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Ancient cities vanished into muddy morass

作者:莫痤    发布时间:2019-03-06 08:13:00    

By Stephanie Pain Two ancient cities that once stood at the mouth of the Nile vanished into a morass of liquid mud when the river burst its banks, according to an analysis of the sediments in Egypt’s Abu Qir bay. The research suggests that the cities sank into the bay when turbulent floodwaters transformed the soft, unstable ground beneath them into a soup of sediment. The disappearance of these cities has been blamed on earthquakes, subsidence and rising sea levels. But Jean-Daniel Stanley, a coastal geoarchaeologist at the Smithsonian Institution in Washington DC, blames the Nile. “A powerful flood would bring a lot of water carrying a lot of sediment – enough to cause failure of the ground at the river mouth,” says Stanley. The ruins of the two long lost Greek cities of Eastern Canopus and Herakleion were uncovered in 1999 and 2000 by marine archaeologist Franck Goddio of the European Institute for Underwater Archaeology in Paris. Hi-tech surveys of the seafloor revealed the substantial remains of Eastern Canopus 1.6 kilometres offshore and buried under five metres of mud. The city of Herakleion lies beneath seven metres of mud 5.4 kilometres from the shore. Today the nearest branch of the Nile lies more than 20 kilometres to the east of Abu Qir bay. But the surveys show that both cities once stood at the mouth of a now-extinct branch of the Nile – where they could control incoming vessels and tax goods being shipped upriver. “You’d think the Greeks would have thought twice about building on low, soft sediment. But it was clearly profitable,” says Stanley. Excavations at the two sites indicate that both cities were damaged by earthquakes before they disappeared. But this doesn’t explain why the land subsided so catastrophically beneath them, says Stanley. Slumping caused by a quake would be widespread in the bay. Instead, it is restricted to the margins of the lost river. Cores taken at the site of Eastern Canopus show clear signs of liquefaction – a process that disrupts the normal layers of sediment. “As the ground turned to liquid some buildings would sink in, others would be pushed up,” says Stanley. Collapse would be rapid. Stanley has pinpointed the flood that did for Eastern Canopus. The discovery of two Arabic coins dating from the 730s suggests the city had not sunk by then. Written records of the highs and lows of the Nile note significant flooding in 741-742 AD, with the river rising a metre higher than the normal flood level. This was almost certainly the flood that buried Canopus, says Stanley. Analysis of cores taken around Herakleion this spring should soon reveal whether it suffered the same fate. The youngest artefacts from the site date from the first century AD, suggesting that the city disappeared soon after. As a city built on mud, Herakleion would have been vulnerable to subsidence during a big flood. “I’m not ruling out earthquakes,” says Stanley. “But flooding would be a strong contender.” Journal reference: Nature (vol 412,

 

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