Attachment: Science published the cooperation papers of experts from South China Institute.
On February 24th, the online edition of Science magazine published a group of isotopic age and systematic carbon isotope data measured by a joint research team of China and the United States based on rock samples from the Three Gorges area in Yichang, and put forward new explanations for scientific issues such as global ice age, abnormal changes in chemical composition of seawater, origin and evolution of multicellular animals before the Cambrian explosion. These scientists believe that the largest ice age in the history of the earth ended almost at the same time on all continents at a very fast speed, that is, 635 million years ago, 580 million years ago, a little ice age occurred, and then primitive animals appeared on the earth. The emergence and prosperity of multicellular organisms, especially animals, is the direct cause of the large-scale abnormal events of seawater composition in the world 550 million years ago. The accelerated prosperity of living things led to a sharp increase in oxygen content in the atmosphere, which laid the groundwork for the later CAMBRIAN life explosion.
It is reported that this research group is composed of experts from Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences and Massachusetts Institute of Technology. Dr. Zhu, the second author of the paper, pointed out that exploring the relationship between life and environmental evolution is one of the most basic scientific problems faced by human beings for knowledge and survival. The evolution of environment and life in geological period from 750 million to 530 million years ago is a hot scientific topic concerned by earth and life scientists. Because this period is a very critical turning point in the history of the earth, the scientific community calls it the transition period between cryptozoic and Phanerozoic, which means that the earth is a key stage in the transition from a barren state without visible life to a modern blue planet with suitable environment and flourishing life. During this period, complex multicellular organisms and animals germinated, followed by a rapid increase in animal diversity, which is often called the "Cambrian explosion". Before animals appeared, the earth's environment was extremely cold, and there were at least three major ice ages. One of the largest glaciers actually extends to the equatorial region, which is an unprecedented extreme climate anomaly on earth. At the same time, geological records show that during this period, the chemical composition of global seawater has undergone abnormal changes with high frequency and large scale. There may be an inevitable internal relationship between staged large-scale ice age and environmental geological events such as the appearance of multicellular organisms and abnormal changes in seawater chemical conditions, which is the "snowball hypothesis" widely concerned by the scientific community in recent years. However, there are great disputes in the scientific community about the occurrence process of environmental geological events such as the number and duration of the ice age, the origin model and exact age of animals, the changes of seawater composition and atmospheric oxygen during this period. The reason is that at present, scientific research results come from different rock formations all over the world, and the sequence of various events that have been discovered in geological time coordinates is not clear. Therefore, reliable and accurate isotopic dating is the most basic scientific basis for finding the correct answer to the relationship between environment and life evolution during the geological period from 750 million to 530 million years ago.
Geological age can be calculated by measuring the contents of parent elements and daughter elements of radioactive isotopes in minerals, because the decay half cycle of radioactive isotopes is fixed. At present, the scientific community can adopt different isotopic dating methods according to different research purposes and research objects. Volcanic ash produced by volcanic eruption in geological period contains a very stable mineral "Zircon" (ZrSiO4), which is the best material for accurately dating geological age. Researchers can calculate the age of zircon formation, that is, the age of volcanic ash, by measuring the contents of uranium isotope 238U and lead isotope 206Pb (the half cycle of 238U decay to 206Pb is 4.5 billion years). MIT's Thermoion Mass Spectrometry Laboratory has always been an internationally recognized laboratory for this age research.
The change of carbon in the process of natural material circulation affects the change of the earth's environment, because carbon is one of the two components of greenhouse gas carbon dioxide, and the change of carbon dioxide content in the atmosphere is the key to the earth's climate change. Studying the cycle law of carbon in nature is an important method to reveal the changes of environmental factors on the earth. As we all know, carbon is one of the most basic elements of living matter, and life activity is the most important factor affecting the circulation of carbon in nature. Because the content of "light carbon" (12C isotope) in living organic matter is higher than that of "heavy carbon" (13C isotope), scientists can understand the relationship between life activities and carbon cycle in geological period by studying the change of carbon isotope composition ratio in rocks, thus revealing the changing process of atmospheric and marine environmental factors.
So, why choose South China as the research area? Because South China has a complete geological record and typical glacial strata in this geological period, geologists call it Nantuo moraine conglomerate, and there are various strata rich in fossils. At present, Weng 'an biota containing the earliest animal fossils, Chengjiang biota representing the CAMBRIAN explosion, He Miao biota rich in large multicellular algae, Gaojiashan biota containing large animal remains and weak skeletal animal fossils and small shell biota have been discovered. Therefore, South China is one of the key areas of life evolution and environmental changes in the world during this period. However, there is no basis for either the glacial strata in South China or the age of these fossil groups including the earliest animal fossils, and the time sequence of environmental change events (including carbon isotope changes) with the glacial period and biota is also uncertain. Therefore, determining its geological age has become the key to distinguish the evolution process of ice age, biology and environment.
According to Dr. Zhu, not all zircon in volcanic ash samples can be used to determine the age. Some zircons come from older strata, and their age cannot represent the time of volcanic eruption. Fortunately, the research team finally found a large number of contemporaneous zircons in volcanic ash of three layers on two geological profiles in Yichang Three Gorges area. Zircon crystals used for dating are generally only about 80 ~ 150 micron in length. The results of thermal ion mass spectrometry analysis obtained high-precision age. First of all, the volcanic ash in the middle of carbonate rocks covering Nantuo moraine complex is 635.2 million years old. This age is very close to the age found at the top of the moraine complex in northern Namibia, South Africa (635.5 million years). At that time, South China was in the low latitude, and Namibia was in the northern and southern hemispheres, far apart. However, the comparison between the two eras shows that the Nantuo Ice Age in South China is simultaneous on all continents in the world, which proves that the Nantuo Ice Age is the largest ice age in the history of the earth. 635 million years ago, this great ice age came to a rapid end on a global scale. The carbonate rocks covered by Nantuo Glacier Complex all over the world (usually called "cap carbonate rocks") are rapidly formed in the ocean under the greenhouse effect environment caused by the sharp increase of CO2 concentration in the atmosphere, which is the direct reason for the rapid end of the ice age.
Secondly, the new study of rock isotopes reveals that a huge negative carbon isotope event occurred in the He Miao biota, and the age value of a layer of volcanic ash above the He Miao biota of 55 1 100 million years completely changed people's previous understanding of the biological and environmental changes at this stage. In the past, it was thought that the negative carbon isotope anomaly of He Miao biota was related to another ice age after Nantuo Ice Age 580 million years ago. The new era shows that this carbon isotope negative anomaly is a global event, which is the same as similar carbon isotope negative anomalies in other continents (North America, Australia and Africa), and has nothing to do with the global ice age, but is caused by the appearance and prosperity of multicellular organisms, especially animals, 5.565438 billion years ago. With the rise and prosperity of multicellular organisms, a large amount of organic carbon is stored. The accelerated prosperity of organisms leads to a sharp increase in oxygen content in the atmosphere, and a large amount of stored organic carbon is oxidized into the atmosphere and seawater to participate in the new carbon cycle. The rapid increase of light carbon (12C isotope) in seawater led to a huge negative carbon isotope event in global seawater around 555 million years. At the same time, the prosperity of living things brought abundant atmospheric oxygen, which laid the foundation for the subsequent Cambrian life explosion.
Surprisingly, the research results reached a very important scientific conclusion: the earliest animals appeared on the earth not earlier than 580 million years. Because the "Weng 'an Biota" containing the earliest animal fossils was found in the rock strata between 635 million and 55.65438 billion years ago, the strata under the "Weng 'an Biota" have obvious "karst karst" surface, which represents a large sea level decline event, presumably caused by another ice age after the Nantuo Ice Age. This ice age was an ice age with typical geological records in Newfoundland, Canada and Europe 580 million years ago. This conclusion not only obtains the comprehensive stratigraphic age correlation results made by other methods, but also wants to match. If this conclusion is reliable, the earliest animals on the earth appeared after the end of an ice age after the Nantuo Great Ice Age, indicating that the "Weng 'an Biota" representing the earliest animal fossil record in South China was almost at the same time as the rich "Ediacara Biota" in other parts of the world at that time, which changed the understanding that "Weng 'an Biota" was earlier than "Ediacara Biota". In this way, the patterns and processes of animal origin and early evolution need to be re-recognized in new time coordinates.
Dr. Zhu said that the higher the precision and density of geological dating, the higher the understanding of the relationship between different geological environments and biological evolution events. This time, the ages they announced only confirmed a series of important environmental and biological evolution events that occurred in key geological periods between 750 million and 530 million years ago. There are still many important events in this period that need to be revealed by different research methods, and the age and time of the events need to be determined successively. For example, the duration and interval of each of the three ice ages in this stage have not yet been determined, and the determination of the time when animals first appeared is obtained by comparing with the ages in other regions. At the same time, the new era will urge geologists to put forward new plans for the time division of this critical turning stage in the history of the earth, so the related research will continue.
Chinese Academy of Sciences