Search

High‐resolution clay mineralogy and major element geochemistry of Core SO18383‐3 collected off the Mekong River mouth in the southern South China Sea have been investigated, aiming to reconstruct sediment provenance and chemical weathering intensity since the last glaciation. The chronology is based on foraminiferal accelerator mass spectrometry 14C dates. Clay mineral analysis suggests that the Mekong River is a major sedimentary source for the studied area. The values of chemical index of alteration (CIA) indicate moderate chemical weathering in the Mekong River basin. Smectite/(illite + chlorite) and smectite/kaolinite ratios coupled with TiO2/K2O ratio reveal the temporal variation in chemical weathering intensity in the Mekong River basin. The lower ratios around the Last Glacial Maximum (29.9–14.7 cal ka BP), the Heinrich Event 1 (~14.8 cal ka BP), the Younger Dryas interval (12.4–11.3 cal ka BP), and middle to late Holocene (7.6–1.1 cal ka BP) indicate weak chemical weathering intensity. In contrast, the higher ratios occurring during the Bølling‐Allerød period (14.7–12.7 cal ka BP) and the period of main deglaciation (14.7–7.6 cal ka BP) indicate stronger chemical weathering in the Mekong River basin. The good correlations between these proxies and the available proxy records of the East Asian summer monsoon intensity suggest that the intensity of chemical weathering in the Mekong River basin over the last 30 ka is significantly controlled by the summer monsoon. This study greatly increases our understanding of the principal forcing factor on continental weathering in the Mekong River basin since the Last Glacial Maximum. Key Points: The Mekong River is the major terrigenous sediment contributor to the southwestern South China SeaChemical weathering intensity in the Mekong River basin has been moderate since the Last Glacial MaximumIntensity of chemical weathering in the Mekong River basin is controlled by the East Asian summer monsoon evolution [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]

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]

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]

• 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]

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]