Sea butterflies, the unsung heroes in the climate crisis

In the open ocean, far from the coast, live very special creatures called sea butterflies. These tiny animals carry a shell that the sea butterflies make from the mineral aragonite. The shell is very sensitive to climate change. As CO2 in the atmosphere increases, it causes acidification of the oceans. The sea butterfly's shell has to take the first hit. Because of this acidification, aragonite, and thus the shell, dissolves more easily and it takes more energy to build up the shell. Calcite is another mineral used by many other organisms, like mussels and oysters, to build their shells, and also dissolves more quickly due to acidification. Researchers at Utrecht University have been studying how exactly the dissolution process of these two minerals takes place on the seafloor, where all shells sink to when the animal has died.

Sea butterfly (Limacina helicina). Credit: Alexander Semenov / Science Photo Library.

The research was carried out by researchers from Utrecht University, Université de Liège, University of Lincoln and University of Leeds, and funded by the Dutch Ministry of Education via the Netherlands Earth System Science Centre (NESSC). Olivier Sulpis, first author of the paper, explains what he and his colleagues did in this study: "We were surprised to find calcite everywhere on the seafloor, but very little aragonite, although recent studies show that aragonite is produced in huge quantities by sea butterflies at the surface of the ocean. In this study, we have created a new computer model that simulates the seafloor and the chemical reactions that take place there. With this model for individual particles, we were able to show that aragonite dissolves a lot at the seafloor surface, before it can be buried in the marine sediments

Sea butterflies are seen as the canary in the coal mine

When aragonite dissolves, it partially cancels out the acidification of the ocean, this is called a buffering effect. This means that this mineral, and therefore the sea butterfly, plays a very important role in regulating the climate. "Sea butterflies are sometimes seen as the canary in the coal mine. They show how far climate change has already progressed," explains Sulpis. "In addition, the buffering effect of dissolving this mineral also has a protective effect for the calcite that manages to be preserved at the seafloor. This explains the fact that calcite grains, rather than sea butterfly aragonite shells, cover about a third of the seafloor and form the limestones that we see on land and that compose many ancient buildings. Due to acidification of the oceans, the sea butterfly makes thinner shells, and may decline in numbers. If the sea butterfly disappears completely, calcite would take over this buffer function, but this cannot continue indefinitely. That is why it is of great importance that we do something now to bring down the CO2 content in the atmosphere."


Aragonite dissolution protects calcite at the seafloor. Sulpis, O., Agrawal, P., Wolthers, M. et al. Aragonite dissolution protects calcite at the seafloor. Nat Commun 13, 1104 (2022).