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?.

Sediment grainsize is an important factor in sedimentology, and tells a lot about the environment in which the gravel, sand or mud was deposited. It tells us about current flow velocities, distance from shore and water depths. It tells us stories about sea level change as the sediment change from coarse grains to fine grains or from fine grains to coarse grains.

Mudstones were long thought to record low-energy conditions of offshore and deeper water environments. Geologists thought that constant, rapid water flow prevented mud's constituents, silts and clays, from coalescing and gathering at the bottoms of rivers, lakes and oceans. As mudstones make up the majority of the geological record - they constitute up to two-thirds of the sedimentary record - it is important to get things right. Mudstones are, however, arguably the most poorly understood type of sedimentary rocks.

New research based on laboratory experiments shows muds will accumulate even when currents move swiftly. The findings appear in the journal Science of 14 December 2007. During their experiments the researchers found that mud beds accumulate at flow velocities that are much higher than what anyone would have expected. The mud accumulates slowly at first, in the form of heart- or arrowhead-shaped ripples that point upstream. These ripples slowly move with the current while maintaining their overall shapes.

A key issue in mudstone sedimentation is flocculation. Flocculation is a process by which individual particles of clay aggregate into clotlike masses or precipitate into small lumps. Such small, loosely held masses or aggregates of fine particles are called floccules. Flocculation enhances the deposition rate of fine-grained sediments, and its understanding is critical for modelling the behaviour of mud in sedimentary environments.

The observations from the experiments do not support the notion that muds can only be deposited in quiet environments with only intermittent weak currents. Another interesting point is the formation of ripples. That ripples have not been observed in sediments so far is probably because they get extremely flat during compaction. Floccule ripples are spaced 30 to 40 cm apart, and ancient sediments of this origin will therefore likely appear parallel-laminated.

Understanding the mechanisms of mudstone deposition is a.o. relevant for oil and gas exploration, maintenance of water reservoirs, and several branches of engineering. The authors think that a re-evaluation of the sedimentary history of large portions of the geologic record nay be necessary.

A big question is of course whether the rest of the geological research environment agree with this viewpoint.


• http://www.sciencemag.org/cgi/content/abstract/318/5857/1760
• http://www.sciencemag.org/cgi/content/full/318/5857/1734
• http://www.terradaily.com/reports/As_Waters_Clear_Scientists_Seek_To_End_A_Muddy_Debate_999.html




PS of 18 December 2007: See Brian's post at Clastic Detritus

Scientific publications are written by experts for their colleague experts within the same field. There is nothing wrong with that, but it makes most of them extremely difficult (if not impossible) to read for the rest of us. I find this a pity with all the important research going on with implications for everybody and relevant as basis for political decisions.

There is, for example, a pressing need to understand the mechanisms of rapid climate change. Research in that direction is reported in the journal Science vol. 317 of 27 July 2007 under the title Four Climate Cycles of Recurring Deep and Surface Water Destabilizations on the Iberian Margin. Let me quote one of the easier sentences in that important report: “In these cores, the ∂¹⁸O record of benthic foraminifera resembled the Antarctic temperature signal, whereas the ∂¹⁸O of planktic foraminifera exhibited changes similar to those found in Greenland ice cores.”

We are talking about deep sea sediment cores taken off the cost of Southern Portugal (that is what is called the Iberian margin in the title). The data collected made it possible to construct a detailed picture of the climate history of the past 420,000 years - that is covering four climate cycles each of around 100,000 years.

I have the impression that most of my readers are geologists having no problems with the sentence I quoted. I, for myself, happened once upon a time to have taken a one year university course in oceanography, so I have no problem either. If however you don’t know anything about foraminifera, benthos, plankton, and isotopes (like ∂¹⁸O), you may have a serious problem with this in itself simple sentence.

Foraminifera are microorganisms living in the sea. They typically produce a calcareous shell (called a test) composed of calcium cabonate. When they die their shells sink to the bottom of the ocean to become a substantial part of the deep sea sediments. Foraminifera have been studied more extensively than any other group of oceanic microfossils. The microscopic formaninifera are the most important for sediemtological studies, although larger foraminifera (up to 16 mm) do exist. See my post on Ocean Drilling for Earth's Climate History.

Some foraminifera are bottom-dwelling. Organisms which live on, in, or near the seabed are called Benthos. So
benthic foraminifera are bottom-dwelling microorganisms.

Other foraminifera drift with ocean currents. They are plankton. So planktic foraminifera live in surface water.

When their shells sink to the bottom of the ocean they will end up in sediment layers side by side with shells of bottom-dwelling foraminifera, which make them ideal for study of (currents in) surface waters and deepsea waters at a specific time.

The chemical formula for calcium carbonate is CaCO₃ (calcium + carbon + oxygen). Here we shall concentrate on the oxygen. The oxygen is extracted from water, H₂O (hydrogen + oxygen).

Oxygen isotopes. Isotopes are not necessarily radioactive. Isotopes of an element have nuclei with the same number of protons but different numbers of neutrons. Oxygen has seventeen known isotopes. Three are stable, ¹⁶O, ¹⁷O, and ¹⁸O, of which ¹⁶O is the most abundant (over 99.7%). That they are stable means that they don’t change after their formation. Oxygen-16 or ¹⁶O has 8 protons and 8 neutrons, whereas Oxygen-18 has 8 protons and 10 neutron, which makes it heavier. Oxygen isotope analysis considers the ratio of O-18 to O-16 present in a core sample taken from deposits in the ocean floor. In fact it is more like a ratio of a ratio, because the delta values (∂¹⁸O) are calculated on the basis of ratios of the ratio of oxygen-16 and oxygen-18 in the sample and the ratio of oxygen-16 and oxygen-18 in average ocean water (Standard Mean Ocean Water, SMOW).

Today the ∂¹⁸O values for rainwater (or meteoric water as it is called by geologists) range from close to -50 per mil at the South Pole to close to zero per mil in some tropical areas. The observed ∂¹⁸O concentration in average annual precipitation is proportional to the mean annual air temperature. The isotopic composition of seawater does not only depend on temperature, but to a larger degree on other factors like continental ice volume.

Now: In samples from deep sea sediments off the coast of Portugal analysis of the oxygen content of bottom-dwelling microorganisms pointed to temperatures at the Antarctic, while the oxygen content in microorganisms, that had lived near the surface, was more like in Greenland ice cores.

Apart from a story about temperatures and ice caps. this also told that the surface current came from the north (Greenland) and the bottom current came from the south (Antarctic Bottom Water, AABW - and NOT North Atlantic Deep Water. NADW - see my post on the Gulf Stream).

• http://www.sciencemag.org/cgi/content/short/317/5837/502
  
• http://www.pro-physik.de/Phy/leadArticle.do?laid=9352 (in German)