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Mud is messy. For some, it’s a plaything. To many, it can mean real hardship. Mud, though, is often overlooked, particularly when it lies out of sight. Deep down at the bottom of the sea, it is one of the most important natural archives of Earth’s past – holding clues of shifting climates, coastlines, ocean conditions and carbon storage.

is the first to use computer models to trace how thick, carbon-rich mud patches on the seafloor have formed over thousands of years – helping to locate hidden carbon stores and understand the seafloor’s long-term role in the climate system. This mud is a carbon time capsule.

Vast amounts of organic carbon settle on the seafloor each year, coming from decaying marine life and from living (or once living) material washed in from land. When stored in the marine environment, this can stay locked away for centuries or millennia.

Marine sediments are the planet’s , particularly in huge underwater mud patches, making them a vital component of the global carbon cycle. because organic matter sticks to the tiny silt and clay particles and gets deposited in dense sediment, protecting it from oxygen – attributes that larger particles such as sand don’t have.

But no two mud patches are the same. Each holds a unique story about when it formed, how it got there, and how much carbon it stores. Across the world’s , scientists still don’t know where all these muddy deposits are or how extensive they might be. Our research shows that computer modelling past ocean conditions can help predict the location and age of carbon-rich mud – all without getting our toes wet.

This is a new way to bring old mud into the blue carbon conversation.

We examined three mud‐rich areas in the shallow north-west European shelf seas: the Fladen Ground, Celtic Deep and western Irish Sea mud belts. Using computer models of ocean tides over the past 21,000 years – back to the peak of the last ice age – we found that each of these mud patches formed at different times.

In the Celtic Deep and western Irish Sea mud belts, mud has accumulated over the past several thousands of years and continues to do so today, especially in the latter. In the Fladen Ground, deposits are ancient relics, preserved by calm tidal conditions since the muds were laid down between 17,000 and 5,000 years ago.

The beauty of this modelling approach is that it can be applied to other shelf seas too. While direct scientific data (as opposed to computer models) is best, sampling the seabed is costly and time-consuming, especially in remote places. That’s why the seafloor remains one of the .

Our work shows that models of past ocean conditions can help identify carbon-rich areas and guide more efficient sample collection. Mapping the size of muddy deposits and understanding the history of how and when they got there helps scientists to better consider how the seabed can store carbon and act as a buffer against climate change.

Scratching below the surface

Until now, assessments of blue carbon in offshore sediments have focused only on the surface of the seabed (typically the top 10cm). But that’s literally just scratching the surface.

A 2024 estimated that 244 million tonnes of organic carbon are stored long-term in the surface of the UK’s seabed, with over 98% of it in that thin upper layer of sediment (the rest in saltmarshes and seagrass meadows). However, carbon buried below 10cm is “largely unquantified and relatively old”, the report noted.

Ignoring that buried carbon risks underestimating the seafloor as a long-term carbon “sink” – an area that stores more carbon than it releases. The seafloor is a key player in climate regulation, as the carbon locked away in seabed sediments would otherwise contribute to atmospheric carbon dioxide.

Disturbing the stored carbon – through offshore trawling, dredging or construction – risks mobilising carbon that has been locked away for centuries or millennia. When seafloor mud is disturbed, the organic carbon it holds can be exposed to oxygen-rich seawater where microbes may break it down, . Some of this carbon dioxide dissolved in the seawater may then find its way .

That’s why we are digging deeper in the , a five-year global research programme exploring blue carbon – asking questions like: where is it, how and when did it get there, and where did it come from? Alongside the computer modelling, we’re studying the sediment record – using long tubes of mud extracted from the seafloor to measure how carbon storage has changed over time.

With atmospheric carbon dioxide levels at a , it’s vital to understand the risks of disturbing underwater carbon stores. Only then can we make smarter decisions about how to protect the ocean, and the carbon stored in its depths.

We often think about protecting the ocean in terms of its marine life. But these muddy sediments, quietly building up on the seafloor, are vital in the fight against climate change. As in his latest film, : “The ocean is our planet’s life-support system, and our greatest ally against climate catastrophe.”