olelog

Googling the combination “euxinic conditions” gave me 2060 hits, and the combination “euxinic sediments” 875 hits, so the term is obviously used, and it has been in use in geology at least since 1930.

Euxinic literally means ‘pertaining to the Black Sea’. The word euxinic comes from the old Roman (and ultimately from the Greek) name for the Black Sea (the Romans called it the Euxine Sea, Pontus Euxinus). So, before I go on, let us have a look at the conditions in the Black Sea.

The waters of the Black Sea are strongly stratified with an upper oxidised layer and a lower anoxic layer. Freshwater (green arrow) flows into the sea from rivers like the Danube, Dniester, Dniepr and Don. Sea-water (blue arrow) flows into the Black Sea from the Mediterranean via the street of Bosporus. Because of the different salinities and densities, the freshwater and sea-water mixing is limited to the uppermost 100-150m. The mixing between surface water and bottom water is strongly restricted, and the whole bottom water is exchanged only once in a 1000 years. Oxygen is needed for rotting of organic matter, so under anoxic conditions organic matter doesn't rot. As a result black, organic rich, sediments accumulate on the bottom. The Black Sea has got its name because such black sediments make the sea water dark. Unlike the Mediterranean, where visibility extends down to a depth of about 30 meters, visibility reaches only as far as about 5 meters in the Black Sea.

Rotting is a bacterial process, and occurs under aerobic conditions, aerobic means occurring only in the presence of oxygen. You also have bacterial activity under anaerobic conditions, that is it occurs in the absence of oxygen. During anaerobic conditions at the bottom of the Black Sea sulphate reducing bacteria strip the oxygen from sulphate and dump hydrogen sulphide (H₂S) as a waste product (a sulphate ion consists of a central sulphur atom surrounded by four equivalent oxygen atoms). Some of the hydrogen sulphide may react with iron to form pyrite (FeS). Increase of pyrite in the sediments is an indication of the activity of sulphate-reducers.

So the term euxinic has to do with an environment of restricted circulation and stagnant or anaerobic conditions. Euxinic conditions are at the same time both anoxic, anaerobic and sulphidic. Euxinic conditions may lead to deposition of euxinic sediments like sapropel. Now there is another nice foreign word. Sapropel (a contraction of the ancient Greek words sapros and pelos, meaning putrefaction and mud, respectively) is a term used in marine geology to describe dark-coloured sediments (mud, slime, or ooze) that are rich in organic matter. Organic carbon concentrations in sapropels commonly exceed 2% in weight.

I have checked the indexes of a few textbooks on oceanography and marine geology. They seem to do quite nicely without using the term euxinic. I have to know the term however to be able to read some of those $@^§* scientific papers!

Papers such as this one:
Euxinia as the cause of the end-permian mass extinction: Evidence from sulfur isotope chemostratigraphy
• http://gsa.confex.com/gsa/2005ESP/finalprogram/abstract_88807.htm
Relevant for my post a few days ago on How to kill 95% of all life?.
Oh ah, and yes, I forgot to say at the start: Google gave me 3,340 results for euxinia! - euxinia means euxinic anoxic conditions.

Euxenite, on the other hand, has nothing to do with euxinia, but is a lustrous, blackish-brown rare-earth mineral consisting primarily of cerium, erbium, titanium, uranium, and yttrium.

Words, words, words.



PS of 30 March 2008
Kim over at All of My Faults Are Stress-Related has started a discussion on possibly unnecessary or outmoded geology terms in the post Geology terms overdue for retirement?.