Tracking carbon from the ocean surface into the depths

Tracking carbon from the ocean surface into the depths

This NOAA mooring off the coast of Washington carries a variety of carbon-related sensors. Fassbender would like to augment these with a new instrument she and other researchers are developing. Credit: Richard Feely, NOAA PMEL

As disastrous storms, floods, and fires become more common in the US and elsewhere, humans are just beginning to appreciate some of the impacts of global warming. But these impacts would be much worse if the ocean hadn’t absorbed roughly 45 percent of the carbon dioxide humans have released since the beginning of the industrial revolution. Although scientists have long known that the oceans are taking up a lot of carbon dioxide, the details of this process are still fuzzy. MBARI marine chemist Andrea Fassbender is trying to bring this process into focus by studying when, where, and how carbon moves between the atmosphere, upper ocean, and deep sea.

The basics of this carbon-cycling process are relatively well understood. When concentrations of carbon dioxide are higher in the atmosphere than in the surface waters of the ocean, carbon dioxide from the atmosphere will dissolve into the ocean. Some of this carbon dioxide is used by microscopic algae that incorporate carbon into their bodies as they grow and reproduce in the sunlit surface waters.

When the microscopic algae are consumed by animals and microbes, the carbon in their bodies is transferred to these organisms, which carry the carbon within their bodies or release it as waste into the surrounding water. Most of this carbon stays within about 100 meters of the sea surface, where it can easily return to the atmosphere, especially during the winter months, when the ocean waters are more churned up and the concentration of algae is lower.

However, a small but vitally important amount of this carbon sinks to deeper water, hundreds to thousands of meters below the sea surface. Some of this carbon is carried down into the depths in the form of marine snow—tiny flecks of dead algae and animals, waste material, and mucus. The farther down this carbon sinks, the longer it is likely to be stored within the ocean before coming in contact with the atmosphere again.

If the carbon sinks deep enough that it is unlikely to be carried back to the surface by winter mixing, it is considered to have been exported from the surface waters. If the carbon reaches depths at which it is unlikely to be carried back to the surface for hundreds of years or more, it is considered to be sequestered in the deep sea.

Oceanographers call this vertical carbon transport process the biological pump, and it is at the core of much of Fassbender’s research. Although the overall concept of the biological pump is relatively simple, the details are extremely complicated and involve many interrelated chemical, biological, and physical processes, which vary from place to place and over time scales ranging from minutes to millennia. The biological pump is also an important component in the computer models that scientists use to predict global warming.

To fully understand the biological pump, oceanographers need to measure carbon in the ocean in all its various forms, including:

  • Dissolved inorganic carbon—Carbon in the form of simple molecules dissolved in seawater, including carbon dioxide, carbonic acid, bicarbonate, and carbonate
  • Particulate organic carbon—Carbon in the form of particles suspended in seawater that are larger than about one half micron (about one 90th of the diameter of a human hair) across
  • Dissolved organic carbon”—Carbon in the form of very tiny particles (less than about one half micron across) and in dissolved organic compounds such as those released during the breakdown of dead animals, algae, and marine snow

Over the past year Fassbender has been actively involved in a number of research projects and publications focusing on carbon cycling in the ocean, with an emphasis on the biological carbon pump and the processes that control how the oceans absorb human-generated carbon. The following text describes some of this groundbreaking work.

Kim Fulton-Bennett, Monterey Bay Aquarium Research Institute (via Phys.org), 27 November 2018. Full article.

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