It forms the ancient lifeline of Egypt. But since when has the Nile been bringing water from the interior of Africa to the Mediterranean? One study shows that the famous river has followed its bed for 30 million years. Evidence for this are rock analyzes and a model simulation, according to which the gradient of the Nile from its source region in the Ethiopian highlands to its mouth in the Mediterranean was almost as strong as it is today 30 million years ago. The reason for this is a current in the Earth’s mantle that maintains this gradient to this day, as the researchers have discovered.
The Nile is more than 6,800 kilometers long, making it one of the longest rivers on earth. Along its course from south to north through the eastern Sahara, the first human cultures emerged thousands of years ago and later the Egyptian Empire. But as important as this river was for human history, its origin is obscure. It is clear that the Nile is fed by two source rivers, the longer White Nile, which rises in Burundi, and the more water-rich Blue Nile, which comes from the highlands of Ethiopia. But since when the water from these source rivers has followed the current course of the Nile into the Mediterranean Sea has been controversial for years. Many scientists assume that the Nile only flows into the Mediterranean after the Mediterranean dried up around six million years ago and the resulting gradient. Previously, according to the theory, a large part of the water of the Nile flowed westward and flowed into the catchment area of the Congo. But there are also some researchers who believe that the Nile could have formed millions of years earlier.
Early connection between Ethiopia and the Mediterranean
Claudio Faccenna from the University of Roma Tre in Rome and his colleagues have now unearthed new indications of a very “old” Nile. For their study, they first examined sediment drill cores from the mouth of the Nile. With the help of comparative chemical analyzes, they were able to reconstruct where the origin of the different old sediments lies. It turned out that minerals from the Ethiopian highlands had been deposited in the mouth of the Nile long before the temporary drying out of the Mediterranean. According to the researchers, this suggests that water must have flowed from the source of the Blue Nile to the Mediterranean around 30 million years ago. “The sediment analyzes from the Nile Delta document the connection from the Ethiopian highlands to the Nile Delta as early as the Oligocene,” stated Faccenna and his team.
But in order to make this connection possible, the Nile must already have had enough gradient to the north at that time – otherwise this water would have flowed west into the Congo Basin instead. The researchers checked whether this was the case with the help of a geophysical model simulation. In this they reconstructed the tectonic changes in northwest Africa and also took into account data on the volcanic origin of the Ethiopian highlands. The simulation showed that a volcanic hotspot began to gradually raise the Ethiopian plateau around 40 million years ago. At the same time, however, the eastern Mediterranean and northern Egypt slowly sank. According to the model, the cause of this sinking is a current in the earth’s mantle that has been pulling material down into the depths for around 35 million years.
Stable course for 30 million years
In summary, this means that there has been a kind of tectonic “conveyor belt” for at least 30 million years, which causes the source region of the Blue Nile to rise and the mouth of the Nile to sink. As a result, there is a persistent difference in altitude between the source and the mouth of around 1500 meters, as Faccenna and his colleagues report. The convection cell in the earth’s mantle responsible for this is still active today, they explain. They conclude that the Nile must be significantly older than some geoscientists previously assumed. “The Nile may have flowed in a similar course as it does today since the Oligocene, say the researchers. “Despite many small-scale changes in the last 30 million years, the river has existed without interruption since then, connecting the Ethiopian highlands with the Mediterranean.”
Source: Claudio Faccenna (Roma Tre University, Rome) et al., Nature Geoscience, doi: 10.1038 / s41561-019-0472-x