Laboratory Observation of the Buffering Effect of Aragonite Dissolution at the Seafloor

Carbon dioxide entering and acidifying the ocean can be neutralized by the dissolution of calcium carbonate, which is mainly found in two mineral forms. Calcite is the more stable form and is often found in deep-sea sediments, whilst aragonite is more soluble and therefore rarely preserved. Recent research shows aragonite may account for a much larger portion of marine calcium carbonate export to the ocean interior via the biological pump than previously thought, and that aragonite does reach the deep sea and seafloor despite rarely being buried. If aragonite is present and dissolving at the seafloor it will raise local pH and calcium and carbonate concentrations, potentially enough to inhibit calcite dissolution, representing a deep-sea, carbonate version of galvanization. Here, we test this hypothesis by simulating aragonite dissolution at the sediment-water interface in the laboratory and measuring its effects on pH using microsensors. We show that the addition of aragonite to calcite sediment, overlain by seawater undersaturated with respect to both minerals, results in an unchanged alkalinity flux out of the dissolving sediment, suggesting a decrease the net dissolution rate of calcite. In combination with a diagenetic model, we show that aragonite dissolution can suppress calcite dissolution in the top millimeters of the seabed, locally leading to calcite precipitation within 1 day. Future research efforts should quantify this galvanization effect in situ, as this process may represent an important component of the marine carbon cycle, assigning a key role to aragonite producers in controlling ocean alkalinity and preserving climatic archives. Carbon dioxide (CO2) concentrations are rising due to human influences and the ocean takes up around a third of these CO2 emissions, causing ocean acidification. This can be counteracted by dissolving calcium carbonate (CaCO3), which neutralizes CO2. CaCO3 makes up the shells of organisms living in surface waters, which sink to the bottom after dying. Most deep-sea sediments are made up of a stable form of CaCO3 called calcite. However, there is increasing evidence that a more soluble form of CaCO3 called aragonite also reaches the seafloor. If aragonite does reach the seafloor, its dissolution could protect the surrounding calcite grains from dissolving. This has been shown in a computational study by Sulpis, Agrawal, et al. (2022), , and is further demonstrated in this laboratory study, where we dissolved aragonite on a calcite bed and measured the surrounding seawater chemistry at sub-millimeter scale. Our study suggests that aragonite not only protects calcite from dissolution, but also leads it to grow. This indicates that aragonite plays an important role in CO2 neutralization and should be included in any representation of the marine carbon cycle. The effects of aragonite dissolution at the sediment-water interface were measured during short-term laboratory experiments Measured porewater pH micro-profiles and a diagenetic model were used to isolate the effects of each reaction on porewater pH Our experiments and simulations suggest that aragonite dissolution, through its buffering effect, slows, and eventually suppresses calcite dissolution