In 2017, an interdisciplinary group of early career scientists visited Costa Rica’s subduction zone, where the ocean floor sinks beneath the continent, to find out if subterranean microbes can affect geological processes that move carbon from Earth’s surface into the deep interior. According to their new study  in Nature, the answer is yes.
The study shows that microbes consume and trap a small but measurable amount of the carbon sinking into the trench off Costa Rica’s Pacific coast. The microbes may also be involved in chemical processes that pull out even more carbon, leaving cement-like veins of calcite in the crust. These unexpected findings have important implications for how much carbon moves from Earth’s surface into the interior, especially over geological timescales. The research is part of the Deep Carbon Observatory’s Biology Meets Subduction project and includes DCO members Peter Barry (Woods Hole Oceanographic Institution, USA; formerly at University of Oxford, UK), Maarten de Moor (National University of Costa Rica), Donato Giovannelli (University of Naples Federico II, Italy), and Karen Lloyd (University of Tennessee, Knoxville, USA), among many others.
“With the exception of the great oxidation event, when microbes first produced oxygen, we usually think of geology as something that happens independently of life, and life just adjusts to the geology,” said Lloyd. “But we found that microbes can impact major geological processes happening on Earth today.”
During the 12-day expedition, the interdisciplinary group collected water samples from thermal springs throughout Costa Rica. Along with other DCO collaborators, they performed analyses on the samples to understand how biological and geochemical processes affect the movement of carbon through the subduction zone.
When the edge of the oceanic plate, called the subducting slab, sinks towards the mantle, the extreme pressure and heat releases the plate’s carbon, like wringing out a wet washcloth. It is well known that some of that carbon escapes back to the surface through the resulting volcanic arc, but scientists often overlook processes occurring in the forearc, which is the cooler region between the trench (the place where the subducting slab enters the subsurface) and the volcanic front.
The Biology Meets Subduction team showed that low temperatures in the forearc support microbial life and water-rock interactions that divert the down-going carbon from the subducting slab and trap it in the crust. The researchers estimate that about 94 percent of that redirected carbon transforms into calcite minerals and microbial biomass in the forearc region.
“We found that a small but substantial amount of carbon is being trapped in the non-volcanic forearc region, instead of escaping through volcanoes or sinking into Earth’s interior,” said Barry.
“It is amazing to consider that tiny microbes can potentially influence geological processes on similar scales as these powerful and visually impressive volcanoes, which are direct conduits to Earth’s interior. The processes that we have identified in the forearc are less obvious, but they are important because they are operating over huge spatial areas in comparison to volcanoes,” said de Moor.
Next, the researchers plan to investigate other forearc regions 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.
Overall, the study shows that biology has the power to affect carbon recycling in subduction zones. “We already knew that microbes altered geological processes when they first began producing oxygen from photosynthesis,” said Giovannelli. “I think there are probably even more ways that biology has had an outsized impact on geology, we just haven’t discovered them yet.”
Additional DCO researchers involved in the new study include Matthew Schrenk, Hannah Miller (both at Michigan State University, USA), Daniel Hummer (Southern Illinois University, USA), Taryn Lopez (University of Alaska, Fairbanks, USA), Katie Pratt (University of Rhode Island, USA), Yemerith Alpízar Segura, Gino González, Carlos Jose Ramírez (all at Volcanes Sin Fronteras, Costa Rica), Patrick Beaudry, Shuhei Ono (both at Massachusetts Institute of Technology, USA), Maria Martínez (National University of Costa Rica), Kate Fullerton (University of Tennessee Knoxville, USA), Estaban Gazel (Cornell University, USA), Justin Kulongoski (California Water Science Center, USGS, USA), Francesco Smedile (Rutgers University, USA), Mustafa Yücel (Middle East Technical University, Turkey) Christopher Ballentine (University of Oxford, UK), Tobias Fischer (University of New Mexico, USA), and David Hilton (Scripps Institution of Oceanography, USA).
Watch this video for a glimpse of the 2017 field campaign, including the final descent into Póas Volcano’s active crater. Credit: Deep Carbon Observatory/CoLab Productions
Main image: Donato Giovannelli and Karen Lloyd collect samples from the crater lake in Poás Volcano. Credit: Katie Pratt