Ocean density identified as a key driver of carbon capture by marine plankton

Locations of the core top samples and schematic representation of the surface currents, upwelling regions of subtropical gyres (in green) and major Atlantic Ocean fronts, many of which also operate at thermocline depths. Credit: Royal Society Open Science (2024). DOI: 10.1098/rsos.240179.




New findings, published in Royal Society Open Sciencehave shown that changes in the density of the oceans significantly affect the rate at which marine plankton absorb carbon into their shells. This has profound implications for the carbon cycle and the ocean’s ability to absorb CO from the atmosphere2 in response to climate change.


Until now, researchers have focused on how ocean chemistry and acidification affect the biomineralization of marine plankton. This study, led by Dr. Stergios Zarkogiannis, from the University of Oxford’s Department of Earth Sciences, is breaking new ground by highlighting the crucial role of physical ocean properties – particularly density – in influencing this process.


Foraminifera, abundant microscopic shell-bearing organisms, play a crucial role in the carbon cycle, due to their ability to store carbon dioxide in their calcium carbonate shells (a process called calcification). These sink to the ocean floor when they die, contributing to long-term carbon storage. Yet, the factors that cause calcification remain poorly understood.


This new study focused on Trilobatus trilobus, an abundant species of planktonic foraminifera. The findings show that this species is highly sensitive to changes in ocean density and salinity – and not just chemistry – and refines the calcification process in response. A major reason for this is that T. trilobus – like other planktonic foraminifera – cannot actively move itself and relies on buoyant forces (a function of ocean density) to maintain its position in the ocean. water column.


According to the new results, as the ocean’s density decreases (and its buoyancy forces decrease), T. trilobus reduces calcification to reduce its weight and prevent it from sinking. This ultimately makes the surface water more alkaline and increases its ability to absorb CO22.


The results have important implications for climate change. When ice caps melt, it introduces fresh water into the oceans, reducing ocean density. Reduced calcification in less dense waters, expected in a future ocean affected by ice sheet melting and freshening due to climate, could increase the ocean’s alkalinity and increase its ability to absorb CO2.


For short-term climate cycles there is an increased uptake of CO2 by the oceans would have a greater influence than the reduced uptake of carbon in planktonic foraminifera (which store carbon for longer cycles).


Dr. Stergios Zarkogiannis said: “Our findings show how planktonic foraminifera adapt their shell architecture to changes in seawater density. This natural adaptation, which potentially regulates atmospheric chemistry for millions of years, underlines the complex interplay between marine life and the global climate system.”


In the study, Dr. analyzed Zarkogiannis modern (late Holocene) fossil shells of T. trilobus collected from deep-sea sediment sites along the Mid-Atlantic Ridge in the central Atlantic Ocean.


Using advanced techniques such as X-ray microcomputed tomography (which rotates samples to capture thousands of X-ray images), reconstructing them in three dimensions to reveal hidden anatomical details, and the geochemistry of shell trace elements, the study linked calcification patterns to variations in the salinity. , density and carbonate chemistry.




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The results showed that the species produces thinner, lighter shells in equatorial waters and thicker, heavier shells in the denser subtropical regions.


According to Dr. Zarkogiannis’s study reformulates the story surrounding ocean calcification, showing that physical ocean changes, such as thickness and salinity play as important a role as chemical factors. These findings provide a critical view of how marine ecosystems are adapting to climate change.


Dr. Zarkogiannis added: “Although planktonic organisms can passively float in the water column, they are far from passive participants in the carbon cycle. By actively adjusting their calcification to control buoyancy and ensure survival, these organisms also regulate the ability of the ocean to absorb CO.2. This dual role underlines their key importance in understanding and addressing climate challenges.”


While this study reveals critical insights into how T. trilobus adjusts its calcification, more research is needed to determine whether buoyancy regulation influences calcification in other groups of organisms that contribute to the regulation of ocean and atmosphere chemistry, such as coccolithophores .


Furthermore, it remains unclear whether this is a universal process affecting all planktonic organisms, including those that form shells using silica or organic materials. Future studies by Dr. Zarkogiannis will investigate whether these principles apply to diverse groups and oceanic regions.



More information:

Calcification and ecological depth preferences of the planktonic foraminifer Trilobatus trilobus in the central Atlantic Ocean, Royal Society Open Science (2024). DOI: 10.1098/rsos.240179. royalsocietypublishing.org/doi/10.1098/rsos.240179




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