A giant abandoned river system the size of the Nile was recently discovered under the eastern Mediterranean. The ancient river flowed for only about 100,000 years, but the evidence it left behind is helping scientists understand what happened in the region in the late Miocene during the Messinian Salinity Crisis (MSC), when a diminished Mediterranean Sea was isolated from the Atlantic Ocean.
About 6 million years ago, the Strait of Gibraltar, the narrow, rocky channel between present-day Spain and Morocco that connects the Mediterranean and Atlantic, closed. With no water flowing in from the ocean, the Mediterranean slowly evaporated until tectonic activity reopened the channel more than 600,000 years later. The extent of the inland sea’s evaporation during this period has been contested since the MSC was discovered nearly 50 years ago. The new findings, reported in Geology, shed light on just how little water remained.
“In order for there to be a huge river system there…the Mediterranean [basin] would have had to be exposed a fair amount,” says lead author Andrew Madof, a geologist at the Chevron Energy Technology Company in Houston. “Maybe 75 to 80 percent [of the basin] was dried out, but there was probably a lake that this river system was flowing into.”
Madof and his colleagues used two- and three-dimensional seismic data collected offshore Cyprus, Israel, Lebanon, and Syria to image the subsurface of the eastern Mediterranean and map the river’s remains, which constituted a collection of sedimentary deposits they named Nahr Menashe. They determined that the river flowed out of what are today Turkey and Syria, draining into a lake in the dry Mediterranean basin. Sedimentary deposits in the seismic imagery indicated that the deposits had been left along riverbanks instead of undersea.
A series of small lobes at the former mouth of the river also shows evidence of backstepping, a process that leaves deposits of former shorelines as a sea’s water level rises. The backstepped lobes found in the new study chronicled the lake’s rising water level at the end of the MSC and matched evidence from other river systems around the Mediterranean, such as the Nile and the former Eosahabi, offshore Libya. Evidence from these systems suggests that at the end of the MSC, contributions from these rivers maintained a lake that covered about a fourth of the Mediterranean’s current extent.
“People have predicted that [rivers] should have been flowing into this part of the Mediterranean, but nobody had ever seen it before,” Madof says. “This is the first major discovery [of a paleoriver] in decades.”
Changes in Climate and Tectonic Shifts
Today, no major river system feeds into the eastern Mediterranean. Shifts of the African, Arabian, and Anatolian plates have resulted in much of the region’s water flowing into the Persian Gulf.
But back in the Miocene and possibly before, the prevailing tectonics in the area dictated that water flowed into the Mediterranean. Madof says it’s possible the newly discovered river system was an ancestral Euphrates River, which today flows from Turkey south through Syria and Iraq to the Persian Gulf.
Climate during the MSC likely also contributed to the formation of the river by causing increased rainfall over Turkey and Syria.
Lisa Murphy Goes is a paleoclimatologist at the University of Miami in Florida who has previously modeled the region’s paleoclimate. Her findings indicate that a reduced Mediterranean would have induced atmospheric circulation changes that could have brought increased precipitation to higher-elevation regions in southern Europe, including Turkey, and an entirely evaporated Mediterranean may have led to a drought from southern Europe to northern Africa. The results of such simulations are dependent on just how much the sea dried up, however, she says.
“The question of whether or not the basin, particularly in the east, dried up has been controversial for quite some time,” Murphy Goes says. “In terms of the atmospheric response to such a cataclysmic event, only when we truly know the specifics of the sea level change, can we be confident that there must have been significant hemispheric-wide impacts.”
Madof’s findings—that up to 80% of the basin dried up and a major river fed into the Mediterranean’s small remainder—support Murphy Goes’s findings of increased precipitation in the region.
Although the new results help refine the sea level extent during the MSC, further research is required to determine the full extent of climatic effects the shrunken sea might have had.
—Mara Johnson-Groh ([email protected] ), Freelance Science Writer
This article is the first in a series made possible through the generous collaboration of the writers and editors of Earth magazine, formerly published by the American Geosciences Institute.