How Does Carbon Get Into the Ocean?

Carbon reaches the ocean through several distinct pathways, but the biggest by far is direct absorption of carbon dioxide from the atmosphere at the sea surface. The ocean currently takes up roughly 2.6 billion metric tons of human-produced CO₂ every year, accounting for about 30% of our total emissions. That gas exchange is only part of the story, though. Rivers carry carbon washed off the land, living organisms shuttle it from surface waters to the deep, volcanic vents inject it from below the seafloor, and even groundwater seeping through coastal sediments delivers a surprising amount. Each pathway operates on different timescales and moves carbon to different depths, which determines how long that carbon stays out of the atmosphere.

Absorbing CO₂ Across the Sea Surface

The single largest route for carbon into the ocean is gas exchange at the air-sea interface. When the concentration of CO₂ in the atmosphere is higher than in the surface water, the gas dissolves into the ocean. Wind and waves are the main drivers of how fast this transfer happens. Stronger winds churn the surface, thin the boundary layer between air and water, and create breaking waves that trap bubbles, all of which speed up gas absorption. Research on the physics of this transfer shows that wave-related processes at the surface layer are the dominant control on CO₂ transfer velocity, even though scientists often use wind speed as a convenient shorthand for those processes.¹

An underappreciated wrinkle is that the ocean surface is often coated with a thin film of organic compounds, sometimes called the sea-surface microlayer. These natural surfactants can slow down gas exchange, especially in calm conditions when the film sits undisturbed. At higher wind speeds, turbulence and breaking waves disperse the film faster than it can rebuild, so the suppression effect fades.² The practical upshot is that gas transfer rates vary a lot from place to place and season to season, depending on local wind patterns, wave conditions, and biology at the surface.

Temperature also matters. Cold water holds more dissolved gas than warm water, so high-latitude oceans in the North Atlantic and around Antarctica are especially efficient carbon sinks. Warm tropical waters, by contrast, tend to hold less CO₂ and can even release it back to the atmosphere. This temperature-driven gradient helps explain why some of the most important regions for ocean carbon uptake are in the cold, windy parts of the world.

What Happens Once CO₂ Dissolves

When CO₂ enters seawater, it does not just float around as dissolved gas. It reacts with water to form carbonic acid, which quickly splits into bicarbonate and carbonate ions. The vast majority of the carbon in the ocean exists as bicarbonate, not as dissolved CO₂ gas. This chemical conversion is what gives the ocean such an enormous capacity to absorb carbon: each molecule of CO₂ that dissolves triggers a chain of reactions that effectively makes room for more. The ocean’s buffering capacity, sometimes described through a parameter called the Revelle factor, quantifies how sensitive surface CO₂ levels are to changes in the total amount of dissolved carbon.³ As the ocean absorbs more CO₂ and its chemistry shifts, that buffering weakens, meaning each additional ton of CO₂ becomes a little harder for the ocean to soak up. This is one of the key concerns about future climate change.

Rivers and Groundwater

Not all ocean carbon arrives from the sky. Rivers carry organic carbon from soils, wetlands, and decaying vegetation on land into coastal waters and eventually the open ocean. A recent global assessment estimated that rivers deliver roughly 420 million metric tons of organic carbon to the ocean each year, split almost evenly between particles suspended in the water and carbon dissolved in it.⁴ That may sound modest next to the billions of tons absorbed from the atmosphere, but it is a critical input for coastal ecosystems. Much of this riverine carbon gets processed in estuaries, where microbes break it down and release CO₂, while the rest makes it out to deeper waters.

Less obvious but locally significant is submarine groundwater discharge, where water percolating through coastal rock and sediment seeps directly into the sea below the surface. This groundwater carries dissolved carbon with it. In some coastal bays, the carbon delivered by groundwater can dwarf what nearby rivers contribute. One study in a semi-enclosed bay found that the dissolved inorganic carbon flux from groundwater was 19 to 39 times larger than the riverine input, making up more than a quarter of the bay’s total carbon supply.⁵ Globally, tracer studies confirm that submarine groundwater discharge provides important fluxes of carbon, nutrients, and metals to coastal waters.⁶

The Biological Pump

Life in the ocean acts as a conveyor belt for carbon. Tiny photosynthetic organisms called phytoplankton live in the sunlit surface layer and, like plants on land, use sunlight to convert dissolved CO₂ into organic matter. When these organisms die or are eaten, their carbon enters the marine food web. Some of that carbon gets respired back into CO₂ near the surface, but a fraction sinks toward the deep ocean as dead cells, fecal pellets, and other organic debris. This downward shuttle of carbon is called the biological carbon pump, and it is one of the main reasons the deep ocean holds so much more carbon than the surface.⁷

Much of this sinking material clumps together into aggregates known as marine snow, fluffy particles that drift slowly downward through the water column. Marine snow mediates the global carbon cycle by carrying carbon to the deep sea, where it can remain for centuries or longer, and simultaneously feeds the organisms that live in the dark ocean below.⁸ How much carbon actually reaches the deep seafloor depends partly on what the particles are made of. Mineral ballast like calcium carbonate shells and silica from diatoms can weigh down aggregates, helping them sink faster and resist decomposition along the way.⁹

Here is an interesting wrinkle with those calcium carbonate shells. When organisms like coccolithophores build shells out of calcium carbonate, the chemical reaction actually releases CO₂ back into the water. So the “carbonate pump” partly counteracts the organic carbon pump by raising surface CO₂ levels. At the same time, the dense mineral shells help organic particles sink faster, potentially increasing the amount of carbon that reaches the deep. Scientists are still working out the net balance between these two competing effects.¹⁰

Zooplankton and the Active Transport

The biological pump is not only about passive sinking. Huge numbers of zooplankton, small animals ranging from copepods to krill, perform a daily vertical migration that is one of the largest synchronized movements of biomass on Earth. They feed near the surface at night and retreat to deeper waters during the day to avoid predators. While at depth, they respire CO₂, excrete dissolved organic matter, and produce fecal pellets, all of which deposit carbon hundreds of meters below where it was originally fixed by photosynthesis.¹¹ Measurements in the Sargasso Sea found that dissolved organic carbon excretion accounted for about a quarter of the total carbon these migrators metabolized at depth, adding a previously underappreciated boost to the downward carbon flux.¹²

The Microbial Carbon Pump

Bacteria and other microbes do something different from the biological pump’s sinking-particle pathway. As they process organic matter in the water, they transform easily degradable carbon compounds into forms that are chemically resistant to further breakdown. These recalcitrant molecules can persist in seawater for thousands of years, representing a huge, slow-turnover carbon reservoir. This concept, known as the microbial carbon pump, highlights that carbon storage in the ocean is not only about moving material to the deep seafloor; it also involves converting carbon into forms that resist decomposition regardless of depth.¹³

Deep Circulation and Long-Term Storage

Getting carbon from the surface into the deep ocean is the bottleneck that determines how much the ocean can store long-term. The physical process that does this most efficiently is deep water formation: in a few key regions, cold, dense surface water sinks to great depth, carrying its dissolved carbon with it. The North Atlantic and the Southern Ocean around Antarctica are the primary sites where this happens. Once carbon-laden water descends into the ocean interior, it can circulate for centuries before returning to the surface. The rate at which this overturning circulation transports carbon from the mixed layer into the interior is the rate-limiting step for how fast the ocean fills up with human-produced CO₂.¹⁴

Changes in this circulation have big consequences. In the Southern Ocean, shifting wind patterns are altering the overturning circulation, with winds intensifying and pushing poleward. Eddies partially compensate for these changes, but the net result is a reorganization of how carbon moves between the surface and the deep in one of the ocean’s most important carbon sink regions.¹⁵

When the Ocean Gives Carbon Back

The ocean is not a one-way carbon sink. In regions where deep, carbon-rich water rises to the surface through upwelling, the ocean can become a source of CO₂ to the atmosphere. Today, poorly ventilated North Pacific waters supply much of the carbon and nutrients that upwell in the Southern Ocean, and the biological uptake at the surface cannot keep pace, fueling CO₂ outgassing.¹⁶ This tug-of-war between absorption and outgassing means that the net amount of carbon the ocean takes up is the balance between regions that are sinks and regions that are sources. Tropical upwelling zones, parts of the Southern Ocean, and certain equatorial regions tend to release CO₂, while cold, high-latitude waters and biologically productive areas tend to absorb it.

Volcanic Vents on the Seafloor

A small but geologically fascinating source of ocean carbon comes from below. Hydrothermal vents along mid-ocean ridges and volcanic arcs inject CO₂ directly into deep ocean water. Vent fluids are highly enriched in CO₂ and acidic compared to normal seawater.¹⁷ One especially striking example is a site called Champagne on NW Eifuku volcano in the Mariana Arc, where researchers discovered both hot hydrothermal fluid and cold droplets of liquid CO₂ venting at about 1,600 meters depth. The hot fluid contained up to 2.7 moles per kilogram of CO₂, the highest concentration ever reported for submarine hydrothermal fluids.¹⁸ Globally, hydrothermal carbon inputs are small compared to the atmospheric exchange, but they have been a continuous source over geological time and influence the chemistry of the deep ocean around vent sites.

Sea Ice as a Carbon Pump

In polar regions, the seasonal cycle of sea ice formation and melting creates its own carbon pump. When seawater freezes, salts and dissolved carbon are rejected from the growing ice into the underlying water, increasing its density. This brine rejection contributes to the formation of deep and intermediate water masses. Research in the Arctic has shown that dissolved inorganic carbon is rejected along with brine and that the low temperatures alter the chemistry in ways that lower the CO₂ concentration of the surface water left behind. The net effect is that sea ice formation can draw down surface CO₂ and help sequester carbon in the deep ocean.¹⁹ As Arctic sea ice declines with warming, this mechanism may weaken, though predicting the net effect is complicated by simultaneously increasing areas of open water that can absorb CO₂ directly from the atmosphere.

Blue Carbon Along the Coasts

Coastal ecosystems like mangrove forests and seagrass meadows are sometimes called blue carbon habitats because they capture and bury organic carbon in their sediments at high rates. Mangroves, in particular, trap organic matter in waterlogged, low-oxygen soils where decomposition is slow, locking carbon away for decades to centuries.²⁰ Restored mangrove stands in Mauritius, for example, showed clear increases in carbon stocks with age, with older stands holding nearly double the organic carbon of recently planted ones and contributing net gains of about 14 metric tons of carbon per hectare over the restoration period.²¹ These coastal habitats occupy a tiny fraction of the ocean’s total area, but their per-area carbon burial rates far exceed those of the open ocean floor. Protecting and restoring them is one of the more cost-effective climate mitigation strategies available, though it is worth noting that mangrove sediments also release methane, which partially offsets their carbon burial benefit.

Burial in Deep-Sea Sediments

The ultimate long-term fate for some ocean carbon is burial in seafloor sediments, where it can be locked away for millions of years. Most organic carbon that sinks through the water column gets consumed or decomposed before it reaches the bottom, but a small fraction survives and accumulates in sediments. Over geological timescales, this burial is what has kept Earth’s atmosphere from accumulating runaway CO₂. A compilation of hundreds of sediment cores showed that organic carbon burial in the deep sea was consistently about 50% higher during ice ages than during warmer interglacial periods, driven by changes in ocean circulation and biological productivity.²²

The Modern Anthropogenic Carbon Sink

Since the Industrial Revolution, the ocean has been absorbing a substantial share of the extra CO₂ humans have added to the atmosphere. Between 1994 and 2007, the ocean took up about 34 billion metric tons of anthropogenic carbon, averaging roughly 2.6 billion metric tons per year and capturing about 31% of human emissions during that period.²³ Independent modeling work has arrived at consistent numbers, estimating the total accumulated human-produced carbon stored in the ocean at 160 to 166 billion metric tons by 2012.²⁴ This service comes at a cost. As the ocean absorbs more CO₂, its pH drops, a process called ocean acidification. The same buffering chemistry described earlier means the ocean’s ability to keep absorbing carbon at the same rate is gradually declining, which could shrink this natural carbon sink in the future.

Engineered Approaches to Speed Things Up

Given the ocean’s enormous natural capacity to hold carbon, researchers are exploring ways to enhance that uptake deliberately. Two approaches that receive the most attention are ocean iron fertilization and ocean alkalinity enhancement. Iron fertilization aims to boost phytoplankton growth in iron-limited regions, increasing the biological pump. Alkalinity enhancement involves adding minerals to seawater to increase its chemical capacity to absorb CO₂, essentially reversing some of the buffering decline caused by acidification.²⁵ Some newer technologies attempt to combine both strategies in a single system, using electrochemical processes to release iron and increase alkalinity simultaneously while also producing hydrogen gas.²⁶ These marine carbon dioxide removal techniques remain largely experimental. The challenges are not only technical but also ecological: fertilizing one part of the ocean can starve downstream regions of nutrients, and adding alkalinity at scale raises questions about effects on marine life that are still poorly understood.