Six million years ago, the Mediterranean Sea was cut off from the Atlantic Ocean. The collision of the African and Eurasian tectonic plates caused the uplift of the Rif and Betic Mountains (in northern Morocco and southern Spain, respectively), closing at least two large straits or channels that connected the water bodies at the time.
This event caused a sudden increase in salinity in the Mediterranean, known as the Messinian Salinity Crisis (MSC), which in turn led to the formation of massive gypsum and salt deposits throughout the Mediterranean.
Scientists are still debating about what happened to sea levels in the Mediterranean after its western waterways closed. One possibility is that water evaporated quickly and sea levels fell by thousands of meters. Only a series of salty or brackish lakes might have survived. Eroded channels and sediment deposits in what might have been the ancient surface of the exposed Mediterranean basin seem to support this theory.
Zanclean Flood Hypothesis
The period of the dry Mediterranean might have ended quickly when the Strait of Gibraltar reopened 5.33 million years ago, causing a massive flood that refilled the empty basin in just 2 years. Seismic reflection imaging, a technique that relies on sound waves to image what lies underground, has revealed that beneath kilometer-deep Quaternary sediments, a huge erosion channel extends across the strait. The ancient channel, several hundred meters deep and 390 kilometers long, stretches from the Gulf of Cádiz (in the Atlantic) to the Algerian Basin (in the Mediterranean). Researchers think that this channel might have been carved during the flood as Atlantic waters rushed in to refill the Mediterranean.
[1/3] These two videos summarize our interpretation of what happened to the Mediterranean ~5 million years ago based on geological & geophysical data. First, the connections between the Atlantic and the Mediterranean may have closed due to tearing and sinking of the lithosphere pic.twitter.com/Jz0YZi29KI
— ∆(Garcia-Castellanos) (@danigeos) February 3, 2020
Looking for evidence to support this theory (called the Zanclean flood hypothesis), an international group of researchers recently identified buried sediments that could have been deposited by the flooding waters. “One of the recurrent questions was ‘Where did all the sediments accumulate after the flood?’” said Daniel Garcia-Castellanos, a geophysicist at the Institute of Earth Sciences Jaume Almera in Barcelona, Spain, who led the search and has studied the Zanclean flood hypothesis over the past decade.
Using computer simulations to recreate the opening of the strait and the subsequent flood, Garcia-Castellanos and his team identified low-flow areas that were likely to accumulate sediments. Then, they looked at seismic profiles of these areas to see if they could find the sediments.
On the lee side of an ancient volcanic cone in the predicted path of the flooding, they found a large sediment deposit. It appears as an amorphous blob in the seismic profile, in contrast to the neatly stratified layers of marine sediments around it, suggesting rapid sedimentation. Its shape and size also seem to match the likely direction of the flood.
“This shape is compatible with the numerical models and with what we see in other megaflood settings,” Garcia-Castellanos said.
In 2018, the same group of researchers found similar deposits near a deep underwater gorge in the Malta Escarpment, the natural barrier separating the eastern and western Mediterranean basins. This gorge, known as the Noto Canyon, is the most likely location where the flood might have spilled over the escarpment to refill the eastern Mediterranean basin.
The main limitation of these simulations is the lack of knowledge about ancient seafloor topography. “We used the present-day seafloor morphology since we don’t know how it was in the past,” explained Garcia-Castellanos. “Nevertheless, these simulations give us a quantitative idea of how far can such a flow transport each grain size.”
The researchers published their findings in a paper reviewing the Zanclean flood hypothesis in Earth-Science Reviews.
To Flood or Not to Flood
The Zanclean flood hypothesis has its detractors. Many researchers studying the MSC are not even sure the Mediterranean actually desiccated much below its modern-day level.
One of the main problems with the desiccation theory is the utterly massive amounts of salt and gypsum thought to have been deposited during the MSC. If its waters had instantly evaporated, the present-day Mediterranean Sea would leave behind a layer of salt no thicker than 30 meters. In comparison, the salt layers associated with the MSC are up to a kilometer thick in some areas.
To accumulate so much salt, critics of the Zanclean hypothesis say, water had to be able to flow into the sea from the open ocean, albeit in a restricted way that could limit the escape of the denser, briny water. This dense water would sink to the deepest parts of the sea, where minerals could precipitate.
Another controversial line of evidence is the networks of ancient channels thought to be the remains of ancient river systems. Current research suggests that these channels could have been carved by the flow of dense, salty water as it sank to the deepest areas of the sea. A similar effect could have formed the erosion channel at the bottom of the Strait of Gibraltar.
Looking for a different way to assess the problem, Javier Garcia Veigas, a researcher at the University of Barcelona not involved in the new research, looked at the isotopic signatures of Messinian gypsum deposits at several points in the Mediterranean. Biological activity and other factors can change the ratios of heavy to light isotopes of certain chemical species in seawater. These differences should be measurable in the minerals formed in aquatic environments with different isotopic compositions. However, Garcia Veigas didn’t find significant isotopic differences between rocks formed in the eastern and western Mediterranean basins, suggesting that they remained connected.
“I think that the consensus in the community is slowly drifting away from the megaflood hypothesis and increasingly moving towards a scenario where the Mediterranean did not fully desiccate,” Garcia Veigas said. “However, we won’t have definitive proof until a drilling campaign can reach the deep Mediterranean basin and confirm that what we see in the seismic profile is really salt formed during the Messinian.”
Garcia-Castellanos acknowledges that not everybody is on board with the Zanclean flood hypothesis. “All of the evidence that has been summarized in this article may have other possible interpretations,” he said. “So this debate won’t end tomorrow.”
—Javier Barbuzano (@javibarbuzano), Freelance Science Journalist