Scientists explore oceans role as carbon sink

Deep sea vent. Image: Ironman/NSF/NOAA/Jason/ WHOI

Hydrothermal vents in the ocean floor have been identified as a source of dissolved black carbon for the first time, furthering understanding of the role of oceans as a carbon sink.

A carbon sink is anything that absorbs more carbon from the atmosphere than it releases – for example, plants and soil. The ocean is one of the largest dynamic carbon sinks in the world, and is susceptible to increased carbon emissions from human activities.

There are even proposals to use the ocean to sequester carbon in an effort to reduce carbon emissions. However, many of the ways in which the ocean works as a carbon sink are not fully understood.

“One of the largest carbon pools on the Earth’s surface is the dissolved organic carbon in the ocean.”

Professor Hiroshi Ogawa, University of Tokyo

A team of scientists in Japan have revealed conclusive evidence that hydrothermal vents are a previously unknown source of dissolved black carbon in the deep ocean. The discoveries by Associate Professor Youhei Yamashita and graduate student Yutaro Mori at Hokkaido University, along with Professor Hiroshi Ogawa at AORI, the University of Tokyo, have been published in the journal Science Advances.

“One of the largest carbon pools on the Earth’s surface is the dissolved organic carbon in the ocean,” explains Ogawa. “We were interested in a portion of this pool, known as dissolved black carbon (DBC), which cannot be utilised by organisms.

"The source of DBC in the deep sea was unknown, although hydrothermal vents were suspected to be involved.”

The stable chemical structure of DBC, along with its resistance to microbial degradation, and use as a major sink of atmospheric CO2 in the global carbon cycle makes it an important area of scientific research. The distribution of DBC in the ocean basins of the North Pacific Ocean and Eastern South Pacific Ocean were analysed by the researchers who compared the data with previously reported concentrations of a helium isotope associated with hydrothermal vent emissions, as well as oxygen use in these regions.

Research vessel Hakuho Maru conducted the observations used for this study.

Their findings showed that hydrothermal vents were an important source of DBC in the Pacific Ocean. This is most likely formed due to the mixing of the hot fluids from hydrothermal vents with cold seawater, and is transported over long distances — thousands of kilometres.

“Most importantly, our research indicates that the DBC from hydrothermal vents is an important source of dissolved organic carbon in the deep ocean. In terms of DBC inputs to the ocean, hydrothermal vents may contribute up to half as much DBC as that which is formed by biomass burning or fossil fuel combustion and subsequently transported via rivers or atmospheric deposition,” concluded Yamashita.

Further research is required to understand exactly how DBC is formed from hydrothermal vents.