Dissolution Kinetics of Biogenic Magnesian Calcites

Abstract:

Ocean acidification (OA) is a serious concern for the health of calcifying ecosystems in the near future. During the past century, surface ocean pH has decreased by ~0.1 pH units, and is expected to decrease further by 0.3-0.4 pH units by the end of this century. The process of OA will likely result in both decreased calcification rates and increased rates of carbonate mineral dissolution, particularly involving the magnesian calcite (Mg-calcite) calcifiers found in shallow-water reef and other carbonate environments. Many Mg-calcite compositions are the most soluble of the carbonate phases commonly found in reef environments (often comprising much of the cementation and structure within a reef), and are therefore potentially the most susceptible to dissolution processes associated with OA. However, the dissolution kinetics of these phases is poorly known, limiting our ability to understand their behavior in nature. Laboratory experiments designed to investigate the mechanisms and dissolution rates of biogenic Mg-calcite mineral phases in distilled water and seawater over a range of CO₂ and T conditions were conducted employing both batch and fluidized-bed reactor systems and using a variety of cleaned and annealed biogenic Mg-calcite phases.

Our initial results have shown that the dissolution rate at 298 K and a pCO₂ of ~350 ppm of the crustose coralline alga Amphiroa rigida (~20 mol% MgCO₃) in seawater undersaturated with respect to this phase is 3.6 µmol g^-1 hr^-1, nearly 50% greater than that under similar conditions for aragonite. This rate and the derived experimental rate law are consistent with the preliminary findings of Walter and Morse (1985). Additional kinetic (and also solubility) data will be presented for the following species: Chiton tuberculatus (~0-4 mol% MgCO₃); Echinometra mathei and/or Lytechinus variegatus (~8-12 mol% MgCO₃); Homotrema rubrum (12-16 mol% MgCO₃); and Lithothamnion sp. (~18-24 mol% MgCO₃). Quantification of the rates of dissolution for a broad range of Mg-calcite phases, along with determination of sediment mineralogy, is necessary to allow managers to model and predict quantitatively the impacts of OA on a variety of coral reef and other carbonate-dominated sedimentary environments.