The Messinian Salinity Crisis was a dramatic geological event during which the Mediterranean Sea experienced repeated cycles of partial or nearly complete desiccation. This transformation highlights how geological forces and climate can reshape large bodies of water over geologically short periods.
The Mediterranean’s Dramatic Transformation
The Messinian Salinity Crisis refers to the repeated drying and refilling of the Mediterranean Sea. This occurred during the Messinian Age of the Miocene epoch, from approximately 5.97 to 5.33 million years ago, lasting about 640,000 years.
The basin reached depths of 3 to 5 kilometers (1.9 to 3.1 miles) below normal sea level in some areas, leaving scattered hypersaline pockets resembling the modern Dead Sea. This event was not a single, continuous drying but involved multiple stages of desiccation, environmental changes, and depositional events.
Unraveling the Causes
The primary cause of the Messinian Salinity Crisis was the restriction or closure of connections between the Mediterranean Sea and the Atlantic Ocean. Tectonic activity played a significant role, causing the uplift of land bridges and the narrowing of the Gibraltar Strait. This tectonic constriction, combined with the Mediterranean’s high evaporation rates, led to a water deficit.
The Mediterranean Sea experiences more evaporation than precipitation and river inflow, requiring a constant influx of water from the Atlantic to maintain its level. When the Strait of Gibraltar became restricted or closed, the Atlantic inflow was reduced or cut off. This isolation, coupled with the arid climate, led to rapid evaporation, causing the sea level to drop by as much as 1.5 kilometers.
Numerical modeling suggests that the uplift of the Gibraltar Arc seaway, occurring at rates of a few millimeters per year, was counteracted by erosion from the Atlantic inflow, sustaining a shallow connection for an extended period. This interplay between uplift and erosion helps explain the long duration of salt precipitation. Eventually, the connection was fully blocked, leading to a near-complete desiccation of the basin within approximately a thousand years.
A Salt-Filled Legacy
As the Mediterranean Sea dried, it left an environment drastically different from its present state. The most striking consequence was the formation of vast evaporite deposits, including layers of salt (halite), gypsum, and anhydrite, which accumulated on the exposed seafloor. These “salt giants” can be hundreds of meters to kilometers thick, with over 1 million cubic kilometers of salt deposited.
The basin transformed into a series of hypersaline lakes, similar to the Dead Sea, and later into larger pockets of brackish water resembling the Caspian Sea, a phase known as the “Lago Mare” event. This environment had a profound impact on marine life, with only a small percentage of Mediterranean endemic species surviving the hypersaline conditions. The desiccation may also have created temporary land bridges, allowing for the migration of terrestrial animals between Africa and Europe.
The Great Flood and Aftermath
The Messinian Salinity Crisis ended with the dramatic and rapid refilling of the Mediterranean Sea, known as the Zanclean Flood. This inflow occurred approximately 5.33 million years ago, marking the beginning of the Zanclean Age of the Pliocene epoch. The refilling was triggered by the sudden reopening of the Strait of Gibraltar, allowing Atlantic waters to cascade into the desiccated basin.
The volume of water involved was immense, with discharge rates estimated at 100 million cubic meters per second, roughly 1,000 times that of the modern Amazon River. Ninety percent of the Mediterranean Basin refilled, likely over a period ranging from a few months to two years, with sea level rising at rates exceeding 10 meters per day. Lower water discharges may have preceded the main flood for several thousand years.
Deciphering Earth’s Ancient Record
Scientists confirmed the Messinian Salinity Crisis through various lines of geological evidence. Early seismic reflection profiles in the late 1960s revealed extensive, buried salt structures beneath the Mediterranean seafloor, initially thought to be Triassic but later identified as Late Miocene. These seismic images provided the first large-scale view of the evaporite deposits.
Deep-sea drilling expeditions, particularly the Deep Sea Drilling Project (DSDP) Leg 13 in 1971, brought up cores containing thick sequences of evaporite minerals, including gypsum and halite, and fossil evidence. These cores provided direct samples of the ancient seabed, allowing scientists to analyze the mineralogy and sedimentary structures. Geochemical markers in sediments and the presence of erosional surfaces, such as deep canyons carved by rivers flowing into the desiccated basin, supported the desiccation hypothesis.