麻豆传媒

THE largest landslides on Earth happen in the oceans, and an ooze of dead plankton may be responsible. If so, it could help us predict the risk of devastating tsunamis triggered by these events.

Far beneath the waves, huge 鈥渕egaslides鈥� can transport 3000 cubic kilometres of sediment at speeds of up to 80 metres per second.

The largest such event on record was the Storegga Slide 8150 years ago off the coast of Norway. Dwarfing every slide known on land, it caused a tsunami that flooded coastlines around the North Sea . This may have been devastating for the prehistoric inhabitants of the area.

Nobody knows what triggers megaslides. The one clue was that past events had a smooth surface underlying them, suggesting the sediment must have slid over some kind of layer of weakness. But there the trail went cold.

鈥淭he problem has been that this weak zone vanishes with the landslide,鈥� says at the Geomar Helmholtz Centre for Ocean Research in Kiel, Germany.

Now Urlaub and her colleagues believe that the cause of the weak zone is ooze, a 鈥渇luffy鈥� substance made of dead single-celled organisms called diatoms. It forms when diatoms 鈥� a major component of plankton 鈥� die and drift down to the seafloor.

Urlaub鈥檚 team got lucky, she says, because a now defunct international research effort called the once collected a core from marine sediments off the north-west coast of Africa. It was right next to the site of the Cap Blanc Slide, a megaslide that happened about 149,000 years ago. This gave the team access to deep-sea sediments, where they discovered a 10-metre-thick layer of ooze (, ).

鈥淭he one clue was that past megaslides had a smooth surface underlying them. There the trail went cold鈥�

鈥淲e found that this diatom ooze layer is also covered by clay,鈥� says Urlaub. The clay probably trapped water beneath it, in the porous diatom layer, and this water may have lubricated the megaslide.

A layer of wet ooze could help explain why megaslides can happen on shallow gradients of less than 3 degrees. Someone walking on that 鈥渨ouldn鈥檛 even notice that there鈥檚 a slope at all鈥�, says Urlaub.

The idea that diatom ooze could trigger a megaslide is not new, but this is the first time ooze has been found associated with a slide.

Nevertheless, 鈥渙ne case is not sufficient to make it a scientific truth鈥�, says geologist at the MARUM Center for Marine Environmental Sciences in Bremen, Germany. Although Urlaub鈥檚 study has 鈥済ot the stone rolling鈥�, he says, 鈥渨e need to find and analyse more slides that were triggered by the same mechanism she describes鈥�.

Urlaub agrees, and plans to study other submarine landslide sites to see whether they also have layers of ooze. She also wants to find out if there are particular species of diatom that make the 鈥渞ight鈥� kind of ooze, and how large a volume of diatoms is needed to make a layer thick enough to set off a megaslide.

Once the science is settled, it could be in our interest to find out which parts of the seabed have lots of ooze 鈥� because megaslides in those areas could cause tsunamis. 鈥淥oze layers might be used as a tool to estimate better which slopes might be prone to failure,鈥� says Urlaub.

This article appeared in print under the headline 鈥淭he tiny creatures that cause giant 鈥榣andslides'鈥�