Dating a massive undersea slide

From San Francisco, at a meeting of the American Geophysical Union

Pieces of moss buried in debris deposits along the Norwegian coast have enabled geologists to better peg the date of an ancient tsunami and the immense underwater landslide that triggered it. Carbon dating of the newly unearthed moss suggests that the landslide occurred about 8,100 years ago.

Sometime after the end of the last ice age, the largest landslide known to geologists took place off the coast of Norway. Called the Storegga slide, this slump of seafloor sediments included about 3,000 cubic kilometers of material. That’s enough mud to cover the entire United States to a depth of about 30 centimeters, says Stein Bondevik, a geologist at the University of Tromsø in Norway.

The tsunami created by the slide scoured coastal sites in Norway, England, Scotland, and Greenland, in some places to heights of 20 meters above sea level. Scientists have previously used carbon dating of seeds, twigs, and other organic material in sediment layers deposited by the tsunami to date the Storegga slide. However, the organisms in those samples could have been long dead when the tsunami occurred and therefore might have provided artificially old date estimates, says Bondevik.

Now, he and his colleagues report that they have unearthed material that was alive when the tsunami buried it. The pieces of moss, found within an 80-cm-thick layer of sand and broken shells at two sites along the western coast of Norway, were still green when the researchers uncovered them. Chlorophyll typically decomposes rapidly if it’s exposed to light and oxygen, but sudden burial by the tsunami sealed off the material, say the researchers. Also, the acidity of the sediments was low because some shell fragments dissolved and released carbonate ions—another factor that preserved the chlorophyll.