Dissolution of Abiogenic and Biogenic Calcium Carbonate under Ocean Acidification Conditions

Abstract

Under ocean acidification conditions, the chemistry of the seawater will change including a decrease in pH, a decrease in carbonate ion concentration and a decrease in the calcium carbonate saturation state of the water (Ω). This has implications for solid marine calcium carbonates including calcifying organisms and carbonate sediments. The dissolution kinetics of marine carbonates are poorly understood, therefore modelling of the future ocean under ocean acidification scenarios is hampered.

The goal of this research was to provide an increased understanding of the kinetics of marine carbonate dissolution, including dependence of the dissolution rate of calcium carbonate mineral phases (calcite, calcite-aragonite, low Mg-calcite) on conditions relevant to ocean acidification, and then to apply this to biogenic samples (Pāua, kina and oyster). The effects of saturation state (Ω), surface area, and temperature were studied. Two methods were refined and used to collect and analyze the dissolution data – a pH-stat method and a pH free-drift method, with manipulation of the carbonate chemistry by addition of NaHCO3 and HCl. A LabVIEW® based program was developed for instrument control and automation and for data acquisition. The empirical equation R = k(1-Ω)n, was used to determine the reaction rates (R), the rate constants (k) and the reaction orders (n) for the each of the mineral phases and shellfish species.

For the pH-stat method, the dissolved mass of carbonate was calculated from HCl consumption; in an open cell configuration the CO2 produced in the dissolution reaction is vented to the atmosphere and any change in pH or alkalinity due to dissolution is restored by acid addition and there is no change in CT (the dissolved inorganic concentration. However, in a closed cell configuration where there is no venting, the calculation procedure must take into account the change in CT, and requires a “correction factor”, (d(CT)/d(HCl)), which is dependent on the initial CT and AT at each stage of the dissolution reaction.

For the pH free-drift method, a numerical simulation technique was developed for easy and accurate estimation of the carbonate concentration at each step in the dissolution reaction.

These chemistry based methods were applied on short time scale experiments (time frames of minutes), which had the advantage of making this work independent of the commonly applied CO2 bubbling method usually used to maintain a constant pCO2 and the application of highly stable electrodes necessary for long scale runs.

The results from the pH-stat and pH free-drift methods are not significantly different, and even close to equilibrium conditions (Ω=1), the dissolution rate determined using the pH free-drift method had an accuracy similar to that of the pH-stat method. For abiogenic calcium carbonate, Iceland spar showed a higher dissolution rate in artificial seawater compared with marble at the same value of Ω. For crushed marble, the dissolution rate constant increases in accordance with a rise in temperature, and a high apparent activation energy for the dissolution reaction was calculated (37.6 kJ mol-1) using the Arrhenius equation (K=Ae-Ea/(RT)), which indicated that dissolution was dominated by surface controlled processes.

The dissolution rate of both crushed and cubic samples decreased with increasing saturation state, and as the loading mass of calcite (a proxy for surface area) increased, the dissolution rate increased. Different reaction orders were obtained in different ranges of saturation state, and this is attributed to the presence of different reaction mechanisms. The history of the sample influenced the dissolution rate, indicating that the kinetics of dissolution depends not only on the chemistry of the solution but also on the surface characteristics of each sample over time.

The study of the dissolution rates of biogenic carbonates focused on three species of particular economic and cultural value to New Zealand. Kina (Sea-urchin - Evechinus Chloroticus) have spines composed of low magnesium calcite; Pāua (New Zealand abalone - Haliotis Iris) have bi-mineral shells made of calcite and aragonite, and Oyster (Tiostrea chilensis) shell is primarily calcite.

All crushed biogenic samples (oyster, sea-urchin spine and Pāua), owing to their complex microstructure and the higher amount of available active surface area, have higher dissolution rates than the abiogenic carbonate samples (marble and Iceland spar). Pāua shells, containing fractions of fine crystalline aragonite in the calcite structure, dissolved faster than the sea-urchin spines composed of low Mg-calcite. That was because grain microstructure complexity overrode the mineral stability and caused a selective dissolution of the more stable mineral. The pure calcite oyster shell dissolved with a rate between that of Pāua and sea-urchin spines.

Marine calcifying species under the impact of ocean acidification have to allocate more energy on shell formation and maintenance processes. This has potential long-term consequences for well-being or survival of some of those species. It has been argued that larval shell growth is solely dependent on seawater saturation state and it effect on the earliest life stages of marine bivalves is the main constriction for successful development into adult populations, and that is one of the most near future threats of ocean acidification to marine organisms.

In subantarctic the water off the south east coast of New Zealand, measured Ω Aragonite ranges from 1.9 to 2.6, and Ω Calcite ranges from 3.0 to 4.1. Saturation state values under 3 makes it difficult for juvenile spices to compose hard shell and generally this condition imposes stress on the calcifier marine organisms. The waters off New Zealand’s South Island are expected to experience the impact of ocean acidification from 2040 onwards.

Our results provide valuable insight into the dissolution rates of marine organisms under acidified seawater conditions and thus also provide perspective on how these organisms may be influenced by future OA.