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 14 C 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 TiO 2 /K 2 O 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.

Clay mineralogy and major-element geochemistry of Miocene sedimentary rock and modern river sediment samples collected from the Ba River basin in central Vietnam are used to evaluate the chemical weathering processes during the Miocene and the present time. The results show that Miocene andesitic sedimentary rocks consist of high smectite (average 72%) with moderate kaolinite (24%), while Miocene felsic sedimentary rocks display abundant kaolinite (65%) with moderate smectite (25%). In comparison, modern river sediments are characterized by moderate smectite (43%) and kaolinite (37%). The typical distribution of clay minerals in the Ba River basin can be resulted from abundant occurrence of felsic intrusive rocks and volcanic rocks along with weak tectonic uplift and the tropical East Asian monsoon climate during the Miocene and in the present time. Despite their different clay mineralogical compositions, major elements of these Miocene sedimentary rocks and modern river sediments display stronger depletion of Ca, Na and Mg than of K and Si during the chemical weathering. The chemical index of alteration (CIA) combined with kaolinite/illite ratio demonstrated moderate chemical weathering during the Miocene and in the present time in central Vietnam, demonstrating similar tectonic activity and climatic conditions occurred during these two periods.

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The continental margin of Indochina represents the westernmost margin of the South China Sea, a wide continental rift that in the west is mostly submerged. In the region of south to central Vietnam, the location of this study (Figure 1), the continental margin extends offshore for >100 km before oceanic crust is reached. The extension that led to continental break-up has yet to be fully explained but is known to be related to the subduction of the paleo-Pacific plate, which at the time formed a wide Cretaceous magmatic arc with widespread calc-alkaline magmatism across south to central Vietnam and southern China. Subduction was directed to the northeast beneath southern China and westwards below Vietnam, the eastern margin of the Indochina block (Hall, 2012;Hennig-Breitfeld et al., 2021;Zhu et al., 2004).Extension is imprinted on the landscape and geology of Vietnam but to what extent is not yet clear. It also remains uncertain as to when surface uplift took place. Was uplift staggered over a long time or did it occur within a short

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