stratigraphic evolution of the late holocene ganges brahmaputra lower delta plain sediment cores from the ganges brahmaputra delta in bangladesh were examined for sedimentological character clay mineralogy elemental trends c n s and 14c geochronology to develop a model for the sedimentary sequence resulting from lower delta plain progradation in the late holocene a widespread facies succession from muddy sand to interbedded mud records progradation of shoal island complexes and the transition from subtidal to intertidal conditions mangrove vegetated islands and peninsulas represent the final phase of progradation a mottled mud that is deposited by penetration of turbid coastal water into the mangroves during high water events organic matter preservation is generally low 1 toc in most of these well drained deposits that are characterized by a permeable silt dominated granulometry clay mineralogy in the cores records the relative influence of smectite and kaolinite rich ganges sediments and illite and chlorite rich brahmaputra material the lower delta plain west of the modern river mouths was deposited as a ganges dominated delta in three phases since 5000 cal years bp with brahmaputra influence confined to the meghna estuary area and to the supratidal section of western delta deposits evolution of the lower delta plain in the late holocene was influenced by regional subsidence patterns in the tectonically active bengal basin which controlled distributary channel avulsion and migration and the creation of accommodation space

late quaternary sedimentary environments and landforms in the ganges delta abstract late quaternary sediments deposited since the lowest stand of sea level during the last glacial maximum in the ganges delta and its surrounding region were classified into five units according to their sedimentary facies these are the lowest unit of sandy gravels the lower unit of sand with a few gravels and the middle upper and uppermost units consisting mainly of sand and silt with occasional peat layers the contact between the lower and middle units is fairly sharp and the upper part of the lower unit is oxidized in some places this suggests a period of subaerial exposure after the deposition of the lower unit during the low stand of sea level about the time of the last glacial maximum rivers in the ganges delta and its surrounding region dissected the surfaces of the region and deposited the lowest unit on the valley floors the lower unit was deposited over the lowest unit during an early stage of the post glacial transgression between ca 12 000 yr bp and 10 000 yr bp the surface of the delta was slightly dissected and the top of the lower unit was weathered and oxidized following this period the middle and upper units of alluvial or deltaic sand and silt with occasional peat were deposited the coastline in the early holocene retreated towards the central part of the present ganges delta in the middle and late holocene the silt and clay with occasional peat layers of the uppermost unit indicate that the lowland gradually became marshy and poorly drained as the rate of transgression became slower

effects of tectonic deformation and sea level on river path selection theory and application to the ganges brahmaputra meghna river delta the set of active rivers of the ganges brahmaputra meghna gbm delta in bangladesh overlies an active plate boundary that continually modifies the landscape of the delta by deformation the response of rivers to spatially variable subsidence from tectonic tilting or other causes has been thought to include preferred occupation of regions of higher subsidence in this paper we develop further the theoretical framework for analysis of the interplay of tectonics and river dynamics and apply this model to conditions in the gbm delta first we examine the overall competition between variable subsidence and channel dynamics and find that tilting in bangladesh should be strong enough to influence river path selection we then present new theory for the effect of subsidence that is spatially not temporally variable we find a constant residence timescale on different parts of the delta and differing frequencies of avulsion to these locations and describe the effects of incision or floodplain deposition on these quantities we present estimates of the channel residence timescale of the jamuna brahmaputra river reconstructed from the lithology provenance and dating of sediment cores we apply our framework to a map of regional subsidence to predict the effects on avulsion for the jamuna river comparison between our predicted 2150 years and our stratigraphically based estimates of avulsion timescale 1800 years shows encouraging consistency

plutonism in three orogenic pulses eastern blue ridge province southern appalachians department of geological sciences university of north carolina mitchell hall chapel hill nc 27599 3315

itrax description and evaluation of a new multi function x ray core scanner a new automated multi function core scanning instrument named itrax has been developed that records optical radiographic and elemental variations from sediment half cores up to 1 8 m long at a resolution as fine as 200 m an intense micro x ray beam focused through a flat capillary waveguide is used to irradiate samples to enable both x radiography and x ray fluorescence xrf analysis data are acquired incrementally by advancing a split core via a programmable stepped motor drive through the flat rectangular section x ray beam traditional xrf determination of element composition in sediments provides high quality data but it takes a considerable time and normally consumes gram quantities of material that is often only available in limited quantities the itrax core scanner non destructively collects optical and x radiographic images and provides high resolution elemental profiles that are invaluable for guiding sample selection for further destructive detailed sampling this paper presents a description of the construction characteristics and capabilities of the itrax system high resolution itrax data obtained from sediment cores are also presented and compared with results from traditional wavelength dispersive xrf analysis at lower resolution finally some recent technical developments linked to the second generation itrax are presented

monsoon source shifts during the drying mid holocene biomarker isotope based evidence from the core monsoon zone cmz of india abstract a better understanding of past variations of the indian summer monsoon ism that plays a vital role for the still largely agro based economy in india can lead to a better assessment of its potential impact under global climate change scenarios however our knowledge of spatiotemporal patterns of ism strength is limited due to the lack of high resolution continental paleohydrological records here we reconstruct centennial scale hydrological variability during the holocene associated to changes in the intensity of the ism based on a record of lipid biomarker abundances and compound specific stable isotopic composition of a 10 m long sediment core from saline alkaline lonar lake situated in the core monsoon zone of central india we identified three main periods of distinct hydrology during the holocene in central india the period between 10 1 and 6 cal ka bp was likely the wettest during the holocene lower average chain length acl index values 29 4 28 6 and negative 13 c wax values 34 8 to 27 8 of leaf wax n alkanes indicate the dominance of woody c 3 vegetation in the catchment and negative d wax values concentration weighted average 171 to 147 suggest a wet period due to an intensified monsoon after 6 cal ka bp a gradual shift to less negative 13 c wax values particularly for the grass derived n c 31 and appearance of the triterpene lipid tetrahymanol generally considered as a marker for salinity and water column stratification mark the onset of drier conditions at 5 1 cal ka bp an increasing flux of leaf wax n alkanes along with the highest flux of tetrahymanol indicate a major lowering of the lake level between 4 8 and 4 cal ka bp we find evidence for a transition to arid conditions indicated by high and strongly variable tetrahymanol flux in addition a pronounced shift to less negative 13 c wax values in particular for n c 31 25 2 to 22 8 during this period indicates a change of dominant vegetation to c 4 grasses in agreement with other proxy data such as deposition of evaporite minerals we interpret this period to reflect the driest conditions in the region during the last 10 1 ka this transition led to protracted late holocene arid conditions after 4 ka with the presence of a permanent saline lake supported by the sustained presence of tetrahymanol and more positive average d wax values 122 to 141 a late holocene peak of cyanobacterial biomarker input at 1 3 cal ka bp might represent an event of lake eutrophication possibly due to human impact and the onset of cattle livestock farming in the catchment a unique feature of our record is the presence of a distinct transitional period between 4 8 and 4 cal ka bp which was characterized by some of the most negative d wax values during the holocene up to 180 when all other proxy data indicate the driest conditions during the holocene these negative d wax values can as such most reasonably be explained by a shift in moisture source area and or pathways or rainfall seasonality during this transitional period we hypothesize that orbital induced weakening of the summer solar insolation and associated reorganization of the general atmospheric circulation as a possible southward displacement of the tropical rainbelt led to an unstable hydroclimate in central india between 4 8 and 4 ka our findings shed light onto the sequence of changes during mean state changes of the monsoonal system once an insolation driven threshold has been passed and show that small changes in solar insolation can be associated with major hydroclimate changes on the continents a scenario that may be relevant with respect to future changes in the ism system

shelf sedimentation off the ganges brahmaputra river system evidence for sediment bypassing to the bengal fan the nature of shelf sedimentation seaward of the world9s largest sediment dispersal system is examined by using sedimentological and geochronological techniques on a unique suite of sediment cores and grab samples sediments from the ganges brahmaputra river system are currently accumulating on the shelf in water depths of less than about 80 m forming a clinoform like deposit similar to subaqueous deltas found off other major river systems the highest sediment accumulation rates on the shelf occur near the head of the swatch of no ground a major submarine canyon that indents the shelf west of the present river mouths this observation together with textural data suggests that river sediments are transported seaward and westward and that the swatch of no ground is currently a major conduit for the transport of sediments from the bengal shelf

chronology of relict lake deposits around the suwalong paleolandslide in the upper jinsha river se tibetan plateau implications to holocene tectonic perturbations abstract the suwalong paleolandslide is located in the batang zhongza reach of the upper jinsha river at the southeast margin of the tibetan plateau where tectonic activity is very strong and has formed a great deal of large paleolandslides or their relicts the relict deposits that have been exposed around the suwalong paleolandslide provide an opportunity to understand the formation process and potential origin of a former landslide dammed lake in the active belt region two level platforms of ancient dammed lake deposits were found on both the upstream and downstream sides of the relict landslide dam the chronology of the landslide and its surrounding relict lake deposits were studied by using 14 c dating and osl dating the minimum age of the suwalong landslide blocking the jinsha river was shown to be about 1355 yr bp and those of its surrounding two platforms of lacustrine sediments were shown as 1 8 and 1 3 ka bp the recordings of the soft sediment deformations induced by seismic liquefaction were found in the dammed lake sediments thus providing substantial evidence for the existence of the paleoseismic events we can infer that the suwalong landslide may have been triggered by a paleoearthquake or combination of paleoearthquake and rainfall events the chronology of the landslide dammed lake deposits around the relict suwalong landslide dam reveals that the large scale landslides in the batang zhongza reach of the upper jinsha river were formed during numerous periods namely the large scale landslides may have been triggered by several paleoearthquake events occurring at around 1900 1355 and 1115 yr bp indicating that the jinsha river faults in the area may have been quite active since the late holocene

morpho sedimentary records at the brahmaputra river exit ne himalaya climate tectonic interplay during the late pleistocene holocene morphological and sedimentary records at the exit of brahmaputra river at pasighat in the ne himalaya inform about the climate tectonic interplay during the past ca 15 ka the geomorphology of the area comprises 1 fan terrace t3 2 a high angle fan 3 terrace t2 4 terrace t1 and 5 a low angle fan geomorphic consideration suggests that the fan terrace t3 and high angle fans are the oldest units and were coeval the low angle fan is the youngest geomorphic unit sedimentological studies and optically stimulated luminescence chronology suggest that i fan terrace t3 formed between 13 and 10 5 ka and comprised multiple events of debris flows separated by the aggradation as channel bars in a braided river environment ii the high angle fan formed during 15 10 ka and comprises channel bar aggradation in braided river conditions iii terrace t2 formed during 10 8 ka due to aggradation in a braided channel environment with lesser events of debris flows iv terrace t1 formed during 7 and 3 ka took place as bars of the braided river sudden coarsening of the sediment indicated a tectonic rejuvenation in the provenance region between 7 and 3 ka and v the low angle fans dated to 3 ka formed due to aggradation in a small tributary joining the brahmaputra river this implies a phase when the main channel of the brahmaputra did not flood regularly and the tributaries were actively aggrading the sedimentation style and incision of these geomorphic units responded to contemporary climatic changes and uplift in the siwalik range along the himalayan frontal fault copyright 2008 john wiley sons ltd

cyclones and tides as feeders of a submarine canyon off bangladesh extremely high annual sedimentation rates of about 50 cm yr at the head of the submarine canyon swatch of no ground which connects the submarine delta of the ganges brahmaputra to the bengal deep sea fan were estimated by 210 pb and 137 cs gamma spectrometry the sediment in the canyon at 228 m water depth consists of graded sand and silt layers and laminated mud the sand and silt layers are assumed to have been deposited by tropical cyclones whereas the intercalated laminated mud is thought to have been deposited by daily variations of tidal currents the graded layers correlate well with the historical record of cyclones and allow an annual fine tuning the rapid infill of the canyon head is caused by trapping of suspended sediment that has been mobilized by storms and tides on the delta topset the sediment is stored only temporarily in the canyon head slides slumps and earthquakes occasionally remove and transfer it by turbidity currents to the deep sea fan the focusing effect of the canyon and the subsequent stepwise export of sediment probably combined with erosion of the canyon floor explain the long term regional stability of this canyon and probably other shelf canyons

rapid coupling between ice volume and polar temperature over the past 150 000 years a new approach to dating a continuous sea level record using speleothem u th ages shows that past variations in global ice volume occurred within centuries of polar climate change with rates of sea level rise reaching at least 1 metre per century

the bengal fan external controls on the holocene active channel turbidite activity the eastern levee of the active channel in the bengal fan has been investigated in order to better understand the history of turbidite activity in this channel during the holocene in the context of

geology and depositional history of cenozoic sediments of the bengal basin of bangladesh abstract the bengal basin lies on the eastern side of the indian subcontinent and occupies the northern part of the bengal geosyncline sedimentation started with the breakup of gondwanaland cretaceous fluvial and deltaic sedimentation was slowly taken over by open marine and deep sea fan sedimentation by the mid eocene the basin was witnessing the maximum marine transgression and most of the stable shelf was in carbonate regime as part of the extended tethyan carbonate shelf the deeper part of the basin was dominated by deep sea fan sedimentation fed by clastic turbidites from the northeast with limited carbonate turbidites from the shelf the collision of the indian plate with the tibetan plate and with the burmese plate in the miocene resulted in a rapid switch in sedimentation pattern in the bengal basin the basin structure and sedimentation were both strongly influenced by the collision pattern of the plates and by the uplift of the himalayas with the miocene collision of the indian plate there was a change from flysch to molasse sedimentation the present day basin configuration with the ganges bramaputra delta on the north and the bengal deep sea fan on the south was established in the pliocene the stratigraphic and tectonic framework of the bengal basin suggest that the deposition of hydrocarbon source rocks was associated with the flysch sedimentation whereas formation of traps and deposition of most of the reservoir rocks in the basin was associated with mid pliocene tectonic movements and with the molasse sedimentation the pre mid miocene sediments now deeply buried in the deeper part of the bengal basin and in the eastern folded belt are the major source of gas of the eastern folded belt of the basin

pore structure of cambrian shales from the sichuan basin in china and implications to gas storage abstract the microstructure of black siliceous shale from the lower cambrian niutitang formation sichuan basin in china was investigated by the combination of field emission scanning electron microscope fe sem and argon ion beam milling the nanometer to micrometer scale pore systems of shales are an important control on gas storage and fluid migration in this paper the organic porosity in shale samples within oil and gas window has been investigated and the formation mechanism and diagenetic evolution of nanopores have been researched fe sem reveals five pore types that are classified as follows organic nanopores pores in clay minerals nanopores of framework minerals intragranular pores in microfossils and microfractures numerous organic nanopores are observed in shales in the gas window whereas microfractures can be seen within the organic matter of shales in the oil window microfractures in oil window shales could be attributed to pressure buildup in the organic matter when incompressible liquid hydrocarbon are generated and the orientation of microfractures is probably parallel to the bedding and strength anisotropy of the formation pores in clay minerals are always associated with the framework of clay flakes and develop around rigid mineral grains because the pressure shadows of mineral grains protect pores from collapse and the increasing of silt content would lead to an increase in pressure shadows and improve porosity nanopores of rock framework are probably related to dissolution by acidic fluids from hydrocarbon generation and the dissolution related pores promote permeability of shales porosity in the low toc low thermal maturity shales contrast greatly with those of high toc high thermal maturity shales while the high toc shales contain abundant organic microporosity the inorganic pores can contribute a lot to the porosity of the low toc shales

controls on facies distribution and stratigraphic preservation in the ganges brahmaputra delta sequence abundant sediment supply and accommodation space in the bengal basin have led to the development of a major late quaternary delta sequence this sequence has formed in a tectonically active setting and represents an important example of a high energy marine and fluvial high yield continental margin deposit recent studies have detailed the delta s stratigraphy and development noting that tectonics and sediment supply control the ganges brahmaputra more significantly than in many other delta systems these ideas are developed here through a discussion of the effects that spatial and temporal variations in tectonics and sediment supply have had on deltaic processes and sequence character unique and differing stratigraphies are found within thedeltasystem suchthatfine grainedsedimentpreservationisfavoredinareasofactivetectonicprocessessuchasfolding block faulting and subsidence coarse grained deposits dominate the stratigraphy under the control of high energy fluvial processes and mixed fine coarse stratigraphies are found in areas dominantly influenced by eustatic sea level change overlaid upon these spatially varying stratigraphic patterns are temporal patterns related to episodic events e g earthquakes and rivers avulsions and long termchangesinclimate andsediment supply modelingisalsousedtoinvestigatethe influenceofavariable sedimentsupply on sequence character results show that the timing and magnitude of sediment input relative to sea level rise is a significant control on the subaerial extent of the delta and the relative dominance of alluvial and marine facies within the sequence d 2002 elsevier science b v all rights reserved

indian monsoon variations during three contrasting climatic periods the holocene heinrich stadial 2 and the last interglacial glacial transition in contrast to the east asian and african monsoons the indian monsoon is still poorly documented throughout the last climatic cycle last 135 000 years pollen analysis from two marine sediment cores nghp 01 16a and nghp 01 19b collected from the offshore godavari and mahanadi basins both located in the core monsoon zone cmz reveals changes in indian summer monsoon variability and intensity during three contrasting climatic periods the holocene the heinrich stadial hs 2 and the marine isotopic stage mis 5 4 during the ice sheet growth transition during the first part of the holocene between 11 300 and 4200 cal years bp characterized by high insolation minimum precession maximum obliquity the maximum extension of the coastal forest and mangrove reflects high monsoon rainfall this climatic regime contrasts with that of the second phase of the holocene from 4200 cal years bp to the present marked by the development of drier vegetation in a context of low insolation maximum precession minimum obliquity the historical period in india is characterized by an alternation of strong and weak monsoon centennial phases that may reflect the medieval climate anomaly and the little ice age respectively during the hs 2 a period of low insolation and extensive iceberg discharge in the north atlantic ocean vegetation was dominated by grassland and dry flora indicating pronounced aridity as the result of a weak indian summer monsoon the mis 5 4 glaciation also associated with low insolation but moderate freshwater fluxes was characterized by a weaker reduction of the indian summer monsoon and a decrease of seasonal contrast as recorded by the expansion of dry vegetation and the development of artemisia respectively our results support model predictions suggesting that insolation changes control the long term trend of the indian monsoon precipitation but its millennial scale variability and intensity are instead modulated by atmospheric teleconnections to remote phenomena in the north atlantic eurasia or the indian ocean

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

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

shelf to canyon connections transport related morphology and mass balance at the shallow headed rapidly aggrading swatch of no ground bay of bengal abstract the swatch of no ground song canyon in the bay of bengal is a shelf incising submarine canyon that is actively aggrading in its upper reaches despite regular gravity driven transport and mass wasting although the canyon lies 150 km downdrift of its main sediment source the ganges brahmaputra meghna gbm river mouth high sedimentation rates 5 50 cm year 1 are sustained by both progradation of the subaqueous delta into the canyon head and the conveyance of shelf generated hyperpycnal flows to the canyon floor this rapid accretion appears to be largely balanced by mass failures triggered by regularly occurring storms and less frequently by major earthquakes here we use high resolution sub bottom sonar data to elucidate dominant sediment dispersal pathways and their transport related morphology at the canyon head these include 1 a laterally prograding clinoform that intersects the canyon head at water depths of 20 120 m 2 several shelf incising bypass gullies that originate in 50 m the clinoform deposits reflect westward progradation of the gbm subaqueous delta into the upper canyon where its axis normal orientation leaves it dissected by cross cutting gullies and mass failures the morphology and acoustic stratigraphy of the gullies coupled with strong bed shear and high suspended sediment concentrations on the inner shelf suggest that these features are sustained by the regular conveyance of gravity driven fluid muds that are formed in shallow water

relative sea level changes during the holocene in bangladesh abstract this paper presents a reconstruction of the holocene paleo environment in the central part of bangladesh in relation to relative sea level changes 200 km north of the present coastline lithofacies characteristics mangal peat diatoms and paleophysiographical evidence were considered to reconstruct the past position and c 14 ages were used to determine the time of formation of the relative sea level during the holocene with standard reference datum the required m s l at the surface of five sections was calculated the relative sea level rsl curve suggests that bangladesh experienced two mid holocene rsl transgressions punctuated by regressions the curve shows an rsl highstand at approximately 7500 cal bp although the height of this highstand could not be determined because the transgressive phase was observed in a bioturbated sand flat facies the curve shows a regression of approximately 6500 cal bp and the rsl was considerably lower perhaps 1 2 m than the present m s l the abundant marine diatoms and mangrove pollens indicate the highest rsl transgression in bangladesh at approximately 6000 cal bp being at least 4 5 to 5 m higher than the modern m s l after this phase the relative sea level started to fall and consequently a freshwater peat developed at approximately 5980 5700 cal bp the abundant mangrove pollens in the salt marsh succession shows the regression at approximately 5500 cal bp when it was 1 2 m higher than the modern sea level the curve indicates that at approximately 5000 cal bp and onwards the rsl started to fall towards its present position and the present shoreline of bangladesh was established at approximately 1500 cal bp and has not noticeably migrated inland since