MOSCOW, May 29, Vladislav Strekopytov. The hypothesis that Lake Cheko in Eastern Siberia is a crater formed by the fall of the Tunguska meteorite has not been confirmed. In the same area, two more similar reservoirs were found, which arose long before the famous catastrophe. Tunguska in Eastern Siberia flew by a huge fireball. Then there was an explosion, the shock wave from which was recorded by seismic stations around the world.
It was immediately assumed that it was a large meteorite. However, the expedition that arrived at the crash site did not find anything resembling an impact crater. The forest was tumbled down like a fan, and at the epicenter some trees remained standing — without branches and bark.
Another hypothesis was put forward: a cosmic body exploded in the atmosphere at an altitude of about ten kilometers. Most likely, a stone asteroid that crumbled under the pressure of gases in microcracks. An icy comet would hardly have reached the earth, and fragments characteristic of metallic meteorites were not found.
Lake Cheko
In 1999, scientists from Italy came to Tunguska. They drew attention to the lake eight kilometers from the epicenter.
“When a celestial body falls, traces should remain,” says Enrico Bonatti, a well-known Italian geologist, a foreign member of the Russian Academy of Sciences. “Together with colleagues from the University of Bologna, we explored the bottom of Lake Cheko, whose morphology differs from other Siberian water bodies. It cannot be explained by ordinary erosion processes.”
The Italians conducted hydroacoustic, radar and hydrological studies, built a bathymetric map of the bottom, performed a geochemical analysis of lake sediments and developed their stratigraphic model. Also, using the annual rings method, we determined the age of the trees surrounding the lake.
Acoustic sounding showed that at the bottom of the lake there is a funnel-shaped shape resembling a crater. The depth reaches 50 meters, which is unusual for local water bodies, mainly swamps and swamps. Another interesting point is that the seismic profile shows that the signs of deformation in the underlying rocks extend 100 meters down and follow the contours of the bottom.
After analyzing the magnetic and seismic anomalies, the researchers found that about ten meters below the bottom in the center of the lake is a large object. Possibly a fragment of a stone asteroid.
A model of an explosion in the atmosphere of a cosmic body with a diameter of 60 meters that is heterogeneous in composition and density was built. A small part of it — six to ten meters — hit the ground, and Lake Cheko subsequently formed there.
Scientists believe that explosions of cosmic bodies in the atmosphere in geological history occurred much more often than impact (that is, shock) events. Stone meteorites and asteroids ranging in size from several tens to 150 meters, as a rule, are completely destroyed when they enter the dense layers of the Earth's air envelope, and their fragments evaporate. Often this happens very quickly, resulting in a low-altitude explosion accompanied by a shock wave.
So it was with the Chelyabinsk meteorite in 2013. It is estimated to have been 20 meters across. In the dense layers of the atmosphere, it collapsed, which was accompanied by a very bright glow, a condensation trail in the sky, a shock wave and a loud sound. =»photo» data-crop-ratio=»0.963545981772991″ data-crop-width=»600″ data-crop-height=»578″ data-source-sid=»» class=»lazyload» lazy=»1″/
Age determination
The meteorite hypothesis of the Tunguska event would be confirmed if it was possible to prove that the lake arose precisely in 1908. However, the study of precipitation, pollen, peat, tree rings, radiocarbon analysis gave conflicting results.
Lead and cesium isotope dating of lake sediments made it possible to establish that the sedimentation rate is about one centimeter per year, and the lake sequence itself is relatively young, it is really about 100 years old. But dendrochronological analysis shows that some of the trees on the shore grew before 1908. It is unlikely that they would have survived the blow, after which a funnel with a diameter of 300 meters remained. On the other hand, Italian scientists have recorded a bend in the trunks of many of them, which formed just at that time.
In 2016, Russian researchers took two cores from the bottom of Lake Cheko and estimated their age by the activity of cesium, lead and radium isotopes. The results did not match those of the Italians. The sedimentation rate, determined by three independent methods, turned out to be 3.2-4.5 millimeters per year, and the age of the sedimentary rocks was from 280 to 390 years.
Evidence
Having studied the deposits of another lake in the same region — Zapovednoye, Siberian geologists discovered a layer of silt from 1908-1910. It is lighter and has a higher content of potassium, titanium, rubidium, yttrium, and zirconium, which are indicators of the entry of a substance of terrigenous origin into the lake. It also contains traces of combustion products and fine dust material. The age of the deposits, estimated by radioisotope methods, corresponds to the time of the Tunguska event.Researchers believe that the formation of an anomalous layer is associated with increased washout processes due to disturbance of the soil cover, massive forest fall and subsequent fires resulting from the disaster.
Sanctuary Lake is located 40 kilometers from the epicenter, but the Upper Lakura River flows through it, collecting runoff in the area of the Tunguska event.
«It is deep, and the silt deposits in it do not mix, but settle and store the history of climate change and catastrophic events,» the press release from the Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences quotes one of the authors of the study, Deputy Director for Research at the Tungussky Reserve, Arthur Meidus .
Scientists are planning to purposefully search for particles of extraterrestrial origin in the dated layers of bottom sediments of lakes in the region, using modern methods, including X-ray fluorescence analysis with synchrotron radiation. This will allow you to determine the content of the elements with an accuracy of millionths of a percent.
Twin lakes
Recently, Siberian researchers have studied the shape of the bottom and the thickness of the bottom sediments of Zapovednoye Lake and its neighboring Lake Peyungda using echolocation and seismoacoustic profiling. It turned out that in morphology they are almost identical to Cheko, the depth is 60 and 34 meters, respectively, and the age of the bottom sediments exceeds several thousand years.
«The similarity of the shape of the three reservoirs and their location <…> hypotheses about the impact origin of Lake Cheko,» the authors of the article write.
Most likely, the lakes were formed as a result of thermokarst processes — due to the collapse of the underlying rocks.
«In addition, all three lakes are flowing, they in riverbeds,” adds the first author of the study, leading researcher at the Institute of Biophysics of the FRC KSC SB RAS, professor at the Siberian Federal University, Doctor of Biology Denis Rogozin, “If these are craters, then meteorites are too accurate. These rivers do not flow along a flat plain, but in fairly deep valleys. The channel after the hypothetical impact could not change. This indicates the terrestrial origin of the reservoirs. There are several options, but geologists should check them. hypothesis.» In their opinion, a whole range of studies is required to fully understand the nature of the Tunguska phenomenon. First of all, it is necessary to date and study in detail the bottom sediments of the region's lakes to see if they contain geochemical anomalies.