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The seeding of cells into an organ is an important step in cell therapy because the final functional properties of the organ are related to the initial cell distribution throughout the organ. However, vessel occlusion is a serious problem that prevents uniform distribution of the cells in the entire organ. Understanding the mechanism of vessel occlusion can help optimize the seeding process. In this study, the vessel occlusion phenomenon under perfusion conditions during cell seeding was investigated. First, we applied a microfluidic system that enabled the observation of the occlusion events during injection. Second, we applied a multiphase numerical model that can describe the cell–cell interactions and cell–fluid interactions to investigate the vessel occlusion phenomenon during the seeding process. In particular, the effects of cell concentration and flow rate were investigated. The results indicate the importance of cell–cell interactions and cell–vessel interactions for the occurrence of vessel occlusion. In addition, it is found that the probability of occurrence of vessel occlusion increases with the increase in cell concentration and decrease in flow rate. The simulation model can help determine the optimum parameters to enhance cell seeding efficiency. [ABSTRACT FROM AUTHOR]

Since the 1990s the Mekong River delta has suffered a large decline in sediment supply causing coastal erosion, following catchment disturbance through hydropower dam construction and sand extraction. However, our new geological reconstruction of 2500-years of delta shoreline changes show that serious coastal erosion actually started much earlier. Data shows the sandy coast bounding river mouths accreted consistently at a rate of +2 to +4 km2/year. In contrast, we identified a variable accretion rate of the muddy deltaic protrusion at Camau; it was < +1 km2/year before 1400 years ago but increased drastically around 600 years ago, forming the entire Camau Peninsula. This high level of mud supply had sharply declined by the early 20th century after a vast canal network was built on the delta. Since then the Peninsula has been eroding, promoted by the conjunction of mud sequestration in the delta plain driven by expansion of rice cultivation, and hysteresis of long-term muddy sedimentation that left the protrusion exposed to wave erosion. Natural mitigation would require substantial increases in sediment supply well above the pre-1990s levels. [ABSTRACT FROM AUTHOR]

• The electrical analog model is expanded to permeable decellularized organs. • The resistance calculation of the permeable flow is identified. • The permeability model can indicate the pressure and flow rate at specific vessels in the decellularized kidney. • The permeability model reveals dynamics that influence the cell seeding process. In recellularization, cell-seeding efficiency refers to the uniform distribution of cells across the decellularized organ, which should be enhanced to ensure effective functioning. During cell seeding, flow dynamics influence the distribution of cells because the driving force of cell movement is the fluid force. However, after decellularization, because of flow permeability through the vessel wall, the fluid pressure and velocity in the vessels of vascular trees are significantly reduced compared with those in the native organ, which might affect cell seeding efficiency. Therefore, it is necessary to assess the flow characteristics in the vessels of decellularized organs to select appropriate seeding conditions. Although electrical analog models have been widely used to investigate the flow distribution in organs, current models do not reflect the permeable conditions. This study proposes a model to extend the conventional electrical analog model to simulate the flow characteristics in decellularized organs. A resistor reflecting permeable flow was added to the original electrical analog model to describe the permeable conditions in the decellularized organs. Decellularization and pressure drop measurements of the kidney were also conducted for model development, calibration, and validation. The developed model was then applied to a decellularized kidney to reveal changes in flow characteristics. The resistance calculation of permeable flow was determined for each generation of vascular trees. The coefficient of permeability can be indicated by the measured flow exiting through the outlet or the pressure drop across the decellularized organ. The developed permeability model had a qualitative and quantitative agreement with the experimental data without calibration. The results of the permeability model for the decellularized kidney indicated significant reductions of up to 70% in the flow rate and pressure of the organ compared to the native kidney. The developed model can simulate the flow characteristics in each individual vessel of decellularized organs. The results from the model can be used to assess the optimal flow rate condition for the cell seeding process. [ABSTRACT FROM AUTHOR]

Abstract: The aim of the study was to characterize a variety of sedimentary facies from the Mekong River delta (MRD) and to determine their geotechnical properties. The paper furthermore discusses the influences of sedimentary environment and conditions on geotechnical properties of the sedimentary facies. A core at the Vinhlong province, MRD, sufficiently presented the sedimentary facies of the area. Eight successive facies were identified based upon sedimentary properties. Characteristics of the unit showed development of sedimentary facies. Each sedimentary facies was formed under a different environment and revealed typical geotechnical properties. Estuarine channel, estuarine marine, delta front-mouth bar, and sub- to inter-tidal flat facies were formed under strong hydrodynamic conditions. The sediments are from clay to coarse sand and even pebbles, and the sedimentary structures are plentiful, such as intercalated clay, silt beddings and fine to coarse sand beddings with very different thicknesses. Strengths of these sedimentary facies vary significantly, which can be observed by cone penetration test (CPTU) results, but not by standard penetration test (SPT). The normalized values from CPTU show saw–tooth graphs with large variations, especially delta front-mouth bar and sub- to inter-tidal flat facies at shallower depth. The soil-behavior-types determined from the normalized values are plentiful from cohesionless to cohesive soils. As an example, an estuarine channel facies experiencing a large consolidation pressure, in relatively long-time (over 9920yr BP) has geotechnical properties with heavily overconsolidated conditions and high strength. On the other hand, marsh, open bay, pro-delta and March/flood plain facies, which formed under relatively low hydrodynamic conditions, have simple and homogenous sedimentary properties. The sediments are commonly silt and clay, and sedimentary structures are very faint interbedded clay and silt laminae, and rarely very fine sand laminae. Strengths of these sedimentary facies increase linearly with depth, the normalized values are all rather constant with depth, and their soil behavior-type is only normally consolidated clays from CPTU results. It could be said that the particular sedimentary conditions in the late Pleistocene–Holocene deposits of MRD can be reasonably estimated by the CPTU test. [Copyright &y& Elsevier]

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 +2m) is 51.5m 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 20–30km 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 8.0ka, 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 7.5ka after a rapid rise of sea level from 8.8 to 8.2ka. During the further sea-level rise from 7.5 to 7.0ka, sediments aggraded, forming topset sedimentary deposits. [Copyright &y& Elsevier]

The Mekong River Delta, southern Vietnam, is a typical mixed tide and wave energy delta with a wide delta plain formed during the last 6 ka and is one of the largest deltas in the world. Three cores were taken from Bentre Province in the lower delta plain, with the objective to describe the sediment facies and to clarify the changes from tide-dominated to tide-wave-dominated delta of the Mekong River Delta during the Late Holocene.Three cores (BT1, BT2, and BT3) were obtained in 1997. Holocene sediments in BT1 and BT3, with thickness ranging from 10 to 20 m, were mainly composed of deltaic sediments unconformably overlying the Late Pleistocene sediments. The BT2 core, however, consisted of transgressive estuarine sediments covered by deltaic sediments infilling the paleo-Mekong River incised valley, which is more than 70 m deep and was formed during the last glacial period.The deltaic sediment facies and succession in the BT2 and BT3 cores taken from the outer delta plain, with beach ridges, were different from those of the BT1 core taken from the inland delta plain. The BT2 and BT3 cores provide a good example of tide- and wave-dominated delta sediments and were characterized by a coarsening-upward delta front facies covered by a fining-upward subtidal to intertidal flat facies, followed by a coarsening-upward foreshore/dune facies. These changes from the inland delta plain to the beach ridge delta plain were accompanied by changes in progradation rates and subaqueous delta topography caused by increased wave influence. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

Recent 200+ km progradation of the Mekong River delta over the last 6000 years has provided a sequence of incised-valley fills with insights into the sedimentary response of a large river system to sea-level changes since the Last Glacial Maximum (LGM). This study provides an analysis of comprehensive synthesis of the incised-valley and successive progradational deposits of the paleo-Mekong River across a regional area spanning from the Cambodian lowland to the Vietnamese shelf. The study is based on the analysis of the depositional facies, diatom and foraminiferal assemblages, and radiocarbon ages of three new cores and previously published cores. The depositional succession observed in these cores is divided into the lower and upper units representing Pleistocene weathered sediment and incised valley fills and delta sediment since the LGM, respectively. The upper unit shows an upward-deepening succession from the fluvial channel to estuary and subtidal deposits, followed by an upward-shallowing succession consisting of progradational deltaic deposits. This unit can be divided into four phases: the fluvial channel sedimentation (before ~13.4 ka); retrogradational and aggradational estuarine system (~13 to ca. 8 ka), aggradational and progradational delta system (ca. 8 to 6 ka), progradational delta system (6 ka to present). Tides were dominant from 13 to ~3.5 ka, followed by wave- and tide-dominated environments until present at its river-mouth and coast. The data from three new cores, along with existing data, enable paleogeography reconstruction of the incised valley and subsequent delta progradation in four stages at 10, 8, 6, and 4 ka. The results indicate the Mekong River incised valley was ‐−35 to −40.0 m deep and 25 km wide in comparison with surrounding interfluve areas at 6 ka. Additionally, it was filled mostly during transgression from 13 to 8 ka and early highstand periods including its shelf valley beneath the present delta plain, which was filled completely by 4 ka. In the upper delta plain in Vietnam, the succession formed during 8–6 ka appears to have been removed and then deposited by subsequent fluvial channel/tidal river facies dated 4.4 to 2.2 ka. • Sedimentary facies changes, evolution of the paleo-Mekong River Incised-valley and paleogeography of Mekong River Delta in Late Pleistocene to Holocene. • Latest Pleistocene - Holocene sequence of incised valley fill. • Three phases of incised valley fill and delta sedimentation after the fluvial channel phase (before ~13.4 ka): retrogradational and aggradational estuarine system (~13 to ca. 8 ka), aggradational to progradational delta system (ca. 8 to 6 ka), and progradational deltaic system (6 ka to present). • Paleogeography reconstruction of the Mekong incised valley and subsequent delta progradation in four stages 10, 8, 6, 4 ka. [ABSTRACT FROM AUTHOR]

The area of coastal rivers with a combination of fluvial, tidal and wave processes is defined as the fluvial to marine transition zone and can extend up to several hundreds of kilometres upstream of the river mouth. The aim of this study is to improve the understanding of sediment distribution and depositional processes along the fluvial to marine transition zone using a comprehensive dataset of channel bed sediment samples collected from the Mekong River delta. Six sediment types were identified and were interpreted to reflect the combined action of fluvial and marine processes. Based on sediment‐type associations, the Mekong fluvial to marine transition zone could be subdivided into an upstream tract and a downstream tract; the boundary between these two tracts is identified 80 to 100 km upstream of the river mouth. The upstream tract is characterized by gravelly sand and sand and occasional heterolithic rhythmites, suggesting bed‐load supply and deposition mainly controlled by fluvial processes with subordinate tidal influence. The downstream tract is characterized by heterolithic rhythmites with subordinate sand and mud, suggesting suspended‐load supply and deposition mainly controlled by tidal processes with subordinate fluvial influence. Sediment distributions during wet and dry seasons suggest significant seasonal changes in sediment dynamic and depositional processes along the fluvial to marine transition zone. The upstream tract shows strong fluvial depositional processes with subordinate tidal influence during the wet season and no deposition with weak fluvial and tidal processes during the dry season. The downstream tract shows strong coexisting fluvial and tidal depositional processes during the wet season and strong tidal depositional processes with negligible fluvial influence during the dry season. Turbidity maxima are present along the downstream tract of the fluvial to marine transition zone during both wet and dry seasons and are driven by a combination of fluvial, tidal and wave processes. [ABSTRACT FROM AUTHOR]

The fluvial to marine transition zone (FMTZ) is the area of coastal rivers in which sedimentation is controlled by the interaction of fluvial and marine processes. This study examines the FMTZ of the Mekong River delta, along a total channel length of ~660 km. Methods consist of collection and analysis of channel bed sediment samples, measurements of channel morphological parameters, and recognition of mangrove, molluscan, and diatom species. The process regime, salinity, morphological, and sedimentary trends recognized were used to define two main tracts for this FMTZ: an upstream, fluvial-dominated tract and a downstream, tide-dominated tract. In more detail, they allow the identification of four subzones, from upstream to downstream: 1) fluvial-dominated, tide-affected; 2) fluvial-dominated, tide-influenced; 3) tide-dominated, fluvial-influenced; and 4) tide-dominated, fluvial-affected. Tide-induced water-level changes affect the entire study area and extend into Cambodia. Measured salinity intrusion extends ~15 km upstream of the river mouth during wet season, and ~50 km during dry season. Brackish water species of mangroves, mollusks, and diatoms, however, occur landward of these limits, suggesting that highly diluted brackish water may reach ~160 km upstream of the river mouth during the dry season. In the fluvial-dominated tract, channels are sinuous and show a seaward-deepening trend, whereas width is relatively constant. In the tide-dominated tract, channels are straight, and show seaward-widening and seaward-shallowing trends. Natural levees are present in the fluvial-dominated, tide-affected subzone, but are replaced by mangroves elsewhere along the FMTZ. In the fluvial-dominated tract, mud content is low, sand grain size fines seaward, and gravelly sand and sand are the dominant facies. In the tide-dominated tract, mud content is high, sand grain size is constant, recycled sand is common, and tidal rhythmites are the dominant facies. Mud pebbles are common in sediments throughout a large part of the FMTZ. These trends characterizing the FMTZ of the Mekong River delta seem to be present in other systems and likely represent a general FMTZ pattern. Nonetheless, minor differences may be observed between different types of systems, or because of differences in local conditions. The comprehensive description of trends and their mutual relationships along the FMTZ presented herein provides critical information that can form the basis of a general conceptual model and can help to better understand these complex zones. [ABSTRACT FROM AUTHOR]

To better understand the sedimentary facies of tide‐dominated deltas, a core dataset from the Ba Lai palaeochannel in the Mekong River Delta was obtained and studied. Nine sedimentary facies were identified and interpreted as representing the Late Holocene evolution of the Ba Lai palaeochannel, including its pre‐abandonment and post‐abandonment phases, as well as pre‐channel phases. The channel formed at 2.6 ka as a distributary channel connected to the deltaic network and was abruptly abandoned and rapidly infilled with sediment at 0.7 ka. The channel deposits are up to ca 11 m thick and overlie shelfal shell layers, which, in turn, overlie Mid‐Holocene and Pleistocene deposits. The active‐channel fill and mouth‐bar deposits consist of sand and mud with cyclical patterns, bidirectional lenses and abundant mud layers, suggesting deposition mainly driven by river and tidal processes. The abandoned‐channel fill consists entirely of organic‐rich mud, suggesting a predominance of tidal processes. Other sedimentary facies include tidal‐flat and marsh deposits; they mostly consist of mud and formed in shallow to subaerial areas near the channel margins or on barforms. Depending on the exact location of the core in this depositional setting, three possible stratigraphic successions and facies models are presented herein. Near channel margins, the deposits show an upward gradual change from heterolithic to mud with a well‐developed fining‐upward trend. Away from the channel margins, the deposits show an upward sharp change from heterolithic to mud due to the channel abandonment. The mouth‐bar‐area facies model shows an upward gradual change from heterolithic to heterolithic/sand to mud deposits with a coarsening‐upward to fining‐upward trend. Although differences among systems likely exist, the authors suggest that the sedimentary facies described in this study and the resulting facies models should be used to better understand tide‐dominated deltaic systems and to improve their interpretation in the geological record. [ABSTRACT FROM AUTHOR]

The Ba Lai distributary channel of the Mekong River Delta was abandoned and infilled with sediment during the Late Holocene, providing a unique opportunity to investigate the sediment fill, timing and mechanisms of channel abandonment in tide-dominated deltaic systems. Based on analysis and age dating of four sediment cores, we show that the channel was active since 2.6 ka and was abandoned at 0.7 ka as marked by the abrupt disappearance of the sand fraction and increase in organic matter and sediment accumulation rate. We estimate that the channel might have been filled in a time range of 45–263 years after detachment from the deltaic network, with sediment accumulation rates of centimetres to decimetres per year, rapidly storing approximately 600 Mt of organic-rich mud. We suggest that the channel was abandoned due to a sediment buildup favoured by an increase in regional sediment supply to the delta. This study highlights that mechanisms for abandonment and infilling of tide-dominated deltaic channels do not entirely fit widely used models developed for fluvial-dominated environments. Their abandonment might be driven by autogenic factors related to the river-tidal and deltaic dynamics and favoured by allogenic factors (e.g., human impact and/or climate change). [ABSTRACT FROM AUTHOR]

As the world's third largest delta and one of the world's most important biodiversity hotspots, the Mekong Delta provides both ecological and food security for its inhabitants. Nevertheless, the delta has been threatened by climate change and human activities, particularly the proliferation of hydropower development across the Mekong Basin since the 1990s. However, compared to the well-studied Holocene Mekong Delta, our understanding of the recent 50-year evolution of the Mekong Delta is not sufficient to address these threats. In this study, we used 43-year Landsat images from 1973 to 2015 to investigate the entirety of the Mekong Delta's shoreline, land area and geomorphological changes. We compiled a new and comprehensive GIS database of the dams and irrigations of the Mekong Basin. The goal was to improve our knowledge of the recent evolution of the Mekong Delta and to link the potential impacts of dams and other factors. Our results show that the Mekong Delta is experiencing a significant decrease in the shoreline progradation rate. Currently, 66% of the entire delta shoreline is under erosion. The erosion segments are mainly located on the eastern side of the Ca Mau Peninsula and northwestern side of the delta in the Gulf of Thailand (GoT). Most parts of the shorelines in the estuarine area are still growing, although river sediment continues to decrease. Geomorphological asymmetries, discontinuous shoreline shifts, and sandy beach-ridge sets can be observed in multitemporal images. The entire Mekong Delta experienced a shift from growing to shrinking around 2005 with the gradual increase of the total accumulated installed capacity of the dams over the entire river basin. In the near future, the realization of planned dams, extension of irrigation, groundwater withdrawal, uncontrolled riverbed mining, delta subsidence, sea level rise, and other factors will accelerate the ongoing delta erosion. [ABSTRACT FROM AUTHOR]

Amino accelerators and antioxidants (AAL/Os) have become a suite of contaminants of emerging concern recently due to the accumulating evidence for their environmental occurrence and associated toxic potential. Nevertheless, data on sedimentary deposition of AAL/Os has remained scarce, particularly for regions beyond the North America. In the present study, we elucidated spatial distribution of fifteen AAL/Os and five AAO transformation products (AAOTPs) in seventy-seven sediments from the Dong Nai River System (DNRS), Vietnam. Total concentrations of AAL/Os (∑AAL/Os) ranged from 0.377 to 51.4 ng/g (median: 5.01 ng/g). 1,3-Diphenylguanidine and 4,4′- bis (1,1-dimethylbenzyl) diphenylamine were the two most prevalent congeners, both with detection frequencies >80%. Additionally, AAOTPs were quantifiable in 79% of the DNRS sediments with a median ∑AAOTPs at 2.19 ng/g, dominated by N, N′ -diphenylbenzidine and 2-nitrodiphenylamine. Higher sediment-associated levels of AAL/Os and AAOTPs were spotted in downstream and the primary tributary of the DNRS compared to the upstream, implying their cumulative sedimentation towards the estuarine region. The distribution patterns of AAL/Os and AAOTPs across individual transects also demonstrated the influence of human activities (e.g., urbanization and agriculture), hydrodynamics, and decontamination by mangrove reserves. Meanwhile, characteristics of sediments, i.e., total organic carbon (TOC) content and grain sizes, exhibited significant correlations with the burdens of these compounds, indicating their preferential partitioning into the fine and TOC-rich matter. This research sheds light on environmental behavior of AAL/Os and AAOTPs beneath Asian aquatic system, and highlights the need for further evaluation of their impacts on the wildlife and public health. [Display omitted] • AAL/Os and AAOTPs were broadly distributed in the Dong Nai River System sediments. • DPG, diAMS, DPB, and 2-NO 2 -DPA were the most prevalent congeners. • Human activities and characteristics of sediments shaped the spatial trends. • This is the first comprehensive report on AAL/Os and AAOTPs in Asian sediments. [ABSTRACT FROM AUTHOR]

Abstract: Sedimentary processes on the inner Mekong Shelf were investigated by examining the characteristics of sediments sampled in gravity cores at 15 locations, including grain size, clay mineralogy, sediment accumulation rates, and the elemental and stable carbon isotopic composition of organic matter (atomic C/N ratios and δ13C). Deltaic deposits exhibit contrasting characteristics along different sides of the delta plain (South China Sea, SCS hereafter, to the east and Gulf of Thailand, GOT hereafter, to the west) as well as on and off the subaqueous deltaic system. On one hand, cores recovered from the subaqueous delta in the SCS/GOT are consisted of poorly/well sorted sediments with similar/different clay mineral assemblage with/from Mekong sediments. Excess 210Pb profiles, supported by 14C chronologies, indicate either “non-steady” (SCS side) or “rapid accumulation” (GOT side) processes on the subaqueous delta. The δ13C and C/N ratio indicate a mixture of terrestrial and marine-sourced organic matter in the deltaic sediment. On the other hand, cores recovered from areas with no deltaic deposits or seaward of the subaqueous delta show excess 210Pb profiles indicating “steady-state” accumulation with a greater proportion of marine-sourced organic matter. Core analysis’s relevance with local depositional environment and previous acoustic profiling are discussed. [Copyright &y& Elsevier]

Abstract: The initial growth stage of a river delta can be defined either as the beginning geomorphologic expansion of the river-dominated coastal lowland or as the starting basinward-directed progradation of the geological/stratigraphic delta body. Differentiating between these two perspectives is necessary for understanding the sensitive reaction of a deltaic system to dynamic changes in sea-level and sediment availability. The Holocene evolution of the northeastern part of the Mekong River delta (Southern Vietnam) offers a great chance to depict these two different definitions and relate them genetically to each other since the transition between an old abrasion platform (inner delta) and the continental-shelf basin (outer delta) is well developed in this marginal part of the delta. The early stage of delta growth taking place during transgression-related inundation between 8calkaBP (maximum flooding) and 5.7 (6.0–4.8) calkaBP (sea-level highstand) was characterized by tide-and saltwater-influenced nearshore conditions allowing extensive mangrove and tidal-flat deposits to aggradate on the wide abrasion platform. With the onset of regression around 4.8calkaBP the depositional center shifted immediately beyond the seaward margin of this estuarine platform and thus true deltaic progradation could develop leading to rapid construction of a delta body (and, in addition, to subsidence/tilting of the inner delta zone). Directly at the transition of these two deposystems, remains of shore-related deposits indicate the sudden breakdown of the estuarine platform conditions around 4.6calkaBP. [Copyright &y& Elsevier]

Abstract: A mesotidal beach on the Mekong River delta coast exhibits unique patterns of river sediment discharge and experiences reversals of ocean wave directions in response to the summer and winter monsoons. We analyzed long-term changes of the shoreline since 1936 from past topographic maps and satellite images, and short-term changes in geomorphology and sedimentology by undertaking repeated field surveys between November 2005 and February 2008 along six shore-normal beach transects. The shoreline of the Mekong River delta coast has changed asymmetrically over the last 70years in response to net southwestward sediment transport related to dry northeasterly winter monsoons. During the summer rainy season, large volumes of sediments are discharged from the river; at this time, wave direction is reversed in response to the relatively weak southwesterly summer monsoon. Mud and very fine sand in the surface sediments of the northeastern (updrift) beach transects tend to be removed during winter, suggesting that the sediment supplied from the river during summer is temporarily deposited near the river mouth and later transported southwestward during the winter monsoon. The relief of intertidal bars on the beach increased during winter in response to higher waves. However, previous studies have suggested that higher waves flatten bars. We hypothesize that the increase in relief that we observed is related to a wide and shallow subtidal delta-front platform that attenuates ocean waves at all times other than at high tide. The beach profile varies longshore: the accreting to stable beach dips seaward at a gradient of about 1/80 with up to three longshore intertidal bars, classified here as either slip-face bars or low-amplitude ridges. In contrast, the eroding beach profile is much steeper (1/20–1/40) and linear without intertidal bars, and is similar in form to a low-tide terrace. Because there is little variation of wave size and grain size within the study area, we consider that the spatial variation of the beach profile is related to the availability of sediment accumulated on the beach. [Copyright &y& Elsevier]

Having developed in the later Quaternary Period, the Mui Ne dunefield, representing the upper-most portion of a major coastal barrier complex in SE Vietnam, formed by wind regimes associated with the Asian monsoon climate. The barrier complex is 100 km long and higher than 150 m in elevation but despite its large dimensions, research on its geomorphological and geological characterization is limited. Several active dune fields extend 2–10 km landwards from the coast while the rest of the barrier surface is stabilized. We applied ground-penetrating radar (GPR) and quartz optically-stimulated luminescence (OSL) dating to the transverse dunes of the Mui Ne dunefield to characterize the morphostratigraphy and recent aeolian sedimentation history. Currently the dunes have a relief of typically 5–10 m and a wavelength of 50–100 m. They are driven by the northeasterly winter monsoon winds and reversed with minor relief in response to the southwesterly summer monsoon. GPR profiles and OSL ages define three morphostratigraphic units: the modern, Holocene, and Pleistocene dune units. The GPR data of the modern dune unit is dominated by a series of steep reflections that dip exclusively towards the west reflecting the winter monsoon and is punctuated by reactivation surfaces. Evidence for the wind reversal is not preserved in the sediment successions but is represented by reactivation surfaces. Thus a subunit bounded by the reactivation surfaces defines annual deposition. The preservation potential of a subunit varies with the migration rate of the transverse dune, and in the fastest migrating dunes, approximately one subunit was preserved every year (1.1 ± 0.6 subunit/yr). The modern dunes are generally younger than 100 years and have migrated WSW at net rates of 0.6–2.5 m/yr. The Holocene dune unit is as thick as the modern unit although it spans a period, ten times longer, reflecting the lower preservation potential of the transverse dunes as sedimentary records, due to erosion associated with their migration. The rate of dune migration suggests that an aeolian contribution of coastal sands has occurred following the sea level highstand of the Middle Holocene, accounting for the extent of the active dunefields. This mode of the barrier accumulation is considered as an analogue for earlier highstand events. However, the dunefields only cover a fraction of the entire barrier. It is thus inferred that barrier formation occurred over a long period, probably encompassing several glacial-interglacial cycles. Sand accumulation during the lowstand, either by transport of sand from the subaerially exposed shelf and/or reworking of sand emplaced during the prior highstand, may contribute significantly to the barrier development. • Mui Ne dunefield represents large barrier development during sea-level highstand. • GPR and OSL dating reveals morphostratigraphy of transverse dunes. • Bimodal dune migration under monsoons results in annual units defined by GPR. • Modern dune fields cover only a limited fraction of the large barrier. • Barrier evolution spans at least for a few glacial-interglacial cycles. [ABSTRACT FROM AUTHOR]

Studies on the sediment distribution along the fluvial to marine transition zone are rare and have focused on large-scale systems. Here, we report a case study from the Dong Nai River System (DNRS), a small-scale, tide-dominated river delta from southern Vietnam. The study is based on 80 channel bed sediment samples, together with channel depth, temperature, and salinity measurements, identification of mangroves and mollusks, and grain-size analysis. Year-round meter-scale water-level fluctuations due to tides are present across the entire study area, whereas saline-water intrusion affects the entire studied part of the Vam Co Dong–Vam Co River and the majority of the studied part of the Dong Nai–Soai Rap River. The sediment distribution of both channels highlights the presence of a downstream area characterized by soft mud indicating active deposition, and upstream areas characterized by sand and gravel sediments with abundant bedrock clasts and other evidence of the extremely limited sedimentation (e.g., encrusting biostructures, borings, coating, and agglutinated mats). The coarser-grained sand and gravel are likely reworked from local bedrock and transported for short distances as also suggested by their angularity and the similarities with Pleistocene sedimentary and igneous rocks in the study area. The sediment of the DNRS is overall coarser and muddier than the sediment of the Mekong, likely reflecting the smaller scale of this system, the lower discharge, and the local sediment supply from the bedrock. • The interactions of riverine and marine processes control this system. • An upstream non-depositional area and a downstream depositional area are identified. • A large part of the sediment is supplied locally from bedrock erosion. [ABSTRACT FROM AUTHOR]

Understanding the relationship between depositional processes and their products in the fluvial–marine transition zone in tide-dominated depositional systems is fundamental to improving environmental and stratigraphic interpretations of long-term sedimentary records. The distributary channels of the Mekong Delta represent a typical example of a complex sedimentary environment characterized by strong fluvial and tidal interactions, and seasonal variability in hydro-sedimentary dynamic processes, particularly the migration of a salt wedge front linked to seasonal fluctuations in freshwater discharge. For this study, we analyzed grain size, magnetic susceptibility, and organic elemental geochemistry of channel-bed surficial sediments collected in 2015 from all distributary channels during the dry season and from the Mekong–Co Chien River during the flood season. This paper reports spatial and seasonal differences in the characteristics of channel-bed surficial sediments, which are closely linked to sediment sources and depositional processes during the dry and flood seasons. Distributary channels are characterized by coarse-grained sediments in the upper reaches and fine-grained sediments in the lower reaches, in both dry and flood seasons, reflecting the difference between the upstream fluvial-dominated environment and the downstream tide-dominated environment. Flood season samples from the Co Chien River show higher magnetic mineral contents and a terrestrial source of organic carbon compared with dry season samples, indicating sourcing from the drainage basin and trapping of suspended materials in the distributary channel due to the barrier effect of the saltwater wedge at the river mouth. Compared with the flood season, during the dry season the mud content was lower in the downstream reach close to the river mouth and higher in the upstream reach of the Co Chien River, and the content of magnetic minerals was lower in both reaches, indicating dissolution of magnetic minerals during early diagenesis. Furthermore, the lower C/N ratios (<10) and slightly enriched δ13C values of the dry season samples from the Co Chien River imply an additional contribution of organic carbon from a marine source. These various features imply a landward import of mud into the Co Chien River induced by stronger salinity intrusion during the dry season. Along the downstream tract of the Bassac River, two mud-dominated reaches were identified during the dry season. The magnetic mineral contents, C/N ratios and δ13C values indicate that the mud-dominated reach near the Bassac River mouth is also supplied from resuspended sediments by estuarine circulation and tidal pumping, whereas the mud in the reach that is transitional from fluvial to tide dominance has a riverine supply. The results suggest that the Mekong distributary channels are important sinks for suspended sediments during both the flood and dry seasons, controlled by a sediment trapping mechanism provided by either estuarine processes or the morphological configuration of the channels, both of which are controlled by fluvial–tidal dynamics. These depositional processes may be compensating for the incision of channels induced by sand mining in recent decades. • High content of magnetic minerals and terrestrial OC in the flood season • Dissolution of magnetic minerals, degraded or marine-sourced OC in the dry season • Trapping of riverine suspended sediments in tide-dominated channels in flood season • Mud imported from the downstream channel and delta front in dry season [ABSTRACT FROM AUTHOR]