Until, when he reached (بلغ) the setting of the sun, he found it (وجدها) set in a spring of murky water (تغرب في عين): Near it (the water) he found (ووجد عندها) a People: We said: "O Dhul-qarnain! (thou hast authority,) either to punish them, or to treat them with kindness." ([018:086]
(look at the picture of sunset at black sea)
Few Hypotheses...
1. The Black Sea's deep waters are much less saline as a result the microalgae concentration is much richer, causing the colour to become darker.
2. There is lack of oxygen in the water that prevents the development of microorganisms, making it appearance black. Black Sea has meromictic(having layer of water that don’t mix.) basin. It means the movement of water between the lower and upper layers of water is very least. It results in acute shortage of oxygen in the lower layer. The lower layers absolute free of oxygen and hence, inactive.
3. The intense fog develops over the Black Sea especially in winter that absorbs the light, thus making the water appear black. . Visibility in the Black Sea is about five meters as compared to thirty-five meters in the Mediterranean Sea.
CONCLUSION
The black sea is a strongly stratified body of water because its upper layer is less saline than deep water layer. The upper layer is dilute (less saline) due to fresh water in flow of the river mainly Danube and other rivers. The deep layer is concentrated (more saline) due to inflow of sea water from the Aegean Sea through the lower part of the Bosporus strait. The difference of densities of upper and lower layer retards vertical mixing in the black sea. Consequently, little atmosphere oxygen reaches the deeper layer which becomes anoxic (oxygen less). This anoxic lower layer allows organic rich sediment to accumulate on the bed of the Black sea without decomposition as oxygen is required for decomposition. This sedimentation also contains Iron sulphide (black coloured solid) and other sulphide which are mostly black. This black sedimentation makes the appearance darker. That is why the Black Sea is not blue rather murkier that other seas. Then, it has no major connection to any other bigger seas/ocean so dead fish; corals, etc. just sit on the floor and makes its appearance murkier.
(Reference: WHY THE BLACK SEA IS CALLED THE “BLACK SEA”? )
