Ocean surface microlayer: Researchers study ocean-atmosphere interface

The ocean-atmosphere interface, where the air and the sea interact, is known as the marine microlayer. Understanding how nutrients, pollutants and organic matter are exchanged between the air and the sea is also important for better understanding the roles they play in regulating the carbon cycle and climate.

The problem, however, is that studying the marine surface microlayer is an extremely time-consuming task and can involve many challenges.

To better understand the atmosphere-ocean interactions of biogeochemically important components, such as gases and various particles, researchers from the University of Delaware and the University of Georgia (UGA) recently conducted a research cruise aboard UD’s research vessel, the Hugh R. Sharp, to explore the surface microlayer of the North Atlantic Ocean.

The research team has previously conducted two cruises, but these were cut short due to bad weather and the COVID-19 pandemic.

Andrew Wozniak, associate professor in the School of Marine Science and Policy (SMSP), will serve as the UD project leader, and Amanda Frossard, associate professor in the UGA Department of Chemistry, will serve as the UGA project leader.

Wozniak said the ocean’s surface microlayer is a unique microenvironment.

“You have a buildup of organic material that makes it to the air-sea interface and accumulates there, creating interesting physical properties that affect the way materials are exchanged,” Wozniak said.

“The idea was that if we could better understand how ocean processes and biology change the chemistry of organic matter in space and time, we could better understand how these gases are exchanged and how the particles they release into the atmosphere affect atmospheric chemistry.”

Frossard explained that the team is interested in studying compounds called surfactants, which reduce the surface tension of liquids such as seawater and accumulate in surface waters and ocean microlayers due to their affinity for surfaces and interfaces, such as rising air bubbles in the ocean.

One of the main goals of this project is to investigate and better understand these surfactants.

“We want to know what surfactants are in the ocean, how they distribute into the microlayers, and what influences their concentration and composition,” Frossard said. “We’re collecting samples here, some of them are being processed on board, but they’ll all be brought back to the lab for a range of analyses.”

“We will use these results to better understand the ocean surface microlayer and, in turn, gas exchange between the atmosphere and ocean, as well as particle emissions from the ocean to the atmosphere.”

Glass Panel

Studying the ocean surface microlayer is never easy, and oceanographic work requires perseverance in the face of many challenges, as previous expeditions have shown us, where unexpected difficulties have been encountered.

During the experiment, researchers on board carried out a series of biological and chemical analyses, which required large amounts of material.

The surface microlayer is incredibly thin, about 100 micrometres thick – roughly the thickness of a sheet of copy paper. One way the researchers collected the material was to immerse a glass slab in water and then scrape the water off the slab with a squeegee into a bottle.

“As you can imagine, this takes a very long time,” Wozniak says. “You need about 1.5 liters of water, and you get about 7 milliliters of water per run.”

The team worked with R/V Sharp crew members John Swallow, Timothy Deering and James Warrington to refine collection techniques to improve sample collection methods.

They used a device called a rosette sampler, which is used to take water samples at great depths. This type of sampler is usually mounted on the side of a boat equipped with a bottle, and after a dive, the person resurfaces with the bottle of water.

However, in this experiment, a piece of glass was attached to the rosette.

“We extend the sampler out the back of the boat, dip it in the water, lift it back onto the boat, and then scrape the glass,” Wozniak says. “We do this over and over again to collect enough water to take a sample. This takes about two hours. We count how many times we do this and keep a record of how many times it takes to take a sample.”

When the glass plate sinks in water and then rises again, the last thing it comes into contact with is the material on its surface, Wozniak says. The surfactant molecules are attracted to the glass and stick to it, and most of the material that drips off as the plate emerges is the water underneath.

“They’re called surfactant molecules because they reduce surface tension, a property that affects turbulence at the air-water interface,” Wozniak says. “Turbulence is a key factor in how gas moves back and forth. The more turbulence there is, the more material exchange there is.”

Currently, methods to quantify how substances such as carbon dioxide are exchanged are based on wind speed, as wind influences turbulence, but these models have large errors that are thought to be due to the influence of other factors such as surfactants.

“By chipping this away from the glass, we were able to solidify a thin layer,” Wozniak says. “This is our solution for studying microscopic layers on the ocean’s surface, and it’s a great example of sailors and scientists working together to solve a real problem.”

Once samples were collected, the researchers analyzed them either on board the ship or back in laboratories back home.

UD students on the expedition included Felix Agbulemanyo, a doctoral student in oceanography in UD’s School of Marine Science and Policy Studies, who is a member of Wozniak’s lab group and conducted the research as part of his doctoral thesis.

The UD research team includes undergraduate student Audrey Tong, who is on board the expedition to help with science and communications research; Tia Ouyang, a member of the Wozniak lab and a doctoral student; and Ava Grove, an undergraduate student new to oceanographic research.