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Posted on 01/05/2007 4:42:11 PM PST by blam
Dating a massive undersea slide
Sid Perkins
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.
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Me too!! Too bad he also said he would become a Christian when he turned 80 years old. Don't think he made it. Uh, bio says he didn't.
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Added tsunami to the keywords. :') Thanks for the others, and for this fascinating topic.
"...Shouldn't evidence of that tsunami still be present in modern England and Scotland?
Has erosion altered the landscape that much in 8100 years to erase all evidence of deposits?..."
Sea level may have been lower then...some of the evidence (scouring, erosion scars, etc) may be underwater at present.
I did a short paper on my interpretation, as a marine navigator, of a Spanish ship wreck on the Oregon Coast.
Old charted anchor bearings did not match the waters of the bay until I matched what the bay was supposed to look like 500 years earlier. Thanks for the info from Oregon State University Hatfield Marine Science Center.
I probably should post a "comment" but this tsunami was caused by a combination of factors: 1 - since the last Ice Age the oceans have risen 150 metres (450')... yes... and toward the end of that Atlantic Ocean water finally made it over a ridge that separated the warm Atlantic from the cold Arctic water off the coast of Norway 2 - much of the extended coast of Norway, now underwater as part of its "continental shelf," was frozen tundra and when it was submerged it started to thaw. The warm Atlantic water hitting it accelerated that thaw 3 - frozen in the tundra (just like in Siberia and northern Canada now) were large amounts of methane clathrate, a form of methane that can expand with explosive rapidity... like "burps" 4 - laid over a bed of methane clathrate was a huge post-glacial deposit of gravel, mud and stone... perched at the edge of the deep ocean.
A big methane "burp" somehow got this huge mass moving downhill, and the largest landslide in known history spread down the slope and across the deep ocean floor - displacing an enomous amount of water and causing tsunamis in all the adjacent coastal areas.
Read all about it (and see the video) at http://www.fettes.com/Shetland/tsunami%20deposits.htm
Excellent addition, thanks.
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