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Earth now and 2.5 billion years ago: New study of air helps understanding both

August 08, 2018

Sulfur isotope has helped reveal surprising information about both the origins of life on Earth and modern sources of air pollution in China, according to a new study from an international collaboration of researchers published on Monday, August 6 in the Proceedings of the National Academy of Sciences.

"For the origin of life on Earth, perhaps one of the most important questions is how it started and evolved," said lead author Mark H. Thiemens, the Distinguished Professor of Chemistry and Biochemistry and the Chancellors Associate Chair at the University of California San Diego.

"This new work provides, in an unexpected way, data that allows us to zoom in on those mechanisms in greater detail," Thiemens added.

The team built upon a method of tracking the origin and evolution of life through changes in atmospheric oxygen using stable sulfur isotopes. In the Earth's early days, the surface was violent, with volcanic eruptions spewing heat and sulfur into atmosphere. At that time, sulfur was far more abundant than oxygen.

As the Earth's surface settled, the atmospheric composition shifted. Oxygen became more plentiful as the dust calmed and the temperatures cooled, but sulfur isotope signatures were trapped in the rocks of billions years old. From these signatures, researchers can mark each shift of the atmosphere's composition to follow biological evolution.

"This method was also utilized in analyzing ancient Mars meteorites to understand the interaction between the atmosphere and surface on the red planet," said LIN Mang, an author of the paper and a Japan Society for the Promotion of Science Research Fellow who conducts research in the School of Materials and Chemical Technology & Earth-Life Science Institute of the Tokyo Institute of Technology.

"Recently, we found that similar isotopic signatures were widely observed in present-day Earth's atmosphere--in the polluted air of China--a phenomenon that we did not expect," LIN said.

The problem, LIN added, was that no one knew the origins of such atmospheric signatures. To solve the mystery, the researchers measured all sulfur isotopes in several representative coal and air samples across China to support their interpretation of high-sensitivity measurements of sulfur isotopes in today's atmosphere.

"We discovered an additional, previously unknown sulfur isotope effect," LIN said.

In addition to the four stable and one radiogenic sulfur isotopes in sulfur samples from Earth's atmosphere and surface, the researchers also saw unusual chemical measurements in the air samples. Together, the sulfur isotope signature with the distinct coal combustion chemicals could be used to identify sources of air pollution in China.

"Air pollution has caused serious problems in public health and in the Chinese economy. The new data suggest that biomass burning may have played a significant role in aerosol formation," said paper author SHEN Yanan, a professor of biogeochemistry at the School of Earth and Space Sciences of the University of Science and Technology of China.

The researchers said that the unique signature of this sulfur isotope effect, as LIN called it, may provide a new tool for understanding where sulfate aerosols in the modern atmosphere originate and how they chemically transform to further pollute the air.

Next, the researchers plan to design and conduct experiments to further explore the chemical mechanisms of their observations. They also plan to model the sulfur isotopic effect to fully analyze how it might influence--or be influenced by--other variables to not only understand Earth's current atmosphere but to also better understand Earth's evolution.

"We hope to quantify this sulfur isotopic effect on an experimental and theoretical basis, which can be incorporated into model analysis to sharpen our interpretations of the Earth's early atmosphere and life record," LIN said.

The researchers are also looking toward the future. "We aim to fully understand how aerosols are formed and transported in the atmosphere in China and globally. We hope that our results will help in formulating policies and taking measures to mitigate polluted air and, ultimately, to take back a clear and blue sky," SHEN said.

University of Science and Technology of China

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