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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]

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]