Relentless. The refilling of the
Mediterranean may have required the largest river that ever
flowed.
CREDIT: ROGER PIBERNAT
By Phil Berardelli
ScienceNOW Daily News
9 December 2009
All it took to get started was a trickle. Researchers have found
evidence that the Atlantic Ocean refilled the dried-out
Mediterranean basin about 5.3 million years ago in a gigantic
flood, which started slowly but accomplished most of the task
within as little as several months.
Today, the crystal-blue waters of the Mediterranean Sea cover some
2.5 million square kilometers. But 5.3 million years ago, that same
area was a desert peppered by extremely salty lakes with depths
down to 2.7 kilometers below current sea level. The sea's waters
had evaporated about 300,000 years earlier, when the
Mediterranean's only inlet at the time, the Strait of Gibraltar,
blocked the flow of seawater from the Atlantic Ocean. Earlier
research had suggested that a major ice age had caused sea levels
to drop, exposing a land bridge between Europe and Africa.
Then, when the ice age ended, the sea rose again. Eventually, it
breached the land bridge, creating what was called the Gibraltar
Falls. Some scientists had described thousands of times more water
flowing over the falls than at Niagara. But the evidence for this
scenario has remained sketchy and the ice-age explanation has been
questioned.
Now a team of researchers has provided a more-nuanced picture of
the breaching of that natural dam: Instead of collapsing, it eroded
over thousands of years. Daniel Garcia-Castellanos, a geophysicist
of Institut de Ciències de la Terra Jaume Almera in Barcelona, part
of the Spanish National Research Council, and colleagues reviewed
all available data from boreholes and seismic studies and analyzed
that data using models designed to simulate how fast-moving rivers
can carve out channels.
Based on that analysis, the team concludes, the flood began as a
relative trickle, as the land bridge subsided a bit. Over the next
several thousand years, the rapidly growing stream carved a notch
deeper and deeper into the land, until the inflow of seawater
became overwhelming--containing possibly 1000 times more water than
the Amazon River does today.
Although the flood built up gradually, it nevertheless grew to
enormous proportions. Up to 90% of the refilling took place in less
than 2 years--perhaps only a few months--raising the water level in
the Mediterranean basin by more than 10 meters a day at its peak,
the scientists argue tomorrow
in Nature. The action of that much water carved a
200-kilometer-long channel right through the center of the straits,
deepening it by as much as 0.4 meter a day. “As soon as the first
trickle of Atlantic water found the way through it, the feedback
between erosion and water flow led to [that] enormous discharge in
a short period," Garcia-Castellanos says.
The remaining mystery is the process that blocked the Mediterranean
in the first place and then refilled it. "Our models do not help
much in knowing what brought on the topographic divide between both
seas," Garcia-Castellanos admits, but he suspects the cause was
subsidence of tectonic plates.
It's a "very convincing" bit of research, says geologist Philip
Gibbard of the University of Cambridge in the United Kingdom. The
idea of a gentle influx growing into a gigantic torrent seems
"perfectly reasonable." But to be sure, Gibbard suggests testing
the erosion model further, by conducting lab experiments or perhaps
applying it to data related to similar refillings of other basins
in the geological record, such as the Black Sea.