Shedding Light on Natural Carbon Sequestration Process
An international group of scientists has shed new light on the processes behind natural sequestration of carbon at a subduction zone where there is communication between the Earth's surface and interior. They, furthermore, confirmed that microbes can affect the cycle of carbon moving from the Earth's surface into the deep interior. This is revealed in a new article in the latest edition of Nature. Sæmundur Ari Halldórsson, research scholar at the University of Iceland's Institute of Earth Sciences is one of the authors.
Carbon plays a vital part in our existence, travelling between the atmosphere, living creatures, soil, oceans, and rock in the so-called carbon cycle. Carbon also plays a considerable part in one of today's most dire environmental threats because when it bonds with oxygen it forms carbon dioxide (CO2), the greenhouse gas considered to be the main cause of global warming.
Carbon is also present when two tectonic plates collide and the denser plate sinks, transporting material from the surface into the Earth's interior. Where an oceanic plate and a continental plate collide the former is subsumed under the latter, creating what is known as a subduction zone. This process transports chemicals from the surface into he Earth's mantle, including carbon, which can then later resurface in eruptions; as volcanic activity is often characteristic of subduction zones.
The interdisciplinary team, comprised of 37 researchers from six nations, focused on carbon sequestration in Costa Rica’s subduction zone, where the ocean floor sinks beneath the continent and volcanoes tower above the surface. Thermal springs in the Central-American country contain water with deeply-seated carbon. By analysing carbon isotopes and their relationships with other chemicals in the water the scientists could establish that the majority of the carbon originally mostly bound in the oceanic crust is released at the continental edge of Costa Rica, in a so-called forearc region, and is sequestered in the Earth's crust in the form of calcite (a pure variety of calcite, almost only found in Iceland is known as Iceland spar).
Microbes affect the carbon cycle
The team, furthermore, established that near-surface microbes are capable of taking carbon dissolved in water and converting it into biomass within the crust. This is the first evidence that subterranean life plays a considerable role in removing carbon from subduction zones, even though the concept that microbes might affect the carbon cycle had been pointed out previously.
Comparable processes are in place in Iceland, in Hellisheiði geothermal power plant, where an international group of scientists is capturing the carbon in our atmosphere and pumping it deep into the basaltic rock in the CarbFix project. The carbon dioxide, dissolved in water, is pumped into the ground and research has shown that it is sequestered into basaltic rock within two years.
The study in Nature sheds clearer light on the processes at play during natural carbon sequestration and enables scientists to paint a clearer picture of how the Earth's carbon source can develop over long timescales.
"One of the most remarkable elements in these results is the considerable amount of carbon that seems to be sequestered in the Costa Rican forearc. High quality data of this type has hitherto been unavailable to estimate the amount of carbon potentially left in the crust. It seems incredible that organisms are such a vital part of the process and that they can bind such large amounts of carbon in the crust as is evident here. It was only with the participation of an interdisciplinary team in the study that it became possible to isolate and analyse the extent of carbon sequestration in this natural laboratory," says Sæmundur Ari Halldórsson, one of the scientists involved in the study.
The team of scientists involve in the study, 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. "If further data collection from other subduction zones, for example from the area off the coast of Indonesia shows similar results, we will have to re-examine our ideas concerning the carbon cycle in the Earth's mantle. If this is true it is clear that path for carbon into the deep mantle is not easy and that the carbon present there is only renewed to a limited extent," Sæmundur adds.
