Long-term sediment decline causes ongoing shrinkage of the Mekong megadelta, Vietnam

Since the 1990s the Mekong River delta has suffered a large decline in sediment supply causing coastal erosion, following catchment disturbance through hydropower dam construction and sand extraction. However, our new geological reconstruction of 2500-years of delta shoreline changes show that serious coastal erosion actually started much earlier. Data shows the sandy coast bounding river mouths accreted consistently at a rate of +2 to +4 km2/year. In contrast, we identified a variable accretion rate of the muddy deltaic protrusion at Camau; it was < +1 km2/year before 1400 years ago but increased drastically around 600 years ago, forming the entire Camau Peninsula. This high level of mud supply had sharply declined by the early 20th century after a vast canal network was built on the delta. Since then the Peninsula has been eroding, promoted by the conjunction of mud sequestration in the delta plain driven by expansion of rice cultivation, and hysteresis of long-term muddy sedimentation that left the protrusion exposed to wave erosion. Natural mitigation would require substantial increases in sediment supply well above the pre-1990s levels

Sequence of Multiple Slope Failures in the Headwall Area of the Giant Sahara Slide Complex at the NW African Continental Margin

Abstract Submarine mega‐slides involving hundreds of cubic kilometers of slope material pose a major threat due to their potential to destroy offshore infrastructure and trigger devastating tsunamis. The Sahara Slide Complex affected about 50,000 km2 of the northwestern (NW) African continental margin. Previous studies focused either on its distal depositional zone or the uppermost headwall area, but failed in reconstructing the succession of individual slide events within the entire headwall area. New hydroacoustic data reveal a complex slide morphology including three main acoustic facies, large scale slide blocks, linear troughs, multiple glide planes and three major headwall scarps (the upper, southern and lower headwall). The evacuated slide scar hosts chaotic slide deposits that cover stratified sediments in the upper and southern headwall area, but are vertically stacked onto older slide deposits in the lower headwall area. Based on these observations, and dating of recently collected sediment samples, we reconstructed the history of slope failures that led to the formation of the structurally and morphologically complex headwall area of the Sahara Slide. Slope instability initiated when the lower headwall failed at ∼60 kyr, followed by the failure of the northeastern upper headwall at ∼14 kyr. Around 6 kyr, a major slide within the upper headwall area took place, followed by a series of smaller events within the southern and most‐proximal upper headwall area. The youngest of these slides occurred around 2 kyr. This scenario suggests a long‐lasting history of successive slope failures for the Sahara Slide Complex along the NW African continental slope