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Paleoclimatology Liveblogs: Two Problematic Bathtubs – The Mediterranean Ahh, the Mediterranean: sun, sea, a beautiful subtropical climate, last bastion of the Tethys ocean—

--thing that requires different handling of some paleotemperature proxies because it’s different from other oceans†—

--and ticking time bomb of climatological weird.

You see, on an oceanic scale, the Med is small. And it’s kinda like two bathtubs stuck together. Two bathtubs with problems.

Two Problematic Bathtubs: The Shape of the Med The Med has two pinch points: The narrow-and-shallow Strait of Gibraltar and the deeper-and-wider Strait of Sicily (though the Strait of Sicily is still relatively small).

A crossectional view of the Mediterranean, made up of two deep basins, with the Strait of Sicily dividing them

Here’s a picture (which I’d love to have a source for, but I can’t find one right now) of the rough shape in cross section of the Mediterranean. See the two basins, connected by a shallow strait? Yep, it’s two bathtubs.

And this bathtub-y natures causes some Problems. The first, obvious one is: what if those straits got cut off? Which has happened! Admittedly it happened to the Strait of Gibraltar’s parents and grandparents, but it did happen, and the Mediterranean got cut off from the wider ocean. And because the Med is right under a dry Hadley cell and thus has a higher rate of evaporation than precipitation—bye-bye Med! (Which I will cover in a later section.)

But the fact that there’s a separation between the east and west due to Sicily causes other problems, of a circulatory kind.

A cartoon of the Mediterranean as a T-shaped basin. Shallow water flows from East to West, sinks in the East, and meets water from the Adriatic Sea (the leg of the T), and sinks further and travels west.

Here’s a cartoon of circulation in the Mediterranean. Surface waters generally travel west to east. As they flow, they get evaporated, and get saltier and denser. Eventually, they reach the Eastern Med, where more evaporation happens, and they get salty+cold+dense. Which makes it sink, and become Intermediate Mediterranean water. First thing to note about Mediterranean circulation: If the surface water in the east isn’t salty, everything from here on out gets borked.

This intermediate water travels west, until it meets cold water from the Adriatic Sea (to the East of Italy.) These waters sinks deeper, and become Mediterranean Deep water. Second thing to note: You need the water from the Eastern Med and the Adriatic sea to make Deep water. They can only make intermediate water on their own.

This process of making deep water happens on a roughly yearly cycle, and how dense each year’s water varies year to year. Which leads to the Mediterranean having a layer cake of different deep waters, with the densest waters on the bottom. This layer caking is important for how the deep water circulates up and over the straits. One way that deep water can get moved over the straits is by being pushed up by a denser layer of water going underneath it—the layer cake gets re-arranged, and some of the layers go up high enough that they are pushed over the straits.

This is important in both Gibraltar and Sicily. Gibraltar has another thing that can move deep water over it: the outflow of the water that’s been pushed up can shear/suck up water underneath it, and then that water goes over the strait. (The Strait of Sicily is the wrong shape for that to work.)

This leads to a big thing in why the Med is Two Problematic Bathtubs: it’s really easy for the circulation to get effed up (especially in the Eastern Med.) There’s so many ways the circulation can get weakened, or stopped.

The Green Sahara

If you look at sediment cores from the Mediterranean, you will find, at semi-regular intervals, black layers of sapropel. Sapropel is organic carbon—which is a bit weird to find, because you’d expect any organic carbon in the ocean to be either eaten, or oxidised. But this isn’t. Something stopped this organic carbon from rotting or oxidising.

And it’s all because the Sahara got wet.

So, Africa has a monsoon (which is much simpler and more well behaved than the Asian monsoon, because there aren’t ruddy great mountains in the way.) The general trigger of the monsoon is like so: In summer, the land heats up more than the ocean, because the ocean has a kind of absurd specific heat capacity/ability to absorb heat. The land heats up the air above it, which makes the air rise. This creates an area of low pressure, which draws in humid ocean air. This humid ocean air also gets pulled up, which makes the water in it condense, and voilá: rain!

When the summer insolation (the amount of sunlight energy) in the Northern hemisphere is at it’s peak, the African monsoon can go quite far North‡ into the Sahara. This makes the Sahara much less… desert-y, and also creates new rivers, and makes other rivers like the Nile stronger. And where do these rivers, and the rain run off, go?

The Mediterranean. So during Green Sahara periods, the Mediterranean has a layer of fresh, high nutrient water on top of it. This causes some Stuff to Happen.

Without a salty dense layer on top of it, deep water production in the East stops. And this definitely kills of circulation in the east, and often kills off circulation in the west.
With this influx of nutrients, all the little microscopic critters go crazy, which produces a lot of organic carbon, and in the deeper waters, starts using up the oxygen.
Without the circulation, the deep water goes stagnant. Without having the addition of surface water, it can’t gain any oxygen, and so it starts losing it. Until, in many cases, the oxygen runs out.
…so when all the organic carbon from the critter-party sinks down, it doesn’t get oxidised, and is preserved as a layer of sapropel.

The Messinian Salinity Crisis: I Could Have Sworn I Saw A Sea Here!

Back about 6 million years ago, the Mediterranean didn’t have a Strait of Gibraltar. Instead, it had the Betic Strait and the Riffian Corridor. Which, uh, closed. Leaving the Mediterranean isolated from the Atlantic.

…meaning it dried out.

But not all at once. And in a slightly weird way.

See, people have done the math of ‘If you have so much water, with yea much salt, you should get evaporite layers with blah thickness.’ In general, you get some thin-ish layers of calcium carbonate and gypsum, then a thick layer of NaCl (aka the salt that you can eat), and lastly a thin layer of bittern salts (aka salt that you really shouldn’t eat.) And the Med largely follows this pattern—but the layer of NaCl is too thick for the size of the Med. So, the Med had to get extra water from somewhere, which was enough to stop it drying out completely, but not so much that it stopped being salty enough to deposit salt. The current main suspects are the other Paratethys seas (aka other last bastions of the Tethys ocean, like the Black Sea) and the African Monsoons.*

…and eventually the Gibraltar strait formed, and the level of the Mediterranean rose until it met it. (Which means that the old idea of a dramatic waterfall as the Gibralter strait opened up probably isn’t true.)

† There is a type of archaea/single celled organism called Thaumarcheota, where you can use what chemicals it produces to tell the temperature. And in the Mediterranean, it doesn’t quite behave how you’d expect.

‡ CONTROVERSIAL STATEMENT ALERT: How much monsoons can move is, uh, a matter of great debate. That involves a lot of atmospheric dynamicists saying ‘It’s simple, really!’ while paleoclimatologists hold their head in their hands saying ‘My entire life is a lie.’ So, take any statement about monsoons moving any great distance with a grain of salt. (To my understanding, a lot of the Green Sahara period is less the monsoon moving, and more it getting strong, but I could be wildly wrong here.)

*CONTROVERSIAL STATEMENT ALERT: There is some evidence that the Mediterranean completely dried up, like the hippo bones found in some Mediterranean islands. Those hippos could have crossed dry salt flats, but not brackish water. So, ‘how dry did the Mediterranean?’ get does not have a cut-and-dried answer.