viernes, 3 de diciembre de 2010
Ancient Mega-Lake Found in Egyptian Desert
* A mega-lake in today's hyper-arid western Egypt might have been fed by the earliest Nile floods.
* The discovery helps explain fish fossils in the desert.
* An alternative hypothesis is that the mega-lake fed the Nile, not the other way around.
The hyper-arid deserts of western Egypt were once home to a lush mega-lake fed by the Nile River's earliest annual floods.
Fossil fish and space shuttle radar images have defined the bed and drainage channels of the long lost lake, which at times was larger than Lake Michigan, stretching as far as 250 miles west of the Nile in southwestern Egypt.
The discovery pushes back the origin of the "Gift of the Nile" floods to more than a quarter million years ago and paints a drastically different picture of Egypt's environment than is seen today. It also explains the longstanding puzzle of the fossilized fish found in the desert -- fish that are of the same kinds that live in today's Nile River.
It took a lot of staring at the high-resolution radar topographic maps from the 1980s and 1990s -- and tinkering with the colors of those maps -- to make sense of it all.
"It just struck me that: 'Hey, maybe that was the level for the lake,'" said Ted Maxwell of the Smithsonian Institution's Center for Earth and Planetary Studies.
Although the topography is compelling, the evidence by itself isn't sufficient to prove a lake was there or how it was created.
For one thing, there needs to be a source for all the water. That would likely have been Wadi Tushka, a pass to the west of the Nile, which is low enough for the Nile River to have flooded through provided there was more rainfall and larger annual floods than are known today.
Then there are the fish fossils, which are unmistakable evidence for there having been Nile-related water water filling the basin.
There are also archeological sites, said Maxwell, that help to roughly confine the dates of the lake's surface elevation in more recent times. Maxwell and his coauthors Bahay Issawi and C. Vance Haynes, Jr., published their study in the December issue of the journal Geology.
Despite the radar maps, fish and archeology, however, there is a lot of evidence that should be there, but isn't, said Maxwell.
Take the old shorelines, or "bathtub rings," that are often the definitive evidence of long-lost mega-lakes of wetter times in other parts of the world, including in Utah, Nevada and California. In Egypt those shorelines have probably been sandblasted away over the millennia by scouring winds, Maxwell said.
And what about some distinctively lake-formed sediments in the basin itself?
"The problem is there is no sedimentary evidence," Maxwell told Discovery News. And so the mega-lake is harder to prove than most.
Despite this, other researchers looking at the area find there are more reasons for believing that a long lineage of great lakes filled the basin that might not have needed the Nile as their source.
"Other possibilities for the source of the water include drainage from the highlands to the west, groundwater recharge from the south, and local rains, potentially from different directions and sources," said researcher Christopher Hill of Boise State University.
The older sedimentary remnants associated with springs and archeological artifacts seem to point to local rains or groundwater creating lakes that were smaller and smaller over time and not from the Nile, said Hill. Those sources could have, perhaps, enlarged the lake enough to join and flow into the the Nile and allow the fish to move upstream into the lake, without Nile flooding. That would mean the water at Wadi Tushka would have been flowing east into the Nile rather than west into the lake.
In other words, the lake certainly existed, but the jury is still out on how it got there.