Great Moments in Earth History: The Messinian Salinity Crisis and the Zanclean Flood

Author’s note: This post is the first in a series of great Earth history moments. Stay tuned for a new post every other week.

Around 6 million years ago, the Mediterranean Sea became separated from the Atlantic. Cut off from the world’s oceans, it began to evaporate. By 5.3 million years ago, there was literally no sea left. 1000 years later, it was refilled in a geologic instant.

A number of discoveries led to the conclusion that the Mediterranean dried out completely sometime in the past. The first came in the 1960s, when seismic studies of the floor of the Mediterranean revealed a unique layer – dubbed the M reflector – across the whole basin. Scientists interpreted it to be a large layer of salt distributed evenly across the seafloor.

The Glomar Challenger discovered a thick layer of evaporite minerals deep in the Mediterranean seafloor.

The Glomar Challenger discovered a thick layer of evaporite minerals deep in the Mediterranean seafloor. Credit: Ocean Drilling Program, Texas A&M University.

Later, in 1970, a leg of the Deep Sea Drilling Project cored deep into the Mediterranean seabed. They found what the seismic data predicted: a hard layer of evaporites – rocks composed of salts.

The only way to get evaporite rocks at the base of a sea is to evaporate water until it becomes so concentrated with salts that they can no longer be dissolved. This forces them to precipitate into a solid form.

Just as enigmatic as the salt layer, engineers mapping the base of the Nile River in preparation for the construction of the Aswan Dam around this time found that carved deep beneath the silty floor of the Nile was a canyon whose ancient base was well below sea level.

The only way  for a canyon to be carved into bedrock is for a river to flow through it. But a river won’t cut lower than sea level. This deep canyon meant that Medteranian sea level must have been dramatically lower in the past.

In 1972, Kenneth Hsu, the primary investigator on the Deep Sea Drilling Leg that cored the Mediterranean, authored a paper in Nature concluding that the sea must have evaporated nearly completely to produce such an anomalous layer of evaporite minerals and to have cut canyons so deep. In the paper he admitted it was a “preposterous idea,” but stated that no other explanation presented itself.

That remains true to this day. Researchers who study this event, dubbed the Messinian Salinity crisis after the geologic age in which is occurred, continue to debate the implications of these discoveries. There are still questions about when the basin lost its water, and whether it happened at the same time in all parts of the basin.

3D Reconstruction of the Messinian Salinity Crisis. Credit: Daniel Garcia-Castellanos

What geologists generally agree upon is that sometime around 6 million years ago, the Mediterranean became sporadically cut off from the Atlantic, going through cycles of partial evaporation and refilling. Around 5.7 million years ago the Mediterranean was closed off for good. Researchers estimate that it then took about 1000 years for the sea to evaporate nearly completely.

Nobody agrees on why the sea became closed off, but most researchers agree that tectonics must have played a role. The major inlet to the Mediterranean lay at the intersection of the African and European tectonic plates, and their collision is responsible for creating the Gibraltar arc, a series of mountains in southern Iberia and Africa. Through processes not entirely understood, this event is thought to have uplifted enough land to separate the Atlantic from the Mediterranean.

The Straight of Gibraltar as viewed from the International Space Station. Credit: NASA

The Straight of Gibraltar as viewed from the International Space Station. Credit: NASA

As awe-inspiring as this event is, its conclusion is even more dramatic. Around 5.3 million years ago the Atlantic breached the straight of Gibraltar. This was the beginning of what has been termed the Zanclean flood. Scientists think that the entire ocean was refilled in only about 40 years, with the majority of the refilling occurring in just a decade.

Emptying and refilling the basin was so significant that it literally altered Earth’s crust, making it first rebound, like a trampoline, from the weight of the water that was lost and then rapidly sink with the weight of the returned water. This would have been enough, some scientists argue, to reverse the direction of many rivers that had flowed into the Mediterranean.

The event also left its mark on evolution. Many African species migrated into the former seabed. Hippopotami are a notable example. When the Zanclean flood refilled the basin, they were left to survive on islands where, through evolutionary processes that often occur on islands, they shrank in size. Until their extinction millions of years later, there had been species of dwarf hippopotami on Malta, Sicily, and Cyprus as a direct result of the Messinian Salinity Crisis.

This crisis – and it was a crisis for both the inhabitants of the evaporating sea, as well as those species that made the former seabed their home – was precipitated by relatively minor shifts in tectonics in a very small portion of the globe. But that shift lead to the destruction of an entire sea, forced Earth’s ridged crust to rebound hundreds of meters, and precipitated what may have been the largest flood in Earth history. Despite the awesome nature of these processes, the only clues it left behind are a layer of salty rocks and deep, hidden canyons.

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