Between 538 million and 635 million years ago, deep seawater may have begun to oxidize on a large scale. During this period, a large number of complex multicellular organisms appeared in the ocean, which is considered to be a key turning point in the evolution of life on Earth.
However, past studies have always believed that the deep ocean was still hypoxic.
A reporter from Science and Technology Daily learned from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences in early February that the latest research results by associate researcher Wang Wei and others at the institute showed that deep seawater from 538 million to 635 million years ago may have begun to oxidize on a large scale. Published online in the internationally renowned journal "Geology" at the end of January.
Oxygen is one of the important conditions for the survival and reproduction of complex multicellular organisms.
In modern oceans, there is enough oxygen from shallow to deep waters for the survival of animals and plants. For example, in the Mariana Trench, which is 11,000 meters deep in the Western Pacific Ocean, it has been confirmed that fish and other animals have been found to survive at about 8,500 meters, but in 500 million This was not the case in the ancient ocean years ago.
It is generally believed that the embryonic form of the modern oxidizing ocean was completely formed after the plants appeared in the Phanerozoic (about 540 million years ago to the present). The previous marine environment always repeated between oxidation and hypoxia.
In recent years, scientists have found a large number of fossil biota composed of complex multicellular life in the Ediacaran strata, such as: "Lantian Biota", "Weng'an Biota", "Miaohe Biota" and "Miaohe Biota" on the Yangtze Platform in South China. "Shibantan Biota" and so on.
The life activities of complex multicellular organisms need to consume a lot of oxygen. Their appearance represents the increase of oxygen content in the ocean environment at that time, but this is contrary to the results of past research.
Associate researcher Wang Wei, Assistant Researcher Guan Chengguo, and Researcher Zhou Chuanming of Nanjing Institute of Geology and Palaeontology, and colleagues from the Institute of Geology and Geophysics, Chinese Academy of Sciences, used the in-situ microanalysis method of pyrite sulfur isotope, combined with petrology and mineralogy analysis , Revealing that the sulfate pool capacity in the Ediacaran ocean is higher than previously estimated, indicating that the deep seawater may have begun to oxidize on a large scale at that time.
The sulfur isotope method is one of the important methods to restore the ancient marine environment.
The restoration of the paleo-ocean environment during the geological historical period mostly relied on geochemical means, and sulfur isotope was one of the most commonly used geochemical indicators.
Under the condition that atmospheric oxygen content is generally low, terrestrial sulfate ions are important ocean oxidants, which play a key role in the oxidation of ancient ocean deep water areas. An adequate sulfate reservoir is an important prerequisite for ancient ocean deep water oxidation.
The sulfate concentration in the ancient ocean in the geological history period cannot be directly measured, and its concentration level is generally characterized by the degree of sulfur isotope fractionation in the ancient ocean.
Wang Wei believes that the application of traditional sulfur isotope methods in the past mostly adopted whole-rock analysis methods, lacking systematic petrological and mineralogical analysis, and did not fully consider the complexity of sedimentary sulfide (such as pyrite) formation process and its later diagenesis Sex.
Early methods may cause the extracted isotopic signals in ancient seawater to be superimposed on signals from other media (such as pore water, groundwater in the late diagenesis period), resulting in a deviation in our understanding of the redox state of ocean waters at that time.
The researchers took fresh Lantian core samples as the research object, carefully sliced and polished the rock samples, carefully observed the morphological characteristics of pyrite under an optical microscope, and found that pyrite has different morphological characteristics.
They also used scanning electron microscopy to observe the morphology of pyrite at the micrometer scale, and used SIMS mass spectrometry to test the isotopic composition of pyrite in situ micro-area.
Based on the experimental data, the research team used mathematical models to initially estimate the underestimation of the sulfate concentration in the ocean at that time, and came to the conclusion that the sulfate concentration in the Ediacaran deep water area was severely underestimated before, and the sulfuric acid in the ocean The capacity of the root reservoir may be sufficient for the oxidation of deep seawater, which indicates that the deep seawater of the ancient ocean may have started large-scale oxidation after the end of the "Snowball Earth Event", thus providing a guarantee for the development of complex multicellular life.
The results of this research overturn the previous studies on the oxidation capacity of deep waters of the Ediacaran ancient ocean from a new perspective. It is believed that the deep waters of the ancient seas 600 million years ago have the ability to undergo large-scale oxidation, which is the emergence of complex life in deep waters. It provides a favorable guarantee, and also explains at a certain level why the "Lantian Biota" appeared in the 600 million-year-old ocean deep water.
At the same time, this study points out that the whole rock sulfur isotope index has certain limitations in the reconstruction of paleoenvironment, and provides corresponding solutions.
Our reporter Zhang Ye