Where does carbon come from? Ancient plants that look like chocolate cones give the answer →

As a common fossil energy, coal produces a large amount of carbon dioxide when burned, making it a veritable high-carbon fuel. Where does the carbon in coal come from?

In May 2024, Chinese researchers discovered the most complete mineralized specimen of Lepidoptera in the excavation of paleontological fossils in China so far, as well as the largest individual Corda wood fossil in the country in the Carboniferous strata in Yangquan City, Shanxi Province. The mystery was finally solved.

Wood fossils found in Yangquan, Shanxi (Photo source: Xinhuanet)

01What are Lepidium and Corda?

Lepidoptera belongs to the tree-like lycophytes , mainly appearing in tropical swamps in the Carboniferous (about 355-295 million years ago) and Permian (about 295-250 million years ago) of the Paleozoic Era. It is one of the most representative trees in the coal-accumulating period of the Paleozoic Era. The stem of Lepidoptera is upright, up to 40 meters high, and 2-3 meters thick at the base. The upper part of the trunk is often dichotomously branched, with thin and long leaves on the branches. The base of the leaves is swollen, and after the leaves fall off, they leave spirally arranged diamond-shaped leaf seats on the surface of the stem, which resembles fish scales, hence the name. Some people also joked that it "looks like a chocolate crispy cone", which is really very vivid!

Lepidocarpus (Image source: Earth Science Dictionary)

In the late Paleozoic Era, in the warm and humid regions of the Northern Hemisphere, coastal swamps were large in scale, and tropical jungles appeared for the first time in Europe, North America, and the North China Platform of China, forming the first large-scale coal-forming period in the history of the Earth. At this time, Lepidoptera was the main plant in the swamp forests of Europe, America, and East Asia, and was the main coal-forming plant in the coastal swamp phase of the Late Carboniferous, and therefore was also the main source of raw materials for carbon-forming movement.

Corda wood is a general term for plants of the genus Corda, an important group of extinct gymnosperms. It is a woody fossil with a tall trunk, up to 50 meters high, and leaves on the stem axis in strips and tightly spiraled, which is widely distributed in the Carboniferous strata.

02The oldest forest ecosystem in North China

The Yangquan plant fossils were discovered in Shuiquangou, Shibuzui Village, the mining area of ​​the city, and Yinying Town in the suburbs, located in the Benxi Formation of the Carboniferous System of the Paleozoic Era. Geologists believe that this is the earliest fossil wood found in China in recent years, dating back to the early coal-accumulating period of the Late Carboniferous Period, 310 million years ago, and is also the oldest terrestrial forest ecosystem found in North China so far. At the same time, the largest individual Corda wood fossil in China, dating back about 295 million years, was also discovered here. It is estimated to be about 43.5 meters high at the time, correcting the erroneous understanding of European and American paleobotanists that there were no tall trees in the Cathaysian flora during the deposition of the Benxi Formation of the Late Carboniferous Period in China .

Researchers dissected the wood fossils under a microscope in the laboratory and found that the wood cell structures in the cross-section of the scaly wood fossils were densely arranged like a honeycomb, while the cross-section of the Koda wood fossils radiated from the center to the surrounding areas. At the same time, the dissection also found a considerable number of fossil fungi with exquisite structures and a large number of fossils suspected to be oribatid feces inside the trees, as well as the earliest "nursery wood" phenomenon, that is, the phenomenon of plants taking root inside another tree. Based on the fossils found, scientists speculate that the thickest part of the trunk of this Koda wood tree was about 4 times the waist circumference of an average adult man when it was alive, and the height of the entire trunk reached 43.5 meters, about the height of a 15-story building. It is the largest individual Koda wood fossil found in China so far, and trees of this height have only been found in European and American plant areas in the past.

Moreover, scientists have discovered that the wood fossils composed of the stems of large Cordae plants have no annual rings, but there are coal seams exposed above and below the strata containing the fossils. This indicates that the Yangquan area was in a humid tropical environment from the Late Carboniferous to the Early Permian, with no obvious seasonal changes. This provides important scientific basis for geologists to understand the local paleogeography and paleoclimate environment hundreds of millions of years ago.

03 Similarities and differences with Uda's "Plant Pompeii"

In fact, Yangquan City is mostly mountainous, with well-exposed geological strata, making it one of the ideal areas for fossil resource exposure and preservation. The wood fossils here are widely distributed and span a wide range of time. They have been found in 14 rock strata deposited from the late Carboniferous to the end of the Permian. Today, in the "Treasures of Jin" exhibition hall of Yangquan Museum, more than a dozen plant fossil specimens such as reed wood fossils, pyroxene wood fossils, Koda wood fossils, and scaly wood fossils are silently telling people about their past and present lives.

Not long ago, three geological heritage sites in my country were selected into the "Second Batch of 100 World Geological Heritage Sites", including the "Wuda Permian Vegetation Fossil Origin" in Inner Mongolia Autonomous Region. This place is also famous for its rich number of plant fossils. It is known as the "Plant Pompeii" because it preserves a tropical rainforest nearly 300 million years ago underground. What is the difference between the Yangquan plant fossil group and it?

Table 1: Differences between the Wuda "Plant Pompeii" and the Yangquan plant fossils (Table created by Ma Zhifei)

04 Conclusion

Where does the carbon in coal come from? This is a very old but puzzling topic. The sun, as the primary power, drives plants to absorb carbon dioxide from the air to form nutrients and store them, forming a wood layer. After the plants die, they are buried in the strata and transformed into coal under high temperature and high pressure over hundreds of millions of years. Humans burn coal to generate heat and release a large amount of carbon dioxide into the atmosphere, thus forming a carbon cycle spanning hundreds of millions of years. Among them, the sun, plants, strata, coal, humans, and the atmosphere together constitute this cycle, maintaining the carbon cycle and dynamic balance of the ecosystem.

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However, it must be noted that different plants have different photosynthesis and carbon fixation capabilities, and the carbon dioxide content in the atmosphere is different in different eras. Carbon dioxide in the atmosphere promotes the growth, development, and prosperity and evolution of plants. At the same time, the prosperity and evolution of plants will in turn promote changes in the concentration of carbon dioxide in the atmosphere, thereby promoting the evolution of regional and global climate systems. The two are mutually reinforcing and mutually reinforcing. In a certain period of time, the structure of plants and atmospheric carbon dioxide can always reach a certain balance. Therefore, through the structural morphology of plants in this period, we can infer the atmospheric concentration at that time, infer the climate system at that time, and predict future climate evolution trends. This is also the significance of our study and research on plant fossils.

Author : Ma Zhifei, a popular science writer of geology and geography, and a member of the Beijing Science and Technology Popularization Creation Association

Reviewer: Liu Hanbin, Senior Engineer, Shanxi Coal Geology Bureau

Produced by: Science Popularization China

References:

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[3] Gu Chao. Listening to the sounds of Yangquan’s ancient forests[N]. Yangquan Evening News. 2024.05.22.

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