Diatom ooze: the weak link in submarine landslides?

Submarine landslides are the largest, and probably the least well-understood mass movements, on Earth.  As I have noted previously, they have the potential to move hundreds of cubic kilometres of materials for hundreds of kilometres on slopes with gradients of less than 3º.  The mechanics of these slides have represented a substantial challenge – the low slope gradients suggest extremely weak layers must control the deformation, but the origin of these low strengths has not been clear.

In a new paper just published in the journal Geology (Urlaub et al. 2018), Morelia Urlaub and colleagues have examined ocean drilling data from the periphery of the Cap Blanc slide, which is a a 149,000 years old, large submarine landslide situated off the coast of NW Africa. The paper includes this seismic refraction line across the slide area, which provides details of the key features of the failure:-

Seismic refraction data for the Cap Blanc submarine landslide, highlighting the scarp of the landslide that may have been generated by weak diatom ooze. Image via: Morelia Urlaub and colleagues, and Geology

.

By correlating the ocean drilling data with seismic refraction data, Urlaub et al. (2018) found thick layers of diatom ooze located at the base of the submarine landslide.  Importantly, these layers were capped with a layer of clay-rich sediment.  Urlaub et al. (2018) suggest that the saturated diatom layers are highly compressible, generating high pore water pressures that are trapped beneath the clay layers, inducing failure in the clay or at the interface between the clay and the diatom ooze.  The authors note that diatom ooze itself has high frictional resistance, so is unlikely to form the sliding surface.  Thus, it is the combination of the diatom ooze and the clay cap that is key to the generation of these slope failures.

Urlaub et al. (2018) have proposed an intriguing hypothesis that would explain the mechanisms behind at least some submarine landslides.  They note that many continental margins have layers of diatom ooze.  The challenge now is to recreate the mechanisms experimentally – this looks to be a task for the dynamic back pressured shearbox apparatus that we developed with GDS Instruments.  This machine is ideal for exploring the behaviour of submarine landslide systems, and is the subject of our ongoing work in New Zealand.

Reference

Morelia Urlaub, Jacob Geersen, Sebastian Krastel, Tilmann Schwenk. 2018. Diatom ooze: Crucial for the generation of submarine mega-slides? Geology; DOI: 10.1130/G39892.1