Ocean Acidification

A story of sacrifice in the deep sea

Article: Sulpis O, Agrawal P, Wolthers M, Munhoven G, Walker M, Middelburg JJ (2022) Aragonite dissolution protects calcite at the seafloor. Nat Commun 13:1–8. https://doi.org/10.1038/s41467-022-28711-z

More than just a pretty face

When you pick up a seashell on the beach or dive into a pile of baked clams to eat, these shells all maintain their strength, shape, and structure through a mineral called calcium carbonate, CaCO3. While it might seem unimportant to us humans on land, this CaCO3 plays an important role in regulating the chemistry of the Earth’s oceans and, in effect, our global climate. When CaCO3 dissolves in seawater, it represents the ultimate natural sink for anthropogenic (human produced) carbon dioxide, CO2, because CO2 is effectively neutralized by its reaction with carbonate (CO32-) to produce bicarbonate (HCO3-), which is not acidic.

A high-resolution image of the types of shells found in deep sea sediment (Image:
Alfred Wegener Institute, Wikimedia Commons).

Many organisms in the ocean are composed of CaCO3, including corals, molluscs, and microscopic plankton, the passive drifters in the sea. When these organisms die, their shells sink and accumulate in the ocean sediment. For those shelled animals that live in the open ocean, their shells can sink for miles down to the deep sea, where they are preserved in the sediment for centuries to millennia.

But not all CaCO3 is created equal. There are two different forms of CaCO3, calcite and aragonite, that dissolve differently based on their crystalline structures. Aragonite is considered the more soluble, or more easily dissolvable, form of CaCO3 because its crystalline structure is not as stable compared to calcite’s. In effect, more calcite is preserved in deep-sea sediment compared to aragonite. Therefore, it was thought that animals that formed calcite were more important in removing CO2 than animals that formed aragonite.

True sacrifice

A pteropod, open ocean sea snails, whose shell, made of aragonite, is beginning to dissolve (Image: NOAA).

This year, a study by a group of European scientists found that animals that form aragonite do, indeed, have a crucial role in the deep sea by essentially sacrificing themselves to create a more habitable environment for the calcite shells to be preserved. In the open ocean, these animals that dominate aragonite production include shelled pteropods and heteropods, which are abundant free swimming sea snails. When the aragonite (CaCO3) dissolves, it releases calcium and carbonate ions, which creates a halo of calcite supersaturation around the nearby calcite shells. The scientists named this process the galvanization of carbonate because the dissolved aragonite provides a protective coating to preserve calcite, just like when zinc is applied around steel or iron to prevent it from rusting and corroding.

Since it is difficult to study processes like ‘galvanization’ miles below in the deep ocean, the scientists built a 3D model to visualize the different mechanisms at play. They also utilized high-resolution 3D X-ray imagery of shells composed of calcite and aragonite so that they could better quantify dissolution rates among different CaCO3 shells.

The scientists concluded that the proposed aragonite ‘galvanization’ could act as another, previously undescribed, negative feedback mechanism regulating the Earth’s climate. However, pteropods, the aragonite producers, are particularly vulnerable to ocean acidification, which decreases the formation of aragonite shells and promotes its dissolution. Therefore, a reduced aragonite transfer to the deep ocean caused by ocean acidification may weaken the proposed galvanizing action, reducing the buffering capacity of the deep-sea sediment and decreasing the natural sink for anthropogenic CO2.

Only the future will tell, but the scientists stress there are still very few measurements of CaCO3 dissolution throughout the global ocean, particularly when considering several carbonate minerals simultaneously (e.g., aragonite and calcite), so future research is needed to focus on future model-, field- and laboratory-based studies about marine CaCO3 dynamics. By studying these dynamics, we can further unlock the role these pretty seashells play in preserving Earth’s ocean chemistry and climate.

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