Development of minerals on organic residues. Fossils
Fossils, or biomorphoses(Russian: biomorphosis, English: biomorph, German: biomorphose) - pseudomorphoses minerals and their aggregates from organic remains of animals (zoomorphosis) or plants (phytomorphosis).
Based on what can we learn about what animals lived in prehistoric times, what they looked like and what paths the evolution of the animal world took? - This is the most interesting science, paleontology. Based on finds of mollusk shells, fish bones, parts of dinosaur skeletons and others ancient organisms, paleontologists restore not only appearance and the structure of extinct animals, but also the age of the rocks in which organic remains were buried, conditions on the planet in different geological eras, and much more. By the way, dinosaur bones exhibited in paleontological museums are no longer bones, but stones in the shape of bones, since bone collapsed and was replaced by mineral matter millions of years ago, leaving the so-called. "fossils". Fossilized bones arose as a result of the saturation of the bone remains of ancient animals with mineral substances from aqueous solutions, which gradually filled the pores and deposited certain minerals in them over a long period of fossilization (from the English “fossil” - “fossil”, “fossil”), while maintaining the external shape of the skeleton and the internal structure of the tissues. Most often, fossil remains of ancient marine animals are found, because their remains, quickly sinking into the muddy bottom, were reliably preserved from decomposition under the influence of bacteria by layers of geological sediments. Imprints of hard tissues imprinted on stone in dense sedimentary rocks are also found.
In sedimentary rocks, organic residues can either literally be replaced by mineral matter or play the role of a kind of active seed, on (around) which concentration and selective precipitation of certain minerals occurs. Thus, in the Jurassic clays of central Russia, pyrite biomorphoses, pyritized shells of mollusks, in particular ammonites, belemnite rostra, etc., are widespread. And in the underlying carbonate rocks of Carboniferous age, biomorphoses of calcite and minerals of its group on the shells of ancient mollusks and stalks of crinoids are common, as well as biomorphoses of minerals of the silica group (quartz, chalcedony, opal) or flint on single and colonial corals, bryozoans, mollusk shells, sea urchin needles, algae colonies, etc. Often there are also remains of organisms (shells, bones), different parts of which are replaced several different minerals at the same time.
Ammolite is a mother-of-pearl layer of fossil ammonite shells with iridescence in green and red tones, which is used as a rare gemstone. It is mined in the eastern foothills of the Rocky Mountains in the United States and Canada. In 1981, ammolite was officially given the status of a gemstone, after which its industrial mining began in the Bear paw deposit in the south of the Canadian province of Alberta.
Pseudofossils are false fossils. Natural formations that, having a structure or mineral composition of inorganic origin may resemble and be mistaken for fossil organic remains. For example, the phenomena of selective growth of concentric-zonal silica aggregates on the surface of a number of pseudomorphs are widespread. (Palaeontologist, be vigilant! - Internet publication about rhythmic aggregates of chalcedony on belemnite rostra, brachiopod shell valves, etc.).
With a broader interpretation of the term, many more can be conventionally classified as boimorphoses. nodules, formed around some biogenic formation, creating around itself a geochemical environment favorable for the deposition of minerals. For example, the presence of pyrite in sedimentary rocks is a sign of the presence of organic matter in them.
According to the results of research by academician. N.P. Yushkina (1966, 1968), the role of microorganisms in the formation of mineral aggregates can manifest itself even at the stage of formation of crystalline nuclei. In particular, under exogenous conditions, a microbiological method of generation of native sulfur, goethite (limonite), manganite, todorokite and some other minerals is carried out; in this case, the mineral substance either accumulates in the cell, completely mineralizing and replacing it, or is either released by the cell into the external environment in the form of tiny crystals and concretions. For example, in fields where there is modern education sulfur, thiobacteria cells secrete microscopic, but already completely crystallized sulfur crystals.
The role of microbial cells as seed particles and condensation centers during the nucleation of minerals and the formation of small mineral bodies is also great. Along with the microbiological one, the macrobiological path of mineral formation, associated with higher plants and animals (crystallization of minerals in plant tissues, the formation of shells and skeletons, mother-of-pearl and pearls, and many others).
Anoxic conditions promote the accumulation of organic matter, which participates in the microbiological reduction of sulfates according to the reaction: SO2- + 3C + 2H2O → 2CO32- + H2S. This is accompanied by a decrease in Eh, an increase in pH and the precipitation of carbonate after the water is saturated with bicarbonate and carbonate ions. As a result, in particular, on the walls of the voids that were air chambers in the body of ammonites, drusy crusts of calcite crystals are formed (see photo).
In the presence of hydrogen sulfide (H2S), it precipitates iron almost completely from solutions. Therefore, the usual companions, in particular, of coal-bearing rocks - carbonaceous shales, black clays or bauxites, are fossils in the form of pseudomorphs of organic remains and (or) nodules of sulfides such as pyrite and marcasite developing around them. Crystals of these minerals often also cover the walls of voids in large fossils and air chambers in ammonites.
A I Herzen
Often in rocks find various traces of life. In them you can find the remains of fossil mollusks, corals, sea lilies, algae and other organisms that lived in the seas, lakes and rivers. In some cases they are inconspicuous due to poor preservation, in others they look as if geological periods lasting hundreds of millions of years separate the time of their burial from the present day. And sometimes the traces of life are so veiled that the nature of the rocks could only be established after the advent of new methods research This was the case, for example, with white writing chalk, the origin of which became clear after studying it with an electron microscope.
TRACES OF LIFE
Various remains and traces of the life of ancient creatures are called fossils. In most cases, an animal or plant, after death, becomes food for other living organisms or is dried out by the sun, and wind and water, completing the destruction, carry away decaying particles. One way or another, a huge mass of dead animals and plants disappears, and organic matter dissipates. And only under favorable conditions in the bowels of the Earth does it turn into oil, peat, coal and oil shale.
And yet, visible traces of ancient organisms remain. They are found mainly in marine sediments. Rivers carry sand and silt particles into the sea, which then settle to the seabed. The remains of animals and plants are buried under them. Very slowly, over hundreds of thousands and millions of years, marine sediments accumulate. Their upper part serves as a kind of covering for the underlying sediments, impeding and then stopping the access of oxygen. This means that organic residues in such conditions do not oxidize. Under such an oxygen-impenetrable layer of sediments, the remains of animals and plants are preserved. They are saturated with minerals circulating in the sediments. solutions, mineralize and turn into fossils.
Fossils are extremely diverse. Most often, hard parts of animals are preserved - bones and teeth of vertebrates, shells of mollusks, shells of crayfish, etc. But soft tissues of organisms are also fossilized. Sometimes bacteria are even found in a fossil state. Among the pyrite ores of the deposits of Kazakhstan and the Urals, “mineralized” bacteria were found in in the form of tiny balls no larger than 50 microns in size.
Among the fossils there are many casts. For example, after the burial of a mollusk and the transformation of the surrounding sediment into a fairly dense rock, the calcareous shell may dissolve groundwater and a void appears. The cavity is filled with a mineral mass and an exact “casting” of the disappeared organism is obtained, a kind of natural sculpture.
Fossils also include prints of animals and plants, paw marks, crawling grooves, etc. Unique dinosaur tracks were discovered in 1969 in the southeast of Turkmenistan on the slope of the ridge. Kugitangtau, traces of these large ancient reptiles (Fig. 4) were traced over a distance of several kilometers. In places, in the marl - former calcareous-silt deposits of the coastal strip of the Late Jurassic (160 million years ago) sea - there are traces of 35 individuals. Most often, three-toed prints left by bipeds are found dinosaurs. The length of these traces is from 40 to 70 cm. Researchers called one area a “playground” due to the abundance of small traces. Paleontologists also discovered traces of the tails of ancient animals - peculiar triangular prints.
Perhaps one of the most amazing recent achievements in the study of the soft remains of ancient animals is the X-ray photographs of organisms that lived about 400 million years ago, taken by B. Stürmer. It is known that sulfur in the protein of the soft tissues of animals upon decomposition gives the mineral pyrite (FeS 2) Such pyritized fossils have been studied in soft x-ray radiation And imagine the joy of a scientist! In the radiographs obtained, the tentacles of cephalopods and fine details of the structure of fragile starfish and crinoids A, photographs of trilobites clearly showed details of the structure of the eyes, including previously unknown connecting fibers from the eyes to the middle of the head.
Paleontologists involved in the study of ancient organisms have long believed that the organic world of past geological eras can only be studied in strata formed since the Cambrian time (approximately 570 million years ago). Older strata were considered devoid of organic remains and were called "mute" because in those years there was no reliable way to determine their relative geological age.
But then in Precambrian metamorphic rocks in different countries discovered organic remains. What seemed unthinkable happened - the “silent” strata of metamorphic rocks “speaking”.
The “first-born” in this regard were limestone shelled buildings in the form of stone bushes, consisting of many calcite convex crusts. Because of their bright red color and whimsical pattern, they were called stromatolites, which translated from Greek means carpet stone.
Stromatolites are not skeletons of organisms or even casts of them. These are waste products of large colonies of algae, but their shape can also be used to judge the age of the surrounding rocks. The oldest algae in the form of mucus covered the rocky bottom of the ocean and deposited calcareous material on their surface. During each year, they appeared two-layer seasonal crusts (one layer in summer, the other in winter) Over hundreds and thousands of years, layered structures were formed in the form of stone bushes, cones, etc. The first stromatolites appeared a very long time ago, about 3 billion years ago, but their heyday occurred in the Riphean and Vendian periods (1650-570 million years ago).
Amazing discoveries have been made in Precambrian layers that, at first glance, contradict common sense. For example, jellyfish prints. Everyone knows that it is not so easy to remove a jellyfish from the water; the watery, gelatinous body cannot be held in the hands; it slips between the fingers. And yet traces of Precambrian jellyfish have been discovered. In order for the prints of soft-bodied jellyfish to survive to this day, absolutely exceptional conditions were needed for the burial of the organisms and subsequent transformations of sedimentary rocks.
In this regard, the Ediacara region in southern Australia is unique. In the metamorphosed sandstones, which lie much lower than the Cambrian layers, at the end of the 50s, many imprints of non-skeletal organisms were discovered. Not all of them can be identified and classified. But it was established that jellyfish and organisms similar to them lived in the Riphean Sea on modern sea feathers (alcyonarians - a detachment from the class coral polyps) At first they were classified as coelenterates, but it has now been established that these organisms belong to a completely special group of extinct animals, identified in special type petalons Some of them lived at the bottom and were attached to the ground, others moved freely. Also found were annelids (annelids) with enlarged head shields, strange bilaterally symmetrical animals resembling worms, and several species of soft-bodied animals that had never been seen before.
One should not think that the skeletonless fauna of the Ediacaran type is unique. At the end of the 70s, on the Winter coast of the White Sea, in Vendian (680-570 million years ago) clays and fine-grained sandstones, Soviet paleontologists found more than 1000 magnificent imprints of various Precambrian organisms. Among them, coelenterates (most of them), flatworms, annelids, arthropods and, possibly, echinoderms were found. At least 70 species of non-skeletal multicellular animals have been identified. This is how researchers imagine the now “lifeless” Precambrian!
Dramatic events of the distant past are also depicted on the stone. Once, American geologists published a photograph of stone tiles; this photo was republished by Komsomolskaya Pravda. On the stone you can see the imprint of a perch trying to swallow an oversized herring.
What happened to these fish? About 40 million years ago, in the territory where the unit is now located. Wyoming in the USA, the waters of a large lake splashed in which fish lived, similar to those swimming in modern rivers and lakes. And it so happened that the predatory perch pounced, as happened before, on the prey, but did not notice that it was large and... choked.
A tragic case for fish and an interesting one for us has come down to our time thanks to a successful coincidence of circumstances. The dead fish sank to the bottom and quickly became covered with silt. And the silt, under the weight of new sediments, compacted over many millions of years and turned into durable stone. The fish bones buried in it were saturated with mineral salts and left a trace of the events of the distant past on the stone slabs, which is rare in terms of clarity.
No less dramatic is the duel of dinosaurs captured in stone, which took place approximately 75 million years ago. In the Tugrikin-Shire cliff in the south of the Mongolian People's Republic, paleontologist R. Barsbold discovered two skeletons of dinosaurs in the Upper Cretaceous rocks, locked in mortal combat. Death found the predator Velociraptor and the victim of the Protoceratops in the moment when the fight reached its highest tension: the velociraptor grabbed the head and stomach of the victim with sharp hook-shaped claws. The outcome of the fight was not in doubt, but the battle was not over. Why didn’t an adult strong predator about 170 cm long defeat the prey, which was almost one and a half times smaller than it? Probably, in a fierce struggle, the opponents fell into the water, where they were sucked into the swamp or got stuck in the viscous bottom of the lake. The Tugrikinsky find is a unique, one-of-a-kind paleontological document that recreates a moment in the life of dinosaurs with extraordinary expression.
Let's give one more interesting story associated with traces of ancient life. In the 50s of our century, paleontologists found the skulls of animals that lived about 100 million years ago Special attention attracted by the skulls of lizards with round holes similar to bullet marks. Science fiction writers assumed that these animals were killed by some hunters. But, since in the Cretaceous period of geological history the development of the organic world on Earth led only to the appearance of the simplest mammals, they began to talk about aliens from other planets that flew to Earth 100 million years ago and hunted dinosaurs.
The solution turned out to be very prosaic. Experts recalled worms and borer mollusks, which can handle even such strong rocks as dense limestone. To prove this, several skulls of cows and pigs were thrown into one of the bays of the Black Sea. And the stone cutters of our day dealt with the heads of large animals no worse than their ancient relatives with the skulls of dinosaurs.
Mineral formations are unique, outwardly similar to human internal organs and even to the brain. Sometimes they were mistaken for real fossils, and then fierce disputes arose among researchers. In 1925, anatomist N. A. Grigorovich found in clay railway station Odi-tsovo near Moscow is a yellowish-brown flint, in shape and size no different from the human brain. Experts saw in it hemispheres separated by a longitudinal groove, the cerebellar vermis, the cerebellum itself and other details. Of course, on the surface of the fossilized brain there were also convolutions located exactly just like the convolutions of the human brain.
True, the Odintsovo fossil showed slight differences on the underside. But they were easily explained by carrying out a simple experiment. When real human brain put in a plaster mold and pressed lightly from above, a situation arose as if the brain was under pressure at a shallow depth underground. Then the cerebellum moved slightly and took exactly the same position as on the fossil.
In 1926, a plaster copy of the Odintsovo fossil was shown to many specialists abroad, including at the University of Berlin and the Institute for Brain Research, scientists from Leipzig, Heidelberg, Bonn, Paris, Liege and other cities. Dozens of specialists carefully studied the fossil - and only four expressed doubt that it was a fossil human brain.
It should be noted that doctors, while working on the Odintsovo fossil, completely missed such a important question, as the conditions for its presence in nature. It was impossible to understand how such a delicate substance as brain tissue turned into flint. This amazing phenomenon, if it really happened, should have been explained by geologists.
Well-known geologists, professors S. A. Yakovlev and G. F. Mirchink, having familiarized themselves with the conditions of occurrence of the Odintsovo fossil, came to the conclusion that it was found in interglacial deposits and was redeposited. This means that during the interglacial period various rocks washed out came into the river and lake valleys from the surrounding glacial deposits, these stones were captured by the glacier while moving through the territory located north of Moscow. Academician A P Pavlov had substantiated data that allowed him, at a meeting of the Consultative Meeting of the Main Science in 1926, to decisively reject the assumption of the organic origin of the Odintsovo fossil: “The bedrock sedimentary deposits along which the ice cover moved to the Moscow region belong to the Cretaceous, Jurassic and Carboniferous systems. In the deposits of the Cretaceous and Jurassic systems, flint intergrowths and silicified organic remains are not found, but they are very abundant in the limestone deposited in the Carboniferous sea. This indicates that the flint mass found at Odintsov, similar to a human brain, was formed in Carboniferous limestone, and if it is a fossilized human brain, it must have ended up in sediment deposited at the bottom of the Carboniferous sea.
But man did not exist during the Carboniferous period, and, therefore, geological data do not allow us to recognize the flint mass found in Odintsovo as a silicified human brain.”
Under favorable conditions, plants also petrify. In this regard, the stone forest discovered in one of the mines of the Vorkuta coal deposit is of exceptional interest. For several hundred meters, the coal seam is overflowing with vertical petrified stumps of large fossil trees - cordaites, horsetails and ferns. Looking at the stumps of the same height - 20-30 cm, you would think that someone cut down the forest during the Carboniferous period more than 280 million years ago.
Petrified stumps are found in a layer of coal 3-5 cm above a layer of carbonaceous clay, which was once soil. The stumps are impregnated with calcium carbonate, and the cellular structure of the wood is perfectly preserved in them.
The history of the Vorkuta stone forest is complex. The vertical position of the stumps definitely indicates that the trees were buried at the place of growth, and were not brought into an ancient peat bog. The same height of the stumps is associated with the same water level in the coastal reservoir; the upper parts of the trees that were above the water rotted, and the lower ones , protected by water from rotting, were preserved. And since the coal layers go around the stumps, we can say that the lower parts of the tree trunks petrified before they were covered by peat. This was caused by the subsidence of the area and the penetration of the sea into this area. The calcium of salt water absorbed into the stumps replaced the wood and preserved the remains of these ancient plants.
The “stone forest” in Bulgaria remains a mysterious geological monument. These are not fossilized trees well known to science. On both sides of the Varna-Sofia highway, near Dikilitash, numerous limestone vertical columns rise 6-7 m high and up to 1.5 m in diameter (Fig. 5). Many of them are hollow, they look like thick pipes. The pillars sometimes stand in groups, sometimes, as if at a parade, lined up in even rows. Vertical grooves give them a resemblance to Doric columns, and at times it may seem that you are among the ruins of an ancient city.
Near the town of Gramada, Vnda district in northwestern Bulgaria, a smaller stone forest is known, consisting of short hollow limestone pillars up to 80 cm high. The area looks like a cleared forest, of which only stumps remain
The formation of such a stone forest has not yet been explained. Of course, these are not petrified trees; there are no signs of plant origin in the stone pillars. The columns consist of limestone with the remains of fossil mollusks of the Paleogene period (65-23 million years ago). It has been suggested that the pillars represent a kind of calcareous concretions in sandstone. But then it is not clear why they are located only vertically. Professor L. Sh. Davitashvili and the Bulgarian geologist K. R. Zaharieva-Kovacheva suggest that in the place of the stone forest in the geological past there was a shallow sea with thickets of large perennial plants, most likely, huge brown algae or trees like modern mangroves. They secreted calcium carbonate, which, like a shell, enveloped the trunks. After the death of the plant and its decomposition, a calcareous shell remained in the form of a stone pillar.
Probably, the Bulgarian scientists E. Bonchev and S. Tonchev came closest to solving the origin of the Dikilitash stone forest. About 50 million years ago, three layers were deposited in the sea on this territory: the lower one - clayey-calcareous sandstone, the middle one - sand and the upper one - limestone. After the sea receded, the limestone began to be dissolved by rainwater. Filtering through the sand, these waters left calcium carbonate that cemented it. So, step by step, limestone pillars were formed and gradually sank down. Then the sand between the stone pillars was washed away, and a “stone forest” appeared on the surface.
REEFS AND LIMESTONES
Limestones must, of course, be placed in first place in terms of prevalence among organogenic rocks. They often form thick layers and strata stretching for tens of kilometers.
Limestones are familiar to many readers. Most often these are dense rocks with a crystalline structure invisible to the naked eye. These are chemogenic limestones formed during the precipitation of carbonate sediment from sea water as a result of chemical and bio chemical reactions. The color of limestones is very variable and is associated with impurities. Pure limestones are white. Organic matter and clayey material can give them a gray or even black color. The brown and reddish color is caused by iron oxides. But, whatever the color of limestones, the line they leave on a stronger stone (i.e., rock powder) is always white. A drop of any acid makes limestone boil as if bubbles are released so abundantly carbon dioxide The hardness of limestones is average, they are easily scratched with a steel knife.
Organogenic limestone always contains fossil remains of mollusks, corals, bryozoans, crinoids and other marine organisms. If the fossils are small and can only be seen with a microscope, such as the remains of many algae, then the organogenic nature of the limestones is revealed only after special studies.
Limestones usually form extended, often thick layers. But there are also non-layered limestones in the form of large tower-shaped and cone-shaped bodies. These are reef limestones, evidence of the steady subsidence of the seabed.
Let's start our conversation about reefs and reef limestones with geologically recent events. In the central part of the Pacific Ocean, several million years ago there were small islands and vast shallow areas - banks. Many of them crowned the tops of underwater volcanoes, sometimes forming underwater ridges. Since that time, coral activity began on a colossal scale. These colonial animals, leading an attached lifestyle, lived, as they do now, in warm oceans and seas, where the temperature does not drop below +20 °C throughout the year. They lived in clean water with normal salinity at depths of no more than 50-100 m.
The corals grew in the form of bizarre bushy colonies, died off, and new ones grew on them. The calcareous skeletons quickly compacted and transformed into durable limestone with coral remains in the form of round tubes and branches with radial partitions. And since the corals grew gradually, there is no layering in coral limestones; these are massive homogeneous rocks.
In the tropical zones of the Pacific, Atlantic and Indian oceans, in addition to coral islands, reefs and shoals located on the surface or at shallow depths, coral structures are found at a depth of several kilometers. How could it be that coral islands ended up at such depths if their creators could only live in shallow waters? This question has occupied the minds of scientists for more than a century and a half. The great naturalist Charles Darwin believed that coral islands were a kind of monuments built by billions of tiny builders in the place where shallows and islands sank into the sea.
Not only Charles Darwin's theory of the evolution of the organic world, but also his explanation of the emergence of coral islands caused a lively discussion. Supporters of Darwin's hypothesis had to prove that coral structures are not some kind of “caps” on the shallows, but bodies going deep under the water.
The first drilling was carried out in last years XIX century on the coral atoll of Funafuti from the Ellis Islands group in the Pacific Ocean. The well had a depth of about 300 m, but never came out of the limestone. The next well, drilled on the Borodino Islands south of Japan, was brought to 432 m. And here geologists were unable to drill the coral structure to the “bottom”.
In 1946, on the Bikini Atoll, the drill penetrated more than 780 m and again stopped in the limestone layer. But geophysical research made it completely clear - the real thickness of coral accumulations on this island is approximately 1300 m. Later, geophysical methods established that the thickness of the coral structure of Enewetak Atoll even more - about 1.5 km This means that here the ocean floor dropped by 1500 m - a very impressive amount. In past geological eras, corals flourished and were distributed almost throughout the planet. But since corals are heat-loving organisms, this means that in those times the sea were warmer than now, and the climate was milder.
Huge tracts of coral limestone remain from past geological eras blessed for corals. Mount Ai-Petri, a true decoration of the southern coast of Crimea, with a crown of stone peaks (Fig. 6) is a typical reef massif. The massive non-layered reef limestones of Ai-Petri are approached on both sides by ordinary layered
There are other remarkable fossil reefs in Crimea - in the vicinity of Sudak (Fig. 7), in the Kerch region, etc. Cape Kazantip, located on the northern coast of the Kerch Peninsula, is shaped like a huge rocky circle. Like other hills of the Kerch Peninsula, it consists of densely cemented skeletons of bryozoans - microscopic organisms that lived in colonies. Externally, the ring ridge of Kazantip is similar to an ancient atoll, and the flat bottom of the basin is similar to the dried bottom of a lagoon. However, this idea of the structure of the cape, based on its external shape, is incorrect. In reality, Cape Kazantip is ovoid in shape a fold with an inflection pointing upward, with a gentle slope of the layers on the wings.
In the core of the Kazantip fold, the most ancient rocks of this region - clays of the Sarmatian stage - were brought to the surface. The wings of the fold are composed of younger Upper Sarmatian - Lower Eotic reef limestones, clays and marls. The distribution of bryozoan reef limestones is quite complex. In the upper part of the cape they form a ring ridge. Along the outer slope of the cape, side ridges branch off from it, similar to giant tree roots, extending radially from the trunk. The space between the side ridges is occupied by clays and marls.
The reef cape arose when the seabed was raised in the Sarmatian and Maeotic centuries. Initially, there was a sandbank on the seabed in place of the cape, which soon turned into an island. Along its circumference at a shallow (20-40 m) depth, where the waves of the sea were no longer affecting, colonies of bryozoans settled, surrounding the island in the form of an underwater ring. As the island rose, some colonies of bryozoans found themselves above the water, died off and turned into limestone. And under water, in conditions favorable for life, at a depth of several tens of meters, other new colonies developed. Consequently, Cape Kazantip is a ring reef formed when the seabed slowly rises and the sandbank turns into an island.
But the organogenic origin of rocks is not always as clearly visible as in the case of coral, bryozoan and other limestones. Perhaps the most interesting example of an organogenic rock is chalk. That white writing chalk, without which not a single educational institution can do.
MYSTERIOUS WRITING CHALK
Writing chalk is a dazzling white, weakly compacted rock with an earthy fracture. It consists of tiny particles of calcium carbonate, loosely bound together, and therefore easily breaks between the fingers and writes on any surface. Chalk sticks to the tongue, which is explained by the huge number of tiny pores - their total volume reaches 45-55% of the volume of the entire rock.
Writer's chalk is a unique rock in many respects. Its exceptionally wide distribution is striking. A strip of chalk deposits can be traced across the territory of the USSR from the banks of the Emba through the Lower and Middle Volga region, Penza, Voronezh, Tambov and Kursk regions, Ukraine, Moldova, Belarus, the southern part of the Baltic states and beyond in Poland, northern France and southern Great Britain. The total length of a continuous strip of writing chalk in Europe is about 4000 km. In the outlying parts of the strip, the thickness of the Cretaceous strata varies from 10 to 100 m, in the central parts it is much greater, reaching 700 m near Kharkov. It is not surprising that such a widely developed rock gave its name to an entire period in the history of the Earth.
Another unique feature of writing chalk is its external homogeneity. Not only in samples, but also in huge outcrops along the banks of the Seversky Donets, chalk gives the impression of a completely homogeneous rock. But this external impression is deceptive. If the chalk surface, cleaned with a knife, is impregnated, for example, with transformer oil, then it is clear the complex structure of the rock is revealed. Numerous winding tubes, passages of mud-eating worms, thin layering, and some thin veins are revealed.
In the writing chalk, the remains of fossil marine animals are quite often noted: calcareous shells of the bivalve mollusks inoceramus, skeletons of the cephalopod mollusks belemnites in the form of massive pointed rods (in common parlance “devil’s fingers”), parts of the shells and needles of sea urchins, etc. But there are few and no large fossils determine the composition of chalk.
We will obtain additional information about the nature of chalk by examining thin rock plates (sections) in a polarizing microscope. At a magnification of 250 - 300 times, a fine-grained mass is visible, consisting of microscopic crystals and lumps of calcium carbonate (calcite mineral) and calcareous foraminiferal shells scattered in it. At the highest possible magnifications in a light microscope - up to 1000 times - calcareous shells of unicellular coccolithophorid algae are sometimes distinguished among calcite crystals.
What is the nature of microscopic crystals of calcite, the predominant constituent of chalk, and how they were formed. Perhaps they precipitated as a result of chemical reactions from sea water (and such a process occurs in modern shallow seas of the tropics and subtropics)? Or did the smallest particles of calcite arise from the limestone shells of sea animals, then crushed by mud eaters?
The answer to these questions is given by studying the powdery part of the chalk using an electron microscope. Even with a magnification of 7-10 thousand times, it is clearly visible that the fine-grained mass of chalk consists of the shells of coccolithophores and their fragments. Each cell of a coccolithophoride is protected by a complex shell - a coccosphere, formed by a number of calcareous shields - coccoliths After the organism dies, the coccosphere disintegrates into its component calcareous shields.
This means that chalk is an organogenic rock, almost entirely composed of ultramicroscopic shells of coccolithophores, organisms that lived in the surface layer of sea water and were transported by currents. From the disintegrated shells of coccolithophores, calcareous silt arose, abundantly populated by silt-eating worms. They passed all the silt through themselves, “plowed up” it entirely, without leaving a single particle in place, continuing the physical and chemical destruction of the calcareous shells. It is not surprising that the silt eaters completely mixed the sediment and destroyed the layering in it.
Writer's chalk is found in flat areas with a primary undisturbed horizontal bedding of layers. It was not covered by thick layers of sedimentary rocks, was not influenced by elevated temperature and pressure, and therefore was not noticeably compacted. The weak compaction of the coccolith mud is also indicated by the slight flattening of the mud-eater passages. Many of them were round in cross section, but under the pressure of the overlying strata they acquired an elliptical shape (the degree of flattening against a circle is 1.5-2). For these reasons, the writing chalk did not recrystallize and the smallest particles of calcite never “grew”; the original high porosity was “preserved” in it and the fossils with their very complex figured surface were perfectly preserved. And the slight compaction of the rock explains the weak connection between the chalk particles, its softness and earthy fracture.
Thus, writing chalk is an organogenic rock, and all its features, from the appearance of sediment to the transformation into rock, are due to the vital activity of several groups of organisms.
The exceptional distribution of writing chalk in Cretaceous deposits requires explanation. Indeed, why did not such a widespread and massive formation of this specific rock occur on Earth either before or after the Cretaceous period? The “secret” of chalk has been unraveled by geologists. During the Cretaceous period, when movements earth's crust were especially slow, and the era of the last mountain building was left far behind, the continents were leveled and low, the oceans expanded and oceanic waters transgressed onto land. In shallow epicontinental seas at depths of tens and hundreds of meters, special favorable conditions for the propagation of calcareous algae. After their death, coccolith silt began to form. The areas of its accumulation were sufficiently remote from the land, and the silt was not “diluted” with clayey material that rivers brought to the sea from the weakly destroyed low land. But closer to the shore, clay particles were already entering the coccolith silt and therefore chalk in the direction towards the land it gives way to marl and then sand.
Speaking about the origin of chalk, it is not without interest to turn to oceanological data. It turns out that the calcareous silts of modern oceans and seas are not identical to the silts of the Cretaceous period. In our time, purely coccolithic oozes are not formed; a significant number of foraminiferal shells are necessarily found in them. And, what is very important, the distribution of modern coccolith-foraminiferal oozes is insignificant. For example, such silt occupies only 2.4% of the area of the Atlantic Ocean and it is found in different conditions: not at shallow and medium depths (50-500 m), like the Cretaceous coccolith silt, but at much greater depths (1000-4500 m). As we see , the modern geological era is unfavorable for the formation of homogeneous silt, which, after petrification, would turn into writing chalk.
In paleobotany, it is customary to divide all fossil plants according to the form of preservation into two main groups (according to Meyen into three): the first, when organic matter is partially and rarely almost completely preserved (petrifications and phytoleims) and the second, when organic matter is not preserved (imprints, casts, kernels).
* further, simplified names will be used on the site - petrified wood (phytoleims (uglificir) and petrification) and plant prints (prints, casts, cores)Petrifications(English) petfaction) (more "Petrified wood") - are called remnants whose tissues are completely or partially replaced by mineral matter while maintaining the cellular structure. These are fossilized trunks with preserved wood structure, remains in coal buds (carbonate nodules in coal seams), volcanic rocks, calcareous sandstones, etc. Petrifications come in various forms mineralogical composition- ferruginized, silicified, calcified, pyritized, etc. Petrifications are studied using etched or polished grinds and replicas. To do this, three different cuts of the stem are prepared: transverse, radial and tangential (the cut runs along the chord). In these sections, the structural elements of the plant’s conducting system are studied. Thus, the main research methods in paleobotany are organographic, epidermal-cuticular, palynological, paleocarpological and paleoxylological.
Gallery of collectible petrified wood specimens:
Imprint of plants is a plant residue that has left its imprint on the rock after it has completely decayed. This is not just a mechanical imprint of a plant on a sediment that has not yet hardened, but the result of a complex physical and chemical process. The residue releases decomposition products into the surrounding mineral environment (matrix) and creates a unique geochemical environment around itself. Imprints on coarse sandstone are often covered with a thin mineral crust, revealing the smallest details. Often, impressions are contrasted with cavities left by volumetric remains, casts and stone cores. In reality, these are different expressions of the same type of preservation with the disappearance of plant matter. The imprint of the sheet is a strongly flattened cavity, and the cast (stone core) is the imprint of a certain internal surface of the residue.
Gallery of plant prints :
Phytoleims(from English bulk maceration) are the charred remains of plants that have preserved organic matter to varying degrees, and are flattened to varying degrees. They are usually classified as imprints if they do not form coal layers. Typical phytoleims include seeds and fruits, spores and pollen grains of plants preserved in rocks. Phytolemes also include clusters of plant cuticles. The main method in the study of phytoleims is maceration in oxidizing mixtures and the production of transfer preparations. During the maceration process, organic matter is removed using nitric acid and alkali. After processing, spores and pollen or cuticular films remain, which are then studied under light and electron microscopes. If a whole piece of rock is macerated, from which plant remains are extracted as a result of its destruction, then this method is called volumetric maceration.
Typically, you will only find one side of the print (positive or negative) with no traces of carbonation. Although, sometimes even leaf prints are quite three-dimensional.
Neuropteris leaves
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On the other hand, I have found quite a few lycopod imprints with a fairly thick layer of carbonized mass covering the ornamental bark of, say, lepidodendron.
Lepidodendron veltheimi (negative) with remains of carbonized mass
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Several successive layers in a sample with a lepidodendron branch
Another example of charcoal on lepidodendron bark (positive)
Thin branch carbonation
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Example of a barrel fragment with traces of carbonation
Example for Sigillaria bark. In the red rectangle you can see the outer and inner layers, between which there is a thin layer (0.5 mm) of carbonized mass.
If we talk about three-dimensional prints, then in 99% of cases from my practice they are flattened to an almost flat state (especially calamite stems, see photo) and only sometimes you can find a three-dimensional print of an almost circular cross-section of a branch or stem.
Kalamite stem on a split
Calamite stem in rock.
This is the same after separating the excess rock.
Stigmaria 3D imprint (positive)
Fragment of a trunk (presumably lycopodoid)
Carbonated organic residues are still not always present in samples; in the vast majority of cases, you find only negative or positive without traces of a carbon layer. For cases where organic matter has been completely destroyed, three-dimensional prints are usually divided into negative - mold - (essentially, these are voids formed in the layer of sediment after the disappearance of organic matter) and positive - cast - (i.e., voids of negatives filled with sediment). Sometimes you can find both at the same time in the same sample.
The simultaneous presence of both the positive and negative imprint of the lepidodendron cortex on this sample can only be explained by assuming that the initially cylindrical fragment of the branch was compressed to an almost flat state. As a result, you can see both the outer cortex (cast) and its imprint (mold) in two parallel planes.
Another example of a split where there are well-defined negative + positive
Splitting of a young lepidodendron branch
As for the “petrified wood” variety, in this case the internal anatomical structure of the plant is preserved (at the cellular level). I know of two varieties - complete petrification and partial (permineralization). Samples of petrified wood can be viewed in the galleries of many forum participants (Andreas, Ceratodus). In my galleries there are only examples of petrified wood from the Devonian (Upper Devonian - Lower Carboniferous boundary) and Permian periods.
These arguments may be incorrect in some ways. If someone corrects me, I will be very grateful.
Ecology
When we find common fossils of ancient shells on the beach, they are very easy to recognize. However, there are fossils of very ancient living creatures that are difficult to recognize even for specialists.
The problem also lies in the fact that many of them are poorly preserved or have come to us in incomplete form. It's not surprising that until better specimens are found, fossils of long-extinct creatures will often be mistaken for completely different species. We invite you to learn about these mysterious fossils that... different time took on mysterious things.
1) Ammonites
Ammonites are common in fossils, but have been misidentified for a long time. Also in Ancient Greece They believed that these were ram horns. They were named after the Egyptian god Amon, who wore such horns. In ancient China they were called horn-stones for the same reason. In Nepal, they were considered as holy relics left by the god Vishnu. The Vikings believed that ammonites were the sacred offspring of the serpent Jormungandr, who turned into stone.
In the Middle Ages in Europe they were called snake stones, were believed to be the fossilized bodies of coiled snakes that were turned into stones by Christian saints. Some enterprising traders even carved snake heads from ammonite fossils and sold them as souvenirs.
Today we know that these are just fossilized shells of squid-like creatures that lived on our planet 400 million years ago and lived until the death of the dinosaurs. More complex fossils include more than just shells. Fossil shells can be found along with protruding tentacles and misshapen heads that resemble modern nautilus mollusks.
2) Fish teeth
The fossilized remains of fish teeth have been interpreted in different ways. Some ancient fish had hard, flat molars that allowed them to crush mollusk shells. In Greece and later in Europe, these fossils were thought to be magical jewelry, they were often called toad stones, since people believed that large toads wore them as decorations on their heads. The teeth were used to make talismans; it was believed that they could cure epilepsy and poisoning.
In Japan, fossils of flat shark teeth have been identified as discarded. terrible monsters Tengu claws. In Europe, shark teeth were seen as hardened tongues of the devil.
It was only in the 17th century that the Danish anatomist Niels Stensen seriously studied these fossils and concluded that most of the "devil's tongues" found were just shark teeth. He also realized that fossils did not appear spontaneously in the earth and that they were located next to the remains of ancient animals long dead.
3) Trees
Lepidodendron- an ancient tree-like plant with bark resembling a pine cone, which has long been extinct. The leaves of this plant looked like grass stems and lepidodendron was still closer to herbs than to modern trees. Most of European coal deposits are the remains of these ancient plants. Lepidodendron fossils are very interesting. Long tree trunks were often preserved entirely in fossils; such a trunk could reach 30 meters in height and about a meter in width.
At fairgrounds in the 19th century, these fossils were often displayed as the bodies of scaly snakes and dragons. People could pay a small fee to admire the ancient "monsters" and listen to fictional tales of their dramatic fate. Various Christian saints could also appear in the stories. More complete fossils might include not only trunks, but also branches, roots, leaves and cones, which provided evidence that these were once trees and not mysterious fairy-tale creatures.
4) Foraminifera
On the Pacific coast of southern Japan you can sometimes find unusual grains of sand. Many of them are shaped like tiny stars, less than 1 millimeter in size. Local legends say that these are the remains of unfortunate children from the divine union of two stars. These “children” died because they fell to Earth or were killed by sea monsters living off the coast of the Japanese island of Okinawa. Their fragile skeletons wash up on the shore, and this is all that remains of the poor creatures.
In fact, these are the remains of various forms of earthly life, creatures similar to amoebas, which are called foraminifera. These creatures and their modern descendants are single-celled creatures that build themselves a protective shell. When they die, their needle-like shells remain, and if you look through a microscope, you can see the tiny chambers and structures in great detail.
5) Protoceratops
Dinosaurs called protoceratops were relatives of more famous Triceratops. They walked on 4 legs and were comparable in size to a large dog, although they were somewhat heavier. They definitely had a large skull with a bird's beak, in the back of which there was a bony outgrowth with holes.
Protoceratops lived in large herds, so they left behind a large number of fossils. For many people who were not yet familiar with dinosaurs, the found skulls seemed like the remains of fantastic and strange creatures. Because of their size, it was believed that Protoceratops were small lions. However distinctive feature The skulls of these animals suggested that they were lions with curved beaks, like those of eagles. The animals' feet resembled the paws of eagles with claws rather than the paws of lions. People thought the creature was a mixture of a lion and an eagle. Apparently, legends about these creatures most likely appeared after people found fossils of Protoceratops.
6) Belemnites
Belemnites are extinct ancient animals that resemble modern squids. Unlike squids, belemnites had 10 “arms” of equal length, which were covered with tiny hooks, and, remarkably, these sea creatures had a skeleton. Belemnites lived during the age of dinosaurs and are well preserved in fossils.
The most commonly found fossilized remains of their skeletons are cylindrical objects with a tapered end without any structures such as tentacles. These fossilized skeletons are shaped like a bullet.
In Europe, they were believed to be "thunderbolts" - objects that fell to earth from the heavens, producing the sound of thunder when they struck the surface of the earth. They were associated with various thunder gods. Many people kept them in different parts of their homes in order to divert lightning. Others believed that belemnites were associated with elves, not gods. They believed that these were the fingers of elves. People used them in various superstitious medicinal practices, such as to treat snake bites or relieve headaches. They applied the fossils to the affected area of the body and cast various spells.
7) Ankisaurs
Ankysaurs were one of the groups of early dinosaurs. These herbivores had long necks and tails and were relatives of the more familiar ones brontosaurus And diplodocus. Ankysaurs were smaller in size than their later ancestors and grew no more than 2 meters in length. They evolved from bipedal ancestors and did not stand entirely on 4 legs, although their front legs were well adapted for locomotion. They reared up on their hind legs when needed and used their front paws to grasp things.
Ankysaurs have attracted particular interest because they were initially misidentified. They were confused with the creature that would seem to be the least like a dinosaur: a human. Strangely, the long neck and tail, lizard-like body, reptile-like skull, and other features were simply ignored! Just the fact that the creature was the size of a man helped make everyone believe that these were the remains of our ancestor.
After other fossils of these creatures were found over the course of several decades, the name "dinosaur" was coined and people recognized that these fossils were not of humans at all, but of reptiles. The fact that you can confuse a lizard with a person shows how people can be mistaken.
8) Mastodons and mammoths
Just a few thousand years ago, mastodons and mammoths roamed the icy land. They looked like elephants, but had warm fur and tusks several meters long. Mass species extinction, climate change and hunting have led to their extinction. Like modern elephants, these animals had very strong muscles in their trunks that were stronger than other muscles in their body.
The trunk of mammoths and mastodons required that there be a hole in the middle of the animal's skull. Modern elephants have the same feature. People who live in areas where elephants live have seen animal skulls more than once, so they know this feature. Others who found skulls of ancient relatives of elephants with giant holes in the middle imagined this creature as a huge humanoid giant with one eye socket. The legend of the Cyclops seems to have its roots in a time when people found skulls of ancient animals outside of Africa.
9) Sea urchins
Sea urchins are spiny, round-shaped creatures whose fossils are commonly found off the coast. They belong to a group of animals called echinoderms. These creatures have lived on our planet for hundreds of millions of years, and their distant ancestors left behind a lot of fossils. Although ancient sea urchins have much in common with modern types, their fossils have long been mistaken for completely different creatures.
In England, they were believed to be supernatural crowns, loaves of sacred bread, or magical snake eggs. In Denmark, they were believed to be “thunderstorm” stones: it was believed that they began to release moisture before storms, which helped people predict inclement weather.
The five lines found on many sea urchin fossils were considered a good omen and were kept as a good luck charm in India. Magical powers associated with sea urchins, reflected how each culture interpreted them. They were believed to be able to cure snake bites, help prepare bread, protect against storms, and bring good luck.
10) Hominids
Many of man's relatives, the apes, left behind fossils. These fossils were often misinterpreted before people began to think about human evolution. Fossils that were found in Europe and America sometimes “proved” the existence of various mythical characters mentioned in the same Bible, such as giants or demons. Others said that these were the ancestors of apes, although modern apes have very different features.
Some are sure that these skeletons belong to aliens, and not fairy-tale monsters. Apparently, fossils found in Asia inspired people to create legends about the Yeti. Some believe that some hominids could have coexisted with humans, so the creators of the legends were inspired not by their fossils, but by these living creatures themselves.