Representative sporopollen fossils from Triassic to Jurassic in Xuanhan area, Sichuan Province

Photo courtesy of respondents

About 200 million years ago, a very important evolutionary event happened on the earth. Europe-North America-South America-Africa, a unified ancient continent that was originally connected, disintegrated and the Atlantic Ocean split in the middle. In the surging sea water, hundreds of kilometers of volcanic groups emerged, erupting day and night, covering the sky with smoke and ash, and the extinction event at the end of Triassic in geological history began.

In the past, the scientific community has formed the knowledge that the extinction at the end of Triassic caused heavy damage to marine life. However, there are different opinions about the changes of the vast land ecology. Scientists have only found a few well-preserved continuous continental sedimentary strata in Europe and America, and the fragmented fossil evidence makes the extinction event at the end of Triassic doubtful. It is puzzling whether it is a regional land biological replacement or a global land ecological change.

Since 2007, Wang Yongdong, a researcher from Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, has led the research team of Mesozoic plants and environment, and conducted multidisciplinary and in-depth research on Triassic-Jurassic strata in Sichuan Basin, and made a series of new progress.

Recently, an important research achievement was published in the international geoscience academic journal "San Gu": Dr. Li Liqin and researcher Wang Yongdong from Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, cooperated with their foreign counterparts and found abundant sporopollen fossils in the Qilixia section of Xuanhan in the northeast margin of Sichuan Basin. It is these microfossils that cannot be directly seen by the naked eye, and the relevant information obtained from them is consistent with the research results of many profiles around the world. These studies reveal the mystery of paleovegetation succession and paleoclimate change at the turn of Triassic-Jurassic.

Sichuan basin was once full of vitality in the late Triassic.

Triassic is the first period of Mesozoic, from 250 million years ago to 20 1 100 million years ago. Reptiles dominated this period, and gymnosperms also rose rapidly.

Since the birth of the 4 billion-year-old earth, the plate movement has never stopped, and the form of the mainland and the ecology of the earth are constantly reconstructed in the process of separation and integration.

During the Triassic period more than 200 million years ago, the East Asian continent and the European continent were not connected, separated by the vast Tethys Ocean. After the Hercynian movement, many geosynclines were transformed into mountain systems, and the land area expanded, resulting in some inland basins in the platform area. Sichuan Basin in South China and Junggar Basin in Northwest China are both located on the eastern edge of Tethys Ocean, and there is a vast water area between North China Plate and South China Plate.

Sichuan basin and Junggar basin are typical representatives of that period, and the geographical conditions of these basins led to further changes in sedimentary facies and biology.

Pteridophyte, also known as pteridophyte, belongs to spore plants. There are about 12000 species of pteridophytes in the world, which are widely distributed all over the world, especially in tropical and subtropical regions. There are about 2600 species in China, mainly distributed in the provinces south of the Yangtze River. The earliest gymnosperms can be traced back to Devonian from 340 million years ago to 395 million years ago. They mainly reproduce by wind pollination, and a few by insects.

Most common ferns are low in appearance, while Ginkgo biloba, conifers and Cycas are perennial woody plants, mostly tall trees, which have different requirements for the environment. Among gymnosperms, conifers are mainly distributed in temperate and cold zones, Cycas belong to tropical and subtropical plants, while Ginkgo plants prefer subtropical warm and cool environments. Most ferns like warm and humid environment, and a few are drought-tolerant. Generally speaking, ferns are more tenacious and more tolerant to extreme environment.

"Our research found that rivers, shoals and peat bogs in Xuanhan, Guangyuan, Hechuan and other places in Sichuan were densely populated at that time, and the periodic fluctuation of water flow made spores and pollen of many plants fall into the water and gather together, which provided good conditions for us to study the terrestrial plant ecology at that time." Li Liqin said.

Micron-scale sporopollen fossils reveal paleoenvironmental changes

One of the five extinction events in Phanerozoic occurred at the turn of Triassic-Jurassic 200 million years ago, and the global land and sea ecosystem suffered heavy losses. Compared with the ocean, the exploration of major biological events in the terrestrial ecosystem at the turn of Triassic-Jurassic is relatively weak, especially the response to the East Asian terrestrial ecosystem in East Tethys. The important reason is that marine strata are more continuous and complete than terrestrial strata, and the preservation conditions of plant fossils are more stringent. It is sometimes difficult for scientists to restore the changes of terrestrial ecological environment at that time only by relying on large plant fossils.

When the researchers were in the field, they made a detailed investigation of Xujiahe Formation-Zhenzhuchong Formation, and took samples from the strata and brought them back to the laboratory for analysis and observation.

"Spores and pollen can be well preserved because they are very small and the outer wall is hard enough. In addition, the current washes them together and is easily buried by underwater sand. " Li Liqin explained.

These spores and pollen are smaller than 100 micron, and the size is only about 30 micron. They can only be seen clearly with a microscope of 400 times, and they can be clearly imaged and photographed with a magnification of 600 times. But it is these microfossils that have become the decoders to reveal the ancient environment.

"We noticed that although the environment in Sichuan Basin was generally warm and humid about 200 million years ago, the number and species of fern spores and Lepidoptera pollen fluctuated greatly in different strata, which indicated that they were dominant in different periods." Wang Yongdong said.

After further detailed geological investigation and fossil specimen analysis, they summed up a rule: in the Late Triassic, true ferns were dominant, with Diptera as the representative, conifers, cycads and ginkgo plants flourished, and seed ferns, lycopodes, mosses and wedge-leaved plants were also small. At the end of the late Triassic, conifers, cycads and ginkgo plants were dominant, conifers began to appear, and ferns were rare. During the transitional period from Triassic to Jurassic, the plant diversity was low, and only a few ferns were dominant. After the early Jurassic, cycads and ginkgo plants revived, conifers and cypresses were abundant in Pinaceae, and ferns were represented by Cyatheaceae/Pteristaceae.

At the end of Triassic, the global climate changed dramatically.

Wang Yongdong said: "At the end of Triassic, the European continent and the African continent on the west coast of Tethys Ocean experienced large-scale volcanic eruption. These areas are covered by volcanic ash, and a large amount of sulfur dioxide enters the sea, causing seawater acidification and hypoxia, and more than 90% of marine life is extinct. Sunlight was blocked by smoke and volcanic ash, which caused the temperature to drop rapidly. Then, due to the continuous accumulation of carbon dioxide ejected from the volcano in the atmosphere, the temperature rose rapidly in a short time. "

Although China's Sichuan Basin and Junggar Basin are located on the east coast of Tethys Ocean, volcanic ash has gradually drifted here, resulting in abrupt climate changes in these areas, further affecting the terrestrial ecosystem.

The trend of paleoclimate change found in this study, such as the peak of pteridophyte trilobites, short-term cooling at the end of late Triassic and warming in early Jurassic, is consistent with the research results of several representative profiles in the world, especially in the western margin of Tethyan Ocean, revealing the global terrestrial paleovegetation and paleoclimate succession changes at the turn of Triassic-Jurassic.

This study is the most detailed and high-resolution record of sporopollen flora in South China at the turn of Triassic-Jurassic, and provides important evidence for the terrestrial ecosystem response to major geological events at the turn of Triassic-Jurassic in the low latitude area of East Asia on the eastern margin of the Paleo-Tethys Ocean.