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.
Further up-core, the tsunami deposit is a mixture of plant fragments, twigs, bark, sand, and other re-deposited mud. The sand may contain marine diatoms, fragments of marine shells, and sea-urchins.
Radiocarbon samples which obtain their carbon from a different source (or reservoir) than atmospheric carbon may yield what is termed apparent ages.
A shellfish alive today in a lake within a limestone catchment, for instance, will yield a radiocarbon date which is excessively old.
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.
[A Conventional Radiocarbon Age or CRA, does not take into account specific differences between the activity of different carbon reservoirs.
We think that the rapid burial of the plants within the shell-bearing sand provided a high p H, 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.A CRA is derived using an age calculation based upon the decay corrected activity of the absolute radiocarbon standard (1890 AD wood) which is in equilibrium with atmospheric radiocarbon levels (as mentioned previously, 1890 wood is no longer used as the primary radiocarbon standard, instead Oxalic Acid standards I and II were correlated with the activity of the original standard).In order to ascertain the ages of samples which were formed in equilibrium with different reservoirs to these materials, it is necessary to provide an age correction.The reason for this anomaly is that the limestone, which is weathered and dissolved into bicarbonate, has no radioactive carbon.Thus, it dilutes the activity of the lake meaning that the radioactivity is depleted in comparison to 14C activity elsewhere.The average difference between a radiocarbon date of a terrestrial sample such as a tree, and a shell from the marine environment is about 400 radiocarbon years (see Stuiver and Braziunas, 1993).