal shortening are complementary processes that have been successively dominant throughout the india eurasia collision history at smaller scales one may distinguish between independent histories at the subcontinental and basin scales this permits a comparison of the relative importance of tectonic and climatic erosion processes affecting the different mountain belts of asia during the cenozoic

miocene sedimentation and subsidence during continent continent collision bengal basin bangladesh abstract the bengal basin a complex foreland basin south of the eastern himalayas exhibits dramatic variability in neogene sediment thickness that reflects a complicated depositional and tectonic history this basin originally formed as a trailing margin se of the indian continental crust complicated by convergence with asia to the north and oblique convergence with burma to the east newly compiled isopach data and previously reported seismic data show evidence of thickening of basin fill toward the south opposite of the pattern typically seen in foreland basins this is presumably due to sedimentary loading of voluminous deltaic sediments near the continent ocean boundary and basinward downfaulting analogous to that in the gulf of mexico isopach data show that there is considerable vertical relief along the base of the miocene stratigraphic sequence probably due to down to the basin faulting caused by focused deltaic sedimentation and associated crustal flexure in contrast when viewed in east west profile basin shape is more typical of a foreland basin with strata thickening eastward toward the indo burman ranges which reflects east west convergence with southeast asia comparison of the lateral and vertical extent of the bhuban and boka bil formations with the bouguer anomaly map of bangladesh suggests that considerable subsidence of the sylhet trough in the northeastern part of the bengal basin which has the lowest gravity value of the region had not taken place by the end of the miocene this post miocene subsidence is attributed to tectonic loading from southward thrusting of the shillong plateau along the dauki fault relatively uniform miocene isopachs across the sylhet trough confirm that this began in the pliocene consistent with results of recent research on sediment provenance in the northwest in the region south of the siwalik foreland basin continental crust has not as yet been loaded allowing relatively little accommodation space for sediment accumulation the miocene here is very thin deltaic progradation across most of bangladesh during the miocene followed earlier more proximal progradation across assam immediately northeast of the bengal basin and has been followed by continued progradation into the southern bangladesh coastal and offshore region

locked and loading megathrust linked to active subduction beneath the indo burman ranges it is unclear whether subduction is still active beneath the indo burman mountain range analyses of gps measurements from this region reveal a locked megathrust fault implying that subduction is active and could generate a large earthquake

polycyclic aromatic hydrocarbons pahs in late eocene to early pleistocene mudstones of the sylhet succession ne bengal basin bangladesh implications for source and paleoclimate conditions during himalayan uplift abstract distribution and possible sources of polycyclic aromatic hydrocarbons pahs have been investigated in 23 late eocene to early pleistocene mudstones from the sylhet succession of the northeastern bengal basin bangladesh paleoclimatic conditions in the southern himalaya region throughout the himalayan uplift were reconstructed based on combustion derived pahs and aromatic land plant derived biomarkers phenanthrene fluoranthene fla pyrene py benz a anthracene baan chrysene triphenylene chry tpn benzofluoranthenes bflas benzo e pyrene bepy benzo a pyrene bapy perylene pery indeno 1 2 3 cd pyrene inpy benzo ghi perylene bghip coronene cor and retene ret were the identified pahs fla fla py ratios 0 5 and inpy inpy bghip 0 2 from almost all sylhet samples suggest occurrence of natural wildfires low contents of baan and bapy indicate decomposition by long exposure to sunlight before sedimentation or early diagenetic weathering increased cor inpy and bghip contents suggest occurrence of larger high temperature wildfires correlation coefficients of the pahs and p values for statistical hypothesis testing showed that the positive and negative correlations within the pahs may be indicative of high or low temperatures in wildfires fungi derived pery showed negative correlations with py r 0 67 p 4 6 10 4 and fla r 0 56 p 5 0 10 3 but not with cor bflas inpy and bghip based on the correlation coefficients for all pahs and their p values five statistical groups py fla cor bflas inpy bghip baan chry tpn bapy pery and ret were recognized these groups are probably correlated with origins and depositional processes according to the results the sylhet succession was deposited in three differing paleoclimatic regimes 1 first phase late eocene to early miocene early to middle stage of himalayan uplift high contents of combustion derived pahs fla py and bepy significant gymnosperm derived ret and low pery abundances in the jaintia and barail groups indicate arid climatic conditions although wildfires could often occur 5 or 6 ring combustion pahs cor inpy and bghip contents are low suggesting that the wildfires were relatively low temperature 2 second phase middle to late miocene middle to late stage of himalayan uplift combustion derived pahs and fungi derived pery were dominant in the surma group the climate was arid to humid and seasonal with a dry season giving conditions suitable for combustion abundant fla bflas bepy cor inpy and bghip imply high temperatures in large wildfires however frequency of the wildfires decreased because of wet climate 3 third phase late miocene to pleistocene late stage of himalayan uplift moderate to high pery contents and low fla py and bepy abundances in the tipam and dupitila groups indicate establishment of more humid climate inpy bghip cor bflas and bapy were predominant intensified humid and seasonal climate arising from the himalayan monsoon decreased the incidence and frequency of general wildfires but increased the ratio of large to small wildfires

internal waves of the second vertical mode in a stratified lake a time series of measurements of thermal structure and near bottom currents in a stratified lake revealed both first and second vertical mode internal seiching the transition into second mode seiching occurred after first mode waves usually generated by pulses of strong winds died down we describe the major features of the second mode seiche and discuss its effects on mixing in the lake below the thermocline on the basis of calculations of eddy diffusion coefficients there is only weak mixing in the interior of the hypolimnion scaling arguments suggest that mixing at the lake s edges due to motion of the isotherms is not significant despite the larger amplitudes of the second mode seiche the most important mixing region appears to be a turbulent bottom boundary layer driven by seiche currents however resuspension of bottom sediments did not occur probably due to a combination of sediment nature and low current speeds

volume balance of erosional loss and sediment deposition related to himalayan uplifts the volume of sediment in the foreland basins deltas and fans which are a consequence of the collision of india and asia appears to require the removal of a vertical section c 35km high if the sediments were entirely derived from the himalaya it is argued that this is implausible and that it is likely that the himalayan hinterland e g karakoram and tibet was an important source of sediment prior to c 20 ma when significant himalayan uplift commenced

spatial temporal variability in surface layer deepening and lateral advection in an embayment of lake victoria east africa vertical and horizontal exchanges in pilkington bay a shallow 9 m embayment of lake victoria were determined from a surface energy budget time series measurements of temperature and quasi synoptic transects of conductivity temperature and depth conducted over a 2 d period the surface energy budget is the first from a tropical lake over a diurnal timescale strong stratification developed during morning and early afternoon 40 cycles h 21 but was eroded beginning in the afternoon by the combination of wind and heat loss surface heat losses contributed 70 of the energy for surface layer deepening 82 of the time from midafternoon until midmorning circulation times of the surface layer were 2 min as it deepened to 1 5 m in the afternoon and were 12 min at night even when mixing extended to the lake bottom spatial differences in the rates of heating and cooling and in the depth of wind mixing caused fronts to develop on spatial scales of kilometers within the bay convergence of these fronts led to downwelling of surface waters and upwelling of deep waters during the stratified period horizontal pressure gradients due to differential heating contributed to thermocline downwelling lateral movement of deep anoxic waters and generation of high frequency internal waves all of which contribute to vertical and horizontal transports although wind and heat loss at one location generally determine the depth of the surface layer and thermocline the depths of these key features may be strongly influenced by rates of heating and cooling elsewhere in a basin the temporal and spatial scales of advection and vertical mixing determine the flux paths of particles and solutes in aquatic ecosystems and ultimately lacustrine productivity time series measurements of surface meteorology from which surface energy budgets can be calculated and water column temperatures can be used as an indirect approach to determine these pathways the data also can be used to determine internal wave dynamics the energetics of mixing and the role of intrusions and density driven flows in advection with this information the rates of circulation within the upper mixed layer and vertical and horizontal transports can be assessed the diurnal cycle of stratification and mixing is deter

the indus clastics forearc basin sedimentation in the ladakh himalaya india abstract the mid cretaceous to early tertiary clastic sediments of the indus group represent the forearc basin succession of the transhimalayan arc trench system in the late albian to early eocene a shallowing upward megasequence tar turbidites to nummulitic limestones and alluvial to deltaic sediments basgo and temesgam formations were deposited respectively on the southern and northern flanks of the basin clastic petrography points to provenance from the largely undissected volcanic cover of the massif with more important plutonic detritus recorded in the northern alluvial units after collision with the indian passive margin in the early eocene sedimentation in the whole basin was characterized by huge alluvial fans nurla formation and then by lacustrine deposits formed in an intermontane depression nimu formation a rapid change in petrographic and geochemical composition is recorded in post early eocene units with sharp increase in plutonic detritus and trace elements typically associated with ophiolites testifying to the uplift of both arc massif and subduction complex sources after collision with the indian passive margin the petrography of the indus basin clastics and the evolution of detrital modes compare well with volcano plutonic suites deposited in modern and ancient forearc basins the composition of sandstone units contained within the accretionary prism instead suggests provenance from an undissected magmatic arc and deposition in abyssal plain and trench settings nindam volcaniclastic turbidites reworking of arc derived sandstones took place in small basins perched on the accretionary trench slope the asian continental margin of the neotethys ocean was thus an active pacific type arc trench system until the eocene when it collided with india underwent polyphase fold thrust deformation and was finally uplifted several kilometers to form the indus suture zone of the himalayas

holocene and modern sediment budgets for the ganges brahmaputra river system evidence for highstand dispersal to flood plain shelf and deep sea depocenters the partitioning of fluvial sediment load across continental margins is an important control on strata formation and sequence development however few quantitative sediment budgets that encompass entire dispersal systems exist for the ganges brahmaputra river system sediment discharge is estimated to be 10 9 t yr at gauging stations 300 km inland of the coast but little has been known of the downstream fate of this material geochronological geophysical and stratigraphic investigations of the lowland flood plain delta plain and shelf help to delineate the extent of holocene fill and allow calculation of a first order sediment budget results reveal that 1500 10 9 m 3 of sediment fill has been sequestered within the flood plain and delta plain since ca 7000 yr b p or about one third of the annual discharge the remaining load appears to be apportioned between the prograding subaqueous delta 1970 10 9 m 3 and transport to the deep sea bengal fan via a nearshore canyon modern 100 yr budget estimates based on short term accretion rates indicate a similar dispersal pattern and show that contemporaneous deposition continues within these disparate depocenters the roughly equal partitioning of sediment among flood plain shelf and deep sea settings reflects the respective influence of an inland tectonic basin a wide shelf and a deeply incised canyon system the findings also support new sequence stratigraphic models for these settings and indicate the important insight that modern river deltas can provide for ancient margin systems furthermore results affirm that values of riverine sediment flux to the oceans may be considerably overestimated by not accounting for loss to the flood plains downstream of the gauging stations

geochemical study of arsenic release mechanisms in the bengal basin groundwater 1 to investigate arsenic mobility in the bengal basin groundwater we sampled water wells and sediments throughout the region there are strong correlations among high levels of dissolved arsenic and iron ammonia and methane especially in samples from a single site laxmipur no linkage is seen between as and agricultural tracers such as phosphate the association of as and fe occurs because arsenic strongly adsorbs onto feooh particles in river water they flocculate with other fine grained particles at the freshwater saltwater transition zone subsequent bacterially mediated reduction of feooh in the clay releases the adsorbed arsenic weathering of as bearing mica plays a significant role in the as budget the correlated presence of as ch4 and nh4 in water supply wells is the result of diffusion out of organic rich clay into the more permeable zones arsenic is mainly released from recent sediments at 50 m depth deposited in the gbr floodplain as sea level rose throughout the holocene

crustal structure and tectonics of bangladesh new constraints from inversion of receiver functions abstract an understanding of the sedimentary and crustal structure of the bengal basin and of the tectonics deforming it remains elusive due to lack of seismic data from bangladesh taking advantage of recently available data from 11 seismic stations deployed over bangladesh we determine the crustal structure beneath each station using 2768 high quality receiver functions rfs inversion of the rfs reveals a highly variable thickness of the overlying sediments beneath the bengal basin the thickness of the sediments increases dramatically across the hinge zone of the early cretaceous passive margin from 3 to 17 km the thick sediments partly represent progradation of the continental margin due to the influx of clastic sediments from the himalayas the moho shallows across the region this reflects thinning of the crystalline crust from 38 km in the indian craton to 34 km at the hinge zone to 4 0 km s at lower crustal levels supports an influence of the kerguelen plume igneous activity during rifting we invert data for a station near the dauki fault which marks the southern boundary of the uplifted shillong plateau for dip and anisotropic effects our results show the dauki as a north dipping thrust fault at jaflong with a dip angle of 32 and strike 110 close to its surficial expression a strong anisotropy 15 and the sense of shear plunge 24 trend 79 compliment the dipping geometry and deformation seems to be related to the initiation of the dauki fault

petrologic case for eocene slab breakoff during the indo asian collision comment and reply comment kohn and parkinson 2002 1 proposed a conceptual model invoking eocene slab breakoff to explain the generation of eocene eclogites and miocene leucogranites in the himalaya and late eocene k rich magmas in southeastern tibet kohn and parkinson further argued that the slab breakoff and its

earth observation based approach for delineating geomorphology guided geoenvironmental zones and its utility in regional planning an analysis in parts of bengal basin west bengal india in this paper we have analysed active and passive earth observation data for identifying the different geomorphic sub provinces associated with coastal and fluvial geomorphic processes in the south western part of bengal basin for this purpose variations in the spectral response of these sub provinces in irs p6 liss iii data are enhanced using principal component pc and independent component ic methods to spatially delineate the geomorphic sub provinces and associated quaternary sediments it has been observed that the false colour composite derived using ic first second and third ics are effective in enhancing fluvial sub provinces while ic band 4 band 3 and band 2 are suitable to delineate coastal sub provinces on the other hand pc composite derived using band 4 band 3 and band 1 are suitable to delineate different quaternary sediments deposited within the spatial extent of geomorphic sub provinces fused image enhanced products of irs indian remote sensing satellite liss iii band and alos advanced land observing satellite palsar phased array l band synthetic aperture radar polarisation bands are used to supplement and complement the variations observed for each geomorphic province in optical data as radar interactions are sensitive to variation in moisture and surface cover although there is a strong synergy between the spatial extent of geomorphic sub provinces and the quaternary units in terms of spatial disposition different quaternary units are also found within the same geomorphic province therefore we have segmented each geomorphic sub provinces based on the spatial extent of the different quaternary deposits to identify few geoenvironmental termed here as geomorpho environmental zones in this regard eleven zones are identified each zone has been characterised based on geomorphic stability potentiality for agriculture practice scope for urban development and reclamation these geoenvironmental zones may provide important input to planners

the internal wave field in sau reservoir observation and modeling of a third vertical mode water withdrawal from mediterranean reservoirs in summer is usually very high because of this stratification is often continuous and far from the typical two layered structure favoring the excitation of higher vertical modes the analysis of wind temperature and current data from sau reservoir spain shows that the third vertical mode of the internal seiche baroclinic mode dominated the internal wave field at the beginning of september 2003 we used a continuous stratification two dimensional model to calculate the period and velocity distribution of the various modes of the internal seiche and we calculated that the period of the third vertical mode is 24 h which coincides with the period of the dominating winds as a result of the resonance between the third mode and the wind the other oscillation modes were not excited during this period

pathways of river water to the surface layers of stratified reservoirs this study uses tracer experiments to characterize the fate of a cold plunging river into a mediterranean reservoir under different stratification conditions three tracer experiments in lake beznar spain demonstrate that a significant fraction of the inflow may be quickly entrained into the surface mixed layer sml rather than flowing to lower depths we observe that a fraction and possibly all of river inflow entrains into the sml when the density current forms intrusions at the top of the metalimnetic layer intrusions near the base of the sml form due to lighter interfacial fluid at the top of the density current with intermediate properties between river reservior water when there is sufficient dilution of the cold inflow and abrupt density changes at the base of the sml consequently a layer of fluid splits from the denser part of the current and intrudes just below the sml while any remaining fluid flows to lower depths we parameterize this splitting behavior in terms of the transition richardson number ri12 2 1 0 1 where 0 1 and 2 are densities of the inflow the sml and the metalimnetic layer underneath respectively and is the total dilution through the plunge zone and subsequent entrainment splitting occurs when ri12 1 consistent with previous laboratory experiments we use this theory to predict the seasonal fate of river water during the stratification of lake beznar and identify the conditions that favor the formation of intrusions near the base of the sml

modeling circulation in lakes spatial and temporal variations the influence of spatial and temporal variations in wind forcing on the circulation in lakes is investigated using field data and the three dimensional estuary and lake computer model elcom applied to lake kinneret lake kinneret field data from six thermistor chains and eight wind anemometers deployed during july 2001 are presented internal wave motions are well reproduced by the numerical model when forced with a spatially uniform wind taken from a station near the lake center however simulated seiche amplitudes are too large especially vertical mode 2 and lead observations by 3 10 h for a 24 h period wave at different locations around the lake consideration of the spatial variation of the wind field improves simulated wave amplitude and phase error at all stations is reduced to less than 1 5 h this improvement is attributable to a better representation of the horizontally averaged wind stress and can be reproduced with a spatially uniform wind that has the same horizontally averaged wind stress as the spatially varying wind field however a spatially varying wind field is essential for simulating mean surface circulation which is shown to be predominantly directly forced by the surface layer averaged wind stress moment

structure of the crystalline basement in the west bengal basin india as determined from dss studies summary deep seismic sounding data were acquired in the west bengal basin india along two profiles i bishnupur palashi kandi along a line about 227 km long in the northsouth direction and ii taki arambagh along a line about 120 km long in the east west direction seismic refraction and wide angle reflection data were recorded by continuous profiling using two 60 channel digital seismic units dfs v with an 80 m geophone group interval and 4 ms sampling rate these data were interpreted in order to delineate the basement configuration the 2 d models of the seismic data both indicate a five layer velocity structure above the archaean crystalline basement 5 9 6 2 km s i a low velocity layer 4 0 km s is inferred immediately above the basement in the shelf region of the basin corresponding to the gondwana sediments upper carboniferous to lower triassic below the rajmahal traps upper jurassic to lower cretaceous of 4 6 to 4 8 km s velocity which is also confirmed from the nearby well data the results along the taki arambagh profile and the drilling results at the jaguli j 1 well are used to investigate whether gondwana sediments and the rajmahal traps exist in the deep part of the bengal basin an additional layer of velocity 5 2 5 3 km s delineated in the palashi kandi profile overlying the basement may correspond to the singhbhum group of rocks of the proterozoic a structural contour map of the basement prepared from the present results indicates a south easterly dip of the basement in general the depth of the basement on the stable shelf of the basin gently increases to about 8 km and dips steeply plunging to a maximum depth of 14 km in the deep basin no structural high that can be related to the calcutta gravity high is found in the basement around the hooghly river

mixing dynamics at the confluence of two large rivers undergoing weak density variations simulations of tracer experiments conducted with a three dimensional primitive equation hydrodynamic and transport model are used to understand the processes controlling the rate of mixing between two rivers ebro and segre with distinct physical and chemical properties at their confluence upstream of a meandering reservoir ribarroja reservoir mixing rates downstream of the confluence are subject to hourly scale oscillations driven partly by changes in inflow densities and also as a result of turbulent eddies that develop within the shear layer between the confluent rivers and near a dead zone located downstream of the confluence even though density contrasts are low at most o 10 1 kg m 3 difference among sources and almost negligible from a dynamic point of view compared with inertial forces they are important for mixing mixing rates between the confluent streams under weakly buoyant conditions can be of up to 40 larger than those occurring under neutrally buoyant conditions the buoyancy effects on mixing rates are interpreted as the result of changes in the contact area available for mixing distortion of the mixing layer for strong density contrasts though when the contact area between the streams becomes nearly horizontal larger density differences between streams will lead to weaker mixing rates as a result of the stabilizing effect of vertical density gradients

sedimentation and basin fill history of the neogene clastic succession exposed in the southeastern fold belt of the bengal basin bangladesh a high resolution sequence stratigraphic approach abstract the tertiary basin fill history of the bengal basin suffers from oversimplification the interpretation of the sedimentary history of the basin should be consistent with the evolution of its three geo tectonic provinces namely western northeastern and eastern each province has its own basin generation and sediment fill history related mainly to the indo burmese and subordinately to the indo tibetan plate convergence this paper is mainly concerned with facies and facies sequence analysis of the neogene clastic succession within the subduction related active margin setting oblique convergence in the southeastern fold belt of the bengal basin detailed fieldwork was carried out in the sitapahar anticline of the rangamati area and the mirinja anticline of the lama area the study shows that the exposed neogene succession represents an overall basinward progradation from deep marine through shallow marine to continental fluvial environments based on regionally correlatable erosion surfaces the entire succession 3000 m thick has been grouped into three composite sequences c b and a from oldest to youngest composite sequence c begins with deep water base of slope clastics overlain by thick slope mud that passes upward into shallow marine and nearshore clastics composite sequence b characteristically depicts tide dominated open marine to coastal depositional systems with evidence of cyclic marine regression and transgression repetitive occurrence of incised channel tidal inlet tidal ridge shoal tidal flat and other tidal deposits is separated by shelfal mudstone most of the sandbodies contain a full spectrum of tide generated structures e g herringbone cross bedding bundle structure mud couplet bipolar cross lamination with reactivation surfaces tidal bedding storm activities appear to have played a subordinate role in the mid and inner shelf region rizocorallium rosselia planolites and zoophycos are the dominant ichnofacies within the shelfal mudstone this paralic sedimentation of neogene succession in the study area can serve as a good point of reference for tide dominated regressive shelf depositional systems the top of the composite sequence b is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of continental fluvial depositional systems represented by composite sequence a in this composite sequence stacked channel bars of low sinuosity braided rivers gradually pass upsequence into high sinuosity meandering river deposits a sequence stratigraphic approach has been adopted to interpret the basin fill history with respect to relative sea level changes and to subdivide the rock record into several sequences and units systems tracts and parasequences based on identified bounding discontinuities such as transgressive erosion surface tes regressive erosion surface res marine flooding surface mfs and incised valley floor ivf this approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove vital to the oil companies engaged in exploration activities in the bengal basin it is strongly recommended here that the traditional lithostratigraphic classification of this part of the basin which is based on the assam stratigraphy be abandoned or at least revised a tentative allostratigraphic scheme is presented and it is suggested that to formalize the scheme further study both surface and subsurface is needed

a comparative study on hydrochemical evolution and quality of groundwater in coastal areas of thailand and bangladesh abstract in recent years thailand and bangladesh have been drawing more attention due to groundwater pollution but there is rarely a comparative study on hydrochemical evolution and quality assessment of groundwater in the areas in this paper end member mixing and gibbs model are used to study the evolution of groundwater hydrochemistry based on the 74 water samples due to the seawater intrusion the evolution path of groundwater in bangkok bay is ca mg hco3 na ca mg hco3 and na ca mg cl na cl which led to increased na k and hco3 concentrations in groundwater and enrichment of fluorine in groundwater caused fluorine pollution in contrast although the evolution path of groundwater in southern bangladesh shows a similar pattern as ca mg hco3 na ca mg hco3 and ca mg cl na cl the principal influence factors are residential agricultural and industrial pollutants the scope of seawater intrusion is only limited to estuary areas of the southern estuary whereas most groundwater is received recharge of polluted surface water which resulted in the enrichment of fe and mn in addition compared with shallow groundwater deep groundwater is updated slowly and evolved to higher degree and is in the na ca mg hco3 and ca mg cl stage for improving groundwater conditions in southern bangladesh enhancing the supply capacity of surface potable water and significantly reducing groundwater exploitation are priority strictly controlling the generation storage and treatment of domestic waste industrial and agricultural pollutants are also important in bangkok bay reducing the groundwater exploitation and dispersing the exploitation area is required

the seasonal evolution of wind internal wave resonance in lake kinneret data from a thermistor chain and wind sensor collected over an annual stratification cycle in lake kinneret israel during 2000 were used to investigate the seasonal evolution of wind internal wave resonance internal wave periods determined from an analytical model were compared with observations and we show that resonance during 2000 occurred during three distinct times of the year at the onset of stratification march during the heating phase june and during the cooling phase november in all cases resonance was between the wind and the dominant radial azimuthal and vertical mode one cyclonic kelvin wave previously observed in lake kinneret internal waves are a ubiquitous feature of lakes existing because of wind forcing acting on a stratified water column where the stratification is generally due to temperature wind applied to the surface can cause a surface setup of water at the downwind end this pressure force is balanced by a tilting of the metalimnion in the opposite sense that is downward at the downwind end spigel and imberger 1980 when the wind forcing relaxes the water surface oscillates a barotropic wave as does the metalimnion a baroclinic wave the two motions can effectively be considered to be decoupled monismith 1985 because they form orthogonal solutions to the wave equation it is the internal baroclinic waves that are of interest in the present study because they provide much of the energy and motion for biogeochemical processes in lakes in large lakes the picture becomes complicated because the effects of the earth s rotation begin to influence the frequency and structure of the internal waves internal gravity waves in large lakes take the form of cyclonic kelvin and or poincare or anticyclonic poincare waves antenucci et al 2000 antenucci and imberger 2001 where the wave crests rotate around the perimeter of the lake basin antenucci and imberger 2001 showed that the frequency as well as the ratio of potential kinetic energy of internal gravity waves in approximately circular or elliptic lakes was dependent on the aspect ratio radial mode azimuthal mode and burger number the burger number is defined asc lf where c is the phase speed of internal waves in a nonrotating system l is some length scale characterizing the basin width and f is the coriolis parameter for a small burger number rotation dominates over gravity and waves approach the oceanic case known as inertial oscillations in which the motion is dominated by the kinetic energy signal for a large acknowledgments we acknowledge the field support and technical staff from the kinneret limnological laboratory kll for providing assistance particularly tzahi rosenberg for maintaining the lake station and helping in the transfer of data from kll to the centre for water research and peter yeates who provided the time series of the lake number

the paleogene record of himalayan erosion bengal basin bangladesh a knowledge of himalayan erosion history is critical to understanding crustal deformation processes and the proposed link between the orogen s erosion and changes in both global climate and ocean geochemistry the most commonly quoted age of india asia collision is 50 ma yet the record of paleogene himalayan erosion is scant either absent or of low age resolution we apply biostratigraphic petrographic geochemical isotopic and seismic techniques to paleogene rocks of the bengal basin bangladesh of previously disputed age and provenance our data show that the first major input of sands into the basin in the 1 km thick deltaic barail formation occurred at 38 ma our biostratigraphic and isotopic mineral ages date the barail formation as spanning late eocene to early miocene and the provenance data are consistent with its derivation from the himalaya but inconsistent with indian cratonic or burman margin sources detrital mineral lag times show that exhumation of the orogen was rapid by 38 ma the identification of sediments shed from the rapidly exhuming southern flanks of the eastern central himalaya at 38 ma provides a well dated accessible sediment record 17 myr older than the previously described 21 ma sediments in the foreland basin in nepal discovery of himalayan detritus in the bengal basin from 38 ma 1 resolves the puzzling discrepancy between the lack of erosional evidence for paleogene crustal thickening that is recorded in the hinterland 2 invalidates those previously proposed evidences of diachronous collision which were based on the tenet that himalayan derived sediments were deposited earlier in the west than the east 3 enables models of himalayan exhumation e g by mid crustal channel flow to be revised to reflect vigorous erosion and rapid exhumation by 38 ma and 4 provides evidence that rapid erosion in the himalaya was coincident with the marked rise in marine 87sr 86sr values since 40 ma whether 38 ma represents the actual initial onset of vigorous erosion from the southern flanks of the east central himalaya or whether older material was deposited elsewhere remains an open question

internal seiche climatology in a small lake where transversal and second vertical modes are usually observed from the analysis of thermistor chain data recorded in a small clongatcd lake compared with the results of numerical simulations second vertical modes of the internal scichcs are shown to be active during more than twothirds of the whole stratifcd period although no rcsonancc with the wind is observed on the other hand due to the wind pattern transversal seiches mainly the first vertical mode are also usually excited and they somctimcs dominate at the end of the stratified period however when the thermocline is very sharp first vertical modeslongitudinal and transversal are the only ones that are excited finally due to the wind a forced oscillation of the whole lake is excited after mortimer 1952 who brought up to date both the earlier descriptions of internal seiches in loch ness watson 1904 and their further interpretations schmidt 1908 wedderburn 1907 wedderburn and young 1915 internal seiches have been recognized as an everyday feature of movement in stratified water systems subject to wind action marmorino and mortimer 1978 hutter 1993 korgen 1995 now the relevance of the internal seiches is widely documented the structure of the horizontal velocities in the water column may determine the regions where the shear is strong enough to create instabilities leading to turbulence that might facilitate the resuspension of bottom sediments wiegand and chamberlain 1987 the periodic vertical displacements of the internal seiches can advect the suspended algal cells forcing them to be exposed to variations in light intensity webster 1990 gaedke and schimmele 1990 and internal scichcs greatly support the flux path of the transference of energy from the wind to the hypolimnion imberger 1994

bathymetry stratification and internal seiche structure internal seiches play a significant role in a broad range of physical chemical and biological processes in lakes a detailed assessment of the impact of seiching requires an understanding of seiche structure which is determined by bathymetry and stratification in this study internal seiche solutions are evaluated for arbitrary bathymetry and continuous stratification using a two dimensional numerical model formulated in terms of a stream function the model produces a finite set of linear internal wave eigenmodes and allows the computation of the complete velocity field over a grid associated with each seiche mode several idealized configurations of continuous stratification and variable bathymetry are used to explore the effect of nonuniform systems on internal wave structure in particular we focus on bed velocity distribution and the resulting potential impact on scalar fluxes sediment transport and internal wave damping model results are also compared to thermistor chain data collected in the upper mystic lake uml winchester massachusetts using an idealized description of the uml bathymetry and density profiles which emulate the seasonal variation of stratification in the lake the evolution of bed velocities during the autumnal breakdown in stratification is assessed providing insight into the fate of the contaminants entering the lake

a base level stratigraphic approach to determining holocene subsidence of the ganges meghna brahmaputra delta plain abstract relative sea level history which is the result of the combined effects of land subsidence sediment supply and absolute sea level history may be reconstructed from preserved sediment thicknesses however variations in the preserved sediment thicknesses between different sedimentary environments strongly limit the accuracy of this type of geological approach particularly in fluvial channelized systems such as delta plains to address this we apply three different and independent stratigraphic approaches to the case of the ganges brahmaputra meghna delta gbmd our approach has been made possible by a broad dataset of geological archives we have collected which includes more than 400 hand drilled stratigraphic wells 198 radiocarbon ages and river seismic reflection data 255 km of high resolution multichannel seismic images the seaward gradient of accommodation and the limit of the subsiding delta plain are estimated assuming that the delta is near or at the base level which is considered to be the relative sea level first a statistical analysis of the variability of preserved sediment thicknesses is used to derive the average pattern of accommodation from the holocene isopach secondly the preserved sediment thicknesses are analyzed by geomorphotectonic domains to estimate an average pattern of accommodation thirdly the burial history of the seismically imaged last glacial incision of the brahmaputra river is reconstructed results suggest that the variability of preserved sediment thicknesses can be up to 35 in a delta plain between river channel and flood plain deposits for the same relative sea level history taking this variability into consideration the holocene relative sea level history of the gbmd and the most likely pattern of subsidence are determined results provide evidence of moderate holocene subsidence over the delta gently increasing seaward from

the seasonal evolution of high vertical mode internal waves in a deep reservoir the causal mechanism and seasonal evolution of the internal wave field in a deep warm monomictic reservoir are examined through the analysis of field observations and numerical techniques the study period extends from the onset of thermal stratification in the spring until midsummer in 2005 during this time wind forcing was periodic with a period of 24 h typical of land sea breezes and the thermal structure in the lake was characterized by the presence of a shallow surface layer overlying a thick metalimnion typical of small to medium sized reservoirs with deep outtakes basin scale internal seiches of high vertical mode ranging from mode v3 to v5 were observed in the metalimnion the structure of the dominant modes of oscillation changed as stratification evolved on seasonal timescales but in all cases their periods were close to that of the local wind forcing i e 24 h suggesting a resonant response nonresonant oscillatory modes of type v1 and v2 became dominant after large frontal events which disrupted the diurnal periodicity of the wind forcing

chemical evolution in the high arsenic groundwater of the huhhot basin inner mongolia pr china and its difference from the western bengal basin india elevated as concentrations in groundwater of the huhhot basin hb inner mongolia china and the western bengal basin wbb india have been known for decades however few studies have been performed to comprehend the processes controlling overall groundwater chemistry in the hb in this study the controls on solute chemistry in the hb have been interpreted and compared with the well studied wbb which has a very different climate physiography lithology and aquifer characteristics than the hb in general there are marked differences in solute chemistry between hb and wbb groundwaters stable isotopic signatures indicate meteoric recharge in the hb in a colder climate distant from the source of moisture in comparison to the warm humid wbb the major ion composition of the moderately reducing hb groundwater is dominated by a mixed ion ca na hco3 cl hydrochemical facies with an evolutionary trend along the regional hydraulic gradient molar ratios and thermodynamic calculations show that hb groundwater has not been affected by cation exchange but is dominated by weathering of feldspars allitization and equilibrium with gibbsite and anorthite mineral weathering and mobilization of as could occur as recharging water flows through fractured argillaceous metamorphic or volcanic rocks in the adjoining mountain front areas and deposits solutes near the center of the basin in contrast wbb groundwater is ca hco3 dominated indicative of calcite weathering with some cation exchange and silicate weathering monosiallitization

seismic stratigraphy and cenozoic evolution of west sumatra forearc basin late tertiary neogene sediments deposited in the sunda forearc reveal the stratigraphic and structural evolution of this active plate margin a dense grid of multichannel seismic reflection data and information from 17 exploratory wells were used to establish a detailed seismic stratigraphic framework of the forearc region from 0 to 6 n several important tectonic cycles are recognized paleogene orogeny neogene subsidence and late tertiary tectonism superimposed on these regional tectonic events are three major transgressive regressive cycles of sedimentation related to changes in sea level and provenance paleogene and older metasedimentary and metamorphic rocks comprise basement beneath the landward inner margin of the forearc basin both basement rocks and lower tertiary sedimentary rocks were deformed and eroded approximately 25 to 30 ma the continental shelf was exposed to subaerial erosion and basin deposits were restricted offshore coincident with a worldwide lowstand of sea level in the oligocene the paleogene orogeny probably occurred prior to the erosional event that cut the prominent angular unconformity on the shelf the neogene history of forearc basin development is characterized by subsidence and nearly continuous sedimentation a basal transgression began in latest oligocene time and culminated in the middle miocene alternating sequences of limestone and shale comprise two second order cycles of sedimentation that are superimposed on an overall transgressive event a major regressive sequence followed in the pliocene owing to an influx of siliciclastic clay silt and sand derived from sumatra sedimentation rates were high and large volumes of terrigenous material were deposited in deltaic systems on the shelf the shelf slope break prograded basinward nearly 10 km 6 mi through lateral accretion and aggradation during a relative highstand or stillstand of sea level

collision of the ganges brahmaputra delta with the burma arc implications for earthquake hazard abstract we take a fresh look at the topography structure and seismicity of the ganges brahmaputra delta gbd burma arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential the gbd the world s largest delta has been built from sediments eroded from the himalayan collision these sediments prograded the continental margin of the indian subcontinent by 400 km forming a huge sediment pile that is now entering the burma arc subduction zone subduction of oceanic lithosphere with 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land the prism starts at an apex south of the gbd shelf edge at 18 n and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit the front of the prism is blind buried by the gbd sediments thus the deformation front extends 100 km west of the surface fold belt beneath the comilla tract which is uplifted by 3 4 m relative to the delta this accretionary prism has the lowest surface slope of any active subduction zone the gradient of the prism is only 0 1 rising to 0 5 in the forearc region to the east this low slope is consistent with the high level of overpressure found in the subsurface and indicates a very weak detachment since its onset the collision of the gbd and burma arc has expanded westward at 2 cm yr and propagated southwards at 5 cm yr seismic hazard in the gbd is largely unknown intermediate size earthquakes are associated with surface ruptures and fold growth in the external part of the prism however the possibility of large subduction ruptures has not been accounted for and may be higher than generally believed although sediment clogged systems are thought to not be able to sustain the stresses and strain weakening behavior required for great earthquakes some of the largest known earthquakes have occurred in heavily sedimented subduction zones a large earthquake in 1762 ruptured 250 km of the southern part of the gbd suggesting large earthquakes are possible there a large but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism and is the only known candidate for a rupture of the plate boundary along the subaerial part of the gbd burma arc collision zone

δ13c of organic carbon in the bengal fan source evolution and transport of c3 and c4 plant carbon to marine sediments carbon isotopic measurements on organic carbon oc in sediment cores from the bengal fan odp leg 116 show a dramatic 10 increase beginning ca 7 ma ago and a rapid decrease after 0 9 ma these shifts reflect changes in the mixing ratio of terrigenous carbon derived from c3 and c4 plants the rapid increase in 13c of bengal fan oc at 7 ma shows that late miocene expansion of c4 plants already documented in the siwaliks was widespread over all the himalayan foreland after 7 ma relations of 13c with sedimentological parameters show that c4 plants dominate in the foreland whereas c3 plants remain abundant in the mountain range variations in the source of the sediments and of the oc appear to be sensitive to climate hydrologic conditions in the basin major changes in the isotopic composition of the carbon flux in one of the worlds largest river systems modify the isotopic budgets of both marine dissolved carbon and the sedimentary carbon mass

evidence of high vertical wave number behavior in a continuously stratified reservoir boadella spain high vertical wave number modes clearly dominate the internal wave field during the stratification period in boadella reservoir in northeast spain in this period the extraction of hypolimnetic water due to summer irrigation brought the surface level down by 6 m in one month and the epilimnetic water progressively occupied the whole water column the temperature profile with the exception of a few meters at the surface layer presented an almost constant temperature gradient of about 0 7 c m the period of the main vertical mode is 24 h with an amplitude of around 1 m thermistor chain records and meteorological data allow us to deduce that this mode is at least a third vertical mode forced by the wind which normally has a typical periodicity of 24 h however when the wind changes direction from south to north the circulation cells developed due to this forced nonstationary oscillation are destroyed when this occurs the bulk richardson number is ri db 1 similar vertical structures as a response to wind forcing should be expected in similar systems although this has not been reported in the literature

groundwater flow system in bengal delta bangladesh revealed by environmental isotopes a total number of 328 groundwater samples are analysed to evaluate the groundwater flow systems in bengal delta aquifers bangladesh using environmental isotope 2h 18o 13c 3h and 14c techniques a well defined local meteoric water line lmwl 2h 7 7 18o 10 7 is constructed applying linear correlation analyses to the monthly weighted rainfall isotopic compositions 18o and 2h the 18o and 2h concentrations of all groundwater samples in the study area are plotted more or less over the lmwl which provides compelling evidence that all groundwaters are derived from rainfall and floodwater with a minor localized evaporation effects for the shallow groundwaters tritium concentration is observed in 40 samples out of 41 with values varying between 0 3 and 5 0 tu which represents an evidence of young water recharge to the shallow and intermediate aquifers a decreasing trend of 14c activity is associated with the heavier 13c values which indicates the presence of geochemical reactions affecting the 14c concentration along the groundwater flow system both vertical and lateral decrease of 14c activity toward down gradient show the presence of regional groundwater flow commencing from the unconfined aquifers which discharges along the coastal regions finally shallow intermediate and deep groundwater flow dynamics has revealed in the bengal delta aquifers bangladesh

near surface lithology and spatial variation of arsenic in the shallow groundwater southeastern bangladesh this study investigated the relationship between near surface lithology and the spatial variability of as concentrations using sediment grain size analysis and electromagnetic induction survey in the southeast bangladesh it has been observed that the aquifers overlain by finer sediments have higher concentrations of as in groundwater whereas as concentrations are remarkably low in aquifers having permeable sandy materials or thinner silt clay layer at the surface the near surface lithology acts as a controlling factor for spatial distributions of groundwater as within the very shallow depths 15 m shallow alluvial aquifers can provide low as drinking water in many areas of the country when tube wells are properly installed after investigation of the overlying near surface sediment attributes and hydraulic properties

characterisation of organic matter in a shallow reducing arsenic rich aquifer west bengal abstract elevated arsenic in groundwaters extensively exploited for irrigation and drinking water in west bengal and bangladesh is causing serious impacts on human health a key mechanism for the genesis of arsenic as in these waters is microbially mediated reductive transformation of arsenic bearing fe iii hydrated oxides the role of organic c in this process whether from in situ organic matter om i e om from within the sediment or from other sources is widely recognised despite this there is a paucity of data about the characteristics of om in these as rich aquifers extraction and analysis of the polar and apolar fractions from seven different sediments from a known groundwater as hotspot in west bengal revealed om characteristic of the original terrestrial depositional environment however this was overprinted by abundant hydrocarbons with thermally mature e g petroleum distributions these hydrocarbons included abundant high molecular weight n alkanes and unresolved complex mixtures ucms as well as thermally mature distributions of hopanes and steranes additionally at certain depths surface sands 8 and 13 m deeper sands 25 m the om appeared to be biodegraded with the preferential removal of petroleum sourced n alkanes suggesting that indigenous microbes within the aquifer can utilise this carbon source the presence of this previously unreported source of bioavailable organic carbon is of importance as it has the potential to promote microbial activity and subsequent as release in the aquifers

physical controls on the spatial evolution of a dinoflagellate bloom in a large lake a set of satellite images of chlorophyll a concentration for lake kinneret israel from the period february to april 2007 captured the temporal evolution of horizontal patchiness that developed during the typical spring bloom of the dinoflagellate peridinium gatunense narrow bands of high concentration located adjacent to the shoreline at the start of the bloom were followed by progressive propagation of this high concentration region from the shoreline toward the center of the lake as the bloom proceeded a three dimensional 3 d hydrodynamic numerical model and an analytical model for a flat bottomed elliptical basin together explain the observed phenomenon the spatial structure of kelvin waves which were demonstrated to most likely be present in february and april due to resonance produced regions of high velocity gradient contributing to horizontal dispersion close to the shoreline this region of high dispersion was narrow and close to the shoreline at the start of the bloom when the burger number was relatively small but it widened over the course of the season as the burger number increased due to increasing stratification in addition an increase in the dispersion rate with time in the lake interior was inferred from numerical simulation due to increasing wind speeds and a thinner surface layer physical processes in this case horizontal dispersion as a result of both kelvin waves and wind can play an important role in governing the dynamics and spatial evolution of dinoflagellate blooms in lakes spatial heterogeneity of phytoplankton is a ubiquitous occurrence in lakes coastal waters and the open ocean phytoplankton patchiness can be defined by localized abruptly elevated concentrations phytoplankton patches may occur at a broad range of spatial scales and such

a time slice approach to the history of cenozoic sedimentation in the indian ocean abstract results from drilling at over 100 sites have been combined with a new plate tectonic model to produce a series of palinspastic maps which represent snap shots of the changing patterns of sediment accumulation in the indian ocean through the cenozoic the maps clearly delineate the main sedimentary depocenters and show how the sedimentary record has responded on an oceanwide scale to major changes in paleogeography tectonics paleocirculation and or climate paleogene sediment accumulation rates were generally low with significant accumulation in only limited areas suggesting weak ocean circulation and stable well stratified conditions by contrast the vigorous thermohaline circulation of the neogene resulted in substantial widespread sedimentation accumulation rates were generally high and displayed a wide range of values following rapid growth of the east antarctic ice sheet in the mid miocene carbonate sedimentation has been progressively restricted to the shallow ridges and plateaus of the western indian ocean but accumulates there with locally high rates the link between tectonics and terrigenous sedimentation is exemplified by the major deep sea fans off east africa zambezi fan and india indus and bengal fans which developed in response to uplift and erosion beginning in late oligocene time in the east african rift system and the himalayas respectively although maps from only five widely spaced times are shown the methodology is such that a continuous series of maps at intervals as close as 1 m y can readily be generated ultimately the combination of such time slice maps with the results of detailed time series studies will provide a powerful tool for understanding past oceanic environments

seasonal evolution of the basin scale internal wave field in a large stratified lake the response of the water column to varying conditions of stratification and wind forcing was investigated in lake kinneret israel using data collected from thermistor chains and acoustic doppler current profilers during 1997 and 1998 the strong daily sea breeze was found to generate a vertical mode 1 internal kelvin wave and basin scale internal poincarewaves of vertical modes 1 2 and 3 the kelvin wave the dominant component of the internal wave field was responsible for alongshore velocities in the nearshore regions in the upwind nearshore regions velocities were dominated by the forced response to the wind and were cross shore in nature in the lake interior the kelvin wave effect on the horizontal velocity field was minor compared to the higher vertical mode poincarewaves the kelvin wave is shown to exist in resonant and nonforced states with the wind whereas the vertical mode 1 poincarewave energy remained relatively constant despite large variability in the forcing condi tions the energy in the higher mode poincarewaves varied greatly both on daily and seasonal timescales the results demonstrate that the wind energy forces multiple basin scale internal wave modes and that prior motion in the water column must be considered when determining the subsequent internal wave response in periodically forced systems lake kinneret israel has long been an important regional resource providing a substantial supply of drinking water as well as supporting a commercial fishery the monomictic lake is forced daily during the summer by a powerful sea breeze with stratification and wind patterns showing little variation from year to year these factors along with a fa vorable bathymetric pattern combine to make lake kinneret an ideal place for the field investigation of large scale inter nal waves and related phenomena such as the benthic bound ary layer horizontal transport and vertical mixing processes in addition to providing substantial insight into the local pro cesses and hence influencing local management study of the lake also provides fundamental knowledge of the dynam ics of enclosed basins on a geophysical scale during summer the internal rossby radius in lake kin neret is typically one half the basin width hence large scale motion in the lake will be affected by the earth s rotation and basin scale internal waves may be of topographic ross by kelvin or poincareform topographic rossby waves are sub inertial frequency oscillations that exist only in rotating fluids with variable bathymetry mysak et al 1985 stocker and hutter 1987 and these waves result from changes in depth affecting the relative vorticity of fluid elements in the absence of vertical boundaries wavelike solutions to the lin ear inviscid equations of motion for a flat bottomed region of homogeneous fluid with a constant rate of rotation are 1

geologic geomorphic and hydrologic framework and evolution of the bengal basin india and bangladesh abstract the bengal basin the largest fluvio deltaic sedimentary system on earth is located in bangladesh and three eastern states of india sediment accumulates in the basin from the ganges brahmaputra and meghna gbm river systems and is dispersed into the bay of bengal forming the largest submarine fan in the world the basin is located in the himalayan foreland at the junction of the indian eurasian and burmese plates the basin is bounded by the indian craton on the west and the indo burmese fold belts on the east it can be broadly divided into a stable shelf and a foredeep separated by a deep seismic hinge zone basin sediments overlie gondwanan basement and vary in thickness from a few kilometers on the stable shelf to more than 16 km in the foredeep the basin was initiated at the breakup of gondwanaland in the late mesozoic and evolved through the formation of the proto gbm delta to the present delta starting around 10 5 ma the stratigraphy of the different parts of the basin differs considerably because of contrast in depositional history within the several sub basins that were produced by intra plate tectonic activities associated with ongoing himalayan orogeny the present day geomorphology is dominated by the extensive holocene gbm floodplain and delta the vertical succession of the deltaic plain can be classified into five units on the basis of differences in grain size which reflect differing depositional environments the initiation of the modern gbm delta at the onset of the pleistocene glacial maximum and its evolution to the present configuration are intricately related to holocene fluvio dynamic processes eustatic sea level changes and tectonic movements the sedimentology and mineralogy of the different parts of the basin reflect differences in sediment provenance the mineralogy is dominated by detrital quartz some feldspar and minor amounts of carbonates illite and kaolinite are the main clay minerals the basin has profuse groundwater resources but the architecture of the aquifers is not yet well resolved different classification schemes have been proposed on the basis of lithology regional groundwater flow follows a low hydraulic gradient from north to south but pumping for irrigation during the past several decades appears to have severely distorted regional flow recharge occurs primarily during the monsoon season groundwater chemistry is dominated by anoxic ca hco 3 type water with relatively high concentrations of fe mn and as carbonate dissolution silicate weathering feooh reduction and mixing with saline water are the primary processes controlling hydrochemistry

an overview of the sedimentary geology of the bengal basin in relation to the regional tectonic framework and basin fill history abstract the bengal basin in the northeastern part of indian subcontinent between the indian shield and indo burman ranges comprises three geo tectonic provinces 1 the stable shelf 2 the central deep basin extending from the sylhet trough in the northeast towards the hatia trough in the south and 3 the chittagong tripura fold belt due to location of the basin at the juncture of three interacting plates viz the indian burma and tibetan eurasian plates the basin fill history of these geo tectonic provinces varied considerably precambrian metasediments and permian carboniferous rocks have been encountered only in drill holes in the stable shelf province after precambrian peneplanation of the indian shield sedimentation in the bengal basin started in isolated graben controlled basins on the basement with the breakup of gondwanaland in the jurassic and cretaceous and northward movement of the indian plate the basin started downwarping in the early cretaceous and sedimentation started on the stable shelf and deep basin and since then sedimentation has been continuous for most of the basin subsidence of the basin can be attributed to differential adjustments of the crust collision with the various elements of south asia and uplift of the eastern himalayas and the indo burman ranges movements along several well established faults were initiated following the breakup of gondwanaland and during downwarping in the cretaceous by eocene because of a major marine transgression the stable shelf came under a carbonate regime whereas the deep basinal area was dominated by deep water sedimentation a major switch in sedimentation pattern over the bengal basin occurred during the middle eocene to early miocene as a result of collision of india with the burma and tibetan blocks the influx of clastic sediment into the basin from the himalayas to the north and the indo burman ranges to the east rapidly increased at this time and this was followed by an increase in the rate of subsidence of the basin at this stage deep marine sedimentation dominated in the deep basinal part while deep to shallow marine conditions prevailed in the eastern part of the basin by middle miocene with continuing collision events between the plates and uplift in the himalayas and indo burman ranges a huge influx of clastic sediments came into the basin from the northeast and east throughout the miocene the depositional settings continued to vary from deep marine in the basin to shallow and coastal marine in the marginal parts of the basin from pliocene onwards large amounts of sediment were filling the bengal basin from the west and northwest and major delta building processes continued to develop the present day delta morphology since the cretaceous architecture of the bengal basin has been changing due to the collision pattern and movements of the major plates in the region however three notable changes in basin configuration can be recognized that occurred during early eocene middle miocene and plio pleistocene times when both the paleogeographic settings and source areas changed the present basin configuration with the ganges brahmaputra delta system on the north and the bengal deep sea fan on the south was established during the later part of pliocene and pleistocene and delta progradation since then has been strongly affected by orogeny in the eastern himalayas pleistocene glacial activities in the north accompanied sea level changes in the bay of bengal

the significance of large sediment supply active tectonism and eustasy on margin sequence development late quaternary stratigraphy and evolution of the ganges brahmaputra delta abstract borehole data from the ganges brahmaputra delta system unveil an intriguing late quaternary history controlled by immense sediment discharge tectonics and eustasy sea level first intersected a major portion of the lowstand surface 10 000 11 000 cal yr bp marking the onset of sediment trapping and delta growth despite rapid sea level rise 1 cm yr sediment load was sufficient to maintain relative shoreline stability during this time by 7000 cal yr bp accommodation was quickly infilled with slowed sea level rise leading to upstream channel migration and widespread dispersal of sands this forced coastal progradation along the western delta front where the ganges was situated and much of the river s fine grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf concurrently brahmaputra sediments were sequestered to an inland tectonic basin thus starving the adjacent shoreline and leading to transgression along the eastern delta by 5000 cal yr bp the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river mouth estuary present delta configuration appears to have developed by 3000 cal yr bp evolution of the tectonically active ganges brahmaputra g b system reveals important similarities and differences with other deltas overall facies succession follows that of basic models progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain however the timing thickness and controls of these deposits differ immense sediment discharge from the ganges and brahmaputra rivers allowed initial delta growth 2000 3000 yr earlier than the global average despite debouching onto a canyon incised high energy margin subsequently a thick 50 m transgressive systems tract was formed during the early holocene phase of delta aggradation contrasting with extensive shoreline retreat documented along most margins highstand progradation of coastal and shelf sequences ensued by the middle holocene but subsiding inland basins also favored accumulation of a thick to 40 m highstand sequence in the lower floodplains unlike many other delta systems subsidence in the subaerial g b delta is not dominated by compaction but rather by plate driven tectonic processes that generate rates up to 4 mm year overall the huge sediment load tectonic subsidence major seismic events and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta shelf and deep sea bengal fan throughout the late quaternary

a paleo brahmaputra subsurface lithofacies analysis of miocene deltaic sediments in the himalayan bengal system bangladesh abstract the bengal foreland basin contains a succession of up to 16 km of dominantly deltaic deposits eroded from the eastern himalayas and the indo burman ranges and carried by major river systems similar to the present day ganges and brahmaputra analysis of electric logs and core descriptions acquired during oil and gas exploration in bangladesh allows construction of lithofacies maps which constrain depositional patterns of miocene strata compilations of sand thickness and sand shale ratio of the miocene surma group show that lower to middle miocene strata of the bhuban formation accumulated in a large elongate trough sand thickness and percentage both decrease markedly away from this depocenter which describes a large scale bend running initially westward from rashidpur northeast bengal basin and curving southward toward the bengal fan middle to upper miocene strata of the boka bil formation show a similar geographic trend in deposition of coarsest and thickest sediment but the major depocenter had shifted northward relative to that of the bhuban formation by some 30 km passing near beani bazar these trends suggest that deltaic deposits of the surma group filled the sylhet trough of the northeast bengal basin from the east published seismic data from western bangladesh show that additional large channels also contributed materials to the bengal basin from the northwest during the late miocene but these channels resulted in very little accumulation in the northwestern part of the basin probably due to restricted subsidence of underlying continental crust this study suggests that there was a major drainage system similar to the modern brahmaputra river during miocene time which carried orogenic sediments eroded from the uplifted terranes of the eastern himalayas and indo burman ranges to the eastern bengal delta

spatial structure of the dominant basin scale internal waves in lake kinneret field data and numerical simulations were used to investigate the effects of basin shape continuous stratification and rotation on the three dimensional structure of the dominant natural basin scale internal wave modes in lake kinneret a stratified lake large enough so that the earth s rotation influences the wave motion the structure of the modes was inferred from power spectral density of measured and simulated isotherm vertical displacements and from rotary power spectral density of isopycnal velocities obtained from numerical simulations the shape of the lake at the level of the thermocline in conjunction with the dispersion relationship determines the horizontal configuration of the natural modes the dominant response to wind forcing was an azimuthal and vertical mode 1 kelvin wave with a natural period of 22 6 h that propagated around the entire basin the second most dominant response was a vertical mode 1 wave with a natural period close to 10 5 h and composed of two counter rotating

impact of glacial lake paleofloods on valley development since glacial termination ii a conundrum of hydrology and scale for the lowstand brahmaputra jamuna paleovalley system

observations of the surface boundary structure within the 23 may 2007 perryton texas supercell abstractin situ data collected within a weakly tornadic high precipitation supercell occurring on 23 may 2007 near perryton texas are presented data were collected using a recently developed fleet of 22 durable rapidly deployable probes dubbed sticknet as well as four mobile mesonet probes kinematic and thermodynamic observations of boundaries within the supercell are described in tandem with an analysis of data from the shared mobile atmospheric research and teaching radar observations within the rear flank downdraft of the storm exhibit large deficits of both virtual potential temperature and equivalent potential temperature with a secondary rear flank downdraft gust front trailing the mesocyclone a primarily thermodynamic boundary resided across the forward flank reflectivity gradient of the supercell this boundary is characterized by small deficits in virtual potential temperature coupled with positive perturbations of equivalent potential temperature the opposing thermodynamic perturbatio

elemental and biomarker characteristics in a pleistocene aquifer vulnerable to arsenic contamination in the bengal delta plain india an elevated level of arsenic as in the indo gangetic delta plain aquifers has been reported since the 1990s organic matter om present in groundwater and aquifer sediments supports the microbial communities in these aquifers during installation of a drinking water well 26 sediment intervals of 6 m each were retrieved up till 156 m from a pleistocene brown sand aquifer bsa grain size distribution sequential extraction of metals and total extractable lipids were analyzed in each sample these parameters were statistically correlated in order to establish relationship between the physical vs inorganic and organic characteristics and how these properties affected the distribution of as in bsas the aquifer sediments consisted of medium to coarse sand except the surface sediments and those at the bottom of the well which had high clay and slit content arsenic as concentration in sediments ranged from 2 to 21 mg kg and indicated a strong correlation with grain size arsenic was mostly associated with crystalline oxides and silicate rich minerals arsenic showed significant correlation with fe in all fractions and suggests presence of pyrite bound as bearing minerals in these sediments the diagnostic sedimentary lipid biomarkers indicated presence of compounds derived from vascular plants and microbial cell wall this inference was supported by various diagnostic lipid ratios the biomarkers were abundant in surface and deeper layers which had high clay and silt content the bsa sediments indicated preferential preservation of n alkanes over other functional compounds which were more reactive and subject to degradation the thick clay layer at 132 156 m contained visible plant fragments and om in this layer indicated preferential preservation of organic carbon most likely due to the absence of specific microbial communities that degraded these compounds and mobilized as statistical analyses indicated the influence of selective inorganic and organic components as fe and fatty acids controlling the co distribution of various inorganic and organic components in the aquifer

sedimentology of the bengal shelf bangladesh comparison of late miocene sediments sitakund anticline with the modern tidally dominated shelf abstract characterisation of latest miocene facies of the bay of bengal reveals a range of tidal and fluvially dominated sediments that represent sedimentation on a broad shallow shelf that was strongly influenced by tidal and seasonal cyclicity these tidally influenced sediments are excellently exposed within growth anticlines of the westward migrating chittagong tripura fold belt ctfb and permit detailed study of the late miocene bengal shelf for comparison with the modern subaqueous deltaic sediments at the outcrop scale facies associations record a subtidal shelf migrating intertidal subtidal bedforms tidal and incised channels the subtidal shelf comprises extensive sand sheets interbedded with thick clay horizons while the migrating intertidal subtidal bedforms exhibit a decrease in bedform size from large dunes to small ripple scale cross stratification that reflect a decrease in current velocity and sand silt grade sediment supply as the bedforms migrated rootlet horizons within these intertidal subtidal bedforms may represent a lower delta plain environment the tidal channel facies consists of metre scale sandstone beds that are rich in locally derived clay clasts with up to 0 5 m of incision into a rhizolith horizon below incised channel facies records a coarse pebbly sandstone with clasts derived from a reworked clastic source with metre scale scour at the base of this facies association into laminated and rhizolith clays a 534 m one and two dimensional section within the sitakund anticline north of chittagong is characterised by 20 100 m thick tidally dominated progradational parasequences mudstones with isolated sand lenses and abundant trace fossils of the cruziana ichnofacies coarsen upwards to fine medium grained sandstones deposited in shallower high energy environments these sands were deposited under both uni directional and bi directional currents resulting in dune scale cross stratification and are overlain by 2 5 cm thick clay drapes considered to be of a thickness greater than that typical of deposition from a single slackwater episode the main controls on sedimentation are the seasonally influenced river and sediment discharge onto the shelf from the himalayan source region combined with the influence of daily spring neap tides channel and bedform migration and minimal wave action over the shallow shelf the modern bay of bengal shelf provides an analogue for interpreting processes recorded within the miocene succession allowing construction of a depositional model for the late miocene shelf of the brahmaputra ganges delta system with sedimentation dominated by tidal and seasonal cyclicity