Ancient marine molecules, bursting from the sea

Waves crash in the ocean and inject tiny particles into the air, which contain molecules of organic carbon more than 5,000 years old. New research published in Science Advances by Steven Beaupré of Stony Brook University’s School of Marine and Atmospheric Sciences (SoMAS) and a national team of scientists, helps to solve a long-standing mystery about what finally happens to these ancient marine molecules:

Decades of measurements have revealed that nearly all of the organic matter dissolved in the oceans is surprisingly old – up to several thousand years. But scientists do not completely understand how this ancient organic matter is eventually destroyed. Beaupré and colleagues performed experiments to show that some of this ancient organic matter can be removed from seawater when wave-generated air bubbles burst on the ocean surface and inject aerosols into the atmosphere. By directly measuring the radiocarbon (¹⁴C) ages of the aerosols, they estimate that somewhere between 19 and 40 percent of the aerosol organic matter is at least 5,000 years old.

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“Our new study shows that a significant proportion of that organic matter is indeed quite old, and that the formation of aerosols by breaking waves could be a significant removal pathway for the old carbon found in the sea,” says Beaupré, lead author and Assistant Professor.

The overall goal of the research is to understand the transfer of organic carbon from the ocean to the atmosphere. The article is based on collaborative research conducted during a research cruise in the North Atlantic Ocean in October and November of 2016 studying the production of primary marine aerosol particles

"Breaking wind waves on the surface of the ocean trap air below the sea surface that rises in the form of bubbles. Upon reaching the surface, the bubbles burst and emit particles into the atmosphere, commonly referred to as sea spray," said Amanda Frossard, assistant professor of chemistry at UGA and co-author on the study. "These aerosol particles range in composition based on their size and contain both salt and organic components. Sea spray particles are important to the atmosphere because they can scatter sunlight, producing the haze observed in ocean regions, and can grow into cloud droplets, depending on their size and composition."

In the study, radiocarbon measurements confirmed that a fraction of the organic carbon in the aerosol particles produced from breaking waves is thousands of years old. The production of aerosol particles from breaking waves is thus a significant removal pathway of old carbon from the ocean. The work also identifies old carbon as a component of these sea spray aerosol particles, though it has been commonly thought that most carbon in particles produced from breaking waves has been recently generated at the ocean surface. 

In one companion study published in Environmental Science and Technology with Frossard as lead author, the focus was on specific types of organic compounds measured in the seawater and aerosol particles, called surfactants. These molecules are commonly used in shampoos and cleaners but can also have natural sources. The study showed that surfactants in the seawater were stronger than those in the aerosol particles produced from that seawater. Surfactants in aerosol particles are important because they may influence how easily particles can grow into cloud droplets. This is part of the ongoing work in the Frossard Lab in Chemistry at UGA.    

Both studies measured properties of aerosol particles produced from seawater using a custom-built marine aerosol generator. This generator and measurements from the research cruise are described in a study in the Journal of Geophysical Research. The generator produces bubbles in a glass chamber, using a stream of air and seawater. The sizes of the bubbles are similar to those produced from breaking waves in the open ocean.

Image: Photo of breaking waves by Amanda Frossard.