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

由二氧化碳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可用古气候档案，其中最著名的是洞穴。这些洞穴碳酸盐沉积具有重建陆地古气候的巨大潜力，因为它们的同位素组成反映了年平均地表温度，并且可以准确地测定其形成年龄。然而，这种类型的碳酸盐形成涉及各种动力学过程，使其同位素组成的解释变得复杂。这阻碍了这些古气候档案的全部潜力的实现。碳酸盐“块状同位素”温度计的最新发展为根据这些碳酸盐恢复古温度提供了新的机会。与传统的碳酸盐-水氧同位素温度计不同，这种新型的集束式同位素温度计可以确定碳酸盐的形成温度，而不需要假设碳酸盐沉淀出的水的同位素组成。但这种温度计也未能逃脱动力学效应的影响。将这种温度计直接应用于自然洞穴，导致了对其形成温度的系统性高估。这一建议试图通过一系列具有良好特征的实验室实验，系统地确定与二氧化碳脱气引起的碳酸盐形成有关的同位素效应，特别是块状同位素效应。该项目的总体目标是为解释自然界中这类碳酸盐的同位素组成提供一个定量框架，从而能够根据其同位素组成进行可靠的古温度重建。在这个项目的过程中，本科生将接触到最先进的碳水化合物化学测量和稳定同位素分析技术。具体地说，研究人员建议确定(1)不同溶解无机碳物种与水之间聚集的同位素交换的动力学，以及(2)与碳酸氢盐(HCO3-)脱水和脱羟基反应相关的动力学同位素分馏因子--将溶解的HCO3-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块状同位素地球化学和水生碳酸盐化学方面的各自专业知识。使用最先进的碳酸盐溶液化学测量技术，可以将溶液碳酸盐化学的变化与同位素测量结合起来，从而能够详细研究碳酸盐沉淀物的同位素组成的动力学控制。本项目中将要确定的参数是理解碳酸盐水体系同位素体系的基础，并将在未来的许多研究中应用。

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.