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June 11, 2019

Microbes may act as gatekeepers of Earth’s deep carbon

Two years ago, a team of scientists, including Michigan State University geomicrobiologist Matt Schrenk, visited Costa Rica’s subduction zone, where the ocean floor sinks beneath the continent and volcanoes tower above the surface. They wanted to find out if microbes can affect the cycle of carbon moving from the Earth’s surface into the deep interior. 

During the 12-day expedition, the 25-person group of multi-disciplinary scientists collected water samples from thermal springs throughout Costa Rica. Scientists have long predicted that these thermal waters spit out ancient carbon molecules, subducted millions of years before. By comparing the relative amounts of two different kinds of carbon – called isotopes – the scientists showed that the predictions were true and that previously unrecognized processes were at work in the crust above the subduction zone, acting to trap large amounts of carbon. 

The new study was recently published in Nature.

This groundbreaking research shows that microbes consume and—crucially—help trap some of the sinking carbon in this zone. This finding has important implications for understanding Earth’s fundamental processes and for revealing how nature can potentially help mitigate climate change.

At a subduction zone there is communication between Earth’s surface and interior. Two plates collide and the denser plate sinks, transporting material from the surface into Earth’s interior. Showing that the microbes at the near surface are playing a fundamental role in how carbon and other elements are being locked up into the crust provides a profound new understanding of Earth processes and helps researchers model how Earth’s interior may develop over time.

“The balance between carbon buried in the deep Earth and that released through volcanoes as gases plays an important role in regulating climate over geologic time scales,” said Schrenk, an assistant professor in the Department of Earth and Environmental Sciences in the MSU College of Natural Science and study co-author. “This study shows that a substantial amount of subducted carbon emitted in groundwater in the forearc region [the area between the subduction zone and the volcanic chain] is ‘filtered’ by geochemical and microbial processes before it enters the atmosphere. This insight was aided by the holistic, interdisciplinary study of the Costa Rican arc, and is one of the first studies to show the impact of subsurface microorganisms upon global-scale biogeochemical processes.”

This is the first evidence that subterranean life plays a role in removing carbon from subduction zones. It has been well established that microbes are capable of taking carbon dissolved in water and converting it into a mineral within the rocks. This work showed that this happens on a large scale across a subduction zone. It is a natural CO2 sequestration process that can control the availability of carbon on Earth’s surface. 

Lead author Peter Barry, who carried out the research while at Oxford University’s Department of Earth Sciences said the team found that a substantial amount of carbon is being trapped in non-volcanic areas instead of escaping through volcanoes or sinking into Earth’s interior.

“Until this point, scientists had assumed that life plays little to no role in whether this oceanic carbon is transported all the way into the mantle,” Barry said, “but we found that life and chemical processes work together to be the gatekeepers of carbon delivery to the mantle.”

Following their analyses, the scientists estimated that about 94 percent of that carbon transforms into calcite minerals and microbial biomass.

The researchers now plan to investigate other subduction zones to see if this trend is widespread. If these biological and geochemical processes occur worldwide, they would translate to 19 percent less carbon entering the deep mantle than previously estimated.

The research is part of the Deep Carbon Observatory’s Biology Meets Subduction project. The interdisciplinary team included 25 researchers from six nations belonging to each of the Deep Carbon Observatory Science Communities: Deep Life, Extreme Chemistry and Physics, Reservoirs and Fluxes, and Deep Energy.

By: Val Osowski