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Climate change and related sea-level rise pose significant threats to lowland coasts. However, the role of key controlling factors responsible for the frequency and landward extent of extreme storm surges is not yet fully understood. Here, we present a high-resolution sedimentary record of extreme storm surge flooding from the non-tidal southern Baltic Sea, spanning two periods: 3.6–2.9 ka BP and 0.7 ka BP until present. Sediments from coastal wetland, including sandy event layers, were analyzed by sedimentological (grain size, loss-on-ignition, micromorphology), geochronological (14C), geochemical (XRF), mineralogical (heavy minerals) and micropaleontological (diatoms) methods. The results show that both periods were characterized by high-frequency of storm surge flooding, in order of 1.3–4.2 events per century. These periods correlate with phases of enhanced storminess in northwest Europe and took place during both rising and fluctuating sea levels. The study shows that the frequency and landward extent of coastal inundation, largely depended on the development of natural barriers (e.g. beach ridges and aeolian foredunes). Thus, in the context of the future coastal storm-surge hazard, the protection of existing coastal barriers and their morphology is essential. [ABSTRACT FROM AUTHOR]

Climate change and related sea-level rise pose significant threats to lowland coasts. However, the role of key controlling factors responsible for the frequency and landward extent of extreme storm surges is not yet fully understood. Here, we present a high-resolution sedimentary record of extreme storm surge flooding from the non-tidal southern Baltic Sea, spanning two periods: 3.6–2.9 ka BP and 0.7 ka BP until present. Sediments from coastal wetland, including sandy event layers, were analyzed by sedimentological (grain size, loss-on-ignition, micromorphology), geochronological (14C), geochemical (XRF), mineralogical (heavy minerals) and micropaleontological (diatoms) methods. The results show that both periods were characterized by high-frequency of storm surge flooding, in order of 1.3–4.2 events per century. These periods correlate with phases of enhanced storminess in northwest Europe and took place during both rising and fluctuating sea levels. The study shows that the frequency and landward extent of coastal inundation, largely depended on the development of natural barriers (e.g. beach ridges and aeolian foredunes). Thus, in the context of the future coastal storm-surge hazard, the protection of existing coastal barriers and their morphology is essential. [ABSTRACT FROM AUTHOR]

Abstract Continuous coastal erosion has been observed on the shoreline of major river deltas worldwide as a result of relative sea level rise and anthropogenic impacts. However, the relative contribution of sea-level rise on deltaic coastline retreat (i. e. separately from anthropogenic interference) has not been clearly defined, due to a lack of research on naturally developing deltas. The Parnaíba River Delta, located on the NE Brazilian coast is one of the few examples of a river delta that is still developing in almost natural conditions in contrast to many heavily impacted deltas worldwide. The wave- and tide-dominated delta is subjected to delta switching processes with two active distributaries in the east and remnant tidal channels in the west. To address the decadal evolution of the delta in terms of suspended sediment transport and shoreline changes remote sensing techniques applied on orbital sensors were used. Satellite images from LANDSAT missions 2 to 8 were used to assess shoreline change between 1972 and 2016. Moreover, maps of suspended sediment concentration calculated from water leaving reflectance revealed the importance of Parnaíba River as major sediment supplier for the adjacent coast. Sufficient sediment supply from the Parnaíba River (PR) maintains shoreline stability and progradation of the delta in the region of the distributaries despite of a reservoir located 700 km upstream, which is supplied by only 25% of the drainage basin. Westward longshore drift of the sediment yielded by the Parnaíba River is an important transporting agent distributing river sediment along the coast. During ebb-phase, the PR discharge pushes the suspended sediment plume as far as 10 km offshore and the runoff forms a hydrodynamic barrier preventing alongshore transport of sediment from the east to the west part of the delta shoreline. During flood-phase and high tide, the river sediment plume is pushed shoreward by tidal forces and westward by longshore currents. A high turbidity zone extends from the river-mouth plume of the main distributary to the western tidal channels. Turbid water enters the tidal channels during flood-phase and leaves during ebb-phase. Coastal accretion or stability characterizes the eastern active part of the delta while moderate erosion prevails in the western abandoned part damped by the westward directed longshore sediment transport from the PR mouth. Highlights • Rivers are the major sediment suppliers to coastlines, especially for river deltas. • Sea-level rise and anthropogenic impacts are considered the major causes for coastline retreat worldwide. • Major river deltas of the world suffering of severe coastal erosion and shoreline retreat. • Satellite image is an important tool for short term (decadal) shoreline change mapping. • How does sea level rise affects a naturally developing delta? [ABSTRACT FROM AUTHOR]

The late Pleistocene Holocene stratigraphic architecture on the northeastern Brazilian continental shelf off the Parnaíba Delta has been explored by high-resolution seismic profiles. The seismic surveys reveal the widespread distribution of incised valleys of different size in offshore continuation of the present-day Parnaiba delta. According to morphology two channel types can be distinguished: U-shaped channels in the eastern part and V-shaped channels in the western part. The stratigraphic successions were grouped into four seismic units separated by different seismic boundaries. The characteristics of the seismic boundaries and internal reflectors of the seismic units were used to distinguish between marine and riverine deposits. The incised-valleys architectural elements were used to link sedimentation processes and variations in base level from late Pleistocene channel avulsion and channel infill in the lowermost course of the paleo-Parnaíba River to marine sediments of the present-day inner shelf. The change of the depositional environments in relation to deglacial sea-level rise is compared to incised valley infills of the Mekong River and Red River systems in Southeast Asia. [ABSTRACT FROM AUTHOR]

Aquino da Silva, A.G.; Stattegger, K.; Schwarzer, K.; Vital, H., and Heise, B., 2015. The influence of climatic variations on river delta hydrodynamics and morphodynamics in the Parnaíba Delta, Brazil. In this work, we present a baseline study on the geo-environmental, morphodynamical, and hydrodynamical conditions of the Parnaíba River and its mouth, the Parnaíba River Delta (PRD). The outline of the PRD indicates that waves are the major forcing factor for delta development. Furthermore, the shape of the delta suggests that there is a considerable degree of asymmetry in its formation. The asymmetry is the result of coastal current influence on the reworking of the sediments that are discharged by the river. To investigate the hydrodynamic conditions of the area, field measurements were conducted on the river, offshore of the PRD, and on the tidal channels of the west part of the delta. At these locations, the currents, salinity, temperature, and tidal dynamics were measured. In addition, a time series of rainfall, river discharge, and suspended sediment concentration (SSC) were obtained from Brazilian governmental agencies. The interpretation of all of the data was used to determine how they interact to characterize the hydro- and morphodynamics of the PRD. Harmonic analysis was of high importance in the analysis of the time series, and it was employed to identify longer periods presenting similarities in terms of meteorological conditions. This allowed determination of the interconnection between rainfall, river discharge, and SSC with the occurrence and intensity of the El Niño-Southern Oscillation. The influence of river discharge, rainfall, and SSC on shoreline change was demonstrated through the analysis of decadal shoreline evolution using a series of LANDSAT images covering the period from 1981 to 2009. The analysis revealed that most changes occurred westward from the PR mouth. Changes in spit migration velocity were correlated to variations in riverine sediment yielded as a result of variations of river discharge and SSC. [ABSTRACT FROM AUTHOR]

Sedimentary processes and the evolution of the wave- and tide-dominated, asymmetric Parnaíba Delta during the late Holocene were investigated based on geochemical and sedimentological analyses of sediment cores collected in 2010, as well as satellite images and historical maps. This is a rare case of pristine deltas essentially unaffected by human activities worldwide. The lowermost part of the main Parnaíba River distributary exhibits several low-sinuosity bends and several anastomosing bifurcation patterns in the east, whereas three NW-SE-oriented tidal channels drain a large mangrove area in the west. Dating of various materials in sediment cores from the tidal flats, tidal channels and supratidal marshes revealed that the oldest sediment (4,853 to 4,228 cal. years BP) is paleo-mangrove soil from the main river distributary. Present-day mangroves and marshes up to 200 years old exhibit high sedimentation rates reaching 3.4 cm/year. The asymmetry of the delta is explained not only by the wind- and wave-induced westward-directed longshore drift but also by neotectonic processes, as revealed by satellite images. Faulting and eastward tilting may have triggered delta lobe switching from west to east. This would explain the erosional character and unusual updrift orientation of the main river-mouth channel. Consistent with existing knowledge on mangrove ecosystems worldwide, sediment carbon and nitrogen signatures lie in the range of freshwater or marine dissolved organic carbon and C terrestrial plants. In the western tidal channels, the low C/N ratios (16-21) of young mangrove soil (deposited in the last 16 years) reflect a stronger influence of marine plants compared to older mangroves (1,390-1,525 cal. years BP; ratios of 20-37). Thus, there would have been a greater influence of the Parnaíba River on tidal-channel sedimentology 1,400 to 1,500 years ago, entailing a natural connection between the present-day tidal channels and the river in ancient times, which was abandoned later during delta lobe switching. This is substantiated by historical maps that indeed show this connection between the main distributary and the tidal-channel system. [ABSTRACT FROM AUTHOR]

The late Pleistocene–Holocene stratigraphic architecture on the steep and narrow shelf off Nha Trang, central Vietnam has been explored by high resolution seismic profiles integrated with sediment core data. Sequence stratigraphic results reveal five major seismic units and three bounding surfaces which are composed of two distinctive sequences. Those sequences are bounded by two regional unconformities (SB1, SB2) which have been formed in respond to different sea-level regimes. The revealed relict beach–ridge deposits at water depth of about ∼130 m below the present water depth indicate that the Last Glacial Lowstand (LGM) sea-level in this area was lower than in neighboring areas and it probably resulted from subsidence due to high sedimentation rate and/or neotectonic movements of the East Vietnam Fault System. The late Pleistocene high amplitude of sea-level change during a long fourth-order and superimposed by shorter fifth-order cycle is the principal factor in reorganizing the formation of the Nha Trang continental shelf sequence. Other local controlling factors as fluctuations in sediment supply, morphological variations of the LGM surface, subsidence rate and hydrodynamic conditions provided the distinctive features of the Nha Trang shelf sequence stratigraphic model in comparison with neighboring other areas. [ABSTRACT FROM AUTHOR]

Highlights: [•] The Tarfaya basin sediments were deposited in a passive margin tectonic setting. [•] The Tarfaya basin sediments were originated from felsic source rocks. [•] The Tarfaya basin sediments demonstrate a change in provenance over time. [ABSTRACT FROM AUTHOR]

The petrography, heavy mineral analysis, major element geochemical compositions and mineral chemistry of Early Cretaceous to Miocene-Pliocene rocks, and recent sediments of the Tarfaya basin, SW Morocco, have been studied to reveal their depositional tectonic setting, weathering history, and provenance. Bulk sediment compositional and mineral chemical data suggest that these rocks were derived from heterogeneous sources in the Reguibat Shield (West African Craton) including the Mauritanides and the western Anti-Atlas, which likely form the basement in this area. The Early Cretaceous sandstones are subarkosic in composition, while the Miocene-Pliocene sandstones and the recent sediments from Wadis are generally carbonate-rich feldspathic or lithic arenites, which is also reflected in their major element geochemical compositions. The studied samples are characterized by moderate SiO contents and variable abundances of AlO, KO, NaO, and ferromagnesian elements. Binary tectonic discrimination diagrams demonstrate that most samples can be characterized as passive continental marginal deposits. AlO/NaO ratios indicate more intense chemical weathering during the Early Cretaceous and a variable intensity of weathering during the Late Cretaceous, Early Eocene, Oligocene-Early Miocene, Miocene-Pliocene and recent times. Moreover, weathered marls of the Late Cretaceous and Miocene-Pliocene horizons also exhibit relatively low but variable intensity of chemical weathering. Our results indicate that siliciclastics of the Early Cretaceous were primarily derived from the Reguibat Shield and the Mauritanides, in the SW of the basin, whereas those of the Miocene-Pliocene had varying sources that probably included western Anti-Atlas (NE part of the basin) in addition to the Reguibat Shield and the Mauritanides. [ABSTRACT FROM AUTHOR]

Abstract: The abrupt transition from fluvial to marine deposition of incised-valley-fill sediments retrieved from the southeast Vietnamese shelf, accurately records the postglacial transgression after 14ka before present (BP). Valley-filling sediments consist of fluvial mud, whereas sedimentation after the transgression is characterized by shallow-marine carbonate sands. This change in sediment composition is accurately marked in high-resolution X-ray fluorescence (XRF) core scanning records. Rapid aggradation of fluvial sediments at the river mouth nearly completely filled the Mekong incised valley prior to flooding. However, accumulation rates strongly reduced in the valley after the river-mouth system flooded and stepped back. This also affected the sediment supply to deeper parts of the southeast Vietnamese shelf. Comparison of the Mekong valley-filling with the East Asian sea-level history of sub- and inter-tidal sediment records shows that the transgressive surface preserved in the incised-valley-fill records is a robust sea-level indicator. The valley was nearly completely filled with fluvial sediments between 13.0 and 9.5 kaBP when sea-level rose rather constantly with approximately 10 mm/yr, as indicated by the East Asian sea-level record. At shallower parts of the shelf, significant sediment reworking and the establishment of estuarine conditions at the final stage of infilling complicates accurate dating of the transgressive surface. Nevertheless, incised-valley-fill records and land-based drill sites indicate a vast and rapid flooding of the shelf from the location of the modern Vietnamese coastline to the Cambodian lowlands between 9.5 ka and 8.5 kaBP. Fast flooding of this part of the shelf is related with the low shelf gradient and a strong acceleration of the East Asian sea-level rise from 34 to 9 meter below modern sea level (mbsl) corresponding to the sea-level jump of melt water pulse (MWP) 1C. [Copyright &y& Elsevier]

The narrow shelf along the coast of central Vietnam is seasonally supplied by large amounts of sediment from the adjacent mountainous hinterland following monsoonal precipitation. This study examines the fate of these sediments, and their accumulation rates along two transects across the shelf, based on analyses of radionuclides (210Pb, 137Cs), sediment texture and structure, as well as carbonate content. The inner shelf is covered by sands, and probably serves as bypass zone for fine sediments transported offshore. Sediment characteristics suggest that the transport to the mid and outer shelf is related to flood events. Averaged over the last century, the 210Pb-based mud mass accumulation rates on the mid and outer shelf vary between 0.25 g cm −2 and 0.56 g cm −2 year −1 (corresponding to linear sediment accumulation rates of 0.20–0.47 cm year −1). Along with high excess 210Pb inventories, these high accumulation rates suggest a significant sediment depocentre on the mid shelf. The 210Pb-derived sediment accumulation rates were found to be several times higher than 14C-derived rates previously reported for the Holocene, at the same location on the outer shelf. This is probably due to the incompleteness of the Holocene record, and an overestimation of the modern rate. Another explanation would be increased erosion within the rivers’ drainage basins, due to 20th century deforestation. This hypothesis is supported by the difference between recent (less sand, more lithic grains in the sand fraction) and older sediments. In terms of modern sedimentation processes and rates, the central Vietnam shelf, although being a part of a narrow passive continental margin, is similar to active flood-dominated continental margins. [ABSTRACT FROM AUTHOR]

Abstract: The Holocene sea-level history of the mesotidal northern Rio Grande do Norte coast, NE Brazil, was reconstructed from sedimentology and AMS radiocarbon dating of beachrock and tidal lagoonal sediments. At 7000 cal. year BP at −3 m sea level stood and passed the modern sea level 6700 cal. year ago. The Holocene highstand reached 1.3 m above modern sea level at 5900 cal. year BP as determined by beachrock samples from the uppermost part of an intertidal foreshore deposit. A 5700 cal. year old shell midden overlies this beachrock that represents the spring-tide sea level. After the highstand, sea level dropped to its present position. Beachrock samples from intertidal upper foreshore deposits were used for elevation and age control. Beachrock positions with their error bars are all within the tidal-range envelope. Therefore, no secondary oscillations during the sea level fall can be confirmed. A linear sea-level fall of this magnitude can be explained by the mechanism of equatorial ocean syphoning. [Copyright &y& Elsevier]

Regressive siliciclastic deposits from the central Sunda Shelf (Southeast Asia) were investigated by seismic surveying and sediment coring. High-resolution stratigraphy is favoured by the low shelf gradient and by a strong and focused sediment supply during periods of lower sea level. Three types of regressive deposits formed over the past fifty thousand years can be distinguished: (1) thick lens-shaped clinoform sediment ‘bodies’ up to 30 km wide which fill gentle depressions; (2) thin horizontally bedded deposits between the lens-shaped ‘bodies’; and (3) a thick progradation sediment wedge on the outer shelf and upper slope. The sediment cores reached the upper part of the regressive deposits and contain a regressive–transgressive succession of terrestrial, tidal and shallow-marine sediment facies. The lens-shaped sediment ‘bodies’ are interpreted as regressive deposits detached from each other, probably reflecting the interplay of low morphological gradient and minor sea-level fluctuations during Marine Isotope Stage 3, which resulted in the reduction of accommodation space and local depositional stacking. The Late Pleistocene sedimentation on the outer shelf was partly controlled by differential subsidence of the margin or by wave and wind-induced current action. [Copyright &y& Elsevier]

Discusses a study which presented a record of the late-glacial transgression of Sunda Shelf, the largest shelf area outside the polar regions between the Indonesian archipelago and Vietnam. Indications of accelerator mass spectrometry radiocarbon dates of organic material; Description of the Sunda Shelf; Resolution to the rapid flowing in the shelf.

• Small mountainous rivers are the major sediment sources on the Vietnam Shelf. • Four stages of weathering and erosion occurred in central Vietnam since Younger Dryas. • Weathering and erosion of central Vietnam are controlled by the East Asian monsoon. Clay mineralogy and elemental geochemistry of Core SO18401 on the Vietnam Shelf are used to reconstruct the history of weathering and erosion in central Vietnam over the past 13.2 ka. The variability of the clay species across the 13.2 ka BP together with the age model allows a subdivision into four temporal stages. Provenance analysis based on clay mineralogy indicates that small mountainous rivers in southern-central Vietnam are the major sources of terrigenous sediments on the central Vietnam Shelf. Smectite and kaolinite originated from the chemical weathering of Mesozoic and Neogene-Quaternary basaltic rocks and Paleo-Mesozoic felsic intrusive rocks, respectively, under warm and humid climate conditions; whereas illite and chlorite were mainly derived from physical weathering of Paleo-Mesozoic felsic intrusive rocks and Precambrian metamorphic rocks in central Vietnam. Accordingly, smectite/(illite + chlorite) ratio, together with elemental ratios of Al 2 O 3 /K 2 O, TiO 2 /Al 2 O 3 , and SiO 2 /Al 2 O 3 , can be utilized to reflect the relative importance between chemical weathering and physical weathering/erosion of the parent rocks in southern-central Vietnam. Higher smectite/(illite + chlorite) and Al 2 O 3 /K 2 O ratios indicate a more important role of chemical weathering, and higher TiO 2 /Al 2 O 3 and SiO 2 /Al 2 O 3 ratios suggest enhanced contribution of physical weathering and erosion. A combination of these mineralogical and elemental proxies allows the distinction of four stages of weathering and erosion evolution in southern-central Vietnam over the Holocene and Younger Dryas periods. Enhanced physical weathering and erosion took place during the periods ∼13.2–11.0 ka BP (Stage I) and ∼3.9–2.0 ka BP (Stage III), associated with relatively arid conditions. By contrast, strengthened chemical weathering occurred during the periods ∼11.0–3.9 ka BP (Stage II) and 2.0 ka BP to present (Stage IV), corresponding to more warm and humid climatic conditions. Our study suggests that the weathering and erosion history of southern-central Vietnam over the Holocene is mainly controlled by the East Asian monsoon climate. [ABSTRACT FROM AUTHOR]

The São Francisco delta in eastern Brazil is considered a classical example of a wave-dominated delta constructed in a glacial isostatic adjustment (GIA) far-field region. Although this delta is widely mentioned in the international literature, few details about its sedimentary architecture have been published. In this paper, we use a new dataset consisting of 15 boreholes and 26 radiocarbon datings to discuss the 3D architecture of the delta and the role played by GIA far-field effects in delta construction. The delta plain consists of a 10–12 m thick littoral sand sheet that progrades over a muddy subaqueous clinoform, forming a compound clinoform. Delta construction began between 8.4 and 7.2 ka BP and included a stage of normal regression that lasted until 5.7 ka BP, followed by a stage of forced regression. The transition between these two stages is marked by the highest position of the sand–mud transition in the boreholes. This forced regression is due to a 3–4-m drop in relative sea level since the mid-Holocene, which is characteristic of GIA far-field regions. Additionally, as a consequence of this drop in relative sea level, there is a progressive increase in the altitude of the beach-ridge deposits continentward, which combined with the development of extensive dune fields in the later stages of delta construction, contributes to the increased resilience of the delta to ongoing sea-level rise. The 3D architecture of the delta was strongly influenced by antecedent topography. The delta was built on a bathymetric depression on the shelf that almost entirely traps the sediment load of the São Francisco River. The uneven relief inside this bathymetric depression ultimately controlled lateral variations in the total thickness of the delta edifice. • Two-stage delta construction in GIA far-field regions: normal and forced regression. • Drop in sea level since mid-Holocene in GIA far-field regions increases delta resilience to ongoing sea-level rise. • Antecedent topography strongly controlled the delta 3D architecture. • 3D architecture of the São Francisco wave-dominated delta is presented for the first time. [ABSTRACT FROM AUTHOR]

A combination of detrital zircon U-Pb geochronology and rare earth element geochemistry was employed to investigate the "source to sink" pathways of Eocene--lower Miocene strata in the northern South China Sea (SCS) taking into account the evolution of the surrounding drainage systems in comparison to the basin sediments. Drastic provenance transformation on the SCS sedimentary patterns has been first discovered that source of Central Vietnam was replaced by Pearl River networks. Rapidly changing local provenances of different source rocks were discerned during Eocene and early Oligocene times. Briefly, the Pearl River drainage system gradually evolved by regional tectonic processes into the present scale and exerted its influence on the northern SCS basins. During early Oligocene, only limited areas received sediments from Pearl River tributaries on the South China continent, while the majority of the southern areas were controlled by the Central Vietnam provenance before the late Oligocene. After the early Miocene, significant amounts of sediment from the inner South China continent were transported into northern SCS basins by the western Pearl River tributaries. Meanwhile, an impressive "Kontum-Ying-Qiong" river system (K-Y-Q) had delivered huge amounts of sediments from Central Vietnam mafic-toultramafic source rocks to the regions farther east since the early Oligocene or even Eocene. This remarkable K-Y-Q traversing the whole Qiongdongnan Basin was gradually replaced by a transport system from the adjacent Hainan Island, in modern-day China, providing acidic detritus after the early Miocene. Admittedly, Tethyan subduction beneath Sundaland makes the paleo-reconstruction work tough and elusive. However, we laid the emphasis on possible basin depositions to unravel the mystery and our discovery of the impressive conduit from Central Vietnam of the Indochina Block, transferring abundant basic sediments derived from the Indosinian event, surely would be of remarkable signifi- cance on research of (1) paleo-geographical evolution by Tethys-southern Eurasia Plate collisional effects and (2) tectonic reconstruction for the entire southeastern margin of the Eurasian continent. [ABSTRACT FROM AUTHOR]

The sedimentological character of material in transport and on the riverbed were investigated in order to record the sedimentary dynamicsin the lowermost Amazon. Suspended sediment concentration measurements show that net deposition occurs on the delta plain (between Obidosand the Xingu River), while downstream suspended sediment transport remains approximately constant. Lateral sediment input is compensatedby erosion in deeply incised channels keeping the region in a dynamic equilibrium. The surficial distribution of bedload sediments carried by the Amazon shows a corridor of sands (channel deposits) borderedby silty sediments at the margins. It consists chiefly of very fine sands, which become fine and medium-coarse sands laterally (Figure 6). The development of bedforms on the sand surface reflects strong bottom currents and sediment transport, as well as tidal effects. Nearshore sedimentary facies identified were interlaminated fine and coarsesilt, interlaminated silt and sand, faintly laminated mud, and mottled mud. [ABSTRACT FROM AUTHOR]

In this study, 11 CBERS 2B and 1 LANDSAT 5-TM satellite images from 2008 were used to estimate the suspended sediment concentration and the total suspended sediment load of the Parnaíba River (NE-Brazil). The calculation of the amount of sediment in suspension was performed using Tassan’s algorithm, which was originally developed for use on LANDSAT 5-TM images; therefore, the CBERS 2B images were spectrally calibrated using LANDSAT 5-TM at-satellite radiance. The application of atmospheric correction to the images was necessary to account for meteorological influence on the spectral data prior to the calculation of the suspended sediment concentration. Three types of dark object subtraction and the 6S model were tested, and one type of dark object subtraction was chosen as the appropriate atmospheric correction method. Tassan’s algorithm requires in situ calibration; therefore, suspended sediment concentrations measured in water samples from the Parnaíba River mouth were used to calibrate the algorithm. The results revealed that the variation of suspended sediment concentration was strongly influenced by seasonal precipitation. In 2008, the suspended sediment released by the Parnaíba River was approximately 2.54 × 10 6 tons. The discharged sediment formed a sediment plume on the inner continental shelf. The extension of the plume depended on the specific hydrodynamic conditions that were forced mainly by the strength of river runoff, longshore currents, tidal currents and amplitudes, and wind and wave climate. [ABSTRACT FROM AUTHOR]

This study aims to reconstruct the Holocene evolution of a semiarid tidal channel system in response to the deglacial sea-level rise and modern coastal processes. With a maximum water depth of approximately 10 m, this very shallow system is in the easternmost part of the Brazilian equatorial margin along a tide-dominated coast in a semidiurnal, mesotidal regime. The channel system is interconnected with the Atlantic Ocean through a tidal inlet formed by the Galinhos spit. This work comprised ship and land-based fieldwork with sedimentological and stratigraphic approaches. Four seismic units (U1-U4) and seismic horizons (H1-H4) were identified by reconstructing the stratigraphic architecture. The results show that: (1) the formation of paleochannels in the current spit may have started before the middle Holocene; (2) there was a network of pre-existing channels with a different configuration filled by units U2 and U3, constituting the transgressive and highstand systems tracts; (3) spit migration is the key factor to open and close inlets; (4) internal configuration patterns show a less energy and monotonous depositional environment above H4, within unit U4 (which started at ~5000 cal yrs. BP); (5) the current system configuration has been active since the formation of the U3 unit since the closure of the paleochannels; and (6) the Galinhos region shows progressive sedimentary infilling of a lagoon behind a coastal spit via sediment accumulation from marine sources and its ultimate transformation into a coastal swamp with tidal channels. • Holocene semi-arid tidal channel system. • Equatorial spit-barrier island system, in a semidiurnal, mesotidal regime • Identification of four Holocene units and four boundaries • Definition of the Holocene seismic architecture framework of the Galinhos tidal channels system. • Identification of transgressive, highstand and lowstand system tracts • Sedimentary infilling of a lagoon behind a coastal spit. [ABSTRACT FROM AUTHOR]

Economic development of the São Francisco River basin has increased by building seven river dams since the 1950s. Subsequently, strong coastal erosion reaching maximum rates of 98.6 m/year has been recorded at the river mouth, also leading to the destruction of a coastal village. This work combines discharge data with new sediment and hydro-acoustic records, collected in January 2009, to verify the impact of river damming on processes, controlling coastline evolution. Data analysis is focused on discharge evolution, morphology and sediment composition of subaqueous dunes as well as on the impact by tidal currents. Discharge has been strongly regulated since 1986, when the last river dam was implemented. Again, bedform morphology seems to be not affected neither by reduced river-discharge nor by reduced sediment supply. However, migration rates of these dunes, as calculated, are lower due to decreased discharge. At the same time, tidal impact is strengthened at the lowermost part of the river. From this we assume, that the replenishment of sediment at the coast near the river-mouth might be time-delayed, which could support coastal erosion. [ABSTRACT FROM AUTHOR]

• Depositional sequences and sedimentary facies/units were established on core records and seismic profiles. • Depositional sequences were primarily constrained by the sea-level change over the past 140 kyr. • Regressive systems tract is dominant compared to transgressive systems tract in a complete sequence. Two 40 m-long sediment cores BL01 and SC02 were drilled in the lower Mekong River Delta plain and compared to high-resolution seismic profiles from the adjacent shelf documenting the presence of two depositional sequences (SQ1, SQ2) for the past 140 kyr. SQ2 is a fourth-order sequence and covers a complete sea-level cycle of 120 kyr. SQ1 is a fifth-order sequence comprising a sea-level half cycle in the past 20 kyr. These two sequences are bounded by the sequence boundaries (SB1, SB2) and the topographic surface. The transgressive systems tract (TST2) of SQ2 was probably formed during the sea-level rise from the Marine Isotope Stage (MIS) 6 lowstand to the MIS 5e highstand, indicated by riverine-estuarine and shallow-marine sediments. The regressive systems tract (RST2) of SQ2 corresponds to the sea-level lowering from MIS 5e to 3 with deltaic and alluvial plain sediments prevailing. These sediments were then gradually exposed to tropical subaerial processes to generate the formation of patchy reddish-orange stiff sandy silt and clay. The overlaying sequence SQ1 contains the lowstand systems tract (LST1), TST1, and RST1. LST1 consists of deltaic sediments at the shelf edge deposited during the MIS 2 sea-level lowstand. TST1 includes retrograding-fluvial, estuarine, and shallow-marine sediments deposited during the deglacial sea-level rise 19–8 cal kyr BP and the maximum flooding period 8–6 cal kyr BP before the advance of the modern Mekong River Delta. RST1 comprises the modern delta-facies association formed during the past 8 kyr and is restricted to the water depth of 25 m. [ABSTRACT FROM AUTHOR]

• Micaceous, limonites and opaque heavy minerals are useful diagnostic features. • Lim, Lim-Mi-Chl, Opq/Lim-Chl-Tur, Opq-Tur-(Ep-Zr) and Opq-Amp-(Ep) heavy minerals provinces distinguished on the shelf. • Mineral groups: Lim-Chl, Amp-Ep, and Opq-Tur-Zr are attributed to specific source areas and dominant transport paths. • Dominant sediment transport from the Mekong River over the continental shelf is southwestward. • Vast mixing zone of different modern and relict sediments is east of the Mekong River mouth. The continental shelf off southern Vietnam is relatively well studied regarding the dominant transport paths of bulk and fine-grained sediments discharged by the Mekong River. Thus, this region is particularly suitable for testing the applicability of heavy mineral analysis in determining the extent and origin of the sandy fraction of continental shelf deposits supplied mainly by a large river and their distribution over the adjacent extensive shelf. The fine sand fraction of 105 sediment samples from the Mekong River Delta and neighbouring continental shelf is investigated, applying two approaches: i) considering only the transparent non-micaceous mineral assemblage and ii) considering all the identified heavy minerals. The geological complexity of the source areas and local dominance of micaceous and non-transparent heavy minerals require the inclusion of all heavy mineral components when interpreting sediment provenance and dominant transport paths. The most diagnostic heavy minerals, amphibole, chlorite, epidote, limonite, opaques, tourmaline and zircon, are used for this purpose. The investigated continental shelf can be divided into two main regions characterised by various heavy mineral assemblages, namely, SW and NE off the Mekong River Delta. The south-western region is dominated by chlorite and limonite, indicating prevailing southward sediment transport from the Mekong River, whereas the north-eastern region is characterised by amphibole, epidote, opaques, tourmaline and zircon, indicating various sources from SE Vietnam and relict sediments partly covering the continental shelf. Between the two main regions, a large mixing zone is found where sediments from all sources are present, including Mekong River sediments transported eastward. [ABSTRACT FROM AUTHOR]

The Gulf of Tonkin coastline migrated at an average rate of ca 60 m year−1 landward during Holocene sea-level rise (20 to 8 ka). Due to a combination of rapid coastline migration and undersupply of sand, neither coastal barriers nor tidal sand bars developed at the mouth of the Red River incised valley. Only a 30 to 80 cm thick sandy interval formed at the base of full-marine deposits. Thus, the river mouth represented a mud-dominated open funnel-shaped estuary during transgression. At the base of the valley fill, a thin fluvial lag deposit marks a period of lowered sea-level when the river did not reach geomorphic equilibrium and was thus prone to erosion. The onset of base-level rise is documented by non-bioturbated to sparsely bioturbated mud that occasionally contains pyrite indicating short-term seawater incursions. Siderite in overlying deposits points to low-salinity estuarine conditions. The open funnel-shaped river mouth favoured upstream incursion of seawater that varied inversely to the seasonal strongly fluctuating discharge: several centimetres to a few tens of centimetres thick intervals showing marine or freshwater dominance alternate, as indicated by bioturbational and physical sedimentary structures, and by the presence of Fe sulphides or siderite, respectively. Recurrent short-term seawater incursions stressed the burrowing fauna. The degree of bioturbation increases upward corresponding to increasing marine influence. The uppermost estuarine sediments are completely bioturbated. The estuarine deposits aggraded on average rapidly, up to several metres kyr−1. Siphonichnidal burrows produced by bivalves, however, document recurrent episodes of enhanced deposition (>0·5 m) and pronounced erosion (<1 m) that are otherwise not recorded. The slope of the incised valley affected the sedimentary facies. In steep valley segments, the marine transgressive surface (equivalent to the onset of full-marine conditions) is accentuated by the Glossifungites ichnofacies, whereas in gently sloped valley segments the marine transgressive surface is gradational and bioturbated. Marine deposits are completely bioturbated. [ABSTRACT FROM AUTHOR]

Three sedimentary systems tracts (LST, TST, and HST) have been identified in Nha Phu Bay and the adjacent shelf, Khanh Hoa Province, Vietnam. These have been deposited since the Last Glacial Maximum (LGM) and interpreted in this study via the analysis of high-resolution acoustic data and sediment cores. The lowstand systems tract (LST) comprises sandy sediments exposed at seabed on the outer shelf as sand ridges, and underlying the transgressive systems tract (TST) on the middle shelf. The LST developed under shallow-marine settings during the sea-level lowstand and early sea-level rise before 14.6 cal kyr BP. The TST comprises primarily fine-grained sediments deposited during the transgression between 14.6 and 8.0 cal kyr BP in an incised trough of the outer shelf, on the middle shelf, and in Nha Phu Bay. Erosion of the seaward portion of the TST on the middle shelf might occur during the late transgression (9.0–8.0 kyr BP). The highstand systems tract (HST) comprises sediments deposited during the sea-level highstand between 8.0 and 0.0 cal kyr BP and is distributed widely in the bay, on the inner-middle shelf, and partially on the outer shelf. A paleo-coast zone, located in 100–120 m present-day water depth, is recognized through the appearance of sand ridges and sandy sediments. The seafloor sediment distribution is mapped via four major acoustic echo types, combined with seabed sediment samples. The coarse-grained sediment distribution, subaqueous outcrops, and thin deglacial sediment thickness on the inner shelf indicate erosion-dominated processes under vigorous hydrodynamic conditions. Nha Phu Bay and the middle shelf are interpreted as the depocentres. The estimated sediment load deposited in Nha Phu Bay is 0.18 Mt./yr, accounting for 15–35% of the annual sediment discharge from the Dinh River. The seabed morphology, muddy sediment distribution, and deglacial sediment thickness reveal a dominant sediment-transport pathway, across the inner shelf to the middle shelf, near the northern shoreline of Nha Phu Bay under the influence of the southwest monsoon. • Sequence-stratigraphic framework for the study area has developed under global sea-level changes since the LGM. • Sedimentary processes were interpreted on sequence stratigraphy, sediment distribution, seabed morphology, and hydrodynamics. [ABSTRACT FROM AUTHOR]

The South China Sea offers an excellent case for studying source-to-sink transport processes of fluvial sediments among the global marginal seas. This study synthesizes existing clay mineralogical and geochemical data from ~ 1500 samples from the seafloor and surrounding rivers, deepwater mooring observation results, and high-resolution glacial–cyclic clay mineralogy records from six high-quality sediment cores. Source-to-sink sediment transport from the river mouth to the continental shelf and then to the abyssal basin is investigated at two time scales: modern process and Late Quaternary glacial cycles. The results firstly show the high diversity of clay mineralogical and geochemical compositions in riverbed surface sediments surrounding the South China Sea, e.g., dominant illite and chlorite in Taiwan, overwhelming majority of smectite in Luzon, dominant kaolinite in South China, and similar amounts of the four clay mineral species in Red and Mekong river systems. The formation of these fluvial sediments through chemical weathering in surrounding drainage systems is controlled principally by the East Asian monsoon climate with warm temperature and high precipitation, and subordinately by tectonic activity and specific lithological character. The basin-wide distribution of clay mineral assemblages combined with neodymium and strontium isotopic compositions reflects strong provenance control and differential settling effects. The differential settling of kaolinite in slightly saline and proximal regions relative to smectite in more saline and distal regions is well demonstrated in the South China Sea. Through combining clay mineralogical distributions on the seafloor with observed oceanic current systems, the modern transport pathways can be well established, e.g., in the northern South China Sea, illite and chlorite from Taiwan are mainly carried by contour currents with the strong influence of mesoscale eddies. High-resolution sediment dynamic analysis for the Late Quaternary reveals different sediment transport patterns in the northern, western, and southern South China Sea when the land–sea configuration dramatically changed during glacial conditions. In the north, the terrigenous dispersal is mainly controlled by provenance supply and oceanic current transport. In the west, the clay mineral assemblage reflects variations of the prevailing surface current influenced by monsoon winds. In the south, the clay mineral input indicates intensive chemical weathering during interglacial periods and strengthened physical erosion during glacial periods. The transport of terrigenous sediments since the last glaciation is quantitatively reconstructed through studying two deepwater sediment cores located in the northern South China Sea. The relative contributions from three provenances (South China, Luzon, and Taiwan) highlight their distinct variability over the last 28 ka, implying that the southward shift of the inter-tropical convergence zone (ITCZ) at 16 ka BP caused an increased sediment contribution from South China, the intensified influence of the Kuroshio Current intrusion controls more efficient westward transport of Luzon sediments, and the stronger deepwater current transports Taiwan-sourced sediments further westward from the last glaciation to the Holocene. [ABSTRACT FROM AUTHOR]

Marine productivity is a key process in controlling the global carbon cycle. To understand the paleo-productivity behavior in the southern South China Sea, multiple geochemical analyses on Core SO18383–3 recovered from the continental slope off the Mekong River were integrated to reveal the response of paleo-productivity to environmental change. The elemental ratios of Bio/Al, bBa/Al, Zn/Al, and Sr/Al allowed the reconstruction of paleo-productivity evolution. The results show that the paleo-productivity was low around the Last Glacial Maximum (LGM) and early deglaciation (29.9–14.8 cal ka BP), then increased during the main deglaciation period (14.8–8.0 cal ka BP), before gradually declining during the Holocene (8.0–1.1 cal ka BP). This variation in paleo-productivity intensity was significantly controlled by the change in phosphorus input driven by the East Asian summer monsoon. Moreover, the opal/CaCO 3 proxy, which reflects the relative abundance of siliceous versus calcareous phytoplankton, showed a higher siliceous phytoplankton dominance in the latest Pleistocene (29.9–10.8 cal ka BP) than in the Holocene. This was possibly governed by the upwelling process with cold waters forced by a cold-core eddy during the strengthening of the East Asian winter monsoon. This study greatly increased our understanding of the principal forcing factors on paleo-productivity in the southern South China Sea over the last 30 ka. • Productivity variations on the continental slope off the Mekong River show three general stages: low productivity level during the period around LGM, gradual increase in productivity level during the main deglaciation period, and declining level of productivity in the Holocene. • Paleo-productivity in this area was significantly related to phosphorus supply that could be controlled by the East Asian summer monsoon. • The dominance of siliceous phytoplankton was found during the latest Pleistocene (29.9–14.8 cal ka BP) whereas biogenic carbonate prevailed during the Holocene. [ABSTRACT FROM AUTHOR]

The Parnaíba River Delta (PRD) is the second major delta in NE Brazil and occupies approximately 100 km of the Brazilian equatorial coast. This delta is highly asymmetric due to a large tidal channel complex in the western portion that has at present time no natural connection to the Parnaíba River (PR) and the actual PR mouth in the eastern part of the PRD. A sedimentological, stratigraphic, and mineralogical study was performed on vibracores acquired in late Holocene sediments from the PRD to investigate how this environment developed during the Holocene. The core acquired in the western portion of the delta (VC-10/16) contains a medium sand layer with fluvial facies between depths of 1.50 and 0.52 m, dated by the radiocarbon method at 4.057 ± 22 BP. However, above a depth of 0.52 m, a transition to silt with tidal channel facies and higher organic matter contents (OM) and calcium carbonate (CaCO 3) is observed. Moreover, a fining-upward trend was also observed in the other four cores located farther west, and the mineral assemblage at the bases of these cores has strong similarities with cores collected in the eastern portion of the PRD (VC-07/16) in the main channel of the PR. However, the core-tops show significant variations in the assemblage of the key minerals (cordierite, cummingtonite, and garnet), indicating that in the past, the PR was connected to the western portion of the delta. The sedimentary provenance and the variations in the depositional environment indicate that a lobe switching process changed the main channel's course and caused the isolation of the delta's western portion during the late Holocene. During this period, the main channel ran through the western portion, and the large tidal channels of the delta are the ancient main channels that were abandoned during the lobe switching process probably caused by tectonics influence. • Mineral assemblages to identify delta lobe switching. • Parnaíba River Delta sedimentology and evolution during Holocene. • Vibrocores in shallow sediments. • Age dating, compositional and textural features of sediments. [ABSTRACT FROM AUTHOR]

Abstract: Land–ocean interactions in the coastal zone are severely influenced by tidal processes. In regions of high sediment discharge like the Mekong River Delta in southern Vietnam, these processes are even more significant. Three cruises in 2006, 2007 and 2008 were carried out to investigate the sediment suspension and their spatial distribution. Additionally, we investigated the influence of the tidal currents in relation to the suspended sediment. Therefore, all cruises took place during the inter-monsoon season between March and May where wave and wind influences are not dominant in contrast to the summer monsoon (May to early October) and winter monsoon season (November to early March). Suspended sediment concentrations (SSCs) in the particle-size range between 2.5 and 500μm were measured with an LISST-instrument (Laser In Situ Scattering and Transmissiometry). Current velocities and directions were recorded with an Acoustic Doppler Current Profiler (ADCP). Additionally, data of different tidal gauge stations in the Mekong River Delta were correlated and compared to the mixed semidiurnal–diurnal tidal cycle. Our results show significant areas of SSCs greater than 25μl/l in the Mekong River branches and its subaqueous delta during the inter-monsoon season. 20% of all measured SSCs in the subaqueous Mekong Delta exceed 100μl/l. Highest concentrations occur close to the seabed. SSCs decrease at the transition to the open shelf. The shelf region contains only low suspension loads, especially on the south-eastern shelf (99% of all samples <25μl/l). However, in the southern shelf region around Ca Mau Cape the suspension load is also higher (>25μl/l) close to the seabed in water depths of 20–25m. Two surveys lasting 25h each were performed on mooring stations in 12m (Mooring 1) and 26m (Mooring 2) water depth and located 3.2km apart on the subaqueous delta slope. Similar patterns of SSC over time show that concentrations of suspension load correlate with the tidal current velocities. High tidal current velocities of up to 0.6m/s near the sea bottom generate increasing SSCs of more than 25μl/l in the water column. Additionally a significant trend of decreasing SSC from the near-seabed to the upper part of the water column can be observed. In terms of sediment transport the ebb phase dominates the tidal cycle by its higher tidal current velocities but the flood phase has the longer duration. The switch of the tidal current direction from ebb to flood phase occurs rapidly against which the change from flood to ebb phase requires up to 3h. This leads to an asymmetry of the tidal ellipses and may cause a net-sediment transport from the shelf into the subaqueous Mekong Delta. In the subaqueous Mekong Delta and adjacent shelf, seven transects show similar patterns of SSCs dependent to the tidal phase. A hypopycnal sediment plume from the subaqueous Mekong Delta into the shelf region was not observed. Our results imply that resuspension by tidal currents dominates the sediment transport in the subaqueous Mekong Delta and adjacent shelf regions during the inter-monsoon season. [Copyright &y& Elsevier]

Abstract: Pollen and phytoliths from sediment cores SO18300, 18302 and 18323 on the continental shelf of the southern South China Sea are analyzed, with special attention to reconstructing vegetation and climate changes on the Sunda Shelf during the Last Glacial Maximum (LGM). The LGM pollen assemblages are characterized by high percentages of pollen from the lowland rain forests and lower montane rainforests, suggesting that the exposed shelf was covered with humid vegetation. A marshy vegetation (with plants of sedges, reeds, bamboo, etc.) developed in the valley along the North Sunda River, and around the marshes were distributed palms and a variety of ferns including tree ferns (Cyathea). The climate during the LGM inferred from the vegetation was cooler than that at the present day, but no significant decrease in humidity was recorded, at least the change was not sufficient to prevent the growth of rainforest. Finally, the observed fluctuations in percentage of mangrove pollen are considered as a sensitive indicator of coastline migrations on the Sunda Shelf. [Copyright &y& Elsevier]

We present a compilation and analysis of 1099 Holocene relative shore-level (RSL) indicators located around the Baltic Sea including 867 relative sea-level data points and 232 data points from the Ancylus Lake and the following transitional phase. The spatial distribution covers the Baltic Sea and near-coastal areas fairly well, but some gaps remain mainly in Sweden. RSL data follow the standardized HOLSEA format and, thus, are ready for spatially comprehensive applications in, e.g., glacial isostatic adjustment (GIA) modelling. We apply a SQL database system to store the nationally provided data sets in their individual form and to map the different input into the HOLSEA format as the information content of the individual data sets from the Baltic Sea area differs. About 80% of the RSL data is related to the last marine stage in Baltic Sea history after 8.5 ka BP (thousand years before present). These samples are grouped according to their dominant RSL tendencies into three clusters: regions with negative, positive and complex (transitional) RSL tendencies. Overall, regions with isostatic uplift driven negative tendencies dominate and show regression in the Baltic Sea basin during the last marine stage. Shifts from positive to negative tendencies in RSL data from transitional regions show a mid-Holocene highstand around 7.5–6.5 ka BP which is consistent with the end of the final melting of the Laurentide Ice Sheet. Comparisons of RSL data with GIA predictions including global ICE-5G and ICE-6G_C ice histories show good fit with RSL data from the regions with negative tendencies, whereas in the transitional areas in the eastern Baltic, predictions for the mid-Holocene clearly overestimate the RSL and fail to recover the mid-Holocene RSL highstand derived from the proxy reconstructions. These results motivate improvements of ice-sheet and Earth-structure models and show the potential and benefits of the new compilation for future studies. • A first standardized and publicly available Holocene relative sea-level database for the Baltic Sea is presented. • The database holds 1099 revised data points with an estimation of vertical and chronological uncertainties. • Negative RSL tendencies prevail over the positive and complex tendencies in the Baltic Sea Basin. • Mid-Holocene RSL highstand occurred around 7.5–6.5 ka BP being consistent with the end of the final melting of the LIS. • The contribution of ice loading in the eastern Baltic Sea Basin is likely overestimated in the ICE-5G and ICE-6G_C models. [ABSTRACT FROM AUTHOR]

This paper presents a 40000-year-long high-resolution mangrove record from sediments of Core 18300, 18302 and 18323 on the continental shelf of the southern South China Sea and reconstructs the coastline changes on Sunda Shelf since the last 40000 years. In the period Marine Isotope 3, the old Sunda Shelf had low sea level, and it was partly exposed. Flourishing vegetations grew on the exposed old land. Mangroves developed along the coastline. On the Last Glacial Maximum, the sea level dropped greatly, coastline moved from inner shelf to outer shelf, the Old Sunda Land exposed further, and the lowering sea level induced the gradual disappearing of mangroves from the inner Sunda Shelf to the outer Sunda Shelf. And pioneer vegetation ferns covered the broadly exposed old land immediately. At the time of the last Deglaciation, sea level rose greatly, the coastline moved to the sea and the Sunda Shelf was drown again. Mangroves were emergent again from outer shelf to inner shelf and developed quickly. [ABSTRACT FROM AUTHOR]

Abstract: Variations in clay mineral composition of sediment cores from the margin and continental slope of the Sunda Shelf (southern South China Sea, SE Asia) covering the past 17,000 yr reflect changing influences of sediment sources together with clay mineral partitioning processes in shallow waters. We identify the deglacial sea level rise as the principal factor driving these changes. During the late glacial, high values of kaolinite are interpreted to reflect a higher contribution of clays from soils that have formed on the exposed Sunda Shelf and in the southern archipelagos of Indonesia. At this time core sites were located in close proximity to the mouths of the Sunda Shelf palaeo-drainage systems on the emerged shelf (“Sundaland”). The progressive landward displacement of the coastline and breakdown of these vast drainage systems during deglaciation led to a decrease in influence of the kaolinite-rich southern sources. When the coastline had retreated closely to its present-day position in mid-Holocene times, the former dominance of southern sources was replaced by a stronger influence of illite-rich sources (e.g. Borneo). The overriding control of sea level changes on the clay mineral distribution patterns precludes a definite climatic interpretation of clay mineral data in terms of climatic/monsoonal changes in such highly dynamic sedimentary environment. [Copyright &y& Elsevier]

An integrated study based on ground penetrating radar (GPR) profiles, vibracore descriptions, water-well logs, and radiocarbon dating in a coastal deposit located in the northern region of Rio Grande do Norte State, northeastern Brazil, allowed us to identify Holocene transgressive and regressive barriers. The construction process for the studied coastal barrier is different from that proposed for the Holocene coastal plains along the eastern Brazilian coast, where the hydraulic barrier set up by large rivers for sediments transported by longshore currents has caused a strongly positive longshore sediment imbalance. In the study area, interpretation of the GPR images, within the constraints of vibracores data, allowed us to interpret five radar facies and four radar boundary sequences for these coastal deposits, which were built up during the Holocene coastal evolution of the region. As a result, the geometry of the coastal barrier was reconstructed. Based on barrier geometry, sediment ages, stratigraphic records, and sedimentation patterns, we propose a barrier evolutionary model for the Holocene for the study region. During the Holocene highstand, a transgressive barrier was deposited and a lagoon extended landward. During the sea-level fall soon after the Holocene highstand, the deposition of a regressive barrier (forced regression) started. This deposition was induced by the coastal geometry and high amounts of eolian sediments supplied by east-northeast winds. Also during this period of sea-level fall, the beach face became wider, and thus more subjected to wind action, facilitating the deposition of the first eolian deposits. These sediments were transported to the nearly formed embayment, providing a surplus for the construction of the regressive barrier. During the regressive phase, tidal channels closed and the lagoon became isolated from the open sea. The geometry of both the regressive and transgressive barriers as well as the stratigraphic relation between the sedimentary deposits suggest that the Holocene highstand in this region was not more than 1.4 m above present-day mean sea level. [ABSTRACT FROM AUTHOR]

• Bent incised valleys formed at times of relatively stable, but lowered sea level. • Bent-incised valley-infill deposits display distinct fill architecture. • Fluvial, estuarine and marine conditions differ in depositional style. • Vertically stacked aggrading strata record pronounced sea-level rise. During times of lowered sea level, Mekong River and Red River incised valleys into the ancient coastal plains of the exposed shelves of the western South China Sea. The deglacial fill history of the incised valley was investigated by seismic surveys and sediment cores. The channels mainly exhibit a low-sinuosity course, but some channel segments are bent. The oldest part of the channel-bend fill exhibits shingled reflectors in Parasound seismic records documenting lateral channel migration typical of meandering rivers. Above, vertically stacked reflectors, which extend from the inner-bend side onto the cut-bank side document that the river-mouth channel turned to a mainly depositional mode. Vertical aggradation started when sea level was ~ 1–2 m below river water-level. During this phase, about two-third of the channel depth was filled by "fluvial-to-estuarine" mud having negative log(Ti/Ca) values as evidenced in XRF core scan data. Typically estuarine conditions developed when river water-level was approximate to sea level. Today channel bends form in estuaries within the zone of bedload convergence. Therefore, it is suggested that the studied channel bends represent antecedent, inherited features that formed during phases of prolonged phases of lowered, but relatively stable sea level to allow bends morphologically to develop. In fact, the bends occur not only in the western South China Sea but also in other areas of the world within a depth range that corresponds to times of retarded sea-level fall during MIS 5b, 5d, 4, and 3. [ABSTRACT FROM AUTHOR]