A quarter of the surface of the Earth is covered by marine sediments rich in calcium carbonates, and their dissolution acts as a giant antacid tablet protecting the ocean against human-made acidification caused by massive CO2 emissions. In the open ocean, calcium carbonates are mainly found in two mineral forms. Calcite, the least soluble form, is widespread at the seafloor. Aragonite, that is secreted by pteropods, also known as sea butterflies, is more soluble and, thus, rarely preserved in marine sediments. How much aragonite is present in the ocean is virtually unknown, as illustrated by published estimates of the fraction of aragonite in calcium carbonate marine production which ranges from 10 to 90%. Thus, marine aragonite cycling is very poorly understood and aragonite is often neglected from biogeochemical models predicting future Earth climates. 

Because of their high solubility, it is generally assumed that aragonite grains entirely dissolve in the upper water column, before entering the deep ocean. Yet, over the past few years, observational evidence that aragonite grains are able to reach the seafloor, even at abyssal depths, has accumulated. If large quantities of aragonite settle at the seafloor, this represents a large, unaccounted, source of alkalinity that buffers the deep ocean and favours the preservation of less soluble calcium carbonate forms. This could have important consequences on predictions of future Earth climates. For instance, since pteropods are particularly vulnerable to ocean acidification, their extinction could suppress the aragonite buffering role at the seafloor and indirectly boost calcite dissolution. ASPERGE is present a three-year research plan that blends global data analyses, model simulations and laboratory experiments, in order to determine the effects of aragonite dissolution at the seafloor, its importance in the global calcium carbonate cycle, and the role it may play in the Anthropocene ocean acidification event.