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Puzzles of the Black Sea

АвторДАТА ПУБЛИКАЦИИ: 07 сентября 2018
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By Vladimir AVILOV, Dr. Sc. (Tech.), Svetlana AVILOVA, Dr. Sc. (Biol.), senior researchers, RAS Institute of Oceanography named after P. Shirshov

Having spent your vacation at a Black Sea resort just once, you will probably never forget its fine beaches and inviting crystal-clear and tender waves. This kind of emotions, no doubt, were shared by our forefathers hundreds and thousands years ago. So far so good. But what about the future? This question is suggested by piles of junk and refuse along the coast, fishermen dragging in empty nets and the shocking stink of sulfur dioxide on the beach. Experts investigating these phenomena warn us of a partial degradation of the ecosystem of the region and it is up to them to try and work out ways and means of dealing with the situation.

Articles in this rubric reflect the opinion of the author.- Ed.

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Back in the 6th century B.C. the Greek historian Hekateus from Miletus marked in the center on the earliest surviving map of the world surrounded by the River Ocean, two inland seas-Thallass (the Mediterranean) and Pontus Euxinus (the Black Sea). Ever since that time the Black Sea has always been, to a greater or lesser degree, in the focus of attention of seafarers and historians, military leaders, artists and scholars. This being so, it belongs to the category of the best studied bodies of water in the world. As for Russia, in the middle of the 19th century studies in the region were conducted by Admiral Stepan Makarov, one of the founding fathers of Russian oceanography, and his example was followed by many prominent Russian and foreign researchers. Numerous expeditions investigated problems of local hydrology, biology, geology and what we call chemistry of the basin. Having said that, let us take a closer look at some of the basic facts and figures, especially those which allow of different interpretations.

To begin with, the Black Sea is the world's biggest body of water of what is called the meromectic type: its upper layers of water down to the depth of 130 - 150 m are saturated with oxygen, and down below close to 90 percent of water is void of this vital gas. This division was established in 1890 by Prof. Nikolai Andrusov (later Member of the St. Petersburg Academy) who established the existence of a benthic zone of hydrogen sulfide. In the following years the existence of this division has been confirmed by other researchers with the next problem on the agenda being the dynamics of the process-whether the "watershed" between the two zones is sinking or lifting with time, or remains unchanged? And the snag of the matter is that no such "watershed" really exists. There is an intermediate layer of water- what specialists call the redox-zone, in which oxygen and hydrogen sulfide coexist and where intense oxidation reactions take place. The thickness of this layer of water amounts to tens, sometimes 70 - 100 m, and its position changes in time and space depending on the time of the year, weather conditions and hydrology of the region.

Most scientists investigating problems of the Black Sea, such as academicians Mikhail Ivanov and Mikhail Vinogradov, are of the opinion that the border of hydrogen sulfide propagation has remained practically unchanged over a long period (of over 100 years) of observations. According to the Institute of Marine Hydrophysics of the Academy of Sciences of the Ukraine (Sebastopol) its position along the whole basin really remains unchanged-at an average depth of 125 - 135 m. Some probes, however, detected hydrogen sulfide only tens of meters below the surface in the central sectors of the sea-a fact that prompted apprehensions about an upward drift of the hydrogen sulfide layer and a threat of an ecological disaster. Such apprehensions were seemingly supported by the circumstances of the Crimean earthquake of 1927. There is documentary evidence of giant tongues of fire seen over the surface of the sea during the quake which were described for the first time by Dr. S. Popov in 1928. He wrote of tongues of fire soaring up over the waves to a height of up to 500 m and over stretches of water of up to 1.5 miles. Flares and expositions occurred against and again during 2 to 3 hours and were registered by observer from the Lukull outpost (near Eupatoria) and the Sebastopol beacon, whose bearings crossed near the foot of the continental slope (sea depth of some 900 - 1,500 m). These phenomena were unfortunately overlooked by the scientific community and received no proper explanation.

As it is, the geological history of the Black Sea offers us more questions than it supplies answers. Its level of salinity drops and rises and what we call benthic hydrogen sulfide was not always there. Experts believe that the layer came into being rather recently, some 4.5 mln years ago, when the Alpine folded area was formed. The bottom of the ancient sea dropped down by several kilometers and the depression began to be filled with sediments. Subsequent tectonic processes broadened and deepened the bowl. In the opinion of Dr. Yuri Neprochny and Dr. Ivan Elnikov of the RAS Institute of Oceanography, the earth crust underlying the central part of the Black Sea is lacking a granite layer and there is but a thin layer of basalt there only. Figuratively speaking, the basin looks like a cup with the walls of granite and a thin basalt bottom located over the hot mantle. The cup is filled with 14 km of sediments over which there are 2 kilometers of water.

Now, what about water. Back in the mid-Quaternary Period (300 - 75 thous. years ago) the region was occupied by

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the ancient fresh-water "sea-lake". It was later connected with the Mediterranean and turned salty. During the glacial period (22 - 10 thous. years ago) the New-Euxinus basin was formed and the level of water salinity dropped considerably again. And some 8 thous. years ago the Black Sea was finally linked with the ocean through the Bosphorus and its salinity started rising again. From that time experts distinguished two periods in the life of this basin-the ancient one and the modern-of the last three thous. years. Changes in water "chemistry" have been reflected in the composition of what is called the bottomset bed. As compared with the present, the ancient deposits contained more organic matter. And what was the reason for that? One theory is that back in those years the Black Sea was "more productive". And if that is true, the current period is not the best in the history of the Black Sea. The ecosystem of the basin has considerably deteriorated and its degradation continues. Millennia ago what we call the "hydrogen sulfide zone" must have been located at a much greater depth. If, according to current theories, it started forming with the opening up of the Bosphorus and the influx of salty sea-water, the mean historical "rate of lift" of the hydrogen sulfide "boundary" amounts to 0.25 m/year. And that means that during the favorable ancient

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period hydrogen sulfide did not rise above the depth of 900 m and did no harm to the ecosystem of the sea.

Today a vast anoxic, or oxygen-free, part of the Black Sea is practically lifeless, with nothing but microorganisms able to survive therein. According to Dr. Yuri Sorokin of the USSR Academy of Sciences Institute of Oceanography (1970) their biomass at the depth of 500 m over the continental shelf off Yalta amounts to 5 mcg/l, which is 1 - 2 orders of magnitude less than in oxygen-filled water. According to our own observations, in the deep-sea sector of the sea (1982) the concentration of ATP as an indicator, the live biomass of microorganisms is within 10 - 16 ng/l and corresponds to their biomass of 2 - 3 mcgC/l. Similar figures (10 ng ATP/l) were obtained by Prof. D. Karl (1978, USA). That is why, by their biochemical parameters, we regard this water as biologically passive. At the same time, using microbiological methods (seeding on nutrient media), we have identified the main physiological groups of the local bacteria: heterotrophic and methane-oxidating, of nitrogen and sulfuric cycles. But their specific productivity (the ratio of yield to mass) is low and does not exceed 0.02 - 0.03 mcgC/l per day. That means that their metabolism is very low, that they exist in a depressed, "drowsy" state. But this community of organisms, while retaining its diversity of species, is ready to produce adequate responses to external impacts- something we established later. One should note at this point that from an ecological standpoint this low level of activity of microorganisms deep in water is a norm for what we call the hydrogen sulfide zone.

The unique state of deeper water layers in the Black Sea is not limited to the presence of hydrogen sulfide only. They are literally saturated with methane with levels of up to 0.5 ml/l which is 3 - 4 orders higher than the equilibrium concentration with atmospheric air. Also present there are ethane, ethylene, hydrogen and carbon dioxide. Gaseous nitrogen reaches 105 percent saturation. The aforesaid components show a marked nonuniformity of distribution around the water area with their levels increasing with depth. Such facts, however, can not be explained simply by hydrodynamics or diffusion mass transfer. Incidentally, despite this anomaly, water raised from depth does not burn, no matter how one tries to "set it on fire, although it spreads out the specific "aroma". This being so, attempts by some analysts to explain tongues of fire seen on the surface of the sea in 1927 by the "surfacing" of water from deeper layers, to say nothing of their "self-ignition", really hold no water, to use the expression.

As for the bottomset deposits, they can burn. What specialists usually find in the gas mixture in such sediments are methane, hydrogen sulfide, nitrogen, and less often- ethane, ethylene and carbon dioxide. A mosaic picture of distribution of these components is observed along the bottom area and their levels differ in the vertical direction. And there are also anomalous areas with increased presence of gases. Deep- sea cores raised on board* in such places visibly changed their shape, swelled and released considerable amounts of gas which could be ignited. This gas consist mainly of methane (up to 90 percent) and also hydrogen sulfide and carbon dioxide. In sediments from an area to the south-west of Sebastopol - in about the same zone where the sea was "on fire" in 1927, we also detected the presence of hydrogen (up to 5 percent).

And there is still no agreement on the origin, or genesis, of these gases. The microorganisms are certainly involved, but how many and to what extent? In the western deep-sea part of the sea we measured abnormally high ATP concentrations (up to 1,070 ng/g) in the ground layer 70 - 80 cm under the bottom. In the same area at the level of 10 cm Dr. D. Karl also found high ATP levels (up to 380 ng/g). According to Dr. M. Ivanov, the community of microorganisms in the bottom sediments includes saprophyte, sulfate-reducing, methane-forming an methane- oxidating bacteria. And the question is-which of them prevail?

And one more puzzle. In 1959, Dr. Anatoly Kriss discovered in the upper layer of the bottom mud in the benthic zone of the Black Sea thionic bacteria, which need oxygen, and came to the conclusion that there must be some active oxidation processes going on at the bottom of the hydrogen sulfide zone. How this could really occur remained an open question for a long time.

* See: V. Avilov, S. Avilova, "These Mysterious Black Sea Sediments", Science in Russia, No. 2, 1995. -Ed.

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Now, let us try and analyze the processes involved. The central one, although the most debatable of them, is the formation and maintenance of the world's largest body of oxygen-free water. Its basic diagram is this: the deep-sea sections, or layers, are saturated with gases rising to the surface from the bottom sediments; when they enter atmospheric oxygen they become neutralized.

So, the central events occur in the sediments and the redox-zone, and as a result what becomes the lifeless 90-percent mass of water turns kind of a "culprit", acting like a passive conductor of gas streams.

In actual fact there occur complex physicochemical and biochemical processes in the better studied redox-zone. It is there that the gases "escape" from the water of the Black Sea, and are "consumed" through the oxidation of methane and hydrogen sulfide. The volumes of discharge can be assessed by the rate of oxidation which directly depends on the rate of oxygen release into the intermediate layer, something determined by many factors of nature. Full-scale observations bear out this functional dependence. Increased rates of oxygen influx push down the border of the hydrogen sulfide layer which "sinks" to the depth of 200 - 250 m in the areas of coastal currents due to a greater rate of vertical mixing of surface water and an intense transfer of the dissolved oxygen.

The processes taking place in the sediments are also very complex, but even less studied because of the difficulty of reproducing the conditions in the places where they occur (in sediments themselves). But it was back in 1893 that Prof. Nikolai Zelinsky, later Member of the Russian Academy, discovered the phenomenon of generation of hydrogen sulfide by some specific bacteria dwelling in the silts of the Black Sea. Prevailing since that time has been the concept of an influx of hydrogen sulfide from sediments as a result of sulfate-reduction. Numerous subsequent studies proved that the most intensive generation of hydrogen sulfate occurs in anaerobic conditions due to the activities of these sulfate-reducing bacteria. This takes place, however, only in the top layer of sediments of 1 - 2 cm in thickness and rapidly "dies down" at depths of 10 - 20 cm. Certain amounts of hydrogen sulfide are generated in putrid processes of protein decomposition. The community of microorganisms in sediments also performs some other important functions, including destruction of organic matter (heterotrophs), production and oxidation of methane. But most often these processes on the surface of the seabed are slowed down.

Now, let us compare the rates of gas generation in the sediments and of oxidation in the intermediate zone of deep-sea zones. Direct experiments and theoretical assessments put the generation of hydrogen sulfite in bottom sediments at about 20 g/m 2 . Similar assessments in the intermediate zone indicate much higher rates-of the order of 100 - 200 g/m 2 a year. This striking difference calls into question the very concept of what we call hydrogen sulfide genesis.

And the situation with methane is even more complicated. Analyses at the top border of the hydrogen sulfide zone, in a layer of 200 - 400 m, have revealed an unusually active microflora of bacteria present therein which is similar to the situation in the water of lakes in the silt of which intense methane fermentation was observed. But no such phenomenon has been detected in the Black Sea sediments. What is more, according to data of Dr. M. Ivanov, some core samples, taken from the western continental slope, contained only methane-oxidizing microorganisms and no methane- forming or sulfate-reducing ones were found. According to our own findings: in the western deep-sea basin at depths of 2 - 4 m the methane level increases by 2 - 3 orders, but ATP concentration drops, which cannot be linked with the formation of methane in the layers under investigation. On the contrary, this rather indicates a process of methane oxidation in situ.

Summing it up, the present level of our knowledge has only revealed an obvious discrepancy between the rates of generation and oxidation of gases, but failed to resolve the problem as such. Our figures indicate an expected "deepening" of the hydrogen sulfide boundary-something which is denied by the available facts. This makes logical a conclusion about some additional gas source. In other words, the processes occurring in the silts on the bottom of the Black Sea "supply" into the water only a fraction of hydrogen sulfide and methane from the total amount necessary for "sustaining" the oxygen-free mass of water. The rest should be looked for in the deeper layers of sediments and must be more likely associated with the formation of gas and oil. The Black Sea

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is not some swamp or shallow lake, and a collision of fluid fluxes from the lithosphere and atmosphere dominates in the formation of its water structure.

In a bid to find the truth and the answers to many puzzles of the Black Sea one should rely on what we know about the formation of oil and natural gas. Of the greatest interest are the finding obtained in 1970 by the Laboratory of Oil and Gas Resources of the World Ocean of our Institute which was set up by Corresponding Member of the Russian Academy, Artem Geodekyan; it is headed now by Academician Anatoly Dmitrievsky. Their historic-genetic method of comprehensive interpretation of geological, geochemical and geophysical parameters has confirmed the existence of foci of oil and gas formation within the sediments of the Black Sea basin, or bowl. Developed with our participation have been technologies of on-site gasobiochemical studies and modeling of bathygenic processes.

We have experimentally confirmed the processes of gas formation within layers of sediments. We used soil samples from the bottom of the Black Sea obtained under the International Deep-Sea Drilling Project from board the Global Challenger at Spot Number 379 from horizons of 50 - 100 m. Using some equipment we imitated their submersion to 3 and 5 km (temperature of up to 250C, geostatic pressure of up to 2,000 atm). We observed releases from rock of large amounts of gas, increasing with depth. We determined the composition of the newly formed gas mixture as containing hydrocarbons (methane and homologues), hydrogen and carbon dioxide; but no helium was found.

In another series of laboratory experiments we have come across a hitherto unknown phenomenon of concentration of a scattered flux of natural gases in the upper layers of sediments under the impact of deep-sea pressure. In modeling upward flows of helium, hydrogen and carbon dioxide we discovered for the first two their accumulation within the layer of Black Sea silt. Gas bubbles within clayey deposits and the places of their release on the surface of the sediments were observed only along the periphery of the experimental chamber.

While studying contemporary Black Sea bottom sediments we reproduced the phenomenon of chemolithoautotrophy. When a scattered flux of gas mixture (hydrogen and carbon dioxide) was supplied into the reaction chamber there occurred a surprising metamorphosis - the initially biologically passive silts were activated by several times, and the sea water above them-by an order of magnitude (ATP content rose from 110 to 1,230 ng/l). As a result of an intense multiplication of microorganisms with the chemolithoautotrophic type of metabolism we detected the formation of methane.

On-site measurements in the Black Sea have put new facts at the disposal of science. Thus, contrary to the common view that a thin benthic layer of seawater is saturated only with hydrogen sulfide, we also discovered there the presence of water-solved oxygen. Samples were taken many times (in 1977, 1980, 1984) with the help of a specially designed bottom sampler, designed by Dr. Vladimir Avilov, at a horizon of 1 -1.5m from the bottom at depths down to 2,200 m. It was established that oxygen content in a narrow near-bottom layer of water at depths of 2 km reached 0.5 - 0.7 ml/1, and near the bottom of a deep-sea Danube canyon with depths of 400 - 800 m it was from 0.3 to 1.6 ml/l. Also observed was a reduction of methane content in the near-bottom water and sediments, and we explained the existence of oxidation processes in such "anomalous" places.

This confirms the hypothesis of Dr. A. Kriss (1959) and explains the prolific activities of tionic bacteria in the surface silt at the expense of the actually present oxygen. What is important for our own conclusions is that on some stretches of the bottom surface sulphatoreduction is interrupted, that is the amounts of generated hydrogen sulfide in such places are lower than the calculated values. This is a serious argument for the concept of the prevalence of deep-lying sources supplying hydrogen sulfide to the sea.

Serious doubts were cast concerning the opinion about low biological activity of the benthic oxygen-free water. In tests of the water area south of the Crimean Peninsula in 1993 an instrument called integrating bathometer identified whole layers of water at depths of 500 to 1,500 m with increased biological activity. There, in the "focal point" of the Black Sea waters, ATP level amounted to 75 - 90 ng/l (biomass of active live microorganisms of 17 - 24 mcgC/l). This is 5 to 7 times greater than our previous mea-

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surements and the measurements of Dr. Yu. Sorokin, D. Karl and other researchers, and 2 to 3 times more than the ATP concentration for deep waters of the ocean which have quite enough oxygen for the propagation of destructive processes with the participation of heterotrophs.

Naturally enough such phenomena are ruled our in the oxygen-free zone of the Black Sea. This being so, how can one explain the increased biological activity of the "core" of its waters? And one can't help thinking about pollution of coastal areas with industrial and household wastes, promoting fermentation. Pollutants getting into oxygen-free bathygenic water layers trigger off an "awakening" and intense breeding of bacteria. The fermentation of an organic substratum produces, among other things, gases-carbon dioxide and hydrogen, which in their turn are used by the methane- forming and sulphate-reducing bacteria. And part of the pollutants, which cannot ferment in anaerobic conditions (saturated hydrocarbons, steroids and such like), cause toxication of the environment, producing a negative impact on the ecosystems of the mass of water and bottom sediments. In a word, such technogenic impact additionally saturates with gases the oxygen-free zone, increasing the risk of its floating up to the surface.

Vfe discovered for the first time in the sediments foci of high biological activity. Microorganisms multiplied in the streams of gasiform fluids. For example, a torrent of methane promotes the multiplication of methane-oxidating bacteria. Measurements of abnormally high ATP levels have pointed to their activation in the top layers of sediments and their decrease with depth down to the horizons of 1 - 3 m. This regularity we called distribution of the 1 st type. And if hydrogen and carbon dioxide predominate in the gas streams, there develop in the sediments chemolithoautotrophs, which provide for the microbiological generation of methane by the mechanism of reduction of carbon dioxide by hydrogen. This process in the ground is of a more "deep-lying" nature: it begins at the level of 0.7 - 1 m and continues to

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the lowest point of a heavy ground pipe, or corer. We single out this kind of anomaly as distribution of the 2nd type. Apart from that, with greater depth we observe in the sediments a growing size of the zone of anomaly, and in its bulk it looks like a buried pyramid.

Summing it up, according to our measurements, methane, carbon dioxide, hydrogen and considerable amounts of hydrogen sulfide get into the benthic zone from deeper layers of sediments. This occurs not all over the seabed, but in what we call local foci of discharge, that is what we are dealing with is a geological phenomenon. And that disproves the views of researchers who associated the production of, say, methane with the decomposition of organic matter in the silt of the ancient Black Sea period. Also wrong are the assumptions that the gas is produced by biochemical synthesis in the present-day sediments.

The aforesaid, in addition to the earlier known facts, makes it possible to suggest the following picture of the processes occurring in the Black Sea. The main "motive force" involved are active inner processes in the lithosphere. Peculiarities in the structure of the earth crust, and the proximity of the high-temperature mantle cause abnormally high heating of the bottom sediments, increasing the zone of gas generation. Figuratively speaking, the whole basin (the bowl with the sediments and water) has been strongly warmed up by the heat of the mantle. Abundant gas generation was and is taking place in the lower layers accompanied by the destruction of the scattered organic matter of rocks. High in-depth gas pressure initiates what we call a scattered gas flow at the expense of filtration through the higher lying layers of sediments. Due to tectonic damage of the thin basalt layer mantle gases break through into the bowl, later turning into scattered fluxes. Finally, in favorable geological conditions gases can accumulate and form primary deposits at depths of several kilometers. The latter, when they disintegrate, again produce scattered gas fluxes.

The scattered abyssal flux of a mixture of natural gases, including hydrocarbons (methane with heavy isotope composition), hydrogen sulfide, hydrogen and carbon dioxide, as they move upwards, undergo phase of physico-chemical transformations, in-termittently producing secondary deposits and breaking up again. Later on their bulk gets into the zone of biochemical transformations. Its lower border can go down by several kilometers-which depends on temperature and humidity-which is "acceptable" for the vital activities of microorganisms (for example, light isotope composition of methane-70-^60 %c was observed at depths of up to 0.5 km, which, in the opinion of Academician Erik Galimov, is a sign of its microbiological origin).

Prevailing everywhere in the zone of biochemical transformations in the community of microorganisms are chemolithoautophs, capable of using inorganic donor electrons (above all hydrogen) and obtain nearly all carbon through the fixation of carbon dioxide. Usually their vital activities are accompanied by generation of methane, and less often-of hydrogen sulfide. The former blends with abyssal catagenic methane, forming a powerful vertical scattered flux, which gets into water through filtration and produces in the top layer of sediments anomalies with distribution of the 1st type. And that means that the "forefathers" of the Black Sea methane of the hydrogen sulfide zone are abyssal fluxes of carbon dioxide, hydrogen and catagenic methane.

Also present there is a parallel effect of accumulation of the scattered fluxes of abyssal gases and biochemical methane. These produce bubbles in sediments under the seabed chiefly located along the walls of the bowl. These formations are periodically "unloaded" into water, leaving in the surface of the bottom sediments craters-funnels, or escape through mud volcanoes along channels of tectonic breaks of gas decompaction. Such local phenomena, according to on-site observations, are very common in the abyssal zone (bottom zone) of the basin and most commonly occur on the continental slope and at its foot. In this case observed in the anomalies were distributions of the 2nd type.

With all that it now becomes clear why the Black Sea "went on fire" in 1927. The Crimean earthquake accelerated (as confirmed by our experiments) the escape of gas bubbles. They floated up to the surface. And those of them which contained hydrogen produced an explosive mixture with air. Then they blew up and set on fire the main mass of gas-methane.

And there is another important thing here. The large volumes and high rate of the influx of methane from the sediments mass brought up the geochemical boundary of the anaerobic-aerobic divide from sediments into water, leading to the formation of the redox-zone. What is known as the hydrogen dioxide pollution of the abyssal water strata of the Black Sea is secondary with respect to the methane one. And the meromec-tic type of the whole water basin makes it possible to regard it as a common ecosystem whose condition depends on the tectonic features of the region and is determined, above all, by the levels of geological processes. At the same time large- scale technogenic impacts, superimposed upon the natural stresses, can cause further lifting up of the redox-zone towards the surface of the sea and the annihilation of biota.

Naturally enough, purposeful human efforts should be directed at evacuating hydrocarbons and hydrogen sulfide from the depths of the Black Sea and preventing any reverses. It is necessary to try and reduce the rate of methane and hydrogen sulfide seepage from wastes into the water so that the basin be gradually cleared of them. An effective technical solution should provide for the removal of these gases from the surface of the seabed in areas of their local discharge. But before experts find some original ecological solution, it is necessary to begin by doing something obvious-take care of the safety of the gas pipelines located on the bottom of the Black Sea and restrict the dumping of wastes into its deep-sea sectors.






 

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