Storyboard: Ocean Acidification in the Southern Ocean

The ocean: a critical carbon sink

The ocean is one of the largest sinks of atmospheric CO₂, absorbing about 25% of anthropogenic carbon emissions. This uptake of carbon through the ocean-atmosphere interface impacts the chemical balance of the surface waters. CO₂ enters the ocean, dissolves and reacts with water, leading to higher hydrogen ion levels and making the water more acidic.

Ocean acidification (OA) not only causes a decrease in pH but also affects the saturation states of calcium carbonate minerals, particularly aragonite, by lowering the abundance of carbonate ions.

Impact on marine ecosystems

Many marine organisms, most notably corals but also lesser-known ones like foraminifera and coccolithophores, rely on calcium ions (Ca2+) and carbonate ions (CO₃2-) to form their calcium carbonate (CaCO₃) shells and skeletons. The reduced availability of these minerals makes it more difficult for them to build and maintain their structures. OA may cause some organisms to die, reproduce later or migrate to different areas. The decline of these organisms, which form the foundation of many ecosystems, threatens the stability of the marine food web.

Ocean acidification in the Southern Ocean

The Southern Ocean (SO) is recognised as a net sink of anthropogenic CO₂. In addition,  CO₂ solubility increases with colder temperatures and carbon concentrations increase due to upwelling of carbon-rich deep waters to the surface.  These factors make the SO particularly vulnerable to ocean acidification.

The acidification process primarily affects the surface and subsurface waters, which are the habitat for phytoplankton species forming the base of the marine food web, and many pelagic organisms.

Over the last decades, both models and observations have shown a decrease in pH in the SO. Coastal regions around Antarctica experience the lowest pH values, while large pH changes are observed along the Antarctic Circumpolar Current (ACC) with strong horizontal gradients across the frontal regions.

Aragonite saturation

There are several OA metrics, with pH being the most common one. It provides information about the hydrogen ion concentration in the ocean. While pH is an important parameter describing the seawater chemistry, it does not offer a complete picture of the effects of OA on marine ecosystems. A valuable metric for this purpose is the aragonite saturation (Ωₐ), defined as the ratio between the Ca2+ and CO₃2 concentrations and the saturation constant (Ksp):

Ω = [Ca²⁺][CO₃²⁻] / Kₛₚ

• If Ω > 1, then CaCO₃ precipitates
• If Ω = 1, then equilibrium
• If Ω < 1, then CaCO₃ dissolves

When Ωₐ drops below 1, the calcium carbonate used by corals and other marine organisms begins to dissolve, making it harder for these organisms to build and maintain their skeletons and shells. The climatology of the aragonite saturation horizon (depth at which Ωa=1) shows large spatial variability, with the deepest values along the ACC and south of the polar front. However, a shallowing of the aragonite saturation horizon by more than 200 m is observed across the entire SO..

Physics and the aragonite saturation

In PolarRES, we use the output from the high-resolution (0.1°) FOSI-HR model (Krumardt et al., 2024). Based on the model results, a ubiquitous decrease in Ωa in the water column can be observed. We show that undersaturation conditions have reached the upper layers of the SO as early as the 2010-2019 decade.

The largest changes in the aragonite saturation horizon are found along the ACC, where the fronts are key features influencing the exchanges between the ocean, the atmosphere, and the cryosphere. Therefore, impacting water mass characteristics and distribution. As part of the PolarRES project, we are investigating the links between these physical characteristics and the observed changes in the Ωar distribution in the water column.