Sinking organic matter changes seabed records

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The remnants of microscopic plankton blooms in near-shore ocean environments slowly sink to the seabed, triggering processes that forever alter an important record of Earth’s history, according to research from geoscientists including David Fike of the ‘University of Washington at St. Louis.

Fike is the co-author of a new study published July 20 in Nature Communications.

“Our previous work has identified the role that changing sedimentation rates have on local and global controls over the geochemical signatures we use to reconstruct environmental changes,” said Fike, professor of Earth and Planetary Sciences and director of environmental studies in Arts and Sciences.

“In this study, we looked at the load of organic carbon, or how much organic matter – which drives subsequent microbial activity in the sediment – is delivered to the seabed,” Fike said. “We are able to show that this too plays a critical role in regulating the types of signals that are retained in sediments.

“We have to be aware of this when trying to extract records of past ‘global’ environmental changes,” he said.

Scientists have long used information from sediments on the ocean floor – layers of rock and microbial mud – to reconstruct conditions in the oceans of the past.

A critical challenge in understanding the evolution of the Earth’s surface is to differentiate between signals preserved in sedimentary records that reflect global processes, such as the evolution of ocean chemistry, and those that are local, representing the deposition environment and the history of sediment burial.

The new study is based on analyzes of a mineral called pyrite (FeS2) which forms in marine sediments influenced by bacterial activity. Scientists examined the concentrations of carbon, nitrogen and sulfur and the stable isotopes of glacial-interglacial sediments on the seabed along the continental margin off today’s Peru.

The varying rates of microbial metabolic activity, regulated by regional oceanographic variations in oxygen availability and the flow of sinking organic matter, appear to have resulted in the observed variability of pyrite sulfur on the Peruvian margin, the researchers found. scientists.

The study was led by Virgil Pasquier, postdoctoral researcher at the Weizmann Institute of Science in Israel, and co-authored by Itay Halevy, also of the Weizmann Institute. Pasquier previously worked with Fike at the University of Washington. Together, the collaborators raised concerns about the common use of sulfur isotopes from pyrite to reconstruct the evolving oxidative state of the Earth.

“We are looking to understand how the Earth’s surface environment has changed over time,” said Fike, who is also director of the International Center for Energy, Environment and Sustainability at the University of Washington. . “To do this, it is essential to understand the types of processes that can influence the records we use for these reconstructions.”

“In this study, we identified an important factor – the local delivery of organic carbon to the seabed – that alters the geochemical signatures preserved in the sedimentary pyrite records,” he said. “It overprints potential records of the global biogeochemical cycle with information about changes in the local environment.

“This observation offers a new window to reconstruct past local environmental conditions, which is quite exciting,” said Fike.

Reference: Pasquier V, Fike DA, Halevy I. Sedimentary isotopes of pyrite sulfur follow the local dynamics of the Peruvian minimum oxygen zone. Nat Common. 2021; 12 (1): 4403. doi: 10.1038 / s41467-021-24753-x

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