Seeking Salt Fingers in the Tropical Atlantic

What are salt fingers? 
Physical processes keep the water stratified. Warm, salty water is less dense than the frigid bottom water and thus sits atop it. When scientists take vertical profile data in this region, they often observe visible steps, known as thermohaline staircases. These staircases result from a fine-scale mixing process known as salt finger mixing, which forms due to double-diffusive mixing. 

The driving force behind double-diffusive mixing is density, the measure of how much mass occupies a specific volume. Cold water is denser than warm water because its molecules are more tightly packed. Salinity also makes the water denser as additional dissolved molecules in the water, like salt, increase the mass per unit volume. 

Double-diffusive mixing occurs when two different variables, like heat and salt, affect diffusion.  Heat diffuses 100 times faster than salt, resulting in different mixing rates. These different rates create salt fingers that look like swirling patterns moving up and down in the water column. This double-diffusive mixing carries warm, salty water downward while also moving cold, fresh Antarctic water upward towards the surface. This water is likely rich in nutrients necessary for phytoplankton and could be a key factor in supporting life in an otherwise nutrient-poor environment. 

The intersection of chemistry, physics, and biology
Gradone, Selden, and the team hypothesize that double-diffusive mixing helps transport cold, nutrient-rich water upward toward the surface, providing sustenance for the base of the ocean’s food web: phytoplankton. 

Phytoplankton are microscopic algae that live near the water’s surface, where sunlight is abundant. Roughly half of global primary production occurs in the Ocean thanks to phytoplankton. This fuels marine food webs that support much of the life in the Ocean, including fisheries important to human food security and global commerce.

Importantly, some of the organic matter produced by phytoplankton sinks into the depths in a process known as the biological carbon pump. Once in the deep sea, the carbon carried by phytoplanktic matter, be it decay, feces, or in the body of another animal, can remain sequestered for months to millennia. The biological carbon pump plays a critical role in regulating global climate by removing CO2 from water in contact with the atmosphere—about as much as humans emit annually. 

Phytoplankton grow when nutrient concentrations, including nitrogen, phosphorus, and iron, are at the correct levels. Nutrients can enter sunlit surface waters from land (e.g., rivers or dust carried from land by wind) or from the deep sea via vertical mixing. Once present, nutrients are often recycled by organisms in the upper ocean. In the open ocean, waters tend to be nutrient-depleted, a state known as oligotrophy, and consequently rarely support highly productive ecosystems.   

The primary nutrient that limits phytoplankton growth in the Tropical Atlantic is nitrogen. Selden and Gradone hypothesize that salt finger mixing helps transport nitrogen from the deep ocean to the surface. The quantity and frequency of nitrogen transport towards the surface help regulate productivity in the region. However, little is known about the rate and magnitude of nitrogen delivery by salt fingers to the surface, and about the planktonic ecosystems that might rely on salt finger mixing. 

An Uncertain Future
Salt finger mixing and its impact on primary production are currently missing from models used to predict changes in global primary production, and might be a significant, but overlooked variable. 

One of the largest sources of uncertainty in projecting the ocean’s future carbon storage capacity, and thus global climate, is the uncertain response of marine primary production to climate change. Satellite records suggest that ocean productivity has begun to decline, and computational models predict increasing losses, particularly in the tropics and subtropics. It is widely hypothesized that these declines are due to increased stratification. In other words, the separation of seawater layers is caused by climate change-induced increases in surface temperature and salinity. This is because stratification reduces the turbulent diffusion of nutrients from deep waters into the upper ocean. Yet, salt finger mixing might potentially offset these changes, as increased temperature and salinity may enhance double diffusion. 

As the upper ocean warms and salinity patterns change, this might increase the rate of double-diffusive mixing and deliver more nutrients to the surface, thereby enhancing phytoplankton growth relative to current scientific model predictions. However, current biogeochemical models lack data collected directly from the field, and so double diffusion is often left out when examining the future of primary productivity in the tropical Atlantic. This data represents a significant source of uncertainty in predicting how ocean ecosystems, and the ocean’s carbon storage capacity in turn, may respond to climate change

Artist-at-Sea, Robertina Šebjanič
From the artist’s website

Robertina Šebjanič is an artist and researcher whose practice drifts through the fluid thresholds of hydro-ecology, (geo)politics, and arts, attuned to the rhythms and ruptures of aquatic worlds. Her award-winning works navigate the intertidal zones between species, bringing to the surface the submerged voices of oceans and rivers.  In her analysis of the Anthropocene and its theoretical framework, the artist uses the terms “aquatocene” and “aquaforming” to refer to the human impact on aquatic environments. 

Her artwork Aurelia 1+Hz / proto viva generator (artist proof) has been part of the .NewArt{collection;}_ Electronic Art Collection, Spain since 2019. She is an Associate Professor at the School of Arts, University of Nova Gorica.                                                                              

Her works received awards, honorary mentions, and nominations at Prix Ars Electronica, Starts Prize, Falling Walls, Re: humanism.