Ocean Uptake: How Marine Systems Absorb Carbon Dioxide

The ocean plays a crucial role in regulating Earth’s climate by absorbing large amounts of carbon dioxide (CO2) from the atmosphere. This helps mitigate human-induced emissions but also alters seawater chemistry, impacting marine life and ecosystems.

Understanding how CO2 moves through marine systems is essential for predicting climate changes and assessing risks to biodiversity. Scientists examine physical, chemical, and biological processes to determine how effectively the ocean can continue acting as a carbon sink.

CO2 Dissolution Chemistry

When CO2 enters the ocean, it dissolves at the air-sea interface, driven by the partial pressure gradient between the atmosphere and seawater. This exchange is influenced by wind speed, wave turbulence, and temperature. Once dissolved, CO2 reacts with water to form carbonic acid (H₂CO₃), which quickly dissociates into bicarbonate (HCO₃⁻) and hydrogen (H⁺) ions. This process contributes to ocean acidification, altering seawater chemistry.

Bicarbonate can further dissociate into carbonate ions (CO₃²⁻) and additional hydrogen ions, though this occurs less frequently under typical oceanic pH conditions. The interplay between these chemical species forms the carbonate system, which helps buffer pH changes in seawater.

Henry’s Law states that gas solubility in a liquid is proportional to its atmospheric partial pressure. As CO2 levels rise, more dissolves in seawater, shifting carbonate system equilibrium. However, excessive absorption weakens seawater’s buffering capacity, reducing its ability to neutralize acidity. This decline in carbonate availability affects marine organisms that rely on calcium carbonate structures.

Carbonate Equilibrium

Dissolved CO2 establishes a balance between carbonic acid, bicarbonate, and carbonate ions, regulating ocean pH and carbonate availability. Seawater pH typically ranges from 7.8 to 8.3, where bicarbonate dominates while carbonate ions are less abundant. Even small shifts in acidity can disrupt this equilibrium.

Seawater’s buffering capacity relies on the reversible nature of these reactions. When excess hydrogen ions enter the system, bicarbonate absorbs them, forming carbonic acid, which then dissociates into CO2 and water. This helps stabilize pH, but increased CO2 absorption shifts equilibrium toward bicarbonate formation, reducing carbonate ion availability. Organisms relying on calcium carbonate structures, such as corals and shellfish, are particularly affected.

Calcium carbonate solubility depends on carbonate equilibrium, with saturation states determining whether it dissolves or precipitates. Aragonite and calcite, two primary forms, have different solubility properties, with aragonite being more vulnerable to dissolution. In colder, deeper waters, naturally higher CO2 levels further reduce carbonate ion availability, weakening marine ecosystems.

Role of Marine Organisms

Marine organisms play a crucial role in carbon cycling. Phytoplankton absorb dissolved CO2 through photosynthesis, converting it into biomass that supports marine food webs. Some of this carbon is respired back into seawater, while a portion sinks as particulate organic matter, a process known as the biological carbon pump. As this material descends, microbial decomposition releases CO2 into deeper waters, where it can remain for centuries.

Calcium carbonate-producing organisms, such as corals, mollusks, and plankton, extract carbonate and calcium ions from seawater to build shells and skeletons. Some of these structures become part of marine sediments, contributing to long-term carbon storage. However, under changing chemical conditions, these structures can dissolve, reintroducing carbon into the water column. The balance between calcification and dissolution affects carbon sequestration in the ocean.

Temperature and Salinity Dynamics

Temperature and salinity significantly influence CO2 absorption and retention. Warmer waters hold less dissolved CO2, as gas solubility decreases with rising temperature. This weakens the ocean’s role as a carbon sink, especially in tropical and subtropical regions. In contrast, colder waters enhance CO2 solubility, making polar and subpolar regions major carbon reservoirs.

Salinity affects water density and circulation, which in turn influence CO2 distribution. High-salinity regions, often found in areas of high evaporation, have denser water that facilitates deep mixing and carbon transport. Conversely, freshwater influx from melting ice caps and heavy rainfall reduces salinity, creating stratified layers that limit vertical mixing. This traps CO2 near the surface, preventing its movement into deeper waters for long-term storage.

Physical Circulation Processes

Ocean currents, driven by wind, temperature gradients, and Earth’s rotation, redistribute CO2-rich waters across depths and latitudes. This movement affects both short-term atmospheric exchange and long-term deep-ocean sequestration. Thermohaline circulation, or the global conveyor belt, transports surface waters to the deep ocean, locking away absorbed CO2 for centuries. Cooling and sinking waters in the North Atlantic and Southern Ocean serve as key carbon entry points.

Upwelling and downwelling also regulate dissolved carbon distribution. Upwelling brings CO2-rich deep waters to the surface, temporarily increasing atmospheric carbon levels in certain regions, such as the coasts of Peru and California. Downwelling, on the other hand, transports surface waters—and their CO2—into the ocean’s interior, aiding long-term storage. Climate-driven shifts in circulation patterns could alter the ocean’s carbon sink capacity. A slowdown in deep-water formation would reduce CO2 sequestration efficiency, increasing atmospheric concentrations and accelerating global warming.

Sediment Exchanges

Carbon also interacts with marine sediments, where long-term storage occurs. Organic material and calcium carbonate structures sinking to the seafloor become part of sediment layers, removing carbon from circulation. In deep-sea environments with slow decomposition rates, carbon can remain buried for millions of years, forming carbonate rock deposits that stabilize atmospheric CO2 over geological timescales.

However, stored carbon can re-enter the ocean under certain conditions. In low-oxygen areas, benthic microbes break down organic matter, releasing CO2 and methane. This process is particularly relevant in coastal zones with high biological productivity. Human activities, such as bottom trawling and deep-sea mining, can also disturb sediments, resuspending stored carbon. Additionally, ocean acidification may accelerate calcium carbonate sediment dissolution, further complicating carbon cycle predictions.