About six million years ago, the Mediterranean Sea was cut off from the Atlantic Ocean. Evaporation far outpaced inflow, and the basin partly dried out, leaving behind a kilometer-thick layer of salt and gypsum — an event known as the Messinian Salinity Crisis (MSC). For over 600,000 years, the Mediterranean alternated between brackish lakes and hypersaline depressions. The big question has been: when the Atlantic finally broke back in 5.33 million years ago, did the sea refill slowly, or catastrophically?
Our new study in PNAS led by Udara Amarathunga provides confirmation for a cataclysmic Zanclean megaflood, the most abrupt marine flooding event in Earth history.
We studied cores from Ocean Drilling Program Sites 969 and 967 in the eastern Mediterranean, focusing on the so-called “mystery sapropel” — an unusually long organic-rich sediment layer at the Miocene–Pliocene boundary. Normally, Mediterranean sapropels form during peaks in Northern Hemisphere summer insolation, when monsoon rains increase freshwater input, stratifying the basin and suppressing deep-water ventilation. Each sapropel spans less than one orbital precession cycle (~21,000 years).
But the mystery sapropel formed in the earliest Pliocene (right after the MSC) breaks this rule: it spans two insolation maxima and the intervening minimum (~33,000 years total). That makes it unique in the Neogene record and impossible to explain with orbital forcing alone.
Geochemical proxies — including biomarkers, X-ray fluorescence, and molybdenum/aluminum ratios — reveal that when Atlantic water first cascaded through the reopened Gibraltar Strait and over the Sicily sill, it was turbulent and oxygen-rich. This aerated the stratified basin, allowing only the most resistant organic matter to accumulate. Over the next 8–12 thousand years, however, oxygen was consumed by remineralization of sinking organic matter, leading to widespread anoxia. Stratification persisted for ~33 thousand years until mixing finally restored normal marine conditions.
The findings provide direct evidence that the MSC ended not with a trickle, but with a flood; not in thousands of years as imagined in the 70's and 80's but in just a few years. A big claim as such an unprecedented flood demanded building stronger evidence before this hypothesis being seriously considered in other disciplines (e.g., paleoecology). Today we are closer to establish that an extraordinary event reshaped the Mediterranean’s chemistry and ecosystems, leaving behind a geologic fingerprint — the mystery sapropel — that still puzzles and informs geoscientists today.
Related papers:
Land-to-sea indicators of the Zanclean megaflood
https://www.nature.com/articles/s43247-024-01972-w
A terminal Messinian flooding of the Mediterranean evidenced by contouritic deposits on Sicily
https://onlinelibrary.wiley.com/doi/10.1111/sed.13074
Transient oxygenation of the Mediterranean after the Zanclean megaflood
https://www.pnas.org/doi/10.1073/pnas.2505429122
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