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

碳酸盐矿物在地球表面或接近地球表面的各种环境中从水溶液中生长。随着它们的生长，它们以对温度、溶液酸度或碱度以及溶液化学敏感的方式从环境中吸收杂质。这种关系使地球科学家能够利用碳酸盐岩来重建地质时期海洋、生物圈和大气的化学变化。虽然这样的重建是基于可靠的理论和实验原理，但许多重要的细节还没有解决，部分原因是不同的环境变量会相互干扰。该项目将进行一系列详细的实验，在实验室中，碳酸盐在极其良好的控制条件下生长，使团队能够确定任何一个环境变量如何在碳酸盐晶体中留下印记。这些结果将提高我们从碳酸盐岩中提取古环境信息的能力，并有可能更好地预测地球将如何应对不断向地球大气中输入的二氧化碳。这项研究将与教学和实践活动相结合，向K-12学生和主修地质学的大四本科生介绍晶体生长和古气候概念。碳酸盐和水之间稳定同位素的分配取决于晶体是在接近平衡的条件下缓慢生长，还是在远离平衡的条件下快速生长。当晶体快速生长或晶体生长是由生物介导时，氧和碳同位素分配会受到生长晶体与溶解无机碳（DIC）之间的反应动力学以及DIC与溶液中H2O之间的反应动力学的影响。基于他们最近在消除这些动力学效应方面的成功，研究团队为该项目设定了四个目标：(1)确定不同盐度如何影响氧和碳同位素的行为；(2)确定可变平衡的DIC池如何在碳酸盐的氧和碳同位素组成中表达；(3)比较方解石和文石两种不同碳酸盐的行为；(4)应用所获得的见解来解释碳酸盐钙华、凝灰岩、珊瑚、脉、结核和沉积物的自然稳定同位素变化。该方法使用最先进的实验设备在固定的pH值和生长速率下生长晶体。然后用质谱法分析晶体和溶液。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.