Carbonate Formation Induced by CO2 Degassing: Controls on the Isotopic Compositions and Implications for Clumped Isotope Thermometry

由二氧化碳（CO2）脱气引起的碳酸钙形成在自然界中普遍存在，并产生了一些最重要的古气候档案，最值得注意的是洞穴沉积物。这些洞穴碳酸盐沉积具有巨大的潜力，重建陆地古气候，因为它们的同位素组成反映了年平均表面温度和它们的形成年龄可以精确地定年。然而，在这种类型的碳酸盐形成中涉及各种动力学过程，使其同位素组成的解释复杂化。这阻碍了这些古气候档案的全部潜力的实现。最近发展的碳酸盐“凝块同位素”温度计提供了新的机会，重建古温度的基础上，这些碳酸盐。与传统的碳酸盐-水氧同位素温度计不同，这种新型的聚块同位素温度计可以确定碳酸盐形成温度，而无需假设碳酸盐沉淀的水的同位素组成。但是这个温度计并没有逃脱动力学效应的影响。直接将这种温度计应用于天然洞穴沉积物，导致系统高估了它们的形成温度。该建议旨在通过一系列表征良好的实验室实验，系统地确定与CO2脱气诱导的碳酸盐形成相关的同位素效应，特别是聚集同位素效应。该项目的总体目标是提供一个定量框架，用于解释自然界中这种类型的碳酸盐岩的同位素组成，从而能够根据其同位素组成进行可靠的古温度重建。在本项目的过程中，本科生将接触到最先进的水碳酸盐化学测量和稳定同位素分析技术。具体而言，研究人员建议确定（1）不同溶解无机碳物种和水之间的聚集同位素交换动力学，以及（2）与碳酸氢盐（HCO 3-）脱水和脱羟基反应相关的动力学同位素分馏因子-这是将溶解的HCO 3-转化为CO2的关键途径。这项拟议的工作将汇集两名研究人员在聚集同位素地球化学和水生碳酸盐化学方面的各自专门知识。使用最先进的技术进行溶液碳酸盐化学测量将使其可行的配对的变化，在溶液碳酸盐化学与同位素测量，从而允许详细调查的动力学控制的同位素组成的碳酸盐沉淀。本项目所确定的参数对了解碳酸盐岩水相体系的同位素体系学具有重要意义，并将在今后的许多研究中得到应用。

Calcium carbonate formation induced by carbon dioxide (CO2) degassing is ubiquitous in nature and produces some of the most important paleoclimate archives available, most notably speleothems. These cave carbonate deposits hold huge potential for reconstructing terrestrial paleoclimate, because their isotopic compositions reflect mean annual surface temperatures and their formation age can be precisely dated. However, various kinetic processes are involved in this type of carbonate formation, complicating the interpretation of their isotopic compositions. This impedes the realization of the full potential of these paleoclimate archives. Recent development of the carbonate "clumped isotope" thermometer offers new opportunities for reconstructing paleotemperatures based on these carbonates. Unlike the conventional carbonate-water oxygen isotope thermometer, this novel clumped isotope thermometer can determine carbonate formation temperatures without assumptions about the isotopic compositions of the water from which carbonate precipitates. But this thermometer has not escaped the influence of kinetic effects. Direct application of this thermometer to natural speleothems has resulted in systematic overestimations of their formation temperatures. This proposal seeks to systematically determine the isotope effects, especially clumped isotope effects, associated with carbonate formation induced by CO2 degassing, through series of well-characterized laboratory experiments. The overarching goal of this project is to provide a quantitative framework for interpreting the isotopic compositions of this type of carbonates in nature and thus enable robust paleotemperature reconstructions based on their isotope compositions. Undergraduate students will be exposed to the state-of-the-art technologies for aqueous carbonate chemistry measurements and stable isotope analysis during the course of this project.Specifically, the investigators propose to determine (1) the kinetics of the clumped isotope exchange between different dissolved inorganic carbon species and water, and (2) the kinetic isotope fractionation factors associated with bicarbonate (HCO3-) dehydration and dehydroxylation reactions - the key pathways for converting dissolved HCO3- to CO2. This proposed work will bring together the respective expertise of two investigators in clumped isotope geochemistry and aquatic carbonate chemistry. The use of state-of-the-art technologies for solution carbonate chemistry measurements will make it feasible to pair the changes in solution carbonate chemistry with isotope measurements, and thus allow detailed investigation of the kinetic controls on the isotopic compositions of carbonate precipitates. The parameters to be determined in this project are fundamental for understanding the isotope systematics of aqueous carbonate system, and will be applicable in many future studies.