This groundbreaking study, published in Nature, shows that microbes consume and - crucially - help trap a small amount of 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.
Co-author, Professor Chris Ballentine, Head of the Department of Earth Sciences at the University of Oxford, said: 'What we've shown in this study is that in areas that are critically important for putting chemicals back down into the planet - these big subduction zones - life is sequestering carbon. On geological timescales life might be controlling the chemicals at the surface and storing elements like carbon in the crust.'
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 study demonstrates that the process is happening on a large scale across a subduction zone. It is a natural CO2 sequestration process which can control the availability of carbon on Earth's surface.
Lead author, Dr Peter Barry, who carried out the research while at the Department of Earth Sciences, Oxford University, said: 'We 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, but we found that life and chemical processes work together to be the gatekeepers of carbon delivery to the mantle.'
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.
Following their analyses, the scientists estimated that about 94 percent of that carbon transforms into calcite minerals and microbial biomass.
Senior author, Karen Lloyd, Associate Professor of Microbiology at the University of Tennessee, Knoxville, said: 'These microbes are literally sequestering carbon. Scientists are actively working on carbon sequestration to mitigate climate change and carbon capture and storage as a means to bury greenhouse gases over long time periods. Our study is a really good example of where this is happening naturally, and it was previously unrecognised. This study shows that this happens on a big, reservoir scale.'
Maarten de Moor, co-author and professor at the National University of Costa Rica's Observatory of Volcanology and Seismology, said: '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 this study are less obvious, but they are important because they are operating over huge spatial areas in comparison to volcanoes.'
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.
Co-author Donato Giovannelli, Assistant Professor at the University of Naples Federico II and affiliated scientist at the CNR-IRBIM and Rutgers University, said: 'There are likely even more ways that biology has had an outsized impact on geology, we just haven't discovered them yet.'
Dr Peter Barry, now an Associate Scientist at Woods Hole Oceanographic Institution, added: 'We have people from three different fields working together and only with such an interdisciplinary approach can you make such breakthroughs. Moving forward, this will change how people look at these systems. For me that is thrilling.'
###
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 (DCO) Science Communities: Deep Life, Extreme Chemistry and Physics, Reservoirs and Fluxes, and Deep Energy.
For more information or to request interviews and images, please contact the University of Oxford press office at ruth.abrahams@admin.ox.ac.uk / 01865 280730.
Read the full article: 'Forearc carbon sink reduces long-term volatile recycling into the mantle' in Nature when the embargo ends on Wednesday 24 April at 1800 London time (BST): https://doi.org/10.1038/s41586-019-1131-5.
See a short video about the research expedition: https://www.youtube.com/watch?v=NAf7MvYZfwY.
Notes to editors
About the University of Oxford
Oxford University has been placed number 1 in the Times Higher Education World University Rankings for the third year running, and at the heart of this success is our ground-breaking research and innovation. Oxford is world-famous for research excellence and home to some of the most talented people from across the globe. Our work helps the lives of millions, solving real-world problems through a huge network of partnerships and collaborations. The breadth and interdisciplinary nature of our research sparks imaginative and inventive insights and solutions.
Journal
Nature