Morphometric signature of sediment particles reveals the source and emplacement mechanisms of submarine landslides

The long travel distances recorded by most submarine landslides indicate that changes in particle attributes (shape and size) may occur during their movement. Yet, little is known about the magnitude of such changes, and their underlying physical drivers. In order to understand the failure characteristics of submarine landslides, a dynamic image analyzer was used to characterize the particle size(s), aspect ratio, sphericity, and convexity of 200 samples recovered from two IODP sites (C0018 and C0021) offshore Nankai (SE Japan). The two IODP sites drilled a series of submarine landslides, and undisturbed slope sediment, previously characterized in detail on 3D seismic data. The results of this work reveal no perceptive differences for particle size and shape between landslide and undisturbed slope strata, suggesting that remobilized sediment, sourced from marine deposits accumulated on the upper slope of Nankai do not change through slope instability. This lack of particle attribute differences between remobilized (landslide) and undisturbed sediment, and between the two IODP Sites, suggests limited interaction between particles during their movement. A fluidization mechanism is therefore proposed whereby the soft and saturated state of deep-sea sediment leads to the development of excess pore water pressure. This mechanism maintains movement and inhibits particle-to-particle contact, limiting subsequent particle shape, and size variations in the failed sediment.