After the mass extinction, how many years will it take for the lake to recover ecologically?
Our reporter Zhang Ye
If the planet is extinct, how long will it take for lakes and terrestrial ecosystems to recover?
If this question is difficult to answer, you may wish to look back to 252 million years ago.
At that time, the Earth's end of the Permian occurred the largest Phanerozoic bio-extinction event, triggering a serious crisis of marine and terrestrial ecosystems, and about 75% of terrestrial biological species disappeared.
The latest research by the scientific research team of the Institute of Geology and Palaeontology, Chinese Academy of Sciences found that after the end of the Permian extinction, it may take 10 million years for lakes and forest ecosystems to recover significantly. The relevant results were published online in Geology recently.
Ancient lakes buried under the Ordos Basin
Under the guidance of Researcher Wang Bo and Researcher Zhang Haichun, the graduate students of the Modern Terrestrial Ecosystem Origin and Early Evolution Research Group of the Institute of Geology and Palaeontology, Chinese Academy of Sciences, Dr. Zhao Xiangdong and Dr. Zhang Dachun cooperated with the engineers of Xie Guwei of Changqing Oilfield and others. The Triassic lacustrine sedimentary strata have been systematically studied.
They conducted high-precision stratigraphy, sedimentology, and paleontology studies on three oil shale and tuff-rich sections (Bawangzhuang, Mazhuang, and Yishicun) on the southern edge of the Ordos Basin. They also analyzed the volcanic ash, tuff, and coagulation in the sections. The limestone sandstone was zircon-uranium-lead dating, and finally the bottom age of the lacustrine oil shale was locked at about 242 million years, which was attributed to the Middle Triassic Tongchuan Formation. This set of oil shale is the earliest known deep lake facies deposit after the end of the Permian extinction, and it has advanced 5 million years from the previous record.
The research team found abundant plants, ostracods, leaf limbs, limulus, insects, fish and fish dung in the oil shale of the Tongchuan Formation. The largest spiral fish manure fossil is 77 mm long, indicating that there were large predatory fish in the lake at that time. By slicing some fish manure fossils, the research team discovered the large jaws of diptera insects.
Fossil studies have shown that complex multi-layer nutritional levels have appeared in the lakes at that time: the primary producers were various algae; the primary consumers were mainly algae-feeding animals such as insects and insects; secondary consumption Those include various types of aquatic carnivorous insects and fish; the top consumers are large predatory fish.
This ecological type is significantly different from the general absence of diptera larvae and scarcity of aquatic beetles in Paleozoic lakes, and represents a typical Mesozoic lake ecosystem.
Little is known about the ecological recovery model of lakes in the early and middle Triassic
In the early Triassic, the earth was still in an extreme climate, and land and lake ecosystems had to recover slowly.
Researcher Wang Bo said that the recovery time of different species is different, the fastest among them are foraminifera, ammonite and dentate stone, which will recover to the level before the mass extinction after 1 to 2 million years. It takes approximately 10 million years for land forests to complete restoration, which is much longer than the restoration time for plant communities inferred from palynological data.
Due to the lack of deep lake sedimentary strata and fossil records in the early and middle Triassic, scientists have little knowledge about the recovery model and time of the lake ecosystem.
Judging from the current geological exploration data, within the first 10 million years of the Triassic period, coal beds were generally missing in terrestrial strata around the world, so this period is also referred to as the "seam missing period".
"It is generally believed that the re-emergence of the Middle Triassic coal seam represents a clear restoration of the forest ecosystem after the mass extinction." Researcher Wang Bo said.
In the Ordos Basin, the oldest known Triassic coal seam is produced from the uppermost part of the Ermaying Formation, and its age is slightly older than that of the Tongchuan Formation oil shale. The results of the study indicate that the recovery of the complex lake ecosystem coincides with the end of the "seamless period", indicating that the lake and forest ecosystems may be closely linked through biological, physical, and chemical effects.
The researchers believe that the hot climate of the early Triassic period will limit the dissolved oxygen content in the lake, thus hindering the recovery of the ecosystem. However, a large increase in carbon burial in the Anni period ocean may lead to a decrease in atmospheric carbon dioxide content and global cooling, thereby improving the ecological conditions of the lake. In addition, volcanic ash brings rich nutrients to the lake, which may significantly increase the primary productivity of ancient lakes in the Ordos Basin. Therefore, global temperature reduction and volcanic ash nutrient input may jointly promote the prosperity of the ancient lake ecosystem in the Ordos Basin.
At the same time, the research horizon is also an important oil-producing layer of Changqing Oilfield. The results of isotopic chronology and stratigraphy also provide new dating and fossil evidence for precise stratigraphic correlation and oil and gas exploration.