EARTH × LIFE
Euxinia: the Sulfidic Ocean
~1.8 to 0.8 billion years ago
Proterozoic · Paleoproterozoic · Statherian
Euxinia, an absence of oxygen combined with dissolved hydrogen sulfide, affected parts of the ocean between 1.8 and 0.8 billion years ago and was produced by bacteria that breathe sulfate.
Euxinia is the state of a body of water that contains no oxygen but does contain dissolved hydrogen sulfide. The air contained little oxygen, and sulfate, a compound of sulfur and oxygen, washed off the continents into the sea. Sulfate-reducing bacteria respire sulfate as other organisms respire oxygen, and release sulfide as waste. These bacteria turned large parts of the deep sea euxinic. The iron that had remained dissolved in the deep sea precipitated as sulfide, that is, left the water as a solid. This precipitation ended the banded iron formations, rocks made of alternating layers of iron and silica. The same precipitation removed from solution the trace metals that many enzymes require, molybdenum and copper among them, and these metals became scarce. Trace metals are metals present in tiny amounts, and enzymes are the proteins that carry out the reactions of cells.
Why it matters
Fixing atmospheric nitrogen, that is, converting the nitrogen of the air into a form that living things can use, becomes difficult in a sea poor in molybdenum. This difficulty is one of the explanations proposed for the billion-year delay in the development of complex life.
Dating & uncertaintyDisputed
In the original model of Canfield (1998), the whole deep ocean turned sulfidic, another word for euxinic, after the banded iron formations stopped. Researchers have since drawn up trace metal budgets, which compare the amounts of these metals entering and leaving the ocean. These budgets indicate that sulfidic water covered only a small fraction of the sea floor. Most of the deep ocean remained anoxic, that is, without oxygen, but rich in iron.
Earth history as one day
14:29:049 h 31 min before midnight
if Earth’s 4.54 billion years were compressed into a single day