Aragonite dissolution protects calcite at the seafloor - PubMed (original) (raw)

Aragonite dissolution protects calcite at the seafloor

Olivier Sulpis et al. Nat Commun. 2022.

Abstract

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.

© 2022. The Author(s).

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1

Fig. 1. Dissolution of natural marine CaCO3 grains after a minute in suspension in water.

The top row shows the water saturation state of calcite (a–c) and aragonite (d) while on the bottom row the corresponding calcite (e–g) and aragonite (h) dissolution rates are displayed.

Fig. 2

Fig. 2. Effects of the dissolution of a pteropod shell on the saturation state with respect to calcite across the sediment–water interface.

a Depth profile of the saturation state with respect to calcite. The blue circle represents the bottom-water value. The black depth profile stands for a case without aragonite, the red depth profile represents the situation with aragonite shown on the (b) panel. Each depth profile is computed as the mean amongst all data points within the central 850 μm × 850 μm column, which corresponds to the size of the pteropod shell, plus and minus one standard deviation. The extent of the colored envelope surrounding the mean profiles stands for the standard deviation. b Depth transect of water saturation state with respect to calcite, with contours for three selected saturation state values.

Fig. 3

Fig. 3. Dissolution of calcite grains in a sediment bed capped with a dissolving aragonite pteropod.

The pteropod is shown in a white mesh. Color gradients indicate surface calcite dissolution rates. The white lining represents a saturation state with respect to calcite of unity, i.e., a transition from undersaturation to supersaturation with respect to calcite that is caused by the dissolving aragonite pteropod.

Fig. 4

Fig. 4. Likely main loci of aragonite-based calcite galvanization.

a Map showing in blue the surface area of seafloor that is located below the current calcite saturation depth and the preindustrial calcite compensation depth, i.e., the calcite lysocline; both fields are from ref. . The four yellow stars in the North Pacific correspond to the three sites where ref. observed aragonite in the water column below the aragonite saturation depth and the site where ref. measured pteropod genetic material in bottom waters. b Zonal integral of the seafloor surface area within the calcite lysocline. c Zonal integrals of the surface productions of aragonite (in red) and calcite (in black) in the uppermost 100 m of the surface ocean in ESM2M (solid) and ESM2G (dashed), averaged over 100 yr after a 1000 yr spinup; taken from ref. .

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