The Earth Changes But Persists

Depending upon whether one follows the expansionist or the reductionist perspective, there are as many as eighteen and as few as four known species of Homo that have lived on this planet. In either case, there is only one living here today.{1} As the climate changes dramatically, a collective fear is that it will mean the end for this last species, for Homo sapiens. That is an unanswerable question today, but what the current dramatic climate changes won’t mean is an end to planet Earth.

With many attributable causes, impressive changes in climate have had equally impressive effects on the Earth’s features throughout its nearly 4.6 billion year history. These effects were not only significant, they were subject to ongoing change. As a complex living system, the Earth is always in flux- at the geological level, at the biological level, and at the atmospheric level. As the Earth’s systems work together to produce climate changes, the effects can also be seen at all of these levels. An incredible example of this occurred between the African and Eurasian landmasses millions of years ago.

Around 6 million years ago, the ancient Tethys ocean- what is now known as the Mediterranean Sea- was undergoing dramatic change in terms of its landmasses and shorelines. For example, the island today known as Crete was still attached to the mainland- the Peloponnesian peninsula that is now Greece. In the late Miocene, the separation was occurring and the Aegean Basin was formed. The separation of the island from the mainland had occurred but the connective bridges with the Peloponnese were still intact until 5 million years ago. During this time period of the Miocene era, an extraordinary event took place, a catastrophic episode initiated by a further advancement toward each other of the African and Eurasian tectonic plates. This massive movement significantly diminished the opening between the Atlantic Ocean and the inland sea until, ultimately, the sea was isolated.  

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As a result, around 5.6 million years ago, in one of the most extensive evaporation events in the history of the planet, today’s Mediterranean Sea was nearly completely desiccated in what is known as the Messinian Salinity Crisis. The sea evaporated into a salty lakebed in a process that, from beginning to end, covered 667,000 years. In her book Ancient Bones, Madelaine Bohme imagined a first-hand experience of this extreme event: “Instead of encountering cool breezes blowing off the blue Mediterranean you are confronted by an endless desert slashed by deep canyons. The path down one of these canyons leads into a flickering, glistening void, a moonscape where salt crystals, some as large as you are, cover the ground. The abyss opening up in front of you is at least 1.25 miles (2 kilometers) deep. Down there the temperature is over 122 degrees Fahrenheit (50 degrees centigrade) and there is absolutely no sign of life.”  

The Messinian Salinity Crisis was first discovered by scientists in the early 1970s when deep sea drilling confirmed a layer of salt beneath the floor of the Mediterranean. But the full sweep and dynamics of this oceanographic process has only come to be understood in the twenty-first century, with some aspects still being debated and much yet to be understood. Through a combination of profound effects, including tectonic movements of the African and Eurasian continents and the cooling of the climate at 6.3 million years ago that caused increased glaciation and thus lowered sea levels around the planet, the movement of water between the Atlantic Ocean and the Mediterranean Sea gradually ceased. In the process of getting to this point, from 5.97 million years ago to approximately 5.33 million years ago, the salinity of the sea rose as the sea’s level dropped. Over time, the imbalance made life for anything living in the sea unsustainable.  

Around 5.6 million years ago, the protracted event was nearing its midpoint. The desiccation was approaching its high with massive deposits of salt and gypsum supplanting the previous marine condition even though the connection to the Atlantic Ocean had not completely closed off yet. Many species migrated within Eurasia and between Eurasia and Africa during the period of the crisis. In many cases, species permanently relocated.  

Photo credit: Giuseppe Fallica/500px

Evidence of the Messinian Salinity Crisis can be seen clearly on the shores and underground in Sicily today. In the town of Realmonte, there is a salt mine with 45 miles (25 kilometers) of tunnels at 300 feet (100 meters) below sea level. The walls of the mine were formed over 5 million years ago during the desiccation of the sea surrounding what is now Sicily. Marked with magnificent, alternating bands of light and dark rocks in swirling striped and circular patterns, the walls are a natural display of the seasonal cycles (known geologically as varves) during the crisis. Temperature variation between winter and summer determine the type, and therefore color, of the salt deposit at that time. Geological processes later caused deformations in the varve deposits and this resulted in the undulating shapes seen in the walls today.

These walls at Realmonte serve as important archives describing some of the most intense conditions of the pan-Mediterranean salinity crisis. The onshore salt deposits at the mine are accessible to researchers while most other deposits from the period are off-shore in the deep Mediterranean basins. Nearly in the center of the Mediterranean Sea, southwestern Sicily contains the Caltanissetta Basin and this is where the Realmonte salt mines are located. Overall, the deposits are a record of the long-term cyclical processes of the desiccation of the basin. The minerals analyzed in the salt slabs give an indication that the salinity of the brine in the basin at the time of the crisis exceeded saturation point which is more than 70 times that of seawater.

About 500 thousand tons of salt are removed from the mine every year in the form of rock salt and kainite. The salt is used in food production, animal food, and tannery and dying processes. The mine also houses the Salt Cathedral, which accommodates as many as 800 people for a service and was built in the early 2000s by the miners working in the mine. The crucifix, the altar, two bas-reliefs, and the statue for the patron saint of miners, Santa Barbara, were all carved from salt.

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Above ground, the shores of Sicily bear evidence of the end to this dramatic period in the region. On the coast of the southern municipality of Realmonte sit the rocky cliffs of Scala dei Turchi (“Scale of the Turks”). These gleaming cliffs are composed of soft limestone and “marl,” a white sedimentary rock formed of planktonic foraminifera{2} deposited after the Zanclean flood. Approximately 5.33 million years ago, the Messinian salinity crisis was ended with a massive refilling of the sea when the Atlantic waters finally found a way through the newly formed Strait of Gibraltar.

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While the desiccation of the sea occurred over a long period of time and in cycles of intermittent advance and retreat, the Zanclean was a brutally fast refilling of the area happening all at once. The flooding may have started as a trickle that continued for several thousand years, but the geological evidence indicates that 90 percent of the water moved from the Atlantic into the Mediterranean over a few months to no more than two years and raised sea levels by more than 33 feet (10 meters) per day. In this brief mega-flood period, researchers project discharges of 100 million cubic meters per second (roughly a thousand times the current flow of the Amazon River), creating the largest known mega-flood deposit in the history of the Earth.

Since the refilling of the sea, the human history in the vicinity of the Mediterranean has included the advancement of foundational civilizations, epic conflicts and marine crusades from Carthage to Rome and Athens to Alexandria. It is still a semiarid climate with low precipitation and high-evaporation, and coastal cities that are still important homes to many human endeavors. Brought about by dramatic climate changes, the changes to the Mediterranean itself were as extreme as can be imagined. Still we see in this incredible event how the Earth does persist. In fact, over its long timespans, it thrives.


1- DNA studies show that the one species remaining today is not separate from the others but in fact carries within its genome a number of the earlier Homo species, having merged with them rather than having driven them to extinction.

2- Planktonic foraminifera are tiny single-celled zooplankton that float on the surface of the ocean. Their shells are made of calcium carbonate and are extremely long-lasting so they contribute to the fossil record in terms of ocean temperatures and processes over millions of years.

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