Mechanically mysterious microfossils: Diatoms and their role for submarine landslides

• Applicant: Dr. Gauvain Wiemer
• Subject Area: Palaeontology ; Geotechnics, Hydraulic Engineering
• Term: from 2018 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 410551087

Submarine landslides have much greater dimensions than mountain slides. Because of their great number, frequency and wide distribution, these have a great societal significance. The stability of a continental slope depends on the geotechnical properties of the slope sediments. A slide can be triggered when the shear resistance of the sediment body significantly decreases in a short time. This may occur due to a rapid increase in pore pressure, which is the fluid pressure in the tiny pockets between the sediment grains.According to a relatively new hypothesis, high pore pressure may be observed systematically in buried layers of diatomaceous ooze. Diatom microfossils are hollow skeletons left by unicellular aquatic phytoplankton. They grow out from a circular or elongated center during valve formation by consuming dissolved silica. When diatoms die, their skeleton sinks to the ground and form layers of diatomaceous ooze. With time, these layers are buried deeper in the stratigraphy and the overburden stress increases. Following the hypothesis, the overburden stress reaches a point where the diatom valves start crushing. If the water within the valves cannot be drained, the pore water takes up the overburden stress and excess pore pressure occurs. Since water barely resists shear stresses, submarine landslides may occur even on gently inclined slopes. In 2018 scientists from the Geomar - Helmholtz Centre for Ocean Research - in Kiel published a case study in the Journal Geology on a submarine mega-slide that occurred on the northwest African continental slope, which was apparently triggered by exactly this mechanism.The aim of the proposed research is to test the hypothesis of collapsing diatoms as trigger for submarine slides using geotechnical methods. The geotechnical properties of generic sample material with varying diatom contents shall first be characterized in detail. The sample materials will then be systematically subjected to different loading and shearing procedures. The measurement of pore pressure evolution during shearing, combined with the analysis of changes in e.g. grain size and structure related to loading and shearing will reveal the conditions under which diatoms start crushing and thereby induce excess pore pressure. In a second step, we propose to compare the results of the generic study with observations that will be made on the sediment involved in the mega-slide that occurred on the northwest African continental slope.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Achim Kopf