A. Lisitsyn, A. Sagalevich, (c)
by Academician Alexander LISITSYN and Anatoly SAGALEVICH, Dr. Sc. (Tech.), P.P. Shirshov Institute of Oceanology, RAS
Thirty years ago or so marine geologists hit upon fantastic ore structures and a just as fantastic fauna near hot springs on the ocean floor. Quite an exotic phenomenon! Yet such wonders were found to be of wide occurrence on the bottom of seas and oceans. Taking a hard second look, marine researchers saw they were dealing with global matter- and-energy exchange processes between the internal and external spheres of the earth, the processes that act upon all facets of its life.
More than a hundred years ago two English oceanographers-J. Murray (Corresponding Member of the St. Petersburg Academy of Sciences) and A. Renard-published their book, Report on the Deep-sea Deposits, Based on the Specimens, Collected During the Voyage of H. M. S. Challenger in the Years 1872 to 1876 (Edinburgh, 1891). This study heralded the birth of a new discipline, marine geology. For a long time marine explorers were puzzled over the enigma: why such high concentration of iron, 42 percent, in the samples of bottom sediments (42 percent of abiogenic matter)? Such wondrous sediments were detected again only in 1958 by an expedition of the USSR Academy of Sciences that, aboard the research vessel Ob, crossed the Pacific Ocean in what is now known as the East Pacific Rise; the Ob was then on her homeward voyage from Antarctica. A few bottom rock samples were taken onboard and assayed. Apart from iron, they were found to be rich in manganese, copper, lead and other metals. These finds drew much interest from two American researchers Kurt Bostrem and Mer Petterson who then went on an expedition to this part of the Pacific. And they saw that such sediments were covering vast areas of the ocean floor. The tentative suggestion was that the seepage of metal-rich fluids from the deep mantle could be responsible for the phenomenon.
In 1972 one of the authors of the present article headed a Science Academy expedition on board the research ship Dmitry Mendeleyev, out to explore the East Pacific Rise. We made a detailed study of the mysterious bottom sediments and their composition. The results were thrilling indeed-so much so that another expedition of the USSR Academy of Sciences went thither in 1975, on the same vessel. Well, metalliferous sediments were found to take huge territories, at least five times as large as that of France. The chemical and mineral composition of the metal-bearing sediments confirmed the initial hypothesis: yes, indeed, fluids of deep mantle matter were seeping through, and in immense amounts at that. Even though no outflows could be pinpointed, marine researchers, proceeding from the geochemical aureoles of scattering in the water, surmised that such outflows should be concentrated somewhere in the axial and most uplifted part of the Mid-Pacific Ridge. Metal-bearing sediments were detected afterwards in the Galapago (Colon) Rise to the east and in the Hess Trench. So the
outflows of endogenic plutonic matter were to be sought there. Research workers of our Institute (Vyacheslav Gordeyev, Lyudmila Dyomina and others) discovered anomalous concentrations of certain elements in benthic waters. Soon after, an American expedition searching there located what looked like outflows of thermal springs (hydrothermae) as well as a large body of sulfide ores.
In 1979, inspecting the same mid-oceanic ridge in the Pacific (21 0 N), our research crews found yet another hydrothermal field with the outflowing waters at t 0 above 300 0 C. The "towers" built of copper-zinc ores and covered with queer animals had puffs of black "smoke" rising above and carried over into the dark water depths (hence the name, black fumaroles, or "black smokers").
It was an owe-inspiring sight: for the first time ever we could watch the picture of large ore deposits being born on the ocean floor. A most spectacular picture in the abundance and forms of organic life deep amidst the barren wastes of the main! It's really incredible - red-hot lava, hydrogen sulfide and heavy metals bursting forth from the bowels of the earth; and quite nearby, such exotic live oases! It looked as if we got into some fancy land or were carried millions of years back.
Meanwhile we had another major expedition, PIKAR, working in the Red Sea (which marine geologists think to be a nascent ocean); that expedition ran three research vessels equipped with two submersibles dubbed Pisces. (Incidentally, all the way back in 1960s sulfide ore outcrops and hot salt brines were detected over there, in the Red Sea.) Well, our people explored fourteen trenches and made an estimate of grade ore deposits: just one of them had as much as 90 min tons.
In the 1980s Soviet research vessels fitted out with the Canadian- built submersibles Pisces-VII and Pisces-XI (capable of diving to a depth of 2 km) as well as Mir-1 and Mir-2 (built in Finland for our Institute of Oceanology, maximum submersion depth 6 km) were at work in the Atlantic (the underwater ridge Reikjanes), in the Gulf of Aden (Tajura Rift), in the Indian Ocean, in the eastern Pacific (the Juan de Fuca Ridge and the Guaymas Trough in the Gulf of California) and in the rift zone of the Mid-Atlantic Ridge. * In 1990 we first managed to study hydrothermal activity in the areas of sea-floor spreading, or stretching, beyond festoon islands in the western Pacific (Papua-New Guinea), as far north as the Bering Sea, and also in the eastern Pacific. Previously we had been studying hot springs on mid-oceanic ridges only.
HEAT ENGINE ON THE OCEAN FLOOR
According to the plate tectonics theory postulated in the 1960s, lithosphere crustal plates at the boundary with neighboring plates accrete at the expense of hot basalt matter, and the bed expands; this process is accompanied by the simultaneous submergence (subsidence) of the opposite parts of the plates, or subduction. One had to find proofs positive of such accretion: at that time the very idea looked like a wild guess for a system of mid-oceanic ridges extending, overall, for as much as 60 thousand kilometers. That is why the initial data on the wide occurrence of metal-bearing sediments attracted a good deal of attention; but one explanation seemed cogent enough for the accumulation of such sediments - the entry of endogenic deep mantle matter. The discovery of hot lava outflows and associated "black smokers" and ore bodies plus the metalliferous stratum detected in the lower parts of drill cores on the border with basalts - all that argued in favor of the new theory of global tectonics. What we needed was quantitative evidence.
Hot springs and ore structures similar to those discovered on the Galapago Rift and on the mid-oceanic ridge in the Pacific (at 21 0 N) should likewise be present throughout the entire extension of the ridges. Actually, however, their occurrence is rather rare, with hundreds of kilometers in between. Some of these spots are hot and active, while others are dead, with the extinct fauna partway brought in by bottom sediments. At first we couldn't tell why thermal springs occurred only in the Pacific but did not show up in the Atlantic and Indian Oceans. But then we learned: that was due to the low rates of spreading (1-2 cm yearly) there. The axial zone of mid-oceanic ridges revealed rift gorges to a depth of 1-1.5 km, and so the puffs of "black smokers" (usually rising as high as 300-600 m above the ore cones) stayed within. Consequently, ore bodies were hard to find. But subsequently the accumulation of metalliferous sediments within such rifts was proved after all. In ridges with high rates of spreading, thermal springs are clustered topmost, while metal-bearing sediments cover the largest areas possible.
Now, what does it all mean - the discovery of more than 150 hydro-thermal fields in the ocean? First, this means that about 15 km 3 or 40 min tons of basalt lava, flows out onto the ocean floor a year. From the physical-chemical angle, hydrothermae operate as heat engines where the magmatic chamber (in the earth's crust) serves as a heater, while the sea water - as a heat-transfer medium and coolant, and basalt rifts - as intake and offtake conduits. Under the effect of the aggressive sea medium (electrolyte), high temperature (1,200 0 C) and pressure (300-400 atm.), basalt loses some of its chemical elements and, simultaneously, gains other elements and compounds captured from the sea water. Thus high-temperature reactions proceed on the ocean floor, within the water/basalt system. The integral performance of this "engine" is in its heat output rated 12-17x10 18 cal/year at least.
One way of studying the scope of submarine hydrothermal activity is to determine changes in the upper layer of basalt sample cores under
* See: A. Lisitsyn, A. Sagalevich, "Underwater Dimension of the Academy", Science in Russia, No. 2, 1997 .- Ed.
the action of marine water. By the data obtained in the course of this research, every year hydrothermal effluvia from basalt rock carry into the World Ocean as much as 577 mln tons of silicon, 314 mln tons of calcium, 118 mln tons of iron, 485 mln tons of manganese and large amounts of other elements besides. Direct assays of thermal flows and their composition, laboratory experiments simulating this natural process as well as other methods produce analogous figures.
To gauge the full significance of the discovery of submarine volcanism, it will make sense to compare it with land volcanism as best studied. Judging by independent data, the scope of submarine volcanicity is tenfold as large. That means that the bulk of molten matter gets onto the sea and ocean floor at a depth of 3 to 4 km. That is, dry land is not the chief taker. It's an all-important factor: eruptive activities on land do not display as high pressures and temperatures as those in the contact of sea water and molten lava.
We have also estimated the total flow of marine water through the newly formed basalts, hot and cooling alike - 29 to 30 bln tons each year. By the rule-of-thumb estimate, the whole water of the World Ocean should pass through this giant chemical "reactor" within 3 to 10 million years, which is rather fast from a geologist's standpoint. It would be in place to note here that our calculations apply to hydrothermae at t higher than 150 0 C, when basalts capture magnesium from sea water, and it changes its characteristics fast. However, our investigations of thermal flows and floor explorations with the use of submersibles showed the far more frequent occurrence of medi-
um- and low-temperature springs which are said to contribute ten times as much of the heat given off. And what concerns the chemical elements, their presence in the effluvia is appreciably less at low temperatures. Here are some of the plausible figures: the seas and oceans are getting in about 70 percent of manganese, 50 percent of silica, 35 to 40 percent of iron, all of 3 He (helium isotope), hydrogen sulfide, hydrogen and a significant part of complex metals. The concentration of volatile components, in particular, of helium, hydrogen sulfide and methane in hydro-thermal fluids is thousands and dozens of thousands of times higher than in sea water. Incidentally, these gases are of special significance for living nature as being vital to bacterial chemosynthesis.
Lately yet another source of hydro-thermal matter in the ocean has been found. These are ultrabasic formations. In the Atlantic Ocean they are associated with the thermal springs of the Logachev and Rainbow fields recently discovered by Russian research expeditions. Here a wider range of elements is getting into ocean water from the earth's interior - for instance, cobalt and nickel as well as elements of the platinum group. A somewhat different picture is obtained in the vicinity of festoon islands of the Pacific, in areas of the off-island-arc spreading and of anterior volcanoes. Not only heat "reactors" arc present there, but also their andesite and rhyolite counterparts. Accordingly, the "menu" of elements fed into the ocean is different: enhanced concentrations of gold and silver are detected in sea
water. True, the exploration of these remote regions exhibiting a complex tectonic picture is yet on its initial leg; thus far we have managed with just one voyage into an area stretching all the way from the Tonga (Friendly) Islands to New Guinea, where we have identified and studied several new hydrothermal fields.
"BLACK SMOKERS" IMPLICATED IN GLOBAL EXCHANGE
Even though the discovery of submarine thermal springs, "black smokers" and outflows of hot fluids in the 1970s and 1980s was a sensation, few, if any, were aware of the real scope of such phenomena. Today we know: a mere 5 percent of the incoming hydrothermal material goes for the building of the wonder ore "towers"; the rest - 95 percent (gases, dissolved elements, suspensions) - dissipate in the oceanic waters and floor sediments. The effect of this colossal mass of endogenic matter has not been studied well enough. Apart from the many chemical elements, it contains huge amounts of sorbents which, among other things, eliminate harmful heavy metals.
As to gases, they dissolve under high pressure in sea water wherefrom they must certainly escape into the atmosphere. This is best seen in the example of the helium isotope 3 He which serves as an excellent marker of the process. This isotope was inherited by the plutonic strata of the earth back in the times when our planet was still being formed from cosmic particles. Thence it travels all the way to molten basalts that rise upwards to the ocean floor and break
through. That is how 3 He gets into the "black smokers". Their torches (flares) scatter it abroad far and wide, and it shows up in places beyond the reach of other marker elements. In time the substance of the flares dissipates and intermixes with water. From it the helium isotope enters the atmosphere and little by little escapes into the cosmic void, just where it came from billions of years ago.
The penetration of plutonic matter into the external spheres of the earth (the hydro-, bio-, atmo- and lithosphere) is a two-way process: basalts move into sea water and back, in a give-and-take fashion. Thus the salt composition of sea water (which is the principal part of the hydrosphere) is largely regulated by the endosphere on the ocean floor: sea water loses some elements and compounds and, simultaneously, it captures other substances and then, in due course, after a long stay in the bottom rock, these substances sink into the earth's depths in the plate subduction zones.
A major part of the elements getting into sea water from thermal springs persists in it; others are precipitated to form sedimentation rock on the floor. This is how the primary substance of the endosphere moves into the hydro- and lithosphere.
This endogenic matter has manifold ties with the biosphere; however, the pattern of these ties is yet to be studied in full. We have already mentioned the capture of hydrogen sulfide, methane, and hydrogen for bacterial chemosynthesis. But these gases are not sufficient for sustaining life in the hydrothermae: by presentday data, there should be no less than 50 different elements implicated in chemosynthesis.
Taking stock of the body of relevant information available to date - information far from complete, even fragmentary now and then - and collating it with what we know about land volcanicity, we obtain a wondrous mosaic picture of how deep mantle matter makes its way into the external spheres of the earth. The chief component of this process, the oceanic one, is now studied with a wide use of geophysics and deep-sea drilling. Furthermore, today we can look into the interaction of the exterior and interior spheres of the earth not only
at the present stage or within the confines of one particular region: we can take in a period as long as 160 million years and even more, and on the global scale at that-an undertaking without peer in the history of science.
So at the end of the 20th century geology became a global science exploring both the continents and the bedrock underneath oceanic waters. Obviously, our science is but part of a broader realm of knowledge now in the formative stage. Our aim is to get to know the global interaction between the internal and external spheres of the earth-now and in the distant past too. Hence the importance of a geosphere-biosphere program that now involves many countries. This program should be augmented by priority research areas least studied thus far; one such area is the oozing of plutonic matter into the World Ocean.
"ARCHEAN TIME PARK"
The life of the flora surrounding us is possible largely due to chlorophyll. In the process of photosynthesis plants convert solar energy, carbon dioxide and water into the original (primary) organic matter. Implicated in this process are some other elements besides. Such is the initial link in the food pyramid on dry land and in the upper layers of the ocean. However, man has encountered a different food pyramid and a different source of organic matter on the ocean floor. Here chemosynthetic bacteria are the head and source of life: utilizing the energy released in the oxidation of ammonia, hydrogen, sulfur compounds and other substances, they convert carbon dioxide into organic matter. Paleontologists, studying relics of pristine organisms, know all too well: suchlike biosystems were the origin of organic life on our planet - chlorophyll came much latter. Incidentally, it was the Russian microbiologists Sergei Vinogradsky, Corresponding Member of the St. Petersburg Academy of Sciences, who in 1887 discovered the phenomenon of chemosynthesis.
According to estimates made by our Russian scientists for eight hydrothermal fields of the Pacific and Atlantic Oceans, the average content of primary bacterial produce is at about 150 mgC/m 2 in 24 hours (variation between 20 to 300 mgC/m 2 ). The animals living there subsist on bacteria whose colonies are found on the seabed in what is known as bacterial mats; also, they feed on benthic organisms in hydrothermal torches (flares), e.g. shrimps and other planktonic organisms. Many such benthonic animals filter water as they consume bacteria. But holding the highest rank among them according to the nutrition pattern are Vestimentifera, which are giant vermiform creatures. Originally predators, they have shifted to cultivating bacteria within their own body, in specialized trophosomal cells. That's incredible, but it's a fact: Vestimentifera have red hemoglobin blood, which means that hydrogen sulfide is lethal to them. Yet chemosynthetic bacteria cannot live without it! And so these giant benthic worms, the Vestimentifera, fetch this harmful gas as "feed" for the bacteria in their trophosomes. But we
cannot tell how they do it, that's a big enigma. They do it anyway, the rest is silence...
Hydrothermae, you understand, are a very hostile medium for creatures of the solar world. But today we know: more than 500 animal species inhabit the ocean depths, and their biomass amounts to 50 kg/m 2 , i.e. it is close to maximum values for shallow seas. Chemosynthetic bacteria can endure in temperatures to 110 0 C, though a cooler medium than that is better to them.
But how could such a big group of living creatures adjust to a hard medium like that-to high temperatures and pressures and, worse, to the enhanced concentration of poisonous heavy metals, radon radioactivity, hydrogen sulfide and sulfur compound? How come? Evidently these animals have built up protective systems, the more so that many of them live under ordinary conditions as well. This capability is a tantalizing problem to biologists and now also to people researching in microbiology, biotechnology, gene engineering and in medicine. Denizens of hydrothermal regions cannot but develop malignancies, lesions in the hereditary apparatus and other maladies under like conditions. A similar picture, by the way, is obtained in the aftermath of human activities nowadays-say, the heightened concentration of heavy metals, radiation and so on down the line. That is why pharmaceutical firms abroad are all set to search for hitherto unknown enzymes and other substances protecting the organism against the deleterious action of an unfriendly environment. Hot benth-ic springs are one of the objects of this search.
But let us look and ponder: life based on bacterial chemosynthesis appeared billions of years ago, probably in the Archean; and so the need of studying it on present-day biological objects has prodded the launching of an international project, "Archean Time Park". This is not to mean that chemosynthetic bacteria and other organisms have persisted immutable since the Archean. Not at all. The heart of the matter is to get to understand the physiology and genetics of the transition of many of them from chemosynthesis to photosynthesis, and comprehend the ecology and interconnections among organisms in this newly emerging biocenosis.
Within each hydrothermal structure (or field) there may be regions of an anaerobic (oxygen-free), reducing medium and those of an aerobic (oxygen-containing), oxidizing medium; each has a characteristic community of organisms. On the whole transitions from one region to another correspond to evolution on the geological time scale - from the anaerobic, deoxidizing medium rich in sulfur via that containing small amounts of oxygen down to a sufficiently oxygen-rich medium similar to the contemporary one. Therefore the unique and varied flora of hydrothermae can be regarded as a laboratory enabling us to gain an insight into biological processes that unfolded at the early stages of the evolution of the earth.
"UPPER" AND "LOWER" WORLDS
It's intriguing to compare two worlds, or kingdoms, of organisms: those that owe their life to chlorophyll and those that live owing to chemosynthesis. The former, chlorophyll-dependent kingdom that has propagated on dry land and in the surface layers of the ocean has for millions of years been utilizing the substance of the external spheres (atmospheric gases, water of the hydrosphere, biogenic elements of the lithosphere). These processes are staggering in their scope: to sustain photosynthesis the plants would capture the entire reserve of carbon dioxide in the atmosphere within just 300 or 400 years and, in about 3 thousand years, give off as much oxygen as that contained in the oxygen envelope of the earth. The scope of water consumption is just as astonishing: to keep photosynthesis going oceanic plants (diatoms for the most part) could let the entire body of ocean water pass through their cells ("cell juice") in a matter of 5 to 6 million years. This volume is close to that obtained in the high-temperature "reactor" of the ocean floor.
And here are some other comparative data. The primary organic produce in the photosynthesis zone is estimated to be close to 70- 100 bln t/year. In the zone of chemosynthesis, however (i.e. on mid-oceanic ridges and in regions of the offisland-arc spreading), this figure is slightly lower, ca. 0.1 percent under the above value. Since the "upper" and "lower" worlds are separated by more than 3,000 m of water, most of
the organic matter is lost in the oxidizing marine medium and only a fraction (percentagewise, <1 percent) of the amount produced by thermal springs is sedimented on the seabed. This is clearly distinct also in the distribution of the benthonic biomass - from minimum amounts on the vast expanses of the pelagic zone to 50 kg/m 2 in the areas of "black smokers". So it is owing to chemosynthesis that peculiar "oases" - that is conglomerations of organisms and organic matter-appear on the bottom.
The above-cited figures point to a significant scope of the material exchange between the internal and external spheres of the earth and to the tight interdependence of this trade-off. All that is part and parcel of one single mechanism. The internal spheres were the pace-maker of this mechanism, especially in the early history of the earth: their substance was redistributed and transformed in the external spheres and thereupon, via natural circulation, came back. This means that in real time the pattern of development was both cyclic and directional. The two kingdoms in the oceans - the "upper" and the "lower" one - might serve as datum marks of such development.
The "upper" world has seen many perturbations and cataclysms time and again, with the organisms inhabiting it becoming extinct, all of them. Such disasters were precipitated by the fall of large meteorites: we know of the "iridium catastrophe" just between the Mesozoic and the Cenozoic (60 to 70 min years ago) that ushered in an "asteroid winter" believed to be the cause of the death of dinosaurs. Yet all such cataclysms notwithstanding, the "lower" kingdom persisted intact. Some part of its inhabitants must have survived since time out of mind. At any rate, the composition of microinclusions in the sulfides and sulfates of hydrothermae in the southern Urals (once under ocean water) - and the age of such inclusions is in a range of 400-450 million years - shows that in many characteristics they are similar to those detected in thermal springs on the ocean floor today. Also the flora (mollusks, Vestimentifera and other organisms) discovered in the ore deposits of the Urals is much alike to what we find on the floor of present-day oceans.
Needless to say, all this evidence should be verified by new facts, especially with respect to the endogenic plutonic matter and its types. The point is that the southern part of the World Ocean has not been studied yet in practical terms, and the body of evidentiary material on its northern part is by no means sufficient. Besides, regions of in-depth research are too wide apart. By our estimates no more than 1 percent of the planet's hydrothermal system is within the scope of scientific research.
The discovery of hydrothermal structures, "black smokers", oceanic ores and peculiar benthic biocenoses is but the tip of an iceberg. Here's what adds to the importance of the discovery: we have found the missing link in the global exchange between the two spheres of the earth, specifically, the vital region where the matter and energy of the earth's interior are released and where the exchange between the internal and external spheres of the planet is taking place. Huge amounts of plutonic heat escape there. But we are yet on the initial stretch of our journey in studying the mechanism and scale of the global exchange of matter.
Still, the full significance of the discovery told in the present article is becoming ever more apparent. At first it all looked like an exotic picture-all these ore "towers" scattered here and there on the ocean floor, and wonder animals living through chemosynthesis. But subsequently, as we came across very large structures about the same size as ore deposits on dry land, it dawned upon us: over there, on the ocean floor, was a laboratory of ore formation, an operating model of events unfolding in the dim and distant past. And this is important for upgrading the present prospecting and surveying methods.
Furthermore, not only solid matter escapes into the open, but also that in solutions and in gases. Solid substances cause the edges of lithospheric plates to accrete; the liquid phase largely determines the composition of the hydrosphere and, respectively, of the biosphere; and the gaseous phase impacts the characteristics of the atmosphere. That is to say, endogenic matter which-let us stress it again-is ejected for the most part on the bottom of the oceans-acts, directly or indirectly, on all sides of our planet's life. Here we are dealing not only with the earth's geological past: this action continues today too, and on about the same scale as hundreds of millions of years ago. Deep sea drilling data and geological findings on continents are proof positive of that.
Quite a few years will be needed to comprehend and evaluate this grandiose natural process on the ocean floor in all its implications. But one thing is clear beyond doubt: today we cannot explain many processes occurring in the exterior spheres of the earth without taking due account of the deep endogenic matter and energy fed onto the ocean floor.
Опубликовано на Порталусе 07 сентября 2018 года
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