CAREER: Toward an inorganic calcite reference frame for interpreting the stable isotope composition of biogenic carbonates

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Carbonate minerals grow from aqueous solutions in a wide variety of settings at or near Earth's surface. As they grow, they incorporate impurities from the environment in ways that are sensitive to temperature, solution acidity or alkalinity, and solution chemistry. Such relationships allow Earth scientists to use carbonate rocks for reconstructing changes in the chemistry of the oceans, biosphere, and atmosphere through geologic time. Although such reconstructions are based on sound theoretical and experimental principles, many important details have yet to be worked out, in part because different environmental variables can interfere with one another. This project will carry out a detailed set of experiments where carbonates are grown in the laboratory under extremely well-controlled conditions, enabling the team to determine how any one environmental variable leaves its imprint in a carbonate crystal. The results will improve our ability to extract paleo-environment information from carbonate rocks and could potentially lead to better forecasts of how the Earth will respond to ongoing anthropogenic inputs of carbon dioxide to Earth's atmosphere. The research will be integrated with teaching and hands-on outreach activities that introduce crystal growth and paleoclimate concepts to K-12 students and undergraduate seniors majoring in geology. The partitioning of stable isotopes between carbonates and water depends on whether the crystals grow slowly, under near-equilibrium conditions, or rapidly, under far-from-equilibrium conditions. When crystals grow rapidly or when crystal growth is biologically mediated, as is often the case in nature, oxygen and carbon isotope partitioning can be affected by reaction kinetics between the growing crystal and dissolved inorganic carbon (DIC), and also by reaction kinetics among the DIC species and H2O in solution. Building on their recent successes towards deconvoluting these kinetic effects, the research team has four objectives for the project: (1) determine how varying salinity influences oxygen and carbon isotope behavior, (2) determine how a variably-equilibrated DIC pool is expressed in the oxygen and carbon isotope composition of carbonates, (3) compare the behavior of two different carbonates, calcite and aragonite, and (4) apply the insights gained to interpreting natural stable isotope variations in carbonate travertines, tufas, corals, veins, nodules, and sediments. The approach uses a state-of-the-art experimental apparatus for growing crystals at fixed pH and growth rate. The crystals and solutions are then analyzed using mass spectrometry.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.