The event you are referring to is one of the most dramatic geological and climatic events in Earth’s history: the Messinian Salinity Crisis (MSC).
Occurring during the late Miocene epoch, roughly 5.96 to 5.33 million years ago, the Mediterranean Sea became completely disconnected from the Atlantic Ocean. Because the Mediterranean experiences a higher rate of evaporation than it receives in freshwater from rivers and rainfall, cutting off its oceanic lifeline caused it to evaporate almost entirely.
The transformation of a vast sea into a scorching, deep salt desert had profound and drastic consequences for the local and global climate, as well as the biosphere. Here is a detailed breakdown of the consequences.
1. The Geographic Transformation
Before understanding the climatic and ecological impacts, it is vital to understand the physical state of the basin. * The Deep Desert: The Mediterranean basin dropped to 3 to 5 kilometers (roughly 2 to 3 miles) below global sea level. * Massive Salt Deposits: As the water evaporated, it left behind vast expanses of evaporite minerals (salt and gypsum). In some areas, these salt deposits are up to 3 kilometers thick. * Deep Canyons: Rivers flowing into the basin, such as the Nile and the Rhône, suddenly had to drop thousands of meters to reach the new "sea level." This caused rapid, violent erosion, carving massive canyons into the continental shelves that rivaled the Grand Canyon in size.
2. Drastic Climatic Consequences
The evaporation of the Mediterranean altered the climate on both a regional and a global scale.
Regional Climate: * An Extreme Furnace: Because the basin floor was miles below sea level, the atmospheric pressure at the bottom was significantly higher (similar to the depths of the Dead Sea today, but much more extreme). This extreme pressure trapped heat. Summer temperatures in the basin floor are estimated to have reached a blistering 80°C (176°F), creating a landscape too hot to support most terrestrial life. * The Albedo Effect: The vast plains of white salt reflected massive amounts of solar radiation back into the atmosphere (a high albedo). This altered regional air pressure systems, drastically changing wind and weather patterns across Europe and North Africa, generally driving severe aridification (drying) of the surrounding lands.
Global Climate: * Altering Ocean Salinity: The Mediterranean trapped an estimated 6% of the dissolved salt of the entire global ocean in its basin. Consequently, the salinity of the world’s remaining oceans dropped significantly. * Disrupting Global Currents: Ocean circulation (the thermohaline circulation) is driven by differences in water temperature and salinity. Less salty oceans meant that seawater was less dense. This disruption altered deep-water formation and global ocean currents, which are responsible for distributing heat around the planet. * Global Cooling: Because less salty water freezes at a higher temperature, the reduced oceanic salinity likely contributed to the expansion of sea ice at the poles, contributing to a period of global cooling.
3. Drastic Ecological Consequences
The biological impacts were immediate, catastrophic, and long-lasting.
Marine Ecosystem Collapse: * Mass Extinction: The thriving, diverse marine ecosystem of the Mediterranean was entirely wiped out. Dolphins, whales, fish, coral reefs, and marine flora perished. * Hypersaline Pockets: The only remnants of the sea were isolated, shallow, hypersaline lakes (similar to the modern Great Salt Lake or Dead Sea). Only highly specialized, extreme-salt-tolerant (halophilic) bacteria and microorganisms could survive in these toxic brine pools.
Terrestrial Ecosystems and Migration: * The "Stepping Stones" of Migration: With the water gone, land bridges formed between North Africa, Europe, and Asia. Animals that previously could not cross the sea began to migrate. African species like elephants, hippos, and primates walked across the hot, dry salt flats into Southern Europe, while European species moved south. * Island Endemism Wiped Out: Islands like Sicily, Corsica, Sardinia, and Cyprus were suddenly transformed into towering mountain peaks standing above the salt plains. The unique, isolated species living on these islands were suddenly exposed to mainland predators and competitors, completely altering island ecosystems.
The Resolution: The Zanclean Flood
The Messinian Salinity Crisis lasted for roughly 600,000 years. It ended abruptly 5.33 million years ago in an event known as the Zanclean Flood.
A combination of tectonic subsidence and global sea-level rise caused the Atlantic Ocean to breach the land bridge at the Strait of Gibraltar. Water cascaded into the Mediterranean basin in a cataclysmic flood. It is estimated that at its peak, the water flowed in at a rate 1,000 times greater than the modern Amazon River, dropping tens of meters a day.
The Mediterranean Sea was likely refilled entirely within a few months to two years, instantaneously drowning the land bridges, restoring the global oceanic salt balance, and allowing Atlantic marine life to recolonize the basin, shaping the Mediterranean as we know it today.