Subduction zones, where one tectonic plate sinks beneath another, are the main points of entry for surface carbon moving into the deep subsurface. But the system has leaks: some carbon returns to the atmosphere through volcanic and hydrothermal activity. And when an oceanic plate plunges into the mantle, the entire seafloor doesn’t sink smoothly into a marine trench. Sometimes layers of sediments get scraped off like frosting from a birthday cake. In many parts of the world, variations in the amount of sediment that actually enter the subduction zone make it difficult to quantify exactly what fraction of the subducted carbon cycles back to the atmosphere through volcanoes.
DCO Reservoirs and Fluxes Community members Brian House (Scripps Institution of Oceanography, USA), Gray Bebout (Lehigh University, USA), and David Hilton (deceased, Scripps Institution of Oceanography, USA) took a novel approach to estimate carbon input at the Sunda margin, a subduction zone along the south and west coast of the Indonesian archipelago. They calculate that a much smaller amount of sediment carbon sinks into the subduction zone than previously thought, which is insufficient to balance the amount of carbon returning to the surface through volcanoes. The researchers suggest that carbonate from the underlying oceanic crust could be one of the missing carbon sources. This new approach could be used to clarify how much carbon enters other modern subduction zones and to make better global estimates of the carbon exchange between the surface and deep Earth. The researchers report the findings of their US National Science Foundation-funded project in a new paper in Geology .
“What we’ve learned from the Sunda margin will be applicable widely across Earth,” said House. “We hope that people in the scientific community will apply similar methods to the other subduction zones in the world.”
Much of the work on subduction zone carbon has occurred in Central America in Costa Rica and at the Izu-Bonin-Mariana subduction zones near Japan, where there is a fairly straightforward relationship between the sediment carbon that enters the subduction zone and how much comes back up as emissions from the volcanic arc that lines the trench. At these two sites, the sediment is not scraped off, and so the entire sediment section appears to subduct deeply.
For the new paper, the researchers thought, “Let’s try something that’s a bit of a headache, but is also more representative of a lot of other complicated subduction zones globally,” said House.
The west coast of Sumatra was the perfect “headache” to try out their new approach. The sediments that have accumulated on the seafloor along the length of the margin vary greatly. On the northern side, erosion from the Himalayas has created a layer of thick, organic carbon-rich sediment. The southern side has sediments that are high in inorganic carbon from the buildup of calcium carbonate shells of sea creatures.
To see how these variations affect the amount of carbon that enters the subduction zone, the researchers collected sediment samples from deep-sea sediment cores drilled off the coast of Sumatra and analyzed them back in the lab at Lehigh University. They measured the amounts of organic and inorganic carbon and also measured their isotopes, which are molecules with a different number of neutrons in the nucleus. The ratios of different isotopes can be used like a fingerprint, to track the movement of carbon through an environment.
The researchers used existing seismic profiles of seafloor sediments to determine the depth of sediment along the margin and the proportion of the sediment stuck to the underlying plate. These seismic profiles are from previous surveys, where researchers detonated dynamite or shot off air guns in the ocean and measured how the sound waves moved through the subsurface. By combining the amount of carbon in the sediments with their thickness, the researchers could estimate how much and what kind of carbon was reaching the trench at points along the Sunda margin’s 4000-kilometer length.
According to their calculations, only one sixth to one twelfth of the carbon that approaches the trench enters the subduction zone because the majority of the sediments get scraped off. This is about ten times less than previous estimates. The researchers also found that much of the organic carbon-rich sediments is scraped off, while deeper sediments high in carbonates likely stay attached to the sinking plate. This changes the proportions of organic and inorganic carbon that potentially leak out through the volcanic arc, relative to the carbon that initially subducts.
“We found that the bulk carbon isotopic composition of the sediment entering the trench was really different from that of the volcanic gases coming out,” said House. He suspects that some of the carbon in the volcanic emissions could be from the oceanic crust underlying the sediments and from the crust overlying the subduction zone.
These methods can be applied at other subduction zone “headaches” to estimate local carbon flux and also to make more accurate global estimates of how much carbon moves between the surface and subsurface. Such estimates are vital to understanding the balance of carbon between the atmosphere and the solid Earth, which controls the global climate.
“Let’s see if we can reduce the uncertainty of the global estimate of the carbon coming into trenches and coming out of volcanoes,” said House. “If we want to know what people are doing to the carbon dioxide content of the atmosphere, it really helps to know what Earth does naturally.”
Main image: The ship’s crew saw groups of mahi-mahi, flying fish, and dolphins and experienced sunsets over the Indian Ocean. At night, squid, attracted by the ship’s lights, darted below the surface. Credit: Brian House