The depth misconception: why carbon sequestration in water isn’t about depth

With climate action a pressing concern, carbon removal strategies are under scrutiny to determine which methods offer truly permanent sequestration. 

When it comes to water, one common assumption is that carbon permanence in aquatic environments requires depths of 1,000 meters or more. In other words, if carbon isn't sinking into the abyss, it won’t stay stored. However, quite a lot of scientific evidence is challenging this idea, demonstrating that shallow waters, too, can support long-term carbon storage. A key study, Persistent Organic Matter in Oxic Subseafloor Sediment, highlights how long term carbon sequestration is driven by a complex set of factors, not just depth. In fact, while depth plays a role in ‘improving’ some of these factors that contribute to carbon retention, it’s not a determining factor in and of itself. In some ways, shallow waters may even offer advantages over deeper, more remote waters.

But let’s start with a fundamental question: does deep always mean permanent? The answer is clearly no. A couple of years ago, a group of researchers in Louisiana dropped an alligator’s carcass into a 2,000 meters deep abyss in the Gulf of  Mexico accompanied by cameras to see their fate. For those unfamiliar with the gruesome footage, the depths are teeming with life. Within days, the entire carcass, bones included, was consumed.  Permanent carbon removal in water is the result of multiple complex variables and depth alone is not one of them. 

In their study, Durbin et al. studied organic carbon preserved for extensive periods in relatively shallow subseafloor sediments, even when oxygen was present—a surprising result, given the widely held belief that oxygen accelerates carbon breakdown. The presence of carbon in these oxic, shallow sediments underscore that organic matter's longevity is influenced by a range of factors beyond oxygen levels or depth alone. These findings shift our understanding and challenge the notion that deeper necessarily equals more permanent.

This misconception about depth and permanence largely stems from the assumption that deeper waters offer greater stability due to limited biological activity, lower oxygen levels, and reduced exposure to disturbance. These conditions may sometimes be favorable for preserving carbon, but they aren’t exclusive to deep water. Organic carbon stability depends on factors such as sediment type, microbial activity, and chemical composition of the surrounding environment. As Durbin et al.’s research demonstrates, shallow sediment can also support long-term carbon storage under the right conditions – to the tune of 24 million years!

One challenge with deep water carbon storage is that carbon must travel vast distances to reach great depths, increasing the likelihood of degradation along the way. Organic matter formed in the photic zone, near the ocean surface, particularly from photosynthetic organisms, faces a longer journey to deeper waters where it’s more exposed to microbes, oxygen or marine animals that can break it down or consume it. 

Studies indicate that only 1% (!) of all organic matter generated in the photic zone will find its way to the sediment, if it is 1,000 meters below. In shallow waters, however, carbon reaches the sediment much faster, reducing exposure to elements that might otherwise degrade it. In addition, shallow waters are far more productive and can support dense populations of photosynthetic organisms like phytoplankton, leading to higher rates of carbon capture through natural processes. The open ocean is a ‘desert’, with minimal biological productivity, further reducing its potential as a carbon sink. As a result, shallow regions can potentially deliver greater amounts of organic carbon to the sediment—an advantage that is often overlooked.

The persistence of carbon in sediment depends, for example, on the interaction between organic matter and sediment minerals, which can ‘shield’ organic carbon from microbial decomposition. In other instances, algal blooms generate vast, complex structures of polysaccharides that are hard to degrade. Finally, the sheer volume of biomass that can ‘rain’ in shallow water but will never develop, let alone reach the depths of the ocean, serves a key component in biochemical fixation processes that will delay or withhold biodegradation over very long timescales. These different biological, chemical and physical interactions can occur at any depth where suitable conditions exist, meaning that, under the right circumstances, carbon can be removed, literally – forever.

Rather than depth, a mix of biological, chemical, and physical parameters collectively influence carbon stability in the aquatic sediment. Depth may aid in reducing some factors that lead to carbon degradation, but it is not a prerequisite for carbon permanence. Shallow waters can offer significant advantages in carbon volume and reduced travel distances, facilitating conditions that can be just as effective for long-term carbon storage, if not superior, to those found at great depths. 

Recognizing the ability of water to store carbon is crucial to fighting climate change, yet relying on incorrect assumptions could hinder our progress in addressing this challenge. By dispelling this depth misconception, we can begin to approach aquatic carbon sequestration more holistically, recognizing the critical role that diverse ecosystems play in the global carbon cycle and the potential they hold for climate solutions. The good news is that water can be a great medium for carbon removal and climate change reversal in ways that go beyond previous hypotheses. As the largest natural regulator of carbon in the atmosphere, water, whether in shallow or deeper systems, offers scalable, economic, and environmentally sound solutions to combat climate change.

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