Characterization of aragonite and calcite solubility products in seawater using modern CO2 system measurement techniques

University Of South Florida, Tampa FL

Investigators

Abstract

Calcium carbonate is a mineral (solid) produced by economically and environmentally important types of marine organisms (including clams and oysters) for structural support and protection from predation. Calcium carbonate, the structural mineral of coral reefs, is becoming increasingly susceptible to dissolution (chemically-induced decomposition) due to the ocean’s uptake of carbon dioxide from the atmosphere. This project is devoted to improving the accuracy of predictions and interpretations of the stability of different forms of calcium carbonate that are uniquely important to the oceanic ecosystem. The stability or instability of solid calcium carbonate in seawater is routinely evaluated from (a) measurements of the concentrations of dissolved chemical components in collected seawater samples and (b) calculations (mathematical models) of the amounts of dissolved calcium and dissolved carbonate in seawater that are required to prevent solid calcium carbonate from dissolving. The necessary calculations for modeling calcium carbonate stability are generated from experiments in which solid calcium carbonate particles are added to seawater samples and the particles are allowed to dissolve until dissolution ends and no further changes in the particles are observed thereafter. The models of calcium carbonate stability in seawater produced through such experiments are important to the shellfish industry and are important to understanding the natural chemical cycles of carbonate-bearing organisms that serve as food for economically-important marine organisms. Over the past decade, the economic interests of the shellfish industry have stimulated monitoring efforts to assess day-to-day changes in the stability of calcium carbonate in hatchery waters. It is also recognized that models of calcium carbonate stability are important to understanding the availability of very small organisms in the surface ocean that serve as an essential food for early-life-stage salmon. Current models of the stability of solid calcium carbonate in seawater are more than thirty-five years old and were generated using measurement techniques that are considered antiquated by modern standards. Modern state-of-the-art methods will be used in this project to determine the chemical conditions required for stability of solid calcium carbonate in seawater, and these measurements will then be used to create quantitative models of calcium carbonate stability over a wide range of salinities and temperatures. For outreach activities, the scientist plans to communicate results from the study to the public via interactive modules at the St. Petersburg Science Festival and the Oceanography Camp for Girls, as well as prepare a presentation for the St. Petersburg SciCafe series about this project and how it relates to ocean acidification. This research will support the dissertation research of one graduate student and an undergraduate summer intern. This project is a state-of-the-art investigation of the solubility of calcium carbonate (CaCO3), a uniquely important mineral in the global ocean. Weathering of terrestrial CaCO3 minerals adds alkalinity to seawater in the form of bicarbonate (HCO3-) and carbonate [CO3(2-)]. Marine calcifiers at the ocean surface precipitate solid CaCO3 for structural support and protection from predation. The biogenic CaCO3 produced by marine calcifiers settles into and dissolves within deeper waters. These processes, sometimes called the calcium carbonate pump, are important for transferring carbon from the surface ocean to depth. As such, calcium carbonate, in both its solid form and dissolved components, plays an integral role in the global carbon cycle. Assessments of marine CaCO3 solubility are essential for a quantitative understanding and interpretation of transformations between solid CaCO3 and its dissolved components. CaCO3 solubility is quantitatively expressed in terms of the product (Ksp) of the dissolved Ca2+ and CO3(2-)concentrations (mol/kg seawater) in seawater at equilibrium with a specified crystalline polymorph of CaCO3. Ksp is influenced by seawater temperature (T), salinity (S), and pressure (P) and is used to quantify saturation state (Ω), the degree to which seawater is under- or over-saturated with dissolved Ca2+ and CO3(2-). In view of the decreasing saturation states of CaCO3 in the global ocean, a consequence of ocean acidification, accurate quantitative assessments of Ksp are essential. In addition to the importance of Ksp characterizations in models of oceanic carbon cycling, Ksp parameterizations are essential to assessments of water chemistry in the shellfish industry. Over the past decade, the economic interests of shellfish growers have stimulated research into the effects of Ω on shellfish health. CaCO3 solubility is also important to assessing the economics of certain fisheries. Models of calcium carbonate stability are important to understanding the availability of carbonate-bearing organisms in the surface ocean that serve as an essential food for early-life-stage salmon. Current models of the stability of solid calcium carbonate in seawater are more than thirty-five years old and were generated using measurement techniques that are considered antiquated by modern standards. In this project, Ksp will be determined using modern state-of-the-art procedures for measurements of pH, alkalinity, and carbonate ion concentrations. These measurements will then be used to create quantitative models of Ksp over a wide range of salinities and temperatures. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.