Carbon storage study provides fresh mystery for ocean scientists
A study led by the UK National Oceanographic Centre has turned all we once thought we knew about the ocean carbon storage mechanism on its head, providing new insight into the role that micro algae known as diatoms play in the process of drawing carbon to the deep sea.
New research from the UK National Oceanography Centre (NOC) has left experts questioning almost everything they thought they knew about the intricacies of carbon storage in the ocean, putting stock back into the old axiom that the more you know, the less you understand.
The new study has, more specifically, placed a major question mark over the role a unique group of tiny microscopic plants – a type of plankton or marine algae, called diatom – plays in driving the ocean’s carbon storage cycle – a process better known as the biological carbon pump.
These diatoms 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, these diatoms have a dense, silica-based exoskeleton – described as being like a miniature glass house – which were previously thought to give them the weight, or ballast, that makes them prone to sinking and, therefore, a key way carbon is transported to the deep ocean.
However, findings from this most recently – based on data captured from two major expeditions to the Southern Ocean’s under-explored ‘twilight zone’ (the region between around 100 metres and 1,000 metres deep) found that the diatom skeletons actually lingered nearer the sea surface, while carbon made its way to the deep ocean by other means.
And these are findings that may just be about to turn everything researchers once thought they knew about the ocean’s carbon storage process on its head.
“The ocean plays a key role in the global carbon cycle, with tiny, microscopic plants taking up billions of tonnes of carbon from the atmosphere every year,” said NOC’s research lead, Dr Sari Giering.
“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 process 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,” continued 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.”
What this means, Dr Giering has underscored, is that there are “unknown” or “poorly measured” processes happening in the deep ocean “that we need to learn more about.”
A concern among climate scientists has been that ocean warming could impact diatom productivity and, therefore, reduce the strength of the biological carbon pump in the Southern Ocean.
The results, however, suggest that these changes “may not impact the strength of the Southern Ocean carbon storage as much as previously thought,” said Jack Williams, a post-graduate researcher at the University of Southampton, and lead author on the study.
“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,” he added.
“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.”
Over the course of the 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 full paper has been published in Nature Geoscience.

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