When using pyrite to understand the Earth and the atmosphere of the earth: think local, not global

The seabed is large and varied and makes up more than 70% of the earth’s surface. Scientists have long used information from sediments at the bottom of the ocean – low rock and microbial grinding – to reconstruct the conditions in the oceans of the past.

These reconstructions are important to understand how and when oxygen became available in the Earth’s atmosphere and eventually increased to the levels that support life as we know it today.

According to geoscientists, including David Fike in the arts and sciences, at Washington University in St. Louis. Louis, conducts reconstructions that rely on signals from sedimentary rocks but ignore the impact of local sedimentary processes.

Their new study was published on February 26 Scientific progress is based on analyzes of a mineral called pyrite (FeS₂) formed in the presence of bacteria. With its chemically reduced iron (Fe) and sulfur (S), the burial of pyrite in marine sediments is one of the most important controls on oxygen levels in the Earth’s atmosphere and oceans.

The researchers compared pyrite in sediments collected in a borehole drilled in the shelf just off the east coast of New Zealand, with sediments drilled from the same basin but hundreds of miles into the Pacific Ocean.

“We were able to find a gradient from deep to deep sediments and compare the differences between the isotopic compositions in pyrite between the sections,” said Fike, professor of Earth and planetary sciences and director of environmental studies at Washington University.

“We demonstrate that for this one basin in the open ocean you get many different signals between shallow and deep water, which is prima facie to argue that these signals are not the global fingerprint of oxygen in the atmosphere,” Fike said . , who is also director of Washington University’s International Center for Energy, Environment and Sustainability (InCEES).

Instead of pointing directly to oxygen, the same signals from pyrite can be reinterpreted when related to other important factors, Fike said, such as sea level change and plate tectonics.

Fike and first author Virgil Pasquier, a postdoctoral fellow at the Weizmann Institute of Sciences in Israel, first questioned the way pyrite was used as a proxy in a study published in 2017 in PNAS and the Mediterranean sediments used. Pasquier collaborates with Professor Itay Halevy at the Weizmann Institute to understand the different controls on the isotopic composition of pyrite. Their results raise concerns about the common use of pyrite sulfur isotopes to reconstruct the evolving oxidation state of the earth.

“Strictly speaking, we are investigating the linked cycles of carbon, oxygen and sulfur, and the controls on the oxidation state of the atmosphere,” Pasquier said.

“It’s much sexier for a paper to reconstruct changes in ocean chemistry in the past than to focus on burying rocks or what happened during the funeral,” he said. ‘But I find this part even more interesting. Because most microbial lives – especially when oxygen initially accumulated in the atmosphere – occurred in sediments. And if our ultimate goal is to understand the oxygen of the oceans, we must understand it. “

For this study, the team conducted 185 sulfur isotope analyzes of pyrite along the two boreholes. They found that changes in the signals in pyrite from the nearby borehole are more controlled by changes in local sedimentation at sea level, rather than by any other factor.

In contrast, sediments in the deeper borehole were immune to changes at sea level. Instead, they recorded a signal related to the long-term reorganization of ocean currents.

“There is a threshold of water depth,” said Roger Bryant, co-author and Ph.D. graduated from Fike’s laboratory at Washington University, now a postdoctoral fellow at the University of Chicago. “Once you get below the water depth, sulfur isotopes are apparently not sensitive to things like climate and environmental conditions in the surface environment.”

Fike added: “The earth is a complex place, and we need to remember that when we try to reconstruct how it has changed in the past, there are a number of different processes that affect the types of signals that are preserved. As we try to better understand the Earth’s long-term evolution, we need to have a more nuanced view of how to extract information from the signals. ‘