Study reveals secrets to Southern Ocean’s critical role in slowing climate change revealed

Insights on one of the most important factors in the Southern Oceanic carbon cycle.

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The Southern Ocean (SO) plays a central role in mitigating global warming, accounting for ~40% of the ocean’s anthropogenic CO2 absorption and 60 to 90% of ocean warming caused by human activities. It is unclear how the production of each biogenic carbon pool contributes to the contemporary air–sea CO2 exchange.

An important component of the Southern Oceanic carbon cycle, the “biological pump,” where carbon is used by organisms at the surface and moved to ocean depths, away from atmospheric interaction, is discussed in the new research.

The NOAA Pacific Marine Environmental Laboratory’s Yibin Huang and Andrea Fassbender, along with Seth Bushinsky, an assistant professor of oceanography at the University of Hawaii at Manoa School of Ocean and Earth Science and Technology (SOEST), led the scientific team.

To quantify for the first time the role that microscopic organisms called phytoplankton play in the Southern Ocean CO2 absorption from the atmosphere through their creation of various types of biogenic (caused by living organisms) carbon, researchers looked at data gathered from more than 60 autonomous profiling floats over ten years. The effects of various biogenic carbon types on carbon export and the exchange of CO2 between the atmosphere and ocean vary.

Lead author Huang, a Cooperative Institute for Marine and Atmospheric Research scientist at the Pacific Marine Environmental Laboratory, said that simultaneously monitoring the three different types of carbon produced by biological activity has posed a longstanding challenge for oceanographers. Due to the complexity of traditional methods, scientists usually treat the total carbon production as a black box.

“Our study applies a recently developed method for estimating the production and export of distinct biogenic carbon pools in a cost-effective way and at ocean basin scales to monitor how marine ecosystems function and their response to future climate change,” said Huang.

Regional biogeochemistry is shaped by the interaction of physical and biological processes through the distinctive Southern Ocean circulation south of 35°South Latitude, which affects the interior of the world’s oceans. Deep waters rich in dissolved inorganic carbon come into contact with the atmosphere due to an upwelling that is now occurring south of the Antarctic Circumpolar Current.

The deep waters are also nutrient-rich, which supports biological activity, which is at its height in the spring and summer. Some phytoplankton species use dissolved inorganic carbon to make their exoskeletons, and they transport it to depth when they die.

Although plankton thrives in this warm, rich water, they cannot effectively use the nutrients and dissolved inorganic carbon that upwelling brings to the top. Locally, some of the dissolved inorganic carbon is released into the air. Large-scale circulation then transports the wasted nutrients toward the equator, powering a significant portion of the biological production in the subtropics and tropics. The slowing in phytoplankton growth during the winter, when deep-water mixing is most active, shapes the seasonal pattern of carbon cycling in the Southern Ocean.

The research focuses on estimating the amount of dissolved inorganic carbon utilized by these tiny organisms and exploring how the natural process of carbon export impacts the modern air-sea exchange of CO2.

The researchers discovered that whereas particle inorganic carbon production reduces CO2 uptake by around 270 million tonnes annually, organic carbon production captures nearly 3 billion tonnes of carbon annually, comparable to about 25% of all human emissions. The biological pump’s effects on the local air-sea CO2 exchange are influenced by variations in the amounts of each type of carbon produced from north to south across the Southern Ocean.

The Southern Ocean would be a source of CO2 to the atmosphere if plankton weren’t eating carbon throughout the growing season in the southern hemisphere.

Fassbender said, “The significant role played by phytoplankton in the modern Southern Ocean carbon sink suggests that understanding year-to-year variability in biogenic carbon production may be of central importance to understanding variability in the overall Southern Ocean carbon sink.”

“Expanding persistent year-round observations from biogeochemical profiling floats would be a cost-effective way to monitor the biological pump throughout the Southern Ocean and globally.”

Journal Reference:

  1. Yibin Huang, Andrea Fassbender et al. Biogenic carbon pool production maintains the Southern Ocean carbon sink. PNAS. DOI: 10.1073/pnas.2217909120

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