The Storegga Slide Tsunami - Deposits, Run-up Heights and Radiocarbon Dating of the 8000-Year-Old Tsunami in the North Atlantic
One of the largest Holocene landslide mapped on Earth is the Storegga Slide offshore Norway. Between 2500 and 3400 km3 material were mobilized in a retrogressive process, generating a huge tsunami dated to about 7300 14C years BP, or ca 8100 calendar years BP. The tsunami is documented from onshore deposits along the entire Norwegian coast, on the Faeroe Islands, the Shetland Islands, in Scotland and possibly also on Greenland. Of these, the tsunami deposits in Shetland reach the highest elevation, indicating a run-up of at least 20 m. Coastal lakes have a high potential for preserving deposits from tsunamis and Storegga tsunami deposits are now documented from more than 40 lakes and submarine basins. Such deposits typically show an erosive, sharp, lower boundary against the underlying lake- or sea-floor mud. Coarse sand and fine gravel particles (2-6 mm) rest on this boundary. The sand often contains rip-up clasts of both gray silt and brown organic lake mud. Further up-core, the tsunami deposit is a mixture of plant fragments, twigs, bark, sand, and other re-deposited mud. The upper boundary is very gradual. The sand may contain marine diatoms, fragments of marine shells, and sea-urchins. This clearly demonstrates that material was brought into the fresh water lakes from the sea during the tsunami event. Based on these findings we have established a sedimentological model for tsunami deposits in lakes and shallow marine basins. Accurate radiocarbon dating of paleotsunamis are problematic because of erosion of the underlying strata, re- deposition of organic material within the tsunami deposit and also re-deposition of organic matter for many years after the tsunami event. Unexpectedly, we discovered plant material within the tsunami sand that is excellent for radiocarbon dating - these are green moss stems that we know were killed by the tsunami. The green color is analyzed to be chlorophyll. Chlorophyll in dead plants degrades rapidly with exposure to light and oxygen. The reason to why the chlorophyll is still present in these 8000-year-old moss stems is surprising. We think that the rapid burial of the plants within the shell-bearing sand provided a high pH, and that the tsunami deposit was subsequently sealed with marine silt preventing it from oxygen and light. The green-colored moss cannot have been re-deposited, but must have been alive when the tsunami struck the coast. Six different samples of such green-colored moss from different cores give an age of 7295±23 years BP, which calibrated to calendar years is 8030-8170 (2 σ range). We compared the field observations of run-up heights with maximum surface elevations from new numerical simulations of the Storegga slide generated tsunami. Such a comparison between observations and simulations help us to estimate important parameters for the slide; the initial acceleration, the maximum velocity, and the mass mobilization time during the slide process. Best fit between field observations and simulations is obtained for a retrogressive slide having a maximum velocity below 35 m/s, probably around 25- 30 m/s. The time lag between the individual slide blocks could not have been more than 15-20 seconds, indicating that the total mass was mobilized in approximately 40 - 60 minutes.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- 1105 Quaternary geochronology;
- 3070 Submarine landslides;
- 4564 Tsunamis and storm surges