When using pyrite to understand Earth’s ocean and atmosphere: think local, not global

The ocean floor is vast and varied, representing over 70% of the Earth’s surface. Scientists have long used information from sediments at the bottom of the ocean – layers of rock and microbial mud – to reconstruct the conditions of the oceans of the past.

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

Yet reconstructions that rely on signals from sedimentary rocks but ignore the impact of local sedimentary processes do so at their own peril, according to geoscientists including David Fike in Arts & Sciences at Washington University in St. Louis. .

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

The researchers compared pyrite in sediment collected from a borehole drilled in the plateau just off the east coast of New Zealand with sediment drilled in the same ocean basin but hundreds of miles away in the Pacific.

“We were able to get a shallow to deep sediment gradient and compare the differences between these isotopic compositions in pyrite between these sections,” said Fike, professor of earth and planetary sciences and director of environmental studies at the University of Washington.

“We demonstrate that, for this deep sea basin, you get very different signals between shallow and deep water, which is prima facie evidence that these signals are not the overall oxygen fingerprint in the water. ‘atmosphere,’ said Fike, who is also director of the International Center for Energy, Environment and Sustainability (InCEES) at the University of Washington.

Instead of pointing directly to oxygen, the same signals from pyrite could be reinterpreted as they relate 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 how pyrite was used as a proxy in a study published in PNAS in 2017 on sediments from the Mediterranean Sea. For his postdoctoral research, Pasquier worked with Professor Itay Halevy of 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 evolutionary oxidation state of the Earth.

“Strictly speaking, we are studying the coupled cycles of carbon, oxygen and sulfur, as well as controls on the oxidative state of the atmosphere,” Pasquier said.

“It’s much sexier for an article to reconstruct past changes in ocean chemistry than it is to focus on burying rocks or what happened during burial,” he said. . “But I find this part even more interesting. Because most of the microbial life – especially around the time when oxygen initially accumulated in the atmosphere – occurred in the sediment. And if our ultimate goal is to understand the oxygenation of the oceans, then we have to understand it. “

For this study, the team performed 185 analyzes of the sulfur isotopes of pyrite along the two boreholes. They determined that changes in pyrite signals from near-shore drilling were more controlled by sea-level-related changes in local sedimentation, rather than any other factor.

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

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

Fike added, “Earth is a complicated place, and we have to remember that when we try to reconstruct how it has changed in the past. There are a number of different processes that impact the types of signals that are preserved. to better understand the long-term evolution of the Earth, we need to have a more nuanced view of how to extract information from these signals. ”