Micron pollen fossils tell you what happened in the Sichuan Basin 200 million years ago

Representative pollen fossils at the turn of the Triassic-Jurassic period in the Xuanhan region of Sichuan

Courtesy of respondents

About 200 million years ago, a very important evolutionary event occurred on Earth. The originally connected Europe-North America-South America-Africa united paleonttomy disintegrated, and the Atlantic Ocean cracked in the middle. In the midst of the gushing sea, hundreds of kilometers of volcanic groups emerge, they erupt day and night, thick smoke and volcanic ash cover the sky, and the end-Triassic mass extinction event in geological history began.

In the past, there was a scientific consensus on the devastation of marine life caused by the mass extinction at the end of the Triassic. However, there are many opinions about the changes in the ecology of the vast land, and there is no consensus. Scientists have found only a few well-preserved continuous continental sedimentary strata in Europe and the Americas, and fragmented fossil evidence has made the end-Triassic mass extinction event suspicious, whether it is a regional land biological replacement or a global terrestrial ecological change, which is confusing.

Since 2007, Wang Yongdong, a researcher at the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences, has led a mesozoic plant and environment research team to carry out multidisciplinary in-depth research in the Triassic-Jurassic strata of the Sichuan Basin, and has made a series of new progress.

Recently, the international geological academic journal "Sangu" published an important research result: Dr. Li Liqin and Wang Yongdong, researchers of the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences, cooperated with foreign counterparts to find rich spore pollen fossils in the section of Xuanhan Qili Gorge on the northeast edge of the Sichuan Basin. It is precisely these microchemical stones that cannot be directly seen with the naked eye that obtain relevant information consistent with the results of multiple cross-sectional studies around the world, which are unraveling the mystery of paleograve succession and paleoclimate change at the turn of the Triassic-Jurassic.

The Sichuan Basin of the Late Triassic Period was once full of life

The Triassic period, dating from 250 million to 201 million years ago, was the first epoch of the Mesozoic Era, with reptiles dominating the period and gymnosperms rapidly emerging.

The 4 billion-year-old earth, since the day it was born, the plate movement has not stopped, dividing and merging, so that the form of the continent and the ecology of the earth are constantly reconstructed.

In the Triassic Period more than 200 million years ago, the East Asian continent was not connected to the European continent, and they were separated by the vast Tethys Ocean. After the Haixi Movement, many troughs were converted into mountain systems, the land area expanded, and some inland basins were produced in the platform area. The Sichuan Basin in South China and the Junggar Basin in Xinjiang in northwest China are both located on the eastern edge of the Tethys Ocean, and there is also a vast body of water between the North China Plate and the South China Plate.

The Sichuan Basin and the Junggar Basin are typical examples of that period, and the geographical conditions of these basins led to further changes in the sedimentary facies and biological communities.

"Although the latitudes of these two basins are different, they both have a humid climate and fertile soil, where the grass is luxuriant, and tall gymnosperms and low ferns have bred a huge family here, such as new reeds, double-fan ferns, cycads, cycads, cycads, etc., as well as some moss and stone pine plants also appear in the forest." Li Liqin told the Science and Technology Daily reporter that after hundreds of millions of years of geological movement, these plants were buried hundreds of meters or even thousands of meters underground, and eventually formed coal.

Ferns, also known as sheep tooth plants, belong to spore plants. There are about 12,000 species of ferns 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 and regions south of the Yangtze River. The earliest gymnosperms date back to the Devonian Period, between 340 million and 395 million years ago, and they mainly rely on wind pollination to breed offspring, with a few insect pollinations.

Common ferns are mostly low in appearance, while ginkgo biloba, conifer, cycads and other ferns are perennial woody plants, mostly tall trees, and the environmental requirements of the two are different. The conifers in gymnosperms are mainly distributed in temperate and cold zones, cycads belong to tropical and subtropical plants, and ginkgo biloba plants prefer subtropical and cool environments. Most ferns prefer warm, moist environments, while a few are drought tolerant. In general, ferns are more tenacious and more tolerant of extreme environments.

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

Micron-sized pollen fossils reveal paleoenvironmental changes

At the turn of the Triassic-Jurassic period 200 million years ago, one of the five major extinction events of the Ezoogene occurred, and the global sea and land ecosystems were severely damaged. Compared with the ocean, the exploration of major biological events at the turn of the Triassic-Jurassic of terrestrial ecosystems is relatively weak, especially for the response of terrestrial ecosystems in East Asia in the Eastern Tethys Ocean. The important reason is that the marine strata are more continuous and complete than the continental strata, and the storage conditions of plant fossils are more demanding, and it is sometimes difficult for scientists to restore the changes in the terrestrial ecological environment at that time only through large plant fossils.

During the field expedition, the researchers conducted a detailed investigation of the Formation of the Xujiahe Formation and the Pearl Chong Formation, and took samples of the rocks in the formation and brought them back to the laboratory for analysis and observation.

"The spores and pollen are well preserved because they are very small and the outer walls are hard enough, and the water rushes them together, which is easily buried and preserved by the sand under the water." Li Liqin explained.

These spores and pollen are less than 100 microns large and only about 30 microns, and must be seen clearly with the help of a microscope of 400 times, and can be clearly imaged after 600 times magnification. But it is these micro-fossils that have become decoders for revealing the ancient environment.

"We noticed that although the environment in the Sichuan Basin was generally warm and humid about 200 million years ago, the number and type of pollen of fern spores and ferns fluctuated significantly in different strata, indicating that the two had a dominant position at different times." Wang Yongdong said.

After further detailed geological investigation and fossil specimen analysis, they concluded a law: in the Late Triassic, euthanasian ferns were absolutely dominant and represented by the double fan fern family, conifers and cycads, ginkgo biloba plants flourished, and a small number of seed ferns, stone pines, mosses and wedge leaf plants. At the end of the Late Triassic, conifers and cycads and ginkgo biloba plants predominated, and the palm scales of conifers began to appear, and there were fewer ferns. By the Triassic-Jurassic transition, plant diversity was low, with only a few ferns predominating. After entering the Early Jurassic Period, cycads and ginkgo biloba resurged, and the coniferous palm scale fir family was abundant, and the ferns were represented by the quercus/mussel shell fern family.

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

"In the past, many people believed that at the end of the Triassic Period, the Earth began to have a greenhouse environment, which led to the extinction of a large number of animals and plants. Our study more accurately analyzed climate change in the 6 million years before the mass extinction and found that the region experienced curve fluctuations of warm humidity (short-term cooling)-dry cooling-temperature recovery. Wang Yongdong told reporters.

Wang Yongdong introduced: "At the end of the Triassic Period, there were large areas of volcanic concentrated eruptions on the European continent and the African continent on the west coast of the Tethys Ocean, these areas were shrouded in volcanic ash that covered the sky, a large amount of sulfur dioxide entered the sea, causing seawater acidification and hypoxia, more than 90% of marine life became extinct, the sun was blocked by soot and volcanic ash, resulting in a rapid drop in temperature, and then due to the continuous accumulation of carbon dioxide emitted by volcanoes in the atmosphere, the temperature rose rapidly in a short period of time. ”

Although China's Sichuan Basin and Junggar Basin are on the east coast of the Tethys Ocean, volcanic ash has gradually drifted here, resulting in sudden climate changes in these areas, which in turn affected terrestrial ecosystems.

The paleoclimate changes of the triassic-Jurassic transition period with significant peaks of fern trispora spores, short-term cooling at the end of the Late Triassic period, and early Jurassic warming coincided with the results of multiple representative profiles around the world, especially in the western margin of the Tethys Ocean, revealing global terrestrial paleoclimate and paleoclimate successional changes at the intersection of triassic and Jurassic.

This study is the most informative high-resolution record of pollen flora at the intersection of triassic-Jurassic in South China, and provides important evidence from the low latitudes of East Asia on the eastern margin of the Guttes Ocean for the response of terrestrial ecosystems to major geological events at the turn of the Triassic-Jurassic.

(Reporter Zhang Ye)