Geologists from Utrecht, London and Granada have found the first direct proof of the largest known mega-flood that ever occurred on earth. This mega-flood ended what is known as the ’ Messinian Salinity Crisis’ , a period around six million years ago when the water level in the Mediterranean Sea sank by around 1.5 km, causing an extreme environmental crisis in the region. The authors studied sandstone that was exposed along the southern coast of Sicily and concluded that it was formed by a powerful flow from the western Mediterranean Sea towards the east. The water flowed over a barrier that separated what were then isolated basins in the western and eastern Mediterranean region. Their research was recently published in the journal Sedimentology.
The waters of the Mediterranean Sea and the Atlantic Ocean flow back and forth and mix via the narrow Straits of Gibraltar. During the Messinian era, around six million years ago, this flow was disrupted by a tectonic closure of the straits. As a result, much of the Mediterranean Sea evaporated faster than rivers could supply more water. A kilometre-thick layer of evaporite (gypsum and salt) gradually formed over the entire Mediterranean Sea basin. At the beginning of the Zanclean, around 5.33 million years ago, the Mediterranean filled once again to today’s current level when the connection with the Atlantic Ocean was restored. Models indicate that this was due to an enormous flood entering the Mediterranean basin via Gibraltar. However, until recently there was no geologic proof of such a mega-flood in the geological record of the Mediterranean Sea region.
Between gypsum and marl
Now, rocks that were formed during the flood have been found along the southern coast of Sicily, explains Wout Krijgsman , Professor at Utrecht University and driving force behind the study. Krijgsman had been working on the Messinian Crisis for several years, when in 2020 he became intrigued by a few meters of sandstone that had become exposed along the southern coast of Sicily, at the transition between a layer of gypsum (a relic from the dry period) and marl (created after the area was covered with water). A year later, he revisited the area with a team of geologists from Utrecht University, the Royal Holloway University in London and the University of Granada. The team also included two of his Master’s students.
The researchers carefully studied the sand and saw that the composition and size of the grains changed every few centimetres. "We soon recognised the structures of fossilised current ripples in the sand", recalls Gijs van Dijk, one of Krijgsman’s former Master’s students at Utrecht University and main author of the new study. "Those structures form when fast-flowing water carries sand. We’re all familiar with the pattern, for example where the sand has been exposed to strong wind or fast water currents for a longer period." The team took photos of the structures, and from their geometry the researchers derived the ancient flow directions and conditions that shaped the sand 5.3 million years ago. They discovered that the sand had not been carried by local rivers or deltas, but that it was formed by a strong underwater current that flowed from west to east in water that was deeper than had previously been assumed.
This new study is significant because it provides the first physical geological proof for a mega-flood that flowed from the western Mediterranean Sea to the east. "Now, for the first time, we can directly prove and quantify one of the most catastrophic periods of environmental change on our planet, which until now we had only been able to describe in geophysical models", Van Dijk explains. "Moreover: geologists are trained to use contemporary processes on the earth’s surface to interpret what we observe in rocks. But here we can’t rely on that knowledge, because we don’t know of any similar phenomena from at least the past 100 million years." According to Van Dijk, that is precisely what makes the work and the results so intriguing.
Publication: Van Dijk, G., Maars, J., Andreetto, F., Hernández-Molina, F.J., Rodríguez-Tovar, F.J. and Krijgsman, W., 2023. A terminal Messinian flooding of the Mediterranean evidenced by contouritic deposits on Sicily. Sedimentology. https://doi.org/10.1111/sed.13074