Aragonite dissolution protects calcite at the seafloor

Results from a new model suggest that a deep-sea, carbonate version of galvanization, in which aragonite sacrifies itself to protect the underlying calcite, could explain the predominance of calcite over aragonite in the sediment record. In the open ocean, calcium carbonates are mainly found in two mineral forms. Calcite, the least soluble, is widespread at the seafloor, while aragonite, the more soluble, is rarely preserved in marine sediments. Despite its greater solubility, research has shown that aragonite, whose contribution to global pelagic calcification could be at par with that of calcite, is able to reach the deep-ocean. If large quantities of aragonite settle and dissolve at the seafloor, this represents a large source of alkalinity that buffers the deep ocean and favours the preservation of less soluble calcite, acting as a deep-sea, carbonate version of galvanization. Here, we investigate the role of aragonite dissolution on the early diagenesis of calcite-rich sediments using a novel 3D, micrometric-scale reactive-transport model combined with 3D, X-ray tomography structures of natural aragonite and calcite shells. Results highlight the important role of diffusive transport in benthic calcium carbonate dissolution, in agreement with recent work. We show that, locally, aragonite fluxes to the seafloor could be sufficient to suppress calcite dissolution in the top layer of the seabed, possibly causing calcite recrystallization. As aragonite producers are particularly vulnerable to ocean acidification, the proposed galvanizing effect of aragonite could be weakened in the future, and calcite dissolution at the sediment-water interface will have to cover a greater share of CO2 neutralization.

Automated summary

Research suggests that a deep-sea carbonate version of galvanization, where aragonite sacrifies itself to protect calcite, could explain the predominance of calcite over aragonite in marine sediments. The dissolution of aragonite at the seafloor provides alkalinity that buffers the deep ocean and favors the preservation of less soluble calcite. However, the proposed galvanizing effect of aragonite may weaken in the future due to ocean acidification.