A one million-year-long sequence of marine terraces on Sumba Island, Indonesia

International audienc

Climatic and tectonic controls on chemical weathering in tropcial Southeast Asia (Malay Peninsula, Borneo and Sumatra)

International audienceClay mineralogy and major-element geochemistry were investigated on 58 surface sediment samples collected in 27 rivers draining Malay Peninsula, Borneo, and Sumatra to assess the chemical weathering process and its controlling factors in tropical Southeast Asia. The clay mineral assemblages show high kaolinite contents in Malay Peninsula (average 80%), Sumatra (58-78%), and South Borneo (41-55%), high illite contents in North Borneo (47-77%), and moderate smectite contents in Sumatra (6-29%). Intensive chemical weathering is identified in all three investigated regions from both clay mineralogical and geochemical (bulk and clay-fraction) results regardless of their various lithological and tectonic settings. Relatively, the chemical weathering intensity increases gradually from North Borneo to South Borneo, and further to Malay Peninsula and Sumatra. Our results suggest that the East Asian-Australian monsoon climatic condition with constant warm temperature and abundant precipitation throughout the year is the principal forcing factor on the chemical weathering, and the tectonic activity including mountain uplift, earthquakes, and volcanic eruptions and their specific lithological characters is only a subordinate factor on the intensity of chemical weathering in tropical Southeast Asia

Climatic and tectonic controls on chemical weathering in tropcial Southeast Asia (Malay Peninsula, Borneo and Sumatra)

International audienceClay mineralogy and major-element geochemistry were investigated on 58 surface sediment samples collected in 27 rivers draining Malay Peninsula, Borneo, and Sumatra to assess the chemical weathering process and its controlling factors in tropical Southeast Asia. The clay mineral assemblages show high kaolinite contents in Malay Peninsula (average 80%), Sumatra (58-78%), and South Borneo (41-55%), high illite contents in North Borneo (47-77%), and moderate smectite contents in Sumatra (6-29%). Intensive chemical weathering is identified in all three investigated regions from both clay mineralogical and geochemical (bulk and clay-fraction) results regardless of their various lithological and tectonic settings. Relatively, the chemical weathering intensity increases gradually from North Borneo to South Borneo, and further to Malay Peninsula and Sumatra. Our results suggest that the East Asian-Australian monsoon climatic condition with constant warm temperature and abundant precipitation throughout the year is the principal forcing factor on the chemical weathering, and the tectonic activity including mountain uplift, earthquakes, and volcanic eruptions and their specific lithological characters is only a subordinate factor on the intensity of chemical weathering in tropical Southeast Asia

Morphometric assessment of uplifting coral reef sequences, Sumba Island, Indonesia.

International audienc

Morphometric assessment of uplifting coral reef sequences, Sumba Island, Indonesia.

International audienc

Coral 13C/12C records of vertical seafloor displacement during megathrust earthquakes west of Sumatra

The recent surge of megathrust earthquakes and tsunami disasters has highlighted the need for a comprehensive understanding of earthquake cycles along convergent plate boundaries. Space geodesy has been used to document recent crustal deformation patterns with unprecedented precision, however the production of long paleogeodetic records of vertical seafloor motion is still a major challenge. Here we show that carbon isotope ratios (δC13) in the skeletons of massive Porites corals from west Sumatra record abrupt changes in light exposure resulting from coseismic seafloor displacements. Validation of the method is based on the coral δC13 response to uplift (and subsidence) produced by the March 2005 Mw 8.6 Nias-Simeulue earthquake, and uplift further south around Sipora Island during a M~8.4 megathrust earthquake in February 1797. At Nias, the average step-change in coral δC13 was 0.6±0.1‰/m for coseismic displacements of +1.8 m and -0.4 m in 2005. At Sipora, a distinct change in Porites microatoll growth morphology marks coseismic uplift of 0.7 m in 1797. In this shallow water setting, with a steep light attenuation gradient, the step-change in microatoll δC13 is 2.3‰/m, nearly four times greater than for the Nias Porites. Considering the natural variability in coral skeletal δC13, we show that the lower detection limit of the method is around 0.2 m of vertical seafloor motion. Analysis of vertical displacement for well-documented earthquakes suggests this sensitivity equates to shallow events exceeding Mw~7.2 in central megathrust and back-arc thrust fault settings. Our findings indicate that the coral C13/C12 paleogeodesy technique could be applied to convergent tectonic margins throughout the tropical western Pacific and eastern Indian oceans, which host prolific coral reefs, and some of the world's greatest earthquake catastrophes. While our focus here is the link between coral δC13, light exposure and coseismic crustal deformation, the same principles could be used to characterize interseismic strain during earthquake cycles over the last several millennia

Source-to-sink transport processes of fluvial sediments in the South China Sea

International audienceThe 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

Source-to-sink transport processes of fluvial sediments in the South China Sea

International audienceThe 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