Ocean Science · Tropical Atlantic

Tropical Atlantic Expedition Tests Whether Salt Finger Mixing Feeds Plankton

Two early-career Rutgers oceanographers led a 25-day survey aboard Schmidt Ocean Institute’s R/V Falkor (too) to measure a centimetre-scale mixing process that may carry nitrogen up to the sunlit surface. Results are expected over the coming year.

By SEVENSEAS Media | Ocean Literacy

In the western tropical Atlantic, warm and salty surface water sits on top of cooler, fresher water that has travelled north from the Antarctic. From 9 August to 2 September, an international team sailing from Bridgetown, Barbados, aboard Schmidt Ocean Institute’s research vessel Falkor (too) worked the boundary between those layers. They were measuring a process called salt finger mixing and trying to establish whether it supplies nutrients to the plankton living above it.

The expedition was led by Dr Joseph Gradone and Dr Corday Selden of Rutgers University, both on their first voyage as chief scientists. Schmidt Ocean Institute describes the survey as the most comprehensive study of salt finger mixing in the tropical Atlantic in more than 20 years, and says early-career researchers made up most of the team.

How salt fingers form

Temperature and salt content largely set the density of seawater. Heat makes it lighter and salt makes it heavier, and heat diffuses through water roughly 100 times faster than salt, according to Schmidt Ocean Institute. Where warm, salty water lies over cool, fresher water, a small parcel nudged downward gives up its warmth to the surrounding water long before it gives up its salt. It ends up denser than its neighbours and keeps sinking, while parcels nudged upward do the reverse. What forms is a field of thin, alternating columns of sinking and rising water. Oceanographers call the process double-diffusive convection, and the columns are the fingers.

In this part of the Atlantic the process is strong enough to show up in vertical profiles as a thermohaline staircase, a stack of well-mixed layers separated by sharp steps in temperature and salinity. The region was the site of an earlier landmark study, the Salt Finger Tracer Release Experiment, in which a Woods Hole Oceanographic Institution team released a harmless chemical tracer about 400 metres down near Barbados in 2001 and returned months later to map how far it had spread. The results, published in Science in 2005, showed salt and tracer mixing about twice as much as heat, the pattern expected from salt fingers rather than ordinary turbulence.

The nitrogen question

Surface waters of the open tropical Atlantic are short of nutrients, and nitrogen is the one that most limits phytoplankton growth there, according to the expedition’s project description. The cooler water underneath is richer. Gradone and Selden’s hypothesis is that salt fingers carry nitrate upward across the boundary, helping to sustain plankton in water that would otherwise support little. The team says the rate and size of that nitrogen delivery are largely unknown, as are the plankton communities that might depend on it.

The biology team ran its own measurements alongside the physics. Selden and PhD student Dylan Buchmiller sealed seawater from the ship’s CTD rosette in bottles, drew out air bubbles by syringe and added nitrogen gas enriched with a heavy stable isotope. The isotope works as a tracer for nitrogen fixation, the conversion of dissolved nitrogen gas into usable forms by microbes known as diazotrophs. PhD student Becca Horwitz measured chlorophyll a, the pigment phytoplankton use to capture light, working in the dark because the pigment is light-sensitive. PhD student Mya Sharpe examined plankton from net tows under a microscope. All three students are at Rutgers.

Instruments for a faint signal

“Measuring fine-scale ocean mixing is like measuring a whisper.”

Dr Joseph Gradone, Rutgers University, expedition co-chief scientist

Gradone said the aim is to capture how many of those whispers there are, which would show how widespread the process really is. The main tool was a Vertical Microstructure Profiler, an untethered instrument that falls freely through the water taking centimetre-scale measurements of turbulence and other physical properties, then returns to the surface for recovery. The VMP 5500 used on this expedition is rated to 5,500 metres.

Two robotic gliders stayed in the water for weeks, sending back temperature, current, dissolved oxygen, microstructure turbulence and zooplankton data that the team used to pick sampling stations as the cruise went on. A nitrate sensor and the CTD rosette, a frame of Niskin bottles that close at set depths to capture water, completed the set.

What it could mean for climate projections

Salt finger mixing is currently missing from the models used to project changes in ocean productivity, the expedition team says. Those models expect productivity to fall in the tropics and subtropics as surface water warms and the water column becomes more stratified, which cuts off the supply of nutrients from below. The team notes that satellite records suggest the decline has already begun.

Gradone and Selden’s hypothesis cuts partly against that projection. Warmer, saltier surface water sharpens the contrast with the cooler water below, which could strengthen salt fingers, raise the upward supply of nutrients and offset some of the expected loss in this region. That has not been shown yet, and the expedition was designed to collect the field measurements that biogeochemical models have so far lacked.

The carbon question follows from the biology. Between 1994 and 2007 the ocean took up about 31 per cent of the carbon dioxide released by human activity, a 2019 global study in Science found. Phytoplankton are part of how the ocean stores carbon: when they are eaten or die, some of their carbon sinks into the deep sea, where Schmidt Ocean Institute says it can stay for anything from months to millennia. If salt fingers change how much phytoplankton grow, the amount of carbon sinking could change too. The expedition has not yet put numbers on either.

Schmidt Ocean Institute’s executive director, Dr Jyotika Virmani, said the measurements address a major gap in knowledge of ocean physics and should help improve computer models of larger systems such as the Atlantic Meridional Overturning Circulation. That is the same current system that carries the cooler, Antarctic-origin water into the region.

What happens next

The team now has a year of laboratory and data work ahead. Selden said they will compare plankton growth, diversity and grazing rates with the physical measurements of salt finger mixing “to determine how the process feeds surface ecosystems.” She expects the findings to clarify how fine-scale mixing drives marine food webs and potential carbon export to the deep sea.

Collaborating institutions listed by Schmidt Ocean Institute include Woods Hole Oceanographic Institution, the University of South Florida, the University of Southampton, the Caribbean Institute for Meteorology and Hydrology in Barbados and the Voice of the Ocean Foundation. The team included Dr Philip Leadbitter of the University of Southampton, and the artist Robertina Šebjanič sailed as the institute’s artist-at-sea.

Expedition details and the full science team are on Schmidt Ocean Institute’s Seeking Salt Fingers in the Tropical Atlantic page.

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