Newswise — Scientists have been curious about the fate of both organic and inorganic carbon carried by Earth's Pacific Plate as it plunges into the planet's interior along the Ring of Fire, a region known for its abundance of volcanoes.
A recent study proposes that a significant portion of this subducted carbon is actually released back into the atmosphere instead of descending further into Earth's mantle. This discovery has the potential to enhance our long-term understanding of Earth's climate patterns.
Conducted by a scientist from the Geophysical Institute at the University of Alaska Fairbanks, the study reveals that the volcanoes in the Aleutian-Alaska Arc emit more subducted carbon dioxide into the atmosphere than previously estimated. This occurs through a process referred to as recycling.
The lead author of the research, Taryn Lopez, a research associate professor, published the findings in the journal Science Advances. The study involves 12 co-authors from various institutions across California, New Mexico, New York, Rhode Island, Washington state, Washington, D.C., Italy, and New Zealand
"While we currently possess a reasonably good understanding of the amount of carbon that enters Earth's interior through subduction and the quantity released into the atmosphere by volcanoes, there is still much to learn about the fate of subducted carbon and the proportion that ultimately returns to the atmosphere," explained Lopez.
The Aleutian-Alaska Arc, spanning from Cook Inlet to the Aleutian Islands, exhibits a lower abundance of carbon sources in its crust and subducted slab compared to most volcanic arcs worldwide. This unique characteristic enables scientists to trace the movement of carbon throughout the subduction process and gain a more accurate understanding of the quantity of subducted carbon that volcanoes release into the atmosphere.
The recycling of carbon in arc volcanoes, which are situated above zones where an oceanic plate subducts, originates from three distinct sources: the subducted oceanic plate or slab itself, the mantle wedge situated above the descending slab, and the overriding crust.
The main focus of Lopez's study was to refine the estimation of the amount of carbon originating from the subducted slab.
Organic carbon primarily settles on the seafloor, representing the surface of the oceanic crust slab as it heads towards subduction. This organic carbon includes remnants of marine organisms, as well as terrestrial plants and animals that have been transported into the ocean.
Inorganic carbon, derived from seawater, can precipitate as minerals on the surface of the oceanic crust slab.
To conduct the research, Lopez and her colleagues collected gas samples from 17 volcanoes, building upon data obtained during previous research. Additionally, they utilized data from ocean drill cores obtained from four locations near the Aleutian Trench, an area where the Pacific Plate submerges beneath the North American Plate.
Armed with this valuable data, the researchers employed chemical modeling techniques to track the proportion of organic and inorganic carbon that reenters the atmosphere after being subducted at the Aleutian Trench. They carefully monitored the carbon's journey from subduction all the way to its release through volcanic outgassing.
The study primarily concentrated on the central and western regions of the Aleutian-Alaska Arc, which consist of oceanic crust.
"In the area extending from the Alaska Peninsula to the west, we have observed that the overriding crust does not contain a significant amount of carbon," explained Lopez. "Consequently, we can infer that the carbon emitted by the volcanoes originates from either the mantle or the subducted slab."
To determine the proportion of carbon isotopes present in the volcanic gases, Lopez and her team analyzed the carbon-12 and carbon-13 isotopes. Carbon-12, which constitutes nearly 99% of Earth's carbon, was contrasted with carbon-13, a rarer isotope with an additional neutron in its nucleus, accounting for only about 1% of carbon. Inorganic carbon, organic carbon, and mantle carbon possess distinct ratios of these isotopes.
By identifying the average isotopic composition of carbon in the volcanic gases and comparing it with the isotopic composition and overall carbon input from the subducted slab, the researchers calculated the amount of subducted carbon that is released into the atmosphere through degassing of the Aleutian-Alaska Arc volcanoes.
Contrary to previous studies, which suggested minimal amounts of organic and inorganic carbon from ocean floor sediments and the subducted slab crust were returned to the atmosphere, Lopez and her colleagues made a remarkable discovery.
Their findings revealed that in the central Aleutians, approximately 43% to 61% of organic carbon derived from sediments is actually released into the atmosphere through volcanic degassing. In addition, in the western Aleutians, approximately 6% to 9% of inorganic carbon originating from the slab crust is returned to the atmosphere via volcanic degassing.
Furthermore, the scientists observed that the proportions of recycled organic and inorganic carbon through arc volcanoes appeared to be influenced by various characteristics of the subduction zone, including the speed of subduction and the temperature of the slab.
"These results challenge our previous understanding, indicating that less carbon is returned to the deep mantle," noted Lopez. "By shedding light on the fate of subducted carbon, these findings contribute to the refinement of global climate models, enhancing our understanding of Earth's climate dynamics."
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