Diatoms, a type of plankton or marine algae, have a significant role in drawing carbon down into the deep, especially in the Southern Ocean, which takes up about a third of organic carbon stored in the ocean.
Uniquely, diatoms have dense, silica-based exoskeletons—like miniature glass houses—which were thought to provide ballast, making them prone to sinking and, therefore, a key way carbon is transported to the deep ocean.
But the new study, based on data from two major expeditions to the Southern Ocean’s underexplored twilight zone—the region between ~100 m to 1,000 m deep—found that the diatom skeletons lingered near the sea surface while carbon made its way to the deep ocean by other means.
“The ocean plays a key role in the global carbon cycle, with tiny, microscopic plants taking up billions of tons of carbon from the atmosphere every year,” explains research lead Dr. Sari Giering, Research Lead at NOC. “For years, it has been believed that this group of plankton—diatoms—play a crucial role in efficiently transporting carbon to the deep ocean, where it is held out of contact with the atmosphere.
“The surprising discovery that diatoms’ silica skeletons stay near the surface while carbon makes it down to the deep ocean forces us to rethink the ecological processes in what we call the biological carbon pump.”
The biological carbon pump describes a collection of processes in which plankton take up carbon in surface waters and shunt this carbon to the deep ocean. These natural processes store billions of tonnes of carbon in the ocean each year.
“Previous studies have looked at what has ended up at the seabed, which shows carbon is making its way there typically with the aid of ballast material, such as diatom’s silica-based skeletons,” adds Dr. Giering.
“But our research, looking at what happens within the twilight zone before carbon reaches the seabed, shows that diatoms are at times not contributing as heavily to the Southern Ocean’s carbon pump as had been thought. This means there are unknown or poorly measured processes happening in the deep ocean that we need to learn more about.”
A concern has been that ocean warming could impact diatom productivity and, therefore, reduce the strength of the biological carbon pump in the Southern Ocean.
“The Southern Ocean is vulnerable to ocean warming, which may alter the availability of nutrients and reduce diatom numbers in the future,” says lead author Jack Williams, a post-graduate researcher at the University of Southampton. “But our results suggest these changes may not impact the strength of Southern Ocean carbon storage as much as previously thought.
“On the other hand, carbon is still making its way to the deep, so there are unresolved processes at play in the Twilight Zone that we need to learn more about. Understanding these processes and how they govern carbon uptake in this hugely important part of the ocean is crucial for accurately predicting how the oceans may store carbon in the future.”
The research was conducted as part of two major NOC-led initiatives, Controls over Ocean Mesopelagic Interior Carbon Storage (COMICS) and Carbon Uptake and Seasonal Traits in Antarctic Remineralization Depth (CUSTARD). These projects were funded by the UK’s Natural Environment Research Council (NERC) and the
European Research Council grants Advancing Novel imaging Technologies and data analyses in order to understand Interior Ocean Carbon Storage (ANTICS).
Over two expeditions, each lasting more than five weeks at sea, NOC scientists and international colleagues studied the twilight zone at four different sites in the Atlantic and Pacific sectors of the Southern Ocean.
This included iron-rich waters around a remote chain of islands and nutrient-starved waters in the open ocean. The collaborative team used a combination of innovative techniques, including ship-based measurements, mooring arrays, and autonomous underwater technologies.
Read the full paper here.
