Julie Meilland (MARUM, co-PI)
Olivier Sulpis (CEREGE, co-PI)
Sonia Chaabane (CEREGE, data management expert)
Deborah Wall-Palmer (Naturalis, member)
Katja Peijnenburg (Naturalis, member)
Fabio Benedetti (ETH Zurich, member)
Nina Bednarsek (Oregon State U., CIMERS, member)
Ralf Schiebel (Max Planck Institute for Chemistry, member)
Mark Ohman (Scripps, member)
Clara Manno (The British Antarctic Survey, member)

The ocean mitigates climate change by taking up a quarter of the annual CO2 emissions. Part of this carbon is stored in seawater in its dissolved form, and part of it is transformed into particulate matter, that eventually leaves the surface ocean and reaches the seafloor. Planktonic organisms secreting a calcium carbonate (CaCO3) shell occupy a key, ambivalent role in this scheme. First, the precipitation of their shell generates CO2, thereby reducing the ocean CO2 sink, while the sinking of their shell constitutes a direct export of C to the deep ocean. Meanwhile, the dissolution of their shells generates alkalinity, which in turn boosts the capacity of seawater to take up more CO2 from the atmosphere. CaCO3 is present in the ocean under two main mineral forms: calcite and aragonite. Because of their different arrangements of atoms, aragonite is much more soluble than calcite. The distribution, abundance and diversity of oceanic calcite producers (mostly foraminifera and coccolithophores) is relatively well understood and quantified, notably thanks to FORCIS, a recently-funded CESAB working group. Yet, we know surprisingly little about the sources of aragonite in the ocean. Aragonite is produced in today’s oceans by pteropods, heteropods and janthinids, groups of pelagic snails, but the magnitude of this production remains unquantified. For example, published estimates of the contribution of aragonite to global CaCO3 production in the modern ocean span a very wide range: from 1% to 98%. In addition, the spatial distribution of aragonite production is unknown, although it is thought that pteropods are more abundant at high-latitudes. This serious lack of knowledge stems from the inherent challenges of working with these highly seasonal, non-cultivable organisms and the great fragility and solubility of their shells. Because aragonite is rarely preserved in the sediment record, sedimentary clues about aragonite cycling are also rare. As a consequence, aragonite is typically neglected in biogeochemical models used to predict and reconstruct Earth's climate. The present project aims to remedy this by bringing together an international group of scientists with complementary expertise in the taxonomy, ecology and biology of aragonite producers but also in marine ecosystem and geochemical modelling and carbonate chemistry. Together, these experts will synthesize and compile a unique and comprehensive database of pteropods, heteropods and janthinids to unveil the dynamics of the production and export of aragonite shells in the global ocean today and over the past decades.