How the Earth self-regulates after volcanic eruptions

■ Reporter Shen Chunlei

Recently, a series of violent eruptions have occurred in Tonga's undersea volcanoes, triggering a huge tsunami.

"Tonga's undersea volcanoes have a huge impact on the world through the release of matter and energy." Shen Jun, a researcher at China University of Geosciences (Wuhan), told China Science Daily, "About 200 million years ago, at the turn of the Triassic and Jurassic, there were also huge volcanic eruptions (the Mid-Atlantic Igneous Province), and their eruption intensity was far greater than the scale of the Tonga volcanic eruption. ”

Recently, an international team of scientific researchers found significant volcanic records in the area away from the volcanic eruptions of the igneous provinces in the Mid-Atlantic Ocean, further revealing that they have produced significant fluctuations in the amount of carbon on the Earth's surface, carbon isotopes and terrestrial chemical weathering, which have a very large impact on the global climate. The relevant research results were published in Nature-Communications, and Shen Jun is the first author and corresponding author of the paper.

Tracking is a difficult point

The intersection of the Triassic and Jurassic periods was the most significant geological abrupt change in earth's evolution, with frequent volcanic activity, climate upheavals, and large-scale extinction events.

Wang Yongdong, one of the corresponding authors of the paper and a researcher at the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (hereinafter referred to as the Nanjing Institute of Paleontology), told China Science Daily: "This mass extinction event is believed to be related to the large-scale volcanic lava eruption in the igneous province of the Mid-Atlantic Ocean during the geological history period. Understanding and revealing the triggers of this mass extinction event has always been one of the hot frontier scientific issues. We hope to find corresponding evidence of volcanic activity in East Asia. ”

Volcanic eruptions at the turn of the Triassic and Jurassic periods span nearly 600,000 years, forming large-scale magmatic deposits across North America and the adjacent United Continent (Pangu For short), and causing a series of climate and ecological upheavals and disturbances.

"Volcanic activity emits large amounts of carbon dioxide and methane gases rich in light carbon into the atmosphere, causing a negative bias of carbon isotopes on a global scale." Yu Jianxin, one of the corresponding authors of the paper and a professor at China University of Geosciences (Wuhan), introduced that the increasing amount of greenhouse gases such as carbon dioxide has triggered climate warming, ocean hypoxia and seawater acidification, and led to the continuous enhancement of continental chemical weathering in the early Jurassic.

However, the intrinsic relationship between volcanicity, carbon isotope perturbations and continental chemical weathering has not been strongly supported by evidence.

Studies have shown that a series of environmental anomalies caused by volcanic activity (such as ozone layer destruction, high temperatures, acidification, etc.) first affect terrestrial ecosystems. However, due to the sedimentary environment and preservation, tracking the recording of volcanic activity in sediments in terrestrial formations far from volcanic eruptions has been a difficult point in academic research.

Start with mercury records

Because mercury can be transported over long distances in the atmosphere, it is very conducive to the preservation of large areas of land and sea strata, so it can be used as a good indicator of tracking traces of volcanic activity. In recent geological studies, mercury has been used to broadly indicate the paleovolcan record in Earth's historical sediments.

Shen Jun et al. began to carry out relevant collaborative research in 2015. They found that at the turn of the Triassic and Jurassic periods, reports of mercury records in sediments were mainly concentrated in the marine sedimentary area of the western edge of Tethys, then known as the Pan continent, while the continental region of the eastern edge of Tethys, far from the Pan continent, was poorly studied and documented, especially with very little attention paid to the recording of mercury in continental sediments during this period.

Subsequently, the research team targeted east Asia, which was located on the eastern edge of the Tethys Ocean at that time, and selected two developing continuous Triassic and Jurassic stratigraphic profiles representing high latitudes (Haojiagou in the Junggar Basin of Xinjiang) and low-middle latitudes (Xuanhan region of the Sichuan Basin), and carried out a variety of methods of analysis and research, including organic carbon isotopes, mercury concentrations and isotopes, chemical alteration indexes and clay minerals.

Shen Jun told China Science Daily: "The purpose of our research is to explore the record of volcanic activity in the land sediments of the Triassic and Jurassic boundary and the impact on land chemical weathering. These profiles are far from the Mid-Atlantic IGN Province, and the influence of the Mid-Atlantic IGN Province on the area away from its vents can be explored during this period. ”

The results show that near the mass extinction line at the turn of the Triassic and Jurassic, both cross-sections of the Junggar Basin and the Sichuan Basin in East Asia showed significant enrichment of mercury concentrations accompanied by negative deviations of carbon isotopes, indicating that volcanic eruptions had a huge impact on mercury and the carbon cycle.

In addition, the team also found that the intensification of chemical weathering in the Junggar Basin at high latitudes occurred almost simultaneously with the peak of mercury concentrations, while in the Sichuan Basin at mid-to-low latitudes, the increase in chemical weathering intensity lagged behind the peak of mercury concentrations by about 200,000 years.

Shen Jun believes that the above findings show that the increase in chemical weathering intensity caused by volcanic activity reacts faster at high latitudes, similar to the "high-latitude amplification effect" of the Earth's surface layer today. "The LOSCAR model analysis showed that the intensification of terrestrial chemical weathering had a very important regulatory effect on atmospheric carbon dioxide at that time."

"Enhanced chemical weathering lasted about 2 million years, which is consistent with the predictions of the results of carbon cycle models that it will take time for excess carbon dioxide in the atmosphere to decrease after a carbon release event." Shen Jun explains, "This means that the earth's self-regulating greenhouse effect took about 2 million years at this turning point. ”

The Earth self-regulates

Speaking of this research result, Shen Jun revealed to China Science News: "We have carried out cross-cooperative research with the Nanjing Institute of Paleontology to carry out high-precision sampling and research on the well-developed and continuous Triassic and Jurassic stratigraphic profiles, which is very critical to obtaining scientific research data. ”

As early as 2010, Wang Yongdong's team conducted research on geological events and terrestrial ecosystem responses during the Triassic and Jurassic periods in south China. "Carbon dioxide was the main driver of climate change during the epoch of geological history," he said. At present, the data on the change of carbon dioxide concentration in the geological historical period span a long time and the accuracy is not high, which requires new data accumulation and verification, especially in the global comparative study under the high-precision chronological frame. ”

Wang Yongdong's team has been concerned about whether there are records of volcanic activity, such as volcanic ash, in the geological sediments at the turn of the Triassic and Jurassic periods in the Sichuan Basin of southern China. "This is critical to understanding the effects of mass extinction events on terrestrial ecosystems at the time and changes in carbon dioxide in the paleo-atmosphere."

In addition, significant signals of volcanic activity have been found in the eastern rim of Tethys, far from the Pancontinent, suggesting that the effects of volcanic activity at the turn of the Triassic and Jurassic may have expanded globally.

Previous studies have proved that volcanic activity is an important source of atmospheric carbon dioxide in geological history, and the atmospheric carbon dioxide concentration at the turn of the Triassic and Jurassic periods is about 10 times that of modern times. On this basis, the international scientific research team combined with the volcanic activity records and geological models of the Junggar Basin in Xinjiang, a high-latitude region, revealed that volcanic action can promote an increase in carbon dioxide concentration, resulting in an increase in the earth's temperature, which in turn causes the chemical weathering of the continent to continue to increase, the carbon dioxide concentration decreases, and the Earth system achieves self-regulation.

Source: China Science Daily