OCEAN: 1.5bya to 1.4bya
The Black Sea is a highly unusual body of water.
Of course, currently it is unusual for geopolitical reasons, because it forms the southern coast of Ukraine. Grain shipments leaving the port of Odesa, travel across the Black Sea south-west into the Aegean and Mediterranean Seas.
But that is not 'our' reason why the Black Sea is unusual.
It is unusual for us because it resembles the oceans of the Proterozoic.
Let me explain.
The Black Sea has a surface area of 436,402 km2, which is twice the size of Britain. It has a maximum depth of over 2 kilometres.
Yet its only exit is via the Bosporus which is a narrow strait running through Istanbul. At its narrowest, the Bosporus is only 700 metres across, and its depth varies between 110 metres and a mere 13 metres.
If the Black Sea were the size of a human head, the Bosporus would be one-tenth of the diameter of a human hair.
At the end of the last ice age, a land bridge connected what are now the two halves of Istanbul. The Black Sea was, therefore, separated physically from the Mediterranean.
According to one theory, this land bridge collapsed around 6,800BC, causing the Mediterranean to inundate the Black Sea, with a force exceeding 200 times the flow of Niagara Falls. It is possible that this was the basis of the Noah's Flood story.
All of which is interesting, but it still isn't the reason why the Black Sea is unusual.
It is unusual because the Black Sea has two distinct layers which do not mix.
The top 200 metres is well oxygenated and has a rich ecosystem.
Beneath that, though, the remaining 90% is stagnant and virtually lifeless, other than for sulphide-munching bacteria. There is no oxygen down there.
As a result, the Black Sea is, by far, the largest body of poorly oxygenated water on Earth.
To put this in context, nowadays, even at the bottom of the oceans, you can still find oxygen-reliant creatures – fish, crustaceans etc. But not in the Black Sea.
The Black Sea was known to the Ancient Greeks as the Euxine, which means 'hospitable sea', presumably based on their experience of the surface waters. But, weirdly, this is now the word used to describe the inhospitable deep waters. 'Euxinic' refers to any waters that are sulphidic and oxygen-depleted.
In 1998, the geochemist Donald Canfield argued that the deep ocean during the Proterozoic was also euxinic. This theory is known as the Canfield Ocean.
Subsequently, others have adjusted the theory to argue that the oceans had three layers: a surface layer of weakly oxygenated water, a mid-layer of iron-filled waters and a thin euxinic layer at the bottom.
But, either way, the message is the same, namely that the oceans were barely oxygenated, and the deep oceans not at all.
They were not, therefore, conducive to the expansion of eukaryotic life. If complex life was to form on Earth, it was clear that something had to change.
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PHOTO: © Ocean
Professor at Northern Arizona University
8mothis is good work. near real time with a nice write up. i hope school age children and people living in the Atlas can see your work, too.