Comprehensive Study Measures How Salt Finger Mixing Feeds Marine Life

In the tropical Atlantic Ocean, scientists onboard Schmidt Ocean Institute’s R/V Falkor (too) have completed the most comprehensive survey in over two decades on an ocean physics phenomenon called salt finger mixing, which plays a critical role in ocean mixing but is extremely difficult to study.

Salt finger mixing occurs when warm, salty surface water sits atop cool, less salty water. The driving forces behind this phenomenon are diffusion and density. Heat makes water less dense, and salt makes it denser. Because heat diffuses through water 100 times faster than salt, the mixing waters form tiny alternating “fingers” of sinking and rising fluid. 

Salt finger mixing is especially prominent in the tropical Atlantic because warm surface waters collide with cooler waters carried by deep-ocean currents. The scientists suspect this mixing carries critical nutrients to the surface, feeding phytoplankton in an otherwise nutrient-poor ecosystem.

A Vertical Microstructure Profiler (VMP) 5500 sinks through the water column. The instrument profiles ocean mixing by descending through the water column to take centimeter-scale measurements before returning to the surface. © Alex Ingle / Schmidt Ocean Institute

“Measuring fine-scale ocean mixing is like measuring a whisper,” said Dr. Joseph Gradone, an assistant research professor at Rutgers University and co-chief scientist for the expedition. “The fine-scale ocean mixing we are interested in is as difficult to measure, but if we can capture just how many whispers there are, we might learn how truly widespread it is.”

A Vertical Microstructure Profiler 5500 is recovered aboard the R/V Falkor (too). The instrument profiles ocean mixing by descending through the water column to take centimeter-scale measurements before returning to the surface. From the left, Expedition Co-chief Scientist Dr. Joseph Gradone (Rutgers University) and Dr. Philip Leadbitter (University of Southampton). © Alex Ingle / Schmidt Ocean Institute

A better understanding of salt finger mixing is important for improving climate models. Scientists hypothesize that climate change may increase the rate of salt finger mixing by sharpening the contrast in water temperature and salinity between tropical surface waters and the deeper polar waters that flow into the Atlantic. 

Early-career researchers, including the two co-chief scientists, Gradone and Dr. Corday Selden from Rutgers University, comprised most of the team. To study the phenomenon, they used an array of underwater technologies, including a nitrate sensor, a Niskin bottle rosette for water sampling, robotic gliders, and a vertical microstructure profiler. The vertical microstructure profiler, a device for measuring subtle changes in turbulence and other physical properties of the water, can reach depths of 5500 meters and quantify fine-scale ocean mixing. They also examined nitrogen consumption and carbon fixation, which are vital components of phytoplankton growth.

“Over the coming year, our team will process the data collected during the expedition to gain insights into the relationship between salt finger mixing and plankton,” said Selden, an assistant professor at Rutgers University. “We will assess plankton growth, diversity, and grazing rates against physical measurements of salt finger mixing to determine how the process feeds surface ecosystems. These findings will clarify how fine-scale ocean mixing drives both marine food webs and potential carbon export to the deep sea.” 

The ocean absorbs approximately 30% of all human-created carbon emissions. Much of the absorbed carbon dioxide is captured by phytoplankton during photosynthesis and converted into organic carbon. When these phytoplankton are consumed or die, their remains slowly sink towards the seafloor, exporting carbon to deeper parts of the ocean, and preventing it from returning to the atmosphere for thousands of years. If salt finger mixing contributes to phytoplankton growth, the amount of carbon sinking could change.