Collaborative Research: Constraining rates of C-O bond reordering in biogenic calcite: Implications for clumped isotope thermometry

温度是气候的一个核心方面，但重建地质时间尺度上地球表面的温度历史仍然是一个具有挑战性的目标。 利用化石碳酸盐（如贝壳）的氧同位素组成已经取得了很大进展，但这些组成既取决于碳酸盐矿物生长期间的温度，也取决于矿物生长的水的氧同位素组成。 因此，氧同位素古温度计需要估计古沃茨的氧同位素组成，如果这些估计不正确，重建的温度将是错误的。 碳酸盐岩“凝聚同位素”测温法是一种新的测温方法，由于不需要对过去水的同位素组成进行假设，而且该方法能够重建过去的温度和过去水的同位素组成，因此在地球科学界引起了广泛的兴趣。 温度信息并不包含在矿物的整体同位素组成中，而是包含在重同位素碳-13和氧-18彼此成键的优先“聚集”中。 然而，虽然这一特征使该方法很有希望解决古气候学、地质生物学、构造学和岩石学中长期存在的问题，但同样的特征也导致了关于原始同位素信号保存的不便事实：从化学和动力学上来说，在埋藏过程中改变碳-13-氧-18键的丰度比改变体碳或氧同位素组成的丰度更重要。 碳-13-氧-18键的丰度可以通过矿物的简单埋藏加热来改变，该矿物的简单埋藏加热导致碳和氧原子通过称为固态扩散的过程通过矿物晶格迁移。本研究采用实验室和自然实验相结合的方法，以腕足动物壳为研究对象，研究了这种碳氧键重排的动力学。 实验室实验将使用从实验岩石学借来的方法来确定Arkyius参数，从而预测固态C-O键重排的温度依赖性速率。 自然实验将有助于评估实验室实验结果，并将重点放在来自北美的3亿年前的腕足动物化石上。 腕足动物是进行这种研究的理想材料，因为它们广泛用于古气候研究，它们具有近似已知的初始温度和形成时间，它们抗重结晶，并且因为可以比较对比的埋葬历史。 实验室和自然实验的一个主要目标是确定温度-时间域，在该温度-时间域中原始的成团同位素组成可以被保存。 换句话说，研究人员试图回答这样的问题：“在什么样的埋藏温度下，化石壳开始由于固态重排而失去其原始的聚集同位素组成。“拟议的工作将导致至少两名研究生和两名本科生的科学培训。 德克萨斯农工大学（TAMU）和约翰霍普金斯大学（JHU）的研究生将分别访问Perez-Huerta在亚拉巴马大学的实验室，进行电子背散射衍射分析，学生将访问合作机构（TAMU或JHU），学习同位素聚集、电感耦合等离子体质谱、阴极发光和研究中使用的其他技术。 学生们将在国际会议上介绍研究结果，并准备发表结果。 这项工作是定量的，将提供与地球科学的学术和应用方面有关的培训。

Temperature is a central aspect of climate, yet reconstructing the temperature history at Earth's surface over geological timescales has remained a challenging goal. Much progress has been made using the oxygen isotopic compositions of fossil carbonates such as shells, but these compositions depend both on the temperature during growth of the carbonate mineral, and on the oxygen isotopic composition of the water in which the mineral grew. Thus the oxygen isotope paleothermometer requires estimates of the oxygen isotopic composition of ancient waters, and the reconstructed temperatures will be in error if these estimates are incorrect. Carbonate "clumped isotope" thermometry is a new method that has generated wide interest within the geoscience community because it does not require assumptions about past water isotopic compositions, and moreover the method is capable of reconstructing both past temperatures and past water isotopic compositions. The temperature information is contained not in the overall isotopic composition of the mineral, but in the preferential "clumping" of the heavy isotopes carbon-13 and oxygen-18 into bonds with each other. However, while this feature lends the method great promise for solving long-standing questions in paleoclimate, geobiology, tectonics, and petrology, the same feature also leads to an inconvenient truth about preservation of the original isotopic signal: It is far easier, chemically and kinetically, for the abundances of carbon-13 ¬ oxygen-18 bonds to be altered during burial than it is for the bulk carbon- or oxygen-isotopic composition to be altered. The abundances of carbon-13 ¬ oxygen-18 bonds can be altered by simple burial heating of the mineral that causes carbon and oxygen atoms migrate through the mineral lattice through a process called solid-state diffusion. This research investigates the kinetics of such C-O bond reordering using a combination laboratory and natural experiments focusing on brachiopod shells. The laboratory experiments will use methods borrowed from experimental petrology to determine Arrhenius parameters allowing prediction of the temperature-dependent rates of solid state C-O bond reordering. The natural experiments will help to evaluate the laboratory experimental results, and will focus on 300 million-year-old brachiopod fossils from North America. Brachiopods are an ideal material for such a study because they are widely used in paleoclimate studies, they have approximately-known initial temperatures and times of formation, they are resistant to recrystallization, and because contrasting burial histories can be compared. A major goal of the laboratory and natural experiments is to define the temperature - time domain in which original clumped isotope compositions can be preserved. Stated differently, investigators seek to answer questions such as "at what burial temperature does a fossil shell begin to loose its original clumped isotope composition due to solid state reordering." The proposed work will result in the scientific training of at least two graduate students and two undergraduate students. The Texas A&M University (TAMU) and Johns Hopkins University (JHU) graduate students will each visit Perez-Huerta's lab at the University of Alabama to conduct electron backscatter diffraction analysis, and the students will visit the collaborating institution (TAMU or JHU) to learn clumped isotope, inductively coupled plasma mass spectrometry, cathodoluminsecence, and other techniques utilized in the study. The students will present findings at international meetings and prepare results for publication. The work is quantitative in nature and will provide training relevant both to academic and applied aspects of geoscience.