By Will Dunham
June 12 (Reuters) - Uruk, the world's first metropolis and the birthplace of written language, was nourished by the Euphrates River, as was Babylon, ancient Mesopotamia's grandest city. The fertile plain between the Euphrates and its companion waterway, the Tigris River, was one of humankind's cradles of civilization.
Researchers now have deciphered how the Euphrates first formed, looking much further back in time than the era of these great urban centers that arose mere millennia ago.
Guided by seismic images of buried sediments and other data, they said the Euphrates appears to have been born between 3.6 million and 1.6 million years ago as two earlier river systems merged due to tectonic activity in the Taurus Mountains in the southern part of modern-day Turkey.
The Euphrates, the longest river in southwest Asia, extends about 1,700 miles (2,800 km), originating in Turkey and flowing through Syria and Iraq before emptying into the Gulf. Present-day cities on the banks of the Euphrates include Birecik in Turkey, Raqqa in Syria and Ramadi, Fallujah and Nasiriyah in Iraq. Ancient cities on the Euphrates also included Ur and Mari.
While the Euphrates has long shaped the region's geology, the timing of its origin and the evolution of its present course had remained enigmatic. The researchers said decoding the river's backstory was important for understanding the milestones in human culture in agriculture, writing, urban development and other areas that occurred on its floodplains.
Geologists using subsurface seismic data while trying to identify possible gas reserves under the Mediterranean spotted buried channel-like features dating to a time more than 5 million years ago when large parts of the sea had dried up, an event called the Messinian salinity crisis.
They determined that two separate rivers - predecessors to the modern-day Karasu and Murat rivers in Turkey - flowed across a region spanning Turkey and Syria and emptied into the Mediterranean basin.
The scientists believe tectonic activity in this earthquake-prone region caused the Murat predecessor to divert toward the Gulf, with the Karasu predecessor later joining it. This, they said, formed a powerful single river system that became the Euphrates.
A key tool used by the scientists was seismic imaging, a technique that creates detailed two- and three-dimensional maps of Earth's subsurface by recording how sound waves travel through and bounce off underground rock layers. It was through seismic imaging that they detected the features that were ancient river channels hidden beneath the seabed.
"This technology is the equivalent of using ultrasound to image fine details of a developing baby or an arthritic knee, but in this case we use it to image buried gravels, sands, mud, limestone and salt that have been compacted and turned into rock," said University of Western Australia geoscientist Simon Lang, who helped with the study published in the journal Nature Geoscience, along with geologist Andrew Madof of Chevron.
The scientists, through modeling of the underground features, determined that these two ancient rivers had flow rates exceeding the modern-day Nile and Tigris-Euphrates.
Working backward, the scientists traced the two ancient rivers to onshore Turkey. By examining geological data from sediments in valleys and coal deposits now uplifted into the Taurus Mountains, they determined that the present-day Karasu and Murat rivers were likely the original sources of those buried channels.
"But somehow they had got cut off from their western lower valley reaches, and had joined together to form what is now the modern-day Euphrates River. That 'somehow' was, of course, tectonic activity across eastern Anatolia," Lang said.
"Today, the waters along the Euphrates and the Tigris join near Basra to form a huge delta at the head of the Persian Gulf. They have filled in a large area of Mesopotamian plain upon which early agriculture developed including early city-states, and the development of cuneiform writing so vital to early human development," Lang said.
The flow of even the mightiest rivers can be changed by dramatic geological events.
"The biggest river on the planet - the Amazon - used to flow west towards modern-day Colombia and Peru towards its paleo-Pacific deltaic coast before the rise of the Andes. As the Andes grew with huge uplifts, all the former distributaries reversed direction over millions of years, and now the Amazon flows into the Atlantic," Lang said.
(Reporting Will Dunham in Washington; Editing by Daniel Wallis)
