late quaternary environmental changes in the pearl river mouth region china abstract this study presents findings concerning late quaternary environmental changes in the pearl river mouth region china based on the study of over 300 boreholes out of these 35 boreholes are selected for the reconstruction of transects across the deltaic plain and estuary 39 radiocarbon dates obtained from these boreholes are used to help define the chronology of stratigraphic units present including two terrestrial units t1 and t2 and two marine units m1 and m2 diatom assemblages are studied in 6 boreholes revealing similarities and differences between the two marine units before the area was inundated by the last interglacial sea mis 5 an older terrestrial unit of sand and gravel t2 was laid down in a number of palaeo valleys during the last interglacial period an older marine unit of silt and clay m2 was laid down which was subsequently subaerially exposed when sea level regressed during the last glacial period mis 4 2 causing the uppermost section to be weathered during the same period a younger terrestrial unit of sand and gravel t1 was deposited along palaeo river channels around the early holocene before 8 2 cal ka bp the postglacial rise in sea level initiated a new phase of sedimentation in the outer part of the estuary characterised by the high percentages of marine diatoms in the sediments in the early holocene strong monsoon freshwater discharge resulted in sedimentation of a fine sand layer in the inner part of the estuary after 8 2 cal ka bp rapid rises in sea level caused widespread marine inundation and sedimentation the diatom data suggest that the relative sea level associated with the m2 and m1 units were both at similar levels because the m2 unit is typically recorded at altitudes of 15 m and 20 m below the present sea level this is likely to be a result of long term subsidence although numerical dating of the pre m1 units has not been made in the present study the chronology of these units can be inferred from uranium series ages and optically stimulated luminescence dating obtained from the adjacent coastal waters of hong kong the t1 unit has yielded ages of about 30 0 cal ka bp while the m2 unit has yielded ages about 130 0 cal ka bp confirming their mis 4 2 and mis 5 ages respectively

origin of groundwater salinity and hydrogeochemical processes in the confined quaternary aquifer of the pearl river delta china summary the pearl river delta prd located in the coastal area of south china contains a large confined quaternary aquifer this aquifer has been identified to have high concentrations of total dissolved solids ranging from 1 g l in the inland area to 26 8 g l near the southeast shoreline this work aims to investigate the origin of groundwater salinity and the major hydrogeochemical processes controlling the groundwater evolution in the confined basal aquifer of the prd a total of 40 boreholes were drilled in the prd during 2007 2009 and groundwater samples were collected for major ion ca 2 mg 2 na k nh 4 cl so 2 4 and hco 3 and isotope d 18 o d 2 h 3 h d 13 c and 14 c analyses these analyses demonstrate that groundwater in the confined basal aquifer is recharged by local rainwater and seawater and seawater is the major salinity source for groundwater the contribution of seawater varies from less than 1 in the inland areas to 74 near the shoreline so 2 4 reduction and methanogenesis occur in this reducing confined aquifer the results of ion exchange show that mg 2 exchanges mainly with the nh 4 ion which is derived from the mineralization of sedimentary organic matter ion exchange between na

worldwide initiation of holocene marine deltas by deceleration of sea level rise radiocarbon dated deltaic sequences of holocene age from different parts of the world began to accumulate within a restricted time range from about 8500 to 6500 years ago evaluation of major delta processes indicates that deceleration in sea level rise was the key factor in holocene delta formation within many deltas there is as much as a 2000 year age range between basal deposits in seaward and landward cores this age difference records the progressive landward migration of near mean sea level depositional environments during the lower to mid holocene establishment of a chronostratigraphic framework for holocene delta development provides a fundamental global baseline for distinguishing sea level change from vertical land motion by tectonism and isostasy and for evaluating rates of future marine incursion into low lying deltas

offshore quaternary sediments and their engineering significance in hong kong yim w w s 1994 offshore quaternary sediments and their engineering significance in hong kong in e f j de mulder editor engineering geology of quaternary deposits eng geol 37 31 50 investigation works undertaken for coastal engineering projects in hong kong has provided abundant information on the quaternary sedimentary sequence beneath the sea floor although the significance of such information may not have been realized at the time of these projects valuable lessons have often been learnt afterwards some examples of the important engineering applications concerning offshore quaternary sediments include 1 the provision of information on water movement sediment transport and sedimentation rates during the holocene period 2 the identification of exploitable offshore sand and gravel resources 3 the interpretation of marine geophysical data through the availability of reliable borehole descriptions 4 the enhancement of engineering soil testing results and their interpretation 5 the prediction of rates of long term ground settlement in coastal land reclaimations 6 the selection of offshore sites for waste disposal 7 the reduction in cost of coastal engineering projects through choosing the most cost effective site 8 the provision of baseline data to assist environmental impact assessment a new geological model is proposed for the offshore quaternary sedimentary sequence of hong kong the engineering properties of the sediments are found to be closely related to the palaco environmental changes

stratigraphy and paleoenvironmental changes in the yangtze delta during the late quaternary the yangtze delta area may be subdivided into paleo valley and paleointerfluves of the last glaciation the postglacial transgressive sedimentary cycle ptsc on the front zones of the paleointerfluves is composed of marsh nearshore and barrier lagoon shallow marine and nearshore tidal flat units with a basement in stiff clay the ptsc in back zones of the paleointerfluves consists of lacustrine marsh deposits the ptsc in the incised valley contains river channel floodplain estuary estuarine shallow marine and deltaic units with an erosional surface at its bottom the stiff clay and the erosional surface constitute the ptsc lower boundary the stiff clay consisting of a paleosol experienced deposition alternating with pedogenesis during the falling of sea level during 18o stage 3 ongoing pedogenesis in the sea level lowstand of 18o stage 2 and early diagenesis after paleointerfluve inundation by the sea level rise of 18o stage 1 the climate during the paleosol formation was temperate with more than 500 mm a 1 rainfall and frequently fluctuating groundwater the yangtze river incised its course during falling sea level of stage 3 and a huge incised valley was formed during the sea level lowstand stage 2 the filling of the incised valley took place during the postglacial sea level rise and delta formation occurred at a late stage of ptsc development

halloysite smectite mixed layered clay in fluvio volcanic soils at the southern foot of mount kilimanjaro tanzania abstract the formation of kaolinite smectite mixed layered clays results from weathering of smectite in leached and silica depleted environments halloysite smectite mixed layered clays have been proposed to form from weatherable minerals in volcanic ash soils through simultaneous formation of 1 1 and 2 1 units in silica rich environments here we report a mineralogical study of weathered b horizon material from soils developed on late quaternary fluvio volcanic deposits of basaltic origin at the southern foot of mount kilimanjaro in northern tanzania this area is characterized by a strong seasonality two months of highly humid conditions followed by a 10 month pronounced dry season during which mineral authigenesis takes place the weak leaching conditions are reflected in the high base saturation of the soils and by their high content of amorphous silica the soils contain no allophane the clay fractions contain dioctahedral 1 1 layer silicates trioctahedral mica gibbsite and feldspar as well as a substantial amount of 1 1 2 1 mixed layered clays with predominance of 1 1 layers the latter consist of dehydrated halloysite that only partially expands after formamide intercalation because of its history of drying in field conditions the interstratified phases are high charge halloysite smectite mixed layered clays cec 30 72 cmol kg 1 clay that are predominantly very fine grained

the role of sea level rise monsoonal discharge and the palaeo landscape in the early holocene evolution of the pearl river delta southern china abstract the early holocene history of the pearl river delta is reconstructed based on a series of sediment cores obtained from one of the main palaeo valleys in the basin sedimentary and microfossil diatom analyses combined with radiocarbon dating provide new evidence for the interactions between sea level rise antecedent topography and sedimentary discharge changes within the deltaic basin since the last glacial these new records show that river channels of last glacial age incised down to c 40 m into an older possibly mis5 age marine sequence which forms the floor of the deltaic basin and exists primarily at c 10 m 15 m below present mean sea level rapid postglacial sea level rise flooded the incised valleys by the beginning of the holocene and prior to c 9000 cal years bp marine inundation was largely confined within these incised valleys the confined available accommodation space of the incised valleys combined with strong monsoon driven freshwater high sediment discharge and a period of rapid rising sea level meant that sedimentation rates were exceptionally high towards c 8000 cal years bp as sea level rose to about 5 m marine inundation spilled out of the incised valleys and the sea flooded the whole deltaic basin as a result the mouth of the pearl river was forced to retreat to the apex of the deltaic basin water salinity within the basin increased markedly as the previously confined system dispersed across the basin and the sedimentation changed from fluvial dominated to tidal dominated sea level continued to rise albeit at a much reduced rate between 8000 and 7000 cal years bp and deltaic sedimentation was concentrated around the apex area of the basin during the last 7000 cal years bp the delta shoreline moved seawards and the sedimentary processes changed gradually from tidal dominated to fluvial dominated

early holocene initiation of the mekong river delta vietnam and the response to holocene sea level changes detected from dt1 core analyses abstract the early holocene initiation of the mekong river delta vietnam was investigated based on sedimentary facies microfossil analyses and radiocarbon dating of dt1 core taken from the upper delta plain the dt1 core 105 38 51 e 10 17 02 n elevation 2 m is 51 5 m long and located in fill of a buried incised valley this incised valley of the paleo mekong river formed during the last glacial maximum which is 20 30 km wide has been traced spatially from the cambodian border to the south china sea coast by compilation of existing core data the dt1 core records the history of the estuarine channel and delta systems of the mekong river during the early to middle holocene the sedimentary facies determined particularly from sedimentary structures diatom and foraminifera assemblages grain size variation and the accumulation rate clearly show an upward deepening succession in the lower part of the core from marsh or intertidal flat to subtidal inner bay facies and an overlying upward shallowing succession from prodelta to intertidal flat floodplain the boundary between the two successions indicating the maximum flooding surface occurred at approximately 8 0 ka at the time that the mekong river delta was initiated the delta initiation and sediment facies succession are closely linked to holocene sea level changes particularly a standstill from 8 0 to 7 5 ka after a rapid rise of sea level from 8 8 to 8 2 ka during the further sea level rise from 7 5 to 7 0 ka sediments aggraded forming topset sedimentary deposits

cyclic formation and stabilization of archean lithosphere by accretionary orogenesis constraints from ttg and potassic granitoids north china craton this study is financially supported by the national natural science foundation of china grant nos 41502179 41530207 41472165 and central university basic scientific research business expenses of china university of geosciences beijing grant no 2652015038 pac acknowledges support from australian research council grant fl160100168

sedimentary facies and holocene progradation rates of the changjiang yangtze delta china abstract the changjiang yangtze river one of the largest rivers in the world has formed a broad tide dominated delta at its mouth during the holocene sea level highstand three boreholes cm97 js98 and hq98 were obtained from the changjiang delta plain in 1997 1998 to clarify the characteristics of tide dominated delta sediments and architecture based on sediment composition and texture and faunal content core sediments were divided into six depositional units in ascending order they were interpreted as tidal sand ridge prodelta delta front subtidal to lower intertidal flat upper intertidal flat and surface soil deposits the deltaic sequence from the prodelta deposits to the delta front deposits showed an upward coarsening succession overlain by an upward fining succession from the uppermost part of the delta front deposits to the surface soil thinly interlaminated to thinly interbedded sand and mud sand mud couplets and bidirectional cross laminations in these deposits show that tide is the key factor affecting the formation of changjiang deltaic facies sediment facies and their succession combined with ams 14c dating revealed that isochron lines cross unit boundaries clearly and delta progradation has occurred since about 6000 to 7000 years bp when the rising sea level neared or reached its present position the average progradation rate of the delta front was approximately 50 km kyear over the last 5000 years the progradation rate however increased abruptly ca 2000 years bp going from 38 to 80 km kyear the possible causes for this active progradation could have been an increase in sediment production in the drainage basin due to widespread human interference and or decrease in deposition in the middle reaches related to the channel stability caused by human activity and climatic cooling after the mid holocene

radiocarbon dating chronologic framework and changes in accumulation rates of holocene estuarine sediments from chesapeake bay abstract rapidly accumulating holocene sediments in estuaries commonly are difficult to sample and date in chesapeake bay we obtained sediment cores as much as 20 m in length and used numerous radiocarbon ages measured by accelerator mass spectrometry methods to provide the first detailed chronologies of holocene sediment accumulation in the bay carbon in these sediments is a complex mixture of materials from a variety of sources analyses of different components of the sediments show that total organic carbon ages are largely unreliable because much of the carbon including coal has been transported to the bay from upstream sources and is older than sediments in which it was deposited mollusk shells clams oysters and foraminifera appear to give reliable results although reworking and burrowing are potential problems analyses of museum specimens collected alive before atmospheric nuclear testing suggest that the standard reservoir correction for marine samples is appropriate for middle to lower chesapeake bay the biogenic carbonate radiocarbon ages are compatible with 210pb and 137cs data and pollen stratigraphy from the same sites post settlement changes in sediment transport and accumulation is an important environmental issue in many estuaries including the chesapeake our data show that large variations in sediment mass accumulation rates occur among sites at shallow water sites local factors seem to control changes in accumulation rates with time our two relatively deep water sites in the axial channel of the bay have different long term average accumulation rates but the history of sediment accumulation at these sites appears to reflect overall conditions in the bay mass accumulation rates at the two deep water sites rapidly increased by about fourfold coincident with widespread land clearance for agriculture in the chesapeake watershed

stratigraphy and holocene evolution of the mud dominated chao phraya delta thailand abstract the central plain of thailand was formed over the last 8 7 kyr mainly by the deltaic processes of two major rivers the chao phraya and the mae klong the delta plain is the third largest delta plain in southeast asia after that of the mekong and the irrawaddy on the basis of sedimentological and paleontological analyses of samples from three boreholes and an open pit survey the evolution of the delta was clarified with high resolution 14 c dating after the maximum transgression at between 8 and 7 cal kyr bp the delta system migrated southward into the paleo gulf of ayutthaya a large mud shoal the sananivate mud shoal formed near the mouth of the paleo gulf between 7 and 3 cal kyr bp and facilitated its infilling as a result the delta has prograded rapidly particularly during the last 2 kyr deltaic sediment volume for the last 7 5 0 5 kyr shows that the average rate of sedimentation was 23 1 3 6 million t yr which is nearly the same as the present total sediment discharge from both rivers the comparatively wide delta plain relative to sediment discharge resulted from the stable sea level the shallow paleo gulf acting as a receiving basin and effective sediment trapping because of the mud shoal

pollen record in the northwestern continental shelf of the south china sea in the past 82 ka paleoenvironmental changes in the last glacial period abstract the northwestern continental shelf of the south china sea scs is in a geographic location that was sensitive to the global paleoenvironmental change during the last glacial period here we present a high resolution palynological record of core zbw 100 65 m in length from the continental shelf of the scs in the past 82 ka in contrast to results for the northern scs the core s pollen assemblage was dominated by arboreal trees between mis5a and mis1 even during the glacial period pollen provenance analyses indicated it was mainly from the drainage basin of the red river the yungui plateau and tibetan plateau between 66 and 64 ka the contribution of pinus fell while that of cyclobalanopsis and trilete spores increased indicating a drier and cooler climate conversely during mis3 the contributions of pinus and lowland trees were high indicating a warm and wet climate the sedimentary environment in core zbw alternated between coastal marine and river alluvial or shore facies a sedimentary hiatus between 42 and 13 ka coincided with a significant fall in sea levels seismic data indicate that the hainan paleo river delta hprd process occurred at the core site between 65 and 56 ka with extremely high rates of sediment deposition pollen data indicate the hprd process was coincided with a transition period from cooler and drier climate to warmer and wetter climate and lower sea levels to higher sea levels

modern fine grained sedimentation spatial variability and environmental controls on an inner pericontinental shelf hong kong abstract fine grained sediments in hong kong form part of an extensive cover that extends from nearshore locations to the mid continental shelf this featureless mud blanket is associated with sediment inputs from the pearl river and has generally been considered to be homogeneous in its characteristics however detailed sedimentological and geochemical analyses have revealed that the superficial deposits show systematic spatial variations that reflect contrasts in deposition between the pearl river estuary to the west and fully marine conditions to the east results indicate significant differences in particle size accumulation rates biogenic silica organic carbon content c n ratios and stable carbon and nitrogen isotopes that reflect location with respect to the pearl river delta calcium carbonate content varies widely tending to be higher close to land major controls over spatial variability include rainfall patterns and the supply of estuarine suspended particulate matter the effects of palaeotopography and modern water depth resuspension induced by monsoon winds and typhoons and human impacts on the sea floor

the history and variability of the east asian paleomonsoon climate abstract changes in the east asian paleomonsoon reflect interactions between the global atmosphere ocean land and ice systems and are also an expression of their combined effect within the boundary conditions imposed by the east asian continent and solar radiation the history of the east asian monsoon is an alternation between dominance by the dry cold winter and warm humid summer monsoons high resolution eolian sequences preserved in the chinese loess plateau reveal that the east asian monsoon may have commenced at least 7 2 ma ago they also provide evidence indicating that the pulsed uplift of the tibetan plateau at about 3 4 and 7 2 ma may have played an important role in inducing climate change the palaeoclimatic records of the last glacial cycle show high frequency variability on time scales of 1000 year or even shorter and instability of the east asian paleomonsoon system the high frequency variability could be due to a non linear response to orbital forcing or a result of the coupling processes between different components of the global system cold air activity in northern high latitudes the trans equatorial air streams from the southern hemisphere and possibly enso may have played an important role in east asian monsoon variability the synchroneity of all the palaeoclimatic events along the polar equator polar pep transect is still an open question correlation of limited palaeoclimatic records for the last 30 kyr obtained from east asia and australia suggests that the trans equatorial air streams driven by the monsoon and trade winds may have had an influence on opposite hemispheres

geomorphologic changes in the lower pearl river delta 1850 2015 largely due to human activity abstract we use 165 years of navigational and bathymetric data and 60 years of sediment discharge data to document and explain geomorphic changes in the pearl river delta prd and its estuary which in recent years has experienced rapid urbanization and explosive economic growth a series of british and chinese navigation charts dating back to 1850 show that since then the prd has gained 1160 km2 of new land most of it reclaimed land derived from disposal of dredge spoils accordingly the prd estuary has decreased by 35 in ocean area and by 39 in water volume changes were most dramatic in the western part of the delta where the modaomen and huangmao seas decreased in ocean area by 62 and 39 respectively to the east upper lingding bay ldb decreased in ocean area by 36 in contrast the lower part of ldb decreased by only 3 in area although shoaling more by than 2 m some of which may come via influx of sediment from the south china sea since 1995 the rate of land accretion in the prd has declined to 40 of what it was over the previous 145 years and to

quaternary sea level change on the continental shelf of hong kong high resolution seismic reflection profiles and continuously sampled boreholes have been used to investigate the quaternary offshore sequence that overlies weathered bedrock in hong kong four formations have been recognized the lowermost chek lap kok formation comprises dominantly fluvial sand and mud much of which probably accumulated prior to 130 000 years bp during sea level lowstands of middle pleistocene oxygen isotope stage 6 age and earlier the upper surface of the chek lap kok formation is defined by a fluvial tidal entrenchment surface that is overlain by transgressive fine grained marine sediments of the sham wat formation these were deposited during the eemian interglacial stage 5c at a time when sea level was similar to that of today in southeastern hong kong waters a late weichselian stage 5c ravinement surface is overlain by submerged shoreface sands succeeded by a back stepping wedge of marine sediments that comprise the waglan formation fluvial tidal entrenchment of the top of this formation occurred prior to the deposition of the uppermost dominantly marine hang hau formation deposited during the late weichselian to holocene transgression the main phase of which is represented by a ravinement surface

postglacial sea level changes in the northern south china sea continental shelf evidence for a post 8200 calendar yr bp meltwater pulse abstract a postglacial sea level record reconstructed through the study of 25 cores and 10 grab samples obtained from the siliciclastic dominated northern south china sea continental shelf between hong kong and dongsha is presented the seven sedimentary facies recognized are 1 pleistocene overconsolidated mud facies affected by palaeosol development due to sub aerial exposure during the last glacial maximum lgm 2 bioclastic sandy gravel facies occurring at a present day seabed depth exceeding 120 m 3 lgm bioclastic beach dune sand facies at a present day seabed depth between 110 and 120 m 4 postglacial siliciclastic beach dune sand facies formed during at least two episodes of sea level stillstands at present day seabed depths between 80 and 90 m and 30 and 50 m respectively 5 postglacial transgressive sand facies of late pleistocene to early holocene age radiocarbon dating shows that this facies was deposited in two stages before and after the 8200 calendar yr bp cold event 6 postglacial sand and gravel dominated facies with acoustic turbidity at a present day seabed depth of about 50 m 7 postglacial mud facies based on the radiocarbon ages obtained from samples located above and below the holocene pleistocene hiatus in the two innermost shelf cores and large variations of atmospheric radiocarbon concentration found during the last glacial period in a submerged stalagmite from the bahamas beck et al 2001 science 292 2453 2458 ages exceeding 8200 calendar yr bp are likely to represent minimum values ages postdating this limit which are less affected by variations of atmospheric radiocarbon concentration indicate the presence of a post 8200 calendar yr bp cold event meltwater pulse caused by eustatic sea level rise from approximately 40 m below present to the present level by about 6000 calendar yr bp inaccuracies of the pre 8200 calendar yr bp radiocarbon ages may be attributed to large variations of atmospheric 14 c during the postglacial period with greenhouse gases released from the sub aerially exposed continental shelf probably accounting for a significant proportion

erosion and sedimentation associated with the last sea level rise offshore hong kong south china sea abstract high resolution seismic data reveals an extensive planar seaward dipping reflector offshore south of hong kong this is interpreted as a ravinement surface created during the last transgression and is associated with ca 2 5 m of shoreface sand which is overlain by a 6 15 m thick sheet of prodelta marine shelf sediments on seismic profiles the shoreface sand is characterised by a chaotic reflector pattern and the sheet by low amplitude parallel flat or low angle continuous reflectors particle size data reveal a westward coarsening and decrease in sorting which reflects increasing proximity to the pearl river foraminiferal and diatom assemblages demonstrate the same overall westward shallowing trend with increase in inner shelf foraminifera and brackish planktonic diatom species a phase of early holocene shoreface sand and intertidal silt deposition in eastern and central areas culminated in a brief period of subaerial exposure this was succeeded by an extensive period of shelf and inner shelf deposition brought about by variation in balance between the rate of sea level rise and accommodation and the discharge and sediment supply from the pearl river

coastal gia processes revealed by the early to middle holocene sea level history of east china abstract in order to examine relative sea level responses to the postglacial ice volume change and the glacio hydro isostatic adjustments gia this study investigated the inner part of the hangzhou bay east china a tectonically relatively stable far field location and reconstructed the early to middle holocene sea level history this investigation has established the elevational relationship between modern saltmarsh mudflat and tidal levels based on diatom analysis for sea level indicative meaning estimates produced 17 high quality sea level index points and simulated gia processes for the study site these results reveal that the relative sea level rose from 38 3 1 6 m in c 10 000 cal a bp to the present height by c 7000 cal a bp and the average rate of sea level rise decreased gradually from 19 6 2 6 mm a to 2 3 1 5 mm a during the 3000 years this period of sea level history was punctuated by two episodes of accelerated rise around 8200 and 7500 cal a bp the relative sea level rose to 0 8 1 4 m above msl by c 6500 cal a bp followed by a gradual fall back to the present height at 4500 cal a bp implying a different response to the potential additional ice melting between 7000 and 4000 cal a bp a comparison of the sea level histories between the inner and outer hangzhou bay indicates the coastal levering effect due to the marine inundation of the continental shelves a further comparison between sea level data from china and malay peninsula reveals different gia effects between the cathaysia yangtze blocks and the sundaland block

sedimentary evidence of late holocene human activity in the pearl river delta china this study examines the sedimentary evidence of human activities during the last 4000 years in the pearl river deltaic area the analyses are focused on indentifying agricultural signatures present in the sedimentary record and establishing the timing of a change from a simple rice based agriculture to a more advanced diverse agriculture the examination is based on modern sediment and plant samples and a sediment core collected from the deltaic area the analyses include particle size and diatom analysis to determine the environmental conditions that were associated with the period of human activities organic carbon isotope ratios and major metal elements reveal an expansion in commercial crop production and metal smelting in the pearl river delta area about 2000 years ago the input of organic matter from introduced sugarcane a c4 plant elevates the bulk organic carbon isotope values in the estuarine sediments above that represented by other common agricultural crops in the study area including rice banana and lotus which are all c3 plants the increase in bulk organic isotopic value coincides with the rise in the concentration of copper iron and lead in the sedimentary sequence suggesting a wider use of metal tools these results indicate that advanced agriculture started about 2000 years ago as an expansion in human population took place in the area this record also provides sedimentary evidence that help ascertain the timing and type of human activities that are linked to subsequent land reclamation on the deltaic plain resulting in rapid shoreline advancement in the last 2000 years copyright 2010 john wiley sons ltd

reconstruction of holocene monsoon history from the pearl river estuary southern china using diatoms and carbon isotope ratios this study employs a multiproxy approach in the reconstruction of holocene monsoon history from sedimentary sequences obtained from the pearl river estuary southern china a set of modern surface sediment samples were collected within and outside the estuary and a sediment core was obtained from the mouth of the estuary a range of modern environmental variables were recorded including water salinity summer winter and annual average and water depth the diatom results show a good relationship with water salinity with marine diatoms dominating the more distal samples from outside the main estuary and freshwater diatoms dominating the proximal samples influenced by higher freshwater flux this pattern is matched by the bulk organic carbon isotopes the isotope values vary between 21 1 0 3 0 for samples from the fully marine environment between 23 2 0 8 and 23 7 0 8 for samples from the mid to outer estuary brackish water environment and less than 24 7 1 3 for samples from the inner estuary close to the freshwater sources similarly the c n ratios vary from below 7 0 6 in the marine end members to over 14 8 3 0 in the freshwater end members both the diatom and carbon data from the sediment core reveal a significant increase in freshwater flux from 8500 cal yr bp resulting from the enhanced summer monsoon regime in early holocene the strength of freshwater flux reached its highest between 7500 and 6000 cal yr bp in the last 6000 years freshwater flux decreased towards present reflecting a gradual weakening of the summer monsoon

sedimentary responses to holocene sea level change in a shallow marine environment of southern china abstract this study has examined how the sedimentary processes in the prevalent non deltaic coastal marine environments along the southeast coast of china have responded to sea level change and other environmental drivers during the holocene three sediment cores collected from a shallow marine embayment of the lantau island hong kong on the northern coast of the south china sea were analyzed the results of particle size total organic carbon content organic carbon and nitrogen ratios organic carbon stable isotope ratios abundance of alkenones and radiocarbon dating of these cores have helped reconstruct the sedimentary history of this site this history shares the similarity in sedimentary processes within the deltaic environment for the early holocene because palaeo incised channels exist in both environments the first 1500 years of deposition involved rapid channel infill caused by the rapidly rising sea level and strong monsoon driven discharge this was followed by a period of decelerated sedimentation under strong marine influence as the sea flooded the wide accommodation space in the subsequent two millennia major differences between the two environmental settings occur in the past 7000 years vertical accretion was seen in the non deltaic coastal environment whilst lateral shoreline advance was the dominant process in the deltaic basin the progressively slowing down in sedimentation rate in the non deltaic site during the middle and late holocene was caused by the reduced monsoon discharge intensified disturbance from tropical cyclone activity and coastal currents

dating of debris flow fan complexes from lantau island hong kong china the potential relationship between landslide activity and climate change five debris flow fan complexes bordering the coastal foothills of western lantau island hong kong have been the subject of a stratigraphic and dating study to explore the potential relationship between past periods of climate change and landslide activity luminescence osl ages from the fan complexes supported by a few calibrated radiocarbon 14c ages on duplicate samples suggest six main periods of accumulation 28 000 20 000 ka 14 500 10 500 ka 5800 5000 ka 4370 4230 ka 3560 3320 ka and 1350 1050 ka the younger periods 10 000 ka appear to be dominated by relatively thin 0 5 1 5 m units deposited mainly by debris flood events whereas the older periods are dominated by thicker 1 0 3 5 m units deposited by debris flow events we hypothesise that immediately prior to the last glacial maximum lgm i e 21 000 ka the climate was cool and dry upland areas were probably devoid of vegetation providing favourable conditions for sediment production in source areas these debris laden source areas are likely to have been mobilised during periodic rainstorm activity rapidly channelised over relatively short drainage courses and then deposited as thick sediment rich debris flow deposits along the coastal foothills during the early to middle holocene the climate was warmer and more humid pluvial conditions dominated forests occupied the upland source areas leading to possibly reduced sediment yields more frequent flash floods and deposition dominated by debris floods compared with recent studies on the weathering and erosion history of the pearl river delta our landslide age data are indicative of a potential link between increased landslide activity and intensification of the east asian monsoon during the early to middle holocene

an evolutionary model for the holocene formation of the pearl river delta china this paper reconstructs the evolutionary history of the pearl river delta over the last 9000 years and investigates land sea interaction in a large deltaic complex which formed under the influence

an inflection in the rate of early mid holocene eustatic sea level rise a new sea level curve from singapore this study presents a sea level curve from 9500 to 6500 cal bp for the farfield location of singapore on the sunda shelf in southeast asia the curve is based on more than 50 radiocarbon dates from elevations of 1 43 m to 15 09 m representing sea level index points in intertidal mangrove and shallow marine sediments deposited by sea level rise accompanying deglaciation the results indicate that mean sea level rose rapidly from around 17 m at 9500 cal bp to around 3 m by 8000 cal bp after this time the data suggest but do not unequivocally prove that the rate of sea rise slowed for a period of 300 500 years centred on 7700 cal bp shortly after the cessation of meltwater input to the oceans from the northern hemisphere renewed sea level rise amounting to 3 5 m began around 7400 cal bp and was complete by 7000 cal bp the existence of an inflection in the rate of sea level rise with a slow down centred on 7700 cal bp is broadly consistent with other available sea level curves over this interval and is supported by evidence of stable shorelines and delta initiation elsewhere at this time as well as evidence of comparatively rapid retreat of the west antarctic ice sheet beginning around 7500 cal bp stepped sea level rise occurring shortly after 7500 cal bp and also earlier during deglaciation may have served to focus significant post glacial episodes of human maritime coastal dispersal into comparatively narrow time intervals

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

late holocene depositional environments and coastal evolution of the mekong river delta southern vietnam abstract the mekong river delta in southern vietnam is a tide dominated delta and has been formed by the mekong river system the sediments are dominantly fine grained and were deposited in the receiving basin with slight inclination of pre existing deposits in the south china sea and gulf of thailand around 6000 to 5000 yr bp maximum holocene transgression occurred and seawaves surrounded uplands of the late pleistocene terrace basement rock and weathered land in the northern parts of the delta double notches were undercut by seawaves on the limestone hill in hatien at about 4 5 and 2 5 m above present sea level marine deposits prevailed throughout the delta during the highstand and regressions of relative sea level over the last 4550 yr bp delta progradation has produced a great flat plain of 62 520 km 2 this extremely fast progradation could be due to a very high sediment supply depositing in the slight inclination of the receiving basin b neotectonic movements and relative sea level changes c widespread mangrove forests playing an important role to enhance sediment accumulation former mangrove marshy deposits were significantly developed in the plain of reeds and longxuyen quadrangle mangrove marshes have also been developing well in the camau peninsula and present coastlines moreover regressions over the last 4550 yr bp have produced many distinct sets of relict beach ridges which are useful in distinguishing the former coastlines and evolutionary history of the delta

when and where did india and asia collide timing of the collision between india and asia is the key boundary condition in all models for the evolution of the himalaya tibetan orogenic system thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from tibetan plateau uplift through continental extrusion across eastern asia as well as our understanding of global climate change during the cenozoic although an abrupt slowdown in the rate of convergence between india and asia around 55 ma is widely regarded as indicating the beginning of the collision most of the effects attributed to this major tectonic episode do not occur until more than 20 ma later refined estimates of the relative positions of india and asia indicate that they were not close enough to one another to have collided at 55 ma on the basis of new field evidence from tibet and a reassessment of published data we suggest that continent continent collision began around the eocene oligocene boundary 34 ma and propose an alternative explanation for events at 55 ma

a preliminary study on the offshore stratigraphy in hong kong and its hydrogeological implications offshore groundwater reserves commonly sequestered in relict fluvial systems have been reported globally in hong kong submarine terrestrial sediments are the evidence of local fluvial systems during glacial periods and they probably sequester relatively fresh water this study aims to locate these sand deposits an offshore vibrocore in the southwestern waters of hong kong was analysed for its geophysical and elemental properties in order to deduce the palaeoenvironment also marine seismic profiling was adopted at two selected sites to depict the distribution of sand and gravel deposits and to estimate the average thickness of these aquifer materials a four unit sequence was noted with increasing marine influence since 8 ka bp this agrees with the hypothesis that the offshore groundwater reserves are the relics of flow systems during glacial periods the extensive sand deposits in the study areas are often shielded by sufficiently thick marine clay against vertical salinisation allowing fresh groundwater to be preserved the average thickness of these aquifer materials is about 11 m at the study sites if these sand deposits are recharged through onshore outcrops the reserves may be a sustainable water resource

holocene delta evolution and sediment discharge of the mekong river southern vietnam abstract evolutionary changes delta progradation and sediment discharge of the mekong river delta southern vietnam during the late holocene are presented based on detailed analyses of samples from six boreholes on the lower delta plain sedimentological and chronostratigraphic analyses indicate clearly that the last 3 kyr were characterized by delta progradation under increasing wave influence southeastward sediment dispersal decreasing progradation rates beach ridge formation and steepening of the face of the delta front estimated sediment discharge of the mekong river for the last 3 kyr based on sediment volume analysis was 144 36 million t yr 1 on average or almost the same as the present level the constant rate of delta front migration and stable sediment discharge during the last 3 kyr indicate that a dramatic increase in sediment discharge owing to human activities as has been suggested for the yellow river watershed did not occur although southeast asian rivers have been considered candidates for such dramatic increases in discharge during the last 2 kyr the mekong river example although it is a typical large river of this region does not support this hypothesis therefore estimates of the millennial scale global pristine sediment flux to the oceans must be revised

the huanghe yellow river and changjiang yangtze river deltas a review on their characteristics evolution and sediment discharge during the holocene abstract today asian rivers play an important role in delivering sediment from the land to the oceans however past sediment discharge before human activities modified the landscape is not well known and is estimated to have been roughly one third to one half of the present level chinese rivers have been strongly influenced by human activities and their sediment discharge has changed over the last 6000 years for the study of the long term human impact on rivers and catchment basins chinese rivers provide excellent examples because continuous historical records are available here the characteristics evolution and sediment discharge during the holocene of the deltas of two large chinese rivers huanghe yellow river and changjiang yangtze river are summarized the huanghe and changjiang deltas are different with respect to coastal environments water and sediment discharge the pattern of development of the delta lobe thickness of holocene sediments and basal topography of the incised valleys formed during the last glacial period the huanghe delta is characterized by a high concentration of sediment huge sediment discharge thin holocene deltaic sediments a lateral delta lobe shift and a steep longitudinal profile in its lower reaches the changjiang delta is characterized by a large water discharge a large interseasonal water level change a tide dominated coastal environment continuous seaward progradation with isolated river mouth sand bodies thick holocene sediments and a deep incised valley formed during the last glacial period based on the sediment volume of prograding deltaic sediments over the last 6000 years the sediment discharge of huanghe 1000 years ago is estimated to have been 1 10 8 tons year 1 10 of the present level an abrupt increase occurred at ca 1000 years ago as a consequence of human activities on the loess plateau especially involving cultivation and deforestation moreover the estimated sediment load of huanghe at sanmenxia station prior to 1000 years was 2 7 4 0 10 8 tons year or about 20 15 25 of the present level our recent study estimated that the sediment discharge of changjiang before 2000 years bp was ca 2 4 10 8 tons year total sediment discharge of both rivers before 2000 years bp is estimated to have been 3 4 10 8 tons year or only about 20 of the present level moreover the total sediment load to the lower reaches is estimated to have been 5 6 10 8 tons year or one fourth the present level

mid holocene sea level highstand along the southeast coast of china various sea level curves have been proposed for the coast of china in the past two decades these sea level curves indicate a complex history of holocene sea level and so the debate on whether or not a higher mid holocene sea level highstand exists in the coast of china has continued this paper aims to re examine the holocene sea level history for the low latitude part of the china coast between 18 n and 32 n by re assessing all the sea level data available from the east to south coasts and separating them according to geological settings in order to examine the influences of global and local factors the reconstructed sea level histories from different coastal sectors of the china coast reveal a certain degree of variability in the timing and height of mid holocene sea level highstand within large river deltas the mid holocene sea level highstand occurred earlier by almost 1000 years than that from other coastal sites the highstand from large river deltas appears also lower in altitude a few metres below the present day sea level due probably to the local factors of subsidence and sediment consolidation in geologically stable coastal sites the highstand is recorded at the same altitude as the present day sea level a 1 2 m higher highstand is found from sites where tectonic uplift is observed

holocene evolution in weathering and erosion patterns in the pearl river delta sediments in the pearl river delta have the potential to record the weathering response of this river basin to climate change since 9 5 ka most notably weakening of the asian monsoon since the early holocene approximate to 8 ka cores from the pearl river delta show a clear temporal evolution of weathering intensity as measured by k al k rb and clay mineralogy that shows deposition of less weathered sediment at a time of weakening monsoon rainfall in the early mid holocene 6 0 2 5 ka this may reflect an immediate response to a less humid climate or more likely reduced reworking of older deposits from river terraces as the monsoon weakened human settlement of the pearl river basin may have had a major impact on landscape and erosion as a result of the establishment of widespread agriculture after around 2 5 ka weathering intensity sharply increased despite limited change in the monsoon but at a time when anthropogenic pollutants e g cu zn and pb increased and when the flora of the basin changed sr 87 sr 86 covaries with these other proxies but is also partly influenced by the presence of carbonate the sediments in the modern pearl river are even more weathered than the youngest material from the delta cores we infer that the spread of farming into the pearl river basin around 2 7 ka was followed by a widespread reworking of old weathered soils after 2 5 ka and large scale disruption of the river system that was advanced by 2 0 ka

holocene evolution of the song hong red river delta system northern vietnam abstract the song hong red river delta located on the western coast of the gulf of bac bo tonkin in the south china sea formed as a result of the song hong sediment discharge throughout the holocene the river s sedimentary basin upstream from the delta plain is not large the delta plain comprises emerged tidal mangrove flats formed during the sea level highstand at 2 3 m 6 4 cal kyr bp and a beach ridge strandplain with straight to lobate beach ridges on the landward and seaward sides of the delta plain respectively the delta affords us the opportunity to examine river mouth morphodynamics comprehensively in relation to sediment discharge and sea level changes in this paper we describe the holocene evolution of the song hong delta system and the river mouth morphodynamics on the basis of seven sediment cores each 30 70 m long taken from the delta plain during 1999 2001 and 101 radiocarbon dates obtained from the core sediments sediments from the seven cores consisting of incised valley fills since the last glacial maximum can be divided in ascending order into fluvial sediments composed of gravelly sand and mottled clay tide influenced estuarine sediments containing shell and wood fragments and deltaic sediments composed of tide influenced sand and mud deposits in which the contents of sand and wood fragments increase upward by applying sequence stratigraphic concepts a transgressive surface can be identified at the gravelly sand mottled clay boundary and the maximum flooding surface can be identified at the estuarine deltaic sediments boundary the lowstand systems tract transgressive systems tract and highstand systems tract record 15 15 9 and 9 0 cal kyr bp respectively during the past 9 kyr the majority of the sediment discharged by the song hong accumulated in the incised valley and enhanced progradation at the river mouth the river mouth prograded from the bay head toward the gulf and its morphology changed from funnel shaped 9 6 cal kyr bp to straight 6 2 cal kyr bp and finally to lobate 2 0 cal kyr bp the morphological change from a funnel shaped to straight coast is interpreted to have been closely related to a hydrologic regime shift from a tide dominated bay head setting to a wave influenced open coast setting the cessation of the sea level rise at 6 cal kyr bp may have played only a small role in the change of river mouth morphology because the progradation rate decelerated from 22 to 4 m yr instead of accelerating at that time the river mouth morphodynamic change from straight to lobate was largely caused by an increase in the song hong sediment discharge from 17 27 9 2 cal kyr bp to 49 million t yr 2 0 cal kyr bp as a result of anthropogenic deforestation along the upper reaches of the song hong