EFFECTS OF INCREASING PCO2 AND SEA SURFACE TEMPERATURE ON THE DISSOLUTION KINETICS OF MG-CALCITES: A LABORATORY STUDY

Quantifying current and future ocean acidification (OA) impacts on calcifying ecosystems is important, especially for reefs of the Hawaiian archipelago already periodically stressed by coral bleaching events, runoff & pollution, impacts of recreational use & fishing, etc. Since the industrial revolution, average surface ocean pH has decreased by approximately 0.1 pH units, and is expected to decrease further (0.3 to 0.4 pH units) by the end of the 21st century. OA is expected to result in both decreased calcification rates and increased rates of carbonate mineral dissolution; specifically, with regards to the various magnesian calcite (Mg-calcite) calcifiers found in shallow-water reef and other carbonate environments. Along with being the second most abundant carbonate mineral found in carbonate shelf sediments, high Mg-calcite compositions are some of the most soluble carbonate phases commonly found in reef environments due to the high degree of Mg substitution (~18-24 mol % Mg), and are therefore potentially the most susceptible to OA-related dissolution. The dissolution thermodynamics and kinetics of biogenic Mg-calcite phases are not well constrained therefore limiting our ability to predict how they will respond to OA. Quantification of the dissolution rates for high Mg-calcite phases in response to OA, along with determination of sediment mineralogy, is necessary to allow resource managers to model and predict quantitatively the impacts of OA on a variety of coral reef and other carbonate-dominated sedimentary environments. I will present results of laboratory experiments employing batch reactors designed to investigate the solubility, mechanisms, and dissolution rates of the biogenic magnesian calcites Amphiroa rigida and Lithoamnion sp. (commonly found in Hawaii's nearshore carbonate environments) in distilled water and seawater over a range of present day and projected CO2 and T conditions.