次生碳酸钙氧同位素和clumped同位素温度计的动力学效应研究

The CaCO3-H2O oxygen isotopic thermometer and the newly-developed clumped-isotope thermometer are widely used in paleoclimatic/paleo-environmental studies. A prerequisite of the two thermometers is that CaCO3-H2O system should be in isotopic equilibrium. However, an increasing number of studies have shown that natural CaCO3 often precipitate in chemical and/or isotopic disequilibrium. The disequilibrium can only be understood by examining the kinetics of related basic chemical and isotopic reactions or processes. To understand the kinetic isotope effect, I propose to explore the oxygen- and clumped- isotope compositions of dissolved carbonate species in following three critical kinetic steps in the carbonate system: 1) CO2 degassing, 2) DIC-H2O isotopic exchange, and 3) CaCO3 precipitates. In this proposal, a systematic and long-term monitoring campaign will be carried out in active travertine-depositing system and speleothem system. I expect to reveal a coupled response of the two thermometers to kinetic effects associated with these three steps and to develop a set of criteria for determining disequilibrium. In addition, by comparing the isotopic fractionations among a large variety of samples from these carefully selected natural carbonate systems, I will be able to place a constraint on the applicability of the two thermometers. These natural-system derived results will constitute the basis for extracting useful paleoclimate information from speleothems and travertines, offering what laboratory-scale experiments could not. Moreover, the results from this proposed study will offer potential solutions to several important problems concerning speleothem research such as paleo-temperature reconstruction and controls of water composition or temperature on δ18OCaCO3.

碳酸钙-水氧同位素温度计和最近发展起来的clumped同位素温度计在古气候环境研究中应用广泛。使用两种温度计的前提条件是系统处于同位素平衡状态。而许多研究表明，自然界的碳酸钙沉积系统常处于化学和同位素非平衡状态，研究碳酸钙沉积过程中基元化学反应的同位素动力学效应是理解和校正同位素非平衡的关键。本项目拟以地表钙华沉积系统以及洞穴石笋沉积系统为研究对象，重点讨论CO2脱气、溶解无机碳(DIC)与水同位素交换以及碳酸钙沉积等过程对DIC和碳酸钙氧同位素及clumped同位素组成的影响，并定量描述两种温度计对以上动力学过程的耦合响应。在此基础上，提出判断石笋和钙华等次生碳酸钙沉积系统同位素平衡与否的新准则以及两种温度计的适用条件，为准确解译次生碳酸钙中的古气候环境信息提供可靠的科学依据。本项目的结果也可为解决第四纪陆地温度重建、石笋δ18O值变化的“水-温”之争等重要问题提供科学支撑。

越来越多的研究表明，实验室和自然界许多环境下沉淀的方解石都未与溶液达到同位素分馏平衡。因此，如何判别什么样的环境下方解石结晶时与溶液达到同位素平衡？这对于其氧碳同位素组成（18O和13C）和clumped同位素组成（47）重建沉积时环境的温度和其他环境信息至关重要。另一方面，对于处在同位素非平衡的方解石沉积系统，不同元素同位素之间的关系是由什么因素引起，反映了怎样的环境参数，也是目前稳定同位素领域研究的前沿课题。方解石的沉淀过程涉及到气液固三相反应。其中，CO2脱气和方解石的快速结晶过程均能导致方解石的18O、13C和47偏离平衡值，造成同位素解译的气候变化失真。针对以上问题，2016年至2017年对云南香格里拉白水台和四川黄龙钙华沉积点、贵州夜郎洞滴水点的水样和方解石样品进行系统性的采样分析，结合实验室碳酸钙快速沉淀实验，取得了以下成果：（1）首次论证了CO2快速脱气过程中碳同位素存在巨大的动力学分馏，最高可达30‰，主要受脱气速率控制；（2）首次发现沉积物表面的扩散边界层厚度对方解石的碳同位素组成和元素比（Mg/Ca和Sr/Ca）有重要影响，表现为边界层越厚，13C和Mg/Ca越高，而Sr/Ca越低，该发现很好地解释了洞穴沉积系统和钙华沉积系统碳同位素分馏的差异；（3）发现溶解无机碳（DIC）与H2O的同位素交换速率决定了方解石18O和13C的相互关系，而它们之间的交换速率与温度有关；（4）发现土地利用类型会影响岩溶区地下水的同位素组成。