Messinian Salinity Crisis
The sea dried up, covering the former bottom with a layer of salt. The temperature reached 80 degrees, and the pressure was 1.5 times higher than at sea level. This is not a description of a post-apocalyptic future, and this is not the planet Plyuk (two times “coo”!). This happened about 6 million years ago, where tourists now bask by the warm Mediterranean Sea.
It could be said that it all started in the 1960s, when geologists in the Mediterranean decided to go for it and see what would happen. More precisely, to observe the reflection of acoustic waves from any underground.
It turned out that some layers at a depth of 100-300 meters reflect the waves perfectly. And this kind of disgrace was happening all over the Mediterranean Sea. It was completely unclear what it was, because they had never seen anything like it before. They decided to drill down to this layer and find out what it was. They started drilling and got gravel. This was already surprising, because gravel should form in continental layers. And here it was found a hundred miles from Barcelona. And then there was more. They drilled down to the reflecting layer and it turned out that it was the upper boundary of evaporites, i.e. minerals that fall out of a saturated solution. And they found them in the middle of the sea. And the minerals themselves turned out to be unusual. They contained anhydrite, CaSO4. That is, almost like gypsum, only without bound water molecules. In order for anhydrite, not gypsum, to form, a high temperature is needed. What kind depends on the concentration of table salt in the water. With a saturated solution, the critical temperature for this is +35. If the salt solution is unsaturated, then an even higher temperature is needed for anhydrite to precipitate. Even in the Dead Sea at +40, anhydrite does not form. True, at high pressure, gypsum can transition to anhydrite, but this is unlikely to be the case. Finally, fossil plants and animals, such as cyanobacterial mats, indicated that these layers were at a shallow depth. And now in these places the depth is more than 2 km. What happens? Did the Mediterranean Sea once dry up so much that only puddles remained?
The presence of redeposited evaporites showed that they had been worked on by air erosion. That is, part of the sea surface was then above its level.
Next, diatoms were found, which live in fresh or brackish water. This means that at some point the surface was flooded with fresh water.
Also found was the fauna of the raukushkovykh, which live in brackish seas, and on the bottom. In addition, it turned out that the river canyons continue far into the sea, i.e. earlier the rivers flowed on land where now the waves splash.
It turned out that at a certain point the Mediterranean Sea dried up, stood dry for some time, and then filled up again.
There are many questions further. Why was the sea closed off from the inexhaustible waters of the Atlantic at that time? Was it completely closed or did water still flow in? How, why and when did it open? As far as I understand, this is still being debated.
The chronology of these events, oddly enough, is a bit uncertain. Milankovitch cycles and magnetization cycles came to the rescue. Let me remind you that the Earth's axis changes its position with a cycle of just over 20,000 years in a period. This leads to insolation cycles, which in turn affects the nature of the deposits. So it is possible to estimate the age of the deposits with an accuracy of 0.02 million years. The direction of the Earth's magnetic field also changes, and the magnetization of rocks can also determine the time of their formation.
So, when the salinity crisis, called Messina (this is the name given to this event, after the city in Sicily), began (5.97 million years ago) and when it ended is more or less clear.
It’s more difficult to answer “why did it start?”
The first thought is that it was due to climate. Then there was just another ice age, the water was cheerfully and actively turning into ice, depositing itself in caps, and the ocean level was falling. The trouble is that this ice age was not the last and the ocean level was falling far from record-breaking, but neither before nor after did the Mediterranean Sea dry up. So, something was happening with tectonics?
It definitely happened, because Gibraltar didn't exist yet. But instead of Morocco there was an archipelago. And there was a cloudless sea over southern Spain… because what the hell are clouds in the sea?
At one fine (or not) moment, the Atlantic's connection with this former Tethys and the future Mediterranean Sea began to be interrupted due to the rise of the land. To what extent is a question. At first, it was estimated that now, if the flow of water from the Atlantic ceased, the Mediterranean Sea would dry up in a thousand years. But how much water was brought by the rivers is unclear. According to plankton deposits, it is clear that during the Messinian salinity crisis, the sea did not dry up immediately.
Apparently, at first, even before the salinity crisis (and therefore the drying up of the sea), the mixing of layers decreased, which destroyed part of the plankton. This is evident from the shales, which are formed under conditions of oxygen deficiency and the presence of hydrogen sulfide, as is now the case in the Black Sea. In turn, this indicates weak mixing of water.
Then the change in acidity finished off the survivors. And it is connected with the change in insolation, and that – with the precession cycles. That is, all this lasted for thousands of years.
Finally, according to estimates, the amount of salt deposited at that time would be enough for 50 (fifty!) modern seas. That is, the water flowed in and out of the drying sea for a long time. If anyone thought about communication with the Indian Ocean, then I will disappoint you. By that time, the Suez Isthmus already existed, but the Suez Canal did not yet exist.
So what was the territory of the modern Mediterranean Sea like at the peak of the crisis?
Initially, it was estimated that the temperature reached 80 degrees Celsius, the sea dried up completely and was replaced by a terrible lifeless desert. And with increased pressure (after all, the earth's surface is much lower than the level of the world ocean). But these estimates were made by stupid linear interpolation with gradients at the modern surface of the sea. If this works for pressure (since it is determined by gravity, and it does not change due to the drying up of the sea), then temperature is a delicate nature and can depend on a bunch of factors. It would seem that geologists can at least determine whether the sea dried up completely or not… but drilling through all the sediments of that era in the central part of the sea was not done. And it seems that because they are afraid that oil will start gushing there, like the clumsy BP in the Gulf of Mexico. However, I do not believe that this is the very reason, if there were such concerns, everyone would have rushed to drill. In addition, oil can be expected where there was dry land, and before the deposition of evaporites in these places there was a sea. True, in the area of Sicily there are also outlets of continental crust, there may be oil there.
On the other hand, one cannot help but recall the school example with Death Valley. It is a gorge in North America, located among the mountains. A long time ago (no, not in a distant galaxy) there was also a lake there, but it dried up. It would seem that cold air should accumulate in the basins among the mountains, as, for example, happens at the Pole of Cold in Oymyakon. But in Death Valley there is a different mechanism of climate formation. The sun heats the air, it rises and does not let cold streams flow into the basin. Having cooled down a little, it descends into the same valley, heats up again, rises again… as a result, far from the southernmost place in North America holds records for maximum temperature. Could a similar mechanism have appeared in the place of the basin from the dried up Mediterranean Sea? I don’t know, I couldn’t find any articles about climate models for it. They only write that there is simply nothing to compare with, since there are currently no huge areas kilometers below sea level.
One way or another, but in the existing deposits, andhydrites were found (not to be confused with anhydrides). And this means that there were at least some places with very high temperatures. Now they fall only in hot places, where the land level is only slightly above sea level and during high tide and storms these surfaces are flooded. The Arabs call them sabkha or sebkha. So, maybe the climate in the drying sea was not so different from the modern one. Or maybe it was different, for example, we still do not know for sure how much this sea dried up. There are disputes about whether some deposits are terrestrial or whether this can also form in a shallow sea.
Anyway, about 5.5 million years ago the drying out reached its peak… and then part of the sea was covered with brackish water. Where did it come from? Perhaps it overflowed from Hungary. Yes, on the territory now occupied by Hungary, until relatively recently, there was a huge freshwater lake. Now only Balaton remains.
Since the water that flooded the Mediterranean Sea became brackish, this whole thing is called the “Lago Mare event”, i.e. “lake-sea”.
And about 5 million years ago, communication with the Atlantic was resumed (what a waterfall it was!). In geological terms, instantly. According to various estimates, it continued to flood with salt water for 2 to 2,000 years. The absence of evaporites (i.e., certain deposits that appear during evaporation) in the Gibraltar area speaks in favor of a shorter period and a catastrophic development of the situation. Apparently, they were simply washed away. On the other hand, a little to the east of Sicily, where there is a large ledge, many rocks were carried off it. That is, the flow was strong.
In the end, everything returned to roughly where it started. True, while the Mediterranean was a mega-Aral, desert hobbits (crossed out) and all sorts of animals managed to run into Europe through the isthmus. No, humans have nothing to do with it, our ancestors only climbed down from trees in the region of Ethiopia and Kenya.
As for the prospects, the movement of the lithospheric plates that make up Europe and Africa has not been cancelled, so it is quite possible that something similar will happen in the future. Can you imagine how easy it will be in a few million years for migrants from Africa to get to Europe on their native camels? Unless the desert becomes radioactive by that time…
Author: Nikita Barinov
