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• 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]

• 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]

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]

• 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]