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Long-term data prompts rethink on regional differences in ocean carbon sequestration

A new publication by researchers from the Department of Earth Sciences at the University of Oxford shows that the relationship between water temperature and the main biological mechanism by which the ocean captures atmospheric carbon dioxide (CO₂) is far more complicated than previously thought.

Drawing on long-term time-series data from oceanographic stations such as the Bermuda Atlantic Time-Series, the research highlights how the quality of currently available data limits our understanding of this critical mechanism in the carbon cycle.

The biological carbon pump (BCP) is a critical part of Earth’s carbon cycle, removing CO₂—the main greenhouse gas responsible for global warming—from the atmosphere and locking it away in the deep ocean. This occurs when microscopic organisms called phytoplankton take in CO₂ during photosynthesis or by creating calcium carbonate shells, then die and sink through the water column, carrying the CO₂ with them.  

Known as “marine snow”, this shower of biogenic particulate material transfers an estimated 10 billion tonnes of carbon every year into the ocean's interior, roughly the same amount as that emitted annually by fossil fuel burning. As a result, it plays an important role in regulating the amount of CO₂ in the atmosphere and hence Earth’s climate.

Up to now, it has been presumed by ocean scientists that water temperature is the main factor determining how efficient the BCP transfers atmospheric carbon to the deep ocean, and therefore that it would vary geographically with latitude. However, the results from the new study put this into question.

The researchers used long-term, good-quality data from six locations across the globe, enabling them to account for seasonal variations. This was combined with data from a range of national and international research projects, using a variety of marine particle sampling methods –including sediment traps, radiometric techniques, and underwater cameras.

Using this amalgamation of data, the research team were unable to definitively conclude that variations in the transfer efficiency of the BCP over the global ocean are driven by temperature. This is largely because variations in the data –stemming from differences in experimental methods or natural fluctuations (just as the atmosphere, the ocean also has “weather”)– masked any potential trends, raising the question of whether such trends exist at all.

Lead researcher Dr Anna Rufas (Department of Earth Sciences, University of Oxford) said: “Potentially the absence of clear patterns can be explained by the differences between methods across research projects, principally because there are no standard method protocols. Therefore, there are large differences between the approaches of research groups studying this problem.”

This could be addressed by protocol standardisation for the collection of marine particle data. The researchers also call for improved data collection in traditionally undersampled areas, such as polar regions in the winter, which are crucial for ocean carbon sequestration.

Co-author Professor Samar Khatiwala commented: “Anyone who has been on a ship knows that the ocean is a pretty ‘noisy’ place. That makes identifying patterns incredibly challenging, even more so for complex processes such as the BCP, where the data are spotty at best and often collected using very different measurement techniques. A relationship may well exist in nature but this careful analysis makes us question some long-held assumptions.”

Co-author Professor Heather Bouman adds: “The BCP provides an essential ecosystem service, naturally removing excess CO₂ from the atmosphere and regulating the temperature of our planet. In a world where CO₂ removal techniques are becoming increasingly important, understanding this fundamental ocean process is vital for recognising its potential contributions in the coming decades.”

Notes:

For media enquiries and interview requests, contact Dr Anna Rufas anna.rufasblanco@earth.ox.ac.uk

The study ‘Can We Constrain Geographical Variability in the Biological Carbon Pump's Transfer Efficiency From Observations?’ is available to read in Geophysical Research Letters at https://doi.org/10.1029/2024GL111203.

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